Infrared spectra and stability of CO and H2O sorption over Ag-exchanged ZSM-5 zeolite: DFT study

The infrared spectra and stability of CO and H2O sorption over Ag-exchanged ZSM-5 zeolite were investigated by using density function theory (DFT). The changes of NBO charge show that the electron transfers from CO molecule to the Ag+ cation to form an σ-bond, and it accompanies by the back donation of d-electrons from Ag+ cation to the CO (π*) orbital as one and two CO molecules are adsorbed on Ag-ZSM-5. The free energy changes ΔG, −5.55 kcal/mol and 6.52 kcal/mol for one and two CO molecules, illustrate that the Ag+(CO)2 complex is unstable at the room temperature. The vibration frequency of C-O stretching of one CO molecule bonded to Ag+ ion at 2211 cm−1 is in good agreement with the experimental results. The calculated C-O symmetric and antisymmetric stretching frequencies in the Ag+(CO)2 complex shift to 2231 cm−1 and 2205 cm−1 when the second CO molecule is adsorbed. The calculated C-O stretching frequency in CO-Ag-ZSM-5-H2O complex shifts to 2199 cm−1, the symmetric and antisymmetric O-H stretching frequencies are 3390 cm−1 and 3869 cm−1, respectively. The Gibbs free energy change (ΔGH2O) is −6.58 kcal/mol as a H2O molecule is adsorbed on CO-Ag-ZSM-5 complex at 298 K. The results show that CO-Ag-ZSM-5-H2O complex is more stable at room temperature.