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Die Fähigkeit, mehrere Reaktanten an derselben aktiven Stelle zu koordinieren ist eine wichtige Voraussetzung für die breite Anwendbarkeit der Einzelatom‐Katalyse. Modellkatalysatoren sind ideal, um den Zusammenhang zwischen der Geometrie der aktiven Stelle und der Bindung der Reaktanten zu untersuchen, da die Struktur der Einkristalloberflächen genau bestimmt werden kann. Die Adsorbate können mit Hilfe der Rastertunnelmikroskopie (STM) abgebildet werden; dadurch sind direkte Vergleiche mit der Dichtefunktionaltheorie möglich. In dieser Studie verfolgen wir mit Hilfe von STM‐Bildsequenzen einzelne Rh1‐Adatome und Rh2‐Dimere auf Fe3O4(001), während CO bei Raumtemperatur dosiert wird. Die CO‐Adsorption auf Rh1‐Adatomen führt ausschließlich zu stabilen Rh1CO‐Monocarbonylen, bei denen das Rh‐Atom die bevorzugte pseudo‐quadratisch planare Koordination erzielt. In geringem Maß bilden sich auch Rh1(CO)2 Gem‐Dicarbonyl‐Adsorbate; diese entstehen jedoch ausschließlich durch das Aufbrechen von Rh2 Dimeren; der Reaktionsweg führt über ein instabiles Rh2(CO)3 Zwischenprodukt. Unsere Ergebnisse veranschaulichen, wie Minderheitenspezies, die für (über die Probenfläche mittelnde) Spektroskopie‐Methoden unsichtbar sind, eine wichtige Rolle in katalytischen Systemen spielen können, und zeigen, dass die Zersetzung von Dimeren oder kleinen Clustern ein Weg sein kann, um reaktive, metastabile Konfigurationen in der Einzelatom‐Katalyse zu erzeugen. [ABSTRACT FROM AUTHOR]

The local environment of metal-oxide supported single-atom catalysts plays a decisive role in the surface reactivity and related catalytic properties. The study of such systems is complicated by the presence of point defects on the surface, which are often associated with the localization of excess charge in the form of polarons. This can affect the stability, the electronic configuration, and the local geometry of the adsorbed adatoms. In this work, through the use of density functional theory and surface-sensitive experiments, we study the adsorption of Rh1, Pt1, and Au1 metals on the reduced TiO2(110) surface, a prototypical polaronic material. A systematic analysis of the adsorption configurations and oxidation states of the adsorbed metals reveals different types of couplings between adsorbates and polarons. As confirmed by scanning tunneling microscopy measurements, the favored Pt1 and Au1 adsorption at oxygen vacancy sites is associated with a strong electronic charge transfer from polaronic states to adatom orbitals, which results in a reduction of the adsorbed metal. In contrast, the Rh1 adatoms interact weakly with the excess charge, which leaves the polarons largely unaffected. Our results show that an accurate understanding of the properties of single-atom catalysts on oxide surfaces requires a careful account of the interplay between adatoms, vacancy sites, and polarons. [ABSTRACT FROM AUTHOR]

Direct synthesis of carboxylic-phosphinic mixed anhydrides has been achieved by treating carbon dioxide with N-phosphine oxide-substituted imidazolylidenes (PoxIms) that contain both nucleophilic carbene and electrophilic phosphorus moieties. This novel mixed anhydride was efficiently derivatized into an ester, an amide, and an unsymmetrical ketone via transformation into its corresponding imidazolium salt followed by a dual substitution reaction. The presented work used well-designed multifunctional carbene reagents to establish a novel utility for carbon dioxide in organic synthesis. [ABSTRACT FROM AUTHOR]

Direct synthesis of carboxylic-phosphinic mixed anhydrides has been achieved by treating carbon dioxide with N-phosphine oxide-substituted imidazolylidenes (PoxIms) that contain both nucleophilic carbene and electrophilic phosphorus moieties. This novel mixed anhydride was efficiently derivatized into an ester, an amide, and an unsymmetrical ketone via transformation into its corresponding imidazolium salt followed by a dual substitution reaction. The presented work used well-designed multifunctional carbene reagents to establish a novel utility for carbon dioxide in organic synthesis. [ABSTRACT FROM AUTHOR]

Direct synthesis of carboxylic-phosphinic mixed anhydrides has been achieved by treating carbon dioxide with N-phosphine oxide-substituted imidazolylidenes (PoxIms) that contain both nucleophilic carbene and electrophilic phosphorus moieties. This novel mixed anhydride was efficiently derivatized into an ester, an amide, and an unsymmetrical ketone via transformation into its corresponding imidazolium salt followed by a dual substitution reaction. The presented work used well-designed multifunctional carbene reagents to establish a novel utility for carbon dioxide in organic synthesis. [ABSTRACT FROM AUTHOR]

Direct synthesis of carboxylic-phosphinic mixed anhydrides has been achieved by treating carbon dioxide with N-phosphine oxide-substituted imidazolylidenes (PoxIms) that contain both nucleophilic carbene and electrophilic phosphorus moieties. This novel mixed anhydride was efficiently derivatized into an ester, an amide, and an unsymmetrical ketone via transformation into its corresponding imidazolium salt followed by a dual substitution reaction. The presented work used well-designed multifunctional carbene reagents to establish a novel utility for carbon dioxide in organic synthesis. [ABSTRACT FROM AUTHOR]

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.

Water-solid interfaces pervade the natural environment and modern technology. On some surfaces, water-water interactions induce the formation of partially dissociated interfacial layers; understanding why is important to model processes in catalysis or mineralogy. The complexity of the partially dissociated structures often makes it difficult to probe them quantitatively. Here, we utilize normal incidence X-ray standing waves (NIXSW) to study the structure of partially dissociated water dimers (H 2 O-OH) at the α-Fe 2 O 3 (012) surface (also called the (11̅02) or "R-cut" surface): a system simple enough to be tractable yet complex enough to capture the essential physics. We find the H 2 O and terminal OH groups to be the same height above the surface within experimental error (1.45 ± 0.04 and 1.47 ± 0.02 Å, respectively), in line with DFT-based calculations that predict comparable Fe-O bond lengths for both water and OH species. This result is understood in the context of cooperative binding, where the formation of the H-bond between adsorbed H 2 O and OH induces the H 2 O to bind more strongly and the OH to bind more weakly compared to when these species are isolated on the surface. The surface OH formed by the liberated proton is found to be in plane with a bulk truncated (012) surface (-0.01 ± 0.02 Å). DFT calculations based on various functionals correctly model the cooperative effect but overestimate the water-surface interaction., Competing Interests: The authors declare no competing financial interest., (© 2024 The Authors. Published by American Chemical Society.)

Single-atom catalysts are potentially ideal model systems to investigate structure-function relationships in catalysis if the active sites can be uniquely determined. In this work, we study the interaction of C 2 H 4 with a model Rh/Fe 3 O 4 (001) catalyst that features 2-, 5-, and 6-fold coordinated Rh adatoms, as well as Rh clusters. Using multiple surface-sensitive techniques in combination with calculations of density functional theory (DFT), we follow the thermal evolution of the system and disentangle the behavior of the different species. C 2 H 4 adsorption is strongest at the 2-fold coordinated Rh 1 with a DFT-determined adsorption energy of -2.26 eV. However, desorption occurs at lower temperatures than expected because the Rh migrates into substitutional sites within the support, where the molecule is more weakly bound. The adsorption energy at the 5-fold coordinated Rh sites is predicated to be -1.49 eV, but the superposition of this signal with that from small Rh clusters and additional heterogeneity leads to a broad C 2 H 4 desorption shoulder in TPD above room temperature., Competing Interests: The authors declare no competing financial interest., (© 2024 The Authors. Published by American Chemical Society.)

The ability to coordinate multiple reactants at the same active site is important for the wide-spread applicability of single-atom catalysis. Model catalysts are ideal to investigate the link between active site geometry and reactant binding, because the structure of single-crystal surfaces can be precisely determined, the adsorbates imaged by scanning tunneling microscopy (STM), and direct comparisons made to density functional theory. In this study, we follow the evolution of Rh 1 adatoms and minority Rh 2 dimers on Fe 3 O 4 (001) during exposure to CO using time-lapse STM at room temperature. CO adsorption at Rh 1 sites results exclusively in stable Rh 1 CO monocarbonyls, because the Rh atom adapts its coordination to create a stable pseudo-square planar environment. Rh 1 (CO) 2 gem-dicarbonyl species are also observed, but these form exclusively through the breakup of Rh 2 dimers via an unstable Rh 2 (CO) 3 intermediate. Overall, our results illustrate how minority species invisible to area-averaging spectra can play an important role in catalytic systems, and show that the decomposition of dimers or small clusters can be an avenue to produce reactive, metastable configurations in single-atom catalysis., (© 2024 The Authors. Angewandte Chemie International Edition published by Wiley-VCH GmbH.)

The adsorption/desorption of ethene (C 2 H 4 ), also commonly known as ethylene, on Fe 3 O 4 (001) was studied under ultrahigh vacuum conditions using temperature-programmed desorption (TPD), scanning tunneling microscopy, X-ray photoelectron spectroscopy, and density functional theory (DFT)-based computations. To interpret the TPD data, we have employed a new analysis method based on equilibrium thermodynamics. C 2 H 4 adsorbs intact at all coverages and interacts most strongly with surface defects such as antiphase domain boundaries and Fe adatoms. On the regular surface, C 2 H 4 binds atop surface Fe sites up to a coverage of 2 molecules per (√2 × √2)R45° unit cell, with every second Fe occupied. A desorption energy of 0.36 eV is determined by analysis of the TPD spectra at this coverage, which is approximately 0.1-0.2 eV lower than the value calculated by DFT + U with van der Waals corrections. Additional molecules are accommodated in between the Fe rows. These are stabilized by attractive interactions with the molecules adsorbed at Fe sites. The total capacity of the surface for C 2 H 4 adsorption is found to be close to 4 molecules per (√2 × √2)R45° unit cell., Competing Interests: The authors declare no competing financial interest., (© 2023 The Authors. Published by American Chemical Society.)

The local environment of metal-oxide supported single-atom catalysts plays a decisive role in the surface reactivity and related catalytic properties. The study of such systems is complicated by the presence of point defects on the surface, which are often associated with the localization of excess charge in the form of polarons. This can affect the stability, the electronic configuration, and the local geometry of the adsorbed adatoms. In this work, through the use of density functional theory and surface-sensitive experiments, we study the adsorption of Rh 1 , Pt 1 , and Au 1 metals on the reduced TiO 2 (110) surface, a prototypical polaronic material. A systematic analysis of the adsorption configurations and oxidation states of the adsorbed metals reveals different types of couplings between adsorbates and polarons. As confirmed by scanning tunneling microscopy measurements, the favored Pt 1 and Au 1 adsorption at oxygen vacancy sites is associated with a strong electronic charge transfer from polaronic states to adatom orbitals, which results in a reduction of the adsorbed metal. In contrast, the Rh 1 adatoms interact weakly with the excess charge, which leaves the polarons largely unaffected. Our results show that an accurate understanding of the properties of single-atom catalysts on oxide surfaces requires a careful account of the interplay between adatoms, vacancy sites, and polarons., Supplementary Information: The online version contains supplementary material available at 10.1007/s11244-022-01651-0., (© The Author(s) 2022.)

Direct synthesis of carboxylic-phosphinic mixed anhydrides has been achieved by treating carbon dioxide with N-phosphine oxide-substituted imidazolylidenes (PoxIms) that contain both nucleophilic carbene and electrophilic phosphorus moieties. This novel mixed anhydride was efficiently derivatized into an ester, an amide, and an unsymmetrical ketone via transformation into its corresponding imidazolium salt followed by a dual substitution reaction. The presented work used well-designed multifunctional carbene reagents to establish a novel utility for carbon dioxide in organic synthesis., (© 2016 The Authors. Published by Wiley-VCH Verlag GmbH & Co. KGaA.)