The influence of impregnating pH on the postnatal and steam reforming characteristics of a Co-Ni/Al 2O 3 catalyst

Abstract

Bimetallic alumina supported Co-Ni catalysts were prepared by impregnation under low (2) and high (8) pH values. Support dissolution due to acid attack appeared to be responsible for the low BET surface area for catalyst obtained at pH 2. However, this low-pH catalyst possesses higher dispersion and superior metal surface area. This is ascribed to the charged-induced migration of metal cations towards the grain centre where adsorption sites are located as a result of the formation of positively charged alumina surface at low pH. Ammonia NH 3-TPD analysis showed that the surface of both catalysts was populated with weak Lewis acid sites though a higher site concentration was found on the high-pH catalyst. TEM images further revealed an eggyolk profile for the catalyst impregnated at pH 2 with metal species located in the particle interior; while in the catalyst synthesised at pH 8, the impregnant metal is concentrated around the external surface of the particle. XRD analysis of the catalysts before and after reduction indicates that the basic catalyst was more difficult to reduce probably because of higher metal aluminate content. This was further confirmed by the lower degree of reduction shown for this catalyst during the thermogravimetric TPR–TPO runs. Solid-state kinetic data of the catalyst calcination, reduction and oxidation conformed with the Avrami–Erofeev model. In particular, the ratios of the associated kinetic rate constants for calcination and oxidation parallel those obtained for the deactivation and steam reforming constants, respectively, in both catalysts. Thus, it may be possible to have an a priori knowledge of the comparative reaction and deactivation behaviour of different catalysts from the temperature-programmed kinetics of their nascent solid states.