Adsorption of Transition Metal Catalysts on Carbon Supports: A Theoretical Perspective
Adsorption of Transition Metal Catalysts on Carbon Supports: A Theoretical Perspective
Understanding the interaction between catalyst and catalyst supports
Adsorption is a fundamental process which takes place on a catalyst surface before it dissociates, diffuses over the surface and recombines with other adsorbed species to form the final product. Therefore, in theoretical chemistry understanding of the local geometrical and electronic properties of the adsorbed species on the catalyst surface has been a topic of core focus. In this short review we briefly summarise some of the important developments on theoretical studies related to the adsorption properties of transition metal (TM) catalysts on graphene and graphene-related carbon materials. Prior to this, we will present a discussion on various forms of carbon materials used as catalyst supports, which will be followed by a brief discussion of the fundamentals of the density functional theory (DFT).
As early as 1834 Michael Faraday performed some of the original studies on adsorption, which is a fundamental surface phenomenon involving the interaction of atoms and molecules from vapour phases or a solution onto the surfaces and pores of solids (1). It is now well-known that adsorption can be either physisorption (also referred as Van der Waals adsorption or physical adsorption) or chemisorption. In physisorption, as the name suggests, the weak physical Van der Waals forces are involved, and the heat evolved during this process is usually very small. Chemisorption on the other hand, first studied by Irving Langmuir in 1916, occurs due to the formation of chemical bonds between the adsorbate and adsorbent and the heat evolved during this process is significantly higher (in the range of 100–500 kJ mol–1). So far numerous theoretical studies have been performed to understand the adsorption properties of atoms, clusters and molecules on catalyst surfaces (2–12). However, in this short review we will focus only on the DFT based studies on the adsorption properties of TM catalysts on graphene and discuss the future of similar studies on amorphous carbon materials where graphene is the basic structural unit (BSU). First, we will present a brief summary of the physical and chemical features of various carbon materials widely used as catalyst supports, which will be followed by a brief discussion on DFT.
1.1 Graphite and Graphene
The word graphene refers to a single atomic plane of graphite and the word graphite has its origin in the Greek word ‘Γραφɛιη’ (pronounced as graphein), which means to draw or to write (13). Graphite is one of the first known materials to be studied using X-ray diffraction (XRD) by A. W. Hull in 1917 (14). He proposed that the normal structure of graphite has hexagonal D6h symmetry (14). Later this structure was confirmed by J. D. Bernal, O. Hassel and Z. H. Mark in 1924 (15, 16). According to their model of graphite, it consists of a stack of parallel hexagonal net planes of carbon atoms with C–C bond length of 1.42 Å and these net planes are separated by a distance of 3.35 Å and the unit cell contains four atoms in a planar stacking sequence ABABA… However, later reports based on X-ray spectroscopy accounted for these hexagonal planes being stacked in ABCABC… sequence (17–22). These reports revealed that in ordinary temperatures and pressures natural graphite occurs in two crystal structures: (a) Bernal with an ABABA… sequence of arrangements and (b) rhombohedral with an ABCABC… sequence of arrangements (23).
During the early period of research on graphite, many studies on its physical properties were also reported. For example, in 1915 G. E. Washburn measured the electrical resistance parallel to the basal plane of a perfect sample approximating to single crystals or highly oriented polycrystalline samples (24). Similar reports on electrical resistance measurements were reported by Ryschkewitsch in 1923 (25). Measurements related to the temperature-dependence of the electrical resistance parallel to the layers were made by D. E. Roberts in 1913 and W. Meissner, H. Franz and H. Westerhof in 1932. In these reports the resistivity was found in the range of 4–8 × 10–5 ohm cm. Later, G. E. Washburn, N. Ganguli and K. S. Krishnan reported that the resistivity perpendicular to the basal plane was 102–103 times greater than that of the parallel planes (26). Simultaneous theoretical studies to explore the electronic structure of graphite were also carried out. The first known energy band structure of graphite crystal was studied as early as 1935 by Hund and Mrowka (27, 28). However, the first known theoretical calculations with an attempt to relate them to properties such as electrical conductivity of graphite were made by Wallace and Coulson in 1947 (29, 30). Wallace employed ‘tight-binding’ approximation to develop the electronic energy bands and Brillouin zones of graphite (31).
In addition to the investigations on the structural, physical and electronic properties, the chemical properties of graphite such as the reaction of graphite with gases, particularly oxygen and carbon dioxide in presence of catalysts were also reported, but the use of graphite as catalyst support was seen much later (32). For example, Brownlie et al. used graphite as a catalyst support to investigate how properties of metals are influenced by its interaction with the surface (31). Under standard temperatures and pressures, graphite is the most stable allotrope of carbon but due to its low surface area it was used very little as catalyst support (33).
In recent years however, many studies on the use of expanded graphite as catalyst supports have been reported (33, 34). We note that a modified graphite having an enlarged interlayer lattice distance of 4.3 Å, and having a long-range-order layered structure of graphite is referred to as expanded graphite (35). For example, Chen et al. recently showed that platinum-cobalt nanoparticles supported on expanded graphite have higher electrocatalytic activity and anti-poisoning ability and long-term stability for methanol oxidation (34). The expanded graphite also finds its application in fuel cell electrode material due to its good electrical and thermal conductivity, chemical stability and excellent mechanical properties (36). Other interesting experimental studies have been also reported on the use of graphite nanoplatelets (37).
Recently, Novoselov et al. reported the isolation of graphene by employing the technique of micromechanical cleavage, which involves mechanical exfoliation of small mesas of highly oriented pyrolytic graphite (38, 39). In recent years, the use of graphene as catalyst support has been also a topic of extensive research especially because of its interesting properties, which include excellent electronic and thermal conductivity, mechanical and thermal stability and high surface area (as compared to graphite) (40, 41). However, in addition to the well-structured materials such as fullerene, carbon nanotubes, graphene and graphite there are many other forms of carbon material such as carbon black, graphite oxide, activated carbon and carbon fibres (CFs) in which graphene is the building block. These carbon materials have wide range of applications, for example carbon blacks find their use as catalyst supports in fuel cells, in car tyres and as fillers in polymers used in conductive packaging for electronic components. Similarly, graphene oxide (GO), activated carbon and CF also find their application in catalyst support (42–51). However, theoretical studies on structural and chemical properties of these amorphous carbons have not yet been done in greater detail. In the following sections a brief discussion on the structural and chemical properties of these amorphous graphene-related carbon materials is presented.
1.2 Carbon Black
Since the early 1900s, several models of the internal structure of carbon blacks have been proposed and in this regard, the review paper by Donnet provides an excellent overview of some of these models (52). One of the first models of carbon blacks was proposed by Riley, which was based on the X-ray studies that basically corresponded to the average structure of particles reflecting the random distribution of crystallites and it represented the organised part of the carbon black particles (Figure 1(a)). The improved models were proposed by Bouland (Figure 1(b)) and Shultz (Figure 1(c)) in which the skin of the particle was believed to be more organised than its core. A more systematic model was proposed by Hekman (Figure 1(d)), which was based on the X-ray and electron microscopy. In this model the skin of the particle was also proposed to be better organised than its randomly arranged crystallites in the core. Later, on the basis of results obtained from lattice fringe imaging technique, a more detailed structure of carbon black was proposed by Heidenreich, Hess and Ban (53). In this model, (Figure 2) they also proposed that the ordering of the crystallite size and their ordering towards the centre of the particle are less than on the surface. According to all these models the commercial carbon blacks are made up of crystallites which are on average four graphite layer planes. The layers are roughly parallel and equidistant but do not exhibit the ABA stacking of hexagonal graphite. These graphite layer planes are referred to as BSUs. The distance between the layers in the BSUs is larger than 3.35 Å. The size of these BSUs in the plane can differ between a few Ångströms and 20–30 Å. If the samples become more amorphous, the number of stacked layers decreases until only one layer is left and simultaneously, the number of condensed rings in the plane also decreases (54). Recently Jäger et al. reported that the carbon black samples condensed in 10 mbar and 100 mbar argon gas atmosphere were similar to the Riley model as shown in Figure 1(a) (54). They further reported that the samples produced by them at 100 mbar argon were close to the paracrystalline model (see Figure 3) where the curved and strongly distributed graphene layers are stacked together to form a particle.
In addition to the studies on the structural features of carbon black since the 1950s it is known that the surface of these particles have functional groups, which include hydroxyl, carboxyl, aldehydes, ketones and quinones (52, 55, 56). Several researchers including Donnet et al., Garten et al., Studebaker et al., Puri et al. and Boehm et al. in the 1950s and 1960s independently arrived at this conclusion (52, 57–61). Since then various spectroscopic studies have also led to similar conclusions. For example, the recent Fourier transform infrared (FTIR) spectral studies by Juan et al. have shown that in the carbon black structures there are lots of aromatic C–C bonds and a large amount of oxygen containing organic functional groups (OFGs) (62). Other recent studies by Pantea et al. have shown that in addition to oxygen containing groups, sulfur containing groups may be also present (42, 63).
1.3 Graphite Oxide
Preparation of graphite oxide was first reported in 1850, which was obtained by oxidising graphite with KClO3/HNO3 (64, 65). Since then several other preparation methods of graphite oxides have been established and some of the most widely known methods are by Brodie, Staudenmaier, Hummers and Offeman. All these methods use strong oxidising agents such as KClO3, KMnO4 with HNO3 (66–68). While the preparation methods for graphite oxide are well established, its structural features are still not fully understood. In this regard, several models have been proposed, which include:
In another study, Lerf et al. employed 13C and 1H nuclear magnetic resonance (NMR) techniques to propose a newer structural model of graphite oxide, which consists of two kinds of regions i.e., (a) aromatic regions with unoxidised benzene rings and (b) regions containing aliphatic six-membered rings (70). In this model, the aromatic entities, double bonds and epoxide groups give rise to a nearly flat carbon grid and only the carbons attached to –OH groups are in a slightly distorted tetrahedral configuration, resulting in some wrinkling of the layers. It was further reported that the functional groups lie above and below the carbon grid forming a layer of oxygen atoms of variable concentration. Like graphite itself, graphite oxide has a layered structure, but its carbon layers contain large quantities of functional groups, mainly –OH and –CHO (71). We note that modifications of graphite oxide with soft donor atoms of sulfur have been also reported by Talanov et al. (72). Further to this, recent studies on the graphite oxide structure, using various microscopy and spectroscopic techniques, proposed a new model of GO, which indicates the presence of –O–, –OH and –C=O groups on the surface. In these models, the two neighbouring –OH groups are located on opposite sites in order to reduce the electrostatic repulsions in between them and for the same reason the –OH groups are located away from each other. The epoxy and ketone groups are also away from the –OH groups (see Figure 4) (73). Additionally, the presence of five- and six-membered-ring lactols has also been proposed (74).
1.4 Activated Carbon
The surface structure of activated carbon is highly complex and depends on the raw material used and method adopted for their production. It is believed that activated carbon generally consists of graphite crystallites with highly disordered, irregular, rough and heterogeneous surfaces (75). There are many experimental studies which have reported the surface chemistry of activated carbon and methods to evaluate it (76, 77). The infrared study on the surface structure of activated carbons has indicated the presence of oxygen-containing OFGs such as C=O in lactones and carboxylic anhydrides, quinine and aceto-enol groups and C–O group in ethers, lactones, phenols and carboxylic anhydrides (78). The structure of activated carbons is still not fully understood.
1.5 Carbon Fibres
CFs are generally in the range of 5–50 μm, which are commercially produced by converting a carbonaceous precursor into fibre. The polyacrylonitrile-based (PAN-based) and mesophase pitch-based CFs are two of the dominant CFs (79). Several studies have shown that the PAN-based CFs have extensively folded and interlinked turbostratic layers (i.e., the sheets of carbon are haphazardly folded and crumbled together) with interlayer spacing larger than those of graphite (79, 80). Mesophase pitch-based CFs on the other hand may have radial (graphene planes radiating out from the centre of the fibre), random (with random orientation of graphitic planes) or onion or quasi-onion like transverse microstructures (79, 81). The mesophase-pitch based CFs usually have a larger diameter (10–15 μm) as compared to the PAN-based CFs (5–7 μm) (80). In addition to understanding the detailed structural features of CF there have also been numerous experimental studies on the characterisation of the surficial reactive functional groups in CFs (82–84). In this regard, the studies by Szabó et al. showed that CFs consist of mainly –OH groups (~97%) and small amounts of –NH2 (~2.7%) and –COOH (~0.3%) groups (84).
Finally, we briefly comment on the structural and chemical features of soot, as carbon black is sometimes confused with soot even though they are very different materials (85). While carbon blacks are made under closely controlled conditions, soot is produced as a byproduct of incomplete combustion of a hydrocarbon or pyrolysis (86, 87). The gas-phase soot contains polycyclic aromatic hydrocarbons (PAHs). It has been found that soot is made of spherical-like particles with diameters in the 20–50 nm range. These particles are composed of graphite-type microcrystallite layers which are concentrically arranged in an onion-like structure. The graphitic layers are found to contain surface OH and C=O groups (88).
2. A Brief Discussion on Density Functional Theory
After having briefly reviewed the structural and chemical features of some interesting ordered and amorphous carbon materials, in the next section we present an outline of the DFT.
2.1 The Hohenberg-Kohn Theorem
In 1964, Pierre Hohenberg and Walter Kohn proposed and proved that for a nondegenerate ground state all the ground state electronic properties are determined by the ground state electron density (ρ0), which could be mathematically represented by Equation (i) (89, 90):
Here, Vext is the Coulomb potential energy between electrons and the nuclei, and Ψ0 is a unique ground state wave function (91). The electron density is dependent only on three spatial variables (r) whereas the wave function depends on 3N variables for a system of N electrons.
Thus, the ground state energy can be written as a functional of the ground state electron density (Equation (ii)),
Here, , and are the average kinetic energy, potential energy due to nucleus-electron and electron-electron interactions, respectively. The term in Equation (ii) can be further written as Equation (iii):
where v (ri ) is the nuclear-electron potential energy for the ith electron.
The term in Equation (iv) is unknown.
In the Hohenberg-Kohn variational theorem, Hohenberg and Kohn further proved that the true ground state electron density minimises the energy functional Ev [ρtr], where ρtr is the trial density function that satisfies ∫ρtr (r) dr = N and ρtr ≥ 0 for all r and the inequality E0 ≤ Ev [ρtr] holds where Ev is the energy functional in the Equation (iv). Hohenberg and Kohn proved their theorems only for nondegenerate ground states and later Levy proved the theorems for degenerate ground states (92).
While the Hohenberg-Kohn theorem tells us that if we know the ground-state electron density ρ(r), we can determine the ground state wave function, it however does not tell how to calculate E0 from ρ0. It also does not tell how to obtain ρ0 without finding the wavefunction (89, 93). In 1965 Kohn and Sham revisited this problem, which is briefly outlined below.
2.2 The Kohn-Sham Method
Kohn and Sham considered a fictitious system of N non-interacting electrons, which experiences an external potential energy vs(ri) (index s is reserved here for the fictitious systems) (93). It was further assumed that the electron density of this reference system ρs(r) is equal to the exact ground state electron density ρ0(r) of the system of N interacting electron system under consideration (molecule). The Hamiltonian of this system is given by Equation (v):
where, is the one-electron Kohn-Sham Hamiltonian for the fictitious system (Kohn-Sham Hamiltonian). Additionally, since this system consists of noninteracting electrons, the ground state wave function Ψs,0 is the anti-symmetrised product (using the Slater determinant) of all the lowest energy one-electron spin-orbitals, which are the product of one-electron spatial orbital and a one-electron spin function. Further to this, they defined the difference between the kinetic energy of the interacting N-electron system and that of the fictitious system as Equation (vi):
where and are the average electronic kinetic energies of the interacting N electron system and the fictitious noninteracting N electron system respectively. Similarly, the quantum mechanical many-body electron-electron interaction energy minus the classical one can be represented as Equation (vii):
Here the term ½ is added so that the repulsion energy between the charges is not counted twice.
which can be also rewritten as Equation (ix):
Where Equation (x),
which is referred to as the exchange-correlation (xc) energy functional (93) and it is responsible for all the many-body electron-electron interaction. The term Exc can be also written as Equation (xi):
where Ex and Ec are exchange and correlation energy respectively.
We note that even though the xc energy is a small part of the total energy of a typical system it plays the vital role in binding atoms together and therefore, Perdew coined it as “nature’s glue” (94). This quantum mechanical phenomenon arises as electrons move in such a way to avoid one another, which in turn, lowers the expectation value of the electron-electron Coulomb interaction. The exchange energy in the xc functional (Equation (xi)) is a consequence of the system obeying the Pauli principle and is free from the spurious self-interaction of an electron, when the exact Fock exchange is used. The correlation energy (Ec = Exc − Ex) should account for the remaining effects of spatial and spin correlation in the many electron system.
Some of the popular exchange and correlation functionals are summarised in Table I, which is not a complete list of all the exchange and correlation functionals currently available.
3. Adsorption of Transition Metal Atoms
3.1 Adsorption of Transition Metal Catalyst on Single and Multi-Layered Graphene
The earlier theoretical studies on the adsorption of TM adatoms on carbon materials were mainly reported on a single layer of sp2 hybridised carbon support with some reports on a few layers of graphene and these studies we‐re mainly focused on structure, bonding, magnetic properties of the adsorbed TMs on graphene and their migration to the high symmetry sites (117). Duffy et al. for example, used the linear combination of atomic orbitals approach as implemented in DMol package to study the effect of the surface on the 3d TM adatoms and dimers on a cluster model of graphite (118–121). In their calculation they used the local spin density approximation using the Vosko-Wilk-Nusair (VWN) exchange and correlation functional (95). They reported that the preferable adsorption sites of scandium, titanium, chromium and manganese are above the carbon atoms while iron, cobalt and nickel prefer over the rings. Due to the hybridisation between these metals with the π-orbitals of graphite there is a small electron transfer. It was also seen that the total magnetic moments are higher for scandium, titanium and vanadium by 1μB than the free atom and lower by 2μB for iron, cobalt and nickel, while chromium and manganese have reversed their free atom values. To understand this behaviour, they considered the local density of states of the iron/graphite (with C6v symmetry: iron stable over the hole-position) and vanadium/graphite (with C3v symmetry: vanadium stable over-atom position) systems. In the iron/graphite system, the molecular orbitals (MOs) mainly comprise 3d and 4s orbitals and a small admixture of 4p orbitals. These MOs are labelled by the C6v group i.e., a1, a2, b1, b2, e1 and e2. However, only the a1, e1 and e2 orbitals have metallic as well as carbon π-orbital components. A careful analysis of these orbitals showed that the adsorbed atom has the tendency to shift the 4s orbital energy up with respect to the 3d orbitals meaning an electron configuration of 3d n+24s0 is preferred over 3d n4s2 where 1 ≤ n ≤ 8 of the elements under consideration (note that chromium has an electron configuration of 3d 54s1 due to half-filled electron configuration being more stable). From such analysis they were able to explain the reduction of the atomic spin moment by 2μB in iron. Similar analyses were done on vanadium/graphite system, which showed that there is also a tendency for increased population of 3d related MOs but due to its enhanced hybridisation with the graphite surface this effect is reduced, which leads to an increase in the spin moment by 1μB.
In another study, Valencia et al. performed periodic spin polarised DFT calculations using a projector-augmented wave (PAW) scheme, and the PW91 exchange and correlation functional as implemented in the VASP code to explore the trends in adsorption of the 3d TM atoms on graphene (122). They found that scandium, titanium, iron and cobalt could remain isolated on the graphene surface but other metals in this series may diffuse with a possibility to form aggregates. They employed density of states and Bader charge analysis to explain the observed trends. Another interesting aspect of this study was the use of the organometallic M(η6-C6H6) MOs to bridge the language used to describe molecular states and that used to describe solid states. Figure 5 shows the qualitative interaction orbital diagram for the M(η6–C6H6) C6v system using Hückel calculations by means of the Computer Aided Composition of Atomic Orbitals (CACAO) program (123–126). This simplified MO diagram shows six 2Π-orbitals of benzene and the valence orbitals of the 3d TM atom and as described above, these orbitals are labelled by the C6v group. Of the six 2Π-orbitals, three with a1 and e1 symmetries are occupied, which interacts with the 4s and 3d orbitals of similar symmetries giving three bonding orbitals mainly localised on the benzene participating in the M–C bonds directing downward. The three antibonding orbitals, which remain unoccupied are mainly localised on the metal 4s and 3d orbitals and directed upward. The metallic group orbitals are shown in the centre of the interaction diagram and they are formed of 1e2 (from mixing of 3d x2 − y2 and 3dxy orbitals, with antibonding π*CC orbitals of benzene participating in metal-carbon δ bonding), 2a1 (from mixing of 4s, 3dz2, 4pz orbitals and the in-phase πCC MO, which leads to a nonbonding σ MO), 2e1 (formed by mixing of 3dyz, 4px and 4py orbitals interacting with the bonding πCC ) and 3a1 (derived mainly from the metal 4s orbital). They employed this qualitative M(η6-C6H6) MO description along with the calculated partial density of states for the periodic M@graphene to show that the energy levels follow a global order i.e., 1e2 ~ 2a1 < 2e1 < 3a1 and they used this scheme to clarify the observed trends on chemisorption of 3d TM with an exception of half-filled chromium and manganese and fully filled copper 3d atoms, which physisorb on the graphitic surface. Further to this, they used qualitative MO diagrams and charge transfer considerations to explain the trends observed in computed magnetism. They also concluded that scandium, titanium, iron and cobalt are stable as isolated atoms on the graphitic surface and other metals diffuse to eventually form aggregates.
In another interesting study Nakada et al. reported the adsorption and migration energies of adatom with atomic numbers 1–83 on graphene at the local density approximation (LDA) level of theory using the PAW method as implemented in VASP (127). In this study they reported that the TM elements mainly adsorb on the hollow sites. They noted that the adsorption types can be of two types i.e., with fixed adsorption and those with no fixed sites. For the systems with no fixed adsorption sites they estimated the migration energy barrier as 0.5 eV where the minimum limit of the migration energy was obtained by taking the difference between the adsorption energies for each site. They concluded that for metal-graphene junctions in addition to the adsorption energies the migration energy plays an important role. Recent studies by Manadé et al. also reported a systematic study on the adsorption of 3d, 4d and 5d TM adsorbed on graphene. In this study they also concluded that TMs prefer the hollow sites when chemisorbed. However, if they are physisorbed, for example in the case of d5 and d10 configuration, then they prefer the bridge or the top sites. Their study showed that inclusion of dispersive forces simply increases the adsorption energies of the TM by ~0.35 eV. They also reported the electron transfer behaviour of the TMs and found that it decays along the series because of the increase in electronegativity (128).
In addition to the adsorption of TMs on a single layer graphene, studies on the adsorption of metal adatoms have been reported on bilayer and trilayer graphene (129–132). For example, Hardcastle et al. employed plane-wave DFT to investigate the adsorption properties of gold, chromium and aluminium atoms on the armchair and zigzag sites of monolayer graphene and on the adsorption sites of single, bi- and tri-layer graphene. They investigated the adatom mobility on the pristine substrates (132, 133). They used the CASTEP code with Perdew-Burke-Ernzerhoff (PBE) exchange and correlation functional in conjunction with Tkatchenko-Scheffler Van der Waals correction scheme (134). In this study they concluded that the graphene sublayers make a significant contribution to the total binding energy of the adatom and the adatoms are extremely mobile on graphene at room temperature. In recent years, the trends of theoretical studies on the interaction of TMs on graphene have moved towards exploring the interaction of more realistic metal nanoparticles on graphene (135, 136). For example, Engel et al. recently systematically studied the electronic properties of gold clusters on graphene using DFT-D+U (135). Further to this, the interactions of molecules for example, NO2, H2S and glucose on TM atoms and clusters supported on pristine graphene have been also reported (137–139).
3.2 Adsorption of Transition Metal Catalyst on Graphene with Vacancies
Graphene can display single or multiple vacancies but it is worth mentioning that the filling of such a vacancy may occur due to the reservoir of loose carbon atoms readily available near these vacancies (140, 141). The single vacancy (SV) refers to a missing carbon atom from the lattice and the double vacancy can be created by combining two SVs or by removing two neighbouring carbon atoms. Double vacancies may result in different types of configuration i.e., two pentagons and one octagon (5-8-5), three pentagons and three heptagons (555-777) and four pentagons, one hexagon and four heptagons (555-6-7777) (140). Therefore, in addition to the interaction of TMs on pristine graphene, studies on graphene with single and double vacancies have been also reported, which is not only interesting from the catalysis point of view but also from the context of spintronics and Kondo physics (142). Krasheninnikov et al. for example, presented a DFT study on the structure, bonding and magnetic properties of the first TM series, platinum and gold on the single and double vacancies of single graphene sheets (142). They found that on the SVs, all the metal atoms form covalent bonds with the under-coordinated carbon atoms and as the atomic radii of TMs are larger than carbon, the metal atom bulges out of the graphene plane. The TM-carbon bond lengths were seen to decrease from scandium to iron due to the decrease of the TM radii and then it increases as bonding becomes weaker. The calculated binding energies of the TMs on the SVs were typically about –7 eV with an exception of fully filled d-shell elements such as copper and zinc. They further noted that SV complexes are magnetic for vanadium, chromium, manganese, cobalt and copper and nonmagnetic for iron and nickel. On the other hand, for the TMs on double vacancies, all the TMs from vanadium to cobalt are magnetic, which is related to the fact that on double vacancies due to the larger ‘hole’ the interactions of the TMs are weak and therefore, the complexes are in higher spin states.
In another study Krasheninnikov et al., using strain fields, further demonstrated that metal atoms have high affinity to the non-perfect and strained regions of the graphene with defects (143). In this work they visualised the strain fields as the difference between the bond lengths between the graphene sheet with and without defects, which suggested that all vacancies result in strain fields in the vicinity of the defects in the range of 2–3 nm or even more. In 2013, Robertson et al. used aberration corrected transmission electron microscopy (ACTEM), high‐resolution transmission electron microscopy (HR‐TEM), electron microscopy and DFT to show that single iron atoms can be trapped on the single and double vacancies of graphene, forming covalent bonds and causing significant displacement, up to 0.5 Å, of the surrounding carbon atoms. They further reported that iron on such vacancies are more stable than when they are incorporated into the graphene edge.
In recent years, theoretical studies have moved towards understanding the interaction of small TM clusters on the single and double vacancies of graphene. In this regard, Sen et al. investigated the stability and electronic properties of Pdn (n = 1–5) clusters on different types of double vacancies mentioned above using DFT and molecular dynamic simulations (144). Their study revealed that the adsorption of Pd4 clusters on the defect bridge site of double vacancy of (555-777) is favourable, which they concluded is because of the hybridisation between carbon 2p and palladium 4d and 5s orbitals and higher charge transfer to the graphene sheet. Further to this, the latest studies in this area are focused on understanding the interaction of molecules on TM clusters adsorbed onto the graphene lattice vacancies. For example, Hamamoto et al. used DFT to demonstrate that the local electronic properties of CO adsorbed on Pt4 clusters supported on graphene with one to four vacancies are remarkably altered due to its interaction with the dangling bonds of carbon atoms in the defect sites (145).
3.3 Adsorption of Transition Metal Catalysts on Doped Graphene
The introduction of heteroatoms such as nitrogen and boron into the carbon framework of graphene can be used to tailor the electronic and local geometrical properties of graphene for applications in catalysis (146–152). The substitution of nitrogen (n-type doping) in the graphene lattice usually leads to four common bonding configurations, which include: (a) quaternary nitrogen (obtained by substituting the carbon atoms of the hexagonal ring by the nitrogen atoms); (b) pyrrolic nitrogen (obtained by nitrogen atoms due to the bonding on the five-membered rings as in pyrrole); (c) pyrazolic nitrogen (obtained by the insertion of an aromatic N2 moiety in a five-membered ring as in pyrazole); and (d) pyridinic nitrogen (obtained by substitution of the nitrogen atoms at the defect sites or on the edge of graphene as in pyridine) (146, 153, 154). Similarly, it has been shown that doping with boron (p-type doping) or more complex functional groups such as boronic esters and boronic acids can be also done on graphene (155). Such incorporation of dopant atoms leads to significant alteration of the electronic properties of graphene, for example Zhang et al. theoretically showed that substitution of nitrogen on the graphene framework introduces asymmetric spin density and atomic charge density making it favourable to oxygen reduction reactions (156). Recently, reports on doping of graphene with beryllium, sulfur, silicon and phosphorus have been also made (157, 158). It is worth mentioning that doping on graphene can be also achieved by electron exchange between adsorbed species on graphene surfaces (148).
It has been now shown that the durability and activity of TM catalysts such as platinum increases significantly on nitrogen-doped carbon supports (159). In this regard, numerous theoretical studies have also been performed. For example, Groves et al. used DFT as implemented in the Gaussian 03 package on 42 carbon atom cluster models to calculate the binding energy between nitrogen-doped graphene structure and platinum adatoms in order to explore the MO and natural bonding orbital data (160). They showed that nitrogen-doping increases the binding energy of platinum to the substrate and the binding energy increases if the nitrogen-atoms are closer to the carbon atom bonded directly to the platinum adatom. In another study Zhou et al. employed the DMol3 code to study the effect of nitrogen and boron doping on the stability of defective graphene supported Nin (n = 1–6) clusters (2). They found that the binding energies of these clusters on doped defective graphene were higher than on pristine graphene meaning the stability of nickel clusters was further improved. It was also seen that Nin clusters were more stable on the boron-doped defective graphene than the nitrogen-doped ones. The probable reason for this was attributed to the fact that boron heteroatoms promote stronger hybridisation between carbon atoms and nickel atoms. Another interesting study by Kropp et al. also found that nitrogen doping resulted in increased binding energies of TMs (161). These studies clearly suggested that nitrogen and boron-doped graphene can be good catalyst support. We note that in recent years there have also been some studies in which the adsorption of small gas molecules on TM doped graphene have been reported (162).
3.4 Adsorption of Transition Metal Catalysts on Amorphous Carbon
Theoretical studies on the geometrical and electronic properties of graphene (mainly graphene nanoflake (GNF) cluster models) with OFGs to model the BSUs of carbon black, GO, activated carbon and soot have been reported (7, 163–167). For example, Efremenko et al. reported the adsorption properties of phenol on activated carbon, Hamad et al. reported the adsorption of water on soot particles and we have also investigated the detailed electronic properties of GNFs with OFGs to mimic the BSUs of amorphous carbon materials discussed earlier in Section 1 with various shapes and sizes (75, 88, 163–165). In this regard, we investigated the influence of orientation and stacking patterns of the model BSUs on the electronic properties. In these models, the oxygen containing OFGs such as –OH, –CHO and –COOH were decorated on the periphery of the GNF models. In one of these studies we used –OH and –SH groups to understand the effect of chemical constitution of these materials (Figure 6). The models constituted of pristine GNF models (referred to as G in Figure 6(a)), with only –OH groups (i.e., G(OH)), with only –SH groups (i.e., G(SH)) and models with mixed –OH and –SH groups (i.e., G(OS-C3h) and G(OS-Cs); C3h and Cs refers to the symmetry of the models). Additional studies on the effect of different stacking conformations with two, three and four layers of G(OH) and G(SH) models on the electronic properties were undertaken (see Figure 7). From these studies it was seen that in the fully relaxed geometries, the CSH angles (due to the –S…HS– bonds) were closer to 90° when compared with the COH angles (due to the –O…HO–bonds), which was related to different hybridisation abilities of the oxygen and sulfur atoms and due to the charge-charge interaction in O…H and charge-quadrupole interaction in S…H. Further to this, it was concluded that by substitution of different OFGs and by using different stacking patterns (such as AAA or ABA) the electronic properties of these systems could be significantly altered. Based on these studies, we also investigated the adsorption properties of platinum-adatom and Pt4 cluster on GNFs with varying concentration of hydroxyl groups (f-GNFs) (7). In this study, based on the DFT results it was proposed that the loading of platinum catalyst on carbon supports could be engineered by controlling the concentration of oxygen containing OFGs.
In addition to the studies on decorating graphene with oxygen containing OFGs there are several studies in which the oxygen-doped graphene have been used to investigate the adsorption properties of TM clusters, which include Pt4, and Pt3M (M = scandium, titanium, vanadium, chromium, manganese, iron, cobalt and nickel) (168, 169). In one of these studies, Cui et al. reported that Pt3Ni supported on the oxygen-doped graphene exhibit enhanced catalytic performance (169). In some recent work, more complex studies of TM adsorption on the OH decorated nitrogen-doped graphene have been reported. For example, Sahoo et al. used transmission electron microscope (TEM), HR-TEM, hard X-ray photoelectron spectroscopy (HAXPES), combined with DFT calculations to report the charge transfer of palladium catalyst to pyrrolic nitrogen-doped reduced GO (NRGO) (170). In this study, they showed that strong bond between palladium and pyrrolic-fraction of the nitrogen-moieties can be formed, which can be further strengthened by the presence of oxygen containing functional groups. This study further reported that the charge-transfer from the palladium 3d-orbitals to the 2p-orbitals of carbon, nitrogen and oxygen of NRGO results in high oxygen reduction reaction. In another recent study Rout et al. synthesised highly active and well-defined gold-copper nanoparticles supported on reduced GO (171). These nanoparticles were then characterised using various techniques, which include ultraviolet-visible (UV-vis), XRD, HR-TEM, scanning TEM (STEM) Raman and X-ray photoelectron spectroscopy (XPS). They found that Au3Cu1 bimetallic clusters exhibited superior catalytic activity. In this study, they used DFT on cluster models of reduced GO to clarify the reasons for the high catalytic activity of these nanoparticles. They concluded that efficient adsorption of the highly dispersed Au3Cu1 on reduced GO led to the enhanced catalytic activity.
As we can see from the above studies, the models on the amorphous carbons such as carbon black, GO, activated carbon and soot are based on a single layer of graphene. This is especially because of the complexity in modelling these systems. However, there are some interesting studies made towards modelling of complex amorphous carbons. For example, recently Caro et al. reported a systematic study in which they combined machine learning and DFT to theoretically understand the surface chemistry of amorphous carbons (172). In another study Ranganathan et al. employed molecular dynamics simulation to produce models of amorphous carbons (173). Clearly, more research in this area is required to reproduce as realistic models of amorphous carbons as those of the experiments and to understand the interaction between these catalyst supports and catalysts to design novel and efficient catalysts.
4. Concluding Remarks and Future Direction
In this short review, we presented a discussion on the structural and chemical features of graphene related carbon materials, which are widely used as catalyst supports. We summarised that so far numerous theoretical studies have been done on the adsorption properties of TM adatoms and clusters on graphene and related systems. The current trend of these studies is moving towards modelling more and more realistic TM nanoparticles interacting with graphene. Most of these theoretical studies are however done on pristine or modified graphene models. Clearly, more theoretical research is required in this area to reproduce realistic models of amorphous carbon supports to complement the models reported in the experimental studies to further our understanding on the interaction between catalyst and catalyst supports to design novel and efficient catalysts. Here, we reviewed some of the important studies which employed DFT, however, to gain deeper understanding of the structure property relationship of these industrially important carbon materials and their interaction with catalysts, multiscale modelling will be required. Artificial intelligence and machine learning tools will be particularly beneficial in this pursuit.
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We thank Ikutaro Hamada for his helpful discussion during the preparation of the manuscript. We also thank Cirrus UK national Tier-2 Service at Edinburgh Parallel Computing Centre (EPCC) (funded by University of Edinburgh, UK, and Engineering and Physical Sciences Research Council (EPSRC) (EP/P020267/1)), Athena at HPC Midlands+ (Funded by EPSRC, EP/P020232/1) and ARCHER2 (via Materials Chemistry Consortium and EPSRC (EP/L000202)) and Supercomputer Wales.
Molecular Surface Chemistry Group at the University of Lincoln, UK LINK https://www.molsurfchem.org/
Arunabhiram Chutia received his PhD in theoretical and computational chemistry from the Graduate School of Engineering, Tohoku University, Japan. Thereafter, he undertook his postdoctoral research in materials science and heterogenous/homogenous catalysis at WPI-Advanced Institute of Materials Research, Institute of Fluid Science (Tohoku University), at the Fuel Cell Nanomaterials Centre (Yamanashi University, Japan) and jointly at University College London and the UK Catalysis Hub in the UK. Currently, he is a senior lecturer in chemistry and leads the Molecular Surface Chemistry Group at the University of Lincoln, UK.