Catalysts for Hydrogen Removal - Kinetic Paradox and Functioning at High Concentration of Hydrogen

Platinum metals dispersed on a porous carrier, e.g. -Al2O3, are used as catalysts for removal of small amounts of hydrogen from the air, where the excess of oxygen is significant.The recombination reaction of H2 and O2 on smooth platinum proceeds at a high rate only in gas mixes with an excess of hydrogen. When the concentration of oxygen exceeds that of hydrogen, in terms of stoichiometric ratio, the process slows down sharply and eventually stops completely.In research undertaken at the Karpov Institute of Physical Chemistry (Moscow) forty years ago the electrochemical mechanism of red-ox reactions was proposed as an explanation for this inhibition by excess oxygen. The results of ellipsometric analysis pointed to the formation of a protective monolayer of PtO molecules on the Pt surface in an oxygen-rich atmosphere. It was observed that the recombination reaction proceeds at a high rate with the use of a porous catalyst at any concentrations of reactant gases. The reason for that lies in the mechanism of the catalysis: the reaction proceeds at a certain depth in the porous body of the catalyst. Hydrogen, which has higher mobility, penetrates in larger quantity than oxygen, thus creating there the stoichiometric excess.To test the proposed mechanism of recombination, the catalytic reaction was studied ) with porous carriers of various thicknesses and b) with metal grids of various porosities covering the catalyst. The data obtained have confirmed unequivocally the earlier hypothesis of hydrogenation of a porous catalyst.Such insight has allowed the authors to develop more effective prototypes of catalyst for removal of hydrogen. In particular, by using a porous grid cover to remove excess heat in the reaction zone of the catalyst plate we achieved a considerable expansion of the region of hydrogen concentrations where the catalyst is both effective and reliable.