Ceramic Monoliths Research Articles

Carbon-Based Honeycomb Monoliths for Environmental Gas-Phase Applications

Honeycomb monoliths consist of a large number of parallel channels that provide high contact efficiencies between the monolith and gas flow streams. These structures are used as adsorbents or supports for catalysts when large gas volumes are treated, because they offer very low pressure drop, short diffusion lengths and no obstruction by particulate matter. Carbon-based honeycomb monoliths can be integral or carbon-coated ceramic monoliths, and they take advantage of the versatility of the surface area, pore texture and surface chemistry of carbon materials. Here, we review the preparation methods of these monoliths, their characteristics and environmental applications.

Methane partial oxidation over Pt–Ru catalyst: An investigation on the mechanism

The mechanism of the partial oxidation of methane has been investigated over a bimetallic Pt–Ru catalyst. Dedicated experiments aimed to separate and quantify the relative contribution of single reactions included in the so-called “combustion and reforming” mechanism using the same catalyst. The catalyst is 0.5% Ru and 0.5% Pt (w/w) supported on mixture of alumina, ceria and zirconia (75/4.4/20.6%, w/w), washcoated on a ceramic monolith. Steam reforming, dry reforming, direct and reverse water-gas shift reactions were investigated. The temperature range investigated is 300 < T < 800 °C, while the space velocity range is 25,000 < GHSV < 100,000 h −1. Conditions at which single side reactions are expected to occur during the partial oxidation process, were approximated by tuning the reactant composition. The experimental results are also compared with thermodynamic equilibrium calculations. The CO and H 2 yields of partial oxidation have been quantitatively connected with steam and dry reforming, while the persistent water-gas shift reaction always rearranges the products and intermediates.

Structured zeolite NaX coatings on ceramic cordierite monolith supports for PSA applications

Novel structured adsorbents in the form of thin zeolite films grown on substrates designed for low pressure drop have a great potential to improve pressure swing adsorption (PSA) processes. In the present work, template free films of NaX zeolite were grown on the walls of ceramic cordierite supports using a seeding technique. The supports had 400 parallel channels per square inch. Films were grown both from a gel and a clear synthesis solution. The materials were analyzed by scanning electron microscopy, X-ray diffraction, N 2 adsorption/desorption measurements, Hg-porosimetry as well as CO 2 breakthrough experiments. When a gel was used for film growth, a film consisting of well intergrown crystals with a thickness of about 1 μm was obtained. However, a large amount of sediments were deposited on top of the film, which resulted in a dispersed CO 2 adsorption breakthrough front. Zeolite films grown in one longer hydrothermal treatment in a clear solution were less intergrown and consisted of both NaX and hydroxysodalite crystals and, in addition, some sediments were deposited on top of the film, which again resulted in a dispersed breakthrough front. By using a multiple-step synthesis procedure and a clear synthesis solution, well intergrown NaX films, free from sediments and with only a very small fraction of hydroxysodalite crystals could be prepared. The CO 2 breakthrough front for the latter adsorbent was sharper than the front for an empty adsorption column and only shifted in time. This indicates that the flow distribution in the adsorbent is even and that the mass transfer resistance in the film is very low due to the small film thickness and high effective diffusivity for CO 2 in the NaX film and still, the adsorption capacity is considerable. The even flow distribution, very low mass transfer resistance and low pressure drop in combination with considerable adsorption capacity in this adsorbent indicates that it is a promising adsorbent for PSA applications. The findings from the present work will be important for the development of structured adsorbents to use as a competitive alternative to traditionally used adsorbents in PSA.

The influence of chemical and thermal aging on the catalytic activity of a monolithic diesel oxidation catalyst

Chemical and thermal aging effects and their influence on the catalytic activity were analyzed for a commercial engine aged diesel oxidation catalyst (DOC). Samples were taken from the inlet (DOCin), the middle (DOCmid) and the outlet (DOCout) area of the ceramic monolith and were compared with a new (DOCnew) and thermal aged (DOC800°C) diesel oxidation catalyst. In all aged DOCs the platinum particles were sintered with particle diameters typically ranging between 10 and 20nm. DOCmid, DOCout and DOC800°C exhibited platinum particle morphology with edges and facets. Round platinum particles without discernable edges and facets were detected in the case of DOCin. DOCin showed two types of continuous, μm-thick contaminant deposits (up to 5 and 20μm) with differing elemental compositions covering the washcoat. Catalytic activity towards the abatement of CO, propene and NO did not show any substantial difference between the different engine aged DOC samples. Thermal aging seems to be the primary cause for the observed loss in catalytic activity compared to DOCnew. However, NO2 production above 275°C was increasing along the monolith in the order of DOCin<DOCmid<DOCout revealing a possible influence of the type of aging on activity towards NO oxidation.

Catalytic Combustion of Toluene over a Copper‐Manganese‐Silver Mixed‐Oxide Catalyst Supported on a Washcoated Ceramic Monolith

AbstractMonolithic catalysts were prepared by washcoating an alumina sol and then impregnating Cu‐Mn‐Ag mixed oxides onto cordierite substrates. The effects of the preparation parameters including the Ag/Cu/Mn ratio, the total amount of active phase and the loading of washcoat, and the reaction conditions, e.g., the space velocity and the oxygen/toluene ratio on the catalytic performance for the combustion of toluene were investigated. It is shown that the Cu‐Mn‐Ag oxides are very active for the combustion of toluene and that the highest catalytic activity is achieved over a monolithic catalyst containing 14.7 wt % of washcoat and 21.2 wt % of active phase with a Ag/Cu/Mn molar ratio of 13.8/43.1/43.1. It is also seen that the optimum catalyst has a good catalytic stability and exhibits an excellent activity not only at a rather high space velocity but also within a wide range of oxygen/toluene ratios.

H 2O-tolerant monolithic catalysts for preferential oxidation of carbon monoxide in the presence of hydrogen

Pt–Fe/mordenite (4 wt% Pt–0.5 wt% Fe) powder catalysts were wash-coated onto ceramic straight-channel monoliths by using silica- and/or alumina-sol as a binder, and were evaluated for the preferential oxidation of carbon monoxide (PROX) in a hydrogen-rich gas. In a synthetic reformate gas (1% CO, 1% O 2, 5% H 2O, 20% CO 2, and balance H 2), the CO concentration was reduced to less than 20 ppm at temperatures ranging from 100 to 130 °C. After a certain period of the PROX reaction, condensation of H 2O in the pores of the mordenite-support occurred over the monolithic catalyst, which was wash-coated with alumina-sol, in the lower temperature range (100–120 °C), resulting in a rapid increase in CO concentration. The monolithic catalyst wash-coated with silica-sol, however, showed an excellent tolerance against H 2O condensation and offered a stable catalytic performance, maintaining a CO concentration of ca. 20 ppm for 200 h. The H 2O-tolerant characteristic was attributed to the relatively small adsorption amount of H 2O over the silica-modified monolithic catalyst.

Microwave-Assisted Self-Propagating Combustion Synthesis for Uniform Deposition of Metal Nanoparticles on Ceramic Monoliths

A new microwave-assisted gel-combustion synthesis in self-propagating mode was developed for the deposition of adhesive metal nanoparticles on ceramic substrates. Pt particles with diameters less than 10 nm uniformly distributed on corundum foams were prepared by the fast and reliable combustion method with no additional calcination or reduction steps. The results are rationalized in terms of complexation of the Pt precursor by glycerol, reduction of the Pt precursor to colloidal Pt nanoparticles, followed by carburization of the organic ligands and rapid carbon burn off preserving the high dispersion and uniformity of the Pt nanoparticles on the foam support.

Structuration of Pd(2 wt %)/Fe−Al Oxide Catalysts on Ceramic and Metallic Monoliths: Physicochemical Characterization, Effect of the Nature of the Slurry, and Comparison with LaMnO3Catalysts

The main objective of this study has been twofold: (i) to investigate the effect of the nature of the slurry used to structurate a catalyst oil monoliths; (ii) to study the influence of the nature of the monoliths oil the physicochemical properties and on the catalytic activities in the combustion of methane. The slurry used in this work was made with an acidified solution of Pd(2 wt %)/Fe-Al oxide catalyst. This catalyst was prepared by the citrate method and deposited oil ceramic and metallic monoliths according to a dip coating or an orbital stirring procedure. Both types of structured catalysts were studied using the following physicochemical techniques: specific surface area measurements (BET), scanning electron microscopy (SEM), X-ray diffraction (XRD), and X-ray photoelectron spectroscopy (XPS). The Pd(2 wt %)[Fe-Al oxide catalyst used for monolith coating does not undergo any textural or crystallinity modification during the deposition procedure. Catalytic activities obtained for powder and monolithic catalysts are compared. The Surface of the support plays an important role in the structuration of catalysts. The amount of deposited catalyst has an influence on the catalytic activity. The amount of deposited catalysts depends oil the nature of the monolith, oil the coating technique, and oil the nature of the Slurry. Comparisons between structured Pd(2 wt %)/Fe-Al oxide and structured LaMnO3 on ceramic and metallic monoliths (previous work) were made to clarify the influence of the nature of the slurry oil the amount of deposited catalysts. The amount of deposited catalysts oil ceramic monolith using Pd(2 wt %)[Fe-Al oxide catalyst is lower than that deposited using LaMnO3, In the case of metallic monoliths, the difference is less significant. The influence of a diffusion limitation of the reaction rate in structured catalysts with Pd(2 wt %)/Fe-Al oxide seems to be less important than with LaMnO3. Higher catalytic activities were observed for Pd(2 wt %)/Fe-Al oxide on the ceramic monoliths at high temperature. Coating oil the metallic monoliths led to a better activity at low temperature compared to the corresponding powder catalyst.

Composite ceramic monoliths with poly(ester acrylate) and poly(ester acrylate)-poly(N-vinylcaprolactam) coatings

Polymerization of a series of oligo(ester acrylates) on the surface of ceramic monoliths was studied with the aim to prepare composite monolithic sorbents, highly porous permeable ceramic materials with polymeric coatings. A chelating organoceramic sorbent was prepared by copolymerization of an oligoester with N-vinylcaprolactam on a ceramic support.

Catalytic partial oxidation of n-butane over Rh catalysts for solid oxide fuel cell applications

Catalytic partial oxidation (CPOx) of n-butane in O 2/Ar mixtures at ratios of 1/3.76 has been investigated over Rh catalysis impregnated in γ-Al 2O 3 washcoat supports coated on α-Al 2O 3 ceramic foam monoliths. The impact of heat loss on Rh-based CPOx reactor performance for small-scale SOFC application is tested here by varying reactor lengths, insulation, and flow conditions. Results for a C/O ratio = 1.0 and T in = 300 °C indicate the n-butane conversion and H 2 and CO selectivity depend strongly upon reactor temperature and thus heat lost through the external reactor walls. Because heat loss does not scale with flow rate, higher flow rates for catalyst contact times approaching 15 ms increase fuel conversion and syngas selectivity. Furthermore, shortening catalyst length (from 10 to 5 mm) also reduces heat loss and increases fuel conversion and H 2 selectivity. CO selectivity, however, drops with the shorter length. These results suggest that for the conditions tested, butane conversion and H 2 production occur rapidly within the first couple of mm in the reactor, whereas CO production occurs more broadly throughout the reactor length. This study provides valuable insight into the conditions needed for good CPOx performance for fuel processing in small-scale SOFC applications.

Influence of the preparation procedure of NSR monolithic catalysts on the Pt-Ba dispersion and distribution

The preparation procedure of a structured NO x storage/reduction catalyst is investigated by optimizing washcoating of ceramic monoliths and subsequent incorporation of Pt and Ba as the active phases. Endurance and firm adherence of the alumina washcoat to the substrate has been achieved. Incorporation of platinum from Pt(NH 3) 4(NO 3) 2 has been studied by impregnation (filling the monolith channels) and adsorption from solution (ion exchange). Highly dispersed and well distributed platinum were obtained by the adsorption procedure at pH 11–12 and calcination temperature no higher than 450–500 °C. Barium was incorporated by dry or wet impregnation. The dry impregnation procedure led to a more homogeneous barium distribution on the surface of washcoat. The sample prepared by adsorption from solution and dry impregnation, for incorporation of Pt and Ba respectively, showed the best storage capacity, ca. around 2.1 μmol NO x per mg of BaO for a feedstream with 380 ppm NO and 6% O 2 in nitrogen, and a space velocity of 32,100 h −1, and also complete release and reduction of the stored NO x when oxygen was shifted to 2.3% H 2. Thus, a relation between good NO x storage behavior and Pt dispersion and Pt-Ba distribution, and the preparation procedure has been established.

Cu-ZSM5 based monolith reactors for NO decomposition

Micronic ZSM5 (Si/Al = 25) powder has been deposited on cordierite monoliths by dip-coating technique using colloidal alumina or silica as binders with different zeolite/binder ratios in aqueous suspension determining the parameters to obtain the best adhesion and mechanical resistance of the washcoat layer. For monolithic catalysts with washcoat containing 10 wt% SiO 2 the following alternative preparation procedures have been used: (i) deposition of a pre-exchanged Cu-ZSM5 powder on the ceramic monolith; (ii) deposition of a H-ZSM5 powder on the ceramic monolith followed by copper exchange of the structured catalyst. The monolithic catalysts have been investigated by morphological (SEM), physical (BET and porosity), chemical (ICP-MS, EDX, XRD, H 2 TPR) and catalytic (NO decomposition tests in flow reactor under dry and wet conditions) analyses. Results have been compared with those obtained on the corresponding Cu-ZSM5 powder sample with the same copper content and Si/Al ratio in order to define the effect of the preparation procedure on the chemical and catalytic properties of the monoliths.

Pd/Al2O3-cordierite and Pd/Al2O3-Fecralloy monolithic catalysts for the hydrogenation of sunflower oil

Abstract Monolithic catalysts were prepared using cordierite and Fecralloy as substrates, with Pd/Al 2 O 3 catalyst deposited by a washcoating method. The samples were characterized and studied for the partial hydrogenation of sunflower oil at 373 K and 413 kPa. In comparative tests, at equal operating conditions and conversion, the structured catalysts presented a lower trans -isomers formation than the powder catalyst. After three consecutive tests, the ceramic monolith showed no change in activity or selectivity, whereas the metallic monolith showed a partial deactivation (10%), which is attributed to a loss of catalyst.

Towards practical application of lanthanum ferrite catalysts for NO reduction with H 2

A series of perovskite-type catalysts (LaFeO 3, La 0.8Sr 0.2FeO 3, Pd/La 0.8Sr 0.2FeO 3, La 0.8Sr 0.2Fe 0.9Pd 0.1O 3, La 0.7Sr 0.2Ce 0.1FeO 3, Pd/La 0.7Sr 0.2Ce 0.1FeO 3 and La 0.7Sr 0.2Ce 0.1Fe 0.9Pd 0.1O 3) has been prepared by the solution combustion synthesis method and fully characterized by XRD, BET, FESEM and TPD/R analyses. The performance of these catalysts towards the NO reduction mechanism by H 2 has been evaluated in a temperature-programmed reaction apparatus (TPRe) in the absence and in the presence of oxygen. The catalysts have been studied in the 25–350 °C temperature range, and significant catalytic activities were measured at 150–250 °C. Among the catalysts screened, La 0.8Sr 0.2Fe 0.9Pd 0.1O 3, showed the best performance. Hence, it was deposited directly over a ceramic honeycomb monolith by in situ SCS, tested in a lab-scale test rig, then submitted to the specific ageing protocol (thermal treatment at 500 °C for 96 h) in the presence of water vapour and sulfur dioxide, two potentially deactivating species present in diesel exhaust gases, and it showed a good compromise between satisfactory catalytic activity and stability. A mechanistic analysis is presented concerning the relationship between the observed activity and the reducibility of the B site, determined through TPR experiments. Some final conclusions are drawn on the perspective of the practical application of the investigated aftertreatment route for diesel exhaust gases.

Comparison of ceramic honeycomb monolith and foam as Ni catalyst carrier for methane autothermal reforming

Structured ceramic honeycomb monolith and open cell foams were employed as Ni catalyst carrier for methane autothermal reforming (CH 4-ATR). The catalysts were prepared through washcoating of the supports with a commercial CeO 2–Al 2O 3-based slurry and further impregnation into aqueous solution of nickel acetate tetrahydrate. A quite good reproducible preparation procedure was individuated in terms of both washcoat loading and resistance to ultrasonic vibrations and thermal shock tests. The catalysts were characterized by means of several techniques such as XRD, SEM, N 2 adsorption at −196 °C, Hg porosimetry and H 2-TPR. The catalytic activity towards CH 4-ATR was tested in kW-scale autothermal reforming reactor which is able to produce up to 5 m 3(stp)/h of H 2. The reactor is thermally integrated with two heat exchangers for the preheating of the air and liquid water fed to the system, thus being completely self-sustained and able to perform without any external heat source. The tests carried out aiming to evaluate the influence of the O 2/CH 4 ratio on catalytic performance showed the higher activity of foam structured samples which can provide a lower average temperature in the catalytic bed.

NO SCR reduction by hydrogen generated in line on perovskite-type catalysts for automotive diesel exhaust gas treatment

This paper deals with the preparation (by solution combustion synthesis, SCS), characterization (by XRD, BET, FESEM and TPD/R analyses), catalytic activity evaluation (in a temperature-programmed reaction—TPRe apparatus), and the assessment of the reaction mechanism of NO reduction by H 2 in the presence of oxygen on a series of perovskite-type catalysts belonging to the LaFeO 3 family (LaFeO 3, La 0.8Sr 0.2FeO 3, Pd/La 0.8Sr 0.2FeO 3, La 0.8Sr 0.2Fe 0.9Pd 0.1O 3, La 0.7Sr 0.2Ce 0.1FeO 3, Pd/La 0.7Sr 0.2Ce 0.1FeO 3, La 0.7Sr 0.2Ce 0.1Fe 0.9Pd 0.1O 3). The catalysts have been studied in the 25–350 °C temperature range. Significant catalytic activities were measured at 150–250 °C. Among the catalysts screened, La 0.8Sr 0.2Fe 0.9Pd 0.1O 3, showed the best performance. Hence, it was deposited directly over a ceramic honeycomb monolith by in situ SCS and then tested in a lab-scale test rig. A mechanistic analysis is presented concerning the relationship between the observed activity and the reducibility of the B site, determined from TPR experiments, as well as the correlation between the observed oxygen inhibition and the proposed NO x reduction mechanism. Some final conclusions are drawn on the perspective of the practical application of the investigated after-treatment route for diesel exhaust gases.

Feasibility study of In Situ CO2 capture on an integrated catalytic CO2 sorbent for hydrogen production from methane

Abstract The capture of CO 2 on the novel CaO on γ-Al 2 O 3 sorbent was demonstrated experimen tally in the temperature regime of the steam meth ane reforming process. Thermogravimetric analysis illustrated that both capture and regeneration of CO 2 can take place at 650 ∘ C; however, the kinetics of the two processes are observed to be very dissimilar. The sorbent was successfully wash-coated on a ceramic monolith and a flow-through reactor test of the monolith illustrated the feasibility of CO 2 capture from a simulated gas stream.

Hierarchically Porous Ceramics from Diatomite Powders by Pulsed Current Processing

Hierarchically porous ceramic monoliths have been fabricated by pulsed current processing (PCP) of diatomite powders. The partial sintering behavior of the porous diatomite powders during PCP or spark plasma sintering was evaluated at temperatures between 600° and 850°C. Scanning electron microscopy and mercury porosimetry measurements showed that the PCP method was able to bond the diatomite powder together into relatively strong monoliths without significantly destroying the internal pores of the diatomite powder at a temperature range of 700°–750°C. Little fusion at the particle contact points occurred at temperatures below 650°C while the powder showed partial melting and collapse of both the interparticle pores and the internal structure at temperatures above 800°C.

High-Temperature Nanoporous Ceramic Monolith Prepared from a Polymeric Bicontinuous Microemulsion Template

Nanoporous ceramic with a unique pore structure was derived from an all-hydrocarbon polymeric bicontinuous microemulsion (BmuE). The BmuE was designed to allow facile removal of one phase, resulting in a nanoporous polymer monolith with BmuE-like structure. The pores were filled with a commercially available, polymeric precursor to nonoxide, Si-based ceramics. Pyrolysis resulted in a monolith of nanoporous ceramic, stable to at least 1000 degrees C, with a BmuE-like pore structure. The pore structure is disordered and 3-D continuous. Microscopy and gas sorption measurements suggest a well-defined pore size distribution spanning roughly 60-100 nm, sizes previously unattainable through related techniques.

Effect of chromium migration from metallic supports on the activity of diesel exhaust catalysts

Fe–Cr–Ni and Fe–Cr–Al sintered spheres are explored as alternative support structures for automotive exhaust catalysts. Initially the activity of Pt supported on the metal support and on ceramic monoliths (cordierite) for the oxidation of CO and NO is comparable. However, after thermal aging at elevated temperatures (800°C) and extended reaction times (24h) the activity for the oxidation of NO of the catalysts supported on the Fe–Cr–Ni spheres decreases significantly, which is attributed to the migration of chromium from the metal bulk phase into washcoat. In the contrary, the aluminium present in the Fe–Cr–Al steel alloy formed a protective alumina barrier inhibiting the degradation of the catalyst after aging by blocking the migration of chromium to the surface.