Defect-Site Promoted Surface Reorganization in Nanocrystalline Ceria for the Low-Temperature Activation of Ethylbenzene

Abstract

The study of the defective surface sites of many oxides has received considerable interest, as these sites are coordinately unsaturated and exhibit extraordinary activity in many catalytic reactions. 1 Most of the research work in this particular area is mainly focused on theoretical simulations. 2 However, the influence of the defect sites on the chemical nature of the oxide surface and their role in the mechanism of activation of a substrate have not yet been fully explored. In this Communication, we take the example of ceria and provide direct evidence wherein the defect-site-enriched oxide promotes the surface reorganization and thereby influences the activation of ethylbenzene (EB) in the oxidative dehydrogenation in presence of N 2 O.Ceria is an interesting oxide and is one of the constituents in many catalyst formulations. 3 The method of preparation strongly influences the structural and surface properties of ceria. 4 We intend to explore its catalytic activity in the oxidative dehydrogenation (ODH) of ethylbenzene using N 2 O. A nanocrystalline ceria sample enriched with Ce 3+ -O --Ce 4+ -type defect sites was prepared by the alcoholysis method (ceria-A). For comparison, three other ceria samples were prepared, one by conventional precipitation and the other two by a solution-combustion method using urea and glycine as fuels (ceria-P,-U and -G), respectively, as reported elsewhere. 4 Interestingly, the nanocrystalline ceria prepared by the alcoholysis method exhibits Ce 3+ -O --Ce 4+ -type defect sites predominantly confined to the surface, as evidenced from diffuse reflectance UVvisible spectroscopy (a weak band at around 650 nm). 5 A rough estimate of the surface to bulk defect ratio follows the order Ceria-A > Ceria-G > Ceria-U/P based on the EPR spectroscopic analysis 6 (Supporting Information, Figure S1) and the temperature programmed reduction 7 of the samples ( Figure S2). These defect sites promote the dehydrogenation of EB using N 2 O at 598 K, which is far lower than the temperatures normally encountered in the existing conventional processes for the dehydrogenation of EB (873 K). An equilibrium conversion of 45 mol % and a styrene selectivity of 94%, comparable to that obtained in commercial practice are achieved by the ceria catalyst prepared by the alcoholysis method. However, the ceria samples prepared by conventional and combustion methods showed comparable activity and selectivity to styrene at much higher activation temperatures (Ceria-G at 648 K and Ceria-U/P at 723 K).To address the role of defect sites in governing the temperature of activation, EPR investigations were done on the ceria samples by subjecting each one of them to hydrogen treatment, as the dehydrogenation process involves interaction of hydrogen atoms with the catalyst surface. Figure 1 shows the direct correlation between the concentrations of Ce 3+ -O --Ce 4+ -type defect sites [at g ⊥ ) 1.96 and g | ) 1.933 (D signal) and 1.940 (A signal)] and the temperature of activation. In the case of defect-site-enriched ceria-A...

Ceria: Recent Results on Dopant-Induced Surface Phenomena

“…Recent investigations revealed a high concentration and a high stability of reactive surface Ce 3+ ions over a wide range of temperature and of oxygen partial pressure on ceria surfaces [58], and Ce 3+ and oxygen vacancies are generally believed to be the active sites on ceria surfaces and therefore refer to the Sad adsorption sites [59][60][61]. It is generally assumed that one of the different reaction steps will be rate determining, with the charge transfer step leading to dissociation being the most likely rate determining step [56,62].…”

“…The following reaction scheme would be conceivable in accordance with the current ideas on surface exchange from pure oxygen atmosphere [56,57] CO 2 (g) + S ad ⇒ CO 2 (ad) (10) where S ad represents a CO 2 adsorption site. Recent investigations revealed a high concentration and a high stability of reactive surface Ce 3+ ions over a wide range of temperature and of oxygen partial pressure on ceria surfaces [58], and Ce 3+ and oxygen vacancies are generally believed to be the active sites on ceria surfaces and therefore refer to the S ad adsorption sites [59][60][61].…”

Selective Hydrogenation of Phenol Catalyzed by Palladium on High-Surface-Area Ceria at Room Temperature and Ambient Pressure

“…20 Ceria is the most industrially significant rare earth oxide catalyst mainly due to its use in three-way catalytic converters (TWC) and fluid catalytic cracking (FCC). 19,21 Recently, ceria-based materials have been investigated for use in soot removal from diesel engine exhausts, 22 volatile organic compound (VOC) degradation, 23 fuel cell technology, 24 water-gas shift reaction, 25,26 preferential CO oxidation (PROX), 27 oxidative dehydrogenation, 28 and selective hydrocarbon oxidations. 29,30 The success of ceria and ceria-based materials in catalysis is oftentimes due to facile Ce 4+ /Ce 3+ redox cycling without disruption of the fluorite lattice structure.…”

Probing surface & transport phenomena in energy materials under operating conditions.

“…16 Zr 0.84 O 1.92 (YSZ) (100) single crystal (MTI Corp.) and patterned via metal-liftoff photolithography. The metal pattern consists of line patterns 20 µm in width and spaced 140 µm apart.…”

“…15 Understanding the behavior of surface reactive species is crucial towards establishing the reaction mechanism. Ce 3+ and oxygen vacancies are thought to be the active sites on ceria surfaces [16][17][18] in reactions such as hydrolysis: ( 2 Several studies have shown that the surface concentrations of these active species can be higher than the bulk, potentially explaining ceria's high chemical activity [16][17][18][19] However, the behavior of these active surface species is not well-understood, in part because studies involving mixed-valent materials are complicated by the fact that both the bulk and the surface participate in the catalytic reactions.…”