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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]

The aim of the present work is to synthesize a zeolite‐based catalyst with a hollow morphology and highly dispersed metal nanoparticles (NPs) encapsulated inside the zeolite micropores. For this purpose, we have studied a treatment using tetraalkylammonium (TAA) bromides for the selective removal of a large Pt particle from the outer surface of a hollow Beta zeolite. TEM analysis reveals that we succeeded in the synthesis of a hollow beta zeolite single crystal with encapsulated particles, with a high dispersion of 50–60 %. The molecular‐sieve‐type mechanism of the obtained catalysts was evaluated in the model reaction of toluene and mesitylene catalytic hydrogenation. Thanks to the high dispersion. a 10‐fold activity enhancement has been obtained with respect to hollow beta zeolites with encapsulated NPs recently described in the literature. The design of an "ideal" Pt‐supported zeolite catalyst is presented with the following features: 1) a high metal loading without NPs at the outer surface, 2) a high metal dispersion and 3) a short critical length (wall thickness <150–200 nm). Thanks to the high dispersion, a 10‐fold activity enhancement has been obtained with respect to hollow Beta zeolites with encapsulated NPs recently described in the literature. [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]

Transition-metal nanoparticles (Co,Ni, and Cu) encapsulated inhollow zeolite single crystals were prepared by recrystallizationof impregnated bulk MFI crystals in the presence of tetrapropylammonium(TPAOH) solutions. The size and number of particles in hollow MFIdepended mainly on the aluminum content. The encapsulation of thenanoparticles prevented them from growing, thus enabling the controlof particle size even after high temperature treatments. For low metalloadings (<3 wt %), the mean particle sizes for Co, Ni, and Cuin hollow silicalite-1 were 3.5 ± 0.3, 3.1 ± 0.5, and 1.5± 0.2 nm, respectively. In the case of hollow ZSM-5, higher loadings(∼8 wt %) could be obtained with mean particle sizes of 17± 2 nm, 13 ± 2 nm, and 15 ± 2 nm for Co, Ni, and Cusystems. The mechanism of transition metal nanoparticle formationwas markedly different from that of noble metals. At high pH values,transition-metal cations first reacted with dissolved silica speciesyielding fibrous metal phyllosilicates that were located inside thecrystal cavities. The metal phyllosilicates were then converted intonanoparticles upon reduction under H2at high temperature(500–750 °C). Silicalite-1 encapsulated Ni particles wereused in the catalytic hydrogenation of substituted benzenes and showedan outstanding size-selectivity effect. Ni particles were accessibleto toluene but not to mesitylene, confirming that the activity isdirectly related to the diffusion properties of molecules throughthe zeolite membrane. [ABSTRACT FROM AUTHOR]