Measurement of Diffuse Reflection from Catalyst Powders

Importance of Topic

The characterization of catalyst powders under reaction conditions is essential for understanding active phase formation, dispersion, and thermal stability. Diffuse reflectance UV-Vis-NIR spectroscopy provides in situ insight into electronic transitions of supported metal species and ligand interactions, guiding optimization of catalyst preparation and performance.

Study Objectives and Overview

This application note demonstrates how the Agilent Cary 5000 UV-Vis-NIR spectrophotometer coupled with the Praying Mantis accessory and high-temperature reaction chamber can monitor thermal transformations of Ni/Al2O3 powders between 20 °C and 250 °C. The goal is to follow nickel speciation and support dehydration in real time.

Methodology and Instrumentation Used

Measurements were performed with:

• Agilent Cary 5000 UV-Vis-NIR spectrophotometer
• Praying Mantis diffuse reflectance accessory with downward-looking optics
• High-temperature reaction chamber fitted with SiO2 or UV quartz windows
• Small volume powder cells (∼0.1 cm3) under controlled air flow

Spectra were recorded as percent reflectance and converted to Kubelka–Munk units F(R) for correlation with concentration and state changes.

Key Results and Discussion

Thermal evolution of Ni[H2O]6(NO3)2 supported on Al2O3 revealed:

• Visible bands at ~400 nm and ~600 nm corresponding to Ni2+ octahedral transitions, shifting with temperature up to ~110 °C indicating formation of nickel hydroxynitrate.
• NIR band around 1153 nm shifting to 1240 nm, alongside loss of the –OH overtone at ~1450 nm, marking dehydration of the salt and support.
• Emergence of NiO features above ~230 °C.

Benefits and Practical Applications of the Method

This approach enables high-precision analysis of small powder samples under reaction conditions, capturing both electronic and ligand vibrational information. It supports catalyst design by providing real-time data on metal speciation, support dehydration, and phase changes without extensive sample preparation.

Future Trends and Potential Applications

Further developments may include:

• Integration with gas chromatography or mass spectrometry for complementary product analysis.
• Time-resolved studies of photocatalytic or gas-solid reactions using the reaction chamber’s viewing port.
• Extension to other supported metal systems and mixed oxide supports.

Conclusion

The Cary 5000 with Praying Mantis accessory offers a versatile, sensitive platform for diffuse reflectance studies of catalyst powders over a wide temperature range. It reliably tracks speciation and support transformations, making it a valuable tool in heterogeneous catalyst research and development.

References

• Marceau E., Perier C., Burt T., Agilent Application Note “Measurement of Diffuse Reflection from Catalyst Powders Using the Praying Mantis Accessory and Agilent Cary 5000 UV-Vis-NIR Spectrophotometer,” Agilent Technologies, 2023.