Energy zyxwvut & Fuels zyxwvu 1994,8, zyxwvu 1233-1237 1233 A TPD Study of Chromium Catalysts Supported on an Oxidized and Nonoxidized Activated Carbon zyxwvutsrqponmlkjihgfedcbaZYX C. Moreno-Castilla,* M. A. Ferro-Garcia, J. Rivera-Utrilla, and J. P. Jolyt Departamento de Quimica Inorganica, Facultad de Ciencias, Universidad de Granada, 18071 Granada, Spain, and Laboratoire $Application de la Chimie a l%nvironnement, Universitk Claude Bernard, Lyon, France Received March zyxwvuts 31, 1994. Revised Manuscript Received July 21, 1994@ A commercial activated carbon was oxidized with nitric acid and both the untreated and the oxidized activated carbon were used as supports for Cr catalysts, which was deposited on them by adsorption from aqueous solution using either chromium nitrate or ammonium chromate as precursor salts. The samples so obtained were heat treated in He flow up to 1200 K following the evolution of CO and COZ by mass spectrometry. Results found show that during this treatment the CO disproportionation reaction takes place only in those supported catalysts obtained from the oxidized activated carbon. This reaction was catalyzed by Cr203 which is formed during the heat treatment. After He treatment up to 1200 K, the samples were treated in a He-02 flow (10 vol % of oxygen) at 573 K for zyxwv 5 min. Results found show that Crz03 supported on the untreated carbon presented a higher dispersion than that supported on the oxidized one. Introduction Adsorption of metallic salts from aqueous solutions on activated carbons is one of the most important methods of preparing supported metal catalysts, be- cause one can control the dispersion and, thereby, the metal particle size of the catalysts by controlling the factors that affect the adsorption process.1-6 Among these factors should be cited the surface area, pore texture and surface chemical nature of the support, and the type of salt, because the metal can be in a cationic or anionic form, and the pH of the solution. In these adsorption systems, the surface chemical nature of the activated carbons determined by the amount and nature of the surface complexes (essentially oxygen complexes), has been in general, to exert a greater influence than the surface area and porosity of the adsorbent because these surface complexes can act as specific sites to adsorb the metal ions or even change their oxidation state. After preparation of the supported cataysts, they are generally heat treated in different atmospheres to obtain the active phase. In this process the oxygen surface complexes are, to a greater or lesser extent, destroyed and essentially evolve as CO, COS,and HzO from the supported catalysts. Moreover, these gases can undergo secondary gas-phase reactions, as CO dispro- * Author to whom the correspondence should be addressed. + Universite Claude Bernard. @ Abstract published in Advance ACS Abstracts, September 1,1994. zyxwvut (1) Rodriguez-Reinoso, F.; Rodriguez-Ramos, I.; Moreno-Castilla, C.; (2) Prado-Burguete, C.; Linares-Solano, A.; Rodriguez-Reinoso, F.; (3) Burch, R. Catal. Today 1990, 7. (4) Solar, J. M.; Leon y Leon, C. A.; Osseo-Asare, K.; Radovic, L. R. (5) Solar, J. M.; Derbyshire, F. J.; de Beer, V. H. J.; Radovic, L. R. (6)Kim, K. T.; Chung, J. S.; Lee, K. H.; Kim, Y. G.; Sung, J. Y. (7) Bautista-Toledo, I.; Rivera-Utrilla, J.; Ferro-Garcia, M. A.; Guerrero-Ruiz, A.; L6pez-Gonz&lez, J. D. zyxwvutsr J. Catal. 1986, 99, 171. Salinas-Martinez de Lecea, C. J. Catal. 1989, 115, 98. Carbon 1990,28, 369. zyxwvutsrqpon J. Catal. 1991, 129, 330. Carbon 1992,30, 467. Moreno-Castilla, C. Carbon, 1994, 32, 93. 0887-0624/94/2508-1233$04.50/0 zyxwvu 0 portionation and water-gas shift, catalyzed by the metal compound which constitutes the active phase of the catalyst. Therefore, during the heat treatment of the supported catalysts, and due to these two facts, viz., gas evolution and its possible catalyzed reaction, some changes in the metallic phase can take place. The effect of the oxygen complexes of the activated carbons on Cr adsorption from aqueous solutions was one of the objectives of a previous and the aim of this study is to investigate the effect of heat treatment in He up to 1200 K on Cr catalysts supported on an oxidized and nonoxidized activated carbon. Experimental Section A commercial activated carbon supplied by Merck (sample M) was used in this study. A portion of this sample was oxidized with HN03 to introduce oxygen surface complexes, sample MO. The procedure was as follows: 50 cm3 of concen- trated nitric acid was added to 5 g of carbon and the suspension was heated at 353 K until dryness. The residue was washed with distilled water till no nitrates were present and dried overnight in an oven at 383 K. These two activated carbon samples, M and MO, were studied to determine their pore texture, surface area, and surface chemical nature before they were used as adsorbents of CdIII) and CdVI) ions from aqueous solutions. The results obtained were the subject of a previous paper' in which they were discussed in detail. Some of the results there obtained are summarized in Table 1. The supported Cr catalysts were prepared by adsorption from aqueous solutions of either Cr(NO&9H20 or (NH&Cr04, respectively, on the activated carbons M and MO. The suspensions, 1 g of carbon and 500 cm3 of Cr solution with a concentration of 0.2 g of CrL, were shaken and thermostated at 298 K for 48 h. The Cr solution concentration was that necessary to obtain the maximum Cr uptake by the supports. Subsequently, the suspensions were filtered out and the carbon was dried in an oven at 383 K. The Cr content of the samples was determined by burning off a portion in a thermobalance and weighing the residue as Cr203. These Cr contents are also shown in Table 1. 1994 American Chemical Society