Dual-site Mechanism Research Articles

Recent Research on Iridium-Based Electrocatalysts for Acidic Oxygen Evolution Reaction from the Origin of Reaction Mechanism.

As the anode reaction of proton exchange membrane water electrolysis (PEMWE), the acidic oxygen evolution reaction (OER) is one of the main obstacles to the practical application of PEMWE due to its sluggish four-electron transfer process. The development of high-performance acidic OER electrocatalysts has become the key to improving the reaction kinetics. To date, although various excellent acidic OER electrocatalysts have been widely researched, Ir-based nanomaterials are still state-of-the-art electrocatalysts. Hence, a comprehensive and in-depth understanding of the reaction mechanism of Ir-based electrocatalysts is crucial for the precise optimization of catalytic performance. In this review, the origin and nature of the conventional adsorbate evolution mechanism (AEM) and the derived volcanic relationship on Ir-based electrocatalysts for acidic OER processes are summarized and some optimization strategies for Ir-based electrocatalysts based on the AEM are introduced. To further investigate the development strategy of high-performance Ir-based electrocatalysts, several unconventional OER mechanisms including dual-site mechanism and lattice oxygen mediated mechanism, and their applications are introduced in detail. Thereafter, the active species on Ir-based electrocatalysts at acidic OER are summarized and classified into surface Ir species and O species. Finally, the future development direction and prospect of Ir-based electrocatalysts for acidic OER are put forward.

Structure engineering and surface reconstruction enabling MoNi4 hollow nanocube for efficient urea-assisted water electrolysis

Nickel-based electrocatalysts are identified as the promising candidates for both urea oxidation reaction (UOR) and hydrogen evolution reaction (HER) in urea-assisted water electrolysis to accelerate energy saving hydrogen production. However, the high occupation of Ni 3d orbital leads to their constrained adsorption for reactant and multiple intermediates during UOR process. Herein, considering the d-band electron complementation, the bimetallic MoNi4 hollow nanocubes self-assembled with numerous ultrathin nanosheets was constructed by introducing molybdenum with relatively empty d orbital and synergistically boost UOR/HER activities. Notably, the MoNi4 requires a lower cell voltage of 1.462 V to drive a current density of 10 mA cm−2 in urea electrolyzer, superior to that in water electrolyzer (1.796 V) and most of the reported nickel-based electrocatalysts, as well as the urea degradation rate is impressive. In situ technologies including electrochemical impedance spectroscopies and Raman spectra reveal that the favorable UOR pathway can be initiated directly by the reconstructed NiOOH species. Density functional theory (DFT) calculations prove that the urea molecular are preferential absorbed on Ni site and then collaborate with the adjacent Mo site to carry out the subsequently dehydrogenation, N-N coupling, and desorption in energetically favorable pathways, proving their dual site mechanism (synergy between Mo and Ni).

Kinetics of the CH4/O2 reaction over supported palladium catalysts on K-doped manganite perovskite: Impact of potassium on the nature of Pd-support interface and related reaction mechanisms

The kinetics of the CH4/O2 reaction, occurring on post-combustion catalysts to treat the exhaust gases of natural gas-powered engines, has been investigated in lean conditions on model Pd catalysts supported on K-doped LaMnO3. Experimental and predicted reaction rates have been compared according to a single site on Pd active sites and dual site reaction mechanism on active sites located at the Pd-perovskite interface. Comparisons with a benchmark Pd/LaMnO3 emphasized significant changes induced by thermal aging at 750 °C in wet atmosphere (10 vol% H2O in 5 vol% O2). Indeed, the contribution of the single site mechanism grows on aged Pd/LaMnO3 emphasizing a deterioration of the Pd-LaMnO3 interface. The opposite trend is observed on Pd/La1−xKxMnO3 with kinetics obeying to a single site mechanism on pre-reduced catalyst and then shifting to a quasi-exclusive dual site mechanism after aging. Such kinetic features have been discussed with respect to bulk and surface physicochemical characterization pointing out the key role of potassium in the stability of the Pd-support interface.

Pd nanoflakes epitaxially grown on defect MoS2 nanosheets for enhanced nitroarenes hydrogenation to anilines

Hydrogenation of nitroarenes to anilines under mild conditions is attractive from both theoretical and practical viewpoints. Here, high-performance catalyst with layered Pd nanoparticles epitaxially growing on ultrathin MoS2 nanosheets (Pd/MoS2) were prepared. It shows surprisingly high catalytic activity, selectivity and stability for hydrogenation of various nitroarenes to corresponding anilines; the turnover frequency is one or two magnitudes higher than conventional Pd-based catalysts. Surface sulfur vacancies are formed on MoS2 nanosheets with low formation energy barrier. These S vacancies provide efficient positions for adsorbing Pd atoms, which enables facile cleavage of N–O bond as result of strong interfacial electron interaction that decreases activation energy by 0.41 eV in comparison with conventional Pd catalysts. In situ spectroscopy and DFT calculation results reveal a dual-site mechanism, in which surface Pd sites adsorb and dissociate H2 into active H*, and then readily reacts with nearby activated nitroarene to form aniline on interfacial Pd-MoS2 sites.

Dual receptor-sites reveal the structural basis for hyperactivation of sodium channels by poison-dart toxin batrachotoxin

The poison dart toxin batrachotoxin is exceptional for its high potency and toxicity, and for its multifaceted modification of the function of voltage-gated sodium channels. By using cryogenic electron microscopy, we identify two homologous, but nonidentical receptor sites that simultaneously bind two molecules of toxin, one at the interface between Domains I and IV, and the other at the interface between Domains III and IV of the cardiac sodium channel. Together, these two bound toxin molecules stabilize α/π helical conformation in the S6 segments that gate the pore, and one of the bound BTX-B molecules interacts with the crucial Lys1421 residue that is essential for sodium conductance and selectivity via an apparent water-bridged hydrogen bond. Overall, our structure provides insight into batrachotoxin’s potency, efficacy, and multifaceted functional effects on voltage-gated sodium channels via a dual receptor site mechanism.

Dual-site OER mechanism exploration through regulating asymmetric multi-site NiOOH.

Asymmetric nickel oxyhydroxide (NiOOH) possesses multi-OH and O active sites on different surfaces, (001) and (001̄), which possibly causes a complicated catalytic process. Density functional theory (DFT) calculations reveal that the unconventional dual-site mechanism (UDSM) of the oxygen evolution reaction (OER) on NiOOH (001) and (001̄) exhibits significantly lower overpotentials of 0.80 and 0.77 V, compared to 1.24 and 1.62 V for the single-site mechanism (SSM), respectively. Through chemical doping or heterojunction modifications, the constructed NiOOH@FeOOH (001̄) heterojunction reduces the thermodynamic overpotential to 0.49 V from original 0.77 V undergoing the UDSM. Although Fe/Co-doping or physical compression yield similar or slightly higher overpotentials and are not conductive to facilitating the OER process by the UDSM, all dual-site paths exhibit obviously lower overpotentials than the SSM for pristine and regulated NiOOH (001) and (001̄) from the whole viewpoint. This work identifies a more reasonable and efficient dual-site OER mechanism, which is expected to help the rational design of highly-efficient electrocatalysts.

(Invited) Understanding and Optimization of Versatile Molecular and Coordination Polymer-Based 3d Transition Metal Oxygen Evolution Reaction Catalysts

The demanding multi-electron transfer process renders the oxygen evolution reaction (OER) a bottleneck for achieving efficient clean hydrogen generation via water electrolysis.[1] Over the past decades, two main categories of catalysts, namely, homogeneous molecular and heterogeneous catalysts, have been implemented for the OER. However, due to the sluggish reaction kinetics and the aggressive reaction media of the OER, the structural integrity of both homogeneous molecular and heterogeneous catalysts faces the dramatic challenges. This calls for a thorough understanding and close monitoring of OER catalysts under their operando reaction conditions.Over the last years, we have been combining a variety of in situ/operando spectroscopy approaches with computational studies toward the comprehensive understanding of our designed catalysts at the atomic level. With this information in hand, we established a full identification of catalytically active species and sites for several systems, some of which are discussed here.First, inspired by nature’s {Mn4CaOx} OER complexes, we recently reported on the design of a tetramer Cu-bipyridyl complex for the OER.[2] Structural characterizations demonstrated a new defect-cubane structure of our designed complex, [Cu4(pyalk)4(OAc)4](ClO4)(HNEt3). We found that this Cu-bipyridyl complex can further undergo structural transformations into two unique complexes under different solution conditions, namely Cu-dimer and Cu-monomers, as revealed by in situ UV-vis and ERP characterizations as well as electrospray ionization mass spectrometry. Specifically, the Cu-monomers can be only formed in presence of carbonate buffer (pH 10.5). Otherwise, the structural transformation into a Cu-dimer complex [Cu2(pyalk)2(OAc)2(H2O)] is dominant under solution conditions. Furthermore, electrochemical characterizations revealed an overpotential of 960 mV to reach a current density of 0.1 mA/cm2 of our designed Cu-dimer catalysts, which is comparable with significant Cu-based OER electrocatalysts. To gain in-depth insights into their conversion processes, postcatalytic characterizations of Cu-based molecular catalysts were carried out based on X-ray photoelectron/absorption (XAS/XPS) spectroscopy appraoches. The results showed that nanosized Cu-oxide-based species were formed in situ in Cu-based molecular catalysts after the OER. Our study highlights the crucial role of the structural integrity of molecular catalysts in solutions for their efficient design.In parallel, we explored the structural transformations of heterogeneous electrocatalysts during the OER. As a typical example, we developed a cost-effective and high-performance NiFe-based coordination polymer (referred to as NiFe-CP) as OER electrocatalyst, which is being investigated as the best-known bimetallic combination for the OER.[3] A central element of our study is the monitoring of true catalytically active species. Results from spectroscopic characterizations revealed a kinetic restructuring of NiFe-CPs into NiFe (oxy)hydroxides during the OER. To further improve the OER activity, we introduced a facile NaBH4-assisted reduction strategy to prepare low-crystalline reduced NiFe-CP (denoted as R-NiFe-CP) OER electrocatalysts with rich structural deficiencies. These catalysts can maintain a very low overpotential of 225 mV at 10 mA/cm2 for over 120 h without any performance decline, outperforming many recent reported bimetallic OER electrocatalysts. As revealed by XAS characterizations and density functional theory (DFT) calculations, engineering of structural deficiencies not only tunes the local electronic structure but also optimizes the rate-determining step towards facilitated OH- adsorption. Noteworthy, the true OER active sites of R-NiFe-CPs originate from the in situ reconstructed Ni-O-Fe motifs. However, fundamental questions, as to (a) the role of engineered structural deficiencies in the generation of active species and (b) facilitating the formation of catalytically active dual oxygen-bridged moieties, need to be answered. Combination of time-resolved operando XAS monitoring and DFT calculations enables the tracking and understanding of the kinetic changes of active species and sites under the operando reaction conditions. We found that the OER of R-NiFe-CPs relies on the in situ formation of crucial high-valent NiIV-O-FeIVO moieties.[4] Furthermore, an anionic engineering strategy through heteroatom sulfur incorporation was carried out to obtain S-R-NiFe-CP showing faster OER kinetics. Importantly, engineered sulfur content promotes the generation of catalytically active S-NiIVO-FeIVO motifs prior to the OER. This offers a lower onset potential to trigger the OER of S-R-NiFe-CPs compared to sulfur-free R-NiFe-CPs. Moreover, our results also suggest a dual-site mechanism pathway of S-R-NiFe-CPs during the OER, in which the O-O bond formed atop the S-NiIVO-FeIVO moieties. Such an anionic modulation strategy for promoting the formation of catalytically active structural moieties and for optimizing the OER kinetics opens an avenue to optimize a wide range of heterogeneous catalysts for the OER.[1] Zhao, Y. et al. Chem. Rev. 2023 , doi.org/10.1021/acs.chemrev.2c00515.[2] Adiyeri Saseendran, D. P. et al. Chem. Comm. 2023, In Revision.[3] Zhao, Y. et al. Adv. Energy Mater. 2020, 10, 2002228.[4] Zhao, Y. et al. ACS Nano 2022, 16, 15318-15327.

Insights into CO Oxidation in Cu/CeO2 Catalysts: O2 Activation at the Dual‐Interfacial Sites

AbstractCu clusters supported on CeO2 have been reported as a promising and active catalyst for CO oxidation. However, the identification of interfacial interactions and active sites is still a great challenge. In this work, we demonstrated that interfacial chemistry can be understood and predicted by using a simple descriptor of adsorbate‐surface interactions that uncovers structure‐activity relationships. The catalytic activity of CeO2 supported Cu clusters for CO oxidation was studied by density functional theory. The Cu−Ce dual site mechanism enables the Cu/CeO2 catalyst to have a much lower reaction energy barrier than the Mars‐van Krevelen (M‐vK) mechanism and the Cu‐only mechanism. The reaction energy barriers of Cu/CeO2‐x with an oxygen vacancy on the CeO2 surface were 0.10, 0.37 and 0.77 eV, respectively. The excellent performance of Cu/CeO2 catalysts is related to the interfacial interaction between Cu and CeO2 and their synergistic redox behaviors, and oxygen vacancies facilitate the generation and stabilization of active Cu+ species through the interaction with Cu clusters. And we have identified the binding energy of O2* can describe the CO oxidation activity of Cu/CeO2. Our study provides insight into the nature of active sites for Cu/CeO2 catalysts and guidance for high‐performance.

N2-to-NH3 conversion by excess electrons trapped in point vacancies on 5f-element dioxide surfaces.

Ammonia (NH3) is one of the basic chemicals in artificial fertilizers and a promising carbon-free energy storage carrier. Its industrial synthesis is typically realized via the Haber-Bosch process using traditional iron-based catalysts. Developing advanced catalysts that can reduce the N2 activation barrier and make NH3 synthesis more efficient is a long-term goal in the field. Most heterogeneous catalysts for N2-to-NH3 conversion are multicomponent systems with singly dispersed metal clusters on supporting materials to activate N2 and H2 molecules. Herein, we report single-component heterogeneous catalysts based on 5f actinide dioxide surfaces (ThO2 and UO2) with oxygen vacancies for N2-to-NH3 conversion. The reaction cycle we propose is enabled by a dual-site mechanism, where N2 and H2 can be activated at different vacancy sites on the same surface; NH3 is subsequently formed by H- migration on the surface via associative pathways. Oxygen vacancies recover to their initial states after the release of two molecules of NH3, making it possible for the catalytic cycle to continue. Our work demonstrates the catalytic activities of oxygen vacancies on 5f actinide dioxide surfaces for N2 activation, which may inspire the search for highly efficient, single-component catalysts that are easy to synthesize and control for NH3 conversion.

Regulating the scaling relationship for high catalytic kinetics and selectivity of the oxygen reduction reaction

The electrochemical oxygen reduction reaction (ORR) is at the heart of modern sustainable energy technologies. However, the linear scaling relationship of this multistep reaction now becomes the bottleneck for accelerating kinetics. Herein, we propose a strategy of using intermetallic-distance-regulated atomic-scale bimetal assembly (ABA) that can catalyse direct O‒O radical breakage without the formation of redundant *OOH intermediates, which could regulate the inherent linear scaling relationship and cause the ORR on ABA to follow a fast-kinetic dual-sites mechanism. Using in situ synchrotron spectroscopy, we directly observe that a self-adjustable N-bridged Pt = N2 = Fe assembly promotes the generation of a key intermediate state (Pt‒O‒O‒Fe) during the ORR process, resulting in high reaction kinetics and selectivity. The well-designed Pt = N2 = Fe ABA catalyst achieves a nearly two orders of magnitude enhanced kinetic current density at the half-wave potential of 0.95 V relative to commercial Pt/C and an almost 99% efficiency of 4-electron pathway selectivity, making it one of the potential ORR catalysts for application to the energy device of zinc‒air cells. This study provides a helpful design principle for developing and optimizing other efficient ORR electrocatalysts.

A Unique NiOOH@FeOOH Heteroarchitecture for Enhanced Oxygen Evolution in Saline Water.

The development of highly efficient non-precious metal electrocatalysts for the oxygen evolution reaction (OER) in low-grade or saline water is currently of great importance for the large-scale production of hydrogen. In this study, by using an electrochemical activation pretreatment, metal oxy(hydroxide) nanosheet structures derived from self-supported nickel-iron phosphide and nitride nanoarrays grown on Ni foam are successfully fabricated for OER catalysis in saline water. It is demonstrated that the different NiOOH and NiOOH@FeOOH (NiOOH grown on FeOOH) structures are generated from nickel-iron nitride and phosphide, respectively, after electrochemical activation. In particular, the NiOOH@FeOOH heteroarchitecture shows outstanding electrocatalytic performance with an ultralow overpotential of 292mV to drive the current density of 500mAcm-2 . An unconventional dual-sites mechanism (UDSM) is proposed to address the OER process on NiOOH@FeOOH and show that the FeOOH underlayer plays a critical role regarding the enhanced OER activity of NiOOH. The new possible UDSM involving two reaction sites presents a different understanding of the OER process on multi-OH layer complexes, which is expected to guide the design of heteroarchitecture electrocatalysts.

Optimized NiFe-Based Coordination Polymer Catalysts: Sulfur-Tuning and Operando Monitoring of Water Oxidation.

In-depth insights into the structure-activity relationships and complex reaction mechanisms of oxygen evolution reaction (OER) electrocatalysts are indispensable to efficiently generate clean hydrogen through water electrolysis. We introduce a convenient and effective sulfur heteroatom tuning strategy to optimize the performance of active Ni and Fe centers embedded into coordination polymer (CP) catalysts. Operando monitoring then provided the mechanistic understanding as to how exactly our facile sulfur engineering of Ni/Fe-CPs optimizes the local electronic structure of their active centers to facilitate dioxygen formation. The high OER activity of our optimized S-R-NiFe-CPs outperforms the most recent NiFe-based OER electrocatalysts. Specifically, we start from oxygen-deprived Od-R-NiFe-CPs and transform them into highly active Ni/Fe-CPs with tailored sulfur coordination environments and anionic deficiencies. Our operando X-ray absorption spectroscopy analyses reveal that sulfur introduction into our designed S-R-NiFe-CPs facilitates the formation of crucial highly oxidized Ni4+ and Fe4+ species, which generate oxygen-bridged NiIV-O-FeIV moieties that act as the true OER active intermediates. The advantage of our sulfur-doping strategy for enhanced OER is evident from comparison with sulfur-free Od-R-NiFe-CPs, where the formation of essential high-valent OER intermediates is hindered. Moreover, we propose a dual-site mechanism pathway, which is backed up with a combination of pH-dependent performance data and DFT calculations. Computational results support the benefits of sulfur modulation, where a lower energy barrier enables O-O bond formation atop the S-NiIV-O-FeIV-O moieties. Our convenient anionic tuning strategy facilitates the formation of active oxygen-bridged metal motifs and can thus promote the design of flexible and low-cost OER electrocatalysts.

Carbon tube-graphene heterostructure with different N-doping configurations induces an electrochemically active-active interface for efficient oxygen electrocatalysis

• An active-active heterointerface is in-situ constructed by urea-assisted pyrolysis strategy. • The active-active interface with directional electron transfer induces dual active sites. • N-SMCTs@N-rGO exhibits excellent electrocatalytic activities and stability for ORR/OER. • N-SMCTs@N-rGO as air-cathode delivers remarkable performance for zinc-air batteries. To accelerate the sluggish kinetics for oxygen reduction/evolution reaction (ORR/OER) of metal-free carbon catalysts, we here report an electrochemically active-active interface for promoting ORR/OER kinetics, which is constructed by in-situ coupling N-doped sub-micron carbon tubes (N-SMCTs, electron donors) with N-doped reduced graphene oxide (N-rGO, electron acceptors). We find that N-SMCTs@N-rGO yields superior bifunctional ORR/OER activity with high durability ( E 1/2 of 0.87 V for ORR, η 10 of 351 mV for OER). Moreover, the zinc-air battery with this material as an electrode displays high power density of 126 mW cm −2 with excellent cycle stability. Density functional theory (DFT) unveils a dual-site mechanism: the carbon adjacent to graphitic-N in N-SMCTs is the most active site for *OOH intermediate while the carbon neighboring to pyridinic-N in N-rGO is beneficial for the adsorption of *O/*OH intermediates. The unique active-active interface with dual-site mechanism suggests its potential to overcome energy barrier bottleneck in the traditional bifunctional oxygen catalysts.

Construction of Ru/Fecop Heterointerface to Drive Dual Active Sites Mechanism for Efficient Overall Water Splitting

Construction of Ru/Fecop Heterointerface to Drive Dual Active Sites Mechanism for Efficient Overall Water Splitting

Construction of Ru/FeCoP heterointerface to drive dual active site mechanism for efficient overall water splitting

The water-splitting efficiency is improved by Ru/FeCoP heterointerface to drive dual-active site mechanism. The η100 of Ru@FeCoP are 0.0741 V for HER and 0.314 V for OER, which is reduced by 63.8% and 55.2% over Pt/C and RuO2, respectively.

Non-precious metal activated MoSi2N4 monolayers for high-performance OER and ORR electrocatalysts: A first-principles study

Developing high-performance electrocatalysts for oxygen evolution reaction (OER) and oxygen reduction reaction (ORR) is crucial for energy conversion and storage. Recently, a new type of two-dimensional material MoSi2N4 was successfully synthesized and received considerable attention because of novel properties and potential applications. Herein, by means of first principles calculation, the OER/ORR activities of 3d transition metal (TM) atoms embedded MoSi2N4 (TM@MSN) were investigated. The calculated results indicate that TM atoms on MoSi2N4 exhibit good electrochemical stability. On TM sites, Ti@MSN shows the highest OER activity with an overpotential of 0.48 V, whereas Cr@MSN is the most active ORR catalyst with an overpotential of 0.48 V. The Si site (Si−N1−Cu) of Cu@MSN follows the dual-site mechanism, exhibiting the same OER/ORR overpotential as that of N site (0.55/0.65 V). Interestingly, the outer N site (Zn−N1) of Zn@MSN achieves the lowest OER overpotential of 0.38 V, better than that of the state-of-the-art RuO2 catalyst. We demonstrate that 3d TM atoms not only serve as active sites themselves but also activate the host atoms to improve OER/ORR performance of MoSi2N4. Our work opens new windows of opportunity for developing novel catalysts beyond the precious metal-based electrocatalysts for efficient energy conversion and storage.

An updated study on NO catalytic oxidation over activated carbon: The effect of pore structure and a dual-site mechanism

NO catalytic oxidation (NOCO) to NO2 is widely believed to be a fundamental step in selective catalytic reduction (SCR) over activated carbon (AC). Thus, the enhancement of NOCO over AC is significant. The effect of the pore structure on NOCO is discussed first. A series of microporous AC samples with various microporous surface areas were prepared, in which a strong positive correlation existed between NO conversion and the microporous surface area. With an increase in the microporous surface area, the NO conversion increased from 13.10% for MAC1 to 41.18% for MAC4. Concurrently, a hierarchical AC sample with an approximate hierarchy degree (the ratio of Vmes+mac to Vt) of 50% demonstrated higher NO conversion than microporous AC when the microporous surface areas were similar. HAC2 exhibited the largest NO conversion (51.58%). Therefore, the microporous surface area and degree of hierarchy are key factors that enhance NO conversion. NOCO has been demonstrated to be an O-enriched process. In situ diffuse reflectance infrared Fourier transform spectroscopy (DRIFT) results revealed that carbonyl groups were generated during the NOCO process. Density functional theory calculations were then performed. Carbonaceous models with unsaturated C atom, carbonyl, and lactone groups were constructed, and the corresponding NOCO reaction paths were calculated. A dual-site NOCO mechanism was proposed by combining the experimental and calculation results. Two types of active sites existed: unsaturated C atoms and high-spin density C atoms that affected the kinetic and steady-state stages of NOCO, respectively. The total amount of high-spin density C atoms strongly depends on the microporous surface area. The study of NOCO was updated, which contributed to the preparation of highly efficient AC for NOCO.

Abundant Lewis acidic sites of peculiar fibrous silica zeolite X enhanced toluene conversion in side chain toluene methylation

Side chain toluene methylation reaction (SCTM) is a reliable reaction to produce styrene and ethylbenzene from toluene. Nevertheless, due to the lack of efficient catalyst caused by less Lewis acid sites and mesoporosity properties, the challenges in this SCTM reaction are still ongoing. Hence, this work is accomplished to gain a better understanding in designing a robust catalyst for side chain toluene methylation. Herein, this work emphasizes the uniqueness of top-notch metal-free fibrous silica zeolite X (FZX) by microemulsion method. This FZX showed superior catalytic activity towards side chain methylation with 15.9% yield of styrene and ethylbenzene, which was 16-fold higher compared to the commercial zeolite X (CZX) catalyst. This was achieved by the excellent properties of dendrimeric silica fibers of FZX possessing high concentration of basic sites and Lewis acid sites as well as high mesoporosity, as proved by the FTIR-Pyrrole, FTR-Pyridine and nitrogen physisorption analyses respectively. The presence of intermediates was confirmed by in-situ FTIR toluene-methanol. 27Al MAS NMR analysis proved that the presence of excessive extraframework alumina (EFAl) species could act as additional Lewis acid sites in FZX to enhance toluene adsorption on the FZX catalyst, thus improving the toluene conversion and desired styrene and ethylbenzene products. Silica MCM41 exhibited lower yield of styrene and ethylbenzene by the fact imbalance of acid base property of catalyst. The kinetic study proved that the side chain toluene methylation occurred via dissimilar dual site mechanism of Langmuir-Hinshelwood. This rationally designed metal-free FZX demonstrates its high potential in polymer and/or petrochemical as well as other industrial applications.

Catalytic site requirements for N2O decomposition on Cu-, Co-, and Fe-SSZ-13 zeolites

N2O decomposition is investigated on Cu, Co and Fe-exchanged SSZ-13 zeolite catalysts at relatively low metal loadings. The catalysts are synthesized by solution ion exchange, and subjected to X-ray diffraction (XRD), temperature-programed-reduction by H2 (H2-TPR), temperature-programed-reaction of N2O (N2O-TPR) coupled with in-situ transmission FTIR, and finally steady-state flow reaction tests. At low N2O pressures (<0.05 kPa), all catalysts display pseudo first-order kinetics. From Arrhenius analysis, Cu and Fe-SSZ-13 display very different apparent activation energies but similar pre-exponential factors, suggesting their similar reaction mechanisms. N2O decomposition follows a dual-site mechanism, occurring on dimeric M-O-M sites in these catalysts, and O2 is formed by the combination of two O ad-atoms from two vicinal metal sites. Under low N2O pressure (0.05 kPa) and first-order kinetic regime, the reaction is limited by N-O cleavage on bare metal active sites. In comparison to Cu-SSZ-13, the much higher N2O decomposition rate over Fe-SSZ-13 is attributed to the much lower activation barriers for the N-O cleavage step. N2O decomposition occurs on isolated Co2+ ions in Co-SSZ-13. The rate-limiting step is N-O cleavage on an O-occupied Co site in the low-pressure first order kinetic regime. This single-site mechanism leads to much higher pre-exponential factors as compared to the dual-site mechanism. This beneficial factor for reaction rate enhancement, however, is compromised by the much higher activation barriers over this catalyst.

Nitrogen Activation and Transformation on Monometallic Niobium Boron Oxide Cluster Anions at Room Temperature: A Dual-Site Mechanism.

Dinitrogen activation and transformation at room temperature is a goal that has been long sought after. Despite that, it remains underdeveloped due to being a challenging research area and the need for a better mechanistic understanding. Herein, we report that well-defined NbB3O2- gas-phase clusters can activate one N2 molecule and generate the products B3N2O- and B3N2O2-, as applying mass spectrometry and theoretical calculations. This unusual N2 activation reaction results from the different functions of the Nb and B3O2 moieties in NbB3O2-. Theoretical calculations suggest that a catalytic cycle can be completed by the recovery of NbB3O2-, which is achieved through the reactions of Nb and NbO with B3O2- and B3O-, respectively. This is the first example of N2 efficient transformation at a monometallic cluster, and this method for generating dual active sites by designing proper ligands may open the way toward the development of more effective N2 fixation and functionalization methodologies.