Adsorption heats of hydrogen, oxygen and carbon monoxide on iridium catalysts and their correlation with surface chemical composition

Catalytic Ring Opening of Cycloalkanes on Ir Clusters: Alkyl Substitution Effects on the Structure and Stability of C–C Bond Cleavage Transition States

“…These Ir cluster size differences (0.7–7 nm) can influence Δ H ⧧ (eq ) only through changes in the properties of the adsorbed species (i.e., H ⧧ and H H* ) because those for gaseous reactants ( H C n H 2 n (g) ) and products ( H H 2 (g) ) are unaffected by the nature and binding properties of catalyst surfaces. Microcalorimetry has shown that the heat of dissociative H 2 adsorption decreases with increasing Ir cluster size (1–10 nm) , and that H* binds 15–25 kJ mol –1 more strongly on edge and corner atoms prevalent on smaller clusters (∼1 nm) than on terrace sites (>10 nm) at near-saturation surface coverages (e.g., 1 ≤ H*/Ir s ≤ 1.5). , Consequently, the process of generating vacancies, required to bind a transition state, is more endothermic on the surfaces of small clusters than it is for large clusters. Prevalent descriptions for the binding of adsorbed species on metal surfaces suggest that as the size of Ir clusters decreases, the stability of H* will increase, as will the stability of hydrogenolysis transition states that bind via C–M bonds .…”

“…These Ir cluster size differences (0.7−7 nm) can influence ΔH ⧧ (eq 15) only through changes in the properties of the adsorbed species (i.e., H ⧧ and H H* ) because those for gaseous reactants (H C n H 2n (g) ) and products (H H 2 (g) ) are unaffected by the nature and binding properties of catalyst surfaces. Microcalorimetry has shown that the heat of dissociative H 2 adsorption decreases with increasing Ir cluster size (1−10 nm) 54,55 and that H* binds 15−25 kJ mol −1 more strongly on edge and corner atoms prevalent on smaller clusters (∼1 nm) than on terrace sites (>10 nm) at near-saturation surface coverages (e.g., 1 ≤ H*/Ir s ≤ 1.5). 54,55 Consequently, the process of generating vacancies, required to bind a transition state, is more endothermic on the surfaces of small clusters than it is for large clusters.…”

Mechanistic analysis of chemisorption and surface interaction in batch calorimetric systems: a new approach

“…Chemisorption of oxygen on palladium-/alumina-supported catalysts (97) and adsorption of hydrogen, oxygen, and CO on iridium-/alumina-supported catalysts (98) are probably equilibrium processes; however, small fluctuations in heats are noted. Monotonically declining heats are observed in the case of oxygen adsorption on palladium-/magnesia-supported catalysts (99), while fallingstepped heats are recorded for hydrogen chemisorption on the same catalysts (99).…”