Supported palladium catalyst prepared from amorphous palladium–zirconium. Structural properties and catalytic behaviour in the hydrogenation of carbon dioxide

Abstract

Amorphous Pd₁Zr₂ has been used as a precursor for the preparation of CO₂ hydrogenation catalysts. The active catalysts were obtained by exposing the amorphous alloy to CO₂ hydrogenation conditions. During this exposure the bulk and surface structure of the alloy changed drastically. The most important changes were: (i) partial crystallization of the amorphous alloy; (ii) the formation of palladium metal particles; and (iii) the oxidation of zirconium to zirconium dioxide. These changes were accompanied by an increase in both the B.E.T. surface area and the palladium surface area of more than two orders of magnitude. Palladium was found to exist in two discernible phases, as pure metal and as solid solution with hydrogen. Two types of palladium particles were distinguishable, small disordered particles and larger crystalline particles of ca. 12 nm mean size. The catalytic behaviour of such catalysts was compared with the corresponding behaviour of a Pd/ZrO₂ catalyst prepared by wet impregnation of crystalline zirconia with a palladium salt. The activity and selectivity behaviours of the two catalyst types were largely different. Palladium contained in the catalyst derived from amorphous Pd₁Zr₂ was more than an order of magnitude more active for CO₂ conversion than palladium in the catalyst prepared by wet impregnation. Over Pd/ZrO₂ derived from Pd₁Zr₂ the products were metane, methanol amd CO, whereas over the conventionally prepared Pd/ZrO₂ no substantial methanol formation was observed.