Accessing the C-C transition state energy on transition metals

The search for catalysts that can efficiently convert large hydrocarbons has been an active area of research for decades. To gain insight into those reactions, electronic structure calculations are playing an increasing role but the screening efforts are impeded by the complexity of the reaction networks that can contain hundreds of elementary steps, presenting a large number of computationally expensive transition state barrier calculations. A large number of the sub reactions in the network involve C-C bond dissociation, a step that has been identified as rate determining in many studies. The purpose of this article is to present a methodology that allows for accurate and rapid assessment of transition state energies for C-C bond breaking in any hydrocarbon based on a small number of simple calculations. Our model significantly enhances the capability of expanding the search space for new and efficient catalysts.

Understanding trends in C-H bond activation in heterogeneous catalysis

While the search for catalysts capable of directly converting methane to higher value commodity chemicals and liquid fuels has been active for over a century, a viable industrial process for selective methane activation has yet to be developed. Electronic structure calculations are playing an increasingly relevant role in this search, but large-scale materials screening efforts are hindered by computationally expensive transition state barrier calculations. The purpose of the present letter is twofold. First, we show that, for the wide range of catalysts that proceed via a radical intermediate, a unifying framework for predicting C-H activation barriers using a single universal descriptor can be established. Second, we combine this scaling approach with a thermodynamic analysis of active site formation to provide a map of methane activation rates. Our model successfully rationalizes the available empirical data and lays the foundation for future catalyst design strategies that transcend different catalyst classes.

Mechanistic insights into heterogeneous methane activation

A framework for predicting whether a catalyst will activate methane through the radical or surface-stabilized pathway is presented.

Formation of Reversible Clusters with Controlled Degree of Aggregation

We develop a reversible colloidal system of silica nanoparticles whose state of aggregation is controlled reproducibly from a state of fully dispersed nanoparticles to that of a colloidal gel and back. The surface of silica nanoparticles is coated with various amino silanes to identify a silane capable of forming a monolayer on the surface of the particles without causing irreversible aggregation. Of the three silanes used in this study, N-[3-(trimethoxysilyl)propyl]ethylenediamine was found to be capable of producing monolayers up to full surface coverage without inducing irreversible aggregation of the nanoparticles. At near full surface coverage the electrokinetic behavior of the functionalized silica is completely determined by that of the aminosilane. At acidic pH the ionization of the amino groups provides electrosteric stabilization and the system is fully dispersed. At basic pH, the dispersion state is dominated by the hydrophobic interaction between the uncharged aminosilane chains in the aqueous environment and the system forms a colloidal gel. At intermediate pH values the dispersion state is dominated by the balance between electrostatic and hydrophobic interactions, and the system exists in clusters whose size is determined solely by the pH. The transformation between states of aggregation is reversible and a reproducible function of pH. The rate of gelation can be controlled to be as fast as minutes while deaggregation is much slower and takes several hours to complete.