Effect of Cu doping on the SCR activity of CeO2 catalyst prepared by citric acid method

CeO₂–CuO catalyst prepared by citric acid method was investigated for selective catalytic reduction of NO with NH₃. The activity of the CeO₂ catalyst was enhanced about 8–27% in the temperature range of 125–225 °C at a space velocity of 28,000 h⁻¹ by the addition of Cu. It was found that the state of Cu species had great impact on the SCR performance of CeO₂–CuO catalyst. Cu²⁺ can enhance the low temperature activity of SCR reaction, while CuO would promote NH₃ oxidation before SCR reaction at high temperature, which would cause the decrease of its high temperature SCR activity.     

Dispersion and valence state of MnO2/Ce(1 − x)ZrxO2–TiO2 for low temperature NH3-SCR

Zr-doped MnO_x/CeO₂–TiO₂ for high temperature stability was investigated in terms of its dispersion and oxidation state. Aggregation of cerium oxide was observed in MnO_x/CeO₂–TiO₂, but the Zr-doped catalyst was well dispersed. An increase of the Mn^4 +/Mn^3 + ratio was confirmed with a Zr addition through valence state analysis. De-NOx efficiency of the catalyst was increased to 40–45% by Zr addition at low temperature (150–200 °C). The substitution of Zr led the catalyst to improve the de-NOx efficiency, with a high dispersion and MnO₂ ratio.     

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.     

A study of the temperature effect on Hantzsch reaction selectivity using Mn and Ce oxides under solvent-free conditions

In this communication, four materials (CeO₂, CeO₂(Cu), MnO_x, and MnO_x(Cu)) were prepared, characterized and tested as catalysts, in solvent-free conditions, for the multicomponent Hantzsch reactions to obtain alternatively the 1,4-dihydropyridine or 2-phenylpyridine depending on the reaction conditions. 1,4-Dihydropyridine 4 was the main product formed at 80 °C (76%), and 2-phenylpyridine 7 was the main product at 40 °C (91%), in oxidant-free conditions, using CeO₂ catalyst. It is the first report that shows that, not only the temperature but also the nature of the catalyst may change the product selectivity in Hantzsch reactions.     

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.     

Preparation of promoted nickel catalysts supported on mesoporous nanocrystalline gamma alumina for carbon dioxide methanation reaction

Nickel-based catalysts supported on mesoporous nanocrystalline gamma Al₂O₃ promoted with various promoters (CeO₂, MnO₂, ZrO₂, La₂O₃) were prepared and employed in the carbon dioxide methanation reaction. It was found that the addition of promoters to the catalyst varied the specific surface area from 139.4 to 147.4 m²/g and CeO₂ and MnO₂ improved the reducibility of catalyst. The catalytic results showed that the catalyst with 2 wt% of cerium promoter possessed high activity and stability in CO₂ methanation reaction and showed a high CO₂ conversion of 80.3% at 350 °C.     

Effects of WO3 doping on stability and N2O escape of MnOx–CeO2 mixed oxides as a low-temperature SCR catalyst

MnO_x–WO_x–CeO₂ catalysts synthesized using a sol–gel method were investigated for the low-temperature NH₃-SCR reaction. Among them, W_0.1Mn_0.4Ce_0.5 mixed oxides exhibited above 80% NO_x conversion from 140 to 300 °C. In addition, this catalyst exhibited high stability and CO₂ tolerance in a 50 h activity test at 150 °C. Substantially reduced N₂O production and enhanced N₂ selectivity were achieved by WO₃ doping, which was due to the weakened reducibility and increased number of acid sites. The decreased SO₂ oxidation activity as well as the reduced formation of ammonium and manganese sulfates resulted in a high SO₂ resistance of this catalyst.     

One-step synthesis of ternary MnO2–Fe2O3–CeO2–Ce2O3/CNT catalysts for use in low-temperature NO reduction with NH3

In this article, a facile one-step strategy for the synthesis of ternary MnO₂–Fe₂O₃–CeO₂–Ce₂O₃/carbon nanotubes (CNT) catalysts was discussed. The as-prepared catalysts exhibited 73.6–99.4% NO conversion at 120–180 °C at a weight hourly space velocity (WHSV) of 210 000 ml·g_cat^− 1·h^− 1, which benefited from the formation of amorphous MnO₂, Fe₂O₃, CeO₂, and Ce₂O₃, as well as high Ce^3 + and surface oxygen (O_ε) contents. The mechanism of formation of MnO₂–Fe₂O₃–CeO₂–Ce₂O₃/CNT catalysts was also proposed.     

Steam reforming of methanol over copper–manganese spinel oxide catalysts

The steam reforming of methanol was studied over a series of copper–manganese spinel oxide catalysts prepared with the urea–nitrate combustion method. All catalysts showed high activity towards H₂ production with high selectivity. Synthesis parameters affected catalyst properties and, among the catalysts tested, the one prepared with 75% excess of urea and an atomic ratio Cu/(Cu + Mn) = 0.30 showed the highest activity. The results show that formation of the spinel Cu_xMn_3 − xO₄ phase in the oxidized catalysts is responsible for the high activity. Cu–Mn catalysts were found to be superior to CuO–CeO₂ catalysts prepared with the same technique.     

Hydrogen production from ethanol steam reforming over Rh/CeO2 catalyst

Hydrogen production from ethanol steam reforming over an Rh/CeO₂ catalyst was investigated with a stoichiometric feed composition. Ethanol was entirely converted to hydrogen and C₁ products (CO, CO₂, CH₄) at 400 °C due to the remarkable C–C bond cleavage capacity of Rh species. The Rh/CeO₂ catalyst exhibited stable activity and selectivity without the obvious deactivation during 70 h on stream test. Structural analysis of the aged catalysts indicated that the strong interaction between Rh and ceria support efficiently inhibited Rh particles sintering (stable at around 2 nm) and coke formation to guarantee catalyst stability.     

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.     

Experimental evidence and mechanism of the oxygen storage capacity in MnO2-Ce(1−x)ZrxO2/TiO2 catalyst for low-temperature SCR

The oxygen storage capacity of amorphous CeO₂ and its mechanism were investigated in a Zr-doped MnO_x-CeO₂/TiO₂ catalyst at low temperatures. The oxygen storage capacity of several catalysts was determined by the release of lattice oxygen upon reduction of Ce⁴⁺ to Ce³⁺. We designed a temperature programmed reduction analysis using ammonia gas to measure the amount of lattice oxygen release and identify a decrease in reaction-onset temperature from 136 °C to 75 °C upon doping the catalyst with Zr. Additional reduction was observed in Zr-doped MnO_x-CeO₂/TiO₂ and this was attributed to an increase in temperature sensitivity of thermal vibrations of the first Ce–O coordination shell. The temperature dependence of the thermal vibrations was identified by examining the behavior of the Debye–Waller factor as a function of temperature with fitting the extended X-ray absorption fine structure.     

Selective oxidation of p-chlorotoluene with Co(OAc)2/MnSO4/KBr in acetic acid–water medium

Selective oxidation of p-chlorotoluene catalyzed by Co(OAc)₂/MnSO₄/KBr with molecular oxygen has been studied. Acetic acid–water is used as the reaction medium in place of pure acetic acid to avoid the nuisance of acetic acid separation. It is found that when MnSO₄ is used in place of the commonly used Mn(OAc)₂, MnO₂ barely forms and the activity of the composite catalyst greatly enhances. Under the optimized conditions (Co/(Co + Mn) mole ratio 0.2, Br/(Co + Mn) mole ratio 0.3, and reaction temperature 106 °C), 22.4% yield of p-chlorobenzaldehyde was obtained at 33.7% conversion of p-chlorotoluene with 66.6% selectivity.     

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.     

Thermally stable ceria–zirconia catalysts for soot oxidation by O2

Ce–Zr mixed oxides calcined at 1000 °C are more active catalysts for soot oxidation than pure CeO₂ calcined at the same temperature, both in loose and tight contact between soot and catalyst. 1000 °C sinterised-CeO₂ presents a very low surface area (2 m²/g), a large crystal size (110 nm) and a lack of surface redox properties. Ce–Zr mixed oxides present higher BET surface areas (typically 17–19 m²/g), smaller crystal sizes and enhanced redox properties. The Zr molar fraction does not affect appreciably the catalytic activity of Ce–Zr mixed oxides in the range studied (Zr molar fraction from 0.11 to 0.51).     

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.     

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.     

Screening of doped MnOx–CeO2 catalysts for low-temperature NO-SCR

The effect of different dopants including niobium, iron, tungsten and zirconium oxide on the low-temperature activity of MnO_x–CeO₂ catalysts for the selective catalytic reduction (SCR) of NO_x with ammonia has been studied with coated cordierite monoliths in model gas experiments. A clearly higher activity and particularly superior nitrogen selectivity was obtained with the niobium-doped catalyst in comparison with the MnO_x–CeO₂ reference system. At 200 °C, the DeNO_x was 80% while the N₂ selectivity reached more than 96%. In contrast, a decrease of the SCR activity was observed when iron, zirconium or tungsten oxides were added to MnO_x–CeO₂. However, the addition of niobium oxide did not improve the resistance of the catalyst against SO₂ poisoning. A strong and irreversible deactivation occurred after exposure to SO₂.     

Honeycomb supported Co3O4/CeO2 catalyst for CO/CH4 emissions abatement: Effect of low Pd–Pt content on the catalytic activity

A structured Co₃O₄–CeO₂ composite oxide, containing 30% by weight of Co₃O_4, has been prepared over a cordieritic honeycomb support. The bimetallic, Pd–Pt catalyst has been obtained by impregnation of the supported Co₃O₄–CeO₂ with Pd and Pt precursors in order to obtain a total metal loading of 50 g/ft³.CO, CH₄ combined oxidation tests were performed over the catalyzed monoliths in realistic conditions, namely GHSV = 100,000 h⁻¹ and reaction feed close to emission from bi-fuel vehicles. The Pd–Pt un-promoted Co₃O₄–CeO₂ is promising for cold-start application, showing massive CO conversion below 100 °C, in lean condition.A strong enhancement of the CH₄ oxidation activity, between 400 and 600 °C, has been observed by addition to the Co₃O₄–CeO₂ of a low amount of Pd–Pt metals.