The results of kinetic tests and ambient-pressure X-ray photoelectron spectroscopy (AP-XPS) show the important role played by a ZnO-copper interface in the generation of CO and the synthesis of methanol from CO hydrogenation. The deposition of nanoparticles of ZnO on Cu(100) and Cu(111), θ < 0.3 monolayer, produces highly active catalysts. The catalytic activity of these systems increases in the sequence: Cu(111) < Cu(100) < ZnO/Cu(111) < ZnO/Cu(100). The structure of the copper substrate influences the catalytic performance of a ZnO-copper interface. Furthermore, size and metal-oxide interactions affect the chemical and catalytic properties of the oxide making the supported nanoparticles different from bulk ZnO. The formation of a ZnO-copper interface favors the binding and conversion of CO into a formate intermediate that is stable on the catalyst surface up to temperatures above 500 K. Alloys of Zn with Cu(111) and Cu(100) were not stable at the elevated temperatures (500-600 K) used for the CO hydrogenation reaction. Reaction with CO oxidized the zinc, enhancing its stability over the copper substrates.

The interaction between a catalyst and reactants often induces changes in the surface structure and composition of the catalyst, which, in turn, affect its reactivity. Therefore, it is important to study such changes using in situ techniques under well-controlled conditions. We have used ambient pressure X-ray photoelectron spectroscopy to study the surface stability of a Pt/Cu(111) single-atom alloy in an ambient pressure of CO. By directly probing the Pt atoms, we found that CO causes a slight surface segregation of Pt atoms at room temperature. In addition, while the Pt/Cu(111) surface demonstrates poor thermal stability in ultrahigh vacuum conditions, where surface Pt starts to diffuse to the subsurface layer above 400 K, the presence of adsorbed CO enhances the thermal stability of surface Pt atoms. However, we also found that temperatures above 450 K cause restructuring of the subsurface layer, which consequently strengthens the CO binding to the surface Pt sites, likely because of the presence of neighboring subsurface Pt atoms.

The hydrogenation of ethylene on Pt(111) single-crystal surfaces was studied by combining measurements of the kinetics of reaction using mass spectrometry detection with the simultaneous characterization of the species present on the surface using reflection− absorption infrared spectroscopy. The kinetics measured by us matches past reports on the same system, with zero-and first-order dependence on the partial pressures of ethylene and hydrogen, respectively, and extensive H−D exchange if D 2 is used instead of H 2 . The reaction takes place in the presence of an alkylidyne surface layer, which forms immediately upon exposure of the clean surface to the reaction mixture and can be removed by hydrogen or another olefin but at rates 1−2 orders of magnitude slower than the ethylene-toethane conversion. The nature of the alkylidyne surface species changes slightly upon being exposed to high pressures of hydrogen, with the carbon in the terminal methyl moiety acquiring some sp 2 character. Moreover, the alkylidyne hydrogenation rate shows an inverse relationship with H 2 pressure and is reduced by the presence of olefins in the gas phase. Turnover frequencies for the olefin hydrogenation reaction under pressures in the Torr range are high, as reported repeatedly in the past, but the corresponding reaction probabilities are quite low, below the 10 −4 range. In contrast, almost unit reaction probability was observed here in effusive collimated molecular beam experiments emulating intermediate pressure conditions.

The reducibility of metal oxides, when they serve as the catalyst support or are the active sites themselves, plays an important role in heterogeneous catalytic reactions. Here we present an integrated experimental and theoretical study that reveals how the addition of small amounts of atomically dispersed Pt at the metal/oxide interface dramatically enhances the reducibility of a Cu2O thin film by H2. X-ray photoelectron spectroscopy (XPS) and temperature-programmed desorption (TPD) results reveal that, upon oxidation, a PtCu single-atom alloy (SAA) surface is covered by a thin Cu2O film and is, therefore, unable to dissociate H2. Despite this, in situ studies using ambient-pressure (AP) XPS reveal that the presence of a small amount of Pt under the oxide layer can, at the single-atom limit, promote the reduction of Cu2O by H2 at room temperature. We built two density functional theory based surface models to better understand these experimental findings: a Cu2O/Cu­(111)-like surface oxide layer, known as the “29” oxide, in which Pt is alloyed into the Cu(111) surface, as well as a PtCu SAA. Our calculations suggest that the increased activity is due to the presence of atomically dispersed Pt under the surface oxide layer, which weakens the Cu–O bonds in its immediate vicinity, thus making the interface between subsurface Pt and the surface oxide a nucleation site for the formation of metallic Cu. This initial step in the reduction process results in the presence of surface Pt atoms surrounded by metallic Cu patches, and the Pt atoms become active in H2 dissociation, which consequently accelerates the reduction of the oxide layer. This work demonstrates how isolated Pt atoms at the metal/oxide interface of a Cu-based catalyst accelerate the reduction of the oxide and, therefore, help maintain the active, reduced state of the catalyst under the reaction conditions.

A chemical approach to the deposition of thin films on solid surfaces is highly desirable but prone to affect the final properties of the film. To better understand the origin of these complications, the initial stages of the atomic layer deposition of titania films on silica mesoporous materials were characterized. Adsorption–desorption measurements indicated that the films grow in a layer-by-layer fashion, as desired, but initially exhibit surprisingly low densities, about one-quarter of that of bulk titanium oxide. Electron microscopy, X-ray diffraction, UV/visible, and X-ray absorption spectroscopy data pointed to the amorphous nature of the first monolayers, and EXAFS and 29Si CP/MAS NMR results to an initial growth via the formation of individual tetrahedral Ti–oxide units on isolated Si–OH surface groups with unusually long Ti–O bonds. Density functional theory calculations were used to propose a mechanism where the film growth starts at the nucleation centers to form an open 2D structure.

The kinetics of the hydrogenation of ethylene on platinum surfaces was studied by using high-flux effusive molecular beams and reflection-absorption infrared spectroscopy (RAIRS). It was determined that steady-state ethylene conversion with probabilities close to unity could be achieved by using beams with ethylene fluxes equivalent to pressures in the mTorr range and high (≥100) H2:C2H4 ratios. The RAIRS data suggest that the high reaction probability is possible because such conditions lead to the removal of most of the ethylidyne layer known to form during catalysis. The observations from this study are contrasted with those under vacuum, where catalytic behavior is not sustainable, and with catalysis under more realistic atmospheric pressures, where reaction probabilities are estimated to be much lower (≤1 × 10(-5)).

The catalytic oxidation of CO on transition metals, such as Pt, is commonly viewed as asharp transition from the CO-inhibited surface to the active metal, covered with O. However,w ef ind that minor amounts of Oa re present in the CO-poisoned layer that explain why,surprisingly,COdesorbs at stepped and flat Pt crystal planes at once,r egardless of the reaction conditions.Using near-ambient pressure X-ray photoemission and ac urved Pt(111) crystal we probe the chemical composition at surfaces with variable step density during the CO oxidation reaction. Analysis of Cand Ocore levels across the curved crystal reveals that, right before light-off,subsurface Ob uilds up within (111) terraces.T his is key to trigger the simultaneous ignition of the catalytic reaction at different Pt surfaces:aCO-Pt-O complex is formed that equals the CO chemisorption energy at terraces and steps,l eading to the abrupt desorption of poisoning CO from all crystal facets at the same temperature.

The effect of carbonaceous deposits on the performance of Pt(111) surfaces as catalysts for the hydrogenation of ethylene was tested by decoupling their preparation, which was done beforehand in an ultrahigh vacuum (UHV) environment, from the catalytic runs, which were carried out in an enclosed "high pressure" cell. The time evolution of the gas composition during reaction was followed continuously by mass spectrometry, and the nature of the surface species was determined by in situ reflection−absorption infrared absorption spectroscopy (RAIRS). Reaction rates were seen to vary by up to approximately 40% depending on the type of molecules preadsorbed at room temperature, with the maximum activity seen with a propylidyne layer, the result of propylene adsorption. The rate with ethylidyne-covered Pt( 111) is reduced by ∼20%, with butylidyne by ∼35%, and with benzyl moieties by ∼40%. These changes are reversible: the surface regains the activity expected when starting with the clean substrate after one or two catalytic runs to full conversion. RAIRS data show that this is because the initial species are slowly replaced by a new layer of the adsorbate that forms with the olefin in the reaction mixture (ethylidyne for ethylene hydrogenation). More significant changes are seen if the adsorbed layers are dehydrogenated at higher temperatures: the turnover frequencies for ethylene hydrogenation are reduced by more than an order of magnitude upon the conversion of propylidyne to C n H(ads) species, by annealing at temperatures between 500 and 650 K. Some activity is regained upon annealing to even higher temperatures, presumably because of the formation of graphitic layers, which have a smaller footprint on the surface. It was concluded that although the main role of carbonaceous deposits in catalytic hydrogenations is to block surface sites, their influence is also affected by the reversibility of their bonding to the surface in a hydrogenation atmosphere and by the structure of their carbon skeleton.