Modifying the Charge-Density of Tetrahedral Cobalt(II) Centers through Carbon-Layer Modulation Promotes C-H Activation in the Propane Dehydrogenation Reaction (PDH).

The electronic structure of active metal centers plays an indispensable role in regulating catalytic reactivity in heterogeneous catalysis, developing other metals as promoters to decorate electronic state is a common strategy, while non-metal component of carbon as electronic additives to regulate d-band center has rarely been studied in thermal-catalysis field. Herein, we report electron-deficient tetrahedral Co(II) (Td-cobalt(II)) centers through carbon-layer modulation for propane dehydrogenation (PDH). It is indicated that bifunctional sites of both Td-cobalt(II) and metallic-cobalt are designed, and the in-situ generated carbon through the disproportionation of CO on metallic-cobalt can cover the inactive metallic-cobalt and tailor d-band of active Td-cobalt(II) simultaneously. More importantly, the pre-deposited carbon-layer is proposed to decrease electron density of Td-cobalt(II) and make d-band center closer to Fermi level, consequently promotes C-H activation in PDH reaction. This study provides new perspective for the utilization of inactive carbon as electronic promoters and unlocks new opportunity to fabricate efficient PDH and other heterogeneous catalysts.

Enhancing AsH3 Detoxification via Electron-Deficient [NiIII-OH (µ-O)] in a Nickel-Modified NaY Zeolite: A Pathway toward As0 Products.

The transformation of toxic arsine (AsH3) gas into valuable elemental arsenic (As0) from industrial exhaust gases is important for achieving sustainable development goals. Although advanced arsenic removal catalysts can improve the removal efficiency of AsH3, toxic arsenic oxides generated during this process have not received adequate attention. In light of this, a novel approach for obtaining stable As0 products was proposed by performing controlled moderate oxidation. We designed a tailored Ni-based catalyst through an acid etching approach to alter interactions between Ni and NaY. As a result, the 1Ni/NaY-H catalyst yielded an unprecedented proportion of As0 as the major product (65%), which is superior to those of other reported catalysts that only produced arsenic oxides. Density functional theory calculations clarified that Ni species changed the electronic structure of oxygen atoms, and the formed [NiIII-OH (µ-O)] active centers facilitated the adsorption of AsH2*, AsH*, and As* reaction intermediates for As-H bond cleavage, thereby decreasing the direct reactivity of oxygen with the arsenic intermediates. This work presents pioneering insights into inhibiting excessive oxidation during AsH3 removal, demonstrating potential environmental applications for recovery of As0 from toxic AsH3.

Comparative Study of α- and ß-MnO2 on Methyl Mercaptan Decomposition: The Role of Oxygen Vacancies.

As a representative sulfur-containing volatile organic compounds (S-VOCs), CH3SH has attracted widespread attention due to its adverse environmental and health risks. The performance of Mn-based catalysts and the effect of their crystal structure on the CH3SH catalytic reaction have yet to be systematically investigated. In this paper, two different crystalline phases of tunneled MnO2 (α-MnO2 and ß-MnO2) with the similar nanorod morphology were used to remove CH3SH, and their physicochemical properties were comprehensively studied using high-resolution transmission electron microscope (HRTEM) and electron paramagnetic resonance (EPR), H2-TPR, O2-TPD, Raman, and X-ray photoelectron spectroscopy (XPS) analysis. For the first time, we report that the specific reaction rate for α-MnO2 (0.029 mol g-1 h-1) was approximately 4.1 times higher than that of ß-MnO2 (0.007 mol g-1 h-1). The as-synthesized α-MnO2 exhibited higher CH3SH catalytic activity towards CH3SH than that of ß-MnO2, which can be ascribed to the additional oxygen vacancies, stronger surface oxygen migration ability, and better redox properties from α-MnO2. The oxygen vacancies on the catalyst surface provided the main active sites for the chemisorption of CH3SH, and the subsequent electron transfer led to the decomposition of CH3SH. The lattice oxygen on catalysts could be released during the reaction and thus participated in the further oxidation of sulfur-containing species. CH3SSCH3, S0, SO32-, and SO42- were identified as the main products of CH3SH conversion. This work offers a new understanding of the interface interaction mechanism between Mn-based catalysts and S-VOCs.

Unraveling the Synergistic Reaction and the Deactivation Mechanism for the Catalytic Degradation of Double Components of Sulfur-Containing VOCs over ZSM-5-Based Materials.

The competitive adsorption behavior, the synergistic catalytic reaction, and deactivation mechanisms under double components of sulfur-containing volatile organic compounds (VOCs) are a bridge to solve their actual pollution problems. However, they are still unknown. Herein, simultaneous catalytic decomposition of methyl mercaptan (CH3SH) and ethyl mercaptan (C2H5SH) is investigated over lanthanum (La)-modified ZSM-5, and kinetic and thermodynamic results confirm a great difference in the adsorption property and catalytic transformation behavior. Meanwhile, the new synergistic reaction and deactivation mechanisms are revealed at the molecular level by combining with in situ diffuse reflectance infrared spectroscopy (in situ DRIFTS) and density functional theory (DFT) calculations. The CH3CH2* and SH* groups are presented in decomposing C2H5SH, while the new species of CH2*, active H* and S*, instead of CH3* and SH*, are proved as the key elementary groups in decomposing CH3SH. The competitive recombining of SH* in C2H5SH with highly active H* in dimethyl sulfide (CH3SCH3), an intermediate in decomposing CH3SH, would aggravate the deposition of carbon and sulfur. La/ZSM-5 exhibits potential environmental application due to the excellent stability of 200 h and water resistance. This work gives an understanding of the adsorption, catalysis, reaction, and deactivation mechanisms for decomposing double components of sulfur-containing VOCs.

The nature of K-induced 2H and 1T'-MoS2 species and their phase transition behavior for the synthesis of methanethiol (CH3SH).

The one-step reaction approach from syngas with hydrogen sulfide (CO/H2/H2S) over potassium (K) promoted Molybdenum disulfide (MoS2) materials can provide alternatives for the synthesis of methanethiol (CH3SH). However, the direct confirmation and determination of the real active nature of K-induced 2H and 1T'-MoS2 for this reaction and the corresponding phase transformation behavior and origin of K-induced 2H-MoS2 from/to 1T'-MoS2 remains unclear. Herein, we proved at the atomic level the precise position of K over 1T'-MoS2 and 2H-MoS2 species using the technique of HAADF-STEM. A relationship between K-induced 1T' and 2H-MoS2 phases and the catalytic property to synthesize CH3SH was established, and K-intercalated 1T'-MoS2 phase was confirmed to have excellent catalytic performances. Moreover, the behavior, origin, and influencing factors of phase transformation of 2H-MoS2 from/to 1T'-MoS2 in the existence of K were well proved.

Investigation of defect-rich CeO2 catalysts for super low-temperature catalytic oxidation and durable styrene removal.

Spherical cerium dioxide (CeO2-S) nanoparticles were successfully prepared using a solvothermal method, and their performances in catalytic oxidation reactions were studied. The CeO2-S catalyst showed superior low-temperature catalytic activity for styrene removal (T90 = 118 °C, GHSV = 18,000 h-1) compared with commercial CeO2. The characterization results showed that there were numerous oxygen defects in CeO2-S that were key to its catalytic performance at low temperatures, high redox properties, and high adsorption capacity for the reaction gases (O2 and styrene). Moreover, the catalytic performance of CeO2-S was highly stable (132 h), and the particles were reusable. FTIR and in-situ DRIFTS results showed that the type of intermediates formed during the oxidation of styrene determined the CeO2 catalytic stability, and the main intermediates were bidentate carbonate species that accumulated on the surface of deactivated CeO2-S and were not thermally stable. Moreover, the soft carbon that also deposited on CeO2-S during the reaction was easily decomposed at higher temperatures. The role of the oxygen vacancies on the CeO2-S catalyst was further revealed by correlating the concentration of oxygen vacancies and the accumulation of coke on the catalyst surface.

Nanoarchitectonics of Ni/CeO2 Catalysts: The Effect of Pretreatment on the Low-Temperature Steam Reforming of Glycerol.

CeO2 nanosphere-supported nickel catalysts were prepared by the wetness impregnation method and employed for hydrogen production from glycerol steam reforming. The dried catalyst precursors were either reduced by H2 after thermal calcination or reduced by H2 directly without calcination. The catalysts that were reduced by H2 without calcination achieved a 95% glycerol conversion at a reaction temperature of only 475 °C, and the catalytic stability was up to 35 h. However, the reaction temperature required of catalysts reduced by H2 with calcination was 500 °C, and the catalysts was rapidly inactivated after 25 h of reaction. A series of physicochemical characterization revealed that direct H2 reduction without calcination enhanced the concentration of oxygen vacancies. Thus, the nickel dispersion was improved, the nickel nanoparticle size was reduced, and the reduction of nickel was increased. Moreover, the high concentration of oxygen vacancy not only contributed to the increase of H2 yield, but also effectively reduced the amount of carbon deposition. The increased active nickel surface area and oxygen vacancies synergistically resulted in the superior catalytic performance for the catalyst that was directly reduced by H2 without calcination. The simple, direct hydrogen reduction method remarkably boosts catalytic performance. This strategy can be extended to other supports with redox properties and applied to heterogeneous catalytic reactions involving resistance to sintering and carbon deposition.

Study of the Metal-Support Interaction and Electronic Effect Induced by Calcination Temperature Regulation and Their Effect on the Catalytic Performance of Glycerol Steam Reforming for Hydrogen Production.

Steam reforming of glycerol to produce hydrogen is considered to be the very promising strategy to generate clean and renewable energy. The incipient-wetness impregnation method was used to load Ni on the reducible carrier TiO2 (P25). In the process of catalyst preparation, the interaction and electronic effect between metal Ni and support TiO2 were adjusted by changing the calcination temperature, and then the activity and hydrogen production of glycerol steam reforming reaction (GSR) was explored. A series of modern characterizations including XRD, UV-vis DRS, BET, XPS, NH3-TPD, H2-TPR, TG, and Raman have been applied to systematically characterize the catalysts. The characterization results showed that the calcination temperature can contribute to varying degrees of influences on the acidity and basicity of the Ni/TiO2 catalyst, the specific surface area, together with the interaction force between Ni and the support. When the Ni/TiO2 catalyst was calcined at 600 °C, the Ni species can be produced in the form of granular NiTiO3 spinel. Consequently, due to the moderate metal-support interaction and electronic activity formed between the Ni species and the reducible support TiO2 in the NiO/Ti-600C catalyst, the granular NiTiO3 spinel can be reduced to a smaller Ni0 at a lower temperature, and thus to exhibit the best catalytic performance.

Toxic chromium treatment induce amino-assisted electrostatic adsorption for the synthesis of highly dispersed chromium catalyst.

Removal of toxic Cr (VI) from aqueous solutions using silicon-based adsorbents has been widely investigated. Meanwhile, contradictory between highly dispersed active Cr species and high Cr loading over commercial Cr-based catalyst was inevitable. In this work, amino-assisted electrostatic adsorption from toxic Cr (VI) treatment was developed to prepare highly dispersed Cr oxides catalysts supported on MCM-41. The Cr loading was as high as 15 wt%, and structure characters of the catalysts were well-reserved. As a result, electrostatic adsorption and subsequent complexation from negatively charged Cr (VI) species and positively charged ammonium groups made a positive contribution to the appearance of highly dispersed mono Cr species, which gave rise to improved non-oxidative propane dehydrogenation (PDH) activity. In contrast, the agglomeration of Cr species and lower PDH activity were observed on the sample synthesized using the traditional wet impregnation method. Besides, the transformation of Cr (VI) to active Cr (III) sites over the catalyst was proved by the designed in-situ H2-TPR, ex-situ UV-vis and Raman spectra results. This procedure reflects a new avenue of green chemistry, which can recycle waste Cr adsorbents as efficient PDH catalysts.

Flowing-Air-Induced Transformation to Promote the Dispersion of the CrOx Catalyst for Propane Dehydrogenation.

Highly dispersed chromium (Cr)-based catalysts are promising candidates for the catalytic dehydrogenation of propane (DHP). However, the easier aggregation of Cr species into crystalline Cr2O3 at the high-temperature calcination and reaction process is a big challenge, which severely restricts the improvement of activity and stability of the DHP reaction. Herein, a flowing-air-induced transformation method was first proposed, and the catalytic performance of the prepared Cr/MCM-41 catalysts was found to be significantly improved compared to that of the Cr-based catalyst prepared by the traditional calcination method, even better than that of most of the reported Cr-based catalysts and some noble metal-based catalysts. X-ray absorption spectroscopy and in situ Raman spectroscopy as well as other characterization techniques demonstrated that the in situ calcination in flowing air could not only effectively restrain the conversion of Cr(VI) into Cr(III) but also largely improve the dispersion of Cr species. Furthermore, DHP activity is found to have a positive correlation with the amount of monomeric Cr(VI) species, which is proved to be the precursor of active coordinatively unsaturated Cr sites. Our proposed flowing-air-induced transformation method provides a general strategy for preparing the highly dispersed Cr-based catalysts and other metal oxide materials with varied valence and exhibits potential application prospects in industry.

Lateral monolayer MoS2 homojunction devices prepared by nitrogen plasma doping.

Monolayer MoS2 possesses good electron mobility, structural flexibility and a direct band gap, enabling it to be a promising candidate for flexible and wearable optoelectronic devices. In this article, the lateral monolayer MoS2 homojunctions were prepared by a nitrogen plasma selective doping technique. The monolayer MoS2 thin films were synthesized by chemical vapor deposition and characterized by photoluminescence, atom force microscope and Raman spectroscopy. The electronic and photoelectric properties of the lateral pn and npn homojunctions were discussed. The results showed that the rectifying ratio of the pn homojunction diode is ∼103. As a photodetector of pn homojunction, the optical responsivity is up to 48.5 A W-1, the external quantum efficiency is 11 301%, the detectivity is ∼109 Jones and the response time is 20 ms with the laser of 532 nm and the reverse bias voltage of 10 V. As a bipolar junction transistor of npn homojunction, the amplification coefficient reached ∼102. A controllable plasma doping technique, compatible with traditional CMOS process, is utilized to realize the monolayer MoS2 based pn and npn homojunctions, and it propels the potential applications of 2D materials in the electronic, optoelectronic devices and circuits.

A critical review on the applications and potential risks of emerging MoS2 nanomaterials.

Molybdenum disulfide (MoS2) nanomaterials have been widely used in various fields such as energy store and transformation, environment protection, and biomedicine due to their unique physicochemical properties. Unfortunately, such large-scale production and use of MoS2 nanomaterials would inevitably release into the environmental system and then potentially increase the risks of wildlife/ecosystem and human beings as well. In this review, we first introduce the physicochemichemical properties, synthetic methods and environmental behaviors of MoS2 nanomaterials and their typical functionalized materials, then summarize their environmental and biomedical applications, next assess their potential health risks, covering in vivo and in vitro studies, along with the underlying toxicological mechanisms, and last point out some special phenomena about the balance between applications and potential risks. This review aims to provide guidance for harm predication induced by MoS2 nanomaterials and to suggest prevention measures based on the recent research progress of MoS2' applications and exerting toxicological data.

Photoresponse-Bias Modulation of a High-Performance MoS2 Photodetector with a Unique Vertically Stacked 2H-MoS2/1T@2H-MoS2 Structure.

Monolayer 2H-phase MoS2-based photodetectors exhibit high photon absorption but suffer from low photoresponse, which severely limits their applications in optoelectronic fields. The metallic 1T phase of MoS2, while transporting carriers faster, shows negligible response to visible light, which limits its usage in photodetectors. Herein, we propose an ultrafast-response MoS2-based photodetector having a channel that consists of a 2H-MoS2 sensitizing monolayer on top of 1T@2H-MoS2. The 1T@2H-MoS2 layer has a thickness of several nanometers and is a mixture of metallic 1T-MoS2 and semiconducting 2H-MoS2, imparting metal-like properties to the photodetector. Compared with the monolayer 2H-MoS2 photodetector, we observed a drastic increase in the photoresponse of the 2H-MoS2/1T@2H-MoS2 vertically stacked photodetector to a value of 1917 A W-1. Owing to the presence of metallic 1T-MoS2 within the metal-like 1T@2H-MoS2, the performance of the 2H-MoS2/1T@2H-MoS2 vertically stacked photodetector is voltage bias-modulated with an external quantum efficiency (EQE) of up to 448,384% and a specific detectivity of up to ∼1011 Jones. The higher carrier density and higher mobility of the 1T@2H-MoS2 layer explain the better bias-modulated performance. In addition, the interface between 2H-MoS2 and 1T@2H-MoS2 ensures fewer dangling bonds and reduced lattice mismatching. Thus, this study presents an exclusive vertically stacked MoS2-based photodetector that lays the foundation for the development of photodetectors exhibiting higher photoresponse.

Low-damaged p-type doping of MoS2 using direct nitrogen plasma modulated by toroidal-magnetic-field.

Low damaged doping of two-dimensional (2D) materials proves to be a significant obstacle in realizing fundamental devices such as p-n junction diodes and transistors due to its atom layer thickness. In this work, the defect formation energy and p-type conduction behavior of nitrogen plasma doping are investigated by first principle calculation. Low damaged substitutional p-type doping in MoS2 using low energy nitrogen plasma composed of N+ and N2 + is achieved by a novel toroidal magnetic field (TMF). The TMF helps to raise the concentration of N2 + ions at low RF power condition. The electrical characteristics of double-layer MoS2 field-effect transistors (FETs) clearly show an efficient p-type doping behavior. Atomic force microscope is applied to verify the slight damage in MoS2. X-ray photoelectron spectroscopy, photoluminescence and Raman spectroscopy confirm the effective p-type doping characteristic with weak damage. These findings provide a low damage technology for efficient carrier modulation of MoS2 and other homogeneous TMDC materials, which overcomes barriers in developing 2D electronic and optoelectronic devices.

The identification of active chromium species to enhance catalytic behaviors of alumina-based catalysts for sulfur-containing VOC abatement.

As to the treatment of sulfur containing VOCs (examples are compounds of CH3SH and C2H5SH), finding a catalyst with high performance is necessary. In this work, Cr(x)-Al2O3 (x = 1.0, 2.5, 5.0, 7.5 and 10 wt%) catalysts were synthesized, and their behaviors toward CH3SH and C2H5SH abatement were investigated. The results indicated that Cr(7.5)-Al2O3 exhibited higher activity than other samples and the reported catalysts, on which CH3SH could be almost completely converted at 375 °C, while the temperature for the reported catalysts was above 450 °C. Moreover, there was no obvious deactivation during 30 h on stream over Cr(7.5)-Al2O3, while only about 10 h was found on the reported CeO2 and HZSM-5 catalysts. The improvement in the catalytic performance could be explained by the important role of the Cr6+ species, while the state of Cr3+ was suggested to be ineffective in the degradation process. The identification of the active Cr sites was proved by the characterization measurements, and the control experiments by using mechanical mixtures of CrO3 or Cr2O3 with Al2O3 as well as the comparison studies between spent Al2O3 and spent Cr(7.5)-Al2O3 catalysts.

Investigation of the reaction pathway for synthesizing methyl mercaptan (CH3SH) from H2S-containing syngas over K-Mo-type materials.

The reaction pathway for synthesizing methyl mercaptan (CH3SH) using H2S-containing syngas (CO/H2S/H2) as the reactant gas over SBA-15 supported K-Mo-based catalysts prepared by different impregnation sequences was investigated. The issue of the route to produce CH3SH from CO/H2S/H2 has been debated for a long time. In light of designed kinetic experiments together with thermodynamics analyses, the corresponding reaction pathways in synthesizing CH3SH over K-Mo/SBA-15 were proposed. In the reaction system of CO/H2S/H2, COS was demonstrated to be generated firstly via the reaction between CO and H2S, and then CH3SH was formed via two reaction pathways, which were both the hydrogenation of COS and CS2. The resulting CH3SH was in a state of equilibrium of generation and decomposition. Decomposition of CH3SH was found to occur via two reaction pathways; one was that CH3SH first transformed into two intermediates, CH3SCH3 and CH3SSCH3, which were then further decomposed into CH4 and H2S; another was the direct decomposition of CH3SH into C, H2S and H2. Moreover, the catalyst (K-Mo/SBA-15) prepared with co-impregnation exhibits higher catalytic activities than the catalysts (K/Mo/SBA-15 and Mo/K/SBA-15) prepared by the sequence of impregnation. Based on characterization of the oxidized, sulfided and spent catalysts via N2 adsorption-desorption isotherms, XRD, Raman, XPS and TPR, it was found that two K-containing species, K2Mo2O7 and K2MoO4, were oxide precursors, which were then converted into main K-containing MoS2 species. The CO conversion was closely related to the amount of edge reactive sulfur species that formed the sulfur vacancies over MoS2 phases.