EXAFS study of model Pd/Ga2O3 catalysts for the liquid-phase hydrogenation of acetylene before and after the reaction

The catalytic characteristics of Pd/Ga₂O₃ samples in the liquid-phase hydrogenation of acetylene to ethylene are studied. The structure of the catalyst’s components before and after the reaction is examined by EXAFS spectroscopy. The activity of the original (after drying) samples results from the reduction of the original palladium oxide to a metallic state in the reaction medium. According to EXAFS, the catalysts reduced at 200°C contain small amounts of Pd-Ga alloy that was preserved in the reaction medium during the liquid-phase hydrogenation of acetylene and presumably increased the activity and selectivity of the samples.     

Selectivity maximization of ethylene epoxidation via NEMCA with zirconia and β″-Al2O3 solid electrolytes

The silver-catalyzed epoxidation of ethylene is a reaction of great technological importance and also represents one of the most challenging and thoroughly studied catalytic systems. It was found that the catalytic activity and selectivity of polycrystalline Ag for the epoxidation and complete oxidation of ethylene can be affected in a pronounced and reversible manner by electrochemically supplying or removing oxygen ions O^2- or Na⁺ to or from the silver catalyst surface in ZrO₂ (8 mol%Y₂O₃) or β″-Al₂O₃ solid electrolyte cell reactors and in the presence or absence of traces of chlorinated hydrocarbons in the gas phase. The steady-state changes in catalytic rates of formation of C₂H₄O and CO₂ are typically 10 to 100 times larger than the corresponding rate of ion transport to or from the catalyst surface, i.e., the reaction exhibits the effect of Non-Faradaic Electrochemical Modification of Catalytic Activity (NEMCA) or “Electrochemical Promotion”. The selectivity to C₂H₄O can be very significantly altered, relative to open circuit conditions. Under fuel rich conditions, temperatures near 250°C and in the presence of traces of 1,2-C₂H₄Cl₂ in the gas phase selectivity values as high as 88% can be obtained, well above the ones reported in the open literature. The observed phenomena are discussed and interpreted within the framework of previous NEMCA studies and the currently prevailing ideas regarding the mechanism of ethylene epoxidation. Paper presented at the 1st Euroconference on Solid State Ionics, Zakynthos, Greece, 11 – 18 Sept. 1994     

Electrochemical promotion of catalyst surfaces deposited on ionic and mixed conductors

The effect of non-Faradaic electrochemical modification of catalytic activity (NEMCA) or electrochemical promotion (EP) was investigated on Pt films deposited on Y₂O₃-stabilized-ZrO₂ (YSZ), an O²⁻ conductor, TiO₂, a mixed conductor, and Nafion 117 solid polymer electrolyte (SPE), a H⁺ conductor and also on Pd films deposited on YSZ and β″-Al₂O₃ a Na⁺ conductor. Four catalytic systems were investigated, i.e. C₂H₆ oxidation on Pt/YSZ, C₂H₄ oxidation on Pd/YSZ and Pd/β″-Al₂O₃, C₂H₄ oxidation on Pt/TiO₂ and H₂ oxidation on Pt/Nafion 117 in contact with 0.1 M aqueous KOH solution. In all cases pronounced and reversible non-Faradaic electrochemical modification of catalytic rates was observed with catalytic rate enhancement up to 2000% and Faradaic efficiency values up to 5000. All reactions investigated exhibit a pronounced electrophobic behaviour which is due to the weakening of chemisorptive oxygen bond at high catalyst potentials. Ethane oxidation, however, also exhibits electrophilic behaviour at low potentials due to weakened binding of carbonaceous species on the surface. The general features of the phenomenon are similar for all four cases presented here showing that the NEMCA effect is a general, electrochemically induced, promoting catalytic phenomenon not depending on the reaction and the type of supporting electrolyte. Paper presented at the 2nd Euroconference on Solid State Ionics, Funchal, Madeira, Portugal, Sept. 10–16, 1995     

Structural phase transition of Rh2O3 studied via PAC

The structural phase transition from α-Rh₂O₃ to β-Rh₂O₃ has been studied via the PAC method on dilute¹¹¹Cd impurity atoms implanted into α-Rh₂O₃. The measured antishielding factor β=112 (2) in the α-phase is similar to that found for¹¹¹Cd in other oxides with corundum structure (Al₂O₃. Cr₂O₃, Fe₂O₃). Oxygen with different crystallographic matrices show a pronounced correlation of the electric field gradient with the NN oxygen coordination.     

Study on ultrasound-assisted precipitation for preparing Ni/Al2O3 catalyst

With the rapid development of oil hydrogenation industry, the development of oil hydrogenation catalyst has also become a research hotspot. In this paper, ultrasound-assisted precipitation technique is used to prepare Ni/Al₂O₃ catalyst. The effect of ultrasonic output power on catalyst performance is investigated. The prepared catalyst is applied to the hydrogenation reaction of castor oil. It is found that the prepared catalyst shows the best hydrogenation performance when ultrasonic output power, frequency and ultrasonic treatment time are 80 W, 40 kHz and 600 min respectively. It also indicates that ultrasound-assisted precipitation technique can reduce the particle size and increase the specific surface area of Ni/Al₂O₃ catalyst so that its activity is improved. In addition, six important elements that should be considered in the development of industrial oil refining catalysts are discussed, and the effects of these factors on the catalyst performance are discussed. Finally, new way for improving catalyst performance is given, and the application of some new materials and methods in oil refining is introduced.     

Effect of Calcination Temperature on La-Modified Al2O3 Catalysts for Vapor Phase Hydrofluorination of Acetylene to Vinyl Fluoride

利用沉积沉淀法制备出了La改性Al₂O₃催化剂,研究不同焙烧温度对La改性Al₂O₃催化剂用于乙炔气相氢氟化合成氟乙烯反应性能的影响．利用NH₃-TPD、Pyridine-FTIR、XRD和Raman等技术对不同温度焙烧的催化剂进行表征,发现焙烧过程能改变催化剂结构的同时也能调变催化剂表面的酸量．经400 °C焙烧的催化剂显示出最高的乙炔转化率(94.6%)、最高的氟乙烯选择性(83.4%)和较低的积炭选择性(0.72%)．催化剂的高活性与其表面的高酸量有关,同时积炭的选择性也与其表面的酸中心数量有关．     

Surface modification of HMS material with silica sol leading to a remarkable enhanced catalytic performance of Cu/SiO2

Cu/SiO₂ catalysts with different bimodal pore structures adjusted by the ratio of HMS and silica sol were prepared via modified impregnation method. Structure evolutions of the catalyst were systematically characterized by N₂-physisorption, X-ray diffraction, H₂ temperature-programmed reduction, N₂O titration and X-ray photoelectron spectroscopy. The results show that the composite silica supported copper catalysts showed remarkably enhanced catalytic performance in the selective hydrogenation of dimethyl oxalate to ethylene glycol compared to the individual silica supported ones obtained by the same method. The dimethyl oxalate conversion and the ethylene glycol selectivity can reach 100% and 98% at 473 K with 2.5 MPa H₂ pressure and 1.5 h⁻¹ liquid hour space velocity of dimethyl oxalate over the optimized Cu/SiO₂ catalyst. The remarkably enhanced catalytic performance of Cu/SiO₂ catalysts might be attributed to the homogeneous dispersion and uniformity of the active copper species and to the larger copper surface areas attained on the HMS supports with large pore diameters and surface areas.     

Electrochemical promotion in emission control catalysis

Electrochemical promotion for the catalytic reduction of NO by CO and of NO by ethylene over a Pt catalyst are reported for the first time. Both reactions are of importance in the catalytic control of automotive emissions and both exhibit strong rate enhancement when Na is pumped to the Pt catalyst electrode from a β″ solid electrolyte. Complementary data obtained with a Pt(111)/Na model system indicate that electrochemically-pumped Na acts by inducing dissociation of chemisorbed NO, which is the reaction initiating step. Paper presented at the 1st Euroconference on Solid State Ionics, Zakynthos, Greece, 11 – 18 Sept. 1994.     

Influence of transition metals (Cr, Mn, Fe, Co and Ni) on the methane combustion over Pd/Ce-Zr/Al2O3 catalyst

The effects of transition metals (Cr, Mn, Fe, Co and Ni) on the catalytic properties of Pd/Ce-Zr/Al₂O₃ catalyst for methane combustion have been investigated. The supported Pd catalysts are characterized by BET, XRD, TEM, TPR, TPO and TPSR measurements. Activity tests in methane combustion show that Pd/Ce-Zr-Ni/Al₂O₃ has the highest catalytic activity and thermal stability among all catalysts. The results of TEM show that the addition of Ni to Pd/Ce-Zr/Al₂O₃ increases the dispersion of Pd component and inhibits the site growth. The results of TPO and TPSR show that the addition of Ni inhibits the decomposition of PdO particles and improves the reduction-reoxidation properties of the active PdO species, which increases the catalytic activity and thermal stability of the Pd/Ce-Zr/Al₂O₃ catalyst.     

Electrochemical promotion of CH4 oxidation on Pd

The effect of Non-Faradaic Electrochemical Modification of Catalytic Activity (NEMCA effect) or Electrochemical Promotion (EP) was used to promote the methane oxidation reaction to CO₂ and H₂O over Pd polycrystalline films interfaced with yttria-stabilized zirconia in galvanic cells of the type: CH₄, O₂, CO₂, Pd/YSZ/Au, CH₄, O₂, CO₂ It was found that by applying positive potentials or currents and thus, supplying O²⁻ onto the catalyst surface, up to 90-fold increases in CH₄ oxidation catalytic rate can be obtained. The induced changes in catalytic rate were two orders of magnitude higher than the corresponding rate of ion transfer to the catalyst-electrode surface, i.e. faradaic efficiency Λ values above 100 can be attained. The reaction exhibits electrophobic behavior under the experimental conditions of the investigation. The results can be rationalized on the basis of the theoretical considerations invoked to explain NEMCA behavior, i.e. the effect of changing work function on chemisorptive bond strengths of catalytically active electron donor or acceptor adsorbates. Paper presented at the 5th Euroconference on Solid State Ionics, Benalmádena, Spain, Sept. 13–20, 1998.     

Thermoluminescence experiments to study lattice defects in aluminosilicates

Summary The thermoluminescent emissions of β-eucryptite and β-spodumene have been recordedvs. temperature and wavelength. Arguments are advanced which allow the observed emission to be ascribed to point defects originated by the exchange of Si⁺⁴ and Al⁺³ ions in (Si, Al)O₄ tetrahedra.     

Study on Ion-dynamics in solids by hole burning spectroscopy

The importance of hole burning spectroscopy for studying elementary migration processes of ions in superionic conductors is being discussed using two examples. In β″-alumina, the potential energy for the conduction of ions along the conduction path, as obtained from the analysis of the results of hole burning spectroscopy, showed that there are two different bare-potentials along the ionic conduction path. One of them is higher than the activation energy for ionic conductivity. This result gives direct evidence that an ionic interaction among the conducting ions plays an important role for ion migration in β″-alumina. In YSZ with various concentration of Y₂O₃, we have observed persistent holes in Pr³⁺ ions doped samples. Their disordered nature will be discussed using the bandwidth considerations. Paper presented at the 9th EuroConference on Ionics, Ixia, Rhodes, Greece, Sept. 15–21, 2002.     

Cavitation intensifying bags improve ultrasonic advanced oxidation with Pd/Al2O3 catalyst

Advanced oxidation processes can potentially eliminate organic contaminants from industrial waste streams as well as persistent pharmaceutical components in drinking water. We explore for the first time the utilization of Cavitation Intensifying Bags (CIB) in combination with Pd/Al₂O₃ catalyst as possible advanced oxidation technology for wastewater streams, oxidizing terephthalic acid (TA) to 2-hydroxyterephthalic acid (HTA). The detailed characterization of this novel reaction system reveals that, during sonication, the presence of surface pits of the CIB improves the reproducibility and thus the control of the sonication process, when compared to oxidation in non-pitted bags. Detailed reaction kinetics shows that in the CIB reactor the reaction order to TA is zero, which is attributed to the large excess of TA in the system. The rate of HTA formation increased ten-fold from ~0.01 μM*min⁻¹ during sonication in the CIB, to ~0.10 μM*min⁻¹ for CIB in the presence of the Pd/Al₂O₃ catalyst. This enhancement was ascribed to a combination of improved mass transport, the creation of thermal gradients, and Pd/Al₂O₃ catalyst near the cavitating bubbles. Further analysis of the kinetics of HTA formation on Pd/Al₂O₃ indicated that initially the reaction underwent through an induction period of 20 min, where the HTA concentration was ~0.3 μM. After this, the reaction rate increased reaching HTA concentrations ~6 μM after 40 min. This behavior resembled that observed during oxidation of hydrocarbons on metal catalysts, where the slow rate formation of hydroperoxides on the metal surface is followed by rapid product formation upon reaching a critical concentration. Finally, a global analysis using the Intensification Factor (IF) reveals that CIB in combination with the Pd/Al₂O₃ catalyst is a desirable option for the oxidation of TA when considering increased oxidation rates and costs.     

Fourier transform infrared studies of cyclohexane and benzene adsorbed on Pt/Al2O3: Evidence for π- and σ-bonded chemisorbed species

Fourier transform infrared spectroscopy has been applied to the study of cyclohexane adsorbed on Al₂O₃ and Pt/Al₂O₃ surfaces. Earlier studies of benzene on these same materials have also been extended to include benzene adsorbed on a Pt/Al₂O₃ surface which contains structured carbon residues. The data provide indirect evidence for the formation of a carbon residue on Pt/Al₂O₃ which retains the six-membered cyclic structure of the parent adsorbates. The carbon residue can be formed upon vacuum heating of the parent C₆ ring molecules chemiorbed on Pt/Al₂O₃. There is spectroscopic evidence that cyclohexane dehydrogenates on Pt/Al₂O₃ at 300 K to form two different chemisorbed species; a π-bonded benzene and a dissociated σ-bonded benzene. These two chemisorbed species have CH stretching vibrations centered at 3030 and 2947 cm^?1, respectively. Benzene added to a clean catalyst surface forms only a π-bonded benzene. However, benzene added to Pt/Al₂O₃ with ordered carbon residues forms both π- and σ-bonded benzenes. The addition of H₂ at 300 K to any of the π- or σ-bonded benzenes or to the carbon residue results in the formation of cyclohexane physisorbed on the catalyst. The absence of CH₃ groups upon hydrogenation suggests the lack of CC bond breaking during adsorption or hydrogenation. Simultaneous infrared and thermal desorption studies on chemisorbed deuterated benzene (from C₆D₁₂) indicate that the a-bonded species exchange H from the surface OH groups of the alumina support more readily than does the π-bonded benzene. In addition to hydrogen exchange with the support, thermal desorption experiments indicate the oxidation of a portion of the chemisorbed hydrocarbons and/or carbon residue by oxygen from the alumina support. Therefore, the support is capable of playing a direct role in reactions occurring on the catalyst surface.     

Atomic resolution scanning tunneling microscopy imaging of Pt electrodes interfaced with β″-Al2O3

Solid electrolytes can be used as active catalyst supports to induce significant and reversible catalytic activity and selectivity enhancement via the effect of Non-Faradaic Electrochemical Modification of Catalytic Activity (NEMCA effect) or Electrochemical Promotion which has been recently reported for over fourty catalytic reactions. Atomically resolved Scanning Tunneling Microscopy was used to image the reversible electrochemically controlled dosing (backspillover) of sodium on Pt(111) interfaced to β″-Al₂O₃ at atmospheric pressure, which has been proposed as the cause of the NEMCA effect in the case of Na⁺ conductors. It was found that electrical current application between the Pt(111) monocrystal and a counter electrode also in contact with the β″-Al₂O₃ Na⁺-conducting solid electrolyte causes reversible migration (backspillover and spillover) of sodium which forms a (12×12) hexagonal structure on the Pt(111) surface. In addition to explaining the phenomenon of Electrochemical Promotion in Heterogeneous Catalysis when using Na-β″-Al₂O₃ solid electrolyte these observations provide the first STM confirmation that:

1. spillover-backspillover phenomena can take place over enormous (~mm) atomic distances, and
2. promoters can form ordered structures on catalyst surfaces under ambient conditions relevant to industrial practice.

     

Preparation and characterization of novel Pd/SiO2 and Ca-Pd/SiO2 egg-shell catalysts with porous hollow silica

Novel egg-shell structured monometallic Pd/SiO₂ and bimetallic Ca-Pd/SiO₂ catalysts were prepared by an impregnation method using porous hollow silica (PHS) as the support and PdCl₂ and Ca(NO₃)₂·4H₂O as the precursors. It was found from transmission electron microscope (TEM), scanning electron microscope (SEM), energy dispersive spectroscopy (EDS) and X-ray diffraction (XRD) that Pd was loaded on PHS with a particle size of 5-12 nm in Pd/SiO₂ samples and the Pd particle size in Ca-Pd/SiO₂ was smaller than that in Pd/SiO₂ since Ca could prevent Pd particles from aggregating. X-ray photoelectron spectroscopy (XPS) analyses exhibited that Pd 3d_5/2 binding energies of Pd/SiO₂ and Ca-Pd/SiO₂ were 0.2 and 0.9 eV lower than that of bulk Pd, respectively, as a result of the shift of the electron cloud from Pd to oxygen in Pd/SiO₂ and to both oxygen and Ca in Ca-Pd/SiO₂. The activity of Ca-Pd/SiO₂ egg-shell catalyst for CO hydrogenation and the selectivity to methanol, with a value of 36.50 mmol_CO mol⁻¹_Pd s⁻¹ and 100%, respectively, were much higher than those of the catalysts prepared with traditional silica gel as the support, owing to the porous core-shell structure of the PHS support.     

Sonochemically synthesized core-shell structured Au–Pd nanoparticles supported on γ-Fe2O3 particles

A sample of Au–Pd bimetallic nanoparticles supported on γ-Fe₂O₃ was synthesized in a sonochemically one-pot process. The structural analyses of the synthesized sample were performed by the techniques of X-ray Absorption Fine Structure (XAFS), X-ray Diffraction (XRD), Transmission Electron Microscopy (TEM) and UV–vis spectrometry. Results indicated that the synthesized sample formed a core-shell structure in which a gold core was surrounded by a thin palladium shell. The reaction rate constant for the hydrogenation of cyclohexene of the present sample showed higher value than that of Pd nanoparticles supported on γ-Fe₂O₃ and core-shell structured Au–Pd nanoparticles supported on SiO₂. The present sample is a promising catalyst material which has a high catalytic activity.     

Electrochemical promotion of the NO reduction by C2H4 on Pt/YSZ and by CO on Pd/YSZ

The effect of electrochemical promotion was investigated for the catalytic reduction of nitric oxide with ethylene and carbon monoxide on polycrystalline Pt and Pd, respectively, deposited on yttria-stabilized zirconia (YSZ). It was found in both cases that applying negative potentials and thus lowering the catalyst work function results in a pronounced increase in the catalytic rate and in the selectivity to nitrogen. A 7-fold increase was observed for the NO+C₂H₄ reaction on Pt while a 2-fold increase was obtained for the NO+CO reaction on Pd. The induced changes in catalytic rates were found to be 7 to 50 times higher than the rates of ion transfer from the catalyst surface. In both reactions, the observed electrophilic behavior can be attributed to the strengthening of the chemisorptive NO bond and concomitant enhanced dissociation of NO as the catalyst potential and work function is decreased. Forced periodic oscillations of the applied current was investigated and resulted in a enhanced production of CO₂, but an intermediate selectivity towards N₂, as compared to constant current application. The effect of the cycling waveform, frequency and amplitude was studied and provided evidence that the synergy observed during the cycling experiment results from a favorable transient coverage of adsorbed species on the catalyst surface as the catalyst potential oscillates from negative to positive values. Paper presented at the 3rd Euroconference on Solid State Ionics, Teulada, Sardinia, Italy, Sept. 15–22, 1996     

Structural and catalytic properties of ZnO and Al<Subscript>2</Subscript>O<Subscript>3</Subscript> nanostructures loaded with metal nanoparticles

Nanostructured zinc oxide (ZnO) nanobelts and aluminum oxide (Al₂O₃) nanoribbons have been grown successfully from the vapor phase. XRD results confirmed the purity and the high quality of the formed crystalline materials. TEM images showed that ZnO nanostructures grew in the commonly known tetrapod structure with nanobelts separated from the tetrapods with an average width range of 10–30 nm and a length of about 500 nm. Al₂O₃ nanostructures grew in the form of nanoribbons with an average width range of 20–30 nm and a length of up to 1 μm. The catalytic oxidation of CO gas into CO₂ gas over the synthesized nanostructures is also reported. Higher catalytic activity was observed for Pd nanoparticles loaded on the ZnO nanobelts (100% conversion at 270 °C) and Al₂O₃ nanoribbons (100% conversion at 250 °C). The catalytic activity increased in the order Cu < Co < Au < Pd for the metal-loaded nanostructures. The preparation methods could be applied for the synthesis of novel nanostructures of various materials with novel properties resulting from the different shapes and morphologies.     

Electrical properties of Na<Subscript>2</Subscript>SO<Subscript>4</Subscript>-based composite systems

Composite electrolytes are well-known multiphase systems and exhibit maxima in the conductivity at certain second-phase concentration. An attempt has been made to investigate a number of sodium sulfate (Na₂SO₄)-based composite systems. The dispersoids that have been used are MgO, Al₂O₃, and SiO₂. The samples have been characterized using impedance spectroscopy, X-ray diffraction, and differential scanning calorimetry. The maximum conductivity has been observed for MgO dispersed system, and the percolation threshold has been observed at 30-mol% dispersoid, MgO concentration. Interestingly, two maxima have been observed in case of the Na₂SO₄–SiO₂ and Na₂SO₄–Al₂O₃ composite systems. In the Na₂SO₄–SiO₂ system, the first maximum occurs at lower concentration, i.e., in the range between 10 and 20 mol%, whereas the second occurs at the 40-mol% dispersoid concentration. For the Na₂SO₄–Al₂O₃ system, although slightly indistinguishable, two peaks in the conductivity vs composition plot have been observed around 12- and 30-mol% Al₂O₃ concentrations.