Formation, structure and catalytic activity of evaporated palladium-silver films

Abstract

Palladium-silver alloy films were prepared in vacuum either by slow simultaneous evaporation of the component metals from separate wire sources, or from an alloy wire, on to a hot glass substrate, or alternatively by successive evaporation of layers of the metals which were homogenized by heating. X-ray techniques were used to study the effects of the preparative conditions on the degree of homogeneity produced through the bulk of the films and to determine their compositions over the extended area of the reaction vessel. In preliminary catalytic experiments, activation energies and kinetics for CO oxidation over Pd/Ag alloy films, evaporated and annealed at 400"C, corresponded to those for similar alloy systems in massive form.

Hydrogen promotion of the palladium-catalyzed carbon monoxide oxidation

“…Moss and Thomas reported that prior exposure to hydrogen increased the activity of palladium foils for the batchwise oxidation of carbon monoxide. 2 Modell suggested that palladium-hydrogen alloys decrease the activation energy for the oxidation.3 Changes in the infrared spectra of CO chemisorbed on platinum4 and nickel5 samples, brought about by the the addition of hydrogen, have been reported. However, these separate kinetic and spectral changes were not correlated because each employed different catalyst samples.…”

The formation of homogeneous copper - nickel alloy films in ultra-high vacuum

“…Clearly surface cleanliness can be improved directly by increasing pumping speed. Moss et al (1970) have been successful in preparing Pd-Ag alloy films in this way. In the present work pumping speed is limited by the 2.25 inch bore bakeable all-metal cock, the presence of which is necessary for gas-handling in planned adsorption work.…”

“…Phase separation, which may occur at clean surfaces because rapid atom migration can permit approach there to the thermodynamically more stable two-phase condition, proves to be more difficult to establish experimentally. LEED has failed to demonstrate such an effect (Ertl and Kiippers 1971) but this technique may not be well-suited to yielding a firm decision for the copper-nickel system (Mozer et al 1968). The existence of the (related) clustering of nickel atoms, however, appears well established and, while debate continues as to its magnitude for variously prepared alloy samples (Seib and Spicer 1970), results to date suggest that it may be at least a contributory reason for the lack of success in relating adsorptive and catalytic activity of copper-nickel alloys to their electronic structure (Dowden 1970, McMahon et al 1971).…”

“…A more detailed examination of film homogeneity was undertaken for those films which gave acceptable lattice parameters. Comparison of 11 1 and 222 x-ray diffractometer intensity profiles for each sample (Moss and Thomas 1964) was not possible because the latter were weak. However, we have followed a procedure which emphasizes (i) agreement of the lattice constant with the accepted literature value for a composition based on loss of weight of the evaporation sources, and (ii) a symmetrical resolved 11 1 x-ray diffractometer profile, as sufficient criteria for homogeneity.…”