Prof. Dr. Ulrich Heiz
Academic Research Focus
- Cluster Catalysis and Advanced Spectroscopy
- Clusters at the Solid-Liquid Interface
- Spectroscopy and Reactivity of Supported Clusters
- Cluster Structure and Dynamics by Scanning Probe Microscopy
- UHV & ambient pressure Photocatalysis
- Clusters at the Solid-Liquid Interface
- Spectroscopy and Reactivity of Supported Clusters
- Cluster Structure and Dynamics by Scanning Probe Microscopy
- UHV & ambient pressure Photocatalysis
Fields of Application
Photo-(electro)catalysis
Publications
27.
P Petzoldt, L Mengel, S Mackewicz, C A Walenta, M Tschurl, U Heiz
The Role of Defects in the Photocatalytic Conversion of Alcohols on Rutile TiO2(110) Journal Article
In: Acs Catalysis, vol. 15, no. 15, pp. 12859-12869, 2025, ISSN: 2155-5435.
@article{nokey,
title = {The Role of Defects in the Photocatalytic Conversion of Alcohols on Rutile TiO2(110)},
author = {P Petzoldt and L Mengel and S Mackewicz and C A Walenta and M Tschurl and U Heiz},
url = {\<Go to ISI\>://WOS:001529526500001},
doi = {10.1021/acscatal.5c03149},
issn = {2155-5435},
year = {2025},
date = {2025-08-01},
journal = {Acs Catalysis},
volume = {15},
number = {15},
pages = {12859-12869},
abstract = {The role of defects is controversially discussed in photocatalysis. Commonly, they are seen as trap states for photon-generated charge carriers, which either promote charge separation and enhance the activity of the photocatalyst or act as recombination centers and lower the photocatalytic performance of the material. The present work illustrates the crucial role of defects as potential reaction centers in photocatalysis. We show that the photocatalytic activity of a rutile TiO2(110) single crystal toward alcohols in the gas phase can be varied by its degree of reduction. Specific heat treatments of the semiconductor lead to the formation of different concentrations of defect states, in particular bridge-bonded oxygen vacancies (BBOV), to which we assign the role of photoactive sites. Here, the dissociative adsorption of the alcohol and its subsequent photoconversion occurs. Our evaluation of the catalytic methanol photooxidation under different reaction conditions further reveals the importance of alcohol surface diffusion, whose influence on the catalytic activity is rationalized based on a capture zone around each BBOV. If a methanol molecule hits the TiO2 surface in this zone, it can be photoconverted even when it does not land directly on the photoactive site. Finally, an analysis of the return to the dark state of reactant and products upon interrupting the illumination enables us to determine methoxy coverages on the photocatalyst prior to illumination, which also scale with the concentration of BBOVs. We find that methoxies abound on Pt-loaded titania in the dark, which explains the increased product formation in the first seconds of photocatalysis.},
keywords = {},
pubstate = {published},
tppubtype = {article}
}
The role of defects is controversially discussed in photocatalysis. Commonly, they are seen as trap states for photon-generated charge carriers, which either promote charge separation and enhance the activity of the photocatalyst or act as recombination centers and lower the photocatalytic performance of the material. The present work illustrates the crucial role of defects as potential reaction centers in photocatalysis. We show that the photocatalytic activity of a rutile TiO2(110) single crystal toward alcohols in the gas phase can be varied by its degree of reduction. Specific heat treatments of the semiconductor lead to the formation of different concentrations of defect states, in particular bridge-bonded oxygen vacancies (BBOV), to which we assign the role of photoactive sites. Here, the dissociative adsorption of the alcohol and its subsequent photoconversion occurs. Our evaluation of the catalytic methanol photooxidation under different reaction conditions further reveals the importance of alcohol surface diffusion, whose influence on the catalytic activity is rationalized based on a capture zone around each BBOV. If a methanol molecule hits the TiO2 surface in this zone, it can be photoconverted even when it does not land directly on the photoactive site. Finally, an analysis of the return to the dark state of reactant and products upon interrupting the illumination enables us to determine methoxy coverages on the photocatalyst prior to illumination, which also scale with the concentration of BBOVs. We find that methoxies abound on Pt-loaded titania in the dark, which explains the increased product formation in the first seconds of photocatalysis.
26.
G D Zhou, C C Aletsee, A Lemperle, T Rieth, L Mengel, J Y Gao, M Tschurl, U Heiz, I D Sharp
Water Oxidation and Degradation Mechanisms of BiVO4 Photoanodes in Bicarbonate Electrolytes Journal Article
In: Acs Catalysis, vol. 15, no. 15, pp. 13048-13058, 2025, ISSN: 2155-5435.
@article{nokey,
title = {Water Oxidation and Degradation Mechanisms of BiVO4 Photoanodes in Bicarbonate Electrolytes},
author = {G D Zhou and C C Aletsee and A Lemperle and T Rieth and L Mengel and J Y Gao and M Tschurl and U Heiz and I D Sharp},
url = {\<Go to ISI\>://WOS:001530023100001},
doi = {10.1021/acscatal.5c03025},
issn = {2155-5435},
year = {2025},
date = {2025-08-01},
journal = {Acs Catalysis},
volume = {15},
number = {15},
pages = {13048-13058},
abstract = {The photoelectrochemical hydrogen peroxide evolution reaction (HPER) has attracted increasing attention as an environmentally friendly approach to generate a commercially and industrially valuable water oxidation product. BiVO4 photoanodes operated in bicarbonate-containing electrolytes have been shown to offer remarkable performance characteristics for HPER, with HCO3 - serving as a reaction mediator. However, the factors affecting the stability of both the semiconductor photoanode and the aqueous electrolyte remain poorly understood. Here, we investigated BiVO4 photoanodes to quantitatively assess the roles of electrolyte composition, bias potential, and illumination on competitive reaction pathways associated with HPER, oxygen evolution reaction, and photocorrosion. Our results confirm that HCO3 - serves as a highly efficient mediator, leading to rapid hole extraction and near complete suppression of interfacial recombination on BiVO4. In addition, these favorable hole transfer kinetics significantly decrease the rate of photocorrosion, leading to dramatically enhanced stability compared to bicarbonate-free electrolytes. While the elevated pH of unbuffered bicarbonate electrolyte leads to gradual chemical attack of BiVO4, the stability is greatly enhanced in near-neutral buffered bicarbonate electrolytes. Finally, we confirm that HCO3 - is regenerated during the photoanodic reaction, though pH swings during operation in an unbuffered electrolyte can lead to electrolyte instabilities. Overall, we find that BiVO4 photoanodes operating in buffered bicarbonate-containing solutions exhibit significantly enhanced stability and can efficiently drive water oxidation reactions, including HPER, thus providing a route to robust production of high value oxidation products.},
keywords = {},
pubstate = {published},
tppubtype = {article}
}
The photoelectrochemical hydrogen peroxide evolution reaction (HPER) has attracted increasing attention as an environmentally friendly approach to generate a commercially and industrially valuable water oxidation product. BiVO4 photoanodes operated in bicarbonate-containing electrolytes have been shown to offer remarkable performance characteristics for HPER, with HCO3 - serving as a reaction mediator. However, the factors affecting the stability of both the semiconductor photoanode and the aqueous electrolyte remain poorly understood. Here, we investigated BiVO4 photoanodes to quantitatively assess the roles of electrolyte composition, bias potential, and illumination on competitive reaction pathways associated with HPER, oxygen evolution reaction, and photocorrosion. Our results confirm that HCO3 - serves as a highly efficient mediator, leading to rapid hole extraction and near complete suppression of interfacial recombination on BiVO4. In addition, these favorable hole transfer kinetics significantly decrease the rate of photocorrosion, leading to dramatically enhanced stability compared to bicarbonate-free electrolytes. While the elevated pH of unbuffered bicarbonate electrolyte leads to gradual chemical attack of BiVO4, the stability is greatly enhanced in near-neutral buffered bicarbonate electrolytes. Finally, we confirm that HCO3 - is regenerated during the photoanodic reaction, though pH swings during operation in an unbuffered electrolyte can lead to electrolyte instabilities. Overall, we find that BiVO4 photoanodes operating in buffered bicarbonate-containing solutions exhibit significantly enhanced stability and can efficiently drive water oxidation reactions, including HPER, thus providing a route to robust production of high value oxidation products.
25.
K Bertrang, T Hinke, S Kaiser, M Knechtges, F Loi, P Lacovig, M Jahangirzadeh Varjovi, F Esch, A Baraldi, S Tosoni, A Kartouzian, U Heiz
The Interaction of Sub-Monolayer Ta Adatoms and Clusters with Oxygen at the Pt(111) Interface Journal Article
In: The Journal of Physical Chemistry C, vol. 129, no. 13, pp. 6511-6523, 2025, ISSN: 1932-7447.
@article{nokey,
title = {The Interaction of Sub-Monolayer Ta Adatoms and Clusters with Oxygen at the Pt(111) Interface},
author = {K Bertrang and T Hinke and S Kaiser and M Knechtges and F Loi and P Lacovig and M Jahangirzadeh Varjovi and F Esch and A Baraldi and S Tosoni and A Kartouzian and U Heiz},
url = {https://doi.org/10.1021/acs.jpcc.5c00699},
doi = {10.1021/acs.jpcc.5c00699},
issn = {1932-7447},
year = {2025},
date = {2025-04-03},
journal = {The Journal of Physical Chemistry C},
volume = {129},
number = {13},
pages = {6511-6523},
abstract = {The interaction of submonolayer quantities of size-selected and soft-landed Tan (n = 4, 5, 6, 8, 13) clusters with Pt(111) is investigated employing high-resolution X-ray photoelectron spectroscopy (HR-XPS), scanning tunneling microscopy (STM), and density functional theory (DFT) simulations. The deposited clusters are monodispersed and stable under ultrahigh vacuum (UHV) conditions at 40 K. They display a size-specific trend in photoemission spectra, which is reasoned in terms of the distinct in plane coordination of Ta atoms in the clusters. Both the Ta coordination number and distance from the Pt surface influence its Bader charge and, accordingly, the oxidation state of the atoms in the Ta cluster. They already fragment in the presence of low amounts of oxygen and form a common oxidation product observed for all cluster sizes. Based on our observations, we propose an oxidation mechanism in the example of Ta8 clusters, which is closely comparable to the one discussed in gas-phase studies on the oxidation of cationic Ta clusters of similar size. Concomitant to oxidation-induced fragmentation, the agglomeration into Ta-oxide islands with Ta in an oxidation state of +5 is observed. However, the strong interaction with the Pt surface leads to Ta 4f orbital photoemission features that differ from those commonly observed for Ta2O5. Computational insights concerning the structure of the Ta-oxide islands indicate flat agglomerates that agree with STM observations. They suggest distinct Ta 4f photoemission contributions from interfacial and surface-related Ta configurations. The respective HR-XPS spectra display specific core-level shifts as a function of bonding configuration and vicinity to the Pt surface. By annealing at 900 K in UHV, we observe oxygen loss and concomitant intermixing of Ta atoms with the Pt subsurface lattice to which results in the formation of a Ta\textendashPt alloy. These species, Ta-oxide islands, and Ta\textendashPt alloy, can reversibly interconvert by oxidative surface segregation and reductive intermixing.},
keywords = {},
pubstate = {published},
tppubtype = {article}
}
The interaction of submonolayer quantities of size-selected and soft-landed Tan (n = 4, 5, 6, 8, 13) clusters with Pt(111) is investigated employing high-resolution X-ray photoelectron spectroscopy (HR-XPS), scanning tunneling microscopy (STM), and density functional theory (DFT) simulations. The deposited clusters are monodispersed and stable under ultrahigh vacuum (UHV) conditions at 40 K. They display a size-specific trend in photoemission spectra, which is reasoned in terms of the distinct in plane coordination of Ta atoms in the clusters. Both the Ta coordination number and distance from the Pt surface influence its Bader charge and, accordingly, the oxidation state of the atoms in the Ta cluster. They already fragment in the presence of low amounts of oxygen and form a common oxidation product observed for all cluster sizes. Based on our observations, we propose an oxidation mechanism in the example of Ta8 clusters, which is closely comparable to the one discussed in gas-phase studies on the oxidation of cationic Ta clusters of similar size. Concomitant to oxidation-induced fragmentation, the agglomeration into Ta-oxide islands with Ta in an oxidation state of +5 is observed. However, the strong interaction with the Pt surface leads to Ta 4f orbital photoemission features that differ from those commonly observed for Ta2O5. Computational insights concerning the structure of the Ta-oxide islands indicate flat agglomerates that agree with STM observations. They suggest distinct Ta 4f photoemission contributions from interfacial and surface-related Ta configurations. The respective HR-XPS spectra display specific core-level shifts as a function of bonding configuration and vicinity to the Pt surface. By annealing at 900 K in UHV, we observe oxygen loss and concomitant intermixing of Ta atoms with the Pt subsurface lattice to which results in the formation of a Ta–Pt alloy. These species, Ta-oxide islands, and Ta–Pt alloy, can reversibly interconvert by oxidative surface segregation and reductive intermixing.
24.
C C Aletsee, P Neumann, I Chorkendorff, M Tschurl, U Heiz
Tertiary Alcohols as Mechanistic Probes for Photocatalysis: the Gas-Phase Reaction of 2-Methyl-2-Pentanol on Titania P25 in a Microphotoreactor Journal Article
In: ACS Catalysis, vol. 15, no. 3, pp. 2584-2594, 2025.
@article{nokey,
title = {Tertiary Alcohols as Mechanistic Probes for Photocatalysis: the Gas-Phase Reaction of 2-Methyl-2-Pentanol on Titania P25 in a Microphotoreactor},
author = {C C Aletsee and P Neumann and I Chorkendorff and M Tschurl and U Heiz},
url = {https://doi.org/10.1021/acscatal.4c07001},
doi = {10.1021/acscatal.4c07001},
year = {2025},
date = {2025-02-07},
journal = {ACS Catalysis},
volume = {15},
number = {3},
pages = {2584-2594},
abstract = {Despite intense research in heterogeneous photocatalysis, a lack of mechanistic understanding still hinders the rational design of efficient photocatalysts to make them competitive with thermal processes that currently dominate the industry. This study elucidates the underlying mechanism of photoreactions by employing tertiary alcohols as probe molecules on a titania P25 catalyst for the understanding of photocatalytic reactions on a molecular scale. We show that the reactions do not follow the commonly assumed reaction mechanism of separate but coupled redox reactions. Instead, the gas-phase reaction occurs selectively via a homolytic bond cleavage of the long alkyl chain, leading to the formation of the corresponding ketone and an alkane, as exemplified for 2-methyl-2-pentanol at ambient pressure. The alkane stems predominantly from the recombination of the alkyl-moiety with surface hydrogen. Additionally, we demonstrate that the alkyl moiety can also undergo a dimerization reaction forming a long chain alkane, which is facilitated on bare TiO2. The high time-resolution enabled by the used microreactor allowed us to confirm that this side reaction is a higher-order process, which is governed by the alcohol surface coverage on TiO2. The parallels of the observed reaction properties with studies performed on a TiO2(110) single crystal in vacuum reveal that no significant pressure and material gap exists. On the one hand, this strongly suggests that also the reaction mechanism for the conversion of other alcohols must be reconsidered on titania-based photocatalysts and, on the other hand, demonstrates the potential of tertiary alcohols as mechanistic probes in photocatalysis. Moreover, the highly selective reactions of tertiary alcohols may open up alternative routes for chemical synthesis.},
keywords = {},
pubstate = {published},
tppubtype = {article}
}
Despite intense research in heterogeneous photocatalysis, a lack of mechanistic understanding still hinders the rational design of efficient photocatalysts to make them competitive with thermal processes that currently dominate the industry. This study elucidates the underlying mechanism of photoreactions by employing tertiary alcohols as probe molecules on a titania P25 catalyst for the understanding of photocatalytic reactions on a molecular scale. We show that the reactions do not follow the commonly assumed reaction mechanism of separate but coupled redox reactions. Instead, the gas-phase reaction occurs selectively via a homolytic bond cleavage of the long alkyl chain, leading to the formation of the corresponding ketone and an alkane, as exemplified for 2-methyl-2-pentanol at ambient pressure. The alkane stems predominantly from the recombination of the alkyl-moiety with surface hydrogen. Additionally, we demonstrate that the alkyl moiety can also undergo a dimerization reaction forming a long chain alkane, which is facilitated on bare TiO2. The high time-resolution enabled by the used microreactor allowed us to confirm that this side reaction is a higher-order process, which is governed by the alcohol surface coverage on TiO2. The parallels of the observed reaction properties with studies performed on a TiO2(110) single crystal in vacuum reveal that no significant pressure and material gap exists. On the one hand, this strongly suggests that also the reaction mechanism for the conversion of other alcohols must be reconsidered on titania-based photocatalysts and, on the other hand, demonstrates the potential of tertiary alcohols as mechanistic probes in photocatalysis. Moreover, the highly selective reactions of tertiary alcohols may open up alternative routes for chemical synthesis.
23.
J Reich, S Kaiser, A Bourgund, M Krinninger, U Heiz, F Esch, B A J Lechner
Exploring the atomic-scale dynamics of Fe3O4(001) at catalytically relevant temperatures using FastSTM Journal Article
In: Surface Science, vol. 752, pp. 122634, 2025, ISSN: 0039-6028.
@article{nokey,
title = {Exploring the atomic-scale dynamics of Fe3O4(001) at catalytically relevant temperatures using FastSTM},
author = {J Reich and S Kaiser and A Bourgund and M Krinninger and U Heiz and F Esch and B A J Lechner},
url = {https://www.sciencedirect.com/science/article/pii/S0039602824001857},
doi = {https://doi.org/10.1016/j.susc.2024.122634},
issn = {0039-6028},
year = {2025},
date = {2025-02-01},
urldate = {2025-02-01},
journal = {Surface Science},
volume = {752},
pages = {122634},
abstract = {Surfaces and interfaces of functional nanoscale materials are typically highly dynamic when employed at elevated temperatures. Both, lateral surface and vertical bulk exchange diffusion processes set in, which can be relevant for applications such as heterogeneous catalysis. Time-resolved scanning tunneling microscopy (STM) is being pushed to ever faster measurement modes to follow such dynamic phenomena in situ. Here, we present FastSTM movies monitoring a range of atomic-scale dynamics of a prototypical reducible oxide catalyst support, Fe3O4(001), at elevated temperatures. Antiphase domain boundaries between two domains of the reconstructed surface exhibit local mobility from around 350 K, while Fe-rich point defects, in a stable equilibrium with the bulk, appear to diffuse in a peculiar zigzag pattern above 500 K. Finally, exploiting the diffusivity of Fe interstitials, we follow the propagation of step edges in the topmost atomic layer of the Fe3O4(001) surface in an oxygen atmosphere.},
keywords = {},
pubstate = {published},
tppubtype = {article}
}
Surfaces and interfaces of functional nanoscale materials are typically highly dynamic when employed at elevated temperatures. Both, lateral surface and vertical bulk exchange diffusion processes set in, which can be relevant for applications such as heterogeneous catalysis. Time-resolved scanning tunneling microscopy (STM) is being pushed to ever faster measurement modes to follow such dynamic phenomena in situ. Here, we present FastSTM movies monitoring a range of atomic-scale dynamics of a prototypical reducible oxide catalyst support, Fe3O4(001), at elevated temperatures. Antiphase domain boundaries between two domains of the reconstructed surface exhibit local mobility from around 350 K, while Fe-rich point defects, in a stable equilibrium with the bulk, appear to diffuse in a peculiar zigzag pattern above 500 K. Finally, exploiting the diffusivity of Fe interstitials, we follow the propagation of step edges in the topmost atomic layer of the Fe3O4(001) surface in an oxygen atmosphere.
22.
J Pittrich, K Liang, L Dörringer, R Kienberger, U Heiz, A Kartouzian, H Iglev
From molecules to materials: SHG-CD microscopy of structured chiral films Journal Article
In: Applied Surface Science, vol. 680, pp. 161331, 2025, ISSN: 0169-4332.
@article{nokey,
title = {From molecules to materials: SHG-CD microscopy of structured chiral films},
author = {J Pittrich and K Liang and L D\"{o}rringer and R Kienberger and U Heiz and A Kartouzian and H Iglev},
url = {https://www.sciencedirect.com/science/article/pii/S0169433224020464},
doi = {https://doi.org/10.1016/j.apsusc.2024.161331},
issn = {0169-4332},
year = {2025},
date = {2025-01-30},
journal = {Applied Surface Science},
volume = {680},
pages = {161331},
abstract = {The interplay between molecular and structural chirality as a function of local sample morphology determines the nonlinear optical properties of many organic and hybrid organic\textendashinorganic thin films. Here, we used second harmonic generation circular dichroism (SHG-CD) microscopy of thin molecular films of 1,1′-bi-2-naphthol (R-BINOL) as a research model. Our results show that the SHG signal measured at frequencies close to the electronic transition of BINOL molecules is resonantly enhanced by more than an order of magnitude compared to the non-resonant case. The extracted resonant SHG-CD signal is dominated by the chiral response of the molecule. In contrast, structural chirality determines the non-resonant SHG-CD images. We see clear evidence that the interference of the SHG signals caused by the molecular and structural chirality can lead to a decrease in the overall SHG intensity when both SHG signals are out of phase. These findings highlight the intricate relationship between molecular and structural chirality on the one hand and structural morphology on the other hand and pave the way for novel applications by exploiting the chiroptic properties of thin films.},
keywords = {},
pubstate = {published},
tppubtype = {article}
}
The interplay between molecular and structural chirality as a function of local sample morphology determines the nonlinear optical properties of many organic and hybrid organic–inorganic thin films. Here, we used second harmonic generation circular dichroism (SHG-CD) microscopy of thin molecular films of 1,1′-bi-2-naphthol (R-BINOL) as a research model. Our results show that the SHG signal measured at frequencies close to the electronic transition of BINOL molecules is resonantly enhanced by more than an order of magnitude compared to the non-resonant case. The extracted resonant SHG-CD signal is dominated by the chiral response of the molecule. In contrast, structural chirality determines the non-resonant SHG-CD images. We see clear evidence that the interference of the SHG signals caused by the molecular and structural chirality can lead to a decrease in the overall SHG intensity when both SHG signals are out of phase. These findings highlight the intricate relationship between molecular and structural chirality on the one hand and structural morphology on the other hand and pave the way for novel applications by exploiting the chiroptic properties of thin films.
21.
J Reich, S Kaiser, U Heiz, J-D Grunwaldt, M M Kappes, F Esch, B A J Lechner
A Critical View on the Quantification of Model Catalyst Activity Journal Article
In: Topics in Catalysis, vol. 67, no. 13, pp. 880-891, 2024, ISSN: 1572-9028.
@article{nokey,
title = {A Critical View on the Quantification of Model Catalyst Activity},
author = {J Reich and S Kaiser and U Heiz and J-D Grunwaldt and M M Kappes and F Esch and B A J Lechner},
url = {https://doi.org/10.1007/s11244-024-01920-0},
doi = {10.1007/s11244-024-01920-0},
issn = {1572-9028},
year = {2024},
date = {2024-02-19},
urldate = {2024-02-19},
journal = {Topics in Catalysis},
volume = {67},
number = {13},
pages = {880-891},
abstract = {The conversion of reactants, reaction rate referred to catalyst mass, and turnover frequency (TOF) are values typically employed to compare the activity of different catalysts. However, experimental parameters have to be chosen carefully when systems of different complexity are compared. In order to characterize UHV-based model systems, we use a highly sensitive sniffer setup which allows us to investigate the catalytic activity by combining three different measurement modes: temperature-programmed desorption, continuous flow, and pulsed-reactivity experiments. In this article, we explore the caveats of quantifying catalytic activity in UHV on the well-studied and highly defined reference system of CO oxidation on Pt(111), which we later compare to the same reaction on Pt19 clusters deposited on Fe3O4(001). We demonstrate that we can apply fast heating ramps for TOF quantification, thus inducing as little sintering as possible in the metastable clusters. By changing the reactant ratio, we find transient reactivity effects that influence the TOF, which should be kept in mind when comparing catalysts. In addition, the TOF also depends on the surface coverage that itself is a function of temperature and pressure. At a constant reactant ratio, in the absence of transient effects, however, the TOF scales linearly with total pressure over the entire measured temperature range from 200 to 700 K since the reaction rate is dependent on both reactant partial pressures with temperature-dependent reaction order. When comparing the maximum TOF at this particular reactant ratio, we find a 1.6 times higher maximum TOF for Pt19/Fe3O4(001) than for Pt(111). In addition, pulsed-reactivity measurements help identify purely reaction-limited regimes and allow for a more detailed investigation of limiting reactants over the whole temperature range.},
keywords = {},
pubstate = {published},
tppubtype = {article}
}
The conversion of reactants, reaction rate referred to catalyst mass, and turnover frequency (TOF) are values typically employed to compare the activity of different catalysts. However, experimental parameters have to be chosen carefully when systems of different complexity are compared. In order to characterize UHV-based model systems, we use a highly sensitive sniffer setup which allows us to investigate the catalytic activity by combining three different measurement modes: temperature-programmed desorption, continuous flow, and pulsed-reactivity experiments. In this article, we explore the caveats of quantifying catalytic activity in UHV on the well-studied and highly defined reference system of CO oxidation on Pt(111), which we later compare to the same reaction on Pt19 clusters deposited on Fe3O4(001). We demonstrate that we can apply fast heating ramps for TOF quantification, thus inducing as little sintering as possible in the metastable clusters. By changing the reactant ratio, we find transient reactivity effects that influence the TOF, which should be kept in mind when comparing catalysts. In addition, the TOF also depends on the surface coverage that itself is a function of temperature and pressure. At a constant reactant ratio, in the absence of transient effects, however, the TOF scales linearly with total pressure over the entire measured temperature range from 200 to 700 K since the reaction rate is dependent on both reactant partial pressures with temperature-dependent reaction order. When comparing the maximum TOF at this particular reactant ratio, we find a 1.6 times higher maximum TOF for Pt19/Fe3O4(001) than for Pt(111). In addition, pulsed-reactivity measurements help identify purely reaction-limited regimes and allow for a more detailed investigation of limiting reactants over the whole temperature range.
20.
K Liang, F Ristow, K Li, J Pittrich, N Fehn, L Dörringer, U Heiz, R Kienberger, G Pescitelli, H Iglev, A Kartouzian
Negative Nonlinear CD–ee Dependence in Polycrystalline BINOL Thin Films Journal Article
In: Journal of the American Chemical Society, 2023, ISSN: 0002-7863.
@article{nokey,
title = {Negative Nonlinear CD\textendashee Dependence in Polycrystalline BINOL Thin Films},
author = {K Liang and F Ristow and K Li and J Pittrich and N Fehn and L D\"{o}rringer and U Heiz and R Kienberger and G Pescitelli and H Iglev and A Kartouzian},
url = {https://doi.org/10.1021/jacs.3c12253},
doi = {10.1021/jacs.3c12253},
issn = {0002-7863},
year = {2023},
date = {2023-12-13},
journal = {Journal of the American Chemical Society},
abstract = {Generally, the relationship between the observed circular dichroism and the enantiomeric excess in chiral systems (CD\textendashee dependence) is linear. While positive nonlinear behavior has often been reported in the past, examples of negative nonlinear (NN) behavior in CD\textendashee dependence are rare and not well understood. Here, we present a strong NN CD\textendashee dependence within polycrystalline thin films of BINOL by using second-harmonic-generation circular dichroism (SHG-CD) and commercial CD spectroscopy studies. Theoretical calculations, microscopy, and FTIR studies are employed to further clarify the underlying cause of this observation. This behavior is attributed to the changing supramolecular chirality of the system. Systems exhibiting NN CD\textendashee dependence hold promise for highly accurate enantiomeric excess characterization, which is essential for the refinement of enantio-separating and -purifying processes in pharmaceuticals, asymmetric catalysis, and chiral sensing. Our findings suggest that a whole class of single-species systems, i.e., racemate crystals, might possess NN CD\textendashee dependence and thus provide us a vast playground to better understand and exploit this phenomenon.},
keywords = {},
pubstate = {published},
tppubtype = {article}
}
Generally, the relationship between the observed circular dichroism and the enantiomeric excess in chiral systems (CD–ee dependence) is linear. While positive nonlinear behavior has often been reported in the past, examples of negative nonlinear (NN) behavior in CD–ee dependence are rare and not well understood. Here, we present a strong NN CD–ee dependence within polycrystalline thin films of BINOL by using second-harmonic-generation circular dichroism (SHG-CD) and commercial CD spectroscopy studies. Theoretical calculations, microscopy, and FTIR studies are employed to further clarify the underlying cause of this observation. This behavior is attributed to the changing supramolecular chirality of the system. Systems exhibiting NN CD–ee dependence hold promise for highly accurate enantiomeric excess characterization, which is essential for the refinement of enantio-separating and -purifying processes in pharmaceuticals, asymmetric catalysis, and chiral sensing. Our findings suggest that a whole class of single-species systems, i.e., racemate crystals, might possess NN CD–ee dependence and thus provide us a vast playground to better understand and exploit this phenomenon.
19.
M Krinninger, N Bock, S Kaiser, J Reich, T Bruhm, F Haag, F Allegretti, U Heiz, K Köhler, B A J Lechner, F Esch
On-Surface Carbon Nitride Growth from Polymerization of 2,5,8-Triazido-s-heptazine Journal Article
In: Chemistry of Materials, vol. 35, no. 17, pp. 6762-6770, 2023, ISSN: 0897-4756.
@article{nokey,
title = {On-Surface Carbon Nitride Growth from Polymerization of 2,5,8-Triazido-s-heptazine},
author = {M Krinninger and N Bock and S Kaiser and J Reich and T Bruhm and F Haag and F Allegretti and U Heiz and K K\"{o}hler and B A J Lechner and F Esch},
url = {https://doi.org/10.1021/acs.chemmater.3c01030},
doi = {10.1021/acs.chemmater.3c01030},
issn = {0897-4756},
year = {2023},
date = {2023-08-23},
urldate = {2023-08-23},
journal = {Chemistry of Materials},
volume = {35},
number = {17},
pages = {6762-6770},
abstract = {Carbon nitrides have recently come into focus for photo- and thermal catalysis, both as support materials for metal nanoparticles as well as photocatalysts themselves. While many approaches for the synthesis of three-dimensional carbon nitride materials are available, only top-down approaches by exfoliation of powders lead to thin-film flakes of this inherently two-dimensional material. Here, we describe an in situ on-surface synthesis of monolayer 2D carbon nitride films as a first step toward precise combination with other 2D materials. Starting with a single monomer precursor, we show that 2,5,8-triazido-s-heptazine can be evaporated intact, deposited on a single crystalline Au(111) or graphite support, and activated via azide decomposition and subsequent coupling to form a covalent polyheptazine network. We demonstrate that the activation can occur in three pathways, via electrons (X-ray illumination), via photons (UV illumination), and thermally. Our work paves the way to coat materials with extended carbon nitride networks that are, as we show, stable under ambient conditions.},
keywords = {},
pubstate = {published},
tppubtype = {article}
}
Carbon nitrides have recently come into focus for photo- and thermal catalysis, both as support materials for metal nanoparticles as well as photocatalysts themselves. While many approaches for the synthesis of three-dimensional carbon nitride materials are available, only top-down approaches by exfoliation of powders lead to thin-film flakes of this inherently two-dimensional material. Here, we describe an in situ on-surface synthesis of monolayer 2D carbon nitride films as a first step toward precise combination with other 2D materials. Starting with a single monomer precursor, we show that 2,5,8-triazido-s-heptazine can be evaporated intact, deposited on a single crystalline Au(111) or graphite support, and activated via azide decomposition and subsequent coupling to form a covalent polyheptazine network. We demonstrate that the activation can occur in three pathways, via electrons (X-ray illumination), via photons (UV illumination), and thermally. Our work paves the way to coat materials with extended carbon nitride networks that are, as we show, stable under ambient conditions.
18.
M Eder, M Tschurl, U Heiz
In: The Journal of Physical Chemistry Letters, vol. 14, no. 26, pp. 6193-6201, 2023.
@article{nokey,
title = {Toward a Comprehensive Understanding of Photocatalysis: What Systematic Studies and Alcohol Surface Chemistry on TiO2(110) Have to Offer for Future Developments},
author = {M Eder and M Tschurl and U Heiz},
url = {https://doi.org/10.1021/acs.jpclett.3c00504},
doi = {10.1021/acs.jpclett.3c00504},
year = {2023},
date = {2023-06-29},
journal = {The Journal of Physical Chemistry Letters},
volume = {14},
number = {26},
pages = {6193-6201},
abstract = {Heterogeneous photocatalytic systems are usually described based on electrochemistry, which the vast majority of interpretations and strategies for optimizing photocatalysts rely on. Charge carrier dynamics are usually in the spotlight, whereas the surface chemistry of the photocatalyst is neglected. This is unjustified, because studies on alcohol photoreforming on metal-decorated rutile single crystals revealed that the electrochemical reaction model is not generally applicable. Hence, many photocatalytic reactions may proceed in a different manner and the thermal chemistry needs to be accounted for. The new mechanism is particularly relevant for reactions in gaseous environments in the absence of solvated ionic species. Here, we compare both mechanisms and highlight their differences and consequences for photocatalysis. Based on alcohol photochemistry, we demonstrate the importance of thermal reactions in photocatalytic mechanisms and the relevance of systematic studies in different environments for a holistic understanding of photocatalysis.},
keywords = {},
pubstate = {published},
tppubtype = {article}
}
Heterogeneous photocatalytic systems are usually described based on electrochemistry, which the vast majority of interpretations and strategies for optimizing photocatalysts rely on. Charge carrier dynamics are usually in the spotlight, whereas the surface chemistry of the photocatalyst is neglected. This is unjustified, because studies on alcohol photoreforming on metal-decorated rutile single crystals revealed that the electrochemical reaction model is not generally applicable. Hence, many photocatalytic reactions may proceed in a different manner and the thermal chemistry needs to be accounted for. The new mechanism is particularly relevant for reactions in gaseous environments in the absence of solvated ionic species. Here, we compare both mechanisms and highlight their differences and consequences for photocatalysis. Based on alcohol photochemistry, we demonstrate the importance of thermal reactions in photocatalytic mechanisms and the relevance of systematic studies in different environments for a holistic understanding of photocatalysis.
17.
S Kaiser, J Plansky, M Krinninger, A Shavorskiy, S Zhu, U Heiz, F Esch, B A J Lechner
Does Cluster Encapsulation Inhibit Sintering? Stabilization of Size-Selected Pt Clusters on Fe3O4(001) by SMSI Journal Article
In: ACS Catalysis, vol. 13, no. 9, pp. 6203-6213, 2023.
@article{nokey,
title = {Does Cluster Encapsulation Inhibit Sintering? Stabilization of Size-Selected Pt Clusters on Fe3O4(001) by SMSI},
author = {S Kaiser and J Plansky and M Krinninger and A Shavorskiy and S Zhu and U Heiz and F Esch and B A J Lechner},
url = {https://doi.org/10.1021/acscatal.3c00448},
doi = {10.1021/acscatal.3c00448},
year = {2023},
date = {2023-04-21},
urldate = {2023-04-21},
journal = {ACS Catalysis},
volume = {13},
number = {9},
pages = {6203-6213},
abstract = {The metastability of supported metal nanoparticles limits their application in heterogeneous catalysis at elevated temperatures due to their tendency to sinter. One strategy to overcome these thermodynamic limits on reducible oxide supports is encapsulation via strong metal\textendashsupport interaction (SMSI). While annealing-induced encapsulation is a well-explored phenomenon for extended nanoparticles, it is as yet unknown whether the same mechanisms hold for subnanometer clusters, where concomitant sintering and alloying might play a significant role. In this article, we explore the encapsulation and stability of size-selected Pt5, Pt10, and Pt19 clusters deposited on Fe3O4(001). In a multimodal approach using temperature-programmed desorption (TPD), X-ray photoelectron spectroscopy (XPS), and scanning tunneling microscopy (STM), we demonstrate that SMSI indeed leads to the formation of a defective, FeO-like conglomerate encapsulating the clusters. By stepwise annealing up to 1023 K, we observe the succession of encapsulation, cluster coalescence, and Ostwald ripening, resulting in square-shaped crystalline Pt particles, independent of the initial cluster size. The respective sintering onset temperatures scale with the cluster footprint and thus size. Remarkably, while small encapsulated clusters can still diffuse as a whole, atom detachment and thus Ostwald ripening are successfully suppressed up to 823 K, i.e., 200 K above the H\"{u}ttig temperature that indicates the thermodynamic stability limit.},
keywords = {},
pubstate = {published},
tppubtype = {article}
}
The metastability of supported metal nanoparticles limits their application in heterogeneous catalysis at elevated temperatures due to their tendency to sinter. One strategy to overcome these thermodynamic limits on reducible oxide supports is encapsulation via strong metal–support interaction (SMSI). While annealing-induced encapsulation is a well-explored phenomenon for extended nanoparticles, it is as yet unknown whether the same mechanisms hold for subnanometer clusters, where concomitant sintering and alloying might play a significant role. In this article, we explore the encapsulation and stability of size-selected Pt5, Pt10, and Pt19 clusters deposited on Fe3O4(001). In a multimodal approach using temperature-programmed desorption (TPD), X-ray photoelectron spectroscopy (XPS), and scanning tunneling microscopy (STM), we demonstrate that SMSI indeed leads to the formation of a defective, FeO-like conglomerate encapsulating the clusters. By stepwise annealing up to 1023 K, we observe the succession of encapsulation, cluster coalescence, and Ostwald ripening, resulting in square-shaped crystalline Pt particles, independent of the initial cluster size. The respective sintering onset temperatures scale with the cluster footprint and thus size. Remarkably, while small encapsulated clusters can still diffuse as a whole, atom detachment and thus Ostwald ripening are successfully suppressed up to 823 K, i.e., 200 K above the Hüttig temperature that indicates the thermodynamic stability limit.
16.
C C Aletsee, D Hochfilzer, A Kwiatkowski, M Becherer, J Kibsgaard, I Chorkendorff, M Tschurl, U Heiz
In: Review of Scientific Instruments, vol. 94, no. 3, pp. 033909, 2023.
@article{nokey,
title = {A re-useable microreactor for dynamic and sensitive photocatalytic measurements: Exemplified by the photoconversion of ethanol on Pt-loaded titania P25},
author = {C C Aletsee and D Hochfilzer and A Kwiatkowski and M Becherer and J Kibsgaard and I Chorkendorff and M Tschurl and U Heiz},
url = {https://aip.scitation.org/doi/abs/10.1063/5.0134287},
doi = {10.1063/5.0134287},
year = {2023},
date = {2023-03-23},
journal = {Review of Scientific Instruments},
volume = {94},
number = {3},
pages = {033909},
abstract = {Despite numerous advancements in synthesizing photoactive materials, the evaluation of their catalytic performance remains challenging since their fabrication often involves tedious strategies, yielding only low quantities in the μ-gram scale. In addition, these model catalysts exhibit different forms, such as powders or film(-like) structures grown on various supporting materials. Herein, we present a versatile gas phase μ-photoreactor, compatible with different catalyst morphologies, which is, in contrast to existing systems, re-openable and \textendashuseable, allowing not only post-characterization of the photocatalytic material but also enabling catalyst screening studies in short experimental time intervals. Sensitive and time-resolved reaction monitoring at ambient pressure is realized by a lid-integrated capillary, transmitting the entire gas flow from the reactor chamber to a quadrupole mass spectrometer. Due to the microfabrication of the lid from borosilicate as base material, 88% of the geometrical area can be illuminated by a light source, further enhancing sensitivity. Gas dependent flow rates through the capillary were experimentally determined to be 1015\textendash1016 molecules s−1, and in combination with a reactor volume of 10.5 μl, this results in residence times below 40 s. Furthermore, the reactor volume can easily be altered by adjusting the height of the polymeric sealing material. The successful operation of the reactor is demonstrated by selective ethanol oxidation over Pt-loaded TiO2 (P25), which serves to exemplify product analysis from dark-illumination difference spectra.},
keywords = {},
pubstate = {published},
tppubtype = {article}
}
Despite numerous advancements in synthesizing photoactive materials, the evaluation of their catalytic performance remains challenging since their fabrication often involves tedious strategies, yielding only low quantities in the μ-gram scale. In addition, these model catalysts exhibit different forms, such as powders or film(-like) structures grown on various supporting materials. Herein, we present a versatile gas phase μ-photoreactor, compatible with different catalyst morphologies, which is, in contrast to existing systems, re-openable and –useable, allowing not only post-characterization of the photocatalytic material but also enabling catalyst screening studies in short experimental time intervals. Sensitive and time-resolved reaction monitoring at ambient pressure is realized by a lid-integrated capillary, transmitting the entire gas flow from the reactor chamber to a quadrupole mass spectrometer. Due to the microfabrication of the lid from borosilicate as base material, 88% of the geometrical area can be illuminated by a light source, further enhancing sensitivity. Gas dependent flow rates through the capillary were experimentally determined to be 1015–1016 molecules s−1, and in combination with a reactor volume of 10.5 μl, this results in residence times below 40 s. Furthermore, the reactor volume can easily be altered by adjusting the height of the polymeric sealing material. The successful operation of the reactor is demonstrated by selective ethanol oxidation over Pt-loaded TiO2 (P25), which serves to exemplify product analysis from dark-illumination difference spectra.
15.
S Kaiser, J Plansky, M Krinninger, A Shavorskiy, S Zhu, U Heiz, F Esch, B A J Lechner
Does cluster encapsulation inhibit sintering? Stabilization of size-selected Pt clusters on Fe $ _3 $ O $ _4 $(001) by SMSI Journal Article
In: arXiv preprint arXiv:2301.10845, 2023.
@article{nokey,
title = {Does cluster encapsulation inhibit sintering? Stabilization of size-selected Pt clusters on Fe $ _3 $ O $ _4 $(001) by SMSI},
author = {S Kaiser and J Plansky and M Krinninger and A Shavorskiy and S Zhu and U Heiz and F Esch and B A J Lechner},
url = {https://arxiv.org/abs/2301.10845},
doi = {https://doi.org/10.48550/arXiv.2301.10845},
year = {2023},
date = {2023-01-25},
urldate = {2023-01-25},
journal = {arXiv preprint arXiv:2301.10845},
abstract = {The metastability of supported metal nanoparticles limits their application in heterogeneous catalysis at elevated temperatures due to their tendency to sinter. One strategy to overcome these thermodynamic limits on reducible oxide supports is encapsulation via strong metal-support interaction (SMSI). While annealing-induced encapsulation is a well-explored phenomenon for extended nanoparticles, it is as yet unknown whether the same mechanisms hold for sub-nanometer clusters, where concomitant sintering and alloying might play a significant role. In this article, we explore the encapsulation and stability of size-selected Pt5, Pt10 and Pt19 clusters deposited on Fe3O4(001). In a multimodal approach using temperature-programmed desorption (TPD), x-ray photoelectron spectroscopy (XPS) and scanning tunneling microscopy (STM), we demonstrate that SMSI indeed leads to the formation of a defective, FeO-like conglomerate encapsulating the clusters. By stepwise annealing up to 1023 K, we observe the succession of encapsulation, cluster coalescence and Ostwald ripening, resulting in square-shaped crystalline Pt particles, independent of the initial cluster sizes. The respective sintering onset temperatures scale with the cluster footprint and thus size. Remarkably, while small encapsulated clusters can still diffuse as a whole, atom detachment and thus Ostwald ripening are successfully suppressed up to 823 K, i.e. 200 K above the H\"{u}ttig temperature that indicates the thermodynamic stability limit.},
keywords = {},
pubstate = {published},
tppubtype = {article}
}
The metastability of supported metal nanoparticles limits their application in heterogeneous catalysis at elevated temperatures due to their tendency to sinter. One strategy to overcome these thermodynamic limits on reducible oxide supports is encapsulation via strong metal-support interaction (SMSI). While annealing-induced encapsulation is a well-explored phenomenon for extended nanoparticles, it is as yet unknown whether the same mechanisms hold for sub-nanometer clusters, where concomitant sintering and alloying might play a significant role. In this article, we explore the encapsulation and stability of size-selected Pt5, Pt10 and Pt19 clusters deposited on Fe3O4(001). In a multimodal approach using temperature-programmed desorption (TPD), x-ray photoelectron spectroscopy (XPS) and scanning tunneling microscopy (STM), we demonstrate that SMSI indeed leads to the formation of a defective, FeO-like conglomerate encapsulating the clusters. By stepwise annealing up to 1023 K, we observe the succession of encapsulation, cluster coalescence and Ostwald ripening, resulting in square-shaped crystalline Pt particles, independent of the initial cluster sizes. The respective sintering onset temperatures scale with the cluster footprint and thus size. Remarkably, while small encapsulated clusters can still diffuse as a whole, atom detachment and thus Ostwald ripening are successfully suppressed up to 823 K, i.e. 200 K above the Hüttig temperature that indicates the thermodynamic stability limit.
14.
J Lengyel, N Levin, M Ončák, K Jakob, M Tschurl, U Heiz
Direct Coupling of Methane and Carbon Dioxide on Tantalum Cluster Cations Journal Article
In: Chemistry – A European Journal, vol. n/a, no. n/a, 2022, ISSN: 0947-6539.
@article{nokey,
title = {Direct Coupling of Methane and Carbon Dioxide on Tantalum Cluster Cations},
author = {J Lengyel and N Levin and M On\v{c}\'{a}k and K Jakob and M Tschurl and U Heiz},
url = {https://chemistry-europe.onlinelibrary.wiley.com/doi/abs/10.1002/chem.202203259},
doi = {https://doi.org/10.1002/chem.202203259},
issn = {0947-6539},
year = {2022},
date = {2022-11-20},
journal = {Chemistry \textendash A European Journal},
volume = {n/a},
number = {n/a},
abstract = {Understanding molecular-scale reaction mechanisms is crucial for the design of modern catalysts with industrial prospect. Through joint experimental and computational studies, we investigate the direct coupling reaction of CH4 and CO2, two abundant greenhouse gases, mediated by Ta1,4+ ions to form larger oxygenated hydrocarbons. Coherent with proposed elementary steps, we expose products of CH4 dehydrogenation [Ta1,4CH2]+ to CO2 in a ring electrode ion trap. Product analysis and reaction kinetics indicate a predisposition of the tetramers for C\textendashO coupling with a conversion to products of CH2O, whereas atomic cations enable C\textendashC coupling yielding CH2CO. Many of the experimental findings are supported by thermodynamic computations, connecting structure, electronic properties, and catalyst function. Moreover, the study of bare Ta1,4+ compounds indicates that methane dehydrogenation is a significant initial step in the direct coupling reaction, enabling new, yet unknown reaction pathways.},
keywords = {},
pubstate = {published},
tppubtype = {article}
}
Understanding molecular-scale reaction mechanisms is crucial for the design of modern catalysts with industrial prospect. Through joint experimental and computational studies, we investigate the direct coupling reaction of CH4 and CO2, two abundant greenhouse gases, mediated by Ta1,4+ ions to form larger oxygenated hydrocarbons. Coherent with proposed elementary steps, we expose products of CH4 dehydrogenation [Ta1,4CH2]+ to CO2 in a ring electrode ion trap. Product analysis and reaction kinetics indicate a predisposition of the tetramers for C–O coupling with a conversion to products of CH2O, whereas atomic cations enable C–C coupling yielding CH2CO. Many of the experimental findings are supported by thermodynamic computations, connecting structure, electronic properties, and catalyst function. Moreover, the study of bare Ta1,4+ compounds indicates that methane dehydrogenation is a significant initial step in the direct coupling reaction, enabling new, yet unknown reaction pathways.
13.
P Petzoldt, M Eder, S Mackewicz, M Blum, T Kratky, S Günther, M Tschurl, U Heiz, B A J Lechner
In: The Journal of Physical Chemistry C, vol. 126, no. 38, pp. 16127-16139, 2022, ISSN: 1932-7447.
@article{nokey,
title = {Tuning Strong Metal\textendashSupport Interaction Kinetics on Pt-Loaded TiO2(110) by Choosing the Pressure: A Combined Ultrahigh Vacuum/Near-Ambient Pressure XPS Study},
author = {P Petzoldt and M Eder and S Mackewicz and M Blum and T Kratky and S G\"{u}nther and M Tschurl and U Heiz and B A J Lechner},
url = {https://doi.org/10.1021/acs.jpcc.2c03851},
doi = {10.1021/acs.jpcc.2c03851},
issn = {1932-7447},
year = {2022},
date = {2022-09-29},
urldate = {2022-09-29},
journal = {The Journal of Physical Chemistry C},
volume = {126},
number = {38},
pages = {16127-16139},
abstract = {Pt catalyst particles on reducible oxide supports often change their activity significantly at elevated temperatures due to the strong metal\textendashsupport interaction (SMSI), which induces the formation of an encapsulation layer around the noble metal particles. However, the impact of oxidizing and reducing treatments at elevated pressures on this encapsulation layer remains controversial, partly due to the “pressure gap” between surface science studies and applied catalysis. In the present work, we employ synchrotron-based near-ambient pressure X-ray photoelectron spectroscopy (NAP-XPS) to study the effect of O2 and H2 on the SMSI-state of well-defined Pt/TiO2(110) catalysts at pressures of up to 0.1 Torr. By tuning the O2 pressure, we can either selectively oxidize the TiO2 support or both the support and the Pt particles. Catalyzed by metallic Pt, the encapsulating oxide overlayer grows rapidly in 1 × 10\textendash5 Torr O2, but orders of magnitude less effectively at higher O2 pressures, where Pt is in an oxidic state. While the oxidation/reduction of Pt particles is reversible, they remain embedded in the support once encapsulation has occurred.},
keywords = {},
pubstate = {published},
tppubtype = {article}
}
Pt catalyst particles on reducible oxide supports often change their activity significantly at elevated temperatures due to the strong metal–support interaction (SMSI), which induces the formation of an encapsulation layer around the noble metal particles. However, the impact of oxidizing and reducing treatments at elevated pressures on this encapsulation layer remains controversial, partly due to the “pressure gap” between surface science studies and applied catalysis. In the present work, we employ synchrotron-based near-ambient pressure X-ray photoelectron spectroscopy (NAP-XPS) to study the effect of O2 and H2 on the SMSI-state of well-defined Pt/TiO2(110) catalysts at pressures of up to 0.1 Torr. By tuning the O2 pressure, we can either selectively oxidize the TiO2 support or both the support and the Pt particles. Catalyzed by metallic Pt, the encapsulating oxide overlayer grows rapidly in 1 × 10–5 Torr O2, but orders of magnitude less effectively at higher O2 pressures, where Pt is in an oxidic state. While the oxidation/reduction of Pt particles is reversible, they remain embedded in the support once encapsulation has occurred.
12.
P Petzoldt, M Eder, S Mackewicz, M Blum, T Kratky, S Günther, M Tschurl, U Heiz, B A J Lechner
In: The Journal of Physical Chemistry C, vol. 126, no. 38, pp. 16127-16139, 2022, ISSN: 1932-7447.
@article{nokey,
title = {Tuning Strong Metal\textendashSupport Interaction Kinetics on Pt-Loaded TiO2(110) by Choosing the Pressure: A Combined Ultrahigh Vacuum/Near-Ambient Pressure XPS Study},
author = {P Petzoldt and M Eder and S Mackewicz and M Blum and T Kratky and S G\"{u}nther and M Tschurl and U Heiz and B A J Lechner},
url = {https://doi.org/10.1021/acs.jpcc.2c03851},
doi = {10.1021/acs.jpcc.2c03851},
issn = {1932-7447},
year = {2022},
date = {2022-09-16},
urldate = {2022-09-16},
journal = {The Journal of Physical Chemistry C},
volume = {126},
number = {38},
pages = {16127-16139},
abstract = {Pt catalyst particles on reducible oxide supports often change their activity significantly at elevated temperatures due to the strong metal\textendashsupport interaction (SMSI), which induces the formation of an encapsulation layer around the noble metal particles. However, the impact of oxidizing and reducing treatments at elevated pressures on this encapsulation layer remains controversial, partly due to the “pressure gap” between surface science studies and applied catalysis. In the present work, we employ synchrotron-based near-ambient pressure X-ray photoelectron spectroscopy (NAP-XPS) to study the effect of O2 and H2 on the SMSI-state of well-defined Pt/TiO2(110) catalysts at pressures of up to 0.1 Torr. By tuning the O2 pressure, we can either selectively oxidize the TiO2 support or both the support and the Pt particles. Catalyzed by metallic Pt, the encapsulating oxide overlayer grows rapidly in 1 × 10\textendash5 Torr O2, but orders of magnitude less effectively at higher O2 pressures, where Pt is in an oxidic state. While the oxidation/reduction of Pt particles is reversible, they remain embedded in the support once encapsulation has occurred.},
keywords = {},
pubstate = {published},
tppubtype = {article}
}
Pt catalyst particles on reducible oxide supports often change their activity significantly at elevated temperatures due to the strong metal–support interaction (SMSI), which induces the formation of an encapsulation layer around the noble metal particles. However, the impact of oxidizing and reducing treatments at elevated pressures on this encapsulation layer remains controversial, partly due to the “pressure gap” between surface science studies and applied catalysis. In the present work, we employ synchrotron-based near-ambient pressure X-ray photoelectron spectroscopy (NAP-XPS) to study the effect of O2 and H2 on the SMSI-state of well-defined Pt/TiO2(110) catalysts at pressures of up to 0.1 Torr. By tuning the O2 pressure, we can either selectively oxidize the TiO2 support or both the support and the Pt particles. Catalyzed by metallic Pt, the encapsulating oxide overlayer grows rapidly in 1 × 10–5 Torr O2, but orders of magnitude less effectively at higher O2 pressures, where Pt is in an oxidic state. While the oxidation/reduction of Pt particles is reversible, they remain embedded in the support once encapsulation has occurred.
11.
F Ristow, K Liang, J Pittrich, J Scheffel, N Fehn, R Kienberger, U Heiz, A Kartouzian, H Iglev
Large-area SHG-CD probe intrinsic chirality in polycrystalline films Journal Article
In: Journal of Materials Chemistry C, vol. 10, no. 35, pp. 12715-12723, 2022, ISSN: 2050-7526.
@article{nokey,
title = {Large-area SHG-CD probe intrinsic chirality in polycrystalline films},
author = {F Ristow and K Liang and J Pittrich and J Scheffel and N Fehn and R Kienberger and U Heiz and A Kartouzian and H Iglev},
url = {http://dx.doi.org/10.1039/D2TC01700H},
doi = {10.1039/D2TC01700H},
issn = {2050-7526},
year = {2022},
date = {2022-08-01},
journal = {Journal of Materials Chemistry C},
volume = {10},
number = {35},
pages = {12715-12723},
abstract = {We used second harmonic generation (SHG) spectroscopy to study the chiroptical properties of R-, S- and racemic (RAC-) 1,1′-bi-2-naphthol (BINOL) films with various thicknesses, incidence angles and degrees of crystallization in the film. The SHG intensity measured at 337 nm increases for thicker films and upon crystallization, while the extracted SHG-circular dichroism (SHG-CD) shows two different regimes depending on the size of the investigated sample area. The data measured at small beam areas, compared to the supramolecular domain size, show strong variation of SHG-CD values depending on the local crystalline structure. In contrast, the anisotropy values measured for beam areas larger than the domain size are almost independent of local sample morphology, film thickness and incidence angle. The SHG-CD values change their sign upon sample flipping or going from R- to S-BINOL. Most interestingly, the observed SHG-CD almost coincides with the value reported for molecular monolayers. Our study reveals that SHG-CD measured with large beam areas can nullify the in-plane anisotropy by averaging over many domains, and thus directly probes intrinsic chirality of the sample. Molecule- and enantiospecific SHG-CD values for BINOL were obtained: ±1.06 for R- and S-BINOL, and 0.01 for racemic BINOL.},
keywords = {},
pubstate = {published},
tppubtype = {article}
}
We used second harmonic generation (SHG) spectroscopy to study the chiroptical properties of R-, S- and racemic (RAC-) 1,1′-bi-2-naphthol (BINOL) films with various thicknesses, incidence angles and degrees of crystallization in the film. The SHG intensity measured at 337 nm increases for thicker films and upon crystallization, while the extracted SHG-circular dichroism (SHG-CD) shows two different regimes depending on the size of the investigated sample area. The data measured at small beam areas, compared to the supramolecular domain size, show strong variation of SHG-CD values depending on the local crystalline structure. In contrast, the anisotropy values measured for beam areas larger than the domain size are almost independent of local sample morphology, film thickness and incidence angle. The SHG-CD values change their sign upon sample flipping or going from R- to S-BINOL. Most interestingly, the observed SHG-CD almost coincides with the value reported for molecular monolayers. Our study reveals that SHG-CD measured with large beam areas can nullify the in-plane anisotropy by averaging over many domains, and thus directly probes intrinsic chirality of the sample. Molecule- and enantiospecific SHG-CD values for BINOL were obtained: ±1.06 for R- and S-BINOL, and 0.01 for racemic BINOL.
10.
M Eder, C Courtois, P Petzoldt, S Mackewicz, M Tschurl, U Heiz
Size and Coverage Effects of Ni and Pt Co-Catalysts in the Photocatalytic Hydrogen Evolution from Methanol on TiO2(110) Journal Article
In: ACS Catalysis, pp. 9579-9588, 2022.
@article{nokey,
title = {Size and Coverage Effects of Ni and Pt Co-Catalysts in the Photocatalytic Hydrogen Evolution from Methanol on TiO2(110)},
author = {M Eder and C Courtois and P Petzoldt and S Mackewicz and M Tschurl and U Heiz},
url = {https://doi.org/10.1021/acscatal.2c02230},
doi = {10.1021/acscatal.2c02230},
year = {2022},
date = {2022-07-21},
journal = {ACS Catalysis},
pages = {9579-9588},
abstract = {In the past decade, hydrogen evolution from photocatalytic alcohol oxidation on metal-loaded TiO2 has emerged as an active research field. While the presence of a metal cluster co-catalyst is crucial as a H2 recombination center, size and coverage effects on the catalyst performance are not yet comprehensively understood. To some extent, this is due to the fact that common deposition methods do not allow for an independent change in size and coverage, which can be overcome by the use of cluster sources and the deposition of size-selected clusters. This study compares size-selected Ni and Pt clusters as co-catalysts on a TiO2(110) single crystal and the resulting size- and coverage-dependent effects in the photocatalytic hydrogen evolution from alcohols in ultrahigh vacuum (UHV). Larger clusters and higher coverages of Ni enhance the product formation rate, although deactivation over time occurs. In contrast, Pt co-catalysts exhibit a stable and higher activity and size-specific effects have to be taken into account. While H2 evolution is improved by a higher concentration of Pt clusters, an increase in the metal content by the deposition of larger particles can even be detrimental to the performance of the photocatalyst. The acquired overall mechanistic picture is corroborated by H2 formation kinetics from mass spectrometric data. Consequently, for some metals, size effects are relevant for improving the catalytic performance, while for other co-catalyst materials, merely the coverage is decisive. The elucidation of different size and coverage dependencies represents an important step toward a rational catalyst design for photocatalytic hydrogen evolution.},
keywords = {},
pubstate = {published},
tppubtype = {article}
}
In the past decade, hydrogen evolution from photocatalytic alcohol oxidation on metal-loaded TiO2 has emerged as an active research field. While the presence of a metal cluster co-catalyst is crucial as a H2 recombination center, size and coverage effects on the catalyst performance are not yet comprehensively understood. To some extent, this is due to the fact that common deposition methods do not allow for an independent change in size and coverage, which can be overcome by the use of cluster sources and the deposition of size-selected clusters. This study compares size-selected Ni and Pt clusters as co-catalysts on a TiO2(110) single crystal and the resulting size- and coverage-dependent effects in the photocatalytic hydrogen evolution from alcohols in ultrahigh vacuum (UHV). Larger clusters and higher coverages of Ni enhance the product formation rate, although deactivation over time occurs. In contrast, Pt co-catalysts exhibit a stable and higher activity and size-specific effects have to be taken into account. While H2 evolution is improved by a higher concentration of Pt clusters, an increase in the metal content by the deposition of larger particles can even be detrimental to the performance of the photocatalyst. The acquired overall mechanistic picture is corroborated by H2 formation kinetics from mass spectrometric data. Consequently, for some metals, size effects are relevant for improving the catalytic performance, while for other co-catalyst materials, merely the coverage is decisive. The elucidation of different size and coverage dependencies represents an important step toward a rational catalyst design for photocatalytic hydrogen evolution.
9.
N Levin, J T Margraf, J Lengyel, K Reuter, M Tschurl, U Heiz
CO2-Activation by size-selected tantalum cluster cations (Ta1–16+): thermalization governing reaction selectivity Journal Article
In: Physical Chemistry Chemical Physics, vol. 24, no. 4, pp. 2623-2629, 2022, ISSN: 1463-9076.
@article{nokey,
title = {CO2-Activation by size-selected tantalum cluster cations (Ta1\textendash16+): thermalization governing reaction selectivity},
author = {N Levin and J T Margraf and J Lengyel and K Reuter and M Tschurl and U Heiz},
url = {http://dx.doi.org/10.1039/D1CP04469A},
doi = {10.1039/D1CP04469A},
issn = {1463-9076},
year = {2022},
date = {2022-01-06},
urldate = {2022-01-06},
journal = {Physical Chemistry Chemical Physics},
volume = {24},
number = {4},
pages = {2623-2629},
abstract = {The reactions of tantalum cluster cations of different sizes toward carbon dioxide are studied in an ion trap under multi-collisional conditions. For all sizes studied, consecutive reactions with several CO2 molecules are observed. This reveals two different pathways, namely oxide formation and the pickup of an entire molecule. Supported by calculations of the thermochemistry of TanO+ formation upon reaction with CO2, changes in the branching ratios at a particular cluster size are related to heat effects due to the vibrational heat capacity of the clusters and the exothermicity of the reaction.},
keywords = {},
pubstate = {published},
tppubtype = {article}
}
The reactions of tantalum cluster cations of different sizes toward carbon dioxide are studied in an ion trap under multi-collisional conditions. For all sizes studied, consecutive reactions with several CO2 molecules are observed. This reveals two different pathways, namely oxide formation and the pickup of an entire molecule. Supported by calculations of the thermochemistry of TanO+ formation upon reaction with CO2, changes in the branching ratios at a particular cluster size are related to heat effects due to the vibrational heat capacity of the clusters and the exothermicity of the reaction.
8.
S Kaiser, F Maleki, K Zhang, W Harbich, U Heiz, S Tosoni, B A J Lechner, G Pacchioni, F Esch
Cluster Catalysis with Lattice Oxygen: Tracing Oxygen Transport from a Magnetite (001) Support onto Small Pt Clusters Journal Article
In: ACS Catalysis, vol. 11, no. 15, pp. 9519–9529, 2021.
@article{,
title = {Cluster Catalysis with Lattice Oxygen: Tracing Oxygen Transport from a Magnetite (001) Support onto Small Pt Clusters},
author = {S Kaiser and F Maleki and K Zhang and W Harbich and U Heiz and S Tosoni and B A J Lechner and G Pacchioni and F Esch},
url = {https://pubs.acs.org/doi/10.1021/acscatal.1c01451},
year = {2021},
date = {2021-07-16},
urldate = {2021-07-16},
journal = {ACS Catalysis},
volume = {11},
number = {15},
pages = {9519\textendash9529},
keywords = {},
pubstate = {published},
tppubtype = {article}
}
7.
M Eder, C Courtois, T Kratky, S Günther, M Tschurl, U Heiz
Nickel clusters on TiO2(110): thermal chemistry and photocatalytic hydrogen evolution of methanol Journal Article
In: Catalysis Science & Technology, vol. 10, no. 22, pp. 7630-7639, 2020, ISSN: 2044-4753.
@article{nokey,
title = {Nickel clusters on TiO2(110): thermal chemistry and photocatalytic hydrogen evolution of methanol},
author = {M Eder and C Courtois and T Kratky and S G\"{u}nther and M Tschurl and U Heiz},
url = {http://dx.doi.org/10.1039/D0CY01465F},
doi = {10.1039/D0CY01465F},
issn = {2044-4753},
year = {2020},
date = {2020-09-09},
journal = {Catalysis Science \& Technology},
volume = {10},
number = {22},
pages = {7630-7639},
abstract = {In heterogeneous photocatalysis, noble metals such as Au, Pt, or Pd are most commonly used as co-catalysts to facilitate H2 evolution, yet their costs are problematic for applications on a large scale. In this work, we show that the cheaper, more abundant transition metal nickel as co-catalyst material reacts accordingly, when being deposited as small clusters onto rutile TiO2. Different to noble metal systems the photocatalysts undergo photocorrosion, depicted in a declining activity during the photoreforming of methanol. The reaction being performed in an ultra-high vacuum environment allows for a more detailed elucidation of the deactivation processes. Supported by reactivity studies under different conditions, Auger electron spectroscopy reveals that coking of the clusters occurs, while nickel oxide formation is not observed. The study thus shows that nickel co-catalysts are indeed prospective systems for the photocatalytic hydrogen evolution reaction, similar to platinum clusters, but instead may also feature unexpected photon-driven deactivation pathways.},
keywords = {},
pubstate = {published},
tppubtype = {article}
}
In heterogeneous photocatalysis, noble metals such as Au, Pt, or Pd are most commonly used as co-catalysts to facilitate H2 evolution, yet their costs are problematic for applications on a large scale. In this work, we show that the cheaper, more abundant transition metal nickel as co-catalyst material reacts accordingly, when being deposited as small clusters onto rutile TiO2. Different to noble metal systems the photocatalysts undergo photocorrosion, depicted in a declining activity during the photoreforming of methanol. The reaction being performed in an ultra-high vacuum environment allows for a more detailed elucidation of the deactivation processes. Supported by reactivity studies under different conditions, Auger electron spectroscopy reveals that coking of the clusters occurs, while nickel oxide formation is not observed. The study thus shows that nickel co-catalysts are indeed prospective systems for the photocatalytic hydrogen evolution reaction, similar to platinum clusters, but instead may also feature unexpected photon-driven deactivation pathways.
6.
C Courtois, C A Walenta, M Tschurl, U Heiz, C M Friend
Regulating Photochemical Selectivity with Temperature: Isobutanol on TiO2(110) Journal Article
In: Journal of the American Chemical Society, vol. 142, no. 30, pp. 13072-13080, 2020, ISSN: 0002-7863.
@article{,
title = {Regulating Photochemical Selectivity with Temperature: Isobutanol on TiO2(110)},
author = {C Courtois and C A Walenta and M Tschurl and U Heiz and C M Friend},
url = {\<Go to ISI\>://WOS:000557854400021},
doi = {10.1021/jacs.0c04411},
issn = {0002-7863},
year = {2020},
date = {2020-07-29},
urldate = {2020-07-29},
journal = {Journal of the American Chemical Society},
volume = {142},
number = {30},
pages = {13072-13080},
abstract = {Selective photocatalytic transformations of chemicals derived from biomass, such as isobutanol, have been long envisioned for a sustainable chemical production. A strong temperature dependence in the reaction selectivity is found for isobutanol photo-oxidation on rutile TiO2(110). The strong temperature dependence is attributed to competition between thermal desorption of the primary photoproduct and secondary photochemical steps. The aldehyde, isobutanal, is the primary photoproduct of isobutanol. At room temperature, isobutanal is obtained selectively from photo-oxidation because of rapid thermal desorption. In contrast, secondary photo-oxidation of isobutanal to propane dominates at lower temperature (240 K) due to the persistence of isobutanal on the surface after it is formed. The byproduct of isobutanal photo-oxidation is CO, which is evolved at higher temperature as a consequence of thermal decomposition of an intermediate, such as formate. The photo-oxidation to isobutanal proceeds after thermally induced isobutoxy formation. These results have strong implications for controlling the selectivity of photochemical processes more generally, in that, selectivity is governed by competition of desorption vs secondary photoreaction of products. This competition can be exploited to design photocatalytic processes to favor specific chemical transformations of organic molecules.},
keywords = {},
pubstate = {published},
tppubtype = {article}
}
Selective photocatalytic transformations of chemicals derived from biomass, such as isobutanol, have been long envisioned for a sustainable chemical production. A strong temperature dependence in the reaction selectivity is found for isobutanol photo-oxidation on rutile TiO2(110). The strong temperature dependence is attributed to competition between thermal desorption of the primary photoproduct and secondary photochemical steps. The aldehyde, isobutanal, is the primary photoproduct of isobutanol. At room temperature, isobutanal is obtained selectively from photo-oxidation because of rapid thermal desorption. In contrast, secondary photo-oxidation of isobutanal to propane dominates at lower temperature (240 K) due to the persistence of isobutanal on the surface after it is formed. The byproduct of isobutanal photo-oxidation is CO, which is evolved at higher temperature as a consequence of thermal decomposition of an intermediate, such as formate. The photo-oxidation to isobutanal proceeds after thermally induced isobutoxy formation. These results have strong implications for controlling the selectivity of photochemical processes more generally, in that, selectivity is governed by competition of desorption vs secondary photoreaction of products. This competition can be exploited to design photocatalytic processes to favor specific chemical transformations of organic molecules.
5.
C Courtois, M Eder, S L Kollmannsberger, M Tschurl, C A Walenta, U Heiz
Origin of Poisoning in Methanol Photoreforming on TiO2(110): The Importance of Thermal Back-Reaction Steps in Photocatalysis Journal Article
In: Acs Catalysis, vol. 10, no. 14, pp. 7747-7752, 2020, ISSN: 2155-5435.
@article{,
title = {Origin of Poisoning in Methanol Photoreforming on TiO2(110): The Importance of Thermal Back-Reaction Steps in Photocatalysis},
author = {C Courtois and M Eder and S L Kollmannsberger and M Tschurl and C A Walenta and U Heiz},
url = {\<Go to ISI\>://WOS:000551549800025},
doi = {10.1021/acscatal.0c01615},
issn = {2155-5435},
year = {2020},
date = {2020-07-17},
urldate = {2020-07-17},
journal = {Acs Catalysis},
volume = {10},
number = {14},
pages = {7747-7752},
abstract = {Alcohol photoreforming on titania represents a perfect model system for elucidating fundamental processes in the heterogeneous photocatalysis of semiconductors. One important but open question is the origin of poisoning during the photoreaction of primary alcohols on a bare, reduced rutile TiO2(110) crystal under ultrahigh vacuum conditions. By comparing the photocatalytic properties of methanol and 2-methyl-2-pentanol, it is demonstrated that the fading activity in methanol photoreforming does not originate from the often-assigned increase of trap states for photon-generated charge carriers. Instead, we attribute the apparent catalyst poisoning to an increased rate of thermal back reactions, particularly to that of the photochemical oxidation step. While overall back reactions are generally considered in photocatalysis, back reactions of individual steps are largely neglected so far. Our work shows that their inclusion in the reaction scheme is inevitable for the comprehensive modeling of photocatalytic processes.},
keywords = {},
pubstate = {published},
tppubtype = {article}
}
Alcohol photoreforming on titania represents a perfect model system for elucidating fundamental processes in the heterogeneous photocatalysis of semiconductors. One important but open question is the origin of poisoning during the photoreaction of primary alcohols on a bare, reduced rutile TiO2(110) crystal under ultrahigh vacuum conditions. By comparing the photocatalytic properties of methanol and 2-methyl-2-pentanol, it is demonstrated that the fading activity in methanol photoreforming does not originate from the often-assigned increase of trap states for photon-generated charge carriers. Instead, we attribute the apparent catalyst poisoning to an increased rate of thermal back reactions, particularly to that of the photochemical oxidation step. While overall back reactions are generally considered in photocatalysis, back reactions of individual steps are largely neglected so far. Our work shows that their inclusion in the reaction scheme is inevitable for the comprehensive modeling of photocatalytic processes.
4.
C A Walenta, C Courtois, S L Kollmannsberger, M Eder, M Tschurl, U Heiz
In: Acs Catalysis, vol. 10, no. 7, pp. 4080-4091, 2020, ISSN: 2155-5435.
@article{,
title = {Surface Species in Photocatalytic Methanol Reforming on Pt/TiO2(110): Learning from Surface Science Experiments for Catalytically Relevant Conditions},
author = {C A Walenta and C Courtois and S L Kollmannsberger and M Eder and M Tschurl and U Heiz},
url = {\<Go to ISI\>://WOS:000526395000011},
doi = {10.1021/acscatal.0c00260},
issn = {2155-5435},
year = {2020},
date = {2020-04-03},
urldate = {2020-04-03},
journal = {Acs Catalysis},
volume = {10},
number = {7},
pages = {4080-4091},
abstract = {Photocatalytic hydrogen evolution from methanol is a standard test reaction for photocatalyst materials. Surprisingly, the exact chemical mechanism is still widely discussed in the literature. In order to disentangle photochemical from thermal reaction steps and gain insights on the atomic level, we use a Pt cluster-loaded TiO2(110) photocatalyst in very well-defined environments. Using Auger electron spectroscopy, temperature-programmed desorption/reaction, isotopic labeling, and isothermal photoreactions, it is possible to identify the surface species present on the catalyst under photocatalytic conditions. Furthermore, an initial conditioning of the photocatalyst is observed and attributed to thermal dehydrogenation of methanol to CO species on the cluster. The analysis of the isothermal photoreactions reveals that the photo-oxidation kinetics are not significantly affected by cocatalyst loading. The observed conversion and product distribution of formaldehyde and methyl formate can be rationalized with kinetic parameters gained from the bare TiO2(110) crystal. The work leads to a detailed mechanistic understanding of the surface species and paves the way for an educated microkinetic modeling approach, which may be extended to a variety of noble metal cocatalysts and other TiO2 modifications.},
keywords = {},
pubstate = {published},
tppubtype = {article}
}
Photocatalytic hydrogen evolution from methanol is a standard test reaction for photocatalyst materials. Surprisingly, the exact chemical mechanism is still widely discussed in the literature. In order to disentangle photochemical from thermal reaction steps and gain insights on the atomic level, we use a Pt cluster-loaded TiO2(110) photocatalyst in very well-defined environments. Using Auger electron spectroscopy, temperature-programmed desorption/reaction, isotopic labeling, and isothermal photoreactions, it is possible to identify the surface species present on the catalyst under photocatalytic conditions. Furthermore, an initial conditioning of the photocatalyst is observed and attributed to thermal dehydrogenation of methanol to CO species on the cluster. The analysis of the isothermal photoreactions reveals that the photo-oxidation kinetics are not significantly affected by cocatalyst loading. The observed conversion and product distribution of formaldehyde and methyl formate can be rationalized with kinetic parameters gained from the bare TiO2(110) crystal. The work leads to a detailed mechanistic understanding of the surface species and paves the way for an educated microkinetic modeling approach, which may be extended to a variety of noble metal cocatalysts and other TiO2 modifications.
3.
C A Walenta, M Tschurl, U Heiz
Introducing catalysis in photocatalysis: What can be understood from surface science studies of alcohol photoreforming on TiO2 Journal Article
In: Journal of Physics-Condensed Matter, vol. 31, no. 47, 2019, ISSN: 0953-8984.
@article{,
title = {Introducing catalysis in photocatalysis: What can be understood from surface science studies of alcohol photoreforming on TiO2},
author = {C A Walenta and M Tschurl and U Heiz},
url = {\<Go to ISI\>://WOS:000482603900001},
doi = {10.1088/1361-648X/ab351a},
issn = {0953-8984},
year = {2019},
date = {2019-08-23},
urldate = {2019-08-23},
journal = {Journal of Physics-Condensed Matter},
volume = {31},
number = {47},
keywords = {},
pubstate = {published},
tppubtype = {article}
}
2.
C Courtois, M Eder, K Schnabl, C A Walenta, M Tschurl, U Heiz
Reactions in the Photocatalytic Conversion of Tertiary Alcohols on Rutile TiO2(110) Journal Article
In: Angewandte Chemie International Edition, vol. 58, no. 40, pp. 14255-14259, 2019, ISSN: 1433-7851.
@article{nokey,
title = {Reactions in the Photocatalytic Conversion of Tertiary Alcohols on Rutile TiO2(110)},
author = {C Courtois and M Eder and K Schnabl and C A Walenta and M Tschurl and U Heiz},
url = {https://onlinelibrary.wiley.com/doi/abs/10.1002/anie.201907917},
doi = {https://doi.org/10.1002/anie.201907917},
issn = {1433-7851},
year = {2019},
date = {2019-08-07},
journal = {Angewandte Chemie International Edition},
volume = {58},
number = {40},
pages = {14255-14259},
abstract = {Abstract According to textbooks, tertiary alcohols are inert towards oxidation. The photocatalysis of tertiary alcohols under highly defined vacuum conditions on a titania single crystal reveals unexpected and new reactions, which can be described as disproportionation into an alkane and the respective ketone. In contrast to primary and secondary alcohols, in tertiary alcohols the absence of an α-H leads to a C−C-bond cleavage instead of the common abstraction of hydrogen. Surprisingly, bonds to methyl groups are not cleaved when the alcohol exhibits longer alkyl chains in the α-position to the hydroxyl group. The presence of platinum loadings not only increases the reaction rate but also opens up a new reaction channel: the formation of molecular hydrogen and a long-chain alkane resulting from recombination of two alkyl moieties. This work demonstrates that new synthetic routes may become possible by introducing photocatalytic reaction steps in which the co-catalysts may also play a decisive role.},
keywords = {},
pubstate = {published},
tppubtype = {article}
}
Abstract According to textbooks, tertiary alcohols are inert towards oxidation. The photocatalysis of tertiary alcohols under highly defined vacuum conditions on a titania single crystal reveals unexpected and new reactions, which can be described as disproportionation into an alkane and the respective ketone. In contrast to primary and secondary alcohols, in tertiary alcohols the absence of an α-H leads to a C−C-bond cleavage instead of the common abstraction of hydrogen. Surprisingly, bonds to methyl groups are not cleaved when the alcohol exhibits longer alkyl chains in the α-position to the hydroxyl group. The presence of platinum loadings not only increases the reaction rate but also opens up a new reaction channel: the formation of molecular hydrogen and a long-chain alkane resulting from recombination of two alkyl moieties. This work demonstrates that new synthetic routes may become possible by introducing photocatalytic reaction steps in which the co-catalysts may also play a decisive role.
1.
A Bourgund, B A J Lechner, M Meier, C Franchini, G S Parkinson, U Heiz, F Esch
Influence of Local Defects on the Dynamics of O-H Bond Breaking and Formation on a Magnetite Surface Journal Article
In: Journal of Physical Chemistry C, vol. 123, no. 32, pp. 19742-19747, 2019, ISSN: 1932-7447.
@article{,
title = {Influence of Local Defects on the Dynamics of O-H Bond Breaking and Formation on a Magnetite Surface},
author = {A Bourgund and B A J Lechner and M Meier and C Franchini and G S Parkinson and U Heiz and F Esch},
url = {\<Go to ISI\>://WOS:000481568900054},
doi = {10.1021/acs.jpcc.9b05547},
issn = {1932-7447},
year = {2019},
date = {2019-07-17},
urldate = {2019-07-17},
journal = {Journal of Physical Chemistry C},
volume = {123},
number = {32},
pages = {19742-19747},
keywords = {},
pubstate = {published},
tppubtype = {article}
}