Cooperative Atom Motion in Ni-Cu Nanoparticles during the Structural Evolution and the Implication in the High-Temperature Catalyst Design

Abstract

Bimetallic nanoparticles (NPs) are widely used in catalysis for a wide range of applications. Like other catalysts, their catalytic performance may also degrade during reactions. The structural evolution and the interfacial segregation of one constituting element are among the many degradation mechanisms. Understanding the atomic motions during this dynamics process promises to serve as a reference in designing a better catalyst. In this work, we used molecular dynamics simulation to examine the interfacial dynamics during the surface enrichment of Cu in an ∼4 nm Ni-Cu bimetallic NP. The Ni/Cu ratio on the surface after segregation was quantified and plotted as a function of the nominal composition in the bulk. Interestingly, the "outward" migration of Cu atoms and the "inward" migration of Ni were not all independent; rather, it showed the simultaneous motion by consecutive atoms along a string. Such collective motion comprised of two elements facilitated the surface segregation, intensified the shape reconstruction, and was rarely observed in previous studies. The simulation results also enabled us to rationally take advantage of the Cu segregation phenomena and design a high-performance yet robust 94Ni6Cu/Al2O3 catalyst for methane dry reforming (DRM) reactions at 850 °C. The proportion of Cu on the surface was more than 50% higher than that in the bulk, offering complementary performances of coking resistance and high activity. The combined computational-experimental study provided one of the possible reasons explaining the large inconsistency in the literature regarding the optimal Ni/Cu ratio of the Ni-Cu DRM catalyst and might shed light on the de novo design of high-temperature bimetallic catalysts.