Effect Of Alloy Formation Research Articles

Selective catalytic oxidation of ammonia to nitrogen over zeolite-supported Pt-Au catalysts: Effects of alloy formation and acid sites

The selective catalytic oxidation of ammonia (NH3-SCO) to N2 and H2O at low temperatures was studied to remove low-concentration (50 ppm) NH3 from the air. A series of mordenite (M20)-supported Pt-Au alloy catalysts with different Pt:Au ratios were synthesized and tested in the NH3-SCO. The NH3 conversion and N2 selectivity both depended on the ratio of Pt to Au. The 0.52 wt% Pt84Au16/M20 catalyst showed the highest NH3 conversion (90% at 144 °C) even better than Pt100/M20, which was ascribed to the combination of electron-deficient Pt and electron-rich Au in the alloy nanoparticles. The effects of acid sites on the NH3-SCO were investigated by NaOH treatment of Pt84Au16/M20 catalyst. Results suggested that the acid sites could help to increase NH3 conversion, and that Brønsted acid sites are essential for the selective formation of N2.

PdZn catalysts for CO2 hydrogenation to methanol using chemical vapour impregnation (CVI).

The formation of PdZn bimetallic alloys on ZnO, TiO2 and Al2O3 supports was investigated, together with the effect of alloy formation on the CO2 hydrogenation reaction. The chemical vapour impregnation (CVI) method produced PdZn nanoparticles with diameters of 3-6 nm. X-ray photoelectron spectroscopy and X-ray diffraction revealed the changes in the structure of the PdZn alloy that help stabilise formate intermediates during methanol synthesis. PdZn supported on TiO2 exhibits high methanol productivity of 1730 mmol kgcat-1 h-1 that is associated with the high dispersion of the supported PdZn alloy.

Developing Pt based bimetallic and trimetallic carbon supported catalysts for aqueous-phase reforming of biomass-derived compounds

In the present work, a series of Pt-based bimetallic and trimetallic activated carbon (AC) and multiwalled carbon nanotube (MWCNT) supported catalysts at a 10 wt% total metal loading were prepared by wetness impregnation method to evaluate gasification performance of lignocellulosic biomass hydrolyzate and glucose solution for hydrogen-rich gas production by aqueous-phase reforming (APR) process. The Ru, Ni and Sn were added as promoter to platinum based supported catalysts with a 1:1 atomic ratio for bimetalic catalysts and 1:1:1 for trimetallic catalysts. The interactions between metals and the effect of alloy formation on gasification activity of the catalysts were investigated. The catalytic activity of Pt-MWCNT catalyst increased 21.2% by the addition of Ru metal. Bimetallic PtRu-MWCNT catalyst had a significantly higher catalytic activity and hydrogen selectivity compared to monometallic Pt-MWCNT for APR of biomass compounds. Since Ni and Ru metals promoted the activity of platinum catalysts, those catalysts could be used for displacement of platinum catalysts due to their low cost and relatively good catalytic activity in gasification of biomass-derived compounds.

ChemInform Abstract: The Surface Science Approach Toward Understanding Automotive Exhaust Conversion Catalysis at the Atomic Level

This review focuses on the reactions and catalysts used for control of emissions of exhaust gases for gasoline-fueled automobiles with emphasis on fundamental understanding of the surface processes. Attention is paid to three-way catalysts, which simultaneously enhance the conversion of CO, hydrocarbons, and nitrogen oxides. The mechanisms of the CO oxidation and nitrogen oxide reactions, the specific differences in behavior of Pt, Pd, and Rh, the effect of alloy formation, and the role of ceria used as additive in three-way catalysts are discussed. Results of surface science studies are compared with results reported for supported catalysts. For CO oxidation, there is excellent agreement between results obtained for single crystal surfaces and supported catalysts. The kinetics of the reactions on pure metals can be understood on the basis of the kinetics parameters obtained from single-crystal studies. For the NO reduction reactions, there is qualitative agreement between results obtained with single-crystal and supported catalysts. The major effects of alloy formation can be understood on the basis of the surface composition.

Pt−Co Bimetallic Catalyst Supported on Single-Walled Carbon Nanotubes: Effect of Alloy Formation and Oxygen Containing Groups

Pt monometallic and Pt−Co bimetallic catalysts have been prepared on single-walled carbon nanotubes (SWNT) with and without HNO3 treatment. The HNO3 treatment introduced oxygen containing groups (OCGs), which affect both the structure and activity of the catalyst. The introduction of OCGs does not affect the structure of Pt monometallic catalysts but increased the dispersion in the bimetallic catalysts. The aqueous phase re-forming (APR) activity of the bimetallic catalysts is also affected by the OCGs, because the local concentration of the reactant around the SWNT support with OCGs is less than the case without OCGs. The two effects act on activity in opposing directions so the bimetallic catalysts on the two supports give similar APR yields, but this discovery gives us direction and a basis for the future design and improvement of SWNT supported catalysts.

Mixing enthalpies in Ag–Ca, Ag–Eu and Ag–Yb liquid alloys

Abstract The enthalpies of mixing in liquid Ag – Ca, Ag – Eu and Ag – Yb alloys were measured by isoperibolic calorimetry at 1300 K. The experimental data were used to calculate the mixing enthalpies within the total composition region. The results of the investigation show exothermic effect of alloy formation. The minimum integral enthalpies of mixing are – 23.5 ± 1.0 kJ · mol – 1 (the Ag – Ca system); –16.5 ± 1.5 kJ · mol – 1 (the Ag – Eu system) and – 21.5 ± 1.9 kJ · mol – 1 (the Ag – Yb system) and are located in the silver-rich composition regions.

Ethanol reforming processes over ZnO-supported palladium catalysts: Effect of alloy formation

ZnO- and SiO 2-supported palladium catalysts were evaluated in the ethanol steam-reforming and oxidative ethanol steam-reforming reactions in the temperature range 548–723 K. The catalysts were characterized before and after reaction by X-ray diffraction, X-ray photoelectron spectroscopy, high-resolution transmission electron microscopy and infrared spectroscopy using CO as a probe molecule. On silica-supported Pd catalyst, ethanol decomposes into H 2, CO and CH 4 in both steam-reforming and oxidative steam-reforming reactions. In contrast, ZnO-supported catalysts containing the PdZn phase exhibit a better catalytic performance for hydrogen production through dehydrogenation of ethanol into acetaldehyde and ulterior reforming.

Electrochemical Impedance and X‐Ray Absorption Spectroscopy (EXAFS) as In‐Situ Methods to Study the PEMFC Anode

AbstractThe effect of alloy formation on the performance of carbonsupported Pt and Ru fuel cell catalysts has been investigated by electrochemical impedance spectroscopy (EIS), X‐ray absorption spectroscopy (EXAFS) and fuel cell tests. The CO tolerance of the respective anode electrocatalysts has been tested in both reformate and methanol operation. This paper describes results for mixtures of carbon‐supported Pt and carbon‐supported Ru catalysts. The particle sizes of the active metals have been modified by heat‐treatment in nitrogen. It has been observed that the size of Pt and Ru particles has a significant influence on the electrochemical performance, the activity improving for smaller Ru particles. The performance of certain Pt‐Ru mixtures in a single cell fuel cell is comparable with an alloy catalyst.

CO oxidation over the Pt-Rh system. 2. Reaction on an alloy

The oxidation of CO over thin films of a Pt-Rh alloy prepared by sequential vacuum evaporation of the metals on an inert support has been investigated at low pressures (P < 2''10-5 mbar). The results are compared with the data on the individual surfaces of Pt and Rh. It is found that the activity of the alloy is intermediate between the activities of the individual metals. The effect of alloy formation on the catalytic activity in CO oxidation is discussed.

Effect of a vacuum-deposited metal film on the CV of the Li insertion/extraction reaction at a graphitized carbon fiber electrode

A metal film of silver, copper, or gold was vacuum-deposited on the surface of mesophase pitch-based carbon fiber prepared at 3100 °C, and the Li insertion/extraction behaviour was examined by measuring the cyclic voltammogram (CV) for various film thicknesses in a non-aqueous electrolyte containing 1 M LiClO 4. The peak height of the CV was examined in detail for the deposited silver film. The peak height increased at first with increasing film thickness up to 150 Å, but then began to decrease. At around 200 Å, a minimum peak height which was nearly the same as that of the pristine sample was observed. The peak height then increased again up to a film thickness of 400 Å, where it was two times higher than that of pristine sample, then decreased gradually. Such a complicated CV could not be observed with a gold or copper film where the enhancing effect was monotonous and more remarkable with a thinner film. X-ray diffraction patterns of the silver film deposited on the carbon fiber showed a strong crystal orientation to the carbon substrate like epitaxial growth, which was found to be dependent on both the thickness and the deposition rate. The CV peak height correlated well with the silver crystal orientation, implying that the electrochemical reaction rate differs on different crystal faces. Such could not be found in the case of gold or copper. The peak enhancing effect and the cycle behaviour were elucidated in view of the nature of solid electrolyte interphase (SEI), conductivity, and the effect of alloy formation.

Effect of alloy formation in Ag−Au system on electrical double layer structure and hydrogen evolution kinetics

The variations of the zero-charge potential, specific surface charge, and free interface energy in the Ag−Au|F−(H2O) system are studied with the use of impedance measurements in a wide range of alloy compositions. In terms of the concept of a finitely thick interface layer, a theoretical isotherm of the surface composition of the alloy, i.e., the concentration dependence of the activity coefficients of the components is plotted. It is shown that, in Ag−Au alloys at 298 K, gold is surface active, apparently, due to its higher hydrophilicity. The exchange current of the overall reaction of hydrogen evolution on Ag−Au alloys from aqueous sulfuric-acid solutions depends nonlinearily on the surface concentration of gold.

Studies on CuIn precursor for the preparation of CuInSe2 thin films by the selenization technique

Abstract The effect of temperature on the degree of alloy formation between Cu and In layers has been studied for the preparation of CuInSe 2 films by the selenization technique. The mechanism of alloy formation is different in bilayers annealed at temperatures lower and higher than the melting point of In, or prepared by deposition of Cu and In at 200°C and 150°C, respectively. While the annealing of the precursor up to 200°C produces an alloy in the interface region of Cu and In layers, annealing at 500°C completely changes its morphology. The effect of alloy formation in the precursor on the selenized CuInSe 2 films is studied using structural, morphological, optical and compositional properties.

Particle size and structural effects in platinum electrocatalysis

Of the several factors which influence electrocatalytic activity, particle size and structural effects are of crucial importance, but their effects and mechanism of interaction,vis-a-vis overall performance, have been, at best, vaguely understood. The situation is further aggravated by the use of a wide range of experimental conditions resulting in non-comparable data. This paper attempts systematically to present the developments to date in the understanding of these structural interactions and to point out areas for future investigation. The entire content of this review has been examined from the context of the highly dispersed Pt electrocatalyst, primarily because it has been examined in the greatest detail. In the first two sections a general idea on the correlations between surface microstructure and geometric model is presented. Subsequently, indicators of a direct correlation between particle size and catalyst support synergism are considered. The structural and particle size effect on electrocatalysis is examined from the point of view of anodic hydrogen oxidation and cathodic oxygen reduction reactions. The hydrogen and oxygen chemisorption effects, presented with the discussion on the anodic and cathodic electrocatalytic reactions, provide important clues toward resolving some of the controversial findings, especially on the dependence of particle size on the anodic hydrogen oxidation reaction. Finally, the effect of alloy formation on the cathodic oxygen reduction reaction is discussed, providing insights into the structural aspect.