Free Energies of Catalytic Species Adsorbed to Pt(111) Surfaces under Liquid Solvent Calculated Using Classical and Quantum Approaches

Abstract

Solvent plays an important role in liquid phase heterogeneous catalysis; however, methods for calculating the free energies of catalytic phenomena at the solid–liquid interface are not well-established. For example, solvent molecules alter the energies of catalytic species and participate in catalytic reactions and can thus significantly influence catalytic performance. In this work, we begin to establish methods for calculating the free energies of such phenomena, specifically, by employing an explicit solvation method using a multiscale sampling (MSS) approach. This MSS approach combines classical molecular dynamics with density functional theory. We use it to calculate the free energies of solvation of catalytic species, specifically adsorbed NH*, NH2*, CO*, COH*, CH2OH*, and C3H7O3* on Pt(111) surfaces under aqueous phase and under a mixed H2O/CH3OH solvent. We compare our calculated values with analogous values from implicit solvation for validation and to identify situations where implicit solvation is sufficient versus where explicit solvent is needed to compute adsorbate free energies. Our results indicate that explicit quantum-based methods are needed when adsorbates form chemical bonds and/or strong hydrogen bonds with H2O solvent. Using MSS, we further separate the calculated free energies into energetic and entropic contributions in order to understand how each influences the free energy. We find that adsorbates that exhibit strong energies also exhibit strong and negative entropies, and we attribute this relationship to hydrogen bonding between the adsorbates and the solvent molecules, which provides a large energetic contribution but reduces the overall mobility of the solvent.

Solvation Free Energies and Adsorption Energies at the Metal/Water Interface from Hybrid QM-MM Simulations

“…The MMsolv bulk solvation energies ∆ h G MM (M) are -4.8, -4.9 and -2.3 kcal•mol −1 . While these solvation free energies are not highly accurate, although in agreement with previous reports,27,29 the error remains in the 1-2 kcal•mol −1 range, which is acceptable in our context and comparable to PCM values. Such errors have also been proposed to originate from missing polarization and charge-transfer effects at the MM level 29.…”

“…While these solvation free energies are not highly accurate, although in agreement with previous reports,27,29 the error remains in the 1-2 kcal•mol −1 range, which is acceptable in our context and comparable to PCM values. Such errors have also been proposed to originate from missing polarization and charge-transfer effects at the MM level 29. These computations also demonstrate that the computational setup is reasonable, i.e., (a) a sound combination of atomic charges and Lennard-Jones parameters and (b) that the direct space computation of the Coulomb interaction does not introduce major inaccuracies or inconsistencies.Since MM naturally includes the size of the solvent molecules, we can expect that the coverage-dependence of adsorption energies differs between PCM and MMSolv.…”

“…Independently and in parallel to our work, Getman and co-workers developed a similar approach to asses adsorption free energies of CO and sugars on Pt (111) and compared it with implicit solvation and ice-like structures optimisations. 28,29 The accuracy of any kind of QM/MM heavily relies on the quality of the forcefield used to sample the phase-space of the solid/liquid interface. Even though several forcefields enjoy great popularity, [30][31][32][33] it is only recently that we have introduced GAL17, the first qualitatively correct force field for the interaction between a water molecule and a Pt(111) surface.…”

Dependency of solvation effects on metal identity in surface reactions

“…To confirm this hypothesis, we have investigated the aqueous-phase effects on the initial C-H and O-H bond cleavages of EG over the (111) facet of six transition metal surfaces (Ni, Pd, Pt, Cu, Ag, Au) using our explicit solvation method, eSMS. The choice of reaction systems is motivated by (i) EG being a commonly studied surrogate molecule of biomassderived polyols, (ii) the selected transition metals are relatively stable and commonly used for aqueous-phase processing of biomass-derived oxygenates 61 , (iii) early dehydrogenation steps of EG over Pt and Ni/Pt catalysts have previously been found to control the overall reaction rate [62][63][64] , (iv) at least over Pt (111) in the vapor phase, initial C-H and O-H bond cleavage are competitive (although O-H bond cleavage is believed to be somewhat favored) 34,64 , and finally (v), explicit solvation approaches have recently been used to demonstrate that for bond cleavage reactions of alcohols over Pt (111), aqueous solvation effects are large and can currently not be described by implicit solvation models 55,65 . Figure 1 illustrates the aqueous-phase effects on the activation free energy barrier of the O-H (CH 2 OHCH 2 OH** + * ↔ CH 2 OCH 2 OH** + H*) and C-H bond cleavages (CH 2 OH-CH 2 OH** + * ↔ CHOHCH 2 OH ** + H*) of EG at 423 K computed by eSMS (see Table 1 for specific numbers).…”

Solvation Free Energies and Adsorption Energies at the Metal/Water Interface from Hybrid QM-MM Simulations

“…A similar approach was used later by Getman and co-workers to asses adsorption free energies of CO and sugars on Pt (111) and compare it with implicit solvation and ice-like structures optimisations. 28,29 The accuracy of any kind of QM/MM heavily relies on the quality of the forcefield used to sample the phase-space of the solid/liquid interface. Even though several forcefields enjoy great popularity, [30][31][32][33] it is only recently that we have introduced GAL17, the first qualitatively correct force field for the interaction between a water molecule and a Pt(111) surface.…”