750
Langmuir 1994,10, 750-755
Preparation of Platinum Supported on Pregraphitized Carbon Blacks F. Coloma, A. Septilveda-Escribano,J. L. G. Fierro,? and F. Rodriguez-Reinoso’. Departamento de Qulmica Inorghnica, Universidad de Alicante, Apartado 99, E-03080 Alicante, Spain Received October 6,1993. In Final Form: January 3,1994” Three samples of pregraphitized carbon blacks with the same porous texture but increasing amounts of oxygen surface groups have been wed to prepare carbon-supported platinum catalysts (about 1w t 5% loading). The carbon surfacebecomes oxidizedupon impregnationwith hexachloroplatinicacid in aqueous solution,whereas platinum ie reduced from Ptw to Ptn, as evidenced by X-ray photoelectron spectroscopy ( X P S ) and temperature-programmed desorption (TPD) experiments. TPD profiles of fresh catalysts show a peak of carbon monoxide evolutionat low temperatures (623-723 K)which does not appear in those of the parent supports,pointingout decompositionof the oxygen surfacegroupscreated upon impregnation with the metal precursor. The platinum-support interaction in the reduced catalysb is diminished by the presence of oxygen surface complexes, leading to a lower metal dispersion and a lower resistance to sintering. I. Introduction The continuous increase in the use of carbonaceous materials as catalyst supports makes necessary a deep knowledge of the preparation variables determining the properties and performance of the final catalysts. Carbonsupported platinum is widely used in a great variety of reactionsincludinghydrogenation,’-8 hydrogenolysis,and isomerization4and also in fuel-cell systems.6 In these applications,highly dispersedplatinum is required in order to achieve a better utilization of the noble metal. Several works have been carried out which relate platinum dispersion to support characteristics such as surface heterogeneity? porous structure? and content of oxygen surface groups on the s u p p ~ r t Solar . ~ et aLehave recently shown that interactions of the carbon surface with both the metal precursor and the solvent during the impregnation step in the process of catalyst preparation greatly influence the metal uptake and dispersion. These interactions are governed by the polarity of the solvent,the pH of the impregnatingsolution,the cationicor anionicnature of the metal precursor, and the surface charge in solution of the carbon support, identified by ita isoelectric point (IEP). Surface characteristics of the carbon can be tailored by partial oxidation in order to create oxygen surface groups which can act as anchoring centers for the metal precursor. To whom correspondence should be addressed. + h t i t u t o de CaWsis y Petroleoqulmica, CSIC Campus UAM, Cantoblanco, E-28049Madrid, Spain. 0 Abstract published in Advance ACS Abstracts, February 16, 1994. (1) Melillo, D.; Cvetovic, R. J.; Ryan, K. M.; Sletzinger, M. J. Org. Chem. 1986,61,1498. (2) Giroir-Fendler, A.; Richard, D.; Gallezot, P. Catal. Lett. 1990,5, 175. (3) Beason, M.;Bullivant, L.; Nicolaus, N.; Gallezot, P. J. Catal. 1993, 140,30. (4) Rodriguez-Reioeo, F.; Rodriguez-Ram-, I.; Moreno-Castilla, C.; Guerrero-Rub, A.; Mpez-Godlez, J. D. J. Catal. 1987,107, 1. (5) Honji, A.; Mori, T.; Hiehinuma, Y.J. Electrochem. SOC.1990,137,
20a. (6)Ehrburger, P.; Mahajan,0.P.; Walker, P. L., Jr. J. Catal. 1976,43, 61. (7) Rodriguez-Reinwo, F.; Rodriguez-Ram-, I.; Moreno-Castilla, C.; Guerrero-Rub, A.; Mpez-Godez, J. D.J. Catal. 1986,99,171. (8) Prado-Burguete, C.; Linaree-Solano, A.; Rodrlguez-Reinoeo, F.; Salinas-Martlnez de k e a , C. J. Catal. 1989,115,98. (9) Solar, J. M.; Leon y Leon, C. A.; Oeaeo-Asare, K.; Radovic, L. R. Carbon 1990,28,369.
0743-7463/94/2410-0750$04.50/0
In aqueous solution, the oxygen functional groups behave as amphoteric oxides, being dissociated or protonated depending on the pH of the solution, according to the following equations:”J C-OH2+ F! C-OH + H+ C-OH
F!
C-0-
+ H+
(1) (2)
Thus, impregnation with an anionic precursor like PtCk” is favored in acidic solution, whereas cationic precursors such as Pt(NHs)42+should be used in basic solutions. In this way, Richard and Gallezotll obtained very small carbon-supported platinum particles by functionalizinga graphite and a carbon black by liquid-phase oxidation with a solutionof sodiumhypochloriteand then exchanging the H+ions of the functionalized supports with Pt(NHs)r2+ cations in an ammoniacal solution. Another important parameter to bear in mind is the isoelectric point of the carbon support. Partial oxidation of a carbon with aqueous solutionsof oxidant reagents (HzOz,HNOs) mainly creates acidic oxygen surface complexesthat lower the isoelectric point of the support.12 The importance of the oxygen surface groups has been evidenced in a previous work from this laboratory, when showinga direct relationship between platinum dispersion and the amount of oxygen surface groups desorbing as carbon monoxide (CO complex) when a carbon black treated under hydrogen at 1273 K and then subjected to different oxidationdegreeswas used as the support.6Later on, it was also observed that higher platinum dispersions were obtained when carbon black supports were heattreated in helium at 2273 K, without any further oxidation treatment, before impregnationwith HzPtC&.’s Although in this case the process of PtC&” adsorption would be favored by the higher IEP of the pregraphitized supports, the absence of oxygen surface groups in the carbon led to the idea of the existence of other anchoring sites for the platinum precursor. These sites could be T sites in the (10) Noh, J. S.; Schwarz, J. A. Carbon 1990,28,675. (11) Richard, D.; Gallezot, P. 1nA.eparationojCatalystsZV;Dalmon, B.,et al., Me.;Elsevier: Amsterdam, 1987; p 71. (12) Lau, A. C.;Furlong, D. N.; Healy, T. W.; Grieeer, F. ColloidsSurf. 1986, 18, 93. (13) Prado-Burguete, C.; Linarea-Solano, A.; Rodriguez-Reinoso, F.; Salinas-Martfnez de k e a , C. J . Catal. 1991,128,397.
Q 1994 American Chemical Society
Langmuir, Vol. 10, No. 3, 1994 751
Platinum Supported on Pregraphitized Carbon Blacks
basal planes of the graphite crystallites, which are capable of acting as electron donors to form a coordination bond with the platinum precursor.14 A further study on the effect of the pregraphitized temperature of the carbon black support on platinum dispersion showed that it was maximum in the catalyst supported on a carbon black pregraphitized in helium at 2073 K.15 In this context, the aim of this paper is to increase the knowledge of interactions taking place during the preparation of carbon-supported platinum catalysts by studying the impregnation of a pregraphitized carbon black, with and without oxygen surface groups but with the same porous texture, with an aqueous solution of hexachloroplatinic acid. 2. Experimental Section 2.1. Supports. The starting material for support preparation was a furnace carbon black (CC-40.220) supplied by Columbian Chemical Co., with a mean particle diameter of 18 nm (carbon S). Support 52 was prepared by thermal treatment of this carbon black at 2073 K for 1 h under a helium flow. Then, support 52 was subjected to an oxidation treatment with Ha02 in order to create oxygen surface groups: 1 g of the support was immersed into an aqueous solution of HzOz (12 N), and the slurry so formed was stirred for 48 h at room temperature. Finally, the oxidized sample (hereafter called 5203 was washed with deionized water to eliminate the excess of HzOz and dried under air at 373 K. One more support (S2hT) was obtained by heat treatment (He flow, 773 K, 1 h) of 520, in such a way that the less stable oxygen surface groups are eliminated. The characterization of the porous texture of the carbons was carried out by physical adsorption of nitrogen at 77 K and carbon dioxide at 273 K in a conventional gravimetric system using silica spring balances. The determination of the amount and type of oxygen surface groups in the carbons was accomplished by temperatureprogrammed desorption (TPD) under helium. Samples (between 150 and 200 mg) were placed in a U-shaped quartz cell and treated at 373 K for 1 h under a helium flow (60 cmasmin-1). Then, the temperature was raised at 50 Ksmin-1 to 1250 K. The decomposition products (carbon monoxide and carbon dioxide) were measured by mass spectroscopy. 2.2. Catalysts. Catalysts were prepared by impregnation of the supports with an aqueous solution (10 cmS/gof support) of HzPtCle.6H20 (reagent for synthesis, from Merck), with the appropriate concentration to obtain a Pt load of about 1 w t 5% Pt. The excess of solvent was removed by flowing nitrogen through the suspension, and the remaining solid was dried overnight at 393 K and then kept in a desiccator until use. The whole preparation process was carried out in the dark. The platinum content was measured by burning away the carbon in air at 973 K and analyzingby UV spectrophotometry (wavelength 261.8 nm) the residue dissolved in aqua regia. The number of Pt surface atoms on the catalysts was determined by hydrogen chemisorption at 298 Kin a volumetric system. A previous common treatment of the catalysts (in the same experimental system) was as follows: (i) 20 min at room temperature in high vacuum, (ii) 12 h under a hydrogen flow (50 cmsemin-1) at 623 K, (iii) 1 h at 573 K under high vacuum, and (iv) cooling in vacuum to 298 K, the temperature of the chemisorption. In some cases, the catalysts were heat-treated for 12-36 h in hydrogen over the temperature range 623-773 K in order to increase the Pt particle size by sintering. Hydrogen chemisorption at 298 K was carried out following the conventional procedure previously described.13 Chemisorption on supports was found to be nil in all cases, as measured under the same experimental conditions. Hydrogen uptakes were used to determine the metal dispersion, D, by assuming that one (14) van
Dam,H.E.;van Bekkum, H. J. Catal. 1991,131,336.
(16) Coloma, F.;Prado-Burguete,C.; Rodrfguez-Reinoeo, F. In New Frontiers in Catalysis; Guczi, L., et al., Eds.;Elsevier: Amsterdam,1993; p 2103.
_
_
Table 1. Surface Area of the Supports ~
_
support
SMC* (mW) 630 52 155 S2h 153 0 Calculated from the BET equation for N z at 77 K. Calculated from the Dubinin-Ftadushkevich (DR)equation for COz at 273 K. hydrogen atom is chemisorbed on a surface platinum atom. The averagePt particle size, d, was calculated from d = 1.08/D (nm).lb Transmission electron microscopy (TEM) was used to check chemisorptionresulta. Experimenta were carried out with a Zeiss EM10 electron microscope. Samples were dispersed ultrasonically in toluene and spread over self-perforated microgrids. X-ray photoelectron spectra were acquired with a Fissons ESCALAB 200R spectrometer provided with a hemispherical electron analyzer and Mg KCYX-ray radiation source (hv = 1263.6 eV), powered at 12 kV and 10 mA. Fresh samples were mounted onto a manipulator which allowed the transfer from the preparation chamber into the analysis chamber. The reduction treatment was carried out *in situ", by heating the fresh samples under a hydrogen flow at 623 K for 1 h. The binding energy (BE) of the C 1s peak of the support at 284.9 eV was taken as an internal standard. The accuracy of the BE values is k0.2 eV.
S
S e d (m2& 956 300 325
*
3. Results and Discussion 3.1. Supports. Table 1reports the Brunauer-EmmettTeller (BET) surface areas of carbons S, S2, and S20,, calculated from nitrogen adsorption at 77 K, as well as the surface areas obtained from carbon dioxide adsorption at 273 K by using the Dubinin-Radushkevich (DR) equation, which characterizes only the narrower microporosity (
