Ceramic Monoliths Research Articles

Porous chromium-based ceramic monoliths: oxides (Cr2O3), nitrides (CrN), and carbides (Cr3C2)

In this research, we have successfully prepared hierarchically porous chromium-based non-oxide ceramic monoliths (chromium nitride (CrN) and chromium carbide (Cr3C2)) by facile two-step methods. By utilizing an epoxide-mediated sol–gel reaction accompanied by phase separation, a well-defined macroporous structure is tailored in the chromium (oxy)hydroxide–urea composites in the presence of poly(acrylic acid). Urea works as a nitrogen and/or carbon source through the following heat-treatment, whereas poly(acrylic acid) behaves as a phase separation inducer as well as a network-supporting constituent. The macroporous morphology of xerogels is controllable by changing the starting composition. Heat-treatment for thus obtained xerogels under an air atmosphere provides chromium oxide (Cr2O3), whereas that under a nitrogen atmosphere leads to the crystallization of CrN at 700 °C and Cr3C2 at 900 °C without any deterioration of the porous morphologies. The heat-treated samples possess a high specific surface area up to 450 m2 g−1 as well as high porosity over 80%, preserving good mechanical strength. This facile and novel synthesis strategy has a potential extendability to other macroporous monoliths with high surface area based on various metal nitrides and carbides, which are useful in various applications such as catalysts, semiconductor devices, magnetic materials, and refractory materials.

CO2 Conversion into Valuable Fuels Using Chromium Based Supports

Abstract CO2 utilization by direct catalytic conversion of CO2 driven by solar energy is an attractive approach for producing alternative value added products suitable for end-use infrastructure. In order to fully harness the solar spectrum and increase photocatalytic activity and selectivity, Cr-TiO2 based films were deposited on ceramic honeycomb monoliths with varying concentrations synthesized by sol-gel technique and dip coating route. The improved photocatalytic activity of the Cr-TiO2 monoliths in the visible light region compared to pure TiO2 can be attributed to increased visible light absorption and accessible active metal sites arising from the appropriate metal dispersion and loading amount.

Green Propulsion: Catalysts for the European FP7 Project GRASP

The Green Advanced Space Propulsion (GRASP) project investigated the issue of green propulsion and the possibility of replacing presently used toxic hydrazine propellant by green propellants. This project was financed by the European Commission in the 7th Framework Program (FP7). A large data base of about 100 green propellants was compiled including physical and material properties, toxicity and performance data. A preliminary selection was conducted to identify the most promising green propellant candidates for experimental work. Catalytic ignition, a very simple and robust subsystem with restart possibility, was thoroughly investigated focusing on the catalyst–propellant couple. For the whole project, more than 50 catalyst variations have been prepared comprising ceramic pellets, ceramic monoliths, metallic foams or gauzes. Very promising green propellant candidates, e.g. ammonium dinitramide-based monopropellants, have been methodically investigated leading to the development and manufacturing of advanced pellet-based catalysts for ionic liquid decomposition. The decomposition of concentrated hydrogen peroxide (87.5 wt%) for bipropellant applications led to a thorough investigation of monolithic catalysts. Examination of monolith parameters (channel shape, channel density, material) and preparation parameters (washcoating procedure, active phase precursor) led to the development of very efficient catalysts showing high activity and stability: 17.9 kg of H2O2 could be decomposed for the first time by a single monolithic catalyst.

Bio-fuel reformation for solid oxide fuel cell applications. Part 3: Biodiesel–diesel blends

The auto-thermal reforming (ATR) performance of diesel blended with biodiesel (e.g., B5, B10, B20, B40, and B80) was investigated and compared to pure diesel and biodiesel ATR in a single-tube reformer with ceramic monolith wash-coated rhodium/ceria–zirconia catalyst. The initial operating condition of the ATR of all studied fuels was set as total moles of oxygen from air, water, and fuel per mole of carbon (O/C) = 1.47, moles of water to carbon (H2O/C) = 0.6, and gas hourly space velocity = 33,950 h−1 at 1223 K reformer temperature, to achieve the same syngas (H2 + CO) production rate. A direct photo-acoustic micro-soot meter was applied to quantify the dynamic evolution of carbon formation and a mass spectrometer was used to measure the gas composition of reformer effluents. The blends with more biodiesel content were found to have a lower syngas production rate and reforming efficiency, and require more air and higher reformer temperature to avoid carbon formation. Strong correlations between ethylene and solid carbon concentration were observed in the reformation of all the fuels and blends, with more biodiesel content tending to have higher ethylene production. This study is one component of a three-part investigation of bio-fuel reforming, also including fuel vaporization and reactant mixing (Part 1) and biodiesel (Part 2).

Methane combustion modelling of wire gauze reactor coated with Co3O4–CeO2, Co3O4–PdO catalysts

An evaluation of a designed wire gauze reactor was performed. The reactor was based on the Ce3O4 catalyst which was doped with CeO2 and PdO to increase the catalytic combustion of CH4. The cerium oxide catalyst was obtained by an impregnation method. Analysis of the prepared catalysts was performed using XRF quantitative method. The determination of the reaction kinetics was obtained by CSTR catalytic combustion experiments. The performance of wire gauze structured reactors with various metal oxide catalysts with regard to the catalytic combustion of methane originated from biogas fuelled power turbines was assessed through mathematical modelling. A one-dimensional plug-flow model of structured catalytic reactor was applied. The modelling results have proven that the wire gauzes can be regarded as a serious alternative to the ceramic monoliths.

Design studies for monolithic high temperature shift catalysts: Effect of operational parameters

In this study, the performance of a commercial high temperature shift (HTS) catalyst has been investigated. The catalyst was a wash-coated ceramic monolith type and used to adjust the H2/CO ratio in a simulated syngas stream. In the experiments, the effects of inlet gas composition, gas hourly space velocity, inlet steam to CO ratio and reaction temperature have been investigated. The results showed that all these parameters have considerable influence on the design of HTS reactor. Precious metal based monolith catalysts enable working under high space velocities and thus reduce the volume of HTS reactor. Specifically defined selectivity, as ratio of the total amount of CO2 and H2 to the amount of CO2 formed during the process, seems to be a good measure of possible unwanted side reactions that might occur. It was found that the selectivity ratio should be ideally about two for HTS process, minimizing the formation of side reactions. Selectivity values below 2.0 indicate the presence of unbalanced H2 which is incompatible with WGS reaction stoichiometry. Results showed that steam is an effective parameter in determining the probability of side reactions, especially the reactions leading to the catalyst deactivation through coke deposition. Another point is that coking tendency of the catalyst is more severe at lower operating temperatures. The formation of methane, an unwanted by-product, was seen to be favored by lower gas hourly space velocities, possibly via the reaction with H2. The formation of methane results in hydrogen consumption to some extent and consequently alters the product composition. The optimum operating conditions of the wash-coated monolith type HTS catalyst studied were found to be as follows: Temperature=~375–400°C, GHSV=~50,000h−1 and the inlet steam to CO ratio=~2.0.

Short-Channel Structured Reactor as a Catalytic Afterburner

Short-channel structures of triangular and sinusoidal channel cross-sectional shape were examined experimentally to find their heat (or mass) transfer and flow friction (flow resistance) characteristics. A particular emphasis was placed on influence of the channels length (referred to its hydraulic diameter) on transport and hydrodynamic characteristics. The transport and friction properties were correlated in terms of dimensionless channel length with satisfactory accuracy. The dimensionless channel length was proved to be a key factor that determines the transport and friction coefficients. Comparison with ceramic monolith has shown possibility of using the structures in automotive catalytic converters.

세라믹 모노리스에 담지된 CuO와 ZnO계 촉매에 의한 이산화탄소의 메탄올로의 전환에 관한 연구

Methyl alcohol is one of the basic intermediates in the chemical industry and is also being used as a fuel additive and as a clean burning fuel. In this study, conversion of carbon dioxide to methyl alcohol was investigated using catalytic chemical methods. Ceramic monoliths (M) with were used as catalyst supports. Monolith-supported CuO-ZnO catalysts were prepared by wash-coat method. The prepared catalysts were characterized by using ICP analysis, TEM images and XRD patterns. The catalytic activity for carbon dioxide hydrogenation to methyl alcohol was investigated using a flow-type reactor under various reaction temperature, pressure and contact time. In the preparation of monolith-supported CuO-ZnO catalysts by wash-coat method, proper concentration of precursors solution was 25.7% (w/v). The mixed crystal of CuO and ZnO was well supported on monolith. And it was known that more CuO component may be supported than ZnO component. Conversion of carbon dioxide was increased with increasing reaction temperature, but methyl alcohol selectivity was decreased. Optimum reaction temperature was about under 20 atm because of the reverse water gas shift reaction. Maximum yield of methyl alcohol over CuO-ZnO/M catalyst was 5.1 mol% at and 20 atm.

Fabrication of Highly Porous Alumina Prepared by Gelation Freezing Route with Antifreeze Protein

Macroporous alumina ceramic monolith was fabricated by the novel gelation freezing method with antifreeze protein (AFP). The gelation freezing route with AFP is a simple and efficient way to produce macrocellular ceramics with tailored and uniform pore architecture as well as high porosity.

Autothermal Reforming of Biodiesel–Ethanol–Diesel Blends for Solid Oxide Fuel Cell Applications

The autothermal reforming (ATR) performance of diesel, biodiesel, diesel blended with 25% biodiesel, and diesel blended with both 25% biodiesel and 10% ethanol was investigated under different operating environments in a single-tube reformer with rhodium/zirconia wash-coated ceramic monoliths. A customized nozzle integrated with a microsize porous device was designed to finely atomize the input heavy hydrocarbon fuels and enhance the homogeneous mixtures of hydrocarbon fuel with steam and air. The optimum operating conditions with high syngas yields and carbon-free content for ATR of each studied fuel were identified, from both thermodynamic analysis and experimental measurements. After establishing the test points for the reformation of the studied fuels under the same initial operating environment [total O/C = 1.47, S/C = 0.6, and gas hourly space velocity at standard temperature and pressure (GHSV) = 34 120 h–1, at 950 °C reformer temperature] with the goal of achieving the same rate of syngas producti...

Single crystalline brookite titanium dioxide nanorod arrays rooted on ceramic monoliths: a hybrid nanocatalyst support with ultra-high surface area and thermal stability

In this study, a cost-effective solution synthesis of commercial scale single crystalline titanium dioxide (TiO2) nanorod arrays has been successfully achieved on the inner walls of cordierite monoliths, which may provide a new class of nanostructured hybrid support for catalysis and energy applications. The morphological evolution has been observed from dandelion TiO2 nanorod clusters to vertically aligned nanorod arrays while increasing the solution treatment time. A heterogeneous nucleation–dissolution–crystallization mechanism is proposed for the growth of the brookite structured TiO2 nanorod arrays on the inner walls of honeycomb monoliths. The brookite structured TiO2 nanorod arrays have a high surface area up to ~250 m2 g−1, with an overall specific surface area in the hybrid nanostructured monolith increasing by 1.8 and 2.5 times after TiO2 nanorods coating for 4 h and 24 h, respectively. The thermal stability tests using differential scanning calorimetry (DSC), thermogravimetric analysis (TGA) and hydrothermal stability tests under boiling water for 24 h suggest that the TiO2 nanorods coated hybrid monoliths are very stable. An as-prepared 9-cm-long TiO2 nanorod-array monolithic device was demonstrated as a continuous flow fixed-bed absorbent with efficient performance in Rhodamine B removal from aqueous solution.

Porous ceramic monoliths assembled from microbeads with high specific surface area for effective biocatalysis

Most bio-technological applications such as enzyme immobilization for biocatalysis or biosensing require highly suitable carriers that offer high accessible surface area and good liquid flow permeability at the same time. Porous monolithic adsorbents (MAds), combining large specific surface areas with high liquid flow permeabilities, are prepared in a fast and versatile two step synthesis via the ionotropic gelation route. The obtained MAds consist of 3-dimensionally assembled porous microbeads with tailorable specific surface areas up to 50 m2 g−1 after sintering. The pores inside microbeads and MAds are highly interconnected with hierarchical pore diameters between 15 nm and 105 μm. MAds permeability constants are: k1 = 6 × 10−9 m2 and k2 = 4 × 10−4 m. The obtained MAds are excellent platforms for further bio-chemical surface modifications applying established silanization protocols. We perform an amino functionalization of the MAds which is subsequently used to covalently link the model enzyme laccase to the MAds. The applied amino molecules are situated across the whole cross-sectional area of each microbead, and enzymes like laccase with a mass of approximately 70 kDa are able to completely access the intra-bead pores. The maximum effectiveness factor η is 0.90 for freshly immobilized laccase. Furthermore, laccase keeps about 85% of its activity as compared to the free enzyme in solution, and after 9 days of storage the activity is still 80%. An additional flux test confirms that the immobilized enzyme remains active under continuous flux conditions for at least 16 h.

Performance and Kinetics of Flame-Made Vanadia/Titania Catalyst Nanoparticles in the Partial Oxidation of o-Xylene

Vanadia/titania catalyst particles were made by flame-spray pyrolysis and deposited onto ceramic sponge monoliths either by direct deposition of the flame-made particles or by a dip-coating technique. In the partial oxidation of o-xylene, the influence of the coating thickness and porosity on the catalytic performance was investigated. It was found that the highly porous coatings obtained by direct deposition exhibit insufficient heat transfer properties, while dip-coated layers are prone to internal mass transfer limitations if a certain thickness of the coating is exceeded. In the absence of transport limitations, kinetic experiments were carried out to derive a reaction network and to develop a quantitative kinetic model. The resulting model describes well the influences of reactant concentrations and temperature on the product distributions in the oxidation of o-xylene to phthalic anhydride over the novel flame-made catalyst and can be used for reactor simulations.

Comparative Evaluation of Thermal Conductivity of Zirconia Solid and Honeycomb Structures

Porous zirconia ceramic monoliths have been extensively used in thermo-structural applications due to their inherent low thermal conductivity in combination with their adaptability to form complicated shapes through advanced ceramic processing techniques. However, extruded cellular honeycomb structures made from these materials have been less explored for thermal management applications. There exist large potential applications due to their unique configurations, resulting in better heat-management mechanisms. Some of the studies carried out on zirconia honeycombs are safeguarded through patents due to its technical importance, or the information is not in the public domain. In the present study, for the sake of comparison, honeycomb specimens with varying wall thicknesses and unit cell lengths maintaining almost same bulk density of around 90% theoretical and relative density of 0.34–0.37 were prepared and subjected to thermal conductivity evaluation along with the solid samples with relative density of 1.0 using monotonic heating regime methodology. In addition, the effect of channel shape was also evaluated using square and triangular channeled honeycombs with the same relative densities. The results obtained from these specimens were correlated with their configurations to bring out the advantages accrued by using the honeycomb with these configurations. It was observed that a significant decrease in thermal conductivity was achieved in honeycombs, which can be attributed to the behavior of various heat transfer mechanisms that are operative at high temperatures in combination with the considerable reduction in thermal mass and the consequent conduction through the solids.

Catalytic ozonation of metolachlor under continuous operation using nanocarbon materials grown on a ceramic monolith

The catalytic ozonation of the herbicide metolachlor (MTLC) was tested using carbon nanomaterials as catalysts. Multiwalled carbon nanotubes were used in semi-batch experiments and carbon nanofibres grown on a honeycomb cordierite monolith were tested in continuous experiments. The application of the carbon catalyst was shown to improve the mineralization degree of MTLC and to decrease the toxicity of the solution subject to ozonation. Degradation by-products were also followed in order to compare the two processes. The application of the carbon coated monolith to the continuous ozonation process was shown to have potential as it improved the TOC removal from 5% to 35% and decreased the inhibition of luminescent activity of Vibrio Fischeri from 25% to 12%.

A novel method to regulate the density of BaTiO3 ceramics for positive temperature coefficient resistance via tape casting

BaTiO3 ceramics for positive temperature coefficient resistance (PTCR) were fabricated via tape casting technique and reduction/reoxidation process. A novel method was used to regulate the density of the ceramics via changing binder amount in tape casting green bodies. The influences of different amounts of the binder on density, electrical properties, and microstructure of ceramics were investigated. The BaTiO3 ceramic monolith without internal electrodes with an addition of binder of 0.84 wt% was sintered in reducing atmosphere at 1150 °C and reoxidized at 800 °C for 1 h showed a resistivity of 56 Ω·cm and a PTCR jump about 3.2 orders of magnitude. The green and sintered density of the samples decreased with the increasing binder amount. The room temperature resistivity increased monotonically with the binder amount. However, the correlation of the PTCR jump with binder amount showed a contrary tendency. These electrical properties can be explained by the Jonker model. The influences of sintering temperature on the density of BaTiO3 ceramics with different binder amount were also discussed. In addition to the ceramic monoliths without internal electrodes, multilayer elements with internal electrodes were also prepared.

Measurement of heat flux in dense air-mist cooling: Part I – A novel steady-state technique

This work presents a technique for characterizing water air-mist cooling of surfaces at temperatures from 200–1200°C while subjected to intense spraying conditions, so that the heat flux could reach values in excess of 10MW/m2. In this new steady-state method, a local region of an air-mist/spray impinges upon the exposed surface of a hot Pt-disk (8mm ∅ by 2.5mm thickness) which is surrounded by an induction coil while both are embedded in a cast ceramic monolith. A digital controller adjusts the output power of a high-frequency generator to balance the induction heating of the Pt-specimen with the heat removed by the boiling of the spray droplets impinging on its active surface, in order to maintain the control temperature of the sample at the predetermined set-point. Measurement of the RMS current flowing through the coil to maintain this temperature, together with the solution of a two-dimensional axi-symmetric computational model of the electromagnetic field and heat conduction equations, enabled estimation of the heat extracted locally by the mist. A rigorous experimental procedure was performed to determine the boiling curves from 200°C to 1200°C and then back to 200°C. The curves for both trajectories revealed that strong boiling hysteresis occurred in the nucleate and transition boiling regimes and that this was almost absent in the stable film boiling regime. On the other hand, hysteresis was not present in thermal loops involving surface temperatures only between 600 and 1200°C. The existence of hysteresis, in a certain temperature interval, points out the importance of considering the thermal history of actual cooling processes when simulating them in the laboratory. The ability of the new technique to maintain the sample temperature for prolonged periods, e.g., hours, enables future studies to improve the understanding of boiling phenomena during intense heat-removal processes. In Part II of this article, the effects that the mist droplet size, droplet velocity and water impact density have on the heat extracted under steady-state conditions from a surface with large superheat are investigated for a wide variety of those characteristics.

Building up Multiwall Carbon Nanotubes Nanostructures inside Millimetric Channels of Ceramic Monoliths

Multiwall carbon nanotubes were grown inside the millimetric channels of commercial ceramic materials, cordierite monoliths, using a modified catalytic chemical vapor deposition method. Fe(CO)5 liquid catalyst precursor was introduced in gas phase at the same time as the acetylene carbon source to allow a more uniform distribution of carbon nanotubes on the substrate surface. Different techniques were used for the characterization of the obtained multiwall carbon nanotubes such as scanning and transmission electron microscopies, N2 adsorption isotherms, Raman spectroscopy and thermogravimetric analysis. The results show that the surface of the ceramic monolith is progressively covered with multiwall carbon nanotubes reaching total coverage for longer times under reaction. The carbon nanotubes were built up with a tree-like morphology. So, firstly larger carbon nanotubes are formed as the tree trunks on the cordierite surface, followed by the growing on them of narrower and longer nanotubes suggesting the tree branches.

Evolution of throttle-channel dual pores in YSZ ceramic monolith through in situ grown nano carbon wedges

Abstract Interconnectivity of the pore channels and their size distribution in a ceramic monolith influence the flow dynamics of permeating fluid. This work proposes an innovative pore-forming mechanism that begins with the growth of poly(p-phenylene terephthamide) (PPTA) in a ceramic green body of yttrium-doped zirconia (YSZ). The spatial confinement drove the formation of PPTA through an extrusion manner, resulting in submicron PPTA rods embedded in the matrix. Theses rods were carbonized under Ar between 800 and 1200 °C for 2 h before oxidizing atmosphere was introduced to facilitate their removal and sintering of YSZ. This dwelling temperature is crucial to the pore size distribution in the sintered ceramic due to the chemical and dimensional changes of the carbon wedges and YSZ grains. The sintered ceramic is characterized by interconnected throttle-to-channel pore structure, which manifested a marked improvement on fluid permeability and a capability of stretching viscous fluids passing through it.

Numerical and Experimental Study on a Honeycomb Ceramic Monolith

Honeycomb ceramic is the key component of the regenerative system. The numerical simulation was performed using FLUENT, a commercial computational fluid dynamics (CFD) code, to compare simulation results to the test data. The regenerative process of a honeycomb ceramic regenerator was simulated under different conditions. Experiments were carried out on honeycomb regenerators that are contained in a methane oxidation reactor. The calculated temperatures of flue gas inlet were compared with the ones measured. The tendency of the temperature is the same as the experiment.