Kamil Czelej

Insight on the Interaction of Methanol-Selective Oxidation Intermediates with Au- or/and Pd-Containing Monometallic and Bimetallic Core@Shell Catalysts.

Using density functional theory (DFT), the interaction of crucial molecules involved in the selective partial oxidation of methanol to methyl formate (MF) with monometallic Au and Pd and bimetallic Au/Pd and Pd/Au core@shell catalysts is systematically investigated. The core@shell structures modeled in this study consist of Au(111) and Pd(111) cores covered by a monolayer of Pd and Au, respectively. Our results indicate that the adsorption strength of the molecules examined as a function of catalytic surface decreases in the order of Au/Pd(111) > Pd(111) > Au(111) > Pd/Au(111) and correlates well with the d-band center model. The preadsorption of oxygen is found to have a positive impact on the selective partial oxidation reaction because of the stabilization of CH3OH and HCHO on the catalyst surface and the simultaneous intensification of MF desorption. On the basis of a dynamical matrix approach combined with statistical thermodynamics, we propose a simple route for evaluating the Gibbs free energy of adsorption as a function of temperature. This method allows us to anticipate the relative temperature stability of molecules involved in the selective partial oxidation of methanol to MF in terms of catalytic surface.

Toward a Comprehensive Understanding of Enhanced Photocatalytic Activity of the Bimetallic PdAu/TiO2 Catalyst for Selective Oxidation of Methanol to Methyl Formate

Photocatalytic selective oxidation of alcohols over titania supported with bimetallic nanoparticles represents an energy efficient and sustainable route for the synthesis of esters. Specifically, the bimetallic PdAu/TiO2 system was found to be highly active and selective toward photocatalytic production of methyl formate (MF) from gas-phase methanol. In the current paper, we applied the electronic structure density functional theory method to understand the mechanistic aspects and corroborate our recent experimental measurements for the photocatalytic selective oxidation of methanol to MF over the PdAu/TiO2 catalyst. Our theoretical results revealed the preferential segregation of Pd atoms from initially mixed PdAu nanoclusters to the interface of PdAu/TiO2 and subsequent formation of a unique structure, resembling a core@shell architecture in close proximity to the interface. The analysis of the calculated band gap diagram provides an explanation of the superior electron-hole separation capability of PdAu nanoparticles deposited onto the anatase surface and hence the remarkably enhanced photocatalytic activity, in comparison to their monometallic counterparts. We demonstrated that facile dissociation of molecular oxygen at the triple-point boundary site gives rise to in situ oxidation of Pd. The in situ formed PdO/TiO2 is responsible for total oxidation of methanol to CO2 (no MF formation) in the gas phase. Our investigation provides theoretical guidance for designing highly selective and active bimetallic nanoparticles-TiO2 catalysts for the photocatalytic selective oxidation of methanol to MF.

Atomistic insight into the electrode reaction mechanism of the cathode in molten carbonate fuel cells

In an era of increasing energy demand challenges combined with simultaneous environmental protection, molten carbonate fuel cells (MCFCs) have emerged as an interesting candidate to overcome both of these issues. Although the macroscopic parameters of MCFCs have been successfully optimized, the microscopic understanding of the electrochemical catalytic reactions, which determine their performance, remains challenging due to their chemical complexity and high operation temperatures. In this paper, we propose a top-down approach to unravel the hitherto unreported electrode reaction mechanism of the cathode in MCFCs using density functional theory (DFT). The oxygen-terminated octopolar NiO(111) is predicted to facilitate cathodic transformation of carbon dioxide to carbonate anions through sequential Mars-van Krevelen (MvK) and Eley-Rideal (ER) mechanisms. This theoretical work opens up new prospects in the atomic scale computational design of the cathode material for MCFCs.

Titanium-related color centers in diamond: a density functional theory prediction

Transition metal-related paramagnetic centers in diamond exhibiting bright photoluminescence are increasingly important defects for realizing high quality solid state single photon sources. Recently, advanced ab initio calculations of single nickel-related NE4 (nickel-vacancy) and NE8 (nickel-vacancy-nitrogen) complexes in nanodiamond provided an insight into the nature of optical transitions and demonstrated their potential for in vivo biomarker applications. For other transition metal-related defects in diamond, however, a comprehensive understanding of photoluminescence is rather scarce. Here we used first principles, hybrid density functional theory analysis to investigate the electronic structure and magneto-optical properties of titanium-related point defects in diamond. Our theoretical results including the paramagnetic S = 1/2 ground state, the calculated zero-phonon lines, quasi-local vibrational modes associated with Ti atoms, and hyperfine coupling parameters provide strong evidence that the neutral Ti–N and TiV–N complexes are indeed the experimentally observed N3 (titanium–nitrogen) and OK1 (titanium-vacancy–nitrogen) color centers. In addition, we predicted another low energy excitation in the spin minority channel of the TiV–N0 defect that needs further experimental verification and might be an interesting candidate for a robust solid state single color emitter in the near IR region. In the case of a yet unobserved, neutral TiV (titanium-vacancy) defect we found a high symmetry D3d configuration in the triplet 3Eu ground state and we calculated the magneto-optical parameters to mediate its future identification. We emphasize the possibility of the dynamic Jahn–Teller effect for some centers and its impact on the experimentally observed hyperfine structure.