Xiaofang Yang

Improving Electrocatalysts for O2 Reduction by Fine-Tuning the Pt−Support Interaction: Pt Monolayer on the Surfaces of a Pd3Fe(111) Single-Crystal Alloy

We improved the effectiveness of Pt monolayer electrocatalysts for the oxygen-reduction reaction (ORR) using a novel approach to fine-tuning the Pt monolayer interaction with its support, exemplified by an annealed Pd(3)Fe(111) single-crystal alloy support having a segregated Pd layer. Low-energy ion scattering and low-energy electron diffraction studies revealed that a segregated Pd layer, with the same structure as Pd (111), is formed on the surface of high-temperature-annealed Pd(3)Fe(111). This Pd layer is considerably more active than Pd(111); its ORR kinetics is comparable to that of a Pt(111) surface. The enhanced catalytic activity of the segregated Pd layer compared to that of bulk Pd apparently reflects the modification of Pd surfaces electronic properties by underlying Fe. The Pd(3)Fe(111) suffers a large loss in ORR activity when the subsurface Fe is depleted by potential cycling (i.e., repeated excursions to high potentials in acid solutions). The Pd(3)Fe(111) surface is an excellent substrate for a Pt monolayer ORR catalyst, as verified by its enhanced ORR kinetics on PT(ML)/Pd/Pd(3)Fe(111). Our density functional theory studies suggest that the observed enhancement of ORR activity originates mainly from the destabilization of OH binding and the decreased Pt-OH coverage on the Pt/Pd/Pd(3)Fe(111) surface. The activity of Pt(ML)/Pd(111) and Pt(111) is limited by OH removal, whereas the activity of Pt(ML)/Pd/Pd(3)Fe(111) is limited by the O-O bond scission, which places these two surfaces on the two sides of the volcano plot.

Molybdenum Carbide as Alternative Catalysts to Precious Metals for Highly Selective Reduction of CO2 to CO

Rising atmospheric CO2 is expected to have negative effects on the global environment from its role in climate change and ocean acidification. Utilizing CO2 as a feedstock to make valuable chemicals is potentially more desirable than sequestration. A substantial reduction of CO2 levels requires a large-scale CO2 catalytic conversion process, which in turn requires the discovery of low-cost catalysts. Results from the current study demonstrate the feasibility of using the non-precious metal material molybdenum carbide (Mo2C) as an active and selective catalyst for CO2 conversion by H2.

Development of a new plasticizer for poly(ethylene oxide)-based polymer electrolyte and the investigation of their ion-pair dissociation effect

Abstract One approach to increasing the ionic conductivity of polymer electrolyte is to add plasticizers to the polymer-salt complexes. Recently, we have synthesized a new plasticizer, modified carbonate (MC3), by attaching three ethylene oxide units to the 4-position of ethylene carbonate. A.c. impedance studies showed that an ionic conductivity of 5 × 10 −5 S cm −1 could be achieved at room temperature, by adding 50 wt.% of MC3 into poly(ethylene oxide) (PEO)-LiCF 3 SO 3 complex. This is two orders of magnitude higher than that found in PEO-LiCF 3 SO 3 electrolyte without a plasticizer, and one order of magnitude higher than that found when using same amount of propylene carbonate (PC) as plasticizer. Temperature-dependent conductivity measurement and thermal analysis show that this new plasticizer increased the ionic conductivity throughout the entire complex system, while the conventional plasticizers only create a high conductivity pathway through the plasticizer itself. The samples are free-standing films with good mechanical properties. When MC3 is used as a plasticizer, the ionic conductivity increase is much higher than using PC as a plasticizer at high temperature (65 °C), implying an increase in the number of charge carriers. Therefore, we believe that MC3 has a stronger ion-pair dissociation effect than PC, when used as a plasticizer. The ion-pair dissociation effect was studied by Raman, Fourier-transform infrared spectroscopy, and near-edge X-ray absorption fine structure spectroscopy.

Low Pressure CO2 Hydrogenation to Methanol over Gold Nanoparticles Activated on a CeOx/TiO2 Interface

Capture and recycling of CO2 into valuable chemicals such as alcohols could help mitigate its emissions into the atmosphere. Due to its inert nature, the activation of CO2 is a critical step in improving the overall reaction kinetics during its chemical conversion. Although pure gold is an inert noble metal and cannot catalyze hydrogenation reactions, it can be activated when deposited as nanoparticles on the appropriate oxide support. In this combined experimental and theoretical study, it is shown that an electronic polarization at the metal-oxide interface of Au nanoparticles anchored and stabilized on a CeO(x)/TiO2 substrate generates active centers for CO2 adsorption and its low pressure hydrogenation, leading to a higher selectivity toward methanol. This study illustrates the importance of localized electronic properties and structure in catalysis for achieving higher alcohol selectivity from CO2 hydrogenation.

CO2 Hydrogenation over Oxide-Supported PtCo Catalysts: The Role of the Oxide Support in Determining the Product Selectivity

By simply changing the oxide support, the selectivity of a metal-oxide catalysts can be tuned. For the CO2 hydrogenation over PtCo bimetallic catalysts supported on different reducible oxides (CeO2 , ZrO2 , and TiO2 ), replacing a TiO2 support by CeO2 or ZrO2 selectively strengthens the binding of C,O-bound and O-bound species at the PtCo-oxide interface, leading to a different product selectivity. These results reveal mechanistic insights into how the catalytic performance of metal-oxide catalysts can be fine-tuned.

Activity of pure and transition metal-modified CoOOH for the oxygen evolution reaction in an alkaline medium

A new electrode structure enabling low overpotentials for the oxidation of water, based on three-dimensional arrays of CoOOH nanowires, is presented. The electrocatalytic activities of pure and M-modified cobalt oxyhydroxides (M = Ni or Mn) nanowires have been investigated in detail for the oxygen evolution reaction (OER) in an alkaline environment. The pure, Ni-, and Mn-modified nanowires, with preferentially exposed low-index surfaces, were fabricated directly on stainless steel mesh current collectors using an inexpensive and scalable chemical synthesis procedure. The unique electrode structure ensures excellent substrate–catalyst electrical contact and increases the surface area accessible to the electrolyte. The OER activity of CoOOH nanowires is shown to be significantly improved through incorporation of Ni. Specifically, optimal OER activity is obtained for CoOOH nanowires with 9.7% surface Ni content, which corresponds to four-times greater current density compared to pure CoOOH. In contrast, Mn modification of the CoOOH nanowires did not improve the OER activity. Tafel analysis suggests Ni incorporation leads to change in the OER rate-determining step based on an observed decrease in the Tafel slope. Electrochemical impedance spectroscopy reveals that Ni incorporation improves the ability of the catalysts to stabilize surface intermediates, whereas Mn incorporation impedes intermediate stabilization. This study provides new insights regarding the influence of transition metal impurities on the OER activity of CoOOH and provides a clear strategy for the optimization of CoOOH-based OER catalysts in alkaline electrolytes.

Stabilization of Catalytically Active Cu+ Surface Sites on Titanium–Copper Mixed‐Oxide Films

The oxidation of CO is the archetypal heterogeneous catalytic reaction and plays a central role in the advancement of fundamental studies, the control of automobile emissions, and industrial oxidation reactions. Copper-based catalysts were the first catalysts that were reported to enable the oxidation of CO at room temperature, but a lack of stability at the elevated reaction temperatures that are used in automobile catalytic converters, in particular the loss of the most reactive Cu(+) cations, leads to their deactivation. Using a combined experimental and theoretical approach, it is shown how the incorporation of titanium cations in a Cu2O film leads to the formation of a stable mixed-metal oxide with a Cu(+) terminated surface that is highly active for CO oxidation.

Synchrotron x‐ray diffraction studies of the structural properties of electrode materials in operating battery cells

Hard x rays from a synchrotron source were utilized in diffraction experiments which probed the bulk of electrode materials while they were operating in situ in battery cells. Two technologically relevant electrode materials were examined; an AB2‐type anode in a nickel–metal–hydride cell and a LiMn2O4 cathode in a Li‐ion ‘‘rocking chair’’ cell. Structural features such as lattice expansions and contractions, phase transitions, and the formation of multiple phases were easily observed as either hydrogen or lithium was electrochemically intercalated in and out of the electrode materials. The relevance of this technique for future studies of battery electrode materials is discussed.

Role of Surface Iron in Enhanced Activity for the Oxygen Reduction Reaction on a Pd3Fe(111) Single-Crystal Alloy†

Applications of polymer electrolyte membrane fuel cells (PEM-FCs) require a continued search for an optimal catalyst with attributes of high activity, durability, low cost, and resistance to negative effects of impurities in the fuel. Platinum-based bimetallic catalyst systems are widely utilized in PEM-FCs because of their good performance in both the cathode and the anode. Alloying with inexpensive metals can reduce the loading of platinum and lower the cost of the fuel cell, and in some cases the catalytic activity is maintained or even becomes higher. Surprisingly, some non-platinumbased alloy catalysts (e.g., palladium–iron and palladium– cobalt) appear to have even better performance than platinum-based alloy catalysts. 8] Recently, higher electrocatalytic activity and stability were observed when metals or alloys were used as supports for a platinum or palladium monolayer. Development of improved cathode catalysts would be aided by a fundamental understanding of the oxygen reduction reaction (ORR) mechanism. However, the ORR system is complex and ultimately requires considering fully the effects of water, solvent ions, changing electrical potentials, and a detailed description of the composition and structure of all the chemical phases present at the electrode surface. Thus, progress has been limited in elucidating the origin of the enhanced performance of various bimetallic catalysts. An important factor that can be isolated is the surface composition and structure of the alloy responsible for the ORR kinetics. Herein, we report studies on a bimetallic alloy single crystal, Pd3Fe(111), that combine surface analytical techniques including low-energy ion scattering (LEIS) and scanning tunneling microscopy (STM), electrochemical analysis, and quantum chemistry calculations to investigate the origin of the enhanced ORR properties of Pd–Fe alloys at a molecular level. We find excellent ORR performance for the Pd3Fe(111) crystal after heating to high temperatures in ultrahigh vacuum (UHV) and establish a strong correlation of this performance to the presence of surface Fe atoms. The lower surface energy of Pd causes significant Pd segregation to the topmost surface layer of Pd–Fe alloys. Pd3Fe(111) surfaces with different structures can be prepared by heating at different temperatures in UHV, and two surface structures are shown by STM images in Figure 1. These two surfaces were studied for catalyzing the oxygen reduction reaction.

A new family of anion receptors and their effect on ion pair dissociation and conductivity of lithium salts in non-aqueous solutions

A new family of anion receptors based on aza-ether compounds have been synthesized. Since the anion complexation of these compounds is not based on either positively charged sites or hydrogen bonding, they have a potential to be used in lithium batteries as electrolyte additives. When these compounds are added into nonaqueous electrolytes using lithium salts, such as LiCl/BF or LiBr/THF, the ionic conductivity can be dramatically increased. Near Edge X-ray Absorption Fine Structure (NF-XAFS) spectroscopy studies show that Cl{sup {minus}} anions are completed with the nitrogen groups in these compounds. The increase in ionic conductivity and the degree of complexation, are both related to the number of R=CF{sub 3}SO{sub 2} groups that are used to substitute the amine hydrogen atoms in these aza-ether compounds.