Chemisorption of isocyanate (-NCO) on Pd (100) at different coverages

P. G. BELELLI, G. R. GARDA, M. M. BRANDA, R. M. FERULLO, N. J. CASTELLANI

Varna, Bulgaria

Conferencia; 12th International Conference on Theoretical Aspects of Catalysis; 2008

Facultad de Química, Universidad de Sofía

With the purpose of diminishing environmental pollution, the interest to improve the catalytic converter technology to control the automobile exhaust emissions is increasing. In reactions involving NOx reduction, the formation of NCO species was detected on Pt, Pd, Rh, Ir and Ru supported catalysts. The IR spectroscopy has determined that isocyanate forms on metal, linked by the N atom, and then spills over to the support where it stabilizes. On Pd(100), NCO was detected at 160 K by RAIR techniques remaining stable up to 300 K. On this surface, the frequency shifts for different monolayer coverages on Pd(100) surface were analyzed (Németh et al., J. Phys. Chem. C 111 (2007) 1424). In this work we have studied, using a periodic DFT method, the NCO adsorption on top, bridge and hollow sites on Pd(100) at different coverages (q=0.11, 0.25, 0.5 and 1) by changing the unit cell sizes and the number of adsorbed NCO. The calculations have been performed using the VASP code. The adsorption energies for low coverage, near -3 eV, denote an important adsorbate substrate interaction for the three sites studied. The lateral repulsive interaction between neighbouring NCO species is very strong at complete coverage, weak at q=0.50 and almost neglected at low coverages. NCO adsorbs preferentially on bridge site at both high (q=1) and low (q=0.11) coverages, although for the second case the hollow site is only slightly less stable. At q=0.25, the NCO asymmetric stretching mode are 2280, 2229 and 2195 cm-1 for top, bridge and hollow sites, respectively. Different combinations involving two adsorbed NCO at q=0.5 were obtained. From these, the most stable superstructure was 2´Ö2 R45° (on bridge sites). In this case the vibrational frequencies corresponding to the two adsorbed isocyanate species by unit cell differ about 100 cm-1 corresponding to the complete coupled and uncoupled modes. A band width of this order was observed in IR spectra (Németh et al., 2007). The electronic structure of NCO/metal bond was analyzed using DOS curves. The N atom bands become less stable in relation to the free radical as the coordination number with the metal surface is lower, thus becoming more evident for the ?NCO top.