Microscopic mechanisms of cooperative communications within single nanocatalysts

Abstract

Catalysis is a method of accelerating chemical reactions that is critically important for fundamental research as well as for industrial applications. It has been recently discovered that catalytic reactions on metal nanoparticles exhibit cooperative effects. The mechanism of these observations, however, remains not well understood. In this work, we present a theoretical investigation on possible microscopic origin of cooperative communications in nanocatalysts. In our approach, the main role is played by positively charged holes on metal surfaces. A corresponding discrete-state stochastic model for the dynamics of holes is developed and explicitly solved. It is shown that the observed spatial correlation lengths are given by the average distances migrated by the holes before they disappear, while the temporal memory is determined by their lifetimes. Our theoretical approach is able to explain the universality of cooperative communications as well as the effect of external electric fields. Theoretical predictions are in agreement with experimental observations. The proposed theoretical framework quantitatively clarifies some important aspects of the microscopic mechanisms of heterogeneous catalysis.

How Heterogeneity Affects Cooperative Communications within Single Nanocatalysts

“…Then, the effective characteristic length of this exponential decay (l is the size of each segment), = l y ln (9) can be associated with the communication lengths in catalytic cooperativity. 28 This is because this length describes the region of increased local concentration due to the produced charged holes, and this increased concentration of charged holes leads to a higher probability of the reaction occurring for the active sites within this region. This exactly corresponds to the correlation length as was measured in experiments.…”

“…Also, in the system with the homogeneous death rates, r 1 = r 2 = r, we obtain τ = 1/r, in agreement with previous theoretical evaluations of catalytic communications for homogeneous systems. 28 The lifetime of the catalytic cooperativity is an important factor that affects catalytic processes in the system. If these lifetimes are very short (fast death rates), which corresponds to r 1 and r 2 being much larger than u 1 and u 2 , one can obtain from eqs 7 and 8,…”

“…To explain the microscopic mechanisms of catalytic communications in single nanoparticles, we recently proposed a novel discrete-state stochastic framework to study catalysis of redox chemical reactions. 28 Single-molecule microscopy experiments have reported both intraparticle and interparticle communications in catalytic reactions on Pd-or Au-based nanocatalysts through redox processes. 19 As experimental evidence suggest that a positively charged species is responsible for cooperative catalytic communication within single nanocatalysts by moving along oxide surfaces of nanoparticles, it was assumed that the probability for a chemical reaction to occur at the given site depends on the local concentration of positively charged holes, 29−31 and each chemical reaction also creates new charged holes.…”

“…The idea was that the increased local concentration of charged holes forces them to move into the areas of the nanoparticle with smaller concentrations, and this stimulates chemical reactions at neighboring sites due to incoming charged particles increasing the concentrations of charged holes around these sites. 28 A quantitative model based on these ideas has been developed, allowing for explicit estimates of communication lifetimes and distances. Excellent agreement with all available experimental observations has been found.…”

“…Now, a heterogeneous discrete-state stochastic model for catalytic communications can be easily developed as a generalization of the original homogeneous approach. 28 In this model, the average time for the created charged hole to stay in the system corresponds to the communication lifetime, while the average distance that this charged hole moves after creation and before disappearing is associated with the communication length scale.…”