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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 O2and H2on the SMSI-state of well-defined Pt/TiO2(110) catalysts at pressures of up to 0.1 Torr. By tuning the O2pressure, we can either selectively oxidize the TiO2support or both the support and the Pt particles. Catalyzed by metallic Pt, the encapsulating oxide overlayer grows rapidly in 1 × 10-5Torr O2, but orders of magnitude less effectively at higher O2pressures, 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.

Iron oxides (FeOx) are among the most common support materials utilized in single atom catalysis. The support is nominally Fe2O3, but strongly reductive treatments are usually applied to activate the as‐synthesized catalyst prior to use. Here, Rh adsorption and incorporation on the (11¯02) surface of hematite (α‐Fe2O3) are studied, which switches from a stoichiometric (1 × 1) termination to a reduced (2 × 1) reconstruction in reducing conditions. Rh atoms form clusters at room temperature on both surface terminations, but Rh atoms incorporate into the support lattice as isolated atoms upon annealing above 400 °C. Under mildly oxidizing conditions, the incorporation process is so strongly favored that even large Rh clusters containing hundreds of atoms dissolve into the surface. Based on a combination of low‐energy ion scattering (LEIS), X‐ray photoelectron spectroscopy (XPS) and scanning tunneling microscopy (STM) data, as well as density functional theory (DFT), it is concluded that the Rh atoms are stabilized in the immediate subsurface, rather than the surface layer. [ABSTRACT FROM AUTHOR]

Laut Lehrbüchern sind tertiäre Alkohole inert gegenüber Oxidationen. Die Photokatalyse tertiärer Alkohole auf einem Titandioxideinkristall unter definierten Vakuumbedingungen offenbart unerwartete und neue Reaktionen, die Disproportionierung in ein Alkan und das entsprechende Keton. Im Gegensatz zu primären und sekundären Alkoholen führt die Abwesenheit eines α‐H zu einem C−C‐Bindungsbruch statt der üblichen Wasserstoffabspaltung. Die Bindung zu einer Methylgruppe wird nicht gespalten, wenn sich im Alkohol eine höhere Alkankette in α‐Stellung zur Hydroxylgruppe befindet. Eine Platinbeladung erhöht nicht nur die Reaktionsrate, sondern ermöglicht einen neuen Reaktionsweg: die Bildung molekularen Wasserstoffs und eines langkettigen Alkans, das sich durch Rekombination zweier Alkylspezies bildet. Diese Arbeit zeigt, dass neue synthetische Routen durch Einführungen photokatalytischer Reaktionsschritte möglich gemacht werden, in denen der Cokatalysator eine entscheidende Rolle spielt. [ABSTRACT FROM AUTHOR]

Keywords: catalyst deactivation; incorporation; iron oxides; redispersion; single atom catalysis EN catalyst deactivation incorporation iron oxides redispersion single atom catalysis 1 1 1 04/26/21 20210423 NES 210423 Rhodium atoms adsorbed on a model iron oxide surface sinter to nanoparticles already at room temperature. Single Atom Catalysts: Surface Reduction State Determines Stabilization and Incorporation of Rh on -Fe2O3(1102) (Adv. Catalyst deactivation, incorporation, iron oxides, redispersion, single atom catalysis. [Extracted from the article]

Determining the local coordination of the active site is a prerequisite for the reliable modeling of single-atom catalysts (SACs). Obtaining such information is difficult on powder-based systems and much emphasis is placed on density functional theory computations based on idealized low-index surfaces of the support. In this work, we investigate how Pt atoms bind to the (11̅02) facet of α-Fe 2 O 3 ; a common support material in SACs. Using a combination of scanning tunneling microscopy, X-ray photoelectron spectroscopy, and an extensive computational evolutionary search, we find that Pt atoms significantly reconfigure the support lattice to facilitate a pseudolinear coordination to surface oxygen atoms. Despite breaking three surface Fe-O bonds, this geometry is favored by 0.84 eV over the best configuration involving an unperturbed support. We suggest that the linear O-Pt-O configuration is common in reactive Pt-based SAC systems because it balances thermal stability with the ability to adsorb reactants from the gas phase. Moreover, we conclude that extensive structural searches are necessary to determine realistic active site geometries in single-atom catalysis.

Infrared Reflection Absorption Spectroscopy (IRAS) on dielectric single crystals is challenging because the optimal incidence angles for light-adsorbate interaction coincide with regions of low IR reflectivity. Here, we introduce an optimized IRAS setup that maximizes the signal-to-noise ratio for non-metals. This is achieved by maximizing light throughput and by selecting optimal incidence angles that directly impact the peak heights in the spectra. The setup uses a commercial Fourier transform infrared spectrometer and is usable in ultra-high vacuum (UHV). Specifically, the optical design features sample illumination and collection mirrors with a high numerical aperture inside the UHV system and adjustable apertures to select the incidence angle range on the sample. This is important for p-polarized measurements on dielectrics because the peaks in the spectra reverse the direction at the Brewster angle (band inversion). The system components are connected precisely via a single flange, ensuring long-term stability. We studied the signal-to-noise ratio (SNR) variation in p-polarized IRAS spectra for one monolayer of CO on TiO2(110) as a function of incidence angle range, where a maximum SNR of 70 was achieved at 4 cm-1 resolution in a measurement time of 5 min. The capabilities for s polarization are demonstrated by measuring one monolayer D2O adsorbed on a TiO2(110) surface, where a SNR of 65 was achieved at a peak height ΔR/R0 of 1.4 × 10-4 in 20 min., (© 2024 Author(s). All article content, except where otherwise noted, is licensed under a Creative Commons Attribution (CC BY) license (https://creativecommons.org/licenses/by/4.0/).)

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.

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., (© 2019 Wiley-VCH Verlag GmbH & Co. KGaA, Weinheim.)