Understanding the mechanisms behind chemical transformations is key to developing improved catalytic systems. To this end, operando spectroscopy has contributed significantly to the advancement of our molecular view of many reactions at the heart of the chemical industry. In this thesis, we show the application of time-resolved spectroscopic techniques to elucidate the mechanism of a reaction that has always occupied a central role in emission control research - the selective catalytic reduction (SCR) of nitrogen oxides with ammonia. By performing our measurements away from steady-state conditions, we have been able to capture molecular and kinetic information that transcends the boundaries of conventional spectroscopy. Therefore, transient experimentation opens up new avenues for structure-activity correlation that will ultimately pave the way for rational catalyst design and process improvement.