RESUMEN
Recent theoretical predictions indicate that functional groups and additives could have a favorable impact on the hydrogen adsorption characteristics of sorbents; however, no definite evidence has been obtained to date and little is known about the impact of such modifications on the thermodynamics of hydrogen uptake and overall capacity. In this work, we investigate the effect of two types of additives on the cryoadsorption of hydrogen to mesoporous silica. First, Lewis and Brønsted acid sites were evaluated by grafting aluminum to the surface of mesoporous silica (MCF-17) and characterizing the resulting silicate materials' surface area and the concentration of Brønsted and Lewis acid sites created. Heat of adsorption measurements found little influence of surface acidity on the enthalpy of hydrogen cryoadsorption. Secondly, platinum nanoparticles of 1.5 nm and 7.1 nm in diameter were loaded into MCF-17, and characterized by TEM. Hydrogen absorption measurements revealed that the addition of small amounts of metallic platinum nanoparticles increases by up to two-fold the amount of hydrogen adsorbed at liquid nitrogen temperature. Moreover, we found a direct correlation between the size of platinum particles and the amount of hydrogen stored, in favor of smaller particles.
RESUMEN
Pt, Rh, and Pd nanoclusters stabilized by PAMAM dendrimer are used for the first time in a gas flow reactor at high temperature (150-250 °C). Pt nanoclusters show a very high activity for the hydrogenation of the methylcyclopentane (MCP) at 200-225 °C with turnover freqency (TOF) up to 334 h-1 and selectivity up to 99.6% for the ring opening isomerization at very high conversion (94%). Rh nanoclusters show different selectivity for the reaction, that is, ring opening isomerization at 175 °C and cracking at higher temperature whereas Pd nanoclusters perform ring enlargement plus dehydrogenation, while maintaining a high activity. The difference in these results as compared to unsupported/uncapped nanoparticles, demonstrates the crucial role of dendrimer. The tunability of the selectivity of the reaction as well as the very high activity of the metal nanoclusters stabilized by dendrimer under heterogeneous conditions open a new application for dendrimer catalysts.
RESUMEN
The Fischer-Tropsch process, or the catalytic hydrogenation of carbon monoxide (CO), produces long chain hydrocarbons and offers an alternative to the use of crude oil for chemical feedstocks. The observed size dependence of cobalt (Co) catalysts for the Fischer-Tropsch reaction was studied with colloidally prepared Co nanoparticles and a chemical transient kinetics reactor capable of measurements under non-steady-state conditions. Co nanoparticles of 4.3â nm and 9.5â nm diameters were synthesized and tested under atmospheric pressure conditions and H2 /CO=2. Large differences in carbon coverage (ΘC ) were observed for the two catalysts: the 4.3â nm Co catalyst has a ΘC less than one while the 9.5â nm Co catalyst supports a ΘC greater than two. The monomer units present on the surface during reaction are identified as single carbon species for both sizes of Co nanoparticles, and the major CO dissociation site is identified as the B5 -B geometry. The difference in activity of Co nanoparticles was found to be a result of the structure sensitivity caused by the loss of these specific types of sites at smaller nanoparticle sizes.
RESUMEN
Hydrogenations of CO or CO2 are important catalytic reactions as they are interesting alternatives to produce fine chemical feedstock hence avoiding the use of fossil sources. Using monodisperse nanoparticle (NP) catalysts, we have studied the CO/H2 (i.e., Fischer-Tropsch synthesis) and CO2/H2 reactions. Exploiting synchrotron based in situ characterization techniques such as XANES and XPS, we were able to demonstrate that 10 nm Co NPs cannot be reduced at 250 °C while supported on TiO2 or SiO2 and that the complete reduction of cobalt can only be achieved at 450 °C. Interestingly, cobalt oxide performs better than fully reduced cobalt when supported on TiO2. In fact, the catalytic results indicate an enhancement of 10-fold for the CO2/H2 reaction rate and 2-fold for the CO/H2 reaction rate for the Co/TiO2 treated at 250 °C in H2 versus Co/TiO2 treated at 450 °C. Inversely, the activity of cobalt supported on SiO2 has a higher turnover frequency when cobalt is metallic. The product distributions could be tuned depending on the support and the oxidation state of cobalt. For oxidized cobalt on TiO2, we observed an increase of methane production for the CO2/H2 reaction whereas it is more selective to unsaturated products for the CO/H2 reaction. In situ investigation of the catalysts indicated wetting of the TiO2 support by CoO(x) and partial encapsulation of metallic Co by TiO(2-x).
RESUMEN
The interaction of the metal and support in oxide-supported transition-metal catalysts has been proven to have extremely favorable effects on catalytic performance. Herein, mesoporous Co3O4, NiO, MnO2, Fe2O3, and CeO2 were synthesized and utilized in CO oxidation reactions to compare the catalytic activities before and after loading of 2.5 nm Pt nanoparticles. Turnover frequencies (TOFs) of pure mesoporous oxides were 0.00020.015 s(1), while mesoporous silica was catalytically inactive in CO oxidation. When Pt nanoparticles were loaded onto the oxides, the TOFs of the Pt/metal oxide systems (0.1500 s(1)) were orders of magnitude greater than those of the pure oxides or the silica-supported Pt nanoparticles. The catalytic activities of various Pt/oxide systems were further influenced by varying the ratio of CO and O2 in the reactant gas feed, which provided insight into the mechanism of the observed support effect. In situ characterization using near-edge X-ray absorption fine structure (NEXAFS) and ambient-pressure X-ray photoelectron spectroscopy (APXPS) under catalytically relevant reaction conditions demonstrated a strong correlation between the oxidation state of the oxide support and the catalytic activity at the oxidemetal interface. Through catalytic activity measurements and in situ X-ray spectroscopic probes, CoO, Mn3O4, and CeO2 have been identified as the active surface phases of the oxide at the interface with Pt nanoparticles.
RESUMEN
The surface structure of Pt(557) during the catalytic oxidation of hydrogen was studied with in situ scanning tunneling microscopy and X-ray photoelectron spectroscopy. At 298 K, the surface Pt oxide formed after exposing Pt(557) to approximately 1 Torr of O2 can be readily removed by H2, at H2 partial pressures below 50 mTorr. Water is detected as the product in the gas phase, which also coadsorbs with hydroxyl groups on the Pt(557) surface.
RESUMEN
Carbon dioxide capture and use as a carbon feedstock presents both environmental and industrial benefits. Here we report the discovery of a hybrid oxide catalyst comprising manganese oxide nanoparticles supported on mesoporous spinel cobalt oxide, which catalyses the conversion of carbon dioxide to methanol at high yields. In addition, carbon-carbon bond formation is observed through the production of ethylene. We document the existence of an active interface between cobalt oxide surface layers and manganese oxide nanoparticles by using X-ray absorption spectroscopy and electron energy-loss spectroscopy in the scanning transmission electron microscopy mode. Through control experiments, we find that the catalyst's chemical nature and architecture are the key factors in enabling the enhanced methanol synthesis and ethylene production. To demonstrate the industrial applicability, the catalyst is also run under high conversion regimes, showing its potential as a substitute for current methanol synthesis technologies.
RESUMEN
The appropriate combination of titania and silica, sulfating and non-sulfating support, respectively, results in Pd catalysts with improved water and sulfur tolerance in methane combustion. For the first time the catalyst recovers the initial activity after one cycle under lean-burn conditions without additional regenerating treatments.