SIZE AND SHAPE DEPENDANCY OF CO ADSORPTION ON Pd14 AND Pd55 NANOCLUSTERS: DFT AND FTIR STUDIES
Palladium is a very effective catalyst for the oxidation of CO. This reaction is very important for reducing automotive and industrial emissions and considered to be a good model for fundamental studies of transition metal catalysts. It is known that different types of facets as well as size and shape of a metal nanoparticle have an effect on catalyst properties. The scope of our work is to investigate how the reactivity of Pd nanoparticles towards CO oxidation depends on their size and shape. The keys to the solution are ab-initio DFT calculations in combination with in-situ FTIR spectroscopy. For theoretical part, we have chosen the set of Pd clusters of different size and respective shape. After optimization of their geometry, the adsorbed CO molecules were relaxed at the top-on positions of cluster surface. That allowed us to generate IR spectra to be compared with experimental ones of CO adsorbed on Pd nanoparticles. In order to be closer to the industrial-type processes, a special setup allowing in-situ FTIR measurements was designed as well.
In our theoretical studies we used both molecular orbital (ADF-2014) and periodic band structure (VASP 5.2) DFT calculation schemes to model small metallic Pd clusters. The basis in calculations was chosen as Triple Zeta with one polarization function and frozen core. We used standard GGA approximation within PBE scheme. Occupations steepest descend method was used for better geometry convergence. All the parameters mentioned above with Scalable SCF gave good convergence in all criteria. The cutoff energy for periodic plane wave pseudopotential simulation was set to 400eV. The cell around Palladium clusters was constructed as a cubic one with 30 nm edge. A single K-point was used in simulation. In this work we constructed and optimized geometry of Pd14 and Pd55 clusters with octahedral symmetry. The relaxed interatomic distances (Angstom) of Palladium 55 cluster with CO molecules placed on top hollow sites of Pd obtained by both ADF and VASP DFT calculation schemes are presented in Table 1.
Table 1. The relaxed interatomic distances (Å) of Pd-55 cluster with CO
ADF VASP 5.2 Pd-C 2.052 2.042 C-O 1.198 1.200
IR spectra for different configurations were obtained during the calculation. For example main peaks are at 2107 and 503 cm-1. Both CO adsorption and oxidation on Pd nanoparticles have been studied extensively with different experimental techniques [R1]. It is assumed that reaction of CO oxidation follows a Langmuir-Hinshelwood mechanism [R2], where both reactants, CO and O2, adsorb on the catalyst surface prior to reaction between the adsorbed species to form CO2. There were also found some differences in the kinetics among the (111), (110), and (100) planes in studies on single crystals of Pd [R3]. Significant number of experimental studies of CO adsorption on surface of Pd have been performed by using Diffuse Reflectance Infrared Fourier Transform Spectroscopy (DRIFTS). DRIFTS is a useful technique for probing catalysts at catalytically relevant temperatures and pressures without intense sample preparation [R4]. Our experimental setup for in-situ DRIFT studies of CO adsorption on Pd nanoparticles as main components includes Vertex 70 FTIR spectrometer, Praying Mantis Diffuse Reflection Accessory equipped with Low and High Temperature Reaction Chambers and a two-channel gas mixing system assembled by using the Swagelok tube fittings.
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