Insights into Different Products of Nitrosobenzene and Nitrobenzene. Hydrogenation on Pd(111) under the Realistic Reaction Condition

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1 Insights into Different Products of Nitrosobenzene and Nitrobenzene Hydrogenation on Pd(111) under the Realistic Reaction Condition Lidong Zhang a, Zheng-Jiang Shao a, Xiao-Ming Cao a,*, and P. Hu a,b,* a Key Laboratory for Advanced Materials, Center for Computational Chemistry and Research Institute of Industrial Catalysis, East China University of Science and Technology, Shanghai , P. R. China b School of Chemistry and Chemical Engineering, The Queen s University of Belfast, Belfast, BT9 5AG, U. K. 1. Details for Gibbs free adsorption energy (G ad ) calculation The chemical potentials of PhNO 2 and PhNO should be obtained at first because the PhNO 2 and PhNO are liquid under the experiment conditions of the reported reaction temperature of K and the hydrogen pressure set as 3 bar 1. The chemical potentials of liquid NB or NSB were calculated as following equation: µ l = µ g = G g + RT ln P P (1) where µ l and µ g mean the chemical potentials of the reactants at the liquid and gas phase, respectively. G g and R mean the molar Gibbs free energy of gas phase at the reaction temperature of K and gas constant, respectively. P and P represent the saturated vapor pressure of liquid adsorbates at the reaction temperature of K and the standard vapor pressure, respectively. µ l was calculated according to the fact that the chemical potential of liquid phase equals to that of gas phase because of liquid-gas equilibrium under the reaction temperature T. The standard molar Gibbs free energy G g of gas phase could be calculated as follows: G g = E g + ZPE + U 0 K T - T S g + RT (2) where E g is the total energy of gas-phase molecule by DFT calculations. ZPE, represent zero point energy, U 0 K T, S g denote molar internal energy variation from 0 K to the reaction temperature and molar entropy of gaseous adsorbates at the corresponding temperature, respectively. The gas phase thermodynamic state functions were calculated based on three-dimensional (3D) ideal gas model, which s detailed formulae have been presented in our previous work. 2-5 The Gibbs free energy for surface adsorbate G ads* was calculated at the same way: * Corresponding Authors: address: xmcao@ecust.edu.cn (Xiao-Ming Cao) and p.hu@qub.ac.uk (P. Hu) S1

2 G ads* = E ads* + ZPE + U 0 K T - TS ads* (3) where E ads* is the total interaction system of adsorbate on Pd(111) surfaces. ZPE is zero-point energy, U 0 K T and S ads* are molar internal energy increased from 0 K to the reaction temperature and molar entropy of adsorbate on Pd(111) surface, respectively. The entropy of surface adsorbate, different from gas phase, was calculated based on the model of 2D lattice ideal gas with hindered translator and rotator recently proposed by Campbell et al. 6-9, which shows better results for adsorbates entropy. According to Campbell s model, the standard entropy of surface adsorbate S ads* is calculated as follows S ads* = S g -S g,1d-trans -R ln hv k B T (4) S ads* is calculated by deducting 1D-translational entropy (S g,1d-trans ) from the gaseous entropy and additionally including the correction due to the hindered 2D-translational and rotational freedom. S g,1d-trans is assumed as one-third of the total 3D translational entropy and v denotes the desorption pre-exponential factor. k B represents Boltzmann s constant and h is Planck s constant. The v was calculated following: v = k B T h exp[(0.30s gas + 3.3R-S g,1d-trans )/R] (5) which shows good agreement with the experimental result from Campbell s investigation 7. The Gibbs free adsorption energy G ad of the unit area for the NB or NSB was defined as follows: G ad = E ad + ZPE + U - T S + RT + RT ln P P A (6) where ZPE, ( U) and S respectively represent the variation of zero-point energy, internal energy variation, entropy of the surface adsorbate with respect to gas phase at the reaction condition 1. A equivalent to the surface area of the modeled supercell. The positive G ad reveals that the adsorption process is thermodynamically favorable. And the greatest value of G ad corresponds to the highest adsorbate coverage under the realistic reaction conditions. Since the reactants are liquid phase and H 2 must dissolve in the reactants before the reaction, the solubility of H 2 in the liquid should be considered in its gaseous chemical potential. Therefore, the adsorption free energy G ad of H with respect to 1/2 H 2 was calculated as follows: S2

3 G ad = G H *+1/2 µ H2 +E slab = E ad + ZPE + U - T S + 1/2RT + 1/2RT ln C /A (7) where ZPE, ( U) and S respectively denote the variation of zero-point energy, internal energy variation, entropy of adsorbed H * with respect to 1/2 H 2 at the K. C means the concentration of H 2 in the liquid methanol, which is obtained at correspond hydrogen pressure and temperature in the liquid methanol Coadsorption with Hydrogen Except for the coverage effect of PhNO 2 and PhNO, the coverage of surface hydrogen is also important for the hydrogenation reaction. Over Pd catalyst surfaces, since the dissociation of hydrogen molecule is facile, we directly investigated the coverage of surface atomic hydrogen (H * ) under the realistic condition. Although there is no surface space to further promote the coverage of big PhNO 2 or PhNO molecules as discussed above, the surface Pd atoms are still capable of adsorbing H atoms. The differential free energy of hydrogen atoms, G ad, is calculated to identify the coverage of H* as follows: G ad = G(adsorbate * + (n-1)h * ) - 1/2µ H2 + G(adsorbate * + nh * ) (8) G (nh * +adsorbate * ) is the free energy of co-adsorbtion of H * and adsorbate *, and µ H2 denotes the chemical potential of hydrogen dissolved in the liquid phase. As displayed in Fig. S5, coadsorbed with 1/9 ML PhNO 2, the differential free adsorption energy of H * keeps growing with the increase of its coverage until the coverage of H * is beyond 5/9 ML. The further promotion of H * coverage would lead to the negative differential free adsorption energy and reduce the thermodynamic stability of the reaction system. Moreover, the G ads for co-adsorption of 1/9 ML PhNO 2 and 5/9 ML H arrives at mev/å 2, which is more negative than the Gads of mev/å 2 for 1 ML H*. This rules out the possibility that the surface is totally occupied by H * and suggests that under the realistic reaction conditions, 1/9 ML PhNO 2 at parallel adsorption mode and 5/9 ML H* at fcc sites would co-adsorb on the Pd(111) surface. For adsorption of PhNO on the Pd(111) surface, the G ads of 1/4 ML PhNO* is more negative than 1 ML H*. So it could exclude the possibility that the surface is fully occupied by H atoms. Co-adsorbed with 1/4 ML PhNO*, the differential free adsorption energy for the first co-adsorbed H* is 0.19 ev. However, the differential free adsorption energy for the second hydrogen atom would turn to be positive. S3

4 Therefore, under the reaction conditions, 1/4 ML PhNO* can be co-adsorbed on the Pd(111) surface with 1/4 ML H*. The G ads of co-adsorbed PhNO* and H* reached mev/å 2. Hence the adsorption configurations and coverages of PhNO 2 * and PhNO* and their influence on co-adsorbed H* are rather different on Pd(111) under the same reaction conditions. 3. Microkinetic Simulation and Analysis In order to identify the dominate pathways of PhNO 2 and PhNO hydrogenation, a systematic microkinetic analysis 2, 11 was performed on the basis of the calculated energetics for each elementary step (Tables 3 and 4). In our microkinetic model, the reactor was simulated using the ideal continuous-stirred tank reactor (CSTR). The involved important reaction steps were summarized in Tables S6~S8, where the rate constants of the surface reactions were calculated following Transition State Theory (TST) and the collision theory was used to deal with those of the adsorption/desorption process. The net rate (ri = ri+ - ri-), the reversibility (Zi), and the degree of rate control (XRC,i) of each step i in nitrobenzene (nitrosobenzene) hydrogenation over Pd(111) were calculated at the steady state. The higher XRC is, the step is more important to determine the reaction rate It is clear from Tables S6 and S7 that the direct route is the dominate pathway for nitrobenzene hydrogenation no matter in high vacuum conditions or the realistic reaction conditions. However, the process of the direct route is different, under the high vacuum conditions, it is more likely to form PhNO* through PhNOOH* de-hydroxylation (R5) while PhNOH* is the initial product (R6) for the first N-O bond rupture under the realistic reaction conditions. It is intriguing that the formed PhNO* would still be hydrogenated to produce PhNOH*. The microkinetic results are in agreement with experiments findings 1,12. Moreover, the rate-determing step is R12 (PhNHOH* + * PhNH* + OH*), which has the highest value of XRC,I (0.65 and 0.85 for high vacuum conditions and the realistic reaction conditions respectively) Comparing the Tables S8 and S9, it could be found that the condensation route is the dominate pathway for nitrosobenzene hydrogenation since the TOF from condensation route towards azoxybenzene (PhNONPh) production is 4 orders of magnitude greater than the direct route for PhNH 2 production. S4

5 Table S1. The reaction energy and energy barrier of each elementary step included in the condensation route of PhNO 2 reduction under the realistic reaction condition and high vacuum condition No. Reactions Activation barrier* E a (ev) Reaction Energy* E(eV) PhNO*+PhNO* PhN(O)N(O)Ph* PhN(O)N(O)Ph*+H* PhN(OH)N(O)Ph* PhN(OH)N(O)Ph* PhNN(O)Ph*+OH* 0.76(1.64) 0.27(0.54) 1.22(0.75) 0.43(1.61) -0.24(0.17) -1.06(-1.44) 4 5 PhNOH*+PhNOH* PhN(OH)N(OH)Ph* PhN(OH)N(OH)Ph* PhN(OH)NPh*+OH* 1.08(1.13) 0.80(0.54) 0.89(0.17) -0.59(-0.68) *Zero point energy correction is included in the calculated activation energy and reaction energy. The data out and in bracket respectively represent the results under the realistic reaction condition and high vacuum condition. Table S2. The reaction energy and energy barrier of each elementary step included in the condensation route of PhNO reduction under the realistic reaction condition No. Reactions Activation barrier Ea (ev) Reaction energy* E (ev) PhNO* + PhN* PhN(O)NPh* PhN(O)NPh* PhNNPh* + O* PhN(O)NPh* + H* PhN(OH)NPh* PhNO* + PhNO* PhN(O)N(O)Ph* PhN* + PhN* PhN=NPh* PhN(OH)NPh* PhN=NPh* + OH* *Zero point energy correction is included in the calculated activation energy and reaction energy. S5

6 Table S3. The imaginary vibrational frequency value of TS for each element step in nitrobenzene hydrogenation. No. Reactions imaginary vibrational frequency* (cm -1 ) PhNO 2 * PhNO* + O* (1a TS(1a/1l) 1l) PhNO 2 *+H* PhNOOH* (1a TS(1a/1b) 1b) PhNOOH*+H* PhN(OH) 2 * (1b TS(1b/1c) 1c) PhNOOH* PhNO* + OH* 1b TS(1b/1d) 1d PhNO* PhN* + O* (1f TS(1f/1m) 1m) PhN*+H* PhNH* (1n TS(1n/1j) 1j) PhN(OH) 2 * PhNOH* + OH* (1c TS(1c/1e) 1e) H* + OH* H 2 O(g) (PhNO) (1d TS(1d/1f) 1f) PhNO* + H* PhNOH* (1f TS(1f/1g) 1g) PhNO* + H* PhNHO* (1f TS(1f/1o) 1o) H* + OH* H 2 O* (PhNOH) (1e TS(1e/1g) 1g) PhNOH* + H* PhNHOH* (1g TS(1g/1h) 1h) PhNHO* + H* PhNHOH* (1o TS(1o/1h) 1h) PhNHOH* PhNH* + OH* (1h TS(1h/1i) 1i) H* + OH* H 2 O(g) (PhNH) (1i TS(1i/1j) 1j) S (318.61) (841.69) (662.43) (271.25) (273.73) (858.95) (209.54) (683.16) (382.97) (736.24) (683.16) (444.35) (861.05) (356.46) (683.16) 16 PhNH* + H* PhNH 2 * (1j TS(1j/1k) 1k) (939.11) 17 PhNOH* PhN* + OH* (1g TS(1g/1l) 1l) (264.72) 18 PhNOH* + PhNO* PhN(OH)N(O)Ph* ( ) 19 PhN(OH)N(O)Ph* PhNN(O)Ph* + OH* (462.32) * The data out and in bracket respectively represent the results under the realistic reaction condition and high vacuum condition.

7 Table S4. The imaginary vibrational frequency value of TS for each element step in nitrosobenzene hydrogenation. No Reactions PhNO* PhN*+O* (2a TS(2a/2i) 2i) PhNO*+H* PhNOH* (2a TS(2a/2b) 2b) PhNO*+H* PhNHO* (2a TS(2a/2j) 2j) PhNOH*+H* PhNHOH* (2b TS(2b/2c) 2c) PhNHO*+ H* PhNHOH* (2j TS(2j/2c) 2c) PhNHOH* PhNH*+OH* (2c TS(2c/2e) 2e) H*+OH* H 2 O*(PhNH) (2e TS(2e/2g) 2g) PhNOH* PhN*+OH* (2b TS(2b/2d) 2d) H*+OH* H 2 O(g)(PhN) (2d TS(2d/2f) 2f) PhN*+H* PhNH* (2f TS(2f/2g) 2g) PhNH*+H* PhNH 2 * (2g TS(2g/2h) 2h) imaginary vibrational frequency (cm -1 ) Table S5. The imaginary vibrational frequency value of TS for each element step in nitrosobenzene hydrogenation. No. Reactions imaginary vibrational frequency (cm -1 ) 1 PhNO* + PhN* PhN(O)NPh* PhN(O)NPh* PhNNPh* + O* PhN(O)NPh* +H* PhN(OH)NPh* PhNO* + PhNO* PhN(O)N(O)Ph* PhN* + PhN* PhN=NPh* PhN(OH)NPh* PhN=NPh* + OH* S7

8 Table S6. The net reaction rate r i, reversibility Z i and degree of rate control X RC,i of each step of direct route during PhNO 2 hydrogenation over Pd(111) high vacuum condition realistic reaction condition step r i /s -1 Z i X RC,i r i /s -1 Z i X RC,i PhNO 2 (g) + * R1 3.25E E E E PhNO 2 * R2 H 2 (g) + 2* 2H* 9.76E E E E R3 R4 R5 R6 R7 R8 R9 R10 R11 R12 R13 PhNO 2 * + H* 3.25E E E E PhNOOH* + * PhNOOH* + H* 2.65E E E E PhN(OH) 2 * + * PhNOOH* + * 2.99E E E E PhNO* + OH* PhN(OH) 2 * + * 2.65E-04 PhNOH* + OH* 9.87E E E H* +OH* 6.51E-03 H 2 O* + * 1.32E E E PhNO* + H* 2.99E-03 PhNOH* + * 3.31E E E PhNO* + H* -6.35E E E E PhNHO* + * PhNOH* + H* 3.25E E E E PhNHOH* + * PhNHO* + H* -6.35E E E E PhNHOH* + * PhNHOH* + * 3.25E-03 PhNH* + OH* 5.83E E E PhNH* + H* 3.25E-03 PhNH 2 * + * 1.39E E E R14 H 2 O* H 2 O + * 6.51E E E E R15 PhNH 2 * PhNH 2 + * 3.25E E E E S8

9 Table S7. The net reaction rate r i, reversibility Z i and degree of rate control X RC,i of each step of condensation route during PhNO 2 hydrogenation over Pd(111) high vacuum condition realistic reaction condition step r i /s -1 Z i X RC,i r i /s -1 Z i X RC,i PhNO 2 (g) + * R1 1.88E E E E PhNO 2 * R2 H 2 (g) + 2* 2H* 2.81E E E E R3 R4 R5 R6 R7 R8 R9 R10 R11 PhNO 2 * + H* 1.88E-22 PhNOOH* + * 4.82E E E PhNOOH* + H* 1.33E-23 PhN(OH) 2 * + * 9.88E E E PhNOOH* + * 1.74E-22 PhNO* + OH* 1.12E E E PhN(OH) 2 * + * 1.33E-23 PhNOH* + OH* 1.44E E E H* +OH* 2.81E-22 H 2 O* + * 2.12E E E PhNO* + H* 1.89E-14 PhNOH* + * 9.39E E E PhNO* + H* -1.89E-14 PhNHO* + * 1.60E E E PhNOH* + H* 1.89E-14 PhNHOH* + * 1.00E E E PhNHO* + H* -1.89E-14 PhNHOH* + * 4.14E E E R12 H 2 O* H 2 O + * 1.41E E E E PhNO* + PhNO* R E E E E PhNONOPh* + * PhNONOPh* + H* R14 PhNONOHPh* 2.15E E E E * PhNO* + PhNOH* R15 PhNONOHPh* 9.38E E E E * PhNONOHPh* + * R E E E E PhNONPh + OH* S9

10 R17 PhNONPh* PhNONPh + * 9.38E E E E Table S8. The net reaction rate r i, reversibility Z i and degree of rate control X RC,i of each elementary step of direct route during PhNO hydrogenation over Pd(111) under the realistic reaction condition step direct route r i /s -1 Z i X RC,i R1 PhNO(g) + * PhNO* 1.10E E R2 H 2 (g) + 2* 2H* 2.20E E R3 PhNO* + H* PhNOH* + * 1.10E E R4 PhNO* + H* PhNHO* + * 4.56E E R5 PhNOH* + H* PhNHOH* + * 5.61E E R6 PhNHO* + H* PhNHOH* + * 4.56E E R7 PhNOH* + * PhN* + OH* 1.09E E R8 PhN* + H* PhNH* + * 1.09E E R9 PhNHOH* + * PhNH* + OH* 5.62E E R10 PhNH* + H* PhNH 2 * + * 1.10E E R11 H 2 O* H 2 O + * 1.10E E R12 PhNH 2 * PhNH 2 + * 1.10E E Table S9. The net reaction rate r i, reversibility Z i and degree of rate control X RC,i of each elementary step of condensation route during PhNO hydrogenation over Pd(111) under the realistic reaction condition step condensation route r i /s -1 Z i X RC,i R1 PhNO(g) + * PhNO* 3.56E E R2 H 2 (g) + 2* 2H* 1.78E E R3 PhNO* + H* PhNOH* + * 1.78E E R4 PhNOH* + * PhN* + OH* 1.78E E R5 PhNO* + PhNO* PhNONOPh* + * -2.25E E R6 PhNO* + PhN* PhNONPh* + * 1.78E E R7 PhN* + PhN* PhNNPh* + * 4.37E E R8 PhNONOPh* + H* PhNONOHPh* -4.50E E R9 PhNONOHPh* + * PhNONPh* + OH* -4.50E E R10 PhNONPh* PhNONPh + * 1.78E E R11 PhNNPh* PhNNPh + * 4.37E E S10

11 R12 H* + OH* H 2 O* + * 1.78E E R13 H 2 O* H 2 O + * 1.78E E S11

12 Figure S1 Vertical adsorption mode of nitrobenzene on Pd(111) at the coverage of 1/36ML. The pictures are composed of the top and front views of each configuration. The cyan balls in CPK style represent Pd atoms and the red, blue, grey and white balls represent O, N, C and H atoms respectively. The right profiles are the DOS and COHP plots of the possible bonding atoms in the corresponding configurations. The red and blue lines respectively represent the DOS of the s and p orbitals of O atoms and the d orbital of correspond bonding Pd atoms. The black line represents the corresponding COHP curve and the ICOHP value up to the Fermi level is also marked. These illustrations are utilized throughout this paper. S12

13 Figure S2 Paralleled adsorption mode of nitrobenzene on Pd(111) at the coverage of 1/36ML. The adsorption configurations are divided into two categories: bridge30 and bridge0. S13

14 Figure S3 Vertical adsorption mode of nitrosobenzene on Pd(111) at the coverage of 1/36ML. The adsorption configurations are identified as vertical-1, vertical-2 and vertical-3 respectively. S14

15 Figure S4 Paralleled adsorption mode of nitrosobenzene on Pd(111) at the coverage of 1/36ML. The adsorption configurations are divided into two categories: bridge30 and bridge0. S15

16 Figure S5 The differential free energy adsorption energies and the top and side views of the corresponding adsorption configurations with the increased coverage of H coadsorbing with 1/9 ML nitrobenzene on Pd(111). One increased hydrogen atom in the supercell of p(3 3) corresponds to 1/9 ML hydrogen coverage (θ H ) under the reaction condition (T = K, P H2 = 3 bar, P NB = bar, solubility of hydrogen in methanol is 14 mol/m 3 ). S16

17 Figure S6 The differential free energy adsorption energies and the top and side views of the corresponding adsorption configurations with the increased coverage of H coadsorbing with 1/4 ML nitrosobenzene on Pd(111). One increased hydrogen atom in the supercell of p(2 2) corresponds to 1/4 ML hydrogen coverage (θ H ) under the reaction condition (T = K, P H2 = 3 bar, P NSB = bar, solubility of hydrogen in methanol is 14 mol/m 3 ). S17

18 Figure S7 Energy profiles of hydrogenation reaction from nitrobenzene to aniline on clean Pd(111) surface. The black line represents the PhNOOH* PhNO* PhNOH* PhNHOH* PhNH* PhNH 2 * pathway and the red line presents the PhNOOH* PhN(OH) 2 * PhNOH* PhNHOH* PhNH* PhNH 2 * pathway. S18

19 Figure S8 parallel (a) and vertical (b) adsorption mode of PhN* on Pd(111). S19

20 Figure S9 Adsorption configuration of PhN* and PhNO* on Pd(111) during nitrosobenzene reduction. S20

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