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2008, 112, 1755-1758 Published on Web 01/24/2008
On the Mechanisms of Hydrogen Spillover in MoO3 Liang Chen Ningbo Institute of Materials Technology and Engineering, Chinese Academy of Science, Ningbo, Zhejiang 315201, P.R. China, and Computational Modeling Center, Air Products and Chemicals, Inc., 7201 Hamilton BouleVard, Allentown, PennsylVania 18195
Alan C. Cooper, Guido P. Pez, and Hansong Cheng* Computational Modeling Center, Air Products and Chemicals, Inc., 7201 Hamilton BouleVard, Allentown, PennsylVania 18195 ReceiVed: December 19, 2007; In Final Form: January 8, 2008
Hydrogen spillover on the MoO3 (010) surface in the presence of a platinum catalyst was modeled using periodic density functional theory (DFT). The migration of H from a saturated Pt6 cluster to the MoO3 (010) surface was found to undergo a transition from repulsive electrostatic to attractive proton-oxygen interactions. The hydrogen is able to move nearly freely on the surface and diffuse into the bulk lattice at ambient temperatures, leading to the formation of hydrogen molybdenum bronze. We show that the high proton mobility is largely attributed to the massive H-bonding network in the MoO3 lattice.
It has been widely recognized that one of the key components of the incipient hydrogen economy is hydrogen storage technology, which has become one of the most active areas of research in materials science recently.1-5 A wide variety of novel materials, ranging from light metal hydrides6,7 and chemical hydrides2 to many carbon-based complexes,3-5 have been proposed in the past few years, of which storage via the socalled “hydrogen spillover” phenomena has received particular attention because of the unusually high reported hydrogen capacity.8,9 In this process, dihydrogen molecules first undergo dissociative chemisorption upon interacting with precious metal catalysts and subsequently hydrogen atoms are spilled over onto the substrates via an adsorption process. It is essential that the adsorbed hydrogen atoms be nearly free to move to other adsorption sites far from where the catalysts reside in order to achieve the reported high storage capacity. Unfortunately, to date, the detailed mechanistic processes have been poorly understood. In this Letter, we report for the first time the detailed theoretical studies on the structures and energetics of hydrogen spillover onto a well-known hydrogen bronze material, HxMoO3. This system has been extensively studied experimentally, but the detailed spillover mechanisms have not been fully unraveled.10-12 We expect that mechanistic understanding of the known hydrogen spillover system would shed light on the spillover process in carbon-based materials. The study of hydrogen bronze materials (e.g., HxWO3 and HxMoO3) has been of interest since the early 1970s.10-13 Hydrogen molybdenum bronze, as a potential hydrogenation, dehydration, and reduction catalyst, has received extensive attention. It can be prepared effectively via hydrogen spillover in the presence of Pt or Pd catalyst. Many experimental studies on the formation, reactivity, and other chemical properties of * Corresponding author. E-mail:
[email protected].
10.1021/jp7119137 CCC: $40.75
HxMoO3 have been carried out. Four types of HxMoO3 phases have been identified depending on the hydrogen concentration. Ritter and co-workers have applied NMR solid-state techniques to study the bonding properties, location, and mobility of hydrogen in HxMoO3.14 They found that the activation energies of the hydrogen diffusion are on the order of 15 to 30 kJ/mol. They also concluded that hydrogen atoms reside on a line connecting the vertex sharing oxygen atoms within the (MoO6)n layers.15,16 An atomic force microscopy study by Smith and Rohrer unraveled the evolution of the MoO3 (010) surface during reduction in H2-N2 mixtures at 700 K.17,18 The results demonstrate that MoO3 intercalates H atoms during the gasphase reactions, resulting in protonation, which leads to precipitates of hydrogen molybdenum bronze. Unfortunately, to our best knowledge, there has been very little theoretical study on the formation of HxMoO3 precipitates; thus, the detailed mechanisms of hydrogen bronze formation still remain unclear. The present study addresses two key steps of hydrogen molybdenum bronze formation: H spillover on the Pt/MoO3 (010) interface and H migration from the surface into the MoO3 bulk.19 The phase of HxMoO3 (x ) 0.125) was chosen in this study, which allows us to examine H migration/diffusion behavior in a sufficiently large supercell to gain insight into hydrogen spillover mechanisms. Density functional theory calculations were performed using the spin polarized generalized gradient approximation as implemented in the Vienna ab initio simulation package (VASP).20,21 The Perdew-Wang (PW91)22 exchange-correlation functional was employed with the electron-ion interactions described by the projector augmented wave (PAW)23,24 pseudopotentials. The Brillouin zone was sampled within a 2 × 2 × 1 Monkhorst-Pack mesh.25 An energy cutoff of 400 eV was used in all calculations. All atoms were fully relaxed with the © 2008 American Chemical Society
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Figure 1. Spillover of hydrogen from Pt6 to the MoO3 (010) surface. The insets show the initial (lower) and final (upper) states. The blue spheres are Pt atoms, and the white spheres are H atoms. The line is drawn as a guide to the eye.
forces converged to less than 0.03 eV/Å. Electron smearing was employed using the Methfessel-Paxton26 technique, with a smearing width of σ ) 0.1 eV, in order to minimize the errors in the Hellmann-Feynman forces due to the electronic free energy. A supercell containing one (2 × 2) MoO3 (010) layer and 18 Å vacuum space was selected in all calculations, as shown in Figure 2. The validity of one layer slab model is sustained by the fact that the MoO3 interlayer interactions are dominated by the van der Waals forces. Furthermore, our calculated Mo-O bond lengths in the relaxed single slab are virtually identical to those in MoO3 bulk. The periodic boundary conditions were imposed in all directions. The nudged elastic band (NEB)27,28 method of Jo´nsson and co-workers was used to examine the activation energy profile along prescribed diffusion pathways. Initial and final states were chosen based on the adsorption calculation, and the number of images was chosen to achieve smooth curves. We begin by investigating hydrogen spillover from the Pt catalyst to the MoO3 (010) surface. In a heterogeneous catalytic system, the catalyst is usually dispersed on support materials as nanoclusters and thus exhibits a wide variety of surface structures. The supported Pt catalyst plays a vital role as a source of hydrogen atoms upon dissociative chemisorption of H2. Under realistic catalytic conditions, the surfaces of the Pt catalyst are fully covered by hydrogen atoms. Our previous study shows that H2 molecules undergo sequential dissociative chemisorption on the Pt6 cluster with a Pt/H ratio up to 1:4, at which the cluster is fully saturated with H atoms.29 The H atoms are negatively charged with electrons transferred from the Pt-5d orbitals to the H2-1σ* orbital. Our calculated sequential H desorption energy at full H-coverage is 2.44 eV, which is the minimum energy required to pull an H atom out of the cluster. Admittedly, the size of the Pt cluster used in the present study is much smaller than the size of real catalyst particles, which may contain thousands of Pt atoms. Computationally, it is prohibitively difficult to utilize the real size of catalyst particles in our theoretical model because, in addition to particle size, various shapes and orientations of catalyst particles and H coverages also need to be considered. Recent study by Zhou et al. on H2 dissociative chemisorption on other small-sized Pt clusters indicates that the H desorption energy at full H-saturation is not too sensitive to the cluster size.30 Nevertheless, we expect that the H migration barrier at the interface between real catalyst particles and the substrate might be smaller than the value
Letters obtained in this study because smaller metal particles tend to have stronger binding with H atoms. It is therefore anticipated that the reported value here represents the upper bound of the migration barrier. In the present work, we carefully placed a fully H-saturated Pt6 cluster obtained from our previous study with the bottom four H atoms removed on the MoO3 (010) slab followed by energy minimization.31 All Pt atoms in the Pt6 cluster, except the bottom Pt atom, are saturated with H atoms. Subsequently, we performed minimum energy path calculation to allow an H atom at the bottom to migrate onto the substrate to form an O-H bond with a terminal O atom. The calculated energy profile is shown in Figure 1. The figure shows that an H atom desorbs readily from the H20/Pt6 cluster and diffuses onto the terminal oxygen to form an O-H bond with a lowenergy barrier of 0.37 eV, indicating that H-desorption energy from the Pt6 cluster can be reduced significantly upon the cluster interaction with the substrate. It is understood that there are numerous possible adsorption configurations for catalyst particles; however, our extensive computational tests suggest that the calculated H-migration barriers fluctuate only in a small range around 0.35 eV. The Bader charge analysis32 revealed that this barrier is a result of charge flows and protonation. In the initial state, each hydrogen atom adsorbed on Pt6 gains 0.050.15 electrons from Pt-5d bands, depending on its location on Pt6. The terminal oxygen atoms in MoO3 are 0.8-1.2 electrons negatively charged. Thus, initially, the interaction between hydrogen and terminal oxygen atoms is electrostatically repulsive. Upon the H migration onto the terminal oxygen, the electrons begin to transfer from the H-1s orbital to the O-2p orbital. The migrated H atom loses its electron and turns out to be positively charged by +0.96. Indeed, it becomes virtually a proton in the final state. It is also noteworthy that the remaining hydrogen atoms adsorbed on Pt6 all partially lose their electron with charges on the atoms ranging from +0.05 to +0.15 electrons. Accordingly, the oxygen atoms gain extra electrons and thus loosen the Mo-O bond strength. The system undergoes a transition from repulsive O-H interactions to attractive proton-oxygen interactions along the migration pathway. As a result, the total energy is reduced. The calculated low activation energy clearly demonstrates the effectiveness of spillover in the H-migration process from the catalyst to the substrate. We next examine the H adsorption on the MoO3 (010) slab. A MoO3 slab comprises two symmetric Mo-O sheets (upper and lower) linked by strong Mo-O bonds. There are three structurally distinct lattice oxygen atoms on each sheet, that is, terminal oxygen, symmetric bridging oxygen, and asymmetric bridging oxygen. Figure 2 shows the optimized MoO3 (010) slab structure and the different binding modes considered in this work. The calculated binding energies, defined as Eb ) E(MoO3) + E(H) - E(H/MoO3), as well as O-H bond lengths are summarized in Table 1, where the three terms on the righthand side are the energies of the bare MoO3 slab, an isolated H atom, and the slab with the adsorbed H atom, respectively. On the terminal oxygen, H is bound at an angle of 60° from the surface normal and thus interacts with two O-2p orbitals simultaneously. On the symmetric bridging oxygen, H is bound in the plane of the surface with the weakest binding energy. Alternatively, two binding modes are identified on the upper asymmetric oxygen: bound perpendicularly to the surface, noted as Aupp,p in Figure 2, or bound at an angle directly pointing toward the lower asymmetric oxygen, noted as Aupp,i. Finally, the binding mode Alow on lower asymmetric oxygen is equivalent to Aupp,i considering the symmetry of MoO3 slab.
Letters
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Figure 2. Side views of the optimized one-layer model of the MoO3 (010) surface. The red spheres are oxygen atoms, and the yellow spheres are Mo atoms. The distinct oxygen atoms and binding modes are noted. The arrows represent the orientation of the H-O bond.
TABLE 1: Binding Sites and Energy of H Adsorbed on MoO3 (010) binding site and mode
Eb (eV)
dO-H bond length (Å)
terminal oxygen Supp oxygen Aupp,p oxygen Aupp,I oxygen Alow oxygen
2.45 2.10 2.67 2.91 2.91
0.98 0.98 0.97 0.97 0.97
Among the five binding modes above, Aupp,i and Alow are the most favorable. Hydrogen is not only bonding with the upper (or lower) asymmetric oxygen with a strong covalent O-H bond of 0.97 Å lengths but also interacting with the lower (or upper) asymmetric oxygen via a weaker H-bonding of 1.7 Å lengths. This result is consistent with the previous NMR and neutron diffraction studies by Ritters14,15 and Dickens16 who identified that protons reside between the lower and upper asymmetric oxygens. The Bader charge analysis indicates that the absorbed H is virtually a proton. The strong binding also changes the Mo-O bond lengths drastically. For example, the H on the terminal O elongates the underneath Mo-O bond length from 1.67 to 1.88 Å. We also notice that adsorption right above the asymmetric oxygen is more stable than that on the terminal oxygen, by 0.22 eV, which is opposite to what Chen et al. reported using local density functional theory (LDA).33 The discrepancy might be due to the overbinding nature of the LDA method used by Chen et al. We now turn to the second key step of HxMoO3 bronze formation: hydrogen migration from the surface into the MoO3 bulk. Figure 3 shows the calculated activation barriers of hydrogen diffusion following a selected pathway. The first barrier corresponds to the diffusion on two adjacent terminal oxygen atoms. The calculated activation energy is 0.51 eV. This relatively low activation barrier suggests that the H migration on the surface can be facile at moderate temperatures. For example, the hopping rate of hydrogen between two adjacent terminal oxygen atoms is about 2.0 s-1 at room temperature, assuming a typical prefactor of 1013 s-1.34 The second barrier corresponds to the diffusion from the terminal oxygen onto the upper asymmetric oxygen. The calculated activation energy is
Figure 3. Migration of proton on the MoO3 (010) surface and from the surface into the bulk.
0.6 eV. This slightly higher energy indicates that the diffusion from the surface (terminal) oxygen into the bulk is the ratelimiting step for the hydrogen bronze formation. As shown in the adsorption calculation, two binding modes have been identified on the asymmetric oxygen: Aupp,p and Aupp,i. Our calculation on the H migration in the lattice reveals that the proton is able to rotate around the direction by 120° from Aupp,p to Aupp,i with nearly zero activation energy. The last activation barrier, corresponding to the migration from Aupp,i to Alow, is found to be 0.35 eV. The NMR experiments have shown that in the low hydrogen content HxMoO3 the protons reside between the lower and upper asymmetric oxygen sites and the diffusion activation energy is in the range of 0.1-0.3 eV.14,35 Our calculated H diffusion barrier and the predicted relative thermodynamic stability of H adsorption sites in the lattice are in good agreement with the experimental observations. In the entire proton migration pathway, the highest activation barrier is no more than 0.6 eV, which clearly explains the high mobility of H in hydrogen bronze. To understand the low barrier diffusion mechanism, we monitored the O-H distance in the H diffusion process following the proton migration pathway. The calculated O-H
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Letters spillover phenomena, such as spillover onto carbon-based materials for hydrogen storage. Acknowledgment. This work is supported in part by the U.S. Department of Energy via Hydrogen Sorption Center of Excellence. References and Notes
Figure 4. O-H bond distance distribution function during migration.
distance distribution is displayed in Figure 4. The figure reveals a massive H-bonding environment along the proton migration pathway in the lattice. The first peak at around 1.0 Å is readily attributed to the strong O-H bond at the stable binding modes, as shown in the Table 1. The second peak at 1.5 Å, which indicates a weaker O-H interaction, can be attributed to the hydrogen bonding at or near the transition states. Indeed, the H-bonding plays a critical role in facilitating the proton migration in the lattice. It helps to reduce the activation energy barriers and to increase the H mobility. The shoulder at 1.82.0 Å and the broad bands at distance greater than 2.0 Å are attributed to a range of oxygen-proton interactions with various strengths. The extensive H-bonding network in the lattice provides a low-energy barrier diffusion channel and thus enables the hydrogen atom to flow nearly freely at ambient temperatures. In summary, we performed extensive quantum-mechanical calculations to model the hydrogen spillover phenomenon that leads to the formation of H bronze in MoO3. The spillover occurs upon adsorption of H atoms coming out of a Pt catalyst fully saturated with H atoms onto the MoO3 (010) surface followed by H diffusion in the lattice. For the first time, the detailed mechanisms of H-insertion into MoO3 were unraveled. We show that the H atoms can readily migrate from the Pt catalyst to the MoO3 (010) surface, followed by protonation, to form strong covalent bonds with the terminal oxygen atoms. Subsequently, the protons can readily migrate into the lattice via low-energy barrier pathways to form hydrogen bronze. Our results suggest that the H adsorption and migration are facilitated by the massive H-bonding network in the lattice and the hydrogen bronze formation can occur at moderate temperatures. The calculated diffusion barriers are in quantitative agreement with the data measured by NMR, and the predicted relative thermodynamic stability of adsorption sites is consistent with experimental observations. The study reveals that H atoms supplied locally by a Pt catalyst can nearly freely flow into the entire lattice, which is precisely the essence of the so-called spillover phenomenon. The unraveled spillover mechanisms presented in this study provide useful insight into other hydrogen
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