Abstract The electrochemical CO2 reduction reaction (CO2RR) has gathered widespread attention in the past decade as an enabling component to energy and fuel sustainability. Copper (Cu) is one of the few electrocatalysts that can convert CO2 to higher-order hydrocarbons. We report the CO2RR on polycrystalline Cu from 5 °C to 45 °C as a function of electrochemical potential. Our result shows that selectivity shifts toward CH4 at low temperature and H2 at high temperature at the potential values between −0.95 V and −1.25 V versus reversible hydrogen electrode (RHE). We analyze the activation energy for each product and discuss the possible underlying mechanism based on their potential dependence. The activation barrier of CH4 empirically obeys the Butler–Volmer equation, while C2H4 and CO show a non-trivial trend. Our result suggests that the CH4 production proceeds via a classical electrochemical pathway, likely the proton-coupled electron transfer of surface-saturated COad, while C2H4 is limited by a more complex process, likely involving surface adsorbates. Our measurement is consistent with the view that the adsorbate–adsorbate interaction dictates the C2+ selectivity.
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Surface basicity controls C–C coupling rates during carbon dioxide-assisted methane coupling over bifunctional Ca/ZnO catalysts
Carbon dioxide-assisted coupling of methane offers an approach to chemically upgrade two greenhouse gases and components of natural gas to produce ethylene and syngas. Prior research on this reaction has concentrated efforts on catalyst discovery, which has indicated that composites comprised of both reducible and basic oxides are especially promising. There is a need for detailed characterization of these bifunctional oxide systems to provide a more fundamental understanding of the active sites and their roles in the reaction. We studied the dependence of physical and electronic properties of Ca-modified ZnO materials on Ca content via X-ray photoelectron and absorption spectroscopies, electron microscopy, and infrared spectroscopic temperature-programmed desorption (IRTPD). It was found that introduction of only 0.6 mol% Ca onto a ZnO surface is necessary to induce significant improvement in the catalytic production of C2 species: C2 selectivity increases from 5% on unmodified ZnO to 58%, at similar conversions. Evidence presented shows that this selectivity increase resultsfrom the formation of an interface between the basic CaO and reducible ZnO phases. The basicity of these interface sites correlates directly with catalytic activity over a wide composition range, and this relationship indicates that moderate CO2 adsorption strength is optimal for CH4 coupling. These results demonstrate, for the first time to our knowledge, a volcano-type relationship between CO2-assisted CH4 coupling activity and catalyst surface basicity, which can inform further catalyst development.
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- Award ID(s):
- 2034647
- NSF-PAR ID:
- 10476617
- Publisher / Repository:
- Royal Society of Chemistry
- Date Published:
- Journal Name:
- Physical Chemistry Chemical Physics
- Volume:
- 25
- Issue:
- 14
- ISSN:
- 1463-9076
- Page Range / eLocation ID:
- 9859 to 9867
- Format(s):
- Medium: X
- Sponsoring Org:
- National Science Foundation
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Abstract MXenes are a new family of two-dimensional carbides and/or nitrides. Their 2D surfaces are typically terminated by O, OH and/or F atoms. Here we show that Ti3C2T
x —the most studied compound of the MXene family—is a good acid catalyst, thanks to the surface acid functionalities. We demonstrate this by applying Ti3C2Tx in the epoxide ring-opening reaction of styrene oxide (SO) and its isomerization in the liquid phase. Modifying the MXene surface changes the catalytic activity and selectivity. By oxidizing the surface, we succeeded in controlling the type and number of acid sites and thereby improving the yield of the mono-alkylated product to >80%. Characterisation studies show that a thin oxide layer, which forms directly on the Ti3C2Tx surface, is essential for catalysing the SO ring-opening. We hypothesize that two kinds of acid sites are responsible for this catalysis: In the MXene, strong acid sites (both Lewis and Brønsted) catalyse both the ring-opening and the isomerization reactions, while in the Mxene–TiO2composite weaker acid sites catalyse only the ring-opening reaction, increasing the selectivity to the mono-alkylated product. -
null (Ed.)A series of Al2O3-supported Fe-containing catalysts were synthesized by incipient wetness impregnation. The iron surface density was varied from 1 to 13 Fe atoms/nm2 spanning sub- and above-monolayer coverage. The resulting supported Fe-catalysts were characterized with N2 physisorption, ex situ XRD, PDF, XAS, AC-STEM and chemically probed by H2-TPR. The results suggest that over this entire range of loadings, Fe was present as dispersed species, with only a very small fraction of Fe2O3 aggregates, at the highest Fe loading. The in situ sulfidation of Fe/Al2O3 resulted in the formation of a highly active and selective PDH catalyst. The highest activity with 52% propane conversion and ~99% propylene selectivity at 560 °C was obtained for the 6.4 Fe/Al2O3 catalyst suggesting that this is the highest amount of Fe that could be fully dispersed on the support in sulfided form. XRD and AC-STEM indicated the absence of any crystalline iron sulfide aggregates after sulfidation and reaction. H2-TPR results indicated that the amount of the reducible Fe sites in the sulfided catalyst remained constant above monolayer coverage, and increasing loading did not increase the number of reducible Fe sites. Consistent with these results, the reactivity per gram of catalyst showed no increase with Fe loading above monolayer coverage, suggesting that additional Fe remains conformal to the alumina surface.more » « less
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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.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/d3cp00332a
