2194
J. Phys. Chem. 1980, 84, 2194-2199
TABLE IV: T, Values for Different Systems Tc,K ref system ionic surfactants in water 319 13 nonionic surfactants in water 222 13 SDS and CTAB in NMA, FA, -314 present studies Me,SO, and DMF SDS and CTAB in water 283 present studies
work of adhesion of alkanes in water transfer of argon from water to aqueous solutions of lower aliphatic alcohols binding of SCN- to human
hemoglobins
550 -300
290
15
19 20, also for
other systems see ref 11
likely to be much different from those of the structured species in the bulk, the value of T, is inexplicably highera10,15 Therefore, we emphasize that due caution must be exercized in assuming values of T, to be indicative of any particular type of compensation phenomenon. Nonetheless, it can be reasonably argued on the basis of observed compensation phenomena for micelle formation processes in the aqueous and nonaqueous solvents alike and from the identical slopes obtained for the compensation lines for all the solvents that presumably the destruction or promotion of some structured species of the solvent does take place. Though this process of creation or destruction of structured species may be thermodynamically similar in all the solvents giving rise to compensation lines of similar slopes, it may be mechanistically quite different
for each solvent solute system thus giving a unique compensation line for each solvent. Acknowledgment. Thanks are due to Mr. R. Usmani for checking some conductivity measurements and also to the Council of Scientific and Industrial Research, India, for providing financial assistance to R.P.S. References and Notes (1) G. C. Kreshec, "Water a Comprehensive Treatise", F. Franks, Ed., Vol. 4, Plenum Press, New York, 1975. (2) A. Ray, J. Am. Chem. SOC.,91, 6511 (1969). (3) H. N. Singh, S. Singh, and K. C. Tewari, J . Am. Oil Chem. Soc., 52, 436 (1975). (4) H. N. Singh and S. Swarup, Bull. Chem. SOC. Jpn., 51, 1534 (1978). (5) H. N. Singh, S. Swarup, and S. M. Saleem, J . Colloid. Interface Sci., 68, 128 (1979). (6) R. Gopal and J. R. Singh, J . Phys. Chem., 77, 554 (1973). (7) R. Gopal and J. R. Singh, J . Indian Chem. Soc., 49, 667 (1972). (8) J. M. Corkill, J. F. Goodman, and S. P. Harrold, Trans. Faraday Soc., 60, 202 (1964). (9) E. H. Crook, G. F. Trebbi, and D. B. Fordyce, J . Phys. Chem., 68, 3592 (1964). (10) G. Stainsby and A. E. Alexander, Trans. Faraoby Soc., 46, 587 (1950). (11) R. Lumry and S. Rajender, Biopolymers, 9, 1125 (1970). (12) H. C. Longwet-Huggins, Proc. R. SOC. London, Ser. A , 205, 247 (1951). (13) C. Jolicoeur and P. R. Philip, Can. J . Chem., 52, 1834 (1974). (14) C. Tanford in "The HydrophobicEffect", C. Tanford, Ed., Wiley, New York, 1973. (15) R. Aveyard and S. M. Saleem, J . Chem. Soc., Faraday Trans. I, 73, 896 (1977). (16) G. R. Leader, J. Am. Chem. Soc., 73, 856 (1951). (17) G. R. Leader and J. F. Gormley, J. Am. Chem. Soc., 73, 5731 (1951). (18) Landolt-Bornstein,"Zahlenwerte und Funktionen", 6th ed, Springer-Verlag, Berlin, W. Germany, 1959, p 613. (19) A. Ben-Naim. Trans. Faradav Soc.. 66. 2749 (1970). (20) A. C. Anusiem, J. G. Beetlestbne,and D: H. Irvine, J.'Chem. SOC. A , 960 (1968).
Physicochemical Characterization and Reducibility of Nickel Oxide Supported on a Wide Range of Silica-Aluminas M. Houalla" and B. Delmon Groupe de Physico-Chimie Minerale et de Catalyse, Universit6 Cathoiique de Louvain, 1348 Louvain-la-Neuve, Belgium (Received: July 24, 1979; In Final Form: March 25, 1980) Publication costs assisted by Services de Programmation Politique Scientifique (Belgium)
XPS measurements, reflectance spectroscopy, and X-ray analysis have been used to investigate the influence of the carrier on the dispersion and active phase-support interaction of nickel oxide supported on a wide range of silica-aluminas. The active phase was kept constant in all the samples (10 wt % NiO) and the silica content of the carrier was varied from 100 to 15 wt '70. It was found that, on silica carrier, the active phase is present Increasing as a poorly dispersed,well-crystallizedNiO. It is characterized by a low XPS intensity ratio IN,/lcanier. the alumina content leads to an increase of the dispersion of Ni2+species manifested by an enhancement of the XPS intensity ratio ZNi/lcarrier. The results were used to account for the difference in the reducibility of the supported oxides. 1. Introduction The dependence of the activity of supported metal catalysts on the nature of the carrier has initiated a large number of studies devoted to a better understanding of the way the carrier may modify the properties of the active phase. Since the obtaining of these catalysts involves, usually, the reduction of the oxide precursor, attention has been focused on the role of the carrier on the reactivity of the metal oxide ion. Most often, the influence of the carrier has been attributed to its interaction with the active phase. Indeed, 0022-3654/80/2084-2194$01 .OO/O
recent studies have shown quite convincingly that, in the case of NiO/A1203,a surface spinel is formed and that the difference of the reactivity of the supported oxide system, at least when no segregation of NiO is observed, can be explained by the ratio Ni2+(octahedral)/Ni2+(tetrahedral) present in the surface ~pinel.l-~ Compound formation has also been invoked to explain the difference in reducibility of nickel oxide supported on SiOz, Si02-A1,03, and A&03.4-7
However, the study of the effect of promoters like Cu in the reduction of NiO/Si02 has given additional insight 0 1980 American Chemical Society
Reducibility of NiO Supported on Silica-Alumina
TABLE I: Chemical Composition and Surface Areas of the Silica and Silica-Aluminas Carriers
-
samplea
SA100 SA95 SA55 SA40 SA15
surface area, 452 408 388 369 373 m'lg a The number following SA indicates the approximate SiO, content (wt a).
into the phenomena and shed light upon other parameters which can explain the low reducibility of NiOISiO,; these interpretations are based simply on the difference in the dispersion of the oxide over the It seems, thus, that a better understanding of the carrier effect necessitates further investigations. Both the state and the textural distribution of the nickel oxide on the support must be determined. The study reported in this publication aims to define, by a combination of various techniques, X-ray analysis, UV reflectance spectra, and XPS measurements, the state of the substance deposited on the carrier. We used, as a support, a series of silica-aluminas which have been already extensively characterized.11-13 We hoped thus to get valuable clues to the variation of the nickel oxide-carrier interactions and to the way the dispersion of the deposited oxide on the carrier varies with the composition of the latter. 2. Experimental Section 2.1. Materials. The synthesis of the silica-aluminas series, covering the range from pure silica to pure alumina, their surface, and their bulk characterization have been described el~ewhere.l'-~~ Four different silica-aluminas from this series, together with a pure silica sample, were chosen for this study. In order to facilitate the comparison, we used supports which all had comparable surface areas (Table I). The catalyeks were prepared by pore volume impregnation of the support with a solution of nickel nitrate. The powder was subsequently dried for 2 h at 110 "C and calcined at 500 "C in air for 6 h. Determination of the nickel content was made by atomic absorption with a Varian Techtron PTY Ltd. spectrometer, after dissolution of the samples by HF and HC1. The color of the nickel oxide supported catalysts is black for silica-rich samples and green for alumina-rich specimens. The suppork is identified by the sign SA followed by its SiOz weight percentage. The supported catalyst series is designated by NiO-SAX where X indicates the SiOz weight percentage previously defined. In all this study, nickel loading expressed as weight percent of NiO in the final solid is around 10 wt 70. For the sake of comparison, two unsupported NiO samples have been prepared by calcining pickel nitrate at 500 and 950 "C, respectively, for 6 and 24 h. 2.2. Measurement Procedures. 2.2.1. Physicochemical Characterization. X-ray diffraction patterns have all been obtained under the same conditions, using a Philips diffractometer with Cu K a radiation (Ni filtered). We obtained diffuse reflectance spectra with a Beckman Acta IV spectrophotometer using, as 21 reference specimen, the carrier of the corresponding NiO-supported sample. The measurements were made in the 800-350-nm wavelength range. XPS measuirements were performed by use of a Vacuum Generators ESCA 2 equipped with an aluminum anode (hv = 1486.6 eV) operated at 50 mA and 10 kV. The samples were finely ground and the powder was dusted and pressed
The Journal of Physical Chemistry, Vol, 84, No. 17, 1980 2195
on double-sided Scotch tape. A Tracer Northern NS560 signal averager was used, in order to improve the signalto-noise ratio. An energy range of 15.7 or 31.4 eV was scanned. This range was divided into 254 channels; measurements were made by repeated scanning of the interval with accumulated time per channel typically equal to 2.5 s for Cis, Sizp, Alzs, 1.25 s for Ols, and 30 s for NizP3. Peaks were smoothed manually and decomposed by &e mirror technique when asymmetrical. Binding energies (BE) were corrected for sample charging by reference to gold evaporated onto the sample by vacuum deposition. A binding energy value of 83.8 eV for Au 4fTp level has been used. Similar values were obtained when the C1, line (BE = 285 eV) has been used occasionally as a reference. The signal intensity is obtained by measuring the area under the peaks and normalizing to unit attenuation and time. The background produced by the inelastically scattered electrons is assumed to vary linearly with energy. The intensity of NiZPsjz level measured includes the associated satellite peak. XPS intensity associated with the carrier may be presented by the sum of Alzs and Sipppeaks intensities normalized to take into account the difference in sensitivity of the detector toward Alzsand Sizplevels. This may be accounted for by measuring the peak intensities of Alzsand Sizpin a given carrier, for instance SA55 Thus, if we know the chemical composition of the support, the sensitivity factor for Alzs and Sizpmay be readily obtained. Our measurements indicate a similar response for both elements, which is not surprising, taking into account the fact that Alzaand Si$ have a comparable photoionization cross section14and a similar kinetic energy. It has been checked that the IAl/( [si + IN) ratio is linear in A1 content with the intercept at origin. 2.2.2. Reduction Experiments. Reduction measurements were carried out in a McBain balance equipped with a circulation pump and a cold trap to remove the water evolving during reduction. All the experiments were carried out'at 400 "C and a hydrogen pressure of PH2N 1 atm. Sample weight was 120 mg. The procedure was as follows. The specimen was outgassed at the reduction temperature and a residual pressure of torr. Hydrogen was then admitted into the circulation loop and the progress of reduction measured until the rate became very low. The system was then pumped at the reduction temperature. The extent of reduction (a)was defined as the fraction of NiO reduced to Ni. The reduction time is usually around 6 h. 3. Results 3.1. X-Ray Data. The X-ray diffraction spectra of NiO supported on SiOz and different Si02-A1203are illustrated in Figure 1. Examination of the diagrams reported in Figure 1 indicates the presence of crystallized NiO in the Si02-rich samples. No detectable shift of the X-ray lines was observed. Less crystalline NiO is observed in the A1203-rich samples, and NiO-SA15 shows practically the same diffraction pattern as the support. No new lines or bands which may be attributed to a new compound between carrier and nickel oxide have been found. Table I1 illustrates the characteristics of the diffraction line intensities of the NiO-SAX catalysts. 3.2. Reflectance Spectra. Figure 2 shows the reflectance spectra of the NiO-SAX series. For comparison, the reflectance spectrum of a gray green nickel monoxide obtained by decomposing nickel nitrate in air at 950 "C is
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Houalla and Delmon
TABLE 111: UV Reflectance Line Positions of the NiO-SAX Series samde NiO Ni0-SA100 Ni0-SA95 Ni0-SA55 NiO-SA40 NiO-SA15
lines oosition. nm 715
415
380
540
730 730
430 430 430
380 380
540 540 540
630
TABLE I V : XPS Parameters of NiZPo,2Level in the NiO-SAX Series
BE, eV SIP Sl(S t P) S - P, eV
fwmh, eV
NiOSA100
NiOSA95
NiOSA55
NiOSA40
NiOSA15
856.11 0.755 0.43 6.4 4.48
856.08 0.64 0.39 6.2 4.62
856.63 0.572 0.365 6.35 5.22
856.8 0.49 0.327 6.1 4.62
857.02 0.54 0.35 6.1 4.48
n
I
I
65
55
45
(28)
35
Figure 1. X-ray diffraction spectra of silica- and silica-alumina-supported nickel oxide.
850
860 870 B i n d i n g energy ( e V )
Figure 3. N,i XPS spectra of unsupported and silica- or sillca-alumina-supported nickel oxide. 500
350
650
nm
800
Figure 2. UV reflectance spectra of silica- and silica-alumina-supported nickel oxide.
TABLE 11: X-Ray Diffraction Lines Positions (d, A ) and Intensitiesa of the NiO-SAX Series NiO(ASTM) Ni0-SA100 Ni0-SA95 Ni0-SA55 NiO-SA40
2.410 vs 2.4169 vs 2.4138 vs 2.4106
2.088 vs 2.0895 vs 2.0895 vs 2.0895
vs
vs
1.476
1.259
1.206
S
W
W
1.4796
1.2592 1.2056
S
W
1.4796
1.262
1.2094
S
W
W
W
1.4796
1.2592 1.2056
S
W
W
2.4106
2.0895 1.4796 ms ms m diffraction bands attribuable to carrier
NiO-SA15 vs = very strong; s = strong; ms = medium strong; m = medium; w = weak.
reported. In the latter case, a MgO pellet was used as a standard. The energies of the optical transition are listed in Table 111. Examination of Figure 2 and Table I11 yields the following remarks. The reflectance spectrum of NiO is in good agreement with that reported by Klier et al.15 It corresponds to an electronic transition in a Ni2+ion placed in an octahedral field. The weak line at 540 nm which cannot be predicted
on the basis of Ni2+in an octahedral environment has been attributed to Ni3+.15 The spectra of Ni0-SA100 and Ni0-SA95 show continuous absorption in the visible range which makes it impossible to detect any fine structure. Increasing the alumina content leads to the appearance of a strong band at 430 nm and a weak one at 730 nm, both attributed to Ni2+in an octahedral environment.16 Reflectance spectra of the alumina-rich sample NiOSA15 shows, besides the strong band at 430 nm, a large and weak band around 630 nm, which may be assigned to Ni2+ in a tetrahedral envir0nment.l' 3.3. ESCA Measurements. 3.3.1. General Observation. Before examining the XPS spectra of supported NiO, it is necessary to define the XPS features of unsupported NiO. Figure 3, curve a, shows the Ni$ XPS spectrum of a NiO sample prepared by decomposition of nickel nitrate under the same conditions used for supported catalysts. A broad satellite peak at about 6 eV above the main peak can be readily distinguished; it has been attributed to an electron shake-up transitional8The decomposition of the Ni2 main peak indicates clearly the presence of a second pe 2 eV above the principal line, which has been ascribed to multiplet splitting.18 The XPS spectra of silica and silica-alumina supported nickel oxide (Figure 3, curves b-f) show equally the 6-eV shake-up satellite, while decomposition of the Nizmiz profile seems to indicate the disappearance of the 2-eV peak. On
The Journal of Physical Chemistry, Vol. 84, No. 17, 1980 2197
Reducibility of 140 Supported on Silica-Alumina
0.E. l
856 100
O
3
P
i
”
.
0
50 ‘10 S A
Flgure 4. Variation of the binding energy of the Ni2P3/2 level as a function of the silica content of the carrier.
I 532.5 - B.E.
Ih
% SA
or w t
% Si02
Figure 6. Variatlon of the binding energy (BE) of the S,i level as a function of the silica content of the carrier: (0)BE of the S,I level of the carrier; (0)BE of the S,i level of the finished catalyst (NiO/carrier).
0 1s I
8. E.
100
0
50 % SA
Flgure 5. Variation of the binding energy (BE) of the OIs level as a function of the silica content of the carrier: (0)BE of the O,, level of the carrier; (0)BE of the O,,level of the final catalyst (NiOkarrier).
the other hand, examination of the XPS parameters of Nizpa/?indicates that satellite splitting (S-P and satellite to primary peak ratio [ S / ( S P ) ] )slightly decreases as the alumina content of the carrier increases (Table IV). Moreover, Nizpyzpeaks are significantly narrower at bsth ends of the composition studied. 3.3.2. Binding Energies. NiQy The variation of the BE values of the Ni,,, level is illustrated in Figure 4. A shift up to 1 eV to higher BE is observed when the support becomes increasingly richer in alumina. The same tendency is obtained when BE values are referenced to the C1, .line (BE = 285 eV) instead of the Au line (BE = 83.8 eV). O1,. Figure 5 shows the BE variation of the 01,level vs. silica content of the NiO-SA series and the expected variation due to the support itself, taken from ref 19. It can be readily seen that the observed variation for the NiO-supported series may be attributed to the support itself and do not reflect any significant contribution originating eventually from the 01,level of nickel oxides. SCzp.Analysis of the BE variation of Sizpis hindered by the interference at high A120, content of an Auger transition peak due to aluminum.m Nevertheless, for silica-rich carriers, the variations of the BE of the Sizpline may well be accounted for as originating from the support (Figure 6). No significant variation of AlzsBE has been observed for samples containing up to 55 wt % silica (Figure 7). 3.3.3. Linie Intensities. Comparison of absolute intensities of the IESCA peaks for independent samples cannot be reliable, since the XPS line intensities are dependent upon several factors, namely, carbon contamination, detector efficiency, etc. Significant measurements require the use of intensity ratios, more precisely the ratio of the
+
118 100
50
0
% S A or w t % S i 0 2
Flgure 7. Variation of the binding energy (BE) of the AI,, level as a function of the silica content of the carrier in silica-alumina-supported nickel oxide.
01 100
0
50 % SA
or w t
% 50,
Flgure 8. Variation of XPS intensities ratios INiW,J(ISlb -I-IAln)as a function of the silica content of the carrier.
intensity of the line of interest to a reference line occurring in all samples. In the case of supported catalysts, the ratio of the peak intensity of the supported substance over that of the support provides a semiquantitative measurement of the dispersion of the metal phase over the ~ a r r i e r . ~ lThis -~~ can give a valuable information in the case of our NiOSAX series, where all solids have the same NiO loading and a comparable carrier surface area. In Figure 8, the ratio ZNi+3,z/(Zsi2,, + ZAlb) is plotted vs. the silica content of the carrier. It can be readily seen that there is a steady increase in the dispersion of Ni2+as the alumina content of the carrier increases. 3.4. Reduction Studies. The results of reduction experiments are shown in Figure 9, which gives the fraction
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a’
No.
17, 1980
7 0
SA-55
G 0
SA-40
4
SA-I5 SA-I5
25 25
0 0
t
5 5
h
Figure 9. Reduction isotherms of silica- and silica-alumina-supported nickel oxide: T = 325 O C ; p y = 1 atm; a = fraction of NiO reduced to Ni.
100
50
0 “Io
SA
Figure I O . Variation of the degree of reducibility of silica- and silicaalumina-supported nickel oxide as a function of the silica content of the carrier: T = 325 O C ; pH2= 1 atm, time of reduction 6 h; a = fraction of NiO reduced to Ni.
-
(a)of NiO reduced to Ni vs. time. Under these conditions, Ni0-SA100 sample is reduced so rapidly that the reduction rate cannot be followed. There is no evidence of a nucleation period, as observed with unreduced NiO; this is in agreement with previous observations reported for silica-supported nickel oxide.8 Figure 10 shows clearly a decrease in the extent of reduction with an increase of alumina content of the carrier. The NiO-SA15 sample is barely reduced.
4. Discussion 4.1. General Considerations. As indicated in the In-
troduction, two parameters of a different nature can explain the differences in the properties and, especially, the ability to be reduced (Le., activated), of supported oxides. These parameters are, on one hand, the dispersion, because less nucleus forming sites are likely to be present in smaller crystallites and, on the other hand, the formation of surface or bulk compounds or associations with the support.’” In this respect, impregnation of a carrier with nickel nitrate followed by drying and calcination may lead to two different situations: (i) formation of a NiO phase, more or less dispersed on the support; and (ii) surface or bulk compound formation between NiO and the carrier. We shall successively consider our results along those two different lines and finally discuss how the two distinct effects converge for explaining the depressed reducibility of supported NiO. Certain results cannot discriminate between the various possibilities. This is the case for X-ray diffraction data. Indeed, the decrease in the intensity of the X-ray lines of NiO may be attributed to two different phenomena: an increase in the amount of Ni2+which combines with the support as N203 increases to form an ill-crystallized surface
Houalla and Delmon
compound undetected by X-ray analysis and the formation of well-dispersed X-ray amorphous NiO over the support favored by an increase in the Alz03content of the carrier. 4.2. Dispersion. The ESCA results (intensities) show a general trend toward a better dispersion of the Ni species (Figure 8) when the A1 content of the support increases. However, as such, the results do not distinguish between better dispersed NiO crystallites or Ni species bound to the support. However, electron microscopy observations and high-resolution electron probe microanalysis (EPMA)24 show that, as the alumina’content of the carrier increases, both phenomena occur, namely, the formation of smaller NiO aggregates and the appearance of a Ni “dispersed” or combined phase. 4.3. NiO-Support Interaction. 4.3.1. Summary of the Properties of Silica-Alumina Supports. In order to rationalize the influence of the support, one must briefly summarize the properties of the silica-aluminas, as taken from ref 13: There is no enrichment of either A1203or SiO, near the surface of the grain constituting the powdered silica-alumina. A mixed silica-alumina phase develops below 25% A1203, which contains tetrahedral aluminum and corresponds to an increase in the number of Bronsted acid sites. Between 25 and 50% A1203 the intrinsic properties of the surface practically do not change. Above 50% A1203, the mixed phase is progressively diluted by alumina, which appears as a distinct phase above 60-70% A1203, and is responsible for a further increase in the surface density of acid sites. 4.3.2. UV Reflectance. The reflectance spectra results indicate that Ni2+ exists mainly in an octahedral environment. Failure to observe reflectance spectra for Si02-rich samples (Ni0-SA100 and Ni0-SA95) can be readily explained by a strong absorption due to Ni3+ generated by the presence of excess oxygen in (oxide) NiO. A coverage of 5% of the surface of NiO by oxygen has been reported to be sufficient to eliminate any fine structure or band in NiO visible reflectance spectra.15 No evidence of surface spinel NiA1204formation can be deduced from the reflectance spectra of the alumina-rich samples. Only in the case of the NiO-SA15 specimen does a weak indication of Ni2+in tetrahedral coordination exist. 4.3.3. XPS Data. The XPS data give much information on the possible interaction between nickel and the support. An important observation is the shift to higher BE of the Ni,,,,, line observed with increasing alumina contents. This comes in agreement with the results reported in ref 25, where a similar BE increase, although of smaller magnitude, was observed for a series of catalysts where nickel oxide was supported on the following carriers: Ti02-Si02, SiO2-AlZO3,A1203,MgO. This effect indicates an electron transfer from the active phase to the support. A possible explanation would be that the electron transfer is a consequence of the acidity of the carrier. This would explain, in our case, the variation of the shift as a function of alumina content, Figure 11 actually shows a striking parallelism of the variations of the surface acids sites density determined by XPS and IR ~pectroscopyl~ and of the BE variation of Ni2!, vs. silica content. However, the explanation might be different. A positive XPS BE shift has been directly attributed to dispersion,% presumably because of a smaller relaxation energy for atomically dispersed phase as compared with metal aggregates. The variations of the Niz ,line width as a function of Alz03content may be relatec! k~the multiplicity of the Ni2+
Reducibility of NiO Supported on Silica-Alumina
The Journal of Physical Chemistry, Vol. 84, No. 17, 1980 2199
resistance to reduction, as it is known that nickel silicates or aluminates reduce less easily than Ni0.992sThis explains the decrease of the ultimate degree of reduction of our catalysts (Figure 10).
"lo
SA
Flgure 11. Variation of Ni2PV2binding energy as a function of silica content of the carrier in silica- and silica-alumina-supported nickel oxide (0). Variation of acid sites density (per 100 A*) as a function of silica content of the carrier ( 0 ) . 1 3
chemical states present. One can expect that carriers containing Silo2or A1203present less surface heterogeneity as compared to silica-alumina. In other words, the chemical environrnent of Ni2+ becomes more complex as we move from one end of the composition range toward the middle. Consequently,the Ni2 line, having different BE on silica and alumina, wouldl3/ieflect these effects and present a maximum width for the carriers which contains comparable amount of Si02and Al21O,. It must, however, be noted that various factors as differential charging and multiplet splitting may well contribute to Nizpa line broadening and preclude any unambiguous interpreiation. 4.4. Overall View, It is clear that, when the nature of the support is changed, both dispersion of the pure oxide phase (NiO) and the amount of surface or bulk compounds between the supported phase and the carrier change. There is a clear interdependence of the two phenomena. The observed increase of the dispersion of the Ni2+ species when alumina is incorporated into silica or present as a separate phase, illustrated by the increase of INiJ(Is,, IMP) XPS ]peak intensities ratios, can be rationalized as a consequence of the type of carrier-active phase interaction involved. In effect, incorporating alumina into silica enhances the support deposited substance interaction and, thus, inhibits migration and sintering during the drying and calcination steps of the preparation of the solid. In light of these results, the variation of the reducibility of NiO-SAX series can be explained. Well crystallized, noninteracting, NiO present in NiOSA100 and NO-SA95 may be easily reduced at 400 "C. The increase of A1203 content of the silica support promotes carriel. NiO interaction and decreases the aggregation state of the NiO particles. Both effects will hinder reducibility. With a certain amount of oversimplification, we might detail the interpretation more. I t has been showngl0 that, below a certain size, the decrease of the size of the crystallites of NiO brings about a decrease olf the rate of reactions lbecause of a decrease of the probability (or rate) of nucleation. This is one of the effects observed in our experiments (Figure 9): the rate of reduction decreases with increasing A1203 content. On the other hand, an increase of'the extent of surface compound formation corresponds to an increase of the
+
5. Conclusion The characterization of the texture and structure of a series of nickel oxide supported on silica and various silica-aluminas leads to the following conclusions: On silica-rich carriers, the active phase is primarily a well-crystallized nickel oxide. Increasing the alumina content leads to stronger interaction between NiO and support, with the former preserving essentially an octahedral environment. Alumina rich sample contains no crystalline NiO. The dispersion of Ni2+species increases steadily with the enrichment of alumina of the carrier. Simultaneously, the reducibility of supported NiO diminishes considerably. The inhibition of the reduction may be attributed to the intervention of two different phenomena, which do not mutually exclude each other, namely, the formation of an ill-crystallized unreducible surface compound and the inhibition of the nucleation related to the dispersion of the NiO phase.
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