- Email: [email protected]
Preparation of Metal Distributions within Catalyst Supports
B, Delmon, P, Grange, P,A, Jacobs and G. Poncelet (Editors), Preparation of Catalysts IV © 1987 Elsevier Science Publishers B.V., Amsterdam - Printed in The Netherlands
PREPARATION OF METAL DISTRIBUTIONS WITHIN CATALYST SUPPORTS M.S. Heisel and J.A. Schwarz 2 lCurrent Address W.R. Grace and Co. 2Department of Chemical Engineering and Materials Science, Syracuse University, Syracuse, NY 13244 SUMMARY The objective of catalyst design is to obtain the optimum metal profile for a particular reaction system. This is accomplished by the addition of certain ingredients to the impregnating solution, which are selected largely on the basis of empirical evidence. This paper proposes a classification scheme for these ingredients founded on three predominant interfacial effects. The first class of ingredients affects the electrostatics of the solution near the surface of the support. Ingredients in the second class alter the pH of the solution and consequently the potential of the support surface. The third class includes ingredients that adsorb onto the support and compete with the catalytic metal for adsorption sites. The conceptual classification scheme developed allows one to predict adsorption profiles for both uniform and nonuniform metal distributions. The concepts are completely general and thus provide a theoretical as well as a practical basis for the preparation of catalysts. INTRODUCTION Studies of the relationship between catalytic performance and metal profiles have been primarily directed towards the oxidation of carbon monoxide and hydrocarbons in automobile exhaust. Improvements in the activity and poison resistance of oxidation catalysts have been achieved by modifying the depth of the metal impregnation in the catalyst support (1-4). One way to alter the distribution of metal in the support is to add ingredients to the impregnating solution. Maatman (5) showed that the impregnation profile of hexachloroplatinic acid on alumina could be changed from an eggshell profile to a uniform profile by adding HC1, HN03, or various inorganic nitrates to the impregnating solution. Similarly, Benesi, Curtis, and Studer (6) demonstrated that the adsorption profiles of metal cations could be altered by changing the pH of solution. The first comprehensive study on the effects of adding various chemical ingredients to the impregnating solution was performed by Shyr and Ernst (7). They obtained an eggshell profile for the adsorption of hexachloroplatinic acid on gamma-alumina in the absense of other ingredients. The individual addition of fourteen salts and acids produced nine distinct adsorption profiles. This illustrates the diversity which can be achieved through the
2
addition of ingredients to the impregnating solution, and reflects the need for an understanding of the physical and chemical phenomena involved in the impregnation process. EXPERIMENTAL This section describes two experimental techniques that were employed to determine the metal profile and the effects of added ingredients on this profile. These procedures will allow for a qualitative validation of the classification scheme developed in the folloWing section. Impregnation Profile Experiments The impregnation procedure shown in Figure 1 involves contacting a smooth end of a dried gamma-alumina pellet with a solution of hexach1oroplatinic acid and the ingredient under study. The solution is drawn up by capillarity and the ingredients are adsorbed onto the outer channels or pores of the pellet.
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~,~ SOLUTION
CALCINATIO'I
4i1O't
J
I'tCIllCJW'HIC
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MICROQENS/lllMETER TRACINGS
.-
Fig. 1. Impregnation technique including steps (a) impregnation of alumina pellets with solution containin9 the active ingredient,(b) calcination of the platinum/alumina catalyst, (c) photography of the calcined sample, and (d) transmission results from the microdensitomer tracing. Solutions of hexach1orop1atinic acid were prepared by dissolving H2PtC16'(6H20), obtained from Eng1ehard Industries in deionized and distilled water. Experiments were performed in the dark, since significant decomposition of chloroplatinate was observed in room light in less than an hour. The support used was cylindrical gamma-alumina extrudate characterized as follows: 0.3 cm, diameter; 190 m2/g, surface area; 0.68 cm 3/g, pore volume; 2.25x10-6 cm, pore radius.
3
The adsorbed platinate colors the white support yellow, which turns dark gray upon calcination. Calcination is carried out for four hours at 450 degrees C, with a low initial heating rate to minimize metal displacement in this step. The intensity of the color of the support is directly related to the amount of platinum adsorbed. Photographic negatives of the calcined pellets were then taken, which show adsorbed platinum as white. A scanning microdensitometer was then used to analyze the negatives. The extent of transmission was plotted as a function of axial distance from the dipped end of the pellet. The percent transmission is directly related to the platinum concentration and the area under the curve is proportional to the total amount of platinum adsorbed. In all experiments, 20.0 ml of solution was contacted with the alumina pellets for four hours. Additional ingredients were checked to make sure that they did not color the support upon calcination. Ingredients which did color the support (i.e. large organic acids) were reduced in flowing hydrogen for four hours. The photography was done with a platinum standard and a control (an unimpregnated pellet). This ensured that the experimentally measured profiles would be unaffected by minor differences in the transmission results of the photographic negatives. For each experiment, four pellets were scanned and the tracings were averaged to obtain a composite profile. Adsorption Experiments Experiments were run at the pore-filling time of twenty minutes with no prior heat treatment. One hundred mg of dry alumina pellets was added to various impregnating solutions while the platinum concentration in solution was monitored with a Beckman DB-GT spectrophotometer. Ultraviolet scans were run at 262 nm for the platinum (IV) complex (8). The amount adsorbed was determined by the difference between initial and final concentrations. CLASSIFICATION SCHEME It is proposed that ingredients added to the impregnating solution can be classified according to their effect on three interfacial phenomena. The first class of ingredients consists of simple inorganic electrolytes such as NaN03' NaCl, and CaC12' which affect the electrostatics at the solution-surface interface. The second class of ingredients includes simple inorganic acids and bases such as HC1, HN03' and NaOH, which affect the pH of the system. These compounds alter the chemistry of the surface by changing the surface potential. The interfacial effects associated with the first two classes are not the result of specific adsorption. Instead, the affinity of the metal ion for the
4
surface is altered by changing the number of available surface sites in the case of class two and by changing the accessibility of the metal to those sites in the case of class one. The third type of ingredient is one that can compete with the metal ion for possible adsorption sites. Although many compounds will adsorb onto the surface, the strongest and most effective are those that contain hydroxyl, carboxyl, and phosphoryl groups. If this type of ingredient is added to the impregnating solution. it will affect the metal adsorption in a chromatographic manner. These ingredients can also introduce significant pH and electrostatic effects into the system. Class 1 Ingredients Simple inorganic salts such as NaND3. NaCl. and CaC12 do not adsorb strongly enough on alumina to compete with the platinum ion for adsorption sites (9). It is therefore apparent that the cations and anions of these salts have a higher affinity for the aqueous phase. They modify the adsorption of platinum by altering the charge distribution near the surface of the support. Adsorption experiments were run at an initial platinum concentration of 5.4xlO-4M. With no Class 1 additions, approximately 0.55 wt% platinum was deposited on the alumina. Four different Class 1 ingredients were added to the impregnating solution: sodium chloride, sodium nitrate, calcium chloride, and calcium nitrate. Figure 2 shows that the addition of NaCl and NaN03. univalent Class 1 ingredients. produces similar effects on platinum adsorption. Since at any given concentration these 1:1 electrolytes introduce the same amount of electrostatics into the solution, they should produce the same effect on the amount of platinum adsorbed. The effects of the 2:1 electrolytes, CaC12 and Ca(ND3)2, are shown in Figure 3. As expected. these ingredients show similar results. For a given concentration, the 2:1 electrolytes show much less platinum adsorption than the 1:1 electrolytes. The 2:1 electrolytes do not show any significant platinum adsorption up to 1.OxlO-1M. This discrepancy can be accounted for by Poisson-Boltzmann theory. The extension of the electric field of the surface into the bulk solution is determined by the ionic strength of the solution, varying inversely with its square root. Because the ionic strength of a 2:1 electrolyte is three times that of a 1:1 electrolyte for a given concentration. a 2:1 electrolyte is a more effective site blocking agent. Figure 4 shows the dependence of the amount of platinum adsorbed on the ionic strength of solution for Class 1 ingredients.
5
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Fig. 2. Effect of univalent Class 1 ingredients on the amount of platinum adsorption on gamma-alumina pellets. Initial hexachloroplatinic acid concentration of 5.46xlO- 4 molar.
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2
4
6
8
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Fig. 3. Effect of divalent Class 1 ingredients on the amount of platinum adsorption on gamma-alumina pellets.
6
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08
0.7
0.6
0.5
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Fig. 4. Relation of the mean activity coefficient of the platinum ion in solution and the amount of platinum adsorbed on gamma-alumina pellets. Initial hexachloroplatinic acid concentration of 5.46xlO-4 molar. The experimental profiles for Class 1 ingredients are shown in Figures Sa-Sc. The initial concentration of platinum was fixed at 2.SxlO- 3M. Figure Sa shows the platinum standard, no Class 1 ingredient added. As NaN03 is added to the solution, the amount of platinum adsorbed decreases and the profiles became uniform. The electrolytes effectively decrease the number of active sites on the alumina surface by electrically screening them from the bulk solution. This effort occurs uniformly down the length of the support pore, causing the platinum coating to be thinner and thus extend deeper into the pore.
7
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Fig. 5. Experimental platinum adsorption profiles produced by impregnating gamma-alumina pellets with an initial hexachloroplatinic acid concentration of 2.5xlO-3 molar and various initial concentrations of sodium nitrate: (a) C02 = 0.0 - platinum - platinum standard, (b) C02 = 2.5xlO-3 molar, and C02 = 2.5xlO-2 molar. Class 2 Ingredients Work done by Maatman (5) showed that uniform profiles could be obtained for platinum deposition from hexachloroplatinic acid on an alumina support by adding simple inorganic acids such as HCl and HN03' It was assumed that the anions competed with the platinum ion for adsorption sites. However, no significant chloride or nitrate ion adsorption can be measured on alumina. Since these anions are not binding to the surface, the pH and electrostatics in the system are affecting the platinum distribution.
8
Figure 6 shows the amount of platinum adsorption as a function of the pH of the solution. The amount of platinum adsorbed drops off rapidly with the addition of base to the system. The zero point of charge occurs at a pH of 8.2 (10). No adsorption occurs past this point. Although the alumina surface is positively charged below a pH of about eight, at a pH of three or less, the dissolution of alumnina is significant (11). High concentrations of acids decrease the amount of platinum adsorbed by decreasing the number of active sites on the alumina surface. As expected, the addition of HCl and HN03 show the same results since they both introduce the same pH and electrostatic effects to the solution.
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9
Figures 7a-7d shows the effects on the platinum profile caused by the addition of base to the solution. Up to the neutralization point, a hydroxide ion concentration of 5xlO- 3M, the profiles resemble that of the platinum standard. The height and the length of penetration decrease slightly with increasing base additions. This corresponds to a gradual decrease in the area under the profile, which represents a decrease in the amount of platinum adsorbed. At the acid-base neutralization point, Figure 7c, the shape of the adsorption profile changes rapidly. The shape changes from an eggshell to a linearly decreasing profile. The length of penetration and the amount adsorbed have decreased. Well past the neutralization point, at a concentration of 7.0xlO- 3M, the height of the platinum profile has decreased by a factor of four. A very small amount of platinum has been deposited at the pore mouth. At a concentration of added hydroxide of 1.OxlO- 2M, no platinum is adsorbed.
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Fig. 7. Experimental platinum adsorption profiles produced by impregnating gamma-alumina pellets with an initial hexachloroplatinic acid concentration of 2.5xlO- 3 molar and various initial concentrations of sodium hydroxide: (a) C02 = 0.0 - platinum standard, (b) C02 = 4.5xlO- 3 molar, (c) C02 = 5.0xlO- 3 molar, and (d) C02 = 7.0xlO- 3 molar.
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Fig. 8. Experimental platinum adsorption profiles produced by impregnating gamma-alumina pellets with an initial hexachloroplatinic acid concentration of 2.5xlO-3 molar and various initial concentrations of nitric acid: (a) C02 0.0 - platinum standard, (b) C02 = 5.0xlO-3 molar, and (c) C02 = 1.OxlO- 2 molar. Figures 8a-8c show how the addition of high concentrations of nitric acid affects platinum adsorption profiles. The metal profiles are uniform with a length of penetration that increases with increasing acid concentrations. The addition of an acidic Class 2 ingredient is shown to be an efficient method of producing uniform profiles with a high degree of penetration into the pore. Class 3 Ingredients In the previous sections, it was shown that the electrostatic (Class 1) and pH (Class 2) effects could each be reduced to a single parameter that was independent of the added ingredients. Class 1 and Class 2 additions control the amount of platinum adsorption and the depth that the platinum penetrates into the support pore. These ingredients can be used to obtain eggshell, uniform, or linearly decreasing platinum profiles. However, depending on reaction conditions, a core or other types of nonuniform metal profiles may be desired. These profiles are obtained by the addition of Class 3 ingredients. which complete with the platinum ion for adsorption sites on the alumina surface.
11
Class 3 ingredients that have a higher affinity for the surface than the active ingredient will adsorb to the surface at the front of the pore. As the solution flows down the pore, the concentration ratio of the active ingredient to the added ingredient becomes large, and the active ingredient will adsorb. In this manner the amount of Class 3 ingredient added and its relative affinity for the surface controls the distribution of the active ingredient. Whether the Class 3 ingredients are added as acids, bases, or salts will affect the resulting adsorption profile by introducing significant Class 1 or Class 2 effects into the system. Figures 9a-9c show the effect of adding a competing phosphate ion to the impregnating solution. The addition of a small amount of phosphoric acid, Figure 9b, yields a platinum profile that is linearly decreasing. The amount of platinum adsorbed at the pore mouth is suppressed because the phosphate ion has taken up adsorption sites. Farther down the pellet more and more platinum is adsorbed. Here, the amount of phosphate in solution has decreased enough to allow the platinum to compete effectively for adsorption sites.
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Fig. 9. Experimental platinum adsorption profiles produced by impregnating gamma-alumina pellets with an initial hexachloroplatinic acid concentration of 2.5xlO-3 molar and various initial concentrations of phosphoric acid: (a) C02 = 0.0 - platinum standard, (b) C02 = 3.7xlO-3 molar, (c) C02 = 7.4xlO-3 molar.
12
At a much higher phosphate concentration, Figure 9c, the length of the platinum profile is about twice as great as that of the platinum standard, Figure 9a; the platinum distribution shows a long uniform profile with a sharp peak at the end. These experimental profiles illustrate the wide variety of profiles that can be produced by a Class 3 ingredient. Changing the ratio of the amount of Class 3 ingredient to active ingredient can control the amount of adsorption. the depth of penetration. and the overall shape of the profile. CONCLUSIONS The classification scheme proposed in this study divides the ingredients in an impregnating solution according to their effects on the impregnation process. By relating the changes in metal distributions to measurable solution parameters, adsorption-transport modeling can be developed. In conclusion, the proposal developed here provides a methodology for producing a wide variety of metal-supported catalysts. The ultimate goal is to relate impregnation modeling to reaction kinetics. This would then allow for the prediction of catalytic behavior based on the composition of the impregnating solution. ACKNOWLEDGEMENT One of us (J.A.S.) acknowledges the support of the Division of Chemical Sciences, U.S. Department of Energy, Basic Energy Science under Contract DE-AC02-84ER13158 during the preparation of this manuscript. REFERENCES 1 E.R. Becker and J. Wei, J. Catal. 46, 365 (1977). 2 E.R. Becker and J. Wei, J. Catal. 46, 372 (1977). 3 D.P. McArthur, Advan. Chern. Ser. 143, 85 (1975). 4 J.C. Summers and L.L. Hegedus, J. Catal. 51, 185 (1978). 5 R.W. Maatman, Ind. Eng. Chern. 51(8), 913 (1959). 6 H.A. Benesi, R.M. Curtis and H.P. Studer, J. Catal. 10, 328 (1968). 7 Y. Shyr and W.R. Ernst, J. Catal. 63,425 (1980). 8 F.R. Harty, The Chemistry of Platinum and Palladium, Wiley-Interscience, New York, 1973. 9 H.L. Bohn, B.L. McNeal and G.A. O'Connor, Soil Chemistry, Wiley-Interscience, New York, 1979. 10 J.A. Schwarz, C.T. Driscoll and A.K. Bhanot, J. Colloid Inter. Sci., Vol. 97, No.1, Jan. 1984. 11 H.M. May, P.A. Helmke and M.L. Jackson, Geochim. Cosmochim Acta 43, 861 (1979).
13
DISCUSSION B. DELMON : This refers to your class I ingredients: Na, Ca are often unwanted. They permanently modify the support. Volatile ions are preferred NHt, substituted ammonium, N0 3, organic acid anions. Van der Waals adsorption on the support modifies its apparent charge. The effect of those ions corresponds to what you describe correctly for class I ingredients, but with additional effects. One has sort of second order effects. One might also say that those ions are somehow situated between class I and class III. I suppose you observed that influence of atomic weight (Rb, Cs, Sr, instead of Na, Cal or molecular weight. Could you comment on that remark? May I add that I like very much your classification. J.A. SCHWARZ: Your question is indeed relevant and the points you raise demonstrate the limitations of any universal classification scheme for predicting the distribution of catalyst precursors within porous supports. In addition to your comments regarding "size" effects, there are other effects which I did not have time to comment upon which further demonstrate the subtle overlapping of the induced effects of added ingredients. For example, NaBr would normally be considered a class I ingredient. However, bromide ions exhange readily with chloride in the PtC1 62 anion, thus giving rise to speciation effects; each of the chloro-bromo complexes absorb at different rates. If the catalysis community is willing to recognize that there will always be exceptions to the rule, I do think that the proposed classification scheme does provide a useful guideline for manipulating the position of catalytic metal precursor within the support structure. G.M. PAJONK : As, in general, catalysts are rarely monomodal in their pore radii distribution, do you think that this distribution will exert an influence beside your three classes of added ingredients? J.A. SCHWARZ: We have confined our studies to a single catalyst support which has a specific pore size distribution. This has eliminated any confusion that might arise when trying to compare impregnation profiles obtained from different supports under similar experimental impregnation conditions. The answer to your question is that catalysts with different pore size distributions will likely lead to qualitatively different profiles. The single pore model assumes the support is comprised of pores with a constant radius, R. The convective transport is modeled by the methods described by Washburn (Phys. Rev. 17, 273 (1921)) which assumes that the flow of the liquid column passes througn an infinite succession of steady states. The expression for the radial average velocity of the liquid front is V p
= ~ (6~)~
t-~
where the pressure drop is a constant and equals 2A/R; A is the surface tension of the solution; ~ is the solution viscosity; and R is the pore radius. Thus the fluid velocity depends upons R~ and of course will change the qualitative behavior of the metal distributions. I might mention that we have carried out a similar study of the impregnation of Pt(NH4)4(OH)2 onto a high surface area silica and have demonstrated that the effect of a cnange in average pore size did not influence the predicted metal distribution based on the proposed classification scheme.
PREPARATION OF METAL DISTRIBUTIONS WITHIN CATALYST SUPPORTS M.S. Heisel and J.A. Schwarz 2 lCurrent Address W.R. Grace and Co. 2Department of Chemical Engineering and Materials Science, Syracuse University, Syracuse, NY 13244 SUMMARY The objective of catalyst design is to obtain the optimum metal profile for a particular reaction system. This is accomplished by the addition of certain ingredients to the impregnating solution, which are selected largely on the basis of empirical evidence. This paper proposes a classification scheme for these ingredients founded on three predominant interfacial effects. The first class of ingredients affects the electrostatics of the solution near the surface of the support. Ingredients in the second class alter the pH of the solution and consequently the potential of the support surface. The third class includes ingredients that adsorb onto the support and compete with the catalytic metal for adsorption sites. The conceptual classification scheme developed allows one to predict adsorption profiles for both uniform and nonuniform metal distributions. The concepts are completely general and thus provide a theoretical as well as a practical basis for the preparation of catalysts. INTRODUCTION Studies of the relationship between catalytic performance and metal profiles have been primarily directed towards the oxidation of carbon monoxide and hydrocarbons in automobile exhaust. Improvements in the activity and poison resistance of oxidation catalysts have been achieved by modifying the depth of the metal impregnation in the catalyst support (1-4). One way to alter the distribution of metal in the support is to add ingredients to the impregnating solution. Maatman (5) showed that the impregnation profile of hexachloroplatinic acid on alumina could be changed from an eggshell profile to a uniform profile by adding HC1, HN03, or various inorganic nitrates to the impregnating solution. Similarly, Benesi, Curtis, and Studer (6) demonstrated that the adsorption profiles of metal cations could be altered by changing the pH of solution. The first comprehensive study on the effects of adding various chemical ingredients to the impregnating solution was performed by Shyr and Ernst (7). They obtained an eggshell profile for the adsorption of hexachloroplatinic acid on gamma-alumina in the absense of other ingredients. The individual addition of fourteen salts and acids produced nine distinct adsorption profiles. This illustrates the diversity which can be achieved through the
2
addition of ingredients to the impregnating solution, and reflects the need for an understanding of the physical and chemical phenomena involved in the impregnation process. EXPERIMENTAL This section describes two experimental techniques that were employed to determine the metal profile and the effects of added ingredients on this profile. These procedures will allow for a qualitative validation of the classification scheme developed in the folloWing section. Impregnation Profile Experiments The impregnation procedure shown in Figure 1 involves contacting a smooth end of a dried gamma-alumina pellet with a solution of hexach1oroplatinic acid and the ingredient under study. The solution is drawn up by capillarity and the ingredients are adsorbed onto the outer channels or pores of the pellet.
r
IMPREGNATION
~,~ SOLUTION
CALCINATIO'I
4i1O't
J
I'tCIllCJW'HIC
NEGATIII£S
1
MICROQENS/lllMETER TRACINGS
.-
Fig. 1. Impregnation technique including steps (a) impregnation of alumina pellets with solution containin9 the active ingredient,(b) calcination of the platinum/alumina catalyst, (c) photography of the calcined sample, and (d) transmission results from the microdensitomer tracing. Solutions of hexach1orop1atinic acid were prepared by dissolving H2PtC16'(6H20), obtained from Eng1ehard Industries in deionized and distilled water. Experiments were performed in the dark, since significant decomposition of chloroplatinate was observed in room light in less than an hour. The support used was cylindrical gamma-alumina extrudate characterized as follows: 0.3 cm, diameter; 190 m2/g, surface area; 0.68 cm 3/g, pore volume; 2.25x10-6 cm, pore radius.
3
The adsorbed platinate colors the white support yellow, which turns dark gray upon calcination. Calcination is carried out for four hours at 450 degrees C, with a low initial heating rate to minimize metal displacement in this step. The intensity of the color of the support is directly related to the amount of platinum adsorbed. Photographic negatives of the calcined pellets were then taken, which show adsorbed platinum as white. A scanning microdensitometer was then used to analyze the negatives. The extent of transmission was plotted as a function of axial distance from the dipped end of the pellet. The percent transmission is directly related to the platinum concentration and the area under the curve is proportional to the total amount of platinum adsorbed. In all experiments, 20.0 ml of solution was contacted with the alumina pellets for four hours. Additional ingredients were checked to make sure that they did not color the support upon calcination. Ingredients which did color the support (i.e. large organic acids) were reduced in flowing hydrogen for four hours. The photography was done with a platinum standard and a control (an unimpregnated pellet). This ensured that the experimentally measured profiles would be unaffected by minor differences in the transmission results of the photographic negatives. For each experiment, four pellets were scanned and the tracings were averaged to obtain a composite profile. Adsorption Experiments Experiments were run at the pore-filling time of twenty minutes with no prior heat treatment. One hundred mg of dry alumina pellets was added to various impregnating solutions while the platinum concentration in solution was monitored with a Beckman DB-GT spectrophotometer. Ultraviolet scans were run at 262 nm for the platinum (IV) complex (8). The amount adsorbed was determined by the difference between initial and final concentrations. CLASSIFICATION SCHEME It is proposed that ingredients added to the impregnating solution can be classified according to their effect on three interfacial phenomena. The first class of ingredients consists of simple inorganic electrolytes such as NaN03' NaCl, and CaC12' which affect the electrostatics at the solution-surface interface. The second class of ingredients includes simple inorganic acids and bases such as HC1, HN03' and NaOH, which affect the pH of the system. These compounds alter the chemistry of the surface by changing the surface potential. The interfacial effects associated with the first two classes are not the result of specific adsorption. Instead, the affinity of the metal ion for the
4
surface is altered by changing the number of available surface sites in the case of class two and by changing the accessibility of the metal to those sites in the case of class one. The third type of ingredient is one that can compete with the metal ion for possible adsorption sites. Although many compounds will adsorb onto the surface, the strongest and most effective are those that contain hydroxyl, carboxyl, and phosphoryl groups. If this type of ingredient is added to the impregnating solution. it will affect the metal adsorption in a chromatographic manner. These ingredients can also introduce significant pH and electrostatic effects into the system. Class 1 Ingredients Simple inorganic salts such as NaND3. NaCl. and CaC12 do not adsorb strongly enough on alumina to compete with the platinum ion for adsorption sites (9). It is therefore apparent that the cations and anions of these salts have a higher affinity for the aqueous phase. They modify the adsorption of platinum by altering the charge distribution near the surface of the support. Adsorption experiments were run at an initial platinum concentration of 5.4xlO-4M. With no Class 1 additions, approximately 0.55 wt% platinum was deposited on the alumina. Four different Class 1 ingredients were added to the impregnating solution: sodium chloride, sodium nitrate, calcium chloride, and calcium nitrate. Figure 2 shows that the addition of NaCl and NaN03. univalent Class 1 ingredients. produces similar effects on platinum adsorption. Since at any given concentration these 1:1 electrolytes introduce the same amount of electrostatics into the solution, they should produce the same effect on the amount of platinum adsorbed. The effects of the 2:1 electrolytes, CaC12 and Ca(ND3)2, are shown in Figure 3. As expected. these ingredients show similar results. For a given concentration, the 2:1 electrolytes show much less platinum adsorption than the 1:1 electrolytes. The 2:1 electrolytes do not show any significant platinum adsorption up to 1.OxlO-1M. This discrepancy can be accounted for by Poisson-Boltzmann theory. The extension of the electric field of the surface into the bulk solution is determined by the ionic strength of the solution, varying inversely with its square root. Because the ionic strength of a 2:1 electrolyte is three times that of a 1:1 electrolyte for a given concentration. a 2:1 electrolyte is a more effective site blocking agent. Figure 4 shows the dependence of the amount of platinum adsorbed on the ionic strength of solution for Class 1 ingredients.
5
30 o NoCI
I::. NoN~
o
o
I::. 0 Q>
Fig. 2. Effect of univalent Class 1 ingredients on the amount of platinum adsorption on gamma-alumina pellets. Initial hexachloroplatinic acid concentration of 5.46xlO- 4 molar.
30 • cocI 2 4CO(N~12
o o
I
2
4
6
8
10
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Fig. 3. Effect of divalent Class 1 ingredients on the amount of platinum adsorption on gamma-alumina pellets.
6
o'--_.......
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0.7
0.6
0.5
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0.3
Fig. 4. Relation of the mean activity coefficient of the platinum ion in solution and the amount of platinum adsorbed on gamma-alumina pellets. Initial hexachloroplatinic acid concentration of 5.46xlO-4 molar. The experimental profiles for Class 1 ingredients are shown in Figures Sa-Sc. The initial concentration of platinum was fixed at 2.SxlO- 3M. Figure Sa shows the platinum standard, no Class 1 ingredient added. As NaN03 is added to the solution, the amount of platinum adsorbed decreases and the profiles became uniform. The electrolytes effectively decrease the number of active sites on the alumina surface by electrically screening them from the bulk solution. This effort occurs uniformly down the length of the support pore, causing the platinum coating to be thinner and thus extend deeper into the pore.
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I
0.8
~
u 0.6 ..J
~ 2 0.4 I-
u
oct
a:
"- 0.2
0.2
0.3
0.4
AXIAL LENGTH
Fig. 5. Experimental platinum adsorption profiles produced by impregnating gamma-alumina pellets with an initial hexachloroplatinic acid concentration of 2.5xlO-3 molar and various initial concentrations of sodium nitrate: (a) C02 = 0.0 - platinum - platinum standard, (b) C02 = 2.5xlO-3 molar, and C02 = 2.5xlO-2 molar. Class 2 Ingredients Work done by Maatman (5) showed that uniform profiles could be obtained for platinum deposition from hexachloroplatinic acid on an alumina support by adding simple inorganic acids such as HCl and HN03' It was assumed that the anions competed with the platinum ion for adsorption sites. However, no significant chloride or nitrate ion adsorption can be measured on alumina. Since these anions are not binding to the surface, the pH and electrostatics in the system are affecting the platinum distribution.
8
Figure 6 shows the amount of platinum adsorption as a function of the pH of the solution. The amount of platinum adsorbed drops off rapidly with the addition of base to the system. The zero point of charge occurs at a pH of 8.2 (10). No adsorption occurs past this point. Although the alumina surface is positively charged below a pH of about eight, at a pH of three or less, the dissolution of alumnina is significant (11). High concentrations of acids decrease the amount of platinum adsorbed by decreasing the number of active sites on the alumina surface. As expected, the addition of HCl and HN03 show the same results since they both introduce the same pH and electrostatic effects to the solution.
E0 ~
~40 III
0 NoOH 0 HCl
g Q.l
/:1 HN~
.5- 30
CJ:)
z
0
i=
ft20
~
0 <1 ~
:::>
z
i=
<1 ...J
Q.
0
10
0
0
4
6
pH
8
10
12
Fig. 6. Effect of acidic and basic Class 2 ingredients on the amount of platinum adsorbed on gamma-alumina pellets. Initial hexachloroplatinic acid concentration of 5.46xlO-4 molar.
9
Figures 7a-7d shows the effects on the platinum profile caused by the addition of base to the solution. Up to the neutralization point, a hydroxide ion concentration of 5xlO- 3M, the profiles resemble that of the platinum standard. The height and the length of penetration decrease slightly with increasing base additions. This corresponds to a gradual decrease in the area under the profile, which represents a decrease in the amount of platinum adsorbed. At the acid-base neutralization point, Figure 7c, the shape of the adsorption profile changes rapidly. The shape changes from an eggshell to a linearly decreasing profile. The length of penetration and the amount adsorbed have decreased. Well past the neutralization point, at a concentration of 7.0xlO- 3M, the height of the platinum profile has decreased by a factor of four. A very small amount of platinum has been deposited at the pore mouth. At a concentration of added hydroxide of 1.OxlO- 2M, no platinum is adsorbed.
10
1,0 (bl
(a)
wO,8
08
w 6°6 u
0,6
~0,4
0,4
~ a:
.
...J
>= ~
If 0,2
0,2
0.° 0
0,/
0,0
02
°
01
PORE AXIAL LENGTH
I.
Idl
(c)
0,8 0,6
;J. z
. e:
0,4 -
Q04 tu 0.2 0,0
0,2
°
0,3
0.4
00
°
0,1
0,2
03
0,4
PORE AXIAL LENGTH
Fig. 7. Experimental platinum adsorption profiles produced by impregnating gamma-alumina pellets with an initial hexachloroplatinic acid concentration of 2.5xlO- 3 molar and various initial concentrations of sodium hydroxide: (a) C02 = 0.0 - platinum standard, (b) C02 = 4.5xlO- 3 molar, (c) C02 = 5.0xlO- 3 molar, and (d) C02 = 7.0xlO- 3 molar.
10
1.0
'"cr:
0
CJ ..J
z
0
;:: CJ
cr: "-
(0)
1.0
(bl
10 -
08
0.8
06
0.6
0.4
0.4
0.2
02
0.0
0 0.1
0.2 0.3 0.4 0.5
0.0 0
(e)
or
02
0.3
0.4 0.5
PORE AXIAL I£NGTH
Fig. 8. Experimental platinum adsorption profiles produced by impregnating gamma-alumina pellets with an initial hexachloroplatinic acid concentration of 2.5xlO-3 molar and various initial concentrations of nitric acid: (a) C02 0.0 - platinum standard, (b) C02 = 5.0xlO-3 molar, and (c) C02 = 1.OxlO- 2 molar. Figures 8a-8c show how the addition of high concentrations of nitric acid affects platinum adsorption profiles. The metal profiles are uniform with a length of penetration that increases with increasing acid concentrations. The addition of an acidic Class 2 ingredient is shown to be an efficient method of producing uniform profiles with a high degree of penetration into the pore. Class 3 Ingredients In the previous sections, it was shown that the electrostatic (Class 1) and pH (Class 2) effects could each be reduced to a single parameter that was independent of the added ingredients. Class 1 and Class 2 additions control the amount of platinum adsorption and the depth that the platinum penetrates into the support pore. These ingredients can be used to obtain eggshell, uniform, or linearly decreasing platinum profiles. However, depending on reaction conditions, a core or other types of nonuniform metal profiles may be desired. These profiles are obtained by the addition of Class 3 ingredients. which complete with the platinum ion for adsorption sites on the alumina surface.
11
Class 3 ingredients that have a higher affinity for the surface than the active ingredient will adsorb to the surface at the front of the pore. As the solution flows down the pore, the concentration ratio of the active ingredient to the added ingredient becomes large, and the active ingredient will adsorb. In this manner the amount of Class 3 ingredient added and its relative affinity for the surface controls the distribution of the active ingredient. Whether the Class 3 ingredients are added as acids, bases, or salts will affect the resulting adsorption profile by introducing significant Class 1 or Class 2 effects into the system. Figures 9a-9c show the effect of adding a competing phosphate ion to the impregnating solution. The addition of a small amount of phosphoric acid, Figure 9b, yields a platinum profile that is linearly decreasing. The amount of platinum adsorbed at the pore mouth is suppressed because the phosphate ion has taken up adsorption sites. Farther down the pellet more and more platinum is adsorbed. Here, the amount of phosphate in solution has decreased enough to allow the platinum to compete effectively for adsorption sites.
10
10
10 (0
w
I 08
'"
(c)
(b)
0.8
II:
W
0.6
~ 06 u
..J
~
>=
u
II:
"-
04
0.4 0.2
0.2 00
0
02 00
0
01
PORE AXIAL LENGTH
Fig. 9. Experimental platinum adsorption profiles produced by impregnating gamma-alumina pellets with an initial hexachloroplatinic acid concentration of 2.5xlO-3 molar and various initial concentrations of phosphoric acid: (a) C02 = 0.0 - platinum standard, (b) C02 = 3.7xlO-3 molar, (c) C02 = 7.4xlO-3 molar.
12
At a much higher phosphate concentration, Figure 9c, the length of the platinum profile is about twice as great as that of the platinum standard, Figure 9a; the platinum distribution shows a long uniform profile with a sharp peak at the end. These experimental profiles illustrate the wide variety of profiles that can be produced by a Class 3 ingredient. Changing the ratio of the amount of Class 3 ingredient to active ingredient can control the amount of adsorption. the depth of penetration. and the overall shape of the profile. CONCLUSIONS The classification scheme proposed in this study divides the ingredients in an impregnating solution according to their effects on the impregnation process. By relating the changes in metal distributions to measurable solution parameters, adsorption-transport modeling can be developed. In conclusion, the proposal developed here provides a methodology for producing a wide variety of metal-supported catalysts. The ultimate goal is to relate impregnation modeling to reaction kinetics. This would then allow for the prediction of catalytic behavior based on the composition of the impregnating solution. ACKNOWLEDGEMENT One of us (J.A.S.) acknowledges the support of the Division of Chemical Sciences, U.S. Department of Energy, Basic Energy Science under Contract DE-AC02-84ER13158 during the preparation of this manuscript. REFERENCES 1 E.R. Becker and J. Wei, J. Catal. 46, 365 (1977). 2 E.R. Becker and J. Wei, J. Catal. 46, 372 (1977). 3 D.P. McArthur, Advan. Chern. Ser. 143, 85 (1975). 4 J.C. Summers and L.L. Hegedus, J. Catal. 51, 185 (1978). 5 R.W. Maatman, Ind. Eng. Chern. 51(8), 913 (1959). 6 H.A. Benesi, R.M. Curtis and H.P. Studer, J. Catal. 10, 328 (1968). 7 Y. Shyr and W.R. Ernst, J. Catal. 63,425 (1980). 8 F.R. Harty, The Chemistry of Platinum and Palladium, Wiley-Interscience, New York, 1973. 9 H.L. Bohn, B.L. McNeal and G.A. O'Connor, Soil Chemistry, Wiley-Interscience, New York, 1979. 10 J.A. Schwarz, C.T. Driscoll and A.K. Bhanot, J. Colloid Inter. Sci., Vol. 97, No.1, Jan. 1984. 11 H.M. May, P.A. Helmke and M.L. Jackson, Geochim. Cosmochim Acta 43, 861 (1979).
13
DISCUSSION B. DELMON : This refers to your class I ingredients: Na, Ca are often unwanted. They permanently modify the support. Volatile ions are preferred NHt, substituted ammonium, N0 3, organic acid anions. Van der Waals adsorption on the support modifies its apparent charge. The effect of those ions corresponds to what you describe correctly for class I ingredients, but with additional effects. One has sort of second order effects. One might also say that those ions are somehow situated between class I and class III. I suppose you observed that influence of atomic weight (Rb, Cs, Sr, instead of Na, Cal or molecular weight. Could you comment on that remark? May I add that I like very much your classification. J.A. SCHWARZ: Your question is indeed relevant and the points you raise demonstrate the limitations of any universal classification scheme for predicting the distribution of catalyst precursors within porous supports. In addition to your comments regarding "size" effects, there are other effects which I did not have time to comment upon which further demonstrate the subtle overlapping of the induced effects of added ingredients. For example, NaBr would normally be considered a class I ingredient. However, bromide ions exhange readily with chloride in the PtC1 62 anion, thus giving rise to speciation effects; each of the chloro-bromo complexes absorb at different rates. If the catalysis community is willing to recognize that there will always be exceptions to the rule, I do think that the proposed classification scheme does provide a useful guideline for manipulating the position of catalytic metal precursor within the support structure. G.M. PAJONK : As, in general, catalysts are rarely monomodal in their pore radii distribution, do you think that this distribution will exert an influence beside your three classes of added ingredients? J.A. SCHWARZ: We have confined our studies to a single catalyst support which has a specific pore size distribution. This has eliminated any confusion that might arise when trying to compare impregnation profiles obtained from different supports under similar experimental impregnation conditions. The answer to your question is that catalysts with different pore size distributions will likely lead to qualitatively different profiles. The single pore model assumes the support is comprised of pores with a constant radius, R. The convective transport is modeled by the methods described by Washburn (Phys. Rev. 17, 273 (1921)) which assumes that the flow of the liquid column passes througn an infinite succession of steady states. The expression for the radial average velocity of the liquid front is V p
= ~ (6~)~
t-~
where the pressure drop is a constant and equals 2A/R; A is the surface tension of the solution; ~ is the solution viscosity; and R is the pore radius. Thus the fluid velocity depends upons R~ and of course will change the qualitative behavior of the metal distributions. I might mention that we have carried out a similar study of the impregnation of Pt(NH4)4(OH)2 onto a high surface area silica and have demonstrated that the effect of a cnange in average pore size did not influence the predicted metal distribution based on the proposed classification scheme.