J . Chem. SOC., Faraday Trans. 1, 1986, 82, 61-68 The Support Effect of CuO Catalyst for the Reduction of Nitric Oxide with Hydrogen or Ammonia Tokio Iizuka" and Hideo Ikeda Department of Chemistry, Faculty of Science, Hokkaido Unicersity, Sapporo 060, Japan Susumu Okazaki Department of Industrial Chemistry, Faculty of Engineering, Ibaraki University, Hitachi 316, Japan The reduction of nitric oxide (NO) with H, or NH, over CuO, catalysts supported on ZrO, or TiO, has been investigated. The activity and selectivity of NO reduction were very sensitive to the types of support oxide. A gradual increase in activity of CuO/ZrO, for the reduction of NO with NH, was observed in the course of the reaction, while the activity of CuO/TiO, was rapidly reduced as the reaction proceeded under the same reaction conditions.For the reaction of NOfNH, in the presence of oxygen, however, CuO/ZrO, was less active than CuO/TiO,. Several states of Cu2+ on the surfaces of ZrO, or TiO, were detected by means of e.s.r. spectroscopy. Cu2+ ions on ZrO, were reduced more easily than those on TiO, and hence this catalyst was concluded to show a high activity in NO+NH, or NO+H, reactions . The catalytic reduction of NO is of considerable practical importance and of great interest in fundamental studies of heterogeneous catalysis. Various types of NO reduction reactions, e.g. those with H,, NH, or CO as a reductant, have been widely investigated to obtain a highly active cata1yst.l In many cases, the support of a catalyst often plays an important role in its catalytic activity.For the reduction of N,O with H,, MOO, supported on TiO, prepared from Ti(SO,), was reported to exhibit an anomalously high activity., For the reduction of NO with NH,, TiO, for Fe and V catalysts3? and active carbon for Cu catalysts were reported as effective supports.5 In this work, the catalytic activities of CuO, supported on TiO, or ZrO, were examined for the reduction of NO with H, or NH, and e.s.r. experiments were performed to elucidate the effect of the supports on the catalytic activity. Experiment a1 'The support oxide, ZrO,, was obtained by calcination of Zr(OH), at 500 "C for 2 h in air. Zirconium hydroxide was prepared by adding aqueous ammonia to an aqueous solution of ZrOCl,. The resulting precipitate was filtered off, washed with deionized water until no Cl- ions were detected in the washing water, and was then dried at 120 "C for 24 h.Titanium dioxide was obtained by calcination of titanic acid at 500 "C for 2 h in air. Titanic acid was prepared by pouring TiCl, into cold water, adding aqueous ammonia, filtering, washing, and drying at 120 "C for 24 h. The CuO/ZrO, catalyst was prepared by mounting the copper ammine complex by the ion exchange method followed by calcination at 350 "C for 2 h [CuO/ZrO,(i)]. Two kinds of CuO/TiO, were prepared in copper ammine complex solution; one by the ion exchange method [CuO/TiO,(i)] and 616 2, Reduction ofNO with H, or NH, l o o 3 A 0 5 15 30 4 5 60 reaction time/min N,, 1 N,O on CuO/TiO,(d). Fig. 1. Reduction of NO with H, at 280 "C. N,, A NH, on CuO/ZrO,(i); 0 N,, 0 N,O on CuO/TiO,(i); the other by the conventional impregnation method [Cul/TiO,(d)].These catalysts were heated in air at 350 "C for 2 h before use. The reactions NO+H, and NO+NH, were studied in a closed recirculation reactor having a volume of 400 cm3. A mixture containing 10 Torrt NO and 30 Torr H, (or NH,) was allowed to react at 220-280 "C. The reaction mixture was periodically analysed by gas chromatography. For the reaction of NO+NH, in the presence of air, a conventional flow reactor was used. The reactant flow rate was 35 cm3 min-l NO+ 35 cm3 rnin-l NH,+ 1350 cm3 min-l air and the space velocity was 20000 h-l. The conversion of NO was observed by a NO, analyser. The e.s.r. spectra were obtained by a Varian E3 spectrometer at room temperature or at liquid nitrogen temperature.Reaction Results The results of the NO + H, reaction are shown in fig. 1. Over CuO/ZrO,, the activity was very high and the products were only N, and H,O in the initial stage, but a gradual increase of NH, formation was observed in the course of the reaction. In contrast, the activity was low and the formation of N,O was observed along with N, over CuO/TiO,(i). Over CuO/TiO,(d), though the activity was almost the same as that of CuO/TiO,(i), the product was mainly N,O, and N, formation was negligible. The results of the NO + NH, reaction are shown in fig. 2. The product was exclusively N, over the all catalysts. The activity of CuO/ZrO, was also very high for this reaction. When the reaction was repeated after the complete conversion to N, was achieved over CuO/ZrO,, a gradual increase of activity was observed.However, the catalysts CuO/TiO,(i) and CuO/TiO,(d) showed low activities, and the decrease in activity was observed over both catalysts by the repetition of the NO+NH, reaction. The conversion of NO in the NO+NH, reaction in the presence of air at various reaction temperatures is depicted in Fig. 3. For this reaction, CuO/TiO,(d) showed the highest activity and CuO/ZrO, was least active. The activity of CuO/ZrO, showed two maxima at ca. 150 and 300 "C. t 1 Torr z 133.3 Pa.T. lizuka, H. Ikeda and S. Okazaki 63 2 reaction time/min Fig. 2. Reduction of NO with NH, at 220 "C. 0 CuO/ZrO,(i), 0 CuO/TiO,(i), a CuO/ TiO,(d). Samples were evacuated at 350 "C between each run.loo 1 100 200 300 4 00 TI" C Fig. 3. Reduction of NO with NH, in the presence of air at various temperatures. 0 CuO/ ZrO,(i), 0 CuO/TiO,(i), (j CuO/TiO,(d). E.S.R. Study Typical e.s.r. spectra of CuO/TiO,(d) are shown in fig. 4. After the oxidation of the sample at 350 "C with oxygen, a broad signal and axial symmetrical signal with parallel hyperfine structure of 78 G were observed in superposition [fig, 4(a)]. In addition to those signals, a pair of small peaks [P,(B)] were observed at both sides of the perpendicular position. When the sample was reduced at 220 "C with H,, the broad signal decreased in intensity and new signals appeared [fig. 4(b)]. The new signals are axial 3 F A R 164 Reduction ufNO with H, or NH, \ g I I (B) = 2.4 3 v ( b ) g I I (A) = 2 .3 5 ] Y V Fig.4. E.s.r. spectra of CuO/TiO,(d), (a) after the oxidation at 350 "C, (b) reduced with H, at 220 "C, ( c ) reduced at 270 "C, ( d ) after 75 min in NO + H, reaction at 280 "C, (e) after 15 min in NO + NH, reaction at 220 "C over the oxidized sample. symmetrical ones with larger hyperfine splitting in the parallel portion than that of the former peak and show another pair of peaks [P,(A)] inside the former pair [P,(B)]. With higher spectrometer gain, two sets of peaks with half the splitting constant of each parallel hyperfine structure of axial symmetrical species were observed. The pair peaks of P,(A) decreased, but P,(B) peaks did not show any change after the reduction at 270 "C with H, as shown in fig. 4(c). When the sample was treated with NO (10 Torr) + H, (10 Torr) at 280 "C for 75 min after the oxidation at 350 "C, the e.s.r.spectrum was almost the same as that obtained after the reduction at 220 "C with H, as shown in fig. 4(d). The same spectrum was also obtained after the reaction of NO+NH, at 220 "C for 15 min over the oxidized sample [fig. 4(e)]. In the case of CuO/TiO,(i), only a broad unsymmetrical signal was observed after oxidation at 350 "C as shown in fig. 5(a). When this sample was reduced with H, at 170 "C, an axial symmetrical species whose parallel hyperfine constant was 122 G and P,(A) pair peaks were observed [fig. 5(h)]. In the reaction of NO+H, at 280 O C , CuO/TiO,(i) was greatly reduced and P,(A) pair peaks increased distinctly with small peaks of Pl(B) as shown in fig.5 (c). When the oxidized CuO/TiO,(i) was contacted withT. Iizuka, H . Ikeda and S. Okazaki 65 YT g,= 2.05 + g1 Fig. 5. E.s.r. spectra of CuO/TiO,(i), (a) after oxidation at 350 "C, (b) reduced with H, at 170 "C, (c) after 30 min NO+ H, reaction at 280 "C over the oxidized sample, (d) after 15 min NO + NH, reaction at 220 "C over the oxidized sample. NO + NH, at 220 "C for 15 min, a broad unsymmetrical peak with a hyperfine constant of 160 G at gll = 2.21 was observed with small peaks of P,(A) [fig. 5(d)]. Oxidized CuO/ZrO, gave only a broad unsymmetrical peak at g z 2.1 as shown in fig. 6(a). The broad signal decreased in intensity drastically upon reduction when T > 100 "C. After the reduction at 220 "C for 15 min, the broad signal almost dis- appeared and a sharp axial symmetrical peak at g = 2.05 and gI1 = 2.30 was observed.In addition to this signal, a sharp singlet peak was superimposed in the perpendicular position of the Cu2+ signal. The spectrum is depicted in fig. 6(b). In the reaction of NO+H, at 280 "C, the catalyst was further reduced as shown in fig. 6(c). When the oxidized CuO/Zr02 was treated with the mixture of NO and NH, at 220 "C for 15 min, the sample was considerably reduced and the same hyperfine structure was observed in the parallel position as CuO/TiO,(i) in NO+NH,. The spectrum is shown in fig. 6(d). The intensity changes of Cu2+ on the catalysts after various treatments at different temperatures are shown in fig. 7. Discussion From the e.s.r. result, it is clear that at least two different states of Cu2+ exist on the surface of CuO/TiO,(d).The species which shows the parallel hyperfine structure of 78 G (B) was insensitive to the reduction and oxidation treatments. The pair of peaks which was denoted as P,(B) can be ascribed to the spin-exchanged pair species which had been observed in Cu(CH,COO), H,06> and CuY zeolite.8 The species of P,(B) was also insensitive to the reduction-oxidation treatments. Thus P,(B) species would be in the same state as the isolated Cu2+ which showed the hyperfine structure of 78 G. This 3-266 Reduction of NO with H, or NH, g=2.05 ( c ) Fig. 6. E.s.r. spectra of CuO/ZrO,(i), (a) after oxidation at 350 "C, (b) reduced with H, for 15 min at 220 "C, (c) after 30 min NO+H, reaction at 280 "C over the oxidized sample, ( d ) after 15 min NO + NH, reaction at 220 "C over the oxidized sample.4 0 o P 2 100 800 600 LOO 2 00 \ \ I I I I I 1 I I I I I I I 20 70 120 170 220 20 70 120 170 220 20 70 120 170 220 T/'C Fig. 7. E.s.r. intensity changes of the Cu2+ ion after the various treatments at different temperatures. 0 CuO/ZrO,(i), 0 CuO/TiO,(i), CuO/TiO,(d). ( a ) H,, (b) NO, (c) NO+NH,. state could be denoted as species B. After the reduction of CuO/TiO, (d), the broad rather symmetrical signal disappeared and the pair species P,(A) and the isolated species A which had the hyperfine structure of 122 G appeared. The broad symmetrical signal was ascribed to a non-linear pair in CuY zeolite by Chao and Lunsford.8 In this pair, it is thought that the exchange interaction involves many spins.On CuO/TiO,(d), itT. lizuka, H. Ikeda and S. Okazaki 67 would be reasonable to think that the many-spin-exchanged cluster of Cu2+ changed to the spin-exchanged pair and isolated Cu2+(A) upon reduction. Thus, the pair peaks of P,(A) would be ascribed to the spin-exchange of the Cu2+(A) pair. Species (A) was very sensitive to reduction-oxidation treatment in contrast to species (B). The hyperfine coupling constant of Cu2+(A) (122 G) was larger than that of Cu2+(B) (78 G) owing to the stronger interaction of Cu2+(B) with TiO, support than Cu2+(A). On CuO/TiO,(i), the main species was Cu2+(A), and the Cu2+(B) species was almost negligible. Thus, CuO/TiO,(i) is more easily reduced with H, compared to CuO/TiO,(d). The sample of CuO/ZrO,(i) showed a larger hyperfine coupling constant ( 1 57 G) than CuO/TiO,(i), and was reduced easier than CuO/TiO,(i).The order of reducibility of Cu2+ species corresponds well with the magnitude of the hyperfine coupling constant, Cu2+/Zr0, > Cu2+(A) > Cu2+(B). In the reaction of NO+H,, the catalyst was reduced gradually in the course of the reaction and the reaction rate became higher over the reduced catalyst. For this reaction, a redox mechanism on CuO or/and Cu+ has been propo~ed.~ The order of activity for the NO+H, reaction, CuO/ ZrO,(i) > CuO/TiO,(i) > CuO/TiO,(d), corresponds well with the reducibility of the catalyst. NO was reduced even to NH, in the final stage of reaction over CuO/ZrO,(i). At this stage it is probable that part of the Cu2+ was reduced to metallic Cu.In the e.s.r. spectrum of reduced CuO/ZrO,(i), a sharp singlet signal appeared in the perpendicular signal position of Cu2+. Though we cannot ascribe this signal to Cu metal because a free Cu atom is known to have an isotropic hyperfine splitting of 2100 G,1° the appearance of the sharp signal would have some correlation with the reduction of Cu2+ to the metallic state. On CuO/TiO,, neither the appearance of this signal nor the formation of NH, in the reaction of NO + H, were observed. Over CuO/TiO, (d), NO was reduced only to N,O. Since the species of Cu2+(B) was not reduced even at high temperature, only a small part of the Cu2+ would be responsible for the redox cycle in the reaction over this catalyst. For the reaction of NO+NH, it has been reported that the loss of activity is caused by the reduction of Cu2+ to Cu+ in the course of the reaction.11.12 Actually, over CuO/TiO,(i) and CuO/TiO, (d), the activity gradually decreased along with the colour change of catalyst from bright blue to red-brown in the reaction.In contrast to this, over CuO/ZrO,(i) the activity increased in the repeat reaction and the colour of the catalyst changed from bright blue to black. Since CuO/ZrO,(i) is more easily reduced in comparison to CuO/TiO,(i) or CuO/TiO,(d), metallic Cu has been formed in the course of the reaction and acts as an active site for the reduction of NO. The metallic state of Cu has been reported to show a high activity for this reaction via the dissociation of NH, over the surface.13~ l4 Thus, the activity over CuO/ZrO,(i) increased along with the reduction of Cu in the reaction.However, CuO/TiO,(i) and CuO/TiO,(d) were not reduced to the metallic state and lost their activity (probably as a result of the appearance of the Cu+ state). After exposure to NO+NH, gas, the hyperfine splitting of A , , = 160 G and A l = 25 G was observed on CuO/TiO,(i) and CuO/ZrO,(i), but was not seen on CuO/TiO,(d). This spectrum has been ascribed to the formation of the Cu(NH,), complex in zeolite-Y.ll It is probable that ammonia molecules can access Cu(A) on TiO, and Cu ion on ZrO, easily, but over CuO/TiO,(d), the interaction of Cu(B) with TiO, is strong and the ammine complex will not form. On the other hand, for the reduction of NO with NH, in the presence of air, CuO/TiO,(d) and CuO/TiO,(i) were active because the oxidized state of Cu was stable over their surfaces.Two maxima of the activity in the case of CuO/ZrO,(i) for the reduction of NO with NH, in the presence of air were observed. This phenomenon might have a correlation with the redox nature of the catalyst. Over CuO/ZrO,, the Cu2+ ion was greatly diminished in NO + NH, in the absence of oxygen at around 200 "C and was insufficiently oxidized at the same temperature in pure NO. At ca. 200 "C the Cu ion68 Reduction ofNO with H, or NH, on ZrO, was probably not oxidized completely and the activity was kept low around this temperature. The authors thank Prof. K. Tanabe and Prof. H. Hattori for helpful discussions. This work was supported by a Grant-in-Aid for Environmental Science No. 59035001 from the Ministry of Education, Science and Culture of Japan. References 1 M. Shelef, Catal. Rev., 1975, 11, 1. 2 S. Okazaki, N. Ohsuka, T. Iizuka and K. Tanabe, J. Chem. SOC., Chem. Commun., 1976, 654. 3 S. Kasaoka and T. Yamanaka, J . Chem. Soc. Jpn, 1977, 907. 4 S. Kasaoka, E. Kasaoka, T. Yamanaka and M. Ono, J . Chem. SOC. Jpn, 1978, 874. 5 F. Nozaki, K. Yamazaki and T. Inomata, Chem. Lett., 1977, 521. 6 B. Bleaney and K. D. Bowers, Proc. R. Soc. London, Ser. A , 1952, 214, 451. 7 H. Abe and J. Shimada, Phys. Rev., 1953,90, 316. 8 C. C. Chao and J. H. Lunsford, J. Phys. Chem., 1972,76, 1546. 9 J. W. London and A. T. Bell, J . Catal., 1973, 31, 96. 10 P. H. Kasai and D. Mcleod Jr, J . Chem. Phys., 1971, 55, 1566. 1 1 W. B. Williamson and J. H. Lunsford, J. Phys. Chem., 1976, 80, 2664. 12 M. Mizumoto, N. Yamazoe and T. Seiyama, J . Catal., 1979, 59, 319. 13 K. Otto, M. Shelef and J. T. Kummer, J . Phys. Chem., 1971, 75, 875. 14 K. Otto and M. Shelef, J. Phys. Chem., 1972,76, 37. Paper 51210; Received 5th February, 1985