Catalytic Lifetime Research Articles

Selective and Efficient Gd-Doped BiVO4 Photoanode for Two-Electron Water Oxidation to H2O2

Photoelectrochemical oxidation of water presents a pathway for sustainable production of hydrogen peroxide (H2O2). Two-electron water oxidation toward H2O2, however, competes with the popular four-electron process to form oxygen and one-electron water oxidation to form OH radical. To date, bismuth vanadate (BiVO4) has been shown to exhibit promising selectivity toward H2O2, especially under illumination, but it suffers from high overpotential and notoriously poor stability. Herein, using density functional theory calculations, we predict that doping BiVO4 with optimal concentrations of gadolinium (Gd) not only enhances its activity for H2O2 production but also improves its stability. Experimentally, we demonstrate that intermediate amounts of Gd doping (6–12%) reduce the onset potential of BiVO4 for H2O2 production by ∼110 mV while achieving a Faradaic efficiency of ∼99.5% under illumination and prolonging the catalytic lifetime by more than a factor of 20 at 2.0 V vs RHE under illumination.

Control of Hierarchical Structure and Framework-Al Distribution of ZSM-5 via Adjusting Crystallization Temperature and Their Effects on Methanol Conversion

Incorporating mesoporosity into zeolite catalysts has been regarded as an innovative technology that improves diffusivity and catalytic lifetime. Here, we propose a facile synthesis of the hierarch...

Epitaxial Growth of Layered-Bulky ZSM-5 Hybrid Catalysts for the Methanol-to-Propylene Process

The hierarchical layered-bulky ZSM-5 (LBZ5) hybrid composites with a core–shell structure were synthesized by epitaxial growth of layered ZSM-5 nanosheets over bulky ZSM-5 crystals. The systematic balance of zeolitic microporosity and interlayered mesoporosity was achieved by elaborate regulation of the ratio between layered phase and bulky phase, resulting in a series of layered-bulky ZSM-5 hybrid composites with the controllable thickness of the layered shell ranging from 98 to 307 nm. The evaluation of catalytic performance in the methanol-to-propylene (MTP) reaction indicated the LBZ5 materials to be candidated catalysts for the MTP process with prolonged catalytic lifetime, superior coke toleration, and enhanced propylene selectivity compared with the parent bulky ZSM-5 (BZ5) and layered ZSM-5 (LZ5) due to their well-retained zeolitic framework, hierarchical meso-/microporosity by the layered-bulky composite structure, and appropriate strong acidity.

All Wrapped up: Stabilization of Enzymes within Single Enzyme Nanoparticles.

Enzymes are extremely useful in many industrial and pharmaceutical areas due to their ability to catalyze reactions with high selectivity. In order to extend their lifetime, significant efforts have been made to increase their stability using protein- or medium engineering as well as by chemical modification. Many researchers have explored the immobilization of enzymes onto carriers, or entrapment within a matrix, framework or nanoparticle with the hope of constricting the movement of the enzyme and shielding it from aggressive environments, thus delaying the denaturation. These strategies often balance three competing interests: (i) maintaining high enzymatic activity, (ii) ensuring good long-term stability against temperature, dehydration, organic solvents, and or aggressive pH, and (iii) enabling a tuning or reversible switching of enzyme activity. In most cases, multiple enzymes will be contained within a single nanoparticle or matrix, but in recent years researchers have begun to wrap up individual enzymes within single enzyme nanoparticles (SENs). In these nanoparticles the enzyme is stabilized by a thin shell, typically a polymer, prepared either by in situ polymerization from the enzyme surface or by assembling a preformed polymer around it. Because of the increased control over the environment directly around the enzyme, and the possibility of more directly controlling substrate diffusion, many SENs show remarkable stability while retaining high initial activities even for quite fragile enzymes. Moreover, the activity of the enzyme can often be more easily fine-tuned by adjusting the layer properties. We postulate that this emerging field will offer exciting and elegant opportunities to both extend the catalytic lifetime of enzymes in aggressive solvents, temperatures and pH, and enable their activity to be switched on and off on demand by modulation of the outer material layer.

Synthesis of sub-micrometric SAPO-34 by a morpholine assisted two-step hydrothermal route and its excellent MTO catalytic performance.

SAPO-34 with a sub-micrometer crystal size was synthesized by a double hydrothermal treatment employing cost-effective morpholine as a structure directing agent, which presented an enhanced catalytic lifetime (nearly 3 times the conventional one) in the reaction of methanol to olefins with a higher light olefin selectivity (total selectivity of 97.1%). Detailed studies of the sample after different time intervals in the second crystallization with and without additional morpholine were carried out, which offered insight into crystal degradation and re-crystallization phenomena. The samples with different morpholine concentrations during the second hydrothermal treatment were also prepared, in which the sample with 80% MOR aqueous solution exhibited the smallest crystal size and the longest MTO lifetime. Furthermore, the investigation on the additional amount of mother liquor (from the first crystallization) required for the second crystallization showed that the presence of half the amount of the mother liquor (nutrients) could give us the required results effectively.

Facile creation of hierarchical nano-sized ZSM-5 with a large external surface area via desilication–recrystallization of silicalite-1 for conversion of methanol to hydrocarbons

Mesoporous ZSM-5 with large external surface and strong acidity were created through desilication–recrystallization of nano-sized silicalite-1, and the catalytic lifetime of its regenerated catalyst is 2.5 times longer than fresh catalyst.

Synthesis and characterization of MFI zeolite nanosheets for optimizing the dominant operational conditions of methanol to propylene process

Nanosheets of MFI zeolite were successfully synthesized by hydrothermal method and were properly characterized by XRD, FE-SEM, EDX-dot mapping, TEM, FT-IR, N2 adsorption/desorption, NH3-TPD, ICP-AES and TGA techniques. The characterizations of MFI zeolite nanosheets suitably confirmed well synthesized MFI nanostructure, superior specific surface area (695.96m2.g-1) and great mesopores volume. Currently, the major concern in commercialized technologies of methanol to propylene (MTP) process is to discover the best operational conditions for maximizing propylene selectivity. Therefore, in present investigation the effectiveness of more significant operational conditions such as reaction temperature, methanol molar percent in feedstock and methanol WHSV on propylene selectivity was considered. For first time, conventional response surface methodology and an artificial neural network model coupled with genetic algorithm were appropriately employed to optimize MTP operational conditions. For providing the data bank of optimization, MTP reaction was carried out over the synthesized MFI zeolite nanosheets at various operational conditions. It was concluded that neuro-genetic method showed more reliable performance in optimizing MTP operational conditions with R2 value of 0.9998. Furthermore, time on stream examination was conducted over MFI zeolite nanosheets at optimal operational conditions which were suggested by neuro-genetic approach. At optimal operational conditions the catalytic life-time was sufficiently enhanced (101h).

Gem-Dimethyl-substituted bis(imino)dihydroquinolines as thermally stable supports for highly active cobalt catalysts that produce linear PE waxes.

Six types of 2,8-bis(imino)-7,7-dimethyl-5,6-dihydroquinoline, 2-(ArN[double bond, length as m-dash]CMe)-8-(ArN)-7,7-Me2C9H6N (Ar = 2,6-Me2C6H3L1, 2,6-Et2C6H3L2, 2,6-iPr2C6H3L3, 2,4,6-Me3C6H2L4, 2,6-Et2-4-MeC6H2L5, 2,4,6-tBu3C6H3L6), distinguishable by their steric and electronic profile, are described that can readily undergo complexation with cobaltous chloride to form their corresponding LCoCl2 chelates, Co1-Co6. The molecular structures of Co2 and Co3 reveal square pyramidal geometries with ring puckering a feature of the gem-dimethyl section of their unsymmetrical N,N,N'-ligands. On activation with either methylaluminoxane (MAO) or modified methylaluminoxane (MMAO), all the cobalt complexes exhibited exceptionally high activities for ethylene polymerization with levels reaching up to 1.19 × 107 g PE per mol (Co) per h for mesityl-containing Co4. Significantly, these catalysts exhibited good thermal stability by displaying their optimal performance at temperatures up to 70 °C whilst also maintaining appreciable catalytic lifetimes. With the exception of that obtained using the most sterically hindered Co6 (2,4,6-t-butyl), the polyethylenes are of low molecular weight (Mw≤16.0 kg mol-1) and of narrow dispersity (Mw/Mn≤3.4). Moreover, end-group analysis of these highly linear polymer waxes reveals evidence for unsaturated as well as various levels of fully saturated materials highlighting the role of both β-H elimination and chain transfer to aluminum as termination pathways.

Effects of Light on Catalytic Activities and Lifetime of Plasmonic Au Catalysts in the CO Oxidation Reaction

The catalytic oxidation of CO to CO2 allows for the removal of poisonous CO to improve indoor and outdoor air quality. Currently, industrial CO oxidation catalysts are not effective at low temperatures. A group of the most promising choices, Au catalysts supported on Mg(OH)2 and MgO, are highly active even at −70 °C, but show decreased activity between 20 and 100 °C. These catalysts also have short lifetimes. Here, we utilize knowledge of plasmonic catalysis in the study of the CO oxidation reaction over plasmonic Au catalysts. We hypothesized that light interacting with catalysts via surface plasmon resonances has the potential to dramatically improve such catalytic reactions. Initial experiments demonstrated a significant improvement of the catalytic activity and lifetime under light. Using the particular features in the temperature dependent conversion curve of this catalyst, we developed a method to separately examine the thermal and nonthermal contributions of light. Detailed studies on the relations...

Nickel diselenide nanoflakes give superior urea electrocatalytic conversion

Urea electro-oxidation reaction (UOR) is a promising but challenging renewable energy production technology, which generally requires lower electrochemical potential, and shows the capabilities in eliminating a potentially harmful substance from wastewater and/or converting human waste into treasure. However, the state-of-the art UOR suffers from the sluggish kinetics because of the 6 e− transfer process involvement. Herein, as a proof-of-concept study, we synthesized the NiIII -rich nickel diselenide nanoflake through a facile hydrothermal reaction. Compared to the counterpart nickel (oxy) hydroxide, the selenylation processing of nickel successfully produces synergetic effects of more exposed active sites and effective electron transports. Particularly, due to the NiIII –rich nature of the as-synthesized nickel diselenide, the reduction/oxidation reactions between NiII and NiIII is greatly accelerated, which in turn effectively decreases the required overpotentials, prolongate the catalytic lifetime, giving rise to the significantly enhanced UOR performances. In-depth mechanistic analysis evidences that the restructure of nickel diselenide (vs. nickel oxide hydroxide) could be the main reason for affording the decent UOR activity as the oxidation from NiII to NiIII was simultaneous occurred during the selenide species disappear process, a result that may be extended to other selenide -based systems.

Analysis of long term catalytic performance for isobutane alkylation catalyzed by NMA–AlCl3 based ionic liquid analog

Abstract Isobutane alkylation with 2-butene to produce high-quality gasoline was catalyzed by N-methylacetamide (NMA)–AlCl3 based ionic liquid (IL) analog with a NMA/AlCl3 molar ratio of 0.75 and CuCl modification, which was marked as CuCl-modified 0.75NMA–1.0AlCl3. The long-term experiment was carried out in the autoclave operated in continuous mode to investigate the distribution of alkylate under different experimental nodes. The result indicated that the long-term alkylation was divided into three stages: rising, stable, and descending regions. C8 selectivity and molar ratio of trimethylpentanes (TMPs) to dimethylhexanes (DMHs) reached the highest level in the stable region, and research octane number (RON) of alkylate was as high as 97. Anionic Al species ([Al2Cl7]−, [AlCuCl5]−) and cationic Al species ([AlCl2L]+) from IL analog as two active Lewis acidic species played a catalytic role in the long-term alkylation, whereas the neutral Al species did not participate into the alkylation. Moreover, the structure of CuCl-modified 0.75NMA–1.0AlCl3 was destroyed after the deactivation, and CuCl was enriched in the CD2Cl2-insoluble substance, resulting in a decreasing TMP/DMH ratio. The catalytic lifetime of IL analog was similar with CuCl-modified 0.55Et3NHCl–1.0AlCl3 IL, but IL analog had a lower cost.

In-situ ion exchange electrocatalysis biological coupling (i-IEEBC) for simultaneously enhanced degradation of organic pollutants and heavy metals in electroplating wastewater.

The goal of this research was to develop a new process for simultaneously removing organics and heavy metals of electroplating wastewater by in-situ ion exchange electrocatalysis biological coupling (i-IEEBC). The study evaluated the removal efficiency of coexisting refractory organics and heavy metal ions, and investigated the effects of current density (CD) on the removal performance of the i-IEEBC method. The results indicated the i-IEEBC reactor exhibited higher average removal rates of COD, TOC, Cr and Cu ions, i.e. 87.23%, 80.42%, 91.25%, and 95.97% in that order, which represented an increase by 32.59%, 40.10%, 31.86%, and 33.82%, respectively, compared with BAF. The optimum CD for simultaneously removing organics and heavy metals of electroplating wastewater in i-IEEBC was 0.40 mA/(cm)2. The change of biodegradability and the presence of short chain organic compounds also indirectly confirmed the excellent removal organic pollutants performance of i-IEEBC at the optimum CD. In addition, the composition and construction of CER before and after the application, under the optimum CD, SEM, EDS and FT-IR spectroscopy also showed that the cation exchange properties of CER improved the catalytic lifetime of the particle electrodes. This research provides a highly efficient new alternative to electroplating wastewater treatment technology.

Seed-fused ZSM-5 nanosheet as a superior MTP catalyst: Synergy of micro/mesopore and inter/external acidity

Abstract ZSM-5 nanosheet (NS) has an optimal diffusivity due to b-axis oriented two-dimensional structure, but the extra large external acidity results in the deactivation of the catalyst in methanol to propylene (MTP) reaction. The seed-fused ZSM-5 nanosheets (CNS-x) were synthesized by a seed induced method, the performance of samples was compared with one without seed. Further, the B-incorporation sample (B-CNS-5) was prepared with 5 wt% seed and boron acid. Revealing the importance of fusion effect of ZSM-5 seed on the nanosheet, especially the change in microstructure properties and its structure-effect relationship, which is not explicitly documented in previous work. With the seed amount increasing from 5 to 30 wt % in the gel, the crystallization time is largely shortened, along with the change in the resulting nanosheet, such as reduction of particle size and acid strength/external surface acidity, an increase of over 50% micropore volume. The CNS-x samples perform longer catalytic lifetime (168–226 h) compared to NS (103 h). CNS-5 displays an extremely high selectivity to propylene (C = 3) ∼54% and total light olefins (C = 2-C = 4) of∼84%. A super sample B-CNS-5 exhibits longest lifetime (302 h), achieving highest C = 3 yield due to change in the acid sites distribution and increase of extra weak acid sites. For the converted methanol (g/gcatalyst), the result is B-CNS-5 (966) > CNS-5 (525)> NS (324). In converse, the average coke selectivity (Sav, C) is B-CNS-5 (0.18‰)

Facet Effects of Ag3 PO4 on Charge-Carrier Dynamics: Trade-Off Between Photocatalytic Activity and Charge-Carrier Lifetime.

Silver phosphate (Ag3 PO4 ) is a promising visible-light-driven photocatalyst with a strong oxidation power and exceptionally high apparent quantum yield of O2 evolution. Although engineering Ag3 PO4 facets is widely known to enhance its photocatalytic activity, most studies have explained its facet effect by calculating surface energies. Herein, the charge carrier dynamics in three kinds of Ag3 PO4 crystals (mixed facets, cubic, and tetrahedral structures) were first investigated using single-particle photoluminescence microscopy and femtosecond time-resolved diffuse reflectance spectroscopy. As a result, we clarified that the disagreement between the photocatalytic activities (dye degradation and O2 evolution) of different Ag3 PO4 facets are the consequence of trade-off between catalytic activity and lifetime of photogenerated charge carriers in addition to surface energy.

Fast synthesis of hierarchical CHA/AEI intergrowth zeolite with ammonium salts as mineralizing agent and its application for MTO process

Fast solvent-free synthesis of hierarchical SAPO-34/18 zeolite was realized by combining oil-bath heating and the addition of catalytic amount of ammonium salt. The obtained zeolite samples were systematically characterized by XRD, SEM, XRF, BET, NH3-TPD, and NMR. XRD and SEM results showed that high-quality SAPO-34/18 nano-crystals with plate morphology can be obtained in only 60 min. The addition of ammonium salt served as mineralizing agent which speeded up the crystallization process in solvent-free precursor. The fast heat transfer of oil-bath heating and seeding also contributed to the ultrafast synthesis. Nitrogen adsorption–desorption analysis indicated the presence of high BET surface area and micropore volume, as well as abundant mesopores and macropores. NH3-TPD and 29Si NMR results showed that the samples exhibited appropriate acid strength and concentration. The obtained SAPO catalysts showed excellent performance in MTO reaction with significantly longer catalytic lifetime and higher propylene and butylene selectivities, compared with micron-sized SAPO-34 catalyst made with the conventional hydrothermal method. The extended lifetime could be attributed to the thin-plate crystal morphology and hierarchical pore structure, which greatly alleviated the diffusion constrain and coke deposition in MTO reaction. Higher propylene and butylene selectivities could be ascribed to the larger size of the AEI cage, which allows the easy diffusion of longer alkenes. The realization of ultrafast synthesis with solvent-free precursor not only reduces the waste disposal and energy consumption, but also increases the autoclave efficiency significantly.

External surface modification of as-made ZSM-5 and their catalytic performance in the methanol to propylene reaction

Post-synthetic treatment of high-silica as-made ZSM-5 with organic template in the micropores was explored to reduce/remove the external surface acid density of ZSM-5. It is found that Na2H2EDTA treatment can selectively remove the surface Al atoms, but generates new acid sites (likely silanol nests) on the external surface. H3PO4 treatment is unable to remove surface Al atoms, while small amount of P is left on the external surface, which effectively decreases the acid density. The catalytic performance of the resultant materials is evaluated in the methanol conversion reaction. H3PO4 treatment can effectively improve both the catalytic lifetime and the stability of propene selectivity. This occurs due to a combination of the increased tolerance to the external coke deposition and the depressed coking rate (reduced side reactions). Na2H2EDTA treatment only prolongs the catalytic lifetime, resulting from the improved tolerance to the external coke deposition. Under the optimized H3PO4 treatment condition, the resultant ZSM-5 gives a catalytic lifetime of about 1.5 times longer than the precursor. Moreover, the propene selectivity is improved, showing a slight increasing trend until the deactivation.

Controllable synthesis of nano-ZSM-5 catalysts with large amount and high strength of acid sites for conversion of methanol to hydrocarbons

Abstract Nano-ZSM-5 with large amount and high strength of acid sites were successfully synthesized by hydrothermal crystallization of pure-silica nucleus in NaAlO2 alkali solution. The concentration of NaAlO2 alkali solution was decreased by increasing the H2O/Al ratio from 200 to 2600, which strongly determined the acidic properties and crystal size of the obtained nano-ZSM-5. Low concentration of NaAlO2 alkali solution was found unfavorable for the incorporation of Al into the bulk of ZSM-5 crystal in crystallization process. Hence, more Al distributed at the exterior of crystals, which enhanced the accessibility of the acid sites during reaction. Large amount and high strength of acid sites were produced for nano-ZSM-5 by increasing H2O/Al ratio. When the H2O/Al ratio increased from 200 to 2600, the total amount of acid sites increased from 0.59 to 0.81 mmol g−1, much higher than conventional value of 0.50 mmol g−1. In addition, the acid strength increased and the ratio of Bronsted/Lewis acid sites were sharply increased from 2.3 to 4.1 when the H2O/Al ratio increased. Moreover, the size of nano-crystal was also decreased by above decreasing the concentration of the NaAlO2 alkali solution. When the H2O/Al ratio was 2600, the obtained sample possessed total acid amount of 0.81 mmol g−1 and crystal size of 60 nm, respectively. Methanol to hydrocarbon (MTH) reaction results showed that the increased acid sites of the catalyst decreased its catalytic lifetime to some extent, but the aromatics selectivity was tunable obviously. As for the catalyst prepared with H2O/Al ratio of 2000, the aromatics selectivity of the obtained catalyst reached to 66.0%. This paper provided a method for the fabrication of small-size nano-ZSM-5 with large amount and high strength of acid sites and regulated Al distribution for MTH reaction.

Corrosion engineering towards efficient oxygen evolution electrodes with stable catalytic activity for over 6000 hours

Although a number of nonprecious materials can exhibit catalytic activity approaching (sometimes even outperforming) that of iridium oxide catalysts for the oxygen evolution reaction, their catalytic lifetimes rarely exceed more than several hundred hours under operating conditions. Here we develop an energy-efficient, cost-effective, scaled-up corrosion engineering method for transforming inexpensive iron substrates (e.g., iron plate and iron foam) into highly active and ultrastable electrodes for oxygen evolution reaction. This synthetic method is achieved via a desired corrosion reaction of iron substrates with oxygen in aqueous solutions containing divalent cations (e.g., nickel) at ambient temperature. This process results in the growth on iron substrates of thin film nanosheet arrays that consist of iron-containing layered double hydroxides, instead of rust. This inexpensive and simple manufacturing technique affords iron-substrate-derived electrodes possessing excellent catalytic activities and activity retention for over 6000 hours at 1000 mA cm-2 current densities.

In Situ Fabrication of Hierarchical MTW Zeolite via Nanoparticle Assembly by a Tailored Simple Organic Molecule.

Amphiphilic surfactants are widely used as templates to synthesize hierarchically structured zeolites due to their multiple functions; however, piloting such new dual-functional templates is limited by their time-consuming nature and high cost. Herein, a simple organic molecule, without a long hydrophobic alkyl chain, was tailored from a gemini-type, poly-quaternary ammonium surfactant, and effectively used as a dual-porogenic template to synthesize hierarchical MTW zeolite. Upon a range of synthesis parameter optimizations, our detailed characterization suggested that the hierarchical MTW zeolite would completely crystallize within 36 hours from the surface to the inside of quasi-spherical particles through in situ consumption of amorphous silicon and aluminum species; much faster than most of the hierarchical MTW zeolites generated by conventional methods. Moreover, the as-prepared hierarchical MTW zeolite exhibited 4 times higher catalytic performance and lifetime of benzene-propene alkylation compared to conventional MTW zeolite, while the introduced crystalline mesopores are of benefit to diffuse reactants, products, and coke depositions. Our strategy broadens the design of new templates in more effective ways to facilely synthesize versatile hierarchical zeolites for diverse applications, especially for those in which macromolecules are involved.

Catalytic Enhancement of SAPO-34 for Methanol Conversion to Light Olefins Using in Situ Metal Incorporation

Parent and MeAPSO-34 (Me = Cu, Ca, and W) catalysts were synthesized for methanol conversion to more valuable products such as light olefins. The effect of metal presence on the catalytic performance and physicochemical properties were investigated. The comprehensive characterizations, XRD, EDX, FESEM, FTIR, N2-BET, NH3-TPD, and DR UV–vis, have been applied to characterize the metal modified SAPO-34. Even though the crystallinity of MeAPSO-34s decreased compared to that of parent SAPO-34, the diffusion and acidity properties were improved. Good activity and selectivity for olefin production were observed for all synthesized catalysts. Metal incorporation to the framework of SAPO-34 enhanced the catalytic lifetime and propylene yield. The methanol conversion was stable for 1 h over MeAPSO-34 with W/F = 6.6 g h/mol at 350 °C and 1 bar. The deactivation rate was higher for the parent SAPO-34 as compared to MeAPSO-34. CaAPSO-34 favored propylene production, while CuAPSO-34 favored ethylene.