Investigating the oxidation of a cobalt-based catalyst using X-ray photoelectron spectroscopy

Importance of Topic

Surface oxidation of metal‐based catalysts directly affects their activity and lifetime in processes such as hydrodesulfurization. X‐ray photoelectron spectroscopy (XPS) offers a surface‐sensitive, quantitative method to monitor chemical states within the top 10 nm of catalyst surfaces. By tracking oxidation of active phases, researchers can optimize catalyst preparation, storage, and handling to maintain performance.

Objectives and Study Overview

This study compares the fresh and air‐exposed states of a γ-alumina supported Co(Ni)MoS hydrodesulfurization catalyst. The primary goals are to:

• Identify surface chemical state changes upon brief air exposure
• Quantify the relative amounts of sulfide and oxide phases of cobalt and molybdenum
• Demonstrate the capability of the Thermo Scientific™ K-Alpha™ XPS System for rapid catalyst characterization

Methodology

The catalyst pellet was handled under hexane and mounted on copper tape. It was introduced into the XPS system via load-lock to minimize atmosphere exposure, and surveyed for elemental and chemical state information. After initial analysis, the pellet was exposed to ambient air for three minutes, then re-analyzed. Comparative spectra provided data on oxidation changes.

Used Instrumentation

• Thermo Scientific™ K-Alpha™ X-ray Photoelectron Spectroscopy System
• Avantage data system with multicomponent peak fitting

Main Results and Discussion

Cobalt 2p3/2 spectra revealed an increase in Co(II) oxide and Co9S8 phases after air exposure, with Co9S8 rising from 13.4 % to 19.0 % and Co(II) oxide from 24.4 % to 38.0 %, while CoMoS decreased from 62.2 % to 43.0 %.

In the Mo 3d region, Mo(II) sulfide dropped from 87.2 % to 47.2 %, whereas Mo(IV) oxide increased from 4.7 % to 28.5 % and Mo(VI) oxide from 8.1 % to 24.3 %.

Full chemical state quantification confirmed that alumina support remained constant (~24 at %), while cobalt, molybdenum and sulfur species underwent significant oxidation. The appearance of oxysulfide and elevated oxygen signals in C 1s and O 1s regions illustrated surface contamination and oxide growth.

Benefits and Practical Applications

• Rapid assessment of catalyst surface quality and oxidation level
• Guidance for storage and handling procedures to preserve active phases
• Support for catalyst formulation and deactivation studies in industrial QA/QC

Future Trends and Opportunities

Advances may include in situ and operando XPS to monitor catalysts under reaction conditions, higher spatial resolution for probing heterogeneity, and coupling with complementary techniques such as XANES and TEM for comprehensive surface characterization.

Conclusion

The K-Alpha XPS System provides fast, accurate identification and quantification of surface oxidation in Co(Ni)MoS catalysts. This enables researchers to evaluate catalyst integrity after environmental exposure and improve performance through informed material handling.

References

• Gandubert et al., Oil & Gas Science and Technology – Rev. IFP, Vol. 62 (2007) No. 1, pp. 79–89