Surface characterization of alumina-supported ceria

Abstract

The rdf for the 21.4 wt % sample differs from the ones for the samples with lower loading, especially with respect to the relatively high intensity of the peak at 0.3 nm. This points to a high degree of polymerization, which is in accordance with the expected formation of precipitates. This rdf also differs from that of the AHM reference compound. This suggests that the precipitate has a different structure. No attempt was made to unravel this structure. The amount of Mo(IV) present in this sample is still consistent with the adsorption model, however.During the preparation of catalysts by impregnation, the time allowed to reach equilibrium is often about 1 h or even less. In the case of the AHM/y-Al203 system, the EXAFS results indicate that 1 h is too short for equilibrium to be attained, since it was found that in the impregnated sample less Mo(IV) is present than in the material obtained in an adsorption experiment. ConclusionsThe present EXAFS study has resulted in a modification of the model describing the adsorption of AHM onto 7-Al203. It appears that two adsorption sites are necessary: (1) basic OH groups, leading to adsorption of Mo(IV); (2) coordinatively unsaturated Al3+ sites, leading to adsorption of Mo(VI).Of the site-blocking reactions employed here, the F exchange of basic OH groups gives results in line with the above adsorption model, but the reaction of Al3+ sites with acacH leads to more ambiguous results.A normal catalyst impregnation time of about 1 h is too short for the reaction of AHM with the basic OH groups to proceed to completion.

Origin of the redox deactivation phenomena in modified alumina‐supported Ce/Pr mixed oxide

“…In this phase, which is quite stable, lanthanide cations are in the trivalent oxidation state, and can be consequently responsible of the blocking of Ce 3+ described in this article. [9,10] On the other hand, XRD diagram for P20C/Al 2 O 3 -Si reduced at 900 • C and reoxidized at 200 • C is quite different. In this case, perovskite-type, as well as other fluorite-related reoxidable phases coexist.…”

“…[8] In alumina-supported mixed oxides, redox deactivation can be attributed to the incorporation of reduced trivalent cations into the structure of alumina. [9,10] This produces the formation of an aluminate with perovskite-like structure. Reduced trivalent cations are thus stabilized and their reoxidation to the tetravalent oxidation state is avoided.…”

Phosphoric acid adsorbed on silica-ceria matrix obtained by sol-gel method: studies of local structure, texture and acid property

“…The SCP samples present cerium as Ce 4+ . [30][31][32][33][34] Phosphorus presented only one photoemission peak, which is assigned to the P 2p 3/2 binding energy at ca. 134.0 eV (average value).…”

COMPLETE OXIDATION OF METHANE OVER PT/CEO2-Al2O3CATALYSTS

“…The peak at the lowest temperature, approximately 125 C, is assigned to elimination of water physisorbed on the catalyst. The peak at 250 C could be attributed to the reduction of both Pt and superficial CeO 2 , and the peak at 450 C can be attributed to the formation of nonstoichiometric cerium oxide (Shyu et al, 1988aSantos et al, 2005. It is observed that the peak attributed to a reduction of superficial ceria is moved slightly to greater temperature as the content of ceria is increased.…”