Insights into SO2 Interaction with Pd/Co3O4–CeO2 Catalysts for Methane Oxidation

The catalytic performance and the sulphur resistance of a Pd (0.7 wt%) catalyst supported over Co₃O₄(30 wt%)–CeO₂(70 wt%) mixed oxide were investigated in the oxidation of methane under stoichiometric and lean conditions. The catalytic behaviour was compared with that of two reference catalysts, palladium supported over pure Co₃O₄ and CeO₂. Catalysts were characterized by XRD, BET, XPS and FTIR measurements. Regeneration by a CH₄-reducing treatment at 600 °C was investigated.     

Pt–Pd bimetallic catalysts for methane emissions abatement

Bimetallic Pd–Pt catalysts with constant 2 wt% metal loading and varying Pt/Pd ratios were prepared, characterized and studied in the catalytic combustion of methane at low temperature under lean conditions in view of their use for CH₄ abatement from lean-burn NGV heavy duty vehicles exhausts. The influence of mild steam ageing featuring long-term use of the catalysts was also addressed together with their tolerance to H₂S. Catalysts were characterised by Transmission Electron Microscopy and Temperature Programmed Oxidation experiments. Experimental data agreed to suggest an interaction between Pd and Pt in Pd-rich catalysts, thus explaining their improved catalytic activity, even after mild ageing, compared to reference Pd/Al₂O₃. This interaction has no effect on the sulfur tolerance.     

Low-Temperature Combustion of CH4 over CeO2–MO x Solid Solution (M = Zr4+, La3+, Ca2+, or Mg2+) Promoted Pd/γ–Al2O3 Catalysts

The catalytic behavior of Pd (2 wt%) catalysts supported on γ-Al₂O₃ and promoted with CeO₂ ? MO _x (M = Zr⁴⁺, La³⁺, Ca²⁺, or Mg²⁺) solid solution was investigated for methane combustion. The results demonstrated that Pd/γ-Al₂O₃CeO₂ MO _x catalysts can be effective for the low-temperature catalytic combustion of methane and are comparable in activity to other conventional catalysts for this reaction. The XPS and XRD results indicated that an enhanced mobility of lattice oxygen induced by the perturbation of Ce–O lattice was responsible for an increased catalytic performance during oxidation reaction. The most active sites in the catalyst system involve contacts between Pd and the CeO₂–MO _x mixed oxide component. Meanwhile, pre-treatment conditions have significant effect on the catalytic activity in methane combustion.     

Pd/Co3O4 catalyst for CH4 emissions abatement: study of SO2 poisoning effect

A catalyst with 0.7 wt% Pd load supported over Co₃O₄ oxide was investigated in the methane oxidation by operating under CH₄/O₂ stoichiometric conditions. The effect of the noble metal addition on the activity of bare Co₃O₄ was evaluated. Samples were characterized by BET, XRD, TPR and XPS analyses. The SO₂ poisoning of Pd catalyst and Co₃O₄ was studied by performing CH₄ oxidation tests under stoichiometric conditions in SO₂ (1 ppm or 10 ppm)_. Experiments evidenced that in our conditions the low amount of SO₂ doesn’t influence the Pd behaviour, whereas in presence of 10 ppm of SO₂ some deactivation occurs that becomes more evident above 450 °C at which the catalyst doesn’t reach 100% of methane conversion. Catalytic tests performed over Co₃O₄ and the Pd supported catalyst, after a treatment at 350 °C for 15 h in 10 ppm SO₂/He, suggest that Co₃O₄ is a sulphating support, as confirmed by XPS analysis. Therefore, an important role in lowering the sulphur poisoning of Pd may be played by Co₃O₄.     

Influence of PdO content and pathway of its formation on methane combustion activity

The methane oxidation reaction is known to induce changes in the surface structure and composition of Pd catalysts; making it extremely arduous to relate the methane oxidation activity to specific catalyst properties by conventional methods (continuous flow reactor studies). To circumvent this, methodical pulse reactor studies have been undertaken to obtain correlations between the initial methane combustion activity and the catalyst properties (Pd⁰/PdO content and path of PdO formation). While the initial methane combustion activity (at 160–280 °C) continuously increased with increasing PdO concentration (0–100%) in the catalyst, it continuously decreased with increasing Pd⁰ content (0–100%). Controlled studies were undertaken to obtain catalysts with identical PdO content by two pathways: (i) by controlled partial oxidization of Pd⁰/Al₂O₃ and (ii) by controlled partial reduction of PdO/Al₂O₃. Interestingly, for a given PdO content, the catalysts obtained by partial oxidation of Pd⁰/Al₂O₃ showed a significantly superior performance to the catalyst obtained by partial reduction of PdO/Al₂O₃ for all the temperatures investigated. These studies unambiguously show that along with the relative concentration of PdO, the PdO formation pathway is also critical in deciding the methane combustion activity of the catalyst.     

Hydrocracking of vegetable oil on boron-containing catalysts: Effect of the nature and content of a hydrogenation component

Catalysts based on metals (Pt, Pd) and metal oxides (NiO, Co₃O₄, MoO₃, WO₃), supported on the surface of borate-containing aluminum oxide (B₂O₃–Al₂O₃), in the hydrocracking of sunflower oil at a temperature of 400°C, a pressure 4.0 MPa and a mass hourly space velocity MHSV 5.0 h^–1 are compared. H₂ TPR and IR spectroscopy of adsorbed CO and ESDR show that the hydrogenation catalyst components are Pt⁰ and Pd⁰, a mixture of Ni²⁺ + Ni⁰, Co²⁺ + Co⁰, or a mixture of the highest and partially reduced oxides of Mo and W. It is established that catalysts containing Pt, Pd, NiO and Co₃O₄, ensure complete oil hydrodeoxygenation. The main oxygen removal reactions in Ptand Pd-systems are decarboxylation and hydrodecarbonylation. For catalysts with NiO and Co₃O₄, characteristic reactions are reduction and methanation. The highest yield of the diesel fraction was obtained on Pt/B₂O₃-Al₂O₃ catalysts with metal contents of 0.3–1.0 wt %. Along with n-alkanes, the diesel fractions obtained on these catalysts include cycloalkanes and iso-alkanes (up to around 40 wt %) and aromatic hydrocarbons present in trace amounts. Hydrocracking on the Pt system at 400°C for 20 h with MHSV of 1.0 h^–1 produces a diesel fraction with a yield of at least 82.0 wt % and the content of iso-alkanes at least 76.1 wt %.     

Performance of H‐ZSM‐5‐supported bimetallic catalysts for the combustion of polluting volatile organic compounds in air

The performance of H‐ZSM‐5‐supported bimetallic catalysts with chromium as the base metal in the combustion of ethyl acetate and benzene is reported. A reactor operated from 100 to 500 °C at a gas hourly space velocity (GHSV) of 32 000 h^?1 was used for study of the activity. A combination of 1.0 wt% chromium and 0.5 wt% copper yielded a catalyst (Cr_1.0Cu_0.5/Z) with improved conversion and carbon dioxide yield. Cr₂O₃ (Cr³⁺) and CuO (Cu²⁺) were the predominant metal species in the catalyst. In agreement with the Mars–van Krevelen model, improved reducibility of Cr³⁺ in the presence of Cu²⁺ led to an improvement in activity. The copper content in Cr_1.0Cu_0.5/Z also favored the formation of deep combustion products. Condensation and subsequent growth of coke precursors in the catalyst pores led to the formation of a softer and less aromatic coke fraction while dehydrogenation activity on acid sites formed a harder and more aromatic coke fraction. The use of Cr_1.0Cu_0.5/Z favored the formation of lower molecular weight intermediates, leading to reduction in formation of softer coke. Copyright © 2005 Society of Chemical Industry     

Catalytic activity of Pd/Al2O3 toward the combustion of methane

This work focuses on the improving of the activities and stabilities of Pd/Al₂O₃ catalysts for lean methane catalytic combustion. The influence of preparation conditions on performance of Pd/Al₂O₃ catalyst has been studied. Results showed that excellent performance of the catalyst was attributed to high hydrothermal stability at the support calcination temperature of 1100 °C. In addition, the catalytic activity was enhanced due to high dispersion of active species at lower catalyst calcination temperature. The catalysts were studied by XPS analysis. Results showed that the active phase of Pd/Al₂O₃ was Pd or Pd/PdO mixture. And the state transformation of Pd species resulted in the deactivation of Pd/Al₂O₃.     

Influence of oxygen and promoting effect of barium on the reduction of NOx by ethanol on Pd/ZrO2 catalyst

The NO reduction by ethanol over barium promoted Pd/ZrO₂ catalyst and the effect of the oxygen on the selectivity were studied. The catalysts were prepared by incipient wetness impregnation with 14.3% of Ba over zirconia and 1% of palladium. The specific surface areas were 58 and 47 m²/g and the dispersions of Pd were 37% and 30% for the Pd/ZrO₂ and Pd–Ba/ZrO₂ catalysts, respectively. The X-ray diffraction patterns indicate the presence of monoclinic zirconia phase on the support and BaCO₃, which is decomposed at 715 and 815 °C. Temperature programmed desorption profiles of NO on Pd/ZrO₂ and Pd–Ba/ZrO₂ catalyst showed a huge amount N₂ formation for the promoted Ba catalyst. Catalytic results showed high NO conversion even at low temperature, in accordance with the TPD results and an increasing selectivity to N₂ when compared with Pd/ZrO₂. The effect of O₂ in the NO_x reduction with ethanol provoked less NO dissociation and lower selectivity to methane.     

Nano-gold supported on Fe2O3: A highly active catalyst for low temperature oxidative destruction of methane green house gas from exhaust/waste gases

A number of nano-gold catalysts were prepared by depositing gold on different metal oxides (viz. Fe₂O₃, Al₂O₃, Co₃O₄, MnO₂, CeO₂, MgO, Ga₂O₃ and TiO₂), using the homogeneous deposition precipitation (HDP) technique. The catalysts were evaluated for their performance in the combustion of methane (1 mol% in air) at different temperatures (300–600 °C) for a GHSV of 51,000 h⁻¹. The supported nano-gold catalysts have been characterized for their gold loading (by ICP) and gold particle size (by TEM/HRTEM or XRD peak broadening). Among these nano-gold catalysts, the Au/Fe₂O₃ (Au loading = 6.1% and Au particle size = 8.5 nm) showed excellent performance. For this catalyst, temperature required for half the methane combustion was 387 °C, which is lower than that required for Pd(1%)/Al₂O₃ (400 °C) and Pt(1%)/Al₂O₃ (500 °C) under identical conditions. A detailed investigation on the influence of space velocity (GHSV = 10,000–100,000 cm³ g⁻¹ h⁻¹) at different temperatures (200–600 °C) on the oxidative destruction of methane over the Au/Fe₂O₃ catalyst has also been carried out. The Au/Fe₂O₃ catalyst prepared by the HDP method showed much higher methane combustion activity than that prepared by the conventional deposition precipitation (DP) method. The XPS analysis showed the presence of Au in the different oxidation states (Au⁰, Au¹⁺ and Au³⁺) in the catalyst.     

Dependence of Synergetic Effect of Palladium–Manganese-Hexaaluminate Combustion Catalyst on Nature of Palladium Precursor

A synergetic effect in the methane oxidation activity of palladium and manganese hexaaluminate was studied over Pd-modified manganese-hexaaluminate catalysts, prepared by incipient wetness impregnation and calcined at 1,200?°C. The magnitude of the synergetic effect is found to be depends on the palladium precursor: it is higher for palladium nitrate and palladium acetate than for tetrachloropalladic acid. The Pd/MnLaAl₁₁O₁₉ catalysts were characterized by X-ray diffraction, X-ray microanalysis, transmission electron microscope and temperature-programmed reduction with hydrogen. These data were compared with the properties of Pd/Al₂O₃ catalysts. At variation of Pd-precursors, a minor trend to the decrease of the Pd particle size was observed at transition from the ex-chloride Pd/MnLaAl₁₁O₁₉ catalyst with uniform Pd-distribution profile to the ex-nitrate and ex-acetate catalysts with egg-shell Pd-distribution. Slightly smaller size of metal palladium particles in the ex-nitrate and ex-acetate catalysts leads to the formation of larger amount of PdO dispersed on their surface during oxygen-pretreatment in H₂-TPR experiments (Pd/PdO atomic ratio was 1/4) and under methane-oxidation mixture in comparison with ex-chloride catalysts (Pd/PdO?=?4/1). The palladium addition to manganese-hexaaluminate changes strongly its redox properties, as result Mn³⁺ reduction to Mn²⁺ take place about 100?°C below that of pure hexaalunimate. The latter indicate probably on the higher oxygen mobility in Pd-modified manganese-hexaaluminate. A higher PdO/Pd ratio formed in the ex-nitrate and ex-acetate Pd-modified manganese-hexaaluminate catalysts together with the high oxygen mobility provide the synergetic effect in methane oxidation activity at light-off temperature region. The high catalytic activity of manganese-hexaaluminate ensures methane combustion efficiency of the Pd-modified manganese-hexaaluminate catalysts at temperature above 700?°C.     

Catalytic dehydrogenation of propane on chromia,palladium and platinum supported catalysts

The dehydrogenation of propane to propylene over Cr₂O₃/Al₂O₃, Pd/Al₂O₃ and Pt/SiO₂ has been investigated in the temperature range 580–618°C. Runs were performed on propane, alone or in the presence of nitrogen (as a diluent), with complete analysis of the reaction products. The reaction was carried out in a fixed bed reactor at space velocities from 450–800 h^?1 which are close to industrial values and at pressures from 0.3 to 1 atm. A set of runs was made over a commercial chromia-alumina catalyst (10% Cr₂O₃) and over a promoted catalyst prepared in the laboratory by impregnation (16.8% Cr₂O₃ + 2% K₂O). The latter catalyst showed high selectivity and stability even when subjected to continuous cycles of dehydrogenation, regeneration and purging. Of the two noble metal supported catalysts used, reduced Pd/Al₂O₃ showed higher activity than Pt/SiO₂ at 618°C. The former catalyst gave a propylene yield of around 98% at 20% conversion level.     

Methane partial oxidation to synthesis gas over bimetallic cobalt/tungsten carbide catalysts and integration with a Mn substituted hexaaluminate combustion catalyst

Co_0.2W_0.8C_x and supported Co_0.2W_0.8C_x catalysts are shown to be active for the partial oxidation of methane to synthesis gas. The catalyst stability is improved by operating at elevated pressure, or in the presence of excess methane. At atmospheric pressure the Co_0.2W_0.8C_x catalysts deactivate by oxidation, as seen by X-ray diffraction. Manganese substituted hexaaluminate catalysts with different Mn contents have been tested as catalysts for the total combustion of methane. In particular BaMn₂Al₁₀O₁₉ is active and stable for the combustion reaction. The temperature rise observed in the reactor was up to 300 K, depending on the reaction conditions, and complete conversion of oxygen in the feed was achieved. A process for stabilising the carbide catalysts is demonstrated, combining the manganese substituted hexaaluminate total oxidation catalyst, in series before the carbide reforming catalyst: this process leads to stable operation, with no carbon formation in the reactor and no carbide catalyst oxidation observed.     

Oxidative dehydrogenation of ethane by carbon dioxide over sulfate‐modified Cr2O3/SiO2 catalysts

The oxidative dehydrogenation of ethane into ethylene by carbon dioxide over unsupported Cr₂O₃, Cr₂O₃/SiO₂ and a series of Cr₂O₃/SiO₂ catalysts modified by sulfate was investigated. The results show that Cr₂O₃/SiO₂ is an effective catalyst for dehydrogenation of ethane and CO₂ in the feed promotes the catalytic activity. Sulfation of silica will influence the catalytic behavior of Cr₂O₃/SiO₂ in dehydrogenation of ethane with carbon dioxide depending on the amount of sulfate. Cr₂O₃/6 wt% SO ₄ ^2- –SiO₂ catalysts exhibit an excellent performance for this reaction, giving an ethylene yield of 55% at 67% ethane conversion at 650°C. Characterizations indicate that addition of sulfate changes the bulk and surface properties of Cr₂O₃/SiO₂, promoting the reduction of Cr⁶⁺ to Cr³⁺ and favoring the catalytic conversion. This revised version was published online in July 2006 with corrections to the Cover Date.     

Methane combustion over copper chromite catalysts

A study of the activity and durability of two different copper chromite catalysts in methane combustion is presented. The catalysts, a massive (CAT-E) and a supported (CAT-I) copper chromite, were characterized by different techniques in order to investigate morphological properties (N₂ adsorption), crystalline structure (X-ray diffraction, XRD) and surface composition (X-ray photoelectron spectroscopy, XPS). Among the different crystalline phases identified, CuCr₂O₄ spinel represented the common phase in both the catalysts. The Cr^VI/Cr^III surface ratio was almost the same for the two catalysts, while the Cu^II/Cu^I surface ratio was much higher on the massive catalyst than on the supported one. The activity for CH₄ combustion was studied in the temperature range 300-700°C at constant CH₄:air ratio of 1:30 and constant methane content, 1.2%. The activity was higher for CAT-I and CAT-E showed better stability. A kinetic study from the catalytic data, collected at different contact times in the interval 0.047-0.315 s as a function of temperature, provided a value of about 110 kJ/mol for the activation energy. This value was obtained for various degrees of methane converted for the two catalysts. The reaction rates were between 10^-3 and 10^-4 (mol_CH4)_conv/(g h) in the temperature interval 550-700°C, for both the catalysts. This revised version was published online in July 2006 with corrections to the Cover Date.     

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.     

On the La–Cr–O Phase Change During Methane Catalytic Combustion Investigated with in situ HT-XRD

La₂CrO₆ (Cr6), LaCrO₃ (Cr3), LaCrO₃–La₂CrO₆ (Cr6–Cr3) and LaCrO₃–La₂O₃ (Cr3-L) catalysts were synthesised and investigated with in situ X-ray diffraction (ISXRD) during methane catalytic combustion in order to characterise the solid phases present under reactants and to determine the effect of chromium oxidation state on the catalytic activity. Methane conversion was evaluated over a temperature range of 300–800 °C, using oxygen-to-methane ratio of 4 and GHSV of 8,000 h^?1. The TPR provided information about the oxygen depletion temperatures characteristic of lattice oxygen mobility in the samples and ISXRD results evidenced a cooperative effect of Cr3 and Cr6 phases at low temperatures (<725 °C) and of Cr3 and LaCrO₄ (Cr4) phases at high temperatures (>750 °C). The relative phase composition determined the oxygen activation capability and hence the corresponding activity for the oxidation of methane. It was observed that while direct and back Cr6 ? Cr4 transition temperatures were unaffected by Cr6 content in the samples, the methane conversion was strongly modified. This suggests that Cr³⁺/Cr⁶⁺ and Cr³⁺/Cr⁵⁺ species involved substantial modification of the surface chemistry which affected the catalytic activity. These results provide the first direct evidence of the presence of Cr4 metastable phase during methane combustion over La–Cr–O 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.     

COMPARATIVE STUDY OF Ag2O/CO3O4 CATALYSTS PREPARED BY THE SOL-GEL AND CO-PRECIPITATION TECHNIQUES: CHARACTERIZATION AND CO ACTIVITY STUDIES

In this study effects of the preparation method on the characteristic properties and CO oxidation activities of Ag₂O/Co₃O₄ catalysts were investigated. Catalysts were prepared by two different methods: sol gel and co-precipitation. N₂ physisorption measurements, X-ray diffraction, and scanning electron microscopy measurements were used to characterize the catalysts. CO oxidation activity tests were carried out under 1% CO, 21% O₂, and the remainder He feed condition between 20° and 200°C. According to the N₂ physisorption measurements, catalysts prepared by the co-precipitation method have a higher surface area than the catalysts prepared by the sol-gel method. Co₃O₄ and AgCoO₂ phases were obtained from catalysts prepared by both techniques. In addition to these phases, metallic silver peaks were obtained by increasing calcination temperature. SEM micrographs of the catalysts showed that catalysts have uniform particles. Increasing the calcination temperature caused the formation of different-sized agglomerates and an increase in the gaps between agglomerates. The best activity was obtained from the Ag₂ O/Co₃ O₄ catalyst calcined at 200°C and prepared by the co-precipitation method. This catalyst gave 50% CO conversion at 106°C. The other two catalysts gave 100% CO conversion at a higher temperature of 200°C.     

Methane combustion over Pd/ZrO2/SiC,Pd/CeO2/SiC,and Pd/Zr0.5Ce0.5O2/SiC catalysts

The performances of different promoters (CeO₂, ZrO₂ and Ce_0.5Zr_0.5O₂ solid solution) modified Pd/SiC catalysts for methane combustion are studied. XRD and XPS results showed that Zr⁴⁺ could be incorporated into the CeO₂ lattice to form Zr_0.5Ce_0.5O₂ solid solution. The catalytic activities of Pd/CeO₂/SiC and Pd/ZrO₂/SiC are lower than that of Pd/Zr_0.5Ce_0.5O₂/SiC. The Pd/Zr_0.5Ce_0.5O₂/SiC catalyst can ignite the reaction at 240 °C and obtain a methane conversion of 100% at 340 °C, and keep 100% methane conversion after 10 reaction cycles. These results indicate that active metallic nanoparticles are well stabilized on the SiC surface while the promoters serve as oxygen reservoir and retain good redox properties.