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The infrared spectrum of an adsorbate formed from NH3 on polyerystalline Pt (111)
Spectmchimico Acta. Vol. 37A, No. 9, pp. 815-817. Printed in Great Britain.
1981
0584-8539/s I/0908 I5-03$02.oo/0 @ 1981 Pergamon Press Ltd.
The infrared spectrum of an adsorbate formed from NH, on polycrystalline Pt (111) DOUGLAS S. DUNN, MARK W. SEVERSON, W. G. GOLDEN* and JOHN OVEREND Department of Chemistry, University of Minnesota, Minneapolis, MN 55455, U.S.A. (Received
22 December
1980)
Abstract-We have studied the infrared reflection-absorption spectrum of an adsorbate formed when an annealed polycrystalline Pt foil, presumably having a (111) surface structure, is exposed to ammonia gas at pressures in the range 0.1-10 torr. Isotopic substitution establishes that the adsorbate contains Nz in addition to NH, species. There is also CN present in the adsorbate, presumably formed by reaction of nitrogen with carbon impurities on the surface. Although the Pt catalyzed oxidation of NH3 to NO or NZ is a well-known reaction and has been used
for many years in the commercial production of nitric acid, there has been virtually no direct characterization of the structures of the molecular species adsorbed at the catalytic surface. In this paper we report the results of a study of the infrared reflection-absorption spectrum (IRRAS) of an adsorbate formed when a well annealed polycrystalline Pt foil (thickness 25 pm), presumably having preponderately a (111) surface structure [ l-33, is exposed to ammonia gas at pressures in the range 0.1-10 torr. The experimental apparatus and the method of cleaning the substrate have been described previously[4]. In the present set of experiments, the reactor was operated in a batch (static) mode. After the experiments were concluded, the Pt foil was demounted from the reactor and examined by AES and SEM. The results of this examination showed that the surface of the foil was contaminated with carbon, oxygen, and a small amount of chlorine. No evidence of calcium or any other metal was found. The SEM examination did not reveal any obvious grain boundaries, even at a magnification of 50000 and, in this respect, the substrate we used differed from that reported by COLLINS and SPICER[2] and COMRIE, WEINBERG and LAMBERT[~].
heating the previously cleaned substrate[4] to ca. 1100 K in the presence of the adsorbant at a pressure of 10 torr. The IRRAS spectrum of the adsorbate was recorded at room temperature (300 K) in the presence of the adsorbant gas at pressures in the range 0.1-10 torr and was found to be independent of adsorbant pressure over this pressure range. The spectra of 14NH, and “NH, in the wavenumber range 2200-12OOcm-’ are shown in Fig. 1. We also recorded the spectrum of 14NH, in the range 3300-28OOcm-’ and the spectrum of 14ND, in the range 2200-1200 cm-’ [5].
\ w D56
1914
I
w
1377
Sk0
However,
our studies of NO on the same substrate[41 make us confident that the surface was essentially (111) and successful electron channeling experiments indicated that the grain size was relatively large. The sample of 14NH3 was taken from a lecture bottle supplied by Matheson (99.99%); the sample of 14NDX was prepared by reacting Mg,N, with D,O and purified by distillation in 2racuo and the sample of 15NHs was supplied by Prochem-lsotopes. All samples were passed through a dryice/acetone trap immediately prior to admission to the reactor. The adsorbates were established by 5600 Cattle Road,
15NH3
.05 %
030
1046
1372
1650
a
1
1
I
20001800 1600
cm *Present address: IBM Corporation, E41, San Jose, CA 95193, U.S.A.
“NH,
1
I
1
1400
-1
Fig. 1. IRRAS spectra of adsorbates formed from 14NH, and “NHr on polycrystalline Pt (111).
815
816
D. S. DUNN et al.
The significant features found in the IRRAS spectrum of 14NH, are summarized in Table 1. The band at 3150cm-’ is presumably assignable to an NH stretching mode. That at 2900cm-’ may be a softened NH stretching mode [6] or possibly a CH stretching mode due to a species formed by reduction of surface carbon. The band at 1377 cm-’ is absent from the spectrum of 14ND3, presumably having been shifted outside the range of our instrument. We have therefore assigned this band to an NH deformation mode (which would be expected to shift to below 1000 cm-’ on deuteration) although it is at present unclear whether the species responsible for this spectral feature is NHs, NH* or NH. Although the wavenumber of 1377 cm-’ observed by us does not match the single feature at 1090cmreported by GLAND, SEXTON and KOLLIN[~] for an adsorbate formed from NH3 on Pt (Ill), their observations were made under ultrahigh vacuum conditions at 100 K after annealing the adsorbate at 173 K and it is quite possible that our higher temperature system is very much different from theirs, especially since they did not see any other feature in the EELS spectrum. The other features in the spectrum of r4NH3 at 2056, 1914 and 1650cm-’ are unshifted in the spectrum of 14ND, and they are therefore attributed to vibrations of molecular species which do not contain hydrogen. When we first noted the band at 2056 cm-’ we believed it to be due to CO impurity which has a very intense IRRAS spectrum [8]. However, the experiments with “NH, clearly establish an isotope shift for this feature of 26cm-’ which is consistent with an assignment to a species containing only a single N atom. The observed wavenumber of 2056 indicates a triple bond or a structure with two adjacent double bonds similar to CO2 or N,O and we have chosen to attribute this band to CN, formed by reaction of N atoms from dissociatively adsorbed NH, with impurity C atoms on the surface. The feature at 1914 cm-’ in 14NH, is found at 1846 cm-’
in “NHp, an isotope shift of 68 cm-‘. This large isotope shift is consistent with the responsible species containing two N atoms and the wavenumber suggests that these are connected by a triple bond, i.e., that the species is dinitrogen. It should be noted that this does not correspond with the wavenumber of 2238 cm-’ reported by SHIGEISHI and KING191 for a low-temperature (120K) adsorbate formed when a Pt foil was exposed to Nz gas at IO-’ torr. They also reported that this adsorbate desorbed at 180K. If the feature we observe at 1914cm-’ is due to dinitrogen, then it must be adsorbed in a state different to that studied by SHIGEISHI and K1NG[9]. MUMMEY and SCHMIDT[~~] have recently studied the desorption spectra of adsorbates from the adsorption of ammonia on an annealed Pt foil. For a low ammonia exposure, IOL, (1L = 1 Langmuir = 106torr-s), H, was the predominate desorbing species. The N, that desorbed under the low exposure conditions had a desorption maximum at 635 K. For much greater ammonia exposures (109L) Nz was the predominate desorbing species with a large desorption peak at 520 K and a shoulder at 635 K. This change in the major surface species from hydrogen at low surface exposures to nitrogen at high exposures and SCHMIDT[ lo] to led MUMMEY suggest that the higher temperature desorption peak is due to nitrogen from decomposing ammonia fragments while the lower temperature N, peak is due to atomic or molecular nitrogen which successfully competes for adsorption sites under high exposure conditions. In any case, the desorption temperature of this nitrogen species is much higher than that found by SHIGEISHI and KING[9]. Our present results support the interpretation of Mummey that NH, is dissociatively adsorbed on Pt and further suggest that there is combination of the N atoms to form Nz as a surface species. Clearly many more experiments are needed before a completely unambiguous description of
Table 1. Observed wavenumbers, in cm-‘, and assignments of features in the spectra of an adsorbate formed from “NH1 on annealed polycrystalline Pt. Isotope shifts are for “NH3 CalCUlated isotope shift
3150
Assignment
v(NH)
2900
"(PI) or "(NH)
zns6
26
51
v(CzN)
1914
6R
65
"(NZN)
16VP
0
7
1377
5
3
7 6(NH)
“This feature falls in the wavenumber range expected for adsorbed NO. However, the calculated isotope shift for NO is 30 cm-’ which does not match the observed shift of 0 cm-’ and we are left in doubt as to the assignment of this particular feature.
Infrared spectrum of an NH, adsorbate this complex now
planning
system
can be realized
further
range of temperatures
experiments
[l I]. We over
are
a wider
and pressures. REFFRENCW
[II R. A. SHIGEISHIand D. A. KING, Surf Sci. 58, 379
(1976).
VI D. M. COLLINS and W. E. SPICER, Surf. Sci. 69,8S
(1977). 131 C. IvL COMRIE, W. H. WEINBERGand R. M. LAM.
BERT.Surf. Sci. 57.619 (1976). f41 D. S:DUNN, M. W: SEVERSON,W. G. GOLDENand J. OVEREND,J. Catal. 65,271 (1980). VI The present experiment is restricted to wavenum-
bers above 1200 cm-’ by the particular photoelastic modulator used. This restriction can, in principle, be overcome by replacing the modulator.
817
[61 J. E. DEMUTH, H. IBACH and S. LEHWALD, Whys. Rev. Lett. 40, 1044 (1978). 171 J. L. GLAND, B. A. SEXTON and E. B. KOLLIN, submitted for publication. [8] W. G. GOLDEN, D. S. DUNN, C. E. PAVLIK and J. OVEREND,J. Chem. Ph. 70.4426 (1979). 191 R. A. SHIGEISHIand D: A. KING, surf. kci. 62, 379 (1977). [IO] (a) M. J. MUMMEYand L. D. SCHMIDT,Surf. Sci. 91, 301 (1980); (b) M. J. MUMMEY, Ph.D: Thesis, University of Minnesota (1980). [ 111 Preliminary experiments‘ under clean conditions only show the bands at 1377, 1650 and 1915cm-’ confirming the assignment of the 2056 cm-’ band to a surface impurity. Experiments studying C2N2 adsorption on a Pt substrate show a band at 2060 cm-’ supporting the assignment of the 2056 cm-’ band to adsorbed CN.
1981
0584-8539/s I/0908 I5-03$02.oo/0 @ 1981 Pergamon Press Ltd.
The infrared spectrum of an adsorbate formed from NH, on polycrystalline Pt (111) DOUGLAS S. DUNN, MARK W. SEVERSON, W. G. GOLDEN* and JOHN OVEREND Department of Chemistry, University of Minnesota, Minneapolis, MN 55455, U.S.A. (Received
22 December
1980)
Abstract-We have studied the infrared reflection-absorption spectrum of an adsorbate formed when an annealed polycrystalline Pt foil, presumably having a (111) surface structure, is exposed to ammonia gas at pressures in the range 0.1-10 torr. Isotopic substitution establishes that the adsorbate contains Nz in addition to NH, species. There is also CN present in the adsorbate, presumably formed by reaction of nitrogen with carbon impurities on the surface. Although the Pt catalyzed oxidation of NH3 to NO or NZ is a well-known reaction and has been used
for many years in the commercial production of nitric acid, there has been virtually no direct characterization of the structures of the molecular species adsorbed at the catalytic surface. In this paper we report the results of a study of the infrared reflection-absorption spectrum (IRRAS) of an adsorbate formed when a well annealed polycrystalline Pt foil (thickness 25 pm), presumably having preponderately a (111) surface structure [ l-33, is exposed to ammonia gas at pressures in the range 0.1-10 torr. The experimental apparatus and the method of cleaning the substrate have been described previously[4]. In the present set of experiments, the reactor was operated in a batch (static) mode. After the experiments were concluded, the Pt foil was demounted from the reactor and examined by AES and SEM. The results of this examination showed that the surface of the foil was contaminated with carbon, oxygen, and a small amount of chlorine. No evidence of calcium or any other metal was found. The SEM examination did not reveal any obvious grain boundaries, even at a magnification of 50000 and, in this respect, the substrate we used differed from that reported by COLLINS and SPICER[2] and COMRIE, WEINBERG and LAMBERT[~].
heating the previously cleaned substrate[4] to ca. 1100 K in the presence of the adsorbant at a pressure of 10 torr. The IRRAS spectrum of the adsorbate was recorded at room temperature (300 K) in the presence of the adsorbant gas at pressures in the range 0.1-10 torr and was found to be independent of adsorbant pressure over this pressure range. The spectra of 14NH, and “NH, in the wavenumber range 2200-12OOcm-’ are shown in Fig. 1. We also recorded the spectrum of 14NH, in the range 3300-28OOcm-’ and the spectrum of 14ND, in the range 2200-1200 cm-’ [5].
\ w D56
1914
I
w
1377
Sk0
However,
our studies of NO on the same substrate[41 make us confident that the surface was essentially (111) and successful electron channeling experiments indicated that the grain size was relatively large. The sample of 14NH3 was taken from a lecture bottle supplied by Matheson (99.99%); the sample of 14NDX was prepared by reacting Mg,N, with D,O and purified by distillation in 2racuo and the sample of 15NHs was supplied by Prochem-lsotopes. All samples were passed through a dryice/acetone trap immediately prior to admission to the reactor. The adsorbates were established by 5600 Cattle Road,
15NH3
.05 %
030
1046
1372
1650
a
1
1
I
20001800 1600
cm *Present address: IBM Corporation, E41, San Jose, CA 95193, U.S.A.
“NH,
1
I
1
1400
-1
Fig. 1. IRRAS spectra of adsorbates formed from 14NH, and “NHr on polycrystalline Pt (111).
815
816
D. S. DUNN et al.
The significant features found in the IRRAS spectrum of 14NH, are summarized in Table 1. The band at 3150cm-’ is presumably assignable to an NH stretching mode. That at 2900cm-’ may be a softened NH stretching mode [6] or possibly a CH stretching mode due to a species formed by reduction of surface carbon. The band at 1377 cm-’ is absent from the spectrum of 14ND3, presumably having been shifted outside the range of our instrument. We have therefore assigned this band to an NH deformation mode (which would be expected to shift to below 1000 cm-’ on deuteration) although it is at present unclear whether the species responsible for this spectral feature is NHs, NH* or NH. Although the wavenumber of 1377 cm-’ observed by us does not match the single feature at 1090cmreported by GLAND, SEXTON and KOLLIN[~] for an adsorbate formed from NH3 on Pt (Ill), their observations were made under ultrahigh vacuum conditions at 100 K after annealing the adsorbate at 173 K and it is quite possible that our higher temperature system is very much different from theirs, especially since they did not see any other feature in the EELS spectrum. The other features in the spectrum of r4NH3 at 2056, 1914 and 1650cm-’ are unshifted in the spectrum of 14ND, and they are therefore attributed to vibrations of molecular species which do not contain hydrogen. When we first noted the band at 2056 cm-’ we believed it to be due to CO impurity which has a very intense IRRAS spectrum [8]. However, the experiments with “NH, clearly establish an isotope shift for this feature of 26cm-’ which is consistent with an assignment to a species containing only a single N atom. The observed wavenumber of 2056 indicates a triple bond or a structure with two adjacent double bonds similar to CO2 or N,O and we have chosen to attribute this band to CN, formed by reaction of N atoms from dissociatively adsorbed NH, with impurity C atoms on the surface. The feature at 1914 cm-’ in 14NH, is found at 1846 cm-’
in “NHp, an isotope shift of 68 cm-‘. This large isotope shift is consistent with the responsible species containing two N atoms and the wavenumber suggests that these are connected by a triple bond, i.e., that the species is dinitrogen. It should be noted that this does not correspond with the wavenumber of 2238 cm-’ reported by SHIGEISHI and KING191 for a low-temperature (120K) adsorbate formed when a Pt foil was exposed to Nz gas at IO-’ torr. They also reported that this adsorbate desorbed at 180K. If the feature we observe at 1914cm-’ is due to dinitrogen, then it must be adsorbed in a state different to that studied by SHIGEISHI and K1NG[9]. MUMMEY and SCHMIDT[~~] have recently studied the desorption spectra of adsorbates from the adsorption of ammonia on an annealed Pt foil. For a low ammonia exposure, IOL, (1L = 1 Langmuir = 106torr-s), H, was the predominate desorbing species. The N, that desorbed under the low exposure conditions had a desorption maximum at 635 K. For much greater ammonia exposures (109L) Nz was the predominate desorbing species with a large desorption peak at 520 K and a shoulder at 635 K. This change in the major surface species from hydrogen at low surface exposures to nitrogen at high exposures and SCHMIDT[ lo] to led MUMMEY suggest that the higher temperature desorption peak is due to nitrogen from decomposing ammonia fragments while the lower temperature N, peak is due to atomic or molecular nitrogen which successfully competes for adsorption sites under high exposure conditions. In any case, the desorption temperature of this nitrogen species is much higher than that found by SHIGEISHI and KING[9]. Our present results support the interpretation of Mummey that NH, is dissociatively adsorbed on Pt and further suggest that there is combination of the N atoms to form Nz as a surface species. Clearly many more experiments are needed before a completely unambiguous description of
Table 1. Observed wavenumbers, in cm-‘, and assignments of features in the spectra of an adsorbate formed from “NH1 on annealed polycrystalline Pt. Isotope shifts are for “NH3 CalCUlated isotope shift
3150
Assignment
v(NH)
2900
"(PI) or "(NH)
zns6
26
51
v(CzN)
1914
6R
65
"(NZN)
16VP
0
7
1377
5
3
7 6(NH)
“This feature falls in the wavenumber range expected for adsorbed NO. However, the calculated isotope shift for NO is 30 cm-’ which does not match the observed shift of 0 cm-’ and we are left in doubt as to the assignment of this particular feature.
Infrared spectrum of an NH, adsorbate this complex now
planning
system
can be realized
further
range of temperatures
experiments
[l I]. We over
are
a wider
and pressures. REFFRENCW
[II R. A. SHIGEISHIand D. A. KING, Surf Sci. 58, 379
(1976).
VI D. M. COLLINS and W. E. SPICER, Surf. Sci. 69,8S
(1977). 131 C. IvL COMRIE, W. H. WEINBERGand R. M. LAM.
BERT.Surf. Sci. 57.619 (1976). f41 D. S:DUNN, M. W: SEVERSON,W. G. GOLDENand J. OVEREND,J. Catal. 65,271 (1980). VI The present experiment is restricted to wavenum-
bers above 1200 cm-’ by the particular photoelastic modulator used. This restriction can, in principle, be overcome by replacing the modulator.
817
[61 J. E. DEMUTH, H. IBACH and S. LEHWALD, Whys. Rev. Lett. 40, 1044 (1978). 171 J. L. GLAND, B. A. SEXTON and E. B. KOLLIN, submitted for publication. [8] W. G. GOLDEN, D. S. DUNN, C. E. PAVLIK and J. OVEREND,J. Chem. Ph. 70.4426 (1979). 191 R. A. SHIGEISHIand D: A. KING, surf. kci. 62, 379 (1977). [IO] (a) M. J. MUMMEYand L. D. SCHMIDT,Surf. Sci. 91, 301 (1980); (b) M. J. MUMMEY, Ph.D: Thesis, University of Minnesota (1980). [ 111 Preliminary experiments‘ under clean conditions only show the bands at 1377, 1650 and 1915cm-’ confirming the assignment of the 2056 cm-’ band to a surface impurity. Experiments studying C2N2 adsorption on a Pt substrate show a band at 2060 cm-’ supporting the assignment of the 2056 cm-’ band to adsorbed CN.