New concepts in low-temperature catalytic hydrogenation and their implications for process intensification

The study of dynamic catalytic processes occurring at nanoscale provides necessary information to innovate in process intensification (PI). The present study addresses the catalytic performance of Rh, Cu, and Ru-supported nanoparticles, under mild reaction conditions (< 250 °C, < 500 kPa), for three reactions: (i) the hydrogenation of CO2 to methane, (ii) the hydrogenation of CO2 to methanol, and (iii) the hydrogenation of N2 to ammonia. In all systems, the activity and selectivity are promoted by the presence of large metal nanoparticles, and therefore, a broad distribution of sizes is needed for high activity. The enhanced performance of Rh and Ru nanocatalysts results from catalytic cooperation between small and larger metal nanoparticles. Larger particles provide small ones with the hydrogen needed for an efficient hydrogenation of adsorbed reaction intermediates. Large Cu particles promote methanol formation on Cu/ZnO catalysts, indicating that a similar catalytic cooperation might proceed between large Cu particles and the sites adsorbing reaction intermediates. Some particular crystal planes and defects developed in larger nanoparticles would facilitate the activation and transfer of hydrogen, giving rise to a cooperation mechanism via hydrogen supply. More accurate kinetic models can be developed from a deep understanding of dynamic processes occurring at nanoscale. The new models can be later applied in the design and development of high-performance units and plants. In addition, a multidisciplinary PI approach including the study of nanoscale catalytic processes would certainly lead to important improvements on the existing process technologies