Catalytic Activity Of Alloys Research Articles

Methylviologen mediated electrochemical synthesis of catalytically active ultrasmall bimetallic PdAg nanoparticles stabilized by CTAC

Efficient electrosynthesis of the monodisperse ultrasmall spherical mono- and bimetallic nanoparticles (NP) of Ag and Pd stabilized in the CTAC shell in solution bulk was carried out by methylviologen mediated reduction of equimolar amounts of Pd(II) and Ag(I) (1.5 mM) in the presence of CTAC (75 mM) at controlled potentials of the MV2+/MV•+ redox couple in water/0.1 M NaCl medium at room temperature. Metal ions are quantitatively converted to NP upon consumption of the theoretical amount of electricity. Sizes of isolated PdNP and AgNP are 5 ± 1 nm and 18 ± 5 nm, respectively. Bimetallic PdAgNP were obtained in three ways: (i) by preliminary synthesis of PdNP and subsequent reduction of AgCl (Pd/AgNP); (ii) by preliminary synthesis of AgNP and subsequent reduction of PdCl2 (Ag/PdNP); (iii) by joint reduction of PdCl2 and AgCl (Ag-PdNP). In all cases, a solid-solution alloy PdAgNP is obtained. The driving force for the alloy formation is the energy gain of ∼33 kJ/mol of an alloy. In the first way, the NP size (8 ± 2 nm) and the Pd content in the alloy (76%) are slightly higher than in the other two methods (6 ± 1 nm, 50 and 68%), but the alloy crystallite size (2–5 nm) in NP is the same. PdNP and alloys NP exhibit a high catalytic activity toward the p-nitrophenol reduction and the Suzuki-Miyaura cross-coupling reactions. In the reduction reaction, an addition of silver to palladium decreases the catalytic activity of NP. The catalytic activity of alloys in the coupling reaction correlates with the Pd content in an alloy. And the catalytic activity of the alloy Pd/AgNP obtained in the first method of the NP preparation is higher than that of PdNP.

Effect of Coverage on Catalytic Selectivity and Activity on Metallic and Alloy Catalysts; Vinyl Acetate Monomer Synthesis

The repulsive lateral interactions that occur at the high coverages present on catalysts under reaction conditions are likely to exert a significant influence on the reaction energetics. The effect of such lateral interactions are explored for the synthesis of vinyl acetate monomer (VAM) on Pd(111) and Au/Pd(111) alloy model single-crystal catalysts, where the reaction kinetics are monitored by reflection–absorption infrared spectroscopy (RAIRS). It is shown by comparing the reactivity of ethylene and its deuterated isotopomers, and by trapping reaction intermediates, that VAM is formed on Pd(111) by a mechanism first proposed by Samanos, where the reaction is initiated by coupling between ethylene and adsorbed acetate species to form an acetoxyethyl intermediate, followed by a rate-limiting β-hydride elimination step to produce VAM. The reaction comprises two sequential steps; a bond-forming reaction, followed by a bond-breaking step. Repulsive lateral interactions are expected to facilitate bond-forming reactions that reduce the total coverage, but inhibit bond-breaking steps, so that VAM formation provides an ideal reaction to explore these effects. Density functional theory (DFT) calculations confirm the proposed coverage effects and yield energy barriers for the reaction of ethylene with acetate-saturated Pd(111) surfaces that are in excellent agreement with experiment. It is also found that bond-breaking in VAM decomposition is inhibited on a crowded Pd(111) surface, indicating that high adsorbate coverages influence both reactivity and selectivity. VAM formation is also explored on Au/Pd(111) alloys. The catalytic activity of alloys is conventionally discussed by invoking ensemble (geometric) and electronic (ligand) effects. However, the acetate coverage also decreases with increasing gold content of the Au/Pd(111) alloy, thereby decreasing the repulsive lateral interactions. The coverage effects on alloys are found to be as large as the ensemble + electronic effects and induce a change in the rate-limiting step for VAM formation from β-hydride elimination to the ethylene-acetate coupling step.

Electrodeposition of Catalytically Active Ni-Mo Alloys / Elektroosadzanie Aktywnych Katalitycznie Stopów Ni-Mo

The work presents results of researches over the deposition of Ni-Mo alloys through the electrolysis conducted in galvanostatic conditions. In order to improve adhesion and aesthetic properties of the deposited alloys, SLS was added to the bath as a surface active agent. The influence of electrolyte pH was tested as well as concentrations of particular components on the composition, structure, cathodic current efficiency of the electrolysis process and catalytic activity of alloys in the process of hydrogen evolution. A positive influence of addition of molybdenum on catalytic activity was observed occurring in lowering the Tafel’s slope within the activation control range from 122 mV/dec (Ni) to 21 mV/dec (Ni-28.5Mo) for a reaction of water molecules reduction in 8 M NaOH at 90°C.

Effect of surface segregation on the catalytic activity of alloys: CO hydrogenation on Pd–Ni(111) surface

A semi-empirical model has been presented for studying the effect of surface segregation and local environment on the catalytic activity of the PdNi(111) surface for CO hydrogenation. Large Pd segregation to the surface has been found to affect the activation barrier for dissociation of molecules on the surface. It also affects the activation barriers for different reaction steps in the CO hydrogenation process to form methanol on the Pd–Ni surface. It is argued that for the Pd–Ni system, hydrogenation of the adsorbed HCO s species is the rate-limiting step. It has then been shown that an addition of even 5 at% of Pd in bulk Ni may increase the activity by as much as four orders of magnitude. So far as the local environmental effect is concerned, it is found that the most active centres for the HCO s hydrogenation step are the centre hollow sites having three-fold symmetry with 3 Pd atoms as nearest neighbours followed by centre hollow sites with two Pd atoms and one Ni atom as nearest neighbours.

Catalysis by amorphous metal alloys. Part 4.—Structural modification towards metastable states and catalytic activity of amorphous NI62B38 ribbon alloy

Alloys in the three different states, i.e. precrystallisation, metastable and stable crystallised states have been prepared by the structural modification of amorphous Ni62B38 ribbon alloy by thermal treatment. The catalytic activities of the alloys in these states for the hydrogenation of ethene and hydrogenolysis of ethane and cyclopropane have been correlated to the surface states which were characterised by XRD, DTA, SEM, ESCA, electrical resistivity and hydrogen chemisorption measurements. The maximum catalytic activity observed in the precrystallisation state is caused by the structural relaxation (rearrangement in short range) involving the change in the strength of the chemical bonding among nickel, boron and oxygen atoms. The aggregation of nickel atoms and the increase in the surface content of boron oxides, which is observed in the metastable state, reduce the number of active sites change the electronic state of surface nickel and bring about a great decrease in the catalytic activity. Further aggregation eventually brings about the crystallisation of the bulk alloy, leading to very low catalytic ability.

Influence of the composition and conditions of heat treatment of palladium-rhodium alloys on their catalytic activity with respect to the dehydrogenation of cyclohexane

1. The catalytic activity with respect to the dehydrogenation of cyclohexane on metallurgical alloys of Pd, containing from 7 to 20% Rh, decreases with the removal of H2 from the alloy and is entirely restored after treatment of the alloys with H2. The catalytic activity of the alloys increases when the temperature of their treatment with air is increased from 350 to 750‡. 2. An increase in the catalytic activity of Pd-Rh alloys was detected after experiments conducted at 450–470‡ in a stream of argon, evidently associated with the removal of firmly adsorbed hydrogen.