Support Induced Effects on the Ir Nanoparticles Activity, Selectivity and Stability Performance under CO2 Reforming of Methane.

The production of syngas (H2 and CO)-a key building block for the manufacture of liquid energy carriers, ammonia and hydrogen-through the dry (CO2-) reforming of methane (DRM) continues to gain attention in heterogeneous catalysis, renewable energy technologies and sustainable economy. Here we report on the effects of the metal oxide support (γ-Al2O3, alumina-ceria-zirconia (ACZ) and ceria-zirconia (CZ)) on the low-temperature (ca. 500-750 ∘C) DRM activity, selectivity, resistance against carbon deposition and iridium nanoparticles sintering under oxidative thermal aging. A variety of characterization techniques were implemented to provide insight into the factors that determine iridium intrinsic DRM kinetics and stability, including metal-support interactions and physicochemical properties of materials. All Ir/γ-Al2O3, Ir/ACZ and Ir/CZ catalysts have stable DRM performance with time-on-stream, although supports with high oxygen storage capacity (ACZ and CZ) promoted CO2 conversion, yielding CO-enriched syngas. CZ-based supports endow Ir exceptional anti-sintering characteristics. The amount of carbon deposition was small in all catalysts, however decreasing as Ir/γ-Al2O3 > Ir/ACZ > Ir/CZ. The experimental findings are consistent with a bifunctional reaction mechanism involving participation of oxygen vacancies on the support's surface in CO2 activation and carbon removal, and overall suggest that CZ-supported Ir nanoparticles are promising catalysts for low-temperature dry reforming of methane (LT-DRM)

Effect of support oxygen storage capacity on the catalytic performance of Rh nanoparticles for CO2 reforming of methane

The effects of the metal oxide support on the activity, selectivity, resistance to carbon deposition and high temperature oxidative aging on the Rh-catalyzed dry reforming of methane (DRM) were investigated. Three Rh catalysts supported on oxides characterized by very different oxygen storage capacities and labilities (γ-Al 2O 3, alumina-ceria-zirconia (ACZ) and ceria-zirconia (CZ)) were studied in the temperature interval 400–750 °C under both integral and differential reaction conditions. ACZ and CZ promoted CO 2 conversion, yielding CO-enriched synthesis gas. Detailed characterization of these materials, including state of the art XPS measurements obtained via sample transfer between reaction cell and spectrometer chamber, provided clear insight into the factors that determine catalytic performance. The principal Rh species detected by post reaction XPS was Rh 0, its relative content decreasing in the order Rh/CZ(100%)>Rh/ACZ(72%)>Rh/γ-Al 2O 3(55%). The catalytic activity followed the same order, demonstrating unambiguously that Rh 0 is indeed the key active site. Moreover, the presence of CZ in the support served to maintain Rh in the metallic state and minimize carbon deposition under reaction conditions. Carbon deposition, low in all cases, increased in the order Rh/CZ < Rh/ACZ < Rh/γ-Al 2O 3 consistent with a bi-functional reaction mechanism whereby backspillover of labile lattice O 2− contributes to carbon oxidation, stabilization of Rh 0 and modification of its surface chemistry; the resulting O vacancies in the support providing centers for dissociative adsorption of CO 2. The lower apparent activation energy observed with CZ-containing samples suggests that CZ is a promising support component for use in low temperature DRM

CO2 Hydrogenation to Methanol over La2O3-Promoted CuO/ZnO/Al2O3 Catalysts: A Kinetic and Mechanistic Study

The hydrogenation of CO2 to methanol has been investigated over CuO/ZnO/Al2O3 (CZA) catalysts, where a part of the Al2O3 (0, 25, 50, 75, or 100%) was substituted by La2O3. Results of catalytic performance tests obtained at atmospheric pressure showed that the addition of La2O3 generally resulted in a decrease of CO2 conversion and in an increase of methanol selectivity. Optimal results were obtained for the CZA-La50 catalyst, which exhibited a 30% higher yield of methanol, compared to the un-promoted sample. This was attributed to the relatively high specific surface area and porosity of this material, the creation of basic sites of moderate strength, which enhance adsorption of CO2 and intermediates that favor hydrogenation steps, and the ability of the catalyst to maintain a large part of the copper in its metallic form under reaction conditions. The reaction mechanism was studied with the use of in situ infrared spectroscopy (DRIFTS). It was found that the reaction proceeded with the intermediate formation of surface formate and methoxy species and that both methanol and CO were mainly produced via a common formate intermediate species. The kinetic behavior of the best performing CZA-La50 catalyst was investigated in the temperature range 190&ndash;230 &deg;C as a function of the partial pressures of H2 (0.3&ndash;0.9 atm) and CO2 (0.05&ndash;0.20 atm), and a kinetic model was developed, which described the measured reaction rates satisfactorily

Mechanistic and kinetic study of solar-light induced photocatalytic degradation of Acid Orange 7 in aqueous TiO2 suspensions

The photocatalytic degradation of aqueous solutions of Acid Orange 7 in TiO2 suspensions has been investigated with the use of a solar light simulating source. It has been found that, when the full range of emitted photons is used, decolorization and complete mineralization of the solution is achieved with satisfactory rates, depending on initial dye concentration. A reaction pathway is proposed according to which degradation of the dye molecules adsorbed on the photocatalyst surface takes place via a series of oxidation steps, which lead to decolorization and formation of a number of intermediates, mainly aromatic and aliphatic acids, which are further oxidized toward compounds of progressively lower molecular weight. Eventually, mineralization is achieved leading to the formation of gas phase CO2 and inorganic ions in solution. Kinetic results show that the initial rate of decolorization depends strongly on surface coverage and on incident photon energy. Visible light photons contribute to decolorization, via the photosensitization mechanism, but with reaction rates which are more than two orders of magnitude lower than the corresponding ones induced by photons of energy higher than that of the band gap of the semiconductor. In both cases, the initial rate of decolorization is significantly reduced for dye coverages close to monolayer

Performance of Particulate and Structured Pt/TiO₂-Based Catalysts for the WGS Reaction under Realistic High- and Low-Temperature Shift Conditions

The water–gas shift (WGS) activity of Pt/TiO2-based powdered and structured catalysts was investigated using realistic feed compositions that are relevant to the high-temperature shift (HTS) and low-temperature shift (LTS) reaction conditions. The promotion of the TiO2 support with small amounts of alkali- or alkaline earth-metals resulted in the enhancement of the WGS activity of 0.5%Pt/TiO2(X) catalysts (X = Na, Cs, Ca, Sr). The use of bimetallic (Pt–M)/TiO2 catalysts (M = Ru, Cr, Fe, Cu) can also shift the CO conversion curve toward lower temperatures, but this is accompanied by the production of relatively large amounts of unwanted CH4 at temperatures above ca. 300 °C. Among the powdered catalysts investigated, Pt/TiO2(Ca) exhibited the best performance under both HTS and LTS conditions. Therefore, this material was selected for the preparation of structured catalysts in the form of pellets as well as ceramic and metallic catalyst monoliths. The 0.5%Pt/TiO2(Ca) pellet catalyst exhibited comparable activity with that of a commercial WGS pellet catalyst, and its performance was further improved when the Pt loading was increased to 1.0 wt.%. Among the structured catalysts investigated, the best results were obtained for the sample coated on the metallic monolith, which exhibited excellent WGS performance in the 300–350 °C temperature range. In conclusion, proper selection of the catalyst structure and reaction parameters can shift the CO conversion curves toward sufficiently low temperatures, rendering the Pt/TiO2(Ca) catalyst suitable for practical applications

Propane Steam Reforming over Catalysts Derived from Noble Metal (Ru, Rh)-Substituted LaNiO3 and La0.8Sr0.2NiO3 Perovskite Precursors

The propane steam reforming (PSR) reaction was investigated over catalysts derived from LaNiO3 (LN), La0.8Sr0.2NiO3 (LSN), and noble metal-substituted LNMx and LSNMx (M = Ru, Rh; x = 0.01, 0.1) perovskites. The incorporation of foreign cations in the A and/or B sites of the perovskite structure resulted in an increase in the specific surface area, a shift of XRD lines toward lower diffraction angles, and a decrease of the mean primary crystallite size of the parent material. Exposure of the as-prepared samples to reaction conditions resulted in the in situ development of new phases including metallic Ni and La2O2CO3, which participate actively in the PSR reaction. The LN-derived catalyst exhibited higher activity compared to LSN, and its performance for the title reaction did not change appreciably following partial substitution of Ru for Ni. In contrast, incorporation of Ru and, especially, Rh in the LSN perovskite matrix resulted in the development of catalysts with significantly enhanced catalytic performance, which improved by increasing the noble metal content. The best results were obtained for the LSNRh0.1-derived sample, which exhibited excellent long-term stability for 40 hours on stream as well as high propane conversion (XC3H8 = 92%) and H2 selectivity (SH2 = 97%) at 600 °C

Effect of the nature of the support on the catalytic performance of noble metal catalysts for the water–gas shift reaction

Summarization: The catalytic activity of supported noble metal catalysts (Pt, Rh, Ru, and Pd) for the WGS reaction is investigated with respect to the physichochemical properties of the metallic phase and the support. It has been found that, for all metal-support combinations investigated, Pt is much more active than Pd, while Rh and Ru exhibit intermediate activity. The turnover frequency (TOF) of CO conversion does not depend on metal loading, dispersion or crystallite size, but depends strongly on the nature of the metal oxide carrier. In particular, catalytic activity of Pt and Ru catalysts, is 1-2 orders of magnitude higher when supported on “reducible” (TiO2, CeO2, La2O3, and YSZ) rather than on “irreducible” (Al2O3, MgO, and SiO2) metal oxides. In contrast to what has been found in our previous study over Pt/TiO2 catalysts, catalytic activity of dispersed Pt does not depend on the structural and morphological characteristics of CeO2, such as specific surface area or primary crystallite size.Παρουσιάστηκε στο: Catalysis Toda

Effects of alkali promotion of TiO2 on the chemisorptive properties and water–gas shift activity of supported noble metal catalysts

Summarization: The effects of alkali promotion of TiO2 on the chemisorptive properties and water–gas shift (WGS) activity of dispersed noble metal catalysts (NM = Pt, Ru, Pd) have been investigated over NM/X–TiO2 samples of variable promoter type (X = Li, Na, K, Cs) and loading (0.0 to 0.68 wt.%). Results of H2-TPD experiments show that addition of alkalis does not affect appreciably the population and chemisorption strength of hydrogen adsorbed on the surface of dispersed metal crystallites indicating that these sites are not influenced by the presence of the promoter. In contrast, the desorption temperature of hydrogen adsorbed on sites located at the metal-support interface shifts monotonically toward lower temperatures with increasing alkali content. This has been attributed to alkali-induced reduction of Ti4+ surface species and creation of a new type of sites at the perimeter of the dispersed metal crystallites, which are in contact with the support. These sites are of the form of NM–□s–Ti3+, where □s denotes an oxygen defect vacancy, and provide the necessary dual-function sites required for the WGS reaction to proceed. Catalytic performance tests and kinetic measurements show that activity depends appreciably on the nature and loading of added alkali, whereas the apparent activation energy (Ea) of the reaction does not vary to a large extent. For Pt/X–TiO2 catalysts, a volcano-type dependence of intrinsic reaction rate on the chemisorption strength of Pt–□s–Ti3+ sites toward hydrogen has been found to exist. Optimized catalysts are about three times more active than unpromoted Pt/TiO2. Results are discussed with respect to the alkali-induced modifications of the physicochemical properties of the support on the population, chemisorptive properties, and catalytic activity of sites located at the metal-support interface.Παρουσιάστηκε στο: Journal of Catalysi

Effects of alkali additives on the physicochemical characteristics and chemisorptive properties of Pt/TiO2 catalysts

Summarization: The effects of alkali additives on the physicochemical and chemisorptive properties of 0.5% Pt/TiO2 have been investigated over catalysts promoted with variable amounts of Na (0–0.2 wt%) or Cs (0–0.68 wt%) with the use of diffuse reflectance infrared spectroscopy (DRIFTS) and temperature-programmed (TPD, TPR) techniques. It has been found that addition of alkalis does not affect adsorption of CO and H2 on the surface of Pt crystallites, indicating the absence of strong electronic-type interactions between these sites and the promoters. However, the presence of alkalis results in the creation and population of new sites with increased electron density, proposed to be located at perimetric sites of Pt crystallites, which are in contact with the support. The adsorption strength of these sites toward CO increases with increasing alkali content, which is evidenced by the development of new, low-frequency IR bands in the ν(CO) region. In contrast, addition of alkali results in weakening of hydrogen adsorption on sites located at the metal/support interface, which is reflected to a significant shift of the corresponding TPD peak toward lower temperatures. Results of CO-TPD experiments indicate that CO adsorbed on Pt interacts with hydroxyl groups associated with the support to yield formate, which decomposes during TPD to CO2 and H2. Thermal decomposition of formate is accomplished at lower temperatures in the presence of alkali. Finally, CO-TPR experiments indicate that the reducibility of TiO2 is enhanced in the presence of alkali, which can be related to the creation of the new sites at the metal/support interface.Presented on: Journal of Catalysi

Investigation of the water-gas shift reaction over alkali-promoted Pt/TiO2 catalysts

Μη διαθέσιμη περίληψηNot available summarizationΠαρουσιάστηκε στο: 7th European Congress on Catalysi