545765 Understanding the Role of Titanium in the Stability of Supported Palladium Catalysts for the Oxidation of Ventilation Air Methane

Wednesday, June 5, 2019
Texas Ballroom Prefunction Area (Grand Hyatt San Antonio)
Matthew Drewery1, Hadi Hosseiniamoli2, Meng-Jung Li1, Eric M. Kennedy1, Adesina Adesoji A.3 and Michael Stockenhuber1, (1)Chemical Engineering, The University of Newcastle, Newcastle, Australia, (2)Western Sydney University, Sydney, Australia, (3)ATODATECH LLC, Pasadena, CA

Understanding the role of titanium in the stability of supported palladium catalysts for the oxidation of ventilation air methane

M Drewery, H Hosseiniamoli, M. J. Li, E. M. Kennedy, A. A. Adesina and M. Stockenhuber*

*Corresponding author

The growth and rate of increase in greenhouse gas emissions (GHG) represents a serious environmental challenge. The need to decrease emissions to minimise environmental effects, combined with increasing societal pressure has led to a significant increase in research focused on these problems. Ventilation air methane (VAM), the subject of this study, contributes approximately 70% of all coal-mining GHG emissions due to the significantly high flowrates required, however the low methane concentration (<1%) is below the lower flammability limit for methane and is thus unable to be oxidised to decrease the impact of emissions. Additionally, the stability of catalysts for long-term applications is a significant obstacle for the commercial adoption of technology, particularly for the expected humidified feeds.

The current study investigates the catalytic combustion of a humidified lean methane feed stream over Pd/TS-1 and Pd/Silicalite-1 at temperatures below 500°C. A time-on-stream analysis of Pd/TS-1 identified a resistance to deactivation for over 100 hours, with this hydrothermal stability investigated via comparisons with the silicalite catalyst. The silicalite catalyst displayed a gradual deactivation over 30 hours which was not evident in Pd supported on TS-1, which coincided with the agglomeration of Pd on the catalyst surface combined with carbon deposition. Further characterisation and analysis using XAS, XPS and in situ IR spectroscopy confirmed that Ti in the TS-1 framework plays an anchoring role, inhibiting the sintering of the catalyst by Pd migration and agglomeration. Despite the lean conditions of the feed, carbonaceous deposits were found to contribute to deactivation.

Introduction

It has been identified that catalytic activity during VAM oxidation is dependent on a number of factors including dispersion, electronic and coordination structure, support properties and interaction of the active metal with the support 1-4. Palladium is considered the most active metal for the catalytic combustion of methane 3,5,6, however the assessment of Pd catalyst performance is complicated due to the influence of a variety of important factors 2,5,7. Catalyst deactivation is a major challenge associated with the implementation of catalytic processes, including VAM mitigation, however no clear agreement concerning the cause of catalyst deactivation during VAM oxidation has been reached. Literature has identified two primary causes which affect catalyst activity and stability, namely water vapour inhibition and catalyst sintering.

Previous studies have reported that water vapour competed with surface active sites on Pd-based catalysts via the transformation of palladium oxide into palladium hydroxide in the presence of steam 8-13. Supporting Pd on zeolites provides a potential mitigation solution for water deactivation. The presence of Al in the zeolite framework leads to a strong affinity for water vapour, with research showing that increasing the Si/Al ratio resulted in the catalyst becoming more hydrophobic leading to higher stability 1,2,14. Further to this, aluminium free zeolites, such as TS-1 and silicalite-1, exhibit enhanced hydrophobic properties compared to high silica zeolites 15.

Experimental

TS-1 catalyst was prepared adopting the method reported by Taramassoet 16. Silicalite-1 was prepared using the same procedure while omitting the addition of a titanium source. 1.2 wt% Pd was loaded onto both supports via incipient wetness with Pd(II) nitrate solution and calcined at 550°C prior to reaction. Catalytic combustion of lean methane was performed in a tubular, stainless steel fixed bed micro reactor. The inlet methane concentration was fixed at 0.7% in an air stream containing 3.4% water vapour, reacting at a GHSV of 100 000 h-1. The average water vapour concentration was determined using a humidity probe and was determined to be constant, with an approximate relative humidity (RH) of 80%.

Results

The light off curves of Pd/TS-1 and Pd/Silicalite-1, assessed under the same reaction conditions, shows that Pd/TS-1 displays a higher activity with a 25°C difference at T90. Palladium was identified as the active species, with no methane oxidation noted for unloaded supports.

The stability of both catalysts was also evaluated under humid simulated VAM feed at a constant temperature of 400°C, with the water vapour present in the feed stream and produced during oxidation predicted as the primary factors for catalyst deactivation 8.The high stability observed is attributed to the hydrophobicity as a result of the Brønsted acid sites 15,17. This inhibits the accumulation of hydroxyl groups on the support and increases the amount of available oxygen 5. Additionally, it has been reported that such high oxygen mobility supports can accelerate hydroxyl desorption impeding accumulation 9. Furthermore, carbon deposits were shown to contribute to the deactivation of the catalyst.

The source of Pd/silicalite deactivation was investigated via a number of characterisation techniques. The process of agglomeration, with concomitant migration of palladium, was identified by analysis with TEM. It can be seen that both the unused Pd/TS-1 and Pd/Silicalite-1 are covered with 2 to 4 nm diameter, highly dispersed palladium particles. While no significant change was identified on Pd/TS-1 after 100 h operation, Pd/Silicalite-1 displayed the agglomeration of palladium particles up to 15 nm, accompanied by much lower dispersion. These results suggest that the deactivation of the silicalite catalyst is a result of agglomeration causing the loss of dispersion, indicating the oxidation reaction is structurally sensitive. This type of deactivation was observed for acid zeolites, which also accumulated carbon deposits under very lean conditions 5,8,18. The role of acidity and the formation of these deposits is discussed in the paper.

Conclusion

The stability of Pd/TS-1 and Pd/Silicalite-1 was evaluated for catalytic combustion VAM. The long term stability of both catalysts is attributed to the hydrophobicity of these aluminium-free zeolites. However, Pd/Silicalite-1 displayed a gradual deactivation over 30 h of time-on-stream. This deactivation occurred coincident with the agglomeration of palladium atoms on the surface of this catalyst as well as carbon deposition (confirmed by TEM), whereas this phenomenon was not observed for the Pd/TS-1 material. It is concluded that the dispersion of palladium in Pd/Silicalite-1 decreased remarkably compared to the slight change of dispersion observed for Pd/TS-1. It is suggested that the Ti in TS-1 framework form the sites for palladium particles and inhibits the sintering of catalyst through migration and agglomeration of palladium particles.

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