Measurement of Diffuse Reflection from Catalyst Powders

Importance of the Topic

The analysis of diffuse reflectance UV-Vis-NIR spectra is critical for understanding the chemical transformations that occur on catalyst supports such as alumina. Monitoring the speciation of nickel on Al2O3 during thermal treatment provides insights into active phase dispersion and catalytic activity. In situ studies under controlled temperature and gas atmospheres enable optimization of catalyst preparation and performance in reactions like desulfurization and hydrogenation.

Objectives and Study Overview

This application note aims to demonstrate how the Agilent Cary 5000 UV-Vis-NIR spectrophotometer, equipped with the Praying Mantis diffuse reflectance accessory and a high-temperature reaction chamber, can be used to track the thermal evolution of Ni[H2O]6(NO3)2 supported on Al2O3. The study covers measurements from 20 to 250 °C, focusing on speciation changes and dehydration processes.

Methodology and Instrumentation

This study employed a Praying Mantis accessory mounted on a Cary 5000 UV-Vis-NIR spectrophotometer. Samples were loaded into a small-volume powder cell within a reaction chamber fitted with SiO2 windows. Reflectance measurements used a PTFE reference at ambient conditions, with air flow for sample scans. Spectra were reported in Kubelka-Munk units to correlate reflectance with concentration. The downward-looking geometry allowed analysis of small powder volumes (~0.1 cm3) under controlled temperature and atmosphere.

Main Results and Discussion

• In the visible region, Ni2+ in octahedral coordination displayed bands near 390 and 600 nm. The 390 nm band shifted with temperature, indicating conversion from hydrated nickel nitrate to nickel hydroxynitrate up to 110 °C.
• In the NIR region, a band at 1153 nm moved to 1240 nm between room temperature and 110 °C, reflecting hydroxynitrate formation. Loss of the –OH overtone at 1450 nm signified dehydration of both the nickel salt and the alumina support.
• Additional absorption features at 1950 and 2300 nm were attributed to residual hydroxyl groups and adsorbed water on alumina, with intensity decreasing upon heating to 250 °C.
• Above 230 °C, spectral changes confirmed the formation of NiO, marking the decomposition of the nickel precursor.

Benefits and Practical Applications

• Enables real-time, in situ monitoring of catalyst precursor transformations under reactive conditions.
• Requires only small sample volumes while maintaining high signal-to-noise ratio.
• Permits analysis across UV-Vis and NIR ranges, providing complementary information to mid-IR techniques.
• Flexible geometry and high thermal stability of the reaction chamber support studies of heterogeneous catalysis and gas-solid interfaces.

Future Trends and Opportunities for Use

Emerging directions include integration with photochemical irradiation for light-driven catalyst studies, coupling with advanced temperature-programmed techniques, and applying data analytics or machine learning for rapid interpretation of complex spectral data. Such developments will further enhance understanding of active phase dynamics and support the design of more efficient catalysts.

Conclusion

The combination of the Agilent Cary 5000 UV-Vis-NIR spectrophotometer with the Praying Mantis accessory and high-temperature reaction chamber provides a powerful platform for small-volume diffuse reflectance studies. It offers high sensitivity and precise monitoring of catalytic precursor transformations, dehydration processes, and active phase formation, making it a valuable tool in catalyst research and development.

Reference

• Marceau E, Perier C, Burt T. Measurement of Diffuse Reflection from Catalyst Powders Using the Praying Mantis accessory and Agilent Cary 5000 UV-Vis-NIR spectrophotometer. Agilent Technologies, 2023.