skip to main content

Title: Oxo dicopper anchored on carbon nitride for selective oxidation of methane
Abstract Selective conversion of methane (CH 4 ) into value-added chemicals represents a grand challenge for the efficient utilization of rising hydrocarbon sources. We report here dimeric copper centers supported on graphitic carbon nitride (denoted as Cu 2 @C 3 N 4 ) as advanced catalysts for CH 4 partial oxidation. The copper-dimer catalysts demonstrate high selectivity for partial oxidation of methane under both thermo- and photocatalytic reaction conditions, with hydrogen peroxide (H 2 O 2 ) and oxygen (O 2 ) being used as the oxidizer, respectively. In particular, the photocatalytic oxidation of CH 4 with O 2 achieves >10% conversion, and >98% selectivity toward methyl oxygenates and a mass-specific activity of 1399.3 mmol g Cu −1 h −1 . Mechanistic studies reveal that the high reactivity of Cu 2 @C 3 N 4 can be ascribed to symphonic mechanisms among the bridging oxygen, the two copper sites and the semiconducting C 3 N 4 substrate, which do not only facilitate the heterolytic scission of C-H bond, but also promotes H 2 O 2 and O 2 activation in thermo- and photocatalysis, respectively.  more » « less
Award ID(s):
Author(s) / Creator(s):
; ; ; ; ; ; ; ; ; ; ; ; ;
Date Published:
Journal Name:
Nature Communications
Medium: X
Sponsoring Org:
National Science Foundation
More Like this
  1. In this report, CeO 2 and SiO 2 supported 1 wt% Ru catalysts were synthesized and studied for dry reforming of methane (DRM) by introducing non-thermal plasma (NTP) in a dielectric barrier discharge (DBD) fixed bed reactor. From quadrupole mass spectrometer (QMS) data, it is found that introducing non-thermal plasma in thermo-catalytic DRM promotes higher CH 4 and CO 2 conversion and syngas (CO + H 2 ) yield than those under thermal catalysis only conditions. According to the H 2 -TPR, CO 2 -TPD, and CO-TPD profiles, reducible CeO 2 supported Ru catalysts presented better activity compared to their irreducible SiO 2 supported Ru counterparts. For instance, the molar concentrations of CO and H 2 were 16% and 9%, respectively, for plasma-assisted thermo-catalytic DRM at 350 °C, while no apparent conversion was observed at the same temperature for thermo-catalytic DRM. Highly energetic electrons, ions, and radicals under non-equilibrium and non-thermal plasma conditions are considered to contribute to the activation of strong C–H bonds in CH 4 and C–O bonds in CO 2 , which significantly improves the CH 4 /CO 2 conversion during DRM reaction at low temperatures. At 450 °C, the 1 wt% Ru/CeO 2 nanorods sample showed the highest catalytic activity with 51% CH 4 and 37% CO 2 conversion compared to 1 wt% Ru/CeO 2 nanocubes (40% CH 4 and 30% CO 2 ). These results clearly indicate that the support shape and reducibility affect the plasma-assisted DRM reaction. This enhanced DRM activity is ascribed to the surface chemistry and defect structures of the CeO 2 nanorods support that can provide active surface facets, higher amounts of mobile oxygen and oxygen vacancy, and other surface defects. 
    more » « less
  2. Abstract

    Methane over‐oxidation by copper‐exchanged zeolites prevents realization of high‐yield catalytic conversion. However, there has been little description of the mechanism for methane over‐oxidation at the copper active sites of these zeolites. Using density functional theory (DFT) computations, we reported that tricopper [Cu3O3]2+active sites can over‐oxidize methane. However, the role of [Cu3O3]2+sites in methane‐to‐methanol conversion remains under debate. Here, we examine methane over‐oxidation by dicopper [Cu2O]2+and [Cu2O2]2+sites using DFT in zeolite mordenite (MOR). For [Cu2O2]2+, we considered the μ‐(η22) peroxo‐, and bis(μ‐oxo) motifs. These sites were considered in the eight‐membered (8MR) ring of MOR. μ‐(η22) peroxo sites are unstable relative to the bis(μ‐oxo) motif with a small interconversion barrier. Unlike [Cu2O]2+which is active for methane C−H activation, [Cu2O2]2+has a very large methane C−H activation barrier in the 8MR. Stabilization of methanol and methyl at unreacted dicopper sites however leads to over‐oxidation via sequential hydrogen atom abstraction steps. For methanol, these are initiated by abstraction of the CH3group, followed by OH and can proceed near 200 °C. Thus, for [Cu2O]2+and [Cu2O2]2+species, over‐oxidation is an inter‐site process. We discuss the implications of these findings for methanol selectivity, especially in comparison to the intra‐site process for [Cu3O3]2+sites and the role of Brønsted acid sites.

    more » « less
  3. The oxidative coupling of methane to ethylene using gaseous disulfur (2CH4+ S2→ C2H4+ 2H2S) as an oxidant (SOCM) proceeds with promising selectivity. Here, we report detailed experimental and theoretical studies that examine the mechanism for the conversion of CH4to C2H4over an Fe3O4-derived FeS2catalyst achieving a promising ethylene selectivity of 33%. We compare and contrast these results with those for the highly exothermic oxidative coupling of methane (OCM) using O2(2CH4+ O2→ C2H4+ 2H2O). SOCM kinetic/mechanistic analysis, along with density functional theory results, indicate that ethylene is produced as a primary product of methane activation, proceeding predominantly via CH2coupling over dimeric S–S moieties that bridge Fe surface sites, and to a lesser degree, on heavily sulfided mononuclear sites. In contrast to and unlike OCM, the overoxidized CS2by-product forms predominantly via CH4oxidation, rather than from C2products, through a series of C–H activation and S-addition steps at adsorbed sulfur sites on the FeS2surface. The experimental rates for methane conversion are first order in both CH4and S2, consistent with the involvement of two S sites in the rate-determining methane C–H activation step, with a CD4/CH4kinetic isotope effect of 1.78. The experimental apparent activation energy for methane conversion is 66 ± 8 kJ/mol, significantly lower than for CH4oxidative coupling with O2. The computed methane activation barrier, rate orders, and kinetic isotope values are consistent with experiment. All evidence indicates that SOCM proceeds via a very different pathway than that of OCM.

    more » « less
  4. Abstract

    Several ceria‐zirconia supported mono and bi‐metallic transition metal oxide clusters containing Fe, Cu, and Ni are synthesized by dry impregnation. Through XRD, H2‐TPR, NH3‐TPD, pyridine adsorption followed by FTIR spectroscopy and XAS, the well‐dispersed nature of the transition metal oxide clusters is revealed, and the Lewis acidity of the catalysts is assessed. In‐situ FTIR spectroscopy is used to monitor the methane activation on catalyst surfaces. All catalysts activate methane at 250 °C forming methyl, alkyl, and methoxy species on the catalyst surface. By co‐feeding steam and oxygen together with methane, continuous direct oxidation of methane to methanol can be achieved, with the complete oxidation to CO2as the other reaction path. Methoxy species are found to be a key intermediate for methanol production. Lowering the methane conversion improves the methanol selectivity. By extrapolation, it is estimated that methanol selectivity close to unity can be achieved below a threshold of methane conversion at about 0.002 %. The formation of CuO and NiO mixed metal oxides produces stronger Lewis acid sites and yields higher methanol selectivity.

    more » « less
  5. Abstract

    Copper‐exchanged zeolites are useful for stepwise conversion of methane to methanol at moderate temperatures. This process also generates some over‐oxidation products like CO and CO2. However, mechanistic pathways for methane over‐oxidation by copper‐oxo active sites in these zeolites have not been previously described. Adequate understanding of methane over‐oxidation is useful for developing systems with higher methanol yields and selectivities. Here, we use density functional theory (DFT) to examine methane over‐oxidation by [Cu3O3]2+active sites in zeolite mordenite MOR. The methyl group formed after activation of a methane C−H bond can be stabilized at a μ‐oxo atom of the active site. This μ‐(O−CH3) intermediate can undergo sequential hydrogen atom abstractions till eventual formation of a copper‐monocarbonyl species. Adsorbed formaldehyde, water and formates are also formed during this process. The overall mechanistic path is exothermic, and all intermediate steps are facile at 200 °C. Release of CO from the copper‐monocarbonyl costs only 3.4 kcal/mol. Thus, for high methanol selectivities, the methyl group from the first hydrogen atom abstraction stepmust bestabilizedawayfrom copper‐oxo active sites. Indeed, it must be quickly trapped at an unreactive site (short diffusion lengths) while avoiding copper‐oxo species (large paths between active sites). This stabilization of the methyl group away from the active sites is central to the high methanol selectivities obtained with stepwise methane‐to‐methanol conversion.

    more » « less