Measurement of Diffuse Reflection from Catalyst Powders

Significance of the topic

Alumina (Al2O3) is a versatile support material in heterogeneous catalysis, influencing dispersion and activity of metal nanoparticles. Monitoring the speciation and dehydration of nickel-based catalysts under thermal treatment provides critical insights into catalyst performance and synthesis optimization.

Objectives and overview of the study

This study demonstrates the use of an Agilent Cary 5000 UV-Vis-NIR spectrophotometer equipped with a Praying Mantis diffuse reflectance accessory and high-temperature reaction chamber to investigate thermal transformations of Ni[H2O]6(NO3)2 on Al2O3 in situ from 20 °C to 250 °C.

Methodology and Instrumentation

All spectra were acquired in Kubelka-Munk units using:

• Agilent Cary 5000 UV-Vis-NIR spectrophotometer
• Praying Mantis accessory with downward-looking geometry
• High-temperature reaction chamber with SiO2 windows and PTFE reference
• Ambient air flow and controlled heating from room temperature up to 250 °C

Main results and discussion

Visible-range spectra revealed three Ni2+ absorption bands (around 390 nm, 600 nm, and a broader feature in the NIR). The 390 nm band shifted up to 110 °C, indicating conversion of hydrated nickel nitrate to hydroxynitrate. NIR analysis showed:

• Shift of the 1153 nm band to 1240 nm with temperature, reflecting hydroxynitrate formation
• Dehydration evidenced by diminishing overtone bands at 1450 nm and 1950 nm
• Emergence of NiO spectral signatures above 230 °C

Benefits and practical applications

This approach enables qualitative in-situ monitoring of catalyst precursors with minimal sample volume (~0.1 cm3), high signal-to-noise ratio, and flexible temperature/gas control. It aids in optimizing catalyst preparation and understanding reaction intermediates in heterogeneous catalysis.

Future trends and potential applications

Extensions of this methodology may involve:

• Operando studies under reactive gases or photochemical conditions
• Time-resolved analysis for kinetic insights
• Application to other metal-support systems and complex multicomponent catalysts

Conclusion

The combined Agilent Cary 5000 and Praying Mantis system provides a robust platform for tracking thermal and chemical changes in catalyst powders, offering valuable information for catalyst design and process optimization.