Dry reforming of methane over nickel supported on Nd–ceria: enhancement of the catalytic properties and coke resistance

Nickel (5 wt%) supported on Nd-doped ceria was studied as catalysts in the DRM reaction at stoichiometric conditions in the range of 600–800 °C. Ce_1−xNd_xO_2−δ supports with different Nd contents (x = 0, 0.05, 0.1 and 0.2) were successfully synthesized. The role of oxygen vacancies by the incorporation of Nd³⁺ into the ceria lattice was investigated. These species were quantified by XRD and Raman spectroscopy, showing a linear dependence as a function of Nd content. Ni/Nd–ceria catalysts were prepared by wet impregnation. Although formation of oxygen vacancies, as well as microstructural features of the support (smaller crystallite sizes, higher surface area, and developed mesoporous structure) were improved as a function of the Nd content, no significant differences were observed in the catalytic properties of Ni/Nd–ceria in the DRM reaction. Despite this, compared to undoped ceria, all the Nd-doped CeO₂ catalysts present an enhanced activity and stability, and the best catalytic performance was observed in the Ni/Ce_0.95Nd_0.05O_2−δ sample. Quantification of carbon residues in spent catalysts showed, as expected, lower amounts in the Ni/Nd–ceria samples; nevertheless, among them, the catalyst with the higher amount of oxygen vacancies, is the one with the higher carbon residues. Incorporation of Nd in ceria changes the acid/base properties, diminishing the gasification capacity of the carbonaceous species. These results emphasize that the activity and stability in the Ni/Nd–ceria catalysts for the DRM reaction depend on two key factors, the redox and the acid/base properties of the CeO₂ supports, offering insights about the necessary and adequate balance between these properties.

The Nd-doped CeO₂ support enhances the reactivity of the catalysts, selectivity toward hydrogen and stability by improving coke deposition resistance.     

Effects of the addition of CeO2 on the steam reforming of ethanol using novel carbon-Al2O3 and carbon-ZrO2 composite-supported Co catalysts

Novel carbon-Al₂O₃ and carbon-ZrO₂ composite-supported Co catalysts were prepared using the sol–gel method with polyethylene glycol (PEG) as a carbon source, and the effects of the addition of CeO₂ to catalysts on the steam reforming of ethanol were investigated. The reactions were carried out in a fixed bed reactor with H₂O/EtOH = 12 (mol/mol) and a temperature range of 300 °C to 600 °C. The catalyst characterization was performed by XRD, nitrogen adsorption and desorption isotherms, TG-DTA, XRF and TEM. Although the carbon-Al₂O₃ composite-supported Co catalysts exhibited a higher conversion of ethanol than the carbon-ZrO₂ composite-supported Co catalysts, the effect of the addition of CeO₂ was hardly observed for catalysts with Al₂O₃. In contrast to the case of catalysts with Al₂O₃, the effect of the addition of CeO₂ to catalysts with ZrO₂ on the conversion and the hydrogen yield was observed, and the hydrogen yield at 600 °C exceeded that of catalysts with Al₂O₃. 16Co42C31.5Ce10.5Zr exhibited the highest hydrogen yield of 89% at 600 °C. Fine Co metal species were observed for the used ZrO₂-based catalysts, while Co₃O₄ peaks were observed for the used Al₂O₃-based catalysts. The development of the carbon nanotube-like structure with a diameter of 50 nm was observed with particles having diameters of 30 nm to 50 nm, suggesting that the carbon deposition might occur so as not to deactivate the catalyst.

For the ideal reaction routes in steam reforming of ethanol catalyzed by Co/CeO₂–ZrO₂, as Al₂O₃ was used instead of ZrO₂, the effect of CeO₂ did not appear, suggesting that the configuration of CeO₂ and cobalt species on ZrO₂ would be important.     

Development of a thermally stable Pt catalyst by redispersion between CeO2 and Al2O3

For catalytic systems consisting of Pt as the active component and CeO₂–Al₂O₃ as the support material, the metal–support interaction between the Pt and CeO₂ components is widely applied to inhibit aggregation of Pt species and thus enhance the thermal stability of the catalyst. In this work, a highly thermostable Pt catalyst was prepared by modifying the synthesis procedure for conventional Pt/CeO₂/Al₂O₃ (Pt/Ce/Al) catalyst, that is, the CeO₂ component was introduced after deposition of Pt on Al₂O₃. The obtained CeO₂/Pt/Al₂O₃ (Ce/Pt/Al) catalyst exhibits significantly different aging behavior. During the hydrothermal aging process, redispersion of Pt species from the surface of Al₂O₃ to the surface of CeO₂ occurs, resulting in a stronger metal–support interaction between Pt and CeO₂. Thus, the formed Pt–O–Ce bond could act as an anchor to retard aggregation of Pt species and help Pt species stay at a more oxidative state. Consequently, excellent reduction capability and superior three-way catalytic performance are acquired by Ce/Pt/Al-a after hydrothermal aging treatment.

Ce/Pt/Al undergoes redispersion of Pt upon hydrothermal aging, resulting in higher dispersion and consequently superior three-way catalytic performance of Ce/Pt/Al-a.     

Enhanced properties of Pd/CeO2-nanorods modified with alkaline-earth metals for catalytic oxidation of low-concentration methane

A series of Pd/CeO₂-nanorods catalysts modified with alkaline-earth metals were prepared by the incipient impregnation method. Their catalytic properties in low-concentration methane oxidation were also investigated. The catalysts were characterized by X-ray diffraction, scanning electron microscopy, transmission electron microscopy and H₂-temperature-programmed reduction techniques. The catalytic results show that the Ca element doped in Pd/CeO₂, with optimum molar ratio of Pd to Ca of 2 and calcination temperature of 450 °C, improved the properties of the Pd-based catalyst remarkably, which is attributed to strong Pd–support interaction and high oxygen mobility. Therefore, calcium is more suitable as a promoter for enhancing the activities of the Pd/CeO₂ catalyst in methane oxidation.

A series of Pd/CeO₂-nanorods catalysts modified with alkaline-earth metals were prepared by the incipient impregnation method.     

Cerium and tin oxides anchored onto reduced graphene oxide for selective catalytic reduction of NO with NH3 at low temperatures

A series of cerium and tin oxides anchored on reduced graphene oxide (CeO₂–SnO_x/rGO) catalysts are synthesized using a hydrothermal method and their catalytic activities are investigated by selective catalytic reduction (SCR) of NO with NH₃ in the temperature range of 120–280 °C. The results indicate that the CeO₂–SnO_x/rGO catalyst shows high SCR activity and high selectivity to N₂ in the temperature range of 120–280 °C. The catalyst with a mass ratio of (Ce + Sn)/GO = 3.9 exhibits NO conversion of about 86% at 160 °C, above 97% NO conversion at temperatures of 200–280 °C and higher than 95% N₂ selectivity at 120–280 °C. In addition, the catalyst presents a certain SO₂ resistance. It is found that the highly dispersed CeO₂ nanoparticles are deposited on the surface of rGO nanosheets, because of the incorporation of Sn⁴⁺ into the lattice of CeO₂. The mesoporous structures of the CeO₂–SnO_x/rGO catalyst provides a large specific surface area and more active sites for facilitating the adsorption of reactant species, leading to high SCR activity. More importantly, the synergistic interaction between cerium and tin oxides is responsible for the excellent SCR activity, which results in a higher ratio of Ce³⁺/(Ce³⁺ + Ce⁴⁺), higher concentrations of surface chemisorbed oxygen and oxygen vacancies, more strong acid sites and stronger acid strength on the surface of the CeSn(3.9)/rGO catalyst.

A series of cerium and tin oxides anchored on graphene oxide (CeO₂–SnO_x/rGO) catalysts are synthesized for selective catalytic reduction of NO with NH₃ in the temperature range of 120–280 °C.     

CuO and CeO2 assisted Fe2O3/attapulgite catalyst for heterogeneous Fenton-like oxidation of methylene blue

In this paper, CuO and CeO₂ were screened as co-catalyst components for Fe₂O₃/attapulgite (ATP) catalyst, and three new catalysts (CuO–Fe₂O₃/ATP, CeO₂–Fe₂O₃/ATP and CuO–CeO₂–Fe₂O₃/ATP) were prepared for degradation of methylene blue (MB). The three catalysts'' characteristics were determined by BET, XRD, FT-IR, SEM and XPS. MB degradation in different systems and at different pH values was also studied. Under the conditions of H₂O₂ concentration of 4.9 mmol L⁻¹, catalyst dosage of 5 g L⁻¹, pH of 5, reaction temperature of 60 °C and MB initial concentration of 100 mg L⁻¹, the as-synthesized catalysts showed much greater reaction rate and degradation efficiency of MB than Fe₂O₃/ATP catalyst. In addition, the reusability of the as-prepared composites was evaluated. The intermediate products of MB degradation were identified by LC-MS and the possible degradation process of MB was put forward.

A novel heterogeneous catalyst CuO–CeO₂–Fe₂O₃/ATP was synthesized for MB degradation and the catalytic mechanism was put forward.     

Tuning the metal-support interaction in the thermal-resistant Au–CeO2 catalysts for CO oxidation: influence of a mild N2 pretreatment

Pretreatment is very important for altering the catalytic properties of the supported noble metal catalysts in many heterogeneous reactions. In this study, a simple and mild pretreatment with N₂ has been reported to re-activate the Au–CeO₂ catalysts that were prepared by a deposition–precipitation method followed by calcination at 600 °C. Upon N₂ pretreatment at 200 °C, the metal-support interaction between Au nanoparticles (NPs) and CeO₂ was observed with the evidence of particular coverage of Au nanoparticles by CeO₂, electronic interactions and changes in CO adsorption ability. As a result, the CO oxidation activity of the pretreated Au–CeO₂ catalysts largely improved compared with those without any pretreatment and even with those subjected to H₂ and O₂ pretreatments. N₂ pretreatment also makes the Au NPs more resistant to sintering at high temperature. Furthermore, this mild pretreatment strategy can provide a potential approach to improve the thermal stability of other supported noble metal catalysts.

The degree of encapsulation for Au–CeO₂ catalysts was identical to the catalysts exhibiting metal-support interaction, which improved the CO oxidation activity.     

Barium promoted Ni/Sm2O3 catalysts for enhanced CO2 methanation

Low temperature CO₂ methanation is a favorable pathway to achieve high selectivity to methane while increasing the stability of the catalysts. A Ba promoted Ni/Sm₂O₃ catalyst was investigated for CO₂ methanation at atmospheric pressure with the temperature ranging from 200–450 °C. 5Ni–5Ba/Sm₂O₃ showed significant enhancement of CO₂ conversion particularly at temperatures ≤ 300 °C compared to Ni/Sm₂O₃. Incorporation of Ba into 5Ni/Sm₂O₃ improved the basicity of the catalysts and transformed the morphology of Sm₂O₃ from random structure into uniform groundnut shape nanoparticles. The uniformity of Sm₂O₃ created interparticle porosity that may be responsible for efficient heat transfer during a long catalytic reaction. Ba is also postulated to catalyze oxygen vacancy formation on Sm₂O₃ under a reducing environment presumably via isomorphic substitution. The disappearance of a high temperature (∼600 °C) reduction peak in H₂-TPR analysis revealed the reducibility of NiO following impregnation with Ba. However, further increasing the Ba loading to 15% formed BaNiO₃–BaNiO_2.36 phases which consequently reduced the activity of the Ni–Ba/Sm₂O₃ catalyst at low temperature. Ni was suggested to segregate from BaNiO₃–BaNiO_2.36 at high temperature thus exhibiting comparable activity with Ni/Sm₂O₃ at 450 °C.

Low temperature CO₂ methanation on 5Ni–5Ba/Sm₂O₃ is a favorable pathway to achieve high selectivity to methane while increasing the stability of the catalysts.     

Effect of Re promoter on the structure and catalytic performance of Ni–Re/Al2O3 catalysts for the reductive amination of monoethanolamine

In this paper, Ni/Al₂O₃ catalysts (15 wt% Ni) with different Re loadings were prepared to investigate the effect of Re on the structure and catalytic performance of Ni–Re/Al₂O₃ catalysts for the reductive amination of monoethanolamine. Reaction results reveal that the conversion and ethylenediamine selectivity increase significantly with increasing Re loading up to 2 wt%. Ni–Re/Al₂O₃ catalysts show excellent stability during the reductive amination reaction. The characterization of XRD, DR UV-Vis spectroscopy, H₂-TPR, and acidity–basicity measurements indicates that addition of Re improves the Ni dispersion, proportion of octahedral Ni²⁺ species, reducibility, and acid strength for Ni–Re/Al₂O₃ catalysts. The Ni15 and Ni15–Re2 catalysts were chosen for in-depth study. The results from SEM-BSE, TEM, and CO-TPD indicate that smaller Ni⁰ particle size and higher Ni⁰ surface area are obtained in the reduced Ni–Re/Al₂O₃ catalysts. Results from in situ XPS and STEM-EDX line scan suggest that Re species show a mixture of various valances and have a tendency to aggregate on the surface of Ni⁰ particles. During reaction, the Ni⁰ particles on the Al₂O₃ support are stabilized and the sintering process is effectively suppressed by the incorporation of Re. It could be concluded that sufficient Ni⁰ sites, the collaborative effect of Ni–Re, and brilliant stability contribute to the excellent catalytic performance of Ni–Re/Al₂O₃ catalysts for the reductive amination of monoethanolamine.

Re promoters improve the catalyst performance and stability of Ni–Re/Al₂O₃ catalysts for the reductive amination of monoethanolamine.     

Influence of zirconia addition in TiO2 and TiO2–CeO2 aerogels on the textural,structural and catalytic properties of supported vanadia in chlorobenzene oxidation

This paper studies the effect of the direct incorporation of ZrO₂ in TiO₂ and TiO₂–CeO₂ aerogel supports prepared by sol–gel route on the physico–chemical and catalytic properties of supported vanadia catalysts in the total oxidation of chlorobenzene. The obtained catalysts have been characterized by means of ICP-AES, N₂ adsorption–desorption at 77 K, XRD, XPS, H₂-TPR and NH₃-TPD. The results revealed that Zr-doped V₂O₅ based catalyst is beneficial for the improvement of catalytic properties in chlorobenzene total oxidation. In particular, in the absence of cerium groups, this beneficial effect is correlated with the better acidic properties or/and the stabilization of the V₂O₅ active phase in a higher oxidation state. However, in the case of cerium rich catalyst, this positive effect is much stronger thanks to the enhanced redox properties of V₂O₅/TiO₂–CeO₂–ZrO₂.

Chlorobenzene conversion over vanadia supported catalysts.     

Facile synthesis of ordered mesoporous zinc alumina catalysts and their dehydrogenation behavior

Ordered mesoporous Zn/Al₂O₃ materials with varying Zn content were simply prepared via an evaporation-induced self-assembly (EISA) method. Dehydrogenation of isobutane to isobutene was carried out on these materials; an isobutane conversion of 45.0% and isobutene yield of 39.0% were obtained over the 10%Zn/Al₂O₃ catalyst at 580 °C with 300 h⁻¹ GHSV. The obtained materials with Zn content up to 10% possess large specific surface area and big pore volume and zinc species can be highly dispersed on the surface or incorporated into the framework. The acidity of these catalysts was changed by the introduction of Zn, the decrease of strong acid sites is conducive to the promotion of isobutene selectivity and the weak and medium acidic sites played an important role in isobutane conversion. In addition, these catalysts exhibited good catalytic stability, due to the effective inhibition of coke formation by the ordered mesoporous structure.

Facile synthesis of ordered mesoporous zinc alumina catalysts that exhibited great catalytic activity (45.0% isobutane conversion, 86.7% isobutene selectivity) and stability.     

Preparation of Ni based mesoporous Al2O3 catalyst with enhanced CO2 methanation performance

A Ni based mesoporous γ-Al₂O₃ (MA) catalyst was prepared via partial hydrolysis without organic surfactants and employed in the carbon dioxide methanation reaction. The obtained catalysts were characterized by N₂ adsorption–desorption, H₂-TPR, XRD, XPS, TG, SEM and TEM-EDS techniques. CO₂ methanation was performed in a fixed-bed reactor. A high surface area of MA with excellent hydrothermal stability was obtained, which promoted the dispersion of nickel species, producing a better catalytic performance. Incorporation of more NiO species into the Ni/MA catalyst increased the amount of active metallic Ni sties, further improving the catalytic activity and CH₄ selectivity. Moreover, the monolithic skeleton of MA with fabric-like walls suppressed the aggregation of active metallic Ni sites and carbon deposition, enhancing the catalyst''s stability, which provides a new insight for potential industrial applications.

A Ni based mesoporous γ-Al₂O₃ (MA) catalyst was prepared via partial hydrolysis without organic surfactants and employed in the carbon dioxide methanation reaction.     

Role of oxide support in Ni based catalysts for CO2 methanation

The CO₂ methanation reaction of reduced and unreduced Ni based CeO₂, Al₂O₃, TiO₂ and Y₂O₃ supported catalysts was investigated. The Ni/CeO₂ and Ni/Y₂O₃ catalysts exhibited similar CO₂ conversions at all reaction temperatures. The catalysts were studied by X-ray diffraction (XRD), H₂ chemisorption, H₂ temperature-programmed reduction (TPR), and in situ diffuse reflection infrared Fourier transform spectroscopy (DRIFTS); the results suggested that the reducibility of both metal and support at low temperature, strong metal support interaction and small Ni particle size are important factors for low-temperature CO₂ methanation. Based on the DRIFT studies, the difference in the CO₂ adsorption properties and reaction pathway depending on the reduced and unreduced Ni based supported catalysts was discussed.

The effect of metal–support interaction and role of support on catalytic performances during Ni based CO₂ methanation reaction were investigated.     

Dehydrocyclization–cracking of methyl oleate by Pt catalysts supported on a ZnZSM-5–Al2O3 hierarchical composite

The dehydrocyclization–cracking of methyl oleate was performed by ZnZSM-5–Al₂O₃ hierarchical composite-supported Pt catalysts in the range of 450–550 °C under 0.5 MPa hydrogen pressure. Most catalysts converted methyl oleate completely and produced aromatics with more than 10 wt% yield as well as valuable fuels even at 450 °C. The reactivity of catalysts changed remarkably depending on the addition method of Pt, while supporting Pt of 0–0.16 wt% did not affect the pore structure of each catalyst. When Pt was introduced into the composite support by the conventional impregnation method, remarkable hydrocracking proceeded through the decarboxylation and decarbonylation of methyl oleate and the successive conversion of C17 fragments and gave the significant amounts of gaseous products. Nevertheless, the selectivity for the aromatics of the gasoline fraction was relatively high and the yields of aromatics reached maximum 19% at 500 °C under 0.5 MPa, suggesting that gaseous olefins would be cyclized through the Diels–Alder reaction on ZnZSM-5 in the composite support. In contrast, when Pt was introduced into catalysts by ion-exchange with ZnZSM-5, the significant conversion of methyl oleate was inhibited and produced liquid fuels in a wide range.

The ideal reaction route in the dehydrocyclization–cracking of methyl oleate catalyzed by Pt/ZnZSM-5–Al₂O₃ is to produce xylene, toluene, and hydrogen through decarboxylation.     

Support effects on catalysis of low temperature methane steam reforming

Low temperature (<500 K) methane steam reforming in an electric field was investigated over various catalysts. To elucidate the factors governing catalytic activity, activity tests and various characterization methods were conducted over various oxides including CeO₂, Nb₂O₅, and Ta₂O₅ as supports. Activities of Pd catalysts loaded on these oxides showed the order of CeO₂ > Nb₂O₅ > Ta₂O_5. Surface proton conductivity has a key role for the activation of methane in an electric field. Proton hopping ability on the oxide surface was estimated using electrochemical impedance measurements. Proton transport ability on the oxide surface at 473 K was in the order of CeO₂ > Nb₂O₅ > Ta₂O_5. The OH group amounts on the oxide surface were evaluated by measuring pyridine adsorption with and without H₂O pretreatment. Results indicate that the surface OH group concentrations on the oxide surface were in the order of CeO₂ > Nb₂O₅ > Ta₂O_5. These results demonstrate that the surface concentrations of OH groups are related to the proton hopping ability on the oxide surface. The concentrations reflect the catalytic activity of low-temperature methane steam reforming in the electric field.

Low temperature (<500 K) methane steam reforming in an electric field was investigated over various catalysts.     

Synergy effect of CuO on CuCo2O4 for methane catalytic combustion

Spinel oxides (AB₂O₄) have been widely studied as catalysts for methane combustion. Increasing attention was focused on the catalysis properties of the [B₂O₃] octahedron; however, the role of the [AO] tetrahedron in the catalytic activity was seldom discussed. Herein, a series of (CuO)_x–CuCo₂O₄ (x = 0, 0.1, 0.2) composite oxides were synthesized by a solvothermal method. The structure, morphology, and physicochemical properties of the as-synthesized samples were characterized by the XRD, SEM, BET, and XPS techniques. The results of the catalytic activity tests showed that the coexistence of CuO with CuCo₂O₄ can improve the catalytic activity. The XPS results demonstrated that there were remarkable Cu⁺ ions present in the composite oxides, which can cause increases in the number of oxygen vacancies on the surface of the catalysts. In addition, the redox of Cu⁺ and Cu²⁺ may improve the oxygen exchange capacity for methane oxidation.

CuO and CuCo₂O₄ exhibit a synergistic effect in catalyzing methane combustion, which increases the oxidation rate of methane on the surface of (CuO)_0.2–CuCo₂O₄ composite oxide and decreasing the methane combustion temperature.     

A mesoporous tungsten carbide nanostructure as a promising cathode catalyst decreases overpotential in Li–O2 batteries

Lithium–oxygen (Li–O₂) batteries as promising energy storage devices possess high gravimetric energy density and low emission. However, poor reversibility of electrochemical reactions at the cathode significantly affects the electrochemical properties of nonaqueous Li–O₂ batteries, and low charge–discharge efficiency also results in short cycle-life. In this work, functional air cathodes containing mesoporous tungsten carbide nanoparticles for improving the reversibility of positive reactions in Li–O₂ cells are designed. Mesoporous tungsten carbides are synthesized with mesoporous carbon nitride as the reactive template and carbon source. And mesoporous tungsten carbides in cathode materials display better electrochemical performance in Li–O₂ cells in comparison with mesoporous carbon nitride and hard carbon. Tungsten carbide-1 (WC-1) with larger specific surface area promotes reversible formation and decomposition of Li₂O₂ at the cathode and lower charge overpotential (about 0.93 V) at 100 mA g⁻¹, which allows the Li–O₂ cell to run up to 100 cycles. In addition, synergistic interaction between WC-1 and LiI could further decrease the charging overpotentials of Li–O₂ cells and improve the charge–discharge performances of the Li–O₂ cells. These results indicate that mesoporous electrocatalysts can be utilized as promising functional materials for Li–O₂ cells to decrease overpotentials.

Tungsten carbide with large specific surface area catalyzes reversible formation/decomposition of Li₂O₂ with low overpotential in a Li–O₂ cell.     

The influence of composition on the functionality of hybrid CuO–ZnO–Al2O3/HZSM-5 for the synthesis of DME from CO2 hydrogenation

A series of CuO–ZnO–Al₂O₃/HZSM-5 hybrid catalysts with different Cu/Zn ratios and disparate Al₂O₃ doping were prepared and characterized by XRD, BET, H₂-TPR, NH₃-TPD and XPS techniques. The optimal Cu/Zn ratio is 7 : 3, and the introduction of a suitable amount of Al₂O₃ to form hybrid catalysts increased the BET specific area and micropore volume, facilitated the CuO dispersion, decreased the CuO crystallite size, increased the interaction between CuO and ZnO, enhanced the number of weak acid sites, altered the copper chemical state and improved the catalytic performance consequently. The highest CO₂ conversion, DME selectivity and DME yield of 27.3%, 67.1% and 18.3%, respectively, were observed over the CZA₇H catalyst. The suitable temperature of 260 °C and the appropriate space velocity of 1500 h⁻¹ for one-step synthesis of dimethyl ether (DME) from carbon dioxide (CO₂) hydrogenation were also investigated. The 50 h stability of the CZA₇H catalyst was also tested.

The introduction of Al₂O₃ increased the number of weak acid sites, altered the copper chemical state and improved the catalytic performance and stability consequently.     

Pd catalyst supported on CeO2 nanotubes with enhanced structural stability toward oxidative carbonylation of phenol

Ordered CeO₂ nanotubes (CeO₂-T) were prepared via a hydrothermal synthesis process using the triblock copolymer polyethylene oxide-polypropylene oxide-polyethylene oxide (P123) as a morphology control agent. CeO₂-T characterization demonstrated the formation of single crystal structures having lengths between 1–3 μm and diameters < 100 nm. A supported Pd catalyst (Pd/CeO₂-T) was also prepared through hydrothermal means. H₂-temperature reduction profile and Raman spectroscopy analyses showed that the oxygen vacancies on the CeO₂ surface increased and the reduction temperature of the surface oxygen decreased after Pd loading onto CeO₂-T. Pd/CeO₂-T was employed as a catalyst toward the oxidative carbonylation of phenol and the reaction conditions were optimized. Phenol conversion was 53.2% with 96.7% selectivity to diphenyl carbonate under optimal conditions. The integrity of the tubular CeO₂ structure was maintained after the catalyst was recycled, however, both activity and selectivity significantly decreased, which was mainly attributed to the Pd active component significantly leaching during the reaction.

Schematic representation of formation of Pd/CeO₂-T.     

Improvement of thermal stability of highly active species on SiO2 supported copper-ceria catalysts

CuO–CeO₂/SiO₂ catalysts lose activity when they are calcined at 600 °C and temperatures above. This loss of activity was related to a decrease in the amount of highly dispersed Cu species interacting with Ce (CuO–CeO₂ interface) over the SiO₂ support. These species are highly active in CO oxidation, so this reaction was selected to conduct this study. In order to avoid the activity loss in CuO–CeO₂/SiO₂ catalysts, the effect of high Ce loads (8, 16, 24, and 36%) on the thermal stability of these catalysts was studied. The results reveal that when increasing calcination temperature from 500 to 700 °C, the catalysts with Ce load equal to or higher than 24% increase the formation of highly dispersed Cu interacting with Ce and therefore the activity (90% of CO conversion at 120 °C). In catalysts with Ce load below 24%, Cu species agglomerate and decrease the activity (less than 5% of CO conversion at 120 °C).

CuO–CeO₂/SiO₂ catalysts with Ce loading of 24% and above keep high activity after calcination at 700 °C. Therefore, a catalyst with high thermal stability of CuO–CeO₂ interface can be obtained able to work in a higher range of temperatures.