Theoretical probing into the stabilizing interplay between metal catalysts and nitrogen-doped carbon supports

Abstract

Global chemical processes are heavily dependent on heterogeneous catalysis. These metal catalysts have been introduced to support materials in various manners to create advanced catalytic systems that are defined by metal-support interactions (MSI). Carbon materials have grown in prominence due to their higher flexibility and inertness but their MSI are seen to weaken over time through degradation mechanisms. Doping nitrogen into the carbon supports has strengthened MSI leading beneficial properties. However, the extrapolation of structure-property relationships from experimental means faces challenges as well as the fundamental chemical nature behind MSI has not been fully elucidated. Furthermore, there is a vastness of structural support components that affect MSI. Density Functional Theory was employed to establish a theoretical framework that probes the nature of the MSI, forms models that can reliably suggest experimental targets, and explores different parameters that influence MSI, starting with single-atom-based catalysts and various different nitrogen defects. Different MSI models were reviewed to address gaps in defect design as well as offer a direct comparison of MSI between a set of twelve different transition metals for experimental reference. Truncated models were shown to be representative of periodic boundary condition models, and both presented that structural design choices affected the calculated adsorption energy and MSI. MSI were dictated by the metal’s valency, along with other periodic relations, where electronic density reorganization from the d- to s-orbitals takes place upon metal interaction with the graphitic defect carbon support. The parameter space was increased to similarly examine the impacts of pyridinic defect in the carbon support where it was found pyridinic defect’s MSI is dramatically different than graphitic defect’s. The resulting conclusions screened for potential metal-nitrogen-carbon candidates for further experimental work. Moreover, graphitic and pyridinic defect combinations were surveyed to address incomplete knowledge in platinum stabilization. Catalyst size was varied as different atomically-sized platinum clusters were investigated over seven different nitrogen defect carbon support combinations. The investigation suggested that higher concentration and density of graphitic defects benefited platinum’s stability while pyridinic defects provided greater stabilization than graphitic defects, unless destabilized by a close-by graphitic defect. Ultimately, it was observed that d-band properties were adept in characterizing MSI.