Journal of Catalysis 262 (2009) 119–126 Contents lists available at ScienceDirect Journal of Catalysis www.elsevier.com/locate/jcat CO hydrogenation on lanthana and vanadia doubly promoted Rh/SiO 2 catalysts Jia Gao, Xunhua Mo, Andrew Chang-Yin Chien, Walter Torres, James G. Goodwin Jr. ∗ Department of Chemical and Biomolecular Engineering, Clemson University, Clemson, SC 29634, USA article info abstract Article history: Received 30 July 2008 Revised 9 December 2008 Accepted 11 December 2008 Available online 18 January 2009 Keywords: Rh catalysts La V Promotion CO hydrogenation Ethanol synthesis This paper reports on a study of the combined promoting effect of La and V oxides for ethanol formation during CO hydrogenation on silica-supported Rh catalysts. Non-promoted and La and/or V oxide promoted Rh/SiO 2 catalysts were prepared by sequential or co-impregnation methods and characterized by TEM, CO chemisorption and FT-IR. Their catalytic properties for CO hydrogenation were investigated using a differential fixed bed reactor at 230 ◦ C and 1.8 atm. It was found that, compared to non-promoted Rh/SiO 2 , the singly promoted catalysts, Rh–La/SiO 2 and Rh–V/SiO 2 , showed improved reactivity (3×) and better ethanol selectivities. However, the doubly promoted Rh–La–V/SiO 2 catalysts exhibited even higher activity (9×) and selectivity for ethanol and other C 2+ oxygenates, with the selectivity of total C 2+ oxygenates >30% at these low pressure reaction conditions. The better performance of the Rh– La–V/SiO 2 catalysts appears to be due to a synergistic promoting effect of the combined lanthana and vanadia additions through intimate contact with Rh. Use of just more of each promoter by itself was not able to produce the enhanced catalytic performance.  2009 Elsevier Inc. All rights reserved. 1. Introduction Catalytic synthesis of ethanol and other higher alcohols from CO hydrogenation has been a subject of significant research since the 1980s. Higher alcohols synthesized from syngas derived from natural gas, coal, or biomass can be used as additives to gasoline or as an easily transportable source of hydrogen. Ethanol is especially desirable to produce selectively. Such produced ethanol would not only decrease the demand for imported crude oil but could also have a positive environmental impact [1]. Rh-based catalysts have been shown to have high activity for the synthesis of C 2+ oxygenates due to the unique carbon monox- ide adsorption behavior on Rh [2–6]. Extensive research efforts have been devoted to study the influence of supports and addi- tives including La 2 O 3 [2–6] , SiO 2 [4,5,7–10], TiO 2 [3,8–16], Al 2 O 3 [8,9,11], ZrO 2 [2,11,17], CeO 2 [8,11], MgO [8,18],V 2 O 3 [18–21], al- kali metals [21–25], Fe [26], Mn [27–34], Ag [35] and Mo [36] on the catalytic activity of Rh for CO hydrogenation. SiO 2 has been frequently used as a support since most Rh-based catalysts sup- ported on SiO 2 have shown moderate activity and good selectivity towards C 2 oxygenates during CO hydrogenation [37]. It is widely accepted that CO dissociation and hydrogenation to produce CH x species is likely the first step for the synthesis of C 2+ oxygenates from syngas on Rh-based catalysts. The CH x species then undergoes three possible different reactions. One is * Corresponding author. Fax: +1 864 656 078. E-mail address: jgoodwi@clemson.edu (J.G. Goodwin). to form C 2 oxygenates by CO insertion, the second is to produce CH 4 by hydrogenation, and the third is to undergo chain growth with another CH x to produce C 2+ hydrocarbons [37]. Many studies have suggested that C–O bond dissociation is the rate-limiting step for CO hydrogenation [16,38], although it remains unclear whether C–O bond cleavage occurs through direct breaking of this bond in an adsorbed CO species or by a process involving hydrogen. In order to optimize the activity and selectivity of a catalyst for ethanol formation, the catalyst should have the ability to adsorb CO nondissociatively, to dissociate CO, to hydrogenate moderately, and to insert CO into a Rh–CH x bond. A simple supported Rh cata- lyst does not seem to meet all these requirements optimally. Typi- cal Rh catalysts for ethanol synthesis from syngas in recent studies all contain multiple components, such as Rh–Li–Mn–Fe [39] and Rh–Zr–Ir [40]. Lanthana and other rare earth oxides have been studied by many researchers for enhancing oxygenates synthesis from syn- gas and have shown interesting promotion/support effects on Rh for better ethanol formation [5,17,41–48]. However, their promo- tion mechanism remains unclear—it is unknown whether lan- thana and other rare earth oxides enhance the formation of C 2 - oxygenates by affecting the dispersion of Rh [44,49], by facilitating CO dissociation or insertion [46,47], or by stabilizing reaction in- termediates [17]. The same is true for vanadia promoted Rh/SiO 2 [20,50–55]. While Kip et al. suggested that V enhances reactivity and selectivity towards ethanol by enhancing CO dissociation [55], other researchers have proposed that the function of V is to boost hydrogenation [53,54,56]. 0021-9517/$ – see front matter  2009 Elsevier Inc. All rights reserved. doi:10.1016/j.jcat.2008.12.006