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Abstract: To focus on the influence of the intermetallic compound—oxide interface of Pd-based intermetallic phases in methanol steam reforming (MSR), a co-precipitation pathway has been followed to prepare and subsequently structurally and catalytically characterize a set of nanoparticulate Ga2O3- and In2O3-supported GaPd2 and InPd catalysts, respectively. To study the possible promoting effect of In2O3, an In2O3-doped Ga2O3-supported GaPd2 catalyst has also been examined. While, upon reduction, the same intermetallic compounds are formed, the structure of especially the Ga2O3 support is strikingly different: rhombohedral and spinel-like Ga2O3 phases, as well as hexagonal GaInO3 and rhombohedral In2O3 phases are observed locally on the materials prior to methanol steam reforming by high-resolution transmission electron microscopy. Overall, the structure, phase composition and morphology of the co-precipitated catalysts are much more complex as compared to the respective impregnated counterparts. However, this induces a beneficial effect in activity and CO2 selectivity in MSR. Both Ga2O3 and In2O3 catalysts show a much higher activity, and in the case of GaPd2-Ga2O3, a much higher CO2 selectivity. The promoting effect of In2O3 is also directly detectable, as the CO2 selectivity of the co-precipitated supported Ga2O3-In2O3 catalyst is much higher and comparable to the purely In2O3-supported material, despite the more complex structure and morphology. In all studied cases, no deactivation effects have been observed even after prolonged time-on-stream for 12 h, confirming the stability of the systems.Graphical Abstract: The presence of a variety of distinct supported intermetallic InPd and GaPd2 particle phases is not detrimental to activity/selectivity in methanol steam reforming as long as the appropriate intermetallic phases are present and they exhibit optimized intermetallic-support phase boundary dimensions. [ABSTRACT FROM AUTHOR]

Highlights: [•] An isolated, unsupported Pd-rich GaPd2 intermetallic compound has been prepared. [•] High thermochemical stability under methanol steam reforming conditions. [•] No active redox chemistry of Ga species detected. [•] Unsupported Pd-rich GaPd2 is inactive and unselective in methanol steam reforming. [•] Bi-functional synergism between GaPd2 and Ga2O3 is present. [ABSTRACT FROM AUTHOR]

Abstract: A set of different Pd/Ga2O3 catalysts has been tested as suitable CO2-selective catalysts in methanol steam reforming, formaldehyde steam reforming and formic acid decomposition. Special focus was put on two different polymorphic forms of Ga2O3, namely α-Ga2O3 and γ-Ga2O3 and their comparison to the use of the more common β-Ga2O3 as oxide support of small Pd particles. The formation of eventually CO2-selective intermetallic Pd–Ga phases started at around 523K on both supports. Pd2Ga at low temperatures (between 523K and 673K) and generally PdGa at higher temperatures (at and above 673K) were the only intermetallic phases observed. Both thermodynamically metastable oxide supports transformed into the stable β-Ga2O3 structure at 873K reduction temperature. In contrast to previous studies, a highly CO2-selective state in methanol steam reforming (CO2 selectivity>70%) could be established on both catalysts after reduction at 673K (α-Ga2O3) and 523K (γ-Ga2O3), where the active state of the catalysts can be characterised as Pd2Ga/α-Ga2O3 and Pd2Ga/γ-Ga2O3, respectively. α-Ga2O3 itself is reasonably CO2-selective in methanol steam reforming (∼80%), although not very active. In contrast, the γ-Ga2O3 polymorph is neither CO2-selective, nor active and resembles the behaviour of β-Ga2O3. The high CO2-selectivity of α-Ga2O3 is explained in terms of a higher surface basicity alongside efficient decarboxylation of intermediary formed formic acid. As crucial parameters for the observed CO2-selectivity in methanol steam reforming, decarboxylation of formic acid and oxidation of formaldehyde proceeding efficiently via a low-temperature reaction channel over either oxide-supported Pd2Ga intermetallic catalysts were identified. [Copyright &y& Elsevier]

Abstract: ZnO was tested as possible methanol and – since formaldehyde is one of the key intermediates in methanol conversion reactions – also as formaldehyde steam reforming catalyst. Catalytic experiments in a batch as well as a flow reactor resulted in highly selective steam reforming, though at low specific activities, of formaldehyde and methanol over ZnO toward CO2 (selectivity of 95–99.6%). Comparison of the behavior of ZnPd near-surface intermetallic phases, unsupported intermetallic ZnPd and supported ZnPd/ZnO catalysts reveals that formaldehyde is formed from methanol in parallel with CO2 on the former, while on unsupported intermetallic ZnPd and ZnO-supported ZnPd, it is efficiently reacted toward CO2, thus, a beneficial role of ZnO in oxidizing formaldehyde-derived intermediates toward CO2 is evident. [Copyright &y& Elsevier]

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: Structure, morphology and composition of different tin oxide and germanium oxide thin film catalysts for the methanol steam reforming (MSR) reaction have been studied by a combination of (high-resolution) transmission electron microscopy, selected area electron diffraction, dark-field imaging and electron energy-loss spectroscopy. Deposition of the thin films on NaCl(001) cleavage faces has been carried out by thermal evaporation of the respective SnO2 and GeO2 powders in varying oxygen partial pressures and at different substrate temperatures. Preparation of tin oxide films in high oxygen pressures (10−1 Pa) exclusively resulted in SnO phases, at and above 473K substrate temperature epitaxial growth of SnO on NaCl(001) leads to well-ordered films. For lower oxygen partial pressures (10−3 to 10−2 Pa), mixtures of SnO and β-Sn are obtained. Well-ordered SnO2 films, as verified by electron diffraction patterns and energy-loss spectra, are only obtained after post-oxidation of SnO films at temperatures T ≥673K in 105 Pa O2. Preparation of GeO x films inevitably results in amorphous films with a composition close to GeO2, which cannot be crystallized by annealing treatments in oxygen or hydrogen at temperatures comparable to SnO/SnO2. Similarities and differences to neighbouring oxides relevant for selective MSR in the third group of the periodic system (In2O3 and Ga2O3) are also discussed with the aim of cross-correlation in formation of nanomaterials, and ultimately, also catalytic properties. [Copyright &y& Elsevier]

Abstract: Pd particles supported on SnO2 and GeO2 have been structurally investigated by X-ray diffraction, (High-Resolution) transmission and scanning electron microscopy after different reductive treatments to monitor the eventual formation of bimetallic phases and catalytically tested in methanol dehydrogenation/reforming. For both oxides this included a thin film sample with well-defined Pd particles and a powder catalyst prepared by incipient wetness impregnation. The hexagonal and the tetragonal polymorph were studied for powder GeO2. Pd2Ge formation was observed on all GeO2-supported catalysts, strongly depending on the specific sample used. Reduction of the thin film at 573K resulted in full transformation into the bimetallic state. The partial solubility of hexagonal GeO2 in water and its thermal structural instability yielded Pd2Ge formation at 473K, at the cost of a structurally inhomogeneous support and Ge metal formation at higher reduction temperatures. Pd on tetragonal GeO2 entered a state of strong metal–support interaction after reduction at 573–673K, resulting in coalescing Pd2Ge particles on a sintered and re-crystallized support, apparently partially covering the bimetallic particles and decreasing the catalytic activity. Pd2Ge on amorphous thin film and hexagonal GeO2 converted methanol primarily via dehydrogenation to CO and H2. At 573K, formation of Pd2Sn and also PdSn occurred on the Pd/SnO2 thin film. Pd3Sn2 (and to some extent Pd2Sn) were predominantly obtained on the respective powder catalyst. Strong deactivation with increasing reduction temperature was observed, likely not based on the classical strong metal–support interaction effect, but rather on a combination of missing active structural ensembles on Sn-enriched bimetallic phases and the formation of metallic β-Sn. Correlations to Pd and its bimetallics supported on ZnO, Ga2O3 and In2O3 were also discussed. [Copyright &y& Elsevier]

Abstract: Structural changes of a variety of different SnO, SnO2 and GeO2 catalysts upon reduction in hydrogen were correlated with associated catalytic changes in methanol steam reforming. Studied systems include SnO, SnO2 and GeO2 thin film model catalysts prepared by vapour phase deposition and growth on polycrystalline NaCl surfaces and, for comparison, the corresponding pure oxide powder catalysts. Reduction of both the SnO2 thin film and powder at around 673K in 1bar hydrogen leads to a substantial reduction of the bulk structure and yields a mixture of SnO2 and metallic β-Sn. On the powder catalyst this transformation is fully reversible upon oxidation in 1bar O2 at 673K. Strongly reduced thin films, however, can only be re-transformed to SnO2 if the reduction temperature did not exceed 573K. For GeO2, the situation is more complex due to its polymorphism. Whereas the tetragonal phase is structurally stable during reduction, oxidation or catalytic reaction, a small part of the hexagonal phase is always transformed into the tetragonal at 673K independent of the gas phase used. SnO2 is highly active and CO2 selective in methanol steam reforming, but the initial high activity drops considerably upon reduction between 373 and 573K and almost complete catalyst deactivation is observed after reduction at 673K, which is associated with the parallel formation of β-Sn. In close correlation to the structural results, the catalytic activity and selectivity can be restored upon an oxidative catalyst regeneration at 673K. Tetragonal GeO2 exhibits only a small activity and no pronounced selectivity to either CO or CO2, at least after reduction. In its fully oxidized state release of surface/lattice oxygen results in a non-catalytic formation of CO2 by oxidation of CO originating from catalytic dehydrogenation. [Copyright &y& Elsevier]

Abstract: Two different Pd/In2O3 samples including a thin film model catalyst with well-defined Pd particles grown on NaCl(001) supports and a powder catalyst prepared by an impregnation technique are examined by electron microscopy, X-ray diffraction and catalytic measurements in methanol steam reforming in order to correlate the formation of different oxide-supported bimetallic Pd–In phases with catalytic activity and selectivity. A PdIn shell around the Pd particles is observed on the thin film catalyst after embedding the Pd particles in In2O3 at 300K, likely because alloying to PdIn and oxidation to In2O3 are competing processes. Increased PdIn bimetallic formation is observed up to 573K reduction temperature until at 623K the film stability limit in hydrogen is reached. Oxidative treatments at 573K lead to decomposition of PdIn and to the formation of an In2O3 shell covering the Pd particles, which irreversibly changes the activity and selectivity pattern to clean In2O3. PdIn and Pd2In3 phases are obtained on the powder catalyst after reduction at 573K and 673K, respectively. Only CO2-selective methanol steam reforming is observed in the reduction temperature range between 473K and 573K. After reduction at 673K encapsulation of the bimetallic particles by crystalline In2O3 suppresses CO2 formation and only activity and selectivity of clean In2O3 are measured. [Copyright &y& Elsevier]

Abstract: Structure, morphology and composition of two Pd/Ga2O3 methanol steam reforming catalysts were correlated with the associated activity and selectivity changes in the methanol steam reforming reaction and linked to studies on pure Ga2O3 supports. For both systems, that are, a Pd/Ga2O3 thin film model catalyst and a powder-supported Pd catalyst, we identified a temperature range in which the reduction with hydrogen yields a single Pd–Ga bimetallic on a reduced oxide support, which in turn suppresses methanol dehydrogenation and results in a high CO2 selectivity in methanol steam reforming. For the thin film catalyst, this included the Pd5Ga2 bimetallic present after reduction in the temperature range 523–600K, for the powder-supported catalyst, Pd2Ga, formed after reduction between 523 and 773K, was found to account for the high CO2 selectivity. In contrast to studies on the corresponding Pd/ZnO catalysts, sintering and metal decoration by reduced GaO x species will additionally have to be considered for discussions about structure–activity correlations in Pd/Ga2O3 thin film model catalysts. Reduction at 673K causes catalyst deactivation and loss of CO2 selectivity due to encapsulation of catalytically active bimetallic particles by mobile GaO x species and hampers oxidative catalyst regeneration. No such behaviour has been observed for the powder-supported catalyst. This difference in catalytic activity and selectivity between the two catalysts is interpreted in terms of their different (bi-)metallic–oxide contact areas. Formic acid formation has been observed on pure Ga2O3 supports, which yields additional CO by formic acid decarbonylation. For the CO2-selective methanol steam reforming reaction over a Pd–Ga bimetallic particle, formic acid is not a gas phase species, which indicates a preferential decarboxylation pathway of adsorbed formate species at low temperatures. [Copyright &y& Elsevier]

Abstract: The combination of (high-resolution) electron microscopy, selected area diffraction and X-ray diffraction was applied to study the structural, compositional and morphological alterations of two Pd/Ga2O3 catalysts relevant for methanol steam reforming upon different oxidative and reductive treatments. These systems include well-defined Pd particles grown epitaxially on vacuum-cleaved NaCl(001) single crystals and subsequently covered by a layer of amorphous Ga2O3 (termed as “thin film model catalysts”), as well as, for comparison, a conventional Pd/Ga2O3 powder catalyst prepared by incipient wetness impregnation. Both catalysts were subsequently subjected to similar treatments in O2 (1bar, 1h) and H2 (1bar, 1h) in the temperature range between 373 and 923K. Oxidation of the thin film model catalyst at 773K converts the Pd/Ga2O3 film into a mixture of PdO and Pd supported on Ga2O3. Subsequent reduction at 523K causes the formation of an epitaxial Pd5Ga2 bimetallic phase with suppressed Pd hydride formation, in close correlation to the corresponding Pd/ZnO system. In contrast to the latter, Pd/Ga2O3 thin film model catalysts are prone to sintering at reduction temperatures ≥673K. However, Pd5Ga2 with some Pd probably remaining in the particle cores is still present. The stability of the bimetallic Pd5Ga2 phase in oxygen was observed to depend on the pre-reduction history, that is, oxidation at 673K of a bimetallic formed at 523K causes the complete decomposition of the bimetallic phase and formation of PdO/Pd particles supported on Ga2O3, whereas bimetallics formed at higher reduction temperatures (i.e. ≥673K) are only transformed back to Pd/Ga2O3. According to TEM investigations, this is due to the increased mobility of Ga and the subsequent formation of a Ga2O3 surface layer preventing further oxidation to PdO. Reduction of the corresponding Pd/β-Ga2O3 impregnated powder catalyst between 573 and 773K causes the formation of a Pd2Ga bimetallic phase, which is further converted to PdGa at reduction temperatures of 923K. No formation of a Ga2O3 surface layer covering the particles has been observed upon re-oxidation of the bimetallic. This is explained on the basis of the difference in metal (bimetallic)–support contact area. [Copyright &y& Elsevier]

Abstract: Electron-microscopy suitable In2O3 thin films prepared by thermal deposition of In2O3 powder in 10−2 Pa O2 at 600K and, for comparison, a commercial polycrystalline In2O3 powder catalyst were tested in methanol steam reforming and in both routes of the water–gas shift reaction as a function of reaction temperature. The effect of oxidative (1bar O2, 373–673K, 1h) and reductive (1bar H2, 373–673K, 1h) catalyst pre-treatments was assessed. The resulting structural and morphological changes occurring during catalyst activation and catalytic reaction were monitored by (high-resolution) transmission electron microscopy, scanning electron microscopy and surface area measurements by N2 adsorption according to BET. Both the In2O3 thin film and the powder sample were observed to be structurally stable under typical catalyst pre-treatments in oxygen and hydrogen at temperatures T ≤673K and T <673K, respectively, as well as under typical methanol steam reforming conditions at temperatures T ≤680K. No pronounced catalyst sintering was observed below 673K. Both In2O3 samples were found to be highly active and selective toward CO2 in methanol steam reforming over a broad temperature range (450< T <673K). Selectivities of >95% toward CO2 were usually observed, with at maximum 5% or less CO formed. No dependence of selectivity on either reaction temperature or oxidative/reductive pre-treatment was observed. No catalytic activity in both routes of the water–gas shift reaction as tested in the same temperature region where the catalysts exhibit high reforming activity and selectivity, could be observed. Therefore In2O3 based catalysts offer a broad range of temperature not influenced by unwanted CO formation via the inverse water–gas shift process. [Copyright &y& Elsevier]