Applied Catalysis A: General 209 (2001) 145–154 Formation of two metal phases in the preparation of activated carbon-supported nickel catalysts Lu´ ıs M.S. Silva, José J.M. Órfão, José L. Figueiredo ∗ Departamento de Engenharia Qu´ ımica, Laboratório de Catálise e Materiais, Faculdade de Engenharia da Universidade do Porto, 4050-123 Porto, Portugal Received 25 April 2000; received in revised form 1 August 2000; accepted 1 August 2000 Abstract The preparation, characterisation and activity of activated carbon-supported nickel catalysts prepared by impregnation were studied by a range of temperature-programmed techniques including thermogravimetry (TG), differential scanning calorimetry (DSC), temperature-programmed reduction/desorption (TPR/TPD) and temperature-programmed reaction. The impregnation isotherm was interpreted in terms of two nickel phases. The two main peaks observed in the reduction of the precursor by DSC and TPR are consistent with the presence of these two Ni phases. One of them (“interactive” phase), which is bound to the support by interaction with surface functional groups during the impregnation step, is responsible for the high reactivity observed in carbon gasification at low temperatures (LTR). The other (“non-interactive” phase), which is detected in the TPD experiments by the low temperature H 2 peaks, and which results from the salt precipitation in the drying step of catalyst preparation, is proposed to be the main catalyst agent in carbon gasification at high temperatures (HTR). © 2001 Elsevier Science B.V. All rights reserved. Keywords: Nickel catalysts; Activated carbon; Impregnation; Temperature-programmed techniques 1. Introduction The use of activated carbons as a catalyst support offers some advantages over the more traditional ox- ide carriers, such as stability in acidic and basic me- dia, ease of recovery of precious metals supported on them, and the possibility of tailoring their properties to specific needs [1–3]. It is well recognised that the performance of carbon-supported catalysts depends not only on the textural properties of the support (surface area, poro- sity) but also on its surface chemistry [3]. Thus, in ∗ Corresponding author. Tel.: +351-22-204-1663; fax: +351-22-200-0808. E-mail address: jlfig@fe.up.pt (J.L. Figueiredo). order to obtain a well dispersed metal catalyst, the surface of the support must be accessible in the first place; anchoring sites for the metal precursor must be available and the metal crystallites, once formed, must be stabilised to prevent sintering. In the preparation of supported catalysts by im- pregnation, the sorption of metal ions in solution is influenced by the physico-chemical properties of the support, pH of the impregnating solution and nature of the solvent. In the case of activated carbons, these effects are strongly influenced by the nature and con- centration of the oxygen surface functionalities (car- boxylic, phenol, lactone, ether and carbonyl groups), which are responsible for both the acid/base and the redox properties of the material. The point of zero charge (PZC) plays a major role in the adsorption of 0926-860X/01/$ – see front matter © 2001 Elsevier Science B.V. All rights reserved. PII:S0926-860X(00)00762-6