Catalytic dehydrogenation of isobutane over supported MoO[subscript x]/K-Al[subscript 2]O[subscript
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Cheng, Emily↗
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Surface Fe clusters promote syngas reaction to oxygenates on Rh catalysts modified by atomic layer deposition
Converting syngas to higher oxygenates is a promising strategy for sustainably producing fuels and chemicals used every day. Rh-based catalysts promoted with metal oxides have long been of interest for oxygenate synthesis, but the role of promoters and the structure of the catalyst during reaction are often not well understood. In this work, we characterize a Rh/SiO 2 catalyst modified by Fe 2 O 3 deposited by atomic layer deposition (ALD). Here, we show that Fe 2 O 3 deposits selectively on the Rh nanoparticles and enhances both the turnover frequency and the oxygenate selectivity of the catalysts. A maximum 29% selectivity towards higher oxygenates is achieved with just 1 cycle of Fe 2 O 3 ALD, and we relate the trends in selectivity to the changing availability of surface sites that drive higher alcohol (e.g., ethanol and propanol) formation. Further, we show that the ALD promoter mitigates catalyst sintering, improving the stability of these materials. In-situ X-ray absorption spectroscopy experiments reveal that the Fe 2 O 3 undergoes reduction during the catalyst pretreatment and forms Fe clusters on the surface of the Rh that migrate but remain stable under syngas reaction conditions. We find that modification of the Rh with these surface Fe species is key to the enhanced alcohol production observed in these materials.
37 INORGANIC, ORGANIC, PHYSICAL, AND ANALYTICAL CH↗
Highly selective hydrogenation of phenol to cyclohexanone over a Pd-loaded N-doped carbon catalyst derived from chitosan
A highly stable Pd-loaded N-doped carbon catalyst (ACN pd ) for phenol hydrogenation was prepared from chitosan by hydrothermal carbonization. ACN pd does not require a reduction step before catalytic use due to the Pd in the as-prepared catalyst mainly exists in the form of Pd 0 (80%). The carbon support involves N-containing groups such as pyridinic nitrogen and pyrrolic nitrogen, which could provide basic sites to adsorb phenol effectively. The as-fabricated ACN pd shows high catalytic performance with turnover frequency (TOF) of 29.34 h -1 . Accordingly, a phenol conversion of 100% and a cyclohexanone selectivity of 99.1% are achieved in 5 h at 100 °C and 1 MPa H 2 . This outstanding performance is attributed to the synergetic effects of the Pd particles, the N-functional groups, and the Lewis acid sites on the support. The carbon support presents intrinsic Lewis acid sites due to its electrophilicity, and Pd doping further increases the strength of such acid sites as it causes electron-deficient structural features. Moreover, the Lewis acid sites inhibit the over-hydrogenation from cyclohexanone to cyclohexanol. This study provides new insights into the application of functional biomass-based carbon materials as catalyst supports.
37 INORGANIC, ORGANIC, PHYSICAL, AND ANALYTICAL CH↗
Synergistic Ru Co atomic pair with enhanced activity toward levulinic acid hydrogenation
Development of efficient metal-based catalysts is of great importance for levulinic acid (LA) hydrogenation to γ-valerolactone (GVL). The widely employed Ru-based catalysts are advantageous for H 2 dissociation, however, the steric hindrance for large Ru particles hampers their coordination to C=O moiety in LA, and thereby decreasing the activity. Herein, we report a Ru 1 Co 1 -N-C double single-atom catalyst (DSAC) with synergistic Ru and Co atomic pairs for LA hydrogenation into GVL. The Ru and Co doped zeolitic imidazole frameworks (RuCo-doped ZIF-8) precursor was rationally designed ((Ru+Co)/(Zn+Ru+Co) = 2 at.%), where the Zn node spatially isolates Ru and Co species, expanding the adjacent Ru-Co distance and facilitating the formation of the Ru-Co atomic pair upon pyrolysis, with each atom coordinated with three nitrogen atoms (N 3 -Ru 1 Co 1 -N 3 ). The Ru 1 Co 1 -N-C catalyst exhibits outstanding catalytic activity, with a turnover frequency (TOF) of 1980 h –1 , surpassing previously reported Ru-based catalysts. Experimental investigation and density functional theory (DFT) calculations reveal that the electron-rich Ru induced by less electronegative Co facilitates H 2 dissociation, while atomic Ru in dual-atomic pairs promotes C=O activation, Ru and Co atomic pairs synergistically enhancing LA conversion to GVL. In conclusion, this research will shed light on the precise control of active sites at atomic scale, and also provides a new concept for designing high-performance Ru-based catalysts towards LA hydrogenation to GVL.
Double single-atom catalysts↗
Low-pressure methanol synthesis from CO 2 over metal-promoted Ni-Ga intermetallic catalysts
Ni-Ga and M-Ni-Ga (M = Au, Co, Cu) catalysts were evaluated for methanol synthesis from CO 2 at 10 bar and 200-270°C. The following trend in turnover frequency (TOF) for CO 2 hydrogenation was observed: AuNiGa > CuNiGa > NiGa > CoNiGa, where TOF increased with decreasing catalyst affinity for CO. The presence of a third metal was found to influence both the formation of the Ni-Ga intermetallic phase as well as the number of available sites for CO chemisorption. Phase formation, catalyst composition and stability were evaluated using therm ogravimetric analysis (TGA), X-ray diffraction (XRD), X-ray fluorescence (XRF), scanning transmission electron microscopy and energy dispersive X-ray spectroscopy (STEM-EDX). Au-Ni-Ga, which showed a nearly 4-fold improvement in TOF at 263°C and 10 bar compared to Ni-Ga, consisted of Ni 3 Ga particles decorated with Au, as evidenced by post catalysis characterization.
37 INORGANIC, ORGANIC, PHYSICAL, AND ANALYTICAL CH↗
Tuning proton transfer and catalytic properties in triple junction nanostructured catalyts
Not Available
Chu, Yuanyuan↗
Dirhodium(II,II) Complexes as Panchromatic Single-Molecule Photocatalysts for Hydrogen Evolution
The growing global energy demand and climate change have prompted a shift from carbon-based fuels to sustainable energy sources. Hydrogen production by harnessing solar energy and using abundant proton sources represents an attractive approach to addressing this crisis. Single-molecule single-chromophore photocatalysts, capable of both absorbing the incident photon and catalyzing the chemical transformation, are able to circumvent energy losses present in multicomponent systems that require a photosensitizer and a catalyst, often employing additional redox relay molecules. Here, the series of complexes derived from cis-[Rh 2 (μ-DPhF) 2 (μ-bncn) 2 ] 2+ (1; DPhF = N,N′-diphenylformamidinate, bncn = benzo[c]cinnoline) discussed in this Account presents robust and air-stable single-molecule photocatalysts with panchromatic absorption from the ultraviolet spectral region to the near-infrared (NIR), with high turnover frequencies of ∼20 to 30 h –1 under red light irradiation. For comparison, other single-molecule hydrogen-evolving photocatalysts reported to date exhibit low photocatalytic efficiency, are not operable in the visible or NIR regions, and are unstable under an ambient atmosphere.
Absorption↗
Microstructurally Strained Pyrochlore–Perovskite Biphasic Electrocatalysts for the Oxygen Evolution Reaction
Efficiency of water splitting for hydrogen production is often limited by the sluggish kinetics of multiple electronic transfers required in the heterogeneous oxygen evolution reaction (OER). Catalyst design for reducing the high OER overpotential remains a major scientific challenge. Lattice-strain engineering, a method for tuning the electronic structure and surface geometric configuration of active sites, may greatly affect the interaction between adsorbates and catalytic surfaces for high activity and stability. Here, in this study, we present the synthesis of biphasic oxides of YPrSrRuMnO x , which consists of distinct phases of Y 2 Ru 2 O 7 pyrochlore and (Pr 0.7 Sr 0.3 )MnO 3 perovskite, and the development of a suitable analytical approach to study the strain–catalytic property relationship. Linear sweep voltammetry results reveal that the biphasic oxide exhibits approximately 3.1 times greater mass activity and 2.4 times larger turnover frequency (TOF) than single-phase Y 2 Ru 2 O 7 in the 0.1 M HClO 4 electrolyte. The biphasic catalyst is also about 3 times more stable than the single-phase oxide under acidic conditions. X-ray photoelectron spectroscopy, nitrogen isotherm, and electrochemical surface area analyses indicate that the oxidation state, specific surface area, and electrochemical surface area do not cause enough difference in the observed enhancement of OER performance. We examined the effects of microstrain on electrocatalysis, originating from lattice mismatch between different phases, using three different structural models. Specifically, we compared the Williamson–Hall method, standard stress–strain analysis, and Rietveld refinement in analyzing the structure–property relationship. Strain mapping using geometric phase analysis (GPA) further revealed significant microstrain and lattice dislocations localized near phase boundaries in the biphasic oxide, in contrast to the uniform strain in single-phase materials. The results reveal that the increased microstrain correlates well with the improved OER performance, as the biphasic oxide catalyst exhibits 2–3 times greater microstrain than Y 2 Ru 2 O 7 pyrochlore.
electrocatalysts↗
A Rhenium Bis -tetramethylphenanthroline Catalyst for CO 2 Reduction to Formate
Catalytic CO 2 reduction reactions featuring high selectivity toward formate are relatively rare. In some homogeneous molecular CO 2 -reducing electrocatalysis, using triethylamine (TEA) and isopropanol (IPA) as additives improves catalytic performance in producing formate. In this paper, we investigate whether the rhenium(I) bis-diimine dicarbonyl complexes, cis-[Re(N^N) 2 (CO) 2 ] + , where N^N is 2,2’-bipyridine ([1] + ) or 3,4,7,8-tetramethyl-1,10-phenanthroline ([2] + ), are capable of electrocatalytically reducing CO 2 to formate in acetonitrile containing TEA and IPA. Catalyst [1] + was ineffective at CO 2 reduction, yielding formate quantities comparable to those produced in experiments without the catalyst. Catalyst [2] + , however, is a promising electrocatalyst for the CO 2 reduction reaction in the presence of TEA and IPA, with formate being produced in millimolar concentrations (10.5 mM), as detected by 1 H NMR spectroscopy after 6 h electrolysis (formate Faradaic efficiency = 11%, with the major balance going to H 2 ). Upon more detailed examination, [2] + exhibited a turnover frequency (TOF) of 12 s –1 for formate, comparable to other leading molecular catalysts that competently execute this reduction. Combinations of spectroscopy, electrochemistry, and theory were used to better understand the mechanism of CO 2 reduction by [2] + . Fourier transform infrared spectroelectrochemical (FTIR-SEC) data provided no evidence for CO ligand dissociation or substitution upon one- and two-electron reduction of [2] + , suggesting that a mechanism distinct from one that is metal-hydride-based is operative in catalysis. Computational studies guide mechanistic investigations toward the proposed formation of a hydrophenanthroline-based intermediate responsible for hydride transfer to CO 2 and electrocatalytic formate production from [2] + .
Beverages↗
Modified Energy Span Analysis Reveals Heterogeneous Catalytic Kinetics
Mechanistic modeling is a cornerstone of catalyst development generally conducted with microkinetic models or density functional theory-based energy profiles. We extend the energy span model of homogeneous catalysis to heterogeneous systems by introducing the modified energy span analysis (MESA) model by implementing collision theory and gas-phase concentration effects. We determine analytically turnover frequencies, coverages, rate-determining steps, apparent activation energies, and reaction orders in agreement with microkinetic and kinetic Monte Carlo simulations. The model applies to discrete catalyst state systems, including single-atom catalysts, homogeneous systems, and microporous materials. A generalizable rate expression is derived for reactor modeling or mechanistic insights from solely experimentally measured reaction orders. Here, we illustrate MESA on three published mechanisms and reveal unexpected phenomena absent in traditional energy span profiles.
37 INORGANIC, ORGANIC, PHYSICAL, AND ANALYTICAL CH↗
Efficient Iridium Catalysts for Formic Acid Dehydrogenation: Investigating the Electronic Effect on the Elementary β-Hydride Elimination and Hydrogen Formation Steps
We report herein a series of Cp*Ir complexes containing a rigid 8-aminoquinolinesulfonamide moiety as highly efficient catalysts for the dehydrogenation of formic acid (FA). The complex [Cp*Ir(L)Cl] (HL = N-(quinolin-8-yl)benzenesulfonamide) displayed a high turnover frequency (TOF) of 2.97 × 10 4 h –1 and a good stability (>100 h) at 60 °C. Comparative studies of [Cp*Ir(L)Cl] with the rigid ligand and [Cp*Ir(L')Cl] (HL' = N-propylpypridine-2-sulfonamide) without the rigid aminoquinoline moiety demonstrated that the 8-aminoquinoline moiety could dramatically enhance the stability of the catalyst. The electron-donating ability of the N,N'-chelating ligand was tuned by functionalizing the phenyl group of the L ligand with OMe, Cl, and CF 3 to have a systematical perturbation of the electronic structure of [Cp*Ir(L)Cl]. Experimental kinetic studies and density functional theory (DFT) calculations on this series of Cp*Ir complexes revealed that (i) the electron-donating groups enhance the hydrogen formation step while slowing down the β-hydride elimination and (ii) the electron-withdrawing groups display the opposite effect on these reaction steps, which in turn leads to lower optimum pH for catalytic activity compared to the electron-donating groups.
37 INORGANIC, ORGANIC, PHYSICAL, AND ANALYTICAL CH↗
Electron-Donating para -Substituent (X) Enhances the Water Oxidation Activity of the Catalyst Ru(4'-X-terpyridine)(phenanthroline-SO 3 ) +
Recently, our group has developed a Ru-based water oxidation catalyst (WOC) with pendant sulfonate (1, Ru(4'-X-terpyridine)(phenanthroline-SO 3 )OTf (X = H, 1a) that shows high activity under both sacrificial oxidant (CAN, Ce(NH 4 ) 2 (NO 3 ) 6 , Ce IV ) and electrocatalytic conditions, in both acidic and neutral media. Here, we demonstrate that the functionalization of the 4'-X-terpyridine ligand with an electron-donating substituent X = OEt (1b) makes potentials of Ru II /Ru III redox catalysis more negative, whereas when X = NO 2 (1c) and CF 3 (1d), potentials are more positive. For 1b, full conversion of the sacrificial oxidant Ce IV occurred in 0.4 h (7 h for 1a), with an initial rate of 2.07 μmol O 2 s –1 and a turnover frequency of 7.6 s –1 , which is 30-fold faster than that for 1a at [cat] 0 = 20 μM. Under electrocatalytic conditions, water oxidation by 1b is three times faster than that by the parent catalyst 1a at close to the same potential. Extensive computations have identified differences in the initial PCET steps of the water oxidation by catalysts 1a, 1b, and 1d, and demonstrated the increased probability of the O 2 formation via the oxide relay pathway in the order 1b< 1a < 1d.
14 SOLAR ENERGY↗
Synthesis, Properties, and Electrochemical Proton Reduction of a Homoleptic Tetrathiolato Ni-Site Model of [NiFe]-Hydrogenase
[NiFe]-hydrogenase enzymes process H 2 at a nonplanar tetracysteinato-Ni site, the sole participator in proton binding/redox chemistry during turnover. With the objective of assessing whether a simple tetrahedral/ tetrathiolato-Ni 2+ core could promote H 2 evolution reaction (HER), we synthesized (Et 4 N) 2 [Ni(S-p-CF 3 −Ph) 4 ] (1) employing para-trifluoromethylbenzenethiolate ( − S-p-CF 3 −Ph) as a Ni-site analog of [NiFe]-hydrogenase. Spectroscopic measurements and X-ray crystallography confirm the distorted tetrahedral geometry of 1. Dissolution of 1 results in partial thiolate dissociation and formation of S,S-bridged complexes such as (Et 4 N) 2 [Ni 2 (S-p-CF 3 −Ph) 6 ] (3) among other ill-defined species. Dissociation is further accelerated in the presence of Brønsted acids, complicating the assessment of 1 for proton reduction. However, this dissociation/proton instability is suppressed in the presence of additional thiolate ligand to ensure tetrahedral/tetrathiolato 1 persists in solution. Electrochemical HER activity was evaluated by monitoring the current response of an MeCN solution of 1/ excess thiolate after sequential titration with a weak Brønsted acid (acetic acid). The results suggest that 1 is a modest electrocatalyst for the HER with a turnover frequency of 14.5 ± 3.6 s −1 and an overpotential of 0.72 ± 0.02 V. Control experiments and supplementary DFT computations indicate that 1, or a species derived from 1, is responsible for the HER and suggest an ECCE-type mechanism.
Evolution reactions↗
Electrocatalytic Alcohol Oxidation with Iron-Based Acceptorless Alcohol Dehydrogenation Catalyst
Electrochemical and chemical studies reveal that the amido complex (PNH x P)Fe(CO)(H)(X) (FeN 1, x = 0, X = 0; Fe(H)(NH) 2, x = 1, X = H; PNHP = bis[2-(diisopropylphosphino)ethyl]amine) is active for the electrocatalytic oxidation of isopropanol. At room temperature, the amido FeN 1 dehydrogenates isopropanol to form acetone. The resulting amino hydride complex Fe(H)(NH) 2 is subsequently oxidized by one electron at a low potential (–0.74 V versus ferrocene/ferrocenium, Fc o/+ ) in tetrahydrofuran. In the presence of strong base (phosphazene base P 2 -Et, Et-N = P 2 (dma) 5 , P2), this oxidation process becomes a two-electron, two-proton process that regenerates FeN 1. FeN 1 is active for the electrooxidation of isopropanol in the presence of strong base (i.e., P2) with an onset potential near –1 V versus Fc o/+ . By cyclic voltammetry, fast turnover frequencies of 1.7 s –1 for isopropanol oxidation are achieved with FeN 1. Lastly, controlled potential electrolysis studies confirm that the product of isopropanol electrooxidation is acetone, generated with high Faradaic efficiency (~100%).
37 INORGANIC, ORGANIC, PHYSICAL, AND ANALYTICAL CH↗
Accelerated Steam Methane Reforming by Dynamically Applied Charges
Catalyst design has traditionally focused on tuning active site properties to optimally bind reaction intermediates and balance the kinetic requirements of multiple competing chemical processes, as necessitated by the Sabatier principle. It has recently been proposed that for reactions following certain potential energy landscapes, the activity limit imposed by the Sabatier principle may be overcome by using programmed oscillations of surface electron density at the timescales of surface reactions (i.e., “catalytic resonance”). Here, we use a combination of density functional theory (DFT) simulations and transient kinetic models (TKMs) to simulate the kinetics of steam methane reforming (SMR) on Ru(211) surfaces under statically and dynamically applied charges. DFT-calculated binding energies of SMR intermediates and transition states exhibit strong sensitivity to positively applied charges and follow unique scaling relationships that deviate from linear periodic trends across transition metals. Our simulations demonstrate that applying a small positive charge to Ru dramatically enhances the steady-state turnover frequency (TOF) of SMR by up to 5 orders of magnitude above the TOF observed over neutral Ru. Thus, statically charging Ru catalysts may be an effective strategy to lower the temperature requirements for SMR. Dynamic square-wave oscillations in charge resulted in SMR catalytic resonance with an onset frequency f ∼ 106 Hz and the corresponding average TOFs exceeding the statically charged Ru surface by an additional 15%. Here, based on sensitivity analyses performed for the two end points of oscillation, we propose that dynamic TOF improvement beyond the Sabatier maximum can be expected when the system oscillates between two kinetic regimes that are uniquely controlled by distinct elementary steps.
Catalysts↗
Capturing Surface Coverage Effects in Heterogeneous Catalysis
Adsorbate–adsorbate lateral interactions at relevant surface coverages have a significant effect on chemical kinetics, thereby influencing the activity of a heterogeneous catalyst. Coverage-dependent kinetic and thermodynamic parameters therefore must be included in studies of such complex systems to properly predict the turnover frequencies and kinetic trends. Thus, it becomes extremely important to accurately capture the strength of lateral interactions between neighboring species under realistic reaction conditions. In this Perspective, we discuss the various existing computational and experimental methods for determining adspecies coverage and configurational effects. The choice of the tools and methods employed in such studies depends on factors such as time, length scales, computational cost, the presence of solvents, and reaction conditions. The applications of each method and the respective challenges are also discussed here. As a result, we discuss the recent developments and future of the state-of-the-art for inclusion of surface coverage and configuration into a holistic picture for accurate predictions of catalytic behavior.
09 BIOMASS FUELS↗
First-Principles Insights into the Thermocatalytic Cracking of Ammonia-Hydrogen Blends on Fe(110). 2. Kinetics
Ammonia (NH 3 ) is an energy-rich molecule that is routinely synthesized from nitrogen (N 2 ) and hydrogen (H 2 ). NH 3 ’s more favorable physical properties compared to H 2 suggests it may offer a way to more conveniently store, transport, and, when needed, extract H 2 via thermal decomposition. However, the high kinetic barrier and endoergicity to decompose to H 2 and N 2 require high temperatures. The standard reaction free energy indicates nearly 100% thermodynamic conversion to the diatomic molecules only at ~673 K and higher. However, even at these temperatures, a catalyst, e.g., iron (Fe), is needed for favorable kinetic conversion. Here, in this study, we explore via density functional theory the kinetics of NH 3 decomposition on the most stable facet of body-centered cubic Fe, namely, (110), under typical high-temperature and finite-pressure operando conditions. We predict coverage-dependent energetics of elementary surface reactions, often neglected in atomic-scale modeling. From these models, we find the recombinative desorption of adsorbed N as N 2 is rate-determining at 573.15–773.15 K and even at an extreme case of 1173.15 K. From microkinetic modeling, we find that the steady-state turnover frequencies (TOFs) for N 2 and H 2 generation rates (r$_{H_2}$) depend exponentially on temperature. The catalyst achieves a steady-state TOF of 36.4 s –1 and an r$_{H_2}$ of 0.107 μmol cm –2 s –1 for a feed of 1.8 bar NH 3 with 0.2 bar H 2 at 1173.15 K. However, at 773.15 K, with the same feed composition and velocity, the steady-state TOF and r$_{H_2}$ decrease to 0.14 s –1 and 4.10 × 10 –4 μmol cm –2 s –1 , respectively, as the process is significantly hindered by slow N 2 desorption. Although at first glance counterintuitive, our simulations suggest that surface modifications that reduce Fe’s reactivity toward NH x species should enhance its overall NH 3 decomposition activity.
37 INORGANIC, ORGANIC, PHYSICAL, AND ANALYTICAL CH↗
Fermi-Based Kinetic Model for the Sabatier Reaction: Sabatier Principle and Beyond It
The CO 2 methanation reaction (aka the Sabatier reaction) is one of the important carbon utilization technologies to reduce atmospheric greenhouse gases. In this study, kinetic modeling of the Sabatier reaction proceeding via the CO route has been presented by considering weak and strong chemisorption concepts rooted in electronic theory. Based on the single-route mechanism of the Sabatier reaction, the dependence of the reaction rate on the position of the catalyst Fermi level with respect to the antibonding states of the reagent (CO 2 ) and the product (CH 4 ), is demonstrated with the classic Sabatier volcano-shape. Furthermore, the model was applied to explain experimental results of reaction rates for Ni- and Ru-based catalytic systems and to understand origin of enhancement of turnover frequencies established for dynamic catalysis.
37 INORGANIC, ORGANIC, PHYSICAL, AND ANALYTICAL CH↗