Promoting effect of the addition of Ce and Fe on manganese oxide catalyst for 1,2-dichlorobenzene catalytic combustion

Mn-based mixed-oxide (MnO_x) catalysts were modified with Fe, Ce, and Ce + Fe, and its catalytic oxidation activity was tested by using 1,2-dichlorobenzene (o-DCB) as models of chlorinated volatile organic compounds. Addition of Ce or Ce + Fe into MnO_x promoted their crystals to turn into amorphous powder, enhanced their specific surface area and changed their redox property. The catalytic activity of MnO_x improved remarkably by adding Ce or Ce + Fe indicating Ce plays an important role. Both Mn-Ce and Mn-Ce-Fe catalysts exhibited good stability for catalytic oxidation of o-DCB, indicating that the introduction of promoter is an important method to improve the catalytic performance.     

An excellent support of Pd–Fe–Ox catalyst for low temperature CO oxidation: CeO2 with rich (200) facets

Pd–Fe–O_x catalysts for low temperature CO oxidation were supported on SBA-15, CeO₂ nano-particles with rich (111) facets and CeO₂ nano-rod with rich (200) facets, and characterized by X-ray diffraction, low-temperature nitrogen adsorption, transmission electron microscopy and temperature-programmed reduction. The results showed that when CeO₂ nano-rod was used as a support, Pd–Fe–O_x catalyst exhibits higher activity (T₁₀₀ = 10 °C), resulting from the rich (200) facets of CeO₂ nano-rod, which leads to a formation of large numbers of the oxygen vacancies on the surface of Pd–Fe–O_x catalysts.     

Modulation of selective sites by introduction of N2O,CO2 and H2 as gaseous promoters into the feed during oxidation reactions

Results obtained by adding gaseous promoters (CO₂, N₂O and H₂) into the reaction feed are presented for two different reactions: (i) oxidative dehydrogenation of propane (ODP), and (ii) catalytic combustion of methane (CCM). The ODP is performed on a mixture of NiMoO₄ and CeO₂, by adding 3 vol.% CO₂ into the feed, and on a NiMoO₄/[Si,V]-MCM-41 mesoporous catalyst, in the presence of 1 or 5 vol.% N₂O in the feed. The CCM is carried out (i) on Pd(2 wt.%)/Ce_xZr_1−xO₂ and Pd(2 wt.%)/γ-Al₂O₃ catalysts, on pure CeO₂ and on a mixture of Pd(2 wt.%)/γ-Al₂O₃ and CeO₂ powders, by adding 3 vol.% CO₂ into the feed, and (ii) on a Pd(2 wt.%)/γ-Al₂O₃ catalyst, in the presence of various amounts of H₂ in the feed. It is shown, through all these various examples, that the activity and/or the selectivity of catalysts can be improved by tuning, in a very controlled manner, the oxidation state of active sites via the use of these gaseous promoters.     

Etherification of n-butanol to di-n-butyl ether over H3PMo12 − xWxO40 (x = 0, 3, 6, 9, 12) Keggin and H6P2Mo18 − xWxO62 (x = 0, 3, 9, 15, 18) Wells–Dawson heteropolyacid catalysts

Etherification of n-butanol to di-n-butyl ether was carried out over H₃PMo_12 − xW_xO₄₀ (x = 0, 3, 6, 9, 12) Keggin and H₆P₂Mo_18 − xW_xO₆₂ (x = 0, 3, 9, 15, 18) Wells–Dawson heteropolyacid (HPA) catalysts. Acid strength of H₃PMo_12 − xW_xO₄₀ Keggin and H₆P₂Mo_18 − xW_xO₆₂ Wells–Dawson HPA catalysts was determined by NH₃-TPD (temperature-programmed desorption) measurements. The correlations between desorption peak temperature (acid strength) of the HPA catalysts and catalytic activity revealed that conversion of n-butanol and yield for di-n-butyl ether increased with increasing acid strength of the catalysts, regardless of the identity of HPA catalysts (without HPA structural sensitivity).     

Characterization of Pd impregnated on metal/silica-pillared H-keyaites (M-SPK,M = Ti,Zr) catalysts for partial oxidation of methane to hydrogen

Pd(5) impregnated metal/silica-pillared H-keyaites (M-SPK, M = Ti, Zr) catalysts were prepared for the partial oxidation of methane (POM) to hydrogen. The catalysts were characterized by BET, TEM, SAXS and XPS. In addition, the catalytic yield of the POM to hydrogen over Pd(5) impregnated on M-SPK and Pd(5)/Al₂O₃, commercial catalyst were investigated in a fixed bed flow reactor under (Ed atmosphere. BET-specific surface areas, average pore sizes and nitrogen adsorption/desorption isotherms were 284.3–396.2 m²/g, 3.3–3.8 nm and type B on type IV isotherms for Pd(5)/M-SPK(M = Ti, Zr), and 90.5 m²/g, 8.3 nm and type E on type IV isotherm for Pd(5)/Al₂O₃, respectively. TEM images of SPK and Pd(5)/SPK showed the formation of mesoporous layer compounds, as well as the homogenous dispersion of Pd particles on the surface. SAXS peaks at 0.13 Å for fresh Pd(5)/SPK were maintained without being broken, even after about 53 h in stream at 973 K. XPS showed the existence of two oxidation states for Pd (Pd⁰ and Pd²⁺) on the surface of the catalyst, depending on the carrier, whereas the presence of Ti and Zr in SPK induced a change in the oxidation state (O²⁻, O⁻) of the catalyst. The yield values of the POM to hydrogen over Pd(5)/M-SPK(M = Ti, Zr) were 64.9% and 55.8%, respectively, at 973K, CH₄/O₂ = 2, GHSV = 8.4 × 10⁴ ml/g_cat h, and these values were kept constant even after 70 h in stream. These results confirm that Ti and Zr in SPK frame induced oxidation states of Pd, and that the yield of Pd(5)/M-SPK positively regulates the POM to hydrogen.     

Catalytic activity of nanometer La1−xSrxCoO3 (x = 0, 0.2) perovskites towards VOCs combustion

Combustive oxidation of volatile organic compounds (VOCs), such as propyl alcohol, toluene and cyclohexane, were studied. The combustion was catalyzed by nanoparticles of La_1−xSr_xCoO₃ (x = 0, 0.2) perovskites prepared by a co-precipitation method. The results showed high activities of the perovskite catalysts. Compared to LaCoO₃, in particular, La_0.8Sr_0.2CoO₃ was much higher in catalytic ability. The total oxidation of VOCs followed the increasing order: cyclohexane < toluene < propyl alcohol. The T_99% of cyclohexane was 40 °C lower than that of toluene, which appeared to be determined by the bond strengths of the weakest C–H and C–C bonds. The 100-h stability experiments showed that La_1−xSr_xCoO₃ (x = 0, 0.2) perovskite was highly stable.     

Accurate hierarchical control of hollow nanocube Pd/CeO2 catalysts for the low-temperature oxidation of CO

An effective approach of simultaneous coordinating etching and Pd nano coating technology is employed to prepare hollow Pd/CeO₂ nanocubes as catalysts for the low-temperature oxidation of CO. The activities of Pd/CeO₂ catalysts are higher than that of Pd supported on Al₂O₃, and the activity of 1 wt.% Pd/CeO₂ can be enhanced obviously and its T₉₀ (which denotes the temperatures at which a 90% conversion of the initial reactants is attained) is as low as − 5.6 °C. The intrinsic hollow nature as well as high porosity of the unique nanostructures of CeO₂ support contributes greatly to the formation of large numbers of surface oxygen vacancies on the as-prepared Pd/CeO₂ catalysts, and therefore it exhibits outstanding catalytic activity. This strategy method is simple, of low cost, which may shed light on a new avenue for fast synthesis of hollow cube-like nano functional materials for catalyst, drug delivery, energy storage and other new applications.     

Promoting effect of Pd on H2 production from cellulose pyrolysis over mesocellular-foam-supported Ni catalysts

Noble-metal promoters have been added to catalysts for reactions such as steam-methane reforming, but have rarely been applied to systems that produce H₂ from larger, biomass-derived molecules, such as polyols or cellulose. We have previously found that nickel catalysts supported on mesocellular-foam-(MCF)-type silica catalyze H₂ formation during cellulose pyrolysis, and sought to increase their activity. Thus, palladium-promoted nickel catalysts supported on MCF were prepared, and their activities were tested in cellulose pyrolysis (RT → 800 °C, 40 °C/min) under dry argon. A thermogravimetric analyzer–mass spectrometer (TG–MS) was used to semi-quantitatively monitor the gases, especially H₂, that were released during pyrolysis over catalysts with and without Pd promoters. Although the Pd promoters had little impact on the fraction of H₂ in the product gas, adding ≥ 0.4 wt.% Pd enhanced the H₂ yield from cellulose pyrolysis by increasing the total gas yield from the reaction. Thus the promoter improved H₂ yield by enhancing the tar-cracking activity of the catalyst. A 5%Ni/MCF catalyst that was doped with 0.7 wt.% Pd yielded 85 cm³ H₂/g cellulose, which was 15% more H₂ than was obtained when the catalyst was 5%Ni/MCF.     

Effect of Pd deposition procedure on activity of Pd/Ce0.5Sn0.5O2 catalysts for low-temperature CO oxidation

Two methods for the preparation of Pd/Ce_0.5Sn_0.5O₂ catalysts have been used: solution combustion (SC) of Pd, Ce and Sn precursor mixtures, and incipient wetness impregnation by [Pd(NO₃)₂(H₂O)₂] solution of Ce_0.5Sn_0.5O_2 – δ support, obtained by the SC technique (SC + IWI). The formation of metallic palladium was observed in addition to ionic palladium in a Pd_x(Ce_0.5Sn_0.5)_1 – xO_{2 – x – δ} solid solution due to high-temperature Pd precursor decomposition during the SC process. IWI of Ce_0.5Sn_0.5O_2 – δ support has been demonstrated to lead to a uniform solid solution of Pd^2 + ions in the Ce_0.5Sn_0.5O_2 – δ matrix at 1% Pd content that resulted in high catalyst activity towards CO oxidation. The increase of Pd content to 5% showed no influence on catalytic activity. This observation is explained by the formation of low active PdO species when Pd content is more than 1%. The proposed SC + IWI method allows obtaining highly active Pd/Ce_0.5Sn_0.5O₂ catalysts with low palladium content.     

Preparation of noble metal nanocatalysts and their applications in catalytic partial oxidation of methane

A series of noble metal catalysts (Ru, Rh, Ir, Pt, and Pd) supported on alumina-stabilized magnesia were prepared and employed in partial oxidation of methane. The prepared catalysts were characterized using BET, SEM, TEM and H₂S chemisorption techniques. The results revealed that the Ru and Rh catalysts had the highest activity in catalytic partial oxidation of methane. Based on the obtained results the following order of activity was observed for different catalysts in partial oxidation of methane: Rh ～ Ru > Ir > Pt > Pd. The obtained results also showed a high catalytic stability without any decrease in methane conversion up to 50 h of reaction.     

Synergy between tungsten and palladium supported on titania for the catalytic total oxidation of propane

Titania-supported palladium catalysts modified by tungsten have been tested for the total oxidation of propane. The addition of tungsten significantly enhanced the catalytic activity. Highly active catalysts were prepared containing a low loading of 0.5 wt.% palladium, and activity increased as the tungsten loading was increased up to 6 wt.%. Catalysts were characterised using a variety of techniques, including powder X-ray diffraction, laser Raman spectroscopy, X-ray photoelectron spectroscopy, temperature-programmed reduction and aberration-corrected scanning transmission electron microscopy. Highly dispersed palladium nanoparticles were present on the catalyst with and without the addition of WO_x. However, the addition of WO_x slightly increases the average palladium particle size, and there was some evidence for the Pd forming epitaxial islands on the support in the tungsten-doped samples. Surface analysis identified a combination of Pd⁰ and Pd²⁺ on a Pd/TiO₂ catalyst, whereas all of the Pd loading was found in the form of Pd²⁺ with the addition of tungsten into the catalysts. At low tungsten loadings, isolated monotungstate and some polytungstate species were highly dispersed over the titania support. The concentration of polytungstate species increased as the loading was increased, and it was also promoted by the presence of palladium. The coverage of the highly dispersed tungstate species over the titania also increased as the tungsten loading increased. Some tungstate species were also found to be associated with the palladium oxide particles, and there was an enrichment of oxidised tungsten species at the peripheral interface of the palladium oxide nanoparticles and the titania. Sub-ambient temperature–programmed reduction experiments identified an increased concentration of highly reactive species on catalysts with palladium and tungsten present together, and we propose that the new WO_x-decorated interface between PdO_x and TiO₂ particles may be responsible for the enhanced catalytic activity in the co-impregnated catalysts.     

Oxy-methane reforming over high temperature stable NiCoMgCeOx and NiCoMgOx supported on zirconia–haffnia catalysts: Accelerated sulfur deactivation and regeneration

NiCoMgO_x and NiCoMgCeO_x on commercial low surface area zirconia–haffnia catalysts have unusually high thermal stability (⩾2000 °C) for syngas generation via the methane partial oxidation process (J. Catal., 233, 36, 2005). Herein we report the results on accelerated sulfur deactivation (0.74 mol% sulfur in feed) and corresponding regeneration (at 800 °C in 1:1 O₂ + N₂ flow) over these catalysts. The NiCoMgCeO_x catalyst, due to a larger mobility of lattice oxygen, showed a considerably higher resistance to sulfur poisoning; the higher mobility of the lattice oxygen in case of the NiCoMgCeO_x catalyst may be related to the presence of CeO₂. During the deactivation process, the selectivity for H₂ was decreased to a much greater extent than that for CO. Regeneration studies showed that even after complete deactivation of the catalysts, the original activity/selectivity of both the catalysts could be completely restored after a simple regeneration process. Based on their exceptionally high thermal stability, high activity/selectivity and easily regenerability, the NiCoMgO_x and NiCoMgCeO_x catalysts appear to be very promising candidates for the CPOM process.     

Vapour phase hydrogenation of phenol over Pd/C catalysts: A relationship between dispersion,metal area and hydrogenation activity

A series of palladium supported on activated carbon catalysts, with Pd varying from 0.5 to 6.0 wt%, were prepared via wet impregnation method using PdCl₂ · xH₂O as a precursor salt. The dried samples were further reduced at 573 K in hydrogen and characterized by CO adsorption at room temperature in order to determine the dispersion, metal area and particle size. The catalysts were tested for vapour phase phenol hydrogenation in a fixed-bed all glass micro-reactor at a reaction temperature of 453 K under normal atmospheric pressure. The decrease in metal surface area as well as dispersion with corresponding increase in turn-over frequency (TOF) against palladium loadings suggest the unusual inverse relationship that exist between Pd dispersion and phenol hydrogenation activity over Pd/carbon catalysts. The stability of TOF at larger crystallite size indicates that phenol hydrogenation is less sensitive reaction especially beyond 3 wt% of Pd content. It is evident from the results that structural properties of the catalysts strongly influence the availability of Pd atoms on the surface for CO chemisorption and hence for phenol hydrogenation. A comparison between selectivity and product yield of the reaction against overall phenol conversion indicates that changes in reaction selectivity for cyclohexanone or cyclohexanol is independent of phenol conversion level and either of the product is not formed at the cost of another. The stability of the catalysts with reaction time suggests that coke formation on the surface of the catalyst is less significant and the formation of cyclohexanone remains almost total even at higher reaction temperatures.     

Catalytic performance of P-modified MoO3/SiO2 in oxidative desulfurization by cumene hydroperoxide

Phosphorous-modified MoO₃/SiO₂ catalysts with various P/Mo molar ratios (MoP_xO/SiO₂, x = 0.3 − 3.0) were prepared by co-impregnation and compared with MoO₃/SiO₂ in the oxidation of dibenzothiophene by cumene hydroperoxide. The addition of P significantly enhanced the catalytic performance, and MoP_1.0O/SiO₂ exhibited the highest oxidation activity. XRD characterization indicated that the active phase of MoP_1.0O/SiO₂ was amorphous, whereas ³¹P-NMR measurements revealed that MoP_1.0O/SiO₂ had a similar connectivity of Mo-O-P to H₃PMo₁₂O₄₀. The total sulfur of a hydrotreated diesel was reduced from 298 to 5 ppmw by oxidation with CHP and subsequent dimethylformamide extraction.     

Preparation of Pd/(Ce1 − xYx)O2/γ-Al2O3/cordierite catalysts and its catalytic combustion activity for methane

A series of (Ce_1 − xY_x)O₂ (x = 0,0.15,0.35,0.5) coatings on γ-Al₂O₃ pre-coated cordierite honeycomb were prepared by sol–gel method, and then palladium was loaded by aqueous solution impregnation deposition with Pd(NO₃)₂ as precursor. The structure and morphology of samples were evaluated and the catalytic combustion activity for methane was also discussed. (Ce_1 − xY_x)O₂ synthesized by sol–gel has a single-phase cubic fluorite structure. Increasing the Y/Ce ratio can significantly improve the inner surface morphology of the honeycomb channels and also the coating mechanical stability, and leads to a considerable improvement in the catalytic activity of the prepared catalysts for methane.     

Sunflower oil hydrogenation on Pd in supercritical solvents: Kinetics and selectivities

The partial hydrogenation of sunflower oil on a few supported Pd catalysts in supercritical (SC) dimethyl ether (DME) as reaction solvent was studied to obtain hydrogenates with low trans C 18:1 and stearic contents.The kinetics was determined on eggshell 0.5% Pd/Al₂O₃ and uniform 2% Pd/C catalysts using a sequential experimental design in a continuous, radial-flow, internal recycle reactor. The operating variables were temperature (456–513 K), pressure (18–23 MPa) and the space-velocity (WHSV = 41–975 h⁻¹). The rotation frequency and the molar feed concentration (oil:H₂:DME) were held constant at 157 rad/s and 1:4:95 mol%, respectively. Kinetic scheme was based on that published before. Some reactor runs were simulated using mixed-flow assumption and the kinetics data for both systems with good results. A comparison was established between the eggshell 0.5% Pd/Al₂O₃ in DME and the data for 2% Pd/C in propane with respect to trans production and stearic formation. trans seems to be lower using 2% Pd/C in propane, while the undesired stearic formation is less on the eggshell 0.5% Pd/Al₂O₃ catalyst in DME. An overview is presented on the merits of the catalysts available for the SCF process in terms of linoleic selectivity and trans yield on a few vegetable fats.     

Simultaneous catalytic removal of NOx and soot particulate over Co–Al mixed oxide catalysts derived from hydrotalcites

Co–Al mixed oxides (CAO) was prepared by co-precipitation method from hydrotalcites (HT) as precursors, and their catalytic activity was investigated for the simultaneously catalytic removal of NO_x and diesel soot particulates by the temperature-programmed reaction (TPR) technique. All HT samples present well crystallized, layered structures, no excess phases were detected. A nonstoichiometric spinel phase was formed by calcining the CAO at 500 °C and 800 °C, irrespective of the Co/Al ratio. Both the activity of soot oxidation and the selectivity to N₂ formation of CAO catalysts calcined at 800 °C were higher than that at 500 °C. The observed difference in the catalytic performance was related to the redox properties of the catalysts and the crystallite size of HT precursors. The active species might come from Co₃O₄, which acted for redox-type mechanism for soot oxidation in the NO_x-soot reaction.     

Effective Pd/C catalyst for chlorobenzene and hexachlorobenzene hydrodechlorination by direct pyrolysis of sawdust impregnated with palladium nitrate

Two Pd/C catalysts were prepared by pyrolysis of Pd(NO₃)₂ impregnated sawdust. At equal pyrolysis time slow ramping with shorter isothermal heating resulted in 0.9 wt.% Pd/C-S1 sample comprising carbon support with some oxygen-containing moieties and Pd⁰ with 2.6 nm average particle size (APS) partially decorated with carbon shell, whereas fast temperature ramping and long isothermal heating provided 0.6 wt.% Pd/C-S2 containing Pd⁰ with 3.7 nm APS, with larger fraction of carbon decorated particles. Pd/C-S1 is slightly more efficient than Pd/C-S2 in gas phase chlorobenzene hydrodechlorination to benzene at 100–250 °C. Only Pd/C-S1 provides hexachlorobenzene hydrodechlorination in liquid phase due to lower APS and probably smaller PdC_x content.     

Enhancement of the catalytic oxidation of hydrogen-lean chlorinated VOCs in the presence of hydrogen-supplying compounds

The complete catalytic oxidation of trichloroethylene (TCE) over alumina-supported noble metal catalysts (Pt and Pd) and in the presence of hydrogen-rich compounds, i.e. water, hexane and toluene was evaluated. Experiments were performed at conditions of lean TCE concentration (around 1000 ppm) in air, between 250 and 550°C in a conventional fixed-bed reactor. Hexane and toluene were added to the feedstream in a concentration of around 1000 ppm and water concentration varied from 1000 to 15 000 ppm. TCE oxidation occurred faster in the presence of hexane and toluene over both catalysts. Over palladium catalysts, water did not alter catalytic activity, whereas over platinum catalysts water enhanced TCE oxidation at low temperatures (<400°C) but inhibited it at higher temperatures (>400°C). Selectivity to HCl was much improved by feeding water as a hydrogen-supplying reactant; 7500 ppm of water enhanced HCl outputs from 39.4 to 78.0% with Pd, and from 37.5 to 58.9% with Pt. Selectivities to C₂Cl₄, formed by chlorination of the feed, and Cl₂ were greatly reduced. On the other hand water promoted complete oxidation of TCE to CO₂, and thus reduced selectivity to CO. In the presence of hexane and toluene, formation of HCl was also enhanced. Hexane showed higher inhibition ability than toluene over both catalysts for the C₂Cl₄ and Cl₂ formation. Unlike in the presence of water, selectivity to CO increased, as a consequence of partial oxidation of both hydrocarbons.     

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.