Comparative study of CO and CO 2 hydrogenation over supported Rh–Fe catalysts Makarand R. Gogate, Robert J. Davis ⁎ University of Virginia, Department of Chemical Engineering, 102 Engineers' Way, PO Box 400741, Charlottesville, VA 22904-4741, United States abstract article info Article history: Received 20 January 2010 Received in revised form 18 March 2010 Accepted 26 March 2010 Available online 8 April 2010 Keywords: CO hydrogenation CO 2 hydrogenation Rh/TiO 2 Rh–Fe/TiO 2 Methane Ethanol The hydrogenation of CO, CO + CO 2 , and CO 2 over titania-supported Rh, Rh–Fe, and Fe catalysts was carried out in a fixed-bed micro-reactor system nominally operating at 543 K, 20 atm, 20 cm 3 min -1 gas flow (corresponding to a weight hourly space velocity (WHSV) of 8000 cm 3 g cat -1 h -1 ), with a H 2 :(CO + CO 2 ) ratio of 1:1. A comparative study of CO and CO 2 hydrogenation shows that while Rh and Rh–Fe/TiO 2 catalysts exhibited appreciable selectivity to ethanol during CO hydrogenation, they functioned primarily as methanation catalysts during CO 2 hydrogenation. The Fe/TiO 2 sample was primarily a reverse water gas shift catalyst. Higher reaction temperatures favored methane formation over alcohol synthesis and reverse water gas shift. The effect of pressure was not significant over the range of 10 to 20 atm. © 2010 Elsevier B.V. All rights reserved. 1. Introduction The fixation of CO 2 into valuable chemicals is potentially an im- portant strategy for CO 2 utilization. While CO hydrogenation over transition metal catalysts in general, and Rh in particular, has been studied quite extensively, there have been fewer studies of CO 2 hydrogenation. Our group has recently studied CO hydrogenation over supported and promoted Rh catalysts with a goal to produce ethanol and higher alcohols, forming methane and other light paraffins as less desirable co-products [1]. One possible reaction network for CO hydrogenation includes the elementary steps of (a) dissociation of the adsorbed CO to form adsorbed carbon and oxygen, (b) hydrogenation of the adsorbed carbon to form an adsorbed methyl species, (c) insertion of non-dissociated CO into the adsorbed methyl species to form an adsorbed acyl species, and (d) hydrogenation of the adsorbed acyl species to form the ethanol product [1]. Thus, a properly promoted metal catalyst performs many different elementary reactions. One goal of the current work was to investigate how CO 2 interacts with the reaction network for CO hydrogenation, so we have studied the hydrogenation of CO 2 in the presence and absence of CO. Prior work on CO 2 hydrogenation over Rh-based catalysts includes two relevant reports by Inoue et al. [2] and Trovarelli et al. [3]. Inoue et al. [2] examined the activity of supported Rh catalysts on different supports such as MgO, Nb 2 O 5 , ZrO 2 , and TiO 2 at 473–573 K, 10 atm, H 2 :CO 2 ratio of 3:1, and a WHSV of 2400 cm 3 g cat -1 h -1 . The ZrO 2 - and Nb 2 O 5 -supported catalysts were the most active as measured by the rate of formation of product hydrocarbons and alcohols, however, the selectivity to CH 4 was nearly 100%. The Rh/TiO 2 catalyst was not as active in terms of the rate of hydrocarbon and alcohol formation, but it was a good reverse water gas shift catalyst. The rate of CO formation on Rh/TiO 2 was nearly an order of magnitude greater than that of the hydrocarbon and alcohol formation [2]. Trovarelli et al. [3] studied the effect of reduction temperature of Rh/TiO 2 on the rates of CO and CH 4 formation during CO 2 hydrogenation reactions at 523 K with a CO 2 :H 2 ratio of 1:1. For a catalyst prepared with RhCl 3 as a precursor and reduced at a low temperature (473 K), methane was the primary product, however, for samples reduced at 623 and 723 K, the rate of CO formation increased significantly. For the catalyst reduced at 723 K, the rate of CO formation was about 2 orders of magnitude higher than that of CH 4 formation. At least on Rh/TiO 2 , the extent of CO formation during CO 2 hydro- genation reactions appears to be a function of many different factors. Some reports suggest however, that CO formation during CO 2 hydro- genation is not significant on Rh/TiO 2 . For example, Szailer et al. [4] report that CH 4 is the main product (99%+) at 548 K with a H 2 :CO 2 ratio of 4:1 on Rh/TiO 2 prepared from RhCl 3 precursor (reduced at 673 K for 1 h). In another contribution from the same group [5], the rate of CH 4 formation alone is reported for CO 2 hydrogenation on a 1% Rh/TiO 2 at 473 K, 1 atm, H 2 :CO 2 ratio of 4:1, and with a SV of 6000– 9000 h -1 , so it was not clear if CO was detected. Solymosi et al. [6] suggest that on Rh/Al 2 O 3 and Rh/TiO 2 , the rate of CO formation is less than 1% of the rate of CH 4 formation for catalysts prepared using RhCl 3 as the precursor and at the reaction conditions of 473 and 548 K, 1 atm, space velocities of 3000–6000 h -1 , with a H 2 :CO 2 ratio of 4:1. Solymosi and Erdöhelyi [7] also report that the hydrogenation of CO 2 Catalysis Communications 11 (2010) 901–906 ⁎ Corresponding author. Tel.: +1 434 924 6284; fax: +1 434 982 2658. E-mail address: rjd4f@virginia.edu (R.J. Davis). 1566-7367/$ – see front matter © 2010 Elsevier B.V. All rights reserved. doi:10.1016/j.catcom.2010.03.020 Contents lists available at ScienceDirect Catalysis Communications journal homepage: www.elsevier.com/locate/catcom