AFM studies of Pd silica supported thin film catalysts

AFM has been used to study the effects of pretreatment gases on Pd/SiO₂ supported thin film catalysts during 1,3-butadiene hydrogenation. The Pd/SiO₂ catalyst, treated with O₂ followed by H₂ at 450°C, has an initial conversion of 85% and a surface morphology of 60 x 65 nm² Pd grains and only deactivates slightly. After a second treatment, the reactivity was fully recovered and the surface morphology exhibits a redispersion of the Pd grains. The catalyst with a similar initial reactivity and morphology but only treated in H₂, shows a decrease in activity and coalescence of Pd grains after repeated treatment. XPS studies have shown that the O₂ and H₂ treated Pd/SiO₂ catalyst has a lower Pd binding energy than the H₂ treated Pd/SiO₂. The effect of the substrate thickness and composition is also reported.     

Selective hydrogenation of cinnamaldehyde using Pd catalysts supported on Mg/Al mixed oxides: Influence of the Pd incorporation method

A series of hydrotalcite‐like compounds was synthesized by varying Mg/Al molar ratio with values of 2, 3, and 4. After thermal treatment at 823 K, the corresponding mixed oxides were obtained and used as catalytic supports. The incorporation of a Pd metallic phase (0.5 g/g loading), was carried out by two methods: 1) in situ vapour phase thermal decomposition, and 2) impregnation by organic method. Fresh and calcined samples were characterized by XRD and N₂ sorption experiments. The basic and metal functions were analyzed by CO₂‐TPD and H₂‐TPR. The Pd‐support interaction was studied by FTIR spectroscopy using CO as a probe molecule while the morphology of Pd nanoparticles on the catalysts was studied by SEM, HRTEM, and theoretical simulation using the Fast Fourier Transform (FFT) method. Finally, the catalytic activity results showed a higher conversion towards hydrocinnamaldehyde in the cinnamaldehyde hydrogenation reaction for the catalysts prepared by vapour phase thermal decomposition, compared with those prepared by organic method, showing the significant dependence on the catalytic activity and the Pd incorporation method.     

Experimental procedure for kinetic studies on egg-shell catalysts: The case of liquid-phase hydrogenation of 1,3-butadiene and n-butenes on commercial Pd catalysts

A study of the liquid-phase hydrogenation of 1,3-butadiene on commercial Pd/Al₂O₃ catalysts of the “egg-shell” type has been performed. Experimental conditions (40°C, 4 atm and high conversion of the di-olefin) were selected in accordance to industrial operating conditions employed for selective hydrogenation of 1,3-butadiene. Three experimental schemes were tested: a slurry reactor, a rotating-basket reactor, and a recirculation system with an external fixed-bed reactor. Significant drawbacks shown by the two former devices were mainly derived from the very high activity and the egg-shell structure of the catalysts. Instead, the recirculation system was found to be an excellent alternative.

Although Pd is present only within a very thin external layer (around 50–250 μm), strong diffusion effects impairing selectivity were observed. Plausible kinetic expressions corresponding to nine series–parallel overall reactions are derived from a mechanistic model. To deal with this network of fast reactions, a rather complex set of computational and predictive tools were employed. A worked out example from several replicates demonstrates the capability of both, experimental and data analysis procedures, for inferring kinetic parameters of the proposed model.     

The preparation of silica supported Pd catalysts: the effect of pretreatment variables on particle size

The effect of pretreatment on the dispersion of Pd catalysts supported on silica has been studied. The catalysts were prepared from [Pd(NH₃)₄] (NO₃)₂ as the metal precursor at a pH = 9. The resulting catalysts were characterized using both CO and H₂ chemisorption, transmission electron microscopy, and in-situ UV reflectance spectroscopy. Pretreatment in H₂ resulted in poor Pd dispersions while pretreatment in He or Ar resulted in very high dispersions. Pretreatment in O₂ resulted in moderate dispersions. The results are explained by considering the chemical structure of the adsorbed surface complex under different pretreatment conditions. The chemistry of the decomposition process is considered in detail.     

Mesoporous silica supported cobalt oxide catalysts for catalytic removal of benzene

Two types of mesoporous silica SBA-15 with different pore diameter were synthesized with an ageing temperature of 373 K and an ageing temperature of 308 K, respectively; in addition, mesoporous silica with amorphous structure was synthesized by adding organosiloxane as part of the silica source during the synthesis procedure. Mesoporous silica and conventional alumina supported cobalt oxide catalysts were prepared by incipient wetness impregnation method. These materials were characterized by FT-IR, nitrogen adsorption–desorption, X-ray diffraction (XRD), transmission electron microscopy (TEM) and Temperature programmed reduction (TPR) techniques, and the activity of the supported cobalt oxide catalysts for deep oxidation of benzene were evaluated in a fixed-bed reactor. It seems that the pore diameter of the silica increase with the elevation of the ageing temperature. Mesoporous silica supported cobalt oxide catalysts are more active than conventional alumina supported ones. Cobalt oxide can be relatively better dispersed on the surface of mesoporous silica which has larger pore diameter and surface areas. Meanwhile, more silanol groups exist on the surface of amorphous silica, which could induce a strong interaction with the supported cobalt oxide species, leading to poor activity for benzene oxidation.     

Oxygen effect on the selectivity of ketone hydrogenation reaction on Pt and Pd catalysts

An oxygen deactivated metal catalyst that exhibits less catalytic activity can catalyze an extensive ketone hydrogenation reaction which results in the formation of alkanes, but a fully reduced metal catalyst that exhibits stronger catalytic activity only catalyzes a mild hydrogenation reaction which results in the formation of alcohols.     

Influence of surface support properties on the liquid-phase selective hydrogenation of phenylacetylene on supported nickel catalysts

The role of textural and acid–base properties of supports on metal–support interaction effects has been evaluated through the results obtained in the liquid-phase selective hydrogenation of phenylacetylene carried out on 20 wt% supported nickel catalysts. SiO₂, Al₂O₃, AlPO₄, active carbon and a natural sepiolite were used as supports. The effects of Ni–Cu alloying were also studied. The relative adsorption constants, K__T,D, and the relative reactivities, R_T,D, obtained in the consecutive hydrogenation process, were used to follow the effects of any electron transfer between the nickel atoms and acid–basic sites on the support surface. Structural defects of supported nickel crystallites were determined by X-ray diffraction line broadening analysis and used for measuring the participation of geometric effects of supported nickel crystallites in the catalytic activity. Results obtained may be better explained within a framework of simultaneous electronic and structural metal–support interaction effects. This revised version was published online in July 2006 with corrections to the Cover Date.     

Kinetic analysis of the liquid phase hydrogenation of 2-ethyl-hexenal in the presence of supported Ni,Pd and Ni-S catalysts

The kinetics of the consecutive hydrogenation reactions 2-ethyl-hexenal → 2-ethyl-hexanal → 2-ethyl-hexanol were studied in the liquid phase in presence of commercial Ni, Pd and Ni-S catalysts. The Pd and Ni-S catalysts were extremely selective with respect to the formation of 2-ethyl-hexanal, while the full reaction sequence was readily catalyzed by nickel. A Langmuir-Hinshelwood model with dissociative hydrogen adsorption was found to be the most probable model, for all three catalysts. The proposal of dissociative hydrogen adsorption of the Ni-S catalyst was supported by an independent gas-phase experiment, using the H₂/D₂ exchange reaction as a model reaction for the hydrogen àdsorption process.     

Difference in the reactivity of acetaldehyde intermediates in the dehydrogenation of ethanol over supported Pd catalysts

Catalytic performances of supported Pd catalysts for the dehydrogenation of ethanol were greatly modified upon the formation of Pd alloy phases. Over Pd–Zn, Pd–Ga and Pd–In alloys, acetaldehyde was selectively produced at lower conversion levels. With the increased conversion level, ethyl acetate was produced at the expense of acetaldehyde. The selectivities for the ethyl acetate formation exceeded that over a Cu/ZnO catalyst. Over metallic Pd, the decomposition of ethanol, C₂H₅OH → CO + CH₄ + H₂, occurred to a considerable extent. It was shown that the reactivity of acetaldehyde species over the Pd alloys was markedly different from that over metallic Pd. Over the Pd alloys, acetaldehyde species were stabilized and transformed into ethyl acetate by the nucleophilic addition of ethanol. By contrast, over metallic Pd, aldehyde species were rapidly decarbonylated to methane and carbon monoxide. This revised version was published online in July 2006 with corrections to the Cover Date.     

Partial oxidation of methane over silica supported molybdenum oxide catalysts

Two kinds of MoO₃/SiO₂ catalysts, MoO₃-I and MoO₃-S, were prepared by impregnation and sol-gel method, respectively. When MoO₃ loading was increased, formation of MoO₃ crystals was observed to begin at a MoO₃ loading of 8 and 16 wt% with MoO₃-I and MoO₃-S, respectively. The highest yield of formaldehyde from methane oxidation was attained also at those critical values of MoO₃ loading of 8 and 16 wt% over MoO₃-I and MoO₃-S, respectively. It is suggested that the active species for formaldehyde formation is well dispersed molybdenum oxide clusters on SiO₂ support: the optimum dispersion of the clusters affords the highest activity for formaldehyde formation.     

TiO2负载Pd催化剂的制备及其顺酐加氢性能研究

采用溶胶-凝胶法和乙醇的超临界流体干燥法制备了一种高效的顺酐选择加氢Pd/TiO2催化剂,并作了X射线衍射谱、BET表面测试、透射电镜、程序升温还原、X射线光电子能谱等表征,在溶胶-凝胶法制备的Pd/TiO2催化剂上,反应压力为3 MPa,反应温度为240℃时,顺酐的转化率为100%,丁酸的选择性达到了93.5%;Pd/TiO2催化剂经过高温还原后产生的Pd和TiO2之间的强相互作用是丁酸的高选择性的主要因素,另外反应温度也对丁酸的选择性有很大影响.     

二氧化硅微球负载钌催化剂的制备及其在苯乙酮不对称氢转移反应中的应用

采用一步原位聚合诱导沉淀法制得了形貌多样、结构复杂的二氧化硅介孔微球,将其与Noyori配体RuCl(p-cymene)[(R,R)-Ts-DPEN]络合制得负载型钌催化剂并应用于苯乙酮不对称氢转移反应。利用场发射扫描电镜和氮气等温吸脱附手段证明了二氧化硅微球和负载型钌催化剂的微球型介孔结构,结合透射电镜表明了钌元素均匀分布在二氧化硅微球上;通过红外光谱显示该催化剂与反应底物之间存在氢键作用;在此基础上考察了载体孔结构,反应条件等因素对催化性能的影响。研究结果表明,钌配体负载到具有小孔径、高比表面积的二氧化硅微球上有利于苯乙酮的不对称氢转移反应,由此引出介孔氧化硅微球的孔道限域效应对不对称催化反应的活性和光学选择性存在明显的促进作用。在40℃、0.2ml苯乙酮的条件下反应16h,负载型钌催化剂用于苯乙酮不对称转移加氢反应其转化率和对映选择性最高可分别达到64.1%和93.4%。     

Use of Water as a Solvent in Directing Hydrogenation Reactions of Aromatic Acids over Pd/carbon Nanofibre Catalysts

By conducting the catalytic hydrogenation over water miscible Pd/carbon nanofibre catalysts, selective ring hydrogenation of aromatic acids can be performed in aqueous solution without the need to provide protection for the external acid function.     

Highly selective supported Pd catalysts for steam reforming of methanol

Steam reforming of methanol, CH₃OH + H₂O 3H₂ + CO₂, was carried out over various Pd catalysts (Pd/SiO₂, Pd/Al₂O₃, Pd/La₂O₃, Pd/Nb₂O₅, Pd/Nd₂O₃, Pd/ZrO₂, Pd/ZnO and unsupported Pd). The reaction was greatly affected by the kind of support. The selectivity for the steam reforming was anomalously high over Pd/ZnO catalysts.     

Pd/C催化剂研究进展

介绍了近年来对苯二甲酸加氢精制Pd/C催化剂的研究开发情况,包括催化剂性能及催化剂制备工艺。着重介绍了该催化剂性能改进、催化剂载体活性炭的预处理工艺以及浸渍溶液中添加辅助溶液的研究进展。     

On the deactivation of supported palladium hydrogenation catalysts by thiophene poisoning

The deactivation of supported palladium catalysts for ethylbenzene hydrogenation by thiophene was studied. The presence of Pd ⁿ⁺ species on the catalyst surface, in samples prepared from acid solutions of PdCl₂, and reduced at temperatures below 450 °C, was evidenced by XPS. A correlation between the concentration of electron-deficient Pd species and the sulfur resistance was found. Thus, the higher the value of the Pd ⁿ⁺/Pd⁰ ratio, the higher the sulfur resistance. Catalysts prepared from Pd(NO₃)₂ or from PdCl₂ reduced at 450 °C, which essentially contain Pd⁰, are more quickly deactivated. Furthermore, our results also suggest that while the influence of the metal dispersion on the stability of the palladium catalysts toward sulfur is not significant, the nature of the support, on the other hand, plays an important role.     

Methanol decomposition to synthesis gas over supported Pd catalysts prepared from synthetic anionic clays

Supported Pd or Rh catalysts were prepared by the solid-phase crystallization method starting from hydrotalcite anionic clay minerals based on [Mg₆Al₂(OH)₁₆CO ₂ ²⁻ ]·4H₂O as the precursors. The precursors were prepared by a coprecipitation method from the raw materials containing Pd²⁺ and various trivalent metal ions which can replace each site of Mg²⁺ and Al³⁺ in the hydrotalcite. Rh³⁺ was also used for preparing the catalyst as comparison. The precursors were then thermally decomposed and reduced to form supported Pd or Rh catalysts and used for the methanol decomposition to synthesis gas. Among the precursors tested, use of Mg–Cr hydrotalcite containing Pd²⁺ resulted in the formation of efficient Pd supported catalysts for the production of synthesis gas by selective decomposition of methanol at low temperature. Although Pd²⁺ cannot well replace the Mg²⁺ site in the hydrotalcite, the Pd supported catalyst (Pd/Mg–Cr) prepared by the solid-phase crystallization method formed highly dispersed Pd metal particles and showed much higher activity than that prepared by the conventional impregnation method. When the precursor was prepared under mild conditions, more fine particles of Pd metal were formed over the catalyst, resulting in high activity. It is likely that the high activity may be due to the highly dispersed and stable Pd metal particles assisted by the role of Cr as the co-catalyst. This revised version was published online in November 2006 with corrections to the Cover Date.     

Oxygen-induced activation of silica supported silver in acrolein hydrogenation

The effect of oxygen pre-treatment of Ag/SiO₂ catalysts used in the acrolein hydrogenation has been investigated from sub-atmospheric (266 mbar) to 10 bar reaction pressure. The pre-treatment has a profound influence on the overall hydrogenation activity, inasmuch the activity increased two- to three-fold. Oxygen pre-treatment also enhanced the selectivity towards the desired allyl alcohol. The beneficial effect of oxygen pre-treatment is ascribed to the formation of electronically modified Ag and/or morphology changes of the nanoparticles as no modification in particle size or dispersion was found.     

Control of the product distribution in the hydrogenation of vegetable oils over nickel on silica catalysts

Several nickel on silica catalysts, prepared by impregnation or precipitation/deposition, and a commercial catalyst were tested for activity and selectivity in the sunflower seed oil hydrogenation. An average turn-over frequency of 2.57 s^?1 was found for the catalysts, assuming inaccessibility of nickel in pores smaller than 2 nm and a constant nickel surface concentration poisoned by the reaction mixture. After studying the mass-transfer steps, the effect of temperature (373-453 K) and pressure (101-608 kPa) on reaction rates in the kinetic regime was analyzed, and the corresponding apparent activation energies and reaction orders were obtained. Conclusions on the effect of temperature and pressure on the selectivity to the preferential hydrogenation of polyunsaturates (S_o) and to the formation of trans-isomers ((S_trans)₀) in the kinetic regime were derived from the results. Finally, a similar analysis was carried out when diffusion limitations were known to be present.     

Study on the catalytic synthesis of diethyl carbonate from CO and ethyl nitrite over supported Pd catalysts

A vapor phase synthesis of diethyl carbonate (DEC) from carbon monoxide and ethyl nitrite (EN) was studied in a continuous flow micro fixed-bed reactor at atmospheric pressure. PdCl₂–CuCl₂/AC (activated carbon) catalyst exhibited better catalytic activity compared with other binary catalyst systems. The suitable Pd-loading is about 2.0 wt%, and some additives (LaCl₃, CeCl₃, PrCl₃) are benefit for the DEC yield and selectivity. Influences of various reaction parameters on the DEC yield and selectivity were tested. The optimum reaction temperature lies in 378–388 K and the suitable gas hourly space velocity (GHSV) range is 2500–3000 h⁻¹ considering both factors of DEC production and CO conversion. An optimum CO/C₂H₅ONO mole ratio exists for catalytic activity, which is about 1/1. The stability of PdCl₂–CuCl₂/AC catalyst and PdCl₂–CuCl₂–CeCl₃/AC catalyst was also investigated. The possible reason of the deactivation behavior of catalysts was discussed with the help of XRD.