Oxygen vacancy (OV) defects at reducible metal oxide surfaces play a key role in a heterogenous catalytic oxidation process1–4. An overview has focused on understanding the roles of OVs playing in the oxidation reaction at reducible metal oxide surfaces5. The roles of oxygen atoms and molecules at catalyst surfaces and the properties of OVs have also been the subject of recent reviews6,7.
The importance of OVs has led to the development of numerous strategies for increasing the concentration of OVs in metal oxide catalysts. Some success has been achieved via doping with secondary metal ions and nano structuring8. The dispersal of metal ions on the surfaces of metal oxides has also been demonstrated to increase the concentration of OVs effectively. However, effective methods to improve the performance of metal oxide catalysts are influenced by current characterization technologies. Therefore, it is required to find an effective characterization technology to identify OVs and understand oxidation mechanisms that occur at the surfaces of metal oxides under real reaction conditions.
The present work addresses these issues by combining an operando diffuse reflectance infrared Fourier transform spectroscopy (DRIFTS) with a temperature-programmed reaction (TPR) cell and MS to explore the behaviors of OVs and adsorbed oxygen species at β-MnO2 surfaces during H2 oxidation reaction conducted in the temperature range of 25-400 °C. The roles of OVs in H2 oxidation process are explored according to relations between OVs and oxygen species, which in turn reveal interactions between surface oxygen species with H2 at different reaction temperatures.
The roles of OVs in H2 oxidation process at β-MnO2 surfaces in the presence of O2 can be deduced from the experimental results, and the proposed mechanism is illustrated in Fig. 1. Firstly, when the reaction temperature is in a range of 110-150 °C, the oxygen atom in the bridge-type M+-O2–-M+ can react with H2 to form H2O and OV via steps (1) and (6) in Fig. 1a. According to steps (2) and (7), the oxygen atoms in the terminal-type M2+-O2– and M-OH react with H2 to generate surface M-OH and gaseous H2O. The gaseous O2 adsorbed at the bare M site in M+-□-M+ (step (4)) yields M2+-O22−. M2+-O22− dissociates simultaneously to M+-O2–-M+ and M2+-O2– (step (5), Eq. 1). We can only find the decrease in M+-O2–-M+ and increase in M-□-M in this temperature range as the step (2) is a rate limited reaction.
(O22−)M-□-M → M-O-M=O (1)
As the oxidation process in Fig. 1b, when the temperature is above 150 °C, OH in M-OH becomes reactive enough, the O2 dissociation step (6) is slowest, resulting in accumulation of M-□-M and M2+-O22−.
Fig. 1 Roles and mechanisms of surface oxygen species and OVs in H2 oxidation at β-MnO2 surfaces. a H2 oxidation between 110 and 150 oC. b H2 oxidation at a temperature higher than 150 oC.
In conclusion, using the operando TPR-DRIFTS-MS technology, O2 dissociation, OVs formation, and surface oxygen species conversion have been explored during H2 oxidation at β-MnO2 surface, the differences in the reaction characteristics of bridge-type (M+-O2–-M+) and terminal-type (M2+-O2–) oxygen species have been clearly observed. Accordingly, we expect this technology could provide an important characterization method to understand the roles of surface oxygen species on metal oxide catalysts and enable the rational design of catalysts of OVs with satisfied performance.
For more details, please read our recent publication in Communications Chemistry:
Xu, J. et al. Exploring the roles of oxygen species in H2 oxidation at β-MnO2 surfaces using operando DRIFTS-MS. Commun Chem 5: 97 (2022).
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