Activity and deactivation of Pd/Al2O3 catalysts in hydrogen and oxygen recombination reaction; a role of alkali (Li, Cs) dopant

Abstract

Activity and deactivation of alkali (Li, Cs) doped 2%Pd/Al2O3 catalyst in the exothermic H2 and O2 reaction have been studied in view of potential application in the passive autocatalytic recombiners (PAR), the safety devices applied in the nuclear plant containments to lower the explosion risk associated with hydrogen release. The catalysts have been prepared with impregnation method and characterized by BET, XPS, TEM and STEM techniques. The role of humidity in the H2/O2 recombination reaction has been studied in a flow laboratory reactor using water saturated reaction mixture (0.5 vol% H2). The time-on-stream behavior of catalyst in contact with reaction mixture of high H2 concentration (7.2 vol%) has been studied using Microscal gas-flow through microcalorimeter. The thermal effects accompanying the H2 conversion during slow deactivation of catalysts have been monitored. The pattern of changes in both the heat evolution and the H2 conversion in the Pd-catalyzed H2/O2 recombination reaction seem to reflect the inhibiting effect of humidity present in the reaction mixture and water molecules formed in the reaction. The presence of both Li and Cs enhanced deactivation of Pd/Al2O3 catalyst and Cs-dopant displayed more pronounced effect. On the other hand, the alkali dopants do not essentially affect the amount of heat evolved during the recombination process ranging from 187 to 220 kJ/mol H2, e.g. being lower than the heat of water formation (246 kJ/mol). The detrimental effect of Li,Cs dopant has been attributed to higher affinity to water, promoting retention of water molecules/films on the catalyst surface and blockage of active sites due to enhanced adsorption of surface species, like H2O/OH.