Extended TMC surfaces can be used as catalysts for a considerable number of reactions [1], or as catalytically active supports for metal particles [2], displaying very high activities. But what happens in the opposite case, where a TMC nanoparticle is supported on a metal? Well, it turns out that this is can be great strategy for obtaining highly active catalysts! Let’s see an example.

In a recent article [3], M. Figueras et al. showed that MoCy (y = 0.5-1.3) nanoparticles supported on Au -which is completely inert towards methane- can adsorb and dissociate methane at room temperature and low methane partial pressure.

Why is methane activation important?

Natural gas is a common source of energy for heating, cooking, and electricity generation. In this gas, methane is the major component and its activation and transformation can have a major impact in industrial operations and environmental pollution control, given that methane greenhouse capabilities are about 23 times larger than those of carbon dioxide. Research endeavours have been undertaken to make it possible to use methane as a feedstock for commodity chemicals such as methanol, ethylene, or benzene.

Why is methane activation challenging?

The activation of this hydrocarbon is particularly difficult due to the high strength of the C-H bonds (4.51 eV/mol for the first bond dissociation energy) and the non-polar character of the molecule. In this respect, it is well known that the methane monooxygenase enzyme is able to activate methane at room temperature. However, this biological system cannot be used in industrial-scale operations. Moreover, to avoid the decomposition of the products and competing reactions, methane activation should proceed at low or medium temperatures.

Optimised geometries (top and side views) for the adsorption of methane on different MoCy/Au(111) models. Au, Mo, C, and H atoms are shown as yellow, blue, black, and white spheres, respectively.

Experimental results

A series of X-ray photoelectron spectroscopy (XPS) experiments, combined with thermal desorption mass spectroscopy (TDS) showed that the Au-supported MoCy nanoparticles are able to dissociate methane at room temperature, and the activity, the stability, and the strength of the interaction with CHx species appears to depend on the C/Mo ratio. While C-deficient nanoparticles are very reactive, they feature low stability due to the strong binding of adsorbed CHx species, which leads to an increase in the C/Mo ratio upon annealing. On the other hand, Mo-deficient systems present the right balance of stability and activity. Although they are less reactive, they are still able to dissociate methane at room temperature, ands are stable under an atmosphere of methane.

Insights from density functional theory (DFT) calculations

A series of DFT calculations on a set of supported MoCy nanoparticle models with different C/Mo ratios confirm that these nanoparticles feature much stronger methane adsorption energies than extended MoC or Mo2C surfaces, with adsorption energy values up to -1.16 eV. Moreover, the energy barrier for methane dissociation on the nanoparticles can be as low as 0.08 eV for C-deficient systems, in agreement with the experimental findings.

Overview

All in all, this study opens the way for the preparation of a new family of active catalysts for methane activation and conversion under mild conditions, thus widening the applications of existing natural gas resources, and is a good example of how the activity of TMCs can be boosted by nanostructuring.

References

[1] F. Viñes et al, J. Catal. 2008, 260, 103-112

[2] J. A. Rodriguez et al, J. Catal. 2013, 307, 1162-169

[3] M. Figueras et al, Phys. Chem. Chem. Phys. 2020, 22, 7110-7118