Over the past few years, the number of publications devoted to Ni SACs has increased. More than half of them are related to the study of electrocatalytic reactions and corresponding density functional theory (DFT) calculations. This tendency is expected due to humanity’s attempts to avoid the production of energy partially or completely by releasing it from non-renewable, non-ecofriendly oil and gas, while many advanced clean energy technologies require highly active catalysts to lower the energy barrier and increase the reaction rate with an efficient and stable route.
2. Application of Supported Ni Single-Atom Catalysts
In the Introduction section,
wresearche
rs briefly discussed the historical and modern applications of Ni-based catalysts, which are mainly related to the catalysis of hydrogenation and dehydrogenation processes. When discussing catalysts for such reactions, one cannot fail to mention the competition between palladium and nickel, which was well covered in a recent review by Chernyshov and Ananikov
[106][15]. Indeed, being one under the other in the same group of the periodic table, these metals are analogues; however, the difference in the periods causes differences in redox potentials and the availability of different oxidation states, which provides outstanding opportunities for Ni catalysis for complex multistep transformations and the discovery of new catalytic reactions. Thus,
wresearche
rs will consider both the theoretical and practical results of using Ni SACs in various hydrogenation and dehydrogenation reactions. It should be noted that the development of Ni SACs’ application field is not much different from the historical ones for Ni bulk and Ni nanoparticles: this is the breaking of X–H bonds, where X is various atoms, for example, C or O, and H is a hydrogen atom. In this regard, for convenience,
weresearchers will further consider separately the activity of heterogeneous catalysts based on Ni single atoms in breaking various X–H bonds, and
we researchers will start with the C–H bond.
2.1. C–H Activation
Currently, the most common catalysts for the activation of C–H bonds are Ni-based catalysts. The activity of nickel is associated with the presence of an unfilled
d orbital, which can accept the σ electron of the C–H bond, thus weakening or breaking it
[107][16]. Such an approach to the activation of hydrocarbons can facilitate cross-coupling reactions, leveling the disadvantages known for this reaction: the use of an expensive catalyst (Pd-based), the presence of several stages of pre-functionalization, which entails a multi-stage reaction, and the presence of by-products. However, the activation of the C–H bond also has its drawbacks, in particular, high reaction temperature and high metal loading are needed. The best-known nickel-catalyzed reaction involving C–H activation is the dry reforming of methane (DRM).
Dry Reforming of Methane
DRM is a process for the simultaneous synthesis of H
2 and CO (syngas) via the conversion of carbon dioxide and methane. Even though this reaction may not actually involve hydrogenation and dehydrogenation, its mechanism includes the C–H bond cleavage as an important step of the reaction. The general reaction (1) describing this process is endothermic with a standard enthalpy of 247 kJ mol
–1 and, therefore, requires high temperatures (>500 °C)
[59,60,108][17][18][19].
The mechanism of this reaction has been thoroughly studied
[109,110][20][21]. The reaction begins with the adsorption of a methane molecule on the catalytically active center (2), followed by its activation (3), which is the rate-determining stage of the reaction
[111,112][22][23]:
where ‘(g)’ stands for the gas phase and ‘*’ here and below in the text means the adsorbed species. The methane molecule is a highly symmetrical nonpolar molecule, which should be considered when choosing a catalyst. As shown in the literature
[113][24], the adsorption of methane on a metal single-atom site occurs due to the hybridization of bonding orbitals of the methane with antibonding orbitals of the metal and vice versa. It should be noted that a support on which metal single atoms are located may also play a role in methane adsorption. The stability of a metal atom is ensured by the strength of its bonding with a support, which is related to overlapping their orbitals. The lengthening of the C–H bond, leading to its further cleavage, depends on the adsorption energy of methane and on the charge given back from metal. This bond elongation is most noticeable for the Ni atom, which is the rationale for the effectiveness of Ni SACs in the DRM reaction. Indeed, according to the reports of the past few years
[114[25][26],
115], Ni single-atom sites exhibit superior activity, in particular, as compared to Ni nanoparticles. However, their use often leads to catalyst deactivation, which is associated with the formation of carbon as a result of the Boudouard reaction (4) and the complete cracking of methane (5), catalyzed by nickel
[110][21].
It is interesting that the equilibrium in the Boudouard reaction can be shifted towards the formation of CO if the chosen support can donate electrons to the nickel atom or Lewis basic sites, which are attractive for CO
2 adsorption, are introduced into the catalyst. The former factor was widely studied in
[107][16], where Ni nanoparticles supported by CeO
2 were used as a catalyst after a reduction in hydrogen. As a result of the reduction, oxygen vacancies were formed, which easily released free electrons. These free electrons increased the
d-electron density on the nickel atoms. Such electron-saturated atoms prevented the migration of σ–electrons of the CH
4 molecule to the
d orbital of the Ni atom, thereby reducing the adsorption of CH
4 and, thus, the degree of carbon deposition, and also increasing the adsorption of CO
2 due to including its unoccupied π–orbital.
Quite recently, these assumptions were partly confirmed for a similar Ni/M–CeO
2 system, where M = Mg, Co, or Zn
[114][25]. In this case, CO
2 molecules were adsorbed on oxygen vacancies generated due to the presence of metal cations, whose ionic radius is smaller than that of Ce
4+. The carbon dioxide molecule dissociated into carbon monoxide and surface active oxygen, which easily removed the deposited carbon, thereby maintaining high catalyst activity for more than 150 h at 800 °C. A similar effect was achieved by Ni and Ce co-doping of a hydroxyapatite support
[60][18] and co-anchoring of Rh and Ni atoms on a CeO
2 support
[59][17]. In addition, ZrO
2 possesses the same properties of producing oxygen vacancies
[116][27]. Thus, the first way to modify Ni SACs is based on the selection of supports containing a large number of oxygen vacancies, which are considered to be closely linked to the content of surface adsorbed oxygen species.
The second way to modify the catalyst by introducing additional basic centers into it is to add a metal and/or its oxide to the support. The creation of such centers will enhance the chemisorption of CO
2 on them
[109][20]. As for the Zr and Ce atoms mentioned above, their addition to oxygen-containing supports also increased the mobility of oxygen ions on the surface, making Ni catalysts more stable
[117][28]. As for other metals, there is a report of better catalytic activity and stability of Ni catalysts for 40 h at 700 °C, in which the support was partially coated with MgO
[118][29]. Interestingly, such doping initiated better Ni dispersion on the support surface. However, this method did not work when using MgO as a support for Ni single-atoms
[119][30].
The use of SACs, including Ni catalysts, is not sufficiently developed since this is a rather young area of catalysis. In this regard, there is no confirmation/denial in the literature of the promotion of the DRM reaction previously obtained for Ni nanoparticles in the case of single atoms (adding K
[120][31], Ca
[121][32], Fe
[122][33], Sr
[123][34] or using CaO, BaO
[124][35], La
2O
3 [125][36], etc. as supports
[126,127][37][38]). However, significant progress is expected in this field.
2.2. H–H Activation
Hydrogen is the simplest molecule, and its properties are the most studied. Since this pure resource is available in abundance at a very low cost, catalytic hydrogenation is a mainstream technology in both research and industry. Molecular hydrogen is not very active under ambient conditions, but many positively charged transition metal atoms are capable of bonding and activating H
2. As is well known, bulk Ni and Pd have a very strong hydrogen adsorption affinity, can easily form metal hydrides, and are widely considered good catalysts due to their excellent hydrogen solubility, corrosion resistance, and diffusivity. As indicated in the literature, the adsorption energy of the hydrogen molecule on the Ni single atom
[128,129][39][40] is higher than that on bulk Ni
[130][41]. The initiation of the hydrogenation reaction most often occurs with the dissociation of the hydrogen molecule. Homolytic decomposition yields adsorbed H* atoms, while heterolytic decomposition produces partially charged H
δ+ and H
δ– species. The type of decomposition, the energy barrier, and the overall energy profile of the reaction strongly depend on the structure of the catalytically active Ni site. For example, according to DFT calculations of a catalytically active Ni atom coordinated to three and four carbon atoms in single and double graphene vacancies, H
2 dissociation is an endothermic process with energy barriers of 0.69 and 0.33 eV, respectively
[131][42]. As can be seen, the dissociation of the adsorbed H
2 is an energy-consuming process
[128,131,132][39][42][43]. However, with an increase in the number of Ni atoms to three in a catalytically active site, the dissociation is almost barrier-free
[130][41]. Interestingly, a similar effect was observed when considering the reverse reaction, where the recombination of hydrogen atoms into a molecule took place
[133][44]. After decomposition, hydrogen atoms can spontaneously migrate to support atoms. This effect is called the hydrogen ‘spillover’ process, and it may take place in the metal-carbon support system
[134][45]. Such behavior, in particular, was observed for some nitrogen-containing systems
[128][39] and also for MoS
2 [132][43]. For instance, the addition of Ni single-atoms on MoS
2 support causes an excess of electron density on the nearest sulfur atoms, thus enhancing their activity towards hydrogen adsorption
[32,49][46][47]. Although the activation of a hydrogen molecule can also occur due to an increase in the H–H bond length after adsorption onto the Ni atom. Furthermore, during the co-adsorption of the reagents on the metal atom, the breaking of this bond for the further course of the reaction will be easier.
2.3. O–H Activation
2.3. O–H Activation
The use of hydrogen as a hydrogenating agent has some disadvantages associated with its gaseous state under ambient conditions. In the reactions discussed above, the typical hydrogen pressure was in the range of 1–3 MPa, which introduces difficulties in carrying out reactions both on laboratory and industrial scales. To overcome this, other molecules are used from which the hydrogen atom can be obtained, such as alcohols. Wang et al. studied the effect of the composition and structure of the Ni–N
xC
y site in graphene on the adsorption of isopropyl alcohol and the subsequent detachment of hydrogen from it
[149][48]. The Ni–N
4 and Ni–N
3 sites turned out to be the most energetically favorable. It was shown that, despite the energetic preference for its formation, the flat structure of the Ni–N
4 site sterically prevents the dissociation of the O–H bond and makes the process highly endothermic. On the contrary, the Ni–N
3 site, in which the Ni atom protrudes above the graphene surface, not only easily adsorbs the isopropyl alcohol molecule due to the overlapping of the
p-orbitals of the O atom and the
d-orbitals of the Ni atom but also easily breaks the O–H bond.
2.4. N–H Activation
In addition to the reactions of the formation of hydrogen atoms or molecules by activation of C–H, H–H, and O–H bonds, which are widely covered in the literature, some authors considered the possibility of using single-atom Ni sites for activation of other bonds, for example, N–H. Feng et al. studied the mechanism of reduction of Br–C
6H
5–NO
2 to Br–C
6H
5–NH
2 at Ni–N
4 and Ni–N
3 sites using N
2H
4·H
2O as a hydrogen donor
[155][49]. Since the former catalytically active site exhibited significantly higher activity than the second one, the energy values given below correspond to the Ni–N
3 site. Hydrazine was adsorbed with the formation of the Ni–N bond, and the N–H bond breaking energy was equal to 0.93 eV, with both N
2H
3* and H* species remaining coordinated to the nickel atom. The subsequent adsorption of the R–NO
2 molecule was exothermic. The limiting stage of the reaction was the chemo-selective reduction process of the −NOH group through the recombination of the adsorbed hydrogen atom and the hydrogen atom of the −OH group—with an activation energy of 0.96 eV. The presence of the Br substituent did not adversely affect the reduction of the nitro group; moreover, the authors calculated that 1.18 eV would be required to break the C–Br bond, which makes this process kinetically unfavorable. In practice, the catalyst achieved 100% conversion and selectivity at 60 °C within 3 h of operation, and the calculated TOF was higher than that for the catalyst containing Ni nanoparticles.
It is interesting that theoretical calculations of the decomposition of N
2H
4 at the Ni–C
3 site did not show such good results as in the work
[156][50]. The adsorption of the molecule on the catalytically active Ni atom still occurred due to the overlapping of the 3
d and 2
p orbitals of Ni and N, respectively. However, the authors calculated not only the dissociation of the N–H bond but also the dissociation of the N–N bond and showed that the second way is energetically preferable (0.86 and 0.51 eV, respectively). Moreover, the final state for the system after the N–H bond cleavage was endothermic, while after the N–N bond cleavage, it was exothermic, which indicates the potential unsuitability of the Ni–C
3 site as catalytically active in the production of H* from hydrazine. Although this site still showed potential activity and high selectivity in the formation of ammonia.