Zeolitic imidazolate frameworks (ZIFs) have been widely investigated for numerous applications including energy storage, heterogeneous catalysis, and greenhouse gas adsorption. Much of the early work has focused on the bulk properties of microcrystalline ZIFs. Herein, we focus on identifying the nature of the surface of ZIF-8 by studying a supported ZIF-8 nanoparticle film using surface characterization techniques. We have experimentally identified the presence of a zinc-rich surface terminated by carbonates and water/hydroxyl groups (in addition to the expected methylimidazole terminations) using X-ray photoelectron spectroscopy (XPS). The thermal stability of ZIF-8 thin films was also investigated using scanning electron microscopy (SEM) and temperature-programmed reaction spectroscopy (TPRS). We determined the onset of decomposition of ZIF-8 thin films to be approximately 630 K using TPRS in an ultrahigh vacuum (UHV) environment. This work presents the first characterization steps needed to study the evolution of ZIF surfaces in situ using surface characterization techniques. Such techniques are capable of determining reaction products and tracking intermediates and surface evolution in gas adsorption/reaction studies of thin films.

The adsorption of CO 2 and H 2 O by ZIF-8 thin films was investigated using X-ray photoelectron spectroscopy (XPS) and temperature-programmed desorption (TPD) in situ under low-temperature, low-pressure conditions. Using these two techniques, we demonstrate the ability to clearly distinguish molecules that exhibit significant adsorption in the pore structure of ZIF-8, from molecules that adsorb predominantly at outer surface sites. In particular, CO 2 was found to penetrate into the pore structure, while H 2 O resided predominantly at the surface. CO 2 uptake was quantified, and mobility within the films was investigated. The ability to distinguish surface processes from those that primarily occur in the bulk is key to understanding the properties of nanoporous materials.

The adsorption of methanol by a zeolitic imidazolate framework-8 (ZIF-8) nanoparticle thin film was studied in situ using temperature-programmed desorption and X-ray photoelectron spectroscopy under low-temperature, low-pressure conditions. Partial pore penetration was observed at 90 K, but upon increasing the exposure temperature of the film to 130 K pore penetration was significantly enhanced. Although many studies exist involving bulk powders, this is the first work to our knowledge that demonstrates the ability to control and monitor the entry of a molecule into a metal organic framework (MOF) film in situ using temperature. In this case, nanoparticle films of ZIF-8 were prepared and studied in ultrahigh vacuum. The ability to control and monitor surface adsorption versus pore adsorption in situ is key to future fundamental study of MOFs, for example, in the identification of active sites in reaction mechanisms.

New adsorbent materials, many of which rely on supported Ag nanoclusters or exchanged Ag + ions, have recently been employed in petroleum processing to further reduce sulfur content in fuels following catalytic hydrodesulfurization (HDS). HDS refractory species include aromatic heterocycles, such as dibenzothiophene (DBT) and its methylated derivatives. Herein, we report a fundamental study of the adsorption of two structurally analogous petroleum-relevant molecules, DBT and fluorene, on a clean TiO 2 (110) surface and one with supported Ag nanoclusters. Using thermal desorption spectroscopy, we determined the desorption activation energies to be 106 ± 2 and 103 ± 2 kJ/mol for DBT and fluorene, respectively. The similar desorption activation energies imply that the interaction of DBT on TiO 2 is not strongly dependent upon the S atom (which fluorene lacks). When adsorbed on supported Ag nanoparticles, both desorption activation energies shifted to 111 ± 2 kJ/mol, suggesting a nonselective binding enhancement, which likely involves the π-electron systems. After heating the Ag/TiO 2 (110) surface to 650 K to force agglomeration of the particles, no enhancement in binding was observed for either molecule, suggesting that the cluster size is critical for the observed enhancement. These results point to the importance of the metal particle size in addition to the oxidation state in commercially employed sorbents.