Oxidative Addition of Water to Rhn (n = 1–4) Clusters on Alumina

ARTICLE pubs.acs.org/JPCC

Oxidative Addition of Water to Rhn (n = 1
4) Clusters on Alumina Surfaces and Spontaneous Formation of H2 Tushar K. Ghosh and Nisanth N. Nair* Department of Chemistry, Indian Institute of Technology Kanpur, Kanpur 208016, India

bS Supporting Information ABSTRACT: A systematic investigation of the reactivity of widely used alumina supported Rh catalyst is presented here. By employing a combination of ab inito molecular dynamics techniques and rigorous electronic structure analyses, we scrutinize the reactivity of Rhn (n = 1
4) clusters on alumina surface. Water dissociation at the Rh/alumina interface is shown to be kinetically and thermodynamically preferred, and it progresses by an oxidative addition of water proton to Rh. This results in a reactive hydride species on the surface, which spontaneously abstracts a proton from surfaceadsorbed water, releasing H2 gas. This reactivity is shown to be essentially independent of the phase and surface structure of alumina, but decreases with increasing cluster size. Importance of these results in the framework of developing an efficient water-splitting catalyst is discussed.

1. INTRODUCTION Supported metal catalysis exploits unique reactivity achieved as a result of the high surface area of metal nanoparticles, generation of low coordination sites, and electronic confinement effects by dispersing metals on supports like metal oxides.1
5 Of particular interest are the noble metals supported on basic oxides,2 e.g., rhodium on alumina, which is widely used in industrial catalytic processes6 like partial oxidation of hydrocarbons and steam reforming reactions with low carbon deposition.7 Basic alumina based oxide supports feature large surface area, high dispersion of metals, and good thermal and mechanical stability.2 Detailed understanding of metal/metal oxide interactions, structure, and reactivity is fundamental to the understanding of the supported metal based heterogeneous catalysis.1,5 Despite the availability of powerful experimental techniques to characterize the metal/metal oxide interfaces at reactive catalytic environments,4,8
10 theoretical studies based on quantum mechanics remain inevitable to comprehend the molecular level details of interactions and reactivity.11 Noble metal clusters render complex electronic structures in the gas phase, and they are even more intricate when they are adsorbed on the metal oxides.12 Their geometric as well as electronic structure and dynamics impose many challenges for computational chemistry because of a myriad of local minimum structures on the free energy surface, several nondegenerate local electronic occupations, and metal/metal oxide charge transfer. Combination of quantum mechanical approach with classical molecular dynamics, called ab initio molecular dynamics,13 is a powerful tool to investigate such chemically complex problems, yet it includes finite temperature effects. By performing extensive molecular dynamics simulations in the framework of periodic density functional theory, we address here electronic properties, dynamics, and r 2011 American Chemical Society

reactivity of small Rh clusters deposited on alumina. Our systematic study exposes a fundamental understanding of reactivity of Rhn clusters on alumina: on the basis of this, we report the potential use of this catalyst to produce H2 from H2O. The result of this study could help in scrutinizing molecular level details of reaction mechanisms of catalytic reactions, and may aid in tailoring novel catalysts. The most stable phase of alumina is the R-form that exposes predominantly the (0001) surface in the non-hydroxylated state at catalytically relevant temperatures above 800 K. This Al terminated surface displays considerable surface relaxations14 and large amplitude atomic vibrations compared to the bulk.15 Under ambient conditions, the surface of alumina is Al(OH)3like rather than stoichiometric, with a layer of water on its surfaces.16 Other than the most stable R-phase of alumina, the metastable γ-phase17 is also largely used for catalysis. Studies on adsorption and growth of Ag,18,19 Cu,20,21 Pd,21 and Al22 clusters on the R-alumina surfaces have been reported. Various experimental investigations on alumina are reviewed in ref 23 and are not detailed here. Activities of Rh/alumina catalyst for industrial processes like removal of NOx from automobile exhalation, oxidation of hydrocarbons, and water-gas-shift-reaction have been studied using a large number of spectroscopic techniques; see, e.g., refs 24
28. Ethanol dehydrogenation reactions on Rh and Rh2 adsorbed on alumina have been reported29 recently using density functional theory based calculation. However, we realize that a more detailed understanding on the electronic structure and reactivity of Rh metal clusters on alumina is unavailable in the literature. Received: March 26, 2011 Revised: May 10, 2011 Published: June 01, 2011 15403

dx.doi.org/10.1021/jp202832v | J. Phys. Chem. C 2011, 115, 15403–15409

The Journal of Physical Chemistry C

ARTICLE

Table 1. Surface Relaxation Energy of r-Alumina with Respect to Increase in Cluster Size along the [0001] Direction While Keeping 2  2 Unit Cell Size along a and b Directionsa

a

number of O-layers

relaxation energy (J m
2)

3

2.04

4

1.96

6

1.96

Five atomic layers were relaxed in these calculations.

Our work focuses especially on the oxidative addition of water molecule at the Rh/alumina interface. It is very well-known that small molecules like water undergo oxidative addition to late transition metal complexes. Such oxidative addition of water is not favorable in transition metal monomer complexes because of the large activation barrier involved in the dissociation and destabilization due to OH
coordination with transition metals.30 Stabilization of both OH
and H þ can be achieved by using a solid surface with electrophilic surface sites together with electron rich metal centers. Such an environment is realizable at the interface of Rh metals on alumina surface. The main attraction of oxidative addition of water is that the proton that is created by the water dissociation is converted to a hydride. Thus, this process has many applications in catalysis and, in particular, in the development of solar water-splitting cells.30
32 Here we report a detailed study of structure and reactivity of Rhn (n = 1
4) clusters on non-hydroxylated (0001) R-alumina surface as well as Rh on hydroxylated (0001) R-alumina and nonhydroxylated (110) γ-alumina surfaces. Notably, we show that the oxidative addition of water is kinetically and thermodynamically feasible at the Rh/alumina interface. We also report that the hydride formed by the oxidative addition at the interface can spontaneously react with another water molecule forming H2, and that the reactivity of Rh cluster shows a size-dependent variation.

2. METHODS AND MODEL The first part of this paper presents the structure of Rh atom and clusters (Rhn, n = 1
4) adsorbed on the non-hydroxylated (0001) R-alumina surface, and their reactivity toward water. Subsequently we demonstrate that the same chemistry also holds for Rh atom adsorbed on the metastable (110) γ-alumina and on the fully hydroxylated (0001) R-alumina surface. The hexagonal Al48O72 supercell used here to model the (0001) R-alumina surface was composed of 2  2 units along the a and b axes and 6 atomic oxygen layers along the c axis. The supercell size was 9.56  9.56  25.00 Å3. A proper convergence of surface relaxation (Table 1) and adsorption energies were obtained using this supercell with respect to increasing cluster size along the [0001] direction. Hydroxylated (0001) surface was generated by replacing each surface Al atom by three H atoms saturating the three cleaved O
Al bonds on both surfaces of the supercell. To simulate the most exposed (110) surface of γ-alumina, we used Al64O96 monoclinic supercell of size 8.41  16.14  22.00 Å3, with the bulk γ-alumina atomic positions taken from ref 33. All computations were performed within the framework of periodic density functional theory with its pseudopotential implementation,13 as available in the CPMD program.34 We employed the PBE35 exchange correlation functional together with ultrasoft pseudopotentials36 with a plane
wave cutoff of 30 Ry; see Table SI 1 for details of pseudopotentials.

Figure 1. (a) View along the [0001] direction of the supercell used for modeling alumina (0001) surface. Two types of triangular adsorption sites, T1 and T2, are labeled. Spin density plots for (b) Rh, (c) Rh2, (d) Rh3, and (e) Rh4, adsorbed on the (0001) non-hydroxylated R-alumina surface, (f) Rh on (0001) hydroxylated R-alumina surface, and (g) Rh on (110) γ-alumina surface, with up-spin in blue and down-spin in yellow (using isovalue 0.09e). (h) Diffusion pathway of Rh atom on the (0001) surface at 1000 K, color coded from red to white to blue with increasing time along 1 ps NVT trajectory. Atom color code: red (O), green (Al), white (Rh), yellow (H).

Starting from different geometries of Rhn clusters on surfacerelaxed alumina, manifold electronic spin multiplicities, and localization of electrons, extensive simulated annealing of nuclei and electronic wave functions were performed using the Car
Parrinello (CP) molecular dynamics approach,37 followed by traditional geometry optimizations to search for the global minimum geometry and electronic structure. For each starting structure, simulated annealing was performed for more than 1 ps with gradual cooling from 1000 K. Fictitious masses for the orbitals were assigned 700 au, and hydrogen masses were replaced by deuterium masses to maintain an adiabatic separation between nuclear and electronic degrees of freedom while using a time step of 0.144 fs. Canonical ensemble simulations were carried out using the Nos
e
Hoover chain thermostat.38 Benchmarks on the alumina surface relaxations and ground states electronic/geometric structures of bare Rh clusters in the gas phase show good agreement with available literature data (Tables SI 2
6). To compute the free energy barrier for water dissociation on Rh atom adsorbed on (0001) R-alumina, we employed the metadynamics technique.39,40 Metadynamics is helpful for exploring the underlying free energy surfaces of chemical reactions, thereby obtaining reaction mechanisms along the minimum free energy pathway; for details see reviews in refs 13,41
43. All the technical details of this simulation were followed directly from ref 44 and are not explained here. Two collective coordinates were selected for simulating the water dissociation reaction: (a) coordination number between Rh atom to both water H atoms, C[Rh
Hw], and (b) coordination number of water oxygen to both water H atoms, C[Ow
Hw]. Coordination numbers were defined using rational functions, as in ref 44, with a distance cutoff 15404

dx.doi.org/10.1021/jp202832v |J. Phys. Chem. C 2011, 115, 15403–15409

The Journal of Physical Chemistry C

ARTICLE

Table 2. Adsorption Energy, Eads, of Rhn Clusters on Various Alumina Surfaces, and Water on the Corresponding Rhn Alumina Interface water adsorption Eads (kJ/mol)

Rh adsorption surface

a

n Eads (kJ/mol) M

molb

dissc

(0001)/R

1


151

4


104


178

(0001)/R

2


231

3


90


216

(0001)/R

3


290

4


100


196

(0001)/R

4


296

3


125


228

(0001)/R 1 hydroxylated


95

4

(110)/γ


223

4


110


254

1

a

Ground state electronic multiplicity; b molecular (mol) adsorption; c dissociative adsorption.

Figure 2. Various configurations of thermodynamically most stable dissociated water on (a) Rh/(0001) R-alumina, (b) Rh2/(0001) Ralumina, (c) Rh3/(0001) R-alumina, (d) Rh4/(0001) R-alumina, (e) Rh/(0001) hydroxylated R-alumina, and (f) Rh/(110) γ-alumina. See Figure 1 for atom colors.

of 1.4 Å for both coordinates. The underlying free energy surface was reconstructed by the negative sum of biasing potentials.

3. RESULTS AND DISCUSSION 3.1. Rh1/r-Al2O3. To begin with, we looked at Rh atom adsorption on R-alumina (0001) surface. Adsorption is thermodynamically most favored above the “triangular oxygens” (T1) on the surface (see Figure 1a), where one of the Rh
O bonds (2.24 Å) is stronger than the other two (2.27, 2.33 Å); see Figure 1b. The spin multiplicity of the Rh atom retains at four after adsorption on the surface from the gas phase (see Table SI 7 for other multiplicities). Adsorption is exothermic with adsorption energy, Eads =
151 kJ/mol. The total charge of Rh is unaltered after adsorption; however, depletion of 5s population together with increase in 4p and 4d occupation45 was observed (Table SI 8). Note that the Rh atom is in a formal charge of zero, although L€owdin charge is slightly greater than that (þ0.13e). Remarkably, depletion in the total spin polarization, σ, of Rh is associated with the increase in σ of one of the nearest neighbor Al atoms (Table SI 8), which is also visually evident from the spin-density plot Figure 1b. In short, a push
pull type of electron reorganization occurred after adsorption, without oxidizing Rh. To probe the reactivity of the Rh atom on the surface, we looked at the water adsorption at the interface. Multitude structures of water, including dissociated ones, were considered to find the most stable structure of adsorbed water. Dissociation of water as H2O f H þ þ OH
, with OH
bound to Al atom in the nearest neighborhood of Rh, together with H þ coordinated to Rh is thermodynamically more stable than any other dissociated or molecular (nondissociated) structure (see Table 2 and Figure 2a). To ascertain the kinetics of water dissociation, i.e., the forward reaction barrier, we employed the metadynamics technique. The reconstructed free energy surface for this reaction is presented in Figure 3. The forward barrier for water dissociation is only 22 kJ/mol, suggesting that the water dissociation is kinetically favored at room temperature. The metadynamics simulation had not proceeded until a reverse reaction was observed; thus, the reverse barrier could not be estimated. However, on the basis of the potential energy differences computed (Table:2), the reverse barrier is roughly about 49 kJ/mol. In Figure 3, C[Rh
Hw] ≈ 0.2 corresponds to the state 1.2 where Rh and H are coordinated. It is much lower than the

Figure 3. Reconstructed free energy surface (left) and the free energy profile for the water dissociation on non-hydroxylated Rh/(0001) Ralumina surface based on the metadynamics simulation. Free energies reported here are in kJ/mol. The reverse barrier for the water dissociation cannot be computed from this free energy landscape.

physically meaningful value (of ≈1) because of the small Rh
H cutoff (=1.4 Å) used in the definition of C[Rh
Hw]. However, the free energy estimates are not affected by this since the reactants, products, and transition states are clearly distinguishable in the free energy surface through the coordinate C[Ow
Hw]. The charge of O of the hydroxyl group confirms an OH
species, while the H on Rh shows a hydride-like character (see Table 3 and SI section 5.6). A protic H has a L€owdin charge of þ0.42e (Table SI 16), while the H on Rh is having
0.11e. Electronic structure analysis shows that charge transfer occurred from Rh to H during the adsorption, resulting in partial oxidation of Rh (Table 3): charge of Rh changes from þ0.13e to þ0.42e after the H þ adsorption. The whole process is similar to oxidative addition reactions in Rh based molecular complexes.31,46
48 This is a crucial observation, as the creation of a basic hydride entity by the adsorption of a proton from water at the interface aids in carrying out various important chemical reactions. It is noted in passing that, unlike in Rh complexes, a bare metal supported on the oxide support (in the absence of any coordinating ligands) is the actual reactive center. There are numerous 15405

dx.doi.org/10.1021/jp202832v |J. Phys. Chem. C 2011, 115, 15403–15409

The Journal of Physical Chemistry C

ARTICLE

Table 3. L€ owdin Charges45 of Selected Atoms after Water Dissociation on Rhn/Alumina Interfacea surfaces

n

HRh

Ow

Rh

(0001)/R

1


0.11


0.69

þ0.42

(0001)/R (0001)/R

2 3


0.03
0.01


0.65
0.64

þ0.20 þ0.16

(0001)/R

4

þ0.05


0.63

þ0.13

(0001)/R hydroxylated

1


0.12


0.72

þ0.35

(110)/γ

1


0.10


0.70

þ0.41

a

See Figure 2. Here HRh and Ow indicate the H bound to Rh, and the O of the dissociated water molecule, respectively.

advantages of this. In the case of (mono) Rh complexes, a water molecule coordinated to Rh cannot spontaneously dissociate to the same Rh metal center to generate a hydride species.31,46 However, this is efficiently achieved at the Rh/alumina interface. Alumina surface has the benefit that the hydroxyl groups formed by the water dissociation are stabilized by the positively charged Al centers. Of great importance, the formed hydride could be used to abstract a proton from water to create H2 gas. To verify the feasibility of such a reaction, we adsorbed a molecular water on one of the Al centers next to Rh. Adsorption of a molecular water next to the dissociated water structure is also exothermic (Eads =
176 kJ/mol). During molecular dynamics simulations at 300 K, starting from the minimized structure, the molecular water got dissociated spontaneously by reacting with the hydride (on Rh); see Figure 4. The formed H2 desorbed from the Rh atom subsequently and was released from the surface. The Rh atom still remained in a partially oxidized state; the charge of the Rh atom changed from þ0.42e to þ0.68e after the release of H2 (Table SI 17). Immediately after the desorption of H2, the Rh atom coordinated with both the hydroxyl groups (see Figure 4). 3.2. Rh2/r-Al2O3. We continued looking at Rh clusters, Rhn, on the surface since it is very likely that metal clusters are formed on the surface rather than monodispersed Rh atoms. Due to the extremely complex nature of electronic and geometric structures of Rhn with n > 1, we limited our study up to n = 4. It is crucial to analyze the change in reactivity with increasing cluster size, especially when they are having different electronic structures. In the thermodynamically stable configuration of a Rh2 cluster on the alumina surface, Rh atoms are binding with oxygen atoms of neighboring T1 and T2 sites in an asymmetric fashion, as in Figure 1c. The ground state multiplicity of the Rh2 cluster changes from five to three on adsorption, and Rh
Rh bond distance elongates from 2.27 to 2.49 Å. The pentet electronic state is 28 kJ/mol higher in energy than the triplet state (Table SI 7). Table SI 9 together with Figure 1c shows that each Rh is having an unpaired electron with same spin, unlike in the gas phase where two electrons are localized on each Rh atom. Adsorption energy of Rh2 cluster is
231 kJ/mol, which is 71 kJ/mol less than twice the adsorption energy of Rh atom (Table 2). This indicates that if two Rh atoms are far from each other, they would thermodynamically prefer to stay isolated rather than cluster. A similar observation was reported for Agn clusters on (0001)/R-alumina surfaces.19 Identical to that for the Rh atom adsorption, the total valence population of Rh-coordinated surface O atoms became depleted, and that of Al atoms coordinated to these O atoms got increased to the same extent (Table SI 9). Note that the redistribution of the valence 5s orbital

Figure 4. Distance r[H
H] between the H on Rh formed after water dissociation and H of adsorbed water on the surface during a molecular dynamics simulation at 300 K. A transient back transfer of the water proton to water oxygen results in an increase in r[H
H] around 0.01 ps. Representative snapshots from the simulation are shown in the inset. For atom colors, see Figure 1.

of each Rh occurred by the formation of a Rh
Rh bond, and thus, the depletion of 5s occupation on adsorption is less in magnitude compared to the Rh atom adsorption. A water molecule dissociated on the Rh2 cluster is again thermodynamically favored over the molecular adsorption, and the adsorption energy is
216 kJ/mol (Table 2 and Figure 2b). The proton adsorption on Rh is followed by a partial oxidation of Rh and reduction of H by the charge transfer from Rh (see Table 3 and Table SI 13). The hydridic character, gauged by the L€owdin population of this H (
0.03e), is lower than that of H on a single Rh atom (
0.11e). Thus, a lower affinity toward a molecular water proton can be envisaged in comparison to that of the H bound to Rh1/R-Al2O3. 3.3. Rh3/r-Al2O3. The ground electronic structure of a Rh3 cluster has a multiplicity of 6, but this decreases to 4 after adsorption on the alumina surface (Table SI 7). Each Rh atom has an unpaired electron localized largely in their 4d orbitals (Figure 1d and Table SI 10). Rh3 prefers to adsorb directly above the T1 site (see Figure 1d) with each Rh bonding with surface O atoms, thus forming an equilateral triangle with each side of length 2.60 Å. Adsorption is followed by a considerable increase in bond distances from the gas-phase configuration of the Rh3 cluster where they are 2.39, 2.39, and 2.57 Å. Adsorption is exothermic (Eads
290 kJ/mol), but the adsorption energy per Rh atom is higher than both Rh and Rh2 cases (Table 2). Like in the case of Rh and Rh2, dissociation of water is more stable than molecular adsorption at the Rh3/alumina interface, with one of the water protons adsorbed on the metal cluster (Figure 2c). Charge computations show that the H entity on Rh3 oxidized one of the Rh atoms and Hþ got reduced (Table 3 and Table SI 14). Interestingly, a systematic decrease in the hydridic character, i.e., increasing L€owdin charge of HRh, going from Rh1 f Rh2 f Rh3, is evident. Thus, the affinity of HRh toward a proton decreases in the same order. 3.4. Rh4/r-Al2O3. Rh4 is having a tetragonal molecular geometry in the gas phase and has a multiplicity of 1 at the electronic ground state. All electrons are mutually paired in this structure. On adsorption, the symmetry of the cluster was lost by differential increase in bond distances. In particular, two Rh
Rh bonds were broken after the adsorption, thereby forming a butterfly-like structure. Rh atoms bind to both Al and O atoms on the surface, with two Rh atoms sitting above two neighboring T1 sites, as shown in Figure 1e. Due to the elongation of bond distances, electronic pairing between Rh atoms gets partially 15406

dx.doi.org/10.1021/jp202832v |J. Phys. Chem. C 2011, 115, 15403–15409

The Journal of Physical Chemistry C broken, resulting in a triplet ground state with unpaired electrons distributed among three Rh centers (see Figure 1e, Table SI 7, and Table SI 11). Adsorption energy of the Rh4 cluster is
296 kJ/mol, and as observed for smaller clusters, adsorption energy per Rh decreases (Table 2). Dissociative water adsorption is thermodynamically preferred on Rh4 (Table 2 and Figure 2d), associated by an oxidative addition of water proton to one of the Rh atoms (Table 3 and Table SI 15), analogous to Rhn (n = 1
3) cases. Again, the charge of HRh is more positive in comparison with smaller clusters indicating a lowering affinity toward a water proton (Table 2). 3.5. Rh1/Hydroxylated r-Al2O3. Under ambient conditions, the surface of R-alumina is hydroxylated.16 To verify the feasibility of oxidative addition of water protons, we performed calculations on Rh atom adsorbed on the fully hydroxylated R-alumina (0001) surface. Among numerous configurations tried, the most stable adsorption structure of Rh on the hydroxylated surface is above “triangular oxygens” (T1) (see Figure 1f) on the surface with all the three Rh
O distances equal to 2.37 Å. The electronic ground state remains as quartet analogous to non-hydroxylated surface; however, the adsorption energy is substantially lower:
95 kJ/ mol for the hydroxylated surface but
151 kJ/mol for the nonhydroxylated. This is expected since the surface O atoms that are coordinated with Rh are saturated with protons on the hydroxylated surface. Electronic structure analyses showed that the unpaired electrons are largely localized on Rh atom itself (see Figure 1f). However, no spin densities can be seen on the neighboring Al, unlike on the non-hydroxylated surface. Other than that, the electronic rearrangements due to adsorption are similar to that of the non-hydroxylated surface. More detailed electronic structural analysis is present in Table SI 18. Transfer of a surface hydroxyl proton to Rh is also exothermic (
25 kJ/mol); see Figure 2e. As expected, the binding of the proton on Rh results in oxidation of Rh, very similar to that in the case of the non-hydroxylated surface. The charges of HRh and Rh are
0.12e and þ0.35e, respectively (Table 3 and Table SI 18). In fact, the affinity toward proton is of the same degree as for the non-hydroxylated surface, and clearly, H2 formation by reacting with a water molecule can be anticipated to be spontaneous. 3.6. Rh1/(110) γ-Al2O3. To gauge the stability of the Rh atom on (0001)/R-alumina, we carried out NVT ensemble simulations at 1000 K.49 We have observed diffusion of Rh atom from one T1 triangular site to another through a neighboring T2 site on the sub-picosecond time scale (Figure 1h). This indicates that the interaction between Rh and R-alumina is not strong enough to immobilize the metal centers at catalytically relevant temperatures. However, a strong metal/metal oxide interaction is crucial for a stable catalytic system. This motivated us to look at γ-alumina, a metastable phase of alumina, which is present in freshly prepared samples used in catalysis.50 In the case of γ-alumina, surface Rh atom adsorbs more favorably at the groove on the surface, coordinating with two 2-fold coordinated surface oxygens (2.23, 2.26 Å) and one 3-fold coordinated oxygen (2.34 Å) at the bottom of the groove (Figure 1g). The most stable structure has a quintet electronic ground state. Adsorption is highly exothermic (Eads =
223 kJ/mol), and the magnitude of adsorption energy reflects a much stronger metal/metal oxide interaction on the γ-alumina surface compared to that of R (see Table 2). Akin to the R surface, the total charge of the Rh atom remains the same following

ARTICLE

Figure 5. Overall catalytic cycle for H2 production using Rh/alumina catalyst. Reactions shown by solid arrows are studied here, and the reaction drawn by dotted line requires future investigation. Free energy barriers (in kJ/mol) are indicated above solid arrows.

adsorption, albeit with the rearrangement of valence electrons. Unpaired electrons are largely localized on the Rh atom, with a low degree of spin polarization on the Rh-bonded oxygen atoms (Figure 1g and Table SI 19). Water dissociation follows the same trend as we observed for the Rh1/R-alumina system. Dissociative adsorption is more favored over molecular adsorption, and the adsorption energies are noticeably higher in magnitude compared to R-alumina (Table 2 and Figure 2f). Very interestingly, the hydroxyl group of the dissociated water bridges between two three-coordinated surface Al atoms over the corrugation on the (110) surface (Figure 2f). Of utmost importance, the water dissociation on Rh/ γ-alumina interface also progresses by an oxidative addition of Hþ on Rh. This is manifested by a charge of
0.10e on HRh and þ0.41e on Rh (see Table 3 and Table SI 19). On the basis of this we can infer a spontaneous reaction with the molecular water forming H2 gas. 3.7. Practical Considerations. With all the results, it is clear that water dissociation on Rh atom is spontaneous by an oxidative addition mechanism, eventually leading to the formation of highly basic reactive species, which in turn can abstract a proton from a molecular water releasing H2; see Figure 5. However, several important points have to be considered for the practical application of the ideas deliberated here. 3.7.1. Size of the Cluster. A decrease in charge transfer from Rh to H was observed with the increase in cluster size, which results in systematic lowering in the affinity of H on Rhn cluster to abstract a water proton. Thus, it would be ideal to have monodispersed Rh atom on the surface instead of clusters. Small Rh metal clusters can be synthesized on oxide surfaces,5,51 and in particular, small clusters50 and monodispersed Rh atoms on γ-alumina were also observed experimentally.52,53 It is very likely that there will be a point of saturation of the reactivity of H on Rh with respect to the cluster size, where at the extreme limit, H has largely a protic character rather than a hydridic character, and therefore may not react with another proton. A fully rhodium loaded surface can be expected to have lower activity, not just because of the decreasing affinity with increasing cluster size, but also due to the fact that additional stabilization of dissociated water by the coordination of hydroxyl group with positively charged surface Al is not conceivable in such cases. 3.7.2. Crystal Phase of Alumina. It is known in the literature that the γ-alumina is preferred over R-alumina for catalytic 15407

dx.doi.org/10.1021/jp202832v |J. Phys. Chem. C 2011, 115, 15403–15409

The Journal of Physical Chemistry C purposes.50 In agreement with that our calculations also show that metastable γ-alumina is better suited than the R phase: adsorption energies of Rh and water on the surface of the former phase are noticeably higher than that of the latter. 3.7.3. Surface Hydroxylation. Molecular dynamics simulations starting from a hydride bound to Rh on the nonhydroxylated (0001) R-alumina surface (i.e., without any adsorbed water molecules nearby) showed spontaneous migration of hydride from Rh to surface Al (data not shown). This is due to the strong Lewis acidity of the 3-fold coordinated surface Al centers. One solution to this problem is by taking a fully hydroxylated surface, or by creating a water/metal oxide interface, such that all neighboring Al centers are saturated with water molecules or hydroxyl groups. Our calculations show that oxidative addition occurs also on a hydroxylated surface. 3.7.4. Catalytic Cycle. The final issue is related to the completion of a full catalytic cycle, which requires reduction of Rh atoms that are oxidized after the release of H2 (Figure 5). A step which includes the removal of dissociated water oxygen atom as O2, coupled with reduction of Rh, would be ideal here.32,54 This could conceivably be accomplished by an appropriate photo (co-)catalyst, which can be an oxide material itself.55,56 A recent experimental report on the water-splitting activity by Mn3O4 and Rh/Cr2O3 deposited on GaN/ZnO solid solution57 boosts the significance of our propositions. An ideal co-catalyst should have the standard electrode potential lower than that of the Rh/Al2O3 system. Experimental works are paramount in order to search for the suitable co-catalyst and benchmark the efficiency of various materials. Unfortunately, simulation of such complex photocatalytic processes requires proper treatment of electron transfer and electronic excited states, and therefore can not be applied as a routine procedure to screen several materials.

4. CONCLUSION Overall, our study shows that Rh atom or cluster adsorption on hydroxylated and non-hydroxylated (0001) R- and (110) γ-alumina surfaces is exothermic in nature. They exhibit complex electronic/magnetic structures, which can differ from that of gasphase Rhn clusters. Adsorption energy of Rhn (n = 2
4) per Rh atom decreases with increasing cluster size, showing less affinity to form clusters when they are monodispersed on the surface. We observed that Rh atom is diffusive on the R surface at catalytic temperatures. Rh
alumina interactions are the strongest on the (110) γ-alumina surface. Interestingly, adsorption retains the oxidation state of Rh. Water dissociation at the Rhn/alumina interface is kinetically and thermodynamically favored, and the forward free energy barrier is only 22 kJ/mol. Hydroxyl groups are stabilized at the surface Al Lewis acid sites. The proton transfer to Rh is associated with oxidation of Rh, resulting in a nucleophilic hydride on the surface, which has the potential to spontaneously abstract a proton from an adsorbed molecular water, releasing H2 at room temperature. Although the hydridic character is independent of R and γ surface used and the amount of surface hydroxylation, a systematic decrease of nucleophilic character of the H on Rh was observed with increasing rhodium cluster size. Formation of such basic reactive centers on the surface is useful for carrying out various chemical reactions, and possibly be one of the crucial steps in the Rh/alumina based catalysis. Although oxidative addition is well exploited in Rh based homogeneous catalysis, an oxidative addition of a water proton

ARTICLE

is difficult to achieve in monorhodium complexes. This has considerable importance in tailoring a catalyst for the hydrogen production from water,32 and for the regeneration of ammonia boranes.48 Finally, we have also addressed various practical considerations in in achieving hydrogen gas generation from water using the Rh/alumina catalyst. For an efficient catalysis, small cluster sizes have to be achieved in experiments, most preferably monodispersion of rhodium atoms on the surface. We see that the metastable γ-alumina is a better choice than the most stable R phase. Surface hydroxylation is desirable to prevent diffusion of the formed hydride from Rh to positively charged surface aluminum centers. A final reductive elimination step is inevitable to complete the catalytic cycle, and might be achievable with the help of a metaloxide photo co-catalyst that can reduce the oxidized rhodium atoms. However more future work is necessary in this direction to find a suitable co-catalyst. We hope that the insights on the reactivity of the Rh/alumina interface presented here would expedite the research directed in the search for efficient water splitting catalysts and also help in designing novel metal/metaloxide catalytic systems.

’ ASSOCIATED CONTENT

bS

Supporting Information. Benchmark calculations, detailed energetics, and charge and orbital analysis. This material is available free of charge via the Internet at http://pubs.acs.org.

’ AUTHOR INFORMATION Corresponding Author

*E-mail: [email protected].

’ ACKNOWLEDGMENT The authors are grateful to the Department of Science and Technology (DST) India for their financial support through the DST-Fast Track program (Project SR/FT/CS-024/2009). Part of the calculations were performed using the computer cluster installed at the Department of Chemistry, Indian Institute of Technology Kanpur, through the DST-FIST program. T.K.G. acknowledges the Council of Scientific and Industrial Research (CSIR), India, for his fellowship. ’ REFERENCES (1) Tauster, S. J.; Fung, S. C.; Baker, R. T. K.; Horsley, J. A. Science 1981, 211, 1121–1125. (2) Supported Metals in Catalysis; Fern
andez-García, M., Anderson, J. A., Eds.; Imperial College Press: London, 2005. (3) Santra, A. K.; Goodman, D. W. J. Phys.: Condens. Matter 2002, 14, R31–R62. (4) Goodman, D. W. Chem. Rev. 1995, 95, 523–536. (5) Gates, B. C. Chem. Rev. 1995, 95, 511–522. (6) Catalysis: Concept and Green Applications; Rothenberg, G., Ed.; Wiley-VCH: Weinheim,Germany, 2008. (7) Navarro, R. M.; Pe~ na, M. A.; Fierro, J. L. G. Chem. Rev. 2007, 107, 3952–3991. (8) Habibpour, V.; Wang, Z. W.; Palmer, R. E.; Heiz, U. J. Appl. Sci. 2011, 11, 1164–1170. (9) Kwak, J. H.; Hu, J.; Mei, D.; Yi, C.-W.; Kim, D. H.; Peden, C. H. F.; Allard, L. F.; Szanyi, J. Science 2009, 325, 1670–1673. (10) Kovtunov, K. V.; Beck, I. E.; Bukhtiyarov, V. I.; Koptyug, I. V. Angew. Chem., Int. Ed. 2008, 47, 1492–1495. 15408

dx.doi.org/10.1021/jp202832v |J. Phys. Chem. C 2011, 115, 15403–15409

The Journal of Physical Chemistry C (11) Theoretical Aspects of Heterogenous Catalysis; Davis, B. H., Ed.; Van Nostrand Reinhold: New York, 1990; Vol. 1 (12) Pacchioni, G.; Rosch, N. J. Chem. Phys. 1996, 104, 7329–7337. (13) Marx, D.; Hutter, J. Ab Initio Molecular Dynamics: Basic Theory and Advanced Methods; Cambridge University Press: Cambridge, U.K., 2009. (14) Verdozzi, C.; Jennison, D. R.; Shultz, P. A.; Sears, M. P. Phys. Rev. Lett. 1999, 82, 799. (15) Soares, E. A.; Hove, M. A. V.; Walters, C. F.; McCarty, K. F. Phys. Rev. B 2002, 65, 195405. (16) Eng, P. J.; Trainor, T. P.; Brown, G. E., Jr.; Waychunas, G. A.; Newville, M.; Sutton, S. R.; Rivers, M. L. Science 2000, 288, 1029. (17) Paglia, G.; Buckley, C. E.; Rohl, A. L.; Hunter, B. A.; Hart, R. D.; Hanna, J. V.; Byrne, L. T. Phys. Rev. B 2003, 68, 144110. (18) Meyer, R.; Ge, Q.; Lockemeyer, J.; Yeates, R.; Lemanski, M.; Reinalda, D.; Neuroack, M. Surf. Sci. 2007, 601, 134–145. (19) Nigam, S.; Majumder, C. Langmuir 2010, 26, 18776–18787. (20) Sanz, J. F.; Hernandez, N. C. Phys. Rev. Lett. 2005, 94, 016104. (21) Leodziana, Z.; Nørskov, J. K. J. Chem. Phys. 2001, 115, 11261. (22) Siegel, D. J.; Hector, L. G., Jr.; Adams, J. B. Phys. Rev. B 2002, 65, 085415. (23) Fu, Q.; Wagner, T. Surf. Sci. Rep. 2007, 62, 431. (24) Liu, Y.; F. Y. Huang, J. M. L.; Weng, W. Z.; Luo, C. R.; Wang, M. L.; Xia, W. S.; Huang, C. J.; Wan, H. L. J. Catal. 2008, 256, 192–203. (25) Ishikawa, A.; Iglesia, E. J. Catal. 2007, 252, 49–56. (26) Grunwaldt, J.-D.; Basini, L.; Clausen, B. S. J. Catal. 2001, 200, 321–329. (27) Chuang, S. S. C.; Tan, C.-D. J. Catal. 1998, 173, 95–104. (28) Rasko, J.; Novak, E.; Solymosi, F. Catal. Today. 1996, 27, 115–121. (29) Wu, S.-Y.; Lia, Y.-R.; Ho, J.-J.; Hsieh, H.-M. J. Phys. Chem. C 2009, 113, 16181–16187. (30) Ozerov, O. V. Chem. Soc. Rev. 2009, 38, 83–88. (31) Walter, M. G.; Warren, E. L.; McKone, J. R.; Boettcher, S. W.; Mi, Q.; Santori, E. A.; Lewis, N. S. Chem. Rev. 2010, 110, 6446–6473. (32) Esswein, A. J.; Nocera, D. G. Chem. Rev. 2007, 107, 4022–4047. (33) Digne, M.; Sautet, P.; Raybaud, P.; Euzen, P.; Toulhoat, H. J. Catal. 2004, 226, 54–68. (34) Hutter, J. et al. CPMD, Version 3.13.1; IBM Corp. R€uschlikon, 1990
2011; MPI f€ur Festk€orperforschung: Stuttgart, 1997
2001. See also http://www.cpmd.org. (35) Perdew, J. P.; Burke, K.; Ernzerhof, M. Phys. Rev. Lett. 1996, 77, 3865. Erratum: Phys. Rev. Lett. 1997, 78, 1396. (36) Vanderbilt, D. Phys. Rev. B 1990, 41, 7892. (37) Car, R.; Parrinello, M. Phys. Rev. Lett. 1985, 55, 2471. (38) Martyna, G. J.; Klein, M. L.; Tuckermann, M. J. Chem. Phys. 1992, 97, 2635. (39) Laio, A.; Parrinello, M. Proc. Natl. Acad. Sci. U.S.A. 2002, 99, 12562–12566. (40) Iannuzzi, M.; Laio, A.; Parrinello, M. Phys. Rev. Lett. 2003, 90, 238302–2383024. (41) Laio, A.; Parrinello, M. In Computer Simulations in Condensed Matter: From Materials to Chemical Biology; Ferrario, M., Cicotti, G., Binder, K., Eds.; Springer Verlag: Berlin, 2006; Vol. 1, pp 315
347. (42) Ensing, B.; Vivo, M. D.; Liu, Z. W.; Moore, P.; Klein, M. L. Acc. Chem. Res. 2006, 39, 73. (43) Laio, A.; Gervasio, F. L. Rep. Prog. Phys. 2008, 71, 126601. (44) Nair, N. N. J. Phys. Chem. B 2011, 115, 2312–2321. (45) Charges and orbital occupations are calculated by the projection of wave function on L€owdin orthogonalized atomic orbitals. (46) Peris, E.; Lahuerta, P. Rhodium Organometallics. In Comprehensive Organometallic Chemistry III: From Fundamentals to Applications; Crabtree, R. H., Mingos, D. M. P., Eds.; Elsevier: 2007, Vol. 7, pp 121
236. (47) Simpson, M. C.; Cole-Hamilton, D. J. Coord. Chem. Rev. 1996, 155, 163–207. (48) Mock, W. T.; Potter, R. G.; Camaioni, D. M.; Li, J.; Dougherty, W. G.; Kassel, W. S.; Twamley, B.; DuBois, D. L. J. Am. Chem. Soc. 2009, 131, 14454–14465.

ARTICLE

(49) Oxidative steam reforming reactions using Rh/Al2O3 catalyst are carried out experimentally within a temperature range 800
1100 K. (50) Weng-Sieh, Z.; Gronsky, R.; Bell, A. T. J. Catal. 1997, 170, 62–74. (51) Bhirud, V.; Goellner, J. F.; Argo, A. M.; Gates, B. C. J. Phys. Chem. B 2004, 108, 9752–9763. (52) Dent, A. J.; Evans, J.; Fiddy, S. G.; Jyoti, B.; Newton, M. A.; Tromp, M. Angew. Chem., Int. Ed. 2007, 46, 5356–5358. (53) Newton, M. A.; Dent, A. J.; Fiddy, S. G.; Jyoti, B.; Evans, J. J. Mater. Sci. 2007, 42, 3288–3298. (54) Yin, Q.; Tan, J. M.; Besson, C.; Geletii, Y. V.; Musaev, D. G.; Kuznetsov, A. E.; Luo, Z.; Hardcastle, K. I.; Hill, C. L. Science 2010, 328, 342–345. (55) Harriman, A.; Pickering, I. J.; Thomas, J. M.; Christensen, P. A. J. Chem. Soc., Faraday Trans. 1988, 84, 2795–2806. (56) Kanan, M. W.; Nocera, D. G. Science 2008, 321, 1072–1075. (57) Maeda, K.; Xiong, A.; Yoshinaga, T.; Ikeda, T.; Sakamoto, N.; Hisatomi, T.; Takashima, M.; Lu, D.; Kanehara, M.; Setoyama, T.; Teranishi, T.; Domen, K. Angew. Chem., Int. Ed. 2010, 49, 4096–4099.

15409

dx.doi.org/10.1021/jp202832v |J. Phys. Chem. C 2011, 115, 15403–15409