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.     

n-Hexane isomerization over copper oxide-promoted sulfated zirconia supported on mesoporous silica

Copper oxide-promoted sulfated zirconia (CuSZ) was supported on MCM-41 by the direct impregnation method. n-Hexane isomerization was investigated over the CuSZ/MCM-41 catalysts. 2-MP, 3-MP and 2,3-DMB are the major isomerization products, besides a small amount of 2,2-DMB. The product distribution is comparable to that reported for Pt based catalysts. The optimal CuO loading in these catalysts calcined at 700 °C is around 3.2 wt% and only leads to the formation of catalytic active metastable tetragonal ZrO₂. The improved performance of CuO-promoted SZ/MCM-41 is a trade-off between the sulfur amount and the content of tetragonal ZrO₂ phase.     

n-Hexane isomerization over copper oxide-promoted sulfated zirconia supported on mesoporous silica

Copper oxide-promoted sulfated zirconia (CuSZ) was supported on MCM-41 by the direct impregnation method. n-Hexane isomerization was investigated over the CuSZ/MCM-41 catalysts. 2-MP, 3-MP and 2,3-DMB are the major isomerization products, besides a small amount of 2,2-DMB. The product distribution is comparable to that reported for Pt based catalysts. The optimal CuO loading in these catalysts calcined at 700 °C is around 3.2 wt% and only leads to the formation of catalytic active metastable tetragonal ZrO₂. The improved performance of CuO-promoted SZ/MCM-41 is a trade-off between the sulfur amount and the content of tetragonal ZrO₂ phase.     

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.     

Carbon dioxide reforming of methane over Co-X/ZrO2 catalysts (X = La,Ce, Mn,Mg, K)

Dry reforming of methane has been studied over Co/ZrO₂ catalysts promoted with different metal additives (La, Ce, Mn, Mg, K) aiming to improve the performance of the catalysts and increase their resistance to coking. Scanning electron microscopy studies and different activity levels of the catalysts clearly show that the type of the promoter significantly affected the metal dispersion properties and catalytic performances of Co/ZrO₂ catalysts. La-modified catalyst exhibited high stability, but moderate activity. It showed no severe coke deposition. Ce-doped Co/ZrO₂ displayed the highest activity among all the catalysts prepared and had a very limited activity loss.     

A study of “superacidic” MoO3/ZrO2 catalysts for methane oxidation

A series of zirconia-supported molybdenum oxide catalysts with different molybdenum loadings prepared using conditions reported to generate “superacidity” have been evaluated for their performance as catalysts for methane oxidation. A marked dependence of Mo content on activity has been observed, with the most active material being that with intermediate molybdenum content. 5 wt% MoO₃/ZrO₂ compares favourably with Zr_xCe_1-xO₂ for methane combustion. The presence of MoO₃ is observed to stabilise the tetragonal polymorph of ZrO₂ and, as Mo content is increased, dispersed MoO₃ crystallites are formed as evidenced by Raman spectroscopy. Temperature-programmed reduction studies evidence differences in the reduction behaviour of the materials as a function of loading. The results indicate that molybdenum oxide supported on monoclinic zirconia gives rise to the most active catalyst. It is tentatively suggested that the formation of a MoO₃ monolayer during reaction may be of importance. This revised version was published online in August 2006 with corrections to the Cover Date.     

Electron Paramagnetic Resonance investigation of the nature of active species involved in carbon black oxidation on ZrO2 and Cu/ZrO2 catalysts

To identify the nature of active paramagnetic species involved in the carbon black (CB) oxidation, an Electron Paramagnetic Resonance (EPR) study of (CB–ZrO₂) and (CB–Cu/ZrO₂) loose contact mixtures treated under argon flow has been undertaken. In the presence of pure zirconia catalysts, it was found that ZrO₂ interacts with CB and can be reduced into Zr³⁺ formed in tetragonal phase of this oxide support. In parallel, several EPR signals assigned to carbonaceous radicals were detected: i) carbonaceous radicals on CB surface, ii) radicals located at the CB–ZrO₂ interface and iii) oxygen deficit carbonaceous radicals observed at high treatment temperature. The carbonaceous signals disappeared completely after CB oxidation in agreement with a regeneration of the catalyst treated under air.For copper supported on zirconia catalysts, oxygen surrounding isolated Cu(II) species and oxygen from tetragonal ZrO₂ lattice are involved in carbon black oxidation. The phenomenon is reversible and this catalyst is also regenerated by air. Carbonaceous radical signals were also observed for (CB–Cu/ZrO₂) mixture and their intensity decrease versus temperature appeared in good agreement with the better activity of Cu/ZrO₂ compared to pure ZrO₂.     

Methane Combustion in the Presence of Zirconia-Based Catalysts

The influence of “superacidic” modification has been shown to enhance the methane combustion activity of ZrO₂ at 800 °C. Modification with SO₄ ²⁻, Fe/Mn/SO₄ ²⁻ and MoO₃ has been investigated, with the latter showing the most marked effect although this is a critical function of loading. Ceria–zirconia catalysts are also active, although the compositions studied are not as effective as 5 wt% MoO₃/ZrO₂ which shows a greater mass normalised activity than Fe₂O₃.     

Electrophilic chlorination of methane over superacidic sulfated zirconia

Catalytic chlorination of methane was studied over SO ₄ ^2– /ZrO₂, Pt/SO ₄ ^2– /ZrO₂, and Fe/Mn/SO ₄ ^2– /ZrO₂ solid superacid catalysts. The reactions were carried out in a continuous flow reactor under atmospheric pressure, at temperatures below 240°C, with a gaseous hourly space velocity of 1000 ml/g h and a methane to chlorine ratio of 4 to 1. At 200°C with 30% chlorine converted the selectivity in methyl chloride exceeds 90%. At more elevated temperatures, the selectivity decreases but stays above 80% in methyl chloride at 225°C using the sulfated zirconia catalysts. The selectivity can be enhanced by adding platinum to sulfated zirconia catalysts. An iron and manganese-doped catalyst exhibited excellent selectivities at somewhat lower conversions. Methyl chloride is obtained at 235°C in selectivities greater than 85%. No chloroform or carbon tetrachloride is formed. The electrophilic insertion involves electron-deficient metal-coordinated chlorine into the methane C-H bond.Catalysis by solid superacids, 29. For part 28 see ref. [14].     

Partial Oxidation of Ethanol to Hydrogen over Ni–Fe Catalysts

A study has been conducted to identify the influence of zirconia phase and copper to zirconia surface area on the activity of Cu/ZrO₂ catalysts for the synthesis of methanol from either CO/H₂ or CO₂/H₂. To determine the effects of zirconia phase, a pair of Cu/ZrO₂ catalysts was prepared on tetragonal (t-) and monoclinic (m-) zirconia. The zirconia surface area and the Cu dispersion were essentially identical for these two catalysts. At 548 K, 0.65 MPa, and H₂/CO_x= 3 (x = 1, 2), the catalyst prepared on m-ZrO₂ was 4.5 times more active for methanol synthesis from CO₂/H₂ than that prepared on t-ZrO₂, and 7.5 times more active when CO/H₂ was used as the feed. Increasing the surface area of m-ZrO₂ and the ratio of Cu to ZrO₂ surface areas further increased the methanol synthesis activity. In situ infrared spectroscopy and transient-response experiments indicate that the higher rate of methanol synthesis from CO₂/H₂ over Cu/m-ZrO₂ is due solely to the higher concentration of active intermediates. By contrast, the higher rate of methanol synthesis from CO/H₂ is due to both a higher concentration of surface intermediates and the more rapid dynamics of their transformation over Cu/ZrO₂.     

Well-dispersed Ceria-promoted Sulfated Zirconia Supported on Mesoporous Silica

Ceria-promoted sulfated zirconia (CeSZ) was supported on mesoporous molecular sieve of pure-silica MCM-41 (abbreviated as CeSZ/MCM-41). It was prepared by direct impregnation of metal sulfate onto the as-synthesized MCM-41, followed by solid state dispersion and thermal decomposition. The resultant catalysts were characterized by TG, XRD, nitrogen physisorption and TEM. It was showed that the hollow tubular structure of MCM-41 was retained, even with ZrO₂ loading as high as 60 wt.%. Most of CeSZ was well dispersed on the interior surface of the ordered mesopores, following a slight twist of the channels. The catalytic activity of CeSZ/MCM-41 was studied in the octadecanol oxidation. The improved performance of CeO₂-promoted catalysts was attributed to the high dispersion of sulfated zirconia (SZ) and the introduction of CeO₂ enhancing the oxidation ability of catalysts by retarding the transformation of zirconia from highly catalytic active metastable tetragonal phase to monoclinic phase.     

Chemical interactions between 3 mol% yttria-zirconia and Sr-doped lanthanum manganite

The chemical interactions between porous (La_0.8Sr_0.2)MnO₃ (LSM) film and 3 mol% yttria tetragonal zirconia (TZ3Y) substrate have been investigated over the temperature range of 1300–1500 °C in air. Two distinct reaction layers of fluorite-type cubic zirconia solid solution c-(Zr,Mn,La,Y)O₂ and lanthanum zirconate pyrochlore (La,Sr)₂(Zr,Y)₂O₇ were observed at the interface of LSM/TZ3Y. It has been found that the diffusion/dissolution of Mn ions in TZ3Y leads to the formation of the fluorite-type cubic zirconia solid solution, while the interaction of lanthanum with TZ3Y results in the formation of the lanthanum zirconate pyrochlore phase. Phase studies in the (ZrY)O₂–La₂O₃–Mn₃O₄ system show that the fluorite-type cubic zirconia solid solution phase c-(Zr,Mn,La,Y)O₂, rather than the tetragonal 3 mol% Y₂O₃–ZrO₂ phase, is in equilibrium with LSM perovskite at high temperatures. A ternary phase diagram of the system at the (ZrY)O₂-rich end at 1400 °C in air was proposed based on the experimental results. It is suggested that the fundamental reason for the beneficial effect of A-site non-stoichiometry or Mn excess of LSM in the inhibiting of the lanthanum zirconate formation is due to the fact that Mn₃O₄ does not equilibrate with lanthanum zirconate at high temperatures.     

On the contribution of X-ray absorption spectroscopy to explore structure and activity relations of Pt/ZrO2 catalysts for CO2/CH4 reforming

X-ray absorption spectroscopy (XAS) has proven to be a very useful technique in characterizing metal-based catalysts exposed to extreme operating conditions. The technique allows in situ evaluation of structural parameters (XAFS) and electronic properties (XANES). The elucidation of the nature and state of Pt-based catalysts in dry reforming of methane with carbon dioxide is presented as case study to show the contribution and potential of XAS to explore property/performance relationships for heterogeneous catalysts. Pt/ZrO₂ is an active and stable catalyst for the reaction between CH₄ and CO₂ to synthesis gas (H₂/CO). The activity and stability of the catalyst is strongly influenced by the catalyst pretreatment (calcination/reduction). The combination of hydrogen chemisorption, IR spectroscopy, XPS and XAS is shown to be suitable to track the changes of the state of the catalyst. In particular, it will be demonstrated, how XAFS helped to correctly attribute variations in the chemisorptive properties of Pt/ZrO₂ after severe temperature treatment to partial and reversible decoration of the small Pt particles with fragments of the oxide support. In situ tracking of the reduction of the catalysts by XANES additionally helped to semiquantitatively assess the partial reduction of the ZrO₂. Finally, XANES helped to demonstrate that CO₂ exposure under these severe conditions did not lead to detectable levels of surface oxidation of Pt. Based on XANES, IR spectroscopy and kinetic measurements it is concluded that in dry reforming activation of methane occurs on Pt, while CO₂ is activated on the support and the two entities react at the metal–support interface. This revised version was published online in June 2006 with corrections to the Cover Date.     

Biodiesel production from waste cooking oil over alkaline modified zirconia catalyst

Screening and catalytic activity of alkaline modified zirconia i.e. Mg/ZrO₂, Ca/ZrO₂, Sr/ZrO₂, and Ba/ZrO₂ as heterogeneous catalyst in biodiesel production from waste cooking oil (WCO) have been investigated. The catalysts were prepared via wet impregnation of alkaline nitrate salts supported on zirconia. Physico-chemical characteristics of the catalysts were analyzed by BET surface area, XRD, FESEM and CO₂–NH₃–TPD. Among the catalysts screened, Sr/ZrO₂ exhibited higher catalytic activities. Characterization results disclosed Sr/ZrO₂ catalyst possessed balanced basic and acid site concentrations with its pore volume, surface area as well as pore diameters suitable for biodiesel production. The balanced active sites facilitated simultaneous transesterification and esterification of WCO. A plausible mechanism has been suggested for the simultaneous reactions. The effects of operating process conditions such as methanol to oil molar ratio, reaction temperature and catalyst loading on biodiesel production in the presence of Sr/ZrO₂ were investigated. Methyl ester (ME) yield at 79.7% was produced over 2.7 wt.% catalyst loading (Sr/ZrO₂), 29:1 methanol to oil molar ratio, 169 min of reaction time and 115.5 °C temperature.     

Carbon Dioxide Reforming of Methane Over Nanocomposite Ni/ZrO2 Catalysts

A systematic study of the size effect of zirconia nanocrystals on nickel-catalyzed reforming of methane with CO₂ shows that extremely stable Ni/ZrO₂ catalysts are obtainable by hydrogen reduction of impregnated nickel nitrate on zirconia particles with sizes less than 25 nm. The same preparation method with larger particles of zirconia results in catalyst samples that deactivate rapidly in the reforming reaction. Comprehensive characterization with XRD, TPR/TPD, and TEM shows that the stable Ni/ZrO₂ catalysts are better described as nanocomposites of size comparable to Ni metal (9-15 nm) and zirconia (7-25 nm) nanoparticles. The high percentage of the Ni-zirconia boundary or perimeter in the nanocomposite catalysts is believed to be crucial for the extremely stable catalytic activity.     

The role of crystalline phase of zirconia in catalytic conversion of ethanol to propylene

Zirconia catalysts can selectively convert ethanol to propylene and exhibit excellent catalytic stability. However, the effects of crystalline phase of ZrO₂ on the catalyst active sites and catalytic performance have not been fully recognized. In this work, when Y or La was doped into ZrO₂, the monoclinic to tetragonal phase transition occurred, and the propylene yields were improved to 44.0% and 42.3%, respectively. The effects of different crystalline phases of ZrO₂ on the ethanol to propylene reaction were analyzed by density functional theory. Comparing the monoclinic, tetragonal and cubic phases of ZrO₂, the tetragonal phase ZrO₂ has the lowest oxygen vacancy formation energy and is likely to form oxygen vacancies to convert ethanol to propylene. Moreover, the adsorption energy of ethanol on tetragonal ZrO₂ is moderate, which is not only beneficial to ethanol conversion, but also reduces catalyst deactivation caused by excessive adsorption. Therefore, tetragonal ZrO₂ shows practical significance for catalyzing ethanol to propylene.     

Oxidation of o-Xylene to Phthalic Anhydride on Sb-V/ZrO2 Catalysts

Zirconia-supported and bulk-mixed vanadiumantimonium oxide catalysts were used for the oxidation of o-xylene to phthalic anhydride. X-ray diffraction, Raman spectroscopy and photoelectron spectroscopy were used for characterization. It was found that vanadium promotes the transition of tetragonal to monoclinic zirconia. The simultaneous presence of Sb and V on zirconia at low coverage led to a preferential interaction of individual V and Sb oxides with the zirconia surface rather than the formation of a binary Sb-V oxide, while at higher Sb-V contents the formation of SbVO₄ took place. Sb-V/ZrO₂ catalysts showed high activity for o-xylene conversion and better selectivity to phthalic anhydride as compared to V/ZrO₂ catalysts. However, their selectivity to phthalic anhydride was poor in comparison to a V/TiO₂ commercial catalyst. The improved selectivity of the Sb-containing catalysts is attributed to the blocking of non-effective surface sites of ZrO₂, the decrease of the total amount of acid sites and the formation of surface V-O-Sb-O-V structures.     

High activity, templated mesoporous SO4/ZrO2/HMS catalysts with controlled acid site density for α-pinene isomerisation

Highly active mesoporous SO₄/ZrO₂/HMS (hexagonal mesoroporous silica) solid acid catalysts with tuneable sulphated zirconia (SZ) content have been prepared for the liquid phase isomerisation of α-pinene. The mesoporous HMS framework is preserved during the grafting process as evidenced by the X-ray diffraction (XRD) and porosimetry with all SO₄/ZrO₂/HMS materials possessing average pore-diameters 20 Å. XRD confirms the presence of a stabilized tetragonal phase of nanoparticulate ZrO₂, with no evidence for zirconia phase separation or the formation of discrete crystallites, consistent with a uniform and highly dispersed SZ coating. The activity towards α-pinene isomerisation scales linearly with Zr loading, while the specific activities are an order of magnitude greater than attainable by conventional methodologies (1 versus 0.08 mol h⁻¹ g Zr⁻¹).     

Effect of the composition of an oxide coating and the preparation method of block catalysts on their activity in the deep oxidation of methane

Deep methane oxidation catalysts containing 3d metal (Mn, Co), rare-earth (La) and alkali-earth (Ba, Sr) oxides in the porous matrices of secondary supports (Al₂O₃, ZrO₂, and their binary composition) formed on honeycomb blocks (cordierite, kaolin-aerosil) are studied by means of X-ray powder diffraction, the thermal desorption of nitrogen, and temperature-programmed reduction with hydrogen. It is shown that the activity and stability of the catalysts depend on the method for their preparation and the nature of the active components and secondary and block supports. After life cycle tests, the proposed catalysts with 80–100% conversion of methane into CO₂ at temperatures of 650–750°C can be recommended for use in systems for the catalytic purification of gases containing hydrocarbon admixtures (methane and C₂–C₄ homologues) and the combustion of hydrocarbon fuels in industrial and household catalytic heat generators.     

A contrastive study of the introduction of cobalt as a modifier for active components and supports of catalysts for NH3-SCR

Three catalysts Mn/Ce–ZrO_X, Mn–Co/Ce–ZrO_X and Mn/Co–Ce–ZrO_X were used for low-temperature NH₃-SCR of NO. XRD, TPR and XPS were performed to characterize the physicochemical property of the catalysts. Experimental results showed that Mn/Co–Ce–ZrO_X had a higher dispersion of manganese oxides, a better redox property, more surface acid sites and more surface adsorbed oxygen species and Mn⁴⁺ ion. These facts caused a better low-temperature activity for Mn/Co–Ce–ZrO_X which was 99.0% at 180 °C. Furthermore, Mn/Co–Ce–ZrO_X showed the best resistance to SO₂ and H₂O which mainly because the introduction of cobalt inhibited the formation of sulfate salts and hydroxyls on the surface of Mn/Co–Ce–ZrO_X.