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desorption dynamics over sulfated zirconia and Pt-promoted sulfated zirconia
Applied Catalysis A: General 252 (2003) 57–74
In situ infrared study of pyridine adsorption/desorption dynamics over sulfated zirconia and Pt-promoted sulfated zirconia Robert W. Stevens Jr. a , Steven S.C. Chuang a,∗ , Burtron H. Davis b a
Department of Chemical Engineering, The University of Akron, Akron, OH 44325-3906, USA b Center for Applied Energy Research, University of Kentucky, Lexington, KY 40511, USA Received 19 November 2002; received in revised form 9 May 2003; accepted 9 May 2003
Abstract The dynamic behavior of adsorbed pyridine and the sulfate group has been studied over sulfated zirconia (SZ) and Pt-promoted sulfated zirconia (Pt/SZ) by in situ infrared (IR) spectroscopy coupled with mass spectrometry (MS). IR analysis confirmed that pretreating SZ in flowing He at 500 ◦ C led to the formation of S=O species. Pyridine adsorption caused desorption of sulfur in the form of SO3 . IR and MS analyses coupled with a temperature-programmed desorption (TPD) study confirmed that pyridine adsorbed onto the Lewis acid sites was oxidized to CO2 while the pyridine–Brønsted acid site complexes showed little desorption or oxidation. Addition of Pt onto sulfated zirconia led to enhanced Brønsted acidity when treated with H2 ; higher loading of Pt led to decreased thermal stability of the sulfate group, promoting desorption of SO2 during the TPD. This is the first report that presents dynamic analyses of pyridine adsorption/desorption phenomena over sulfated zirconia samples. © 2003 Elsevier B.V. All rights reserved. Keywords: Sulfated zirconia (SZ); Pt-promoted sulfated zirconia (Pt/SZ); Diffuse reflectance infrared Fourier transform spectroscopy (DRIFTS); In situ; Infrared (IR); Dynamics; Pyridine adsorption; Pyridine desorption
1. Introduction Sulfated zirconia (SZ) has generated much interest due to its ability to isomerize alkanes at low temperatures [1–9]. It has been claimed to be a superacid (having acid strength greater than 100% H2 SO4 ) [2] as well as to possess important acidic [6–10] and redox properties [11,12]. Reports also exist that claim that SZ is no stronger than sulfuric acid [9,12]. The interpretation of the origin of acidity and catalytic activity of this material has been surrounded by a large amount of controversy. ∗ Corresponding author. Tel.: +1-330-972-6993; fax: +1-330-972-5856. E-mail address: [email protected] (S.S.C. Chuang).
Adsorption of pyridine coupled with infrared (IR) analysis has been accepted as a general practice to qualify the types of acids on the surface. Adsorption of pyridine on Brønsted acid sites forms a pyridinium ion, giving rise to infrared bands at 1638, 1611, 1540 and 1486 cm−1 ; pyridine adsorption on Lewis acid sites yields covalently bound species, giving characteristic bands at 1486 and 1445 cm−1 . The results of this technique are usually presented in the form of before/after adsorption [6,8,13,14]. Metal promoters, such as Pt, have been found to enhance the isomerization activity of SZ [15]. H2 addition to the surface of SZ and Pt-promoted sulfated zirconia (Pt/SZ) to enhance Brønsted acidity has been studied by several groups [2,3,13,14]. Mixed results have been reported: Ebitani et al. claimed that the
0926-860X/$ – see front matter © 2003 Elsevier B.V. All rights reserved. doi:10.1016/S0926-860X(03)00375-2
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presence of Pt on SZ in the presence of H2 caused an enhancement of Brønsted acidity through the spillover of adsorbed H atoms from the noble metal to the support [14], whereas Zhang et al. [13] conducted a similar study and reported that H2 had no effect. The controversial results could stem from not only difference in the nature of the catalysts due to different preparation methods but also due to the validity of techniques used for characterization of the acidity of SZ and Pt/SZ. Although infrared spectroscopy of adsorbed pyridine has been widely used to characterize Brønsted/Lewis acidity of SZ [2–4], a lack of understanding of the interaction between pyridine and SZ may contribute to the wide discrepancy in interpretation of the origin of acidity and catalytic activity of SZ and Pt/SZ. The objective of this study is to investigate the interactions of pyridine and hydrogen with SZ, 1 wt.% Pt/SZ, and 4 wt.% Pt/SZ. In situ diffuse reflectance infrared Fourier transform spectroscopy (DRIFTS) was used to determine the dynamic behavior of adsorbed pyridine; mass spectrometry (MS) was employed to determine the composition of the gaseous products during pyridine adsorption and temperature-programmed desorption (TPD) on SZ and Pt/SZ.
2. Experimental 2.1. Catalyst preparation ZrO2 was prepared by a precipitation method: (1) zirconium hydroxide was precipitated at a pH of 10.5 by mixing a 0.3 M aqueous solution of ZrCl4 (Aldrich, >99.9% purity) with an excess amount of ammonium hydroxide (Fisher Scientific, reagent grade); (2) the precipitate was washed thoroughly with deionized water and filtered; and (3) dried at 383 K for 50 h. The dried ZrO2 was sulfated by immersing it into 0.5 M H2 SO4 (Fisher Scientific, reagent grade, 15 cm3 /g of ZrO2 ) and stirred for 2 h. It was then filtered and dried (without washing). Pt/SZ was prepared by impregnating the SZ solid with a solution of H2 PtCl6 ·6H2 O (Sigma–Aldrich, >99.9% purity) in deionized water. Finally, the catalysts (both SZ and Pt/SZ) were calcined in air at 600 ◦ C for 2 h and stored. Total sulfur content was measured with a Leco SC432 instrument and determined to be 3.18, 3.23
and 3.43 wt.%, respectively, for SZ, 1 wt.% Pt/SZ, and 4 wt.% Pt/SZ. BET surface area of the three samples (SZ, 1 wt.% Pt/SZ, and 4 wt.% Pt/SZ) was determined to be 139, 147, and 123 m2 /g, respectively. 2.2. Experimental apparatus Fig. 1 displays the experimental apparatus which consists of three sections: (i) a gas metering section; (ii) a reactor section; and (iii) an effluent gas analysis section. The gas metering section consists of Brooks 5850 mass flow controllers (not shown), which deliver controlled gas flows to the reactor system, and a pyridine saturator. He flow was directed into the pyridine saturator at room temperature through a pair of interconnected three-way valves, delivering a gaseous flow of pyridine (partial pressure ≈28 mmHg) to the reactor. The reactor system consists of an in situ DRIFTS reactor (Spectra-Tech Inc.), which is situated inside an IR bench (Nicolet Magna 550). The DRIFTS reactor features ZnSe crystal windows, which allows transmission of IR from 650 to 4000 cm−1 . Precise temperature control is accomplished via the intimate contact between the catalyst sample and the thermocouple as well as between the heater and catalyst (inset in Fig. 1). The effluent section is analyzed via a quadrapole mass spectrometer (Balzers QMG 112). This arrangement allows one to study the dynamics of the catalyst surface, its adsorbates, and product formation during the course of the reaction study. For each experiment, 75 mg of catalyst was placed into the DRIFTS reactor. All stages of each experiment were analyzed with both IR and MS. IR spectra were collected at a resolution of 4 cm−1 and were a result of 32 co-added scans. The catalyst was pretreated in situ and heated in He flow from room temperature to 500 ◦ C at a rate of 10 ◦ C/min and held for 2 h. Following the pretreatment, the reactor was cooled slowly back to room temperature during which IR spectra of the clean catalyst surface were collected as a function of temperature, hereafter referred to as pretreated backgrounds. The IR bench is a single-beam instrument, thus it is necessary to scan a background spectrum (reference spectrum) of the catalyst prior to the collection of a spectrum of the sample which consist of adsorbed species and solid SZ or Pt/SZ catalysts. Spectra are presented in the form of absorbance,
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Fig. 1. Schematic of experimental apparatus. The inset illustrates details of the DRIFTS reactor.
A(υ) = −log{I(υ)/I0 (υ)}, where I(υ) and I0 (υ) refer to IR intensities of the sample and background spectra, respectively, as a function of frequency (wavenumber). IR data are presented in two formats: (i) relative to an untreated sample background; and (ii) relative to the pretreated sample. The first utilizes a single spectrum at 40 ◦ C prior to any treatment as a background, whereas the second uses a series of spectra taken at different temperatures during cooling back to room temperature in helium pretreatment; this allows the absorbance spectra to be calculated from background and sample spectra of matching temperature (Fig. 2). Absorbance spectra relative to the untreated background were used to observe changes to the sample surface during the He pretreatment as well as shifts in the S=O group during latter adsorption and desorption studies. Typical spectra of this type are shown in Fig. 3(a). Absorbance spectra relative to the pretreated background gave a clean baseline due to the matched temperatures of the sample/background spectra and are further rationalized by the fact that the point of reference is a clean surface. The typical spectra are shown in Fig. 3(b). The Balzers QMG 112 mass spectrometer could measure eight m/e signals simultaneously; m/e values of 79, 80, 64, 48, 44, 2, 4 and 18 were monitored
corresponding to pyridine, SO3 , SO2 , SO, CO2 , H2 , He and H2 O, respectively. Total gas flow rates were maintained at 30 cm3 /min throughout all of the experiments. 2.3. Pyridine adsorption (acidity characterization) Following pretreatment, the reactor was heated to 150 ◦ C. This temperature was selected for the ease of comparison with literature data and to insure that pyridine (boiling point = 116 ◦ C) did not condense on the catalyst surface. He flow was then directed into the pyridine saturator, exposing the SZ surface to pyridine for about 15 min, during which changes in concentration of adsorbate and gaseous species were recorded via IR and MS. Physisorbed pyridine was removed from the surface, leaving only the stronger chemisorbed species, by flowing only He over the surface while maintaining the reactor at 150 ◦ C until no further changes were observed via IR or MS. The reactor was then cooled to 40 ◦ C and a final IR measurement was collected to characterize the acid surface. The Brønsted/Lewis (B/L) sites ratio was calculated from a ratio of the peak heights of bands at 1540 and 1445 cm−1 , respectively.
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Fig. 2. IR (a) and MS (b) analyses during the pretreatment of SZ from 40 to 500 ◦ C. Infrared spectra are relative to the untreated sample at 40 ◦ C. The shaded bar on the temperature axis of (b) indicates constant temperature.
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2.4. Temperature-programmed desorption
3. Results
Following the pyridine adsorption and characterization at 40 ◦ C, the reactor was heated at a rate of 10 ◦ C/min under He flow from 40 to 500 ◦ C. During the temperature-programmed cycle, transient changes to the sample were recorded via IR and MS.
3.1. Pretreatment
2.5. Hydrogen treatment Separate sets of experiments were conducted with fresh catalysts to examine the effect of H2 on the Brønsted acidity of the SZ catalysts. After pretreatment and before pyridine adsorption, He flow was switched with a four-port valve to H2 (30 cm3 /min) for a period of 15 min at 150 ◦ C. The H2 was then switched back to He and the system was allowed to return to a steady state condition (i.e., unchanging IR spectra and MS profiles).
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Fig. 2(a) shows the absorbance IR spectra of SZ as a function of temperature during pretreatment, relative to an untreated IR background at 40 ◦ C. Upon heating from 40 to 100 ◦ C, SZ exhibited the formation of H2 O at 1636 cm−1 and S=O at 1329 cm−1 . Increasing the temperature to 200 ◦ C led to the growth of both the H2 O and S=O bands as well as appearance of terminal OH on the Zr surface and S–OH as evidenced by the bands at 3758 and 3637 cm−1 [16], respectively. This was accompanied by desorption of water and H2 as evidenced by the MS profiles in Fig. 2(b). The difference in the temperatures which yielded a maximum evolution of H2 and H2 O (183 ◦ C versus 135 ◦ C) indicates that the H2 signal is not simply a fragment of H2 O. Further increase in temperature led to a loss at 3693 cm−1 , which is assigned to bridged OH on Zr
Fig. 3. (a) IR relative to an untreated sample background. (b) IR relative to a pretreated background. (c) Peak intensities of selected species. (d) MS analysis during pyridine adsorption over SZ. Times indicated are relative to the time of pyridine introduction.
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Fig. 3. (Continued ).
[16], as well as growth of the S=O bond, which shifted to higher wavenumber as its intensity increases and is ultimately centered at 1394 cm−1 . As the reactor temperature was further increased, the broad OH region and the H2 O band at 1608 cm−1 continually decreased in intensity while the S=O band at 1394 cm−1 increased. Terminal Zr–OH at 3758 cm−1 became visible as the water was removed and remained stable on the surface. No sulfate was removed from the surface during the pretreatment as evidenced from an absence of change in the SOx MS profiles in Fig. 2(b). All other samples gave very similar behavior to that depicted in Fig. 2, thus their individual results are not shown. 3.2. Pyridine adsorption Fig. 3 illustrates the dynamics of pyridine adsorption over SZ at 150 ◦ C as analyzed by IR and MS. Fig. 3(a) illustrates spectra that are relative to an untreated background. Pyridine adsorption on SZ pro-
duced the pyridinium ion (pyridine–Brønsted acid site complex; hereafter referred to as “Pyr-B”) as indicated by IR vibrations at 1638, 1611, 1540 and 1486 cm−1 , as well as covalently bound pyridine (pyridine–Lewis acid site complex; hereafter referred to as “Pyr-L”) at 1486 and 1442 cm−1 concurrently with a decrease in S=O at 1398 cm−1 . Further exposure led to: (i) increasing intensities of all bands; (ii) formation of Pyr-L at 1575 cm−1 ; (iii) losses of Zr–OH and S–OH bands at 3730 and 3639 cm−1 , respectively; and (iv) evolution of H2 O, H2 , and SOx (Fig. 3(d)). Comparison of the MS fragments of SO3 , SO2 and SO suggests that the major sulfur species evolved is SO3 . Fig. 3(b) illustrates absorbance spectra relative to pretreated background spectra and highlight the loss of S=O at 1405 cm−1 and growth of a broad band in the 1339 cm−1 region. The loss of the S=O stretch can be attributed to a loss of sulfate species (i.e., displaced during the adsorption of pyridine as evidenced in Fig. 3(c)) and/or to an interaction between sulfate
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Fig. 3. (Continued ).
and pyridine resulting in a change to the double bond character of S=O [17]. The losses of Zr–OH and S–OH coupled with the formation of the Pyr-B complex band at 1540 cm−1 suggests that both of these species may serve as Brønsted acid sites according to the following scheme:
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The loss in the intensity of S–OH is greater than that of Zr–OH, suggesting that S–OH is a more significant Brønsted acid site. Fig. 3(c) plots the intensity of sulfate and adsorbed pyridine with time. Pyr-B and Pyr-L changed at the same rate, suggesting that adsorption of pyridine on Brønsted and Lewis sites occurred at the same rate. It is interesting to note that the intensities of the adsorbed species became constant after 2 min of pyridine exposure, whereas gaseous SOx , H2 O and H2 continued to increase with pyridine as shown in Fig. 3(d). Continuous evolution of SOx (Fig. 3(d)) without variation of IR profiles (Fig. 3(c)) can be attributed to the fact that the upper portion of the sample, which is analyzed by the DRIFTS, has been saturated with adsorbed pyridine while the remaining sample continues to uptake pyridine as well as desorb SOx . Upon switching from
pyridine to He, the SOx , H2 O and H2 immediately decreased. 3.3. Hydrogen treatment IR analyses of the exposure of SZ to H2 flow (30 cm3 /min) at 150 ◦ C are shown in Fig. 4(a) and (b). Exposure of the pretreated SZ surface to H2 initially resulted in a decrease and a downward shift in S=O at 1409 cm−1 and the formation of adsorbed water at 1603 cm−1 . The change in IR spectra due to H2 O can be clearly discerned by the difference spectra in Fig. 4(b), which subtract the spectrum prior to H2 admission from the subsequent spectra following H2 admission. Increased time of exposure led to: (i) increased growth of adsorbed H2 O and loss of S=O; (ii) formation of bridged OH on zirconia at 3678 cm−1 ;
Fig. 4. (a) IR analysis relative to an untreated sample background showing the SZ surface before/after the H2 treatment. (b) IR analysis relative to a pretreated sample background during the 15 min H2 treatment of SZ at 150 ◦ C. Times indicated are relative to the time of H2 admission. The final spectrum shows the surface after a 1.5 h He purge.
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Fig. 5. Comparison of IR spectra, relative to an untreated sample background, of H2 -treated samples following He flushing.
and (iii) formation of a band at 1667 cm−1 . The lack of formation of terminal Zr–OH (would be seen at 3730 cm−1 ) or S–OH at 3639 cm−1 indicates that the surface was probably saturated with these types of hydroxyls prior to the H2 treatment. No SOx species were observed via MS in the reactor effluent, thus the decrease in S=O intensity is not due to the displacement of SOx from the SZ surface. Removal of H2 O led to an increase in the S=O bond intensity (Fig. 2); addition of H2 led to the formation of H2 O (i.e., H2 O addition). It is therefore not surprising that the addition of H2 will lead to a decrease in the S=O intensity. Fig. 5 illustrates IR spectra of the sample surface, relative to the untreated background, both before and after the H2 treatment over all of the catalysts (i.e., SZ, 1 wt.% Pt/SZ, and 4 wt.% Pt/SZ). In all cases formation of bridged OH on Zr, adsorbed H2 O, and a loss of S=O was observed. The SZ catalyst exhibited the largest formation of bridged OH at 3652 cm−1 and adsorbed H2 O; both Pt/SZ catalysts exhibited equal intensities of both of these species. The largest decrease in S=O was exhibited by 1 wt.% Pt/SZ. Although the
intensity of these various species depends on the Pt loading, the wavenumber of these species, which reflects their nature, remained unchanged. These results suggest that the presence of Pt does not alter the surface chemistry upon H2 exposure. Pyridine adsorption over H2 -treated SZ is illustrated in Fig. 6(a)–(c). The spectrum at time zero in Fig. 6(a) depicts the SZ surface following the H2 treatment. Similar to the results of SZ in Fig. 3(b), exposure of the SZ catalyst to pyridine led to formation of the pyridine complex bands at 1640, 1611, 1575, 1490 and 1442 cm−1 and losses of S=O at 1397 cm−1 , Zr–OH at 3729 cm−1 , and S–OH at 3638 cm−1 as shown in Fig. 6(a) and (b), however, an additional band is formed at 1676 cm−1 . This band existed for a short time and may be assigned to a hydronium ion, H3 O+ [18]. Fig. 6(b) shows that the increase of the 1676 cm−1 band correlates with the depletion of S=O and S–OH and precedes the formation other species on the surface, suggesting that this species is related to the sulfate group. Increased exposure led to the growth of all pyridine complex bands, stronger
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Fig. 6. (a) IR relative to a pretreated sample background. (b) Peak intensities of selected species. (c) MS analysis during pyridine adsorption over H2 -treated SZ. Times indicated are relative to the time of pyridine introduction.
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Fig. 6. (Continued ).
loss of S=O, Zr–OH, and S–OH, and eventual depletion of bridged OH on Zr. The evolution profiles during pyridine adsorption in Fig. 6(c) are virtually identical to those shown in Fig. 3(d), suggesting that H2 pretreatment of the SZ surface does not affect the adsorption chemistry. The delayed decrease of bridged OH on Zr at 3679 cm−1 in Fig. 6(a) suggests that it is not as reactive as Zr–OH or S–OH as a Brønsted acid site. The loss of S=O at 1397 cm−1 due to the adsorption of pyridine is much greater than that which occurred after the H2 treatment; this is clear upon comparison of the first and last spectra in Fig. 6(a). 3.4. Pt effect Fig. 7 depicts pyridine adsorption over the H2 -treated 4 wt.% Pt/SZ catalyst. Exposure of pyridine to the surface of H2 -treated 4 wt.% Pt/SZ exhibited the same types of trends (i.e., increase and decrease in the intensities of adsorbed species) as
those over H2 -treated SZ as those shown in Fig. 6. Differences in the extents decreasing IR intensities can be observed between the surface character in the presence and absence of Pt (Fig. 7 versus Fig. 6, respectively). These differences, following the pyridine adsorption, included less OH loss, smaller S=O consumption, and a smaller amount of Lewis acid sites present after the He purge when the sample contained Pt. This suggests that the presence of Pt did not affect the surface chemistry during H2 treatment, but did alter the extents of the surface chemistry. The H3 O+ species at 1676 cm−1 was once again observed during pyridine adsorption; the identical behavior in its position, duration, and intensity in Figs. 6 and 7 shows that Pt does not influence it. 3.5. Acidity characterization (adsorption summary) A summary of final IR spectra collected at 40 ◦ C following pyridine adsorption and He purging over all samples examined is shown in Fig. 8. An absorbance
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Fig. 7. IR analysis relative to a pretreated sample background during pyridine adsorption over H2 -treated 4 wt.% Pt/SZ. Times indicated are relative to the time of pyridine addition.
spectrum of a mixture of pyridine and sulfuric acid (50/50 (v/v)) is also shown for reference. The mixture generates a pyridinium ion and thus contains IR bands that are akin to those of the pyridine–Brønsted acid site complex. The acid sites on the surface may be characterized by qualifying the predominant acid-type species present; this may be expressed as the B/L ratio. This ratio is calculated from the IR peak intensities of the pyridine–Brønsted acid site complex at 1540 cm−1 and pyridine–Lewis acid site complex at 1445 cm−1 . This calculation is made possible by the fact that the
extinction coefficients for both of these species are reported to be equal [19]; it was also reported that the B/L ratio can be accurately calculated from IR absorbance intensities as opposed to peak areas [20]. The B/L ratio of the samples studied is depicted in Fig. 9 as a function of Pt content for both the H2 -treated and non-H2 -treated samples. The results indicate that over the Pt/SZ catalysts, exposure of the catalyst to H2 prior to pyridine adsorption led to an increase in the Brønsted character of the surface (B/L ratio increases); the Brønsted character was also greater in the case of
Table 1 IR intensity summary of the pyridinium ion on Brønsted sites and covalently bound pyridine on Lewis sites Pt content (wt.%)
IR intensity of Pyr-B (1540 cm−1 )
IR intensity of Pyr-L (1445 cm−1 )
B/L intensity ratio (I1540 /I1445 )
SZ
H2 -treated SZ
SZ
H2 -treated SZ
SZ
H2 -treated SZ
0 1 4
0.217 0.166 0.131
0.186 0.198 0.135
0.170 0.174 0.112
0.221 0.129 0.064
1.276 0.954 1.170
0.842 1.535 2.109
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Fig. 8. Final IR spectra of pyridine absorption over all samples studied. Spectra shown are collected at 40 ◦ C, relative to a pretreated sample background, and are subsequent to the removal of physisorbed pyridine via purging with He.
Fig. 9. Brønsted/Lewis (B/L) sites ratio plot as a function of both Pt content and H2 treatment. The ratio was calculated from a ratio of the peak heights of the IR absorbance vibrations at 1540 and 1445 cm−1 .
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Fig. 10. (a) IR relative to an pretreated sample background. (b) MS analysis during the pyridine TPD from 40 to 500 ◦ C over SZ.
4 wt.% Pt/SZ than the 1 wt.% Pt/SZ catalyst. Absolute intensities of the Pyr-B and Pyr-L complexes for all samples are summarized in Table 1. It is interesting to note that not only the Brønsted character of the Pt/SZ catalysts is enhanced upon H2 treatment, but also the Lewis character is reduced. 3.6. Pyridine TPD The IR and MS analyses as a function of temperature of the TPD from 40 to 500 ◦ C in He flow of the SZ catalyst are shown in Fig. 10. Increasing temperature from 40 to 300 ◦ C resulted in a slight depletion of Zr–OH at 3729 cm−1 , S–OH at 3638 cm−1 , and Pyr-L at both 1575 and 1445 cm−1 (Fig. 10(a)), and was accompanied by evolution of H2 O in the reactor effluent (Fig. 10(b)). Further increasing the temperature from 300 to 500 ◦ C resulted in decreasing all IR intensities and evolution of CO2 . No pyridine desorbed throughout the experiment. Part of adsorbed pyridine is oxidized to CO2 , indicating that pyridine TPD is not a valid method to gauge acid strength. The sulfate groups on the zirconia surface appear to be stable
due to the absence of SOx species in the MS analysis in Fig. 10(b). The reappearance of the S=O band at 1406 cm−1 (becomes less negative) is quite noteworthy. Recall that the exposure of the SZ catalyst to pyridine resulted in the loss of S=O (Fig. 3) and this was accompanied by evolution of SO3 . The increase in S=O suggests that its initial loss was not solely due to the decomposition of surface sulfate but rather, at least in part, due to an interaction of the pyridine molecule with the sulfate group resulting in a change to its chemical bonding (i.e., double bond to single bond). As pyridine is removed, S=O species return. When Pt is present on the surface of the SZ, a different behavior is observed. Fig. 11(a) and (b) depicts IR and MS analyses during the TPD over 4 wt.% Pt/SZ from 40 to 500 ◦ C in He flow. Increasing temperature resulted in the same trends in Fig. 11(a) as in Fig. 10(a), thus will not be discussed. A temperature increase from 40 to 355 ◦ C produced a peak in the CO2 profile in Fig. 11(b); further heating to 500 ◦ C caused a second maximum in CO2 evolution at 456 ◦ C accompanied by increases in
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Fig. 11. (a) IR relative to a pretreated sample background. (b) MS analyses during the pyridine TPD from 40 to 500 ◦ C over H2 -treated 4 wt.% Pt/SZ. The rate of heating used was 10 ◦ C/min.
the SO2 and SO MS profiles. The SO3 profile remained stable at baseline value throughout the full temperature range. Comparing Figs. 10(b) and 11(b) reveals also that much more H2 O was produced over the 4 wt.% Pt/SZ catalyst. These results suggest that Pt affects the thermal stability of the catalyst and that sulfate primarily decomposes as SO2 where the SO profile is a fragment of SO2 on the mass spectrometer. Fig. 12(a)–(c) illustrate normalized IR intensities for S=O, Pyr-L and Pyr-B complexes on SZ, 1 wt.% Pt/SZ, and 4 wt.% Pt/SZ, respectively, during TPD; Fig. 12(d)–(f) shows the corresponding TPD profiles. H2 treatment did not have any appreciable impact on the TPD. Thus, the respective results are not shown here. Fig. 12(a)–(c) show that as temperature increased above 150 ◦ C, Pyr-L decreased, S=O increased, and Pyr-B remained relatively constant in all cases. Fig. 12(d)–(f) show that CO2 evolved with its center at approximately 420 ◦ C. The CO2 evolution profile appears to correspond to a decrease in Pyr-L. Pyr-B only changed appreciably upon the decomposition of sulfate (i.e., Fig. 12(c)).
4. Discussion 4.1. He pretreatment As the sulfated zirconia is heated in flowing helium, H2 O is progressively removed from the surface sample and formation of S=O becomes apparent (Fig. 2). As the H2 O content of the sample decreased, the S=O vibration became more intense and the band shifted to higher wavenumber. The shifting is believed to be related to the sulfate group transforming from an ionic form to that of a covalent structure [16]. It has been suggested that the S=O wavenumber reflects the type of sulfate structure predominant on the zirconia surface [21]. Frequencies below 1400 cm−1 are indicative of an isolated sulfate group (Zr–O)3 –S=O, whereas those above 1400 cm−1 are likely attributed to a polynuclear structure of the sulfate group [(Zr–O)2 (SO)]2 –O. Fig. 5 indicates that all samples showed S=O vibrations at nearly identical frequencies, that being ∼1385 cm−1 , suggesting that the presence of Pt did not affect the sulfate group as well as indicating that the sulfate
72 R.W. Stevens Jr. et al. / Applied Catalysis A: General 252 (2003) 57–74 Fig. 12. Pyridine TPD summary. Normalized IR intensities are shown as a function of temperature over: (a) SZ; (b) 1 wt.% Pt/SZ; (c) 4 wt.% Pt/SZ. Vibrations shown are S=O (䊊, 1370–1405 cm−1 ), Pyr-L (䊐, 1445 cm−1 ), Pyr-B (䉫, 1540 cm−1 ). Mass spectral analysis of the reactor effluent showing compositional changes as a function of temperature over: (d) SZ; (e) 1 wt.% Pt/SZ; (f) 4 wt.% Pt/SZ. Monitored species are shown as SO (䊊, m/e = 48), SO2 (䊐, m/e = 64), SO3 (䉫, m/e = 80), CO2 ( , m/e = 44), and pyridine ( , m/e = 79).
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group on our samples is predominantly of the isolated type. 4.2. Pyridine adsorption The interaction of pyridine with sulfated zirconia resulted in a decrease of S=O (i.e., 1370–1410 cm−1 ) in all cases (Figs. 3 and 6). This may be attributed to: (i) desorption of the sulfate group; or (ii) a reaction of the sulfate (S=O) with pyridine, resulting in a conversion to another species, such as S–O or S–OH. Our results show that both of these events take place. As the sample was exposed to pyridine, the IR band centered at 1405 cm−1 (S=O) decreased and SOx , predominantly SO3 , was eluted as confirmed by MS analysis. Morterra et al. [22] reported that pyridine can ligand displace sulfates and create more sites on the zirconia surface, leading to an over-estimated amount of Lewis acidity. Our results confirm the displacement and further suggest that the sulfate species being displaced may be in the form of polynuclear structure. Water is also eluted from the surface upon pyridine exposure (Fig. 3b) and is coupled with a decrease in Zr–OH and S–OH intensities (Fig. 3a and c), suggesting that the hydroxyl groups are Brønsted acid sites. Over the hydrogen-treated sample (Fig. 6), a band at 1676 cm−1 is briefly visible; this is assigned to H3 O+ [18]. The H3 O+ is presumed to form from the excess amount of H present and is believed to further interact with the pyridine to form the pyridinium ion (1540 cm−1 ): H3 O+ + pyr → (H–pyr)+ + H2 O. Further study is necessary to verify this step. 4.3. Temperature-programmed desorption Simultaneous monitoring of changes in IR spectra and gaseous species evolved from the SZ and Pt/SZ surfaces during TPD allows further elucidation of interaction of pyridine with SZ. During TPD, Pyr-B remained relatively stable and showed a slightly decrease throughout the temperature range, whereas Pyr-L intensity decreased with increasing temperature, indicating that it is the covalently bound pyridine that is oxidized to CO2 . Since Pyr-L is covalently bonded on Zr sites, the formation of CO2 from Pyr-L reflects the oxidative activity of Zr sites. Formation of CO2 from temperature-programmed desorption of
73
pyridine has been reported on SZ catalysts with Fe, Mn and Ni [23–26]. It is important to note that intensity of the S=O band increases in a positive direction, signifying its return (Figs. 10–12). Although Pyr-B is known to result in part from interaction between pyridinium ion with a proton associated with sulfate groups, the change of Pyr-B does not correspond to that of the S=O curve. In the case of SZ, the Pyr-L complex at 1445 cm−1 decreased 74%, while S=O at 1405 cm−1 increased 46% over the course of heating. From these results, it is reasonable to conclude that the bond order of the sulfate group is reduced when pyridine complexes with an adjacent Zr atom. Furthermore, it appears that the loss of Pyr-L contributes more to the recovery of the S=O than that of Pyr-B. The major role of Pt in TPD is to destabilize the sulfate group, leading to SO2 evolution on 4 wt.% Pt/SZ. The stability of the sulfate groups on the 1 wt.% Pt/SZ samples appeared to be identical to the SZ samples, suggesting that a small Pt loading may provide the added benefit of Brønsted acidity enhancement while avoiding the detrimental sulfate stability decrease.
5. Conclusion Simultaneous measurement of adsorbed species and gaseous species evolved from SZ and Pt/SZ surfaces allows elucidation of: (i) the initial state and final state of the catalyst surface; and (ii) the interaction between pyridine and SZ during pyridine adsorption and temperature-programmed desorption. He treatment, which has been referred to as an activation, of fresh SZ and Pt/SZ at 500 ◦ C led to the formation of a prominent S=O species. The wavenumber of the S=O shifted downward and part of polynuclear sulfate desorbed as SO3 upon pyridine adsorption. The desorption of sulfate leads to changes in the nature of surface sites which make interpretation of isomerization activity in terms of the ratio of Pyr-B to Pyr-L unreliable. Temperature-programmed desorption (TPD) study coupled with IR and MS shows oxidation of adsorbed pyridine to CO2 is related to Zr4+ (i.e., Lewis acid sites). The presence of Pt as a promoter on the sulfated zirconium was found to enhance the Brønsted acidity of the sample when treated with H2 and to promote desorption of sulfate as SO2 during the TPD.
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Acknowledgements This work was partially financed by the Ohio Board of Regents Grant R4552-OBR. References [1] M. Scheithauer, E. Bosch, U.A. Schubert, H. Knozinger, T.K. Cheung, F.C. Jentoft, B.C. Gates, B. Tesche, J. Catal. 177 (1998) 137. [2] K. Arata, Appl. Catal. A: Gen. 146 (1996) 3. [3] X. Song, A. Sayari, Catal. Rev. Sci. Eng. 38 (1996) 329. [4] A. Corma, Chem. Rev. 95 (1995) 559. [5] B.-Q. Xu, W.M.H. Sachtler, J. Catal. 165 (1997) 231. [6] D.J. Zalewski, S. Alerasool, P.K. Doolin, Catal. Today 53 (1999) 419. [7] T. Yamaguchi, T. Jin, K. Tanabe, J. Phys. Chem. 90 (1986) 3148. [8] B. Li, R.D. Gonzalez, Catal. Today 46 (1998) 55. [9] B. Umansky, J. Engelhardt, W.K. Hall, J. Catal. 127 (1991) 128. [10] G. Yaluris, R.B. Larson, J.M. Kobe, M.R. Gonzalez, K.B. Fogash, J.A. Dumesic, J. Catal. 158 (1996) 336.
[11] J.M. Kobe, M.R. Gonzalez, K.B. Fogash, J.A. Dumesic, J. Catal. 164 (1996) 459. [12] K.T. Wan, C.B. Khouw, M.E. Davis, J. Catal. 158 (1996) 311. [13] C. Zhang, R. Miranda, B.H. Davis, Catal. Lett. 29 (1994) 349. [14] K. Ebitani, J. Konishi, H. Hattori, J. Catal. 130 (1991) 257. [15] J.E. Tabora, R.J. Davis, J. Catal. 162 (1996) 125. [16] E.E. Platero, M.P. Mentruit, Catal. Lett. 30 (1995) 31. [17] T. Jin, T. Yamaguchi, K. Tanabe, J. Phys. Chem. 90 (1986) 4794. [18] F. Babou, G. Coudurier, J.C. Vedrine, J. Catal. 152 (1995) 341. [19] D.J. Rosenthal, M.G. White, G.D. Parks, AIChE J. 33 (1987) 336. [20] B.H. Davis, R.A. Keogh, S. Alerasool, D.J. Zalewski, D.E. Day, P.K. Doolin, J. Catal. 183 (1999) 45. [21] C. Morterra, G. Cerrato, V. Bolis, Catal. Today 17 (1993) 505. [22] C. Morterra, G. Cerrato, F. Pinna, G. Meligrana, Top. Catal. 15 (2001) 53. [23] E.C. Sikabwe, M.A. Coelho, D.E. Resasco, R.L. White, Catal. Lett. 34 (1995) 23. [24] A. Jatia, C. Chang, J.D. MacLeod, T. Okubo, M.E. Davis, Catal. Lett. 25 (1994) 21. [25] B. Li, R.D. Gonzalez, Ind. Eng. Chem. Res. 35 (1996) 3141. [26] W.M. Hua, C.X. Miao, J.M. Chen, Z. Gao, Mater. Chem. Phys. 45 (1996) 220.
In situ infrared study of pyridine adsorption/desorption dynamics over sulfated zirconia and Pt-promoted sulfated zirconia Robert W. Stevens Jr. a , Steven S.C. Chuang a,∗ , Burtron H. Davis b a
Department of Chemical Engineering, The University of Akron, Akron, OH 44325-3906, USA b Center for Applied Energy Research, University of Kentucky, Lexington, KY 40511, USA Received 19 November 2002; received in revised form 9 May 2003; accepted 9 May 2003
Abstract The dynamic behavior of adsorbed pyridine and the sulfate group has been studied over sulfated zirconia (SZ) and Pt-promoted sulfated zirconia (Pt/SZ) by in situ infrared (IR) spectroscopy coupled with mass spectrometry (MS). IR analysis confirmed that pretreating SZ in flowing He at 500 ◦ C led to the formation of S=O species. Pyridine adsorption caused desorption of sulfur in the form of SO3 . IR and MS analyses coupled with a temperature-programmed desorption (TPD) study confirmed that pyridine adsorbed onto the Lewis acid sites was oxidized to CO2 while the pyridine–Brønsted acid site complexes showed little desorption or oxidation. Addition of Pt onto sulfated zirconia led to enhanced Brønsted acidity when treated with H2 ; higher loading of Pt led to decreased thermal stability of the sulfate group, promoting desorption of SO2 during the TPD. This is the first report that presents dynamic analyses of pyridine adsorption/desorption phenomena over sulfated zirconia samples. © 2003 Elsevier B.V. All rights reserved. Keywords: Sulfated zirconia (SZ); Pt-promoted sulfated zirconia (Pt/SZ); Diffuse reflectance infrared Fourier transform spectroscopy (DRIFTS); In situ; Infrared (IR); Dynamics; Pyridine adsorption; Pyridine desorption
1. Introduction Sulfated zirconia (SZ) has generated much interest due to its ability to isomerize alkanes at low temperatures [1–9]. It has been claimed to be a superacid (having acid strength greater than 100% H2 SO4 ) [2] as well as to possess important acidic [6–10] and redox properties [11,12]. Reports also exist that claim that SZ is no stronger than sulfuric acid [9,12]. The interpretation of the origin of acidity and catalytic activity of this material has been surrounded by a large amount of controversy. ∗ Corresponding author. Tel.: +1-330-972-6993; fax: +1-330-972-5856. E-mail address: [email protected] (S.S.C. Chuang).
Adsorption of pyridine coupled with infrared (IR) analysis has been accepted as a general practice to qualify the types of acids on the surface. Adsorption of pyridine on Brønsted acid sites forms a pyridinium ion, giving rise to infrared bands at 1638, 1611, 1540 and 1486 cm−1 ; pyridine adsorption on Lewis acid sites yields covalently bound species, giving characteristic bands at 1486 and 1445 cm−1 . The results of this technique are usually presented in the form of before/after adsorption [6,8,13,14]. Metal promoters, such as Pt, have been found to enhance the isomerization activity of SZ [15]. H2 addition to the surface of SZ and Pt-promoted sulfated zirconia (Pt/SZ) to enhance Brønsted acidity has been studied by several groups [2,3,13,14]. Mixed results have been reported: Ebitani et al. claimed that the
0926-860X/$ – see front matter © 2003 Elsevier B.V. All rights reserved. doi:10.1016/S0926-860X(03)00375-2
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presence of Pt on SZ in the presence of H2 caused an enhancement of Brønsted acidity through the spillover of adsorbed H atoms from the noble metal to the support [14], whereas Zhang et al. [13] conducted a similar study and reported that H2 had no effect. The controversial results could stem from not only difference in the nature of the catalysts due to different preparation methods but also due to the validity of techniques used for characterization of the acidity of SZ and Pt/SZ. Although infrared spectroscopy of adsorbed pyridine has been widely used to characterize Brønsted/Lewis acidity of SZ [2–4], a lack of understanding of the interaction between pyridine and SZ may contribute to the wide discrepancy in interpretation of the origin of acidity and catalytic activity of SZ and Pt/SZ. The objective of this study is to investigate the interactions of pyridine and hydrogen with SZ, 1 wt.% Pt/SZ, and 4 wt.% Pt/SZ. In situ diffuse reflectance infrared Fourier transform spectroscopy (DRIFTS) was used to determine the dynamic behavior of adsorbed pyridine; mass spectrometry (MS) was employed to determine the composition of the gaseous products during pyridine adsorption and temperature-programmed desorption (TPD) on SZ and Pt/SZ.
2. Experimental 2.1. Catalyst preparation ZrO2 was prepared by a precipitation method: (1) zirconium hydroxide was precipitated at a pH of 10.5 by mixing a 0.3 M aqueous solution of ZrCl4 (Aldrich, >99.9% purity) with an excess amount of ammonium hydroxide (Fisher Scientific, reagent grade); (2) the precipitate was washed thoroughly with deionized water and filtered; and (3) dried at 383 K for 50 h. The dried ZrO2 was sulfated by immersing it into 0.5 M H2 SO4 (Fisher Scientific, reagent grade, 15 cm3 /g of ZrO2 ) and stirred for 2 h. It was then filtered and dried (without washing). Pt/SZ was prepared by impregnating the SZ solid with a solution of H2 PtCl6 ·6H2 O (Sigma–Aldrich, >99.9% purity) in deionized water. Finally, the catalysts (both SZ and Pt/SZ) were calcined in air at 600 ◦ C for 2 h and stored. Total sulfur content was measured with a Leco SC432 instrument and determined to be 3.18, 3.23
and 3.43 wt.%, respectively, for SZ, 1 wt.% Pt/SZ, and 4 wt.% Pt/SZ. BET surface area of the three samples (SZ, 1 wt.% Pt/SZ, and 4 wt.% Pt/SZ) was determined to be 139, 147, and 123 m2 /g, respectively. 2.2. Experimental apparatus Fig. 1 displays the experimental apparatus which consists of three sections: (i) a gas metering section; (ii) a reactor section; and (iii) an effluent gas analysis section. The gas metering section consists of Brooks 5850 mass flow controllers (not shown), which deliver controlled gas flows to the reactor system, and a pyridine saturator. He flow was directed into the pyridine saturator at room temperature through a pair of interconnected three-way valves, delivering a gaseous flow of pyridine (partial pressure ≈28 mmHg) to the reactor. The reactor system consists of an in situ DRIFTS reactor (Spectra-Tech Inc.), which is situated inside an IR bench (Nicolet Magna 550). The DRIFTS reactor features ZnSe crystal windows, which allows transmission of IR from 650 to 4000 cm−1 . Precise temperature control is accomplished via the intimate contact between the catalyst sample and the thermocouple as well as between the heater and catalyst (inset in Fig. 1). The effluent section is analyzed via a quadrapole mass spectrometer (Balzers QMG 112). This arrangement allows one to study the dynamics of the catalyst surface, its adsorbates, and product formation during the course of the reaction study. For each experiment, 75 mg of catalyst was placed into the DRIFTS reactor. All stages of each experiment were analyzed with both IR and MS. IR spectra were collected at a resolution of 4 cm−1 and were a result of 32 co-added scans. The catalyst was pretreated in situ and heated in He flow from room temperature to 500 ◦ C at a rate of 10 ◦ C/min and held for 2 h. Following the pretreatment, the reactor was cooled slowly back to room temperature during which IR spectra of the clean catalyst surface were collected as a function of temperature, hereafter referred to as pretreated backgrounds. The IR bench is a single-beam instrument, thus it is necessary to scan a background spectrum (reference spectrum) of the catalyst prior to the collection of a spectrum of the sample which consist of adsorbed species and solid SZ or Pt/SZ catalysts. Spectra are presented in the form of absorbance,
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Fig. 1. Schematic of experimental apparatus. The inset illustrates details of the DRIFTS reactor.
A(υ) = −log{I(υ)/I0 (υ)}, where I(υ) and I0 (υ) refer to IR intensities of the sample and background spectra, respectively, as a function of frequency (wavenumber). IR data are presented in two formats: (i) relative to an untreated sample background; and (ii) relative to the pretreated sample. The first utilizes a single spectrum at 40 ◦ C prior to any treatment as a background, whereas the second uses a series of spectra taken at different temperatures during cooling back to room temperature in helium pretreatment; this allows the absorbance spectra to be calculated from background and sample spectra of matching temperature (Fig. 2). Absorbance spectra relative to the untreated background were used to observe changes to the sample surface during the He pretreatment as well as shifts in the S=O group during latter adsorption and desorption studies. Typical spectra of this type are shown in Fig. 3(a). Absorbance spectra relative to the pretreated background gave a clean baseline due to the matched temperatures of the sample/background spectra and are further rationalized by the fact that the point of reference is a clean surface. The typical spectra are shown in Fig. 3(b). The Balzers QMG 112 mass spectrometer could measure eight m/e signals simultaneously; m/e values of 79, 80, 64, 48, 44, 2, 4 and 18 were monitored
corresponding to pyridine, SO3 , SO2 , SO, CO2 , H2 , He and H2 O, respectively. Total gas flow rates were maintained at 30 cm3 /min throughout all of the experiments. 2.3. Pyridine adsorption (acidity characterization) Following pretreatment, the reactor was heated to 150 ◦ C. This temperature was selected for the ease of comparison with literature data and to insure that pyridine (boiling point = 116 ◦ C) did not condense on the catalyst surface. He flow was then directed into the pyridine saturator, exposing the SZ surface to pyridine for about 15 min, during which changes in concentration of adsorbate and gaseous species were recorded via IR and MS. Physisorbed pyridine was removed from the surface, leaving only the stronger chemisorbed species, by flowing only He over the surface while maintaining the reactor at 150 ◦ C until no further changes were observed via IR or MS. The reactor was then cooled to 40 ◦ C and a final IR measurement was collected to characterize the acid surface. The Brønsted/Lewis (B/L) sites ratio was calculated from a ratio of the peak heights of bands at 1540 and 1445 cm−1 , respectively.
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Fig. 2. IR (a) and MS (b) analyses during the pretreatment of SZ from 40 to 500 ◦ C. Infrared spectra are relative to the untreated sample at 40 ◦ C. The shaded bar on the temperature axis of (b) indicates constant temperature.
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2.4. Temperature-programmed desorption
3. Results
Following the pyridine adsorption and characterization at 40 ◦ C, the reactor was heated at a rate of 10 ◦ C/min under He flow from 40 to 500 ◦ C. During the temperature-programmed cycle, transient changes to the sample were recorded via IR and MS.
3.1. Pretreatment
2.5. Hydrogen treatment Separate sets of experiments were conducted with fresh catalysts to examine the effect of H2 on the Brønsted acidity of the SZ catalysts. After pretreatment and before pyridine adsorption, He flow was switched with a four-port valve to H2 (30 cm3 /min) for a period of 15 min at 150 ◦ C. The H2 was then switched back to He and the system was allowed to return to a steady state condition (i.e., unchanging IR spectra and MS profiles).
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Fig. 2(a) shows the absorbance IR spectra of SZ as a function of temperature during pretreatment, relative to an untreated IR background at 40 ◦ C. Upon heating from 40 to 100 ◦ C, SZ exhibited the formation of H2 O at 1636 cm−1 and S=O at 1329 cm−1 . Increasing the temperature to 200 ◦ C led to the growth of both the H2 O and S=O bands as well as appearance of terminal OH on the Zr surface and S–OH as evidenced by the bands at 3758 and 3637 cm−1 [16], respectively. This was accompanied by desorption of water and H2 as evidenced by the MS profiles in Fig. 2(b). The difference in the temperatures which yielded a maximum evolution of H2 and H2 O (183 ◦ C versus 135 ◦ C) indicates that the H2 signal is not simply a fragment of H2 O. Further increase in temperature led to a loss at 3693 cm−1 , which is assigned to bridged OH on Zr
Fig. 3. (a) IR relative to an untreated sample background. (b) IR relative to a pretreated background. (c) Peak intensities of selected species. (d) MS analysis during pyridine adsorption over SZ. Times indicated are relative to the time of pyridine introduction.
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Fig. 3. (Continued ).
[16], as well as growth of the S=O bond, which shifted to higher wavenumber as its intensity increases and is ultimately centered at 1394 cm−1 . As the reactor temperature was further increased, the broad OH region and the H2 O band at 1608 cm−1 continually decreased in intensity while the S=O band at 1394 cm−1 increased. Terminal Zr–OH at 3758 cm−1 became visible as the water was removed and remained stable on the surface. No sulfate was removed from the surface during the pretreatment as evidenced from an absence of change in the SOx MS profiles in Fig. 2(b). All other samples gave very similar behavior to that depicted in Fig. 2, thus their individual results are not shown. 3.2. Pyridine adsorption Fig. 3 illustrates the dynamics of pyridine adsorption over SZ at 150 ◦ C as analyzed by IR and MS. Fig. 3(a) illustrates spectra that are relative to an untreated background. Pyridine adsorption on SZ pro-
duced the pyridinium ion (pyridine–Brønsted acid site complex; hereafter referred to as “Pyr-B”) as indicated by IR vibrations at 1638, 1611, 1540 and 1486 cm−1 , as well as covalently bound pyridine (pyridine–Lewis acid site complex; hereafter referred to as “Pyr-L”) at 1486 and 1442 cm−1 concurrently with a decrease in S=O at 1398 cm−1 . Further exposure led to: (i) increasing intensities of all bands; (ii) formation of Pyr-L at 1575 cm−1 ; (iii) losses of Zr–OH and S–OH bands at 3730 and 3639 cm−1 , respectively; and (iv) evolution of H2 O, H2 , and SOx (Fig. 3(d)). Comparison of the MS fragments of SO3 , SO2 and SO suggests that the major sulfur species evolved is SO3 . Fig. 3(b) illustrates absorbance spectra relative to pretreated background spectra and highlight the loss of S=O at 1405 cm−1 and growth of a broad band in the 1339 cm−1 region. The loss of the S=O stretch can be attributed to a loss of sulfate species (i.e., displaced during the adsorption of pyridine as evidenced in Fig. 3(c)) and/or to an interaction between sulfate
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Fig. 3. (Continued ).
and pyridine resulting in a change to the double bond character of S=O [17]. The losses of Zr–OH and S–OH coupled with the formation of the Pyr-B complex band at 1540 cm−1 suggests that both of these species may serve as Brønsted acid sites according to the following scheme:
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The loss in the intensity of S–OH is greater than that of Zr–OH, suggesting that S–OH is a more significant Brønsted acid site. Fig. 3(c) plots the intensity of sulfate and adsorbed pyridine with time. Pyr-B and Pyr-L changed at the same rate, suggesting that adsorption of pyridine on Brønsted and Lewis sites occurred at the same rate. It is interesting to note that the intensities of the adsorbed species became constant after 2 min of pyridine exposure, whereas gaseous SOx , H2 O and H2 continued to increase with pyridine as shown in Fig. 3(d). Continuous evolution of SOx (Fig. 3(d)) without variation of IR profiles (Fig. 3(c)) can be attributed to the fact that the upper portion of the sample, which is analyzed by the DRIFTS, has been saturated with adsorbed pyridine while the remaining sample continues to uptake pyridine as well as desorb SOx . Upon switching from
pyridine to He, the SOx , H2 O and H2 immediately decreased. 3.3. Hydrogen treatment IR analyses of the exposure of SZ to H2 flow (30 cm3 /min) at 150 ◦ C are shown in Fig. 4(a) and (b). Exposure of the pretreated SZ surface to H2 initially resulted in a decrease and a downward shift in S=O at 1409 cm−1 and the formation of adsorbed water at 1603 cm−1 . The change in IR spectra due to H2 O can be clearly discerned by the difference spectra in Fig. 4(b), which subtract the spectrum prior to H2 admission from the subsequent spectra following H2 admission. Increased time of exposure led to: (i) increased growth of adsorbed H2 O and loss of S=O; (ii) formation of bridged OH on zirconia at 3678 cm−1 ;
Fig. 4. (a) IR analysis relative to an untreated sample background showing the SZ surface before/after the H2 treatment. (b) IR analysis relative to a pretreated sample background during the 15 min H2 treatment of SZ at 150 ◦ C. Times indicated are relative to the time of H2 admission. The final spectrum shows the surface after a 1.5 h He purge.
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Fig. 5. Comparison of IR spectra, relative to an untreated sample background, of H2 -treated samples following He flushing.
and (iii) formation of a band at 1667 cm−1 . The lack of formation of terminal Zr–OH (would be seen at 3730 cm−1 ) or S–OH at 3639 cm−1 indicates that the surface was probably saturated with these types of hydroxyls prior to the H2 treatment. No SOx species were observed via MS in the reactor effluent, thus the decrease in S=O intensity is not due to the displacement of SOx from the SZ surface. Removal of H2 O led to an increase in the S=O bond intensity (Fig. 2); addition of H2 led to the formation of H2 O (i.e., H2 O addition). It is therefore not surprising that the addition of H2 will lead to a decrease in the S=O intensity. Fig. 5 illustrates IR spectra of the sample surface, relative to the untreated background, both before and after the H2 treatment over all of the catalysts (i.e., SZ, 1 wt.% Pt/SZ, and 4 wt.% Pt/SZ). In all cases formation of bridged OH on Zr, adsorbed H2 O, and a loss of S=O was observed. The SZ catalyst exhibited the largest formation of bridged OH at 3652 cm−1 and adsorbed H2 O; both Pt/SZ catalysts exhibited equal intensities of both of these species. The largest decrease in S=O was exhibited by 1 wt.% Pt/SZ. Although the
intensity of these various species depends on the Pt loading, the wavenumber of these species, which reflects their nature, remained unchanged. These results suggest that the presence of Pt does not alter the surface chemistry upon H2 exposure. Pyridine adsorption over H2 -treated SZ is illustrated in Fig. 6(a)–(c). The spectrum at time zero in Fig. 6(a) depicts the SZ surface following the H2 treatment. Similar to the results of SZ in Fig. 3(b), exposure of the SZ catalyst to pyridine led to formation of the pyridine complex bands at 1640, 1611, 1575, 1490 and 1442 cm−1 and losses of S=O at 1397 cm−1 , Zr–OH at 3729 cm−1 , and S–OH at 3638 cm−1 as shown in Fig. 6(a) and (b), however, an additional band is formed at 1676 cm−1 . This band existed for a short time and may be assigned to a hydronium ion, H3 O+ [18]. Fig. 6(b) shows that the increase of the 1676 cm−1 band correlates with the depletion of S=O and S–OH and precedes the formation other species on the surface, suggesting that this species is related to the sulfate group. Increased exposure led to the growth of all pyridine complex bands, stronger
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Fig. 6. (a) IR relative to a pretreated sample background. (b) Peak intensities of selected species. (c) MS analysis during pyridine adsorption over H2 -treated SZ. Times indicated are relative to the time of pyridine introduction.
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Fig. 6. (Continued ).
loss of S=O, Zr–OH, and S–OH, and eventual depletion of bridged OH on Zr. The evolution profiles during pyridine adsorption in Fig. 6(c) are virtually identical to those shown in Fig. 3(d), suggesting that H2 pretreatment of the SZ surface does not affect the adsorption chemistry. The delayed decrease of bridged OH on Zr at 3679 cm−1 in Fig. 6(a) suggests that it is not as reactive as Zr–OH or S–OH as a Brønsted acid site. The loss of S=O at 1397 cm−1 due to the adsorption of pyridine is much greater than that which occurred after the H2 treatment; this is clear upon comparison of the first and last spectra in Fig. 6(a). 3.4. Pt effect Fig. 7 depicts pyridine adsorption over the H2 -treated 4 wt.% Pt/SZ catalyst. Exposure of pyridine to the surface of H2 -treated 4 wt.% Pt/SZ exhibited the same types of trends (i.e., increase and decrease in the intensities of adsorbed species) as
those over H2 -treated SZ as those shown in Fig. 6. Differences in the extents decreasing IR intensities can be observed between the surface character in the presence and absence of Pt (Fig. 7 versus Fig. 6, respectively). These differences, following the pyridine adsorption, included less OH loss, smaller S=O consumption, and a smaller amount of Lewis acid sites present after the He purge when the sample contained Pt. This suggests that the presence of Pt did not affect the surface chemistry during H2 treatment, but did alter the extents of the surface chemistry. The H3 O+ species at 1676 cm−1 was once again observed during pyridine adsorption; the identical behavior in its position, duration, and intensity in Figs. 6 and 7 shows that Pt does not influence it. 3.5. Acidity characterization (adsorption summary) A summary of final IR spectra collected at 40 ◦ C following pyridine adsorption and He purging over all samples examined is shown in Fig. 8. An absorbance
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Fig. 7. IR analysis relative to a pretreated sample background during pyridine adsorption over H2 -treated 4 wt.% Pt/SZ. Times indicated are relative to the time of pyridine addition.
spectrum of a mixture of pyridine and sulfuric acid (50/50 (v/v)) is also shown for reference. The mixture generates a pyridinium ion and thus contains IR bands that are akin to those of the pyridine–Brønsted acid site complex. The acid sites on the surface may be characterized by qualifying the predominant acid-type species present; this may be expressed as the B/L ratio. This ratio is calculated from the IR peak intensities of the pyridine–Brønsted acid site complex at 1540 cm−1 and pyridine–Lewis acid site complex at 1445 cm−1 . This calculation is made possible by the fact that the
extinction coefficients for both of these species are reported to be equal [19]; it was also reported that the B/L ratio can be accurately calculated from IR absorbance intensities as opposed to peak areas [20]. The B/L ratio of the samples studied is depicted in Fig. 9 as a function of Pt content for both the H2 -treated and non-H2 -treated samples. The results indicate that over the Pt/SZ catalysts, exposure of the catalyst to H2 prior to pyridine adsorption led to an increase in the Brønsted character of the surface (B/L ratio increases); the Brønsted character was also greater in the case of
Table 1 IR intensity summary of the pyridinium ion on Brønsted sites and covalently bound pyridine on Lewis sites Pt content (wt.%)
IR intensity of Pyr-B (1540 cm−1 )
IR intensity of Pyr-L (1445 cm−1 )
B/L intensity ratio (I1540 /I1445 )
SZ
H2 -treated SZ
SZ
H2 -treated SZ
SZ
H2 -treated SZ
0 1 4
0.217 0.166 0.131
0.186 0.198 0.135
0.170 0.174 0.112
0.221 0.129 0.064
1.276 0.954 1.170
0.842 1.535 2.109
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Fig. 8. Final IR spectra of pyridine absorption over all samples studied. Spectra shown are collected at 40 ◦ C, relative to a pretreated sample background, and are subsequent to the removal of physisorbed pyridine via purging with He.
Fig. 9. Brønsted/Lewis (B/L) sites ratio plot as a function of both Pt content and H2 treatment. The ratio was calculated from a ratio of the peak heights of the IR absorbance vibrations at 1540 and 1445 cm−1 .
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Fig. 10. (a) IR relative to an pretreated sample background. (b) MS analysis during the pyridine TPD from 40 to 500 ◦ C over SZ.
4 wt.% Pt/SZ than the 1 wt.% Pt/SZ catalyst. Absolute intensities of the Pyr-B and Pyr-L complexes for all samples are summarized in Table 1. It is interesting to note that not only the Brønsted character of the Pt/SZ catalysts is enhanced upon H2 treatment, but also the Lewis character is reduced. 3.6. Pyridine TPD The IR and MS analyses as a function of temperature of the TPD from 40 to 500 ◦ C in He flow of the SZ catalyst are shown in Fig. 10. Increasing temperature from 40 to 300 ◦ C resulted in a slight depletion of Zr–OH at 3729 cm−1 , S–OH at 3638 cm−1 , and Pyr-L at both 1575 and 1445 cm−1 (Fig. 10(a)), and was accompanied by evolution of H2 O in the reactor effluent (Fig. 10(b)). Further increasing the temperature from 300 to 500 ◦ C resulted in decreasing all IR intensities and evolution of CO2 . No pyridine desorbed throughout the experiment. Part of adsorbed pyridine is oxidized to CO2 , indicating that pyridine TPD is not a valid method to gauge acid strength. The sulfate groups on the zirconia surface appear to be stable
due to the absence of SOx species in the MS analysis in Fig. 10(b). The reappearance of the S=O band at 1406 cm−1 (becomes less negative) is quite noteworthy. Recall that the exposure of the SZ catalyst to pyridine resulted in the loss of S=O (Fig. 3) and this was accompanied by evolution of SO3 . The increase in S=O suggests that its initial loss was not solely due to the decomposition of surface sulfate but rather, at least in part, due to an interaction of the pyridine molecule with the sulfate group resulting in a change to its chemical bonding (i.e., double bond to single bond). As pyridine is removed, S=O species return. When Pt is present on the surface of the SZ, a different behavior is observed. Fig. 11(a) and (b) depicts IR and MS analyses during the TPD over 4 wt.% Pt/SZ from 40 to 500 ◦ C in He flow. Increasing temperature resulted in the same trends in Fig. 11(a) as in Fig. 10(a), thus will not be discussed. A temperature increase from 40 to 355 ◦ C produced a peak in the CO2 profile in Fig. 11(b); further heating to 500 ◦ C caused a second maximum in CO2 evolution at 456 ◦ C accompanied by increases in
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Fig. 11. (a) IR relative to a pretreated sample background. (b) MS analyses during the pyridine TPD from 40 to 500 ◦ C over H2 -treated 4 wt.% Pt/SZ. The rate of heating used was 10 ◦ C/min.
the SO2 and SO MS profiles. The SO3 profile remained stable at baseline value throughout the full temperature range. Comparing Figs. 10(b) and 11(b) reveals also that much more H2 O was produced over the 4 wt.% Pt/SZ catalyst. These results suggest that Pt affects the thermal stability of the catalyst and that sulfate primarily decomposes as SO2 where the SO profile is a fragment of SO2 on the mass spectrometer. Fig. 12(a)–(c) illustrate normalized IR intensities for S=O, Pyr-L and Pyr-B complexes on SZ, 1 wt.% Pt/SZ, and 4 wt.% Pt/SZ, respectively, during TPD; Fig. 12(d)–(f) shows the corresponding TPD profiles. H2 treatment did not have any appreciable impact on the TPD. Thus, the respective results are not shown here. Fig. 12(a)–(c) show that as temperature increased above 150 ◦ C, Pyr-L decreased, S=O increased, and Pyr-B remained relatively constant in all cases. Fig. 12(d)–(f) show that CO2 evolved with its center at approximately 420 ◦ C. The CO2 evolution profile appears to correspond to a decrease in Pyr-L. Pyr-B only changed appreciably upon the decomposition of sulfate (i.e., Fig. 12(c)).
4. Discussion 4.1. He pretreatment As the sulfated zirconia is heated in flowing helium, H2 O is progressively removed from the surface sample and formation of S=O becomes apparent (Fig. 2). As the H2 O content of the sample decreased, the S=O vibration became more intense and the band shifted to higher wavenumber. The shifting is believed to be related to the sulfate group transforming from an ionic form to that of a covalent structure [16]. It has been suggested that the S=O wavenumber reflects the type of sulfate structure predominant on the zirconia surface [21]. Frequencies below 1400 cm−1 are indicative of an isolated sulfate group (Zr–O)3 –S=O, whereas those above 1400 cm−1 are likely attributed to a polynuclear structure of the sulfate group [(Zr–O)2 (SO)]2 –O. Fig. 5 indicates that all samples showed S=O vibrations at nearly identical frequencies, that being ∼1385 cm−1 , suggesting that the presence of Pt did not affect the sulfate group as well as indicating that the sulfate
72 R.W. Stevens Jr. et al. / Applied Catalysis A: General 252 (2003) 57–74 Fig. 12. Pyridine TPD summary. Normalized IR intensities are shown as a function of temperature over: (a) SZ; (b) 1 wt.% Pt/SZ; (c) 4 wt.% Pt/SZ. Vibrations shown are S=O (䊊, 1370–1405 cm−1 ), Pyr-L (䊐, 1445 cm−1 ), Pyr-B (䉫, 1540 cm−1 ). Mass spectral analysis of the reactor effluent showing compositional changes as a function of temperature over: (d) SZ; (e) 1 wt.% Pt/SZ; (f) 4 wt.% Pt/SZ. Monitored species are shown as SO (䊊, m/e = 48), SO2 (䊐, m/e = 64), SO3 (䉫, m/e = 80), CO2 ( , m/e = 44), and pyridine ( , m/e = 79).
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group on our samples is predominantly of the isolated type. 4.2. Pyridine adsorption The interaction of pyridine with sulfated zirconia resulted in a decrease of S=O (i.e., 1370–1410 cm−1 ) in all cases (Figs. 3 and 6). This may be attributed to: (i) desorption of the sulfate group; or (ii) a reaction of the sulfate (S=O) with pyridine, resulting in a conversion to another species, such as S–O or S–OH. Our results show that both of these events take place. As the sample was exposed to pyridine, the IR band centered at 1405 cm−1 (S=O) decreased and SOx , predominantly SO3 , was eluted as confirmed by MS analysis. Morterra et al. [22] reported that pyridine can ligand displace sulfates and create more sites on the zirconia surface, leading to an over-estimated amount of Lewis acidity. Our results confirm the displacement and further suggest that the sulfate species being displaced may be in the form of polynuclear structure. Water is also eluted from the surface upon pyridine exposure (Fig. 3b) and is coupled with a decrease in Zr–OH and S–OH intensities (Fig. 3a and c), suggesting that the hydroxyl groups are Brønsted acid sites. Over the hydrogen-treated sample (Fig. 6), a band at 1676 cm−1 is briefly visible; this is assigned to H3 O+ [18]. The H3 O+ is presumed to form from the excess amount of H present and is believed to further interact with the pyridine to form the pyridinium ion (1540 cm−1 ): H3 O+ + pyr → (H–pyr)+ + H2 O. Further study is necessary to verify this step. 4.3. Temperature-programmed desorption Simultaneous monitoring of changes in IR spectra and gaseous species evolved from the SZ and Pt/SZ surfaces during TPD allows further elucidation of interaction of pyridine with SZ. During TPD, Pyr-B remained relatively stable and showed a slightly decrease throughout the temperature range, whereas Pyr-L intensity decreased with increasing temperature, indicating that it is the covalently bound pyridine that is oxidized to CO2 . Since Pyr-L is covalently bonded on Zr sites, the formation of CO2 from Pyr-L reflects the oxidative activity of Zr sites. Formation of CO2 from temperature-programmed desorption of
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pyridine has been reported on SZ catalysts with Fe, Mn and Ni [23–26]. It is important to note that intensity of the S=O band increases in a positive direction, signifying its return (Figs. 10–12). Although Pyr-B is known to result in part from interaction between pyridinium ion with a proton associated with sulfate groups, the change of Pyr-B does not correspond to that of the S=O curve. In the case of SZ, the Pyr-L complex at 1445 cm−1 decreased 74%, while S=O at 1405 cm−1 increased 46% over the course of heating. From these results, it is reasonable to conclude that the bond order of the sulfate group is reduced when pyridine complexes with an adjacent Zr atom. Furthermore, it appears that the loss of Pyr-L contributes more to the recovery of the S=O than that of Pyr-B. The major role of Pt in TPD is to destabilize the sulfate group, leading to SO2 evolution on 4 wt.% Pt/SZ. The stability of the sulfate groups on the 1 wt.% Pt/SZ samples appeared to be identical to the SZ samples, suggesting that a small Pt loading may provide the added benefit of Brønsted acidity enhancement while avoiding the detrimental sulfate stability decrease.
5. Conclusion Simultaneous measurement of adsorbed species and gaseous species evolved from SZ and Pt/SZ surfaces allows elucidation of: (i) the initial state and final state of the catalyst surface; and (ii) the interaction between pyridine and SZ during pyridine adsorption and temperature-programmed desorption. He treatment, which has been referred to as an activation, of fresh SZ and Pt/SZ at 500 ◦ C led to the formation of a prominent S=O species. The wavenumber of the S=O shifted downward and part of polynuclear sulfate desorbed as SO3 upon pyridine adsorption. The desorption of sulfate leads to changes in the nature of surface sites which make interpretation of isomerization activity in terms of the ratio of Pyr-B to Pyr-L unreliable. Temperature-programmed desorption (TPD) study coupled with IR and MS shows oxidation of adsorbed pyridine to CO2 is related to Zr4+ (i.e., Lewis acid sites). The presence of Pt as a promoter on the sulfated zirconium was found to enhance the Brønsted acidity of the sample when treated with H2 and to promote desorption of sulfate as SO2 during the TPD.
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