
: Adsorption energies of two relevant single water molecule adsorption modes for various TnOm sites. Water can be either adsorbed on BAS (A, ) or on the Al atom in the antiposition to BAS (B, ) as shown in Fig. 2 of the main text. To investigate the influence of dispersion corrections on the adsorption energies, the adsorption energies were recalculated using PBE+D2 functional. The functionals differ mainly in the water adsorption in mode B, however, both predict the same trend that the adsorption of water in mode B is energetically less favorable than the adsorption in mode A.
Figure S2:
The adsorption energies of two water molecules for various TnOm sites were recalculated using PBE-D2 functional. Four adsorption modes are possible: the adsorption of two water molecules on the BAS proton and on the Al atom in the anti-position to the BAS proton (A', ); on the Al atom in the anti-position to the BAS proton with only one water molecule coordinated on the Al atom (B', ); on the Al atom in the syn-position to the BAS proton and the BAS proton (C', ); and a formation of Zundel ion (D', ). With dispersion included, the adsorption enthalpies of two water molecules are higher than when they are omitted ( Fig. 4 ).
Figure S2:
However, both functionals predict the same trends, namely that the formation of Zundel ion (D') is the most stable configuration as well as they preserve the differences in adsorption energies between different Al sites. Table S1 : To validate the reliability of the PBE functional to correctly describe the reaction profiles, three reaction pathways for T3O4 site (inters.) and both a single and two water molecules models were recalculated using PBE+D2 functional. All geometries were re-optimized, including transition states. The results show that both functionals captures equally well the trends, as both predict the smallest activation energy (Ea) for Mechanism II and the highest for Mechanism I in 2 H2O model. The trends and relative differences are reproduced also for reaction energies ΔE. All data are in kJ/mol. Scheme S1: The schematic representation of the distances (r1-r3) that were used to characterize the stationary points of Mechanism I. S4 : For a single water molecule model a linear relationship between r1 distance (Scheme S1) and imaginary frequency of TS1 has been found. For a two-water molecule model, no correlation between any of r1-r3 parameters has been found. We conclude that the presence of second water molecule alters the potential energy surface and introduces the additional degree of flexibility due to which it is not possible to identify a simple reaction coordinate such as O-H distance anymore. For both models we identify TS1 as a late transition state, structurally resembling the stable intermediate I1 as can be seen from the tabulated r1-r3 distances in Tables   S5-S7 . Scheme S2: The schematic representation of the distances (r1-r3) that were used to characterize the stationary points of Mechanism II. The definition of distance r1 and r3 is identical to the one from Scheme S1. the slope of the linear fit is reversed. We identify TS1 as a late transition state that structurally resembles P1 as can be seen from Table S11 . ) is expected to be the reaction coordinate. However, no linear relationship between the distance r2 and imaginary frequency of the TS3 has been found (left). We expect that the potential energy surface (PES) of water adsorption is very shallow with multiple local minima, due to which the localization of the exact TS3 is difficult and no simple reaction coordinate can be found. The next step is the water dissociation for which we identified a linear relationship between distance r1 (Scheme 3) and imaginary frequency of the TS4 (middle). TS4 is a late transition state with r1 distance being close to the I2 (Table S14 ). The last step of the reaction is the breaking of vicinal di-silanol, where we find a correlation between the distance r3 and imaginary frequency of TS5 (right). Again, we identify TS5 as a late transition state (Table S15 ).
Figure S7:
The analysis of imaginary frequencies of transition states for Mechanism III and two water molecule model. Alike for two water model of Mechanism I, we were not able to find any correlation between imaginary frequencies of transition states TS4, TS4' and TS5 and distances r1-r4. We attribute this to the presence of the additional water molecule, that alters the reaction profile and its presence does not allow the identification of the simple reaction coordinate anymore.
Figure S7:
Low frequencies of the imaginary modes (< 300 cm -1 ) suggest that the PES is very shallow with multiple local minima present. Scheme S4: The schematic representation of the distances (r1-r3) that were used to characterize the stationary points of Mechanism IV for two water molecule model. Figure S8 : A linear relationship between r2 distance (Scheme S4) and imaginary frequency of the TS6 of Mechanism IV has been found. Contrary to all other transition states, TS6 is an early transition state with parameters r1-r3 close to initial configuration (Table S18 ). around T3 site (intersection) for each of the reaction Mechanisms I-IV. The text in parentheses next to the adsorption modes denotes the position of the BAS proton, e.g C1' (O4) means that the proton is bonded to oxygen 4 ( Fig. 1) for the C1' mode, while number 1 means that it is the most stable structure of type C'. -20 -21 In what follows we report free energy rogersmichael@example.net. Notice, that free energies of some transition states are correctly reported to be lower than the starting and final state. This is because the harmonic approximation is too crude approximation for the free energies of very small barriers -such as the ones experienced during the water reorganization. However, this phenomenon has never influenced the activation energies of water reorganization, therefore we consider obtained free energy profiles sufficient/reliable. R represents the empty zeolite, and its energy is corrected for two physisorbed water molecules. means that the proton is bonded to oxygen 4 ( Fig. 1) for the C1' mode, while number 1 means that it is the most stable structure of type C'. reorganization from the most stable adsorption mode D' into one of the active adsorption modes (A', B' or C') around T11 site (straight) for each of the reaction Mechanisms I-IV in analogy to (Fig. 13) , PBE-D2 predicts the higher stability of the initial adsorbates and thus all stationary points, which results in the higher reaction rates on the absolute scale.

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