NH, Oxldation over UV-Irradiated TiO,
precipitation, reverse osmosis, or solvent extraction, although these methods become less effective at low concentrations of copper ions. The photodeposition procedure described here may be applicable to treatment of waste streams, although the effect of other substances in the streams which might interfere with the process must be considered. Treatment of waste streams for removal of CN- and SOz by photocatalytic methods has previously been suggested.1° The method may also find application in the photodeposition of copper contacts on semiconductor substrates and for the preparation of catalyst materials, as previously described for Pt catalyst^.^
Acknowledgment. The support of this research by the Heinrich-Hertz-Stiftung (to H. R.) and by the National
The Journal of Physical Chemistry, Val. 83, No. 17, 1979
2251
Science Foundation is gratefully acknowledged. We are also indebted to the assistance of Dr. Bernard Kraeutler who participated in some of the earlier experiments. References a n d Notes (1) T. Freund and W. P. Gomes, Catal, Rev., 3 ,
1 (1969). (2) A. J. Bard, J . Pbotochem., 10, 59 (1979). (3) B. Kraeutler and A. J. Bard, J. Am. Chem. SOC.,100, 5985 (1978). (4) B. Kraeutler and A. J. Bard, J. Am. Cbem. Soc., 100, 4317 (1978). (5) M. S. Wrlghton, P. T. Wolczanski, aml A. B. Ellis, J. SoM State Chem., 22, 17 (1977). (6) B. Kraeutler and A. J. Bard, Now. J. Cbim., 3 , 31 (1979); J. Am. Chem. Soc., 99, 7729 (1977). (7) R. Nasanen and V. Pentlinen, Acta Chem. Scand., 6, 837 (1952). [8) A. G. Massey, "Comprehensive Inorganic Chemistry", Vol. 3, Pergamon Press, New York, 4973, Chapter 27, p 48. (9) A. W. Fletcher, Cbem. Ind. (London), 414 (1973). (10) S. N. Frank and A. J. Bard, J . Pbys. Cbem., 81, 1484 (1977).
NH3 Oxidation over UV-Irradiated TiOl at Room Temperature Henrl Morranega, Jean-Marie Herrmann, and Pierre Plchat" lnstitut de Recberches sur la Catalyse, C.N.R.S.,69626 Villeurbanne CBdex, France (Received December 11, 1978: Revised Manuscript Received April 17, 1979) Publication costs asslsted by Centre National de la Recherche Scienfifique
NH3 oxidation into Nz and N20 by O2 in the 30-250-torr pressure range has been carried out over UV-irradiated Ti02 (anatase) at room temperature. It has been checked that photons of energy greater than the band gap of TiOz are needed and that the formation rates, rN2and rN,O, in Nz and N20 are both proportional to UV light intensity. The electrical photoconductivity of Ti02 is not affected by NH3, N2 and N20, Le., these compounds do not compete with O2 for surface free electrons. A kinetic study showed that rN2and rNzO both depend on O2 pressure, whereas NH3 pressure influences only rN2. The selectivity is not affected by PO,, the ratio rN2/rNzo being -4 for 100-torr NH3 pressure. The formal kinetics corresponds to a Langmuir-Hinshelwood model with reaction between O2and NH3 adsorbed on different types of sites for N2 formation, and with reaction between O2 and a nitrogen-containing intermediate for N20 formation. A step mechanism taking into account the activation of the catalyst by photons is tentatively proposed.
Introduction The oxidation of ammonia has been extensively studied owing to its use for HN03 synthesis when NO is selectively obtained, as well as because NH3 is an atmospheric pollutant and is also formed during engine operation in the rich mode. It has also been examined in connection with ammoxidation reactions. It forms Nz, NzO, and NO in various ratios over a variety of catalysts (metals and oxides). The photochemical oxidation of NH3 occurs at wavelengths below 220 nm,l which are shorter than those employed for photocatalysis over oxides. Relatively little work has been published on the photocatalytic oxidation of NH3. Over T i 0 2 in aqueous solutions, NO2- ions were formed.2 In the presence of dry TiOz a t room temperature a mixture of gaseous NH3 and O2produced small amounts only of N 2 0 and of surface NO2- ions but no N2.3 Over ZnO a t 25 OC, N 2 0 and HzO were photocatalytically formed and a mechanism involving 0 2 - ions reacting with NH3 to yield HNO species was p r ~ p o s e d . ~Nitrate surface ions, resulting from the photocatalyzed oxidation of NH3 by O2 in the -183-25 O C temperature range over -pA1203pretreated at 700 "C,were observed by IR spectroscopy; however, the gas phase was not a n a l y ~ e d . ~ 0022-365417912083-225 1$01.0010
In this laboratory, the properties of UV-irradiated TiOz at room temperature have been used for the oxidation by Oz of ~ r g a n i c ~and - ~ inorganic molecules,6 and for Oz isotopic exchange.l0 To improve the understanding of these reactions, this article presents a kinetic study of the already pointed out photocatalytic oxidation of NH3 over Ti02,6as well as the effect of NH, and of its oxidation products, N2 and N20, upon T i 0 2 electrical photoconductivity. The various kinetic models, the role of UV light and oxygen species, are discussed. Experimental Section Apparatus and Catalyst. The photocatalytic activities were measured in a differential flow-photoreactor described previously.'J1 The catalyst powder was spread as a thin uniform layer onto a porous fiberglass membrane perpendicular to the UV beam and located at ca. 6.5 cm below the axis of a Philips HPK 125-W lamp. A water-containing cuvet removed the infrared part of the beam. The catalyst received ca. 25 mW cm-2 in the spectral range thus obtained. The reaction mixture flowed through the catalyst layer. The effluents were analyzed by gas chromatography. More details may be found in ref 11. The photoconductivity of T i 0 2 was measured under static conditions, in another cell, as in ref 12. TiOa (50 mg) 0 1979 American Chemical Society
2252
The Journal of Physical Chemistry, Vol. 83, No. 17, 1979
H. Mozzanega, J.-M. Herrmann, and P. Pichat 1
I
PNH3/ Torr
Figure 1. Influence of 0,pressure on N2 formation for various NH3 pressures as indicated (in torr).
was lightly compressed (3.3 kg cm-2) in a frame consisting of two quartz bars and two gold bars used as electrodes, so that good electrical contacts were achieved, Ti02was nonporous anatase (70 m2 g-l, mean particle size -220 A) from Degussa (Frankfurt am Main). Procedures. The measurements of the photocatalytic activity were carried out after the achievement of a stationary state which was reached within ca. 90 min and maintained over 8 h. A fresh sample of unpretreated Ti02 was used in each experiment. The mass of catalyst was such that the photocatalytic activity was proportional to the catalyst mass, i.e., the solid particles were completely UV illuminated and the interparticle diffusion was not rate limiting. Helium was used as the carrier gas. The total flow rate was 50 cm3 min-l, the limit of the gas diffusion regime being 13 cm3 min-l. The reaction mixture was first allowed to flow through the catalyst in the dark for a few minutes and the constancy of the gas chromatographic peaks of the reactants was verified. For the reactant pressures and UV illumination used, the conversion level was low (> 1 in contrast with the preceding paragraph, which is only possible by assuming that there occur two sorts of adsorbed NH3? one forming Nz and the other, corresponding to a greater adsorption coefficient, forming NzO. In fact, different types of acid sites are available only on dehydrated TiO2,l5whereas in the present case NH3 is only weakly and reversibly adsorbed, mainly by hydrogen and van der Waals bonds. It is more reasonable to suggest that the kinetic law for NzO formation corresponds to a Langmuir-Hinshelwood mechanism between Oz and a nitrogen-containing intermediate, resulting from a primary oxidation step of NH3, assuming that the surface coverage 8, of this intermediate at the stationary state is uninfluenced by PNHs. It becomes
H. Mozzanega, J.-M. Herrmann, and P. Pichat
mechanism 1118
+0 NH + 0
NH, NH
+ HNO
where k "$0 = kN2& The two sets of values for k'Nzo and KO; obtained with n = 1 and n = 1 / 2 are given in Table 11. It may be seen that a same value is found for KO, only for n = 1, when formations in N2 or N20 are successively considered. This allows one to infer that the same type of molecularly adsorbed oxygen is involved in the formation of the two products, which is supported by the independence of the selectivity on P o . Detailed Mechanism. Role of UV Eight and Oxygen Species. In the following paragraphs, an attempt is made to propose a more detailed mechanism taking into account the activation mode of the catalyst by photons, the preceding kinetic results, and the previous models of the catalytic oxidation of NH3 over oxides. These models have been classified into two groups:16 mechanism 117 NH3 + 0 NH30 NH30 + 0 HNO + H 2 0 HNO + NHaO Nz + 2Hz0 HNO + HNO NzO + HzO
--
-
2HNO
4
+
+ HzO
HNO
Nz + HzO
+ H2O
NzO
In the present work, the nature of the intermediates containing a nitrogen atom has not been determined, so that reference will be made to the species proposed in the above mechanisms. The mechanism of the photocatalytic oxidation should include the activation of a species by the holes created at the surface of TiOz, an n-type semiconductor, since the reaction rate is related to the energy and intensity of UV light. In preceding articles, arguments were presented suggesting that the activated form of oxygen at the surface of UV-irradiated T i 0 2 a t room temperature is dissociated1°J3and results from the following reactions:13
-
+
(Ti02) hu Oad;
+ P+
p+
-+
+ e-
Oads*
( 1)
(2)
Equation 1shows the absorption by anatase of a photon whose energy is greater than the band gap (-3.2 eV). Equation 2 symbolizes the activation of 0- species on neutralization by photoholes. In this equation and in the following ones, the asterisk indicates entities which possess an excess energy arising from the initial activation by photons. We suggest that the O* species are responsible for the attack of adsorbed ammonia molecules and that the resulting NH30* species (or their tautomer ",OH*) may form activated imide intermediates because of the dehydrating properties of TiOz as in mechanism 11.
"20Hads*
or
NH
-+
-+
"ads*
+ H2Oads
(4)
Concerning the formation of HNO species, which intervene in the synthesis of both N2 and N 2 0 (mechanisms I and 11), we found that the kinetic data imply that they result from the oxidation of nitrogen-containing intermediates by some form of adsorbed molecular oxygen, thus excluding any kind of atomic oxygen such as 02-, 0-,0*,and 0. This molecularly adsorbed oxygen is probably in a neutral state, since 02-species are stable at room temperature at the surface of an n-type semiconductor, and it is assumed that the NH* species are activated enough to compete with electrons in order to react with it: In agreement with mechanism IT, N2 and NzO will be formed by the following reactions: "ads* "Oads
+ "Oads
+ HNOad,
---*
Naads + HPOads
(6)
N~Oads+ H@ads
(7)
From the above equations, it may be readily deduced that, as expected, both the formation rates in N2 and NzO depend upon Oz pressure. However, it is necessary to assume that the rates of eq 3 and 4,corresponding to the formation of NH* species, are far greater than the rate of eq 5, referring to the formation of HNO species, in order to explain that rNzdepends upon NH3 pressure, whereas rND does not. In other words, [",&*I >> [HNOah]. This assumption seems reasonable, since the quantity of Nz formed is approximately 4 times that of NzO, which in-
Kinetics of Oxygen Titration by
CO on Rh
dicates that HNO species are less numerous than NH* species. A greater reactivity of these last species may also account for the N2/N20 ratio. From eq 1-3, which are equivalent t o those considered for isobutane oxidation under the same conditions,l, it may also be seen that NH, consumption is proportional to the generation rate of holes created by the UV photons. At least one photohole is required to transform one NH, molecule, which means that the quantum yield cannot exceed 1. From the mere photonic point of view, this upper limit can be reached only if all photoholes are involved in neutralization of 0- species. 0; species, which also exist at the surface of UV-irradiated Ti02,7J2,19exhibit a reactivity much smaller than that of 0- species,20so that their interaction with holes is less probable.21 Because of the coverage in negative oxygen species the surface is depleted of electrons and the recombination of electron-hole pairs is negligible. In conclusion, the mechanism which is tentatively proposed involves two types of oxygen adsorbed species: a first one, dissociated, activated by the photoholes, and responsible for the attack of the adsorbed NH, molecules; a second one, molecular, which oxidizes the resulting NH radicals into HNO intermediates whose concentration is supposed to be independent of NH3 pressure at the stationary state.
Acknowledgment. The authors thank Dr. F. Juillet who initiated this study while being adviser for the thesis of one of us (H.M.).
The Journal of Physical Chemistry, Vol. 83, No. 17, 1979
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References and Notes (1) S. Z. Levine and J. G. Calvert, Chem. Phys. Lett., 48, 81 (1977). (2) G. Gopalarao, Z . Pbys. Chem. A , 184, 337 (1939); G. Gopalarao and Ch. 1. Varadanam. J . Indian Chem. Soc., 18, 361 (1941). W. R. Mc Lean and M. Richtie, J . Appl. Cbem., 15, 452 (1965). I. E. Den Besten and M. Qasim, J . Catal., 3 , 387 (1964). A. V. Alekseev, D. V. Pozdnyakov, A. A. Tsyganenko, and V. N. Filimonov, React. Kinet. Catal. Lett., 5, 9 (1978). F. Juillet, F. Lecomte, H. Mozzanega, S.J. Teichner, A. Thevenet, and P. Vergnon, Faraday Symp., 7, 57 (1973). M. Formenti, F. Juillet, P. Meriaudeau, and S.J. Teichner, Cafal. h e . Int. Congr., 5tb, 7972, 2, 1011 (1973). N. Djeghri, M. Formenti, F. Juillet, and S.J. Telchner, Faraday L)iscuss., 58, 185 (1974). M.-N. Mozzanega, J.-M. Herrmann, and P. Pichat, Tetrahedron Lett ., 2965 (1977). H. Courbon, M. Formenti, and P. Pichat, J . Phys. Cbem., 81, 550 (1977). H. Mozzanega, These CNAM, Lyon, France, 1975. J.-M. Herrmann, J. Disdier, and P. Pichat, “Proceedings of the 7th International Vacuum Congress and 3rd International Conference on Solid Surfaces”, Vol. 11, R. Dobrozemsky et ai., 1977, p 951. J.-M. Herrmann, J. Dlsdler, M.-N. Mozzanega, and P. Pichat, presented in part at the 6th North American Meeting of the Catalysis Society, Chicago, Ill., March, 1979, J . Catal., in press. N . I. Il’chenko and G. I. Golodets, J . Catal., 38, 57 (1975). M. Primet, P. Pichat, and M.4. Mathleu, J . Pbys. Cbem., 75, 1221 (1971); G. D. Parfitt, J. Ramsbotham, and C. H. Rochester, Trans. Faraday SOC., 87, 841 (1971); N. D. Parkyns, “Symposium on Chemisorption Catalysts”, Institute of Petroleum, London, 1971, p 150. J.-E. Germain and R. Perez, Bull. SOC. Chim. Fr., 2042 (1972). N. Giordano, E. Cavaterra, and D. Zema, J . Catal., 5, 1325 (1966), and references cited in ref 16 and 18. J. Zawadski, Faraday Discuss., 8 , 140 (1950). S. Fukuzawa, K. M. Sancier, and T. Kwan, J. Cafal., 11, 384 (1968). J. H. Lunsford, Catal. Rev., 8, 135 (1973). J.-R. Martin, ThBse, Lyon, France, 1978.
Kinetics of Oxygen Titration by Carbon Monoxide on Rhodium Charles T. Campbell,+ Shei-Kung Shl, and J. M. White” Department of Chemistry, University of Texas, Austin, Texas 78712 (Received February 2, 1979) Publication costs assisted by the Robert A. Welch Foundation
Data for the low pressure transient kinetics of oxygen titration by carbon monoxide on polycrystalline Rh are reported for temperatures between 360 and 779 K. The time dependence of CO pressure and COz evolution are followed when a constant flux of CO is introduced onto a Rh wire predosed in oxygen. The results of a variety of simulations are compared with these experimental transients and it is shown that all the data are satisfactorily accounted for by a complex Langmuir-Hinshelwood reaction path. The model involves three features: (1) an activation energy which depends on oxygen coverage below 529 K, (2) the inhibition of CO adsorption by oxygen coverage above 529 K, and (3) a drastic increase in the activation energy around 529 K.
I. Introduction As one of the simplest examples of a heterogeneous catalytic reaction, the oxidation of carbon monoxide over transition metals has enjoyed a great deal+of careful investigation aimed a t achieving a fundamental understanding of the kinetic and mechanistic details of the reaction pathway. As yet, however, these studies have failed to produce a fully coherent picture of the process. A case in point is the confusion over whether the production of COz occurs via an Eley-Rideal (ER) or a Langmuir-Hinshelwood (LH) combination of CO and oxygen.l-19 The majority of these studies have been of two types: (1)those providing qualitative features over a large National Science Foundation Trainee.
range of experimental variables (temperature, pressure, coverage);l-1° and (2) those providing quantitative determinations of the kinetic parameters over a narrower range of ~ariab1es.ll-l~ In this paper we attempt, by means of simulation, to combine these qualitative and quantitative results into a coherent model which is suitable over a wide range of temperatures and pressures. In this approach various kinetic models are numerically solved in an effort to accurately simulate the results of experimental measurements and furnish predictions at conditions far-removed from those used to determine the parameters. In this report we present a reaction model which adequately simulates the transient COz production rates during the titration of preadsorbed oxygen on Rh over a range of
0022-3654/79/2083-2255$01 .00/0 0 1979 American Chemical Society
