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Dynamics of Hydrogen Spillover on Pt/γ-Al2O3 Catalyst Surface: A Quantum Chemical Molecular Dynamics Study Farouq Ahmed,† Md. Khorshed Alam,† Ai Suzuki,‡ Michihisa Koyama,§ Hideyuki Tsuboi,† Nozomu Hatakeyama,† Akira Endou,† Hiromitsu Takaba,§ Carlos A. Del Carpio,† Momoji Kubo,| and Akira Miyamoto*,†,‡,§ Department of Applied Chemistry, Graduate School of Engineering, Tohoku UniVersity, 6-6-11-1302 Aoba, Sendai, 980-8579 Japan, New Industry Creation Hatchery Center, Tohoku UniVersity, 6-6-10 Aoba, Sendai, 980-8579 Japan, Department of Chemical Engineering, Graduate School of Engineering, Tohoku UniVersity, 6-6-10 Aoba, Sendai 980-8579, Japan, and Fracture and Reliability Research Institute, Graduate School of Engineering, Tohoku UniVersity, 6-6-11-701 Aoba, Sendai, 980-8579 Japan ReceiVed: April 20, 2009; ReVised Manuscript ReceiVed: June 29, 2009
The hydrogen spillover mechanism has earned intensive interest in the past decades because it plays a vital role in emerging technologies for the reduction of NOx in automobile exhausts. Hydrogen spillover arises in hydrogen-catalyzed reactions on a supported metal catalyst. In the present study, we applied quantum chemical molecular dynamics (QCMD) to investigate the mechanism of the hydrogen spillover process on a Pt/γAl2O3 catalyst surface for the first time. The direct observation of dissociative adsorption of hydrogen and diffusion of hydrogen on a Pt/γ-Al2O3 catalyst surface were successfully investigated. The diffusion of the hydrogen atom in the gas phase explains the high reactivity observed in the hydrogen spillover mechanism. Introduction The ability of supported noble metal catalysts to substantially increase the amounts of hydrogen adsorption in the reduction condition, epitomized by the alumina-platinum system (Pt/γAl2O3), has been attracting renewed attention concerning heterogeneous catalytic reactions1 which are namely involved in the reduction of automobile exhaust gases such as NOx.2 Central to this potentiality is the hydrogen spillover effect by means of which dissociated hydrogen chemisorbed on the surface of the metal particle moves on the surface of the support metal oxide, the latter having no or little activity for atomic hydrogen adsorption.3-11 Spillover of hydrogen consists of several elementary steps of reactions such as adsorption, dissociation, diffusion and desoprtion. Since no studies to date have been able to study the spillover step in isolation, all studies interpret experimental data for a combination of sequential steps.12-24 The temperature dependency of catalytic activity will convey some information on the mechanisms. Khoobiar12 as well as Ioannides and Verykios13 showed that the H2 molecule was adsorbed on Pt/ Al2O3 even at room temperature. Baumgarten et al.14-17 have published a series of papers wherein they concluded that spillover hydrogen diffuses into the gas phase even at low temperatures ( bridging >3-fold. Calculated adsorption energy was -97.23 kJ/mol that is supported by experimental as well as theoretical results.58,59 Experimental value of adsorption energy of hydrogen molecule is approximately about -25.08 to -112.86 kJ/mol at different support surfaces and under different experimental conditions.57 Further investigation of bond energies between Ha-Hb, Ha-Pt and Hb-Pt (Figure 3) showed that Ha-Hb bond energies decreased whereas Ha-Pt and Hb-Pt bond energies increased with simulation time. The values for the Ha-Pt bond energies were 0.0, -0.836, -26.33, -5.85, and -20.48 kJ/mol at 0 fs, 58 fs, 114 fs, 140 fs, and 155 fs, respectively. The Hb-Pt bond energies were 0.0, 3.762, -16.3, -83.6, and 0.0 kJ/mol at 0 fs, 58 fs, 114 fs, 140 and 155 fs respectively. The Ha-Hb bond energies were -430.96, -377.87, -428.03, -145.046, and -2.09 kJ/mol at 0 fs, 58 fs, 114 fs, 140 and 155 fs respectively. This analysis suggests the bond dissociation of Ha-Hb within the simulation time. It was also found that the bond population (Table 4) of H-H in the adsorbed state became lower (0.71) than that in the gas phase (0.87). A central issue in the spillover mechanism has been the nature of the activated hydrogen species. For instance, Roland and Roessner60 have proposed several possible spilled-over hydrogen species, such as H atom, ionic particles (H+ or H-) and H3+ species. They concluded that the activated hydrogen (H*) species, acting as electron donors on the support surface, depending on its electronic properties, lead to the coexistence of H atoms and H+ ions. The atomic charges on the spilled-over hydrogen were 0.01, -0.04, -0.02, and 0.04 at 155, 170, 180, and 190 fs respectively shown in Figure 4. In the adsorption state of atomic hydrogen on the γ-Al2O3 (100) surface, shown in Figure 5c, the atomic charge was 0.14. This nature of spilled-over hydrogen was in qualitative agreement with the findings obtained by the previous report.60 Effect of Different Initial Directions of Velocities of Hydrogen. In the present study, a hydrogen molecule was deposited on the Pt/γ-Al2O3 (100) surface with initial velocity 5000 m/s that was calculated from the Boltzmann distribution at 873 K. Here we also consider the adsorption of hydrogen molecule at the kink/apical sites because the positions of kink/ apical sites have been shown to form strongly chemisorbed hydrogen species on platinum single crystals.56 Considering such phenomena, an initial position of hydrogen molecule was set so as to easily adsorb to the kink site of the Pt cluster. Adsorption energy at the kink site was -97.23 kJ/mol calculated by the QCMD method. For measuring the strong adsorption of hydrogen we again consider the effects of molecular rotation of H2 on the dissociative adsorption process. The activation barriers for the perpendicular orientation with Pt/γ-Al2O3 catalyst surface are always much higher than that of the parallel orientation, which indicates that the parallel orientation is favored for H2 adsorption.61 The effect of different initial directions of velocities and position of adsorption on the
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Figure 2. Snapshots of MD calculation results at different simulation steps from 0 to 155 fs: (a) initial structure of H2/Pt/γ-Al2O3 where H2 molecule was initially set above the supported Pt, (b) snapshot at 58 fs where H2 molecule approaches and interacts with Pt surface, (c)-(e), represents the three-step dissociative adsorption of H2 on the Pt surface starting with an (c) on top adsorption, (d) bridge site adsorption, and (e) 3-fold site adsorption at 114 fs, 117 fs, and 128 fs respectively. Finally, (f) snapshot at 155 fs where one of the adsorbed hydrogen atom (Hb) spilled-over from the Pt surface and the other adsorbed hydrogen atom (Ha) remained bonded to the surface.
Figure 3. Change in bond energies with time. Solid and dashed lines represent the bond energy of Ha-Pt and Hb-Pt and those of Ha-Hb, respectively.
dynamics of the deposited H2 molecule was summarized in Tables 10 where details of initial direction of velocity in different axes (x, y and z) are shown. Here in the case of study 1, H2 molecules have parallel as well as dissociative adsorption on Pt/γ-Al2O3 catalyst surface. In the mean time rotation (because of opposite direction of velocity in x axes) of molecule was observed which causes parallel adsorption of H2. In the case of study 2, the H2 molecule approached with perpendicular direction to the Pt/γ-Al2O3 catalyst surface. But no stable
adsorption and dissociation were observed. In this case, no rotation of the molecule was observed because of the same direction of velocity in the x axes. Last of all in the case of study 3, the same phenomenon of study 2 was observed. In Figure 2a (before UA-QCMD simulation started) H2 was initially in a nonparallel (e.g., perpendicular) orientation with Pt/γ-Al2O3 catalyst surface. But as the simulation proceeds it tends to reorient itself to achieve a parallel orientation (Figure 2c). Our findings conclude that parallel adsorption on the Pt/γ-Al2O3 catalyst surface is necessary for dissociation. Adsorption of Atomic Hydrogen on Support Surface. To investigate the adsorption of spilled-over hydrogen (which was spilled over in the gas phase Figure 2f) atom on the surface of γ-Al2O3 support, further UA-QCMD simulations were carried out using the previous simulation condition. Here we only considered the γ-Al2O3 (100) surface consists of 64 aluminum and 96 oxygen atoms, which was modeled a partial surface of Pt/γ-Al2O3 (100) where Pt was not loaded. One hydrogen atom was placed at the vacuum region in the simulation cell at the initial simulated step. As hydrogen spilled over with the velocity 8850 m/s, we applied the same velocity (8850 m/s) of H for the second simulation. The cell parameters for the simulation model shown in Figure 5 were fixed to a ) b ) 15.8 Å, c ) 50 Å, R ) β ) γ ) 90° during the simulation. Snapshot at 0 fs showed the initial position of hydrogen atom in the vacuum region. Snapshots at different simulation time in Figure 5
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Figure 4. Snapshots of spilled-over hydrogen (green color) evolving into the gas phase, which correspond to 155, 170, 180, and 190 fs.
Figure 5. Snapshots of the spilled-over hydrogen on γ-Al2O3 surface with the trajectories of motions of the hydrogen: (a) initial position of atomic hydrogen above the γ-Al2O3 surface (where velocity of hydrogen atom was set to 8850 m/s), (b) snapshot at 188 fs where hydrogen approached the γ-Al2O3 surface, (c) snapshot at 377 fs where hydrogen adsorbed on the γ-Al2O3 surface, (d) introduction of another hydrogen with same property at 390 fs, (e) snapshot at 480 fs where newly introduced hydrogen interacted and formed a bond with adsorbed hydrogen, and (f) snapshot at 545 fs where molecular hydrogen desorbs from the γ-Al2O3 surface.
TABLE 10: Change in Dynamic Behaviors of H2 on the Pt/γ-Al2O3 Model with the Different Velocity Components Given to the H2 Moleculea velocity components(m/s) atom
x
y
z
Ha
3730
0
-3350
Hb
-3730
0
-3350
Ha Hb
3730 3730
0 0
-3350 -3350
Ha Hb
0 0
3730 3730
-3350 3350
rotation
observations adsorption direction
Study 1 yes parallel
dissociative adsorption yes (on kink/ apical pt atom)
Study 2 no perpendicular
no
Study 3 no perpendicular
no
a Initial velocity of the H2 was set to 5000 m/s. Ha and Hb represents the individual hydrogen atoms in the hydrogen molecule. The observations of the hydrogen atoms after 120 fs are listed.
showed that at around 377 fs (Figure 5c) the hydrogen atom was impinged to the γ-Al2O3 surface and was adsorbed on the surface. The readsorption behavior of the diffusing H atom
starting with such a high velocity suggests that the diffusion of H atom in the gas phase which was formed by spillover of hydrogen molecule can spread over the surface, not only on the Pt cluster. This can be explained by considering the high concentration of hydrogen observed in a spillover reaction. After the adsorption of the spillover hydrogen atom on the γ-Al2O3 surface, we placed another hydrogen atom in the same simulation model (Figure 5d) given the initial velocity directing to collide with the preadsorbed hydrogen atom. Further simulation shows that hydrogen atom approached to the adsorbed hydrogen atom and formed a bond at about 480 fs (Figure 5e). At the final stage, desorption of the formed H2 was observed from the γ-Al2O3 surface at about 545 fs, as shown in Figure 5f. From the present UA-QCMD simulation, it would be suggested that the adsorption of the H atom on the γ-Al2O3 support is a metastable state and the atoms were readily reduced by another H atom leading to the formation of the H2 molecules. The molecule may interact again with the metal acting as a source of hydrogen spillover. Adsorption of hydrogen atom will lead to the diffusion from the sites near the metal catalyst particles to distant sites, which will be necessary step for the H atom spillover phenomenon. Conclusions We successfully investigated the dissociative adsorption of hydrogen spillover and unveiled the factors involved in this chemical transformation by our developed UA-QCMD method. Further investigation reveals the possibility of the formation of molecular hydrogen from the γ-Al2O3 surface, which would be used in further reaction of the spillover hydrogen. The changes in structures with the reactions were also investigated. It was indicated that the kinetic energy increased by the dissociation of hydrogen bond on the Pt cluster promoted the diffusion of the spillover hydrogen atom to the gas phase. References and Notes (1) Li, C.; Xin, Q. Stud. Surf. Sci. Catal. 1997, 112, 1. (2) Sermon, P. A.; Bond, G. C. Catal. ReV. 1974, 8, 211. (3) Teichner, S. J. Appl. Catal. 1990, 62, 1. (4) Conner, W. C.; Falconer, J. L. Chem. ReV. 1995, 95, 759. (5) Teichner, S. J.; Inui, T.; Fujimoto, K.; Masai, M. Stud. Surf. Sci. Catal. 1993, 77, 27. (6) Khoobiar, S.; Stiles, A. B. Catal. Supports Supported Catal. 1987, 9, 201. (7) Baumgarten, E.; Wagner, R.; Lentes-Wagner, C. J. Catal. 1987, 104, 307. (8) Miller, J. T.; Meyer, B. L.; Modica, F. S.; Lane, G. S.; Vaarkamp, M.; Koningsberger, D. C. J. Catal. 1993, 143, 395. (9) Pajnok, G. M. Appl. Catal., A 2000, 202, 157. (10) Robell, A. J.; Ballou, E. V.; Boudrat, M. J. Phys. Chem. 1964, 68, 2748. (11) Srinivas, S. T.; Rao, P. K. J. Catal. 1994, 148, 470. (12) Khoobiar, S. J. Phys. Chem. 1964, 68, 411. (13) Ioannides, I.; Verykios, X. E. J. Catal. 1993, 143, 175.
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