Performance of Pd catalysts supported on nanocrystalline α-Al2O3 and Ni-modified α-Al2O3 in selective hydrogenation of acetylene

Nanocrystalline α-Al₂O₃ and Ni-modified α-Al₂O₃ have been prepared by sol–gel and solvothermal methods and employed as supports for Pd catalysts. Regardless of the preparation method used, NiAl₂O₄ spinel was formed on the Ni-modified α-Al₂O₃ after calcination at 1150 °C. However, an addition of NiO peaks was also observed by X-ray diffraction for the solvothermal-made Ni-modified α-Al₂O₃ powder. Catalytic performances of the Pd catalysts supported on these nanocrystalline α-Al₂O₃ and Ni-modified α-Al₂O₃ in selective hydrogenation of acetylene were found to be superior to those of the commercial α-Al₂O₃ supported one. Ethylene selectivities were improved in the order: Pd/Ni-modified α-Al₂O₃–sol–gel > Pd/Ni-modified α-Al₂O₃-solvothermal ≈ Pd/α-Al₂O₃–sol–gel > Pd/α-Al₂O₃-solvothermal  Pd/α-Al₂O₃-commerical. As revealed by NH₃ temperature program desorption studies, incorporation of Ni atoms in α-Al₂O₃ resulted in a significant decrease of acid sites on the alumina supports. Moreover, XPS revealed a shift of Pd 3d binding energy for Pd catalyst supported on Ni-modified α-Al₂O₃–sol–gel where only NiAl₂O₄ was formed, suggesting that the electronic properties of Pd may be modified.     

Effect of CO2 in the feed stream on the deactivation of Co/γ-Al2O3 Fischer–Tropsch catalyst

The influence of CO₂ on the deactivation of Co/γ-Al₂O₃ Fischer–Tropsch (FT) catalyst in CO hydrogenation has been investigated. The presence of CO₂ in the feed stream reveals a negative effect on catalyst stability and in the formation of heavy hydrocarbons. The CO₂ acts as a mild oxidizing agent on cobalt metal during Fischer–Tropsch synthesis. During FT synthesis on Co/γ-Al₂O₃ of 70 h, the CO conversion and C₅₊ selectivity in the presence of CO₂ decreased more significantly than in the absence of CO₂. CO₂ is found to be responsible for the partial oxidation of surface cobalt metal at FT synthesis environment with the co-existence of generated water.     

Effects of the preparation methods on the performance of the Cu–Cr–Fe/γ-Al2O3 catalysts for the synthesis of 2-methylpiperazine

Co-precipitation, impregnation and ultrasonic sol–gel (USG) methods have been used to prepare Cu–Cr–Fe/γ-Al₂O₃ catalysts, which were further used to synthesize 2-methylpiperazine. The catalysts were characterized by XRD, XPS, TG/DSC, BET, TPR, AAS and TEM. It is found that preparation method can greatly impact the catalytic performance of the catalysts, the Cu–Cr–Fe/γ-Al₂O₃ catalyst prepared by the ultrasonic sol–gel method proved to be the most active and stable for this reaction. The dispersion and stabilization of Cu⁰ in the reduced catalysts are attributed to the existence of CuCr₂O₄ and Fe₂O₃. A surprising copper migration was detected by XPS analysis for the Cu–Cr–Fe/γ-Al₂O₃-USG catalyst after the calcination process, which may be crucial to the high activity and stability of this catalyst.     

Study of ruthenium supported on Ta2O5–ZrO2 and Nb2O5–ZrO2 as catalysts for the partial oxidation of methane

Ru-based catalysts supported on Ta₂O₅–ZrO₂ and Nb₂O₅–ZrO₂ are studied in the partial oxidation of methane at 673–873 K. Supports with different Ta₂O₅ or Nb₂O₅ content were prepared by a sol–gel method, and RuCl₃ and RuNO(NO₃)₃ were used as precursors to prepare the catalysts (ca. 2 wt.% Ru). At 673 K high selectivity to CO₂ was found. An increase of temperature up to 773 K produced an increase in the selectivity to syngas (H₂/CO = 2.2–3.1), and this is related with the transformation of RuO₂ to metallic Ru as was determined from XRD and XPS results. At 873 K and with co-fed CO₂ an increase of the catalytic activity and CO selectivity was found. A TOF value of 5.7 s⁻¹ and H₂/CO ratio ca. 1 was achieved over Ru(Cl)/6TaZr. Catalytic results are discussed as a function of the support composition and characteristics of Ru-based phases.     

Effect of Ru addition to Co/SiO2/HZSM-5 catalysts on Fischer–Tropsch synthesis of gasoline-range hydrocarbons

Microporous HZSM-5 zeolite and mesoporous SiO₂ supported Ru–Co catalysts of various Ru adding amounts were prepared and evaluated for Fischer–Tropsch synthesis (FTS) of gasoline-range hydrocarbons (C₅–C₁₂). The tailor-made Ru–Co/SiO₂/HZSM-5 catalysts possessed both micro- and mesopores, which accelerated hydrocracking/hydroisomerization of long-chain products and provided quick mass transfer channels respectively during FTS. In the same time, Ru increased Co reduction degree by hydrogen spillover, thus CO conversion of 62.8% and gasoline-range hydrocarbon selectivity of 47%, including more than 14% isoparaffins, were achieved simultaneously when Ru content was optimized at 1 wt% in Ru–Co/SiO₂/HZSM-5 catalyst.     

Synthesis of DME from syngas on the bifunctional Cu–ZnO–Al2O3/Zr-modified ferrierite: Effect of Zr content

The catalytic activity on the coprecipitated Cu–ZnO–Al₂O₃/Zr-ferrierite (CZA–ZrFER) with different Zr content from 0 to 5 wt.% was investigated for the direct synthesis of dimethylether (DME) from H₂-deficient and biomass-derived model syngas (H₂/CO molar ratio = 0.93). The catalytic functionalities, such as CO conversion and DME selectivity, showed their maxima on the bifunctional catalyst with 3 wt.% Zr-modified ferrierite. Detailed characterization studies were conducted on the catalysts to measure their properties such as surface area, acidity by temperature-programmed desorption of ammonia (NH₃-TPD), reducibility of Cu oxide by temperature-programmed reduction (TPR), copper surface area measurements by N₂O titration method, electronic states of copper by IR analysis and particle size measurement by XRD and TEM analysis. The number of acid sites measured by NH₃-TPD on the bifunctional catalysts decreased monotonously with the increase of Zr content, meanwhile, the acidic strength is found to be minimal on the catalyst showing best performance. The reducibility of copper oxide and the surface area of metallic copper also exhibited their maximum values at the same Zr composition indicating that these are responsible for the optimum functionality of the bifunctional CZA–ZrFER catalyst. The role of easily reducible copper species with small particle size and the suppressed strong acidic sites is also emphasized in the consecutive reaction from syngas to DME on the bifunctional catalyst. The different behavior of intrinsic rate of the bifunctional catalysts is also well correlated with the metallic surface area of copper and the amount of acidic sites with their acidic strength.     

Effect of the hydrogen spillover on the selectivity of dibenzothiophene hydrodesulfurization over CoSx/γ-Al2O3, NiSx/γ-Al2O3 and MoS2/γ-Al2O3 catalysts

Dibenzothiophene (DBT) hydrodesulphurization (HDS) reaction at 3 MPa and 325–375 °C on Mo/γ-Al₂O₃ single-bed and Me/γ-Al₂O₃//SiO₂//Mo/γ-Al₂O₃ (Me = Co or Ni) double-bed catalysts were investigated. Results indicate that ratio cyclohexylbenzene (CHB)/biphenyl (BP) or selectivity is higher when using double-beds rather than a single-bed. Synergy in dibenzothiophene hydrodesulphurization on Co//Mo and Ni//Mo double-beds is also detected. Changes in selectivity and conversion are attributed to the action of spillover hydrogen (Hso) formed in the first bed that reaches the second bed.     

Hydrodesulphurization performance of NiW/TiO2-Al2O3 catalyst for ultra clean diesel

TiO₂-Al₂O₃ binary oxide supports were obtained by sol–gel methods from Tetra-n-butyl-titanate and pseudoboehmite/aluminium chloride resources. The typical physico-chemical properties of NiW/TiO₂-Al₂O₃ catalysts with different TiO₂ loadings and their supports were characterized by means of BET, XRD and UV–vis DRS, etc. The BET results indicated that the specific surface areas of NiW/TiO₂-Al₂O₃ catalysts were as higher as that over pure γ-Al₂O₃ support, and the pore diameters were also large. The XRD and UV–vis DRS analyzing results showed that the Ti-containing supported catalysts existed as anatase TiO₂ species and the incorporation of TiO₂ could adjust the interaction between support and active metal, and impelled the higher reducibility of tungsten. The hydrodesulphurization (HDS) performance of the series catalysts were evaluated with diesel feedstock in a micro-reactor unit, and the HDS results showed that NiW/TiO₂-Al₂O₃ catalysts exhibited higher activities of ultra deep hydrodesulphurization of diesel oil than that of NiW/Al₂O₃ catalyst. The optimal TiO₂ content of NiW/TiO₂-Al₂O₃ catalysts was about 15 m%, and the corresponding HDS efficiency could reach to 100%. The sulphur contents of diesel products over NiW/TiO₂-Al₂O₃ (from pseudoboehmite/AlCl₃) catalysts with suitable TiO₂ content could be less than 15 ppmw, which met the sulphur regulation of Euro IV specification of ultra clean diesel fuel.     

Carbon dioxide reforming of ethanol over Ni/Y2O3–ZrO2 catalysts

Supported nickel catalysts of composition Ni/Y₂O₃–ZrO₂ were synthesized in one step by the polymerization method and compared with a nickel catalyst prepared by wet impregnation. Stronger interactions were observed in the formed catalysts between NiO species and the oxygen vacancies of the Y₂O₃–ZrO₂ in the catalysts made by polymerization, and these were attributed to less agglomeration of the NiO during the synthesis of the catalysts in one step. The dry reforming of ethanol was catalyzed with a maximum CO₂ conversion of 61% on the 5NiYZ catalyst at 800 °C, representing a better response than for the catalyst of the same composition prepared by wet impregnation.     

The functionalities of Pt/γ-Al2O3 catalysts in simultaneous HDS and HDA reactions

A Pt/γ-Al₂O₃ catalyst was tested in simultaneous hydrodesulfurization (HDS) of dibenzothiophene and hydrodearomatization (HDA) of naphthalene reactions. Samples of it were subjected to different pretreatments: reduction, reduction–sulfidation, sulfidation with pure H₂S and non-activation. The reduced catalyst presented the best performance, even comparable to that of Co(Ni)Mo catalysts. All catalyst samples were selective to the HDS reaction over HDA, and to the direct desulfurization pathway of dibenzothiophene HDS over the hydrogenation reaction pathway of HDS. The effect of H₂S partial pressure on the functionalities of the reduced Pt/γ-Al₂O₃ catalyst was studied. The results showed that an increase in H₂S partial pressure does not cause poisoning, but an inhibition effect, without changing the catalyst selectivity. Accordingly, the activity trends were ascribed to adsorption differences between the different reactive molecules over the same catalytic active site. TPR characterization along with a thermodynamics analysis showed that the active phase of reduced Pt/γ-Al₂O₃ is constituted by Pt⁰ particles. However, presulfidation of the catalyst leads to a mixture of PtS and Pt⁰ which has a negative effect on the catalytic performance without changing catalyst functionalities.     

CO2 adsorption and activation over γ-Al2O3-supported transition metal dimers: A density functional study

Catalytic conversion of CO₂ to liquid fuels has the benefit of reducing CO₂ emission. Adsorption and activation of CO₂ on the catalyst surface are key steps of the conversion. Herein, we used density functional theory (DFT) slab calculations to study CO₂ adsorption and activation over the γ-Al₂O₃-supported 3d transition metal dimers (M₂/γ-Al₂O₃, M = Sc–Cu). CO₂ was found to adsorb on M₂/γ-Al₂O₃ negatively charged and in a bent configuration, indicating partial activation of CO₂. Our results showed that both the metal dimer and the γ-Al₂O₃ support contribute to the activation of the adsorbed CO₂. The presence of a metal dimer enhances the interaction of CO₂ with the substrate. Consequently, the adsorption energy of CO₂ on M₂/γ-Al₂O₃ is significantly higher than that on the γ-Al₂O₃ surface without the metal dimer. The decreasing binding strength of CO₂ on M₂/γ-Al₂O₃ as M₂ changes from Sc₂ to Cu₂ was attributed to decreasing electron-donation by the supported metal dimers. Hydroxylation of the support surface reduces the amount of charge transferred to CO₂ for the same metal dimer and weakens the CO₂ chemisorption bonds. Highly dispersed metal particles maintained at a small size are expected to exhibit good activity toward CO₂ adsorption and activation.     

Selective oxidation of propane and ethane on diluted Mo–V–Nb–Te mixed-oxide catalysts

Te-free and Te-containing Mo–V–Nb mixed oxide catalysts were diluted with several metal oxides (SiO₂, γ-Al₂O₃, α-Al₂O₃, Nb₂O₅, or ZrO₂), characterized, and tested in the oxidation of ethane and propane. Bulk and diluted Mo–V–Nb–Te catalysts exhibited high selectivity to ethylene (up to 96%) at ethane conversions <10%, whereas the corresponding Te-free catalysts exhibited lower selectivity to ethylene. The selectivity to ethylene decreased with the ethane conversion, with this effect depending strongly on the diluter and the catalyst composition. For propane oxidation, the presence of diluter exerted a negative effect on catalytic performance (decreasing the formation of acrylic acid), and α-Al₂O₃ can be considered only a relatively efficient diluter. The higher or lower interaction between diluter and active-phase precursors, promoting or hindering an unfavorable formation of the active and selective crystalline phase [i.e., Te₂M₂₀O₅₇ (M = Mo, V, and Nb)], determines the catalytic performance of these materials.     

Size effects on χ- to α-Al2O3 phase transformation

Nano-scaled χ-Al₂O₃ powders with d₅₀ mean particle sizes from 17 to 314 nm were prepared to investigate the size effect on their phase transformation. Structural properties and crystallization behavior as a function of thermal treatments of various-sized χ-Al₂O₃ particles were examined by DTA, XRD and TEM characterizations. It was confirmed that the decrease of particle size allows for stable α-Al₂O₃ formation at relatively low temperature. Furthermore, the phase transformation route of χ-Al₂O₃ to α-Al₂O₃ was also modified due to the decrease of particle size. A critical size of χ-Al₂O₃ that determines the phase transformation behavior was found to be around 40 nm. For particles larger than 40 nm, a transition phase of κ-Al₂O₃ is formed before obtaining final α-Al₂O₃ phase. Nevertheless, for those smaller than the critical size, starting χ-Al₂O₃ particles have to grow to 40 nm and then directly transform to α-Al₂O₃ bypassing κ-Al₂O₃ at a temperature as low as 1050 °C.     

Synergetic effect of combined use of Cu–ZnO–Al2O3 and Pt–Al2O3 for the steam reforming of methanol

Commercial Cu–ZnO–Al₂O₃ catalysts are used widely for steam reforming of methanol. However, the reforming reactions should be modified to avoid fuel cell catalyst poisoning originated from carbon monoxide. The modification was implemented by mixing the Cu–ZnO–Al₂O₃ catalyst with Pt–Al₂O₃ catalyst. The Pt–Al₂O₃ and Cu–ZnO–Al₂O₃ catalyst mixture created a synergetic effect because the methanol decomposition and the water–gas shift reactions occurred simultaneously over nearby Pt–Al₂O₃ and Cu–ZnO–Al₂O₃ catalysts in the mixture. A methanol conversion of 96.4% was obtained and carbon monoxide was not detected from the reforming reaction when the Pt–Al₂O₃ and Cu–ZnO–Al₂O₃ catalyst mixture was used.     

Sintering kinetics of α-Al2O3 powder

The sintering kinetics of α-Al₂O₃ powder are reviewed in this paper. The initial sintering of α-Al₂O₃ micropowder and α-Al₂O₃ nanopowder is all controlled by grain boundary diffusion. The sintering kinetics dominate up to a relative density of 0.77, where the coarsening kinetics dominate during further densification. Herring's scaling law can be used to predict the approximate sintering temperature of α-Al₂O₃ powder and demonstrates that if the particle size can be reduced to <20 nm, sintering below 1000°C may be possible. ©     

Development of a multi-layered micro-reactor coated with Pt–Co/Al2O3 catalyst for preferential oxidation of CO

Pt–Co/Al₂O₃ catalysts were prepared with different Co/Pt weight ratios (0.3–1.8) and their performances for preferential oxidation of CO (PROX) were tested. The activity of the catalyst increased with Co/Pt weight ratio due to the increase of the area of active phase by interaction between Pt and Co species. The 13-layered micro-channel reactor was prepared by stacking the plates coated with Pt–Co/Al₂O₃ catalyst. The reactor was divided into three parts (inlet, middle, and outlet) to evaluate the performance of each part. Most of O₂ supplied was depleted at the inlet part and the temperature gradient of the reactor occurred due to the high exothermicity of oxidations of CO and hydrogen. In order to prevent hot spot and temperature gradient, the reactor with non-uniform distribution of the catalyst (partially coating the catalyst on the channels) was prepared. The prepared reactor showed uniform temperature distribution and exhibited excellent performance for PROX.     

Low-temperature firing and microwave dielectric properties of 16CaO–9Li2O–12Sm2O3–63TiO2 ceramics with V2O5 addition

The sintering behaviors and microwave dielectric properties of the 16CaO–9Li₂O–12Sm₂O₃–63TiO₂ (abbreviated CLST) ceramics with different amounts of V₂O₅ addition had been investigated in this paper. The sintering temperature of the CLST ceramic had been efficiently decreased by nearly 100 °C. No secondary phase was observed in the CLST ceramics and complete solid solution of the complex perovskite phase was confirmed. The CLST ceramics with small amounts of V₂O₅ addition could be well sintered at 1200 °C for 3 h without much degradation in the microwave dielectric properties. Especially, the 0.75 wt.% V₂O₅-doped ceramics sintered at 1200 °C for 3 h have optimum microwave dielectric properties of Kr = 100.4, Q × f = 5600 GHz, and TCF = 7 ppm/°C. Obviously, V₂O₅ could be a suitable sintering aid that improves densification and microwave dielectric properties of the CLST ceramics.     

The effect of sodium on the catalytic activity of ZnO–Al2O3/ZSM-5 and SnO–Al2O3/ZSM-5 for the transesterification of vegetable oil with methanol

In order to elucidate the effect of sodium on the activity of ZSM-5 supported metal oxides catalysts (ZnO–Al₂O₃/ZSM-5 and SnO–Al₂O₃/ZSM-5) for the transesterification of soybean oil with methanol, ZSM-5 supported metal oxides were prepared with and without sodium hydroxide by impregnation. The metal compositions of the ZSM-5 supported metal oxide catalysts and the metal concentrations dissolved from the catalysts to the methylester phase were measured by SEM-EDS and inductive coupled plasma spectroscopy, respectively. The catalytic activity of ZnO–Al₂O₃/ZSM-5 and SnO–Al₂O₃/ZSM-5 containing sodium did not originate from surface metal oxides sites, but from surface sodium sites or dissolved sodium leached from the catalyst surface.     

Dielectric properties and microstructures of CaSiO3 ceramics with B2O3 addition

The effects of B₂O₃ additives on the sintering behavior, microstructure and dielectric properties of CaSiO₃ ceramics have been investigated. The B₂O₃ addition resulted in the emergence of CaO–B₂O₃–SiO₂ glass phase, which was advantageous to lower the synthesis temperature of CaSiO₃ crystal phase, and could effectively lower the densification temperature of CaSiO₃ ceramic to as low as 1100 °C. The 6 wt% B₂O₃-doped CaSiO₃ ceramic sintered at 1100 °C possessed good dielectric properties: _r = 6.84 and tan δ = 6.9 × 10⁻⁴ (1 MHz).     

CO hydrogenation to iso-C4 hydrocarbons over CeO2–TiO2 catalysts

CeO₂–TiO₂ (Ce:Ti = 0.25–9, molar ratio) catalysts were synthesized by a sol–gel method and the catalytic performances were evaluated in the selective synthesis of isobutene and isobutane from CO hydrogenation under the reaction conditions of 673–748 K, 1–5 MPa and 720–3000 h⁻¹. The physical properties, such as specific surface area, cumulative pore volume, average pore diameter, crystal phase and size, of the catalysts were characterized by N₂ adsorption/desorption and XRD. All the CeO₂–TiO₂ composite oxides showed higher surface areas than pure TiO₂ and CeO₂. No TiO₂ phase was detected on the samples of CeO₂–TiO₂ in which TiO₂ contents were in the range of 10–50 mol%. Crystalline Ce₂O₃ was detected in CeO₂–TiO₂ (8:2). The reaction conditions, temperature, pressure and space velocity, had obvious influences on the CO conversion and distribution of the products over CeO₂–TiO₂ (8:2) catalyst.