Catalytic conversion of carbon dioxide (CO 2 ) into liquid fuels has been identified as one of the priority research directions in a recent U.S. Department of Energy Report, ''Basic Research Needs: Catalysis for Energy'' [1]. Recycling CO 2 to make liquid fuels neutralizes CO 2 emissions, thereby alleviating the greenhouse effect caused by CO 2 . However, converting CO 2 to liquid fuel is an energy-intensive process due to the nature of the reactions involved. Carbon dioxide can be used as a reforming reagent for methane to produce synthesis gas (CO + H 2 ) [2-11], which can then be converted to liquid fuels. Fe-, Co-and Ni-based catalysts were found to have reasonable activities for CO 2 reforming of methane [4][5][6][7][8][9][10][11]. Alternatively, CO 2 can be directly hydrogenated, where the products from direct CO 2 hydrogenation depend on the catalysts used. Over the Ni-based catalysts, methane is one of the main products [12][13][14]. On the other hand, methanol will be produced predominantly over the Cu-based catalysts [15,16]. In these heterogeneous CO 2 conversion processes, adsorption of CO 2 and activation of the C-O bond are key steps. A detailed characterization of CO 2 interaction with the catalysts at the molecular level will help us to better understand the underlying mechanisms of the reactions and aid in the elucidation of key factors that affect the performance of the catalysts.
There have been many studies on the interaction between CO 2 and transition metal surfaces [17]. On the basis of combined experimental and computational studies, Vesselli et al. [18] suggested that the CO 2 molecule was adsorbed on the Ni (1 1 0) surface through the carbon atom in a negatively charged and bent configuration. Wang et al. studied the chemisorption of CO 2 on the Ni (1 1 1), Ni (1 0 0) and Ni (1 1 0) surfaces using DFT methods and reported that the C O bond of CO 2 was activated due to electron transfer from the Ni surface into the anti-bonding orbital of CO 2 [19]. The C O bond activation ability of the Ni surfaces was suggested to be in the order of Ni (1 1 0) > Ni (1 0 0) > Ni (1 1 1). UV photoelectron spectroscopy, X-ray photoelectron spectroscopy and Auger electron spectroscopy have all been applied to characterize CO 2 adsorption on potassium-covered Fe (1 1 0) surface at 85 K [20,21]. A small amount of CO 2 was found to dissociate into CO and oxygen whereas the molecularly adsorbed CO 2 was found in a bent configuration.
The mechanisms for CO 2 adsorption and activation over the oxide-supported metal catalysts are expected to be different from that on the pure metal surfaces due to the presence of the oxide support. Aksoylu and O ¨nsan [14] investigated CO 2 adsorption and methanation over oxide-supported nickel catalysts. They found