ARTICLE pubs.acs.org/Langmuir
Effect of O2 on the Adsorption of SO2 on Carbon-Supported Pt Electrocatalysts K. Punyawudho,† S. Ma,‡ J. W. Van Zee,‡ and J. R. Monnier*,‡ † ‡
Department of Industrial Chemistry, Chiang Mai University, Chiang Mai, Thailand Department of Chemical Engineering, University of South Carolina, Columbia, South Carolina 29208, United States ABSTRACT: Adsorption of SO2 in the presence of O2 on Pt/C catalysts often used as electrocatalysts has been investigated by temperature programmed desorption (TPD) and X-ray photoelectron spectroscopy (XPS). The amounts of SO2 adsorption on Pt/C in the presence of O2 were much higher than those in the absence of O2 (SO2N2) and from the carbon support (Vulcan XC-72) alone. Adsorption is dependent on oxygen concentration over the range 020% but reaches saturation at 20% O2. The spillover of SO2 from Pt to the carbon support has been proposed for 10, 20, and 40% Pt loadings, characterized by desorption temperatures of approximately 150 and 260 °C for SO2 adsorbed on Pt and carbon, respectively. Adsorbed PtS, CS, CSOx, and PtSO4 species were identified by XPS as S-containing species on both Pt and carbon. Both TPD and XPS indicate that the carbon support plays a major role in SO2 adsorption, primarily as SOx (x = 3, 4). The bonding of S and SOx on the carbon support was strong enough that back diffusion to the Pt surface did not occur.
’ INTRODUCTION The performance losses due to contamination of SO2 of proton exchange membrane fuel cells (PEMFCs) have been thoroughly investigated,15 but fundamental data for specific adsorption sites on both Pt surfaces and the carbon support are lacking as recently cited by St-Pierre.6 Recently, Mohtadi et al.2 demonstrated that the cell potential dropped from 0.7 to 0.45 V during exposure to air contaminated with 5 ppm SO2; partial recovery of the cell potential was observed following exposure to SO2-free air for 24 h. Even at a lower concentration of 1 ppm SO2, Jing et al.1 showed that cell performance was lower, although 84% recovery was achieved by using cyclic voltammetry (CV) and scanning from open circuit voltage (OCV) to 1.4 V. Punyawudho4 verified that SO2 accumulates on the Pt/C catalyst in PEM fuel cells at OCV; further, SO2 accumulation was dependent on the concentration of water vapor in the fuel cell. The dependence of cell potential on SO2 adsorption and oxidation was also observed by Jie et al.5 who claimed that in order to completely recover performance the cell must be scanned above 1.05 V by CV. In all cases, the degradation of cell performance was due to competitive adsorption of SO2 with O2 adsorption on Pt surface sites. Because it is difficult to study the kinetics and site specificity of SO2 adsorption at electrochemical conditions of cell voltage and in the presence of moisture (ORR conditions), most studies of SO2 adsorption on Pt and carbon-supported Pt surfaces have been limited to conditions more typical of conventional Pt catalyst usage. Coadsorption of SO2 and O2 on Pt(111) surfaces has been investigated by Wilson et al.7 using high resolution electron energy loss spectroscopy (HREELS) at different temperatures. For the surface exposed to only SO2, SO2 (m/e = 64) desorbed from the Pt surface at temperatures of ∼300 K, while SO3 (m/e = 80) was r 2011 American Chemical Society
observed at ∼550 K; for the coadsorption of SO2 and O2, SO2 was observed at ∼300 and ∼550 K from the Pt surface. For the case of SO2 alone, three vibration energy losses were observed at 525, 940, and 1245 cm1, which were assigned as the deformation, symmetric, and asymmetric stretching modes, respectively, of adsorbed SO2. For the case of coadsorbed O2 and SO2, peaks at 1150 and 1300 cm1 were observed at 270 K. Based on the work of Nakamoto,8 the asymmetric stretching frequency of SO3 occurs at 1100 cm1. Thus, Wilson et al.7 suggested that the peaks at 1150 and 1300 cm1 corresponded to the asymmetric stretches of SO4 on Pt (111), formed by SO2 adsorption in the presence of O2 at temperature above 270 K. Vibrational spectroscopic analyses for SO2 adsorbed on other metals have also been conducted. Adsorption of SO2 on O-precovered Ag(110) surfaces gave peaks at 885835 cm1, which were assigned to a monodentate form of SO3 (one oxygen bonded to Ag) as well as peaks at 1180, 875, and 805 cm1 corresponding to a bidentate adsorption geometry (two oxygens bonded to Ag).9,10 An adsorbed SO4 species was also observed, with stretching frequencies at 1275, 1070, and 910 cm1 for monodentate geometry and at 1285 and 915 cm1 for bidentate geometry. For the case of Pd(100), adsorption of SO2 on an O-precovered surface gave stretching modes at 1240 and 995 cm1 that were assigned to SO4.11 Similarly, Burke and Madix11,12 found that SO4 formed on Pd(100) surfaces for SO2 adsorption on both clean and O-precovered surfaces at temperatures > 298 K. Received: January 5, 2011 Revised: April 28, 2011 Published: May 24, 2011 7524
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The effects of equilibrium O2 concentration on O-precovered Pd(100) surface have also been examined. At low coverages of O (θO ∼ 0.1), an η2-SO4 species was identified based on the vibrational spectra data of others.1316 At higher O precoverages (θO ∼ 0.2) an η3-SO4 species was formed that decomposed to SO2(g) at 560 K. Adsorption of SO2 on carbon surfaces has also been studied. Pliego et al.17 have used computation methods and surface models to determine the adsorption structure of SO2 on graphite surfaces. Results indicated that the zigzag carbon edge sites of graphite were the preferred sites for SO2 adsorption. The same zigzag sites were involved in adsorption of SO2 in the presence of O2.18 Two distinct peaks were observed during desorption of SO2 from activated carbon in the presence of O2,19,20 although SO3(ad) was proposed as the actual adsorbate on the activated carbon.19,21 Similarly, using Fourier transform infrared (FTIR) spectroscopy, Zawadzki22 assigned SO3 at 1045 cm1 as the active form of SO2 adsorption on carbon in presence of both O2 and H2O. The possible pathway for SO2 and O2 adsorption on graphite to form adsorbed SO3 and SO4 species is expressed below.18 Note that for this mechanism to proceed there must be some type of surface site that activates the dissociation of O2 to form an adsorbed surface atomic oxygen species. OðadÞ þ SO2 f SO3ðadÞ
Table 1. Pt Weight Loadings and Surface Characteristics of Pt/C Samples from Selective H2 Chemisorption of Catalysts A, B, C, and Da Pt wt
mass
avg Pt
active Pt s
loading
catalyst
size
ites
carbon
catalyst
(%)
(g)
(nm)
(μmol)
surface area (m2)
A
20
0.051
5.9 ( 0.1 10.0 ( 0.2
10.3
B
10
0.101
5.8 ( 0.4 11.5 ( 0.9
23.0
C D
40 20
0.100 0.102
15.9 ( 1.2 14.8 ( 1.1 5.9 ( 0.1 20.1 ( 0.4
15.1 20.6
exposed
a
Pt surface areas calculated using formalisms of Anderson38 and assuming hemispherical Pt particle geometries. Exposed carbon surface area determined by subtracting the cross-sectional area of Pt hemispherical particles from BET surface area of XC-72 carbon support. Error limits for Pt particle sizes and subsequent calculation of the number of active Pt surface sites based on three different chemisorption analyses for each sample.
manuscript, we will discuss the effects of both H2O and O2 on the kinetics of SO2 adsorption/desorption. Because of the complexities of these systems, it is necessary to consider the effects of O2 and H2O separately.
ð1Þ
’ EXPERIMENTAL SECTION
1 SO3ðadÞ þ O2 f SO4ðadÞ 2
ð2Þ
SO4ðadÞ þ H2 O f H2 SO4ðadÞ
ð3Þ
Others23 have found that the amount of adsorbed SO2 increased for carbon-supported Cu when O2 was present. Lopez et al.23 suggested the increase in SO2 capacity was due to Cu-assisted O2 dissociation with subsequent migration of atomic O to the carbon support so that SO2 adsorption on the O-activated carbon could occur; XPS analysis suggested that SO3 was the only species present. The phenomenon of O transfer from the copper surface to the carbon support, commonly referred to as spillover, has been studied widely on heterogeneous catalyst systems.2427 While H spillover from metal surfaces to catalyst supports is generally recognized, there are essentially no reports of O spillover, particularly for the Pt/C system. However, Baumgarten and Schuch27,28 have reported the migration of adsorbed O from Pt to carbon or graphite to give catalytically assisted combustion of coke. The purpose of this work is to examine adsorption of SO2 in the presence of O2 on carbon-supported Pt catalysts that have also been frequently evaluated as cathodic electrocatalysts in PEM fuel cells, specifically, 10, 20, and 40 wt % Pt supported on Vulcan XC-72 carbon. Temperature programmed desorption of SO2 that was adsorbed at different concentrations of SO2 in a gas stream that also contained variable levels of O2 has been used to determine the site specificity and energetics of SO2 adsorption/ desorption. The amounts and kinetics of SO2 desorption are compared to our recent work for the adsorption of SO2 on the same catalysts in the absence of O2. These studies were not conducted under conditions similar to those present in PEM fuel cells (no voltage or H2O vapor present), but because SO2 poisoning of cathodic PEM fuel cell electrocatalysts occurs in the presence of air (20% O2), it is important to understand the kinetics of SO2 adsorption in the presence of O2. In a future
Sample sizes of 0.10 g of Vulcan XC-72 carbon and Pt/Vulcan XC-72 catalysts with loadings of 10, 40, and 20 wt % Pt (E-TEK division of BASF Fuel Cell GmbH) were used for all experiments and are denoted as catalysts B, C, and D, respectively. In addition, 0.05 g of 20% Pt/XC-72 carbon, denoted as catalyst A, was also evaluated for all experiments to determine whether the quantitative results for catalyst D (0.10 g of 20% Pt/carbon) scaled proportionally. This was done as an internal check to determine whether a systematic error in our analytical procedure gave biased results. These materials were characterized for Pt surface site concentrations using selective H2 chemisorption (Micromeritics 2920 Automated Analyzer). From the chemisorption data, the average Pt particle sizes were determined using standard methods detailed in our earlier work29 which also showed that Pt particle sizes calculated from chemisorption data are clearly preferred over the use of X-ray line broadening or HRTEM particle measurements for determining average Pt particle sizes; the BET surface area of the XC-72 support (254 m2/g) was provided by the supplier. These properties and catalyst nomenclature are summarized in Table 1. Mass flow controllers (Brook 5850E) were used to mix the different SO2 concentrations in the gas streams originating from premixed cylinders of SO2/balance UHP grade N2, Ar, or O2. We report data for equilibrium adsorption of 4, 33, 200, 800, 2000 (0.2%), and 10 000 ppm (1%) SO2 in 20% O2 mixtures. The desired gas flows were passed over the Pt/carbon samples until equilibrium was reached (typically 30 min). Exposures to different SO2 concentrations were continued until there was no change in the m/e = 64 ion current for gas phase SO2. The temperature programmed desorption (TPD) apparatus and methodology used to study adsorptiondesorption of SO2 have been described previously.30 Basically, a high precision quadrupole mass spectrometer (QMS; Inficon Transpector 2) was used to continuously monitor all volatile SOx species (such as SO2 and SO3) as well as other common volatile components such as H2O, CO, and CO2 during the TPD experiment. Before each TPD experiment, the QMS was calibrated for SO2 using a known gas composition containing SO2 to give a threepoint calibration of ion current (amps) for SO2þ/torr of SO2. From this response factor (amps SO2þ/torr SO2), it was possible to very accurately determine the molar amounts of SO2 that desorbed during the TPD experiments. A temperature ramp rate of 20 °C/min over the range 7525
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Figure 1. Effect of O2 concentration on SO2 adsorption for 2000 ppm SO2 at Tads = 25 °C for catalyst B. Pt surface site concentration is 114 μmol Pt sites/gcat.
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Figure 3. SO2 adsorption isotherms on catalyst B in the presence of 20% O2/balance Ar. The horizontal dashed line corresponds to concentration of Pt sites =114 umol/gcat (assuming one SO2 per Pt site) measured by chemisorption, and the symbols (0) and (4) correspond to Tads = 80 and 25 °C, respectively. SO2 coverage on Pt is calculated by dividing the left scale by 114 μmol/gcat.
Figure 2. TPD of SO2 (64 amu) for equilibrium adsorption of 2000 ppm SO2 in presence of 20% O2/balance Ar at 25 °C for the carbon support (- - -) and 10% Pt/C (—). Sample weights = 0.10 g for both samples. The left y-axis corresponds to catalyst sample B, and the right yaxis corresponds to Vulcan XC-72 carbon support. 25 550 °C was used for all experiments. The sweep gas used for all TPD experiments was 30 sccm of Ar that had been passed through H2O and O2 traps to remove any possible contaminants. During the adsorption of SO2 in the presence of O2 at 25 and 80 °C, the quadrupole mass analyzer confirmed that no carbon decomposition, or combustion to CO2, occurred. Temperatures in excess of 200 °C are typically required for the Pt-assisted combustion of carbon supports.30 X-ray photoelectron spectroscopy (XPS) measurements were conducted using a Kratos AXIS Ultra DLD XPS system equipped with a monochromatic Al KR source. The monochromatic Al KR source was operated at 15 keV and 150 W. The pass energy was fixed at 40 eV for the detailed scans. The 10% Pt/C catalyst and carbon support were pretreated in the catalyst pretreatment cell attached to the UHV chamber of the XPS by flowing H2 (UHP grade) at 300 °C for 1 h, followed by an additional hour at 300 °C in flowing Ar before cooling to room temperature. The samples were then transferred into the UHV chamber and analyzed by XPS to obtain background spectra. The samples were finally transferred back to the catalyst pretreatment cell for exposure to 1% SO2/20%O2 in Ar (UHP) for 30 min before XPS analysis was again performed.
’ RESULTS AND DISCUSSION SO2 Adsorption in the Presence of O2 on Carbon Support and Pt/C. The effects of O2 concentration on SO2 adsorption on
Figure 4. TPD of SO2 after equilibrium exposure to 0.2% SO2 in the presence of 20% O2/Ar at 25 °C for sample B (10% Pt/C).
catalyst B (10 wt % Pt/carbon) were determined over the range 060% while maintaining SO2 at 2000 ppm. The amounts of SO2 adsorbed at 25 °C in the presence of different O2 concentrations are assumed to be equal to the amount of SO2 detected during thermal desorption (TPD). The molar concentrations of SO2 adsorption at 25 °C shown in Figure 1 were determined from integration of the individual SO2 TPD peaks; the plot indicates that the amount of SO2 adsorbed is strongly dependent on the concentration of O2 below 20%. No additional desorption of SO2 occurred for O2 concentrations between 20 and 60%. Interestingly, the upper limit of SO2 adsorption is the same as O2 concentration in air, making results of this study more relevant to O2 concentrations encountered at fuel cell conditions. The desorption of SO2 from the 10% Pt/carbon sample in Figure 1 reaches a maximum value of 550 μmol SO2/gcat at 20% O2, which is approximately 5 times the concentration of Pt surface sites, suggesting that most of the SO2 is adsorbed on the carbon support or as multilayers on Pt. The formation of SO2 multilayers on the Pt surface can be ruled out, since Anderson and Campbell31 have shown that SO2 multilayers become unstable on Pt(111) surfaces at temperatures as low as 169 K. 7526
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Table 2. Summary of Equilibrium Adsorption of 0.2% SO2 in the Presence of 20% O2 at 25°C on Pt and Carbon Surfaces for Catalysts ADa Pt wt loading
SO2 adsorption on
Pt surface Sites
SO2 adsorption
exposed carbon
sample
(%)
Pt surface (μmol)
(μmol)
θSO2 on Pt
on carbon (μmol)
surface area (m2)
μmol SO2/m2 carbon
A B
20 10
2.66 ( 0.21 2.75 ( 0.32
10.0 ( 0.2 11.5 ( 0.9
0.27 ( 0.02 0.24 ( 0.03
25.72 ( 1.27 53.03 ( 1.67
10.3 23.0
2.50 ( 0.12 2.30 ( 0.07
C
40
4.04 ( 0.57
14.8 ( 1.1
0.27 ( 0.06
48.17 ( 2.03
15.1
3.19 ( 0.13
D
20
5.60 ( 0.45
20.1 ( 0.4
0.28 ( 0.05
52.20 ( 1.89
20.6
2.54 ( 0.09
a
Concentration of Pt surface sites and exposed carbon surface area from Table 1. Error limits for SO2 adsorption based on average of two different TPD analyses.
Figure 5. Comparison of TPD data for 0.2% SO2/20% O2 adsorbed on 10% Pt/C. Normal TPD from 25 to 550 °C (- - -) and TPD from 25 to 550 °C after sample had been heated to 150 °C and then cooled to 25 °C (—). Inset plot shows the same curves with resolution of low temperature SO2 peak.
The TPD profiles following the adsorption of 2000 ppm SO2 in the presence of 20% O2 at 25 °C for catalyst B and the Vulcan XC-72 carbon support are shown in Figure 2. While the TPD profiles are not shown, similar results were obtained for catalysts A, C, and D, as discussed later in the text. Ion currents only for SO2þ (m/e = 64) were detected during TPD; no other gas phase, S-containing species were observed. Desorption of SO2 from catalyst B gives a distinct peak centered at ∼260 °C; two other less intense peaks are observed for the carbon only surface, a lower temperature peak centered at ∼100 °C and a higher temperature peak at ∼300 °C. Based on the work of others,19,32 the two SO2 desorption peaks for carbon can be assigned to the decomposition of SO3 to give SO2 at both temperatures. These two SO2 desorption peaks were not observed previously for SO2 desorption from SO2/N2 gas mixtures,30 indicating that O2 has facilitated SO2 adsorption on the carbon surface. Note also that the SO2 desorption peak in Figure 2 for the Pt/C catalyst has a peak area ∼75 times larger than that from the carbon support alone. Similar results have been reported by Lopez et al. for a Cu/C catalyst.23 Adsorption Isotherms. Figure 3 shows the SO2 adsorption isotherms at 25 and 80 °C calculated from TPD data at equilibrium exposures of 4 ppm, 33 ppm, 200 ppm, 800 ppm, 0.2% (2000 ppm), and 1% (10 000 ppm) SO2 in the presence of 20% O2. Catalyst B was used for all adsorption isotherm data which from chemisorption analysis gave a surface platinum site concentration of 114 μmol/gcat. At an SO2 concentration of 2000 ppm, saturation SO2 coverage was achieved, giving values of
Figure 6. S2p spectra of 1% SO2 in the presence and absence of 20% O2 on both the XC-72 carbon support and 10 wt % Pt/C at 25 °C. For each S species, only the binding energy of the S2p3/2 peak is presented.
approximately 550 and 380 μmol/gcat for adsorption temperatures of 25 and 80 °C, respectively. At lower concentrations, the effect of adsorption temperature was minimal; for example, adsorption of SO2 from a gas stream containing 4 ppm SO2 gave 11.4 and 11.2 μmol SO2/gcat at 25 and 80 °C, respectively. As commented earlier, the amount of adsorbed SO2 at equilibrium exposures greater than 200 ppm is much greater than the concentration of surface platinum sites. The coverage in monolayers (ML) is calculated by dividing the left scale by the measured concentration of Pt sites and using an arbitrary adsorption stoichiometry of one SO2 per Pt site, although it is 7527
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Table 3. Comparison of S2p3/2 Peaks with Literature Values for Platinum and Carbon Surfaces this work at 298 K
species
S2p3/2
S2p3/2
(eV)
(eV)
PtS 162.4
PtSO3a
PtSO4b
167.4
172.9
CS 163.7163.8
CSO3 or CSO4 a
literature values
168.5
surface
temperature (K)
ref
161.5
Pt(111)
208418
Polcik et al.39
161.0
Pt(110)1 2
250650
Zebisch et al.33
161.9
Pt(111)
300400
Sun et al.37
166.7
Pt(111)
300400
Sun et al.37
168.2
activated carbon
85418
Polcik et al.39
172.3
Pt(111)
300
Wilson et al.7
163.9
activated carbon
N/A
Aliev et al.40
162.5
activated carbon
903
Humeres et al.41
163.5
Vulcan XC-72
298
Swinder and Rolison42
163.2164.0
various carbons
298
Chang43
168.2 168.5
activated carbon various carbons
N/A 298
Lopez et al.23 Chang43
In the absence of oxygen. b In the presence of oxygen.
recognized that due to steric crowding and electron repulsion effects of the SdO bonds the saturation coverage of SO2 will likely be much lower than one SO2 per surface Pt site. In fact, the limiting coverage of ∼0.25 SO2/Pt site suggests that four contiguous Pt surface atoms are required for the adsorption of SO2 on the Pt surface. Adsorption of SO2 in a bidentate mode with S and one of the O atoms bound to the Pt surface or both O atoms bonded to the Pt surface as observed by others9,10 would similarly limit the saturation coverage of SO2 to be less than one SO2/Pt site. To further investigate the unusually high level of SO2 adsorption, the TPD curve for adsorption of 0.2% SO2 on catalyst B (from Figure 2) was resolved into two peaks using Gaussian fitting methods to give the two desorption peaks shown in Figure 4. The peak centered at ∼150 °C is consistent with the “weakly bound” SO2 on Pt that was observed following SO2 adsorption from SO2/N2 gas mixtures.30 However, the higher temperature peak at ∼260 °C yields the major component of adsorption. Integration and quantification of the two resolved desorption peaks are summarized in Table 2 for samples AD. The calculated coverages of SO2 on the low temperature Pt sites are all quite similar at 0.240.28, which is also very near the value of θSO2 = 0.23 for SO2 adsorption on Pt surfaces at similar conditions (25 °C and 0.2% SO2, with no O2) in our earlier work.30 We can thus conclude that SO2 which is adsorbed on Pt (even in the presence of O2) desorbs at ∼150 °C. However, the SO2 desorption at ∼260 °C must be associated with the carbon support, suggesting some type of Pt-assisted spillover as previously claimed for Cu/C catalysts23 since carbon alone (Figure 2) exhibited very low capacity for SO2 adsorption. In fact, similar TPD experiments by Lopez et al.23 for SO2 adsorbed in the presence of O2 gave a large SO2 desorption peak centered between 260 and 500 °C that was attributed to decomposition of a CSO3 species on the carbon surface. If we assume the SO2 at 260 °C comes from the carbon support, the amounts of SO2 desorbed from the carbon surface are relatively
similar at 2.33.2 μmol/m2 carbon for samples AD (Table 2), supporting the hypothesis that the strongly bound SO2 comes from the carbon support. SO2 Adsorption on Carbon Support Component of Pt/C Catalysts. As shown in Figure 2, the desorption temperature of ∼260 °C for the CSO2 peak for Pt/C is markedly different from the desorption temperatures of the carbon support, indicating that the presence of Pt either (1) changes the nature of SO2 adsorbed on carbon or (2) modifies the surface of the carbon to become active for the strong adsorption of SO2. Either explanation argues for some type of spillover from Pt to carbon. Explanation (1) would require that the presence of O2 during SO2 adsorption creates a type of mobile SO2 species on Pt that undergoes facile transfer to carbon. This does not seem likely, since SO2 adsorption on Pt in the absence of O230 or the presence of O2 gives rise to a desorption peak at ∼150 °C, indicative of similar SO2 species in both cases. Explanation (2) suggests that dissociative adsorption of O2 occurs on Pt with spillover of atomic oxygen to the adjacent carbon surface. The oxygen-functionalized carbon then becomes active for the strong adsorption of SO2. Because carbon alone is not active for dissociation of O2 at the conditions (25 and 80 °C) of SO2 adsorption, it cannot exhibit oxygen-assisted SO2 adsorption. In order to determine whether CSO2 is a reservoir for recontamination of the Pt surface by reverse spillover from C to Pt to form PtSO2 (at conditions similar to SO2 removal from a contaminated fuel cell), a modified TPD sequence was performed and the results summarized in Figure 5. Catalyst B was first exposed to 0.2% SO2 and 20% O2/balance Ar at 25 °C in the normal manner. An abbreviated TPD sequence was halted at 150 °C to give only SO2 desorption from the Pt surface. After cooling to 25 °C in Ar, a second TPD experiment to 550 °C was conducted (solid line in Figure 5). The PtSO2 desorption peak was not observed during the second TPD scan, indicating the CSO2 species is stable and that no reverse spillover of SO2 occurs. Thus, in a sense, the 7528
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Figure 7. S2p spectra of Pt/C catalyst type B after exposure to 1% SO2/ 20% O2 at 25 °C and raising the temperature successively to 80, 300, and 500 °C. For each S species, the only the binding energy of the S2p3/2 peak is shown.
activated carbon acts as a stable sink for SO2 contaminants when O2 is present during SO2 exposure. XPS Results for SO2 Adsorption. X-ray photoelectron spectroscopy was used to study the structure of the SO2 species adsorbed on the atomic oxygen-activated carbon surface. Figure 6 shows the XPS spectra for different S species taken before and after equilibrium exposure to 1% SO2 in both the presence and absence of O2 at 25 °C in the XPS pretreatment cell. Spectra for both the 10% Pt/C (catalyst B) and XC-72 carbon support are shown. The intensities were scaled by using the C1s area as reference, and binding energies were referenced to the Fermi energy. The spectra for the S2p levels were fitted as doublets for S2p1/2 and S2p3/2 with a spinorbit splitting of 1.2 eV and an intensity ratio of 1:2.3335 The values in column two of Table 3 show the S2p3/2 binding energies obtained in this study for comparison with various S2p3/2 literature values for different S species on platinum and carbon surfaces. Note that only S2p3/2 values for the present study are shown for simplicity. In Figure 6, plots (a) and (b) correspond to the XPS spectra for carbon XC-72 alone, and plots (c), (d), and (e) correspond to spectra for the Pt/C catalyst. The XPS spectra of the carbon support and the Pt/C taken before exposure to SO2 (i.e., plots a and c) show a binding energy
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for S2p3/2 at 163.8 eV for both the carbon and the Pt/C surfaces which are attributed to residual, or adventitious, CS species on the support, consistent with the literature values in Table 3. The source of the adventitious S on the carbon may be due to the source of the material used to make the Vulcan XC-72 carbon. The manufacturer for Vulcan XC-72 claims total sulfur levels may be as high as 0.75% by weight.36 The XPS spectrum for Pt/C (plot c) also shows an additional small peak at a lower BE of 162.4 eV (S2p3/2) which is assigned as PtS. Thus, there is evidence of sulfur contamination on both the carbon and the Pt components of the as-received 10 wt % Pt/C catalyst which was not removed during pretreatment by H2 reduction at 300 °C, which is indicative of S contamination from the carbon support to the Pt particles. After exposure to 1% SO2/20% O2 (plot b), XC-72 carbon gives an additional weak, broad peak with BE centered at 168.5 eV (S2p3/2) that can be assigned as CSOx (where x is most likely 3) in agreement with the work of Lopez et al.28 There is no significant change in intensity for the contaminant CS species on the carbon surface. After exposure of catalyst B to 1% SO2/20% O2 (plot e), an intense, broad peak for S2p is observed with BE values between 172 and 166 eV and a shoulder between 172 and 177 eV. These peaks are not present when 1% SO2 is adsorbed in the absence of O2 (plot d), indicating the importance of O for SO2 adsorption. The S2p peak for SO2 (in the presence of O2) adsorption on Pt/C (plot e) can be resolved to give two peaks, one centered at 168.5 eV, which is in good agreement for the BE of CSOx for carbon alone (plot b). Thus, the XPS data corroborate the TPD results and show that the great majority of SOx (x most likely is 3) is adsorbed on the carbon support of the Pt/C catalyst, but only when both O2 and Pt are present. The weak shoulder at 172.9 eV may also indicate the presence of small amounts of PtSO4 as observed by Wilson et al.7 for SO2O2 coadsorption on Pt(111) at 300 K. The XPS peak intensities for the PtS species between 162 and 166 eV for SO2 adsorption in both the absence and presence of O2 (plots d and e, respectively) are very similar and corroborate the TPD results which indicated that SO2 adsorption on Pt is not dependent on the presence of O2. Finally, the small broad shoulder at 167.1 eV, (plot d) can be assigned to a PtSOx species similar to that observed by Sun et al.37 and Zebisch et al.33 for SO2 adsorption on polycrystalline Pt surfaces. XPS was also used to determine the temperature dependence of SO2 desorption from the 10 wt % Pt/XC-72 carbon catalyst after exposure to 1% SO2/20% O2 at 25 °C. The results are shown in Figure 7. Analyses at higher temperatures give lower peak intensities, particularly for CSOx and PtSO4, which is consistent with SO2 desorption observed during TPD analysis. In addition, no new sulfur XPS peaks appear at higher temperatures, indicating no additional S-containing species on carbon are formed. As temperatures increase from 25 to 80, 300, and 500 °C, the BE of S2p3/2 for CSOx shifts from 168.5 eV at 25 °C to 168.4, 168.3, and 168.2 eV, respectively. The downward shift of the BE indicates a reduction in the oxidation states of sulfur for the adsorbed SOx species, suggesting changes in the compositions of S-containing species, that is, SO4 f SO3 f SO2 f SO. The position of the S2p3/2 peak for PtS shows a slight increase in BE at higher temperatures, consistent with dissociation of SO2 to form PtS species. Thus, at 500 °C, XPS shows primarily CSOx and PtS as the remaining surface species. The PtS moieties are removed as SO2 during the O-assisted desorption cycles following the initial TPD sequence as previously shown in our earlier work.30 7529
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’ CONCLUSIONS SO2 adsorption in the presence of O2 on a family of Pt/C catalysts has been investigated. The amount of SO2 adsorption increases with O2 concentration up to 20%, reaching saturation after 20% O2. The adsorption isotherms of SO2 adsorption (in the presence of 20% O2) at 25 and 80 °C on 10% Pt/C over the range 4 ppm to 1.0% (10 000 ppm) indicates saturation SO2 adsorption is much greater than the available platinum surface sites. From the comparison of SO2 adsorption on Pt/C with SO2 adsorption on the carbon support alone, it is shown that Pt-catalyzed spillover of activated oxygen atoms from Pt to the carbon support is responsible for the greatly enhanced SO2 adsorption to form CSOx (x = 3, 4) species. In the absence of Pt-catalyzed spillover of oxygen atoms, the amount of SO2 adsorbed on the carbon support is negligible. The desorption temperature corresponding to SO2 adsorbed on Pt is ∼150 °C and on the carbon support is ∼260 °C. The amount of SO2 adsorbed on the O-functionalized carbon support is much greater than the amount adsorbed on Pt; the amount of SO2 adsorbed on Pt is not dependent on the presence of O2 during SO2 adsorption and is limited to ∼0.25 ML based on the surface concentration of Pt sites and an adsorption stoichiometry of one SO2 per surface Pt site. The nature of and speciation of adsorbed SO2 were determined by XPS to be PtSO2, CS, CSOx (x = 3, 4) and PtSO4 during adsorption of SO2 in the presence of O2. The bonding of S and SOx on the carbon support was strong and stable. There was no back diffusion of the SOx species from the carbon support to the Pt surface. Thus, in a sense, the activated carbon acts as a stable sink for SO2 contaminants when O2 is present during SO2 exposure to Pt/carbon catalysts. As a corollary, the results from this study suggest that the adsorptive capacity of SO2 on activated carbons can be greatly increased if small amounts of a platinum group metal (Pt, Pd, Rh) are deposited on the carbon to effect dissociation of molecular O2 and subsequent spillover of atomic oxygen to the carbon component. ’ AUTHOR INFORMATION Corresponding Author
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[email protected]. Telephone: 803-777-9543.
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dx.doi.org/10.1021/la2000377 |Langmuir 2011, 27, 7524–7530
