Energy Fuels 2010, 24, 992–1000 Published on Web 11/20/2009
: DOI:10.1021/ef900996f
Investigation of the Deactivation of a NOx-Reducing Hydrocarbon-Selective Catalytic Reduction (HC-SCR) Catalyst by Thermogravimetric Analysis: Effect of the Fuel and Prototype Catalyst J. Rodrı´ guez-Fern andez,† A. Tsolakis,*,‡ M. Ahmadinejad,‡ and S. Sitshebo‡ † Escuela T ecnica Superior (ETS) de Ingenieros Industriales, Departamento de Mec anica Aplicada e Ingenierı´a de Proyectos, Universidad de Castilla-La Mancha, 13071 Ciudad Real, Spain, and ‡School of Mechanical Engineering, University of Birmingham, Birmingham B15 2TT, United Kingdom
Received September 8, 2009. Revised Manuscript Received November 2, 2009
Diesel engines, fuels, and aftertreatment systems have to be optimized together to meet the targets imposed in the engine and vehicle emissions regulations, especially for particulate matter (PM) and nitrogen oxides (NOx). Hydrocarbon-selective catalytic reduction (HC-SCR) over Ag/Al2O3 catalysts is an attractive, cost-effective choice for reducing NOx, especially in the presence of hydrogen, which can be produced on-board in a fuel reformer. However, at low temperatures, the Ag/Al2O3-SCR catalyst activity decays rapidly, indicating a time period of activity loss. In this work, the catalyst deactivation process has been studied using a thermogravimetric analyzer (TGA). The effect of the space velocity, hydrogen addition, and the engine exhaust from the engine operation on gas-to-liquid (GTL), rapeseed methyl ester (RME), and ultra-low sulfur diesel (ULSD) fuels at different operating modes was investigated. In addition, the presence of a prototype oxidation catalyst located in-between the engine out and the SCR catalyst was studied, and its effect on the SCR performance was addressed. Results from the TGA confirm that the accumulation of species on the catalyst is more accentuated at low load. Fueling the engine with GTL fuel can reduce the deposition of poisoning species (i.e., carbonitrates and soot) on the catalyst surface in the less favorable conditions (low temperature and low load). The incorporation of the prototype catalyst in front of the SCR catalyst, although it improved the NOx reduction reaction over the Ag/Al2O3 catalyst, did not significantly alter the deposition of possible poisoning species on the catalyst surface. Similarly, the hydrogen addition in the exhaust gas upstream from the SCR catalyst improved the NOx reduction without significantly affecting the deposition of such species on the catalyst.
include first-generation biodiesel fuels (i.e., transesterified oil)1-3 but whose sustainability has been frequently questioned. More recently, new fuels produced by FischerTropsch processes [i.e., gas-to-liquid (GTL) fuel] have gained attention, because significant engine and aftertreatment benefits have been reported.4-6 The sustainability of FischerTropsch fuels can be easily addressed using residual biomass as raw material, although large investments are required to make this technology competitive. The next NOx limits (Euro 6 in Europe; see Figure 1) will require the introduction of sophisticated aftertreatment techniques from the diesel manufacturers, such as the urea/ ammonia-selective catalytic reduction (SCR), hydrocarbon (HC)-SCR, or lean NOx trap (LNT). HC-SCR is the more cost-effective, simpler to implement alternative, but significantly higher NOx conversion efficiencies are still required for this method to find a final application.
1. Introduction To meet the more stringent emissions regulations (Figure 1), several techniques have been developed and some of them have already been integrated within engine technology. Among these techniques, the most extensively researched ones are (i) new advanced combustion modes, (ii) exhaust gas recirculation (EGR), and more recently, (iii) aftertreatment devices, as well as (iv) the use of hydrogen (produced by fuel reforming) as a combustion or aftertreatment performance improver. In the case of diesel combustion, these techniques have to deal with the well-known diesel particulate matter (PM)-nitrogen oxides (NOx) trade-off and the abatement of NOx and PM is still a challenge. Meeting the emissions standards is also a concern for the fuel industry, because future fuels are supposed to be environmentally friendly fuels, to reduce engine emissions and, at the same time, to improve the performance of aftertreatment technologies. Such fuels
(3) Lapuerta, M.; Rodrı´ guez-Fernandez, J.; Agudelo, J. R. Bioresour. Technol. 2008, 99 (4), 731–740. (4) Tsolakis, A.; Abu-Jrai, A.; Theinnoi, K.; Wyszynski, M. L.; Xu, H. M.; Megaritis, A.; Cracknell, R.; Golunski, S. E.; Peucheret, S. M. SAE Tech. Pap. 2007-01-2044, 2007. (5) Szybist, J. P.; Kirby, S. R.; Boehman, A. L. Energy Fuels 2005, 19 (4), 1484–1492. (6) Li, X.; Huang, Z.; Wang, J.; Zhang, W. Sci. Total Environ. 2007, 382 (2-3), 295–303.
*To whom correspondence should be addressed. Telephone: þ44-(0)121-4144170. Fax: þ44-(0)121-4143958. E-mail: a.tsolakis@ bham.ac.uk. (1) Lapuerta, M.; Armas, O.; Rodrı´ guez-Fernandez, J. Prog. Energy Combust. Sci. 2008, 34 (2), 198–223. (2) Agarwal, A. K. Prog. Energy Combust. Sci. 2007, 33 (3), 233– 271. r 2009 American Chemical Society
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Energy Fuels 2010, 24, 992–1000
: DOI:10.1021/ef900996f
Rodríguez-Fernandez et al.
Figure 1. European Union (EU) emissions standards for (left) passenger cars and (right) heavy-duty diesel engines.
Despite the wide work reported in the literature regarding HC-SCR,7-15 an ideal catalyst is yet to be designed. This should show good activity and selectivity over a wide range of operation temperatures preferably under passive-mode operation (i.e., only with HCs present in the engine exhaust gas) and being tolerant to poisoning species.13,14 Nowadays, the most promising candidate is silver, mainly because it has the potential to reduce NOx with a wide variety of HCs.16-21 Nonetheless, two major problems are associated with this catalyst. First, high temperatures (over 300 °C) are required to attain good NOx reduction efficiency over Ag/Al2O3 catalysts. Although the catalyst activity can be slightly improved by optimizing the injected HCs,19-23 small amounts of hydrogen have to be added to the exhaust gas upstream from the catalyst to enlarge the temperature activity window.23,24 In explaining the hydrogen mechanism, several mechanisms have been proposed in the debate, such as a hydrogen-assisted
promotion of acetate (one of the intermediate compounds in the SCR NOx reduction),25 a decrease in the activation energy of NOx reactions,26 a reduction in the concentration of nitrates (poisoning species),26-28 or a promotion of the NO2 formation, which is effective in oxidizing carbon-rich species.19 Other possible explanations for the hydrogen promotion include an enhanced conversion of cyanide to isocyanate or the recently questioned formation of small Ag clusters, very reactive for NOx conversion.29 The second problem of Ag/Al2O3 catalysts is their activity loss over time at low temperatures because of the accumulation of poisoning species, leading to blocked active sites. However, it is not clear yet if such activity loss is the result of the surface deposition of inactive HCs and soot (present in significant amounts in a diesel exhaust gas), often referred to as catalyst coking, and/or it is more focused on specific compounds, such as carbonitrates, which have been reported to have a negative effect on this type of catalyst.29 Both referred problems but more thoroughly the second one are addressed in the present work. The aim is to perform a comparative study of the effect of different fuels, engine configuration, and operation modes on the performance and activity loss of a silver/alumina HC-SCR NOx reduction catalyst. This is carefully examined in the study, with and without externally added hydrogen, in an attempt to identify if the presence of hydrogen prevents the catalyst from poisoning. The effect of a prototype oxidation catalyst (located just before the SCR catalyst) on the SCR performance is studied as well. This is an innovative work in which we use thermogravimetric analyses (TGAs) of the catalysts used in all of the engine tests to quantify the mass of compounds deposited on the catalyst surface and, thus, determine if the global coking process has an effect on the decline of catalyst NOx reduction activity. When the working conditions in the TGA are selected, it is possible to quantify the organic compounds and C-rich species (i.e., soot) deposited on the catalyst surface.
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(25) Shibata, J.; Shimizu, K.; Satokawa, S.; Satsuma, A.; Hattori, T. Phys. Chem. Chem. Phys 2003, 5, 2154–2160. (26) Shimizu, K.; Shibata, J.; Satsuma, A. J. Catal. 2006, 239, 402– 409. (27) Creaser, D.; Kannisto, H.; Sjoblom, J.; Ingelsten, H. H. Appl. Catal., B 2009, 90, 18–28. (28) Arve, K.; Er€anen, K.; Snare, M.; Klingstedt, F.; Murzin, D. Y. Top. Catal. 2007, 42/43, 399–403. (29) Breen, J. P.; Burch, R. Top. Catal. 2006, 39, 53–58.
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Energy Fuels 2010, 24, 992–1000
: DOI:10.1021/ef900996f
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2. Experimental Section The experiments were carried out using engine exhaust gas from a Lister-Petter TR1 engine, which has been fully described in previous works along with the entire test cell.30,31 The engine is a single-cylinder, direct-injection experimental diesel engine. An electric dynamometer with a motor and a load cell was coupled to the engine and used to load and motor the engine. The EGR ratio was determined volumetrically as the percentage reduction in the volume flow rate of air at the considered operating mode. This work is focused on the performance and the activity loss of the aftertreatment device during the starting period (10 min), which consists of a Ag/Al2O3 catalyst (described later); therefore, the purpose of the engine is to produce exhaust gas whose composition (CO2, THC, NOx, and PM) is similar to the one seen in modern diesel vehicles. This task is perfectly accomplished by the engine used, as can be seen from the emissions values recorded (presented in the next section). A chemiluminescence analyzer was used for the measurements of NOx emissions, with a resolution of 1 ppm, and a correction was made to consider the differences in ambient temperature and humidity. Total hydrocarbons (THCs) were measured using a flame ionization detector (FID). Both analysers were properly calibrated with certified bottled gases. Other emissions were measured from the exhaust gas, including O2 by an electrochemical method and CO and CO2 by nondispersive infrared (NDIR), but not included in the results because they are not important for the discussion. A silver-based SCR catalyst (2 wt %) was prepared by impregnating γ-alumina with an aqueous solution of silver nitrate (AgNO3), before drying (16 h) and calcining (in air for 2 h at 500 °C). It was prepared as a powder by granulating it to a particle size of 250-355 μm for packed-bed testing in a reactor. For each test, 0.6 g of the catalyst was located in the reactor, inside a tubular furnace, whose heating power was controlled by a proportional integral derivative (PID) to fix the desired set-point temperature (250 or 350 °C) in the inlet of the catalyst (measured with a thermocouple 5 mm above the catalyst bed inlet). Although the temperature of the gas sample in the furnace inlet was around 175-225 °C (below the set point), the sample is heated rapidly in the PID-controlled furnace and the temperature at the inlet of the bed catalyst was observed to match the set point (250 or 350 °C) at any time of the tests. The exhaust gas temperature after the prototype catalyst (PC) was slightly increased or not affected. The use of this higher heat in the actual engine operation may further benefit the SCR catalyst activity, but that effect was beyond the scope of this study. The engine back pressure and the pressure drop along the SCR catalyst sample were kept constant. The experimental installation of the catalyst reactor system is shown in Figure 2. A vacuum pump was used to fix a gas flow rate of 4 L/min through the reactor, with gas samples taken downstream from the catalyst bed. One of the aims of the work was to examine the effect of a PC located in the exhaust tailpipe, just upstream from the SCR catalyst (see Figure 2), in substitution of the more usual diesel oxidation catalysts (DOCs). This PC is supported on a ceramic monolith brick (with a cell density of 600 cpsi) and was recently developed by Johnson Matthey Plc for HCs absorption at low temperatures and oxidation of HC, CO, and PM at high temperatures. It was first prepared in an aqueous solution and then uniformly coated onto the ceramic monolith. With regard to the fuels, the engine was run on ultra-low sulfur diesel (ULSD) fuel, GTL fuel, and rapeseed methyl ester (RME). Their main physical and chemical properties are listed in Table 1. Some of the tests were performed with 1500 ppm hydrogen addition from a certified bottled source with a high purity (>99%).
Figure 2. Simplified schematic of the catalyst development rig. Table 1. Fuel Properties cetane number density at 15 °C (kg/m3) viscosity at 40 °C (cSt) 50% distillation (°C) 90% distillation (°C) LHV (MJ/kg) sulfur (mg/kg) aromatics (wt %) C (wt %) H (wt %) O (wt %) H/C ratio
method
ULSD
GTL
RME
ASTM D613 ASTM D4052 ASTM D 445 ASTM D86 ASTM D86
53.9 827.1 2.467 264 329 42.7 46 24.4 86.5 13.5 0 1.88
79 784.6 3.497 295.2 342.1 43.9
