Hence, the decrease in H2O pulse intensity is clearly more pronounced than that of the incoming H2 pulses, based on the decay of the Ar pulses. This points to a relative decrease in reactive water formation during the experiment. Furthermore, the decay in the last two subsequences is less pronounced in the pulse heights than in the integrated intensities, indicative of a slight decay in pulse width during that phase, which was confirmed also by evaluation of the pulse shapes in that region. We can only speculate about possible reasons for this behavior. Most likely, during the sequence the catalyst surface has reached a state where on the one hand H2O formation is less efficient, and on the other hand its interaction with the catalyst surface is weaker (less broadening).