Effect of steam and carbon dioxide pretreatments on methane decomposition and carbon gasification over doped-ceria supported nickel catalyst

Effects of steam (H₂O) and carbon dioxide (CO₂) pretreatments on methane (CH₄) decomposition and carbon gasification over doped-ceria supported nickel catalysts have been studied from 400 to 500 °C. The doped ceria employed were gadolinia-doped ceria and samaria-doped ceria. Results indicate that a drastic increase of both H₂O and CO₂ dissociation activities occurs as the temperature increases from 450 to 500 °C. The formation of the surface hydroxyl species during H₂O treatment inhibits the followed CH₄ decomposition. CO but no CO₂ was formed during CH₄ reaction after H₂O treatment. Carbon deposition during CH₄ decomposition is quite large but can be removed via gasification with afterward CO₂ treatment. However, some of the deposited carbon species is in a form which can not be removed with CO₂ treatment but can be removed with O₂ treatment. And, higher values of the oxygen-ion conductivity and the density of the surface oxygen vacancies lead to higher activities for all dissociation and decomposition reactions.     

Selective oxidation of soot over Cu doped ceria/ceria–zirconia catalysts

Copper doped ceria and ceria–zirconia mixed oxides were prepared using the citric acid sol–gel method. The temperature-programmed oxidation (TPO) results showed that the Cu modification helped to improve the activity and selectivity of ceria and ceria–zirconia for soot catalytic oxidation. The CO-TPR results showed that Cu–Ce had a better reducibility than pure ceria at low temperatures. After ageing at 800 °C for 20 h in flow air, CuO–CeO₂ showed the maximum soot oxidation rate at 378 and 519 °C under tight and loose contact conditions, respectively, achieving a nearly 100% selectivity to CO₂ production. This effect may be attributed to the existence of well dispersed copper oxide species strongly interacting with the ceria surface, which may decrease the activation energy of soot oxidation. A conceivable mechanism of this synergetic effect was proposed.     

Selective Conversion of Ethane to Ethene via Oxidative Dehydrogenation Over Ca-doped ThO2 Using CO2 as Oxidant

Ca-doped ThO₂, synthesized by solution combustion method was tested for dehydrogenation of ethane with CO₂. Doping ThO₂ with Ca resulted in the creation of oxide ion vacancies and an increased conversion of ethane compared to pure ThO₂. On Th_0.75Ca_0.25O₂ selectivity to ethene was 97 at 46% ethane conversion at 725 °C. Well-known reference catalysts like 5%Cr/TS-1 or OMS-2 showed significantly lower selectivity, but the former was more active under the same conditions.     

Roles of Surface and Bulk Lattice Oxygen in Forming CO2 and CO During Methane Reaction over Gadolinia-Doped Ceria

Temperature-programmed reaction of methane and temperature-programmed reduction were performed over gadolinia-doped ceria (GDC). It was found that CO₂ formation can occur at very much lower temperature than CO formation. The surface lattice oxygen acts as the active site for CH₄ adsorption. This active site has a dynamic characteristic due to the mobility of the lattice oxygen. The rates of CO and CO₂ formations can be controlled by the supply rate of the lattice oxygen from the GDC bulk; this supply rate depends on the mobility and the concentration of the bulk lattice oxygen. CO₂ formation is associated with the existing surface lattice oxygen while CO formation depends on the oxygen species coming from the bulk lattice during methane reaction.     

Characterization and Catalytic Behavior of VO x -CeO2 Catalysts for the Oxidative Dehydrogenation of Propane

A series of ceria-supported vanadium catalysts was prepared via impregnation of the support with an ammonium metavanadate solution. The 723 K calcined samples were tested for propane oxydehydrogenation (ODH) activity and selectivity. The sample exhibiting the highest propane conversion was found to be the ceria support material itself, although this showed essentially no selectivity towards propene. An optimum propene yield of 4.2% was obtained at 673 K for the 6 wt% V₂O₅-CeO₂ sample. Conversion decreased with increasing V loading which was attributed to the formation of cerium vanadate (CeVO₄). This phase was found in all samples after calcination, its abundance rising in proportion to the V loading. In the 6 wt% V₂O₅ catalyst hydrated surface VO_x species were present, although they underwent conversion to CeVO₄ at temperatures above 573 K. The low reducibility of these surface vanadates was linked to the oxidation activity. It is inferred that surface polyvanadate species are responsible for the selective ODH of propane with V-O-V and/or V-O-Ce being the active oxygen species.     

Splitting CO2 with a ceria‐based redox cycle in a solar‐driven thermogravimetric analyzer

Thermochemical splitting of CO₂ via a ceria‐based redox cycle was performed in a solar‐driven thermogravimetric analyzer. Overall reaction rates, including heat and mass transport, were determined under concentrated irradiation mimicking realistic operation of solar reactors. Reticulated porous ceramic (RPC) structures and fibers made of undoped and Zr⁴⁺‐doped CeO₂, were endothermally reduced under radiative fluxes of 1280 suns in the temperature range 1200–1950 K and subsequently re‐oxidized with CO₂ at 950–1400 K. Rapid and uniform heating was observed for 8 ppi ceria RPC with mm‐sized porosity due to its low optical thickness and volumetric radiative absorption, while ceria fibers with μm‐sized porosity performed poorly due to its opacity to incident irradiation. The 10 ppi RPC exhibited higher fuel yield because of its higher sample density. Zr⁴⁺‐doped ceria showed increasing reduction extents with dopant concentration but decreasing specific CO yield due to unfavorable oxidation thermodynamics and slower kinetics. © 2016 American Institute of Chemical Engineers AIChE J, 63: 1263–1271, 2017     

Ceria and Gold/Ceria Catalysts for the Abatement of Polycyclic Aromatic Hydrocarbons: An In Situ DRIFTS Study

Gold catalysts supported on nano-crystalline ceria prepared by deposition precipitation have been characterised and tested for the total oxidation of naphthalene. Two different precipitation methods were used to prepare the nano-crystalline ceria supports and it was observed that although both supports were active materials for naphthalene oxidation, ceria synthesized by homogeneous precipitation with urea was markedly more active than CeO₂ precipitated by carbonate. The addition of gold to both active CeO₂ catalysts resulted in different effects for the total oxidation of naphthalene. Gold addition promotes the naphthalene conversion to CO₂ when ceria is prepared by precipitation with carbonates, whilst the light off temperature is shifted towards higher temperatures when gold is added to ceria synthesized by the urea method. This behaviour has been related to a change in the support characteristics and a removal of the carbonate surface species, when gold is deposited onto the ceria support.     

Preferential CO oxidation in H2-rich gas mixtures over Au/doped ceria catalysts

Nanosized gold catalysts supported on doped ceria were prepared by deposition–precipitation method. A deep characterization study by HRTEM/EDS, XRD, FT-Raman, TPR and FTIR was undergone in order to investigate the effect of ceria modification by various cations (Sm³⁺, La³⁺ and Zn²⁺) on structural and redox properties of gold catalysts. Doping of ceria affected in different way catalytic activity towards purification of H₂ via preferential CO oxidation. The following activity order was observed: Au/Zn–CeO₂ > Au/Sm–CeO₂ > Au/CeO₂ > Au/La–CeO₂. The differences in CO oxidation rates were ascribed to different concentration of metallic gold particles on the surface of Au catalysts (as confirmed by the intensity of the band at 2103 cm⁻¹ in the FTIR spectra collected during CO–O₂ interaction). Gold catalysts on modified ceria showed improved tolerance towards the presence of CO₂ and H₂O in the PROX feed. The spectroscopic experiments evidence enhanced reactivity when PROX is performed in the presence of H₂O already at 90 K.     

Ethanol Steam Reforming: Higher Dehydrogenation Selectivities Observed by Tuning Oxygen-Mobility and Acid/Base Properties with Mn in CeO2·MnOx·SiO2 Catalysts

Doping CeO₂–SiO₂ catalysts with Mn was found to increase H₂ selectivity and decrease ethylene selectivity. In the presence of SiO₂, relatively high surface areas (~150–190 m² g^?1 range) were stabilized with these coprecipitated catalysts after calcining at 500 °C for 5 h. By means of a combination of ex-situ (XRD, TPR, CO₂–TPD, OSC measurements) along with in situ (Ce L_III edge XANES, DRIFTS) techniques Mn was proposed to act to improve the ability of ceria to deliver active oxygen surface species essential to the stepwise oxidative dehydrogenation of adsorbed species from ethoxides to acetates. Doping ceria with Mn at low levels improved the carbonate decomposition rate by weakening surface basicity. At high Mn content, acetates become too stable and poison catalytically active sites.     

Promoting effect of carbon dioxide on the dehydrogenation and aromatization of ethane over gallium‐loaded catalysts

Ga₂O₃ and Ga₂O₃/TiO₂ catalysts were found to be effective agents for the dehydrogenation of ethane to ethene in the presence of carbon dioxide at 650 °C. The activity of the Ga₂O₃ and Ga₂O₃/TiO₂ catalysts in the presence of CO₂ was 2–4 times higher than that without CO₂. Ethene yields reached ca. 20–25% and selectivity was ca. 70–90% at 650°C in the 17% ethane and 83% CO₂ feed at an SV of 9,000 ml/(g‐cat h). The presence of CO₂ markedly promoted dehydrogenation of ethane over Ga₂O₃ and Ga₂O₃/TiO₂ catalysts. Furthermore, the promoting effect of CO₂ on the aromatization of ethane and ethene over a Ga₂O₃+H/ZSM‐5 catalyst was also observed above 650 °C. Aromatics yields were higher than those without CO₂. This revised version was published online in July 2006 with corrections to the Cover Date.     

Synthesis of flower-like La or Pr-doped mesoporous ceria microspheres and their catalytic activities for methane combustion

La or Pr-doped flower-like mesoporous ceria microspheres were prepared by a unique calcination of La or Pr-doped Ce(OH)CO₃ microspheres hydrothermally synthesized with the aid of glucose and acrylic acid. Prepared flower-like ceria-based materials have a novel hierarchical architecture with mesoporous structure and high surface area, which can obviously facilitate the catalytic combustion of methane, compared with general La or Pr-doped ceria. The presence of La or Pr can promote the production of oxygen vacancies and improve oxygen mobility, which result in enhancing the oxygen-storage capacity of the flower-like ceria and its catalytic performance for the methane combustion.     

Steam reforming of n-butane on Pd/ceria

We have examined the steam reforming of n-butane on ceria, 1 wt% Pd/ceria, 1 wt% Pd/alumina, and 15 wt% Ni/silica between 573 and 873 K, with H₂O:C ratios between 1.0 and 2.0. No stable rates could be observed on Ni/silica due to rapid coking under these conditions. While rates were stable on the other catalysts, Pd/ceria showed a much higher activity than either Pd/alumina or ceria individually. Of additional interest, CO₂:CO ratios were much higher on Pd/ceria and approached equilibrium. The reaction order for n-butane on Pd/ceria was 0.15. For H₂O, reaction order changed from 0.6 to zero at the stoichiometric, n-butane:H₂O ratio. It is suggested that the high activity of Pd/ceria for this reaction is due to a dual-function mechanism, in which ceria can be oxidized by H₂O and then supply oxygen to the Pd.     

The activity and selectivity of oxygen atoms adsorbed on a Ag/α-Al2O3 catalyst in ethene epoxidation

The bonding of the oxygen species held on a Ag/-Al₂O₃ catalyst has been studied by temperature programmed desorption and their reactivity in ethene epoxidation by temperature programmed reduction using ethene as the reductant. The Ag/-Al₂O₃ catalyst was produced by the thermal decomposition of a Ag oxalate/-Al₂O₃ precursor. Oxygen desorbs from this Ag/-Al₂O₃ catalyst in two states, one (peak maximum temperature 520 K) having a desorption activation energy of 140 kJ mol^–1 – oxygen desorbing from Ag(111), and one (peak maximum temperature 573 K) having a desorption activation energy of 155 kJ mol^–1 – oxygen desorbing from a highly stepped or defected Ag surface. Temperature programmed reduction of the two oxygen states existing on the surface of the Ag/-Al₂O₃ catalyst using ethene as the reductant produced two peaks at 373 and 473 K in which ethene epoxide and CO₂ evolved coincidently. The peak at 373 K derives from the reduction of oxygen atoms adsorbed on Ag(111). The higher temperature peak (473 K) corresponds to the reduction of oxygen atoms adsorbed on highly stepped or defected Ag surface. The selectivity to ethene epoxide for the 373 K peak is ~ 57%, while that of the 473 K peak is 34%. The coincident evolution of ethene epoxide and CO₂ shows that the selective and unselective reaction pathways have a common surface intermediate – probably an oxametallacycle. The higher selectivity of the oxametallacycle formed by the bonding of ethene to the weaker Ag-O bond is considered to result from its having a lower activation energy to cyclisation than that produced by ethene bonding to the higher Ag-O bond.     

Fabrication and characterization of Matrimid/MIL-53 mixed matrix membrane for CO2/CH4 separation

In this study, gas separation properties of Matrimid/MIL-53 mixed matrix membranes with different MOF weight percentages (0–20 wt.%) were investigated. TEM, XRD and DLS analysis were implemented to investigate MIL-53, structure and particles size distribution. SEM, FTIR, DSC and TGA analyses were conducted to characterize the fabricated membranes. The SEM images of these membranes showed good adhesion between polymer and particles, although for 20% MIL-53 loading, particles agglomeration was observed in some areas. Moreover, surface images of the membranes showed adequate dispersion of the particles in the polymer matrix, especially at lower MOF loadings. The permeability of pure CO₂ and CH₄ gases for all membranes were measured and the ideal CO₂/CH₄ selectivity was calculated. CH₄ permeability of membranes increased slightly as the percentage of loading increased. At 20 wt.% MOF loading, void formation led to a significant increase in CH₄ permeability (300% over pure Matrimid). CO₂ permeability showed the same trend; there was a 94% increase in permeability compared to pure Matrimid for 15 wt.% MMMs. CO₂/CH₄ selectivity also increased as MOF loading increased. The highest selectivity was shown for 15 wt.% MOF loading. This membrane had 84% growth in selectivity over pure Matrimid. Although at 20 wt.% MIL-53 loading, membrane separation performance was destroyed.     

Investigation of isosynthesis via CO hydrogenation over ZrO2 and CeO2 catalysts: Effects of crystallite size,phase composition and acid–base sites

This paper investigates the isosynthesis via CO hydrogenation over zirconia and ceria catalysts. Various techniques including XRD, NH₃-TPD, CO₂-TPD and BET surface area were employed for the catalyst characterization. The results showed that not only acid–base properties, but also crystallite size and crystal phase essentially influenced the catalytic performance. It was found that the activity and the selectivity of isobutene in hydrocarbons on nanoscale catalysts were higher than those on the micronscale ones. Moreover, the acid–base properties were dependent on the fraction of tetragonal phase for zirconia, but independent on crystal phase for ceria. The synthesized nanoscale zirconias were more active than the commercial one but less than the nanoscale ceria. From the results, it was indicated that zirconia with 29% tetragonal phase exhibited the highest activity. Furthermore, the presence of tetragonal phase in zirconia played an important role on the selectivity of isobutene in hydrocarbons.     

Ceria‐coated diesel particulate filters for continuous regeneration

The potential of diesel particulate filters wash‐coated with highly dispersed nano‐metric ceria particles for continuous regeneration has been investigated. To this end, catalytic filters were prepared, soot‐loaded (avoiding the formation of the cake layer), and regenerated—under isothermal conditions—at temperature ranging from 200–600°C. Results have shown that catalytic oxidation of soot starts from 300°C and, at all temperatures, the selectivity to CO₂ is higher than 99%. 475°C is the minimum temperature at which the filter is regenerated via catalytic path. At this temperature, the catalytic filter maintains substantially the same performance over repeated cycles of soot loading and regeneration, indicating that the thermal stability of ceria is preserved. This has been further confirmed by comparison between the outcomes obtained from characterization (X‐ray powder diffraction, N₂ adsorption at 77 K, Hg intrusion porosimetry, and scanning electron microscope/energy dispersive X‐ray analysis) of fresh filter and filter subjected to repeated regeneration tests. © 2017 American Institute of Chemical Engineers AIChE J, 63: 3442–3449, 2017     

Synergistic Catalysis of Carbon Dioxide Hydrogenation into Methanol by Yttria-Doped Ceria/γ-Alumina-Supported Copper Oxide Catalysts: Effect of Support and Dopant

Methanol synthesis from carbon dioxide hydrogenation was studied over ceria/-alumina- and yttria-doped ceria (YDC)/-alumina-supported copper oxide catalysts to seek insight into the catalysis at metal–support interfaces. It was found that, in comparison with Cu/-Al₂O₃, the Cu/CeO₂/-Al₂O₃ and Cu/YDC/-Al₂O₃ catalysts exhibited substantial enhancement in activity and selectivity toward methanol formation. The extent of enhancement was augmented by increased ceria loading on -alumina and with increased yttria doping into ceria. The enhancement is inferred to result from the synergistic effect between copper oxide and surface oxygen vacancies of ceria.     

The catalytic activity of CuO–CeO2 mixed oxides for diesel soot oxidation with a NO/O2 mixture

Copper doped ceria and ceria–zirconia mixed oxides were synthesized using the citric acid sol–gel method. The temperature-programmed oxidation (TPO) results showed that the Cu modification improved the low-temperature activity and the selectivity to CO₂ of ceria for soot oxidation in the presence of NO and excess oxygen even after ageing at 800 °C for 20 h in flow air. Meanwhile, not only the segregation of Cu and sintering of CuO, but also the separation of Ce- and Zr-rich phases worsened the activity of the Cu–Ce–Zr catalyst after the high-temperature calcination.     

Reverse water–gas shift reaction over ceria nanocube synthesized by hydrothermal method

A well-defined ceria nanocube with six (100) planes was successfully prepared by a facile hydrothermal method. Hydrogenation tests on the carbon dioxide, and several advanced analysis techniques, were used to investigate the catalytic performance of ceria nanocube for reverse water–gas shift (RWGS) and understand the governing reaction mechanism. The results demonstrated that the obtained ceria was a typical mesoporous material with a fluorite structure, and mainly had cerium with + 4 valence oxidation state. As-obtained ceria nanocube showed good performance for RWGS reaction, while nickel on ceria evidently promoted the hydrogenation of CO₂. An oxygen-transformation and metal-dissociation mechanism for RWGS reaction was proposed. The dissociation of carbon dioxide over ceria by directly oxidized oxygen vacancy was considered as a main reaction pathway of RWGS. Meanwhile, dissociated adsorption of CO₂ and hydrogen over nickel surface directly formed CO and supplied spillover hydrogen to nearby oxygen vacancies, respectively. The neighboring oxygen vacancies at the interface of nickel and ceria were considered as efficient active sites for CO₂ hydrogenation.     

Potential rare-earth modified CeO2 catalysts for soot oxidation

The physico-chemical properties of ceria (CeO₂) and rare earth modified ceria (with La, Pr, Sm, Y) catalysts are studied and correlated with the soot oxidation activity with using O₂ and O₂ + NO. CeO₂ modified with La and Pr shows superior soot oxidation activity with O₂ compared with the unmodified catalyst. The improved soot oxidation activity of rare earth doped CeO₂ catalysts can be correlated to the increased meso/micro pore volume and the stabilisation of the external surface area. On the other hand, unreducible ions decrease the intrinsic soot oxidation activity of rare earth modified ceria with both O₂ and NO + O₂ due to the decreased amount of redox surface sites. The catalyst bulk oxygen storage capacity is not a critical parameter in determining the soot oxidation activity. The modification with Pr shows the best soot oxidation with both O₂ and O₂ + NO compared with all other catalysts.