Metal Agglomeration Research Articles

Oxidation studies on nickel-base superalloy 617 OCC

Nickel-base superalloy 617 OCC has been indigenously developed for potential use in Advanced Ultra-Super Critical (USC) Power Plants. Hot corrosion is one of the serious issues associated with superheater and reheater tubes in the boiler part of the USC power plant. To understand the high temperature corrosion processes coming into play, it is useful to have a good understanding of the oxidation behavior of the alloy at the same temperature in ambient air environment. The present study reports, how the Alloy 617 OCC behaves in laboratory air environment at 700 °C. Studies were carried out at intervals of 500 h for a cumulative exposure time of 5000 h. Thermogravimetry, XRD of the corroded surface, SEM/EDAX analysis of corroded surface and cross-sectional surface were carried out after each of the 10 exposure times. Thermogravimetry revealed that there was essentially no weight change of the test specimen over the entire exposure time range. XRD revealed that chromium oxide is essentially the only corrosion product. SEM/EDAX and XRD of the corroded surface showed the progressive increase in the extent of chromium oxide formation on the surface. A well-developed chromium-depleted region was seen below the chromium oxide layer in the X-ray mapping studies. The present research showed that considerable levels of nickel and cobalt occur above the chromium oxide layer at the surface. There is evidence that metallic agglomerates form above the chromium oxide layer. Titanium present in the superalloy gets preferentially oxidized and gets into the chromium oxide layer at the surface. Preferential surface oxidation of aluminum was also noticed. There was evidence for internal oxidation of aluminium at long exposure times. The above findings will be presented and discussed, bringing out the oxidation mechanisms coming into play for 617 OCC at 700 °C.

High loading single-atom Cu dispersed on graphene for efficient oxygen reduction reaction

Single-atom metal dispersed on graphene materials are highly desired in various fields such as energy conversion/storage, catalysis and nanoelectronics. However, the fabrication of such materials with high loading level is still challenging, as the conventional pyrolysis protocol usually leads to metal agglomeration due to the poor thermal stability of metal precursors and the high surface energy of single-atom metals. Herein, we demonstrate the fabrication of single-atom Cu dispersed on graphene (Cu/G) with ultrahigh Cu loading of 5.4 wt%, using a unique confined self-initiated dispersing protocol. It is revealed that Cu is introduced into graphene matrix via highly active gaseous Cu-containing intermediate, which results in abundant and well-dispersed Cu-containing moieties. This Cu/G material with ultrahigh loading level as an electrocatalyst presents remarkable activity towards the oxygen reduction reaction (ORR) due to the abundant and highly dispersive Cu single atoms, even outperforming the commercial Pt/C. Our findings not only facilitate the development of single-atom metal dispersed on graphene materials but also highlight the importance of tuning active site structures in non-noble metal electrocatalysis.

Catalytic performance of NiMo/Al2O3-USY in the hydrocracking of low-temperature coal tar

A series of NiMo/Al2O3-USY catalysts with different MoO3 contents were prepared through incipient wetness method and further modified with NH4F. The NiMo/Al2O3-USY catalysts were characterized by XRD, XPS, HR-TEM, NH3-TPD, H2-TPR and N2 adsorption and their catalytic performance in the hydrocracking of low-temperature coal tar (LTCT) was investigated in a 200 mL fixed-bed reactor. The results indicate that the appropriate MoO3 content is 15%; higher MoO3 content may lead to the agglomeration of active metals on the support, although it has little influence on the sulfidation degree of Mo species and the conversion of coal tar upon hydrocracking. In addition, the amount of strong acid sites and pore diameter decrease gradually with a further increase in the MoO3 content, which is disadvantageous for deep hydrocracking. The modification of USY zeolite with NH4F solution can enlarge the average pore diameter of resultant NiMo/Al2O3-USY catalysts and then improve the residue conversion of coal tar. However, the amount of strong acid sites decreases obviously when the concentration of NH4F solution exceeds 0.6 mol/L, which may lead to an increase of the sulfur content in the hydrocracking product. Over the NiMo/Al2O3-USY catalyst modified with 0.6 mol/L NH4F solution, the residue conversion of coal tar reaches 87.65%; the sulfur contents in the gasoline fraction (< 180°C) and diesel fraction (180–320°C) are 5.96 and 34.98 mg/kg, respectively.

Highly dispersed nickel catalysts via a facile pyrolysis generated protective carbon layer.

Highly dispersed nickel catalysts were synthesized by simple Ni(acac)2 impregnation followed by pyrolysis of organic ligands, which produces a protective carbon coating on the Ni nanoparticles, reducing the metal mobility and sintering. The synthesized catalyst shows high thermal stability and enhanced CO methanation activity compared to the catalyst prepared by the traditional impregnation/calcination method.

Growth inhibition of harmful cyanobacteria by nanocrystalline Cu-MOF-74: Efficiency and its mechanisms.

Metal-organic Frameworks (MOFs) as a new type of nanomaterials are extensively used in various fields of environment pollution remediation. However, the MOFs are rarely applied in the removal of cyanobacterial blooms, and more fundamental investigation is warrant for more insights into mechanisms for algae inhibition. In this study, Cu-MOF-74 was synthesized by a simple hydrothermal method, and its inhibitory effect on the growth of Microcystis aeruginosa was studied. Furthermore, its mechanisms were explored with respect to metal ion release, agglomeration, shading and algal cell membrane breakage, production of extracellular hydroxyl radical and intracellular reactive oxygen species. The results showed that the inhibition rate of M. aeruginosa was 372% after 24-h exposure when the concentration of Cu-MOF-74 exceeded 1 mg/L. However, the addition of Cu-MOF-74 at the concentration lower than 0.1 mg/L promoted the algal growth. The inhibition of algal growth by Cu-MOF-74 was basically attributed to the presence of hydroxyl radical and intracellular reactive oxygen species, with the released Cu2+ and cell aggregation involved to some extent. Overall, nanocrystalline Cu-MOF-74 is of great potential in the control of harmful cyanobacterial blooms and the inhibition is specific to the concentration of Cu-MOF-74.

Cu−Ni−Al Spinel Oxide as an Efficient Durable Catalyst for Methanol Steam Reforming

AbstractCopper metal catalysts suffer from gradual loss of activity due to the agglomeration and sintering of active metals during the catalytic reaction processes. Herein, we show that enhanced catalytic durability for methanol steam reforming (MSR) can be achieved by using Ni modified Cu−Al spinel solid solution as catalysts without pre‐reduction treatment. In so doing, Cu atoms can be gradually released from spinel structure after the initiating of reaction by non‐spinel CuO entities. Incorporation of Ni in the spinel structure reveals two positive effects: slowing down the Cu releasing rate and producing even smaller Cu nanoparticles as compared to that of Cu−Al spinel. Since Ni2+ ions in spinel framework cannot be reduced during the catalytic process, we suggest that the remaining defective CuaNib□cAldO4 spinel structure plays a crucial role in stabilizing the formed Cu nanoparticles. The synthesized sample with 0.05 mol Ni per mol Cu shows the best catalytic performance as demonstrated by pretty stable activity during the long‐term catalytic testing up to 2000 h. The findings show great commercial application potentials for the production of H2 from MSR.

Palladium Nanoparticles Loaded on TiO2–Graphene Hybrids (Pd/TiO2–Gr) with Enhanced Electrocatalytic Activity in Formic Acid Oxidation

We first report the hybrids of TiO2 and graphene (Gr) used as the support of Pd nanoparticles, showing high performance electrocatalysts for formic acid oxidation. The hybrids are prepared with the solvothermal method, and tetrabutyl titanate and graphene-oxide (GO) are used as the precursors. The initial mass ratios of Ti:GO in the precursors are 0.07, 0.15, 0.26, 0.60, respectively. Compared to Pd/graphene, the as-obtained Pd/TiO2–Gr electrocatalysts exhibit higher catalytic activity and stability. Among them, the Pd/TiO2–Gr (Ti : GO = 0.26, initial mass ratio) exhibit the highest activity. Its current density is about 3.1 times of Pd/graphene, and the agglomeration of Pd metal that can be seen in Pd/graphene has been inhibited in the as prepared Pd/TiO2–Gr. Thus the electrocatalytic performance of Pd for formic acid oxidation is greatly improved by the binary TiO2–graphene support.

Morphological transformations during Fischer-Tropsch synthesis on a titania-supported cobalt catalyst

Deactivation mechanisms for a rhenium-promoted titania-supported cobalt catalyst are investigated during Fischer-Tropsch (FT) synthesis. Bench-scale reactor tests, chemisorption studies, thermogravimetric analyses (TG), and transmission electron microscopy (TEM) are used to probe environmental effects on catalytic activity.Bench-scale reactor studies show a steady decrease in activity with time. While a fraction of this loss can be recovered with a low temperature reduction (rejuvenation) inside the reactor, multiple data indicate this phenomenon is primarily attributable to water-induced oxidation of small cobalt particles.Data from long-term FT runs indicate the presence of three non-rejuvenable deactivation mechanisms: metal agglomeration, strong metal-support interaction (SMSI), and mixed metal oxide formation. High conversion studies implicate byproduct water in the agglomeration process, and ex-situ TEM data conclusively reveals that growth occurs via a coalescence mechanism. Combined kinetic and chemisorption studies reveal that SMSI results from the gradual buildup of titania decoration on the surface of the active cobalt and is exacerbated with multiple rejuvenation cycles. TGA data indicate that mixed metal oxide formation occurs in long-duration, pilot plant runs. In all cases, the aggregate of reactor kinetic, chemisorption, TG, and TEM results point to chemically-assisted deactivation phenomena attributable to the byproduct water.

Deactivation Mechanism and Regeneration Study of Ga–Pt Promoted HZSM-5 Catalyst in Ethane Dehydroaromatization

Direct nonoxidative conversion of ethane to aromatics has become an effective way of upgrading shale gas. Metal-promoted shape selective zeolite catalysts are often used for aromatization. Although the coking issue of the catalysts in ethane aromatization has been reported, the deactivation mechanism and the performance of regenerated Ga–Pt promoted HZSM-5 needs to be further investigated. The objective of this study is to elucidate deactivation mechanism of Ga–Pt promoted HZSM-5 and investigate the feasibility of regenerating deactivated catalysts for commercial viability. When using lower concentration of oxygen (2 vol %) for regeneration, decreased catalyst deactivation rate was observed. The metal particle size, crystalline structures, and acidity are characterized by various analytical instrumentations (TEM, XRD, NH3-TPD). The change of Bronsted acidity was observed on regenerated catalysts. The results showed that metal agglomeration and leaching of Pt from homogeneous Ga–Pt particle were the main causes of deactivation other than coke deposition, indicating that stabilization of bimetallic metal particles on zeolite surface is critical.

Catalytic Etching of Platinoid Gauzes during the Oxidation of Ammonia by Air. Reconstruction of Surface of Platinoid Gauzes at 1133 K in Air, in Ammonia, and in an NH3 + O2 Reaction Medium

The structure, morphology, and chemical composition of the surface and near-surface layers of platinoid wires of polycrystalline gauzes, containing Pt (81 wt %), Pd (15 wt %), Rh (3.5 wt %), and Ru (0.5 wt %) after treatment at 1133 K in various media—in air, in ammonia, and after NH3 oxidation in air—are studied by X-ray diffraction (XRD), scanning electron microscopy (SEM), energy-dispersive X-ray (EDX) spectroscopy, and X-ray photoelectron spectroscopy (XPS). A thin film is found on the surface of the initial gauze containing an oxide layer of Rh2O3 with a thickness of ~2 nm, on the surface of which an inhomogeneous graphite-like layer 10–50 nm thick is located. It is shown that the heat treatment of gauzes in air leads to the partial removal of the surface graphite-like film that forms the reticulated structure on the wire surface. The treatment of gauzes in an ammonia atmosphere leads to the complete removal of the graphite-like and oxide layers and to the growth of metal grains of ~10 μm. After the catalytic reaction of NH3 oxidation, a deep structural rearrangement of the surface layer of the wire takes place, as a result of which crystalline metal agglomerates of ~10 μm are formed. It is supposed that the reaction of NH3 molecules with oxygen atoms penetrated on the defects leads to the local increase of temperature, due to which the metal atoms emerge on the surface and form large crystalline agglomerates and pores in the region of the grain boundaries.

A comparative study on different regeneration processes of Pt-Sn/γ-Al2O3 catalysts for propane dehydrogenation

Three different regeneration processes including hydrogen or nitrogen purging and coke-burning treatment were used to restore the Pt-Sn/γ-Al2O3 catalysts, through which propane dehydrogenation reaction was performed in a consecutive reaction-regeneration mode. It was found that the catalyst using hydrogen regeneration showed the best stability compared with those regenerated by nitrogen purging and coke-burning treatment, suggesting that hydrogen regeneration is an effective approach for maintaining the performance of Pt-Sn/γ-Al2O3 catalysts in propane dehydrogenation reaction. The effect of different regeneration atmospheres on the metal active center and the coke deposition was investigated by XRD, TEM, N2-physisorption, TPO, TG and Raman technologies, and the results revealed that hydrogen or nitrogen regeneration resulted in little impact on the size and structure of metal active center, retaining the effective PtSn phase over the catalyst. Moreover, hydrogen regeneration not only removed the low dense components of the coke, but also altered the property of the residual coke through hydrogenation, leading to a higher mobility of coke, and thus a higher accessibility of the metal active centers. Whereas nitrogen regeneration only removed the low dense components of the coke. Although coke-burning regeneration caused a thorough coke removal, the catalyst subjected to repeated redox exhibited poor stability due to metal agglomeration, phase segregation and the resulting large Pt3Sn particle and core-shell structure with a Sn-rich surface.

Effect of Pt doping on activity and stability of Ni/MgAl2O4 catalyst for steam reforming of methane at ambient and high pressure condition

Effect of addition of Pt noble metal to Ni/MgAl2O4 catalyst for steam reforming of methane was studied at atmospheric pressure and at high pressure (10 bar). Pt loading of the catalyst was varied from 0.01 wt.% to 1.0 wt.%. Catalyst characterization studies showed that the degree of reduction of the catalyst increased from 44.9% for Ni/MgAl2O4 to 67.7% for the catalyst doped with 1.0 wt.% Pt, which was attributed to H2 spill over effect. With increasing Pt loading, the metal dispersion passed through a maxima and the highest dispersion of active metals was obtained with a Pt doping of 0.1 wt.%. Increase in Pt concentration had no effect on the concentration of NiAl2O4 that had already formed during calcination at 850 °C after nickel impregnation. Addition of Pt significantly increased the catalytic activity of Ni/MgAl2O4 for the steam reforming of methane at both reactor pressures. Comparison of Pt doped Ni/MgAl2O4 with Pt/MgAl2O4 showed that only lower loading of Pt produced a synergetic effect. The highest catalytic activity was obtained with 0.1 wt.% Pt doped Ni/MgAl2O4. Above this Pt loading the catalytic activity decreased. The highest activity of 0.1 wt.% Pt doped catalyst was credited to an increase in active metal dispersion, while the decrease in catalytic activity on further addition of Pt was attributed to agglomeration of active metals. High pressures decreased the conversion and stability of each Pt doped catalyst. However, at the same operating conditions there was a marginal increase in the catalyst stability due to Pt doping and the decline in catalyst activity with run time was lowest for 0.1 wt.% Pt doped Ni/MgAl2O4 catalyst.

Iron Exsolution Phenomena in Lanthanum Strontium Ferrite SOFC Anodes

As a mixed ionic and electronic conductor (MIEC), lanthanum strontium ferrite (La0.6Sr0.4FeO3-δ, LSF) is frequently investigated as a new material for SOFCs to lower the operating temperatures. Due to the duality of their conductivities, MIECs are suited for use as electrode as well as electrolyte materials. LSF has been proposed as a possible anode material and a number of investigations have analyzed its electrocatalytic activities. However, it has been shown that iron exsolution occurs in reductive environments at higher temperatures, leading to the formation of Fe rods on the surface.[1] Other experiments revealed that this segregated metallic Fe also appears when applying cathodic biases, causing a pronounced enhancement of the electrochemical water-splitting kinetics.[2] However, this structural instability would render this perovskite unsuitable for the use in SOFCs. It is therefore necessary to investigate the details of the exsolution in order to find ways to eventually suppress it. Utilizing an array of complementary in situ and ex situ techniques such as XPS, in situ XRD, scanning EXAFS, in situ TGA/DSC, and in situ TEM, we characterize this phenomenon regarding its thermodynamic and kinetic properties. According to in situ XRD, there are two distinct processes taking place: in the lower temperature regime, the perovskite exhibits a non-linear shift of the diffraction peaks to larger unit cell volumes, while, at temperatures above 773 K, Fe starts to crystallize. The shift allows the hypothesis to be made that the Fe atoms are relocated from their lattice positions to interstitial sites, thereby possibly forming diffusion channels, before they segregate to the surface and crystallize as metallic iron agglomerates. With XPS, the iron oxidation state in the surface-near region can be observed as a function of the reduction treatment in dry, flowing hydrogen. Starting from a state with only Fe(III) present, the perovskite is continuously reduced at temperatures above 673 K, resulting in the presence of only Fe(II) after reduction at 773 K. However, upon raising the temperature further, the observed Fe(II) concentration drops again: at 873 K, only 57% of the sampled iron is in the form of Fe(II), whereas the amount is zero after reduction at 973 K. This increase in the Fe(III) content is readily explained by TEM investigations: due to the transport of the samples in air, the exsolved iron particles are oxidized at their surface, forming a 2 nm thick oxide overlayer.[1] This shell prevents the surface-sensitive XPS measurements from sampling the metallic core. However, since the surface-near region of the perovskite itself only consisted of Fe(II) at 773 K, any Fe(III) in the spectrum originates from the oxide hull of segregated metallic Fe particles. Since the observed Fe(III) concentration, and thus the amount of segregated iron, increases above 773 K, the XPS results agree well with the in situ XRD measurements. Scanning EXAFS measurements of single LSF particles show that there is a difference in the electronic structure between the surface and bulk regions, corroborating the findings from the XPS analyses. [1] Thalinger, R.; Gocyla, M.; Heggen, M.; Klötzer, B.; Penner, S. J. Phys Chem. C 2015, 119, 22050–22056 [2] Opitz, A. K.; Nenning, A.; Rameshan, C.; Rameshan, R.; Blume, R.; Hävecker, M. Knop-Gericke, A.; Rupprechter, G.; Fleig, J.; Klötzer, B. Angew. Chem. Int. Ed. 2015, 54, 2628­–2632 Figure 1: Temperature-programmed i n situ XRD measurements in hydrogen/argon. Arrows mark the peaks resulting from segregated metallic Fe. Figure 1

Impact of Dislocations on Surface Reactivity of Ceria with Singe Atom Catalysts

Single-atom catalysts, where atomically dispersed transition metals are anchored at solid surfaces, are a new type of low price, high efficiency and nonflammable catalysts for water gas shift and CO2 methanation, where selective oxidation or reduction is needed. Ceria is one of the most promising support materials for these single-atom catalysts, because of its large oxygen capacity and high ionic mobility. The large amount of available lattice oxygen or oxygen vacancies in ceria is beneficial for selective oxidation and reduction, though the performance of pure ceria is limited by its poor surface exchange properties. Mixing or doping transition metals to ceria can activate the surface of ceria. In particular, transition metals anchored as isolated single atoms are found to be highly active. However, it is challenging to enhance the density of these single atoms due to the strong tendency of metal agglomeration. Having understood how dislocations in oxides can serve as traps for charged point defects in our previous studies, we have aimed here to assess whether we can stabilize single metal atoms at dislocations in ceria. To this end, first, a wide range of charge states and configurations of Cu defects in a dislocation free ceria model are calculated to elucidate the defect chemistry for Cu ions in bulk ceria. The formation energies of these defects as a function of chemical potential of oxygen and the Fermi energy indicate that the most preferred Cu defect is Cu1+ interstitial. This finding differs from previous calculations in the literature where only a copper vacancy complex is calculated, but is consistent with reported experimental observations. Moreover, the equilibrium Cu defect concentration in ceria is negligible, indicating a low solubility of Cu in bulk ceria. Second, an edge dislocation model in ceria was constructed. We found that the Cu defect formation energy at the edge dislocation core is 2 eV lower than that in the bulk, indicating a tendency for Cu to enrich at the dislocation compared to the bulk ceria (which has a very low solubility of Cu). More interestingly, and importantly, Cu has a lower oxidation state in the dislocation core than in the bulk. This lower oxidation state is more active for catalytic activity. Both the enrichment of Cu at the dislocation core and the more active oxidation state indicate that dislocations in ceria are beneficial for increasing surface electro-catalytic reaction kinetics with single atom catalysts.

Effect of iron and boron ultrafine powders on combustion of aluminized solid propellants

The paper presents the results of measurement of the burning rate of aluminized composite solid propellants (CSPs) and parameters of sampled condensed combustion products including their particle size distribution, chemical and phase composition. Effect of ultra-fine iron and amorphous boron additives in CSP formulations based on AP, butadiene rubber and Alex (2 wt. %) on the combustion characteristics was studied. The sampled particles of condensed combustion products (CCPs) were classified as oxide particles (less than 55 µm) and agglomerate particles (up to 500 µm). The CCPs particles were subjected to morphological, particle size, chemical and phase analyses. It was found that partial replacement of Alex by 2 wt. % of iron in CSP leads to 1.3-1.4 fold increase in the burning rate in the pressure range of 2.2-7.5 MPa. At the same time the agglomeration of metal fuel is slightly increased: the mean diameter of agglomerate particles in CCPs is increased up to 1.2 fold and their content is increased up to 1.4 fold. The content and mean diameter of oxide particles in CCPs are reduced by 16 % and 13 %, respectively. Upon partial replacement of Alex by 2 wt. % of boron the burning rate is practically unchanged as compared with that for basic propellant with Alex. However the agglomeration is significantly enhanced, which is manifested at the increase in the agglomerate particles content in CCPs by 1.8-2.2 times, increase by 1.6-1.7 times in the agglomerates mean diameter and increase in the unburned metal fraction in agglomerates by 1.6-1.9 times. The content and the mean diameter of the oxide particles are reduced more significantly than in the case of iron introduction, namely, by 20-30 % and 30-40 %, respectively.

Self-forming graphene/Ni patterns on sapphire utilizing the pattern-controlled catalyst metal agglomeration technique

We fabricated graphene/Ni patterns directly on sapphire substrates through a self-forming process utilizing the pattern-controlled catalyst metal agglomeration technique, which was accomplished via a thermal annealing process of rectangular Ni patterns preformed on thin amorphous carbon films on sapphire. It was confirmed that graphene films were synthesized along with the preformed Ni patterns as a result of the progress of Ni agglomeration. Notably, a few-layer graphene film was observed in specific areas along the periphery of the preformed Ni patterns. The self-forming graphene/Ni patterns showed ohmic conductivity with a contact resistance ranging from 4 × 104 to 7 × 104 Ω μm.

Finding a suitable catalyst for on-board ethanol reforming using exhaust heat from an internal combustion engine

Ethanol steam reforming with pure ethanol and commercial bioethanol (S/C = 3) was carried out inside the housing of the exhaust gas pipe of a gasoline internal combustion engine (ICE) by using exhaust heat (610–620 °C). Various catalytic honeycombs loaded with potassium-promoted cobalt hydrotalcite and with ceria-based rhodium–palladium catalysts were tested under different reactant loads. The hydrogen yield obtained over the cobalt-based catalytic honeycomb at low load (F/W < 25 mLliq·gcat−1·h−1, GHSV = 4·102 h−1) was remarkably high, whereas that obtained over the noble metal-based catalytic honeycombs was much superior at high loads (F/W = 25–150 mLliq·gcat−1·h−1, GHSV = 4·102–2.4·103 h−1). At higher reactant loads the overall hydrogen production was limited by heat transfer from the exhaust heat to the reformer inside the housing of the exhaust gas pipe of the ICE. Extensive carbon deposition occurred over the cobalt-based honeycomb, making its use impractical. In contrast, stability runs (>200 h) at high load (F/W = 150 mLliq·gcat−1·h−1, GHSV = 2.4·103 h−1) showed that promotion of the ceria-supported noble metal catalyst with alumina and zirconia is a key element for practical application using commercial bioethanol. HRTEM analysis of post mortem honeycombs loaded with RhPd/Ce0.5Zr0.5O2–Al2O3 showed no carbon formation and no metal agglomeration.

Impact of Lead Impurities in Zinc Dust on the Selective Reduction of a Dibromoimidazole Derivative

A low level of lead in zinc dust was identified as the causative agent responsible for inhibiting selective reduction of a dibromoimidazole derivative via metal agglomeration and lead deposition on the zinc surface.

Effect of Ru Species on N2O Decomposition over Ru/Al2O3 Catalysts

Ru is considered as an effective active species for N2O decomposition; however, there is disagreement about which ruthenium species is key for catalytic activity. In order to understand the role of Ru species in N2O decomposition, Ru/Al2O3 (Ru/Al2O3-H2, Ru/Al2O3-NaBH4, Ru/Al2O3-air) catalysts with different ratios of metallic Ru were prepared and evaluated for their catalytic activities. Various characterizations, especially in situ diffuse reflectance infrared Fourier transform spectroscopy (DRIFTS), were applied to investigate the relationship between activity and different Ru species. The results indicate that the N2O conversion displayed a linear relationship with the amount of metallic Ru. The DRIFTS results of adsorption for N2O show that metallic Ru was the active site. The catalytic processes are put forward based on metallic Ru species. The deactivation with increasing times used is due to the decrease in the amount of metallic Ru and agglomerates of Ru particles on the surface of catalysts.

Development of a Catalytic Combustion Type Gas Sensor with Low Power Consumption

Currently, gas leak alarms using AC power-supply are popular in Japan. However, there is pointed out, such as the hassle of installation property and power cord of the alarm, there was an increasing demand of the alarm of a battery drive system. The gas sensor of very low power consumption was required for realizing the battery operating system. Therefore, we focused on MEMS technology. The heating element occupying power most of for operating the sensor has been developed micro heater using a MEMS technology. The micro-heater has a rapid thermal response of several tens of milliseconds after the supply power. Furthermore, we developed a catalytic combustion type gas sensor reduced to 1/1000 of conventional power consumption by using the detection method of the intermittent operating. In general, miniaturization of the gas sensor would have a negative effect. Those effects were decreasing of the sensitivity / noise ratio and tending to susceptible to environmental factors. In this study, we report on improvement of the sensor for high sensitivity and high gas selectivity. The micro-heater was formed by sputtered platinum pattern on a silicon wafer. Formed platinum membrane was covered with a silicon nitride film. It is an air-bridge structure of the square of 90μm angle, suspended by etching the heater backside. The micro-heater has low power consumption because its thermal capacity is low. The catalytic combustion type micro-gas-sensor was prepared by sintering materials that Pd loaded γ-Al2O3 for a sensing element and only γ-Al2O3 for a reference element. The catalytic combustion type micro-sensor to give amplifies the potential difference generated both elements by a resistance change of the heater according to the catalytic combustion reaction of combustible gases. Measurements of gas sensitivity of the sensor which was heated to approximately 450°C， placed in a gas chamber, it was performed by injecting a test gas having a predetermined concentration. Response carve of the sensor when power was supplied to the micro-heater becomes detectable state at tens of milliseconds. Methane sensitivity was observed with good linearity. However, the hydrogen sensitivity was approximately three times the methane it, methane selectivity of the sensor was found to be low. Material of the reference element in order to improve this problem has been changed. The reference element materials have used γ-Al2O3 does not have catalytic combustion reaction with any combustible gases. The reference element material was to have sensitivity to miscellaneous gases such as hydrogen by changing from γ-Al2O3 in the Pt loaded γ-Al2O3. Miscellaneous gas sensitivity of each element cancelled each other, the selectivity of methane sensitivity was improved. As a result, the sensitivities of H2 / CH4 ratio have become possible to improve the methane selectivity to 1 times or less. The second improvement was to increase the methane gas sensitivity. By the small size of the device due to the low power consumption, the amount of catalyst could be applied would be very small. Further, since the planar structure, the detection target gas could react only one side of the catalyst, the sensitivity obtained by the combustion reaction was low. Therefore, we consider how to increase the gas contact point on the element. As a first method, it was optimize the amount of Pd supported on the catalyst. If the amount of Pd loaded ratio is substituted standard ratio for 1, the sensitivity of the methane was found to be a maximum value in the Pd ratio is 2. On the other hand, the specific surface area was decreased with an increase in the amount of Pd loaded in particular reduction of the specific surface area more than Pd ratio is 2 was significant. When the amount of Pd loaded increases gas contact points, gas sensitivity is improved to increase. However, the sensitivity is found to decrease by a metal agglomeration when Pd loaded content is too high may occur. As a second method, utilizing the characteristics of the sensor elements consists of an air-bridge, in order to encourage gas contact from the heater backside, some slit-shaped through-hole between the heater patterns. It becomes possible to wrap the catalyst heater backside by the structural modification. Then methane gas sensitivity of the sensor by gas contact point of the heater back side increases has been improved by 40% compared with the previous type. These improvements had the prospect of practical use of methane gas sensor of the catalytic combustion type with micro-heater using MEMS technology.