Catalytic removal of perchloroethylene (PCE) over supported chromium oxide catalysts

The oxidation of perchloroethylene (PCE) was investigated over chromium oxide catalysts supported on SiO₂, SiO₂–Al₂O₃, activated carbon, mordenite type zeolites, MgO, TiO₂ and Al₂O₃. Supported chromium oxide catalysts were more active than any other metal oxide catalysts including noble metal examined in the present study. PCE removal activity of chromium oxide catalysts mainly depended on the type of supports and the content of metal loaded on the catalyst surface. TiO₂ and Al₂O₃ containing high surface areas were effective for the high performance of PCE removal, since the formation of well dispersed Cr(VI) active reaction sites for the present reaction system, was enhanced even for the high Cr loading on the catalyst surface. CrO_x catalysts supported on TiO₂ and Al₂O₃ also exhibited stable PCE removal activity at a low feed concentration of PCE of 30 ppm up to 100 h at 350°C. However, significant catalyst deactivation was observed at high PCE concentration of 10 000 ppm. CrO_x/TiO₂ revealed stronger water tolerance than CrO_x/Al₂O₃ due to the surface hydrophobicity.     

Catalytic nitroxidation of para-xylene over chromium oxide based catalysts

The synthesis of para-tolunitrile (PTN) and terephthalonitrile (TPN) by reacting nitrogen monoxide with para-xylene was studied in presence of three chromium oxide catalysts. The Cr₂O₃—Al₂O₃ catalyst was prepared under the form of aerogel or xerogel and shows excellent selectivities in nitriles along with good stability. However, the unsupported Cr₂O₃ catalyst showed much less performance and, in particular, produced no TPN. The beneficial action of alumina was explained in terms of stabilization of the Cr⁵⁺ species. Kinetic results seemed to indicate a redox-type mechanism of the reaction. Moreover the PTN was shown to be an intermediate in TPN formation.     

A pilot plant study for catalytic decomposition of PCDDs/PCDFs over supported chromium oxide catalysts

Chromium oxide catalysts supported on TiO₂ and Al₂O₃ were examined in a fixed-bed flow reactor system for the removal of PCE (perchloroethylene), a simulant of 2,3,7,8-TCDD (2,3,7,8-tetrachlorodibenzo-p-dioxin), and in a pilot plant employing actual flue gas from a sintering plant for the removal of PCDDs/PCDFs (poly-chlorinated dibenzo-dioxin/poly-chlorinated dibenzo-furan). The 12.5 wt.% chromium oxides supported on TiO₂ and Al₂O₃ revealed excellent stability and performance of PCE removal in the feed gas stream containing water vapor. In a pilot plant study, the catalysts washcoated on the honeycomb reactor revealed 93–95% of PCDDs/PCDFs removal activity over CrO_x/Al₂O₃-HC20 (CrO_x/Al₂O₃ catalyst washcoated on 20 cell-honeycomb), and more than 99% of the decomposition activity over CrO_x/TiO₂-HC20 (CrO_x/TiO₂ catalyst washcoated on 20 cell-honeycomb) at 325 °C and 5000 h⁻¹ of reactor space velocity without the de novo synthesis of PCDDs/PCDFs. In particular, CrO_x/TiO₂-HC20 showed 94% of PCDDs/PCDFs decomposition activity even at 280 °C reaction temperature. The catalyst also exhibited significant NO removal activity. The chromium oxide seems to be a promising catalyst for the removal of PCDDs/PCDFs and NO_x contained in the flue gas.     

Partial oxidation of propene over metal oxide catalysts pretreated with NO2

Titania pretreated with NO₂ has been found to catalyze the partial oxidation of propene into oxygenates such as acetone and acrolein at around 623 K, while fresh TiO₂ produces only carbon oxides. Temperature‐programmed desorption (TPD) revealed that nitrogen oxides adsorbed on TiO₂ are stable at temperatures below 673 K. Even after the propene oxidation at 573 K for 2 h, nitrogen oxides were confirmed still to exist on the TiO₂ surface. At temperatures higher than 673 K, however, the desorption and/or the reduction of the adsorbed nitrogen oxides took place, and concomitantly the catalytic ability giving oxygenates disappeared. This revised version was published online in July 2006 with corrections to the Cover Date.     

Perovskite-type oxide monolithic catalysts for combustion of chlorinated hydrocarbons

The catalytic activity of perovskite-coated and perovskite-extruded monolithic catalysts was studied in the total oxidation of several aliphatic chlorinated hydrocarbons. At low reaction temperatures a reversible catalyst deactivation takes place. The complete decomposition of the chlorinated hydrocarbons without formation of by-products depends on the reaction conditions, the kind of chlorinated hydrocarbon and the monolith preparation.     

Catalytic fluorination of 1,1,1‐trifluoro‐2‐chloroethane (HCFC‐133a) over chromium catalysts

Catalytic fluorination of HCFC‐133a was performed over various forms unsupported and supported chromium oxides. Catalytic activity is strongly dependent on the morphological structure of chromia. Increased conversion is observed as the crystallinity of chromia decreases. The best catalytic activity and strongest resistance against deactivation observed with CrOx/MgO is attributed to the presence of highly dispersed chromium species (isolated Cr(VI) and oligomeric Cr–O–Cr species) on the support surface. This revised version was published online in July 2006 with corrections to the Cover Date.     

Effect of perchloroethylene (PCE) and hydraulic shock loads on a membrane‐aerated biofilm reactor (MABR) biodegrading PCE

BACKGROUND: A membrane‐aerated biofilm reactor (MABR) has previously been used to provide both anaerobic and aerobic conditions for mineralisation of perchloroethylene (PCE). However, very little is known about the stability of this reactor under hydraulic and PCE shock loads. An MABR was therefore subjected to sudden hydraulic and PCE shock loads in order to investigate its stability under such conditions. RESULTS: After each shock, the reactor responded with an increase of chemical oxygen demand (COD) and volatile fatty acids (VFA)s, a breakthrough of PCE and its biodegradation intermediates in the effluent, and a decrease in methane production. Although some PCE biodegradation intermediates were found in the effluent during each shock loading, the MABR performance recovered without the accumulation of any particular PCE biodegradation intermediates during PCE shock loads. During the hydraulic shock loads, the MABR was unstable at hydraulic retention times (HRTs) of 6 h with PCE and its biodegradation intermediates detected in the effluent. However, these intermediates were degraded when the HRT was reset to 48 h. CONCLUSIONS: This study suggests that MABRs can withstand fluctuations in influent strength and flows which occur in wastewater treatment works. Copyright © 2009 Society of Chemical Industry     

Effect of support on the conversion of methane to synthesis gas over supported iridium catalysts

A partial oxidation of methane was carried out using iridium catalysts supported on several metal oxides. The productivity of the synthesis gas from methane was strongly affected by the choice of support oxides for the catalysts. The synthesis gas production proceeded basically via a two-step reaction consisting of methane combustion to give H₂O and CO₂, followed by the reforming of methane from CO₂ and steam. Although the combustion and the reforming of methane from steam did not depend upon the catalyst support, a large variation in the catalytic activity for the reforming of methane from CO₂ was observed over Ir catalysts with different supports. The support activity order in the reforming of methane from CO₂ with iridium catalysts was as follows: TiO₂≧ZrO₂≧Y₂O₃>La₂O₃>MgO≧Al₂O₃>SiO₂. The same order was observed in the synthesis gas production from the partial oxidation of methane. This revised version was published online in August 2006 with corrections to the Cover Date.     

Reducibility of supported gold (III) precursors: influence of the metal oxide support and consequences for CO oxidation activity

The origin of CO oxidation performance variations between three different supported Au catalysts (Au/CeO₂, Au/Al₂O₃, Au/TiO₂) was examined by in situ XAFS and DRIFTS measurements. All samples were prepared identically, by deposition-precipitation of an aqueous Au(III) complex with urea, and contained the same gold loading (~1 wt %). The as-prepared supported Au(III) precursors exhibited different reduction behaviour during exposure to the CO/O₂/He reaction mixture at 298 K. The reducibility of the Au(III) precursor was found to decrease as a function of the support material in the order: titania > ceria > alumina. The as-prepared samples were inactive catalysts, but Au/TiO₂ and Au/CeO₂ developed catalytic activity as the reduction of Au(III) to metallic Au proceeded. Au/Al₂O₃ remained inactive. The developed catalytic CO oxidation activity at 298 K varied as a function of the support as follows: titania > ceria > alumina ~ 0. The EXAFS of samples pretreated in air at 773 K and in H₂ at 573 K reveals the presence of only metallic particles for Au/TiO₂ and Au/Al₂O₃. Au(III) supported on CeO₂ remains unreduced after calcination, but reduces during the treatment with H₂. CO oxidation experiments performed at 298 K with the activated samples show that the presence of metallic gold is necessary to obtain active catalysts (Au/CeO₂ is not active after calcination) and that the reducible supports facilitate the genesis of active catalysts, while metallic gold particles on alumina are not active.     

Model oxide supports for studies of catalyst sintering at elevated temperatures

Sintering is one of the most important causes for loss of catalytic activity when catalysts are subjected to elevated temperatures, as in automotive exhaust or during catalytic combustion. Herein we report a novel form of model catalyst for sintering studies at elevated temperatures. These model catalysts based on disks of SiO₂ (quartz) and Al₂O₃ (sapphire) allow sintering temperatures of > 900 °C to be studied. High resolution scanning electron microscopy (SEM) was used for the study of these catalysts. Using modern in-lens backscatter electron detection operating at low voltages, high contrast images can be obtained of the metal catalyst on these model oxide supports. Model catalysts make it possible to observe the same area of the sample, before and after sintering, which is essential to gain mechanistic insights into the sintering phenomena. Our results show that while Ostwald ripening is the dominant mechanism for Pd sintering at 900 °C, we see evidence for the migration of particles as large as 50 nm on the alumina surface.     

Selective oxidation of H2S to elemental sulfur over chromium oxide catalysts

CrO_x and CrO_x supported on SiO₂ have been found to be active for the selective oxidation of hydrogen sulfide to elemental sulfur. The catalysts show maximum sulfur yield at a stoichiometric ratio of O₂/H₂S, 0.5. Amorphous Cr₂O₃ exhibits higher yield of sulfur and has stronger resistance against water than supported Cr/SiO₂, especially at low temperatures. At high temperatures above 300°C, the sulfur yield over the supported catalyst becomes similar to amorphous Cr₂O₃ because the Claus reaction occurring on the silica support removes SO₂ to increase the sulfur yield. Active sites are the amorphous monochromate species that can be detected as a strong temperature programmed reduction (TPR) peak at 470°C. Catalytic activity can be correlated with the amount of labile lattice oxygen and the strength of Cr–O bonding. The reaction proceeds via the redox mechanism with participation of lattice oxygen.     

Supported chromium oxide catalysts using metal carboxylate complexes: dehydrogenation of propane

The present paper constitutes the first stage of a systematic study of the influence of precursor nature and structure on the catalyst behavior in dehydrogenation of propane, by varying the nature of the ligands and the nuclearity of the starting compounds at fixed net surface potentials. We show results obtained by the use of different Cr carboxylates to design the Cr₂O₃/γ-Al₂O₃ catalyst at low loadings and these results are compared with those obtained for chromic acid. The following short chain chromium compounds were selected as precursors: citrate (Cr(III)), dimer monohydrate acetate (Cr(II)), acetyl acetonate (Cr(III)) and hydroxyacetate (Cr(III)). An exhaustive characterization by means of BET surface, XPS, XRD, X-ray fluorescence, laser Raman spectroscopy, EPR and catalytic test in propane dehydrogenation enabled us to draw relevant conclusions. Low metal–support interaction might be the major cause of polymerization in Carbox. The interaction between the chromia phase and the support surface, which stabilizes different oxidation stages and coordinations of the chromia species, defines the surface architecture. Part of the Cr³⁺ is incorporated in the vacant octahedral sites of the spinel surface. The larger extension of this phenomenon in Cral might be responsible for surface inactive species that could cause a loss of catalytic activity. Deactivation between cycles might depend on the catalyst stabilization by the incorporation of Cr³⁺ into the support structure. The surface array of monomer species in a polymer species environment stabilizes the catalyst architecture and confers characteristics of high stability between cycles. The above considerations permit to infer that chromium carboxylates are interesting precursors for the control of surface species in chromia/Al₂O₃ catalysts.     

Volatile organic compounds (VOCs) removal over dual functional adsorbent/catalyst system

This study is focused on the elimination of tetrachloroethylene (PCE) and methylethylketone (MEK) by adsorption/catalytic oxidation in humid conditions using zeolites as adsorbent and catalyst. Adsorption experiments have led to the conclusion that HFAU(17) zeolite was a very efficient adsorbent for the removal of PCE and MEK. Furthermore, complete transformation of PCE into CO₂ and HCl was achieved at 500 °C over 1.2%Pt/HFAU(5). MEK was completely converted into CO₂ at 220 °C over 1%Pt/NaX, without formation of CO. Formation of coke occurs during MEK oxidation reaction, especially over acid catalyst, and at low reaction temperature. Furthermore, it was pointed out that water has a negative effect on MEK conversion. Lastly, adsorption/catalytic oxidation of PCE and MEK was performed over a dual functional HFAU(17)-Pt/FAU adsorbent/catalyst system. The adsorption step was performed at 30 °C with an HFAU(17) adsorbent, and the oxidation step was carried out at 450 °C for PCE with 1.2%PtHFAU(5), and at 250 °C for MEK transformation with 1%PtNaX. It appeared that the adsorbent-catalyst couple remains efficient for MEK transformation during successive adsorption/oxidation cycles.     

Comparison of metathesis activity of catalysts prepared by anchoring of MoO2(acac)2 on various supports

The properties and olefin metathesis activity of molybdena–alumina, molybdena–silica–alumina and molybdena–silica catalysts prepared by anchoring of MoO₂(acac)₂ complex were compared. The specific activity obtained for molybdena–silica–alumina system is higher than that of molybdena–alumina and molybdena–silica catalysts. This statement concerns also the catalysts treated with tetramethyltin     

Methane oxybromination over Rh-based catalysts: Effect of supports

Bromine mediation has been regarded as one of the most efficient ways to activate and convert methane to useful organics. This article reports the effects of active components (Rh, Ru, Pd and Pt) and supports (SiO₂, MgO and Al₂O₃) on the catalysis of methane oxybromination. Among the prepared catalysts, Rh/SiO₂ is the best in performance (CH₄ conversion of ca. 20% and CH₃Br selectivity exceeding 70%). The results reveal that support type has a notable influence on the catalytic performance of Rh, especially on product distribution. The high selectivity to CH₃Br over Rh/SiO₂ is attributed to its low propensity for CH₃Br oxidation. It was found that Rh small in particle size shows high catalytic activity and CH₃Br selectivity. Although silicalite-1 zeolite suffers from a certain degree of structural damage due to the presence of high temperature steam, the use of silicalite-1 as support results in a performance comparable to that of Rh/SiO₂.     

The electronic structure of oxide-supported tungsten oxide catalysts as studied by UV spectroscopy

The UV spectra of tungsten oxide catalysts supported on alumina, titania and zirconia, and on titania–alumina and titania–zirconia mixed oxides are reported and discussed. Evidence is provided for the different electronic structure of supports and catalysts, which could affect the behavior of the tungsten oxide centres in the different cases. On alumina, tungsten oxide centres are “isolated” by the insulating support, while on titania-based materials they are likely in electronic contact with each other and with Ti centres through the support conduction band. In the case of WOx2013;ZrO₂, the 5d levels of tungsten ions fall just below of the lower energy limit of the support conduction band. This revised version was published online in July 2006 with corrections to the Cover Date.     

Selective oxidation of toluene over the new catalyst cobalt tetra (4-hydroxyl) phenylporphyrin supported on zinc oxide

Cobalt tetra(4-hydroxyl)phenylporphyrin/ZnO was prepared and used for the catalytic oxidation of toluene with O₂. Its activity has been increased by 70% over that of the cobalt tetra(4-hydroxyl)phenylporphyrin and the effective reuse of the supported catalyst for toluene oxidation is seven times per 1 mg (1.35 × 10⁻⁶ mol) cobalt tetra(4-hydroxyl)phenylporphyrin, demonstrating the promotion by zinc oxide.     

Effect of hydrocarbon species on no oxidation over diesel oxidation catalysts

The effect of propylene concentration on NO oxidation as a function of temperature and position over a model Pt-Pd/Al₂O₃ diesel oxidation catalyst was investigated. Propylene had an apparent inhibition effect on NO oxidation. This apparent inhibition is a result of NO₂, as the NO oxidation product, acting as an oxidant in the reaction with propylene. This was verified with experiments that included NO₂, and a resulting significant temperature decrease in the onset of NO₂ reduction when propylene was present. Furthermore, increasing amounts of propylene further decreased the NO₂ reduction temperature. Similar results were observed with m-xylene and dodecane addition as well. The results also demonstrate that NO₂ was consumed preferentially relative to O₂ during hydrocarbon oxidation. With low inlet levels of O₂, the addition of NO₂ apparently inhibited propylene oxidation after the onset of NO₂ reduction. This subsequent inhibition was due to the NO formed, demonstrating that propylene results in reduced NO₂ outlet levels while NO inhibits propylene oxidation.     

Effect of chloride on sintering of Pt/Al2O3 catalysts

Reduced Pt/Al₂O₃ catalysts with different chloride contents were treated at different temperatures under oxygen flow. TPR and TPD studies of oxidized species show that at low Cl/Pt atomic ratio (1) PtO₂ is formed at low temperature (400–500 K) and is totally decomposed (900 K) yielding reduced metallic Pt and inducing metal sintering. At high Cl/Pt atomic ratio (6) formation of stable (up to 1000 K) platinum oxichloride avoids metal sintering.