Energetics of van der Waals Adsorption on the Metal-Organic Framework NU-1000 with Zr6-oxo, Hydroxo, and Aqua Nodes.

We report measurements of adsorption isotherms and the determination of the isosteric heats of adsorption of several small gases (H2, D2, Ne, N2, CO, CH4, C2H6, Ar, Kr, and Xe) on the metal-organic framework (MOF) NU-1000, which is one of the most thermally stable MOFs. It has transition-metal nodes of formula Zr6(µ3-OH)4(µ3-O)4(OH)4(OH2)4 that resemble hydrated ZrO2 clusters and can serve as catalysts or catalyst supports. The linkers in this MOF are pyrenes linked to the nodes via the carboxylate groups of benzoates. The broad range of adsorbates studied here allows us to compare trends both with adsorption on other surfaces and with density functional calculations also presented here. The experimental isotherms indicate similar filling of the MOF surface by the different gases, starting with strong adsorption sites near the Zr atoms, a result corroborated by the density functional calculations. This adsorption is followed by the filling of other adsorption sites on the nodes and organic framework. Capillary condensation occurs in wide pores after completion of a monolayer. The total amount adsorbed for all the gases is the equivalent of two complete monolayers. The experimental isosteric heats of adsorption are nearly proportional to the atom-atom (or molecule-molecule) Lennard-Jones well-depth parameters of the adsorbates but ∼13-fold larger. The density functional calculations show a similar trend but with much more scatter and heats that are usually greater (by 30%, on average).

Role of iron carbonyls in the inhibition of oxygen activation for the oxidation of CO catalyzed by iron oxide-supported gold.

Iron oxide-supported gold samples were prepared by co-precipitation from HAuCl(4) and Fe(NO(3))(3). The activities of the samples as CO oxidation catalysts were tested without thermal treatment and following treatments in flows of He and O(2) at various temperatures. It was found that the untreated samples and those treated in a flow of He at 150 °C were more active than samples that had been treated at 400 °C in either a flow of O(2) or of He. Infrared spectra recorded during CO oxidation catalysis indicate the presence of bonded CO molecules to cationic gold on all samples, whereas spectra of the least active catalysts indicate a predominant presence of Fe(2+) carbonyls, which were highly stable under the conditions of our experiments. Our results indicate that in the least active samples the Fe(2+)-bound CO blocks sites that would otherwise be available for oxygen activation.