Effect of Sr Substitution in LaMnO₃(100) on Catalytic Conversion of Acetic Acid to Ketene and Combustion-Like Products

Abstract

The chemistry of acetic acid reactions was studied over La1–x Sr x MnO3(100) surfaces (x = 0, 0.3, or 0.7). Two types of temperature-programmed reaction (TPR) experiments were performed: pre-exposure TPRs as well as continuous-exposure TPRs (CE-TPRs). The main products observed in both types of experiments were CO, CO2, H2O, acetaldehyde, and ketene (ethenone). In acetic acid CE-TPR experiments with 1:1 codosing of oxygen, the surfaces showed ketene production in the order of LaMnO3(100) > La0.7Sr0.3MnO3(100) > La0.3Sr0.7MnO3(100). The presence of oxygen increased the absolute amount of ketene produced. The results are consistent with ketene being formed by a dehydration mechanism, with the pathway suppressed if there are too many surface oxygen vacancies. The effect of Sr substitution can be explained as increasing the ease of (or initial quantity of) oxygen vacancies, thereby decreasing the amount of surface oxygens which ultimately inhibits both ketene formation and formation of the combustion-like products CO, CO2, and H2O. Our interpretation is that both of these routes are catalyzed by sites (or site ensembles) that include surface oxygens, which is consistent with other published studies.

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

“…A second plausible explanation may be related to the stability of the vacancies themselves: the Sr-substituted perovskite may have more easily formed vacancies, but if the vacancies are more stable, they may bind adsorbates less strongly (again leading to less reactivity). The same trend of decreased overall activity with Sr substitution was observed for acetic acid conversion to ketene and combustion-like products . Acetic acid is likely to bind sufficiently strongly to the surface that the latter plausible explanation may be the most credible: Sr substitution may result in vacancies that are less strongly binding and thus less likely to affect the electronic structure of adsorbate molecules.…”

“…A model LaMnO 3 (100) (LaMnO 3 (100)) catalyst was prepared as a thin film grown on 0.05% Nb-doped SrTiO 3 (100) (STO(100)) by pulsed laser deposition (PLD). 14 The film was grown using an LaMnO 3 target with a laser fluence of ∼1.5 J cm −2 , while the substrate was heated up to 650 °C in a background pressure of 200 mTorr O 2 . The film growth was monitored by reflection high-energy electron diffraction (RHEED) that indicated an LaMnO 3 film thickness of ∼20 nm.…”

“…The same trend of decreased overall activity with Sr substitution was observed for acetic acid conversion to ketene and combustion-like products. 14 Acetic acid is likely to bind sufficiently strongly to the surface that the latter plausible explanation may be the most credible: Sr substitution may result in vacancies that are less strongly binding and thus less likely to affect the electronic structure of adsorbate molecules. Such an interpretation is chemically reasonable and has the important (but expected) implication that strategies to increase O-vacancy production by substitution may not always result in greater O-vacancy-mediated catalytic activity.…”

“…The single-crystalline nature of the LaMnO 3 (100) film was determined by thin film X-ray diffraction (XRD), and the film thickness was confirmed by the thickness fringes in the high-resolution θ – 2θ scans of the (002) diffraction peaks (“Epitaxy” software, Panalytical). A La 0.7 Sr 0.3 MnO 3 (100) thin film was grown to a thickness of ∼20 nm on a 0.05% Nb-doped STO(100) by PLD using the same conditions, as reported previously. , Thus, both films were ∼20 nm in thickness. Atomic force microscopy and RHEED images are available in reference .…”

“…A La 0.7 Sr 0.3 MnO 3 (100) thin film was grown to a thickness of ∼20 nm on a 0.05% Nb-doped STO(100) by PLD using the same conditions, as reported previously. , Thus, both films were ∼20 nm in thickness. Atomic force microscopy and RHEED images are available in reference .…”