Water splitting on composite plasmonic-metal/semiconductor photoelectrodes: evidence for selective plasmon-induced formation of charge carriers near the semiconductor surface

A critical factor limiting the rates of photocatalytic reactions, including water splitting, on oxide semiconductors is the high rate of charge-carrier recombination. In this contribution, we demonstrate that this issue can be alleviated significantly by combining a semiconductor photocatalyst with tailored plasmonic-metal nanostructures. Plasmonic nanostructures support the formation of resonant surface plasmons in response to a photon flux, localizing electromagnetic energy close to their surfaces. We present evidence that the interaction of localized electric fields with the neighboring semiconductor allows for the selective formation of electron/hole (e(-)/h(+)) pairs in the near-surface region of the semiconductor. The advantage of the formation of e(-)/h(+) pairs near the semiconductor surface is that these charge carriers are readily separated from each other and easily migrate to the surface, where they can perform photocatalytic transformations.     

Oxidation catalysis by oxide-supported Au nanostructures: the role of supports and the effect of external conditions

Oxide-supported Au nanostructures are promising low-temperature oxidation catalysts. It is generally observed that Au supported on reducible oxides is more active than Au supported on irreducible oxides. Recent studies also suggest that cationic Au(delta+) is responsible for the unique Au/oxide catalytic activity, contrary to the conventional perception that oxide supports donate electronic charge to Au. We have utilized density functional calculations and ab initio thermodynamic studies to investigate the oxidation state of Au nanostructures deposited on reducible and irreducible supports. We find that there are fundamental differences in the electronic structure of Au deposited on the different oxides. We propose a simple model, grounded in the first principles calculations, which can explain the oxide-specific catalytic activity of Au nanostructures and which can account for the presence and the role of cationic Au(delta+).     

High activity carbide supported catalysts for water gas shift

Nanostructured carbides are refractory materials with high surface areas that could be used as alternatives to the oxide materials that are widely used as support materials for heterogeneous catalysts. Carbides are also catalytically active for a variety of reactions, offering additional opportunities to tune the overall performance of the catalyst. In this paper we describe the synthesis of molybdenum carbide supported platinum (Pt/Mo(2)C) catalysts and their rates for the water gas shift reaction. The synthesis method allowed interaction of the metal precursor with the native, unpassivated support. The resulting materials possessed very high WGS rates and atypical Pt particle morphologies. Under differential conditions, rates for these catalysts were higher than those for the most active oxide-supported Pt catalysts and a commercial Cu-Zn-Al catalyst. Experimental and computational results suggested that active sites on the Pt/Mo(2)C catalysts were located on the perimeter of the Pt particles and that strong interactions between Pt and the Mo(2)C surface gave rise to raft-like particles.     

Engineering selectivity in heterogeneous catalysis: Ag nanowires as selective ethylene epoxidation catalysts

Controlling selectivity in heterogeneous catalysis is critical for the design of environmentally friendly catalytic processes that minimize the production of undesired byproducts and operate with high energy efficiency. We show that the Ag nanowire catalysts exhibit higher selectivity in the ethylene epoxidation reaction than conventional spherical particle catalysts. The higher selectivity of the nanowire catalysts was attributed to a higher concentration of the Ag(100) surface facets in the nanowire catalysts compared to the particle catalysts. Density functional theory calculations show that the transformation of the surface oxametallacycle intermediate to form the selective product, EO, is more favorable on the Ag(100) than on Ag(111). The studies show that recent advances in the controlled synthesis of uniform nanostructures with well-defined surface facets might provide an important platform for the design of highly selective heterogeneous catalysts.     

Synthesis, structure, and reactions of stable oxametallacycles from styrene oxide on Ag(111)

Styrene oxide undergoes an activated ring opening on Ag(111) at temperatures above 200 K. The product of this reaction is a stable oxametallacycle intermediate. The structure of this species has been obtained by density functional theory calculations and the computed vibrational spectrum is consistent with the experimental spectrum obtained using high-resolution electron energy loss spectroscopy. The oxametallacycle formed by ring-opening styrene oxide is structurally analogous to that previously observed for ring opening of epoxybutene on Ag(110) and represents the largest member of this adsorbate structure class yet isolated. In both cases, the epoxide ring opens at the carbon bearing the pendant unsaturated group, and the pendant group (phenyl in styrene oxide) is oriented nearly parallel to the surface plane. The oxametallacycle formed from styrene oxide reacts at 485 K to regenerate styrene oxide plus small amounts of phenylacetaldehyde. This peak temperature is similar to that previously reported for generation of styrene oxide from adsorbed styrene and oxygen atoms on Ag(111), suggesting that the epoxidation proceeds via the oxametallacycle intermediate isolated in the present work.     

Formation of a stable surface oxametallacycle that produces ethylene oxide.

Temperature programmed desorption, high-resolution electron energy loss spectroscopy (HREELS), and density functional theory (DFT) were used to investigate the adsorption and reaction of ethylene oxide (EO) on the Ag(111) surface. When EO is dosed onto Ag(111) at 140 K it adsorbs molecularly, desorbing without reaction at approximately 200 K. On the other hand, when EO is dosed at 250 K, the ring-opening of EO is activated, and a stable surface intermediate is formed. This intermediate reacts at 300 K to re-form EO plus a few other products. HREELS and DFT studies suggest that this stable intermediate is a surface oxametallacycle. Moreover, the activation energies observed for the reaction of the oxametallacycle to form EO are in an excellent agreement with the values reported for the steady-state ethylene epoxidation process. This work represents the first demonstration of surface oxametallacycle ring-closure to form EO. Comparison of the spectroscopic results obtained from silver single crystals and supported catalysts strongly suggests that oxametallacycles are important intermediates in silver-catalyzed ethylene epoxidation.     

On the mechanism of Cs promotion in ethylene epoxidation on Ag

We have examined the mechanism by which Cs impacts the selectivity of ethylene epoxidation on silver. The main focus was analysis of promoter-intermediate and promoter-transition state interactions. We show that Cs enhances selectivity to EO by stabilizing the transition state involved in the formation of EO relative to the transition state that is involved in the combustion. It is found that adsorbed Cs induces significant electric fields over Ag(111). This effect can be understood in terms of simple dipole/dipole interactions where the transition state involved in the selective pathway has a favorable dipole orientation as compared to the transition state involved in combustion.     

Control of ethylene epoxidation selectivity by surface oxametallacycles

Surface science experiments, DFT calculations, and kinetic isotope effect data are utilized to understand the elementary steps that govern the selectivity of silver catalysts for the partial oxidation of ethylene to produce ethylene oxide. It is proposed that selective and unselective pathways proceed via a common intermediate, the surface oxametallacycle. The structures of the transition states leading from this intermediate to selective and unselective products are calculated. From the calculated Gibbs free energies of activation for competing pathways, it is possible to predict selectivity to ethylene oxide as well as the magnitude of the kinetic isotope effect. The proposed mechanism is qualitatively and quantitatively in accord with experimental results.     

Controlling carbon surface chemistry by alloying: carbon tolerant reforming catalyst

Steam reforming is a process where a hydrocarbon is converted into hydrogen and oxygenated carbon species. Ni is often used as catalyst for the reaction. Long term stability of steam reforming catalysts is governed by their ability to selectively oxidize C atoms while preventing C-C bond formation. In this communication we demonstrate that C atom chemistry over Ni surfaces can be controlled by surface alloying. We show that bimetallic Sn/Ni catalyst is much more carbon-tolerant that monometallic Ni. The main reason for this is that Sn alloying results in dramatically lower rates of C-C bond formation as compared to C-oxidation. The bimetallic catalyst was identified in quantum computational studies of the underlying atomic-scale phenomena that govern C atom surface chemistry. The catalysts were also characterized with various electron- and X-ray-based microscopies and spectroscopies.