Density Functional Theory Study of Methanol Decomposition on the CeO₂(110) Surface

Abstract

Methanol decomposition on the stoichiometric CeO 2 (110) surface has been investigated using density functional theory slab calculations. Three possible initial steps to decompose methanol by breaking one of three bonds (O-H, C-O, and C-H) of methanol were examined. The relative order of thermodynamic stability for the three possible bond scission steps is C-H > O-H > C-O. We further isolated transition states and determined activation energies for each of the bond-breaking modes using the nudged elastic method. The activation barrier for the most favorable dissociation mode, the O-H bond scission, is 0.3 eV on the (110) surface. An even lower activation barrier (<0.1 eV) has been obtained on the CeO 2 (111) surface for the same bondbreaking mode. We also calculated pre-exponential factors based on the harmonic approximation and obtained overall rate constants at 300 and 500 K for all three initial decomposition steps. In contrast to the order of thermodynamic stability, the calculated bond breaking barriers indicated a different favorable bond breaking order: O-H > C-O > C-H. Our results are consistent with experimental observation that methoxy is the dominant surface species after the stoichiometric CeO 2 surface was exposed to methanol. The experimentally observed methanol chemistry was determined by the kinetics of the initial dissociation steps rather than the thermodynamic stability of product states. The surface coverage of methanol was found to affect the relative stability between molecular and dissociative adsorption modes: Dissociative adsorption modes are preferred for methanol coverages up to 0.5 ML but only molecular adsorption was found to be stable at a full monolayer coverage.

Unraveling the structure sensitivity in methanol conversion on CeO2: A DFT+U study

“…In all the cases, the transition state associated to this process (TS2) involves the reduction of one single Ce 4+ to Ce 3+ , whereas the other unpaired electron resulting from the C-H scission is delocalized over these C and H atoms and the involved surface barrier for the direct formation of I2 from I1 on the (110) was previously reported [38] as 0.9 eV higher than ours. We found that the direct mechanism from I1 to I2 via the transition state TS2 is only favored on the (100) surface.…”

“…Despite all the above and other theoretical works reported in the literature [37][38][39][40][41][42], up to date there is no complete study assessing the selectivity of methanol dehydrogenation on the most representative ceria facets and proving their different behavior as experimentally observed. Herein, we present a thorough mechanistic study on the selective conversion of methanol to formaldehyde and its subsequent conversion to CO. To account for the particular morphology of the three common nanoshapes above, we have studied the three lowest index and energy surfaces (111), (110), and (100) (Fig.…”

The surface chemistry of cerium oxide

“…In contrast, calculations by Mei et al have indicated that the most stable dissociative state on stoichiometric CeO 2(111) involves cleavage of a C-H bond and the formation of a C-O(l) bond [209]. In a subsequent publication these authors indicated that O-H cleavage may be kinetically favored due to much lower activation barrier compared to C-H cleavage, resulting in adsorbed species that agree with experimental observations [210]. The concentration of methoxy on the surface thus determines the number of surface active sites on that surface.…”

Modifying ceria (111) with a TiO2 nanocluster for enhanced reactivity

“…Beste et al 70 found two adsorption modes on the perfect (111) surface from DFT+U calculations: molecular and dissociative as methoxy and hydroxyl, with adsorption energies of −0.76 eV and −0.08 eV. Mei et al 71 also used DFT+U to study methanol adsorption at the clean (111) surface. These authors find a coverage dependent adsorption process.…”