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Peculiarities of the vibratioanl spectra of molecules adsorbed on the surface of heterogeneous catalysts: theoretical analysis
Journal of Molecular Structure, 147 (1986) 193-201 Elsevier Science Publishers B.V., Amsterdam - Printed in The Netherlands
PECULIARITIES OF THE VIBRATIONAL SPECTRA OF MOLECULES ADSORBED ON THE SURFACE OF HETEROGENEOUS CATALYSTS: THEORETICAL ANALYSIS
E. B. BURGINA, E. N. YURCHENKO
and E. A. PAUKSHTIS
Institute of Catalysis, Novosibirsk 630090
(U.S.S.R.)
(Received 24 April 1986)
ABSTRACT The vibrational spectra of adsorption complexes resulting from the adsorption of ammonia on the surface of heterogeneous catalysts are interpreted by computation of frequencies and forms of normal vibrations. The characteristics of normal vibrations are computed from the state of separate fragments of adsorption complexes. It is shown that an interaction of internal coordinates, associated with the adsorption bond, with internal coordinates of the adsorbed molecule leads to a decrease of characteristics of vibrations sensitive to the surface state. A correlation is established between the characteristics of such vibrations and the changes in their frequencies in comparison with the free molecule. INTRODUCTION
When studying surface properties of heterogeneous catalysts as well as the influence of certain surface centers on the state of the adsorbate, the method of vibrational spectroscopy of the adsorbed molecules is most widely used [l-3]. Variations observed in vibrational spectra of adsorbed molecules as compared with individual ones are caused, (i) by changing the force field and the structure of the adsorbed molecule itself, and (ii) by interacting internal coordinates of the molecule with coordinates appearing as a result of the adsorption bond formation. Up to now, only the first reason has been taken into account [3, 41, i.e., all the changes in the molecular spectra following adsorption have been explained on the basis of distortion of the structure of the free molecules and changes of their force constants. As shown previously, the interaction between internal vibrational coordinates considerably influences the values of the characteristic vibration frequencies observed in the spectra of adsorbed ethylene, propylene [5] and carbon monoxide [6] . Now we study the influence of the interaction of ammonia and ammonium ion internal coordinates with those coordinates corresponding to adsorption bonds, on the position of the vibration frequencies most sensitive to the interaction of these molecules with the adsorbent surface. The numerous IR spectra of adsorbed NH; show that such a 0022-2860/86/$03.50
o 1986 Elsevier Science Publishers B.V.
194
frequency is 6KH , whereas Si!& remains practically constant after the interaction of NH; with the protii surface centers. To explain the causes of the different behaviour of these two normal vibrations is one of the aims of the present paper. During the adsorption of NH3 on the surface of carriers with strong Brijnstead centers, the proton transfer from OH groups to the NH: molecule with the formation of NH’, ions is possible. Such ions, as suggested in ref. 3, may be stabilized on the surface due to interaction with one (MO-) and/or two (MO-, OMz) basic centers. In this connection it seemed interesting to determine whether taking into consideration the new coordinates of the adsorption complex and their interaction is important for the description of the adsorbed NW4 vibrational spectrum. ANALYSIS
PROCEDURE
Changes in the vibrational spectra of NH3 and NH: upon adsorption have been modelled by computing the normal vibrations of adsorption complexes (Fig. 1) with further analysis of the relative distribution of the potential energy of the normal vibration on the internal coordinates, which will be designated below as characteristic. All computations were carried out in the harmonic approximation using the programs of Gribov and Dementjev H
H
H
q51ka7 i
i
M
M
M (a)
\
(b)
Fig. 1. Normal coordinates of adsorption complexes: (a) ammonia adsorbed on the OH group of the surface, with the adsorption bond directed along the axis of symmetry of the NH, group; (b) one-center adsorption complex of the ammonium ion where the direction of the absorption bond coincides with that of one of the NH(q)/bonds 01~~ being a coordinate of the angle change on the linear interval [7] ; (c) two-center adsorption complex of the ammonium ion where the adsorption bonds q5 and q6 are in the plane of the NH bonds q3 and q4, bonds q, and qz are in the perpendicular plane, and bond lengths and angles for NH, and NH, fragments are taken as for free ammonia molecules and the ammonium ion, respectively.
195
[7]. Anharmonicity of the light atom vibrations was taken into account by using the spectroscopic masses of hydrogen and deuterium. Initially, the surface of the heterogeneous catalyst for adsorption of NH3 molecules was modelled by a surface fragment of aluminosilicate HO(M03)* (M = Al, Si) whose geometrical parameters and force constants were taken from ref. 8. It follows from computations for adsorption complexes AX,0(M0,)2 (A = C, N; X = Cl, H) that the “breaking” of 6 atoms of oxygen from the fragment O(M03h (i.e., transition to OMz, Fig. l(b)) and the varying of the mass M from 20 to 200 a.u. have insignificant kinematic influences on the frequencies of an absorption complex connected with vibrations of the fragment AX4. Further simplification of the model, i.e. transition to surface modelling by a point mass, leads however to an unjustified increase in the adsorption complex symmetry. Therefore we took the fragments HOM, OMz and OzM3 as models of surface centers which can adsorb NH3 molecules. Fig. l(a) shows the internal coordinates of the adsorption complex of ammonia obtained by the interaction of NH3 with OH groups of the surface. Figs. l(b), (c) show the internal coordinates of one- and two-center adsorption complexes of NH’,, respectively. We use the force fields of the NH3 molecule and the NH’, ion obtained by solving an inverse spectral problem for the frequencies of free NH3, ND3, NH: and ND’, as a zero-order approximation of the force fields of each adsorption complex; force constants of surface fragments were taken from ref. 8 and those of hydrogen bonds from refs. 9 and 10. The experimental frequencies taken from refs. 11 and 12 were used when solving inverse spectral problems for NH3 and ND3 molecules; the force constants obtained are: K = 10.72 X 106, KNH_-NH = 0.05 X 106, KNH_-HNH = 0.4X 106, KHNH = O.fiX 106, KHNH--HNH = -0.07 X lo6 cm-’ [5]. For experimental frequencies ascribed to free NH: and ND’, the values obtained are [ 121: K,, = 8.83 X 106, KNH_-NH = 0.18 X 106, KNH_-HNH = -0.23 X 106, KHNH = 0.75 X 106, KHNH--HNH = -0.03 X lo6 cm-*. Analysis of vibrational spectra of NH4 salts [13] and recent experiments 1141 lead, however, to the conclusion that the frequencies v1 and u3 of free NH: exceed those cited in ref. 12 by approximately 200 cm-‘. So we use v1 = 3250 (A),v2 = 1700 (E), v3 = 3350 (F,) and v4 = 1430 c m-l (F,) from ref. 15 as frequencies of free NH:. The set of force constants: KNH = 10.83 X 106,KNH_NW = 0.05 X 106, KNH-HNH = 0.4 x 106, KHNH = 0.81 X 106, KHNHeHNH = -0.07 X lo6 cm-‘, obtained by solving corresponding inverse spectral problems, was used for the description of the force fields of the NH4 fragment in absorption complexes (Fig. 1). As in the analysis of $‘&, and V& in pseudotetrahedral complexes ZnC12L2 [ 161, we suggest that the abov$ different sensitivity of B&n and Szn to the interaction of NH3 with protic centers of the surface may” be explahed by a different contribution of internal coordinates of the adsorption complex to the corresponding normal vibrations. One can verify this assumption by analyzing the characteristics of normal vibrations of different models of the adsorption complexes with respect to changes of
196
N-H bonds, HNH angles or coordinates of the entire NH3 group. For the quantitative estimation of the characteristics of normal vibrations we use the coefficients of relative distribution of the total potential energy on the internal coordinates of a selected fragment f’. For estimating f’ rather than the relation in ref. 16, we use the following more accurate equation. f’ = (F KtC + Jj Kijltlj)l(
5: KtC
+
II Ktslttfs) tfs
Here i, j are indices of internal coordinates of a fragment, t, s are indices of internal coordinates of the entire molecule (including the selected fragment), K terms are force constants and 1 terms are coefficients of the normal vibration forms in the internal coordinates. This relation takes into account all the members in the expression for potential energy of the normal vibration (diagonal as well as non-diagonal ones), providing an estimate of its characteristics with respect to the change of the arbitrary ith coordinate, as well as of all the coordinates associated with a group of atoms in the molecule. RESULTS
AND DISCUSSION
Changes of 8&n values after adsorption of ammonia were interpreted in ref. 4 as the rest& of the force field changes of the adsorbate molecule itself. This interpretation was based on the fact that the force constant of the N-H bond is approximately a factor of ten greater than that of the bond, and likewise for force constants of internewly-formed N * * -H*(O) actions between N-H and N - * - H* bonds, etc. The explanation of the growth of 6 &n due to increase of the N-H bond force constant brings us, however, to the3following contradiction: the wide range of 6&n? values (v2 = 950-1200 cm-‘) of the adsorbed NH3 requires an essential change of the HNH angle diagonal force constant in order to obtain satisfactory coincidence of the calculated and experimental values of frequencies. This leads inevitably to a notable change in calculated frequency BS 6 (uq = 1627 cm-‘), that is not observed experimentally. This is, in our ozmon, the reason why Kiselev and Lygin [4] explained the discrepancy between their calculation and experimental data by decreasing v2 anharmonicity. If this is so, the anharmonicity of v4 should also decrease, which must lead to an unobservable increase of v4. Thus, in both cases, the frequencies S&n and SF, should increase. We doubt that the anharmonicity of is ‘the only ieason for the frequency shift of the normal vibration 6NH, up to -250 cm-‘, besides which there is no evidence for a change of %H anhbmonicity of the hydrogen atom vibrations in NH3 as the result of the interaction of NH3 with proton donor groups of the surface via the N atom. Let us point out that the cause of v2 change suggested by Kiselev and Lygin [4] implies the invariability of harmonic force constants determining the values of v2 and v4 at the H’ * . - NH3 interaction.
197
From computations of normal vibrations of the adsorption complex (Fig. l(a)) without changing the force constants of the NH3 fragment as compared with those of the free NH, molecule, we conclude that the characteristic of normal vibrations with respect to NH3 atom shifts equals unity for vibrations with frequencies 3342 and 3437 cm-‘; it is approximately unity for frequency 1651 cm-’ and notably less than unity for -950 cm-‘. In the last case, the expression of the form in internal coordinates (see Fig. l(a)) is: -0.15 q1 + 0.82 (Yl + 0.82 (Y2 + 0.82 a3 + 0.82 o4 + 0.82 CY~+ 0.82 Q. Obviously, the characteristic of this normal vibration on the NH3 coordinates (a4 - CY~)decreases with increase in the contribution of the coordinates q,(NH*) and CK~, (Ye,01~ (HNH*), i.e. the stronger the adsorption bond and the higher the associated force constants (Rnn, K HNH *Y K NH*_-HNH*, etc.). Figure 2 illustrates this situation as f’ changes (by K NH* variation only). The computations of harmonic vibration frequencies which involve the entire observed range of AgH values can be performed without changing the force constants of ammonia’itself, but only by changing the parameters of adsorption bonds, e.g., when KNH * increases from 0 to 1.5 and 2.5 6 RH increases from 926 to 1011 and 1086 cm-‘, respectively; when KHNH* i& creases from 0 to 0.1 and from 0.2 to 0.3, Sk, increases from 926 to 1087 and from 1144 to 1199 cm-‘, as well; when dNHLHNH* increases from 0 to 0.01, ?&n, increases from 1199 to 1232 cm-‘. The< other above-mentioned vibration frequencies of the NH3 fragment of the adsorption complex are practically insensitive to the changes of these force constants. In the case of NH3 adsorption on the silica surface, the adsorption bonds
cm-’
Fig. 2. Dependence of the characteristic of normal vibration of the adsorption complex of ammonia, with the major contribution of s(NH,) (according to the change of coordinates of NH, groups, f’(NH), of the adsorption bond, f’(NH*), and of angles with the adsorption bond, f’(HNH*)).
198
were registered at 3322, 3403, 3417, 1635 and 1130 cm-’ [17]. To obtain similar values of frequencies, it is sufficient to take in our model (Fig. l(a)): K NH* = 2.5, KOHsN = 0.2 and KHNH* = 0.2, with values of NH3 group force constants as above. The calculated values of v are 3318, 3415, 3414, 1638 and 1127 cm-‘; those for the deuterocomplex D3N - * * DOMz are: 2392, 2541, 2543, 1206 and 946 cm-‘. The value 6&n, = 950 cm-’ is registered experimentally. Thus, we can describe the registered spectrum changes by taking into account, (i) the interaction of the NH3 group coordinates with those appearing in the adsorption complex (Fig. l(a)), and (ii) the contribution to the potential energy of vibration S&u of the energy changes of these new coordinates. In this description we” do not suppose any changes in anharmonicity of NH3 group vibrations during ammonia adsorption nor in the harmonic force constants of the NH3 group. The question naturally arises as to whether the approximation, in which harmonic force constants of the NH3 group are unchanged after the formation of the adsorption complex, is correct or not. To clear up this question, we have made a quantum-chemical calculation of the NH3 molecule and of our adsorption complex (Fig. l(a)), using the MINDO-3 method with the geometry optimization [US], modified in accordance with the properties of our surfaces*. The computations have shown that as the hydrogen bond No.* H* strengthens (bond length decreases from 1.41 to 1.33 A), the OH* bond weakens (bond length increases from 0.99 to 1.01 A), but the lengths of the N-H bonds do not change. Thus, our assumption that KNH is unchanged during the NH3 adsorption is correct in the first approximation. The weaker is the adsorption bond the more correct is this assumption. We might, in principle, determine the influence of the interaction of the surface with NH3 on force constants of the latter from the solution of the inverse spectral problem if we could obtain the exact values of vibration frequencies of adsorbed ammonia, interpreted as Vnn, SFH . These frequency values [14] are almost the same as those for gaseous ammonia [12]. This confirms the correctness of our first approximation of the force field of the adsorption complex. Computations of one- and two-center adsorption complexes of ammonium ion (Fig. l(b, c)) were made with a similar approximation, i.e. assuming invariability of the force field of the adsorbed molecule. Only the strength of the hydrogen bond (KOH*,KOH*N, KH*OM) was changed. Computations have shown that the vibration forms of the adsorption complex differ essentially from those of the free ion NH: (see Table l), even if this bond is very weak (KOH* = 0.01; KOH*N = O;KH*o, = 0).One can see from Table 1 that the new coordinates qou*, (YMon*,(YNn*o contribute to all types of normal vibrations, thus resulting in the notable splitting of all three times degenerate *The parameter of pseudoatom M determining -8 to -15 [ 18 1. The authors varied from A. G. Pelmencsikov for their help in computations.
the acid of the M-OH are grateful to G. M.
*surface groups Zidomirov and
TABLE 1 Forms of normal vibrations of ammonium ion and of its one- and two-center adsorption complexes in the 1300-3400 NH, *..O,M, b
NH,*.*OM; 32551x-' 354, +35q, +35q, +35q, 3348cm-' 51q,-514, -13a, -1301, + 13or,+13a, 3348cm-' -29q, -29q,+ 3348cm“
589,
+ 15a,-15~1,
-2Oq,
-2Oq, -20qs + 624, + lOa, + lOor,+10a, +1oa, flOcr, +1ocx, 1713cm-' -83~1, +82or,-8301, +82a,
1713cm-' --46a,---48.~~ t95a, -4601, -48~1, t95a, 1400cme' -79cx, t7901, t79a, -79a, 1400cm-' -79a, -79a, +79cY, +79a, 14OOcm+ -112a, t11201,
BSee Fig. 1. bSee Fig. l(c).
3255cm-' 89q,+ 33q2 + 33q, + 33q, +39q, 3339 cm-’ -51q1 +51q, t1201, -12a,-12a, t12a, t 12cl, 3339 cm-' -29q, -29q5 + 599, -14or, t 14CY,t 13a,, 3321cm‘' 609,
-234,
cm-’ region
-239,
-23q,
+23q,
1713cm-' -8201, t82o1, -882~~ t82a, 1713cm-' -47cx.-4701, t95a, -47ff.-47% t 95a; +26,x;--2601; -774~1;~ . 1428cm-' -174, -6501, -65a, -6501, t65a, t 65a, +66a, 141Ocm-' -79a, t79a, +79cY, -79ci, 1410cm' lOq,-45cQ -45a, 91a, t33a,-33cx,
+91a, +45a, +45a,-+102cY,,
3255cm-' 88q,
t
38q,
+ 3Oq,
+ 3Oq,
-28q,
-28q,
3356cm-' -32q,
-32q, t39sa +39c?,--3319,-3ls, t 18a,-l7cr,-l8a,-l8a, 3342cm-' -5lq, +51q, +13a, t13a,--13a,-1301, 3319 cm-' 51qs-5lq, +43q, f43q,-ll~* i-llcx,t lla, -l.la, 173ijcm-' 15q,t15q,-103o1, +49a,t49a,--85a, +49cu,t 49cx,-65a,-65a, +17a,t17a,,t17a,, +1701,, 1690cm-' -82~1, +82a, -820 mst82a,
1463cme' 81ar,+81a,--8101, -81~1, 1402cm-' -32q5 -3324, 85~1,-18or, 1362cm“ -78~1,+78a, 62a, -1601,
+102a, +12Oa, +85a, + -18a,,-18a,,-1801,, +78a, -78ci, +62a,t16a,, -16a,, -16a,,
vibrations at one-center adsorption, i.e. unlike the case of NHB, for the NH’; ion having high symmetry (Td) more frequencies of normal vibrations become more sensitive to interaction with the surface. Obviously enough, intensification of such interactions, i.e. the growth of KOH*, KOH*N, KHsOM, leads to some decrease of value of all the vibrations whose frequencies are underlined in Table 1, with respect to the vibrations of atoms of NH4 groups in the complexes, as well as to the increase of splitting of degenerate vibrations. It is only by increasing the abovementioned force constants up to values 1.5, 0.2, 0.3, respectively (the geometry and force constants of the NH4 group being unchanged) that the splitting of vq(F2) increases up to 130 cm-‘; this frequency splits into three components even in the one-center complex. Such a pattern of splitting of v4 in both types of complexes leads to the conclusion that the criterion for the recognition of one- and twocenter adsorption according to the number of splitting components S&+ (-1400 cm-‘), suggested by Davidov [3], is correct only for very weak adsorption bonds, i.e. when the contributions of new internal coordinates (namely ~1~ and CQ) into the potential energy of the S$n vibration are negligibly small. These coordinates are not transformed b; irreducible representations of the CsV group assumed for the one-center adsorption complex by Davidov [ 31. According to recent data, the stabilization of NH’, ion on the surface of oxide carriers in a one-center complex is unreal; this stabilisation is realised at the two-center adsorption only. In this connection one may ask whether the approximation is acceptable in which force constants of the NH4 group in the NH402M3 complex (Fig. l(c)) are the same as those for an individual NH,‘ ion. The ab initio computations * of a two-center adsorption complex with geometry optimization have shown large changes in the NH4 group geometry of the complex. These estimations provide a negative answer to the above question. Therefore, in computations of real spectra of adsorbed ammonium ion we must take into account both the changes in force constants and geometry of the NH4 group, and the contribution of adsorptionformed bonds and their interactions in the adsorption complex. Thus we conclude that the neglect of even the weak adsorbent-adsorbate bonds may result in mistakes in the theoretical description and understanding of vibrational spectra changes observed for adsorbed NH3 and NH:. To avoid this misunderstanding, it is necessary to consider the vibrations of the total adsorption complex in all cases. Sufficient dimensions of these complexes are determined exclusively by kinematic interactions, which may be analysed precisely. REFERENCES 1 R. Willis (Ed.), Surface Physics. Vibrational Spectroscopy of Adsorbates, Mir, Moscow, 1984 (in Russian). 2 E. N. Yurchenko and E. A. Paukshtis, Usp. Khim., 52 (3) (1983) 426-453. *These computations have been made in collaboration with A. G. Y. D. Pankratiev, G. D. %domirov and are to be published separately.
Pelmencsikov,
201 3 A. A. Davidov, IR-spectroscopy in Chemistry of Oxide Surfaces, Nauka, Novosibirsk, 1984 (in Russian). 4 A. V. Kiselev and V. I. Lygin. IR-Spectra of Surface Compounds, Nauka, Moscow, 1972 (in Russian). 5 E. B. Burgina and E. N. Yurchenko, J. Mol. Struct., 116 (1984) 17. 6 R. F. Klevzova, E. N. Yurchenko, A. A. Glinskya, E. B. Burgina, N. K. Yeremenko and V. V. Badakin. Zh. Strukt. Khim., 26 (1985) 84. 7 L. A. Gribov and V. A. Dementjev, Methods and Algorithms of Computations in the Theory of Vibrational Spectra of Molecules, Nauka, Moscow, 1981 (in Russian). 8 A. N. Lazarev, A. P. Mirgorodsky and I. S. Ignatjev. Vibrational Spectra of the Complex Oxides: Silicates and their Analogues, Nauka, Leningrad, 1975 (in Russian). 9 T. V. Yuchnevitch, E. T. Kokhanova, Zh. Prikl. Spektrosc., 39 (1983) 617. 10 A. I. Finkelshtein, Zh. Prikl. Spektrosc., 41 (1984) 256. 11 Th. Koops, T. Visser and W. M. A. Smith, J. Mol. Struct., 96 (1983) 203. 12 K. Nakamoto, IR Spectra of Inorganic and Coordination Compounds, Mir, Moscow, 1966 (in Russian). 13 J. P. Mathiew and H. Poulet. Spectrochim. Acta, 16 (1960) 696. 14 M. Crofton and T. Oko. J. Chem. Phys., 79 (1983) 3157. 15 V. P. Glushko (Ed.), Thermodynamical Properties of Individual Materials, Nauka, Moscow, 1978 (in Russian). 16 E. N. Yurchenko, E. B. Burgina, L. V. Konovalov, V. G. Pogareva and V. I. Bugaev. Zh. Strukt. Khim., 22 (1981) 81. 17 A. A. Zyganenko and M. A. Bakaeva. Opt. Spectrosc. (USSR), 54 (1983) 1117. 18 A. G. Pelmenshchikov, I. D. Mikheikin and G. M. Zhidomirov, Kinet. Katal., XXII(G) (1981) 1427.
PECULIARITIES OF THE VIBRATIONAL SPECTRA OF MOLECULES ADSORBED ON THE SURFACE OF HETEROGENEOUS CATALYSTS: THEORETICAL ANALYSIS
E. B. BURGINA, E. N. YURCHENKO
and E. A. PAUKSHTIS
Institute of Catalysis, Novosibirsk 630090
(U.S.S.R.)
(Received 24 April 1986)
ABSTRACT The vibrational spectra of adsorption complexes resulting from the adsorption of ammonia on the surface of heterogeneous catalysts are interpreted by computation of frequencies and forms of normal vibrations. The characteristics of normal vibrations are computed from the state of separate fragments of adsorption complexes. It is shown that an interaction of internal coordinates, associated with the adsorption bond, with internal coordinates of the adsorbed molecule leads to a decrease of characteristics of vibrations sensitive to the surface state. A correlation is established between the characteristics of such vibrations and the changes in their frequencies in comparison with the free molecule. INTRODUCTION
When studying surface properties of heterogeneous catalysts as well as the influence of certain surface centers on the state of the adsorbate, the method of vibrational spectroscopy of the adsorbed molecules is most widely used [l-3]. Variations observed in vibrational spectra of adsorbed molecules as compared with individual ones are caused, (i) by changing the force field and the structure of the adsorbed molecule itself, and (ii) by interacting internal coordinates of the molecule with coordinates appearing as a result of the adsorption bond formation. Up to now, only the first reason has been taken into account [3, 41, i.e., all the changes in the molecular spectra following adsorption have been explained on the basis of distortion of the structure of the free molecules and changes of their force constants. As shown previously, the interaction between internal vibrational coordinates considerably influences the values of the characteristic vibration frequencies observed in the spectra of adsorbed ethylene, propylene [5] and carbon monoxide [6] . Now we study the influence of the interaction of ammonia and ammonium ion internal coordinates with those coordinates corresponding to adsorption bonds, on the position of the vibration frequencies most sensitive to the interaction of these molecules with the adsorbent surface. The numerous IR spectra of adsorbed NH; show that such a 0022-2860/86/$03.50
o 1986 Elsevier Science Publishers B.V.
194
frequency is 6KH , whereas Si!& remains practically constant after the interaction of NH; with the protii surface centers. To explain the causes of the different behaviour of these two normal vibrations is one of the aims of the present paper. During the adsorption of NH3 on the surface of carriers with strong Brijnstead centers, the proton transfer from OH groups to the NH: molecule with the formation of NH’, ions is possible. Such ions, as suggested in ref. 3, may be stabilized on the surface due to interaction with one (MO-) and/or two (MO-, OMz) basic centers. In this connection it seemed interesting to determine whether taking into consideration the new coordinates of the adsorption complex and their interaction is important for the description of the adsorbed NW4 vibrational spectrum. ANALYSIS
PROCEDURE
Changes in the vibrational spectra of NH3 and NH: upon adsorption have been modelled by computing the normal vibrations of adsorption complexes (Fig. 1) with further analysis of the relative distribution of the potential energy of the normal vibration on the internal coordinates, which will be designated below as characteristic. All computations were carried out in the harmonic approximation using the programs of Gribov and Dementjev H
H
H
q51ka7 i
i
M
M
M (a)
\
(b)
Fig. 1. Normal coordinates of adsorption complexes: (a) ammonia adsorbed on the OH group of the surface, with the adsorption bond directed along the axis of symmetry of the NH, group; (b) one-center adsorption complex of the ammonium ion where the direction of the absorption bond coincides with that of one of the NH(q)/bonds 01~~ being a coordinate of the angle change on the linear interval [7] ; (c) two-center adsorption complex of the ammonium ion where the adsorption bonds q5 and q6 are in the plane of the NH bonds q3 and q4, bonds q, and qz are in the perpendicular plane, and bond lengths and angles for NH, and NH, fragments are taken as for free ammonia molecules and the ammonium ion, respectively.
195
[7]. Anharmonicity of the light atom vibrations was taken into account by using the spectroscopic masses of hydrogen and deuterium. Initially, the surface of the heterogeneous catalyst for adsorption of NH3 molecules was modelled by a surface fragment of aluminosilicate HO(M03)* (M = Al, Si) whose geometrical parameters and force constants were taken from ref. 8. It follows from computations for adsorption complexes AX,0(M0,)2 (A = C, N; X = Cl, H) that the “breaking” of 6 atoms of oxygen from the fragment O(M03h (i.e., transition to OMz, Fig. l(b)) and the varying of the mass M from 20 to 200 a.u. have insignificant kinematic influences on the frequencies of an absorption complex connected with vibrations of the fragment AX4. Further simplification of the model, i.e. transition to surface modelling by a point mass, leads however to an unjustified increase in the adsorption complex symmetry. Therefore we took the fragments HOM, OMz and OzM3 as models of surface centers which can adsorb NH3 molecules. Fig. l(a) shows the internal coordinates of the adsorption complex of ammonia obtained by the interaction of NH3 with OH groups of the surface. Figs. l(b), (c) show the internal coordinates of one- and two-center adsorption complexes of NH’,, respectively. We use the force fields of the NH3 molecule and the NH’, ion obtained by solving an inverse spectral problem for the frequencies of free NH3, ND3, NH: and ND’, as a zero-order approximation of the force fields of each adsorption complex; force constants of surface fragments were taken from ref. 8 and those of hydrogen bonds from refs. 9 and 10. The experimental frequencies taken from refs. 11 and 12 were used when solving inverse spectral problems for NH3 and ND3 molecules; the force constants obtained are: K = 10.72 X 106, KNH_-NH = 0.05 X 106, KNH_-HNH = 0.4X 106, KHNH = O.fiX 106, KHNH--HNH = -0.07 X lo6 cm-’ [5]. For experimental frequencies ascribed to free NH: and ND’, the values obtained are [ 121: K,, = 8.83 X 106, KNH_-NH = 0.18 X 106, KNH_-HNH = -0.23 X 106, KHNH = 0.75 X 106, KHNH--HNH = -0.03 X lo6 cm-*. Analysis of vibrational spectra of NH4 salts [13] and recent experiments 1141 lead, however, to the conclusion that the frequencies v1 and u3 of free NH: exceed those cited in ref. 12 by approximately 200 cm-‘. So we use v1 = 3250 (A),v2 = 1700 (E), v3 = 3350 (F,) and v4 = 1430 c m-l (F,) from ref. 15 as frequencies of free NH:. The set of force constants: KNH = 10.83 X 106,KNH_NW = 0.05 X 106, KNH-HNH = 0.4 x 106, KHNH = 0.81 X 106, KHNHeHNH = -0.07 X lo6 cm-‘, obtained by solving corresponding inverse spectral problems, was used for the description of the force fields of the NH4 fragment in absorption complexes (Fig. 1). As in the analysis of $‘&, and V& in pseudotetrahedral complexes ZnC12L2 [ 161, we suggest that the abov$ different sensitivity of B&n and Szn to the interaction of NH3 with protic centers of the surface may” be explahed by a different contribution of internal coordinates of the adsorption complex to the corresponding normal vibrations. One can verify this assumption by analyzing the characteristics of normal vibrations of different models of the adsorption complexes with respect to changes of
196
N-H bonds, HNH angles or coordinates of the entire NH3 group. For the quantitative estimation of the characteristics of normal vibrations we use the coefficients of relative distribution of the total potential energy on the internal coordinates of a selected fragment f’. For estimating f’ rather than the relation in ref. 16, we use the following more accurate equation. f’ = (F KtC + Jj Kijltlj)l(
5: KtC
+
II Ktslttfs) tfs
Here i, j are indices of internal coordinates of a fragment, t, s are indices of internal coordinates of the entire molecule (including the selected fragment), K terms are force constants and 1 terms are coefficients of the normal vibration forms in the internal coordinates. This relation takes into account all the members in the expression for potential energy of the normal vibration (diagonal as well as non-diagonal ones), providing an estimate of its characteristics with respect to the change of the arbitrary ith coordinate, as well as of all the coordinates associated with a group of atoms in the molecule. RESULTS
AND DISCUSSION
Changes of 8&n values after adsorption of ammonia were interpreted in ref. 4 as the rest& of the force field changes of the adsorbate molecule itself. This interpretation was based on the fact that the force constant of the N-H bond is approximately a factor of ten greater than that of the bond, and likewise for force constants of internewly-formed N * * -H*(O) actions between N-H and N - * - H* bonds, etc. The explanation of the growth of 6 &n due to increase of the N-H bond force constant brings us, however, to the3following contradiction: the wide range of 6&n? values (v2 = 950-1200 cm-‘) of the adsorbed NH3 requires an essential change of the HNH angle diagonal force constant in order to obtain satisfactory coincidence of the calculated and experimental values of frequencies. This leads inevitably to a notable change in calculated frequency BS 6 (uq = 1627 cm-‘), that is not observed experimentally. This is, in our ozmon, the reason why Kiselev and Lygin [4] explained the discrepancy between their calculation and experimental data by decreasing v2 anharmonicity. If this is so, the anharmonicity of v4 should also decrease, which must lead to an unobservable increase of v4. Thus, in both cases, the frequencies S&n and SF, should increase. We doubt that the anharmonicity of is ‘the only ieason for the frequency shift of the normal vibration 6NH, up to -250 cm-‘, besides which there is no evidence for a change of %H anhbmonicity of the hydrogen atom vibrations in NH3 as the result of the interaction of NH3 with proton donor groups of the surface via the N atom. Let us point out that the cause of v2 change suggested by Kiselev and Lygin [4] implies the invariability of harmonic force constants determining the values of v2 and v4 at the H’ * . - NH3 interaction.
197
From computations of normal vibrations of the adsorption complex (Fig. l(a)) without changing the force constants of the NH3 fragment as compared with those of the free NH, molecule, we conclude that the characteristic of normal vibrations with respect to NH3 atom shifts equals unity for vibrations with frequencies 3342 and 3437 cm-‘; it is approximately unity for frequency 1651 cm-’ and notably less than unity for -950 cm-‘. In the last case, the expression of the form in internal coordinates (see Fig. l(a)) is: -0.15 q1 + 0.82 (Yl + 0.82 (Y2 + 0.82 a3 + 0.82 o4 + 0.82 CY~+ 0.82 Q. Obviously, the characteristic of this normal vibration on the NH3 coordinates (a4 - CY~)decreases with increase in the contribution of the coordinates q,(NH*) and CK~, (Ye,01~ (HNH*), i.e. the stronger the adsorption bond and the higher the associated force constants (Rnn, K HNH *Y K NH*_-HNH*, etc.). Figure 2 illustrates this situation as f’ changes (by K NH* variation only). The computations of harmonic vibration frequencies which involve the entire observed range of AgH values can be performed without changing the force constants of ammonia’itself, but only by changing the parameters of adsorption bonds, e.g., when KNH * increases from 0 to 1.5 and 2.5 6 RH increases from 926 to 1011 and 1086 cm-‘, respectively; when KHNH* i& creases from 0 to 0.1 and from 0.2 to 0.3, Sk, increases from 926 to 1087 and from 1144 to 1199 cm-‘, as well; when dNHLHNH* increases from 0 to 0.01, ?&n, increases from 1199 to 1232 cm-‘. The< other above-mentioned vibration frequencies of the NH3 fragment of the adsorption complex are practically insensitive to the changes of these force constants. In the case of NH3 adsorption on the silica surface, the adsorption bonds
cm-’
Fig. 2. Dependence of the characteristic of normal vibration of the adsorption complex of ammonia, with the major contribution of s(NH,) (according to the change of coordinates of NH, groups, f’(NH), of the adsorption bond, f’(NH*), and of angles with the adsorption bond, f’(HNH*)).
198
were registered at 3322, 3403, 3417, 1635 and 1130 cm-’ [17]. To obtain similar values of frequencies, it is sufficient to take in our model (Fig. l(a)): K NH* = 2.5, KOHsN = 0.2 and KHNH* = 0.2, with values of NH3 group force constants as above. The calculated values of v are 3318, 3415, 3414, 1638 and 1127 cm-‘; those for the deuterocomplex D3N - * * DOMz are: 2392, 2541, 2543, 1206 and 946 cm-‘. The value 6&n, = 950 cm-’ is registered experimentally. Thus, we can describe the registered spectrum changes by taking into account, (i) the interaction of the NH3 group coordinates with those appearing in the adsorption complex (Fig. l(a)), and (ii) the contribution to the potential energy of vibration S&u of the energy changes of these new coordinates. In this description we” do not suppose any changes in anharmonicity of NH3 group vibrations during ammonia adsorption nor in the harmonic force constants of the NH3 group. The question naturally arises as to whether the approximation, in which harmonic force constants of the NH3 group are unchanged after the formation of the adsorption complex, is correct or not. To clear up this question, we have made a quantum-chemical calculation of the NH3 molecule and of our adsorption complex (Fig. l(a)), using the MINDO-3 method with the geometry optimization [US], modified in accordance with the properties of our surfaces*. The computations have shown that as the hydrogen bond No.* H* strengthens (bond length decreases from 1.41 to 1.33 A), the OH* bond weakens (bond length increases from 0.99 to 1.01 A), but the lengths of the N-H bonds do not change. Thus, our assumption that KNH is unchanged during the NH3 adsorption is correct in the first approximation. The weaker is the adsorption bond the more correct is this assumption. We might, in principle, determine the influence of the interaction of the surface with NH3 on force constants of the latter from the solution of the inverse spectral problem if we could obtain the exact values of vibration frequencies of adsorbed ammonia, interpreted as Vnn, SFH . These frequency values [14] are almost the same as those for gaseous ammonia [12]. This confirms the correctness of our first approximation of the force field of the adsorption complex. Computations of one- and two-center adsorption complexes of ammonium ion (Fig. l(b, c)) were made with a similar approximation, i.e. assuming invariability of the force field of the adsorbed molecule. Only the strength of the hydrogen bond (KOH*,KOH*N, KH*OM) was changed. Computations have shown that the vibration forms of the adsorption complex differ essentially from those of the free ion NH: (see Table l), even if this bond is very weak (KOH* = 0.01; KOH*N = O;KH*o, = 0).One can see from Table 1 that the new coordinates qou*, (YMon*,(YNn*o contribute to all types of normal vibrations, thus resulting in the notable splitting of all three times degenerate *The parameter of pseudoatom M determining -8 to -15 [ 18 1. The authors varied from A. G. Pelmencsikov for their help in computations.
the acid of the M-OH are grateful to G. M.
*surface groups Zidomirov and
TABLE 1 Forms of normal vibrations of ammonium ion and of its one- and two-center adsorption complexes in the 1300-3400 NH, *..O,M, b
NH,*.*OM; 32551x-' 354, +35q, +35q, +35q, 3348cm-' 51q,-514, -13a, -1301, + 13or,+13a, 3348cm-' -29q, -29q,+ 3348cm“
589,
+ 15a,-15~1,
-2Oq,
-2Oq, -20qs + 624, + lOa, + lOor,+10a, +1oa, flOcr, +1ocx, 1713cm-' -83~1, +82or,-8301, +82a,
1713cm-' --46a,---48.~~ t95a, -4601, -48~1, t95a, 1400cme' -79cx, t7901, t79a, -79a, 1400cm-' -79a, -79a, +79cY, +79a, 14OOcm+ -112a, t11201,
BSee Fig. 1. bSee Fig. l(c).
3255cm-' 89q,+ 33q2 + 33q, + 33q, +39q, 3339 cm-’ -51q1 +51q, t1201, -12a,-12a, t12a, t 12cl, 3339 cm-' -29q, -29q5 + 599, -14or, t 14CY,t 13a,, 3321cm‘' 609,
-234,
cm-’ region
-239,
-23q,
+23q,
1713cm-' -8201, t82o1, -882~~ t82a, 1713cm-' -47cx.-4701, t95a, -47ff.-47% t 95a; +26,x;--2601; -774~1;~ . 1428cm-' -174, -6501, -65a, -6501, t65a, t 65a, +66a, 141Ocm-' -79a, t79a, +79cY, -79ci, 1410cm' lOq,-45cQ -45a, 91a, t33a,-33cx,
+91a, +45a, +45a,-+102cY,,
3255cm-' 88q,
t
38q,
+ 3Oq,
+ 3Oq,
-28q,
-28q,
3356cm-' -32q,
-32q, t39sa +39c?,--3319,-3ls, t 18a,-l7cr,-l8a,-l8a, 3342cm-' -5lq, +51q, +13a, t13a,--13a,-1301, 3319 cm-' 51qs-5lq, +43q, f43q,-ll~* i-llcx,t lla, -l.la, 173ijcm-' 15q,t15q,-103o1, +49a,t49a,--85a, +49cu,t 49cx,-65a,-65a, +17a,t17a,,t17a,, +1701,, 1690cm-' -82~1, +82a, -820 mst82a,
1463cme' 81ar,+81a,--8101, -81~1, 1402cm-' -32q5 -3324, 85~1,-18or, 1362cm“ -78~1,+78a, 62a, -1601,
+102a, +12Oa, +85a, + -18a,,-18a,,-1801,, +78a, -78ci, +62a,t16a,, -16a,, -16a,,
vibrations at one-center adsorption, i.e. unlike the case of NHB, for the NH’; ion having high symmetry (Td) more frequencies of normal vibrations become more sensitive to interaction with the surface. Obviously enough, intensification of such interactions, i.e. the growth of KOH*, KOH*N, KHsOM, leads to some decrease of value of all the vibrations whose frequencies are underlined in Table 1, with respect to the vibrations of atoms of NH4 groups in the complexes, as well as to the increase of splitting of degenerate vibrations. It is only by increasing the abovementioned force constants up to values 1.5, 0.2, 0.3, respectively (the geometry and force constants of the NH4 group being unchanged) that the splitting of vq(F2) increases up to 130 cm-‘; this frequency splits into three components even in the one-center complex. Such a pattern of splitting of v4 in both types of complexes leads to the conclusion that the criterion for the recognition of one- and twocenter adsorption according to the number of splitting components S&+ (-1400 cm-‘), suggested by Davidov [3], is correct only for very weak adsorption bonds, i.e. when the contributions of new internal coordinates (namely ~1~ and CQ) into the potential energy of the S$n vibration are negligibly small. These coordinates are not transformed b; irreducible representations of the CsV group assumed for the one-center adsorption complex by Davidov [ 31. According to recent data, the stabilization of NH’, ion on the surface of oxide carriers in a one-center complex is unreal; this stabilisation is realised at the two-center adsorption only. In this connection one may ask whether the approximation is acceptable in which force constants of the NH4 group in the NH402M3 complex (Fig. l(c)) are the same as those for an individual NH,‘ ion. The ab initio computations * of a two-center adsorption complex with geometry optimization have shown large changes in the NH4 group geometry of the complex. These estimations provide a negative answer to the above question. Therefore, in computations of real spectra of adsorbed ammonium ion we must take into account both the changes in force constants and geometry of the NH4 group, and the contribution of adsorptionformed bonds and their interactions in the adsorption complex. Thus we conclude that the neglect of even the weak adsorbent-adsorbate bonds may result in mistakes in the theoretical description and understanding of vibrational spectra changes observed for adsorbed NH3 and NH:. To avoid this misunderstanding, it is necessary to consider the vibrations of the total adsorption complex in all cases. Sufficient dimensions of these complexes are determined exclusively by kinematic interactions, which may be analysed precisely. REFERENCES 1 R. Willis (Ed.), Surface Physics. Vibrational Spectroscopy of Adsorbates, Mir, Moscow, 1984 (in Russian). 2 E. N. Yurchenko and E. A. Paukshtis, Usp. Khim., 52 (3) (1983) 426-453. *These computations have been made in collaboration with A. G. Y. D. Pankratiev, G. D. %domirov and are to be published separately.
Pelmencsikov,
201 3 A. A. Davidov, IR-spectroscopy in Chemistry of Oxide Surfaces, Nauka, Novosibirsk, 1984 (in Russian). 4 A. V. Kiselev and V. I. Lygin. IR-Spectra of Surface Compounds, Nauka, Moscow, 1972 (in Russian). 5 E. B. Burgina and E. N. Yurchenko, J. Mol. Struct., 116 (1984) 17. 6 R. F. Klevzova, E. N. Yurchenko, A. A. Glinskya, E. B. Burgina, N. K. Yeremenko and V. V. Badakin. Zh. Strukt. Khim., 26 (1985) 84. 7 L. A. Gribov and V. A. Dementjev, Methods and Algorithms of Computations in the Theory of Vibrational Spectra of Molecules, Nauka, Moscow, 1981 (in Russian). 8 A. N. Lazarev, A. P. Mirgorodsky and I. S. Ignatjev. Vibrational Spectra of the Complex Oxides: Silicates and their Analogues, Nauka, Leningrad, 1975 (in Russian). 9 T. V. Yuchnevitch, E. T. Kokhanova, Zh. Prikl. Spektrosc., 39 (1983) 617. 10 A. I. Finkelshtein, Zh. Prikl. Spektrosc., 41 (1984) 256. 11 Th. Koops, T. Visser and W. M. A. Smith, J. Mol. Struct., 96 (1983) 203. 12 K. Nakamoto, IR Spectra of Inorganic and Coordination Compounds, Mir, Moscow, 1966 (in Russian). 13 J. P. Mathiew and H. Poulet. Spectrochim. Acta, 16 (1960) 696. 14 M. Crofton and T. Oko. J. Chem. Phys., 79 (1983) 3157. 15 V. P. Glushko (Ed.), Thermodynamical Properties of Individual Materials, Nauka, Moscow, 1978 (in Russian). 16 E. N. Yurchenko, E. B. Burgina, L. V. Konovalov, V. G. Pogareva and V. I. Bugaev. Zh. Strukt. Khim., 22 (1981) 81. 17 A. A. Zyganenko and M. A. Bakaeva. Opt. Spectrosc. (USSR), 54 (1983) 1117. 18 A. G. Pelmenshchikov, I. D. Mikheikin and G. M. Zhidomirov, Kinet. Katal., XXII(G) (1981) 1427.