Nanostructured catalysts dynamics

A key aspect of metal oxides is that they possess multiple functional properties acid-base, electron transfer and transport, chemisorption by a and 7i-bonding of hydrocarbons, O-insertion and H-abstraction, etc. This multi-functionality allows them to catalyze complex selective multistep transformations of hydrocarbons, as well as other catalytic reactions (NO,c conversion, for example). The control of the catalyst multi-functionality requires the ability to control not only the nanostructure, e.g. the nano-scale environment around the active site, " but also the nano-architecture, e.g. the 3D spatial organization of nano-entities. The active site is not the only relevant aspect for catalysis. The local area around the active site orients or assists the coordination of the reactants, and may induce sterical constrains on the transition state, and influences short-range transport (nano-scale level). Therefore, it plays a critical role in determining the reactivity and selectivity in multiple pathways of transformation. In addition, there are indications pointing out that the dynamics of adsorbed species, e.g. their mobility during the catalytic processes which is also an important factor determining the catalytic performances in complex surface reaction, " is influenced by the nanoarchitecture. 

In HRTEM studies of complex catalyst structures, complementary multislice image simulations using the dynamical theory of electron diffraction (Cowley 1981) may be necessary for the nanostructural analysis and to match experimental images with theory. 

High-resolution transmission electron microscopy (HRTEM) has matured markedly in the preceding decade and has emerged as a powerful technique for investigation of nanostructured metal catalysts at the atomic level, even under working conditions. The ability to image the dynamic structure and morphology of supported metal nanocluster catalysts in such detail makes HRTEM an essential complement to the arsenal of spectroscopic techniques used for characterization of... 

XRD is a useful yet seldom applied technique for characterization of solid catalysts in the functioning state. Its merits emerge from the unambiguous determination of phases, their dynamics, and their relevant nanostructures under operating conditions. This information should be the basis of every attempt to determine the structure and function of a catalytic material. Speculations about structure and function are much more frequent than XRD investigations of the catalysts under working conditions, leaving many open questions about the nature of active phases in solid catalysts. 

XRD is valuable because it provides unambiguous determinations of phases, their dynamics, and evidence of their structures at the nanoscale, even at high pressures and temperatures. This technique is still not frequently applied for characterization of functioning catalysts, but the advent of ultra-high-brilliance radiation sources such as free-electron lasers and improved synchrotron sources will open new possibilities for determination of time-resolved XRD patterns to establish details of the temporal evolution of structural dynamics and of the nanostructures of functioning catalysts. Examples in this chapter illustrate the value of the method for characterization of mixed oxide catalysts, such as those containing molybdenum oxide structures. 

Nanoscopies supplemented with conventional techniques will allow the rational handling of the catalyst/reactive system based on its knowledge at the atoniic/molecular level. The application of nanoscopies in surface chemistry offers the possibility for determining the nanostructure of solid surfaces, surface reconstruction phenomena, to identify the structure of ionic and molecular adlayers, to study the dynamics of these adlayers in their adsorption and desorption at the submonolayer and monolayer (ML) level. Likewise, they are important tools to follow reactions at sohd surfaces in real time in different environments. The reader can get acquainted with the state of the art on these topics in Refs [5-12]. 

Observation of Dynamic Nanostructural and Nanochemical Changes in Ceria-Based Catalysts During In-situ Reduction, R. Sharma, PA. Crozier, Z.C. Kang and L. Eyring, Philos. Mag., 84, 2731-2747 (2004). 

An important factor affecting the performance of DMFCs is the kinetics of catalyst. Platinum (Pt/C) is the most effective catalyst for oxygen reduction reaction but it is not selective towards ORR in presence of methanol. The addition of yttrium to Pt increases the ORR activity and are promising ORR electrocatalyst [207]. Carbon supported PtY(OH)3 hybrid catalyst are developed with dynamic spillover of metal oxide [208]. Recently, catalyst for DMFC Pt Pd/C NP was prepared by the galvanic displacement reaction between Pt and Pd. A simple synthesis strategy was followed to prepare carbon based [209] and carbon-supported Pd nanostructure [190]. A higher methanol tolerance of Pt Pd/C with less Pt content than Pt/C suggests that it is potential alternative cathode electrocatalyst for DMFCs [190]. 

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