Microcalorimetric measurements of differential heats of adsorption on reactive metal surfaces

Techniques are presented for using heat-flux microcalorimetry to measure differential heats of adsorption on reactive metal surfaces. Samples are prepared ex-situ in ultrapure flowing gases and then sealed in Pyrex capsules. Special microcalorimetric cells are employed to break the sample capsule after thermal equilibration of the sample with the microcalorimeter. In this manner, clean samples can be exposed rapidly to the adsorbing gas, minimizing surface contamination. The present techniques were used to address the function of alkali metals on Ni and Pt, to probe the interactions of {dollar}rm Hsb2, Csb2Hsb4,{dollar} and {dollar}rm Csb2Hsb2{dollar} with Pt before and after exposure to reaction conditions for ethylene hydrogenation, and to investigate the interaction of N{dollar}sb2{dollar} with Fe. The microcalorimetric results show that the addition of alkali metals to Ni and Pt powders promotes CO adsorption by increasing the initial heats, and also by extending the CO adsorption capacity of Pt. The addition of alkali metal salts to Pt/SiO{dollar}sb2{dollar} and Pt/Sn/SiO{dollar}sb2{dollar} catalysts for isobutane dehydrogenation does not increase the initial heats or saturation coverages for CO adsorption, indicating that the alkali species are not in the metallic state on these catalysts. The presence of alkali metal species on Pt/Sn/SiO{dollar}sb2,{dollar} however, decreases the CO saturation coverages and increases the isobutane dehydrogenation selectivities, indicating that the alkali species are associated with the active sites. The microcalorimetric results also show that the presence of ethylidyne species on Pt significantly decreases the adsorption strengths and saturation coverages of H{dollar}sb2{dollar} and {dollar}rm Csb2Hsb4{dollar} on Pt. The presence of adsorbed hydrogen on Pt decreases the adsorption strength and saturation coverage of H{dollar}sb2{dollar} on Pt. Surfaces exposed to ethylene hydrogenation reaction conditions contain significant amounts of reactive hydrogen, since they show formation of {dollar}rm Csb2Hsb6{dollar} upon exposure to {dollar}rm Csb2Hsb4.{dollar} The microcalorimetric results further show that N{dollar}sb2{dollar} adsorbs dissociatively on reduced Fe ammonia synthesis catalysts with an initial strength of 250 kJ/mol, independent of the promoter content. The adsorption strength decreases slowly with coverage, and indicates that N{dollar}sb2{dollar} is adsorbed strongly at high N coverages typically observed for Fe catalysts operating under industrial ammonia synthesis conditions.