Dependence on Size of Supported Rh Nanoclusters in the Decomposition of Methanol

Decomposition of methanol-d4 on Rh nanoclusters supported by thin-film Al2O3/NiAl(100) under near-ambient-pressure conditions

The decomposition of methanol-d4 (CD3OD) on Rh nanoclusters grown by the deposition of Rh vapors onto an ordered thin film of Al2O3/NiAl(100) was studied, with various surface-probe techniques and largely under near-ambient-pressure (NAP) conditions. The results showed a superior reactivity of small Rh clusters (diameter < 1.5 nm) exposed to CD3OD at 5 × 10-3-0.1 mbar at 400 K; the gaseous production of CO and D2 from decomposed methanol-d4 per Rh surface site on the small Rh clusters with diameters of ∼1.1 nm was nearly 8 times that on large ones with diameters of ∼3.5 nm. The promotion of reactivity with decreased cluster size under NAP conditions was evidently greater than that under ultrahigh vacuum conditions. Moreover, the concentration of atomic carbon (C*; where * denotes adsorbate)-a key catalyst poisoner-yielded from the dissociation of CO* from dehydrogenated methanol-d4 was significantly smaller on small clusters (diameter < 1.5 nm). The NAP size effect on methanol-d4 decomposition involved the surface hydroxyl (OH*) from the little co-adsorbed water (H2O*) that was dissociated at a probability dependent on the cluster size. H2O* was more likely dissociated into OH* on small Rh clusters, by virtue of their more reactive d-band structure, and the OH* then effectively promoted the O-D cleavage of methanol-d4, as the rate-determining step, and thus the reaction probability; on the other hand, the OH* limited CO* dissociation on small Rh clusters via both adsorbate and lateral effects. These results suggest that the superior properties of small Rh clusters in both reactivity and anti-poisoning would persist and be highly applicable under "real-world" catalysis conditions.

Investigating the role of undercoordinated Pt sites at the surface of layered PtTe2 for methanol decomposition

Transition metal dichalcogenides, by virtue of their two-dimensional structures, could provide the largest active surface for reactions with minimal materials consumed, which has long been pursued in the design of ideal catalysts. Nevertheless, their structurally perfect basal planes are typically inert; their surface defects, such as under-coordinated atoms at the surfaces or edges, can instead serve as catalytically active centers. Here we show a reaction probability > 90 % for adsorbed methanol (CH3OH) on under-coordinated Pt sites at surface Te vacancies, produced with Ar+ bombardment, on layered PtTe2 - approximately 60 % of the methanol decompose to surface intermediates CHxO (x = 2, 3) and 35 % to CHx (x = 1, 2), and an ultimate production of gaseous molecular hydrogen, methane, water and formaldehyde. The characteristic reactivity is attributed to both the triangular positioning and varied degrees of oxidation of the under-coordinated Pt at Te vacancies.

Decomposition of methanol-d4 on a thin film of Al2O3/NiAl(100) under near-ambient-pressure conditions

We have studied the decomposition of methanol-d4 on thin film Al2O3/NiAl(100) under near-ambient-pressure conditions, with varied surface-probe techniques and calculations based on density-functional theory. Methanol-d4 neither adsorbed nor reacted on Al2O3/NiAl(100) at 400 K under ultrahigh vacuum conditions, whereas they dehydrogenated, largely to methoxy-d3 (CD3O*, * denoting adsorbates) and formaldehyde-d2 (CD2O*), on the surface when the methanol-d4 partial pressure was increased to 10-3 mbar and above. The dehydrogenation was facilitated by hydroxyl (OH* or OD*) from the dissociation of little co-adsorbed water; a small fraction of CD2O* interacted further with OH* (OD*) to form, via intermediate CD2OOH* (CD2OOD*), formic acid (DCOOH* or DCOOD*). A few surface carbonates were also yielded, likely on the defect sites of Al2O3/NiAl(100). The results suggest that alumina not only supports metal clusters but also participates in reactions under realistic catalytic conditions. One may consider accordingly the multiple functions of alumina while designing ideal catalysts.

The normalization of the active surface sites of bimetallic Pd-Pt catalysts, their inhomogeneity, and their roles in methane activation

Multi-metallic catalysts containing Pt species are widely used. As there is no methodology to evaluate the quantity of active surface sites of Pt or other metal species, researchers have only published the total conversion or selectivity of all active surface sites. This study focuses on Pt-Pd bimetallic catalysts and uses both methane reaction kinetics and infrared (IR) spectroscopy to characterize the surface Pd and Pt sites. The surface Pt sites, which were determined from the fitted rate coefficients, were evaluated in the reaction region where the catalyst structure was insensitive to catalytic performance. Another methodology involves IR spectroscopy to normalize the active surface sites. As three typical absorption bands of Pt species were observed during CO chemisorption, spectral deconvolution was conducted to obtain the integrated intensity of the Pd and Pt species, and the quantity of surface Pd and Pt sites was calculated. The two methods have good consistency, and the IR spectra are considered to be more suitable for calculating the quantity of active surface sites. In addition, the IR spectra revealed a correlation between oxidative Pd surface sites and methane reactivity. The ionic Pd sites provide abundant oxygen intermediates in the catalytic reaction and improve the catalytic performance. As for the surface Pd species and bulk Pd species, the XPS results indicate a similar variation in the Pd^δ+/(Pd^δ+ + Pd⁰) ratio vs. Pd/Pt ratio.

Dissociation of water on atomic oxygen-covered Rh nanoclusters supported on graphene/Ru(0001)

We studied the dissociation of water (H₂O*, with * denoting adspecies) on atomic oxygen (O*)-covered Rh nanoclusters (Rh_O^* ) supported on a graphene film grown on a Ru(0001) surface [G/Ru(0001)] under ultrahigh-vacuum conditions and with varied surface-probe techniques and calculations based on density-functional theory. The graphene had a single rotational domain; its lattice expanded by about 5.7% to match the Ru substrate structurally better. The Rh clusters were grown by depositing Rh vapors onto G/Ru(0001); they had an fcc phase and grew in (111) orientation. Water adsorbed on the Rh clusters was dissociated exclusively in the presence of O*, like that on a Rh(111) single-crystal surface. Contrary to the case on Rh(111)_O^* , excess O* (even at a saturation level) on small Rh_O^* clusters (diameter of 30-34 Å) continued to promote, instead of inhibiting, the dissociation of water; the produced hydroxyl (OH*) increased generally with the concentration of O* on the clusters. The difference results from more reactive O* on the Rh_O^* clusters. O* on Rh_O^* clusters activated the dissociation via both the formation of hydrogen bonds with H₂O* and abstraction of H directly from H₂O*, whereas O* on Rh(111)_O^* assisted the dissociation largely via the formation of hydrogen bonds, which was readily obstructed with an increased O* coverage. As the disproportionation (2 OH* → H₂O* + O*) is endothermic on the Rh_O^* clusters but exothermic on Rh(111)_O^* , OH* produced on Rh_O^* clusters showed a thermal stability superior to that on the Rh(111)_O^* surface-thermally stable up to 400 K.

Dependence on Size of Supported Rh Nanoclusters in the Dehydrogenation of Methanol-d4 Obstructed by CO

The size effect on the activity of a catalyst has been a focal issue since ideal catalysts were pursued, whereas that on the degradation of a catalyst, by reaction intermediates such as CO, is little discussed. We demonstrate that the dehydrogenation of methanol-d₄ on supported Rh nanoclusters precovered with CO (Rh_-CO clusters) was obstructed, indicated by a decreased production of CO and D₂; the obstructive effect exhibits a remarkable dependence on the cluster size, with a minimum at a cluster diameter near 1.4 nm. The decreased production arose from a decreased reaction probability controlled by the increased activation energy for each dehydrogenation step (including formation of methoxy-d₃), adsorption energies of CO, and repulsion from the CO array on the Rh_-CO surface. The effects of these factors in deactivating the clusters varied separately with the cluster size. Consequently, the size effect on the CO poisoning should be taken into account in engineering the cluster size to optimize the catalytic performance.

Promoted activity of annealed Rh nanoclusters on thin films of Al2O3/NiAl(100) in the dehydrogenation of Methanol-d4

Annealed Rh nanoclusters on an ordered thin film of Al₂O₃/NiAl(100) were shown to exhibit a promoted reactivity toward the decomposition of methanol-d₄, under both ultrahigh vacuum and near-ambient-pressure conditions. The Rh clusters were grown with vapor deposition onto the Al₂O₃/NiAl(100) surface at 300 K and annealed to 700 K. The decomposition of methanol-d₄ proceeded only through dehydrogenation, with CO and deuterium as products, on Rh clusters both as prepared and annealed. Nevertheless, the catalytic reactivity of the annealed clusters, measured with the production of either CO or deuterium per surface Rh site from the reaction, became at least 2–3 times that of the as-prepared ones. The promoted reactivity results from an altered support effect associated with an annealing-induced mass transport at the surface. Our results demonstrate a possibility to practically prepare reactive Rh clusters, regardless of the cluster size, that can tolerate an elevated reaction temperature, with no decreased reactivity.

Annealing alters the support effect to promote the activity of alumina-supported Rh clusters in methanol dehydrogenation.

Distinct dependence on size of Pt and Rh nanoclusters on graphene/Pt(111) in the decomposition of methanol-d4

Pt and Rh nanoclusters, grown on deposition of Pt and Rh vapors onto graphene/Pt(111), show separate reactivity toward the decomposition of methanol-d₄. The Pt (Rh) clusters had a mean diameter 2.0-3.5 nm (2.1-4.0 nm) and height 0.45-0.94 nm (0.41-0.9 nm) evolving with the coverage; they were structurally ordered, having an fcc phase and growing in (111) orientation, and had lattice constants similar to their bulk values. Methanol-d₄ on the Pt clusters did not decompose but desorbed mostly, disparate from that on Pt(111) surface; the disparity arose as the adsorption energies of methanol-d₄ on most surface sites of the Pt clusters became smaller than their single crystal counterpart. This size effect, nevertheless, did not apply on the Rh clusters, despite their similar atomic stacking; the Rh clusters showed a reactivity similar to that of the Rh(111) surface because the adsorption energies of methanol-d₄ on both Rh clusters and Rh(111) are comparable. The distinct size dependence was rationalized through their electronic structures and charge distribution of Fukui function mapping. Our results suggest that reactive transition metals do not necessarily become more reactive while they are scaled down to nanoscale; their reactivity evolves with their size in a manner largely dependent on their electronic nature.

Investigation of Thermal Stability and Reactivity of Rh Nanoclusters on an Ultrathin Alumina Film

We studied the structural and morphological evolution of Rh clusters on an ordered ultrathin alumina film grown on NiAl(100) in annealing processes, under ultrahigh vacuum conditions and with various surface probe techniques. The Rh clusters, prepared on vapor deposition of Rh onto the alumina film at 300 K, had an fcc phase and grew in the (100) orientation; the annealing altered the cluster structure little—the lattice parameter decreased by a factor <2%—but the cluster morphology significantly. With elevated temperature, small clusters (diameter ≤1.5 nm) decreased little in size; in contrast, large clusters (diameter ≥2.0 nm) varied in a complex manner—their mean diameter decreased to about 1.5 nm on annealing to 450 K, despite their similar height, while it increased to above 2.0 nm at temperature ≥570 K. This atypical decrease in size was governed predominantly by energetics. Such a reduced size enhanced the total surface area as well as the reactivity of the clusters toward methanol decomposition, so increased the production of D2 (H2) and CO from decomposed methanol-d4 (or methanol). The result implies a higher temperature tolerance for Rh clusters on the alumina film and a practical approach to prepare small Rh clusters with high reactivity.

Microscopic evidence for the dissociation of water molecules on cleaved GaN(11[combining macron]00)

The dissociation of water molecules absorbed on a cleaved non-polar GaN(11[combining macron]00) surface was studied primarily with synchrotron-based photoemission spectra and density-functional-theory calculations. The adsorbed water molecules are spontaneously dissociated into hydrogen atoms and hydroxyl groups at either 300 or 130 K, which implies a negligible activation energy (<11 meV) for the dissociation. The produced H and OH were bound to the surface nitrogen and gallium on GaN(11[combining macron]00) respectively. These results highlight the promising applications of the non-polar GaN(11[combining macron]00) surface in water dissociation and hydrogen generation.

Adsorption of an Au atom and dimer on a thin θ-Al2O3/NiAl(100) film: dependence on the thickness of the θ-Al2O3 film

With calculations based on density-functional theory (DFT) we investigated the adsorption of a single Au atom and a dimer on thin θ-Al₂O₃(001) films supported on NiAl(100). The interaction of the Au adsorbates with the surface was shown to depend on the thickness of the film. The adsorption energy for an Au atom on θ-Al₂O₃(001)/NiAl(100) of film thickness ≤four atomic layers was significantly enhanced—over three times that on a bulk θ-Al₂O₃(001) surface, and accompanied with a shortened Au-oxide bond and an uplifted Au-binding Al. The strong Au-surface interaction involved a decreased work function of θ-Al₂O₃(001)/NiAl(100) and consequently drove charge to transfer from the substrate to the adsorbed Au atom; the charge was transferred from NiAl, through alumina, on monolayer θ-Al₂O₃(001)/NiAl(100), but directly from alumina on thicker layers. For an Au dimer, both upright (end-on) and flat-lying (side-on) geometries existed. The flat-lying dimer was preferred on mono- and tri-layer alumina films, having a greater adsorption energy but a weakened Au–Au bond, whereas the upright geometry prevailed for films of other thickness, having a weaker adsorption energy and being less charged, similar to that on a bulk θ-Al₂O₃(001) surface. The results imply an opportunity to control the properties and morphologies of metal clusters supported on an oxide film by tuning its thickness.

The adsorption behavior of a single Au atom and a dimer on thin-film θ-Al₂O₃(001)/NiAl(100) varies with the thickness of the film.

Formation and structures of Au–Rh bimetallic nanoclusters supported on a thin film of Al2O3/NiAl(100)

The crystallinity, orientation, and lattice parameters of Au–Rh bimetallic clusters on Al₂O₃/NiAl(100) are revealed and dominated by Rh.

The interaction of CO molecules on Au–Rh bimetallic nanoclusters supported on a thin film of Al2O3/NiAl(100)

The varied infrared absorption intensity of CO on bimetallic clusters reflects altered electronic states of CO-binding surface atoms.

Dependence on size of supported Rh nanoclusters for CO adsorption

The adsorption and lateral interactions of CO molecules on Rh nanoclusters supported on an ordered thin film of Al₂O₃/NiAl(100) altered with the size of the Rh clusters.