Improved Catalyst Stability Research Articles

Supramolecular Enhancement of Electrochemical Nitrate Reduction Catalyzed by Cobalt Porphyrin Organic Cages for Ammonia Electrosynthesis in Water**

AbstractThe electrochemical nitrate (NO3−) reduction reaction (NO3RR) to ammonia (NH3) represents a sustainable approach for denitrification to balance global nitrogen cycles and an alternative to traditional thermal Haber‐Bosch processes. Here, we present a supramolecular strategy for promoting NH3 production in water from NO3RR by integrating two‐dimensional (2D) molecular cobalt porphyrin (CoTPP) units into a three‐dimensional (3D) porous organic cage architecture. The porphyrin box CoPB‐C8 enhances electrochemical active site exposure, facilitates substrate–catalyst interactions, and improves catalyst stability, leading to turnover numbers and frequencies for NH3 production exceeding 200,000 and 56 s−1, respectively. These values represent a 15‐fold increase in NO3RR activity and 200‐mV improvement in overpotential for the 3D CoPB‐C8 box structure compared to its 2D CoTPP counterpart. Synthetic tuning of peripheral alkyl substituents highlights the importance of supramolecular porosity and cavity size on electrochemical NO3RR activity. These findings establish the incorporation of 2D molecular units into 3D confined space microenvironments as an effective supramolecular design strategy for enhancing electrocatalysis.

Dendritic Organosilica Nanoparticle‐Based Metal‐Enzyme Hybrid Catalyst for Chemoenzymatic Deracemization of Cyclic Amines

AbstractThe synthesis of enantiopure chiral cyclic amines is of critical importance but highly challenging. In this study, a metal‐enzyme hybrid catalyst was fabricated by the co‐immobilization of palladium nanoparticles and monoamine oxidases on dendritic organosilica nanoparticles (DONs) for chemoenzymatic deracemization, furnishing a panel of chiral cyclic amines in high enantioselectivities (96–98 % ee). Catalyst optimization revealed that the ultra‐high specific surface area, mesoporous channels and hydrophobic microenvironment of the DONs are responsible for the improved catalyst stability, activity and catalytic efficiency.

Al2O3 Nanorod with Rich Pentacoordinate Al3+ Sites Stabilizing Co2+ for Propane Dehydrogenation

The search for inexpensive, environmentally friendly, and highly effective catalysts to activate C-H bonds in propane dehydrogenation (PDH) reactions is still a major challenge. Co-based catalysts have gained significant attention in recent years due to their excellent ability to activate C-H bonds and their high selectivity towards olefins, despite being a non-noble and environmentally unfriendly metal. However, further improvements are necessary for practical utilization, particularly in terms of activity and anti-carbon deposition capacity. In this study, we synthesized Al2O3 nanorods with abundant pentacoordinated Al3+ (Al3+penta) sites. The supported Co on the Al2O3 nanorod (Co/Al2O3-NR) exhibited higher selectivity (>96% propylene selectivity) and stability (deactivation rate 0.15 h−1) compared to Co supported on an Al2O3 nanosheet with fewer pentacoordinated Al3+ sites. Various characterizations confirmed that Co(II) mainly exists as CoAl2O4 rather than Co3O4 in the form of Co/Al2O3-NR, which inhibits the reduction of Co(II) to Co0 and accordingly improves catalyst stability.

Electrochemical hydrogen evolution on Pt-based catalysts from a theoretical perspective.

Hydrogen evolution reaction (HER) by splitting water is a key technology toward a clean energy society, where Pt-based catalysts were long known to have the highest activity under acidic electrochemical conditions but suffer from high cost and poor stability. Here, we overview the current status of Pt-catalyzed HER from a theoretical perspective, focusing on the methodology development of electrochemistry simulation, catalytic mechanism, and catalyst stability. Recent developments in theoretical methods for studying electrochemistry are introduced, elaborating on how they describe solid-liquid interface reactions under electrochemical potentials. The HER mechanism, the reaction kinetics, and the reaction sites on Pt are then summarized, which provides an atomic-level picture of Pt catalyst surface dynamics under reaction conditions. Finally, state-of-the-art experimental solutions to improve catalyst stability are also introduced, which illustrates the significance of fundamental understandings in the new catalyst design.

Confinement of Cu2O by in-situ derived NH2-MIL-125@TiO2 for synergetic photothermal-driven hydrogen evolution from aqueous-phase methanol reforming

Photothermal synergism can effectively activate methanol at low operating temperature and significantly reduce the activation energy of the reaction, achieving more efficiently H2 release from methanol reforming. Here, a novel hierarchical heterojunction that integrating photo- and thermal-active Cu2O catalytic species with in-situ derived NH2-MIL-125@TiO2 framework was specifically designed for photothermal-driven aqueous phase reforming of methanol into H2. The afforded Cu2O/NH2-MIL-125@TiO2 realized an outstanding photothermal H2 production activity (apparent quantum efficiency of 22.3 % at 365 nm), ca. 13-times higher than that of thermocatalytic condition. Interestingly, the photothermal effect did confer the Cu2O/NH2-MIL-125@TiO2 with unexpected activity at low temperature subsided to 100 °C and accelerated the activation of MeOH/H2O with an obvious reduction of activation energy from 82.62 kJ·mol−1 to 52.40 kJ·mol−1.The improvement of catalyst stability and the promotion of charge separation also contributed to a long-term photothermal H2 evolution activity with average rate of 1.49x106 μmolgCu-1h−1 and a total turnover number (TON) up to 5971 in 63 h, almost 125-fold promotion compared with Cu2O.

Dynamic Electrification of Dry Reforming of Methane with In Situ Catalyst Regeneration

We report the design and performance of a rapid pulse Joule heating (RPH) reactor with an in situ Raman spectrometer for highly endothermic, reversible reactions. We demonstrate it for methane dry reforming over a bimetallic PtNi/SiO2 catalyst that shows better performance than its monometallic counterparts. The catalyst temperature ramp rate can reach ∼14000 °C/s, mainly owing to the low thermal mass and resistivity of the heating element. Joule heating elements afford temperatures unachievable by conventional technology to enhance performance and more than double the energy efficiency. Dynamic electrification can increase syngas productivity and rate. Extensive characterizations suggest that pulse heating creates an in situ catalyst regeneration strategy that suppresses coke formation, sintering, and phase segregation, resulting in improved catalyst stability, under many conditions. Potentially driven by renewable electricity, the RPH can provide superb process advantages for high-temperature endothermic reactions and lead to negative carbon emissions.

Manipulating the Cobalt Species States to Break the Conversion–Selectivity Trade-Off Relationship for Stable Ethane Dehydrogenation over Ligand-Free-Synthesized Co@MFI Catalysts

Nonoxidative dehydrogenation of low-cost alkanes provides a promising route to produce valuable olefins. Herein, we hydrothermally synthesized various non-noble-metal-based, environment-friendly, and Al-free Co@MFI catalysts without the assistance of any additional coordination agents. The Co2+ species were successfully incorporated into well-crystallized MFI to form a stable and atomically dispersed −Coδ+–Oδ−– structure. The Co@MFI catalyst could show a stably high activity for ethane dehydrogenation with equilibrium-approached conversions at 600 °C and at the same time gave an extremely high selectivity to ethylene (∼99%), which is owed to the relatively unreducible −Co–O– species and its appropriate chemical state at the right reaction-temperature window. However, the Co@MFI catalyst showed equilibrium-deviated conversions at lower temperatures (such as 550 °C) and a suppressed activity in the H2O or CO2 co-feeding tests. Then, with characterizations, density functional theory calculations, and abundant experiments over different catalysts, including impregnated Co/MFI and amorphous Co@MFI, this study has impressively demonstrated that the chemical state of Coδ+ species manipulates the conversion–selectivity trade-off relationship in the conversion of alkanes. It is suggested that Co with a lower valence like Co0 promotes both C–C and C–H bond scissions of alkanes into coke and CH4, while Co with a higher one shows a decreased activity or even inactivity for alkane dehydrogenation. In this study, the possible causes for the success in the synthesis of the ligand-unassisted Co@MFI catalyst, the catalyst deactivation modes and the strategies for improving catalyst stability were also demonstrated in detail. This work not only contributes a performance-advanced Co-based catalyst for alkane dehydrogenation but also provides new insights into incorporation of a metal into zeolites.

Catalytic Performance and Kinetic Modeling of n-Butane Isomerization over Metal Bearing Silicotungstic Acid Supported on Mesoporous KIT-6

In this study, mesoporous KIT-6 support was synthesized hydrothermally. Its structure was improved by impregnation with phosphotungstic acid (HPW), silicotungstic acid (HSiW), and some metals such as Pt, Pd, and Ce. This work converts mesoporous KIT-6 into a bifunctional catalyst containing acid and metal sites. The structures of the synthesized catalysts were characterized by various analyses such as XRD, XRF, FTIR, SEM, EDS, BET, NH3-TPD, and TGA. Furthermore, the catalytic performances of heteropoly acids (HPW and HSiW) and KIT-6 modified samples were investigated in an n-butane isomerization process. Impregnation of HSiW on mesoporous support resulted in higher initial n-butane conversion and isobutane selectivity and improved catalyst stability compared to the unsupported sample. The introduction of 0.5 wt % platinum on the 60%HSiW/KIT-6 led to the highest n-butane conversion (72%) and isobutane yield (56.8%) at 250 °C and weight hourly space velocity (WHSV) of 0.15 h–1, at the beginning of the reaction. The 0.5%Pt/60%HSiW/KIT-6 sample was the most stable catalyst against deactivation compared to other modified samples. Its conversion and its selectivity to isobutane reached 51.8 and 73.8%, respectively, after 7 h of reaction. Also, the influence of different process variables such as reaction temperature, WHSV, and carrier gas on the catalyst activity (n-butane conversion, isobutane selectivity, and yield) was investigated. In addition, the classical bifunctional model was used to describe the kinetic behavior of 0.5%Pt/60%HSiW/KIT-6 in the isomerization reaction. This model inhibits the hydrogen and n-butane pressure effects on the reaction rate and agrees with the ideal bifunctional metal–acid mechanism. The activation energy for this model was found to be 29.64 kJ mol–1. The classical bifunctional model provided a reasonable fit for the experimental data and allowed to specify the kinetic parameters.

Enhancing the reusability of hydroxyapatite by barium modification for efficient isomerization of glucose to fructose in ethanol

The application of synthetic catalysts to displace costly isomerases for glucose isomerization to fructose has been a long-standing goal in biomass valorization, but current catalysts generally suffer from inferior stability and reusability. Here we show that the combination of barium modified hydroxyapatite (Ba/HAP) as catalyst with ethanol as solvent enables highly efficient and selective isomerization of glucose to fructose, affording fructose yield of 35.4% with selectivity up to 93.5% at high glucose loading (10 wt%). Moreover, the Ba/HAP catalyst was reused with high recovery rate and the leaching of ions was greatly inhibited. The composition, structure and catalytic activity of catalyst can be almost completely restored after calcination. Density functional theory simulations revealed that the formation of relatively stable BaCa6(PO4)4O phase is beneficial to the improved catalyst stability. Simultaneously achieving high reaction efficiency and good catalyst reusability in green solvent, our study is an important step towards sustainable catalysis.

Wilkinson-type catalysts in ionic liquids for hydrogenation of small alkenes: understanding and improving catalyst stability

We apply supported ionic liquid phase (SILP) catalysts to hydrogenate small 1-alkenes in a continuous fixed-bed reactor. We rationalize the origin of catalyst decomposition via IR spectroscopy and adapt the catalyst design for improved stability.

Assembling Phenothiazine into a Porous Coordination Cage to Improve Its Photocatalytic Efficiency for Organic Transformations

AbstractPhoto‐catalysis by small‐molecules is often limited by catalyst degradation and low electron‐transfer efficiency. Herein we report a stable N‐phenyl‐phenothiazine (PTH)‐derived porous coordination cage (PCC) as a highly efficient photocatalyst. By the incorporation of the photocatalytic PTH moiety into a PCC, aggregation‐induced quenching (AIQ) was shown to be reduced. An improvement in catalyst stability was discovered, ascribed to the synergistic effects of the PTH moieties. The catalyst, operating through a photolytic single‐electron transfer, was utilized for photo‐catalyzed dehalogenation and borylation. Evaluation of the catalytic mechanism in the borylation reaction showed that the improved performance results from the more efficient formation of the electron donor‐acceptor (EDA) complex with the cage. This discovery provides a potential strategy to improve the photophysical properties and stabilities of small‐molecule organic photocatalysts via supramolecular chemistry.

Assembling Phenothiazine into a Porous Coordination Cage to Improve Its Photocatalytic Efficiency for Organic Transformations.

Photo-catalysis by small-molecules is often limited by catalyst degradation and low electron-transfer efficiency. Herein we report a stable N-phenyl-phenothiazine (PTH)-derived porous coordination cage (PCC) as a highly efficient photocatalyst. By the incorporation of the photocatalytic PTH moiety into a PCC, aggregation-induced quenching (AIQ) was shown to be reduced. An improvement in catalyst stability was discovered, ascribed to the synergistic effects of the PTH moieties. The catalyst, operating through a photolytic single-electron transfer, was utilized for photo-catalyzed dehalogenation and borylation. Evaluation of the catalytic mechanism in the borylation reaction showed that the improved performance results from the more efficient formation of the electron donor-acceptor (EDA) complex with the cage. This discovery provides a potential strategy to improve the photophysical properties and stabilities of small-molecule organic photocatalysts via supramolecular chemistry.

A Homogeneous‐Heterogeneous Kinetic Study of Oxidative Coupling of Methane (OCM) on La2O3/CeO2 Catalyst

AbstractA La2O3/CeO2 catalyst was synthesized and achieved 19 % C2+ yield with 65 % C2+ selectivity at 650 °C. The doping of CeO2 not only enhanced catalytic performance, but also improved catalyst stability in the presence CO2 and H2O. This study investigated the kinetics on La2O3/CeO2 over a wide range of operating conditions. Planar laser‐induced fluorescence was implemented to directly observe the presence of formaldehyde, as the first time reported in OCM. A packed bed reactor model was constructed based on the dimensions of experimental setup and catalyst characterization. Rather than using a tuned gas phase model, the experimentally validated mechanism NUIGMech1.1 was implemented. The proposed homogeneous‐heterogeneous OCM mechanism kinetic model was validated against experimental data under isothermal conditions, and the results show good agreement with measured species. The rate of production (ROP) was performed to identify the important reactions and prove the necessity of surface reactions for the OCM process.

From ensemble electrochemistry to nanoparticle-impact electrochemistry: Altered reaction process and possible applications

Nanoparticle-impact electrochemistry (NIE) is an electroanalytical method based on the stochastic collisions of individual nanoparticles onto an inert ultramicroelectrode. In this article, we overview different characteristics of NIE-based reactions from their ensemble counterpart, that is, enhanced mass transport, intermittent working mode, and reduced reaction time scale. A particular interest is focused on the survey of the resulting changes in the reaction process regarding enhanced reaction efficiency and rate, altered selectivity, and improved catalyst stability. A summary of the application prospect of NIE is followed, and existing challenges are provided.

Iron Dihydride Complex Stabilized by an All-Phosphorus-Based Pincer Ligand and Carbon Monoxide.

PNP-pincer-stabilized iron carbonyl dihydride complexes are key intermediates in catalytic hydrogenation and dehydrogenation reactions; however, decomposition through these intermediates has been observed. This inspires the development of a PPP-pincer system that may show improved catalyst stability. In this work, bis[2-(diisopropylphosphino)phenyl]phosphine (or iPrPPHP) is used to react with FeCl2 under a carbon monoxide (CO) atmosphere to yield trans-(iPrPPHP)Fe(CO)Cl2. A subsequent reaction with NaBH4 produces syn/anti-(iPrPPHP)FeH(CO)Cl or cis,anti-(iPrPPHP)Fe(CO)H2, depending on the amount of NaBH4 employed. The cis-dihydride complex shows catalytic activity for the conversion of PhCHO to PhCH2OH (under H2) or PhCO2CH2Ph (under Ar). It also catalyzes the dehydrogenation of PhCH2OH to PhCHO and PhCO2CH2Ph, albeit with limited turnover numbers. A more efficient catalytic process is the dehydrogenation of formic acid to carbon dioxide (CO2), which can operate under additive-free conditions. Mechanistic investigation suggests that the cis-dihydride complex undergoes protonation with formic acid to release H2 while forming anti-(iPrPPHP)FeH(CO)(OCHO)·HCO2H, in which the CO ligand has shifted and the formate is hydrogen-bonded to formic acid. The hydrido formate complex loses CO2 under ambient conditions, completing the catalytic cycle by reforming the cis-dihydride complex.

(Invited) Membrane Coated Electrocatalysts for Selective and Stable Oxygen Evolution in Seawater

Seawater electrolysis has the potential to be a more sustainable means of hydrogen production compared to conventional water electrolysis which relies on highly pure water. This is particularly true for arid coastal regions with access to seawater and ideal conditions for harvesting solar and wind energy, but where fresh water is already scarce.[1] Seawater electrolysis is challenging due to the large concentration of chloride ions, which can be detrimental to electrocatalyst stability. Furthermore, the presence of chloride ions allows the chlorine evolution reaction (CER) to compete with the oxygen evolution reaction (OER) at the anode. Although Cl2 is of industrial value, global hydrogen production already exceeds chlorine production, and demand for hydrogen is projected to grow more rapidly. Additionally, since Cl2 is toxic and harmful to the environment, implementing seawater electrolysis is simplified if pure oxygen is produced and can be safely vented to the atmosphere. Our group has shown that ultrathin semi permeable oxide overlayers can be designed to selectively transport reactants to the active catalyst at the buried interface.[2-3] Importantly for seawater electrolysis, the oxide overlayer selectively rejected chloride ions while allowing for water transport.[4] Thus, the oxide overlayer acts as a membrane, and the composite material can be referred to as a membrane coated electrocatalyst (MCEC). An additional advantage of the MCEC architecture compared to conventional electrocatalysts is enhanced stability.[5] This makes MCECs particularly attractive for stable and selective OER in seawater. This work describes how MCECs can (i) improve catalyst stability and (ii) enable selectivity for OER over CER by impeding transport of chloride ions to the catalyst at the buried interface. This work explores the fundamental relationships between chloride ion transport through different oxide overlayer materials. This knowledge is then applied to prepare MCECs supported on high surface area porous electrodes.

Development of CoMo-X catalysts for production of H2 and CNTs from biogas by integrative process

In this study, catalytic conversion of biogas into hydrogen and carbon nanotubes is investigated using an integrative process of dry reforming and methane decomposition. The catalytic improvement of cobalt catalysts promoted with various promoters: Mo, Ce, Zr, W, and Ca, is examined to discover the best combination that is resistant to a metal sintering. The addition of Mo shows an improved catalyst stability due to an enhanced metal-support interaction, avoiding metal sintering. The current process shows a remarkable CO2 conversion of 99.4% and a high CH4 conversion of 95.8%, while the gaseous products comprise of H2 up to 77%v/v at 900 °C, GHSV of 4500 ml/g-h. The long-term stability test for 7 h shows that the catalysts promoted with Mo, Zr, and W could provide almost complete CO2 conversion at 900 °C and under a high GHSV of 45,000 ml/g-h. The analysis of spent catalysts shows that deposited carbon composed of well-structured multi-walled carbon nanotubes (MWCNTs) with high graphitization (IG/ID = 2.14), compared to the commercial CNTs (IG/ID = 0.74). The as-synthesized CNTs could achieve 91%w/w purity without any purification. The process test at high temperature of 1000 °C, provides higher graphitization (IG/ID = 3.76) with compensation of lower yield of CNTs. This integrative process demonstrates a promising route to completely convert a renewable biogas into high-value products such as hydrogen and carbon nanotubes with less production of wastes.

Hierarchical CuS@ZnIn2S4 Hollow Double-Shelled p-n Heterojunction Octahedra Decorated with Fullerene C60 for Remarkable Selectivity and Activity of CO2 Photoreduction into CH4.

In this work, a hollow double-shelled architecture, based on n-type ZnIn2S4 nanosheet-coated p-type CuS hollow octahedra (CuS@ZnIn2S4 HDSOs), is designed and fabricated as a p-n heterojunction photocatalyst for selective CO2 photoreduction into CH4. The resulting hybrids provide rich active sites and effective charge migration/separation to drive CO2 photoreduction, and meanwhile, CO detachment is delayed to increase the possibility of eight-electron reactions for CH4 production. As expected, the optimized CuS@ZnIn2S4 HDSOs manifest a CH4 yield of 28.0 μmol g-1 h-1 and a boosted CH4 selectivity up to 94.5%. The decorated C60 both possesses high electron affinity and improves catalyst stability and CO2 adsorption ability. Thus, the C60-decorated CuS@ZnIn2S4 HDSOs exhibit the highest CH4 evolution rate of 43.6 μmol g-1 h-1 and 96.5% selectivity. This work provides a rational strategy for designing and fabricating efficient heteroarchitectures for CO2 photoreduction.

Synthesis and Characterization of Mesoporous Silica Nanoparticles Loaded with Pt Catalysts

Coating the catalyst with a nanoporous layer has been demonstrated to be an effective approach to improve catalyst stability. Herein, we systematically investigate two types of core-shell mesoporous silica nanoparticles with a platinum nanocatalyst using a variety of characterization methods. One of the mesoporous particles has a unique amine ring structure in the middle of a shell (Ring-mSiO2/Pt-5.0/SiO2), and the other one has no ring structure (mSiO2/Pt-5.0/SiO2). Brunauer–Emmett–Teller/Barrett–Joyner–Halenda (BET/BJH) presented a similar surface area for both particles, and the pore size was 2.4 nm. Ultra-Small-Angle X-ray Scattering (USAXS)/ Small-Angle X-ray Scattering (SAXS) showed the size of mSiO2/Pt-5.0/SiO2 and Ring-mSiO2/Pt-5.0/SiO2 were 420 nm and 272 nm, respectively. It also showed that the ring structure was 30 nm above the silica core. Using high-resolution Transmission Electron Microscopy (TEM), it was found that the platinum nanoparticles are loaded evenly on the surface of the silica. In situ SAXS heating experiments and Thermogravimetric Analysis (TGA) indicated that the mSiO2/Pt-5.0/SiO2 were more stable during the high temperature, while the Ring-mSiO2/Pt-5.0/SiO2 had more change in the particle.

Catalytic co-conversion of glycerol and oleic acid to bio-aromatics: Catalyst deactivation studies for a technical H-ZSM-5/Al2O3 catalyst

The catalytic co-conversion of glycerol and oleic acid (45/55 wt%) to bio-based aromatics (benzene, toluene, and xylenes, bio-BTX) over a technical H-ZSM-5/Al2O3 catalyst in a fixed-bed reactor was investigated with special attention to catalyst deactivation phenomena. The spent catalysts after a time on stream (TOS) of 12 h were regenerated by an ex-situ oxidative treatment to remove coke and reused for another run. In total, 5 reaction-regeneration cycles were performed. The performance and characteristics of the fresh and regenerated catalysts were determined to identify and quantify the effects of co-feeding on catalyst stability and regenerability. The performance of the catalyst changes upon the number of reaction-regeneration cycles. Despite a negative effect on the peak BTX yield, a remarkable improvement in catalyst stability was found, leading to higher BTX productivity (1260–1505 gBTX gcat-1 for the regenerated catalysts during a TOS of 12 h vs. 834 gBTX gcat-1 for the fresh one during a life-time of 11 h). Detailed catalyst characterization studies show irreversible changes in the catalyst structure after reaction-regeneration cycle(s), such as textural property, crystal structure, framework, and acidity, which impact catalyst performance.