Formation, Activity and Growth of Copper Nanoparticles in Methanol Synthesis Catalysts

Abstract

Supported metal catalysts play a pivotal role in the production of fuels and chemicals, in the purification of exhaust gases and in electrochemical energy conversion systems. Further improvement of these materials requires a fundamental understanding of the processes involved in the synthesis, the structural characteristics required for maximum activity, and the influence of different parameters on catalyst deactivation. Specifically, the development of supported metal catalysts would greatly benefit from detailed insights into the process of metallic nanoparticle formation, the (synergistic) effect of the (bi)metallic surface structure on catalyst activity, and the growth of metallic nanoparticles during reaction. Here, we investigated these aspects for copper-based methanol synthesis catalysts. Firstly, the formation of copper nanoparticles on a silica support during reduction in H2 of copper phyllosilicate, a solid homogeneous precursor, was investigated. With careful optimization of the imaging strategy, time-resolved in situ TEM provided unique mechanistic and kinetic information about the nucleation and growth of nanoparticles that is representative for large scale nanomaterials synthesis. The size evolution of the particles was well described by a two-step, autocatalytic reduction mechanism involving diffusion of copper species (likely Cu2+ ions) with either diffusion-limited or reaction-limited particle growth. The plate-like structure of the precursor restricted the diffusion of copper and the autocatalytic reduction limited the probability for secondary nucleation. The combination of a uniform size of precursor particles and the autocatalytic reduction thus offers means to synthesize nanoparticles with well-defined sizes in large amounts. Secondly, the sensitivity of the methanol synthesis reaction to the surface structure was investigated for Cu and CuZn catalysts. The activity depended on the promoting effect of zinc, which was affected by the thermodynamic stability of the zinc phase. The turnover frequency decreased by a factor of about 3 going from 8 to 2 nm copper particles for both Cu and CuZn catalysts. In view of recent theoretical studies, the observed effect of the copper particle size on the activity indicates that the methanol synthesis reaction predominantly takes place at surface sites with a unique configuration of several copper atoms such as step-edge sites, which smaller particles cannot accommodate. Thirdly, the impact of the synthesis route, chemical nature of the support surface, particle size distribution and interparticle spacing on copper particle growth in the methanol synthesis reaction were investigated independently from each other. Using precipitation instead of impregnation as a synthesis route resulted in a lower catalyst activity and a higher catalyst stability, probably due to partial entrapment of the copper particles during reduction of the precipitated precursor. Functionalizing the support with aminopropyl groups and having a narrow particle size distribution increased catalyst stability by inhibiting the transport of copper species over the support and decreasing the driving force for Ostwald ripening, respectively. In conclusion, the work described in this thesis presents new fundamental insights into the physicochemical processes involved in the synthesis and performance of copper-based methanol synthesis catalysts and the structural parameters influencing these processes. These insights provide new strategies for the rational design of copper-based methanol synthesis catalysts in particular and supported metal catalysts in general.