A modulated excitation ED-EXAFS/DRIFTS study of hydrothermal ageing of Rh/Al2O3

• Time-resolved ED-EXAFS and DRIFTS are used to determine structural changes induced by hydrothermal ageing of 2 wt% Rh/Al2O3.
• Under CO–NO pulsed conditions CO2 production is maximum at the NO-to-CO switch on all catalysts.
• Oxidized Rh species are beneficial to activity but different species form depending on ageing temperature.
• On the fresh catalyst, surface oxide species on reduced Rh nano-particles are active.
• On the catalyst aged at 1273 K, a RhOAl species is redox active but not for desired chemistry.

The thermal stability of Rh-based catalysts for automotive applications is crucial for NOx emission control. Exposure of Rh/Al2O3 to high temperatures under lean conditions causes the loss of active metal area by reaction of Rh with the Al2O3 support that is detrimental for activity and selectivity. This has been confirmed using X-ray diffraction (XRD) and extended X-ray absorption fine structure spectroscopy (EXAFS) of fresh and hydrothermally aged 2 wt% Rh/Al2O3 at 973 and 1273 K. The nature of the species responsible for the activity of the aged catalysts was investigated by the combination of diffuse reflectance IR spectroscopy (DRIFTS) and energy dispersive EXAFS spectroscopy (ED-EXAFS) with phase-sensitive detection (PSD) in a modulated excitation approach. The observed changes in the IR and ED-EXAFS spectra are correlated with the structural changes induced by ageing. The chemistry of CO and NO on Rh was used to probe such changes. The data reveal that though metallic Rh nano-particles are the base for an active catalyst, a surface oxide species (RhO) is likely necessary for activity. Ageing causes loss of Rh dispersion and formation of different Rh structures, which provide distinct RhO species and therefore different catalytic properties.

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