Complex Magnetic Order in Topochemically Reduced Rh(I)/Rh(III) LaM_0.5Rh_0.5O_2.25 (M = Co, Ni) Phases

Topochemical reduction of the cation-disordered perovskite oxides LaCo0.5Rh0.5O3 and LaNi0.5Rh0.5O3 with Zr yields the partially anion-vacancy ordered phases LaCo0.5Rh0.5O2.25 and LaNi0.5Rh0.5O2.25, respectively. Neutron diffraction and Hard X-ray photoelectron spectroscopy (HAXPES) measurements reveal that the anion-deficient phases contain Co1+/Ni1+ and a 1:1 mixture of Rh1+ and Rh3+ cations within a disordered array of apex-linked MO4 square-planar and MO5 square-based pyramidal coordination sites. Neutron diffraction data indicate that LaCo0.5Rh0.5O2.25 adopts a complex antiferromagnetic ground state, which is the sum of a C-type ordering (mM5+) of the xy-components of the Co spins and a G-type ordering (mΓ1+) of the z-components of the Co spins. On warming above 75 K, the magnitude of the mΓ1+ component declines, attaining a zero value by 125 K, with the magnitude of the mM5+ component remaining unchanged up to 175 K. This magnetic behavior is rationalized on the basis of the differing d-orbital fillings of the Co1+ cations in MO4 square-planar and MO5 square-based pyramidal coordination sites. LaNi0.5Rh0.5O2.25 shows no sign of long-range magnetic order at 2 K – behavior that can also be explained on the basis of the d-orbital occupation of the Ni1+ centers

Pd₂Ga-Based Colloids as Highly Active Catalysts for the Hydrogenation of CO₂ to Methanol

Colloidal Pd₂Ga-based catalysts are shown to catalyze efficiently the hydrogenation of CO₂ to methanol. The catalysts are produced by the simple thermal decomposition of Pd­(II) acetate in the presence of Ga­(III) stearate, which leads to Pd⁰ nanoparticles (ca. 3 nm), and the subsequent Pd-mediated reduction of Ga­(III) species at temperatures ranging from 210 to 290 °C. The resulting colloidal Pd₂Ga-based catalysts are applied in the liquid-phase hydrogenation of carbon dioxide to methanol at high pressure (50 bar). The intrinsic activity is around 2-fold higher than that obtained for the commercial Cu-ZnO-Al₂O₃ (60.3 and 37.2 × 10^–9 mol_MeOH m^–2 s^–1), respectively, and 4-fold higher on a Cu or Pd molar basis (3330 and 910 μmol mmol_Pd or Cu^–1 h^–1). Detailed characterization data (HR-TEM, STEM/EDX, XPS, and XRD) indicate that the catalyst contains Pd₂Ga nanoparticles, of average diameters 5–6 nm, associated with a network of amorphous Ga₂O₃ species. The proportion of this Ga₂O₃ phase can be easily tuned by adjusting the molar ratio of the Pd:Ga precursors. A good correlation was found between the intrinsic activity and the content of Ga₂O₃ surrounding the Pd₂Ga nanoparticles (XPS), suggesting that methanol is formed by a bifunctional mechanism involving both phases. The increase in the reaction temperature (190–240 °C) leads to a gradual decrease in methanol selectivity from 60 to 40%, while an optimum methanol production rate was found at 210 °C. Interestingly, unlike the conventional Cu-ZnO-Al₂O₃, which experienced approximately 50% activity loss over 25 h time on stream, the Pd₂Ga-based catalysts maintain activity over this time frame. Indeed, characterization of the Pd/Ga mixture postcatalysis revealed no ripening of the nanoparticles or changes in the phases initially present

Exploring the Potential of Nitride and Carbonitride MAX Phases: Synthesis, Magnetic and Electrical Transport Properties of V₂GeC, V₂GeC_0.5N_0.5, and V₂GeN

The chemical composition variety of MAX phases is rapidly evolving in many different directions, especially with the synthesis of carbides that contain two or more metals on the M-site of these layered solids. However, nitride and carbonitride MAX phases are still underrepresented, and only a few members have been reported that are for the most part barely characterized, particularly in terms of magnetic and electronic properties. Here, we demonstrate a simple and effective synthesis route, as well as a comprehensive characterization of three MAX phases, (i) V2GeC, (ii) the hitherto unknown carbonitride V2GeC0.5N0.5, and (iii) the almost unexplored nitride V2GeN. By combining a microwave-assisted precursor synthesis with conventional heat treatment and densification by spark plasma sintering, almost phase-pure (carbo)nitride products are obtained. Magnetic measurements reveal an antiferromagnetic-paramagnetic-like phase transition for all samples in the temperature range of 160–200 K. In addition, increasing the amount of nitrogen on the X-site of the MAX phase structure leads to a constant increase in the magnetic susceptibilities while the electrical resistivity is constantly decreasing. Overall, these findings provide crucial insights into how to tune the electronic and magnetic properties of MAX phases by only varying the chemical composition of the X-site. This further substantiates the demand for (carbo)nitride research with the potential to be extended to the remaining elemental sites within the MAX phase structure to push toward controlled material design and to achieve desired functional properties, such as ferromagnetism

Reversible Redox Cycling of Well-Defined, Ultrasmall Cu/Cu₂O Nanoparticles

Exceptionally small and well-defined copper (Cu) and cuprite (Cu₂O) nanoparticles (NPs) are synthesized by the reaction of mesitylcopper­(I) with either H₂ or air, respectively. In the presence of substoichiometric quantities of ligands, namely, stearic or di­(octyl)­phosphinic acid (0.1–0.2 equiv vs Cu), ultrasmall nanoparticles are prepared with diameters as low as ∼2 nm, soluble in a range of solvents. The solutions of Cu NPs undergo quantitative oxidation, on exposure to air, to form Cu₂O NPs. The Cu₂O NPs can be reduced back to Cu(0) NPs using accessible temperatures and low pressures of hydrogen (135 °C, 3 bar H₂). This striking reversible redox cycling of the discrete, solubilized Cu/Cu­(I) colloids was successfully repeated over 10 cycles, representing 19 separate reactions. The ligands influence the evolution of both composition and size of the nanoparticles, during synthesis and redox cycling, as explored in detail using vacuum-transfer aberration-corrected transmission electron microscopy, X-ray photoelectron spectroscopy, and visible spectroscopy