We predict the structures of neutral gas-phase gold clusters (Aun, n = 5-13) at finite temperatures based on free-energy calculations obtained by replica-exchange ab initio molecular dynamics. The structures of neutral Au5-Au13 clusters are assigned at 100 K based on a comparison of experimental far-infrared multiple photon dissociation spectra performed on Kr-tagged gold clusters with theoretical anharmonic IR spectra and freeenergy calculations. The critical gold cluster size where the most stable isomer changes from planar to nonplanar is Au11 (capped-trigonal prism, D3h) at 100 K. However, at 300 K (i.e., room temperature), planar and nonplanar isomers may coexist even for Au8, Au9, and Au10 clusters. Density-functional theory exchangecorrelation functionals within the generalized gradient or hybrid approximation must be corrected for longrange van der Waals interactions to accurately predict relative gold cluster isomer stabilities. Our work gives insight into the stable structures of gas-phase gold clusters by highlighting the impact of temperature, and therefore the importance of free-energy over total energy studies, and long-range van der Waals interactions on gold cluster stability.Metal clusters in the gas phase are widely used as model systems to study fundamental properties of condensed matter (see, e.g., Refs. [1][2][3][4][5][6][7]). At the nanoscale, gold is not a fully noble, nonreactive material; thus gold clusters are of particular interest due to their possible applications in gas sensing, pollution reduction, and catalysis [8][9][10][11][12][13][14][15][16][17]. Gold clusters in the gas phase exhibit many structural isomers of similar energetics and can rapidly interconvert among them [18][19][20][21][22][23][24]. Geometry and size can impact the physicochemical properties of clusters, for example, HOMO-LUMO gap, polarizability, and catalytic activity [25][26][27][28][29][30][31]. Small gold clusters often adopt stable planar geometries [32][33][34][35][36]. The critical size where gold clusters begin favoring nonplanar (three-dimensional, 3D) structures over planar (two-dimensional, 2D) structures has attracted sustained interest [37][38][39][40][41][42][43][44]. Gasphase ion-mobility experiments at room temperature suggest a transition from a 2D to 3D ground state structure at size 12 [45] and 8 [46] for negatively and positively charged clusters, respectively. Computational and experimental evidence supports these ground-state structural assignments for the 2D to 3D transition size [47,48]. For neutral gold clusters, computational studies predict their critical transition size is between Au10 and Au14 at zero kelvin [26,38,44], but experimental evidence is lacking due to the difficulty in spectroscopically characterizing neutral gas-phase clusters relative to charged clusters.Both experimental and computational studies indicate that dynamic structural rearrangements are a common feature among clusters at finite temperatures [22,[49][50][51][52][53][54][55][56][57]. It is clear that small go...
Restructuring of interfaces plays a crucial role in materials science and heterogeneous catalysis. Bimetallic systems, in particular, often adopt very different composition and morphology at surfaces compared to the bulk. For the first time, we reveal a detailed atomistic picture of long-timescale restructuring of Pd deposited on Ag, using microscopy, spectroscopy, and novel simulation methods. By developing and performing accelerated machine-learning molecular dynamics followed by an automated analysis method, we discover and characterize previously unidentified surface restructuring mechanisms in an unbiased fashion, including Pd-Ag place exchange and Ag pop-out, as well as step ascent and descent. Remarkably, layer-by-layer dissolution of Pd into Ag is always preceded by an encapsulation of Pd islands by Ag, resulting in a significant migration of Ag out of the surface and a formation of extensive vacancy pits within a period of microseconds. These metastable structures are of vital catalytic importance, as Ag-encapsulated Pd remains much more accessible to reactants than bulk-dissolved Pd. Our approach is broadly applicable to complex multimetallic systems and enables the previously intractable mechanistic investigation of restructuring dynamics at atomic resolution. File list (2) download file view on ChemRxiv 061220_PdAg_ESI_v5.pdf (13.63 MiB) download file view on ChemRxiv 061220_PdAg_Main_v5.pdf (11.39 MiB)
Vanadium redox flow batteries are a promising technology for energy storage, yet the mechanism of the kinetically limiting V 2+ /V 3+ redox reaction remains poorly understood. Here, we elucidate the impact of anion complexation on V 2+ /V 3+ kinetics on a glassy carbon electrode in three common electrolytes: hydrochloric acid, sulfuric acid, and mixed HCl/H 2 SO 4 . The V 2+ /V 3+ kinetics are ∼2.5 times faster in HCl and have lower apparent activation energies than those in H 2 SO 4 or HCl/H 2 SO 4 . We also identify the presence of [V(H 2 O) 4 Cl 2 ] + species in HCl by UV−vis spectroscopy. We confirm that the V 2+ /V 3+ reaction proceeds via an adsorbed intermediate and propose a bridging mechanism through adsorbed *Cl (in HCl) and *OH (in H 2 SO 4 or HCl/H 2 SO 4 ). A bridging mechanism through *Cl is supported by even faster redox kinetics in HBr than in HCl, possibly due to the higher polarizability of *Br. By measuring the exchange current densities using steady-state current measurements and impedance spectroscopy, we show that the overall reaction is a two-electron process in HCl as opposed to a one-electron process in H 2 SO 4 and HCl/H 2 SO 4 .
Restructuring of interfaces plays a crucial role in materials science and heterogeneous catalysis. Bimetallic systems, in particular, often adopt very different composition and morphology at surfaces compared to the bulk. For the first time, we reveal a detailed atomistic picture of long-timescale restructuring of Pd deposited on Ag, using microscopy, spectroscopy, and novel simulation methods. By developing and performing accelerated machine-learning molecular dynamics followed by an automated analysis method, we discover and characterize previously unidentified surface restructuring mechanisms in an unbiased fashion, including Pd-Ag place exchange and Ag pop-out, as well as step ascent and descent. Remarkably, layer-by-layer dissolution of Pd into Ag is always preceded by an encapsulation of Pd islands by Ag, resulting in a significant migration of Ag out of the surface and a formation of extensive vacancy pits within a period of microseconds. These metastable structures are of vital catalytic importance, as Ag-encapsulated Pd remains much more accessible to reactants than bulk-dissolved Pd. Our approach is broadly applicable to complex multimetallic systems and enables the previously intractable mechanistic investigation of restructuring dynamics at atomic resolution.
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