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From Oxide‐Supported Palladium to Intermetallic Palladium Phases: Consequences for Methanol Steam Reforming
Author(s) -
Lorenz Harald,
Rameshan Christoph,
Bielz Thomas,
Memmel Norbert,
Stadlmayr Werner,
Mayr Lukas,
Zhao Qian,
Soisuwan Soipatta,
Klötzer Bernhard,
Penner Simon
Publication year - 2013
Publication title -
chemcatchem
Language(s) - English
Resource type - Journals
SCImago Journal Rank - 1.497
H-Index - 106
eISSN - 1867-3899
pISSN - 1867-3880
DOI - 10.1002/cctc.201200712
Subject(s) - intermetallic , steam reforming , palladium , catalysis , x ray photoelectron spectroscopy , oxide , materials science , selectivity , chemical engineering , methanol , chemistry , inorganic chemistry , metallurgy , organic chemistry , hydrogen production , alloy , engineering
This Minireview summarizes the fundamental results of a comparative inverse‐model versus real‐model catalyst approach toward methanol steam reforming (MSR) on the highly CO 2 ‐selective H 2 ‐reduced states of supported Pd/ZnO, Pd/Ga 2 O 3 , and Pd/In 2 O 3 catalysts. Our model approach was extended to the related Pd/GeO 2 and Pd/SnO 2 systems, which showed previously unknown MSR performance. This approach allowed us to determine salient CO 2 ‐selectivity‐guiding structural and electronic effects on the molecular level, to establish a knowledge‐based approach for the optimization of CO 2 selectivity. Regarding the inverse‐model catalysts, in situ X‐ray photoelectron spectroscopy (in situ XPS) studies on near‐surface intermetallic PdZn, PdGa, and PdIn phases (NSIP), as well as bulk Pd 2 Ga, under realistic MSR conditions were performed alongside catalytic testing. To highlight the importance of a specifically prepared bulk intermetallicoxide interface, unsupported bulk intermetallic compounds of Pd x Ga y were chosen as additional MSR model compounds, which allowed us to clearly deduce, for example, the water‐activating role of the special Pd 2 Ga‐β‐Ga 2 O 3 intermetallicoxide interaction. The inverse‐model studies were complemented by their related “real‐model” experiments. Structure–activity and structure–selectivity correlations were performed on epitaxially ordered PdZn, Pd 5 Ga 2 , PdIn, Pd 3 Sn 2 , and Pd 2 Ge nanoparticles that were embedded in thin crystalline films of their respective oxides. The reductively activated “thin‐film model catalysts” that were prepared by sequential Pd and oxide deposition onto NaCl(001) exhibited the required large bimetaloxide interface and the highly epitaxial ordering that was required for (HR)TEM studies and for identification of the structural and catalytic (bi)metalsupport interactions. To fully understand the bimetalsupport interactions in the supported systems, our studies were extended to the MeOH‐ and formaldehyde‐reforming properties of the clean supporting oxides. From a direct comparison of the “isolated” MSR performance of the purely bimetallic surfaces to that of the “isolated” oxide surfaces and of the “bimetaloxide contact” systems, a pronounced “bimetaloxide synergy” toward optimum CO 2 activity/selectivity was most evident. Moreover, the system‐specific mechanisms that led to undesired CO formation and to spoiling of the CO 2 selectivity could be extracted.