No abstract

Interactions between catalytically active metal particles and reactant gases depend strongly on the particle size, particularly in the subnanometer regime where the addition of just one atom can induce substantial changes in stability, morphology, and reactivity. Here, time-lapse scanning tunneling microscopy (STM) and density functional theory (DFT)-based calculations are used to study how CO exposure affects the stability of Pt adatoms and subnano clusters at the Fe 3 O 4 (001) surface, a model CO oxidation catalyst. The results reveal that CO plays a dual role: first, it induces mobility among otherwise stable Pt adatoms through the formation of Pt carbonyls (Pt 1 -CO), leading to agglomeration into subnano clusters. Second, the presence of the CO stabilizes the smallest clusters against decay at room temperature, significantly modifying the growth kinetics. At elevated temperatures, CO desorption results in a partial redispersion and recovery of the Pt adatom phase. S ubnanometer metal particles exhibit a range of interesting electronic or catalytic properties that can vary substantially with the removal or addition of a single atom (1-6). Understanding the mechanistic details underlying the rearrangement of the active phase is important because changes in cluster size and shape are known to be commonplace under the conditions used in heterogeneous catalysis (7,8), and because such processes are associated with deactivation phenomena such as sintering. Although sintering is usually regarded as a thermally activated process, there is increasing evidence that adsorbates influence sintering rates in a reactive environment by formation of mobile metal-molecule intermediates (2,. Indeed, in a previous study we demonstrated that the formation of highly mobile Pd 1 -CO species led to enhanced sintering in the Pd/Fe 3 O 4 (001) system (31). Here, we turn our attention to Pt. Mobility is induced in the form of Pt 1 -CO. In addition, CO stabilizes the smallest clusters. When it desorbs, Pt dimers break up into single atoms; thus, the CO is necessary for preserving nuclei that act as seeds for further growth. Using roomtemperature scanning tunneling microscopy (STM), complemented by X-ray photoelectron spectroscopy (XPS) and density functional theory with an on-site Hubbard U (DFT+U), we follow the COinduced diffusion and coalescence of Pt atom-by-atom, creating catalytically active (32) subnano clusters with a well-defined size distribution. On heating, desorption of CO leads to significant redispersion of Pt into the adatom phase. Fig. 1B), the configuration commonly observed for other metal adatoms at this surface (31,35,36,39,40). DFT+U calculations find an adsorption energy ΔE ads (Pt 1 ) of 3.89 eV compared with free Pt atoms in vacuum and little charge transfer to the surface (<0.5 e − ). A second configuration, labeled Pt 1 *, not previously observed for other metals, appears offset to one side in STM images. Our DFT+ U calculations find a stable adsorption site [ΔE ads (Pt 1 *) = 3.01 eV, charge transfer <0.3...

Metal-support interactions are frequently invoked to explain the enhanced catalytic activity of metal nanoparticles dispersed over reducible metal-oxide supports, yet the atomic scale mechanisms are rarely known. Here, we use scanning tunneling microscopy to study a Pt 1-6 /Fe 3 O 4 model catalyst exposed to CO, H 2 , O 2 , and mixtures thereof, at 550 K. CO extracts lattice oxygen at the cluster perimeter to form CO 2 , creating large holes in the metal-oxide surface. H 2 and O 2 dissociate on the metal clusters and spill over onto the support. The former creates create surface hydroxyl groups, which react with the support to desorb water, while atomic oxygen reacts with Fe from the bulk to create new Fe 3 O 4 (001) islands. The presence of the Pt is crucial because it catalyses reactions that already occur on the bare iron-oxide surface, but at higher temperatures. Variations in the CO oxidation activity of nominally similar metal nanoparticles dispersed over different metal oxides [1] are a clear indicator of metal-support interactions, but the mechanisms by which the metal oxide gets involved are difficult to ascertain. The support is known to transfer electrons to (or from) the clusters [2] , modifying their shape and adsorption properties, and adsorb reactants and promoters (e.g. water) that can speed up the reaction [3] . At elevated temperature, the so-called Mars-van Krevelen (MvK) mechanism [1i] can occur, where CO molecules extract lattice oxygen (O lattice ) at the cluster perimeter forming CO 2 , and gas phase O 2 repairs the surface to complete the catalytic cycle. While there is mounting evidence that MvK plays a role when reducible metal oxides are utilized as the support [1i] , there is little atomic-scale information available to better understand how the process occurs.In this paper, we use scanning tunneling microscopy (STM) to follow the evolution of a Pt/Fe 3 O 4 (001) model catalyst exposed to CO, O 2 and H 2 at 550 K. Holes and islands in the vicinity of Pt clusters provide direct evidence of reduction and oxidation of the metal oxide in CO and O 2 rich atmospheres respectively, while dissociation and spillover of H 2 leads to hydroxylation of the support lattice. We interpret our results as the metal catalyzing reactions that otherwise occur between the reactants and support at higher temperatures.An STM image of the Pt/Fe 3 O 4 (001) model catalyst utilized as the basis for this work is shown in Figure 1. The Fe 3 O 4 (001)-support [4] exhibits large, flat terraces ( Fig. 1, inset) characterized by rows of protrusions related to surface Fe atoms in STM images. The rows rotate by 90° from terrace to terrace, a consequence of the spinel structure. Surface O atoms are not imaged because they have no density of states in the vicinity of the Fermi energy [5] . Surface OH groups, formed through the reaction of water and oxygen vacancies during sample preparation [6] , modify the density of states of the neighboring Fe atoms making them appear brighter in STM images [7] . The sur...

Anisotropic bimetallic nanoparticles are promising candidates for plasmonic and catalytic applications. Their catalytic performance and plasmonic properties are closely linked to the distribution of the two metals, which can change during applications in which the particles are exposed to heat. Due to this fact, correlating the thermal stability of complex heterogeneous nanoparticles to their microstructural properties is of high interest for the practical applications of such materials. Here, we employ quantitative electron tomography in high-angle annular darkfield scanning transmission electron microscopy (HAADF-STEM) mode to measure the 3D elemental diffusion dynamics in individual anisotropic Au-Ag nanoparticles upon heating in situ.This approach allows us to study the elemental redistribution in complex, asymmetric nanoparticles on a single particle level, which was inaccessible to other techniques so far. In this work, we apply the proposed method to compare the alloying dynamics of Au-Ag nanoparticles with different shapes and compositions and find that the shape of the nanoparticle does not exhibit a significant effect on the alloying speed whereas the composition does. Finally, comparing the experimental results to diffusion simulations allows us to estimate the diffusion coefficients of the metals for individual nanoparticles.

Dilute Pd-in-Au alloy catalysts are promising materials for selective hydrogenation catalysis. Extensive surface science studies have contributed mechanistic insight on the energetic aspect of hydrogen dissociation, migration and recombination on dilute alloy systems. Yet, translating these fundamental concepts to the kinetics and free energy of hydrogen dissociation on nanoparticle catalysts operating at ambient pressures and temperatures remains challenging. Here, the effect of the Pd concentration and Pd ensemble size on the catalytic activity, apparent activation energy and rate limiting process is addressed by combining experiment and theory. Experiments in a flow reactor show that a compositional change from 4 to 8 atm% Pd of the Pd-in-Au alloy catalyst leads to strong increase in activity, even exceeding the activity per Pd atom of monometallic Pd under the same conditions, albeit with an increase in apparent activation energy. First-principles calculations show that the rate and apparent activation enthalpy for HD exchange increase when increasing the Pd ensemble size from single Pd atoms to Pd trimers in a Au surface, suggesting that the ensemble size distribution shifts from mainly single Pd atoms on the 4 atm% Pd alloy to larger Pd ensembles of at least three atoms for the 8 atm% Pd/Au catalyst. The DFT studies also indicated that the rate-controlling process is different: H 2 (D 2 ) dissociation determines the rate for single atoms whereas recombination of adsorbed H and D determines the rate on Pd trimers, similar to bulk Pd. 2Both experiment and theory suggest that the increased reaction rate with increasing Pd content and ensemble size stems from an entropic driving force. Finally, our results support hydrogen migration between Pd sites via Au and indicate that the dilute alloy design prevents the formation of subsurface hydrogen, which is crucial in achieving high selectivity in hydrogenation catalysis.

Trained neural networks are used to extract the first partial coordination numbers from XANES spectra. In bimetallic nanoparticles, the four local structure descriptors provide rich information on structural motifs.

We studied suspensions of sterically stabilized poly(methyl methacrylate) (PMMA) particles in the solvent cyclohexyl bromide (CHB; εr = 7.92). We performed microelectrophoresis measurements on suspensions containing a single particle species and on binary mixtures, using confocal microscopy to measure the velocity profiles of the particles. We measured the charge of so-called locked PMMA particles, for which the steric stabilizer, a comb-graft stabilizer of poly(12-hydroxystearic acid) (PHSA) grafted on a backbone of PMMA, was covalently bonded to the particle, and for unlocked particles, for which the stabilizer was adsorbed to the surface of the particle. We observed that locked particles had a significantly higher charge than unlocked particles. We found that the charge increase upon locking was due to chemical coupling of 2-(dimethylamino)ethanol to the PMMA particles, which was used as a catalyst for the locking reaction. For particles of different size we obtained the surface potential and charge from the electrophoretic mobility of the particles. For locked particles we found that the relatively high surface potential (∼ +5.1 kBT/e or +130 mV) was roughly constant for all particle diameters we investigated (1.2 μm < σ < 4.4 μm), and that the particle charge was proportional to the square of the diameter.

Composite noble metal-based nanorods for which the surface plasmon resonances can be tuned by composition and geometry are highly interesting for applications in biotechnology, imaging, sensing, optoelectronics, photovoltaics, and catalysis. Here, we present an approach for the oxidative etching and subsequent metal overgrowth of gold nanorods, all taking place while the nanorods are embedded in mesoporous SiO 2 shells (AuNRs@meso-SiO 2 ). Heating of the AuNRs@meso-SiO 2 in methanol with HCl resulted in reproducible oxidation of the AuNRs by dissolved O 2 , specifically at the rod ends, enabling precise control over the aspect ratio of the rods. The etchedAuNRs@meso-SiO 2 were used as a template for the overgrowth of a second metal (Ag, Pd, and Pt), yielding bimetallic, core−shell structured nanorods. By varying the reaction rates of the metal deposition both smooth core−shell structures or gold nanorods covered with a dendritic overlayer could be made. This control over the morphology, including metal composition, and thus the plasmonic properties of the composite rods were measured experimentally and also confirmed by Finite-Difference Time-Domain (FDTD) calculations. The presented synthesis method gives great control over tuning over both plasmonic properties and the particle stability/affinity for specific applications.