Untreated Catalyst Research Articles

Kinetics and mechanism of oxygen electroreduction on PtCoCr/C catalyst containing 20–40 wt % platinum

It is shown that the mechanism of oxygen electroreduction on the PtCoCr/C systems in 0.5 M H2SO4 is similar to that proposed for the Pt/C catalyst. The activity of ternary catalysts is by two and more times higher than that of monoplatinum catalyst. The constant k1 is much larger than k2 (k1 and k2 are the rate constants of O2 reduction to water and H2O2, respectively) for all catalysts studied. This indicates that the catalytic systems are selective with respect to O2 reduction immediately to water in the practically important potential range from 1.0 to 0.6 V. The yield of H2O2 increases with a shift of potential in the cathodic direction (<0.7 V) and does not exceed 1%. The sum of rate constants of further conversion of hydrogen peroxide also increases with a shift of potential in the cathodic direction. After a corrosion attack (a treatment in the acid for 24 h), a ratio between the rate constants (k1/k2) for the PtCoCr/C catalysts increases. This is caused by a considerable increase in k1, which is 2.84 × 10−2 cm/s for the catalyst containing 34 wt % Pt (against 1.5 × 10−2 cm/s for the untreated catalyst). This can be explained by the reaction proceeding on the particle surface, which was enriched in platinum in the course of corrosion treatment. The properties of platinum clusters on the alloy surface differ from those of monoplatinum as a result of the ligand effect. The amount of oxygen chemisorbed from water on this surface is lower than on Pt/C catalyst. This is the main factor determining an increase in the activity and stability of ternary catalysts.

Adsorption Properties of Supported Platinum Catalysts Prepared using Dendrimers

The effect of different oxidation and reducing pretreatments on dendrimer-encapsulated platinum nanoparticles (Pt-DENs) dispersed on a high-surface-area sol-gel-made silica support was assessed by evaluating the capacity of the resulting catalysts to adsorb CO, NO, and acetylene using IR absorption spectroscopy. The untreated catalysts are themselves capable of CO uptake, but only slowly, in a diffusion-controlled process, and into a weak adsorption state. Either oxygen or hydrogen pretreatments are required for stronger adsorption. Under similar temperature and pressure conditions, O(2) pretreatments result in higher uptakes, but also lead to partial oxidation and sintering of the Pt nanoparticles, and still do not fully eliminate the dendrimer matter. Hydrogen pretreatments alone at 525 K proved sufficient to expose the metal nanoparticles to the gas adsorbents and to activate the catalyst for hydrocarbon conversion reactions. NO adsorption is also seen in the H(2)-activated catalysts, much more extensive if adsorption is initiated at 125 K. Acetylene adsorption is via the π bonding favored on surfaces partially covered with carbonaceous deposits, suggesting that the dendrimer moieties that remain on the Pt surface of these catalysts may temper the dehydrogenation activity of the metal and favor hydrogenation and isomerization steps.

ETBE synthesis over silicotungstic acid and tungstophosphoric acid catalysts calcined at different temperatures

Vapor phase ethyl tertiary butyl ether synthesis was investigated using heat treated heteropoly acid catalysts, namely silicotungtsic acid (STA) and tungstophosphoric acid-Keggin (TPA-K) and these results were compared with the results obtained with untreated catalysts. ETBE synthesis experiments showed that heat treatment of TPA-K at temperatures over 473 K had caused significant decrease of its catalytic activity. Activity of STA was more stable and deactivation of this catalyst was observed by heat treatment at 673 K and above. Heat treatment at high temperatures caused loss of constitutional water of STA and TPA-K, causing loss of protons, consequently the loss of acidity of the catalysts, resulting deactivation. FT-IR, TGA–DTA and DRIFTS analyses on pyridine-adsorbed catalysts supported the conclusions related to structural changes of STA and TPA-K with heat treatment. Highest ETBE yields were obtained at around 368 K, while at temperatures over 423 K formation of DEE and ethylene were observed due to dehydration of ethanol.

Study of induction period over K 2CO 3/MoS 2 catalyst for higher alcohols synthesis

The K 2CO 3/MoS 2 catalyst for higher alcohols synthesis with synthesis gas as feedstock was prepared. The catalyst was characterized by TPR, in-situ XPS, XRD and SEM. Effects of pretreatment with H 2, CO or synthesis gas on activity and selectivity of the catalyst were investigated. Results showed that there was a remarkable induction period about 180 h at the initial reaction stage for the un-treated catalyst. The catalytic performances for alcohols synthesis changed notably during the induction period. The induction period was confirmed to be resulted primarily from the sulfur losing and K element dispersion on the surface of ADM catalysts. Pretreatment of the catalyst could remarkably shorten the time of induction period as well as promote the catalytic activity. Furthermore, the higher alcohols (C 2 + OH) content in the liquid products were enhanced after the catalyst pretreated by CO or synthesis gas which could be ascribed to the increasing of Mo 4+ content on the surface of the catalyst.

Effect of Low Concentration of SO x on NO Reduction with Propene Over Ag/Al2O3

Catalytic activity for NO reduction with propene was investigated at 0–80 ppm SO2. NO was reduced more efficiently by propene on SO2-treated than untreated catalyst. Simultaneously, combustion of reductant was observed to lower NO reduction efficiency. Thus, the role of surface-adsorbed SOx species was regarded as depressing reductant combustion. NH3 adsorption revealed that SO2 treatment increased Bronsted acidity of the Ag/Al2O3 catalyst, which promoted propene activation. Reductant activation is a more important step, compared with NO activation to oxidative nitrate species. The NCO species, an index intermediate in NOx reduction, was produced on SO2-adsorbed Ag/Al2O3 at a lower temperature (473 K) than on the untreated catalyst. The reductive intermediates at low temperature are suggested to be alcohol, or aldehyde-adsorbed species, based on observed C=O band.

Promoted metal utilization capacity of alkali-treated zeolite: Preparation of Zn/ZSM-5 and its application in 1-hexene aromatization

Promoted metal utilization capacity of ZSM-5 zeolite has been achieved via the intracrystalline mesopores created by alkali treatment. The untreated and alkali-treated Zn/ZSM-5 catalysts were prepared by the conventional liquid ion exchange and impregnation methods. These catalysts were investigated by several techniques: N 2 adsorption–desorption, transmission electron microscopy, X-ray photoelectron spectroscopy, ammonia temperature-programmed desorption, and Fourier transform infrared spectroscopy. Compared with the untreated catalysts, the alkali-treated catalysts exhibited higher Zn-loading in liquid ion exchange, better metal distribution and more Lewis acid sites in impregnation, owing to the location of Zn species in the created intracrystalline mesopores. The alkali-treated Zn/ZSM-5 catalysts displayed dramatically improved catalytic stability in 1-hexene aromatization. These findings show new potential for developing some advanced bifunctional metal-zeolite catalysts by a facile and low-cost method.

Tuning the support adsorption properties of Pd/SiO 2 by silylation to improve the selective hydrogenation of aromatic ketones

Silylation of Pd/SiO 2 catalysts increases the selectivity toward alcohols in the reduction of aromatic ketones. This work demonstrates that the selectivity is directly related to the adsorption strength of the alcohol on the surface of the support relative to the adsorption strength of the ketone. This observation can be explained by interaction of the support coverage with the metal coverage. Silylation yields a more hydrophobic support, on which the aromatic alcohol adsorbs more weakly relative to the ketone, in turn decreasing the amount of the alcohol adsorbed on the metal and thus suppressing the consecutive reduction of the alcohol. Silylation was carried out by using di-alkyl (dichlorodimethylsilane) and tri-alkyl (hexamethyldisilazane and hexamethyldisilane) silylating agents. Hexamethyldisilazane provided to be the most effective agent in terms of incorporation of methyl groups, catalyst hydrophobicity, and stability. Selective hydrogenation of 4-isobutyl acetophenone (4-IBAP) to 4-isobutylphenyl ethanol (4-IBPE) revealed that not only was the fresh hexamethyldisilazane-silylated Pd/SiO 2 catalyst more selective than the untreated catalyst, but also the silylated catalyst was much more selective after a deactivation–regeneration cycle than the untreated Pd/SiO 2 catalyst. The change in selectivity can be explained by a change in the relative adsorption strength of 4-IBPE over 4-IBAP on silylation.

Effect of carbon surface oxidation on platinum supported carbon particles on the performance of gas diffusion electrodes for the oxygen reduction reaction

The effect of carbon surface oxidation on platinum supported carbon particles (Pt/C) with nitric acid was investigated by cyclic voltammetry, electrochemical impedance spectroscopy, polarization experiments and chronoamperometry. Cyclic voltammograms, polarization curves and electrochemical impedance spectra showed that the treated catalyst had much larger active surface area and higher ionic conductivity than the untreated catalyst, and provided enhanced performance for oxygen reduction. The formation of acidic groups was examined by IR spectra. The Pt/C surface oxidation had a large effect on the performance of a gas diffusion electrode for oxygen reduction reaction.

Modification of acidity of Mo-Fe/HZSM-5 zeolite via argon plasma treatment

The NH3-TPD characterization was conducted to confirm that the acidity of Mo-Fe/HZSM-5 zeolite could be selectively modified via the glow discharge plasma treatment. The plasma catalyst treatment could totally change the distribution of aromatic products with higher methane conversion compared to the untreated catalyst. Some polycyclic aromatics such as anthracene, pyrene and phenanthrene were also produced over the plasma treated catalyst, in addition to benzene, toluene and naphthalene, which were normally obtained over the untreated catalyst.

Effect of Hydrohalogenation of Metal/Zeolite Catalysts for Cyclohexene Hydroconversion II. Rhenium/H-ZSM-5 Catalysts

Abstract A post-preparation treatment of the Re/H-ZSM-5 catalyst with HCl or HF was performed, and its effect on cyclohexene hydroconversion reactions at 50-400 °C in a continuous flow reactor was investigated. The HF treatment significantly improved the catalytic activity for cyclohexene hydrogenation to cyclohexane and methylcyclopentenes to methylcyclopentane in comparison to the HCl treatment. This is principally attributed to the higher Re dispersion and enhancement of the acid site number and strength. Thereupon occurs the highest hydrocracking activity and the lowest dehydrogenation activity for cyclohexene to cyclohexadienes on Re/H-ZSM-5(HF) by virtue of the highest strength of acid sites. The untreated Re/H-ZSM-5 catalyst exhibited the highest dehydrogenation activity for aromatics production, which may be attributed to the lowest quantity of extraframework Al and Si depositing in the pores and cavities of the zeolite support. Industrially, the relative amounts of xylene isomers produced on the hydrohalogenated catalysts are of interest. The amount of p -xylene was higher than m -xylene on the HF-treated catalyst; m - and p -xylenes presented almost equal values on the HCl-treated catalyst; m -xylene exceeded p -xylene on the untreated catalyst. O-xylene was almost equal to the sum of p - and m -xylenes in all cases.

Optimized CuO–CeO2 catalysts for COPROX reaction

CuO–CeO2 catalyst precursors were synthesized by homogeneous thermal decomposition of urea. Catalysts containing different Cu/Ce ratios were obtained after calcination of the precursors at 450∘C and characterized by ICP-OES, N2 adsorption, XRD and TPR. Catalytic tests were performed in the temperature range 100–300∘C to evaluate the activity of these samples for the COPROX reaction. A strong synergy between Cu and Ce is evidenced by the difference in the catalytic activity of the individual oxides (CuO and CeO2), a mechanical mixture of both oxides and a Cu–Ce mixed catalyst. The synergy between Cu and Ce is confirmed by the improvement of the reducibility of both cations in the mixed samples. Nitric acid and ammonium carbonate washed catalysts, especially high-copper-content samples, are more active than the untreated catalysts suggesting that the washing procedure could provoke a destruction of the polycrystalline structure that improves the contact between the reactants and the catalytic sites.

Effect of Hydrohalogenation of Metal/Zeolite Catalysts for Cyclohexene Hydroconversion ‐Part 3‐ Pd/H‐ZSM‐5 Catalysts

AbstractCyclohexene (CHE) hydroconversion was performed in a flow reactor at atmospheric pressure and temperatures of 50–400 °C using: Pd/H‐ZSM‐5, Pd/H‐ZSM‐5(HCl), and Pd/H‐ZSM‐5(HF) catalysts. These catalysts were characterized for acid site strength distribution via NH3 TPD, Pd dispersion via H2 chemisorption, TPR via reduction of the metal oxide in the catalysts and XRD for tracing crystallinity The hydroconversion steps proceeded as follows: CHE → Cyclohexane (CHA); CHE → Methylcyclopentenes (MCPEs) → Methylcyclopentane (MCPA); CHE → Cyclohexadienes (CHDEs) → Benzene → Alkylbenzenes; CHE and others → Hydrocrackedproducts.The overall hydroconversion of CHE was achieved in the catalyst order: Pd/H‐ZSM‐5 &gt; Pd/H‐ZSM‐5(HF) &gt; Pd/H‐ZSM‐5(HCl). CHE hydrogenation step was the major reaction at low temperatures which significantly inhibited via HCl treatment, but slightly enhanced via HF treatment. At medium temperatures, on all catalysts, isomerisation to MCPEs and MCPA increase to a maximum then a decline with a further increase of temperature. The overall isomerisation of CHE was highest on the untreated catalyst. During the higher temperature range, dehydrogenation, alkylation and hydrocracking were increased with temperature. Dehydrogenation of CHE always yielded larger amounts of 1,3‐CHDE than 1,4‐CHDE. These cyclohexadienes were produced in the catalyst order: Pd/H‐ZSM‐5(HF) &gt; Pd/H‐ZSM‐5(HCl) &gt; Pd/H‐ZSM‐5. In general, benzene alkylation to toluene exceeded that of xylenes, indicating that the second methylation is more difficult than the first. However, the catalytic activities for benzene and toluene production were in the order: Pd/H‐ZSM‐5 » Pd/H‐ZSM‐5(HCl) &gt; Pd/H‐ZSM‐5(HF), whereas for xylenes production, Pd/H‐ZSM‐5 » Pd/H‐ZSM‐5(HF) &gt; Pd/H‐ZSM‐5(HCl). Intrapore diffusion plays an important role during the dehydrogenation reactions as well as during the interconversion of individual aromatic hydrocarbons.

CO Adsorption on Gold Clusters Stabilized on Ceria−Titania Mixed Oxides: Comparison with Reference Catalysts

Fourier transform infrared spectra of CO adsorption from 120 K up to room temperature on two gold catalysts supported on different mixed ceria-titania oxides are discussed in comparison with those obtained on Au/TiO(2) and Au/Fe(2)O(3) reference catalysts provided by the World Gold Council. The spectra of adsorbed CO, run on the different samples before preliminary treatment, are shown and compared with those of the untreated catalysts and of the samples reduced either in CO or in hydrogen. Big differences have been found between the ceria-titania supported samples and the reference ones: unusual absorption bands, irreversible to outgassing, have been detected after CO interaction on the untreated and oxidized ceria containing samples. These absorptions are assigned to CO on Au(n)(+) small clusters stabilized at the ceria defects. By reduction in hydrogen, negatively charged Au(n)(-) species are produced on the same sample. Oxidized small particles are present on the reference catalysts, but only on the untreated samples; after treatment, only metallic step sites are evident.

Plasma treatment of Ni and Pt catalysts for partial oxidation of methane

In this work DBD (dielectric barrier discharge) plasma treatments of 10%Ni/Al2O3and 1%Pt/Al2O3catalysts have been conducted to study the principles of plasma treatment of supported catalysts. It was found that 10%Ni/Al2O3and 1%Pt/Al2O3catalysts treated by plasma exhibit a higher catalytic activity and a better stability than the catalysts prepared without plasma treatment. Methane conversion over the plasma treated catalyst is 3-5% higher than on untreated catalysts. The metal species dispersion also increased after plasma treatment, which leads to improvement of the interaction between active species and supports, the catalytic activities and the resistance to carbon deposition.

Plasma-Assisted Activation of Supported Au and Pd Catalysts for CO Oxidation

A nonthermal atmospheric pressure helium plasma was used for catalyst activation. The CO oxidation activity of alumina-supported Au and Pd catalyst was monitored using a gas chromatograph before and after plasma treatment. Catalysts were prepared by incipient wetness impregnation of Au and Pd salts on alumina. The plasma-treated Au catalyst converted 78% CO to CO/sub 2/ as compared to 45% in the case of untreated catalyst at room temperature. Similarly, plasma-treated Pd catalysts demonstrated improved performance at 100/spl deg/C and 150/spl deg/C. At 100/spl deg/C, 16% conversion was achieved with the plasma-treated catalyst whereas there was no conversion with untreated catalyst. The conversion increased to 82% as compared to 44% for the untreated sample at 150/spl deg/C. At 200/spl deg/C both untreated as well as plasma-treated samples achieved nearly 100% conversion.

Surface and catalytic properties of MoO 3/Al 2O 3 system doped with Co 3O 4

Thermal solid–solid interactions in cobalt treated MoO 3/Al 2O 3 system were investigated using X-ray powder diffraction. The solids were prepared by wet impregnation method using Al(OH) 3, ammonium molybdate and cobalt nitrate solutions, drying at 100 °C then calcination at 300, 500, 750 and 1000 °C. The amount of MoO 3, was fixed at 16.67 mol% and those of cobalt oxide were varied between 2.04 and 14.29 mol% Co 3O 4. Surface and catalytic properties of various solid samples precalcined at 300 and 500 °C were studied using nitrogen adsorption at −196 °C, conversion of isopropanol at 200–500 °C and decomposition of H 2O 2 at 30–50 °C. The results obtained revealed that pure mixed solids precalcined at 300 °C consisted of AlOOH and MoO 3 phases. Cobalt oxide-doped samples calcined at the same temperature consisted also of AlOOH, MoO 3 and CoMoO 4 compounds. The rise in calcination temperature to 500 °C resulted in complete conversion of AlOOH into very poorly crystalline γ-Al 2O 3. The further increase in precalcination temperature to 750 °C led to the formation of Al 2(MoO 4) 3, κ-Al 2O 3 besides CoMoO 4 and un-reacted portion of Co 3O 4 in the samples rich in cobalt oxide. Pure MoO 3/Al 2O 3 preheated at 1000 °C composed of MoO 3–αAl 2O 3 solid solution (acquired grey colour). The doped samples consisted of the same solid solution together with CoMoO 4 and CoAl 2O 4 compounds. The increase in calcination temperature of pure and variously doped solids from 300 to 500 °C increased their specific surface areas and total pore volume which suffered a drastic decrease upon heating at 750 °C. Doping the investigated system with small amounts of cobalt oxide (2.04 and 4 mol%) followed by heating at 300 and 500 °C increased its catalytic activity in H 2O 2 decomposition. This increase, measured at 300 °C, attained 25.4- and 12.9-fold for the solids precalcined at 300 and 500 °C, respectively. The increase in the amount of dopant added above this limit decreased the catalytic activity which remained bigger than those of un-treated catalysts. On the other hand, the doping process decreased the catalytic activity of treated solids in isopropanol conversion especially the catalysts precalcined at 300 °C. This treatment modified the selectivities of treated solids towards dehydration and dehydrogenation of reacted alcohol. The activation energies of H 2O 2 decomposition were determined for pure and variously doped solids. The results obtained were discussed in light of induced changes in chemical composition and surface properties of the investigated system due to doping with cobalt oxide.

Effect of pretreatment on properties of TS-1/diatomite catalyst for hydroxylation of phenol by H 2O 2 in fixed-bed reactor

The TS-1/diatomite catalyst was prepared for the hydroxylation of phenol with H 2O 2 in the fixed-bed reactor and the effects of pretreatment on the properties of TS-1/diatomite were studied by FT-IR, XRD, UV–vis, ICP-AES, BET surface area and NH 3-TPD techniques. It is shown when the catalyst is pretreated by the KAc, NaAc, NH 4Ac, NH 4Cl or HNO 3 aqueous solution, the framework structure of TS-1 is not destroyed and titanium in the framework is not removed. The surface area of catalyst has no obvious change compared with that of the untreated catalyst. But the extra-framework TiO 2 can be removed partly, which leads to the slight increase of the crystallinity of catalyst and the decrease of acid concentration on the surface of the TS-1/diatomite catalyst. As a result, the activity, selectivity and utilization of H 2O 2 for hydroxylation of phenol are improved. After the TS-1/diatomite catalyst is pretreated by the NH 3·H 2O, Na 2CO 3 or Na 3PO 4 solution, its framework silicon is dissolved partly in the base solution and the framework structure of TS-1 is destroyed. While the crystallinity and surface area of catalyst decrease and the concentration of acid sites on the surface of catalyst increased slightly. As a result, the catalytic activity of the TS-1/diatomite catalyst for hydroxylation of phenol descended or deactivated completely.

Characterisations of Pd–Ag/Al 2O 3 catalysts for selective acetylene hydrogenation: effect of pretreatment with NO and N 2O

This contribution concerns the surface characterisation of the Pd–Ag/Al 2O 3 catalyst for the selective hydrogenation of acetylene. It is shown that, by means of the temperature programmed reduction (TPR) and temperature programmed desorption (TPD), it is possible to view the surface Pd sites of the catalyst prior to pretreatment with NO or N 2O, as well as seeing the effect of Ag addition. It is shown that hydrogen adsorption on the palladium is effectively inhibited by the presence of Ag. The role of pretreatment on the activity and ethylene gain enhancement reported in our previous communication is investigated by employing transmission infrared spectroscopy. Strong adsorption of nitrate or nitrite species on Pd surface was observed from NO and N 2O pretreatment, resulting in the blockage of Pd sites responsible for direct ethane formation. Accordingly, higher ethylene gain from NO x -treated catalysts was seen over the untreated catalyst.

Effect of electron beam irradiation on CO 2 reforming of methane over Ni/Al 2O 3 catalysts

The effect of electron beam irradiation on the CO 2 reforming of methane over Ni/Al 2O 3 was investigated. The conversion rate of CO 2 and CH 4 forming H 2 and CO using various catalysts irradiated with an absorbed dose greater than 2 MGy was 5–10% higher than when using an untreated catalyst. The Ni/O ratio on the catalyst surface increased after treatment with an electron beam, and was more prominent for catalysts with a higher Ni content. As such, based on XRD and XPS measurements, electron beam treatment was found to result in either the desorption of oxygen from NiO or the removal of OH groups from the outermost surface layer of the catalyst. In addition, the concentration of active sites, such as Ni 2+ and NiO, or surface defects was also found to increase with the absorbed radiation dose, thereby increasing the conversion rate.

Mechanical stress induced activity and phase composition changes in sulfated zirconia catalysts

Sulfated zirconia and Mn-promoted sulfated zirconia (0.5 and 2.0 wt% Mn) catalysts were subjected to mechanical stress. Pressing (10 min 540 MPa), milling (10 min vibrating mill), and grinding (manually, 10 min agate mortar) effected a partial phase transformation from the tetragonal to the monoclinic phase of ZrO 2. The mechanical stress also reduced the n-butane isomerization rate (1 kPa n-butane, 323–378 K, atmospheric pressure) to 30% and less of that measured for untreated catalyst. Standard sample preparation techniques for analytical methods thus alter the structural and catalytic properties of sulfated zirconia. Attempts to correlate data from different methods are futile unless the integrity of the sulfated zirconia is ensured.