Support Effect in Oxide Catalysis: Methanol Oxidation on Vanadia/Ceria

Abstract

Density functional theory is used for periodic models of monomeric vanadia species deposited on the CeO2(111) surface to study dissociative adsorption of methanol and its subsequent dehydrogenation to formaldehyde. Dispersion-corrected PBE+U calculations are performed and compared with HSE and B3LYP results. Dissociative adsorption of methanol at different sites on VO2·CeO2(111) is highly exothermic with adsorption energies of 1.8 to 1.9 eV (HSE+D). Two relevant pathways for desorption of formaldehyde are found with intrinsic barriers for the redox step of 1.0 and 1.4 eV (HSE+D). The calculated desorption temperatures (370 and 495 K) explain the peaks observed in temperature-programmed desorption experiments. Different sites of the supported catalyst system are involved in the two pathways: (i) methanol can chemisorb on the CeO2 surface filling a so-called pseudovacancy and the H atom is transferred to an V-O-Ce interphase bond or (ii) CH3OH may chemisorb at the V-O-Ce interphase bond and form a V-OCH3 species from which H is transferred to the ceria surface, providing evidence for true cooperativity. In both cases, ceria is directly involved in the redox process, as two electrons are accommodated in Ce f states forming two Ce(3+) ions whereas vanadium remains fully oxidized (V(5+)).

Descriptor Analysis in Methanol Conversion on Doped CeO2(111): Guidelines for Selectivity Tuning

“…The methoxy is partially reduced in the associated transition state leading to a radicaloid species, where one electron is delocalized over the C, H, and the surface O, and one single Ce(IV) is reduced to Ce(III), as noted also in ceria and other reducible oxides. 65,[78][79][80] In the final step, formaldehyde desorbs from the surface to the gas phase (R3). For ceria (111), the barrier for the C-H bond breaking is 1.03 eV and the reaction is exothermic by 1.78…”

“…88,89 Acid-base properties have been shown to control adsorption and reactions on metal oxides 90 and metals, 91 whereas the redox character controls the conversion of methanol to formaldehyde on vanadia supported on ceria. 12,79 It is worth noting that these properties are easily accessible. Geometric parameters can be obtained by performing a simple calculation on the optimized cell and can be derived experimentally from the XRD data or more accurately from adsorption measurements (BET).…”

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

“…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 non-innocent role of cerium oxide in heterogeneous catalysis: A theoretical perspective

“…[22,32,21,23,98,33]. Evaporated vanadium on CeO 2 (1 1 1) was known to mimic the reactivity of high-surface area, polycrystalline vanadia/ceria [99], but the atomic structure was then not resolved.…”

“…In many cases, metal ↔ ceria [29][30][31]12,10] and metal-oxide ↔ ceria [32,20,33] interactions have been proven to be crucial to achieving high catalytic performance. In practical terms, one wants to optimize the properties of both the adspecies and the oxide support and manipulate adspecies ↔ support interactions to improve catalytic activity and selectivity.…”