Effects of cesium and chlorine on oxygen adsorption on promoted Ag/α-Al 2O 3 catalysts

Abstract

Integral heats of oxygen adsorption were measured at 443 K on unpromoted and promoted Ag/α-Al 2O 3 catalysts. Two unpromoted catalysts with average Ag particle sizes of about either 200 or 300 nm were utilized while the promoted catalysts had particle sizes in the range of 250–350 nm. The promoted catalysts were prepared with either CsNO 3 to study the effect of cesium, or CsCl to study the combined effect of cesium and chlorine. For both groups, the Cs content was around either 400 or 1200 ppm. Following a low-temperature pretreatment involving calcination at 523 K and reduction in H 2 at 473 K, the catalysts were characterized by volumetric measurements of oxygen chemisorption and subsequent hydrogen titration at 443 K. Unlike the unpromoted catalysts, the promoted samples exhibited varying amounts of reversible oxygen adsorption with a maximum of 30% reversible adsorption on a 1174 ppm Cs-only promoted catalyst; however, the presence of only Cs at either loading altered the total oxygen uptake by only ±10% compared to the unpromoted catalyst. Regardless, the H 2/(O 2) irrev titration ratio was close to the stoichiometric ratio of 2.0 except for the catalyst with the highest Cl content. With this latter sample, there was additional hydrogen uptake beyond the titration of adsorbed oxygen. Several cycles of pretreatment/adsorption measurements were needed to ‘stabilize’ these catalysts and obtain reproducible results. Energy changes were measured with samples derived from these stabilized catalysts, and an exotherm was obtained for both the initial total uptake and the reversible uptake so that the energy change associated only with the irreversible oxygen uptake could be determined. This difference exotherm frequently exhibited a long tail which could not be explained by adsorption processes alone; consequently, time-dependent uptakes and energy changes were used to calculate heats of adsorption. These apparent heat of adsorption values for O 2 were higher for the Cl-containing catalysts than for the unpromoted and Cs-only promoted catalysts. The presence of Cl decreased both the rate of oxygen adsorption and the total oxygen uptake; however, with the catalyst containing the most Cl these two parameters increased as the catalyst was given a series of low-temperature pretreatment/adsorption cycles. In contrast, a single high-temperature pretreatment at 773 K with this sample was adequate to provide uptake rates and total oxygen uptakes comparable to those with the stabilized catalyst. This stabilization process appears to be associated with the removal of Cl from the silver surface.