Aimo Rautiainen

Processing of catalysts by atomic layer epitaxy : modification of supports

Abstract Different supports were modified with titania, zirconia and chromia by the atomic layer epitaxy technique (ALE). In ALE, a metal precursor is bound to the support in saturating gas-solid reactions. Surface oxides are grown by alternating reactions of the metal precursor and an oxidizing agent. Growth mechanisms differ depending on the precursor-support pair and the processing conditions. In this work, the influences of the support, precursor and reaction temperature were investigated by comparing the growth of titania from Ti(OCH(CH 3 ) 2 ) 4 on silica and alumina, titania from TiCl 4 and Ti(OCH(CH 3 ) 2 ) 4 on silica, and zirconia from ZrCl 4 on silica and alumina. The modification of porous oxides supported on metal substrates (monoliths) was demonstrated for the growth of chromia from Cr(acac) 3 .

Preparation of silica-supported cobalt catalysts through chemisorption of cobalt(II) and cobalt(III) acetylacetonate

Silica-supported cobalt catalysts, Co/SiO2, were prepared by the saturating chemisorption of gaseous cobalt acetylacetonate reactants, Co(acac)2 and Co(acac)3. Thermoanalytical investigation indicated that both reactants are suitable for the preparation of cobalt catalysts in the temperature range 170–220 °C. Co(acac)2 and Co(acac)3 gave identical cobalt loading on silica. Moreover, diffuse reflectance Fourier transform infrared spectroscopy showed that the chemisorbed species formed from the two reactants were identical. The identical species are explained by the transformation of Co(acac)3 to Co(acac)2 before reaction with the silica. Ligand exchange reaction between the reactants and the OH groups of silica led to surface-bound Co–acac structures. In addition, the reactants adsorbed molecularly on the surfaces, at least on surfaces that contained a low number of reactive OH groups. Chemisorption of the reactants reached saturation level with an acac ligand density of about 2.8 acac per square nanometer. Calcination at 450 °C removed the acac ligands of the surface complex. Through repetition of the cycles of Co(acac)3 reaction and calcination up to five times, Co/SiO2 catalysts were prepared with 6 to 19 wt.% Co, that is, 2 to 8 Co atoms per nm2.