Hydrogen and methoxy coadsorption in the computation of the catalytic conversion of methanol on the ceria (111) surface

Abstract

Methanol decomposition to formaldehyde catalyzed by the ceria (111) surface has been investigated using the DFT+U method. Our results rationalize experimental temperature programmed desorption experiments on the fully oxidized surface. Particular attention has been paid to the model correctness of methoxy with coadsorbed hydrogen on the surface. This issue has been raised by the experimental observation of water desorption at low temperature removing hydrogen from the system. Our investigation also includes hydrogen diffusion as a means of hydrogen removal. We show that the CH 3 O − /H + ion pair remains at large separation, rendering the coadsorbed model as appropriate in contrast to a neutral isolated methoxy, for which the dehydrogenation barrier becomes negligible. We find that the presence of methoxy reduces the reaction energy for water formation considerably. In

Surface Reactions and Catalytic Activities for Small Alcohols over LaMnO3(100) and La0.7Sr0.3MnO3(100): Dehydrogenation, Dehydration, and Oxidation

“…For eq , the reaction is written as producing an aldehyde in an O-vacancy, as the methanol attributed to eq is observed at a slightly lower temperature than the aldehyde, and this interpretation is consistent with calculations on cerium oxide . The ≥ ∼500 K temperature range is also sufficient that there may a contribution from direct ,,,,− CH bond breaking from an alkoxy in an O-vacancy (eq ). …”

“…58 While we did not systematically study the effects of coverage dependence, we did obtain some data to ensure that the pre-exposures being used were adequate: those data are shown in the Supporting Information and do show the expected trend for a recombination reaction (that lower coverages result in a higher peak maximum temperature). The presence of methoxy from dissociation of methanol is also supported by the observation that water is produced below 300 K, which results from the reaction of surface hydrogens with lattice oxygen (eq 2): 35,56,59,60 such surface hydrogens are generated from dissociative methanol adsorption. 20,21,23,38,57 As a result, the consumption of surface hydrogens creates O vacancies, which may further stabilize methoxy groups present on the surface.…”

“…While we did not systematically study the effects of coverage dependence, we did obtain some data to ensure that the pre-exposures being used were adequate: those data are shown in the Supporting Information and do show the expected trend for a recombination reaction (that lower coverages result in a higher peak maximum temperature). The presence of methoxy from dissociation of methanol is also supported by the observation that water is produced below 300 K, which results from the reaction of surface hydrogens with lattice oxygen (eq ): ,,, such surface hydrogens are generated from dissociative methanol adsorption. ,,,, As a result, the consumption of surface hydrogens creates O vacancies, which may further stabilize methoxy groups present on the surface. ,, The methoxy groups present undergo further reactions at higher temperatures including methoxy disproportionation to form methanal: based on theoretical results obtained over cerium oxide, the mechanism is likely via eq at ∼300 K and likely via eq at ∼500 K . The mechanism proposed for methanal production is a redox mechanism, consistent with prior characterization of methanol reactions on oxides .…”

“…This may be the reason for less production of methanal at higher temperatures (500–600 K) on La 0.7 Sr 0.3 MnO 3 (100) (Figure b), relative to what is observed on LaMnO 3 (100) (Figure a). The decreased H 2 O production (if occurring by eq ) does not necessarily mean decreased thermodynamic reducibility as a result of Sr incorporation: a decrease of eq may be due to a kinetic limitation, ,, regardless of whether the Sr increases the facility of reduction. The CO and CO 2 that are produced above 750 K may result from the oxidation of a small amount of dehydrogenated C species or carbonate type species.…”

“…The differences in temperatures of the major products in PE-TPR suggest that breakage of α C−H bonds is easier than that of C−O or β C− H bonds on both surfaces, similar to published studies on cerium oxide. 21,23,34,35,38,59,63 The extent of products observed increased in the order of methanol < ethanol < propan-2-ol, probably due to a combination of longer alcohols having stronger adsorption strengths and also more electron donation from the carbon backbone. The CE-TPR experiments show the same products as PE-TPR but with sustained production at primarily higher temperature ranges.…”