THEORETICAL PREDICTION OF SINGLE-SITE ENTHALPIES OF SURFACE PROTONATION FOR OXIDES AND SILICATES IN WATER - H+(H3O+) CATALYSIS

Abstract

Surface protonation is the most fundamental adsorption process of geochemical interest. Yet remarkably little is known about protonation of mineral surfaces at temperatures greater than 25°C. Experimentally derived standard enthalpies of surface protonation, ΔHr,1^o, ΔHr,2^o, and ΔHr,ZPC^o, correspond to the reactions >SOH+H+=>SOH2+ >SO-+H+=>SOH >SO-+2H+=>SOH2+ respectively, and provide a starting point for evaluating the role of surface protonation in geochemical processes at elevated temperatures. However, the experimental data for oxides do not have a theoretical explanation, and data are completely lacking for silicates other than SiO2. In the present study, the combination of crystal chemical and Born solvation theory provides a theoretical basis for explaining the variation of the enthalpies of protonation of oxides. Experimental values of ΔHr,1^o, ΔHr,2^o, and ΔHr,ZPC^o consistent with the triple layer model can be expressed in terms of the inverse of the dielectric constant (1/#) and the Pauling bond strength per angstrom (s/rM-OH) of each mineral by equations such as ΔHr,ZPC^o=ΔΩr,Z[(1/#)-(T/#)2(##/#T)]-B'Z(s/rM-OH)+H 'Z. The Born solvation coefficient ΔΩr,Z was taken from a prior analysis of surface equilibrium constants. The coefficients BZ' and HZ' were derived by regression of experimental enthalpies for rutile, γ-alumina, magnetite, hematite, and silica. This approach permits widespread prediction of the enthalpies of surface protonation.Predicted standard enthalpies of surface protonation for oxides and silicates extend over the ranges (in kcal.mole-1): ΔHr,1^o ~ -3 to -15; ΔHr,2^o ~ -0.5 to -18; ΔHr,ZPC^o ~ -4 to -33. Minerals with the largest values of s/rM-OH (e.g., quartz and kaolinite) are predicted to have weakly negative enthalpies and a weak temperature dependence for their protonation equilibrium constants. Conversely, minerals with the smallest values of s/rM-OH (e.g., garnets and olivines) should have strong negative enthalpies and a strong temperature dependence for their protonation equilibrium constants.