| AVS 62nd International Symposium & Exhibition | |
| In-Situ Spectroscopy and Microscopy Focus Topic | Monday Sessions |
| Session IS+AS+SA+SS-MoA |
| Session: | Ambient Pressure X-ray Photoelectron Spectroscopy Studies for Catalytic and Energy Materials in Gas Phase |
| Presenter: | Petra Reinke, University of Virginia |
| Authors: | E. Monazami, University of Virginia J.B. McClimon, University of Pennsylvania N. Johansson, Lund University, Sweden P. Shayesteh, Lund University, Sweden S. Urpelainen, Lund University, Sweden J. Schnadt, Lund University, Sweden P. Reinke, University of Virginia |
| Correspondent: | Click to Email |
Transition metal carbide (TMC) surfaces are coveted as catalytic materials, electrodes and hard protective coatings. A bottleneck in their use is surface oxidation, which leads to a decay in performance. Our work establishes the feasibility of surface recovery by using a carbon-rich WC layer where recarburization of the surface is initiated by an annealing step. Thin carbon-rich tungsten carbide layers are grown by co-deposition of W and C60 on a MgO(001) surface at 1100 K. The MgO substrate serves as a diffusion barrier for carbon, and the films have a well-defined carbon inventory controlled by the deposition rates of the reactants. The film surfaces were studied by in-situ Scanning Tunneling Microscopy and Spectroscopy. Raman spectroscopy confirmed the presence of highly defective graphitic carbon. The oxidation-recarburization (O-R) cycles were studied in the ambient pressure endstation SPECIES at MAX-Lab (J. Synchr. Rad. 701, 19 (2012)) in a pressure of 0.3 mbar of O2. Oxidation with p(O2) of 10-5 mbar were performed at SPECIES for direct comparison to low p(O2) STM experiments.
The carbon-rich WC films exhibit a relatively rough surface, which allows only in a few instances true atomic resolution, but graphite as well as graphene layers can be identified. Oxidation at T>550 K leads to etching of surface carbon and the growth of a W-oxide layer and STS maps show the oxide evolution. The oxidation in the low p(O2) pressure regime progresses slowly and the surface carbide is recovered by annealing.
The use of the SPECIES endstation enabled a quantiative study of the O-R cycles including a detailed analysis of the respective bonding environments which are modified at different times in the O-R cycle. The oxidation in the ambient pressure environment was monitored using the ratio of W-carbide to W-oxide in the W4f core level during the reaction. The steady state thickness of oxide is a function of sample temperature and order of annealing cycles. The surface carbide concentration can be fully recovered in a subsequent annealing step, and repeated O-R cycles were performed. The O-R process is controlled by the interplay between surface oxidation, oxygen and carbon diffusion and our results will be modeled with a simple set of transport equations. We will discuss the role of different bonding environments as we move through the O-R cycle and compare UHV and ambient pressure results up to 800 K. These results clearly illustrate that carbon-rich tungsten carbide materials can be used to achieve a long term use of carbide surfaces in catalysis and fuel cell applications.
supported by NSF-Division of Materials Research (Ceramics) DMR-100580, STINT award.