The effect of coordinatively unsaturated surface Cr 3+ on the adsorption of gases on chromia

Abstract

Clean chromias of smaller and larger degrees of dehydroxylation, i.e., smaller and larger surface concentrations in Cr 3+( cus), were prepared by activation in flowing helium to 315°C (microporous, amorphous) and to 520°C mesoporous, microcrystalline). These chromias were then exposed to argon, hydrogen, methane, or nitrogen at −160 to −78°C, flushed with helium, and the residual amount of adsorbate was determined after various times. From these data and from displacement experiments in which pulses of one gas were passed over chromia on which another gas was adsorbed, the relative strengths of adsorption at Cr 3+( cus) are Ne ⪡ Ar ⪡ H 2 ⪡ CH 4 < N 2. Converting Cr 3+( cus) to Cr 3+CO by adsorption of carbon monoxide or to Cr 3+OH − by rehydroxylation with water somewhat decreased the strength of adsorption of argon and greatly decreased those of nitrogen and methane. On microporous chromia, reduction in the strength of adsorption of nitrogen and methane was less for poisoning by carbon monoxide than it was for poisoning by water. It is suggested that this resulted from Cr 3+CO projecting more into the micropores than Cr 3+OH −.