Preparation,characterization and catalytic activity studies of CeO2–K diesel soot oxidation catalysts loaded on porous Al2O3 substrate using water-immiscible solvent

CeO₂–K catalysts supported on porous alumina substrate have been prepared by using a novel water-immiscible solvent. The advantage of this method is to load the catalyst onto the filter surface by one-time coating and prevent depositing the catalyst into the porous structure of support materials. The catalytic activities of the supported catalysts were evaluated by TPR system and the results showed that the pure CeO₂ displayed a poor catalytic activity for soot oxidation, while the addition of K element into CeO₂ would result in the formation of CeO₂–K solid solution and significant enhancement of catalytic activity. Nevertheless, the variation of K content had a limited effect on soot ignition temperature. The catalyst with a Ce:K molar ratio of 1:2 exhibited an ignition temperature of about 330 °C and the oxidation rates of about 0.16 and 0.28 mg min⁻¹ cm⁻² at temperatures of 370 and 390 °C, respectively.     

Methane decomposition over ceria modified iron catalysts

The catalytic behaviour of ceria supported iron catalysts (Fe–CeO₂) was investigated for methane decomposition. The Fe–CeO₂ catalysts were found to be more active than catalysts based on iron alone. A catalyst composed of 60 wt.% Fe₂O₃ and 40 wt.% CeO₂ gave optimal catalytic activity, and the highest iron metal surface area. The well-dispersed Fe state helped to maintain the active surface area for the reaction. Methane conversion increased when the reaction temperature was increased from 600 to 650 °C. Continuous formation of trace amounts of carbon monoxide was observed during the reaction due to the oxidation of carbonaceous species by high mobility lattice oxygen in the solid solution formed within the catalyst. This could minimise catalyst deactivation caused by carbon deposits and maintain catalyst activity over a longer period of time. The catalyst also produced filamentous carbon that helped to extend the catalyst life.     

Development of cobalt catalysts for the steam reforming of naphthalene as a model compound of tar derived from biomass gasification

The catalytic performances of Co/MgO catalysts for the steam reforming of naphthalene were investigated. The results of characterizations (TPR, XRD, CO adsorption, and CO-TPD) showed that large-sized Co metal particles were formed over the catalysts pre-calcined at 873 K with high Co loading via reduction of Co₃O₄ and MgCo₂O₄ phases. A few Co metal particles were obtained over the catalysts pre-calcined at 1173 K with all Co loading values after reduction.The catalytic performances data showed that 12 wt.% Co/MgO catalyst pre-calcined at 873 K exhibited the best catalytic performance (conv., 23%, 3 h) for the steam reforming of naphthalene among the catalysts tested in this study, due to the existence of Co metal and the low amounts of coke deposition. On the other hand, the data also revealed that the reaction of steam reforming of naphthalene proceeds over all Co-loaded catalyst pre-calcined at 1173 K initially; however, the deposition of the polymer of C_nH_m radicals and the oxidation of catalysts by H₂O led to the decrease of activity.It should be noted that 12 wt.% Co/MgO catalyst pre-calcined at 873 K showed high and stable activity under the low steam/carbon mole ratio (0.6), with H₂ and CO₂ as main products. These two excellent advantages serve to increase the overall biomass gasification system energy efficiency and allow using the product gas for fuel cell system. Thus, Co catalyst is a promising system for the steam reforming of naphthalene derived from biomass gasification as a second fixed catalytic bed.     

Highly efficient Ru/Sm2O3-CeO2 catalyst for ammonia synthesis

A series of Ru/Sm₂O₃–CeO₂ catalysts were prepared by using a co-precipitation (CP) method and characterized by XRD, BET, SEM, H₂-TPD-MS, H₂-TPR and CO chemisorption. The activity test shows that ammonia concentration of the catalyst with 7% Sm is 13.4% at 10 MPa, 10,000 h^− 1, 425 °C, which is 21% higher than that of Ru/CeO₂. Such high catalytic activity was due to three effects: the morphology changes of catalyst, electrodonating property of partially reduced CeO_2 − x to Ru metal and the property of easily hydrogen desorption derived from the presence of Sm³⁺ in ceria.     

Pt-CeO2/C anode catalyst for direct methanol fuel cells

In order to develop a cheaper and durable catalyst for methanol electrooxidation reaction, ceria (CeO₂) as a co-catalytic material with Pt on carbon was investigated with an aim of replacing Ru in PtRu/C which is considered as prominent anode catalyst till date. A series of Pt-CeO₂/C catalysts with various compositions of ceria, viz. 40 wt% Pt-3–12 wt% CeO₂/C and PtRu/C were synthesized by wet impregnation method. Electrocatalytic activities of these catalysts for methanol oxidation were examined by cyclic voltammetry and chronoamperometry techniques and it is found that 40 wt% Pt-9 wt% CeO₂/C catalyst exhibited a better activity and stability than did the unmodified Pt/C catalyst. Hence, we explore the possibility of employing Pt-CeO₂ as an electrocatalyst for methanol oxidation. The physicochemical characterizations of the catalysts were carried out by using Brunauer Emmett Teller (BET) surface area and pore size distribution (PSD) measurements, X-ray diffraction (XRD), X-ray photoelectron spectroscopy (XPS) and transmission electron microscopy (TEM) techniques. A tentative mechanism is proposed for a possible role of ceria as a co-catalyst in Pt/C system for methanol electrooxidation.     

Potassium-modified molybdenum-containing SBA-15 catalysts for highly efficient production of acetaldehyde and ethylene by the selective oxidation of ethane

Mo-incorporated SBA-15 (Mo-SBA-15) catalysts with different Mo:Si molar ratios were synthesized by a direct hydrothermal method, and SBA-15-supported Mo catalysts (Mo/SBA-15 and K-Mo/SBA-15) were prepared by an incipient-wetness impregnation method. The structures of the catalysts were characterized by means of N₂ adsorption–desorption, XRD, TEM, FT-IR, and UV-Raman, and their catalytic performance for the oxidation of ethane was tested. Turnover frequency and product selectivity are strongly dependent on the molybdenum content and catalyst preparation method. Furthermore, the addition of potassium promotes the formation of isolated tetra-coordination molybdenum species and potassium molybdate. The K/Mo-SBA-15 catalysts possess much higher catalytic selectivity to acetaldehyde in the selective oxidation of ethane than the supported molybdenum catalysts (Mo/SBA-15 or K-Mo/SBA-15). The highest selectivity of CH₃CHO + C₂H₄ 68.3% is obtained over the K/Mo-SBA-15 catalyst. Analysis of kinetic results supports the conclusion that ethane oxidation is the first-order reaction and ethane activation may be the rate-determining step for the oxidation of ethane. Ethylene is a possible intermediate for acetaldehyde formation.     

Temperature-induced evolution of reaction sites and mechanisms during preferential oxidation of CO

Active sites responsible for the preferential oxidation of carbon monoxide were investigated using 4 wt.% Cu–CeO₂ catalysts prepared by flame spray pyrolysis. Surface redox properties of the catalyst were assessed using a series of temperature-programmed reduction (CO, H₂ and mixed) experiments, as well as operando infrared spectroscopy. It was demonstrated that CO and H₂ react at identical surface sites, with CO₂ formation proceeding simultaneously via three distinct Cuⁿ⁺–CO carbonyl species. The origin of high catalytic selectivity towards CO at below 150 °C stems from the carbonyl stabilization effect on the catalyst surface, preventing adsorption and subsequent oxidation of H₂. Under non-selective conditions at higher temperatures, a gradual red-shift and loss of intensity in the carbonyl peak was observed, indicating reduction of Cu⁺ to Cu⁰, and the onset of an alternate redox-type oxidation mechanism where CO and H₂ compete for the oxidation sites. These results for Cu–CeO₂ suggest that improved low-temperature catalytic activity will only be achieved at the expense of reduced high-temperature selectivity and vice versa.     

Promotion effect of metal oxides on inverse CeO2/CuO catalysts for preferential oxidation of CO

The MO_x–CeO₂/CuO (M = Co, Mn, Sn and Zn) catalysts were synthesized by the hydrothermal method and characterized by XRD, BET, SEM, H₂-TPR and HRTEM techniques. It is found that the MnO₂–CeO₂/CuO catalyst exhibits the best activity from 75 °C to 115 °C, suggesting that the addition of Mn is the most effective for improving low-temperature activity. The reasons are that MnO₂ improves the dispersion of CeO₂ and the textural property of CeO₂/CuO catalyst. Moreover, the presence of MnO₂ is favorable for preventing the reduction of CuO, and MnO₂ also enhances the interaction between CeO₂ and CuO.     

A novel glass fiber catalyst for the catalytic combustion of ethyl acetate

Supported CuO catalysts were prepared by wet impregnation into novel glass fiber corrugated honeycomb supports, and the catalytic combustion of ethyl acetate and the effect of copper loading were examined. Among the catalysts tested, Cu10/Al₂O₃-M showed the highest activity. For the catalyst, 100% conversion of ethyl acetate was achieved at 300 °C, feed concentration of 1802 mg/m³ and the space velocity of 5000 h^− 1. To reveal these phenomena, the supports and catalysts were characterized by SEM, BET, XRD, H₂-TPR and ethyl acetate-TPD. The catalyst activity was strongly related to the amount of highly dispersed CuO species and suitable porosity.     

Selective catalytic oxidation of carbon monoxide with carbon dioxide,water vapor and excess hydrogen on CuO–CeO2 mixed oxide catalysts

A series of CuO–CeO₂ catalysts with Cu content in the range of 5–50 at.% (the atomic percent of Cu/(Cu + Ce)) were prepared by co-precipitation method. The catalysts were tested for CO oxidation and selective CO oxidation with CO₂, H₂O and excess H₂. The catalysts were highly active in the CO oxidation and also active and remarkably selective in the selective oxidation. CO₂ and H₂O, however, decreased the catalyst activity; between CO₂ and H₂O, H₂O decreased the activity more than CO₂. Among the catalysts, the 10 at.% Cu catalyst outperformed all the other catalysts in the CO oxidation without CO₂ and H₂O in the feed, whereas the 20 at.% Cu exhibited the highest activity in the selective CO oxidation. This change in the optimum Cu content could be ascribed to the adverse effect of H₂O on the activity that was dependent on the Cu content of the catalysts.     

Comparison of Co/MgO and Ni/MgO catalysts for the steam reforming of naphthalene as a model compound of tar derived from biomass gasification

The catalytic performances of 12 wt.% Co/MgO catalyst pre-calcined at 873 K and of Ni catalysts for the steam reforming of naphthalene were investigated. The results of characterizations (TPR, XRD, and CO adsorption) for Ni catalysts showed that Ni metal particles were formed over the catalysts pre-calcined at 873 K with high Ni loading via reduction of NiO–MgO phases. A few Ni metal particles were obtained over the catalysts pre-calcined at 1173 K with all Ni loading values.The catalytic performance data showed that Co/MgO catalyst had higher activity (conv., 23%, 3 h) than any kinds of Ni/MgO catalysts tested in this study, under lower steam/carbon mole ratio (0.6) and higher concentration of fed naphthalene (3.5 mol%) than those used in the other works. The steam reforming of naphthalene proceeded when there was a stoichiometric ratio between the carbon atoms of naphthalene and H₂O over Co catalyst; however, the activation of excess H₂O happened over the Ni catalyst and this phenomenon can lead to having lower activity than Co catalyst. We concluded that these observations should be attributed to different catalytic performances between Co/MgO and Ni/MgO catalysts.     

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.     

Hydrogen production by oxidative steam reforming of ethanol over an Ir/CeO2 catalyst

Oxidative steam reforming of ethanol over an Ir/CeO₂ catalyst was investigated. Ethanol dehydrogenation to acetaldehyde and decomposition to methane and carbon monoxide were the primary reactions at low temperatures, and complete reforming of ethanol was achieved at 773 K with hydrogen, carbon oxides and methane as the only products in the outlet stream. More importantly, stability test revealed that the Ir/CeO₂ catalyst could show rather stable catalytic performance for 60 h time-on-stream without deactivation. The improvement was attributed to the effective prevention of the sintering of the highly dispersed Ir particles through the strong interaction between Ir and CeO₂ and to the significant resistance to coke deposition due to the higher oxygen storage-release capacity of ceria.     

Hydrogen production via methanol steam reforming over Au/CuO,Au/CeO2, and Au/CuO–CeO2 catalysts prepared by deposition–precipitation

The production of hydrogen (H₂) with a low concentration of carbon monoxide (CO) via steam reforming of methanol (SRM) over Au/CuO, Au/CeO₂, (50:50)CuO–CeO₂, Au/(50:50)CuO–CeO₂, and commercial MegaMax 700 catalysts were investigated over reaction temperatures between 200 °C and 300 °C at atmospheric pressure. Au loading in the catalysts was maintained at 5 wt%. Supports were prepared by co-precipitation (CP) whilst all prepared catalysts were synthesized by deposition–precipitation (DP). The catalysts were characterized by Brunauer–Emmett–Teller (BET) surface area, X-ray diffraction (XRD), temperature-programmed reduction (TPR), and scanning electron microscopy (SEM). Au/(50:50)CuO–CeO₂ catalysts expressed a higher methanol conversion with negligible amount of CO than the others due to the integration of CuO particles into the CeO₂ lattice, as evidenced by XRD, and a interaction of Au and CuO species, as evidenced by TPR. A 50:50 Cu:Ce atomic ratio was optimal for Au supported on CuO–CeO₂ catalysts which can then promote SRM. Increasing the reaction time, by reducing the liquid feed rate from 3 to 1.5 cm³ h^?1, resulted in a catalytic activity with complete (100%) methanol conversion, and a H₂ and CO selectivity of ～82% and ～1.3%, respectively. From stability testing, Au/(50:50)CuO–CeO₂ catalysts were still active for 540 min use even though the CuO was reduced to metallic Cu, as evidenced by XRD. Therefore, it can be concluded that metallic Cu is one of active components of the catalysts for SRM.     

Catalytic performance of Ag–Co/CeO2 catalyst in NO–CO and NO–CO–O2 system

A new bimetallic catalyst (Ag–Co/CeO₂) was studied for simultaneously catalytic removal of NO and CO in the absence or presence of O₂. CeO₂ prepared by homogeneous precipitation method was optimized as supports for the active components. The addition of Ag on CeO₂ greatly improved the catalytic activities in the lower temperature regions (⩽300 °C), and the introduction of Co on CeO₂ increased the activities at higher temperatures (⩾250 °C). The bimetallic Ag–Co/CeO₂ catalyst combined the advantages of the corresponding individual metal supported catalysts and showed superior activity due to the synergetic effect. The effect of support, temperature, loading amount, GHSV and oxygen on catalysis was investigated. NO and CO could be completely removed in the temperature range of 200–600 °C at a very high space velocity of 120 000 h⁻¹. No deactivation was observed over 4% Ag–0.4% Co/CeO₂ catalyst even after 50 h test.     

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.     

Impact of metal doping on the activity of Au/CeO2 catalysts for catalytic abatement of VOCs and CO in waste gases

The catalytic performance in the total oxidation of CO and methanol over gold catalysts supported on ceria doped by different metal oxides (Me = Fe, Mn and Co) was studied and a strong influence of the nature of dopant was observed. The activity towards the oxidation of CO and CH₃OH was in the order: AuCeCo > AuCe > AuCeFe > AuCeMn. The characterization by XRD and HRTEM evidenced differences in the average size and the distribution of gold particles. AuCeCo catalyst exhibited superior low-temperature CO oxidation activity (100% conversion degree was obtained at 25 °C) and almost 100% total oxidation of CH₃OH at about 40 °C. Higher hydrogen consumption was estimated by means of TPR over this catalyst. The effect of modification with Co₃O₄ of Au/CeO₂ catalysts on their CO oxidation activity was further studied by varying of the dopant content (5, 10 and 15 wt.% Co₃O₄).     

Compact hollow fibre reactors for efficient methane conversion

In this study, a micro-structured catalytic hollow fiber membrane reactor (CHFMR) has been prepared, characterized and evaluated for performing steam methane reforming (SMR) reaction, using Rh/CeO₂ as the catalyst and a palladium membrane for separating hydrogen from the reaction. Preliminary studies on a catalytic hollow fiber (CHF), a porous membrane reactor configuration without the palladium membrane, revealed that stable methane conversions reaching equilibrium values can be achieved, using approximately 36 mg of 2 wt.%Rh/CeO₂ catalyst incorporated inside the micro-channels of alumina hollow fibre substrates (around 7 cm long in the reaction zone). This proves the advantages of efficiently utilizing catalysts in such a way, such as significantly reduced external mass transfer resistance when compared with conventional packed bed reactors. It is interesting to observe catalyst deactivation in CHF when the quantity of catalyst incorporated is less than 36 mg, although the Rh/CeO₂ catalyst supposes to be quite resistant against carbon formation. The “shift” phenomenon expected in CHFMR was not observed by using 100 mg of 2 wt.%Rh/CeO₂ catalyst, mainly due to the less desired catalyst packing at the presence of the dense Pd separating layer. Problems of this type were solved by using 100 mg of 4 wt.% Rh/CeO₂ as the catalyst in CHFMR, resulting in methane conversion surpassing the equilibrium conversions and no detectable deactivation of the catalyst. As a result, the improved methodology of incorporating catalyst into the micro-channels of CHFMR is the key to a more efficient membrane reactor design of this type, for both the SMR in this study and the other catalytic reforming reactions.     

Influence of SBA-15 support on CeO2–ZrO2 catalyst for the dehydrogenation of ethylbenzene to styrene with CO2

Pure oxides of ceria (CeO₂) and zirconia (ZrO₂) were prepared by precipitation method and a catalyst comprising of 25 mol% of CeO₂ and 75 mol% of ZrO₂ (25CZ) mixed metal oxide was prepared by co-precipitation method and also a catalyst with 25 wt% of 25CZ (25 mol% of CeO₂ and 75 mol% of ZrO₂) and 75 wt% SBA-15(25/25CZS) was prepared by precipitation–deposition method. Aqueous NH₃ solution was used as a hydrolyzing agent for all the precipitation reactions. These catalysts were characterized by X-ray diffraction and nitrogen adsorption–desorption techniques for the confirmation of SBA-15 structural intactness. All these catalysts were found to be effective for the oxidative dehydrogenation of ethylbenzene (ODHEB) to styrene in the presence of CO₂ and also it was observed that there was a sequential enhancement in the catalytic activity from individual oxides to mixed oxides followed by supported mixed oxide catalysts. Of the catalysts studied in this work, the supported 25/25CZS catalyst exhibited the superior activity, which was about 10–20 times higher than the activity of bulk single oxides in terms of turn over frequency.     

A Ce–Sn–Ox catalyst for the selective catalytic reduction of NOx with NH3

A series of Ce–Sn–O_x catalysts prepared by the facile coprecipitation method exhibited good catalytic activity in a broad temperature range from 100 °C to 400 °C for the selective catalytic reduction of NO_x with NH₃ at the space velocity of 20,000 h^− 1. The Ce₄Sn₄O_x catalyst calcined at 400 °C showed high resistance to H₂O, SO₂, K₂O and PbO under our test conditions. The better catalytic performance was associated with the synergistic effect between CeO₂ and SnO₂, which strengthened the NH₃ and NO_x adsorption capacity on the surface of the catalyst.