Langmuir 1985,1, 407-414 dent from these and other reports, as well as from the variety of irregular surfaces analyzed here, that the fradal approach has the potential of offering a solution to irregularity problems. This is certainly true a t least a t the original level of Richardson's interpretation. However, if one looks for self-similarity, then more sensitive tests, of the kind described in this article or analyses of large assemblies should be employed. The simple message of this
407
paper is beware of fractal rabbits (Figure 11).
Acknowledgment. We thank Peter Pfeifer for many stimulating discussions and for critical reading of the first draft of this paper. Financial support of the Israel Academy of Sciences is acknowledged. Supported by the F. Haber Research Center for Molecular Dynamics in Jerusalem.
CO and NO Adsorption on NiO: A Spectroscopic Investigation E. Escalona Platero, S. Coluccia, and A. Zecchina* Istituto di Chimica-Fisica, Corso, M. D'Azeglio 48, 10125 Torino, Italy Received January 7, 1985. I n Final Form: February 22, 1985 The IR spectra of CO and NO adsorbed on NiO polycrystalline samples characterized by values of the specific surface area between 100-150 and 1-3 m2g-' are reported. The spectra become progressively simpler with the decrease of the surface area and the increase of the perfection of the individual cubic crystallites (as shown by the electron micrographs),and their comparison allows the assignment of the modes associated with CO and NO adsorbed on extended (100)faces and geometrical (edges and steps) and atomic defects. Examination of the IR spectra obtained at different coverages reveals that dipole-dipole and chemical effects (similar to those described for metal surfaces) are responsible for marked frequency shifts of the stretching frequency.
Introduction NiO, MgO, and NiO-MgO solid solutions have the same crystalline (rock salt) structure, a similar lattice parameter, and the same cubic habit: hence they represent an ideal family of solids for investigations concerning the surface properties of Mg2+and Ni2+(both isolated and clustered) located in well-defined geometrical situations. The interaction of CO and NO with MgO and MgO-NiO solid solutions has been investigated in detaiP4 by IR spectroscopy and the results can be briefly summarized as follows: (a) CO. (i) CO does not interact a t room temperature with 5-fold coordinated (both isolated and clustered) ions located on (100)facelets of cubic microcrystals; only at 77 K weak Mg2+-C0 (a) and Ni2+-C0 (a-a) adducts are formed which are characterized by stretching frequencies in the 2160-2140-cm-' range. (ii) The room temperature interaction of CO with the ions located on edges and steps (and corners and other defects as well) is much more complex because it involves not only the cations but also the 02-anions. Apparently on both MgO and NiO-MgO solid solutions, the interaction mechanism shows a common feature: it leads to the simultaneous production of reduced and oxidized species following the parallel schemes 202- ( n + 1)co Co:-& (co),2- MgO (1)
--
+ 202- + Ni2+ + 5CO
+
C032-ada+ Nio(C0)4 MgO-NiO (2) where the negative species are in close interaction with the (1) Guglielminotti, E.; Coluccia, S.; Garrone, E.; Cerruti, L.; Zecchina, A. J. Chem. SOC.,Faraday Trans. 1, 1979, 75, 96. (2) Zecchina, A.; Spoto, G.; Coluccia, S.; Guglielminotti, E. J. Chem. SOC.,Faraday Trans. 1, 1984,80, 1875. (3) Zecchina, A.; Spoto, G.; Coluccia, S.; Guglielminotti, E. J. Chem. SOC., Faraday Trans 1, 1984,80, 1891. (4) Escdona Platero, E.; Spoto, G.; Zecchina, A. J. Chem. SOC., Faraday Trans 1, in press.
Mg2+ions and Ni(C0)4forms, with a coordinatively unsaturated Mg2+02-pair, the surface complex
MgZt
whose structure has been characterized by isotopic exchange experiments2 and proved by direct Nio(C0)4 chemisorption on Mg0.5 On concentrated solid solutions also reduced polinuclear species have been observed. The relative proportion of mononuclear and polynuclear species depends not only upon the aggregation state of the Ni2+ions but also upon the CO pressure (which favors the formation of the mononuclear species). The redox process previously described is not the only one occurring on low-coordinated sites: in fact, room temperature stable Mg2+-CO (a) is also formed in a concerted way. At 77 K the reduction of Ni2+ions located on exposed situations does not occur, and surface Ni2+(CO), adducts absorbing a t 2100-2050 cm-' are observed. (b) NO. (i) NO does not interact a t room temperature with Mg2+ions located on (100)faces of both MgO and MgO-NiO solid solutions. At 77 K, on the contrary, the following process occurs
? I
? I
Y
N
(5) Guglielminotti, E.; Zecchina, A,; Boccuzzi, F.; Borello, E. In "Growth and Properties of Metal Clusters"; Elsevier Scientific: Amsterdam, 1980; p 165.
0743-7463/85/2401-0407$01.50/00 1985 American Chemical Society
408 hngmuir, Val. 1. No.4, 1985
Platero. Coluccia, and Zecchina
which leads to cis-N202adsorbed species. (ii) NO forms, with Ni2+located on (1001faces, room temperature stable Ni2+-N0 (UT) adducts characterized by a narrow peak at -1800 Unlike Mg2+-NO adducts, Ni2+-NO complexes located in adjacent positions do not show great tendency to dimerization because the Ni2+-NO bond is stronger than the N-N bond of the dimer. (iii) The interaction of NO with the ions located on more exposed situations (like edges, steps, and corners) is more complex because it simultaneously involves anions and cations. On pure MgO the following sequence of reactions has been evidenced 4 N 0 + 20"
-
-
2N02- + N20;'
N 2 0 ~ ~ - N 2 0 + Oz-
77 K
room temperature
(4) (5)
where the negative NO2- and N202" species are in close interaction with Mg2+ions. The first reaction occurs only at 77 K while the second one becomes important only a t room temperature. The sum of reactions 4 and 5 represents the noncatalytic pathway for N20 formation on MgO and can be considered analogous to reaction 1concerning CO (reduced and oxidized species are in fact simultaneously formed). On MgCbNiO solid solutions the presence of the reducible Ni2* ions (which can act as electron acceptors) gives the alternative pathway 2NO
+ 20" + Ni2+
Nia + 2NO 4N0
-
+
2N02- + Nia
N20 + Ni2++ 0"
+ 02-
+
2NO;
+ N20
(6)
(7) (8)
which well explains the easier formation of N 2 0 on solid solutions. The preferential formation of NOz- on edges (which can act as a bidentate ligand and consequently can saturate the coordination vacancies of Ni" ions in adjacent position) explains the absence of IR manifestation of Ni2+-NO located on edges. On all samples a small concentration of Ni3*-NO groups characterized hy the 1915cm-' stretching frequency have also been observed. Their presence clearly suggests that reaction 7 is followed, a t least partially (at room temperature), by the following: NO
+ N20 + Ni2+
-
Ni3+
+ NO, + N2 t
(9)
leading to NiS+ions (and hence to Ni3+-NO nitrosyls) and to nitrogen. Unlike pure MgO and MgO-NiO solid solutions, the IR spectra of CO and NO adsorbed on pure NiO are less known. The spectra of CO and NO adsorbed on nearly stoichiometric, high surface area NiO (ex-Ni(OH),) has been investigated by Guglieminotti et al.?a Due to the high surface heterogeneity of these samples, the bands of the adsorbed species are broad and extremely intense, so preventing a very detailed assignment. We have investigated the surface properties of a set of NiO samples characterized by increasing dimension and perfection of the microcrystals and decreasing surface area. In fact on these samples the contribution to the overall IR spectra of CO and NO adsorbed on defective situations progressively decreases and that of the species on perfect (l00l (6) G i e b , E ,Garrone, E.;Guglielminotti,E.;k h i n a , A. J. Mol. Catol. 1981,24, 59. ( I ) Guglielminotti, E; Ccrmti.L.;Borello. E.Gmz. Chim. Rol. 1977,
107, 447. (8) Gwlielminoni. E; Carmti.L.; Borello. E.C o n . Chim. ltal. 1977, 107, 503.
I Figure 1. Electron micrograph of the Ni(OH), sample.
faces increases. The comparison of the IR spectra of CO and NO adsorbed on these samples can be of invaluable help for the vibrational assignments. As NiO treated at high temperature in O2can carry quite a substantial excess of oxygen in the extemal layers:Ja m e has been taken to outgass the sample under conditions that are known to favor a nearly stoichiometric situation.
Experimental Section The NiO samples have been prepared by deeompoaingNi(OH), in vacuo at 493 K, following the procedure described in ref 11. In thin way a yellow-green sample is obtained having lO(t150 m2 g-' surface area (sample A). On this sample the adsorption of CO, NO, and Oz(both at room temperature and 77 K) has been investigated by IR spectroscopy. In this paper only the results concerning the first two gases will be discussed, while those concerning 0,will be described in a separate paper. The sample A was then outgassed at 873 K for 1 h, heated in O2at the same T for 1 h (in order to favor the sintering p-), and finally outgassed at the same temperature for 15 min. After this treatment the sample is green and shows a specificsurface area of -40 m2 g-' (sample B). On this sample the adsorption of CO and NO (both at room temwrature and 77 K)has been investigated by 'IR. The CO and NO spectra have been a h obtained on more sintered samoles (C. D. E. F). The samole C has been obtained as follows. A& out&& i0at 1073 K, sample B was contacted with O2 at 1073 K for 2 h and then outgassed at the m e temperature for 15 min: after this treatment the specificsurface area is 10 mz g-'. Further decreases of surfacearea were obtained by extending the time of hating m 0,at 1073 K (samples D. E, F). The surface area of sample F was 1-3 mz g-'. Of course before CO and NO adsorption the D, E, and F samples underwent the usual activation in vacuo at 1073 K. The shape of the microcrystah formed after each stage of the described procedure has been examined by transmission electron microscopy (Philips 300 microscope). The surface area of the samples has been obtained by the conventional BET method. The CO and NO coverage both at room temperature and 77 K bas been measured on a Sartorius microbalance. The lR s p t r a have been carried out on a PE 580 B spectrometer equipped with a data station. High-purity gases (Matheson) have been used. Results Electron Microscope Studies. The sequence of photographs illustrated in Figures 1-5 shows the change in sample morphology with thermal treatment. In Figure 1 (9) Roberts, M. W.; Smart. R St C. Surf. Sci. 1980,100,590. (10) Roberts, M.W.;Smart, R St. C. J. Chem. Soe.. Fomdoy T" I, 1984,80, 2957. (11) Gravelle. P.C.; Teicher. S.J. Ado. Catol. 1969,20, 167.
CO and NO Adsorption on NiO
Langmuir, Vol. 1, No. 4, I985 409
-
Figure 2. Electron micrograph of the NiO (A) sample obtained by decomposition in vacuo at 493 K of Ni(OH),.
!
Figure 3. NiO sintered at 973 K in O2 (Bsample).
7
IE
t
I Figure 4. NiO sintered
.P a t 1073 K in O2 for 2 h
(Dsample).
the starting Ni(OH), material (specific surface area 30 mz g-') is represented: single crystals in the form of hexagonal platelets were observed. In Figure 2, the morphology of the NiO specimens after decomposition of Ni(OH), in vacuo a t 493 K is illustrated (sample A). The hexagonal shape of the original Ni(OH), microcrystals is apparently retained. However, as already reported'* the microcrystal surface appears very rough. The surface area has in the mean time increased 3-5 times. (12) kkins, F. P.; Fensham. P. J.: Sanders, J. V. T".Famday Sm.
IWO.66.1748.
Figure 5. NiO sintered at 1073 K in 0, for 16 h (F sample). This implies that the original hexagonal single crystals have been fragmented into smaller units and that the hexagonal plates of Figure 2 are not single crystals but are constituted of smaller entities. In Figure 3 the electron micrograph of a specimen treated in O2 at 873 K and outgassed at the same T is illustrated (sample B): the hexagonal forms present in the figure appear to be built up of individual microcrystals of cubic shape (as illustrated by the presence of small cubic projections, with -5-nm edge). As the surface area is of the order of 3C-40 m2g-' (i.e. similar to that of the starting Ni(OHI2),we conclude that the individual cubelets form part of a well-sintered polycrystal, which preserves the shape of the starting material. The morphology of a sample sintered under more severe conditions (i.e., similar to those of samples C and D)is illustrated in Figure 4. The hexagonally shaped polycrystals are no longer present. The agglomerates are now thicker and irregularly shaped with dimensions in the 100-200-nm range (in agreement with the -7-10 m2 g-' specific surface area). The surface of these massive polycrystals is made up of (100)facelets with edge dimensions in the 10-15-nm range. Finally in Figure 5 the electron micrograph of a specimen sintered for a very long time at 1073 K (Le., similar to an F sample; specific surface area 1-3 m2 g-') is illustrated the majority of particles are individual highly perfect cubic single crystals only slightly rounded a t the comers and edge positions. The appearance in many cases of darker crystals showing apparent crystal angles 290' is due to cubes and parallelepipeds not parallel to the plane of the grid (and so giving apparent hexagonal shapes). Figures 2-4 show without ambiguity that, on passing from samples A to samples F, an increasing percentage of surface ions is located on 11001 faces of growing perfection and dimensions. Infrared Studies. (1) CO. The amount of CO adsorbed a t room temperature (immediately after contact) on B and D samples corresponds to 0 9 0.1 (8 = 1is the coverage corresponding to 1 CO molecule per Ni2+ion of the surface). A substantial increase in the CO coverage is obtained by decreasing the temperature to 77 K in fact, at this temperature, 55% of the Ni2* ions (average, of three measures) carries one CO molecule irreversibly adsorbed. The 8 = 0.55 figure can be considered as approximate, because of the uncertainties connected with the surface area measurement. The IR soectrum in the 22OC-2050-cm-' ranee of CO adsorbed ai77 K on samples characterized by dkreasing surface area [from 100-150 (A sample) to 1-3 m2 g-' (F samples)], and increasing perfection of the crystallites, is
Platero, Coluccia, and Zecchina
410 Langmuir, Vol. 1, No. 4 , 1985
h
1 2200
,
2150
I
2100
cm-1
2..0
2200
21 50
2100
cm-'
2050
Figure 7. IR spectra of CO at increasing coverages on a sample intermediate between E and F.
these species are irreversible, because they do not disappear by outgassing at 77 K. (iii) The shoulder at -2110 cm-' corresponds to a weakly adsorbed species, because it is clearly observed only at the highest equilibrium pressures. (iv) The broad shoulder initially observed at -2165 cm-' (I9 N 0) is found at -2150 cm-' at 6 N 0.5 (AD N -15 cm-l). (2) NO. The amount of NO adsorbed a t room temperature (p = 40 torr) (immediately after contact) corresponds to 6 = 0.45 (average of two measurements obtained on B and D samples), i.e., nearly the same coverage as before reached by CO at low temperature. This means that the IR spectra of NO adsorbed at, room temperature can be usefully compared with those of CO adsorbed at 77 K. The IR spectra in the 1900-1650-cm-' range of NO adsorbed at room temperature on samples characterized by decreasing surface area and increasing perfection (A, B, C, D, F) are compared in Figure 8. The following results are observed: (i) The spectrum of NO adsorbed on A samples is constituted of an extremely strong peak at 1810-1780 cm-' with a pronounced tail on the low-frequency side (where spulders at -1765, -1735, and 1710 cm-l are evident). (ii) On more sintered samples the whole spectrum becomes more symmetric because the band at 1801 cm-' becomes narrower and the lower frequency shoulders tend to disappear. (iii) On the intermediate B-D samples, a broad and weak band is observed at 1700-1710 cm-', which is not present on F samples. (iv) The half-width of the main peak gradually decreases from 42 (B and C samples) to 25 (D samples) to 15 cm-' (F samples). (v) A weak peak a t 1875 cm-' (absent on A samples) is observed on B and C samples only; on D samples a shoulder at 1850 cm-' is also observed. (vi) Even after very severe sintering treatment the shoulder at 1765-1770 cm-' persists together with an asymmetry at -1825 cm-' (F samples; 1-3 m2 g-'). In Figure 9 the spectra in the 1950-850-cm-' range of increasing doses (I9 = 0-0.45) of NO adsorbed a t room temperature on D sample are illustrated: (i) the intensity of the main peak at 1800 cm-' gradually increases with the NO pressure, gradually undergoing a shift from 1801 (I9 N 0) to 1807 cm-I (6 N 0.45) (An = +6 cm-'); this peak
-
-
CO and NO Adsorption on NiO
Langmuir, Vol. 1, No. 4,1985 411 Discussion (1) Effect of t h e Preparation Conditions on t h e
1DOO
1700
1 BOO
cm-'
Figure 8. IR spectra of NO (1900-1650 cm-') adsorbed at room temperature on A-D and F samples (NO pressure 40 torr).
, 1900
I700
1500
1300
1100
cm-l
900
Figure 9. IR spectra in the 1950-800-cm-' range of NO at increasing coverages on a D sample. is associated with a reversible species because it disappears upon room temperature outgassing. (ii) The satellite a t lower frequency (already present with its maximum intensity a t the lowest 0) shifts from 1740 (0 N 0) to 1710 cm-' (A2 = -30 cm-l); this species is irreversible because it does not disappears upon room temperature outgassing (experiments, not illustrated here for the sake of brevity, demonstrate that this satellite disappears if NO is left in contact with the sample for several hours). (iii) A shoulder a t 1850 cm-' is clearly observed a t the lowest 8. (iv) The shoulder a t -1765 cm-' is observed only a t the highest coverages. (v) In the low-frequency region three weak bands are immediately observed with their maximum intensity a t 1370, 1095, and 885 cm-'; it is most noticeable that although their intensity does not grow with the overall NO coverage, their frequency is, on the contrary, affected (AP = -10, +lo, and -10 em-', respectively). (vi) A second pair of weak bands a t 1215 and 1300 cm-l gradually grows with NO pressure and reaches its maximum intensity a t 0 = 0.45; these bands are reversible because they disappear upon room temperature evacuation.
Shape of NiO Microcrystals. As illustrated by Figures 2-5, samples heated in O2a t increasing temperature and for increasing time tend to sinter and to lose surface area. The original hexagonal single crystals of Ni(OH)2(Figure 1)are fragmented during the topochemical decomposition12 at 493 K and give hexagonal aggregates of microcrystals in close contact (Figure 2). This fact explains the great increase of surface area, on passing from Ni(OH)2 (- 30 m2 g-l) to NiO (A samples) (100-150 m2 g-l).'l The shape of the individual microcrystals cannot be inferred from the electron micrographs, which only show the presence of surface roughness likely associated with high concentration of geometric defects (edges, steps, and corners). By heating A samples in O2 a t 873 K, the surface area of the samples abruptly decreases to -40 m2 g-l. The morphology of the sample (as illustrated in Figure 3) is still characterized by the presence of hexagonal polycrystals: however, the texture is now clearly observable. The polycrystals appear now to be built with individual cubic crystallites partially melted together (cubic projections are clearly evidenced): this implies that already on B samples the { 100)facelets are the predominant surface structures. The tendency to preferentially expose { 100)faces is favored by further sintering (Figure 4) up to the point where only cubes and parallelepipeds are present and hence the (100)face becomes the absolutely overwhelming structure (Figure 5 ) (in agreement with the results reported in ref 13). These results ensure the following: (i) On C-F samples the vast majority of surface ions lies on {loo)faces. (ii) The absolute and relative amounts of ions located on geometrical defects (edges, steps, and corners) rapidly fall to negligible values, on passing from C to F samples. The adopted sintering procedure is consequently suitable for the preparation of samples with controlled concentrations of geometrical defects. Of course electron microscopy cannot give any indication of defects with atomic scale dimensions (like those associated with oxygen excess in the external layers, which can be very abundant after heating in O2 a t high temperature)?JO In order to minimize the amount of these defects, all samples were outgassed in vacuo at high temperature after the sintering step in 02. (2) The Maximum Coverage of CO a n d NO on NiO. From the gravimetric experiments, it has been established that the coverage of CO adsorbed a t 77 K closely corresponds to that of NO adsorbed at room temperature and is approximately only half of the maximum value (estimated on the basis of one adsorbed molecule per each Ni2+ ion). This observation can be explained on the basis of the following considerations: CO and NO are adsorbed on the common (100)faces and on defects (edges, steps, corners, and other defective situations). On highly sintered samples (B-F) the proportion of ions located on (loo] faces is predominant and likely reaches the 98% values on the most sintered ones.14 Hence the B 0.5 coverage observed on sintered NiO samples essentially reflects the surface stoichiometry of the (100)faces. It is concluded that on (100) faces only half of the available Ni2+ ions can adsorb CO and NO.
-
(13) Jones, C. F.; Segall, R. L.; Smart,R. St. C.; Turner, P. S. J. Chem. SOC., Faraday Tram. 1, 1977, 73, 1710. (14)Zecchina, A.; Spoto, G.; Coluccia, S.; Guglielminotti, E. J. Phys. Chem. 1984,88, 2575.
412 Langmuir, Vol. 1, No. 4 , 1985
Figure 10. As will be shown in the next paragraphs, CO and NO form u-T bonded carbonylic and nitrosylic adducts. So the question is: why do only half of the equivalent Ni2+ ions form linear adducts with CO and NO? The most plausible answer is as follows: the formation of one nitrosylic or carbonylic adduct per Ni2+ion of the { 100)plane (where the minimum Ni2+-Ni2+distance is 2.95 8)implies NO-NO and CO-CO distances lower than twice the van der Waals radius,15with subsequent dramatic increase of the repulsive forces;16hence a 1:l stoichiometry is unfavored. In contrast, NO and CO molecules adsorbed on Ni2+ions 4.18-A apart (which is the distance between a Ni2+ion and the second nearest one) do not repel each other: consequently the surface phase a t 0 N 0.5 is stable and the adsorption process practically stops at this coverage. This ordered surface structure closely corresponds to what is observed on metals, where "in registry" structures are very often found in adlayers where the adsorbate-adsorbate distance is larger than twice the van der Waals radius.l' On metal surfaces "out of registry" compressed phases are, however, quite often observed, where the adsorbateadsorbate distance is similar or smaller than twice the van der Waals radius and where some overlap between the orbitals centered on adjacent molecules can occur. Apparently such a situation does not occur on NiO: this is likely due to the intrinsic nature of the (100)surface of NiO where (unlike metal surfaces) the d orbitals centered on Ni2+ions do not overlap each other. If these schematic considerations can explain why chemisorption practically stops at half-monolayer, they do not exclude that near the maximum coverage some disorder can be present in the adsorbed phase and that a small fraction of molecules can be adsorbed on adjacent ions (vide infra). (3) The IR Spectrum of Adsorbed CO on A-F Samples, The narrow peak (AijlI2 = 9 cm-'), which dominates the spectrum of CO adsorbed a t 77 K on F samples (1-3 m2g-l), is unambiguosly assigned to CO on {lo01faces that are highly perfect also on the atomic scale. In fact, (i) on these samples (100)faces are the absolutely predominant structures and (ii) the very small half-width of the peak (comparable to or smaller than those observed for CO on metal single crystals)15implies high surface order. The 2137-cm-' band is due to CO species u-P bonded to Ni2+ ions (as in Figure 10) forming a regular array of parallel oscillators (vide infra). As already discussed, only one out of every two Ni2+ions carries an adsorbed CO molecules. It is most noticeable that Ni2+ions emerging on {loo] faces of NiO-MgO solid solutions give, with CO at 77 K, (15) Hollins, P.; Pritchard, J. Springer Ser. Chem. Phys. 1980, 76,125. (16) Clark, A. "The Theory of Adsorption and Catalysis"; Academic Press: New York, 1970. (17) Van Hove, M. A. 'The Nature of the Surface Chemical Bond"; Rhodin. T. N.. Ertl, G., Eds.; North-Holland Publishing Co. 1979; p 276.
Platero, Coluccia, and Zecchina surface adducts characterized by stretching frequency at -2150 cm-1.2 The stretching frequency of an array of parallel oscillators as described in Figure 10 should be determined by both dynamic (dipole-dipole,18J9and vibrationalZ0)and static ( ~ h e m i c a l ) ' ~lateral J ~ J ~ interactions among CO oscillators. As the lateral interactions are influenced by coverage, this problem will be taken into consideration in paragraph 5 , where the spectra at different 0 are discussed. The shoulders on the high- and low-frequency sides are associated with imperfections. Due to the high geometrical perfection of the individual crystallites of the F samples (less than 2% of the surface atoms are on edges, steps, and corners),14we favor the hypothesis that atomic defects are most plausibly involved. In particular we hypothize that the high-frequency shoulder is associated with CO adsorbed in the proximity of oxygen excess defects, which are probably present even after outgassing at high temperat~re.~JO On passing from F to E, D, C, and B samples the peak associated with CO adsorbed on (100)faces becomes more intense and broad. While the increase of the intensity is simply due to the increase of surface area, the large increment of the half-width is directly related to the concentration increase of surface imperfections (of both geometric and atomic nature) which limit the periodicity of the (loo}face. In fact surface Ni2+ions in the proximity of edges, steps, corners, and defects associated with oxygen excess in the external layer become gradually more and more important as the surface area increases (and the area of the (100)facelets decreases). CO bonded to these different Ni2+ions is characterized by a stretching frequency slightly different with respect to those of the perfect face: hence the broadening of the CO peak is well explained. Along the F-B series, the progressive building up of a complex absorption at 2100-2050 cm-l is observed (at intermediate sintering conditions two peaks are clearly observed). We assign these absorption bands to CO adsorbed on geometrical defects like edges, steps, and corners. In fact, (i) the absorption in the 2100-2050-~m-~ region undergoes the most spectacular increment on passing from samples B to A, i.e., in connection with the increase of the geometric defects (vide supra) and (ii) the stretching frequency of CO adsorbed a t 77 K on Ni2+located on edges and steps of NiO-MgO solid solutions has been found in the 2050-2100-cm-' range (the strongest peak being at 2065 cm-1).2 Due to the complexity of the spectra, we shall not try a detailed assignment: we only mention that the presence of Ni2+(CO), ( n = 2, 3) polycarbonilic species cannot be excluded. (4) IR Spectra (1900-1650 cm-l) of NO on A-F Samples. The narrow peak ( A q 2 = 15 cm-') at 1807 cm-l present on F samples is due to NO linearly bonded to Ni2+ ions located on highly perfect (loo}faces. In fact, (i) on F samples, {loo]faces are predominant, (ii) the half-width of the peak (comparable with that observed for NO on single crystals)21implies high surface ordering, and (iii) the NO stretching of Ni2+-N0 complexes formed on (100)faces of MgO-NiO solid solutions occurs a t nearly identical freq~ency.~ Like for the CO case, a regular array of parallel oscillators is present at the maximum coverage (Figure 11) where only one out of every two Ni2+ions carries a NO (18) Mahan, G . D.; Lucas, A. A. J. Chem. Phys. 1978,68, 1344. (19) Persson, B. N. J.; Ryberg, R. Phys. Reu. E 1981, 24 (12), 6954. (20) Moskovits, M.; Hulse, J. E. Surf. Sci. 1978, 78, 397. (21) Hayden, B. E. Surf. Sci. 1983, 131, 419.
CO and NO Adsorption on NiO
Langmuir, Vol. 1, No. 4,1985 413
?
?
N
M
to-+-+
/
&' -/+ c-+-t-
/
t-
2 +-
$1 / N
+/"
/
f-
Ot
-/+
d/
++/;
Figure 11.
molecule. The parallel oscillators in the ordered array likely interact together via dynamic and static effects: this problem will be discussed in the next paragraph. On passing from F to E, D, C, and B samples the peak associated with NO adsorbed on (100)faces becomes more intense and broad the explanation is the same given for the peak of CO adsorbed on {loo]faces. Along the F-B series we observe also the building up of an absorption a t 1700-1750 cm-'. The assignment is similar to that given for the low-frequency component of CO: the adsorption is due to NO adsorbed on geometric defects (like edges and steps). The low-frequency component disappears when NO is left in contact with the samples for several hours. We explain this fact on the basis of the results obtained on MgO-NiO solid solutions4 (vide reactions 6-8). Very briefly, edge nitrosyls are only the initial (and hence transient) products of the NO interaction because they are consumed as in reactions 6-8 giving NO2-species and N20. The presence of NOz- species even immediately after NO contact a t room temperature is well documented by the low-frequency peaks illustrated in Figure 9 which are readily assigned to NO, specie^.^ Finally the peak at 1875 cm-' (B, C samples) is assigned (like for NiO-MgO solid solution^)^ to Ni3+-NO species formed on very exposed situations by NO interaction with Ni3+ ions generated during the previous exposure to 02. In fact this peak is absent on A samples (because they have never been exposed to oxygen) and D-F samples (because prolonged outgassing in vacuo a t 1073 K likely causes the desorption of the surface oxygen). (5) Adsorbate-Adsorbate Interactions in CO and NO {loo}Overlayers: Preliminary Considerations. The peaks associated with CO and NO adsorbed on { 100) faces shift downward (-16 cm-l) and upwards (+6 cm-l) with increasing coverage. These shifts are caused by the building up of adsorbate-adsorbate interactions. A certain number of explanations has been proposed for these frequency vs. coverage shifts (normally described on metals): i.e., (i) vibrating molecules interact "through space" via a dipole-dipole mechanism,ls (ii) vibrating molecules interact with their own images and with the images of the other mo1ecules,l8(iii) vibrating molecules interact "through solid" via a vibrational coupling mechanism involving common bonding electrons (which allow dynamic "communication" between adsorbed molecules),2° (iv) adsorbed molecules experience a progressive change of the adsorbateadsorbent interaction energy as a function of coverage caused by "chemical" influences transmitted through the solid,'* and (v) adsorbed molecules perturb each other through the electrostatic or solvent effect.22 Effects i-iii are dynamic, while effects iv and v are static.
As shown by many authors,23green NiO prepared at temperatures above 973 K has very low conductivity and hence very low concentration of free carriers a t room temperature and 77 K. As a consequence effect ii (which is relevant for CO and NO adsorbed on metals) can be discarded. Moreover (iii) should be negligible too, because the through-solid vibrational coupling strongly declines when the adsorption enthalpy becomes small (like for room temperature and 77 K reversible species).20Consequently the dipoledipole dynamic contribution is expected to play the major role in the dynamics of the CO and NO overlayers on NiO. As far as the static effects are concerned, the contribution of the electrostatic (or solvent) effect should also be negligible, because even a t the maximum coverage the adsorbed CO and NO molecules are not densely packed [the CO-CO and NO-NO distances in the overlayer (4.2 A) are larger than the van der Waals minimum for two CO molecules with their axes parallel (3.3 A)'5]. From these considerations it comes out that in the CO-NiO and NONiO systems, the frequency shifts caused by the adsorbate-adsorbate interactions should be nearly totally accounted for by dipole-dipole (dynamic) and chemical (static) effects. The separation between the static and the dynamic contributions is usually found by studying the IR spectra of '2CO-'3C0 and 14NO-15N0 mixtures (dilution limit method).24 This method is based on the assumption that adsorbed l2C0 and 14N0oscillators totally surrounded by 13C0and 15N0 (as occurs if diluted '2CO-'3C0 and 14NO15N0 mixtures are dosed) are completely dynamically decoupled (because of the sufficiently large intrinsic difference between the frequency of the 'TO, 14N0and 13C0, 15NO). Consequently the l2C0 and 14N0 oscillators are influenced only by static (in the present case chemical) effects. Decoupling is expected to occur also each time a CO oscillator does not possess the same stretching frequency as the other CO surrounding it (like a CO adsorbed on an atomic defect of an extended face). We have seen that CO and NO adsorbed on edges and steps have stretching frequencies 50-60 cm-' lower than those of CO and NO on (100)faces. Hence they cannot appreciably couple with the oscillators of (100)faces. The shifts caused on the bands of CO and NO adsorbed on edges (steps) by filling the (100)faces are consequently totally chemical in nature. For CO, on edges, the average between the shifts of the two peaks shown in Figure 8 gives AP -26.5 cm-l, while for NO the equivalent shift is AP = -30 cm-'. If we assume that these figures can represent also the chemical shifts for CO and NO on {lOOl faces (which, of course, deserves a demonstration by the isotopic mixtures method), the dynamic shifts on (100)faces can be obtained by difference from the total figures. The result is that the dynamic shifts (on passing from 8 = 0 to 8 N 0.5) for CO and NO on {loo)faces are Anco = +10.5 cm-' AvNO = +36 cm-I Of course, these figures have only a qualitative meaning. These preliminary results are interesting because (i) they show the existence of static effects of the same entity observed on metal^,'*^'^ (ii) the "chemical" shifts are negative, suggesting that CO and NO on (100)faces act as electron donors and that the donated charge goes on the antibonding a* orbitals of CO and NO species located on edges and steps via an inductive m e ~ h a n i s m , ' ~(iii) ,~~ the (23) Jones, C. F.; Segall, R. L.; Smart, R. St. C.; Turner, P. S. J. Chem. SOC.,Faraday Trans 1 1978, 74, 1615.
(22) Griffin, G. B.; Yaks, J. T., Jr. J. Chem. Phys. 1982, 77, 3744.
(24) Crossley, A.; King, D. A. Surf. Sci. 1980, 95, 131.
414
Langmuir 1985,1, 414-420
dynamic shifta are positive, in agreement with the theory.I8 (6) T h e Pressure-Sensitive Shoulders at -21 10 (CO) a n d 1765 (NO) cm-’. As illustrated in Figure 9, the shoulders at -2110 (CO) and -1765 (NO) cm-’ are clearly observed only a t coverages near 0 = 0.5 and are present also on samples mainly exposing (100)faces. Their frequencies are lower than those of CO and NO on (100)faces, suggesting for these species a higher electron density on the s* antibonding orbitals (and hence, following the usual molecular orbital schemes of the metal carbonyls and nitrosyls, stronger Ni2+-CO and Ni2+-NO bonds). This hypothesis is, however, in contrast with their smaller stability. We suggest that these peaks are associated with a small fraction of CO and NO molecules adsorbed on nearest-neighbor Ni2+ ions of the (100) faces and destabilized by repulsive interactions. The minimum Ni2+-Ni2+ distance on (l00l faces is 2.95 A, i.e., smaller than the van der Waals distance: this implies that CO and NO molecules located a t such low distances (forming compressed patches) should show some orbital overlap between adjacent molecules. The most abundant formation of compressed clusters in NO adlayers (in the vicinity of B = 0.5) is likely associated with a spin pairing process causing the stabilization of dimeric forms (which of course is not operating for CO). The whole subject of the hypothetical formation of such compressed patches will be discussed in a subsequent paper.25 (7) The Nitrite (NO2-) Species. As illustrated in Figure 9, two types of NO2- species are formed by NO adsorption on edges (steps) (monodentate and bidentab): (25) Escalona-Platero, E.; Zecchina, A., unpublished results.
Their frequencies are influenced by the filling of (100)faces through inductive effecs, in a way similar to that described for the high-frequency nitrosyls a t 1740 cm-’. The concentration of the bidentate species definitely decreases when the NO pressure decreases, while that of the monodentate species shows an opposite trend (and vice versa). We interpret this reversible behavior as due to the interconversion of the two species caused by the changes of electronic and steric factors brought about by the filling of (100)faces. Very similar effects have been observed on the NiO-MgO solid solutions and have received a similar e~planation.~ Conclusions The comparison of the IR spectra of CO and NO adsorbed on a series of NiO samples characterized by decreasing specific surface area and increasing perfection of the crystallites allows assignment of the bands associated with species adsorbed on extended (100)faces, on edges and steps and atomic defects. The maximum CO and NO coverage on (100)faces is only -0.5: this means that the formation of carbonylic and nitrosylic adducts involves only half of the Ni2+ions and that more compressed phases are unfavored by strong adsorbate-adsorbate repulsions. At 0 = 0.5, CO and NO form “in registry” well-ordered surface phases (similar to those observed on metals), where the CO and NO molecules interact via “dipole-dipole”and chemical effects. Acknowledgment. This investigation has been carried out with the financial support of the Minister0 della Pubblica Istruzione, Progetti di Rilevante interesse Nazionale. Registry No. NiO, 1313-99-1;CO, 630-08-0; NO, 10102-43-9.
High-Purity, Monodisperse TiOz Powders by Hydrolysis of Titanium Tetraethoxide. 1. Synthesis and Physical Properties+ Eric A. Barringer* and H. Kent Bowen* Materials Processing Center, Department of Material Science and Engineering, Massachusetts Institute of Technology, Cambridge, Massacusetts 02139 Received November 15, 1984. I n Final Form: March 15, 1985 Spherical, monodisperse TiOz powders were formed through the controlled hydrolysis of dilute ethanolic solutions of titanium tetraethoxide. Powders having average diameters in the range 300-600 nm were synthesized by using ethoxide and water concentrationsof 0.1-0.2 M and 0.3-0.7 M, respectively. Induction times for precipitation, obtained through measurements of the onset of turbidity, can be discussed in terms of nucleation kinetics and the hydrolysis and condensation reactions. Chemical and physical properties such as particle size, surface area, density (3.1 g/cm3),and crystal structure (amorphous to X-rays) were measured. Of particular interest, the surface structure of the particles, determined through nitrogen adsorption isotherms, varied from mesoporous to atomically smooth and was dependent on precipitate washing procedures and suspension aging. Introduction ~ i t a npowders, i~ widely used in industrial as pigments, opacifiers, photocat&sts, and fillers, have been obtained either directly from titanium-bearing minerals or by precipitation from solutions of titanium salts ‘Research supported by DOE, Contract AC02-80ER10588.
0743-7463/85/2401-0414$01.50/0
or alkoxides. The most common procedures have been based on the hydrolysis of acidic solutions of Ti(1V) salts. In addition, gas-phase oxidation reactions of TiC141-3and (1) Formenti, M.; Juillet, F.; Meriaudeau, P., Teichner, S. J.; Vergnon, P. J. Colloid Interface Sci. 1972, 39, 79. (2) George, A. P.; Murley, R. D.; Place, E. R. Faraday S y m p . Chem. SOC.1973, 7, 63.
0 1985 American Chemical Society
