Microscopic models of PdZn alloy catalysts: structure and reactivity in methanol decomposition

Abstract

We review systematic experimental and theoretical efforts that explored formation, structure and reactivity of PdZn catalysts for methanol steam reforming, a material recently proposed to be superior to the industrially used Cu based catalysts. Experimentally, ordered surface alloys with a Pd : Zn ratio of approximately 1 : 1 were prepared by deposition of thin Zn layers on a Pd(111) surface and characterized by photoelectron spectroscopy and low-energy electron diffraction. The valence band spectrum of the PdZn alloy resembles closely the spectrum of Cu(111), in good agreement with the calculated density of states for a PdZn alloy of 1 : 1 stoichiometry. Among the issues studied with the help of density functional calculations are surface structure and stability of PdZn alloys and effects of Zn segregation in them, and the nature of the most likely water-related surface species present under the conditions of methanol steam reforming. Furthermore, a series of elementary reactions starting with the decomposition of methoxide, CH(3)O, along both C-H and C-O bond scission channels, on various surfaces of the 1 : 1 PdZn alloy [planar (111), (100) and stepped (221)] were quantified in detail thermodynamically and kinetically in comparison with the corresponding reactions on the surfaces Pd(111) and Cu(111). The overall surface reactivity of PdZn alloy was found to be similar to that of metallic Cu. Reactive methanol adsorption was also investigated by in situ X-ray photoelectron spectroscopy for pressures between 3 x 10(-8) and 0.3 mbar.

Steam reforming of methanol on PdZn near-surface alloys on Pd(1 1 1) and Pd foil studied by in-situ XPS, LEIS and PM-IRAS

“…For the specific 1:1 stoichiometry, DFT calculations identified a 1:1 ratio of Pd and Zn as the most stable configuration also in the surface layer, at least from the viewpoint of surface/bulk energetics that were not affected by high gas pressures or elevated temperature [8,10]. Related experimental studies of the Zn/Pd(111) system using XPS reported different stages of the alloying process as a function of temperature, among them the formation of the theoretically predicted 1:1 surface composition [5,11,12]. Studies performed by some of us revealed tetragonal AuCu-type PdZn nanoparticles [2] and a tetragonal PdZn nearsurface alloy formed on Pd(111) at elevated temperatures and high Zn coverages beyond 5 ML, which closely resembled the active tetragonal bulk alloy state [6].…”

“…As highlighted in recent theoretical work [11,16], the presence of the -with respect to clean Pd -electronicallyaltered PdZn surface is certainly crucial for steering the individual dehydrogenation steps from methanol to CO. The enhanced stability and thus preferential formation of formaldehyde on the PdZn(111) surface and the suppression of its dehydrogenation to CO were already predicted in [16].…”

“…Studies performed by some of us revealed tetragonal AuCu-type PdZn nanoparticles [2] and a tetragonal PdZn nearsurface alloy formed on Pd(111) at elevated temperatures and high Zn coverages beyond 5 ML, which closely resembled the active tetragonal bulk alloy state [6]. Since the electronic structure of the 1:1 PdZn alloy is similar to that of metallic copper, a phenomenological interpretation of the observed analogous catalytic performance of both Cu and PdZn was suggested [5,[11][12][13][14][15]. A general concept involving the cooperative action of the Cu-like electronic structure of an extended PdZn "multilayer" surface alloy, in combination with a corrugated 1:1 PdZn surface layer, being capable of efficient water activation, was presented recently by Rameshan et al [14].…”

“…Reduction of Pd supported on ZnO in H 2 at T~523 K leads to the formation of particularly stable 1:1 PdZn nanoparticles crystallizing in the tetragonal AuCu-type lattice [1,2,4]. Both experimental and theoretical studies have already addressed the important issue of surface composition and segregation trends of the Pd-Zn intermetallic system (near-surface alloy on a single crystal and 1:1 PdZn nanoparticles on ZnO [5][6][7][8][9][10][11]) as a function of the Zn:Pd ratio (Zn coverage) and the temperature. For the specific 1:1 stoichiometry, DFT calculations identified a 1:1 ratio of Pd and Zn as the most stable configuration also in the surface layer, at least from the viewpoint of surface/bulk energetics that were not affected by high gas pressures or elevated temperature [8,10].…”

“…CH 2 O(ads)), to create a CO 2 -precursor such as formic acid at the surface. Theory examined the likely forms of formaldehyde and water at the PdZn surface, which are then supposed to interact toward CO 2 [11,16,17]. We may ask the questions where and how water is activated, e.g.…”