Nanoparticles and Their Precursors - American Chemical Society

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J. Phys. Chem. C 2007, 111, 6328-6334

RuS2 Nanoparticles and Their Precursors: A Theoretical Approach Francisco J. Tenorio,† Juvencio Robles,‡ Geonel Rodrı´guez-Gattorno,†,§ and David Dı´az*,† Facultad de Quı´mica, UniVersidad Nacional Auto´ noma de Me´ xico, Coyoaca´ n, 04510, Me´ xico D.F., Me´ xico, and Facultad de Quı´mica, UniVersidad de Guanajuato, Noria Alta S/N. 36050, Guanajuato, Guanajuato, Me´ xico ReceiVed: July 12, 2006; In Final Form: January 22, 2007

Electronic structure calculations of the chemical precursors involved in ruthenium disulfide nanoparticle synthesis are performed, finding the structures of minimum energy for the isomers studied. On the basis of the pyrite-type arrangement observed for ruthenium disulfide, a simple finite cluster model for ruthenium disulfide nanoparticle (NP) was employed to explain the observed reactivity of RuS2 NPs. Moreover, we study some finite cluster models for naked RuS2 NPs, with sulfur vacancy defects and interacting with ammonia, dimethylsulfoxide (DMSO), and tetrathiomolybdate molecules.

Introduction Ruthenium disulfide RuS2 is a very interesting material mainly due to its use as a catalyst in different processes. Sulfur release from fossil fuel feedstocks, as well as in refined oil, is mandated since the resulting fuels burn more cleanly and efficiently. A very well-known way to achieve this is by adding hydrogen in the presence of a catalyst to release hydrogen sulfide in a process known as hydrodesulfurization (HDS). Among all transition metal sulfides, RuS2 has been shown to be the most active catalyst1 for the HDS processes. Compared with molybdenum sulfide, a reference catalyst, RuS2 is 13 times more active for the hydrodesulfurization of thiophene2 and 10 times more active for the hydrogenation of biphenyl. The great interest toward RuS2 has aided in catalysts development not only in HDS but in hydrodenitrogenation (HDN) and olephines hydrogenation reaction systems.3-5 Additionally, RuS2 has been used in photoelectrocatalytic processes, where it is found that a photostable narrow band gap semiconductor is able to photooxidize water under infrared illumination.6 Ruthenium disulfide bulk is a pyrite-like crystal whose lattice is described by the Pa3 cubic space group. Usually RuS2 (laurite) is considered to be formed by Ru(IV)7 species as well as sulfide ions S-2. However it has been reported that crystallized RuS2 is diamagnetic,8 and this behavior can only be explained if this species contains only Ru(II). In this way, ruthenium disulfide can be described as a face-centered cube of Ru(II) atoms with disulfide (S2-2) ions. Each Ru atom is bonded to six disulfide ions in an octahedral arrangement. Each atom has three Ru neighbors, and therefore, each S2 species is bonded to six metal neighbors, see Figure 1. Recently Diaz and co-workers9 reported the preparation of RuS2 nanoparticles (NPs) in dimethylsulfoxide (DMSO) colloidal dispersions and assembled thin films, using commercial RuCl3 as chemical reagent. These nanoparticles (NPs) showed interaction with organic and inorganic Lewis bases. During the first * To whom correspondence should be addressed. E-mail: [email protected]. † Universidad Nacional Auto ´ noma de Me´xico. ‡ Universidad de Guanajuato. § Present Address: Cinvestav-IPN, Unidad Me ´ rida, AP 73 Cordemex, CP 97310, Me´rida, Yucata´n, Me´xico.

Figure 1. RuS2 unit cell, showing the Ru(II) and S2-2 ions. Ruthenium atoms are located in an octahedral arrangement and S2-2 ions in pseudooctahedral coordination. Gray balls represent the Ru(II) ions and yellow balls are the sulfur atoms.

step of RuS2 NPs synthesis a color change of the solution from amber to green can be appreciated. This is associated with the releasing of one chlorine equivalent from the ruthenium coordination sphere, according to electrochemical measurements.9 With these data, a general reaction scheme for the chlorine release was suggested, Scheme 1. Additionally, the interactions and surface modification by triethylamine and ammonium tetrathiomolybdate with RuS2 NPs are studied. Several reports about the electronic structure in bulk-simulated RuS2 using different methodologies are found in the literature.10-19 Most of them have been focused in surface reactions taking place at the different exposed planes, modeling the surface defects and suggesting interactions between RuS2 surfaces and sulfur-containing organic molecules such as benzothiophene. However, neither the effect of surface-modifier molecules such as donor charge systems, nor the precursor molecules formed during the synthesis of these nanoparticles is yet well understood. In this work we present, on the first part, the results of a theoretical study about the proposed species in the synthesis of ruthenium disulfide nanoparticles, as suggested recently by Dı´az and co-workers.9 The study of the precursor molecules was considered as an important key toward the control of size and size distribution of small nanoparticles during the synthetic

10.1021/jp064403i CCC: $37.00 © 2007 American Chemical Society Published on Web 04/12/2007

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SCHEME 1: Precursor Species Suggested in the Synthesis of Ruthenium Disulfide Nanoparticles, as Proposed by Diaz and Co-Workers9

process. This is an aspect usually ignored in nanostructures preparations. On the second part of this work, a simple finite cluster model is used to simulate the interactions between RuS2 NPs and surface modifiers to better understand the electronic processes taking place during surface modification of the nanoparticles and the conditions that should be met for electronic coupling between the semiconductor and a molecular modifier. As such, the capping agent molecules were covalently bonded to the surface of RuS2 nanoparticles, while retaining their identity. Methodology. The study about the precursor molecular species in the RuS2 NPs synthesis, assumed to be present in solution before adding sodium sulfide, was carried out using the Gaussian-98 program.20 Full geometry optimizations without symmetry restrictions were done for these precursors. The hybrid B3LYP21 functional was used with the Los Alamos LANL2DZ effective core pseudopotentials with a split valence double-ζ basis set.22 For the ruthenium disulfide nanoparticle model employed, full geometry optimizations were performed at the ab initio HF level of theory with minimal basis set STO-3G23 as well as with density functional theory (DFT) methods, using the VoskoWilk-Nusair (VWN)26 functional and TZ2P basis set.31 The obtained wave functions and energies were improved through single point calculations at the HF/3-21G* level of theory24 and density functional calculations were also performed. We compare the results for three different functionals: (i) Local spin density exchange functional25 with the VWN26 correlation functional. (ii) Becke exchange functional with density gradient corrections27 and Perdew correlation functional with gradients corrections.28 (iii) Finally, the hybrid B3LYP21 functional was also employed. Both, LANL2DZ Los Alamos effective core potentials with split valence double-ζ22 as well as Stuttgart/ Dresden (SDD) potentials29 were employed with HF and each DFT method. Results and Discussion Precursor Molecules. The different species used in this study are shown in Figure 2. Relative energies for the fac- and mer[RuCl3(DMSO)3] isomers are shown in Table 1. According to the experimental study,9 the composition of commercial RuCl3 is assumed to be close to [Ru(III+x+y)Cl3+y(H2O)3-x-y(OH)x], resulting from the partial hydrolysis of RuCl3‚nH2O. Therefore, commercial RuCl3 contains Ru(III) and significant amounts of Ru(IV). Both oxidation states were taken into account to model the precursor species in an octahedral environment for ruthenium. Therefore, [RuCl3(DMSO)3] fac and mer isomers were calculated for Ru(III) as well as fac and mer [RuCl3(DMSO)3]+ for Ru(IV). The optimized structures show a chlorine preferential orientation to form the mer isomers rather than the fac ones. The structures seem to be those with the lower steric interactions and electrostatic repulsion. These structures are expected to be the most stable. However, the small difference in energy for

Figure 2. Structures of ruthenium disulfide nanoparticles’ precursors, showing [RuCl3(DMSO)3] structures (a) for the fac and (b) for the mer isomer as well as [RuCl2(DMSO)4] structures (c) for the cis and (d) for the trans isomer.

TABLE 1: Relative Energies for Each Oxidation State (kcal/mol) in [RuCl3(DMSO)3] Isomers, at the B3LYP/ LANL2DZ Level of Theory isomer fac mer

[RuCl3(DMSO)3]

[RuCl3(DMSO)3]+

4.2 0.0

2.3 0.0

TABLE 2: Relative Energies for Each Oxidation State (kcal/mol) for the [RuCl2(DMSO)4] Isomers Obtained at the B3LYP/LANL2DZ Level of Theory isomer [RuCl2(DMSO)4] [RuCl2(DMSO4)]+ [RuCl2(DMSO)4]+2 cis trans

31.0 0.0

22.9 0.0

>100 0.0

each oxidation state suggests the possibility that the sample may consist of not only one isomer but of a mixture of them. The occurrence of several isomers quite close in energy suggests that the release of chlorine from each molecule could be a complex process, involving several equilibria, with slow chlorine release. The actual process may involve photocatalytic activation and formation of excited states that in principle may require the use of time dependent DFT methods, the equivalent ab initio ones or broken-symmetry approximations.34 Here, we neglect this possibility. Moreover, ligand substitution processes with Ru(III) in strong field are always very slow. To simulate the species obtained after one chlorine equivalent is released, we computed RuCl2(DMSO)4, [RuCl2(DMSO)4]+, and [RuCl2(DMSO)4]+2 clusters. This is in order to consider Ru(II), Ru(III), and Ru(IV) oxidation states. Ru(II) oxidation state is now included based on the fact that RuS2 NPs are made of this one, but we also considering the other two oxidation states occurring in the reaction. These coordination species exhibit preferred structures in which the chlorine atoms occupy trans positions (Table 2). After geometry optimization, Ru(IV) cis isomer resembles a pentacoordinate ruthenium atom more than in a octahedral environment. This is due to the release of one DMSO molecule. Forcing the geometry to preserve octahedral environment, results in a structure 100 kcal/mol higher than the trans isomer. The preference for leaving a pentacoordinate ruthenium atom with three DMSO molecules

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Figure 3. Schematic representation of the finite cluster moiety chosen as a primary model for the theoretical calculation. For better clarity some S2-2 ions were omitted.

Figure 4. Local environment surrounding ruthenium and sulfur atoms in our finite cluster model. Eight sulfur atoms octahedral surround each ruthenium atom, while sulfur atoms are coordinated by three Ru and one S (bridge) atoms, forming a distorted tetrahedron.

and two chlorine ions makes the optimized structure useless for our analysis. Since there is a preference for the trans isomers, once the chlorine is released, it is suggested that a single precursor species is present in solution and therefore we may conclude that this is a homogeneous reaction environment. Nanoparticle-Modifiers Interaction; Choosing the Model. Small nanoparticles are systems in which the surface is responsible for most of the properties. This motivated us to construct a model based in a RuS2 unit cell, considering the surface atoms in it, Figure 3. In this way, we get a system with four ruthenium atoms bonded by disulfide bridges connecting three ruthenium atoms (µ3-S2H) with hydrogen atoms as capping atoms. Moreover, in order to complete all dangling bonds, the octahedral environment in each ruthenium atom is saturated with S2H- ions, three of them on each ruthenium, as is shown in Figure 4. The notation [Ru4(µ3-S2H)4(S2H)12]-8 is useful to describe stoichiometry, charge, and spatial arrangement in our finite cluster model. The optimized structure for this nanoparticle model is presented in Figure 5. The HF-optimized structure presents longer Ru-S distances (2.67 Å) than the one optimized with the local functional VWN and TZ2P basis set where these distances are 2.44 Å in average. Taking this last structure as basic unit, we proceeded to determine the active sites that may react with surface modifiers and solvent molecules. For this structure, a main requirement should be met in order to be considered a good model for reactivity. Ruthenium(II) has a d6 electronic configuration in a strong field (low-spin, t2g7) environment. Therefore, the electronic structure for the HOMO

Tenorio et al.

Figure 5. Proposed finite cluster model, [Ru4(µ3-S2H)4(S2H)12]-8 unit, consisting of four ruthenium centers in a octahedral environment, with disulfide bridges between the ruthenium atoms and three S2H- groups capping each ruthenium. Gray balls represent the Ru(II) ions, yellow balls are the sulfur atoms, and hydrogen atoms are in white.

should mainly consist of d-type orbitals centered in the ruthenium atoms. Additionally, the LUMO must show active sites in the same centers, arising from empty d orbitals. Having obtained the optimized structures, the search for active sites was performed using HF/3-21G* calculations as well as VWN, BP86, and B3LYP calculations. For density functional calculations, LANL2DZ and SDD effective core potentials were used. The use of 6-311G** basis set for sulfur and hydrogen with LANL2DZ core effective potentials for ruthenium did not show a significant improvement in the results but result in an underestimation for the ruthenium orbitals contribution. VWN and BP86 showed very similar results, suggesting that the choice of either a LDA or GGA functional was not a determinant factor in our calculation. Moreover VWN, BP86, and B3LYP calculations provided very similar results; therefore we show the graphics corresponding to B3LYP/LANL2DZ. HF results for the wavefunctions obtained with 3-21G* basis set, LANL2DZ, and SDD pseudopotentials were consistent, qualitatively, with the results from the DFT methods. The contribution to the frontier orbitals consists mainly in d-type orbitals, as seen in Figures 6 and 7 for the highestoccupied molecular orbital (HOMO) and the next two closer occupied orbitals. This can be clearly appreciated in Figure 6. Moreover, external sulfur atoms close to the ruthenium centers with d contribution appear to be active centers. For the lowest-unoccupied molecular orbital (LUMO) orbital and the two closer unoccupied orbitals, one can see the d-type orbital contribution. The analysis of these orbitals, show a large contribution of the dx2-y2 and dz2 orbitals, eg. Moreover, the contribution of neighbor sulfur atoms is almost negligible, suggesting that ruthenium atoms are the main active sites for attack by Lewis bases species. Physical and chemical behavior of a chemical system can be described by conceptual DFT reactivity indexes such as the chemical hardness (η)32

η)

()

1 ∂µ 2 ∂N

) V(r)

( )

1 ∂ 2E 2 ∂N2

(1)

V(r)

An operational and approximate definition of η involves the ionization potential (IP) and electron affinity (EA) of the system. Within the validity of Koopman’s theorem,33 the frontier orbital energies are given by

-HOMO ) IP; -LUMO ) EA

(2)

And, within this approximation, we can write

η ) (LUMO - HOMO)/2

(3)

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Figure 6. From left to right: HOMO, HOMO-1, and HOMO-2 orbital maps for the [Ru4(µ3-S2H)4(S2H)12]-8 unit, from the B3LYP/LANL2DDZ calculation. It is clear the d-type (t2g) orbitals contribution (centered in ruthenium atoms) to these occupied orbitals.

Figure 7. From left to right: LUMO, LUMO+1, and LUMO+2 orbital maps for the [Ru4(µ3-S2H)4(S2H)12]-8 unit from the B3LYP/LANL2DZ calculation. It is clear the d orbitals (eg) contribution in the ruthenium atoms for these virtual orbitals.

TABLE 3: HOMO and LUMO Energy (eV) Obtained for the [Ru4(µ3-S2H)4(S2H)12]-8 Model Method of calculation

HOMO energy (eV)

LUMO energy (eV)

Eg (eV)

BP86/LANL2DZ BP86/SDD VWN/LANL2DZ VWN/SDD BLYP/LANL2DZ BLYP/SDD B3LYP/LANL2DZ B3LYP/SDD HF/3-21G* HF/LANL2DZ HF/SDD

13.86 13.84 14.26 13.31 14.19 14.09 13.11 13.14 10.65 10.36 10.45

14.78 14.73 14.92 14.18 15.13 14.69 15.32 14.89 18.8 18.26 16.03

0.92 0.89 0.66 0.87 0.94 0.6 2.21 1.75 8.15 7.90 5.58

The energetic arrangement in a number of frontier orbitals for different level of theory calculations on the [Ru4(µ3-S2H)4(S2H)12]-8 model is shown in Table 3. LUMO energies are included also for DFT results for completeness purposes. The energy difference (gap) between HOMO and LUMO is compared with the experimental colloidal dispersion band gap (3.6 eV).9 HF results show clearly the highest values for band gap; however, the B3LYP/LANL2DZ result (2.21 eV) is the closest to the experimental value, as shown in Table 3. Moreover, it is expected that the band gap of the semiconductor will always be larger in magnitude than the corresponding gap in the macrocrystalline compound, due to the implicit confinement created because of our finite cluster model. In this way, HF results are in good agreement with observed gaps. According to previous reports,14-19 the presence of surface defects such as sulfur vacancies is necessary to generate active sites. In our model, such vacancies are not necessarily mandatory, due to the fact that we found active sites even without

defects in the model, and because it is a finite cluster rather than an extended, periodic, system. As the occurrence of surface defects cannot be discarded, we proceeded then to model surface defects in our nanoparticle. To achieve this, we started from a [Ru4(µ3-S2H)4(S2H)11]-7 cluster. This model has an outer S2H group vacancy in a ruthenium center, but the central cubic core is kept. We did not model a ruthenium vacancy mainly because the central cubic structure of the model would be destroyed. After geometry optimization, following the previously described methodology, it could be appreciated that the structure was preserved. All the results obtained under DFT gave similar results and here are shown only for one of them. Through frontier orbital analysis, it can be observed, Figure 8, that the larger HOMO contribution arise from completely capped ruthenium atoms and the sulfur atoms close to it. LUMO inspection shows that ruthenium atom with a S2H vacancy is the most susceptible to participate in reactions with Lewis bases (donor charge agents). Experimentally, nanoparticles are synthesized in DMSO, a good electron-donor solvent. After, they interact with Lewis bases such as triethylamine and ammonium tetrathiomolybdate, which cause surface modification at the NP. Once we model the surface defects in nanoparticles, we may identify this ruthenium atom as an active site to react with donor charge agents. Therefore, we may model the interaction between one nanoparticle prototype and solvent molecule (DMSO) as well as tetrathiomolybdate ion. Moreover, the interaction between a single ammonia molecule (a simple model for triethylamine) and the NP was also modeled. Optimized structures are showed in Figure 9. Natural and Mulliken charges are showed in Table 4. Since there are not equivalent ruthenium atoms in modified nanopar-

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Figure 8. Left, HOMO; right, LUMO for the [Ru4(µ3-S2H)4(S2H)11]-7 system with a vacancy defect at the B3LYP/LANL2DZ level of calculation.

TABLE 4: Condensed Natural and Mulliken Charges for Pure and Modified Nanoparticles. Mulliken Results Are Shown in Parentheses. Nonaffected and Affected Atoms by Substituent Group Are Denoted by Ru1 and Ru2, Respectively systems naked NP NP-NH3

atoms Ru -0.25 S 0.01 (-0.48) (-0.07) Ru1 -0.26 S 0.05 N -1.03

(-0.54) Ru2 -0.14 (-0.5) NP-DMSO Ru1 -0.26 (-0.66) Ru2 -0.09 (-0.60) NP-MoS4 Ru1 -0.24 (-0.9) Ru2 -0.17 (-0.43)

(-0.04) (-0.84) S 0.07 C -0.84 S(DMSO) 1.19 O -0.89 (0.02) (-0.81) (0.83) (-0.61) S 0.06 Mo 0.06 S (MoS4) -0.29 (-0.05) (-0.08) (-0.31)

TABLE 5: Natural Atomic Population for the Models Studied naked NP Figure 9. Optimized structures obtained at the VWN/TZ2P level of calculation for (a) NH3-NP, (b) DMSO-NP, and (c) MoS4-NP interaction.

ticles (with NH3, DMSO, and MoS4), the results show the extreme obtained values: the non-affected (Ru1) and the affected one (Ru2) with the NH3, DMSO, or MoS4 substituent group. It can be seen that, no matter the method employed is, the ruthenium atoms have total negative charge. If we were considering only Mulliken population analysis, these spurious results could not sound so strange. However, we must consider some important features concerning to the studied systems. The first of all and probably the most important is that we are considering charged species. This also leads to a filling of the ruthenium d states. Moreover, this is an indication of the induced electron-acceptor character of ruthenium atoms that can be related with the high HDS activity of RuS2.19 Natural population analysis is showed in Table 5. These results suggest that the donor-charge agents such as NH3 and DMSO affect mainly to d states in ruthenium (R1), through the increase of the electronic population in these systems. The small decreasing in d popula-

NH3 substituted

DMSO substituted

MoS4 substituted

atoms

s

p

d

Ru S Ru1 Ru2 S N Ru1 Ru2 S C S(DMSO) O Ru1 Ru2 S Mo S(MoS4)

0.32 1.68 0.32 0.31 1.68 1.45 0.33 0.31 1.68 1.22 1.45 1.80 0.32 0.31 1.68 0.35 1.87

0.03 4.30 0.03 0.04 4.26 4.56 0.03 0.07 4.23 3.62 3.33 5.07 0.09 0.03 4.25 0.04 4.42

7.86 7.89 7.76 7.87 7.67

7.80 7.80 5.53

tion for the Ru2 atom can be explained as a relative loss of reactivity compared with the other active centers. However, due to the position of surface modifiers in this site, it is immediately discarded seen as reactive site. As can be seen, the presence of available sites for electron donors as well as its availability for back donation is a very important feature for this kind of materials.

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Figure 10. Orbital energy as a function of orbital number, for different nanoparticle-surface modifier model species, obtained at the HF/3-21G*// VWN/TZ2P level of calculation.

The comparison for the 10 highest occupied molecular orbitals and the 10 lowest unoccupied ones at the HF/3-21G*//VWN/ TZ2P level of theory, Figure 10, [HF/SDD//VWN/TZ2P] qualitatively shows that the interaction between the nanoparticle and a soft base such as DMSO solvent molecules generates a softer species (gap ) 7.79 eV) [4.03 eV] than the naked nanoparticle model (gap ) 8.15 eV) [5.58 eV]. Moreover, if the interaction is with a hard base such as ammonia, the resulting structure is the hardest for the nanoparticle with donor charge agents (gap ) 8.41 eV) [5.51 eV]. Tetrathiomolybdate ion generates the softest species observed (gap ) 6.37 eV) [3.50 eV]. It is clear then that the influence of the surface modifier could generate harder or softer species, depending on its nature, being a decisive factor for the electronic structure of this large surface area system. It is important to design new NP systems that, depending on its size and whether they are naked or conveniently surface-modified, exhibit the appropriate band gap to achieve the desired chemical performance.

the resulting structure is just a little bit softer than the naked nanoparticle in the following hardness order: nanoparticle > nanoparticle-ammonia > nanoparticle-DMSO > nanoparticle-MoS4. Finally, the electronic structure and the properties of the RuS2 nanoparticles strongly depend on the nature and the effect of the surface modifier and the properties of the atoms exposed at surface. The resulting new materials can be designed in order that, depending on its size and whether they are naked or conveniently surface-modified, their properties can be tuned to exhibit the appropriate band gap or to achieve the desired chemical performance.

Concluding Remarks

Supporting Information Available: The analysis of the distances for the surface modifier species is included. Rootmean-square data are also presented. This material is available free of charge via the Internet at http://pubs.acs.org.

The precursor species in the synthesis of ruthenium disulfide nanoparticles were studied using DFT methods for computing the minimum energy structures. According to the results, the precursor species are initially formed by isomers very close in energy, suggesting a mixture of isomers in the reactant. These generate a complex system that slowly releases one chlorine atom per molecule. Once the chlorine atom per molecule is released, our computed results suggest that the coordination precursor species present in the reaction media, adopt structures in which the chlorine atoms occupy the trans positions. A simple finite cluster was found to be a useful model to find and identify active sites in ruthenium disulfide nanoparticles. Our DFT calculations show that ruthenium atoms are those mainly involved in the observed reactivity for this nanoparticle. The occurrence of sulfur surface defects was not found as the only factor responsible for this reactivity, but it should increase it. The interaction between nanoparticle and surface modifier molecules was also studied. It was observed that the interaction between the nanoparticle and a soft base such as DMSO solvent generates a softer product than the naked nanoparticle. Moreover, if the interaction is with a hard base such as ammonia,

Acknowledgment. FJT thanks to DGAPA-UNAM for a postdoctoral fellowship. We acknowledge financial support from projects IN100398, IN105102-2, and IN110405 PAPIIT-DGAPAUNAM and CONACyT-SEP E-43662-F and SEP-2003-C0243453. DGSCA-UNAM is also acknowledged for providing computer time.

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