CO_2 methanation over TiO_2–Al_2O_3 binary oxides supported Ru catalysts

TiO_2 modified Al_2O_3 binary oxide was prepared by a wet-impregnation method and used as the support for ruthenium catalyst. The catalytic performance of Ru/TiO_2–Al_2O_3catalyst in CO_2 methanation reaction was investigated. Compared with Ru/Al_2O_3 catalyst, the Ru/TiO_2–Al_2O_3catalytic system exhibited a much higher activity in CO_2 methanation reaction. The reaction rate over Ru/TiO_2–Al_2O_3 was 0.59 mol CO_2·(g Ru)1·h-1, 3.1 times higher than that on Ru/Al_2O_3[0.19 mol CO_2·(gRu)-1·h-1]. The effect of TiO_2 content and TiO_2–Al_2O_3calcination temperature on catalytic performance was addressed. The corresponding structures of each catalyst were characterized by means of H_2-TPR, XRD, and TEM. Results indicated that the averaged particle size of the Ru on TiO_2–Al_2O_3support is 2.8 nm, smaller than that on Al_2O_3 support of 4.3 nm. Therefore, we conclude that the improved activity over Ru/TiO_2–Al_2O_3catalyst is originated from the smaller particle size of ruthenium resulting from a strong interaction between Ru and the rutile-TiO_2 support, which hindered the aggregation of Ru nanoparticles.     

Comparative study of CO and CO2 hydrogenation over supported Rh–Fe catalysts

The hydrogenation of CO, CO + CO₂, and CO₂ over titania-supported Rh, Rh–Fe, and Fe catalysts was carried out in a fixed-bed micro-reactor system nominally operating at 543 K, 20 atm, 20 cm³ min^− 1 gas flow (corresponding to a weight hourly space velocity (WHSV) of 8000 cm³ g_cat^− 1 h^− 1), with a H₂:(CO + CO₂) ratio of 1:1. A comparative study of CO and CO₂ hydrogenation shows that while Rh and Rh–Fe/TiO₂ catalysts exhibited appreciable selectivity to ethanol during CO hydrogenation, they functioned primarily as methanation catalysts during CO₂ hydrogenation. The Fe/TiO₂ sample was primarily a reverse water gas shift catalyst. Higher reaction temperatures favored methane formation over alcohol synthesis and reverse water gas shift. The effect of pressure was not significant over the range of 10 to 20 atm.     

Isosynthesis via CO hydrogenation over SO4–ZrO2 catalysts

Catalytic performances of sulfated zirconia catalysts with various contents of sulfur (from 0.1 to 0.75%) on isosynthesis were studied. It was firstly found that undoped-zirconia synthesized from zirconyl nitrate provided higher activity towards isosynthesis reaction (106 μmol kg-cat^?1 s^?1) compared to that synthesized from zirconyl chloride (84.9 μmol kg-cat^?1 s^?1). Nevertheless, the selectivity of isobutene in hydrocarbons was relatively lower. It was then observed that the catalytic reactivity and selectivity significantly improved by sulfur loading. The most suitable sulfur loading content seems to be at 0.1%, which gave the highest reaction rate and selectivity of isobutene. By applying several characterization techniques, i.e. BET, XRD, NH₃- and CO₂-TPD and SEM, it was revealed that the high reaction rate and selectivity towards isosynthesis reaction of sulfated zirconia catalysts are related to the acid–base properties, Zr³⁺ quantity and phase composition.     

CH4 reforming with CO2 for syngas production over nickel catalysts supported on mesoporous nanostructured γ-Al2O3

Nanostructured γ-Al₂O₃ with high surface area and mesoporous structure was synthesized by sol-gel method and employed as catalyst support for nickel catalysts in methane reforming with carbon dioxide. The prepared samples were characterized by XRD, BET, TPR, TPH, SEM and TPO techniques. The BET analysis showed a high surface area of 204m²g^?1 and a narrow pore-size distribution centered at a diameter of 5.5 nm for catalyst support. The results revealed that an increase in nickel loading from 5 to 15 wt% decreased the surface area of catalyst from 182 to 160 m²g^?1. In addition, the catalytic results showed an increase in methane conversion with increase in nickel content. TPO analysis revealed that the coke deposition increased with increasing in nickel loading, and the catalyst with 15 wt% of nickel showed the highest degree of carbon formation. SEM and TPH analyses confirmed the formation of whisker type carbon over the spent catalysts. Increasing CO₂/CH₄ ratio increased the methane conversion. The BET analysis of spent catalysts indicated that the mesoporous structure of catalysts still remained after reaction.     

Highly selective hydrogenation of phenol and derivatives over Pd catalysts supported on SiO2 and γ-Al2O3 in aqueous media

Pd/Al₂O₃ and Pd/SiO₂ catalysts containing Pd nanoparticles in the size range of 3–13 nm were prepared and investigated in direct selective hydrogenation of phenol to cyclohexanone. Catalysts with 3 nm Pd nanoparticles present highly active and promoted the selective formation of cyclohexanone under atmospheric pressure of hydrogen in aqueous media without additives. Conversion of 99% and a selectivity higher than 99% were achieved within 3 h at 333 K. The generality of Pd/Al₂O₃ catalyst with 3 nm Pd nanoparticles for this reaction was demonstrated by selective hydrogenation of other hydroxylated aromatic compounds with similar performance.     

Furfural hydrogenation over carbon‐supported copper

Furfural hydrogenation over copper dispersed on three forms of carbon – activated carbon, diamond and graphitized fibers – were studied. Only hydrogenation of the C=O bond to form either furfuryl alcohol or 2‐methyl furan occurred at temperatures from 473 to 573 K. Reduction at 573 K gave the most active catalysts, all three catalysts had activation energies of 16 kcal/mol, and turnover frequencies were 0.018–0.032 s^-1 based on the number of Cu⁰ + Cu⁺ sites, which were counted by N₂O adsorption at 363 K and CO adsorption at 300 K, respectively. The Cu/activated carbon catalyst showed no deactivation during 10 h on stream, in contrast to the other two catalysts. A simple Langmuir–Hinshelwood model invoking two types of sites was able to fit all kinetic data quite satisfactorily, thus it was consistent with the presence of both Cu⁰ and Cu⁺ sites. This revised version was published online in July 2006 with corrections to the Cover Date.     

Characterization and reactivity of RHA–Al2O3 composite oxides supported nickel catalysts

RHA–Al₂O₃ composite oxide supports were prepared by impregnation of rice husk ash (RHA) with an aluminum sulfate solution, and were then used to prepare supported nickel catalysts by the ion exchange method. The supports and catalysts were characterized by transmission electron microscopy (TEM), X-ray diffraction (XRD), nitrogen adsorption and temperature-programmed desorption (TPD) of hydrogen. The reactivities of nickel catalysts were tested by CO₂ hydrogenation with H₂/CO₂ ratios of 4/1 for temperatures between 573 and 873 K. These results show that nickel compound with a layer structure was formed after the drying step. Furthermore, the nickel crystallites were finely distributed on the support even at high loading. The BET surface area of the unreduced catalysts decreased with the nickel loading up to 2.44 wt% and then increased with further deposits. The hydrogenation reactivity increases with an increase in nickel loading. Furthermore, the hydrogenation reactivity increases with an increasing reaction temperature up to 773 K and then remained constant.     

Preparation of highly active nickel catalysts supported on mesoporous nanocrystalline γ-Al2O3 for CO2 methanation

In this study, a group of Ni catalysts supported on mesoporous nanocrystalline γ-Al₂O₃ with high surface area and with different Ni contents was prepared and employed in carbon dioxide methanation reaction. The obtained results showed increasing in nickel content from 10 to 25 wt.% decreased the specific surface area of catalysts from 177 to 130 m²/g and increased the nickel crystallite size from 12 to 24 nm. In addition, increasing in nickel content increased the reducibility of catalyst. The catalytic results revealed that the catalyst with 20 wt.% of Ni possessed high activity and stability in CO₂ methanation reaction.     

Isothermal, non‐oxidative, two‐step CH4 conversion over unsupported and supported Ru and Pt catalysts

The isothermal, non‐oxidative, two‐step conversion of CH₄ to C₂₊ hydrocarbons was investigated over unsupported and supported Pt and Ru catalysts at moderate temperatures and elevated pressures. The single‐cycle specific activity (μmol C₂₊/g_cat) and total product yield (μmol C₂₊/μmol surface metal) for Ru powder at 430 K were significantly higher than for Pt powder at 503 K. The activity and total product yield for mixed metal oxide (MMO)‐supported Ru were significantly less than for Ru powder; however, Pt/MMO exhibited similar activity and product yield in comparison to Pt powder. The formation of methylbutane and methylpentane increased with increasing pressure over Ru/MMO. However, increasing pressure favored the formation of C₄ species over Pt/MMO. This revised version was published online in July 2006 with corrections to the Cover Date.     

Lean NOx catalysis over alumina‐supported catalysts

aluminasupported catalysts show promise as lean NO_x catalysts. The role of alumina in influencing the structural and chemical properties of the active phase supported on it is discussed using some effective aluminabased lean NO_x catalysts. These include Ag/Al₂O₃, CoO_x/Al₂O₃ and SnO₂/Al₂O₃. Alumina plays an important role in stabilizing Ag in the oxidic phase and cobalt in the 2+ oxidation state. For SnO₂/Al₂O₃, alumina increases the SnO₂ surface area. On both Ag/Al₂O₃ and SnO₂/Al₂O₃, alumina also participates actively in the NO_x reduction reaction. An active organic intermediate is formed on Ag or Sn oxide which reacts with NO_x subsequently on alumina to form N₂.     

Reduction of NO by CO under oxidizing conditions over Pt and Rh supported on Al2O3–ZrO2 binary oxides

The platinum and rhodium particles supported on Al₂O₃–ZrO₂ binary oxides were prepared by adding the metal precursors to the binary gel. High specific surface areas (220–250 m²/g) and small metallic particle size (∼20 Å) were obtained. In the reduction of NO by CO without oxygen in the reactant flow high levels of N₂O were achieved, whereas in the presence of oxygen the formation of N₂O notably increases. This selectivity behavior was not observed in catalysts prepared by impregnation with the metallic precursors of Al₂O₃–ZrO₂ mixed oxide stabilized after calcination at 500 °C, since in these catalysts the selectivity to N₂O is the higher with or without oxygen. Thus, it is proposed that the metallic impregnation of gels strongly modifies the mechanism by which the NO reduction by CO occurs.     

Intramolecular selective hydrogenation of cinnamaldehyde over CeO2–ZrO2-supported Pt catalysts

Selective liquid phase hydrogenation of cinnamaldehyde is reported, for the first time, over CeO₂, ZrO₂, and CeO₂–ZrO₂-supported Pt catalysts. Cinnamyl alcohol is the selective product. These catalysts are highly active and selective even at 25 °C and found to be superior to most of the hitherto known supported Pt catalysts. Alkali addition (NaOH) has enhanced the performance of these catalysts. At an optimized reaction condition, 95.8% conversion of cinnamaldehyde and 93.4% selectivity of cinnamyl alcohol have been obtained. Acidity of the support (due to the presence of ZrO₂ component) and higher electron density at Pt (due to CeO₂ component) are attributed to be responsible for the superior catalytic activity of Pt supported on CeO₂–ZrO₂ composite material.     

High catalytic activity in CO PROX reaction of low cobalt-oxide loading catalysts supported on nano-particulate CeO2–ZrO2 oxides

Co₃O₄/NP-ZrO₂, Co₃O₄/NP-CeO₂ and Co₃O₄/NP-Ce_0.8Zr_0.2O₂ catalysts were prepared via a reverse microemulsion/incipient wetness impregnation (RM–IWI) method. The catalytic properties for CO preferential oxidation (CO PROX) reaction in H₂-rich stream were investigated. The Co₃O₄/NP-Ce_0.8Zr_0.2O₂ catalyst with 1.8 wt.% Co₃O₄ loading has exhibited higher catalytic activity than that of the other two catalysts. The higher catalytic activity might be attributed to the combination effect of the highly dispersed cobalt oxide, the improvement in CeO₂ reducibility due to ZrO₂ incorporation in CeO₂ structures, and the strong cobalt oxide-support interaction.     

Synthesis of natural gas from CO methanation over SiC supported Ni–Co bimetallic catalysts

A series of SiC supported bimetallic Ni–Co catalysts with different Ni/Co ratios was investigated for CO methanation reaction. Compared with monometallic Ni- and Co-based catalysts, the bimetallic Ni–Co catalysts showed higher methanation activity and the Ni/Co ratios significantly affected the methanation activity. The highest activity was obtained over 6Ni4Co/SiC bimetallic catalyst, which also showed excellent stability during the methanation reaction. Further characterizations revealed that the surface enrichment of metal components occurred on the bimetallic catalysts. The interaction between Ni and Co and higher metal dispersion might improve the adsorption and activation of CO and thus enhance the methanation activity of the bimetallic catalysts.     

Core–shell structured Cu@m-SiO2 and Cu/ZnO@m-SiO2 catalysts for methanol synthesis from CO2 hydrogenation

The core–shell catalysts with Cu and Cu/ZnO nanoparticles coated by mesoporous silica shells are prepared for CO₂ hydrogenation to methanol. With the confined effect of silica shell, the size of Cu nanoparticles is only about 5.0 nm, which results in high activity for CO₂ conversion. The CH₃OH selectivity is enhanced significantly with the introduction of ZnO. The core–shell structured catalysts endow the Cu nanoparticles trapped inside with excellent anti-aggregation and no deactivation is observed with time-on-stream. Therefore, the core–shell Cu/ZnO@m-SiO₂ catalyst exhibits the maximum CH₃OH yield with high stability.     

The partial gas‐phase hydrogenation of benzene to cyclohexene on supported and coated ruthenium catalysts

The conversion of benzene to useful products such as cyclohexene is of industrial interest because of the expected surplus of benzene due to its substitution in gasoline by other nonpolluting components in the next years. Therefore, the partial gasphase hydrogenation of benzene to cyclohexene at atmospheric pressure was performed in order to develop catalysts as an alternative to those used in liquidphase hydrogenation. Two types of rutheniumcontaining catalysts were investigated, viz. supported catalysts with different support materials and coated catalysts with electrolytically formed alumina as support. In order to yield the desired cyclohexene the presence of methanol as a reaction modifier was necessary in the gas phase during the reaction. The hydrogenation on supported Ru catalysts gave selectivities of about 35%, while on coated Ru catalysts selectivities up to 45% were obtained at conversion degrees of 5%. Improved catalyst performance, especially higher selectivity and yield, was obtained at increased partial pressure of methanol and hydrogen and by addition of copper as second metal in the oxide layer of the coated catalysts.     

High‐resolution imaging of nanoparticle bimetallic catalysts supported on mesoporous silica

Although conventional high‐resolution transmission electron microscopy is a powerful method for the elucidation of the structure of mesoporous solids (diameter of pores from 1.5 to 20 nm), it is far less capable than high‐resolution scanning transmission electron microscopy in identifying the spatial distribution of nanocrystals of catalysts encapsulated within the mesopores. Using high‐angle annular dark‐field imaging (either in a 100 or 300 keV STEM system), it is possible to locate precisely individual bimetallic nanoparticles (Ag₃Ru₁₀, Cu₄Ru₁₂ and Pd₆Ru₆ hydrogenation catalysts) supported on mesoporous silica, to determine their size distribution, and to record their characteristic X‐ray emission maps. It is also established that there is little tendency for elemental fragmentation of the bimetallic catalysts, all of which were prepared by decarbonylating, by thermolysis, precursor cluster carbonylate anions: [Ag₃Ru₁₀C₂(CO)₂₈Cl]⁻, [Ru₆C(CO)₁₆Cu₂Cl]²⁻ and [Ru₆Pd₆(CO)₂₄]²⁻. This revised version was published online in July 2006 with corrections to the Cover Date.     

Temperature-programmed reduction and CO oxidation studies over Ce–Sn mixed oxides

The reduction behaviour of Ce–Sn mixed oxides has been studied by a temperature-programmed hydrogen reduction technique and compared with that of pure SnO₂ and CeO₂. The mixed oxides were found to reduce at lower temperature as compared to that of individual oxides. Carbon monoxide oxidation studies showed that mixed oxides have better activity for CO oxidation reaction than the constituent oxides, which is in conformity with their surface reduction behaviour. The improved oxidation activity is attributed to a synergetic effect existing in these mixed oxides.