Synthesis of alumina supported nickel nanoparticle catalysts and evaluation of nickel metal dispersions by temperature programmed desorption

摘要

The purpose of this study was to synthesize highly dispersed Ni/Al2O3 catalysts and to develop a suitable hydrogen -temperature programmed desorption (H,-TPD) method for the determination of nickel metal surface area, dispersion, and crystallite sizes. Several highly dispersed Ni/Al2O3 catalysts with a Ni loading between 15 and 25wt.% were synthesized. The reducibility of catalysts was determined by temperature programmed reduction (TPR) experiments. All catalysts exhibited a single reduction peak with a maximum rate of H, consumption (T-max in TPR) occurring below 450 degrees C. Three different H-2-TPD methods were employed to determine the amount of H, chemisorbed. In TPD-1, a 10% H-2/Ar mixture was used for catalyst pre-reduction and surface saturation by cooling down from T,,,,, in TPR to room temperature. In TPD-2, the catalyst surface after pre-reduction was flushed with Ar at T-max in TPR+ 10 degrees C. The TPD-3 was similar to the TPD-2, but used 100% H, instead of 10% H-2/Ar mixture. In all three TPD methods, the profiles exhibited 2 domains of H-2 desorption peaks, one below 450 degrees C, referred to as type-1 peaks, and attributed to H-2 desorbed from exposed fraction of Ni atoms, and the other above 450 degrees C, denoted as type-2 peaks, and assigned to the desorption of H-2 located in the subsurface layers and/or to spillover H-2. Flushing the reduced catalyst surface in Ar at T-max in TPR+ 10 degrees C in TPD-2 and TPD-3 removed most of the H-2 located in the subsurface layers/spillover H2. The amount of H, chemisorbed to form a monolayer on the reduced Ni/Al2O3 catalysts was determined quantitatively from the TPD peak areas of type-1 peaks in TPD-1, and from both type-1 and type-2 peaks in TPD-2 and TPD-3. The Ni metal surface area, dispersions and crystallite sizes were calculated from the chemisorption data and the values were compared with those obtained using the static chemisorption method. Both TPD-2 and TPD-3 gave chernisorption results similar to that obtained from the static method. (c) 2006 Elsevier B.V. All rights reserved.