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.     

Effects of the structure of CeCu catalysts on the catalytic combustion of toluene in air

CeCu composite oxide catalysts were prepared by a hard-template method (CeCu-HT) and a complex method (CeCu-CA). The prepared CeCu composite oxide catalysts were characterized by X-ray diffraction (XRD), transmission electron microscopy (TEM), and Brunauer–Emmett–Teller (BET) analyses. The catalytic properties of the prepared CeCu composite oxide catalysts were also investigated by the catalytic combustion of toluene in air. XRD results showed that the synthesized CeCu composite oxide catalysts had different phase components and crystallinities but similar CeO₂CuO solid solution phases. Low-angle XRD, TEM, and BET results indicated that the prepared CeCu-HT catalyst had a developed ordered mesoporous structure and a large specific surface area of 206.1 m² g^?1. Toluene catalytic combustion results indicated that the CeCu-HT catalyst had higher toluene catalytic combustion activity in air than the CeCu-CA catalyst. The minimum reaction temperature at which toluene conversion exceeded 90% for toluene catalytic combustion on the CeCu-HT catalyst was 225 °C. The toluene catalytic combustion conversion on the CeCu-HT catalyst at 240 °C exceeded 99.3% with decreased toluene concentration in air to below 70 ppm. On the other hand, the toluene catalytic combustion conversion on the CeCu-CA catalyst was only 92% even when the reaction temperature reached 280 °C. The differences between the toluene catalytic combustion performances of the CeCu composite oxide catalysts prepared by different methods can be attributed to their discrepant compositions and structures.     

Hydroxyls-induced oxygen activation on “inert” Au nanoparticles for low-temperature CO oxidation

The NaOH additive substantially enhances the catalytic activity of Au/SiO₂ catalyst inert in catalyzing CO oxidation at temperatures below 150 °C, and Au/NaOH/SiO₂ catalyst with a NaOH:Au atomic ratio of 6 is active at room temperature. Both the particle size distribution and the electronic structure of Au nanoparticles were found to be similar in Au/SiO₂ and Au/NaOH/SiO₂ catalysts, unambiguously proving that hydroxyls on “inert” Au nanoparticles can induce the activation of O₂ for CO oxidation at room temperature. The accompanying density functional theory (DFT) calculation results reveal the determining role of COOH(a) in hydroxyls-induced activation of O₂ on the Au(1 1 1) surface. Our results successfully elucidate the influence of hydroxyls on the intrinsic activity of Au nanoparticles in CO oxidation, providing novel insights into the role of hydroxyls in the catalytic activity of Au catalysts and advancing the fundamental understanding of oxidation reactions catalyzed by Au catalysts.     

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).     

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).     

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.     

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.     

Hydrogenation of carbon monoxide to methane over mesoporous nickel-M-alumina (M = Fe,Ni, Co,Ce, and La) xerogel catalysts

Mesoporous nickel(30 wt%)-M(10 wt%)-alumina xerogel (30Ni10MAX) catalysts with different second metal (M = Fe, Ni, Co, Ce, and La) were prepared by a single-step sol–gel method for use in the methane production from carbon monoxide and hydrogen. In the methanation reaction, yield for CH₄ decreased in the order of 30Ni10FeAX > 30Ni10NiAX > 30Ni10CoAX > 30Ni10CeAX > 30Ni10LaAX. Experimental results revealed that CO dissociation energy of the catalyst and H₂ adsorption ability of the catalyst played a key role in determining the catalytic performance of 30Ni10MAX catalyst in the methanation reaction. Optimal CO dissociation energy of the catalyst and large H₂ adsorption ability of the catalyst were favorable for methane production. Among the catalysts tested, 30Ni10FeAX catalyst with the most optimal CO dissociation energy and the largest H₂ adsorption ability exhibited the best catalytic performance in terms of conversion of CO and yield for CH₄ in the methanation reaction. The enhanced catalytic performance of 30Ni10FeAX was also due to a formation of nickel–iron alloy and a facile reduction.     

CO removal from reformed fuel over Cu/ZnO/Al2O3 catalysts prepared by impregnation and coprecipitation methods

A composition of Cu/ZnO/Al₂O₃ catalysts prepared by the impregnation method was optimized for water gas shift reaction (WGSR) coupled with CO oxidation in the reformed gas. The optimum composition of the impregnated catalyst for high WGSR activity was 5 wt.% Cu/5 wt.% ZnO/Al₂O₃. The optimum loading amounts of Cu and ZnO in the impregnated catalyst were smaller than those in the coprecipitated catalyst. Its catalytic activity above 200 °C was comparable to that of the conventional coprecipitated Cu/ZnO/Al₂O₃ catalyst. However, the activity of the impregnated Cu/ZnO/Al₂O₃ catalysts was significantly lowered at 150 °C, whereas no deactivation was observed for the coprecipitated catalyst at the same temperature. It was found that deactivation occurred over impregnated catalysts with H₂O and/or O₂ in the reaction gas; it prevented CO adsorption on the surface.     

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.     

CO-NO/O2 redox reactions over Cu substituted cobalt oxide spinels

Catalytic conversion of NO and CO over Cu substituted cobalt oxide spinels show excellent activity for CO-O₂ and NO-CO reactions. Lower concentration of Cu in cobalt oxide spinel is having an enhancing effect on the catalytic conversion. Best activity among the tested catalyst was found for Co_2.9Cu_0.1O₄ and complete conversion (100%) is observed at 93 °C for CO oxidation by O₂ and 209 °C for NO reduction by CO. Prepared catalysts show promising activity compared to few of the precious metal based catalysts reported in the literature. The influence of moisture and oxygen on catalytic conversion has been studied.     

Palladium nanoparticles supported on amino functionalized metal-organic frameworks as highly active catalysts for the Suzuki–Miyaura cross-coupling reaction

Well dispersed Pd nanoparticles supported on amino functionalized metal-organic frameworks MIL-53(Al)-NH₂ (Al(OH)[H₂N-BDC], H₂N-BDC = 2-aminoterephthalic acid, MIL = Materials of Institut Lavoisier) were prepared using a direct anionic exchange approach and subsequent reduction with NaBH₄. The Pd/MIL-53(Al)-NH₂ catalyst exhibitted high activity and good stability for Suzuki–Miyaura cross-coupling reaction.     

Preparation and characterization of CeO2–TiO2 support for Ru catalysts: Application in CWAO of p-hydroxybenzoic acid

The catalytic wet air oxidation of aqueous solutions of p-hydroxybenzoic acid has been carried out over CeO₂–TiO₂ supported ruthenium catalysts (Ru/Ce–Ti) at 140 °C and 50 bar of air. High activity of ruthenium supported catalysts was observed. It was found that the decrease of the molar ratio Ce/Ti from 3 to 1/3, improves the activity of Ru catalysts. The activity of the samples decreases in the following order: Ru/Ce–Ti (1/3) > Ru/CeO₂ ≈ Ru/TiO₂ > Ru/TiO₂DT51. Characterization of samples was performed by means of N₂ adsorption–desorption, XRD, UV–visible, TPR, SEM and TEM.     

Influence of CeO2 morphology on the catalytic oxidation of ethanol in air

Nano-CeO₂ catalysts of different shapes were synthesized at different hydrothermal crystallization temperatures from an alkaline aqueous solution. X-ray diffraction (XRD), transmission electron microscope (TEM), and H₂ temperature-programmed reduction (H₂-TPR) were used to study the synthesized nano-CeO₂ catalyst samples. The catalytic properties of the prepared nano-CeO₂ catalysts for the catalytic oxidation of ethanol in air were also investigated. TEM analysis showed that CeO₂ nanorod and nanocube catalysts have been synthesized at hydrothermal crystallization temperatures of 373 K and 453 K, respectively. XRD results showed that the synthesized nano-CeO₂ catalysts have similar cubic fluorite structures. H₂-TPR results indicated that CeO₂ nanorod and nanocube catalysts exhibit different reduction behaviors for H₂ and that the nanorod catalyst has better low-temperature reduction performance than the nanocube catalyst. Ethanol catalytic oxidation results indicated that oxidation and condensation products (including acetaldehyde, acetic acid, CO₂, and ethyl acetate) have been produced from the prepared catalysts. The ethyl acetate and acetic acid can be ignited by ethanol at low temperature on the CeO₂(R) catalyst to give low catalytic combustion temperature for ethyl acetate and acetic acid molecules. CeO₂ nanorods gave ethanol oxidation conversion rates above 99.2% at 443 K and CO₂ selectivity exceeding 99.6% at 483 K, while CeO₂ nanocubes gave ethanol oxidation conversion rates of about 95.1% until 508 K and CO₂ selectivity of only 93.86% at 543 K. CeO₂ nanorod is a potential low-cost and effective catalyst for removing trace amounts of ethanol to purify air.     

Mesoporous CuO/TixZr1 − xO2 catalysts for low-temperature CO oxidation

Mesoporous CuO/Ti_xZr_1 − xO₂ catalysts were prepared by a surfactant-assisted method, and characterized by N₂ adsorption/desorption, TEM, XPS, in-situ FTIR and H₂-TPR. The catalysts exhibited high specific surface area (S_BET = 241 m²/g) and uniform pore size distribution. XPS and in-situ FTIR displayed that Cu⁺ and Cu²⁺ species coexisted in the catalysts. The CuO/Ti_xZr_1 − xO₂ catalysts presented obviously higher activity in CO oxidation reaction than the CuO/TiO₂ and CuO/ZrO₂ catalysts. Effect of molar ratios of Ti to Zr and calcination temperature on catalytic activity was investigated. The CuO/Ti_0.6Zr_0.4O₂ catalyst calcined at 400 °C exhibited excellent activity with 100% CO conversion at 140 °C.     

Promotional effects of Ce on the activity of MnAl oxide catalysts derived from hydrotalcites for low temperature benzene oxidation

Ce/MnAl and MnAl mixed metal oxides catalysts have been obtained by calcination of layered double hydroxides precursors. The composite oxides catalysts were studied in total oxidation of benzene. Physicochemical properties of all the catalysts were characterized by using a series of specific analytical techniques. The results revealed that the 0.2Ce/MnAl catalyst exhibited the highest catalytic performance with T₉₀ about 210 °C at a high space velocity (SV = 60,000 mL g^− 1 h^− 1), ascribed to its lower-temperature reducibility, the abundant surface lattice oxygen (O_latt), and synergetic effect between Mn^4 + and Ce^3 +/Ce.     

A new preparation method of Au/ferric oxide catalyst for low temperature CO oxidation

A new preparation method of Au/α-Fe₂O₃ catalyst for CO oxidation reaction was proposed in this paper. The method includes only a simple modification of the conventional coprecipitation method, adding HAuCl₄ solution after the growth of iron hydroxide grain to a certain size, but significantly influenced the catalytic activity in the reaction. In the characterization study, XRD (X-ray diffractometer) analysis, TEM (transmission electron microscope) observation, and N₂ adsorption measurement showed similar results among the samples calcined at the same temperature, but the effect of the preparation method appeared in the CO adsorption measurement among the samples calcined at 200 °C. Catalysts having high CO adsorption ability also performed well in CO oxidation tests. The CO adsorption and oxidation studies indicated that the proposed preparation method results in stable and effective loading of Au, compared to the conventional coprecipitation method. In the CO oxidation test, the catalyst prepared by the proposed mixing scheme achieved complete CO conversion for more than 3000 h at 25 °C, space velocity 100,000 h^?1, and 500 ppm CO. The selectivity for the CO oxidation was confirmed using reformed gas containing excess H₂. In addition, the NO reduction reaction was favored over CO oxidation by the catalyst. Thus, we were able to load Au on the α-Fe₂O₃ effectively and demonstrate its potential as an environmental catalyst.     

The catalytic activity for CO oxidation of CuO supported on Ce0.8Zr0.2O2 prepared via citrate method

CuO/Ce_0.8Zr_0.2O₂ catalysts were prepared by citrate method and used for carbon monoxide oxidation. The samples were characterized by XRD, XPS, BET and ICP-AES techniques. The catalytic properties of the catalysts were studied by using a microreactor-GC system. XRD analysis showed Ce_0.8Zr_0.2O₂ was cubic, fluorite structure for all the catalysts. The XPS indicated the valence of Ce atom was +4 and there were reduced copper species presented in the CuO/Ce_0.8Zr_0.2O₂ catalyst. The results showed that the CuO loadings, calcination temperature and calcination time affected the catalytic activity of the catalysts for low-temperature CO oxidation. For comparison, the catalytic activities of CuO/CeO₂ catalysts calcined at different temperatures were also studied. The results indicated that CuO/Ce_0.8Zr_0.2O₂ catalyst had better thermal resistance than CuO/CeO₂ catalyst and had inferior activity than the CuO/CeO₂ catalyst when they were both calcined at 600 °C.     

Porous MOFs supported palladium catalysts for phenol hydrogenation: A comparative study on MIL-101 and MIL-53

Two metal organic frameworks (MOFs), chromium benzenedicarboxylates MIL-101 and MIL-53, have been synthesized and used as the support of palladium catalysts. The palladium catalysts were characterized by XRD, TEM, and CO chemisorption. MIL-101 is highly hydrophilic and beneficial as support for fine Pd nanoparticles with an average size of 2.3 nm. Microporous MIL-53 is relatively hydrophobic and larger Pd particles with an average size of 4.3 nm were formed on the external surface. Pd/MIL-101 showed better phenol selective hydrogenation activity to cyclohexanone (> 98%) under mild reaction conditions because of its smaller particle size.     

Tuning the catalytic property of non-noble metallic impurities in graphene

The stable configuration, electronic structure, magnetic property and catalytic activity of single-atom non-noble-metal (NNM) catalysts on graphene are investigated using the first-principles method. In contrast to the pristine graphene, a vacancy defect in graphene strongly stabilises the NNM adatom and makes it more positively charged. The charging leads to the CO adsorption unfavourable, while facilitate the O₂ adsorption, thus alleviating the CO poisoning and improving the reaction possibility for CO oxidation. Besides, there are more electrons transferred between NNM doped-graphene and O₂ molecule, which enhance their interaction and induce changes in the electronic structures and magnetic properties of the systems. Moreover, the sequential processes of CO oxidation on the Co–, Al– and Zn–graphene systems have lower enough energy barriers (<0.4 eV) by the Langmuir–Hinshelwood (LH) reaction (CO + O₂ → OOCO → CO₂ + O_ads) than that on the Ni–graphene substrate. Among the reaction processes, the rate-controlling step is the breaking of the O–O bond of the OOCO complex to form the CO₂ molecule and the atomic O_ads. The results validate the reactivity of NNM catalysts at the atomic scale and initiate a clue for fabricating graphene-based catalysts with low cost and high activity.