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Solid Titration Microcalorimetry
Guczi, L. ef al. (Editors),New Fronfiers in Cafalysh
Proceedings of the 10th International Congress on Catalysis, 19-24 July, 1992,Budapest, Hungary 0 1993 Elsevier Science Publishers B.V. All rights reserved
SURFACE ENERGETIC CHARACTERIZATION OF SUPPORTED METAL CATALYSTS BY GAS/SOLIDTITRATION MICROCALORIMETRY
J. M.Guy A. P. Masia, A. R. Paniego and J. M.T.Menayo Instituto de Quimica Fisica "Rocasolano", CSIC, Serrano 119,28006 Madrid, Spain
Adsorption microcalorimetry is a technique which has proved its usefulness in the characterization of catalysts, mainly using probe molecules. Its application can be extended to the study of surface reactions, thereby broadening the information that can be obtained. Here we show results of hydrogen titration of preadsorbed oxygen on iridium supported catalysts. Thermochemical calculations with the calorimetric data yielded information on the surface energetics for the two adsorbates.
1. EXPERIMENTAL Hydrogen and oxygen (Sociedad Espaiiola del Oxigeno, Spain), 99.995% pure, were used as adsorbates. Supported iridium catalysts were prepared by the incipient wetness method using hexachloroiridic acid (Alfa Ventron Inorganics.) Silica gel (BASF, D-ll- 11) and y-alumina (Girdler, T-126) were used as supports. Specific surface areas determined by the BET method, after heating in air at 973 K, were 128.0 and 149.1 m2g-', respectively. The following catalysts were prepared: Ir(5 %)/Siq, Ir(0.5 %)/SO2 and Ir(2.5 %)/A120, with 5.45%, 0.53% and 2.55% metallic content. The surface iridium amount, Irsf, was determined as described elsewhere [ 11 and is given in the first row of Table 1. Calorimetric measurements were carried out in a Tian-Calvet microcalorimeter (Model BT, Setaram, France), coupled to a volumetric apparatus with a Baratron manometer (MKS, USA.) Titration calorimetric experiments of preadsorbed oxygen with hydrogen were performed at 315 K. Differential heats of adsorption for hydrogen and oxygen were previously determined since they were needed for the thermochemical calculations described later. Calorimetric measurements were also made on the supports to take into account the adsorption on them. In the following, amounts of substance are expressed as pmol (of atoms in the case of hydrogen and oxygen) per gram of catalyst dried in vacuum at 700 K.
2. RESULTS AND DISCUSSION 2.1 Titration volumetry In figure 1 hydrogen titration isotherms for the two Ir/Siq samples are given as hydrogen uptake divided by surface iridium amount. The isotherms for the two samples nearly coincide. The dispersion degree, and therefore metal particle size, is approximately the same for the two samples (5%: 2.6 nm, 0.5%: 3.5 nm), which suggests similar surface properties. The titration isotherms show an initial vertical segment at zero equilibrium pressure. Afterwards a right angle knee appears followed by a final part where hydrogen uptake in-
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creases smoothly with pressure. Figure 1 ' also depicts a hydrogen adsorption isotherm. The knee and final part are similar to those observed in titration isotherms, which can be ascribed to the same process taking place after adsorption or titration ended. In consequence, the appearance of pressure was taken as the 2 end point of the titration reaction. The ' , same conclusion holds for the adsorption and titration isotherms on the Ir/A1203 sample. The amount of titrating hydrogen, Htit, for the three samples is given in the PIkPo third row Of 1. The amount Of Figure 1 . Hydrogen titration isotherms on Ir/Si02: oxygen taken U P in the run preceding the 0 , 0 5% and X 0.5% samples, Lower line: titration experiment, O,,, is shown in the hydrogen adsorption isotherm on the 5 % sample. second row. (Surface oxidation may proceed beyond adsorption if oxygen is present in the gas phase [l]. The two runs on the 1r/Al2O3sample, Table 1, show this clearly.) The two quantities, Htit and O,,, allow us to calculate the amount of hydrogen that remains on the iridium surface after oxygen reduction and water migration to the support (fifth row in Table 1.) H/Ir stoichiometries obtained (sixth row) are different for different supports [ 13, which justifies the need to calculate them instead of assuming a unique value of 1.
t2
Table 1
Hydrogen titration volumetry results and stoichiometries H , I Irsf
2;
Surface Ir: IrSf 'ad
(plkPa) Had
-f
'ox
hit - 2.0pr
Had / Ir,,
Ir(5%)/Si02 95 111.5 113.3 319.5 319.3 (0.56) (1.33) 96.5 92.7 1.01 0.98
Ir(0.5%)/Si02 7.5
8.2
23.5 (0.18) 7.1 0.95
@no1 of atoms/g)
Ir(2.5%)/Al2O3 80 97.8 79.2 320.4 288.4 (0.22) (0.45) 130.0 124.8 1.63 1.56
2.2 Titration microcalorimetry
The hydrogen titration differential heat per mol of hydrogen atoms consumed vs. amount taken up for various experiments on the Ir/SiO, samples is plotted in Figure 2-A. The titration heat includes the enthalpy change produced in each one of the processes taking place in the overall surface reaction. The dashed line corresponds to the adsorption heat measured in a hydrogen calorimetric isotherm on the 5 % sample. The titration and adsorption curves are in good agreement, which indicates that the hydrogen adsorption involved in the titration run follows the same trend as in an adsorption experiment, i.e. the iridium sites thatflrst become empty after oxygen reduction and water migration to the support, are precisely those of higher hydrogen adsorption energy. The same conclusion is reached for the titration reaction on the Ir/A1203 sample (Figure 2-B.)
2.3 Thennochemical calculations
The following thermochemical cycle can be written for the elementary reactions that take place in the overall titration process:
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where Sup stand for the support and cx is the value of the H/Ir stoichiometry: 1 and 1.6 for Ir/SiO, and Ir/A1,0? catalysts, respectively, as seen in Table 1. This thermochemical cycle was used to calculate the oxygen adsorption heat on these catalysts. (Calorimetric direct measurement of this quantity gave a constant value over most of the coverage range, since mobility of adsorbed oxygen is precluded by the strong 0-Ir bond formed. Therefore oxygen remains in the site where it first adsorbs, so that heats measured are statistical mean values.) From equations (1-5) we obtain: (6)
Adsorption and titration heats depend on coverage, so that application of equation (6) yields oxygen adsorption heats on iridium as function of coverage. A value of 242 kl/mol was used as water formation heat at 315 K in the vapour state [2]. Water adsorption heat on the supports vs. coverage was taken from the literature ([3] for silica and [4] for alumina.) The hydrogen adsorption heat and the hydrogen titration heat of preadsorbed oxygen at different coverages were interpolated from experimental values. Data and results for one experiment on the 1r/A1203 catalyst are given in Table 2. The amounts of oxygen being titrated appear in column 1. The hydrogen consumed in the oxygen titration is given in column 2: each Ir atom adsorbs one 0 atom; upon H titration, two H atoms combine with the 0 atom to form water (column 4) that migrates into the support, and 1.6 hydrogen Figure 2. Hydrogen titration heat: (A) IdSi02: o , 0 5% atoms (column 3) adsorb on the freed and X 0.5% samples. (B) Ir(2.5%)/M2O3: A , V Dashed Ir atom (one atom only in the case of Ir/Si02 samples.) lines: hydrogen adsorption heat.
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Table 2 Thermochemical calculations:
The differential heat values in columns 5, 6 and 7 were obtained by interpolation to the corresponding amounts in the preceding columns, The oxygen adsorption heat, which is the calculated value for breaking the existing O-Ir bond (column 8), increases as the titration reaction progresses. This shows that this reuction proceeds from weaker to stronger 0 - l r bonds. The same kind of calculations were made for the Ir/SiO, samples and similar results were Obtained. Figure 3 depicts Figure 3. Calculated oxygen adsorption heat on: Ir/Si02: Oxygen adsorption heats vs. coverage, 5%and X 0.5% and Ir(2.5%)/A1203:V . Dashed with O n e experiment for each sample. line: measured values on the 5% sample. They are in good accord and reveal the heterogeneity not shown by direct calorimetric measurements (dashed line in Figure 3.) From the two conclusions that have been reached we can deduce that iridium sites which adsorb oxygen weakly adsorb hydrogen strongly, and viceversa. The question arises wether this opposite heterogeneity is a property of the iridium crystallites, or if it produced upon adsorption of oxygen on them, i.e. if the oxygen on the indium surface could induce in it weak adsorption sites for oxygen which would be strong sites for hydrogen. This work was partially supported by the DGICYT, Spanish Ministry of Education and Science, under Project no. PB87-0327. 3. REFERENCES
1. M. Cabrejas Manchado, J.M.Guil and A. Ruiz Paniego, J. Chem. SOC.Faraday Trans. I , 85 (1989) 1775. 2. TRC Thermodynamic Tables, Texas Engineering Experimental Station (199 1). 3. J. Fournier, B. Fubini, V. Bolis and H.PkzCrat, Can. J. Chem., 67 (1989) 289. 4. G. Della Gatta, B. Fubini and G. Venturello, I. Chim. Phys. 70 (1973) 64.
Proceedings of the 10th International Congress on Catalysis, 19-24 July, 1992,Budapest, Hungary 0 1993 Elsevier Science Publishers B.V. All rights reserved
SURFACE ENERGETIC CHARACTERIZATION OF SUPPORTED METAL CATALYSTS BY GAS/SOLIDTITRATION MICROCALORIMETRY
J. M.Guy A. P. Masia, A. R. Paniego and J. M.T.Menayo Instituto de Quimica Fisica "Rocasolano", CSIC, Serrano 119,28006 Madrid, Spain
Adsorption microcalorimetry is a technique which has proved its usefulness in the characterization of catalysts, mainly using probe molecules. Its application can be extended to the study of surface reactions, thereby broadening the information that can be obtained. Here we show results of hydrogen titration of preadsorbed oxygen on iridium supported catalysts. Thermochemical calculations with the calorimetric data yielded information on the surface energetics for the two adsorbates.
1. EXPERIMENTAL Hydrogen and oxygen (Sociedad Espaiiola del Oxigeno, Spain), 99.995% pure, were used as adsorbates. Supported iridium catalysts were prepared by the incipient wetness method using hexachloroiridic acid (Alfa Ventron Inorganics.) Silica gel (BASF, D-ll- 11) and y-alumina (Girdler, T-126) were used as supports. Specific surface areas determined by the BET method, after heating in air at 973 K, were 128.0 and 149.1 m2g-', respectively. The following catalysts were prepared: Ir(5 %)/Siq, Ir(0.5 %)/SO2 and Ir(2.5 %)/A120, with 5.45%, 0.53% and 2.55% metallic content. The surface iridium amount, Irsf, was determined as described elsewhere [ 11 and is given in the first row of Table 1. Calorimetric measurements were carried out in a Tian-Calvet microcalorimeter (Model BT, Setaram, France), coupled to a volumetric apparatus with a Baratron manometer (MKS, USA.) Titration calorimetric experiments of preadsorbed oxygen with hydrogen were performed at 315 K. Differential heats of adsorption for hydrogen and oxygen were previously determined since they were needed for the thermochemical calculations described later. Calorimetric measurements were also made on the supports to take into account the adsorption on them. In the following, amounts of substance are expressed as pmol (of atoms in the case of hydrogen and oxygen) per gram of catalyst dried in vacuum at 700 K.
2. RESULTS AND DISCUSSION 2.1 Titration volumetry In figure 1 hydrogen titration isotherms for the two Ir/Siq samples are given as hydrogen uptake divided by surface iridium amount. The isotherms for the two samples nearly coincide. The dispersion degree, and therefore metal particle size, is approximately the same for the two samples (5%: 2.6 nm, 0.5%: 3.5 nm), which suggests similar surface properties. The titration isotherms show an initial vertical segment at zero equilibrium pressure. Afterwards a right angle knee appears followed by a final part where hydrogen uptake in-
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creases smoothly with pressure. Figure 1 ' also depicts a hydrogen adsorption isotherm. The knee and final part are similar to those observed in titration isotherms, which can be ascribed to the same process taking place after adsorption or titration ended. In consequence, the appearance of pressure was taken as the 2 end point of the titration reaction. The ' , same conclusion holds for the adsorption and titration isotherms on the Ir/A1203 sample. The amount of titrating hydrogen, Htit, for the three samples is given in the PIkPo third row Of 1. The amount Of Figure 1 . Hydrogen titration isotherms on Ir/Si02: oxygen taken U P in the run preceding the 0 , 0 5% and X 0.5% samples, Lower line: titration experiment, O,,, is shown in the hydrogen adsorption isotherm on the 5 % sample. second row. (Surface oxidation may proceed beyond adsorption if oxygen is present in the gas phase [l]. The two runs on the 1r/Al2O3sample, Table 1, show this clearly.) The two quantities, Htit and O,,, allow us to calculate the amount of hydrogen that remains on the iridium surface after oxygen reduction and water migration to the support (fifth row in Table 1.) H/Ir stoichiometries obtained (sixth row) are different for different supports [ 13, which justifies the need to calculate them instead of assuming a unique value of 1.
t2
Table 1
Hydrogen titration volumetry results and stoichiometries H , I Irsf
2;
Surface Ir: IrSf 'ad
(plkPa) Had
-f
'ox
hit - 2.0pr
Had / Ir,,
Ir(5%)/Si02 95 111.5 113.3 319.5 319.3 (0.56) (1.33) 96.5 92.7 1.01 0.98
Ir(0.5%)/Si02 7.5
8.2
23.5 (0.18) 7.1 0.95
@no1 of atoms/g)
Ir(2.5%)/Al2O3 80 97.8 79.2 320.4 288.4 (0.22) (0.45) 130.0 124.8 1.63 1.56
2.2 Titration microcalorimetry
The hydrogen titration differential heat per mol of hydrogen atoms consumed vs. amount taken up for various experiments on the Ir/SiO, samples is plotted in Figure 2-A. The titration heat includes the enthalpy change produced in each one of the processes taking place in the overall surface reaction. The dashed line corresponds to the adsorption heat measured in a hydrogen calorimetric isotherm on the 5 % sample. The titration and adsorption curves are in good agreement, which indicates that the hydrogen adsorption involved in the titration run follows the same trend as in an adsorption experiment, i.e. the iridium sites thatflrst become empty after oxygen reduction and water migration to the support, are precisely those of higher hydrogen adsorption energy. The same conclusion is reached for the titration reaction on the Ir/A1203 sample (Figure 2-B.)
2.3 Thennochemical calculations
The following thermochemical cycle can be written for the elementary reactions that take place in the overall titration process:
1861
where Sup stand for the support and cx is the value of the H/Ir stoichiometry: 1 and 1.6 for Ir/SiO, and Ir/A1,0? catalysts, respectively, as seen in Table 1. This thermochemical cycle was used to calculate the oxygen adsorption heat on these catalysts. (Calorimetric direct measurement of this quantity gave a constant value over most of the coverage range, since mobility of adsorbed oxygen is precluded by the strong 0-Ir bond formed. Therefore oxygen remains in the site where it first adsorbs, so that heats measured are statistical mean values.) From equations (1-5) we obtain: (6)
Adsorption and titration heats depend on coverage, so that application of equation (6) yields oxygen adsorption heats on iridium as function of coverage. A value of 242 kl/mol was used as water formation heat at 315 K in the vapour state [2]. Water adsorption heat on the supports vs. coverage was taken from the literature ([3] for silica and [4] for alumina.) The hydrogen adsorption heat and the hydrogen titration heat of preadsorbed oxygen at different coverages were interpolated from experimental values. Data and results for one experiment on the 1r/A1203 catalyst are given in Table 2. The amounts of oxygen being titrated appear in column 1. The hydrogen consumed in the oxygen titration is given in column 2: each Ir atom adsorbs one 0 atom; upon H titration, two H atoms combine with the 0 atom to form water (column 4) that migrates into the support, and 1.6 hydrogen Figure 2. Hydrogen titration heat: (A) IdSi02: o , 0 5% atoms (column 3) adsorb on the freed and X 0.5% samples. (B) Ir(2.5%)/M2O3: A , V Dashed Ir atom (one atom only in the case of Ir/Si02 samples.) lines: hydrogen adsorption heat.
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Table 2 Thermochemical calculations:
The differential heat values in columns 5, 6 and 7 were obtained by interpolation to the corresponding amounts in the preceding columns, The oxygen adsorption heat, which is the calculated value for breaking the existing O-Ir bond (column 8), increases as the titration reaction progresses. This shows that this reuction proceeds from weaker to stronger 0 - l r bonds. The same kind of calculations were made for the Ir/SiO, samples and similar results were Obtained. Figure 3 depicts Figure 3. Calculated oxygen adsorption heat on: Ir/Si02: Oxygen adsorption heats vs. coverage, 5%and X 0.5% and Ir(2.5%)/A1203:V . Dashed with O n e experiment for each sample. line: measured values on the 5% sample. They are in good accord and reveal the heterogeneity not shown by direct calorimetric measurements (dashed line in Figure 3.) From the two conclusions that have been reached we can deduce that iridium sites which adsorb oxygen weakly adsorb hydrogen strongly, and viceversa. The question arises wether this opposite heterogeneity is a property of the iridium crystallites, or if it produced upon adsorption of oxygen on them, i.e. if the oxygen on the indium surface could induce in it weak adsorption sites for oxygen which would be strong sites for hydrogen. This work was partially supported by the DGICYT, Spanish Ministry of Education and Science, under Project no. PB87-0327. 3. REFERENCES
1. M. Cabrejas Manchado, J.M.Guil and A. Ruiz Paniego, J. Chem. SOC.Faraday Trans. I , 85 (1989) 1775. 2. TRC Thermodynamic Tables, Texas Engineering Experimental Station (199 1). 3. J. Fournier, B. Fubini, V. Bolis and H.PkzCrat, Can. J. Chem., 67 (1989) 289. 4. G. Della Gatta, B. Fubini and G. Venturello, I. Chim. Phys. 70 (1973) 64.