Understanding chemical reactions on solid surfaces covered by two-dimensional materials

A huge fraction of all industrial chemical processes involve heterogeneous catalysis. These processes entail the breaking of atomic bonds and the formation of new ones at the interface between a solid catalyst and its gas or liquid environment. Several environmental challenges we are facing nowadays (e.g., transformation and storage of renewable energies, fuel cell technologies for the car industry, air pollution) could find a solution in the discovery of new catalysts. It has only recently been recognized that heterogeneous catalysis can take place in the confined space between a solid catalyst and a weakly interacting two-dimensional (2D) overlayer. This peculiar space can be regarded as a nanoreactor, in which confined molecule adsorption and surface reactions may occur. Even more enhanced catalytic performance could be exhibited by hybrid layers in which two or more materials are combined. In their efforts to find new materials and their properties, the researchers of the BACH beamline group examined the confinement effects and the reactions in the nanospace between an in-plane heterostructure of graphene and hexagonal boron nitride (h-BNG) and the platinum substrate. 

The structure and composition of a real catalyst, as well as the reaction mechanism of catalysis, are often highly complex. Any direct study of the catalyst and catalysis under working conditions is an almost unachievable task. Therefore, the surface science approach has been employed in order to gain closer insight into the nature of the catalysts and elementary steps involved in heterogeneous catalysis on solid surfaces.
As a model case in this study, the researchers used platinum in the form of plane Pt(111) crystal covered by a hybrid h-BNG 2D layer and they investigated the interaction of this system with carbon monoxide. Conditions under which CO gas molecules intercalate the h-BNG layer and adsorb on the platinum substrate were found.
By employing synchrotron-radiation soft X-ray spectroscopy, this experimental approach yielded a couple of intriguing insights into 2D cover-molecule-catalyst interactions.
It was shown that it is possible to stabilize a densely-packed CO overlayer under the 2D cover, which allows for the use of standard surface science techniques to characterize such structures normally unstable under ultra-high-vacuum conditions at room temperature. The experimental results provided evidence that the cover acted as an effective barrier that confined CO in the interface between the platinum surface and h-BNG, which was demonstrated by a large amount of CO found to be chemisorbed on the Pt surface at temperatures much higher than those of bare Pt(111). Furthermore, repeated cycles of CO intercalation and thermal desorption revealed that CO reacted with the h–BNG cover at an elevated temperatureat the ternary boron nitride–graphene–platinum interface. Part of the intercalated CO molecules dissociated and the atomic oxygen reacted with boron atoms. The reaction was hampered after a couple of cycles and the intercalation–desorption appeared to be stabilized afterwards, although only with half the CO adsorption capacity. Finally, the system was regenerated to a large extent by a high-temperature treatment.
This research demonstrates that mixed 2D covers, such as in-plane graphene and hexagonal boron nitride heterostructure, can not only act as an effective barrier to confine gas molecules in a 2D interface, but also significantly modulate the catalytic performance of the solid substrates underneath. Based on the results obtained, metallic atoms intercalated under the mixed h-BNG layer will open up the opportunity to further tune catalysis under two-dimensional materials.
 

Figure 1.  High-resolution temperature-programmed X-ray photoemission spectroscopy (XPS) for h-BNG covered Pt(111) and bare Pt(111) after CO gas exposures. The difference in CO desorption kinetics from the two surfaces is obvious (dashed line). Moreover, the chemical sensitivity of the XPS technique can also reveal differences in CO desorption under the boron nitride and graphene phases of the 2D layer (compare hBN-CO N1s and Gr-CO C1s panels). The low energy electron diffraction (LEED) patterns at the bottom revealed different CO long-range ordering on the Pt(111) surface with (right) and without (left) the h-BNG cover.

This research was conducted by the following research team:

Igor Píš¹, Elena Magnano^2,3, Silvia Nappini² and Federica Bondino²

¹ Elettra - Sincrotrone Trieste S.C.p.A., Area Science Park - Basovizza, Trieste, Italy.
² CNR-IOM, Laboratorio TASC, Area Science Park - Basovizza, Trieste, Italy.
³ Department of Physics, University of Johannesburg, Auckland Park, South Africa 

Contact persons:

Igor Píš, email:
Federica Bondino, email: