Chemical and Thermal Sintering of Supported Metals with Emphasis on Cobalt Catalysts During Fischer-Tropsch Synthesis.

Abstract

This comprehensive critical review combines, for the first time, recent advances in nanoscale surface chemistry, surface science, DFT, adsorption calorimetry, and in situ XRD and TEM to provide new insights into catalyst sintering. This work provides qualitative and quantitative estimates of the extent and rate of sintering as functions of nanocrystal (NC) size, temperature, and atmosphere. This review is unique in that besides summarizing important, useful data from previous studies, it also advances the field through addition of (i) improved or new models, (ii) new data summarized in original tables and figures, and (iii) new fundamental perspectives into sintering of supported metals and particularly of chemical sintering of supported Co during Fischer-Tropsch synthesis. We demonstrate how the two widely accepted sintering mechanisms are largely sequential with some overlap and highly NC-size dependent, i.e., generally, small NCs sinter rapidly by Ostwald ripening, while larger NCs sinter slowly by crystallite migration and coalescence. In addition, we demonstrate how accumulated knowledge, principles, and recent advances, discussed in this review, can be utilized in the design of supported metal NCs highly resistant to sintering. Recommendations for improving the design of sintering experiments and for new research are addressed.