Kinetics and Thermodynamics of H2O Dissociation on Reduced CeO2

Article pubs.acs.org/JPCC

Kinetics and Thermodynamics of H2O Dissociation on Reduced CeO2(111) Heine A. Hansen*,† and Christopher Wolverton* Department of Materials Science and Engineering, Northwestern University, Evanston, Illinois 60208, United States S Supporting Information *

ABSTRACT: We use density functional theory to investigate the reaction between reduced CeO2−x(111) and water. H2O dissociation to hydroxyl is facile on surface vacancies and lattice oxygen, while subsequent decomposition of hydroxyl into H2 has a high barrier, which results in reversible adsorption of H2O under ultra-high-vacuum conditions. The barrier to H2 formation through hydroxyl decomposition decreases by 0.2 eV, while H2O formation becomes more difficult at high hydroxyl coverage. However, on isolated oxygen vacancies on a hydroxyl covered surface, H2 may be produced through a CeH intermediate with a 1.14 eV barrier. Oxygen vacancies are found to be more stable in the subsurface than in the surface layer at all vacancy coverages and for hydroxyl coverages less than 25−50%. The competition of H2O desorption and vacancy diffusion from the subsurface to the surface may prevent formation of hydroxyl from H2O dosing at low temperature, while the highly stable hydroxyl phase may provide a thermodynamic driving force for further surface reduction in the presence of water. On the basis of our calculations we suggest substitutional doping with a cation that binds H stronger than Ce may improve the decomposition of hydroxyls into hydrogen.

1. INTRODUCTION Ceria is a versatile material with key applications in three-way catalysts,1 the water gas shift reaction,2 solid oxide fuel cell electrodes,3 and solar thermochemical fuel production.4−6 In the latter process, ceria is thermally reduced at high temperatures using concentrated solar thermal power 1/xCeO2 → 1/xCeO2 − x + 1/2O2(g)

A range of ultra-high-vacuum (UHV) experiments on CeO2−x(111) thin films on close packed noble metal surfaces have found that H2O dissociates on surface vacancies (VO) and lattice oxygen (OL) yielding surface hydroxyls according to14,24−26 H 2O(g) + OL + VO → 2OH

(1)

Surface hydroxyls have been reported to be stable up to 400−600 K in temperature-programmed desorption (TPD) and photospectroscopy experiments,15,26 where they are removed through either H2O desorption or H2 production:14,15,24

and may then be reoxidized in H2O to produce H2 according to H 2O(g) + 1/xCeO2 − x → 1/xCeO2 + H 2(g)

(2)

4,5

at lower temperature. For thermochemical fuel production, more than 1000 K difference between the high temperature and low temperature step is required for both steps to be thermodynamically favorable at standard pressures.7 The temperature difference may be reduced by varying the partial pressures of steps (1) and (2).8 Sintering and sublimation of ceria lead to degradation at high temperature,6,9−11 while the fuel production rate may become prohibitively slow if the low temperature step is performed at too low a temperature.12 Therefore, improved kinetics for fuel production would be advantageous. Adding a noble metal catalyst such as Rh, Pt, or Cu improves the H2 production rate,4,12 but is not a practical solution due to catalyst sublimation at the oxide reduction step at high temperature. The interaction between water and ceria has received considerable attention experimentally and theoretically.13−21 Most work on well-defined model surfaces have focused on the (111) facet of ceria, which is found to be the most stable surface termination and is often observed experimentally.4,22,23 © XXXX American Chemical Society

(3)

2OH → H 2(g) + 2OL

(4)

In contrast to the experiments on CeO2−x(111) deposited on close packed noble metal surfaces, H2O TPD experiments by Henderson et al. suggest H2O is adsorbed molecularly rather than dissociatively on CeO2−x(111)/YSZ(111), because H2O dosed at 140 K desorbs with a peak around 190−270 K on both reduced CeO2−x(111)/YSZ(111) and oxidized CeO2(111)/ YSZ(111).13 Additionally, no H2 production from H2O is observed on CeO2−x(111)/YSZ(111).13 More remarkably, the reduced CeO2−x(111) facet on yttria-stabilized zirconia (YSZ) is further reduced by water dosing at 650 K, instead of being oxidized as the bulk.13 The experimental discrepancies between CeO2−x(111) deposited on YSZ and noble metals have been Received: August 27, 2014 Revised: October 28, 2014

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dx.doi.org/10.1021/jp508666c | J. Phys. Chem. C XXXX, XXX, XXX−XXX

The Journal of Physical Chemistry C

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correction has been used to decouple the electrostatic interaction between the periodic images. The first Brillouin zone is sampled using a 3 × 3 × 1 Monkhorst−Pack grid of k points.37 All calculations are spin polarized. For simplicity, we focus on high spin states, because the energy differences between different spin states have been shown to be small ( 0;50 i.e., the pre-exponential factor will be in the range from 4 × 1013 to 4 × 1015 s−1 at 200 K. If CeO2−x(111) is dosed with H2O at low temperature, water will therefore desorb faster from the surface than vacancies will diffuse from the subsurface to the surface as the sample is heated during a TPD experiment. These findings are in agreement with the experiments showing vacuum annealed CeO2−x(111) is largely unreactive when exposed to water at low temperature, and corroborates the observation by Henderson et al. that H2O desorption competes with H2O dissociation on reduced CeO2−x(111).13 However, this competition is very close and hydroxyls could possibly form if there is significant H2O readsorbing from the UHV background during the experiment. Further, we note that if more than 25−50% hydroxyl coverage is present on the surface, vacancies become more stable at the surface than the subsurface (cf. Figure 6). This level of hydroxyl coverage could possibly be left on the surface from chemical reduction or from H2O adsorption and dissociation during sample cooling. 4.1.2. Reversible H2O Dissociation Due to Activated H2 Formation. In the event H2O dissociates into hydroxyls, these hydroxyls are much more likely to be removed from the surface through H2O desorption rather than H2 desorption due to the large barrier for H2 desorption. Therefore, water splitting on CeO2−x(111) is predicted to be reversible in agreement with no H 2 production being observed under H 2 O TPD on CeO2−x(111)/YSZ(111).13 In the following we make a simple kinetic model to quantify the competition between H2 and H2O desorption. The H2O desorption rate is calculated as the reverse rate of H 2O(g) + VO,surf + OL → 2OH

Figure 12. Rates for hydroxyl decomposition into H2O(g) and H2(g) on surfaces with varying levels of hydroxyl coverage. The formation of H2O is faster than the formation H2, although H2O formation becomes slower with increasing hydroxylation. Hydroxyl decomposition becomes irreversibly slow below 450−600 K. An estimated rate for H2 formation by hydrogen spillover to a noble metal catalyst is shown as a dotted line.

The H2O desorption rate from the hydroxylated surface at 650 K is predicted to be 10−4 s−1 at high coverage and 10 s−1 at low coverage. This rate means that experiments performed at elevated temperature and pressure, but cooled down and evacuated, may be sensitive to transients in cooling and evacuation. The H2 desorption rate is predicted to be 10−8 s−1 at 650 K from the hydroxylated surface, and very high temperatures are required to make H2 desorption fast. For example, a temperature of 1050 K is needed to achieve a H2 desorption rate of 1 s−1 (assuming 1 ML OH coverage). Increasing surface reduction was found to shift the H2 desorption peak to lower temperature,14 which is consistent with our thermodynamic finding that OH removal through H2O production becomes more difficult at high OH or vacancy coverage due to repulsive interactions between vacancies and hydroxyl; see Figure 12. Henderson et al. suggested the difference in H2 desorption between CeO2−x(111) on YSZ(111) and CeO2−x(111) on Rh(111) was caused by non-(111) planes, steps, or kinks present on ceria grown on the close packed metal surfaces.13 Although CeO2(111)/Ru(0001) features step edges and domain boundaries,54 Mullins and Overbury have argued that ceria on Ru(0001) is sufficiently smooth to be a good representation of CeO2(111) because the H2O desorption peak is similar between the experiments on CeO2−x(111)/ YSZ(111) and CeO2−x(111)/Ru(0001).15 Due to the high barrier for H2 formation on the hydroxylated surface, we find it likely that H2 desorbs from low coordinated defect sites such as steps or from spillover to unexposed noble metal pits.24 The presence of hydrogen spillover has been demonstrated for Pt on CeO2(111)/Cu(111) exposed to H2,55 and H2 production from hydroxyl decomposition is also shifted to lower temperature by the addition of Rh to CeO2(111)/ Rh(111). 14 H 2 formation through H spillover from CeO2−x(111) to Pt has also been demonstrated in DFT calculations of the water gas shift reaction on Pt10 clusters on CeO2−x(111).56 A kinetic barrier for H2 dissociation into hydroxyl agrees well with the fact that H2 is not adsorbed on pristine CeO2 below 743 K while reduction of ceria by hydrogen is promoted by noble metal catalysts.57−59 An estimated rate for noncatalyzed H2 dissociation into hydroxyl at 700 K is 3 × 10−3 s−1. This rate appears too low considering the fact H2 dissociation is experimentally observed around this temperature, but could

(14)

where the rate constant for H2O desorption is given by k −14 = k14 /K14

(15)

k14 is calculated from ideal gas thermodynamics assuming a sticking coefficient for H2O of 1, and K14 is the equilibrium constant associated with dissociative H2O adsorption:51,52 K14 = exp(−ΔG14 /(kBT ))

(16)

Calculated rates for hydroxyl decomposition into H2O and H2 are shown in Figure 12. A similar rate is obtained if the desorption rate is calculated from 2OH ↔ H 2Oads +VO,surf + OL → H 2O(g) + VO,surf + OL (17)

See the Supporting Information (Figure S4) for details. Due to repulsive interactions between surface vacancies and hydroxyls, the H2O desorption rate becomes lower on the more hydroxylated surface. The rate constants of H2 formation on CeO2−x(111) is calculated for the fully hydroxylated surface using harmonic transition state theory. Additionally, we have estimated a hypothetical “catalyzed” H2 production rate if the reverse barrier for H2 dissociation would be 0.11 eV as on Pt(111), 53 rather than 1.38 eV on the hydroxylated CeO2−x(111). Further details are given in the Supporting Information. I



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be caused by H2 dissociation taking place on non-(111) planes or defects. 4.1.3. Water Induced Surface Reduction. While the inert behavior of CeO2−x(111)/YSZ(111) can be explained by H2O desorption competing with vacancy diffusion to the surface, we will in the following show that further surface reduction of CeO2−x(111)/YSZ(111) at 650 K in the presence of water can be explained by the thermodynamic stability of surface hydroxyls. To assess the thermodynamic stability, we must first consider if the surface is in equilibrium with O2, H2, or H2O in the gas phase or bulk oxygen vacancies. As we find the barrier for vacancy diffusion from the subsurface to the surface is low and as it is found from experiment that further annealing without H2O at 650 K does not result in further surface reduction,13 then the surface vacancies are likely in equilibrium with bulk vacancies. As the barrier for water dissociation on surface vacancies is vanishingly small, while there are large barriers to H2 desorption, then surface H must be in equilibrium with both bulk oxygen vacancies and water in the gas phase. We therefore calculate the thermodynamic stability of vacancies and H at various coverages assuming the surface to be in equilibrium with bulk vacancies and water as a function of the chemical potential of water and oxygen as shown in Figure 13. The phase diagram is calculated employing the method of

Figure 14. Free energy of formation of surface terminations of CeO2−x(111) at 650 K and 1 mbar H2O. In the absence of water, the most stable termination considered is 0.25 ML oxygen vacancies. In the presence of water of 0.50 ML H and 1 ML H are more stable.

therefore provides a thermodynamic driving force for surface reduction by H2O at elevated temperature. This explanation supports an original explanation proposed by Henderson et al.13 However, we note Matolin et al. proposed the enhanced reduction of CeO2−x(111) by water might be caused by an electronic effect of Ce 4f charge accumulation and Ce 5d charge depletion.25 Figure 15 shows the temperature and pressure dependence of the transition between surface hydroxyl and 0.25 ML

Figure 13. Most stable surface terminations on CeO2(111) as a function of the chemical potential of oxygen and water at 650 K. Surface reduction by vacancy formation requires μO < −3.3 eV at 650 K, while in the presence of water reduced hydroxylated surfaces may be formed at larger μO.

Figure 15. Temperature and H2O partial pressure for the transition between 0.25 ML subsurface vacancy and 1/2 and 1 ML surface hydroxyl, respectively. A chemical potential of −3.4 eV has been used for the oxygen chemical potential.

ab initio thermodynamics and includes the changes in vibrational free energy of the surface at 650 K while the effects of configurational entropy have been neglected. At low chemical potentials for water, reducing the oxygen chemical potential leads to increased surface reduction through the creation of oxygen vacancies. Our simulations with 1/4 ML VO have 50% surface Ce3+, which agrees roughly with the 60− 80% surface Ce3+ obtained by annealing under UHV conditions.13 The chemical potential of oxygen in these UHV experiments is therefore probably in the range −4.0 eV < μO < −3.3 eV. At high chemical potentials of water, reducing the oxygen chemical potential leads to increasing hydroxyl coverage due to water dissociation. Figure 14 shows the relative free energy of various surface terminations as a function of surface Ce3+ concentration at a representative chemical potential μO = −3.6 eV and H2O at 1 mbar. It is seen that, starting from a surface with 50% Ce3+ associated with oxygen vacancies, formation of a partially hydroxylated surface with 50% Ce3+ reduces the free energy. The free energy is even further reduced if a fully hydroxylated surface with 100% Ce3+ is formed. Surface hydroxylation

subsurface vacancies for a bulk oxygen chemical potential of −3.4 eV. At 1 bar of H2O, the hydroxylated surface becomes unstable versus 0.25 ML subsurface vacancies above 950−1050 K for 1 and 1/2 ML H, respectively. At 10−12 bar of H2O, hydroxyl becomes unstable versus subsurface vacancies above 400 K. At this choice of oxygen chemical potential, the 1/2 ML OH structure becomes stable at a lower H2O partial pressure than the 1 ML OH structure, although at more reducing conditions 1/2 ML OH might not be stable (cf. Figure 13 at μO = −3.9 eV). Our thermodynamic results for hydroxyl decomposition to H2O shown in Figure 15 are close to a similar analysis by Molinari et al., who calculated hydroxyl to desorb as H2O above 1000 K at 1 bar and 525 K at 10−10 bar using a different computational setup. 4.2. CeO2−x(111) Reoxidation by H2O. On the basis of our calculations we construct the free energy diagrams for the production of H2 on CeO2(111) by the reoxidation of reduced ceria in an H2O atmosphere at 650 and 1200 K. At low temperature the surface is dominated by hydroxyls, and the thermodynamically favored pathway for forming surface J



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energy for the reverse step, H2 dissociation, is independent of hydroxyl coverage. The main effect of increased temperature on the free energy diagram of a single pathway is that the H2O and H2 gas phase species are stabilized. Due to the larger entropy of H2O compared to H2, the overall thermodynamic driving force for H2 production decreases with increasing temperature. At 650 K the formation of hydroxyls from gas phase water and subsurface vacancies is exergonic and the hydroxyls act as a thermodynamic sink, whereas at 1200 K, the surface hydroxyl is thermodynamically unstable versus formation of either water and subsurface vacancies or H2. It is seen from Figure 16a that, at 650 K, it is easier to produce H2 by forming surface vacancies and to go through the H−Ce pathway than it is to go through the 2H−O pathway. At 1200 K, however, it is easier to produce H2 through the 2H−O pathway. The main reason for the change in pathway is that, although vacancy formation becomes more facile at 1200 K, the adsorption of H2O in the H−Ce pathway becomes much more difficult. Based on our identified pathways, we suggest it may be possible to improve H2 production on CeO2−x(111) by adding a substitutional dopant that binds H stronger than cerium in order to stabilize the transition state for hydroxyl decomposition and CeH formation. Another possibility would be to destabilize the hydroxyl groups at the surface while keeping the transition state for hydroxyl recombination to H2 fixed (cf. Figure 16b), which will increase the rate of H2 formation at low temperature and high H2O pressure. However, further studies are needed to identify how such modifications can be realized.

vacancies is through desorption of water followed by vacancy diffusion from the bulk to the surface. The net reaction is VO,bulk + 2OH → 2VO,surf + Obulk + H 2O(g)

(18)

The free energy diagrams, shown in Figure 16, start out with a bulk vacancy and H2O at 1 bar, where the initial energy of the

5. CONCLUSIONS We have studied the thermodynamics and kinetics of water dissociation and H2 production on CeO2−x(111). Oxygen vacancies are more stable in the subsurface layer than in the surface layer for all considered vacancy coverages in the range from 0.25 to 1 ML coverage of vacancies. However, oxygen vacancies become more stable on the surface than in the subsurface if the coverage of surface hydroxyl exceeds 0.25−0.5 ML. The diffusion of vacancies between the subsurface layer and the surface layer is found to have barriers below 0.5 eV. The dissociation of H2O on surface vacancies and lattice oxygen has a negligible barrier to the formation of surface hydroxyl, which is thermodynamically stable at a wide range of conditions. The stability of vacancies in the subsurface may be important for the observed inert behavior of CeO2−x(111)/YSZ(111) upon H2O exposure due to competitive desorption of H2O and vacancy diffusion to the surface. The stability of hydroxyl can provide a thermodynamic driving force for diffusion of vacancies from the bulk to the surface resulting in increased surface reduction by H2O rather than reoxidation. Our calculations find a 2.5−2.7 eV barrier to decomposition of hydroxyl to H2 on CeO2−x(111), which means that hydroxyl is more likely to decompose into H2O than into H2. H2O adsorption on CeO2−x(111) under UHV conditions is therefore reversible, in agreement with experiments on CeO2−x(111)/ YSZ(111). Due to the high barrier to H2 formation we then suggest H2, formed by H2O exposure of CeO2−x(111) on noble metals,14,15 might be caused by H2 desorbing via non-(111) facets or defects or via hydrogen spillover to the noble metal substrate.

Figure 16. Free energy diagrams for H2O dissociation to H2 via hydroxyl (2 H−O) and H−Ce (H−Ce + H−O) intermediates on CeO2(111) at (a) 650 and (b) 1200 K.

H−Ce and 2H−O pathways have been chosen to satisfy (18). The vacancy moves from the bulk to the subsurface and the surface layer with moderate barriers in a diffusion process that is more favorable on the clean surface than on the hydroxylated surface and only weakly dependent on temperature. When the vacancy is at the surface, water adsorbs at the surface in an endergonic step, which is strongly temperature dependent due to the loss of translational entropy of the H2O molecule upon adsorption. On a surface that is not completely covered with hydroxyl, water may then react with lattice oxygen and an oxygen vacancy to produce hydroxyls in a very exergonic reaction before the hydroxyls decompose into H2 against a 2.5−2.7 eV enthalpy barrier. This H2 production step is shown in Figure 16 for both the surface with two OHs and the surface with four OHs. While the activation energy to H2 formation from hydroxyl is slightly lower on the surface with four OHs due to repulsive interactions between hydroxyls, it is seen that the activation K



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We find H2O might dissociate into OH and CeH on surface vacancies on the hydroxylated surface with a 1.14 eV barrier followed by facile decomposition of OH and CeH into H2, while a second order transition state leads directly to H2. We find the pathway involving the CeH intermediate to be the dominant pathway for H2 production in an H2O atmosphere at 650 K, while the OH decomposition into H2 is more likely at very high temperatures. While H2O dissociation on surface vacancies and lattice oxygen conserves the reduced state of ceria, the decomposition of hydroxyl to H2 reoxidizes the ceria surface. We find the transition state for H2 formation occurs when ceria is partly reoxidized, after the first hydroxyl has decomposed into lattice oxygen and an H atom weakly bound to the Ce ions. Accordingly, we suggest the barrier to H2 formation via both hydroxyl decomposition and CeH decomposition can be decreased by adding substitutional cationic dopants that stabilize this hydrogen in the transition state. Alternatively, hydroxyl decomposition into H2 can also be facilitated by destabilizing hydroxyl.

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ASSOCIATED CONTENT

* Supporting Information S

Tables containing formation energies of vacancies. Minimum energy pathways for additional reaction steps. Details on the ab initio thermodynamic approach including vibrational corrections to the free energy and details on the model for estimating H2 and H2O formation rates on hydroxylated ceria. This material is available free of charge via the Internet at http:// pubs.acs.org.

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AUTHOR INFORMATION

Corresponding Authors

*E-mail: [email protected]. *E-mail: [email protected]. Present Address †

H.A.H.: Department of Energy Conversion and Storage, Technical University of Denmark, 4000 Roskilde, Denmark. Author Contributions

The manuscript was written through contributions of all authors. All authors have given approval to the final version of the manuscript. Notes

The authors declare no competing financial interest.

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ACKNOWLEDGMENTS H.A.H. gratefully acknowledges support from a Laboratory Directed Research and Development program at Sandia National Laboratories, in the form of a Grand Challenge project entitled “Reimagining Liquid Transportation Fuels: Sunshine to Petrol”. Sandia is a multiprogram laboratory operated by Sandia Corp., a Lockheed Martin company, for the U.S. Department of Energy, National Nuclear Security Administration, under Contract No. DE-AC04-94AL85000. C.W. acknowledges support by the U.S. Department of Energy, Office of Science, Basic Energy Sciences, under Grant DEFG02-07ER46433.

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