Temperature-programmed Reduction Tests Research Articles

In-situ Ni exsolution from NiTiO3 as potential anode for solid oxide fuel cells

Sample NiTiO3 (NTO) is prepared by the molten salts synthesis route as a potential anode material for solid oxide fuel cell (SOFC) applications. An additional sample impregnated with 5 mol%Ni (N-NTO) is also presented. Structural characterization reveal a pure NiTiO3 phase upon calcination at 850 °C and 1000 °C. Redox characterization by temperature programmed reduction tests indicate the transition from NiTiO3 to Ni/TiO2 at ca. 700 °C. Ni nanoparticles (ca. 26 nm) are exsolved in-situ from the structure after a reducing treatment at 850 °C. Catalytic activity tests for partial oxidation of methane performed in a fixed bed reactor reveal excellent values of activity and selectivity due to the highly dispersed Ni nanoparticles in the support surface. Time-on-stream behavior during 100 h operation in reaction conditions for sample N-NTO yield a stable CH4 conversion. Electrolyte supported symmetrical cells are prepared with both materials achieving excellent polarization resistance of 0.023 Ω cm2 in 7%H2/N2 atmosphere at 750 °C with sample N-NTO. The maximum power density achieved is of 273 mW cm−2 at 800 °C with a commercial Pt ink used as a reference cathode, indicating further improvement of the system can be achieved and positioning the N-NTO material as a promising SOFC anode material.

Efficient synthesis of BiFeO3 by the microwave-assisted sol-gel method: “A” site influence on the photoelectrochemical activity of perovskites

BiFeO3 (BF) and LaFeO3 (LF) perovskites were synthesized using a microwave-assisted (MW) and sol-gel (SG) methods. XRD, XPS, TEM, UV-DRS techniques were applied to study physicochemical properties of perovskites. In addition, Incident Photon-to-Current Efficiency (IPCE) measurements, Linear Sweep Voltammetry (LSV) and impedance spectroscopy were used to characterize electrochemical properties of the materials. The band gap energy increases in the following way: BF-MW (2.05 eV), LF-MW (2.18 eV), BF-SG (2.26 eV) and LF-SG (2.54 eV), demonstrating a remarkable influence of the synthesis method on the optical and electronic properties of the materials. Furthermore, XRD showed a significant impact of the synthesis methods on the crystal structure. Perovskites synthesized under MW irradiation showed a pure crystal structure compared to the perovskites prepared by SG method, which contained some admixtures. IPCE shows that LF-MW has a better charge separation ability compared to BF-MW. However, BF-SG showed the highest activity. Temperature programmed reduction tests (TPR) revealed a better ability of BF-MW to adsorb/desorb oxygen, compared to LF-MW. XPS measurements pointed at the presence of Fe4+. Finally, the photocatalytic activity of the perovskites was tested in solar water-splitting as a function of the synthesis method and presence of Bi and La in “A” sites of the ABO3 perovskites. We postulate, that the Jahn-Teller distortion effect in LF-MW increases its catalytic activity by decreasing the binding energy compared to BF-MW.

TPR studies on NiO–CGO composites prepared by combustion synthesis

NiO–ceria composites, which are promising candidates as anodes for intermediate temperature solid oxide fuel cells (IT-SOFCs) were prepared by urea combustion synthesis (UCS) method. The UCS method is, in general, a highly suitable synthesis method for the production, at low temperatures, of fine and reactive powders. By means of XRD and SEM-EDX techniques, the structural, microstructural and compositional behavior of the as-prepared powders has been studied. In addition, temperature programmed reduction (TPR) tests were performed to investigate the reducibility of the composites. After reduction of the NiO–CGO as-prepared compositions, the combustion powders exhibit the presence of Ni, the fluorite CGO solid solution that remains stable and NiO is no longer present. The morphology and size of the nanoparticles and aggregates of the as-prepared powders make them reactive at intermediate temperatures (400–800°C). TPR tests show wide overlapping peaks which are associated with the two primary reduction stages; one is related to the surface NiO reduction mechanism and the other to the coexistence of interactions between the NiO–CGO surface and bulk reduction processes. Further, after TPR measurements the resulting products have high phase stability and reproducibility.

Vapour phase H2O2 decomposition on Mn based monolithic catalysts synthesized by innovative procedures

Manganese oxide catalysts supported on monolithic yttria stabilized zirconia honeycombs were studied for H2O2 decomposition in view of space propulsion applications. The materials were prepared by impregnation (IM), precipitation (PR) and sol–gel (SG) methods and characterized by N2 adsorption, SEM and H2 temperature programmed reduction (TPR). The catalytic activity for H2O2 decomposition was studied under vapour phase conditions. The tests were carried out in a flow apparatus at T=200°C, gas hourly space velocity (GHSV)=2.00 and 2.67s−1, H2O2 concentrations of 11.3mol%. The redox properties of the catalysts were markedly influenced by the preparation method. In the SG catalyst, a large fraction of Mn was not reducible in the TPR tests differently from the PR and IM materials. Despite this effect, the SG catalyst showed an activity comparable or higher than that of PR and IM, due to a more effective dispersion of Mn species. A too strong effect of space velocity on H2O2 conversion was attributed to an autocatalytic effect. A radical mechanism was hypothesized: it was assumed that a reaction between O2 and Mn species produced radicals that promoted the overall reaction.

Characterization of Ni-Cu-based catalysts for multi-fuel steam reformer

Low-temperature methane and methanol steam-reforming catalysts Ni/Al2O3, Cu/Al2O3 and Ni–Cu/Al2O3 with various loadings of Ni and Cu were prepared using a wet impregnation method. The samples were characterized using scanning electron microscope, surface area (BET) test, X-ray diffraction (XRD), infrared test, CO chemisorption test and temperature-programmed reduction tests. XRD testing showed that NiO and CuO were present. Ni–Cu-alloyed catalyst shows a significant change in the catalyst characteristics compared with those of individual metals. The results presented in this paper show the main changes in the catalyst properties using ex situ testing.

Methane conversion reactions on Ni catalysts promoted with Rh: Influence of support

Ni catalysts supported on γ-Al 2O 3 and Mg(Al)O were prepared with and without Rh as a promoter and tested in the reforming of methane in the presence of excess methane, simulating a model biogas. The effects of adding synthetic air on the methane conversion and the formation of carbon were assessed. The catalysts were characterized by X-ray spectroscopy (EDS), surface area (BET), X-ray diffraction (XRD), Temperature-programmed reduction (TPR), X-ray absorption near-edge structure (XANES) and XPD. The results showed that in catalysts without Rh, the Ni interacts strongly with the supports, showing high reduction temperatures in TPR tests. The addition of Rh increased the amount of reducible Ni and facilitated the reduction of the species interacting strongly with the support. In the catalytic tests, the samples promoted with Rh suffered higher carbon deposition. The in situ XPD suggested that on the support γ-Al 2O 3, the presence of Rh probably led to a segregation of Ni species with time on stream, leading to carbon deposition. On the support MgAlO, the presence of Rh improved the dispersion of Ni, by reducing the Ni 0 crystallite size, suggesting that in this case the carbon deposition was due to a favoring of CH 4 decomposition by Rh.