115,844 research outputs found
Thermal conductance and thermoelectric figure of merit of C-based single-molecule junctions: electrons, phonons, and photons
Motivated by recent experiments, we present here an ab initio study of the
impact of the phonon transport on the thermal conductance and thermoelectric
figure of merit of C-based single-molecule junctions. To be precise, we
combine density functional theory with nonequilibrium Green's function
techniques to compute these two quantities in junctions with either a C
monomer or a C dimer connected to gold electrodes, taking into account
the contributions of both electrons and phonons. Our results show that for
C monomer junctions phonon transport plays a minor role in the thermal
conductance and, in turn, in the figure of merit, which can reach relatively
high values on the order of 0.1, depending on the contact geometry. At the
contrary, phonons completely dominate the thermal conductance in C dimer
junctions and strongly reduce the figure of merit as compared to monomer
junctions. Thus, claims that by stacking C molecules one could achieve
high thermoelectric performance, which have been made without considering the
phonon contribution, are not justified. Moreover, we analyze the relevance of
near-field thermal radiation for the figure of merit of these junctions within
the framework of fluctuational electrodynamics. We conclude that photon
tunneling can be another detrimental factor for the thermoelectric performance,
which has been overlooked so far in the field of molecular electronics. Our
study illustrates the crucial roles that phonon transport and photon tunneling
can play when critically assessing the performance of molecular junctions as
potential nanoscale thermoelectric devices
Determination of the nature of the Cu coordination complexes formed in the presence of NO and NH3 within SSZ-13
Ammonia-selective catalytic reduction (NH3-SCR) using Cu zeolites is a well-established strategy for the abatement of NOx gases. Recent studies have demonstrated that Cu is particularly active when exchanged into the SSZ-13 zeolite, and its location in either the 6r or 8r renders it an excellent model system for fundamental studies. In this work, we examine the interaction of NH3-SCR relevant gases (NO and NH3) with the Cu2+ centers within the SSZ-13 structure, coupling powder diffraction (PD), X-ray absorption spectroscopy (XAFS), and density functional theory (DFT). This combined approach revealed that, upon calcination, cooling and gas exposure Cu ions tend to locate in the 8r window. After NO introduction, Cu-ions are seen to coordinate to two framework oxygens and one NO molecule, resulting in a bent Cu-nitrosyl complex with a Cu-N-O bond angle of similar to 150 degrees. Whilst Cu seems to be partially reduced/changed in coordination state, NO is partially oxidized. On exposure to NH3 while the PD data suggest the Cu2+ ion occupies a similar position, simulation and XAFS pointed toward the formation of a Jahn-Teller distorted hexaamine complex [Cu(NH3)(6)](2+) in the center of the cha cage. These results have important implications in terms of uptake and storage of these reactive gases and potentially for the mechanisms involved in the NH3-SCR process
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