Porous Si/SnO2 nanowires heterostructures for H2S gas sensing

In this work, a low-temperature (100 °C) gas sensor based on heterojunctions of porous Si and SnO₂ nanowires (NWs) is presented. Porous Si was obtained from p-Si wafers by electrochemical etching, and SnO₂ NWs were fabricated by a vapor-liquid-solid route. Different characterization techniques were used to verify the formation of porous Si/SnO₂ NW heterojunctions. H₂S gas sensing results showed enhanced gas sensing performance of the porous Si/SnO₂ NW sensor in comparison with that of a porous Si sensor. The reasons for such enhancement are discussed in detail. This study demonstrates the promising effects of SnO₂ NWs in combination with porous Si to realize low-temperature H₂S gas sensors that are highly compatible with existing Si processing technology.     

Xanthate sensing properties of Pt-functionalized WO3 microspheres synthesized by one-pot hydrothermal method

WO₃ microspheres in a hierarchical nanorod-assembled architecture were prepared by using a facile one-pot hydrothermal method. The morphology and structure of pure, and 1, 3, and 5 mol% Pt-functionalized WO₃ microspheres were characterized by means of SEM, TEM, XRD, XPS and FTIR measurements. Structural characterizations demonstrated that these WO₃ microspheres assembled by numerous one-dimensional WO₃ nanorods were approximately 2–5 µm in diameter. The nanorods with their diameters in the range of 70–90 nm showed a single crystal hexagonal structure. Gas sensors based on pure and Pt-functionalized WO₃ microspheres showed reversible response and outstanding selectivity to xanthate gas at the operating temperature range of 75–175 °C. The sensor response increased with the increase of xanthate gas concentration. The highest response of 102.7 was obtained for the sensor based on 3 mol% Pt-functionalized WO₃ microspheres to 100 ppm xanthate gas at an operating temperature of 100 °C, which could be ascribed to the large effective surface area and high porosity of WO₃ microspheres as well as the catalytic effect of Pt nanoparticles.     

NO2 gas sensing responses of In2O3 nanoparticles decorated on GO nanosheets

Nanomaterials offer a wide range of applications in environmental nanotechnology. Hazardous pollutants in the environment are needed to be detected and controlled effectively to avoid human health risks. In this paper, we described the fine-controlled growth of In₂O₃ nanoparticles embedded on GO nanosheets by a facile precipitation method. The In₂O₃@GO nanocomposites exhibited outstanding gas sensing performance as compared with pure In₂O₃ nanoparticles towards NO₂. At 225 °C, the sensor displayed high selectivity, best response (78) to 40 ppm NO₂, quick response, and recovery times of 106s/42s. The improved sensing performances of the nanocomposite were attributed to large surface area, high gas adsorption-desorption capability, and the formation of p-n heterojunctions between In₂O₃ nanoparticles and GO nanosheets. The excellent gas detecting activities validate In₂O₃@GO nanocomposites as a promising candidate in the NO₂ gas sensor industry.     

Enhanced gas sensing properties of hierarchical SnO2 nanoflower assembled from nanorods via a one-pot template-free hydrothermal method

Well-defined three-dimensional (3D) hierarchical tin dioxide (SnO₂) nanoflowers with the size of about 200 nm were successfully synthesized by a simple template-free hydrothermal method. X-ray diffraction (XRD), scanning electron microscopy (SEM), transmission electron microscopy (TEM) and N₂ adsorption-desorption analyses were used to characterize the structure and morphology of the products. The as-synthesized full crystalline and large specific surface area SnO₂ nanoflowers were assembled by one-dimensional (1D) SnO₂ nanorods with sharp tips. A possible self-assembly mechanism for the formation the SnO₂ nanoflowers was speculated. Moreover, gas sensing investigation showed the sensor based on SnO₂ nanoflowers to exhibit high response and fast response-recovery ability to detect acetone and ethanol at an operating temperature lower than 200 °C. The enhancement of gas sensing properties was attributed to their 3D hierarchical nanostructure, large specific surface area, and small size of the secondary SnO₂ nanorods.     

NO2 inhibits the catalytic reaction of NO and O2 over Pt

The rate equation for the overall reaction of NO and O₂ over Pt/Al₂O₃ was determined to be r=k_f[NO] ^1.05±0.08[O₂]^1.03±0.08[NO₂]^0.92±0.07(1-), with k_f as the forward rate constant, =([NO₂]/K[NO][O₂]^1/2), and K as the equilibrium constant for the overall reaction. An apparent activation energy of 82 kJ mol^–1 ± 9 kJ mol^–1 was observed. The inhibition by the product NO₂ makes it imperative to include the influence of NO₂ concentration in any analysis of the kinetics of this reaction. The reaction mechanism that fits our observed orders consists of the equilibrated dissociation of NO₂ to produce a surface mostly covered by oxygen, thereby inhibiting the equilibrium adsorption of NO, and the non-dissociative adsorption of O₂, which is the proposed rate determining step.     

Hydrothermally grown ZnO nanorods arrays for selective NO2 gas sensing: Effect of anion generating agents

Vertically aligned ZnO nanorods (ZNRs) arrays with various aspect ratios were deposited by using a simple and inexpensive hydrothermal route at relatively low temperature of 90 °C. The influence of hydroxide anion generating agents in the solution on the growth of ZNRs arrays was studied. Hexamethylenetetramine (HMTA) and ammonia were used as hydroxide anion generating agents while polyethyleneimine (PEI) as structure directing agent. The combined effect of these three agents plays a crucial role in the growth of ZNRs arrays with respect to their rod length and diameter, which controls the aspect ratio. The deposited ZNRs exhibited hexagonal wurtize crystal structure with preferred orientation along (002) plane. The highly crystalline nature and pure phase formation of ZNRs was confirmed from FT-Raman studies. The maximum gas response (R_g/R_a) of 67.5 was observed for high aspect ratio ZNRs, deposited with combination of HMTA, ammonia as well as PEI. The enhancement in gas response can be attributed to high surface area (45 cm²/g) and desirable surface accessibility in high aspect ratio ZNRs. Fast response–recovery characteristics, especially a much quicker gas response time of 32 s and recovery time of 530 s were observed at 100 ppm NO₂ gas concentration.     

Fabrication of Ti3C2Tx/In2O3 nanocomposites for enhanced ammonia sensing at room temperature

Ti₃C₂T_x, as a novel two-dimensional material, displays promising prospects in NH₃ detection at room temperature. However, the NH₃ detection limit of pristine Ti₃C₂T_x is still a major research concern. Therefore, it is important to explore new Ti₃C₂T_x-based nanocomposites for better NH₃-sensing performance. In the present experiment, Ti₃C₂T_x/In₂O₃ nanocomposites were successfully synthesized by ultrasonication and characterized by XRD, FESEM, TEM, XPS, and BET. The optimal Ti₃C₂T_x/In₂O₃-based sensor had a high response of 63.8% (30.4 times higher than that of pristine Ti₃C₂T_x) to 30 ppm NH₃ at room temperature. In addition, the optimal Ti₃C₂T_x/In₂O₃-based sensor had stable repeatability, excellent selectivity, and long-term stability, while exhibiting excellent potential for NH₃ detection at room temperature.     

Boosting low-temperature selective catalytic reduction of NO with NH3 of V2O5/TiO2 catalyst via B-doping

A series of B-doped V₂O₅/TiO₂ catalysts has been prepared the by sol-gel and impregnation methods to investigate the influence of B-doping on the selective catalytic reduction (SCR) of NO_x with NH₃. X-ray diffraction, Brunauer-Emmett-Teller specific surface area, scanning electron microscope, X-ray photoelectron spectroscopy, temperature-programmed reduction of H₂ and temperature-programmed desorption of NH₃ technology were used to study the effect of the B-doping on the structure and NH₃-SCR activity of V₂O₅/TiO₂ catalysts. The experimental results demonstrated that the introduction of B not only improved the low-temperature SCR activity of the catalysts, but also broadened the activity temperature window. The best SCR activity in the entire test temperature range is obtained for VTiB_2.0 with 2.0% doping amount of B and the NO_x conversion rate is up to 94.3% at 210 ℃. The crystal phase, specific surface area, valence state reducibility and surface acidity of the active components for the as-prepared catalysts are significantly affected by the B-doping, resulting in an improved NH₃-SCR performance. These results suggest that the V₂O₅/TiO₂ catalysts with an appropriate B content afford good candidates for SCR in the low temperature window.     

气体传感器材料——In2O3纳米线的制备与表征

采用物理气相沉积法,以纯铟粒为原料,在980℃下Si(100)基片上通过改变气流成分制备了一系列氧化铟纳米线,并采用扫描电子显微镜、X射线衍射和透射电子显微镜对其形貌和结构进行了表征.试验表明,随着氩气中氢气含量增加,纳米线的产量降低,10%的含氢量最优.纳米线的直径在50～100nm之间,长度可达几十微米.纳米线的结构是方铁锰矿体心立方单晶结构.纳米线的生长符合VLS模型,铟起到了自催化的作用.     

Cu2O quantum dots modified by RGO nanosheets for ultrasensitive and selective NO2 gas detection

Real-time monitoring of trace NO₂ emission has been an emerging challenge in environment and health sectors lately. Aiming to overcome this challenge, NO₂ gas sensors based on cuprous oxide quantum dots (Cu₂O QDs) anchored onto reduced graphene oxide (RGO) nanosheets serving as a sensitive layer were prepared in this report. Apart from a series of purposive measurements, various characterization techniques such as XRD, Raman, XPS and TEM were employed as well to assist the exploration of sensors performance to NO₂ gas. The experimental results revealed a 580% response enhancement for prepared RGO/Cu₂O sensors compared with pure RGO counterparts, as well as an excellent selectivity. In a specific experiment, the sensing response attained 4.8% and 29.3% toward 20 ppb and 100 ppb NO₂ respectively at 60 °C, which was larger than most Cu₂O based resistive gas sensors. Moreover, further subtle modulation of this RGO/Cu₂O nanocomposites led to a preferable room-temperature response of 37.8% toward 100 ppb NO₂, which also offered a favorable stability of 98.1% response retention after four exposures within ten days. The obtained results imply that the prepared RGO/Cu₂O QDs sensors possess a competitive capability of trace NO₂ detection.     

Solvothermal preparation and gas sensing properties of hierarchical pore structure SnO2 produced using grapefruit peel as a bio-template

Hierarchical pore structure nano-sized SnO₂ was synthesized using a solvothermal method with SnCl₄ as the raw material and grapefruit peel as the bio-template. The products were characterized by powder X-ray diffraction, high resolution scanning electron microscopy, transmission electron microscopy and nitrogen adsorption/desorption measurements. The results show that the SnO₂ prepared from the grapefruit peel bio-template consists of many large size (5–20 µm) interconnected pores with a honeycomb structure and nanosized pores (9.46 nm) on the walls of the large pores. The as-prepared SnO₂ presented a high specific surface area of 42.98 m²/g and the average crystallite size was about 10±0.5 nm. The gas sensing performance of the prepared material toward several volatile organic compounds was investigated. The results show that the hierarchical pore structure nano-sized SnO₂ was highly sensitive and selective to n-butanol, indicating that this material may be a promising candidate for future development as a n-butanol gas sensor.     

Sulfur deactivation and regeneration of Pt/BaO/Al2O3 and Pt/SrO/Al2O3 NO x storage catalysts

Transient experiments were performed to study sulfur deactivation and regeneration of Pt/BaO/Al₂O₃ and Pt/SrO/Al₂O₃ NO _x storage catalysts. It was found that the strontium-based catalysts are more easily regenerated than the barium-based catalysts and that a higher fraction of the NO _x storage sites are regenerated when H₂ is used in combination with CO₂ compared to H₂ only.     

Ultrasensitive NO2 gas sensing performance of two dimensional ZnO nanomaterials: Nanosheets and nanoplates

Highly sensitive NO₂ gas sensors with low detection limit are vital for practical application in air pollution monitoring. Here, the NO₂ gas sensing performance of porous ZnO nanosheets and nanoplates were investigated, with different shape and thickness. It was found that ultra-thin ZnO nanoplates had a higher sensitivity than coral-like ZnO nanosheets. The results were attributed to the high specific surface and very small thickness of the ultrathin nanoplates. The nanoplates have indeed a thickness of 15 nm compared to that of the nanosheets which is 100 nm, and a BET surface area of 75 m²/g, while that of the nanosheets is 6 m²/g. The chemosensor based on ultra-thin ZnO nanoplates shows a response (calculated as the ratio between the resistance of the sensor in the presence of the gas and in its absence) of 76 to 0.5 ppm of NO₂ at 200 °C, with a theoretical detection limit of 3 parts per trillion and a selectivity higher than 760 towards acetone, ethanol, isopropyl alcohol, triethylamine, SO₂ and CO. The specific surface and the small thickness of the ultra-thin nanoplates contribute to its highly improved sensing performance, making it ideal for NO₂ gas sensing.     

Adsorbate-assisted NO decomposition in NO reduction by C3H6 over Pt/Al2O3 catalysts under lean-burn conditions

The rates and product selectivities of the C₃H₆-NO-O₂ and NO-H₂ reactions over a Pt/Al₂O₃ catalyst, and of the straight, NO decomposition reaction over the reduced catalyst have been compared at 240C. The rate of NO decomposition over the reduced catalyst is seven times greater than the rate of NO decomposition in the C₃H₆-NO-O₂ reaction. This is consistent with a mechanism in which NO decomposition occurs on Pt sites reduced by the hydrocarbon, provided only that at steady state in the lean NO _x reaction about 14% of the Pt sites are in the reduced form. However, the (extrapolated) rate of the NO-H₂ reaction at 240C is about 10⁴ times faster than the rate of the NO decomposition reaction thus raising the possibility that NO decomposition in the former reaction is assisted by H_ads. It is suggested that adsorbate-assisted NO decomposition in the C₃H₆-NO-O₂ reaction could be very important. This would mean that the proportion of reduced Pt sites required in the steady state would be extremely small. The NO decomposition and the NO-H₂ reactions produce no N₂O, unlike the C₃H₆-NO-O₂ reaction, suggesting that adsorbed NO is completely dissociated in the first two cases, but only partially dissociated in the latter case. It is possible that some of the associatively adsorbed NO present during the C₃H₆-NO-O₂ reaction may be adsorbed on oxidised Pt sites.     

Synthesis and high sensing properties of a single Pd-doped SnO2 nanoribbon

Monocrystal SnO₂ and Pd-SnO₂ nanoribbons have been successfully synthesized by thermal evaporation, and novel ethanol sensors based on a single Pd-SnO₂ nanoribbon and a single SnO₂ nanoribbon were fabricated. The sensing properties of SnO₂ nanoribbon (SnO₂ NB) and Pd-doped SnO₂ nanoribbon (Pd-SnO₂ NB) sensors were investigated. The results indicated that the SnO₂ NB showed a high sensitivity to ethanol and the Pd-SnO₂ NB has a much higher sensitivity of 4.3 at 1,000 ppm of ethanol at 230°C, which is the highest sensitivity for a SnO₂-based NB. Pd-SnO₂ NB can detect ethanol in a wide range of concentration (1 ~ 1,000 ppm) with a relatively quick response (recovery) time of 8 s (9 s) at a temperature from 100°C to 300°C. In the meantime, the sensing capabilities of the Pd-SnO₂ NB under 1 ppm of ethanol at 230°C will help to promote the sensitivity of a single nanoribbon sensor. Excellent performances of such a sensor make it a promising candidate for a device design toward ever-shrinking dimensions because a single nanoribbon device is easily integrated in the electronic devices.     

Constructing bilayer sensors with Co-MOF-derived Co3O4 porous sensing films and SnO2 catalytic overlayers to enhance room-temperature triethylamine sensing performance

Gas sensors with good repeatability and controllable fabrication method are extremely desired for practical applications. Co-MOF-derived Co₃O₄ is a promising gas-sensing material candidate because of its large surface area, ultrahigh porosities, and abundant oxygen defects. However, the advantages of Co-MOF precursor were limited by the traditional sensor fabrication methods. Moreover, the high resistance and poor surface activity of Co₃O₄ resulted in low gas-sensing performance at room temperature (RT). To overcome these challenges, in-situ sensors based on Co₃O₄ porous films with controlled nanoscale thickness were directly prepared on ceramic substrates by using Co-MOF films as precursors. To further improve the conductivity, SnO₂ catalytic overlayers were introduced on top of Co₃O₄ sensors to construct SnO₂/Co₃O₄ bilayer sensors, which were promising for triethylamine (TEA) detection at RT. As a result, the optimized SnO₂/Co₃O₄ sensor exhibited a fast response/recovery rate (11 s/16 s), high selectivity, and a satisfactory sensitivity (150%) to TEA at RT. The enhanced gas-sensing performance could be attributed to the unique bilayer structures, improved conductivity, and synergistic effects of the SnO₂ catalytic overlayers and Co₃O₄ sensing layers.     

Pt/Al_2O_3催化剂失活分析及再生处理

介绍了苯加氢制环己烷过程中的Pt/Al2 O3 催化剂酸中心的产生、积碳、机械杂质的覆盖及水、硫、一氧化碳等对催化剂活性、选择性、寿命的影响 ,以及催化剂失活后的再生方法     

Study of the low-temperature reaction between CO and O2 over Pd and Pt surfaces

Field electron microscopy (FEM), high-resolution electron energy loss spectroscopy (HREELS), molecular beams (MB) and temperature-programmed reaction (TPR) have been applied to the study of the kinetics of CO oxidation at low temperature, and to determine the roles of subsurface atomic oxygen (O_sub) and surface reconstruction in self-oscillatory phenomena, on Pd(111), Pd(110) and Pt(100) single crystals and on Pd and Pt tip surfaces. It was found that high local concentrations of adsorbed CO during the transition from a Pt(100)-hex reconstructed surface to the unreconstructed 1×1 phase apparently prevents oxygen atoms from occupying hollow sites on the surface, and leads to the appearance of a weakly bound active adsorbed atomic oxygen (O_ads) state in an on-top or bridge position. It was also inferred that subsurface oxygen O_sub on the Pd(110) surface may play an important role in the formation of new active sites for the weakly bound O_ads atoms. Experiments with ¹⁸O isotope labeling clearly show that the weakly bound atomic oxygen is the active form of oxygen that reacts with CO to form CO₂ at T 140–160 K. Sharp tips of Pd and Pt, several hundreds angstroms in diameter, were used to perform in situ investigations of dynamic surface processes. The principal conclusion from those studies was that non–linear reaction kinetics is not restricted to macroscopic planes since: (i) planes as small as 200 Å in diameter show the same non-linear kinetics as larger flat surfaces; (ii) regular waves appear under conditions leading to reaction rate oscillations; (iii) the propagation of reaction–diffusion waves involves the participation of different crystal nanoplanes via an effective coupling between adjacent planes.     

A Study on the Properties and Mechanisms for NO x Storage Over Pt/BaAl2O4-Al2O3 Catalyst

The NO_x storage catalyst Pt/BaAl₂O₄-Al₂O₃ was prepared by a coprecipitation--impregnation method. For fresh sample, the barium mainly exists as the BaAl₂O₄ phase except for some BaCO₃ phase. The BaAl₂O₄ phase is the primary NO _x storage phase of the sample. EXAFS and TPD were used for investigating the mechanism of NO _x storage. It is found that two kinds of Pt sites are likely to operate. Site 1 is responsible for NO chemisorption and site 2 for oxidizing NO to nitrates and nitrites. When NO adsorbs on the sample below 200 °C, it mainly chemisorbs in the form of molecular states. Such adsorption results in an increase of the coordination magnitude of Pt-O, and a decrease of that of Pt-Pt and Pt-Cl. The coordination distance of Pt-Pt, Pt-Cl and Pt-O also increases. When the adsorption occurs above 200 °C, NO can be easily oxidized by O₂, and stored as nitrites or nitrates at the basic BaAl₂O₄. Site 2 is regenerated quickly. A high adsorption temperature is favorable for nitrate formation.     

Combustion of Methane at Low Temperature over Pd and Pt Catalysts Supported on Al2O3, SnO2 and Al2O3-Grafted SnO2

Pd and Pt catalysts supported on alumina, tin(IV) oxide and tin(IV) oxide grafted on alumina were prepared, characterised and tested with respect to the low-temperature combustion of methane after reduction in H₂ and ageing under reactants at 600°C. In the case of Pd, the use of SnO₂ or SnO₂-based supports led to catalysts slightly less active than Pd/Al₂O₃. In contrast, SnO₂ was found to strongly promote the oxidation of methane over Pt catalysts with respect to Pt/Al₂O₃, even after ageing under reactants. When Pt was supported on SnO₂ grafted on Al₂O₃, the activity was found at most similar to or, after ageing, lower than Pt/Al₂O₃. This negative effect was discussed, being partly related to the sintering of SnO₂ under reactants observed by FTIR and XRD.