Metallic monolith supported LaMnO3 perovskite-based catalysts in methane combustion

Metallic monolith supported LaMnO₃ perovskite-based catalysts are characterized by a high activity in methane combustion (95.5% conversion at 745 °C) and by a high thermal resistance. The activity of the catalysts depends on the duration and temperature of LaMnO₃ calcination. The same relation holds for the chemical composition of the catalyst surfaces when they are determined by the XPS method. The shortening of the time of LaMnO₃ perovskite calcination from 12.5 h to 8 h (700 °C) reduces the conversion of methane over a fresh catalyst. This is attributable to the lower amount of manganese (Mn:La = 0.48) on the surface of this catalyst compared to the catalyst whose perovskite was calcined for 12.5 h (Mn:La = 1.8). The extension of calcination time from 8 h to 12.5 h (at 700 °C) brings about a decrease in the specific surface area (SSA) from about 13.7 m²/g to 9.4 m²/g. After approximately 6 h on stream, the activities of the two catalysts become comparable. Aging of the catalyst with an LaMnO₃ active layer at 920 °C for 24 h reduces methane combustion to 82.5% (at 745 °C). The aging process changes the catalyst surface, where Al and C content increases and the Mn:La ratio decreases. The activity of the monolithic LaMnO₃ catalyst rises with the increase in the amount of the active layer from 11.5% to 17.8%. Methane conversion is greater over catalysts with an LaMnO₃ than with an LaCoO₃ active layer, but the LaMnO₃ catalysts show a lower resistance to thermal shocks.     

Catalytic combustion of chlorobenzene on modified LaMnO3 catalysts

Modified LaMnO₃ catalysts with perovskite structure prepared by co-precipitation method were tested in catalytic combustion of chlorobenzene. Characterizations show that the substitutions of different elements for Mn and La species affect significantly surface active oxygen species, H₂ consumption and the ratio of surface oxygen to lattice oxygen of LaMnO₃ catalysts. TOF_SA of LaMnO₃ catalysts is almost proportional to oxygen mobility. LSMO catalyst (substituted by Sr for La) with the largest oxygen mobility presents the highest stable activity which is ascribed to quickly remove the Cl species adsorbed on the surface of catalysts.     

Methane Combustion and CO Oxidation on Zirconia-Supported La,Mn Oxides and LaMnO3 Perovskite

ZrO₂-supported La, Mn oxide catalysts with different La, Mn loading (0.7, 2, 4, 6, 12, and 16 wt% as LaMnO₃) were prepared by impregnation of tetragonal ZrO₂ with equimolar amounts of La and Mn citrate precursors and calcination at 1073 K. The catalysts were characterized by X-ray diffraction (XRD), X-ray absorption spectroscopy (XAS), and BET specific surface area determination. The redox properties were tested by temperature-programmed reduction (TPR), and the catalytic tests were carried out for methane combustion at 650–1050 K and for CO oxidation at 350–800 K. XRD revealed the presence of tetragonal zirconia with traces of the monoclinic phase. LaMnO₃ perovskite was also detected for loading higher than 6%. XAS and TPR experiments suggested that at high loading small crystallites of LaMnO₃, not uniformly spread on the zirconia surface, were formed; while at low loading, La, Mn oxide species interacting with the support, and hard to be structurally defined, prevailed. The catalysis study indicated that the presence of a perovskite-like structure is necessary for the development of highly active sites. Dilute catalysts were in fact poorly active even when considering the activity per gram of La, Mn perovskite-like composition. For methane combustion and CO oxidation, similar trends of the activity as a function of the loading point to a similarity of the active sites for the two reactions on the examined catalytic system.     

Catalytic performance of NO oxidation over LaMeO3 (Me = Mn,Fe, Co) perovskite prepared by the sol–gel method

The catalytic performance of LaMeO₃ (Me = Mn, Fe, Co) perovskite prepared by a sol–gel method was studied. These catalysts were characterized by X-ray diffraction (XRD), N₂ adsorption (BET), H₂ temperature programmed reduction (TPR), NO temperature programmed desorption (TPD) and CO–O₂ pulse. LaCoO₃ exhibited the best activity than that of LaFeO₃ and LaMnO₃ even after hydrothermal ageing. The activity sequence is in accordance with the reducibility of the samples. The activated oxygen species and adsorbed NO play key roles in the NO oxidation reaction.     

Effect of preparation method and particle size on LaMnO3 performance in butane oxidation

LaMnO₃ with different crystal domain sizes and surface areas were prepared by citrate and sol-gel combustion methods and tested as catalysts for butane total oxidation reaction. The catalysts were characterized by N₂ physisorption, XRD and SEM. LaMnO₃ with crystal domain sizes in the range of 30–90 nm were detected by XRD characterization when high calcination temperature, at least 500 °C for sol-gel combustion method and 700 °C for citrate method, was required to prepare pure nanocrystalline phase. Although LaMnO₃ prepared by these two methods had similar crystal domain sizes, BET surface areas of samples by citrate method were significantly larger than that of samples prepared by combustion method. The difference of surface area lies in the morphology differences between the two series of samples (SEM micrographs) generated by strong sintering of samples prepared by combustion method. The catalytic activity of LaMnO₃ in butane total oxidation increased with increasing surface area being higher for materials prepared by citrate method. Thus, citrate method showed significant advantages over combustion method in preparation of perovskite catalysts.     

Pd-Perovskite Catalysts for Methane Emissions Abatement: Study of Pd Substitution Effects

Several perovskite-type oxide catalysts (LaMnO₃, LaMn_0.95Pd_0.05O₃, LaMn_0.9Pd_0.1O₃, LaMn_0.85Pd_0.15O₃, 6wt%Pd-LaMnO₃) were prepared, characterized, and tested as catalysts for methane oxidation. The half conversion temperature of methane over the best catalyst (LaMn_0.85Pd_0.15O₃) selected was 425 °C respect to 485 °C for LaMnO₃. This catalyst and the 6wt%Pd-LaMnO₃ one were then deposited on cordierite monoliths and tested. Half methane conversion (T ₅₀) was achieved at about 300 °C (GHSV = 10000 h^?1) for both catalytic converters. Conversely, the perovskite catalyst substituted with Pd showed a better thermal-proof property than that supporting dispersed Pd.     

结构型LaMnO3钙钛矿催化剂的催化燃烧活性和热稳定性

为降低堇青石载体对钙钛矿催化剂活性和稳定性的影响,以堇青石蜂窝陶瓷为基材,采用原位沉淀和悬浮浸渍技术分别制备了SiO₂和La₂O₃为涂层的结构型LaMnO₃催化剂,通过甲苯催化燃烧反应考察了催化剂的活性和热稳定性。结果表明,原位沉淀技术虽然可以均匀和高强度地在载体表面负载La、Mn活性组分,但无法在表面形成LaMnO₃钙钛矿的活性相。悬浮浸渍技术则可以保持LaMnO₃催化剂的结构和活性,结构催化剂与粉末LaMnO₃表现出相似的活性规律。La₂O₃涂层比SiO₂涂层可以更有效地保持LaMnO₃在蜂窝陶瓷载体表面的高活性和热稳定性。     

Deactivation of Diesel Oxidation Catalysts by Sulphur in Laboratory and Engine-Bench Scale Aging

The activity of sulphur–water- and water-treated PtPd/Al₂O₃- and Pt/Al₂O₃-based monolith catalysts was investigated. The catalysts were characterized by X-ray photoelectron fluorescence, X-ray photoelectron spectroscopy, transmission electron microscopy and BET–BJH. The sulphur poisoning had a diminishing effect on the catalyst activity. The correlation between the laboratory-poisoned and engine-bench-aged catalyst activity was detected and found to have a relatively good correspondence.     

Influences of A- or B-site substitution on the activity of LaMnO<Subscript>3</Subscript> perovskite-type catalyst in oxidation of diesel particle

LaMnO₃ was partially substituted at A- or B-site by Sol-Gel method and characterized by XRD, SEM and BET. Perovskite oxides were formed in all substitutions. The catalytic activities of substituted catalysts on carbon black oxidation were measured by Temperature Programming Oxidation (TPO). Experimental results showed that all substitutions increased the catalytic activity of LaMnO₃, and La_0.8Cs_0.2MnO₃ showed the highest catalytic activity. Under tight contact, the activity enhancement of different substitutions decreased in the order Cs>K>V>Ce>Co>Cu>Fe. Dynamic analysis showed that partial substitutions increased the pre-exponential factor and the catalytic activity by increasing the oxygen vacancy on the catalyst surface. The active components on the surfaces of La_0.8Ce_0.2MnO₃ and LaMn_0.8V_0.2O₃ included CeO₂ and LaVO₄, which changed the apparent activities and dynamic parameters of these two catalysts. This work was presented at the 6 ^th Korea-China Workshop on Clean Energy Technology held at Busan, Korea, July 4–7, 2006.     

Pd (1?wt%)/LaMn0.4Fe0.6O3 Catalysts Supported Over Silica SBA-15: Effect of Perovskite Loading and Support Morphology on Methane Oxidation Activity and SO2 Tolerance

Catalysts of palladium (1?wt%) deposited over silica SBA-15 supported LaMn_0.4Fe_0.6O₃ perovskite (with perovskite loading of 10, 30 and 40?wt%), characterized by several techniques (BET, SAXS, XRD, TPR) are tested in the combustion of methane. Bulk LaMn_0.4Fe_0.6O₃ with the corresponding supported Pd catalyst are also considered for comparison purpose. Dispersing LaMn_0.4Fe_0.6O₃ oxide over silica SBA-15 improves the activity of the supported palladium catalysts to an extent depending on the perovskite loading. After ageing at 600?°C for 14?h, Pd catalysts supported over SBA-15 loaded with 30 and 40?wt% of LaMn_0.4Fe_0.6O₃, deactivate less as compared to Pd over bulk perovskite. Moreover, during catalytic tests carried out in the presence of 10?vol. ppm SO₂ these catalysts exhibit better sulphur tolerance and higher regeneration capability as compared to the Pd/LaMn_0.4Fe_0.6O₃. The superior performance of such catalysts is attributed to the good dispersion of the LaMn_0.4Fe_0.6O₃ over the SBA-15, with consequent increase of the perovskite surface area with respect to bulk perovskite. In addition, the porous structure of the silica contributes to a better stabilization of the active species against sintering and acts as a chemical sink during the catalyst exposure to SO₂.     

NO2-aided Soot Oxidation on LaMn0.7Ni0.3O3 Perovkite-type Catalyst

The LaMnO₃-based perovskite-type mixed oxides were studied for both trapping of NO _x and combustion of diesel soot. The LaMn_0.7Ni_0.3O₃ (LMN3) perovskite shows high NO _x adsorption capacity, quick adsorption rate and efficient adsorbed species. After the catalyst interacts with NO at low temperature around 325 °C, decomposition of the nitrates leads to the decrease of the maximum soot oxidation temperature to 430 °C. The fine crystallite size, increased surface area and readily reducibility at low temperature also favor the oxidation of soot over LMN3 under loose contact conditions.     

Influence of Catalyst Pretreatments on Propane Oxidation Over Ru/γ-Al2O3

The influence of different treatments (in H₂ or in O₂ at 250 or 600 °C) of alumina supported Ru catalysts on the total oxidation of propane was investigated. Ruthenium catalysts were prepared using RuCl₃ as metal precursor and characterized by H₂ chemisorption, O₂ uptake, BET, XRD and TEM. The presence of chloride on the catalyst surface was found to exert an inhibiting effect on the activity of Ru. The reduced Ru/γ-Al₂O₃ catalysts after partial removing chlorine ions were more active than the same samples oxidized at 250 °C. The higher activity of the reduced Ru/γ-Al₂O₃ catalysts was attributed to the presence of a large amount of active sites on small Ru _x O _y clusters without well defined stoichiometry or on a poorly ordered layer of a ruthenium oxide on the larger Ru particles. The formation of highly dispersed, but in some extent crystallized RuO₂ phase in catalysts oxidized at 250 °C, leads to slightly lower activity of the Ru phase. Strong decline of the activity was found for catalysts oxidized at 600 °C. At this temperature, the Ru particles were completely oxidized to well-crystallized RuO₂ oxide, and the mean crystallite size of the Ru oxide phase was much higher (9–25 nm) than that of after oxidation at 250 °C (~4 nm). The effect of the regeneration treatment in H₂ on the activity of the Ru/γ-Al₂O₃ catalysts was also studied. The active ruthenium species for propane oxidation were discussed based on the catalytic and characterization data both before and after activity tests.     

Investigation of alumina-supported Ni and Ni-Pd catalysts by partial oxidation and steam reforming of n-octane

Aseries of nickel and nickel-palladium supported upon alumina catalysts were prepared in order to obtain a suitable catalyst that could be used in the process of producing hydrogen by partial oxidation and steam reforming of n-octane. Hydrogen production by partial oxidation and steam reforming (POSR) of n-octane was investigated over alumina-supported Ni and Ni-Pd catalysts. The process occurred by a combination of exothermic partial oxidation and endothermic steam reforming of n-octane. It was found that Ni/Al₂O₃ catalyst activity was high at high temperatures and increased with the Ni loadings. Its activity, however, was not obviously increased when Ni loadings were over 5.0 wt%. Compared with nickel catalyst, the bimetallic catalyst of Ni-Pd/A1₂O₃ showed markedly increased activity and hydrogen selectivity at experimental conditions. The catalytic performance also became more stable when the palladium was added, which indicated that palladium plays an essential role in the catalytic action. The used catalysts of Ni-Pd/A1₂O₃ were regenerated three times by using air at space velocity of 2,000 h⁻¹ to obtain a long duration catalyst. Also, the typical catalyst was characterized by using SEM, BET, TG and ICP methods in detail.     

Partial oxidation of methane to synthesis gas over some titanates based perovskite oxides

Ca_1–x - _x Sr _x TiO₃-based mixed oxide catalysts containing chromium, iron, cobalt or nickel were prepared and used in the oxidation of methane. The catalyst containing cobalt or nickel showed high activity for the synthesis gas production from methane. In the case of nickel containing catalyst, nickel oxide originally separated from the perovskite structure was easily reduced to nickel metal, which showed synthesis gas production activity. In the case of the cobalt containing catalyst, pretreatment with methane was required for high activity. Reduced metallic cobalt was formed from the perovskite structure, which revealed relatively high selectivity for the oxidative coupling of methane, and afforded synthesis gas production. Both the catalysts also catalyzed carbon dioxide reforming of methane and especially both high activity and selectivity were observed over the nickel containing catalyst.     

The Stability Study of Au/La-Co–O Catalysts for CO Oxidation

Perovskite oxide LaCoO₃ and the mixture oxides of La₂O₃ + Co₃O₄ were prepared by sol–gel method. Then Au/La–Co–O catalysts were prepared by deposition- precipitation (DP) method and characterized by means of XRD, BET, XPS, TEM and IR. The catalytic performance for CO low-temperature oxidation and stability over these catalysts were compared. The results of experiment showed gold catalysts supported on perovskite oxides have higher catalytic activity and stability than that of supported on the simple oxides.     

Properties and Catalytic Performance for Methane Combustion of LaMnO3 Perovskite Prepared in Oil–Water Two-phase System

LaMnO₃ catalysts with high catalytic activity for methane combustion have been prepared in an oil–water two-phase system. The procedure was carried out using oleic acid as phase-transfer agent, and ammonia as co-precipitation agent. In the meanwhile, H₂O₂ is employed to control the valence of Mn ions when precipitation. The control results in perovskites with similar phases having three O₂-TPD patterns, and induces the catalysts possessing distinct catalytic activities. The structure and properties of the catalysts were characterized by BET, TG-DSC, XRD and O₂-TPD techniques.     

Fe-doped LaCoO<Subscript>3</Subscript> perovskite catalyst for NO oxidation in the post-treatment of marine diesel engine’s exhaust emissions

New post-treatment process for marine diesel engine exhaust emissions was proposed by combining NO oxidation and wet scrubbing technology for the simultaneous removal of SO_X, NO_X and PM. NO, insoluble in aqueous scrubbing absorbent, is preferentially oxidized to NO₂, which then turns fully soluble in it. Fe substituted LaCo_1-xFe_xO₃ perovskite catalysts were developed for NO oxidation to NO₂. The catalysts were prepared by co-precipitation method and analyzed with XRD, XRF, BET, FT-IR, NO-TPD and XPS techniques. Crystal structure change from rhombohedral to orthorhombic was observed with the increased amount of Fe substituted in the B site of the perovskite by XRD analysis. From FT-IR and NO-TPD analysis, nitrate on perovskite species was found to be the active species for NO oxidation. Quantitative analysis was performed within the prepared catalysts. Catalytic activity was measured using a packed bed reactor operated at 150–400 °C, atmospheric pressure and with gas hourly space velocity (GHSV) of 20,000 h^-1 using a simulated exhaust gas composed of NO 400 ppm, O₂ 10% balanced with N2. Formation of Fe⁴⁺ cation enhanced the redox property as well as the mobility of the lattice oxygen present in the perovskite catalysts, confirmed by XPS analysis. Reaction mechanism of NO oxidation on Fe substituted LaCo_1-xFe_xO₃ was discussed based on Mars-van Krevelen mechanism.     

Selective oxidation of isobutane to methacrolein over MoVTe mixed oxide supported on SBA-3 and SiO2

SBA-3 and SiO₂-supported MoVTe mixed oxide catalysts have been prepared by impregnation and/or direct synthesis methods and tested for selective oxidation of isobutane to methacrolein (MAL). It was found that the supported catalysts showed much higher activity than the bulk MoVTe mixed oxide for the reaction. Among the supported catalysts, better isobutane conversion and MAL yield were achieved on the 3% MoV_0.8Te_0.23O_x/SBA-3 catalyst prepared by the impregnation method. The catalysts were characterized with BET, XRD, Raman, H₂-TPR, XPS and FT-IR of pyridine adsorption. The good performance of the SiO₂ and SBA-3 supported MoV_0.8Te_0.23O_x catalysts was attributed to a combination of different properties: (i) formation of well dispersed active phases on large surface areas of SiO₂ and SBA-3 supports, which is beneficial for the isolation of active site and preventing the further oxidation of unstable reaction intermediate as well as product; (ii) improved activity for hydrogen abstraction of C-H bond of isobutane due to the formation of isolated pseudotetrahedral VO₄ species.     

Enhancement in oxidative property on amorphous rare earth doped Mn catalysts

Rare earth metal (Ce, La, or Pr) doped Mn-based catalysts were prepared to obtain amorphous Mn–Ce–O_x, Mn–La–O_x and Mn–Pr–O_x. Promotional effects of NO conversion at low temperature were observed after rare earth metal doping. The Mn–Ce–O_x catalyst had the best oxidation performance and the maximum NO oxidation conversion was 94.0% at the reaction temperature of 239 °C. Among the reported Mn-based catalysts for NO oxidation, the Mn–Ce–O_x catalyst showed superior low-temperature activity. The SEM, XRD, BET and XPS analyses further confirmed that the amorphous structure of the catalyst contributed a lot to the enhancement of activity.     

Ammoxidation of 3-picoline to nicotinonitrile over vanadium phosphorus oxide-based catalysts

Ammoxidation of 3-picoline to nicotinonitrile was investigated on vanadium phosphorus oxide (VPO), VPO/SiO₂ and additive atom (Cu, Zr, Mn and Co) incorporated VPO catalysts under atmospheric pressure and at 673 K. For the purpose of comparison a conventional V₂O₅–MoO₃/Al₂O₃ catalyst was also studied under identical conditions. These catalysts were characterized by means of X-ray diffraction, electron spin resonance, infrared, ammonia chemisorption and BET surface area methods. The VPO-based catalysts show better performance than the V₂O₅–MoO₃/Al₂O₃ catalyst. Further, the VPO/SiO₂ and VPO catalysts exhibit better conversion and product selectivities than the additive-containing VPO catalysts. Better activity of VPO and VPO/SiO₂ catalysts was related to their high active surface area, higher surface acidity and lower oxidation state of vanadium. The redox couple between (VO)₂P₂O₇ (V⁴⁺) and α_I-VOPO₄ (V⁵⁺) phases appears to be responsible for the ammoxidation activity of VPO catalysts. © 1998 SCI.