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.

Catalytic Properties of Ceria and CeO2-Containing Materials

The infrared spectral performance of the N x O y species observed on oxide surfaces [N2O, NO−, NO, (NO)2, N2O3, NO+, NO2 − (different nitro and nitrito anions), NO2, N2O4, N2O5, NO2, and NO3 − (bridged, bidentate, and monodentate nitrates)] is considered. The spectra of related compounds (N2, H-, and C-containing nitrogen oxo species, C─N species, NH x species) are also briefly discussed. Some guidelines for spectral identification of N x O y adspecies are proposed and the transformation of the nitrogen oxo species on catalyst surfaces are regarded.

Identification of Neutral and Charged N x O y Surface Species by IR Spectroscopy

Automotive catalytic converters: current status and some perspectives

The utilization of ceria in industrial catalysis

Reactions on mineral dust.

The oxidative adsorption of SO 2 on ceria has been studied using gravimetry and IR spectroscopy. SO 2 oxidation gives rise to two main types of sulfates: surface and bulklike species. The former are characterized by IR bands in the 1400-1340 cm −1 range while the latter lead to a very broad band near 1200 cm −1 . Their relative amount depends on the sample surface area and the total sulfate amount formed. The sulfate thermal stability and reducibility are compared to those of sulfate species formed on other oxides. It is also found that oxidation of SO 2 adsorbed species into sulfate can occur without O 2 addition, a concomitant ceria reduction then being evidenced by UV-visible reflectance spectroscopy.

Study of ceria sulfation

Acidic properties and stability of sulfate-promoted metal oxides

SO2 adsorption on different metal oxides (MgO, CeO2, ZrO2, MgAl2O4, TiO2-anatase, TiO2-rutile, Al2O3 and Na–Al2O3) has been studied using various techniques (thermogravimetry, temperature-programmed desorption, IR spectroscopy). Several types of species are formed and the results are discussed in terms of their thermal stability. Weakly adsorbed species can result from the coordination of SO2 on Lewis acid sites such as Al2O3 or TiO2. However, such interactions are quite weak. On more basic oxides, such as Na–Al2O3, SO2 acts as an electron acceptor and adsorbs on either weakly basic O2– sites or on the basic OH– groups. In this latter case, the formation of hydrogen sulfite species is suggested. More strongly adsorbed species occur on all the oxides. These are characterized by strong absorption bands between 1100 and 800 cm–1, and are due to sulfite species. Several types generally occur. Gravimetric measurements allow us to compare the thermal stability and the amount of the different types of adsorbed SO2 species. The quantity of SO2 irreversibly adsorbed after evacuation at T < 370 K gives rise to an evaluation of the number of basic sites and therefore to a scale of basicity of the different oxides.The results are compared with those obtained using other probe molecules such as carbon dioxide or hexafluoroisopropanol. A special case is that of ceria for which it is observed that heating under vacuum transforms sulfite species into sulfates, preventing the use of TPD for the determination of its relative basicity.

Comparative Study of SO2 Adsorption on Metal Oxides.

The nature, thermal stability, and reducibility in H{sub 2} of sulfate species on copper-on-alumina and their mechanism of formation during interaction of the sorbent-catalyst with SO{sub 2}-containing flows were studied in a flow microreactor and by Fourier-transform infrared spectroscopy. Spectra of the sulfate species formed on Al{sub 2}O{sub 3}, CuO, CuAl{sub 2}O{sub 4}, and CuO/Al{sub 2}O{sub 3} samples either by impregnation with various amounts of sulfate salts or by direct sulfation with SO{sub 2} + O{sub 2} are compared. The results indicate that, on pure alumina, two types of surface sulfate species form, one more stable at low surface coverage attributed to a type with only one double S{double bond}O bond and the second less stable and more easily decomposed by water vapor, attributed to a SO{sub 3} group linked to an Al-O pair site or to an oligomer species as S{sub 2}O{sub 7}. Sulfation of CuO leads to bulklike CuSO{sub 4}, whereas sulfation of copper aluminate leads to three types of sulfate species, one linked to Al{sup 3+} ions, another to Cu{sup 2+} ions, and the third to sulfate species in interaction both with Al{sup 3+} and Cu{sup 2+} ions. The analysis of the formation of sulfate species onmore » copper-on-alumina sorbent-catalysts suggests the following mechanism: in the presence of gaseous oxygen, copper performs catalytically the first step of oxidation of SO{sub 2} to SO{sub 3} which then forms a stable surface sulfate at either the copper site or the aluminum site. During the first cycle of interaction of the sorbent catalyst with the SO{sub 2}-containing flow, a sulfate linked mainly to Al sites forms in an amount (about 300-400 {mu}mol/g) equivalent to the limiting value of sulfate species on pure Al{sub 2}O{sub 3}. During the first cycle of reduction, copper aluminate sites are reduced to metallic Cu which give rise to the formation of surface species that are completely regenerated in the consecutive treatment with H{sub 2} at 420C.« less

Nature and mechanism of formation of sulfate species on copper/alumina sorbent-catalysts for sulfur dioxide removal

Moroccan Alfa plant (Stipatenacissima L) was investigated using several techniques: chemical composition, Fourier transform infrared (FTIR), crystallinity index determined by X-ray diffraction and scanning electron microscope (SEM). The raw fiber contained 39.53 wt% of cellulose, 27.63 wt% of hemicellulose, and 19.53 wt% of lignin. The longitudinal view by SEM confirmed the bundle shape. Moreover, a homogeneous distribution of various particles called trichomes with regular forms was recorded. The aim of the research is to improve the properties of Alfa fibers via a mild and effective technique in order to proceed their utilization in construction. To modify the surface, fibers were treated by sodium hydroxide (6 wt%) and hydrothermal treatment at different times. A comparison was carried on the untreated and treated Alfa fibers by several techniques. Enhanced properties were obtained with 6 h of treatment by sodium hydroxide and 1 h by hydrothermal treatment. Both treatments exhibit a modification of the fibers microstructure and morphology; FTIR and chemical analysis confirmed the hemicellulose and lignin reduction after 6 h of alkali treatment and 1 h of hydrothermal treatment. As a result, an improvement of the crystallinity index was noticed. SEM micrographs also confirmed an enhancement of the fibers roughness after treatment.

Mohamed Waqif

Papers

Study of ceria sulfation

Acidic properties and stability of sulfate-promoted metal oxides

Comparative Study of SO2 Adsorption on Metal Oxides.

Nature and mechanism of formation of sulfate species on copper/alumina sorbent-catalysts for sulfur dioxide removal

Modifications of Alfa fibers by alkali and hydrothermal treatment