Eleanor M. Crabb

Carbon Monoxide Electro‐oxidation Properties of Carbon‐Supported PtSn Catalysts Prepared Using Surface Organometallic Chemistry

A series of platinum-tin catalysts supported on carbon have been prepared from organometallic precursors using surface organometallic chemistry (SOMC). The catalysts were characterized using chemisorption, transmission electron microscopy (TEM), energy dispersive X-ray analysis (EDX), X-ray photoelectron spectroscopy (XPS), and cyclic voltammetry. The addition of tin to Pt/C suppresses chemisorption of both hydrogen and carbon monoxide, with a rapid decrease on addition of just a small amount of tin, leveling off to give a plateau at higher loading. TEM, EDX, and XPS provide evidence that the platinum and tin occur together on the support and that on exposure to air the catalysts consist mainly of metallic platinum in association with tin oxide. A catalyst prepared using SOMC and another of similar loading prepared using conventional precipitation were compared for the electro-oxidation of adsorbed carbon monoxide using cyclic voltammetry. The catalyst prepared using SOMC showed enhanced activity with a larger decrease in the onset potential of carbon monoxide oxidation compared to Pt/C. Comparison of a range of carbon support platinum-tin catalysts with different loading prepared using SOMC showed a decrease in the onset potential for the catalyst at low loading with no further significant decrease in potential on addition of further tin. The peak intensities, however, changed significantly with an increase in loading.

Synthesis and characterisation of carbon-supported PtGe electrocatalysts for CO oxidation

Platinum-germanium catalysts supported on carbon were synthesised from organometallic precursors using surface organometallic chemistry (SOMC). The catalysts were characterised using chemisorption, X-ray absorption spectroscopy, transmission electron microscopy, energy dispersive X-ray analysis, X-ray diffraction and cyclic voltammetry. Physical characterisation provided evidence for the selective deposition of the germanium with the platinum using SOMC, existing mainly as platinum metal in association with GeO2. From anodic stripping cyclic voltammetry it can be inferred that the presence of GeO2, at the platinum sites changes the CO oxidation profile of strongly adsorbed CO, increasing its oxidation at 0.68 V versus RHE relative to the main oxidation at 0.85 V versus RHE. The peak potentials appear to be independent of the germanium loading; however, significant changes were observed in the peak current densities, with an increased promotional effect at higher germanium content. We believe the change in the relative distribution between these sites is mainly due to changes in the surface structure of the platinum on addition of GeO2. Further promotional activity is seen, with a limited oxidation activity evident at an onset of 0.35 V versus RHE

Controlled modification of carbon supported platinum electrocatalysts by Mo

A carbon-supported platinum electrocatalyst was modified with molybdenum using surface organometallic chemistry. The catalyst was characterized using transmission electron microscopy, cyclic voltammetry (CV), polarization studies, and in situ fluorescence extended X-ray absorption fine structure (EXAFS) studies. The CV and polarization studies show that CO oxidation starts at low overpotentials, similar to those of a conventially prepared PtCoMo/C electrocatalyst. EXAFS at the Mo K edge recorded at 0.65 V show that the Mo exists as an oxide species associated with the Pt surface with a Mo-O distance of 1.75 A. At 0.05 V this oxide is reduced with the formation of a metal-metal bond between the Mo and Pt, with a bond distance of 2.63 A. ©2001 The Electrochemical Society. All rights reserved.

Effect of Ru surface composition on the CO tolerance of Ru modified carbon supported Pt catalysts

A series of ruthenium modified carbon supported catalysts have been prepared by surface organometallic chemistry (SOMC) with the following nominal Ru∶Pt surface ratios, (1∶4)RuPt/C, (1∶2)RuPt/C, (3∶4)RuPt/C and (1∶1)RuPt/C. The catalysts were characterised using X-ray diffraction (XRD), extended X-ray absorption fine structure (EXAFS), cyclic voltammetry (CV), and half-cell polarisation studies. The XRD measurements showed that a bulk PtRu alloy was not formed following SOMC modification. However, the EXAFS measurements indicated that a surface alloy is formed upon electrochemical reduction of the Ru modified catalysts. The CV studies show that the electrooxidation of CO on the Ru modified Pt/C catalysts occurs at lower potentials than on the unmodified Pt/C catalysts, but at higher potentials than on an alloyed PtRu/C with a bulk composition of 1∶1 Pt∶Ru. Half cell polarisation measurements in 100 ppm CO in H2 show that the CO tolerance of the SOMC RuPt/C catalysts approached that of the conventional PtRu/C alloy catalyst. The results therefore indicate that a bulk alloy phase is not an essential factor in the improvement in CO tolerance of PtRu/C catalysts over that of Pt/C.

Properties of alumina supported Pd-Fe and Pt-Fe catalysts prepared using surface organometallic chemistry

Abstract The Pd-Fe and Pt-Fe catalysts supported on alumina were prepared by a surface organometallic route using ferrocene. The materials were characterised using hydrogen chemisorption, TEM, EDX and EPR spectroscopy and evaluated for the hydrogenation of 1,3-butadiene. Addition of iron suppresses the chemisorption of hydrogen for both the palladium and platinum catalysts. The EDX analysis of Pd-Fe/Al2O3 indicated that the palladium and iron exist together on the support and EPR studies for both the iron doped palladium and platinum catalysts showed a peak at g=2.1 which can be interpreted as metallic iron possibly in interaction with the noble metal, with another at g=4.3 attributed to ferric iron. These results provide evidence for a selective reaction between the ferrocene and surface of the reduced monometallic catalyst to give iron in association with the platinum group metal. The catalysts were believed to consist mainly of palladium or platinum in close association with the iron, as MFe0 and/or the metal with an overlayer of FeOx. In the hydrogenation of 1,3-butadiene, addition of iron appeared to suppress total hydrogenation, in particular of 1-butene. This was particularly evident for the platinum catalyst with a large decrease in n-butane formation for Pt-Fe/Al2O3 at the same activity compared to Pt/Al2O3. This enhancement in the selectivity for the Pt-Fe catalyst may be attributed to both geometric and electronic effects.

Preparation, structure, and stability of Pt and Pd monolayer modified Pd and Pt electrocatalysts

A controlled surface reaction technique has been successfully employed to prepare a series of Pt modified Pd/C (Pt/Pd/C) and Pd modified Pt/C (Pd/Pt/C) catalysts. The resulting catalyst materials were characterised by TEM, XRD, electrochemistry, and EXAFS techniques. In the case of the Pd/Pt/C carbon catalysts, core-shell structural arrangements were found, with a 0.04 A contraction of the Pd-Pd bond distance for the 1 Pd/Pt/C being observed. A greater degree of alloying was found for the Pt/Pd/C catalysts where the surface had a mixed composition with a large proportion of the Pt in the interior of the nanoparticle. However, strong Pt characteristics were exhibited in the voltammetry of Pt/Pd/C catalysts, most notably a large increase in the stability with respect to the electrochemical environment compared to Pd alone.

To alloy or not to alloy

The cathode electrocatalysts for proton exchange membrane (PEM) fuel cells are commonly platinum and platinum based alloy nanoparticles dispersed on a carbon support. Control over the particle size and composition has, historically, been attained empirically, making systematic studies of the effects of various structural parameters difficult. The controlled surface modification methodology used in this work has enabled the controlled modification of carbon supported Pt nanoparticles by Cr so as to yield nanoalloy particles with defined compositions. Subsequent heat treatment in 5% H2 in N2 resulted in the formation of a distinct Pt3Cr alloy phase which was either restricted to the surface of the particles or present throughout the bulk of the particle structure. Measurement of the oxygen reduction activity of the catalysts was accomplished using the rotating thin film electrode method and the activities obtained were related to the structure of the nanoalloy catalyst particles, largely determined using Cr K edge and Pt L3 edge XAS.

In Situ XAS Studies of Core-Shell PEM Fuel Cell Catalysts: The Opportunities and Challenges

Core-shell electrocatalysts are of increased interest in PEM fuel cells as a means of improving activity and reducing costs. In situ x-ray absorption spectroscopy (XAS) provides a unique opportunity to provide characterization of the structure of such materials under operating conditions, enabling the effects of the electrochemical environment on the structures to be explored. However, such measurements present numerous challenges in terms of both data collection and analysis. Here we present two case studies that illustrate both the opportunities and challenges; (i) the modification of a Pt catalyst by Ru and the potential dependent formation of a surface alloy and (ii) a Pt shell - Pd core catalyst with varying shell thickness.

Phase Transitions of Ruthenium-Doped Iron Oxide Studied by 57Fe Mössbauer Spectroscopy at Elevated Temperatures

The 57Fe Mössbauer spectra recorded in situ from 5% ruthenium-doped maghemite show parameters typical for maghemite up to 600 K and a hyperfine field distribution indicating that the process of doping is to some extent, inhomogeneous and the materials are formed with a range of particle size. At 700 K a phase transition of maghemite to hematite is detected and further studies show that Ru-doped hematite had a Morin transition temperature at ca. 390 K. Further heating of the sample to 800 K accelerates the maghemite-hematite and broadens the temperature range over which the Morin transition takes place.

Reduction properties of Ce in CeOx/Pt/Al2O3 catalysts

Abstract A controlled surface reaction (CSR) technique has been successfully employed to prepare a series of CeOx modified Pt/Al2O3 catalysts, offering a unique system to specifically probe the relationship between Ce and Pt without any bulk CeO2 present. Ce L3 edge X-ray absorption near edge structure (XANES) analysis was used to ascertain the oxidation state of the Ce in the catalyst materials in atmospheres of air, H2 (g), and CO (g) at room temperature. The XANES data showed that the Ce was present as both Ce3+ and Ce4+ oxidation states in an atmosphere of air, becoming predominantly present as Ce3+ in H2 and CO. The results indicate the role of Pt in the process, and show that with the absence of bulk CeO2, changes in Ce oxidation state can be observed at non-elevated temperatures. The CeOx/Pt/Al2O3 catalysts were tested for their performance toward the water gas shift (WGS) reaction and showed improved performance compared to the unmodified Pt/Al2O3, even at very low concentrations of Ce (∼0.35 wt-%).