Carbon Catalysts - American

Langmuir 1996, 12, 379-385

379

Structure Sensitivity of CO2 Hydrogenation Reaction Catalyzed by Pt/Carbon Catalysts M. C. Romań-Martıńez, D. Cazorla-Amoro´s, C. Salinas-Martıńez de Lecea, and A. Linares-Solano* Departamento de Quı´mica Inorga´ nica, Universidad de Alicante, Apdo. 99, Alicante, Spain Received April 25, 1995X Structure sensitivity of CO2 hydrogenation reaction catalyzed by Pt/carbon and Pt-M/carbon (M ) Ca, Mg) catalysts has been studied. Catalytic activity experiments have been carried out by TPR (temperature programmed reaction). Various techniques like H2-TPD (hydrogen temperature programmed desorption), TEM (transmission electron microscopy), and XRD (X-ray diffraction) have been used to study the evolution of platinum particles structure. The effects of a heat treatment at high temperature (1200 K) under different atmospheres and the catalyst reduction conditions on the catalytic activity and on the catalyst structure have been analyzed. The results of catalytic activity in relation to particle size have been analyzed with the concept of reaction dimension. The results show that CO2 hydrogenation catalyzed by carbonsupported platinum catalysts is highly sensitive to the structure of the platinum particles. The reaction dimension obtained is close to 0.06. A heat treatment to high temperature (1200 K) produces a large increase in activity. This behavior has been explained considering that at these temperatures, carbon atoms can dissolve in platinum producing a change in the structure, as a result of which the number of active sites increases noticeably.

1. Introduction Heterogeneous catalysis is usually a surface phenomenon; consequently the catalytic activity is directly related to the amount of exposed atoms in the catalyst particles. Therefore, a higher surface area, i.e. a higher dispersion (ratio between the number of surface atoms and total atoms) means, in principle, a higher catalytic activity. However, in a considerable number of cases the type of interaction occurring between reactants and/or products and the catalyst, and hence catalyst activity, depends on the surface structure of the particles that constitute the active phase. On the other hand, such surface structure may be deeply affected by the interaction with the reactants and the products, as suggested by the present view of surfaces as systems having a great flexibility.1 In this way, the surface can suffer structural modification in various time scales: that of the chemisorption process, of the catalyzed reaction, or at longer time scales. The temperature, the gaseous atmosphere, and the interaction with the support, in other words, the chemical environment, may cause a significant modification in the structure of the active species.2-4 As a result of all this, and in the specific case of supported platinum, we may find such diverse structures as spheres,5-7 cubo-octahedrons,7,8 or flat shapes of different geometries.5,7,9-11 X Abstract published in Advance ACS Abstracts, December 1, 1995.

(1) Somorjai, G. Catal. Lett. 1992, 12, 17. (2) Ponec, V. In Metal-Support and Metal-Additive Effects in Catalysis; Imelic, B., et al., Eds.; Elsevier: Amsterdam, 1982; p 63. (3) Winterbottom, W. L. Acta Metall. 1967, 15, 303. (4) Lin-Cai, J.; Pletcher, D. J. Electroanal. Chem. 1983, 149, 237. (5) Baker, R. T. K.; Prestidge, E. B.; Garten, R. L. J. Catal. 1979, 56, 390. (6) Atamny, F.; Duff, D.; Baiker, A. Catal. Lett. 1995, 34, 305. (7) Yacamań, M. J. In Catalytic Materials, relationship between structure and reactivity; Whyte, T. E., Jr., Dalla Betta, R. A., Derouane, E. G., Baker, R. T. K., Eds.; ACS Symposium Series 248; American Chemical Society: Washington, DC, 1984; p 335. (8) Yacamań, M. J.; Domıńguez, J. M. J. Catal. 1980, 64, 213. (9) Yeung, K. L.; Wolf, E. E. J. Catal. 1992, 135, 13. (10) Richard, D.; Bergeret, G.; Leclercq, C.; Gallezot, P. J. Microsc. Spectrosc. Electron. 1989, 14, 377. (11) Epell, S.; Chottiner, G.; Scherson, D.; Pruett, G. Langmuir 1990, 6, 1316.

0743-7463/96/2412-0379$12.00/0

Depending on the effect of the particle structure upon the activity and selectivity of the reactions, these are divided into two large groups:12 (a) non-structure-sensitive reactions, those in which the specific catalytic activity is not affected by the changes in the size of the catalyst particles, and (b) structure sensitive reactions, where the specific catalytic activity depends on the size of the catalyst particles. In previously obtained results13 on the catalysis of CO2 hydrogenation by carbon-supported platinum catalysts, it was found that the turnover frequency (TOF) depends on metal dispersion. This fact means that the reaction studied is structure-sensitive. There are few studies about the structure sensitivity of the CO2 hydrogenation reaction. In the literature, no study was found dealing with this subject when the reaction is carried out in gaseous phase. However, a few studies have been found analyzing, by means of voltammetry, the CO2 reduction in aqueous solution over platinum single crystals of different orientations.14,15 These studies show that CO2 reduction is structure sensitive: the reaction practically does not occur in the plane (111), whereas it does occur when the electrodes show the (100) and (110) orientations. The mentioned platinum surfaces would have the following order in reactivity: Pt(110) > Pt(100) . Pt(111).15c The present paper studies in depth the structure sensitivity of the CO2 hydrogenation reaction in Pt/C and Pt-M/C catalysts (M ) Ca, Mg). For this purpose, an interpretation is made of the results previously obtained in this system13 from the point of view of the structureactivity relationship and, furthermore, an analysis is made of the evolution of the activity of these samples when they are submitted to various processes modifying the structure of the platinum particles. Thus, the study carried out (12) Boudart, M. AIChE J. 1972, 18, 465. (13) Romań-Martıńez, M. C.; Cazorla-Amoro´s, D.; Linares-Solano, A.; Salinas-Martıńez de Lecea, C. Appl. Catal. A 1994, 99, 187. (14) Moralloń, E.; Va´zquez, J. L.; Pe´rez, J. M.; Aldaz, A. J. Electroanal. Chem. 1994, 380, 47. (15) (a) Rodes, A.; Pastor, E.; Iwasita, T. J. Electroanal. Chem. 1994, 369, 183. (b) Rodes, A.; Pastor, E.; Iwasita, T. J. Electroanal. Chem. 1994, 373, 167. (c) Rodes, A.; Pastor, E.; Iwasita, T. An. Quı´m. 1993, 89, 458.

© 1996 American Chemical Society

380

Langmuir, Vol. 12, No. 2, 1996

Roma´ n-Martıńez et al.

Table 1. Platinum and Alkaline-Earth Content, Platinum Dispersion sample

% Pt

Pt/A Pt/A4 Pt/A2 Pt-Ca/A2 Pt-Mg/A2

0.62 1.05 0.96 0.91 1.20

%M

D(Pt)

2.4 2.1

0.70 0.40 0.28 0.20 0.26

focuses on the following aspects: (i) the effect of gaseous atmosphere during the catalyst reduction stage and of heat treatment at higher temperatures, (ii) the estimation of the type of structural changes suffered by the particles as a result of heat treatments, and (iii) an approach to the structure sensitivity of the reaction from the point of view of the concept of reaction dimension, as developed by Avnir et al.16 2. Experimental Procedure 2.1. Supports. Three supports have been prepared, via the following procedure: the support labeled A was prepared by carbonization of a phenolformaldehyde polymer resin, in nitrogen flow, at 1273 K (heating at 5 K/min). The carbon obtained is a highly pure material, which has a high surface area (SBET of almost 600 m2/g) and a well-developed porosity.17 The support named A2 was obtained from A by HNO3 15 M oxidation at 353 K up to dryness. The oxidizing treatment causes an increase of about 150 m2/g in the BET surface of the support. A selective elimination of surface complexes from support A2, by means of a heat treatment at 800 K in nitrogen produced the support labeled A4. The removal of the less stable oxygen complexes, mainly carboxylic groups (which decompose as carbon dioxide), gives the support A4 a surface area of about 850 m2/g. 2.2. Catalysts. The Pt/C catalysts were prepared by impregnation with an aqueous solution of the [Pt(NH3)4]Cl2 complex, in a suitable concentration so as to obtain a platinum content of 1%. Impregnation was performed in excess of solution (10 mL of solution/g of carbon), then evaporating the solvent by flushing a nitrogen flow through the suspension. After this, the catalysts were dried at 383 K during the night and kept in a desiccator until they were used. In order to prepare the Pt-M/C catalysts, Ca2+ and Mg2+ ions were introduced in the A2 support (which has a high proportion of oxygen groups on its surface) by means of an ion exchange process from the aqueous solution of the corresponding acetates.17 The samples prepared in this way, which were labeled Ca/A2 and Mg/A2, were impregnated with the aqueous solution of [Pt(NH3)4]Cl2 in the conditions above described. The platinum and alkaline-earth content of the catalysts is shown in Table 1. Platinum dispersion was measured by hydrogen chemisorption, according to the following procedure: (i) a treatment at 673 K for 2 h in helium, with a 60 mL/min flow (this treatment is performed to prevent the platinum precursor decomposition in a hydrogen atmosphere, conditions in which the mobile hydride [Pt(NH3)2]H2 would be formed, thus causing an agglomeration of the platinum particles18,19); (ii) a heat treatment in hydrogen (60 mL/min) at 623 K for 12 h, so as to complete platinum reduction; (iii) vacuum outgassing for an hour at 573 K and cooling to 298 K; (iv) hydrogen dosage. The adsorption isotherms are linear in the pressure range used (50-250 Torr) and the monolayer of chemisorbed hydrogen was calculated by extrapolating the isotherm at zero pressure. Platinum dispersion for the various catalysts is shown in Table 1. 2.3. TPR Experiments. Catalytic activity of the different samples has been studied by means of TPR (temperatureprogrammed reaction) experiments. The equipment used for such purpose basically consists of a flow microreactor, which (16) Farin, D.; Avnir, D. J. Am. Chem. Soc. 1988, 110, 2039. (17) Salinas-Martıńez de Lecea, C.; Almela-Alarcoń, M.; LinaresSolano, A. Fuel 1990, 69, 21. (18) Dalla Betta, R. A.; Boudart, M. Proceedings of the 5th International Congress on Catalysis; Hightower, J., Ed.; North Holland: Amsterdam, 1973; Vol. 1, p 1329. (19) Rodrı´guez-Reinoso, F.; Rodrı´guez-Ramos, I.; Moreno-Castilla, C.; Guerrero-Ruiz, A.; Lo´pez-Gonza´lez, J. D. J. Catal. 1986, 99, 171.

operates at atmospheric pressure, coupled to a VG Quadrupoles mass spectrometer (MS). The equipment also includes a system of valves which allows the gas entering the reactor to be changed quickly (in a few seconds) with no alteration in the total flow. At the reactor outlet, a portion of the gas is sent to the MS through a typical introduction system, formed by a capillary tube, a rotary vacuum pump, and a porous leak. The raw MS data are corrected for background levels, fragmentation, and MS sensitivity. The results are expressed as molar fraction per gram of sample. As the system used is an open flow-through one, the reaction rate is proportional to the molar fraction and the figures show, indeed, specific rates. Before the TPR experiments, the catalysts were reduced in situ in the conditions described above, i.e., first, platinum precursor decomposition in helium (60 mL/min) at 673 K; then treatment in hydrogen flow (60 mL/min) at 623 K, for 12 h. The samples were cooled down in hydrogen and at room temperature the hydrogen was replaced by the mixture CO2/H2/He (3/10/87), also with a 60 mL/min flow. When the gases had a stable level, heating was initiated at 20 K/min up to 1200 K. Once the maximum temperature was reached, the reactant gas was replaced by a helium flow and the sample was allowed to cool down. After this heat treatment at high temperature, further TPR experiments were performed on the sample. The spectra obtained display the evolution of the H2, CO2, CO, H2O, and CH4 levels. Besides, for each of the spectra the b1 ) CO2 + CO and b2 ) CO2 + 1/2CO balances have been calculated, corresponding to the CO2 + H2 a CO + H2O (hydrogenation) and CO2 + C a 2CO (carbon gasification) reactions, respectively. As was explained before,13 the fitting of the TPR spectrum to one of these balances allows us to determine the predominance of one of these two possible reactions. 2.4. Hydrogen Desorption Experiments. In order to perform the hydrogen adsorption-desorption experiments, the same experimental device as for the TPR experiments was used. The procedure was as follows: The sample, previously reduced in the conditions described above, was treated at room temperature in a pure hydrogen flow (60 mL/min) for 5 min. Then hydrogen was replaced by helium (60 mL/min) and, after waiting a few minutes for the hydrogen residual pressure in the mass spectrometer to level out, the heating at 100 K/min up to 773 K was started. 2.5. X-ray Diffraction. Both the samples obtained after the reduction treatment (for dispersion measurements) and those obtained after the first TPR experiments were studied by X-ray diffraction. The size of the platinum crystal was calculated from the widening of the diffraction peak by applying the Scherrer equation. The samples were slightly ground to perform these measurements. The experiments were carried out using a Seifert JSO Debye-Flex 2002 diffractometer, with a Cu cathode and a Ni filter (35 mA and 42 kV), being the scanning rate 2 deg/min. 2.6. Transmission Electron Microscopy. For observation with the transmission electron microscope, the samples were prepared as follows: A small portion of the ground catalyst was dispersed in toluene (about a 3 mg sample in 200 mL of toluene) with an ultrasonic bath; a drop of this suspension was placed on a copper grid coated with a perforate carbon film, which is used as a slide in the microscope. The equipment used is a Zeiss EM-10 transmission electron microscope, which allows a magnification higher than 100000×.

3. Results and Discussion 3.1. Effect of the Heat Treatment on the Catalyst Activity. The TPR experiments obtained with the supports have been discussed previously.13,30 In summary, these experiments show that there is a noticeable interaction between hydrogen and the support, depending on the surface chemistry of the supports. It is also observed that the gasification reaction starts above 1073 K. It must be pointed out that no reaction between hydrogen and carbon dioxide occurred during the TPR experiment. Figure 1 shows, as an example, the TPR spectra obtained for the Pt/A2 sample. Figure 1a corresponds to the first TPR (TPR-I) and Figure 1b to a second TPR (TPR-II), performed after TPR-I. With this sample, and with all

CO2 Hydrogenation Reaction

Langmuir, Vol. 12, No. 2, 1996 381

Figure 2. CO evolution in three consecutive TPR experiments, sample Pt/A2. Table 3. Platinum Dispersion and TOF (Turnover Frequency) for the Catalysts before and after the First Heat Treatment in the Reaction Mixture TOF (s-1) × 103

TOF (s-1) × 103 TPR-I

Table 2. Reaction Rate, Referred to CO Formation, for Three Consecutive TPR Experiments r (mol of CO/mol of Pt/s) × 103 TPR-I

TPR-II

TPR-III

sample

573 K

673 K

573 K

673 K

573 K

673 K


2.8 0.0 0.0 8.0 0.0

18.0 3.1 0.0 37 8.5

18.3 5.9 2.3 4.1 7.1

63.0 38.1 19.9 33.0 36.1

16.2 8.2 3.0 3.4 7.1

63.0 45.5 24.0 33.1 32.2

the cases analyzed, it has been found that, after the heat treatment, there is a considerable increase in activity and the catalyst selectivity suffers no changes (basically CO and H2O are produced). The TPR-II profiles of all the catalysts studied are very similar. Table 2 summarizes reaction rate (mol of CO produced/mol of Pt/s) at 573 and 673 K in the TPR-I and TPR-II experiments. It also includes the reaction rate in a third TPR (TPR-III), performed after two consecutive TPR experiments. The results show that the Pt/A catalyst (the one with the highest initial platinum dispersion) exhibits the highest reaction rate after the heat treatment, whereas the Pt/A2 sample (the catalyst containing only platinum with the lowest dispersion) continues to be the least active. In other words, the same trend observed in catalyst activity before the heat treatment13 is shown in the heat treated catalysts. The only exception is found in the Pt-Ca/A2 catalyst. This sample, which is the one with the highest activity in the TPR-I, suffers a loss in activity after the heat treatment at high temperature. If, after the TPR-II experiment, the samples are submitted to a further TPR experiment (TPR-III), it can be found that there is no significant change in catalyst activity (Table 2). Seemingly, after a heat treatment at 1200 K the catalyst reaches a considerable particle size,

TPR-III

D

573 K

673 K

D*a 573 K 673 K 573 K 673 K


0.70 0.40 0.28 0.20 0.26

4.4 0.0 0.0 40.0 0.0

25.7 7.7 0.0 185.0 32.7

0.21 0.19 0.16 0.19 0.19

a

Figure 1. TPR spectra of sample Pt/A2 (b1 ) CO2 + CO; b2 ) CO2 + 1/2CO): (a) TPR-I; (b) TPR-II (see the text).

TPR-II

sample

87.3 31.2 14.1 21.5 37.5

300.2 200.5 124.6 173.6 189.9

79.1 42.9 18.5 18.1 37.5

300.2 239.5 149.8 174.0 169.7

D*, catalyst dispersion after TPR-I.

the structure of which is almost not modified with further heat treatments. As an example of this phenomenon, Figure 2 shows the CO evolution for the Pt/A2 sample in the three consecutive TPR experiments. Table 3 shows Dsinitial platinum dispersion (after the reduction treatment)sand D*splatinum dispersion of the samples submitted to the TPR-I treatmentstogether with the corresponding turnover frequency (TOF). As has been observed by X-ray diffraction and H2 chemisorption, a heat treatment results in an increase of particle size. In all cases, final dispersion (D*) is near 0.20. With these dispersion values, the turnover frequency of the treated catalyst was calculated and then compared with that of the nontreated catalysts. The results shown in Table 3 underline an important fact: the heat treatment at high temperature causes a noticeable increase (by a factor over 10) in specific activity in all the catalysts, except in the above mentioned case of the Pt-Ca/A2 sample. Such a catalyst shows in the TPR-I a turnover frequency similar to that of the most active catalysts for CO2 hydrogenation.13 The heat treatment allows us to reach, in all cases, a similar and even higher activity (Pt/A sample) than that of the nontreated Pt-Ca/A2 catalyst. This indicates that platinum, after a suitable treatment, is an adequate catalyst for the CO2 hydrogenation reaction, with a high selectivity toward CO formation. These results confirm that the CO2 hydrogenation reaction is sensitive to the catalyst structure. Such sensitivity had been detected when studying electrochemical CO2 hydrogenation (in an aqueous solution) in different planes of platinum single crystals.14,15 On the other hand, the results obtained show the structural change suffered by the platinum particles as a result of a heat treatment. After such a treatment, the particles develop structures that must contain a higher concentration of active sites for the CO2 hydrogenation reaction. The differences existing between the treated catalysts can be more clearly observed by comparing TOF values.

382


Figure 3. CO evolution during a TPR experiment after different treatments, sample Pt/A4.

Thus, we notice that the Pt/A catalyst shows the highest activity, Pt/A2 is the least active and the Pt/A4, Pt-Ca/ A2, and Pt-Mg/A2 catalysts show similar values of turnover frequency. This evolution indicates that the redistribution suffered by the platinum particles as a result of the heat treatment depends on the initial catalyst dispersion and also that the period of the heat treatment performed is not enough to obtain in all cases particles with an identical structure. The results suggest that in the case of the most disperse catalysts, structural modification is basically determined by particle sintering. In catalysts with an initially low dispersion (D = 0.2), since such dispersion practically does not change after the heat treatment, the structural modification must be determined by the mobility of atoms both within and on the surface of the platinum particles. 3.2. Effect of the Atmosphere during Catalyst Reduction and Heat Treatment. The atmosphere in which the catalyst is treated during or after the reduction stage may have an influence on the structure of the particles formed. The interaction of the gas with the surface of the particles affects in a different way the surface energy of the platinum planes.20,21 In order to analyze this question, different treatments were performed on the samples, and the catalyst activity was compared. Thus, the effect of the following treatments was studied: (a) reduction in normal conditions (two steps), first, treatment in He (673 K) and then in H2 (623 K) (experiments described in section 3.1); (b) direct reduction in H2 at 623 K (as indicated in the experimental procedure, a direct reduction treatment in H2 yields a mobile hydride which in turn leads to a significant agglomeration of platinum catalysts with D = 0.05 are obtained); (c) treatment in CO at 673 K for 1 h after reduction in normal conditions ((a) treatment); (d) treatment in He up to 1200 K after reduction in normal conditions. Figure 3 shows, as an example, CO evolution in TPR experiments with the Pt/A4 sample after being submitted to each of the treatments we have described. Figure 3 shows that when the sample is directly reduced in H2 (a treatment leading to a very low dispersion (D ≈ 0.05)), the activity is very low. It follows, therefore, the trend (activity versus dispersion) found in the catalysts that only contain platinum and have been reduced in two stages (He + H2) (see Table 3). The sample treated in CO at 673 K does not differ significantly from the catalyst which was not submitted to such a treatment, which indicates that the temperature used has not been high enough to obtain an important structural modification. (20) Lombardo, S. J.; Bell, A. T. Surf. Sci. Rep. 1991, 13, 1. (21) Shi, A.-C.; Masel, R. I. J. Catal. 1989, 120, 421.


Figure 4. CO evolution in two consecutive TPR experiments with sample Pt/A4 directly reduced in hydrogen (without the previous treatment in helium).

When the catalyst, after being reduced, is submitted to a heat treatment in He up to 1200 K (Figure 3), a substantial increase in activity is achieved. However, in this case, the activity is lower than that obtained after a treatment in the reactant mixture (see Figure 3). This indicates that both the presence of hydrogen, CO2, and CO (the latter resulting from the reaction) and the partial gasification suffered by the support enable the particles to have their structure modified, which in turn leads to a higher catalyst activity. It must be pointed out that, in all cases, a heat treatment in the reactant mixture causes again a very significant increase in catalyst activity. Figure 4 shows, as an example, CO evolution in the Pt/A4 sample directly reduced in hydrogen, during two consecutive TPR experiments. The TPR-I curve is that shown in the previous figure (Figure 3), which shows the low activity of this sample, and the TPR-II curve proves the marked increase in activity which is obtained after a heat treatment in the reactant mixture (CO2/H2/He). In short, these results suggest that the structure formed after a high temperature heat treatment is the one containing the largest number of active sites for the reaction studied and that this structure is substantially different from that achieved after the reduction treatment at 623 K. It may be underlined that, in accordance with what was previously discussed,22 the support has a relevant effect upon the structure of the platinum particles. 3.3. Temperature-Programmed Desorption (TPD) after Hydrogen Chemisorption. TPD experiments after chemisorption of a gas, such as H2, are greatly useful to monitor in situ the evolution of catalyst dispersion after various treatments. Also, from the different chemisorbed states that may occur, these experiments may allow us to obtain data on the surface structure and particle shape.23 In the case of H2 chemisorption in platinum single crystals,24-26 polycrystalline platinum,27 and carbonsupported platinum,28 the TPD spectra show well-defined peaks appearing over a wide range of temperatures, both above and under 298 K. The relative proportion of the various adsorption states and the corresponding desorption energy are typical of the surface structure studied. (22) Romań-Martıńez, M. C.; Cazorla-Amoro´s, D.; Linares-Solano, A.; Salinas-Martıńez de Lecea, C.; Yamashita, H.; Anpo, M. Carbon 1995, 33, 3. (23) Poppa, H. Catal. Rev.-Sci. Eng. 1993, 35, 359. (24) Christmann, K.; Ertl, G.; Pignet, T. Surf. Sci. 1976, 54, 365. (25) Pennemann, B.; Oster, K.; Wandelt, K. Surf. Sci. 1991, 249, 35. (26) Shern, C. S. Surf. Sci. 1992, 264, 171. (27) Thrush, K. A.; White, J. M. Appl. Surf. Sci. 1985, 24, 108. (28) Scholten, J. J. F.; Pijpers, A. P.; Hustings, A. M. L. Catal. Rev.Sci. Eng. 1985, 27, 151.


Langmuir, Vol. 12, No. 2, 1996 383 Table 4. Platinum Dispersion Hydrogen Desorption (A) in TPD Spectra and HS/HW Ratio before the heat treatment sample

D


0.70 0.40 0.28 0.20 0.26

a

Figure 5. H2 desorption in a TPD experiment (100 K/min) after hydrogen chemisorption at 298 K, sample Pt/A.

The states desorbing at temperatures below 298 K are related to the so-called weakly bonded hydrogen, whereas those desorbing at higher temperature correspond to what is known as strongly bonded hydrogen.29 The experimental device used here to perform the TPD experiments does not allow carrying out H2 chemisorption at low temperatures (of around 150 K), which in turn makes it impossible to obtain the complete TPD spectrum. The experimental procedure followed consisted of H2 chemisorption at 298 K, replacement of H2 by He to remove weakly bonded hydrogen, and then the TPD experiment. This allows a determination of the strongly bonded hydrogen, which may be compared for the various catalysts. The information obtained in this way is complementary to that of the chemisorption isotherms, from which we can calculate the total hydrogen adsorbed at 298 K. Figure 5 shows as an example of the TPD spectra obtained, those corresponding to the Pt/A catalyst, after the reduction stage (before TPR-I) and after being treated up to 1200 K in the reactant mixture (after TPR-I). The spectra show an asymmetrical desorption peak (with a smaller slope after the maximum), the maximum of which appears between 433 and 453 K. Also, a substantial H2 desorption can be observed after 623 K, related to the hydrogen transferred to the support during the reduction stage.30 Both the desorption energies and the reaction order of the TPD experiments have been estimated from an analysis of the peak width and the skewness index.31 The desorption energy values obtained for the various spectra vary from 35 to 50 kJ/mol, which are within the range found for the desorption states observed in platinum catalysts.28 The calculated skewness index (X1/2 ) 7-20) is higher than what might be expected for a second-order desorption (X1/2 ) 3-5). This result, that proves the existence of several overlapping peaks, is in accordance with the various adsorption states observed at high temperatures, both in platinum catalysts28 and in well-defined planes of platinum single crystals.24-26 Table 4 includes the desorption area (µmol of H2/g) calculated in the TPD spectra for the samples after the reduction stage and after a high-temperature heat treatment in the reactant mixture. The table also shows the ratio between strongly and weakly bonded hydrogen (HS/ (29) Prado-Burguete, C.; Linares-Solano, A.; Rodrı´guez-Reinoso, F.; Salinas-Martıńez de Lecea, C. J. Catal. 1991, 128, 397. (30) Romań-Martıńez, M. C.; Cazorla-Amoro´s, D.; Linares-Solano, A.; Salinas-Martıńez de Lecea, C. Carbon 1993, 31, 895. (31) Falconer, J. L.; Schwarz, J. A. Catal. Rev.-Sci. Eng. 1983, 25, 141.

Aa (µmol/g) HS/HW 7.3 3.3 1.7 6.8 2.1

0.50 0.18 0.13 3.10 0.15

after the heat treatment D 0.21 0.19 0.16 0.19 0.19

Aa (µmol/g) HS/HW 1.6 1.6 1.7 2.1 2.6

0.31 0.20 0.29 0.35 0.31

A, desorption area expressed as µmol of H2 per gram of catalyst.

HW), a parameter which is related to the surface structure of the platinum particles. The HS/HW ratio obtained for the catalysts containing only platinum (Pt/A, Pt/A4, and Pt/A2) depends, as has also been observed previously,29 on the catalyst dispersion. This ratio decreases with decreasing platinum dispersion. For high dispersions and, presumably, due to interaction with the support, the structure shows substantial differences when compared to that found for lower dispersions. The XAFS experiments carried out in these samples32 confirm that the platinum particles structure depends on the interaction with the support. The Pt/A sample possesses a structure in which the platinum atoms have an important coordination with atoms from the support, which is in accordance with a flattened structure.10 With decreasing dispersion, the coordination by platinum atoms increases, until finally the typical EXAFS spectrum of bulk platinum can be observed (as in the case of the Pt/A2 sample32). All this shows that there is a transition toward structures which possess a higher tridimensionality and/ or a higher crystallinity. In the case of catalysts containing the alkaline earth (Pt-Ca/A2 and Pt-Mg/A2), two clearly different situations occur. The HS/HW ratio reaches a high value in the case of the catalyst containing calcium. In the catalyst containing magnesium, however, this ratio is close to those obtained for the catalysts containing only platinum with similar dispersions. These results suggest that in the Pt-Ca/A2 catalyst, the Pt-Ca interaction is important, indicating that in this case it must have developed a structure for the platinum particles very different from that observed in the other cases. Such modification may be responsible for the high activity found with this catalyst (see Table 3). In the Pt-Mg/A2 catalyst, the Pt-Mg interaction is less important, and the improvement in activity due to the presence of this alkaline earth, though it does exist, is small compared with that observed in the Pt-Ca/A2 sample. For catalysts treated up to 1200 K, the HS/HW ratio is about 0.3 in all cases, except for the Pt/A4 catalyst. These values differ from those found for nontreated catalysts, thus showing that the structure obtained after the treatment up to 1200 K in the reactant mixture is substantially different. It is interesting to point out that after the heat treatment, the HS/HW ratio obtained for the Pt-Ca/A2 catalyst is 0.35 (see Table 4), which indicates that the interaction existing between Pt and Ca has disappeared, perhaps as a result of the segregation that occurs when Pt and CaO sinter. It may be concluded, therefore, that a high-temperature treatment causes a certain homogenization of structures. In short, the TPD experiments after H2 chemisorption, supplemented with the chemisorption isotherms, yield information on the structure of platinum particles and its evolution in relation with the dispersion and the treatments to which the catalysts are submitted. The results (32) Romań-Martıńez, M. C. PhD Thesis, University of Alicante, 1995.

384


Figure 6. TEM photograph of the non-heat-treated sample Pt/A2: magnification, 100000×; scale, 1 cm ) 122 nm.

obtained show that the catalysts, after the reduction stage (He + H2) (before the heat treatment), have platinum particles with a structure that varies according to catalyst dispersion. It can be also deduced that after a heat treatment all the catalyst particles show structures that are different from the previous ones and that, according to the TPR experiments results, these particles have the largest number of active sites for CO2 hydrogenation. 3.4. TEM Study of Treated and Nontreated Catalysts. TEM photographs obtained for the nontreated catalysts show a certain degree of heterogeneity in particle size. Though a few particles can be found with a size of about 10 nm, most of them have a much smaller size (e5 nm), which in some cases, depending on the sample analyzed, cannot be detected with the magnification used (100000×). Figure 6 shows, as an example, a photograph obtained for the reduced catalyst Pt/A2 nontreated at high temperature (i.e., after the reduction stage). We may observe a particle the size of which is 9 nm (marked in Figure 6 with an arrow), whereas the rest have an average size of 3.6 nm (which corresponds, considering spherical particles, to a dispersion of 0.30). The shape of these particles is very difficult to determine; only in the case of larger particles it is possible to consider that we are dealing with rounded shapes, which might correspond to spheres. The TEM photographs of the catalysts subjected to a heat treatment (TPR) show that in such catalysts there is a noticeably higher proportion of larger particles. As an example, Figures 7 and 8 show the TEM images corresponding to the treated samples Pt/A2 and Pt-Ca/ A2, respectively. In these figures it may be observed that although there is a heterogeneous distribution of particle size, ranging between 2 and 25 nm, there is a larger number of particles having a size close to 5 nm. This observation agrees with the chemisorption results, according to which the platinum dispersion in these samples is =0.2 (that corresponds, considering spherical particles, to a particle size of about 5 nm). In all cases, the TEM technique confirms that the heat treatment causes an increase in particle size that normalizes the resulting size distribution. The particles have various shapes; in general, they are rounded shapes, though in some cases we can find particles with straight sides (marked in Figure 8 with an arrow), from which we may deduce that some crystallographic planes have developed more than others. In spite of these observations, the results of the TEM study do not allow us to conclude that after the heat treatment a specific structure of platinum particles has developed.


Figure 7. TEM photograph of the sample Pt/A2 heat-treated in the reactant mixture: magnification, 100000×; scale, 1 cm ) 122 nm.

Figure 8. TEM photograph of the sample Ca-Pt/A2 heattreated in the reactant mixture: magnification, 100000×; scale, 1 cm ) 122 nm.

3.5. Application of Reaction Dimension Concept. Avnir et al.16 have developed the concept of reaction dimension as applied to dispersed catalysts. The aim is to find a parameter typical of the catalyzed reaction, so as to establish quantitative comparisons of sensitivity to structure. In relation with this idea, turnover frequency (TOF) varies according to a power law with particle size

TOF ∝ RDr-2

(1)

where 2R is particle size and Dr is reaction dimension. Figure 9 shows the plot of ln(TOF), for TOF calculated at 673 K, versus ln(R) for the various catalysts studied. The conclusions that may be drawn from these results are similar to those deduced from the TPD experiments and TEM microscopy. It may be observed that for catalysts treated at T e 673 K, except for Pt-Mg/A2 and Pt-Ca/A2 catalysts, a good linearization of specific activity may be reached, with a slope close to 2, which means a dimension of Dr ) 0.06. A geometrical interpretation of this value of Dr leads to the conclusion that it is mainly the atoms in the corners that show activity for the CO2 hydrogenation reaction.19 The deviation observed for catalysts containing two catalytic species (platinum and alkaline earth) reveals the positive effect of the alkaline earth in catalyst activity, either due to Pt-Ca interaction as is suggested in section 3.3 or as a result of the strong interaction existing between CO2 and CaO.13 Finally, the performance of the catalysts treated at 1200 K (catalysts after a TPR experiment and


Langmuir, Vol. 12, No. 2, 1996 385

The existence of dissolved carbon atoms in platinum, even forming a substitutional alloy, leads to a structural change very different in nature from that occurring in nontreated samples with a different dispersion degree. This may be the reason for the high deviation observed in the plot of ln(TOF) versus ln(R) in the case of treated samples. It is possible that the structural change, which takes place as a result of the presence of carbon atoms in the platinum particles, leading to the appearance of a primitive cubic superstructure,34 might lead to the formation of more active sites toward CO2 hydrogenation. 4. Conclusions

Figure 9. Plot of ln(TOF) versus ln(R) for different catalysts and treatments.

sample Pt/A2 treated up to 1200 K in He) deviates from that of nontreated catalysts, which means that these catalysts do not follow a structural evolution similar to that in catalysts which have not been treated. Platinum particles may change in shape, as several authors have pointed out,7,33,34 after being submitted to a heat treatment. Thus, Yacamań has observed that after a methanation reaction at 1123 K, particle shape tends to be more flat,7 whereas Baker et al. have noticed that platinum particles on carbon change from a hemispherical into a flat morphology when heated in hydrogen at 925 K and then recover their shape at 1120 K.33 Such structural changes, typical of the Pt/C system, seem to be related to carbon solution in platinum particles, in such a way that a substitutional alloy is formed.34 It is likely that the reason for the increase in activity observed after the thermal treatment may be connected with these observations. Thus, during a treatment up to 1200 K in the presence of CO2, support gasification takes place in a certain range of temperatures. This process, when catalyzed by Pt, may occur by means of C-C bonds breaking and C atoms solution in platinum particles.35 (33) Baker, R. T. K.; Sherwood, R. D.; Dumesic, J. A. J. Catal. 1980, 66, 56. (34) Lamber, R.; Jaeger, N. Surf. Sci. 1993, 289, 247.

The results discussed in this paper strongly suggest that the CO2 hydrogenation reaction in gaseous phase is structure-sensitive. It has been found that specific catalytic activity changes with dispersion (and, hence, with particle size), in such a way that the concept of reaction dimension can be applied to this system. The estimated dimension (0.06) indicates that the reaction is highly sensitive to the structure. From the hydrogen desorption experiments it may be deduced, as well, that the structure of the particles in nontreated catalysts depends on the dispersion state; that is, particles which have a different size also possess a different geometrical shape (ranging between forms of varied tridimensionality and/or crystallinity). The structural changes that can take place after a heat treatment (at 1200 K) would lead to a substantial increase in specific catalyst activity. Although a high-temperature treatment is necessary for structural change, the atmosphere in which such treatment is performed also has a clear influence. The creation of active sites caused by a heat treatment is mainly attributed to the formation of certain structures as a result of the solution of carbon atoms in platinum particles. Such a phenomenon takes place at high temperatures as a result of Pt-carbon interaction. Acknowledgment. The authors thank the DGICYT (Project AMB92-1032-CO2-O2) for financial support and the MEC for the Thesis grant of M. C. Romań. LA950329B (35) Owens, W. T.; Rodrı´guez, N. M.; Baker, R. T. K. J. Phys. Chem. 1992, 96, 5048.

Carbon Catalysts - American

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