MOR Catalyst: ONIOM-PM6 Study

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Interaction of CO Molecule with Au/MOR Catalyst: ONIOM-PM6 Study, Active Sites, Thermodynamic and Vibrational Frequencies Anibal Sierraalta,* Rafael An˜ez, Lenin Diaz, and Roberto Gomperts† Laboratorio de Quı´mica Computacional, Centro de Quı´mica, Instituto Venezolano de InVestigaciones Cientı´ficas, Apartado 21827, Caracas 1020-A, Venezuela ReceiVed: December 8, 2009; ReVised Manuscript ReceiVed: May 13, 2010

ONIOM calculations have been carried out to determine geometries, adsorption energies, and vibrational frequencies of CO on a model for Au-exchanged mordenite catalysts, Au/MOR. The CO-calculated vibrational frequencies (υCO) are in good agreement with the reported experimental values. We proposed to interpret the frequency results. CH3COCH3 and CH3SH adsorption enthalpy calculations on Au/MOR model show that the Au/MOR catalyst behaves like a soft acid according to Pearson’s rule. A higher structural deformation degree of the mordenite was found in the calculations with PM6 than with universal force field approach approach. A new pseudopotential (ACEP-121) was developed to improve the Au-Au distance, and Au ionization potential. 1. Introduction Zeolitic materials play an important role as catalytic materials in petroleum refining and the petrochemical industry. Zeolites are interesting host materials due to their ability to disperse transition metals species inside their internal pores and the possibility of controlling the particle size.1-3 Recently, Au has attracted attention in heterogeneous catalysis as well as in colloidal chemistry and nanoparticles systems. The discovery of unique activity of gold nanoparticles in CO oxidation at low temperatures1,4,5 has stimulated the research on gold catalysts. Well-dispersed Au+ species inside NaY and ZSM-5 zeolites exhibit high activity for chemisorption of NO and CO and direct NO decomposition reactions.6-8 In addition, Au+ and Au0 sites in Au/ZSM-5 and Au/mordenite are considered responsible for CO activation and the water gas shift (WGS) reaction at low temperature.9,10 To explain the catalytic activity of Au in zeolites, different species with different oxidation states and structures have been proposed in the literature. Ichikawa et al.9,11 using CO adsorption and Fourier transform IR (FTIR) to study Au/NaY, Au/Namordenite, and Au/Na-ZSM-5, concluded that Au+ is the dominant active site on which the reactions take place. Sachtler et al.12 using of FTIR, X-ray diffraction (XRD), and COtemperature-programmed reduction (TPR), analyzed the Au/ MFI system and concluded that the gold is present mainly as Au+ and Au3+. Other authors5,13-15 have proposed that charged (Aunδ+) or neutral (Aum) gold nanoparticles inside the pores of zeolites are the responsible species for the observed activity. Despite experimental studies, many key questions remain unanswered: Where are located the Au+ ions or the Aunδ+ species into the zeolite? Is Au+ or Aunδ+ the main species that explains the CO IR bands? Is it possible to have gold polycarbonyls species inside the zeolite? Are the gold sites hard or soft acids? In this work, we use the ONIOM methodology * To whom correspondence should be addressed. E-mail: [email protected]. † SGI Mailstop DER-2077, 46600 Landing Parkway, Fremont, California 94538.

to analyze the physical chemistry properties of Au supported on Mordenite in order to understand the nature of the Au/MOR active sites. 2. Computational Details 2.1. Basis Sets and PM6. All geometry optimizations, energy, and frequency calculations were performed using Gaussian-03 program (G03) version RevD.02.16 The lower energy structures were obtained using the two-layer ONIOM2 and three-layer ONIOM3. Universal force field approach (UFF) with no charge assigned to atoms and the PM6 semiempirical method17 were employed for the low-level and medium level calculations, respectively; high-level, calculations were performed using the density functional theory (DFT) approach (B3LYP) with the LANL2DZ basis set and its corresponding pseudopotentials for H, Si, Al, and O atoms belonging to the mordenite model. The small-core Los Alamos (LanL2DZ) pseudopotential18 as well as the readjusted small-core relativistic compact effective potential (ACEP-121, developed in the present work) with their corresponding basis sets were employed for Au. For the CO and CO2 molecules, the full-electron 6-31+G(d) with Rd(C) ) 0.435120 and Rd(O) ) 0.395156 basis set was used.20 B3LYP functional21 along with the relativistic pseudopotential LANL2DZ have been used in the literature to study the interaction between Au3 clusters with organic compounds,22,23 Au+ with pyridine,24 and different Aun clusters, n ) 2-10 or intermetallic clusters, such as AunFe,25 AunNi,26 AunY2,27 AunPd-1,0,+1,28 AunCu and AunAg,29 and so forth, showing acceptable agreement with other high level theoretical methods or experimental results. In this work, we used the PM6 method that is included into the semiempirical quantum chemistry program MOPAC200730 but not in G03 RevD.02. To use the PM6, we develop a script file in perl language for running MOPAC2007 Version 7.221 L as external procedure for ONIOM in Gaussian-03. 2.1.1. ACEP-121. Some authors have questioned the application of DFT calculations related to gold molecules31 because the density functionals do not correctly describe the weak interaction between CO and Au and furthermore, in general, the Au-Au distances calculated using DFT and pseudpotencials

10.1021/jp102458p  2010 American Chemical Society Published on Web 06/10/2010

Interaction of CO Molecule with Au/MOR Catalyst

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TABLE 1: Calculated and Experimental Atomic Au Ionization Potential (Au-IP), Au Diatomic Ionization Potential (Au2-IP), Distance (R), and Total Unsigned Error

method

Au-IP ev

Au2-IP ev

LanL2DZ SDD cc-PP CEP-121 ACEP expa

9.42 9.43 9.41 9.27 9.22 9.23

9.43 9.26 9.27 9.13 9.17 9.16 ( 0.10

a

R Au2 Å R Au2-1 2.573 2.583 2.570 2.572 2.465 2.472

2.737 2.726 2.707 2.701 2.585 2.582

total unsigned Å error 0.716 0.555 0.513 0.289 0.030

Experimental data from ref 31.

or effective core potentials (ECP) are overestimated. To obtain reasonable values of distances and angles in gold compounds, it is necessary to perform the calculation at the MP2 level with atomic f functions on Au atoms,32,33 but this approach is impractical when the molecular structure contains many atoms. To overcome this obstacle, we readjusted the StevensBasch-Krauss ECP (CEP-121)19 for the gold atom using the Nelder-Mead simplex method34 to search the minimum. Details of the procedure, the Au atomic basis set, exponents and contraction coefficients, and the adjusted CEP-121 (ACEP) pseudopotential parameters are given in the Supporting Information. 2.1.2. Benchmarking Results. Table 1 shows the calculated ionization potentials and distances Au-Au values for Au, Au2, and Au2-1 using LanL2DZ, CEP-121, Stuttgart-Dresden (SDD),35 the adjusted CEP-121 (ACEP) pseudopotentials with their corresponding basis sets, and the recalibrated Stuttgart pseudopotential36 with cc-pVDZ-PP Peterson’s correlation consistent basis set with the f-type functions (cc-PP).37 In general, the ACEP shows the best agreement between the calculated and the experimental values, followed by the CEP121 and SDD pseudopotentials. The low LanL2DZ performance could be a consequence of its atomic basis set, which is the smallest one. To test the ACEP performance in the adiabatic detachment energy calculations (ADE), we performed calculations for Aun-1, n ) 1-5 clusters. Experimental ADE values were plotted versus theoretical values. In general, good linear regressions were obtained for all cases (r2 ) 0.9799 for LanL2DZ; 0.9870 for SDD; 0.9822 for CEP-121; 0.9729 for cc-PP, and 0.9940 for ACEP). Table 2 shows the experimental, calculated, and interpolated values. These last values were obtained using the corresponding linear regressions between the calculated and experimental values. Again, the lowest total unsigned error corresponds to ACEP while the highest value corresponds to LANL2DZ. The molecule Au2CO has two stable isomers, linear (C∞V symmetry) and triangular (C2V symmetry). The lower energy isomer corresponds to the lineal geometry. Table 3 reports the lineal isomer geometrical parameters calculated at B3LYP and MP2 level. Comparing the Au-Au distance with the corresponding to the Au2 molecule (Table 1) it is clear that there is not a significant change in the distance. Therefore, the trend in the Au-Au distance is similar to the found for the gold diatomic molecule. It is well-known that f-type polarization functions helps to describe properly the gold-gold interaction.32,38 To test the influence of the f-type polarization function on the calculated properties, calculations with two uncontracted f functions (Rf ) 1.19, 0.20)32 were performed. According to results presented on Table 3, the influence of the f-type functions on the isomer

geometry is stronger at MP2 than at B3LYP level. The results using B3LYP and the ACEP pseudoptential are closer to the MP2 level results than the corresponding to SDD and CEP121. The value of 2.425 Å for the Au-Au distance is in excellent agreement with the value reported by Schwerdtfeger et al.31 (RAu-Au ) 2.429 Å) at MP2 level using an optimized special atomic basis set for gold atom, that has three uncontracted f-functions and one g function (5s3p3d3f1g). Table 4 shows the geometrical parameters for the Au2CO molecule with C2V symmetry. From Tables 1-3, it is clear that the results with CEP-121 are similar to the results obtained with the SDD pseudopotential. Therefore, for sake of brevity Table 4 presents only results with CEP-121 for MPn and CC methods. The Au-Au distance shows an oscillating behavior when going from MP2 to MP3 to MP4(SDTQ) and from CCSD to CCSD(T). This oscillating behavior has been previously reported in the literature and attributed to the electronic correlation degree of each method.39,40 The results obtained with B3LYP/(ACEP or ACEP+2f) are closer to those obtained with MP3 or CCSD/ (CEP-121 or CEP-121 + 2f) than the values calculated with the others pseudopotential, SDD, CEP-121, and LanL2DZ. In general, the quality of the results using B3LYP/ACEP is reasonable compared with those results computed with larger basis sets such as SDD and more sophisticated methods but with less computational cost than those computed with MP2, MP4, CCSD, etc. 2.2. Mordenite Model. Inside the Mordenite (H-MOR) unit cell, there are four different T sites, (T1, T2, T3, and T4). The Brønsted sites are created by introducing the Al atom in the four different T sites and adding a proton to the crystallographically distinct oxygens. The Brønsted sites in the H-MOR structure were located according to van Santen and Li’s work.41 According to Sauer et al.,42 the preferential position of the Al atom into H-MOR structure follows the order T4 > T2 > T1 > T3. That is, Al occupies preferentially the T4 position, followed by the T2 position, and so on. However, the energy difference between Al in the less-stable site T3 in the more-stable site T4 is only 4.4 kJ/mol. On the other hand, Miyamoto and col.43 have shown that Brønsted acid sites at T1, T2, and T4 are stronger than Brønsted acid site at T3 site. The NH3 adsorption energy follows the order T4(0.0 kJ/mol) < T1(+1.5 kJ/mol) < T2(+5.7 kJ/mol). Therefore, we studied T4 and T1, for the following reasons: (a) They are located at the main channel, where the small gold aggregates have enough space to grow. (b) The Al atom is located preferentially at T4. (c) The stability of T1 is between the more-stable T4 and the less-stable T3, but its acidity (measured as NH3 adsorption energy) is similar to T4. Two gold species were studied in this work, one ion Au+ and a charged cluster Au3+. Because the gold atom or the cluster in the mordenite models replaces the Brønsted acid site proton, they have a formal net charge of +1 in all studied models. The H-MOR was modeled with a molecular structure with 1011 atoms (see Figure 1). The terminal oxygen atoms were saturated with H. During the optimization procedure, only the atoms at the high level were permitted to relax. 3. Results and Discussions 3.1. Schemes. To study the electronic properties of Au+ and Au3+ exchanged with H-mordenite, we analyzed the following four schemes: • Scheme A (SA): ONIOM3. Mordenite is represented by one structure with 1011 atoms, 10 tetrahedrons (10T) at high level (B3LYP), 52 tetrahedrons (52T) at medium level (PM6), 202T at low level (UFF), and the pseudopotential LanL2DZ for Au (see Figure 1a).

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TABLE 2: Experimental and Calculated Adiabatic Detachment Energies (ADE) in eV molecule

expa

Au5 Au4Au3Au2Auc AuHtotal unsigned error

3.06 2.70 3.88 1.94 2.31 0.90

-

LanL2DZb 3.19 2.83 3.69 2.05 2.19 0.83

SDDb

3.18 2.82 3.68 2.03 2.17 0.80

3.17 2.83 3.70 1.99 2.27 0.83

0.75

cc-PPb 3.05 2.73 3.55 1.95 2.21 0.87

0.58

3.20 2.87 3.69 2.03 2.09 0.89

CEP-121b 3.03 2.71 3.51 1.89 1.95 0.78

0.81

3.20 2.84 3.70 2.01 2.19 0.86

2.94 2.60 3.41 1.82 1.99 0.74 0.70

ACEPb 3.21 2.64 3.81 1.94 2.28 0.91

2.80 2.29 3.41 1.67 1.97 0.74 0.32

a Experimental data from ref 31 and 56. b Italicized: calculated values. Unitalicized: from linear regression between the experimental and calculated values. c This value corresponds to the vertical detachment energy from ref 57.

TABLE 3: Au2CO Lineal Isomer, Geometrical Parameters, Distances in Angstroms and Angles in Degreesa method

Au-Au

Au-C

∠ AuAu-C

∠ AuC-O

B3LYP/LanL2DZ B3LYP/SDD B3LYP/cc-PP B3LYP/CEP-121 B3LYP/ACEP MP2/SDD MP2/cc-PP MP2/CEP-121 B3LYP/LanL2DZ+2f B3LYP/SDD+2f B3LYP/cc-PP+2f B3LYP/CEP-121+2f B3LYP/ACEP +2f MP2/SDD+2f MP2/cc-PP+2f MP2/CEP-121+2f

2.555 2.561 2.551 2.556 2.459 2.545 2.540 2.541 2.528 2.545 2.537 2.538 2.425 2.480 2.468 2.465

1.952 1.955 1.952 1.953 1.881 1.838 1.833 1.830 1.947 1.949 1.946 1.951 1.878 1.843 1.852 1.850

178.5 178.5 178.5 178.5 178.6 178.5 178.5 178.5 178.6 178.6 178.5 178.6 178.6 178.6 178.6 178.6

178.4 178.4 178.4 178.4 178.7 178.8 178.8 178.8 178.4 178.4 178.4 178.4 178.7 178.8 178.8 178.8

a For C and O, Dunning’s augmented correlation consistent triple valence basis sets were used (aug-cc-pVTZ). Two uncontracted f functions (Rf ) 1.19, 0.20) from ref 32.

TABLE 4: Au2CO C2W Symmetry, Geometrical Parameters, Distances in Angstroms and Angles in Degreesa method

Au-Au

Au-C

∠ Au-C-Au

MP2/CEP-121 MP3/CEP-121 MP4/CEP-121 CCSD/CEP-121 CCSD(T)/CEP-121 MP2/CEP-121+2f MP3/CEP-121+2f MP4/CEP-121+2f CCSD/CEP-121+2f CCSD(T)/CEP-121+2f B3PLYP/LanL2DZ B3LYP/SDD B3LYP/cc-PP B3LYP/CEP-121 B3LYP/ACEP B3PLYP/LanL2DZ+2f B3LYP/SDD+2f B3LYP/cc-PP+2f B3LYP/CEP-121+2f B3LYP/ACEP+2f

3.123 3.241 3.115 3.212 3.176 3.011 3.281 3.033 3.207 3.152 3.379 3.381 3.368 3.382 3.270 3.361 3.366 3.354 3.366 3.242

1.907 1.950 1.913 1.940 1.931 1.910 1.991 1.933 1.968 1.960 2.029 2.036 2.031 2.035 1.963 2.022 2.028 2.054 2.022 1.960

109.97 112.38 109.02 111.78 110.66 104.04 110.98 103.34 109.11 107.08 112.76 112.56 112.07 112.40 112.84 112.43 112.21 111.80 112.43 111.65

a

For C and O, Dunning’s augmented correlation consistent triple valence basis sets were used (aug-cc-pVTZ). Two uncontracted f functions (Rf ) 1.19, 0.20) from ref 32.

• Scheme B (SB): ONIOM2. Mordenite is represented by one structure with 1011 atoms, 10T at high level (B3LYP), 254T at low level (UFF), and the pseudopotential LanL2DZ for Au (see Figure 1b).

• Scheme C (SC): ONIOM2. Mordenite is represented by one structure with 1011 atoms, 10T at high level (B3LYP), and 254T at low level (UFF), and the pseudopotential ACEP-121 for Au (see Figure 1b). • Scheme D (SD): ONIOM2. Mordenite is represented by one structure with 1011 atoms, 25T at high level (B3LYP), 239T at low level (UFF), and Au, ACEP-121 (see Figure 1c). 3.2. Au-MOR. Sites T1 and T4. Table 5 shows the selected geometrical parameters for the minimum energy geometry when Au+ is at T1 site. According to the literature,44 at B3LYP level of calculation the average values of the mean unsigned error (MUE) for bond lengths is 0.05 Å, and 1.3° for the angles. In general, considering the MUE values, the Al-O(x) distances, x ) 1-4, and Al-O(y)-Si angles, y ) 1, 3, 5, 6 (see Figure 2), follow the same trend for all schemes. That is, the Al-O(3) distance ≈ Al-O(1) ≈ Al-O(5) ≈ Al-O(6) in all schemes. On the other hand, the gold atom position into the 6-members ring (Figure 2) depends not only on the gold ECP (schemes SB and SC) but also depends on the number of atoms at high level (schemes SC and SD). Table 5 shows that in general the Au-O distances are similar in SA and SB pointing out that, from the geometrical point of view, both models behaves similar. Consequently, this suggests that the local interaction between gold species and the neighborhood oxygen atoms is strong enough that the methodology employed in the low level does not seems to affect significantly the gold position into the mordenite. This last result is in agreement with published results by Thomson et al.45-47 that showed in studies of Au clusters in zeolites, using ONIOM with DFT at high and UFF at low level, that 11T high-level model is large enough to include most of the local electrostatic interactions that would affect the Au cluster adsorption processes. According to the Au-O distances presented in Table 6 for Au at T4 site (see Figure 3a,b), the gold atom is mainly bonded to the O(7) and O(10) atoms, in all models, although there is not a great difference in the Au position obtained in any of the schemes. The fact that the ONIOM3 with PM6 results looks like the SB, SC, and SD results could be attributed to a very local strong interaction between the Au1+ cation and the Brønsted basic site [AlO4]-1. Consequently, the low or medium level effects on the active site geometry seem to be smaller than the near local interactions included into the high level. In order to estimate the relative gold stability into MOR, the relative energy was calculated as follows

Relative Energy ) [EtotalAu-MOR(T4) Etotal-MOR(T4)] - [EtotalAu-MOR(T1) - Etotal-MOR(T1)] (1) EtotalAu-MOR(T4) is the total energy for Au-MOR minimum energy structure with Au at T4, and EtotalAu-MOR(T1) for Au



Figure 1. (a) T1 site. SA Scheme. Low level, wireframe. Medium level, Tubes. High level, ball and stick. (b) T1 site. SB and SC schemes. Low level, wireframes. High level, ball and stick. (c) T1 site. SD schemes. Low level, wireframes. High level, ball and stick.

at T1. The process with charge separation, reaction 2, was not considered because it tends to overestimate the interaction energy due to the Coulombic charge separation

Au-MOR(0,1) + H+ f H-MOR(0,1) + Au+

(2)

The Au relative energy, calculated using reaction 1 is in all cases, is negative, which means that the Au at T4 is slightly more stable than in T1. The quality of the low level (SA), as well as the size of the high level (SD) affects the magnitude of the relative energy but not the trend. In general for Au, the CEP121 and consequently the ACEP-121 produce bond dissociation energy (BDE) values lower than to those obtained with LanL2DZ pseudopotential, as is illustrated at the bottom legend of the Table 6 for the Au-OH BDE calculation with 6-31+G(d,p) basis set for O and H, and LanL2DZ, CEP-121, and ACEP121 for gold atom. Consequently, the value obtained with the

scheme SC is lower than the corresponding to SB. The results obtained with the scheme SA show that the cage effect would has a larger effect on properties such as bonding energy than on geometry. However, considering the B3LYP binding energy MUE value, (6 kcal/mol, all the schemes give approximately, the same value. 3.3. Au3-MOR. Sites T1 and T4. Table 7 shows selected bond lengths and angles for Au3-MOR models at the T1 and T4 sites. The relative position of the [Au3]+ moiety in all schemes is similar. This can be noticed by the Au(2)-Al distance. The Au-Au distance is not affected by the “low level” and is more or less the same in SA and SB. Qualitatively, all schemes reproduce the same trends. In schemes SC and SD, the Au-Au distances are shorter than in SA and SB due to the different ECPs employed. Increasing the “high level” from 10T to 25T does not affect the Au-Au distances.

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TABLE 5: Selected Bond Length (Å) and Angles (°) for Au Atom at T1 Site schemes SA 1011 atoms

SB 1011 atoms

SC 1011 atoms

SD 1011 atoms

distances angles

B3LYP/LANL2DZ:PM6:UFF (10T:52T:202T)

B3LYP/LANL2DZ:UFF (10T:254T)

B3LYP/ACEP:UFF (10T:254T)


Al-O(1) Al-O(3) Al-O(6) Al-O(5) Au-O(1) Au-O(3) Au-O(4) Au-O(2) Au-Al Al-O(1)-Si Al-O(3)-Si Al-O(6)-Si Al-O(5)-Si O(4)-Au-O(3) Al-O(3)-Au Al-O(4)-Au

1.728 1.802 1.705 1.722 2.910 2.152 2.903 3.034 3.219 163.7 124.1 141.5 144.2 81.7 108.7 44.4

1.738 1.806 1.711 1.727 2.905 2.157 2.880 3.035 3.194 162.4 125.1 141.3 145.2 82.4 107.0 43.4

1.741 1.803 1.707 1.728 2.818 2.127 2.780 2.912 3.113 162.6 125.5 141.3 145.4 84.3 104.5 41.6

1.768 1.783 1.721 1.743 2.352 2.237 2.652 2.716 3.094 166.6 135.9 142.4 146.9 81.8 100.5 43.8

TABLE 6: Selected Bond Length (Å) and Angles (°) for Au Atom at T4 Site and Au-MOR Relative Energy (kcal/mol) between the T1 and T4 Sitesa SA 1011 atoms

SB 1011 atoms

SC 1011 atoms

SD 1011 atoms

distances angles





Al-O(7) Al-O(8) Al-O(9) Al-O(10) Au-O(7) Au-O(10) Au-Al Al-O(7)-Si Al-O(8)-Si Al-O(9)-Si Al-O(10)-Si relative energy

1.820 1.737 1.733 1.718 2.230 2.603 3.190 133.5 127.6 141.0 166.9 -12.2

1.832 1.747 1.738 1.730 2.232 2.602 3.203 134.0 128.2 140.7 167.5 -8.7

1.829 1.746 1.736 1.735 2.196 2.490 3.127 134.0 128.0 141.4 166.3 -6.3

1.849 1.745 1.737 1.742 2.154 2.652 3.120 132.0 129.2 144.4 165.0 -4.9

a

Au-OH BDE at B3LYP: Lanl2DZ (44.9 kcal/mol), CEP-121(28.3 kcal/mol), ACEP-121(29.1 kcal/mol).

3.4. CO Adsorption on H-MOR. FTIR spectroscopy of adsorbed CO and diffuse reflectance UV-visible spectroscopy are widely used techniques for surface characterization of solids as well as metal-containing zeolites.48-53 When the CO molecule

Figure 2. Au on T1 site. Atoms at the low level are not shown for clarity.

interacts (via carbon) with sites that have a positive charge it shifts the C-O stretching frequency from that of the free molecule (2143 cm-l) to higher values. The magnitude of this shift can be related to the electronic properties site. At low temperature (77 K), the CO is adsorbed on H-mordenite and the IR spectra shows two peaks, one centered at 2172 cm-1 and the other at 2138 cm-1.49 The first band is assigned to a CO hydrogen bonded to the bridged Si(OH)Al Brønsted acid and second band is assigned to CO physisorbed inside the mordenite channels. Recently, Marie et al.52 controlled the CO dose and showed that at the beginning (low coverage) two bands centered at about 2180 and 2171 cm-1 appear. When the CO dose is increased, these bands increased in intensity and slightly shifted toward lower wavenumbers (2177 and 2169 cm-1, respectively52). Increasing the CO pressure transforms these two bands into a broader band with maximum at 2175 cm-1. At high coverage, one band around 2137 cm-1 appears. The former is attributed to CO interacting with OH groups in the main channel, while the latter is attributed to the presence of physisorbed CO in the pores or to a pseudoliquid CO inside the zeolite pores.51 To analyze this, we performed quantum mechanical calculations for CO adsorption on T1 and T4 sites of the H-MOR model using scheme SB. The calculated CO vibrational frequencies



Figure 3. (a) Au on T4 site. SB and SC Schemes. Low level, wireframe. High level, ball and stick. (b) Au on T4 site. Only atoms at the high level are shown for clarity.

TABLE 7: Selected Bond Length (Å) Angles (°) for [Au3]+ at T1 and T4 Sitesa schemes

distances/angles

SA 1011 atoms B3LYP/LANL2DZ:PM6:UFF (10T:52T:202T)

SB 1011 atoms B3LYP/LANL2DZ:UFF (10T:254T)

SC 1011 atoms B3LYP/ACEP:UFF (10T:254T)

SD 1011 atoms B3LYP/ACEP:UFF (25T:239T)

Au(2)-Al (T1) Au(2)-Al (T4) Au(1)-Au(2) (T1) Au(1)-Au(2) (T4) Au(2)-Au(3) (T1) Au(2)-Au(3) (T4) Au(1)-Au(3) (T1) Au(1)-Au(3) (T4) Relative energy

3.442 3.300 2.784 2.665 2.705 2.783 2.632 2.660 -7.3

3.425 3.322 2.794 2.666 2.704 2.793 2.635 2.665 -6.3

3.379 3.275 2.667 2.540 2.559 2.660 2.519 2.544 -8.6

3.315 3.324 2.684 2.543 2.563 2.659 2.517 2.544 -10.5

a

Au-Au distance in Au2 molecule. Experimental: 2.472 Å. B3LYP/LanLDZ: 2.573 Å. B3LYP/ACEP: 2.465 Å.

TABLE 8: Selected Bond Lengths (Å), Angles (°), and CO Adsorption Energy (kcal/mol) for Au at T1 and T4 Sites SA 1011 atoms

SB 1011 atoms

SC 1011 atoms

SD 1011 atoms

distances angles





Au-Al (T1) Au-Al (T4) Au-C (T1) Au-C (T4) AuC-O (T1) AuC-O (T4)

MOR Catalyst: ONIOM-PM6 Study

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