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A series of samples where platinum is deposited onto titania P25 using a chemical deposition method was tested in the photo-production of hydrogen using methanol as a sacrificial agent. The optimization of the platinum titania contact was analyzed as a function of the synthesis conditions. The catalysts’ structural and electronic properties were measured using a combination of porosimetry, X-ray diffraction, X-ray photoelectron, photoluminescence, and UV–visible spectroscopies, together with a microscopy study of the noble metal particle size distribution and an in situ infrared study of the catalysts under reaction conditions. The measurement of the optical properties of the catalyst suspensions and the subsequent calculation of the true quantum efficiency were used to show that a significant enhancement of ca. 250% can be achieved through the series of Pt-containing catalysts. The study provides solid evidence concerning the physical roots of the photoactivity behavior. For catalysts having equal Pt properties in terms of primary particle size and electron handling capability, a synergy between the noble metal and the oxide support at interface sites is shown to promote the conversion of carboxylate-type surface species into carbon dioxide and maximize the hydrogen yield of the reaction.

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 (, 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.

A series of WO3–TiO2 composite catalysts with variable quantities of tungsten was prepared by a single-pot microemulsion procedure and evaluated in the gas-phase selective photo-oxidation of the aromatic hydrocarbon styrene. Samples improve the performance of TiO2 by a factor ca. 3 and increase significantly the selectivity to valuable products (as styrene oxide) upon UV and, more importantly, sunlight-type excitation. Optimum performance in terms of both activity and selectivity was achieved under a sunlight-type renewable energy source. A complete structural (bulk and surface) and electronic characterization using Transmission Microscopy, X-ray diffraction (XRD), X-ray Photoelectron (XPS), Raman, and UV–visible spectroscopies and calorimetry with the help of the ammonia probe molecule was carried out. The study was able to detect and quantify the presence of different W entities as a function of the tungsten content of the catalysts. Three species were found to be present throughout the series; oligomeric W species and WO3 nanoparticles in close contact with anatase-TiO2 and non-contacting WO3 platelets. From a spectro-kinetic analysis carried out with the help of electron paramagnetic resonance (EPR), we provide evidence that these W-containing species play different roles in reaction. Activity and selectivity improvements with respect to the bare titania are intimately related with presence of oligomeric W species at anatase surfaces., A. Kubacka and M.J. Muñoz-Batista thank MINECO for support through, respectively, the postdoctoral, Ramón y Cajal” and predoctoral FPI programs. R.R. 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). Financial support by MINECO is also acknowledged (project CTQ2010-14872/BQU).

Trabajo presentado en el Fourth International Conference on Semiconductor Photchemistry, celebrado en Praga (República Checa) del 23 al 27 de junio de 2013., Heterogeneous photocatalysis is considered as a promising route to afford pollutant degradation as well as hydrogen production [1]. Nowadays, the research activity in this field is focused in the development of novel altemative materials capable to use of sunlight as the green energy source. Taking in mind this purpose, the applications of the up-conversion process by phosphor-like systems (e.g., NIR or visible to UV) would optimize the photocatalytic performance of traditiona} UV active photocatalysts, such as Ti02 [2]. In the present work, erbium-doped Ti02 materials were synthesizcd by means of a surfactant­ free hydrothermal method [3]. From the XRD results was observed that when the Er3+ doping increases, a and b lattice parameters of the tetragonal cell progressively increase. Taking into account the ionic radius value of Er3+ and Ti4+, it could be assumed that Er3+ could either replace sorne Ti4+ sites or occupy the interstitial site of anatase structure. From TEM images was observed a homogeneous distribution of roundish nanoparticles (-15 nm). The photocatalytic activity was tested by means of the liquid-phase degradation of phenol, methylene blue dye and for the gas phase of toluene. In order to distinguish the contribution of each region of the lamp spectrum, different experiments were carried out using UV and NIR filters. The best reaction rate was attained for sample doped with 2 at% of Er3+. From the combined use of photocatalytic and luminescence experiments, we suggest a dual-type mechanism which could explain the improved photoactivities in doped systems.

The behaviour of Pd/Al 2 O 3 supported catalysts with metal loading in the 0.5–4 wt.% interval has been examined during cycling CO/NO conditions in an attempt to interpret the size-dependence of the metal chemical activity within a dynamic redox situation. To this end, a synchronous, multitechnique approach using X-ray absorption (XAS) or hard X-ray diffraction (HXRD) in tandem with diffuse reflectance infrared spectroscopy (DRIFTS) and mass spectrometry (MS) was used to establish the size-dependence of chemically significant steps in formation of N 2 and CO 2 . During cycling, EXAFS and HXRD reveal the existence of a reversible Pd morphology and phase change (PdC x formation and removal) phenomena in response to the gas atmosphere reducing/oxidizing nature. This is observed for Pd particles having dispersion values going from ca. 1–0.4. Using EXAFS Pd–Pd coordination numbers, we dynamically normalized the DRIFTS and MS signals obtained during cycling conditions and find that metal size/phase-effects appear of importance in CO 2 formation but can be essentially neglected in N 2 formation. Such (size-dependent) metal behaviour appears far from expectations coming from studies using non-cycling gas mixtures. The chemical relevance of NO, CO activation, dissociation and coupling steps are discussed in relation to the product formation and their size-dependence or independence.

In this report we investigate the structure–activity relationship in Ti–M (M = V, Mo, Nb, W) mixed-metal oxides with anatase structure used for the photoelimination of toluene under sunlight-type excitation. These systems were prepared by a microemulsion method, and their physicochemical properties were characterized by a multitechnique approach using X-ray diffraction–Rietveld, photoelectron spectroscopy, and Raman and UV–visible spectroscopy. The preparation method allowed the incorporation of up to around 20 at% of Mo, Nb, and W, whereas a significantly inferior solubility limit, below 5 at% was observed for V. The presence of nonpunctual defects, intimately related to the existence of MOx clustering, produced electronic mid-gap states involved in charge recombination and appeared to be the most negative factor influencing photoactivity. The maximization of photoactivity occurred for Ti–M samples with the highest doing level, together with a minimum structural disturbance of the anatase-type structure. The physicochemical bases for the photoactivity behavior of the Ti–M samples as function of the M content and, in particular, the presence of partial (for a specific Ti–M series) or global maxima are discussed.

The photocatalytic effects of the platinization on sunlight-active nanosize Ti–W mixed oxide materials with anatase structure were investigated. Platinization was shown to promote the photocatalytic mineralization of toluene, leading to an enhanced reaction rate of ca. 2.3 with respect to the parent Ti–W mixed oxide. This reaction rate increase is significantly higher than that obtained using ultraviolet excitation for titania-based oxides in the degradation of toluene and seems intimately related to the presence of Pt O bonds located at the platinum–anatase interface. The chemical/physical bases of such behavior are discussed on the light of a structural/electronic characterization of the solids with the help of X-ray diffraction and Raman/UV–visible spectroscopy, along with additional in situ experiments run under sunlight excitation using infrared and electron paramagnetic resonance spectroscopy.

Nanosized CeO2 and CuO/CeO2 samples, active for CO-PROX or related processes were comparatively examined by O2 probe electron paramagnetic resonance and in situ Raman and X-ray diffraction techniques. Their behavior toward CO reduction, as well as the oxygen-handling properties of the CO-reduced samples, was explored. An appreciable reduction of the ceria bulk was detected on treatment under CO at 473 K. On the basis of the analysis of the evolution of different oxygen-derived species (superoxide, peroxide, O−) on low-temperature (77–300 K) oxygen chemisorption on the CO-reduced samples, a general picture of the redox properties of the samples is presented. Results demonstrate that the presence of copper promotes completion of the redox cycle under CO/O2 by favoring both ceria reduction and oxidation. This can be relevant to explaining the remarkable oxidation activity and synergetic effects observed for catalysts combining CuO and CeO2.

Pd–Ni catalysts supported on Al2O3, (Ce,Zr)Ox/Al2O3, and (Ce,Zr)Ox are characterized at a nanoscopic level using mainly electron microscopy-related techniques (HREM images, XEDS, and Z-contrast images/EELS spectra done with TEM and STEM instruments). The presence of rather homogeneous Ce–Zr mixed oxide nanostructures is revealed from analysis of HREM pictures. Analysis of the Pd–Ni/(Ce,Zr)Ox/Al2O3 system shows the existence of preferential interactions of Pd and Ni with the (Ce,Zr)Ox and Al2O3 components, respectively, as evidenced mainly by XEDS and confirmed by FMR. Dispersion states of the metals and particle size distributions of palladium are inferred from analysis of Z-contrast images and EELS spectra. The results are complemented by XPS and XAFS results used to analyse the chemical state of the metallic components in the catalysts and their dispersion over the different supports.

Pd–Ni catalysts supported on Al 2 O 3 , (Ce,Zr)O x /Al 2 O 3 , and (Ce,Zr)O x were examined with the principle objective of determining the effects of Ni on catalytic activity for CO oxidation and NO reduction reactions under stoichiometric conditions. Catalytic activity findings for the CO + O 2 and CO + O 2 + NO reactions were analyzed in conjunction with in situ DRIFTS and XANES results to obtain information on the processes occurring in the catalysts during the course of the reactions. The results reveal a significant dependence on the nature of the support in terms of the catalytic changes produced by nickel. In the absence of significant nickel-induced electronic perturbations of palladium, these are related to indirect effects on palladium distribution over the catalysts or to a certain impediment of the interactions between active palladium and Ce–Zr mixed-oxide components. Significant promotion of CO oxidation was observed for the (Ce,Zr)O x /Al 2 O 3 -supported catalyst, which reveals a relevant role for the particle size of the nanostructured Ce–Zr mixed oxide in this reaction.

A combination of in situ Raman, XANES, and EPR spectroscopies is used to study the nature of the interaction between V and Ce in the ceria-supported vanadia catalysts. Vanadium oxide species disperse on ceria up to 9 V atoms/nm 2 of support. Surface V 5+ species closely interacts with ceria support promoting a reduction of surface Ce 4+ to Ce 3+ . Upon heating or during reaction surface vanadia reacts with ceria support forming a CeVO 4 phase. The active site appears to be V 5+ OCe 3+ for both systems. The redox cycle for oxidative dehydrogenation appears to be associated with Ce, rather than with V sites.

The behavior of Pd-based systems supported on alumina and ceria–zirconia and the complex ceria–zirconia on alumina was investigated during light-off tests using a stoichiometric C3H6 + CO + NO + O2 mixture. In situ XANES/DRIFTS studies of the Pd chemical state at the surface and bulk of the materials were conducted to determine the noble metal response to the gas atmosphere during light off. The initial, oxidized PdO-like phase was converted to Pd(0) in all cases except for the ceria–zirconia-supported catalyst. Contact between Pd and bulk ceria–zirconia particles appeared to stabilize Pd clusters in the 373–673 K region with an average Pd(I)-like oxidation state. The presence of hydrocarbon in the gas phase was essential in order to stabilize these Pd(I)-like species. The role of such a phase in providing a source of active oxygen for hydrocarbon oxidation is shown here to strongly decrease light off temperature and this, together with its stability over a wide range of temperatures, may offer a contribution to reducing hydrocarbon emissions during cold start.

The behavior of Pd three-way catalysts promoted with Ce–Zr mixed oxides and supported on alumina subjected to thermal degradation treatments has been examined during light-off tests under stoichiometric CO+NO+O 2 . Attention was paid to the region of aging temperature 1273–1373 K, where phase segregation of the ceria–zirconia promoter occurs. Characterization of the samples was performed using X-ray diffraction and Raman spectroscopy. In situ XANES/DRIFTS studies of the Pd chemical state at the surface and in the bulk of the materials were conducted to determine the noble metal response to the gas atmosphere during light-off. Electron paramagnetic resonance was used to analyze the promoter surface and to study the redox response of the materials in contact with the reactant gases. Thermal degradation appears to sinter both the noble metal and promoter components of the catalyst but also stabilizes oxidized Pd entities in contact with the promoter prior to the occurrence of the Ce–Zr phase segregation reaction. Additionally, it enriches the promoter surface in the lanthanide cation. The influence of these physicochemical phenomena on the catalytic properties of the Pd-based TWC is discussed.

A study of the lean NOx reduction activity with propene in the presence of water over Ag/Al2O3 catalysts with different silver loadings (1.5–6 wt%) has been done using X-ray diffraction, ultraviolet–visible spectroscopy, transmission electron microscopy, and in situ diffuse reflectance infrared and X-ray absorption spectroscopies under reaction conditions. The catalysts were prepared by an impregnation method employing EDTA complexes that allow highly dispersed silver phases to be obtained, which are stabilized under reaction conditions by strong interactions with the support. It is shown that the active species corresponds to silver aluminate-like phases with tetrahedral local symmetry. The role of silver in the reaction mechanism is shown to be mainly in the activation of NOx and propene species. In particular, the silver entities have been found to offer a new reaction path for propene activation which involves generation of acrylate species as a partially oxidized active intermediate. Differences between two active catalysts containing 1.5 and 4.5 wt% of Ag suggest that optimization of the SCR activity can be related to the oxygen lability of the tetrahedral silver aluminate-like phase present in the catalyst. As postulated previously, the high nonselective propene oxidation activity of the highest loaded sample (with 6 wt% Ag) appears to be related to formation of metallic silver surface states at low reaction temperatures which are active for NO dissociation.  2003 Elsevier Science (USA). All rights reserved.

A catalyst composed of copper oxide supported on cerium–zirconium mixed oxide (CuOx/CeZrO4) has been studied with respect to its activity for CO oxidation under stoichiometric conditions employing either oxygen or oxygen with a small amount of NO as oxidant. The nature of copper oxide entities, as well as the redox properties of the catalyst following interactions with CO and O2–NO, has been studied by XPS, EPR, and static NO adsorption infrared spectroscopy while in situ DRIFTS has been employed to follow processes occurring at the catalyst surface under reaction conditions. Characterization of the copper oxide species in both fully oxidized and partially reduced states reveals that they are significantly affected by interactions with the underlying support. On the basis of catalytic activity results and in combination with analysis of the evolution of particular Cu+ carbonyls, comparing the CO oxidation reaction in the presence and absence of NO, it is proposed that the two basic factors affecting the catalytic performance of this type of system are the facility for achieving a partially reduced state for the copper oxide phase at the interfacial zone and the redox properties of the CuOx/CeZrO4 interface.

A series of Pd–Cr bimetallic catalysts supported on a (Ce,Zr)Ox/Al2O3 mixed support or Al2O3 alone have been characterized using a combination of X-ray diffraction, electron paramagnetic resonance, and Raman spectroscopy and employing in situ diffuse reflectance infra-red Fourier transform and X-ray near-edge structure spectroscopy to analyze the redox and chemical processes taking place during the course of the CO+NO+O2 reaction. The catalytic behavior of these bimetallic systems was strongly affected by the nature of the support. In the case of the alumina support, evidence for interactions between Pd and Cr was observed in the calcined state. Under reaction conditions, formation of a mixed oxide phase containing Pd(I) and Cr(III) appears to be responsible for the improvement in CO oxidation and for the detrimental effect in the NO reduction process with respect to a monometallic, Pd reference system. In the case of the (Ce,Zr)Ox/Al2O3 mixed support, the addition of Cr was less influential in terms of catalytic activity. Results for the latter are rationalized mainly on the basis of the absence of interaction between the metal components in the calcined state and the presence of surface entities containing Cr alone when metallic-like Pd particles are formed during the course of reaction.

In this report we investigate the behavior of nanosize Ti–W mixed oxides with anatase structure in the photoelimination of toluene using sunlight-type excitation. These systems were prepared by a microemulsion method and their physicochemical properties characterized by a multitechnique approach using X-ray diffraction and photoelectron spectroscopy, as well as Raman, infrared and UV–vis spectroscopies. The preparation method allows the incorporation of up to ca. 20 at.% of W in substitutional positions of the anatase network. The photoactivity of the substitutionally disordered Ti–W mixed oxides grows in parallel with the W content of the material and improves the performance of a single anatase sample prepared in a similar way and also that of the Degussa P25 material. The chemical/physical basis of such behavior is discussed in light of the characterization results.

A series of Pd–Cu bimetallic catalysts supported on Ce–Zr mixed oxide or on (Ce,Zr)O x /Al 2 O 3 mixed supports (with loadings of 10 and 33 wt% Ce–Zr mixed oxide) have been examined and compared to reference monometallic catalysts to determine the influence of copper on the catalytic activity for CO oxidation and NO reduction using a mixture of either O 2 or O 2 –NO as oxidant under stoichiometric conditions. The samples were characterized by X-ray diffraction, transmission electron microscopy–X-ray energy-dispersive spectrometry, and electron paramagnetic resonance, employing in situ diffuse reflectance Fourier transform spectroscopy and X-ray absorption near-edge spectroscopy techniques to characterize physicochemical processes taking place during the course of the reactions. All the catalysts show Ce–Zr mixed oxide nanoparticles with pseudocubic phases and a Ce/Zr atomic ratio close to 1. The main differences between the samples are attributed to changes in the distributions of the two metallic components over the supports. The beneficial effects of copper on the catalytic activity of alumina-containing samples are related to the formation of an active Pd–Cu alloy in contact with the Ce–Zr mixed oxide component. A preferential interaction between copper and alumina in these samples is proposed to optimize the properties of the alloy phase by decreasing the copper concentration in the latter. However, the destruction of the alloy under reaction conditions at high temperatures, required for NO activation, removes the copper-promoting effects for NO reduction. The catalytic behavior of the Pd–Cu/(Ce,Zr)O x sample is governed by the Cu–(Ce,Zr)O x character of its active sites, which induces both beneficial and detrimental effects, on its catalytic properties, depending on the type of reaction.

The effect of thermal degradation on the catalytic performance in light-off tests of Pd three-way catalysts supported on alumina and promoted with Ce–Zr mixed oxides has been investigated either under synthetic gas mixtures (for catalysts in powder form) or in engine tests (for catalyst monoliths). Accelerated aging of the systems by oven heating at 1273 K produces a significant sintering of both Pd and Ce–Zr mixed oxide active components, indicated by X-ray diffraction, transmission electron microscopy, and diffuse reflectance Fourier transform infrared (DRIFTS) techniques. In spite of these effects, the small differences between the light-off performances of fresh and aged systems show the relatively low relevance (particularly in real conditions under engine emissions) of the thermal degradation on the overall performance of the system. In situ DRIFTS results suggest that this is the result of the relatively low structural sensitivity of the desorption of hydrocarbon fragments adsorbed on the active sites, which act mainly as self-poisons for the catalytic reactions.

Redox processes occurring during the CO–O2 reaction and catalytic properties for CO oxidation have been examined for CuO/CeO2 and CuO/ZrCeO4 catalysts using catalytic activity tests, in situ DRIFTS and EPR spectroscopies, and CO-TPD. EPR results suggest synergetic effects in the reduction–oxidation of these systems, both copper and the support being involved in these redox processes. Both EPR and CO-TPD experiments indicate a more facile reduction by CO of the CuO/CeO2 system, mainly due to the greater reducibility of sites at the interface between the copper oxide clusters and the support for this catalyst. A greater CO oxidation activity is exhibited by the CuO/CeO2 catalyst. A comparison of both catalysts and analysis of the redox properties of the different copper entities present in the catalysts indicate that the active copper sites for CO oxidation are located on the copper oxide clusters. On the basis of the results obtained by the different techniques, it is proposed that CO oxidation in these systems follows a redox mechanism in which both the support and copper oxide clusters are simultaneously reduced or oxidized following interaction with CO or O2, respectively. The limiting step in the reaction is related to the oxidation process.

Two alumina-supported Ce–Zr mixed oxide samples (with 10 and 33 wt% loads) are prepared by a microemulsion method, and their structural and redox properties are investigated using Electron Paramagnetic Resonance (EPR) and Raman spectroscopies, X-ray diffraction, temperature-programmed reduction, and transmission electron microscopy. The results show that the preparation method produces highly dispersed Ce–Zr materials which crystallize with an average particle size around 2 nm, the latter depending only weakly on the loading in the range analyzed. The Ce–Zr mixed oxide particles formed have Ce : Zr atomic ratios close to 1 with a slight enrichment in Ce for the 33 wt% CeO2–ZrO2/Al2O3 sample, and present a t″ phase type tetragonal symmetry. In both samples, EPR results show that a certain amount of the mixed oxide is present on the alumina surface as 2D (two-dimensional) patches which have redox sites with properties different from those existing in 3D, bulk-like mixed oxide particles; the characteristics of these latter also differ from those observable in unsupported Ce–Zr mixed oxide (possibly due to an epitaxial relationship with the alumina surface), as evidenced by the absence of formation of associated oxygen vacancies on the samples subjected to outgassing at 773 K. Redox cycling of these materials shows a decrease in reduction onset temperature, similar to the behavior known for unsupported Zr–Ce oxides.