Deactivation Of Oxide Research Articles

Integration of calcium looping and calcium hydroxide thermochemical systems for energy storage and power production in concentrating solar power plants

Energy storage is a key factor in the development of renewables-based electrical power systems. In recent years, the thermochemical energy storage system based on calcium-looping has emerged as an alternative to molten salts for energy storage in high-temperature concentrated solar power plants. This technology still presents some challenges that could be solved by integrating the thermochemical energy storage system based on calcium hydroxide. This work studies a novel concentrated solar power system integrating calcium-looping and calcium hydroxide thermochemical energy storage systems. The results show that the combined use of hydration-dehydration cycles in the calcination-carbonation processes of the calcium looping for energy storage could partially solve the issue related to the multicyclic deactivation of calcium oxide. The improvement in the conversion of calcium oxide during carbonation is demonstrated experimentally when hydration-dehydration cycles are combined. Numerical simulations demonstrate the technical feasibility of the integrated process, with efficiencies ranging between 38-46%, improved with the increase in calcium oxide conversion in the carbonator, showing the potential of the proposed integration.

Enhanced stability and activity for solvent-free selective oxidation of cyclohexane over Cu2O/CuO fabricated by facile alkali etching method

The easy deactivation of cuprous oxide (Cu2O) have significantly limited their practical applications in catalysis. Formation of composites with other semiconductors including CuO is an crucial strategy for stability improvement of Cu2O. However, this strategy developed so far has often focused on only Cu2O thin film systems. Herein, Cu2O/CuO composites were fabricated via a simple alkali etching method, for the first time, to stabilize the powdered Cu2O in liquid-phase selective oxidation of cyclohexane under solvent-free conditions. The etching method presented remarkable high dispersion of copper species and an appropriate Cu+/Cu2+ ratio on catalyst surface, which leads to superior catalytic activity and stability. Hot filtration experiment confirmed that Cu2O/CuO composite was a heterogeneous catalyst which could be reused at least five times without considerable loss of catalytic activity, whereas the pure Cu2O suffered from severe deactivation caused by oxidative decomposition of Cu2O and copper leaching. Moreover, the amount of Cu+ and Cu2+ sites on catalyst surface are dramatically affected by the amount of alkali during etching process and the catalytic performance can be further tuned by regulating the Cu+/Cu2+ ratio. The Cu2O/CuO with a molar ratio of NaOH/Cu2O = 1:1 exhibited the highest catalytic activity with a cyclohexane conversion of 84.3 % and cyclohexanone selectivity of 77.0 %. The present work provides a promising strategy to stabilize Cu2O catalysts in catalytic oxidation systems.

High-pressure rapid hydrogasification of pinewood for methane production using calcium looping concept

Pressurized hydrogasification of biomass is a direct method for methane generation from renewable energy sources, and the calcium looping gasification concept is usually used in the hydrogen production process. In this paper, the calcium looping gasification concept, using a high addition amount of coarse calcium oxide as catalyst, is proposed for biomass hydrogasification. Results indicate that the methane yield increased by 35% while the carbon dioxide fraction decreased from 12 to 5 vol% with a mass ratio of calcium oxide to biomass of 2 at 800 °C. Meanwhile, more naphthalene and light aromatics were observed after calcium oxide addition. The deactivation of calcium oxide in multicycle tests was mainly due to carbon deposition and calcium carbonate formation rather than sintering. The deactivated catalyst could be recovered by calcination in air at 800 °C.

Catalyst Deactivation of a Silica-Supported Bismuth–Molybdenum Complex Oxide and the Related Complex Oxides for the Oxidative Dehydrogenation of 1-Butene to 1,3-Butadiene

This study was an examination of the catalyst deactivation of a silica-supported bismuth-molybdenum complex oxide, and that of catalysts used in the absence of bismuth, for the oxidative dehydrogenation of 1-butene. Due to the detection of deactivation, the molar ratio of 1-butene against oxygen in the reactant gas was adjusted to a ratio similar to that used in industrial processes where reaction temperatures average 100 K higher. Regardless of the presence or absence of bismuth in the catalysts, the conversion of 1-butene was decreased by 6 h on-stream. Both the progress of the coking from the inlet to the outlet of the catalyst and the reduction of molybdenum in the catalysts directly contributed to the deactivation. X-ray photoelectron spectrometry revealed that a greater reduction of molybdenum in the near-surface region and a smaller partial pressure of oxygen (P(O2)) in the reactant gas, although the molybdenum on the surface was not reduced at all. This indicated that the lattice oxygen was pumped from the near-surface region to the surface during the reaction and the oxygen-poor conditions of the near-surface region both in the gas and catalyst phases were formed at a smaller P(O2), which resulted in the enhancements of both the reduction of molybdenum and that of coking. Based on the thermogravimetric analysis, the silica-supported bismuth-molybdenum complex oxide used at P(O2) = 4.1 kPa (color of the catalyst = black) was increased in weight while that used at P(O2) = 16.4 kPa (color of the catalyst = gray) showed a weight decrease, which indicated that the weight decrease caused by the reduction in molybdenum in the near-surface region used at 4.1 kPa was greater than the weight increase from the coking. It was concluded that the reduction in molybdenum followed by the coking on the catalyst surface were the main factors in the catalyst deactivation.

A New Deactivation Mechanism of Sulfate-Promoted Iron Oxide

The deactivation of sulfate-promoted iron oxide in the esterification of acetic acid and n-butanol was studied. The sulfate-promoted iron oxide was used ten runs and 10 h, continually and accumulatively. After ten-run continual use of the catalyst, a considerable deactivation happened to it. The fresh and the deactivated catalysts were compared by means of many characteristic methods including FTIR, XRD, BET, SEM, TG–DSC, and NH3–TPD, to disclose some possible reasons for the deactivation of sulfate-promoted iron oxide in the esterification. Based on the comparative analyses of IR patterns of the fresh catalyst and the deactivated one, a deactivation mechanism is tentatively proposed. Namely, surface sulfate groups, which are originally coordinated to Fe3+ cations and can so induce and generate strong Lewis acidity of Fe3+ cations, may have been gradually turned into free sulfate groups and sulfate esters arisen from strong Lewis-acidic Fe3+ cations’ being hydrolyzed by H2O and their being alcoholyzed by n-butanol, which leads to a gradual destruction of the originally strong coordination between Fe3+ cations and surface sulfate groups, so leading to the acidity degradation of the catalyst, and so finally leading to the deactivation of it. Emphatically, in the proposed mechanism, the water produced from the esterification may play a key role on the deactivation of the catalyst, because it can directly hydrolyze some strong Lewis-acidic Fe3+ cations of the catalyst and indirectly promote the alcoholysis of them, to form weak Lewis-acidic Fe–OH species. The deactivated catalyst has a larger crystallinity, a smaller specific surface area, a smaller sulfate groups content, a weaker acidity than the fresh. All these phenomena, accompanying the deactivation of sulfate-promoted iron oxide, can be interpreted by the proposed deactivation mechanism very well.

Simultaneous removal of NO and SO2 from combustion fuel gases using supported copper oxide catalysts

Nitrogen oxide (NOx) and sulphur dioxide (SO2) emissions produced by the combustion of fuel in stationary sources, such as power stations, industrial heaters or cogeneration plants, cause significant environmental problems. Selective catalytic reduction (SCR) is a well established process for the control of NOx emissions mainly due to its efficiency, selectivity and economics. High removal efficiencies for both NOx and SO2 can be achieved with the use of copper oxide catalysts, as they act as sorbents for the latter - forming copper sulfate - and catalyze the reduction of the former - to N2, in the presence of NH3. An added advantage is that these catalysts are relatively easy to be regenerated under reducing conditions. This study examines the deactivation and regeneration procedures of copper oxide catalysts/sorbents that are supported on Al2O3, SiO2, CeO2-Al2O3, in the presence of SO2 and identifies the appropriate conditions for the simultaneous removal of NO and SO2. The results obtained indicate that copper oxide supported on alumina, ceria-alumina and silica carriers, can be used as effective catalysts for the simultaneous removal of NO and SO2. Furthermore, they can be easily regenerated with a gas mixture of 1 % NH3/Ar at an optimum temperature of 673 K and they retain their initial activity. Thus, the use of regenerative fixed bed catalytic reactor appears as one of the most promising technologies however, further research focusing in new catalytic materials performance is necessary.

Investigation of the Stability of CeO2, V2O5 and CeV Mixed Oxide on the Partial Oxidation of Propane

The cerium–vanadium mixed oxide was synthesized by complexation method. XRD patterns of the (CeV) mixed oxide showed the formation of CeVO4 phase and segregated CeO2. Incorporation of vanadium into the ceria structure improved the redox ability of ceria. The results showed that the activity of the (CeV) mixed oxide was higher than of the vanadium oxide (V2O5) for the partial oxidation of propane. Both catalysts showed similar hydrogen selectivity but significant deactivation with time on stream. Investigation by XRD diffractograms of V2O5 after reaction showed sintering of V2O5 and different oxidation states after reaction, while the crystallites and the structure of the mixed oxide CeVO4 remained unchanged. The deactivation of the mixed oxide (CeV) with time on stream can be explained by insufficient oxygen supply to restore the CeO2 species at the surface. .

Deactivation of iron oxide used in the steam-iron process to produce hydrogen

The deactivation of iron oxide in the steam-iron process is caused by a decrease in surface area of the material. A sintering model is used to derive a relation for the loss in surface area, based on the conversion degree of the material. Furthermore a kinetic model is used to model the increase in grain size and increase in solids fraction in a particle. In the steam-iron process pure hydrogen can be produced from any hydrocarbon feedstock by using a redox cycle of iron oxide. One of the main problems connected to the use of the iron oxide is the inherent structural changes that take place during oxygen loading and unloading leading to severe deactivation. This deactivation reduces the capability of the material for uptake and release of oxygen, basically due to loss of specific surface area. In this paper a simplified (reactive) sintering model is used to derive a relation for the loss in surface area of the material in the first redox cycles. This model is based on the relative conversion and the resulting swelling of the material during oxidation. Furthermore the grainy pellet model is used to describe the increase in grain size and increase in solid fraction in a particle due to the subsequent cycling of the iron oxide in the redox process. Model predictions are compared with redox experiments of H 2 /H 2 O–Fe/Fe 3 O 4 at 800 °C. Grain growth over subsequent cycles could explain the observed deactivation over about 20 cycles satisfactorily.

Effect of additives on redox behavior of iron oxide for chemical hydrogen storage

The redox behaviors of iron oxides, which were modified with Pd, Pt, Rh, Ru, Al, Ce, Ti and Zr as additives, were investigated using temperature-programmed reaction (TPR) technique. The modified iron oxides were prepared by co-precipitation method using urea precipitant. The role of additives was also examined using XRD and SEM analysis in detail. As a result, Pd, Pt, Rh and Ru additives have an effect on promoting the reduction and lowering the re-oxidation temperature of iron oxide. Especially, it is revealed that the effect of Rh species on lowering the reduction temperature is attributed to decrease of activation energy for H 2 reduction according to Fe 2O 3 → Fe 3O 4 course. Meanwhile, Al, Ce, Ti and Zr additives played an important role in prevention of deactivation of iron oxide by repeated redox cycles. Redox performances of iron oxides were also enhanced due to cooperative effects by co-addition of Rh and another species such as Al, Ce and Zr. Finally, Fe–O/(Rh, Ce, Zr) sample exhibited good performance for H 2 evolution by water-splitting through synergistic effect of component additives.

PdO/Al 2O 3 in catalytic combustion of methane: stabilization and deactivation

In the recently developed catalytically assisted combustors for gas turbines using natural gas, deactivation of the palladium oxide (PdO) catalyst needs to be prevented. The effects of additives such as lanthanum and neodymium in PdO/Al 2O 3 on the catalytic durability at 1123 K were studied using a conventional fixed-bed flow reactor at atmospheric pressure. The surface properties of the catalysts were investigated using CO chemisorption, XRD, and TPD after CH 4 adsorption. The catalyst deactivation during CH 4 oxidation followed the equation φ= r 1[1/(1+ α 1 t)] n1 + r 2[1/(1+ α 2 t)] n2 , where r, α and n are constants, subscripts 1 and 2 are the rapid and slow deactivation species, respectively, and t is time on stream. The PdO/Al 2O 3 catalyst was rapidly deactivated by the transformation of PdO to metallic Pd and slowly deactivated by the particle growth of PdO. The addition of Nd 2O 3 and La 2O 3 to PdO/Al 2O 3 prevented the particle growth of PdO as well as the transformation of PdO to Pd up to high temperature.

Ascorbic acid improves postischemic vasodilatation impaired by infusion of soybean oil into canine iliac artery.

This study was conducted to (a) assess postischemic vasodilatation by changes in the vascular cross-sectional area using simultaneous intravascular two-dimensional and Doppler ultrasound before and after the infusion of Intralipid (Pharmacia & Upjohn, Peapack, NJ, U.S.A.); (b) evaluate how antioxidant ascorbic acid modifies the effects of Intralipid on postischemic vasodilatation: and (c) clarify the changes in plasma nitrite and nitrate (NOx-) levels after the infusion of Intralipid with and without ascorbic acid. Twenty-eight mongrel dogs were used to measure for vascular cross-sectional area and average instantaneous peak velocity in the iliac arteries after the 5-min occlusion of the arteries. Postischemic vasodilatation was impaired after the infusion of Intralipid (20%, 2 ml/kg) and this impaired response was reversed by the co-administration of ascorbic acid (30 mg/kg). NG-monomethyl-L-arginine completely abolished postischemic vasodilatation. Plasma NOx levels were significantly reduced after the infusion of Intralipid compared with baseline (11.6+/-0.4 vs. 12.9+/-0.3 microM, p = 0.025) and after infusion of Intralipid with ascorbic acid compared with baseline (11.8+/-0.5 vs. 13.1+/-0.4 microM, p = 0.047). We concluded that ascorbic acid reverses the endothelial dysfunction induced by Intralipid without increasing plasma NOx- levels and that deactivation of nitric oxide by oxidative stress is a primary contributor to Intralipid-induced impaired vasodilation.

Vitamin C in the Prevention of Nitrate Tolerance

To evaluate the efficacy of vitamin C in preventing the development of nitrate tolerance. A MEDLINE search (from 1966 to July 2000) was conducted to identify relevant articles, with additional references obtained from the bibliographies of these articles. One possible mechanism of nitrate tolerance involves superoxide-induced deactivation of nitric oxide, providing the rationale for the use of antioxidants. Most published research concerning deactivation of nitric oxide has involved vitamin C; a summary of this information is presented here. Preliminary studies seem to support the role of vitamin C in attenuating the development of nitrate tolerance. Considering these findings, larger, long-term trials are necessary to further establish the role of vitamin C in this situation.

A study of the deactivation and service life of Ir oxide anodes supported on Al substrates

The deactivation of Ir oxides supported on Al substrates has been studied in 0.5 M H2SO4. Their electrochemical behaviour and service life was also compared to IrO2 electrodes, similarly prepared, supported on Ti. The Ir oxides were prepared by thermal decomposition of an Ir salt precursor solution. The service life and other oxide properties were found to be influenced by different factors used for preparation of the Ir oxide electrodes, for example, the temperature used for the decomposition process and the solution used to etch the Al substrate. In contrast to the IrO2 anodes supported on Ti, the service lives of the IrO2 anodes supported on Al were found to be very short. The deactivation of the latter anodes appears to be related to poor adhesion between the Ir oxide and the Al substrate. However, it was found that the service life of IrO2 anodes supported on Al is increased when a layer consisting of iridium is electrochemically deposited onto the Al substrate prior to the thermal formation of the IrO2.

Catalyst Deactivation during α-Pinene Isomerization. Fe-Mn-Promoted Sulfated Zirconium Oxide

This works studies the deactivation of zirconium oxide, promoted by iron and manganese catalysts (SZFM), during the α-pinene isomerization. Experiments were done in liquid phase by using a batch reactor, and the solid crystalline structure was studied by XRD while the acidity by FTIR. The deactivated catalyst exhibited coke deposits and lost its Bronsted acidity, though it still kept its crystalline structure. The nature of the coke deposit formed was analyzed by infrared spectroscopy (FTIR) as well as by Temperature Programmed Oxidation (TPO). Regeneration was done in air flow, where the temperature required to restore Bronsted acidity and to totally eliminate the coke deposits was of about 873 K.

NiO-Alkaline Earth Oxide Catalysts for Oxidative Methane-to-Syngas Conversion: Influence of Alkaline Earth Oxide on the Surface Properties and Temperature-Programmed Reduction/Reaction by H2and Methane

Temperature-programmed reaction of pure methane from 100–900°C with NiO-MgO, NiO-CaO, NiO-SrO, and NiO-mixed alkaline earth oxides (viz., MgO-CaO, MgO-SrO, MgO-BaO, CaO-SrO, CaO-BaO, and SrO-BaO) with Ni/alkaline earth element(s) ratio of 1.0 (calcined at 930°C for 4 h) has been studied. Temperature-programmed reduction of these catalysts and NiO-MgO with different Ni/Mg ratios by H2from 100°C to 900°C has also been thoroughly investigated. The NiO-CaO, NiO-MgO-CaO, and NiO-MgO catalysts show increasingly superior performance in the oxidative conversion of methane to CO and H2at a very low contact time (1 ms). However, the presence of higher alkaline earth oxide (SrO and/or BaO) causes a large decrease or almost complete deactivation of the NiO-containing alkaline earth oxide(s) catalyst for the reaction. The catalysts before and after reduction (by H2at 500°C) are characterized for their relative surface composition of Ni and alkaline earth element(s) by XPS. Because of the reduction, the surface Ni/alkaline earth(s) ratio for all the catalysts is decreased; the decrease is, however, very large for the NiO-SrO and all the BaO containing catalysts.

H2 selective gas sensor based on SnO2

A highly selective H2 gas sensor was developed based on a ‘hot wire type’ gas sensor commercially available. Tin oxide semiconductor was sintered in a bead (0.5 mm in diameter) covering over a platinum wire (0.02 mm in diameter) coil. The gas sensor was operated by a bridge electric circuit. A dense layer of ≈0.1 mm in depth near the surface of the porous bead, was formed by chemical vapor deposition of hexamethyldisiloxane (HMDS). The dense layer functioned as a ‘molecular sieve’, thereby the diffusion of gases with large molecular diameters, except for H2, was effectively controlled, resulting in a prominent selectivity for H2. On the other hand, diffusion control of O2 caused a strong H2 reduction of the tin oxide in the inner layer covered with the dense layer, resulting in decay in sensor output and irreversible reduction of the hydrogen sensitivity (deactivation of tin oxide). As a countermeasure against the damage, addition effect of cerium oxide from 0.5 to 5.0 at% was investigated. The H2 selective gas sensor thus obtained, had a significantly minor humidity dependence and a prominent long term stability.

Shielding effect of aluminium on sulphur dioxide deactivation of vanadium oxide on titania-alumina DeNOx catalysts

Vanadium oxide supported on a TiO2–Al2O3 mixed oxide prepared by gel-supported precipitation shows enhanced properties of resistance to deactivation by SO2 during the selective removal of NOX with ammonia in flue gases owing to the transfer of sulphate groups to Al sites.

Deactivation of iron oxide—iron phosphate catalysts in isopropanol dehydration

The progressive deactivation of various iron oxide—iron phosphate catalysts in the isopropanol dehydration reaction was studied at atmospheric pressure and 220–280°C. The deactivation of the catalysts is mainly due to coke, rather than the water present. The decay of activity with time on-stream is satisfactorily expressed by a very simple exponential equation, the parameters of which depend on the amount of acid and on temperature. Coke can be easily burnt off, restoring the behaviour of the fresh catalyst. Coke formation follows the Voohries equation.