Supported iridium catalysts prepared by atomic layer deposition: effect of reduction and calcination on activity in toluene hydrogenation

Abstract

Ligand removal from supported iridium catalysts prepared by atomic layer deposition from Ir(acac) 3 was studied by direct reduction in hydrogen flow and by calcination in oxygen flow followed by reduction in hydrogen flow, as well as by thermogravimetric analysis. Thermal decomposition of acac ligand residuals required high temperatures, and in samples containing no iridium the removal of carbonaceous species was not complete. Metallic iridium particles less than 2 nm in size were formed during direct reduction and larger particles upon calcination followed by reduction. The activity of the catalysts in toluene hydrogenation in most cases depended on particle size.

Beta Zeolite-Supported Iridium Catalysts by Gas Phase Deposition

“…This is analogous to the observation that Hacac does not react with silica, i.e. Si-O-acac bonds cannot be formed [9,11,12]. The high acac-to-iridium ratio on beta zeolite is then proposed to be due to secondary reaction or adsorption of Hacac in the micropores, where this small molecule can easily enter.…”

“…Since the number of acac ligands per nm 2 after blocking was high compared with aluminum content of 0.8 wt % on the beta zeolite, also some other Hacac-related species must be present. The FTIR spectra of blocked samples B400 and IrB400 were characterized by a strong band at 1,643 cm -1 that was not observed in samples without blocking nor in previous studies of Hacactreated alumina and silica-alumina [9,11]. This peak can be assigned to associative adsorption of Hacac in the micropores, an interaction comparable to the strong adsorption of water in zeolites.…”

“…Hacac was used to block active sites on the support and thereby to reduce the amount of iridium deposited on the surface. Hacac did not react with silica but formed Al(acac) x in a reaction with alumina surface [8][9][10][11][12]. The removal of the ligands from Ir(acac) x and Al(acac) x surface species required high reduction temperatures or calcination prior to reduction treatment [10,13].…”

Genesis and Evolution of Surface Species during Pt Atomic Layer Deposition on Oxide Supports Characterized by in Situ XAFS Analysis and Water−Gas Shift Reaction

“…Our findings may extend to the ALD synthesis of other noble metal nanoparticles, such as Pt, Ru, Ir, and Pd. 3,8,9,14,33 This method has several potential applications; the first is the synthesis of more uniform and varied particle sizes, as demonstrated in this study. In addition, the synthesis of bimetallic nanoparticles should also be possible.…”

Temperature Programmed Hydrogenation of Toluene

“…T max has been observed in steady-state hydrogenation of toluene with Ni/Al 2 O 3 catalyst [14] and in TPH of toluene with Co/SiO 2 and Co/Al 2 O 3 catalysts [19]. The occurrence of a maximum reaction rate has also been reported in steady-state hydrogenation of toluene for Pt [15,21,28,29] and in TPH of toluene for Ir catalysts supported on alumina and silica-alumina [30]. Under normal reaction conditions the hydrogenation/dehydrogenation equilibrium is completely on the hydrogenation side [12], and thus thermodynamic limitations are ruled out.…”