Ammonia Activation by Ce Atom: Matrix-Isolation FTIR and Theoretical

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Cite This: J. Phys. Chem. A XXXX, XXX, XXX−XXX

Ammonia Activation by Ce Atom: Matrix-Isolation FTIR and Theoretical Studies Zhen Pu,† Fang Li,‡ Jianwei Qin,† Bingyun Ao,† Peng Shi,† and Maobing Shuai*,† †

Institute of Materials, China Academy of Engineering and Physics, Mailbox No. 9-21, Huafengxincun, Jiangyou 621908, Sichuan, P. R. China ‡ School of Material Science and Engineering, Southwest University of Science and Technology, 59 Middle Section of Qinglong Road, Mianyang 621010, P.R. China S Supporting Information *

ABSTRACT: The activation of ammonia by cerium atom has been investigated in solid argon using infrared spectroscopy and density functional theoretical calculations. The results reveal that the spontaneous formation of CeNH3 complex on annealing is the initial step in the reactions of cerium atoms with ammonia. The CeNH3 complexes rearrange to generate the inserted HCeNH2 molecules on irradiation. A “triplet−singlet” spin conversion occurs along the reaction path in which HCeNH2 (3A″) isomerizes into H2CeNH (1A′). The H2CeNH molecules finally decompose to yield HCeN + H2 upon photolysis. The periodic trend and differences for the M + NH3 (M = Ti, Zr, Hf, Ce, Th) systems are discussed on the basis of the present and previous works. DFT calculations predict that the most stable ground state for HHfNH2 and HThNH2 is singlet due to the stronger relativistic effects in Hf and Th atoms, while that for HTiNH2, HZrNH2, and HCeNH2 is triplet. Besides, the H2-elimination process is different for Ce and M (Ti, Zr, Hf, Th) cases.

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INTRODUCTION As one of the traditional industrial chemicals, ammonia has been widely used in fertilizers, nitric acid production, and ammonium salts. Recently, ammonia has attracted considerable attention as a feedstock for producing hydrogen in fuel cells because it is more easily stored than H2 and is carbon-free.1 In addition, synthesis of amine-containing organic molecules directly from ammonia in pharmaceutical industry can limit the generation of byproducts.2 For all of these applications, it is required to understand the ammonia activation mechanism and the N−H bond cleavage steps. The activation of a strong N−H bond of ammonia (bond dissociation energy = 99.5 kcal/mol3) by main-group elements and transition-metal atoms has been the subject of considerable experimental and theoretical investigations.4−9 It has been shown that laser-ablated metal atoms (M = Sc, Ti, V, Zr, Hf, Cr, Mo, Ta) react with ammonia to generate a MNH3 complex, which further rearranges to form HMNH2 and H2MNH molecules upon different irradiation conditions. The activation of ammonia by f-block metal atoms has also received much attention. Wang and his co-workers reported that the ground-state thorium atom reacted dramatically with ammonia to generate HNThH2 and that 5f orbitals were strongly involved in the ThN bond.10 The UNH3 complex isomerized to produce H2NUH and HN UH2 upon irradiation, and the NU bond in HNUH2 was enhanced by partial triple-bond character.11 Ce, with a 4f15d16s2 ground-state electronic configuration, is the second element in the lanthanide series. From the © XXXX American Chemical Society

viewpoint of the Mendeleev periodic law, Ce could be categorized into an extended “group IV” in the periodic table due to its four-electon valence shell. Thus, Ce should present more or less similar reaction properties with Ti, Zr, Hf, and Th elements. By now, the reactions of ammonia with Ti, Zr, Hf, and Th neutral metal atoms have been investigated by experiments combined with calculations.7,9,10 Therefore, the reaction chemistry of Ce with ammonia becomes central to improve our understanding on the periodic trend of ammonia activation by the extended “group IV” elements. Our previous study on the reactions between Ce and ammonia focuses on the HCeN molecule with an enhanced CeN triple bond.12 However, the comprehensive reaction mechanism has not been figured out yet. We present herein a brief experimental identification of the intermediates and products in reactions between ammonia and Ce atoms in solid argon. This paper focuses on, both experimentally and theoretically, exploring the reaction mechanism. The present work has been compared with the reported works to summarize the periodic trend and differences in “extended group IV” element reactions with ammonia. Received: January 15, 2018 Revised: March 27, 2018 Published: March 27, 2018 A

DOI: 10.1021/acs.jpca.8b00430 J. Phys. Chem. A XXXX, XXX, XXX−XXX

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METHODS

The experimental setup and detailed procedures for ammonia reaction with laser-ablated Ce atoms have been described previously.13 The rotating Ce metal target was ablated by a Nd:YAG laser fundamental beam (1064 nm, 10 Hz repetition rate, and 10 ns pulse width) with typical 10−20 mJ/pulse laser power. The laser-ablated Ce atoms were codeposited with ammonia onto a 4 K CsI window for 1 h at a rate of 3−4 mmol/h. The infrared spectra were recorded in the range 4000−400 cm−1 on a Bruker Vertex 80 V spectrometer at 0.5 cm−1 resolution. The matrix samples were annealed at 25 K and subsequently irradiated by a medium-pressure mercury arc lamp (175 W, globe removed, λ > 220 nm) with different glass filters. More experimental details are presented in the following context. Density functional theory calculations on molecular structures and vibrational frequencies were performed by B3LYP14,15 and BP8614,16−18 methods using the Gaussian 09 program.19 The relativistic energy-consistent small-core SDD pseudopotential with the (14s, 13p, 10d, 8f, 6g)/[10s, 8p, 5d, 4f, 3g] basis set was employed for Ce,20−23 and 6-311+ +G(3df,3pd) basis sets were used for H and N.24−26 The structures of intermediates, products, and transition states were fully optimized. Frequency analysis for all structures was made, and zero-point energy (ZPE) was calculated. Each transition state was determined by only one imaginary frequency. The potential energy surface (PES) was predicted to probe the reaction mechanism along the minimum energy path. Similar calculations on structures and energy for Ti, Zr, Hf, and Th systems were also performed by the B3LYP method to compare the periodic trend and differences. More computational details are presented in the following context.

Figure 1. Infrared spectra of products in the 1400−850 and 750−700 cm−1 ranges, where m is HCeNH2, n is H2CeNH, k is HCeN, j is CeNH3, and h is CeN. (1) 1 h codeposition of laser-ablated Ce atoms with 1% 14NH3 (99% Ar) at 4 K; (2) after 25 K annealing; (3) after 10 min of irradiation at λ > 500 nm; (4) after 10 min of irradiation at λ > 350 nm; (5) after 10 min of irradiation at λ > 220 nm.

Calculations. DFT calculations are performed for the Ce + NH3 reaction system including product geometries, frequencies, and PESs. The calculated frequencies are in good agreement with the observed values as listed in Table S1. For example, the symmetric N−H bending mode in CeNH3 complex is predicted at 1153.4 cm−1 in B3LYP calculations, which is overestimated by only 5.0% as compared to the observed one at 1098.2 cm−1. The optimized structures and PES profiles are shown in Figures 2 and 3, respectively. The HCeNH2 molecule is predicted to have 3A″ ground state with the lowest energy at the B3LYP level of theory, while the isomer H2CeNH molecule is found to have 1A′ ground state with 29.9 kcal/mol higher in energy. Since the spin state between the inserted HCeNH2 and H2CeNH molecules is different, the isomerization process must involve spin crossing between the high-spin 3HCeNH2 and the low-spin 1H2CeNH. The reaction pathways for the singlet and triplet states are examined to investigate the possible spin transition involved in the reactions. The dominant reaction channels will be discussed below in detail. Structure optimizations and PES calculations are also performed for M + NH3 systems (M = Ti, Zr, Hf, Th). The optimal reaction paths on PESs are illustrated in Figure 4. Activation Mechanisms. The PESs predicted by DFTB3LYP calculations for singlet and triplet states are illustrated in Figure 3. The first step of the ammonia activation by groundstate Ce atom is to form the intermediate 3CeNH3 complex along the triplet PES. The CeNH3 bands enhance markedly upon annealing (see Figure 1 and Figure S1 in the Supporting Information), indicating that the formation of CeNH3 complex (reaction 1) requires negligible activation energy. DFT calculations predict that reaction 1 is barrier-free with 8.3 kcal/mol stabilization energy. On λ > 500 nm irradiation, the CeNH3 bands disappear while the HCeNH2 bands increase dramatically. This implies that the intermediate HCeNH2 is generated from CeNH3 complex. Thus, the second step of ammonia activation is the insertion of Ce atom into the N−H bond to produce the intermediate HCeNH2 (reaction 2). As shown in Figure 3, the insertion proceeds through a transition

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RESULTS Infrared Spectra. The products of laser-ablated Ce atoms with natural and isotopic ammonia have been examined in our previous work.12 The typical infrared spectra with selected spectral range are shown in Figure 1. Briefly, a very strong band belonging to HCeNH2 (labeled as m) at 1274.7 cm−1 is observed after deposition. It increases obviously upon λ > 500 nm irradiation but decreases markedly upon λ > 220 nm irradiation. Another band at 1098.2 cm−1 labeled as j is assigned to CeNH3. It increases upon 25 K annealing but disappears upon λ > 500 nm irradiation. A new band at 1224.1 cm−1 (labeled as k, belonging to HCeN) tracked with a site-split doublet at 937.7 and 934.8 cm−1 appears upon λ > 350 nm irradiation, and its intensities increase distinctly upon λ > 220 nm irradiation. A group of weak bands labeled as n at 1296.4, 1243.2, and 757.8 cm−1 increases after 25 K annealing. This group of bands is attributed to H2CeNH. They present no obvious changes upon λ > 500 nm and λ > 350 nm irradiation but decrease markedly upon λ > 220 nm irradiation. The weak band labeled as h at 843.4 cm−1 is assigned to CeN.27,28 Besides, the band at 736.7 cm−1 29 belonging to trace CeO2 is observed as a result of the reaction of Ce atoms with O2 impurity. 15N- and D-isotopic substitution experiments were carried out for product identification. Typical spectra in selected regions are presented in Figures S1 and S2 in the Supporting Information. The observed absorptions in the reactions of Ce with ammonia are summarized in Table 1. The absorption behaviors of product upon stepwise annealing and irradiation will be discussed below in detail. B

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Table 1. Observed Absorptions (cm−1) of Intermediates and Products in Reactions between Group IV M (M = Ti, Zr, Hf, Ce, Th) Atoms and Ammonia Tia

Zr

Hf

MNH3, δsym(NH3) HMNH2, ν(M−H)

1155.6 1531.7

1156.0 1478.4

1158.8 b

1098.2 1274.7

HMNH2, ν(M−NH2) H2MNH, νsym(MH2) H2MNH, νasym(MH2) H2MNH, ν(M−NH) MNH, ν(M−NH) HMN, ν(M−H) HMN, ν(M−N)

1613.1 1582.1 942.9c -

1556.0 1520.3 889.2d -

1614.7 1590.9 898.3 -

516.1 1296.4 1243.2 757.8 1224.1 937.7 934.8 (site)

assignment

Ce

Th 1149.8 1437.0 1445.3 (site) 1398.7 1357.4 793.4 834.3 -

a

The experimental data for Ti, Zr, Hf, and Th are extracted from refs 7, 9, and 10. bThe short dash means the absorptions were too weak to be observed or the corresponding products were not generated. cAbsorption at 942.9 cm−1 was assigned to H248TiNH. Besides, weak absorptions at 947.6, 945.3, 940.7, and 938.3 cm−1 were also observed for H246TiNH, H247TiNH, H249TiNH, and H250TiNH, respectively. dAbsorption at 889.2 cm−1 was assigned to H290ZrNH. Besides, weak absorptions at 887.8 cm−1 for H292ZrNH and 886.4 cm−1 for H294ZrNH were observed as well.

CeNH3 → HCeNH 2

(2)

Note that the subsequent λ > 350 nm and λ > 220 nm irradiations increase the HCeN absorption but decrease the HCeNH2 absorption, suggesting the photoinduced dehydrogenation reaction 3. DFT calculations predict reaction 3 is endothermic by 42.2 kcal/mol. HCeNH 2 → HCeN + H 2

(3)

As shown in Figure 3, the dehydrogenation of HCeNH2 is predicted to proceed along a more detailed reaction path. The triplet HCeNH2 isomerized to yield singlet H2CeNH in reaction 4 via spin crossing. The isomerization through singlet TS2 is endothermic by 29.9 kcal/mol with a 49.0 kcal/mol energy barrier. The subsequent step is the formation of intermediate HCeN(H2) from H2CeNH along the singlet PES (reaction 5), which is also endothermic by 11.2 kcal/mol. The energy barrier through singlet TS3 is predicted to be 24.5 kcal/ mol. The intermediate HCeN(H2) finally decomposes into HCeN and dihydrogen (reaction 6). Unfortunately, the intermediate HCeN(H2) is not trapped in our experiments probably because of the fast rate of reactions from 4 to 6.

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(4)

H 2CeNH → HCeN(H 2)

(5)

HCeN(H 2) → HCeN + H 2

(6)

DISCUSSION According to Mendeleev’s periodic law, element Ce should present a similar reaction trend to the group IV elements, namely, Ti, Zr, Hf, and Th. Up to now, combined with the previous studies, the investigations on ammonia activation by the extended group IV element M (M = Ti, Zr, Hf, Ce, Th) have been completed. Therefore, in order to obtain an improved understanding of the reaction chemistry of M with ammonia, it becomes valuable to summarize the periodic trend and differences. The potential profiles for the reactions of ammonia with M atoms along the energy optimal reaction path are illustrated in Figure 4. Some experimentally observed absorptions for those product molecules are listed in Table 1. As shown in Figure 4, the first step in reactions between M atoms and ammonia is to form MNH3 complexes, which has

Figure 2. Optimized geometries of the intermediates and transition states along the optimal path on the PESs. Bond lengths are in angstroms, and bond angles are in degrees. The ground state for CeNH3, TS1, and HCeNH2 is triplet, while that for TS2, H2CeNH, TS3, HCeN(H2), and HCeN is singlet.

state 3TS1 along triplet PES with an energy barrier of 21.9 kcal/ mol. DFT calculations indicate that reaction 2 is exothermic by 34.5 kcal/mol. Notice that the triplet HCeNH2 is the most stable molecule on Ce + NH3 PESs, which is supported by the strongest absorptions of HCeNH2 observed in experiments. Ce + NH3 → CeNH3

HCeNH 2 → H 2CeNH

(1) C

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Figure 3. Singlet and triplet PESs calculated from Ce + NH3 to the HCeN + H2. Energy is relative to the free ammonia and triplet Ce atom.

Figure 4. PESs for the reactions of ammonia with group IV atoms along the energy optimal reaction paths. M = Ti, Zr, Hf, Ce, and Th. Energy is relative to the free ammonia and triplet M atoms.

Table 2. Calculated Bond Lengths and EBDE of MNH3 Complexes at the B3LYP/6-311++G(3df,3pd)/SDD Level of Theory

been widely reported in similar investigations.7,9,30,31 The absorptions of MNH3 complexes increase upon annealing in experimental observations, indicating that the formation of MNH3 is exothermic and barrier-free. DFT calculations suggest that MNH3 has a 3A″ ground state with C3v symmetry. Since MNH3 complex is the first intermediate in ammonia activation processes and it determines the energy transfers and geometry rearrangements of the following steps, a discussion on the M− N and N−H bond lengths is needed. The geometric parameters of the MNH3 complex are listed in Table 2. The N−H bond length is elongated by about 0.007 Å, implying the weakness of N−H bonding and the initiation of the ammonia activation. The M−N bond length varies from 2.210 to 2.684 Å through the series of complexes. It is found that the M−N bond length of MNH3 complex is very close to the sum of the single-bond covalent radii given by Pyykkö et al.32 The M−N bond length increases in general with the increase of covalent radii from Ti to Th. However, the M−N bond length in CeNH3 is the longest, even longer than that in ThNH3. This indicates the binding between ammonia and Ce is the most unstable. As

MNH3

d(M−N)a (Å)

d(N−H)b (Å)

rc (pm)

EBDEd (kcal/mol)

TiNH3 ZrNH3 HfNH3 CeNH3 ThNH3 NH3

2.210 2.351 2.408 2.684 2.572

1.021 1.021 1.020 1.021 1.021 1.014

136 154 152 163 175 71

21.7 20.6 14.6 8.3 17.0

a

d(M−N) is the bond length between M and N in MNH3 complexes. d(N−H) is the bond length of the N−H bond in MNH3 complexes. c r is the covalent radius for N and M (M = Ti, Zr, Hf, Ce, Th) atoms in single bonded molecules. dEBDE is the bond dissociation energy, EBDE = E(M) + E(NH3) − E(MNH3). b

listed in Table 2, CeNH3 has the smallest bond dissociation energy (BDE), which is consistent with the bond length. MNH3 then isomerizes into the intermediate HMNH2 molecules upon photolysis through transition states with D

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although the energy barriers of H2 elimination were higher than those of isomerization. However, in our opinion, the reaction path of H2 elimination for HZrNH2 and HHfNH2 is a matter of debate. Since H2MNH (M = Zr, Hf) is more stable than HMNH2, and the isomerization is energetically favorable, the reaction from HMNH2 (M = Zr, Hf) should proceed along the isomerizing path. This is supported by strong H2ZrNH and H 2HfNH absorptions upon photolysis in experimental observations. We prefer to consider that H2MNH dissociates to generate MNH + H2. Our additional calculations predict the dissociation energy barrier of H2MNH is lower than that of HMNH2. Unfortunately, no MNH (M = Ti, Zr, Hf) absorption was found in Chen and Zhou’s work; thus, it is difficult to determine the more reasonable reaction path. For Th, Wang et al. suggested ThNH was formed through H2ThNH dissociation.10 It is worth noting that the most important difference in H2 elimination is found in Ce + NH3 reactions. In the present work, HCeN molecules are observed rather than CeNH. As mentioned above, triplet HCeNH2 isomerizes to singlet H2CeNH and then transforms into intermediate HCeN(H2), which finally decomposes into HCeN and dihydrogen.

different energy barriers. The photoinduced rearrangement reactions are also exothermic. As shown in Figure 4, DFT calculations predict the spin multiplicity of HMNH2 is different. HTiNH2, HZrNH2, and HCeNH2 have a triplet state, while HHfNH2 and HThNH2 have a singlet state. Since the periodic trend of multiplicity is first broken for HMNH2, it is necessary to probe into the reasons. It is well-known that relativistic effects have a great influence on chemical bonding and reactions. Usually, the relativistic effect is stronger for heavier elements. The energy level of atomic orbitals for M atom and the corresponding configuration are provided in Figure 5. The

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CONCLUSIONS Herein we report the experimental and theoretical investigations on the ammonia activation by Ce atom in solid argon. The results indicate that the first step to activate ammonia is the spontaneous formation of triplet CeNH3 complexes on annealing with no energy barrier. The subsequent step is the rearrangement from CeNH3 complex to the inserted triplet intermediate HCeNH2 through a transition state. Then, the HCeNH2 (3A″) isomerizes into H2CeNH (1A′) via a triplet− singlet spin crossing. Finally, the H2CeNH is dissociated to yield HCeN and H2 via photolysis. The periodic trend and the differences of the group IV atoms reacting with ammonia have been discussed on the basis of the present and previous works. DFT calculations predict that the binding between ammonia and Ce is the most unstable among MNH3 complexes. The most stable ground state for HHfNH2 and HThNH2 is singlet due to the stronger relativistic effects in Hf and Th atoms, while that for HTiNH2, HZrNH2, and HCeNH2 is triplet. In addition, the H2-elimination process is different. The H2MNH (M = Ti, Zr, Hf, Th) decomposes to generate MNH and dihydrogen, while the H2CeNH molecule dissociates to yield HCeN, rather than CeNH, and dihydrogen.

Figure 5. Energy levels of atomic orbitals for Ti, Zr, Ce, Hf, and Th and corresponding configuration of valence electrons.

valence electron configuration for ground-state M atom is ns2(n − 1)d2 (n = 4−7), while that for Ce is 4f16s25d1. The ns and np orbitals are contracted in the (n − 1)d orbitals in the radial distribution due to the relativistic effect except for Ti. As illustrated in Figure 5, the energy of ns orbitals for Hf and Th atoms is calculated to be much lower than that of their (n − 1)d orbitals (1.94 and 1.10 eV for Hf and Th, respectively). This implies that the double occupied ns2 orbitals prefer to remain in a pair and they are barely involved in bonding in HHfNH2 and HThNH2 molecules. As shown in Figure S3, the paired HOMOs of HHfNH2 and HThNH2 are basically derived from Hf 6s and Th 7s orbitals, respectively. Therefore, the spin state of HHfNH2 and HThNH2 is a singlet. However, the energy between ns and (n − 1)d orbitals for Ti, Zr, and Ce atoms is very close and both ns and (n − 1)d orbitals are involved in bonding in the generation process of HMNH2 molecules. This leads to a triplet state for HMNH2 (M = Ti, Zr, Ce). It can be found in Figure 4 that the next reaction step is HMNH2 undergoes α-H transfer to generate H2MNH. This process is carried out via a triplet−singlet spin crossing for Ti, Zr, and Ce. For Hf and Th, the spin crossing occurs in the formation process of HMNH2 from MNH3. The singlet intermediate H2MNH (M = Zr, Hf, Th) is predicted to be the most stable product in the potential profiles. However, the inserted triplets HTiNH2 and HCeNH2 have the lowest energy for the cases of Ti and Ce. A possible reaction process is the H2 elimination to yield MNH + H2 (M = Ti, Zr, Hf, Th, except for Ce). Chen,9 Zhou,7 and their co-workers reported triplet TiNH was generated directly from triplet HTiNH2 as the energy barrier of HTiNH2 dissociation was lower than that of the rearrangement to H2TiNH. In their view, ZrNH and HfNH were also formed through HZrNH2 and HHfNH2 dissociation, respectively,

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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.jpca.8b00430. Additional information including infrared spectra of Ce atom and 15NH3 (ND3 + NHD2 + NH2D) reaction in solid argon (Figures S1 and S2), molecular orbital diagrams of HMNH2 (M = Ti, Zr, Hf, Ce, Th) (Figure S3), and the observed and calculated fundamental frequencies of the intermediates and products in Ce + NH3 reactions (Table S1) (PDF)

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AUTHOR INFORMATION

Corresponding Author

*E-mail: [email protected]. ORCID

Zhen Pu: 0000-0003-4608-3609 E

DOI: 10.1021/acs.jpca.8b00430 J. Phys. Chem. A XXXX, XXX, XXX−XXX

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The authors declare no competing financial interest.

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ACKNOWLEDGMENTS We gratefully acknowledge financial support from the National Natural Science Foundation of China (No. 21771167) and NSAF (No. U1630118).

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DOI: 10.1021/acs.jpca.8b00430 J. Phys. Chem. A XXXX, XXX, XXX−XXX