The effect of octahedral distortions on the electronic properties and magnetic interactions in O3 NaTMO2 compounds (TM = Ti–Ni & Zr–Pd)

Feeble Single-Atom Pd Catalysts for H2 Production from Formic Acid

Single-atom catalysts are thought to be the pinnacle of catalysis. However, for many reactions, their suitability has yet to be unequivocally proven. Here, we demonstrate why single Pd atoms (PdSA) are not catalytically ideal for generating H2 from formic acid as a H2 carrier. We loaded PdSA on three silica substrates, mesoporous silicas functionalized with thiol, amine, and dithiocarbamate functional groups. The Pd catalytic activity on amino-functionalized silica (SiO2-NH2/PdSA) was far higher than that of the thiol-based catalysts (SiO2-S-PdSA and SiO2-NHCS2-PdSA), while the single-atom stability of SiO2-NH2/PdSA against aggregation after the first catalytic cycle was the weakest. In this case, Pd aggregation boosted the reaction yield. Our experiments and calculations demonstrate that PdSA in SiO2-NH2/PdSA loosely binds with amine groups. This leads to a limited charge transfer from Pd to the amine groups and causes high aggregability and catalytic activity. According to the density functional calculations, the loose binding between Pd and N causes most of Pd's 4d electrons in amino-functionalized SiO2 to remain close to the Fermi level and labile for catalysis. However, PdSA chemically binds to the thiol group, resulting in strong hybridization between Pd and S, pulling Pd's 4d states deeper into the conduction band and away from the Fermi level. Consequently, fewer 4d electrons were available for catalysis.

NaMoO2: a Layered Oxide with Molybdenum Clusters

NaMoO₂ was synthesized as a layered oxide from the reaction between the layered oxide Na_2/3MoO₂ and metal sodium. Its structure was determined from high-resolution powder X-ray diffraction, and it can be described as an α-NaFeO₂ distorted structure in which sodium ions and molybdenum atoms occupy octahedral interstitial sites. Chains of "diamond-like" clusters of molybdenum were evidenced in the [MoO₂] layers resulting from the Peierls distortion expected in a two-dimensional triangular lattice formed by transition metal atoms with a d³ electronic configuration. Molybdenum-molybdenum distances as short as 2.58 Å were found in these clusters. The magnetic moment recorded at low temperatures and at room temperature showed that NaMoO₂ presents a very low magnetic susceptibility compatible with the localization of the 4d electrons in the Mo-Mo bonds. This localization was confirmed by DFT calculation that showed the NaMoO₂ was diamagnetic at 0 K. A sodium battery was built using NaMoO₂ as the positive electrode material, and we found that sodium ions can be reversibly deintercalated and intercalated in NaMoO₂, indicating that this compound is one of the many phases existing in the Na_xMoO₂ system.