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SiO2 catalysts with different crystallite size
Applied Catalysis B: Environmental 38 (2002) 77–90
FTIR study of the reduction reaction of NO by CO over Rh/SiO2 catalysts with different crystallite size Paulo Araya a,∗ , Francisco Gracia a , Joaqu´ın Cortés a , Eduardo E. Wolf b a
Departamento de Ingenier´ıa Qu´ımica, Facultad de Ciencias F´ısicas y, Matemáticas, Universidad de Chile, Casilla 2777, Santiago, Chile b Department of Chemical Engineering, University of Notre Dame, Notre Dame, IN 46556, USA Received 5 July 2001; received in revised form 17 December 2001; accepted 18 January 2002
Abstract In situ IR spectroscopy was used to study NO adsorbed during the NO + CO reaction on two Rh/SiO2 catalysts with 31 and 127 Å crystallite sizes. The reaction was studied at T < 550 K and at variable partial pressures of CO and NO below 0.08 atm. Under these conditions, the reaction is selective to the formation of N2 O and the only NO adsorbed species observed by IR is an anionic species (NO− ). It was found that the N2 O selectivity and the rate per gram of catalysts were similar on both catalysts, resulting in a lower turn over number (TON) for the catalysts with smaller crystallite size. An analysis combining kinetics and infrared results indicates that the reaction rate is determined by the different dissociation rate constant of the adsorbed NO species on each catalyst. © 2002 Elsevier Science B.V. All rights reserved. Keywords: CO–NO reaction; N2 O selectivity; Rh supported catalyst; Crystallite size effect
1. Introduction The mechanism of the reduction reaction of NO by CO over Rh has been the subject of numerous studies because of its importance in pollution control in catalytic converters. Extensive reviews of the literature of these catalytic systems have been reported by Taylor [1] and by Shelef and Graham [2]. Several reaction mechanisms, which differ in the role of the various adsorbed species, have been proposed in the literature [3–13]. The first mechanism suggested for this reaction included the classic CO and NO adsorption steps, followed by the dissociation of NO on the surface to give adsorbed N and O, the reaction between adsorbed CO and O to CO2 and the recom∗ Corresponding author. Tel.: +56-2-671-4284; fax: +56-2-699-1084. E-mail address: [email protected] (P. Araya).
bination of two adsorbed N atoms to yield N2 . The formation of N2 O has caused some disagreement in the literature because in the early studies its presence had not been detected. However, it is now accepted that this species is the main product at low temperatures, while above 300 ◦ C, the main product is N2 [8–10]. The production of N2 O would occur through the reaction pathway, proposed by Hecker and Bell [11], between adsorbed NO and N. The role of the N2 O(a) species [14,15] as well as the formation step of a ␦-N2 species [16] has been the subject of disagreement among various groups. In later works [17–20] the mechanism of the surface reactions on supported Rh has been reviewed to account for previous IR studies in which three different NO adsorbed species were detected on Rh supported catalysts [11,21,22]. These IR studies have shown the formation of bands that have been attributed to different species of adsorbed NO on the rhodium surface (the anionic species Rh–NO−
0926-3373/02/$ – see front matter © 2002 Elsevier Science B.V. All rights reserved. PII: S 0 9 2 6 - 3 3 7 3 ( 0 2 ) 0 0 0 1 9 - X
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(Eq. (1), and the cationic species Rh–NO+ (Eq. (5))), which participate in different steps of the reaction mechanism. The neutral species does not appear to play a role in this revised mechanism. The cationic species is generated by adsorption of NO over Rh+ , while the anionic species is produced by adsorption of NO over Rh0 . Based on these results, Chuang and Tan [20] have proposed the following revised mechanism for the reduction of NO by CO over Rh/Al2 O3 : NO(g) + Rh0 ↔ Rh0 –NO−
(1)
Rh0 –NO− + Rh0 ↔ Rh0 –N + Rh0 –O
(2)
Rh0 –NO− + 2Rh0 ↔ (Rh+ )2 O2− + Rh0 –N
(3)
Rh+ + 2CO ↔ Rh+ (CO)2
(4)
Rh+ + NO(g) ↔ Rh–NO+
(5)
Rh–NO+ + 2CO(g) ↔ Rh+ (CO)2 + NO(g)
(6)
COad + (Rh+ )2 O2− → 2Rh0 + CO2
(7)
Rh+ (CO)2 + 2Oad → Rh+ + 2CO2
(8)
Rh0 –NO− + Rh0 –N → 2Rh0 + N2 O(g)
(9)
Rh0 –N + Rh0 –N → N2(g) + 2Rh0
(10)
Rh0 –N + COad → Rh–NCO
(11)
According to this mechanism, the adsorbed NO− species results in NO dissociation, and it is responsible for the catalyst’s activity for NO reduction. The main difference between the above mechanism and those proposed for unsupported Rh is the participation of the adsorbed Rh(CO)2 species leading to the formation of CO2 gas. In supported Rh there is an effect due to crystallite size on the reaction rate and on the selectivity toward the formation of N2 O. Oh et al. [4] found that the reaction rate per Rh atom exposed to the gas phase (turn over number (TON)) of a supported Rh catalyst is several orders of magnitude lower than that measured on Rh(1 1 1). Later it was reported that the TON increases with increasing size of the Rh crystallites [23]. Selectivity toward N2 O, however, is not affected by crystallite size. Previous IR studies have been done on catalysts with the same crystallite size [11,20]. The single crystal results did not related the TON to a specific adsorbed
species but rather to the crystal structure [31,33], and earlier studies with variable crystallite size did not include IR results [23]. The purpose of this study is to elucidate the effect of crystallite size on the kinetic behavior of the catalysts using in situ FTIR to interpret it in terms of the mechanisms proposed in the literature for supported and unsupported Rh. To this effect, we measured the steady-state kinetic behavior of supported Rh catalysts having different crystallite size, and the concentration of adsorbed NO and CO species by in situ IR. A correlation between selectivity for N2 O formation and the adsorbate concentration is developed and the resulting expression is compared with experimental results.
2. Experimental Two catalysts containing 0.4% (w/w) Rh supported on Aerosil 130 silica were prepared by the dry impregnation method, using the inorganic salt RhCl3 as a precursor. One of them was calcined at 650 ◦ C for 3 h, and the other at 425 ◦ C for the same time. Then both catalysts were reduced at 425 ◦ C for 3 h in a stream of 5% H2 in Ar. Different calcination temperatures were used to obtain catalysts with different crystallite size. The catalyst calcined at 650 ◦ C has a low dispersion of 9.1%, corresponding to an average crystallite size of 127 Å (Rh-127 Å catalyst), while the one calcined at 425 ◦ C has a medium dispersion of 37.5%, with an average crystallite size of 31 Å (Rh-31 Å catalyst). A stable catalyst with higher dispersion was not achieved on a silica support by this method. Catalysts with lower dispersion were not used in this study because the IR signal in those catalysts was too low to get meaningful spectra. For the IR analysis of the adsorbed species, an IR-reactor cell (ISRI, Inc.) was placed inside the chamber of an IR spectrometer (Bruker, Vector 22). The IR-reactor cell has a temperature control system and it is connected to mass-flow controllers, allowing reaction conditions to be varied while the analysis of the species adsorbed on the catalyst was carried out in-situ. The IR spectra were obtained with a resolution of 2 cm−1 and 20 scans per spectrum. The activity and selectivity of the catalysts was determined by analyzing the products of the reaction (CO2 , N2 and N2 O) in
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a gas chromatograph using a Hayasep D column and a thermal conductivity detector. To calculate the reaction rates from the effluent concentration of the IR-reactor cell requires assuming the mixing pattern inside the cell (perfectly mixed or plug flow). However, since low conversions were used (less than 5%), differential conditions were assumed and the rates were calculated from the equation corresponding to a differential flow reactor. The procedure used for the IR studies is the following: the catalyst with a shape of a disk (compressed at 13,000 psi) was placed in the IR-reactor cell and heated to 200 ◦ C, in a stream of 5% H2 in Ar mixture flowing at 12 cm3 /min to prereduce the catalyst. After H2 was desorbed at the same temperature by flushing in pure He for 2 h, the IR-reactor-cell temperature was raised to the reaction temperature. Once the temperature was stabilized, a spectrum of the sample was taken on pure He to be used later as a background value. Then CO flow was started, and 10 min later the NO flow started until a CO partial pressure of 0.05 atm and a NO partial pressure of 0.03 atm balanced with He were reached, with a total flow of 100 cm3 /min. After 10 min, an IR spectrum of the catalyst was taken, and the effluent was sampled for GC analysis. Each measurement in every experiment was repeated three times to assure reproducibility, which was found to be within 5%. The partial pressure of NO was then decreased, at constant CO partial pressure, and the IR spectra and effluent gas composition for the different NO partial pressures were measured. Once this study was completed, the gas composition was returned to the initial NO concentration and the CO partial pressure was decreased while keeping constant the CO partial pressure.
3. Results and discussion The experiments were originally planned to study the effect of the partial pressure of reagents between 0.02 and 0.05 atm for CO and between 0.01 and 0.03 atm for NO, at a temperature of 260 ◦ C. These conditions are similar to those used in single crystal studies [31,33], and enabled us to compare our results with supported catalysts with single crystals’ results under the same conditions. As discussed below, the partial pressure range of NO was extended
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to 0.086 atm to further examine additional effects at full NO coverage. 3.1. Effect of NO partial pressure First, we discuss the effect of the NO partial pressure at constant CO partial pressure on the catalysts’ activity and selectivity at a temperature of 260 ◦ C. It is well known that CO adsorbs rapidly over Rh in the linear, bridged and gem forms, with IR bands of these species well documented in the literature [24–27]. As mentioned earlier, in the case of the adsorption of NO over Rh, the following species have been reported: the NO+ species, characterized by a band at 1930 cm−1 ; the NO− species with a band at 1830 cm−1 ; and the NO species, with a high frequency band between 1770 and 1740 cm−1 , and a low frequency band between 1690 and 1630 cm−1 [11,21,22]. Recently, Chafik et al. using Rh/TiO2 catalysts [28] proposed that the 1830 and 1725 cm−1 bands correspond to the symmetric and asymmetric vibrations of the Rh(NO)2 species, as it had been suggested previously [22,29], and that this species would be responsible for the formation of N2 O. Fig. 1 shows the IR spectra obtained on both catalysts at a constant CO partial pressure of 0.05 atm and with the NO partial pressure varying between 0.03 and 0.01 atm. Fig. 1 also includes the spectrum obtained after allowing CO in He to flow for 10 min at 260 ◦ C (subtracting the CO gas spectrum) before beginning the flow of NO and starting the reaction. The IR bands seen for the two catalysts are very similar. Before starting the reaction, with CO flowing at a partial pressure of 0.05 atm, spectra 1 shows bands at 2055 cm−1 (Rh-127 Å catalyst) and 2048 cm−1 (Rh-31 Å catalyst) corresponding to CO linearly adsorbed on Rh, and a broad band centered at 1920 cm−1 which can be assigned to the bridged adsorption of CO. No band associated with adsorbed gem CO could be detected. This is consistent with the agglomerating effect of CO on Rh at temperatures above 170 ◦ C [30], which eliminates these species observed only al lower temperatures on catalysts with much higher dispersions than the Rh-31 Å catalysts. As expected, the intensity of the CO band is much higher in the Rh-31 Å catalyst than in the Rh-127 Å catalyst, consistent with its higher dispersion. After starting the flow of NO, spectrum 2, the bands corresponding to adsorbed CO disappear
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Fig. 1. IR spectra of the catalysts at P CO = 0.05 atm, variable PNO , T = 260 ◦ C. (A) Rh-127 Å catalyst. (1) P NO = 0 atm; (2) P NO = 0.03 atm; (3) P NO = 0.017 atm; (4) P NO = 0.01 atm, (B) Rh-31 Å catalyst. (1) P NO = 0 atm; (2) P NO = 0.03 atm; (3) P NO = 0.017 atm; (4) P NO = 0.01 atm.
completely and the IR bands corresponding to the anionic species NO at 1692 cm−1 (Rh-127 Å catalyst) and at 1675 cm−1 (Rh-31 Å catalyst) appear. Bands for gas phase NO are located at 1914 and 1843 cm−1 , together with a series of bands above 2000 cm−1 corresponding to gaseous CO, CO2 and N2 O, and to the Si–NCO and Rh–NCO species. The bands corresponding to the cationic (1908 cm−1 ) and neutral (1830 cm−1 ) species are not seen because they may be masked by the bands due to gaseous NO. However, after subtracting the spectrum of NO gas for identical partial pressure and temperature those NO species cannot be identified with certainty, suggesting that, even though if present, their concentration is too low to be detected. As the partial pressure of NO decreases (spectra 2–4) the absorbance band corresponding to NO− decreases in both catalysts,
showing that under the experimental conditions used, the concentration of adsorbed NO depends on the NO pressure. It has been reported that over Rh(1 1 1) at 325 ◦ C, the integrated absorbance of NO is maximum and independent of NO pressure when it is higher than 2 Torr (0.003 atm) [31]. To verify if this effect is also observed in supported catalysts the NO pressure range in our study was increased up to 0.086 atm. The IR spectra obtained at PNO equal to 0.046, 0.073 and 0.086 atm show the same bands seen in Fig. 1, so they are not shown again. Fig. 2A and B show the integrated absorbance of the NO− species as a function of NO partial pressure for Rh-127 and Rh-31 Å catalysts (1692 and 1675 cm−1 , respectively). As seen in Fig. 2A, for the case of the Rh-127 Å catalyst, the integrated absorbance of NO− increases in the PNO range between
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Fig. 2. Effect of NO partial pressure at P CO = 0.05 atm, T = 260 ◦ C. (A) NO− integrated absorbance on Rh-127 Å catalyst. (B) NO− integrated absorbance on Rh-31 Å catalyst. (C) TON of product formation on Rh-127 Å catalyst. (D) TON of product formation on Rh-31 Å catalyst. (E) Selectivity to N2 O formation on Rh-127 Å catalyst. (F) Selectivity to N2 O formation on Rh-31 Å catalyst.
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0.01 and 0.046 atm, and becomes almost constant at higher PNO . In the case of the Rh-31 Å catalyst, due to its higher dispersion, the integrated absorbance of NO− is much higher than that in the Rh-127 Å catalyst and increases between PNO 0.01 and 0.03 atm, remaining constant at higher PNO . The above confirms the hypothesis that at 260 ◦ C NO coverage does not reach its maximum at PNO lower than 0.03 atm, and this, as discussed below, has implications in the kinetic behavior of the catalysts. Fig. 2C and D show activity results expressed as TON (moles of product/moles of exposed Rh per second) toward the formation of CO2 , N2 O and N2 for the Rh-127 and Rh-31 Å catalysts, respectively. It is seen that for the Rh-127 Å catalyst, which has the larger crystallite size, the TON toward the formation of CO2 is almost three times greater than that of Rh-31 Å catalyst. This agrees with the results reported previously [4,32] showing a decrease in the TON of CO2 formation as Rh crystallite size decreases, when the reaction is performed at temperatures above 500 K. In both catalysts, the increase in NO partial pressure between 0.01 and 0.03 atm leads to an increase in activity for the formation of CO2 and N2 O which thereafter remains practically constant at higher PNO . N2 formation, on the other hand, remains nearly constant in the whole NO pressure range. It has been reported that on Rh(1 1 1) [31] and Rh(1 1 0) [33] single crystals the rate of formation of CO2 , N2 O and N2 is practically independent on the NO partial pressure in the range from 2 Torr (0.003 atm) to 40 Torr (0.053 atm). This was explained by the observation that on Rh single crystals the coverage with NO is always high and independent on the concentration of NO in the gas phase [31]. However, on the supported catalysts used in this study, the dependence of the activity on the NO partial pressure is consistent with the increase of the NO coverage observed in the IR spectra, as PNO increases up to 0.03 atm. Fig. 2E and F show experimental results of the N2 O selectivity as a function of PNO obtained on both catalysts. It is seen that the N2 O selectivity increases as the NO partial pressure increases from 0.01 to 0.03 atm, and it remains almost constant above 0.03 atm. This behavior is parallel to the variation of the integrated absorbance of NO− within the same PNO range. The selectivity values obtained on both catalysts at PNO higher than 0.03 atm are of the order of 72%, in good
agreement with those obtained on Rh(1 1 1) at temperatures close to 250 ◦ C [31], and higher than the 60% value observed on Rh(1 1 0) [33]. The reaction mechanism proposed on Rh(1 1 1) and Rh(1 1 0) [31,33] single crystals includes the following steps: non-dissociative CO and NO adsorption on a single site (CO–Rh and NO–Rh), dissociation of adsorbed NO onto adsorbed N–Rh and O–Rh atoms, surface reaction of adsorbed CO–Rh and O–Rh to yield gas phase CO2 , surface reaction of adsorbed NO–Rh and adsorbed N–Rh to yield gas phase N2 O, and recombination of adsorbed N–Rh to yield N2 . This mechanism does not account for the formation of anionic or cationic NO species on the surface, and thus it is a simpler form of the mechanism summarized in Eqs. (1)–(11). The selectivity of the catalyst (SN2 O ) is defined by SN2 O =
rate of N2 O formation rate of N2 O formation + rate of N formation (12)
In both cases, for single crystals and supported catalysts, assuming that the surface reactions are elementary, the selectivity depends on the NO and N coverage according to the expression, SN2 O =
k9 θNO θN k9 θNO θN + k10 θN2
(13)
where k9 and k10 are the kinetic constants in Eqs. (9) and (10). Dividing the numerator and the denominator of Eq. (13) by k9 θ NO θ N the following expression is obtained: 1 SelN2 O = (14) 1 + (k10 θN /k9 θNO ) considering that for Rh(1 1 1) the integrated absorbance increases linearly with NO coverage when θ NO < 70% [31], it is assumed that the concentration of adsorbed NO- is linearly proportional to the integrated absorbance at NO partial pressures equal to or less than 0.03 atm, i.e. θNO =
kIA S0
(15)
where IA is the integrated absorbance, S0 the number of sites available and k is a proportionality constant. If SN is the number of sites occupied by adsorbed
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N, and since θ N = S N /S0 upon substitution Eq. (15) into Eq. (14) and rearranging one obtains IA = IA + KSN SelN2 O
(16)
with K = k10 /(k9 k). Plotting IA/SelN2 O versus IA for the Rh-31 and Rh-127 Å catalysts, it is possible to determine that (KSN )Rh-31 Å = 1.12 and (KSN )Rh-127 Å = 0.22. From Eq. (15) at NO monolayer coverage θNO ml =
kIAml S0 =1⇒k= S0 IAml
(17)
therefore KSN k10 SN k10 SN = = IANO k9 kIANO k9 (S0 /IAml )IANO k10 (SN /S0 ) k10 θN KSN = ≈ or k9 (IANO /IAml ) IANO k9 θNO (18) Then from Fig. 2A and B, at high PNO (IANO )Rh-31 Å = 2.1 and (IANO )Rh-127Å = 0.58, then it follows that KSN 1.12 = = 0.57, IANO Rh-31 Å 2.16 at PNO > 0.03 atm (SelN2 O )Rh-31 Å 1 = ≈ 0.66 1 + 0.57
(19)
and 0.22 KSN = 0.37, = IANO Rh-127 Å 0.58 at PNO > 0.03 atm (SelN2 O )Rh-127 Å 1 = ≈ 0.77 1 + 0.37 The above estimates show that the selectivity achieves a value of 0.6–0.7 for both the Rh-127 and Rh-31 Å catalysts. This occurs because the ratio of the rates of N2 O formation and N2 formation (Eq. (14)) is between 2 and 3 for both catalysts at this low temperature. This implies that the rate of NO dissociation must be low to allow reaction with N–Rh species. At higher temperatures this is not the case and most of the adsorbed NO dissociates to form N–Rh species that recombine as N2 and reduce the N2 O selectivity.
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The results predicted from Eq. (16) are plotted as broken lines in Fig. 2E and F in the range of validity of that expression (i.e. up to 0.03 atm). As it is seen in Fig. 2E, the agreement is excellent for the Rh-127 Å catalyst, which has the larger crystallite size, but it slightly overestimates the results for the catalyst with the smaller crystallite size, Fig. 2F. Nonetheless, Eq. (16) predicts well the experimental behavior of the selectivity with the NO partial pressure. According to Peden et al. [33], catalysts having large crystals should have a behavior similar to that of Rh(1 1 1), while those having small crystals should be similar to Rh(1 1 0). At similar temperature, Rh(1 1 0) has a lower selectivity to N2 O formation than Rh(1 1 1) [33]. The above results agree with the fact that at low NO coverage (P NO = 0.01 atm) the N2 O selectivity of the Rh-31 Å catalyst is lower than that of the Rh-127 Å catalyst. At high NO concentration, however, the N2 O selectivity is very similar on both catalysts, in agreement with Eqs. (18) and (19). We attributed the observed dependence of the activity and selectivity of both catalysts on the NO partial pressure to the observation that coverage with NO is not constant in the whole range of NO pressure studied. To further study this point the reaction temperature was decreased so that the lower limit of PNO at which constant coverage with NO is obtained, should be displaced to lower PNO values making the reaction insensitive to this variable. To verify this prediction, the experiments were repeated at a temperature of 235 ◦ C. For the experiments at 235 ◦ C, new catalysts disks were prepared and loaded into the IR cell and subjected to the same pretreatment as those used at 260 ◦ C. The IR spectra obtained at 235 ◦ C from both catalysts show the same peaks that were seen at 260 ◦ C, so they are not displayed again, and only the integrated absorbance of the NO− species at the different NO partial pressures are shown in Fig. 3. Fig. 3A shows that the NO integrated absorbance on the Rh-127 Å catalyst increases slightly between PNO 0.01 and 0.03 atm, remaining constant at higher PNO . In the case of the Rh-31 Å catalyst, Fig. 3B shows that the increase in the integrated absorbance of NO− is more pronounced than in the Rh-127 Å catalyst in the PNO range between 0.01 and 0.03 atm. As at T = 260 ◦ C, for PNO values greater than 0.03 atm, the integrated absorbance of NO− is the constant. In both catalysts, the change in the integrated absorbance of
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Fig. 3. Effect of NO partial pressure at P CO = 0.05 atm, T = 235 ◦ C. (A) NO− integrated absorbance on Rh-127 Å catalyst. (B) NO integrated absorbance on Rh-31 Å catalyst. (C) TON of product formation on Rh-127 Å catalyst. (D) TON of product formation on Rh-31 Å catalyst. (E) Selectivity to N2 O formation on Rh-127 Å catalyst. (F) Selectivity to N2 O formation on Rh-31 Å catalyst.
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NO is less than that observed at 260 ◦ C. Fig. 3C and D show the TON toward the formation of the three reaction products (CO2 , N2 O and N2 ) for Rh-127 and Rh-31 Å catalysts, respectively, which are lower than those measured at higher temperature, as it was expected. The TONs on both catalysts differ by a factor of about three with respect to those at T = 260 ◦ C, and there is only a slight effect of the NO partial pressure on both catalysts. As seen in Fig. 3C, on the Rh-127 Å catalyst, the TON for the formation of the three reaction products decreases slightly as the NO partial pressure increases from 0.01 to 0.03 atm. In the case of the Rh-31 Å catalyst, in Fig. 3D, the TON is lower than on the Rh-127 Å catalyst in the pressure range between 0.01 and 0.046 atm. At PNO higher than 0.05 atm, the rates remain practically constant on both catalysts. The N2 O selectivity at different PNO , is shown in Fig. 3E and F for each catalyst, respectively. The selectivity toward N2 O on the Rh-127 Å catalyst, Fig. 3E, is practically independent of the NO partial pressure and remains around 62%, paralleling the behavior of the value of the integrated absorbance of the NO− species. However, on the catalyst with the greater dispersion (Rh-31 Å), the selectivity increases from 41 to 62% in the NO pressure range between 0.01 and 0.046 atm, and becomes constant at higher NO pressure, which is consistent with the behavior observed for the integrated absorbance of NO− . Same trend for selectivity as in the case of T = 260 ◦ C is obtained for T = 235 ◦ C using Eqs. (18) and (19) with (KSN )MD = Rh-31 Å = 2.048 and (KSN )LD = Rh-127 Å = 0.481. The theoretical selectivity curves calculated from Eq. (16) are also shown in Fig. 3E and F. Similarly to what was seen at 260 ◦ C, there is excellent agreement in the case of the lower dispersion catalyst, while in the case of the catalyst with the smaller crystallite size (Rh-31 Å), Eq. (17) slightly overestimates the selectivity value as PNO decreases. The behavior of the Rh-127 and Rh-31 Å catalysts at 235 ◦ C is not conclusive to support the proposal of Peden et al. [33] that the catalyst with the larger crystallite size must exhibit similar activity dependence than Rh(1 1 1), while the catalysts with the smaller crystallite size should have activity dependence like Rh(1 1 0). In fact, these authors show that the selectivity to N2 O of Rh(1 1 0) becomes independent of the NO partial pressure at PNO values greater than those
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found for Rh(1 1 1). This agrees with the observation that while the selectivity of the Rh-127 Å catalyst does not depend on PNO in the whole range studied, the selectivity of the Rh-31 Å catalyst is independent of PNO only at NO partial pressures higher than 0.046 atm. However, the selectivity of both catalysts at high PNO is quite similar, which does not agree with the lower selectivity that should be observed for the Rh-31 Å catalyst if it behaved similarly to Rh(1 1 0). This aspect will be discussed again below, when the behavior of the selectivity of both catalysts with temperature is discussed. 3.2. Effect of the partial pressure of CO The study of the effect of the CO partial pressure was done keeping PNO constant at 0.03 atm and varying the partial pressure of CO between 0.02 and 0.05 atm. Fig. 4A shows the IR spectra obtained for the Rh-127 Å catalyst; and Fig. 4B those for the Rh-31 Å catalyst at 260 ◦ C. The bands seen are similar to those described for Fig. 1. It is seen that the reduction of the CO partial pressure at a constant NO pressure of 0.03 atm does not produce important changes in the IR spectra, except for a slight decrease in the absorbance of the NO− species as the CO partial pressure increases. Fig. 5A and B show the integrated absorbance of the NO− species adsorbed on both catalysts. In both cases, the integrated absorbance decreases as PCO increases. A decrease in the absorbance of the NO− species as PCO increases has also been observed for supported Rh at 210 ◦ C [11] and for Rh(1 1 1) at 325 ◦ C [31]. Fig. 5C and D show the activity of formation of the reaction products at the different CO partial pressures for Rh-127 Å catalyst and Rh-31 Å catalyst respectively. It is seen that in both catalysts the TON of all the reaction products increases as the CO partial pressure increases from 0.02 to 0.05 atm. The activity of formation of CO2 increases about 62 and 75% for Rh-127 and Rh-31 Å catalysts respectively, as PCO increases. This behavior agrees qualitatively with that observed on supported Rh at 210 ◦ C for a pressure range similar to that used in this study [11], but it does not agree with the zero order in PCO seen in unsupported Rh. In spite of the changes in the absorbance of the NO− species and in the activity of the catalysts with PCO , the N2 O selectivity remains almost constant, as seen
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Fig. 4. IR spectra of the catalysts at P NO = 0.05 atm and variable PCO , T = 260 ◦ C. (A) Rh-127 Å catalyst. (1) P CO = 0.05 atm; (2) P CO = 0.028 atm; (3) P CO = 0.019 atm. (B) Rh-31 Å catalyst. (1) P CO = 0.05 atm; (2) P CO = 0.028 atm; (3) P CO = 0.019 atm.
in Fig. 5E for the Rh-127 Å catalyst and in Fig. 5F for the Rh-31 Å catalyst. When the reaction temperature is reduced to 235 ◦ C, the IR spectra obtained at different PCO are very similar to those shown in Fig. 3 for the reaction at 260 ◦ C and they are not included here. The integrated absorbance of NO− is shown in Fig. 6A for the Rh-127 Å catalyst and in Fig. 6B for the Rh-31 Å catalyst. In both catalysts the integrated absorbance of NO− decreases as PCO increases, but the change is significantly less than that observed at 260 ◦ C. Fig. 6C and D show the TON as a function of PCO for catalysts Rh-127 and Rh-31 Å, respectively. As at 260 ◦ C, the activity toward product formation in both catalysts increases as CO partial pressure increases, but the change in the activity is also less than at 260 ◦ C. In fact, at 235 ◦ C, the percentage of increase in the TON of CO2 formation, as PCO increases from 0.02 to 0.05 atm, is almost
the half of that observed at 260 ◦ C. Similarly to what was seen at 260 ◦ C, the CO partial pressure does not affect the N2 O selectivity, which remains almost constant at about 62%, as seen in Fig. 6E and F. The positive PCO order observed in the activity of our supported catalysts, and by Hecker and Bell in a catalyst with high Rh content [11] is consistent with the mechanisms proposed for supported and unsupported Rh. In fact, higher PCO must increase the concentration of adsorbed CO leading to higher production rates of CO2 , as it would be expected from Eqs. (7) and (8), or from the reaction between CO linearly adsorbed in Rh0 and oxygen product of NO dissociation for unsupported Rh [31,33]. In addition to the greater activity (TON) shown by the Rh-127 Å catalyst with larger crystallite size, the activity behavior with changes in the partial pressure of reactants is quite similar on both catalysts. The TON
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Fig. 5. Effect of CO partial pressure at P NO = 0.03 atm, T = 260 ◦ C. (A) NO integrated absorbance on Rh-127 Å catalyst. (B) NO integrated absorbance on Rh-31 Å catalyst. (C) TON of product formation on Rh-127 Å catalyst. (D) TON of product formation on Rh-31 Å catalyst. (E) Selectivity to N2 O formation on Rh-127 Å catalyst. (F) Selectivity to N2 O formation on Rh-31 Å catalyst.
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Fig. 6. Effect of CO partial pressure at P NO = 0.03 atm, T = 235 ◦ C. (A): NO integrated absorbance on Rh-127 Å catalyst. (B): NO integrated absorbance on Rh-31 Å catalyst. (C): TON of product formation on Rh-127 Å catalyst. (D): TON of product formation on Rh-31 Å catalyst. (E): Selectivity to N2 O formation on Rh-127 Å catalyst. (F): Selectivity to N2 O formation on Rh-31 Å catalyst.
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of CO2 formation and N2 O selectivity increase on both catalysts with an increase in NO partial pressure until a high NO coverage is achieved. Likewise, on both catalysts, increasing the CO partial pressure increases the activity TON of CO2 formation without substantially changing the N2 O selectivity. The most noticeable difference is found in the greater sensitivity of the N2 O selectivity in the catalyst with smaller crystal size to the change in NO partial pressure, which becomes evident at 235 ◦ C. Some of these results support the proposal of Peden et al. [33] that the catalyst with smaller crystallite size tends to behave more like Rh(1 1 0), while that with a larger crystallite size behaves like Rh(1 1 1). An important difference between Rh(1 1 1) and Rh(1 1 0) is their response to changes on temperature changes. While the selectivity of Rh(1 1 1) increases as the temperature increases, it reaches a maximum at about 400 ◦ C, and then decreases at higher temperatures, in the case of Rh(1 1 0) N2 O selectivity decreases continually as temperature increases [33]. If the N2 O selectivity of both Rh-127 and Rh-31 Å catalysts is compared at 235 and 260 ◦ C, it is seen that at a PCO of 0.05 atm and a PNO not higher than 0.046 atm, when the integrated absorbance of NO− reaches a constant value, the selectivity increases with increasing temperature on both catalysts. The increase in the selectivity at each temperature is practically identical with both catalysts. Thus, regarding the effect of temperature for T lower than 260 ◦ C, both catalysts have a behavior closer to that of Rh(1 1 1) than to that of Rh(1 1 0). With respect to the lower activity of the supported catalysts as compared to that of unsupported Rh, Chuang and Tan [20] have suggested that since on unsupported Rh only the NO− species exists, which is responsible for the dissociation of NO (controlling step), the supported catalysts would have a lower activity because their surface is partially oxidized, increasing the adsorption of the NO+ species, and thereby decreasing the number of Rh0 sites where the NO species is formed. The IR spectra of our catalysts shown in Fig. 1, allow important conclusions to be reached in relation to the latter aspect. The spectrum of CO adsorbed on both catalysts, spectrum (1) in Fig. 1A and B, shows that before the reaction starts there are no oxidized Rh sites, as evidenced by the complete absence of gem CO, and as expected, the number of CO adsorption sites in the low dispersion
89
catalyst is considerably less than in the medium dispersion catalyst. The above follows from the greater absorbance of linear CO in the Rh-31 Å catalyst, even though similar mass of catalysts were used to prepare the disks placed in the IR cell. When NO starts flowing, and adsorbed CO is replaced by NO, spectrum (2) in Fig. 1A and B, it is seen that the integrated absorbance of the NO− species is also substantially greater in the Rh-31 Å catalyst than in Rh-127 Å catalyst. At P CO = 0.05 atm and P NO = 0.03 atm and at a temperature of 260 ◦ C, the integrated absorbance of the NO− species obtained (spectrum 2) were 2.096 and 0.5178, and the rate per gram of catalysts 9.138 × 10−6 and 8.042 × 10−6 for the Rh-31 and Rh-127 Å catalyst, respectively. If the rate constant for dissociation of NO− species is the same on both catalysts, a greater activity per gram of catalyst would be expected on the Rh-31 Å catalyst than on the Rh-127 Å catalyst. It follows that in such case the TON should be the same on both catalysts. Since the rate per gram of catalysts is similar, when dividing by the larger active area of the Rh-31 Å catalysts, its TON is smaller than the TON of the Rh-127 Å catalysts. The above results lead us to conclude that it is not the number of sites available for NO− adsorption that determines the activity of the catalysts, but rather the rate constants for NO− dissociation which are different due to differences in the type of sites existing in each catalyst.
4. Conclusions The results show that even though the activity of the catalyst with larger crystallite size (127 Å) is higher than that of the catalyst with smaller crystallite size (31 Å), its behavior as a function of the partial pressure of the reactants is quite similar within the range of reaction conditions studied. The activity (TON) and selectivity of both catalysts are sensitive to the variation in the NO partial pressure until a high constant coverage with adsorbed NO− is achieved. This was the only adsorbed NO species clearly observed under the experimental conditions used. The most evident difference in the behavior of both catalysts lies in the dependence of the selectivity on PNO shown at a temperature of 235 ◦ C, which can be explained by the different concentrations of NO
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adsorbed on both catalysts as a function of PNO at that temperature. The main difference between the supported catalysts as compared to unsupported Rh is the NO partial pressure needed to reach a high constant coverage with NO. Once the NO coverage reaches a maximum, the dependence of activity and selectivity on PNO is very similar to that seen for Rh(1 1 1). Although under certain conditions (low PNO ), the catalyst with small crystallite size has a behavior that may be associated with that seen for Rh(1 1 0), at high NO coverage both the selectivity value as well as its dependence on temperature are closer to those of Rh(1 1 1). With respect to the effect of PCO on the activity and selectivity of the catalysts, a similar behavior is seen with both catalysts. While the dependence of selectivity on PCO is similar to that observed with unsupported Rh, the catalyst’s activity shows a positive order in PCO that differs from that seen with Rh single crystal. Finally, the results of this work indicate that the decrease in the TON in the catalyst with small crystallites is not related a lower concentration of NO species, but to a lower NO dissociation rate constant, suggesting the existence of different types of active sites on each catalyst. Acknowledgements The financial support of FONDECYT under Grant No. 8000015 is gratefully acknowledged. References [1] K.C. Taylor, Catal. Rev. SCI. Eng. 35 (1993) 457. [2] M. Shelef, G. Graham, Catal. Rev. Sci. Eng. 36 (1994) 433. [3] L. Dubois, P. Hansma, G.A. Somorjai, J. Catal. 65 (1980) 318. [4] S. Oh, G. Fischer, J. Carpenter, D. Goodman, J. Catal. 100 (1986) 360.
[5] S.B. Schwartz, G.B. Fisher, L.D. Schmidt, J. Phys. Chem. 92 (1988) 389. [6] A.A. Chin, A.T. Bell, J. Phys. Chem. 87 (1983) 3700. [7] T.W. Root, L.D. Schmidt, G.B. Fisher, Surf. Sci. 134 (1983) 30. [8] B.K. Cho, B.H. Schanks, J.E. Bailey, J. Catal. 115 (1989) 486. [9] R.W. McCabe, C. Wong, J. Catal. 121 (1990) 422. [10] S.H. Oh, J. Catal. 124 (1990) 477. [11] W.C. Hecker, A.T. Bell, J. Catal. 84 (1983) 200. [12] B.K. Cho, J. Catal. 138 (1992) 225. [13] B.K. Cho, J. Catal. 148 (1994) 697. [14] V.P. Zhdanov, J. Catal. 162 (1996) 147. [15] B.K. Cho, J. Catal. 162 (1996) 149. [16] D.N. Belton, C.L. DiMaggio, S.J. Schmieg, K.Y. Simon, J. Catal. 157 (1995) 559. [17] G. Srinivas, S. Chuang, S. Debnath, J. Catal. 148 (1994) 748. [18] R. Krishnamurthy, S. Chuang, D. Balakos, J. Catal. 157 (1995) 512. [19] R. Krishnamurthy, S. Chuang, J. Phys. Chem. 99 (1995) 16727. [20] S. Chuang, C. Tan, J. Catal. 173 (1998) 95. [21] H. Arai, H. Tominaga, J. Catal. 43 (1976) 131. [22] F. Solymosi, J. Sarkany, J. Appl. Surf. Sci. 3 (1979) 68. [23] S.H. Oh, C.C. Eickel, J. Catal. 128 (1991) 526. [24] A. Yang, C. Garland, J. Phys. Chem. 61 (1957) 1504. [25] J. Yates, T. Duncan, S. Worley, R. Vaughn, J. Chem. Phys. 70 (1979) 1219. [26] R. Cavanagh, J. Yates, J. Chem. Phys. 74 (1981) 4150. [27] C. Rice, S. Worley, C. Curtis, J. Guin, A. Tarrer, J. Chem. Phys. 74 (1981) 6487. [28] T. Chafik, D. Kondarides, X. Verykios, J. Catal. 190 (2000) 446; D. Kondarides, T. Chafik, X. Verykios, J. Catal. 191 (2000) 147; D. Kondarides, T. Chafik, X. Verykios, J. Catal. 193 (2000) 303. [29] E. Hyde, R. Rudham, C. Rochester, J. Chem. Soc., Faraday. Trans. 80 (1984) 531; J. Anderson, G. Millar, C. Rochester, J. Chem. Soc., Faraday. Trans. 86 (1990) 571; J. Liang, H. Wang, L. Spicer, J. Phys. Chem. 89 (1985) 5840. [30] F. Solymosi, M. Pasztor, J. Phys. Chem. 89 (1985) 4789. [31] H. Permana, K.Y. Simon Ng, C. Peden, S. Schmieg, D. Lambert, D. Belton, J. Catal. 164 (1996) 194. [32] J. Kaspar, C. De Leitenburg, P. Fornasiero, A. Trovarelli, M. Graziani, J. Catal. 346 (1994) 138. [33] C. Peden, D. Belton, S. Schmieg, J. Catal. 155 (1995) 204.
FTIR study of the reduction reaction of NO by CO over Rh/SiO2 catalysts with different crystallite size Paulo Araya a,∗ , Francisco Gracia a , Joaqu´ın Cortés a , Eduardo E. Wolf b a
Departamento de Ingenier´ıa Qu´ımica, Facultad de Ciencias F´ısicas y, Matemáticas, Universidad de Chile, Casilla 2777, Santiago, Chile b Department of Chemical Engineering, University of Notre Dame, Notre Dame, IN 46556, USA Received 5 July 2001; received in revised form 17 December 2001; accepted 18 January 2002
Abstract In situ IR spectroscopy was used to study NO adsorbed during the NO + CO reaction on two Rh/SiO2 catalysts with 31 and 127 Å crystallite sizes. The reaction was studied at T < 550 K and at variable partial pressures of CO and NO below 0.08 atm. Under these conditions, the reaction is selective to the formation of N2 O and the only NO adsorbed species observed by IR is an anionic species (NO− ). It was found that the N2 O selectivity and the rate per gram of catalysts were similar on both catalysts, resulting in a lower turn over number (TON) for the catalysts with smaller crystallite size. An analysis combining kinetics and infrared results indicates that the reaction rate is determined by the different dissociation rate constant of the adsorbed NO species on each catalyst. © 2002 Elsevier Science B.V. All rights reserved. Keywords: CO–NO reaction; N2 O selectivity; Rh supported catalyst; Crystallite size effect
1. Introduction The mechanism of the reduction reaction of NO by CO over Rh has been the subject of numerous studies because of its importance in pollution control in catalytic converters. Extensive reviews of the literature of these catalytic systems have been reported by Taylor [1] and by Shelef and Graham [2]. Several reaction mechanisms, which differ in the role of the various adsorbed species, have been proposed in the literature [3–13]. The first mechanism suggested for this reaction included the classic CO and NO adsorption steps, followed by the dissociation of NO on the surface to give adsorbed N and O, the reaction between adsorbed CO and O to CO2 and the recom∗ Corresponding author. Tel.: +56-2-671-4284; fax: +56-2-699-1084. E-mail address: [email protected] (P. Araya).
bination of two adsorbed N atoms to yield N2 . The formation of N2 O has caused some disagreement in the literature because in the early studies its presence had not been detected. However, it is now accepted that this species is the main product at low temperatures, while above 300 ◦ C, the main product is N2 [8–10]. The production of N2 O would occur through the reaction pathway, proposed by Hecker and Bell [11], between adsorbed NO and N. The role of the N2 O(a) species [14,15] as well as the formation step of a ␦-N2 species [16] has been the subject of disagreement among various groups. In later works [17–20] the mechanism of the surface reactions on supported Rh has been reviewed to account for previous IR studies in which three different NO adsorbed species were detected on Rh supported catalysts [11,21,22]. These IR studies have shown the formation of bands that have been attributed to different species of adsorbed NO on the rhodium surface (the anionic species Rh–NO−
0926-3373/02/$ – see front matter © 2002 Elsevier Science B.V. All rights reserved. PII: S 0 9 2 6 - 3 3 7 3 ( 0 2 ) 0 0 0 1 9 - X
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(Eq. (1), and the cationic species Rh–NO+ (Eq. (5))), which participate in different steps of the reaction mechanism. The neutral species does not appear to play a role in this revised mechanism. The cationic species is generated by adsorption of NO over Rh+ , while the anionic species is produced by adsorption of NO over Rh0 . Based on these results, Chuang and Tan [20] have proposed the following revised mechanism for the reduction of NO by CO over Rh/Al2 O3 : NO(g) + Rh0 ↔ Rh0 –NO−
(1)
Rh0 –NO− + Rh0 ↔ Rh0 –N + Rh0 –O
(2)
Rh0 –NO− + 2Rh0 ↔ (Rh+ )2 O2− + Rh0 –N
(3)
Rh+ + 2CO ↔ Rh+ (CO)2
(4)
Rh+ + NO(g) ↔ Rh–NO+
(5)
Rh–NO+ + 2CO(g) ↔ Rh+ (CO)2 + NO(g)
(6)
COad + (Rh+ )2 O2− → 2Rh0 + CO2
(7)
Rh+ (CO)2 + 2Oad → Rh+ + 2CO2
(8)
Rh0 –NO− + Rh0 –N → 2Rh0 + N2 O(g)
(9)
Rh0 –N + Rh0 –N → N2(g) + 2Rh0
(10)
Rh0 –N + COad → Rh–NCO
(11)
According to this mechanism, the adsorbed NO− species results in NO dissociation, and it is responsible for the catalyst’s activity for NO reduction. The main difference between the above mechanism and those proposed for unsupported Rh is the participation of the adsorbed Rh(CO)2 species leading to the formation of CO2 gas. In supported Rh there is an effect due to crystallite size on the reaction rate and on the selectivity toward the formation of N2 O. Oh et al. [4] found that the reaction rate per Rh atom exposed to the gas phase (turn over number (TON)) of a supported Rh catalyst is several orders of magnitude lower than that measured on Rh(1 1 1). Later it was reported that the TON increases with increasing size of the Rh crystallites [23]. Selectivity toward N2 O, however, is not affected by crystallite size. Previous IR studies have been done on catalysts with the same crystallite size [11,20]. The single crystal results did not related the TON to a specific adsorbed
species but rather to the crystal structure [31,33], and earlier studies with variable crystallite size did not include IR results [23]. The purpose of this study is to elucidate the effect of crystallite size on the kinetic behavior of the catalysts using in situ FTIR to interpret it in terms of the mechanisms proposed in the literature for supported and unsupported Rh. To this effect, we measured the steady-state kinetic behavior of supported Rh catalysts having different crystallite size, and the concentration of adsorbed NO and CO species by in situ IR. A correlation between selectivity for N2 O formation and the adsorbate concentration is developed and the resulting expression is compared with experimental results.
2. Experimental Two catalysts containing 0.4% (w/w) Rh supported on Aerosil 130 silica were prepared by the dry impregnation method, using the inorganic salt RhCl3 as a precursor. One of them was calcined at 650 ◦ C for 3 h, and the other at 425 ◦ C for the same time. Then both catalysts were reduced at 425 ◦ C for 3 h in a stream of 5% H2 in Ar. Different calcination temperatures were used to obtain catalysts with different crystallite size. The catalyst calcined at 650 ◦ C has a low dispersion of 9.1%, corresponding to an average crystallite size of 127 Å (Rh-127 Å catalyst), while the one calcined at 425 ◦ C has a medium dispersion of 37.5%, with an average crystallite size of 31 Å (Rh-31 Å catalyst). A stable catalyst with higher dispersion was not achieved on a silica support by this method. Catalysts with lower dispersion were not used in this study because the IR signal in those catalysts was too low to get meaningful spectra. For the IR analysis of the adsorbed species, an IR-reactor cell (ISRI, Inc.) was placed inside the chamber of an IR spectrometer (Bruker, Vector 22). The IR-reactor cell has a temperature control system and it is connected to mass-flow controllers, allowing reaction conditions to be varied while the analysis of the species adsorbed on the catalyst was carried out in-situ. The IR spectra were obtained with a resolution of 2 cm−1 and 20 scans per spectrum. The activity and selectivity of the catalysts was determined by analyzing the products of the reaction (CO2 , N2 and N2 O) in
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a gas chromatograph using a Hayasep D column and a thermal conductivity detector. To calculate the reaction rates from the effluent concentration of the IR-reactor cell requires assuming the mixing pattern inside the cell (perfectly mixed or plug flow). However, since low conversions were used (less than 5%), differential conditions were assumed and the rates were calculated from the equation corresponding to a differential flow reactor. The procedure used for the IR studies is the following: the catalyst with a shape of a disk (compressed at 13,000 psi) was placed in the IR-reactor cell and heated to 200 ◦ C, in a stream of 5% H2 in Ar mixture flowing at 12 cm3 /min to prereduce the catalyst. After H2 was desorbed at the same temperature by flushing in pure He for 2 h, the IR-reactor-cell temperature was raised to the reaction temperature. Once the temperature was stabilized, a spectrum of the sample was taken on pure He to be used later as a background value. Then CO flow was started, and 10 min later the NO flow started until a CO partial pressure of 0.05 atm and a NO partial pressure of 0.03 atm balanced with He were reached, with a total flow of 100 cm3 /min. After 10 min, an IR spectrum of the catalyst was taken, and the effluent was sampled for GC analysis. Each measurement in every experiment was repeated three times to assure reproducibility, which was found to be within 5%. The partial pressure of NO was then decreased, at constant CO partial pressure, and the IR spectra and effluent gas composition for the different NO partial pressures were measured. Once this study was completed, the gas composition was returned to the initial NO concentration and the CO partial pressure was decreased while keeping constant the CO partial pressure.
3. Results and discussion The experiments were originally planned to study the effect of the partial pressure of reagents between 0.02 and 0.05 atm for CO and between 0.01 and 0.03 atm for NO, at a temperature of 260 ◦ C. These conditions are similar to those used in single crystal studies [31,33], and enabled us to compare our results with supported catalysts with single crystals’ results under the same conditions. As discussed below, the partial pressure range of NO was extended
79
to 0.086 atm to further examine additional effects at full NO coverage. 3.1. Effect of NO partial pressure First, we discuss the effect of the NO partial pressure at constant CO partial pressure on the catalysts’ activity and selectivity at a temperature of 260 ◦ C. It is well known that CO adsorbs rapidly over Rh in the linear, bridged and gem forms, with IR bands of these species well documented in the literature [24–27]. As mentioned earlier, in the case of the adsorption of NO over Rh, the following species have been reported: the NO+ species, characterized by a band at 1930 cm−1 ; the NO− species with a band at 1830 cm−1 ; and the NO species, with a high frequency band between 1770 and 1740 cm−1 , and a low frequency band between 1690 and 1630 cm−1 [11,21,22]. Recently, Chafik et al. using Rh/TiO2 catalysts [28] proposed that the 1830 and 1725 cm−1 bands correspond to the symmetric and asymmetric vibrations of the Rh(NO)2 species, as it had been suggested previously [22,29], and that this species would be responsible for the formation of N2 O. Fig. 1 shows the IR spectra obtained on both catalysts at a constant CO partial pressure of 0.05 atm and with the NO partial pressure varying between 0.03 and 0.01 atm. Fig. 1 also includes the spectrum obtained after allowing CO in He to flow for 10 min at 260 ◦ C (subtracting the CO gas spectrum) before beginning the flow of NO and starting the reaction. The IR bands seen for the two catalysts are very similar. Before starting the reaction, with CO flowing at a partial pressure of 0.05 atm, spectra 1 shows bands at 2055 cm−1 (Rh-127 Å catalyst) and 2048 cm−1 (Rh-31 Å catalyst) corresponding to CO linearly adsorbed on Rh, and a broad band centered at 1920 cm−1 which can be assigned to the bridged adsorption of CO. No band associated with adsorbed gem CO could be detected. This is consistent with the agglomerating effect of CO on Rh at temperatures above 170 ◦ C [30], which eliminates these species observed only al lower temperatures on catalysts with much higher dispersions than the Rh-31 Å catalysts. As expected, the intensity of the CO band is much higher in the Rh-31 Å catalyst than in the Rh-127 Å catalyst, consistent with its higher dispersion. After starting the flow of NO, spectrum 2, the bands corresponding to adsorbed CO disappear
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Fig. 1. IR spectra of the catalysts at P CO = 0.05 atm, variable PNO , T = 260 ◦ C. (A) Rh-127 Å catalyst. (1) P NO = 0 atm; (2) P NO = 0.03 atm; (3) P NO = 0.017 atm; (4) P NO = 0.01 atm, (B) Rh-31 Å catalyst. (1) P NO = 0 atm; (2) P NO = 0.03 atm; (3) P NO = 0.017 atm; (4) P NO = 0.01 atm.
completely and the IR bands corresponding to the anionic species NO at 1692 cm−1 (Rh-127 Å catalyst) and at 1675 cm−1 (Rh-31 Å catalyst) appear. Bands for gas phase NO are located at 1914 and 1843 cm−1 , together with a series of bands above 2000 cm−1 corresponding to gaseous CO, CO2 and N2 O, and to the Si–NCO and Rh–NCO species. The bands corresponding to the cationic (1908 cm−1 ) and neutral (1830 cm−1 ) species are not seen because they may be masked by the bands due to gaseous NO. However, after subtracting the spectrum of NO gas for identical partial pressure and temperature those NO species cannot be identified with certainty, suggesting that, even though if present, their concentration is too low to be detected. As the partial pressure of NO decreases (spectra 2–4) the absorbance band corresponding to NO− decreases in both catalysts,
showing that under the experimental conditions used, the concentration of adsorbed NO depends on the NO pressure. It has been reported that over Rh(1 1 1) at 325 ◦ C, the integrated absorbance of NO is maximum and independent of NO pressure when it is higher than 2 Torr (0.003 atm) [31]. To verify if this effect is also observed in supported catalysts the NO pressure range in our study was increased up to 0.086 atm. The IR spectra obtained at PNO equal to 0.046, 0.073 and 0.086 atm show the same bands seen in Fig. 1, so they are not shown again. Fig. 2A and B show the integrated absorbance of the NO− species as a function of NO partial pressure for Rh-127 and Rh-31 Å catalysts (1692 and 1675 cm−1 , respectively). As seen in Fig. 2A, for the case of the Rh-127 Å catalyst, the integrated absorbance of NO− increases in the PNO range between
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Fig. 2. Effect of NO partial pressure at P CO = 0.05 atm, T = 260 ◦ C. (A) NO− integrated absorbance on Rh-127 Å catalyst. (B) NO− integrated absorbance on Rh-31 Å catalyst. (C) TON of product formation on Rh-127 Å catalyst. (D) TON of product formation on Rh-31 Å catalyst. (E) Selectivity to N2 O formation on Rh-127 Å catalyst. (F) Selectivity to N2 O formation on Rh-31 Å catalyst.
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0.01 and 0.046 atm, and becomes almost constant at higher PNO . In the case of the Rh-31 Å catalyst, due to its higher dispersion, the integrated absorbance of NO− is much higher than that in the Rh-127 Å catalyst and increases between PNO 0.01 and 0.03 atm, remaining constant at higher PNO . The above confirms the hypothesis that at 260 ◦ C NO coverage does not reach its maximum at PNO lower than 0.03 atm, and this, as discussed below, has implications in the kinetic behavior of the catalysts. Fig. 2C and D show activity results expressed as TON (moles of product/moles of exposed Rh per second) toward the formation of CO2 , N2 O and N2 for the Rh-127 and Rh-31 Å catalysts, respectively. It is seen that for the Rh-127 Å catalyst, which has the larger crystallite size, the TON toward the formation of CO2 is almost three times greater than that of Rh-31 Å catalyst. This agrees with the results reported previously [4,32] showing a decrease in the TON of CO2 formation as Rh crystallite size decreases, when the reaction is performed at temperatures above 500 K. In both catalysts, the increase in NO partial pressure between 0.01 and 0.03 atm leads to an increase in activity for the formation of CO2 and N2 O which thereafter remains practically constant at higher PNO . N2 formation, on the other hand, remains nearly constant in the whole NO pressure range. It has been reported that on Rh(1 1 1) [31] and Rh(1 1 0) [33] single crystals the rate of formation of CO2 , N2 O and N2 is practically independent on the NO partial pressure in the range from 2 Torr (0.003 atm) to 40 Torr (0.053 atm). This was explained by the observation that on Rh single crystals the coverage with NO is always high and independent on the concentration of NO in the gas phase [31]. However, on the supported catalysts used in this study, the dependence of the activity on the NO partial pressure is consistent with the increase of the NO coverage observed in the IR spectra, as PNO increases up to 0.03 atm. Fig. 2E and F show experimental results of the N2 O selectivity as a function of PNO obtained on both catalysts. It is seen that the N2 O selectivity increases as the NO partial pressure increases from 0.01 to 0.03 atm, and it remains almost constant above 0.03 atm. This behavior is parallel to the variation of the integrated absorbance of NO− within the same PNO range. The selectivity values obtained on both catalysts at PNO higher than 0.03 atm are of the order of 72%, in good
agreement with those obtained on Rh(1 1 1) at temperatures close to 250 ◦ C [31], and higher than the 60% value observed on Rh(1 1 0) [33]. The reaction mechanism proposed on Rh(1 1 1) and Rh(1 1 0) [31,33] single crystals includes the following steps: non-dissociative CO and NO adsorption on a single site (CO–Rh and NO–Rh), dissociation of adsorbed NO onto adsorbed N–Rh and O–Rh atoms, surface reaction of adsorbed CO–Rh and O–Rh to yield gas phase CO2 , surface reaction of adsorbed NO–Rh and adsorbed N–Rh to yield gas phase N2 O, and recombination of adsorbed N–Rh to yield N2 . This mechanism does not account for the formation of anionic or cationic NO species on the surface, and thus it is a simpler form of the mechanism summarized in Eqs. (1)–(11). The selectivity of the catalyst (SN2 O ) is defined by SN2 O =
rate of N2 O formation rate of N2 O formation + rate of N formation (12)
In both cases, for single crystals and supported catalysts, assuming that the surface reactions are elementary, the selectivity depends on the NO and N coverage according to the expression, SN2 O =
k9 θNO θN k9 θNO θN + k10 θN2
(13)
where k9 and k10 are the kinetic constants in Eqs. (9) and (10). Dividing the numerator and the denominator of Eq. (13) by k9 θ NO θ N the following expression is obtained: 1 SelN2 O = (14) 1 + (k10 θN /k9 θNO ) considering that for Rh(1 1 1) the integrated absorbance increases linearly with NO coverage when θ NO < 70% [31], it is assumed that the concentration of adsorbed NO- is linearly proportional to the integrated absorbance at NO partial pressures equal to or less than 0.03 atm, i.e. θNO =
kIA S0
(15)
where IA is the integrated absorbance, S0 the number of sites available and k is a proportionality constant. If SN is the number of sites occupied by adsorbed
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N, and since θ N = S N /S0 upon substitution Eq. (15) into Eq. (14) and rearranging one obtains IA = IA + KSN SelN2 O
(16)
with K = k10 /(k9 k). Plotting IA/SelN2 O versus IA for the Rh-31 and Rh-127 Å catalysts, it is possible to determine that (KSN )Rh-31 Å = 1.12 and (KSN )Rh-127 Å = 0.22. From Eq. (15) at NO monolayer coverage θNO ml =
kIAml S0 =1⇒k= S0 IAml
(17)
therefore KSN k10 SN k10 SN = = IANO k9 kIANO k9 (S0 /IAml )IANO k10 (SN /S0 ) k10 θN KSN = ≈ or k9 (IANO /IAml ) IANO k9 θNO (18) Then from Fig. 2A and B, at high PNO (IANO )Rh-31 Å = 2.1 and (IANO )Rh-127Å = 0.58, then it follows that KSN 1.12 = = 0.57, IANO Rh-31 Å 2.16 at PNO > 0.03 atm (SelN2 O )Rh-31 Å 1 = ≈ 0.66 1 + 0.57
(19)
and 0.22 KSN = 0.37, = IANO Rh-127 Å 0.58 at PNO > 0.03 atm (SelN2 O )Rh-127 Å 1 = ≈ 0.77 1 + 0.37 The above estimates show that the selectivity achieves a value of 0.6–0.7 for both the Rh-127 and Rh-31 Å catalysts. This occurs because the ratio of the rates of N2 O formation and N2 formation (Eq. (14)) is between 2 and 3 for both catalysts at this low temperature. This implies that the rate of NO dissociation must be low to allow reaction with N–Rh species. At higher temperatures this is not the case and most of the adsorbed NO dissociates to form N–Rh species that recombine as N2 and reduce the N2 O selectivity.
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The results predicted from Eq. (16) are plotted as broken lines in Fig. 2E and F in the range of validity of that expression (i.e. up to 0.03 atm). As it is seen in Fig. 2E, the agreement is excellent for the Rh-127 Å catalyst, which has the larger crystallite size, but it slightly overestimates the results for the catalyst with the smaller crystallite size, Fig. 2F. Nonetheless, Eq. (16) predicts well the experimental behavior of the selectivity with the NO partial pressure. According to Peden et al. [33], catalysts having large crystals should have a behavior similar to that of Rh(1 1 1), while those having small crystals should be similar to Rh(1 1 0). At similar temperature, Rh(1 1 0) has a lower selectivity to N2 O formation than Rh(1 1 1) [33]. The above results agree with the fact that at low NO coverage (P NO = 0.01 atm) the N2 O selectivity of the Rh-31 Å catalyst is lower than that of the Rh-127 Å catalyst. At high NO concentration, however, the N2 O selectivity is very similar on both catalysts, in agreement with Eqs. (18) and (19). We attributed the observed dependence of the activity and selectivity of both catalysts on the NO partial pressure to the observation that coverage with NO is not constant in the whole range of NO pressure studied. To further study this point the reaction temperature was decreased so that the lower limit of PNO at which constant coverage with NO is obtained, should be displaced to lower PNO values making the reaction insensitive to this variable. To verify this prediction, the experiments were repeated at a temperature of 235 ◦ C. For the experiments at 235 ◦ C, new catalysts disks were prepared and loaded into the IR cell and subjected to the same pretreatment as those used at 260 ◦ C. The IR spectra obtained at 235 ◦ C from both catalysts show the same peaks that were seen at 260 ◦ C, so they are not displayed again, and only the integrated absorbance of the NO− species at the different NO partial pressures are shown in Fig. 3. Fig. 3A shows that the NO integrated absorbance on the Rh-127 Å catalyst increases slightly between PNO 0.01 and 0.03 atm, remaining constant at higher PNO . In the case of the Rh-31 Å catalyst, Fig. 3B shows that the increase in the integrated absorbance of NO− is more pronounced than in the Rh-127 Å catalyst in the PNO range between 0.01 and 0.03 atm. As at T = 260 ◦ C, for PNO values greater than 0.03 atm, the integrated absorbance of NO− is the constant. In both catalysts, the change in the integrated absorbance of
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Fig. 3. Effect of NO partial pressure at P CO = 0.05 atm, T = 235 ◦ C. (A) NO− integrated absorbance on Rh-127 Å catalyst. (B) NO integrated absorbance on Rh-31 Å catalyst. (C) TON of product formation on Rh-127 Å catalyst. (D) TON of product formation on Rh-31 Å catalyst. (E) Selectivity to N2 O formation on Rh-127 Å catalyst. (F) Selectivity to N2 O formation on Rh-31 Å catalyst.
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NO is less than that observed at 260 ◦ C. Fig. 3C and D show the TON toward the formation of the three reaction products (CO2 , N2 O and N2 ) for Rh-127 and Rh-31 Å catalysts, respectively, which are lower than those measured at higher temperature, as it was expected. The TONs on both catalysts differ by a factor of about three with respect to those at T = 260 ◦ C, and there is only a slight effect of the NO partial pressure on both catalysts. As seen in Fig. 3C, on the Rh-127 Å catalyst, the TON for the formation of the three reaction products decreases slightly as the NO partial pressure increases from 0.01 to 0.03 atm. In the case of the Rh-31 Å catalyst, in Fig. 3D, the TON is lower than on the Rh-127 Å catalyst in the pressure range between 0.01 and 0.046 atm. At PNO higher than 0.05 atm, the rates remain practically constant on both catalysts. The N2 O selectivity at different PNO , is shown in Fig. 3E and F for each catalyst, respectively. The selectivity toward N2 O on the Rh-127 Å catalyst, Fig. 3E, is practically independent of the NO partial pressure and remains around 62%, paralleling the behavior of the value of the integrated absorbance of the NO− species. However, on the catalyst with the greater dispersion (Rh-31 Å), the selectivity increases from 41 to 62% in the NO pressure range between 0.01 and 0.046 atm, and becomes constant at higher NO pressure, which is consistent with the behavior observed for the integrated absorbance of NO− . Same trend for selectivity as in the case of T = 260 ◦ C is obtained for T = 235 ◦ C using Eqs. (18) and (19) with (KSN )MD = Rh-31 Å = 2.048 and (KSN )LD = Rh-127 Å = 0.481. The theoretical selectivity curves calculated from Eq. (16) are also shown in Fig. 3E and F. Similarly to what was seen at 260 ◦ C, there is excellent agreement in the case of the lower dispersion catalyst, while in the case of the catalyst with the smaller crystallite size (Rh-31 Å), Eq. (17) slightly overestimates the selectivity value as PNO decreases. The behavior of the Rh-127 and Rh-31 Å catalysts at 235 ◦ C is not conclusive to support the proposal of Peden et al. [33] that the catalyst with the larger crystallite size must exhibit similar activity dependence than Rh(1 1 1), while the catalysts with the smaller crystallite size should have activity dependence like Rh(1 1 0). In fact, these authors show that the selectivity to N2 O of Rh(1 1 0) becomes independent of the NO partial pressure at PNO values greater than those
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found for Rh(1 1 1). This agrees with the observation that while the selectivity of the Rh-127 Å catalyst does not depend on PNO in the whole range studied, the selectivity of the Rh-31 Å catalyst is independent of PNO only at NO partial pressures higher than 0.046 atm. However, the selectivity of both catalysts at high PNO is quite similar, which does not agree with the lower selectivity that should be observed for the Rh-31 Å catalyst if it behaved similarly to Rh(1 1 0). This aspect will be discussed again below, when the behavior of the selectivity of both catalysts with temperature is discussed. 3.2. Effect of the partial pressure of CO The study of the effect of the CO partial pressure was done keeping PNO constant at 0.03 atm and varying the partial pressure of CO between 0.02 and 0.05 atm. Fig. 4A shows the IR spectra obtained for the Rh-127 Å catalyst; and Fig. 4B those for the Rh-31 Å catalyst at 260 ◦ C. The bands seen are similar to those described for Fig. 1. It is seen that the reduction of the CO partial pressure at a constant NO pressure of 0.03 atm does not produce important changes in the IR spectra, except for a slight decrease in the absorbance of the NO− species as the CO partial pressure increases. Fig. 5A and B show the integrated absorbance of the NO− species adsorbed on both catalysts. In both cases, the integrated absorbance decreases as PCO increases. A decrease in the absorbance of the NO− species as PCO increases has also been observed for supported Rh at 210 ◦ C [11] and for Rh(1 1 1) at 325 ◦ C [31]. Fig. 5C and D show the activity of formation of the reaction products at the different CO partial pressures for Rh-127 Å catalyst and Rh-31 Å catalyst respectively. It is seen that in both catalysts the TON of all the reaction products increases as the CO partial pressure increases from 0.02 to 0.05 atm. The activity of formation of CO2 increases about 62 and 75% for Rh-127 and Rh-31 Å catalysts respectively, as PCO increases. This behavior agrees qualitatively with that observed on supported Rh at 210 ◦ C for a pressure range similar to that used in this study [11], but it does not agree with the zero order in PCO seen in unsupported Rh. In spite of the changes in the absorbance of the NO− species and in the activity of the catalysts with PCO , the N2 O selectivity remains almost constant, as seen
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Fig. 4. IR spectra of the catalysts at P NO = 0.05 atm and variable PCO , T = 260 ◦ C. (A) Rh-127 Å catalyst. (1) P CO = 0.05 atm; (2) P CO = 0.028 atm; (3) P CO = 0.019 atm. (B) Rh-31 Å catalyst. (1) P CO = 0.05 atm; (2) P CO = 0.028 atm; (3) P CO = 0.019 atm.
in Fig. 5E for the Rh-127 Å catalyst and in Fig. 5F for the Rh-31 Å catalyst. When the reaction temperature is reduced to 235 ◦ C, the IR spectra obtained at different PCO are very similar to those shown in Fig. 3 for the reaction at 260 ◦ C and they are not included here. The integrated absorbance of NO− is shown in Fig. 6A for the Rh-127 Å catalyst and in Fig. 6B for the Rh-31 Å catalyst. In both catalysts the integrated absorbance of NO− decreases as PCO increases, but the change is significantly less than that observed at 260 ◦ C. Fig. 6C and D show the TON as a function of PCO for catalysts Rh-127 and Rh-31 Å, respectively. As at 260 ◦ C, the activity toward product formation in both catalysts increases as CO partial pressure increases, but the change in the activity is also less than at 260 ◦ C. In fact, at 235 ◦ C, the percentage of increase in the TON of CO2 formation, as PCO increases from 0.02 to 0.05 atm, is almost
the half of that observed at 260 ◦ C. Similarly to what was seen at 260 ◦ C, the CO partial pressure does not affect the N2 O selectivity, which remains almost constant at about 62%, as seen in Fig. 6E and F. The positive PCO order observed in the activity of our supported catalysts, and by Hecker and Bell in a catalyst with high Rh content [11] is consistent with the mechanisms proposed for supported and unsupported Rh. In fact, higher PCO must increase the concentration of adsorbed CO leading to higher production rates of CO2 , as it would be expected from Eqs. (7) and (8), or from the reaction between CO linearly adsorbed in Rh0 and oxygen product of NO dissociation for unsupported Rh [31,33]. In addition to the greater activity (TON) shown by the Rh-127 Å catalyst with larger crystallite size, the activity behavior with changes in the partial pressure of reactants is quite similar on both catalysts. The TON
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Fig. 5. Effect of CO partial pressure at P NO = 0.03 atm, T = 260 ◦ C. (A) NO integrated absorbance on Rh-127 Å catalyst. (B) NO integrated absorbance on Rh-31 Å catalyst. (C) TON of product formation on Rh-127 Å catalyst. (D) TON of product formation on Rh-31 Å catalyst. (E) Selectivity to N2 O formation on Rh-127 Å catalyst. (F) Selectivity to N2 O formation on Rh-31 Å catalyst.
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Fig. 6. Effect of CO partial pressure at P NO = 0.03 atm, T = 235 ◦ C. (A): NO integrated absorbance on Rh-127 Å catalyst. (B): NO integrated absorbance on Rh-31 Å catalyst. (C): TON of product formation on Rh-127 Å catalyst. (D): TON of product formation on Rh-31 Å catalyst. (E): Selectivity to N2 O formation on Rh-127 Å catalyst. (F): Selectivity to N2 O formation on Rh-31 Å catalyst.
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of CO2 formation and N2 O selectivity increase on both catalysts with an increase in NO partial pressure until a high NO coverage is achieved. Likewise, on both catalysts, increasing the CO partial pressure increases the activity TON of CO2 formation without substantially changing the N2 O selectivity. The most noticeable difference is found in the greater sensitivity of the N2 O selectivity in the catalyst with smaller crystal size to the change in NO partial pressure, which becomes evident at 235 ◦ C. Some of these results support the proposal of Peden et al. [33] that the catalyst with smaller crystallite size tends to behave more like Rh(1 1 0), while that with a larger crystallite size behaves like Rh(1 1 1). An important difference between Rh(1 1 1) and Rh(1 1 0) is their response to changes on temperature changes. While the selectivity of Rh(1 1 1) increases as the temperature increases, it reaches a maximum at about 400 ◦ C, and then decreases at higher temperatures, in the case of Rh(1 1 0) N2 O selectivity decreases continually as temperature increases [33]. If the N2 O selectivity of both Rh-127 and Rh-31 Å catalysts is compared at 235 and 260 ◦ C, it is seen that at a PCO of 0.05 atm and a PNO not higher than 0.046 atm, when the integrated absorbance of NO− reaches a constant value, the selectivity increases with increasing temperature on both catalysts. The increase in the selectivity at each temperature is practically identical with both catalysts. Thus, regarding the effect of temperature for T lower than 260 ◦ C, both catalysts have a behavior closer to that of Rh(1 1 1) than to that of Rh(1 1 0). With respect to the lower activity of the supported catalysts as compared to that of unsupported Rh, Chuang and Tan [20] have suggested that since on unsupported Rh only the NO− species exists, which is responsible for the dissociation of NO (controlling step), the supported catalysts would have a lower activity because their surface is partially oxidized, increasing the adsorption of the NO+ species, and thereby decreasing the number of Rh0 sites where the NO species is formed. The IR spectra of our catalysts shown in Fig. 1, allow important conclusions to be reached in relation to the latter aspect. The spectrum of CO adsorbed on both catalysts, spectrum (1) in Fig. 1A and B, shows that before the reaction starts there are no oxidized Rh sites, as evidenced by the complete absence of gem CO, and as expected, the number of CO adsorption sites in the low dispersion
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catalyst is considerably less than in the medium dispersion catalyst. The above follows from the greater absorbance of linear CO in the Rh-31 Å catalyst, even though similar mass of catalysts were used to prepare the disks placed in the IR cell. When NO starts flowing, and adsorbed CO is replaced by NO, spectrum (2) in Fig. 1A and B, it is seen that the integrated absorbance of the NO− species is also substantially greater in the Rh-31 Å catalyst than in Rh-127 Å catalyst. At P CO = 0.05 atm and P NO = 0.03 atm and at a temperature of 260 ◦ C, the integrated absorbance of the NO− species obtained (spectrum 2) were 2.096 and 0.5178, and the rate per gram of catalysts 9.138 × 10−6 and 8.042 × 10−6 for the Rh-31 and Rh-127 Å catalyst, respectively. If the rate constant for dissociation of NO− species is the same on both catalysts, a greater activity per gram of catalyst would be expected on the Rh-31 Å catalyst than on the Rh-127 Å catalyst. It follows that in such case the TON should be the same on both catalysts. Since the rate per gram of catalysts is similar, when dividing by the larger active area of the Rh-31 Å catalysts, its TON is smaller than the TON of the Rh-127 Å catalysts. The above results lead us to conclude that it is not the number of sites available for NO− adsorption that determines the activity of the catalysts, but rather the rate constants for NO− dissociation which are different due to differences in the type of sites existing in each catalyst.
4. Conclusions The results show that even though the activity of the catalyst with larger crystallite size (127 Å) is higher than that of the catalyst with smaller crystallite size (31 Å), its behavior as a function of the partial pressure of the reactants is quite similar within the range of reaction conditions studied. The activity (TON) and selectivity of both catalysts are sensitive to the variation in the NO partial pressure until a high constant coverage with adsorbed NO− is achieved. This was the only adsorbed NO species clearly observed under the experimental conditions used. The most evident difference in the behavior of both catalysts lies in the dependence of the selectivity on PNO shown at a temperature of 235 ◦ C, which can be explained by the different concentrations of NO
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adsorbed on both catalysts as a function of PNO at that temperature. The main difference between the supported catalysts as compared to unsupported Rh is the NO partial pressure needed to reach a high constant coverage with NO. Once the NO coverage reaches a maximum, the dependence of activity and selectivity on PNO is very similar to that seen for Rh(1 1 1). Although under certain conditions (low PNO ), the catalyst with small crystallite size has a behavior that may be associated with that seen for Rh(1 1 0), at high NO coverage both the selectivity value as well as its dependence on temperature are closer to those of Rh(1 1 1). With respect to the effect of PCO on the activity and selectivity of the catalysts, a similar behavior is seen with both catalysts. While the dependence of selectivity on PCO is similar to that observed with unsupported Rh, the catalyst’s activity shows a positive order in PCO that differs from that seen with Rh single crystal. Finally, the results of this work indicate that the decrease in the TON in the catalyst with small crystallites is not related a lower concentration of NO species, but to a lower NO dissociation rate constant, suggesting the existence of different types of active sites on each catalyst. Acknowledgements The financial support of FONDECYT under Grant No. 8000015 is gratefully acknowledged. References [1] K.C. Taylor, Catal. Rev. SCI. Eng. 35 (1993) 457. [2] M. Shelef, G. Graham, Catal. Rev. Sci. Eng. 36 (1994) 433. [3] L. Dubois, P. Hansma, G.A. Somorjai, J. Catal. 65 (1980) 318. [4] S. Oh, G. Fischer, J. Carpenter, D. Goodman, J. Catal. 100 (1986) 360.
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