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Dispersion of platinum on silica and alumina by chemical extraction
Catalysis, 61 (1990) 75-87 Elsevier Science Publishers B.V., Amsterdam -
75
Applied
Printed in The Netherlands
Dispersion of Platinum on Silica and Alumina by Chemical Extraction YONGNIAN YANG, JINCHENG PAN, NING ZHENG, XINGQUAN LIU and JIAYU ZHANG* Department
of Chemistry,
Peking
University,
Beijing
(Peoples
Republic
of China)
(Received 3 April 1989, revised manuscript received 18 May 1989)
ABSTRACT Chemical extraction combined with transmission electron microscopy (TEM) was used to study the dispersion of platinum in a series of Pt/Si02 and Pt/Al,O, catalysts. With standard EuroPt1 catalyst the results showed that the “percentage extracted” of platinum measured after oxidation at cu. 400 oC was identical with the “percentage exposed’! of platinum evaluated by TEM. Further, interaction between the PtO, formed and the support used could also be explored by this method. A detailed discussion is given of the effects of several factors on the extraction profiles. Keywords: platinum/alumina, (extraction, TEM).
platinum/silica,
EuroPt-1, dispersion, catalyst chracterization
INTRODUCTION
The catalytic properties of metal catalysts in general are closely related to the state of dispersion of the active metals. The need to evaluate the degree of dispersion or “percentage exposed” has led to the development of a variety of methods [ 1,2], one of the most frequently used being hydrogen chemisorption. The main uncertainties of the chemisorption method [ 3-51 include the problem of establishing the actual monolayer coverage and the variability in chemisorption stoichiometry, particularly at sizes < 20 A. Chemisorption is mainly used to determine the number of surface metal atoms. It has been suggested [ 61 that oxidation of surface metal atoms leading to the formation of surface metal ions, followed by extraction, should give a direct measure (in principle) of the number of surface metal atoms without concern for the aforesaid uncertainties of chemisorption. However, reports on the actual application of this method are not available, perhaps owing to the lack of satisfactory results. In this work, chemical extraction combined with transmission electron microscopy (TEM) was used to study the dispersion of platinum in a series of 0166.9834/90/$03.50
0 1990 Elsevier Science Publishers B.V.
76
Pt/SiO, and Pt/A1203 catalysts on a quantitative basis. To establish the feasibility of the extraction method, we used the well known standard catalyst EuroPt-1 (6.3% Pt/SiO,) [7] in order to achieve reliable comparisons. EXPERIMENTAL
Preparation of samples The catalysts tested were standard EuroPt-1, commercial CK303 and several laboratory-made catalysts. The last materials were prepared by wet impregnation of ground alumina or silica with an aqueous solution of platinum compounds, namely H,PtCl, (for Pt/Al,O,) and Pt (NH,),( OH), (for Pt/ SiO*), followed by stirring for 3-4 h at room temperature, drying for 24 h at 110’ C and calcining (only for Pt/A1203 ) for 2 h at 500” C, with subsequent reduction under a hydrogen flow of 50 ml/min for 2 h at 400°C (for Pt/SiO,) or 450°C (for Pt/Al,O,) in a glass U-tube reactor with a heating rate of ca. 8”C/min. Three different aluminas (CK300, Tonerde y-Al,O, and Degussa Al,O,) and laboratory-made silica with a specific surface area of 270 m2/g were used as supports. Extraction procedure and determination of platinum ions A known weight (20-100 mg) of ground reduced sample was oxidized under an oxygen flow of ca. 15 ml/min in a glass U-tube reactor for 1 h at a given temperature. The oxidized sample was transferred into a conical flask to which were added first 10 ml of 1: 1 (v/v) hydrochloric acid and then 10 ml of 23% tin(I1) chloride solution (prepared by dissolving 27.4 g of SnC1,*2H,O in 35 ml of concentrated hydrochloric acid and dilution to 100 ml). The suspension was stirred for 3 h at room temperature or refluxed (under a reflux condenser ) for 1 h. A constant amount of platinum ions was obtained in this period, as shown by preliminary tests. The remaining catalyst was separated from the extract solution by centrifugation and the extinction (log I,,/1 or optical density) of the transparent yellow solution, namely ( PtSn,C1,)4+ [ 8,9], was measured by spectrophotometry (Shidmadzu UV-120-02) at 403 nm. The colour was stable for at least 48 h. Preliminary experiments showed that the colour intensity (extinction) is directly proportional to the platinum concentration (see Fig. 1) and the equation C= l.l9E-0.003 was obtained by computer linear fitting, where C is the platinum concentration in mg per 50 ml (range O-l mgper 50 ml) and E is the extinction (in a l-cm glass cell). The platinum concentration in the extract solution to be measured in general was controlled within 1.0 mg per 50 ml.
17
Fig. 1. Calibration graph for spectrophotometry.
Determination of total platinum content and expression of platinum dispersion The total amount of platinum in catalysts was determined by first dissolving the support in 1: 1 (v/v) sulphuric acid (for alumina) or 40% sodium hydroxide solution (for silica) and then dissolving the black platinum residue in aqua regia, followed by spectrophotometry of (PtSn,Cl,) 4+ as mentioned above. The “percentage exposed” (PE) of platinum was expressed either as the ratio of extracted platinum ion content to the total platinum content of the samples having the same weight or, for brevity, as the ratio of the extinctions between the extract solution and the solution for measurement of the total platinum content. Preparation of TEM specimen A portion of sample was ground in a small agate mortar and the ground material was dispersed in anhydrous ethanol by ultrasonic treatment. A drop of this suspension was evaporated on a carbon-coated grid at room temperature. The electron microscope (JEM ZOOCX) was operated at 200 kV and at a magnification of 200 000. Subsequent photographic enlargement resulted in a total magnification of ca. 600 000, so that a distance of 1 mm on the final print corresponded to ca. 17 A in the sample. RESULTS
The feasibility of the method, termed the oxidation-extraction method, was first examined with standard EuroPt-1 catalyst. The results of measuring the “percentage exposed” (PE ) of platinum in this catalyst over a wide range of oxidation temperatures are shown in Table 1 and Fig. 2. It was found (see Fig. 2) that the two curves (extraction profiles) obtained
Oxidation temperature Extinction 10 (1 cm) Percentage exposed Extinction 10 (1 cm) Percentage exposed
With refluxing
(“C)
the extinction
300 0.492 49.3 0.436 43.1
is 0.997.
200 0.480 48.1 0.450 45.1
of total platinum
100 0.448 44.9 0.428 42.9
temperature
Room temp. 0.448 44.9 0.397 39.8
Values
“Re-reduced at 400°C for 2 h and ground before extraction;
With stirring at room temperature
Parameter
(20 mg) as a function of oxidation
Extraction
PE values of Pt in EuroPt-1”
TABLE 1
400 0.490 49.1 0.440 44.1
500 0.488 48.9 0.426 42.7
600 0.160 16.0 0.098 9.80
79
, - -------I K
\ \ \
under refluxing
G
30.
_- under stirring at room temp. : 20. $
1
\
100
200
300
400
500
\+ : I
600
oxidation temperature,?
Fig. 2. Extraction profiles of EuroPt-1, +, With reluxing;
x , with stirring at room
temperature.
at different extraction temperatures were parallel with each other (the difference in PE values at a given oxidation temperature could be due to the formation of a compacted film of platinum oxide which was difficult to extract completely at room temperature), the PE value increased with increase in oxidation temperature (perhaps owing to the transformation of Pt-0 from covalent to ionic bonding) and remained constant within the oxidation temperature range 300~500’ C (all surface platinum atoms converted into platinum ions ) , and then decreased as the oxidation temperature was increased further to 600 oC (owing to sintering) . These observations indicated that the constant PE value, i.e., 49.1%, obtained from the extraction profile with refluxing could be used as a direct measure of the actual “percentage exposed” of platinum in EuroPt- 1. The PE value of platinum in EuroPt-1 measured by our method was lower than the reported value of ca. 60% [ 71. Additional assessment of the dispersion of platinum was made by TEM. The results of measuring the diameter of 512 Pt particles in this sample are shown in Table 2 and Fig. 4 and the TEM photograph of EuroPt-1 is shown in Fig. 3. The commonly accepted measures of mean particle size [lo] for the distribution shown in Table 2 and Fig. 4 were given as follows: the number-average diameter by d
,‘nidi_21
n Cn;-
Oi *
the surface-average diameter by
where n is the number of particles and d is the particle diameter. Assuming a spherical particle, the PE can be calculated by [lo]
80 TABLE 2 Size distribution of Pt particles in EuroPt-1 Particle diameter (A)=
Number of particles
7.5
7
12.5 17.5 22.5 27.5 32.5 37.5 42.5
82 175 123 74 31 11 9
Percentage of particles 1.4 16.0 34.2 24.0 14.5 6.1 2.1 1.8
“Represents the mid-point of a 5 A increment in particle diameter.
Fig. 3. TEM of EuroPt-1 after re-reduction at 400°C for 2 h. Magnification 600 000.
PE=11.32=44.2%
4
which is in agreement with the oxidation-extraction
result in this study, in-
81
Particle
Size
'A
Fig. 4. Particle size distribution of Pt in EuroPt-1.
TABLE 3 PE values of Pt in different catalysts Catalyst
nominal Pt loading (W.-W)
Percentage exposedb
CK 303 (100 mg)” Pt/CK 300 (50 mg) Pt/Tonerde (50mg) Pt /Degussa (50 mg) Pt/SiOz (100 mg)
0.3 1.0 1.0 1.0 0.5
98.0 96.0 91.0 96.0 73.4
“Reduced at 450” C for 2 h. “Obtained from extraction with refluxing at an oxidation temperature of 300-500 ‘C.
dicating mild aggregation of small platinum particles after ca. 12 years of storage and re-reduction owing to the weak interaction between platinum and inert silica. The platinum PE values of commercial CK303 and several laboratory-made catalysts are given in Table 3 and their extraction profiles are shown in Fig. 5. It is interesting that the extraction profiles of the series of Pt/Al,O, catalysts are different from that of EuroPt-1. At low oxidation temperatures ( < 200’ C), both the extraction profiles are nearly parallel, but diverge from each other with increasing oxidation temperature, indicating an interaction between PtO, and the alumina support. Further, some of the extraction profiles (see Fig. 5a and c) are insensitive to oxidation temperature ( < 5OO”C), perhaps owing to the small size of the platinum particles. The extraction profiles of 0.5% Pt/ SiO, are similar to that of EuroPt-1, but no differences in PE values obtained at different extraction temperatures are observed and , moreover, milder sintering seems to occur during oxidation at 500°C (see Fig, 5d). The factors influencing the extraction profiles were further examined. The
82
results obtained showed (i) with CK300 as the support, an increase in platinum loading from 1% to 6%, followed by sintering at 650” C during hydrogen treatment, decreased the PE of platinum (measured with refluxing) from 96.0% to 51.9%, and the extracted platinum ion content became variable with oxidation temperature (in the range 20-300” C) as shown in Fig. 6, and (ii) with
100 -
C-
#i.____k---__&_
z ;
+
--_ -x-w_ --Ip-
eo-
cz +4
---a \ \
+
\\ I
(a)
B 60E k _
\ \ \ \
40-
I 100
200
400
300
500
600
Oxidation temperature, %
100 t
-A-80.
P k w $ +
_-x.
_--*--
_*c--z-
lso-
-.
--JL _ --I_
\ \ ,
(b)
z
I!
t\ \ \\ \
40-
20 100
200
300
400
\\
\ \ \ \ \ \ \ \ \ \ :
500
Oxidation temperandre,
600 .C
83
\ 20
? 100
200
90 Oxidat ion
400
wo
temperature
600 , ‘C
(d)
I 100
200
300 Oxidation
Fig. 5. Extraction (c) 1% Pt/Degussa
profiles of supported AlSO
400
600
500
lC
temperature, Pt catalysts:
(d) 0.5% Pt/SiOz.
(a) 1% Pt/CKSOO;
Symbols
(b) 1% Pt/Tonerde
Al,O,;
as in Fig. 2.
Tonerde Al,O, as the support, having different properties from that of CK300 [ 11,121, a decrease in the platinum loading from 1.0% to 0.1% increased the PE of platinum slightly from 91.0% to ca. lOO%, causing the extracted platinum ion content to be independent of oxidation temperature (in the range 20600’ C ) , as shown in Fig. 7. These results indicate that both the platinum par-
84
100 +
l
l
+
C--t
1% 80 -
33 2
\
AN
40
+
+---+-
+
6%
2?5 \ 20 I 100
200
700 Oxidation
400
+
500
temperature,
600 %
Fig. 6. Extraction profiles of Pt/CK300 with different Pt loadings.
100 t
t-+
*
+
+
l
t
0.1%
-_A-----+ 8. 8o $2 ;
0 3
60.
40
100
900
300 Oxidation
400
500
temperature,
600
lC
Fig. 7. Extraction profiles of Pt/Tonerde A1203with different Pt loadings.
title size effect and the state of the platinum dispersion may play important roles in influencing the extraction profiles. DISCUSSION
We first discuss on the depth of oxidation of the platinum particles. The principle of the oxidation-extraction method is based on the simple idea that
85
the interaction of oxygen with platinum particles is expected to take place only at the surface of the platinum particles by controlling the oxidation temperature. If oxygen penetrates into the platinum lattice, in other words if bulk oxidation occurs, the “percentage extracted” of platinum should increase with increasing oxidation temperature until it reaches a value of unity, i.e., 100%. However, such a phenomenon was not observed in our experiments. It can be seen from Fig. 2 and Fig. 5d that the highest extracted platinum ion contents, e.g., 49.1% (for EuroPt-1) and 73.4% (for 0.5% Pt/SiO,), evaluated from the plateau of the extraction profiles (with refluxing) are always lower than 100% as confirmed by several repeated tests. It seems a great coincidence that the aforesaid values are in agreement with the PE of platinum evaluated by TEM. On the other hand, it has been reported [ 13,141 that the “as received” EuroPt1 contains substantially a platinum oxide rather than metallic platinum, as shown by extended X-ray absorption fine structure (EXAFS) measurements and temperature-programmed reduction. All of the above-mentioned facts seem to indicate that the extraction removes only the top most layer of the oxidized platinum, whatever the oxidation is in the interior of the platinum particles. It follows that the method used seems to be reliable. The same conclusion is expected to be appropriate also for Pt/A1,03 catalysts. It is noteworthy that high PE values of platinum (nearly unity) are measured even with low-temperature oxidation (say 20’ C ) , as shown in Figs. 6 and 7, with the implication that the platinum is dispersed nearly automatically over the surface of the alumina support. The second point to be discussed concerns the effect of the platinum particle size or state of the platinum dispersion. The results obtained show that (i) with inert silica as support, the two curves (PE vs. oxidation temperature) measured at different extraction temperatures are parallel (for EuroPt-1, Fig. 2) or overlap each other (for 0.5% Pt/SiOz, Fig. 5d) and (ii) with active aluTABLE
4
APE values as a function of Pt dispersion. Oxidation temperature = 500 a C (without sintering) Catalyst
Pt loading (wt.-W)
Pt/Tonerde
Pt/CK
Al,O,
300
“sintering
Percentage
exposed
APE
At 20°C
With refluxing
0.1 1
63.5 70.8
98.0 91.0
34.5 20.2
6”
34.7
47.2
12.5
0.3 (CK303) 1
73.0 78.3
98.0 96.0
25.0 17.7
6”
39.0
52.1
13.1
at 650 ‘C by hydrogen treatment.
86
mina as support, the two curves are parallel at lower temperatures (20-200 ’ C ) but diverge from each other at oxidation temperatures > 200 oC. The latter phenomenon can be attributed to the interaction of the PtO, formed with the alumina support [ 151. Accordingly, the higher the oxidation temperature ( > ZOO’C), the stronger is the PtO,-Al,O, interaction and the lower is the platinum ion content to be extracted at a lower extraction temperature (room temperature). It seems conceivable that the above-mentioned interaction can be described as an interfacial PtO,-Al,O, support interaction. According to the so-called concept of adlineation [ 161, the strength of this interaction should depend on the platinum dispersion in addition to the oxidation temperature. To gain further insight into the role of platinum dispersion, we compared the BPE values (the difference in PE values measured at 20’ C and with refluxing) for catalysts having different platinum loadings. The results (Table 4) confirm the role of platinum dispersion in effecting the interfacial interaction, namely, the higher the platinum dispersion, the larger is APE. Our experimental results can now be summarized as follows: (i) The (oxidation) temperature dependence of the PE of platinum may be attributed to the nature of the Pt-0 bonding, i.e., ionic or covalent, as only the platinum ion can be extracted by tin (II) chloride solution. Exposure of small platinum particles to air leads to complete oxidation and the formation of platinum ions [ 171, i.e., the PE of small platinum particles is independent of temperature. (ii) The fact that the two curves for the extraction of Pt/SiO, run parallel or overlap each other over a wide range of oxidation temperatures can be attributed to the absence of an interaction between the PtO, formed and the silica support [ 151. The difference in the PE of platinum measured at different extraction temperatures may be due to the formation of a compacted film of PtO, which is difficult to extract at room temperature. (iii) The interaction of alumina with PtO, can take place only at the PtAl,O, phase boundary when the oxidation temperature becomes progressively higher ( > 200’ C) , causing the two extraction curves to diverge from each other. The interfacial PtO,-A1203 interaction becomes strongest as the platinum disperses atomically. (iv) An oxidation temperature of ca. 400’ C may be preferable, as the surface platinum atoms can be transformed completely into platinum ions, whereby reproducible data for PE can be obtained. ACKNOWLEDGEMENTS
Financial support by the National Natural Science Foundation of China is gratefully acknowledged. The authors thank Prof. Dr. V. Ponec (Gorlaeus Laboratory, Leiden University, The Netherlands) for providing some of the catalysts (EuroPt-1 and CK303 ) and alumina samples used in this study.
87 REFERENCES
8 9
10 11 12 13 14 15 16 17
J.L. Lemaitre, P.G. Menon and F. Delannay, in F. Delannay (Ed.), Characterization of Heterogeneous Catalysts, Marcel Dekker, New York, 1984, p. 299. L.C. de Menorval, J.P. Fraissard and T. Ito, J. Chem. Sot., Faraday Trans. I, 78 (1982) 403. V. Ponec, personal communication. J.R. Anderson, Sci. Prog. (Oxford), 69 (1985) 461. M. Che and C.O. Bennett, Adv. Catal., in press. Z.V. Luk’yanova, V.I. Shekhobalova and V.S. Boronin, Russ. J. Phys. Chem., 53 (1979) 225. G.C. Bond and P.B. Wells, Appl. Catal., 18 (1985) 221. A.S. Meyer, Jr., and G.H. Ayres, J. Am. Chem. Sot., 77 (1955) 2671. G.C. Bond and P.B. Wells, Appl. Catal., 18 (1985) 225. J.R. Anderson, Structure of Metallic Catalysts, Academic Press, New York, 1975, pp. 35% 360. X.-D. Huang, Y.-N. Yang and J.-Y. Zhang, Appl. Catal., 40 (1988) 291. X.-P. Fan, W. Li, Y.-N. Yang and J.-Y Zhang, Appl. Catal., 38 (1988) 89. R.W. Joyner, J. Chem. Sot., Faraday Trans. I, 26 (1980) 357. G.C. Bond and M.R. Galsthorpe, Appl. Catal., 35 (1987) 169. H.C. de Jongste and V. Ponec, J. Cat& 64 (1980) 228. R. Kramer, and H. Zuegg, 8 ICC, V-275,1984. M.J.P. Botman, L.-Q. She, J.-Y. Zhang, W.L. Driessen and V. Ponec, J. Catal., 103 (1987) 280.
75
Applied
Printed in The Netherlands
Dispersion of Platinum on Silica and Alumina by Chemical Extraction YONGNIAN YANG, JINCHENG PAN, NING ZHENG, XINGQUAN LIU and JIAYU ZHANG* Department
of Chemistry,
Peking
University,
Beijing
(Peoples
Republic
of China)
(Received 3 April 1989, revised manuscript received 18 May 1989)
ABSTRACT Chemical extraction combined with transmission electron microscopy (TEM) was used to study the dispersion of platinum in a series of Pt/Si02 and Pt/Al,O, catalysts. With standard EuroPt1 catalyst the results showed that the “percentage extracted” of platinum measured after oxidation at cu. 400 oC was identical with the “percentage exposed’! of platinum evaluated by TEM. Further, interaction between the PtO, formed and the support used could also be explored by this method. A detailed discussion is given of the effects of several factors on the extraction profiles. Keywords: platinum/alumina, (extraction, TEM).
platinum/silica,
EuroPt-1, dispersion, catalyst chracterization
INTRODUCTION
The catalytic properties of metal catalysts in general are closely related to the state of dispersion of the active metals. The need to evaluate the degree of dispersion or “percentage exposed” has led to the development of a variety of methods [ 1,2], one of the most frequently used being hydrogen chemisorption. The main uncertainties of the chemisorption method [ 3-51 include the problem of establishing the actual monolayer coverage and the variability in chemisorption stoichiometry, particularly at sizes < 20 A. Chemisorption is mainly used to determine the number of surface metal atoms. It has been suggested [ 61 that oxidation of surface metal atoms leading to the formation of surface metal ions, followed by extraction, should give a direct measure (in principle) of the number of surface metal atoms without concern for the aforesaid uncertainties of chemisorption. However, reports on the actual application of this method are not available, perhaps owing to the lack of satisfactory results. In this work, chemical extraction combined with transmission electron microscopy (TEM) was used to study the dispersion of platinum in a series of 0166.9834/90/$03.50
0 1990 Elsevier Science Publishers B.V.
76
Pt/SiO, and Pt/A1203 catalysts on a quantitative basis. To establish the feasibility of the extraction method, we used the well known standard catalyst EuroPt-1 (6.3% Pt/SiO,) [7] in order to achieve reliable comparisons. EXPERIMENTAL
Preparation of samples The catalysts tested were standard EuroPt-1, commercial CK303 and several laboratory-made catalysts. The last materials were prepared by wet impregnation of ground alumina or silica with an aqueous solution of platinum compounds, namely H,PtCl, (for Pt/Al,O,) and Pt (NH,),( OH), (for Pt/ SiO*), followed by stirring for 3-4 h at room temperature, drying for 24 h at 110’ C and calcining (only for Pt/A1203 ) for 2 h at 500” C, with subsequent reduction under a hydrogen flow of 50 ml/min for 2 h at 400°C (for Pt/SiO,) or 450°C (for Pt/Al,O,) in a glass U-tube reactor with a heating rate of ca. 8”C/min. Three different aluminas (CK300, Tonerde y-Al,O, and Degussa Al,O,) and laboratory-made silica with a specific surface area of 270 m2/g were used as supports. Extraction procedure and determination of platinum ions A known weight (20-100 mg) of ground reduced sample was oxidized under an oxygen flow of ca. 15 ml/min in a glass U-tube reactor for 1 h at a given temperature. The oxidized sample was transferred into a conical flask to which were added first 10 ml of 1: 1 (v/v) hydrochloric acid and then 10 ml of 23% tin(I1) chloride solution (prepared by dissolving 27.4 g of SnC1,*2H,O in 35 ml of concentrated hydrochloric acid and dilution to 100 ml). The suspension was stirred for 3 h at room temperature or refluxed (under a reflux condenser ) for 1 h. A constant amount of platinum ions was obtained in this period, as shown by preliminary tests. The remaining catalyst was separated from the extract solution by centrifugation and the extinction (log I,,/1 or optical density) of the transparent yellow solution, namely ( PtSn,C1,)4+ [ 8,9], was measured by spectrophotometry (Shidmadzu UV-120-02) at 403 nm. The colour was stable for at least 48 h. Preliminary experiments showed that the colour intensity (extinction) is directly proportional to the platinum concentration (see Fig. 1) and the equation C= l.l9E-0.003 was obtained by computer linear fitting, where C is the platinum concentration in mg per 50 ml (range O-l mgper 50 ml) and E is the extinction (in a l-cm glass cell). The platinum concentration in the extract solution to be measured in general was controlled within 1.0 mg per 50 ml.
17
Fig. 1. Calibration graph for spectrophotometry.
Determination of total platinum content and expression of platinum dispersion The total amount of platinum in catalysts was determined by first dissolving the support in 1: 1 (v/v) sulphuric acid (for alumina) or 40% sodium hydroxide solution (for silica) and then dissolving the black platinum residue in aqua regia, followed by spectrophotometry of (PtSn,Cl,) 4+ as mentioned above. The “percentage exposed” (PE) of platinum was expressed either as the ratio of extracted platinum ion content to the total platinum content of the samples having the same weight or, for brevity, as the ratio of the extinctions between the extract solution and the solution for measurement of the total platinum content. Preparation of TEM specimen A portion of sample was ground in a small agate mortar and the ground material was dispersed in anhydrous ethanol by ultrasonic treatment. A drop of this suspension was evaporated on a carbon-coated grid at room temperature. The electron microscope (JEM ZOOCX) was operated at 200 kV and at a magnification of 200 000. Subsequent photographic enlargement resulted in a total magnification of ca. 600 000, so that a distance of 1 mm on the final print corresponded to ca. 17 A in the sample. RESULTS
The feasibility of the method, termed the oxidation-extraction method, was first examined with standard EuroPt-1 catalyst. The results of measuring the “percentage exposed” (PE ) of platinum in this catalyst over a wide range of oxidation temperatures are shown in Table 1 and Fig. 2. It was found (see Fig. 2) that the two curves (extraction profiles) obtained
Oxidation temperature Extinction 10 (1 cm) Percentage exposed Extinction 10 (1 cm) Percentage exposed
With refluxing
(“C)
the extinction
300 0.492 49.3 0.436 43.1
is 0.997.
200 0.480 48.1 0.450 45.1
of total platinum
100 0.448 44.9 0.428 42.9
temperature
Room temp. 0.448 44.9 0.397 39.8
Values
“Re-reduced at 400°C for 2 h and ground before extraction;
With stirring at room temperature
Parameter
(20 mg) as a function of oxidation
Extraction
PE values of Pt in EuroPt-1”
TABLE 1
400 0.490 49.1 0.440 44.1
500 0.488 48.9 0.426 42.7
600 0.160 16.0 0.098 9.80
79
, - -------I K
\ \ \
under refluxing
G
30.
_- under stirring at room temp. : 20. $
1
\
100
200
300
400
500
\+ : I
600
oxidation temperature,?
Fig. 2. Extraction profiles of EuroPt-1, +, With reluxing;
x , with stirring at room
temperature.
at different extraction temperatures were parallel with each other (the difference in PE values at a given oxidation temperature could be due to the formation of a compacted film of platinum oxide which was difficult to extract completely at room temperature), the PE value increased with increase in oxidation temperature (perhaps owing to the transformation of Pt-0 from covalent to ionic bonding) and remained constant within the oxidation temperature range 300~500’ C (all surface platinum atoms converted into platinum ions ) , and then decreased as the oxidation temperature was increased further to 600 oC (owing to sintering) . These observations indicated that the constant PE value, i.e., 49.1%, obtained from the extraction profile with refluxing could be used as a direct measure of the actual “percentage exposed” of platinum in EuroPt- 1. The PE value of platinum in EuroPt-1 measured by our method was lower than the reported value of ca. 60% [ 71. Additional assessment of the dispersion of platinum was made by TEM. The results of measuring the diameter of 512 Pt particles in this sample are shown in Table 2 and Fig. 4 and the TEM photograph of EuroPt-1 is shown in Fig. 3. The commonly accepted measures of mean particle size [lo] for the distribution shown in Table 2 and Fig. 4 were given as follows: the number-average diameter by d
,‘nidi_21
n Cn;-
Oi *
the surface-average diameter by
where n is the number of particles and d is the particle diameter. Assuming a spherical particle, the PE can be calculated by [lo]
80 TABLE 2 Size distribution of Pt particles in EuroPt-1 Particle diameter (A)=
Number of particles
7.5
7
12.5 17.5 22.5 27.5 32.5 37.5 42.5
82 175 123 74 31 11 9
Percentage of particles 1.4 16.0 34.2 24.0 14.5 6.1 2.1 1.8
“Represents the mid-point of a 5 A increment in particle diameter.
Fig. 3. TEM of EuroPt-1 after re-reduction at 400°C for 2 h. Magnification 600 000.
PE=11.32=44.2%
4
which is in agreement with the oxidation-extraction
result in this study, in-
81
Particle
Size
'A
Fig. 4. Particle size distribution of Pt in EuroPt-1.
TABLE 3 PE values of Pt in different catalysts Catalyst
nominal Pt loading (W.-W)
Percentage exposedb
CK 303 (100 mg)” Pt/CK 300 (50 mg) Pt/Tonerde (50mg) Pt /Degussa (50 mg) Pt/SiOz (100 mg)
0.3 1.0 1.0 1.0 0.5
98.0 96.0 91.0 96.0 73.4
“Reduced at 450” C for 2 h. “Obtained from extraction with refluxing at an oxidation temperature of 300-500 ‘C.
dicating mild aggregation of small platinum particles after ca. 12 years of storage and re-reduction owing to the weak interaction between platinum and inert silica. The platinum PE values of commercial CK303 and several laboratory-made catalysts are given in Table 3 and their extraction profiles are shown in Fig. 5. It is interesting that the extraction profiles of the series of Pt/Al,O, catalysts are different from that of EuroPt-1. At low oxidation temperatures ( < 200’ C), both the extraction profiles are nearly parallel, but diverge from each other with increasing oxidation temperature, indicating an interaction between PtO, and the alumina support. Further, some of the extraction profiles (see Fig. 5a and c) are insensitive to oxidation temperature ( < 5OO”C), perhaps owing to the small size of the platinum particles. The extraction profiles of 0.5% Pt/ SiO, are similar to that of EuroPt-1, but no differences in PE values obtained at different extraction temperatures are observed and , moreover, milder sintering seems to occur during oxidation at 500°C (see Fig, 5d). The factors influencing the extraction profiles were further examined. The
82
results obtained showed (i) with CK300 as the support, an increase in platinum loading from 1% to 6%, followed by sintering at 650” C during hydrogen treatment, decreased the PE of platinum (measured with refluxing) from 96.0% to 51.9%, and the extracted platinum ion content became variable with oxidation temperature (in the range 20-300” C) as shown in Fig. 6, and (ii) with
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Oxidation temperature, %
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Oxidation temperandre,
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83
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wo
temperature
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(d)
I 100
200
300 Oxidation
Fig. 5. Extraction (c) 1% Pt/Degussa
profiles of supported AlSO
400
600
500
lC
temperature, Pt catalysts:
(d) 0.5% Pt/SiOz.
(a) 1% Pt/CKSOO;
Symbols
(b) 1% Pt/Tonerde
Al,O,;
as in Fig. 2.
Tonerde Al,O, as the support, having different properties from that of CK300 [ 11,121, a decrease in the platinum loading from 1.0% to 0.1% increased the PE of platinum slightly from 91.0% to ca. lOO%, causing the extracted platinum ion content to be independent of oxidation temperature (in the range 20600’ C ) , as shown in Fig. 7. These results indicate that both the platinum par-
84
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Fig. 6. Extraction profiles of Pt/CK300 with different Pt loadings.
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400
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Fig. 7. Extraction profiles of Pt/Tonerde A1203with different Pt loadings.
title size effect and the state of the platinum dispersion may play important roles in influencing the extraction profiles. DISCUSSION
We first discuss on the depth of oxidation of the platinum particles. The principle of the oxidation-extraction method is based on the simple idea that
85
the interaction of oxygen with platinum particles is expected to take place only at the surface of the platinum particles by controlling the oxidation temperature. If oxygen penetrates into the platinum lattice, in other words if bulk oxidation occurs, the “percentage extracted” of platinum should increase with increasing oxidation temperature until it reaches a value of unity, i.e., 100%. However, such a phenomenon was not observed in our experiments. It can be seen from Fig. 2 and Fig. 5d that the highest extracted platinum ion contents, e.g., 49.1% (for EuroPt-1) and 73.4% (for 0.5% Pt/SiO,), evaluated from the plateau of the extraction profiles (with refluxing) are always lower than 100% as confirmed by several repeated tests. It seems a great coincidence that the aforesaid values are in agreement with the PE of platinum evaluated by TEM. On the other hand, it has been reported [ 13,141 that the “as received” EuroPt1 contains substantially a platinum oxide rather than metallic platinum, as shown by extended X-ray absorption fine structure (EXAFS) measurements and temperature-programmed reduction. All of the above-mentioned facts seem to indicate that the extraction removes only the top most layer of the oxidized platinum, whatever the oxidation is in the interior of the platinum particles. It follows that the method used seems to be reliable. The same conclusion is expected to be appropriate also for Pt/A1,03 catalysts. It is noteworthy that high PE values of platinum (nearly unity) are measured even with low-temperature oxidation (say 20’ C ) , as shown in Figs. 6 and 7, with the implication that the platinum is dispersed nearly automatically over the surface of the alumina support. The second point to be discussed concerns the effect of the platinum particle size or state of the platinum dispersion. The results obtained show that (i) with inert silica as support, the two curves (PE vs. oxidation temperature) measured at different extraction temperatures are parallel (for EuroPt-1, Fig. 2) or overlap each other (for 0.5% Pt/SiOz, Fig. 5d) and (ii) with active aluTABLE
4
APE values as a function of Pt dispersion. Oxidation temperature = 500 a C (without sintering) Catalyst
Pt loading (wt.-W)
Pt/Tonerde
Pt/CK
Al,O,
300
“sintering
Percentage
exposed
APE
At 20°C
With refluxing
0.1 1
63.5 70.8
98.0 91.0
34.5 20.2
6”
34.7
47.2
12.5
0.3 (CK303) 1
73.0 78.3
98.0 96.0
25.0 17.7
6”
39.0
52.1
13.1
at 650 ‘C by hydrogen treatment.
86
mina as support, the two curves are parallel at lower temperatures (20-200 ’ C ) but diverge from each other at oxidation temperatures > 200 oC. The latter phenomenon can be attributed to the interaction of the PtO, formed with the alumina support [ 151. Accordingly, the higher the oxidation temperature ( > ZOO’C), the stronger is the PtO,-Al,O, interaction and the lower is the platinum ion content to be extracted at a lower extraction temperature (room temperature). It seems conceivable that the above-mentioned interaction can be described as an interfacial PtO,-Al,O, support interaction. According to the so-called concept of adlineation [ 161, the strength of this interaction should depend on the platinum dispersion in addition to the oxidation temperature. To gain further insight into the role of platinum dispersion, we compared the BPE values (the difference in PE values measured at 20’ C and with refluxing) for catalysts having different platinum loadings. The results (Table 4) confirm the role of platinum dispersion in effecting the interfacial interaction, namely, the higher the platinum dispersion, the larger is APE. Our experimental results can now be summarized as follows: (i) The (oxidation) temperature dependence of the PE of platinum may be attributed to the nature of the Pt-0 bonding, i.e., ionic or covalent, as only the platinum ion can be extracted by tin (II) chloride solution. Exposure of small platinum particles to air leads to complete oxidation and the formation of platinum ions [ 171, i.e., the PE of small platinum particles is independent of temperature. (ii) The fact that the two curves for the extraction of Pt/SiO, run parallel or overlap each other over a wide range of oxidation temperatures can be attributed to the absence of an interaction between the PtO, formed and the silica support [ 151. The difference in the PE of platinum measured at different extraction temperatures may be due to the formation of a compacted film of PtO, which is difficult to extract at room temperature. (iii) The interaction of alumina with PtO, can take place only at the PtAl,O, phase boundary when the oxidation temperature becomes progressively higher ( > 200’ C) , causing the two extraction curves to diverge from each other. The interfacial PtO,-A1203 interaction becomes strongest as the platinum disperses atomically. (iv) An oxidation temperature of ca. 400’ C may be preferable, as the surface platinum atoms can be transformed completely into platinum ions, whereby reproducible data for PE can be obtained. ACKNOWLEDGEMENTS
Financial support by the National Natural Science Foundation of China is gratefully acknowledged. The authors thank Prof. Dr. V. Ponec (Gorlaeus Laboratory, Leiden University, The Netherlands) for providing some of the catalysts (EuroPt-1 and CK303 ) and alumina samples used in this study.
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