In-situ X-ray Absorption Study of Evolution of Oxidation States and Structure of Cobalt in Co and CoPt Bimetallic Nanoparticles (4 nm) under Reducing (H 2 ) and Oxidizing (O 2 ) Environments (original) (raw)
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Catalysis Today, 2012
In situ near edge X-ray absorption fine structure (NEXAFS) spectroscopy and ambient pressure X-ray photoelectron spectroscopy (AP-XPS) were performed to monitor the oxidation state and structure of 4 nm CoPt nanoparticles during the reaction of CO with O 2 -a model oxidation reaction. In addition, reversible changes in the oxidation state of cobalt as a function of cycling CO and O 2 pressure (in the range of millitorr to 60 Torr) were quantified and compared. Turnover frequency reaction data was also obtained for the CoPt nanoparticles and correlated with the oxidation states and structures observed spectroscopically. These findings indicate that separated from the effect of partial pressure of the reactant gases, chemical state and structure changes of the CoPt nanoparticles during CO oxidation are important factors in determining the rate of the reaction.
Topics in Catalysis
Cobalt and platinum–cobalt bimetallic alloy nanoparticles of uniform size distribution where prepared and supported on MCF-17 to produce a controlled and well-characterized model catalyst which was studied under reaction conditions during CO2 hydrogenation. Near edge X-ray absorption fine structure (NEXAFS) spectroscopy was used to elucidate the oxidation state of the catalyst under reaction conditions while the effect of reducing H2 gas on the composition and structure of the bimetallic PtCo nanoparticles was measured using ambient pressure X-ray photoelectron spectroscopy (AP-XPS) and environmental transmission electron microscopy (ETEM). NEXAFS indicates that Pt aids the reduction of Co to its metallic state under relevant reaction conditions, while AP-XPS and ETEM indicate that Pt is enriched at the surface by exchange with subsurface layers which become Pt deficient—in agreement with the “Pt-like” selectivity seen during catalytic testing of these materials.
The Journal of Physical Chemistry C, 2013
We report on the atomic structural changes and diffusion processes during the chemical transformation of ε-Co nanoparticles (NPs) through oxidation in air into hollow CoO NPs and then Co 3 O 4 NPs. Through XAS, XRD, TEM, and DFT calculations, the mechanisms of the transformation from ε-Co to CoO to Co 3 O 4 are investigated. Our DFT calculations and experimental results suggest that a two-step diffusion process is responsible for the Kirkendall hollowing of ε-Co into CoO NPs. The first step is O in-diffusion by an indirect exchange mechanism through interstitial O and vacancies of type I Co sites of the ε-Co phase. This indirect exchange mechanism of O has a lower energy barrier than a vacancy-mediated diffusion of O through type I sites. When the CoO phase is established, the Co then diffuses outward faster than the O diffuses inward, resulting in a hollow NP. The lattice orientations during the transformation show preferential orderings after the single-crystalline ε-Co NPs are transformed to polycrystalline CoO and Co 3 O 4 NPs. Our Co 3 O 4 NPs possess a high ratio of {110} surface planes, which are known to have favorable catalytic activity. The Co 3 O 4 NPs can be redispersed in an organic solvent by adding surfactants, thus rendering a method to create solution-processable colloidal, monodisperse Co 3 O 4 NPs.
We have used grazing incidence X-ray absorption fine structure spectroscopy at the cobalt K-edge to characterize monolayer CoO films on Pt(111) under ambient pressure exposure to CO and O 2 , with the aim of identifying the Co phases present and their transformations under oxidizing and reducing conditions. X-ray absorption near edge structure (XANES) spectra show clear changes in the chemical state of Co, with the 2+ state predominant under CO exposure and the 3+ state predominant under O 2-rich conditions. Extended X-ray absorption fine structure spectroscopy (EXAFS) analysis shows that the CoO bilayer characterized in ultrahigh vacuum is not formed under the conditions used in this study. Instead, the spectra acquired at low temperatures suggest formation of cobalt hydroxide and oxyhydroxide. At higher temperatures, the spectra indicate dewetting of the film and suggest formation of bulklike Co 3 O 4 under oxidizing conditions. The experiments demonstrate the power of hard X-ray spectroscopy to probe the structures of well-defined oxide monolayers on metal single crystals under realistic catalytic conditions.
Low temperature fuel cells are clean, effective alternative fuel conversion technology. Oxygen reduction reaction (ORR) at the fuel cell cathode has required Pt as the electrocatalyst for high activity and selectivity of the four-electron reaction pathway. Targeting a less expensive, earth abundant alternative, we have developed the synthesis of cobalt phosphide (Co2P) nanorods for ORR. Characterization techniques that include total X-ray scattering and extended X-ray absorption fine structure revealed a deviation of the nanorods from bulk crystal structure with a contraction along the b orthorhombic lattice parameter. The carbon supported nanorods have comparable activity but are remarkably more stable than conventional Pt catalysts for the oxygen reduction reaction in alkaline environments.
ACS Catalysis, 2019
The shape of metal nanoparticles can dramatically depend on reaction conditions. While Pt nanoparticles are known to dynamically respond to the partial pressure of CO, going through several reconstructions, in situ TEM images show that, surprisingly, Co nanoparticles do not change their shape under a CO atmosphere. Detailed DFT calculations attribute this contrasting behavior to two factors: 1) CO adsorption has a higher stabilization effect on the high index facets of Pt than on those of Co; 2) the Co surface energy is more sensitive to the coordination number, making high index surfaces less stable relative to Pt. These two factors combined can affect the stability of high-index surfaces as is the case for Pt nanoparticles, which reconstruct already at low CO pressures. In the case of Co nanoparticles, the low index surface remains the most stable even at high CO partial pressures. The robustness of the shape of Co nanoparticles challenges recent proposals that high-index facets, which facilitate direct CO dissociation, are present on Co nanoparticle catalysts under Fischer-Tropsch conditions.
High low-temperature CO oxidation activity of platinum oxide prepared by magnetron sputtering
Applied Surface Science, 2015
a b s t r a c t CO oxidation on platinum oxide deposited by magnetron sputtering on flat (Si) and highly porous (multiwalled carbon nanotubes, MWCNT) substrates were examined using X-ray photoelectron spectroscopy, scanning tunneling microscopy, temperature-programmed desorption and temperature-programmed reaction in both UHV and ambient pressure conditions. Platinum in the freshly deposited thin film is present entirely in the 4+ oxidation state. The intrinsic CO oxidation capability of such catalyst proved to be significantly higher under approx. 480 K than that of pure platinum, presumably due to the interplay between metallic and cationic platinum entities, and the reaction yield can be further enhanced by increasing effective surface area when MWCNT is used as a support. The thermo-chemical stability of the platinum oxide, however, has its limitations as the thin film can be gradually thermally reduced to metallic platinum (with small residuum of stable Pt 2+ species) and this process is further facilitated in the presence of reducing CO atmosphere.
Study of the atomic structure and morphology of the Pt 3 Co nanocatalyst
Journal of Physics: Conference Series, 2009
It has been shown that Pt3Co nanoparticles used as a catalyst for cathode of Proton Exchange Membrane Fuel Cells (PEMFC) enhance oxygen reduction reaction (ORR) activity even by a factor of two compared to pure Pt nanoparticles. The local structure and chemical disorder of a commercially available Pt3Co nanocatalyst supported on high surface area carbon were investigated. High-quality XAFS spectra were collected at the ELETTRA synchrotron XAFS 11.1 beamline. XAFS spectra analysis have been performed accounting for the reduction of the coordination number and degeneracy of three-body configurations, resulting from transmission electron microscopy (TEM) and x-ray diffraction (XRD) extracted mean particles diameter, size distribution and expected surface atom contributions. The presence of a Co-Co first neighbour EXAFS signal is shown to be related to the degree of the alloy's chemical disorder. This is a good starting point for analyzing the atomic structure of Pt3Co nanocrystalline system and their changes as a function of alloy preparation or working conditions when they operate as a catalyst in PEMFC.