Ceria-alumina

For environmental reasons, reaction (Eqn. 21) (NO -> N2) should be promoted, N20 having a dramatic greenhouse gas effect. The different steps of reaction (Eqn. 23) have been investigated in detail, mainly by FTIR spectroscopy [61-63], One of the possible intermediate is isocyanate. NCO species could be formed on the metal and migrate on the support, which may explain the large differences observed when Rh is supported on different oxides (alumina, silica, zirconia, ceria-alumina, etc.). However, the main step should be the dissociative adsorption of NO ... 

Table 90 Performance data of microstructured Pt/ceria-alumina catalysts versus powdered...

VOv supported on ai2o3 VO, supported on silica, ceria, alumina, zirconia, niobia, titania-silica, zirconia-silica VOr supported on alumina, silica Dehydration Dehydration at 773 K in 02/He, methanol adsorption 02/He at 773 K, adsorption of isopropanol... 

Figure 3.15 FTIR spectra of pure powder pressed disks of sulfated ceria-alumina, V205-W03-Ti02 catalyst and W03-Zr02 catalyst.

Furthermore, it was clearly shown that noble metals, rhodium in particular, play an active role in promoting the OSC of the support [1,3-6,31]. It was shown for example that only Rh can really promote OSC on alumina catalysts [32,33]. Nevertheless the situation changes when ceria is added to alumina. A comparative study of alumina and ceria-alumina supported bimetallic catalysts [32,33] demonstrated the differences between those systems. Ceria-alumina catalysts were shown to have higher OSC values which do not depend on the composition of the bimetallic (Fig. 7.3). 

Figure 7J. Effecl of the metallic phase composition on the OSC at 4S0X for alumina (triangles) and ceria-alumina (squares) supported bimetallic PtRh catalysts.

Furthermore, a closer look to experimental data and a tentative balance between CO consumption, CO2 formation and O2 uptake showed the irreversible loss of oxygen on Pt/A Os as well as the significant retention of carbon on ceria-alumina supports. In fact surface carbonated species were clearly identified by FT-IR spectroscopy (1200-1600 cm" ) upon CO adsorption [34-36]. An even more detailed study of Pt/A Oa and Pt/Ce02 systems [37] evidenced that the OSC of alumina systems mainly originates from the WGS reaction with alumina surface OH groups. Furthermore, in the case of ceria-supported catalysts, three contributions to the OSC could be distinguished the OSC related to the metal particle (M/MO), the OSC of ceria and the OSC originating from the metal-ceria contact. 

Figure 7.4. Effect of temperature on the OSC of alumina (open symbols) and ceria-alumina (closed symbols) supported Rh (squares) and Pt (triangles) catalysts.

In Table 1 the BET surface areas are listed for the two samples, mono 1 and 2 and the Pd-Pt/mono2. For comparison the specific values of the bare monolith, the reference ceria-alumina wash-coat and the reference powder Co30Ce are enclosed. From this table, it can be noticed that the sample mono 1, obtained by impregnation method possess lower surface area than mono 2, synthesized by dip-coating, probably because the impregnation route causes in the wash-coat some pore blocking, reducing the specific surface. 

Table 1. BET surface area (m /g) of monolithic catalysts. The specific surface area of the bare cordierite, ceria-alumina washcoat and Co30Ce powder are also...

Ce02 from Rhone-Poulenc and with Sbet 1 10 m /g (sample A) was used as received. Ceria/alumina samples with nominal Ce02 loads of 30% (sample B) and 10% w/w (sample C) were prepared by incipient wetness method using alumina (Condea, high purity Puralox grade, Sbet=210 m2/g, mixture of y and 5 forms this material is called here sample D) and an aqueous solution of Ce(N03)3 (Fluka) the resulting solid was first dried 24 h in air at 395 K, then calcined imder flow of dry air at 773 K during 3 h. In the preparation of the catalyst for experiments in the gas flow reactor, tlie alumina used was in form of small spheres, ca. 1 mm dia. for the specimens used in spectroscopic measurements it was in powder form. Samples B and C presented after calcination Sbet= 165 and 195 m /g respectively. 

Ceria and alumina, which form the intermediate porous layer (the washcoat) between the mechanical support and the supported metals and promotors, are important components in three-way catalysts (TWC) used for car exhaust gas cleaning. Although basie studies have been published on such systems [1], the interactions which can exist between alumina and eeria and which in turn may affect the interactions between the supported metal and the washcoat [2], are not fully understood. In particular, the multiple roles attributed to ceria, like stabilization of the alumina [3], of the supported metals [4], like storage and release of oxygen [2], are veiy probably dependent on preparation methods, activation and reaction conditions. Therefore, selective characterization of each oxide in ceria-alumina can be useful for a better imderstanding of the role played by both alumina and ceria in TWC. The present work describes two "ehemical methods", the adsorption of carbon dioxide followed by IR spectroseopy and the temperature-progranuned reduction (TPR), which lead to an estimation of the surface extent of alumina and ceria respectively. 

Ceria-aluminas were studied by nitrogen adsorption at 77 K, in an automated volumetric set up built in the laboratory after a vacuum desorption at 773 K. All the BET measurements were refered to the initial mass of the sample. 

FTIR spectroscopy (BRUKER IFS 110) study of the adsorption of carbon dioxide on the solids was used to determine the free alumina surface area. More precisely, the adsorption of CO2 onto hydroxyl groups of the alumina produced characteristic bands of hydrogenocarbonate species [6,7], The optical density of the band at 1235 cm l, (5 C-O-H bending mode), after taking into accoimt the bands displayed by ceria, was measured and used to determine the free alumina surface. From this method and by comparison with total BET surface area, a ceria surface has been estimated. Ceria-aluminas, alumina and ceria were used as self supported wafers (0.01 to 0.03 g/cm ) they were thermally treated up to 673 K, under oxygen and then under high vacuum, in situ, before adsorption of CO2 at room temperature. Then, the catalysts were evacuated at 295, 373, 473 and 573 K, for 1 h. IR spectra were recorded at room temperature, after CO2 adsorption and after each desorption temperature. 

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