Article pubs.acs.org/JPCC
Energetic Driving Force of H Spillover between Rhodium and Titania Surfaces: A DFT View Jeanet Conradie,*,†,‡ Jose Gracia,† and J. W. (Hans) Niemantsverdriet† †
Schuit Institute of Catalysis, Eindhoven University of Technology, P.O. Box 513, 5600 MB, Eindhoven, The Netherlands Department of Chemistry, University of the Free State, 9300 Bloemfontein, Republic of South Africa
‡
S Supporting Information *
ABSTRACT: Hydrogen spillover from a rhodium particle, over the most stable (111) surface, to a TiO2 rutile support occurs at low hydrogen coverage because the adsorption energy of H atoms at low hydrogen coverage on rutile is larger than that on rhodium. H diffuses over the support with an activation barrier low enough to allow this. With increased H coverage on the reducible metal oxide support, equilibrium is reached and spillover back to rhodium is feasible.
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INTRODUCTION The phenomenon where H atoms migrate from a hydrogenrich metal to a hydrogen-poor support surface is coined as “spillover”.1,2 In general, spillover involves the transport of an active species, adsorbed or formed on a first surface, onto another surface that does not under the same conditions adsorb or form on this species.3 In the case of hydrogen spillover, H2 is thought to be activated and dissociated on a metal surface, after which the H atoms diffuse to another phase (usually an oxide, activated carbon, or metal organic framework (MOF)). Although there is little doubt that H spillover does happen in certain cases,4 a proper analysis of the energetics involved is needed to understand this process. For example, nanoscale gold dispersed on oxide supports is active for hydrogenation,5 dehydrogenation,6 and oxidation reactions.7 Changes in selectivity depend on the size control of the gold nanoparticles, which affects the activation of H2 and O2 and the coverage of atomic hydrogen and active oxygen intermediates. Faster hydrogenations are related to significant hydrogen coverage on the metal particles. Another example involves the superior activity toward oxidation of CO in the presence of H2 of Au/ MOx (MOx = metal oxide), where hydrogen spillover from gold to the MOx plays an important role. Hydrogen atoms are believed to donate electron density to the MOx, enhancing its activity as an oxidation agent.8 Overall, we must imagine hydrogen spillover as a reversible and dynamic process, which can stimulate certain reactions depending on the dominant thermodynamics of the process. Spillover between metal nanoparticles and a support are most relevant for catalysis,9 catalyst preparation, and H storage.10 In this work, we will model separately, as independent systems, selected MOx and metal surfaces. We assume nanoparticles to be big enough so their electronic structure is not modified by metal support interactions. We have chosen as model systems two TiO2 surfaces as support and Rh as metal particles over the most compact (111) cut. The presence of a metal phase and two TiO2 exposed surfaces, © 2012 American Chemical Society
with different dissociative chemisorption energies and evolution with coverage, will let us simulate the most feasible principles that can rule hydrogen spillover as surface phenomena. All of our calculations are based on density functional theory. Molecular hydrogen activation will be taken as granted on metal active sites, and it will not be considered. We will center on the adsorption energy and mobility of the hydrogen atoms on the metal surfaces as thermodynamic pumps for the migration of atomic hydrogen toward the MOx. Later on, we will compare the mobility and adsorption energy of atomic hydrogen with increasing coverage on the MOx phases. Finally, we will analyze the data from the point of view that spillover is reversible, and some surfaces represent a reservoir of atomic hydrogen ready for reaction.11,12
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RESULTS AND DISCUSSION TiO2 crystallizes in three different phases, rutile (the most stable one), anatase, and brookite.13 Rutile and anatase exhibit a tetragonal structure and brookite exhibits an orthorhombic structure. The most stable form of the three crystallographic forms of TiO2 is rutile (for large particles) and the metastable anatase form (for nanoparticles with diameters up to about 14 nm).14 This is due to the relatively low surface free energy of the anatase stable (101) and (100) faces.15 Anatase particles are often used in industry16 and exhibit high activity for photocatalytic applications.17 Rutile and brookite are mostly used as catalyst supports. The rutile (001) surface is significantly less stable than the (100) and the (110) surfaces, the latter being the most stable.18 TiO2 Rutile (110) and (001) Surfaces. The TiO2 rutile (110) surface is one of the most important surface-models for metal oxides.13 The structure is made of alternative horizontal and vertical TiO chains (Figure 1b).19 The axes of the chains Received: August 16, 2012 Revised: November 2, 2012 Published: November 12, 2012 25362
dx.doi.org/10.1021/jp308175t | J. Phys. Chem. C 2012, 116, 25362−25367
The Journal of Physical Chemistry C
Article
Table 1. Calculated Adsorption Energies for H on Rutile and Rhodium with Respect to H2 in Vacuuma rutile (001)
rutile (110)
Rh (111)
coverage (ML)
adsorption energy per H (eV)
0.25 0.50 0.75 1.00 0.17 0.33 0.50 0.67 0.83 1.00 0.25 0.50 1.00
−2.03 −1.58 −0.98 −0.35 −0.93 −0.54 −0.22 0.08 0.34 0.65 −0.54 −0.53 −0.51
a
H adsorbs in a top-on position on O for rutile and on the fcc hollow site for the rhodium surfaces.
Figure 1. Views of the TiO2 rutile p(1 × 2) unitcell (indicated with black lines left): (a) (001) surface and (b) (110) surface. Red and blue circles represent O and Ti atoms, respectively.
are parallel, but there is an alternation in their orientation; half of the chains lie in the surface plane, whereas the other half is orientated perpendicularly. The TiO2 surface of the p(1 × 2) unit cell exposes two Ti-5c (Ti-5c = five-coordinated Ti), four O-3c, two O-2c atoms, and two underlying Ti-6c. Most striking are the so-called bridging oxygen atoms (O-2c), which form rows along the [010] direction. The surface contains both basic and acidic sites for adsorption.20 The TiO2 rutile (001) surface of the p(1 × 2) unit cell exposes two Ti-4c and four O-2c; see Figure 1a. The layer below the surface consists of two Ti-6c and four O-3c. The third layer is the same as the surface layer. H Adsorption on Rutile Surfaces. Calculations of this study, in agreement with previous studies for the (110) surface,21,22 showed that H atoms adsorb on the rutile (001) and (110) surfaces in an on-top configuration on O. The adsorption energy is dependent on the coverage,22,23 for example, −2.03 eV with respect to H2 in vacuum for a 0.25 ML coverage on the (001) surface. Table 1 gives a summary of the H adsorption values at different coverages on the rutile (001) and (110) surfaces. On the rutile (001) surface, H atoms adsorb on the O-2c that is coordinated to two Ti atoms in the clean surface. During the adsorption process the O is lifted out of the surface and becomes top-on on Ti; see Figure 2a. On the rutile (110) surface the H atoms adsorb on the bridging oxygen atoms (O-2c), and thus the O-atoms become threecoordinated. H Adsorption on Rhodium Surfaces. H atoms can adsorb on several sites of the rhodium (111) surface with a slight preference for fcc-hollow over the hcp-hollow-bonded H (Figure S1 of the Supporting Information). The adsorption energy for Rh (111) fcc-hollow, hcp-hollow, bridge, and top are −0.54, −0.53, −0.43, and −0.16 eV per H, respectively, at 0.25 ML coverage. Published DFT calculated adsorption energies for Rh (111) at 1 ML coverage are 0.57, 0.55, 0.40, and 0.14 eV per H for fcc-hollow, hcp-hollow, bridge, and top, respectively.24
Figure 2. Top and side views of the TiO2 rutile surface with H adsorb on (a) 0.25 ML coverage on the (001) surface and (b) 0.17 ML coverage on the (110) surface. Red, blue, and white circles represent O, Ti, and H atoms, respectively.
H atoms at all coverages will thus be able to diffuse freely over the rhodium surfaces between the fcc hollow and the hcp hollow sites via the bridge adsorption position as transition state (TS) with an energy barrier of ca. 0.11 eV; see Table 1. The adsorption energy does not show a big dependence on coverage. (The difference in energy at low and high coverage is less than 0.03 eV.) The H adsorption energy calculated for rhodium at different H coverages is also given in Table 1. It is clear that the adsorption energy of H on the rhodium is largely independent of the coverage while the adsorption energy of H on rutile changes dramatically with coverage. H Diffusion on Rutile Support Surface. 0.25 ML H on Rutile (001). In evaluating the rutile (001) surface with a 0.25 ML hydrogen adsorption (Figure 3a), we note that it should be possible for the H to diffuse in the [010] direction from the one O to the vacant site on the next O. In the [100] direction, however, the “next O” is at 4.60 Å. We thus assume that the H atom should move in a ca. 45° direction to the next vacant O on the surface. For the H to diffuse in the [100] direction from the one minimum energy position to a crystallographic equivalent minimum position, at least two TSs are thus expected. Figure 3c,d visualizes the two TSs found for H diffusion in the [100] direction with activation energies of 0.87 and 0.84 eV respectively. The activation energy for diffusion in 25363
dx.doi.org/10.1021/jp308175t | J. Phys. Chem. C 2012, 116, 25362−25367
The Journal of Physical Chemistry C
Article
Figure 3. (a) Optimized 0.25 ML H adsorption rutile (001) surface. The green arrow shows the diffusion path in the [010] direction and the purple arrows show the diffusion path in the [100] direction. (b) Visualization of the transition state of H diffusion on 0.25 ML (001) rutile surface in the [010] direction. (c,d) First and second transition states for H diffusion in the [100] direction on the (001) rutile surface. The displacement vector (blue arrow) indicates the movement of the H atom at the imaginary frequency. For clarity, the top layer of O, Ti, and H atoms is shown with red, blue, and white circles, whereas the bottom layers are shown in lighter colors.
Figure 4. (a) Optimized minimum energy 0.50 ML H adsorption rutile (001) surface. The purple arrow shows the first part of the H diffusion path. (b) Optimized 0.50 ML H adsorption rutile (001) surface after H diffusion as indicated in panel a. Energy of the surface in panel b is 0.14 eV higher than the minimum energy surface in panel a. The purple arrow shows the second part of the H-diffusion path. After diffusion, the surface will relax to the surface in panel a with the H half a unit cell displaced. For clarity, the top layer of O, Ti, and H atoms is shown with red, blue, and white circles, whereas the bottom layers are shown in lighter colors.
the [010] direction is smaller, viz. 0.56 eV; see Figure 3b for a visualization of the corresponding TS. The preferred pathway for H diffusion on the rutile (001) surface will thus be in the [010] direction with a barrier of
