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.     

Ce1−xCuxO2−x/Al2O3/FeCrAl catalysts for catalytic combustion of methane

A series of the Ce_1−xCu_xO_2−x/Al₂O₃/FeCrAl catalysts (x = 0–1) were prepared. The structure of the catalysts was characterized using XRD, SEM and H₂-TPR. The catalytic activity of the catalysts for the combustion of methane was evaluated. The results indicated that in the Ce_1−xCu_xO_2−x/Al₂O₃/FeCrAl catalysts the surface phase structure were the Ce_1−xCu_xO_2−x solid solution, -Al₂O₃ and γ-Al₂O₃. The surface particle shape and size were different with the variety of the molar ratio of Ce to Cu in the Ce_1−xCu_xO_2−x solid solution. The Cu component of the Ce_1−xCu_xO_2−x/Al₂O₃/FeCrAl catalysts played an important role to the catalytic activity for the methane combustion. There were the stronger interaction among the Ce_1−xCu_xO_2−x solid solution and the Al₂O₃ washcoats and the FeCrAl support.     

Au/Ce1−xZrxO2 as effective catalysts for low-temperature CO oxidation

The physico-chemical properties and activity of Ce-Zr mixed oxides, CeO₂ and ZrO₂ in CO oxidation have been studied considering both their usefulness as supports for Au nanoparticles and their contribution to the reaction. A series of Ce_1−xZr_xO₂ (x = 0, 0.25, 0.5, 0.75, 1) oxides has been prepared by sol–gel like method and tested in CO oxidation. Highly uniform, nanosized, Ce-Zr solid solutions were obtained. The activity of mixed oxides in CO oxidation was found to be dependent on Ce/Zr molar ratio and related to their reducibility and/or oxygen mobility. CeO₂ and Ce_0.75Zr_0.25O₂, characterized by the cubic crystalline phase show the highest activity in CO oxidation. It suggests that the presence of a cubic crystalline phase in Ce-Zr solid solution improves its catalytic activity in CO oxidation. The relation between the physico-chemical properties of the supports and the catalytic performance of Au/Ce_1−xZr_xO₂ catalysts in CO oxidation reaction has been investigated. Gold was deposited by the direct anionic exchange (DAE) method. The role of the support in the creation of catalytic performance of supported Au nanoparticles in CO oxidation was significant. A direct correlation between activity and catalysts reducibility was observed. Ceria, which is susceptible to the reduction at the lowest temperature, in the presence of highly dispersed Au nanoparticles, appears to be responsible for the activity of the studied catalysts. CeO₂-ZrO₂ mixed oxides are promising supports for Au nanoparticles in CO oxidation whose activity is found to be dependent on Ce/Zr molar ratio.     

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.     

Partial oxidation of methane to syngas over BaTi1 − xNixO3 catalysts

Performances of BaTi_1 − xNi_xO₃ perovskites, prepared using sol–gel method, as catalysts for partial oxidation of methane to syngas have been studied. The catalysts were characterized by XRD, BET and TEM. The experimental studies showed the calcination temperature and Ni content exhibited a significant influence on catalytic activity. Among catalysts tested, the catalyst BaTi_0.8Ni_0.2O₃ exhibited the best activity and excellent stability.     

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.     

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.     

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.     

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.     

Properties of Ca1−xHoxMnO3 perovskite-type electrodes

In this work, Ca_1−xHo_xMnO₃ (x = 0, 0.1 and 0.2) perovskite oxide pelleted electrodes were prepared from the respective powders obtained by the citrate route method at 1173 K.The electrodes exhibit particle size that decreases with the holmium content in the oxide. All the samples reveal semiconductor behaviour and the presence of holmium induces a marked decrease in the electrical resistivity. The results can be well attributed to the changes in the Mn⁴⁺/Mn³⁺ ratio. Electrodes were characterized by cyclic voltammetry and chronopotentiometry. Cyclic voltammetric studies indicate a similar behaviour of the electrodes, irrespective of their composition. Two pairs of peaks were identified and associated, one to the Mn⁴⁺/Mn³⁺ redox couple and the other to the Mn⁷⁺/Mn⁴⁺ and Mn⁶⁺/Mn⁴⁺ redox couples. The voltammetric data provide evidence that the electrodes roughness factor increases with the introduction of Ho-ions in the oxide structure, what is consistent with the crystallite size obtained by X-ray diffraction (XRD) and the morphology observed by scanning electron microscopy (SEM). The Ho-substituted electrodes present higher current density when compared with CaMnO₃ electrodes what can be attributed both to higher electrical conductivity and smaller particle size. The chronopotentiometric studies have shown that the discharge occurs by different mechanisms for the oxide electrodes with and without Ho.     

Catalytic wet oxidation of phenol over PtxAg1−xMnO2/CeO2 catalysts

Catalytic wet oxidation reactions of aqueous phenol over unpromoted, base- and noble-metal promoted MnO₂/CeO₂ catalysts were carried out under mild conditions (80–130°C, 0.5 MPa O₂) in a batch slurry reactor. Even though the catalyst-mediated oxidation was very effective in destroying phenol, only a moderate selectivity toward complete mineralization into CO₂ and H₂O was attained due to parallel formation of deactivating carbonaceous deposits. Promotion of the mixed-oxide catalysts with platinum and/or silver enhanced the mineralization selectivity and reduced appreciably the amount of deposits.     

Catalytic wet peroxide oxidation of phenolic solutions over a LaTi1−xCuxO3 perovskite catalyst

LaTi_1−xCu_xO₃ perovskites are efficient catalysts for the oxidation of aqueous solutions of phenol using hydrogen peroxide as precursor of free radicals. The catalytic results have shown that under mild reaction conditions and oxidant contents lower than stoichiometric, phenol was rapidly removed with a final total organic carbon (TOC) conversion of ca. 90%. Initial rate of TOC removal depends dramatically on temperature, catalyst loading and peroxide concentration. Perovskite catalyst after reaction is completely regenerated by calcination and retains a similar catalytic performance. Finally, the catalytic results demonstrate that leaching of active species during the catalytic oxidation seems to proceed by a complex mechanism in which the presence of hydrogen peroxide in combination with phenol plays an essential role.     

Methane oxidation over vanadium-modified Pd/Al2O3 catalysts

Three different vanadium-modified Pd/Al₂O₃ catalysts were prepared and tested as catalysts for the deep oxidation of methane. Vanadium was added to the palladium catalyst by incipient wetness of palladium catalyst in order to modify its properties and improve its thermal stability and thioresistance. The behaviour of vanadium-modified catalysts depends on the concentration of this compound, being 0.5 wt.% the optimum amount. However, when strong catalyst poisons are present in the gas (SO₂), these modified catalysts do not show a better performance than unmodified catalyst. Bimetallic catalysts were tested with and without further reduction, being observed that reduced bimetallic catalysts perform worse than the non-reduced ones.     

CO oxidation on Pd/CeO2–ZrO2 catalysts

The promotive effects of cerium oxide on commercial three-way catalysts (TWCs) for purification of motor exhaust gases have been widely investigated in recent years. This work shows the cooperative effects of CeO₂–Pd on the kinetics of CO oxidation over Pd/CeO₂–ZrO₂. Under reducing-to-moderately oxidizing conditions, a zero-order O₂ pressure dependence is found which can be interpreted on the basis of a mechanism involving a reaction between CO adsorbed on Pd and surface oxygen from the support. The high oxygen-exchange capability of the CeO₂–ZrO₂ support, as determined from temperature-programmed reduction/oxygen uptake measurements is suggested as being responsible for such a catalytic behavior.     

Electronic and structural properties of SnxTi1−xO2 solid solutions: a periodic DFT study

The structural and electronic properties of selected compositions of Sn_xTi_1−xO₂ solid solutions (x=0, 1/24, 1/16, 1/12, 1/8, 1/6, 1/4, 1/2, 3/4, 5/6, 7/8, 11/12, 15/16, 23/24 and 1) were investigated by means of periodic density functional theory (DFT) calculations at B3LYP level. The calculations show that the corresponding lattice parameters vary non-linearly with composition, supporting positive deviations from Vegard’s law in the Sn_xTi_1−xO₂ system. Our results also account for the fact that chemical decomposition in Sn_xTi_1−xO₂ system is dominated by composition fluctuations along [0 0 1] direction. A nearly continuous evolution of the direct band gap and the Fermi level with the growing value of x is predicted. Ti 3d states dominate the lower portion of the conduction band of Sn_xTi_1−xO₂ solid solutions. Sn substitution for Ti in TiO₂ increases the oxidation–reduction potential of the oxide as well as it renders the lowest energy transition to be indirect. These two effects can be the key factors controlling the rate for the photogenerated electron–hole recombination. These theoretical results are capable to explain the enhancement of photoactivity in Sn_xTi_1−xO₂ solid solutions.     

Co3O4/CeO2 composite oxides for methane emissions abatement: Relationship between Co3O4–CeO2 interaction and catalytic activity

Co₃O₄/CeO₂ composite oxides with different cobalt loading (5, 15, 30, 50, 70 wt.% as Co₃O₄) were prepared by co-precipitation method and investigated for the oxidation of methane under stoichiometric conditions. Pure oxides, Co₃O₄ and CeO₂ were used as reference. Characterization studies by X-ray diffraction (XRD), BET, temperature programmed reduction/oxidation (TPR/TPO) and X-ray photoelectron spectroscopy (XPS) were carried out.

An improvement of the catalytic activity and thermal stability of the composite oxides was observed with respect to pure Co₃O₄ in correspondence of Co₃O₄–CeO₂ containing 30% by weight of Co₃O₄. The combined effect of cobalt oxide and ceria, at this composition, strongly influences the morphological and redox properties of the composite oxides, by dispersing the Co₃O₄ phase and promoting the efficiency of the Co³⁺–Co²⁺ redox couple. The presence in the sample Co₃O₄(30 wt.%)–CeO₂ of a high relative amount of Ce³⁺/(Ce⁴⁺ + Ce³⁺) as detected by XPS confirms the enhanced oxygen mobility.

The catalysts stability under reaction conditions was investigated by XRD and XPS analysis of the used samples, paying particular attention to the Co₃O₄ phase decomposition. Methane oxidation tests were performed over fresh (as prepared) and thermal aged samples (after ageing at 750 °C for 7 h, in furnace). The resistance to water vapour poisoning was evaluated for pure Co₃O₄ and Co₃O₄(30 wt.%)–CeO₂, performing the tests in the presence of 5 vol.% H₂O. A methane oxidation test upon hydrothermal ageing (flowing at 600 °C for 16 h a mixture 5 vol.% H₂O + 5 vol.%O₂ in He) of the Co₃O₄(30 wt.%)–CeO₂ sample was also performed. All the results confirm the superiority of this composite oxide.     

Crystalline structure and electrical properties of YCuxMn1–xO3 solid solutions

Solid solutions belonging to the Mn-rich region of the YCu_xMn_1–xO₃ system have been studied. The powders were prepared by solid-state reaction between the corresponding oxides. Sintered ceramics were obtained by firing at 1100–1325 °C. The incorporation of 30 at.% Cu to the yttrium manganite induces the formation of a perovskite-type phase, with orthorhombic symmetry. Increase of the Cu amount do not appreciably affects the orthorhombicity factor b/a, up to an amount of 50 at.% Cu. Above this Cu amount, a multiphase system has been observed, with the presence of unreacted-Y₂O₃, YMnO₃ and Y₂Cu₂O₅, along with a perovskite phase. DC electrical conductivity measurements have shown a semiconducting behaviour for all the solid solutions with perovskite-type structure. The room temperature conductivity increases with Cu until 33 at.% Cu, and then decreases. Thermally activated small polaron hopping mechanism, between Mn³⁺ and Mn⁴⁺ cations, controls the conductivity in these ceramics. Results are discussed as a function of the Mn³⁺/Mn⁴⁺ ratio for each composition.     

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.     

A correlation between crystal structure and catalytic activity in the solid solutions CdMoxW1−xO4

Two CdMo_xW_1−xO₄ solid solutions with the scheelite and the wolframite structures were synthesised via different techniques and their structures were determined from powder refinements. The limit of the reciprocal solubility of Mo in CdWO₄ has been checked. Several compositions of the solid solutions have been tested in the catalytic mild oxidation of propane or propene and relations between the structures and the catalytic activity are proposed.