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Dry methods for the synthesis of solid materials, where the synthesis is carried out in the absence or with just minimal amounts of solvents, are currently receiving particular attention. Dry synthesis methods include thermal (e.g., heating) and non-thermal (e.g., irradiation, mechanical action) processes. Their convenience is related to the reduced environmental impact and the lower aggregate costs of the procedures compared to traditional solution-based methodologies. Among others, mechanochemical methods, primarily dominated by ball milling, are becoming increasingly popular as an alternative approach for the solid-state synthesis of the most disparate materials. In fact, ball milling has already been successfully applied to synthesize molecular complexes, coordination polymers, extended inorganic solids directly from the elements, nanocomposites, and functional materials. Most importantly, mechanochemistry has also been recognized as a convenient means for the nanostructuring of solid materials for heterogeneous catalysis, including metal and metal oxide nanoparticles, porous materials, supported metal catalysts, and hybrid inorganic-organic materials.
The present dissertation will initially discuss the advantages of the mechanochemical approach for solid catalysts synthesis in comparison with conventional synthetic methodologies, with a focus on supported metal catalysts, the most important class of industrial catalysts. In this way, the state of the art in the preparation of supported metal catalysts via ball milling will be introduced, including the application of synthesized materials in various notable catalytic reactions. For more insight into the strategies implied in the synthesis of solid catalysts by ball milling, or the other means of practical mechanochemistry, the reader is invited to refer to a recent review by the title “Mechanochemical Synthesis of Catalytic Materials” by A. P. Amrute, J. De Bellis, M. Felderhoff, and F. Schüth, from where a significant part of the contents included in this introductory section was extracted.
Among others, one of the most relevant examples is offered by a recent study reporting on a general process for the direct dry synthesis of supported metal catalysts. Briefly, when the coarse powders of the metal and oxide support are ball-milled in a high-energy regime (e.g., vibration high-energy ball mill), the metal is finely comminuted and eventually deposited on the oxide support in the form of nanoparticles. The method proved to have general application as it was extended to different metals (e.g., Au, Pt, Ag, Ni, Cu) and oxide supports (e.g., TiO2, α-Al2O3, α-Fe2O3, and Co3O4). All materials were explored as solid catalysts in CO-oxidation, a simple model reaction, demonstrating a behavior consistent with literature reports, according to the case. The success in the direct synthesis of supported metal nanoparticles encouraged further investigation of the method, particularly the possibility of accessing more complex catalyst formulations (e.g., bimetallic) was explored. As a result of this follow-up study, it was demonstrated that when two metal sources are combined and subjected to the mechanochemical treatment at the same time in the presence of the target oxide support, the formation of supported bimetallic nanoparticles is observed. The synthesis of supported PdAu and CuAu nanoparticles on MgO and yttria-stabilized zirconia as representative oxide supports was studied in detail. Again, all the materials were investigated in CO-oxidation, demonstrating a similar behavior to conventionally synthesized counterparts. Considering the relevance of the mechanochemical synthesis, a systematic study on the methodology was carried out, particularly to address some of the unsolved questions that emerged during the previously mentioned investigation. Indeed, during this preliminary study, it was observed that the metal dispersion depends on the choice of the metal(s) and oxide support upon ball milling under given reaction conditions, suggesting that the outcome of the mechanochemical synthesis can potentially be predicted, which is highly appealing. Thus, the study was extended to many different combinations of metals (e.g., PdAu, PdAg, PdPt, PtAu, and AuAg) and oxide supports (e.g., MgO, MgAl2O4, α-Al2O3, and α-Fe2O3). In addition, the corresponding monometallic counterparts were also investigated to understand better how comminution proceeds upon ball milling, independently of the alloying of metal nanoparticles. In this context, it was observed that the synthesis of supported bimetallic nanoparticles proceeds through the initial comminution of the monometallic metal components and their subsequent deposition on the oxide support as nanoparticles (induction period). Then, alloying is initiated and is carried on through the gradual dissolution of one metal component into the other upon impact. Eventually, a steady-state in the comminution and alloying of metal nanoparticles is attained, and the target material is obtained. XRD and electron microscopy were the primary means to time-resolve the overall process and collect evidence on underlying mechanisms. Notably, it was observed that metals comminute to an extent depending on their relative affinity for the given oxide support under reproducible conditions. A similar trend was also observed for the size distribution of alloy nanoparticles, supporting the notion that the outcome of the mechanochemical synthesis can be predicted and potentially controlled.
The previous study demonstrated that many systems are accessible via ball milling, including systems for which a high-temperature miscibility gap exists (e.g., PdPt and PtAu disordered nanoalloys), as it has already been demonstrated for bulk alloys. However, in all cases, the materials were synthesized using stainless steel milling jars and balls, with the result of including a large amount of iron from the abrasion of milling tools. In general, the presence of iron in the materials is detrimental to their use as solid catalysts for the possible interference in the catalysis and must, therefore, be avoided. In addition, the use of ball milling machines more similar by design to those typically used for large-scale preparations would be beneficial for the future upscale of the synthesis.
To this end, ceramic vessels (e.g., yttria-stabilized zirconia) were explored to synthesize a set of iron-free materials for catalysis applications. In addition, the synthesis was conveniently carried out in planetary-type ball mills, which are easier to use and clean and more practical when upscaling is required. By these means, a series of PdAg catalysts supported on α-Al2O3 was obtained. All the materials were investigated in the selective hydrogenation of acetylene, an industrially relevant catalytic reaction. However, the reaction was not carried out under conventionally applied conditions, where trace amounts of acetylene are typically found in an ethylene-rich stream at normal pressure. Instead, the hydrogenation of highly concentrated acetylene streams in the presence of equimolar amounts of ethylene and under pressure was explored. Such conditions meet the requirements of a hypothetical plant where the plasma-induced methane pyrolysis would serve as the starting point to produce ethylene at a lower energetic cost through the intermediate formation of acetylene. Interestingly, all the mechanochemically-synthesized materials were highly active and selective in the catalytic reaction. In addition, ball-milled materials deactivated appreciably later than conventionally-synthesized counterparts, which had been independently synthesized for comparison.
Overall, the research demonstrates that the direct dry synthesis supported metal catalysts, including bimetallic formulations, is feasible upon mechanochemical activation of suitable solid precursors, which is highly appealing for industrial applications, for its simplicity, lower cost, and environmental impact. Actually, a broad range of materials was obtained during the research work, including some notoriously hard-to-synthesize materials, and the results suggested that many more are potentially accessible. In addition, the mechanochemical synthesis can result in the formation of more performant materials as catalysts, i.e., more active, selective, or less prone to deactivation under reaction conditions, compared to conventionally-synthesized counterparts. Most importantly, the rational design of solid catalysts by ball milling is possible. In fact, it has been demonstrated that the outcome of the mechanochemical synthesis can be predicted. Finally, the mechanochemical synthesis proved to be compatible with different milling devices, including machines similar by design to those typically used for large-scale preparations. Hence, the large-scale production of supported metal catalysts by these means does not seem out of reach.
