Hydrogen activation on (Cu,Zn)/gamma Al2O3 systems as studied by low-temperature hydrogen-deuterium equilibration

The catalytic activity of (Cu, Zn)gamma-Al2O3 systems for the H2-D2 equilibration has been investigated in a flow apparatus and in a circulation apparatus at 78 to 500 K (generally at 90 to 195 K). The following catalysts were studied: a) gamma-Al2O3; b) Cu(n)+/gamma-Al2O3, with different n values; c) a zinc-deficient zinc aluminate (<< defective ZnAl2O4 spinel >>) and ZnO; d) Zn2+/gamma-Al2O3; e) (Cu(n)+, Zn2+)/gamma-Al2O3, with different n values. Before kinetic experiments, the catalysts were heated in dry oxygen at 673 to 773 K with a final evacuation at the temperature of the oxygen treatment. In the experiments directed to test the reduced state of copper, catalysts were reduced to a controlled extent with H2 at 473 to 673 K. The poisoning effects of the reactants or CO2 preadsorbed at 298 K have also been investigated. The results show that the H2-D2 equilibration at low temperature is positively influenced by the presence of zinc ions on alumina but not by copper. The alumina support reduces the self-poisoning of the reactants. The nature of the active sites in the (Cu, Zn)gamma-Al2O3 system is discussed and it is inferred that in these catalysts the active site for the isotopic equilibration is different from that required for the hydrogenation of alkenes around room temperature.