Photocatalysis based on optically active, “plasmonic” metal nanoparticles has emerged as a promising approach to facilitate light-driven chemical conversions under far milder conditions than thermal catalysis. However, an understanding of the relation between thermal and electronic excitations has been lacking. We report the substantial light-induced reduction of the thermal activation barrier for ammonia decomposition on a plasmonic photocatalyst. We introduce the concept of a light-dependent activation barrier to account for the effect of light illumination on electronic and thermal excitations in a single unified picture. This framework provides insight into the specific role of hot carriers in plasmon-mediated photochemistry, which is critically important for designing energy-efficient plasmonic photocatalysts.

http://science.sciencemag.org/content/sci/362/6410/69.full.pdf

Quantifying hot carrier and thermal contributions in plasmonic photocatalysis

Reaction kinetic studies were conducted for aqueous-phase reforming of ethylene glycol over silica-supported Ni, Pd, Pt, Ru, Rh and Ir catalysts at temperatures of 483 and 498 K and at a total pressure of 22 bar. The rates of production of hydrogen, carbon dioxide, carbon monoxide, methane, ethane and higher paraffins were measured for conversion of an aqueous feed solution containing 10 wt.% ethylene glycol. It was observed that the overall catalytic activity for ethylene glycol reforming, as measured by the rate of CO 2 production at 483 K, decreases in the following order for silica-supported metals: Pt∼Ni>Ru>Rh∼Pd>Ir. Silica supported Rh, Ru and Ni showed a low selectivity for production of H 2 and a high selectivity for alkane production. In addition, Ni/SiO 2 showed significant deactivation at the higher temperature of 498 K. Silica-supported Pt and Pd catalysts exhibited relatively high selectivities for production of H 2 , with low rates of alkane production. It appears that catalysts based on Pt and Pd may be promising materials for the selective production of hydrogen by aqueous-phase reforming of oxygenated hydrocarbons, such as ethylene glycol.

Aqueous-phase reforming of ethylene glycol on silica-supported metal catalysts

The reduction of CuO dispersed on fluorite-type oxide catalysts, namely La-doped CeO2 and Y-doped ZrO2 was studied in this work. On both supports distinct copper species were identified as a function of copper content by temperature-programmed reduction (TPR) by H2 and CH4, X-ray photoelectron spectroscopy (XPS), X-ray powder diffraction (XRD) and scanning transmission electron microscopy/energy dispersive X-ray (STEM/EDX) analyses. At low copper loading ( 15 at%), in addition to clusters, larger CuO particles are present which are reduced at higher temperature close to the reduction temperature of bulk CuO. At copper loading lower than ca. 5 at%, copper is present as highly dispersed clusters or isolated Cu ions, which interact strongly with the fluorite-type oxide, thus requiring higher reduction temperature. However, the latter is still below the bulk CuO reduction temperature. Copper is more stabilized when dispersed in Ce(La)O2 than in Zr(Y)O2 matrix, so that reduction of copper oxide species requires lower temperatures on the Zr(Y)O2-based catalysts. The reducibility of the doped ceria is enhanced by the presence of copper in both H2- and CH4-TPR. On the other hand no such interaction is present in CuZr(Y)O2 system. The activity of various copper species for methane oxidation is discussed.

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Reduction characteristics of copper oxide in cerium and zirconium oxide systems

Publisher Summary Metals constitute a wide class of catalysts, and because catalysis occurs on the surface, there is an economic incentive, especially for precious metals, to obtain catalysts in the form of small metal particles. This, however, raises two main problems. One is fundamental in nature and addresses the question as to below which particle size the metallic properties are lost. The other is more practical and concerns the preparation and characterization of very small particles and their catalytic activity. The other problem concerns the practical ways of producing metals with particle sizes around the critical value calculated above. The easiest and most practical way to achieve this goal is to deposit the metal in low concentration on a high-surface-area support. The main purpose of using a carrier is, of course, to achieve a high dispersion of the metal and to stabilize it against sintering. In a number of reactions, however, the metal support is not inert and the overall process is actually a combination of two functions: that of the metal and that of the catalytically active support.

The Influence of Particle Size on the Catalytic Properties of Supported Metals

The present paper provides a synopsis of the Fischer-Tropsch (FT) synthesis for hydrocarbons. Emphasis is placed on issues related to catalyst development and design strategies, contemporary views on reaction kinetics and mechanism and the significant advancement in industrial reactor technology. Activity and product selectivity control still remain major incentives for continued research activities and the recent application of multimetallic catalysts appears promising. The article also highlights the usefulness of periodic operation of FT reactors for product distribution, time-average catalyst longevity and synthesis rates.

Hydrocarbon synthesis via Fischer-Tropsch reaction: travails and triumphs

Etude par chimisorption: Cu, Rh, Pt, Pd, Ni, Ru. Etude par chromatographie. Etude par desorption a temperature programmee de H 2 (pour Ni, Ir, Pt). Etude par XPS

J. J. F. Scholten

Papers

Surface Characterization of Supported and Nonsupported Hydrogenation Catalysts