Spectroscopy and in-situ studies of environmental catalysts

Nitrogen oxides(NOx) are formed in combustion processes and are toxic to humans and harmful to the environment. The selective catalytic reduction (SCR) of NOx is an important catalytic process for limiting emissions. The development of catalysts applicable for low-temperature and automotive deNOx is an active research area as stricter legislation is implemented.

Zeolites, particularly Cu-CHA materials, are applied in the automotive deNOx industry. Electron paramagnetic resonance spectroscopy (EPR) was applied to show that the differences in synthesis methods affected the copper distribution in the synthesized materials. This demonstrates that care must be taken when comparing results across otherwise similar materials.

Using samples synthesized in-house by different methods, an industrially synthesized sample series, and commercially available materials the developed EPR-based methodologies and testing protocols were used to reveal the differences between the materials. Experiments with isotope-labeled NH3 demonstrate the rapid ligand exchange and mobility of Cu in the solid-state.

H2-TPR and EPR were applied to analyze a series of industrial synthesized Cu-CHA materials to elucidate the discrepancy in the literature regarding the quantification of individual Cu sites. For application in automotive exhaust treatment systems, the sensitivity of Cu-CHA materials to SO2 impurities in the exhaust is an important concern.

SO2 is a poison to the catalyst even in low concentrations of a few ppm. The effect of SO2 on the SCR reactivity of Cu-CHA was analyzed by in-situ EPR for the first time. The results showed that the fresh catalyst was able to change its oxidation state rapidly, particularly from Cu+ to Cu2+, even in the presence of trace O2.

In the sulfated and regenerated samples, this reactivity was retarded. The EPR results point to the mobility of Cu on the catalyst being blocked by SO2 coordination. The amount of copper that could be reduced and oxidized was less in the poisoned catalyst than in the fresh catalyst. The regeneration of the catalyst by heating caused some of the previously deactivated copper to become responsive to the gas treatment.

The full reactivity of the fresh sampåle was not regained by the regeneration of the poisoned catalyst. A copper-based metal-organic framework (MOF), investigated in collaborative work, was able to adsorb gaseous NH3 stepwise and reversibly. The reversible and stepwise adsorption was followed by EPR in an experiment designed for this specific purpose resembling the reaction conditions reported by our collaborators.

Post-synthetic modifications resulted in the crystalization of two new materials. The modified MOF with one linker exchanged was compared to the structure reported before showed a disordered structure in SC-XRD. The new MOF was also able to reversibly adsorb NH3, however, it did show signs of less stability compared to the original MOF.

The second new structure was a 1-D polymer of {Cu2(CH3COO)4(tpt)}n. The 1-D zig-zag chain had a free coordination site on the TPT linker. New vanadium-containing poly-oxometalate SCR deNOx catalyst materials, synthesized by Bukowski et.al., were investigated by in-situ EPR. The difference in activity between the material on TiO2 support and Al2O3 support was analyzed by EPR and a difference in interaction with the support could be demonstrated.

The materials were analyzed in gases relevant to the SCR reaction and a difference in the willingness of the materials to change oxidation state was demonstrated. It was suggested that the metal-support interaction was the reason for the difference in activity. Throughout the work presented here, the importance of spectroscopy under realistic and relevant conditions was demonstrated to be crucial to provide useful insights into the changing coordination environment of the active metal centers in the catalysts and materials.

Several times, the analysis method had to be adapted to the specific materials and reactions under investigation. For these types of analysis, operando and in-situ EPR spectroscopy were powerful tools to track even minute changes in the analyzed systems.