Operando Synchrotron X-Ray Absorption and Diffraction Study of Co-based catalysts: Insights into Structural and Microstructural Changes
N. Yigit,a,* L. Lukashuk,a A. Nagl,a P. Hans,a M. Leoni,b K. Föttinger,a G. Rupprechtera
aTechnische Universität Wien, Vienna, 1060, Austria bUniversity of Trento, Trento, 38122, Italy
*Corresponding author: nevzat.yigit@tuwien.ac.at
Catalytic preferential CO oxidation (PrOx), i.e. CO+H2+0.5O2→CO2+H2, is a key reaction for removing traces of CO from the H2-rich stream of hydrogen-fueled proton exchange membrane fuel cells. Transition metal oxides, especially cobalt-based materials, are perspective catalysts for PrOx. However, the current understanding of PrOx over these catalysts is mainly based on pre- and post-catalyst characterization, and open questions include the nature of active sites and reaction pathways. The ultimate goal of this work was to investigate the nature of the active sites/phases of Co-based catalysts in order to understand the reaction mechanism in the preferential oxidation of CO (PrOx). In this case, we compared the structural changes of Co in different chemical compounds and oxidation states (Co3O4, and LaCoO3 perovskite), again aiming at understanding the nature of the active sites for the reaction on both catalysts.
1. Scope
Due to the cost and limited availability of noble metals, in recent years increasing attention has been paid to noble-metal-free alternatives, i.e, transition metal oxides, as catalysts for CO oxidation and PrOx. Among these oxides especially cobalt-based materials have turned out to be perspective catalysts for PrOx1-3. Despite intensive studies, there is still a lack of understanding of driving forces leading to outstanding catalytic performance of cobalt-based catalysts in PrOx: the redox Co3+/Co2+ couple involved in electron exchange, structure modifications or microstructural changes. This hinders further development and commercialization of cobalt-based catalysts for technologically important industrial processes. Therefore, an in depth investigation of the structural, microstructural and electronic changes of cobalt-based catalysts occurring under working catalytic conditions (operando) is crucial.
We have performed an operando investigation (XAS and XRD) to study the structure and oxidation state of two cobalt-based catalysts, i.e. Co3O4 and LaCoO3, during PrOx, while simultaneously monitoring the catalytic activity by mass spectrometry. These catalysts are structurally different (i.e., Co3O4 face centric cubic while LaCoO3 perovskite is rhombohedral) and exhibit different reactivity in PrOx. Apart from the PROX reaction mixture (50% H2, 5% CO, 5% O2 in He), reduction in CO or H2 atmospheres were examined as well as side reactions (i.e., CO oxidation, CO and CO2 methanation, water gas shift) were investigated. To investigate the bulk structure, we utilized operando X-ray absorption spectroscopy (XAS) at the Co K -edge (7709 eV), which was carried out in transmission geometry at the l811 beamline, Max-lab II, Lund, Sweden. Operando time-resolved powder XRD experiments with a time resolution of about one diffractogram per second were performed at the I11beamline, Diamond, UK in order to gain information about the structural and microstructural changes (lattice constant as a function of oxidation state) of catalysts.
2. Results and discussion
For the Co3O4 catalyst, operando XAS and NAP-XPS revealed that under conditions of high PROX selectivity both the bulk and surface of Co3O4 were fully oxidized (up to 250°C). When surface reduction started at higher temperature, the selectivity changed to methanation4. The rather easy surface reduction of Co3O4 in pure CO (starting already at ~100 °C) suggests that in PrOx the adsorbed CO reacts with lattice oxygen, which is replenished by gas phase O2. Reoxidation of the catalysts by O2 during PrOx is fast preventing overall reduction of the catalysts when CO oxidation is dominant.
To explore further the potential of cobalt-based materials for PrOx, we focused on studying LaCoO3 as a model perovskite system. On the contrary to Co3O4, the analysis of operando XANES at the Co K-edge for LaCoO3 revealed that the perovskite phase remained stable in the PrOx reaction mixture up to 400 °C, and no methane production was observed below 400 °C. Thus, preserving cobalt in a high oxidation state (+3) in LaCoO3 during PrOx explains the wider temperature window for the selective CO oxidation in excess of hydrogen as compared to Co3O4. The XRD patterns for the LaCoO3 catalyst recorded during the PrOx process with increasing temperature and isothermally at 200 °C were compared with the XRD patterns recorded in O2 atmosphere (Figure 1). From Figure 1b it can be seen that under PrOx conditions the XRD reflections for LaCoO3 were broadened and shifted to higher 2theta in comparison to the XRD reflections in O2 atmosphere. This is an indication of partial reduction/microstructural changes for LaCoO3 in the PrOx mixture and formation of defects. During PrOx (i.e., 5%CO, 5%O2, 50% H2) the bulk structure of LaCoO3 remained intact up to 300 °C, when CO oxidation was dominant. Further increase of temperature led to structural changes and formation of La2Co2O5 shifting the selectivity of the reaction from CO oxidation to hydrogen oxidation (Figure 1a). The La2Co2O5 structure turned out to be - once formed - the most stable structure under PrOx conditions at 350 °C. The catalyst could not be oxidized again and remained much less selective for CO oxidation in PrOx at 350 °C.
a) PROX b) at 200 °C O2 vs PROX
Figure 1. In-situ XRD during preferential CO oxidation on LaCoO3 with increasing temperature from RT to 350 °C (a). Comparison of LaCoO3 under PROX conditions (5%CO, 5%O2, 50% H2 and 40% He) and in O2 atmosphere at 200 °C (b).
3. Conclusions
The results of our study illustrate that during PrOx the bulk structure of LaCoO3 and Co3O4 remain intact, when CO oxidation is dominant. The fact that cobalt is preserved in a high oxidation state in the case of LaCoO3 during PrOx explains the wide temperature operation window for selective CO oxidation in excess of hydrogen in comparison to Co3O4. Incorporation of Co into the LaCoO3 structure leads to partial reduction of LaCoO3 (i.e., microstructural changes of LaCoO3) in the PrOx mixture and formation of defects during the preferential oxidation of CO to CO2 in a wide temperature range of 100-300 °C. This study underpins the potential of a combined operando XRD and XAS approach for studying catalytic systems and revealing if the structure/microstructure of the catalyst is dynamic under process conditions. The knowledge obtained in this study could help in developing novel catalysts for PrOx application via tuning structural/microstructural properties of materials.
This work was supported by the Austrian Science Fund (FWF) [SFB F4502 FOXSI]. We acknowledge Diamond Light Source for time on Beamline I11 under Proposal [EE13826].
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