Active Site Engineering Research Articles

Host-Guest Chemistry Meets Electrocatalysis: Cucurbit[6]uril on a Au Surface as a Hybrid System in CO2 Reduction.

The rational control of forming and stabilizing reaction intermediates to guide specific reaction pathways remains to be a major challenge in electrocatalysis. In this work, we report a surface active-site engineering approach for modulating electrocatalytic CO2 reduction using the macrocycle cucurbit[6]uril (CB[6]). A pristine gold surface functionalized with CB[6] nanocavities was studied as a hybrid organic–inorganic model system that utilizes host–guest chemistry to influence the heterogeneous electrocatalytic reaction. The combination of surface-enhanced infrared absorption (SEIRA) spectroscopy and electrocatalytic experiments in conjunction with theoretical calculations supports capture and reduction of CO2 inside the hydrophobic cavity of CB[6] on the gold surface in aqueous KHCO3 at negative potentials. SEIRA spectroscopic experiments show that the decoration of gold with the supramolecular host CB[6] leads to an increased local CO2 concentration close to the metal interface. Electrocatalytic CO2 reduction on a CB[6]-coated gold electrode indicates differences in the specific interactions between CO2 reduction intermediates within and outside the CB[6] molecular cavity, illustrated by a decrease in current density from CO generation, but almost invariant H2 production compared to unfunctionalized gold. The presented methodology and mechanistic insight can guide future design of molecularly engineered catalytic environments through interfacial host–guest chemistry.

Active sites engineering of Pt/CNT oxygen reduction catalysts by atomic layer deposition

Abstract Understanding carbon-supported Pt-catalyzed oxygen reduction reaction (ORR) from the perspective of the active sites is of fundamental and practical importance. In this study, three differently sized carbon nanotube-supported Pt nanoparticles (Pt/CNT) are prepared by both atomic layer deposition (ALD) and impregnation methods. The performances of the catalysts toward the ORR in acidic media are comparatively studied to probe the effects of the sizes of the Pt nanoparticles together with their distributions, electronic properties, and local environments. The ALD-Pt/CNT catalysts show much higher ORR activity and selectivity than the impregnation-Pt/CNT catalysts. This outstanding ORR performance is ascribed to the well-controlled Pt particle sizes and distributions, desirable Pt0 4f binding energy, and the Cl-free Pt surfaces based on the electrocatalytic measurements, catalyst characterizations, and model calculations. The insights reported here could guide the rational design and fine-tuning of carbon-supported Pt catalysts for the ORR.

Engineering highly active oxygen sites in perovskite oxides for stable and efficient oxygen evolution

Perovskite oxides, represented by Ba0.5Sr0.5Co0.8Fe0.2O3-δ (BSCF), with O anion partially substituted by F anion are designed. Introduction of F in perovskite lattice initiated the transition of Co(III) and Fe(III) species to lower oxidation states and triggered the surface O anion to be activated to highly oxidative O2−/O‒, which is deemed to be efficient active sites for oxygen evolution reaction (OER) catalysis. As a consequence, the F substituted BSCF (F-BSCF) catalyst exhibits outstanding electrocatalytic activity with overpotential of only 280 mV to deliver 10 mA cm‒2 OER, which is among the results for state-of-the-art metal oxide based catalysts. Strikingly, an stable chronoamperometric response prolonged for 100 h and an impressive cycling stability demonstrate its prominent durability, much superior to the commercial IrO2 catalyst. These findings highlight the promising potential of F substitution as an efficient strategy for active site engineering in traditional precious metal-free OER electrocatalysts.

Surface-engineered cobalt oxide nanowires as multifunctional electrocatalysts for efficient Zn-Air batteries-driven overall water splitting

Active sites engineering of electrocatalysts is an essential step to improve their intrinsic electrocatalytic capability for practical applications. Here, we demonstrate a facile surface phosphidisation into cobalt oxide (Co3O4) nanowires to boost their active sites, where phosphorus substitution for oxygen atoms greatly increases the number of cobalt (II) cations (Co2+) and oxygen vacancies (VO) active sites. First-principles calculations combined with high-resolution X-ray photoemission spectroscopy, electron spin resonance and X-ray absorption spectroscopy characterization identify that the phosphorus heteroatoms and VO alter the electron density of surface Co atoms in Co3O4 nanowires, resulting in the conversion of cobalt (III) cations (Co3+) to more active Co2+. As a result, the multifunctional catalysts of Co3O4 nanowires exhibit superior catalytic performance in overall water splitting with a low cell voltage of 1.61 V at 10 mA cm−2 and zinc-air batteries with a high open-circuit voltage of 1.41 V and power density of 72.1 mW cm−2. Particularly, two-series-connected Zn-air batteries powerfully drive an electrolyzer system without any additional electrode materials. These experimental and theoretical findings offer a promising approach to constructing active sites in non-precious metal catalysts for efficient and stable electrocatalytic activity.

Estimation of CO2 sequestration potential by afforestation in the arid rangelands of Western Australia based on long-term empirical data

Large-scale afforestation is a key measure to mitigate global warming, however, implementation may result in land-use competition with agriculture. To avoid such competition, carbon mitigation methods using arid and semi-arid areas have been proposed, but to our knowledge there is no report of rates of sequestration based on long-term observations from actual experimentation. In this study (1999–2015), in an arid area near Leonora, Western Australia (annual rainfall: 220 mm year−1; pan evaporation: 3400 mm year−1), carbon sequestration was assessed in above and below ground biomass in Eucalyptus camaldulensis under ambient conditions and with active site amelioration (combination of water harvesting with large mounds and hardpan blasting). The carbon sequestration rate was estimated at 7.92 Mg-CO2-e ha−1 year−1 for a total carbon sink of 230 Mg-CO2-e ha−1. Carbon mitigation may thus be a viable option in arid regions, not only in Western Australia but globally, and can be enhanced with active site engineering.

In situ nanoarchitecturing and active-site engineering toward highly efficient carbonaceous electrocatalysts

Oxygen reduction reaction (ORR) lies at the heart of many renewable energy technologies. Development of facile approaches toward highly active carbon materials for ORR is urgently desirable. We accidentally discovered a method for preparing heteroatom-doped carbon catalyst through an in situ self-templating and self-sacrificing strategy during the carbonization of a single polymer precursor. The prepared N,S co-doped and hierarchical porous carbon possesses a large specific surface area up to 2446 m2 g−1, which is the state-of-the-art among the porous carbons prepared by direct pyrolysis of linear polymer precursors. The use of in situ nanoarchitecturing and active-site engineering ensures that the N,S co-doped porous carbon catalyst has sufficient active sites for charge/mass transport and catalysis. As expected, as-prepared carbonaceous electrocatalyst exhibited superior ORR activity in both alkaline and acidic electrolytes and outperformed the state-of-the-art benchmark Pt/C and the best carbonaceous electrocatalysts for ORR in aqueous media so far. Moreover, the N,S co-doped carbon catalyst was demonstrated to be a highly efficient air cathode for Zn–air batteries. Thus, this study provides a new and economical method of fabricating feasible carbon electrocatalysts with exceptional catalytic activities for energy conversion and storage.

Activity Improvements of an Engineered ω-transaminase for Ketones Are Positively Correlated with Those for Cognate Amines

Chiral amines are broadly used as essential building blocks of diverse pharmaceutical drugs. ω-Transaminase (ω-TA) is one of the promising enzymes for biocatalytic preparation of chiral amines owing to its capability for reductive amination of ketones. However, industrial application of ω-TA to asymmetric synthesis of chiral amines is often limited by marginal activities for ketones. Here, we explore whether activity improvements of ω-TA for ketones caused by active site engineering are correlated with those for cognate amines. We examined amino donor reactivities of 20 amines with ω-TA from Ochrobactrum anthropi (OATA) and its engineered variant carrying a W58L mutation (OATAW58L). Consistent with the previously observed activity increases for ketones, OATAW58L showed 3 to 79-fold activity improvements for amines. Docking simulations suggested that the activity improvements resulted from increased proximity of a bound amine to an internal aldimine owing to relocation of a small binding pocket. Activity comparison indicated that the W58L mutation induced activity increases for ketones better than it did those for amines. This result motivated us to carry out Pearson correlation analysis among the activity improvements for amines and ketones, revealing a strong positive correlation (r = 0.84). This suggests that active site engineering beneficial for activity of ω-TA toward a target amine is likely to evoke an activity improvement for its cognate ketone. Our finding is expected to provide a clue to design a robust high-throughput screening method required for creation of an engineered mutant displaying a better activity toward a target ketone.

Photocatalysis: Single‐Atom Engineering of Directional Charge Transfer Channels and Active Sites for Photocatalytic Hydrogen Evolution (Adv. Funct. Mater. 32/2018)

Photocatalysis: Single‐Atom Engineering of Directional Charge Transfer Channels and Active Sites for Photocatalytic Hydrogen Evolution (Adv. Funct. Mater. 32/2018)

Single‐Atom Engineering of Directional Charge Transfer Channels and Active Sites for Photocatalytic Hydrogen Evolution

AbstractEfficiency of layered photocatalysts such as graphitic carbon nitride (g‐CN) is still too low due to the poor utilization of photoexcited‐charge carriers. The major drawback is that the weak van der Waals force among g‐CN layers is unfavorable for the charge transfer between the adjacent layers and the intrinsically π‐conjugated planes with inefficient random in‐plane charge migration. Herein, an atomically dispersed Pd layered photocatalyst with both bridged sites of adjacent layers and surface‐sites of g‐CN is demonstrated, providing directional charge‐transfer channels and targeting active sites for photocatalytic water reduction. Both theoretical prediction and empirical characterizations are conducted to achieve the successful synthesis of single‐atom engineered Pd/g‐CN hybrid and the excellent separation of charge transfer as well as the efficient photocatalytic hydrogen evolution, much better than that of the optimized Pt/g‐CN benchmark. The finding in this work provides a rational way for tailoring the performance and engineering of single‐atomic noble metal.

Active Sites Engineering toward Superior Carbon-Based Oxygen Reduction Catalysts via Confinement Pyrolysis.

Developing efficient and low-cost defective carbon-based catalysts for the oxygen reduction reaction (ORR) is essential to metal-air batteries and fuel cells. Active sites engineering toward these catalysts is highly desirable but challenging to realize boosted catalytic performance. Herein, a sandwich-like confinement route to achieve the controllable regulation of active sites for carbon-based catalysts is reported. In particular, three distinct catalysts including metal-free N-doped carbon (NC), single Co atoms dispersed NC (Co-N-C), and Co nanoparticles-contained Co-N-C (Co/Co-N-C) are controllably realized and clearly identified by synchrotron radiation-based X-ray spectroscopy. Electrochemical measurements suggest that the Co/Co-N-C catalyst delivers optimized ORR performance due to the rich Co-Nx active sites and their synergistic effect with metallic Co nanoparticles. This work provides deep insight for rationally designing efficient ORR catalyst based on active sites engineering.

Rational Engineering of Multiple Active Sites in an Ester Hydrolase.

Effects of altering the properties of an active site in an enzymatic homogeneous catalyst have been extensively reported. However, the possibility of increasing the number of such sites, as commonly done in heterogeneous catalytic materials, remains unexplored, particularly because those have to accommodate appropriate residues in specific configurations. This possibility was investigated by using a serine ester hydrolase as the target enzyme. By using the Protein Energy Landscape Exploration software, which maps ligand diffusion and binding, we found a potential binding pocket capable of holding an extra catalytic triad and oxyanion hole contacts. By introducing two mutations, this binding pocket became a catalytic site. Its substrate specificity, substrate preference, and catalytic activity were different from those of the native site of the wild type ester hydrolase and other hydrolases, due to the differences in the active site architecture. Converting the binding pocket into an extra catalytic active site was proven to be a successful approach to create a serine ester hydrolase with two functional reactive groups. Our results illustrate the accuracy and predictive nature of modern modeling techniques, opening novel catalytic opportunities coming from the presence of different catalytic environments in single enzymes.

Active site engineering by surface sulfurization for a highly efficient oxygen evolution reaction: a case study of Co3O4 electrocatalysts

Enhanced catalytic activity of Co3O4@CoSx through surface sulfurization.

Active site engineering of Fe- and Ni-sites for highly efficient electrochemical overall water splitting

Robust oxygen evolution performance and excellent hydrogen evolution performance were realized by engineering the active Fe sites in Fe-doped Ni2P.

Ternary Pd–Ni–P nanoparticle-based nonenzymatic glucose sensor with greatly enhanced sensitivity achieved through active-site engineering

We develop a unique ternary Pd–Ni–P nanocatalyst for the sensitive enzymefree electrooxidation detection of glucose under alkaline conditions. By reducing the distance between the Pd and Ni active sites in the Pd–Ni–P nanoparticles (NPs) via atom engineering, the catalyst structure is transformed from Pd@Ni–P dumbbells into spherical NPs, greatly enhancing the catalyst sensitivity. The glassy carbon electrode modified with Pd–Ni–P ternary NPs, which behaves as an efficient nonenzymatic glucose sensor, offers excellent electrocatalytic performance with a high sensitivity of 1,136 μA·mM−1·cm−2, a short response time of 2 s, a wide linear range of 0.5 μM to 10.24 mM, a low limit of detection of 0.15 μM (signal-to-noise ratio = 3), and good selectivity and reproducibility. Moreover, owing to its superior catalytic performance, the Pd–Ni–P modified electrode has excellent reliability for glucose detection in real samples of human serum. Our study provides a promising alternative strategy for designing and constructing high-performance multicomponent nanocatalyst-based sensors.

Active Site Engineering of ω-Transaminase Guided by Docking Orientation Analysis and Virtual Activity Screening

Creation of enzyme variants displaying desirable catalytic performance usually necessitates tedious and time-consuming procedures for library generation and selection, which may be circumvented by a computational method based on a precise understanding of the reaction mechanism in the context of active site environment. Despite the great potential of ω-transaminases (ω-TAs) for asymmetric synthesis of chiral amines from ketones, it remains elusive why ω-TAs exhibit marginal activities for most ketones in contrast to their high activities for α-keto acids and aldehydes. To address this puzzling question, crystal structure determination and molecular modeling of ω-TAs were carried out to analyze docking orientations of the amino acceptors in the Michaelis complex. We found that ketones, unlike the reactive substrates, led to nonproductive binding complexes where the bound substrate was hardly accessible to a nucleophilic attack by the pyridoxamine cofactor to initiate reductive amination of the amino accept...

Switching head group selectivity in mammalian sphingolipid biosynthesis by active-site engineering of sphingomyelin synthases

SM is a fundamental component of mammalian cell membranes that contributes to mechanical stability, signaling, and sorting. Its production involves the transfer of phosphocholine from phosphatidylcholine onto ceramide, a reaction catalyzed by SM synthase (SMS) 1 in the Golgi and SMS2 at the plasma membrane. Mammalian cells also synthesize trace amounts of the SM analog ceramide phosphoethanolamine (CPE), but the physiological relevance of CPE production is unclear. Previous work revealed that SMS2 is a bifunctional enzyme producing both SM and CPE, whereas a closely related enzyme, sphingomyelin synthase-related protein (SMSr)/SAMD8, acts as a monofunctional CPE synthase in the endoplasmatic reticulum. Using domain swapping and site-directed mutagenesis on enzymes expressed in defined lipid environments, we here identified structural determinants that mediate head group selectivity of SMS family members. Notably, a single residue adjacent to the catalytic histidine in the third exoplasmic loop profoundly influenced enzyme specificity, with glutamic acid permitting SMS-catalyzed CPE production and aspartic acid confining the enzyme to produce SM. An exchange of exoplasmic residues with SMSr proved sufficient to convert SMS1 into a bulk CPE synthase. This allowed us to establish mammalian cells that produce CPE rather than SM as the principal phosphosphingolipid and provide a model of the molecular interactions that impart catalytic specificity among SMS enzymes.

Expanding Beyond the Micropore: Active-Site Engineering in Hierarchical Architectures for Beckmann Rearrangement

The ability to engineer discrete solid-acid centers within hierarchically porous architectures that contain micropores with interconnected mesopores offers the potential to overcome the spatial restraints and diffusional limitations imposed by conventional microporous zeotype catalysts, which often lead to decreased catalyst lifetimes and restricted substrate scope. By employing a one-step soft-templating approach, coupled with detailed physicochemical and spectroscopic characterization, isolated, solid-acid sites can be suitably tailored and discretely modulated within the micropores and mesopores. The design strategy facilitates a synergistic enhancement in catalytic activity, selectivity, substrate versatility, and longevity, compared to analogous microporous zeotypes that have been extensively employed in the chemical industry, as solid-acid catalysts, for the production of e-caprolactam (precursor for Nylon-6).

Morphology and Active-Site Engineering for Stable Round-Trip Efficiency Li–O2 Batteries: A Search for the Most Active Catalytic Site in Co3O4

For the purpose of finding the catalytic active site for the oxygen reduction reaction (ORR) and the oxygen evolution reaction (OER) in the crystal structure of Co3O4, a {100} faceted Co3O4 cube enclosed by only a Co2+ site and a {112} faceted Co3O4 plate also exposing a Co3+ site was adopted as a catalyst for lithium–oxygen (Li–O2) batteries. Thus, we found that the Co3+ site plays the crucial role of determining the adsorption properties of reactants, enabling high round-trip efficiency and cyclic stability, as well as the morphology of discharge products such as Li2O2.

Active-Site Engineering of ω-Transaminase for Production of Unnatural Amino Acids Carrying a Side Chain Bulkier than an Ethyl Substituent

ω-Transaminase (ω-TA) is a promising enzyme for use in the production of unnatural amino acids from keto acids using cheap amino donors such as isopropylamine. The small substrate-binding pocket of most ω-TAs permits entry of substituents no larger than an ethyl group, which presents a significant challenge to the preparation of structurally diverse unnatural amino acids. Here we report on the engineering of an (S)-selective ω-TA from Ochrobactrum anthropi (OATA) to reduce the steric constraint and thereby allow the small pocket to readily accept bulky substituents. On the basis of a docking model in which L-alanine was used as a ligand, nine active-site residues were selected for alanine scanning mutagenesis. Among the resulting variants, an L57A variant showed dramatic activity improvements in activity for α-keto acids and α-amino acids carrying substituents whose bulk is up to that of an n-butyl substituent (e.g., 48- and 56-fold increases in activity for 2-oxopentanoic acid and L-norvaline, respectively). An L57G mutation also relieved the steric constraint but did so much less than the L57A mutation did. In contrast, an L57V substitution failed to induce the improvements in activity for bulky substrates. Molecular modeling suggested that the alanine substitution of L57, located in a large pocket, induces an altered binding orientation of an α-carboxyl group and thereby provides more room to the small pocket. The synthetic utility of the L57A variant was demonstrated by carrying out the production of optically pure L- and D-norvaline (i.e., enantiomeric excess [ee]>99%) by asymmetric amination of 2-oxopantanoic acid and kinetic resolution of racemic norvaline, respectively.

Heteroatomic Ni, Sn Clusters-Grafted Anatase TiO2 Photocatalysts: Structure, Electron Delocalization, and Synergy for Solar Hydrogen Production

This work shows the molecular engineering of metal active sites on the surface of anatase nanoparticles, based on the fundamental of metal-to-metal charge transfer (MMCT). A series of Sn, Ni-co-modified TiO2 photocatalysts were prepared by use of surface organometallic chemistry. Photocatalytic hydrogen production from ethylenediaminetetraacetic acid disodium salt solution as a model reaction was used to evaluate the photocatalytic properties of the materials. The chemical states of the modifiers were characterized by a combination of various spectroscopic techniques. These results clearly reveal the synergy of atomically isolated Ni and Sn centers for solar-to-hydrogen conversion. A near 10-fold enhancement of hydrogen production is achieved on Sn, Ni-co-grafted TiO2 photocatalysts under solar light irradiation. Detailed characterization suggests that the synergy is closely related to the [−O–Ti–O–Sn–O–Ni–O–Ti−] heteroatomic clusters with the eight-member cyclic structure. An electron-delocalization loop is proposed for efficient separation of charges photogenerated in bulk TiO2 and excitons generated in Ni–O–Ti molecular linkages.