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Langmuir 1999, 15, 1176-1181
IR and XPS Study of NO and CO Interaction with Palladium Catalysts Supported on Aluminosilicates A. M. Venezia,*,† L. F. Liotta,† and G. Deganello†,‡ Istituto di Chimica e Tecnologia dei Prodotti Naturali (ICTPN-CNR), via Ugo la Malfa, 153, 90146 Palermo, Italy, and Dipartimento di Chimica Inorganica, Viale delle Scienze, 90128 Palermo, Italy
P. Terreros,§ M. A. Pen˜a,§ and J. L. G. Fierro§ Instituto de Catalisis y Petroleoquı´mica, CSIC, Campus UAM, Cantoblanco, 28049 Madrid, Spain Received July 31, 1998. In Final Form: October 29, 1998 Adsorption of CO and NO individually and as a mixture of both on palladium catalysts has been investigated by Fourier transform infrared spectroscopy (FTIR) and X-ray photoelectron spectroscopy (XPS). The effect of sodium ions has been considered by comparing Pd/SiO2 catalyst, sodium-doped Pd/ SiO2, and Pd on natural and synthetic silicoaluminates containing sodium in their bulk structure but differing in the surface area of the support. The presence of sodium induces a different adsorption behavior depending on its location, either at the surface of the catalysts or inside the support structure. As a consequence, different species are formed from the reaction between NO and CO at room temperature and at high temperature. Modification of the metal surface of the various catalysts is observed upon the gas treatments. Palladium is oxidized to a certain extent depending on the type of the catalyst support. Moreover, different chemical states of the adsorbed nitrogen species are found.
Introduction The removal of nitric oxide and carbon monoxide from exhaust gases is an important scientific challenge of the present days. The most practical way of removing both is the catalytic reduction of NO with CO or hydrocarbons, present in the exhaust gases, over supported transition metals. Several studies of this reaction over single-crystal or supported polycrystalline platinum, palladium, and rhodium have been carried out.1,2 Differently from rhodium, where large NO dissociation occurs,3 platinum and palladium do not appear to activate such a process.4,5 It is still a matter of debate whether the NO dissociation6,7 or a bimolecular surface reaction between adsorbed NO and CO8,9 is the rate-determining step. The selectivity of the reaction toward CO2, N2O, and N2 changes with the type of catalyst. On Rh only CO2 and N2 were found,3 whereas N2O and CO2 were obtained on supported palladium9 and platinum.10 A recent investigation of the * Corresponding author. † Istituto di Chimica e Tecnologia dei Prodotti Naturali. Telephone ++ 39 91 680 93 72. Fax: ++ 39 91 680 93 99, E-mail:
[email protected]. ‡ Dipartimento di Chimica Inorganica. § Fax: ++34 1 5854760. (1) Taylor, K. C. Catal. Rev.sSci. Eng. 1993, 35, 457. (2) Permana, H.; Peden, K. Y. S.; Schmieg S. J.; Belton, D. N. J. Phys. Chem. 1955, 99, 16344. (3) Tolia, A. A.; Williams, C. T.; Takoudis, C. G.; Weaver, M. J. J. Phys. Chem. 1995, 99, 4599. (4) Wickham, D. T.; Banse, B. A.; Koel, B. E. Surf. Sci. 1987, 179, 332. (5) Ramsier, R. D.; Gao, Q.; Waltenberg, H. B.; Yates, J. T., Jr. J. Chem Phys. 1994, 100, 6837. (6) Banse, B. A.; Wickham, D. T.; Koel, B. E. J. Catal. 1989, 119, 238. (7) Moriki, S.; Inoue, Y.; Miyazaki, E.; Yasumori, Y. J. Chem Soc., Faraday Trans. 1 1982, 178, 171. (8) Xi, G.; Bao, J.; Shao, S.; Li, S. J. Vac. Sci. Technol. 1992, A10, 2351. (9) Grill, C. M.; Gonzalez, R. D. J. Phys. Chem. 1980, 84, 878. (10) Lorimer, D.; Bell, A. T. J. Catal. 1979, 59, 223.
CO-NO reaction under high pressure has detected formation of CO2 and N2 also on palladium films.11 However, few studies have examined the effect of the support on the chemisorption properties and on the outcome of the reaction between the two gases over supported palladium catalysts. Previous investigations on pumice-supported catalysts have established the occurrence of electronic effects of the carrier on the supported palladium. Such effects, displayed as an enrichment of the electronic charge on the metallic centers, were attributed to the presence of alkali ions in the support structure12,13 and were shown to improve the catalytic behavior in some hydrogenation reactions. Results of a surface study of palladium catalysts, exposed to NO, CO, and a mixture of both gases, are described in the present study, aiming to define the role of the alkali ions, when they are at the surface of the catalyst or part of the carrier structure. To investigate the adsorbed and evolved species, and also the chemical changes of the catalyst surface following the gas exposure at different temperatures, infrared spectroscopy (IR) and X-ray photoelectron spectroscopy (XPS) have been used. For catalysts with lowsurface-area supports, for which the IR technique has low sensitivity, only the XPS technique gave useful information. Experimental Section Catalyst Preparation. In this work, natural pumice, synthetic pumice, and silica were used as supports for palladium catalysts (see Table 1). Natural pumice (from Pumex. Spa. Lipari, Italy) is an amorphous aluminosilicate with low surface area. (11) Williams, C. T.; Tolia, A. A.; Chan, H. Y. H.; Takoudis, C. G.; Weaver, M. J. J. Catal. 1996, 163, 63. (12) Venezia, A. M.; Rossi, A.; Duca, D.; Martorana, A.; Deganello, G. Appl. Catal. 1995, 125, 113. (13) Venezia, A. M.; Rossi, A.; Liotta, L. F.; Martorana, A.; Deganello, G. Appl. Catal. 1996, 147, 81.
10.1021/la980959o CCC: $18.00 © 1999 American Chemical Society Published on Web 01/28/1999
NO and CO Interaction with Palladium Catalysts
Langmuir, Vol. 15, No. 4, 1999 1177
Table 1. Oxide Percentage Compositiona and Specific Surface Area S (m2/G)b of the Supports support
SiO2
Al2O3
Na2O
K2O
S
pumice ksynt3 SiO2
87.7 82.1
7.0 14.1
2.0 1.7
3.3 2.1
5 169 200
a Obtained from elemental chemical analysis. analysis.
b
From BET
Table 2. Metal Loading of Different Catalysts (wt %, i.e., Gram of Metal per 100 g of Catalyst), Alkali to Palladium Bulk Atomic Ratios, R(Na/Pd), and Metal Particle Size d catalyst
Pd (wt %)
R(Na/Pd)
da (Å)
Pd/pumice Pd/ksynt3 Pd/SiO2 Na-Pd/SiO2
0.5 1.3 1.5 1.5
13.8 4.4
40 54 50 52
a
6.4
determined by WAXS measurements.
Before being used as support, it underwent a purification treatment with diluted HNO3.14 The synthetic support (ksynt3) prepared by the sol-gel technique described previously15 is also an aluminosilicate with much higher surface area. In Table 1 the composition and the surface area of the supports are listed. Palladium catalysts supported on natural and synthetic pumices were prepared by use of an organometallic precursor and by an ion-exchange method, respectively.15 The first method, particularly suitable for low-surface-area supports, consisted of reacting the hydroxyl groups of pumice with the Pd(C3H5)2 complex in pentane followed by H2 reduction at low temperature. The reduction was carried out at 258 K for 30 min, then at 273 K for 1 h, and finally at 298 K for 30 min. The second method consisted of reacting [Pd(NH3)4](OH)2 with the support in NH3-saturated aqueous solution. The support containing Pd diammine units was then dried at 353 K, then calcined at 573 K to eliminate ammonia, and finally reduced in hydrogen for 10 h at 573 K (0.5 K/min). This method was also used to prepare the undoped and the sodium-doped silica-supported catalysts. In the latter case prior to reduction, impregnation of the catalyst precursor with a sodium nitrate solution was performed.15 The metal content was determined by atomic absorption spectroscopy. The catalysts with the metal weight percentages, the bulk atomic ratios R(Na/ Pd), and the metal particle sizes are listed in Table 2. Catalyst Characterization. The FTIR spectra of selfsupporting wafers were recorded on a Nicolet 5 ZDX FT spectrometer. An all-glass cell equipped with greaseless stopcocks and KBr windows was used for the gas treatments. Previous to any IR measurements, the samples were outgassed at 573 K for 1 h, contacted with H2 at 573 K for 0.5 h, and then outgassed at the same temperature for 0.5 h and cooled to room temperature. The IR spectrum in this stage was taken as reference for all the following experiments to eliminate the support overtone bands. Catalysts were exposed to 20 Torr of CO at 298 K, outgassed for 30 min at 573 K, followed by exposure to 20 Torr of NO at 298 K, and again outgassed at 573 K for 30 min. Subsequently an equimolecular mixture of CO and NO (20 Torr in total) was admitted at 298 K and then heated to 573 K for 30 min. The time of exposure during each gas treatment was 10 min. To eliminate the adsorption bands of the gas phase, the difference spectra between those of the sample plus gas and those of the gas alone after withdrawing the wafer were considered, using the spectrometer software routine. Photoelectron spectra were recorded with a VG ESCALAB 200R spectrometer equipped with a hemispherical electron analyzer and a Mg KR 120 W X-ray source. The powder samples were pressed into small stainless steel cylinders and then mounted on a sample rod placed in a pretreatment chamber, where they underwent the same treatments done in the infrared experiment. The samples were outgassed at 298 K for 1 h prior (14) Venezia, A. M.; Rossi, A.; Floriano, M. A.; Deganello, G. Surf. Interface Anal. 1992, 18, 532. (15) Liotta, L. F.; Venezia, A. M.; Martorana, A.; Rossi, A.; Deganello, G. J. Catal. 1997, 171, 169.
to being introduced in the analysis chamber. The pressure in the ion-pumped analysis chamber was below 1.5 × 10-9 Torr (1 Torr ) 133.3 Pa) during data acquisition. The spectra were collected at a pass energy of 20 eV, which is typical of high-resolution conditions. The position of the peaks was obtained after subtraction of a linear background and fitting the experimental curve with Lorentzian and Gaussian lines of variable proportion. For the metallic component of the Pd 3d spectrum, a curve with an exponential tail on the high-binding-energy side was used in order to take into account the intrinsic asymmetry of the metallic peak.12 The binding energies (BEs) were referred to the binding energy of the internal standard peak Si 2p previously calibrated for the supports alone.13 The accuracy of the BE was estimated as (0.2 eV.
Results and Discussion IR Results. In Table 3 the main absorption frequencies obtained with the different samples subjected to different gas and thermal treatments are reported. The behavior of each catalyst is described in detail below. Adsorption on Pd/SiO2. Figure 1a and b reports the infrared spectra of the sample after exposure to 20 Torr of CO and of NO, respectively, for 10 min. The spectrum of the CO-saturated catalyst surface consists of two bands at 2090 and 1910 cm-1. Previous IR studies of carbon monoxide adsorbed on Pd/SiO2 had reported two main bands centered at 2088 and 1965 cm-1.16 Each of these bands contained many components. The high-frequency bands were attributed to linear CO chemisorbed on sites of different coordination, whereas the lower energy bands were attributed to bridging and multibonded CO species adsorbed on different phases of crystallites. The bands at 2090 and 1910 cm-1 are therefore attributed respectively to linear and bridged chemisorbed CO. Adsorption on different accessible crystal faces, depending on the metal dispersion, may cause the shift of the band from 1965 to 1910 cm-1.17 Evacuation at 573 K leads to the disappearance of all the CO bands. As shown in Figure 1b, the adsorption of 20 Torr of NO produces only one strong peak at 1760 cm-1. A survey of the literature of NO stretching vibrations on palladium offers the possibility to distinguish between metallic Pd0 and ionic Pd+n species. Generally, NO on Pd0 gives rise to bands at 1730-1750 cm-1 of linear NO, at 1650-1670 cm-1 of bent NO, and at 1570 cm-1 of bridged bonded NO. Moreover, NO on Pd+n gives bands at 1810-1815 and 1760-1775 cm-1.5,18 As indicated by XPS, the observation of oxidized Pd upon NO exposure suggests that the band at 1760 cm-1 can be due to NO adsorbed on ionic Pd+2 and the shoulder at lower wavenumber could be assigned to NO adsorbed on Pd. The band disappears after pumping at 573 K for 30 min. The spectra of the CO and NO mixture at room temperature and at 573 K are shown in Figure 2. The adsorption at room temperature produces the CO bands at 2116 and 1887 cm-1 and a strong NO band at 1757 cm-1. As the extinction coefficient of NO is lower than that for CO, the relative intensity of the NO and CO bands indicates a preferential adsorption of NO. Moreover the coadsorption seems to determine a shift of the CO-related bands. In particular, the partial oxidation of Pd by NO,19 proved by the XPS results described later, would produce the high-frequency shift of the linear CO. A lower coverage of CO in the presence of NO could explain the shift of the (16) Liotta, L. F.; Martin, G. A.; Deganello, G. J. Catal. 1996, 164, 322. (17) Szany, J.; Kevin, K. W.; Goodman, D. W. J. Vac. Sci. Technol. 1993, A11, 1969. (18) Brown, M. F.; Gonzales, R. D. J. Catal. 1977, 47, 333. (19) Hoost, T. E.; Otto K.; Laframboise, K. A. J. Catal. 1995, 155, 303.
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Venezia et al.
Table 3. Main Absorption Frequencies (cm-1) and Detected Gas Products after Sample Exposures to 20 Torr of CO, NO, and an Equimolecular Mixture (10 + 10 Torr) of the Two Gases catalyst
CO
NO
NO+CO 298 K
NO+CO 573 K
Pd/SiO2 Na-Pd/SiO2 Pd/ksynt3
2090, 1910 2006, 1870, 1637, 1678 2080, 1914, 1783
1760
