466544 Reduction of CO2 to CO and Methanol on Ceria Based Catalyst: Mechanistic Insights
Carbon dioxide (CO2) utilization via reduction to commodity chemicals and fuels can be accomplished by the catalytic and electrocatalytic reduction reactions. Since CO2 is a thermodynamically stable molecule, it requires high temperature and pressure condition to obtain favorable conversion to a desired product1. Ceria based materials, having high oxygen storage capacity and mixed ionic-electronic conducting property, have been suggested to play an active role in the catalytic and electrocatalytic reduction of CO2. We have performed density function theory (DFT) calculations to study the mechanistic insights of CO2 reduction on CeO2(110) surface. CO2 molecule sitting in the vicinity of oxygen vacancy site on the surface, is activated to form bent carbonate CO2d- species which dissociates into CO via the incorporation of the oxygen atom into the vacancy. The calculated activation barrier and reaction energy for this redox reaction is 258.9 kJ/mole and 238.6 kJ/mole respectively. The effect of lateral interactions were studied by performing calculations for the same reaction step on two oxygen vacancy (di-vacancy) on 2x2 supercell unit. The activation barrier and reaction energy on a di-vacancy were significantly reduced to 134.3 and 127.3 kJ/mole respectively. In presence of hydrogen, CO2 dissociation to CO is assisted by the atomic hydrogen adsorbed on the reduced ceria surface. Reaction energy diagram with intrinsic reaction and activation energy is illustrated in Figure 1A. In presence of a hydrogen atom the CO2 dissociation reaction occurs in two exothermic steps: CO2+H→ COOH (ΔH = -69.2 kJ/mole), COOH→CO+OH (ΔH = -80.4 kJ/mole) with activation barrier of 39.0 and 47.4 kJ/mole respectively. CO2 or CO adsorbed on the ceria surface could hydrogenate to methanol via formate (HCOO) or carboxyl (COOH) mediated mechanisms. Formate species, produced by the hydrogenation of CO2 (CO2+H→ HCOO), was observed to be more stable with binding energy of -222.9 kJ/mole on the stoichiometric ceria surface as compared to the carboxyl intermediate species with binding energy of -36.0 kJ/mole2. Due to its highly stable structure, formate species may act as a spectator and may not participate in further hydrogenation reaction. On reduced surface COOH dissociation occur by the incorporation of carboxyl species into the surface vacancies via redox mechanism. On the other hand, carboxyl mediated mechanism involve exothermic reaction except the dissociation of COOH to CO and OH which is endothermic of -5.0 and -24.4 kJ/mole on stoichiometric and reduced ceria surface. On stoichiometric ceria surface, the endothermic dissociation step (COOH→CO+OH) has the maximum barrier of 55.6 kJ/mole among the elementary steps associated with carboxyl mediated route. On reduced ceria surface, intrinsic activation barriers of elementary reaction steps, associated with carboxyl mediated path, were calculated (Figure 1A). Hydrogen atom co-adsorbed on the surface was observed to assist CO2 dissociation by forming a carboxyl intermediate, CO2+H→COOH (ΔEact = 39.0 kJ/mole, ΔH = -69.2 kJ/mole) which on subsequent dissociation produces CO via the redox mechanism. On hydrogenation, CO is likely to produce methanol. The energetics of CO hydrogenation to produce methanol showed exothermic steps with activation barriers comparable to the DFT calculations reported for Cu(111) and CeO2-x/Cu(111) interface. While on the stoichiometric surface, COOH dissociation COOH→CO+OH (ΔEact = 55.6 kJ/mole, ΔH = 5.7 kJ/mole) is likely to be difficult as compared to rest of the elementary steps, whereas on the reduced surface the energetics of the same step were significantly lowered (ΔEact = 47.4 kJ/mole, ΔH = -80.4 kJ/mole). In comparison, hydrogenation of methoxy, H3CO+H→H3COH, appears to be relatively difficult (ΔEact = 58.7 kJ/mole) on the reduced surface. COOH dissociation step has the maximum barrier (126 kJ/mole) as compared to other hydrogenation elementary steps which implies that the dissociation step could be the rate determining step of this route. The activation energy of this rate determining step was calculated on reduced ceria surface which is lower (by ~50kJ/mole) than that on stoichiometric ceria surface. Ceria based materials have been suggested to possess electrocatalytic activity for CO2 reduction which can be further improved by aliovalent metal dopants. Surface of ceria doped with aliovalent dopants such as gadolinium (Gd), praseodymium (Pr) and samarium (Sm), have been suggested to have mixed ionic-electronic conductive nature. Classical molecular dynamic simulations were utilized to determine the oxide ion diffusivity (D) in the Pr doped ceria (PDC) at different temperatures (Figure 1B). The calculated diffusivity was of 1.15x10-8 cm2s-1 at 973 K. The activation energy of oxide ion diffusion in PDC was estimated to be of 37.0 kJ/mole. We have performed the electrocatalytic reduction to CO2 to CO in a high temperature solid oxide electrolysis cell (SOEC). CuO-PDC composite, was used as an electrocatalyst for CO2 reduction in the SOEC system. Physical characterization of cathode, anode and electrolyte were done by X-Ray diffraction (XRD) pattern (Figure 1C) and scanning electron microscopy (SEM). Electrochemical impedance spectroscopy (EIS) were performed in presence CO2/CO atmosphere to determine the ohmic and polarization losses associated with electrolyte and electrode components of SOEC (Figure 1D). Combined with high catalytic activity and fast oxygen anion transport, ceria materials could be a potential candidate for catalytic or electrocatalytic reduction of CO2.
1. Neetu Kumari, M. Ali Haider, and S. Basu. Mechanism of Catalytic and Electrocatalytic CO2 Reduction to Fuels and Chemicals. CRC Press 2016, Chapter 6, 267286.
2. Kumari, N.; Sinha, N.; Haider, M. A.; Basu, S. CO2 Reduction to Methanol on CeO2(110) Surface: A Density Functional Theory Study. Electrochim. Acta 2015, 177, 2129.
Figure 1 (A) Reaction energy of CO2 reduction to CO and methanol on reduced ceria surface, (B) Mean square displacement plot for PDC at three different temperatures, (C) XRD pattern of PDC cathode of SOEC, and (D) EIS plot in different environment of CO2/CO.