JOURNAL OF CATALYSIS 161, 626–640 (1996) ARTICLE NO. 0225 Characterization of Carbonaceous Species Formed during Reforming of CH 4 with CO 2 over Ni/CaO–Al 2 O 3 Catalysts Studied by Various Transient Techniques M. A. Goula, ∗ A. A. Lemonidou, ∗ and A. M. Efstathiou† ,1 ∗ Chemical Process Engineering Research Institute, CPERI-FORTH, and Department of Chemical Engineering, Aristotle University of Thessaloniki, P.O. Box 1517, Thessaloniki 54006; and †Institute of Chemical Engineering and High Temperature Chemical Processes, ICE/HT-FORTH, P.O. Box1414, University Campus, Patras GR-26500, Greece Received August 1, 1995; revised January 31, 1996; accepted February 12, 1996 Carbon dioxide reforming of methane to synthesis gas at 750 ◦ C over 5 wt% Ni/CaO–Al 2 O 3 catalysts has been investigated with re- spect to effects of support composition (CaO to Al 2 O 3 ratio) on catalyst stability, amount and reactivity of carbon species formed during reaction, and relative proportion of reaction routes that lead to carbon formation (CH 4 vs CO 2 molecule). Temperature- programmed oxidation (TPO) and hydrogenation (TPH) experi- ments, following reforming reaction with 20% CH 4 /20% CO 2 /He and 20% 13 CH 4 /20% CO 2 /He mixtures, have been conducted for the aforementioned carbon characterization studies. Two kinds of carbon species (free of chemically bound hydrogen) were mainly found to accumulate on the catalyst surface, where the amount and reactivity of them are influenced by the CaO/Al 2 O 3 ratio used to deposit the nickel metal. Transient isothermal hydrogenation ex- periments of the carbon species formed during reforming reaction resulted in CH 4 responses, where the time of appearance of the CH 4 peak maximum in hydrogen stream as a function of hydrogenation temperature was used to obtain the intrinsic activation energy of the hydrogenation process. It was found that this activation energy is influenced by the support composition. TPO experiments con- ducted following reforming reaction with 13 CH 4 /CO 2 /He mixture have demonstrated that the relative amount of adsorbed carbon species formed via the CH 4 and CO 2 molecular routes was strongly dependent on support composition. H 2 temperature-programmed desorption, temperature-programmed reduction, and X-ray pho- toelectron spectroscopic measurements conducted over the present catalysts suggest that the nickel particle morphology and its size dis- tribution must be influenced by the support composition, which in turn controls the origin, the kinetics, and the reactivity of carbon de- position under reforming reaction conditions. c  1996 Academic Press, Inc. INTRODUCTION There is a growing interest in the process of carbon diox- ide reforming of methane to synthesis gas which results in a suitable CO/H 2 ratio for the production of higher hydrocar- 1 To whom correspondence should be addressed. Fax: +(3061)-993.255. bons and oxygenated derivatives. This reaction is slightly more endothermic than the steam reforming of methane, the latter being a well-established industrial process for producing synthesis gas rich in hydrogen appropriate for the synthesis of methanol (1, 2). Supported noble metals have shown higher activity and lower sensitivity to coking than other supported metal and metal oxide catalysts (3–8). However, the fact that these noble metals are high in cost and limited in availability makes the development of nickel-based catalysts for appropriate industrial practice a challenge to the catalytic scientific community. There is a limited amount of fundamental research con- cerning the reforming of CH 4 with CO 2 over nickel-based catalysts, where most of the research work concerns the examination of catalytic activity and stability with time on stream (3, 7, 9–17). Gadalla and Sommer (11) have shown that over supported nickel catalysts, deactivation is due to either carbon deposition, metal sintering, or phase transformation, such as the formation of NiAl 2 O 4 in the case of a Ni/γ -Al 2 O 3 catalyst. All these phenom- ena, however, depend on reaction conditions (tempera- ture, CH 4 /CO 2 ratio) and the calcination temperature used during and after preparation of the catalyst. Zhang and Verykios (10) have recently shown that the stability of a 17 wt% Ni/γ -Al 2 O 3 catalyst was improved by the ad- dition of CaO (CaO/Al 2 O 3 = 1/9) in the support compo- sition. Temperature-programmed oxidation experiments performed following reforming reaction have indicated that the reactivity toward oxidation to CO 2 of the carbon species formed during reforming reaction increased in the case of a CaO-promoted catalyst as compared with a Ni/γ -Al 2 O 3 catalyst. It was suggested (10) that the improved stability of the CaO-promoted catalyst may be related to the en- hanced reactivity of carbon formed under reforming re- action conditions and, thus, to the lower amount of accu- mulated carbon. Similarly, Ruckenstein and Hu (15) have observed high stability and activity over a reduced 20 wt% NiO/MgO catalyst as compared with NiO/CaO, Ni/SrO, and 0021-9517/96 $18.00 Copyright c  1996 by Academic Press, Inc. All rights of reproduction in any form reserved. 626