(621ac) CO2 Reduction to Methanol on CeO2(110) Surface: Mechanistic Insight

CO₂ reduction
to methanol on CeO₂(110) surface:
Mechanistic Insight

Neetu Kumari¹,
M. Ali Haider¹, Nishant Sinha²,
Suddhasatwa Basu¹

¹
Chemical Engineering Department, Indian Institute of Technology, New Delhi,
Delhi, India

²Dassault Systemes, Bangalore,
India.

Numerical
basis-set based ab-initio density
function theory (DFT) calculations were performed to study the reaction
mechanism for CO₂ reduction to methanol on the extended surface of CeO₂(110). Due to high oxygen storage capacity
and mixed ionic-electronic conducting property, ceria have been suggested to play
an active role in the catalytic and electrocatalytic
reduction of CO₂.  In this
study, energetics of probable reaction routes involving the formate
(HCOO) and carboxyl (COOH) intermediates, have been analyzed to assess the
formation of methanol on CeO₂(110) surface. Calculations were
performed to determine the most favorable adsorption orientation and
corresponding binding energy of reaction intermediates. On the stoichiometric
ceria surface, adsorbed formate intermediate species
were ascertained to be more stable with higher binding energy (-223 kJ/mole) as
compared to the carboxyl (E_binding = -37
kJ/mole) and is likely to be a spectator. In order to produce methanol via formate mediated routes, HCOO is required to be
hydrogenated to H₂COOH which subsequently dissociates into H₂CO
and OH. The dissociative elementary step of this route is significantly
endothermic (DE_rxn=64 kJ/mole). On
the contrary, the mechanistic routes involving the carboxyl (COOH) intermediate
shows all the way exothermic steps (Figure 1) on the stoichiometric CeO₂
(110) surface to produce methanol. The dissociation step (COOH → CO+OH)
is thermoneutral (DE_rxn~5 kJ/mole)
on stoichiometric ceria and slightly endothermic (DE_rxn=24
kJ/mole) on reduced ceria. Activation barrier for the dissociation step is
estimated to be 127 kJ/mole, which is significantly higher as compared to that
on the reduced ceria surface (E_a= 67.2
kJ/mole). The other suggested rate limiting step (CO₂+H→COOH),
has lower activation barrier of 37 kJ/mole on ceria surface. Experimentally,
electrochemical reduction of CO₂ has been carried out in solid oxide
electrolysis cell (SOEC). Ceria with different dopants (Gd,
Pr, and Sm) have been incorporated as cathode, for CO₂
reduced to CO and/or methanol.   

Reference:
N. Kumari, et al., Electrochim.
Acta (2015), http://dx.doi.org/10.1016/j.electacta.
2015.01.153

Figure  SEQ Figure \* ARABIC 1 Reaction energy diagram of CO₂
reduction to methanol on stoichiometric CeO₂(110)
surface