Rhodium-Based Catalysts
13.1.1 Studies on Reduction of CO and CO2
Rhodium (Rh) is well known for its ability to produce C2+ species such as ethanol, acetaldehyde, and acetic acid; therefore, Rh-based catalysts are the most widely studied catalysts for ethanol production from syngas [13,14]. Most of the studies prior to 2008 in this area used CO or CO2 separately as simpler models for syngas to develop catalysts and reaction conditions, therefore, CO and CO2 reductions will be discussed as fundamental reactions as an introduction to this area of research. Rh occupies an interesting position in the periodic table as it lies between metals that easily dissociate CO (e. g., Fe and Co) and metals that do not dissociate CO to produce methanol (e. g., Pd, Pt, and Ir) [15,16]. In addition to ethanol, Rh can form methane, other alcohols, and oxygenates like aldehydes as well. The general reaction mechanism proposed for
As we know, syngas contains both CO and CO2, therefore reduction of CO2 is also of interest in the conversion of syngas to ethanol. The hydrogenation of CO2 to ethanol and other C2-oxygenates can happen in two different ways. The first route is the reaction of CO2 with hydrogen in the reverse water gas shift (r-WGS) reaction, and then CO reduction to give ethanol and other reduction products.
Water gas shift reaction CO + H2O ^ CO2 + H2
The other route is the decomposition of CO2 to adsorbed C and O on the Rh surface, then after the formation of C and O on the surface the rest of the steps can proceed for the CO hydrogenation. In fact, there is experimental evidence to support this; adsorption of CO2 results in the formation of linearly and bridge-bonded CO, which has been identified by IR spectroscopy on Rh-Mo/ZrO2 [20], Rh/ Al2O3 [21] and Rh-Li/Y [22]. In a Rh on alumina study, Iizuka and Tanaka have shown that presence of hydrogen strongly enhanced the formation of CO, possibly by reacting with the surface O atom formed in the initial adsorption of CO2 and driving the adsorption process forward [21]. As shown in Figure 13.3, this suggests that CO2 hydrogenation proceeds via the dissociative adsorption of CO2 to form CO and O atoms on the surface to adsorbed C and O atoms [23,24], with later steps of the CO reduction according to the mechanism in Figure 13.2.