The kinetics of selective oxidation of propene on bismuth vanadium molybdenum oxide catalysts

We report the results of a systematic investigation of the kinetics of propene oxidation to acrolein over Bi1-x/3V1-xMoxO4. BET isotherms were measured to determine catalyst surface area, and powder X-ray diffraction was used to characterize the bulk structure. Further characterization by X-ray absorption near-edge spectroscopy (XANES) was used to determine the oxidation states of Bi, Mo, and V before and after exposure of the catalyst to propene at 713 K. We find that, contrary to previous discussions of the mechanism of propene oxidation on Bi1-x/3V1-xMoxO4, Bi remains in the 3+ state and only V and Mo undergo reduction and oxidation during reaction. The kinetics of propene oxidation were examined to establish the activation barrier for acrolein formation, and how the partial pressure dependences on propene and oxygen change with the value of x. The data obtained from this study were then used to propose a generalized model for the kinetics of propene oxidation over Bi1-x/3V 1-xMoxO4 that is consistent with our findings about the reducibility of the three metallic elements in the oxide. According to this model, vanadium and molybdenum are randomly distributed to form three types of sites each associated with its own rate parameters. MoV sites are found to exhibit the highest activity. The proposed model provides a good description of the experimental data for all catalyst formulations examined, for a range of propene and oxygen partial pressures, and for temperatures above 653 K. © 2013 Elsevier Inc. All rights reserved

The influence of functionals on density functional theory calculations of the properties of reducible transition metal oxide catalysts

Reducible transition metal oxides (RTMOs) comprise an important class of catalytic materials that are used for the selective oxidation and electro- and photochemical splitting of water, and as supports for metal nanoparticles. It is, therefore, highly desirable to model the properties of these materials accurately using density functional theory (DFT) in order to understand how oxide structure and performance are related and to guide the search for materials exhibiting superior performance. Unfortunately, accurate description of the structural and electronic properties of RTMOs using DFT has proven particularly challenging. The M06-L density functional, which has been shown to be broadly accurate for calculations of gas phase clusters, has recently become available to researchers carrying out calculations in the solid state, but its performance in determining the properties RTMOs has been little investigated. The aim of this work was to assess the performance of the M06-L functional for describing the structural and electronic properties of a family of RTMOs: MoO2, MoO3, and Bi2Mo3O 12. Lattice constants, band gaps, and densities of states calculated using the M06-L functional are compared to those obtained from DFT+U. We have also used the M06-L functional to determine the reaction barrier for propene activation over Bi2Mo3O12, the rate-limiting step in the oxidation of propene to acrolein. We find that while DFT calculations carried out with the M06-L functional are roughly five times more expensive computationally than those performed with DFT+U, the results obtained using the M06-L functional provide sensible results for all properties investigated, while avoiding the necessary trade-off between accurate electronic structure and accurate thermochemistry that occurs in DFT+U. © 2013 American Chemical Society

The influence of functionals on density functional theory calculations of the properties of reducible transition metal oxide catalysts

Reducible transition metal oxides (RTMOs) comprise an important class of catalytic materials that are used for the selective oxidation and electro- and photochemical splitting of water, and as supports for metal nanoparticles. It is, therefore, highly desirable to model the properties of these materials accurately using density functional theory (DFT) in order to understand how oxide structure and performance are related and to guide the search for materials exhibiting superior performance. Unfortunately, accurate description of the structural and electronic properties of RTMOs using DFT has proven particularly challenging. The M06-L density functional, which has been shown to be broadly accurate for calculations of gas phase clusters, has recently become available to researchers carrying out calculations in the solid state, but its performance in determining the properties RTMOs has been little investigated. The aim of this work was to assess the performance of the M06-L functional for describing the structural and electronic properties of a family of RTMOs: MoO , MoO , and Bi Mo O . Lattice constants, band gaps, and densities of states calculated using the M06-L functional are compared to those obtained from DFT+U. We have also used the M06-L functional to determine the reaction barrier for propene activation over Bi Mo O , the rate-limiting step in the oxidation of propene to acrolein. We find that while DFT calculations carried out with the M06-L functional are roughly five times more expensive computationally than those performed with DFT+U, the results obtained using the M06-L functional provide sensible results for all properties investigated, while avoiding the necessary trade-off between accurate electronic structure and accurate thermochemistry that occurs in DFT+U. © 2013 American Chemical Society. 2 3 2 3 12 2 3 1

The kinetics of selective oxidation of propene on bismuth vanadium molybdenum oxide catalysts

We report the results of a systematic investigation of the kinetics of propene oxidation to acrolein over Bi1-x/3V1-xMoxO4. BET isotherms were measured to determine catalyst surface area, and powder X-ray diffraction was used to characterize the bulk structure. Further characterization by X-ray absorption near-edge spectroscopy (XANES) was used to determine the oxidation states of Bi, Mo, and V before and after exposure of the catalyst to propene at 713 K. We find that, contrary to previous discussions of the mechanism of propene oxidation on Bi1-x/3V1-xMoxO4, Bi remains in the 3+ state and only V and Mo undergo reduction and oxidation during reaction. The kinetics of propene oxidation were examined to establish the activation barrier for acrolein formation, and how the partial pressure dependences on propene and oxygen change with the value of x. The data obtained from this study were then used to propose a generalized model for the kinetics of propene oxidation over Bi1-x/3V 1-xMoxO4 that is consistent with our findings about the reducibility of the three metallic elements in the oxide. According to this model, vanadium and molybdenum are randomly distributed to form three types of sites each associated with its own rate parameters. MoV sites are found to exhibit the highest activity. The proposed model provides a good description of the experimental data for all catalyst formulations examined, for a range of propene and oxygen partial pressures, and for temperatures above 653 K. © 2013 Elsevier Inc. All rights reserved

Characterization of Isolated Ga3+ Cations in Ga/H-MFI Prepared by Vapor-Phase Exchange of H-MFI Zeolite with GaCl3

Ga/H-MFI was prepared by vapor-phase reaction of GaCl3 with Brønsted acid O-H groups in dehydrated H-MFI zeolite. The resulting [GaCl2]+ cations in the as-exchanged zeolite are treated in H2 at 823 K to stoichiometrically remove Cl ligands and form [GaH2]+ cations. Subsequent oxidation in O2 and characterization by IR spectroscopy and NH3-temperature-programmed desorption (TPD) suggests that, for Ga/Al ratios ≤0.3, Ga3+ exists predominantly as [Ga(OH)2]+-H+ cation pairs and to a lesser degree as [Ga(OH)]2+ cations at low Ga/Al ratios (∼0.1); while both species are associated with proximate cation-exchange sites, calculated free energies of formation suggest that [Ga(OH)]2+ cations are more stable on cation-exchange sites associated with NNN (next-nearest neighbor) framework Al atoms than on those associated with NNNN (next-next-nearest neighbor) framework Al atoms. Ga K-edge X-ray Absorption Near Edge Spectroscopy (XANES) measurements indicate that, under oxidizing conditions and for all Ga/Al ratios, all Ga species are in the +3 oxidation state and are tetrahedrally coordinated to 4 O atoms. Fourier analysis of Ga K-edge Extended X-ray Absorption Fine Structure (EXAFS) data supports the conclusion that Ga3+ is present predominantly as [Ga(OH)2]+ cations (or [Ga(OH)2]+-H+ cation pairs). For Ga/Al ratios ≤0.3, wavelet analysis of EXAFS data provide evidence for backscattering from nearest neighboring O atoms and from next-nearest neighboring framework Al atoms. For Ga/Al > 0.3, backscattering from next-nearest neighboring Ga atoms is also evident, characteristic of GaOx species. Upon reduction in H2, the oxidized Ga3+ species produce [Ga(OH)H]+-H+ cation pairs, [GaH2]+-H+ cation pairs, and [GaH]2+ cations. Computed phase diagrams indicate that the thermodynamic stability of the reduced Ga3+ species depends sensitively on temperature, Al-Al interatomic distance, and H2 and H2O partial pressures. For Ga/Al ratios ≤0.2, it is concluded that [GaH2]+-H+ cation pairs and [GaH]2+ cations are the predominant species present in Ga/H-MFI reduced above 673 K in 105 Pa H2 and in the absence of water vapor

Characterization of Isolated Ga3+ Cations in Ga/H-MFI Prepared by Vapor-Phase Exchange of H-MFI Zeolite with GaCl3

Ga/H-MFI was prepared by vapor-phase reaction of GaCl with Brønsted acid O-H groups in dehydrated H-MFI zeolite. The resulting [GaCl ] cations in the as-exchanged zeolite are treated in H at 823 K to stoichiometrically remove Cl ligands and form [GaH ] cations. Subsequent oxidation in O and characterization by IR spectroscopy and NH -temperature-programmed desorption (TPD) suggests that, for Ga/Al ratios ≤0.3, Ga exists predominantly as [Ga(OH) ] -H cation pairs and to a lesser degree as [Ga(OH)] cations at low Ga/Al ratios (∼0.1); while both species are associated with proximate cation-exchange sites, calculated free energies of formation suggest that [Ga(OH)] cations are more stable on cation-exchange sites associated with NNN (next-nearest neighbor) framework Al atoms than on those associated with NNNN (next-next-nearest neighbor) framework Al atoms. Ga K-edge X-ray Absorption Near Edge Spectroscopy (XANES) measurements indicate that, under oxidizing conditions and for all Ga/Al ratios, all Ga species are in the +3 oxidation state and are tetrahedrally coordinated to 4 O atoms. Fourier analysis of Ga K-edge Extended X-ray Absorption Fine Structure (EXAFS) data supports the conclusion that Ga is present predominantly as [Ga(OH) ] cations (or [Ga(OH) ] -H cation pairs). For Ga/Al ratios ≤0.3, wavelet analysis of EXAFS data provide evidence for backscattering from nearest neighboring O atoms and from next-nearest neighboring framework Al atoms. For Ga/Al > 0.3, backscattering from next-nearest neighboring Ga atoms is also evident, characteristic of GaO species. Upon reduction in H , the oxidized Ga species produce [Ga(OH)H] -H cation pairs, [GaH ] -H cation pairs, and [GaH] cations. Computed phase diagrams indicate that the thermodynamic stability of the reduced Ga species depends sensitively on temperature, Al-Al interatomic distance, and H and H O partial pressures. For Ga/Al ratios ≤0.2, it is concluded that [GaH ] -H cation pairs and [GaH] cations are the predominant species present in Ga/H-MFI reduced above 673 K in 10 Pa H and in the absence of water vapor. 3 2 2 2 2 3 2 2 2 x 2 2 2 2 2 2 + + 3+ + + 2+ 2+ 3+ + + + 3+ + + + + 2+ 3+ + + 2+