Noble Metal Loading Research Articles

Tailoring of Gas Sensing Characteristics Using Bilayer Sensor with Oxide Semiconductor Sensing Layer and Nanoscale Au Catalytic Overlayer

Introduction Gas sensing characteristics of oxide semiconductor chemiresistors can be enhanced or controlled by loading noble metal catalysts to the entire sensing materials. Although uniform catalyst loading generally promotes the sensing reaction between analyte gas and ionized oxygen on the surface, it may induce other adverse effects such as 1) the excessive increase of sensor resistance depending on work function of metal catalyst, 2) the gas oxidation at the upper part of sensing film, and 3) the gas reforming during the gas transport within sensing film. Excessively high sensor resistance hampers the cost-effective signal measurement using conventional electric circuits, while it is difficult to separate the gas sensing, reforming, and oxidation reactions, leading to low controllability of gas sensing characteristics. Herein, a novel bilayer design with an oxide chemiresistors sensing layer (SnO2, ZnO, and Co3O4) and nanoscale catalytic Au overlayer was suggested to solve above problems. Bilayer sensor design effectively separated the sensing and catalytic reaction, leading to the excellent control of gas sensing characteristics without affecting the sensor resistance in air. Method Sensing materials SnO2 hollow spheres were prepared using ultrasonic spray pyrolysis of mixed solution consisting of 0.1 M of Tin (II) chloride dihydrate (SnCl2•2H2O, ≥ 99.99%) and 0.025 M of citric acid monohydrate (C6H8O7•H2O, ≥ 99.0%). Droplets of this source solution were then generated by six ultrasonic transducers (resonance frequency: 1.7 MHz) and transferred into a quartz tube (inner diameter: 55 mm) in an electric furnace heated to 700 ºC in air (flow rate : 10 L/min) for pyrolysis. As-prepared precursor powders were converted into SnO2 hollow spheres via heat treatment at 600 ºC for 2 h in air. SnO2 sensing films were fabricated by screen printing onto Pt interdigitated electrode (IDE) (electrode gap: 100 mm) patterns on SiO2/Si substrates (size = 8 mm x 8 mm). Catalytic Au nanoparticles were deposited on the SnO2 thick films through e-beam evaporation of Au grains (99.998%) in a vacuum chamber (below 2×10-6 Torr) at a rate of 0.1 Å sec−1 (nominal thicknesses: 0.5, 1, and 3 nm), as calibrated by a quartz crystal microbalance, which were converted into discrete configurations of Au nanoparticles through heat treatment at 550 ºC for 2 h. Additionally, SnO2 thick films with 1-nm-thick Au overlayer were also heat treated at 550 ºC for 0.5 and 4 h to investigate the effects of Au configuration on gas sensing characteristics. Results and Conclusions The configuration of Au nanoparticle overlayer was changed by controlling the Au-coating thickness and annealing time and investigated the gas sensing characteristics of SnO2 sensors with various configurations of Au nanoparticles overlayer. The Au nanoparticle overlayer significantly enhanced the methylbenzene selectivity and response of SnO2 thick film sensor by reforming of less reactive methylbenzenes into more reactive species and suppressing the cross-responses to relatively reactive interference gases (e.g. ethanol and HCHO) through oxidative filtering. Furthermore, the methylbenzene selectivity and response of ZnO and Co3O4 were also significantly enhanced by introducing the Au nanoparticle overlayer, confirming the general validity on the role of Au nanoparticle overlayer. In all the sensing films, the gas transport as well as sensor resistance is hardly influenced by nanoscale catalytic Au overlayer, demonstrating that the bilayer sensor is promising platform to control the gas sensing characteristics by separating catalytic reaction and sensing reaction and by tuning the catalyst-induced gas reforming and oxidation reaction. These sensors can be used for various sensor applications with new functionality.

(Invited) Effects of Noble-Metal Loading Onto Metal-Oxide Electrodes on CO-Sensing Properties and Mechanism of Potentiometric Gas Sensors Utilizing an Anion-Conducting Polymer Electrolyte

IntroductionSome kinds of electrochemical CO sensors using a cation-conducting polymer (e.g., Nafion®) as an electrolyte and carbon-based electrodes loaded with noble metal (mainly Pt) have been presently commercialized as a sensor operated at room temperature, and they practically show attractive CO-sensing properties. On the other hand, many kinds of metal oxides are basically limited to be used as the sensing-electrode materials of the sensors, due to the lack of chemical stability under the strong acidic environment. Recently, anion-conducting polymers (ACP) have been a focus of constant attention, in the field of various electrochemical applications, such as fuel cells and water hydrolysis, because of the large hydroxide-anion conductivity and the relatively long-term stability. We have so far developed various kinds of electrochemical gas sensors utilizing the ACP electrolyte, and we firstly demonstrated that Pt- or Pd-loaded carbon black (CB) was quite promising as a sensing-electrode material to detect H2 [1], CO [2], and CO2 [3]. However, their gas selectivity was relatively poor, and thus we have investigated various metal oxides loaded with noble-metal nanoparticles as a CO-sensing electrode material [4–7]. In this presentation, we will discuss the fundamental CO-sensing properties of their sensors and the CO-sensing mechanism. Experimental n wt% noble-metal (N) nanoparticles were loaded onto metal-oxide (MO) powder by precipitation-deposition (only for Au) or general impregnation (for other noble metals) technique, respectively, and they were heat-treated at elevated temperatures in air or H2 for 1 h. The obtained powder (nN/MO) was mixed with an appropriate amount of ACP solution (AS-4, Tokuyama Corp.) and then the paste obtained was applied on the surface of both sides of an ACP membrane (A201, Tokuyama Corp.) as sensing and counter electrodes by blade coating, and then EC(nN/MO(Tm)) sensors (T: annealing temperature (°C), m: annealing atmosphere (air or H2)) were obtained. The sensor was sandwiched with Au meshes (100 mesh) as a current collector and was set up in a gas-sensing measurement system with two electrode compartments. Electromotive force (E) of all sensors to 500 ppm CO or H2 balanced with synthetic air (O2: 20%, N2: 80%), which flowed over the sensing electrode, was measured at 30°C, while synthetic air was flowed over the counter electrode. Relative humidity (RH) of these flowing gases was controlled to be 0–80% (generally, 57%) at 30°C by bubbling the gases into distilled water. The magnitude of response was defined as a change in E value induced by a sample gas (ΔE SG, SG (sample gas): CO or H2). CO selectivity against H2 was defined as a ratio of CO response to H2 response (ΔE CO/ΔE H 2). Results and Discussion Different combinations of various N (Au, Ir, Ru, Pd, Pt, etc.) and MO (Bi2O3, CeO2, In2O3, TiO2, SnO2, V2O5, etc.) have been estimated as a CO-sensing electrode material. Among them, we confirmed that only three kinds of nN/MO, nAu/In2O3, nAu/SnO2, and nPt/SnO2, were promising candidates as the sensing-electrode material of highly sensitive CO sensors, together with relatively excellent operating stability. Response transients of representative EC(2Au/In2O3(Tm)), EC(2Au/SnO2(Tm)), and EC(2Pt/SnO2(Tm)) sensors to 500 ppm CO and H2 are shown in Fig. 1. Both the EC(2Au/In2O3(400air)) and EC(2Au/SnO2(400air)) sensors showed larger CO response than H2 response, but the CO selectivity against H2 (2.6 for EC(2Au/In2O3(400air)), 2.3 for EC(2Au/SnO2(400air))) was smaller than we expected. The annealing of the 2Au/In2O3 and 2Au/SnO2 at 250°C in H2 decreased the H2 response and thus enhanced the CO selectivity against H2 (28 for EC(2Au/In2O3(250H2)), 35 for EC(2Au/SnO2(250H2))). However, the small H2 response gradually increased and thus the CO selectivity against H2 decreased with their operating time, even though the CO response remained almost unchanged during the same period. On the other hand, the EC(2Pt/SnO2(500air)) sensor showed the largest CO selectivity against H2 (8.3) among all the EC(2N/MO(Tair)) sensors, together with the excellent long-term stability. On the other hand, the heat treatment of 2Pt/SnO2 powder at 250°C in H2 enhanced the responses to both CO and H2 and consequently reduced the CO selectivity against H2 (1.9 for EC(2Pt/SnO2(250H2))). The reasons for such variations in sensing property and the gas-sensing mechanism on the N/MO surface will be discussed in our presentation.

High performance and cost-effective supported IrOx catalyst for proton exchange membrane water electrolysis

A multistep synthesis method is introduced to fabricate high performance IrOx/W-TiO2 catalyst for noble metal reduction in PEM water electrolyzers. The performances of the IrOx/W-TiO2 catalyst for oxygen evolution reaction (OER) are measured in half-cells and single-cells and compared with unsupported IrOx and commercial IrO2. In half-cell tests, IrOx/W-TiO2 displays an improved Ir mass activity compared with the unsupported IrOx, demonstrating a positive effect of the support material. In single-cell tests, the performance of IrOx/W-TiO2 also exceeds both the unsupported and commercial IrO2. The cell voltage reaches 1.602V at 1 A cm−2 (Nafion 212) with total noble metal loading less than 0.2 mg cm−2, suggesting that significant Ir reductions are possible by using IrOx/W-TiO2 without sacrificing the cell performance. It also shows improved durability than other unsupported catalysts at reduced Ir loading. In-situ electrochemical test reveals that the performance is associated with the enhanced conductivity of the catalyst layer.

Controlling phase transfer of molybdenum carbides by various metals for highly efficient hydrogen production

The α phase Mo carbide has been widely investigated recently for its high activity in hydrogen production from water gas shift (WGS) reaction. However, high loading of noble metals as well as high economic and environmental cost derived from high-temperature ammonification and carbonization process will lead to high cost of hydrogen production. Thus, the efficient controlling of phase transfer is promising. Herein, metals (Au, Pt, Rh, Cu) with a wide range of loadings were impregnated on flame spray pyrolysis (FSP) made MoO3 to produce Mo carbides by one-step carbonization. A breakthrough high metal-normalized hydrogen production rate of 213 mmol H2·gmetal−1·s−1 was achieved on 0.025 wt% Rh/MoCx, which was much higher than Pt and Au based Mo carbides ever reported. The addition of trace Rh induced direct MoO3 transformation to high purity α-MoC1-x in one-step carbonization instead of two-steps ammonification and carbonization process. In comparison to Rh, the addition of Pt, Au and Cu tend to transfer MoO3 into β-Mo2C at the same conditions. Besides, the one with 2 wt% Rh exhibited high stability in WGS reaction even at high temperature (300 °C) due to its inhibition on carbides oxidation induced by H2O. We demonstrate that it is feasible to control phase transfer of Mo carbides even by trace amount of metals to simplify the production process of catalysts. The catalytic performance improved by Rh in aspects of both activity and stability provides a guide for producing more stable Mo carbides catalysts.

Control and Impact of Metal Loading Heterogeneities at the Nanoscale on the Performance of Pt/Zeolite Y Catalysts for Alkane Hydroconversion.

The preparation of zeolite-based bifunctional catalysts with low noble metal loadings while maintaining optimal performance has been studied. We have deposited 0.03 to 1.0 wt % Pt on zeolite H-USY (Si/Al ∼ 30 at./at.) using either platinum(II) tetraammine nitrate (PTA, Pt(NH3)4(NO3)2) or hexachloroplatinic(IV) acid (CPA, H2PtCl6·6H2O) and studied the nanoscale Pt loading heterogeneities and global hydroconversion performance of the resulting Pt/Y catalysts. Pt/Y samples prepared with PTA and a global Pt loading as low as 0.3 wt % Pt (nPt/nA = 0.08 mol/mol, where nPt is the number of Pt surface sites and nA is the number of acid sites) maintained catalytic performance during n-heptane (T = 210–350 °C, P = 10 bar) as well as n-hexadecane (T = 170–280 °C, P = 5 bar) hydroisomerization similar to a 1.0 wt % Pt sample. For Pt/Y catalysts prepared with CPA, a loading of 0.3 wt % Pt (nPt/nA = 0.08 mol/mol) sufficed for n-heptane hydroisomerization, whereas a detrimental effect on n-hexadecane hydroisomerization was observed, in particular undesired secondary cracking occurred to a significant extent. The differences between PTA and CPA are explained by differences in Pt loading per zeolite Y crystal (size ∼ 500 nm), shown from extensive transmission electron microscopy energy-dispersive X-ray spectroscopy experiments, whereby crystal-based nPt/nA ratios could be determined. From earlier studies, it is known that the Al content per crystal of USY varied tremendously and that PTA preferentially is deposited on crystals with higher Al content due to ion-exchange with zeolite protons. Here, we show that this preferential deposition of PTA on Al-rich crystals led to a more constant value of nPt/nA ratio from one zeolite crystal to another, which was beneficial for catalytic performance. Use of CPA led to a large variation of Pt loading independent of Al content, giving rise to larger variations of nPt/nA ratio from crystal to crystal that negatively affected the catalytic performance. This study thus shows the impact of local metal loading variations at the zeolite crystal scale (nanoscale) caused by different interactions of metal precursors with the zeolite, which are essential to design and synthesize optimal catalysts, in particular at low noble metal loadings.

Ultralow-Loading Ruthenium Catalysts by Plasma-Enhanced Atomic Layer Deposition for a Solid Oxide Fuel Cell

Designing and fabricating highly active and thermally stable catalysts with minimal noble-metal loading is crucial for solid oxide fuel cells that operate with direct methane fuel. In this study, u...

Recent Advance of Transition‐Metal‐Based Layered Double Hydroxide Nanosheets: Synthesis, Properties, Modification, and Electrocatalytic Applications

AbstractNowadays, to alleviate the growing pressure of energy shortages and environmental pollution, electrochemical energy conversion is seen as a fossil‐free, secondary pollution‐free, and highly efficiency pathway. Electrocatalysts, as the “heart” of the electrochemical conversion process, play a key role in accelerating reaction rates and promoting efficiency. Transition‐metal‐based layered double hydroxides (TM LDHs), one of the famous electrocatalysts, show unique 2D supermolecular‐stratified structure, excellent electronic properties, and their raw materials are easily available. In this review, electrocatalysis‐related properties of TM LDHs and the state‐of‐the‐art synthesis methods of TM LDH‐based electrocatalysts are summarized. Modifications for promoting the development of TM LDH‐based electrocatalysts are included, such as carbon materials modification, defect engineering, and noble metal loading. Then, the application of TM LDH nanosheets in the field of electrocatalysis is reviewed in detail. Finally, recent challenges, ongoing improvements, and future expectations for TM LDH nanosheets are presented.

Two‐Dimensional Metal‐Organic Framework Nanosheet Supported Noble Metal Nanocrystals for High‐Efficiency Water Oxidation

AbstractIt still remains a challenge to simultaneously optimize the nanostructure and electronic structure of electrocatalysts of noble‐metal‐based electrocatalysts to improve their performance and reduce their noble metal loadings, in which the dispersion and activity of the noble metal components play an important role. A series of hybrids of noble metal nanocrystals (NMNCs) and two‐dimensional metal‐organic framework (2D MOF) nanosheets are designed and synthesized by a facile, fast, and general approach, in which NMNC dispersion is quickly mixed with MOF precursors at room temperature. The NMNCs/2D MOF hybrids, denoted as M‐Ni‐NS (M = Ir, Ru or Pt, etc.), are assembled by taking advantage of abundant O‐atom arrays on the surface the of 2D MOF nanosheets and they enable excellent dispersity and stability. Experimental and computational results show that the M‐O‐Ni bridging bonds realize successful tuning of the electronic structure of active sites and modify their activity. Taken together with the merits of the nanostructure of the 2D MOF support, this multiscale optimization strategy produces Ir‐Ni‐NS electrocatalysts with excellent performance for the oxygen evolution reaction, achieving a low overpotential of 270 mV at 10 mA cm−2 in alkaline media and long time stability.

Effect of operating conditions and TWC parameters on emissions characteristics of a China-VI stoichiometric natural gas engine

This study involved conducting an experimental and numerical investigation on the effects of the air-to-fuel ratio (AFR), engine speed, and engine load on the inlet gas component of a three-way catalyst (TWC) and on the effects of noble metal loading, noble metal ratio, and carrier pore density on the emission conversion efficiency. The results showed that AFR can significantly affect the raw emissions of NOx and THC, and better emission conversion efficiency of a TWC can be reached when AFR is controlled between 0.995 to 1. Compared with engine speed, engine load has a relatively small effect on exhaust temperature but greatly affects the flow velocity and NOx and THC emissions. Increasing the content of Pt in the catalyst can improve the THC conversion efficiency. For low Pt and Pd-Rh catalysts, the THC conversion effect is significantly deteriorated. The content of Rh affects the NOx conversion, and NOx conversion efficiency at high speeds is significantly reduced when Rh content is reduced. Higher carrier pore density can slightly improve the catalytic reaction rate and emission conversion efficiency at high engine speeds. However, high conversion efficiency can be maintained even after aging.

One-pot, ligand-free, room-temperature synthesis of Au/Pd/ZnO nanoclusters with ultra-low noble metal loading and synergistically improved photocatalytic performances

Au/Pd/ZnO nanoclusters with ultra-low noble metal loadings were prepared by a one-step and ligand-free method at room temperature. HRTEM, ICP-MS, XPS, and elemental mapping analysis confirmed that the obtained Au/Pd/ZnO nanoclusters were composed of ZnO nanoclusters decorated with well-dispersed AuPd nanoparticles. Au/Pd/ZnO nanoclusters exhibited higher photocatalytic activity compared with those of pristine ZnO, Au/ZnO and Pd/ZnO. Moreover, the high catalytic activity of Au/Pd/ZnO nanoclusters could be maintained even after 5 cycles of photocatalytic reaction. A mechanism for the enhanced photocatalytic performance was also suggested, which was in light of the synergistic effects of the SPR effect from Au and the improved photogenerated charge carrier separation from Pd.

Mesoporous CdxZn1-xS with abundant surface defects for efficient photocatalytic hydrogen production

The practical application of photocatalytic water splitting for hydrogen evolution hinges on the development of high-efficient and low-cost photocatalysts. Defects engineering has emerged as a promising strategy to enhance photocatalytic activity effectively. Herein, a facile and versatile co-precipitation method is proposed to fabricate mesoporous Cd-Zn-S solid solutions (E-CdxZn1-xS) with abundant surface defects by the inorganic salts formed in the reaction system as self-template. Compared with Cd-Zn-S solid solutions (W-Cd0.65Zn0.35S) prepared by the traditional co-precipitation method, the enhanced specific surface area and abundant surface defects endow E-Cd0.65Zn0.35S with more accessible active sites and effective separation of electron-hole pairs for the photocatalytic water splitting reaction. The E-Cd0.65Zn0.35S solid solution exhibits hydrogen evolution rate of 5.2 mmol h−1 g−1 without loading noble metal as cocatalyst under visible light, which is 1.13 times higher than that of W-Cd0.65Zn0.35S sample. The present work provides a simple, low-cost and prospective strategy for the synthesis of defective Cd-Zn-S solid solutions, and it also delivers guidance to design and develop the advanced visible-light photocatalyst in the future.

(Vittorio de Nora Award Address) Analysis of the Catalyst Requirements with Regards to Catalyst Structure and Catalyst Durability Studies for PEM Water Electrolysis

Proton exchange membrane (PEM) based water electrolyzers are a promising technology for the large-scale production of hydrogen from renewable energy. The most efficient and durable catalysts for PEM water electrolysis are platinum to catalyze the hydrogen evolution reaction (HER) and iridium oxide to catalyze the oxygen evolution reaction (OER). While the currently rather high noble metal loadings (≈0.3-0.5 mgPt/cm2 and ≈1-3 mgIr/cm2) have only a minor impact on the overall water electrolyzer system cost [1], the large-scale global implementation of PEM water electrolysis would require a substantial reduction of the noble metal loadings due to the limited availability of Pt and Ir. This is easy to achieve for the Pt cathode catalyst, owing to its fast HER kinetics and the fact that a high dispersion of Pt is readily achieved with carbon-supported Pt (Pt/C). However, lowering the Ir loading below ≈0.5 mgIr/cm2 is not possible with currently available Ir catalysts (Ir black, nano-IrO2, or IrO2 supported on a TiO2 substrate), as it results in too thin, non-contiguous electrode layers with poor in-plane conductivity, owing to their low iridium packing density of ≈2.3 gIr/cm3 electrode (this is in contrast to ≈0.05-0.10 gPt/cm3 electrode for a typical Pt/C catalyst) [2]. Thus, to reach the long-term target of an Ir-specific power density of ≤0.01 gIr/kW [2], novel catalysts with a lower iridium packing density are required, which could be achieved, e.g., by supporting iridium or iridium oxide nanoparticles on conductive supports or by other approaches [3]. A detailed analysis leading to this conclusion will be discussed in this presentation.The development and implementation of such novel catalysts also requires the evaluation of their long-term stability. Conventionally, this is either done by measurements in liquid electrolyte, typically using the rotating disk electrode (RDE) technique, or testing in an actual PEM electrolyzer. Unfortunately, recent studies revealed that the RDE based approach does not yield reliable catalyst durability data, since for yet unknown reasons, the catalyst life-times obtained by the RDE method are three to four orders of magnitude lower than what is observed in PEM electrolyzers [3, 4]. On the other hand, catalyst durability testing in a PEM electrolyzer requires very long measurement times, so that accelerated durability test protocols are necessary [5]. These aspects will also be discussed.

(Invited) Ruthenium/Samaria-Doped Ceria Cermet Anode through Plasma Enhanced Atomic Layer Deposition

Direct methane-fueled solid oxide fuel cells (DM-SOFCs) operating at low temperature range (<450 C) are promising next-generation energy conversion devices, in which, however, anode reactions should be facilitated for high performance even at low temperatures. Obtaining catalyst with high activity, thermal stability and carbon coking resistance is essential for high-performance SOFC operating with hydrocarbon fuel. Noble metal catalyst, e.g., Ru, can be effective for direct methane oxidation in DM-SOFC anode. However, their high costs prevent them from being extensively applied to such devices as SOFCs, and, therefore, significant efforts have been made to lower the noble metal loading in designing fuel cell electrodes.Herein, we report the design and fabrication of heterogeneous catalyst with ultra-low-loading Ru nanostructures via plasma enhanced atomic layer deposition (PEALD) on SDC backbone anode. We confirmed the presence of highly dispersed PEALD Ru particles over the SDC backbone compared to sputtered Ru deposited on only porous sputtered SDC surface. We found out that PEALD Ru of 100 ALD cycles, or approximately 5nm in thickness with Ru loading of only 0.004 mg/cm2, was sufficient for the use of anode catalyst with methane fuel. Furthermore, compared to the cell with 100 nm-thick sputtered Ru/sputtered SDC anode, the cell with PEALD Ru/sputtered SDC anode showed 45 % higher peak power density with only 1/20 of Ru loading. Moreover, after 4 hours operation with dry methane at 450 C, the activation resistance of the cell with the sputtered Ru composite anode increased by 7 times, while that of the cell with the PEALD Ru composite anode increased by only 2 times. This result suggests that, using PEALD, one can simultaneously reduce Ru loading, thereby reducing cost, and improve the performance and the thermal stability of the anode for DM-SOFC.

Study of spatial and temporal aging characteristic of catalyzed diesel particulate filter catalytic performance used for diesel vehicle

Catalyzed diesel particulate filters (CDPFs) have been widespread used as a technically and economically feasible mean for meeting increasingly stringent emissions limits. An important issue affecting the performance of a CDPF is its aging with using time. In this paper, the effects of noble metal loadings, regions and using mileage on the aging performance of a CDPF were investigated by methods of X-ray diffraction (XRD), X-ray photoelectron spectroscopy and catalytic activity evaluation. Results showed that aging of the CDPF shifted the XRD characteristic diffraction peaks towards larger angles and increased the crystallinity, showing a slowing downward trend with the increase of the noble metal loadings. In addition, the increase of the noble metal loading would slow down the decline of Pt and Pt4+ concentration caused by aging. The characteristic temperatures of CO, C3H8 conversion and NO2 production increased after aging, and the more the noble metal loadings, the higher the range of the increase. But noticeably, excessive amounts of noble metals would not present the corresponding anti-aging properties. Specifically, the degree of aging in the inlet region was the deepest, the following is the outlet region, and the middle region was the smallest, which were also reflected in the increase range of crystallinity, characteristic temperatures of CO, C3H8 conversion and NO2 production, as well as the decrease range of Pt and Pt4+ concentrations. The increase of aging mileage reduced the size of the aggregates of the soot and ash in CDPFs, however, improved the degree of tightness between particles. Meanwhile Carbon (C) concentration in the soot and ash increased with the aging mileage.

Hydrogen Oxidation Artifact During Platinum Oxide Reduction in Cyclic Voltammetry Analysis of Low-Loaded PEMFC Electrodes

An artifact appearing during the cathodic transient of cyclic voltammograms (CVs) of low-loaded platinum on carbon (Pt/C) electrodes in proton exchange membrane fuel cells (PEMFCs) was examined. The artifact appears as an oxidation peak overlapping the reduction peak associated to the reduction of platinum oxide (PtOx). By varying the nitrogen (N2) purge in the working electrode (WE), gas pressures in working and counter electrode, upper potential limits and scan rates of the CVs, the artifact magnitude and potential window could be manipulated. From the results, the artifact is assigned to crossover hydrogen (H2X) accumulating in the WE, once the electrode is passivated towards hydrogen oxidation reaction (HOR) due to PtOx coverage. During the cathodic CV transient, PtOx is reduced and HOR spontaneously occurs with the accumulated H2X, resulting in the overlap of the PtOx reduction with the oxidation peak. This feature is expected to occur predominantly in CV analysis of low-loaded electrodes made of catalyst material, whose oxide is inactive towards HOR. Further, it is only measurable while the N2 purge of the WE is switched off during the CV measurement. For higher loaded electrodes, the artifact is not observed as the electrocatalysts are not fully inactivated towards HOR due to incomplete oxide coverage, and/or the currents associated with the oxide reduction are much larger than the spontaneous HOR of accumulated H2X. However, owing to the forecasted reduction in noble metal loadings of catalyst in PEMFCs, this artifact is expected to be observed more often in the future.Graphical abstract

Effect of Operating Conditions and TWC Parameters on Emissions Characteristics of a Stoichiometric Natural Gas Engine

This study involved conducting an experimental and numerical investigation on the effects of the air-to-fuel ratio (AFR), engine speed, and engine load on the inlet gas component of a three-way catalyst (TWC) and on the effects of noble metal loading, noble metal ratio, and carrier pore density on the emission conversion efficiency. The results showed that AFR can significantly affect the raw emissions of NOx and total hydrocarbon (THC), and better emission conversion efficiency of a TWC can be reached when AFR is controlled between 0.995 to 1. Compared with engine speed, engine load has a relatively small effect on exhaust temperature but greatly affects the flow velocity and NOx and THC emissions. Increasing the content of Pt in the catalyst can improve the THC conversion efficiency. For low Pt and Pd-Rh catalysts, the THC conversion effect is significantly deteriorated. The content of Rh affects the NOx conversion, and NOx conversion efficiency at high speeds is significantly reduced when Rh content is reduced. Higher carrier pore density can slightly improve the catalytic reaction rate and emission conversion efficiency at high engine speeds. However, high conversion efficiency can be maintained even after aging.

Vectorial carriers interfacial migration of Cd1-xZnxS heterophase junctions for visible-light water splitting

Twin structure in Cd1-xZnxS solid solution based on sphalerite/wurtzite (ZB/WZ) heterophase junction is considered as a quite promising approach to enhance the charge separation efficiency and inhibit the charge recombination during the photocatalytic process. Herein, Cd1-xZnxS nanocrystals based on ZB/WZ heterophase junction with different molar ratios of Cd/Zn were synthesized by one-step solvothermal process. XRD analysis and HRTEM images verified the formation of nano-twin structures in the Cd0·6Zn0·4S solid solution. Photoluminescence (PL) spectra, time-resolved photoluminescence (TRPL) spectra, Electrochemical impedance spectroscopy (EIS) test and Photocurrent response revealed that the recombination of photogenerated charge carriers has been reduced due to the presence of ZB/WZ heterophase junction. The experimental results showed that the photocatalytic activity of Cd0·6Zn0·4S without noble metal load was the best under visible light irradiation, and the hydrogen production efficiency was up to 472 μmol h−1 and an apparent quantum efficiency of 9.8% at 420 nm. The periodic type-Ⅱ staggered energy band arrangement formed between ZB and WZ phases can orientate the photoinduced electrons and holes to ZB and WZ regions, respectively, and redox reactions can occur on the surface. Meanwhile, Cd0·6Zn0·4S exhibited good stability for hydrogen production. The result provides an available progress for the development of photocatalytic mechanism in heterophase junction between ZB and WZ phases, leading to the vectorial interfacial transfer of photogenerated e−/h+ pairs and prolonged carrier lifetime which can effectively enhance the photocatalytic reactivity.

Metal-free plasmonic boron phosphide/graphitic carbon nitride with core-shell structure photocatalysts for overall water splitting

In this work, we report a novel metal-free plasmonic boron phosphide/graphitic carbon nitride (BP@g-C3N4) catalyst for photocatalytic overall water splitting. The BP@C3N4 catalyst exhibits broadened spectral adsorption in UV–vis to NIR region and shows the activity for hydrogen evolution even under 730 nm illumination, which is close to NIR region of solar irradiation. The highest H2 production rate reaches 31.5 μmol g−1 h−1 under visible light irradiation, which is 34.3 and 112.6 times higher than corresponding data of Pt/g-C3N4 and g-C3N4 catalysts respectively without noble metal loading. The finite element frequency-domain simulations indicate that the significantly improved photocatalytic water splitting performance is due to the BP plasmon resonance leading to effectively photo-induced carrier separation and transfer, which is crucially for solar energy harvesting and utilization. The metal-free earth-abundant plasmonic BP@g-C3N4 can catalyze photocatalytic overall water splitting by the plasmonic effect under visible light and near infrared irradiation efficiently.

Thin film and noble metal loading effects on the photocatalytic reactivity of helium-plasma-induced nanostructured tungsten oxides

Helium-plasma irradiation induces nanostructure formation on metal surfaces. This study investigated the photocatalytic performance of tungsten (W) oxide nanostructures formed on a thin film and the loading effects of noble metals. Methylene blue (MB) degradation was performed, and the performance of different nanostructured samples formed on thin-film and sheet W with changing the oxidization temperatures was compared; the differences were discussed in terms of the oxidation degree and vacancies. Moreover, noble metals (Pt, Ag, Au) were loaded onto the nanostructured sample oxide by magnetron sputtering as cocatalysts. The MB degradation ability was significantly enhanced by Pt loading. The results showed that nanostructuring and Pt support by using dry processes are novel routes for fabricating highly active photocatalysts.

Sputter-etching treatment of proton-exchange membranes: Completely dry thin-film approach to low-loading catalyst-coated membranes for water electrolysis

Simultaneous plasma etching of a proton-exchange membrane (PEM) and deposition of a cerium oxide layer during reactive magnetron sputtering leads to the formation of a pronounced fiber-like structure on its surface. The level of structural porosity can be adjusted by varying the working pressure during the process. A PEM treated this way can be subsequently coated with a thin layer of iridium, forming an anode-side catalyst-coated membrane (CCM) for applications in water electrolysis. Due to the significantly enlarged surface of the membrane, there is no necessity for any additional, potentially corroding, support nanoparticles to achieve efficient in-cell operation. Moreover, utilizing a rotatory frame-shaped substrate holder and a multitarget deposition apparatus, the sputter-etching process can be used in the preparation of a full anode/cathode thin-film CCM in a single vacuum entry. This structure yields remarkable performance characteristics in an electrolyzer cell, considering its low combined noble metal loading of just 220 μg cm−2. Using this completely dry process for CCM manufacturing may facilitate efficient large-scale future production.