No abstract

We report on the continuous fine-scale tuning of band gaps over 0.4 eV and of the electrical conductivity of over 4 orders of magnitude in a series of highly crystalline binary alloys of two-dimensional electrically conducting metal–organic frameworks M3(HITP)2 (M = Co, Ni, Cu; HITP = 2,3,6,7,10,11-hexaiminotriphenylene). The isostructurality in the M3(HITP)2 series permits the direct synthesis of binary alloys (M x M′3–x )­(HITP)2 (MM′ = CuNi, CoNi, and CoCu) with metal compositions precisely controlled by precursor ratios. We attribute the continuous tuning of both band gaps and electrical conductivity to changes in free-carrier concentrations and to subtle differences in the interlayer displacement or spacing, both of which are defined by metal substitution. The activation energy of (Co x Ni3–x )­(HITP)2 alloys scales inversely with an increasing Ni percentage, confirming thermally activated bulk transport.

Iron- and nitrogen-doped carbon (Fe-N-C) materials are leading candidates to replace platinum catalysts for the oxygen reduction reaction (ORR) in fuel cells; however, their active site structures remain poorly understood. A leading postulate is that the iron-containing active sites exist primarily in a pyridinic Fe-N4 ligation environment, yet, molecular model catalysts generally feature pyrrolic coordination. Herein, we report a molecular pyridinic hexaazacyclophane macrocycle, (phen2N2)Fe, and compare its spectroscopic, electrochemical, and catalytic properties for ORR to a typical Fe-N-C material and prototypical pyrrolic iron macrocycles. N 1s XPS and XAS signatures for (phen2N2)Fe are remarkably similar to those of Fe-N-C. Electrochemical studies reveal that (phen2N2)Fe has a relatively high Fe(III/II) potential with a correlated ORR onset potential within 150 mV of Fe-N-C. Unlike the pyrrolic macrocycles, (phen2N2)Fe displays excellent selectivity for four-electron ORR, comparable to Fe-N-C materials. The aggregate spectroscopic and electrochemical data demonstrate that (phen2N2)Fe is a more effective model of Fe-N-C active sites relative to the pyrrolic iron macrocycles, thereby establishing a new molecular platform that can aid understanding of this important class of catalytic materials.

Alloying is an important strategy for the design of catalytic materials beyond pure metals. The conventional alloy catalysts however lack precise control over the local atomic structures of active sites. Here we report on an investigation of the active-site ensemble effect in bimetallic Pd–Au electrocatalysts for CO2 reduction. A series of Pd@Au electrocatalysts are synthesized by decorating Au nanoparticles with Pd of controlled doses, giving rise to bimetallic surfaces containing Pd ensembles of various sizes. Their catalytic activity for electroreduction of CO2 to CO exhibits a nonlinear behavior in dependence of the Pd content, which is attributed to the variation of Pd ensemble size and the corresponding tuning of adsorption properties. Density functional theory calculations reveal that the Pd@Au electrocatalysts with atomically dispersed Pd sites possess lower energy barriers for activation of CO2 than pure Au and are also less poisoned by strongly binding *CO intermediates than pure Pd, with an intermediate ensemble size of active sites, such as Pd dimers, giving rise to the balance between these two rate-limiting factors and achieving the highest activity for CO2 reduction.

Supported nanoparticles are broadly employed in industrial catalytic processes, where the active sites can be tuned by metal-support interactions (MSIs). Although it is well accepted that supports can modify the chemistry of metal nanoparticles, systematic utilization of MSIs for achieving desired catalytic performance is still challenging. The developments of supports with appropriate chemical properties and identification of the resulting active sites are the main barriers. Here, we develop two-dimensional transition metal carbides (MXenes) supported platinum as efficient catalysts for light alkane dehydrogenations. Ordered Pt3Ti and surface Pt3Nb intermetallic compound nanoparticles are formed via reactive metal-support interactions on Pt/Ti3C2Tx and Pt/Nb2CTx catalysts, respectively. MXene supports modulate the nature of the active sites, making them highly selective toward C–H activation. Such exploitation of the MSIs makes MXenes promising platforms with versatile chemical reactivity and tunability for facile design of supported intermetallic nanoparticles over a wide range of compositions and structures.

Electrochemical capacitors (ECs) have emerged as reliable and fast-charging electrochemical energy storage devices that offer high power densities. Their use is still limited, nevertheless, by their relatively low energy density. Because high specific surface area and electrical conductivity are widely seen as key metrics for improving the energy density and overall performance of ECs, materials that have excellent electrical conductivities but are otherwise nonporous, such as coordination polymers (CPs), are often overlooked. Here, we report a new nonporous CP, Ni 3 (benzenehexathiolate) (Ni 3 BHT), which exhibits high electrical conductivity of over 500 S/m. When used as an electrode, Ni 3 BHT delivers excellent specific capacitances of 245 F/g and 426 F/cm 3 in nonaqueous electrolytes. Structural and electrochemical studies relate the favorable performance to pseudocapacitive intercalation of Li + ions between the 2D layers of Ni 3 BHT, a charge-storage mechanism that has thus far been documented only in inorganic materials such as TiO 2 , Nb 2 O 5 , and MXenes. This first demonstration of pseudocapacitive ion intercalation in nonporous CPs, a class of materials comprising thousands of members with distinct structures and compositions, provides important motivation for exploring this vast family of materials for nontraditional, high-energy pseudocapacitors.

Catalysts based on earth-abundant elements, such as Ni and Mo, that can be used for the conversion of lignin-derived compounds are desirable. However, they usually exhibit low activity and/or selectivity toward the target reaction, hydrodeoxygenation (HDO). For example, conversion of m-cresol in H 2 over a typical Ni/SiO 2 leads to ring hydrogenation at low temperatures and C−C hydrogenolysis to CH 4 at high temperatures. Here, we report that a bimetallic Ni−Mo/SiO 2 catalyst with Ni:Mo ratio ≈ 1 reduced at an optimized temperature can be very active and selective for HDO of m-cresol to toluene over a wide range of reaction temperatures (250−350 °C) and 1 atm of H 2 . This behavior is explained in terms of the surface structure of Mo oxides on the surface of Ni nanoparticles. Detailed characterization (XRD, Raman, TPR, EXAFS, and XPS) indicates that, after calcination, NiMoO 4 is the predominant phase. However, after subsequent reduction, metallic Ni nanoparticles segregate out of the partially reduced MoO x . Interestingly, while no significant structural/electronic modifications are detected for the bulk of the metallic Ni particles, the surface chemistry is clearly altered (i.e., no hydrogenolysis/hydrogenation, weak CO/H 2 adsorption, and lower electron density in the d band of Ni). These results suggest that after reduction, in contrast to the formation of NiMo alloy, the Ni surface gets decorated by reduced MoO x moieties, a phenomenon similar to that previously observed on reducible oxides (so-called SMSI), which is essential for maximizing HDO and inhibiting hydrogenolysis.

Bi-Oxazoline (biOx) has emerged as an effective ligand framework for promoting nickel-catalyzed cross-coupling, cross-electrophile coupling, and photoredox-nickel dual catalytic reactions. This report fills the knowledge gap of the organometallic reactivity of (biOx)Ni complexes, including catalyst reduction, oxidative electrophile activation, radical capture, and reductive elimination. The biOx ligand displays no redox activity in (biOx)Ni(I) complexes, in contrast to other chelating imine and oxazoline ligands. The lack of ligand redox activity results in more negative reduction potentials of (biOx)Ni(II) complexes and accounts for the inability of zinc and manganese to reduce (biOx)Ni(II) species. On the basis of these results, we revise the formerly proposed "sequential reduction" mechanism of a (biOx)Ni-catalyzed cross-electrophile coupling reaction by excluding catalyst reduction steps.