Abstract
Starting from subsurface Zr0-doped “inverse” Pd and bulk-intermetallic Pd0Zr0 model catalyst precursors, we investigated the dry reforming reaction of methane (DRM) using synchrotron-based near ambient pressure in-situ X-ray photoelectron spectroscopy (NAP-XPS), in-situ X-ray diffraction and catalytic testing in an ultrahigh-vacuum-compatible recirculating batch reactor cell. Both intermetallic precursors develop a Pd0–ZrO2 phase boundary under realistic DRM conditions, whereby the oxidative segregation of ZrO2 from bulk intermetallic PdxZry leads to a highly active composite layer of carbide-modified Pd0 metal nanoparticles in contact with tetragonal ZrO2. This active state exhibits reaction rates exceeding those of a conventional supported Pd–ZrO2 reference catalyst and its high activity is unambiguously linked to the fast conversion of the highly reactive carbidic/dissolved C-species inside Pd0 toward CO at the Pd/ZrO2 phase boundary, which serves the role of providing efficient CO2 activation sites. In contrast, the near-surface intermetallic precursor decomposes toward ZrO2 islands at the surface of a quasi-infinite Pd0 metal bulk. Strongly delayed Pd carbide accumulation and thus carbon resegregation under reaction conditions leads to a much less active interfacial ZrO2–Pd0 state.
Highlights
IntroductionHigher H2 contents can in principle be achieved via membrane reactor operation or a combination with the water-gas shift reaction and/or methane steam reforming to attain the optimum H2 /CO ratio of 2 required for the synthesis of renewable fuels [4,5]
Among multiple worldwide and local approaches and strategies toward the mitigation of global warming, the methane dry reforming reaction (DRM) is regarded as a potential method to simultaneously deal with the two major climate-harming greenhouse gases, methane and carbon dioxide, and to further convert them to useful syngas following the reaction CH4 + CO2 → 2H2 + 2CO.At 100% selectivity, a H2 :CO = 1:1 ratio is obtained, which is directly suitable for follow-up reactions like carbonylation or hydro-formylation processes [1,2,3]
State shows a mix of bulkand ultrathin zirconia and already a minor contribution of intermetallic
Summary
Higher H2 contents can in principle be achieved via membrane reactor operation or a combination with the water-gas shift reaction and/or methane steam reforming to attain the optimum H2 /CO ratio of 2 required for the synthesis of renewable fuels [4,5] In this respect, loss of H2 selectivity at elevated pressures due to the water-gas shift equilibrium [6], as well as coking issues, especially on Ni-based catalysts [7,8,9,10], represent the major application-oriented obstacles. Attempts to enhance the Ni coking resistance, while simultaneously keeping a high activity, yielded promising bimetallic DRM catalysts In this respect, the NiPd/ZrO2 system stood out, in terms of the desired combination of excellent catalytic performance and coking resistance [19]. Despite the promising empirical data that have been compiled on the latter, the mechanistic benefit of alloying of Ni especially with Pd has not been resolved yet
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