J . Chem. SOC., Faraday Trans. 1, 1984, 80, 1595-1604 Ammonia Decomposition on (1 1 10) and (0 0 1) Surfaces of Ruthenium BY CHIKASHI EGAWA, TETSUYA NISHIDA, SHUICHI NAITO* AND KENZI TAMARU Department of Chemistry, Faculty of Science, University of Tokyo, Hongo, Bunkyo-ku, Tokyo 1 13, Japan Received 21st September, 1983 The decomposition of ammonia on stepped Ru(1 1 10) and flat Ru(0 0 1) surfaces has been investigated by Auger electron spectroscopy, low-energy electron diffraction, thermal-desorption studies and kinetic studies. The reaction takes place at ca. 400 K, and N, and H, are formed stoichiometrically. At lower temperatures (< 500 K) the reaction proceeds through the com- bination of N*(ads), which is equilibrated with H, and NH, in the gas phase, and the rate of N, formation is expressed as follows : rNO a exp Cpe,,).The reaction takes place via a typical Temkin-Pyzhev mechanism in which an isotope effect for the hydrogen atoms in the ammonia molecule is observed, i.e. the rate of NH, decomposition is 1.5 times as fast as that of ND,. The formation of N*(ads), which is observed as a peak at 570 K in the thermal-desorption spectra for N,, takes place preferentially on the stepped sites. Therefore, in the transient state the rate of N, formation on the stepped Ru( 1 1 10) surface is an order of magnitude faster than that on the flat Ru(O0 1) surface, although the steady-state reaction rate is twice that on the flat surface. This means that stepped sites on the Ru surface play an important role in breaking the N-H bond.On the other hand, at higher temperatures (> 600 K) the rate of reaction is linearly dependent on ammonia pressure only and independent of hydrogen and nitrogen pressures; the amount of adsorbed hydrogen on the surface was negligible. Since no isotope effect was observed, the reaction is thought to proceed through the recombination of N(ads) on the surface. Amano et aZ.l have reported that Ru metal, like Fe, has a high activity for both ammonia synthesis and decomposition reactions, with surface nitrogen being formed according to the Temkin-Pyzhev mechanism. On the other hand, Danielson et aL2 have reported that molecularly adsorbed ammonia at 100 K on an Ru(0 0 1) surface was desorbed at 133 and 186 K from the surface as molecular ammonia, while atomic nitrogen was formed through an activation process with a very small sticking probability at temperatures between 300 and 500 K, giving a (2 x 2) LEED pattern on dissociation. Ammonia decomposition on flat Pt(1 1 1) and stepped Pt(5 5 7) surfaces was recently investigated by the molecular-beam technique ;3 the decomposi- tion rate on the stepped surface is 16 times faster than that on the flat surface. The step sites on the Pt surface are therefore considered to be effective for N-H bond scission.As reported el~ewhere,~ it has been observed in X-ray and ultraviolet photoelectron spectra that NH(ads) species, as well as atomic nitrogen, was preferen- tially formed on the Ru(1 1 10) surface during ammonia decomposition. Many studies of ammonia decomposition on various transition metals have been rep~rted.~-l~ In this study the reaction mechanism of ammonia decomposition on flat Ru(0 0 1) and stepped Ru( 1 1 10) surfaces, the latter one step per five atomic distances, has been elucidated by studying the behaviour of surface nitrogen species, in a similar I5951596 DECOMPOSITION OF NH, ON RU SURFACES way to the reaction of NO and H, reported in ref.(15), where atomic nitrogen exhibited heterogeneous behaviour and its desorption obeyed the Zeldovich-Rodinsky equation. EXPERIMENTAL The experiments were performed in a stainless-steel ultrahigh-vacuum system equipped with 4-grid LEED-A.e.s. optics and a mass filter, as described previously.16 The Ru(0 0 1) and Ru(1 1 10) planes were cut from a single-crystal rod (99.999% purity, Metal Research Ltd) and prepared by standard methods.The Ru(0 0 1) surface has a hexagonal close-packed structure, while the Ru(1 1 10) surface has a stepped structure, identified as described e1~ewhere.l~ Edge effects were determined in the following manner. After the sample surface was treated with 5 langmuir of adsorbed H,S, both planes were cleaned by Ar-ion bombardment (1.3 kV and 4 pA for 30 min). The turnover frequency of the reaction in the steady state was obtained from the change in partial pressure of the product species from reaction temperature to room temperature for flow-reactor conditions, taking the effective pumping speed and the number of Ru atoms per cm2 into consideration. This completely eliminated any possible effects on the filament of a mass-filter or the wall of the stainless-steel chamber.The amount of adsorbed species on the surface during the reaction was estimated from the area under the desorption peak in thermal desorption spectra in the same manner as the reaction rates. A change of heating rate or a sudden decrease of ammonia pressure did not have an influence on the amount of adsorbed species on the surface. The sample was mounted on a sample holder constructed of oxygen-free copper by spot- welded tantalum wires 0.3 mm in diameter, through which the sample was resistively heated using direct current. A preliminary study employing this sample holder without a sample re- vealed that the Ta wire was inactive in the reaction and had no influence on the measurements of the reaction rate or the amount of adsorbed species.The sample temperature was measured by a Pt-Pt/Rh (13%) thermocouple spot-welded at the centre edge of the sample. The isotope 15NH3 was employed in this study to avoid the overlapping of N, and CO mass peaks. RESULTS AND DISCUSSION The decomposition of ammonia on the Ru(0 0 1) surface took place at ca. 400 K and nitrogen and hydrogen were produced stoichiometrically in the steady state as shown in fig. 1, where the temperature dependence of the rates of N, and H, formation, together with surface concentrations of nitrogen and hydrogen, are given. The rate of ammonia decomposition increased up to ca. 560 K, where the reaction probability for the collision of ammonia molecules was ca. 0.05, and thereafter decreased at higher temperatures under 1.9 x Pa of ammonia.Both nitrogen and hydrogen coverage, which were estimated from the amount of desorbed N, and H, by thermal desorption studies, also decreased with temperature, approaching zero at 770 and 560 K, respectively. Hydrogen is considered to be adsorbed as NH,(ads) species because its desorption temperature is higher than that from the dissociated adsorption of H,. The reaction temperature giving the maximum reaction rate shifted to higher temperatures with increasing ammonia pressure, as shown in fig. 2(A). The temperature at which ammonia decomposition started did not change appreciably with ammonia pressure, but the rate of N, formation on the low-temperature side of the peak in fig. 2(A) depended slightly on ammonia pressure, while that on the higher-temperature side increased with ammonia pressure.The dependence of the rate of N, formation at 520 to 633 K upon ammonia pressure is plotted in the range of ammonia pressure between lop5 and Pa ammonia pressure. However, it became less dependent at higher ammonia pressures and lower reaction temperatures. A similar dependence was obtained on the Pa in fig. 2(B) and shows a linear dependence at ca. 633 K andC. EGAWA, T. NISHIDA, S. NAITO AND K. TAMARU 1597 Fig. 1. Rate of N, and H, formation and surface coverages as a function of temperature during ammonia decomposition on Ru(0 0 1). pNHa = 1.9 x loF5 Pa. 0, N, formation rate; A, H, formation rate; , nitrogen coverage; A, hydrogen coverage. - I d 1 0 E C 0 .- c p 10' Fig. 2. (A) Relation between rate of N, formation and reaction temperature for various ammonia pressures on Ru(0 0 1).pNH,/Pa: (a) 2 x ( d ) 4 x lo-* and (e) 1.2 x Pa. (B) Dependence on ammonia pressure of rate of N, formation during ammonia decomposition on Ru(0 0 1) (open symbols) and Ru (1 1 10) (closed symbols). 0, 0, 520; 0, ., 562; A, A, 598 and 0, +, 633 K. (b) 5 x ( c ) 1.5 x1598 DECOMPOSITION OF NH, ON Ru SURFACES 1 2 nitrogen coverage (X lo-' ) Fig. 3. Rate of N, formation plotted against nitrogen coverage during ammonia decomposition on Ru(0 0 1). 0, 520; 0, 562; A, 598 and 0 : 633 K. Ru( 1 1 10) surface, as shown in fig. 2(B). These results suggest that a different reaction mechanism for ammonia decomposition may be operating at different temperatures. The rate of N, formation as well as nitrogen coverage in the steady state were independent of N, pressure up to 1 O-, Pa at 520-600 K under 3 x Pa of ammonia, although its coverage was much less than saturation.In fact, the dissociative adsorption of molecular nitrogen above room temperature could not be observed, at least up to lo5 langmuir exposure. Consequently, it is concluded that during NH, decomposition surface nitrogen is not equilibrated with molecular nitrogen in the gas phase. As to the effect of hydrogen pressure, both the rate of N, formation and nitrogen coverage were independent of H, pressure at 598 K. Since a negligible amount of hydrogen is adsorbed at higher temperatures in the steady state, N, formation is thought to proceed through the recombination of N(ads) on the surface, as in the cases of W16 and M017 surfaces.In contrast, at lower temperatures the rate of N, formation was reduced, together with nitrogen coverage, with increasing hydrogen pressure. This dependence is more predominant on the stepped surface, as will be discussed later in detail. The rate of N, formation at 520-633 K was plotted as a function of nitrogen coverage in the steady state under various ammonia pressures. As is demonstrated in fig. 3, the rate of N, formation is exponentially proportional to nitrogen coverage, which is similar to NO hydrogenation on Ru, whose rate is one order of magnitude faster than that during ammonia decomposition at the same nitrogen coverage.15 The rate of the reaction and nitrogen coverage in the period before reaching steady state were also investigated.At higher temperatures the rate of the reaction increased with time in accordance with nitrogen coverage on both the flat and stepped surfaces, as shown in fig. 4. On the other hand, as shown in the lower half of fig. 4, at lowerC. EGAWA, T. NISHIDA, S. NAITO AND K. TAMARU 1599 t1103 s Fig. 4. Changes in the rate of N, formation and nitrogen coverage for the transient process up to steady state for ammonia decomposition on Ru(0 0 1). pNHB = 2.7 x lop5 Pa. Upper part, T = 578 K; lower part, T = 520 K. temperatures the rate of N, formation showed a maximum at first and then decreased to its steady-state value, although the total nitrogen coverage increased monotonically toward steady state. This behaviour was more prominent on the stepped surface, as demonstrated in fig.5. The change in the rate in the initial stages, which were not observed on the flat surface for which edge effects had been eliminated, is therefore a characteristic feature of the stepped surface with step sites. To attain steady state even at higher temperatures on Ru(0 0 1) required longer times than on Ru(1 1 lo), which is consistent with activated adsorption as reported by Danielson et al., The initial rate on Ru(1 1 lo), in particular, was one order of magnitude faster than that on Ru(0 0 l), although that on Ru(1 1 10) at steady state was about twice that on Ru(0 0 l), as shown in fig. 2 (B). The changes in the rate of ammonia decomposition on Ru(1 1 10) at lower temperatures were studied in more detail by the thermal desorption of N(ads) during the reaction.As is shown later in fig. 10, a shoulder at 570 K "*(ads)], in addition to the main peak at 750 K [Ns(ads)], was observed during the reaction at 450 K. In the transient state at the beginning of the reaction the amount of N*(ads) showed a maximum in a similar manner to that of the rate of N, formation, although the amount of Ns(ads) grew with time until the steady state was reached. Accordingly, it is suggested that N*(ads) plays an important role in N, formation at lower temperatures and that the formation of N*(ads) may be inhibited by the presence of Ns(ads) on the surface at a later stage of the reaction.1600 DECOMPOSITION OF NH, ON R U SURFACES 2 0 1 t/103 2 Fig. 5. Changes in the rate of N, formation for the transient process up to steady state of ammonia decomposition on Ru(0 0 1) and Ru(1 1 10).pNHI = 2.7 x Pa, T = 520 K. (a) Ru(0 0 1) (edge effects eliminated); (b) Ru(1 1 10). --. Y Q) 2 E .4 Y E 2 z J l -0 -0 - I m .75 3 2 a z 0 5 10 t/min Fig. 6. Changes in the rates of HD and N, formation for the transient process up to steady state of ammonia decomposition on Ru(1 1 10). pNHs = 3.5 x Pa, pHz = pDz = 2.4 x lo-’ Pa, T = 450 K. H,-D, exchange was also investigated on both surfaces during ammonia decom- position. As is shown in fig. 6, the rate of HD formation, which was larger than that on a clean surface at the initial stage in the transient state, was reduced rapidly with the increase of N, formation and then reached a steady-state value that was a quarter of the rate of H,-D, exchange on a clean surface (dashed line).This suggests that the exchange between NH,(ads) formed on step sites and D, proceeds effectively at the beginning of the reaction, because the rate of HD formation on Ru(0 0 1) did not exceed the dashed line. In addition, the decrease in the rate of formation of HD in the later stage indicates that the formation of NH,(ads) is hindered on the surface, which is correlated with the change in the amount of N*(ads) and Ns(ads) mentioned before. The different behaviour of surface nitrogen species formed during ammonia decomposition was obtained on both surfaces in X-ray and ultraviolet photoelectronC. EGAWA, T. NISHIDA, S. NAITO AND K. TAMARU 1601 0 Fig. 7. Effect of H, partial pressure on ammonia decomposition at lower temperatures on Ru(1 1 10).pNH3 = 4.1 x lops Pa, T = 447 K. 0, N, formation rate; A, ON, and 0, nitrogen coverage (X (6N*) Fig. 8. Rate of N, formation plotted against nitrogen coverage (ON*) during ammonia decomposition on Ru( 1 1 10). T = 447 K. spectroscopic measurements as reported elsewhere.* NH(ads) species characterized by an 8.8 eV peak and atomic nitrogen at 5.6 eV below the Fermi level were observed in the u.p. spectra during the reaction at 360 K on Ru(1 1 lo), while atomic nitrogen was detected -on Ru(O0 1) in an amount only half that on Ru(1 1 10). It is thus considered that the activation energy for N-H bond breaking is lowered on step sites and accordingly NH,(ads) species, including atomic nitrogen, are effectively formed on step sites.The relationship between the rate of N, formation and the amount of N*(ads) was investigated under various H, pressures at 450 K on Ru(1 1 10) as shown in fig. 7. The amount of Ns(ads) on the surface was unchanged under different H, pressures; however, both the rate of N, formation and the amount of N*(ads) decreased with increasing H, pressure. This indicates that N*(ads) plays a decisive role in N, formation at lower temperatures. As is demonstrated in fig. 8, the rate of N, formation may be1602 DECOMPOSITION OF NH, ON Ru SURFACES Fig. 9. Isotope effect Open symbols NH,; I n I 1 oi4 1 015 collision frequency/molecule s-l cm-2 on rate of N, formation during ammonia decomposition on Ru(O0 1). filled symbols ND,. 0, a, 520; 0, +, 541; 0, a, 562 and A, A, 598 K.expressed as follows; rN2 GC exp(PB,*). Under the conditions of 520 K and Pa of ammonia the reaction order of ammonia decomposition may be expressed as & & J I E ~ ~ . Accordingly, it is suggested that the formation of N, at lower temperatures proceeds through the combination of N(ads), which is equilibrated with H, and NH, in the gas phase. In other words, the reaction takes place via a typical Temkin-Pyzhev mechanism. Isotope effects on ammonia decomposition were examined by employing NH, and ND,. As is shown in fig. 9, at lower temperatures the rate of N, formation during NH, decomposition was 1.5 times as fast as that during ND, decomposition (the abscissa is expressed as ammonia collision frequency). In contrast, N, formation at higher temperatures is similar for both NH, and ND, decomposition, which is consistent with the independence of N, formation on hydrogen pressure.According to the Temkin-Pyzhev mechanism, the kinetic isotope effect on ammonia decompo- sition would be obtained as (KH/KD)O-,, where KH(KD) is the equilibrium constant 2NH,(ND,) + 2N(ads) + 3H,(D,) for the reaction if the intermediate is N(ads), taking the reaction order described above into account. Its value can be calculated to be 1.4 at 520 K, which is in good agreement with the observed isotope effect. However, if the reaction intermediate is NH(ads), its value would be 1.2, which is a little lower than the experimental value, Finally, the effect of oxygen on the reaction is briefly discussed.As mentioned before, the rate of N, formation during the NO+H, reaction is one order of magnitude faster than that during ammonia decomposition at the same nitrogen coverage. This is in agreement with the different peak temperatures in the N, thermal- desorption spectra; i.e. the desorption temperature of N(ads) during the NO + H, reaction is ca. 650 K, which is 100 K lower than that during ammonia decomposition. Addition of 0, below 1 langmuir during ammonia decomposition on Ru(0 0 1) caused a second peak at 650K, together with the enhancement of N, desorption. It is therefore concluded that the rate of N, formation during the NO+H, reaction is enhanced by the presence of oxygen on the surface (below 1 % of a monolayer). On the other hand, as is demonstrated in fig.10, the presence of oxygen on Ru(1 1 lo), which is preadsorbed by introducing a small amount of 0, (below 0.3 langmuir) at high temperatures (ca. 1200 K), affects the rate of nitrogen desorption in a similar wayC. EGAWA, T. NISHIDA, S. NAITO AND K. TAMARU 1603 (0 ) 450 570 770 TIK Fig. 10. Effect of preadsorbed oxygen on N, thermal-desorption spectra during ammonia decomposition on Ru(1 1 10). 0, exposure: (a) 0, (b) 0.02, (c) 0.07, (d) 0.13 and (e) 0.27 langmuir. Thermal desorption from 450 K, pNHB = 4 x Pa. to the formation of N*(ads). The peak at 570 K in the thermal-desorption spectra was also observed during NO+H, reaction on Ru(1 1 10) at 450 K, where its rate was 1.5 times as fast as that on Ru(0 0 1). However, the nature of the oxygen species on the stepped surface has not been explored in detail in this study.CONCLUSIONS In this study ammonia decomposition has been investigated on stepped Ru( 1 1 10) and flat Ru(0 0 1) surfaces. At lower temperatures (< 500 K) the reaction proceeds through the combination of N*(ads), which is equilibrated with H, and NH, in the gas phase; i.e. the reaction takes place via a typical Temkin-Pyzhev mechanism, where the rate of N, formation may be expressed as rN, cc exp (PON*). The isotope effect of hydrogen in the ammonia molecule is also observed under these conditions. The rate of N, formation during NH, decomposition was 1.5 times as fast as that during ND, decomposition. The formation of N*(ads), which is observed as a peak at 570 K in N, thermal desorption spectra, takes place preferentially on step sites.Especially in the transient state, the rate of N, formation on Ru(1 1 10) is one order of magnitude faster than that on Ru(0 0 1) for which edge effects have been eliminated. Accordingly, it is concluded that step sites on the Ru surface play a decisive role in breaking the N-H bond in ammonia. On the other hand, at higher temperatures (> 600 K) the reaction rate was nearly proportional to ammonia pressure and independent of hydrogen and nitrogen pressures, and no isotope effect was observed. It is thus interpreted that the reaction proceeds through the recombination of N(ads) on the surface, since the amount of hydrogen adsorbed on the surface during the reaction is negligible.1604 DECOMPOSITION OF NH, ON R U SURFACES A. Amano and H. Taylor, J. Am. Chem. Soc., 1954,76,4201. W. L. Guthrie, J. D. Sokol and G. A. Somojai, SurJ Sci., 1981, 109, 390. C . Egawa, S. Naito and K. Tamaru, Surf. Sci. in press. S. R. Logan and C. Kemball, Trans. Faruduy Soc., 1960,56, 144. M. Grunze, F. Bozso, G. Ertl and M. Weiss, Appl. Surf. Sci., 1978, 1, 241. M. Weiss, G. Ertl and F. Nitschke, Appl. Surf. Sci., 1979, 2, 614. 3, 217. K. Kishi and M. W. Roberts, Surf. Sci., 1977, 62, 252. * L. R. Danielson, M. J. Dresser, E. E. Donaldson and J. T. Dickinson, Sug. Sci., 1978, 71, 599. * M. 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