Coverage-Dependent N2 Adsorption and Its Modification of Iron

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Coverage Dependent N Adsorption and Its Modification of Iron Surfaces Structures Tao Wang, Xinxin Tian, Yong Yang, Yong-Wang Li, Jianguo Wang, Matthias Beller, and Haijun Jiao J. Phys. Chem. C, Just Accepted Manuscript • DOI: 10.1021/acs.jpcc.5b11953 • Publication Date (Web): 15 Jan 2016 Downloaded from http://pubs.acs.org on January 18, 2016

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The Journal of Physical Chemistry Coverage Dependent N2 Adsorption and Its Modification of Iron Surfaces Structures a

b

b

b

b

a

a,b,

Tao Wang, Xinxin Tian, Yong, Yang, Yong-Wang Li, Jianguo Wang, Matthias Beller, Haijun Jiao *

a) Leibniz-Institut für Katalyse e.V. an der Universität Rostock, Albert-Einstein Strasse 29a, 18059 Rostock, Germany; b) State Key Laboratory of Coal Conversion, Institute of Coal Chemistry, Chinese Academy of Sciences, Taiyuan, Shanxi, 030001, PR China.

E-mail address: [email protected]

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The Journal of Physical Chemistry

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Abstract Spin-polarized density functional theory calculations were performed to investigate N2 dissociative adsorption on iron (100), (110), (111), (210), (211), (310) and (321) surfaces. An ordered c (2×2) structure was found on Fe(100) at 0.5 ML coverage, which is in excellent agreement with the experiment; and a c (4×2) ordered structure is also found at 0.75 ML saturation coverage. Strong surface reconstruction is found on Fe(110) upon nitrogen adsorption, where the dense packed (110) is reconstructed into (100)-alike. Under the consideration of temperature and N2 partial pressure, the estimated N2 desorption temperature on Fe(100) at 925 K agrees with the experimentally detected 920-950K. In addition, N2 pretreatment results in Fe(100) to be mostly exposed; while that of H2 pretreatment favors Fe(110). Further direct comparison of N2 and H2 adsorptions has been made to show their difference and similarity.


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Introduction Iron-based catalysts are widely used in large scale industrial processes, such as ammonia synthesis

1,2

and Fischer-Tropsch synthe-

3

sis (FTS). Their attractive characters come from their low prices and high activity. For understanding the reaction mechanisms of ammonia synthesis, systematic ultra-high vacuum (UHV) surface science studies of N2,

4-14

15-20

and H2

interactions with iron surfaces

were performed. These studies clearly revealed the adsorption states, desorption temperatures and energies of these molecules on the Fe(110), Fe(100) and Fe(111) low Miller index iron surfaces. There are also studies about the activity of ammonia synthesis. For 21

example, Spencer et al., found that the relative rates of ammonia formation on Fe(111), Fe(100) and Fe(110) were 418 : 25 : 1 at 22

798 K and a total pressure of 20 atmosphere. Apart from the clean iron surfaces, Parker et al., found that K promoter can increase 23

the activity of all the iron surfaces, but it does not produce surfaces of equal activity. However, Strongin et al., reported that K had no effect on the activity of ammonia synthesis of the Fe(110) surface but enhanced the rates of the Fe(111) and (100) surfaces. Most 24

importantly, the results from Rayment et al., proposed that we should reconsider the role of Fe(111), where other active phases were possible. Apart from the experimental studies, the adsorption, dissociation and diffusions of hydrogen on iron catalyst are computed.

25-30

The first theoretical calculation of N atom adsorption on iron surfaces was reported by the Nøskov group, they found that the Fe(100) surface has much stronger adsorption strength than the Fe(110) and (111) surfaces. The c(2×2)-N/Fe(100) structure was predicted to 31

form on the (111) and (110) surfaces. To explain the inconsistent experimental findings about N2 activation activity, they further calculated the detailed N2 adsorption and dissociation on the Fe(111) surface, where two dissociation channels were identified; i.e., one through all the molecular states sequentially with low energy barrier but high entropy barrier, and one via a direct channel into a new precursor, which is highly activated. They concluded that the low barrier channel is expected to dominate the reaction, while 32

the high barrier channel may become effective at high temperatures. They further investigated N2 dissociation on the Fe(110) and Fe/Ru(0001) surfaces by using high-pressure thermal experiments and supersonic molecular beam measurements as well as DFT cal33

culations, where the surface steps and defects were identified to dominate the activity. The strain effect on N2 dissociation on Fe surfaces was also calculated by using the fcc-Fe(111) surface, where the strained surface was found to have higher activity.

34

35

Motivated by the discrepancy of N2 dissociation barrier on the Fe (110) surface between experiment (0.28 eV) and theory (1.1~1.2 eV),

33,34

36

Goikoetxea et al., reported a theoretical study of the N2/Fe(110) dynamics. It is found that N2 dissociation has en-

ergy barrier of about 1.1 eV and takes place along a very narrow reaction path, which verified the low reactivity of this surface. However, they also found two N2 adsorption wells on the Fe(110) surface and the preferences of these two types of adsorption sites is dependent on the incident energy of the molecule and the surface temperature. Apart from N2 adsorption and dissociation, nitrogen 37

diffusion into bulk iron also attracted great attentions. Pedersen et al., studied the diffusion of individual N atom on the Fe(100) surface by using scanning tunneling microscopy and DFT calculations, and they found that the diffusion of N atom has barrier of 38

0.92±0.04 eV and the diffusion is coupled with strong lattice distortions. Wu et al., also calculated the adsorption, absorption and diffusion of N atoms on the Fe(100) and Fe(110) surfaces and found N to prefer staying on surface sites instead of diffusing into bulk. Despite of the weak adsorption energy of N on the surface, the Fe(110) surface has much lower N diffusion barrier than the Fe(100) 39

surface. To have a full nitridation mechanism of bcc Fe by nitrogen molecule, Yeo et al., systematically calculated the adsorption, dissociation, penetration and diffusion of N2 on and in the Fe(100) and (110) surfaces. The penetration step was identified to be rate determining because of its highest energy barrier. To interpret the low initial sticking probabilities of nitrogen on iron single-crystal 40

planes, Panczyk applied statistical rate theory to describe the dissociation adsorption kinetics of N on the Fe(100) and Fe(111) surfaces. The Fe(100) surface has stronger N adsorption strength but lower initial sticking probability than the Fe(111) surface, and the



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main difference in activity of these two surfaces is attributed to the different activation barriers. Apart from pure metallic iron sur41

face, Šljivančanin et al., studied N2 fixation on the MgO supported Fe nanoclusters. On the basis of the potential energy surface of N2 hydrogenation, the nitrogenated Fe cluster was found to be a promising potential catalyst for ammonia synthesis at low temperature. In addition to N2 adsorption and dissociation, NH3 decomposition on iron surfaces also attracted theoretical interests cently because of its applications in the production of CO free H2 and prevention of environmental pollution.

42,43

re-

44-46

On the basis of the importance of nitrogen interaction with iron surfaces, these theoretical investigations have provided useful information for understanding the initial steps of ammonia synthesis and explained many experimental findings at microscope scale. However, some other factors should also be considered; i.e., (a) coverage dependent nitrogen adsorption on iron surfaces is not well addressed; (b) high Miller index surfaces that can be used to model the stepped and kinked structures of iron catalysts were hardly applied; (c) the effect of temperature and N2 partial pressure on N surface coverage was not included; (d) the influences of N adsorption on the surface morphology of iron catalyst was not yet mapped, despite of the extensive studies of surface reconstructions. To account for the above considerations, we applied DFT calculation in combination with atomistic thermodynamics to study N2 dissociative adsorptions on seven iron surfaces at different coverage and conditions. Our goal is the understanding of coverage dependent N2 dissociative adsorption on iron catalysts and the modification of the surface morphology by nitrogen adsorption. Theoretical Methods (a) Methods: All calculations were performed by applying the plane-wave based density functional theory (DFT) method with the Vienna Ab Initio Simulation Package (VASP) tor augmented wave (PAW) method,

49,50

47,48

and periodic slab models. The electron ion interaction was described with the projec-

while the electron exchange and correlation energy was solved within the generalized gra51

dient approximation in the Perdew-Burke-Ernzerhof formalism (GGA-PBE). Spin-polarization was included for iron systems to cor52

rectly describe magnetic properties and this was essential for an accurate description of adsorption energy. An energy cut-off of 53

400 eV and a second-order Methfessel-Paxton electron smearing with σ = 0.2 eV were used to ensure accurate energies with errors less than 1 meV per atom. Geometry optimization was done when forces became smaller than 0.02 eV/Å and the energy difference was lower than 10

–4

eV. The vacuum layer between periodically repeated slabs was set as 10 Å to avoid interactions

among slabs. (b) Models: The calculated lattice constant of α-Fe bulk crystal structure is 2.84 Å with a local spin magnetic moment of 2.214 μB, 54

and they agree well with the experiment. The p(4×4), p(4×4), p(3×3), p(3×2), p(4×2), p(3×2) and p(2×3) super cells were used to 30

model the (100), (110), (111), (210), (211), (310) and (321) surfaces, which are the same as used in our previous work. The 3×3×1 Monkhorst-Pack k-point grid was applied in all the surfaces for sampling the Brillouin zone. (c) Thermodynamics: Ab initio atomistic thermodynamics is a convenient tool in solving problems referring to real reaction con55,56

ditions

and has been widely applied in many catalytic systems.

57-63

The detailed description of the method can be found in the

Supporting Information. Results (a) Surface structures and adsorption sites: In contrast to the early calculations which focused mainly on the (110), (100) and (111) low Miller index surfaces for the basic structures; we included the (210), (211), (310) and (321) higher Miller index surfaces for the stepped and kinked structures (Figure 1). Each surface has different adsorption sites; i.e., (100) has top (T), bridge (B) and 4-fold hollow (4F) sites; (110) has top (T), short-bridge (SB), long-bridge (LB) and 3-fold (3F) sites; (111) has top (T), shallow-hollow (SH), deep-hollow (DH) and 4-fold hollow (4F) sites; (210) has three top (T1, T2, T3), three bridge (B1, B2, B3), four 3-fold (3F1, 3F2, 3F3, 3F4) and one 4-fold (4F) sites; (211) has one top (T), one bridge (B), two 3-fold (3F1, 3F2) and one 4-fold (4F) sites; (310) has one top (T),


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one bridge site (B), two 3-fold (3F1, 3F2) and one 4-fold (4F) sites; and (321) has three top (T1, T2, T3), two bridge (B1, B2), six 3-fold (3F1, 3F2, 3F3, 3F4, 3F3, 3F4) and one 4-fold (4F) sites. Figure 1: Schematic top and side views of seven iron surfaces and possible adsorption sites (T for top, B for bridge, F for fold, LB for long bridge, SB for short bridge, SH for shallow hollow and DH for deep hollow)

(b) Nitrogen adsorption and surface reconstruction To get the stable adsorption configuration of dissociated N2 at different coverage, stepwise adsorption method was used, i.e.; one N atom was added to the previous most stable state for getting the next most stable state after considering different possibilities. It should be noted that during the identification of the most stable configurations at different coverage, we firstly tested all possible structures by adding one more N atom on the previously most stable coverage on the basis of the individual adsorption energies at different adsorption sites. In addition, we also tested other different arrangements of those N atoms at the corresponding coverage. Our goal is to find the most stable one by considering as many as different possibilities. We applied the stepwise adsorption energy to determine the saturation coverage, ΔEads = E(N)n+1/slab - [E(N)n/slab + 1/2EN2], where a positive ΔEads for n+1 adsorbed N atoms indicates the saturation adsorption with nN atoms. Total energies of all the considered N adsorption configurations on seven iron surfaces at different coverage were given in supporting information (Tables S1-S7). On Fe(100) (Figure S1), the most stable adsorption of one N atom is located in the 4F site with adsorption energy of -1.50 eV. With the increase of N coverage, the stepwise adsorption energies generally decrease despite of some changes. However, such changes did not affect our identification of stable coverage at different conditions, because the total energies of different coverage are the basis for our atomistic thermodynamics method. The 4F hollow site is still most favorable with increasing coverage. The saturation coverage has 12N atoms (3/4 ML), and all are located at the 4F sites. Obviously, no reconstruction of (100) upon N



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adsorption was found. More interestingly, regular arrangement of N atoms at different coverage was found. As shown in Figure 2, the adsorbed N atoms form an ordered c(2×2) structure at 0.5 ML coverage (8 N atoms). Actually, this result is in excellent 64

agreement with the LEED study of nitrogen adsorption on Fe(100), where a c(2×2) structure was proposed. In addition, the calculated saturation coverage on the Fe(100) surface is 0.75 ML and a c(4×2) structure is also formed at this coverage. Figure 2: The clean (a), c(2×2) –8N (b)and c(4×4)-12N (c) structures of adsorbed N atoms on Fe(100) surface

On Fe(110) (Figure S2), the most stable adsorption of one N atom is located on the LB site (-1.31 eV) and the adsorption energy decreases with increasing coverage, where the saturation coverage has 10N atoms (5/8 ML) on the surface. At 0.5 ML (8N) coverage, a regular c(2×2) structure (Figure 3a) was also found, which is the same as that Fe(100). However, the most interesting finding is the strong reconstructions of Fe(110) upon nitrogen adsorption. As shown in Figure 3b-3d, (110) is expanded and becomes very similar with (100). Figure 3: Reconstructions of Fe(110) Surface with different numbers of N atoms. (Blue balls for Fe atoms and red balls for N atoms)

On Fe(111) (Figure S3), the most stable adsorption of one N atom is located on the 4F site (-0.99 eV). With further increase of N coverage, the 4F hollow site is still most favorable. The saturation coverage has 10N (10/9 ML) atoms and all are located at the 4F sites. Interestingly, at 1 ML (9N) coverage in Figure 4, all N atoms are located at the 4F sites and form a regular c(1×1) structure. However, this structure had not been detected experimentally and only complex LEED patterns without exact nitrogen adsorption 64

configurations were found by Bozso et al. In addition, reconstruction was also found on this surface; i.e.; at the coverage of 10N, part of Fe(111) also reconstructs into a Fe(100)-like structure and the 4F site reconstructs into a square. This finding is also in line 64

with the experiment by Bozso et al., where Fe(111) was predicted to transform into another atomic arrangement although the detailed transformation information was not outlined. Our calculated structural and energetic information clearly identified these related configurations. Figure 4: Structures of 1N (a), 4N (b), c(1×1)-9N (c) adsorption and surface reconstruction at 10N (d) on Fe(111)


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On Fe(210) (Figure S4), the most stable adsorption of one N atom is located at the 4F hollow site (-0.94 eV). However, with the increase of N coverage, both the 4F and 3F hollow sites became possible. The saturation coverage has 10 N (5/3 ML) atoms in 3F2, 3F3 and 4F adsorption configurations coexisting on the surface. At saturation coverage, the surface structure was only slightly affected by adsorbed N atoms without obvious and regular reconstructions. On Fe(211) (Figure S5), the most stable adsorption of one N atom is located at the 4F hollow site (-1.04 eV) and the saturation coverage has 6N (3/4 ML) atoms and all are located at the 4F sites. There is no obvious surface reconstruction upon N adsorption. On Fe(310) (Figure S6), the most stable adsorption of one N atom is located at the 4F hollow site (-1.30 eV) and the saturation coverage has 6N (1 ML) atoms in 4F and 3F1 adsorption configurations coexisting on the surface. There is also no obvious surface reconstruction upon N adsorption. On Fe(321) (Figure S7), the most stable adsorption of one N atom is located at the 4F hollow site (-1.04 eV). The 3F hollow site became favorable with the increase of N coverage and the 3F site reconstructs in to the 4F site, similar with that on Fe(100). The saturation coverage has 6N (1 ML) atoms in 4F and reconstructed 3F adsorption configurations coexisting on the surface. On the basis of the adsorption energy of one N atom, Fe(100) has the strongest N adsorption followed by (110), (310), (211), (321), (111) and (210), which is in line with early theoretically reported results.

31,39

In addition, the adsorption of nitrogen can cause

obvious and regular reconstruction on (110) and (111). (c) Gibbs free energy with temperature and pressure: Based on the energies of N adsorption at different coverage from standard DFT calculations, we would like to include the effects of temperature and nitrogen partial pressure, where the adsorption Gibbs free energy was chosen as the criteria. The phase diagrams in Figure 5 present the relationship between the stable N coverage with temperatures and nitrogen partial pressure on seven iron surfaces. It clearly revealed that the stable N coverage decreases upon temperature increase at given pressures, while increases with N2 partial pressure at given temperature. On the basis of these phase diagrams of nitrogen coverage, one can directly get the stable nitrogen coverage under any given T and pN2. Further inspections showed that each surface has many regions, which represent different stable N coverage. The stable nitrogen coverage on these seven surfaces is quite different even under the same temperature and N2 partial pressure, which in turn rationalize the experimentally observed different ammonia activities of iron surfaces. These phase diagrams are also of great significance to UHV surface science studies, micro-kinetics modeling as well as practical experimental applications. Moreover, desorption temperatures of nitrogen at any given pressure on each surface can be compared with the experimentally detected temperature-programmed desorption data. Since extensive early UHV surface science studies of nitrogen interaction with iron surfaces were performed at pressures not -4

-4

-7

exceeding 4×10 Torr, we here chose pN2 = 10 Torr (10 atm) to discuss the N2 desorption temperatures on each surface. On the



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basis of our thermodynamic results, the full desorption of the adsorbed nitrogen atoms at this chosen pressure will take place at about 925K on Fe(100), which is in good agreement with experiment N2-TPD result, where a sharp N2 desorption peak at about 920-4

950K was detected after exposing the Fe(100) surface to 4×10 Torr of N2 at 700K for 12h.

64

Figure 5: Equilibrium phase diagrams of stable N coverage on seven iron surfaces.

(d) Morphology of iron particles under different conditions: Experimentally, the iron catalyst is always pretreated with N2/H2 65

mixtures before performing ammonia synthesis. Therefore, the temperature and N2 partial pressure should have important effects on the surface morphology of iron catalyst. As shown in the atomistic thermodynamics in Supporting Information, nitrogen adsorption influences the surface energies of iron surfaces. On the basis of the surface free energy after nitrogen adsorption (Tables S8 and S10), Wulff construction

66,67

was performed to model crystal shapes of iron catalyst at different temperatures and nitrogen partial

pressure. As shown in Figure 6, the Wulff particle can expose all seven surfaces with different area ratio under ideal condition. However, the surface morphology and ratios of the exposed surfaces were dramatically affected by nitrogen adsorption. For example, at 400K and with increasing N2 partial pressure, the proportion of (100) surface increased while those of the other surfaces decreased. At pN2 = 50 atm and with increasing temperature, the proportion of (100) surface decreased while that of the (110) surface increased. Within a wide range of temperature and N2 partial pressure, Fe(100) is the most favorably exposed surface, which is quite different from the morphology of iron particle under H2 atmosphere, where Fe(110) is the most exposed surface. distinct reactivity of catalyst pretreated at different temperature, pressure as well as gas compositions.


30,68

This finding verified the

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Figure 6: Wulff shapes of iron catalyst under N atmosphere at different temperature

Discussion 30

On the basis of our previous results about hydrogen interactions on iron surfaces and current results about nitrogen adsorption, we can have a general picture about the similarity and difference of the interaction of hydrogen and nitrogen on different iron surfaces. Table 1 lists the computed surface energies of seven iron surfaces as well as binding energies, saturation coverage and desorption temperatures of hydrogen and nitrogen on these surfaces for direct comparison. Generally nitrogen has much larger binding energies and also much higher desorption temperatures than hydrogen. However, hydrogen has generally much higher satura-



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tion coverage than nitrogen. In addition, there is no general correlation between surface stabilities and adsorption strengths of hydrogen or nitrogen because of their quite different adsorption configurations on each surface. Most importantly, hydrogen and nitrogen affect the surface morphologies of iron catalyst in quite different manners, e.g.; under hydrogen atmosphere, Fe(110) is most exposed, while under nitrogen atmosphere Fe(100) surface is most exposed. -2

Table 1: Surface energies (J/m ) of seven iron surfaces as well as binding energies (Eb, kcal/mol), saturation coverage and full de-9

sorption temperatures (T) at p = 10 atmosphere of H (reference 30) and N atoms on each surface

Surfaces

Saturation coverage

Eb

γ

-9

T at p = 10 atm

H

N

H

N

H

N

Fe(100)

2.53

63

155

7/6 ML

3/4 ML

310K

875K

Fe(110)

2.47

68

150

1 ML

5/8 ML

425K

725K

Fe(111)

2.70

64

143

7/3 ML

10/9 ML

350K

575K

Fe(210)

2.57

66

141

3 ML

5/3 ML

350K

550K

Fe(211)

2.58

67

144

2 ML

3/4 ML

400K

560K

Fe(310)

2.55

67

150

2 ML

1 ML

425K

710K

Fe(321)

2.62

67

144

3 ML

1 ML

400K

625K

Different from hydrogen adsorption, nitrogen adsorption on iron surfaces resulted in obvious surface reconstructions, which might be of great relevance to their ammonia synthesis activities. Experimentally, Fe(111) is widely regarded as the most active sur21

face for ammonia synthesis, followed by Fe(100) surface while Fe(110) is least active. It is known that Fe(111) is least stable surface of iron catalyst, which also should be the least abundant surface experimentally. In principle, increasing the proportion of Fe(111) surface in iron catalyst should be an wise way to improve the ammonia synthesis activity. There is indeed available ways to transfer 69

24

the stable Fe(110) and (100) surfaces into active Fe(111) surface. However, Rayment et al., has also reminded us to reconsider the role of Fe(111) surface in ammonia synthesis since 1985. In this respect, our results might provide some insights into this confused question. We found that the least active Fe(110) has the strongest surface reconstruction upon nitrogen adsorption, which resulted in the formation of Fe(100) surface-like structure. This should also cause a significant increase in ammonia synthesis activity, because Fe(100) is proved to be 25 times higher in activity than Fe(110) surface.

21

In addition, based on Wulff constructions, we find that at given N2 partial pressure and increasing temperature, the proportion of exposed Fe(100) surface decreases while that of (110) surface increases, which finally causes a decrease in (100)/(110) ratio. Indeed, 70

it is found that the decrease in XRD intensity values of Fe(100)/Fe(110) surface is a sign of successful activation This agreement between our results and experimental findings verified our predicted iron morphologies under nitrogen atmosphere, where we can propose that the Fe(100) should play a very important role in ammonia synthesis since it is the most abundant exposed surface under a wide range of conditions. Conclusions Spin-polarized density functional theory calculations along with ab initio atomistic dynamics were performed to investigate N2 dissociative adsorption on the iron (100), (110), (111), (210), (211), (310) and (321) surfaces. Among the these surfaces, nitrogen has the strongest adsorption on the Fe(100) surface and an ordered c(2×2) structure was

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formed at 0.5 ML coverage (8 N atoms), in excellent agreement with the LEED study. In addition, an ordered c(4×2) structure is also found at 0.75 ML saturation coverage. Strong surface reconstruction was found on the Fe(110) surface upon nitrogen adsorption, where the dense packed (110) surface is reconstructed into a (100)-alike surface. Since the Fe(100) surface has 25 times higher activity in ammonia synthesis than Fe(110) surface, this surface reconstruction should be of great significance in improving activity of ammonia synthesis reactions Under the consideration of temperature and N2 partial pressure, the computed N2 desorption temperature at 925 K on Fe(100) is in perfect agreement with the experimentally observed temperature-programmed desorption result at ultra-high vacuum condition. We also computed the N2 desorption temperatures, but there are no available experimental data for comparison. On the basis of Wulff construction, with increasing temperature at given N2 pressure, the proportion of exposed Fe(100) surface decreases while that of (110) surface increases, which finally causes a decrease in (100)/(110) ratio. Actually, this finding is proved to be a sign of successful activation of iron catalyst in ammonia synthesis experimentally. Under N2 pretreatment, Fe(100) is the most exposed surface, which should play a very important role in ammonia synthesis. This is quite different from that of H2, where Fe(110) is most exposed under H2 pretreatment. Our systematic investigations of N2 and H2 interaction mechanisms on seven iron surfaces provide useful and interesting insights into their different interaction manners, which are of great significance in academic research and industrial applications.

Acknowledgment: This work was supported by National Natural Science Foundation of China (No. 21273262), Chinese Academy of Sciences and Synfuels CHINA. Co., Ltd. We also acknowledge general finical support from the BMBF and the state of MecklenburgWestern Pommerania.

Supporting Information Available: Detailed description of atomistic thermodynamics method; energies of all the considered N adsorption configurations on seven iron surfaces (Table S1-S7); surface free energies of iron surfaces at different condition (Table S8S10) as well as structures and energies of N adsorption configurations on seven iron surfaces (Figure S1-S7) are included. This material is available free of charge via the internet at http://pubs.acs.org.



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Coverage-Dependent N2 Adsorption and Its Modification of Iron

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