Hydrogenation of Hydrogen Cyanide to Methane and Ammonia by a

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Hydrogenation of Hydrogen Cyanide to Methane and Ammonia by a Metal Catalyst: Insight from First-Principles Calculations Ming-Kai Hsiao, Wen-Ting Lo, Jia-Hui Wang, and Hui-Lung Chen J. Phys. Chem. C, Just Accepted Manuscript • DOI: 10.1021/acs.jpcc.6b06490 • Publication Date (Web): 08 Sep 2016 Downloaded from http://pubs.acs.org on September 11, 2016

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The Journal of Physical Chemistry

Hydrogenation of Hydrogen Cyanide to Methane and Ammonia by a Metal Catalyst: Insight from First-Principles Calculations

Ming-Kai Hsiao, Wen-Ting Lo, Jia-Hui Wang, and Hui-Lung Chen* Department of Chemistry and Institute of Applied Chemistry, Chinese Culture University, Taipei, 111, Taiwan

*Corresponding author E-mail: [email protected] Tel: +886-2-28610511 ext 25313 Fax: +886-2-28617006

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Abstract The adsorption and hydrogenation behaviors of hydrogen cyanide to methane and ammonia formation by W(111) catalyst were systematically investigated using the density functional theory method. Based on our calculated consequences, it is found that the WHCN(T,T-µ2-C,N) is calculated to be the most stable conformer, possessing an adsorption energy of -49.8 kcal/mol, among all calculated structures of HCN/W(111) system. To comprehend the electronic property of its interaction between the adsorbate and substrate, we calculated the electron localization functions, local density of states, and Bader charges; our results were consistent and explicable. Reaction paths in all possible mechanisms were explored in detail, involving the hydrogenation on different orientations of each adsorbate and the scission of the carbon-nitrogen bond. Before forming an imine intermediate (H2CNH(a)), two adsorbed hydrogen atoms will sequentially react with the nitrogen and then carbon atoms in the first and second hydrogenation steps, and the corresponding activation barriers are calculated to be 37.4 and 16.3 kcal/mol, respectively. After yielding an imine intermediate (H2CNH(a)), however, the breaking of carbon-nitrogen bond is likely to proceed at this stage with a pertinent barrier height of 27.5 kcal/mol, forming CH2(a)+NH(a). At elevated temperatures, these adsorbates could be desorbed by further consecutive hydrogenations to generate the final products of methane and ammonia. Our findings provide atomistic-level insights into the new pathways for methane and ammonia syntheses via the facile hydrogenation of HCN by effectively catalytic surface of W(111).

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1. Introduction The gaseous hydrogen cyanide (HCN) in atmosphere, a general endothermic species, is generated primarily in biomass burning through pyrolysis of N-containing compounds. Even though it has a apparent natural mutability in HCN emissions and within a single or resembling fire kinds, it is usually employed as a tracer of pollution deriving from conflagrations1,2 to deconvolute mixtures of urban and biomass burning emissions.3 In addition to biomass flaming, the gaseous hydrogen cyanide could be composed in lightening disturbed air in the troposphere of the progressive earth,4,5 constructing lightening a supplementary beginning of HCN.6 The examination of the constructions for CN-containing molecules adsorbing on metallic substrates is very significant to understand their reactivity and characteristics in catalysis and other phenomena of varied surfaces. The gaseous HCN molecule is the simplest nitrile form including carbon-nitrogen triple bond. Therefore, researches of the interaction of HCN with a series of metal surfaces can provide profitable insights into the C-N, C-H, and N-H bonds activation behaviors. Hydrogenation utilizations for several kinds of nitriles under liquid phase and with raised pressure of hydrogen were utilized by a lot of chemical factories to manufacture varied significant amines. As primary amines were found to be the useful products in many fields of applications, some effective catalysts such as nickel and cobalt have been discussed in detail by catalytic reactions.7 Even though the activity and selectivity for above catalysts were shown to be varied, some possible causes of discrepancies are still did not appropriately comprehend.8 About the reaction mechanisms of these processes, it was generally shown that the hydrogen addition on nitriles could lead to obtain primary amines progressing via an intermediate imine, 3



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which could condense to the primary amine together with the formation of byproducts of secondary amines.9 In experiment, some interaction behaviors between hydrogen cyanide with varied stoichiometric materials have been investigated using many up-to-date techniques. Two adsorption characteristics were already found and fully discussed for the HCN/W(100) system: (1) under the lower temperature condition, the HCN will be easily anchored on tungsten surface by perpendicular types, and (2) parallel kinds of adsorptions will be found at higher temperature instead.10 There are still two different types of surfaces, Cu(100) and Pt(111),11,12 were used to explore the adsorption behaviors for HCN molecule; they both found that the HCN molecule only has the perpendicular type of adsorption on these metal surfaces. It was shown that the catalytic interaction between HCN and Pt(111) leads to production of NCH2 and CN, nevertheless the further bond scission phenomena of CN bond can be effortlessly achieved by Ni(111),13 Ru(0001)14 and γ-Al2O3.15 Besides the hydrogen addition on gaseous molecule of nitrile, the hydrogen addition of CH3CN was also fully discussed by of nickel catalyst with various supports.16,17 In addition, computational studies of hydrogen additions on nitriles are considerably scanty; it is found that there is a literature carrying out the hydrogen addition of CH3CN onto nickel surface with (111) facet,18 and two references discussing the hydrogenation of HCN on Ni(111) and Co(111) surfaces using the DFT calculations.19,20 Comprehending the catalytic behaviors of HCN on varied surfaces for geometric and electronic structures will obtain insight into which surface characteristics control reaction selectivity. So far, there were no computational approaches exploring the hydrogen addition for nitriles on specific metallic surface with body-centered-cubic lattice. Therefore, our objective of this study is to address the fully 4


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pictures of both adsorption and hydrogenation perspectives for HCN onto W(111) substrate. The adsorption constructions and energetics, site favorableness, relevant stability for HCN and its hydrogenated derivatives, and relevant barrier heights are methodically represented.

2. Computational Methods The adsorption and hydrogenation behaviors of HCN with H2 on a W(111) surface as well as the detailed electronic properties have been explored through the spin-polarized density-functional theory (DFT)21 with the projector augmented wave (PAW) method22,23 and the plane-wave basis set with a cutoff energy of 400 eV, in which the convergence criterion of all optimizations were set to 10-4 Hartree/Bohr for the total energy. Calculations were performed by using the Vienna ab initio simulation package (VASP) program.24-28 The exchange-correlation effects were set up for within the revised Perdew–Burke–Ernzerhof scheme (rPBE) in the generalized gradient approximation (GGA) in the total energy calculation.29,30 The Brillouin-zone was sampled by means of the Monkhorst-Pack grid, in which the (4 × 4 × 4) and (4 × 4 × 1) k-point meshes for bulk and surface, respectively, were setting in the calculations. Each slab was composed of 6 atomic layers and constructed by the unit cell of 3 × 3 for the W(111), while its bottom three atomic layers were kept fixed and the remaining layers were fully movable during the calculations. In addition, in the direction perpendicular to each metal facet was separated by a vacuum space greater than 15 Å, making sure that its periodic slabs exist no mutual interaction. With regard to the calculated methodology for the adsorption energy predictions, we used the general formula as follows: ∆Eads = E[substrate + adsorbate] – (E[substrate] + E[adsorbate]) 5



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where ∆Eads is the adsorption energy, E[substrate + adsorbate] the total energy of the adsorbate/W(111) system in the equilibrium state, E[substrate] the total energy of the W(111) substrate alone, and E[adsorbate] the total energy of the free adsorbate alone. A negative value of ∆Eads represents an exothermic process of adsorption. Transition states along a reaction course were examined by three procedures: (1) to obtain the transition states, the possible reaction paths were refined using the climbing image variant of the nudged elastic band method (CI-NEB),32,33 typically with creating at least eight images between the reactant and product, to find out the believable transition state. (2) each likely transition state was then relaxed using a quasi-Newton algorithm to make the forces on the atoms less than 0.05 eV/Å. (3) the frequency analysis was performed to be a confirmation of each transition state structure, in which there is only one imaginary frequency was found for each of them.

3. Results and Discussion 3.1. Accuracy of the method and model In order to determine whether satisfactory results may be obtained from our computational method, we firstly calculated some significant characters of bulk tungsten, gas-phase HCN, NH3 and CH4 molecules (they are important reactant and products in the titled reaction). Previously, Chen et. al.34 have already predicted the lattice parameter of bulk tungsten at GGA-rPBE level of theory, and their result represented 3.181 Å, which agrees accordingly with the value of 3.165 Å by experiment.35 Besides, the structural parameters, vibrational frequencies and dissociation energies of relevant isolated gaseous molecules, HCN, NH3 and CH4, are explored by enclosing them into a huge unit cell of 25 × 6


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25 × 25 Å3 dimensions. As shown in Table 1, all our calculated results are in general good agreement with the experimental observations,36-41 even though there are still some little inconsistent data being found in vibrational frequencies of ammonia molecule. Consequently, above examined results indicate that the chosen method and model herein are suitable for describing the behaviors of HCN adsorption and hydrogenation on W(111) surface. 3.2. Adsorptions of HCN and its hydrogenated species Firstly we investigated seven important adsorbed species for the probable hydrogenation processes of HCN on W(111) surface, such as HCN, CH2, CH, NH2, NH, N, and H, respectively. Three probable adsorption states onto the W(111) substrate by varying the conformation of aforementioned species are considered, labeled as top (T), 3-fold-shallow (S), and 3-fold-deep (D), respectively, are given in Figure 1(b). About the specific label of these notations (T, S, and D) throughout the manuscript, one can refer to our previously published studies.42,43 To discuss more easily, we labeled HCN/W(111), CH2/W(111), CH/W(111), NH2/W(111), NH/W(111), N/W(111), and H/W(111), respectively, to symbolize adsorptions for HCN, CH2, CH, NH2, NH, N, and H onto W(111) surface. The electronic localization function (ELF) contour diagrams of W(111) surface were also shown in Figures 1(c) and 1(d) for side and top views, respectively. The ELF contour diagram apparently shows that the values of ELF is approach to zero within shallow and deep sites, implying the behaviors of delocalized electron distribution were clearly presented. Conversely, the top site makes its electron density highly localized, showing that there exists a certain extent of electron localization at the top site onto W(111) surface. 7



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These results could reflect the fact that the tungsten metal in top site would donate electrons to the gaseous molecule as they adsorbed and make the surface oxidized and potentially electron donors. As a result, all these charge transferring phenomena (especially on top site position) between metal and gaseous molecule could promote adsorption and activation behaviors between HCN and W(111). The structures of HCN adsorption onto W(111) substrate are illustrated in Figure 2, and their relevant calculated results are tabulated in Table 2. The resulting HCN/W(111) structures (shown in Figure 2) are written as follows: WHCN(T-η1-N), WHCN(T-η2-C,N), and WHCN(T,T-µ2-C,N) sites of adsorption, respectively. As seen from Table 2, the isomer WHCN(T,T-µ2-C,N) is the most energetically stable among all calculated HCN/W(111) structures with the energy of adsorption about -49.8 kcal/mol. As shown in Figure 2, the bond lengths of C−W, N−W, and C−N for isomer WHCN(T,T-µ2-C,N) are 2.164 Å, 1.962 Å, and 1.332 Å, respectively. We observe that the C−N bond of the adsorbed HCN lengthen ca. 16.2% as compared to the optimized C−N bond length of 1.146 Å for a gaseous HCN alone. In consequence, one could conclude that the structure of adsorbed HCN is obviously twisted by W(111) surface. Compared to the similar systems of HCN on Ni(111) and Co(111) surfaces,19,20 it is found that our predicted adsorption energy of WHCN(T,T-µ2-C,N) is somewhat higher than theirs (ca. -35 and -40 kcal/mol for Ni(111) and Co(111), respectively), indicating a stronger interaction is observed between HCN and W(111) surface. The possible reason could be explained by the closely packed of first-layer metals for Ni(111) and Co(111) surfaces, which give rise to weaken their extent of chemisorption between HCN and substrate. 8


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Delbecq et al.44 remarked that the energy of adsorption could be partitioned by three principal parts– the energy of relaxation of W(111) substrate, the energy of distortion of HCN, and the mutual interaction energy of between HCN and W(111). All these detailed calculated outcomes were also listed in Table 2. As compared to obtained outcomes for WHCN(T-η1-N), WHCN(T-η2-C,N), and WHCN(T,T-µ2-C,N), we discovered that their energies of relaxation and distortion are nearly close when comparing the data contributed by the portions of interaction energies. The energy of mutual interaction for WHCN(T,T-µ2-C,N) is 32.1 and 8.3 kcal/mol larger than those of WHCN(T-η1-N) and WHCN(T-η2-C,N) counterparts, implying that the gas-surface restructuring for WHCN(T,T-µ2-C,N) configuration could strongly stabilize its whole structure. To further analyze the bonding nature of HCN on W(111) surface, we also calculated the electronic local density of states (LDOS) of some particular configurations projected on the orbitals for the adsorbate and the W(111) substrate (shown in Figure 3). Figure 3(a) exhibits the LDOS before the HCN−W(111) interaction, and Figure 3(b) represents the LDOS of energetically the highest adsorption structure, WHCN(T,T-µ2-C,N), respectively. In Figure 3(a), these four peaks derive from HCN located either sides of the Fermi level representing the bonding (1π), non-bonding (5σ), and anti-bonding (4σ* and 2π*) orbitals. As the adsorption proceeds (see Figure 3(b)), the Fermi level of HCN will be shifted right after adsorption, which illustrates the increase of charge density in adsorbed HCN. The outcome is in excellent accord with our aforementioned consequence of ELF that the adsorbed HCN will be more reduced on top sites than on shallow and deep sites by receiving more electrons. It is also partially populated due to the back-donation of tungsten 9



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d electrons to the unoccupied orbital 2π* of HCN, as indicated by the slight increase in the bond length of C−N (ca. 0.19 Å). In addition, it is found that two specific states (5σ and 2π*) reduced abruptly after adsorption of HCN on W(111), indicating that the larger molecular adsorption energy of HCN onto W(111) will be obtained through stronger hybridization mutually. In order to explore the minimum energy paths (MEPs) of hydrogen addition mechanisms for HCN onto W(111) substrate, we then carried out the optimizations of adsorbed geometries and adsorption energies for CH2/W(111), CH/W(111), NH2/W(111), NH/W(111), N/W(111), and H/W(111), respectively. As expected, the coordination of CH2, CH, NH2 and NH fragments adsorbing on the W(111) surface could form several isomeric intermediates of CH2/W(111), CH/W(111), NH2/W(111), NH/W(111), N/W(111), and H/W(111), respectively (shown in Figure 4). The computed results (see Tables 3 and 4) show that the possible coordinates of three isomers for both the CH2 and NH2 adsorbing on W(111) are WCH2(T,S-µ2-C)-a, WCH2(T,S-µ2-C)-b, and WCH2(S-η1-C) (with the corresponding adsorption energies of -94.5, -92.7 and -85.5 kcal/mol), as well as WNH2(T-η1-N)-a, WNH2(T-η1-N)-b, and WNH2(T,S-µ2-N) (with the adsorption energies of -94.8, -93.3 and -93.4 kcal/mol), respectively. The C atom of CH2 favors to adsorb between the top and shallow sites of W(111), whereas the N atom of NH2 prefers to adsorb at the top site. Our calculated results also show that the highest adsorption energies of CH2 and NH2 on the W(111) surface are very similar (-94.5 and -94.8 kcal/mol). For the CH and NH fragments, they are preferred to form the tridentate, WCH(T,T,S-µ3-C), and unidentate, WNH(T-η1-N), constructions with the adsorption energies of -142.5 and -143.3 kcal/mol, 10


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respectively. However, we note that the distances of C-W and N-W bonds are 2.075 and 1.794 Å, respectively, which are slightly shorter than that of CH2/W(111) and NH2/W(111) counterparts (2.079 and 1.964 Å). indicating that the CH and NH fragments are more stabilized effectively by W(111). At last, we examined adsorptions of nitrogen and hydrogen atoms on the W(111), which lead to three different adsorbed isomers: WX(T-η1-X), WX(T,S-µ2-X), and WX(T,T,S,D-µ4-X), in which X represents either N or H (as shown in bottom part in Figure 4). As shown in Table 5, each atomic adsorbate (N and H atoms) can anchor strongly on W(111). With regard to each adsorption site, those between top, shallow and deep sites−WX(T,S-µ2-X), and WX(T,T,S,D-µ4-X)−are more favored (for N, -168.3 ~ -176.9 kcal/mol, and H, -65.1 ~ -68.4 kcal/mol) than those of the top sites, WX(T-η1-X) (for N, -133.0 kcal/mol, and H, -52.8 kcal/mol).

3.3. Reaction network and potential energy surfaces for HCN hydrogenation As the WHCN(T,T-µ2-C,N) construction is the most stable one among all calculated structures of HCN/W(111) system, we selected it as a beginning configuration (labeled as IM1) to explore the hydrogenation of HCN on W(111). The proposed possible subsequent paths for the hydrogenation of HCN onto a W(111) surface are schematically depicted in Figure 5; the calculated corresponding potential-energy surfaces are drawn in Figures 6 and 7, and the geometrical illustrations of all adsorbed intermediates (IM1~IM13) and transition states (TS1~TS9) using the GGA-rPBE level of theory are all portrayed in Figure 8. As Figure 6 indicates, the resulting profile shows that the formation of IM1 takes place smoothly through the MEP on elongating its C−N bond from 1.146 to 1.332 Å and shortening 11



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both C−W and N−W bonds, with no well defined transition state and with 49.8 kcal/mol exothermic. The operation of HCN hydrogenation was investigated by adding a co-adsorption H atom at its neighboring site of the adsorbed HCN on W(111), forming HCN(a)+H(a) (denoted as IM2), with an energy descending 6.9 kcal/mol as compared to IM1. It should be noted that the location of co-adsorption H atom was placed between the top and shallow site since it will be greatly stable and possess the highest co-adsorption energy. In first elementary step, our prediction for the possible reaction pathways are hydrogenation of HCN(a) to HCNH(a) (via TS1), hydrogen addition on HCN(a) to H2CN(a) (via TS2), or scission of the carbon-nitrogen bond (via TS3). The energies of activation for the formation of HCNH(a) (IM3), H2CN(a) (IM4), and CH(a)+N(a)+H(a) (IM5) are 37.4, 53.0 and 45.4 kcal/mol, respectively. In all cases discussed above, we can therefore suggest that it is much easier to hydrogenate the nitrogen atom of HCN and to form HCNH(a) at this stage, and this course is endothermic by 3.2 kcal/mol. Successively, after co-adsorbing one more H atom from IM3 to IM6 by 13.8 kcal/mol exothermic, the second hydrogenation steps of this co-adsorbate might proceed through the association of H atom and C or N atoms of HCNH(a) on crossing two different transition states TS4 or TS6 with corresponding activation barriers of 16.3 and 57.4 kcal/mol, forming H2CNH(a) (IM7) and HCNH2(a) (IM9), respectively, in which they are both slightly endothermic (5.1 and 5.0 kcal/mol, respectively) with respect to IM6. For scission of carbon-nitrogen bond (via TS5) from IM6 to form CH(a)+NH(a)+H(a) (IM8) was also considered, the barrier height is somewhat large, 29.2 kcal/mol. Based on the results given above, we found that the hydrogen addition occurring on carbon atom of HCNH(a) will be neither much easier than on nitrogen atom nor C−N 12


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bond cleavage. Therefore, we could anticipate that the interaction of repulsive force between approaching nitrogen lone pair and hydrogen atom is what causes hydrogen addition on N atom to be extra arduous. Oliva et al.19,20 investigated theoretically the hydrogenation of HCN onto the Ni(111) and Co(111) surfaces, showing that the hydrogenation reaction will proceed through an imine intermediate, H2CNH(a), which could be obtained by hydrogenating either on carbon atom of the HCNH or on nitrogen atom of the H2CN. With regard to the adsorbed imine intermediate (H2CNH(a)), some experimentalists had been found that the accumulation of the imine intermediate would be clearly observed in both HCN/cobalt and HCN/nickel systems.7,9 On the other hand, imines were already found to be quite useful and significant intermediates in pharmaceutical, biological, and chemical syntheses.45 For example, Taggi et al.46 already confirmed that the varied imines can directly synthesize the β-Lactams by asymmetric catalysis, in which the β-Lactams have been utilized for many urgent non-antibiotic treatments. In the next stage (shown in Figure 7), the hydrogen addition of H2CNH(a) intermediate resulted from preceding step would further give rise to either H2CNH2(a) or H3CNH(a). With the subsequent co-adsorbing one H atom from IM7 to IM10, which is exothermic by 14.5 kcal/mol, the third hydrogenation steps of this co-adsorbate could proceed through the combination of H atom and C or N atoms of H2CNH(a) on passing two different transition states TS7 or TS8 with corresponding barrier heights of 46.8 and 38.4 kcal/mol, forming H2CNH2(a) (IM13) and H3CNH(a) (IM12), respectively, in which the IM13 is highly endothermic (22.6 kcal/mol) with respect to IM10, whereas the IM12 is slightly endothermic (6.2 kcal/mol). However, it is interesting to note that the energy barrier for the 13



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scission of C−N bond to form CH2(a)+NH(a)+H(a) (IM11) is not only smaller than those of aforementioned predictions, only 27.5 kcal/mol for TS9, but apparently smaller (about 11 kcal/mol less) than the hydrogenation at C atom of H2CNH(a). Similar simulated system for methylamine (H3CNH2) on varied surfaces was also theoretically reported,47 they already predicted activation energies of C−N bond cleavage with a range of ca. 34.4~81.4 kcal/mol, clearly greater than the barrier height calculated in our work. We hence predicted the scission of the C−N bond would occur at this stage, and found especially that the formation of co-adsorbate IM11 being slightly exothermic by1.2 kcal/mol. Finally, about the desorption processes, such as reactions CH2(a)+2H(a)CH4(g) and NH(a)+2H(a)NH3(g), are expected to be fired up successfully by a pertinent entropy term at a specific temperature, implying that these final steps of desorption could present no significant problem. However, since there is no experimental observation available for this catalytic reaction system the importance of such study needs to be experimentally confirmed. Consequently, our proposed overall minimum energy pathway, HCN(g)+3H2(g)+W(111)→IM1→IM2→TS1→IM3→IM6→ TS4→IM7→ IM10→TS9→IM11→CH4(g) + NH3(g) + W(111), is calculated to be exothermic by 74.9 kcal/mol. Compared to other final products of HCN hydrogenations on Ni(111) and Co(111) surfaces,19,20 the production of H3CNH2 is found to be both more favorable, due to the fact that the further successive hydrogenation on imine intermediate (H2CNH(a)) would much easier on Ni(111) and Co(111) than our simulated W(111) surface. As a consequence, the existence and stability of H2CNH(a) are the key intermediates to the superior activity of the

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varied metal catalysts toward methylamine, methane and ammonia syntheses from HCN hydrogenation.

3.4. Electronic analyses of each intermediate in HCN hydrogenation To further analyze the electronic interactions between adsorbate of HCN and the W(111) upon adsorption, we calculated the charge density difference (CDD) for some important specific structures. Figure 9 illustrates drawing of contour surface for CCD, ∆ρdiff = ρ[substrate + adsorbate] – ρ[substrate] – ρ[adsorbate], for each adsorbate/surface system of the minimum energy pathway for HCN hydrogenation: (a) before adsorption, (b) IM1, (c) IM3, and (d) IM7, in which the blue and yellow parts represent the accumulation and depletion electrons, respectively. In addition, the charges of the chosen atoms are predicted by employing the Bader charge analysis48,49 with a code designed from Henkelman et al..50 As shown in the CDD from Figures 9(a) to 9(b), the charge transferred from the W(111) surface (mainly from top layer) to the HCN adsorbate was obviously observed, 1.83 |e|. This phenomenon is in excellent agreement with our foregoing observations of ELF and LDOS results that the tungsten metal in top site would donate electrons to the adsorbed HCN(a). while the first hydrogenation occurs, 1.64 |e| was shifted from tungsten metals to its adsorbed HCNH(a) intermediate (see Figure 9(c), IM3). Finally, in the second hydrogenation process forming H2CNH(a) (IM7), we found this behavior of charge transfer (only 1.11 electron) become somewhat indistinct, suggesting that the p electrons of adsorbed imine, H2CNH(a), could also partially feed back to W(111) surface. Consequently, one can therefore believe

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that a remarkable charge transfer between the adsorbate and tungsten surface would play a important role in promoting the catalytic efficiency for HCN hydrogenation.

Summary In conclusion, the methane and ammonia syntheses from HCN hydrogenation on the W(111) surface with the high activity have been analyzed with theoretical approach. The origins of discrepancy in reactivity and selectivity are revealed in terms of adsorption structures, charge distributions, and activation energies analyses of some key adsorbed intermediates. Our calculated results showed that the hydrogenation of HCN to imine intermediate H2CNH(a) is both thermodynamically and kinetically favorable on W(111). Subsequently, the scission of C−N bond of H2CNH(a) to form CH2(a)+NH(a) is more favorable than those of further hydrogenations to H2CNH2(a) or H3CNH(a), indicating that the new pathway for methane and ammonia syntheses will make it possible for future experimental design. To the best of our knowledge, there are no previous experimental or theoretical studies of this reaction for HCN/W(111) system. The comparisons with two similar theoretical studies for the analogous reactions on Ni(111) and Co(111) surfaces were done to qualitatively explain the different selectivity of final product formation. All these novel results derived by computational techniques will be arduous to achieve through experimental observations, indicating that our study can help for future strategic design of prospective catalysts with high catalytic activity and regioselectivity for many important conversion reactions.

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Acknowledgement This research was supported by (1) Ministry of Science and Technology in Taiwan, with the Grant Number of MOST 105-2113-M-034-002, and (2) the research foundation from Chinese Culture University, and (3) the use of CPU time from National Center for High-Performance Computing, Taiwan. In addition, I would like to take this opportunity to express my deepest gratitude to my previous supervisor, Prof. Jia-Jen Ho, who was already retired in this year. Without his persistent encouragement and continuous guidance, I could not get any achievement and scholarship now. Wishing you all the best on your life after your retirement, thank you sincerely.

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(36) Harmony, M. D.; Laurie, V. W.; Kuczkowski, R. L.; Schwendeman, R. H.; Ramsay, D. A.; Lovas, F. J.; Lafferty, W. J.; Maki, A. G. Molecular Structures of Gas‐Phase Polyatomic Molecules Determined by Spectroscopic Methods. J. Phys. Chem. Ref. Data 1979, 8, 619-720. (37) Maki, A. G.; Blaine, L. R. Infrared Spectra of HCN from 2000 to 3600 cm−1 J. Mol. Spectrosc. 1964, 12, 45-68. (38) Blanksby, S. J.; Ellison, G. B. Bond Dissociation Energies of Organic Molecules. Acc. Chem. Res. 2003, 36, 255-263. (39) Herzberg, G. Molecular Spectra and Molecular Structure. Iii.: Electronic Spectra and Electronic Structure of Polyatomic Molecules; Van Nostrand Reinhold Co.: New York, 1966. (40) Guelachvili, G.; Abdullah, A. H.; Tu, N.; Rao, K. N.; Urban, Š.; Papoušek, D. Analysis of High-Resolution Fourier Transform Spectra of 14NH3 at 3.0 µm. J. Mol. Spectrosc. 1989, 133, 345-364. (41) Herranz, J.; Thyagarajan, G. The Rotational Structure of the ν(e) Fundamental Infrared Band of Tetrahedral XY4 Molecules. J. Mol. Spectrosc. 1966, 19, 247-265. (42) Hsiao, M.-K.; Wu, S.-K.; Chen, H.-L. Adsorption and Dehydrogenation Behaviors of the NH3 Molecule on the W(111) Surface: A First-principles Study. J. Phys. Chem. C 2015, 119, 4188-4198. (43) Hsiao, M.-K.; Su, C.-H.; Wu, S.-K.; Chen, H.-L. Computational Investigation of NH3 Adsorption and Dehydrogenation on W-modified Fe(111) surface. Phys. Chem. Chem. Phys. 2015, 17, 30598-30605.

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Table 1. Calculated and experimental values of the geometrical parameters (bond lengths in Å and angles in degree), vibrational frequencies (v, in cm-1) and dissociation energies (Do in kcal/mol, included ZPVE)a of gaseous HCN, NH3 and CH4 molecules HCN calculated r (Å) θ (deg) vasym vsym Do a

f

calculated b

1.070(H–C)/1.146(C–N) 1.063(H–C)/1.155(C–N) 180.0 180.0 3369.7 1924.6 120.7

3311.5c 2096.9c 124.8±0.2d

CH4

experimental

experimental

1.017 107.8e

1.098 109.5

1.094e 109.5e

3788.2 3606.6 101.4

3443.8f 3336.6f 99,e 106.1±0.1d

1582.7 3085.7 98.4

1533.0g 3018.9g 103.49±0.03d

Reference 36. Reference 38.

Reference 39.

Reference 40.

g

Reference 41.

24


e

calculated

1.022 106.8

Reference 37.

d e

experimental

The dissociation energies of H−CN, H−NH2, and H−CH3, respectively.

b c

NH3

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Table 2. Calculated adsorption energies, relaxation energies, distortion energies, interaction energies (kcal/mol), and geometrical parameters (Å) of adsorbed HCN molecule on W(111) surface adsorption site For HCN molecule WHCN(T-η1-N) WHCN(T-η2-C,N) WHCN(T,T-µ2-C,N) a

adsorption energy

relaxation energy

distortion energy

interaction energy

d(W-N or C)a

d(C-N)

-17.6

0.4

1.9

-19.9

2.117

1.170

-41.3 -49.8

0.4 0.3

2.0 1.9

-43.7 -52.0

2.083 1.962

1.261 1.332

The shortest distance between the adsorbed atom (C or N ) and the corresponding adsorption site of W(111)surface.

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Table 3. Calculated adsorption energies, relaxation energies, distortion energies, interaction energies (kcal/mol), and geometrical parameters (Å) of adsorbed CH2 and CH molecules on W(111) surface adsorption site

a

adsorption energy

relaxation energy

distortion energy

interaction energy

d(W-C)a

d(C-H)b

For CH2 molecule WCH2(T,S-µ2-C)-a WCH2(T,S-µ2-C)-b WCH2(S-η1-C)

-94.5

0.2

0.2

-94.9

2.079

1.100 / 1.034

-92.7 -85.5

0.2 0.2

0.3 0.6

-93.2 -86.3

2.128 2.174

1.113 / 1.094 1.137 / 1.104

For CH molecule WCH(T-η1-C)

-139.0

0.3

0.5

-139.8

1.854

1.095

WCH(T,S-µ2-C) WCH(T,T,S-µ3-C)

-140.4 -142.5

0.3 0.5

-0.1 0.6

-140.6 -143.6

2.004 2.075

1.099 1.096

The shortest distance between the adsorbed atom (C) and the corresponding adsorption site of W surface.

b

The bond lengths of CH1 / CH2 are presented.

26


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Table 4. Calculated adsorption energies, relaxation energies, distortion energies, interaction energies (kcal/mol), and geometrical parameters (Å) of adsorbed NH2 and NH molecules on W(111) surface adsorption energy

relaxation energy

distortion energy

interaction energy

d(W-N)a

d(N-H)b

-94.8

8.4

1.1

-104.3

1.964

1.021 / 1.018

-93.3 -93.4

10.1 8.4

1.0 0.7

-104.4 -102.5

1.974 2.125

1.019 / 1.018 1.023 / 1.022

For NH molecule WNH(T-η1-N)

-143.3

9.4

0.6

-153.3

1.794

1.017

WNH(T,S-µ2-N) WNH(T,T,S-µ3-N)

-142.9 -142.5

2.5 1.4

0.4 0.4

-145.8 -144.3

1.968 2.004

1.021 1.023

adsorption site For NH2 molecule WNH2(T-η1-N)-a WNH2(T-η1-N)-b WNH2(T,S-µ2-N)

a

The shortest distance between the adsorbed atom (N) and the corresponding adsorption site of W surface.

b

The bond lengths of NH1 / NH2 are presented.

27



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Table 5. Calculated adsorption energies and geometrical parameters (Å) of adsorbed N and H atoms on W(111) surface For N atom WN(T-η1-N) WN(T,S-µ2-N) WN(T,T,S,D-µ4-N) For H atom WH(T-η1-H) WH(T,S-µ2-H) WH(T,T,S,D-µ4-H) a

adsorption energy

d(W-N or H)a

-133.0 -168.3 -176.9

1.966 1.904 1.989

-52.8

1.766

-68.4 -65.1

1.907 1.870

The shortest distance between the adsorbed atom (N or H) and the corresponding adsorption site of

surface.

28


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(a)

(b)

T

T S D

3

2

1 3

2

1 3

2

T

1

5 6

(c)

4

5 6

4 6

5

D

D

D

S

4

D T

D

T S

D T

S

T

D

S

T

T S

T

T S

T

T S

T S

D T

T

(d)

0.40

0.00

Figure 1. Graphical representations and the corresponding electronic localization function (ELF) contour maps of W(111) surface used in the present study:(a)(c) side views, and (b)(d) top views, respectively.

29



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Page 30 of 38

1.072 1.094

1.171 2.117

WHCN(T –η1-N)

2.109

2.083

WHCN (T-η2-C,N)

1.106 2.164

1.332 1.962

WHCN(T,T-µ2-C,N)

Figure 2. Optimized adsorption structures of HCN molecule on W(111) surface and their important geometry parameters calculated at the GGA-rPBE level of theory. The bond lengths are given in angstroms.

30


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Figure 3. Local density of states (LDOS) for HCN adsorption on W(111) surface: (a) before adsorption, and (b) energetically the highest adsorption structure, WHCN(T,T-µ2-C,N). The black and red lines represent W(d), HCN(s, p), respectively. The dashed line represents the Fermi level.

31



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Figure 4. Optimized adsorption structures of CH2, CH, NH2 and NH molecules, as well as N and H (in parenthesis) atoms on W(111) surface and their important geometry parameters calculated at the GGA-rPBE level of theory. The bond lengths are given in angstroms. 32


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H2CN(a) HCN(a)

HCN(a) + H(a)

HCNH(a) CH(a) + N(a) + H(a)

HCNH2(a) HCNH(a) + H(a)

H2CNH(a)

H3CNH(a) 2H2(g) H2CNH(a) + H(a)

CH(a) + NH(a) + H(a)

CH2(a) + NH(a) + H(a) H2CNH2 (a)

Figure 5. Schematic diagram of proposed possible reaction network for the HCN hydrogenation pathways on W(111) surface.

33


CH4(g) + NH3(g)


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

Figure 6. The calculated potential energy diagrams (in kcal/mol) for the W(111) catalyzed sequential hydrogenation of hydrogen cyanide to imine intermediate (H2CNH(a)). The black lines correspond to the hydrogenation processes onto N atom, the blue lines correspond to the hydrogenation processes onto C atom, whereas the red lines refer to the scission processes of C–N bond of HCN(a), respectively. 34


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Figure 7. The calculated potential energy diagrams (in kcal/mol) for the W(111) catalyzed sequential hydrogenation starting from imine intermediate (H2CNH(a)) to methane and ammonia. The black lines correspond to the hydrogenation processes onto N atom, the blue lines correspond to the hydrogenation processes onto C atom of, whereas the red lines refer to the scission processes of C–N bond of H2CNH(a), respectively. 35



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

IM1

IM2

IM3

Page 36 of 38

IM4

HCN(a)

HCN(a)+H(a)

HCNH(a)

H2CN(a)

IM5

IM6

IM7

IM8

HCNH(a)+H(a)

H2CNH(a)

CH(a)+NH(a)+H(a)

IM9

IM10

IM11

IM12

HCNH2(a)

H2CNH(a)+H(a)

CH2(a)+NH(a)+H(a)

H3CNH(a)

IM13

TS1

CH(a)+N(a)+H(a)

H2CNH2 (a) TS4

HCNH(a)+H(a) TS8

H2CNH(a)+H(a)

TS2

HCN(a)+H(a)

HCN(a)+H(a)

TS5

TS6

CH(a)+NH(a)+H(a)

HCNH(a)+H(a)

TS3

CH(a)+N(a)+H(a) TS7

H2CNH(a)+H(a)

TS9

CH2(a)+NH(a)+H(a)

Figure 8. Geometrical illustration of all intermediates (IM1~IM13) and transition states (TS1~TS9) for the HCN-W(111) interactions using the GGA-rPBE level of theory. 36


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(b)

(a)

Sum = -1.83|e| 0.09|e| 0.68|e|

-2.60|e| 0.71|e|

(c)

(d)

Sum = --1.64|e| 0.02|e| 0.56|e|

0.05|e| -2.27|e|

Sum = --1.11|e| 0.65|e| 0.07|e| 1.00|e| -0.08|e| -2.75|e|

Figure 9. Illustration of charge-density difference for sequential hydrogenation of HCN on W(111) surface via the proposed minimum-energy pathway: (a) before interaction, (b) IM1, (c) IM3, and (d) IM7, respectively.

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H

C

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Hydrogenation of Hydrogen Cyanide to Methane and Ammonia by a

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