Charged FeCn clusters: A comparison with (TM=Sc, Ti, V, Co and Zn, n=1–8) systems

Probing electronic and vibrational structures of TaCn-/0 (n = 2-4) using high-resolution photoelectron spectroscopy and theoretical calculations

We report the high-resolution photoelectron spectroscopy of transition metal carbide cluster anions TaCn- (n = 2-4) using a cryogenic ion trap combined with the slow electron velocity imaging (cryo-SEVI) technique. From the vibrationally resolved photoelectron spectra and associated ab initio calculations, the electron affinities of TaCn (n = 2-4) were determined with high precision: 1.818(2), 2.202(5), and 2.431(2) eV, respectively. The electronic and vibrational structures observed in the photoelectron spectra were interpreted using density-functional theory and coupled-cluster singles and doubles with perturbative triples calculations. Both the neutral TaCn clusters and their anions exhibit planar C2v structures, where the Ta atom bridges each C atom. Furthermore, we observed the spin-orbit splitting in the ground state of TaC2 (X̃4B1), with a measured splitting of 256(25) cm-1. This splitting is well explained by the calculated E1/2(±3/2)-E1/2(±1/2) splitting of 216 cm-1, obtained using the MRCI+SOC method.

Formation of Acetylene in the Reaction of Methane with Iron Carbide Cluster Anions FeC3- under High-Temperature Conditions

The underlying mechanism for non-oxidative methane aromatization remains controversial owing to the lack of experimental evidence for the formation of the first C-C bond. For the first time, the elementary reaction of methane with atomic clusters (FeC₃^- ) under high-temperature conditions to produce C-C coupling products has been characterized by mass spectrometry. With the elevation of temperature from 300 K to 610 K, the production of acetylene, the important intermediate proposed in a monofunctional mechanism of methane aromatization, was significantly enhanced, which can be well-rationalized by quantum chemistry calculations. This study narrows the gap between gas-phase and condensed-phase studies on methane conversion and suggests that the monofunctional mechanism probably operates in non-oxidative methane aromatization.

How far away are iron carbide clusters from the bulk?

Combining the basin hopping structure searching algorithm and density functional theory, the iron carbide clusters, Fe_xC_y (x ≤ 8 and y ≤ 8), and clusters with various stoichiometries (Fe_2nC_n, Fe_3nC_n, Fe_nC_2n, Fe_nC_3n and Fe_nC_4n (n = 1-7), Fe_5nC_2n, and Fe_4nC_n (n = 1-5)) are predicted. The stable structures of iron rich carbide clusters are composed of C-C dimers or single C atoms on the surface of the clusters, which are remarkably different from their corresponding bulk structures, where the carbon atoms are atomically distributed in the iron matrix. The most stable carbon rich clusters are highly diverse in topology (bowl, basket, plane, shoe, necklace, etc.) with long carbon chains. The Bader charge analysis shows that the size effect on iron carbide clusters is an electronic tuning. Large carbon-rich clusters appear even under low carbon chemical potentials, whereas small iron-rich clusters are only energetically stable in high carbon chemical potentials, which indicates that changing the carbon chemical potential can tune the morphology (size and stoichiometry) of the iron carbide clusters. These results may help us understand the catalytic activity of iron and iron carbides in reactions such as the Fischer-Tropsch synthesis and the carbon nanotube formation process.

Methane Activation by Iron-Carbide Cluster Anions FeC6(-)

Laser-ablation-generated and mass-selected iron-carbide cluster anions FeC6(-) were reacted with CH4 in a linear ion trap reactor under thermal collision conditions. The reactions were characterized by mass spectrometry and density functional theory calculations. Adsorption product of FeC6CH4(-) was observed in the experiments. The identified large kinetic isotope effect suggests that CH4 can be activated by FeC6(-) anions with a dissociative adsorption manner, which is further supported by the reaction mechanism calculations. The large dipole moment of FeC6(-) (19.21 D) can induce a polarization of CH4 and can facilitate the cleavage of C-H bond. This study reports the CH4 activation by transition-metal carbide anions, which provides insights into mechanistic understanding of iron-carbon centers that are important for condensed-phase catalysis.