Influence of preparational variables on the crystallite size distribution of nickel catalysts

Nickel catalysts were prepared by precipitation--deposition of nickel in an aqueous solution containing the suspended silica support. The solution was well stirred and maintained at 90 [degrees] C. At this temperature, urea added to the solution slowly decomposes to release hydroxyl ions throughout the aqueous suspension. The increase of pH causes the nickel ions to precipitate uniformly and deposit onto the silica carrier. After washing and drying the nickel content of the catalyst was determined chemically. The crystallite size distribution (CSD) of the catalysts prepared by this method was determined from low-field magnetic measurements. The influence of some preparational variables on the CSD of the catalysts was investigated. They included the precipitation time, composition of the solution, nature of the support, initial pH, reduction time, temperature and flow rate and effect of calcination. The effect of adsorbed hydrogen and passivation by oxygen on the magnetization of the catalysts was also studied on selected samples. The results indicate that the preparation method is particularly suitable for control of the CSD of the catalysts. Uniformly dispersed catalysts with very small crystallites can be prepared in a reproducible manner. Variables affecting the CSD to a great extent are the total nickel content (governed by composition of the solution and precipitation time), the nature of the support and the reduction temperature. Longer reduction times and higher hydrogen flow-rates result in increased amounts of reduced nickel without increasing the crystallite sizes.