Molecularly Imprinted Ru Complex Catalysts Integrated on Oxide Surfaces

Selective catalysis is critical for the development of green chemical processes, and natural enzymes that possess specialized three-dimensional reaction pockets with catalytically active sites represent the most sophisticated systems for selective catalysis. A reaction space in an enzyme consists of an active metal center, functional groups for molecular recognition (such as amino acids), and a surrounding protein matrix to prepare the reaction pocket. The artificial design of such an integrated catalytic unit in a non-enzymatic system remains challenging. Molecular imprinting of a supported metal complex provides a promising approach for shape-selective catalysis. In this process, an imprinted cavity with a shape matched to a template molecule is created in a polymer matrix with a catalytically active metal site.In this Account, we review our studies on molecularly imprinted metal complex catalysts, focusing on Ru complexes, on oxide surfaces for shape-selective catalysis. Oxide surface-attached transition metal complex catalysts not only improve thermal stability and catalyst dispersion but also provide unique catalytic performance not observed in homogeneous precursors. We designed molecularly imprinted Ru complexes by using surface-attached Ru complexes with template ligands and inorganic/organic surface matrix overlayers to control the chemical environment around the active metal complex catalysts on oxide surfaces. We prepared the designed, molecularly imprinted Ru complexes on SiO₂ surfaces in a step-by-step manner and characterized them with solid-state (SS) NMR, diffuse-reflectance (DR) UV-vis, X-ray photoelectron spectroscopy (XPS), Brunauer–Emmett–Teller isotherm (BET), X-ray fluorescence (XRF), and Ru K-edge extended X-ray absorption fine structure (EXAFS). The catalytic performances of these Ru complexes suggest that this process of molecular imprinting facilitates the artificial integration of catalytic functions at surfaces. Further advances such as the imprinting of a transition state structure or the addition of multiple binding sites could lead to systems that can achieve 100% selective catalysis

Surface Functionalization of Supported Mn Clusters to Produce Robust Mn Catalysts for Selective Epoxidation

A robust heterogeneous Mn catalyst for selective epoxidation was prepared by the attachment of a Mn₄ oxonuclear complex [Mn₄O₂(CH₃COO)₇(bipy)₂]­(ClO₄)·3H₂O (<b>1</b>) on SiO₂ and the successive stacking of SiO₂-matrix overlayers around a supported Mn cluster. The structures of supported Mn catalysts were characterized by means of FT-IR spectroscopy, diffuse-reflectance UV/vis spectroscopy, Raman spectroscopy, X-ray photoelectron spectroscopy, and Mn K-edge X-ray absorption fine structure. A ligand exchange reaction between the CH₃COO ligand of <b>1</b> and surface silanol group produced a SiO₂-supported Mn cluster (<b>2</b>), whose coordination structure was similar to <b>1</b>. Subsequent heating of <b>2</b> under vacuum yielded supported Mn clusters (<b>3</b>, <b>4</b>) through the partial elimination of CH₃COO ligands. The surface-attached Mn clusters of <b>2</b>, <b>3</b>, and <b>4</b> were easily released to a reaction solution under epoxidation conditions (Mn leaching: approximately 50%), although they were active for epoxidation of <i>trans</i>-stilbene (the conversion of <i>trans</i>-stilbene, 99%, and the selectivity of <i>trans</i>-stilbene epoxid, 96%, for 6 h on <b>3</b>). We found that the functionalization of the supported Mn cluster on <b>2</b> with surface SiO₂-matrix overlayers altered the reactivity of the supported Mn cluster. Dimeric Mn species (<b>5c</b>) with reduced Mn oxidation state and coordination numbers was formed together with a reaction nanospace surrounded by the SiO₂-matrix overlayers. By optimizing the stacking manner of the SiO₂-matrix overlayers, the durability of the Mn catalyst was remarkably improved from leaching (the Mn leaching reached the minimum value of 0.01%), and active and stable epoxidation performances were successfully achieved in the heterogeneous phase (the conversion of <i>trans</i>-stilbene, 97%, and the selectivity of <i>trans</i>-stilbene epoxide, 91%, for 31 h on <b>5c</b>)

Ceria-Doped Ni/SBA-16 Catalysts for Dry Reforming of Methane

Ni-ceria nanoparticles (Ni/Ce = 1/1) in the cage-like pores of SBA-16 were prepared and evaluated in methane dry reforming reactions. Coexistence of ceria in NiCe/SBA-16 resulted in forming uniformly sized Ni particles (av. 5.7 nm) within the mesopores of SBA-16, because of the confinement effect from the framework of SBA-16 and the strong interaction between Ni and ceria. Ceria addition facilitated the reduction of NiCe/SBA-16 compared with Ni/SBA-16, and Ce³⁺ was the dominant species in both fresh and used NiCe/SBA-16 catalysts, as determined by Ce L_III-edge X-ray absorption near-edge structure (XANES). The methane conversion was much more stable on NiCe/SBA-16 than on Ni/CeO₂ and Ni/SBA-16 in the methane dry reforming at 973 K during a 100 h reaction period; the deactivation of the Ni catalyst and the collapse of the SBA-16 framework were preferably suppressed for NiCe/SBA-16 under the reaction conditions. The remarkable effect of ceria on the structural stability of both the active Ni particles and the SBA-16 framework led to the consistent catalytic performance of NiCe/SBA-16 in methane dry reforming

[Fe₄] and [Fe₆] Hydride Clusters Supported by Phosphines: Synthesis, Characterization, and Application in N₂ Reduction

Multiple iron atoms bridged by hydrides is a common structural feature of the active species that have been postulated in the biological and industrial reduction of N₂. In this study, the reactions of an Fe­(II) amide complex with pinacolborane in the presence/absence of phosphines afforded a series of hydride-supported [Fe₄] and [Fe₆] clusters Fe₄(μ-H)₄(μ₃-H)₂{N­(SiMe₃)₂}₂(PR₃)₄ (PR₃ = PMe₃ (<b>2a</b>), PMe₂Ph (<b>2b</b>), PEt₃ (<b>2c</b>)), Fe₆(μ-H)₁₀(μ₃-H)₂(PMe₃)₁₀ (<b>3</b>), and (η⁶-C₇H₈)­Fe₄(μ-H)₂{μ-N­(SiMe₃)₂}₂{N­(SiMe₃)₂}₂ (<b>4</b>), which were characterized crystallographically and spectroscopically. Under ambient conditions, these clusters catalyzed the silylation of N₂ to furnish up to 160 ± 13 equiv of N­(SiMe₃)₃ per <b>2c</b> (40 equiv per Fe atom) and 183 ± 18 equiv per <b>3</b> (31 equiv per Fe atom). With regard to the generation of the reactive species, dissociation of phosphine and hydride ligands from the [Fe₄] and [Fe₆] clusters was indicated, based on the results of the mass spectrometric analysis on the [Fe₆] cluster, as well as the formation of a diphenylsilane adduct of the [Fe₄] cluster

[Fe₄] and [Fe₆] Hydride Clusters Supported by Phosphines: Synthesis, Characterization, and Application in N₂ Reduction

Multiple iron atoms bridged by hydrides is a common structural feature of the active species that have been postulated in the biological and industrial reduction of N₂. In this study, the reactions of an Fe­(II) amide complex with pinacolborane in the presence/absence of phosphines afforded a series of hydride-supported [Fe₄] and [Fe₆] clusters Fe₄(μ-H)₄(μ₃-H)₂{N­(SiMe₃)₂}₂(PR₃)₄ (PR₃ = PMe₃ (<b>2a</b>), PMe₂Ph (<b>2b</b>), PEt₃ (<b>2c</b>)), Fe₆(μ-H)₁₀(μ₃-H)₂(PMe₃)₁₀ (<b>3</b>), and (η⁶-C₇H₈)­Fe₄(μ-H)₂{μ-N­(SiMe₃)₂}₂{N­(SiMe₃)₂}₂ (<b>4</b>), which were characterized crystallographically and spectroscopically. Under ambient conditions, these clusters catalyzed the silylation of N₂ to furnish up to 160 ± 13 equiv of N­(SiMe₃)₃ per <b>2c</b> (40 equiv per Fe atom) and 183 ± 18 equiv per <b>3</b> (31 equiv per Fe atom). With regard to the generation of the reactive species, dissociation of phosphine and hydride ligands from the [Fe₄] and [Fe₆] clusters was indicated, based on the results of the mass spectrometric analysis on the [Fe₆] cluster, as well as the formation of a diphenylsilane adduct of the [Fe₄] cluster

Size Regulation and Stability Enhancement of Pt Nanoparticle Catalyst via Polypyrrole Functionalization of Carbon-Nanotube-Supported Pt Tetranuclear Complex

A novel multiwall carbon nanotube (MWCNT) and polypyrrole (PPy) composite was found to be useful for preparing durable Pt nanoparticle catalysts of highly regulated sizes. A new pyrene-functionalized Pt₄ complex was attached to the MWCNT surface which was functionalized with PPy matrix to yield Pt₄ complex/PPy/MWCNT composites without decomposition of the Pt₄ complex units. The attached Pt₄ complexes in the composite were transformed into Pt⁰ nanoparticles with sizes of 1.0–1.3 nm at a Pt loading range of 2 to 4 wt %. The Pt nanoparticles in the composites were found to be active and durable catalysts for the <i>N</i>-alkylation of aniline with benzyl alcohol. In particular, the Pt nanoparticles with PPy matrix exhibited high catalyst durability in up to four repetitions of the catalyst recycling experiment compared with nonsize-regulated Pt nanoparticles prepared without PPy matrix. These results demonstrate that the PPy matrix act to regulate the size of Pt nanoparticles, and the PPy matrix also offers stability for repeated usage for Pt nanoparticle catalysis

Rate Enhancements in Structural Transformations of Pt–Co and Pt–Ni Bimetallic Cathode Catalysts in Polymer Electrolyte Fuel Cells Studied by in Situ Time-Resolved X‑ray Absorption Fine Structure

In situ time-resolved X-ray absorption fine structure spectra of Pt/C, Pt₃Co/C, and Pt₃Ni/C cathode electrocatalysts in membrane electrode assemblies (catalyst loading: 0.5 mg_metal cm^–2) were successfully measured every 100 ms for a voltage cycling process between 0.4 and 1.0 V. Systematic analysis of in situ time-resolved X-ray absorption near-edge structure and extended X-ray absorption fine structure spectra in the molecular scale revealed the structural kinetics of the Pt and Pt₃M (M = Co, Ni) bimetallic cathode catalysts under polymer electrolyte fuel cell operating conditions, and the rate constants of Pt charging, Pt–O bond formation/breaking, and Pt–Pt bond breaking/re-formation relevant to the fuel cell performances were successfully determined. The addition of the 3d transition metals to Pt reduced the Pt oxidation state and significantly enhanced the reaction rates of Pt discharging, Pt–O bond breaking, and Pt–Pt bond re-forming in the reductive process from 1.0 to 0.4 V

Kinetics and Mechanism of Redox Processes of Pt/C and Pt₃Co/C Cathode Electrocatalysts in a Polymer Electrolyte Fuel Cell during an Accelerated Durability Test

The degradation of Pt electrocatalysts in membrane electrode assemblies (MEAs) of polymer electrolyte fuel cells under working conditions is a serious problem for their practical use. Here we report the kinetics and mechanism of redox reactions at the surfaces of Pt/C and Pt₃Co/C cathode electrocatalysts during catalyst degradation processes by an accelerated durability test (ADT) studied by operando time-resolved X-ray absorption fine structure (XAFS) spectroscopy. Systematic analysis of a series of Pt L_III-edge time-resolved XAFS spectra measured every 100 ms at different degradation stages revealed changes in the kinetics of Pt redox reactions on Pt/C and Pt₃Co/C cathode electrocatalysts. In the case of Pt/C, as the number of ADT cycles increased, structural changes for Pt redox reactions (charging, surface, and subsurface oxidation) became less sensitive because of the agglomeration of catalyst particles. It was found that their rate constants were almost constant independent of the agglomeration of the Pt electrocatalyst. On the other hand, in the case of Pt₃Co/C, the rate constants of the redox reactions of the cathode electrocatalyst gradually reduced as the number of ADT cycles increased. The differences in the kinetics for the redox processes would be differences in the degradation mechanism of these cathode electrocatalysts

Potential-Dependent Restructuring and Hysteresis in the Structural and Electronic Transformations of Pt/C, Au(Core)-Pt(Shell)/C, and Pd(Core)-Pt(Shell)/C Cathode Catalysts in Polymer Electrolyte Fuel Cells Characterized by in Situ X‑ray Absorption Fine Structure

Potential-dependent transformations of surface structures, Pt oxidation states, and Pt–O bondings in Pt/C, Au­(core)-Pt­(shell)/C (denoted as Au@Pt/C), and Pd­(core)-Pt­(shell)/C (denoted as Pd@Pt/C) cathode catalysts in polymer electrolyte fuel cells (PEFCs) during the voltage-stepping processes were characterized by in situ (operando) X-ray absorption fine structure (XAFS). The active surface phase of the Au@Pt/C for oxygen reduction reaction (ORR) was suggested to be the Pt₃Au alloy layer on Au core nanoparticles, while that of the Pd@Pt/C was the Pt atomic layer on Pd core nanoparticles. The surfaces of the Pt, Au@Pt and Pd@Pt nanoparticles were restructured and disordered at high potentials, which were induced by strong Pt–O bonds, resulting in hysteresis in the structural and electronic transformations in increasing and decreasing voltage operations. The potential-dependent restructuring, disordering, and hysteresis may be relevant to hindered Pt performance, Pt dissolution to the electrolyte, and degradation of the ORR activity

Protracted Relaxation Dynamics of Lithium Heterogeneity in Solid-State Battery Electrodes

The lithium (Li) heterogeneity formed in the composite electrodes has a significant impact on the performance of solid-state batteries (SSBs). Whereas the influence of various factors on the Li heterogeneity, such as (dis)charge currents, ionic and/or electronic conductivity of the constituent materials, and interfacial charge transfer kinetics, is extensively studied, the influence of the relaxation on the Li heterogeneity in SSB electrodes is largely unexplored, despite its unignorable impact on the battery performance. Here, we performed a three-dimensional operando evaluation of the relaxation dynamics of the electrode-scale Li heterogeneity in a composite SSB electrode under open-circuit conditions after charging using the computed tomography combined with X-ray absorption near-edge structure spectroscopy (CT-XANES). In contrast to the electrode for the liquid-based Li-ion batteries, the Li heterogeneity formed in the composite SSB electrode during charging was not fully relaxed, even after a long open-circuit hold, leaving both higher and lower Li content regions. Such protracted relaxation dynamics in the composite SSB electrode may be due to the high interfacial resistance between active material particles as well as between active material and solid electrolyte particles and is potentially an essential issue for SSBs. This work demonstrated that our CT-XANES technique can three-dimensionally resolve the relaxation dynamics of Li heterogeneity within SSB electrodes, which has only been analyzed indirectly by conventional electrochemical methods such as electrochemical impedance spectroscopy. Our technique can be a valuable tool for identifying detrimental factors affecting the battery performance, ultimately contributing to the development of high-performance SSBs