Barr Halevi

Density functional theory calculations of XPS binding energy shift for nitrogen-containing graphene-like structures

Our results validate the use of independent DFT predicted BE shifts for defect identification and constraining ambient pressure XPS observations for Me-Nx moieties in pyrolyzed carbon based ORR electrocatalysts. This supports the understanding of such catalysts as vacancy-and-substitution defects in a graphene-like matrix.

Structural and mechanistic basis for the high activity of Fe–N–C catalysts toward oxygen reduction

The development of efficient non-platinum group metal (non-PGM) catalysts for oxygen reduction reaction (ORR) is of paramount importance for clean and sustainable energy storage and conversion devices. The major bottleneck in developing Fe–N–C materials as the leading non-PGM catalysts lies in the poor understanding of the nature of active sites and reaction mechanisms. Herein, we report a scalable metal organic framework-derived Fe–N–C catalyst with high ORR activity demonstrated in practical H2/air fuel cells, and an unprecedented turnover frequency (TOF) in acid in rotating disk electrode. By characterizing the catalyst under both ex situ and operando conditions using combined microscopic and spectroscopic techniques, we show that the structures of active sites under ex situ and working conditions are drastically different. Resultantly, the active site proposed here, a non-planar ferrous Fe–N4 moiety embedded in distorted carbon matrix characterized by a high Fe2+/3+ redox potential, is in contrast with those proposed hitherto derived from ex situ characterizations. This site reversibly switches to an in-plane ferric Fe–N4 moiety poisoned by oxygen adsorbates during the redox transition, with the population of active sites controlled by the Fe2+/3+ redox potential. The unprecedented TOF of the active site is correlated to its near-optimal Fe2+/3+ redox potential, and essentially originated from its favorable biomimetic dynamic nature that balances the site-blocking effect and O2 dissociation. The porous and disordered carbon matrix of the catalyst plays pivotal roles for its measured high ORR activity by hosting high population of reactant-accessible active sites.

The CO oxidation mechanism and reactivity on PdZn alloys.

The effect of Zn on the CO adsorption and oxidation reaction is examined experimentally and theoretically on two PdZn catalysts with different compositions, namely the intermetallic 1:1 β-PdZn and α-PdZn as a solid solution of 9 at% Zn in Pd. These bimetallic catalysts, made using an aerosol derived method, are homogeneous in phase and composition so that the measured reactivity excludes support effects. Both specific reactivities for CO oxidation on these two PdZn catalysts were measured. It was found that the initial rates are high and different between these catalysts, presumably due to the weakening of the CO adsorption and easier binding of oxygen to Pd sites modified by Zn. However, the rates decrease with time and become comparable to that on Pd at the steady state. With the help of density functional theory, it was suggested that the transient kinetics are due to the oxidation of Zn during the catalysis, which yields pure Pd where the reaction takes place.

Effect of pyrolysis pressure on activity of Fe–N–C catalysts for oxygen reduction

Iron and nitrogen doped carbon, Fe–N–C, catalysts are synthesized by high pressure pyrolysis of Ketjenblack carbon, melamine and iron acetate precursor mixture in a closed, reusable scale-up stainless steel reactor. The effects of precursor loading with constant precursor ratios on obtained pressure, nitrogen retention and oxygen reduction reaction (ORR) activities are studied. The results indicate that higher precursor loading increases the gas phase pressure and improves nitrogen retention and ORR activity. Furthermore, a relationship is found between active site density, nitrogen retention and pressure that suggests that the limiting reaction may be an adsorption process driven via high pressure of volatile intermediates from the melamine.

Application of the Discrete Wavelet Transform to SEM and AFM Micrographs for Quantitative Analysis of Complex Surfaces

The discrete wavelet transform (DWT) has found significant utility in process monitoring, filtering, and feature isolation of SEM, AFM, and optical images. Current use of the DWT for surface analysis assumes initial knowledge of the sizes of the features of interest in order to effectively isolate and analyze surface components. Current methods do not adequately address complex, heterogeneous surfaces in which features across multiple size ranges are of interest. Further, in situations where structure-to-property relationships are desired, the identification of features relevant for the function of the material is necessary. In this work, the DWT is examined as a tool for quantitative, length-scale specific surface metrology without prior knowledge of relevant features or length-scales. A new method is explored for determination of the best wavelet basis to minimize variation in roughness and skewness measurements with respect to change in position and orientation of surface features. It is observed that the size of the wavelet does not directly correlate with the size of features on the surface, and a method to measure the true length-scale specific roughness of the surface is presented. This method is applied to SEM and AFM images of non-precious metal catalysts, yielding new length-scale specific structure-to-property relationships for chemical speciation and fuel cell performance. The relationship between SEM and AFM length-scale specific roughness is also explored. Evidence is presented that roughness distributions of SEM images, as measured by the DWT, is representative of the true surface roughness distribution obtained from AFM.

Pt7Sn3 catalysts for ethanol electro-oxidation: correlation between surface structure and catalytic activity

Ethanol electro-oxidation has attracted great attention in the fuel cell technology for direct ethanol fuel cells (DEFCs). Active and inexpensive electrocatalysts are required to efficiently break C-C bond and completely convert ethanol to CO2. Recent studies showed that PtxSn1-x catalysts have promising catalytic activity for ethanol electro-oxidation in both acidic and alkaline solutions [1, 2]. Despite the numerous studies on PtSn catalysts, several issues regarding their catalytic activity and stability remain to be addressed. Surface composition and chemistry of bimetallic catalyst is complex and may be influenced by several parameters, e.g., synthesis method, particle size, catalyst structure, surface chemistry, therefore investigation of the surface structure of bimetallic catalysts and its correlation to the catalytic performance is an important task. In the present work PtSn nanostructured catalysts with the atomic ratio of Pt to Sn of 70:30 at. % were synthesized and tested for ethanol electro-oxidation in alkaline and acidic solutions. X-ray photoelectron spectroscopy has been chosen to study the surface chemistry of carbon-supported Pt7Sn3 catalysts and correlate it with electrocatalytic activity. The ability to discriminate between different chemical environments, not just elemental compositions, is one of the primary advantages of XPS in the characterization of catalysts. Synthesis of the bimetallic Pt7Sn3 catalysts supported on carbon (Vulcan XC-72) is described in details elsewhere [4]. Table 1 summarizes the synthesis conditions and some characteristics of Pt7Sn3 nanoparticles. Carbon-supported Pt7Sn3 catalysts were analyzed by KRATOS Axis Ultra DLD X-ray Photoelectron Spectrometer. The XPS analysis was conducted at 140 W and at pass energy of 20 eV. The peak positions were corrected for sample charging by setting the maximum of C 1s peak to binding energy of 284.7 eV. Data analysis and quantification was performed using CasaXPS software. Correlation of XPS structural information with catalytic performance for ethanol electro-oxidation in acidic and alkaline solutions, particle size and structural characteristics is accomplished by application of Multivariate statistical methods of data analysis (MVA). [3] Principal Component Analysis (PCA) and Partial Least Squares Discriminant Analysis (PLSDA) are used herein as an analysis tools to find samples which are globally correlated or anti-correlated, and to facilitate visualization of the variables responsible for the correlations and to highlight differences between two categories of structures of catalysts obtained. Several observations were found from the structureto-property correlations for the carbon-supported Pt7Sn3 catalysts : 1. Best performing catalysts for ethanol electrooxidation in acidic solution do not have largest absolute amounts of Pt and Sn on the surface, whereas samples with largest amount of total Pt and Sn have the worst catalytic activity (current density, i) and largest electrochemical active surface area (EASA). 2. Relative distribution of types of Pt and Sn is more important than the absolute amounts. Best performing samples have small amounts of both metals, but have largest relative amount of both metallic Pt and metallic Sn. The same best performing samples have fewest amounts of all types of oxides, i.e. PtO, PtO2 and SnOx. Ongoing electrochemical evaluations of Pt7Sn3 catalysts for ethanol oxidation in alkaline media will be correlated to XPS structural information and discussed along with their catalytic activities.

Effect of Alloying Pd with Oxophillic Metals on Electro-Oxidation of Alcohols in Alkaline Media

The effect of alloying Pd with Zn for use in electro-oxidation of methanol and ethanol in alkaline media was studied. Pd, Zn, and PdZn supported on Vulcan XC72R were synthesized, tested for alcohol electro-oxidation and compared to unsupported PdZn. It was found that alloying Pd with Zn had a synergistic effect on the oxidation of ethanol but not of methanol.