Effect of protonation and deprotonation on surface charge density of Nb2O5 A thermodynamic approach

The protonation and deprotonation of the Nb2O5 surface has been followed in order to understand the reactions of surface of this catalyst. The simultaneous potentiometric and conductometric titrations had been carried by using 50 mL of water suspension of Nb2O5 40 g L-1. The oxide was entirely deprotonated when adding 0.4 mL NaOH 1 mol L-1, and later titrated with 0.1 mol L-1. The titration had supplied K (1) and K (2) and the obtained values were 3.24 x 10(-3) and 4.17 x 10(-8), respectively. The zero point charge was pH(pcz) = 4.94. The thermodynamic studies were carried out by using 50 mL of a 40 g/L Nb2O5 aqueous suspension with the pH adjusted to pH(PZC) value. The suspension was titrated with 0.5 mol/L of HNO3 or NaOH for protonation or deprotonation studies, respectively, in an isoperibol calorimeter CSC ISC-4300. Thus, the obtained thermodynamic values of the protonation and deprotonation of Nb2O5 were Delta(dp) G = -37.60 kJ/mol, Delta(dp) H = -23.72 kJ/mol and Delta S-dp = 47 J/(mol K).