The Catalytic Decomposition of Hydrogen Peroxide Vapour by Oxides and Mixed Oxides

The vapour phase decomposition of hydrogen peroxide has been studied on a wide range of metal oxide, mixed oxide and promoted metal oxide surfaces. The investigation was conducted mainly in a flow system, in the temperature range 38 - 184 C using high-purity nitrogen as the carrier gas, and H2O2 partial pressures not greater than 1 mm of Hg. Oxide surfaces were held on vacuum flashed metal films, or "specpure" metal slips or else were presented to the gas stream as a compressed slip of the bulk oxide. There have been many studies of H2O2 decomposition in solution but few in the vapour phase and these have been confined almost exclusively to work with inactive catalysts such as glass, SiO2, SnO2 and Al2O3. No difficulty was found in obtaining reproducible results with active oxides for the decomposition efficiency (d) and also for the temperature coefficient (E A). On the basis of these two values the catalytic efficiencies of the oxides have been compared. In order of decreasing efficiency we find for single oxides:-Mn2O3 > PbO> Ag2O > CoO/CO2O3> Cu2O > NiO "green" > CuO > Fe2O3 >CdO > ZnO = MgO > Sn O2 > alpha-Al2O3 > glass (pyrex) This order taken in conjunction with the work of Garner, Stone, Wagner, Schwab and others on the decomposition of N2O and the combination of CO and O2 leads us to classify the oxides on the tendency they exhibit to absorb or reject oxygen which is in turn related to the defect-type of the oxide. We state then in the terminology of the semi-conductor field:- p type (PbO, CU2O etc.) > n-type (CuO, ZnO etc. ) > insulators Mg O, alpha-Al2O3) as catalysts. The classification is examined, with reference to the treatment of stoichiometric NiO with Li2O and Ga2O3, which produce strongly p-type and n-type oxide mixtures respectively. Two series of oxides mixed in stoichimetric proportion are also examined. These are the pre-spinel mixtures Mn2O3/XO, where X=Cd, Zn, Pb, Ni, and CuO/Y2O3 where Y=Al, Fe, Co. The effect of annealing temperature is carefully examined in the former case and it is shown that the best catalysts are those in which interdiffusion of ions has begun between oxide crystallites, but no stable spinel has yet formed. The most striking effects observed were in the synergesis obtained with the Fe+++ and Cu++ oxide mixture and the fact that inert ZnO seemed to promote the already extremely active On all catalysts, temperature coefficients were measured by making fast temperature changes. These values were, in most cases, dependant to some extent on the temperature from which the change was made - the higher temperature surface giving the highest temperature coefficient. Energies of activation calculated from temperature coefficients range from 4 - 34 K cals/mol. Kinetic studies at constant temperatures cm both p and n-type oxides viz:- Fe2O3, CoO, NiO yield. A mechanism is put forward to explain this. It is suggested that catalysis depends on cyclic transfer of electrons between the surface and H2O2 or its radicals and ions. O2- or O- adsorbed at p-type oxides (with attendant positive holes), or lattice oxygen vacancies (with attendant electrons) in the surface of n-type oxides, are suggested as suitable sites for the catalytic cycle. Increase of oxygen pressure accelerated the catalysis both on n-type and p-type oxides - probably acting by raising the surface concentration of such species as O-2. The catalyst surface is in slowly attained thermal equilibrium and also is subject to change with varying H2O2 pressure. These effects are observed and discussed. To assess the possibility of linking the flow results with those of previous workers with static systems, a brief investigation has been conducted in a static apparatus. The conclusion is that comparison between static and dynamic systems is not easy to make. Information on the physical properties of the oxide systems has been obtained from magnetic susceptibility, surface area and X-ray measurements. The significance of which is fully discussed