Faisal Mehmood

Increased Silver Activity for Direct Propylene Epoxidation via Subnanometer Size Effects

Silver Cluster Catalysts for Propylene Oxide The formation of ethylene oxide—in which an oxygen atom bridges the double bond of ethylene—can be made directly and efficiently from ethylene and oxygen with the aid of silver catalysts (typically comprising a small silver cluster on aluminum oxide). Similar approaches are not so successful for making propylene oxide—an important starting material for polyurethane plastics, which are made from chlorinated intermediates. Lei et al. (p. 224) report that silver trimers, Ag3, deposited on alumina are active for direct propylene oxide formation at low temperatures with only a low level of formation of CO2 by-product, unlike larger particles that form from these clusters at higher temperatures. Density functional calculations suggest that the open-shell nature of the clusters accounts for the improved reactivity. Clusters of three silver atoms deposited on alumina are active for the low-temperature direct formation of propylene oxide. Production of the industrial chemical propylene oxide is energy-intensive and environmentally unfriendly. Catalysts based on bulk silver surfaces with direct propylene epoxidation by molecular oxygen have not resolved these problems because of substantial formation of carbon dioxide. We found that unpromoted, size-selected Ag3 clusters and ~3.5-nanometer Ag nanoparticles on alumina supports can catalyze this reaction with only a negligible amount of carbon dioxide formation and with high activity at low temperatures. Density functional calculations show that, relative to extended silver surfaces, oxidized silver trimers are more active and selective for epoxidation because of the open-shell nature of their electronic structure. The results suggest that new architectures based on ultrasmall silver particles may provide highly efficient catalysts for propylene epoxidation.

Subnanometre platinum clusters as highly active|[nbsp]|and selective catalysts for the oxidative dehydrogenation of propane

Small clusters are known to possess reactivity not observed in their bulk analogues, which can make them attractive for catalysis. Their distinct catalytic properties are often hypothesized to result from the large fraction of under-coordinated surface atoms. Here, we show that size-preselected Pt(8-10) clusters stabilized on high-surface-area supports are 40-100 times more active for the oxidative dehydrogenation of propane than previously studied platinum and vanadia catalysts, while at the same time maintaining high selectivity towards formation of propylene over by-products. Quantum chemical calculations indicate that under-coordination of the Pt atoms in the clusters is responsible for the surprisingly high reactivity compared with extended surfaces. We anticipate that these results will form the basis for development of a new class of catalysts by providing a route to bond-specific chemistry, ranging from energy-efficient and environmentally friendly synthesis strategies to the replacement of petrochemical feedstocks by abundant small alkanes.

Comparative study of CO adsorption on flat, stepped, and kinked Au surfaces using density functional theory

Our ab initio calculations of CO adsorption energies on low-Miller-index [(111) and (100)], stepped (211), and kinked (532) gold surfaces show a strong dependence on local coordination with a reduction in Au atom coordination leading to higher binding energies. We find trends in adsorption energies to be similar to those reported in experiments and calculations for other metal surfaces. The (532) surface provides insights into these trends because of the availability of a large number of kink sites which naturally have the lowest coordination (6). We also find that for all surfaces an increase in CO coverage triggers a decrease in the adsorption energy. Changes in the work function upon CO adsorption, as well as the frequencies of the CO vibrational modes, are calculated, and their coverage dependence is reported.

Comparative Density Functional Study of Methanol Decomposition on Cu4 and Co4 Clusters

A density functional theory study of the decomposition of methanol on Cu(4) and Co(4) clusters is presented. The reaction intermediates and activation barriers have been determined for reaction steps to form H(2) and CO. For both clusters, methanol decomposition initiated by C-H and O-H bond breaking was investigated. In the case of a Cu(4) cluster, methanol dehydrogenation through hydroxymethyl (CH(2)OH), hydroxymethylene (CHOH), formyl (CHO), and carbon monoxide (CO) is found to be slightly more favorable. For a Co(4) cluster, the dehydrogenation pathway through methoxy (CH(3)O) and formaldehyde (CH(2)O) is slightly more favorable. Each of these pathways results in formation of CO and H(2). The Co cluster pathway is very favorable thermodynamically and kinetically for dehydrogenation. However, since CO binds strongly, it is likely to poison methanol decomposition to H(2) and CO at low temperatures. In contrast, for the Cu cluster, CO poisoning is not likely to be a problem since it does not bind strongly, but the dehydrogenation steps are not energetically favorable. Pathways involving C-O bond cleavage are even less energetically favorable. The results are compared to our previous study of methanol decomposition on Pd(4) and Pd(8) clusters. Finally, all reaction energy changes and transition state energies, including those for the Pd clusters, are related in a linear, Brønsted-Evans-Polanyi plot.

Exploring Computational Design of Size-Specific Subnanometer Clusters Catalysts

Computational design of catalysts is currently an area of significant interest. While this area has made great strides in recent years, these methods have mainly been applied to solid heterogeneous catalysts. An emerging class of catalysts with very promising properties is that constructed from clusters of atoms at or below the nanoscale. The use of computational catalyst design methods for the construction and optimization of subnanometer clusters, however, has not yet been extensively explored. In this review, we discuss recent work on subnanometer catalysts in our group and discuss how computational catalyst design principles are being explored for this class of materials. Specifically, the origin of activity and selectivity for supported metal clusters that catalyze the production of propene and propylene oxide are discussed along with the implications of these studies for implementing a descriptor-based catalyst optimization. The extension of these ideas for designing a catalyst for methanol decomposition is then discussed and an application of a descriptor-based scheme for the optimization of methanol decomposition by subnanometer catalyst is shown.

Energetics of CO on stepped and kinked Cu surfaces : A comparative theoretical study

Our ab initio calculations of CO adsorption on several low and high miller index surfaces of Cu show that the adsorption energy increases as the coordination of the adsorption site decreases from 11 to 6, in qualitative agreement with experimental observations. On each surface the adsorption energy is also found to decrease with increase in coverage, although the decrement is not uniform. Calculated vibrational properties show an increase in the frequency of the metal-C mode with decrease in coordination whereas no such effect is found in the frequency of the CO stretch mode. Examination of the surface electronic structure shows a strong local effect of CO adsorption on the local density of state of the substrate atoms. We also provide some energetics of CO diffusion on Cu(111) and Cu(211).

Electronic and optical properties of titanium nitride bulk and surfaces from first principles calculations

Prediction of the frequency-dependent dielectric function of thin films poses computational challenges, and at the same time experimental characterization by spectroscopic ellipsometry remains difficult to interpret because of changes in stoichiometry and surface morphology, temperature, thickness of the film, or substrate. In this work, we report calculations for titanium nitride (TiN), a promising material for plasmonic applications because of less loss and other practical advantages compared to noble metals. We investigated structural, electronic, and optical properties of stoichiometric bulk TiN, as well as of the TiN(100), TiN(110), and TiN(111) outermost surfaces. Density functional theory (DFT) and many-body GW methods (Greens (G) function-based approximation with screened Coulomb interaction (W)) were used, ranging from G0W0, GW0 to partially self-consistent sc-GW0, as well as the GW-BSE (Bethe-Salpeter equation) and time-dependent DFT (TDDFT) methods for prediction of the optical properties. Str...

C and S induce changes in the electronic and geometric structure of Pd(533) and Pd(320)

We have performed ab initio electronic structure calculations of C and S adsorption on two vicinal surfaces of Pd with different terrace geometries and widths. We find that both adsorbates induce a significant perturbation of the surface electronic and geometric structure of Pd(533) and Pd(320). In particular, C adsorbed at the bridge site at the edge of a Pd chain in Pd(320) is found to penetrate the surface to form a sub-surface structure. The adsorption energies show an almost linear dependence on the number of adsorbate–metal bonds, and lie in the ranges 5.31–8.58 eV for C and 2.89–5.40 eV for S. A strong hybridization between adsorbate and surface electronic states causes a large splitting of the bands, leading to a drastic decrease in the local densities of electronic states at the Fermi level for Pd surface atoms neighbouring the adsorbate, which may poison catalytic activity of the surface. Comparison of the results for Pd(533) with those obtained earlier for Pd(211) suggests a local character of the impact of the adsorbate on the geometric and electronic structures of Pd surfaces.