par Lambeets, Sten ;Barroo, Cédric ;Kruse, Norbert ;Visart de Bocarmé, Thierry
Référence Interdisciplinary Surface Science Conference (20: 2015/04/02: Birmingham, UK)
Publication Non publié, 2015-04-02
Communication à un colloque
Résumé : The valorisation of CO2 gas into useful products for the industry seems to beis one key way to address the issue of global warming. Yet, its application has to be economically viable. For instance, heterogeneous catalysis is used to produce methanol by CO2 hydrogenation. Heterogeneous Catalysis is a surface phenomenon; hence most of the catalysts are shape as nano-sized nanoparticles dispersed on a support. Development of new catalysts design needing an increase understanding of mechanisms leading the reaction on surface, and the synergy between the reaction and the nanoparticle surface structure at the molecular scale. In order to improve the efficiency of such catalysts, a deep understanding of the reaction processes at the molecular level is needed. Catalytic particle shape, its size and its surface composition are some of the features that fill influence the activity and that should be determined with time so as to unravel the detailed mechanism the this reaction.This work reports on the study ofof the adsorption of CO2 adsorption as well as the interaction of H2/CO2 gas mixtures on over nano-sized rhodium crystals. This system has been studied at the nanoscale using field emission techniques including Field Ion Microscopy (FIM) and Field Emission Microscopy (FEM) [1]. The FIM/FEM device is able to image in real time the surface of a conductive material, conditioned as a thin tip, at the nanoscale with 0,2 nm lateral resolution in FIM mode and 2 nm in the FEM mode. The structure of the rhodium nanocrystals have been characterised by FIM with atomic lateral resolution, whereas CO2 adsorption and dissociation have been followed by FEM. Brightness analysis is used to monitor the reaction in while it proceeds. The brightness intensity of the FEM pattern depends on the amount ofcurrent density of electrons emitted from the nanocrystal tip apex and thus, on the local work function of the surface. The introduction of pure CO2 gas during the imaging causes the brightness to decrease due to the dissociative adsorption of CO2 gas to O(ads) and CO(ads) species. Upon increase of the hydrogen pressure, reaction phenomena were observed from 650 K to 700 K. The increasing brightness testifies the occurrence of a reaction between adsorbed hydrogen and adsorbed oxygen. Results from the brightness analysis have been compared to literature data which allow to propose a chemical scenario explaining these observations and identify the reaction as the Reverse Water Gas Shift (CO2(g) + H2(g)→ CO(g) + H2O(g)) [2,3]. These assumptions are in line with direct local chemical analysis performed by atom probe techniques [4]. The latter consist to in the coupling of a FIM device with a Time of Flight mass spectrometer operated during the ongoing processes using field pulses to desorb surface species as ions during the ongoing reaction.