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[Display omitted] • The surface of cobalt was monitored by operando IR under CO 2 -CO-H 2 mixtures. • The selectivity to methane decreased monotonously with CO surface coverage. • The state of the cobalt metallic surface remained unaffected by the CO/CO2 ratio. An operando IR study was carried out to investigate the loss of selectivity to higher hydrocarbons observed over cobalt-based Fischer-Tropsch catalysts when CO is replaced with CO 2. Our data unambiguously show that the structure and oxidation state of the surface of metallic cobalt remains the same whether CO 2 or CO are cofed with H 2. The selectivity to CH 4 appears to increase monotonously with decreasing surface coverage of adsorbed CO. The switch from Fischer-Tropsch synthesis to methanation using CO 2 -rich feeds over metallic cobalt can therefore be assigned to surface coverage effects only, assuming that most, if not all, hydrocarbons products are formed on metallic cobalt. [ABSTRACT FROM AUTHOR]

• Flame Sprayed Pyrolysis (FSP) cerium-zirconium oxides show high catalytic activities. • Coprecipitated (CP) and FSP mixed oxides exhibits different surface properties. • FSP enables a surface enrichment in cerium oxide at the top layer. • Cerium enrichment gives higher concentration of adsorbed methanol at the surface. The direct synthesis of Dimethyl carbonate (DMC) from methanol and CO 2 is a green process which allows CO 2 valorization. Among efficient catalysts, ceria, zirconia and cerium-zirconium mixed oxides are often reported as the most active catalysts. In a recent report, we discovered that cerium-zirconium mixed oxides prepared by Flame Spray Pyrolysis (FSP) show greater catalytic activities than those prepared by precipitation, although both exhibit very similar surface area and bulk features. The objective of this study was to find out the origins of the superior catalytic activities obtained by flame spray pyrolysis synthesis method by a deeper analysis of bulk and surface properties. We have opted to focus on mixed ceria-zirconia of equimolar composition (Ce 0.5 Zr 0.5 O 2) as it exhibits maximum catalytic activity for both synthesis methods. Combining bulk and surface characterization as well as surface adsorption measurements using probe molecules, we propose that flame spray pyrolysis enables a surface enrichment in cerium oxide still in interaction with zirconium oxide that leads to a high concentration of adsorbed methanol at the surface, which might explain the greater activity of the catalysts prepared using this method. Beyond the application of DMC synthesis, we can anticipate that Flame Spray Pyrolysis synthesis should generate relative high surface area mixed oxides with different catalytic performances with respect to mixed oxides prepared at lower temperature owing their metastable nature. [ABSTRACT FROM AUTHOR]

[Display omitted] • 3,3′-dimethyl-1-butene isomerization activity is correlated to the concentration of moderate Brønsted acid sites. • 2,6-di-tert-butylpyridine is are very sensitive IR probe for moderate Brønsted acid sites such as alumina supported catalysts. • A correlation between catalytic performance and acidity characterization is demonstrated for reforming catalysts. The characterization of surface acid sites is a major issue for the mechanistic understanding and activity prediction of a solid catalyst, especially for applications that require the use of bi-functional materials. The concentration of Brønsted acid sites is measured by two main methods: adsorption of a probe molecule, and catalytic activity in a model reaction. In a previous study, we reported that the adsorption of 2,6-di- tert -butylpyridine on the surface of alumina-supported reforming catalysts (PtSn/Al 2 O 3 –Cl) was very effective in determining the density of Brønsted acid sites. In this work, the acidity of reforming catalysts was studied through their activity in the model reaction of 3,3-dimethyl-1-butene isomerization. We have demonstrated that the activity of reforming catalysts is clearly correlated with the surface chlorine density and more extensively with the density of Brønsted acid sites as measured by the adsorption of 2,6-di- tert -butylpyridine. As a result, this study highlights the relevance of using 2,6-di- tert -butylpyridine adsorption in the characterization of the Brønsted acidity of an alumina with no metal phase interference. [ABSTRACT FROM AUTHOR]

Pt nanoparticles supported on ZSM-5 nanoshells have been prepared by a mild desilication method. The zeolite support is a ZSM-5 crystal with extremely thin walls (ca. 15 nm) that contain meso/microporous defects. The 2–3 nm diameter Pt nanoparticles are entrapped and highly dispersed in these ZSM-5 nanoshells. The particles are stable up to 750 °C in H 2 owing to this immobilization. Catalytic activities for toluene hydrogenation were compared with those of reference Pt-based catalysts. The reaction rates obtained on the novel Pt-ZSM-5 nanoshells catalyst were about 100 times higher than those measured over conventional Pt-ZSM-5 catalysts. The highly defective zeolite nanoshell allowed even bulky molecules such as mesitylene to access all the Pt particles. This was in contrast to Pt nanoparticles encapsulated in ZSM-5 hollow single crystals, which were totally inactive due to molecular sieving. This novel Pt-ZSM-5 nanoshell catalyst shows outstanding sintering-resistance and mass transfer properties owing to its hierarchical design. [ABSTRACT FROM AUTHOR]

The present work examines the different parameters that affect desilication of nanometric Beta zeolite crystals in alkaline solutions. The influences of different synthesis parameters such as the nature of the zeolite (size of the crystals, composition), the alkalinity, the temperature and the amount of added Al species on the morphology and properties of hollow structures are discussed. Addition of sodium aluminate to the alkaline solution protects the surface from desilication and favors dissolution at the center of the crystals, leading to the formation of hollow structures. The size, morphology and crystallinity of those structures depend mainly on zeolite composition, treatment temperature and alkalinity of the solution. If hollow structures are totally amorphous when desilication is carried out under stirring, their crystallinity and thermal stability are significantly improved in the presence of tetraethyammonium (TEA+) cations. Hollow Beta zeolites have been obtained by controlled desilication of bulk crystals in the presence of protective aluminum species under mild conditions. The best materials associating hollow structures with crystalline walls were prepared in the presence of tetraethylammonium cations, which favored partial recrystallization of dissolved species on the zeolite surface. Image 1 • Beta nanocrystals have been desilicated in the presence of protective Al species. • Desilication of nanocrystals at moderate temperature produces hollow structures. • Hollow structures contain zeolite domains in an amorphous aluminosilicate matrix. • The crystallinity of hollow structures is improved in the presence of TEA+ cations. [ABSTRACT FROM AUTHOR]