Elucidating the CO2 Activation in the Context of the Reverse Water Gas Shift Reaction
Elucidating the CO2 Activation in the Context of the Reverse Water Gas Shift ReactionFigure 1: Comparision of a theoretical (DFT) Raman spectrum of a non-defective primitive In2O3 cell (red) and an in situ 385 nm Raman spectrum (blue) of In2O3 recorded at 25 °C in 25% O2/H2. The asterisks (*) mark bands originating from the CaF2 window, while the sharp feature at around 650 cm–1 results from cosmic rays.
Marc ZiembaEinleitung
The aim of the last project was to understand the CO2 activation over Au and Cu loaded ceria and bare indium oxide. In the framework of the reverse water-gas shift reaction (rWGSR), which is of interest for the production of syn gas, we have investigated this reaction experimentally. However, it is important to understand the interactions of the reactants with the support or metal in more detail to be able to identify possible reaction intermediates. Therefore, a combination of density functional theory (DFT), Raman spectroscopy and DRIFTS will be used to allow for interpretations at the molecular level. Within this combination of theory and experiment it is possible to gain new insight into reaction mechanisms and the influence of the metal, the support as well as the metal support interactions.
Methoden
The general procedure is to perform a structure relaxation with tight convergence criteria within the PBE framework. Then, a vibrational analysis is performed, which involves the calculation of normal modes employing a method based on density functional perturbation theory (DFPT), followed by the calculation of IR and/or Raman activities. The calculation of Raman intensities is computationally demanding due to the fact that it depends on the third order derivative of the energy. These calculations, i.e., the simulations of the Raman and IR spectra, run in close interaction with the corresponding experiments to provide a direct interpretation of the experimental results.
Ergebnisse
In this project, the aim was to better understand our experimental findings during the rWGSR over Au/CeO2(111) catalysts. In this context, we would like to briefly discuss the interaction of H2 with gold deposited on CeO2(111). Transient DRIFTS measurements were able to detect bands that could be assigned to specific hydrogen species adsorbed on Aux/CeO2(111) using DFT. More precisely, we could follow the H2 dissociation over Au/CeO2, since both species (H2-Au and H-Au) have characteristic bands. The latter H-Au vibration has previously been proposed to be associated with a band at 2134 cm–1. This assignment could be confirmed theoretically by our calculations. The H2-Au band (1943 cm–1) has not been observed before. In summary, it can be said that our calculations allowed a better interpretation of different vibrational bands and thus a better mechanistic understanding of overall reaction. Another part addressed the interpretation of Raman spectra of cubic In2O3 and its defect chemistry. In this context, we calculated Raman spectra of clean as well as defect-rich In2O3 and were able to unambiguously assign the nature as well as its symmetry of defect-associated bands for the first time. This opens an important basis for a better interpretation of In2O3 spectra in catalytically relevant processes. Finally, the interactions of H2 or CO with In2O3(111) could be investigated in more detail, which will be linked to experimental results in the near future.
Diskussion
It should be mentioned that the calculations are/were done within the PBE+U or PBE framework, which is a reasonable approximation in terms of vibrational modes. This is, however, different for electronic structures, for which the use of hybrid functionals (HSE06) is essential, which should also be a topic in the future. Furthermore, mainly bulk properties of In2O3 were calculated as the first step, but for catalytic processes the properties of the surface are of more relevance. Therefore, in the future mainly two surface terminations of In2O3 will be considered in more detail, the 111 and the 110 termination. Furthermore, previous studies have shown that metallic indium occurs under reaction conditions, so its influence will also be considered in more detail. Due to the symmetry-induced size of the systems, these systems are computationally quite elaborate, hence the use of a high performance computer is essential.
