Methanol partial oxidation on carbon-supported Pt and Pd catalysts

Different Pt and Pd catalysts supported on an activated carbon were prepared by using different metal precursors. Prepared catalysts were pretreated at 400 °C under different atmospheres to decompose the precursor compound and reduce the metal. After pretreatments, the supported catalysts were characterized by H₂ chemisorption, X-ray diffraction, transmission electron microscopy and X-ray photoelectron spectroscopy to know their metal dispersion, particle size, distribution and oxidation state. Afterwards, the catalysts were tested in methanol partial oxidation with two different O₂/CH₃OH molar ratios. Results obtained in this reaction were compared with those obtained for methanol decomposition in inert atmosphere. For Pt catalysts, there was an increase in methanol conversion and hydrogen production and a decrease in carbon monoxide production under oxidizing conditions. Both methanol conversion and partial oxidation reactions appear to be sensitive to Pt particle structure in the particle size range studied. Results obtained under oxidizing conditions differed between Pd and Pt catalysts. Finally, catalytic activity in methanol partial oxidation was more affected by Pt than Pd particle size in the size range studied.     

Towards the selective fixation of palladium on composite oxide carriers

The selective metallization of a semiconducting oxide (ceria) supported on an insulating oxide (silica) is investigated through new methods involving optical and electronic properties of the components of the composite. Supported ceria particles in the nanometer range are studied. Two methods of Pd deposition are applied: (i) electroless deposition via laser activation of the composite suspension in the Pd precursor solution; (ii) UV photoreduction of Pd on the composite. Electroless deposition results in a wide distribution of metal particle size, from 2–3 run to 600 nm, while some ceria particles are not metallized at all. On the contrary, photoreduction fixes the Pd on ceria selectively, yielding Pd particles with sizes comparable to that of the ceria. The influence of various parameters governing the metal deposition is discussed.     

Selective Hydrogenation catalysts. Influence of the support on the sulphur-resistance

The influence of the support acidity on the activity, selectivity and sulphur-resistance of Pd/Al₂O₃ catalysts has been studied, using the selective hydrogenation of styrene to ethylbenzene as a model reaction. It was found that the addition of sodium to the support causes a decrease in sulphur resistance and an increase in the activity and the selectivity to ethylbenzene, proportional to the amount of alkali added. The Pd electronic state was followed by X-ray photoelectron spectroscopy (XPS), and the average metal particle size of the catalysts was determined by Transmission Electronic Microscopy (TEM). The XPS analysis of samples demonstrated that there is a relation between the support acidity and Pd electronic state.     

Continuous mode linoleic acid hydrogenation on Pd/sibunit catalyst

Catalytic hydrogenation of linoleic acid and oleic acid to stearic acid over palladium on mesoporous carbon sibunit (Pd/sibunit) catalyst was studied in a continuous trickle-bed reactor with the weight hourly space velocity 5.4 h⁻¹ at 120°C and 30 bar using tall oil fatty acid (TOFA) as reactor feed. Stearic acid preparation using TOFA as a raw material is of industrial importance. Pd/sibunit catalysts with spherical particle shape of the size 1.62 mm were synthesized with the palladium loadings 0.5, 1, and 2 wt %. The metal dispersion (%), metal particle size (nm), as well as metallic surface area (m²/g metal) of the three synthesized Pd/Sibunit samples were measured by CO chemisorption. The Pd/C catalysts were tested in linoleic acid hydrogenation, showing promising behavior in terms of activity, selectivity and stability to be used in fixed bed applications. The product stream from the fixed bed reactor was saved and analyzed by X-ray fluorescence (XRF) and direct current plasma (DCP) spectroscopy. The catalyst activity increased with the Pd loading. The lowest metal loading of 0.5 wt % gave the least prone to initial deactivation and thus the most stable catalyst. This catalyst can be recommended for farther pilot testing.     

载体对不饱和醛/酮中C=O选择性加氢催化剂性能的影响

研究载体活性炭对沉淀法制备的Pd/C催化剂催化活性的影响,在不饱和醛/酮加氢反应中通过考察载体粒度和试剂改性等条件对催化剂催化活性以及C=O选择性的影响。结果表明,将粒度(100~200)目的活性炭在80℃的10%HNO_3溶液中浸泡120 min,洗涤干燥后得到活性炭载体制备的Pd/C催化剂具有较高的活性和C=O选择性,不饱和醛转化率达95.2%,不饱和醇选择性达96.3%。     

Catalytic enhancement of platinum supported on zeolite beta for toluene hydrogenation by addition of palladium

This work focused on the preparation, characterization and catalytic performance of a bimetallic platinum–palladium catalyst for toluene hydrogenation. A catalyst with 3 wt% loading of each metal was prepared by co-impregnation on zeolite beta in proton form and denoted as 3Pt3Pd/HBEA. The structure of HBEA was retained after catalyst preparation and the metal occupied strong acidic sites of the zeolite. Compared to monometallic 3Pt/HBEA, the 3Pt3Pd/HBEA exhibited smaller Pt particle size due to better dispersion on the support. The catalytic performance of the bimetallic catalyst at various temperatures indicated that the presence of Pd enhanced toluene hydrogenation of Pt catalyst at high temperature. The most suitable temperature for toluene hydrogenation on 3Pt3Pd/HBEA was 150 °C for which a complete toluene conversion was obtained with methylcyclohexane as the only product.     

The influence of acidity of carbon nanofibre-supported palladium catalysts in the hydrogenolysis of hydroxymatairesinol

The lignan hydroxymatairesinol (HMR, extracted from Norway spruce knots) was hydrogenolysed to matairesinol (MAT) over palladium supported by carbon nanofibres (Pd/CNF) in 2-propanol at 70 °C under hydrogen flow. The influence of support acidity on the activity and the selectivity to MAT was studied. The acidity of the Pd/CNF catalyst was varied by heat-treatment at different temperatures in nitrogen flow. The catalysts were characterized by transmission electron microscopy (TEM), inductively coupled plasma emission mass-spectrometry (ICP-MS, metal content), H₂-chemisorption (dispersion, metal particle size), and titration using NaOH. A more acidic support material was more active and selective to the desired product MAT. The major byproduct was 7-iso-propoxymatairesinol resulting from a reaction of the solvent with HMR over the acid sites on the support. The hydrogenolysis of HMR to MAT requires both the presence of metal and acidity.     

Efficient Anchorage of Palladium Nanoparticles on the Multi‐Walled Carbon Nanotubes as Electrocatalyst for the Hydrazine Electrooxidation in Strong Acidic Solutions

A facile homogeneous precipitation–reduction reaction method, which involves PdCl₂ → PdO · H₂O → Pd⁰ reaction path, is used to synthesize the multi‐walled carbon nanotubes (MWCNTs) supported Pd nanoparticles (Pd/MWCNTs) catalysts. The particle size of Pd/MWCNTs catalysts can be easily tuned by controlling the hydrolysis temperature of PdCl₂. X‐ray diffraction (XRD) and transmission electron microscopy (TEM) measurements show the particle size of Pd/MWCNTs catalysts increases with hydrolysis temperature of PdCl₂, which is ascribed to the fact that the particle size of PdO · H₂O nanoparticles increases with hydrolysis temperature of PdCl₂. At the lower hydrolysis temperature, the as‐prepared Pd/MWCNTs catalyst possesses the higher dispersion and the smaller particle size. Consequently, the resultant Pd/MWCNTs catalyst exhibits the big electrochemical active surface area and the excellent electrocatalytic performance for hydrazine electrooxidation in strong acidic solutions. In addition, the electrochemical measurement indicate that particle size effect of Pd‐NPs occurs during the N₂H₄ electrooxidation. In brief, the mass activity and specific activity of the Pd/MWCNTs catalyst increases and decreases with decreasing the particle size of Pd‐NPs for the N₂H₄ electrooxidation, respectively.     

The interaction of oxygen with alumina-supported palladium particles

Utilizing a combination of molecular beam techniques and scanning tunneling microscopy (STM) under ultrahigh vacuum (UHV) conditions we have studied the interaction of oxygen with an alumina-supported Pd model catalyst as well as the influence of the oxygen pretreatment on the kinetics of the CO oxidation reaction. The Pd particles were deposited by metal evaporation in UHV onto a well-ordered alumina film prepared on a NiAl(110) single crystal. The particle density, morphology and structure are determined by STM both immediately after preparation and after oxygen adsorption and CO oxidation. The oxygen sticking coefficient and uptake in the temperature regime between 100 and 500 K and the kinetics of the CO oxidation reaction are quantitatively probed by molecular beam techniques. It is found that starting at temperatures below 300 K the Pd particles rapidly incorporate large amounts of oxygen, finally reaching stoichiometries of PdO_>0.5. STM shows, that neither the overall particle shape nor the dispersion is affected by the oxygen and CO treatment. Only after saturation of the bulk oxygen reservoir are stable CO oxidation conditions obtained. In the low-temperature regime (<500 K), only the surface oxygen, but not the bulk and subsurface oxygen is susceptible to the CO oxidation. The activation energies for the Langmuir–Hinshelwood step of the CO oxidation reaction were determined both in the regime of high CO coverage and high surface oxygen coverage. A comparison shows that the values are consistent with previous Pd(111) single crystal results. Thus, we conclude that, at least for the particle size under consideration in this study (5.5 nm), the LH activation energies are neither affected by the reduced size nor by the oxygen pretreatment.     

Reactivation of spent Pd/AC catalyst by supercritical CO2 fluid extraction

In this article, we reported a nondestructive and environmentally friendly method for the reactivation of a spent Pd/AC catalyst for the hydrogenation of benzoic acid by using supercritical CO₂ (scCO₂) fluid extraction. The effects of reactivation conditions, such as extraction temperature, pressure, CO₂ flow rate, and time, on the activity of the reactivated Pd/AC catalyst, were presented. The catalyst was characterized by N₂ physisorption, laser particle size analysis, and transmission electron spectroscopy, and the liquid extract was analyzed by GC-MS. It is found that scCO₂ fluid extraction was very efficient in eliminating organic substances blocking the pores of the catalyst, while did not affect noticeably the granule size of the catalyst and the particle size of Pd. The reactivated Pd/AC catalyst regained more than 70% of the activity of the fresh 5.0 wt % Pd/AC catalyst, and has been successfully used in an industrial unit for the hydrogenation of benzoic acid. © 2009 American Institute of Chemical Engineers AIChE J, 2009     

Complete benzene oxidation over Pt-Pd bimetal catalyst supported on γ-alumina: influence of Pt-Pd ratio on the catalytic activity

Pt-Pd bimetal catalysts were prepared in order to develop and investigate catalysts with excellent activity and stability for benzene destruction. In the reaction results, the addition of Pt to Pd/γ-Al₂O₃ catalyst brought about the increase of catalytic activity. Moreover, it was effective in preventing the deactivation of the catalysts in benzene combustion. The addition of some amount of Pt made Pd particles available for better benzene combustion. On the contrary, the addition of Pt beyond a certain amount decreases activity because of the Pd active sites overlapped with the Pt active sites. The activity of the catalysts is related to oxidation state of metal, Pd/Al ratio and particle size on γ-Al₂O₃. These effects of Pt addition to Pd catalysts were studied by XPS, XRD, and TEM analyses.     

Effect of pH Value and Temperatures on Performances of Pd/C Catalysts Prepared by Modified Polyol Process for Formic Acid Electrooxidation

The highly active Pd/C catalysts for formic acid electrooxidation have been prepared by a modified polyol process at different pH values of reaction solutions and different reducing temperatures, respectively. Their physical properties have been characterised by energy dispersive analysis of X‐ray, X‐ray diffraction and transmission electron microscopy. Their electrochemical performances for formic acid electrooxidation have been tested by cyclic voltammetry and amperometric i–t curves. The results of physical characterisations show that all the Pd/C catalysts present an excellent face centered cubic crystalline structure. Their particle sizes are decreasing firstly and then increasing with the increasing of the pH values of reaction solutions. The reducing temperatures also markedly affect the Pd particle sizes. And their nanoparticles have narrow size distributions and are highly dispersed on the surface of carbon support, and Pd metal loading in Pd/C catalyst is similar to the theoretical value of 20 wt.%. The results of electrochemical measurements present that the Pd/C catalyst prepared by waterless polyol process at the pH value of 10 and the reducing temperature of 120 °C has the smallest particle size of about 5.6 nm, and exhibits the highest catalytic activity (1172.0 A · g_Pd<?h‐2.85>^–1<?h.8>) and stability for formic acid electrooxidation.     

Synthesis of palladium nanoparticles on carbon nanotubes and graphene for the chemoselective hydrogenation of para-chloronitrobenzene

We have studied the synthesis of palladium nanoparticles over carbon nanotubes (Pd/CNT) and graphene (Pd/G) and we have tested their catalytic performance in the liquid phase chemoselective hydrogenation of para-chloronitrobenzene at room temperature. The catalysts were characterized by N₂ adsorption/desorption isotherms, TEM, X-ray diffraction, infrared and X-ray photoelectron spectroscopy and ICP-OES. The palladium particle size on Pd/G (3.4 nm) and Pd/CNT (2.8 nm) was similar though the deposition was higher on Pd/G. Pd/CNT was more active which can be ascribed to the different surface area and electronic properties of the Pd nanoparticles over CNT, while the selectivity was 100% to the corresponding haloaniline over both catalysts and they were quite stable upon recycling.     

络合还原法制备的Pd/C催化剂对甲酸氧化的电催化性能

分别以硼氢化钠和乙二醇为还原剂,经络合还原法制备了炭载钯（Pd/C）催化剂。透射电镜（TEM）和X射线粉末衍射谱（XRD）结果表明,以乙二醇为还原剂制备的Pd/C催化剂中Pd粒子具有较小的粒径、均匀的粒径分布和较大的相对结晶度,Pd粒子的平均粒径和相对结晶度分别为4.2±2 nm和1.88。电化学测试结果显示,以乙二醇为还原剂制备的Pd/C催化剂具有较大的电化学活性面积,对甲酸氧化表现出较高的电催化活性和稳定性。     

Studying the Metal-Support Interaction in Pd/γ-Al2O3 Catalysts by Atomic-Resolution Electron Energy-Loss Spectroscopy

A 2 wt% Pd/-Al₂O₃ catalyst, reduced in hydrogen at different temperatures, has been studied by combined atomic resolution Z-contrast imaging and electron energy loss spectroscopy (EELS) techniques. Z-contrast images have been used primarily to evaluate the particle -size distribution and morphology as a function of the catalyst reduction temperature. Energy-loss spectra obtained from specific locations in and around the metal particles show a shift in the Pd M_2,3 edge, which is an indication of the Pd nanoparticles getting negatively charged. This acquired charge on the Pd particles is the result of electron transfer from the alumina support and appears to be dependent on the size of the Pd nanoparticles. No aluminum signal is observed on the surface of the metallic nanoparticles, and the observed oxygen signal on the Pd nanoparticles is probably due to oxygen that gets adsorbed on the reduced catalyst.     

Vapor‐phase selective hydrogenation of citral over Pd/bentonite: effect of reduction method

BACKGROUND: Well‐dispersed nanoparticles of palladium were synthesized by wet impregnation technique over bentonite followed by three different reduction methods (H₂ or NaBH₄ or ethanol) and characterized by transmission electron microscopy, X‐ray diffraction, temperature‐programmed reduction and atomic absorption spectroscopy. Hydrogenation of citral over Pd‐supported bentonite catalysts was studied in vapor phase using a micro‐reactor. The effect of reduction method and metal loading on the conversion of citral and selectivity towards nerol and geraniol were examined. RESULTS: Among the catalysts evaluated in the vapor phase, Pd/bentonite reduced by ethanol was found to give the highest conversion and Pd/bentonite reduced by NaBH₄ was found to give the highest selectivity towards nerol and geraniol. This may be attributed to the smallest particle size of Pd in the former catalyst and presence of boron species on the latter catalyst, respectively. CONCLUSION: The presence of boron in proximity to palladium particles polarized C?O bond and helped C?O adsorption, thereby yielding nerol and geraniol (the unsaturated alcohols). Copyright © 2008 Society of Chemical Industry     

Complete methane oxidation over Pd catalyst supported on α-alumina. Influence of temperature and oxygen pressure on the catalyst activity

The influence of the reaction parameters including temperature, oxygen concentration, and of in situ hydrogen reduction on the Pd catalyst activity towards complete methane oxidation is studied experimentally.Zero porosity α-alumina plates are used as a support for Pd catalyst. This lowers the influence of metal–support interaction on the catalyst state as confirmed by UV–visible spectroscopy. A plug flow reactor with a high linear gas velocity is used to measure the reaction rate. Overall conversion is kept low for most of the experiments so that the reaction is in the kinetically limited regime. The oxidation state of the catalyst before and after the reaction is determined using UV–visible reflectance spectroscopy of the plate surface. Changes in the catalyst activity with time are monitored after stepwise changes in the reaction parameters.Activity was found to decrease with time at low temperatures and high oxygen concentrations (condition when PdO phase is stable) and to increase with time at high temperatures and low oxygen concentrations (conditions when Pd is stable). A sharp increase in conversion was observed after the in situ hydrogen reduction of the sample.The experimental data is consistent with the reduced Pd form of the catalyst being more active towards methane oxidation than the oxidized PdO form at high temperatures. Possible particle size and morphology effects are discussed.     

Self-oscillations of methane oxidation rate over Pd/Al2O3 catalysts: Role of Pd particle size

Oscillations of the methane oxidation rate were studied under methane-rich conditions on Pd/Al₂O₃ catalysts differing in Pd particle size. It was demonstrated that the temperature interval where oscillations occur narrows from 300–360 °C for the catalyst with Pd particle aggregates from 50–100 nm to 345–355 °C for the catalyst with isolated Pd particles of ~ 5 nm in size. At the same time, the period of oscillations showed ~ 6-fold increase. Structural transformations of Pd in the oscillation cycle were similar to those observed on bulk Pd used as a catalyst in the same reaction.     

Hydrogenation of carbon dioxide over metal catalysts prepared using microemulsion

This paper relates to a novel preparation method of metal supported catalysts using microemulsions. The size distribution of metal particles in the catalysts, thus, prepared was appreciably narrow and the average particle size was much smaller than that of the conventional catalyst prepared from impregnation. It was found that the particle size could be controlled by the conditions of microemulsions regardless of metal content. The Rh, Pd and Pt catalysts prepared from microemulsions were found to exhibit a much higher activity for the hydrogenation of carbon dioxide than those from impregnation.     

Incorporation of Pd nanoparticles in mesostructured silica

Monodisperse Pd nanoparticles were prepared by controlled reduction in organic phase and subsequent transfer to aqueous phase. A systematic study was carried out to finely tune nanoparticle size and optimize particle size distribution. The use of 4-dimethylaminopyridine as a transfer agent allowed for the easy and quantitative extraction of the Pd to the aqueous phase. The quaternary amine-functionalized metal nanoparticles were then used as metallic micelle replicas to grow silica around them. This novel and facile metal incorporation method provided an excellent dispersion and homogeneity of Pd nanoparticles on silica supports. In addition, cationic surfactants, such as cetyltrimethylammonium bromide, can be used to produce mesoporosity in these nanoparticles-containing silicas. As an alternative, the metal nanoparticles were functionalized with alkoxysilanes by covalent binding using mercaptopropyltriethoxysilane and then co-polymerized with tetraethoxysilane via basic-catalyzed hydrolysis in the presence of cationic surfactants.