Gold Grafted to Mesoporous Silica Surfaces, a Molecular Picture

(10) The silica slab consists of more than 120 atoms (Si27O54·13H2O), which enables us to model a correct representation of a hydrated silica surface...
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J. Phys. Chem. C 2009, 113, 13855–13859

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Gold Grafted to Mesoporous Silica Surfaces, a Molecular Picture A. Wojtaszek,†,‡,§ I. Sobczak,‡ M. Ziolek,‡ and F. Tielens*,†,‡ UPMC UniV Paris 06, UMR 7197, Laboratoire de Re´actiVite´ de Surface, Tour 54-55, 2e`me e´tage - Casier 178, 4, Place Jussieu, F-75005 Paris, France, CNRS, UMR 7609, Laboratoire de Re´actiVite´ de Surface, Tour 54-55, 2e`me e´tage - Casier 178, 4, Place Jussieu, F-75005 Paris, France, and Adam Mickiewicz UniVersity, Faculty of Chemistry, Grunwaldzka 6, 60-780 Poznan´, Poland ReceiVed: April 21, 2009; ReVised Manuscript ReceiVed: June 1, 2009

The adsorption of HAuCl4, the typical precursor for gold grafting on metal oxide surfaces, is investigated on amorphous silica support. The adsorption of the precursor is investigated as a function of its chlorinated form, depending on the pH, using periodic DFT. The results are supported with experimental data based on catalytic probe reactions (acetonylacetone cyclization and oxidation of methanol). The location of chloride ions neighboring hydroxyls gives rise to generation of Brønsted basicity demonstrated by the methylcyclopentenone formation from acetonylacetone and CO2 from methanol. The most stable grafting complex is formed by the AuCl(OH)3- precursor form on a chloride-free silica surface. The presence of Si-Cl groups at the surface destabilizes the grafting complex. Introduction

Computational Details

Gold-containing catalysts are a unique class of materials, which attracted a lot of attention due to their unexpected behavior. Until recently, gold was considered to be a relatively uninteresting element in chemistry but was very much appreciated as decoration. Now, every day new facets of its chemistry are discovered and a very rich chemistry of gold is revealed, in contradiction with its noble reputation. A compilation of the gold chemistry is presented in the reviews of Pyykko¨.1-3 Haruta demonstrated the low-temperature oxidation of CO4,5 and Hutchings the hydrochlorination of acetylene to vinyl chloride.6,7 As a catalyst, gold is dispersed as nanosized particles supported on metal oxide surfaces. Two classes of oxide supports are used, reducible and inert ones. Silica belongs to the inert supports and shows high activity when nano size gold particles are highly dispersed in mesoporous silica.8

The mesoporous material MCM-41 is modeled using an amorphous hydrated silica slab, designed, described, and characterized by our group.10 The silica slab consists of more than 120 atoms (Si27O54 · 13H2O), which enables us to model a correct representation of a hydrated silica surface (Figure 1). The same model has been used with success in studies concerning vanadium oxide grafting11 and amino acid adsorption.12,13 Experimentally, the HAuCl4 molecule is adsorbed on the surface under different forms depending on the pH of the solution. The gold precursor is dechlorinated forming AuCln(OH)4-n species following the reaction in aqueous solution:14,15

AuMCM-41 hexagonally ordered mesoporous silica containing gold exhibited unique properties that have been described for the first time very recently.9 Gold species surrounded by chloride were generated in silicate MCM-41 matrix prepared by one-step synthesis with silicon (sodium silicate) and gold (HAuCl4) sources. This sample revealed basic character resulting from the presence of chloride. Moreover, this catalyst exhibited an excellent performance in the electron transfer to oxygen in NO-SCR reaction with propene. The role of chloride anions as a promoter in this behavior was evidenced. The above findings have been an inspiration to the calculations undertaken in this work because a molecular picture of the catalysts is still lacking. The aim of this work is to examine the interactions of gold precursors on amorphous silica surfaces in the presence of chloride using first principle calculation methods in combination with catalytic probe reactions. * To whom correspondence should be addressed. E-mail: frederik.tielens@ upmc.fr. † UPMC Univ Paris 06, UMR 7197, Laboratoire de Re´activite´ de Surface. ‡ CNRS, UMR 7609. § Adam Mickiewicz University, Faculty of Chemistry.

+ HAuCl4 + H2O f AuCl4 + H3O AuCl4 + H2O f AuCl3(OH) + HCl

AuCl3(OH)- + H2O f AuCl2(OH)2 + HCl

(1)

AuCl2(OH)2 + H2O f AuCl(OH)3 + HCl AuCl(OH)3 + H2O f Au(OH)4 + HCl

Simultaneously one can have surface silanol groups, which transform in Si-Cl groups due to the liberation of Cl- ions. + HAuCl4 + (Si - (OH)m) f AuCl4 - (Si - (OH)m - H )

(2) where m is the number of surface silanols, restricted to three in our calculations (Figure 2). Silanol nests of four silanol groups are too improbable for the surface silanol density experimentally observed (5 nm-2). The brute formula of the surface is written in italics. Combining reactions 1 and 2, Cl- groups in HAuCl4 can exchange with OH- groups from the aqueous solution or from the silica surface, forming AuCln(OH)4-n species. In our model, AuCl4- was considered as the starting species instead of the neutral HAuCl4. To keep the whole unit cell neutral in our

10.1021/jp9036815 CCC: $40.75  2009 American Chemical Society Published on Web 07/14/2009

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Figure 1. Silica model surface used in the calculations, showing the silanol nest (group of three silanol groups).

Figure 2. Schematic representation of the model site used in the presented calculations. The A and B sites are exchanged by OH or Cl groups.

calculations, the missing proton was placed on a silanol group at the bottom side of the amorphous silica slab. In general one can thus write: + AuCl4 - (Si - (OH)m - H ) + qOHaq f

AuCl4-qOHq - (Si - (OH)m - H+) + qClaq

(3)

where q is the number of chloride groups exchanged with OHgroups from the aqueous solution. The leaving Cl- group can be left in the aqueous solution (eq 1) or exchanged with a surface silanol: + AuCl4 - (Si - (OH)m - (Cl)3-m - H ) f

AuCl4-rOHr - (Si - (OH)m-r - (Cl)3-m+r - H+)

(4)

where r is the number of chloride groups exchanged with OHgroups from surface. As a starting geometry for the geometry optimizations, the precursor was placed in the neighborhood of a silanol nest at the silica surface (Figure 1). On the basis of the possible reactions shown above, a scheme is presented in Figure 2 summarizing the different situations, that is AuA4- species, with A ) Cl or OH, corresponding to the gold precursor used experimentally, are adsorbed on the

hydrated silica surface containing Si-B groups, with B ) Cl or a OH. All combinations were screened giving in total 20 structures. All calculations are performed using ab initio plane-wave pseudopotential calculations implemented in VASP.16,17 The Perdew-Burke-Ernzerhof (RPBE) functional18-20 has been chosen to perform the periodic DFT calculations with an accuracy on the overall convergence tested elsewhere.21-24 The valence electrons are treated explicitly and their interactions with the ionic cores are described by the Projector Augmented-Wave method (PAW),25,26 which allows us to use a low-energy cutoff equal to 400 eV for the plane-wave basis. The integral over the first Brillouin zone is performed using the gamma point. The position of all of the atoms in the super cell are relaxed in the potential energy determined by the full quantum mechanical electronic structure until the total energy differences between the loops decrease below 10-4 eV. Experimental Details 1. Catalysts’ Preparation. Mesoporous molecular sieve of MCM-41 type containing gold was synthesized by the hydrothermal method in the same manner as conventional MCM-41.27,28 Sodium silicate (27% SiO2 in 14% NaOH, Aldrich) was used as a silicon source and cetyltrimethylammonium chloride (Aldrich) was the surfactant template. The solution of hydrogen tetrachloroaurate(III) hydrate (HAuCl4 - Johnson Matthey, UKUSA) as the source of gold was subsequently added into the formed gel (molar gel ratios ) 1 SiO2: 0.75 NaOH: 6.5 CTMACl: 103.75 H2O). The mixture was stirred for 0.5 h. The pH was decreased from 12.5 to 11 with HCl, after which distilled water was added. The gel was loaded into a stoppered polypropylene (PP) bottle and heated without stirring at 373 K for 24 h. The mixture was then cooled down to room temperature and the pH level was adjusted to 11 with HCl. This reaction mixture was heated again to 373 K for 24 h. The assumed Si/Au atom ratio was 256 (corresponding to 1 wt % of Au). The resulting precipitated product was washed with distilled water, dried in air at ambient temperature, and the template in the catalysts was removed by calcination at 823 K, 2 h in helium flow, and 14 h in the air under static conditions. 2. Acetonylacetone Cyclization/Dehydration. The catalysts were tested for acetonylacetone (AcAc) cyclization as a probe reaction. A tubular, down-flow reactor was used in experiments that were carried out at atmospheric pressure, using nitrogen as the carrier gas. The catalyst bed (0.05 g) was first activated for 2 h at 723 K under nitrogen flow (40 cm3 min-1). Subsequently, a 0.5 cm3 of acetonylacetone (Fluka, GC grade) was passed continuously over the catalyst at 623 K. The substrate was delivered with a pump system and vaporized before being passed through the catalyst with the flow of nitrogen carrier gas (40 cm3 min-1). The reaction products were collected for 30 min downstream of the reactor in the cold trap (solid CO2) and analyzed by gas chromatography (GC 8000 Top equipped with a capillary column of DB-1, operated at 353 K, attached to a FID). 3. Methanol Oxidation. The methanol oxidation reaction was performed in a fixed-bed flow reactor. The catalyst (0.02 g), with a size fraction of 0.5 < θ < 1 mm, was placed into the reactor. The samples were activated in helium flow (40 cm3 min-1) at 723 K for 2 h. The rate of heating was 15 K/min. Next, the temperature decreased to the temperature of the reaction (523 K). A 40 mL/min He/O2/MeOH (88/8/4 mol %) flow was used as a reactant mixture. The reactor effluent was analyzed using an on line gas chromatograph (GC 8000 Top

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TABLE 1: Adsorption Energy for the Different Forms of the Gold Precursor on Different Cl-Containing Silica Surfaces, Values in eV precursor silica silica-Cl silica-ClCl silica-ClClCl

AuCl4 AuCl3OH- AuOH2Cl2- AuClOH3- AuOH4-2.80 -2.06 -1.69 -1.59

-2.42 -1.61 -1.93 -1.87

-2.79 -1.92 -1.62 -1.62

-3.10 -2.38 -2.06 -1.57

-0.09 0.70 1.04 2.15

equipped with a capillary column of DB-1 -FID detector and Porapak Q and 5A molecular sieves columns-TCD detector). Helium was applied as a carrier gas. Results and Discussion The adsorption of the gold precursor was studied systematically as a function of the chloride content (Table 1). The form of the precursor at pH 9-14 is Au(OH)4-. The adsorption of this form on the silica surface is slightly exothermic, thanks to the formation of hydrogen bonds between the precursor OHgroups and the surface silanols. The presence of Cl on the surface destabilizes the adsorption complex and the adsorption energy becomes endothermic. The Au-OH · · · Cl-Si interactions are thus a destabilizing agent. The next form of the precursor, AuCl(OH)3-, appears between pH 6-9. Compared with the first form, the adsorption energy increases considerably (becomes more negative). Adsorption energies around -2.50 eV are calculated, keeping the same trend in relation with the Cl content on the surface as in the former case. The interaction is mainly due to H bonds between OH groups of the precursor and the silica surface. The presence of a Cl group on the Au atom is expected to be responsible for the enhanced interaction. The Cl group (electron acceptor group) on the gold atom polarizes the precursor and the remaining OH groups, which improves the H-bond formation, with the surface silanols. Going down the pH scale, the next form of the precursor is AuCl2(OH)2-. The adsorption energy increases (becomes less negative) independently of the Cl content of the silica slab. The increase does not become as important as that in the Au(OH)4case. This result indicates that the presence of Cl on the precursor introduces two effects that are in competition with each other. Besides improving the polarizability of the precursor molecule, it also has a repulsive effect with the surface due to the Cl-Cl repulsion. This can be seen in Figure 3 because the presence of Si-Cl groups at the surface accentuates this effect.

Figure 3. Adsorption energy for the different forms of the gold precursor calculated on different Cl-containing silica slabs. The precursor form is related to the pH in an aqueous solution. Values are in eV.

Figure 4. Optimized geometry of the most stable precursor form adsorbed at the silica surface.

Increasing the Cl content further, as is the case for the precursor form between pH 4-6, the adsorption energy decreases further, reaching a maximum (worst adsorption) for the low Cl-containing silica slabs (pure SiOH slab and Cl slab). For high Cl-containing slabs, this maximum was reached before (vide infra). Two trends are observed in Figure 3 depending on the Cl content on the silica slab. The high Cl-content silica slab reaches earlier the maximum; increasing further the Cl content, the adsorption energy does not improve considerably nor does it become worse. When the total number of Cl groups exceeds four, the repulsive effect between the Cl groups of the precursor and the silica slab seems to have reached its maximum. For the complete chlorinated form of the precursor, as it is found at low pH, the adsorption energy is intermediate. The adsorption energy improves slightly for the low Cl-containing surface and decreases slightly for the high Cl-containing slabs. This result indicates that an Au-Cl · · · HO-Si interaction is stronger than a Au-OH · · · Cl-Si one. In summary, the adsorption trend as a function of the Cl content is dominated by two attractive and two repulsive interactions. The attractive interactions: Au-OH · · · HO-Si (improved when there is a Cl group on the Au atom) and the Au-Cl · · · HO-Si, the latter interaction being stronger than the former. The repulsive interactions: Au-Cl · · · HO-Si and Au-Cl · · · Cl-Si, the latter repulsion being weaker than the former. The form of the precursor that balances the repulsive and attractive forces optimally is the AuCl(OH)3- form of the precursor (Figure 4). This result is what experimentally is observed. Indeed, the optimum pH for high activity for gold grafting was found to be 9, at which the gold precursor HAuCl4 is anionic and which has almost all its chloride removed (i.e., AuCl(OH)3-).15 There is some direct and indirect experimental evidence for the connection of gold with chloride in AuMCM-41 material prepared in one-pot synthesis from the sources of silicon (sodium silicate) and gold (HAuCl4) and cetyltrimethylammonium chloride as a template. 1. The Presence of Chlorine on the Surface of AuMCM41. The presence of chloride ions in the surrounding of gold centers in the mesoporous AuMCM-41 catalyst was detected by the TOF-SIMS method on the basis of the AuCl-/Auintensity ratio, which gives information about the amount of gold bonded to or surrounded by chloride ions. These results have been published in ref 9. A combination of TOF-SIMS measurements and the DTA/DTG analyses allows us to suggest

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TABLE 2: Catalytic Activity of AuMCM-41 in AcAc Cyclization and MeOH Oxidation AcAc conversion at 623 K, %

MCP/ DMFa

MeOH conversion at 523 K

CO2 selectivity, %

31

104

52

20

a

MCP, methylcyclo-pentenone; DMF, dimethyl-furan.

the mechanism of Au-Cl formation. It has been proposed that the cationic surfactant (cetyltrimethylammonium cations) interacts with [AuCl4]- ions from HAuCl4 and stabilizes Au-Cl species. Taking into account the results of first principle calculations presented above, one can state that not [AuCl4]but rather AuCl(OH)3- takes part in this interaction. During calcination, the surfactant is eliminated whereas the AuCl(OH)3species became grafted onto the walls of the MCM-41 material. It indicates that the gold precursor, HAuCl4, is the source of chloride ions on the surface of AuMCM-41. One should point out that in TOF-SIMS study, ions indicating Si-Cl bonding are not emitted, confirming the theoretical results presented above. Au-OH species are not emitted either but probably they are not stable enough for emission together with AuCl, which is more stable. 2. Indirect Evidence for the Presence of Cl in Cl-Au-OH Species - The Catalytic Activity. AuMCM-41 sample revealed high activity in acetonylacetone (AcAc) cyclization to MCP (methylcyclo-pentenone) showing basic character of the surface.9 It is often reported that OH- groups are responsible for these active sites.29,30 To explain the source of basicity in one-pot synthesized AuMCM-41, gold-impregnated MCM-41 material was studied for comparison. Both of the catalysts exhibit almost the same amount of OH groups as judged from the IR spectra (∼3740 cm-1). The main difference between Au/MCM-41 (impregnated sample) and AuMCM-41 (one-pot synthesis) is due to the various amount of chloride, which is approximately 1 order of magnitude higher for AuMCM-41 than for Au/MCM41.9 Taking this into account, there is no doubt that the presence of Au-Cl species in the neighbor of hydroxyls is responsible for the very high Brønsted basicity of AuMCM-41, leading to a very high MCP/DMF ratio (>100). DMF, which is formed on Brønsted acid centers, was produced in very low amounts, whereas selectivity to MCP was very high (Table 2). The observed difference in conversion of AcAc (from 35% for the impregnated sample to 38% for one-pot synthesized AuMCM-41) is negligible and could be caused by small differences in the texture parameters of the materials and the size of Au metal particles.9 Interestingly, impregnated sample reveals acidic properties. Thus, the behavior of OH groups changes from acidity to basicity when chloride ions are neighboring hydroxyls. Moreover, the presence of basic sites on the surface of AuMCM-41 was deduced from CO2 selectivity in MeOH oxidation reaction.31 Similarly, to the AcAc cyclization reaction the activity and selectivity of AuMCM-41 sample in this reaction was compared with Au/MCM-41 material. AuMCM-41 showed much higher selectivity to CO2 (20% on AuMCM-41 and 8% on Au/MCM-41). This behavior indicates that the presence of Au-Cl species in AuMCM-41 enhances the basicity responsible for CO2 selectivity. Moreover, the activity of the sample in which Au was introduced during the synthesis AuMCM-41 is much higher than that of the impregnated materials (6% for the impregnated sample and 52% for AuMCM-41 at 523 K). Total oxidation to CO2 occurs according to the radical mechanism, which requires easy electron transfer.32 Such

electron transfer is necessary also in the other redox processes. Therefore, excellent performance of AuMCM-41 catalyst in the electron transfer to oxygen was found in the NO-SCR reaction with propene. FTIR studies after propene adsorption and oxygen admission showed the total oxidation of propene to CO2.9 Taking into account the above TOF-SIMS evidence for the presence of chloride in this catalyst, one can conclude that chloride ions play the role of promoter taking part in the electron transfer during oxidation of propene with oxygen to CO2 without the presence of NO molecules. On the other hand, exposure of AuMCM-41 pretreated with propene and oxygen to NO produces intense bands from NO2- species chemisorbed on gold.9 The production of nitrites proves the presence of active oxygen ions, which could result from the electron transfer from chloride to oxygen. Conclusions The grafting mechanism of gold to amorphous silica is investigated using first principle calculation techniques and discussed by means of probe reactions. The presence of chloride ions at the silica surface and in the precursor molecule is investigated in detail. It is found that the grafting of gold to the silica surface occurs through H bond complexes. The formation of multiple H bonds stabilizes the grafting, which is the result of a subtle interplay between attractive Au-OH · · · HO-Si (improved when there is a Cl group on the Au atom) and Au-Cl · · · HO-Si interaction, and repulsive Au-Cl · · · HO-Si and Au-Cl · · · Cl-Si interactions. The Cl-Au-OH · · · HO-Si interaction being crucial to graft the gold complex to the silica surface. Such interaction changes the catalytic behavior of OH groups giving rise to basicity observed experimentally in test reactions (AcAc cyclization and oxidation of MeOH). The most stable grafting complex is formed for the AuCl(OH)3- precursor form on a chloride-free silica surface. The presence of Si-Cl groups at the surface destabilizes the grafting complex. The absence of Si-Cl species in TOF-SIMS experiments is in agreement with the theoretical result. The results from the catalytic activity indicate that the presence of Cl on the silica support originates from Au-Cl species, also in line with the presented theoretical results. The presence of Au-OH species could not yet be evidenced experimentally. Two scenarios are possible to explain this: a) the Au-OH are indeed present but due to the low stability it is very difficult to detect them with TOF-SIMS, or b) the calcinations process modified the grafted precursor so that the Au-OH groups are eliminated. The next step in the characterization of the gold silica catalysts is the effect of calcination on the grafted precursor and the formation of metallic gold clusters grafted at the surface. Acknowledgment. The computation facilities are provided by IDRIS, CINES, and by CCRE (Universite´ Pierre et Marie Curie). A significant part of the computing time was provided by the Barcelona Supercomputing Centre. A.W. thanks COST action D36, WG No D36/0006/06, and the Polish Ministry of Science (Grant No. 118/COS/2007/03) for financial support. References and Notes (1) Pyykko¨, P. Angew. Chem., Int. Ed. 2004, 43, 4412. (2) Pyykko¨, P. Inorg. Chem. Acta 2005, 358, 4113. (3) Pyykko¨, P. Chem. Soc. ReV. 2008, 37, 1967. (4) Haruta, M.; Kabayashi, T.; Samo, H.; Yamada, N. Chem. Lett. 1987, 405. (5) Haruta, M.; Tsubota, S.; Kobayashi, T.; Kageyama, H.; Genet, M. J.; Delmon, B. J. Catal. 1993, 144, 175. (6) Hutchings, G. J.; Joffe, R. Appl. Catal. 1986, 20, 215. (7) Hutchings, G. Gold Bull. 2004, 37, 3.

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