Preparation by Atomic Layer Deposition and Characterisation of Catalyst Supports Surfaced with Aluminium Nitride (2002)

Catalyst supports with novel chemical and physical properties are needed for producing new families of catalytic materials. The goals of this work were to demonstrate the preparation of aluminium nitride type materials and evaluate their properties as catalyst supports.\n\nTo obtain aluminium nitride in high-surface-area form suitable for catalyst applications, porous silica and alumina supports were surfaced with aluminium nitride by the atomic layer deposition (ALD) technique by repeating the separate, saturating reactions of gaseous trimethylaluminium (TMA) and ammonia. The reaction temperatures of TMA and ammonia were 150 and 550 °C, respectively. Six reaction cycles led to an average growth of 2.4 aluminium atoms per cycle per square nanometre, and, according to low-energy ion scattering, 74% coverage. The aluminium nitride species were shown to be evenly distributed on the silica. The aluminium nitride appeared amorphous in X-ray diffraction, but 27Al nuclear magnetic resonance (NMR) spectroscopy confirmed its formation.\n\nInsight into the growth mechanism of aluminium nitride was obtained by investigation of the individual steps leading to the growth. The surface reaction products after the TMA and ammonia reactions were identified by diffuse reflectance Fourier transform infrared spectroscopy and 1H, 13C and 29Si NMR, and they were quantified by elemental analysis and 1H NMR. TMA reacted through ligand exchange with hydrogen atoms present on the surface in hydroxyl groups (OH) and amino groups (NHx), releasing methane, and further through dissociation in siloxane bridges, coordinatively unsaturated aluminium-oxygen pairs and nitrogen bridges. Steric hindrance imposed by the methyl ligands defined the saturation of the surface with adsorbed species; at saturation, there were five to six methyl groups per square nanometre. The ammonia reaction replaced the methyl groups present on TMA-modified surfaces with NHx groups (x = 2, 1 or 0). The results for the TMA reaction agreed quantitatively with the results obtained by others for the ALD growth of aluminium oxide thin films. A model was derived that relates the size and reactivity of the metal reactant to the growth per cycle of the oxide by ALD.\n\nThe properties of the AlN/oxide materials as catalyst supports were evaluated for cobalt hydroformylation and chromium dehydrogenation catalysts. In the preparation of the catalysts from cobalt(III) and chromium(III) acetylacetonate by ALD, the factor defining the saturation of the reaction was the same as on the respective oxides: the steric hindrance imposed by the acetylacetonate (acac) ligands. Dissociative and associative reactions of the metal acetylacetonate reactants and of the Hacac released in ligand exchange reaction took place on the AlN/oxide supports. This was a disadvantage for the Co/AlN/silica catalysts, as the high acac/Co ratio of the surface complex led to the desorption of cobalt(II) acetylacetonate during catalytic testing in hydroformylation.\n\nAfter removal of the remaining acac ligands with ammonia, the activity of the Cr(III)/alumina and Cr(III)/AlN/alumina catalysts was evaluated in isobutane dehydrogenation at 580 °C. The aluminium nitride modification of the support decreased the dehydrogenation activity of the chromium catalysts. Pairs of chromium and oxygen ions seem to be required for active chromium catalysts, and replacing the neighbouring oxygen with nitrogen was a disadvantage.\n\nAlthough no improvements were observed in catalytic performance of the prepared catalysts relative to conventional systems, the information obtained will be useful in the future investigations of the growth of aluminium nitride and of other materials by ALD and in the identification of the active sites on dehydrogenation catalysts.