Adsorption and decomposition of NO on carbon and carbon-supported catalysts

Carbon 40 (2002) 119–124

Adsorption and decomposition of NO on carbon and carbon-supported catalysts ´ J. Zawadzki*, M. Wisniewski Faculty of Chemistry, Nicholas Copernicus University, Gagarina 7, 87 -100 Torun´ , Poland Received 19 January 2001; accepted 25 March 2001

Abstract The interactions of NO with carbon and carbon-supported catalysts have been investigated by means Fourier transform infrared spectroscopy. Nitric oxide direct decomposition over carbon-supported catalysts (Cu, Pt) was studied in a temperature ranging from 473 to 623 K. NO conversion increased with increasing reaction temperature in the whole temperature range. The carbon-supported Pt catalyst has a very high activity for the decomposition of NO in the absence of oxygen. As a result of NO chemisorption isocyanate (–NCO) species on the surface of carbon containing Cu were observed. When the reaction temperature was increased, the –NCO band at 2229 cm 21 became more intense.  2002 Elsevier Science Ltd. All rights reserved. Keywords: A. Carbon films; B. Impregnation; C. Infrared spectroscopy; D. Catalytic properties

1. Introduction Nitrogen oxides (NO x ) in exhaust gases are a harmful material that should be excluded in order to protect the earth from acid rain, or other environmental concerns. Such concerns about our environment have simulated much extensive research on catalysts for: 1. Direct reduction of NO [1–10] 2. Selective catalytic reduction [11–19] The decomposition of NO is thermodynamically favorable but kinetically hindered, and is therefore only achieved with catalyst. Direct decomposition of NO into its constituents, N 2 and O 2 , without using a reducing agent appears to be a very attractive option. This would avoid the complications linked to the addition of a reductant and the associated emission of this reductant. The selective catalytic reduction (SCR) of NO x is one of the most effective means. Much attention, therefore, has been focused on the developing new catalysts, which can degrade the gas to non-toxic ones as N 2 or CO 2 . Extensive work is being carried out to improve their properties as well as to

*Corresponding author. E-mail address: [email protected] (J. Zawadzki).

characterize them and to understand the mechanism through which nitric oxide decomposition occurs. Generally, NO reduction is realized using reducing agents like CO [11–13], hydrocarbons [14–16] and ammonia [17–21]. Utilization of carbon and carbon-supported catalysts for removal of NO x has also been proposed [19–27]. An NO molecule has an unpaired electron and is paramagnetic. The presence of an unpaired electron in the molecular 2p * orbital results, on the one hand in a relatively low binding energy, and on the other hand the ability to either donate or accept an electron in forming a chemical bond. According to Kaneko at al. [28], NO molecules adsorbed on surface of carbon are dimerized even above room temperature. The NO dimmers in the micropores were formed by an adsorbate–adsorbate interaction enhanced by the micropore fields. The reaction between NO and carbon surface whose mechanism has not yet been completely elucidated, is promising with respect to the decomposition of NO to N 2 and O 2 . The present investigation was undertaken with the aim of developing a clearer picture of the species adsorbed on carbon-supported catalysts when they are exposed to NO in the absence of oxygen. In situ infrared spectroscopy has been widely used to study adsorbates on the surface of catalysts. It has been proven to be a powerful tool, which provides useful information concerning the nature and the

0008-6223 / 02 / $ – see front matter  2002 Elsevier Science Ltd. All rights reserved. PII: S0008-6223( 01 )00081-1

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role of the observed intermediate species in the reaction network.

2. Experimental Carbon films used in this study were prepared from cellulose [29]. The raw material used for carbonization was cellophane, which was previously purified. The following procedure was used to prepare an oxidized carbon film (C ox ). Samples outgassed at 873 K (a standard pre-treatment) were exposed to 1000 hPa of pure O 2 at 573 K for 1 h. In the present studies carbon films differing in the chemical structure of surface functional groups were used. Infrared (IR) spectroscopic studies were carried out in a vacuum cell described previously [29]. The cells’ construction enabled the thermal treatment of the carbon film up to 1200 K in any controlled atmosphere or in a vacuum. IR spectra were recorded for the samples after they were cooled down to room temperature. Spectral changes accompanying the adsorption of NO were established by comparing IR spectra of the same film recorded in a vacuum and those recorded under a definite NO pressure. Catalysts for Fourier transform infrared (FTIR) analysis were prepared using oxidized carbon film as a support. The C ox carbon film was immersed in 0.1 M aqueous solution of Cu (NO 3 ) 2 . The oxidized carbon film containing Pt was prepared using a 3% solution of H 2 PtCl 6 . Metal-loaded C ox Cu and C ox Pt powdered carbon samples (for catalytic tests) were prepared by the impregnation procedure. The impregnations were made with quantities of salts chosen in order to obtain 1 wt.% of Pt or Cu. In order to perform an impregnation, it is necessary to consider that the true ion-exchange reactions between ions of the precursor in solution and the surface of the support occur during impregnation. Metal-loaded unoxidized carbon samples (C 873 Cu and C 873 Pt) were prepared by outgassing metal-loaded C ox at 873 K. Catalytic activity measurements for the NO decomposition and kinetic studies were conducted at atmospheric pressure in a microcatalytic reactor under a steady-state conditions, by passing a reactant gas containing 1720 ppm NO in Ar. Nitric oxide (98.5%) from Aldrich and a high purity of Ar (99.99%) were used. The reactor was a 4 mm i.d. quartz tube. Samples weighting 49–51 mg were used for the activity and for the kinetic studies. Flow rates of the feed gas (25–125 ml min 21 ) were governed by Tylan mass flow controller. Prior to each experiment the catalyst was pre-treated at 623 K in flowing Ar for at least 3 h and then cooled to the desired temperature. Prior to the catalytic activity measurement the gas mixture (1720 ppm of NO in Ar) was passed through carbon containing trap to remove traces of NO 2 before it was allowed to come in contact with the catalysts. In order to examine the reaction of NO with carbon, we take advantage of the fact that the reaction of carbon with NO 2

is very rapid. The FTIR spectrum of this ‘prepurified’ nitric oxide does not show NO 2 absorption bands. Week bands of N 2 O were present. As molecular absorption coefficient of NO is noticeably week compared with that of the NO 2 or N 2 O band, the NO 2 was not present in the gas mixture or its concentration was negligibly small. FTIR analysis for nitrogen oxides employed multiple reflection gas cell, which gave an accuracy of 0.5 ppm for NO 2 and 20 ppm for NO. In order to calculate the amount of nitrogen produced, it was assumed that the nitric oxide reacts to form either nitrogen or remains in the form of oxides (NO, N 2 O). Since FTIR spectrometry is capable of speciating nitrogen oxides, the amount of nitrogen formed would be equal to the amount of nitrogen missing from the oxide form. The effluent gases (NO and N 2 O) were analyzed using Mattson FTIR Genesis II system fitted with a 2m (Graseby Specac, Sirocco Series) gas cell.

3. Results and discussion Fig. 1 shows the infrared spectra obtained after exposing the carbon sample containing Cu to NO at increasing temperatures. Extensive studies [30] have established the relation between the state of adsorption sites and the wavenumber of adsorbed NO, resulting in the widespread use of IR spectroscopy of adsorbates for catalysts characterization. The adsorption of NO on copper containing catalysts followed by IR spectroscopy has been carried out [13,31,32] with the aim to determine the oxidation state of copper on supported copper catalysts. The shift of the nNO 21 band with respect to the gaseous NO (1876 cm ) will depend on the electron donation from non-bonding NO orbitals to empty d-orbitals of catalyst as well as the back donation from the d-orbitals of catalysts to the antibonding orbitals of NO. IR spectra show that the physical adsorption of NO on C ox Cu is very low. Only very weak 21 21 signals around 1900 cm and 1845 cm are visible. In the case of oxidized carbon film, strong absorption in the 21 region 1800–1500 cm and thermal decomposition of surface functional groups made complex the interpretation 1 of very small bands attributable to NO adsorbed on Cu 21 and Cu surface species. Since the dissociation of NO is accepted by several investigators [5,31,33] as the required first step in NO reduction, the formation of small band at 21 1590 cm may be caused by chemisorption of oxygen, which was formed during dissociation of NO according to the scheme: NO ( g ) 1 C (s) → C (s) [NO] (a) C (s) [NO] (a) → N 2 ( g ) 1 C (s) [O] (a) The increased intensity of the 1600 cm 21 band confirm

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Fig. 1. FTIR spectra recorded after exposure of C ox Cu film to NO. (1) C ox Cu film outgassed at 473 K; after contact with NO at (2) 298 K, (3) 473 K, (4) 573 K, (5) 623 K; (6) after outgassing at 473 K.

that in the initial phase of oxygen chemisorption, the ion-radical surface compounds suggested previously [29] were formed. The weak IR bands between 1580 cm 21 and 1650 cm 21 observed here would also be assigned to n(–NO 2 ) and n(–ONO) of an organic nitro and nitrite compounds. As a result of NO chemisorption at 473 K the sample displays band at 2229 cm 21 , which may be attributed to isocyanate (–NCO) species adsorbed on the carbon containing Cu surface. As the reaction temperature was increased, this band became more intense. Several of the investigators [12,34–36] who used IR spectroscopy of adsorbed NO have extrapolated their findings to construct an elaborate mechanism in which the role of the reaction intermediate is assigned to some species such as isocyanates. One possible mechanism for –NCO formation is as follows: NO ( g ) → NO (a) → N (a) 1 O (a) N (a) 1 CO (a) → NCO (a) The intensity of the NCO band on carbon supported Cu increased by heating the sample up to 623 K. CO is formed as a result of chemisorption of oxygen on the surface of carbon as well as the small thermal decomposition of surface functional C=O acidic groups. With increasing temperature (Fig. 1) the band at 1760 cm 21 decreases in intensity. The NCO species has been generally regarded as the spectator species in the NO1CO reaction to produce N 2 . However, it is not clear whether these isocyanate species

are intermediates in the main reaction, or if they are simply undesired side products hampering the efficiency of the decomposition process. Oxygen, originating from the NO decomposition reaction, can react with NO molecules to produce NO 2 . In the case of carbon supported catalysts metal-bound oxygen species can also contribute to the formation of NO 2 molecules that are adsorbed strongly on carbon surface. Because of the complex nature of the NO decomposition reaction, a number of adsorbed N x Oy species form even in the absence of gas phase oxygen. All of these N x Oy species can then interact with the surface functional groups of carbon, forming adsorbed entities with intrinsically different reactivities. It was interesting to check the thermal stability of –CNO species. No substantial changes (Fig. 1) in the intensity of the 2229 cm 21 band have been observed after 30-min evacuations at 473 K (spectrum 6), i.e. the NCO species are stable at this temperature. To further clarify the surface species formed during the reaction, the NO decomposition was investigated on C 873 Pt. It has long been established that the decomposition of NO on Pt is retarded by product oxygen [5,37]. Lim at al. [5] reported that for the case of Pt / carbon, the inhibition product oxygen is removed continuously from the platinum surface by the surrounding carbon support. Fig. 2 shows the IR spectra of adsorbed NO on C 873 Pt after heating at 623 K. A comparison of the behavior of adsorbed NO on C ox Cu (Fig. 1) and C 873 Pt (Fig. 2) shows distinct differences in chemisorptive and reactivity properties. NO molecules are readily dissociated on the platinum surface, producing nitrogen and leaving oxygen atoms to remain on the

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Fig. 2. FTIR spectra recorded after exposure C 873 Pt film to NO. (1) C 873 Pt film outgassed at 473 K; (2) after contact with NO at 623 K; (3) IR spectrum 2 from which the spectrum 1 has been subtracted. Fig. 3. FTIR spectra of the product stream from reaction of 1720 ppm NO in Ar with C 873 Pt at 598 K recorded at a different flow rate.

surface of carbon. IR spectra presented in Fig. 2 indicate that chemisorption of oxygen (formed as the result of dissociation of NO) leads to a decrease of absorption coefficient value of carbon and to increased intensity of the 1600 cm 21 band. The IR spectra presented above confirm the formation, in the initial phase of oxygen chemisorption, of ion-radical surface compounds suggested previously [29]. Strong evidence for the formation of a week bands at 1600–1800 cm 21 region (spectrum 3) was obtained by subtracting the spectrum of catalyst before exposure to NO (spectrum 1) from the spectrum obtained after the reaction of NO with C 873 Pt at 623 K (spectrum 2). The bands at 1598 and 1667 cm 21 (spectrum 3) are assigned to n(NO 2 ) and n(ONO) of nitro and nitrito surface compounds, respectively. It is believed that nitro and nitrito species could influence NO decomposition. The new band at 1763 cm 21 corresponds to the vibration of dinitrosyl (NO) d2 2 species. The band at 1717 cm 21 is probably due to surface C=O groups. It should be noted that the band at 2229 cm 21 is not present in the spectrum. Fig. 3 shows FTIR spectra of the gaseous product stream from the catalytic decomposition of NO over C 873 Pt catalyst at 598 K recorded at different flow rate. The infrared spectrum of pure nitric oxide (NO) gas is difficult to obtain, in part because nitric oxide is thermodynamically unstable with respect to disproportionation at room temperature 3NO → N 2 O 1 NO 2 IR bands at 1903, 1875 and 1853 cm 21 are respectively

due to the R-, Q-, and P-branch of NO. Upon raising the temperature (Fig. 3) these bands are almost completely removed while the bands of N 2 O at 2237 and 2211 cm 21 become more intense. Nearly complete conversion of NO was achieved at temperatures at 623 K. The absence of NO 2 and CO bands (Fig. 3) indicates that they are not present in the ‘prepurified’ reactant stream as well as in the product of the reaction (the concentration of these compounds is too small to be detected by IR in experimental conditions). The absorption bands in the 2300–2380 cm 21 region are due to the CO 2 . It is a possibility that oxygen, liberated during NO dissociation, react with the carbon surface or CO 2 might arise as a result of thermal decomposition of the surface functional groups. This means that when temperature reaches a certain level the carbon becomes not only a support, but also a reactant in NO decomposition. The changes in the intensities of N 2 O bands reached the maximum at 573 K probably because of its reaction intermediate character. Nitric oxide direct decomposition over the supports (oxidized and nonoxidized carbon) and carbon supported catalysts C ox Pt, C 873 Pt, C ox Cu; C 873 Cu was studied in a temperature ranging from 473 to 623 K. Fig. 4 shows the temperature dependence of the degree of NO conversion on carbon-supported catalysts. The addition of only 1% of Pt to the unoxidized carbon gave a considerable improvement in NO decomposition (it

´ / Carbon 40 (2002) 119 – 124 J. Zawadzki, M. Wisniewski

Fig. 4. NO decomposition over carbon and carbon containing catalysts as a function of temperature.

reached 98% decomposition already in 623 K). NO conversion increased with increasing reaction temperature in the whole temperature range. The pure carbons (oxidized and nonoxidized) have a very low activity toward NO decomposition.

Fig. 5. Effect of the contact time (t 5 m /F[g*h / mol]) on the decomposition of NO at different temperatures.

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Fig. 6. Selectivity towards N 2 defined as [N 2 / N 2 1N 2 O] of carbon supported Pt (C 873 Pt) upon varying total flow rate.

Fig. 5 shows the NO decomposition over C 873 Pt at different temperatures as a function of modified contact time. C 873 Pt possess very high activity for the direct decomposition of NO in the absence of oxygen. The variations in catalytic activity are accompanied by changes in contact time. An almost complete decomposition (98%) of NO is achieved above 573 K at contact time 1.5 g s mmol 21 . The rates of NO reduction and activation energies over carbon-supported Pt catalysts are shown in Table 1. The data in Table 1 show that the activity of carbon supported Pt catalyst was independent of the surface area. The results clearly indicate that oxidized carbon containing Pt is less reactive than carbon outgassed at 873 K. The active sites for NO decomposition appear to be produced over carbon supported Pt by outgassing at a high temperature. The unsaturated valency of the surface introduced by the elimination of the oxygen containing surface functional groups can constitute the active centers. In the absence of any other reductant in gas phase, NO is reduced by reaction with the carbon atoms. Fig. 6 shows the selectivity behavior of carbon sup-

Table 1 Comparison of rates reduction and activation energies over carbon supported Pt at 573 K Sample

SBET (m 2 g 21 )

NO conv. (%)a

Rate of NO disappearance (mmol s 21 g 21 )

EA (kcal mol 21 )

C ox Pt C 873 Pt

476 491

7.6 73.5

1.31310 22 1.85310 21

54.73 32.85

a

Flow rate 50 ml min 21 , [NO] o 51720 ppm.

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ported Pt catalyst as a function of the reaction temperature for different total flow rate of gases. The selectivity towards N 2 defined as [N 2 ] / [N 2 ]1[N 2 O] increases with increasing temperature. The N 2 selectivity decreases with the increase of the feed gas flow rate. For the lowest flow rate (25 ml min 21 ) increase of selectivity from 0.35 to 0.98 is observed in the 523–623 K intervals. The selectivity profiles of the downstream products during NO decomposition at short contact time are typical for consecutive reaction, indicating that N 2 O is an intermediate in the decomposition of NO; i.e. the reaction pathway for nitrogen formation can be expended by NO→N 2 O→N 2 .

4. Conclusions In this work the interaction of NO with carbon and carbon-supported catalysts has been studied by in situ infrared spectroscopy combined with continuous flow microreactor. IR spectra show that the physical adsorption of NO on carbon and carbon-supported catalysts is very low. The interaction of NO is markedly dependent on the chemical structure of carbon surface. The activity of carbon-supported catalysts towards NO decomposition was practically independent of the surface area. Pt loaded carbon outgassed at 873 K showed high catalytic activity for NO decomposition, even in the absence of oxygen. The selectivity towards N 2 increases with increasing temperature and with the decrease of the feeds gas flow rate. When temperature reaches a certain level the carbon becomes not only a support, but also a reactant in NO decomposition. Heating the C ox Cu sample with NO to 473 K resulted in the formation of 2229 cm 21 band, which may be attributed to isocyanate (–NCO) species. As the reaction temperature was increased, this band become more intense. The formation of isocyanate species can be explained by the NO–CO reaction mechanism in which (i) NO adsorbed on carbon surface dissociates to form adsorbed nitrogen and oxygen and (ii) adsorbed oxygen further reacts with carbon to produce CO. However, it is not clear whether these isocyanate species are intermediates in the main reaction, or if they are simply undesired side products hampering the efficiency of the decomposition process.

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