Acidity of the aqueous rutile TiO2(110) surface from density functional theory based molecular dynamics

Jun Cheng, Michiel Sprik

Research output: Contribution to journalArticle

121 Citations (Scopus)

Abstract

The thermodynamics of protonation and deprotonation of the rutile TiO2(110) water interface is studied using a combination of density functional theory based molecular dynamics (DFTMD) and free energy perturbation methods. Acidity constants are computed from the free energy for chaperone assisted insertion/removal of protons in fully atomistic periodic model systems treating the solid and solvent at the same level of theory. The pK(a) values we find for the two active surface hydroxyl groups on TiO2(110), the bridge OH (Ti2OH+), and terminal H2O adsorbed on a 5-fold Ti site (TiOH2) are -1 and 9, leading to a point of zero proton charge of 4, well within the computational error margin (2 pK(a) units) from the experimental value (4.5-5.5). The computed intrinsic surface acidities have also been used to estimate the dissociation free energy of adsorbed water giving 0.6 eV, suggesting that water dissociation is unlikely on a perfect aqueous TiO2(110) surface. For further analysis, we compare to the predictions of the MultiSlte Complexation (MUSIC) and Solvation, Bond strength, and Electrostatic (SBE) models. The conclusion regarding the MUSIC model is that, while there is good agreement for the acidity of an adsorbed water molecule, the proton affinity of the bridging oxygen obtained in the DFTMD calculation is significantly lower (more than 5 pKa units) than the MUSIC model value. Structural analysis shows that there are significant differences in hydrogen bonding, in particular to a bridging oxygen which is assumed to be stronger in the MUSIC model compared to what we find using DFTMD. Using DFTMD coordination numbers as input for the MUSIC model, however, led to a pK(a) prediction which is inconsistent with the estimates obtained from the DFTMD free energy calculation.

Original languageEnglish
Pages (from-to)880-889
Number of pages10
JournalJournal of Chemical Theory and Computation
Volume6
Issue number3
DOIs
Publication statusPublished - Mar 2010

Keywords

  • protonation equilibrium-constants
  • space gaussian pseudopotentials
  • bond-valence methods
  • PK(A) prediction
  • redox properties
  • water interface
  • music model
  • adsorption
  • charge
  • dependence

Cite this

Acidity of the aqueous rutile TiO2(110) surface from density functional theory based molecular dynamics. / Cheng, Jun; Sprik, Michiel.

In: Journal of Chemical Theory and Computation, Vol. 6, No. 3, 03.2010, p. 880-889.

Research output: Contribution to journalArticle

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abstract = "The thermodynamics of protonation and deprotonation of the rutile TiO2(110) water interface is studied using a combination of density functional theory based molecular dynamics (DFTMD) and free energy perturbation methods. Acidity constants are computed from the free energy for chaperone assisted insertion/removal of protons in fully atomistic periodic model systems treating the solid and solvent at the same level of theory. The pK(a) values we find for the two active surface hydroxyl groups on TiO2(110), the bridge OH (Ti2OH+), and terminal H2O adsorbed on a 5-fold Ti site (TiOH2) are -1 and 9, leading to a point of zero proton charge of 4, well within the computational error margin (2 pK(a) units) from the experimental value (4.5-5.5). The computed intrinsic surface acidities have also been used to estimate the dissociation free energy of adsorbed water giving 0.6 eV, suggesting that water dissociation is unlikely on a perfect aqueous TiO2(110) surface. For further analysis, we compare to the predictions of the MultiSlte Complexation (MUSIC) and Solvation, Bond strength, and Electrostatic (SBE) models. The conclusion regarding the MUSIC model is that, while there is good agreement for the acidity of an adsorbed water molecule, the proton affinity of the bridging oxygen obtained in the DFTMD calculation is significantly lower (more than 5 pKa units) than the MUSIC model value. Structural analysis shows that there are significant differences in hydrogen bonding, in particular to a bridging oxygen which is assumed to be stronger in the MUSIC model compared to what we find using DFTMD. Using DFTMD coordination numbers as input for the MUSIC model, however, led to a pK(a) prediction which is inconsistent with the estimates obtained from the DFTMD free energy calculation.",
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N2 - The thermodynamics of protonation and deprotonation of the rutile TiO2(110) water interface is studied using a combination of density functional theory based molecular dynamics (DFTMD) and free energy perturbation methods. Acidity constants are computed from the free energy for chaperone assisted insertion/removal of protons in fully atomistic periodic model systems treating the solid and solvent at the same level of theory. The pK(a) values we find for the two active surface hydroxyl groups on TiO2(110), the bridge OH (Ti2OH+), and terminal H2O adsorbed on a 5-fold Ti site (TiOH2) are -1 and 9, leading to a point of zero proton charge of 4, well within the computational error margin (2 pK(a) units) from the experimental value (4.5-5.5). The computed intrinsic surface acidities have also been used to estimate the dissociation free energy of adsorbed water giving 0.6 eV, suggesting that water dissociation is unlikely on a perfect aqueous TiO2(110) surface. For further analysis, we compare to the predictions of the MultiSlte Complexation (MUSIC) and Solvation, Bond strength, and Electrostatic (SBE) models. The conclusion regarding the MUSIC model is that, while there is good agreement for the acidity of an adsorbed water molecule, the proton affinity of the bridging oxygen obtained in the DFTMD calculation is significantly lower (more than 5 pKa units) than the MUSIC model value. Structural analysis shows that there are significant differences in hydrogen bonding, in particular to a bridging oxygen which is assumed to be stronger in the MUSIC model compared to what we find using DFTMD. Using DFTMD coordination numbers as input for the MUSIC model, however, led to a pK(a) prediction which is inconsistent with the estimates obtained from the DFTMD free energy calculation.

AB - The thermodynamics of protonation and deprotonation of the rutile TiO2(110) water interface is studied using a combination of density functional theory based molecular dynamics (DFTMD) and free energy perturbation methods. Acidity constants are computed from the free energy for chaperone assisted insertion/removal of protons in fully atomistic periodic model systems treating the solid and solvent at the same level of theory. The pK(a) values we find for the two active surface hydroxyl groups on TiO2(110), the bridge OH (Ti2OH+), and terminal H2O adsorbed on a 5-fold Ti site (TiOH2) are -1 and 9, leading to a point of zero proton charge of 4, well within the computational error margin (2 pK(a) units) from the experimental value (4.5-5.5). The computed intrinsic surface acidities have also been used to estimate the dissociation free energy of adsorbed water giving 0.6 eV, suggesting that water dissociation is unlikely on a perfect aqueous TiO2(110) surface. For further analysis, we compare to the predictions of the MultiSlte Complexation (MUSIC) and Solvation, Bond strength, and Electrostatic (SBE) models. The conclusion regarding the MUSIC model is that, while there is good agreement for the acidity of an adsorbed water molecule, the proton affinity of the bridging oxygen obtained in the DFTMD calculation is significantly lower (more than 5 pKa units) than the MUSIC model value. Structural analysis shows that there are significant differences in hydrogen bonding, in particular to a bridging oxygen which is assumed to be stronger in the MUSIC model compared to what we find using DFTMD. Using DFTMD coordination numbers as input for the MUSIC model, however, led to a pK(a) prediction which is inconsistent with the estimates obtained from the DFTMD free energy calculation.

KW - protonation equilibrium-constants

KW - space gaussian pseudopotentials

KW - bond-valence methods

KW - PK(A) prediction

KW - redox properties

KW - water interface

KW - music model

KW - adsorption

KW - charge

KW - dependence

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DO - 10.1021/ct100013q

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VL - 6

SP - 880

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JO - Journal of Chemical Theory and Computation

JF - Journal of Chemical Theory and Computation

SN - 1549-9618

IS - 3

ER -