Performance of La1−xSrxAl1−y−y′FeyMgy′O3−δ perovskites in methane combustion: effect of aluminum and magnesium content

Perovskites of different La_1−xSr_xAl_1−y−y′Fe_yMg_y′O_3−δ compositions (x=0, 0.1, 0.15, 0.2 and y=0.1, 0.3, 0.5, 0.8) were prepared from a reactive precursor slurry of hydrated oxides. Each sample was aged between 16 and 26 h up to 1473 K. Activity in methane combustion (1%/air) was determined in a plug-flow reactor, with 1 g catalyst and 24 l/h flowrate. Gradual decrease in activity due to thermal aging was observed, the degree of activity loss being composition dependent. Nevertheless, activity of samples aged at 1370 K was nearly independent of composition. The best thermal stability showed LaAl_0.65Fe_0.15Mg_0.2O₃ perovskite. None of the magnesium substituted perovskites performed better than a La_0.85Sr_0.15Al_0.87Fe_0.13O₃ reference sample.     

La(1−x)SrxCo1−yFeyO3 perovskites prepared by sol–gel method: Characterization and relationships with catalytic properties for total oxidation of toluene

La_(1−x)Sr_xCo_(1−y)Fe_yO₃ samples have been prepared by sol–gel method using EDTA and citric acid as complexing agents. For the first time, Raman mappings were achieved on this type of samples especially to look for traces of Co₃O₄ that can be present as additional phase and not detect by XRD. The prepared samples were pure perovskites with good structural homogeneity. All these perovskites were very active for total oxidation of toluene above 200 °C. The ageing procedure used indicated good thermal stability of the samples. A strong improvement of catalytic properties was obtained substituting 30% of La³⁺ by Sr²⁺ cations and a slight additional improvement was observed substituting 20% of cobalt by iron. Hence, the optimized composition was La_0.7Sr_0.3Co_0.8Fe_0.2O₃. The samples were also characterized by BET measurements, SEM and XRD techniques. Iron oxidation states were determined by Mössbauer spectroscopy. Cobalt oxidation states and the amount of O⁻ electrophilic species were analyzed from XPS achieved after treatment without re-exposition to ambient air. Textural characterization revealed a strong increase in the specific surface area and a complete change of the shape of primary particles substituting La³⁺ by Sr²⁺. The strong lowering of the temperature at conversion 20% for the La_0.7Sr_0.3Co_(1−y)Fe_yO₃ samples can be explained by these changes. X photoelectron spectra obtained with our procedure evidenced very high amount of O⁻ electrophilic species for the La_0.7Sr_0.3Co_(1−y)Fe_yO₃ samples. These species able to activate hydrocarbons could be the active sites. The partial substitution of cobalt by iron has only a limited effect on the textural properties and the amount of O⁻ species. However, Raman spectroscopy revealed a strong dynamic structural distortion by Jahn–Teller effect and Mössbauer spectroscopy evidenced the presence of Fe⁴⁺ cations in the iron containing samples. These structural modifications could improve the reactivity of the active sites explaining the better specific activity rate of the La_0.7Sr_0.3Co_0.8Fe_0.2O₃ sample. Finally, an additional improvement of catalytic properties was obtained by the addition of 5% of cobalt cations in the solution of preparation. As evidenced by Raman mappings and TEM images, this method of preparation allowed to well-dispersed small Co₃O₄ particles that are very efficient for total oxidation of toluene with good thermal stability contrary to bulk Co₃O₄.     

Surface properties and catalytic performance of La1−xSrxCrO3 perovskite-type oxides for CO and C3H6 combustion

Catalytic CO oxidation and C₃H₆ combustion have been studied over La_1−xSr_xCrO₃ (x = 0.0–0.3) oxides prepared by solid-state reaction and characterised by X-ray diffraction (XRD), nitrogen adsorption (BET analysis) and X-ray photoelectron spectroscopy (XPS). The expected orthorhombic perovskite structure of the chromite is observed for all levels of substitution. However, surface segregation of strontium along with a chromium oxidation process, leading to formation of Cr⁶⁺-containing phases, is produced upon increasing x and shown to be detrimental to the catalytic activity. Maximum activity is achieved for the catalyst with x = 0.1 in which mixed oxide formation upon substitution of lanthanum by strontium in the chromite becomes maximised.     

Effects of iron and cerium in La1−yCeyCo1−xFexO3 perovskites as catalysts for VOC oxidation

Perovskite-type mixed oxides La_1−yCe_yCo_1−xFe_xO₃ with high specific surface area were prepared by reactive grinding. These catalysts were characterized by N₂ adsorption, X-ray diffraction, oxygen storage capacity (OSC), H₂-temperature-programmed reduction (TPR-H₂), O₂-, and CH₃OH-temperature-programmed desorption (TPD). The catalytic performance of the samples for volatile organic compounds (VOC), CH₃OH, CO and CH₄ oxidation was evaluated. Cerium allows an enhancement of the reducibility of the B-site cations in perovskite structure during OSC and TPR-H₂ and an increase in the amount of β-O₂ desorbed during TPD-O₂. As opposed to cerium, the addition of iron in the perovskite structure causes a drop in B-site cations reducibility and a decrease of the oxygen mobility in the bulk. As a consequence, the catalytic activity in VOC oxidation is enhanced by introduction of cerium and weakened by iron in the lattice.     

Catalytic combustion of CH4 and CO on La1−xMxMnO3 perovskites

The activities of perovskites depend on compositions and preparation methods. Various perovskites, La_1−xM_xMnO₃ (M=Ag, Sr, Ce, La), have been prepared by two different methods (co-precipitation and spray decomposition). The new preparation method, spray decomposition, produced perovskites of a high surface area of over 10 m²/g. The catalytic activities for CH₄ and CO oxidation have been studied on a series of catalysts, La_1−xM_xMnO₃. The perovskite-type oxide, La_0.7Ag_0.3MnO₃, shows the highest catalytic activity: the complete conversion of CO and CH₄ at 370 and 825 K, respectively.     

NO decomposition and reduction by CH4 over Sr/La2O3

Both NO decomposition and NO reduction by CH₄ over 4%Sr/La₂O₃ in the absence and presence of O₂ were examined between 773 and 973 K, and N₂O decomposition was also studied. The presence of CH₄ greatly increased the conversion of NO to N₂ and this activity was further enhanced by co-fed O₂. For example, at 773 K and 15 Torr NO the specific activities of NO decomposition, reduction by CH₄ in the absence of O₂, and reduction with 1% O₂ in the feed were 8.3·10⁻⁴, 4.6·10⁻³, and 1.3·10⁻² μmol N₂/s m², respectively. This oxygen-enhanced activity for NO reduction is attributed to the formation of methyl (and/or methylene) species on the oxide surface. NO decomposition on this catalyst occurred with an activation energy of 28 kcal/mol and the reaction order at 923 K with respect to NO was 1.1. The rate of N₂ formation by decomposition was inhibited by O₂ in the feed even though the reaction order in NO remained the same. The rate of NO reduction by CH₄ continuously increased with temperature to 973 K with no bend-over in either the absence or the presence of O₂ with equal activation energies of 26 kcal/mol. The addition of O₂ increased the reaction order in CH₄ at 923 K from 0.19 to 0.87, while it decreased the reaction order in NO from 0.73 to 0.55. The reaction order in O₂ was 0.26 up to 0.5% O₂ during which time the CH₄ concentration was not decreased significantly. N₂O decomposition occurs rapidly on this catalyst with a specific activity of 1.6·10⁻⁴ μmol N₂/s m² at 623 K and 1220 ppm N₂O and an activation energy of 24 kcal/mol. The addition of CH₄ inhibits this decomposition reaction. Finally, the use of either CO or H₂ as the reductant (no O₂) produced specific activities at 773 K that were almost 5 times greater than that with CH₄ and gave activation energies of 21–26 kcal/mol, thus demonstrating the potential of using CO/H₂ to reduce NO to N₂ over these REO catalysts.     

The effects of La2O3 on the structural properties of La2O3–Al2O3 prepared by the sol–gel method and on the catalytic performance of Pt/La2O3–Al2O3 towards steam reforming and partial oxidation of methane

The effect of La₂O₃ content on the structural properties and catalytic behavior of Pt/xLa₂O₃–Al₂O₃ catalysts in steam reforming of methane and partial oxidation of methane was investigated. There was a decrease in the density of Pt sites with the increase of La₂O₃ loadings according to Fourier transform infrared spectroscopy of adsorbed CO and to dehydrogenation of cyclohexane results. However, transmission electron microscopy data indicates an opposite trend. This apparent disagreement could be due to the partial coverage of Pt sites by LaO_x species. CH₄ turnover rates and specific rates of steam reforming of methane increased for higher La₂O₃ loadings. The Pt/Al₂O₃ catalyst was strongly deactivated during partial oxidation of methane, while La₂O₃-containing catalysts exhibited higher stability. The increase of activity observed during the reactions was ascribed to the ability of the [LaPt_xO]Pt⁰-like species to promote the gasification of coke. This cleaning mechanism led to higher accessibility of the active sites to CH₄.     

Preparation and characterization of LaFe1−xMgxO3/Al2O3/FeCrAl: Catalytic properties in methane combustion

MnO_x–CeO₂ mixed oxides prepared by sol–gel method, coprecipitation method and modified coprecipitation method were investigated for the complete oxidation of formaldehyde. Structure analysis by H₂-TPR and XPS revealed that there were more Mn⁴⁺ species and richer lattice oxygen on the surface of the catalyst prepared by the modified coprecipitation method than those of the catalysts prepared by sol–gel and coprecipitation methods, resulting in much higher catalytic activity toward complete oxidation of formaldehyde. The effect of calcination temperature on the structural features and catalytic behavior of the MnO_x–CeO₂ mixed oxides prepared by the modified coprecipitation was further examined, and the catalyst calcined at 773 K showed 100% formaldehyde conversion at a temperature as low as 373 K. For the samples calcined below 773 K, no any diffraction peak corresponding to manganese oxides could be detected by XRD measurement due to the formation of MnO_x–CeO₂ solid solution. While the diffraction peaks corresponding to MnO₂ phase in the samples calcined above 773 K were clearly observed, indicating the occurrence of phase segregation between MnO₂ and CeO₂. Accordingly, it was supposed that the strong interaction between MnO_x and CeO₂, which depends on the preparation route and the calcination temperature, played a crucial role in determining the catalytic activity toward the complete oxidation of formaldehyde.     

Inhibiting and deactivating effects of water on the selective catalytic reduction of nitric oxide with ammonia over MnOx/Al2O3

The effect of water on the selective catalytic reduction (SCR) of nitric oxide with ammonia over alumina supported with 2–15 wt.-% manganese oxide was investigated in the temperature range 385–600 K, with the emphasis on the low side of this temperature window. Studies on the effect of 1–5 vol.-% water vapour on the SCR reaction rate and selectivity were combined with TPD experiments to reveal the influence of water on the adsorption of the single SCR reactants. It turned out that the activity decrease due to water addition can be divided into a reversible inhibition and an irreversible deactivation. Inhibition is caused by molecular adsorption of water. TPD studies showed that water can adsorb competitively with both ammonia and nitric oxide. Additional kinetic experiments revealed that adsorbed ammonia is present in excess on the catalyst surface, even in the presence of water. Reduced nitric oxide adsorption is responsible for the observed reversible decrease in the reaction rate; the fractional reaction order changes from 0.79 in the absence of water to 1.07 in its presence. Deactivation is probably due to the dissociative adsorption of water, resulting in the formation of additional surface hydroxyls. As the amount of surface hydroxyls formed is limited to a saturation level, the deactivating effect on the catalyst is limited too. The additional hydroxyls condense and desorb in the temperature range 525–775 K, resulting in a lower degree of deactivation at higher temperature. A high temperature treatment at 775 K results in a complete regeneration. The amount of surface hydroxyls formed per unit surface area decreases at increasing MnO_x-loading. The selectivity to the production of nitrogen is enhanced significantly by the presence of gas phase water.     

Electrical resistivity and thermopower of (La1−xSrx)MnO3 and (La1−xSrx)CoO3 at elevated temperatures

Electrical resistivity and Seebeck (S) measurements were performed on (La_1−xSr_x)MnO₃ (0.02x0.50) and (La_1−xSr_x)CoO₃ (0x0.15) in air up to 1073 K. (La_1−xSr_x)MnO₃ (x0.35) showed a metal-to-semiconductor transition; the transition temperature almost linearly increased from 250 to 390 K with increasing Sr content. The semiconductor phase above the transition temperature showed negative values of S. (La_1−xSr_x)CoO₃ (0x0.10) showed a semiconductor-to-metal transition at 500 K. Dominant carriers were holes for the samples of x0.02 above room temperature. LaCoO₃ showed large negative values of S below ca. 400 K, indicative of the electron conduction in the semiconductor phase.     

Catalytic oxidation of nitrogen monoxide over La1−xCexCoO3 perovskites

This paper presents an investigation on the NO oxidation properties of perovskite oxides. La_1−xCe_xCoO₃ (x = 0, 0.05, 0.1, 0.2, 0.3, 0.4) perovskite-type oxides were synthesized through a citrate method and characterized by XRD, BET and XPS. The catalytic activities were enhanced significantly with Ce substitution, and achieved the best when x was 0.2, but decreased at higher x values. The performed characterizations reveal that the adsorbed oxygen on the surface plays an important role in the oxidation of NO into NO₂. The surface compounds after the co-adsorption of NO and O₂ at room temperature, were investigated by DRIFTS and TPD experiments. Three species: the bridging nitrate, the hyponitrites and the monodentate nitrate, were formed on the surface. The order of thermal stabilities was as follows: monodentate > hyponitrite > bridging. Among them, only the monodentate nitrate which decomposed at above 300 °C, would desorb NO₂ into the gas phase. When Ce was added, the temperature of monodentate nitrate desorption became low and the adsorption of the other two species decreased. This might be related to the oxidation state of Co on the surface. Analysis by synthesizing the characterization results and catalytic activity data shows that large amounts of adsorbed oxygen, small amount of inactive compounds on the surface and low NO₂ desorption temperature are favorable for the oxidation of NO.     

Decomposition of nitric oxide over perovskite oxide catalysts: effect of CO2, H2O and CH4

Direct nitric oxide decomposition over perovskites is fairly slow and complex, its mechanism changing dramatically with temperature. Previous kinetic study for three representative compositions (La_0.87Sr_0.13Mn_0.2Ni_0.8O_3−δ, La_0.66Sr_0.34Ni_0.3Co_0.7O_3−δ and La_0.8Sr_0.2Cu_0.15Fe_0.85O_3−δ) has shown that depending on the temperature range, the inhibition effect of oxygen either increases or decreases with temperature. This paper deals with the effect of CO₂, H₂O and CH₄ on the nitric oxide decomposition over the same perovskites studied at a steady-state in a plug-flow reactor with 1 g catalyst and total flowrates of 50 or 100 ml/min of 2 or 5% NO. The effect of carbon dioxide (0.5–10%) was evaluated between 873 and 923 K, whereas that of H₂O vapor (1.6 or 2.5%) from 723 to 923 K. Both CO₂ and H₂O inhibit the NO decomposition, but inhibition by CO₂ is considerably stronger. For all three catalysts, these effects increase with temperature. Kinetic parameters for the inhibiting effects of CO₂ and H₂O over the three perovskites were determined. Addition of methane to the feed (NO/CH₄=4) increases conversion of NO to N₂ about two to four times, depending on the initial NO concentration and on temperature. This, however, is still much too low for practical applications. Furthermore, the rates of methane oxidation by nitric oxide over perovskites are substantially slower than those of methane oxidation by oxygen. Thus, perovskites do not seem to be suitable for catalytic selective NO reduction with methane.     

Direct NO decomposition over La2−xBaxNiO4 catalysts containing BaCO3 phase

Catalytic and in situ Fourier Transform Infrared (FTIR) spectroscopic studies were conducted to investigate the adsorption and oxidation of o- and p-chlorophenol over a 3.6 wt.% V₂O₅/TiO₂ catalyst. At a space velocity of approximately 53,000 cm³ g⁻¹ h⁻¹, this catalyst was found to be active for the oxidation of o- and p-chlorophenol at temperatures as low as 200 °C, yielding CO₂ and HCl as the main products. Trace amounts of higher molecular weight products were also detected at the reactor outlet indicating the operation of additional condensation, coupling and chlorination/dechlorination side reactions in parallel to the main complete oxidation scheme. The in situ FTIR studies revealed that different phenols adsorb on the V₂O₅/TiO₂ catalyst through their hydroxyl group. Furthermore, the formation of similar surface species (i.e., maleates, acetates, formates and an aldehyde-type species) was observed. The results were compared with those of previous studies on the oxidation of m-dichlorobenzene (m-DCB) and benzene and suggest that a similar reaction mechanism is operating in all cases, although the relative kinetic significance of the different steps varies with the presence and the position of the hydroxyl and chlorine groups on the aromatic ring.     

三元复合钙基材料CaO-Ca3Al2O6-MgO的合成及其CO2吸附性能

采用Al₂O₃和MgO同时掺杂改性的方法制备了CaO-Ca₃Al₂O₆-MgO复合钙基高温吸附CO₂材料。复合钙基材料孔隙发达,活性物相为CaO,惰性骨架物相为Ca₃Al₂O₆和MgO。Ca₃Al₂O₆/MgO质量比偏小的材料,表面微粒粒径较小。在10%（体积分数,下同）CO₂和90% N₂的混合气气氛下,采用热重分析仪测量了复合钙基材料吸附CO₂容量、碳化反应速率以及循环碳化（670℃）/煅烧（900℃）过程的稳定性。结果发现,复合钙基材料CaO-Ca₃Al₂O₆-MgO具有较好的吸附CO₂性能,提高Ca₃Al₂O₆/MgO质量比,合成材料的循环稳定性较好;降低Ca₃Al₂O₆/MgO质量比,合成材料的碳化反应速率加快,CaO转化率提高。最后,通过对不同循环次数下复合钙材料的比表面积、孔径分布、微观形貌、表面元素分布,晶相、晶粒大小进行研究分析,对合成材料的失活以及掺杂物质对烧结的抑制机理进行了讨论。     

Selective catalytic reduction of NO by hydrocarbons on Ga2O3/Al2O3 catalysts

Catalytic properties of supported gallium oxides have been examined for the selective reduction of NO by CH₄ in excess oxygen. The activity was greatly affected by the support; Ga₂O₃/Al₂O₃ (Al₂O₃ supported Ga₂O₃) and Ga₂O₃–Al₂O₃ mixed oxide exhibited high activity and selectivity as comparable to Ga-ZSM-5, while unsupported Ga₂O₃ and the other supported Ga₂O₃ were ineffective. For Ga₂O₃/Al₂O₃, the activity changed with Ga₂O₃ content, and was highest at about 30 wt% Ga₂O₃, which corresponds to a theoretical monolayer coverage. Gallium oxide highly dispersed on Al₂O₃ is considered to be responsible for the high activity and selectivity. The reaction characteristics of Ga₂O₃/Al₂O₃ were studied and compared with Ga-ZSM-5 and Co-ZSM-5. Ga₂O₃/Al₂O₃ exhibited the highest activity and selectivity at high temperature. In addition, Ga₂O₃/Al₂O₃ showed higher tolerance against water than Ga-ZSM-5. C₃H₈ and C₃H₆ were also evaluated as reducing agents, and Ga₂O₃/Al₂O₃ showed higher activity than Ga-ZSM-5 above 723 K achieving almost complete reduction of NO to N₂.     

Sol-gel synthesis of BaTiO3 and Ba1−x−yCaySrx(ZryTi1−y)O3 perovskite powders

A BaTiO₃ powder has been prepared by the sol-gel process from the hydrolysis of a solution of barium acetate and titanium ethylate in the presence of acetic acid as a catalyst. Supplementary constituents in the form of Ca(CH₃COO)₂, Zr(OC₃H₇)₄, Sr(NO₃)₂ also can be used. Intermediate phases of barium acetate and barium carbonate have been identified by differential thermal analysis, X-ray diffraction, infra-red and scanning electron microscopy. BaTiO₃ with perovskite structure synthesizes in the temperature range from 600 to 1000°C.     

Synthesis and properties of lanthanum hexaaluminate ceramic La1−xCaxAl11−y−zMgyTizO18

The aim of the present work is to obtain ceramic materials with a hexagonal structure and high density, hardness and mechanical strength at lower synthesis temperature. Ceramic samples with nominal composition La_1−xCa_xAl_11−y−zMg_yTi_zO₁₈ (x=0–1; y=0–3; z=0–3,5) are prepared. The samples are sintered at temperature 1500 °C by one-stage and two-stage ceramic technology. By X-ray diffraction and scanning electron microscopy, predominant phase LaAl₁₁O₁₈ and second phases LaAlO₃ and -Al₂O₃ are identified. Ceramic materials are characterized with high physico-mechanical properties and may be find application for production of mill bodies and materials for immobilization of nuclear waste.     

The promoting effect of Nb2O5 addition to Pd/Al2O3 catalysts on propane oxidation

Pd/Nb₂O₅/Al₂O₃ catalysts were investigated on propane oxidation. Diffuse reflectance spectroscopy (DRS) and X-ray photoelectron spectroscopy (XPS) analysis suggested that monolayer coverage was attained between 10 and 20 wt.% of Nb₂O₅. Temperature programmed reduction (TPR) evidenced the partial reduction of niobium oxide. The maximum propane conversion observed on the Pd/10% Nb₂O₅/Al₂O₃ corresponded to the maximum Nb/Al surface ratio. The presence of NbO_x polymeric structures near to the monolayer could favor the ideal Pd⁰/Pd²⁺ surface ratio to the propane oxidation which could explain the promoting effect of niobium oxide.     

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.     

NO reduction by CH4 in the presence of O2 over La2O3 supported on Al2O3

Dispersing La₂O₃ on δ- or γ-Al₂O₃ significantly enhances the rate of NO reduction by CH₄ in 1% O₂, compared to unsupported La₂O₃. Typically, no bend-over in activity occurs between 500° and 700°C, and the rate at 700°C is 60% higher than that with a Co/ZSM-5 catalyst. The final activity was dependent upon the La₂O₃ precursor used, the pretreatment, and the La₂O₃ loading. The most active family of catalysts consisted of La₂O₃ on γ-Al₂O₃ prepared with lanthanum acetate and calcined at 750°C for 10 h. A maximum in rate (mol/s/g) and specific activity (mol/s/m²) occurred between the addition of one and two theoretical monolayers of La₂O₃ on the γ-Al₂O₃ surface. The best catalyst, 40% La₂O₃/γ-Al₂O₃, had a turnover frequency at 700°C of 0.05 s⁻¹, based on NO chemisorption at 25°C, which was 15 times higher than that for Co/ZSM-5. These La₂O₃/Al₂O₃ catalysts exhibited stable activity under high conversion conditions as well as high CH₄ selectivity (CH₄ + NO vs. CH₄ + O₂). The addition of Sr to a 20% La₂O₃/γ-Al₂O₃ sample increased activity, and a maximum rate enhancement of 45% was obtained at a SrO loading of 5%. In contrast, addition of SO⁼₄ to the latter Sr-promoted La₂O₃/Al₂O₃ catalyst decreased activity although sulfate increased the activity of Sr-promoted La₂O₃. Dispersing La₂O₃ on SiO₂ produced catalysts with extremely low specific activities, and rates were even lower than with pure La₂O₃. This is presumably due to water sensitivity and silicate formation. The La₂O₃/Al₂O₃ catalysts are anticipated to show sufficient hydrothermal stability to allow their use in certain high-temperature applications.