Active Sites and Mechanism for the Water-Gas Shift Reaction on Metal and Metal/Oxide Catalysts

Current industrial catalysts for the water-gas shift reaction are commonly mixtures of Fe-Cr and Zn-Al-Cu oxides, used at temperatures between 350-500oC and 180250oC, respectively. These oxide catalysts are pyrophoric and normally require lengthy and complex activation steps before usage. Improved catalysts are being sought, particularly for lower temperature (e.g., at T<150oC, equilibrium lowers the Active Sites and Mechanism for the Water-Gas Shift Reaction on Metal and Metal/Oxide Catalysts (BNL FWP CO-027) Figure 1. Left: TEM image of an inverse CeOx/CuO catalysts. Right: In-situ time-resolved PDF data for a CeO2/CuO catalyst during the WGS. Rodriguez – Brookhaven National Laboratory Hydrogen Fuel Cells 2 DOE Hydrogen and Fuel Cells Program 2013 Annual Merit Review and Peer Evaluation Meeting synthesis process. The active phase of catalysts which combine Cu, Ni, Au or Pt with oxides such as CeO2, TiO2 and CeOx/TiO2 essentially involved nanoparticles of the reduced metals. The oxide support underwent partial reduction and was not a simple spectator, facilitating the dissociation of water and in some cases modifying the chemical properties of the supported metal. Therefore, to optimize the performance of these catalysts one must take into consideration the properties of the metal and oxide phases. Figure 1 shows a TEM image and PDF data for an inverse CeOx/CuO powder catalysts. In the TEM image, taken for the as-prepared catalysts, one can see crystallites that in many cases exhibit a (111) surface termination. The PDF results for water-gas shift reaction conditions show a simultaneous disappearance of the Cu-O vector of CuO with the appearance of a Cu-Cu vector for metallic copper. These data, and in-situ results obtained for other catalysts in our group, indicate that a WGS metal/ oxide catalyst is a dynamic entity that changes with reaction conditions. Metal-oxide interactions and the activity of water-gas shift catalysts A series of model catalysts {CeOx/Cu(111), CeOx/ Au(111), Pt/CeO2(111), Ni/CeO2(111), Pt/TiO2(110), Pt/CeOx/ TiO2(110)} was used to study fundamental aspects of the water-gas shift reaction. These studies revealed that the oxide component of the catalyst can affect the reaction process in two different ways. First, the presence of O vacancies in the oxide greatly facilitates the dissociation of water. Second, the electronic properties of the metal can be affected by interactions with the oxide producing special chemical properties. This is the case in the Ni/CeO2(111), Pt/ CeO2(111) and Pt/CeOx/TiO2(110) systems. In Figure 2, small coverages of Ni on CeO2(111) are highly active for the water-gas shift reaction and do not produce methane, although bulk Ni is a very good catalyst for the methanation of CO. The electronic properties of Ni and Pt nanoparticles deposited on CeO2(111) and CeOx/TiO2(110) have been examined using core and valence photoemission. The results of valence photoemission point to a new type of metal-support interaction which produces large electronic perturbations for small Ni and Pt particles in contact with ceria. The Ni/CeO2(111), Pt/ CeO2(111) and Pt/CeOx/TiO2(110) systems exhibited a density of metal d states near the Fermi level that was much smaller than that expected for bulk metallic Ni or Pt. The electronic perturbations induced by ceria on Ni made this metal a very poor catalyst for CO methanation, but transformed Ni into an excellent catalyst for the production of hydrogen through the water-gas shift reaction(Figure 2) and the steam reforming of ethanol. Furthermore, the large electronic perturbations seen for small Pt particles in contact with ceria significantly enhanced the ability of the admetal to adsorb and dissociate water made it a highly active catalyst for the water-gas shift reaction(Figure 3). The behaviour seen for Ni/CeO2(111), Pt/CeO2(111) and Pt/CeOx/TiO2(110) systems product CO concentration into ranges suitable for direct fuel cell use without further purification.). Ceria (Au, Cu or Pt on CeO2), titania (Au, Pd or Pt on TiO2) and molybdena (Ni, Cu and Au on MoO2) based nanostructured catalysts are very promising new candidates for high activity, lower temperature operation in WGS systems. However, the design and optimization of these and other metal/oxide WGS nanocatalysts are hindered by controversy about basic questions regarding the nature of the active sites and the reaction mechanism. Here, we follow a coordinated experimental and theoretical effort to better understand these promising metal/oxide catalysts, and to develop concepts for their improvement. The project combines research in three thrusts: (i) in-situ studies to determine catalyst structure, oxidation state and chemistry under reaction conditions; (ii) studies of relevant model systems, primarily based on nanoparticles supported on single crystal substrates; and (iii) computational modeling. By closely linking these three research thrusts, our goal is to provide strongly validated mechanistic and structural conclusions for the future design and optimization of nanostructured WGS catalysts.