Xiaochun Zhou

Size-dependent catalytic activity and dynamics of gold nanoparticles at the single-molecule level.

Nanoparticles are important catalysts for petroleum processing, energy conversion, and pollutant removal. As compared to their bulk counterparts, their often superior or new catalytic properties result from their nanometer size, which gives them increased surface-to-volume ratios and chemical potentials. The size of nanoparticles is thus pivotal for their catalytic properties. Here, we use single-molecule fluorescence microscopy to study the size-dependent catalytic activity and dynamics of spherical Au-nanoparticles under ambient solution conditions. By monitoring the catalysis of individual Au-nanoparticles of three different sizes in real time with single-turnover resolution, we observe clear size-dependent activities in both the catalytic product formation reaction and the product dissociation reaction. Within a model of classical thermodynamics, these size-dependent activities of Au-nanoparticles can be accounted for by the changes in the adsorption free energies of the substrate resazurin and the product resorufin because of the nanosize effect. We also observe size-dependent differential selectivity of the Au-nanoparticles between two parallel product dissociation pathways, with larger nanoparticles less selective between the two pathways. The particle size also strongly influences the surface-restructuring-coupled catalytic dynamics; both the catalysis-induced and the spontaneous dynamic surface restructuring occur more readily for smaller Au-nanoparticles due to their higher surface energies. Using a simple thermodynamic model, we analyze the catalysis- and size-dependent dynamic surface restructuring quantitatively; the results provide estimates on the activation energies and time scales of spontaneous dynamic surface restructuring that are fundamental to heterogeneous catalysis in both the nano- and the macro-scale. This study further exemplifies the power of the single-molecule approach in probing the intricate workings of nanoscale catalysts.

Single-Molecule Catalysis Mapping Quantifies Site-Specific Activity and Uncovers Radial Activity Gradient on Single 2D Nanocrystals

Shape-controlled metal nanocrystals are a new generation of nanoscale catalysts. Depending on their shapes, these nanocrystals exhibit various surface facets, and the assignments of their surface facets have routinely been used to rationalize or predict their catalytic activity in a variety of chemical transformations. Recently we discovered that for 1-dimensional (1D) nanocrystals (Au nanorods), the catalytic activity is not constant along the same side facets of single nanorods but rather differs significantly and further shows a gradient along its length, which we attributed to an underlying gradient of surface defect density resulting from their linear decay in growth rate during synthesis (Nat. Nanotechnol.2012, 7, 237-241). Here we report that this behavior also extends to 2D nanocrystals, even for a different catalytic reaction. By using super-resolution fluorescence microscopy to map out the locations of catalytic events within individual triangular and hexagonal Au nanoplates in correlation with scanning electron microscopy, we find that the catalytic activity within the flat {111} surface facet of a Au nanoplate exhibits a 2D radial gradient from the center toward the edges. We propose that this activity gradient results from a growth-dependent surface defect distribution. We also quantify the site-specific activity at different regions within a nanoplate: The corner regions have the highest activity, followed by the edge regions and then the flat surface facets. These discoveries highlight the spatial complexity of catalytic activity at the nanoscale as well as the interplay amid nanocrystal growth, morphology, and surface defects in determining nanocatalyst properties.

Available hydrogen from formic acid decomposed by rare earth elements promoted Pd‐Au/C catalysts at low temperature

The usage of hydrogen as a clean, efficient power carrier for stationary and mobile applications is attracting more and more attention. Much effort has been made towards hydrogen application technologies, especially in fuel cells. Nevertheless, the production and storage of hydrogen is the bottleneck of hydrogen economy. In transportable energy applications, hydrogen is generally produced from reforming organic molecules, such as gasoline, methanol, ethanol and so on. Hydrogen production suffers from various problems such as low efficiency, high operating temperature, huge volume, weight loading, and excessive formation of CO. On the other hand, hydrogen storage technologies are limited by low efficiency and possible danger. Notably, formic acid is a promising hydrogen carrier with advantages of considerable hydrogen content (4.4 wt %), and non-toxic and non-flammable properties. It has been reported that Au-based, Pd-based, 10] Pt-based, and metal (e.g. , Ru, Ir, Rh, Fe) complex catalysts can be used for the decomposition of formic acid (DCFA). The hydrogen from the DCFA also has been used in proton exchange membrane fuel cell (PEMFC). 21] In our previous study, the Au or Ag additive overcame the deactivation of Pd catalyst. Furthermore, the addition of Ce further improved the activity of the Pd–Au and Pd–Ag catalysts. Then, it is necessary to understand the promotion effect of other rare earth elements (REs) and design new and highly active catalysts. Here, we systematically studied the promotion effect of three REs (Dy, Eu, and Ho) on the Pd–Au/C catalysts in the DCFA reaction. In addition, the application of reforming gas in fuel cell is studied. Figure 1 a shows the output rates of reforming gas from DCFA catalyzed by Pd–Au/C, Pd–Au–Dy/C, Pd–Au–Eu/C, and Pd–Au–Ho/C. All the REs (Dy, Eu, Ho) could significantly promote the activity of Pd-Au/C catalyst. The activity order of the four catalysts was Pd–Au–Dy/C>Pd–Au–Eu/C>Pd–Au–Ho/C> Pd–Au/C. All activities increased with the temperature exponentially. In addition, these catalysts were even active at room temperature temporarily and above 325 K steadily. The activation energies for the DCFA reaction on the prepared catalysts were also calculated according to the Arrhenius equation. Figure 1 b and Table 1 show that all the REs-promoted Pd–Au/C catalysts have lower activation energies of DCFA than Pd–Au/C. Among the REs catalysts, Pd–Au–Eu/C had the lowest value of 84.2 7.4 kJ mol . However, the most active was Pd–Au–Dy/C, which had a decomposition rate of 1198 mL min 1 g 1 Pd and a turnover frequency (TOF) of 269 202 h 1 at 365 K. This catalytic performance of Pd–Au–Dy/C can provide output power of 106 W g 1 Pd theoretically, which is promising to be used in portable applications. Generally, promotion effect comes from three aspects, that is, distribution improvement of nanoparticles, electronic effect, and synergistic effect. The promotion effect of REs in these three aspects is stated as follows. Firstly, the particle size distributions of the prepared catalysts were measured by transmission electron microscopy (TEM), as shown in Figure 2 A and Figure 2 B. The relationships among the average particle size, activity, TOF, and activation energy are shown in Figure 3. The activity of REs promoted Pd–Au/C catalysts increased from 431 to 1198 mL min 1 g 1 Pd with the size decrease from 4.6 1.5 to 2.0 1.5 nm (Figure 3 a). The activity of REs-promoted catalysts can be improved by decreasing the particle size, likely due to the increasing surface-tovolume ratio. However, the TOF and activation energy Ea are not clearly dependent on the particle size as shown in Figure 3 b and Figure 3 c. Interestingly, Figure 3 d shows that TOF increased with decreasing activation energy, indicating that the activation energy determines the catalytic activity of the Figure 1. a) The activity of Pd–Au/C, Pd–Au–Dy/C, Pd–Au–Eu/C, and Pd–Au– Ho/C catalysts at different temperatures; the activity is expressed by the output gas per minute and per gram Pd. b) lnk vs T 1 plot for the DCFA reaction according to Arrhenius equation. The activation energies are shown in Table 1.

Single-molecule electrocatalysis by single-walled carbon nanotubes.

We report a single-molecule fluorescence study of electrocatalysis by single-walled carbon nanotubes (SWNTs) at single-reaction resolution. Applying super-resolution optical imaging, we find that the electrocatalysis occurs at discrete, nanometer-dimension sites on SWNTs. Single-molecule kinetic analysis leads to an electrocatalytic mechanism, allowing quantification of the reactivity and heterogeneity of individual reactive sites. Combined with conductivity measurements, this approach will be powerful to interrogate how the electronic structure of SWNTs affects the electrocatalytic interfacial charge transfer, a process fundamental to photoelectrochemical cells.

How Does a Single Pt Nanocatalyst Behave in Two Different Reactions? A Single-Molecule Study

Using single-molecule microscopy of fluorogenic reactions we studied Pt nanoparticle catalysis at single-particle, single-turnover resolution for two reactions: one an oxidative N-deacetylation and the other a reductive N-deoxygenation. These Pt nanoparticles show distinct catalytic kinetics in these two reactions: one following noncompetitive reactant adsorption and the other following competitive reactant adsorption. In both reactions, single nanoparticles exhibit temporal activity fluctuations attributable to dominantly spontaneous surface restructuring. Depending on the reaction sequence, single Pt nanoparticles may or may not show activity correlations in catalyzing both reactions, reflecting the structure insensitivity of the N-deacetylation reaction and the structure sensitivity of the N-deoxygenation reaction.

Interpreting single turnover catalysis measurements with constrained mean dwell times

Observation of a chemical transformation at the single-molecule level yields a detailed view of kinetic pathways contributing to the averaged results obtained in a bulk measurement. Studies of a fluorogenic reaction catalyzed by gold nanoparticles have revealed heterogeneous reaction dynamics for these catalysts. Measurements on single nanoparticles yield binary trajectories with stochastic transitions between a dark state in which no product molecules are adsorbed and a fluorescent state in which one product molecule is present. The mean dwell time in either state gives information corresponding to a bulk measurement. Quantifying fluctuations from mean kinetics requires identifying properties of the fluorescence trajectory that are selective in emphasizing certain dynamic processes according to their time scales. We propose the use of constrained mean dwell times, defined as the mean dwell time in a state with the constraint that the immediately preceding dwell time in the other state is, for example, less than a variable time. Calculations of constrained mean dwell times for a kinetic model with dynamic disorder demonstrate that these quantities reveal correlations among dynamic fluctuations at different active sites on a multisite catalyst. Constrained mean dwell times are determined from measurements of single nanoparticle catalysis. The results indicate that dynamical fluctuations at different active sites are correlated, and that especially rapid reaction events produce particularly slowly desorbing product molecules.

Nanobubbles: An Effective Way to Study Gas-Generating Catalysis on a Single Nanoparticle

Gas-generating catalysis is important to many energy-related research fields, such as photocatalytic water splitting, water electrolysis, etc. The technique of single-nanoparticle catalysis is an effective way to search for highly active nanocatalysts and elucidate the reaction mechanism. However, gas-generating catalysis remains difficult to investigate at the single-nanoparticle level because product gases, such as H2 and O2, are difficult to detect on an individual nanoparticle. Here, we successfully find that nanobubbles can be used to study the gas-generating catalysis, i.e., H2 generation from formic acid dehydrogenation on a single Pd-Ag nanoplate, with a high time resolution (50 ms) via dark-field microscopy. The research reveals that the nanobubble evolution process includes nucleation time and lifetime. The nucleation rate of nanobubbles is proportional to the catalytic activity of a single nanocatalyst. The relationship between the catalytic activity and the nucleation rate is quantitatively described by a mathematical model, which shows that an onset reaction rate (ronset) exists for the generation of nanobubbles on a single Pd-Ag nanoplate. The research also reveals that a Pd-Ag nanoplate with larger size usually has a higher activity. However, some large-sized ones still have low activities, indicating the size of the Pd-Ag nanoplate is not the only key factor for the activity. Notablely, further research shows that Pd content is the key factor for the activity of single Pd-Ag nanoplates with similar size. The methodology and knowledge acquired from this research are also applicable to other important gas-generating catalysis reactions at the single-nanoparticle level.

Flexible and Lightweight Fuel Cell with High Specific Power Density

Flexible devices have been attracting great attention recently due to their numerous advantages. But the energy densities of current energy sources are still not high enough to support flexible devices for a satisfactory length of time. Although proton exchange membrane fuel cells (PEMFCs) do have a high-energy density, traditional PEMFCs are usually too heavy, rigid, and bulky to be used in flexible devices. In this research, we successfully invented a light and flexible air-breathing PEMFC by using a new design of PEMFC and a flexible composite electrode. The flexible air-breathing PEMFC with 1 × 1 cm2 working area can be as light as 0.065 g and as thin as 0.22 mm. This new PEMFC exhibits an amazing specific volume power density as high as 5190 W L-1, which is much higher than traditional (air-breathing) PEMFCs. Also outstanding is that the flexible PEMFC retains 89.1% of its original performance after being bent 600 times, and it retains its original performance after being dropped five times from a height of 30 m. Moreover, the research has demonstrated that when stacked, the flexible PEMFCs are also useful in mobile applications such as mobile phones. Therefore, our research shows that PEMFCs can be made light, flexible, and suitable for applications in flexible devices. These innovative flexible PEMFCs may also notably advance the progress in the PEMFC field, because flexible PEMFCs can achieve high specific power density with small size, small volume, low weight, and much lower cost; they are also much easier to mass produce.

A nonmonotonic dependence of standard rate constant on reorganization energy for heterogeneous electron transfer processes on electrode surface

In the present work a nonmonotonic dependence of standard rate constant (k(0)) on reorganization energy (lambda) was discovered qualitatively from electron transfer (Marcus-Hush-Levich) theory for heterogeneous electron transfer processes on electrode surface. It was found that the nonmonotonic dependence of k(0) on lambda is another result, besides the disappearance of the famous Marcus inverted region, coming from the continuum of electronic states in electrode: with the increase of lambda, the states for both Process I and Process II ET processes all vary from nonadiabatic to adiabatic state continuously, and the lambda dependence of k(0) for Process I is monotonic thoroughly, while for Process II on electrode surface the lambda dependence of k(0) could show a nonmonotonicity.

Revealing the Activity Distribution of a Single Nanocatalyst by Locating Single Nanobubbles with Super-Resolution Microscopy

It is challenging to uncover the catalytic activity at different locations of a single nanocatalyst for gas-generating reactions in real time. This research uses super-resolution microscopy to localize the center of single nanobubbles and reveal the local activity distribution at several to tens of nanometers accuracy. The distances between the centers of the nanobubbles and the center of the nanoplate usually distribute in a certain range from 0 to 500 nm, with the maximum population exhibiting at ∼200 nm. This research also shows that more nanobubbles appear near the tips of the Pd-Ag nanoplate compared with the edges, which indicates higher activity at the tips. In addition, the relationship between the location, lifetime, and turnover rate of the nanobubbles was also carefully studied. This work presents an effective, high-resolution method to localize the activity distribution of nanocatalysts during gas-generating reactions, such as photocatalytic water splitting, dehydrogenation, and electro-oxidation.