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Determination of the metallic surface area of nickel and its dispersion on a silica support by means of a microbalance
Applied Cutalysi.s, l(l981)3-l
ElsevierGcientiiicPub~ingCompany,Ameterdam-_RintedinBelgium
DETERMINATION OF THE METALLIC SURFACE AREA OF NICKEL AND ITS DISPERSIONON A
SILICA SUPPORT BY MEANS OF A MICROBALANCE 6. LOHRENGEL and M. BAERNS Lehrstuhl fUr Technische Chemie, Ruhr-UniversitltBochum, Postfach 102148, D-4630 Bochum, G.F.R. (Received 4 September 1980, Accepted 26 November 1980) INTRODUCTION A means of characterizationof supported metal catalysts is the determination of total metal content and its dispersion on a support of high surface area. The required measurements are usually performed in the following way:Determination of the total metal content by chemical analysis Measurement of the BET-surface by volumetric or gravimetric means Determination of metal surface area by chemisorption of a suitable gas 11.21. Metal dispersion is then derived from these measurements. Determination of total as well as metallic surface area, metal content and dispersion of a supported nickel catalyst by means of a microbalance, as proposed in this paper, has several advantages:Measurements of chemisorption and quantitative determination of nickel are performed in the balance with the same sample, about 50 to 500 mg being required. (The problem of highly toxic nickel carbonyl disposal has to be carefully considered.) Completeness of reduction of an oxide precursor can be controlled before chemisorption by constancy of weight of the catalyst in a hydrogen atmosphere at elevated temperature. Surface area is determined with higher accuracy as compared to volumetric procedures 131. EXPERIMENTAL Measurements of adsorption and nickel determination by carbonyl formation were performed in a vacuum microbalance (Sartorius No. 4433) having a maximum sensitivity down to 0.001 mg. The balance was connected to a vacuum system consisting of an oil diffusion pump with a trap cooled by liquid nitrogen which was backed up by a 01669834/81/0000+000/$02.60O1981Elsevier6cientificPublishingCompany
4
mechanical rotary pump. The ultimate vacuum attainable amounted to 5~10-~ Pa. Gas analysis in the system was possible by a quadrupole mass spectrometer (Balzers QMG 311) attached to the balance via a special gas dosing system. The gases used were of high purity, being 99.999 vol % and 99.995 vol % hydrogen and carbon monoxide respectively.The catalyst precursor containing nickel oxide was first reduced in a flow system outside the balance under hydrogen flow at 600°C for 18 h; this can of course also be done in the balance. The sample was then transferred to the balance and re-exposed to hydrogen at 550°C and lo5 Pa until the weight was constant. Subsequently the system was evacuated at the elevated temperature, ascertaining that all hydrogen was removed, and then the system was cooled to -196'C for measuring the total BET-surface (190 m* 9-l) by nitrogen adsorption or to 23'C for chemisorptionmeasurements. Chemisorption of hydrogen and carbon monoxide was performed up to a final pressure of lo5 Pa. At each constant pressure level the increasing weight of the catalyst was noted with time until constant weight was reached. The carbonyl formation reaction Ni + 4C0 _+
Ni(C0)4
was performed after adsorption of CO and increasing the temperature to 100°C at a CO pressure of lo5 Pa. At the end of this reaction the gas was analysed by massspectrometry in order to test whether nickel carbonyl was still being formed. To control the nickel content as obtained by carbonylation a chemical analysis was performed by dissolving the nickel in nitric acid and titration with 0.1 m EDTA and Murexide as indicator at pH 10. RESULTS Adsorption of hydrogen The adsorption isotherm for hydrogen does not approach a saturation region for the Ni-Si02 catalyst as shown in Figure 1. The linear increase of the isotherm might be due to processes like a change of adsorption stoichiometry,penetration of hydrogen into the bulk or even reactive interaction of hydrogen with the support. To determine the chemisorptive hydrogen uptake the linear part of the isotherm is extrapolated to zero pressure; (the additional adsorption capacity of the support or the nickel itself, as derived from the slope of the isotherm was 6.053~10-~ mg/ torr g-catalyst). Chemisorption of hydrogen on the nickel catalyst amounts to 0.31 mg H2/g-catalyst or 4.85~10~~ H2 molecules per cm* BET-surface area respectively. This results in an active nickel surface area of approximately 12 m*/g-catalyst, if dissociative hydrogen adsorption and the following stoichiomatry is assumed: H2 + 2Ni #
LNi...H
4 -3 w
-2
O,l-
-1
0 0
FIGURE 1
1
2
3
4
5
6
7
8
9
'O~,O~&
Adsorption isotherms of hydrogen and carbon monoxide on silica-supported
nickel at 23'C Adsorption of carbon monoxide Adsorption occurs via a fast process (minutes) which is followed by a slow one (hours). The first process is attributed to a molecular adsorption of CO; the corresponding isotherm is also shown in Figure 1. The maximum chemisorptioncapacity for carbon monoxide was calculated for the fast adsorption process by using the Langmuir equation for one-site adsorption; 7.24 mg CO/g-catalyst or 8.2~10'~ CO molecules per cm2 BET-surface area respectivelywere adsorbed. This results in an active nickel surface area of 10 m2/g-catalyst, assuming molecular CO adsorption and a one to one adsorption stoichiometry: Ni + CO+Ni...CO Carbonylation of nickel The weight decrease of the catalyst during carbonylation at JO5 Pa and 100°C is plotted versus time in Figure 2. After about 150 h the change in catalyst weight had approached zero; (the time can be reduced by applying higher temperatures).The decrease of 34 mg corresponds to the total amount of nickel that had been removed by carbonylation.Thus, the nickel content of the reduced catalyst sample equals 18.6 wt %; this is in good agreement with the value of 18.0 wt % which was obtained
6
by titrimetric chemical analysis.
catalyst weight
[mgl 190 180 -
150 * 0
FIGURE 2
20
* LO 60 80 100 120 140 160 t[hl
Nickel removal by carbonylation at carbon monoxide pressure of 1015 Pa
and 1OO'C. Active surface area and dispersion of nickel on the support The active nickel amounts to 9.7x1013 nickel atoms per cm2 by assuming complete dissociative hydrogen adsorption or 0.063 m2 active nickel per m2 total BET-surface area of the catalyst. However, only 8.2~10'~ nickel atoms per cm2 would result on the basis of molecular adsorption of carbon monoxide. The difference may be explained by accounting for a small proportion of bridged besides mainly linear bonding of the carbon monoxide to the nickel surface. This proportion can be derived from the above data; it amounts to 15.5 %. This result supports earlier investigationsof Brooks and Christopher who reported that the surface area of a nickel-alumina catalyst derived from a) dissociative hydrogen adsorption and b) molecular, linear carbon monoxide adsorption may differ by about 30 % [41. The degree of dispersion, defined as active nickel atoms on the surface per total nickel atoms, amounts to 9.6 %.
CONCLUSION A vacuum microbalance is a suitable device to determine the chemisorptive capacity of a catalyst and to evaluate its active metal surface area, assuming a certain surface stoichiometry of the adsorbed gas. In the case of nickel it is recotmnendedto use hydrogen, which is believed to be completely dissociated on nickel at room temperature. Carbon monoxide is not in a uniformly defined chemisorptive state since it may be adsorbed using one or two nickel atoms depending on linear or bridged bonding: adsorption of carbon monoxide should only be applied if quantitative determination of hydrogen adsorption is not within the range of experimental accuracy. Carbon monoxide is, however, suited to determine the nickel content of the catalyst by nickel carbonyl formation. REFERENCES 1 2 3 4
R.J. Farrauto,AIChESymp. Ser. No. 143, 79 (1974) 9. J. Muller, Rev. Pure and Appl. Chem., 19 (1969) 151. D.A. Cadenhead and N.J. Wagner in Experimental Methods in Catalytic Research II, R.B. Anderson, Academic Press, London and New York. C.S. Brooks and G.L.M. Christopher, J. Catal. 10 (1968) 211.
ElsevierGcientiiicPub~ingCompany,Ameterdam-_RintedinBelgium
DETERMINATION OF THE METALLIC SURFACE AREA OF NICKEL AND ITS DISPERSIONON A
SILICA SUPPORT BY MEANS OF A MICROBALANCE 6. LOHRENGEL and M. BAERNS Lehrstuhl fUr Technische Chemie, Ruhr-UniversitltBochum, Postfach 102148, D-4630 Bochum, G.F.R. (Received 4 September 1980, Accepted 26 November 1980) INTRODUCTION A means of characterizationof supported metal catalysts is the determination of total metal content and its dispersion on a support of high surface area. The required measurements are usually performed in the following way:Determination of the total metal content by chemical analysis Measurement of the BET-surface by volumetric or gravimetric means Determination of metal surface area by chemisorption of a suitable gas 11.21. Metal dispersion is then derived from these measurements. Determination of total as well as metallic surface area, metal content and dispersion of a supported nickel catalyst by means of a microbalance, as proposed in this paper, has several advantages:Measurements of chemisorption and quantitative determination of nickel are performed in the balance with the same sample, about 50 to 500 mg being required. (The problem of highly toxic nickel carbonyl disposal has to be carefully considered.) Completeness of reduction of an oxide precursor can be controlled before chemisorption by constancy of weight of the catalyst in a hydrogen atmosphere at elevated temperature. Surface area is determined with higher accuracy as compared to volumetric procedures 131. EXPERIMENTAL Measurements of adsorption and nickel determination by carbonyl formation were performed in a vacuum microbalance (Sartorius No. 4433) having a maximum sensitivity down to 0.001 mg. The balance was connected to a vacuum system consisting of an oil diffusion pump with a trap cooled by liquid nitrogen which was backed up by a 01669834/81/0000+000/$02.60O1981Elsevier6cientificPublishingCompany
4
mechanical rotary pump. The ultimate vacuum attainable amounted to 5~10-~ Pa. Gas analysis in the system was possible by a quadrupole mass spectrometer (Balzers QMG 311) attached to the balance via a special gas dosing system. The gases used were of high purity, being 99.999 vol % and 99.995 vol % hydrogen and carbon monoxide respectively.The catalyst precursor containing nickel oxide was first reduced in a flow system outside the balance under hydrogen flow at 600°C for 18 h; this can of course also be done in the balance. The sample was then transferred to the balance and re-exposed to hydrogen at 550°C and lo5 Pa until the weight was constant. Subsequently the system was evacuated at the elevated temperature, ascertaining that all hydrogen was removed, and then the system was cooled to -196'C for measuring the total BET-surface (190 m* 9-l) by nitrogen adsorption or to 23'C for chemisorptionmeasurements. Chemisorption of hydrogen and carbon monoxide was performed up to a final pressure of lo5 Pa. At each constant pressure level the increasing weight of the catalyst was noted with time until constant weight was reached. The carbonyl formation reaction Ni + 4C0 _+
Ni(C0)4
was performed after adsorption of CO and increasing the temperature to 100°C at a CO pressure of lo5 Pa. At the end of this reaction the gas was analysed by massspectrometry in order to test whether nickel carbonyl was still being formed. To control the nickel content as obtained by carbonylation a chemical analysis was performed by dissolving the nickel in nitric acid and titration with 0.1 m EDTA and Murexide as indicator at pH 10. RESULTS Adsorption of hydrogen The adsorption isotherm for hydrogen does not approach a saturation region for the Ni-Si02 catalyst as shown in Figure 1. The linear increase of the isotherm might be due to processes like a change of adsorption stoichiometry,penetration of hydrogen into the bulk or even reactive interaction of hydrogen with the support. To determine the chemisorptive hydrogen uptake the linear part of the isotherm is extrapolated to zero pressure; (the additional adsorption capacity of the support or the nickel itself, as derived from the slope of the isotherm was 6.053~10-~ mg/ torr g-catalyst). Chemisorption of hydrogen on the nickel catalyst amounts to 0.31 mg H2/g-catalyst or 4.85~10~~ H2 molecules per cm* BET-surface area respectively. This results in an active nickel surface area of approximately 12 m*/g-catalyst, if dissociative hydrogen adsorption and the following stoichiomatry is assumed: H2 + 2Ni #
LNi...H
4 -3 w
-2
O,l-
-1
0 0
FIGURE 1
1
2
3
4
5
6
7
8
9
'O~,O~&
Adsorption isotherms of hydrogen and carbon monoxide on silica-supported
nickel at 23'C Adsorption of carbon monoxide Adsorption occurs via a fast process (minutes) which is followed by a slow one (hours). The first process is attributed to a molecular adsorption of CO; the corresponding isotherm is also shown in Figure 1. The maximum chemisorptioncapacity for carbon monoxide was calculated for the fast adsorption process by using the Langmuir equation for one-site adsorption; 7.24 mg CO/g-catalyst or 8.2~10'~ CO molecules per cm2 BET-surface area respectivelywere adsorbed. This results in an active nickel surface area of 10 m2/g-catalyst, assuming molecular CO adsorption and a one to one adsorption stoichiometry: Ni + CO+Ni...CO Carbonylation of nickel The weight decrease of the catalyst during carbonylation at JO5 Pa and 100°C is plotted versus time in Figure 2. After about 150 h the change in catalyst weight had approached zero; (the time can be reduced by applying higher temperatures).The decrease of 34 mg corresponds to the total amount of nickel that had been removed by carbonylation.Thus, the nickel content of the reduced catalyst sample equals 18.6 wt %; this is in good agreement with the value of 18.0 wt % which was obtained
6
by titrimetric chemical analysis.
catalyst weight
[mgl 190 180 -
150 * 0
FIGURE 2
20
* LO 60 80 100 120 140 160 t[hl
Nickel removal by carbonylation at carbon monoxide pressure of 1015 Pa
and 1OO'C. Active surface area and dispersion of nickel on the support The active nickel amounts to 9.7x1013 nickel atoms per cm2 by assuming complete dissociative hydrogen adsorption or 0.063 m2 active nickel per m2 total BET-surface area of the catalyst. However, only 8.2~10'~ nickel atoms per cm2 would result on the basis of molecular adsorption of carbon monoxide. The difference may be explained by accounting for a small proportion of bridged besides mainly linear bonding of the carbon monoxide to the nickel surface. This proportion can be derived from the above data; it amounts to 15.5 %. This result supports earlier investigationsof Brooks and Christopher who reported that the surface area of a nickel-alumina catalyst derived from a) dissociative hydrogen adsorption and b) molecular, linear carbon monoxide adsorption may differ by about 30 % [41. The degree of dispersion, defined as active nickel atoms on the surface per total nickel atoms, amounts to 9.6 %.
CONCLUSION A vacuum microbalance is a suitable device to determine the chemisorptive capacity of a catalyst and to evaluate its active metal surface area, assuming a certain surface stoichiometry of the adsorbed gas. In the case of nickel it is recotmnendedto use hydrogen, which is believed to be completely dissociated on nickel at room temperature. Carbon monoxide is not in a uniformly defined chemisorptive state since it may be adsorbed using one or two nickel atoms depending on linear or bridged bonding: adsorption of carbon monoxide should only be applied if quantitative determination of hydrogen adsorption is not within the range of experimental accuracy. Carbon monoxide is, however, suited to determine the nickel content of the catalyst by nickel carbonyl formation. REFERENCES 1 2 3 4
R.J. Farrauto,AIChESymp. Ser. No. 143, 79 (1974) 9. J. Muller, Rev. Pure and Appl. Chem., 19 (1969) 151. D.A. Cadenhead and N.J. Wagner in Experimental Methods in Catalytic Research II, R.B. Anderson, Academic Press, London and New York. C.S. Brooks and G.L.M. Christopher, J. Catal. 10 (1968) 211.