J. Phys. Chem. 1985, 89, 77-80
77
Infrared Study of the Reaction between NO and 0, and of the Adsorption of NO, on Platinum B. A. Morrow,* Richard A. McFarlane, and L. E. Moran Department of Chemistry, University of Ottawa, Ottawa, Ontario, Canada, K l N 9B4 (Received: September 6, 1984)
Using silica-supported platinum, infrared spectroscopy has been used to study (a) the adsorption of NO,, (b) the reaction between adsorbed NO and 02,and (c) the reaction between preoxidized Pt and NO. NO, adsorption at very low coverage gives rise to v(N0) bands near 1785 (band A) and 1620 cm-' (band C) which are characteristic of NO adsorption on Pt(ll0) or polycrystalline Pt, thus indicating that NO, initially dissociates on Pt/Si02 to yield PtNO species and adsorbed 0 oxygen. At higher coverages, bands A and C are reduced in intensity as new bands, called B' at 1710 cm-' and D at 1545 cm-', eventually become the dominant spectral features. The same B' and D bands are also very prominent features after adding excess 0, to preadsorbed N O on Pt, or after adding NO to a Pt surface which has been pretreated with excess 02. Bands A and C have been previously attributed to linear PtNO and either bridged Pt2N0 or bent PtNO species by ourselves and others, whereas we assign the B' band to linear PtNO adsorbed on partially oxidized Pt and the D band to a bidentate nitrato species Pt02N=0. The spectra show that the surface compositions after saturation coverage under conditions a, b, or c are remarkably similar.
Although the adsorption of NO and Pt has been very extensively studied by infrared spectrosocopyl-10 and electron energy loss spectroscopy11+12 it is surprising that there is almost no published data concerning the interaction between oxygen and NO on Pt7-8 nor of the adsorption of NO2on Pt.' It is well-known that at high temperatures (e.g., in a catalytic converter for control of NO emissions) NO dissociates on Pt to yield N2and surface 0xide~3'*'~ (the dissociation is essentially complete at 475 K), and it has also been shown that preadsorbed oxygen on Pt inhibits the adsorption Further, although the dissociation and decomposition of of NO hardly occurs a t all at temperature below 320 K on Pt(1 1l),-(1 lo), or -(loo) surfaces, at least one high index surface has been shown to be active for NO decomposition at ambient temperatures.16 Finally, recent nonspectroscopic work has shown that NO, readily dissociates on Pt at low temperatures."J8 It is apparent that the chemistry of NO 0,and of NO2 on Pt is quite complex and that a spectroscopic study of these systems might yield new information concerning the functioning of Pt catalysts for NO reduction in general. In this paper we discuss the results of a transmission infrared study of these systems using silica supported Pt. N0.8913
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Experimental Section The experimental p r o c e d ~ r eand ~ ~ the , ~ ~infrared celIl9 have been described previously. Briefly, the catalysts were prepared by impregnation of Cab-0-Si1 HS5 (325 m2 g-I) with H2PtC1, dissolved in acetone. Disks containing 40 mg/cm2 of the dried mixture were pressed in a stainless steel die, mounted in the cell, and reduced in H2 at 673 K. Reduced samples contained 16.2 wt % Pt and the quantity of O2and H2 irreversibly adsorbed a t ambient temperatures was 3.941 and 0.248 mg/g of catalyst, respectively. Assuming 1 oxygen atom per surface Pt and assuming each Pt atom occupies2' 8.4 A2, we calculate the Pt surface area to be 76.9 m2/g of Pt. I R spectra were recorded on a modified Perkin-Elmer Model 13G presample chopped ratio recording spectrometer. We have determinedm that the nominal sample temperature under ambient conditions is about 325 K when using black supported metal samples, and this temperature is assumed to apply to all of the data obtained in this study. A compensating silica disk which was degassed a t 673 K in a separate cell was placed in the reference beam of the spectrometer so as to cancel the silica absorption in the 2 100-1 250-cm-' spectral region. Weak residual features are still evident in most background spectra (see Figure 2a), and the baseline generally slopes to higher percent trans-
* Member of the Ottawa-Carleton Institute for Graduate Studies and Research in Chemistry. 0022-3654/85/2089-0077$01.50/0
mission at lower wavenumber because the light scattering characteristics are different for the Pt/SiO, and SiO, samples. Reduced samples generally weighed about 165 mg, and the IR cell had a volume of 300 mL. All gas dosages are reported as micromoles (pmol) per gram of catalyst. On a bare catalyst, the maximum NO or NO, coverage corresponded to 185 and 186 pmol g-I, respectively. Extra careMwas taken to purify NO which was from Matheson. 15N0and 180(99% 2 of isotopic enrichment from Merck Isotopes and Prochem, respectively) were used without further purification.
Results and Discussion The infrared spectrum of excess NO adsorbed on silica supported Pt a t 325 K, after evacuation of excess NO, is shown in Figure la. The origin of major absorption bands, A at 1785 cm-' and C at 1620 cm-', have been discussed in detail previously.I-l2 Briefly, A has been assigned to a linear PtNO species whereas C, which is relatively more intense at low fractional coverage, has been attributed by us to bent PtNO,Io although this may also be due to bridged NO or to vibrationally coupled chains of NO m ~ l e c u l e s . ' - ~(The ~ ~ ~exact ~ ~ ~assignment ~~'~ need not concern us here.) The shoulder B at 1690 cm-' is weak for adsorption at ambient temperatures but is the dominant spectral feature fol(1) Dunn, D. S.; Golden, W. G.; Severson, M. W.; Overend, J. J. Phys. Chem. 1980,84, 336. (2) Severson, M. W.; Overend, J. J . Chem. Phys. 1982, 76, 1584. (3) Dunn, D. S.; Severson, M. W.; Hylden, J. L.; Overend, J. J. Catal. 1982, 78, 225. (4) Hayden, B. E. Sur. Sct. 1983, 131, 419. (5) Brown, M. F.; Gonzalez, R. D. J. Catal. 1976, 44, 4771. (6) Primet, M.; Basset, J. M.; Garbowski, E.; Mathieu, M. V. J . Am. Chem. SOC.1975, 97, 3655. (7) Ghorbel, A, Primet, J. J . Chim. Phys. 1976, 73, 89. (8) DeJong, K.P.; Meima, G. R.; Geus, J. W. Appl. Surf.Sci. 1983,14, 73. (9) Fang, S.M.; White, J. M. J . Carol. 1983, 83, 1. (10) Morrow, B. A.; Chevrier, J. P.; Moran, L. E. J. Cat& in press. (1 1) Gorte, R. J.; Schmidt, L. D. Surf.Sci. 1981, 1 1 1 , 260 and references therein. (12) Gorte, R. J.; Gland, J. L. Surf.Sci. 1981, 102, 348. (13) Mummey, M. J.; Schmidt, L. D. Surf.Sci. 1981, 109, 29, 42. (14) Gorte, R. J.; Schmidt, L.D.; Gland, J. L. Surf.Sci. 1981, 109, 367. (15) Gland, J. L.; Sexten, B. A. Surf.Sci. 1980, 94, 355. (16) Park, Y. 0.; Masel, R. I.; Stolt, K. Sut$ Sci. 183, 131, L385. (17) Dahlgren, D.; Hemminger, J. C. Surf. Sci. 1982, 123, L739. (18) Segner, J.; Vielhaber, W.; Ertl, G. Isr. J. Chem. 1982, 22, 375. (19) Morrow, B. A.; Ramamurthy, P.; J . Phys. Chem. 1973, 77, 3052. (20) Morow, B. A.; Moran, L. E., J . Catal. 1980, 62, 294. (21) Benson, J. F.; Boudart, M. J . Catal. 1965, 4, 704.
0 1985 American Chemical Society
78 The Journal of Physical Chemistry, Vol. 89, No. 1, 1985 A
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Morrow et al. C
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c m-’ Figure 1. (a) IR spectrum after adsorption of NO at saturation at 325 K on reduced Pt/Si02 and evacuation of excess NO. The other curves
show the spectrum after adding the following incremental doses (pmol g-’ of catalyst) of gaseous O2to the cell for 5 min and evacuating the
sample for 10 min: b, 18; c, 9; d, 54; e, 409; f, 1800. The %T scale refers to curves a and b and the other curves have been displaced downwards for clarity of presentation.
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Figure 2. IR spectra of NO2 adsorbed on Pt/Si02 after adding the following incremental doses in pmol g-’ of catalyst and evacuation for 5 min or more: a, background; b, 5 ; c, 5; d, 15; e, 30.
lowing adsorption of N O at 140-160 K and has been assigned to a weakly held form of adsorbed NO on Pt.l0 The shoulder B’ a t 1710 cm-l is only observed when excess N O has been added and the assignment of this feature forms part of the basis of this work. The spectral changes which occurred when successive doses of O2were added to preadsorbed NO are shown in Figure lb-f. The most noticeable change with the first small dose of O2 (Figure 1b) was the decrease in intensity of the A and B bands and an increase in intensity of the B’ and C bands. Bands A, B‘, and C also shifted to higher wavenumber. However, with further doses of O2 all peaks eventually decreased in intensity albeit to different extents, the shifts to higher wavenumber continued, and a new band, called D, eventually appeared at 1545 cm-I. The final frequencies (Figure If) were A at 1810 cm-l, B’ at 1723 cm-I, C at 1640 cm-I, and band B had disappeared. Figure 2 shows the spectra which were observed when various quantities of NOz were adsorbed on reduced Pt/SiO,. At low coverage the spectra were virtually identical with those of N O
Figure 3. IR spectra after adding NO to Pt/Si02 which had been pretreated with excess O2 (>lo3 pmol g-l of catalyst) at 325 K, evacuation of O2for at least 10 min, and then adding the following incremental doses (pmol g-l) of NO followed by evacuation for 10 min: a, background; b, 2.5; c, 2.5; d, 9; e, 36; f, 450 (excess).
on Pt at low coverage except a very weak feature near 1710 cm-I was evident. Band A near 1800 cm-’ reached its maximum intensity after addition of 25 hmol g-’ of NOz and band D started to appear. With further doses, A decreased in intensity and D grew until, at saturation, the spectrum is very similar to that shown in Figure If. Therefore, we conclude that the surface composition on Pt after adsorption of excess NOz is the same as occurs after adding excess O2 to preadsorbed NO. The spectra observed after the adsorption of N O on a Pt sample which had been pretreated with an excess of O2 (excess O2 evacuated) are shown in Figure 3. This clearly shows that the B’ band is the dominant spectral feature at low coverage. Its peak position starts out at 1710 cm-’ and at saturation coverage it was at 1724 cm-’. The evidence overwhelmingly suggests that the B’ band in Figures 1-3 is due to an adsorbed N O species which requires the presence of surface oxygen. We assume that this also applies to the case of excess N O adsorbed on reduced Pt/Si02 (Figure la) and we conclude that, at 325 K, some N O dissociates to yield surface oxide, but only at near saturation N O coverage. In support of this, we have observed that, after saturation adsorption of 15N0 on Pt/SiOz, evacuation of excess ISNO,and leaving the cell under static vacuum for 1 h, traces of lsN2 (about 1% of the total I5N adsorbed) could be detected by mass spectrometry. DeJong et a1.8 also reported that up to 5% of N O can dissociate on Pt/SiOz. Thus the extent of dissociation of N O on Pt/SiOz at saturation is very small and although the evidence of others strongly suggests that N O does not dissociate on 111, 110, or 100 Pt at 298 K, our experimental T i s 325 K, our sample is polycrystalline, and since N O is reported to partially dissociate on a Pt(410) surface,I6 our conclusions regarding NO/Pt/Si02 are not surprising. Assignments. The bands A and C were previously assigned by us to linear PtNO and bent PtNO, respectively, on reduced Pt (the latter might also be assigned to a bridged PtzNO species or vibrationally coupled chains of N O molecules). Whatever the exact assignment, we will assume that the bands in this spectral region can be assigned to NO adsorbed on Pt, albeit perhaps the species are perturbed by the presence of surface oxygen. (There are no striking discontinuities in Figure 1 as O2 is added to Pt/NO/Si02 except for the growth of band D.) Bands B’ and D are clearly associated with excess oxygen and, since the ratio of the intensity of B’/D varies greatly, we assume that different surface species are responsible for these bands. With this in mind, then we notice that NO2 on Pt yields, at low coverage, a spectrum that is very similar to that which is due to NO on Pt/Si02; Le., the bands A and C are dominant although
IR Study of NO
+ O2 Reaction
The Journal of Physical Chemistry, Vol. 89, No. 1, 1985 79 40
B‘
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. L c
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%T
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c m-’ Figure 5. IR spectra after adding the indicated incremental doses (pmol g-l) of a 1:l mixture of 14NL60/15N160 to Pt/Si02 which had been pretreated with excess I6O2.The bottom curve is the background.
1001
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c m-’ Figure 4. IR spectra after adding the indicated incrementaldoses (pmol g-I) of I4Nl6Ofor 5 min, followed by evacuation for 10 min, to Pt/Si02 which had been preexposed to excess I6O2(top curve) or ‘*02 (the remainder). The QTscale refers to the bottom curve, the others have been
displaced upward for clarity of presentation.
B’ appears weakly. Only at high coverages do B’ and D become important. We conclude that NO2 initially dissociates on Pt to yield PtNO (bands A and C) and that the surfaces oxygen has little perturbing influence. Only at higher coverage do we eventually see the effects of this oxygen as the B’ and D bands appear. A very similar result was also found for NOz on Ni/ Si02.20 Finally, we note that NO2 dissociation on Pt at ambient temperatures has also been detected by other technique^.'^-'^ Band B‘. The band near 1710 cm-I has a frequency which is too high to be attributed to a nitrate speciesm3 (see further below). Therefore, we looked a t the effect of using oxygen-18. Figure 4 shows the spectra observed when I4NO was added to a Pt/Si02 sample which has previously been exposed to an excess of I8O2 (99% oxygen-18). Even at the lowest dose, two B‘ bands are observed, the low wavenumber band being due to the incorporation of l80into the species responsible for this band. However, the spectra are difficult to interpret because,with increasing coverage, the oxygen 16 peak dominates. The I8Oshift is 46 cm-’ whereas that observed for gaseous NO is 50 cm-’ and it is 48 cm-I for the nitrosyl halides.24 W e can only assume that the B’ peak can be assigned to NO, probably in a linear form, adsorbed on an oxidized surface, and that exchange between surface oxygen and NO is possible to a limited extent. Further speculation is not warranted because we believe that, as has been found for CO on PtZSand for N O on bare Pt,14 there is dipolar coupling between adsorbed NO molecules and that the relative intensities do not reflect the 180/160 ratio. We show for comparison in Figure 5 the spectrum of a 1:l mixture of 1sNO/14N0adsorbed, a t low coverage, on an I6O2pretreated surface. Although the resolution is poor, the relative peak heights of the A, B’, and C bands are not 1:1, nor (22) Nakamoto, K. ‘Infrared Spectra of Inorganic and Coordination Compounds”; Wiley-Interscience: New York, 1970. (23) Blyholder, G.; Allen, M.C. J. Phys. Chem. 1966, 70, 352. (24) Pinchas, S.; Laulicht, I. “Infrared Spectra of Labelled Compounds”; Academic Press: New York, 1971. (25) Woodruff, D. P.; Haydon, B. E.; Prince, K Bradshaw, A. M.Surf. Sci. 1982, 123, 391.
are they the same in each case. Band D. This band is uniquely due to the presence of N O and 02,or from NO2 at high coverage where we have already suggested that decomposition must have started. The frequency, 1545 cm-I, is too high to be attributed to an ionic nitrite or nitrate 22*23 (and such species are not expected on Pt), or to a nitro (PtNO,) or nitrito (Pt-.ONO) complex. It is also too high to be attributed to a monodentate nitrato species (PtONO), but it could be in the range expected for a bidentate nitrato complex
V
for which the high-frequency v(N0) mode can be in the range 1475-1600 cm-’. Since this band only becomes a major feature after an excess of oxygen is present, an assignment to a nitrato species, whatever the structure, is reasonable. To our knowledge, only two other vibrational studies of the adsorption of N O and 02,or of NO2on Pt have been reported7p8 and, in both of these, the adsorption of NO on reduced Pt was the major point of the study. Ghorbel and Primet7 used alumina-supported Pt and found that the spectrum of NO adsorbed on preoxygen treated Pt had bands at 2230,2215,1780,1570-1620, 1320, and 1220 cm-’. The latter three bands were attributed, correctly, to nitrate species on the support. The 1780-cm-I band was the strongest feature and almost certainly corresponds to our species A. The weak doublet at 2230/2215 cm-I was attributed to a small quantity of gas-phase N 2 0 . We did not observe this in our work. Adsorbed NO2 also gave rise to the nitrate bands below 1630 cm-’, and also a strong band at 1845 cm-I. Only one spectrum, at saturation, was reported, and this band must also correspond to band A in this study, which is also at higher frequency when oxygen is present on the surface (1810 cm-I, Figure lf). They did not find a band near 1710 cm-’ corresponding to our B’. Since the A1203support clearly played a strong role in their system, we do not feel that further discussion is warranted. Finally, DeJong et a1.8 showed one spectrum of N O adsorbed on a Pt/SiOz sample which had been preoxidized in O2at 400 OC. Spectral features which resembled ours (bands A, B’, C, and D) were observed, although the spectrum was very weak (maximum absorbance of about O.l), and they simply attributed the C and D bands to “oxidized forms of NO, e.g. nitrates”. Uniquely, however, they observed a weak band at 1935 cm-I which was attributed to adsorbed N O perturbed by surface oxygen. We did not observe this band. Otherwise, our spectra for the stated experimental conditions were similar. Conclusions The intense infrared bands due to linear PtNO (A) and bent or bridged NO on Pt (C) decrease in intensity when 0,is added and are partially replaced by new bands which grow near 1710
J. Phys. Chem. 1985, 89, 80-85
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(B’) and 1545 cm-’ (D). These are assigned to linear P t N O on an oxidized Pt surface and to a bidentate nitrate species Pt0,N=O, respectively. NO2 at low coverages dissociates on Pt to yield a spectrum which is similar to that of NO on Pt/Si02. However, at higher coverages the bands B’ and D assigned above intensify and eventually become major spectral features. When N O is adsorbed on Pt/SiO, which had been pretreated with excess 02,the sharp band B’ is observed first and remains themost intense feature, although accompanied by weaker A, C, and D bands, during subsequent dosages of NO up to saturation. The band B’ is also observed as a weak shoulder at saturation coverage for NO on Pt/Si02 and our results clearly show that
this species is associated with N O and oxygen on Pt. Therefore, we conclude that a small quantity of N O dissociates on Pt/SiO, at saturation coverage at ambient temperatures. The final surface compositon at 325 K is very similar for the following systems using Pt/Si02: (a) PtNO (maximum coverage) O2gas (excess); (b) Pt NO2 gas (excess); (c) PtO (maximum coverage) N O (excess).
+
+
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Acknowledgment. We are grateful to the National Sciences and Engineering Research Council of Canada and Imperial Oil Ltd. for financial support. Registry NO. NO*, 10102-44-0; NO, 10102-43-9; 02,7782-44-7; Pt, 7440-06-4.
Reduction-OxMation and Catalyttc Properties of 12-MoiybdophosphoricAcid and Its Alkali Salts. The Role of Redox Carriers in the Bulkt Noritaka Mizuno, Tetsuji Watanabe, and Makoto Misono* Department of Synthetic Chemistry, Faculty of Engineering, The University of Tokyo, Hongo, Bunkyo- ku, Tokyo 1 1 3, Japan (Received: September 12, 1984)
The catalytic oxidation of CO and H2over 12-molybdophosphoricacid and its Cs and Na salts has been studied at 350 “C in a closed circulating system. By the quantitative comparison of these rates with the rates of stoichiometric reduction of catalysts by CO and H2 and reoxidation by 02,it was demonstrated that (i) these catalytic oxidations proceeded by a redox mechanism, and (ii) in these reactions the difference in the redox carriers in the bulk was closely reflected, as was found previously for stoichiometric reactions. For example, the rate of catalytic oxidation of CO was proportional to the surface area of catalyst, while the rate of catalytic oxidation of H2 depended very little on the surface area. In the oxidation of CO, the surface polyanions are mainly involved in catalysis, owing to slow diffusion of the lattice oxide ion, Le., the redox carrier. On the other hand, in the latter reaction, the redox carriers are protons and electrons which migrate in the bulk, so that all polyanions in the bulk can take part in the redox cycles. Therefore, although the order of the catalytic activity differed considerably between the two reactions, it decreased monotonously with the alkali content (both Na and Cs) when the rate of CO oxidation was normalized to the surface area and the rate of Hzoxidation to the weight. It was further found that linear relationships exist respectively for CO and H2 between the rate of catalytic oxidation and the rate of stoichiometric reduction. These results were discussed on the basis of a redox mechanism.
Introduction Catalytic oxidation reactions over mixed oxides are often believed to proceed by the repetition of reduction and reoxidation of catalysts. This mechanism is called “redox” or the Mars-Van Krevelen mechanism. In this mechanism, the lattice oxide ion is directly involved in the oxidation reaction, so that the reactivity and mobility of the lattice oxide ion are usually very important. Since the participation of the lattice oxide ion in the catalyst bulk was demonstrated by Keulks et al.’ and Hockey et aL2 for the oxidation of propylene over bismuth molybdate catalyst, quite a few studies have been published about the role of the lattice It was reported that the rate of supply of lattice oxide ion to the surface active site played important roles in catalytic oxidation of propylene over bismuth molybdate4 and multicomponent bismuth molybdate catalysts6 and some role in the oxidation of C O over La,-,Sr,CoO, catalyst^.^ In these cases, the lattice oxide ion was incorporated into the products by catalytic oxidation (oxygen-addition reactions), and the lattice oxide ion is the redox carrier in the catalyst bulk. For heteropoly acids, protons and electrons as well as the lattice oxide ion can be redox carriers in the bulk? We previously found that the redox carrier differed depending on the kind of reactions for stoichiometric (noncatalytic) reduction of heteropoly compounds and their reoxidation.lOJ1 The polyanions not only on the surface but also in the bulk were able to take part in the Catalysis by Heteropoly Compounds. VIII.
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oxidative dehydrogenation reactions, as the redox carriers were protons and electrons. On the other hand, polyanions only on or near the surface were utilized when the lattice oxide ions (and electrons) are the carriers, e.g., oxygen-addition reactions, due to the slow diffusion of oxide ion. In the present work, catalytic oxidations of CO and H2 over 12-molybdophosphoric acid and its alkali salts were chosen as typical examples, and we investigated how the above differences in the redox carriers found for stoichiometric reactions were reflected in the catalytic reactions and attempted to clarify the mechanism of catalytic oxidation over heteropoly compounds and the role of the redox carriers in the (1) Keulks, G. W. J . Caral. 1970, 19, 232. (2) Wragg, R. D.; Ashmore, P. G.; Hockey, J. A. J . Catal. 1971, 22, 49. (3) Keulks, G. W.; Krenzke, L. D. Proc. Inr. Connr. Catal., 6th. 1976, 1977, 2, 806. (4) Christie, J. R.; Taylor, D.; McCain, C. C. J. Chem. SOC.,Faraday Trans. 1 1976, 72. 334. Pendleton. P.; Taylor. D. Ibid. 1976. 72. 1114. ( 5 ) Sakata, K.; Nakamura, T.; Misono, M.; Yoneda, Y. Chem. Lett. 1979, 273. (6) Ueda, W.; Moro-oka, Y.; Ikawa, T. J . Catal. 1981, 70,409. (7) Grasselli, R. K.; Burrington, J. D. Adu. Caral. 1981, 30, 133. (8) Sleight, A. W. ‘Advanced Materials in Catalysis”; Academic Press: New York, 1977; p 181. (9) Misono, M.; Sakata, K.; Yoneda, Y.; Lee, W. Y. Proc. Inr. Congr. Catal., 7rh, 1980 1981, 1047. (10) Komaya, T.; Misono, M. Chem. Lett. 1983, 1177. (1 1) Misono, M.; Mizuno, N.; Komaya, T. Proc. In?. Congr. Caral., 8th, 1984 1984, 5,487.
0 1985 American Chemical Society
