Search

Abstract: Microporous materials such as zeolites, metal organic frameworks, activated carbons and aluminum phosphates are suitable for catalysis and separation applications. These high surface area materials are invariably produced in particulate forms and need to be transformed into hierarchically porous structures for high performance adsorbents or catalysts. Structuring of porous powders enables an optimized structure with high mass transfer, low pressure drop, good heat management, and high mechanical and chemical stability. The requirements and important properties of hierarchically porous structures are reviewed with a focus on applications in gas separation and catalysis. Versatile powder processing routes to process porous powders into hierarchically porous structures like extrusion, coatings of scaffolds and honeycombs, colloidal processing and direct casting, and sacrificial approaches are presented and discussed. The use and limitations of the use of inorganic binders for increasing the mechanical strength is reviewed, and the most important binder systems, e.g. clays and silica, are described in detail. Recent advances to produce binder-free and complex shaped hierarchically porous monoliths are described and their performance is compared with traditional binder-containing structured adsorbents. Needs related to better thermal management and improved kinetics and volume efficiency are discussed and an outlook on future research is also given. [Copyright &y& Elsevier]

Abstract: We report here our theoretical investigations of the interaction of ammonia (NH3) with pure and tungsten–doped ceria surfaces. The objective of the work was to understand the modification of surface acidity via the use of dopants, at the fundamental level. Surface acidity is an important property that is frequently used to characterize the reactivity of surfaces. Interest in ammonia stems from its use as a probe molecule in experimentally determining the acidity of surfaces, while ceria and tungsten oxide are examples of metal oxides widely used in catalytic systems. Density functional theory calculations were performed using the Vienna ab-initio simulation package (VASP 4.6.2). The surfaces were modeled using finite number of layers, cleaved from the relaxed bulk structures. The geometries were optimized and resulting binding energies of ammonia were used to correlate to acid site strength. Our calculations show that ammonia binds to ceria at the Lewis and Brønsted acid sites, however this binding is very weak (<5kcal/mol). In the presence of tungsten, the binding to the Lewis acid sites continues to be weak, while the binding to the Brønsted acid sites is increased significantly (to ∼10kcal/mol). We have used the H-terminated surface to study the Brønsted acidity and as a corollary, we also report the significantly different binding of hydrogen to the pure and tungsten–doped ceria surfaces. [Copyright &y& Elsevier]