Zhiyao Duan

Tunability of the Adsorbate Binding on Bimetallic Alloy Nanoparticles for the Optimization of Catalytic Hydrogenation

In this paper, we show that PtAu and PdAu random alloy dendrimer-encapsulated nanoparticles with an average size of ∼1.6 nm have different catalytic activity trends for allyl alcohol hydrogenation. Specifically, PtAu nanoparticles exhibit a linear increase in activity with increasing Pt content, whereas PdAu dendrimer-encapsulated nanoparticles show a maximum activity at a Pd content of ∼60%. Both experimental and theoretical results suggest that this contrasting behavior is caused by differences in the strength of H binding on the PtAu and PdAu alloy surfaces. The results have significant implications for predicting the catalytic performance of bimetallic nanoparticles on the basis of density functional theory calculations.

Efficient CO Oxidation Using Dendrimer-Encapsulated Pt Nanoparticles Activated with <2% Cu Surface Atoms

In this paper, we show that the onset potential for CO oxidation electrocatalyzed by ∼2 nm dendrimer-encapsulated Pt nanoparticles (Pt DENs) is shifted negative by ∼300 mV in the presence of a small percentage (<2%) of Cu surface atoms. Theory and experiments suggest that the catalytic enhancement arises from a cocatalytic Langmuir-Hinshelwood mechanism in which the small number of Cu atoms selectively adsorb OH, thereby facilitating reaction with CO adsorbed to the dominant Pt surface. Theory suggests that these Cu atoms are present primarily on the (100) facets of the Pt DENs.

O2 activation at the Au/MgO(001) interface boundary facilitates CO oxidation

Density functional theory calculations reveal that the work function of Au supported on MgO(001) is substantially reduced because of an interfacial dipole moment formed at the Au/MgO interface. Consequently, the Au/MgO interface plays an active role in the activation of O2 molecules by promoting charge transfer to the O2 2π* orbital. The presence of F-centers in the MgO substrate can further promote the charge transfer and bonding of O2 at the interface boundary. However, O2 dissociation is kinetically hindered. The system is then able to catalyze CO oxidation at low temperature as adsorbed CO and O2 readily react to form CO2 with a low energy barrier.

A combined theoretical and experimental EXAFS study of the structure and dynamics of Au147 nanoparticles

In this study, we present a framework for characterizing the structural and thermal properties of small nanoparticle catalysts by combining precise synthesis, extended X-ray absorption fine structure (EXAFS) spectroscopy, and density functional theory (DFT) calculations. We demonstrate the capability of this approach by characterizing the atomic structure and vibrational dynamics of Au147. With the combination of EXAFS spectroscopy and DFT, the synthesized Au147 nanoparticles are determined to have an icosahedral structure. A decrease in the Einstein temperature of the Au147 particles compared to their bulk value was observed and interpreted in terms of softer vibration modes of surface bonds.

Computationally Assisted STEM and EXAFS Characterization of Tunable Rh/Au and Rh/Ag Bimetallic Nanoparticle Catalysts

The acceleration of rational catalyst design by computational simulations is only practical if the theoretical structures identified can be synthesized and experimentally verified. Of particular interest are bi-functional/bimetallic catalysts, which can have the potential to exceed the selectivity and efficiency of a single-component system [1]. However, adding a second metal greatly increases the complexity of the system; variation in the elements’ mixing patterns and reconfiguration can affect the reaction mechanisms and thus catalytic performance [2].

The interplay between ceria particle size, reducibility, and ethanol oxidation activity of ceria-supported gold catalysts

The structure of a support material can have profound impacts on the behavior of a catalyst, altering the activity and selectivity of chemical reactions. In this article, we investigate the influence of the support materials structure on the activity of Au/CeO2 catalysts for selective oxidation of ethanol in a fixed-bed flow reactor. By doping the ceria support with Al, La, and Zr during synthesis and by altering the temperature of pretreatment in air after synthesis, ceria particles varying in size between 3 nm and 22 nm were prepared. The smaller ceria particles exhibited higher oxygen storage capacities as determined by temperature programmed reduction testing and resulted in more active catalysts for ethanol oxidation. We note a linear correlation between oxygen storage capacity and catalytic activity for ethanol oxidation.

Formation of HONO from the NH3-promoted hydrolysis of NO2 dimers in the atmosphere

Significance As the primary source of “detergent” OH radicals, nitrous acid (HONO) plays an essential role in the chemistry of the atmosphere. Despite extensive studies, the source of HONO is still elusive. Although recent studies have shown the importance of reactive nitrogen compounds during aerosol formation, mechanistic insight into how these compounds react is still missing. Herein, based on Born–Oppenheimer molecular-dynamics simulations and free-energy sampling, we identified a formation mechanism for HONO via the NH3-promoted hydrolysis of NO2 dimer (ONONO2) on water clusters/droplets. The near-spontaneous formation of HONO at the water–air interface sheds light on the catalytic role of water droplets in atmospheric chemistry. This finding provides not only a missing HONO source but also insight into HONO chemistry. One challenging issue in atmospheric chemistry is identifying the source of nitrous acid (HONO), which is believed to be a primary source of atmospheric “detergent” OH radicals. Herein, we show a reaction route for the formation of HONO species from the NH3-promoted hydrolysis of a NO2 dimer (ONONO2), which entails a low free-energy barrier of 0.5 kcal/mol at room temperature. Our systematic study of HONO formation based on NH3 + ONONO2 + nH2O and water droplet systems with the metadynamics simulation method and a reaction pathway searching method reveals two distinct mechanisms: (i) In monohydrates (n = 1), tetrahydrates (n = 4), and water droplets, only one water molecule is directly involved in the reaction (denoted the single-water mechanism); and (ii) the splitting of two neighboring water molecules is seen in the dihydrates (n = 2) and trihydrates (n = 3) (denoted the dual-water mechanism). A comparison of the computed free-energy surface for NH3-free and NH3-containing systems indicates that gaseous NH3 can markedly lower the free-energy barrier to HONO formation while stabilizing the product state, producing a more exergonic reaction, in contrast to the endergonic reaction for the NH3-free system. More importantly, the water droplet reduces the free-energy barrier for HONO formation to 0.5 kcal/mol, which is negligible at room temperature. We show that the entropic contribution is important in the mechanism by which NH3 promotes HONO formation. This study provides insight into the importance of fundamental HONO chemistry and its broader implication to aerosol and cloud processing chemistry at the air–water interface.

Experimental and Theoretical Structural Investigation of AuPt Nanoparticles Synthesized Using a Direct Electrochemical Method

In this report, we examine the structure of bimetallic nanomaterials prepared by an electrochemical approach known as hydride-terminated (HT) electrodeposition. It has been shown previously that this method can lead to deposition of a single Pt monolayer on bulk-phase Au surfaces. Specifically, under appropriate electrochemical conditions and using a solution containing PtCl42-, a monolayer of Pt atoms electrodeposits onto bulk-phase Au immediately followed by a monolayer of H atoms. The H atom capping layer prevents deposition of Pt multilayers. We applied this method to ∼1.6 nm Au nanoparticles (AuNPs) immobilized on an inert electrode surface. In contrast to the well-defined, segregated Au/Pt structure of the bulk-phase surface, we observe that HT electrodeposition leads to the formation of AuPt quasi-random alloy NPs rather than the core@shell structure anticipated from earlier reports relating to deposition onto bulk phases. The results provide a good example of how the phase behavior of macro materials does not always translate to the nano world. A key component of this study was the structure determination of the AuPt NPs, which required a combination of electrochemical methods, electron microscopy, X-ray absorption spectroscopy, and theory (DFT and MD).

Electrocatalytic Study of the Oxygen Reduction Reaction at Gold Nanoparticles in the Absence and Presence of Interactions with SnOx Supports

Here we report that density functional theory (DFT) can be used to accurately predict how Au nanoparticle (NP) catalysts cooperate with SnO x ( x = 1.9 or 2.0) supports to carry out the oxygen reduction reaction (ORR). Specifically, dendrimers were used to encapsulate AuNPs and prevent their interactions with the underlying SnO x supports. After removal of the dendrimers, however, the AuNPs are brought into direct contact with the support and hence feel its effect. The results show that both SnO1.9 and SnO2.0 supports strongly enhance the electrocatalytic properties of AuNPs for the ORR. In the case of AuNP interaction with a SnO1.9 support, the number of electrons involved in the ORR ( neff) increases from 2.1 ± 0.2 to 2.9 ± 0.1 following removal of the dendrimers, indicating an increased preference for the desired four-electron product (water), while the overpotential decreases by 0.32 V. Similarly, direct interactions between AuNPs and a SnO2.0 support result in an increase in neff from 2.2 ± 0.1 to 3.1 ± 0.1 and a reduction of the overpotential by 0.28 V. These experimental results are in excellent agreement with the theoretically predicted onset potential shift of 0.30 V. According to the DFT calculations, the observed activity enhancements are attributed to the existence of anionic Au resulting from electron transfer from surface oxygen vacancies within the SnO x supports to the AuNPs. This theoretical finding was confirmed experimentally using X-ray photoelectron spectroscopy. Importantly, the calculations reported here were performed prior to the experiments. In other words, this study represents an unusual case of theory accurately predicting the electrocatalytic manifestation of strong metal support interactions.

Thermal properties of size-selective nanoparticles: Effect of the particle size on Einstein temperature

Characterizing size related thermal properties of nanoclusters is challenging due to the requirement to accurately control both their average sizes and the size distributions. In this work, temperature-dependent Extended X-ray Absorption Fine Structure spectroscopy and the phenomenological bond-order-length-strength (BOLS) model were employed to investigate the size-dependent Einstein temperature of Au nanoclusters. Theoretical calculations of Einstein temperature and average bond distance for clusters with different sizes agree quantitatively with experiment. The BOLS model is thus useful for predictive understanding of structure and thermal properties in well-defined metal clusters.