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Catalyst Sintering Kinetics Data: Is There a Minimal Chemical Mechanism Underlying Kinetics Previously Fit by Empirical Power-Law Expressions—and if So, What Are Its Implications?

Finney, Eric E., Finke, Richard G.
Industrial & engineering chemistry process design and development 2017 v.56 no.37 pp. 10271-10286
catalysts, equations, kinetics, models, nanoparticles, process design
Catalyst sintering is an undesired, but general, and hence practically important catalyst deactivation process. Understanding sintering kinetics and, then, the associated mechanism(s) is an important goal, one crucial to being better able to limit and otherwise control catalyst sintering rationally. However, and despite the availability of atomic-based sintering models, the kinetics of sintering of practical catalysts are to this day most often accounted for by curve-fitting with empirical power laws. Such empirical kinetics treatments are, unfortunately, devoid of rigorous mechanistic insight because they lack the balanced chemical equations that are required to define the rate constants and to also define the proper concepts and associated words that, in turn, are crucial for being able to describe correctly the sintering process physically. Hence, addressed herein is the key, previously unanswered question: is there a disproof-based, Ockham’s razor-obeying, hence mechanistically rigorous, minimal chemical mechanism that can be used to curve-fit sintering kinetics data previously accounted for by empirical power law expressions? If so, then what are its implications? The results provided demonstrate that literature catalyst sintering data, previously fit using empirical power laws, can instead be quantitatively accounted for by a simple, deliberately minimalistic, two-step kinetic model consisting of bimolecular nucleation of agglomeration, B + B → C (rate constant k₃), followed by autocatalytic agglomeration, B + C → 1.5C (rate constant k₄), in which B is the average starting nanoparticle, and C is the average larger, agglomerated nanoparticle. The results and findings compellingly demonstrate that the two-step mechanism can account for a variety of sintering kinetics data previously fit only by empirical power laws. Evidence is presented that the kinetic model appears to correspond to what has been called Particle Migration and Coalescence (PMC) in the prior literature. Ten conclusions and hypotheses, as well as four caveats, are listed in the Conclusion section, along with suggestions for further research.