Promotional Effect of Carbon on Fe Catalysts for Ammonia

Article pubs.acs.org/IECR

Promotional Effect of Carbon on Fe Catalysts for Ammonia Decomposition: A Density Functional Theory Study Jian Ji,† Xuezhi Duan,*,† Xueqing Gong,† Gang Qian,† Xinggui Zhou,† De Chen,‡ and Weikang Yuan† †

State Key Laboratory of Chemical Engineering, East China University of Science and Technology, 130 Meilong Road, Shanghai 200237, China ‡ Department of Chemical Engineering, Norwegian University of Science and Technology, Trondheim 7491, Norway S Supporting Information *

ABSTRACT: Density functional theory calculations have been performed to investigate the recombinative desorption of N on terraced Fe(100) and Fe(110) as well as stepped Fe(111) and Fe(211) surfaces. The results showed that the stepped surfaces, especially the Fe(111) surface, are more active than the terraced surfaces, which could be related to the presence of so-called C7 sites for the stepped surfaces. Carbon atoms were found to stabilize the thermodynamically unstable stepped surfaces to be the energetically favored facets and thus facilitate the preferential exposure. As these carbon atoms incorporated into the stepped surfaces in the form of surface or subsurface carbon, the activation energy for the recombinative desorption of N was lowered, which was mainly ascribed to the decrease in the d band center of surface Fe atoms. In addition, a relation between the electronic and structural properties of Fe catalysts in the absence or presence of carbon and ammonia decomposition activity was also correlated.

1. INTRODUCTION Ammonia decomposition over transition metal catalysts has been extensively studied to obtain a fundamental understanding of the nature of active sites and the reaction mechanism, and the process has found numerous (potential) applications in the energy and environmental industries.1−3 Among the commonly used catalysts, Fe catalyst is more attractive in view of the low cost and abundant resource, despite having an activity lower than those of Ru and Ni catalysts.4−11 Therefore, many efforts have been made to improve the activity of Fe catalysts by manipulating the electronic and structural properties, for example, adding other metals and metal oxides,12−17 lowering the Fe particle size,5 and tuning the Fe particle shape.8−10 It is well-known that ammonia decomposition proceeds by stepwise dehydrogenation of NH3, followed by the recombination of N and H to form N2 and H2, respectively.18,19 In the past decades, a great amount of work has been done on NH3 adsorption and/or dehydrogenation over Fe clusters and surfaces (e.g., (100), (110), (111), and (211)).20−25 However, in these studies, the recombinative desorption of N, which is considered as the rate determining step,21,26 was of less concern. Conducting a comparative study of the recombinative desorption of N on different Fe surfaces will help us not only identify Fe active facets and sites, but also understand the role of Fe electronic and structural properties. Recently, dispersed Fe particles incorporated into carbon nanofibers (Fe-CNFs), prepared by the catalytic chemical vapor deposition (CCVD) method, have been found to be active for ammonia decomposition, especially for Fe particles on the tops of platelet carbon nanofibers.8−10 In particular, when the resulting carbon atoms from Fe catalytic CO decomposition decreased by lowering the CO partial pressure, the as-prepared Fe catalyst had polyhedral shape and showed higher activity.10 This indicates that carbon atoms possibly facilitate the faceting © 2013 American Chemical Society

of Fe active surfaces. An investigation on how to induce the reconstruction of Fe crystal facets by carbon atoms is therefore needed. Moreover, for the Fe-CNFs catalysts prepared by the CCVD method, the carbon atoms inevitably existed in the Fe particles according to the growth mechanism of carbon nanofibers,3,9,10 and thus whether or not these carbon atoms influence the electronic properties and the activity of the Fe catalyst is worth discussion. In this work, a comparative study of the recombinative desorption of N atoms on terraced Fe(100) and Fe(110) as well as stepped Fe(111) and Fe(211) surfaces was first performed by periodic spin-polarized density functional theory (DFT), in which the stepped surfaces were found to be more active than the terraced surfaces. Surface energies of different Fe surfaces under different carbon coverages were calculated, and then the corresponding equilibrium Fe shapes were obtained by Wulff construction to elucidate effect of carboninduced reconstruction of Fe crystal facets. Moreover, effects of surface or subsurface carbon incorporation on recombinative desorption of N on Fe-stepped surfaces were further studied. Finally, a relation between the d-band center of surface Fe atoms and activation energy of recombinative desorption of N is correlated to understand the role of Fe electronic and structural properties in ammonia decomposition.

2. COMPUTATIONAL DETAILS All periodic spin-polarized DFT calculations were carried out using the Perdew−Burke−Ernzerhof functional27 and projector-augmented wave potentials28,29 as implemented in the Received: Revised: Accepted: Published: 17151

September 12, 2013 November 11, 2013 November 17, 2013 November 17, 2013 dx.doi.org/10.1021/ie4030165 | Ind. Eng. Chem. Res. 2013, 52, 17151−17155

Industrial & Engineering Chemistry Research

Article

Figure 1. The most stable adsorption configurations of N and the transition states of the recombinative desorption of N on Fe(100), Fe(110), Fe(111), and Fe(211) surfaces. Small balls denote N atoms, and large blue, blue-green and yellow balls correspond to the Fe atoms in the first, second and third layer, respectively. The C7 sites were marked by red hexagons.

Vienna Ab-initio Simulation Package (VASP),30−32 in which the cutoff kinetic energy of 400 eV was applied to the plane waves. For calculations of N adsorption energy and activation energy of recombinative desorption of N, the detailed parameters of the slab models for the four Fe surfaces were listed in Supporting Information, Table S1, and the transition states of the recombinative desorption of N were determined using the dimer method.33 In these calculations, the structural optimization was performed using the conjugated-gradient method with the force convergence criterion of 0.03 eV·Å−1,34 and the energies included zero-point energy (ZPE) corrections. In addition, the methods reported by Huo et al.35 were employed to calculate the surface energies of clean and carbon adsorbed Fe(100), Fe(110), Fe(111), and Fe(211) surfaces, in which the detailed parameters of the slab models were shown in Supporting Information, Figure S1.

Figure 2. Activation energies of the recombinative desorption of N on the four Fe surfaces. All the energies included ZPE corrections.

3. RESULTS AND DISCUSSION 3.1. Structure Sensitivity. Figure 1 shows the most stable adsorption configurations of N on Fe(100), Fe(110), Fe(111), and Fe(211) surfaces. Clearly, all the N atoms prefer to locate at the 4-fold hollow sites, in accordance with the previous results,24,36 in which N atoms bind five Fe atoms on the Fe(100) surface while binding four Fe atoms on the other three surfaces. Moreover, the adsorption energies of N on the four Fe surfaces slightly depend on the crystallographic orientation, which are −6.61, −6.39, −6.26, and −6.17 eV, respectively. Bozso et al. also found similar N adsorption energies over three Fe single-crystal surfaces (i.e., Fe(100), Fe(110), and Fe(111)) using auger electron spectroscopy and thermal desorption techniques.37,38 The transition states of the recombinative desorption of N on the four Fe surfaces are also shown in Figure 1. It can be seen that the two N atoms locate at the C7 sites of stepped Fe(111) and Fe(211) surfaces. Encouraged by the findings that the C7 sites of Fe are the active sites for ammonia synthesis,39−41 we surmise that the C7 sites of Fe may also be the active sites for ammonia decomposition. To verify this point, the activation energies for the recombinative desorption of N on the four Fe surfaces are calculated, and the results are shown in Figure 2. Clearly, the stepped Fe(111) and Fe(211) surfaces have much lower Ea than the terraced Fe(100) and Fe(110) surfaces, indicating that the recombinative desorption of N easily occurs on the stepped surfaces with the presence of

C7 sites. Unfortunately, the stepped surfaces (e.g., Fe(111) and Fe(211)) usually have a relatively high surface energy and thus are unstable. Enhancing the stability of the stepped Fe surfaces and thus the ratio of active surfaces could be an effective way to improve the activity. 3.2. Carbon-Induced Faceting of Fe Stepped Surfaces. Along the above line, carbon adsorbates were proposed to enhance the stability of the active Fe-stepped surfaces and thus the exposure ratio. At carbon coverage of 1/24 ML on the four Fe surfaces, the carbon adsorption preferentially locates at the 4-fold hollow sites (Supporting Information, Figure S1). However, the corresponding adsorption energies depend on the crystallographic orientation, which are −8.45, −7.90, −7.62, and −7.73 eV, respectively. This implies that C adsorption can stabilize the Fe surfaces to different degrees. To reveal the stabilization effect of C adsorption, we calculated the surface energies of the four Fe surfaces under different carbon coverages. As shown in Table 1, for the clean Fe surfaces, the calculated surface energies are in good agreement with the experimental estimation for a polycrystalline surface (2.41 J· m−2)42 as well as the calculated values by Huo et al.35 and Blonski and Kiejn.43 With the increase of carbon coverage, all the surface energies decrease, and thus all the surfaces become more stable. Notably, when the C/Fe ratio increases to 1/12, the stepped surfaces are even more stable than the terraced surfaces. This indicates that carbon can stabilize the active surfaces to be the energetically favored surfaces. 17152

dx.doi.org/10.1021/ie4030165 | Ind. Eng. Chem. Res. 2013, 52, 17151−17155


Article

Table 1. Surface Energies (Esurf, J·m−2) for the Fe(hkl)−C System C/Fe (100) (110) (111) (211)

0 2.48 2.36 2.61 2.53

(2.4435 (2.3735 (2.6035 (2.4935

and and and and

2.4743) 2.3743) 2.5843) 2.5043)

1/24

1/12

1.78 1.42 1.51 1.48

1.07 0.96 0.42 0.43

(Supporting Information, Figure S3 and Table S2). There is a volcano-type relationship between N adsorption energy and the activity of catalyst for ammonia decomposition, in which among single metals Ru has the optimal N adsorption energy and Fe has higher N adsorption energy.18,21 Therefore, lowering the N adsorption energy over the Fe catalyst toward that over the Ru catalyst may be effective to enhance the activity. As expected, the surface carbon and subsurface carbon incorporations can lower the activation energies of the recombinative desorption of N on Fe(111) and Fe(211) surfaces and thus promote ammonia decomposition. In addition, Figure 4 gives a relation between the d-band center of surface Fe atoms and the activation energy of

Motivated by the fact that the contribution of the active surface to exposed surface area relates to both the surface energy and the orientation,44 we employed Wulff construction to examine the equilibrium shapes of Fe catalysts at different carbon coverages based on the calculated surface energies. The results were shown in Figure 3. For the clean Fe surfaces, the

Figure 4. Activation energies of the recombinative desorption of N on Fe(100), Fe(110), Fe(111), and Fe(211) surfaces in the absence or presence of carbon as a function of the d-band center. All the energies included ZPE corrections.

recombinative desorption of N to understand the role of Fe electronic and structural properties for ammonia decomposition, in which the d-band center was calculated by the d-band model of Hammer-Nørskov,47 and the corresponding projected density of states (DOS) pictures were shown in Supporting Information, Figure S4. Apparently, there is a linear dependence between the activation energy of recombinative desorption of N on the d-band center of Fe surfaces except the Fe(100) surface, indicating that the electronic effect is responsible for the difference in the activities. However, for the Fe(100) surface, the activation energy of recombinative desorption of N is still higher desipite the lower d-band center, which could be due to the unique geometrical effect, for example, the adsorbed N atom being influenced by the subsurface Fe atom (Figure 1).

Figure 3. Equilibrium shapes of Fe catalysts at different carbon coverages obtained by Wulff construction (a, b, and c) and the contribution of each surface to the total surface area (d).

thermodynamically most stable Fe(110) surface has the largest contribution to the total surface area with a percentage of 52.4%, followed by Fe(211) (27.4%), Fe(100) (18.0%), and Fe(111) (2.2%). The increase in the carbon coverage leads to the remarkable increase in the total surface area of exposed active surfaces. These results indicate that carbon adsorbates can promote the reconstruction of Fe crystal facets and induce faceting of Fe-stepped surfaces. 3.3. Structure−Activity Relationship. In section 3.2, carbon adsorbates were found to induce faceting of active Festepped surfaces. These carbon atoms will inevitably exist on the surface and subsurface of faceted Fe particles. Supporting Information, Figure S2 shows the most stable configurations of carbon adsorption on Fe-stepped surfaces for the surface carbon at the 4-fold hollow site and the subsurface carbon at the subsurface octahedral site. Notably, previous studies showed that the surface and subsurface carbon incorporated into Pd catalysts remarkably influence the selective hydrogenation events of the unsaturated hydrocarbon.45,46 An attempt is thus necessary to clarify the effect of surface and subsurface carbon on recombinative desorption of N on the stepped surfaces. These carbon atoms were found to lower the N adsorption energy compared to the clean Fe stepped surfaces

4. CONCLUSIONS On the basis of periodic spin-polarized density functional theory calculations, the stepped Fe(111) and Fe(211) surfaces were found to be more active than the terraced Fe(110) and Fe(100) surfaces. This difference possibly is due to the presence of C7 sites in the stepped surfaces. Carbon adsorbates were proposed to stabilize the thermodynamically unstable stepped surfaces, making them energetically favored surfaces. According to the obtained equilibrium shapes of Fe catalysts at different carbon coverages, carbon adsorbates can induce the faceting of the stepped surfaces, and Fe particles show preferential exposure of Fe active facets. These carbon atoms in principle existing on the surface and subsurface of Fe(111) 17153



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and Fe(211) can further promote the recombinative desorption of N. Moreover, there is an apparently linear relationship between the d-band center of surface Fe atoms, except for the Fe(100) surface, and the activation energy of the recombinative desorption of N. These theoretical results can help us understand the promotional effect of carbon on Fe catalysts for ammonia decomposition and further guide the catalysts design in a future study.

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ASSOCIATED CONTENT

S Supporting Information *

Parameters of the slab models for Fe surfaces (Table S1), and adsorption energies of N and activation energies of recombinative desorption of N on Fe(111) and Fe(211) in the absence or presence of carbon (Table S2). Top and side views of Fe(100), Fe(110), Fe(111), and Fe(211) surfaces with the carbon coverage of 1/24 ML as well as the corresponding carbon adsorption energies (Figure S1), optimized configurations of surface or subsurface carbon incorporated Fe(111) surfaces (Figure S2), the most stable adsorption configurations of N and the transition states of the recombinative desorption of N on surface or subsurface carbon incorporated Fe(111) and Fe(211) surfaces (Figure S3), and DOS projected onto the dband of surface atoms of Fe(100), Fe(110), Fe(111), and Fe(211) surfaces in the absence or presence of carbon (Figure S4). This material is available free of charge via the Internet at http://pubs.acs.org.

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AUTHOR INFORMATION

Corresponding Author

*E-mail: [email protected]. Tel.: +86-21-64250937. Fax: +86-21-64253528. Notes

The authors declare no competing financial interest.

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ACKNOWLEDGMENTS This work is financially supported by the Natural Science Foundation of China (21306046), the Shanghai Natural Science Foundation (12ZR1407300), the China Postdoctoral Science Foundation (2012M520041 and 2013T60428), and the Fundamental Research Funds for the Central Universities (WA1214020, WG1213011 and WA1114006).

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Promotional Effect of Carbon on Fe Catalysts for Ammonia

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