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Recent advances in single atom catalysis have sparked interest in their use as low-cost and high-efficiency catalysts in a wide variety of reactions. One such reaction that has been heavily studied with single atom catalysts is the water gas shift reaction. In addition, water participates in a rich variety of other industrially important catalytic processes, such as steam reforming reactions. However, much debate surrounds the structure and activity of single atoms toward water gas shift chemistry. By taking a model study approach, we determine the influence of atomically dispersed Pt atoms on the activation of water. Using a thin film Cu 2 O/Cu(1 1 1) support, water activation is probed via isotopic scrambling temperature programed desorption experiments. We determine that the presence of supported single Pt atoms on the thin Cu 2 O film will facilitate the low-temperature activation of water. Theory offers a viable water scrambling pathway on the supported Pt atoms, detailing the dynamic relationship between water, the Pt atom, and the support. The results reveal that single Pt atoms are capable of O-H bond activation when water interacts with such a catalyst. [ABSTRACT FROM AUTHOR]

Chemical-vapor deposition (CVD) of a Zr(O-tBu) 4 precursor on different Cu substrates was used to prepare model systems for ZrO x H y –Cu catalysts and to test their reactivity and selectivity in methanol steam reforming (MSR). A partially hydroxylated and initially fully oxidized submonolayer ZrO x H y surface species results, exhibiting a pronounced catalytic synergism between the ZrO x H y overlayer and Cu only with respect to partial methanol dehydrogenation to formaldehyde. Thus, it differs strongly from in situ grown ZrO x H y layers on Cu formed from an initially bimetallic mixed Zr/ZrO x state under MSR conditions. CVD-grown Zr–OH groups are not stable under MSR conditions; thus reversible in situ hydroxylation and water-activating reaction channels are suppressed. Comparison of the two model systems indicates that only a dedicated Cu–ZrO x H y interface with in situ formed and reversibly hydroxylated sites (accessible only from initially (inter)metallic Cu/Zr species at the surface) leads to water activation, total oxidation of intermediate formaldehyde, and enhanced CO 2 selectivity. [ABSTRACT FROM AUTHOR]

Abstract: In situ X-ray photoelectron spectroscopy (in situ XPS) was used to study the structural and catalytic properties of Pd–In near-surface intermetallic phases in correlation with previously studied PdZn and PdGa. Room temperature deposition of ∼4 monolayer equivalents (MLEs) of In metal on Pd foil and subsequent annealing to 453K in vacuum yields a ∼1:1 Pd/In near-surface multilayer intermetallic phase. This Pd1In1 phase exhibits a similar “Cu-like” electronic structure and indium depth distribution as its methanol steam reforming (MSR)-selective multilayer Pd1Zn1 counterpart. Catalytic characterization of the multilayer Pd1In1 phase in MSR yielded a CO2-selectivity of almost 100% between 493 and 550K. In contrast to previously studied In2O3-supported PdIn nanoparticles and pure In2O3, intermediate formaldehyde is only partially converted to CO2 using this Pd1In1 phase. Strongly correlated with PdZn, on an In-diluted PdIn intermetallic phase with “Pd-like” electronic structure, prepared by thermal annealing at 623K, methanol steam reforming is suppressed and enhanced CO formation via full methanol dehydrogenation is observed. To achieve CO2-TOF values on the isolated Pd1In1 intermetallic phase as high as on supported PdIn/In2O3, at least 593K reaction temperature is required. A bimetal-oxide synergism, with both bimetallic and oxide synergistically contributing to the observed catalytic activity and selectivity, manifests itself by accelerated formaldehyde-to-CO2 conversion at markedly lowered temperatures as compared to separate oxide and bimetal. Combination of suppression of full methanol dehydrogenation to CO on Pd1In1 inhibited inverse water–gas-shift reaction on In2O3 and fast water activation/conversion of formaldehyde is the key to the low-temperature activity and high CO2-selectivity of the supported catalyst. [Copyright &y& Elsevier]

Abstract: In situ X-ray photoelectron spectroscopy and low-energy ion scattering were used to study the preparation, (thermo)chemical and catalytic properties of 1:1 PdGa intermetallic near-surface phases. Deposition of several multilayers of Ga metal and subsequent annealing to 503–523K led to the formation of a multi-layered 1:1 PdGa near-surface state without desorption of excess Ga to the gas phase. In general, the composition of the PdGa model system is much more variable than that of its PdZn counterpart, which results in gradual changes of the near-surface composition with increasing annealing or reaction temperature. In contrast to near-surface PdZn, in methanol steam reforming, no temperature region with pronounced CO2 selectivity was observed, which is due to the inability of purely intermetallic PdGa to efficiently activate water. This allows to pinpoint the water-activating role of the intermetallic/support interface and/or of the oxide support in the related supported Pd x Ga/Ga2O3 systems, which exhibit high CO2 selectivity in a broad temperature range. In contrast, corresponding experiments starting on the purely bimetallic model surface in oxidative methanol reforming yielded high CO2 selectivity already at low temperatures (∼460K), which is due to efficient O2 activation on PdGa. In situ detected partial and reversible oxidative Ga segregation on intermetallic PdGa is associated with total oxidation of intermediate C1 oxygenates to CO2. [Copyright &y& Elsevier]

Abstract: The CO2 selectivity in methanol steam reforming was investigated for a “multilayer” PdZn 1:1 surface alloy (thickness of ∼1.3nm) and for a subsurface-Zn diluted “monolayer” Pd–Zn surface alloy, both exhibiting a 1:1 composition in the surface layer. Despite having almost the same surface layer stoichiometry, these two types of near-surface alloys exhibit different corrugations and electronic structures. The CO2-selective multilayer alloy features a lowered density of states close to the Fermi edge and surface ensembles of PdZn exhibiting a “Zn-up/Pd-down” corrugation, acting as bifunctional active sites both for reversible water activation as ZnOH and for reaction of methanol (via formaldehyde+ZnOH) toward CO2. The thermochemical stability limit of the multilayer alloy at around 573K was determined in-situ at elevated pressures of water, methanol and CO, applying in-situ XPS, PM-IRAS spectroscopy, LEIS and AES. Above 573K, the coordination of the surface 1:1 PdZn layer with subsurface-Zn gradually decreased by bulk diffusion of Zn “escaping” from the second and deeper layers, resulting in a transition from the CO2-selective PdZn “multilayer” state to the unselective “monolayer” state, which only catalyzes methanol dehydrogenation to CO. [ABSTRACT FROM AUTHOR]