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).

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.

Synthesis and characterization of CoOx/BiVO4 photocatalysts for the degradation of propyl paraben.

Cobalt-promoted bismuth vanadate photocatalysts of variable cobalt content (0-1.0 wt.%) were synthesized and characterized with various techniques including BET, XRD, DRS, XPS and TEM. BiVO4 exists in the monoclinic scheelite structure, while cobalt addition improves the absorbance in the visible region although it does not affect the band gap energy of BiVO4. Cobalt exists in the form of well-dispersed Co3O4 nanocrystallites, which are in intimate contact with the much larger BiVO4 nanoparticles. Photocatalytic activity was evaluated for the degradation of propyl paraben (PP) under simulated solar radiation. The activity of pristine BiVO4 is significantly improved adding small amounts of cobalt and is maximized for the catalyst containing 0.5 wt.% Co. PP degradation in ultrapure pure water increases with increasing photocatalyst loading (100 mg/L to 1.5 g/L), and decreasing PP concentration (1600-200 µg/L). Experiments in bottled water, as well as in pure water spiked with bicarbonate and chloride ions showed little effect of non-target inorganics on degradation. Conversely, degradation is severely impeded in secondary treated wastewater. The enhancement of the photocatalytic activity of the synthesized catalysts is attributed to efficient electron-hole separation, achieved at the p-n junction formed between the p-type Co3O4 and the n-type BiVO4 semiconductors.

Photodegradation of ethyl paraben using simulated solar radiation and Ag3PO4 photocatalyst.

In this work, the solar light-induced photocatalytic degradation of ethyl paraben (EP), a representative of the parabens family, was studied using silver orthophosphate, a relatively new photocatalytic material. The catalyst was synthesized by a precipitation method and had a primary crystallite size of ca 70nm, specific surface area of 1.4m2/g and a bandgap of 2.4eV. A factorial design methodology was implemented to evaluate the importance of EP concentration (500-1500µg/L), catalyst concentration (100-500mg/L), reaction time (4-30min), water matrix (pure water or 10mg/L humic acid) and initial solution pH (3-9) on EP removal. All individual effects but solution pH were statistically significant and so were the second-order interactions of EP concentration with reaction time or catalyst concentration. The water matrix effect was negative (all other effects were positive) signifying the role of humic acid as scavenger of the oxidant species. Liquid chromatography-time of flight mass spectrometry revealed the formation of methyl paraben, 4-hydroxybenzoic acid, benzoic acid and phenol as primary transformation by-products; these are formed through dealkylation and decarboxylation reactions initiated primarily by the photogenerated holes. Estrogenicity assays showed that methyl paraben was more estrogenic than EP; however, parabens are slightly estrogenic compared to 17ß-estradiol.

Kinetics of ethyl paraben degradation by simulated solar radiation in the presence of N-doped TiO2 catalysts.

Ethyl paraben (EP), an emerging micro-pollutant representative of the parabens family, has been subject to photocatalytic degradation under simulated solar radiation at a photon flux of 1.3·10(-4) E/(m(2) s). Six nitrogen-doped titania catalysts synthesized by annealing a sol-gel derived TiO2 powder under ammonia flow and their un-doped counterparts, calcined in air at different temperatures in the range 450-800 °C, were compared under solar and visible light and the most active one (N-doped TiO2 calcined at 600 °C) was used for further tests. Experiments were performed at EP concentrations between 150 and 900 µg/L, catalyst loadings between 100 and 1000 mg/L, pH between 3 and 9, different matrices (ultrapure water, water spiked with humic acids or bicarbonates, drinking water and secondary treated wastewater) and hydrogen peroxide between 10 and 100 mg/L. For EP concentrations up to 300 µg/L, the degradation rate can be approached by first order kinetics but then shifts to lower order as the concentration increases. The rate increases linearly with catalyst loading up to 750 mg/L and hydrogen peroxide up to 100 mg/L. Near-neutral (pH = 6.5-7.5) and alkaline conditions (pH = 9) do not affect degradation, which is reduced at acidic pH. The presence of humic acids at 10-20 mg/L impedes degradation due to the competition with EP for the oxidizing species and this is more pronounced in actual wastewater matrices. UPLC-ESI-HRMS and HPLC-DAD were employed to follow EP concentration changes, as well as identify and quantify transformation by-products during the early stages of the reaction. Five such products were successfully detected and, based on their concentration-time profiles, a reaction network for the degradation of EP is proposed. Hydroxyl radical reactions appear to prevail during the initial steps as evidenced by the rapid formation of hydroxylated and dealkylated intermediates.

Solar light-responsive Pt/CdS/TiO2 photocatalysts for hydrogen production and simultaneous degradation of inorganic or organic sacrificial agents in wastewater.

Photocatalytic degradation of waste material in aqueous solutions and simultaneous production of hydrogen was studied with the double purpose of environmental remediation and renewable energy production. Both powdered and immobilized Pt/CdS/TiO(2) photocatalysts were used to oxidize model inorganic (S(2-)/SO(3)(2-)) and organic (ethanol) sacrificial agents/pollutants in water. Powdered Pt/CdS/TiO(2) photocatalysts of variable CdS content (0-100%) were synthesized by precipitation of CdS nanoparticles on TiO(2) (Degussa P25) followed by deposition of Pt (0.5 wt %) and were characterized with BET, XRD, and DRS. Immobilized photocatalysts were deposited either on plain glass slides or on transparent conductive fluorine-doped SnO(2) electrodes. The results show that it is possible to produce hydrogen efficiently (20% quantum efficiency at 470 nm) by using simulated solar light and by photocatalytically consuming either inorganic or organic substances. CdS-rich photocatalysts are more efficient for the photodegradation of inorganics, while TiO(2)-rich materials are more effective for the photodegradation of organic substances.

Mechanistic aspects of the ethanol steam reforming reaction for hydrogen production on Pt, Ni, and PtNi catalysts supported on gamma-Al2O3.

Mechanistic aspects of ethanol steam reforming on Pt, Ni, and PtNi catalysts supported on gamma-Al(2)O(3) are investigated from the analysis of adsorbed species and gas phase products formed on catalysts during temperature-programmed desorption of ethanol and during ethanol steam reforming reaction. DRIFTS-MS analyses of ethanol decomposition and ethanol steam reforming reactions show that PtNi and Ni catalysts are more stable than the Pt monometallic counterpart. Ethanol TPD results on Ni, Pt, and NiPt catalysts point to ethanol dehydrogenation and acetaldehyde decomposition as the first reaction pathways of ethanol steam reforming over the studied catalysts. The active sites responsible for the acetaldehyde decomposition are easily deactivated in the first minutes on-stream by carbon deposits. For Ni and PtNi catalysts, a second reaction pathway, consisting in the decomposition of acetate intermediates formed over the surface of alumina support, becomes the main reaction pathway operating in steam reforming of ethanol once the acetaldehyde decomposition pathway is deactivated. Taking into account the differences observed in the mechanism of ethanol decomposition, the better stability observed for PtNi catalyst is proposed to be related with a cooperative effect between Pt and Ni activities together with the enhanced ability of Ni to gasify the methyl groups formed by decomposition of acetate species. On the contrary, monometallic catalysts are believed to dehydrogenate these methyl groups forming coke that leads to deactivation of metal particles.

Adsorption of Acid Orange 7 on the surface of titanium dioxide.

The adsorption of a model textile azo-dye, Acid Orange 7 (AO7), on the surface of titanium dioxide was extensively investigated in aqueous TiO(2) suspensions over wide ranges of AO7 concentrations (1 x 10(-4)-3 x 10(-3) M) and pH values (2-10). Results obtained with the use of a variety of techniques, including potentiometric titrations, adsorption isotherms, adsorption edges, and microelectrophoresis, were used for the description of the "AO7 solution/TiO(2) surface" interface. This has been achieved by taking into account the effects of pH on the speciation of the dye in solution and on the nature and population of the surface groups of TiO(2). Results could be modeled very well with the use of the recently introduced CD-MUSIC approach. According to the model employed, the TiO(2) surface is not considered homogeneous but is characterized by the presence of different types of surface groups, namely singly (TiOH(-1/3)), doubly (Ti(2)O(-2/3)), and triply (Ti(3)O(0)) coordinated. Surface complexes are not treated as point charges, but their charge is spatially distributed in the interfacial region. It has been found that adsorption of AO7 on the TiO(2) surface occurs to a significant extent only at pH values lower than 7, via the sulfonic group of the azo-dye, through the formation of a bidentate innersphere surface complex. The determination of the adsorption mode of TiO(2), which is supported by ex situ FTIR results, as well as of the adsorption constant, K(ads), allowed the description of the pH dependency of the AO7 adsorption over large pH and AO7 concentration ranges.