Surface Activation of Transition Metal Nanoparticles for Heterogeneous Catalysis: What We Can Learn from Molecular Dynamics

Abstract

Many heterogeneous reactions catalyzed by nanoparticles occur at relatively high temperatures, which may modulate the surface morphology of nanoparticles during reaction. Inspired by the discovery of dynamic formation of active sites on gold nanoparticles, we explore theoretically the nature of the highly mobile atoms on the surface of nanoparticles of various sizes for 11 transition metals. Using molecular dynamics simulations, on a 3 nm Fe nanoparticle as an example, the effect of surface premelting and overall melting on the structure and physical properties of the nanoparticles is analyzed. When the nanoparticle is heated up, the atoms in the outer shell appear amorphous already at 900 K. Surface premelting is reached at 1050 K, with more than three liquid atoms, based on the Lindemann criterion. The activated atoms may transfer their extra kinetic energy to the rest of the nanoparticle and activate other atoms. The dynamic studies indicate that the number of highly mobile atoms on the surface increases with temperature. Those atoms with a high Lindemann index, usually located on the edges or vertices, attain much higher kinetic energy than other atoms and potentially form different active sites in situ. When the temperature passes the surface premelting temperature, a drastic change in the coordination number (SCN) of the surface atoms occurs, with attendant dramatic broadening of the distribution of the SCN, suppling active sites with more diverse atomic coordination numbers. The electronic density of states of a nanoparticle tends to “equalize”, due to the breaking of the translational symmetry of the atoms in the nanoparticle, and the d-band center of the nanoparticle moves further away from the Fermi level as the temperature increases. Besides Au, other nanoparticles of the transition metals, such as Pt, Pd, and Ag, may also have active sites easily formed in situ.

Preparation of Cobalt Nanoparticles

“…Most of these applications require chemically stable, well‐dispersed, and uniform‐sized particles, hence the rapid development of methods for the synthesis of metal NPs [80–85] . In particular, NPs of 3d metals have found application in engineering, chemical and petrochemical industries, aviation, and space technology [86–92] . The research in this field is associated with the discovery of very interesting and practically useful chemical and physical properties of iron, cobalt, and nickel NPs [93–98] .…”

Structural trends in the dehydrogenation selectivity of palladium alloys

“…Formation of such a defect through surface pre-melting in nanoparticles at high temperature is well documented. 70 While the same partial occupancy can occur for similar reasons in 1 : 1 alloys, the occurrence of fourfold Pd ensembles by elemental exchange is less likely because multiple atom exchanges would need to occur in close proximity to produce atomic ensembles with maximum propensity for hydrogenolysis. The initial segregation analysis performed using DFT further illustrates this point (Table S9 †); the 1 : 1 alloys have higher thermodynamic barriers for undergoing metastable exchanges of surface M atoms with sub-surface Pd atoms, as compared to 3 : 1 alloys, further leading to their superior selectivity.…”

Supported Metal Nanoparticles Assisted Catalysis: A Broad Concept in Functionalization of Ubiquitous C−H Bonds

“…Catalytic activity of monometallic NPs is lesser compared to homogeneous counterparts owing to a smaller number of catalytic active metal atoms present on the surface of NPs and no utilization of some metal atoms present in the core part of the NPs [109,110] . However, bimetallic (BM) NPs encompass a greater number of catalytic active metal atoms on the surface of NPs and also found to be a highly reactive, stable, reusable and economic catalyst due to its advantageous synergistic effect.…”