Nature of the mixed-oxide interface in ceria-titania catalysts: Clusters, chains, and nanoparticles

The ceria-titania mixed metal oxide is an important component of catalysts active for the production of hydrogen through the water-gas shift reaction (CO + H2O → H2 + CO2) and the photocatalytic splitting of water (H2O + hv → H2 + 0.5O 2). We have found that ceria-titania catalysts prepared through wet chemical methods have a unique hierarchal architecture. Atomic resolution imaging by high-angle annular dark field scanning transmission electron microscopy (HAADF STEM) reveals that ceria supported on titania exhibits a range of morphologies. One can clearly identify ceria structures involving clusters, chains, and nanoparticles, which are distributed inhomogeneously on the titania support. These structures are often below the sensitivity limit of techniques such as X-ray diffraction (XRD), which in this case identifies the average particle size of the ceria and titania nanoparticles (via the Debye-Scherer equation) to be 7.5 and 36 nm, respectively. The fluorite-structured ceria grows epitaxially on the anatase-structured titania, and this epitaxial growth influences the morphology of the nanoparticles. The presence of defects in the ceria - such as dislocations and surface steps - was routinely observed in HAADF STEM. Density functional theory (DFT) calculations indicate an energetic preference for the formation of O vacancies and the corresponding Ce 3+ sites at the ceria-titania interface. Experimental corroboration by soft X-ray absorption spectroscopy (SXAS) does suggest the presence of Ce3+ sites at the interface. © 2013 American Chemical Society.The research carried out at the Center for Functional Nanomaterials and the Chemistry Department of Brookhaven National Laboratory was supported by the U.S. Department of Energy, Office of Basic Energy Sciences, under Contract No. DE-AC02-98CH10886. The theoretical studies were funded by the Ministry of Economy and Competitiveness (Spain, grants MAT2012-31526 and CSD2008-0023) and EU FEDER. Computational resources were provided by the Barcelona Supercomputing Center/Centro Nacional de Supercomputación (Spain).Peer Reviewe

Tungsten as an interface agent leading to highly active and stable copper-ceria water gas shift catalyst

A series of W–Cu–Ce mixed oxide catalysts prepared by microemulsion was evaluated in the water-gas shift (WGS) reaction. At low temperatures (<350 °C), the total conversion of CO on the W–Cu–Ce systems was two times larger than on binary Cu–Ce mixed oxides which are well known catalysts for the WGS. In addition and in contrast with Cu–Ce, W–Cu–Ce catalysts were stable and no signs of deactivation were found after 10 h of reaction time. The rationale for the excellent catalytic performance presented by the W–Cu–Ce ternary oxide was elucidated from the viewpoint of a complete structural (e.g. analysis of the long and short range order) and redox behavior characterization using in situ, time-resolved X-ray diffraction (XRD) as well as X-ray absorption (XAS), infrared (diffuse reflectance Fourier transform DRIFTS) and Raman spectroscopies. From a single phase fluorite-type structure, the catalysts show significant structure/redox evolution under reaction conditions as a function of the W and Cu content. As it occurs in the parent Cu–Ce system, the dominant presence of metallic Cu and fluorite-type oxide phases is detected under reaction conditions for the ternary systems. An outstanding promotion of catalytic properties is nevertheless evidenced for samples with W content above 10 at.% and is shown to be related to the presence of oxidized W–Cu local entities. Such local entities, which are obviously characteristic of the ternary system, greatly enhance fluorite redox properties and play an interfacial role between the main metallic Cu and fluorite-type oxide phases. As a consequence of all these effects, incorporation of W into the initial material leads to efficient WGS catalysts, most promising for their application in the so-called low temperature region, e.g. below 350 °C.A.K. would like to thank the Ramón y Cajal Project of MICINN (Spain) for a Postdoctoral Fellowship. P.M. wants to acknowledge EU (Project no. UDA-POKL.04.01.01-00-029/10-00) for financial support of a stay at Madrid (ICP-CSIC). The work performed at BNL was supported by the U.S. Department of Energy, Office of Basic Energy Sciences, Chemical Science Division (DE-AC02-98CH10886). The NSLS is supported by the Chemical and Materials Science Divisions of the U.S. Department of Energy. Financial support by Spanish “Plan Nacional” Projects CTQ2010-14872 and CTQ2009-14527 and by the Comunidad de Madrid(Project DIVERCEL, Ref.: S2009/ENE-1475) is acknowledged.Peer Reviewe

Surface reduction mechanism of cerium-gallium mixed oxides with enhanced redox properties

The doping of CeO2 with different types of cations has been recognized as a significant factor in controlling the oxygen vacancies and improving the oxygen mobility. Thus, the catalytic properties of these materials might be determined by modifying the redox properties of ceria. A combined experimental and theoretical study of the redox properties of gallium-doped cerium dioxide is presented. Infrared spectroscopy and time-resolved X-ray diffraction were used for temperature programmed reduction (H2) and oxidation (with O2 and H2O) studies. Additionally, X-ray absorption near edge spectroscopy shows that only Ce4+ is reduced to Ce3+ in the ceria–gallia mixed oxides when annealed up to 623 K. The oxygen storage capacity (OSC) measurements show a pronounced enhancement on the reduction of ceria by gallium doping. Theoretical calculations by density functional theory (DFT) confirm the higher reducibility of gallium-doped ceria oxides and give a molecular description of the stabilization of the doped material. On the basis of infrared spectroscopic measurements, a novel mechanism is proposed for the surface reduction of Ce4+ to Ce3+ where Ga–H species are suggested to be directly involved in the process. In addition, the reoxidation by H2O was precluded in the gallium-doped ceria oxide.This work has been financed by Eulanest 042, PME 2006 311, CAID 2009 J379, and MINCyT-ECOS A09E01. The work at BNL was financed by the US DOE, Office of BES (DE-AC02-98CH10086). M.C. and F.T. are grateful to HPC GENCICINES/IDRIS (grant 2011-x2011082131) and the CCRE-DSI of UniversitéP. M. Curie for computational resources.Peer Reviewe

Pulse Studies to Decipher the Role of Surface Morphology in CuO/CeO2 Nanocatalysts for the Water Gas Shift Reaction

The water-gas shift reaction (WGS, CO + H2O → H2 + CO2) was studied over CuO/CeO2 catalysts with two different ceria particle morphologies, in the form of nanospheres (ns) and nanocubes (nc). To understand the strong dependence of the WGS reaction activity on the ceria nanoshapes, pulses of CO (without and with water vapor) were employed during in situ X-ray diffraction and X-ray absorption near edge structure measurements done to characterize the catalysts. The results showed that CuO/CeO2 (ns) exhibited a substantially better activity than CuO/CeO2 (nc). The higher activity was associated with the unique properties of CuO/CeO2 (ns), such as the easier reduction of highly dispersed CuO to metallic Cu, the stability of metallic Cu and a larger concentration of Ce3+ in CeO2 (ns).The work performed at Brookhaven National Laboratory, was supported by the U.S. Department of Energy, Office of Science, Office of Basic Energy Sciences, and Catalysis Science Program under contract DE-AC02-98CH10886 contract. This work used resources of the National Synchrotron Light Source, which is a DOE Office of Science User Facility. The financial support from the National Natural Science Foundation of China (Grant 21303272) is gratefully acknowledged. Financial support from MINECO (CTQ2012-32928 project) and EU COST CM1104 action is also acknowledged.Peer Reviewe

Superior performance of Ni-W-Ce mixed-metal oxide catalysts for ethanol steam reforming: Synergistic effects of W- and Ni-dopants

The ethanol steam reforming (ESR) reaction was studied over a series of Ni-W-Ce oxide catalysts. The structures of the catalysts were characterized using in situ techniques including X-ray diffraction, pair distribution function, X-ray absorption fine structure, and transmission electron microscopy; while possible surface intermediates for the ESR reaction were investigated by diffuse reflectance infrared Fourier transform spectroscopy. In these materials, all the W and part of the Ni were incorporated into the CeO2 lattice, with the remaining Ni forming highly dispersed nano-NiO (<2 nm) outside the Ni-W-Ce oxide structure. The nano-NiO was reduced to Ni under ESR conditions. The Ni-W-Ce system exhibited a much larger lattice strain than those seen for Ni-Ce and W-Ce. Synergistic effects between Ni and W inside ceria produced a substantial amount of defects and O vacancies that led to high catalytic activity, selectivity, and stability (i.e., resistance to coke formation) during ethanol steam reforming.The research carried out at National Synchrotron Light Source, Brookhaven National Laboratory, was supported by the U.S. Department of Energy, Office of Science, Office of Basic Energy Sciences (DE-AC02-98CH10886 contract). STEEM-EELS data were obtained at the Center for Functional Nanomaterials, supported by the U.S. Department of Energy, Office of Basic Energy Sciences under Contract No. DE-AC02-98CH10886. The financial support from the National Natural Science Foundation of China (Grant 21303272) and China Scholarship Council (File No. 201208420304) is gratefully acknowledged. Anna Kubacka thanks Spanish MINECO for a “Ramón y Cajal” postdoctoral fellowship.Peer Reviewe

Water-gas shift reaction on Ni-W-Ce catalysts: Catalytic activity and structural characterization

The water-gas shift reaction (WGS, CO + H2O → H2 + CO2) was studied over a series of W-Ce, Ni-Ce, and Ni-W-Ce mixed-metal oxide catalysts. The structure of the catalysts and the WGS reaction intermediates were characterized using in situ techniques including X-ray diffraction (XRD), X-ray absorption near edge structure (XANES), scanning transmission electron microscopy (STEM), and diffuse reflectance infrared Fourier-transform spectroscopy (DRIFTS). XANES showed the existence of Ni 2+ and W6+ inside the ceria lattices. The coexistence of Ni and W inside of ceria led to a large lattice strain, not seen for Ni-Ce and W-Ce, that facilitated the reduction of Ni-W-Ce and gave this oxide special catalytic properties. A Ni0.2W0.1Ce0.7O 2 catalyst displayed the highest catalytic activity among all the mixed oxides, followed by a Ni0.2W0.2Ce 0.6O2 catalyst. Besides high activity, the Ni-W-Ce catalysts also displayed the effective suppression of the methanation reaction (CO + 3H2 → CH4 + H2O) under WGS conditions compared to W-free Ni-Ce catalysts. The introduction of W in the lattice of Ni-Ce favored the formation of O vacancies that facilitated the dissociation of water, preventing the dissociation of CO and the formation of methane. Because of the special chemical properties of Ni-W-Ce, monodentate formates and carbonates, which could be chemically active species for the WGS reaction, appear on the surface of these catalysts. Synergistic interactions between Ni and W give Ni-W-Ce unique structural and chemical properties not seen for W-Ce or Ni-Ce mixed-metal oxides.The research carried out at National Synchrotron Light Source, Brookhaven National Laboratory, was supported by the U.S. Department of Energy, Office of Science, Office of Basic Energy Sciences (DE-AC02-98CH10886 contract). STEEM-EELS data were obtained at the Center for Functional Nanomaterials, supported by the U.S. Department of Energy, Office of Basic Energy Sciences, under Contract DE-AC02-98CH10886. The financial support from the National Natural Science Foundation of China (Grant 21303272) and China Scholarship Council (File No. 201208420304) is gratefully acknowledged. Anna Kubacka thanks Spanish MINECO for a “Ramon y Cajal ́ ” postdoctoral fellowship.Peer Reviewe

Pulsed-reactant in situ studies of ceria/CuO catalysts using simultaneous XRD, PDF and DRIFTS measurements

The transient reduction–oxidation (redox) response of a ceria/CuO catalyst has been studied with pulsed gas techniques. The experimental setup allowed for simultaneous measurement of X-ray diffraction (XRD) and diffuse reflectance infrared Fourier transform spectroscopy (DRIFTS) coupled to mass spectrometric (MS) analysis of reactants and products. The DRIFTS showed the bonding of CO to CuxO and formation of carbonates on ceria. The CO stretch signal was observed in the DRIFTS as the Cu2O phase appeared in the simultaneous XRD pattern. This signal was still observed when XRD indicated that the Cu2O phase had converted to Cu(0). This unexpected result could arise from an amorphous surface of Cu2O which could not be detected by XRD or because the surface sensitivity of the DRIFTS technique is much higher than the surface sensitivity of the XRD technique. Time-resolved pair distribution function (PDF) analysis was useful for tracking the appearance or disappearance of key structural features (CuO, CeO, CeCe, and CuCu) during the pulse experiments. The in situ PDF analysis provided new evidence that oxygen vacancies in the ceria could be detected with XRD data.Financial support from Ministerio de Economía y Competitividad (project CTQ2012-32928 and EU COST CMT104) is acknowledged.Peer Reviewe

Hierarchical heterogeneity at the CeOx-TiO2 interface: Electronic and geometric structural influence on the photocatalytic activity of oxide on oxide nanostructures

Mixed oxide interfaces are critical for delivering active components of demanding catalytic processes such as the photocatalytic splitting of water. We have studied CeOx–TiO2 catalysts with low ceria loadings of 1, 3, and 6 wt % that were prepared with wet impregnation methods to favor a strong interaction between CeOx and TiO2. In these materials the interfaces between CeOx–TiO2 have been sequentially loaded (1%, 3%, and 6%), with and without Pt (0.5 wt %). The structure and properties of the catalysts were characterized using several X-ray and electron based techniques including XRD, XPS, UPS, NEXAFS, UV–vis, and HR-STEM/STEM-EELS to unravel the local morphology, bulk structure, surface states, and electronic structure. The combination of all these techniques allows us to analyze in a systematic way the complete structural and electronic properties that prevail at the CeOx–TiO2 interface. Fluorite structured nanocrystallites of ceria on anatase-structured titania were identified by both XRD and NEXAFS. A sequential increase of the CeOx loading led to the formation of clusters, then plates, and finally nanoparticles in a hierarchical manner on the TiO2 support. The electronic structures of these catalysts indicate that the interaction between TiO2 and CeO2 is closely related to the local morphology of nanostructured CeO2. Ce3+ cations were detected at the surface of CeO2 and at the interface of the two oxides. In addition, the titania is perturbed by the interaction with ceria and also with Pt. The photocatalytic activity for the splitting of H2O using UV light was measured for these materials and correlated with our understanding of the electronic and structural properties. Optimal catalytic performance and photoresponse results were found for the 1 wt % CeOx–TiO2 catalyst where low dimensional geometry of the ceria provided ideal electronic and geometrical properties. The structural and electronic properties of the interface were critical for the photocatalytic performance of this mixed-oxide nanocatalyst system.The research carried out in this manuscript was performed at Brookhaven National Laboratory, supported by the U.S. Department of Energy, Office of Science, Office of Basic Energy Sciences, and Catalysis Science Program under Contract DE-SC0012704. L.B. also acknowledges financial support from the JAE-CSIC grant programPeer Reviewe

Unraveling the dynamic nature of a CuO/CeO2 catalyst for CO oxidation in Operando: A combined study of XANES (fluorescence) and drifts

The redox chemistry and CO oxidation (2CO + O2 → 2CO2) activity of catalysts generated by the dispersion of CuO on CeO2 nanorods were investigated using a multitechnique approach. Combined measurements of time-resolved X-ray absorption near-edge spectroscopy (XANES) and diffuse reflectance infrared Fourier transform spectroscopy (DRIFTS) in one setup were made possible with the development of a novel reaction cell in which fluorescence mode detection was applied to collect the XANES spectra. This is the first reported example using XANES in a similar technique combination. With the assistance of parallel time-resolved X-ray diffraction (XRD) measurements under operando conditions, we successfully probed the redox behavior of CuO/CeO2 under CO reduction, constant-flow (steady-state) CO oxidation and during CO/O2 cycling reactions. A strong copper ↔ ceria synergistic effect was observed in the CuO/CeO2 catalyst. Surface Cu(I) species were found to exhibit a strong correlation with the catalyst activity for the CO oxidation reaction. By analysis of phase transformations as well as changes in oxidation state during the nonsteady states in the CO/O2 cycling reaction, we collected information on the relative transformation rates of key species. Elementary steps in the mechanism for the CO oxidation reaction are proposed based on the understandings gained from the XANES/DRIFTS operando studies.The research carried out at the Chemistry Department, the National Synchrotron Light Source (NSLS), and the Center for Functional Nanomaterials (CFN), at Brookhaven National Laboratory (BNL) was supported by the Division of Chemical Sciences, Geosciences, and Biosciences, Office of Basic Energy Sciences of the U.S. Department of Energy (Contract No. DEAC02-98CH10886). Use of the Advanced Photon Source, an Office of Science User Facility operated for the U.S. Department of Energy (DOE) Office of Science by Argonne National Laboratory, was supported by the U.S. DOE under Contract No. DE-AC02-06CH11357. The financial support from China Scholarship Council (File No. 201206010107) is gratefully acknowledged. Financial support from MINECO (Plan Nacional Project No. CTQ2012-32928) and EU COST CM1104 action is also acknowledged. Thanks are also due to ICP-CSIC Unidad de Apoyo for SBET measurement.Peer Reviewe