КИНЕТИКА И КАТАЛИЗ, 2013, том 54, № 5, с. 611-619
УДК 542.941.7:546.172.6-31:546.681 '623'47-31
MULTI-COMPONENT CATALYSTS WITH SPINEL STRUCTURE FOR THE SELECTIVE REDUCTION OF NITROGEN OXIDE BY ETHYLENE
IN LEAN-EXHAUST GAS STREAMS
© 2013 Md. Hasan Zahir1, *, K. Alhooshani1, Mohammad A. Jafar Mazumder1, Toshio Suzuki2
1Chemistry Department, King Fahd University of Petroleum and Minerals, Dhahran 31261, Saudi Arabia 2Advanced manufacturing Research Institute, National Institute of Advanced Industrial Science and Technology,
Nagoya, Japan *E-mail: hzahir@kfupm.edu.sa Received 26.07.2012 г.
The Ga2O3—Al2O3—ZnO (GAZ) multi-component spinel powders with incorporated Cu2+, Co2+, Fe2+, Ni2+, Mn2+ and In2+ metal cations were synthesized by co-precipitation method from a mixed solution of nitrate salts. Spinel crystal structure of each composition was confirmed by XRD measurements. The multi-component oxide powders were tested in the reduction of nitrogen oxide (NO) under lean conditions. Among the catalysts tested, In2O3-containing GAZ with a pure spinel phase structure showed promising catalytic activity in the NO reduction in the presence of 10% H2O vapor. In addition, the effect of H2O vapor and SO2 on the selective reduction of NO over In2O3—GAZ/cordierite and In2O3—GAZ/metal honeycombs catalysts has been investigated.
DOI: 10.7868/S0453881113050195
Nitrogen oxides (NOx = NO + NO2) emitted from the gasoline and diesel exhaust represent one of the major environmental problems. Selective catalytic reduction (SCR) of NOx by hydrocarbons (HC) is an effective way to remove NO from the exhaust gases. Over the last decade, there have been considerable efforts to investigate the performance of various catalysts in HC-SCR [1, 2]. It is well known that the systems based on three-way catalysts and zeolite catalysts are unstable in real exhaust conditions particularly in the presence of O2 and H2O vapor. Therefore, various attempts have been made to develop more suitable catalysts. Among all studied catalysts, metal oxide based catalysts were found promising due to their high thermal stability. Hamada et al. [3] used Ga2O3—Al2O3 (GA) and/or GA systems doped with Ni2+, Ag2+, Co2+, Cu2+, In2+ and Sn2+ on y-Al2O3 for NOX reduction. They found that Ga2O3 and In2O3 oxides carried by acidic supports, such as y-Al2O3, TiO2, and SiO2— Al2O3, showed high de-NOx activity [4]. The authors also demonstrated the effect of double dopants (Ga2O3 and ZnO) into Al2O3 (i.e., Ga2O3—Al2O3—ZnO (GAZ) system)) with spinel structure, and found good catalytic activity and selectivity similar to Ga-ZSM-5 for SCR of NO [5, 6]. The GAZ system with spinal structure showed exceptionally high NO reduction activity even after calcinations at 1000°C [7]. However, decreasing NO reduction activity in the presence of H2O limits the applicability of GAZ system.
Among the y-Al2O3-supported materials, In-Al2O3 and In-GA nanocomposites revealed the highest per-
formance in terms of the N2 yield and hydrocarbon selectivity [8]. Catalysts based on In2O3 supported by y-Al2O3 have been thoroughly investigated over the years [4, 8]. However, no studies have been reported on In2O3 containing Al2O3 with spinel type catalysts. In general, y-Al2O3 show a phase transition to a-Al2O3 around 800° C and once the phase transition occurrs the catalytic activity of y-Al2O3 based catalysts decreases significantly. Although the phase transition temperature of y-Al2O3 is higher than the temperature of exhaust gases, the thermal stability of the y-Al2O3 based catalyst is unreliable because of the possibility of local increases temperature in practical use. Therefore, the In2O3 containing Al2O3 with spinel structure might be a promising NO reduction system particularly in the presence of H2O vapor at high temperature. Generally, most of the investigations focus on single-component catalysts. So far no attempts have been made to study multi-component catalyst formulations consisting of several active elements on the alumina support. Mixed metal oxides catalysts, containing more than one kind of metal atom are widely used in several fields of research related to catalysis [9, 10]. The addition of multivalent and/or multi-component transition metal ions may adversely affect selectivity because these ions are active in the catalytic oxidation of hydrocarbons [9]. However, NO reduction activity over multi-component metal oxides system is scant and it is desirable to find a way to enhance the activity [9].
Most of the NOX catalysts have been prepared in the form of powders or pellets, which could not meet
Physical properties of washcoated* cordierite and metal honeycombs employed in the present study
Cell type: circular parallel channels (cordierite), zigzag square channel (metal)
Cell density : 900 cpsi** (for cordierite and metal honeycombs)
Washcoated catalyst: In doped-GAZ spinel
Average thickness of washcoat: 18 p.m (Cordierite) measured by SEM
* Washcoats — GAZ + alumina binder
** cpsi = number of cellsin-2
requirements of practical applications. The catalytic performance over honeycomb system washcoated with the In-GAZ, which plays an important factor in the development of automotive catalysts, has not been explored yet. Typically, both metal and ceramic monoliths are employed for automotive catalysts nowadays. The popular design for the automotive exhaust catalysts is the monolith catalysts. A monolith has a honeycomb structure with parallel and usually straight channels or cells, and the catalyst particles are dispersed on the wall of the channels. Compared with packed-bed reactors loaded with pellet catalysts, the monolith has several advantages such as low pressure-drop (up to two orders of magnitude) [11], high-geometric surface, robustness, low weight and better heat transfer rate [12]. Traditionally, cordierite monoliths have been used quite extensively, due to their lower production cost. Metal monoliths have other merits such as high mechanical strength, high thermal conductivity, and durability [13].
In this study, the effect of Ni2+, Co2+, Cu2+, Fe2+, Mn2+ and In2+ metal oxide on Ga2O3-Al2O3-ZnO with spinel structure has been investigated for the selective reduction of NO with C2H4. These studies were aimed at improving the activity of GAZ system in the presence of H2O vapor and SO2. NO reduction activity over cordierite and metal honeycombs systems wash-coated with the In2O3—GAZ multi-component spinel catalyst system has also been investigated in order to verify, whether the catalysts were good candidate for practical lean NO catalysts.
EXPERIMENTAL
Materials and Synthesis
The loading of Ga2O3 was fixed at 30 mol %, while that of ZnO was 30 mol % in the case of Ga2O3— Al2O3—ZnO system. In2+, Co2+, Cu2+, Fe2+, Mn2+ and Ni2+ were selected as the metal oxide additive. The composition of starting nitrates was M : Ga : Al : Zn = = 5 : 30 : 35 : 30 mol %, and it is known that the catalyst containing 30 mol % of Ga2O3 gives the highest
catalytic activity in the de-NOx reactions in the Ga2O3—Al2O3 (GA) system [5]. Appropriate amounts of starting nitrates were dissolved in distilled water and aqueous ammonium carbonate solution was added to the solution to co-precipitate metallic ions. The pH of the solution was maintained at ca. 8.5 and the solution was vigorously stirred for 24 h. The obtained precipitate was washed thoroughly with distilled water, followed by drying at 110°C and heat treatment at 800°C for 5 h in air.
Preparation of Honeycombs Washcoated with Infl3—GAZSpinel
The washcoating slurry was prepared by mixing the In2O3—GAZ, alumina sol and distilled water in the weight ratio of14.3 : 8.4 : 77.3. At present, synthetic or natural alumosilicate-cordierite, with a low specific surface area, not exceeding 0.2—0.4 m2 g-1, is most frequently applied to form a honeycomb structure. In this study, alumina sol (Alumina sol 520, Nissan Chemical Industries, Ltd.) was used, which acted as an inorganic binder. The commercial cordierite and metal honeycombs were immersed in the slurry and then withdrawn, followed by drying. The excess suspension inside the channels of the cordierite substrate was blown off. Before the introduction of wash coat of Al2O3, the metal honeycomb was pretreated by thermal oxidation at 800°C for 5 h in static air atmosphere. Since the suspension cannot be coated sufficiently by a single impregnation, multi-impregnation was required. The coated honeycombs were heat treated at 800°C for 5 h. The physical properties of the washcoated honeycombs employed in the present study are listed in table.
Catalytic Activity Measurements
The catalytic activity of the resulting powder and honeycomb washcoated with GAZ spinel was measured using a fixed bed flow pyrex glass tubular reactor with an internal diameter of 10 mm for powder and 32 mm for monolith catalysts. For both cases, the samples were placed between quartz wool plugs in the reactor.
A commercial cordierite honeycomb composed of MgO-Al2O3-SiO2 and a metal monolith composed of Fe-Cr-Al, were also tested. Honeycombs (cordierite and metal) of the same size (50 mm in length, 25 mm in width, and 40 mm in height) were used as the substrate. The reactor space velocity was defined as the ratio of the total volumetric flow rate to the volume of the monolith reactor. However, in the case of powders samples, the space velocity was defined as the total flow rate related to the weight of catalysts. Dead space between the honeycomb catalyst and the reactor wall was filled with an inert material to prevent breakthrough of gas. All the lines were heated to avoid possible water condensation.
The flow rate of each gaseous reactant was regulated by a mass-flow controller (STEC Inc.). The flow of each gas was fixed and held constant with a 4-channel electronic mass-flow controller (UCAR Instruments, MFB-21-1). A temperature controller (Leeds & Northrup Instruments, Electromax V Plus) was used both to
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