Catalytic wall reactor: Catalytic coatings of stainless steel by VOx/TiO2 and Co/SiO2 catalysts

Catalytic wall (structured) reactors and structured supports are suitable to study the catalytic properties of nanosized materials. The coating of metallic (aluminum and stainless steel) plates by thin layers of active phase is presented in two cases, VO_x/TiO₂ and Co/SiO₂, catalysts used in the oxidative dehydrogenation (ODH) of propane and in Fischer–Tropsch synthesis (FTS) of clean fuels, respectively. The preparation of coated plates and their characterisation by various methods of physicochemical analysis are described. Both chemical and physical methods were used for coating. VO_x/TiO₂ layers were obtained by grafting of Ti (on Al or stainless-steel plates) and V (on TiO₂) alkoxides and use of sol–gel media or suspension. A silica primer was deposited (on stainless-steel plate) by plasma-assisted chemical vapour deposition (PACVD) onto which Co oxide and silica were coprecipitated from sol–gel. The catalytic experiments in the respective reactions were carried out in special plate reactors and compared with those of catalytic powders. The study shows that the coating of a metallic substrate by a catalyst is not straightforward and requires specific studies dealing with both chemistry (chemical affinity between substrate and catalytic layers) and catalytic engineering (catalytic performance in taylor-made reactors).     

Low-temperature SCR of NO with NH3 over AC/C supported manganese-based monolithic catalysts

Metal oxide/active carbon/ceramic (MO_x/AC/C) monolithic catalysts were prepared by impregnation method for selective catalytic reduction (SCR) of NO_x with NH₃ at low-temperature, and they also had been characterized by elemental analysis, N₂-BET, XRD, SEM and NO-TPD. The adsorption capability of the monolithic catalyst was greatly enhanced due to the attached active carbon. An ultrasonic treatment was used to improve the impregnation process, and which can increase their catalytic activities. More than 90% NO_x conversion could be achieved over the Mn-based monolithic catalysts at low-temperature, and which could be improved further by doping Ce, from 30% to 78% at 100 °C. Mn–Fe–Ce and Mn–V–Ce monolithic catalysts had better tolerance to SO₂ than Mn or Mn–Ce monolithic catalysts.     

Deactivation studies of the SCR of NOx with hydrocarbons on Co-mordenite monolithic catalysts

The catalytic reduction of NO_x with hydrocarbons (butane or methane) on CoMOR washcoated monolithic catalysts was studied in the presence of steam and excess oxygen. The significant changes observed in the catalytic behavior of CoMOR powder and monoliths depended essentially on the hydrocarbon nature (carbon number) and the concentration of water in the feed. When the reducing agent was methane, a low concentration of water (2%) decreased the NO to N₂ conversion. However, when butane was used instead of methane, the maximum NO_x conversions increased from 50 to 58% and from 52 to 64% for the CoMOR powder and monolith, respectively. The presence of water inhibited the NO adsorption when the reducing agent was methane but when butane was used, water helped to remove the surface-carbon deposits as indicated by TPO and XPS results. This fact explains the increase observed in the NO_x conversion. The characterization with TPR and UV–vis spectroscopy showed that the main Co species present in the selective catalysts were the Co(II) ions exchanged at different sites of the mordenite and highly dispersed Co_xO_y moieties. More rigorous reaction conditions, i.e. 10% of water, led to the irreversible deactivation with both reductants. The Co₃O₄ phase was detected in all the deactivated powder and monolithic catalysts. The Co₃O₄ spinel was formed from the cobalt ion migration, which was promoted in wet atmosphere. In addition, for monolithic catalysts washcoated with CoMOR, the silica binder inhibited the water deactivation effect probably due to the silica–cobalt interaction, as a Co_xO_ySi silicate.     

Hysteresis in activity of microporous lean NOx catalysts in the presence of water vapor

Increasingly stringent ambient air quality standards coupled with the need to improve fuel economy has drawn significant attention to the search for emission control systems for lean burn engine vehicles. Much of the focus has been on zeolite-based catalysts for the conversion of NO_x to N₂ for automotive exhaust emission control. Under certain conditions, these catalysts are highly active catalysts for the reduction of NO_x using hydrocarbons as the reductant. However, many of these catalysts suffer from a variety of deactivation processes such as irreversible poisoning by SO_x or hydrothermal dealumination.

In addition to these deactivation processes, a recent focus of our research has been on the influence of water vapor on the activity of zeolite-based catalysts at low operating temperatures. We observe a hysteresis in catalytic activity of lean NO_x reduction (NO feed concentrations <100 ppm) upon increasing and decreasing temperature ramps at the low end of the operating window, that being from 100 to 300 °C using hydrocarbons as reductants. We describe these reversible influences of water vapor and the implications for this hysteresis in catalytic activity for the application of zeolite-based catalysts in lean NO_x catalysis, and compare these results to the instance of using ammonia as reductant.     

规整结构催化剂及反应器研究进展

介绍了一种新颖的规整结构催化剂。它在废气排放处理等气相催化反应中已表现出优于常规催化剂的优良催化性能,在气液固多相催化反应领域的研究也表明它能够应用于更多的催化反应。将这种催化剂和反应器结构特性与常规催化反应中应用的固体催化剂和反应器进行了比较,结果表明它有可能替代浆态床和固定床反应器,具有良好的应用前景。     

On the promotional effect of Pd on the propene-assisted decomposition of NO on chlorinated Ce0.68Zr0.32O2

The selective catalytic reduction (SCR) of NO_x assisted by propene is investigated on Pd/Ce_0.68Zr_0.32O₂ catalysts (Pd/CZ), and is compared, under identical experimental conditions, with that found on a Pd/SiO₂ reference catalyst. Physico-chemical characterisation of the studied catalysts along with their catalytic properties indicate that Pd is not fully reduced to metallic Pd for the Pd/CZ catalysts. This study shows that the incorporation of Pd to CZ greatly promotes the reduction of NO in the presence of C₃H₆. These catalysts display very stable deNO_x activity even in the presence of 1.7% water, the addition of which induces a reversible deactivation of about 10%. The much higher N₂ selectivity obtained on Pd/CZ suggests that the lean deNO_x mechanism occurring on these catalysts is different from that occurring on Pd⁰/SiO₂. A detailed mechanism is proposed for which CZ achieves both NO oxidation to NO₂ and NO decomposition to N₂, whereas PdO_x activates C₃H₆ via ad-NO₂ species, intermediately producing R-NO_x compounds that further decompose to NO and C_xH_yO_z. The role of the latter oxygenates is to reduce CZ to provide the catalytic sites responsible for NO decomposition. The proposed C₃H₆-assisted NO decomposition mechanism stresses the key role of NO₂, R-NO_x and C_xH_yO_z as intermediates of the SCR of NO_x by hydrocarbons.     

Pillared clay and zirconia-based monolithic catalysts for selective catalytic reduction of nitric oxide by methane

New monolithic catalysts based on zirconia and pillared clays (PILC) have been studied for NO_x removal by CH₄ in the presence of oxygen. A comparative study of the influence of ZrO₂ from various commercial sources for the system Pd–ZrO₂ and the effect of the noble metal chosen for the system NM–PILC was carried out, trying to correlate the catalytic activity with the physico-chemical properties of these catalysts. The obtained results indicate that structure and surface acidity of the support plays an important role on the selectivity to NO_x reduction, although properties such as the surface area or pore volume could also determine the overall activity of the monolithic catalysts.     

Analysis of the thermodynamic feasibility of NOx decomposition catalysis to meet next generation vehicle NOx emissions standards

Free energy minimization calculations are used to determine the thermodynamic equilibrium concentrations of NO_x and other species in stoichiometric and lean gas mixtures over a range of temperatures and compositions. Under lean (excess N₂ and O₂) conditions, the NO decomposition (NO↔(1/2)N₂+(1/2)O₂) and NO oxidation (NO+(1/2)O₂↔NO₂) equilibria impose lower bounds on the NO_x concentrations achievable by thermodynamic equilibration or NO_x decomposition, and these equilibrium NO_x concentrations can be practically significant. Assuming a perfect isothermal catalyst acting on a representative diesel exhaust stream collected over the federal test procedure (FTP) cycle, equilibrium NO_x levels exceed upcoming California Low Emission Vehicle II (LEV-II) and Tier II NO_x emissions standards for automobiles and trucks at temperatures above approximately 800 K. Consideration of a perfect adiabatic catalyst acting on the same diesel exhaust shows that equilibrium NO_x values can fall below NO_x emissions standards at lower temperatures, but to achieve these low concentrations would require the catalyst to attain 100% approach to equilibrium at very low temperatures. It is concluded that NO_x removal based on a thermodynamic equilibrating catalyst under lean exhaust conditions is not practically viable for automotive application, and that to achieve upcoming NO_x standards will require selective NO_x catalysts that vigorously promote NO_x reactions with reductant and do not promote NO decomposition or oxidation. Finally, the ability of a selective NO_x catalyst system to reduce NO_x concentrations to or below thermodynamic equilibrium values is proposed as a useful measure for selective catalytic reduction (SCR) activity.     

Engineering the electronic and geometric structure of VOx/BN@TiO2 heterostructure for efficient aerobic oxidative desulfurization

Particle size governs the electronic and geometric structure of metal nanoparticles (NPs), shaping their catalytic performances in heterogeneous catalysis. However, precisely controlling the size of active metal NPs and thereafter their catalytic activities remain an affordable challenge in ultra-deep oxidative desulfurization (ODS) field. Herein, a series of highly-efficient VO_x/boron nitride nanosheets (BNNS)@TiO₂ heterostructures, therein, cetyltrimethylammonium bromide cationic surfactants serving as intercalation agent, BNNS and MXene as precursors, with various VO_x NP sizes were designed and controllably constructed by a facile intercalation confinement strategy. The properties and structures of the prepared catalysts were systematically characterized by different technical methods, and their catalytic activities were investigated for aerobic ODS of dibenzothiophene (DBT). The results show that the size of VO_x NPs and V⁵⁺/V⁴⁺ play decisive roles in the catalytic aerobic ODS of VO_x/BNNS@TiO₂ catalysts and that VO_x/BNNS@TiO₂-2 exhibits the highest ODS activity with 93.7% DBT conversion within 60 min under the reaction temperature of 130 °C and oxygen flow rate of 200 mL·min^–1, which is due to its optimal VO_x dispersion, excellent reducibility and abundant active species. Therefore, the finding here may contribute to the fundamental understanding of structure-activity in ultra-deep ODS and inspire the advancement of highly-efficient catalyst.     

Bench-scale demonstration of an integrated deSoot–deNOx system

A catalytic deSoot–deNO_x system, comprising Pt and Ce fuel additives, a Pt-impregnated wall-flow monolith soot filter and a vanadia-type monolithic NH₃-SCR catalyst, was tested with a two-cylinder DI diesel engine. The soot removal efficiency of the filter was 98–99 mass% with a balance temperature (stationary pressure drop) of 315 °C at an engine load of 55%. The NO_x conversion ranged from 40 to 73%, at a NH₃/NO_x molar ratio of 0.9. Both systems were measured at a GHSV of 52 000 l/(l h). The maximum NO_x conversion was obtained at 400 °C. The reason for the moderate deNO_x performance is discussed. No deactivation was observed after 380 h time on stream. The NO_x emission at high engine loads is around 15% lower than that of engines running without fuel additives.     

Correlation between the surface properties and deNOx activity of ceria-zirconia catalysts

The catalytic activities of ceria-zirconia mixed oxides Ce_xZr_1−xO₂ (x = 0.17, 0.42, 0.62 and 0.8) are determined by isothermal steady-state experiments using a representative mixture of exhaust gases of coal combustion. Results show that all supports are active in deNO_x reaction in the presence of the previous hydrocarbons. However, their catalytic activity varies with the content of cerium and goes through a maximum for x = 0.62, leading to 27% NO_x consumption. This activity was correlated with physicochemical properties determined by a linear solvation energy relationship (LSER) approach. Moreover, a mechanism of HC assisted reduction of NO is proposed on ceria-zirconia supported catalysts. This mechanism is divided in three catalytic cycles involving: (i) the oxidation of NO into NO₂, (ii) the reaction of NO₂ and the hydrocarbons leading to RNOx species and C_xH_yO_z, and finally (iii) the decomposition of NO assisted by these latter C_xH_yO_z species.     

Influence of support acid pretreatment on the behaviour of CoOx/γ-alumina monolithic catalysts in the CH4-SCR reaction

The effect of treatment with different mineral acids (H₂SO₄, H₃PO₄, HNO₃ and HCl) on the activity of monolithic CoO_x/γ-Al₂O₃ catalysts in the reduction of nitric oxide with methane in the presence of oxygen (CH₄-SCR of NO_x) was studied. Their behaviour in the methane oxidation reaction in both the presence and absence of NO_x was determined in order to interpret the results in terms of intrinsic activity and competition between both processes. Depending on the nature of the acid used, significant differences were observed in the catalytic activities which were related to the textural states, surface acidities and the nature of the detected species. The best results were obtained after treatment with H₂SO₄, which increased the activity towards NO_x elimination compared to the other catalysts. This behaviour was attributed not only to an increase in surface acidity but also to the stabilisation of the active Co²⁺ species, thus avoiding the formation of Co₃O₄ spinel that is responsible for the strongly adsorbed NO_x species that lead to NO₂ formation which increase the rate of the undesired methane oxidation reaction at high temperatures.     

Influence of zirconia raw materials on the development of DeNOx monolithic catalysts

Pd-zirconia-based monolithic catalysts were prepared with various commercial zirconia raw materials and a natural magnesium silicate binder, sepiolite, for the selective catalytic reduction (SCR) of NO with CH₄ in oxygen excess. The different textural properties, metastable tetragonal zirconia phase stability, surface acidity, Pd dispersion and catalytic properties of these monoliths were compared to select the most suitable structured catalyst for NO_x control in natural gas-fired power plants. The influence of operating temperature in the two reactions, NO reduction and CH₄ combustion, with the monolithic catalysts was determined. A 0.4 wt.% Pd-zirconia catalyst, manufactured from a sulphated zirconium hydroxide raw material, was selected as the most appropriate in the reaction under study, reaching a maximum NO conversion at 400 °C.     

Experimental methods for activity measurements in environmental catalysis

The very specific operating conditions of catalytic processes for the abatement of pollutants from mobile or fixed sources, require an unusual experimental set-up to determine the catalyst performance. Selected examples of environmental catalysis applications such as deNO_x with NH₃ or hydrocarbons, deSO_x with either powder or monolith catalysts, low temperature soot oxidation, are discussed with respect to the experimental methods to be employed for catalytic activity measurements.     

Selective catalytic reduction of NOx with ethanol and other C1-4 oxygenates over Ag/Al2O3 catalysts: A review

Research results regarding selective catalytic reduction (SCR) of NO_x with ethanol and other C_1-4 oxygenates as reductants over silver-alumina catalysts are summarized. The aspects of the process mechanism, nature of active sites, role of alumina and silver (especially in the formation of bifunctional active sites), effects of reductants and reaction conditions are discussed. It has been determined that key stages of the process include formation of reactive enolic species, their interaction with NO_x and formation of nitroorganic compounds which transform to NCO_ads species and further to N₂. The results obtained over various silver-alumina catalysts demonstrate the perspectives of their application for reducing the level of nitrogen oxides in engine emissions, including in the presence of water vapor and sulfur oxides. Ways to improve the catalysts for the SCR of NO_x with C_1-4 oxygenates are outlined.     

Nature of nitrogen specie in coke and their role in NOx formation during FCC catalyst regeneration

NO_x emission during the regeneration of coked fluid catalytic cracking (FCC) catalysts is an environmental problem. In order to follow the route to NO_x formation and try to find ways to suppress it, a coked industrial FCC catalyst has been prepared using model N-containing compounds, e.g., pyridine, pyrrole, aniline and hexadecane–pyridine mixture. Nitrogen present in the FCC feed is incorporated as polyaromatic compounds in the coke deposited on the catalyst during cracking. Its functionality has been characterized using XPS. Nitrogen specie of different types, namely, pyridine, pyrrolic or quaternary-nitrogen (Q-N) have been discriminated. Decomposition of the coke during the catalyst regeneration (temperature programmed oxidation (TPO) and isothermal oxidation) has been monitored by GC and MS measurements of the gaseous products formed. The pyrrolic- and pyridinic-type N specie, present more in the outer coke layers, are oxidized under conditions when still large amount of C or CO is available from coke to reduced NO_x formed to N₂. “Q-N” type species are present in the inner layer, strongly adsorbed on the acid sites on the catalyst. They are combusted last during regeneration. As most of the coke is already combusted at this point, lack of reductants (C, CO, etc.) results in the presence of NO_x in the tail gas.     

Kinetic modelling of sulfur deactivation of Pt/BaO/Al2O3 and BaO/Al2O3 NOx storage catalysts

In this work, a kinetic model is constructed to simulate sulfur deactivation of the NO_x storage performance of BaO/Al₂O₃ and Pt/BaO/Al₂O₃ catalysts. The model is based on a previous model for NO_x storage under sulfur-free conditions. In the present model the storage of NO_x is allowed on two storage sites, one for complete NO_x uptake and one for a slower NO_x sorption. The adsorption of SO_x is allowed on both of these NO_x storage sites and on one additional site which represent bulk storage. The present model is built-up of six sub-models: (i) NO_x storage under sulfur-free conditions; (ii) SO₂ storage on NO_x storage sites; (iii) SO₂ oxidation; (iv) SO₃ storage on bulk sites; (v) SO₂ interaction with platinum in the presence of H₂; (vi) oxidation of accumulated sulfur compounds on platinum by NO₂. Data from flow reactor experiments are used in the implementation of the model. The model is tested for simulation of experiments for NO_x storage before exposure to sulfur and after pre-treatments either with SO₂ + O₂ or SO₂ + H₂. The simulations show that the model is able to describe the main features observed experimentally.     

Environmental catalysis: trends and outlook

Environmental catalysis has continuously grown in importance over the last 2 decades not only in terms of the worldwide catalyst market, but also as a driver of advances in the whole area of catalysis. The development of innovative “environmental” catalysts is also the crucial factor towards the objective of developing a new sustainable industrial chemistry. In the last decade, considerable expansion of the traditional area of environmental catalysis (mainly NO_x removal from stationary and mobile sources, and VOC conversion) has also occurred. New areas include: (i) catalytic technologies for liquid or solid waste reduction or purification; (ii) use of catalysts in energy-efficient catalytic technologies and processes; (iii) reduction of the environmental impact in the use or disposal of catalysts; (iv) new eco-compatible refinery, chemical or non-chemical catalytic processes; (v) catalysis for greenhouse gas control; (vi) use of catalysts for user-friendly technologies and reduction of indoor pollution; (vii) catalytic processes for sustainable chemistry; (viii) reduction of the environmental impact of transport. Therefore, a significant change has occurred in the last decade in the areas of interest regarding environmental catalysts and in the modality of approaching the research. This review, based on but not limited to the workshop “Environmental Catalysis: A Step Forward” (Maiori, Italy, May 2001), introduces the proceedings of this workshop reported in this issue of Catalysis Today and has the objective of providing an overview to the topic and setting the basis for a step forward in environmental catalysis research.     

Electronic and chemical properties of mixed-metal oxides: basic principles for the design of DeNOx and DeSOx catalysts

Correlations between the electronic and chemical properties of perovskites, molybdates, and metal-doped MgO or CeO₂ are examined. Simple models based on band-orbital mixing can explain trends found for the interaction of these catalytic materials with adsorbates: the less stable the occupied levels of a mixed-metal oxide, the higher its chemical reactivity. Metal↔oxygen↔metal interactions are common in mixed-metal oxides and can lead to substantial changes in the electronic and chemical properties of the cations. This is particularly true in the case of ABO₃ perovskites (A=Pb, Ca, Sr, Li, K, Na; B=Ti, Zr, Nb), and it is an important phenomenon that has to be considered when mixing AO and BO₂ oxides for catalytic applications. In systems like Ce_1−xZr_xO₂ and Ce_1−xCa_xO₂, the structural stress induced by the dopant (Zr or Ca) leads to perturbations in the electronic properties of the Ce cations. The trends in the behavior of metal-doped MgO illustrate a basic principle in the design of mixed-metal oxide catalysts for DeNO_x and DeSO_x operations. The general idea is to find metal dopants that upon hybridization within an oxide matrix remain in a relatively low oxidation state and at the same time induce occupied electronic states located well above the valence band of the host oxide. Electronic effects should not be neglected a priori when explaining the behavior or dealing with the design of mixed-metal oxide catalysts.     

Abatement of diesel-exhaust pollutants: NOx storage and soot combustion on K/La2O3 catalysts

Potassium-loaded lanthana is a promising catalyst to be used for the simultaneous abatement of soot and NO_x, which are the main diesel-exhaust pollutants. With potassium loadings between 4.5 and 10 wt.% and calcination temperatures between 400 and 700 °C, this catalyst mixed with soot gave maximum combustion rates between 350 and 400 °C in TPO experiments, showing a good hydrothermal stability. There was no difference in activity when it was either mixed by grinding in an agate mortar or mixed by shaking in a sample bottle (tight and loose conditions, respectively). Moreover, when the K-loaded La₂O₃ is used as washcoat for a cordierite monolith, there were found no significant differences in the catalytic behaviour of the system, which implies its potentiality for practical purposes.

The influence of poisons as water and SO₂ was investigated. While water does not affect the soot combustion activity, SO₂ slightly shift the TPO peak to higher temperature. Surface basicity, which is a key factor, was analysed by measuring the interactions of the catalytic surface with CO₂ using the high frequency CO₂ pulses technique, which proved to be very sensitive, detecting minor changes by modifications in the dynamics of the CO₂ adsorption–desorption process. Water diminishes the interaction with CO₂, probably as a consequence of an adsorption competition. The SO₂ treated catalyst is equilibrated with the CO₂ atmosphere more rapidly if compared with the untreated one, also showing a lower interaction. The lower the interaction with the CO₂, the lower the activity.

Differential scanning calorimetric (DSC) results indicate that the soot combustion reaction coexists with the thermal decomposition of hydroxide and carbonate species, occurring in the same temperature range (350–460 °C). The presence of potassium increases surface basicity shifting the endothermic decomposition signal to higher temperatures.

We also found that NO₂ strongly interacts with both La₂O₃ and K/La₂O₃ solids, probably through the formation of monodentate nitrate species which are stable under He atmosphere until 490 °C. These nitrate species further react with the solid to form bulk nitrate compounds. The addition of Cobalt decreases the nitrates stability and catalyses the NO_x to N₂ reduction under a reducing atmosphere, which is a necessary step for a working NO_x catalytic trap. Preliminary studies performed in this work demonstrated the feasibility of using these catalysts to simultaneously remove NO_x and soot particles from diesel exhausts. The nitrate formation is still observed during the catalytic combustion of soot in the presence of NO_x, making our K/La₂O₃ a very interesting system for practical applications in simultaneous soot combustion and NO_x storage in diesel exhausts.