The preparation of high surface area CeO2-ZrO2 mixed oxides by a surfactant-assisted approach
TL;DR: In this paper, the authors investigated the preparation of high surface area, three-way catalysts (TWC)-like, ceria-zirconia mixed oxide and showed that under basic conditions cationic surfactants effectively incorporate into hydrous oxides of cerium and zirconium.
Abstract: The study investigates the preparation of high surface area, three-way catalysts (TWC)-like, ceria–zirconia mixed oxide. It is shown that under basic conditions cationic surfactants effectively incorporate into hydrous oxides of cerium and zirconium. The presence of cerium inhibits the action of alkyl-trimethyl-ammonium salts as true templating agents and there is no formation of a regular pore structure. The elimination of surfactants upon calcination gives rise to the formation of high surface area, fluorite-structured CeO2–ZrO2 solid solution characterized by a fairly good compositional homogeneity. Surface areas in excess of 230 m2/g were obtained after calcination at 723 K, which drop to ca. 40 m2/g following treatment at 1173 K.
TL;DR: In this paper, the use of CeO 2 -based materials in the automotive three-way catalysts (TWCs) is considered, and the multiple roles of COO 2 as a TWC promoter and in particular the oxygen storage/release capacity (OSC) are critically discussed.
Abstract: In the present paper the use of CeO 2 -based materials in the automotive three-way catalysts (TWCs) is considered. The multiple roles of CeO 2 as a TWC promoter and in particular the oxygen storage/release capacity (OSC) are critically discussed. Attention is focused on the advanced OSC materials containing ZrO 2 , which are employed in the last generation of catalytic automotive converters.
TL;DR: In this article, the authors illustrate the technology for abatement of exhaust emissions by analysing the current understanding of TWCs, the specific role of the various components, the achievements and the limitations.
Abstract: Automotive three-way catalysts (TWCs) have represented over the last 25 years one of the most successful stories in the development of catalysts. The aim of this paper is to illustrate the technology for abatement of exhaust emissions by analysing the current understanding of TWCs, the specific role of the various components, the achievements and the limitations. The challenges in the development of new automotive catalysts, which can meet future highly demanding pollution abatement requirements, are also discussed.
TL;DR: In this article, a series of La 3+ -doped CeO 2 catalysts (La 3+ loading between 5 and 50 wt%) have been studied for soot oxidation by O 2.
Abstract: The catalytic behaviours of CeO 2 and a series of La 3+ -doped CeO 2 catalysts (La 3+ loading between 5 and 50 wt%) have been studied for soot oxidation by O 2 . XRD and Raman spectroscopy characterisation indicated that solid solutions are formed in the studied Ce/La ratio, in which La 3+ cations replace Ce 4+ cations in the CeO 2 lattice. Thermogravimetric analysis showed that La 3+ significantly improves CeO 2 catalytic activity for soot oxidation with O 2 . The best catalytic activity was found with 5 wt% La 3+ -doped CeO 2 catalyst (CeO 2 -5La), in both loose and tight contact conditions. This improvement seems to be related to the increase in BET surface area and the change in the catalyst redox properties of CeO 2 brought about by doping with La 3+ . La 3+ decreases the onset temperature of Ce 4+ to Ce 3+ reduction by H 2 from 580 °C (CeO 2 ) to 325 °C (CeO 2 -5La) and increases the amount of Ce 4+ that can be reduced by H 2 (maximum amount for CeO 2 -5La catalyst). An advanced TAP reactor is used for the first time to study catalysed soot oxidation with labelled oxygen. In the absence of catalyst, oxidation starts above 500 °C, and mainly labelled oxidation species (CO and CO 2 ) were found. In the presence of catalyst, it is shown that the gas-phase labelled oxygen replaces nonlabelled lattice oxygen, creating the highly active nonlabelled oxygen. This highly active nonlabelled oxygen reacts with soot, giving CO and CO 2 . The creation of such active oxygen species starts from 400 °C and thereby decreases the soot oxidation temperature. CeO 2 -5La produces more such active species, for example, leading to 98% oxygen conversion at 400 °C compared with 37% over CeO 2 alone under identical circumstances.
TL;DR: In this paper, the textural and structural properties of catalysts and supports were studied in their calcined, reduced and used state by N 2 adsorption-desorption, XRD, UV-vis DRS, TPR, SEM-EDS and TPH.
Abstract: Nickel catalysts supported on binary CeO 2 –ZrO 2 carriers (28–100% CeO 2 molar content) were prepared and evaluated regarding their catalytic performance for the CO 2 reforming of CH 4 (Dry Reforming of Methane, DRM). The textural and structural properties of catalysts and supports were studied in their calcined, reduced and used state by N 2 adsorption–desorption, XRD, UV–vis DRS, TPR, SEM–EDS and TPH. Zirconium improves the textural properties of the CeO 2 –ZrO 2 supports and the corresponding catalysts and enhances their textural stability under thermal reductive treatment. XRD analysis shows the formation of Ce x Zr 1− x O 2 solid solution for all Ce/(Ce + Zr) ratios. Considerable alterations in the electronic environment of the cations and increased lattice defects in the binary solid solutions were detected by UV–vis DR spectroscopy. A significant increase in the reducibility of both supports and catalysts is observed in the presence of Zr. Compared to the zirconia-free sample, the Ni/CeO 2 -ZrO 2 catalysts exhibited much higher activity for the title reaction, accredited to the increase of the surface concentration of the active sites. However, the amount of carbonaceous deposits is not straightforward related to the activity but depends on the Ce/Zr ratio. Among the zirconium containing catalysts, the zirconium-rich one exhibited the higher activity and the stronger resistance to the formation of carbonaceous deposits.
TL;DR: In this article, high surface area ceria (CeO 2) was synthesized by a surfactant-assisted approach, which has useful dry reforming activity for H 2 and CO production under solid oxide fuel cells (SOFCs) conditions.
Abstract: High surface area ceria (CeO 2 (HSA)), synthesized by a surfactant-assisted approach, was found to have useful dry reforming activity for H 2 and CO production under solid oxide fuel cells (SOFCs) conditions. The catalyst provides significantly higher reforming reactivity and excellent resistance toward carbon deposition compared to Ni/Al 2 O 3 and conventional low surface area ceria (CeO 2 (LSA)) under dry reforming conditions. These enhancements are due to the high redox property of CeO 2 (HSA). During the dry reforming process, the redox reactions between the gaseous components in the system and the lattice oxygen (O x ) take place on ceria surface. Among these reactions, the rapid redox reactions of carbon compounds such as CH 4 , and CO with lattice oxygen (CH 4 + O x → CO + H 2 + O x −1 and CO + O x = CO 2 + O x −1 ) can prevent the formation of carbon species from the methane decomposition and Boudard reactions even at low inlet carbon dioxide concentration. In particular, the dry reforming rate over CeO 2 (HSA) is proportional to the methane partial pressure and the operating temperature. Carbon dioxide presents weak positive impact on the methane conversion, whereas both carbon monoxide and hydrogen inhibit the reforming rate. The activation energies and reforming rates under the same methane concentration for CeO 2 toward the dry reforming are almost equal to the steam reforming as previously reported [1–4] . This result suggests the similar reaction mechanisms for both the steam reforming and the dry reforming over CeO 2 ; i.e., the dry reforming rate is governed by the slow reaction of adsorbed methane, or surface hydrocarbon species, with oxygen in CeO 2 , and a rapid gas–solid reaction between CO 2 and CeO 2 to replenish the oxygen.
TL;DR: In this paper, the synthesis of mesoporous inorganic solids from calcination of aluminosilicate gels in the presence of surfactants is described, in which the silicate material forms inorganic walls between ordered surfactant micelles.
Abstract: MICROPOROUS and mesoporous inorganic solids (with pore diameters of ≤20 A and ∼20–500 A respectively)1 have found great utility as catalysts and sorption media because of their large internal surface area. Typical microporous materials are the crystalline framework solids, such as zeolites2, but the largest pore dimensions found so far are ∼10–12 A for some metallophosphates3–5 and ∼14 A for the mineral cacoxenite6. Examples of mesoporous solids include silicas7 and modified layered materials8–11, but these are invariably amorphous or paracrystalline, with pores that are irregularly spaced and broadly distributed in size8,12. Pore size can be controlled by intercalation of layered silicates with a surfactant species9,13, but the final product retains, in part, the layered nature of the precursor material. Here we report the synthesis of mesoporous solids from the calcination of aluminosilicate gels in the presence of surfactants. The material14,15 possesses regular arrays of uniform channels, the dimensions of which can be tailored (in the range 16 A to 100 A or more) through the choice of surfactant, auxiliary chemicals and reaction conditions. We propose that the formation of these materials takes place by means of a liquid-crystal 'templating' mechanism, in which the silicate material forms inorganic walls between ordered surfactant micelles.
TL;DR: A survey of the use of cerium oxide and CeO2-containing materials as oxidation and reduction catalysts is presented in this paper, with a special focus on catalytic interaction with small molecules such as hydrogen, carbon monoxide, oxygen, and nitric oxide.
Abstract: Over the past several years, cerium oxide and CeO2-containing materials have come under intense scrutiny as catalysts and as structural and electronic promoters of heterogeneous catalytic reactions. Recent developments regarding the characterization of ceria and CeO2-containing catalysts are critically reviewed with a special focus towards catalyst interaction with small molecules such as hydrogen, carbon monoxide, oxygen, and nitric oxide. Relevant catalytic and technological applications such as the use of ceria in automotive exhaust emission control and in the formulation of SO x reduction catalysts is described. A survey of the use of CeO2-containing materials as oxidation and reduction catalysts is also presented.
TL;DR: In this article, a generalized approach to the synthesis of periodic mesophases of metal oxides and cationic or anionic surfactants under a range of pH conditions is presented.
Abstract: THE recent synthesis of silica-based mesoporous materials1,2 by the cooperative assembly of periodic inorganic and surfactant-based structures has attracted great interest because it extends the range of molecular-sieve materials into the very-large-pore regime. If the synthetic approach can be generalized to transition-metal oxide mesostructures, the resulting nanocomposite materials might find applications in electrochromic or solid-electrolyte devices3,4, as high-surface-area redox catalysts5 and as substrates for biochemical separations. We have proposed recently6 that the matching of charge density at the surfactant/inorganic interfaces governs the assembly process; such co-organization of organic and inorganic phases is thought to be a key aspect of biomineralization7. Here we report a generalized approach to the synthesis of periodic mesophases of metal oxides and cationic or anionic surfactants under a range of pH conditions. We suggest that the assembly process is controlled by electrostatic complementarity between the inorganic ions in solution, the charged surfactant head groups and—when these charges both have the same sign—inorganic counterions. We identify a number of different general strategies for obtaining a variety of ordered composite materials.
TL;DR: A neutral templating route for preparing mesoporous molecular sieves is demonstrated based on hydrogen-bonding interactions and self-assembly between neutral primary amine micelles (S�) and neutral inorganic precursors (l�).
Abstract: A neutral templating route for preparing mesoporous molecular sieves is demonstrated based on hydrogen-bonding interactions and self-assembly between neutral primary amine micelles (S degrees ) and neutral inorganic precursors (l degrees ). The S degrees l degrees templating pathway produces ordered mesoporous materials with thicker framework walls, smaller x-ray scattering domain sizes, and substantially improved textural mesoporosities in comparison with M41S materials templated by quaternary ammonium cations of equivalent chain length. This synthetic strategy also allows for the facile, environmentally benign recovery of the cost-intensive template by simple solvent extraction methods. The S degrees 1 degrees templating route provides for the synthesis of other oxide mesostructures (such as aluminas) that may be less readily accessible by electrostatic templating pathways.