J. Chem. Soc., Faraday Trans. I, 1984, 80, 29-35 Fourier-transform Infrared Spectroscopic Study of Adsorption and Decomposition of Ammonia over Magnesium Oxide BY SETSUKO KAGAMI, TAKAHARU ON IS HI*^ AND KENZI TAMARU Department of Chemistry, Faculty of Science, The University of Tokyo, Hongo, Bunkyo-ku, Tokyo 113, Japan Received 24th January, 1983 The adsorption and decomposition of ammonia over magnesium oxide have been studied by means of infrared spectroscopy. The decomposition of ammonia proceeds at 573 K to form N, and H,. The following mechanism of decomposition of ammonia over MgO has been elucidated : (1) (2) (3) at room temperature 1 NH,(g)=NH,(a) NH,(a)eNH,(a)+H(a) NH,(a)+NH(a)+H(a) at 473 K NH(a) s N(a) + H(a) ') (4) Many studies have been carried out on the synthesis and decomposition of ammonia over supported transition metals by means of infrared spectroscopic methods.Nakata and Matsushita found the NH, adsorbed species on the iron-supported silica catalyst . I Pozdnyakov and Filimonov2 investigated the adsorption of ammonia on metal catalysts such as Fe, Ni, Pt, Pd and Ru supported on SiO, or MgO and reported that ammonia was adsorbed on metal surfaces at least in two forms; one of the adsorbed forms is coordinated ammonia and the other is dissociatively chemisorbed. Brill et aZ.3 studied ammonia synthesis from nitrogen and hydrogen over an iron cata- lyst supported on magnesia and found that the chemisorbed species on the Fe catalyst has a hydrazine-like structure. Okawa et aL4 reported that after catalytic decomposition of ammonia on iron catalysts supported on magnesia, two infrared bands were observed at 2220 and 2050 cm-l; these were both assigned to the N-N stretching vibration of molecularly adsorbed nitrogen.However, they did not study the chemisorption and decomposition of ammonia on the MgO carrier in detail. The adsorption of ammonia on various oxides such as al~mina,~ silica-alumina,8 zinc oxideg and magnesium oxidelo* l1 has been studied by infrared spectroscopy in connection with Lewis- and Brarnsted-acid sites. However, no studies were carried out on the decomposition of ammonia over these oxides. Nagatsuta, Yokohama 227, Japan. t Present address: Research Laboratory of Resources Utilization, Tokyo Institute of Technology, 2930 I.R. STUDY OF AMMONIA DECOMPOSITION ON MgO In this paper the adsorption and decomposition of ammonia over magnesium oxide studied by means of infrared spectroscopy are reported.In this study nitrogen species adsorbed on the oxide were observed for the first time. EXPERIMENTAL The high-purity MgO used in this experiment was obtained from Merck. The major impurities were Na (ca. 2.079, Ca (ca. 0.0279, carbonate (ca. 1.5%) and Fe (ca. 0.005%), and its surface area was 89 m2 g-l. Ca. 40 mg of MgO was pressed into a self-supporting disc 20 mm in diameter. The sample was placed in an i.r. cell which was attached to a closed gas-circulation system as described previously.'* The oxidized catalyst was obtained by evacuating MgO at 773 K for 1 h and subsequently oxidized under 13 kPa of oxygen for 5 h in the cell; the reduced catalyst was prepared by reducing the oxidized sample under 13 kPa of hydrogen at 773 K for 1 h.15N-substituted ammonia (96 at. % purity) used was obtained from M.S.D. Canada Ltd. These gases were used without purification. Infrared spectra were recorded with a JEOL JIR 10 Fourier-transform infrared spectrometer using a liquid-nitrogen-cooled HgCdTe detector. Infrared spectra of the adsorbed species were recorded at room temperature in the region 3600-800 cm-l, after gaseous ammonia had been removed at the reaction temperature by a liquid-nitrogen trap. 1.r. spectra were usually obtained with 256 scans and the spectral resolution was 2 cm-l. Ratio-recorded spectra were obtained by first measuring the background produced by MgO stored as a reference and then the adsorbed gas samples.3600 3200 2800 2400 2000 1600 1200 800 wavenum berlcrn-' Fig. 1. Fourier-transform infrared spectra of ammonia adsorbed on MgO. (a) Ammonia (1.6 kPa) was introduced at 298 K for 20 min and the gas phase was removed by a liquid-nitrogen trap; (b) after heating for 1 h at 473 K in gaseous ammonia; (c) at 573 K for 1 h; (d) at 673 K for 1 h; (e) at 773 K for 1 h.S. KAGAMI, T. ONISHI AND K. TAMARU 31 RESULTS AND DISCUSSION Fourier-transform infrared spectra of species adsorbed over reduced MgO were obtained after the adsorption and decomposition of ammonia (and deuterated ammonia) at various temperatures and are shown in fig. 1 and 2. At room temperature characteristic absorption bands were observed at 3370 (2610), 3230 (2510), 1600 (I 170) and 1160 (875) cm-l as shown in fig.1 (a) and 2(a). 3600 3200 2800 2LOO 2000 1600 1200 800 wavenum ber/cm -' Fig. 2. Fourier-transform infrared spectra of deuterated ammonia adsorbed on MgO. (a) ND, was introduced at room temperature for 20min and the gas phase was removed by a liquid-nitrogen trap; (b) after heating for 1 h at 473 K in gaseous ND,; (c) at 573 K for 1 h; ( d ) at 673 K for 1 h; (e) at 773 K for 1 h. The assignments of these bands are given in table 1 together with related data. As shown in the table, most ammonia was molecularly adsorbed over MgO at room temperature. The i.r. spectra of ammonia adsorbed over oxidized MgO at room temperature are shown in fig. 3. In this case characteristic absorption bands were observed at 3310 and 1550 cm-l in addition to absorption bands of the species NH,(a).The intensity of the band at 3750 cm-l due to an OH stretching vibration increased as the temperature was raised. The 33 10 and 1550 cm-l bands can be assigned to the NH,(a) species on the basis of the data given in table 2. It may be concluded that over oxidized MgO a small amount of ammonia is dissociatively adsorbed to produce NH,(a) even at room temperature : NH, -+ NH,(a) (at room temperature) (1) NH, + NH,(a) + H(a) (partial). (2)32 I.R. STUDY OF AMMONIA DECOMPOSITION ON MgO Table 1. Comparison of vibrational frequencies (cm-l) of adsorbed NH, (ND,) NH,(ND,)/ dehydrated NH,/ NH,/ NH,/ assignment MgOa NH3(s)13 MgO'O A120,, Zn09 Fe-Mg02 Pt"'l4* vd(NH3) 3370 3378 v, (NH,) 3230 3223 sd(NH3) 1600 1646 6, (NH,) 1160 1060 (2610) (2510) (1 170) (875) 3340 3400 3320 3270 3340 (2500) 3280 3335 3270 3200 3240 3210 (2350) 1603 1620 - 1610 1600 (1180) - 1140 1140 1110 - (930) a This work: over the reduced surface at room temperature; * at low coverage (8 < 0.4).A 3600 3200 2800 2LOO 2000 1600 1200 800 wavenum ber/cm -* Fig. 3. Fourier-transform infrared spectra of ammonia adsorbed on oxidized MgO. (a) Ammonia was introduced at room temperature for 20 min and the gas phase was removed by a liquid-nitrogen cold trap; (b) after heating for 40 min at 473 K in gaseous ammonia. Table 2. Comparison of vibrational frequencies (cm-l) of amido compounds ~~~ ~~ assignment NH3/MgOa Hg(NH2)+Cl-l5 NH,/A1,0,5 Co complex(I)lsv Co complex(II)16* v[NH,(a)] 3370 - 3386 3200 - v[NH,(s)] 3310 - 3335 - - W H , 1 Pw (NH2 1 Pt (NH,) Pr (NH, 1 1550 1534 1510 1560 1560 1110 1022 - 1049 1068 - - - - - - (978) - 668 - a This work: over the oxidized surface at room temperature; [Co(NH,) (NX3)lo] (NO,),; K O , (NH,) (NH, )8 CW2O)I c1,.S. KAGAMI, T.ONISHI AND K. TAMARU 33 Over the reduced catalyst these bands were weak and appeared as a shoulder on the NH,(a) band. When the adsorbed ammonia was heated to 473 K the absorption bands gradually decreased and a band at 1410 cm-1 appeared which was tentatively assigned to the deformation vibration of NH(a) on the basis of reference spectra of imino metal complexes, as shown in table 3: NH,(a) -+ NH(a) + H(a) (at 473 K). Table 3. Comparison of vibrational frequencies (cm-l) of imido compounds (3) assignment NH3/MgOa 0 s [Fe(NO), NH],18 V(NH) 3370 2994 3376,3304 W H ) 1410 1414 1458 a This work at 473 K; [Os,(NH)Cl,biPY,] (ClO,).At 573 K two sharp bands at 2220 and 2195 cm-l and a weak broad band at 2080 cm-l appeared; at 673 K these three bands became strong in intensity, as shown in fig. 1 ( c ) and (d) and 2(c) and (d). At 773 K the former two bands became weaker and the 2080 cm-l band became stronger. When gaseous ammonia was introduced to this surface at room temperature, the former two peaks were reduced in intensity while the 2080 cm-l peak was unchanged. Three bands at 2220,2195 and 2080 cm-l were observed at the same position when ND, was used instead of NH,. When the decomposition of 15NH, proceeded at 773 K, these bands shifted to the lower-frequency region, as shown in table 4.Table 4. Isotope shift of adsorbed N, species after ammonia decomposition at 773 K on reduced MgO gases used wavenumber/cm-l NH3 2220 2 195 2080 ND3 2220 2 195 2080 2200 2 1 d0 2060 15NH3 The surface preadsorbed species was obtained by decomposition of 14NH, at 773 K; after the gas phase was evacuated, 15NH, was introduced into the system at the same temperature for 30min. The absorption bands of adsorbed species due to 14NH, disappeared and those due to 15NH, appeared. Infrared spectra of dinitrogen adsorbed on metal and oxide catalysts have been studied and it was found that i.r. bands due to adsorbed dinitrogen species appear in the region 1900-2300 cm-I.l9 The three bands observed are related to dinitrogen species adsorbed on the magnesium oxide.However, the isotope shifts of these bands are smaller than the calculated values. In table 5 i.r. data for the isotope shift of dinitrogen adsorbed on various catalysts are shown. In the case of dinitrogen species adsorbed on a K-Al,O, catalyst the isotope shift is also too while the same species adsorbed on a34 I.R. STUDY OF AMMONIA DECOMPOSITION ON MgO Table 5. Infrared data of dinitrogen species adsorbed on various catalysts wavenumber/cm-' Ni/Si021g 2195 2123 K-A120,20 2030 2010 2220 2200 (present work) 2195 2180 2080 2060 MgO Ni/SiO, catalyst was observed at the expected wavenumber.20 The reason for the small isotope shift is not clear at present. The former two sharp bands at 2220 and 2195 cm-1 can be assigned tentatively to the N E N stretching vibrations of dinitrogen species weakly adsorbed on MgO.The broad and stable band at 2080 cm-' was due to dinitrogen species adsorbed strongly on a magnesium metal site. The decomposition of ammonia over MgO proceeded slowly at 573 K to form nitrogen and hydrogen and more rapidly at high temperature: NH(a) -+ N(a) + H(a) ) (4) l (at 573-773 K). 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