Ceria-based oxides as supports for LaCoO3 perovskite; catalysts for total oxidation of VOC

Supported LaCoO₃ perovskites with 10 and 20 wt.% loading were obtained by wet impregnation of different Ce_1−xZr_xO₂ (x = 0–0.3) supports with a solution prepared from La and Co nitrates, and citric acid. Supports were also prepared using the “citrate method”. All materials were calcined at 700 °C for 6 h and investigated by N₂ adsorption at −196 °C, XRD and XPS. XRD patterns and XPS measurements evidenced the formation of a pure perovskite phase, preferentially accumulated at the outer surface. These materials were comparatively tested in benzene and toluene total oxidation in the temperature range 100–500 °C. All catalysts showed a lower T₅₀ than the corresponding Ce_1−xZr_xO₂ supports. Twenty weight percent LaCoO₃ catalysts presented lower T₅₀ than bulk LaCoO₃. In terms of reaction rates per mass unit of perovskite calculated at 300 °C, two facts should be noted (i) the activity order is more than 10 times higher for toluene and (ii) the reverse variation with the loading as a function of the reactant, a better activity being observed for low loadings in the case of benzene. For the same loading, the support composition influences drastically the oxidative abilities of LaCoO₃ by the surface area and the oxygen mobility.     

Effect of LaCoO3 perovskite deposition on ceria-based supports on total oxidation of VOC

Supported LaCoO₃ perovskites with 10 wt.% loading were prepared by impregnation of different supports containing ceria with a solution of La and Co nitrates and citric acid. All precursors were calcined at 700 °C for 5 h. XRD investigations indicated the perovskite formation via “citrate” precursor only on ceria support. All catalysts were tested for toluene total oxidation in the temperature range 100–600 °C. In spite of a large surface area, alumina-supported perovskites showed a lower global activity. It appears then the necessity of the presence of a perovskite phase for good oxidative activity. In terms of reaction rates higher reaction rates per perovskite weight were observed for all supported catalysts when compared to bulk LaCoO₃.     

Partial oxidation of methane over rhodium catalysts for power generation applications

The partial oxidation of methane (POM) to syngas, i.e. H₂ and CO, over supported Rh catalysts was investigated at atmospheric pressure. The influence of support material, Rh loading and the presence of water vapor on the methane conversion efficiency and the product gas composition was studied. The catalysts containing ceria in the support material showed the highest activity and formation of H₂ and CO. By increasing the Rh loading, a decrease of the ignition temperature was obtained. The addition of water vapor to the reactant gas mixture was found to increase the ignition temperature and the formation of hydrogen, which is favorable for combustion applications where the catalytic POM stage is followed by H₂-stabilized homogeneous combustion.     

Low-temperature total oxidation of methane by pore- and vacancy-engineered NiO catalysts

Designing methane combustion catalysts operated under low temperature (<400°C) remains a huge challenge, especially for noble-metal–free catalytic systems. With NaCl as a crystalline scaffold, NiO catalyst with abundant oxygen vacancies and an ultra-high–specific surface area of 181 m² g⁻¹ is obtained. The mesoporous NiO exhibits outstanding CH₄ combustion performance (T₉₀ = 370°C at the weight hourly space velocity (WHSV) = 20,000 mL g⁻¹ h⁻¹). X-ray photoelectron spectroscopy (XPS), H₂-temperature-programmed reduction (TPR), kinetic measurements, and O¹⁸ isotope-labeling experiments together disclose the key role of surface lattice oxygen and reaction mechanism by NiO catalysts. More importantly, the excellent stability of NiO by doping La was obtained (low-temperature thermal stability: 385°C, 400 h, 4 vol% H₂O).     

Supported nanocomposite catalysts for high-temperature partial oxidation of methane

In order to utilize the vast potential of nanoparticles for industrial catalysis, it is necessary to develop methods to stabilize these particles at realistic technical conditions and to formulate nanoparticle-based catalysts in a way that facilitates handling and reduces health and safety concerns. We have previously demonstrated that metal nanoparticles can be efficiently stabilized by embedding them into a high-temperature stable nanocomposite structure. Building onto these results, we report here on the next step towards a simple, hierarchically structured catalyst via supporting platinum barium-hexaaluminate (Pt-BHA) nanocomposites onto a range of different conventional and novel support structures (monoliths, foams, and felts). The catalysts were characterized via SEM, TEM, XRD, porosimetry, chemisorption, and reactive tests in catalytic partial oxidation of methane to synthesis gas (CPOM), and compared to conventionally prepared Pt-catalysts. In particular silica felt supported Pt-BHA showed excellent activity and selectivity combined with good stability and very low noble metal requirement at the demanding high-temperature conditions of short-contact time CPOM. Overall, we see great potential for these supported nanocomposite catalysts for use in demanding environments, such as high-temperature, high-throughput conditions in fuel processing and similar energy-related applications.     

AFeO3 (A=La, Nd, Sm) and LaFe1−xMgxO3 perovskites as methane combustion and CO oxidation catalysts: structural, redox and catalytic properties

Catalytic methane combustion and CO oxidation were investigated over AFeO₃ (A=La, Nd, Sm) and LaFe_1−xMg_xO₃ (x=0.1, 0.2, 0.3, 0.4, 0.5) perovskites prepared by citrate method and calcined at 1073 K. The catalysts were characterized by X-ray diffraction (XRD). Redox properties and the content of Fe⁴⁺ were derived from temperature programmed reduction (TPR). Specific surface areas (SA) of perovskites were in 2.3–9.7 m² g⁻¹ range. XRD analysis showed that LaFeO₃, NdFeO₃, SmFeO₃ and LaFe_1−xMg_xO₃ (x·0.3) are single phase perovskite-type oxides. Traces of La₂O₃, in addition to the perovskite phase, were detected in the LaFe_1−xMg_xO₃ catalysts with x=0.4 and 0.5. TPR gave evidence of the presence in AFeO₃ of a very small fraction of Fe⁴⁺ which reduces to Fe³⁺. The fraction of Fe⁴⁺ in the LaFe_1−xMg_xO₃ samples increased with increasing magnesium content up to x=0.2, then it remained nearly constant. Catalytic activity tests showed that all samples gave methane and CO complete conversion with 100% selectivity to CO₂ below 973 and 773 K, respectively. For the AFeO₃ materials the order of activity towards methane combustion is La>Nd>Sm, whereas the activity, per unit SA, of the LaFe_1−xMg_xO₃ catalysts decreases with the amount of Mg at least for the catalysts showing a single perovskite phase (x=0.3). Concerning the CO oxidation, the order of activity for the AFeO₃ materials is Nd>La>Sm, while the activity (per unit SA) of the LaFe_1−xMg_xO₃ catalysts decreases at high magnesium content.     

Catalytic oxidation of methane over hexaaluminates and hexaaluminate-supported Pd catalysts

An aqueous (NH₄)₂CO₃ coprecipitation method, based on that of Groppi et al. [Appl. Catal. A 104 (1993) 101–108] was used to synthesize Sr_1−xLa_xMnAl₁₁O₁₉₋ hexaaluminates. These materials were first synthesized by alkoxide hydrolysis. This synthesis route requires special handling of the starting materials and is not likely to be commercially practical. The materials prepared by (NH₄)₂CO₃ coprecipitation have similar surface areas as those prepared by the alkoxide hydrolysis method. Their CH₄ oxidation activity, measured as the temperature needed for 10% conversion of methane, is higher than those prepared by alkoxide hydrolysis. The La-substantiated material, LaMnAl₁₁O₁₉₋, shows high surface area with 19.3 m²/g after calcination at 1400°C for 2 h. It is active for CH₄ oxidation with T_10% at 450°C using 1% CH₄ in air and 70 000 cm³/h g space velocity. The stability and activity of LaMnAl₁₁O₁₉₋ prepared by (NH₄)₂CO₃ coprecipitation method is a simple and important step forward for the application of CH₄ catalytic combustion for gas turbines.     

The nature of active sites for the oxidation of methane on La-based perovskites

Highly crystalline, monophasic LaFeO₃ and LaCoO₃ perovskites, prepared by the explosion method, are shown to be heterogeneous at surface level. The outmost atomic layers of these perovskites contain high concentrations of carbonate-type species. Their specific activities for methane combustion are in fact identical to La₂O₂CO₃ and air-exposed La₂O₃. These results compared with pertinent data from the literature hint that surface heterogeneity may be often present in mixed oxides catalysts.     

Kinetics of methane combustion over CVD-made cobalt oxide catalysts

The present investigation provides the required kinetic parameters to evaluate and to predict the rate of the catalytic combustion of methane over cobalt oxide. For this purpose, monolithic cordierites with low specific surface area were uniformly coated with cobalt oxide thin films of controlled thickness using the chemical vapor deposition (CVD) process. The obtained catalysts were tested in the catalytic combustion of methane in oxygen-deficient and -rich conditions. Catalysts with loadings above 0.46 wt.% are active starting at a temperature of 250 °C and completely convert methane to CO₂ below 550 °C where the conversion rate reaches 35 μmol (CH₄)/g_cat s. The involvement of the bulk-oxide-ions in the catalytic reaction was supported by the constant value of the normalized reaction rate to the weight of deposited cobalt oxide. The experimental data fit well to the Mars–Van Krevelen redox model and can be approximated with a power rate law in oxygen-rich mixtures. The resulting activation energies and frequency factors allow the identification of the rate-limiting step and accurately reproduce the effect of the temperature and partial pressure of the reactants on the specific reaction rate.     

Support effect on complete oxidation of volatile organic compounds over Ru catalysts

Catalytic combustion of ethyl acetate, acetaldehyde, and toluene was investigated on various supported Ru catalysts prepared by the impregnation method, and the effect of reduction treatment on the activity was examined. Among the as-calcined catalysts tested, Ru/CeO₂ showed the highest activity for all tests regardless of the pre-treatment in hydrogen atmosphere. The catalytic activity of Ru/SnO₂ was significantly degraded by the reduction treatment, whereas the activity of Ru/ZrO₂ and Ru/γ-Al₂O₃ was enhanced. To reveal these phenomena, the as-calcined and reduced catalysts were characterized by X-ray diffraction (XRD), X-ray photoelectron spectroscopy (XPS), temperature-programmed reduction (TPR), transmission electron microscopy (TEM), and BET surface area. The dispersion of ruthenium on the supports was evaluated by chemisorption methods of carbon monoxide. The catalytic activity was strongly related to ruthenium species easily oxidizable and reducible at low temperatures. Such ruthenium species were loaded on CeO₂ in a highly dispersed state, resulting in the highest activity.     

AMnO3 (A=La, Nd, Sm) and Sm1−xSrxMnO3 perovskites as combustion catalysts: structural, redox and catalytic properties

Catalytic combustion of methane has been investigated over AMnO₃ (A = La, Nd, Sm) and Sm_1−xSr_xMnO₃ (x = 0.1, 0.3, 0.5) perovskites prepared by citrate method. The catalysts were characterized by chemical analysis, XRD and TPR techniques. Catalytic activity measurements were carried out with a fixed bed reactor at T = 623–1023 K, space velocity = 40 000 N cm³ g⁻¹ h⁻¹, CH₄ concentration = 0.4% v/v, O₂ concentration = 10% v/v.

Specific surface areas of perovskites were in the range 13–20 m² g⁻¹. XRD analysis showed that LaMnO₃, NdMnO₃, SmMnO₃ and Sm_1−xSr_xMnO₃ (x = 0.1) are single phase perovskite type oxides. Traces of Sm₂O₃ besides the perovskite phase were detected in the Sm_1−xSr_xMnO₃ catalysts for x = 0.3, 0.5. Chemical analysis gave evidence of the presence of a significant fraction of Mn(IV) in AMnO₃. The fraction of Mn(IV) in the Sm_1−xSr_xMnO₃ samples increased with x. TPR measurements on AMnO₃ showed that the perovskites were reduced in two steps at low and high temperature, related to Mn(IV) → Mn(III) and Mn(III) → Mn(II) reductions, respectively. The onset temperatures were in the order LaMnO₃ > NdMnO₃ > SmMnO₃. In Sm_1−xSr_xMnO₃ the Sr substitution for Sm caused the formation of Mn(IV) easily reducible to Mn(II) even at low temperature. Catalytic activity tests showed that all samples gave methane complete conversion with 100% selectivity to CO₂ below 1023 K. The activation energies of the AMnO₃ perovskites varied in the same order as the onset temperatures in TPR experiments suggesting that the catalytic activity is affected by the reducibility of manganese. Sr substitution for Sm in SmMnO₃ perovskites resulted in a reduction of activity with respect to the unsubstituted perovskite. This behaviour was related to the reduction of Mn(IV) to Mn(II), occurring under reaction conditions, hindering the redox mechanism.     

Pd catalysts supported on silicon nitride for the combustion of methane: Influence of the crystalline and amorphous phases of the support and of the preparation method on the catalytic performances

Non-oxide refractory materials, such as silicon nitride having high thermal stability and thermal conductivity can be used as catalytic supports. The influence of the Si₃N₄ support nature and of the chemical compounds used for preparations on the physical-chemistry and catalytic properties of the palladium systems in the total oxidation of methane was investigated. A strong influence of the phase composition and the crystalline state of supports on the catalytic properties in the total oxidation of methane of the Pd catalysts was found. The activity of Pd catalysts increases with the -Si₃N₄ content and crystallization state of the support. The catalytic activity of Pd/-Si₃N₄ is also strongly affected by the preparation procedure. The Pd/-Si₃N₄ catalyst obtained by aqueous impregnation is less active and less stable. It was proposed that if water is used as an impregnation solvent, the surface acid-based properties of Si₃N₄ support and/or of the Pd active phase are irreversibly damaged. Pd supported on -Si₃N₄, prepared by impregnation of the Pd precursors in toluene solutions are found to be the most active and stable under reaction conditions.     

CO, H2 or C3H8 assisted catalytic combustion of methane over supported LaMnO3 monoliths

The effect of the addition of a second fuel such as CO, C₃H₈ or H₂ on the catalytic combustion of methane was investigated over ceramic monoliths coated with LaMnO₃/La-γAl₂O₃ catalyst. Results of autothermal ignition of different binary fuel mixtures characterised by the same overall heating value show that the presence of a more reactive compound reduces the minimum pre-heating temperature necessary to burn methane. The effect is more pronounced for the addition of CO and very similar for C₃H₈ and H₂. Order of reactivity of the different fuels established in isothermal activity measurements was: CO>H₂≥C₃H₈>CH₄. Under autothermal conditions, nearly complete methane conversion is obtained with catalyst temperatures around 800 °C mainly through heterogeneous reactions, with about 60–70 ppm of unburned CH₄ when pure methane or CO/CH₄ mixtures are used. For H₂/CH₄ and C₃H₈/CH₄ mixtures, emissions of unburned methane are lower, probably due to the proceeding of CH₄ homogeneous oxidation promoted by H and OH radicals generated by propane and hydrogen pyrolysis at such relatively high temperatures.

Finally, a steady state multiplicity is found by decreasing the pre-heating temperature from the ignited state. This occurrence can be successfully employed to pilot the catalytic ignition of methane at temperatures close to compressor discharge or easily achieved in regenerative burners.     

钙钛矿复合氧化物的介孔化及用于CH4催化燃烧

利用有序介孔立方相(Ia3d)乙烯基三乙氧基硅烷(TEVS)为模扳,通过纳米组装法在硅基硬模板的介孔中填入La - Fe -柠檬酸络合物,经焙烧和碱洗去除硅基模板,制备具有有序介孔结构且高比表面积的LaFeO3钙钛矿(ABO3),并与软模板法和自燃烧法制备的LaFeO3钙钛矿催化荆作对比,运用XRD、TEM和BET对合...     

Rh–La(Mn,Co)O3 monolithic catalysts for the combustion of methane under fuel-rich conditions

Novel Rh–La(Mn,Co)O₃ structured catalysts were developed for the partial oxidation of methane to syngas intended as a preliminary conversion step in combustion systems such as power turbines and utility burners employing a fuel-rich fuel-lean approach to reduce NO_x formation. Active components were impregnated on La–γ-Al₂O₃ washcoated honeycomb monoliths and the catalysts were characterised by BET, SEM/EDS, H₂-TPR, and in situ FT-IR under reaction conditions. Catalytic partial oxidation of methane was tested under both pseudo-isothermal and pseudo-adiabatic conditions showing that the process can be conducted with high yield and selectivity: improved and stable performances were found especially in the case of Rh–LaMnO₃ catalyst, due to the synergism between active sites and to the stabilization of the noble metal.     

La–Ag–Co perovskites for the catalytic flameless combustion of methane

Ag represents an interesting dopant for the highly active LaCoO₃ perovskites used for the catalytic flameless combustion (CFC) of methane, due to its ability to adsorb and activate oxygen and to the possibility of incorporation into the framework as Ag⁺ or Ag²⁺, with formation of oxygen vacancies. In the present work we compared the catalytic activity and resistance to sulphur poisoning of a series of LaCoO₃, x%Ag/LaCoO₃, La_1−xAg_xCoO₃ samples (nominal composition), the latter two notations indicating post-synthesis Ag loading or direct incorporation during the synthesis, respectively. The samples were prepared by flame pyrolysis (FP) and by the sol–gel (SG) method, leading to different particle size and possibly to different incorporation degree of the dopant, quantified by Rietveld refinement of XRD patterns.Higher activity was observed, in general, with fresh catalysts synthesised by FP. The SG samples demonstrated a slightly better resistance to sulphur poisoning when considering the conversion decrease between the fresh and the poisoned samples, due to lower surface exposure. However, interesting data have been obtained with some of the Ag-doped poisoned FP samples, performing even better than the fresh SG-prepared ones.Ag addition led to a complex change of activity and resistance to poisoning. The activity of FP-prepared samples doped with a small amount of Ag (e.g. 5 mol%) was indeed lower than that of the undoped LaCoO₃. By contrast, a further increase of Ag concentration led to increasing catalytic activity, mainly when big extraframework Ag particles were present. By contrast, for SG samples a low Ag amount was beneficial for activity, due to an increased reducibility of Co³⁺.     

Preparation by flame spray pyrolysis of ABO3±δ catalysts for the flameless combustion of methane

Perovskitic mixed oxides prepared through flame-spray pyrolysis possess a good stability in high-temperature application, viz. the catalytic flameless combustion of methane. Some preparation operating parameters are here analysed, such as O₂ pressure drop along the spraying nozzle, O₂ discharge velocity and flow rate. These parameters have been correlated with specific surface area, activity and durability of the prepared samples, as well as with flame temperature, varied by using different fuel mixtures. It was found that specific surface area increases with increasing O₂ velocity and flow rate and with decreasing the combustion enthalpy of the solvent mixture. This reflects on both activity and durability of the catalyst.     

Performance and characterization of supported metal catalysts for complete oxidation of formaldehyde at low temperatures

Activities of a series of metals (Pt, Pd, Rh, Cu, Mn) supported on TiO₂ were investigated for the catalytic oxidation of formaldehyde. Among them, Pt/TiO₂ was found to be the most promising catalyst. Nitrogen adsorption, hydrogen chemisorption, X-ray diffraction (XRD), transmission electron microscopy (TEM) and temperature programmed reduction (TPR) by H₂ were used to characterize the platinum catalysts. Using Ce_0.8Zr_0.2O₂, Ce_0.2Zr_0.8O₂, SiO₂ as supports instead of TiO₂, the activity sequence of 0.6 wt.% platinum with respect to the supports is TiO₂ > SiO₂ > Ce_0.8Zr_0.2O₂ > Ce_0.2Zr_0.8O₂, and this appears to be correlated with the dispersion of platinum on supports rather than the specific surface areas of the catalysts. Platinum loading on TiO₂ has a great effect on the catalytic activity, and 0.6 wt.% Pt/TiO₂ catalyst was observed to be the most active, which could be attributed to the well-dispersed platinum surface phase. The reduction temperature greatly affects the particle size and, consequently, the catalytic activity. The smaller particle size of platinum, due to its high dispersion on support, has a positive effect on catalytic performance. Increasing formaldehyde concentration and space velocity exhibits an inhibiting effect on the catalytic activity.     

Cobalt-containing catalysts for the high-temperature combustion of methane

Cobalt was supported on ZrO₂, La-doped ZrO₂ and La₂O₃ through atomic layer epitaxy (ALE) and wet impregnation. The rate data obtained at 770 K is compared with literature information about cobalt inserted in other matrixes. The ALE technique using ZrO₂ and La-doped ZrO₂ yielded the best cobalt-containing catalysts. Bulk and surface characterization techniques provided key clues to understand the origin of the large difference in catalytic activity reported for cobalt-containing formulations.     

负载型臭氧氧化催化剂研究进展

多相催化臭氧化技术是一种高效的污水净化技术。其中负载型臭氧氧化催化剂因其催化效果好、易分离、重复率用率高等优点而成为研究热点之一。依据不同的载体类型,将负载型臭氧氧化催化剂进行归纳分类,并综述了不同载体的催化剂在催化臭氧氧化水处理中的最新应用。