Understanding metal-graphene interactions

Figure 1. Fundamental model.

September 04, 2018 – A new study examining graphene-metal interactions and was carried out in the Group of Electron Microscopy for Materials Science at Ulm University, Germany and the Nanoscience & Nanotechnology Centre at Nottingham University, UK. The study uses carbon nanotubes as miniature test tubes to systematically examine the interaction of transition metals with the side walls at atomic resolution.

Metal-graphene interactions studied at atomic resolution is a current aim in high resolution TEM [1], and combined with confinement of metal atoms inside carbon nanotubes provides a powerful strategy for studying structural and chemical properties in detail.[2] The ever-changing nature of the metal nanoclusters during the chemical reactions creates a significant obstacle to the rational design of new nanocatalysts. The only way to assess the relationship between the atomic structure of the nanocluster and its catalytic activity is imaging the dynamic transformations promoted by the metal nanocluster, ideally at the atomic scale. Transmission electron microscopy (TEM) nowadays offers atomic spatial and high temporal resolution, inaccessible in other microscopy methods. With the help of the in-situ TEM techniques, a new study now considers the influence of the e-beam, which has been largely neglected in previous studies.[3-5] It has a dramatic effect on the dynamics and performance of nanocatalysts, as the rates of the reactions are directly proportional to the dose rate of the e-beam.

In the new study, TEM images have been combined to form a stop-frame movie bearing information on reaction dynamics and mechanisms at the atomic level.[6] The reactions were driven by isolated events in which direct momentum transfer from a fast incident electron to an atom results in a shift of the atom from its equilibrium position. The exact amount of kinetic energy transferred from the e-beam to the atom has been exactly calculated. The catalytic activity has been studied by observing the restructuring of a surrounding nanotube. For a defect to emerge in the nanotube sidewall, a C–C bond must first be broken, followed by the formation of bond between the metal nanocluster and the dangling bonds of the nanotube (Fig. 1 left). This is depending on the metal nanocluster because: (i) it reduces the energy threshold for carbon atom displacement (Fig. 1 (right) and (ii) it stabilizes the vacancy defect due to the formation of the metal carbon bond (Fig. 1 right). While the latter appears to be more significant, both roles are linked by the Bell-Evans-Polanyi principle—relating the activation energy E_d' to the enthalpy for reactions ∆H' of similar type — which, if applied to the ChemTEM study, (e.g. the lower ∆H' — the lower E_d'; Fig. 1 right), is ultimately defined by the bonding of the metal nanocluster with carbon.

It has been theoretically and experimentally demonstrated previously that the presence of metals can facilitate formation of defects under e-beam irradiation such as mono-vacancy [7, 8] and di-vacancy defects[9, 10] in the graphitic lattice [11, 12]. The authors already conducted ChemTEM studies of individual metals (Ni[13, 14], Re[15]) or metal triads (W-Re-Os[16, 17], Fe-Ru-Os[18]) and found a strong correlation between the observed dynamics of defect formation in carbon structures and the position of the metal in the period or group.[16, 17] Now, in order to analyze this in detail, they conducted a global comparison for the transition metals, revealing fundamental trends across the periodic table.

Using the low-voltage but atomic-resolution TEM equipment to analyze the specially prepared samples, the researchers were able to get a detailed look at the mechanisms of catalysis of nanocatalysts, which play a key role in many industrial processes such as the water-gas shift reaction or methanol synthesis on Cu, and oxygen reduction in fuel cells on Pt/Co, just to name a few. Recent estimates indicate that catalytic chemical reactions account for 30-40% of global gross domestic product (GDP), making the catalytic behavior of nanoclusters at the atomic level an urgent task.

For the study, published Sep. 4 in Nature’s Communications, the researchers analyzed a very large dataset, comprising 14 different transition metals, which allows to identify the systematic changes in the properties for those compounds.

They found why the dynamics type of transition metals and their interaction with carbon nanotubes (catalytic activity) could not be explained so far. The measurements demonstrate no significant correlation between the efficiency of interaction with carbon and the metal’s position in the periodic table. This suggests a complex interplay of the element-specific valence electron configuration, electronegativity and the size of the atom—as discussed recently in comparative theoretical studies of transition metals on carbon,[6-9]. In the new experiments with spatial and temporal resolution, which could not previously be achieved with any other method, the scientists were now able to visualize changes in the atomic structure of the metal nanocluster simultaneously with the carbon structure growth. The momentum transferred by the incident electrons directly to the atoms could purposefully influence the changes.

In general, most of the 14 transition metal nanoclusters which were examined show more dynamic when they begin bonding to carbon. For example, the W and Ni nanoclusters continually and rapidly change as the new carbon structures grow from their surface (Fig. 2, Supplementary Fig. 8, Supplementary Movies 2, 3). This strikingly illustrates the key challenge of nanocatalysis—metal nanoclusters changing their atomic structures during the reaction, that is very similar to the behavior predicted theoretically. [19, 20]

The new observations provide direct evidence of dynamic single-atom catalysis, which was also predicted theoretically.[21] For example, in the arresting behavior of the Pt nanocluster in process 3, the nanocluster acts as a viscous liquid dispersing to individual atoms (Fig. 2, 118 s, marked with the green arrow) and then re-clustering over time (Fig. 2, 238 s). Such individual atoms were expected to coexist with nanocatalysts and play an important role owing to their high surface energy[22].

Figure 2. Nanocluster activities.

By analyzing the TEM series (Fig. 2), it was found that the activity of metal nanoclusters towards defect formation in NTs is indeed related to the bonding energy between the nanocluster and carbon, and a volcano-shaped plot was obtained (Fig. 3) — a graphical representation of the Sabatier principle that defines an effective catalytic surface to be neither too reactive nor too inert[23], so that it both binds effectively to the reactants and readily releases the products of the reaction.

The Sabatier principle has become firmly established in the foundations of heterogeneous catalysis[24], guiding the discovery of new catalysts,[25] including those for carbon nanostructure growth.[26] However, because of the highly dynamic surface of metal nanoclusters, there was until now no certainty that this principle would apply also for nanocatalysis. The scientists could now prove that for the 14 transition metals this is indeed the case (Fig. 2), thus verifying that series 1 (Fig. 2) is connected to the relative nanocluster-carbon bonding energy, and that still for nanocatalysts with their tiny size (20–60 atoms) the reaction is closer to the heterogeneous rather than homogeneous catalysts.

Figure 3. Comparison of transition metals.

The filling of d-orbitals along a period of transition elements results in a sharp decrease in the contribution of the d-orbitals to metallic bonding for Ni, Pd and Pt as compared to other metals.[27] The very high occupancy of the d-orbitals in group 10 allows only transient interactions with surrounding carbon and high diffusivity of carbon atoms through the metal nanocluster, such that no well-defined interface can exist between these metal nanoclusters and carbon structures under demonstrated ChemTEM conditions, thus explaining why the group 10 metals Ni, Pd and Pt appear to disobey the Sabatier principle.

Recent advancements in TEM together with the use of nano test tubes offer a radically new way to study and compare chemical processes at the atomic level.28 “In past studies, they had less information on atomic reactions promoted by nanocatalysts. With filming in the transition electron microscope, we can determine for the first time see the atomic changes of the nanocatalyst and its environment with atomic accuracy. For over six centuries scientists have been using laboratory glassware (test tubes, retorts, flasks, beakers etc.) to discover and investigate the properties of chemical elements and the mechanisms of chemical reactions. These traditional methods work well for reactions where the key parameters stay the same, but in nanocatalysis owing to the largely unpredictable and dynamic behavior of the active center, new approaches are required.” say the study’s senior authors, Ute Kaiser, professor of electron microscopy at Ulm University and Andrey Khlobystov, professor of chemistry at Nottingham University.

ChemTEM methodology applied to transition metals inside carbon nano test tubes provides atomic-level nanocluster dynamics analysis enabled to compare, categorize and order the group of transition metals with respect to their relative catalytic activity and their bonding with carbon. The use of a precisely shaped e-beam to supply the energy to drive chemical reactions and capture their progress in real time coupled with a detailed understanding of the mechanisms [29-31] enables the determination of fundamental thermodynamic parameters[12, 31] and reveals mechanisms of chemical reactions.[15, 32, 33].

“The new study covers the whole block of early and middle transition metals consisting of three periods and five groups, which, for the first time, enabled a detailed analysis of global trends within the family of transition elements.” Says Kecheng Cao from Ulm University, first author of the study. “It reveals that the metal nanoclusters become more dynamic once they are engaged in reactions with carbon—one very unexpected outcomes of this analysis, having significant implications for understanding the atomistic workings of catalytic cycles at the nanoscale. Equally important are our findings that the tiny clusters of metals obey the Sabatier principle as heterogeneous catalysts (with three important exceptions), despite the absence of a well-defined surface, and that different metals exhibit drastically different tendencies towards defect formation in the carbonic lattice of the nanotube.”

This knowledge provides a new guide for the development of better catalysts involving C–C bond dissociation that could be rationally designed and tested on the basis of ChemTEM atomic-scale experiments. Importantly, the strategy of utilizing a confined environment such as a nano test tube to stabilize metal nanoclusters could be utilized in the development of more durable and reusable nanocatalysts. The filming of the highly dynamic nature of metal nanoclusters during reactions is a promising approach to improve the performance of nanocatalysts in the future.[34]

Highlighted Topics

Single-atom catalysis

Inside the nanotube metal atoms are engaged in M-C bonding which perturbs the C-C bonds making them easier to break. Transferred energy (E_T) from the e-beam to the carbon atoms of the nanotube can provide the energy required to break the C-C bonds and eject individual carbon atoms, creating a defect within the nanotube structure which is stabilised by M-C -bonding. Theoretical calculations (the numbers above the curly arrows indicate the activation energy required from the e-beam to enable the transformation) reveal that interactions between a single metal atom (Ru) and the sp2-hybridised carbon atoms of a graphitic lattice significantly reduce the energy threshold for carbon atom ejection from 22.3 eV for the metal-free case to 19 eV in the presence of a metal atom. The metal also significantly stabilises the mono-vacancy structure after carbon atom ejection from 7.85 eV to 1.39 eV which affects the energy threshold to ejection of carbon atoms. A further C-atom can be ejected resulting in formation of a di-vacancy defect which is similarly stabilised by the metal (c.f. formation energy of 2.33 eV compared to 7.88 eV for the metal-free system). The momentum is transferred to the carbon atom highlighted in red and yellow for first and second impacts respectively from below, perpendicular to the graphitic lattice. The nature of the M-C interactions for different metals determines the stability of the defect and thus dictates how quickly carbon atoms are knocked out in the vicinity of the metal atom and evolve into new carbon nanostructures (e.g. protrusions, carbon shells, internal nanotubes).

Figure 4. Theoretical calculations demonstrate the metal atom catalyzing defect formation in the graphenic lattice due to the metal-carbon bonding.

Resource: Cao, K., Zoberbier, T., Biskupek, J., Botos, A., McSweeney, R. L., Kurtoglu, A., Stopiello, C. T., Markevich, A. V., Besley, E., Chamberlain, T. W., Kaiser, U., & Khlobystov, A. N. (2018). Comparison of atomic scale dynamics for the middle and late transition metal nanocatalysts. Nature communications, 9: 3382, doi: 10.1038/s41467-018-05831-z, [PDF], see also the supporting information.