The reactivity of various Pd ensembles on the Au—Pd(100) alloy catalyst toward CO oxidation was investigated by using density functional theory (DFT). This study was prompted by the search for efficient catalysts operating at low temperature for the CO oxidation reaction that is of primary environmental importance. To this aim, we considered Pd modified Au(100) surfaces including Pd monomers, Pd dimers, second neighboring Pd atoms, and Pd chains in a comparative study of the minimum energy reaction pathways. The effect of dispersion interactions was included in the calculations of the O₂ dissociation reaction pathway by using the DFT-D3 scheme. The addition of the dispersion interaction strongly improves the adsorption ability of O₂ on the Au—Pd surface but does not affect the activation energy barriers of the Transitions States (TSs). As for O₂ to dissociate, it is imperative that the TS has lower activation energy than the O₂ desorption energy. DFT-D3 is found to favor, in some cases, O₂ dissociation on configurations being identified from uncorrected DFT calculations as inactive. This is the case of the second neighboring Pd configuration for which uncorrected DFT predicts positive Gibbs free energy (ΔG) of the O₂ adsorption, therefore an endergonic reaction. With the addition of D3 correction, ΔG becomes negative that reveals a spontaneous O₂ adsorption. Among the investigated Au—Pd (100) ensembles, the Pd chain dissociates most easily O₂ and highly stabilizes the dissociated O atoms; however, it has an inferior reactivity toward CO oxidation and CO₂ formation. Indeed, CO strongly adsorbs on the palladium bridge sites and therefore poisoning the surface Pd chain. By contrast, the second neighboring Pd configuration that shows somewhat lower ability to dissociate O₂ turns out to be more reactive in the CO₂ formation step. These results evidence the complex effect of Pd ensembles on the CO oxidation reaction. Associative CO oxidation proceeds with high energy barriers on all the considered Pd ensembles and should be excluded, in agreement with experimental observations.

In this paper, the size-dependent changes in energetic, vibrational, and electronic properties of C–O gas molecule interacting with surface Pd atom of a variety of AuPd nanoalloy structures are investigated by means of first principles calculations. The variation in C–O adsorption energies, C–O vibration frequencies (ν_C−O), and Pd d-bond centers (ε_d) on a series of non-supported Au_n−1–Pd₁ nanoparticles (with n varying from 13 to 147) and on two semi-finite surfaces are inspected with cluster size. We demonstrate for the first time that, with small AuPd bimetallic three-dimensional clusters as TOh₃₈, one can reach cluster size convergence even for such a sensitive observable as the adsorption energy on a metal surface. Indeed, the results show that the adsorbate-induced perturbation is extremely local and it only concerns the isolated Pd interacting with the reactive gas molecule. Except for 13 atom clusters, in which molecular behaviour is predominant, no finite-size effects are observed for surface Pd atom substituted in AuPd free nanoclusters above 38 atoms.

Iridium (Ir) modified Silicon (Si) (001) surface is studied with Scanning Tunneling Microscopy/Spectroscopy (STM/STS) and Density Functional Theory (DFT). A model for Ir-silicide nanowires based on STM images and ab-initio calculations is proposed. According to our model, the Ir adatom is on the top of the substrate dimer row and directly binds to the dimer atoms. I-V curves measured at 77 K shows that the nanowires are metallic. DFT calculations confirm strong metallic nature of the nanowires.

Understanding the spin distribution in FeN₄-doped graphene nanoribbons with zigzag and armchair terminations is crucial for tuning the electronic properties of graphene-supported non-platinum catalysts. Since the spin-polarized carbon and iron electronic states may act together to change the electronic properties of the doped graphene, we provide in this work a systematic evaluation using a periodic density-functional theory-based method of the variation of spin-moment distribution and electronic properties with the position and orientation of the FeN₄ defects, and the edge terminations of the graphene nanoribbons. Antiferromagnetic and ferromagnetic spin ordering of the zigzag edges were considered. We reveal that the electronic structures in both zigzag and armchair geometries are very sensitive to the location of FeN₄ defects, changing from semi-conducting (in-plane defect location) to half-metallic (at-edge defect location). The introduction of FeN₄ defects at edge positions cancels the known dependence of the magnetic and electronic proper-ties of undoped graphene nanoribbons on their edge geometries. The implications of the reported results for catalysis are also discussed in view of the presented electronic and magnetic properties.