21368
J. Phys. Chem. C 2009, 113, 21368–21375
Structure and State of Copper Oxide Species Supported on Yttria-Stabilized Zirconia Vera P. Pakharukova,*,†,‡ Ella M. Moroz,† Vladimir V. Kriventsov,† Tatyana V. Larina,† Andrey I. Boronin,†,‡ Lidiya Yu. Dolgikh,§ and Peter E. Strizhak§ BoreskoV Institute of Catalysis, SB RAS, Pr. LaVrentieVa 5, 630090 NoVosibirsk, Russia; NoVosibirsk State UniVersity, PirogoVa Street 2, 630090 NoVosibirsk, Russia; and PisarzheVskii Institute of Physical Chemistry of the NAS, Pr. Nauki 31, 03039 KieV, Ukraine ReceiVed: August 8, 2009; ReVised Manuscript ReceiVed: October 27, 2009
The structure and state of the active component-copper species in zirconia-supported copper catalysts with yttria-stabilized zirconia as a support were investigated. Catalysts with different copper contents (2-10 wt % of Cu) prepared by an impregnation technique were characterized by X-ray diffraction (XRD) analysis, radial distribution function (RDF) of electronic density method, X-ray absorption fine structure (XAFS) analysis, UV-visible diffuse reflectance spectroscopy (UV-vis DRS), and X-ray photoelectron spectroscopy (XPS). It was established that a small amount of copper could be incorporated in the zirconia fluorite lattice, while the main portion of the copper was situated on the support surface as finely dispersed copper oxide chain clusters. Introduction Zirconia-supported copper (Cu/ZrO2) catalysts have attracted considerable attention due to their promising catalytic activity in a number of reactions: methanol synthesis from hydrogen and carbon oxides,1,2 NO reduction by CO,3,4 CO oxidation,5,6 steam reforming of methanol,7,8 and water-gas-shift reaction.9 Zirconia used as a support exhibits three different polymorphs: stable monoclinic (m-ZrO2), metastable tetragonal (t-ZrO2), and cubic (c-ZrO2). It is expected that Cu/ZrO2 systems with enhanced catalytic activity may be optimized by variation of the structural modifications of ZrO2 among cubic, tetragonal, and monoclinic forms. Thus, it has been found that the activity of Cu/ZrO2 catalysts depends on the phase structure of ZrO2. Jung and Bell reported that Cu/ZrO2 catalysts based on tetragonal and monoclinic zirconia with nearly identical surface area exhibited different activity for methanol synthesis from both CO and CO2 hydrogenation: the catalyst with m-ZrO2 as the support was 4.5-7.5 times more active.2 At the same time, the Cu/ZrO2 catalyst with the tetragonal zirconia showed better CO oxidation activity than the catalyst with the monoclinic zirconia as it follows from the work.5 However, understanding of the effect of zirconia polymorphs on the state and structure of supported copper species is still lacking. More articles are devoted to investigation of the copper state on the tetragonal and cubic zirconia. Ma et al. characterized Cu/ZrO2 catalysts with amorphous, monoclinic, and tetragonal zirconia prepared by the impregnation method and found that despite the lowest surface area, the tetragonal zirconia supported copper catalyst showed the highest copper dispersion.10 It was shown by several authors that in the Cu/ZrO2 catalysts prepared by coprecipitation technique the cubic or tetragonal zirconia were the major phase, while the CuO phase was not detected by XRD analysis in catalysts with copper loading up to 20 at.%.9,11,12 The absence of CuO reflections in the catalysts * To whom correspondence should be addressed. Phone: +7 (383) 326 95 32. Fax: +7 (383) 330 80 56. E-mail:
[email protected]. † Boreskov Institute of Catalysis. ‡ Novosibirsk State University. § Pisarzhevskii Institute of Physical Chemistry of the NAS.
diffraction patterns has been attributed to incorporation of copper ions into zirconia with substitutional solid solution formation or a fine dispersion of copper species on the zirconia surface. Kundakovich and Flytzani-Stephanopoulos studied the copper species in catalysts prepared by coprecipitation technique with yttria-stabilized zirconia as the support. Temperature programmed reduction by H2, XPS, and scanning transmission electron microscopy/energy dispersive X-ray analysis were used.11 It was shown that at low copper loading (
