Reduction of NO on copper and its poisoning by SO2 a mechanistic study

CatalystDeactivation 1999 B. DelmonandG.F. Froment(Editors) 9 1999ElsevierScienceB.V. All rightsreserved.

249

Reduction of NO on copper and its poisoning by SO2, a mechanistic study C.M. Pradier a*, H. Lu b, P. Dubot ~ "Physico-Chimie des Surfaces, ESA 7045, Ecole Nationale Sup6rieure de Chimie de Paris, 11 rue P. & M. Curie, 75005- Paris, France, e-mail [email protected]" b Materials Physics, KTH, Stockholm, Sweden CECM, 15 rue G. Urbain, 94400- Vitry sur Seine, France *to whom correspondence should be addressed

Abstract A combination of catalytic tests and surface studies has been undertaken to better understand the poisoning effect of sulfur dioxide upon the reduction of NO by a hydrocarbon. The characterization of the copper surface by photoelectron spectroscopy, at various stages of the reaction, made clear an initial oxidation of the catalyst accompanied by the formation of an oxygenated intermediate; it is followed by a reduction of the surface and a rapid reaction of NO with this species. In situ surface infrared measurements have been made on a model copper surface, Cu(110), under 10.5 Torr of SO2 or SO2 + O= revealing the formation of molecular adsorbed compounds when the surface is oxidized, slightly covered with oxygen or hydroxylated. SO 2 is a drastic poison of the catalytic NO reduction; it reacts with the oxidized surface leading to sulfite and sulfate groups, it interacts with water, leading to hydrogenosulfoxide adsorbates which can obviously block a number of surface sites. It also dissociates on the metallic copper and occupies surface sites. It does not block the hydrocarbon adsorption completely but severely reduces the formation of the active intermediate. 1. INTRODUCTION The unavoidable presence of trace amounts of SO2 in hydrocarbon streams is responsible for a deactivation of copper-based catalysts [1]. It is of crucial importance in deNOx catalysis when exhaust gases contain relatively large quantities of oxygen and sulfurcontaining compounds. According to Iwamoto et al., on Cu-ZSM-5 zeolites, the catalytic NO decomposition is drastically poisoned upon addition of sulfur dioxide whereas, at 773K, the reduction of NO with propene is only slightly affected [2]. The poisoning effect of SO2 hence appears to be dependent upon the mechanism of the reaction of NO reduction and it is important to characterize the interaction of SO2 with the copper catalyst surface under conditions as close as possible to the ones of the reaction. The removal of sulfur oxide from fuel combustion is also an important subject because emission of such compounds is responsible for air pollution. Today, the storage-recovery process on a copper oxide catalyst seems to be the most promising approach. Questions remain to be solved in order to optimize reactivity of SO2 in the presence of oxygen and a hydrocarbon.

250 A basic study of the reactivity of a copper planar surface in the reduction of NO by a hydrocarbon was previously reported. It underlined the role of an oxygenate intermediate and the poisoning of its formation by sulfur dioxide [3, 4] Interaction of SO2 with copper and copper oxide surfaces has been investigated both on dispersed and model surfaces. As an example, the mechanism of formation of sulfate species on copper/alumina has been studied by J.C. Lavalley using IR. The authors suggest that, in the presence of gaseous oxygen, copper oxidizes SO2 into SO3 which then forms a stable surface sulfate [5]. On polycrystalline Cu, Cu20 and CuO surfaces, the occurrence of Sads, SOads, SO2ads , SO3ads as well as SO4ads has been deduced from XPS data, depending on the oxide stoichiometry, the temperature and SO2 pressure [6]. The role of surface defects upon SOx dissociation has been made clear both on polycrystalline copper and on Cu(111) [7]. On copper single crystal surfaces, Cu(110) and Cu(100), a recent study has shown, by in situ infrared analysis, the formation of oxidized compounds, depending on the oxidation level and on the structure of the surface [8]. This paper aims at clarifying the role of sulfur dioxide in the NO reduction catalytic process by correlating catalytic results to surface characterization data. First, the poisoning effect of sulfur dioxide upon the catalytic reaction of NO reduction in the presence of isobutene has been investigated on a plane copper surface. Second, the interaction of SO2 and oxygen with a model copper surface has been characterized, at a molecular level by use of surface science techniques, XPS and IRRAS. Both studies have been achieved with the objective of determining the surface intermediates, understanding the reaction paths and eventually the poisoning mechanism when trace amounts of SO2 are in the reactant feed. 2. EXPERIMENTAL

The NO reduction was studied in a 18 d m 3 closed, batch-reactor. The catalyst, a planar polycrystalline, 70 cm 2 area, copper foil, 5N purity, was placed in the reactor which was inside an oven so that both gas and catalyst were heated to the desired temperature. A small, constant, leak to a mass spectrometer (Balzers, QMA 125) enabled to monitor the gas phase composition as a function of time. The catalytic reactor was connected to a UHV chamber equipped with a VG photoelectron spectrometer for surface analyses. All spectra were recorded using a Mg kot (1253.6 eV) X-ray source with a pass energy of 20 eV. The copper surface was mechanically polished and rinsed in alcohol before being cleaned in the reactor itself. The copper sample was reduced in hydrogen at 900 K for 2h before each catalytic test. The interaction of SO2 with a model copper surface has been investigated at room temperature, by in situ Infrared Reflection Absorption Spectroscopy (IRRAS), in a UHV chamber equipped with ZnSe windows which enable the IR beam to reach the sample at grazing incidence (6~ A Fourier-transformed infrared spectrometer (NICOLET Magna 550) was used with a MCT detector. A typical spectrum was obtained, at 4cm ~ resolution, by ratioing the signal over a reference spectrum taken immediately before gas admittance. A Cu(110) surface was chosen in these in situ IRRAS analyses because it is rough enough, at the atomic scale, to mimic the structure of a polycrystalline sample. It also has a controlled and well known structure which makes possible the discussion of the nature and of the coordination of the surface species at a molecular level.

251 3. R E S U L T S 3.1 Catalytic tests The reduction of NO was studied in the presence of oxygen, isobutene and trace amounts of sulfur dioxide under the following conditions: PNO = PO~ = 1 Tom Pi-C4Hs = 0.5 Torr, 0 < PSO~ < 10 .4 Torr, T = 770 K. These pressure ratios correspond to slightly reducing conditions regarding the complete oxidation of the hydrocarbon. Figure 1 shows the gas phase composition (main constituents) as a function of reaction time for an initial pressure PSO~ = 10 .6 Torr, i.e; 0.2 ppm SO~ in the gas mixture. . . . . . . . . . . . . . .

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Figure 1. Time course of the reaction of NO conversion. T = 770K, PSO2 = l x l 0 "6Torr, PNO= PO2 = 1 Torr, Pi-C4H8 - 0.5 Torr (reactants and main products). The conversion of NO proceeds in two stages, an initial stage during which the main catalytic process is the consumption of 02, HC and NO to the benefit of CO2, H20 and iC4H60 and a second stage marked by a rapid conversion of NO. No obvious changes in the reaction products were detected when PSO 2 was varied up to 10 .4 Torr. Noticeable is the rapid decrease of the SO 2 pressure in the gas phase though neither sulfides, sulfites nor sulfates could ever be detected by mass spectrometry throughout the reaction. Let us note also the simultaneous decrease of NO, and of i-C4H60 in stage 2 of the reaction. The latter has been identified as an active intermediate of the reaction. The rate of conversion of NO, in stage 2, considered as representative of the catalytic activity under a given set of conditions, drastically decreases when PSO2 passes from 0 to 10 .4 Torr as reported in Table 1.

252 Table 1 9 Catalytic activity for different values of PSO2 in the initial gas mixture

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3.2.Surface characterization In order to interpret these catalytic results at a molecular level, a careful characterization of the catalytic surface has been carried out both by photoelectron spectroscopy (XPS) at various stages of the reaction and by in situ Infrared Reflection Absorption Spectroscopy (IRRAS) in another series of experiments. XPS characterization At stages marked A, B and C, on Figure 1, the catalytic chamber was pumped out and the copper sample was transferred under vacuum to the analysis chamber. We will focus on the O ls and S 2p spectra which are relevant to understand the surface mechanism. These spectra are shown in Figure 2. One can see important, and non independent, changes in the oxygen and sulfur peaks. Corresponding to the <> stage of the reaction, an intense oxygen peak is observed which can be fitted with a main contribution at 530.5 ascribed to oxygen from the copper oxide and possibly from SOx species, plus a weak shoulder at ca 532.6 eV. The S 2p peak is broad, with at least two contributions at ca 169 and 170.0 eV and, a very small additional peak at ca 161.5 eV. These two peaks are drastically modified in the further stage of the reaction. The surface is almost depleted in oxygen while the sulfur peak is now centered at 161.5 eV. In the final stage of the reaction, the oxygen contribution is still weak and at least three peaks are detected for the S 2p level, 161.5, 170.0 and 171.9 eV. S2p

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253 Note that the copper signals, Cu 2p and CULMMAuger lines indicated that the surface was oxidized to Cu20 in stage A and in a metallic state, Cu ~ in stages B and C (spectra not shown). IRRAS measurements IRRAS spectra have been taken on the Cu(ll 0) surface, either in the metallic state or slightly oxidized to Cu20 in its superficial layers, in the presence of 10.5 Torr of SO2. No molecular species could be detected by interaction of SO2 with the metallic surface suggesting a complete dissociation of sulfur dioxide. Figure 3 shows the reflection absorption spectrum of the oxidized Cu(110) surface upon SO2 + 02 interaction. These conditions mimic, in spite of the absence of hydrocarbon, the surface reactivity at the initial stage of the reaction, i.e. when the copper surface is still oxidized to Cu20. Vibration bands appearing in the S-O stretching region, have been attributed to the co-existence of sulfates and sulfites resulting from a nucleophillic attack of the surface oxygen atoms towards the sulfur atom of SO2 groups. The position and the number of vibration bands suggest that sulfites are monodentate, coordinated via one oxygen atom (bands at 870, 940, 965 and 1000 cm l) and sulfates are bidentate, resulting from SO2 interaction with 2 surface oxygen atoms (bands at 1030, 1130, 1150 and 1250 cm "l) [9].

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Figure 4. IRRAS spectrum of the hydroxylated Cu(110) surface under SO2 + H20 (Ptot = 10.4 Torr)

Because the presence of water together with $02 in the gas phase is likely to alter the nature of the surface intermediates, the interaction of SOz + H20 was also studied on a hydroxylated and partially oxidized Cu(110) surface (see Figure 4). A new broad band at 1280-1300 cm "t indicates the formation of hydrogenosulfates on the surface [ 10]. Finally, the interaction of SOz + isobutene was investigated on the hydroxylated Cu(110) surface by in situ IRRAS. The spectrum is shown in Figure 5.

254

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Figure 5. IRRAS spectrum of the hydroxylated Cu(110) surface under an equimolecular gas mixture, PSO2 = Pi-C4H8 = 5xl 0.5 Torr Vibration bands in the v(S-O) region are a~ain detected; those are weak and co-exist with a slightly more intense band at ca 1660 cm", characteristic of the stretching vibration mode v(C=C), which identifies the existence of some adsorbed isobutene together with sulfite and sulfate groups.

4. DISCUSSION The XPS spectra show distinct changes in the surface chemical composition throughout the catalytic process. In agreement with the literature, the low BE S2p peak can be unambiguously attributed to dissociated sulfur whereas the peak at high BE is characteristic of adsorbed SO2 or SOx species, x being equal to 3 or 4. This finding is in agreement with what has been observed by Baxter et al. i.e. after an initial formation of SO3 surface complexes, by interaction of SO2 with Cu20, SO3 dissociates or disproportionates at 673 K leading to CuxS surface species [ 11 ]. These authors propose the following mechanism: Cu20 + SO2(g) ~ Cu2SO3 (a) --4 Cu2S + 3/2 02 In our experiments, the presence of isobutene in the gas phase is likely to promote the reduction of surface sulfoxides into surface sulfides. Note that we never detected any shift of the copper Auger lines. Cu2S may not appear because it is only a surface compound on a bulk metallic sample. The oxygen signal is dominated by the Cu20 and SOx contribution at the beginning of the reaction; the shoulder at higher BE is attributed to adsorbed OH groups, ti20 being a primary product of the reaction. These data suggest that, on an oxidized surface, SO2 essentially binds to surface oxygen atoms which are strong Lewis base centers. This is confirmed by the IRRAS data that make clear the formation of molecular SOx species when O atoms are preadsorbed. SO2 deactivates the catalytic process, in the initial stage of the reaction, by competing, in the adsorbed phase, with the hydrocarbon for reacting with oxygen. It is a chemical effect. The formation of both sulfites and sulfates, on an oxidized Cu(110) surface, can be explained by the presence of oxygen in the gas phase. Sulfites result from a direct interaction of SO2 with one surface oxygen, whereas the formation of sulfates obeys the following mechanism : oxygen molecules react with surface anions of the copper oxide, providing 2 oxygen atoms, at short distance from each others, which then interact with the S atom of impinging SO2 molecules.

255 Note that the surface concentration in sulfites and sulfates can be estimated to several tenths of a monolayer knowing that a monolayer of sulfite corresponds to an absorbance of 4 to 20x10 "4 [12]. This is of course a rough estimation considering the orientation sensitivity of the signal intensity inherent to the IRRAS technique. Sulfoxide species, Cu2SO3 or CuSO4, are l~aown to decompose at T>400 K under low-pressure conditions, and of course, our conclusions, based on surface analyses at RT, assume that these adsorbates are stable at 770 K under a reactive pressure of a few Torr. The presence of disulfides, $2052, resulting from the reaction, 02. + 2SO2 ~ $2052", is also possible though we could not detect them (bands at v < 700 cm "l [ 13]). The reaction of SO2 with water, a product of the reaction, leading to hydrogenosulfite ions HSO3-, cannot be excluded. The conversion of SO2 into HSO3, promoted by water, is often encountered in the Claus reaction [14]. It is also an intermediary step of the formation of surface sulfite ions on copper. These complexes, which have been identified by in situ IRRAS (Figure 4), may contribute to the surface poisoning by a steric blocking of the surface sites. The 2 "d stage of the catalytic process, stage B, corresponds to the complete consumption of oxygen from the gas phase and to the reduced state of the catalytic surface; At that time, SO2 dissociates on the surface in agreement with what has been shown by IRRAS on a metallic Cu(110) surface. In other words, the surface is, in the most "active" stage of the reaction, occupied by atomic sulfur coming from SO2 dissociation, the oxygen atoms are probably involved in a reaction process with the hydrocarbon. The poisoning effect of SO2 is, at that stage, essentially due to surface occupancy (steric effect) rather than to a strong oxidation of the copper surface as suggested by Iwamoto on copper dispersed catalysts. Note that, at 700 K, sulfur is likely to diffuse into the bulk copper, explaining the progressive consumption of gas-phase SO2. At the very end of the reaction, the growth of a new S2p contribution at high BE is ascribed to complex species upon interaction of surface sulfur and the oxygenated intermediate i-CaH60. Finally, note that the poisoning by sulfur dioxide is reversible ; the catalytic properties of the copper surface could be recovered after a treatment at 900 K in hydrogen. At that temperature, sulfur diffuses out of the bulk and desorbs. Conclusion

The poisoning effect of SO2 upon the reduction of NO by isobutene has been clarified thanks to a combination of catalytic tests and surface characterization. SO2 molecules tend to dissociate on a metallic copper surface whereas they lead to complex and sterically big adsorbed species on an oxygen-enriched surface. Under the initial reaction conditions, SO2 reacts with oxygen, in competition with the hydrocarbon, and induces a decrease of the rate of formation of the active intermediate. Our results also show that surface sulfoxide species are not stable. They tend to dissociate or disproportionate, leading to adsorbed sulfur which sterically poisons the catalytic surface in the most active stage of the reaction. References 1 M. Iwamoto, H. Yahiro and K. Tanda, Stud. Surf. Sci. Catal., 37, (1988) 219. 2 M. Iwamoto, H.Yahiro, S.Shundo, Y. Yu-U and N. Mizuno, Appl. Catal., 69 (1991) LI5. 3 H. Lu, C.M.Pradier, and A.S. Flodstr6m, J. of Mol. Cat., A, 112, (1996) 447. 4 H. Lu, C.M.Pradier, and A.S. FlodstrOm, J. of Mol. Cat., A, 112, (1996) 459. 5 M. Waqif, O. Saur, J.C.Lavalley, S. Perathoner, and G. Centi, J. phys. Chem. 95, (1991)4051.

256 6 7 8 9 10 11 12 13 14

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