Speaker
Description
Heterogeneous catalysis is strongly influenced by surface features of the respective catalysts, such as the presence, amount and nature of defects, the electronic structure and compositional variations. As an example, we found in collaboration with the Interface Science Department that nanoscale composition inhomogeneities of Co$_{2}$FeO$_{4}$ impacts the oxygen evolution performance [1] Here, we present an in-depth look on the surfaces of cobalt-based spinel and perovskite oxides as catalysts for the oxidation of carbon monoxide (CO) and the selective oxidation of 2-propanol. Especially the latter is an excellent model reaction as it is sensitive to the chemical nature of the catalyst surface, such as acid/base or redox properties, which can control the reaction towards different products such as acetone or propene.
By means of identical location imaging and spectroscopy (ILIAS) in the transmission electron microscope (TEM) and near-ambient pressure X-ray photoelectron spectroscopy (NAP-XPS), we characterize the electronic, crystallographic and morphological surface structures of the spinel oxide Co$_{3}$O$_{4}$ before and after different low-temperature pre-treatments in presence and absence of oxygen. The nanostructured features resulting from these procedures are correlated with catalytic CO oxidation results obtained after identical treatments: pre-reducing in N$_{2}$ leads to stepped surfaces that are inactive, while pre-oxidation creates terraces and causes pronounced activity in CO oxidation even at room temperature suggesting the presence of electrophilic oxygen.
Using a combination of operando microscopy and spectroscopy, we investigate the selective oxidation of 2-propanol in the gas phase at temperatures up to 300 °C. This combination of techniques reveals that the surface is far from being a static entity during the reaction, roughening up by exsolution of nanoparticles that continue to form a smooth overlayer at increased temperatures before roughening up again at 300 °C. At the same time, the fraction of Co(II) increases continuously, as evidenced by NAP-XPS and NEXAFS, correlating with increases in acetone formation and also total oxidation rates. These reaction-induced changes are not limited to the surface, as electron diffraction experiments indicate a continuous shrinkage of the unit cell volume, which can be explained by a bulk reduction (and loss of oxygen), before a phase transition to the rock-salt-structured CoO finally occurs above 325 °C.
For the perovskite oxide LaFe$_{x}$Co$_{1-x}$O$_{3}$, we investigate the effects of dry and wet 2-propanol oxidation feeds on the electronic structure and adsorbates by operando NAP-XPS and NEXAFS[2]. Again, cobalt is reduced to Co(II) in both reactant mixtures, which is in correlation with the observed acetone production. Deconvolution of the O 1s and C 1s spectra reveals significant differences in the amount of hydroxidic and carbonylic/carbonate species adsorbed on the surface: in dry feed, the C 1s signal is dominated by a strong C=O contribution, which is diminished upon introduction of water, with a signal most likely originating from carbonates, appearing instead [2].
In all cases, we show that these oxide surfaces undergo severe dynamic changes during the thermal pre-treatment and catalytic reactions, be electronic, morphological or structural nature.
References:
1 F.T. Haase et al. JACS 2022, 144, 12007-12019.
2 Dreyer, M.et al. Chem. – Eur. J. 2021, 27 (68), 17127–17144.
| Abstract Number (department-wise) | AC 2.3 |
|---|---|
| Department | AC (Schlögl) |
