Ceramic Monoliths Research Articles

Synthesis of composite materials of carbon nanofibres and ceramic monoliths with uniform and tuneable nanofibre layer thickness

Composite materials consisting of ceramic monoliths and carbon nanofibres (CNFs) have been synthesized by catalytic growth of CNFs on the γ-alumina washcoating layer covering the walls of a ceramic monolith. The composites possess a relatively uniform mesoporous layer of CNFs of relatively small diameter. The thin alumina washcoating (ca. 0.1 μm) prevents the CNFs from being trapped inside the alumina pores and hence the CNFs grow freely throughout the washcoating layer to form a uniform layer of CNFs that completely covers the surface of the monolith walls. The growth temperature is found to control the thickness of the CNF layer (2–4 μm), the growth rate of the nanofibres, and the mechanical strength of the resulting CNF–monolith composite. At ideal conditions, a complete adhesion of the CNF layer and higher mechanical strength than the original cordierite monolith can be obtained. The CNF layer has an average pore size of 17 nm with absence of microporosity which renders these monoliths promising candidates for the use as catalyst supports, especially for liquid phase reactions. The CNFs have small diameters (5–30 nm) due to the high dispersion of Ni particles in the growth catalyst and the CNFs exhibit an unusual branched structure.

N2O Decomposition over Perovskite Catalysts

Four perovskite catalysts LaBO3 (where B = Cr, Mn, Fe, and Co) were prepared via a highly exothermic and self-sustaining reaction, the so-called “solution combustion synthesis (SCS)”, and characterized by means of X-ray diffraction, BET, field-emission scanning electron microscopy−energy-dispersive spectrometry, and H2-temperature-programmed reduction (TPR) analyses. The performance of these catalysts toward the decomposition of N2O to N2 and O2 was evaluated in a temperature programmed reaction (TPRe) apparatus in the absence and the presence of different oxygen concentrations. Among the catalysts screened, LaCoO3 showed the best performance, with 50% conversion of N2O at 455 °C and 490 °C in the absence and presence of 5% of oxygen, respectively. The LaCoO3 catalyst was deposited by in situ SCS directly over a ceramic honeycomb monolith and then tested in a lab-scale test rig. The coated ceramic monolith gave 50% N2O conversion performance similar to that obtained on powder for GHSV values of industrial interest (10 000−30 000 h-1). The correlation between the observed oxygen inhibition and the proposed N2O decomposition mechanism as well as the relationship between the observed activity and the reducibility of the B site, determined from TPR experiments, is discussed.

Glycerin Steam Reforming for Hydrogen Production

Biodiesel production is expected to grow steadily in the future. In converting vegetable oils into biodiesel, approximately 10% (w/w) of glycerin is produced as a by-product. With the increasing production of biodiesel, there would be glut of glycerin in the world market; therefore, it is essential to find useful applications for this by-product. Glycerin is a potential feedstock for hydrogen production because one mole of glycerin can produce up to four moles of hydrogen. The objective of this study was to develop, test, and characterize promising catalysts for hydrogen production from steam reforming of glycerin. The following five catalysts were prepared on ceramic foam monoliths (92% Al2O3, 8% SiO2): rhodium-cerium, rhodium-cerium-platinum, ruthenium-yttrium, nickel, and iridium. The catalysts were prepared by the incipient wetness technique. This article discusses selectivity data of the selected gases resulting from the glycerin steam reforming process. The study found that the rhodium-cerium-platinum catalyst was the most selective towards hydrogen under the experimental conditions investigated (700C, 1 atm, and gas hourly specific velocity of 9.2 104 h-1). The glycerin conversion was 100% with all the catalysts tested.

Structured catalytic reactor for VOC combustion

Structured catalytic reactor for VOC combustion VOC emission has recently become a challenge for environmental protection. Catalytic combustion seems a promising method of VOC removal, however, there are still more specific demands concerning the process. The reactor design has to assure enhanced transfer properties accompanied by low flow resistance to reduce the pumping costs. Neither the packed beds nor the classic ceramic monoliths are able to fulfil the requirements. To solve the problem we propose a wire gauze structure composed of several gauze sheets staked. A number of mass transfer and flow resistance experiments were performed for two gauze types and correlations were derived for the Sherwood number and the friction factor. The results were found to be in a reasonable agreement with the literature available. The studied gauze structures were compared with a classic monolith for the assumed case study. The calculated length of the gauze structured reactor was significantly shorter, up to ten times, when compared with a classic ceramic monolith, but the pressure drop was higher.

Development of a Novel Monolith-Bioreactor for the Treatment of VOC-polluted Air

A novel bioreactor packed with ceramic monolith colonized by a toluene-degrading culture was investigated in order to assess its suitability for waste gas treatment. Operational parameters that were considered included start-up of the bioreactor, toluene loading rate, changes in gas flow rate, liquid feed mode, and steady-state operation. This is the first report on the treatment of toluene-polluted air in such a biological monolith reactor. Data on performance and stability have been obtained showing that this system is suitable for waste gas treatment. Removal efficiencies around 90% could be maintained at different gas flow rates, although this value gradually dropped when increasing the load above 30 g m−3 h−1. Interestingly, omitting the continuous feed of a liquid trickling phase improved the reactor's performance. One potential drawback that needs to be minimized is related to clogging after long term operation. Further studies aimed at optimizing this novel application would allow reaching a high performance under long-term, stable conditions.

Review on Monolithic Ceramic Catalysts in Different Applications: Support Materials, Incorporation Techniques and Physical Properties

A broad spectrum of the fundamental principles of ceramic monoliths as supports in different applications based on catalytic reactions is presented in this review. Importance of selecting right ceramic structure, carrier materials etc has been discussed in detail focusing on several major studies performed since ‘50s till date. The large array of preparation technology using various incorporation techniques of active phase on the support meeting suitable characteristics of a good monolithic catalyst has also been illustrated at length. The choice of a suitable ceramic catalyst for a particular application strongly depends on emphasizing the physical properties of the catalyst, i.e. active surface area, adhesion properties, pore volume and particle size. The significance of these properties have also been reviewed in this article. Ceramic monoliths are especially well-known due to their cost-effectiveness, low pressure drop and requisite high mechanical strength for the long run harsh conditions encountered in environmental applications. Several emerging applications especially in gas phase catalytic reactions are discussed briefly, emphasizing on the implication of ceramic catalysts in hydrogen production and fuel cell technology, the main debate in alternative energy nowadays.

Effect of rare-earth element oxides (La 2O 3, Ce 2O 3) on the structural and physico-chemical characteristics of Pd/Al 2O 3 monolithic catalysts of nitrogen oxide reduction by methane

It is shown that introduction of additives of rare-earth element oxides (La 2O 3, CeO 2) enables regulating the structural and functional characteristics of Pd/Al 2O 3-catalysts (applied on ceramic monoliths of honeycomb structure) of nitrogen oxide reduction by methane. Modifying additives provide increase of thermal stability of porous structure of both highly dispersed Al 2O 3, as the second support, and the catalyst as a whole. Contribution of La 2O 3 and CeO 2 in increasing the thermal stability is of an additive nature, and lanthanum oxide shows the higher efficiency than cerium one. According to X-ray phase analysis data, stabilizing action is conditioned by occurrence of rare-earth element oxides into lattice of Al 2O 3, which retards diffusional processes leading to phase transitions of low-temperature crystalline modifications of alumina into high-temperature ones with a low specific surface. For the catalyst samples modified with La 2O 3 an effect of thermal activation is observed, which is revealed by increase in catalytic activity as a result of annealing at 850 °C. Such a phenomenon, as shown by means of X-ray photoelectron spectroscopy technique, can be explained via stabilization of palladium in singly charged state in the form of groups of Pd +O 2 − and corresponding increase in concentration of active centers.

Tungsten bronze-based nuclear waste form ceramics. Part 1. Conversion of microporous tungstates to leach resistant ceramics

The effective immobilization of Cs + and/or Sr 2+ sorbed on hexagonal tungsten oxide bronze (HTB) adsorbent materials has been achieved by heating in air at temperatures in the range 500–1000 °C. Crystalline powdered HTB materials formed by heating at 800 °C displayed leach characteristics comparable to Cs-containing hot-pressed hollandites in the pH range from 0 to 12. If the Cs-loaded HTB sorbents were pressed into pellets prior to calcination, ceramic monoliths could be prepared with negligible Cs volatilization losses. Heating to temperatures in excess of 1250 °C under dynamic air flow resulted in the melting of the sorbent to form phase assemblages consisting of millimetre-sized crystals of bronzoid phases. Up to 5 wt% mass loss was observed for small scale samples of melted materials under dynamic air flow. Both the calcined and melted bronzoid waste forms are multiphase ceramics in which Cs + remains bound within, and appears to stabilize, the hexagonal bronze phase, even after complete melting at 1300 °C. The leachability of Sr from the phases prepared by heating appears to be somewhat worse than that of Cs. Saturation of the HTB adsorbents with lanthanide elements (Nd, La, Ce) gave rise to cubic bronze phases in which we propose that the lanthanides substitute at the tungsten or molybdenum sites rather than the tunnel positions. The lanthanides were rather easily leached from the calcined phases in 0.1 M HNO 3 at 150 °C.

Short-channel structured reactor for catalytic combustion: Design and evaluation

Emission control of hazardous air pollutants, including volatile organic compounds (VOC), is one of the priorities for environmental catalysis. Metallic structured short-channel reactors of various geometries are regarded as an alternative to ceramic monoliths. The paper presents the coupled results on reactor structure modelling and the catalyst testing. It is expected, that mass transfer coefficients for laminar flow partially developing in short-channels can be increased up to 10 times comparing to standard monoliths, but the increased transfer properties involve elevated pressure drop. Therefore, performance evaluation criteria (PEC) have been developed to properly estimate the structure performance. The criteria derived cover both the general case for the comparable mass transport and kinetics of the process and the restricted case for the diffusional process limitation. In comparison with monoliths, the best structures (sinusoidal and triangular) seem to be from 1.5 times upto 4 times more efficient. Chromium–aluminium (CrAl) steel was surveyed in terms of its applicability for carrier manufacturing and catalyst depositing. Alumina washcoat and cobalt catalyst were deposited as organic precursors using Langmuir–Blodgett method (LB). Additional noble metal promoters were deposited by chemisorption. Catalytic tests have proved that the small amounts of highly dispersed Co 3O 4 are sufficiently active in combustion of diluted n-hexane with 100% selectivity to CO 2.

R-curve modeling for Ni/Al 2O 3 laminates

A metal/ceramic laminate has increased fracture toughness as compared to the ceramic monoliths. The toughness enhancement is controlled by the closure (‘bridging-stresses’) exerted by the metallic bridging layers (‘ligaments’) astride the crack in the ceramic layers. The crack-opening-displacement (COD) carries information on the level and location of these stresses. If the COD profile is known, the bridging-stresses and consequently, the R-curve behavior of the composite can be modeled. In this study, a weight-function-based approach is used to generate the R-curve and is compared with experimental results for Al 2O 3/Ni multilayer laminates.

Bulk Consolidation of Non-Oxide Ceramic Powders Derived from Polymer Precursors

Consolidation of pyrolyzed powders has been tried by hot isostatic pressing (HIP) without sintering additives, in order to obtain dense non-oxide ceramic bulk materials derived from polymer precursors. Si1.0C1.6N1.3 ceramic powders were derived from a polyvinylsilazane polymer. The polymer was thermally crosslinked at 250oC and pyrolyzed at 1050oC under Ar atmosphere. The pyrolyzed powders were die-pressed into rectangular bars at room temperature and densified by HIP at 1400oC-900 MPa and 1500oC-950 MPa. Dense ceramic monolith, in which pores were not observed by optical microscopy, was obtained by the HIP consolidation at 1500oC-950 MPa. The microstructure of the ceramic monolith was a nano-composite structure consisted of α-Si3N4 and graphite phases. In the compression tests of the HIP-treated sample, slight plastic deformation was observed at 1400 and 1500oC in spite of high compressive stress over 1000 MPa. On the other hand, the sample showed a compressive strain of about 7% at 1000 MPa at 1600oC. The compressive strain of about 11% was achieved at 1700oC.

CO and H 2 oxidation on a platinum monolith diesel oxidation catalyst

This paper presents experimental and modelling results for the oxidation of mixtures of hydrogen and carbon monoxide in a lean atmosphere. Transient light-off experiments over a platinum catalyst (80 g/ft 3 loading) supported on a washcoated ceramic monolith were performed with a slow inlet temperature ramp. Results for CO alone agree with earlier results that predict self-inhibition of CO; that is an increasing light-off temperature with increasing CO concentration. Addition of hydrogen to the feed causes a reduction in light-off temperature for all concentrations of CO studied. The most significant shift in light-off temperature occurs with the addition of small amounts of hydrogen (500 ppm, v/v) with only minor marginal enhancement occurring at higher hydrogen concentrations. Hydrogen alone in a lean atmosphere will oxidise at room temperature. In mixtures of hydrogen and CO, the CO was observed to react first until a conversion of about 50% was observed, at which point the conversion of hydrogen rapidly went from 0 to 100%. Simulations performed using literature mechanistic models for the oxidation of these mixtures predicted that hydrogen ignites first, followed by CO, a direct contradiction of the experimental evidence. Upon changing the activation energy between adsorbed hydrogen and oxygen, the CO was observed to oxidise first, however, no enhancement of light-off was predicted. The effect cannot be explained by the mechanistic model currently under discussion.

Effective diffusivities and pore-transport characteristics of washcoated ceramic monolith for automotive catalytic converter

Using the chromatographic technique (carrier-gas: N 2; tracer-gases: He, Ar) the effective diffusivity in single-pellet string columns (SPSC) packed with porous slab particles was studied. Dispersion due to extra-column effects was eliminated via convolution of column responses for two lengths. The measurements were done for cordierite particles and for cordierite coated with alumina-based washcoat. The effective diffusion coefficients for two tracer–carrier pairs were evaluated by fitting column response (chromatographic) peaks in time-domain. Mean transport parameters were evaluated for both types of porous particles. The obtained mean transport pore radii are in reasonable agreement with pore-size distribution from mercury porosimetry.

Structured Reactors for Enzyme Immobilization: A Monolithic Stirrer Reactor for Application in Organic Media

The use of a Monolithic Stirrer Reactor (MSR) for carrying out enzyme-catalysed reactions in organic media, is presented. The MSR is a convenient tool to compare monolithic catalysts in the absence of external mass transfer limitations. Ceramic honeycomb monoliths have been functionalized with different carbonaceous and polymeric carriers to provide adsorption sites for a lipase from Candida antarctica (CALB). The enzyme-loaded monoliths were employed as stirrer blades in a monolithic stirrer reactor for the acylation of butanol with vinyl acetate in toluene. Two types of ceramic honeycombs were examined and compared; a classical cordierite honeycomb and a new high-porosity mullite advanced ceramic material (ACM), having an open pore structure on the micrometer scale. The ACM and cordierite monoliths were compared in terms of immobilization capacity, activity and stability. The results show that using the MSR, internal and external mass transfer limitations are absent, and that all of the immobilized enzyme is used effectively. The open structure of the ACM monoliths allows for a higher enzyme loading and leads to a higher activity per monolith volume compared to classical cordierite monoliths. No effect of the monolith microstructure on intrinsic enzyme activity was observed; for all carriers, 35% of the free enzyme activity is retained after immobilization. The use of monoliths as stirrer blades allows easy reuse of both the enzyme and the supports-carrier combination.

Preferential oxidation of carbon monoxide with iron-promoted platinum catalysts supported on metal foams

A series of 5 wt% Pt/0.5 wt% Fe/γ-Al 2O 3 catalysts supported on metal foams of different geometries were synthesized and tested for preferential oxidation of a low CO concentration in the presence of a high H 2 concentration. The catalysts were tested in a fixed bed adiabatic reactor at a total pressure of 0.2 MPa (absolute) to simulate fuel processor operating pressure. The inlet temperature was varied from 80 °C to 170 °C, and the gas hourly space velocity ranged from 5000 h −1 to 45,000 h −1. The inlet gas composition to the reactor reproduced that of the effluent stream from the water-gas-shift reactor in a typical fuel processor: H 2 42%, CO 2 9%, H 2O 12%, CO 1.0%, O 2 0.5–1.0%, and N 2 35–35.5%. The geometry of a foam is characterized by the volume fraction of solid material (cell density) and by the number of pores per inch. The catalysts with lower cell densities generally exhibited higher CO conversions and selectivities. Under most operating conditions, the CO conversion and selectivity of the best metal foam catalysts were comparable to those of a 400 cells per square inch, ceramic straight-channel monolith with the same nominal catalyst loading. Both the reverse water-gas-shift (r-WGS) reaction and transport resistances affected the performance of these catalysts. Under adiabatic conditions, the r-WGS reaction made it impossible to achieve low outlet CO concentrations. The effects of space velocity and linear velocity were studied independently using various catalyst lengths and volumetric gas flow rates. At a constant space velocity, the CO conversion increased with higher linear velocities, suggesting a significant mass transfer resistance between the bulk gas and the catalyst surface.

ポリビニルシラザン由来の非晶質 Si-C-N モノリスの作製と圧縮変形

Consolidation of non-crystalline Si-C-N powders has been tried by hot isostatic pressing without sintering additives, in order to fabricate dense non-oxide ceramic monoliths without the crystallization. A polyvinylsilazane polymer was thermally cross-linked at 250°C and pyrolyzed at 1050°C under Ar atmosphere, which resulted in the formation of the ceramic state with a chemical composition of Si1.0C1.6N1.3 in atomic ratio. The pyrolyzed powders were die-pressed into rectangular bars at room temperature and consolidated by hot isostatic pressing at 1400°C-900 MPa. A ceramic monolith with a level of almost full density was obtained by the consolidation, although a few pores of about 1 μm still remained in the cross-sectional structure. The amorphous phase was significantly maintained in the consolidated sample. In the compression tests of the sample, only slight plastic deformation was observed at 1400°C and 1500°C in spite of high compressive stress over 1600 MPa. On the other hand, the sample showed plastic deformation at 1600°C.

The fracture toughness of Ni/Al 2O 3 laminates by digital image correlation II: Bridging-stresses and R-curve models

A composite of metal and brittle ceramic layers have increased fracture toughness as compared to ceramic monoliths. The property controlling the toughness enhancement is the, ‘bridging-stress’, exerted by the ductile phase astride the crack in the ceramic. This bridging-stress is a function of the crack-opening displacement (COD) which is a function of the size of the crack and the position along its profile. Depending on the accuracy of estimation of the bridging-stress, the modeled R-curve and experimental one match. In this study, a weight function based approach to generate the R-curve is reported and compared with the experimental results for Al 2O 3/Ni multilayer laminates.

Si 3N 4/BN fibrous monoliths: Mechanical properties and tribological responses

Ceramic fibrous monoliths (FMs) consist of fiber-like cells that surround a weaker matrix phase called the cell boundary. FMs based on Si 3N 4/BN exhibit many excellent mechanical properties, and much work has been done to characterize and understand the relations among their processing, microstructure, and properties. The following body of data and understanding for Si 3N 4/BN FMs are discussed in this paper: processing, elastic constants, in-plane fracture and modeling of fracture, thermal stresses, interfacial shear strength and tailoring of gross interface structure, creep, impact erosion resistance, and sliding wear resistance. Possibilities to improve their properties are also presented.

Effect of alkali metal additives on the activity and selectivity of structured silver catalysts in epoxidation of ethylene by nitrogen(I) oxide

We have shown that introducing Cs, K, or Na additives into the composition of silver catalysts (supported on ceramic honeycomb monoliths) for epoxidation of ethylene by nitrogen(I) oxide leads to an increase in the yield of the target product (ethylene oxide) by a factor of 1.5–2, while the presence of lithium in the composition of the composites inhibits the process. The optimal content of alkali promoters in the composition of the catalysts decreases in the series Na, K, Cs antibatically relative to the ionic radii of the elements.

Vanadium supported on carbon coated honeycomb monoliths for the selective catalytic reduction of NO at low temperatures: Influence of the oxidation pre-treatment

Mesoporous carbon–ceramic honeycomb monoliths were prepared for catalyst supports by a polymer blend approach. In these monoliths, vanadium was supported by ion exchange of the cationic precursor with the protons of oxygenated surface groups. Previously to vanadium incorporation, the carbon–ceramic monoliths were oxidized using different agents under various conditions to get different oxidation degrees. The texture and surface chemistry of the as-prepared carbon–ceramic monolithic supports were exhaustively characterized by N 2 physisorption, Fourier transform infrared spectroscopy, temperature programmed decomposition and photoelectron spectroscopy. The resulting vanadium on carbon coated monoliths were tested in the selective catalytic reduction of NO with NH 3 at 453 K. The chemical and textural characterization of the samples after the different oxidation treatments revealed some of the reasons leading to a higher vanadium loading and dispersion. Among the oxidation treatments used, that with 10% O 2 at 603 K gave rise to the highest vanadium loading and a high dispersion. Treatments with HNO 3 also brought about high vanadium dispersion while that with ozone led to the lowest dispersion. It was observed that vanadium loading did not only depend on the amount of oxygen-containing groups but also on the textural properties resulting after the oxidation treatment.