Mechanical Processing – Experimental Tool or New Chemistry?

Abstract

The article highlights the preparation and modification of hydrogen-rich materials by mechanical processing of solids at ambient conditions in the absence of solvents. Possible mechanisms of mechanically induced chemical transformations in solids ranging from metal alloys to organic materials are discussed. Although exact mechanisms of particular processes have to be handled on a case-by-case basis, mechanically facilitated reactions of different solids share common features, which define mechanical processing as an enabling technique for a new type of chemistrythe Mechanochemistry. INTRODUCTIONFor more than a decade, hydrogen as an alternative to traditional energy sources such as oil, coal and natural gas has been the focus of research and development efforts in all technologically advanced countries of the world. It is strongly believed that hydrogen-based economy can resolve energy-related problems and slow down global climate change. Hydrogen can be produced from a variety of sources including renewables and photolytic water splitting. 1. It is non-toxic and, as an energy carrier, extremely environmentally benign. Water is the only product formed during the conversion of hydrogen into energy by oxidation.Despite apparent benefits, an immediate incorporation of hydrogen into the world economy faces serious challenges. Unlike oil and natural gas, hydrogen has no large-scale infrastructure supporting its transportation. Although it is routinely used by chemical and refining industries, the cost of hydrogen storage and delivery is still too high for energy applications.Storing hydrogen in solids -metal hydrides, composites or porous materials -offers a unique opportunity for its convenient and safe use in a variety of automotive, portable and stationary applications. Unfortunately, none of the materials currently on the market fully satisfies the needs of end users. 3 Therefore, search for new hydrogen storage media remains the focus of hydrogen-related research.The article below provides a brief overview of an experimental approach, the mechanical processing, that has been used by practically every experimentalist involved in hydrogen storage research. The approach uses mechanical force in the form of milling or grinding for nano-scale design and modification of solid hydrogen storage media. The paper focuses on chemical events facilitated by mechanical processing and shows that mechanical milling and grinding have distinct chemical effect enabling a new type of chemistry -the Mechanochemistry.

Mechanochemistry of inorganic and organic systems: what is similar, what is different?

“…Various supramolecular organometallic, coordination and inclusion compounds were synthesised recently by co-grinding by several research groups. 33,37,39,40,45,[47][48][49][50][51][52][54][55][56][57][58]60 In the cases of organic and coordination compounds, quite often no high-energy ball-milling is required, but simple manual co-grinding in a mortar may be sufficient to achieve a complete transformation. This holds, for example, for formation of many organic salts and molecular co-crystals.…”

Development of Catalyst‐Enhanced Sodium Alanate as an Advanced Hydrogen‐Storage Material for Mobile Applications

“…suggest that Ti hydrides might be formed during the doping or hydrogenation process because of favorable thermodynamics during the reaction of metallic Ti with hydrogen and that these Ti hydrides contribute to the catalytically enhanced reversible hydrogen storage of NaAlH 4 . [99] Direct experimental evidence for this hypothesis is provided by Wang et al,w ho use XRD to observe the formation of nanocrystalline Ti hydrides duringt he mechanical milling of NaAlH 4 with metallic Ti.T hese authors find that Ti exhibits ac atalytic effect that is similar to that exhibited by commercial TiH 2 . [46] Recently,Z hang et al have also detected the formation of TiH 2 and TiH 0.71 during the hydrogenation of aT iO 2 @C-doped samplea t1 00-120 8C; they have established that the catalytic activities of the Ti-basedc atalytic species increase in the order TiO 2 < TiH 2 < TiH 0.71 < Al-Ti.…”

Formation of Stable Radicals by Mechanochemistry and Their Application for Magic Angle Spinning Dynamic Nuclear Polarization Solid-State NMR Spectroscopy

“…Ball milling is known to create radicals in polymers at low temperatures and in an inert gas atmosphere . We note that compressive forces have also been shown to create radicals in both synthetic and natural polymers. − Mechanochemical processing of solids has recently found numerous applications in organic and inorganic synthesis, − particle size adjustment, isotopic enrichment, − preparation of multicomponent pharmaceuticals, − catalysis, − and other areas of solid-state chemistry and materials science. − The wide variety of possible mechanochemistry applications and its simplicity make it appealing. Furthermore, since mechanochemistry uses minimal or no solvents, it is often more environmentally friendly than traditional solution-phase reactions.…”