Oxidative Degradation of Multi-Carbon Substrates by an Oxidic Cobalt Phosphate Catalyst

The development of heterogeneous catalysts to affect the activation of recalcitrant biomolecules has applications for biomass processing, biomass fuel cells, and wastewater remediation. We demonstrate that a cobalt oxygen evolution catalyst (Co-OEC) can catalyze the oxidation of carbon feedstocks completely to CO₂. A quantitative analysis of the product distribution from the oxidative degradation of the C₂ compound, ethylene glycol, is elaborated and a reaction sequence is proposed. The Co-OEC is also found to be competent for oxidatively degrading C₂₊ compounds, including glucose and lignin, to carbon dioxide at consequential Faradaic efficiencies

Oxygen Reduction Catalysis at a Dicobalt Center: The Relationship of Faradaic Efficiency to Overpotential

The selective four electron, four proton, electrochemical reduction of O₂ to H₂O in the presence of a strong acid (TFA) is catalyzed at a dicobalt center. The faradaic efficiency of the oxygen reduction reaction (ORR) is furnished from a systematic electrochemical study by using rotating ring disk electrode (RRDE) methods over a wide potential range. We derive a thermodynamic cycle that gives access to the standard potential of O₂ reduction to H₂O in organic solvents, taking into account the presence of an exogenous proton donor. The difference in ORR selectivity for H₂O vs H₂O₂ depends on the thermodynamic standard potential as dictated by the p<i>K</i>_a of the proton donor. The model is general and rationalizes the faradaic efficiencies reported for many ORR catalytic systems

Iodide-Mediated Control of Rhodium Epitaxial Growth on Well-Defined Noble Metal Nanocrystals: Synthesis, Characterization, and Structure-Dependent Catalytic Properties

Metal nanocrystals (NCs) comprising rhodium are heterogeneous catalysts for CO oxidation, NO reduction, hydrogenations, electro-oxidations, and hydroformylation reactions. It has been demonstrated that control of structure at the nanoscale can enhance the performance of a heterogeneous metal catalyst, such as Rh, but molecular-level control of NCs comprising this metal is less studied compared to gold, silver, platinum, and palladium. We report an iodide-mediated epitaxial overgrowth of Rh by using the surfaces of well-defined foreign metal crystals as substrates to direct the Rh surface structures. The epigrowth can be accomplished on different sizes, morphologies, and identities of metal substrates. The surface structures of the resulting bimetallic NCs were studied using electron microscopy, and their distinct catalytic behaviors were examined in CO stripping and the electro-oxidation of formic acid. Iodide was found to play a crucial role in the overgrowth mechanism. With the addition of iodide, the Rh epigrowth can even be achieved on gold substrates despite the rather large lattice mismatch of ∼7%. Hollow Rh nanostructures have also been generated by selective etching of the core substrates. The new role of iodide in the overgrowth and the high level of control for Rh could hold the key to future nanoscale control of this important metal’s architecture for use in heterogeneous catalysis

Probing Edge Site Reactivity of Oxidic Cobalt Water Oxidation Catalysts

Differential electrochemical mass spectrometry (DEMS) analysis of the oxygen isotopologues produced by ¹⁸O-labeled Co-OEC in H₂¹⁶O reveals that water splitting catalysis proceeds by a mechanism that involves direct coupling between oxygens bound to dicobalt edge sites of Co-OEC. The edge site chemistry of Co-OEC has been probed by using a dinuclear cobalt complex. ¹⁷O NMR spectroscopy shows that ligand exchange of OH/OH₂ at Co­(III) edge sites is slow, which is also confirmed by DEMS experiments of Co-OEC. In borate (B_i) and phosphate (P_i) buffers, anions must be displaced to allow water to access the edge sites for an O–O bond coupling to occur. Anion exchange in P_i is slow, taking days to equilibrate at room temperature. Conversely, anion exchange in B_i is rapid (<i>k</i>_assoc = 13.1 ± 0.4 M^–1 s^–1 at 25 °C), enabled by facile changes in boron coordination. These results are consistent with the OER activity of Co-OEC in B_i and P_i. The P_i binding kinetics are too slow to establish a pre-equilibrium sufficiently fast to influence the oxygen evolution reaction (OER), consistent with the zero-order dependence of P_i on the OER current density; in contrast, B_i exchange is sufficiently facile such that B_i has an inhibitory effect on OER. These complementary studies on Co-OEC and the dicobalt edge site mimic allow for a direct connection, at a molecular level, to be made between the mechanisms of heterogeneous and homogeneous OER

Probing Edge Site Reactivity of Oxidic Cobalt Water Oxidation Catalysts

Differential electrochemical mass spectrometry (DEMS) analysis of the oxygen isotopologues produced by ¹⁸O-labeled Co-OEC in H₂¹⁶O reveals that water splitting catalysis proceeds by a mechanism that involves direct coupling between oxygens bound to dicobalt edge sites of Co-OEC. The edge site chemistry of Co-OEC has been probed by using a dinuclear cobalt complex. ¹⁷O NMR spectroscopy shows that ligand exchange of OH/OH₂ at Co­(III) edge sites is slow, which is also confirmed by DEMS experiments of Co-OEC. In borate (B_i) and phosphate (P_i) buffers, anions must be displaced to allow water to access the edge sites for an O–O bond coupling to occur. Anion exchange in P_i is slow, taking days to equilibrate at room temperature. Conversely, anion exchange in B_i is rapid (<i>k</i>_assoc = 13.1 ± 0.4 M^–1 s^–1 at 25 °C), enabled by facile changes in boron coordination. These results are consistent with the OER activity of Co-OEC in B_i and P_i. The P_i binding kinetics are too slow to establish a pre-equilibrium sufficiently fast to influence the oxygen evolution reaction (OER), consistent with the zero-order dependence of P_i on the OER current density; in contrast, B_i exchange is sufficiently facile such that B_i has an inhibitory effect on OER. These complementary studies on Co-OEC and the dicobalt edge site mimic allow for a direct connection, at a molecular level, to be made between the mechanisms of heterogeneous and homogeneous OER

Yolk–Shell Nanocrystal@ZIF‑8 Nanostructures for Gas-Phase Heterogeneous Catalysis with Selectivity Control

A general synthetic strategy for yolk–shell nanocrystal@ZIF-8 nanostructures has been developed. The yolk–shell nanostructures possess the functions of nanoparticle cores, microporous shells, and a cavity in between, which offer great potential in heterogeneous catalysis. The synthetic strategy involved first coating the nanocrystal cores with a layer of Cu₂O as the sacrificial template and then a layer of polycrystalline ZIF-8. The clean Cu₂O surface assists in the formation of the ZIF-8 coating layer and is etched off spontaneously and simultaneously during this process. The yolk–shell nanostructures were characterized by transmission electron microscopy, scanning electron microscopy, X-ray diffraction, and nitrogen adsorption. To study the catalytic behavior, hydrogenations of ethylene, cyclohexene, and cyclooctene as model reactions were carried out over the Pd@ZIF-8 catalysts. The microporous ZIF-8 shell provides excellent molecular-size selectivity. The results show high activity for the ethylene and cyclohexene hydrogenations but not in the cyclooctene hydrogenation. Different activation energies for cyclohexene hydrogenation were obtained for nanostructures with and without the cavity in between the core and the shell. This demonstrates the importance of controlling the cavity because of its influence on the catalysis

Nanoscale-Phase-Separated Pd–Rh Boxes Synthesized via Metal Migration: An Archetype for Studying Lattice Strain and Composition Effects in Electrocatalysis

Developing syntheses of more sophisticated nanostructures comprising late transition metals broadens the tools to rationally design suitable heterogeneous catalysts for chemical transformations. Herein, we report a synthesis of Pd–Rh nanoboxes by controlling the migration of metals in a core–shell nanoparticle. The Pd–Rh nanobox structure is a grid-like arrangement of two distinct metal phases, and the surfaces of these boxes are {100} dominant Pd and Rh. The catalytic behaviors of the particles were examined in electrochemistry to investigate strain effects arising from this structure. It was found that the trends in activity of model fuel cell reactions cannot be explained solely by the surface composition. The lattice strain emerging from the nanoscale separation of metal phases at the surface also plays an important role