Insights into Different Products of Nitrosobenzene ... - ACS Publications

Subscriber access provided by University of South Dakota

C: Surfaces, Interfaces, Porous Materials, and Catalysis

Insights into Different Products of Nitrosobenzene and Nitrobenzene Hydrogenation on Pd(111) under the Realistic Reaction Condition Lidong Zhang, Zhengjiang Shao, Xiao-Ming Cao, and Peijun Hu J. Phys. Chem. C, Just Accepted Manuscript • DOI: 10.1021/acs.jpcc.8b05364 • Publication Date (Web): 13 Aug 2018 Downloaded from http://pubs.acs.org on August 26, 2018

Just Accepted “Just Accepted” manuscripts have been peer-reviewed and accepted for publication. They are posted online prior to technical editing, formatting for publication and author proofing. The American Chemical Society provides “Just Accepted” as a service to the research community to expedite the dissemination of scientific material as soon as possible after acceptance. “Just Accepted” manuscripts appear in full in PDF format accompanied by an HTML abstract. “Just Accepted” manuscripts have been fully peer reviewed, but should not be considered the official version of record. They are citable by the Digital Object Identifier (DOI®). “Just Accepted” is an optional service offered to authors. Therefore, the “Just Accepted” Web site may not include all articles that will be published in the journal. After a manuscript is technically edited and formatted, it will be removed from the “Just Accepted” Web site and published as an ASAP article. Note that technical editing may introduce minor changes to the manuscript text and/or graphics which could affect content, and all legal disclaimers and ethical guidelines that apply to the journal pertain. ACS cannot be held responsible for errors or consequences arising from the use of information contained in these “Just Accepted” manuscripts.

is published by the American Chemical Society. 1155 Sixteenth Street N.W., Washington, DC 20036 Published by American Chemical Society. Copyright © American Chemical Society. However, no copyright claim is made to original U.S. Government works, or works produced by employees of any Commonwealth realm Crown government in the course of their duties.

Page 1 of 48 1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 18 19 20 21 22 23 24 25 26 27 28 29 30 31 32 33 34 35 36 37 38 39 40 41 42 43 44 45 46 47 48 49 50 51 52 53 54 55 56 57 58 59 60

The Journal of Physical Chemistry

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 200237, P. R. China b

School of Chemistry and Chemical Engineering, The Queen’s University of Belfast,

Belfast, BT9 5AG, U. K.

ABSTRACT Selective hydrogenation of nitroarene compounds are applied in many fields such as agrochemicals, pharmaceuticals and dyes. Pd catalyzing hydrogenation of nitrobenzene (PhNO2) and nitrosobenzene (PhNO) could exhibit different selectivities. It was regarded as the evidence to challenge Haber mechanism for PhNO2 hydrogenation in which PhNO is an important intermediate. In this study, we systematically investigate their hydrogenation mechanisms under the realistic reaction condition based on first-principle calculations. It is found that the weak bonding between nitro group and the Pd(111) surface leads to the flat-lying chemisorption configuration of PhNO2 and the other intermediates during PhNO2 hydrogenation. In contrast, the strong bonding between nitroso group and the surface makes PhNO to switch its chemisorption mode from the flat-lying adsorption under the ultra-high *

Corresponding Authors:

E-mail address: [email protected] (Xiao-Ming Cao) and [email protected] (P. Hu)

1

ACS Paragon Plus Environment

The Journal of Physical Chemistry 1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 18 19 20 21 22 23 24 25 26 27 28 29 30 31 32 33 34 35 36 37 38 39 40 41 42 43 44 45 46 47 48 49 50 51 52 53 54 55 56 57 58 59 60

vacuum condition to the vertical adsorption under the reaction condition. For the flat-lying PhNO2, the chemisorbed phenyl group makes hydrogenation easier but hinders N-O bond breaking, resulting in the production of PhNH2 via direct pathway. Conversely, without the hinderance of chemisorbed phenyl group, N-O bond breaking and N-N coupling becomes more favorable during the reduction of vertical PhNO* towards the formation of azoxy compound on Pd(111). These results unveil the fact that the difference between the selectivities of PhNO2 and PhNO hydrogenation are independent on the formation of PhNO* but dependent on the phenyl group adsorption mode.

2


Page 2 of 48

Page 3 of 48 1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 18 19 20 21 22 23 24 25 26 27 28 29 30 31 32 33 34 35 36 37 38 39 40 41 42 43 44 45 46 47 48 49 50 51 52 53 54 55 56 57 58 59 60


1. INTRODUCTION Heterogeneous hydrogenation of nitroarene is industrially significant to synthesize valuable intermediates such as aniline, hydroxylamine, azo and azoxy compounds for the manufacture of agrochemicals, pharmaceuticals, dyes and pigments1-4. Its selectivity generally strongly relies on the adsorption structure and reaction mechanism5-12. The wisdom used to prevail that the hydrogenation of nitroarene to aniline derivative should follow Haber mechanism13 in which the reaction could occur via either a direct route or a condensation route. Based on the electrochemical study, Haber proposed a three-step process in the direct route, which successively passes through

nitrosobenzene

compound

and

the

corresponding

hydroxylamine

intermediates rapidly followed by the rate determining step of the final hydrogenation of hydroxylamine to the aniline compounds. The condensation route denotes the condensation of two intermediate species to form azoxybenzene, azobenzene or hydrazobenzene followed by the stepwise hydrogenation to produce aniline. Nitrosobenzene is regarded as the primary intermediate in both two routes. However, in the recent years, Haber mechanism starts to be challenged and is not universally accepted any longer14-17. The most arguable issue is whether nitrosobenzene

compound

is the surefire

intermediate for the

nitroarene

hydrogenation to hydroxylamine and aniline. The most intuitive method to verify it is to perform the contrast test utilizing the investigated intermediate as the reactant, i.e. the hydrogenation of nitrosobenzene compound. Gelder et al.15 compared the 3



hydrogenation of nitrobenzene (PhNO2) and nitrosobenzene (PhNO) on a palladium/carbon catalyst. They found that the turnover frequency (TOF) of the hydrogenation of PhNO2 remarkably significantly outperforms that of PhNO hydrogenation. In addition, their kinetic isotope results suggested that the hydrogenation of PhNO to aniline mainly occurs via the condensation route while the hydrogenation of PhNO2 to aniline (PhNH2) does not undergo N-N coupling process. This gives rise to the discrepancy between the hydrogenation of nitrosobenzene and nitrobenzene. Therefore, it is plausible that the hydrogenation of PhNO2 could directly pass though phenylhydroxylamine (PhNHOH) without undergoing PhNO. They also proposed the formation of PhNHOH mainly rises from open shell PhNOH intermediates instead of PhNO. Interestingly, the similar result was found by Corma group

14

for the hydrogenation of PhNO2 on Au/TiO2 as well. The in situ IR

experiments suggest that the hydrogenation of PhNO2 also predominantly occurs via the direct route. Moreover, they also found that the hydrogenation of PhNO2 to PhNHOH is more efficient than the hydrogenation of PhNO on Au/TiO2. Therefore, a one-step mechanism from PhNO2 to PhNHOH is proposed. The mechanism of nitrobenzene reduction to aniline over Pt(111) was also studied by Sheng et al.18 based on DFT calculations upon the ultra-high vacuum and 0 K condition. They computationally identified a predominant pathway without passing through PhNO as PhNO2 → PhNOOH → PhN(OH)2 → PhNOH → PhNHOH → PhNH → PhNH2. The above mentioned experimental and theoretical studies appear to demonstrate that PhNO could possibly not be the important intermediate for the hydrogenation of 4


Page 4 of 48

Page 5 of 48 1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 18 19 20 21 22 23 24 25 26 27 28 29 30 31 32 33 34 35 36 37 38 39 40 41 42 43 44 45 46 47 48 49 50 51 52 53 54 55 56 57 58 59 60


PhNO2. This also provides an explanation for the different products for PhNO2 and PhNO hydrogenation. Yet the coverage effect appears to be overlooked. Since the reaction mechanism frequently varies with the surface coverage19-21, especially for the reactants with multi-functional groups6. The coverage effect could possibly remarkably influence the interaction between the surface metal atoms and the functional group, giving rise to the various adsorption configurations and the resulting selectivity. Recently, our group reported that the adsorption configuration of nitroarene does be sensitive to its coverage which depends on the realistic reaction condition5. Moreover, as found in Corma’s work14, the coverage of PhNO remained lower during the PhNO2 hydrogenation, which could possibly be significantly lower than that in the controlled trial of the hydrogenation of PhNO. This indicates that the contrast trial does not exclude the coverage factor. The contrast trial utilizing the hydrogenation of PhNO might be questionable. More importantly, the selectivities for PhNO2 and PhNO hydrogenation are rather different. Even if PhNO is not the key intermediate during PhNO2 hydrogenation, the possible intermediate PhNOH* is not so distinctive from PhNO. The understanding on the mechanism leading to the selectivity discrepancy would also facilitate the design of catalysts for specific products. Except for the intrinsic properties of the material and reactants22-24, the surface coverage intimately associates with the reaction temperature and pressure which could significantly influence the adsorption free energy and chemical potential of the surface species25-28. In general, DFT calculations upon the ultra-high vacuum and 0 K 5



condition could not reflect the role of reaction condition in the reaction mechanism. In the recent years, the first-principle thermodynamics with the combination of ab initio calculations and atomistic thermodynamics were attempted to plot adsorption phase diagrams under more realistic conditions of temperature and pressure29-31. This makes it possible to further explore the mechanistic study under the realistic condition. Regarding the possible important role of reaction conditions in the selectivity, we performed a comparative mechanistic study on the hydrogenation of PhNO2 and PhNO over Pd(111) under the reaction condition (T = 323.15 K, pH2 = 3 bar) upon Gelder et al.’s work15 to clarify the reliability of Haber mechanism and to understand the origin to the different selectivities to the hydrogenation of PhNO2 and PhNO utilizing DFT calculations combined with atomistic thermodynamics. A four-step strategy was proposed to understand the reaction mechanism under the realistic condition. The possible adsorption configurations of PhNO2 and PhNO were firstly compared under the low coverage. The interactions between the functional group at PhNO2 and PhNO and surface Pd atoms and their influence on the adsorption strength were first resolved based on Crystal Orbital Hamiltonian Population (COHP) analysis32-36. Then the adsorption configurations of PhNO2 and PhNO on Pd(111) at a series of coverages were systematically investigated. The adsorption configurations and coverages of PhNO2 and PhNO as well as H were identified under the realistic reaction condition. Finally, their hydrogenation pathways from the identified adsorption configurations were computationally investigated to understand the influence of the reaction condition on the selectivity and the role of PhNO in the 6


Page 6 of 48

Page 7 of 48 1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 18 19 20 21 22 23 24 25 26 27 28 29 30 31 32 33 34 35 36 37 38 39 40 41 42 43 44 45 46 47 48 49 50 51 52 53 54 55 56 57 58 59 60


PhNO2 hydrogenation.

2. COMPUTATIONAL DETAILS All the DFT calculations in this work were carried out using Vienna ab initio simulation package (VASP)37-40 within the periodic plane wave framework. The electron exchange-correlation term was described by the GGA-PBE41. The Projector-Augmented-Wave (PAW)42 was used to model the electron-ion interaction with an energy cut-off of 500 eV for expanded plane wave basis sets. Van der Waals interactions were determined by the PBE-D3(BJ)43-44. According to the previous study5, the scheme of PBE-D3(BJ) could precisely describe the adsorption energies for nitroarenes on noble metal surfaces. The calculated adsorption energy of benzene on Pd(111) surface was -2.01 eV by PBE-D3(BJ), which is in good agreement with experiment results45 (-2.04 eV). A variety of adsorption configurations were first explored in the p(6×6) Pd(111) slab avoiding steric hindrance and adsorption competition. The COHP32-36 for each adsorption configuration was plotted and the integration of the contribution of the energy bands up to the Fermi level based on the projected crystal orbital Hamilton population (IpCOHP) analysis indicates the bonding strength between the functional groups of adsorbates and surface. The more positive the -IpCOHP is, the stronger bonding exists. To obtain the coverages of adsorbates under the reaction condition, the Gibbs free adsorption energies of the unit area (Gad) for adsorbates were computed 7



Page 8 of 48

based on DFT results and thermodynamic correction, the adsorption energy Eads is calculated by the following equation: Eads = Ex* - Esurf - Ex

(1)

where Ex*, Esurf and Ex are the total energies of the adsorbates on the surface, bare surface and gas-phase molecules. The asterisk represents the adsorbate on the surface. The Gibbs free adsorption energy Gad of the unit area for the adsorbate was defined as follows: Gad = Eads + ∆ZPE + ∆∆U - T∆ ° + RT + RT lnP⁄P° ⁄A

(2)

where ∆ZPE, ∆(∆U) and ∆ ° respectively represent the variation of zero-point energy, internal energy variation, entropy of the surface adsorbate with respect to gas phase at the reaction condition15. P and P° represent the saturated vapor pressure of liquid adsorbates at the reaction temperature of 323.15 K and the standard vapor pressure, respectively. The gas phase thermodynamic state functions were calculated based on three-dimensional (3D) ideal gas model. The detailed formulate have been presented in our previous work.46-49 The entropy of surface adsorbate was calculated based on the Campbell model. 25-26, 50-51

The negative Gad reveals that the adsorption process is thermodynamically

favorable and the most negative value of Gad corresponds to the highest adsorbate coverage under the realistic reaction conditions. The p(1×1), p(1×2), p(2×2), p(2×3), p(3×3), p(3×4), p(4×4), p(4×5), p(5×5), p(5×6), and p(6×6) supercells were utilized to model the coverages corresponding to 1 ML, 1/2 ML, 1/4 ML, 1/6 ML, 1/9 ML, 1/12 ML, 1/16 ML, 1/20 ML, 1/25 ML, 1/30 ML, 8


Page 9 of 48 1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 18 19 20 21 22 23 24 25 26 27 28 29 30 31 32 33 34 35 36 37 38 39 40 41 42 43 44 45 46 47 48 49 50 51 52 53 54 55 56 57 58 59 60


and 1/36 ML. The corresponding k-point mesh were 10 × 10 × 1, 10 × 5 × 1, 5 × 5 × 1, 5 × 4 × 1, 3 × 3 × 1, 3 × 3 × 1, 3 × 3 × 1, 3 × 2 × 1, 2 × 2 × 1, 2 × 2 ×1, and 2 × 2 ×1 in the Monkhorst−Pack scheme, which have been verified to be accurate enough for these systems. The Pd(111) surfaces were modeled by a four-layer slab. The bottom two layers were fixed during the geometry optimization, whereas the upper two layers and adsorbates were allowed to relax. The slab separation was provided normal to the surface by inclusion of a 19 Å vacuum region. The fitted lattice constant of bulk fcc Pd was found to be 3.885 Å, which is in agreement with the experimental value of 3.89 Å52. The geometry convergence criterion was set as 0.05 eV/Å for the maximal component of force. The transition state (TS) in reaction pathway was searched within a constrained optimization scheme53. It was achieved when all the forces remained lower than the convergence criterion. Each TS was further verified as a first-order saddle point with only one imaginary vibrational frequency and the corresponding vibrational mode along the reaction coordination which was visually confirmed by the software of wxDragon54 on the basis of a numerical vibrational frequency analysis.

3. RESULTS Since the adsorption of reactant frequently determines the selectivity, especially for multi-functional group compounds, we started with the investigation on possible adsorption configurations of PhNO2 and PhNO over Pd(111). 9



3.1. Adsorption of Reactants on Pd(111) 3.1.1. Adsorption of PhNO2 and PhNO on Pd(111)

Similar to the case of Pt(111) studied before5, the adsorption configurations of PhNO2 and PhNO on the Pd(111) surface could be classified to two modes upon the angle between the benzene ring and Pd surface: vertical and parallel adsorption, e.g. vertical adsorption refers to the configuration where the benzene ring is perpendicular to the Pd(111) surface while parallel adsorption refers to the configuration where the benzene ring parallels to the Pd(111) surface. We firstly explored the interaction modes between functional groups and surface Pd atoms at low coverage without incorporating adsorbates interaction. Adsorption energies for for PhNO2 and PhNO on Pd(111) at different configurations at the coverage of 1/36 ML are listed in Tables 1 and 2.

Vertical adsorption mode of PhNO2 Analogical to the results on Pt(111)5, the vertical adsorption mode corresponds to the chemisorption of PhNO2 via the oxygen atoms of nitro compound. According to the number of bound oxygen, the vertical adsorption mode can be further classified to monodentate and bidentate adsorption configurations. As summarized in Table 1, the adsorption energies of bidentate configuration (-0.89 eV) is slightly prior to monodentate adsorption (-0.84 eV). The further COHP analysis also suggests that there are obvious bonding areas in the range of overlap between the Pd d orbitals and O s and p orbitals below the Fermi level. 10


Page 10 of 48

Page 11 of 48 1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 18 19 20 21 22 23 24 25 26 27 28 29 30 31 32 33 34 35 36 37 38 39 40 41 42 43 44 45 46 47 48 49 50 51 52 53 54 55 56 57 58 59 60


Moreover, the -IpCOHP values between O-Pd at bidentate configuration is 1.10 eV (Figure 1), further indicating that the formed chemical bond between the nitro and Pd surfaces.

Parallel adsorption mode of PhNO2 When the PhNO2 molecules lies on the Pd(111) surface, both the benzene ring and the nitro group are likely to be bonded to the surface. The adsorption models can be divided into bridge, hcp and fcc according to the position of the geometrical center of benzene ring on the surface. Similar to that found in previous study on Pt(111)5, the adsorption energies of the configurations with benzene ring located at bridge site are evidently stronger than those at hcp and fcc sites. Therefore, we focused on the adsorption configurations with benzene ring located at bridge. Upon the angle between carbon-carbon bonds and bridge, they could be further classified to two categories: bridge0° with carbon-carbon bonds along the short bridge and bridge30° with carbon-carbon bonds along the long bridge. Due to the different interaction between nitro group and Pd surfaces, several types of bridge30° and bridge0° could be further refined. As listed in Table 1, the adsorption energies of bridge30° configurations are significantly stronger than those at bridge0° adsorption configurations. For three bridge30° adsorption configurations, the adsorption energies are very similar. The -IpCOHP values corresponding to the bonding between phenyl group and surface neighboring Pd atoms are far greater than those between nitro group and surface neighboring Pd atoms. The chemical bonding between nitro group and surface Pd atoms could almost be negligible (-IpCOHP = 11



0.29-0.60 eV). This indicates that the chemisorption of PhNO2 depends on the bonding between phenyl group and surface Pd atoms predominantly at parallel adsorption mode. Consequently, the maximized interaction between phenyl group and neighboring surface Pd atoms (-IpCOHP = 6.30 eV) makes bridge30°-3 configuration to possess the strongest adsorption energy (Figure 1). On the basis of the above results, we could find that the bonding between phenyl group and Pd surface atoms is evidently stronger than that between nitro group and Pd surface atoms. Hence, at low coverage without adsorbates interaction, PhNO2 preferentially lie on Pd(111) at bridge30°-3 configuration. Moreover, the bonding between nitro group and Pd surfaces at vertical mode is stronger than that parallel mode.

Vertical adsorption mode of PhNO Similar to PhNO2, PhNO could be adsorbed vertically on the Pd(111) surface via a nitroso functional group. However, different from PhNO2, both N and O at PhNO would form the chemical bonding with the surface Pd atoms at the vertical adsorption mode. As listed in Table 2, among three vertical adsorption configurations, the vertical-2 configuration at which N is located at the bridge site and O is located at off-top site possesses the strongest adsorption energy of -1.89 eV, which is about 1 eV higher than the vertical adsorption of PhNO2. This indicates the significantly stronger bonding between nitroso group and surface Pd atoms compared to the nitro group. The further COHP analysis also confirmed this result. The -IpCOHP value of 3.30 eV at vertical-2 configuration of PhNO is 12


Page 12 of 48

Page 13 of 48 1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 18 19 20 21 22 23 24 25 26 27 28 29 30 31 32 33 34 35 36 37 38 39 40 41 42 43 44 45 46 47 48 49 50 51 52 53 54 55 56 57 58 59 60


evidently greater than that of 1.10 eV at bidentate adsorption configuration of PhNO2 (Figure 1).

Parallel adsorption mode of PhNO Similar to PhNO2, the adsorption of phenyl group is of prime importance for PhNO at parallel adsorption mode. Bridge30°-1 at which the interaction between phenyl group and Pd atoms is maximized (-IpCOHP = 6.06 eV, Figure 1) and nitroso group adsorbs over surface with the di-σ configuration enjoys the greatest adsorption energy of -3.28 eV. Different from PhNO2, the interaction between nitroso group and surface is still strong at parallel adsorption mode. Notably, the N-O bond is stretched from 1.24 Å of gas phase to 1.33 Å at bridge30°-1 configuration. Moreover, the -IpCOHP value between nitroso group and surface at bridge30°-1 is as large as 2.25 eV, which is very close to the vertical adsorption value and significantly larger than that of nitro group at PhNO2 at parallel adsorption. It confirms that the nitroso functional group possesses a significantly stronger adsorption capacity than the nitro functional group. Yet the -IpCOHP value between nitroso group and Pd surface atoms at bridge30°-1 is slightly lower than the other parallel adsorption configurations, indicating that maximizing the phenyl group adsorption strength would weaken the bonding between nitroso group and surface Pd atoms to some extent. Although both PhNO2 and PhNO chemisorb over Pd(111) preferentially at parallel mode at lower coverage, the evidently stronger bonding exists between nitroso group and Pd atoms compared to nitro group. This indicates that the vertical adsorption mode of PhNO is likely to compete with its parallel 13



Page 14 of 48

adsorption mode at high coverage since the parallel chemisorption of phenyl group needs to occupy more than 6 surface Pd atoms sites.

Table 1. Adsorption energies, average geometric parameters for PhNO2 at different adsorption configurations on Pd(111) at the coverage of 1/36 ML Adsorption

Adsorption

Ead

mode

configuration

(eV)

Gas phase

-

Vertical adsorption

Parallel adsorption

Nitro group

Phenyl group

d(O-Pd)

d(N-O)

d(C-Pd)

d(C-C)

Å

Å

Å

Å

-

-

1.24

-

1.40

bidentate

-0.89

2.16

1.28

5.60

1.40

monodentate

-0.84

2.34

1.28

5.62

1.40

bridge30°-1

-2.23

2.77

1.25

2.19

1.45

bridge30°-2

-2.24

2.32

1.25

2.19

1.45

bridge30°-3

-2.25

3.26

1.25

2.18

1.45

bridge0°-1

-1.88

2.87

1.26

2.35

1.44

bridge0°-2

-1.88

3.19

1.25

2.26

1.44

Table 2. Adsorption energies, average geometric parameters for PhNO at different adsorption configurations on Pd(111) at the coverage of 1/36 ML Adsorption

Adsorption

Ead

mode

configuration

(eV)

Gas phase

-

Nitroso group

Phenyl group

d(O-Pd)

d(N-O)

d(C-Pd)

d(C-C)

Å

Å

Å

Å

-

-

1.24

-

1.40

bidentate

-1.89

2.06

1.30

4.50

1.40

Mono-1

-2.12

2.05

1.33

4.46

1.40

Mono-2

-2.00

2.30

1.31

4.52

1.40

Parallel

bridge30°-1

-3.28

2.12

1.33

2.19

1.45

adsorption

bridge30°-2

-3.12

2.10

1.33

2.31

1.44

Vertical adsorption

14


Page 15 of 48 1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 18 19 20 21 22 23 24 25 26 27 28 29 30 31 32 33 34 35 36 37 38 39 40 41 42 43 44 45 46 47 48 49 50 51 52 53 54 55 56 57 58 59 60


bridge0°-1

-3.05

2.09

1.33

2.26

1.45

bridge0°-2

-2.93

2.11

1.32

2.23

1.45

Figure 1. The most stable vertical and parallel adsorption configurations of nitrobenzene (a) and nitrosobenzene (b) on Pd(111) at the coverage of 1/36 ML. 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.

15



3.1.2. Coverage Effect on the Adsorption of PhNO2 and PhNO on Pd(111) After understanding the influence of the functional group bonding with the surface Pd atoms on the adsorption behavior of PhNO2 and PhNO on Pd(111), we further incorporated the adsorbates interaction to systematically investigate the adsorption behaviours of PhNO2 and PhNO at different coverages. The Gibbs free adsorption energies per unit area (Gads), i.e., the normalized integral free adsorption energies of the optimal vertical and parallel adsorption configurations were compared at a series of coverages (Figure 2 and 3). The coverage of adsorbates and adsorption mode under the reaction condition (T = 323.15 K, the saturated vapor pressure of PhNO2 pPhNO2 = 0.038 bar) could be identified in terms of the most negative Gads It is clear from Figure 2 that the Gads of the parallel adsorption mode is always prior to that of vertical adsorption mode for PhNO2 at the same coverage when PhNO2 is able to adsorb on Pd(111) at parallel mode. This indicates that PhNO2 does not adsorb vertically on the Pd (111) surface through the nitro group under the realistic reaction conditions. The Gads of PhNO2 keeps becoming more negative with the increase of its coverage until it reaches its maximum value at 1/9 ML. The further increase of the coverage causing the significantly increasing intermolecular repulsion results in the more positive Gads for parallel adsorption mode. When the coverage is higher than 1/6 ML, the too strong intermolecular repulsion makes PhNO2 unable to adsorb on Pd(111) at parallel mode. Despite the weaker intermolecular repulsion at vertical mode, the weak nitro group bonding capacity still makes its Gads more 16


Page 16 of 48

Page 17 of 48 1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 18 19 20 21 22 23 24 25 26 27 28 29 30 31 32 33 34 35 36 37 38 39 40 41 42 43 44 45 46 47 48 49 50 51 52 53 54 55 56 57 58 59 60


positive than the most negative value of parallel mode. Therefore, in the practical reaction environment, PhNO2 would chemisorb on Pd(111) at the parallel mode and the coverage of PhNO2 on the Pd (111) surface is most likely to be 1/9 ML at which the system is thermodynamically most stable. Furthermore, based on Gads calculation, 1/9 ML PhNO2 at parallel adsorption mode could co-adsorb with 5/9 ML H* on the Pd(111) surface as well under the realistic reaction conditions (Figure S5). Owing to the smaller volume of PhNO compared to PhNO2, its Gads of parallel adsorption mode could keep becoming more negative until 1/6 ML. PhNO could not adsorb on Pd(111) at higher coverage via the parallel adsorption mode. Unlike PhNO2, the strong chemical bonding between nitroso group and surface Pd atoms makes its most negative Gads to be obtained via vertical adsorption at coverage of 1/4 ML (Figure 3). This shows that the optimal adsorption mode of PhNO would be switched from parallel mode to vertical mode with the increase of coverage. Consequently, opposite to PhNO2, PhNO preferentially chemisorbs vertically on the Pd(111) surface with a coverage of 1/4 ML under the realistic reaction condition, maximizing the stability of the reaction system. Moreover, 1/4 ML PhNO* can be co-adsorbed on the Pd(111) surface with 1/4 ML H* under the realistic reaction conditions (Figure S6).

17



Figure 2. The Gibbs free energy adsorption energy of unit area (Gads) of nitrobenzene at the vertical adsorption configuration via -NO2, the parallel mode as well as H with respect to 1/2 H2 on Pd(111) at different coverages (θ) under the reaction condition (T = 323.15 K, pH2 = 3 bar, pPhNO2 = 0.038 bar, solubility of hydrogen in methanol is 14 mol/m3).

Figure 3. The Gibbs free energy adsorption energy of unit area (Gads) of nitrosobenzene at the vertical adsorption configuration via -NO, the parallel mode as well as H with respect to 1/2 H2 on Pd(111) at different coverages (θ) under the reaction condition (T = 323.15 K, pH2 = 3 bar, pPhNO2 = 0.036 bar, solubility of hydrogen in methanol is 14 mol/m3).

18


Page 18 of 48

Page 19 of 48 1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 18 19 20 21 22 23 24 25 26 27 28 29 30 31 32 33 34 35 36 37 38 39 40 41 42 43 44 45 46 47 48 49 50 51 52 53 54 55 56 57 58 59 60


3.2. Hydrogenation of PhNO2 over Pd(111) For the hydrogenation to PhNO2 to PhNH2, it must undergo two deoxygenation steps. Regardless of the direct route or condensation route, it must start with the first N-O bond breaking. The controversial point between Haber mechanism 13and Gelder et al.’s work15 is to form PhNO or PhNOH catalyzed by Pd catalysts after the first N-O bond scission. To clarify it, we proceeded the mechanistic study for the first N-O bond breaking from the identified co-adsorption configuration of 1/9 ML flat-lying PhNO2* and 5/9 ML H* under the realistic reaction condition. The dissociation of N-O bond could be launched directly or with the assistance of hydrogen. As listed in Table 3, for the direct N-O bond dissociation of PhNO2* over Pd(111), the energy is as high as 1.23 eV due to the cleavage of both σ bond and delocalized π bond of nitro group while the hydrogenation on O terminal of N-O to generate PhNOOH* (1b) only needs to climb over an energy barrier of 0.66 eV. Therefore the N-O bond breaking over Pd(111) relies on the assistance of hydrogen. As displayed in Figure 4, the resulting PhNOOH* is likely either to be directly dehydroxylated to generate PhNO* (1d) or to proceed the hydrogenation to PhN(OH)2* (1c) followed by the dehydroxylation to generate PhNOH* (1e). The formation of PhN(OH)2* only requires a hydrogenation barrier of 0.21 eV and its formation would release the energy of 0.29 eV. Although the energy barrier of the subsequent dehydroxylation to PhNOH* (0.76 eV) is 0.16 eV higher than that of the dehydroxylation of PhNOOH* to PhNO* (1e), the corresponding TS(1c/1e) is energetically very close to and even 0.13 eV more stable than TS(1b/1d) due to the 19



exothermic process of the formation of PhN(OH)2*. Considering approximate 5/9 ML coverage of H*, the production rates for PhN(OH)2* and PhNO* could be competed but PhNOH* which is related to the coverage of surface H* would be slightly prior to the formation of PhNO*. After the first N-O breaking, the produced OH* coadsorbed with PhNO* or PhNOH* could be readily hydrogenated to water, overcoming an energy barrier of about 0.40 eV followed by the exothermic process of water desorption under the realistic reaction condition. For the second N-O bond breaking, the direct N-O breaking energy barriers for PhNO* and PhNOH* are respectively as high as 1.56 eV and 1.36 eV. Hence the N-O bond breaking must rely on the hydrogen assistance over Pd(111). The hydrogenation of PhNO* could form either PhNHO* or PhNOH*. The energy barrier of 0.59 eV for PhNOH* formation is 0.32 eV lower than that of PhNHO*. Thus the hydrogenation of PhNO* would preferentially form PhNOH*. Hence, PhNOH* must be the intermediate for the formation of PhNH2*. The energy barrier of the subsequent hydrogenation of PhNOH* to PhNHOH* (0.61 eV) is similar to the hydrogenation of PhNO* to PhNOH*. Due to the strong interaction between phenyl group and Pd, PhNHOH* (1h) is difficult to be desorbed. It would tend to rupture the N-O bond to form PhNH* although this elementary step needs to climb over an energy barrier of 1.08 eV. The resulting OH* is also readily to be hydrogenated with an energy barrier of 0.56 eV followed by the desorption of water. The subsequent hydrogenation of PhNH* (1j) would form PhNH2* (1k), which is required to conquer an energy barrier of 0.70 eV and is also exothermic. As displayed 20


Page 20 of 48

Page 21 of 48 1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 18 19 20 21 22 23 24 25 26 27 28 29 30 31 32 33 34 35 36 37 38 39 40 41 42 43 44 45 46 47 48 49 50 51 52 53 54 55 56 57 58 59 60


in Figure 4, starting from PhNO*, the subsequent steps are all exothermic. The energy profile goes downhill to PhNH2* step by step in the direct route. The PhNHOH* to PhNH* enjoys the highest energy barrier during the whole hydrogenation process, indicating that it is the rate determining step for the hydrogenation of PhNO2 to PhNH2. It is well consistent with Haber mechanism in which the final hydrogenation of hydroxylamine to the aniline compounds is the rate determining step. We further considered the possibility of the condensation route starting from PhNO* or PhNOH*. Although the energy barriers of N-N coupling for PhNO* and PhNOH* are not rather high (0.76 eV), the N-N coupling process is endothermic (Figure 5). Moreover, the energy barrier of the dehydroxylation step for the produced PhN(O)-N(OH)Ph* to PhN(O)-NPh* is as high as 1.22 eV. The TS of N-O bond breaking in the condensation route is rather unstable. Similarly, the N-N coupling for two PhNO* are not rather difficult (0.76 eV) but the subsequent N-O breaking is formidable (Table S1). In addition, the N-N coupling for two PhNOH* is not rather easy (1.08 eV). Hence, the condensation route would be a minor route towards the formation of PhNH2 from PhNO2. We also calculated the results on the clean Pd(111) surface for the direct route corresponding to the high vacuum condition without lateral interaction. Notably, as listed in Table 3, compared to the results on the Pd(111) surface under the realistic conditions, the hydrogenation barriers are increased while the energy barriers of N-O bond breaking are lowered under the high vacuum condition. The N-O breaking of PhNOOH* to PhNO* is pretty easy, which is evidently exothermic and only needs to 21



climb over an energy barrier of 0.14 eV. The energy barrier of the N-O bond breaking of PhN(OH)2* is also reduced (0.49 eV) but still 0.35 eV higher than the dehydroxylation of PhNOOH*. Importantly, its preliminary step of the hydrogenation of PhNOOH* to PhN(OH)2* becomes more difficult. Its energy barrier is increased to 0.30 eV from 0.21 eV. More importantly, the step releases less energy (-0.06 eV). Consequently, TS(1c/1e) would be energetically less stable than TS(1b/1d) under the high vacuum condition. The opposite conclusion under different reaction conditions could rise from the coverage effect. Under the realistic reaction condition, the competitive adsorption partially weakens the bonding between each adsorbate and surface. The weakened bonding between adsorbates and surface is more beneficial to the association reaction such as hydrogenation. Hence the hydrogenation barrier becomes lower under the realistic reaction condition. On the contrary, the high coverage of H* occupies more sites, hindering the direct N-O bond dissociation reaction for the formation of the intermediates with the higher valence which requires more sites after bond breaking. This leads to the higher energy barrier of N-O bond breaking. With the increase of the surface coverage, the hydrogen-assisted N-O bond breaking turns to be more favorable compared to the direct breaking route. In addition, notably, the hydrogenation starting with the PhNO2* at parallel adsorption mode leads to the result that the produced PhNO* remains the parallel adsorption configuration due to the high desorption energy (3.20 eV on the bare Pd(111) surface and 1.76 eV coadsorbed with H*) which forbids the switch to vertical adsorption mode. The further microkinetic analysis (SI) based on the DFT results confirms that the 22


Page 22 of 48

Page 23 of 48 1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 18 19 20 21 22 23 24 25 26 27 28 29 30 31 32 33 34 35 36 37 38 39 40 41 42 43 44 45 46 47 48 49 50 51 52 53 54 55 56 57 58 59 60


second N-O bond breaking at PhNHOH* is the rate determining step for direct route during PhNO2 hydrogenation since it has the greatest degree of rate control XRC (Table S6). In addition, under the high-vacuum condition, the net reaction rates of PhNOOH* hydrogenation and the N-O breaking of PhN(OH)2* are lower than the N-O breaking of PhNOOH* (Table S6), indicating that PhNO* is the passing intermediate for PhNH2 formation. On the contrary, under the current reaction condition, the reaction pathway through PhNO* is less favorable for PhNH2 production. The reaction condition could determine whether PhNO* is the important intermediate for PhNH2 production.

23



Figure 4. Energy profiles of hydrogenation reaction from PhNO2* to PhNH2* on Pd(111) with the structures of intermediates and transition states under the realistic reaction condition. The black line represents the PhNOOH* → PhNO* → PhNOH* → PhNHOH* → PhNH* → PhNH2* pathway and the red line presents the PhNOOH* → PhN(OH)2* → PhNOH* → PhNHOH* → PhNH* → PhNH2* pathway.

24


Page 24 of 48

Page 25 of 48 1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 18 19 20 21 22 23 24 25 26 27 28 29 30 31 32 33 34 35 36 37 38 39 40 41 42 43 44 45 46 47 48 49 50 51 52 53 54 55 56 57 58 59 60


Figure 5. Energy profiles of N-N coupling between PhNO* and PhNOH* on Pd(111) during PhNO2 hydrogenation. The black and red lines respectively represent the reactions upon the H covered surface and the clean surface.

25



Page 26 of 48

Table 3. The reaction energy and energy barrier of each elementary step in the PhNO2 hydrogenation under the realistic reaction condition and high vacuum condition

Step 1

Energy Barrier*

Reaction Energy*

Ea (eV)

∆E (eV)

1.23(1.15)

1.05(0.50)

0.66(0.71)

0.11(0.41)

0.21(0.30)

-0.29(-0.06)

0.60(0.14)

0.26(-1.11)

2.32(1.76)

2.01(0.00)

0.99(1.13)

-0.99(-0.49)

0.76(0.49)

0.22(-0.65)

0.41(0.76)

-0.92(-0.20)

0.59(0.89)

-0.46(0.40)

0.91(1.11)

-0.27(0.54)

0.40(0.76)

-1.05(-0.20)

0.61(0.98)

-0.37(0.00)

0.54(0.97)

-0.42(-0.16)

Reactions PhNO2* → PhNO* + O* (1a → TS(1a/1l) → 1l)

2

PhNO2*+H* → PhNOOH* (1a → TS(1a/1b) → 1b)

3

PhNOOH*+H* → PhN(OH)2* (1b → TS(1b/1c) → 1c)

4

PhNOOH* → PhNO* + OH* 1b → TS(1b/1d) → 1d

5

PhNO* → PhN* + O* (1f → TS(1f/1m) → 1m)

6

PhN*+H* → PhNH* (1n → TS(1n/1j) → 1j)

7

PhN(OH)2* →PhNOH* + OH* (1c → TS(1c/1e) → 1e)

8

H* + OH*→ H2O(g) (PhNO) (1d → TS(1d/1f) → 1f)

9

PhNO* + H* → PhNOH* (1f → TS(1f/1g) → 1g)

10

PhNO* + H* → PhNHO* (1f → TS(1f/1o) → 1o)

11

H* + OH* → H2O* (PhNOH) (1e → TS(1e/1g) → 1g)

12

PhNOH* + H* → PhNHOH* (1g → TS(1g/1h) → 1h)

13

PhNHO* + H* → PhNHOH* 26


Page 27 of 48 1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 18 19 20 21 22 23 24 25 26 27 28 29 30 31 32 33 34 35 36 37 38 39 40 41 42 43 44 45 46 47 48 49 50 51 52 53 54 55 56 57 58 59 60


(1o → TS(1o/1h) → 1h) PhNHOH* → PhNH* + OH*

14

1.08(0.73)

-0.14(-1.10)

0.56(0.76)

-0.68(-0.20)

0.70(0.99)

-0.67(-0.03)

1.36(1.25)

0.33(0.27)

(1h → TS(1h/1i) → 1i) 15

H* + OH* → H2O(g) (PhNH) (1i → TS(1i/1j) → 1j) PhNH* + H* → PhNH2*

16

(1j → TS(1j/1k) → 1k) 17

PhNOH* → PhN* + OH* (1g → TS(1g/1l) → 1l)

18

PhNOH* + PhNO* → PhN(OH)N(O)Ph*

0.76(1.49)

0.53(1.19)

19

PhN(OH)N(O)Ph* → PhNN(O)Ph* + OH*

1.22(0.75)

-1.06(-1.44)

*Zero point energy correction is included in the calculated energy barrier and reaction energy. The data out and in bracket respectively represent the results under the realistic reaction condition and high vacuum condition.

3.3. Reduction of PhNO over Pd(111) The condensation route was found as the main route and azoxybenzene was proposed as a primary product for the hydrogenation of PhNO in Gelder’s work. Interestingly, although PhNO* is possible not to be the most important intermediate during the hydrogenation of PhNO2 towards PhNH2, it could be found from our calculations above that the hydrogenation of flat-lying PhNO* would still produce PhNH2 via the direct route. This is opposite to Gelder’s work. Hence, it is valuable to achieve a better understand of the hydrogenation of PhNO under real reaction conditions. Based on the adsorption configuration analysis under the realistic condition above, 27



we further performed a mechanistic study of the PhNO reduction on Pd (111) surfaces with 1/4 ML PhNO* and 1/4 ML H*. Notably, different from PhNO2 hydrogenation, the reaction begins with the vertical adsorption of PhNO* utilizing PhNO as the reactant under the realistic condition. For the first step of hydrogenation of PhNO* (2a) to generate PhNOH* (2b), the energy barrier is increased to 0.95 eV, which is almost twice that of the hydrogenation of paralleled PhNO* coadsorbed with H*. Moreover, this step turns to be endothermic. Yet the energy barriers for PhNHO* formation (1.33 eV) and the direct N-O bond breaking to PhN* (1.65 eV) are higher, so the PhNOH* would be the primary product for the initial hydrogenation. The higher energy barrier and endothermic reaction could be attributed to the stronger bonding between the nitroso functional group and the Pd surface at its vertical adsorption compared to its paralleled adsorption. Interestingly, different from the parallel PhNOH*, the subsequent hydrogenation of PhNOH* to PhNHOH* (1.07 eV) is evidently higher than the dehydroxylation of PhNOH* to generate PhN* (0.49 eV). This indicates that PhN* (2d) instead of PhNHOH* (2c) would be formed during PhNO hydrogenation on Pd(111). The energy barrier of the subsequent hydrogenation of the concomitant OH* with PhN* to water is 0.71 eV, which is more difficult than those during PhNO2 hydrogenation. Notably, the energy barrier of PhN* hydrogenation to PhNH* is as high as 1.34 eV, which would be the rate determining step for the production of PhNH2. Moreover, the step is endothermic. This indicates that it is very difficult to produce PhNH2 through the direct hydrogenation mechanism starting with PhNO*. 28


Page 28 of 48

Page 29 of 48 1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 18 19 20 21 22 23 24 25 26 27 28 29 30 31 32 33 34 35 36 37 38 39 40 41 42 43 44 45 46 47 48 49 50 51 52 53 54 55 56 57 58 59 60


Therefore, we further explored the possibility of the condensation route or the formation of azo compound. Interestingly, the energy barrier of N-N coupling for PhNO* and PhN* to form azoxybenzene (PhNN(O)Ph) is only 0.90 eV, which is evidently lower than that in PhNO2 hydrogenation. Moreover, it is exothermic (-0.38 eV). This could be attributed to their vertical adsorption configurations where the phenyl groups would not be the obstacle for the molecular movement at TS. As listed in the Table 5, the N-N coupling of two PhNO* is formidable (2.05 eV), implying the importance of PhN* in condensation route. In addition, the conversion from azoxybenzene to azobenzene is not easy. The energy barrier of direct deoxygenation of PhNN(O)Ph* is as high as 1.60 eV, much higher than the energy barrier for dehydroxylation of PhNN(OH)Ph* (0.30 eV). However, the preliminary step to form PhNN(OH)Ph* through the hydrogenation of PhNN(O)Ph* is endothermic and has to climb over a barrier of 1.10 eV, indicating that azoxybenzene could possibly be the product. Furthermore, PhNNPh* could not be obtained from the N-N coupling of two PhN* directly, either. This process has to overcome a formidable barrier as high as 2.15 eV Therefore, compared to higher energy barrier for the hydrogenation of PhN*, azoxybenzene is more easily to be produced during the hydrogenation of PhNO. Hence, the calculation results of PhNO hydrogenation are in good agreement with Gelder’s finding.

29



Figure 6. Energy profiles of hydrogenation reaction from PhNO* to PhNH2* on Pd(111) with the structures of intermediates and transition states under the realistic reaction condition. The black line represents the PhNO* → PhNOH* → PhNHOH* → PhNH* → PhNH2* pathway and the red line presents the PhNO* → PhNOH* → PhN* → PhNH* → PhNH2* pathway.

30


Page 30 of 48

Page 31 of 48 1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 18 19 20 21 22 23 24 25 26 27 28 29 30 31 32 33 34 35 36 37 38 39 40 41 42 43 44 45 46 47 48 49 50 51 52 53 54 55 56 57 58 59 60


Figure 7. Energy profiles of N-N coupling between PhN* and PhNO* towards the production of PhNN(O)Ph and PhN=NPh on Pd(111) during PhNO hydrogenation.

31



Page 32 of 48

Table 4. The reaction energy and energy barrier of each elementary step in the PhNO hydrogenation under the realistic reaction condition. No. 1

Energy Barrier*

Reaction energy*

Ea (eV)

∆E (eV)

1.65

0.18

0.95

0.39

1.33

0.52

1.07

-0.30

0.90

-0.10

0.67

-0.59

0.92

-0.46

0.49

-0.90

0.71

-0.72

1.34

0.26

0.80

-0.62

Reactions PhNO*→PhN*+O* (2a→TS(2a/2i)→2i)

2

PhNO*+H*→PhNOH* (2a→TS(2a/2b)→2b)

3

PhNO*+H*→PhNHO* (2a→TS(2a/2j)→2j)

4

PhNOH*+H*→PhNHOH* (2b→TS(2b/2c)→2c)

5

PhNHO*+ H*→PhNHOH* (2j→TS(2j/2c)→2c)

6

PhNHOH*→PhNH*+OH* (2c→TS(2c/2e)→2e)

7

H*+OH*→H2O*(PhNH) (2e→TS(2e/2g)→2g)

8

PhNOH*→PhN*+OH* (2b→TS(2b/2d)→2d)

9

H*+OH*→H2O(g)(PhN) (2d→TS(2d/2f)→2f)

10

PhN*+H*→PhNH* (2f→TS(2f/2g)→2g)

11

PhNH*+H*→PhNH2* (2g→TS(2g/2h)→2h)

*Zero point energy correction is included in the calculated energy barrier and reaction energy.

32


Page 33 of 48 1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 18 19 20 21 22 23 24 25 26 27 28 29 30 31 32 33 34 35 36 37 38 39 40 41 42 43 44 45 46 47 48 49 50 51 52 53 54 55 56 57 58 59 60


Table 5. The reaction energy and energy barrier of the coupling step in the PhNO hydrogenation No.

Energy Barrier*

Reaction Energy*

Ea (eV)

∆E (eV)

Reactions

1

PhNO* + PhN* → PhN(O)NPh*

0.90

-0.38

2

PhN(O)NPh* → PhNNPh* + O*

1.60

0.32

3

PhN(O)NPh* +H* → PhN(OH)NPh*

1.10

0.73

4

PhNO* + PhNO* → PhN(O)N(O)Ph*

2.05

1.52

5

PhN* + PhN* → PhN=NPh*

2.15

1.25

6

PhN(OH)NPh* → PhN=NPh* + OH*

0.30

-0.38

*Zero point energy correction is included in the calculated energy barrier and reaction energy.

4. DISCUSSION The adsorption and hydrogenation pathways of PhNO2 and PhNO under realistic reaction conditions were calculated systematically. Notably, the calculation results show that when PhNO2 and PhNO are fed as reactants, the reactions would proceed along the direct route and the condensation route (Scheme 1), respectively. Although the reaction pathway via PhNO* is slightly inferior to that via PhNOH* under the reported reaction condition during PhNO2 hydrogenation, the reaction pathways starting from PhNO* are pretty different between PhNO2 hydrogenation and PhNO hydrogenation. PhNO* lies parallel to the Pd(111) surface in the PhNO2 hydrogenation while it is vertical to the surface in the PhNO hydrogenation under the realistic reaction condition. Notably, the different adsorption configurations should account for the different selectivities. Since the selectivities are closely related to three types of crucial 33



processes: hydrogenation, N-O activation and N-N coupling, we performed the further analysis on the role of the adsorption configuration in these steps.

Scheme 1 Pathways for the reduction of PhNO2 and PhNO

4.1. Influence of Adsorption Configuration on the Hydrogenation Steps Comparing the energy barriers of all the hydrogenation steps from PhNO* during PhNO2 and PhNO hydrogenation listed in Tables 3 and 4, we could found that the energy barriers in the PhNO2 hydrogenation are always lower than those corresponding to the same steps in the PhNO hydrogenation. This could be attributed to the competitive adsorption between phenyl group and N containing group at parallel adsorption mode during PhNO2 hydrogenation. Taking PhNO* hydrogenation to PhNOH* as an example, as stated in Section 3.1, both phenyl and nitroso group are bonded to the Pd(111) surface when PhNO* chemisorbs on Pd(111) at parallel mode while only nitroso group is

34


Page 34 of 48

Page 35 of 48 1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 18 19 20 21 22 23 24 25 26 27 28 29 30 31 32 33 34 35 36 37 38 39 40 41 42 43 44 45 46 47 48 49 50 51 52 53 54 55 56 57 58 59 60


bonded to the Pd(111) surface when PhNO* chemisorbs on Pd(111) at vertical mode. Hence, the -IpCOHP for NO-Pd at vertical mode is 1.05 eV greater than that at the parallel mode (Figure 11(b)), i.e. the competition adsorption between phenyl group and nitroso group results in the weakened bonding between nitroso group and Pd(111) surface at the parallel adsorption mode compared to that at the vertical adsorption mode. Since the hydrogenation generally requires the energy to partially break the bonding between intermediates and surface atoms for the formation of the bonding between intermediate and hydrogen, the weaker the bonding between intermediates and surface atoms, the lower the hydrogenation barrier is. Hence, PhNO* chemisorbed at parallel mode is more beneficial to hydrogenation compared to that at vertical mode. Thus the hydrogenation steps are easier during PhNO2 hydrogenation than those in PhNO hydrogenation, especially for the direct route.

4.2. Formation of PhN* Apart from the influence on the hydrogenation, the adsorption configuration also remarkably affects the dehydroxylation of PhNOH* to produce PhN*. The energy barrier of flat-lying PhNOH* dehydroxylation is as high as 1.36 eV during PhNO2 hydrogenation, implying that the PhN* would not be produced. Interestingly, the energy barrier for vertical PhNOH* dehydroxylation is significantly reduced to 0.49 eV during PhNO hydrogenation. Regarding more difficult hydrogenation processes for vertical PhNOH* compared to flat-lying 35



PhNOH*, the reaction pathways are therefore extended into two different ways from PhNOH* for PhNO2 and PhNO hydrogenation. The COHP analysis on the TS of PhNOH* dehydroxylation were further performed to understand the reason why PhNOH* dehydroxylation is so different during PhNO2 and PhNO hydrogenation. Owing to the stronger adsorption of phenyl group, the -NOH* group of flat-lying PhNOH* is away from the surface. Similarly, the N moiety is far away from the Pd surface at TS of the dehydroxylation of flat-lying PhNOH* (Figure 8). This leads to negligible interaction between the N atom and the surface Pd atoms at TS (-IpCOHP = 0.18 eV, Figure 8(a)). Thus the N moiety with a higher valence could not be stabilized due to its low coordination. Consequently, the N-O bond breaking of PhNOH* becomes formidable at parallel adsorption mode. Different from parallel adsorption mode, without the interference of phenyl group, the N moiety could bond with two Pd atoms at the TS of vertical PhNOH* dehydroxylation. The -IpCOHP value for N-Pd is dramatically increased to 2.62 eV. Thus the strong N-Pd bonding makes the N-O bond breaking easy. Moreover, the -IpCOHP value of N-Pd for the formed PhN* at vertical adsorption mode is also 1.61 eV higher than that at the parallel adsorption mode (Figure S8), further indicating that the dehydroxylation of PhNOH* would occur at the vertical adsorption mode. Hence, flat-lying PhNOH* prefers to be further hydrogenated while the vertical one tends to form vertical PhN*. 36


Page 36 of 48

Page 37 of 48 1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 18 19 20 21 22 23 24 25 26 27 28 29 30 31 32 33 34 35 36 37 38 39 40 41 42 43 44 45 46 47 48 49 50 51 52 53 54 55 56 57 58 59 60


Figure 8. TSs of PhN-OH in parallel mode (a) and vertical mode (b), Pictures are composed of the top and front views of each configuration. The black lines denote corresponding COHP curves and the -IpCOHP value up to the Fermi level is also marked.

4.3. N-N Coupling Pathway As discussed above, since the second N-O bond breaking is rather difficult with the interference of strongly chemisorbed phenyl group during PhNO2 hydrogenation, N-N coupling would occur between PhNO* and PhNOH*. However, no matter what is the final product, azoxybenzene, azobenzene or aniline, their formation has to undergo the N-O bond breaking. The dehydroxylation of formed intermediate PhN(O)N(OH)Ph* after N-N coupling 37



has to encounter the hindrance from two flat-lying adsorbed phenyl group. This is even poorer than PhNOH* dehydroxylation. Hence, flat-lying PhNO2 hydrogenation would not prefer to produce aniline via condensation route or produce azoxy or azo compounds over Pd(111). During PhNO hydrogenation, due to the vertical adsorption configuration, PhN* could be formed. With the formation of PhN*, PhN=NPh and PhN(O)NPh* could respectively be directly produced via the self-coupling of PhN* and the coupling of PhN* and PhNO*. Because of the higher hydrogenation barrier from PhN* to PhNH2, the lower N-N coupling barrier would facilitate the formation of azobenzene or azoxybenzene. Interestingly, we found that the coupling of PhNO* and PhN* significantly outperforms that of PhN* or PhNO* self-coupling. The COHP analysis of N-N coupling TS were further carried out to clarify the origin to the optimal N-N coupling route. As displayed in Figure 9, among these TSs, the TS of PhNO* and PhN* coupling possesses the maximal -IpCOHP value for N-N bonding, indicating the strongest N-N bonding among these TSs. This could be attributed to the fact that the N moiety at PhN* with higher valence is possible to form stronger bonding compared to the nitroso moiety at PhNO*. In addition, the intermediate requires the energy to migrate to the lower coordination site for N-N bonding. For PhN* and PhNO* coupling, the PhN* needs to migrate to the bridge site from fcc site and to react with PhNO* at TS. Compared to adsorbed PhN* and PhNO* (Figure S9), the 38


Page 38 of 48

Page 39 of 48 1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 18 19 20 21 22 23 24 25 26 27 28 29 30 31 32 33 34 35 36 37 38 39 40 41 42 43 44 45 46 47 48 49 50 51 52 53 54 55 56 57 58 59 60


-IpCOHPs of N-Pd (PhN*) and of NO-Pd (PhNO) are respectively reduced from 4.16 eV to 2.42 eV and from 2.59 eV to 1.63 eV at TS. For two saturated PhNO* self-coupling, the bonding breaking between one PhNO* and surface is required for the formation of N-N bond. Hence, compared to adsorbed PhNO*, the -IpCOHPs of NO-Pd for two PhNO molecules are respectively reduced to 0.32 and 1.83 eV. For the self-coupling of PhN*, for the N=N double bond formation for azobenzene, two PhN* have to migrate to top sites at TS. As a consequence, compared to adsorbed PhN*, the -IpCOHP of N-Pd for two PhN molecules are dramatically reduced to 1.93 and 1.88 eV at TS. Moreover, surprisingly, the -IpCOHP of N-N is only 0.50 eV. This may be attributed to its longer N-N distance at TS constrained by two neighbor Pd atoms. In sum, the less weakened N(NO)-Pd bonding compared to adsorption states and the stronger formed N-N bonding at TS make the coupling of PhNO* with lower valence and PhN* with higher valence to be the best pathway for N-N coupling.

39



Figure 9. TSs of PhNO*-PhN* coupling (a), PhN*-PhN* coupling (b), PhNO*-PhNO* coupling (c) and PhN(O)N(OH)Ph* dehydroxylation in parallel mode (d). The pictures are composed of the top and front views of each configuration. The black lines denote corresponding -COHP curves and the -IpCOHP value up to the Fermi level is also marked.

5. CONCLUSION The mechanisms of nitrobenzene and nitrosobenzene reduction over Pd(111) catalyst under the realistic reaction conditions have been systematically investigated utilizing DFT calculations at PBE-D3(BJ) level. The different reaction pathways and products of PhNO2 and PhNO reduction have been computationally identified, which are consistent with experimental results. The main conclusions could be drawn as follows: 1. Due to the weak bonding between nitro group and the surface, PhNO2 always prefers to chemisorb on the Pd(111) surface at parallel adsorption mode, leading to formed flat-lying PhNO* during PhNO2 hydrogenation; 2. The stronger bonding between nitroso group and the surface makes the reactant PhNO to chemisorb over the Pd(111) surface at vertical adsorption mode under the realistic condition during PhNO hydrogenation; 3. Due to the interference of phenyl group leading to variation of the bonding between N-containing group and the surfaces, the reduction of flat-lying and vertical PhNO* could respectively produce PhNH2 via direct pathway and the N-N coupling products, azoxybenzene. Hence, utilizing PhNO as the 40


Page 40 of 48

Page 41 of 48 1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 18 19 20 21 22 23 24 25 26 27 28 29 30 31 32 33 34 35 36 37 38 39 40 41 42 43 44 45 46 47 48 49 50 51 52 53 54 55 56 57 58 59 60


reactant could not demonstrate whether PhNO* is the intermediate for PhNO2 hydrogenation; 4. PhNOH* must be the intermediate for the PhNO2 hydrogenation towards PhNH2. The reaction condition governs whether the optimal pathway passes through PhNO* during PhNO2 hydrogenation over Pd(111). At high hydrogen pressure, the pathway without PhNO* is slightly favourable while the Haber mechanism undergoing PhNO* is more favourable under the high vacuum condition.

ASSOCIATED CONTENT Supporting Information The Supporting Information is available free of charge on the ACS Publications website, including details for Gibbs free adsorption energy (Gad) calculation, microkinetic simulation and analysis, energetics of condensation route for PhNO2 and PhNO hydrogenation, the imaginary vibrational frequency value of TS for each element step, adsorption configuration and COHP analysis of PhNO2, PhNO and PhN.

AUTHOR INFORMATION

Corresponding Authors *E-mail address: [email protected] *E-mail address: [email protected] 41



NOTES There are no conflicts to declare

ACKNOWLEDGEMENTS The authors gratefully acknowledge the financial support from NSFC (21333003 and 21673072) and Program of Shanghai Subject Chief Scientist (17XD1401400).

REFERENCES (1) Patai, S. The Chemistry of Amino, Nitroso, and Nitro Compounds and Their Derivatives; Wiley: New York, 1982; Vol. 29. (2) Adams, J. P.; Paterson, J. R. Nitro and Related Compounds. J. Chem. Soc., Perkin Trans. 1 2000, 3695-3705. (3) Nishimura, S. Handbook of Heterogeneous Catalytic Hydrogenation for Organic Synthesis; Wiley: New York, 2001. (4) Blaser, H. U.; Steiner, H.; Studer, M. Selective Catalytic Hydrogenation of Functionalized Nitroarenes: An Update. Chemcatchem 2009, 1, 210-221. (5) Zhang, L.; Cao, X.-M.; Hu, P. Insight into Chemoselectivity of Nitroarene Hydrogenation: A DFT-D3 Study of Nitroarene Adsorption on Metal Surfaces under the Realistic Reaction Conditions. Appl. Surf. Sci. 2017, 392, 456-471. (6) Boronat, M.; Concepcion, P.; Corma, A.; Gonzalez, S.; Illas, F.; Serna, P. A Molecular Mechanism for the Chemoselective Hydrogenation of Substituted Nitroaromatics with Nanoparticles of Gold on TiO2 Catalysts: A Cooperative Effect between Gold and the Support. J. Am. Chem. Soc. 2007, 129, 16230-16237. (7) Makosch, M.; Sa, J.; Kartusch, C.; Richner, G.; van Bokhoven, J. A.; Hungerbuhler, K. Hydrogenation of Nitrobenzene over Au/MeOx Catalysts—a Matter of the Support. Chemcatchem 2012, 4, 59-63. (8) Richner, G.; van Bokhoven, J. A.; Neuhold, Y. M.; Makosch, M.; Hungerbuhler, K. In Situ Infrared Monitoring of the Solid/Liquid Catalyst Interface During the Three-Phase Hydrogenation of Nitrobenzene over Nanosized Au on TiO2. Phys. Chem. Chem. Phys. 2011, 13, 12463-12471. (9) Serna, P.; Concepcion, P.; Corma, A. Design of Highly Active and Chemoselective Bimetallic Gold-Platinum Hydrogenation Catalysts through Kinetic and Isotopic Studies. J. Catal. 2009, 265, 19-25. 42


Page 42 of 48

Page 43 of 48 1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 18 19 20 21 22 23 24 25 26 27 28 29 30 31 32 33 34 35 36 37 38 39 40 41 42 43 44 45 46 47 48 49 50 51 52 53 54 55 56 57 58 59 60


(10) Liu, X.; Li, H. Q.; Ye, S.; Liu, Y. M.; He, H. Y.; Cao, Y. Gold-Catalyzed Direct Hydrogenative Coupling of Nitroarenes to Synthesize Aromatic Azo Compounds. Angew. Chem. Int. Ed. Engl. 2014, 53, 7624-7628. (11) Liu, X.; Ye, S.; Li, H. Q.; Liu, Y. M.; Cao, Y.; Fan, K. N. Mild, Selective and Switchable Transfer Reduction of Nitroarenes Catalyzed by Supported Gold Nanoparticles. Catal. Sci. Technol. 2013, 3, 3200-3206. (12) Corma, A.; Serna, P.; Concepcion, P.; Calvino, J. J. Transforming Nonselective into Chemoselective Metal Catalysts for the Hydrogenation of Substituted Nitroaromatics. J. Am. Chem. Soc. 2008, 130, 8748-8753. (13) Haber, F. Gradual Electrolytic Reduction of Nitrobenzene with Limited Cathode Potential. Z. Elektrochem 1898, 22, 506. (14) Corma, A.; Concepcion, P.; Serna, P. A Different Reaction Pathway for the Reduction of Aromatic Nitro Compounds on Gold Catalysts. Angew. Chem. Int. Ed. Engl. 2007, 46, 7266-7269. (15) Gelder, E. A.; Jackson, S. D.; Lok, C. M. The Hydrogenation of Nitrobenzene to Aniline: A New Mechanism. Chem. Commun. 2005, 0, 522-524. (16) Makaryan, J. A.; Savchenko, V. I. N-Arylhydroxylamines Transformation in the Presense of Heterogeneous Catalysts. Stud. Surf. Sci. Catal. 1993, 75, 2439-2442. (17) Visentin, F.; Puxty, G.; Kut, O. M.; Hungerbuhler, K. Study of the Hydrogenation of Selected Nitro Compounds by Simultaneous Measurements of Calorimetric, Ft-Ir, and Gas-Uptake Signals. Ind. Eng. Chem. Res. 2006, 45, 4544-4553. (18) Sheng, T.; Qi, Y.-J.; Lin, X.; Hu, P.; Sun, S.-G.; Lin, W.-F. Insights into the Mechanism of Nitrobenzene Reduction to Aniline over Pt Catalyst and the Significance of the Adsorption of Phenyl Group on Kinetics. Chem. Eng. J. 2016, 293, 337-344. (19) Bradley, J. M. Control of a Biphasic Surface Reaction by Oxygen Coverage : The Catalytic Oxidation of Ammonia over Pt(100). J. Phys. Chem. 1995, 99, 17032-17042. (20) Xu, L.; Stangland, E. E.; Mavrikakis, M. A DFT Study of Chlorine Coverage over Late Transition Metals and Its Implication on 1,2-Dichloroethane Hydrodechlorination. Catal. Sci. Technol. 2018, 8, 1555-1563. (21) Didas, S. A.; Sakwa-Novak, M. A.; Foo, G. S.; Sievers, C.; Jones, C. W. Effect of Amine Surface Coverage on the Co-Adsorption of CO2 and Water: Spectral Deconvolution of Adsorbed Species. J Phys Chem Lett 2014, 5, 4194-4200. (22) He, L.; Wang, L. C.; Sun, H.; Ni, J.; Cao, Y.; He, H. Y.; Ean, K. N. Efficient and Selective Room-Temperature Gold-Catalyzed Reduction of Nitro Compounds with CO and H2O as the Hydrogen Source. Angew. Chem. Int. Ed. Engl. 2009, 48, 9538-9541. (23) Serna, P.; Boronat, M.; Corma, A. Tuning the Behavior of Au and Pt Catalysts for the Chemoselective Hydrogenation of Nitroaromatic Compounds. Top. Catal. 2011, 54, 439-446. (24) Corma, A.; Serna, P. Chemoselective Hydrogenation of Nitro Compounds with 43



Supported Gold Catalysts. Science 2006, 313, 332-334. (25) Campbell, C. T.; Sellers, J. R. The Entropies of Adsorbed Molecules. J. Am. Chem. Soc. 2012, 134, 18109-18115. (26) Campbell, C. T.; Sellers, J. R. Enthalpies and Entropies of Adsorption on Well-Defined Oxide Surfaces: Experimental Measurements. Chem. Rev. 2013, 113, 4106-4135. (27) Ihm, H.; Ajo, H. M.; Gottfried, J. M.; Bera, P.; Campbell, C. T. Calorimetric Measurement of the Heat of Adsorption of Benzene on Pt(111). J. Phys. Chem. B 2004, 108, 14627-14633. (28) Wellendorff, J.; Silbaugh, T. L.; Garcia-Pintos, D.; Norskov, J. K.; Bligaard, T.; Studt, F.; Campbell, C. T. A Benchmark Database for Adsorption Bond Energies to Transition Metal Surfaces and Comparison to Selected DFT Functionals. Surf. Sci. 2015, 640, 36-44. (29) Zhang, R.; Hao, X.; Duan, T.; Wang, B. Adsorption and Activation of CO and H2, the Corresponding Equilibrium Phase Diagrams under Different Temperature and Partial Pressures over Cu(100) Surface: Insights into the Effects of Coverage and Solvent Effect. Fuel Process. Technol. 2017, 156, 253-264. (30) Shi, Q.; Sun, R. Adsorption Manners of Hydrogen on Pt(100), (110) and (111) Surfaces at High Coverage. Comput. Theor. Chem. 2017, 1106, 43-49. (31) Hellstrom, M.; Behler, J. Surface Phase Diagram Prediction from a Minimal Number of DFT Calculations: Redox-Active Adsorbates on Zinc Oxide. Phys. Chem. Chem. Phys. 2017, 19, 28731-28748. (32) Dronskowski, R.; Bloechl, P. E. Crystal Orbital Hamilton Populations (COHP): Energy-Resolved Visualization of Chemical Bonding in Solids Based on Density-Functional Calculations. J. Phys. Chem. 1993, 97, 8617-8624. (33) Deringer, V. L.; Tchougreeff, A. L.; Dronskowski, R. Crystal Orbital Hamilton Population (COHP) Analysis as Projected from Plane-Wave Basis Sets. J. Phys. Chem. A 2011, 115, 5461-5466. (34) Maintz, S.; Deringer, V. L.; Tchougreeff, A. L.; Dronskowski, R. Analytic Projection from Plane-Wave and Paw Wavefunctions and Application to Chemical-Bonding Analysis in Solids. J. Comput. Chem. 2013, 34, 2557-2567. (35) Maintz, S.; Deringer, V. L.; Tchougreeff, A. L.; Dronskowski, R. Lobster: A Tool to Extract Chemical Bonding from Plane-Wave Based DFT. J. Comput. Chem. 2016, 37, 1030-1035. (36) Maintz, S.; Esser, M.; Dronskowski, R. Efficient Rotation of Local Basis Functions Using Real Spherical Harmonics, 2016; Vol. 47, p 1165. (37) Kresse, G.; Hafner, J. Ab Initio Molecular Dynamics for Liquid Metals. Phys. Rev. B 1993, 47, 558. (38) Kresse, G.; Hafner, J. Ab Initio Molecular-Dynamics Simulation of the Liquid-Metal–Amorphous-Semiconductor Transition in Germanium. Phys. Rev. B 1994, 49, 14251. (39) Kresse, G.; Furthmüller, J. Efficient Iterative Schemes for Ab Initio Total-Energy Calculations Using a Plane-Wave Basis Set. Phys. Rev. B 1996, 54, 11169. (40) Kresse, G.; Furthmüller, J. Efficiency of Ab-Initio Total Energy Calculations for 44


Page 44 of 48

Page 45 of 48 1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 18 19 20 21 22 23 24 25 26 27 28 29 30 31 32 33 34 35 36 37 38 39 40 41 42 43 44 45 46 47 48 49 50 51 52 53 54 55 56 57 58 59 60


Metals and Semiconductors Using a Plane-Wave Basis Set. Comp. Mater. Sci. 1996, 6, 15-50. (41) Perdew, J. P.; Burke, K.; Ernzerhof, M. Generalized Gradient Approximation Made Simple. Phys. Rev. Lett. 1996, 77, 3865. (42) Joubert, D. From Ultrasoft Pseudopotentials to the Projector Augmented-Wave Method. Phys. Rev. B 1999, 59, 1758- 1775. (43) Grimme, S.; Antony, J.; Ehrlich, S.; Krieg, H. A Consistent and Accurate Ab Initio Parametrization of Density Functional Dispersion Correction (DFT-D) for the 94 Elements H-Pu. J. Chem. Phys. 2010, 132, 154104. (44) Grimme, S.; Ehrlich, S.; Goerigk, L. Effect of the Damping Function in Dispersion Corrected Density Functional Theory. J. Comput. Chem. 2011, 32, 1456-1465. (45) Lee, A. F.; Lambert, R. M.; Goldoni, A.; Baraldi, A.; Paolucci, G. On the Coverage-Dependent Adsorption Geometry of Benzene Adsorbed on Pd(111): A Study by Fast XPS and NEXAFS. J. Phys. Chem. B 2000, 104, 11729-11733. (46) Cao, X.-M.; Burch, R.; Hardacre, C.; Hu, P. An Understanding of Chemoselective Hydrogenation on Crotonaldehyde over Pt (111) in the Free Energy Landscape: The Microkinetics Study Based on First-Principles Calculations. Catal. Today 2011, 165, 71-79. (47) Sun, X.; Cao, X.; Hu, P. Theoretical Insight into the Selectivities of Copper-Catalyzing Heterogeneous Reduction of Carbon Dioxide. Sci. Chi. Chem. 2015, 58, 553-564. (48) Wang, Z.; Cao, X. M.; Zhu, J.; Hu, P. Activity and Coke Formation of Nickel and Nickel Carbide in Dry Reforming: A Deactivation Scheme from Density Functional Theory. J. Catal. 2014, 311, 469-480. (49) Hu, W.; Lan, J.; Guo, Y.; Cao, X.-M.; Hu, P. Origin of Efficient Catalytic Combustion of Methane over Co3O4(110): Active Low-Coordination Lattice Oxygen and Cooperation of Multiple Active Sites. ACS Catalysis 2016, 6, 5508-5519. (50) Campbell, C. T.; Sprowl, L. H.; Árnadóttir, L. Equilibrium Constants and Rate Constants for Adsorbates: Two-Dimensional (2D) Ideal Gas, 2D Ideal Lattice Gas, and Ideal Hindered Translator Models. J. Phys. Chem. C 2016, 120, 10283-10297. (51) Sprowl, L. H.; Campbell, C. T.; Árnadóttir, L. Hindered Translator and Hindered Rotor Models for Adsorbates: Partition Functions and Entropies. J. Phys. Chem. C 2016, 120, 9719-9731. (52) Haynes, W. M. CRC Handbook of Chemistry and Physics; CRC press, Florida, 2014. (53) Alavi, A.; Hu, P.; Deutsch, T.; Silvestrelli, P. L.; Hutter, J. CO Oxidation on Pt(111): An Ab Initio Density Functional Theory Study. Phys. Rev. Lett. 1998, 80, 3650-3653. (54) Eck, B., Wxdragon, 1.7. 2. RWTH Aachen: Aachen, Germany 2011.

45



46


Page 46 of 48

Page 47 of 48 1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 18 19 20 21 22 23 24 25 26 27 28 29 30 31 32 33 34 35 36 37 38 39 40 41 42 43 44 45 46 47 48 49 50 51 52 53 54 55 56 57 58 59 60


Table of Contents Graphic

47



Table of Contents 36x17mm (300 x 300 DPI)


Page 48 of 48

Insights into Different Products of Nitrosobenzene ... - ACS Publications

Recommend Documents