The Effect of H 2 O on the Use of Ethanol as Reductant in the SCR System

In this experimental study, effects of H 2 O on the use of ethanol as reducing agents were investigated on the Ag-Pt-W-Ti/Cordierite catalyst. Ag-Pt-W-Ti/Cordierite catalyst was synthesized using the impregnation method for selective catalytic reduction (SCR) of NO x . To investigate the effects of H 2 O addition on the NO x conversion ratios, tests were carried out at 200-270 ° C under 30000 h -1 GHSV using three different reductants (ethanol, 5% H 2 O - 95% ethanol, 10% H 2 O - 90% ethanol). The catalytic activity of the catalyst increased with increase in exhaust gas temperature. The highest NO x conversion ratio was 89.9% at 270 o C with ethanol. The tests showed that when the H 2 O content of the reductant increased from 5% to 10%, the NO x conversion ratios significantly decreased at temperatures below 240 o C. NO x conversion ratios enhanced as engine load increased.


INTRODUCTION
Nitrogen oxides (NO x ) emitted from vehicles are major air pollutants that require strict environmental regulations because of their toxicity [1,2,3]. Selective catalytic reduction (SCR) of NO x using reductant is a one of the most efficient and low cost technology for elimination of NO x from engine exhaust gases [4,5,6].
The catalysts are an important factor for SCR technology [7]. A catalyst includes a substrate material, active components such as transition, noble and rare metals and different supported materials such as zeolites, alumina, TiO 2 , active carbon [1,8].
Cordierite (2Al 2 O 3 .5SiO 2 .2MgO) is commonly used as monolith substrate due to strong thermal stability, economic feasibility and low thermal expansion [9,10]. The major disadvantage of cordierite is its low surface area. The surface area of cordierite is about 0.5 m 2 /g. Surface area of cordierite should be increased to provide strong interaction between active components and supports, to obtain high dispersion and highly active catalyst. One way to increase the surface area is coating of supported materials, including alumina, TiO 2 , CeO 2 , active carbon, another way is to pre-treatment of cordierite [11,12]. Surface area of cordierite increases with acid pre-treatment such as nitric acid, hydrochloric acid, sulfuric acid, acetic acid, oxalic acid and EDTA [12,13]. In addition, catalytic activity of catalyst enhances with acid pre-treatment.
Among the supported materials, TiO 2 is predominantly used due to provide large surface area and porosity, which is beneficial for catalytic reaction and catalytic activity [14]. CeO 2 supported materials are used due to their special redox ability and high oxygen storage capacity. However, CeO 2 has small surface area and agglomeration. Therefore, it is not preferred as a support material [15].
The principle of NO x conversion reaction is to reduce NO x to N 2 by using ammonia (NH 3 ), urea, hydrocarbons (HC), and oxygenated hydrocarbons (OHC) and with the help of a catalyst. Ammonia is the commonly used as reductant in the SCR system [3,14,16]. The main NH 3 -SCR reactions are as follows [14,16] NO + NO 2 + 2 NH 3 → 2 N 2 + 3 H 2 O "Fast SCR reaction" (5) In the NH 3 -SCR systems as the source of ammonia are used urea-water solutions. However, urea-SCR systems cause ammonia slip, require heavy urea tank and include complex and costly injection system. These problems can be solved by using HC as reductant in the SCR systems [17,18].

Catalyst synthesis
The low surface area of cordierite was increased for catalyst synthesis. For this purpose, 200 cm 3 a cordierite material was pre-treated with 40 % hot oxalic acid solution for 3 hours. Then, it was washed with distilled water, dried at 110 o C and calcined at 550 o C. Thus it made ready for coating.
The catalyst was synthesized using the impregnation method. For this purpose, 2.5 g AgNO 3 (≥99% Sigma-Aldrich), 2 g (NH 4 ) 10 H 2 (W 2 O 7 ) 6 (99.9% metal basis Sigma-Aldrich), 2 g Pt (NH 3 ) 4 (NO 3 ) 2 (≥50.0% Pt basis Sigma-Aldrich), 50 g TiO 2 (anatase, ≥99% trace metal basis) and 0.5 g SiO 2 (nano powder, 99.5% trace metals basis) were added into 250 ml distilled water. The mixture was mixed using an ultrasonic stirrer. The pre-treated cordierite was dipped in this mixture and its clogged pores were opened. After the coating, it was dried at 120 o C for 1 hour and calcined at 550 o C for 3 hours. Thus, Ag-Pt-W-Ti/Cordierite catalyst was synthesized. Surface area of the synthesized catalyst was 30.38 m 2 /g. Picture of cordierite used in catalyst production is given in Figure  1. Catalyst preparation flow scheme was given in Figure 2.

Performance test system
A performance test system was used in the laboratory of the Automotive Department of Çukurova University to investigate the effect of H 2 O on the use of ethanol as a reductant and the NO x reduction efficiency of Ag-Pt-W-Ti/ Cordierite catalyst. The test system includes a two cylinder V type AKSA diesel engine, an engine loading system and a designed exhaust system. Schematic diagram of the performance test system was given in Figure 3. The technical properties of the test engine were shown in Table 1. The loading unit used consists of 10 resistances, each providing 1 kW loading. The power amount of system pulled from engine has been kept under control using the ammeter and voltmeter on the system. In the test system, an orifice plate was used to determine the exhaust gas flow rate and a heater to adjust the exhaust gas temperatures. Exhaust gas tem- peratures were determined using two K type thermocouple temperature sensors. NO x emission reduction ratios were detected using two Continental model NO x sensors and the data was monitored by computer. Accuracy values of measuring device are ± 10 ppm for CO, ± 1% for CO 2 and ± 1 ppm for NO x .

NO x conversion ratios
The catalytic performance of the Ag-Pt-W-Ti/Cordierite catalyst was examined under gas hourly space velocity (GHSV) of 30000 h −1 . The NO x conversion ratios of the catalyst were shown in Figure 4-7.
The activity of Ag-Pt-W-Ti/Cordierite is highest for NO x reduction using ethanol as reductant.    The Effect of H2O on the Use of Ethanol as Reductant in the SCR System