TMCs have been attracting an increasing amount of interest in the last few decades in the field of heterogeneous catalysis due to all their nice properties described in a previous post. However, apart from the use of TMCs per se as active catalysts, they can also be used as catalytically active supports [1]. This line of research originated from the theoretical discovery that TiC can modify the electronic structure of supported Au particles, thereby drastically increasing their catalytic activity through strong metal-support interactions (SMSIs) between Au and TiC [2].
In this post, we list the main findings of our latest research paper just published in Journal of Materials Chemistry A [3], where we study the atomic and electronic properties of small metal clusters of precious metals (Rh, Pd, Pt and Au) and more affordable metals (Co, Ni and Cu) supported on TMCs with 1:1 stoichiometry (TiC, ZrC, HfC, VC, NbC, TaC, MoC and WC). Our study employs periodic DFT calculations within a high-throughput screening framework to obtain the structural and electronic properties of these materials.
Bulk and slab models
Figure 1. Bulk and slab models. The systems belonging to each bulk or slab model are indicated below the drawing.
For all carbides under consideration except MoC and WC, the most stable phase corresponds to a face-centred cubic (fcc) crystal packing [4]. The lowest energy surfaces for all these carbides have been shown to be the (001) faces. For MoC and WC, fcc and hexagonal closed packed (hcp) phases can be synthesised by employing different carburising agents. In the case of the MoC hexagonal phase, the Mo- and C-terminated (001) faces have been theoretically predicted to be the lowest energy ones, with similar stability [5], but for the WC hcp phase, the lowest energy surface is the W-terminated (001) face.
Atomic structure
- for both cubic and hexagonal TMCs, supported metal clusters bind to surface C rather than surface metal atoms
This result is observed by comparing the total energies of several possible configurations for 3 and 4 atom clusters on top of the TMC slabs. In fact, the higher melting points of metal carbides compared to the corresponding metals or metal alloys indicates that C-metal bonds are stronger than metal-metal bonds. Although the reason for this result is not entirely clear, it is probably because the mixing of the d states of the supported cluster with the C p states is stronger than with the metal d states of the TMC. Interestingly, previous XPS experiments on Au/TiC confirmed the formation of stronger bonds of Au with C than with Ti, as evidenced by the large Au-induced shift in the C 1s core level, while only small changes in peak position were observed in the Ti 2p peaks after the deposition of Au [6].
The following results are also observed:
- there is also a broad preference for the formation of 2D clusters over 3D, except for Au clusters, where the tC-th tetrahedral configuration is normally lower in energy than the flat ones
- in many 4-atom clusters, there is a large vertical displacement of the nearest surface C atom, placing itself in the centre of the metal cluster
Binding strength
Figure 2. A) Averages of the adsorption energy per atom (top), and formation energy of the cluster (bottom). Averages by metal and by support are plotted in the left and right panels, respectively. In the left panels, the averages have been separated in three categories: fcc (blue), hcp M-terminated (orange) and hcp C-terminated (green) TMC supports. Note that the latter does not correspond to an average, as there is only one data point (i.e., h-MoC(001)-C). B) Scatter plot of the adsorption energy per atom against the cluster formation energy. Only the most stable configurations for each cluster-support pair are considered in all plots.
The stability of the metal clusters on TMC surfaces is evaluated by calculating the adsorption energy per atom and the formation energy of the cluster (see their definitions in our work). The adsorption energy is a measure of the binding strength of the cluster to the TMC surface, so that negative values correspond to favourable adsorption, and the more negative the stronger the binding. On the other hand, the formation energy is a measure of the stability of the clusters compared to the bulk metal, and therefore it can be used as a descriptor for resistance to metal aggregate formation (coarsening). A negative formation energy for a given cluster indicates that the cluster configuration is thermodynamically preferred over the bulk.
- Clusters bind more strongly to hexagonal (001) TMC surfaces than to cubic (001) ones (Figure 2A)
Interestingly, hexagonal (001) surfaces resemble cubic (111) surfaces, which are less stable than cubic (001) surfaces but are generally more active, due to the existence of more surface states near the Fermi level [7] and higher d-band centres [8]. Thus, they interact more strongly with the supported clusters.
- For the hexagonal (001) surfaces, the C-termination interacts more strongly with the clusters than the M-termination (Figure 2A)
This result can be explained by the fact that, in TMCs, metal-C bonds are in general stronger than the metal-metal bonds.
- The strongest binding is found for Rh and Pt clusters, while the lowest is found for Cu and Au clusters (Figure 2A)
Cu and Au are coinage metals, and therefore have filled d states that are lower in energy. This leads to a higher occupation of antibonding states when interacting with the d bands of carbides, and therefore weaker binding.
Figure 3. A) Histogram of the shifts in the atomic adsorption energies of the metal clusters due to the presence of surface C vacancies (negative values indicate stronger binding with C vacancies). B) Distribution of net atomic Bader charges for the supported clusters on the stoichiometric TMC (top) and in the presence of surface C vacancies (bottom).
- Surface C vacancies increase the binding strength of the supported clusters (Figure 3A)
In the absence of a surface C atom, supported clusters can pull more electron density from the carbide, leading to a higher charge transfer and therefore stronger binding. In fact, the clusters end up being more reduced when supported on C-vacancy sites.
As a final remark, note that the scatter plot in Figure 2B shows that Co, Ni and Rh clusters supported on the C-terminated h-MoC(001) surface are the more stable systems, from a purely energetic point of view. The opposite corner of the plot contains most Cu and Au clusters on cubic TMCs. Despite that, small Cu and Au particles in contact with TiC(001) were predicted theoretically and demonstrate experimentally to be good catalysts for CO2 activation and the catalytic synthesis of methanol [9]. Therefore, we expect that all the small metal clusters supported on TMCs considered in this work will remain anchored at a given site, without tending to aggregate into larger clusters and, more importantly, without any tendency to escape from the TMC surface.
Charge transfer
Figure 4. A) Heatmap of net atomic Bader charges for each cluster-TMC pair. B) Average net atomic Bader charges by metal (left panel) and by support (right panel). All Bader charges are calculated for the lowest energy configurations.
Figure 5. A) Electronegativity of all elements involved in the present study. B) Net atomic Bader charges of surface C atoms in the clean TMCs as a function of the electronegativity of the TMC metal atoms. C) Net atomic Bader charges of the supported clusters plotted against net atomic Bader charges of surface C atoms in the clean TMCs. All Bader charges are calculated for the lowest energy configurations.
- The charge transfer resulting from cluster adsorption is much higher -in absolute value- in hexagonal (001) surfaces than in cubic (001) ones (Figure 4)
In C-terminated hexagonal surfaces, the more electronegative surface C (Figure 5) atoms pull electron density from the supported cluster, oxidising it. Similarly, in M-terminated hexagonal surfaces, the less electronegative surface metal atoms yield electron density to the supported cluster, reducing it. These cases correspond to the two extreme situations in which the difference in electronegativity between the surface atoms and the supported cluster atoms is highest, leading to higher charge transfer. However, in cubic surfaces, half of the surface atoms correspond to C and the other half to metals, leading to a lower electronegativity difference between surface atoms and cluster atoms and, therefore, lower charge transfer.
- In hexagonal surfaces, supported clusters are oxidised in the C-termination and reduced in M-termination (Figure 4)
As discussed above, the C-termination grasp electron density from the cluster (C is more electronegative), whereas the M-termination yields electron density to the cluster (surface metal atoms are less electronegative).
- Supported clusters become more reduced (or less oxidised) when going down a group in the periodic table (Figure 4B)
Indeed, the electronegativity of cluster atoms increases when going down a group, explaining this result.
- The net atomic charges of supported clusters are correlated with the net atomic charges of surface C before cluster adsorption (Figure 5C)
The higher the electron density of the C atoms before adsorption, the higher the density the metal cluster can pull (or the lower density it can yield) once the cluster and the TMC interact. Thus, those TMCs with a higher ionic character (e.g. ZrC or HfC) are more prone to yielding electron density to metallic clusters.
Polarisation of the electron density
Figure 6. Charge density difference plots for Cu, Co, Pd and Pt clusters supported on selected TMC facets. The isosurface level is taken as 0.0015 e·bohr-3. Yellow regions denote accumulation of charge density, while blue regions denote charge density depletion.
Despite no significant charge transfer in most of the studied systems, the TMC support always induces a significant polarisation of the cluster electron density, resulting in an accumulation of charge density at the interface between the cluster and the support and a charge depletion on top of the cluster atoms. Moreover, we observe that:
- The most polarised cluster are those of Pt, Pd and Rh, while the least polarised ones are those of Co and Cu.
Larger atoms such as Pt, Pd and Rh, which have more loosely held electrons and more diffuse orbitals, have a higher polarisability than smaller atoms such as Co and Cu, which have tightly bound electrons.
Conclusions
The deposition of small metal particles on TMCs can lead to stable catalysts with unique catalytic properties. By carefully selecting elements with desired electronegativity for the host TMC and the metal cluster, it is possible to manipulate the charge of the supported cluster and tune the amount of charge density on the cluster hollow sites, which can facilitate the bonding of certain molecules. Moreover, we identify Pt, Pd and Rh clusters supported on hexagonal TMC (001) facets as the candidates with the highest potential catalytic activity and stability, as estimated from the degree of polarisation of the electron density and the values for the adsorption energy and formation energy. We hope that the trends identified in this study will provide a solid theoretical background from which potential catalytic activity and stability can be estimated and understood, paving the road for further studies on the interaction and catalytic conversion of chemical species with TMC-supported metallic nanoclusters.
References
[1] J. Catal. 2013, 307, 162-169; [2] J. Chem. Phys. 2007, 127, 211102; [3] J. Mat. Chem. A 2022, Accepted Manuscript (DOI 10.1039/D1TA08468B); [4] Phys. Chem. Chem. Phys. 2018, 20, 6905-6916; [5] Phys. Chem. Chem. Phys. 2013, 15, 12617-12625; [6] J. Chem. Phys. 2007, 127, 211102; [7] Surf. Sci. Rep. 1995, 21, 177; [8] Phys. Chem. Chem. Phys. 2018, 20, 6905; [9] J. Phys. Chem. Lett. 2012, 3, 2275-2280
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