(461d) 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 CO₂^d-
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.

Reference

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, 267–286.

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, 21–29.

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.