Transition Metal Atoms Embedded in Graphene: How Nitrogen Doping

Letter Cite This: ACS Catal. 2019, 9, 6864−6868

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Transition Metal Atoms Embedded in Graphene: How Nitrogen Doping Increases CO Oxidation Activity Thomas Kropp* and Manos Mavrikakis* Department of Chemical and Biological Engineering, University of WisconsinMadison, 1415 Engineering Drive, Madison, Wisconsin 53706, United States

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S Supporting Information *

ABSTRACT: We study 14 transition metals on pristine and N-doped graphene using density functional theory. For double vacancies, nitrogen doping increases the binding strength of harder transition metals to the support and reduces their oxygen affinity. Inversely, the oxygen affinity of softer metals increases. Since O2 binding energies are correlated with the CO oxidation barrier in a volcano-like trend, doping also affects the activity of the single-atom catalyst. Among these systems, Fe atoms embedded in N-doped graphene are the most active CO oxidation catalysts. These insights can be used to guide the synthesis of highly active oxidation catalysts from nonprecious metals.

KEYWORDS: CO oxidation, nitrogen doping, density functional theory, graphene, single-atom catalyst, transition metals

P

(VASP);29,30 exchange−correlation energies were obtained using the functional by Perdew, Burke, and Ernzerhof (PBE).31 Additional computational settings are provided in the Supporting Information. We find that N doping increases the binding strength of harder transition metals (i.e., metals with low electron affinities and small atomic radii such as Co, Fe, and Ni that tend to form more ionic bonds), while it decreases the binding strength of softer transition metals (Ir, Mo, Rh, and Ru). By increasing the binding strength to the support, N doping reduces the oxygen affinity of the harder metal atoms. Inversely, it increases the oxygen affinity of softer metals. Since the binding energy of O2 is correlated with the barrier for CO oxidation in a volcano-like trend, N doping also affects the activity of the single-atom catalysts. Among the systems studied here, Fe atoms embedded in N-doped graphene were found to be the most active CO oxidation catalyst. Binding energies (BEs) of metal atoms on different adsorption sites are calculated using

orous carbon has been widely used as a sorbent, as an energy storage medium, and as a support material in heterogeneous catalysis.1−3 For catalytic applications, most studies refer to supported transition metal nanoparticles, though atomic dispersion has been achieved as well.4 Such single-atom catalysts with metal atoms stabilized in vacancytype defects of graphene offer high catalytic activity at low metal loadings, making them highly desirable synthesis targets.5 Graphene-supported single-atom catalysts have been synthesized using Au,6 Co,7−9 Fe,10−13 Mn,14 Ni,15 Pd,16,17 Pt,18 and Ru.19 In most cases, metal atoms were found to bind to double vacancy sites that contain up to four N substitutions, leading to a porphyrin-like coordination environment. These compounds catalyze hydrogen evolution,7,8 oxygen reduction,9−11 oxidation of benzene12 and methanol,18 and decomposition of formic acid.17 Additional transition metals were considered in computational studies on pristine20−25 and N-doped26−28 graphene supports. In agreement with experimental studies, density functional theory (DFT) predicts that metal atoms bind preferably inside vacancy-type defects.20 While most transition metals bind more strongly to single vacancies, electron-rich metals (Au, Cu, and Zn) bind more strongly to double vacancies.21 To gain further insight into the catalytic properties and stability of graphene-supported single atoms, we study CO oxidation on 14 atomically dispersed transition metals (groups 6−11) on pristine and N-doped graphene using DFT under periodic boundary conditions. Spin-polarized calculations were performed using the Vienna ab initio simulation package © XXXX American Chemical Society

BE = E(Me/G) − E(Me bulk ) − E(G)

(1)

where E(Me/G), E(Mebulk), and E(G) are the total energies of the metal atom supported on graphene, the metal atom in the bulk, and the undecorated graphene support, respectively. While binding energies are typically calculated relative to metal atoms in the gas phase, the definition adopted here represents the relative stability of the single-atom catalyst with respect to the bulk metals. Negative binding energies indicate that atomic Received: May 10, 2019 Revised: June 8, 2019 Published: July 1, 2019 6864

DOI: 10.1021/acscatal.9b01944 ACS Catal. 2019, 9, 6864−6868

Letter

ACS Catalysis

Figure 1. Binding energies (eq 1) for transition metal atoms (Me) on pristine graphene (black circles), single vacancies (red squares), and double vacancies (blue diamonds); hollow symbols refer to the same structures after substituting one C atom with one N atom (Table S3). Metals are sorted by their binding energies to pristine graphene; lines are drawn to guide the eye. The corresponding structures are shown as top views next to the diagram. The purple arrow refers to the dimerization of two Me1/G-V1C species to Me2/G-V2C, which is exothermic for all metals. The green arrow refers to the (555777) reconstruction of G-V2C.

Inside unreconstructed double vacancies (G-V2C), binding energies for all transition metals but Ag are negative (blue diamonds in Figure 1). However, when the reconstructed double vacancy (green arrow in Figure 1) is used as a reference, only Co, Cu, Ni, Pt, and Ru single sites are more stable than metal particles. Atomic dispersion is also more stable than dimer formation at the double vacancy (Me2/GV2C in Table S2), indicating that cluster growth is unfavorable at low coverage. N doping (blue, hollow diamonds in Figure 1) increases the binding strength for the harder transition metals (Ag, Au, Co, Cr, Cu, Fe, Mn, Ni, Pd, and Pt), while it decreases the binding strength of softer transition metals (Ir, Mo, Rh, and Ru), which is in agreement with Pearson’s acid− base theory, as nitrogen is harder than carbon and, therefore, binds more strongly to harder transition metals. N doping also decreases the energy gain for the reconstruction of the double vacancy, thereby significantly increasing the stability of atomically dispersed transition metals. Thus, with the exception of Mo, all transition metals considered in this work are predicted to form stable single-atom catalysts on Ndoped graphene. To assess the catalytic activity of these materials, CO oxidation was chosen as a model reaction. While reaction conditions and coverage may affect catalytic properties by changing the relative stability of various intermediates and transition states, we limit our investigation to the low-coverage limit where the reactive environment is the simplest. Different reaction mechanisms have been proposed,22−24,27 but the dominant pathway has not yet been identified. Furthermore, it is difficult to compare the reported barriers due to the different computational methods that were employed. As suggested by Deng et al.,27 CO may react with preadsorbed oxygen (on most metals, a superoxide is formed; the charge for each O2n− species is provided in Table S3), forming CO2 and a MeO species. Then, a second CO molecule may react with the remaining O atom, closing the catalytic cycle (black path in Figure 2a). The first oxidation has a higher barrier on all transition metals other than Co and Ni (Table S4). On Co and Ni, the O atom inserts into a CMe bond, forming a Me OC species that is less active than MeO toward CO oxidation. Figure 2b shows the barriers for the first oxidation step as a function of the O2 binding energies (black circles); whenever a different reaction is predicted to be rate-limiting,

dispersion is more stable than particle formation, whereas positive values indicate that the single-atom catalyst is metastable. Three different adsorption sites were considered: pristine graphene films (G), single vacancies (G-V1C), and double vacancies (G-V2C). In agreement with previous DFT studies,32 undecorated double vacancies were found to be more stable than single vacancies (Figure S1). Since migration barriers for single vacancies are relatively low (135 kJ/mol),32 double vacancies are expected to be the more common type of defects after annealing (synthesis conditions typically exceed 600 °C). Molecular dynamics simulations show that two single vacancies may indeed form a double vacancy, while the reverse reaction is not observed.33 N doping (GNm; the subscript m is the number of N atoms per unit cell) lowers the vacancy formation energies, but it does not change the relative stability of single and double vacancies. Furthermore, doping decreases the energy gain for the reconstruction of the double vacancy (green arrow in Figure 1; ΔE = −141 and −71 kJ/mol for GV2C and GN1-V2C, respectively). A detailed discussion of these structures can be found in the Supporting Information. On pristine graphene films, positive binding energies are obtained for each transition metal (black circles in Figure 1), indicating that metal particle formation, not atomic dispersion, is thermodynamically favored. This has been explicitly shown for Ni, Pd, and Pt using DFT.34 As previously reported by Foster and co-workers,21 transition metal atoms bind strongly to single vacancies in graphene (G-V1C, red squares in Figure 1). We find that binding energies for all transition metals but Ag and Au are negative on single vacancies (Table S2), indicating that atomic dispersion is more stable than particle formation. For these transition metals, atomic dispersion is also more stable than dimer formation at the single vacancy (Me2/G-V1C), which indicates that cluster growth at the single vacancy is unfavorable at low coverage. However, the dimerization of Me1/G-V1C to Me2/G-V2C is exothermic for all transition metals (purple arrow in Figure 1; dimerization energies are compiled in Figure S2). Thus, atomically dispersed transition metals on single vacancies are metastable. For N-doped graphene, the formation energy of single vacancies is lower (Figure S1), which leads to less exothermic metal atom binding energies (red, hollow squares in Figure 1). 6865

DOI: 10.1021/acscatal.9b01944 ACS Catal. 2019, 9, 6864−6868

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ACS Catalysis

Figure 2. (a) Energy profile for CO oxidation on Fe/G-V2C (see Figure S4 for other metals) starting with preadsorbed O2. Minima along the black path are shown below (C - black, Fe - gray, and O - red). Hollow bars refer to transition states with barriers given in kJ/mol for elementary steps going from reactants (left of the transition state) to products (right of the transition state). The red path refers to O2 dissociation prior to CO oxidation; the blue path corresponds to carbonate formation. (b) Barriers for the first CO oxidation step (black pathway) are plotted against O2 binding energies for Me/G-V2C and some Me in N-doped G-V2C. The black lines correspond to a linear fit (MAE = 13 kJ/mol, R2 = 0.64). If a different reaction is predicted to be rate-limiting, that barrier is shown as well with an arrow connecting both circles (color code: blue for carbonate decomposition, red for CO oxidation by graphene oxide, and green for CO poisoning).

transition metals, carbonate formation is endothermic and not predicted to occur during CO oxidation. As N doping impacts the stability of the single-atom catalysts, it also affects their ability to bind oxygen. Nitrogen stabilizes the highest occupied d orbital of the metal center relative to the Fermi level (density of states shown in Figure S5) and, therefore, decreases the energy gain for the electron transfer into the O2 π orbital. On pristine graphene, the Fe atom relaxes out of the graphene plane to accommodate the On− 2 species, but as N doping increases the binding energy of Fe, this relaxation becomes less exothermic. Overall, the more strongly the metal binds to the vacancy, the less strongly it binds O2 (Figure 3). Thus, N doping shifts Co, Cr, Fe, and Mn single-atom catalysts toward the peak of the volcano (BEO2 = −84 kJ/mol based on the linear fit in Figure 2b). For Co/GN4V2C and Fe/GN4-V2C, O2 binding energies (−69 and −94 kJ/

that barrier is shown as well with an arrow indicating the increase/decrease in activity. On the basis of the oxidation barrier, Pd and Pt are predicted to be the most active catalysts. However, CO adsorption was found to be more exothermic than O2 adsorption on Ni, Pd, and Pt (Table S4). Thus, O2 has to replace a CO molecule first, increasing the overall oxidation barrier (green circles in Figure 2b refer to oxidation barriers starting from preadsorbed CO). Therefore, the ideal CO oxidation catalyst has an oxygen affinity similar to Pt/G-V2C but does not suffer from CO poisoning. Since most transition metals bind O2 more strongly than Pt/G-V2C, lowering their oxygen affinity would increase their oxidation activity. This may be achieved by stabilizing the metals in higher oxidation states, as recently suggested by Tour and co-workers19 for oxygen reduction on Ru atoms embedded in N-doped graphene. Oxygen might also dissociate prior to CO oxidation (red path in Figure 2a), which is highly exothermic for the undoped single-atom catalysts considered in this work. However, O2 dissociation barriers exceed CO oxidation barriers on all transition metals other than Ag and Fe. On Fe/G-V2C, the reaction of CO with dissociated oxygen is less exothermic than the reaction with molecular O2 and, therefore, involves a higher barrier (Figure 2a). Thus, oxygen dissociation leads to catalyst deactivation at low temperatures. In contrast, dissociated oxygen is more reactive than molecular oxygen on Ag/G-V2C with an oxidation barrier of 99 kJ/mol compared to 136 kJ/mol for O*2 . Carbonate formation (blue path in Figure 2a) is also considered, as a previous DFT study23 suggested carbonate as a possible intermediate for CO oxidation on Fe/G-V1C. We find that carbonate formation is only exothermic on Ir and Rh. On these metals, the barrier for carbonate formation is lower than the CO oxidation barrier. Thus, carbonate formation is predicted to occur at the temperatures required for CO oxidation, and carbonate decomposition becomes the ratelimiting step (blue circles in Figure 2b). On the other

Figure 3. Binding energies for Fe atoms to GNm-V2C (0 ≤ m ≤ 4) from eq 1 using the most stable vacancy structures as references (black) as well as O2 binding energies to the Fe atom (red) for different N substitution patterns; dots are connected to guide the eye. The dashed line corresponds to the ideal O2 binding energy from Figure 2b. Figure S3 shows the data for the remaining metals. 6866

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ACS Catalysis mol, respectively) are particularly close to the peak, indicating high activity. To verify this prediction, CO oxidation barriers were calculated: the resulting values (66 and 64 kJ/mol for Co/GN4-V2C and Fe/GN4-V2C, respectively) are close to the DFT barriers reported by Deng et al.27 for CO oxidation on phthalocyanine complexes of Co2+ and Fe2+ (63 and 70 kJ/ mol, respectively), which exhibit a similar coordination environment. An even lower barrier (13 kJ/mol) has been recently reported for CO oxidation on Fe/GN4-V2C following the Langmuir−Hinshelwood mechanism,28 though we find that the coadsorption of CO and O2 at the active site is not favorable. By lowering the oxygen affinity of the single-atom catalysts, the dopant also suppresses O2 dissociation (235 kJ/ mol endothermic on Fe/GN4-V2C), thereby preventing catalyst deactivation via graphene oxide formation. These findings indicate high oxidation activity for the Fe catalyst, which has been experimentally shown for benzene oxidation on Fe atoms embedded in N-doped graphene.12 The binding energies for CO are similarly affected by N doping (Table S3). Therefore, the preferred adsorption species is unaffected by N doping on the harder transition metals. On the softer metals (Ir, Rh, and Ru), which are destabilized by the dopant, CO now binds more strongly than O2. Since oxygen binding energies were also found to be a good descriptor for the activity toward oxygen reduction reaction (ORR),35 we can elucidate the impact of N doping on the ORR activity of these single-atom catalysts as well. On the basis of their Sabatier analysis, Nørskov and co-workers35 conclude that the ideal ORR catalyst should have a binding energy for atomic O similar to Pt(111), which amounts to −124 kJ/mol (relative to 1/2 O2) when using the PBE functional. The value for Fe/GN4-V2C (−119 kJ/mol), therefore, indicates high ORR activity, which is in agreement with experimental results.10−13 On pristine graphene, O binds much more strongly to Fe atoms (−204 kJ/mol), lowering the ORR activity substantially. Thus, N doping increases the activity of the single-atom catalyst by lowering the oxygen affinity of the Fe atoms, which explains the observed dependence on the N content of the support. While Rossmeisl and co-workers26 already identified the high oxygen reduction activity of Fe/GN4-V2C, they did not comment on the impact of N doping. In summary, N doping was found to increase the binding energies of transition metals in double vacancies of graphene, enabling the formation of stable single-atom catalysts. As the transition metal binds more strongly to the vacancy, it binds oxygen less strongly. As a result, N doping affects the activity of the catalysts toward CO oxidation as well as oxygen reduction. Fe atoms embedded in N-doped graphene have a similar oxygen affinity as metallic platinum, leading to high catalytic activity that is unaffected by CO poisoning. These insights can be used to guide the synthesis of highly active catalysts from nonprecious metals. They also highlight parallels between heterogeneous and enzymatic catalysis, as the active site of these single-atom catalysts is similar to Fe porphyrin complexes, which are used in biological systems for O2 transport (heme) and oxidation catalysis involving O 2 (cytochrome P450).

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Computational methods; defects in pristine and Ndoped graphene; structure parameters of transition metals supported on graphene; and barriers and reaction energies for CO oxidation (PDF)

AUTHOR INFORMATION

Corresponding Authors

*E-mail: [email protected]. *E-mail: [email protected]. ORCID

Thomas Kropp: 0000-0002-9166-566X Manos Mavrikakis: 0000-0002-5293-5356 Notes

The authors declare no competing financial interest.

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ACKNOWLEDGMENTS This work has been supported by the U.S. Department of Energy (DOE) through the Office of Basic Energy Sciences (BES) by grant DE-FG02-05ER15731. Computational work was performed using supercomputing resources at the UW Madison Center for High Throughput Computing (CHTC), the Center for Nanoscale Materials (CNM) at Argonne National Laboratory under contract number DE-AC0206CH11357, and the National Energy Research Scientific Computing Center (NERSC) under contract number DEAC02-05CH11231. T.K. is grateful for partial financial support by the Alexander von Humboldt Foundation.

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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/acscatal.9b01944. 6867

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