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Abstract The complex structure of the catalytic active phase, and surface‐gas reaction networks have hindered understanding of the oxidative coupling of methane (OCM) reaction mechanism by supported Na2WO4/SiO2catalysts. The present study demonstrates, with the aid of in situ Raman spectroscopy and chemical probe (H2‐TPR, TAP and steady‐state kinetics) experiments, that the long speculated crystalline Na2WO4active phase is unstable and melts under OCM reaction conditions, partially transforming to thermally stable surface Na‐WOxsites. Kinetic analysis via temporal analysis of products (TAP) and steady‐state OCM reaction studies demonstrate that (i) surface Na‐WOxsites are responsible for selectively activating CH4to C2Hxand over‐oxidizing CHyto CO and (ii) molten Na2WO4phase is mainly responsible for over‐oxidation of CH4to CO2and also assists in oxidative dehydrogenation of C2H6to C2H4. These new insights reveal the nature of catalytic active sites and resolve the OCM reaction mechanism over supported Na2WO4/SiO2catalysts.
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Synthesis and molecular structure of model silica-supported tungsten oxide catalysts for oxidative coupling of methane (OCM)
The molecular and electronic structures and chemical properties of the active sites on the surface of supported Na 2 WO 4 /SiO 2 catalysts used for oxidative coupling of methane (OCM) are poorly understood. Model SiO 2 -supported, Na-promoted tungsten oxide catalysts (Na–WO x /SiO 2 ) were systematically prepared using various Na- and W-precursors using carefully controlled Na/W molar ratios and examined with in situ Raman, UV-vis DR, CO 2 -TPD-DRIFT and NH 3 -TPD-DRIFT spectroscopies. The traditionally-prepared catalysts corresponding to 5% Na 2 WO 4 nominal loading, with Na/W molar ratio of 2, were synthesized from the aqueous Na 2 WO 4 ·2H 2 O precursor. After calcination at 800 °C, the initially amorphous SiO 2 support crystallized to the cristobalite phase and the supported sodium tungstate phase consisted of both crystalline Na 2 WO 4 nanoparticles (Na/W = 2) and dispersed phase Na–WO 4 surface sites (Na/W < 2). On the other hand, the catalysts prepared via a modified impregnation method using individual precursors of NaOH + AMT, such that the Na/W molar ratio remained well below 2, resulted in: (i) SiO 2 remaining amorphous (ii) only dispersed phase Na–WO 4 surface sites. The dispersed Na–WO 4 surface sites were isolated, more geometrically distorted, less basic in nature, and more reducible than the crystalline Na 2 WO 4 nanoparticles. The CH 4 + O 2 -TPSR results reveal that the isolated, dispersed phase Na–WO 4 surface sites were significantly more C 2 selective, but slightly less active than the traditionally-prepared catalysts that contain crystalline Na 2 WO 4 nanoparticles (Na/W = 2). These findings demonstrate that the isolated, dispersed phase Na–WO 4 sites on the SiO 2 support surface are the selective-active sites for the OCM reaction.
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- Award ID(s):
- 1706581
- PAR ID:
- 10188883
- Date Published:
- Journal Name:
- Catalysis Science & Technology
- Volume:
- 10
- Issue:
- 10
- ISSN:
- 2044-4753
- Page Range / eLocation ID:
- 3334 to 3345
- Format(s):
- Medium: X
- Sponsoring Org:
- National Science Foundation
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The experimentally validated computational models developed herein, for the first time, show that Mn-promotion does not enhance the activity of the surface Na 2 WO 4 catalytic active sites for CH 4 heterolytic dissociation during OCM. Contrary to previous understanding, it is demonstrated that Mn-promotion poisons the surface WO 4 catalytic active sites resulting in surface WO 5 sites with retarded kinetics for C–H scission. On the other hand, dimeric Mn 2 O 5 surface sites, identified and studied via ab initio molecular dynamics and thermodynamics, were found to be more efficient in activating CH 4 than the poisoned surface WO 5 sites or the original WO 4 sites. However, the surface reaction intermediates formed from CH 4 activation over the Mn 2 O 5 surface sites are more stable than those formed over the Na 2 WO 4 surface sites. The higher stability of the surface intermediates makes their desorption unfavorable, increasing the likelihood of over-oxidation to CO x , in agreement with the experimental findings in the literature on Mn-promoted catalysts. Consequently, the Mn-promoter does not appear to have an essential positive role in synergistically tuning the structure of the Na 2 WO 4 surface sites towards CH 4 activation but can yield MnO x surface sites that activate CH 4 faster than Na 2 WO 4 surface sites, but unselectively.more » « less
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- Free Publicly Accessible Full Text
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Accepted Manuscript1.0
- Journal Article:
- https://doi.org/10.1039/D0CY00289E
