The Ba-hexaaluminate doped with CeO2 nanoparticles for catalytic combustion of methane

The Ba-hexaaluminate doped with CeO₂ nanoparticles with high surface area for catalytic combustion have been prepared by using the alumina sol as the (NH₄)₂CO₃ coprecipitation precursor and supercritical drying method. The catalysts are composed of the rod-like particles and granular ones. The CeO₂/BaAl₁₂O_19−α catalyst possesses the highest surface area (83.5 m²/g) and the smallest CeO₂ mean crystallite size (24.3 nm). Introduction of transition metal ion into the Al₂O₃ spinel leads to the increase of the catalytic activity. Nevertheless the hexaaluminate cannot be obtained when further increasing the introduction, the components of the main crystalline phases are Al_2.267O₄ and CeO₂. The CeO₂/BaFeMnAl₁₀O_19−α catalyst possesses the lowest complete conversion of methane temperature, probably due to the high surface area and the excellent performance of activating oxygen.     

Nanocatalysis on Supported Oxides for CO Oxidation

Active gold and palladium nanoparticles supported on a variety of oxides (CeO₂, ZrO₂, Al₂O₃, SiO₂, MgO and ZnO) were synthesized using laser vaporization and microwave irradiation methods. The catalytic activities for CO oxidation on the nanoparticle catalysts were evaluated and compared among different oxide supports. The effect of shape on the catalytic activity is demonstrated by comparing the activities of the Au and Pd catalysts deposited on MgO nanocubes and ZnO nanobelts. The Au/CeO₂ nanoparticles deposited on MgO nanocubes exhibit high catalytic activity and stability. The enhanced catalytic activity is attributed to the presence of a significant concentration of the corner and edge sites in MgO nanocubes. The Au- and Pd-doped Mn₂O₃ nanoparticles show promising results for the low temperature CO oxidation. Several approaches for incorporating the Au and Pd nanocatalysts within mesoporous oxide supports are presented and discussed.     

Improving the dispersion of CeO2 on γ-Al2O3 to enhance the catalytic performances of CuO/CeO2/γ-Al2O3 catalysts for NO removal by CO

Acetic acid (HAc) aqueous was used as solvent in wetness impregnation to prepare CeO₂-modified γ-Al₂O₃ support. With the help of HAc, the dispersion of CeO₂ on γ-Al₂O₃ is significantly improved and the size of CeO₂ nanoparticles can be controlled through tuning the concentration of HAc aqueous. XPS analysis shows that the percentages of Ce^3 + in CeO₂ nanoparticles will vary with the size. Then we load CuO on the as-prepared CeO₂-modified γ-Al₂O₃ support and choose NO reduction with CO as a probe reaction to investigate the influences of impregnation solvent on the catalytic properties. The results demonstrate that the CuO/CeO₂/γ-Al₂O₃ prepared in the solvent with volume ratio of 20:1 (H₂O:HAc) has the highest activity in NO + CO reaction. Combing the structural characterizations and catalytic performances, we think that the size of the CeO₂ nanoparticles should be a key factor that affects the activities of CuO/CeO₂/γ-Al₂O₃. Furthermore, CuO dispersed on CeO₂ nanoparticles with an average size of ca. 5 nm should be the highest active sites for NO + CO reaction.     

Structure-activity Relation of Fe2O3–CeO2 Composite Catalysts in CO Oxidation

A series of Fe₂O₃–CeO₂ composite catalysts were synthesized by coprecipitation and characterized by X-ray diffraction (XRD), BET surface area measurement, Raman spectroscopy, and X-ray photoelectron spectroscopy (XPS). Their catalytic activities in CO oxidation were also tested. The Fe₂O₃–CeO₂ composites with an Fe molar percentage below 0.3 form solid solutions with the CeO₂ cubic fluorite structure, in which the doped Fe³⁺ initially substitutes Ce⁴⁺ in fluorite cubic CeO₂, but then mostly locate in the interstitial sites after a critical concentration of doped Fe³⁺. With an Fe molar percentage between 0.3 and 0.95, the Fe₂O₃–CeO₂ composites are mixed oxides of the cubic fluorite CeO₂ solid solution and the hematite Fe₂O₃. XPS results indicate that CeO₂ is enriched in the surface region of Fe₂O₃–CeO₂ composites. The Fe₂O₃–CeO₂ composites have much higher catalytic activities in CO oxidation than the individual pure CeO₂ and Fe₂O₃, and the Fe_0.1Ce_0.9 composite shows the best catalytic performance. The structure-activity relation of the Fe₂O₃–CeO₂ composites in CO oxidation is discussed in terms of the formation of solid solution and surface oxygen vacancies. Our results demonstrate a proportional relation between the catalytic activity of cubic CeO₂-like solid solutions and their density of oxygen vacancies, which directly proves the formation of oxygen vacancies as the key step in CO oxidation over oxide catalysts.     

NO oxidation over supported cobalt oxide catalysts

NO oxidation was conducted over cobalt oxides supported on various supports such as SiO₂, ZrO₂, TiO₂, and CeO₂. The N₂ physisorption, an inductively coupled plasma-atomic emission spectroscopy (ICP-AES), X-ray diffraction (XRD), NO chemisorptions, the temperature-programmed desorption (TPD) with a mass spectroscopy after NO or CO chemisorptions were conducted to characterize catalysts. Among tested catalysts, Co₃O₄ supported on ceria with a high surface area showed the highest catalytic activity. This catalyst showed superior catalytic activity to unsupported Co₃O₄ with a high surface area and 1 wt% Pt/γ-Al₂O₃. For ceria-supported Co₃O₄, the catalytic activity, the NO uptake at 298 K and the dispersion of Co₃O₄ increased with increasing the surface area of CeO₂. The active participation of the lattice oxygen in NO oxidation could not be observed. On the other hand, the lattice oxygen participated in the CO oxidation over the same catalyst. The deactivation was observed over Co₃O₄/CeO₂ and 1 wt% Pt/γ-Al₂O₃ in the presence of SO₂ in a feed. 1 wt% Pt/γ-Al₂O₃ was deactivated by SO₂ more rapidly compared with Co₃O₄/CeO₂.     

Promotion and Inhibition of the Oxidase-Mimicking Activity of Nanoceria by Phosphate,Polyphosphate, and DNA

Nanoceria (CeO₂ nanoparticles) is an extensively studied nanozyme with interesting oxidase-mimicking activity. As they can work in the absence of toxic and unstable H₂O₂, CeO₂ nanoparticles have been widely used in biosensing. CeO₂ nanoparticles often encounter phosphate-containing molecules that can affect their catalytic activity, and various reports exist in the literature showing both promoted and inhibited activity. In this work, we systematically studied five types of phosphate: orthophosphate, pyrophosphate, triphosphate, trimetaphosphate, and a polyphosphate with 25 phosphate units (Pi₂₅). In addition, DNA oligonucleotides of various length and sequence. DNA was included as they contain a phosphate backbone that can strongly adsorb on nanoceria. We observed that a high concentration of DNA in acetate buffer inhibited activity, whereas a low concentration of DNA in phosphate buffer increased activity. The change of activity was also related to the type of substrate and related to the aggregation of CeO₂. These discoveries provide an important understanding for the further use of CeO₂ nanoparticles in biosensor development, materials science, and nanotechnology.     

Sol–gel derived CeO2/α‐Al2O3 bilayer thin film as an anti‐coking barrier and its catalytic coke oxidation performance

A CeO₂/α‐Al₂O₃ bilayer was coated on a high temperature alloy (Incoloy 800H) by sol–gel dip‐coating and was evaluated for its potential as an anticoking barrier and coke oxidation catalyst. The bilayer effectively functioned as a barrier to metal surface catalyzed coking. The film prevented filamentous catalytic coking via blocking surface active metallic sites on the Incoloy substrate. Furthermore, the bilayer reduced the oxidation temperature of pyrolytic coke deposited on the film surface as compared to a bare oxidized Incoloy substrate, mostly owing to the oxidation catalytic activity of the CeO₂ layer. Finally, it is demonstrated that the presence of the α‐Al₂O₃ buffer layer is critically important to the overall performance. Without the α‐Al₂O₃ layer, a CeO₂ layer nearly completely lost both its barrier and oxidation catalytic functions. It is presumed that metallic species migrating from the substrate during high temperature treatments are responsible for the CeO₂ deactivation, likely by blocking catalytic sites on the CeO₂ surface. © 2018 American Institute of Chemical Engineers AIChE J, 64: 4019–4026, 2018     

Effect of support on the apparent activity of palladium oxide in catalytic methane combustion

The support effect on the low temperature catalytic oxidation of methane over palladium catalysts was studied by comparing a series of metal oxides as the support. Samples of 0.010 g/g Pd catalysts supported on different grades and/or phases of TiO₂, Al₂O₃, and ZrO₂ were prepared via incipient impregnation and their catalytic activity was evaluated using a laboratory plug-flow reactor. The specific surface area of the supports determined by nitrogen adsorption varied from about 13-220 m²/g. Initial experiments conducted with titania (anatase) as a support showed a low apparent activity and a poor thermal stability. Focusing on anatase, we have successfully improved its thermal stability by additions of Al₂O₃ or by doping with CeO₂, or La₂O₃. However, contrary to expectations based on some information in the literature, we have found that the activity decreased in the sequence of Al₂O₃ > ZrO₂ > TiO₂, and was not a direct function of specific surface area. This was especially evident in the case of titania. The surface structure of the support and the nature of its interaction with the active component PdO seem to play a far more important role in activity than the apparent specific surface area. Moreover, anatase-supported catalysts present a very rapid deactivation, whereas rutile-supported catalysts are relatively stable. The observed phenomena could potentially be related to the interaction between support and the active phase of palladium. Several models have been proposed to describe the strong metal-support interaction, but either charge transfer or encapsulation seems to be the most probable.     

Reduction of CO<Subscript>2</Subscript> to CO via reverse water-gas shift reaction over CeO<Subscript>2</Subscript> catalyst

CeO₂ catalysts with different structure were prepared by hard-template (Ce-HT), complex (Ce-CA), and precipitation methods (Ce-PC), and their performance in CO₂ reverse water gas shift (RWGS) reaction was investigated. The catalysts were characterized using XRD, TEM, BET, H₂-TPR, and in-situ XPS. The results indicated that the structure of CeO₂ catalysts was significantly affected by the preparation method. The porous structure and large specific surface area enhanced the catalytic activity of the studied CeO₂ catalysts. Oxygen vacancies as active sites were formed in the CeO₂ catalysts by H₂ reduction at 400 °C. The Ce-HT, Ce-CA, and Ce-PC catalysts have a 100% CO selectivity and CO₂ conversion at 580 °C was 15.9%, 9.3%, and 12.7%, respectively. The highest CO₂ RWGS reaction catalytic activity for the Ce-HT catalyst was related to the porous structure, large specific surface area (144.9 m²?g^?1) and formed abundant oxygen vacancies.     

Preparation and characterization of nanostructured Ce0.9Cu0.1O2−δ solid solution with high surface area and its application for low temperature CO oxidation

Nanostructured Ce_0.9Cu_0.1O_2−δ solid solution with high surface area was prepared by improved citrate sol–gel method with incorporation of thermal treatment under N₂. The sample was characterized by TG–DSC, BET nitrogen adsorption, XRD and H₂-TPR. Its catalytic activity for CO oxidation was tested. It was found that the improved method offered catalysts with higher surface area and smaller crystallite size, which led to higher catalytic activity for low temperature CO oxidation. H₂-TPR measurement indicated that there were three CuO species in the Ce_0.9Cu_0.1O_2−δ solid solutions: finely dispersed CuO species on the surface of CeO₂, partial Cu²⁺ penetrated into CeO₂ lattice and bulk CuO phase. The finely dispersed CuO species was regarded as the active site for the low temperature CO oxidation.     

The Effect of Acidic and Redox Properties of V2O5/CeO2–ZrO2 Catalysts in Selective Catalytic Reduction of NO by NH3

V₂O₅ supported ZrO₂ and CeO₂–ZrO₂ catalysts were prepared and characterized by N₂ physisorption, XRPD, TPR, and NH₃-TPD methods. The influence of calcination temperature from 400 to 600 °C on crystallinity, acidic and redox properties were studied and compared with the catalytic activity in the selective catalytic reduction (SCR) of NO with ammonia. The surface area of the catalysts decreased gradually with increasing calcination temperature. The SCR activity of V₂O₅/ZrO₂ catalysts was found to be related with the support crystallinity, whereas V₂O₅/CeO₂–ZrO₂ catalysts were also dependent on acidic and redox properties of the catalyst. The V₂O₅/CeO₂–ZrO₂ catalysts showed high activity and selectivity for reduction of NO with NH₃.     

The Effect of Modifiers on the Performance of Ni/CeO2 and Ni/La2O3 Catalysts in the Oxy–Steam Reforming of LNG

This work interrogates for the first time the catalytic properties of various monometallic Ni catalysts in the oxy-steam reforming of LNG. Various research techniques, including X-ray diffraction (XRD), specific surface area and porosity analysis (BET method), scanning electron microscopy with X-ray microanalysis (SEM-EDS), temperature-programmed desorption of ammonia (TPD-NH₃), temperature-programmed reduction (TPR-H₂) and the FTIR method, were used to study their physicochemical properties. The mechanism of the oxy-steam reforming of LNG is also discussed in this paper. The high activity of monometallic catalysts supported on 5% La₂O₃–CeO₂ and 5% ZrO₂–CeO₂ oxides in the studied process have been proven and explained on the basis of their acidity, specific surface area, sorption properties in relation to the reaction products, the crystallite size of the metallic nickel and their phase composition.     

Effect of CeO2 on Supported Pd Catalyst in the SCR of NO: A DRIFT Study

The activity of Pd/Al₂O₃ and Pd/Al₂O₃–CeO₂ samples has been tested in the selective catalytic reduction of NO by propene. It is found that the activity of Pd/Al₂O₃ decreases with calcination temperature, while the activity of Pd/Al₂O₃–CeO₂ increases abnormally with increasing calcination temperature. Surface-area measurement shows both samples suffer a linear decrease in their surface area, so it is reasonable to attribute the activity enhancement to the effect of CeO₂. The adsorption behavior and state of surface-active sites have been characterized by diffuse reflectance FTIR spectroscopy using CO and NO as probes and the effect of CeO₂ has been revealed. The CeO₂ component increases and stabilizes the dispersion of surface Pd species to prevent it from aggregating at high temperature. CeO₂ may also act as a buffer during the redox cycle of Pd, lengthen the period of Pd redox procedure and render Pd a property of inertia in its redox process, thus increasing the activity of the Pd/Al₂O₃–CeO₂ sample. The essential feature of both effects is the strong interaction between Pd and CeO₂. The intensity of interaction increases linearly with calcination temperature and so does the sample activity.     

Synthesis,characteristics, and redox properties of Zr/Sn-doped CeO2 nanoparticles in H2 gas at high temperatures

Metal(M = Zr, Sn)-doped CeO₂ nanoparticles were synthesized by a hydrothermal process to develop PdO@M-doped CeO₂ catalysts. The average particle size of both M-doped CeO₂ nanoparticles was under 10 nm, whereas the particle size reduced as the dopant concentration increased in the M-doped CeO₂ nanoparticles. The largest specific surface area was 226 m²/g in the Zr-doped CeO₂ nanoparticles. The particle morphology showed a spherical shape in both M-doped CeO₂ nanoparticles. The PdO@M-doped CeO₂ catalysts were then prepared by adsorbing Pd(OH)₂ onto the surface of M-doped CeO₂ nanoparticles by a precipitation method and synthesizing the catalysts by calcining at 500 °C for 3 h. The H₂ consumptions of the PdO@M-doped CeO₂ catalysts were characterized as the oxygen storage capacity at various temperatures. The results show that the oxygen storage capacities of the PdO@M-doped CeO₂ catalysts are superior to that of the pure CeO₂ catalyst at temperatures higher than 550 °C. The oxygen storage capacity of the PdO@Sn-doped CeO₂ catalyst is better than that of the PdO@Zr-doped CeO₂ catalyst.     

High temperature desulfurization over nano-scale high surface area ceria for application in SOFC

In the present work, suitable absorbent material for high temperature desulfurization was investigated in order to apply internally in solid oxide fuel cells (SOFC). It was found that nano-scale high surface area CeO₂ has useful desulfurization activity and enables efficient removal of H2S from feed gas between 500 to 850°C. In this range of temperature, compared to the conventional low surface area CeO₂, 80–85% of H2S was removed by nano-scale high surface area CeO₂, whereas only 30–32% of H2S was removed by conventional low surface area CeO₂. According to the XRD studies, the product formed after desulfurization over nano-scale high surface area CeO₂ was Ce₂O₂S. EDS mapping also suggested the uniform distribution of sulfur on the surface of CeO₂. Regeneration experiments were then conducted by temperature programmed oxidation (TPO) experiment. Ce₂O₂S can be recovered to CeO₂ after exposure in the oxidation condition at temperature above 600°C. It should be noted that SO₂ is the product from this regeneration process. According to the SEM/EDS and XRD measurements, all Ce₂O₂S forming is converted to CeO₂ after oxidative regeneration. As the final step, a deactivation model considering the concentration and temperature dependencies on the desulfurization activity of CeO₂ was applied and the experimental results were fitted in this model for later application in the SOFC model.     

CuO and CuO-ZnO catalysts supported on CeO2 and CeO2-LaO3 for low temperature water-gas shift reaction

This paper describes an investigation on CuO and CuO-ZnO catalysts supported on CeO₂ and CeO₂-La₂O₃ oxides, which were designed for the low temperature water-gas shift reaction (WGSR). Bulk catalysts were prepared by co-precipitation of metal nitrates and characterized by energy-dispersive spectroscopy (EDS), X-ray diffraction (XRD), surface area (by the BET method), X-ray photoelectron spectroscopy (XPS), and in situ X-ray absorption near edge structure (XANES). The catalysts' activities were tested in the forward WGSR, and the CuO/CeO₂ catalyst presented the best catalytic performance. The reasons for this are twofold: (1) the presence of Zn inhibits the interaction between Cu and Ce ions, and (2) lanthanum oxide forms a solid solution with cerium oxide, which will cause a decrease in the surface area of the catalysts. Also the CuO/CeO₂ catalyst presented the highest Cu content on the surface, which could influence its catalytic behavior. Additionally, the Cu⁰ and Cu¹⁺ species could influence the catalytic activity via a reduction-oxidation mechanism, corroborating to the best catalytic performance of the Cu/Ce catalyst.     

Catalytic Efficiency of Ceria–Zirconia and Ceria–Hafnia Nanocomposite Oxides for Soot Oxidation

The catalytic oxidation of soot particulates has been investigated over CeO₂, CeO₂–ZrO₂ and CeO₂–HfO₂ nanocomposite oxides. These oxides were synthesized by a modified precipitation method employing dilute aqueous ammonia solution. The prepared catalysts were characterized by X-ray diffraction (XRD), transmission electron microscopy (TEM), Raman spectroscopy, UV-Vis diffuse reflectance spectroscopy (UV-Vis DRS) and BET surface area methods. The soot oxidation has been evaluated by a thermogravimetric method under ‘tight contact’ conditions. The XRD results revealed formation of cubic CeO₂, Ce_0.75Zr_0.25O₂ and Ce_0.8Hf_0.2O₂ phases in case of CeO₂, CeO₂–ZrO₂ and CeO₂–HfO₂ samples, respectively. TEM studies confirm the nanosized nature of the catalysts. Raman measurements suggest the presence of oxygen vacancies, lattice defects and oxide ion displacement from normal ceria lattice positions. UV-Vis DRS studies show presence of charge transfer transitions Ce³⁺←O^2? and Ce⁴⁺←O^2? respectively. The catalytic activity studies suggest that the oxidation of soot could be enhanced by incorporation of Zr⁴⁺ and Hf⁴⁺ into the CeO₂ lattice. The CeO₂–HfO₂ combination catalyst exhibited better activity than the CeO₂–ZrO₂. The observed high activity has been related to the nanosized nature of the composite oxides and the oxygen vacancy created in the crystal lattice.     

Hydrogen Production by Steam Reforming of Fusel Oil Using a CeCoOx Mixed-Oxide Catalyst

Various CeCoO_x mixed-oxide catalysts with different Ce/Co ratios were prepared by surfactant-assisted template precipitation of CeO₂ and Ce₃O₄. The obtained catalysts were characterized by X-ray diffraction, X-ray photoelectron spectra, hydrogen temperature-programmed reduction, nitrogen physisorption, and transmission electron microscopy. In general, the mixed-oxide CeCoO_x catalysts showed well-dispersed CeO₂ and Co₃O₄ and good catalytic characteristics including a high specific surface area and porous structure. The effectiveness of the prepared catalysts on the hydrogen (H₂) production from steam reforming of fusel oil was studied in a packed-bed reactor. Co played an important role in C–C scission to break down the large C_2–5 molecules into smaller species resulting in H₂ formation. Ce could provide supplementary active oxygen to prevent coke formation on Co, resulting in a more stable activity of the mixed-oxide catalyst throughout the reaction course.     

Au/Ce1−xZrxO2 as effective catalysts for low-temperature CO oxidation

The physico-chemical properties and activity of Ce-Zr mixed oxides, CeO₂ and ZrO₂ in CO oxidation have been studied considering both their usefulness as supports for Au nanoparticles and their contribution to the reaction. A series of Ce_1−xZr_xO₂ (x = 0, 0.25, 0.5, 0.75, 1) oxides has been prepared by sol–gel like method and tested in CO oxidation. Highly uniform, nanosized, Ce-Zr solid solutions were obtained. The activity of mixed oxides in CO oxidation was found to be dependent on Ce/Zr molar ratio and related to their reducibility and/or oxygen mobility. CeO₂ and Ce_0.75Zr_0.25O₂, characterized by the cubic crystalline phase show the highest activity in CO oxidation. It suggests that the presence of a cubic crystalline phase in Ce-Zr solid solution improves its catalytic activity in CO oxidation. The relation between the physico-chemical properties of the supports and the catalytic performance of Au/Ce_1−xZr_xO₂ catalysts in CO oxidation reaction has been investigated. Gold was deposited by the direct anionic exchange (DAE) method. The role of the support in the creation of catalytic performance of supported Au nanoparticles in CO oxidation was significant. A direct correlation between activity and catalysts reducibility was observed. Ceria, which is susceptible to the reduction at the lowest temperature, in the presence of highly dispersed Au nanoparticles, appears to be responsible for the activity of the studied catalysts. CeO₂-ZrO₂ mixed oxides are promising supports for Au nanoparticles in CO oxidation whose activity is found to be dependent on Ce/Zr molar ratio.     

Water–Gas Shift Reaction over CuO/CeO2 Catalysts: Effect of the Thermal Stability and Oxygen Vacancies of CeO2 Supports Previously Prepared by Different Methods

A series of CuO/CeO₂ catalysts were prepared through a two-step process: (1) CeO₂ supports were firstly prepared by precipitation (P), hydrothermal (HT) and sol-gel (SG) methods, respectively; and (2) CuO was deposited on the above CeO₂ supports by deposition-precipitation method. The as-synthesized CeO₂ supports and CuO/CeO₂ catalysts were characterized by N₂-physisorption, XRD, XPS, Raman, and H₂-TPR. The CuO/CeO₂ catalysts were examined with respect to their catalytic activity for the water–gas shift reaction, and their catalytic activities are ranked as: CuO/CeO₂-P > CuO/CeO₂-HT > CuO/CeO₂-SG. The results suggest that the CeO₂ prepared by precipitation (i.e., CeO₂-P-300) has the best thermal stability and the most amounts of surface oxygen vacancies, which make the corresponding CuO/CeO₂-P catalyst present the largest pore volume, the smallest crystal size of CuO, the highest microstrain (i.e., the highest surface energy) and the most amounts of active sites (i.e., the moderate copper oxide (crystalline) interacted with surface oxygen vacancies of ceria). Therefore, the catalytic activity of CuO/CeO₂ catalysts, in nature, depends on the thermal stability and the number of surface oxygen vacancies of the CeO₂ supports previously prepared by different methods.