Reverse Water鈥揋as Shift on Interfacial Sites Formed by Deposition of Oxidized Molybdenum Moieties onto Gold Nanoparticles

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• 作者：Ronald Carrasquillo-Flores ; Insoo Ro ; Mrunmayi D. Kumbhalkar ; Samuel Burt ; Carlos A. Carrero ; Ana C. Alba-Rubio ; Jeffrey T. Miller ; Ive Hermans ; George W. Huber ; James A. Dumesic
• 刊名：Journal of the American Chemical Society
• 出版年：2015
• 出版时间：August 19, 2015
• 年：2015
• 卷：137
• 期：32
• 页码：10317-10325
• 全文大小：611K
• ISSN：1520-5126

文摘

We show that MoO_x-promoted Au/SiO₂ catalysts are active for reverse water鈥揼as shift (RWGS) at 573 K. Results from reactivity measurements, CO FTIR studies, Raman spectroscopy, and X-ray absorption spectroscopy (XAS) indicate that the deposition of Mo onto Au nanoparticles occurs preferentially on under-coordinated Au sites, forming Au/MoO_x interfacial sites active for reverse water鈥揼as shift (RWGS). Au and AuMo sites are quantified from FTIR spectra of adsorbed CO collected at subambient temperatures (e.g., 150鈥?70 K). Bands at 2111 and 2122 cm^鈥? are attributed to CO adsorbed on under-coordinated Au⁰ and Au^未+ species, respectively. Clausius鈥揅lapeyron analysis of FTIR data yields a heat of CO adsorption (螖H_ads) of 鈭?1 kJ mol^鈥? for Au⁰ and 鈭?4 kJ mol^鈥? for Au^未+ at 33% surface coverage. Correlations of RWGS reactivity with changes in FTIR spectra for samples containing different amounts of Mo indicate that interfacial sites are an order of magnitude more active than Au sites for RWGS. Raman spectra of Mo/SiO₂ show a feature at 975 cm^鈥?, attributed to a dioxo (O鈺?₂Mo(鈭扥鈥揝i)₂ species not observed in spectra of AuMo/SiO₂ catalysts, indicating preferential deposition of Mo on Au. XAS results indicate that Mo is in a +6 oxidation state, and therefore Au and Mo exist as a metal鈥搈etal oxide combination. Catalyst calcination increases the quantity of under-coordinated Au sites, increasing RWGS activity. This strategy for catalyst synthesis and characterization enables quantification of Au active sites and interfacial sites, and this approach may be extended to describe reactivity changes observed in other reactions on supported gold catalysts.