Reprint of: Oxidative dehydrogenation of cyclohexane and cyclohexene over supported gold, –palladium catalysts

Supported gold, palladium and gold–palladium catalysts have been used to oxidatively dehydrogenate cyclohexane and cyclohexenes to their aromatic counterpart. The supported metal nanoparticles decreased the activation temperature of the dehydrogenation reaction. We found that the order of reactivity was Pd ≥ Au–Pd > Au supported on TiO₂. Attempts were made to lower the reaction temperature whilst retaining high selectivity. The space-time yield of benzene from cyclohexane at 473 K was determined to be 53.7 mol/kg_cat/h rising to 87.3 mol/kg_cat/h at 673 K for the Pd catalyst. Increasing the temperature in this case improved conversion at a detriment to the benzene selectivity. Oxidative dehydrogenation of cyclohexene over AuPd/TiO₂ or Pd/TiO₂ catalysts was found to be very effective (conversion >99% at 423 K). These results indicate that the first step in the reaction sequence of cyclohexane to cyclohexene is the slowest step. These initial results suggest that in a fixed-bed reactor the oxidative dehydrogenation in the presence of oxygen, palladium and gold–palladium catalysts are readily able to surpass current literature examples and with further modification should yield even higher performance.     

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.     

Dehydrogenation of propane over ZnMOR. Static and dynamic reaction energy diagram

The dehydrogenation of propane over Zn²⁺-exchanged mordenite has been studied theoretically using ab initio density-functional calculations at different levels of theory. We compare (i) total-energy calculations based on semilocal exchange-correlation functionals with those adding semi-empirical corrections for dispersion forces, (ii) calculations based on a large periodic model of the zeolite with calculations based on small and large finite cluster models, and (iii) calculations of the free energies of activation and of the reaction rates based on harmonic transition state theory (hTST) with those based on thermodynamic integration over free-energy gradients determined by constrained ab initio molecular dynamics. Dehydrogenation proceeds in four steps: (i) adsorption of propane on the Zn²⁺ cation, (ii) dissociation of a hydrogen atom leading to the formation of a Zn-propyl complex and a Brønsted acid site, (iii) reaction of the acid proton and a β–H atom of propyl, resulting in the elimination of a hydrogen molecule, and (iv) desorption of propene from the Zn²⁺ cation.The periodic calculations demonstrate that the dispersion corrections increase the adsorption/desorption energies from 70 to 107 kJ/mol for propane and from 177 to 233 kJ/mol for propene and decrease the activation energy for H-dissociation from 73 to 61 kJ/mol, while the activation energy for the heterolytic dehydrogenation is almost unaffected with 132 kJ/mol. Hence, dispersion corrections are of foremost importance for lowering the activation energy for H-dissociation below the desorption energy of propane. While according to the periodic calculations the highest activation energies are predicted for the heterolytic dehydrogenation and the desorption of propene, cluster calculations predict a higher activation energy for H-dissociation than for H₂ elimination. Both hTST and thermodynamic integrations show that both activation processes lead to a loss of entropy because the transition state configurations admit for a lower degree of disorder than the initial and intermediate states. hTST consistently underestimates the loss of entropy, the anharmonic corrections are most important for the H-dissociation step.     

Phenol degradation by Fenton's process using catalytic in situ generated hydrogen peroxide

The recent reported pathway using oxygen and formic acid at ambient conditions has been utilized to generate hydrogen peroxide in situ for the degradation of phenol. An alumina supported palladium catalyst prepared via impregnation was used for this purpose. Almost full destruction of phenol was carried out within 6 h corresponding to the termination of 100 mM formic acid at the same time. In addition, a significant mineralization (60%) was attained. A simulated conventional Fenton process (CFP) using continuous addition of 300 ppm H₂O₂ displayed maximum 48% mineralization. Study of different doses of formic acid showed that decreasing the initial concentration of formic acid caused faster destruction of phenol and its toxic intermediates. The catalytic in situ generation of hydrogen peroxide system demonstrated interesting ability to oxidize phenol without the addition of Fenton's catalyst (ferrous ion). Lower Pd content catalysts (Pd1/Al and Pd0.5/Al) despite of producing higher hydrogen peroxide amount for bulk purposes, did not reach the same efficiency as the Pd5/Al catalyst in phenol degradation. The later catalyst showed a remarkable repeatability so that more than 90% phenol degradation along with 57% mineralization was attained by the used catalyst after twice recovery. Higher temperature (45 °C) gave rise to faster degradation of phenol resulting to almost the same mineralization degree as obtained at ambient temperature. Meanwhile, Pd leaching studied by atomic adsorption proved excellent stability of the catalysts.     

A DFT Study on the Adsorption of Formic Acid and Its Oxidized Intermediates on (100) Facets of Pt,Au, Monolayer and Decorated Pt@Au Surfaces

Abstract  

In this work we have systematically characterized the adsorption of formic acid, its decomposed intermediates and products on the (100) surfaces of Pt, Au, monolayer and decorated Pt@Au surfaces. The calculated thermodynamic results validate our previous experimental results that the decorated Pt@Au surface may facilitate formic acid oxidation compared with the benchmark Pt catalyst; while the monolayer Pt@Au surface is not suitable for formic acid oxidation.     

Vapour phase hydrogenation of olefins by formic acid over a Pd/C catalyst

It has been found that ethylene and propylene could be effectively hydrogenated by formic acid vapour over a Pd/carbon catalyst at low temperatures (<440 K). Surface hydrogen formation from formic acid is the rate-determining step for this hydrogenation reaction. Interaction of this hydrogen with the olefins is then fast. The conversion of formic acid in the presence of either of the olefins at any temperature is higher than in their absence. This has been explained by a much lower surface hydrogen concentration in the presence of the olefins. Direct experiments have confirmed that hydrogen inhibits the formic acid decomposition. Water vapour addition has a small positive effect on the decomposition of formic acid as well as on the hydrogenation of the olefins with formic acid. Catalysts consisting of gold supported on carbon or titania are both active in the production of hydrogen from formic acid. However, in contrast to the Pd/C catalyst, neither gives hydrogenation of the olefins with this acid.     

Photolysis of HCOOH over Rh Deposited on Pure and N-Modified TiO2

Abstract  

The photo-induced vapor-phase decomposition of formic acid was investigated on pure, N-doped and Rh-promoted TiO₂. The bandgap of TiO₂ was narrowed by 0.82–1.04 eV as a result of the incorporation N into TiO₂. Adsorption of formic acid on pure TiO₂ produced strong absorption bands due to formate species, the intensity of which decreased by illumination. The photodecomposition of formic acid on pure TiO₂ at 300 K occurs to only a limited extent: on N-doped TiO₂, however, it is enhanced by a factor of 2–4. The N-modified TiO₂ catalyzes the photoreaction even in the visible light, which is attributed to the prevention of electron–hole recombination. The deposition of Rh on TiO₂ markedly increased the extent of photodecomposition. The conversion is complete in 200 min, while the extent of decomposition reaches only ~30% on pure TiO₂. The effect of Rh is explained by a better separation of charge carriers induced by illumination and by enhanced electron donation to the adsorbed formate species. On TiO₂ samples both the dehydrogenation and dehydration reactions occurred, on Rh/TiO₂ only a trace amount of CO was formed. Addition of water to formic acid eliminated this CO, but exerted no other influence on the occurrence the photoreaction.     

Size-controlled synthesis and impedance-based mechanistic understanding of Pd/C nanoparticles for formic acid oxidation

This work provides a detailed electrochemical impedance study for formic acid electro-oxidation on size-controlled Pd/C nanoparticles, the synthesis of which was done by a simple protocol using ethylene glycol as a reducing agent. By controlling KOH concentration, this strategy provides a synthesis method for Pd nanoparticles with a selective size range of 3.9–7.5 nm. The as-prepared Pd nanoparticles exhibited size-dependent electrochemical property and electrochemical characterizations of four different Pd/C nanocatalysts (3.9, 5.2, 6.1, and 7.5 nm) showed that Pd particle with average size of 6.1 nm has the highest formic acid oxidation activity. Electrochemical impedance-based characterizations of formic acid oxidation on Pd/C suggested that at high potentials the adsorbed oxygen species could block the catalyst surface and inhibit the oxidation reaction, as reflected by the negative polarization resistance. Unlike Pd/C, the intermediate adsorbed CO species (CO_ads) plays a critical role for formic oxidation on Pt/C and thus the impedance spectra of Pd/C and Pt/C appear different potential-dependent patterns in the second quadrant. The issue of CO was investigated by an impedance investigation of Pd/C in a mixture of formic acid containing dissolved CO.     

Pyrolysis of synthetic polymers and plastic wastes. Kinetic study

Thermal decomposition of natural polystyrene, recycled plastics, low density polyethylene, acrylonitrile butadiene styrene, polyenterophthalate of ethylene, and polypropylene has been carried out. Both isothermal and dynamic experiments at different heating rates have been performed in a thermobalance with the objective of determining the kinetic parameters. Also, a first set of experiments, performed in a cylindrical stainless-steel atmospheric pressure reactor, was designed to evaluate the products of the pyrolysis process. The effects of N₂ flow rate (200–300 cm³ min^− 1), initial mass fed to the reactor (15–75 mg), temperature (415–490 °C), and heating rate (5–30 K min^− 1) were studied. By application of a first order kinetic model, the activation energy for every plastic has been determined. The values of activation energy in isothermic and dynamic regimen are the following: natural polystyrene: 136, and 168 to 286 kJ/mol. Recycled plastic: 250, and 150 to 290 kJ/mol. Low density polyethylene: 285, and 220 to 259 kJ/mol. Acrylonitrile butadiene styrene: 118, and 104 to 251 kJ/mol. Polyenterophthalate of ethylene: 161, and 117 to 255 kJ/mol. Polypropylene: 169, and 153 to 265 kJ/mol.     

Relationship among the physicochemical properties, electrocatalytic performance and kinetics of carbon supported Pt catalyst for ethanol oxidation

A new carbon supported Pt (Pt/C(b)) catalyst was prepared by reducing H₂PtCl₆ in glycol solution using formic acid as a reducing agent, and has been found in this work to be highly active and stable for the electrochemical oxidation of ethanol. The preparation produces highly dispersed Pt particles, of 2.6 nm average size, and with high electrochemical surface area, 98 m²/g. The apparent activation energy of ethanol oxidation over the Pt/C(b) catalyst electrode is low, 10–14 kJ/mol, over the range of potentials from 0.3 to 0.6 V.     

DFT study of nitrided zeolites: Mechanism of nitrogen substitution in HY and silicalite

We have performed embedded-cluster calculations using density functional theory to investigate mechanisms of nitrogen substitution (nitridation) in HY and silicalite zeolites. We consider nitridation as replacing Si–O–Si and Si–OH–Al linkages with Si–NH–Si and Si–NH₂–Al, respectively. We predict that nitridation is much less endothermic in HY (29 kJ/mol) than in silicalite (132 kJ/mol), indicating the possibility of higher nitridation yields in HY. To reveal mechanistic details, we have combined for the first time the nudged elastic band method of finding elusive transition states, with the ONIOM method of treating embedded quantum clusters. We predict that nitridation of silicalite proceeds via a planar intermediate involving a

ring with pentavalent Si, whereas nitridation of HY is found to proceed via an intermediate similar to physisorbed ammonia. B3LYP/6-311G(d,p) calculations give an overall barrier for silicalite nitridation of 343 kJ/mol, while that in HY is 359 kJ/mol. Although the overall nitridation barriers are relatively high, requiring high temperatures for substitution, the overall barriers for the reverse processes are also high. As such, we predict that once these catalysts are made, they remain relatively stable.

Graphical abstract

The mechanism of nitridation in HY and silicalite is revealed using denstiy functional theory. The barriers for forward and backward processes are large, indicating that nitrided zeolites are stable once formed.

1. Download : Download high-res image (57KB)
2. Download : Download full-size image

     

Reaction of Formic Acid on Zn-Modified Pd(111)

The decomposition of formic acid on Zn/Pd(111) was studied using Temperature Programmed Desorption and High Resolution Electron Energy Loss Spectroscopy. On Pd(111), HCOOH decomposes via both dehydration and dehydrogenation pathways to produce CO, CO₂, H₂ and H₂O. Small amounts of Zn (<0.1 mL) incorporated the Pd(111) surface were found to increase the stability of formate species and alter their decomposition selectivity to favor dehydrogenation, resulting in an increase in CO₂ production. This difference in reactivity appears to be caused by relatively long range electronic interactions between surface Pd and Zn atoms and may be important in Pd/ZnO methanol steam reforming catalysts which exhibit high selectivities to CO₂ and H₂.     

Molybdenum Oxoanions as Dispersing Agents in the Preparation of Pd/C Catalysts for the Selective Oxidation of Glyoxal

Abstract  

Catalysts Pd/C were prepared in the presence of Mo oxoanions. The size of Mo precursor and the electrostatic interactions with the Pd precursor during the synthesis were found to be responsible for high Pd dispersions. These catalysts were very active for glyoxal oxidation into glyoxalic acid.     

Thermochemical investigation of lithium borate glasses and crystals

Lithium borate (LB) glasses and crystals with x = Li/(Li + B) = mole fraction of Li₂O of 0.2–0.5 have been synthesized by the quenching method. The thermodynamics of these materials were analyzed by high-temperature oxide melt solution calorimetry. The formation enthalpies from oxides of glasses range from −33.6 to −67.3 kJ/mol and those of crystals range from −42.1 to −77.4 kJ/mol, where compositions are given on the basis of one mole of (Li₂O + B₂O₃). The formation enthalpies of both glasses and crystals become more negative with increasing Li₂O mole fraction up to 0.5. The enthalpies of formation of glasses can be fit over the entire composition range (0 < x < 1) by a quadratic polynomial). The vitrification enthalpies were derived for x = 0.2 to 0.5 and ranged from 8.5 to 17.6 kJ/mol. The main factors controlling energetics are the strongly exothermic acid–base reaction between the network former (B₂O₃) and the network modifier (Li₂O) and the formation of tetrahedrally coordinated boron in the glasses and crystals.     

硝酸浓度对水热处理活性炭制备Pd/AC催化剂性能的影响

采用水热硝酸氧化法对活性炭表面基团进行处理,考查了硝酸浓度对活性炭的孔结构、表面含氧基团的影响,对催化剂进行了N_2物理吸附、NH_3-TPD、FTIR、TEM等表征,并评价Pd/AC催化剂催化甲酸分解的性能。结果表明,随着硝酸浓度的提高,活性炭上含氧基团增加,浓度过高会破坏活性炭孔结构,使负载Pd粒子尺寸有所增大,催化剂活性降低。当活性炭在硝酸浓度为1.0 mol·L~(-1),150℃下水热处理4h,负载Pd催化剂的催化活性最好,在甲酸浓度为0.01 mol·L~(-1),90℃反应1 h情况下,甲酸分解率达到92.22%。     

Relationship among the physicochemical properties,electrocatalytic performance and kinetics of carbon supported Pt catalyst for ethanol oxidation

A new carbon supported Pt (Pt/C(b)) catalyst was prepared by reducing H₂PtCl₆ in glycol solution using formic acid as a reducing agent, and has been found in this work to be highly active and stable for the electrochemical oxidation of ethanol. The preparation produces highly dispersed Pt particles, of 2.6 nm average size, and with high electrochemical surface area, 98 m²/g. The apparent activation energy of ethanol oxidation over the Pt/C(b) catalyst electrode is low, 10–14 kJ/mol, over the range of potentials from 0.3 to 0.6 V.     

NiPd/TiO2催化剂的制备及其催化甲酸分解制氢性能

高效、清洁且无毒无害的催化剂是实现以甲酸(HCOOH)为化学储氢材料分解制氢的重点。本文采用水热法在453K的条件下制备TiO₂载体,再通过浸渍法向其中加入总量为0.1 mmol的NiCl₂.6H₂O和K₂PdCl₄金属溶液,将活性组分Ni、Pd负载到TiO₂载体上合成NiPd/TiO₂催化剂,并探究其对催化甲酸分解制氢的性能的影响。探究结果表明,在光照条件下,NiPd/TiO₂催化剂中,当金属Ni:Pd比例为2:8时,催化剂的反应转化频率(TOF)值最大,此时催化剂的 TOF 为3528 h_-1^,且该催化剂上甲酸分解的活化能(E_a)为53.9 kJ/mol。关键词：镍钯催化剂;甲酸;分解制氢;二氧化钛;光照中图分类号：TQ630 文献标识码： A 文章编号：     

Synthesis of high-stability acidic Ce3 + (La3 + or Sm3 +)~β/Al-MCM-41 and the catalytic performance for the esterification of oleic acid

Ce^3 + (La^3 + or Sm^3 +)~β/Al-MCM-41 molecular sieves were synthesized by impregnation and used to catalyze the esterification of oleic acid with short chain alcohols, such as methanol, ethanol, isopropanol, and isobutanol, to obtain biodiesel. Ce^3 +(La^3 + or Sm^3 +)~β/Al-MCM-41 was found to exhibit excellent catalytic activity and stability. The effect of rare earth elements on the acidity of catalysts was examined in detail by NH₃-TPD and Py-FTIR. The optimum conditions for the esterification of oleic acid with methanol were determined. Moreover, the kinetics of the esterification showed that the average reaction order (n) was 1.92, with an activation energy of 51.46 kJ/mol.     

Silicalite‐Filled PEBA Membranes for Recovering Ethanol from Aqueous Solution by Pervaporation

Pervaporation (PV) performances of silicalite‐filled polyether‐block‐amide (PEBA) membranes for separation of ethanol/water mixtures have been studied. The effects of silicalite content, ethanol concentration in feed, and feed temperature on the PV performances of the membranes have been investigated. It is found that addition of silicalite can improve PV performances of PEBA membranes. When the silicalite content is 2.0 wt %, both permeation flux and separation factor reach the maximum values, which are 833 g/m²h and 3.6, respectively. With increasing of ethanol in the feed and feed temperature, both separation factor and total flux increased. The higher permeation activation energy of ethanol (E_ethanol = 21.62 kJ/mol) compared to that of water (E_water = 18.33 kJ/mol) for the 2.0 wt% silicalite‐filled PEBA membrane accounts for the increase of the separation factor with feed temperature.     

Microcalorimetric studies of interactions of ethene, isobutene, and isobutane with silica-supported Pd, Pt, and PtSn

Microcalorimetric measurements were made of the interaction of hydrogen, ethene, isobutene and isobutane at 300 K with silica- supported Pt, Pd, and PtSn catalysts. The initial heats of hydrogen adsorption on silica-supported Pd and Pt are 104 and 95 kJ/mol, respectively. The presence of Sn decreases the saturation uptake of hydrogen on the PtSn sample. The initial heats of ethene interaction with Pd/silica and Pt/silica are 170 and 145 kJ/mol, respectively. The presence of Sn decreases the initial heat to 115 kJ/mol on the PtSn sample. The initial heats of isobutene interaction with silica-supported Pd and Pt are 160 and 190 kJ/mol, respectively. The presence of Sn decreases the initial heat to 125 kJ/mol on the PtSn sample. It appears that ethene and isobutene adsorb dissociatively on silica-supported Pd and Pt to form alkylidyne species at 300 K, with an average strength of carbon-metal bonds for these species near 230 kJ/mol. Ethene and isobutene adsorb on silica-supported PtSn to form di- σ- and π-bonded alkene species at 300 K, with an average strength of carbon-metal bonds for these species near 190 and 130 kJ/mol, respectively. Isobutane appears to adsorb dissociatively on a small number of sites on silica-supported Pd and Pt, and this dissociation is also inhibited by Sn on PtSn samples. This revised version was published online in July 2006 with corrections to the Cover Date.