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Molecular Reactions of O2 and CO2 on Ionically Conducting Catalyst Yi-Lin Huang, Christopher Pellegrinelli, Mann Sakbodin, and Eric D. Wachsman ACS Catal., Just Accepted Manuscript • DOI: 10.1021/acscatal.7b03467 • Publication Date (Web): 26 Dec 2017 Downloaded from http://pubs.acs.org on December 26, 2017
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ACS Catalysis
Molecular Reactions of O2 and CO2 on Ionically Conducting Catalyst Yi-Lin Huanga,b, Christopher Pellegrinellia,b, Mann Sakbodina,c, Eric D. Wachsmana,b* a
Maryland Energy Innovation Institute, University of Maryland, College Park, MD, 20742 Department of Materials Science & Engineering, University of Maryland, College Park, MD, 20742 c Department of Chemical and Biomolecular Engineering, University of Maryland, College Park, MD, 20742 b
Corresponding Author: Eric D. Wachsman:
[email protected] 1 ACS Paragon Plus Environment
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Abstract The presence of CO2, an unavoidable component in air and fuel environments, is known to cause severe performance degradation in oxide catalysts. Understanding the interactions between O2, CO2, and ion conducting oxides is critical to developing energy conversion devices. Here, surface reaction kinetics of Ba0.5Sr0.5Co0.8Fe0.2O3-δ (BSCF) with the presence of both O2 and CO2 is determined using gas phase isotope exchange. BSCF actively reacts with CO2 and the incorporation of oxygen from CO2 to the lattice of BSCF is directly observed as low as 50 °C. Above 200 °C the reaction between CO2 and the BSCF surface dominates and is independent of the oxygen partial pressure. In addition, CO2 compete with O2 for binding to vacancy sites, forming surface intermediate species. Surprisingly, these surface intermediate species offer its oxygen to exchange with oxygen in gaseous O2 and CO2, inhibiting the interactions between O2 and the solid surface. This work provides fundamental insight into functioning oxide catalysts and the results can be applied to the design of improved oxide catalysts.
Keywords: BSCF, surface reaction kinetics, isotope exchange, CO2, heterogeneous catalysis 2 ACS Paragon Plus Environment
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Introduction Ionically conducting oxide catalysts, conducting ions and catalyzing surface reactions simultaneously, are widely used for a number of energy conversion applications, such as oxygen permeation membranes,1, 2 solid oxide fuel cell (SOFC) cathodes,3, 4 hydrogen production from water-splitting and metal-air batteries.5 However, for such devices atmospheric CO2, unavoidable under real operating conditions, is known to cause severe degradation on oxide catalysts.6-9 Fundamental understanding of oxygen reaction kinetics and CO2 exchange on oxide catalysts is crucial to further improve the stability of this material and advance design of new materials. However, the intricacy of gas-solid reactions and the complexity of temperatureoxygen partial pressure (pO2) dependence on the catalytic activity of non-stoichiometric metal oxide surfaces limits the investigation of surface molecular reactions on oxide catalysts using conventional characterization techniques. In this work, surface reaction kinetics on Ba0.5Sr0.5Co0.8Fe0.2O3-δ (BSCF) is investigated via gas phase oxygen isotope exchange. Although BSCF has demonstrated to have remarkable performance due to extraordinary surface catalytic activity for both the oxygen reduction and evolution reactions (ORR/OER),10,5, 9, 11-15 the stability of BSCF in CO2 environments is still a major issue for practical applications. For BSCF, it was found previously that CO2 prefers to bond to the alkaline-earth metals, Ba and Sr, occupying the A-site in the ABO3 perovskite. As a consequence, carbonates, such as BaxSr1-xCO3, may be formed on the surface, stopping the oxygen permeable flux in BSCF.16-18 Thus, understanding the interactions between O2, CO2, and the BSCF surface is important to determine the CO2 deactivation kinetics on BSCF at the molecular level.
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Our study mainly focuses on kinetics of the conversion of gaseous reactants to products, paramount to many energy conversion applications, to gain insights for the mass transport between gaseous oxygen-containing molecules and oxide catalysts.19, 20 The schematic diagram for gas phase isotopic oxygen exchange is shown in Figure 1. BSCF powder is used to achieve a high surface area for catalytic reactions to occur. Gaseous reactants, O2 and CO2 molecules, interact with the BSCF surface and can be tagged with isotopically labeled 18O, as illustrated in the enlarged view of the BSCF surface in Figure 1. These gaseous products, including oxygen isotopologues,
16
O2(m/z = 32),
16
O18O(m/z = 34),
18
O2(m/z = 36), and CO2 isotopologues,
C16O2(m/z = 44), C16O18O(m/z = 46), C18O2(m/z = 48), can be monitored by quadrupole mass spectrometer (QMS). Hence, any interactions between the BSCF surface and gaseous molecules can be observed. Here we conducted two different types of isotope exchange experiments by either operating with a ramp up of temperature under a desired pO2 to qualitatively survey the reactivity of O2 and CO2 over a wide range of temperature and pO2/pCO2, or operating under steady state conditions (isothermal and isobaric) to quantitatively extract kinetic parameters of materials by analyzing transient isotopic signals.
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ACS Catalysis
Figure 1. Gas phase isotopic oxygen exchange on BSCF. (a) Schematic drawing of isotope exchange on the BSCF surface. A controlled inlet gas environment in different partial pressures (pO2 and pCO2) and different isotope gasses isotope gasses (18O2 or 16O2) can flow across sample powder and oxygen exchange kinetics can be back-calculated based on the “products” after surface exchange. Results and Discussion To determine the binding energy of CO2 on the BSCF surface, temperature programmed desorption (TPD) on BSCF was conducted after exposing to 1% CO2 (AIRGAS, balanced with helium) for 30 minutes at different adsorption temperatures (TAd): 200, 350, 650, and 800 °C. The corresponding O2 and CO2 signals are shown in Figure 2 (a) and (b), respectively. As baselines, O2 and CO2 TPD on fresh BSCF (pretreated at 800 °C for 30 minutes in pO2=0.05 atm) were performed using helium as carrier gas, as shown in Figure 2 (c) and (d), respectively. An O2 desorption peak can be observed at 450°C, consistent with results from McIntosh et al.21 and Vente et al.11 The amount of desorbed O2 at 450°C decreases with increasing TAd for CO2. At TAd =200 °C, there is no detectable CO2 adsorption on the BSCF surface. Above 350°C, a clear CO2 desorption peak is observed around 750 °C, and this peak shifts to higher temperature with the increase of TAd, suggesting the bonding strength between CO2 and BSCF increases with the 5 ACS Paragon Plus Environment
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increase in TAd. The desorption energy of CO2 on BSCF is a function of TAd, consistent with the results reported by Yan et al.22 and Zhang et al.23, where the CO2 binding energy is 200-260 kJ/mol for TAd=350-800 °C with a reaction order of one.
Figure 2. O2 and CO2 signals of TPD on BSCF after CO2 adsorption. (a) O2 signal and (b) CO2 signal of CO2 TPD on BSCF after pretreated in 1% CO2 for 30 minutes at different adsorption temperatures (Tad). (c) O2 and (d) CO2 signal of TPD on fresh BSCF, pretreated at 800°C in PO2=0.05 atm.
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We designed two different temperature programmed isotope exchange techniques based on the source of tracer
18
O to differentiate transport pathways of oxygen with the presence of
multiple gases. In temperature programmed exchange (TPX), where 18
O2, reactions between
18
18
O comes from gaseous
O2 and the solid surface can be observed to determine the catalytic
activity of BSCF towards oxygen exchange. In isotope saturated temperature programmed exchange (ISTPX),
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where inlet gasses contain only normal isotope (16O2 and C16O2) and the
solid lattice is pre-labeled with 18O, heterogeneous gas-solid reactions can be directly observed by tracking the movement of 18O from the solid to gaseous O2 and CO2 molecules. Figure 3 (a) shows TPX of BSCF in 25000ppm 16
18
O2 only. Above 250~300°C, gaseous
O2 and 16O18O, are observed, indicating that above this temperature BSCF can exchange its 16O
lattice with oxygen in 18O2 molecules. Representative ISTPX of BSCF with only 1250ppm CO2 is shown in Figure 3 (b). Surprisingly, BSCF lattice oxygen actively exchanges with oxygen in CO2 at temperatures as low as 50°C. Based on the amount of CO2 that is exchanged to isotopically labeled CO2 and the total surface area of BSCF testing powder, the exchange process is not limited to only surface oxygen. All lattice oxygen participates in the exchange due to the high oxygen diffusivity of BSCF. The formation of singly exchanged C16O18O is observed at low temperature (
