From this simple picture of different impurities, one can derive the following conclusions to rationalise the differences between type I and II. (i) The peaks C1, C3 and A1 observed in type I are related to an impurity which is present in a higher concentration in the batches with type I than in batches of type II. The presence of these species could block the surface for other processes, such as those observed with type II in peaks C2/A2 and C4. (ii) Likewise, an impurity inducing the processes related to peaks C2 and C4 could block the surface for the processes related to peaks C1, C3, and A1.