Probing Cu(I)-exchanged zeolite with CO: DFT modeling and experiment.

Addition of CO on Cu-exchanged zeolite was investigated by means of quantum chemical calculations based on density functional theory. The aim of this investigation was to get insights about changes of electronic properties of a copper site with zeolite composition by using a CO probe molecule. Calculated nu(CO) frequency values show that various Si/Al ratios of faujasite zeolite reproduce the expected experimental decrease of the nu(CO) values with decreasing Si/Al ratio. These calculations predict that H/Na ratio variations also induce changes in the nu(CO) values. These results illustrate that different compositions of the zeolite change the electronic properties of copper that are reflected in the nu(CO) frequency values. DFT results showed also that different structures and CO adsorption energies are obtained due to various Si/Al and H/Na ratios of the zeolite. Finally, these calculations evidence the possibility for CO to be connected at the same time to Cu(I) and to a close Na cation, Cu being at site II and Na at site II in Cu(I)-exchanged faujasite. A DRIFT experiment on two samples of faujasite, Cu(28)H(51)NaY and Cu(25)H(0)NaY, supports nu(CO) displacements to higher energy values with increasing H/Na ratio.

Selective catalytic reduction of NO by NH3 on Cu-faujasite catalysts: an experimental and quantum chemical approach.

The selective catalytic reduction (SCR) of NO by NH3 in the presence of O2 on Cu-faujasite (Cu-FAU) has been studied. Substitution of some Cu2+ with H+ and Na+ cations, compensating for the negative charge of the zeolite framework, forms the various CuHNa-FAU studied. The amount of Cu was held constant and the proportion of H+ and Na+ varied in the sample. The substitution of Na+ for H+ increases sharply the SCR rate by lowering the temperature of reaction by about 150 K. It is proposed that the rate increase mainly comes from an unhindered migration of Cu from hidden to active sites and a modification of the redox properties of Cu species. The former was demonstrated by diffuse reflectance IR spectroscopy of adsorbed CO. The change in redox properties was demonstrated by a faster oxidation of Cu+ to Cu2+ (rate-determining step). Quantum chemical calculations on model clusters of CuHNa-FAU indicate that the faster rate of oxidation can be explained by a higher lability of protons in the absence of Na, which can be then removed from the catalyst more easily to yield H2O during the oxidation process.