Activity versus Selectivity of the Methanol Oxidation at Ceria Surfaces: A Comparative First-Principles Study

Abstract

Ceria nanoparticles may expose (100) and (111) facets depending on the preparation method. Motivated by that, we study the reactions of methanol with the CeO 2 (100) surface using dispersion-corrected PBE+U subject to periodic boundary conditions and compare with results for the CeO 2 (111) surface. At (100) facets, the oxidative dehydrogenation of methanol to formaldehyde occurs with an intrinsic barrier of only 0.94 eV (91 kJ/mol), which is significantly lower than the corresponding barrier of 1.23 eV (119 kJ/mol) at the (111) surface. The lower barrier maps to lower formaldehyde desorption temperatures as observed in temperature-programmed desorption spectroscopy. The higher activity is in accordance with predicted lower oxygen defect formation energy in the (100) surface. Thus, defect formation energies are valid reactivity descriptors for oxidation reactions following a Mars-van Krevelen mechanism. At both surfaces, formaldehyde either desorbs or forms a bridging dioxymethylene intermediate, which can be further oxidized into carbon oxides. However, formaldehyde desorption from the (100) surface is significantly more endothermic. Thus, methanol oxidation at the (100) surface is predicted to yield both formaldehyde and carbon oxides, whereas at the (111) surface, formaldehyde can easily desorb. Therefore, the desorption step appears to be the key to the selectivity of methanol oxidation to formaldehyde.

Shape Engineering of Oxide Nanoparticles for Heterogeneous Catalysis

“…Nanorods that preferentially expose the reactive {110} facets are typically more active and/ors elective for CO oxidation [16,29] and methanold ehydrogenation than catalysts with othere xposed facets. [30] Nanocubes, exclusively exposing the polar {100} facets, are usually more active for soot combustion, [31] hydrogen oxidation, [32] and the water-gas shift (WGS) reaction than catalysts with other shapes and different exposed facets. [33] For CO oxidation, the activity usuallyf ollows the order nanorods > nanocubes > octahedra, and this has been straightforwardly correlated with preferential exposure of the {110}, {100}, and{ 111}f acets on these particles ( Figure 2).…”

“…On the (100) facet, methanol oxidation to formaldehydeo ccurs fast with an activation energy of 91 kJ mol À1 ,w hich is lower than that (119 kJ mol À1 )o nt he (111)p lane. [30] This is due to the easier formation of oxygen defectso nt he (100) surface. However, formaldehyde desorption from the (100) surface is significantly more endothermic,a nd the product is predicted to be am ixture of formaldehydea nd carbon oxides.…”

“…Ceria shows a clear shape‐dependent effect in catalysis. Nanorods that preferentially expose the reactive {110} facets are typically more active and/or selective for CO oxidation and methanol dehydrogenation than catalysts with other exposed facets . Nanocubes, exclusively exposing the polar {100} facets, are usually more active for soot combustion, hydrogen oxidation, and the water‐gas shift (WGS) reaction than catalysts with other shapes and different exposed facets .…”

“…Ceria nanoparticles may expose the {100} or {111} facets, depending on their shape. On the (100) facet, methanol oxidation to formaldehyde occurs fast with an activation energy of 91 kJ mol −1 , which is lower than that (119 kJ mol −1 ) on the (111) plane . This is due to the easier formation of oxygen defects on the (100) surface.…”

Synthesis, Structure and Catalytic Properties of Faceted Oxide Crystals

“…Cu 2 O and CeO 2 are examples of reducible metal oxides whose catalytic performances in various oxidation reactions has been proposed to proceed through Mars‐van Krevelen (MvK) cycles involving lattice oxygen removal and regeneration [113,114] . The surface reduction step of the MvK cycle has been correlated with catalytic performance.…”