The presence of water (2%) in the propellant-grade hydrazine used in these experiments has been ignored. Because a certain amount condenses out in the apparatus a t high pressure, its contribution to the flow cannot be determined accurately anyway.
h c k e t Research CorP.1 “Development of Design and Scaling Criteria for Monopropellant Hydrazine Reactors Employing Shell 405 Spontaneous Catalyst” (RRC-66-R-76, Vol. I, 11) (work performed under NBSA Contract NAS 7-372), Sayer, Rocket C, Research F,, Corp., Seattle, No, Wash., Jan 19, 1967. 6th Propulsion
literature Cited
Joint Specialist Conference, American Institute of Aeronautics and Astronautics, New York, N. Y., June 1970. Vancini, C. A., “Synthesis of Ammonia,” CRC Press, Cleveland, Ohio, 1971.
Bryant, J. T., Wood, S. E., A I A A J., in press. Nielsen, A., “An Investigation of Promoted Iron Catalysts for the Synthesis of Ammonia,” 3rd ed, Jul. Gjellerupb Forlag, Copenhagen, 1968
RECEIVED for review September 7, 1972 ACCEPTEDDecember 15, 1972 This work was supported by the Naval Ship Systems Command
Study of Impregnated Chromia on Alumina Catalysts with an Electron Probe Microanalyzer Hong-Chiu Chen’ and Robert B. Anderson* Department of Chemical Engineering and Institute for Jiaterials Research, Mdiaster University,Hamilton, Ontario, Canada
An electron probe microanalyzer was used to determine concentration profiles in porous y-alumina spheres impregnated with aqueous solutions of chromium(ll1) nitrate or chromic acid. Chromium from the nitrate was deposited near the outside of the sphere whereas from chromic acid solutions the chromium penetrated deeper. Dry alumina spheres were impregnated in three ways: with a volume of solution equal to the pore volume, with half of ihe pore volume, and b y storage in excess solution. For the second method, the chromium was deposited in the outer portion of the sphere.
T h e preparation of supported catalysts by impregnation is a simple method and has been used for many years. However, the detailed physical and chemical processes involved in the impregnation are complex and have not been thoroughly investigated. The method of impregnation usually involves the evacuation of a dry support, adding solution, drying, and calcining. The same sequence may be repeated one or more times. Preparations of nickel on silica-alumina (Ciapetta and Hunter, 1953) and chromia on alumina (Ciapetta aiid Plank, 1953) have been reported. The physical and chemical processes involved in the impregiiatioii were discussed for t’he dispersion of platinum on porous supports (Maatman and Prater, 1957; Maatman, 1959). The partial exclusion of electrolytes from the pores of porous supports has been reported; this effect has been attributed to the size of the hydrated ions being too large to ent’er the smallest pores that water can enter or to the inability of solute species larger than the water molecule to be as concentrated near a solid surface as i t is in the bulk pore solution (Dalton, et al., 1962). Aqueous electrolytes reacted with the surface of alumina by ion exchange JTith surface X13+, OH-, and H+. If equivalent, amounts of cation aiid anion exchange with H aiid 01%-,respectively, the process gives the appearance of “molecular” adsorption (Fischer and Kulling, 1956; Umland, 1956). +
Taken in part, from the doctoral theyis of I-Iong-Chiu Chen, Department of Chemical Engineering, RlcRlaster University, 1972. 122 Ind. Eng. Chem. Prod. Res. Develop., Vol. 1 2 , No. 2, 1973
After calcination chromia, Cr203, is usually the form of chromium in chromia-alumina. Numerous studies have been reported on the microstructure of chromia-alumina catalysts (Eischens and Selwood, 1948; Weller and Voltz, 1954; Matsunage, 1957, 1958; Bridges, et al., 1961; Poole and hIacIver, 1967). Adsorption, magnetic, and X-ray diffraction investigations showed that the chromia is present as clusters of very small crystallites that only partly cover the alumina. The present paper considers the dispersion of chromia on alumina on a larger scale, as the resolution of the electron microprobe is about the same as that of the optical microscope. The electron probe microanalyzer provides nondestructive analyses for volumes of material the order of a few cubic microns. X beam of electrons is impinged on a polished section of the unknown. The characteristic X-rays of the elements in the specimen are produced and are measured quantitatively to yield, after correction, a reasonably accurate quantitative analysis. The electron beam has an accelerating voltage about twice the X-ray absorption edge of the heaviest element of the specimen and a diameter of about 1 p . Due to penetration and scattering of electrons, the X-rays emerge from a hemisphere the order of 3-4 in diameter. This scattering of electrons limits the resolution of the instrument to several microns. Thus, the electron probe is a blunt tool for examining the fine structural details of catalysts, but it is useful in measuring interesting phenomena on a scale of microns. This instrument has proved useful in examining the ammonia synthesis catalysts (Chen, 1972; Chen and Anderson, 1972), regarding the general morphology of the catalysts, the distribution of
current measured by impinging the electron beam into a Faraday cage with no electrons back-scattered was 150 nA. Periodic readings of specimen current were made during the experiment to ensure that the results were obtained under stable prohe conditions. Point counting was performed along the diameter of the circular cross section at 52-p intervals. X-Rays of AI and Cr were measured simultaneously. After corrections of the X-ray data for atomic number effects, X-ray absorption, and flnorescence (Haworth, 1968), the weight ratios of Cr to AI were plotted agaimt the radius of the cross section. The weight ratios were used for expressing concentratioas because the concentration of aluminum should remain constant in the preparation of the catalysts. An adsorption isotherm for Nz a t 77°K was determined on
used as the volume of solution required to fill the micropores of the alumina. Scanning electron micrographs of unpolished sections of the h-..L--
.._.- "h.....
:- lX-
ml..-:nm.m--,-,-,
t'0.40-
0.22 -
I
l
_I
-
I
1000.0
MICRONS I
CHROMIC R C I D (
',
:,\,!
3
-.
0 LL
'..
ri 0 0.20
(
I
2000,O
Figure 4. Concentration profiles for particles impregnated with the chromic acid solutions of different concentrations by the PV method
M I
+-
3
1000.0
0.0
RADIUS
Figure 2. Concentration data on a particle impregnated with 5 M chromic acid solution by the PV method
..,..............
I
I
-2000.0
-3000.0
'-
-
- .- ,-,
0
+-
3 0.18
-
0.16
-
0.14
I
-3000.0
+3
0.20
-
0.10
-
.......... '..
\a'
............
I -L000.0
-1000.0
ROD I U 5
1000.0 [
2000.0
3000.0
YICRONS 1
0.03
-3000.0
-2000.0
-1000.0
C
R A D I U S ( MICRONS I
Figure 3. Smoothed curves for the concentration data of Figure 2: solid curve, using four-constant, seven-point equation; dotted curve, using four-constant, five-point equation
Figure 5. Concentration profiles for particles impregnated once, twice, and three times, respectively, with 2.5 M chromic acid solution by the PV method with drying and calcination after each impregnation
tions make the examination of concentration profiles difficult; therefore, the data were smoothed using standard leastsquares procedures for equally spaced points. Smoothing equations using two- and four-constant polynomials and five and seven points were applied four times to these and other data. The third-degree polynomial using seven points was chosen as the best method, because widely scattered points mere eliminated without "washing out" the characteristics of the concentration profile, and the data presented subsequently are the smoothed curves obtained by this method. Figure 3 shows smoothed curves for the data of Figure 2 using the four-constant equation fitted to five and seven points (Hershey, et al., 1967). The remaining undulations in the concentration profiles seem significant, as most of them appear in both the raw and smoothed data. The period of the undulations was a t least 10 times larger than the alumina aggregates shown in Figure 1; therefore, these factors seem unrelated. The total amount of chromium impregnated on the particle of Figure 2 mas 0.119 g of Cr/g of alumina and the amount found in the particle was 0.116 g of Cr/g of alumina. As nil1 be shown subsequently, the amount of chromium
introduced and that found by the probe analyses agreed satisfactorily. The concentration profiles in duplicate experiments were similar but not identical. The concentration curves were usually reasonably symmetrical. Figure 4 shows the results on four particles impregnated with the chromic acid solution of different concentrations and with a n amount of solution equal to the pore volume of each particle (PV). The weight ratio decreases slowly from the periphery to the center of each particle. For a concentration of 0.4 Jf the central portion of the particle contains no chromium; all the solute is adsorbed on the outer portion of the particle before the solution reaches the center. Chromium is found in the central portion for higher concentrations. As t h e concentration increases, the entire concentration curve is increased by more or less a n equal amount. The solid curve in Figure 4 is the same as the solid curve in Figure 3. I n Figure 5 the low, middle, and high curves are concentration profiles for three particles impregnated by the PV method with a 2.5 Jf chromic acid solution once, twice, and three times, respectively. After each impregnation the particle was dried a t 110' for 8 hr and calcined a t 500' for 4 hr to
124 Ind.
Eng. Chern. Prod. Res. Develop., Vol. 1 2 ,
No. 2, 1973
CHROMIUM N I T R R T E ( 1.8M 1 0.30
I AMOUNT
GP ECLUTION
EXCESS FORE V G L b M E
0.26
c
1/2 PORE VOLUME
_-----_
, ,,
-
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.............
,,
-
0.18
r
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-
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a
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U
0
.
+ =
0.10
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0.080
0 LL
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3
0.040 0,020 0.000
I
m PORE e s YGLUMK s
-
i-
-3000.0
..........
I
I
-2000.0
-1000.0
0.0
RADIUS
R A D I U S I MICRONS 1
Figure 6. Concentration profiles for particles impregnated with 2.5 M chromic acid solution by XS, PV, and HPV methods
-i
1
W U N T OF SOLUTION
[
1000.0
2000.0
3000.0
MICRONS 1
Figure 8. Concentration profiles for particles impregnated with 1.8 M chrornium(ll1) nitrate solution by XS and PV methods
CHROMIUM N I T R R T E [ 1.2M
CHROMIUM N I T R A T E
I
0.14
-! EXCBSS PO13 " O J J J ! I ~
0.12
............. I
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.
2
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0.0
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200010
3000.0
I
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7.0
XJOO.O
ZOOO.O
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Figure 7. Concentration profiles for particles impregnated with chromium(ll1) nitrate solutions of different concentrations b y the PV method
Figure 9. Concentration profiles for particles impregnated with 1.2 M chromium(ll1) nitrate solution by XS and PV methods
ensure that the chromium would not redissolve in the subsequent impregnation. Multiple impregnations with drying and calcination between them provide a means of accumulating a large amount of chromium across the particle. The technique is useful when the solubility of the solute is limited and the use of a concentrated solution to accumulate more solute in the particle as shown in Figure 4 is not possible. Figure 6 illustrates the use of different volumes of the impregnating solution to give different dispersions of chromium on the alumina particles. The concentration of the solution was 2.5 M. The upper curve corresponds to a particle immersed in a n excess solution (XS) for 9 days. The middle curve shows the particle impregnated b y the PV method and the lower curve corresponds to a particle impregnated with a n amount of solution equal to 50% of the pore volume (HPV). I n a spherical particle half of the particle volume is contained in a shell near the periphery with a thickness of about one-fifth of the particle radius. However, in the lower curve of Figure 6 (HPV method) the chromium concentration drops to zero at a distance from the periphery greater than one-fifth of the radius. Apparently, part of the pore volume in the external shell was not filled during the impregnation.
The chromium content in the shell is lower than in the corresponding portions when the PV method is used. Figure 7 illustrates the results on three particles impregnated by the PV method with chromium(II1) nitrate solutions of different concentrations. The weight ratio in each particle changes in a similar fashion, decreasing sharply from a large value near the periphery followed by a gradual change and another sharp decrease to zero near the center of the particle. The more concentrated the impregnating solution the larger the weight ratio and the thicker the shell. We may infer that chromium(II1) nitrate is more strongly adsorbed by the alumina than chromic acid. The solid line in Figure 8 shows the change of the weight ratio in a particle impregnated with 1.8 Jf chromium(II1) nitrate solution by the XS method and the dotted line is for a particle prepared by the PV method. Both particles have approximately equal chromium content in the outer portions. Soaking the particle in a n excess solution caused the accumulation of chromium in the central portion but gave little increase of chromium content in the outer shell. Figures 9 and 10 give similar comparisons between particles prepared with 1.2 and 0.6 -11chromium(II1) nitrate solutions, Ind. Eng. Chem. Prod. Res. Develop., Vol. 1 2 , No. 2, 1973
125
0.070
0.060
CHROMIUM NITRRTE
c
0.6M 1
1
I
_1
a
0.050
-
;0.040
-
LL 0
.
these features do not seem to be intimately related to the macrostructure of the support; possibly they result from the roughness of the polished cross section. The X-ray powder diffraction patterns of impregnated particles showed only broad peaks corresponding to y a l u mina; therefore the chromia was poorly crystallized or the particle size was too small to give distinctive peaks.
3
CL
0.030
Discussion
-
LL
0
;0.020 3 0.010
t
-
0.000 -3000.0
-2000.0
0.0
-1000.0
HRDIUS
[
1000.0
2000.0
3000.0
MICRONS 1
Figure 10. Concentration profiles for particles impregnated with 0.6 M chromium(lll) nitrate solution by XS and PV methods
Table I. Comparison of Chromium Content from Microprobe Analyses w i t h Amount Used in Preparation Figure no.
4 4 4 4 5 5 5 6 6 6
7 7 7 8 9 10
Type of curve
--......
-----_.-.-. __...*.. -----___ ...... -----------
......
---
___ ___ ___
Chromium content, g of Cr/g of A1203 Found Introduced
0 0 0 0 0 0 0 0 0 0 0 0 0 0 0 0
1161 0134 0773 0487 0667 1475 2464 0658 0266 1147 0160 0307 0426 0468 0341 0202
0 0 0 0 0 0 0 0 0
1194 0101 0793 0399 0596 1192 1787 0596 0298
0 0143 0 0287 0 0429
respectively. Similar to Figure 8, approximately equal chromium content was found in the outer portions of the particles and soaking a particle in excess solution brought chromium by diffusion to the central portion of the particle. The total amount of chromium in the particles, i.e., the average concentration, was obtained by numerical integration using Simpson's rule on the smoothed concentration curves. Each curve is divided into two portions by the origin (the center of the particle), and each portion was used to obtain the chromium content in its hemisphere. The total amount of chromium found in each particle is compared with with the amount impregnated in Table I. Since the integration was carried out on a n absolute basis, the results could be seriously affected by failure to section the particle at its center. I n view of this factor and the inherent uncertainties of analyses by the electron probe, the agreement seems satisfactory in most instances. For particles impregnated with excess solution, the amount of chromium introduced was not determined. The concentration curves determined by the probe analyses often had wavelike patterns. These undulations have periods tenfold larger than the sizes of alumina aggregates. Thus, 126 Ind. Eng. Chem. Prod. Res. Develop., Vol. 12, No. 2, 1973
A variety of processes are involved in the impregnation of a dry alumina particle, including the penetration of solution by capillary forces, the adsorption of solute on the pore walls, and the diffusion of solute in the pore solution. When the impregnated particle is dried, the evaporation of water near the periphery concentrates the pore solution, causing more diffusion of solute toward the center of the particle. If the concentration of the pore solution exceeds the solubility, the solute precipitates on the alumina. The alumina support consists of roughly spherical porous aggregates of sizes between about 5 and 30 p . The spaces between these aggregates are very large pores with diameters about the size of the aggregates. Most of the surface area and the fine pores are within the alumina aggregates. The average radiusoof the fine pores as estimated from nitrogen isotherms is 72 A. The hydrated radii of Cr042- and Cr3+ are 3.75 and 4.61 A, respectively (Nightingale, 1959). Therefore, the penetration of these ions into the pores of the alumina spheres was not hindered. As a dry alumina particle is impregnated with a n amount of solution equal to the pore volume of the particle (PV), the solution penetrates the particle to fill the pores by capillary forces. The length of time required for the complete penetration is proportional to the viscosity of the solution and inversely proportional to the surface tension of the solution and the pore radius (Maatman and Prater, 1957). When the solution enters a pore, part of the solute adsorbs on the pore walls and the concentration of the solution decreases. The amount of solute adsorbed is determined by the adsorption isotherm of the solute from its solution. For strong adsorption a large portion of the solute adsorbs on the pore walls but for weak adsorption a large portion of the solute is in the pore solution. Both the solute adsorbed on the pore walls and the solute in the pore solution contribute to the total amount of the solute in the particle. For a strongly adsorbed chemical, if the rate of penetration of the solution is low and the rate of adsorption of the solute on the pore walls is high, the solute should accumulate near the pore mouth. For a weakly adsorbed chemical, if the rate of penetration of the solution is high and the rate of adsorption of the solute on the pore walls is low, the solute is likely to penetrate deep into the pores. The distribution of a n element on a given support is a function of the chemical used and its adsorption isotherm from solution, as well as the amount, concentration, and viscosity of the solution. If the alumina particle is placed in a n excess amount of solution (XS), the pores are first filled according to the processes described above, a concentration gradient of the solute in the pore solution is produced between the periphery and the center of the particle, and more solute is transferred into the particle by diffusion. However, if a particle is impregnated with a volume of solution less than its pore volume, the solution penetrates only a short distance from the periphery, but usually deeper than the thickness of a shell containing a pore volume equal to the amount of solution. I n this situation it is possible that larger pores are filled and the smaller pores
are not filled or the pore walls were covered b y multilayers of the pore solution rather than the pores being filled. The drying step in preparing impregnated catalysts is more complicated than the impregnation, because i t involves heat and mass transport and interfacial tensions of solutions t h a t are dependent on temperature and concentration. If the particles are heated uniformly, e.g., by a stream of heated gas, the temperatures of the outer portions of the catalyst will exceed those of inner parts. Within the particle a temperature gradient is established that is a function of gas temperature and flow, the thermal conductivity of the particle, the heat of vaporization of water from the solution, time, and other factors. Because temperature increases from the center to the periphery and the interfacial tensions of the solutions vary in the opposite way, there should be no tendency for the liquid to be drawn to the periphery of the particle during drying, and if the porous particle is not completely filled with solution, the interfacial forces should pull the solution toward the center. The evaporation of water starts at the periphery of the particle and proceeds preferentially from larger pores where the vapor pressure is larger. As water evaporates, the concentration of the solution in the pores increases causing diffusion of solute toward the center of the particle or center of the aggregate. The adsorption of solute on the support increases with increasing concentration and when the solubility is exceeded, precipitation occurs. I n this final section the results of this paper are summarized with respect to methods for preparing a catalyst with a concentration gradient and average concentration appropriate to the requirements of the reaction. We note that using a volume of impregnating solution equal to the pore volume, the PV method, is the simplest procedure. Using excess solution (XS) requires substantial soaking time and additional preparative steps. Using smaller volumes of solution than the pore volume, e.g., the H P V method, would require a device for applying the desired amount of solution to each particle. In the present discussion chromic nitrate is regarded as being strongly adsorbed on alumina and chromic oxide as weakly adsorbed. A . To obtain a concentration profile with only a gradual decrease from periphery to center, a weakly adsorbed solute and the PV method should be used. Increasing the concentration of impregnant, increases the entire concentration curve b y more or less equal amounts, and the same effect is obtained by multiple impregnations by the PV method with drying and calcination between impregnations. Multiple impregnations would be useful when the solubility of the impregnant is small. The XS method introduces more of the active component than the PV procedure, but uniform concentrations were not obtained even after 9 days of soaking for chromic acid on alumina. B. T o obtain a sharply decreasing gradient of concentration from the outside to the center, solutions of a strongly adsorbed solute should be used in the PV method, or either type of solute can be used in the H P V method. I n the PV method with strongly adsorbed solutes, none of the active
component reaches the central part of the particle; however, increasing concentrations of solute increased both the concentration in the outer portion and the depth of penetration of the active component in the support. For the XS method with a strongly adsorbed solute, the active component penetrated to the center of the particle; however, the concentration profile in the outer parts of the particle were essentially the same as t h a t obtained by the PV method. Usually the XS method offers little advantage over the PV method, presumably because diffusion in the solution in pores is very slow. Most, but not all, of the effects of t h e XS method can be achieved with the PV method by changing concentrations of solution or multiple impregnations. Using smaller volumes of solution than required to fill the pore volume, e.g., the H P V method, seems to be a positive way of depositing the active material in the outer portions of the support. Finally, we note that many other factors than concentration profiles are also important in the preparation of impregnated catalysts, such as the presence of particular chemical species, crystallite size, dispersion of the active component on the support, and the absence of certain impurities. literature Cited
Bridges, J. M., MacIver, D. S., Tobin, H. H., Actes Congr. Znl. Catal., 1960, 2 (1961). Brunauer, S., Emmett, P. H., Teller, E., J . Amer. Chem. SOC., 60, 309 (1938).
Chen, H. C., Ph.D. Thesis, Department of Chemical Engineering, McMaster University, May 1972. Chen, H. C., Anderson, R. B., J . Colloid Znterface Sci., 38, 535 f1972). - -,\ - -
Ciapetta, F. G., Hunter, J. B., Ind. Eng. Chem., 45, 147 (1953). Ciapetta, F. G., Plank, C. J., Catalysis, 1 , 315 (1953). Dalton, R. W., McClanahan, J. L., Maatman, R. W., J . Colloid Sci., ’17, 207 ’(1962). Eischens, R. P., Selwood, P. W., J . Amer. Chem. Soc., 69, 1590, 2698 (1947); 70, 2271 (1948).
Fischer, W., Kulling, A., 2. Elektrochem., 60, 680 (1956). Freel, J., Pieters, W. J. M., Anderson, R. B., J . Catal., 16, 281
-,.
1-1”~. 7 n ) \
Haworth, C. W., “Summer School in Electron Microscopy and Microprobe Analysis,” Department of Metallurgy and Materials Science, McMaster University, 1968. Hershey, H. C., Zakin, J. L., Simha, R., Znd. Eng. Chew, Fundam. 6,413 (1967).
Kempling, J. C., Anderson, R. B., Znd. Eng. Chem., Process Des. Develop., 9, 116 (1970).
Lippens, B. C., Linsen, B. G., De Boer, J. H., J . Catal., 3, 32 (1964).
Maatman, R. W., Znd. Eng. Chem., 51, 913 (1959). Maatman, R. W., Prater, C. D., ibid., 49, 253 (1957). Matsunaga, Y., Bull. Chem. SOC.Jap., 30, 868 (1957); 31, 58, 745 (1958).
Nightingale, E. J., Jr., J . Phys. Chem., 63, 1381 (1959). Poole, C. P., Jr., MacIver, D. S., Aduan. Catal., 17, 223 (1967). Satterfield, C. N., Ind. Eng. Chem., 61, No. 6, 4 (1969). Umland, F., 2. Elektrochem., 60, 689, 701, 711 (1956). Weller, S.,Volta, 9. E., J . Amer. Chem. soc., 76, 4695, 4701 (1954).
RECEIVED for review October 5, 1972 ACCEPTEDJanuary 22, 1973 The authors are pleased to acknowledge fellowship and operating funds provided by the National Research Council of Canada.
Ind. Eng. Chem. Prod. Res. Develop., Vol. 12, No. 2, 1973
127
