Paper HC+SS-ThM10
CO₂ Hydrogenation on Rhodium: Comparative Study using Field Emission Techniques and 1-D Atom Probe

Thursday, November 10, 2016, 11:00 am, Room 103A

Valorization of CO₂ into useful products is one way to fulfill current environmental and economic imperatives. This can be done via the selective reduction of CO₂ using heterogeneous catalysts. To get a better understanding of the fundamental processes, we studied the CO₂ adsorption as well as its interaction with H₂ on single nanosized Rh crystals. For this, Field Emission Microscopy (FEM), Field Ion Microscopy (FIM) and 1-D Atom Probe (1DAP) were used. These methods use samples prepared as sharp needles, the extremity of which is imaged with nanoscale (FEM) and even with atomic lateral resolution (FIM).

The structure of the Rh nanocrystals is characterized by FIM, and CO₂ adsorption, dissociation and hydrogenation is studied in FEM mode. The brightness intensity of the FEM pattern depends on the presence and the nature of adsorbates. Probing and analyzing the brightness signal over time allows to qualitatively monitor the variations of surface composition, and thus the presence of surface reactions, during the ongoing processes.

Finally, 1DAP, which corresponds to the combination of a FIM device with mass spectrometry, is used to identify the nature of the different surface species.

The FEM pattern of a clean sample essentially highlights {012} facets. During CO₂ exposure, the brightness of these facets drastically decreases and remains dark, reflecting the CO₂ dissociative adsorption over these facets - leading to the formation of O(ad) species. The presence of O(ad) at the surface induces a new FEM pattern where {113} facets become the most visible. This pattern reflects the formation of subsurface oxygen O(sub) beneath the {113} facets, which is confirmed by comparison with N₂O, O₂ and CO on Rh systems. To study the hydrogenation of CO₂, pure H₂ gas is introduced while the pressure of CO₂ is kept constant. Reaction phenomena, proved by variations in the brightness pattern, were observed from 650 to 734 K.

The adsorption of hydrogen at the surface leads to the formation of H(ad) species reacting with O(ad) to form H₂O(ad). Similar reaction phenomena were also observed with N₂O+H₂/Rh and O₂+H₂/Rh systems in the same temperature range, but not with the CO+H₂/Rh system, proving the role of O(ad) in the mechanism. Our observations allow to identify the reaction as the Reverse Water Gas Shift:

CO₂(g)+H₂(g)→CO(g)+H₂O(g)

These assumptions are in line with direct local chemical first analyses performed by 1DAP. Rhodium oxides species - RhO²⁺ and RhO₂²⁺ - and CO₂ with its dissociation products, i.e. CO₂⁺, CO⁺ and O⁺ , are detected in the first layers of a (115) facet of the Rh nanoparticle during an exposure to pure CO₂ at 325 K.