Selective Catalytic Reduction of NO by Methane on Copper Catalysts : the Effects of Modifying the Catalysts with Acids on γ-alumina

This work compares the performance of copper catalysts modified with that of acids. Hence, γ-alumina is used to support the catalyst in the selective catalytic reduction of nitric oxide by hydrocarbons (HC-SCR) using methane at temperatures from 623-1023 K in the presence of oxygen. The copper catalysts were prepared by the incipient wet impregnation of a modified support of γ-alumina with copper nitrate, and by modifying the using nitric, acetic and phosphoric acids, respectively. The effectiveness of the transformation with nitric oxide over the catalysts, determined by comparing the conversion of nitric oxide with non-modified Cu/γ-Al2O3, to that by modified Cu/γ-Al2O3, implied that the reduction of transformation nitric oxide was most efficient when the catalyst modified with nitric acid. Modifying a copper catalyst with nitric acid markedly was found to increase the efficiency of reduction, whereas treatment with acetic and phosphoric acids did not promote the reduction of NO. However, a higher concentration of nitric acid corresponds to a higher efficiency of transformation nitric oxide; thus, converting nitric oxide using the catalyst modified with nitric acid is favored. Pre-treating the support γ-alumina with a higher concentration of acid solution promotes higher activity and greater acidity of the transforming nitric oxide reaction over a copper catalyst, because such pretreatment with acid enlarges the pores in the support, increasing the internal mass transfer and the reactivity over the catalyst. Notably, the presence of Cu (I) and Cu (II) at active sites may also affect the efficiency of the transformative reaction with nitric oxide. When the inlet gas is methane, the corresponding efficiency of the transformation nitric oxide shows that methane is an effective reducing agent.


Introduction
Nitrogen oxides (NO x ) are a very interesting and important family of air-polluting chemical compounds, and are increasingly attracting the interest of regulators in Taiwan.The primary *Corresponding author: Tel: +886-7-5254410 Fax: +886-7-5254449 E-mail address: loujc@mail.nsysu.edu.twsources of nitrogen oxides, including several stationary industrial fossil fuel combustion processes or mobile sources, are of concern because these pollutants are significant precursors of both acid rain and acidification of the aquatic system, and because the pollutants react in the atmosphere to form ozone and photochemical smog (Huang et al., 1999).The many available technologies for reducing nitric oxide emissions can be classified as dry or wet processes.Complete biological treatment systems have been extensively investigated and implemented.Of these, the catalytic method is the most effective means of meeting present and future requirements, and is used in dry systems, including selective catalytic reduction (SCR), which involves ammonia, and selective non-catalytic reduction (SNCR), which involves ammonia or urea.
Furthermore,modifications to combustion processes have been extensively examined and effectively implemented (Centi and Perathoner, 1995).SCR uses a catalyst and ammonia in the presence of oxygen to reduce nitric oxides to elemental nitrogen and water.It also has great potential for controlling emissions of nitric oxide, and is thus commonly performed in factories.However, its disadvantages include high operating temperatures (300~600 ℃ ), the need for chemical additives, safety hazards and the emission of nitrous oxide (N 2 O), which is a harmful greenhouse gas (Zhu et al.,2000).In spite of these problems, the many benefits of SCR make it an attractive alternative control technique.Catalysts have a finite life in a flue gas, and some ammonia can pass through without reacting.The use of wet-scrubbing agents to adsorb nitric oxide enables alkali in water, water alone, or hydrogen peroxide to be used as the liquid for capturing nitric oxides (Burdeinaya et al., 1996).The biological treatment of nitric oxide is an inexpensive alternative technique, but it is only suitable for use in chemical processes (Liu et al., 1991;Chou and Lin, 1998;Ye et al., 1994).The main drawbacks of such biological treatment processes include the high gas empty-bed retention time, the expense of the required nutrient additives and the degradation of the biofilm.
The catalytic reduction of nitrogen monoxide (NO) in an air stream represents an alternative technique to the fuel combustion and chemical process; it allows the flue gas to flow with minimum resistance and a minimum drop in pressure, maximizing the surface area of the catalyst.Using a catalyst to cause combustion to occur below the NO formation temperature can appropriately limit temperature.Metal oxide catalysts, including iron oxide-based catalysts and vanadium oxide supported on Al 2 O 3 , TiO 2 or SiO 2 have been demonstrated to exhibit a high activity in terms of the SCR of NO in the presence of a large excess of oxygen (Wong and Nobe, 1986;Tufano and Turco, 1993).During recent years, much research on the use of copper-based catalysts in the conversion of nitrogen oxides has been presented.
Most has focused on developing selective NO reduction with hydrocarbons, such as methane, propane and propene.Zeolite-and aluminasupported catalysts have also been extensively studied (Desai et al., 1999;Moretti et al., 1999).
The reduction of NO by methane in the presence of oxygen generally proceeds as follows.Centi and Perathoner (1995)  Consequently, many researchers have begun to focus on NO decomposition (Tomasic et al., 1998;Pietrogiacomi et al., 1999;Chen et al., 1999).
However, these studies involve too much uncertainty about the chemical mechanisms of the catalytic reduction of NO.Additionally, the catalytic reduction of NO deserves further investigation because in air streams, it has yielded only limited results.Appropriate treatments of catalysts with acids modify the acidity and texture of the catalysts to increase both their specific surface area and the number of useful acid active centers (Shouro et al., Wang et al., 1998).Therefore, the treatment of catalysts with acid has been extensively investigated over recent years to accelerate the decomposition of NO under moderate reaction conditions.
However, the decomposition of NO on the acid-modified Cu/γ-Al 2 O 3 catalyst has not been deeply considered.Given the world's extensive supply of natural gas (NG), methane would certainly be a preferred hydrocarbon for use in the non-selective reduction of NO.Hydrocarbons also represent attractive alternative reducing agents to ammonia (Armor, 1995).Thus, this study focuses Accordingly, the activities of the prepared catalysts were evaluated and the superior catalyst was chosen.The catalysts were also characterized by BET, TGA, triethylamine-TPD, XRD and SEM, to provide further insight into the increase in activity associated with treatment with acid.During analysis, the sample was scanned from 20 to 80° at a rate of 0.4°/min.Scanning electron microscopy with an energy dispersive X-ray spectrometer (SEM/EDX, JEOL, JSM-6400, Kevex, DeltaII), elucidated the morphology of the catalysts and provided information on the distribution of copper on the catalyst surface.

Materials and Methods
Hence, copper-based catalysts prepared by incipient wetness impregnation were found to exhibit various activities in the conversion of NO.
The different copper species in these catalysts are responsible for this divergence.Thermogravimetric analysis (TGA) experiments were conducted in a TGA unit (TGA-50, Shimadzu, Kyoto, Japan) at a heating rate of 283 K/min.
The effect of acidity and the characteristics associated with the modification of the catalysts using acid were measured in temperatureprogrammed desorption (TPD) experiments on triethylamine.These experiments were conducted in a vertical tube furnace with He as the carrier gas, according to a method similar to that of Shouro et al. (2000).Triethylamine is favorable as the probe used in the TPD analysis of mesoporous materials since it can easily migrate into the mesopores.A 0.1 g sample was placed in a quartz tube and dehydrated in a stream of helium at 1023 K for 3 hr.
Triethylamine was adsorbed at 373 K for 2 hr and physisorbed triethylamine was extracted (at 373 K for 2 hr).The system was heated to 773 K at a rate of 60 K/min in the stream of helium, and the amount of desorbed triethylamine was analyzed using a chromatograph (GC-14A, Shimadzu, Kyoto, Japan) equipped with a thermal conductivity detector.
Experiments were performed on a tubular fixed-bed flow quartz reactor (TFBR).Three flowing gases, NO, CH 4 and O 2 , were used in the preparation of the feeding mixture and the diluent's gas, helium, at the inlet of the reactor.Each gas was independently controlled using a mass flow regulator.Highly pure, dry air was used as the carrier gas, and its flow rate was controlled using a mass flow meter (830 Series Side-Trak TM , Sierra, Monterey, CA, USA) at 8-13 L/min.The mass of the catalysts was 1g (at an empty bed volume of approximately 1.2 cm 3 ).External mass transfer and inter-particle diffusion were effectively unlimited in this reactor.The reaction tube (300 mm long with an inner diameter of 28mm) was placed inside a split tube furnace, along with the tube that contained the catalyst.Two thermocouples of type K (KT-110, Kirter, Kaohsiung, Taiwan), each with a diameter of 0.5 mm, were mounted and equally spaced along the catalyst bed.The thermocouples were also connected to a PID controller (FP21, Shimaaen, Tokyo, Japan) that kept the temperature in the tube within ±0.5 %.The feed gas (GHSV, 108000 ml/h-g) was maintained at concentrations of 1000 ppm NO and 1000 ppm CH 4 , and 2% O 2 .The catalyst was not deactivated during testing.
Samples before and after the reaction were

Results and Discussion
The capacity and texture of a catalyst's exterior surface strongly affects its adsorptive and catalytic behavior.Table 1 summarizes  Test conditions : 1000 ppm NO in He, 1000 ppm CH 4 in He, O 2 ＝2%, GHSV＝108000 ml/h-g.consequently higher loadings resulted in metal sintering, and a consequent reduction in catalytic activity.
Figure 2 shows the effect of copper loading on the conversion of γ-Al 2 O 3 at various temperatures.
The figure shows that the maximum activity increases with copper loading in the range of 2 to 10%, and the the reaction conditions strongly affect the relationship between the activity of the catalyst and copper content.However, the conversion of NO is a maximum over a loading of 8 wt.% Cu/γ-Al 2 O 3 and is lower at excess copper loading (10%), because the side reaction of hydrocarbon combustion along with higher loadings cause metal sintering.Besides, the literature states that bulk CuAl 2 O 4 species can be formed by increasing the copper content to 10 wt.% (Chen et al, 1999).
However, the copper species may change as a function of copper loading.Centi (1995) showed   The results were observed in the CH 4 -SCR reaction using the 8 wt.% Cu/γ-Al 2 O 3 catalyst, wherein the NO conversion rate exceeded 71.2% at 873 K.
Consequently, higher calcination temperatures correspond to a higher conversion yield.
Additionally, the textures of the catalysts were observed also to depend on the copper loading and the calcination temperature.However, the effect of texture on activity requires further investigation.
Recently, Centi et al. (1995) observed isolated copper ions and highly dispersed Cu 2+ clusters in samples calcined between 400 and 600℃. of NO and its reduction over oxides, increasing the rate of conversion of NO; the activity later declines but remains higher than in the absence of oxygen.
Accordingly, copper oxides based on alumina have been found to be active in the selective reduction of NO with hydrocarbons/O 2 (HC-SCR).However, the effect of oxygen concentration must be considered.show that the conversion of NO increased sharply with the concentration of nitric acid from 0.5 N to 6 N, when a modified catalyst was used at temperatures between 800 K and 1023K.Nitric acid is a strong oxidizing reagent with a strong electrochemical tendency, corresponding to the generation of acidic functional groups on the surface of alumina when treated with this acid (Teng et al., 2001).The reduction of NO causes the extent of conversion to increase with the number of acidic sites on the surface of the alumina, resulting  (Xie et al., 2000).
Table 3 presents the pH of the acid-modified aqueous slurry.The γ-Al 2 O 3 is seen to exhibit basic properties.The sample treated with 6 M nitric acid is the most acidic, which result is consistent the literature (Zhu et al., 2000).Consequently, the activity of the catalyst modified with 6 M nitric acid was considerably improved, as shown by conversion rates of over 90% from nitric oxide to nitrogen at 1023 K, with a spatial velocity of 108,000 h -1 ; the addition of methane clearly promotes the conversion of NO.   these complexes evolve into triethylamine at low temperatures (＜773 K).Similar results have been reported elsewhere (Teng et al., 2001;Zhu et al., 2000).Thus, pre-treating the support alumina with a more concentrated acidic solution increases

Conclusions
This study examines the effects of treating, with published an excellent review of the chemistry of copper-based catalysts, in which they indicated avenues for future research.Copper-containing catalysts (zeolite-and oxide-based catalysts) can be used as the active catalytic decomposition phase in catalytic processes because they are shape-selective in a broad variety of reactions.Accordingly, catalytic reduction is a new process for reducing nitrogen oxides, and the catalysts employed are effective at higher temperatures than conventional catalysts.
on the involvement of metal oxide catalysts, which exhibit significant catalytic activity in the selective reduction of NO by hydrocarbons, (HC-SCR) , in this case methane on Cu/γ-Al 2 O 3 catalysts.The effects of three types and three different concentrations of inorganic acid, including nitric acid, phosphoric acid and acetic acid solution used to treat a γ-alumina support, on the reduction of NO, were also investigated.This study presents the results of laboratory studies of the catalytic reduction of NO over copper catalysts in a stream of air, and reports the activity of Cu/γ-Al 2 O 3 with various loadings of copper, as prepared by the incipient wetness impregnation method.

Figure 2 .
Figure 2. Effect of copper loading on the Cu/γ-Al 2 O 3 catalyst for the conversion of NO with CH 4 .

Figure 3
Figure 3 presents the effect of NO conversion at various calcination temperatures.The figure shows that the rate of conversion of NO increases with the calcination temperature in the range 823-873 K.

Figure 7
Figure 7 plots the effects of the 8 wt.% Cu/γ-Al 2 O 3 catalyst modified with nitric acid at various concentrations on the conversion of NO by methane at various temperatures under the same operational conditions as listed in Fig.6.The results

Figure 8
Figure 8 plots the variations of TPD desorption intensity associated with 8 wt.% Cu/γ-Al 2 O 3 modified with various inorganic acids.The temperature at which the peak desorption occurs is related to the adsorption strength.Nitric acid caused intense surface oxidation, revealed by the evolution of an amount desorption from the surface of the tested catalysts, implying that the conversion rate of NO is high in reduction over Cu-oxide catalysts modified with 2 M nitric acid.At temperatures above 773 K, the amounts of triethylamine evolved with the four catalysts are similar.The difference between the extents of the

Figure 8 .
Figure 8. TPD of triethylamine profiles of 8 wt.% Cu/γ-Al 2 O 3 catalysts that are untreated or treated with various inorganic acids.

Temperature
De-NO reaction over the copper catalyst.Hence, copper-based catalysts modified with nitric acid are active in transforming nitric oxide.Notably, the active sites of the surface copper species the catalysts were determined by X-ray diffraction (XRD) to be Cu (I) and Cu (II), which might also determine the efficiency of the De-NO reaction.NO was adsorbed onto Cu + cations and decomposed in situ into molecular nitrogen and Cu 2+ -coordinated oxygen.Oxygen that bridges two copper sites is easy to abstract.Typically, hydrocarbons regenerated at the active site may react vigorously with surface oxygen.Recent results published by Moretti et al. (1999) indicate that the most active copper sites might be dimeric (Cu 2 O 2 2+ ) or those of Cu + …Cu 2 + …O - species, probably bonded to next-nearest-neighbor framework AlO 4 -species.As a result, the efficiency of De-NO mixing obtained when the inlet gas is mixed only with methane implies that methane is a good reducing agent.Accordingly, in this study, alumina-supported copper catalysts modified with acid are shown to be highly active in the catalytic decomposition of NO in a reductive environment.The surface morphological changes of copper loaded γ-Al 2 O 3 were elucidated by scanning electron microscopy to provide further information concerning the status of the copper species in these catalysts.Figure 10 illustrates the surface structure of an acid-treated Cu/γ-Al 2 O 3 catalyst.Figure 10 (a) shows that the surface of the Cu/γ-Al 2 O 3 catalyst is more aggregated and crystalline than that depicted in Fig. 10 (b). Figure 10 (b) shows that the disaggregation and dispersion phases were formed by the direct removal of metal ions by the acidification of the surface of the catalyst, implying that the porosity of the particles changed.These crystal phases may explain the high activity of the catalysts.The results also confirm that the dispersion of the catalyst increases the efficiency of removal of NO by methane.

Figure 10 .
Figure 10.SEM photograph of the outward appearance of dispersed metal white point of 8 wt.% Cu/γ-Al 2 O 3 catalyst that is (a) untreated and (b) that is treated with 6 M nitric acid.
three inorganic acids at various concentrations, a series of copper-based catalysts, on the conversion associated with decomposition of NO with hydrocarbon (HC-SCR).Reactions with methane are investigated, methane is found to support the efficient conversion of NO.The 8 wt.% Cu/γ-Al 2 O 3 catalyst modified with nitric acid is more active in modifying the catalyst than one modified by either of the other two acids.The rate of conversion of NO increased sharply with the concentration of nitric acid from 0.5 M to 6 M in a modified catalyst at temperatures from 800 K to 1023 K.The extent of promotion of NO decomposition may be associated with the number of additional acid centers on the acidified catalyst that can trap NO molecules.A higher concentration of nitric acid is associated with greater desorption from the surface of the test catalysts, and thus more extensive conversion during the reduction of NO than that associated with a copper-based catalyst modified with nitric acid at a concentration of 6 M.This enhancement is related to the additional acid sites and greater acidity of the γ-Al 2 O 3 that adsorbs NO molecules.A reduction of about 93.2% of the NO can be achieved over the catalyst modified with nitric acid at a concentration of 6M at a temperature of 1023 K.

Table 1 .
Surface area and texture of untreated catalysts, determined by BET.
catalysts.The distribution of pore sizes was determined as follows.First, the specific surface areas of the catalysts were measured by nitrogen adsorption-desorption, using a BET surface area analyzer(ASAP 2000, Micromeritics, Norcross,   Georgia).Table1presents the compositions and properties of the prepared catalysts.X-ray

Air Compressor Mass Flow Meter Pressure Regulator Filter Dewater Mixer GAS Analysis GC TCD Preheater Thermocouple Furnace Thermocouple To Vent To Vent Catalyst Bed Reactor He H 2 O Water bath Mass Flow Meter Saturated Water Vapor Producer Mass Flow Meter CH 4 Mass Flow Meter NO
Figure 1.Schematic diagram of the tubular fixed-bed reaction (TFBR) system.Neckarsulm, Germany), which was (linked to the designed location to enable continuous monitoring during combustion.Gas samples with known concentrations of NO were used to calibrate the analyzer.

Table 2
2 /g); the one modified with acetic acid (168 m 2 /g) had the next highest surface area, and the one modified with phosphoric acid (162 m 2 /g) had the least surface area.Modification with nitric acid was also more effective than with

Table 2 .
Area and texture of catalysts after treatment with acid, determined by BET.