Effect of form of the surface reactivity of differently prepared zinc oxides
The relationships between the from of crystallites and their adsorption properties have been investigated on ZnO samples of various origins by means of electron microscopy, X-ray diffraction and adsorption calorimetry. Three polycrystalline ZnO samples have been studied: one obtained by ignition of zinc metal (Kadox) and the others obtained by decompositon of zinc carbonate and oxalate. The higher surface reactivity found on Kadox ZnO in comparison with that found on ex-salt ZnO is not due to the preferential development of crystal planes at the surface but to the presence of better defined single microcrystals with sharp edges. In all the cases examined the preparation route leads to a difference in morphology and consequently a difference in reactivity. Carbon monoxide and hydrogen have been used as surface probes for active sites: CO is coordinated onto cations exposed at the edges between the (0001) and (1010) planes; H2 is adsorbed in different forms, one of which, type I, occurs on sites located at the same edge as CO. Prereduction of ZnO reduces the adsorption activity towrds both gases; the extent of this reduction also depends upon the actual morphology.