Noble metal

About: Noble metal is a research topic. Over the lifetime, 15113 publications have been published within this topic receiving 337947 citations.

Role of Brønsted and Lewis acid sites on Ni/TiO2 catalyst for vapour phase hydrogenation of levulinic acid: Kinetic and mechanistic study

Abstract: The TiO2 supported Ni catalysts were investigated for vapour phase hydrogenation of aqueous levulinic acid at ambient pressure. Ni/TiO2 demonstrated high selectivity of γ-valerolactone (GVL) compared to noble metal (Pt, Pd, Ru) catalysts. The surface acid sites played an important role on the product distribution. Pyridine adsorbed DRIFT spectra revealed that the Lewis acid sites on Ni/TiO2 were responsible for high selectivity of γ-valerolactone. In contrast, the Bronsted acid sites are prone to ring opening of γ-valerolactone to produce valeric acid and hydrocarbons. The physicochemical characteristics of Ni/TiO2 were correlated with γ-valerolactone rates. Based on the kinetic study a mechanism has been proposed.

Hydrogen Generation upon Nanocatalyzed Hydrolysis of Hydrogen-Rich Boron Derivatives: Recent Developments.

Abstract: ConspectusProduction of hydrogen from nonfossil sources is essential toward the generation of sustainable energy. Hydrogen generation upon hydrolysis of stable hydrogen-rich materials has long been proposed as a possibility of hydrogen disposal on site, because transport of explosive hydrogen gas is dangerous. Hydrolysis of some boron derivatives could rapidly produce large amounts of hydrogen, but this requires the presence of very active catalysts. Indeed, late transition-metal nanocatalysts have recently been developed for the hydrolysis of a few hydrogen-rich precursors.Our research group has focused on the improvement and optimization of highly performing Earth-abundant transition-metal-based nanocatalysts, optimization of remarkable synergies between different metals in nanoalloys, supports including positive synergy with nanoparticles (NPs) for rapid hydrogen generation, comparison between various endo- or exoreceptors working as homogeneous and heterogeneous supports, mechanistic research, and comparison of the nanocatalyzed hydrolysis of several boron hydrides.First, hydrogen production upon hydrolysis of ammonia borane, AB (3 mol H2 per mol AB) was examined with heterogeneous endoreceptors. Thus, a highly performing Ni@ZIF-8 nanocatalyst was found to be superior over other Earth-abundant nanocatalysts and supports. With 85.7 molH2·molcat-1·min-1 at 25 °C, this Ni nanocatalyst surpassed the results of previous Earth-abundant nanocatalysts. The presence of NaOH accelerated the reaction, and a remarkable pH-dependent "on-off" control of the H2 production was established. Bimetallic nanoalloys Ni-Pt@ZIF-8 showed a dramatic volcano effect optimized with a nanoalloy containing 2/3 Ni and 1/3 Pt. The rate reached 600 molH2·molcat-1·min-1 and 2222 molH2·molPt-1·min-1 at 20 °C, which much overtook the performances of both related nanocatalysts Ni@ZIF-8 and Pt@ZIF-8. Next, hydrogen production was also researched via hydrolysis of sodium borohydride (4 mol H2 per mol NaBH4) using nanocatalysts in ZIF-8, and, among Earth-abundant nanocatalysts, Co@ZIF-8 showed the best performance, outperforming previous Co nanocatalysts. For exoreceptors, "click" dendrimers containing triazole ligands on their tripodal tethers were used as supports for homogeneous (semiheterogeneous) catalysis of both AB and NaBH4 hydrolysis. For both reactions, Co was found to be the best Earth-abundant metal, Pt the best noble metal, and Co1Pt1 the best nanoalloy, with synergistic effects. Based on kinetic measurements and kinetic isotope effects for all of these reactions, mechanisms are proposed and the hydrogen produced was further used in tandem reactions. Overall, dramatic triple synergies between these nanocatalyst components have allowed hydrogen release within a few seconds under ambient conditions. These nanocatalyst improvements and mechanistic findings should also inspire further nanocatalyst design in various areas of hydrogen production.

Water-gas shift: in situ spectroscopic studies of noble metal promoted ceria catalysts for CO removal in fuel cell reformers and mechanistic implications

Abstract: In situ, steady-state diffuse reflectance infrared Fourier transform spectroscopy (DRIFTS) measurements for adsorption of CO and for water-gas shift (WGS) reaction conditions indicate that formates are present on the surface of reduced ceria, being formed by reaction with geminal OH groups that are present after reduction of the ceria surface shell. The process of surface shell reduction was strongly catalyzed by the presence of metal, while changing very little if at all the catalysis of bulk reduction. Gold was found to reduce the surface ceria at a lower temperature than that of platinum, but platinum gave a slightly higher degree of surface shell reduction. Under steady-state WGS at a high H 2 O/CO ratio, the concentrations of surface formates are strongly limited at high CO conversions, while metalCO was not. Since under these conditions, CO exhibits a first order rate dependency, the active site should move to sparser coverages of CO, indicating that a formate mechanism is more likely the correct one. At low temperatures and conversions, the formates were close to the equilibrium adsorption/desorption coverages obtained from only CO adsorption. In situ X-ray absorption near edge spectroscopy (XANES) directly links the metal to its ability to aid in catalyzing reduction of the surface shell of ceria. After surface shell reduction of ceria by hydrogen, addition of water to the hydrogen stream gave no indication of reoxidation whatsoever, as would be necessary under a ceria-mediated redox process. The reoxidation of ceria by water under helium alone was very slow, and only slight changes were recorded at 350 °C. Therefore, the results strongly favor a formate mechanistic scheme for low temperature water-gas shift. To date, most researchers have claimed a ceria-mediated redox process operating to describe the mechanism. Both mechanisms require reduction of the ceria surface.

A layered automotive catalytic composite

Abstract: This invention relates to a catalytic composite for treating an exhaust gas to minimize the hydrogen sulfide content thereof and comprising a first support which is a refractory inorganic oxide having dispersed thereon at least one noble metal component and having dispersed immediately thereon an overlayer comprising at least one oxygen storage component and optionally a second support which is a refractory inorganic oxide. The first support may be selected from the group consisting of alumina, silica, titania, zirconia and aluminosilicates with alumina being preferred. The noble metal component may be selected from the group consisting of platinum, palladium, rhodium, ruthenium and iridium. The oxygen storage component is an oxide of a metal which includes cerium, iron, nickel, cobalt lanthanium, neodymium, praseodymium, etc. and mixtures thereof. Cerium oxide is a preferred oxygen storage component. The second support may be selected from the group consisting of alumina, silica, titania, zirconia and aluminosilicates, with alumina preferred.