Dispersion and reduction of copper oxide supported on WO3-modified Ce0.5Zr0.5O2 solid solution
ABSTRACT XRD (X-ray diffraction), BET (Brunauer-Emmett-Teller), LRS (laser Raman spectra), XPS (X-ray photoelectron spectroscopy), and TPR (temperature-programmed reduction) are used to investigate the surface properties of CuO/WO3/Ce(0.5)Zr(0.5)O2 samples. The results indicate that (1) tungsten oxide can be highly dispersed on Ce(0.5)Zr(0.5)O2 (denoted as CZ hereafter) solid solution, with a dispersion capacity of about 0.8 mmol WO(3)/(100 m2 CZ), and comparatively, the supported tungsten oxide species are preferentially interacted with ceria component on the surface of CZ; (2) for CuO/WO3/CZ samples with a half-monolayer WO3 loading, i.e., xCu-0.4W-CZ series, the surface of CZ is only partially covered by the preloaded WO3) and the supported copper oxide species are dispersed on the remaining surface vacant sites on CZ as well as on top of the preloaded tungsten oxide, while for the samples preloaded with a full-monolayer WO3, i.e., xCu-0.8W-CZ series, only dispersed on the top of the preloaded tungsten oxide monolayer; (3) the effect of the loading amount of WO3 on the reduction property of Cu2+ ions in a series of CuO/WO3/CZ samples has been observed and tentatively attributed to the formation of WO3 monolayer on CZ and the different coordination environments of the dispersed Cu2+ ions are discussed on the basis of the consideration of the incorporation model proposed previously (Chen, Y.; Zhang, L. Catal. Lett. 1992, 12, 51).
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ABSTRACT: WO3/CexZr1–xO2 materials were evaluated as a possible NH3-selective catalytic reduction (SCR) active catalyst in a NOx storage reduction (NSR) + SCR combined system. The effect of the support composition was investigated at a constant WO3 loading (9.1 wt % of WO3). The impact of WO3 promotion over textural, structural, acid–base, and redox properties of SCR samples was characterized by means of nitrogen adsorption–desorption isotherms, XRD, NOx storage capacity, NH3 temperature programmed desporption, pyridine adsorption followed by FTIR, and H2-TPR. Catalytic activities in NH3-SCR and NH3-SCO reactions as well as corresponding kinetics parameters are also discussed. All WO3/Ce–Zr materials are active and fully selective in N2 for NOx reduction by NH3 and ammonia oxidation by O2. For the SCR reaction, the rate of NO conversion is found approximately half-order with respect to NO, and negative to nearly zero-order with respect to NH3. Tungstated ceria–zirconia materials were then associated downstream with a model Pt–Ba/Al NSR catalyst. Whatever the WO3–Ce/Zr catalyst, the global NOx conversion and N2 yield are significantly enhanced by the addition of the SCR catalyst. Special attention is paid to the influence of the ceria content of the SCR catalyst on the ammonia reactivity in the combined NSR + SCR system. It is demonstrated that NH3 produced during the regeneration step of Pt–Ba/Al catalyst can react either with NOx (NH3-SCR) or with O2 (NH3-SCO). This reactivity depends both on the Ce/Zr ratio of support and on the temperature. Finally, it is demonstrated that at high temperature (i.e., 400 °C), the strength of the acid sites of the WO3–Ce/Zr catalysts is not sufficient to ensure no ammonia slip.ACS Catalysis 04/2013; 3(6):1120–1132. DOI:10.1021/cs3008329 · 7.57 Impact Factor
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ABSTRACT: A series of M/TiO2 and M/TiO2-SiO2 (with M = Mn, Cu and Ce) catalysts were prepared by adopting a wet-impregnation method and investigated for the selective catalytic reduction (SCR) of NOx in the temperature range of 100-500 oC with excess (10 vol.%) oxygen in the feed at industrially relevant conditions. Our XRD results suggest that the growth of crystalline TiO2 phase is strongly inhibited due to the SiO2 migration into the TiO2 lattice. The increase of SiO2 molar content in TiO2-SiO2 support led to the decrease in anatase phase of titania peak intensity of XRD spectrum and also exhibiting a lower crystallinity of TiO2 with no phase transition of anatase to rutile. Our XPS depth profile analysis illustrate that the surface atomic ratio of Cu1+/ Cu2+ greatly enhanced with increase in TiO2 content in the TiO2-SiO2 support, and these results are consistent with the H2-TPR results where the additional reduction peak evolved at 200 oC for copper loaded titania-rich (Cu/TiO2) catalyst. The high activity of Cu-based TiO2 formulations has been assigned to the enhancement in the formation of Cu1+ active sites, existence of surface Cu2+, Cu1+ species and the increment of reduction potentials of surface copper species. The Ce3+/ Ce4+ and Ce3+/Cen+ atomic ratio (1.14 and 0.53, respectively) in Ce/TiO2 catalyst calculated from deconvoluted XPS spectra are much higher than that of Ce/TiO2-SiO2 (1:1) and Ce/TiO2-SiO2 (3:1). The existence of higher Ce3+ surface species over CeO2/TiO2 illustrates the increment of surface oxygen vacancies and thus facilitates for the adsorption of oxygen species or activates reactants in the SCR reaction. The relative atomic percentage value of Mn4+/Mn3+ characterized by deconvoluted XPS was significantly high (Mn4+/Mn3+ = 1.98) for the Mn/TiO2 compared to Mn/TiO2-SiO2 catalysts (Mn4+/Mn3+ = 1.23, 1.75). When ceria was supported on pure TiO2, the low-temperature reduction peak was broad and less defined, and the reducibility in the low temperature range was much less pronounced. On the other hand, the addition of ceria to titania with strong reciprocal interaction is generally perceived as a shift in the bulk reduction temperature to lower values, to about 500-650 °C. As bigger Ce4+ ions enter the lattice structure to proxy the Ti4+ ions with smaller ionic radii (2.48 and 2.15 Å, respectively), the lattice could become highly strained. The NOx conversions and the apparent kinetic constant of the catalyst kac, over the Cu, Mn, Ce-loaded on different supports TiO2 and TiO2-SiO2 (3:1 and 1:1) catalysts measured under steady-state conditions results demonstrated higher activity of the Ti-rich materials.Industrial & Engineering Chemistry Research 02/2015; DOI:10.1021/ie504709j · 2.24 Impact Factor
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ABSTRACT: Noble metals-promoted tungstated oxides have been shown to be profitable in a wide variety of catalytic reactions of environmental interest but to be detrimental in the hydrogenation of aromatics. The origin of the deleterious effect of tungstates on the hydrogenation performance of noble metals is still being debated. To provide further insights into this, the location and the oxidation state of Rh were investigated as a function of the W surface density (0–10 W/nm2) of Rh/WOx–Ce0.62Zr0.38O2 (Rh/W–CZ) catalysts after high-temperature reduction. For that purpose, a thorough characterization of the oxide phases was performed through N2-sorption, X-ray diffraction, Raman spectroscopy, NOx temperature-programmed desorption, and X-ray photoelectron spectroscopy (XPS), whereas the metallic phases were characterized by low-temperature H2 chemisorption, XPS, N2 Fourier transform infrared spectroscopy and benzene hydrogenation. It was found that Rh deposited on both tungstates and CZ, and did not sinter with increasing W surface densities. The observed linear decrease in the Rh hydrogenation performance of the WOx-promoted Ce0.62Zr0.38O2 below pseudo monolayer coverage of CZ (4.8 W/nm2CZ) was assigned to a strong metal support interaction effect between the Rh particles and the nonreducible underlying WOx phase, resulting in the formation of electron-deficient Rh species (Rhδ+).The Journal of Physical Chemistry C 03/2014; 118(14):7386–7397. DOI:10.1021/jp410848f · 4.84 Impact Factor