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Oxygen Dissociation Kinetics of Concurrent Heterogeneous Reactions on Metal Oxides Yi-Lin Huang, Christopher Pellegrinelli, and Eric D Wachsman ACS Catal., Just Accepted Manuscript • DOI: 10.1021/acscatal.7b01096 • Publication Date (Web): 18 Jul 2017 Downloaded from http://pubs.acs.org on July 18, 2017
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Oxygen Dissociation Kinetics of Concurrent Heterogeneous Reactions on Metal Oxides Yi-Lin. Huang, Christopher Pellegrinelli, and Eric D. Wachsman* University of Maryland Energy Research Center, University of Maryland, College Park, MD, 20742
*Corresponding author: Eric D. Wachsman;
[email protected]
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Abstract The high activity of oxide catalysts toward the oxygen reduction reaction (ORR) attracts unwanted
interactions
with
other gaseous
oxygen-containing species
in
air.
Understanding the interaction between oxygen-containing species, mainly water and carbon dioxide, and oxides is important for many energy applications. However, the oxygen self-exchange process and the high temperature operating condition limit the investigation on these concurrent reactions. Here we report a direct observation of impacts of water and carbon dioxide on dissociation rates of ionically conducting catalysts, La0.6Sr0.4Co0.2Fe0.8O3-δ(LSCF) and (La0.8Sr0.2)0.95MnO3±δ(LSM), using gas phase isotope exchange. The concurrent heterogeneous reactions of oxygen and other oxygen-containing species on oxide catalysts can either promote or hinder oxygen dissociation rates, depending on the participation of lattice oxygen. LSCF appears to be much more active in exchange with these oxygen-containing species while LSM shows relatively little exchange. Oxygen-containing species exhibit site-blocking effects and inhibit the reaction on LSCF. In contrast, water and CO2 promote the oxygen dissociation rate on LSM, likely due to prominence of homoexchange, where intermediate surface species play an important role. Our study provides insights into the reaction mechanism of oxygen dissociation and the impact of co-existing ambient air oxygen-species.
Keywords: (dissociation, isotope, water, carbon dioxide, kinetics) 2
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Introduction The oxygen reduction reaction (ORR) is an important electrochemical reaction with many technology applications.1-16 By reducing oxygen from gas phase molecules to solid or surface phase atoms, controllable oxidative processes become possible. The strong OO bond in oxygen molecules requires a highly active catalyst to facilitate the reactions.1724
Due to the high catalytic activity of these oxygen catalysts, other gaseous molecules
can also participate in the reaction, resulting in unwanted reactions. For instance, contaminants in air, such as water and carbon dioxide, have been reported to have direct impacts on the performance of solid oxide fuel cells (SOFC) and lithium-air battery cathodes.14, 25-38 It still lacks fundamental understanding of how water and carbon dioxide interact with the surface from a reaction kinetics point of view. Here, we applied gas phase isothermal isotope exchange with 1:1 ratio of 18
16
O2 and
O2 (1:1 IIE)39, 40 with the presence of water as well as carbon dioxide to determine the
effects of oxygen-containing species on oxygen exchange for perovskite oxide catalysts (La0.8Sr0.2)0.95MnO3±δ (LSM) and La0.6Sr0.4Co0.2Fe0.8O3-δ (LSCF) that have fundamentally different reaction mechanisms.41-57 LSM is a pure electronic conductor and provides an active surface to catalyze the ORR. The concentration of oxygen vacancies in LSM is low and oxygen transport of LSM is limited to the near surface region or grain boundaries.
39, 58, 59
In contrast, LSCF is a mixed ionic electronic conductor (MIEC) with
a high concentration of oxygen vacancies. The analysis of gas phase isotopic oxygen exchange requires a suitable model with distinct boundary conditions, which vary based on the experimental set up. Compared to isotope exchange depth profiling (IEDP) which is an ex situ technique that analyzes the 3
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solid isotopic concentration, gas phase isotope exchange allows higher sensitivity for measuring surface exchange processes.39 The discrepancy between gas phase isotopic oxygen exchange and IEDP can be found in the supporting information. Our approach is based on steady state isotopic transient kinetics analysis (SSITKA),58,
60-62
a well-
established technique to probe surface catalytic reactions, but further takes into account the self-exchange process between reactants and oxide catalysts for a macro-view of molecular exchange on the catalyst surface. While flowing both
16
O2 and
18
O2 isotopic
gases it is possible to separate oxygen dissociation step from the overall ORR and can provide insight on multiple gas-solid reactions as well as changes in isotopic oxygen distribution in the solid-phase, in real time. There are two main types of surface exchange processes,63-65 homo-exchange and hetero-exchange. In homo-exchange the catalyst only provides surface sites for gas-gas exchange to occur, as shown in Figure 1 (a). Oxygen containing reactants are dissociated and re-associated with each other on the catalyst surface without the participation of lattice oxygen. In contrast, hetero-exchange reactions involve the exchange between gas atoms and solid (or surface) atoms. The conversion of gaseous oxygen into solid oxygen consists of a series of reactions. We can simplify the surface exchange process into two different elementary steps,66 as shown in Figure 1 (b). The first step is dissociation, describing the gas-solid reaction involving breaking of the O-O molecular bond. The second step is incorporation, describing the transport of oxygen from surface to bulk. A vacancy in the solid phase is required for the dissociated oxygen to incorporate into the oxygen lattice via vacancy exchange mechanism, as highlighted in Figure 1 (b). Thus, the 4
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overall reaction is a combination of heterogeneous catalysis and solid-state diffusion. In both cases, the vacancy concentration, or the oxidation states of terminal ions on the surface, plays an important role. For the homoexchange, surface vacancy concentration represents the active sites for oxygen dissociation. For the heteroexchange, oxygen vacancy concentration determines the rates of incorporation and diffusion. Although
homo-exchange
and
hetero-exchange
are
different
exchange
mechanisms, the observed “products” of isotopically labelled oxygen-containing reactants after exchange are the same, as listed in Figure 1. Therefore, the 18O exchange curves, describing the amount of incorporated
18
O into the solid phase as a function of
exchange time, can be used to identify the participation of lattice oxygen, as shown in Figure S5 in Supporting Information. We can distinguish the effects of oxygen-containing reactants on each reaction step separately. The oxygen dissociation reaction can be shown as40: ex
ex
216 O 2 + 218 O 2 ↔16 O 2
ex− p
+216 O18O ex− p +18 O 2
ex− p
[1]
where [16O2]ex and [18O2]ex are oxygen concentrations participating in the exchange reactions, and [16O2]ex-p, [16O18O]ex-p, and [18O2]ex-p are the products after exchange. In the above reaction, the scrambled product, 16O18O, is produced only after surface exchange. Therefore, the production rate of
16
O18O can uniquely be used to describe the catalytic
activity of materials toward oxygen dissociation and from this we can calculate the apparent activation energy (Ed) for the oxygen dissociation (re-association) reaction.
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Figure 1. Examples of exchange between O2, D2O and the solid surfaces through (a) homo-exchange mechanism on LSM and (b) hetero-exchange mechanism on LSCF.
Introducing oxygen with and without the presence of these other oxygencontaining species, we can directly observe, in situ their effects on both surface catalytic reactions and gas-solid reactions by tracing the movement of
18
O to other oxygen-
containing reactants. In this isotope exchange study, D2O, instead of H2O, is used as water source due to the overlapping of signal with
18
O (m/z=18) in mass spectrometer.
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The exchange reactions between the dissociated oxygen and CO2, or D2O, can be expressed as: x
x
x
O * + D 2 16 O ↔ 16 O * + D 2 x O
[2]
O * + C16 O 2 ↔16 O* + C x O16 O
[3]
O * + y O* + C16 O 2 ↔ 216 O* + C x O y O
[4]
The isotopic oxygen coverage on the catalyst surface, i.e. the possibility of x and y to be either 16 or 18, is determined by the equilibrium of overall kinetics between all gasgas and gas-solid exchange reactions. Therefore, the final concentration of each oxygencontaining isotopologue is a function of the concentrations of all inlet gas species, O2 exchange rates, as well as water and CO2 exchange rates. Two different cases are considered to solve the complex exchange reactions. In the case of homo-exchange where there is no participation of lattice oxygen, the overall reaction can be simplified to just surface catalytic reactions. In the case of heteroexchange, oxygen transport between gas and solid phases needs to be taken into account. The surface exchange coefficient (k) is used to quantitatively describe the changes in the self-exchange rate between gas and solid with the presence of oxygen-containing reactants. 39
Results and Discussion Figure 2 shows representative 1:1 IIE of LSCF with the presence of 3000ppm D2O or 2500ppm CO2 (details in Figure S1 and Figure S2 in Supporting Information). 7
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Due to the experimental limitations, oxygen partial pressure (pO2) of this study was fixed at 0.05 atm.40 Open symbols represent the experimental data for O2, D2O, and CO2 isotopologues as a function of exchange time. The lines are the best-fit results. It is important to note that 18O2 gas is the only source of 18O in the system, and any formation of D218O, C16O18O or C18O2 involves the participation of
18
O2 molecules. The pink
symbols in Figure 2 (e) and (f) represent D216O, while the green symbols represent D218O. As temperature increases from 400°C to 500°C, we can see an increase in the level of D218O. CO2 signals for 1:1 IIE of LSCF at 400°C and 500°C are shown in Figure 2 (g)(h). At 400°C, CO2 starts to participate in the exchange, as seen by the formation of C16O18O, suggesting that at this temperature LSCF has the ability to dissociate both O2 and CO2. Hetero-exchange between O2, CO2, and the LSCF surface, are the dominant surface reactions, rather than homo-exchange between O2 and CO2.
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Figure 2. Representative experimental data (symbols) and fitting results (lines) for 1:1 IIE of LSCF with the presence of water and CO2. Concentrations of O2 were 25000ppm 16O2 and 25000ppm 18O2 for a total of 50000ppm, and the concentration of D2O was 3000ppm and CO2 was 2500ppm. The O2 signals are shown on the left for (a) 400°C, (b) 500°C in water, and (c) 400°C, (d) 500°C with CO2; and the corresponding D2O signals on the right for (e) 400°C, (f) 500°C, and CO2 signals for (g) 400°C, (h) 500°C. 9
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Figure 3. Representative experimental data (symbols) and fitting results (lines) for 1:1 IIE of LSM with the presence of water and CO2. Concentrations of O2 were 25000ppm 16O2 and 25000ppm 18O2 for a total of 50000ppm, and the concentration of D2O was 3000ppm and CO2 was 2500ppm. The O2 signals are shown on the left for (a) 350°C, (b) 650°C in water, and (c) 350°C, (d) 650°C with CO2; and the corresponding D2O signals on the right for (e) 350°C, (f) 650°C, and CO2 signals for (g) 350°C, (h) 650°C. 10
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LSM has a different exchange mechanism in the presence of water and CO2 than LSCF. Representative 1:1 IIE of LSM with the presence of water at different temperatures is shown in Figure 3 (details in Figure S3 and Figure S4 in Supporting Information). Due to the lack of oxygen vacancies in LSM, homogeneous exchange dominates on LSM up to 650 °C, as shown in the O2 signal in Figure 3 (b) and (d). At 350°C, a significant portion of homo-exchange between O2 and D2O or CO2 occurs, as shown in D2O and CO2 signals in Figure 3 (e) and (h). To quantitatively determine the impacts of water and CO2 on oxygen dissociation, we can consider the implications of the level of exchange we are seeing and its relationship to the overall catalytic activity and reaction kinetics of the materials. The steady state concentrations of oxygen isotopologues of LSCF with the presence of water and CO2 (solid symbols) at different temperatures is summarized in Figure 4 (a) and (b), respectively, and the best-fit line is shown in dark yellow. The presence of water and CO2 increases the temperature necessary for LSCF to be able to dissociate the same amount of 18
O2. In contrast, the presence of oxygen-containing reactants on LSM show the opposite
effect, as shown in Figure 4 (c) and (d). With the presence of 3000ppm D2O and 2500ppm CO2, the
16
O18O signal begins to appear at lower temperature. There is no
temperature dependence for the steady state concentrations of oxygen isotopologues at high temperature (~450°C for LSCF and ~700°C for LSM), indicating that the reaction is limited by mass transfer at these temperatures. 39 The effects of water and CO2 on the dissociation rate (rd) are summarized in an Arrhenius plot in Figure 5 (a). Compared with rd without the presence of other oxygen-
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containing reactants (open symbols), CO2 and water on the catalytic activity of different perovskite oxides (closed symbols) show different reaction mechanisms.
Figure 4. Steady state concentrations of 16O2 (18O2) (blue closed symbols) and 16O18O (red closed symbols) for LSCF with the presence of (a) 3000ppm D2O and (b) 2500ppm CO2 or without (open symbols) ) and for LSM with the presence of (c) 3000ppm D2O and (d) 2500ppm CO2 or without (open symbols) at different temperatures with PO2=0.05 atm. While LSCF shows higher levels of dissociated oxygen, it also shows a much greater influence from water and CO2. The
16
O18O level decreases with the presence of
CO2 and water, indicating that both of them play an important role in inhibiting the catalytic ability of LSCF toward the dissociation of oxygen. The apparent activation energy for oxygen dissociation/re-association (Ed) on LSCF is around 77 kJ/mol,40 while
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Ed with the presence of water and CO2 increases to 102 kJ/mol and 88 kJ/mol, respectively. For LSM, the presence of water and CO2 during exchange results in an increase in the production of
16
O18O, depicted in Figure 5 (a). Homo-exchange between O2 and
D2O, or CO2, dominates surface reactions on LSM and the presence of D2O and CO2 seems to accelerate the overall homo-exchange process. With the presence of water and CO2, Ed increases to 86 kJ/mol and 69 kJ/mol, respectively (only O2: 63 kJ/mol).40
Figure 5. (a) Arrhenius plot of CO2 and water effects on dissociation ability of LSM and LSCF. Fit lines for CO2 effect is shown in dash red lines and fit lines for water effect is shown in solid red line. (b) Arrhenius plot of CO2 and water effects on surface exchange coefficient (k) of LSCF with Eex=59 kJ/mol and 46 kJ/mol, respectively. Without the presence of water and CO2 are shown in open symbol with an Eex=45 kJ/mol.39, 40 Experiments carried out at PO2=0.05 atm and concentrations of 2500ppm and 3000ppm for CO2 and D2O, respectively. 13
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Water and CO2 effects on the overall hetero-exchange process has been determined by changes in the 18O lattice fraction. An Arrhenius plot of surface exchange coefficient (k) for LSCF is shown in Figure 5 (b). The change of k for LSCF with the presence of CO2 (Green) and water (Blue) is also shown in Figure 5 (b). CO2 and water on the surface of LSCF limit the overall hetero-exchange. In contrast, the low concentration of oxygen vacancies in LSM at the temperatures studied limits the ability of LSM to incorporate oxygen. The boundary condition for deriving k is based on the volumetric difference between isotopic concentration in gas and solid phase. The nonuniform distribution of 18O in LSM due to the limitation of bulk exchange invalidates this boundary condition. Therefore, k is not suitable to describe oxygen transport in LSM. Isotope exchange results suggest that the presence of CO2 and water affects the surface exchange mechanisms differently for the two materials studied. The concentration of oxygen vacancies in the near surface region has a significant impact on the interaction of oxygen-containing reactants. Here we propose the possible reaction mechanisms to explain effects of oxygen-containing species on the dissociation rates of different perovskite oxide catalysts, as shown in Scheme 1, using water molecule as an example. Scheme 1 (a) shows the hetero-exchange on LSCF without the presence of oxygen-containing species. The participation of oxygen vacancy is necessary for the hetero-exchange to occur. When oxygen-containing species is present, oxygen vacancy can be occupied by these oxygen containing species, as shown in Scheme 1 (b). Water and CO2 physically block the same exchange sites with O2 and actively participates in the ORR, resulting in a decrease of LSCF’s dissociation ability. Blocking of available surface 14
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sites and a decrease in the total accumulated exchange in LSCF limits the heteroexchange rate. The other possible mechanism for this effect is the formation of chemical complexes that change surface bonding energies, leading to the decrease of catalytic activity. 67
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Scheme 1. Proposed reaction mechanisms to explain effects of oxygen-containing species, e.g. water, on perovskite oxide catalysts. A simple model, consisting of one layer of transition metal (TM) and another layer of oxygen lattice (O) is used to visualize steps of the exchange process. (a) The heteroexchange without the presence of oxygen-containing species via vacancy exchange mechanism. (b) The heteroexchange with the presence of water, where the dissociation of oxygen is hindered because active sites for O2 are occupied by water. (c) The model for the enhancement of homoexchange with the presence of water proposed by Peri, 64, 68, 69 where the irregularity of the solid surface after water desorption facilitates the homoexchange process, leading to a higher concentration of 16O18O.
In contrast CO2 and water show a promotion effect on the oxygen exchange properties of LSM than for LSCF. One possible explanation for the enhancement of oxygen dissociation rate on LSM is related to the dominance of the homo-exchange process, as shown in Scheme 1 (b). The catalytic activity of LSM is affected by the interactions of oxygen containing reactants with the solid surface. Peri
64, 68, 69
proposed
that for some oxide materials, hydrated surfaces may have increased rates of 16
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homogeneous exchange due to the formation of surface complexes involving OH-. The irregularity of the surface after desorption of adsorbed oxygen-containing species provides a fast pathway for oxygen dissociation. The effects may be limited to changes in the rate of homo-exchange for LSM. However, under real operating conditions dissociated oxygen atoms can participate in the overall ORR. This shows a fundamental difference in the way these oxygen-containing molecules interact with the surface of LSM and LSCF. The causes may be related to the catalytic properties of Co, Fe and Mn, or the concentration of oxygen vacancies for the two materials, or, even more likely, a combination of all of these factors. Conclusion We investigated the reaction kinetics of oxygen catalysts, including surface reactions and solid-state exchange, in the presence of multiple gasses, using gas phase isotope exchange. Both water and CO2 participate in the ORR on LSCF and LSM. For LSCF, water and CO2 actively exchange with lattice oxygen and prohibits O2 exchange with lattice oxygen. The presence of water and CO2 indicates blocking effects on the LSCF surface possibly due to the competitive surface adsorption or the formation of surface complexes. In contrast, water and CO2 promote the oxygen dissociation rate on LSM, likely due to prominence of homoexchange, where intermediate surface species play an important role. Our results show that water and CO2 has a higher impact in the intermediate temperature range (below 450 °C) and this impact decreases as temperature increases. Because one of the major goals for SOFC technology is lowering operating temperature, the impacts of water and CO2 is inevitable. Using isotope exchange we can distinguish 17
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the different transport mechanisms on ionically conducting catalysts in the presence of multiple gases. Gas phase isotope exchange allows us to better distinguish between homoexchange and hetero-exchange of oxygen molecules, as well as other oxygen-containing species exchange on metal oxides. The combination of isotope exchange with other techniques can potentially provide a well-defined picture of the ORR and multi-gas reaction mechanisms. Experimental section La0.6Sr0.4Co0.2Fe0.8O3-δ (LSCF) (Praxair) and (La0.8Sr0.2)0.95MnO3±δ (LSM) (Fuel Cell Materials) powders were used and the sample weights for each powder were normalized to have equivalent total surface areas of 0.1 m2. A non-circulating plug-flow reactor is selected to observe the effects of oxygen-containing reactants on oxygen exchange under a continuous flow.70 The total flow rate is fixed at 20 SCCM. To mimic real operating conditions, concentrations of water and CO2 are fixed at low levels. The water concentration is fixed at 3000ppm by bubbling the carrier gas through a glass impinger submerged in a temperature controlled water bath. The concentration of CO2 for the range of experiments is fixed at 2500 ppm. Powders were pretreated at 800°C for 30 minutes in normal isotope (16O2) environment with PO2=0.05 atm to remove any adsorbed contaminants and saturate the lattice with 16O, before each 1:1 IIE measurement. The sample was then cooled down to the target temperature and the system equilibrated at a PO2 of 0.05 atm. At t=0, the pneumatic valve was used to rapidly switch between 16O2 and 18O2, resulting in a 1:1 ratio of 16O2:18O2 flowing to the reactor. Changes in concentrations of the gas species, D216O, 18
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D218O,
16
O2,
16
O18O,
18
O2, C16O2, C16O18O, and C18O2, are then analyzed and recorded
using a quadrupole mass spectrometer (QMS) system. Supporting information Experimental set-up, comparison of isotope exchange technique, raw data of isotope exchange measurements with the presence of water or CO2 on LSCF and LSM. Acknowledgments Authors wish to acknowledge the support of the U.S. Department of Energy, NETL, Contract #: DEFE0009084.
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