Acid–Base Reactivity of Perovskite Catalysts Probed via Conversion of 2-Propanol over Titanates and Zirconates

Abstract

Although perovskite catalysts are well-known for their excellent redox property, their acid–base reactivity remains largely unknown. To explore the potential of perovskites in acid–base catalysis, we made a comprehensive investigation in this work on the acid–base properties and reactivity of a series of selected perovskites, SrTiO3, BaTiO3, SrZrO3, and BaZrO3, via a combination of various approaches including adsorption microcalorimetry, in situ FTIR spectroscopy, steady state kinetic measurements, and density functional theory (DFT) modeling. The perovskite surfaces are shown to be dominated with intermediate and strong basic sites with the presence of some weak Lewis acid sites, due to the preferred exposure of SrO/BaO on the perovskite surfaces as evidenced by low energy ion scattering (LEIS) measurements. Using the conversion of 2-propanol as a probe reaction, we found that the reaction is more selective to dehydrogenation over dehydration due to the dominant surface basicity of the perovskites. Furthermore, the adsorption energy of 2-propanol (ΔH ads,2–propanol ) is found to be related to both a bulk property (tolerance factor) and the synergy between surface acid and base sites. The results from in situ FTIR and DFT calculations suggest that both dehydration and dehydrogenation reactions occur mainly through the E1cB pathway, which involves the deprotonation of the alcohol group to form a common alkoxy intermediate on the perovskite surfaces. The results obtained in this work pave a path for further exploration and understanding of acid–base catalysis over perovskite catalysts.

Controlling Reaction Selectivity through the Surface Termination of Perovskite Catalysts

“…This reaction mechanism is denoted as the E 1cB pathway and is expected from the weak acidity of the surface sites in STO. [21] Therate-determining step (RDS) for acetone formation is the cleavage of the C a ÀHbond and for propene formation is the concerted cleavage of the C b À Ha nd C À Ob onds (Figure 4). Calculations show that the Ti-terminated surface of STO favors the production of propene (DE a,propene = 145 kJ mol À1 , DE a,acetone = 155 kJ mol À1 )a nd the Sr-terminated surface favors the production of acetone (DE a,propene = 235 kJ mol À1 , DE a,acetone = 149 kJ mol À1 ), which agrees well with our experimental observations.…”

“…Calculations show that the Ti-terminated surface of STO favors the production of propene (DE a,propene = 145 kJ mol À1 , DE a,acetone = 155 kJ mol À1 )a nd the Sr-terminated surface favors the production of acetone (DE a,propene = 235 kJ mol À1 , DE a,acetone = 149 kJ mol À1 ), which agrees well with our experimental observations. Apparent activation energies were calculated by fitting the Arrhenius equation to kinetic data (see Figure S13 and Table S4) collected under differential conditions (conversion 13 %) [21] and used to compare reactivity data at the same temperature for the five samples:T iO 2 -disk 400 8 8C ,S rO 400 8 8C , STO ðHNO 3 Þ,400 8 8C ,S TO 400 8 8C ,a nd STO 550 8 8C .A pparent activation energies for acetone production on surface-Sr-rich STO (STO 550 8 8C ;1 63 kJ mol À1 )a nd for propene production on as urface-Ti-rich STO sample (STO ðHNO 3 Þ,400 8 8C ;1 30 kJ mol À1 ) showed general agreement with the magnitude of the DFTcalculated activation energies for the RDS,n amely,1 49 and 145 kJ mol À1 ,r espectively.A lthough the good agreement between our DFT barriers of the rate-limiting steps and the experimental apparent activation energies sheds light on the reaction mechanisms at the two different terminations, ap roper reaction kinetic analysis is warranted in the future to firmly establish arelationship between the DFT-predicted mechanism and the experimental kinetic data. Theu niquet unabilityo fr eaction selectivityt hrough induceds urfacet erminationso fS TO is evidentf romacomparison with theindividualsingleoxides.…”

Controlling Reaction Selectivity through the Surface Termination of Perovskite Catalysts

“…The results suggest that both dehydrogenation and dehydration of 2‐propanol involve initial deprotonation to generate the 2‐propanoxy intermediate; then, depending upon the basicity of the adjacent surface oxygen atom, either the C β −H or C α −H bond is cleaved to produce propene or acetone, respectively (see the Supporting Information). This reaction mechanism is denoted as the E 1cB pathway and is expected from the weak acidity of the surface sites in STO . The rate‐determining step (RDS) for acetone formation is the cleavage of the C α −H bond and for propene formation is the concerted cleavage of the C β −H and C−O bonds (Figure ).…”

“…Apparent activation energies were calculated by fitting the Arrhenius equation to kinetic data (see Figure S13 and Table S4) collected under differential conditions (conversion ≤13 %) and used to compare reactivity data at the same temperature for the five samples: TiO 2 ‐disk 400 °C , SrO 400 °C , STO ,400 °C , STO 400 °C , and STO 550 °C . Apparent activation energies for acetone production on surface‐Sr‐rich STO (STO 550 °C ; 163 kJ mol −1 ) and for propene production on a surface‐Ti‐rich STO sample (STO ,400 °C ; 130 kJ mol −1 ) showed general agreement with the magnitude of the DFT‐calculated activation energies for the RDS, namely, 149 and 145 kJ mol −1 , respectively.…”

Solid Base Bi24O31Br10(OH)δ with Active Lattice Oxygen for the Efficient Photo‐Oxidation of Primary Alcohols to Aldehydes

“…Previous studies showed that the acid–base properties of metal‐oxide catalysts (e.g., MgO, MoO 3 , and WO 3 ) determine the alcohol dehydration and/or dehydrogenation processes . While isopropanol is oxidized to acetone via dehydrogenation over basic catalysts, propene is formed via dehydration over acidic catalysts . Therefore, we anticipate that the combination of base catalysis and photocatalysis may promote the photo‐oxidation of primary alcohols.…”