Facilitating the reduction of V–O bonds on VOx/ZrO2 catalysts for non-oxidative propane dehydrogenation†

Supported vanadium oxide is a promising catalyst in propane dehydrogenation due to its competitive performance and low cost. Nevertheless, it remains a grand challenge to understand the structure–performance correlation due to the structural complexity of VOx-based catalysts in a reduced state. This paper describes the structure and catalytic properties of the VOx/ZrO2 catalyst. When using ZrO2 as the support, the catalyst shows six times higher turnover frequency (TOF) than using commercial γ-Al2O3. Combining H2-temperature programmed reduction, in situ Raman spectroscopy, X-ray photoelectron spectroscopy and theoretical studies, we find that the interaction between VOx and ZrO2 can facilitate the reduction of V–O bonds, including V 
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Created by potrace 1.16, written by Peter Selinger 2001-2019
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 O, V–O–V and V–O–Zr. The promoting effect could be attributed to the formation of low coordinated V species in VOx/ZrO2 which is more active in C–H activation. Our work provides a new insight into understanding the structure–performance correlation in VOx-based catalysts for non-oxidative propane dehydrogenation.

S2 S1. Experimental and computational methods

Characterization methods
X-ray diffraction (XRD) measurements were performed on a Rigaku C/max-2500 diffractometer with CuKα radiation. Raman measurements of VZr with different loadings were conducted under ambient condition on a Renishaw inVia reflex Raman spectrometer equipped with visible (532 nm) Ar-ion laser beam. The samples were dried at 300 °C for 2 h before the measurement.
The measurement of specific surface area (SSA) of the samples was conducted on a Micromeritics Tristar 3000 analyzer at −196 °C. The Brunauer-Emmett-Teller (BET) method was applied to calculate SAAs on the basis of the N2 isotherms.
H2-Temperature Program Reduction (H2-TPR) tests were executed on a Micromeritics AutoChem 2920 apparatus. The sample (0.4 g) was purged at 300 °C for one hour under an Ar stream (20 mL/min). After cooling down to 100 °C, H2-TPR was conducted in 10 vol % H2/Ar (30 mL/min) flow. The sample was headed up to 800 °C with a heating rate of 10 °C/min. The signal was detected with a thermal conductivity detector (TCD).
In situ Raman was performed on a Renishaw inVia reflex Raman spectrometer with 325 nm Ar-ion laser beam.
The sample was pretreated with 1% O2/N2 at 550 °C for 1 hour and recorded the signal. Then, the gas was changed to 10% H2/Ar and collected the spectra every minute.
XPS tests were executed on a PHI 1600 ESCA instrument (PE Company) equipped with an Al Kα X-ray radiation source (hν = 1486.6 eV). Before the test, reduced samples were pretreated under H2 atmosphere at 550 °C for 30 min. The binding energies were referenced to the C 1s peak at 284.6 eV.

Reactivity test
Catalytic performance evaluation was carried out in a quartz fixed-bed reactor (8 mm ID) under 0.13 MPa. 0.4 g catalyst mixed with quartz sands were packed into the tube. The sample was heated up to 550 °C under N2 flow (36 mL/min) and then reduced at the same temperature under H2 atmosphere (H2:N2=7:36) for 30 minutes.
Afterward, a mixture of C3H8, N2 and H2 (C3H8:N2:H2=7:36:7) was fed to the reactor at a rate of 50 mL/min. The products were analyzed with an online GC (2060) equipped with a flame ionization detector (Chromosorb 102 column) and a thermal conductivity detector (Al2O3 Plot column). Propane conversion and propylene selectivity based on all products and gas-phase products were calculated from eq(1), eq(2) and eq (3).
Where stands for different carbon product in the gas phase. stands for the number of carbon atom in the molecular. stands for the molar flow rate of species .
Turnover frequency (TOF) and propane conversion rates were determined under a special condition. Total flow rate was determined to eliminate the mass transfer and the conversion below 15% to ensure differential reaction.
TOF was calculated based on the total number of V atom from eq(4).
As VOx could be well dispersed on ZrO2 as a monolayer, TOF based on the number of V atom will be same as S3 TOF based on active sites. This calculation method has been widely applied in many other works 1-6 . Nevertheless, for the catalysts with crystal V2O5, the calculated TOF would be smaller than the actual TOF based on the active sites.

Models and computational details
Both VZr and VAl models and their correspond reduced models were created to investigate reducibility and propane dehydrogenation energy barriers. Monoclinic ZrO2 unit cell was cut along the (1 � 11) plane and γ-Al2O3 unit cell along the (100)  Calculations were performed using Vienna ab initio simulation package (VASP) 7,8 . In order to correct on site Coulomb correlation of occupied V 3d orbitals, we employed the gradient-corrected exchange−correlation functional by Perdew, Burke, and Ernzerhof (PBE) 9 and an effective Hubbard-type U parameter of 3.2 eV. The valence wave functions were expanded by plane wave with a cutoff energy of 400 eV. The atomic core was described by the Projected Augmented Wave (PAW) pseudopotentials 10 .
Due to the overbinding of GGA in the O2 molecule, we used H2(g) and H2O(g) as reference for oxygen vacancy formation energy: In potential energy diagrams, the energy of C3H8 in the gas phase is taken as reference and the energy of intermediates are corrected with H2 in the gas phase. The adsorption energy is defined as: Moreover, the transition states were located by the climbing-image nudged elastic band method (NEB) 11 . The activation barrier Ea was calculated based on following equation: