Carbide-Modified Pd on ZrO2 as Active Phase for CO2-Reforming of Methane—A Model Phase Boundary Approach
Norbert Köpfle, Kevin Ploner, Peter Lackner, Thomas Götsch, Christoph W. Thurner, Emilia A. Carbonio, Michael Hävecker, Axel Knop‐Gericke, Lukas Schlicker, Andrew Doran, Delf Kober, Aleksander Gurlo, Marc‐Georg Willinger, Simon Penner, Michael Schmid, Bernhard Klötzer
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.