Understanding of Selective H2 Generation from Hydrazine

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Understanding of Selective H Generation from Hydrazine Decomposition on Ni(111) Surface Hui Yin, Yuping Qiu, Hao Dai, Li-Yong Gan, Hongbin Dai, and Ping Wang J. Phys. Chem. C, Just Accepted Manuscript • DOI: 10.1021/acs.jpcc.7b11293 • Publication Date (Web): 28 Feb 2018 Downloaded from http://pubs.acs.org on March 2, 2018

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The Journal of Physical Chemistry

Understanding of Selective H2 Generation from Hydrazine Decomposition on Ni(111) Surface Hui Yin, Yu-Ping Qiu, Hao Dai, Li-Yong Gan*, Hong-Bin Dai, and Ping Wang* School of Materials Science and Engineering, Key Laboratory of Advanced Energy Storage Materials of Guangdong Province, South China University of Technology, Guangzhou 510641, P.R. China ∗

Corresponding author. Tel: +86 20 3938 0583

E-mail: [email protected] (L.-Y. Gan); [email protected] (P. Wang).

Abstract Catalytic decomposition of hydrazine monohydrate (N2H4⋅H2O) over Ni-based catalysts has been extensively investigated as a promising hydrogen generation means for onboard or portable applications. Despite the flourishing experimental development of Ni-based catalysts, the mechanistic study of catalytic decomposition process is still in its infancy. Herein, we report a first-principles study of the elementary steps in N2H4 decomposition over Ni(111) surface. The calculations show that the decomposition behaviors of N2H4 strongly depend on the surface coverage. At a higher coverage of 0.25 ML, the calculation results are in excellent agreement with the experimental observation. Our calculations, for the first time, presented a complete picture of catalytic selective H2 generation from N2H4 decomposition over Ni surface. Such mechanistic understanding may lay foundation for the development of high-performance Ni-based catalyst for promoting hydrogen generation from N2H4.

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1. Introduction Hydrazine (N2H4) is a labile liquid compound that shows important applications in space technology, primarily as monopropellant for maneuvering thrusters of spacecraft.1-2 Recently, hydrazine monohydrate (N2H4⋅H2O) has attracted considerable attentions as a potential hydrogen carrier. This is a result of its many favorable attributes, such as high hydrogen density (8 wt%), relatively low cost, compatible with the existing liquid-based distribution infrastructure, and free of solid byproduct in the decomposition reaction. The latter is of particular significance for the compact design of practical hydrogen source.3-11 N H → N + 2H [∆ = −95.4 kJ/mol] (R1) 3N H → N + 4NH [∆ = −157 kJ/mol] (R2) N2H4 is the effective hydrogen storage component of N2H4⋅H2O, which decomposes via two competitive routes.12-13 From a perspective of hydrogen storage, the complete decomposition (R1) must overwhelmingly precede the incomplete decomposition (R2). To this end, a number of group VIII transition metal and/or their alloys have been examined in terms of catalytic performance towards N2H4⋅H2O decomposition. It was found that Ni is a preferred choice of catalytically active element, which possesses moderate activity and relatively high selectivity towards H2 generation from N2H4 (R1).14-15 Alloying Ni with noble metals (i.e., Rh, Ir and Pt) typically results in remarkably improved activity and H2 selectivity in comparison with the monometallic counterparts. A series of Ni-based bimetallic catalysts, like Ni-Ir and Ni-Pt, enabled complete decomposition of N2H4·H2O at moderate temperatures with nearly 100% H2 selectivity.10,

16-29

But in spite of these

encouraging experimental progresses, the mechanistic study of catalytic decomposition of N2H4 over Ni-based catalyst is still in its infancy. So far, there exist only preliminary theoretical studies exploring N2H4 adsorption on Ni30-31 and Ni-based alloy32 surfaces, while a detailed catalytic mechanism of N2H4 decomposition is still absent. Such lack of mechanistic insight into the catalytic decomposition process will inevitably hamper the experimental exploration of high-performance N2H4 decomposition catalysts. Herein, we report a first-principles calculation of the elementary steps of N2H4 decomposition over Ni catalyst. The calculations showed that the decomposition behaviors of N2H4 strongly depend on the surface coverage. At higher coverage, the calculation results are in excellent agreement with the experimental 2


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observation. Our calculations, for the first time, present a complete picture of catalytic selective H2 generation from N2H4 decomposition over Ni surface.

2. Methods 2.1 Theoretical First-principles calculations were performed using Vienna ab-initio Simulation Package with projected-augmented-wave pseudopotentials.33-34 Spin-polarized generalized gradient approximation of Perdew-Burke-Ernzerhof exchange-correlation functional was used35 with a plane wave cutoff energy of 400 eV, which is sufficient to ensure the convergence.30-32 The lattice constant of nickel bulk was optimized to 3.53 Å, consistent with the experimental value (3.52 Å).36 Ni(111) surface was modeled by a five-layers periodic slab with a vacuum thickness of 20 Å. All adsorbed species were placed on one side of the slab. The upper three layers and the adsorbates were allowed to relax till all residual forces declined below 0.02 eV/Å, while the bottom two layers were fixed. A dipole correction was applied due to asymmetric slabs.37 Two supercell sizes of 3×3 and 2×2, corresponding to coverage of 0.11 and 0.25 monolayer (ML), respectively, were used to investigate the coverage effect. Test calculations with larger supercell were performed and showed that the present results are fully converged. The integration of the Brillouin zone was done with Gamma-centered 5×5×1 and 7×7×1 k-point grid for the 3×3 and 2×2 supercell, respectively. Van der Waals interaction is not expected to affect the properties because strong chemical bonds are formed between the adsorbates and the surface, as suggested in Ref. 32. The minimum energy paths (MEPs) and transition states (TSs) were determined by climbing-image nudged elastic band method.38 After standard TS search, the TS is further optimized by the quansi-Newton method until the residual forces were less than 0.02 eV/Å. !"# )

The adsorption energies (

!"#

=

$/%& − $

−

of adsorbates on Ni(111) were calculated according to:

%& (1)

where EX, ENi and EX/Ni are the total energies of gas-phase species and Ni(111) surface without and with X adsorbed. Specifically, EX of N2H4 adopts the energy in gauche-conformation, which is the most stable in gas phase.39 The energy barrier ( '(

=

*+

−

,+ , Δ

=

.+

'( )

−

and reaction energy (Δ ) of each elementary step were calculated as

,+ (2)

3



where

,+ ,

*+ ,

and

.+

are the total energies of the initial state (IS), TS, and final state (FS), respectively.

Zero point energy (ZPE) corrections were considered in all energy barriers and reaction energies in the following except for specialization and were calculated as 1 ZPE = 2 ℎ53 (3) 2 3

2.2 Experimental The supported Ni nanoparticles on Vulcan carbon (XC-72 R) were prepared by a simple impregnation method followed by reduction under a flowing H2 atmosphere at 300 oC. The catalytic decomposition properties of N2H4·H2O over the Ni/C catalyst was examined by a gravimetric water-displacement method. The molar ratio of the catalyst (quantified as metallic Ni) to N2H4·H2O was fixed at 1:10. The details of catalyst preparation and property examination can be found in Refs.10, 24.

3. Results and discussion 3.1 Adsorption of reaction intermediates on Ni(111) In the present study, the chemisorption and decomposition process of N2H4 was investigated on the most stable Ni(111) plane. Since the lateral interaction of adsorbates may play an important role in the adsorption and reaction behaviors, we conducted calculations at two surface coverage, 0.11 and 0.25 ML. First, we studied the chemisorption properties of the reactant, reaction intermediates and products of N2H4 decomposition, and presented the energetically stable structures, adsorption energies, and geometrical properties in Fig. 1 and Table 1. Three adsorption configurations were considered for N2H4: anti-, gauche-, and cis-conformation. In the former two cases, only one N atom bonds to Ni and the N-N axis is tilted from the surface. In the latter, both N atoms are bonded to Ni atoms with the N-N axis parallel to the surface. At 0.11 ML, anti-conformation is the most energetically favorable configuration with Eads of -0.83 eV, which is followed by gauche- (-0.77 eV) and cis-conformation (-0.76 eV). Upon increasing the surface coverage to 0.25 ML, gauche-conformation becomes the most energetically favorable structure with Eads of -0.65 eV. And as expected, chemisorption results in weakened intramolecular bonding strength. For example, the gas-phase N2H4 molecule prefers gauche-conformation with a N-N bond length of 1.436 Å and N-H bond 4


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lengths ranging in 1.022-1.026 Å. Upon adsorption at 0.11 ML in anti-configuration, the N-N bond length is elongated to 1.466 Å and the N-H bond lengths are increased to 1.027-1.034 Å. The adsorbed N2H4 at 0.25 ML in gauche-conformation shows a N-N bond length of 1.452 Å and N-H bond lengths ranging from 1.025 to 1.034 Å. As coverage decreased to 0.06 ML, the relative stability of the three conformations is changed to cis (-0.92 eV) > anti (-0.86 eV) > gauche (-0.79 eV), suggesting that the behavior of N2H4 adsorption on Ni(111) is strongly coverage-dependent. For the N2H3 species, similar adsorption geometries are adopted at 0.11 and 0.25 ML. In both cases, N2H3 prefers to be adsorbed at fcc, wherein the NH2 entity sits at a near-top site and NH nearly occupies a bridge site with its N-N bond titling from the surface. And the intramolecular N-N and N-H bond lengths at 0.11 and 0.25 ML are quite close. Nevertheless, the adsorption energy was found to decrease from -2.05 eV at 0.11 ML to -1.77 eV at 0.25 ML, suggesting a strengthened lateral repulsion at higher coverage. In addition, at both coverage, N2H3 adsorption at hcp shows quite similar features to that at fcc, with only slightly decreased adsorption energy by 20 meV. Intuitively, there are two isomers of N2H2, i.e., NNH2 and HNNH. NNH2 prefers to adsorb at fcc in end-on mode, wherein the N-N axis is perpendicular to the surface with one ending N binding with three Ni atoms and the far ending N-H tilting ~69° from the surface normal. Increasing coverage causes no appreciable change of both bond lengths and adsorption energy (see Table 1). HNNH (diimide) is an important intermediate during N2H4 decomposition, as experimentally detected on Ni(100) in the 200-450 K range.40 On Ni(111), it adopts a side-on mode with each NH fragment sitting close to a bridge site, giving rise to an N-N bond length of 1.352 Å and both N-H bond lengths of 1.031 Å at 0.11ML. The HNNH adsorption is weaker than that of NNH2, particularly at higher coverage. This is due primarily to the enhanced lateral repulsion in side-on adsorption configuration, similar to the N2H4 adsorption in cis-conformation.31 In terms of N2H species, the adsorption configuration is quite similar to that of NNH2, that is, end-on adsorption at fcc site. But the adsorption strength of N2H is much weaker than that of NNH2. For example, the adsorption energy of N2H is only -1.91 eV at 0.25 ML, which is almost 1 eV lower than that of NNH2. The N2 molecule energetically prefers end-on mode at a top site with the adsorption energy of -0.42 eV at 0.11 ML. Increasing the coverage to 0.25 ML hardly alters its adsorption stability, indicative of a negligible 5



lateral interaction at the studied coverage. The N-N bond length of the adsorbed N2 is only slightly longer (by 0.016 Å) than the gas-phase molecule, suggesting that N2 is weakly perturbed on Ni(111). It is noted that the N-N bond lengths of N2Hx (x = 0-4) follow an order of N2H4 > N2H3 > N2H2 > N2H > N2 at both coverage. This implies that the N-N bond gets strengthened with proceeding the dehydrogenation process on Ni(111). Similar results were also obtained in the theoretical studies of N2H4 decomposition on Ir(111) and Rh(111) surfaces.41-42 In addition, except for N2H, the adsorption strength of the N2Hx species on the Ni(111) decreases with increasing the surface coverage due to the lateral interactions. In the case of NH3 adsorption, the top site is energetically preferred with its N atom bonded to surface. As a consequence of the repulsion between the adsorbates, the adsorption of NH3 becomes weaker with increasing the coverage from 0.11 to 0.25 ML. But judging from the N-H bond lengths, the NH3 adsorbate well maintains its gas-phase molecular structure. In the cases of NH2, NH and N, they all bond to the surface via N atom. Their adsorption energies are respectively -2.72, -4.52 and -5.30 eV at 0.11 ML, and are weakly coverage-dependent. It is evident that the binding strength between N and the surface gets enhanced with consecutive removal of an H atom from NH3. H atom adsorption at fcc is most energetically favorable with adsorption energy of -2.79 eV, which does not alter at the two studied coverage. These results are consistent well with previous studies.43-44

3.2 Reaction selectivity of N2H4 decomposition over Ni(111) The decomposition of N2H4 involves N-N and N-H bond cleavage. Different bond cleavage at the initial stage may lead to different decomposition reaction pathways. As a consequence, determination of the energetic preference of N-N and N-H bond cleavage is a key step in prediction of the decomposition behaviors of N2H4. To this end, we first conducted a comparative study of N-H and N-N bond cleavage on the Ni(111) surface. At a coverage of 0.11 ML, the three conformations of N2H4 show slight energy differences within 0.07 eV, as seen in Table 1. For comparison purpose, we selected the energetically most stable anti-conformation and the least stable cis-conformation as representative ISs and the results are shown in Figs. 2a and 2b, respectively. In the anti-conformation, the four N-H bonds can be categorized into two groups. The two N-H pointing to the surface have longer bond lengths than the others pointing to the vacuum 6


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(see Fig. 1) due to the steric attraction from the surface.45 Cleavage of the former type of N-H bonds is exothermic by 0.46 eV with an energy barrier of 0.68 eV. At TSa, the N-H distance increases from the initial 1.034 to 1.535 Å. The dissociated H approaches a bridge site and the N2H3 fragment lays down with its N-N axis nearly parallel to the surface (inset TSa in Fig. 2a). This process ends with N2H3 adsorption at fcc and H at another adjacent fcc site. As a comparison, the stronger N-H bonds pointing to the vacuum show a higher energy barrier by around one third than the former type of N-H bonds. In the case of N-N bond breaking, the bond distance gradually increases to 2.064 Å and each of NH2 approaches a top site at TSb (inset TSb in Fig. 2a), and then they diffuse to bridge sites at FS. This process has a barrier of 0.49 eV and reaction energy of −1.13 eV. In the cis-conformation, the four N-H bond lengths and their surroundings are all the same. Cleavage of a representative N-H bond has a barrier of 0.58 eV, and releases energy of 0.44 eV (Fig. 2b). Meanwhile, the cis-conformation has a more stretched N-N bond than anti- and gauche-conformation probably because both N atoms bond to surface (see Table 1 and Fig. 1). As a result, a smaller barrier height of 0.36 eV is needed to cleave the N-N bond, which is consistent with previous work.46 For the generated NH2 radicals from N-N bond cleavage, there exist two possible scenarios. One is to continue dissociation into NH and H and the other is to abstract an H atom from adjacent N2H4 to form NH3. Our study results unambiguously support the latter one. The dissociation of NH2 needs to overcome an energy barrier of 0.53 eV and the reaction is exothermic by 0.21 eV. The dissociated H atom gets close to an adjacent hcp site while NH diffuses oppositely to a neighboring fcc at TSa′′ (Fig. 2c). In sharp contrast, the intermolecular reaction between NH2 and adjacent N2H4 shows an energy barrier as low as 0.22 eV and reaction energy of −0.16 eV. This low-energy process can be summarized as follows: Upon NH2 attacking, the N2H4 molecule in anti-conformation will firstly convert into cis-conformation via gauche-conformation with a small barrier of 40 meV; then, the N2H4 molecule has its N-H bond, which points to the NH2 radical, stretched by 0.026 Å, and its N-N axis slightly twisted simultaneously at TSb′′ (inset TSb′′ in Fig. 2c). The low barrier of the intermolecular reaction suggests that the formed NH2 radicals from the N2H4 molecule upon N-N bond cleavage dominantly leads to NH3 formation on Ni(111), similar to the findings on Ir(111) and Rh(111).41-42

7



Page 8 of 22

As the coverage of N2H4 increased to 0.25 ML, the gauche-conformation is the most stable and was thus selected as the initial state to study the cleavage of N-H and N-N bonds. In this conformation, the N-H bond pointing to the surface (see inset IS in Fig.2d) is the longest due to the attraction between the H atom and surface. Thus, cleavage of this bond is the most favorable with an energy barrier of 1.08 eV. The reaction is exothermic by 0.14 eV. At TS1, the N-H bond is remarkably stretched (1.813 Å) with the H atom diffusing to a bridge site and the N-N axis of the N2H3 fragment still tilting to the surface. As for N-N bond cleavage at 0.25 ML, the structure at the TS is completely different from that at 0.11 ML due to a larger lateral repulsion between adsorbates: The lower NH2 entity approaches a bridge site while the upper one still suspends above the surface with a slight rotation (inset TS5 in Fig. 2d). This process shows a higher barrier of 1.08 eV as well as less amount of energy release (0.68 eV) than that at 0.11 ML. Coincidentally, the barrier of N-N bond cleavage is identical to that of N-H bond cleavage, suggesting that the two processes are competitive with each other during the decomposition of N2H4. The lateral interactions play an important role in determining the reaction paths. To provide deeper insight into the physical origin of the coverage-dependent energetic preference of N-N and N-H bond cleavage, the calculated barriers (without ZPE) were resolved according to the following formula47: '(

=

(78" 6!#

(78" 6!#

−

!"# 9:;/*+

+

!"# ,+

=

(78" 6!#

−
*+ .

+

is the adsorption energy of A (B) at TS

quantitatively measures the interaction between A and B, including the Pauli repulsion

and the bonding competition effects due to sharing substrate atoms.

!"# ,+ is

the adsorption energy at IS. Only

processes from the most stable ISs were analyzed. Only the three TS-related components are listed in Table 2 because

(78" 6!#

is a constant independent of coverage and the initial states for the two bond cleavage are the

same. According to Eq. 4, stronger

!"# 9/*+

(and

!"# ;/*+)

or smaller

&8> !"# *+ (and ,+ )

gives rise to lower

A comparison of the N-H bond scission at 0.11 and 0.25 ML indicates that the

&8> *+

0.28 eV) at the higher coverage due to the more stretched N-H bond at TS. Meanwhile,

'( .

term is weaker (by !"# %? @A /*+

(by 0.76

eV) decrease more remarkably upon increasing the coverage. As a result, the barrier of the N-H bond 8


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cleavage shows a net increment of 0.33 eV. As for the N-N bond cleavage, the three TS-related components show more notable changes than those of N-H bond cleavage because the structure at TS alters completely with increasing the surface coverage. The adsorption of the lower NH2 ( &8> *+ )

upon increasing the coverage, and the interaction (

!"# %@? /*+)

is enhanced by 0.54 eV

between the two NH2 fragments changes from

repulsion (-0.35 eV) at 0.11 ML to attraction (0.21 eV) at 0.25 ML. Specifically, the adsorption configuration of the upper NH2 (

!"# %@? B/*+ )

fragment changes from strongly bonding (-1.96 eV) with the

surface at 0.11 ML to suspending above the surface with a negligible interaction (-0.10 eV) at 0.25 ML at respective transition state (insets TS1 and TS5 in Fig. 2). Since the contribution of

!"# %@? B/*+ dominates

over

those of the other two items, the barrier of N-N bond cleavage at 0.25 ML is much higher than that at 0.11 ML. From our calculations, N2H4 shows essentially different bond cleavage behaviors at the two studied coverage on Ni(111). At 0.25 ML, cleavage of the N-H turns out to be competitive to that of N-N bond. In contrast, at 0.11 ML, N-N bond cleavage is dominant, resulting in the formation of NH2, which unambiguously leads to NH3 formation via a lower-energy intermolecular reaction path. Such decomposition behavior of N2H4 over Ni(111) is quite similar to that on Ir(111).41 Similar results were also obtained in the theoretical study of N2H4 decomposition over Rh(111) surface.42 Here, it should be noted that the predicted similarity in decomposition behavior is just based on the dynamic analysis of the mechanistic steps. But a theoretical analysis of a catalytic dissociation reaction requires a collective consideration of adsorption and reaction dynamics. The intrinsic rate is governed by the energy difference (Ea) between the gas-phase state and transition state, which is given by the sum of the adsorption energy (Eads) of the reactant and the barrier (Eeb) of the dissociation reaction: Ea= Eeb−Eads.48-49 According to our calculation and literature results, the Ea values were determined to be −0.34, −0.58 and −1.39 eV for Ni(111), Rh(111)42 and Ir(111)41, respectively. As Ea directly correlates with the intrinsic activity of the catalysts: the lower the Ea value, the higher the activity. The theoretically predicted activity variations agree well with the experimental observations: Ni < Rh *+

0.11 ML

0.92

-2.57

-1.34

-0.40

0.63

-1.82

-1.96

-0.35

0.25 ML

1.25

-2.60

-0.58

-0.12

1.19

-2.36

-0.10

0.21

16


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Fig. 1. Adsorption configurations of various species upon N2H4 decomposition on Ni(111) at surface coverage of 0.25 ML (white dashed lines). * denotes an adsorption site.

17



Fig. 2. MEPs of N2H4 decomposition from (a) anti- and (b) cis-conformation. Red and blue represents N-H and N-N bond cleavage, respectively. (c) MEPs of NH2 → NH + H (blue) and NH2 + N2H4 → NH3 + N2H3 (red). (d) MEPs of N-H (blue) and N-N (red) bond cleavage from gauche-conformation. White dashed lines in (a)-(c) and (d) correspond to surface coverage of 0.11 and 0.25 ML, respectively. The barriers and reaction energies (in eV) are shown in brackets.

18


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Fig. 3. Catalytic performance of Ni/C catalyst towards N2H4·H2O decomposition in 4 mL 0.5 mol/L N2H4·H2O at 50 and 60 oC, respectively.

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Fig. 4. Elementary steps of dehydrogenation involved in N2H4 decomposition on Ni(111) at 0.25 ML (white dashed lines). Energy barriers and reaction energies (in eV) are given in brackets.

20


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Fig. 5. Complete MEPs of N2H4 decomposition over Ni(111) via the intramolecular dehydrogenation route.

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Understanding of Selective H2 Generation from Hydrazine

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