Characterisation of ceria supported chromia catalysts

IR spectroscopy in catalysis

Abstract: Infrared (IR) spectroscopy undoubtedly represents one of the most important tools in catalysis research. In this review, recent catalytic applications of the most popular IR techniques will be presented. Each section starts from the very general basis of the spectroscopic method applied. The last section is devoted to the adsorption of chelating compounds on surfaces of mineral oxides. The aim of adding a large number of illustrations and an appendix is to make the presented material more familiar for young researchers and postgraduate students. Most of the papers discussed have appeared in the last 5–6 years, because the older literature has been reviewed in earlier papers.

Application of Spinel-Type Cobalt Chromite as a Novel Catalyst for Combustion of Chlorinated Organic Pollutants

Abstract: Various chromium-containing catalysts were tested for the total oxidation of trichloroethylene (TCE) as a model reaction for the catalytic combustion of chlorinated organic pollutants. A spinel-type cobalt chromite (CoCr2O4) among others was proven to be a very promising catalyst showing higher activity and higher CO2 selectivity than traditional alumina supported chromia. Even if both Cr3+ and Cr6+ species were observed on the surface of CoCr2O4, the Cr6+ species was stable under reducing environment. The presence of Cr3+-Cr6+ pair sites and the effect of redox treatments on the activity were investigated to explain the nature of possible active sites for TCE decomposition. Higher selectivity to CO2 of CoCr2O4 was ascribed to the abundance of its Cr3+ species, together with its activity for water gas shift reaction.

Soot oxidation via CuO doped CeO2 catalysts prepared using coprecipitation and citrate acid complex-combustion synthesis

Abstract: This work deals with soot oxidation under SO2 presence over CuO doped CeO2 prepared using two routes, coprecipitation (CP) and citrate acid complex-combustion synthesis (CA). TPO, XRD, BET, H2-TPR, O2-TPD, FTIR and XPS are utilized to correlate the performance and physicochemical features of catalysts. The results show that solid solutions are formed due to Cu2+ cations entering into the ceria lattice. This reorganization increases the amount of oxygen vacancies and then creates more surface active species (Ce4+/Ce3+ and O*), and increases the reducibility and oxygen storage capacity of catalysts, thus promoting catalytic activity for soot oxidation. Both for Cu0.05Ce0.95-CA and Cu0.05Ce0.95-CP after TPO in the presence of various concentrations of SO2, more Ce4+/Ce3+ redox couples, and more active oxygen species, detected by XPS technique and FTIR, respond to the better activity. When compared to Cu0.05Ce0.95-CP, the better reducibility and greater OSC, and active oxygen species of Cu0.05Ce0.95-CA coincide with its better activity. A possible pathway of soot oxidation over Cu0.05Ce0.95-CA is proposed, based on the relationship of the performance and the physicochemical features of catalysts, which pathway follows the mechanism associated with redox couple and oxygen spill over effect.

Insights into synergistic effect of chromium oxides and ceria supported on Ti-PILC for NO oxidation and their surface species study

Abstract: The insights of synergistic effect between chromium oxides and ceria supported on Ti-PILC were studied for NO oxidation. The aim of this study was to investigate the role of chromium oxides and ceria and their synergistic effect in textural properties, redox performance and surface species over the Cr1−xCex/TP catalysts. These catalysts were investigated in detail by means of Brunauer–Emmertt–Teller (BET) surface area analysis, X-ray diffraction (XRD), transmission electron microscope (TEM), temperature-programmed reduction of H2 (H2-TPR), temperature-programmed desorption (NO-TPD, O2-TPD), photoluminescence spectra (PL), X-ray photoelectron spectroscopy (XPS) and in situ diffuse reflectance infrared Fourier transform spectroscopy (in situ DRIFTS). It has been found that CrOx were beneficial to adsorb and activate NO to form NO+ and then generate nitrates while ceria were inclined to activate O2 via oxygen vacancies to produce nitrates. Besides, the results of in situ DRIFTS further demonstrated that surface species were associated with not only reaction atmosphere but also reaction temperature. Hence, a possible reaction model was tentatively proposed.

Nanosized iron and chromium oxides supported on mesoporous CeO2 and SBA-15 silica: Physicochemical and catalytic study

Abstract: Mesoporous ceria and SBA-15 silica were modified with iron and chromium oxide nanoparticles and characterized by XRD, N2-physisorption, FTIR, UV–vis, Moessbauer spectroscopy and TPR–TG in hydrogen. Their catalytic behaviour in methanol decomposition to CO and hydrogen was also studied. Stabilization of mono- and bi-chromate species, FeOx patches or isolated iron ions as well as Fe2O3 and Cr2O3 nanoparticles in different ratio depending on the nature of the porous matrix was observed. The simultaneous presence of iron and chromium oxides lead to change in their dispersion, providing easier reducibility, higher catalytic activity and stability of the obtained materials in comparison with the corresponding mono-component ones. The “intimate contact” at the interface of both loaded metal oxide nanoparticles and the support was discussed with respect to explain the differences in the state of the active ingredient and its specific catalytic behaviour.

The utilization of ceria in industrial catalysis

Abstract: The main applications of cerium dioxide in industrial catalysis are reviewed, with particular attention to the role played by ceria. The main uses of CeO2 are connected to depollution of noxious compounds from gaseous streams originating from industrial productions and from automobiles (de-SOx in FCC processes; treatment of emissions from spark-ignited and diesel engines), although ceria is also a key component of catalyst formulation for the dehydrogenation of ethylbenzene to styrene. Recent advances in the application of ceria for the removal of organic compound from wastewater through oxidation (catalytic wet oxidation CWO) are also reviewed. # 1999 Elsevier Science B.V. All rights reserved.

Surface Chemistry and Spectroscopy of Chromium in Inorganic Oxides.

Abstract: Focuses on the surface chemistry and spectroscopy of chromium in inorganic oxides. Characterization of the molecular structures of chromium; Mechanics of hydrogenation-dehydrogenation reactions; Mobility and reactivity on oxidic surfaces.

In Situ Raman Spectroscopy of Alumina-Supported Metal Oxide Catalysts

Abstract: The molecular structures of the surface overlayerers of rhenium(VII) oxide, molybdenum(VI) oxide, tungsten(VI) oxide, chromium(VI) oxide, vanadium(V) oxide, niobium(V) oxide, and titanium(IV) oxide on γ-alumina were determined by in situ Raman spectroscopy under dehydrated conditions. It was found that the dehydrated surface metal oxide structures of all the systems under study, except for supported titanium oxide, are different from those under ambient condition where moisture is present on the surface

Vanadium oxide monolayer catalysts. 3. A Raman spectroscopic and temperature-programmed reduction study of monolayer and crystal-type vanadia on various supports

Abstract: Vanadium(V) oxide supported on 7-A1203, GO2, Cr2O3, Si02, Ti02, and Zr02 was studied by X-ray fluorescence, by X-ray diffraction, and especially by the combination of Raman spectroscopy and temperature-programmed reduction (TPR) for qualitative and quantitative structural analysis, respectively. Catalysts were prepared via ion-exchange and wet-impregnation methods. The V contents ranged from - 1 to 40 wt % V. At low surface concentrations only surface vanadate phases of two-dimensional character are observed for all carriers. According to Raman and TPR data the structure of these surface vanadate species is independent of the preparation technique. At medium and high surface concentrations, the webimpregnated samples already contain crystalline V20b At equal surface concentrations the ion-exchanged catalysts contain no V205 crystallites. An exception is Si02 on which also crystalline V206 is formed in both preparation techniques. Monolayer stability toward thermal treatment decreases in the order AZO>3 Ti02> Ce02,w hereas on heating ion-exchanged V/Si02 the crystalline V206 spreads out over the silica surface. The reducibilities of the ion-exchanged catalyats, as measured by TPR, can be used as a measure for the contact interaction between vanadia and the carrier oxides. At temperatures of 500-800 K, this interaction ranges from strong with titania to weak with silica as a carrier.

Predicting molecular structures of surface metal oxide species on oxide supports under ambient conditions

Abstract: The molecular structures of the two-dimensional vanadium oxide overlayers on different oxide supports (MgO, Al 2 O 3 , ZrO 2 , TiO 2 and SiO 2 ) were determined with Raman spectroscopy under ambient conditions. The surface vanadium oxide molecular structures were found to depend on the net pH at which the surface possesses zero surface charge (point of zero charge, pzc). The net surface pH at pzc is determined by the specific oxide support and the surface coverage of the acidic vanadium oxide overlayer