Low-coordinated Surface Sites Research Articles

Morphology-Dependent Interaction between Tetraphenylporphyrins and a Metal Oxide Surface: Electronic Signature, Photo Physics, and Chemical Interaction

Porphyrin-substrate hybrid systems are the building blocks in a series of materials, such as organic light-emitting diodes, chemical sensors, dye-sensitized solar cells and catalysts. Functional layers and 2D nanostructures on surfaces enable sensors, and nanoscale optical and magnetic materials. 3D porphyrin structures are for instance used in controlling and manipulating light at subwavelength dimensions. Understanding and correctly describing the way molecules interact with the substrate and in thin-film structures gives a handle on geometrical and electronic device properties. Indeed, adsorption and film-formation have a relevant effect on the molecular electronic structure even in the absence of strong covalent bonding, namely in terms of level-alignment with substrate states in the former and polarization-induced renormalization of molecular band gaps in the latter.Using the case of Co(II)-tetraphenylporphyrin (CoTPP) on the MgO(100), we shine light on the adsorption at morphology-related low-coordinated surfaces sites and the effect of multilayer film formation from a theoretical perspective in a frame work of hybrid density functional theory and many-body perturbation theory. We simulate the photoemission spectra for relevant adsorption sites as a fingerprint with distinct site-related features in both valence and core-level regions of the electronic structure that are also observed in experiments on MgO(100) films on Ag(100) and thus aid the identification of sites. Effects of the formation of porphyrin films on the electronic and optical properties will be outlined for H2TPP and MgTPP.We also highlight the self-metalation of H2TPP at morphology-related low-coordinated sites. Metalation is an important pathway for functionalization of porphyrin-based organic-inorganic structures. We address the self-metalation mechanism using ab initio molecular dynamics simulation at room temperature. While H2TPP is mobile on the pristine surface as the steric hindrance by phenyl rings prevents the physisorption of the macrocycle at a specific site, step edges or kink sites provide anchor points exposing low-coordinated, reactive oxygen-sites to hydrogens of the macrocycle. We report a spontaneous proton transfer at these sites forming an intermediate complex before the metalation occurs. This highlights the role of low-coordinated surface sites for the chemical interaction with the porphyrin.

Defect-Rich CuZn Nanoparticles for Model Catalysis Produced by Femtosecond Laser Ablation.

Femtosecond laser ablation of Cu0.70Zn0.30 targets in ethanol led to the formation of periodic surface nanostructures and crystalline CuZn alloy nanoparticles with defects, low-coordinated surface sites, and, controlled by the applied laser fluence, different sizes and elemental composition. The Cu/Zn ratio of the nanoparticles was determined by energy dispersive X-ray spectroscopy, X-ray photoelectron spectroscopy, and selected area electron diffraction. The CuZn nanoparticles were about 2-3 nm in size, and Cu-rich, varying between 70 and 95%. Increasing the laser fluence from 1.6 to 3.2 J cm-2 yielded larger particles, more stacking fault defects, and repeated nanotwinning, as evident from high-resolution transmission electron microscopy, aided by (inverse) fast Fourier transform analysis. This is due to the higher plasma temperature, leading to increased random collisions/diffusion of primary nanoparticles and their incomplete ordering due to immediate solidification typical of ultrashort pulses. The femtosecond laser-synthesized often nanotwinned CuZn nanoparticles were supported on highly oriented pyrolytic graphite and applied for ethylene hydrogenation, demonstrating their promising potential as model catalysts. Nanoparticles produced at 3.2 J cm-2 exhibited lower catalytic activity than those made at 2.7 J cm-2. Presumably, agglomeration/aggregation of especially 2-3 nm sized nanoparticles, as observed by postreaction analysis, resulted in a decrease in the surface area to volume ratio and thus in the number of low-coordinated active sites.

Role of Fe decoration on the oxygen evolving state of Co3O4 nanocatalysts.

The production of green hydrogen through alkaline water electrolysis is the key technology for the future carbon-neutral industry. Nanocrystalline Co3O4 catalysts are highly promising electrocatalysts for the oxygen evolution reaction and their activity strongly benefits from Fe surface decoration. However, limited knowledge of decisive catalyst motifs at the atomic level during oxygen evolution prevents their knowledge-driven optimization. Here, we employ a variety of operando spectroscopic methods to unveil how Fe decoration increases the catalytic activity of Co3O4 nanocatalysts as well as steer the (near-surface) active state formation. Our study shows a link of the termination-dependent Fe decoration to the activity enhancement and a significantly stronger Co3O4 near-surface (structural) adaptation under the reaction conditions. The near-surface Fe- and Co-O species accumulate an oxidative charge and undergo a reversible bond contraction during the catalytic process. Moreover, our work demonstrates the importance of low coordination surface sites on the Co3O4 host to ensure an efficient Fe-induced activity enhancement, providing another puzzle piece to facilitate optimized catalyst design.

Dealloying of Pt1Bi2 intermetallic toward optimization of electrocatalysis on a Bi-continuous nanoporous core-shell structure.

Noble metal nanoporous materials hold great potential in the field of catalysis, owing to their high open structures and numerous low coordination surface sites. However, the formation of porous nanoparticles is restricted by particle size. Herein, we utilized a Pt1Bi2 intermetallic nanocatalyst to develop a dealloying approach for preparing nanoparticles with a bi-continuous porous and core-shell structure and proposed a mechanism for the formation of pores. The particle size used to form the porous structure can be <10 nm, which enhances the nanocatalyst's performance for the oxygen reduction reaction (ORR). This study provides a new understanding of the formation of porous materials via a dealloying approach.

Low-coordinated surface sites make truncated Pd tetrahedrons as robust ORR electrocatalysts outperforming Pt for DMFC devices

Low-coordinated surface sites make truncated Pd tetrahedrons as robust ORR electrocatalysts outperforming Pt for DMFC devices

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

The 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 4nm, 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.

Dendritic Silver/Palladium Alloy Arrays for Electrochemical CO2 Reduction

Increased fossil fuel consumption has sparked concerns about developing new energy sources and reducing carbon dioxide emissions. The electrochemical reduction of CO2 using renewable energy sources, e.g. wind and solar is an appealing method for making a range of carbon-based products such as CO even more valuable hydrocarbons.1 There have been several significant advances in CO2 electrochemical reduction in recent years.2 However, currently there are no efficient catalysts for CO2 reduction as activation of CO2 requires a large input of energy; and the reaction usually results in the broad product distribution.3 Thus, the search for new catalysts, which can promote this reaction along a specific reaction pathway, is one of the most important tasks to make electrochemical CO2 reduction successful.Herein, we developed dendritic silver/palladium (Ag/Pd) alloys nanoarrays by a simple galvanic replacement reaction. Figure 1a demonstrates a typical SEM image of Ag/Pd dendrites with feather-like branches. The earlier study proposed the vacancies generated by the displacement reaction attract silver diffusing to the surface and form alloys with Pd.4 Compared with the alloys of Ag80Pd20 or Ag60Pd40, Ag70Pd30 shows the better performance that can selectively convert CO2 to CO with a maximum faradaic efficiency (FE) of 98.6% and a current density of ~ 9 mA/cm2 at the potential of −0.8 V vs. RHE (Figure 1b, c).The high performance of Ag/Pd alloy can be ascribed to the synergistic effects between Pd and Ag. Alloying can adjust electronic state to optimize the adsorption and coupling of intermediate. The calculated Gibbs free energy (ΔG) diagram is depicted in Figure 1d, representing CO2 reduction to CO through elementary reaction steps involving the intermediates of *COOH and *CO according to first principles calculations. The formation of *COOH with an energy barrier of 0.81 eV on Ag70Pd30 alloy surface, which is ~ 0.1 eV lower than on the pure Ag(111) surface. In addition, dendrite with multiple active sites and hydrophobic array might play an indispensable role in improving the catalytic performance. The dendritic vertical arrays structure has an increased active surface area, which will expose more low-coordinated surface sites. Contact angle measurements have confirmed the hydrophobic surface feature of this alloy (Figure 1e), which can enhance the capture of gas, in turn increase the concentration of CO2 on the electrode surface, therefore improve CO2 reduction selectivity.5 Figure 1. (a) SEM image of Ag70Pd30 sample. (b) Potential-dependent faradaic efficiency of different samples on electrocatalytic reduction of CO2 to CO. (c) Stability performance of Ag70Pd30 for CO2 reduction operated at the potential of −0.8 V vs. RHE for 8 h. (d) Free energy diagram for electrochemical CO2 reduction to CO. (e) Contact angel of silver piece and silver/palladium dendrites.

Proton Migration on Perfect, Vacant, and Doped MgO(001) Surfaces: Role of Dissociation Residual Groups

Although migration processes on low-coordination surface sites of a magnesium oxide (MgO) surface are important in technological applications including catalysis in gas interfaces, they are not well understood. In particular, hydrated MgO(001) surfaces present charge transfer and ionic transport phenomena that sometimes do not work properly in technological devices. On the other hand, low-coordination surface sites are chemically attractive. Given that defects and adsorbed species can be characterized by the kinetics of surface processes, in this work, we investigated proton mobility pathways produced from the dissociative adsorption of water molecules on different MgO(001) surfaces: a perfect-terrace, with an anionic vacancy, and with an Al-doped + cationic vacancy. For that purpose, we employed density-functional theory with periodic boundary conditions combined with the climbing imaged nudged elastic band method to compute energy barriers. The dissociative adsorption of water molecules on the perfect-t...

Disclosing the Oxidation State of Copper Oxides upon Electrochemical CO2-Reduction

In recent years, an increasing amount of work has been devoted to study the electrochemical reduction of carbon dioxide, which is regarded as a means to valorize CO2 while possibly closing the carbon cycle. Despite this interest and the large variety of CO2-reduction products that have been reported in the literature, only the reduction towards formate, CO or certain alcohols is expected to be cost competitive if the latter can be produced in sufficiently large volumes.1,2 Specifically, CO2-electroreduction to methanol or ethanol has preponderantly been carried out using catalysts based on copper oxides (CuOx),3 for which the (surface) oxidation state in the course of the reaction and its concomitant effect on the product selectivity remain controversial. This assignment is further complicated by the large surface roughness customarily displayed by these copper oxides (that are often prepared through electrodeposition and thermal treatment steps),4 which upon reduction would lead to an abundance of low-coordinated surface sites that reportedly reduce CO2 to ethanol.5 To untangle these effects, in this study we used DC magnetron sputtering to prepare low roughness Cu2O thin films (TFs) with a controlled oxide thickness of 100 nm, and used these as electrocatalysts for the reduction of CO2. Interestingly, these model electrodes did not display the enhanced selectivity towards ethanol and ethylene usually reported for CuOx, but instead led to a product distribution similar to the one attained with polycrystalline copper. Consistently, grazing angle X-ray diffraction measurements of the TFs prior to and after chronoamperometric (CA) CO2-reduction at - 1.0 V vs. the reversible hydrogen electrode (RHE) suggested the co-reduction of the copper oxide in the course of the reaction. To verify this and additionally study the TFs’ surface oxidation state, complementary X-ray photoelectron spectroscopy (XPS) and Ar-sputtering measurements were performed; chiefly, sample re-oxidation upon disassembly of the electrochemical cell and/or transfer to the XPS were avoided by performing the electrochemical measurements in a N2-filled glovebox and transferring the sample in an air-tight chamber, respectively. As illustrated in Figure 1, this resulted in clear changes in the Cu-2p and -LMM spectra – specifically, whereas in the former no significant differences were observed when the post-CA samples were transferred in air or N2 (likely due to the identical Cu-2p spectra exhibited by Cu0 and Cu2O),6 the Auger spectra were much more sensitive to the transfer means, and the excellent agreement6 between the spectrum of the N2-transferred sample and reports for Cu0 confirmed the complete reduction of the TF upon CO2-electroreduction. Ultimately, these results indirectly point at the surface roughness as the factor determining the enhanced alcohol selectivity of CO2-reduction electrocatalysts derived from (reduced) copper oxides, while highlighting the importance of air-free sample transfer for the post-mortem assignment of surface oxidation states.

Poison Tolerance to the Selective Hydrogenation of Cinnamaldehyde in Water over an Ordered Mesoporous Carbonaceous Composite Supported Pd Catalyst

A coordination-assisted pyrolysis procedure was adopted to encapsulate palladium (Pd) nanoparticles in a mesoporous carbonaceous matrix. X-ray diffraction and transmission electron microscopy measurements revealed that approximately 2.5 nm nanoparticles were highly dispersed inside the well-ordered porous framework. High-resolution TEM and temperature-programmed hydride decomposition analysis demonstrated the formation of interstitial carbon in the Pd lattice. Diffuse reflectance infrared Fourier transform spectroscopy indicated that carbon species could be deposited on low-coordinated surface sites of the Pd particles. This catalyst exhibited high activity in the selective hydrogenation of cinnamaldehyde (CAL) at 80 °C under an H2 pressure of 1.0 MPa (turnover frequency (TOF) of 2.4 s–1) to produce hydrocinnamyl aldehyde with high selectivity (HCAL; approximately 80%) in water and could be reused eight times with no clear activity loss. A trapping agent poisoning experiment using solid SH-SBA-15 revealed...

Functional link between surface low-coordination sites and the electrochemical durability of Pt nanoparticles

A promising strategy for achieving enhanced catalytic activity involves the use of nanoscale electrocatalysts; however, their low stability remains a major challenge. Among the various performance-degradation mechanisms, atomic dissolution is known to cause severe nanoparticle deactivation. To date, the factors influencing these catalysts' durability are not understood. Herein, we assess the role of low-coordination surface sites, focusing on the atomic dissolution of Pt nanoparticles. The density of low-coordination sites was finely controlled, and no significant size change occurred. Based on our findings, we suggest that the initial low-coordination sites trigger metal dissolution, which subsequently accelerates Pt dissolution. We believe that controlling the surface coordination number can open new routes for the design of highly durable nanoscale electrocatalysts.

Effects of structure and size of Ni nanocatalysts on hydrogen selectivity via water-gas-shift reaction—A first-principles-based kinetic study

The effects of structure and size of nickel nanocatalysts on hydrogen production via water-gas shift reaction (WGSR) were investigated using a first-principles-based kinetic model. Using periodic density functional theory and statistical calculations, thermochemistry and kinetics of the WGSR and competing methanation was calculated on Ni(111), Ni(100), and Ni(211) facets. The kinetics of the elementary reactions involving CH, OH, and CO bond was found to fit to a general Brønsted–Evans–Polanyi (BEP) type linear relationship on all Ni facets considered. A mechanism describing the competition between the hydrogen and methane formation routes is constructed for further microkinetic modeling. The hydrogen production turnover frequency (TOF) via the WGSR route suggests the preference to the low-coordinated surface sites with the reaction activities following the order of Ni(211)>Ni(100)>Ni(111) using a simulated feed gas with a molar ratio of CO:H2O=1:2. Due to the methanation, the TOF of methane production follows the same trend of hydrogen production. Consequently, the TOF of hydrogen production decreases with increasing particle diameters, due to the decreasing fractions of low-coordinated surface nickel atoms. It is also found that the presence of H2 in feed gas can largely enhance the methanation reaction.

Increasing Stability and Activity of Core-Shell Catalysts By Preferential Segregation of Oxide on Edges and Vertexes: Oxygen Reduction on Ti-Au@Pt/C

The slow kinetics of the oxygen reduction reaction (ORR) is one of the key obstacles for the widespread commercialization of polymer electrolyte membrane fuel cells (PEMFCs), which has been established as one of the zero-emission power sources for the future [1-4]. Pt and its alloys with transition metals are frequently used as electrocatalysts for the cathodic oxygen reduction. However, the high cost and low utilization of Pt of the state-of-the-art Pt catalysts continues to humper its broad application in PEMFCs. This can be resolved using the Pt monolayer catalysts having low Pt loading, high utilization efficiency, as well as high activity and durability [5, 6]. We recently showed that Pt monolayer shell deposited on Pd or PdAu alloy core has higher activity and stability than pure Pt electrocatalysts [6]. However, total Pt group metal content is still too high for practical application. As a non-Pt group metal, Au is a good candidate for the core material since it is stable under the oxidizing conditions of the ORR. However, the lack of interest in the Au@Pt core-shell catalysts can be attributed to two impeding effects on ORR activity, that are, the strong bonding of OH and O to Pt due to the up-shifts of d-band center for Pt on Au core induced mainly by the strain effects [7], and the blocking of active sites of Pt due to the Au segregation onto the Pt surface [6, 8]. In this contribution we are reporting on a novel Au@Pt core-shell catalyst with the low-coordinated surface sites doped by Ti oxide, e.g. vertex and edges, while Pt occupies the surface facets, Ti-Au@Pt (Fig. 1). Its mass (3.0 A mg–1 Pt) and specific activities (1.32 mA cm–2 Pt) are about 13 and 5 times higher than the corresponding activities of the commercial Pt/C catalyst (0.22 A mg–1 Pt and 0.254 mA cm–2 Pt), respectively (Fig. 2). This mass activity is among the highest reported for the Pt based core-shell nanoparticles under similar testing conditions, while the durability tests show no activity loss after 10,000 potential cycles from 0.6 to 1.0 V (vs. RHE). By correlating electron energy-loss spectroscopy and X-ray photoelectron spectroscopy analyses with electrochemical measurements, we attribute the superior activity and durability of the new Ti-Au@Pt catalyst to its distinct microstructure. The enhanced ORR activity of Ti-Au@Pt/C compared to commercial Pt/C is due to a more effective hydrogenation of OHad on its Pt surface than commercial Pt/C catalyst. This can be attributed to the repulsive interaction between the OHad on Pt and the oxide species on a neighboring Ti. Improved durability can also be attributed to the presence of the Ti which prevents Au atoms from segregating onto the Pt surface and thus can largely retain the electrochemical surface area. This core-shell catalyst points to a new direction for the ORR and similar catalysts design and optimization. Acknowledgment Work at Brookhaven National Laboratory is supported by US Department of Energy, Division of Chemical Sciences, Geosciences and Biosciences Division, under the Contract No. DE-AC02-98CH10886.

Model Approach in Heterogeneous Catalysis: Kinetics and Thermodynamics of Surface Reactions.

Heterogeneous catalysts are widely employed in technological applications, such as chemical manufacturing, energy harvesting, conversion and storage, and environmental technology. Often they consist of disperse metal nanoparticles anchored onto a morphologically complex oxide support. The compositional and structural complexity of such nanosized systems offers many degrees of freedom for tuning their catalytic performance. However, a rational design of heterogeneous catalysts based on an atomistic-level understanding of underlying surface processes has not been fully achieved so far and remains one of the primary goals for catalysis research. In our group, we developed concepts for replacing highly complex real supported catalysts by simplified model systems, which complexity can be gradually increased in order to mimic certain structural aspects of practically relevant catalysts in a controlled way. Well-defined model systems consisting of metal-nanoparticle ensembles supported on planar oxide substrates have proven to provide a successful approach to achieve fundamental insights into heterogeneous catalysis. In this Account, two mechanistic case studies focusing on an atomistic-level understanding of surface chemistry are presented in which we investigate how the nanoscopic nature of metal clusters affects their interaction with the adsorbates and the reactive processes. Particularly, we investigate the effects of the particle size and the flexibility of the atoms constituting metal clusters on the binding energy of gas-phase adsorbates, such as CO and oxygen. We identified two major structural factors determining the binding energy of gas phase adsorbates on metal nanoparticles: the local configuration of the adsorption site and the particle size. While the effect of the local configuration of the adsorption site was found to be adsorbate specific, the reduction of the cluster size results in a pronounced decrease of binding energy for both adsorbates and appears to be a general trend. In the second case study, we address the role of the surface modifiers, such as carbon, on the process of hydrogen diffusion into volume of Pd nanoparticles that was previously identified is an important step in hydrogenation chemistry. We provide for the first time direct experimental evidence that, inline with the recent theoretical predictions, the atomically flexible low-coordinated surface sites on Pd particles play a crucial role in the diffusion process and that their selective modification with carbon results in marked facilitation of subsurface hydrogen diffusion. By virtue of these examples, we demonstrate how model studies on complex nanostructured materials may provide an atomistic view of processes at the gas-solid interface related to heterogeneous catalysis.

Engineering Preferential Adsorption of Single-Walled Carbon Nanotubes on Functionalized ST-cut Surfaces of Quartz

Horizontal alignment during synthesis of single-walled carbon nanotubes has been found experimentally along certain directions of well-defined quartz surfaces. The reasons for such alignment are here examined using first-principles computational analysis, as a function of structure and chemistry of the specific exposed facet, presence and location of OH and H functional groups, and degree of hydration of the surface. It is found that selective functionalization of low-coordinated surface sites may cause exposure of low-coordinated Si atoms that bond strongly to nanotube walls. On the other hand, saturation of low-coordinated oxygen also favors carbon nanotube adhesion to the substrate. As found previously on bare silica surfaces, a chirality preference is confirmed on functionalized surfaces toward zigzag over armchair nanotubes. Magnetization effects on the surface originated by the presence of adsorbed functional groups are found to enhance adsorption of arm-chair nanotubes compared to that on clean surfaces. On the basis of the findings, it is suggested that surfaces may be engineered to favor horizontal adsorption of specific chiralities along preferential directions.

Particle size effects in the catalytic electroreduction of CO₂ on Cu nanoparticles.

A study of particle size effects during the catalytic CO2 electroreduction on size-controlled Cu nanoparticles (NPs) is presented. Cu NP catalysts in the 2-15 nm mean size range were prepared, and their catalytic activity and selectivity during CO2 electroreduction were analyzed and compared to a bulk Cu electrode. A dramatic increase in the catalytic activity and selectivity for H2 and CO was observed with decreasing Cu particle size, in particular, for NPs below 5 nm. Hydrocarbon (methane and ethylene) selectivity was increasingly suppressed for nanoscale Cu surfaces. The size dependence of the surface atomic coordination of model spherical Cu particles was used to rationalize the experimental results. Changes in the population of low-coordinated surface sites and their stronger chemisorption were linked to surging H2 and CO selectivities, higher catalytic activity, and smaller hydrocarbon selectivity. The presented activity-selectivity-size relations provide novel insights in the CO2 electroreduction reaction on nanoscale surfaces. Our smallest nanoparticles (~2 nm) enter the ab initio computationally accessible size regime, and therefore, the results obtained lend themselves well to density functional theory (DFT) evaluation and reaction mechanism verification.

Towards a comprehensive understanding of platinum dissolution in acidic media

Platinum is one of the most important electrode materials for continuous electrochemical energy conversion due to its high activity and stability. The resistance of this scarce material towards dissolution is however limited under the harsh operational conditions that can occur in fuel cells or other energy conversion devices. In order to improve the understanding of dissolution of platinum, we therefore investigate this issue with an electrochemical flow cell system connected to an inductively coupled plasma mass spectrometer (ICP-MS) capable of online quantification of even small traces of dissolved elements in solution. The electrochemical data combined with the downstream analytics are used to evaluate the influence of various operational parameters on the dissolution processes in acidic electrolytes at room temperature. Platinum dissolution is a transient process, occurring during both positive- and negative-going sweeps over potentials of ca. 1.1 VRHE and depending strongly on the structure and chemistry of the formed oxide. The amount of anodically dissolved platinum is thereby strongly related to the number of low-coordinated surface sites, whereas cathodic dissolution depends on the amount of oxide formed and the timescale. Thus, a tentative mechanism for Pt dissolution is suggested based on a place exchange of oxygen atoms from surface to sub-surface positions.

Spatiotemporal catalytic dynamics within single nanocatalysts revealed by single-molecule microscopy

This review discusses the latest advances in using single-molecule microscopy of fluorogenic reactions to examine and understand the spatiotemporal catalytic behaviors of single metal nanoparticles of various shapes including pseudospheres, nanorods, and nanoplates. Real-time single-turnover kinetics reveal size-, catalysis-, and metal-dependent temporal activity fluctuations of single pseudospherical nanoparticles (<20 nm in diameter). These temporal catalytic dynamics can be related to nanoparticles' dynamic surface restructuring whose timescales and energetics can be quantified. Single-molecule super-resolution catalysis imaging further enables the direct quantification of catalytic activities at different surface sites (i.e., ends vs. sides, or corner, edge vs. facet regions) on single pseudo 1-D and 2-D nanocrystals, and uncovers linear and radial activity gradients within the same surface facets. These spatial activity patterns within single nanocrystals can be attributed to the inhomogeneous distributions of low-coordination surface sites, including corner, edge, and defect sites, among which the distribution of defect sites is correlated with the nanocrystals' morphology and growth mechanisms. A brief discussion is given on the extension of the single-molecule imaging approach to catalysis that does not involve fluorescent molecules.

Cobalt/cobalt oxide nanoparticles-assembled coatings with various morphology and composition synthesized by pulsed laser deposition

Nano-particle (NP) catalysts are under intense investigation in the nanotechnology community because of their large surface-to-volume atomic ratio, size- and shape-dependent properties, and high concentration of low-coordinated active surface sites. However, there is no general strategy to synthesize NPs, possibly in one step, of various materials with tailored properties and desired morphologies. In this work, cobalt oxide NPs assembled coatings have been prepared by reactive pulsed laser deposition of Co in O2 atmosphere. Three different deposition parameters have been varied to investigate their relevance in tailoring composition and structure of the synthesized NPs: 1) the target–substrate distance, 2) the oxygen pressure in deposition chamber, and 3) post oxidation of NPs deposited in vacuum condition. It is proved that by appropriate selection of the deposition parameters, several NP types may be obtained in one single step, namely: Co3O4 or CoO NPs, Co–Co3O4 or Co–CoO core shell NPs, with size ranging from a few nm to several tens of nm. Transmission and Scanning Electron Microscopy as well as Raman Spectroscopy have been utilized to carefully investigate the compositional and structural features of the NPs. A correlation is made between final NP properties and laser-induced processes mainly related to phase explosion generating liquid droplets that cool during flight or on the substrate. Photocatalytic properties of this heterogeneous Co3O4 NPs-assembled coating catalyst for degradation of methylene blue solution under light irradiation are assessed and compared to that of homogenous catalyst producing Co2+ ions in methylene blue solution.

Investigating the activity enhancement on PtxCo1−x alloys induced by a combined strain and ligand effect

Due to their high activity for the oxygen reduction reaction (ORR), Pt based alloys are of considerable interest as catalysts for polymer electrolyte fuel cells (PEMFC). We present two synthesis protocols that give PtxCo1−x alloy nanoparticles (NPs) supported on a commercial carbon black support (Ketjen black EC300J) with controlled particle size, Co content and particle distribution for oxygen reduction reaction (ORR). The as prepared catalysts samples can be pre-leached in acid solution to obtain skeleton NPs and heat-treated in a reducing atmosphere to promote alloying as well as to reduce the number of low coordinated surface sites of the NPs. Without heat treatment, the catalysts exhibit an activity increase for the oxygen reduction reaction mainly due to a strain effect, whereas an improved performance due to a combined strain and ligand effect requires thermal treatment of the catalyst.