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.     

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.     

Influence of Ru segregation on the activity of Ru–Co/γ-Al2O3 during FT synthesis: A comparison with that of Ru–Co/SiO2 catalysts

γ-Al₂O₃ and SiO₂ supported Co catalysts, with varying amounts of Ru, were prepared and evaluated for Fischer–Tropsch synthesis (FTS). The composition of Ru for optimum activity was found to be support-dependent. The reducible Co₃O₄ was high in the region of 0–1.64 wt.% of Ru in Co/SiO₂ catalysts. Co/γ-Al₂O₃ displayed a maximum for reducible Co species at 0.42 wt.% Ru. Segregation of Ru occurred beyond this composition decreasing the extent of reduction. Co/γ-Al₂O₃ catalysts showed lower activity and olefin selectivity, in spite of higher Co dispersion, than Co/SiO₂ catalysts. The catalytic performance depends on the amount of reducible Co species, which again depends upon the optimum content of Ru.     

Photocatalytic cyclohexane oxidehydrogenation on sulphated MoOx/γ-Al2O3 catalysts

The photocatalytic oxidative dehydrogenation of cyclohexane on sulphated MoO_x/γ-Al₂O₃ catalysts has been studied in a two-dimensional fluidized bed photoreactor. The influence of Mo loading at similar sulphate content and the effect of catalyst preparation method have been investigated.Considering the influence of Mo loading at similar sulphate content, the highest photoactivity at 2.4 SO₄ wt% was found at MoO₃ loading of 8 wt%. Selectivity to cyclohexene was 100%, irrespective of the Mo content.At fixed MoO_x content, in particular at 50% of theoretical monolayer coverage, the preparation method of catalysts strongly affected the photocatalysts performances, showing in addition a slight decrease in selectivity to cyclohexene due to side-production of benzene. All the catalysts showed a similar equivalent band gap energy. Thermogravimetric analysis evidenced the presence of surface sulphate species of different thermal stabilities. A linear correlation of photoactivity with the surface sulphates amount of lower thermal stability has been found for all sub-monolayer MoO_x sulphated catalysts. The neighboring of surface sulphates to octahedral polymolybdate species appears to be a key parameter for the photoactivity of the catalysts.The catalyst selectivity was related to surface acidity. Higher acidity resulted in increased cyclohexene dark adsorption and consequently in enhanced benzene formation.     

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.     

Examination of de-coking of promoted (Co, Ni) Mo/γ-Al2O3 catalysts by X-ray photoelectron spectroscopy

Promoted (Co, Ni)-Mo/γ-Al₂O₃ hydrodesulphurization catalysts have been decoked using hydrogen gasification. The catalysts were exposed to a benzene, toluene and xylene (BTX) feed for one week until a significant level of coke had been built up on the surface. The catalysts were then decoked by gasifi- cation at 400°C in hydrogen. The decoking was monitored using the X-ray photoelectron spectroscopy C/Al ratio. After 6 h gasification there was a tenfold decrease in the C/A1 ratio indicating a significant amount of decoking. Furthermore, the initial activity was regained as determined by the conversion of a standard BTX feed.     

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.     

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. ©     

Sintering of α-Al2O3/quartz, and α-Al2O3/cristobalite related to mullite formation

Cristobalite and quartz react differently in mixtures with α-Al₂O₃ at 1415°C. With cristobalite, an eutectic liquid forms in accordance with the metastable phase equilibrium diagram for α-Al₂O₃-SiO₂ (cristobalite) in the absence of mullite. With quartz, a liquid first forms on the surfaces of quartz because of the occurence of an intermediate liquid phase on transformation of quartz to cristobalite. These liquids act as precursors to the formation of mullite by reacting with α-Al₂O₃. Mullite was detected earlier in the cristobalite-containing mixtures under similar firing conditions because the growth of mullite becomes significant with the formation of the eutectic liquid at the α-Al₂O₃-cristobalite interface since it is already saturated with Al₂O₃. The kinetics of sintering are affected by the rates of the step reactions.     

HDS of benzothiophene and dihydrobenzothiophene over sulfided Mo/γ-Al2O3

The hydrodesulfurization (HDS) of benzothiophene (BT) and dihydrobenzothiophene (DHBT) was studied over a sulfided Mo/γ-Al₂O₃ catalyst at 5 MPa and 280 and 300 °C. In the absence of H₂S, benzothiophene reacted by hydrogenation to dihydrobenzothiophene and by hydrogenolysis to ethylbenzene (EB), and dihydrobenzothiophene reacted by hydrogenolysis to ethylbenzene. H₂S inhibited both hydrogenation and hydrogenolysis, but the latter much more strongly. The reverse inhibition was observed for 2-methylpiperidine (MPi). In the presence of H₂S and/or 2-methylpiperidine, dihydrobenzothiophene reacted to ethylbenzene as well as by total hydrogenation to octahydrobenzothiophene, and on to ethylcyclohexenes and ethylcyclohexane. Dihydrobenzothiophene did not react back to benzothiophene at and below 300 °C, while the equivalent tetrahydrodibenzothiophene reacted fast to an equilibrium with tetrahydrodibenzothiophene, due to stabilization of the vinylic bond by the alkyl groups. The observed products and kinetic results were explained by a model in which the CS bonds were mainly broken by hydrogenolysis.     

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.     

Low-temperature 1,3-butadiene hydrogenation over supported Pt/3d/γ-Al2O3 bimetallic catalysts

Low-temperature 1,3-butadiene hydrogenation is used as a probe reaction to investigate the hydrogenation activity over several γ-Al₂O₃ supported Pt/3d (3d = Co, Ni, Cu) bimetallic catalysts. Batch and flow reactor studies are employed to quantify the kinetic activity and steady-state conversion, respectively, of each catalyst. Transmission electron microscopy (TEM) is utilized to characterize particle sizes and extended X-ray absorption fine structure (EXAFS) measurements are performed to verify the Pt–3d bimetallic bond formation. Pulse carbon monoxide chemisorption measurements are also performed to characterize the number of active sites. Additionally, density functional theory (DFT) calculations are included to determine the binding energies of 1,3-butadiene and atomic hydrogen on the corresponding model surfaces. The binding energies of the adsorbates are found to correlate with the hydrogenation activity, allowing for use of such correlation to potentially predict hydrogenation catalysts with enhanced activity based on the binding energies of the adsorbates of interest.     

Partial oxidation of methane over VOx/α-Al2O3 catalysts

The catalytic behavior of a series of VO_x/α-Al₂O₃ catalysts for the partial oxidation of methane has been evaluated. Samples with different vanadia loading were prepared from NH₄VO₃ and V(AcAc)₃. Characterization performed by TPR and oxygen uptake measurements indicates that different VO_x species are present on the samples. The catalytic patterns indicate that each V-surface species possesses different activity and selectivity. Isolated vanadates are the most active and selective towards HCHO, while V₂O₅ crystallites are detrimental to the catalytic performance.     

Deep Hydrodesulphurization via Hydrogen Spillover

Abstract  

The synergistic effect between Co//Mo and Co//W stacked bed systems in the hydrodesulphurization (HDS) of 4,6-dimethyldibenzothiophene (4,6-DMDBT) was studied. The reaction was carried out in a high-pressure continuous-flow microreactor at 3 MPa and 300 °C. The observed synergism can only be explained by the action of hydrogen spillover (H_SO). The selectivity in hydrogenation over direct desulphurization pathways (HYD/DDS) of Mo/γ-Al₂O₃ and W/γ-Al₂O₃ catalysts increased when the reaction was carried out in Co//Mo and Co//W stacked bed systems, respectively. The changes in selectivity can be explained by the fact that hydrogen spillover does not only create more active sites, but it also modifies the sites, favouring the HYD pathway. Differences in synergistic effect between Co//Mo and Co//W stacked bed systems were related to the higher degree of sulphurization of the molybdenum compared to the tungsten phase, as detected by XPS.     

Conversion of biomass to fuel: Transesterification of vegetable oil to biodiesel using KF loaded nano-γ-Al2O3 as catalyst

KF-impregnated nanoparticles of γ-Al₂O₃ were calcinated and used as heterogeneous catalysts for the transesterification of vegetable oil with methanol for the synthesis of biodiesel (fatty acid methyl esters, FAME). The ratio of KF to nano-γ-Al₂O₃, calcination temperature, molar ratio of methanol/oil, transesterification reaction temperature and time, and the concentration of the catalyst were used as the parameters of the study. A methyl ester yield of 97.7 ± 2.14% was obtained under the catalyst preparation and transesterification conditions of KF loading of 15 wt%, calcination temperature of 773 K, 8 h of reaction time at 338 K, and using 3 wt% catalysts and molar ratio of methanol/oil of 15:1. This relatively high conversion of vegetable oil to biodiesel is considered to be associated with the achieved relatively high basicity of the catalyst surface (1.68 mmol/g) and the high surface to volume ratio of the nanoparticles of γ-Al₂O₃.     

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.     

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.     

Layered NdBaCo2−xNixO5+δ perovskite oxides as cathodes for intermediate temperature solid oxide fuel cells

The effect of Ni substitution on the crystal chemistry, thermal and electrochemical properties, and catalytic activity for oxygen reduction reaction of the layered NdBaCo_2−xNi_xO_5+δ perovskite oxides has been investigated for 0 ≤ x ≤ 0.6. The oxygen content (5 + δ) and oxidation state of the (Co, Ni) ions in the air-synthesized NdBaCo_2−xNi_xO_5+δ samples decrease with increasing Ni content, accompanied by a structural transition from tetragonal (0 ≤ x ≤ 0.4) to orthorhombic (x = 0.6). Similarly, the thermal expansion coefficient (TEC) and electrical conductivity also decrease with increasing Ni content. The x = 0.2 and 0.4 samples exhibit slightly improved performance as cathodes in single cell solid oxide fuel cell (SOFC) compared to the x = 0 sample, which is in accordance with the ac-impedance data. Among the samples studied, the x = 0.4 sample exhibits a combination of low thermal expansion and high catalytic activity for the oxygen reduction reaction in SOFC.     

Influence of α-Al2O3 addition on sintering and grain growth behaviour of 8 mol% Y2O3-stabilised cubic zirconia (c-ZrO2)

The effect of α-Al₂O₃ addition on sintering and grain growth behaviour of high purity 8 mol% yttria-stabilised cubic zirconia (c-ZrO₂) was investigated. For these purposes, 1 wt.% α-Al₂O₃ was selected as a dopant in c-ZrO₂. The slip-cast specimens were sintered at different temperatures between 1150 and 1400 °C. It was seen that doped c-ZrO₂ had a faster sintering rate and lower sintering temperature than undoped c-ZrO₂. In particular, doped c-ZrO₂ achieved a density of 95% of its theoretical value at 1275 °C, while undoped c-ZrO₂ reached the same value at 1325 °C. The different sinterability of doped c-ZrO₂ and undoped c-ZrO₂ can be attributed to their different behaviour of grain growth. For grain growth measurements, the specimens sintered at 1400 °C were annealed at 1400, 1500 and 1600 °C for 10, 30 and 66 h. It was seen that grain growth rate could be controlled by the deliberate addition of 1 wt.% grain boundary phase of α-Al₂O₃. A grain growth exponent of 2 and activation energy for grain growth of 298 kJ/mol were obtained for undoped c-ZrO₂. The α-Al₂O₃ containing specimens had a grain growth exponent of 3 and activation energy of 361 kJ/mol. The slow grain growth in doped c-ZrO₂ is attributed to solute ions segregation in grain boundary region. The addition of the grain boundary phase results in limiting matter transfer along the grain boundary resulting in slower grain growth.     

The effect of synthesis gas composition on the Fischer–Tropsch synthesis over Co/γ-Al2O3 and Co–Re/γ-Al2O3 catalysts

The Fischer–Tropsch synthesis over Co/γ-Al₂O₃ and Co–Re/γ-Al₂O₃ was investigated in a fixed-bed reactor at 20 bar and 483 K using feed gases with molar H₂/CO ratios of 2.1, 1.5 and 1.0 simulating synthesis gas derived from biomass. With lower H₂/CO ratios in the feed, the CO conversion and the CH₄ selectivity decreased, while the C₅₊ selectivity and olefin/paraffin ratio for C₂–C₄ increased slightly. The water–gas shift activity was low for both catalysts, resulting in high molar usage ratios of H₂/CO (close to 2.0), even at the lower inlet ratios (i.e. 1.5 and 1.0). For both catalysts, the drop in the production rate of hydrocarbons when shifting from an inlet ratio of 2.1 to 1.5 was significant mainly because the H₂/CO usage ratio did not follow the change in the inlet ratio. The hydrocarbon selectivities were rather similar for inlet H₂/CO ratios of 2.1 and 1.5, while significantly deviating from those for an inlet ratio of 1.0. With the studied catalysts, it is possible to utilize the advantages of an inlet ratio of 1.0 (higher selectivity to C₅₊, lower selectivity to CH₄, no water–gas shifting of the bio-syngas needed prior to the FT reactor) if a low syngas conversion is accepted.