HC-SCR: NO x Reduction using Mn and Cu Catalysts Impregnated in Coconut and Palm Kernel Shell Activated Carbon

. The characteristics of catalysts impregnated in coconut shell (CS) and palm kernel shell (PKS) activated carbon were determined as potential precursors of catalysts used in a flue gas denitrification system at low temperature. In this study, Manganese (Mn) and Copper (Cu) with metal loading of 8% were impregnated in the activated carbon (AC) before undergoing low temperature calcination process. The morphological properties of samples was analysed using Scanning Electron Microscopy (SEM) and Brunauer, Emmett and Teller (BET) was used to determine the surface area and pore size of samples. The exhaust gas from a diesel engine at a constant flow rate of 4L/min was passed through in a fixed-bed catalytic reactor containing the catalyst, and the concentration of NO x was measured for temperatures ranging from 150°C to 250°C. It was found that the CS catalysts (CS-Mn and CS-Cu) and PKS catalysts (PKS-Mn and PKS-Cu) have the potential to reduce NO x concentration, and results showed that the metal loading of 8% resulted NO x reduction ranging from ~48% to 64%.


Introduction
Nitrogen oxides (NO x ) are among the severe contaminants released by automobiles and other vehicles, and by manufacturing industries. Diesel-powered vehicles are becoming more sought after due to their excellent fuel efficiency and less emission of CO 2 compared to their stoichiometric spark triggered gasoline counterpart [1]. Three ways catalysts (TWCs), one of the strategies for NO x conversion, is reported to produce excess of oxygen contributing to the incapability of catalyst in reducing NO x from lean burn engine exhaust [2]. Hence, a major challenge today is to minimize NO x emissions from lean-burn diesel engine exhausts.
Selective Catalytic Reduction (SCR) has been experimented and applied as one of the efficient ways to reduce NO x emission into the environment [3]. The most extensively used in the industries are NH 3 -SCR, which uses ammonia gas as reductant, and HC-SCR that is using hydrocarbon as the alternative for reducing agent. In this case, HC-SCR is the most promising method to control the emission of NO x from diesel engine vehicles due to the simplicity of system as it does not require an extra gas tank installed because hydrocarbon (HC) itself can act as reducing agent and onboard fuel as reductant [4]. This means, the system, which operated under lean conditions, exploits the unburned hydrocarbon that are already present in the gas stream of the diesel engine exhaust [5].
Catalytic reduction systems usually use precious metal-based catalysts, for instance, Rhodium(Rh), Platinum(Pt), and Palladium(Pd) supported-oxide. Owing to their abilities that is water vapor resistant and SO x poisoning, these metals are capable of performing efficient NO x abatement in low operation temperature (lower than 300°C). A study claimed that Pt based catalysts, however, exhibit two downsides namely low N 2 selectivity and narrow temperature window system for reduction of NO x [6]. On the other hand, Rh based catalysts are mostly used in three ways catalysts (TWCs) operation as Rh metal promotes the best NO x reduction under low activity temperature; however, it is also said that the catalysis activity involved in synthesis gas conversion process [7].
Other metals that have also been studied for SCR catalysis process are such as Manganese (Mn), Titanium (Ti), Nickel (Ni) and Copper (Cu). These metals are believed to have the capability to remove NO x gas under low operation temperature especially from stationary sources [8]. However, the potential of these metals have yet to be broadly looked into hydrocarbon SCR systems as they are mostly investigated in urea-SCR. In this study, Mn and Cu are selected as metallic catalysts for the removal of NO x under low catalytic activity temperature owing to their chemical properties such as alkali metal resistance of catalysts for diesel exhaust treatment [9].
The mechanism of HC-SCR is complex. As concluded by Lamacz et al. (2013) in a proposed model, when hydrocarbons are unable to undergo oxidation process to CO 2 and H 2 O at the dissociation temperature of NO, which contributing to the formation of N 2 , it is obligatory to make use of three-function catalysts. Oxidation of NO to NO 2 is the first function of this catalyst. The second function denoted by the hydrocarbons undergoing mild oxidation by NO 2 to oxygenate species which is presumed to be vital for regeneration process of catalyst. In the third function, N 2 is being released in the dissociation of NO x (via a dinitrosyl-adsorbed intermediate) prior to the N 2 formation and generation of adsorbed oxygen species. The overall activated reductant (C x H y O z ) oxidation process resulting in the removal of adsorbed oxygen species. As stated by this research group, function 2 and 3 are said to happen simultaneously, however, both of these functions are not in the same cycle of catalysis at molecular level [10].
A novel establishment of an alternatively sourced carbon-supported catalysts, for instance, activated carbon, bio-char and virgin biomass waste residue, has been developed which is believed to contribute to a more environmental friendly and cost effectiveness [11] for managing the emission of NO in SCR de-NO x process under low operation temperature [12]. In this study, a laboratory scale reactor was carried out for the purpose of determining the efficiency of NO x reduction under HC-SCR low temperature using activated carbon derived from coconut and palm kernel shell as catalysts support for Mn and Cu metals. The relation of the physical properties of the catalysts support with the reduction of NO x was also analysed.

Preparation of catalyst
Prior to the impregnation process the coconut shell activated carbon (CSAC) and the palm kernel shell activated carbon (PKSAC) was sieved to give uniform particle size of 1 mm. Both CSAC and PKSAC were impregnated with metal nitrate of an appropriate concentration to obtain 8% of metal content per gram of activated carbon. In this study, manganese (II) acetate tetrahydrate (Mn(CH 3 COO) 2 ·4H 2 O) from ACROS ORGANIC, and copper (II) nitrate-trihydrate (Cu(NO 3 ) 2 .3H 2 O) from SYSTERM ChemAR were used as the metal precursors.
During the impregnation, the solution of metal nitrate was continuously mixed with activated carbon for 5 hours. Then, the samples were heated to 70 °C while being constantly stirred until all the liquid evaporated. After that, the samples were dried in an oven at 110 °C for a period of 12 hours. Finally, the prepared samples were heat-treated at 300 °C for 4 hours. The catalysts that impregnated with 8% (w/v) of metallic catalysts denoted as CS-Mn8, CS-Cu8 and PKS-Mn8, PKS-Cu8.

Catalyst performance test
Experiments were carried out in a fixed-bed reactor that was filled with the catalyst. The schematic diagram of the experimental setup is shown in Figure 1. The prepared catalysts of CS and PKS were each weighed 10 g and placed on borosilicate glass wool (0.5 g) in the center of the sample holder. The sample holder containing the catalyst was placed inside the fixed-bed reactor after the set temperature was reached. Temperature used here was between 150 -200 o C.
A stream gas from diesel engine was passed through the prepared catalyst, which act as sorbent and also as SCR catalyst . The feed flow through the sample holder was controlled at 4L/min. The concentration of NOx in the inlet and outlet of the reactor was measured using Flue Gas Analyzer for a duration of 5 minutes. The reduction efficiency was calculated using the relationship denoted by the following equation: NO x,in denotes the inlet NO x concentration (vol %) in the stream gas introduced into the fixed-bed reactor and NO x,out denotes the outlet NO x concentration (vol %) in the stream gas out of the reactor.

Figure 1
The schematic diagram of the experimental apparatus.

Catalysts characterizations
The properties of the activated carbon that was impregnated with 8% of Mn and 8% of Cu were analysed based on their surface area, pore size and the images showing the morphological properties that obtained from the experimental work done using Brunauer, Emmett and Teller (BET) and Scanning Electron Microscopy (SEM) apparatus. The metallic catalysts derived from palm kernel shells and coconut shells activated carbon denoted as PKS-Mn8, PKS-Cu8, CS-Mn8 and CS-Cu8.  Physical properties of CS and PKS-based catalysts were investigated in BET analysis as shown in Table 1. From the analysis, it can be seen that the specific surface areas and the volume of micropore of both types of activated carbon samples decreased after the process of impregnation of metal. It is presumed that the partial pores were blocked by the particles of metals. This has been proven by several studies saying that metal embedded onto the support filled the micropores but will not affect the performance of the reduction activities [13 -15]. Impregnation method has proved that the metal particles are successfully located in the pores especially in the internal part, hence, plugging the tapered microporosity. As a result, this fact contributes to the first proof of dispersion of metal on the surface of both activated carbons in high degree.

BET analysis
The difference in the surface area and pores in BET analysis was supremely low among the impregnated of both CSAC and PKSAC stipulating the percentage of metal loadings impregnated onto the surface of activated carbons were almost equal. The catalysts' surfaces and morphologies which undergone the process of impregnation of metals are shown in Figure 2. Comparing between the two types of activated carbon support, smoother and better consistency dispersion of metal was observed for the coconut shells activated carbon support than those obtained for palm kernel shells activated carbon. This can be deducted that the nature of the CSAC reacted well with both Mn and Cu metals. Consequently, better dispersion of metals can be observed over the surface found for CSAC, which is also predicted to promote better catalytic efficiency. Tang et al. (2015) had proved that the impregnation method to prepare catalyst provided a better dispersion and formation of metal solution which will encourage the catalytic activity [13].

NO x reduction
In Figure 3, comparing between the performance of raw CSAC and PKSAC, it is shown that CSAC adsorbed NOx better than PKSAC with percentage adsorption of NOx estimated to be 28% and 22%, respectively. However, in this case, only adsorption process can be observed because there is no reaction between metal catalysts with hydrocarbon (HC). This is because the obvious difference in the surface area and size of pores of the AC to facilitate the adsorption of NO x molecules in Table 1. In addition to that, owing to favorable surface area and pores size, the performance of metal catalysts impregnated on CSAC shows better efficiency if compared to that in PKSAC. This trend is aligned with findings by Guera et al. (2016) where limited surface area and low porosity of activated carbon small amount of adsorbed N 2 [14].
According to SCR mechanisms by M. Adamowska-Tessier (2015), there are 3 functions of SCR mechanisms, which is respectively the oxidation of NO to NO 2 in the first function, followed by mild oxidation of hydrocarbon (HC) in the second and, finally, the conversion of NO 2 to N 2 happens in the third function. Figures 4 (a) and (b), show the percentage of NO x reduction versus time. The reading recorded at zero minute is the reading taken for the NO x concentration before the exhaust gas entered the reactor that was filled with catalyst, which gives zero NO x reduction. The concentration of NO x in the exhaust gas that came out from the reactor was measured for 5 minutes, and the % of NO x reduction was found to be higher for the activated carbon impregnated with Cu compared to that impregnated with Mn. The addition of metal onto the activated carbon resulted in the increase in the % of NO x reduction as illustrated by results shown in Figures 3 and 4; The % of NOx reduction for raw CS was found to be about 28%, as shown in Figure 3 while that for CS with Cu and Mn was increased to above than 50%.
Here, the increase in the % of NO x reduction indicates the effect of SCR reaction, which can be observed in the first 30 seconds as the actual conversion of NO x into N 2 , as in the third function of the mechanisms, happened during that time frame. The % of NO x reduction was found to decrease gradually, which may be due to the trapped NO x that filled the pores of the activated carbon. Based on studies conducted by Surjit Singh et al. (2013), he found that the optimum reduction of NO x was only 25% under temperature of 200 °C. The flow rate of the flue gas used in his study was at 1L/min, which may contribute to the low NO x reduction. However, the percentage of NO x reduction in the study increased to 67% as the temperature was elevated to 280 °C, which may indicate higher NO x conversion [12]. The % of NO x reduction for different samples of catalyst analysed in this work was compared as shown in Figure 5.    Figure 4 (a) and (b) showed the results of NO x reduction using the catalyst impregnated onto the activated carbon derived from coconut shells, as shown in Figure 4 (a) and from the palm kernel shells, as shown in Figure 4 (b) with 8% of Mn and Cu. Comparing between the potential of metal Cu and Mn in catalytic activities, it was found that both curves in (a) and (b) showed Cu gives better reduction of NO x; The % of NO x reduction was estimated to be 64% and 58% for CS-Cu8 and PKS-Cu8, respectively.

Effect of metal type
Results obtained here may suggest that Cu that has higher electronegativity has a greater tendency to attract electron toward its atom resulting in higher NO x reduction. Consequently, owing to this nature, Cu makes a better catalytic metal than Mn.

Conclusions
This study emphasized on the effect of manganese and copper impregnated onto biomass waste derived activated carbon combined as Selective Catalytic Reduction (SCR) by using hydrocarbon (HC) as the reduce agent. Based from the observations and results obtained, it can be concluded that coconut shells activated carbon based metal catalysts was found to give better reduction of NO x compared to palm kernel shells activated carbon based catalysts.
CSAC based catalysts shows a catalytic effect towards the removal of NO x as well as active, selective towards NO x and was observed stable in laboratory testing condition. From the results, Cu is a better catalyst for the SCR reaction than Mn as it has been proven that the combination of Cu and coconut shell activated carbon showed better NO x removal.