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.

Optimization of Ni-Based Catalysts for Dry Reforming of Methane via Alloy Design: A Review

“…Although we have known that the performance of Ni something Cu bimetallic catalyst depends on the Cu/Ni ratio, we found that the ratio itself is very complex and difficult to be unified. The research report of Yang et al may provide a reasonable explanation for the significant difference in the optimum Ni/Cu ratio of Ni–Cu DRM catalysts in other literature studies. They used molecular dynamics simulation methods to study the interfacial dynamics of Cu in the surface enrichment process of ∼4 nm Ni–Cu bimetallic nanoparticles and provided a reference for the ab initio design of high-temperature bimetallic catalysts.…”

“…Since the bulk phases of Cu and Ni are both face-centered cubic structures with similar lattice constants (3.62 vs 3.54 Å), the researchers suggested that Ni–Cu bimetallic catalysts may form stable Cu–Ni alloys with high anticoke and DRM activity. − Choi et al investigated the DRM performance of a series of modified catalysts of Ni-M/Al 2 O 3 (M = Co, Cu, Zr, Mn, Mo, Ti, Ag, and Sn). Compared to the but-metal Ni/Al 2 O 3 catalysts, only the Co, Cu, and Zr modified catalysts among the studied second metals showed a slight increase in DRM activity.…”

Single atom alloys vs. phase separated alloys in Cu, Ag, and Au atoms with Ni(111) and Ni, Pd, and Pt atoms with Cu(111): a theoretical exploration

“…These results agree with the experimental observations. 4,10,[19][20][21]26,27,[29][30][31] To obtain better understanding, we analyzed energy changes along the assumed reaction to generate the SAA and PSA of nX/ M(111) from M(111) and n X atoms. This analysis elucidated that the interaction energy E int of M(111) with either n X atoms or X n cluster plays an important role in determining which of the SAA and PSA is produced.…”

“…These findings agree with previously reported experimental results. 19,20,[25][26][27][28][29][30] The SAA structure of Au on Ni(111) and the PSA structure of Cu on Ni(111) have been discussed in previously reported computational studies, [56][57][58] but the relative stabilities of the SAA and PSA have not been compared before and the reasons why Au atoms form the SAA with Ni(111) but Cu atoms form the PSA with Ni(111) have been unclear. In nAg/Ni(111) (n = 2-4), the relative stabilities depend on the Ag coverage on the Ni(111) surface, as follows: in 2Ag/ Ni(111), the SAA is slightly more stable than the PSA, whereas, in 3Ag/Ni(111) and 4Ag/Ni(111), the PSA is slightly more stable than the SAA.…”

“…26 The presence of the Cu phase on the Ni surface was further supported by classical molecular dynamics simulations showing that the Cu migration and enrichment on the Ni surface occurred at a high temperature. 27 On the other hand, the Cu-based SAA of Ni was prepared by depositing slowly Ni atoms on the Cu(111) single crystal at a surface temperature of 433 K. 28 Similarly, Ni 0.01 Cu and Ni 0.001 Cu alloys were prepared by depositing Ni on the Cu nanoparticle surface supported on SiO 2 . The Ni 0.001 Cu alloy exhibited good performance in the non-oxidative dehydrogenation reaction of ethanol.…”