CO adsorption on PdGa(1 0 0), (1 1 1) and (1^¯1^¯1^¯) surfaces: A DFT study

Graphical abstract: - Highlights: • CO adsorption is top on Pd sites for all surfaces. • Our results agree with TDS peak at 210 K for PdGa (1^¯1^¯1^¯) attributed to CO-Pd bond. • Pd-CO bond is formed at expenses of metal-metal bond. No Ga-CO interaction is found. • A back-donation for all surfaces was detected. After adsorption Pd PDOS shift towards lower energies. • IR frequencies for the C-O adsorbed presents a red shift compared to gas phase. - Abstract: CO adsorption on (1 0 0), (1 1 1) and (1^¯1^¯1^¯) planes is analyzed using density functional theory (DFT) calculations. Changes in the electronic structure of these surfaces and CO bond after adsorption are also addressed here. CO is located on Pd atop geometry with a tilted configuration of 7.8° in the (1 0 0) plane, while in the (1 1 1) and (1^¯1^¯1^¯) are perpendicular to the surface. No direct interaction of CO with Ga is detected. The overlap population (OP) of Pd-Pd and Pd-Ga bond decreases as the new Pd-CO bond is formed. In all cases, the C-O bond length changes less than 1% compared to the vacuum but its strength decrease about 50% as determined by the changes in the OP. The effect of CO is limited to its first Pd neighbor. Analysis of orbital interaction reveals that Pd-CO bond mainly involves s-s and s-p orbitals with less participation of Pd 4d orbitals. Computed CO vibration frequencies after adsorption shows a red shift from vacuum towards 1972.9, 1990.4 and 1988.6 cm⁻¹ on (1 0 0), (1 1 1) and (1^¯1^¯1^¯) planes respectively, following the same trend that experimental data on the PdGa intermetallic compound