The parahydrogen conversion on palladium-gold alloys

Abstract

The activation energy of the parahydrogen conversion on palladium-rich alloys is 3-5 kcal./mole and increases abruptly between 40 and 30 atomic per cent. palladium t o the value of 8-5 kcal. found for gold-rich alloys. Magnetic measurements have identified this composition as corresponding t o the complete filling of the d-band, or atomic d-orbitals, with electrons. Vacant d-orbitals are therefore essential for the low-temperature catalysis, and it is suggested that they bond the chemisorbed hydrogen atoms M-H.The reaction goes through a triatomic H, complex formed by entry of an H, molecule from the van der Waals layer,It is postulated that each site exerts two bonds t o the activated complex, employing " atomic " d and hybrid d2.56 sP*.*~ metallic orbitals. The values of temperature-independent factor found agree with earlier calculations for this mechanism. The d-band of palladium may also be filled by electrons from dissolved hydrogen atoms which cause a similar increase in activation energy.This work 1 was initiated in Bristol University in 1945 in an attempt to relate the activity of transition metal catalysts to their electronic structure. I n the past various efforts have been made to provide a theoretical basis for catalyst activity, by relating catalytic activity to lattice spacing. Thus, on the experimental side there are the papers by Balandin,2 Long et aZ.,3 Twigg, Herington and Rideal,'~ 6 and Beeck and his co-workers.6B 7 On the theoretical side, calculations of the chemisorption of hydrogen molecules by the Eyring-Polanyi method indicate a marked effect of the lattice-spacing in the surface on the activation energy of the process.** @ Recent advances in our knowledge of the metallic state now make it possible to begin a more fundamental approach to the problem, and to attempt to relate catalyst activity t o the nature and occupation of the electronic energy levels of the catalyst. All those metals active as hydrogenation catalysts fall into the group of transition elements, with the exception of copper which, in general, has a higher activation energy than the others. The atoms of these elements possess partly empty d-shells and the theory of the bulk metal may be treated by either of two methods. The first method, due to Mott and * Part I in a series of papers on the Electronic Basis of Catalyst Activity.

Synthesis of vinyl acetate on Pd-based catalysts

“…Shortrange ordering in Au-Pd alloys has been reported and reviewed by Allison et al [24]. Early work on the properties of these PdAu catalysts concluded that the 0.6 d-band vacancies of pure Pd fill at an alloy composition of $60% Au, a results that stimulated studies of the catalytic properties of Au-Pd mixtures at this critical composition, e.g., ortho, para-hydrogen conversion [30]. However, in surveying a number of reactions, the consensus is that surface composition is more important for adsorption and catalysis than is the bulk electronic structure [12,24,[31][32][33][34].…”

Dense Ceramic Membranes for Hydrogen Separation

“…It has also been suggested that exchange may occur via triatomic intermediates. These two mecha-nisms are usually referred to as the Bonhoeffer-Farkas and the Rideal-Eley mechanisms [11,12]. The former has been observed to be operative usually at higher temperatures whereas the latter, involving intermediates, is operative only at low temperatures.…”

Is Nanostructuring Sufficient To Get Catalytically Active Au?

“…The reaction takes place between reactant from the gas phase and an adsorbed one (Scheme 1.2). The Eley-Rideal mechanism well describes the kinetics of the hydrogen-deuterium reaction [10].…”