Revealing Size Dependent Structural Transitions in Supported Gold Nanoparticles in Hydrogen at Atmospheric Pressure

Abstract

activity of Au NPs with size smaller than 5 nm has been often attributed to sizedependent physical features such as the fraction of surface atoms. [5][6][7] For instance, in reactions of hydrogenation, it is widely accepted that surface atoms on small particles are much more active than those on large particles since the fraction of low coordinated sites, that is, at edges and corners, which are the hydrogen activation sites, increases when size decreases. [8][9][10][11] This concept has been largely adopted for studying the reactivity of Au NPs, especially by theoreticians and surface science researchers. [12] The common practice is to consider Au nanoparticles as perfect model crystals with well-defined symmetry and static surface facets interacting with low coverage of gas molecules. However, the effect of adsorbates, which induces surface changes, is generally neglected in these studies. [13][14][15] This is a serious drawback that may prevent reliable description of the catalyst reactivity that mainly depends on the configuration of the surface. Aside from structural effects, quantum size effects are also frequently invoked to explain the exceptional activity of Au NPs. [16,17] For instance, using Density Functional Theory, Illas et al. have reported that the minimum energy pathway of O 2 dissociation for different Au NPs is chemically similar whichThe enhancement of the catalytic activity of gold nanoparticles with their decreasing size is often attributed to the increasing proportion of low-coordinated surface sites. This correlation is based on the paradigmatic picture of working gold nanoparticles as perfect crystal forms having complete and static outer surface layers whatever their size. This picture is incomplete as catalysts can dynamically change their structure according to the reaction conditions and as such changes can be eventually size-dependent. In this work, using aberration-corrected environmental electron microscopy, size-dependent crystal structure and morphological evolution in gold nanoparticles exposed to hydrogen at atmospheric pressure, with loss of the face-centered cubic crystal structure of gold for particle size below 4 nm, are revealed for the first time. Theoretical calculations highlight the role of mobile gold atoms in the observed symmetry changes and particle reshaping in the critical size regime. An unprecedented stable surface molecular structure of hydrogenated gold decorating a highly distorted core is identified. By combining atomic scale in situ observations and modeling of nanoparticle structure under relevant reaction conditions, this work provides a fundamental understanding of the size-dependent reactivity of gold nanoparticles with a precise picture of their surface at working conditions.The ORCID identification number(s) for the author(s) of this article can be found under https://doi.org/10.1002/smll.202104571.

Cu segregation in Au–Cu nanoparticles exposed to hydrogen atmospheric pressure: how is fcc symmetry maintained?

“…These 18 observations differ from those of Au NPs in the same range of size, which loses their fcc symmetry upon the interaction with hydrogen. Indeed, while hydrogen is known to weakly interact with Au, it is found to induce drastic changes of both the surface and the core structures leading to a change of the symmetry of the whole particle [29]. In contrast, at first stage, hydrogen seems to affect only the surface atoms in the case of Au-Cu NPs.…”

“…This result may indicate a charge transfer processes from the hydrogenated surface atoms to subsurface atoms, predicted to be stronger in the case of Cu. For pure hydrogenated gold NP [29], the charge transfer was found to take place from the core toward the surface, which revealed a complex and non-intuitive behavior. In summary, if the chemical properties of our small sized Au and Au-Cu NPs, could be determined, to a certain degree, by the local density of d-states lying below the Fermi level, further analysis of the sp-states, which are smaller in value but near the Fermi level, as well as the anti-bonding electron transfers [48] using more accurate level of theory, would be useful.…”

“…In a very recent work [29], combined in situ ETEM observations under hydrogen at atmospheric pressure and varied temperature, and ab initio molecular dynamic (AIMD) simulations showed that monometallic Au NPs of sub-4nm size lose their initial fcc symmetry (fcc symmetry under vacuum vs non-fcc or icosahedral symmetries under hydrogen) and are highly fluctuating. In contrast, no changes were observed on bigger sized Au NPs (> 4 nm).…”

“…In contrast, no changes were observed on bigger sized Au NPs (> 4 nm). The strong structural instability of small sized Au NPs was shown to result from the strong motion of atoms from the center toward the sub-surface layers and vice-versa, induced by the hydrogenated gold surface and the formed H-Au-H-Au crowned lines surrounding the distorted core [29]. In this work, we show that alloying Au by Cu induces stabilization of the NPs and prevents the structural fluctuations under atmospheric hydrogen pressure exposure as the case of monometallic Au counterpart.…”

Realistic Modelling of Dynamics at Nanostructured Interfaces Relevant to Heterogeneous Catalysis

“…These approaches have emerged as powerful tools to explore the conformational changes of metal NPs when exposed to different pressures of molecules. Very recently, Nassereddine et al [54] employed BOMD simulations to provide insights on the atomic-scale mechanisms that underpin the experimentally observed size-dependent structural dynamics of Au NPs exposed to atmospheric hydrogen pressure. These authors monitored the structural evolution of hydrogen-covered Au NPs of 2 nm in size for more than 60 ps of simulation time, in order to demonstrate that the strong Au-H interactions may induce a high mobility of gold atoms, which drastically change the shape and the symmetry of the whole NP at room temperature (Figure 1).…”