HCHO Selectivity Research Articles

Interface Defects and Carrier Regulation in MOF-Derived Co3O4/In2O3 Composite Materials for Enhanced Selective Detection of HCHO.

Gas sensors for real-time monitoring of low HCHO concentrations have promising applications in the field of health protection and air treatment, and this work reports a novel resistive gas sensor with high sensitivity and selectivity to HCHO. The MOF-derived hollow In2O3 was mixed with ZIF-67(Co) and calcined twice to obtain a hollow Co3O4/In2O3 (hereafter collectively termed MZO-6) composite enriched with oxygen vacancies, and tests such as XPS and EPR proved that the strong interfacial electronic coupling increased the oxygen vacancies. The gas-sensitive test results show that the hollow composite MZO-6 with abundant oxygen vacancies has a higher response value (11,003) to 10 ppm of HCHO and achieves a fast response/recovery time (11/181 s) for HCHO at a lower operating temperature (140 °C). The MZO-6 material significantly enhances the selectivity to HCHO and reduces the interference of common pollutant gases such as ethanol, acetone, and xylene. There is no significant fluctuation of resistance and response values in the 30-day long-term stability test, and the material has good stability. The synergistic effect of the heterostructure and oxygen vacancies altered the formaldehyde adsorption intermediate pathway and reduced the reaction activation energy, enhancing the HCHO responsiveness and selectivity of the MZO-6 material.

Highly Sensitive and Selective Defect WS2 Chemical Sensor for Detecting HCHO Toxic Gases

The gas sensitivity of the W defect in WS2 (VW/WS2) to five toxic gases—HCHO, CH4, CH3HO, CH3OH, and CH3CH3—has been examined in this article. These five gases were adsorbed on the VW/WS2 surface, and the band, density of state (DOS), charge density difference (CDD), work function (W), current–voltage (I–V) characteristic, and sensitivity of adsorption systems were determined. Interestingly, for HCHO-VW/WS2, the energy level contribution of HCHO is closer to the Fermi level, the charge transfer (B) is the largest (0.104 e), the increase in W is more obvious than other adsorption systems, the slope of the I–V characteristic changes more obviously, and the calculated sensitivity is the highest. To sum up, VW/WS2 is more sensitive to HCHO. In conclusion, VW/WS2 has a great deal of promise for producing HCHO chemical sensors due to its high sensitivity and selectivity for HCHO, which can aid in the precise and efficient detection of toxic gases.

ZIF-L(Co) derived cobalt doped In2O3 hollow nanofibers with high surface activity for efficient formaldehyde gas sensing

Gas sensors based on semiconductor metal oxides are identified as a highly promising candidate for toxic gas detection, yet they still suffer from high operating temperature due to low surface activity at low temperature. To address this issue, we present an elaborate design for the homogeneous functionalization of ZIF-L(Co) derived cobalt (Co) catalysts into hollow In2O3 frameworks, aiming to highly activate the redox capacity and catalytic efficacy of the In2O3 sensor to HCHO gas. Benefiting from the Co catalysts doping boosts the catalytic activity and generates abundant oxygen vacancies, the optimized 1 wt% Co-doped In2O3 HNFs sensor exhibits a high response of 40.4 toward 100 ppm HCHO at a moderately low temperature of 180 ℃, which is 4 times higher than that of pristine In2O3 HNFs. Moreover, 1 wt% Co-doped In2O3 HNFs sensor has virtues of high selectivity and excellent long-term stability for HCHO sensing. This study highlights the important influence of Co catalyst on the modulation of surface active sites of metal oxides-based sensors, inspiring the development of cost-effective sensors for indoor hazardous gas monitoring.

Ultra-high specific surface area spherical FePOx/SiO2 aerogel with excellent mechanical properties for the highly selective direct oxidation of CH4 to HCHO

Silica aerogels, characterized by their high porosity and substantial specific surface area, are suitable for applications as catalysts or catalyst supports. The simultaneous attainment of a substantial specific surface area and robust mechanical properties in aerogel materials remains a formidable challenge in material synthesis. Spherical FePOx/SiO2 aerogel materials were synthesized employing a combination of heating reflux, the sol-gel technique, and supercritical ethanol drying. These composites demonstrate an exceptional specific surface area, uniformly dispersed active components, shape controllability, and superior mechanical strength. A noteworthy enhancement in both specific surface area (1175 m2/g) and compressive modulus (7.56 MPa) surpasses many findings reported in extant literature. Under conditions of a reaction temperature at 650 °C and a flow rate of 97.5 mL/min, the HCHO selectivity and yield for 4 wt% FePOx/SiO2 aerogel were 18.3 and 4.2 times, respectively, higher than those of 4 wt% FePOx/SiO2 particles. These composites manifest significant selectivity towards the direct catalytic oxidation of CH4 to HCHO.

Oxygen Vacancy-Rich TiO2 as an Efficient Non-noble Metal Catalyst toward Mild Oxidation of Methane Using Hydrogen Peroxide as the Oxidant

Direct functionalization of methane remains a key challenge, especially for using non-noble metal catalysts. We demonstrated that TiO2 nanorods with abundant oxygen vacancies enabled mild oxidation of methane by H2O2 into formaldehyde (HCHO) without light irradiation. The activity of TiO2 nanorods with the concentration gradient of oxygen vacancies (VO) increased with the VO concentration. In H2O2 aqueous solution under 30 bar of CH4 at 70 °C for 1 h, the TiO2 nanorods with the most abundant VO exhibited a total oxygenate yield of 40.80 μmol, among which the selectivity for HCHO was 64.1%. On the basis of the catalytic and spectroscopic data, we identified the reaction intermediates and accordingly mapped the reaction scheme. Specifically, H2O2 is activated on Ti atoms near VO to form surface peroxo intermediates, followed by the activation of CH4 to produce methoxy groups. The methoxy group can react either with water to form methanol or with hydroxyl radicals to form CH3OOH. Methanol is attacked by hydroxyl radicals and dehydrated to form •CH2OH that further reacts with hydroxyl radicals and is dehydrated to HCHO. CH3OOH directly undergoes dehydration to engender HCHO.

Lead-free defective halide perovskites Cs2SnX6 (X = Cl, Br, I) for highly robust formaldehyde sensing at room temperature

In this contribution, all-inorganic lead-free halide perovskites Cs2SnX6 (X = Cl, Br, I) were synthesized by simple room-temperature precipitation. Material characterization demonstrated their feature in rich defects of halogen vacancies. As room-temperature formaldehyde (HCHO) sensing materials, Cs2SnX6 exhibited reliable sensitivity order of Cs2SnCl6 < Cs2SnBr6 < Cs2SnI6. Notably, the Cs2SnI6 sensor showed excellent selectivity for HCHO at room temperature with high response (Sr = 78, 100 ppm), short response/recovery time (3.6 s/7.2 s), good repeatability and long-term stability. Finally, the HCHO sensing mechanism was investigated via in-situ infrared analysis, conforming abundant intermediates of dioxymethylene, formate, and carbonate. The excellent sensing performance is attributed to rich iodine vacancies (VI) as active sites induced boosted stepwise oxidation of HCHO molecules on Cs2SnI6 surface. This work provides a fine candidate for practical HCHO detection at room temperature, and furthermore, gives new insights in designing halide perovskites for next-generation sensory devices.

Vacancy engineered BiVO4 photoanode realizes efficient photoelectrochemical CH3OH oxidation in near-neutral media: Active site regulation improves HCHO selectivity

Photoelectrochemical (PEC) CH3OH oxidation holds great promise for sustainably obtaining high value-added HCHO. Low HCHO yield and selectivity are two major constraints to the development of PEC CH3OH oxidation. Herein, we deliver the first report of efficient PEC CH3OH oxidation catalyzed by vacant BiVO4 (BVO) photoanode derived from electrolyte photooxidation and disclose the oxygen vacancy (OV)-induced catalytic sites shift for HCHO selectivity modulation. The near-neutral electrolyte solution provides an excellent environment for alcohol-aldehyde conversion and stable performance of BVO photoanode. OVs promote photogenerated carrier separation, thus ensuring a high photocurrent density and efficient HCHO production for the BVO photoanode. More importantly, the introduction of OV changes the adsorption site for CH3OH molecules from the Bi site to V site of BVO, which enhances the adsorption of CH3OH molecules, and reduces the key C-H activation energy barrier and the energy required for HCHO desorption, collectively boosting the evolution rate and selectivity of HCHO. Thus, OV engineered BVO photoanode exhibits tunable Faraday efficiencies for CH3OH oxidation of 88.4%, 94.7% and 85.6% at 0.8 VRHE, 1.0 VRHE and 1.2 VRHE, respectively, and achieves a prominent HCHO production rate of 579.9 mmol m-2h−1 and a stable photocurrent density of 3.4 mA cm−2 at 1.2 VRHE in near-neutral electrolyte. This work may inspire the future work for PEC chemicals high value-added conversion by using vacancy engineering and matching suitable electrolyte solution.

Enabling Specific Photocatalytic Methane Oxidation by Controlling Free Radical Type

Selective CH4 oxidation to CH3OH or HCHO with O2 in H2O under mild conditions provides a desired sustainable pathway for synthesis of commodity chemicals. However, manipulating reaction selectivity while maintaining high productivity remains a huge challenge due to the difficulty in the kinetic control of the formation of a desired oxygenate against its overoxidation. Here, we propose a highly efficient strategy, based on the precise control of the type of as-formed radicals by rational design on photocatalysts, to achieve both high selectivity and high productivity of CH3OH and HCHO in CH4 photooxidation for the first time. Through tuning the band structure and the size of active sites (i.e., single atoms or nanoparticles) in our Au/In2O3 catalyst, we show alternative formation of two important radicals, •OOH and •OH, which leads to distinctly different reaction paths to the formation of CH3OH and HCHO, respectively. This approach gives rise to a remarkable HCHO selectivity and yield of 97.62% and 6.09 mmol g-1 on In2O3-supported Au single atoms (Au1/In2O3) and an exceptional CH3OH selectivity and yield of 89.42% and 5.95 mmol g-1 on In2O3-supported Au nanoparticles (AuNPs/In2O3), respectively, upon photocatalytic CH4 oxidation for 3 h at room temperature. This work opens a new avenue toward efficient and selective CH4 oxidation by delicate design of composite photocatalysts.

The effect of VOx species polymerization degree and coordination environments of V-KIT-6 catalysts on the performance for the selective oxidation of methane

V-incorporated mesoporous KIT-6 silica materials with different V content (xV-KIT-6) were used for partial oxidation of methane (POM) with dioxygen under atmospheric pressure. Turnover frequency of CH4 (TOFCH4) is basically indistinguishable for highly dispersed VOx species under (sub-)monolayer coverage on SiO2. However, the polymerization degree of VOx species significant affect HCHO selectivity. At 625 °C, 3V-KIT-6 catalyst exhibited best catalytic performance of POM: methane conversion was 8.1%, formaldehyde selectivity was 26.1% and selectivity of the incomplete oxidation products was 90.5%. Compared with 3V-KIT-6 catalyst, the supported 1.1V/KIT-6 catalyst with same vanadium content and dispersity exhibit poor high-temperature TOFHCHO and low-temperature HCHO selectivity, suggesting that the supported and incorporated VOx species possess different coordination environments, such as VO and VOSi bond length and bond angle. To further investigate their microstructure effect on catalytic performance, in situ and operando UV Raman spectroscopy was used to investigate their different redox properties of VOx species. Under the pure CH4 atmosphere, their VOV or VOSi lattice oxygen are more reducible than VO lattice oxygen, suggesting the former are active sites. Supported VOx species are more easily reduced by CH4 than the incorporated ones, the VO lattice oxygen of 3V-KIT-6 catalyst cannot be completely consumed by CH4 after 1h, at least 30% VO bonds maintained. When 5% O2 was cofed, the VO bonds on 3V-KIT-6 and 1.1V/KIT-6 recovered quickly, indicating that the partially reduced VOx species are oxidized by O2 faster than their reduction by CH4. Thus, the activation of CH4 on VOx species should be the rate-determining step. Some unsaturated VOx species are present on 1.1V/KIT-6 easily, those may generate unselective electrophilic oxygen species for overoxidation.

Challenges and Prospects in the Catalytic Conversion of Carbon Dioxide to Formaldehyde

AbstractFormaldehyde (HCHO) is a crucial C1 building block for daily‐life commodities in a wide range of industrial processes. Industrial production of HCHO today is based on energy‐ and cost‐intensive gas‐phase catalytic oxidation of methanol, which calls for exploring other and more sustainable ways of carrying out this process. Utilization of carbon dioxide (CO2) as precursor presents a promising strategy to simultaneously mitigate the carbon footprint and alleviate environmental issues. This Minireview summarizes recent progress in CO2‐to‐HCHO conversion using hydrogenation, hydroboration/hydrosilylation as well as photochemical, electrochemical, photoelectrochemical, and enzymatic approaches. The active species, reaction intermediates, and mechanistic pathways are discussed to deepen the understanding of HCHO selectivity issues. Finally, shortcomings and prospects of the various strategies for sustainable reduction of CO2 to HCHO are discussed.

Challenges and Prospects in the Catalytic Conversion of Carbon Dioxide to Formaldehyde.

Formaldehyde (HCHO) is a crucial C1 building block for daily-life commodities in a wide range of industrial processes. Industrial production of HCHO today is based on energy- and cost-intensive gas-phase catalytic oxidation of methanol, which calls for exploring other and more sustainable ways of carrying out this process. Utilization of carbon dioxide (CO2 ) as precursor presents a promising strategy to simultaneously mitigate the carbon footprint and alleviate environmental issues. This Minireview summarizes recent progress in CO2 -to-HCHO conversion using hydrogenation, hydroboration/hydrosilylation as well as photochemical, electrochemical, photoelectrochemical, and enzymatic approaches. The active species, reaction intermediates, and mechanistic pathways are discussed to deepen the understanding of HCHO selectivity issues. Finally, shortcomings and prospects of the various strategies for sustainable reduction of CO2 to HCHO are discussed.

Selective oxidation of methane into formaldehyde and carbon monoxide catalyzed by supported thermally stable iron oxide subnanoclusters prepared from a diiron-introduced polyoxometalate precursor

Direct oxidative conversion of methane (CH4) into useful C1 products remains challenging due to the low reactivity of CH4 and its facile overoxidation into carbon dioxide (CO2) under high temperature conditions. Iron oxide (FeOx) cluster catalysts are promising because of their low activation energy for CH4 oxidation. However, preliminary results revealed that FeOx clusters were easily aggregated and deactivated under CH4 oxidation conditions at 873 K. In this study, we used a diiron-introduced polyoxometalate as a precursor to form thermally stable FeOx subnanoclusters on SiO2, which selectively converted CH4 into formaldehyde (HCHO) and carbon monoxide (CO) (CH4 conversion, 2.3%; HCHO and CO selectivity, 87% at 873 K after 1 h). The FeOx subnanocluster catalyst maintained catalytic activity even after 72 h. Various characterizations, such as STEM, X-ray absorption spectroscopy, and X-ray diffraction, revealed that the in situ formed FeOx subnanoclusters were stabilized by WOx nanoclusters originating from the polyoxometalate frameworks.

Effects of Field-Effect and Schottky Heterostructure on p-Type Graphene-Based Gas Sensor Modified by n-Type In2O3 and Phenylenediamine

Over the past decade, advantages of graphene in high-performance gas sensing have been demonstrated, especially for single- or few-layered graphene wherein the theoretical and technical advances are mature. Owing to the complexity of multi-layered graphene (MLG) sensors and the increasing demand for practical applications, there is an urgent need to comprehensively understand the correlation between MLG and its derivatives for developing next-generation gas sensors. Herein, theoretical and empirical strategies for obtaining better gas sensors are developed. These approaches can be divided into three categories: 1) building devices with Fermi level near the Dirac point (EF,Dirac), 2) enhancing the adsorption probability f(x) and driving force (gap between as-prepared and saturated Fermi levels), and 3) accelerating mobility. A device employing p-type reduced graphene oxide (rGO) decorated with n-type indium oxide and phenylenediamine (GIP) was designed and fabricated by adopting approaches 1 and 2 (EF,Dirac and f(x) enhancement). The resulting hole-compensated GIP displayed a remarkable response to formaldehyde (HCHO), which was 66.3 times higher than rGO, with faster response/recovery. GIP also exhibited higher selectivity for HCHO than for ammonia and trimethylamine. We believe that the classification will untangle the complex role of graphene in sensing, helping to design next-generation advanced gas sensors.

CH3OH selective oxidation to HCHO on Z-scheme Fe2O3/g-C3N4 hybrid: The rate-determining step of C–H bond scission

• The unique Z-scheme ZOF@CN NTs photoanode exhibits outstanding CH 3 OH oxidation activity. • ZOF@CN NTs with high selectivity provides a green and clean pathway for CH 3 OH conversion process. • The CH 3 O* via the C–H bond scission to form CH 2 O* is the rate-determining step for the reactions. Fe 2 O 3 as photoanode photoelectrocatalytic (PEC) CH 3 OH conversion is a promising approach for industrial preparation of high value-added HCHO. However, the conversion efficiency is limited by its poor charge separation and sluggish oxidation mechanism. This study reports a direct Z-scheme system consisting of 1D nanotubes oxygen-deficient a -Fe 2 O 3 and g -C 3 N 4 (ZOF@CN NTs) with great enhancement of PEC CH 3 OH oxidation to HCHO activity. The ZOF@CN NTs presents a CH 3 OH oxidation photocurrent density of 1.05 mA cm −2 at 0.6 V vs Ag/AgCl, 5.25 times than that of oxygen-deficient a -Fe 2 O 3 alone (0.20 mA cm −2 ), and HCHO selectivity of 81.5%. Combining by UV–vis, UPS, PL spectroscopy, PEC performance and density functional theory calculations, the mechanism of CH 3 OH oxidation to HCHO on Z-scheme ZOF@CN NTs photoanode was found along following pathway: CH 3 OH*–CH 3 O*- CH 2 O* –CH 2 O (g), which is starting from the O–H bond scission and then followed by C−H scission. The CH 3 O* via the C–H bond scission to form CH 2 O* is the key step for the reaction, and N atoms from g -C 3 N 4 with higher electronegativity compared with O atoms, which is useful for C–H bond scission. This work serves as a solid foundation for understanding Fe 2 O 3 -based Z-scheme system photoanodes for CH 3 OH oxidation to HCHO.

Gold nanorods assisted silver mirror reaction for consecutive color change based on-site visual semi-quantification of indoor formaldehyde

In the present study, a multicolor conversion strategy based on gold nanorods (AuNRs) assisted silver mirror reaction is developed for visual quantification of formaldehyde (HCHO). The method involves the silver deposition on the surface of AuNRs via the reduction reaction of Tollens' reagent in the presence of HCHO. The peak shifts of the longitudinal localized surface plasmon resonance (LSPR) and color variations of AuNRs solution are sensitive to the thickness of silver layer deposited on the surface of AuNRs. Under the optimized conditions, there is a good linear relationship between the peak shift and the concentration of HCHO (for both liquid sample and gas sample) in a wide range of concentration, thus an AuNRs based colorimetric assay for quantitative analysis of HCHO is established. Moreover, a series of consecutive color changes (from mauve to dusty blue, nattier blue, blue, mazarine blue, dark green, clay bank and finally orange red) responding to the presence and amount of HCHO enables visual semi-quantitative determination of HCHO within 15 min, and the clearly discernible dosage scale is as low as 0.10 μM. This assay exhibits desirable selectivity for HCHO over several common interfering organic molecular species and potential gas interferents, which has been successfully applied to measure the concentration of HCHO indoor.

Oximation reaction induced reduced graphene oxide gas sensor for formaldehyde detection

High-performance gas sensors can offer great potentials for monitoring and detection of volatile organic compounds (VOCs) in both domestic and industrial environment. In the present work, a new HCHO gas sensor was constructed with reduced graphene oxide (RGO) induced by the oximation reaction. The gas-sensing performance test results suggested that the RGO hydroxylamine hydrochloride (RGO/HA-HCl) sensor presented a high response of 75% at 16 ppm HCHO at room temperature, and a high selectivity for HCHO suffering little interference with high concentrations of volatile organic compounds, including methanol, ethanol, and methylbenzene, dichloromethane and water. Additionally, the RGO/HA-HCl sensor also showed a good long-term stability with RSD of 5.83% for a 15-day continuous sensing test, and the detection limit (DL) could reach 0.023 ppm under ambient conditions. Moreover, the mechanism for the high sensitivity and selectivity of formaldehyde was further established by in-situ gas chromatography mass spectrometry (GC–MS). This work would provide a reliable new HCHO gas sensor which could be used for monitoring and forewarning the emission of HCHO for a better protection and improvement of our environment.

Vacancy-Assisted Selective Detection of Low-ppb Formaldehyde in Two-Dimensional Layered SnS2

A two-dimensional (2D) layered SnS2 film synthesized by the thermal-chemical vapor deposition method is utilized for detecting formaldehyde (HCHO), which causes a sick building syndrome. A back-gated field-effect transistor (FET)-based SnS2 sensor successfully detects HCHO with concentrations down to 1 ppb in a nitrogen atmosphere. Sensing measurements performed under dry air conditions also exhibit a clear response to 20 ppb of HCHO, which is more sensitive than the previously reported sensors based on other 2D-layered materials. Moreover, it is found that the sensor possesses a high selectivity for HCHO over other organic species. Theoretical calculations suggest that native sulfur vacancies existing in n-type SnS2 crystals play an important role in HCHO detection. Actually, oxygen atoms that are unexpectedly detached from HCHO molecules are found to fill the vacancies, giving rise to p-type doping in SnS2. As a result, decrease in the drain current of SnS2-FET can be found as a signal of HCHO detection. Furthermore, considering the future mass-production of sensors, we demonstrate large-scale growth of the SnS2 film by means of magnetron-sputtering deposition and subsequent annealing in a diluted hydrogen sulfide atmosphere. The sputtered film is also found to exhibit a good sensing ability to HCHO.

Graphene Oxide@3D Hierarchical SnO2 Nanofiber/Nanosheets Nanocomposites for Highly Sensitive and Low-Temperature Formaldehyde Detection.

In this work, we reported a formaldehyde (HCHO) gas sensor with highly sensitive and selective gas-sensing performance at low operating temperature based on graphene oxide (GO)@SnO2 nanofiber/nanosheets (NF/NSs) nanocomposites. Hierarchical SnO2 NF/NSs coated with GO nanosheets showed enhanced sensing performance for HCHO gas, especially at low operating temperature. A series of characterization methods, including X-ray diffraction (XRD), Field emission scanning electron microscopy (FE-SEM), Transmission electron microscope (TEM), X-ray photoelectron spectroscopy (XPS) and Brunauer–Emmett–Teller (BET) were used to characterize their microstructures, morphologies, compositions, surface areas and so on. The sensing performance of GO@SnO2 NF/NSs nanocomposites was optimized by adjusting the loading amount of GO ranging from 0.25% to 1.25%. The results showed the optimum loading amount of 1% GO in GO@SnO2 NF/NSs nanocomposites not only exhibited the highest sensitivity value (Ra/Rg = 280 to 100 ppm HCHO gas) but also lowered the optimum operation temperature from 120 °C to 60 °C. The response value was about 4.5 times higher than that of pure hierarchical SnO2 NF/NSs (Ra/Rg = 64 to 100 ppm). GO@SnO2 NF/NSs nanocomposites showed lower detection limit down to 0.25 ppm HCHO and excellent selectivity against interfering gases (ethanol (C2H5OH), acetone (CH3COCH3), methanol (CH3OH), ammonia (NH3), methylbenzene (C7H8), benzene (C6H6) and water (H2O)). The enhanced sensing performance for HCHO was mainly ascribed to the high specific surface area, suitable electron transfer channels and the synergistic effect of the SnO2 NF/NSs and GO nanosheets network.

Fabrication of heterostructured p-CuO/n-SnO2 core-shell nanowires for enhanced sensitive and selective formaldehyde detection

Highly sensitive and selective gas sensors based on heterostructured p-CuO/n-SnO2 core-shell nanowires (NWs) with precisely controlled shell thickness were synthesized through a sequential process combining a solution processing and atomic layer deposition. The gas sensing devices were fabricated on micro-electromechanical systems, which has triggered great research interest for low power consumption and highly integrated design. The designed p-CuO/n-SnO2 core-shell NW structured gas sensors exhibited superior gas sensing performance, which is closely related to the thickness of the SnO2 shell. Specifically, p-CuO/n-SnO2 core-shell NWs with a 24 nm thick SnO2 shell displayed a high sensitivity (Ra/Rg) of 2.42, whose rate of resistance change (i.e. 1.42) is 3 times higher than the pristine CuO NW sensor when detecting 50 ppm formaldehyde (HCHO) at 250 °C. The enhanced gas sensing performance could be attributed to the formation of p-n heterojunction which was revealed by specific band alignment and the heterojunction-depletion model. Besides, the well-structured p-CuO/n-SnO2 core-shell NWs achieved excellent selectivity for HCHO from commonly occurred reducing gases. In a word, such heterostructured p-CuO/n-SnO2 core-shell NW gas sensors demonstrate a feasible approach for enhanced sensitive and selective HCHO detection.

Copper grafted on SBA-15 as efficient catalyst for the selective oxidation of methane by oxygen

CuO x /SBA-15 catalysts prepared by a grafting approach (denoted as CuO x /SBA-15- gra) have been characterized and studied for the selective oxidation of methane to formaldehyde by oxygen. Our characterizations using XRD, H 2-TPR, UV–vis and ESR suggest that Cu 2+ ions are almost isolated on the surface of SBA-15 at a Cu content ≤0.6 wt%. The increase in the Cu content to >0.6 wt% decreased the fraction of isolated Cu 2+ ions and increased that of aggregated CuO x clusters. On the other hand, the fraction of isolated Cu 2+ ions was much smaller in the CuO x /SBA-15 catalyst prepared by the impregnation method (denoted as CuO x /SBA-15- imp) even with a lower Cu content (0.5 wt%). The CuO x /SBA-15- gra provided significantly higher HCHO selectivity in the selective oxidation of CH 4 than the CuO x /SBA-15- imp, and the CuO x /SBA-15- gra catalyst with a Cu content of 0.6 wt% showed the best performance for HCHO formation. In situ FT-IR spectroscopy of adsorbed CO and XPS measurements suggest the generation of Cu + after the reaction, and the Cu + site is likely responsible for the activation of oxygen, forming an active oxygen species for the selective oxidation of CH 4 to HCHO.