Volcano-Curves for Dehydrogenation of 2‑Propanol and Hydrogenation of Nitrobenzene by SiO₂‑Supported Metal Nanoparticles Catalysts As Described in Terms of a d‑Band Model

To confirm whether the activity trends in multistep organic reactions can be understood in terms of the Hammer–Nørskov d-band model in combination with the linear energy relations, we studied correlations between the reaction rates for dehydrogenation and hydrogenation reactions and the position of the d-band center (ε_d) relative to the Fermi energy (<i>E</i>_F), the ε_d – <i>E</i>_F value, of various metal catalysts. SiO₂-supported metal (M = Ag, Cu, Pt, Ir, Pd, Rh, Ru, Ni, and Co) catalysts with the same metal loading (5 wt %) and similar metal particle size (8.9–11.7 nm) were prepared. The dehydrogenation of adsorbed 2-propanol in a flow of He and the hydrogenation of adsorbed nitrobenzene in a flow of H₂ were tested as model reactions of organic reactions on the metal surface. As a test reaction of H₂ dissociation on the surface, SiOH/SiOD exchange on the M/SiO₂ catalysts in a flow of D₂ is carried out. The liquid phase hydrogenation of nitrobenzene under 3.0 MPa of H₂ is adopted as an organic reaction under realistic conditions. Generally, the activities show volcano-type dependences on the ε_d – <i>E</i>_F value, indicating that the ε_d – <i>E</i>_F value is useful as a qualitative activity descriptor in heterogeneous catalysis of metal nanoparticles for multistep organic reactions

Heterogeneous Ni Catalyst for Direct Synthesis of Primary Amines from Alcohols and Ammonia

This paper reports the synthesis of primary amines from alcohols and NH₃ by an Al₂O₃-supported Ni nanoparticle catalyst as the first example of heterogeneous and noble-metal-free catalytic system for this reaction without additional hydrogen sources under relatively mild conditions. Various aliphatic alcohols are tolerated, and turnover numbers were higher than those of Ru-based homogeneous catalysts. The catalyst was recoverable and was reused. The effects of the Ni oxidation states and the acid–base nature of support oxides on the catalytic activity are studied. It is clarified that the surface metallic Ni sites are the catalytically active species, and the copresence of acidic and basic sites on the support surface is also indispensable for this catalytic system

Heterogeneous Ni Catalysts for N‑Alkylation of Amines with Alcohols

Nickel nanoparticles loaded onto various supports (Ni/MO_<i>x</i>) have been prepared and studied for the N-alkylation of amines with alcohols. Among the catalysts, Ni/θ-Al₂O₃ prepared by in situ H₂-reduction of NiO/θ-Al₂O₃ shows the highest activity, and it acts as reusable heterogeneous catalyst for the alkylation of anilines and aliphatic amines with various alcohols (benzyl and aliphatic alcohols) under additive free conditions. Primary amines are converted into secondary amines and secondary amines into tertiary amines. For the reaction of aniline with an aliphatic alcohol the catalyst shows higher turnover number (TON) than precious metal-based state-of-the-art catalysts. Mechanistic studies suggest that the reaction proceeds through a hydrogen-borrowing mechanism. The activity of Ni catalysts depends on the nature of support materials; acid–base bifunctional supports give higher activity than basic or acidic supports, indicating that acid–base sites on supports are necessary. The presence of basic (pyridine) or acidic (acetic acid) additive in the solution decreased the activity of Ni/θ-Al₂O₃, which suggests the cooperation of the acid–base site of θ-Al₂O₃. For a series of Ni/θ-Al₂O₃ catalysts with different particle size, the turnover frequency (TOF) per surface Ni increases with decreasing Ni mean particle size, indicating that low-coordinated Ni species and/or metal–support interface are active sites. From these results, we propose that the active site for this reaction is metal–support interface, where low-coordinated Ni⁰ atoms are adjacent to the acid–base sites of alumina

Design of Interfacial Sites between Cu and Amorphous ZrO₂ Dedicated to CO₂‑to-Methanol Hydrogenation

We examined the formation mechanism of active sites on Cu/ZrO₂ specific toward CO₂-to-methanol hydrogenation. The active sites on Cu/<i>a</i>-ZrO₂ (<i>a</i>-: amorphous) were more suitable for CO₂-to-methanol hydrogenation than those on Cu/<i>t</i>-ZrO₂ (<i>t</i>-: tetragonal) and Cu/<i>m</i>-ZrO₂ (<i>m</i>-: monoclinic). When <i>a</i>-ZrO₂ was impregnated with a Cu­(NO₃)₂·3H₂O solution and then calcined under air, most of the Cu species entered <i>a</i>-ZrO₂, leading to the formation of a Cu–Zr mixed oxide (Cu_<i>a</i>Zr_1‑<i>a</i>O_<i>b</i>). The H₂ reduction of the thus-formed Cu_<i>a</i>Zr_1‑<i>a</i>O_<i>b</i> led to the formation of Cu nanoparticles on <i>a</i>-ZrO₂, which can be dedicated to CO₂-to-methanol hydrogenation. We concluded that the selective synthesis of Cu_<i>a</i>Zr_1‑<i>a</i>O_<i>b</i>, especially amorphous Cu_<i>a</i>Zr_1‑<i>a</i>O_<i>b</i>, is a key feature of the catalyst preparation. The preparation conditions of the amorphous Cu_<i>a</i>Zr_1‑<i>a</i>O_<i>b</i> specific toward CO₂-to-methanol hydrogenation is as follows: (i) Cu­(NO₃)₂·3H₂O/<i>a</i>-ZrO₂ is calcined at low temperature (350 °C in this study) and (ii) the Cu loading is low (6 and 8 wt % in this study). Via these preparation conditions, the characteristics of <i>a</i>-ZrO₂ for the catalysts remained unchanged during the reaction at 230 °C. The latter preparation condition is related to the solubility limit of Cu species in <i>a</i>-ZrO₂. Accordingly, we obtained the amorphous Cu_<i>a</i>Zr_1‑<i>a</i>O_<i>b</i> without forming crystalline CuO particles