Mechanisms of Pyrrole Hydrogenation on Ru(0001) and Hydrogen

Article pubs.acs.org/JPCC

Mechanisms of Pyrrole Hydrogenation on Ru(0001) and Hydrogen Molybdenum Bronze Surfaces Yongjie Xi,† Liang Huang,‡ and Hansong Cheng*,‡ †

Department of Chemistry, National University of Singapore, 3 Science Drive 3, Singapore 117543, Singapore Sustainable Energy Laboratory, Faculty of Material Science and Chemistry, China University of Geosciences, Wuhan 430074, China

‡

S Supporting Information *

ABSTRACT: Hydrogenation of pyrrole and its derivatives is of fundamental importance in chemical and pharmaceutical processes for which a mechanistic understanding is still lacking. The objective of the present study is to unveil the energetic profiles of pyrrole hydrogenation on Ru(0001) along the prescribed routes by periodic density functional theory (DFT) calculations. It was found that the activation energy of the rate-limiting step of the hydrogenation process is 1.05 eV. The calculated desorption energy of the hydrogenated product is 1.24 eV. We also found hydrogen molybdenum bronze (HxMoO3) to be an efficient catalyst material for pyrrole hydrogenation via detailed mechanistic investigations. HxMoO3 is expected to outperform ruthenium owning to the lower hydrogenation barrier and more facile desorption of the product. We rationalize the more favorable energetics of pyrrole/HxMoO3 by the lower adsorption energy of pyrrole and the protonic nature of H in HxMoO3. The chemistry revealed in this work provides meaningful insight into the design of low-cost hydrogenation catalysts. ces.24−27 In contrast to the extensive experimental studies of pyrrole hydrogenation and adsorption, there is a relative paucity of computational investigations. Pyrrole adsorption modes on Mo(110) have been elaborated with DFT calculations.28 Mechanisms of pyrrole hydrogenation on NiMoS, a catalyst involved in a hydrotreating process whereby sulfur and nitrogen are removed from petroleum fractions, were explored computationally.29 Unfortunately, a fundamental mechanistic understanding of the hydrogenation of pyrrole on surfaces of commonly used transition-metal catalysts is still lacking. Hydrogen bronzes have been perceived as promising hydrogenation catalysts,30−32 of which hydrogen molybdenum bronze (HxMoO3, 0 < x < 2)33 and hydrogen tungsten bronze (HxWO3, 0 < x < 0.6)34 are noticeable examples. The commonly held perception of the formation of hydrogen bronzes via a H spillover mechanism is that H2 can undergo dissociative chemisorption on transition-metal (Pt, Pd, etc.) particles followed by facile H migration from the metal particles to the terminal oxygen atoms on the oxide surfaces and further H diffusion throughout the oxide lattices. It was found that the formation process can be accelerated by water.35−37 Other preparation methods involve mechanical milling of oxides (e.g., WO3) with liquid hydrocarbons or under hydrogen atmosphere. Experimentally, it was demonstrated that ethylene30 and acrolein31 can be hydrogenated by bronze materials with high efficiency. DFT calculations revealed that hydrogenation of

1. INTRODUCTION Catalytic hydrogenation of unsaturated compounds is of fundamental importance for a wide range of commodity chemicals, petrochemicals, and pharmaceuticals.1−6 Design of efficient and selective hydrogenation catalysts using earthabundant elements has been an active areas of research in chemistry for decades. 7,8 Extensive studies have been performed on hydrogenation of aromatic rings on many heterogeneous catalysts.9 With a nitrogen-containing fivemembered aromatic ring, pyrrole has attracted considerable attention, partly due to its negative impact on fuel reforming as an environmental hazard.10 Another aspect showing the importance of pyrrole hydrogenation is the usefulness of partially (pyridines) or fully H-saturated pyrroles (pyrrolidines) in pharmaceutical processes.11−13 Apart from these, hydrogenation of pyrrole can also be used as a probe for studying size and shape effects of metal catalysts.14,15 Hydrogenation of pyrrole and its derivative was achieved over precious metals (Pt, Pd, Ru, Rh) or Raney nickel as early as the 1930s.16−24 Diastereoselection products of hydrogenation of pyrrole derivatives were also obtained by adding a chiral auxiliary to the heterogeneous catalyst.11 The poisoning effect of pyrrole in hydrogenation is attributed to the unshared pair of electrons on N,24 which is commonly eliminated using protic acids.11 The interactions between pyrrole and heterogeneous catalyst were extensively studied by experimental methods including near edge X-ray absorption fine structure (NEXAFS), angle-resolved UV photoelectron spectroscopy (ARUPS), sum-frequency generation (SFG), surface vibrational spectroscopy, and helium spin−echo (HeSE) methods over Pd(111), Pt(111), Rh(111), and Cu(111) crystalline surfa© 2015 American Chemical Society

Received: July 7, 2015 Revised: August 23, 2015 Published: September 15, 2015 22477

DOI: 10.1021/acs.jpcc.5b06486 J. Phys. Chem. C 2015, 119, 22477−22485

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The Journal of Physical Chemistry C

Figure 1. (a) Schematics of the Ru(0001) slab model. Pyrrole in the gas phase and the most favorable adsorption configuration on Ru(0001) (Atop1 mode in Table S2, SI) and different adsorption sites on Ru(0001) are also shown. The bond lengths are labeled in angstroms. (b) Isosurface of charge density difference upon formation of the pyrrole/Ru(0001) interface: pink, charge accumulation; yellow, charge depletion. (c) The slab model of molybdenum oxide. Different oxygen sites are assigned.

using a plane wave basis set with a cutoff energy of 400 eV to describe the valence electrons, and the electron−ion interactions were described by the projector augmented wave (PAW)42,43 method. For calculations of a gas-phase species, a single k-point was used with a box size of 15 × 15 × 15 Å3, whereas for surface cells, the k-point sampling was performed with a 3 × 3 × 1 Monkhorst−Pack44 with a SCF convergence criterion of 5 × 10−5 eV. The force threshold for structural optimization was set to 0.05 eV/Å. The energetic profiles along the prescribed reaction pathways were located using the climbing image nudged elastic band (CI-NEB) method.45 The calculated transition states were verified by computing the vibrational frequencies that yield one imaginary frequency [Table S1, Supporting Information (SI)]. The optimized lattice parameters of the hexagonal closepacked (hcp) Ru were found to be a = 2.72 Å and c = 4.31 Å, in good agreement with the experimental values of a = 2.71 Å and c = 4.28 Å.46 A four-layer (4 × 4) Ru(0001) slab with a vacuum layer of 15.0 Å was constructed to model the reaction pathways of pyrrole on ruthenium. The optimized lattice parameters of a = 3.963 Å, b = 14.031 Å, and c = 3.696 Å for the MoO3 unit cell are in reasonable agreement with the experimental data of 3.963 Å, 13.855 Å, and 3.696 Å, respectively.33 To describe the hydrogenation of pyrrole on HxMoO3, a four-layer (2 × 2) MoO3 (010) slab with a 15.0 Å vacuum layer was constructed, which is in line with a previous study (Figure 1).32 HxMoO3 was described by attaching 2 H atoms to the terminal oxygen of the MoO3 slab comprising 16 MoO3 units, corresponding to x = 0.125. The adsorption energy of a surface species is defined by Eads = Esurface+adsorbate − (Esurface + Eadsorbate).

ethylene over HxMoO3 occurs with a moderate activation energy ranging from 0.4 to 0.8 eV, depending on the hydrogen content of HxMoO3. The calculated thermodynamics was found to be consistent with the experimental observations.32 Therefore, it is reasonable to expect that hydrogen bronzes may display similar catalytic performance for hydrogenation of pyrrole. The aim of the present work is to understand the mechanism of pyrrole hydrogenation on a transition-metal surface and to predict the feasibility of pyrrole hydrogenation by hydrogen bronzes. Specifically, the energetic profiles of pyrrole hydrogenation on Ru(0001), which occurs at 130−150 °C and 25− 30 bar in an experiment,20 were investigated; HxMoO3 was selected as the candidate bronze material since it can be more readily formed38 and may contain a higher hydrogen content than tungsten bronze. Transition-metal-oxide bronzes may serve as inexpensive alternatives of precious metal catalysts for hydrogenation. This paper is organized as follows. The computational methodology is briefly discussed in section 2. The adsorption mode of pyrrole on Ru(0001) is then described in section 3.1 and the calculated possible energetic pathways of pyrrole hydrogenation are presented in section 3.2. The adsorption and hydrogenation of pyrrole on HxMoO3 are treated in the same vein in section 3.3. The results are analyzed and discussed in section 4, and conclusions that may be derived from this work are summarized in section 5.

2. COMPUTATIONAL METHODS AND MODELS The first-principles calculations were carried out using periodic density functional theory, as implemented in the Vienna ab initio simulation package (VASP).39,40 The exchange− correlation effects were incorporated in the spin-polarized generalized gradient approximation (GGA) with the PBE functional.41 The Kohn−Sham equation was solved iteratively

3. RESULTS AND DISCUSSIONS 3.1. Pyrrole Adsorption on Ru(0001). To map out the reaction pathways of pyrrole hydrogenation, we first identified 22478


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as a superscript or subscript denotes the initial state for hydrogenation. (iii) R12 Int−1 denotes the first step of 1,2-addition, where the superscript denotes 1,2 addition and the subscript stands for addition of hydrogen on C1 of IInt. 21 (iv) R21 denotes the consecutive steps of R21 Int−2 and R2−12; i.e., a H is added to C2, which is followed by C1 addition. In principle, partial hydrogenation of pyrrole on Ru(0001) occurs in three ways: forming 1,2-adduct (identical to the 3,4adduct due to symmetry), 1,4-adduct, and 2,3-adduct. The third way, however, forms unstable diradical, which was not considered in our investigations. The complete hydrogenation process can be either 1,2−3,4 addition or 1,4−2,3 addition. The adsorption energy of H on the face-centered cubic (fcc), hcp, and atop sites (Figure 1a) of Ru(0001) were calculated to be −0.67, −0.62, and −0.18 eV, respectively, which agree well with the reported value that the fcc site is 0.05 eV more energetically favorable than the hcp site.47 H adsorption on the bridge site is unstable; after geometry optimization it resides on the fcc site. The activation energy for the dissociative chemisorption of H2 on Ru(0001) was calculated to be only 0.05 eV and the diffusion of H is also facile, with the energy barrier being around 0.1 eV (Figure S1, SI), in accord with the literature.47,48 The dissociative chemisorption of H2 on Ru(0001) (−1.31 eV in Figure S1, SI) is comparable with the adsorption energy of pyrrole (−1.64 eV). Hence, it is reasonable to expect the reaction to follow the Langmuir− Hinshelwood mechanism. Given the fact that the dissociation of H2 and the migration of H on the surface are both facile, it is reasonable to assume that atomic hydrogen is abundant on the surface for the hydrogenation reaction. H atoms are placed at the fcc sites neighboring the C atoms to which the H atoms are supposed to add. We first considered the 12−34 pathways, corresponding to R12−R34, R21−R34, R12−R43, R21−R43. The minimum energy pathways (MEPs), associated with the initial, intermediate, and final states, are displayed in Figure 4. The coadsorption energy of pyrrole and two H atoms (IInt) on Ru(0001) was calculated to be −2.53 eV, slightly higher than the sum of the adsorption energy of pyrrole and two H atoms (−2.95 eV). R12 Int−1 has an activation energy of 0.73 eV, forming I1 with a slight endothermicity of 0.09 eV. R12 is then completed via R12 2−12, which has an energy barrier of 0.82 eV. Overall, R12 has an effective barrier49 of 0.91 eV and is endothermic by 0.44 eV. In parallel, R21 Int−2 needs to surmount a 1.21 eV barrier, forming I2 with an endothermicity of 0.34 eV, which further overcomes a 0.67 eV barrier to form I12 (pyridine). Considering the whole energetic profile, R21 has an effective barrier of 1.21 eV. The noticeable difference of the 21 activation energies of R12 Int−1 and RInt−2 can be understood from the calculated frontier orbitals of pyrrole. Both the HOMO and the LUMO of the molecule have a larger lobe on the α-carbon (C1 or C4) than the β-carbon (C2 or C3) (Figure S2, SI), suggesting a higher reactivity of C1. It is worthy to note that for H addition on the same atom, the activation energy of R21 2−12 is lower than that of R12 Int−1 (corresponding to H addition on C1); 21 also the activation energy R12 1−12 is lower than that of RInt−2 (H addition on C2). This is due to the fact that a one-hydrogenadduct is energetically unstable, forcing the intermediate to undergo further hydrogenation. 34 43 I12 can be further hydrogenated to I12 34 through R or R in 34 43 the presence of another H2. RInt−3 and RInt−4 proceed with the activation barriers of 0.66 and 1.05 eV, respectively, and the 12 intermediate state of I12 3 is energetically lower than that of I4 by

the most favorable adsorption mode. The parallel mode was first taken into account. Four types of adsorption sites are presented in Figure 1. We performed structure relaxations with the N atom located at each of the adsorption sites and other parts of the molecule placed in a symmetric fashion on the Ru(0001) surface, constituting eight initial structures, as displayed in Table S2 (SI), among which the atop mode (Figure 1a; see also Atop-1 in Table S2, SI) with an adsorption strength of −1.64 eV was identified to be the lowest energy structure. Upon adsorption, the carbon atoms of pyrrole favor the sp3 electronic configuration with the four H atoms tilting from the C4N plane. The C−C and C−N bonds are elongated by 0.053−0.085 Å. Figure 1b displays the isosurfaces of the calculated charge density difference of the Atop-1 mode structure. It is evident that the charge accumulates between the pyrrole ring and the Ru surface, which indicates a covalent bonding interaction between the molecule and the substrate and is consistent with the strong calculated adsorption energy. The charge depletion on the pyrrole ring is also indicative of the weakened C−C and C−N bonds. We also considered a few possible vertical adsorption modes. Starting from several putative vertical configurations with the N atom anchored on the Ru(0001) surface, we obtained optimized adsorption structures in parallel to the Ru(0001) surface. Interestingly, these structures are also coincident with the optimized structures with the initial putative parallel configurations. Previous studies on pyrrole adsorption on Pt(111)25 and Pd(111)27 surfaces revealed that the N−H bond undergoes dissociation upon adsorption. Therefore, it is informative to investigate the N−H scission in the case of pyrrole on Ru(0001). Taking Atop-1 as the initial state, however, we obtained a high dissociation barrier of 1.24 eV (Figure 2),

Figure 2. Calculated energetic profile for N−H bond dissociation of pyrrole on Ru(0001). TS = transition state.

indicating that N−H dissociation is unlikely to occur on the Ru(0001) surfce under ambient conditions. On the basis of the above analysis, we chose Atop-1 for the ensuing studies of pyrrole hydrogenation. 3.2. Hydrogenation of Pyrrole on Ru(0001). Partial hydrogenation of pyrrole forms dihydropyrrole, which can be further saturated to tetrahydropyrrole. For clarity, we use the following nomenclature to describe the reaction species and pathways (Figure 3): (i) I and R denotes reaction intermediate and reaction steps, respectively. (ii) I1 denotes the intermediate state for which H is added to C1, while the fully hydrogenated species is denoted by I12 34; Int 22479


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Figure 3. Schematic illustrations of possible reaction pathways, nomenclature for reaction species and pathways, and labeling scheme. C1 and C4, as well as C2 and C3, are in symmetrical positions when pyrrole is in the gas phase. R14 and R23 are identical to R41 and R32 here. Depending on the symmetry of catalyst surfaces, however, C1 and C4 or C2 and C3 may have different environments. Hence, R14, R41, R23, and R32 are all accounted for (in the case of 1,4−2,3 addition for HxMoO3, vide infra).

Figure 4. Energetic profiles of R12−R34, R21−R34, R12−R43, and R21−R43. The H atoms involved in the reactions are highlighted in green; the relative energy of transition states are shown in red. Both the top view and the side view of I12 34 are displayed.

Figure 5. Energetic profiles of R14−R23. H atoms involved in reactions are highlighted in green; the relative energy of transition states is shown in red.

0.13 eV. To produce I12 34 (pyrrolidine), a 0.94 eV barrier is 34 required for R3−34, as compared to 0.57 eV for R43 4−34. Again, we 34 observe that the activation energy of RInt−3 is higher than that 43 of R43 4−34. For H addition on C3, the barrier for RInt−4 is larger than that of R34 3−34 (H addition to C4), in the same vein with the

above-mentioned case of 1,2 addition. Overall, the effective barriers of R34 and R43 are 1.29 and 1.05 eV, respectively. We subsequently investigated the 1,4−2,3 reaction pathways 1 (Figure 5). R14 Int−1 proceeds with a barrier of 1.13 eV to form I , 14 which is further hydrogenated to I upon surmounting a 0.80 eV barrier. The energy of the formed I14 is 0.10 eV higher than 22480


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Figure 6. (a−c) Energetic profiles of pyrrole hydrogenation on H0.125MoO3 along the 1,2−3,4 route; the activation energies of the transition states are shown in red. (d) The optimized surface structures of the reactant, the intermediates, and the product. The H atoms involved in the reactions are highlighted in green. The H−C or H−O bond distances are labeled in angstroms.

limiting step of R14−R23. Compared with the energetic profiles of the 1,2−3,4 addition, we finally identified R12−R43 to be the most favorable reaction pathway for the hydrogenation process. To complete the reaction, however, a 1.24 eV barrier needs to be overcome for pyrrolidine to desorb from the Ru(0001) surface. As shown in Figure 4, the N atom of I12 34 interacts with the Ru surface with the calculated N−Ru bond length of 2.275 Å. The high adsorption energy of pyrrolidine/Ru(0001) can be attributed to the strong interaction between the unshared electron pair of N and the Ru surface, which is in agreement with the experimental observation.24 As mentioned, the poisoning effect of pyrrolidine can be eliminated by using protic acids.11 3.3. Hydrogenation of Pyrrole on HxMoO3(010). Hydrogenation of unsaturated compounds by hydrogen

that of I12, in line with the gas-phase stability in which I12 is energetically more stable than I14 by 0.24 eV. Therefore, 1,2addition is more favorable than 1,4-addition both thermodynamically and kinetically. Two H atoms are placed on symmetrical fcc sites neighboring C2 and C3. The formation 23 of I14 2 via RInt−2 needs to overcome a 0.23 eV barrier [to differentiate I14 2 with the states of ongoing steps, it is denoted as I14 2 (fcc-1) in Figure 5]. Due to the facile migration of H on Ru(0001) (see Figure S1, SI), the other H atom can migrate to a nearby hcp site [denoted as I14 2 (hcp) in Figure 5] and further 14 to another fcc site [I14 2 (fcc-2)]. The I2 (fcc-2) state is 0.1 eV (fcc-2) and is identical to the I12 lower in energy than I14 2 4 state 23 in Figure 4. Thus, R2−23 has the same energetic profile as R43 4−34 with an activation energy of 0.57 eV. Hence, R14 Int−1 with an activation energy of 1.13 eV was identified to be the rate22481


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Figure 7. (a and b) Energetic profiles of pyrrole hydrogenation on H0.125MoO3 along the 1,4−2,3 route; the activation energies of the transition states are shown in red. (c) Optimized structures of the reactant, the intermediates, and the product. The H atoms involved in reactions are highlighted in green. The H−C and H−O bond distances are labeled in Å.

(b) in Figure 6] are −1.11 and −0.83 eV, respectively, which are comparable to the sum of adsorption energies for two H atoms on MoO3 and pyrrole on the surface at respective configurations (−0.86 and −0.93 eV). This suggests that the presence of H on the MoO3 surface does not impede the adsorption of pyrrole. The R12 (a), R12 (b), R21 (a), and R21 (b) reaction routes were considered first, as shown in Figure 6. IInt (a) is formed by attaching two H atoms to O3 and O4 (oxygen sites are labeled in Figure 1) of IInt (a)-pre. Starting from IInt (a), I1 (a) and I2 21 (a) can be formed through R12 Int−1 (a) and RInt−2 (a) with the activation barriers of 0.30 and 0.53 eV (Figure 6a), respectively. Despite the long distance from the H atom on O4 to C2 (3.761 Å, Figure 6d), the relatively weak adsorption of pyrrole (0.24 eV) enables the molecule to adjust its position for more favorable hydrogen attack, accounting for the moderate barrier of R21 Int−2 (a). Again, the same frontier orbital rationale applies for the energy barrier discrimination of H addition on α and β carbons. To form I12 (a), an essentially insurmountable barrier of 2.32 eV was observed for R12 1−12 (a), probably because the open-shell intermediate species I1 (a) is strongly anchored by the HxMoO3 surface with a desorption energy of 1.55 eV, which makes it difficult for I1 (a) to reorient to shorten the long distance (3.642 Å) between C2 and the H on O4. In contrast, R21 2−12 (a) proceeds with a much lower barrier of 0.62 eV. 21 Considering the endothermicity of R21 Int−2 (a) (0.34 eV), R (a) has an effective barrier of 0.96 eV. For IInt (b), H atoms are first placed at O1 and O3 (Figure 1), which are then added to C1 and C2, respectively. R12 Int−1 (b) has a 0.21 eV barrier and is exothermic by −0.23 eV (Figure 6b). Subsequently, I1 (a) undergoes hydrogenation with a 0.78 eV barrier. In parallel, the respective barriers for R21 Int−2 (b) and

bronzes was demonstrated in experiments.31 Herein, we examine the reaction pathways of pyrrole hydrogenation over the H 0.125 MoO3 (010) surface. Starting from the eight configurations, we optimized the adsorption of pyrrole on MoO3(010) (Figure S3, SI). Configuration 1 with the hydrogen bonded nitrogen pointing to oxygen has marginal energetic preference (0.03 eV) over 2, where the hydrogen atoms on α carbon atoms of pyrrole are attracted by the terminal oxygen atoms of the bronze. In both cases, the elongation of C−N or C−C bond lengths upon pyrrole adsorption is within 0.016 Å, much smaller than the value in the pyrrole/Ru(0001) counterpart. The other six configurations exhibit considerably higher energies and are thus less stable. Therefore, 1 and 2 are subjected to ensuing investigations. Two H atoms are then populated to the terminal oxygen of 1 and 2, which are favorable for hydrogenation along the prescribed pathways. This is due to the fact that hydrogen diffusion between the adjacent terminal sites or from the internal sites to the terminal sites has low barriers,37,50 and therefore, H atoms are available whenever possible. The adsorption energies of two H atoms on MoO3(010) were calculated to be −0.43 eV for O1 and O2 (or O3 and O4), −0.53 eV for O1 and O3 (or O2 and O4), and −0.65 eV for O1 and O4 (or O2 and O3), respectively, with different oxygen sites depicted in Figure 1c. We adopt essentially the same labeling scheme used in the pyrrole/ Ru(0001) case, except that the reactions initiated from 1 and 2 are differentiated by an additional (a) and (b), respectively [e.g., I1 (a), I1 (b); also IInt (a)-pre, IInt (b)-pre and IInt (a), IInt (b)], and are used to distinguish the adsorption configurations on H0.125MoO3 and MoO3(010) surface, respectively. The coadsorption energies of two H atoms and pyrrole on the MoO3(010) surface for the two configurations [IInt (a) and IInt 22482


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The Journal of Physical Chemistry C 21 R21 (b) the most 2−12 (b) are 0.23 and 0.72 eV, making R 12 kinetically favorable pathway to form I . 12 We next turn to the formation of I12 34. In principle, both I 12 12 (a) and I (b) should be the starting points for forming I34 in view of the fact that I12 (b) is the kinetic product while I12 (a) is energetically lower than I12 (b) by 0.14 eV. However, the orientation of I12 (a) is unfavorable for further hydrogenation. We calculated the energy barrier for the reorientation of I12 (a) to I12 (b), which is about 0.22 eV, as shown in Figure S4 (SI). Hence, we only treat the hydrogenation of I12 (b) and no differentiation of (a) and (b) is thus needed. Two H atoms are positioned in I12 (b) to form I12 Int (Figure 6c). Notably, the was found to be essentially barrierless, as the formation of I12 3 NEB algorithm failed to locate a transition state along the prescribed pathway, and exothermic by 0.92 eV. R34 3−34 then proceeds with a 0.55 eV barrier to complete the reaction. R43 Int−4 and R43 4−34 have energy barriers of 1.25 and 0.23 eV, respectively. 43 The distinctive energetic profiles of R34 Int−3 and RInt−4 are consistent with the difference of the bond distances of H(O2)− C4 and H(O4)−C3 (3.778 vs 2.274 Å). The shorter bond distance suggests that migration of H atoms from the HxMoO3 surface to I12 is more facile. The desorption of the fully saturated pyrrolidine (I12 34) needs to overcome a moderate energy of 0.67 eV, considerably lower than that of Ru(0001) (1.24 eV). Another possible reaction route of pyrrole hydrogenation on HxMoO3 is the 1,4−2,3 addition route, which also bifurcates since two initial adsorption states are involved (Figure 7). Two H atoms are placed on O2 and O3 of IInt (a)-pre (Figure 7c and 1 in Figure S3, SI) and on O1 and O2 of IInt (b)-pre (Figure 7c and 2 in Figure S3, SI) to form IInt (a) and IInt (b), which allow reactions to proceed along the prescribed pathways. Since C1 and C4 in IInt (a) are essentially in symmetric 14 positions, we only consider the routes R14 Int−1 (a) and R1−14 (a) 14 (Figure 7a). RInt−1 (a) has an activation energy of 0.27 eV, forming I1 (a), which is further hydrogenated via R14 1−14 (a) with a barrier of 0.83 eV. In contrast, C1 and C4 of IInt (b) have distinctive coordination environments, as displayed in Figure 7c. Therefore, the preference of H addition onto C1 and C4 is accounted for. R14 Int−1 (b) requires one to overcome a 0.52 eV barrier and has an endothermicity of 0.38 eV. The reaction is then followed by further hydrogenation via R14 1−14 (b) with a barrier 0.68 eV. Hence, R14 (b) has an effective barrier of 1.01 41 eV. In parallel, the respective barriers for R41 Int−4 (b) and R4−14 (b) are 0.59 and 0.64 eV, with the effective barrier of the overall process being 1.10 eV. We note here that the configuration of R14 (b) is favorable for further hydrogenation while I14 (a) is not. Therefore, the I14 (a) intermediate, despite being formed with a lower activation energy, must be reoriented to I14 (b) to enable further hydrogenation. Starting from I14 (b), two H atoms are positioned at O3 and 14 O4 to form I14 Int. Unlike the case for 2,3 addition of IInt on Ru(0001), C2 and C3 have quite different environments on the HxMoO3 surface; therefore, both R23 and R32 were considered. 23 32 32 We found that R23 Int−2−R2−23 and RInt−3−R3−23 proceed with comparable energetics (Figure 7b). The former has activation energies of 0.70 and 0.16 eV, while the respective values for the latter are 0.77 and 0.12 eV. The effective barriers of the R23 and R32 routes are 0.72 and 0.65 eV, respectively. From the calculated energetic profiles of 1,2-addition, we finally identify that the MEP of pyrrole hydrogenation on HxMoO3 follows the R21 (b)−R34 3−34 route.

4. DISCUSSIONS ON PYRROLE HYDROGENATION OVER Ru(0001) AND HxMoO3(010) We have calculated the energetic profiles of pyrrole hydrogenation on the two crystalline surfaces with distinctively different nature. On the Ru(0001) surface, H atoms are negatively charged for which the calculated Bader charge51 on H is ca. −0.2|e|, and thus, the pyrrole molecule encounters nucleophilic attack by hydrogen. In sharp contrast, on HxMoO3(010), the H atoms are essentially protonic,50 and consequently, pyrrole is under electrophilic attack by hydrogen. The hydrogenation of pyrrole on Ru(0001) necessitates one to overcome a 1.05 eV barrier, and the desorption of the fully saturated product is endothermic by 1.24 eV. In comparison, hydrogenation of pyrrole and the desorption of pyrrolidine on HxMoO3(010) have respective barriers of 0.72 and 0.67 eV, considerably lower than the former case. For pyrrole adsorption on Ru(0001), the unsaturated C atoms are sp3-like and can be readily attacked by hydrogen. Despite that, the aromatic ring is strongly anchored on the surface due to the covalent interaction with the substrate, which makes it difficult to adjust its orientation upon the initial hydrogen addition. Consequently, further hydrogenation requires a relatively high activation barrier. In contrast, the adsorption of pyrrole on HxMoO3(010) is much weaker, which makes it easier for pyrrole to adjust its adsorption configuration. Another key factor responsible for the lower reaction barrier is that the protonic H on HxMoO3 gives rise to a favorable electrostatic interaction with the negatively charged carbon of pyrrole, making the electrophilic attack relatively facile. Usually, the hydrogen content of HxMoO3 is up to x = 2, well above the value used in the present study. It has been shown that the hydrogenation rate of olefins increases with the hydrogen content in bronze materials.32 We can envisage that the hydrogenation efficiency of molybdenum bronzes can be improved at higher hydrogen contents. HxMoO3 can be prepared with several means52 and upon release of hydrogen, HxMoO3 can be regenerated. Thus, MoO3 serves the role of catalyst in the entire reactive process. In particular, diastereoselective hydrogenation by HxMoO3 may be expected with the participation of appropriate chiral auxiliaries.11 5. CONCLUSION Hydrogenation of pyrrole and its derivatives is of fundamental importance in chemical and pharmaceutical processes. A mechanistic understanding of pyrrole catalytic hydrogenation is essential for the design of cost-effective and efficient catalysts. In the present study, we examined the energetic profiles of pyrrole hydrogenation on Ru(0001) along the prescribed pathways of the 1,2−3,4 addition and the 1,4−2,3 addition. By sampling different adsorption configurations and examining the feasibility for N−H bond dissociation, we identified the initial structure for hydrogenation reaction. We concluded the most favorable hydrogenation pathway is R12−R43, which has a 1.05 eV barrier in the rate-limiting step. The desorption energy of the fully hydrogenated product is 1.24 eV. The calculated energetics of the reaction is in accordance with the experimental condition of comparatively high temperature and pressure and also echoes the necessity of adding a protonic acid to the reaction system. We also proposed the hydrogenation of pyrrole by hydrogen molybdenum bronze, for which the reaction pathways were examined in the same vein with pyrrole/Ru(0001). The activation energy of the rate22483


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limiting step along the MEP was calculated to be 0.72 eV, and the desorption energy of the fully saturated product is 0.67 eV. We ascribe the more favorable energetics of pyrrole/Ru(0001) to the lower adsorption energy of pyrrole on HxMoO3 and the protonic nature of H in HxMoO3. This study provides a new avenue to hydrogenate unsaturated compound efficiently with low-cost catalysts. Experiments of pyrrole hydrogenation by HxMoO3 are underway, which will be published in due course.

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ASSOCIATED CONTENT

S Supporting Information *

The Supporting Information is available free of charge on the ACS Publications website at DOI: 10.1021/acs.jpcc.5b06486. Transition state configuration and imaginary vibrational frequency for each reaction step; putative and optimized configurations of pyrrole on different adsorption sites of Ru(0001); desorption, dissociation, and diffusion of hydrogen on Ru(0001) surface; frontier orbitals of pyrrole; optimized adsorption configurations of pyrrole on MoO3(010); energetic profile for the reorientation of I12 (a) to I12 (b), as well as atom coordinates of reaction species in Figure 4−7 (PDF)

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AUTHOR INFORMATION

Corresponding Author

*E-mail: [email protected]. Notes

The authors declare no competing financial interest.

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ACKNOWLEDGMENTS The authors gratefully acknowledge support of a start-up grant from NUS, a POC grant from National Research Foundation of Singapore, a Tier 1 grant from Singapore Ministry of Education, a DSTA grant, and the National Natural Science Foundation of China (No. 21233006 and 21473164).

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REFERENCES

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Mechanisms of Pyrrole Hydrogenation on Ru(0001) and Hydrogen

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