Structural, catalytic/redox and electrical characterization of systems combining Cu-Ni with CeO2 or Ce1−xMxO2−δ (M = Gd or Tb) for direct methane oxidation

The present work analyses bimetallic Cu-Ni formulations, in comparison to monometallic Cu ones, combined with CeO₂ or other structurally related mixed oxides resulting from doping of the former with Gd or Tb, focusing to its possible use as anodes of solid oxide fuel cells (SOFC) for direct oxidation of methane. The main objective is the characterization of the various formulations at structural level as well as with regards to the redox changes taking place in the systems upon interaction with methane, in order to evaluate the effects induced by the presence of dopants. In the same sense, an analysis of thermal expansion and electrical properties of the systems is performed, considering its possible implantation in SOFC single cells. For the mentioned purposes, the systems have been analysed by means of CH₄-TPR tests subsequently followed by TPO tests, as well as by XRD, Raman and XPS, with the aim of exploring structural and redox changes produced in the systems and the formation of carbon deposits during such interactions. The results reveal significant modifications in the structural, catalytic/redox and electrical properties of the systems as a function of the presence of Ni and/or Gd and Tb dopants in the formulation.     

Structural, catalytic/redox and electrical characterization of systems combining Cu-Fe with CeO2 or Ce1−xMxO2−δ (M = Gd or Tb) for direct methane oxidation

The present work analyses bimetallic Cu-Fe formulations combined with CeO₂ or other structurally related mixed oxides resulting from doping of the former with Gd or Tb, focusing to its possible use as anodes of solid oxide fuel cells (SOFC) for direct oxidation of methane. The main objective is the characterization of the various formulations at structural level as well as with regard to redox changes taking place in the systems upon interaction with methane, in order to evaluate the effects induced by the presence of the mentioned dopants. In the same sense, an analysis of thermal expansion and electrical properties of the systems as well as their chemical compatibilities with several electrolytic materials is performed, considering its possible implantation in SOFC single cells. For the mentioned purposes, the systems have been analysed by means of CH₄-TPR tests subsequently followed by TPO tests, as well as by XRD, Raman and XPS, with the aim of exploring structural and redox changes produced in the systems and the formation of carbon deposits during interaction with methane. The results reveal significant modifications in the structural, catalytic/redox and electrical properties of the systems as a function of the presence of Fe and/or Gd and Tb dopants in the formulation.     

Synthesis and characterization of bimetallic Cu–Ni/ZrO2 nanocatalysts: H2 production by oxidative steam reforming of methanol

Cu/ZrO₂, Ni/ZrO₂ and bimetallic Cu–Ni/ZrO₂ catalysts were prepared by deposition–precipitation method to produce hydrogen by oxidative steam reforming of methanol (OSRM) reaction in the range of 250–360 °C. TPR analysis of the Cu–Ni/ZrO₂ catalyst showed that the presence of Cu facilitates the reduction of the Ni at lower temperatures. In addition, this sample showed two reduction peaks, the former peak was attributed to the reduction of the adjacent Cu and Ni atoms which could be forming a bimetallic Cu-rich phase, and the second was assigned to the remaining Ni atoms forming bimetallic Ni-rich nanoparticles. Transmission Electron Microscopy revealed Cu or Ni nanoparticles on the monometallic samples, while bimetallic nanoparticles were identified on the Cu–Ni/ZrO₂ catalyst. On the other hand, Cu–Ni/ZrO₂ catalyst exhibited better catalytic activity than the monometallic samples. The difference between them was related to the Cu–Ni nanoparticles present on the former catalyst, as well as the bifunctional role of the bimetallic phase and the support that improve the catalytic activity. All the catalysts showed the same selectivity toward H₂ at the maximum reaction temperature and it was ∼60%. The high selectivity toward CO is associated to the presence of the bimetallic Ni-rich nanoparticles, as evidenced by TEM–EDX analysis, since this behavior is similar to the one showed by the monometallic Ni-catalyst.     

Dry reforming of methane over nanostructured nickel – Actinide (Th,U) bimetallic oxides

Dry reforming of methane (DRM) can be an efficient alternative to convert greenhouse gases and to produce valuable feedstock. In this work, nanostructured nickel - actinide bimetallic oxides (2NiO.ThO₂ and NiO.UNiO₄) were tested for the first time as DRM catalysts. They were very active and highly selective towards the production of syngas at low temperatures. Their catalytic performance is better than that of a commercial rhodium supported catalyst (5 wt % Rh/Al₂O₃) used as a reference. In particular, the thorium catalyst was more active and selective at 550 °C than the commercial catalyst, which is a significant “low” temperature for DRM. The nanostructured nickel - actinide bimetallic oxides long-term stability was also remarkable and an important improvement if we consider the behavior of others nickel-based catalysts that normally undergoing significant deactivations due to coke or sintering.     

Preparation of Cu-Ni/YSZ solid oxide fuel cell anodes using microwave irradiation

A microwave irradiation process is used to deposit Cu nanoparticles on the Ni/YSZ anode of an electrolyte-supported solid oxide fuel cell (SOFC). The reaction time in the microwave is only 15 s for the deposition of 6 wt% Cu (with respect to Ni) from a solution of Cu(NO₃)₂·3H₂O and ethylene glycol (HOCH₂CH₂OH). The morphology of the deposited Cu particles is spherical and the average size of the particles is less than 100 nm. The electrochemical performance of the microwave Cu-coated Ni/YSZ anodes is tested in dry H₂ and dry CH₄ at 1073 K, and the anodes are characterized with scanning electron microscopy, electrochemical impedance spectroscopy, and temperature-programmed oxidation. The results indicate that preparation of the anodes by the microwave technique produces similar performance trend as those reported for Cu-Ni/YSZ/CeO₂ anodes prepared by impregnation. Specifically, less carbon is formed on the Cu-Ni/YSZ than on conventional Ni/YSZ anodes when exposed to dry methane and the carbon that does form is more reactive.     

Methanation of CO2 over nanostructured nickel-4f block element bimetallic oxides

Nanostructure nickel-4f block element bimetallic oxides were prepared by electrospinning technique using a precursor sol-gel solution containing metal nitrates and a treatment of controlled oxidation under dry air. The heating rate of such treatment allows the control of the product morphology aiming the production of either nanoparticles or nanofibers. Catalytic hydrogenation studies of carbon dioxide were for the first time undertaken using this type of bimetallic oxides as catalysts and remarkably almost all of them present a catalytic activity superior to that of a commercial rhodium catalyst supported on alumina (5 wt % Rh/Al₂O₃) for the production of methane. Last but not the least, the nickel-4f block element bimetallic oxides present also a deactivation resistance for at least 50 h in the gaseous stream, which is an advantage for nickel-based catalysts.     

Effect of sintering aids on the conductivity of BaCe0.9Ln0.1O3−δ

Polycrystalline powders of BaCe_0.9Ln_0.1O_3−δ (Ln = La, Nd, Sm, Gd, Yb, Tb and Y) have been prepared using a freeze-drying precursor route at 1000 °C. In order to decrease the densification temperature, different sintering aids (e.g. Co, Zn, Ni, Fe and Cu) were added by mixing the polycrystalline powders with a nitrate solution containing these metals. This allows obtaining dense ceramics at temperatures as low as 1000 °C, when compared to samples without sintering aids at 1400 °C. The effect of aid content and sintering temperature on the microstructure and electrical conductivity were investigated by scanning electron microscopy and impedance spectroscopy. The addition of small amounts of transition metals does not produce observable structural changes by conventional X-ray powder diffraction. However, the bulk and total conductivities decrease when compared to samples without transition metals. Zn seems to be the most effective sintering element for BaCe_0.9Ln_0.1O_3−δ electrolytes because it does not cause significant changes in the ionic and electronic conductivities.     

Hydrogen production from oxidative steam reforming of methanol: Effect of the Cu and Ni impregnation on ZrO2 and their molecular simulation studies

Cu and Ni were supported on ZrO₂ by co-impregnation and sequential impregnation methods, and tested in the oxidative steam reforming of methanol (OSRM) reaction for H₂ production as a function of temperature. Surface area of the catalysts showed differences as a function of the order in which the metals were added to zirconia. Among them, the Cu/ZrO₂ catalyst had the lowest surface area. XRD patterns of the bimetallic catalysts did not show diffraction peaks of the Cu, Ni or bimetallic Cu–Ni alloys. In addition, TPR profiles of the bimetallic catalysts had the lowest reduction temperature compared with the monometallic samples. The reactivity of the catalysts in the range of 250–350 °C showed that the bimetallic samples prepared by successive impregnation had highest catalytic activity among all the catalysts studied. These results were also confirmed by theoretical calculations. The reactivity of the monometallic and bimetallic structures obtained by molecular simulation followed the next order: Ni_shellCu_core/ZrO₂ ≅ Cu_shellNi_core/ZrO₂ > Ni/Cu/ZrO₂ > Cu/Ni/ZrO₂ > Cu–Ni/ZrO₂ > Cu/ZrO₂ > Ni/ZrO₂. These findings agree with the experimental results, indicating that the bimetallic catalysts prepared by successive impregnation show a higher reactivity than the Cu–Ni system obtained by co-impregnation. In addition, the selectivity for H₂ production was higher on these catalysts. This result could be associated also to the presence of the bimetallic Cu–Ni and core–shell Ni/Cu nanoparticles on the catalysts, as was evidenced by TEM–EDX analysis, suggesting that the OSRM reaction may be a structure–sensitive reaction.     

Catalytic performance of Pd–Ni bimetallic catalyst for glycerol hydrogenolysis

Catalytic conversion of glycerol from biodiesel production to value-added chemicals and fuels is actually of great interest for industrial chemical research. Bimetallic catalysts are confirmed superior to monometallic catalysts in terms of catalytic activity and selectivity for glycerol hydrogenolysis. Accordingly, a series of Pd–M (M = Fe, Co, Ni, Cu, Zn) bimetallic catalysts were prepared in this work via coprecipitation to investigate the promoting effect of Pd. The relationship between the catalytic performance and metal-support interaction was also discussed. Through the catalyst screening, Pd–Ni bimetallic catalyst exhibited moderate activity and the highest selectivity towards ethylene glycol. At 493 K and hydrogen pressure of 6.0 MPa, the glycerol conversion and selectivity of ethylene glycol reached 89% and 22% respectively. XRD and TEM patterns showed that the Pd nanoparticles with an average size of ∼4 nm were uniformly dispersed in the supports. H₂-TPR revealed that the reduction temperatures of metal oxides were significantly decreased by the introduction of Pd component. XPS curves indicated that unique performance of the Pd–Ni bimetallic catalyst might be attributed to the formation of Pd–Ni alloy. And the required metal-support interaction was assumed responsible for the cleavage of C–C bond and generation of ethylene glycol. In the end, hydrogenolysis reactions for the main products of glycerol conversion were carried out over Pd–Ni catalyst to explore the possible reaction pathways for glycerol hydrogenolysis.     

Development of electrolyte-supported intermediate-temperature single-chamber solid oxide fuel cells using Ln0.7Sr0.3Fe0.8Co0.2O3−δ (Ln = Pr, La, Gd) cathodes

Iron-cobalt-based perovskite oxides with general formula Ln_0.7Sr_0.3Fe_0.8Co_0.2O_3−δ (where Ln = La, Pr and Gd) have been investigated for their application as intermediate-temperature cathodes in solid oxide fuel cells (SOFCs). Powdered samples of these materials were synthesized by a novel gel combustion process and then characterized by X-ray powder diffraction (XPD) and scanning electron microscopy (SEM). XPD patterns were satisfactorily indexed with an orthorhombic GdFeO₃-type structure and, for all samples, a mean particle size of less than 1 μm was estimated from the SEM data. Experimental single-chamber SOFCs using with these materials as cathodes and NiO-SDC (samaria-doped ceria) and SDC as anode and electrolyte, respectively, were evaluated at 600 °C in a methane/oxygen mixtures. Peak power densities of 65.4, 48.7 and 46.2 mW cm⁻² were obtained for Ag|Ln_0.7Sr_0.3Fe_0.8Co_0.2O_3−δ|SDC|NiO-SDC|Pt cells with Ln = Pr, La and Gd, respectively. The relatively high power density obtained for the Pr compound shows that it could be an interesting material for cathode of single-chamber SOFCs.     

Influence of metal oxide on LiBH4/2LiNH2/MgH2 system for hydrogen storage properties

In this study, various nanoscale metal oxide catalysts, such as CeO₂, TiO₂, Fe₂O₃, Co₃O₄, and SiO₂, were added to the LiBH₄/2LiNH₂/MgH₂ system by using high-energy ball milling. Temperature programmed desorption and MS results showed that the Li–Mg–B–N–H/oxide mixtures were able to dehydrogenate at much lower temperatures. The order of the catalytic effect of the studied oxides was Fe₂O₃ > Co₃O₄ > CeO₂ > TiO₂ > SiO₂. The onset dehydrogenation temperature was below 70 °C for the samples doped with Fe₂O₃ and Co₃O₄ with 10 wt.%. More than 5.4 wt.% hydrogen was released at 140 °C. X-ray diffraction indicated that the addition of metal oxides inhibited the formation of Mg(NH₂)₂ during ball milling processes. It is thought that the changing of the ball milling products results from the interaction of oxide ions in metal oxide catalysts with hydrogen atoms in MgH₂. The catalytic effect depends on the activation capability of oxygen species in metal oxides on hydrogen atoms in hydrides.     

Development of a Co–Ni bimetallic aerogel catalyst for hydrogen production via methane oxidative CO2 reforming in a magnetic assisted fluidized bed

A novel nano-sized Co–Ni bimetallic aerogel catalyst was synthesized via the sol–gel process followed by the supercritical drying method. The catalyst exhibited at least 28% higher activity and 15% higher H₂ selectivity than those of a monometallic cobalt aerogel catalyst in methane oxidative CO₂ (Oxy-CO₂) reforming. Cold-model experiments revealed that channels were alleviated and the agglomerate size was reduced when the catalyst was applied in a magnetic assisted fluidized bed (MAFB). Owing to the improved fluidization quality of the catalyst in the MAFB reactor, CH₄ conversion was raised by 12% and 7% as compared with those in the fixed bed reactor and the conventional fluidized bed reactor, respectively. Furthermore, the catalytic performance was quite stable during a 50 h reaction. This stable performance can be illustrated both by the superior catalytic property of the Co–Ni bimetallic aerogel catalyst and the intensified gas–solid interaction in the MAFB reactor.     

Biogas reforming over bimetallic PdNi catalysts supported on phosphorus-modified alumina

A series of bimetallic PdNi catalysts supported on alumina modified with different amounts of phosphorus (0.5-5 wt%) were prepared. The effect of phosphorus content on the structure, surface properties and catalytic behavior of supported PdNi catalysts in biogas reforming was studied. The physicochemical properties of the samples were characterized by using different techniques: N₂ adsorption-desorption isotherms, X-ray diffraction (XRD), UV-vis diffuse reflectance spectroscopy (DRS), X-ray photoelectron spectroscopy (XPS), temperature-programmed reduction (TPR), temperature-programmed desorption of ammonia (TPD), thermogravimetric and differential thermal analysis (TG/DTA) and scanning transmission electron microscopy (STEM). The catalytic properties of the catalysts were evaluated in the reaction of reforming of methane with CO₂. It was shown that increasing the P content (≥1 wt%) leads to agglomeration of the metal Ni particles, as well as to increase of the total acidity of the catalysts. Within bimetallic system, the PdNi catalyst with 0.5 wt% phosphorus showed the best performance and stability caused by the presence of highly dispersed nickel particles on the catalyst surface due to the strong interaction between supported species and alumina.     

Syngas production by catalytic partial oxidation of methane over (La0.7A0.3)BO3 (A = Ba,Ca, Mg,Sr, and B = Cr or Fe) perovskite oxides for portable fuel cell applications

Hydrogen is a clean energy carrier for the future. More efficient, economic and small-scale syngas production has therefore important implications not only on the future sustainable hydrogen-based economy but also on the distributed energy generation technologies such as fuel cells. In this paper, a new concept for syngas production is presented with the use of redox stable lanthanum chromite and lanthanum ferrite perovskites with A-site doping of Ba, Ca, Mg and Sr as the pure atomic oxygen source for the catalytic partial oxidation of methane. In this process, catalytic partial oxidation reaction of methane occurs with the lattice oxygen of perovskites, forming H₂ and CO syngas. The oxygen vacancies due to the release of lattice oxygen ions are regenerated by passing air over the reduced nonstoichiometric perovskites. Studies by XRD, temperature-programmed reduction (TPR) and activity measurements showed the enhanced effects of alkaline element A-site dopants on reaction activity of both LaCrO₃ and LaFeO₃ oxides. In both series, Sr and Ca doping promotes significantly the activity towards the syngas production most likely due to the significantly increased mobility of the lattice oxygen in perovskite oxide structures. The active oxygen species and performance of the LaACrO₃ and LaAFeO₃ perovskite oxides with respect to the catalytic partial oxidation of methane are discussed.     

Influence of rare-earth doping on the microstructure and conductivity of BaCe0.9Ln0.1O3−δ proton conductors

Doped barium cerates BaCe_0.9Ln_0.1O_3−δ containing earth-rare dopants with different ionic radii, Ln = La, Nd, Sm, Gd, Yb, Tb and Y, have been investigated as candidate materials for fuel cells and other electrochemical applications. The synthesis of these materials was performed using a precursor method based on freeze-drying, which allows a precise control of the homogeneity of the ceramic powders. Dense ceramic pellets were obtained at 1400 °C under identical sintering conditions. The microstructure of the ceramics exhibits similar features with relative density higher than 95% and the grain size decreasing as the ionic radius of the dopant decreases. Impedance spectroscopy measurements were performed to study separately the different contributions to the total conductivity. The bulk, grain boundary and total conductivities depend on the ionic radius of the dopant, reaching a maximum for Gd-doped samples with a value of 0.02 S cm⁻¹ for the total conductivity at 600 °C.     

Initialization of a methane-fueled single-chamber solid-oxide fuel cell with NiO + SDC anode and BSCF + SDC cathode

The initialization of an anode-supported single-chamber solid-oxide fuel cell, with NiO + Sm_0.2Ce_0.8O_1.9 anode and Ba_0.5Sr_0.5Co_0.8Fe_0.2O_3−δ + Sm_0.2Ce_0.8O_1.9 cathode, was investigated. The initialization process had significant impact on the observed performance of the fuel cell. The in situ reduction of the anode by a methane–air mixture failed. Although pure methane did reduce the nickel oxide, it also resulted in severe carbon coking over the anode and serious distortion of the fuel cell. In situ initialization by hydrogen led to simultaneous reduction of both the anode and cathode; however, the cell still delivered a maximum power density of ∼350 mW cm⁻², attributed to the re-formation of the BSCF phase under the methane–air atmosphere at high temperatures. The ex situ reduction method appeared to be the most promising. The activated fuel cell showed a peak power density of ∼570 mW cm⁻² at a furnace temperature of 600 °C, with the main polarization resistance contributed from the electrolyte.     

Combination of CO2 reforming and partial oxidation of methane to produce syngas over Ni/SiO2 and Ni–Al2O3/SiO2 catalysts with different precursors

Ni/SiO₂ and Ni–Al₂O₃/SiO₂ catalysts were prepared by incipient wetness impregnation using citrate and nitrate precursors and tested with a reaction of combination of CO₂ reforming and partial oxidation of methane to produce syngas (H₂/CO). The catalytic activity of Ni/SiO₂ and Ni–Al₂O₃/SiO₂ greatly depended on interaction between NiO and support. NiO strongly interacted with support formed small nickel particles (about 4 nm for NiSC which is abbreviation of Ni/SiO₂ prepared with Nickel citrate precursor) after reduction. The small nickel particles over NiSC catalysts exhibited a good catalytic performance.     

Stability improvements of Ni/α-Al2O3 catalysts to obtain hydrogen from methane reforming

Ni catalysts supported on commercial α-Al₂O₃ modified by addition of CeO₂ and/or ZrO₂ were prepared in the present work. Since the principal objective was to evaluate the behavior of these systems and the support effect on the stability, methane reforming reactions were studied with steam, carbon dioxide, partial oxidation and mixed reforming. Results show that catalysts supported on Ce–Zr–α-Al₂O₃ composites present better reforming activity and stability noticeably higher than in the case of the reference support. With respect to composites, the presence of mixed oxides of Ce_xZr_1−xO₂ type facilitates the formation of active phases with higher interaction. This fact reduces the deactivation by sintering conferring to the system a higher contribution of adsorbed oxygen species, favoring the deposited carbon elimination. These improvements resulted in being dependent on the Ce:Zr ratio of the composite, thus obtaining more stable catalysts for Ce:Zr = 4:1 ratios.     

Effect of V substitution at B-site on the physicochemical and electrocatalytic properties of spinel-type NiFe2O4 towards O2 evolution in alkaline solutions

Spinel-type ternary transition metal oxides of nickel, iron and vanadium with composition NiFe_2−xV_xO₄ (0 ≤ x ≤ 1) have been synthesized by a hydroxide precipitation method and investigated for their physicochemical and electrochemical catalytic properties towards the oxygen evolution reaction (OER) in alkaline solutions at 25 °C. The OER study indicates that substitution of V from 0.25 to 1.0 mol for Fe in the spinel matrix increases the electrocatalytic activity of the oxide greatly; the activity being the greatest with x = 0.5 mol. At low overpotentials, the Tafel slope and the order for the OER with respect to OH⁻ concentration were found to be ∼40 mV and ∼2, respectively.