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J. Phys. Chem. C 2010, 114, 4950–4960
CO Adsorption on Hydrated Ru/Al2O3: Influence of Pretreatment Diana Gottschalk,† Erin A. Hinson,‡ Adam S. Baird,† Hollins L. Kitts,† and Kathryn A. Layman*,† Department of Chemistry and Center for Materials Science, MSC 4501, James Madison UniVersity, Harrisonburg, Virginia 22807, and Department of Chemistry, Western Carolina UniVersity, Cullowhee, North Carolina 28723 ReceiVed: July 21, 2009; ReVised Manuscript ReceiVed: February 8, 2010
The adsorption of CO on hydrated H2-, O2-, H2S-, and He-pretreated 5 wt % Ru/Al2O3 was investigated using attenuated total reflection IR spectroscopy (ATR-FTIR) to determine how the oxidation state of Ru influences CO-H2O interactions. The frequencies of the three IR bands (high frequency (HF), midfrequency (MF), and low frequency (LF)) that are observed when CO adsorbs on Ru/Al2O3 are influenced by (1) CO coverage (CO-CO interactions), (2) Ru oxidation state, (3) pretreatment gas, and (4) pretreatment time. Water red-shifts the CO features by as much as 116 cm-1and influences the MF/LF ratio in a complex mechanism involving both (1) blockage of CO adsorption on the LF site and (2) the formation of OH---CO interactions. While the LF band position is correlated to the adsorbate electronegativity and the MF band slightly correlates to CO coverage, the HF position is independent of either CO coverage or adsorbate electronegativity. These varying degrees of correlations can be rationalized using the extent of interaction between water and the adsorbed CO species by assuming the LF band is due to CO adsorption on an a-top site that is expected to have minimal interaction with coadsorbed water, while the MF and HF bands most likely arise from Ru(CO)m(OH)n (n, m g 1) and Ru(CO)o(H2O)p (o, p g 1), respectively. Introduction The adsorption of CO on Ru single crystals and supported Ru has been extensively studied in the absence of water.1-13 However, water is known to affect the selectivity and activity of heterogeneous catalysts.14-25 In some cases, water inhibits catalytic activity,18,23-25 while enhancing it in others.19,21,22 This influence is attributed to mechanistic and/or kinetic changes of the catalytic process14-26 including blockage of CO adsorption,14-16 shifting the CO adsorption site,14,15 formation of formates and/ or carbonates,18,20-22,27-29 and formation of H2O-O2 complexes.30 However, recent mechanistic studies have found that the blocking of active sites by carbonates, formates, and water cannot entirely explain catalyst deactivation,15,31-33 suggesting that additional, not yet clearly established mechanisms must also contribute to the effect that water has on the catalytic activity. Water controls the surface properties of metal oxides by influencing (1) the extent of surface hydroxylation, (2) the ratio of Brønsted/Lewis surface sites, and (3) the surface metal oxide structure via surface reconstruction.32 For example, surface reconstruction,31 surface reoxidation,15 and formation of cobalt aluminate34 have recently been observed in XAFS, XANES, XPS, and XRD studies of CO oxidation and FT synthesis over bulk and supported Co3O4. Thus, CO studies performed under dry conditions may not probe the active sites under working conditions. While there are numerous studies of the adsorption of CO or H2O, there are far fewer investigations of the coadsorption of these species. Vibrational studies of the coadsorption of CO * Author to whom correspondence should be addressed. Prof. Kathryn A. Layman, Department of Chemistry, MSC-4501, James Madison University, Harrisonburg, VA 22802, Tel: 540-568-1656, Fax: 540-568-7938, e-mail:
[email protected]. † James Madison University. ‡ Western Carolina University.
and H2O on single crystals7,14,15,31,32,35-45 indicate that the influence of water on CO adsorption depends on the extent of surface water coverage and substrate temperature.14 In a recent study,46 we reported that CO adsorption on hydrated 5 wt % Ru/Al2O3 produced νCO absorbances at ∼2048, 1992, and 1924 cm-1 that are red-shifted by 50-116 cm-1 from those seen in the absence of water (2020-2040, 2080, and 2140 cm-1).1-7,9-12 In contrast, Solymosi et al.11 did not observe a significant shift in the linear-bonded νCO features at 2025, 2075, and 2140 cm-1 when 5 wt % Ru/Al2O3 was exposed to 0.5 Torr of CO at 300 K in the presence of 0.1 Torr of H2O. The frequency of the C-O stretch (νCO) is sensitive to the identity of the metal, metal particle modifications, metal-support interactions, and the surrounding medium.47-49 In the absence of water, Pfnur et al.50 reported that the νCO band shifts continuously from 1984 to 2061 cm-1 as a function of increasing CO coverage and the νCO absorbance was found to blue-shift by 20-30 cm-1 in the presence of coadsorbed oxygen.51-54 Neither the role of CO coverage nor of coadsorbates on CO adsorption has been reported for systems hydrated prior to CO adsorption. Since catalytic investigations show that the mechanism in the presence of water depends on sample pretreatment55 and on the nature of the active sites,55 here we have investigated the role of sample pretreatment on the adsorption of CO on hydrated 5 wt % Ru/Al2O3. These investigations provide additional insight into how the oxidation state of Ru influences CO-H2O interactions. The H2- pretreatment time was extended to 26 h, and temporal IR spectra for CO adsorption on 2 h and 23 h He-degassed hydrated Ru/Al2O3 were collected. Although we previously found that Ru/Al2O3 and hydrated Ru/Al2O3 have similar bulk and surface properties by BET, XRD, and XPS,46 the detailed analysis presented here indicates that the IR spectra for CO adsorption on H2- and He-pretreated Ru/Al2O3 are significantly different. This suggests that water slightly alters
10.1021/jp906916m 2010 American Chemical Society Published on Web 03/01/2010
CO Adsorption on Hydrated Ru/Al2O3 the surface properties (surface oxidation and/or affinity for H atoms) of Ru/Al2O3. To further explore these differences and to increase the relevance of these studies to catalytic systems, CO adsorption was also studied on O2-pretreated hydrated Ru/ Al2O3. (RuO2 is the catalytically active phase for CO oxidation at room temperature.56) Since the electronegativity of sulfur (EN ) 2.5) is between the values for H (EN ) 2.1) and O (EN ) 3.5), CO adsorption was also investigated on H2S-pretreated hydrated Ru/Al2O3 to elucidate the dependence of the νCO features on coadsorbate electronegativity and on the Ru oxidation state. While the νCO features were influenced by CO coverage and sample pretreatment, we find that the amount of adsorbed water is more important than these environmental factors in determining the properties of these absorbances. Attenuated total reflection IR spectroscopy (ATR-FTIR) was used to investigate the gas-phase adsorption of CO on 5 wt % Ru/Al2O3. Since conventional transmission and diffuse reflectance FTIR provide stronger signals than ATR-FTIR, these methods are more commonly employed for gas-phase studies. For diffuse reflectance and transmission IR studies, samples are typically hydrated by exposure to low pressures of water vapor, creating a low surface coverage of water. Although higher pressures and longer exposure times could be used to increase surface coverage, this procedure significantly raises the background pressure since water is not easily pumped from the system. While some water molecules diffuse into the bulk, most adhere to the surface. Although several monolayers of water can be deposited, these surfaces may not accurately represent the hydration by liquid water, since possible structural changes produced in the catalyst by extensive hydration are not produced using this method. ATR has proven to be an effective method for observing CO interactions in liquid water.46 The data presented here were produced by extensive hydration of the catalyst in liquid water prior to gas-phase CO adsorption. While no additional water was added to the sample during CO adsorption, the physical properties kept much of the sample hydrated at the surface of the ATR cell where the results were obtained. The results suggest that ATR-IR can also be used to bridge the gap between gas-phase and liquid-phase studies to investigate interactions on hydrated surfaces. Experimental Methods Chemicals and Gases. Helium (UHP), hydrogen (UHP), 1 mol % oxygen in helium (UHP), 3.03 mol % hydrogen sulfide in hydrogen (UHP), and carbon monoxide (UHP)) were obtained from Robert’s Oxygen Gas Company and dried by flowing through a moisture trap prior to use. Impurities in carbon monoxide were trapped using a pentane-liquid nitrogen bath, as previously described in the literature.57-65 Commercial 5 wt % ruthenium on alumina powder (Ru/Al2O3, reduced and dried) was used as received from Sigma-Aldrich Chemicals, soaked with deionized water, and pretreated in flowing gases as described below. Catalyst Characterization. X-ray Diffraction Measurements. X-ray diffraction (XRD) patterns were acquired on a PANalytical X’Pert Pro θ/θ XRD using a 16 mm or a 27 mm silicon sample plate. The diffractometer is outfitted with a Cu KR source (λ ) 1.5418 Å), an Xcelerator RTMS detector, and is interfaced to a personal computer for data acquisition and analysis using X’pert Highscore Plus software. XPS and BET Surface Area Measurements. X-ray photoelectron spectroscopy (XPS) and BET measurements were also
J. Phys. Chem. C, Vol. 114, No. 11, 2010 4951 conducted. The experimental procedures and results of these studies were described previously.46 Infrared Spectroscopy Measurements. Thin-Film Preparation. The catalyst was mounted on a ZnSe (45°, 80 × 10 × 3 mm3, Thermo Electron) internal reflection element (IRE) using the previously described method.46 Briefly, the catalyst (∼20 mg) was placed in 4 mL of H2O to create an aqueous catalyst slurry. After stirring for 6 h, ∼0.6 mL of the slurry was spread onto the IRE and allowed to dry overnight, creating a thin film. As discussed in more detail in the results section, this “dry” film was still hydrated. The amount of water remaining on the surface varied significantly between samples due to differences in ambient humidity. The sample-coated IRE was mounted into a gas flow cell for ATR-FTIR analysis. The hydrated thin film mass was typically 5-15 mg. In Situ ATR-FTIR Spectroscopy. ATR-FTIR spectra were collected using a Nicolet Nexus 6700 spectrophotometer equipped with a Spectra-Tech ARK horizontal ATR accessory. The experiments were performed using unpolarized light. The spectra were obtained from 4000 to 400 cm-1 using 1024 scans at 4 cm-1 resolution. The sample spectra were ratioed against a background spectrum obtained in flowing He (145 mL/min) following sample pretreatment and prior to CO exposure. Unless otherwise stated, IR spectra were collected at room temperature in the presence of flowing CO (100 mL/min). The IR spectra are presented as obtained without smoothing. Comparison plots were made using the IR spectra collected after 2 h exposure to CO. In situ ATR-FTIR experiments were carried out at room temperature using the previously described Thermo-Nicolet flow cell.46 Since no additional water was used and the peristaltic pump was turned off, the gases diffused through the system. Following mounting in the ATR-FTIR system, the catalyst samples were degassed in He (145 mL/min) for 1 h. The catalysts were then pretreated in H2 (100 mL/min), a 3.03 mol % H2S/H2 gas mixture (100 mL/min), a 1.0 mol % O2/He gas mixture (145 mL/min), or additional He (145 mL/min) at room temperature for two hours. To remove weakly bonded species from the surface of the H2-, H2S-, and O2-pretreated catalysts, the catalyst was then degassed in flowing He (145 mL/min) at room temperature for one hour. The catalysts were never (intentionally) exposed to additional water (vapor or liquid) after this pretreatment. The samples were hydrated during film preparation, and this water was not totally removed by the sample pretreatment and subsequent exposure to CO gas, establishing the conditions for the exposure of CO on hydrated catalysts. Results Catalyst Characterization. The XRD patterns for hydrated 5 wt % Ru/Al2O3 pretreated in H2, O2, H2S, and He are shown in Figure 1. The XRD pattern for the alumina support is also provided for clarity. The XRD patterns are nearly identical, indicating that their bulk structures were not changed during pretreatment. Any differences in the IR spectra for CO adsorption on these samples must arise from differences in the Ru oxidation state, coadsorbed species, and/or other surface properties changed during the pretreatment. Infrared Spectroscopy Measurements. To test for impurities in the system, a clean ZnSe IRE was exposed to flowing CO gas using different CO flow rates and CO exposure times. Only the P and R branches for gas-phase CO at 2105 and 2178 cm-1 were observed in the IR spectra, confirming that the pentane-liquid nitrogen bath had successfully trapped any impurities in the CO
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Figure 3. The integral form of the Ginstling-Brounshtein diffusion model (D4; 1 - (2/3)R - (1 - R)2/3) plotted as a function of time. Calculation of R was achieved by integrating the νOH stretch (∼3406 cm-1) as a function of time to obtain It. Each It was then divided by the intensity observed after pretreatment for over 24 h (I∞).
Figure 1. X-ray diffraction patterns for the alumina support, hydrated 5 wt % Ru/Al2O3 prior to any sample preparation, and 5 wt % Ru/ Al2O3 after pretreatment in H2, H2S, O2, and He for 2 h at 673K. The * indicates the determined Ru peaks for all of the XRD graphs.
Figure 2. IR spectra of the changes in the hydrated Ru/Al2O3 film induced during pretreatment in H2 (150 mL/min) at 298 K.
gas (see Figures S1-S4 in Supporting Information). Therefore, all of the CO bands observed in Figures 4-12 are from CO interactions with Al2O3 and/or Ru. The IR spectra in Figure 2 illustrate the changes in the hydrated Ru/Al2O3 film during pretreatment in H2 (150 mL/ min) at 298 K. The background spectrum was obtained prior to starting gas flow. After acquiring the background spectrum, the sample was exposed to flowing H2 (150 mL/min) and IR spectra
were acquired approximately every 15 min for up to 7 h. An IR spectrum under assumed steady state conditions was collected approximately 24 h after initiating the experiment. Since these spectra were ratioed against a background spectrum obtained following thin film preparation, the negative νOH and δH2O bands at 3406 and 1637 cm-1, respectively, confirm that water was being removed from the sample by the flowing gas. (The negative P and R bands for CO2 at 2329 and 2369 cm-1 also observed in Figure 2 were caused by flushing the CO2 containing air from the cell after the gas flow was started). The intensities of the negative νOH and δH2O bands increased with increasing time indicating that water remains on the surface even after exposure to flowing gas for ∼24 h. Additional negative bands are also observed at 1520 and 1400 cm-1, suggesting that surface carbonates or other species with similar vibrational frequencies were also removed by the flowing H2. Similar results were obtained when the other pretreatment gases (He, Ar, O2, 1 mol % O2 in He, and 3.03 mol % H2S in H2) were used. The rate of water removal from the surface was approximated by using the extent of reaction (R).66 R was found by integrating the νOH stretch at 3406 cm-1, and calculating it from It/I∞, where It is the intensity at time t and I∞ is the intensity after 24 h. The R values were fit to the integral form of several solid state kinetic models.66 While each of the diffusion (D1, D2, D3, and D4) models yielded an acceptable fit, the D4 (Ginstling-Brounshtein) model generally gave the best R2 value (see Figure 3). Dehydration occurs at the air interface forming a barrier layer. The structure of the barrier layer influences the rate, giving rise to the different diffusion models (D1 ) flat plane; D2 ) cylindrical particles; D3 and D4 ) spherical particles).66 The D4 model is consistent with the hydrated Ru/Al2O3 being packed to form a porous, nonuniform surface, in agreement with the SEM images published previously.46 The surfaces of the particles are hydrated, and trapped water most likely fills the voids between particles, suggesting an environment that bridges the gap between gas-phase and liquid-phase adsorption studies. The water removal rate depends on both the gas flow rate and the type of pretreatment gas (Table 1), as expected for diffusion controlled processes. Since the rate of diffusion correlates to the thickness of the film, which should increase with increasing sample mass, the normal procedure is to divide
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TABLE 1: Water Removal Rates from Thin Films of Hydrated Ru/Al2O3 in the Presence of Various Pretreatment Gases at 298 Ka
a
gas
flow rate (mL/min)
removal rate (×10-5 m2/s/mg)
H2 H2 H2 O2 He Ar 1 mol % O2/He
50 100 150 150 150 150 150
2.1-2.6 4.7-7.0 2.4-14.5 0.4-2.2 1.1-6.1 0.6-1.6 2.6-3.0
Rates were obtained using the D4 kinetic model.
the rate constants by the initial sample mass. As shown in Table 1, large variations (a factor of 6) in the calculated rate constants were observed using this procedure while the “unnormalized” samples were in better agreement. This can be rationalized from the cell geometry. The evanescent wave generated by the reflection of the IR beam through ZnSe was approximated to be 0.7 µm,46 a value that is less than the thickness of the Ru/ Al2O3 film (2-30 µm).46 Therefore, the IR beam is monitoring only the bottom of the sample and the measurement volume is largely independent of the total amount of sample since the measurements are for removal of water from the measurement volume; not necessarily for the rate of water removal from the sample. Nevertheless, some differences are expected. Since the sample is composed of packed powder, some differences in pore sizes and percent occupancy in the measurement volumes are expected, which in turn may change the amount of water contained in the sample. Also, the relative humidity as the sample is prepared will affect the rate of water removal. Higher humidity means more water is retained by the sample. While this will lead to higher water removal rates at the surface, it will also increase the time it takes to remove the water near the IRE keeping the sample hydrated longer. Since only the bottom layers of the sample are measured, CO must diffuse through the film to be observed. Initially, CO primarily adsorbs to sites not penetrated by the ATR evanescent wave, while some CO rapidly diffuses into the voids near the IRE, enabling the observation of gas-phase CO in the IR spectra. As the CO exposure time increases, CO adsorption sites of the outermost layers become saturated, causing a greater portion of the CO to diffuse into the voids near the IRE. This explains why the intensities of the νCO bands for both CO gas and adsorbed CO increase with CO exposure time. Equilibrium between the CO in the voids and adsorbed on the surface generates the observed adsorbed bands. The increasing concentration allows a convenient way to determine how the adsorption changes as a function of concentration. Since the measurement volume is relatively constant, samples cannot be compared by normalizing to sample mass so the relative intensities of the three linear/multicarbonyl νCO absorbances at 1924-1948 cm-1 (LF), 1961-2016 cm-1 (MF), and 2026-2075 cm-1 (HF) for CO adsorbed on hydrated supported Ru (Figures 4-12) were used without mass correction. The intensities of these features depend on (1) the number of available adsorption sites, (2) the amount of CO adsorbed on the site, and (3) the molar extinction coefficient for each band. Assuming a linear relationship between coverage and integrated intensity (which only exists when the CO coverage is below 0.33 monolayers (ML) on dry, single crystals)50 and that the molar extinction coefficients for the HF, MF, and LF bands are similar enables the discussion of differences in CO adsorption
Figure 4. Temporal ATR-FTIR spectra of CO adsorption on hydrated 5 wt % Ru/Al2O3 pretreated in He for 2 h. IR spectra collected at 298 K in the presence of CO (100 mL/min).
as a function of time for a given sample. To correct for minor differences caused by sample packing, degree of hydration, and sample thickness, ratios of intensities were also used to compare samples. He-Pretreated Hydrated Ru/Al2O3. Selected temporal IR spectra for CO adsorption on hydrated Ru/Al2O3 pretreated in He for 2 h (2 h He Ru/Al2O3) in the presence of flowing CO (100 mL/min) at room temperature are shown in Figure 4. After 45 min, a low frequency (LF) νCO band at 1929 cm-1 and a (MF) poorly resolved shoulder were observed. The intensity of this band increased as the exposure time increased. After ∼90 min, a high frequency (HF) band appears at 2053 cm-1 and continues to increase in intensity with increasing CO exposure. While the HF band may be due to CO adsorption on the hydrated Al2O3 support,46 it is assigned as a Ru species based on the experimental evidence provided in the Supporting Information section (see Figures S2-S4) showing that the intensity is proportional to the amount of Ru on the surface and previous investigations reporting preferential adsorption of CO on Ru rather than on the support.1-6,9-12 The absorbance feature at ∼1820 cm-1 is from CO adsorption on a bridged or threefold hollow Ru site.3,38,67-72 After 120 min, the MF shoulder can clearly be identified as a potential separate species, with a frequency of ∼1971 cm-1. The frequencies of the HF, MF, and LF features are coverage independent. CO adsorption on 23 h He Ru/Al2O3 (Figure 5) results in MF and LF bands at 1996 and 1942 cm-1 shortly after the CO flow was started. A weak HF band at 2064 cm-1 appeared after 30 min. A weak band associated with bridged bonded CO was also observed at 1824 cm-1. The intensities of these bands increased with increasing exposure to CO. Their frequencies, however, remain constant. The MF/LF and MF/HF ratios decrease while the HF/LF ratio increases with increasing CO exposure time. The MF band has a high frequency shoulder, suggesting that this band is a composite of at least two different species.
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Figure 5. Temporal ATR-FTIR spectra of CO adsorption on hydrated 5 wt % Ru/Al2O3 pretreated in He for 23 h. IR spectra collected at 298 K in the presence of CO (100 mL/min).
Figure 6. ATR-FTIR spectra for CO adsorption on hydrated Ru/Al2O3 pretreatment in He for 2 and 23 h. CO exposure time was 120 min.
For comparison, IR spectra collected after exposing 2 and 23 h He Ru/Al2O3 to CO for 120 min are plotted in Figure 6. The frequencies, HF/LF, and MF/LF values for the two samples are summarized in Table 2. When water was removed from the sample by extending the He pretreatment time to 23 h, the HF, MF, and LF bands were blue-shifted by at least 11 cm-1, and the HF/LF and MF/LF ratios increased. The intensities of the HF, MF, and LF bands are much larger for CO adsorption on 23 h He Ru/Al2O3 (note the 0.1 factor) than for CO adsorption
Gottschalk et al. on 2 h He Ru/Al2O3 indicating that more CO adsorption occurred as the sample lost water. O2-Pretreated Hydrated Ru/Al2O3. IR spectra for the adsorption of CO on 2 h O2-pretreated Ru/Al2O3 (as a function of CO exposure time) are depicted in Figure 7. The HF, MF, and LF bands at 2056, 1986, and 1935 cm-1, respectively, appeared simultaneously on this sample. A weak band associated with bridged bonded CO is also observed at 1826 cm-1. While the frequencies of these bands remained constant, their intensities increased with increasing CO exposure time. The HF/LF, MF/ LF, and MF/HF ratios decreased with increasing CO exposure time indicating that each band was from a different species. CO adsorption on 64 h O2-pretreated Ru/Al2O3 (Figure 8) resulted in HF, MF, and LF bands shifting to 2047, 1977, and 1940 cm-1, respectively, while the bridge-bonded site at 1826 cm-1 did not change. Initially, CO adsorption occurred primarily on the HF, LF, and bridge-bonded sites, but a very weak MF band was also present in the IR spectrum collected after 75 min of exposure. Although the intensities of these bands increased with increasing CO coverage, their frequencies remained constant. The MF/LF, MF/HF, and LF/HF ratios increased with increasing CO exposure time. For comparison, IR spectra collected for CO adsorption on 2 and 64 h O2-pretreated Ru/Al2O3 after exposure to CO for 120 min are plotted in Figure 9. The frequencies, HF/LF, and MF/LF ratios are summarized in Table 2. Increasing the O2pretreatment time decreased the HF/LF and MF/LF ratios, blueshifted the LF band by 5 cm-1 and red-shifted the MF and HF bands by 9 cm-1. These shifts are consistent with previous reports that oxidation blue-shifts the LF band and red-shifts the MF and HF bands.1,3,53,54,73-76 Assuming similar film thicknesses and pore densities, CO adsorption is hindered on 64 h O2pretreated Ru/Al2O3 as evidenced by the longer CO exposure times needed to detect CO adsorption and the lower HF, MF, and LF band intensities relative to the νCO bands for gaseous CO. H2-Pretreated Hydrated Ru/Al2O3. CO adsorption on 2 h H2pretreated Ru/Al2O3 has been described in detail elsewhere.46 Briefly, the HF, MF, and LF bands appeared simultaneously at 2049, 1992, and 1924 cm-1, and a weak feature associated with bridge-bonded CO is observed at 1828 cm-1.46 During the first 45 min, the HF/LF and MF/LF ratios increased. These changes suggest that water blocks the LF site. As the flowing CO removed the water, the CO could adsorb on the LF site and the MF/LF and HF/LF ratios decreased. After 90 min, the MF band increased in intensity faster than the HF band, causing the MF/ HF ratio to also increase. For 63 h H2-pretreated Ru/Al2O3, the HF, MF, and LF bands appeared simultaneously at 2047, 1979, and 1926 cm-1, as indicated in Figure 10. A strong feature associated with bridgebonded CO is also observed at 1816 cm-1, indicating that longer reduction times increase the amount of CO that adsorbs on the bridge-bonded sites. The LF and HF frequencies are not significantly different from those observed for 2 h H2-pretreated Ru/Al2O3, but the MF frequency red-shift of 13 cm-1 is outside the expected experimental error (Figures 10 and 11). Since the MF band for CO adsorption on 63 h H2-pretreated Ru/Al2O3 has a high frequency shoulder, this band could be from more than one species. The HF/LF and MF/LF ratios continually decreased with CO exposure time and both the HF/LF and MF/LF ratios decreased when the H2-pretreatment time increased (Table 2), indicating that the LF species prefers a dry, reduced site.
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TABLE 2: Summary of IR Data for CO Adsorption on Hydrated Ru/Al2O3 Pretreated in He, 1 mol % O2 in He, H2, and 3.03 mol % H2S in H2a sample
ENb
HFb
MFb
LFb
HF/LF
MF/LF
MF/HF
OH/LF
2 h He Ru/Al2O3 23 h He Ru/Al2O3 2 h O2 Ru/Al2O3 64 h O2 Ru/Al2O3 2 h H2 Ru/Al2O3 63 h H2 Ru/Al2O3 2 h H2S Ru/Al2O3
----3.5 3.5 2.1 2.1 2.5
2053 2064 2056 2047 2049 2047 2039
1971 (sh) 1996 1986 1977 1992 1979 1996
1929 1942 1935 1940 1924 1926 1932
0.30 0.51 0.58 0.42 0.58 0.43 1.29
0.40 0.93 0.63 0.16 1.02 0.71 1.11
1.33 1.82 1.09 0.38 1.76 1.65 0.86
0 0.03 0.21 0.76 0.48 0.81 0.53
a
CO exposure time was 120 min. b EN ) electronegativity; LF ) low frequency; MF ) midfrequency; HF ) high frequency.
Figure 7. Temporal ATR-FTIR spectra of CO adsorption on hydrated 5 wt % Ru/Al2O3 pretreated in 1 mol % O2 in He for 2 h. IR spectra collected at 298 K in the presence of CO (100 mL/min).
H2S-Pretreated Hydrated Ru/Al2O3. CO adsorption on 2 h H2S-pretreated Ru/Al2O3 resulted in HF, MF, and LF bands at 2039, 1996, and 1932 cm-1 (Figure 12). While the ∼1820 cm-1 bridge-bonded CO feature was not observed, a weak band was present at 1724 cm-1. While this feature may be from CO adsorption on bridged or threefold hollow Ru sites,3,38,67-72 it could also arise from the formation of a µ-bonded carbonyl complex, where both the C and O of CO are coordinated to the same Ru atom.77,78 Since it is in the carbonyl stretching region, it could also be a C-C or C-O stretch in compounds such as OCS or HCO, indicating reaction of the CO with species on the surface.18,79-83 The H2S pretreatment clearly hindered CO adsorption since it did not occur until after exposure times of 60 min. At this point, the HF, MF, and LF bands appeared simultaneously. For the first 120 min, the HF/LF and MF/LF ratios decreased as CO exposure time increased. After 120 min, these ratios increased. Meanwhile, the MF/HF ratio remained constant throughout the experiment. Extended H2S-pretreatment studies were not conducted, since the 2 h pretreatment significantly hindered CO adsorption and the extended H2S-pretreatment time was expected to further block CO adsorption.
Figure 8. Temporal ATR-FTIR spectra of CO adsorption on hydrated 5 wt % Ru/Al2O3 pretreated in 1 mol % O2 in He for 64 h. IR spectra collected at 298 K in the presence of CO (100 mL/min).
Discussion Similarities to CO Adsorption on Dry Supported Ru Catalysts. As found for dry supported Ru catalysts, CO adsorption on hydrated supported Ru (Figures 4-12) results in three linear/multicarbonyl νCO absorbance features at 1924-1948 cm-1 (LF), 1961-2016 cm-1 (MF), and 2026-2075 cm-1 (HF). The LF band frequency is very sensitive to the amount of coadsorbed water, CO coverage, and Ru oxidation state. The LF feature on the hydrated catalyst blue-shifted as the electronegativity of the coadsorbate increased (Table 2) in agreement with the previous reports for CO adsorption on Ru single crystals and supported Ru under dry conditions.53,54,73,74,84-86 This correlation held regardless of the amount of water remaining on the surface; the samples listed in order of increasing LF position are 2 h H2 Ru/Al2O3 < 63 h H2 Ru/Al2O3 < 2 h H2S Ru/Al2O3 < 2 h O2 Ru/Al2O3 < 64 h O2 Ru/Al2O3 (Table 2). Based solely on this electronegativity argument, the LF frequency would be expected to be more red-shifted for 63 h H2 Ru/Al2O3 than for 2 h H2 Ru/Al2O3. Since the MF/LF ratio for 63 h H2 Ru/Al2O3 is smaller than that for 2 h H2 Ru/Al2O3, more CO is adsorbed on the LF site for this sample. This additional CO is expected to increase the number of CO-CO interactions and blue-shift the LF frequency producing the observed trend.1,50,73,75
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Figure 9. ATR-FTIR spectra for CO adsorption on 2 h and 64 h O2 pretreated hydrated Ru/Al2O3. CO exposure time was 120 min.
Figure 11. ATR-FTIR spectra for CO adsorption on 2 h and 63 h H2 pretreated hydrated Ru/Al2O3. CO exposure time was 120 min.
Figure 10. Temporal ATR-FTIR spectra of CO adsorption on 63 h H2 pretreated hydrated 5 wt % Ru/Al2O3. IR spectra collected at 298 K in the presence of CO (100 mL/min).
Figure 12. Temporal ATR-FTIR spectra of CO adsorption on hydrated 5 wt % Ru/Al2O3 pretreated in 3.03 mol % H2S in H2 for 2 h. IR spectra collected at 298 K in the presence of CO (100 mL/min).
The LF, MF, and HF bands blue-shifted when the pretreatment time increased from 2 to 23 h (He), 63 h (H2) and 64 h (O2). Two changes in the sample could produce these shifts: 1) increased CO-CO interactions caused by the increased CO adsorption on the longer pretreated samples and/or 2) changes in the electrostatic interactions between water and CO caused by less water being adsorbed on the longer pretreated samples. Both are expected to produce blue shifts. Since the amount of
CO increases as water is lost, it is not possible to determine which caused the observed blue shifts. The MF/HF ratio was larger for 2 h H2 Ru/Al2O3 than 2 h O2 Ru/Al2O3 and larger for 63 h H2 Ru/Al2O3 than 64 h O2 Ru/Al2O3. These results suggest that oxidation may increase CO uptake on the HF site, in agreement with the literature for CO adsorption on dry supported catalysts.1,3,73-76,87 Oxidation and sulfidation also hindered total CO adsorption as indicated by the longer CO exposure times needed for CO adsorption on
CO Adsorption on Hydrated Ru/Al2O3 64 h O2 Ru/Al2O3 and 2 h H2S Ru/Al2O3, in agreement with the literature.5,73 Differences from CO adsorption on dry supported Ru/ Al2O3. Water 1) red-shifted the HF, MF, and LF frequencies, 2) hindered CO adsorption, especially on the LF site, 3) reduced the influence of oxidation on band position and intensity, 4) oxidized Ru/Al2O3, and 5) altered the influence of CO coverage on band position. The influence of water on band frequency and intensity differed for each feature. 1. Red-shifted HF, MF, and LF frequencies. As previously reported,46 the HF, MF, and LF frequencies are significantly red-shifted on hydrated samples compared to the frequencies reported in the absence of water. This red-shift was observed regardless of the sample pretreatment employed. All frequencies were found to change slightly as the amount of coadsorbed water changed, in agreement with Baird et al.46 2. Blockage of CO adsorption, especially on LF site. The intensities of the LF, MF, and HF bands significantly increased when the catalyst was dried for 23 h in flowing He compared to samples dried in He for 2 h, indicating that water hinders CO adsorption. In addition, the time required to detect CO adsorption is longer in the presence of water (45 min for 2 h He Ru/Al2O3; 40 min for 2 h H2 Ru/Al2O3) than on dry Ru samples (15 min for 23 h He Ru/Al2O3; 10 min for 63 h H2 Ru/Al2O3). Water has previously been reported to block CO adsorption, especially on the LF sites, on Ru single-crystals, electrodes, and dry supported catalysts.38,88,89 The HF, MF, and LF bands appeared simultaneously in the IR spectra collected for CO adsorption on H2-, O2-, and H2Spretreated Ru/Al2O3 (the MF band is very weak for 64 h O2 Ru/Al2O3). The simultaneous appearance of these three bands at low CO coverage is unique to the hydrated surface since the LF band appears first in anhydrous systems. Since the MF and HF bands typically do not appear until higher CO coverage, the simultaneous appearance of the HF, MF, and LF bands at low coverage suggests that the water preferentially blocks CO adsorption on the LF sites (Ru atop sites). The report by Solymosi et al.11 that water accelerated the development of the HF and MF bands is also in agreement with this result. Confirming this assessment, the HF/LF and MF/LF ratios decreased when the O2 and H2 pretreatment times were extended from 2 h to 63-64 h. In addition, the HF/LF and MF/LF ratios tended to decrease with CO exposure time. During CO adsorption, water is removed from the surface, as indicated by the loss in intensity in the δH2O absorbance at 1632 cm-1. As water is removed from the surface, the LF sites become more occupied decreasing the HF/LF and MF/LF ratios. 3. Reduction of the influence of oxidation state on band position and intensity. The decrease in the MF/LF and HF/LF ratios as the O2 pretreatment time increased is opposite that observed for dry supported Ru catalysts where oxidation increased the HF/LF and MF/LF ratios.1,3,73-76,87 Although oxidation of dry Ru/Al2O3 red-shifts the frequency of the HF absorbance1,3,74-76 and, in some cases, the MF frequency,1,3,74 the HF and MF band positions are not significantly influenced by O2 pretreatment of hydrated Ru/Al2O3. The red-shift in HF and MF band position observed when the oxidation pretreatment increased is most likely due to increased CO coverage since extended H2 pretreatment times also redshifted the HF and MF bands. While the MF band in the IR spectrum for 2 h O2 Ru/Al2O3 is red-shifted from that observed for 2 h H2 Ru/Al2O3, the MF/LF ratio is higher for 2 h H2 Ru/ Al2O3, indicating that these frequency differences are most likely due to different MF site occupancies since water hinders the
J. Phys. Chem. C, Vol. 114, No. 11, 2010 4957 adsorption of CO. Yokomizo et al.75 reported that the MF/LF ratio is coverage dependent for CO adsorption on oxidized Ru/ SiO2, in agreement with this result. 4. Oxidation of Ru/Al2O3. In the IR spectra for 2 h He Ru/ Al2O3 and 23 h He Ru/Al2O3 (Figures 4-6), the LF and MF bands appeared at low CO coverage while the HF band formed at higher CO coverage. This CO adsorption timeline is similar to that reported by Yokomizo et al.75 for CO adsorption on oxidized Ru/SiO2. In addition, the LF band frequency is slightly higher for 2 h He Ru/Al2O3 (1929 cm-1) than for 2 h H2 Ru/ Al2O3 (1924 cm-1). Furthermore, the HF/LF and MF/LF ratios decreased as the H2 pretreatment time was increased from 2 to 63 h (Table 2). Chen et al.1 reported that the MF band was the strongest when CO adsorption occurred in the presence of a coadsorbed species such as O, Cl, or OH, and that the LF band was the strongest when the second species was removed, in agreement with our results. These IR results strongly suggest that water oxidized surface Ru sites. Since water has been reported to oxidize surface Ru sites,77,88 the oxidation of Ru in this study is not surprising. 5. Altered influence of CO coWerage on band position. Since CO-CO interactions are expected to increase as the CO coverage increases and since the CO coverage is initially expected to increase with increasing exposure times, the frequency of the LF absorbance is expected to blue-shift with increasing CO exposure time. However, the LF frequency was independent of CO exposure time for CO adsorption on hydrated supported Ru samples. This frequency independence further supports the conclusion that water limits the amount of CO adsorption. With smaller amounts of CO adsorption occurs, fewer CO-CO interactions would form so the LF does not change. It is possible that as water is removed from the surface, CO islands surrounded by water form.90,91 A significant number of CO-CO interactions would be expected to form within these CO islands. The absence of an observable blue-shift cannot rule out this possibility. Removal of a few water layers could result in a more uniform water film that would create an electrostatic interactionthatwouldfurtherred-shifttheCOfrequencies.38,40,52,92-95 This red-shift could negate the expected blue-shift from CO-CO interactions leading to no observable chemical shift. The MF position and MF/LF ratio are directly correlated and increase in the order of He Ru/Al2O3 < O2 Ru/Al2O3 < H2 Ru/ Al2O3 < H2S Ru/Al2O3 for 2 h pretreated samples. For the extended pretreated samples, the MF position and MF/LF ratio increase in the order of O2 Ru/Al2O3 < H2 Ru/Al2O3 < He Ru/ Al2O3. This correlation strongly suggests that the MF band is influenced by CO coverage. However, no correlation is observed when the 2 h and extended pretreated samples are compared as a single unit. Since the samples in the extended time group are drier than those pretreated for 2 h, this lack of correlation between the MF band and the MF/LF ratio indicates that the MF band position depends on the amount of coadsorbed water and that the position of the MF band is more sensitive to CO coverage than the HF and LF bands. This is surprising since in the absence of water, only the HF and LF band positions depend on the CO coverage.1,50,54,73,75 Since Chen et al.1 reported that the MF band was the strongest when CO adsorption occurred in the presence of OH, the OH region of the ATIR spectra was also analyzed. The most prominent feature in this region is the large negative band produced by the slow drying of the sample. In several IR spectra, sharp features were observed at ∼3740-3750 cm-1 and at ∼3814-3856 cm-1 (Figure 13) that are assigned as the rotational fine structure of νOH stretches of gas-phase water
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Figure 13. ATR-FTIR spectra of the OH region for CO adsorption on 2 h and 63 h H2 pretreated hydrated Ru/Al2O3. CO exposure time was 120 min.
produced by the evaporation of water from the surface (compare to Figure 2). The MF/LF ratio appears to be correlated with the integrated intensity of these bands (OH). Specifically, the MF/LF ratio increases as the OH/LF ratio increases for the 2 h pretreated samples: 2 h He Ru/Al2O3 < 2 h O2 Ru/Al2O3 < 2 h H2 Ru/Al2O3 < 2 h H2S Ru/Al2O3. Although no clear evidence for isolated OH groups on Al2O3 (νOH ) 3800, 3775, 3745, 3730, 3710, and 3690 cm-1)11,96,97 is observed, the IR spectrum of the negative bands is similar to the spectrum observed for hydrated boehmite.11,96,97 Since removing adsorbed water from boehmite produces surface hydroxyls, one possible explanation for this trend is that the MF band arises from the interaction of CO with isolated OH groups. Solymosi et al.11 and Chen et al.1 found that the appearance of MF and HF bands was accompanied by decay of isolated surface adsorbed OH on the alumina support. Zaki and Kno¨zinger98,99 observed that only the isolated OH groups at ∼3730, ∼3710, and ∼3690 cm-1 interacted with CO, and that this interaction shifted their frequency by about 100 cm-1. In this model, the OH groups are expected to arise from the removal of water hydrogen bonded to the surface hydroxyls of Al2O3 and/or the dissociation of water on the support [H2O + Al-O-Al f 2 Al-OH100 or H2O + Al f H2O-Al; H2O-Al + Al f H-Al + Al-OH; H2O-Al + O-Al f Al-OH81] at room temperature. For the extended pretreated samples, the MF/LF ratio decreased as the OH/LF ratio increased when the O2 (H2) pretreatment time was extended from 2 to 64 h (63 h). (23 h He Ru/Al2O3 fails to fit this trend since there was a large CO coverage on this sample.) Since these samples are drier than the 2 h pretreated samples, the extended pretreated samples have isolated OH groups prior to CO exposure. Consequently, the MF sites are occupied and saturated during the early stages of CO exposure. As the CO exposure time increases, water is removed from the LF sites rather than from the support, enabling CO to occupy the LF sites. As a result, MF/LF ratio decreases as the OH/LF ratio increases.
Gottschalk et al. Band Assignments. The HF, MF, and LF bands for CO adsorption on dry and hydrated Ru/Al2O3 have similar CO coverage, oxidation, and site blockage dependences. These similarities suggest that the bands on dry and hydrated Ru/Al2O3 arise from similar species. Since the frequency of the LF band is sensitive to the chemical environment (coadsorbate electronegativity, Ru oxidation state, etc), the LF absorbance is assigned as an atop CO species, in agreement with previous reports.1-13 The assignments of the HF and MF bands are less certain. Several models have been presented to explain the appearance of HF and MF bands when CO adsorbs on supported Ru catalysts. These bands have been assigned as the symmetric and asymmetric stretches of di-3,7-9,11,77,101 or tricarbonyl,6,54,75,75,77,87,102 as CO adsorbed on low coordination edge and corner metal atoms,2,76 as CO adsorbed on a surface oxide or on a Ru atom perturbed by a nearby electronegative element such as O or Cl,1,3,13,73,76,77,87,103,104 and as CO bonded to Ru2+ or Ru3+ ions covalently bonded to oxygen atoms on the support.11,13,76 In this study, the MF/HF ratio, which was typically greater than 1, depended on CO exposure time, pretreatment, and the amount of coadsorbed water. Since the MF/HF ratio (or Isymmetric/ Iasymmetric ratio) is expected to be constant and much less than 1 for ruthenium tricarbonyl,75,104 the HF and MF bands on hydrated Ru are probably not from ruthenium tricarbonyl. For ruthenium dicarbonyl, the bond angle can be calculated using Isymmetric/ Iasymmetric ) cot2 (angle).104 The calculated bond angles using the MF/HF ratio for hydrated Ru were typically between 32° and 44°. However, the bond angle for ruthenium dicarbonyl is expected to be greater than 45°,104 making it unlikely that they are from ruthenium dicarbonyl either. Thus, the HF and MF bands are assigned as arising from two different species. However, these species could still be multicarbonyls. The metal dipole selection rule dictates that only vibrational modes with oscillating dipoles perpendicular to metal surface will be IR active. The asymmetric mode would have a small dipole component perpendicular to the surface in Ru(CO)n structures that have the molecular axes of CO oriented 45° away from the surface normal.1 Consequently, the intensity of the asymmetric stretch would be weak compared to that of the symmetric stretching band.1 Furthermore, resolved symmetric and asymmetric bands were not observed in the IR spectra for the Ru(CO)n(PF3)m series.1,3,105 Finally, the MF band appears to have a shoulder in Figures 5 & 10 and there is indication of splitting of the HF band in some spectra (not shown). Spectra obtained using isotopic mixtures of 12CO and 13CO indicated that the HF band is composed of several peaks,1,75,76 in agreement with these results. On the basis solely of these observations, the assignment of the HF and MF bands for CO adsorption on hydrated Ru/Al2O3 in our previous publication46 has been modified as follows. The species giving rise to the MF band is most likely assigned to Ru(CO)m(OH)n (n, m g 1), since it exhibited a slight, yet variable correlation with CO coverage. The variability in the correlation would be due to differences in value of n. While the HF band did not exhibit a correlation with coadsorbate electronegativity or with CO coverage, the frequency of this band appears to be very sensitive to the amount of coadsorbed water. Therefore, it has been assigned to Ru(CO)o(H2O)p (o, p g 1). These assignments are similar to those proposed by Chen, Zhong, and White,1 where the HF and MF bands were assigned to Ru(CO)nXm (n, m g 1) and Ru(CO)Xp (p g 1), respectively. In these formulas, X represents a coadsorbed species. These assignments would be consistent with our observed results, assuming that X is H2O.
CO Adsorption on Hydrated Ru/Al2O3 Conclusions Explanation of Contrasting Literature Results. Although Chen et al.1 reported that the HF band blue-shifts with increasing CO coverage, most studies indicate that the HF and MF bands are coverage independent in the absence of water.1,54,73,75 In several literature studies, the HF and MF bands developed in parallel, maintaining a constant ratio.3,54,75 In other literature studies, the HF/MF ratio was reported to depend on CO coverage, the presence of coadsorbates, and/or the support.1,11,75 Our results suggest that these differences may be explained by variations in the amount of background water between the experiments. There are complex interactions between water and CO on the surface. Water hinders CO adsorption and forms electrostatic and chemical interactions with CO. On a water-saturated sample, OH groups are expected to hydrogen bond with adsorbed water molecules. Under these conditions, CO adsorption would occur primarily on the HF and LF sites not blocked by water. As water is removed from the surface, isolated OH groups are created. The interaction of CO with these OH groups gives rise to the MF band. As the sample continues to dry, and water is removed from the surface, CO migrates to the LF sites, reducing the MF/ LF and HF/LF ratios. The extent of the water influence is unique for each band. The LF band is only influenced by water via an electrostatic interaction. The MF and HF bands are influenced by both bonding and electrostatic interactions with water. The position of the HF feature varies between 2039 and 2056 cm-1. There is no evidence of a direct correlation between the shift in frequency of this feature with either its intensity or the electronegativity of the coadsorbed species suggesting that this frequency variation is caused by differences in the amount of coadsorbed water. CO Adsorption Depends on Pretreatment; He Different from H2, O2, and H2S. The IR spectra for CO adsorption on 2 h and 23 h He Ru/Al2O3 are uniquely different from the IR spectra for H2, O2, and H2S pretreated Ru/Al2O3 in that only the LF and MF bands are observed at low CO coverage. The CO adsorption time profile for the He pretreated samples more closely resemble those reported for CO adsorption on dry oxidized Ru/SiO2 than the other pretreated Ru/Al2O3. It is surprising that the IR spectra for H2-pretreated Ru/Al2O3 are more similar to those of O2-pretreated Ru/Al2O3 than those of He Ru/Al2O3, especially for the extended pretreatments. Since all three bands were still observed at low CO coverage in the IR spectra for extended O2- and H2-pretreated Ru/Al2O3, these results suggest that H2, O2, and H2S pretreatments altered the surface of hydrated Ru/Al2O3 similarly. However, it is unclear how the surface was altered. Clearly, O2 and H2S pretreatments are expected to leave residual O and S adsorbed on the surface. Perhaps, residual H atoms were also left on the surface following pretreatment in H2, effectively blocking CO adsorption and inducing similar effects as residual O and S. Acknowledgment. This research was supported by a Research Corporation Cottrell College Science Award (CC6552) and by a supplemental award from the Camille and Henry Dreyfus Scholar/Fellow Program for Undergraduate Institutions (SL-05-001). The PANalytical X’Pert Pro θ/θ XRD utilized in this research was funded by the National Science Foundation (DMR-0315345). Acknowledgement is also given to the National Science Foundation (DMR-0353773, CHE-0353807, and CHE-0754521) for undergraduate summer research stipends. The authors would also like to acknowledge the Department of
J. Phys. Chem. C, Vol. 114, No. 11, 2010 4959 Chemistry and Center for Material Science at James Madison University for start-up funds and unlimited access to departmental equipment. Supporting Information Available: IR spectra of CO(gas) flowing over a clean ZnSe IRE at room temperature (Figure S1) are provided to demonstrate the purity of CO used in these studies. The IR spectra in Figures S2 and S3 also indicate that the νCO bands arise from CO interacting with the Al2O3 support and/or Ru on the particles, and not from metal carbonyl impurities. IR spectra for CO adsorption on hydrated 5 wt % Ru/SiO2 (Figure S3) and on hydrated 15 wt % Ru/Al2O3 (Figure S4) strongly indicate that CO is interacting with Ru and not the support. This material is available free of charge via the Internet at http://pubs.acs.org. References and Notes (1) Chen, H. -W.; Zhong, Z.; White, J. M. J. Catal. 1984, 90, 119– 126. (2) Dalla Betta, R. A. J. Phys. Chem. 1975, 79, 2519–2525. (3) Davydov, A. A.; Bell, A. T. J. Catal. 1977, 49, 332–344. (4) Dulaurent, O.; Nawdali, M.; Bourane, A.; Bianchi, D. Appl. Catal., A 2000, 201, 271–279. (5) Guerra, C. R.; Schulman, J. H. Surf. Sci. 1967, 7, 229–249. (6) Hadjiivanov, K. I.; Lavalley, J. C.; Lamotte, J.; Mauge, F.; SaintJust, J.; Che, M. J. Catal. 1998, 176, 415–425. (7) Hadjiivanov, K.; Klissurski, D. G. Bulg. Chem. Comm. 1998, 30, 159–166. (8) Hadjiivanov, K. Appl. Surf. Sci. 1998, 135, 331. (9) Kobayashi, M.; Shirasaki, T. J. Catal. 1973, 28, 289–295. (10) Nawdali, M.; Bianchi, D. Appl. Catal., A 2002, 231, 45–54. (11) Solymosi, F.; Rasko, J. J. Catal. 1989, 15, 107–119. (12) Maroto-Valiente, A.; Cerro-Alarcon, M.; Guerrero-Ruiz, A.; Rodriguez-Ramos, I. Appl. Catal., A 2005, 283, 23–32. (13) Mizushima, T.; Tohji, K.; Udagawa, Y.; Ueno, A. J. Am. Chem. Soc. 1990, 112, 7887–7893. (14) Henderson, M. A. Surf. Sci. Reports 2002, 46, 1–308, and references therein. (15) Jacobs, G.; Das, T. K.; Patterson, P. M.; Li, J.; Sanchez, L.; Davis, B. H. Appl. Catal., A 2003, 247, 335–343. (16) Park, J. W.; Jeong, J. H.; Yoon, W. L.; Rhee, Y. W. J. Power Sources 2004, 132, 18–28. (17) Jacobs, G.; Patterson, P. M.; Das, T. K.; Luo, M.; Davis, B. H. Appl. Catal., A 2007, 270, 65–76. (18) Grillo, F.; Natile, M. M.; Glistenti, A. Appl. Catal., B 2004, 48, 267–274. (19) Calla, J. T.; Davis, R. J. Ind. Eng. Chem. Res. 2005, 44, 5403– 5410, and references therein. (20) Sato, Y.; Soma, Y.; Miyao, T.; Naito, S. Appl. Catal., A 2006, 304, 78–85. (21) Tanaka, K. -I.; Shou, M.; He, H.; Shi, X. Catal. Lett. 2006, 110, 185–190. (22) Shi, X.; Tanaka, K. -I.; He, H.; Shou, M.; Xu, W.; Xiuli, Z. Catal. Lett. 2008, 120, 210–214. (23) Ko, E. -Y.; Park, E. D.; Seo, K. W.; Lee, H. C.; Lee, D.; Sim, S. Catal. Today 2006, 116, 377–383. (24) Park, J. W.; Jeong, J. H.; Yoon, T. H.; Kim, C. S.; Lee, D. K.; Park, Y.-K.; Rhee, Y. W. Int. J. Hydrogen Energy 2005, 30, 209–220. (25) Li, J.; Zhan, X.; Zhang, Y.; Jacobs, G.; Das, T.; Davis, B. H. Appl. Catal., A 2002, 228, 203–212. (26) Jacobs, G.; Ji, Y.; Davis, B. H.; Cronauer, D.; Kropf, A. J.; Marshall, C. L. Appl. Catal., A 2007, 333, 177–191. (27) Jacobs, G.; Chenu, E.; Patterson, P. M.; Williams, L.; Sparks, D. L.; Thomas, G.; Davis, B. H. Appl. Catal., A 2004, 258, 203–214. (28) Jacobs, G.; Patterson, P. M.; Williams, L.; Chenu, E.; Sparks, D.; Thomas, G.; Davis, B. H. Appl. Catal., A 2004, 262, 177–187. (29) Jacobs, G.; Williams, L.; Graham, U.; Sparks, D.; Davis, B. H. J. Phys. Chem. B 2003, 107, 10398–10404. (30) Liu, L. M.; McAllister, B.; Ye, H. Q.; Hu, P. J. Am. Chem. Soc. 2006, 128, 4017–4022. (31) Jacobs, G.; Patterson, P. M.; Das, T. K.; Luo, M.; Davis, B. H. Appl. Catal., A 2004, 270, 65–76. (32) Jansson, J.; Palmqvist, A. E. C.; Fridell, E.; Skoglundh, M.; Osterlund, L.; Thormahlen, P.; Langer, V. J. Catal. 2002, 211, 387–397. (33) Jansson, J.; Skoglundh, M.; Fridell, E.; Thormahlen, P. Top. Catal. 2001, 16/17, 385–389. (34) Pajares, J. A.; Gonzalez de Prado, J. E.; Garcia Fierro, J. L.; Gonzalez Tejuca, L.; Weller, S. W. J. Catal. 1976, 44, 421–428.
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