From Oxide‐Supported Palladium to Intermetallic Palladium Phases: Consequences for Methanol Steam Reforming

Abstract

This Minireview summarizes the fundamental results of a comparative inverse-model versus real-model catalyst approach toward methanol steam reforming (MSR) on the highly CO₂-selective H₂-reduced states of supported Pd/ZnO, Pd/Ga₂O3, and Pd/In₂O3 catalysts. Our model approach was extended to the related Pd/GeO₂ and Pd/SnO₂ systems, which showed previously unknown MSR performance. This approach allowed us to determine salient CO₂-selectivity-guiding structural and electronic effects on the molecular level, to establish a knowledge-based approach for the optimization of CO₂ 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 Pd₂Ga, under realistic MSR conditions were performed alongside catalytic testing. To highlight the importance of a specifically prepared bulk intermetallic[BOND]oxide interface, unsupported bulk intermetallic compounds of Pd_xGa_y were chosen as additional MSR model compounds, which allowed us to clearly deduce, for example, the water-activating role of the special Pd₂Ga-β-Ga₂O₃ intermetallic[BOND]oxide 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, Pd₅Ga₂, PdIn, Pd₃Snv, and Pd₂Ge 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 bimetal[BOND]oxide interface and the highly epitaxial ordering that was required for (HR)TEM studies and for identification of the structural and catalytic (bi)metal[BOND]support interactions. To fully understand the bimetal[BOND]support 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 “bimetal[BOND]oxide contact” systems, a pronounced “bimetal[BOND]oxide synergy” toward optimum CO2 activity/selectivity was most evident. Moreover, the system-specific mechanisms that led to undesired CO formation and to spoiling of the CO2 selectivity could be extracted

Hydrogen production by methanol steam reforming: Catalytic performance of supported-Pd on zinc–cerium oxides’ nanocomposites

“…A plausible explanation is to posit that under the reaction mixture, and upon heating to 623 K, the surface of the metal crystallites became modified, richer in full 3D active PdZn ensembles, most likely owing to an increase of the Pd-Zn alloying which -in turndecreased methanol decomposition. This is entirely consistent with recent literature reports [4,28] and our XPS data, as shown below.…”

“…In particular, the presence of well-dispersed ZnO domains on the support surface and/or as patches onto the PdZn bimetallic crystallites under MSR reaction condition, seems to be crucial for obtaining high turnover frequencies. Nonetheless, other authors have concluded that just the right PdZn alloy (namely, tetragonal PdZn␤ 1 ) suffices to grant high S CO2 and high TOF values [26], albeit under MSR conditions oxidized Zn species seem to be present in their pre-reduced bimetallic particles as was suggested by Lorenz et al [28] based on their relevant XPS spectra, probably because those alloy powders were again exposed to water.…”

“…Under these premises, the best performing catalysts would be highly active, selective and stable materials for which: (i) ZnO were highly dispersed on the surface and -therefore-where the chances of Pd-Zn intimacy prior to pre-reduction were higher (so as to grant the formation of full 3D active PdZn ensembles [28] in as much as possible), and (ii) the catalyst surface area were at least several fold higher than that of pure ZnO.…”

“…This outstanding selectivity to CO 2 suggests that on the Pd/ZnO catalysts the surface Pd atoms are (mostly) isolated or far apart enough on the metallic crystallite surface, most likely due to the formation of a "full 3D" active PdZn ensembles [28] which is responsible of modulating the catalysts' selectivity, as suggested by other workers [3,14,18,23]. In addition, patches of surface palladium not properly alloyed with Zn [27] -or just a fraction of pure Pd crystallites-can explain the appearance of CO (S CO < 35%) from MD on those sites, because the eventual concurrency of MSR + RWGS would demand that water conversion were much smaller than X CH3OH which, at least under 573 K, was not observed.…”

Ultraselective low temperature steam reforming of methanol over PdZn/ZnO catalysts—Influence of induced support defects on catalytic performance

“…Moreover, there was an indication that addition of Zn to Pt or Pd altered the barriers for C-H cleavage and thus changed the stability of various intermediates in MSR [22]. The PdZn/ZnO interface was found to increase water activation at lower temperatures, therefore promoting CO2 selectivity [23]. However, the role of ZnO in the performance of PdZn/ZnO is not yet fully understood.…”