Separation of Geometric and Electronic Effects of the Support on the

The Si modifier resulted in the most acidic and Cs in the most basic Al2O3 support. Application of the new ..... atom pair, potential, σ2 [Å2], S02,...
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J. Phys. Chem. C 2007, 111, 3938-3948

Separation of Geometric and Electronic Effects of the Support on the CO and H2 Chemisorption Properties of Supported Pt Particles: The Effect of Ionicity in Modified Alumina Supports A. Yu. Stakheev,† Y. Zhang,‡,§ A. V. Ivanov,† G. N. Baeva,† D. E. Ramaker,| and D. C. Koningsberger*,‡ Zelinsky Institute of Organic Chemistry, 119991, Leninsky Prosp. 47, Moscow, Russia, Group of Inorganic Chemistry and Catalysis, Department of Chemistry, Utrecht UniVersity, Sorbonnelaan 16,3584 CA Utrecht, The Netherlands, and Department of Chemistry and Material Science Institute, George Washington UniVersity, Washington, DC ReceiVed: August 8, 2006; In Final Form: NoVember 16, 2006

The geometric and electronic properties of supported Pt particles have been altered by modifying the ionicity (acid base properties) of the Al2O3 support via the sol-gel method. The Si modifier resulted in the most acidic and Cs in the most basic Al2O3 support. Application of the new Delta XANES technique shows that, above 373K in vacuum, the Pt surface is covered with hydrogen chemisorbed in an atop site for Pt particles dispersed on an acidic Cl-Al2O3 and mostly in the n-fold sites on Pt particles dispersed on a basic RbAl2O3. Further, FTIR data shows a significant bridged CO coverage in the Rb-Al2O3 case but not in the Cl-Al2O3. At low temperatures, when the coverage of both CO and H should be nearly complete, the Delta XANES results show that the coverage of H on Pt/Cl-Al2O3 is about twice that of Pt/Rb-Al2O3, consistent with the FTIR data which shows a similar reduction of linear CO adsorption on Pt/Rb-Al2O3. This is attributed to the different dispersions of the particles. EXAFS analysis makes clear that this difference in dispersion is mostly due to different particle morphologies, almost flat (for Pt/Cl-Al2O3) versus (hemi)spherical (for Pt/ Rb-Al2O3), although the sizes are also different. The observed changes in CO and H2 chemisorption properties at high temperature and in Pt particle morphology are due to a shift of the Pt valence band to higher binding energy with decreasing ionicity (increasing acidity) of the support, as indicated by the atomic XAFS results. These atomic XAFS results can be directly correlated, assuming the oxygen Madelung potential model, with the XPS shift of the O 1s BE of about 2 eV, showing a decrease of the net electron charge on the support oxygen atoms with decreasing ionicity of the support. The hydrogen Delta XANES results are combined with a three-site (atop, 2- or 3-fold, and ontop H, in order of decreasing bond strength) Langmuir adsorption model for hydrogen chemisorption. This combination accounts for the variation in hydrogen coverage with change in T, P, and support ionicity as described above. The consequences of these results for Pt-catalyzed CO oxidation and hydrogenolysis/hydrogenation reactions are discussed.

1. Introduction The function of a support is primarily to keep the supported metal particles highly dispersed. Previous studies have reported that the support acid/base properties can also have a large influence on the electronic properties of the supported metal particles. We have previously proposed an oxygen Madelung potential model that characterizes the nature of this interaction between the metal particles and the support.1,2,3 This model is based upon data obtained with FTIR (Fourier transform infrared) on chemisorbed CO, XPS (X-ray photoelectron spectroscopy) data, atomic XAFS (X-ray absorption fine structure), and ab initio multiple scattering calculations on supported Pt clusters.1,2,3 The data show that with decreasing ionicity of the * Corresponding author. Address: Group of Inorganic Chemistry and Catalysis, Department of Chemistry, Utrecht University, PO Box 80083, 3508 TB Utrecht, The Netherlands. Tel: +(31) 30 2537400. Fax: +(31) 30 2511027. E-mail: [email protected]. † Zelinsky Institute of Organic Chemistry. ‡ Utrecht University. § Present address: Department of Chemical Engineering, University of California, Berkeley, CA 94720-1462. | George Washington University.

support (with decreasing electron charge on the O atoms; more acidic), the electronic properties of a metal particle are influenced in at least three separate ways: (i) the complete Pt density of states (DOS) shifts to higher binding energy,1,2,3 (ii) the location of the 6s,p interstitial bonding orbital (IBO) moves from the surface to the metal-support interface of the Pt particles,3,4 and (iii) the insulator to metal transition is occurring at lower particle sizes.5 DFT calculations show that on supports with low ionicity (acidic), the Pt-H bond strength is smaller and the Pt-O bond strength larger than those on ionic supports (more basic, with electron rich O atoms).4,6 The DFT calculations imply a lower H coverage for Pt on supports with low ionicity (acidic) under certain H2 pressure conditions.6 This has been confirmed by hydrogen chemisorption data7,8 obtained on Pt/Y zeolite, where the electron richness of the zeolite oxygen atoms was systematically decreased by changing the charge-compensating ion from Na1+, Mg2+, to La3+. The electron richness of the zeolite oxygen atoms was found to be the lowest in Pt/H-USY containing extraframework Al and Bro¨nsted acid sites. The Pt dispersion in these samples was calculated using HRTEM data, which

10.1021/jp0651182 CCC: $37.00 © 2007 American Chemical Society Published on Web 02/15/2007

Support Influence on CO and H allows the amount of chemisorbed H per surface Pt atom to be determined; it increased in the order Pt/H-USY < Pt/LaY < Pt/MgY < Pt/NaY.7 The results of a new Delta XANES procedure have led to new insights into the influence of the ionicity of the support on the type of active sites for H chemisorption.6,9 This procedure is based on taking the difference between the XANES data at the Pt L3 edge of the sample in vacuum (Pt) from the data after chemisorption of hydrogen (H/Pt) (∆µL3 ) µL3(H/Pt)- µL3(Pt)). This difference produces spectral signatures, which in combination with FEFF8 and DFT calculations, allows identification of specific adsorption sites. At very low H2 pressure or high temperature, the strongest bonded H is chemisorbed in an atop position. With decreasing temperature or at higher H2 pressure, only n-fold (n ) 2 or 3) sites are occupied. At low temperature or high H2 pressure, the weakest bonded H is positioned in an “ontop” site, defined as Pt atop sites surrounded by stronger bonded H atoms in the neighboring 3-fold sites. Thus far, no experimental evidence has been reported, which verifies a systematic change in oxygen electron density as a function of the support ionicity to further support the O Madelung potential model. Little is known about the change in particle morphology induced by the support. If indeed the Pt-O bond strength is stronger for Pt particles dispersed on supports with a low ionicity (acidic), one could expect a change in Pt particle morphology from a (hemi)spherical shape to more raftlike. In this study, both the morphology and electronic properties of Pt particles supported on Al2O3 were systematically varied by changing the ionicity of the support through introduction of different modifiers via the sol-gel method. The change in Pt particle morphology could be followed with the help of EXAFS, and by monitoring the geometrical changes induced by H2 desorption. The change in the electron density of the support oxygen atoms as measured by XPS, and the change in the electronic structure of the Pt particles as determined with Atomic XAFS could be correlated using the O Madelung potential model. The changes in hydrogen chemisorption properties of Pt were detected using the Delta XANES technique, and this change was combined with a three-site (atop, 2- or 3-fold, and ontop) Langmuir hydrogen adsorption model to calculate the change in hydrogen coverage of each site as a function of T, P, and support ionicity. The changes in CO and H chemisorption properties at low (due to dispersion effects) and high temperatures (due to electronic effects) are revealed, and their consequences for Pt catalyzed oxidation and hydrogenolysis/ hydrogenation reactions are discussed. 2. Methods 2.1. Experimental. 2.1.1. Catalysts Preparation. Modified Al2O3 supports from the most acidic (Si) to the most basic (Cs) were prepared. The sol-gel method was used in order to achieve a homogeneous distribution of modifier throughout alumina bulk. Alumina sol was prepared in accordance with the procedure described by Yoldas10 using aluminum isopropoxide as alumina source. To achieve the resulting loading with the modifier, a corresponding amount of the metal salt was dissolved in 20 mL of distilled water (except Si-modified alumina, see below) and added dropwise to approximately 100 mL of alumina sol under continuous stirring for 1 h. The resulting mixture was held at 90 °C for 4 h still under continuous stirring. After that, water was removed on a rotary evaporator at 60 °C, and the product was dried overnight at 120 °C and calcined at 600 °C for 4 h.

J. Phys. Chem. C, Vol. 111, No. 10, 2007 3939 TABLE 1: Properties of Supported Pt/Al2O3 Catalysts atomic ratio Z/Al acid-base modifier (Z)

bulk

surfacea

SBET, m2/g

Si W F Cl K Rb Cs

0.10 0.10 0.10 0.10 0.10 0.10 0.10

0.236 0.074 0.116 0.065 0.085 0.113 0.127

280 376 291 293 343 190 197 186

a

-XPS data.

Basic (K, Rb, Cs) modifiers were introduced using corresponding nitrates. The acidic (F, W) additives were introduced using NH4F and (NH4)10W12O41. For the acidic Si modifier, an ethanol solution of Si(C2H5O)4 was utilized. The modifier Al atomic ratio was maintained at 1/10. Pt was introduced onto the alumina samples by incipient wetness impregnation with Pt(NH3)4NO3 (1 wt % Pt). The samples were dried overnight at room temperature and calcined in flowing O2 with the temperature ramped at a rate of 0.5 °C/ min from RT to 500 °C with a hold at 500 °C for 2 h. Thereafter, the samples were purged in Ar, followed by cooling to RT. Reduction was carried out in flowing H2 with the temperature ramped at a rate 8 °C/min from RT to 350 °C with a hold at 350 °C for 2 h. The properties of the different catalysts are given in Table 1. 2.1.2. XPS. X-ray photoelectron spectra were obtained using an XSAM-800 spectrometer (Kratos) with Al KR1,2 radiation for spectra excitation. Binding energies of the peaks were corrected for sample charging by referencing to the C 1s peak at 285.0 eV. The modifier/Al surface atomic ratio was calculated from integral intensities of XPS peaks using Scofield’s photo ionization cross-sections for Al KR1,2 excitation.11 2.1.3. FTIR of Chemisorbed CO. The spectra were recorded with a Nicolet Prote´ge´ 460 IR Fourier spectrometer equipped with a diffuse reflectance attachment at a resolution of 4 cm-1. The pretreated samples were placed in a quartz reactor evacuated at 350 °C and treated with hydrogen for 1 h in hydrogen (100 Torr) at 350 °C with further evacuation at the same temperatures down to a residual pressure of 10-4 Torr and cooled to room temperature. The adsorption of CO (20 Torr) was carried out at room temperature. The spectra were recorded after evacuation at room temperature. The spectra were processed using the Kubelka-Munk equation according to the OMNIC software. Computational treatment included smoothing the spectra, subtraction of the background, and deconvolution of the spectra by approximation with the combination of the Gauss-Lorentz functions. 2.1.4. XAFS Data Collection and EXAFS Data Analysis. XAFS data were collected at the Pt L3 and L2 edge using beamline X1.1 of the HASYLAB synchrotron (Hamburg, Germany) equipped with a Si(311) crystal. The monochromator was detuned to 50% of maximum intensity. All measurements were done in transmission mode using ion chambers filled with a N2/Ar mixture to have an X-ray absorbency of 20% in the first and of 80% in the second ion chamber. At the Pt L3 edge (11 564 eV), the estimated resolution was 2 eV. To decrease low- and high-frequency noise as much as possible, an acquisition time of 0.5 s for the EXAFS data was used with a gradual increase to 1.5 s at high photon energy and the averaging of 3 scans. The samples were pressed into self-supporting wafers (calculated to have a total absorbance of 2.5) and placed in a

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TABLE 2: FEFF7 Parameters Used to Calculate Pt-Pt and Pt-O Theoretical Referencesa atom pair Pt-Pt Pt-O a

potential

σ2 [Å2]

S02

Vr [eV]

Vi [eV]

Dirac-Hara Dirac-Hara

0.00234 0.00342

0.82 0.90

-4.2 0.0

3.0 3.0

The references were calibrated using Pt-foil and Na2Pt(OH)6.15

controlled atmosphere cell operated at 1 atm.12 Subsequently, the catalysts were dried at 150 °C for 1 h, re-reduced in hydrogen flow at 300 °C, and cooled down in hydrogen flow to RT. Then hydrogen supply was disconnected from the cell, and the cell was cooled down to liquid nitrogen temperature to perform XAFS measurement. Subsequently, the samples were evacuated at 100 and 200 °C for 30 min (treatments denoted as VAC100 and VAC200, respectively). After each evacuation, the EXAFS data were collected at liquid nitrogen temperature maintaining a vacuum better than 10-3 Pa sample Extraction of the EXAFS data from the measured absorption spectra was performed with the XDAP code.13 Three scans were averaged, and the pre-edge was subtracted using a modified Victoreen curve. The background was subtracted employing cubic spline routines with a continuously adjustable smooth parameter, which is a prerequisite for a full isolation of the AXAFS oscillations from the atomic background.14 Normalization was per formed by dividing the data by the height of the absorption edge at 50 eV. Data analysis was performed by multiple-shell fitting in R-space using the EXAFS data-analysis program in XDAP, which allows one to minimize the residuals between both the mag nitude and the imaginary part of the Fourier transform of the data and the fit. R-space fitting has significant advantages compared to the usually applied fitting in k -space.15 The difference file technique was applied together with phase- and amplitude-corrected Fourier trans forms to resolve the different contributions in the EXAFS data. In the difference file technique, one simultaneously fits all contributions, but each individual contribution is monitored and optimized with respect to the other contributions present in the EXAFS spectrum.15 This allows for a more accurate determination of the minor contributions to the EXAFS spectrum. Both high-Z (Pt-Pt) and low-Z (Pt-O) contributions are present in the EXAFS data collected on the Pt/Al2 O3 samples.16 The R-space fits have been carried out by applying k1 and k3 weightings in order to be certain that the results are the same for all weightings. A k3 weighting emphasizes the high Z contributions to the spectrum, while a k1 weighting focuses on the low Z contribution. Theoretical phase shifts and backscattering amplitudes for the Pt-Pt and Pt-O absorber-scattering pairs were used in the EXAFS data analysis; these were generated utilizing the FEFF7 code with the parameters listed in Table 2.15 The theoretical references were calibrated with the help of the experimental data from Pt foil and Na2Pt(OH)6 employing the procedures reported in ref 15. 2.2. Delta XANES technique. It is well-known from early work that the Pt L3 XANES is sensitive to the adsorption of H.17,18,19,20 To isolate and identify these rather small changes, difference spectra, ∆µ, are obtained by taking the difference between the L3 spectra with and without H, ∆µL3 ) µL3(H/Pt) - µL3(Pt), with µL3(H/Pt) the L3 edge spectrum in the presence of H2 and µL3(Pt) the L3 edge spectrum in vacuum. Since the absorption µ equals µo(1 + χ), the total change can be expressed as21

∆µ L3 ) µL3(H/Pt) - µL3(Pt) ) ∆µo + ∆(µoχPt-Pt) + µo,H/PtχPt-H (1) with

∆µL3: the L3 edge difference spectrum µL3 (H/Pt): the L3 edge spectrum in the presence of H2 µL3 (Pt): the L3 edge spectrum in vacuum ∆µo: changes in the atomic L3 XAFS with H coverage ∆(µoχPt-Pt): changes in the Pt-Pt total scattering induced by H2 chemisorption µo,H/Pt: the free atom L3 absorption (including atomic XAFS) in the presence of H2 χPt-H: the additional Pt-H scattering The FEFF8 code22 was designed to calculate the absorption, µ, the same µ quantity as that obtained in experiment. FEFF8 performs real-space full multiple scattering calculations utilizing a muffin-tin potential calculated with Hedin-Lundquist exchange correlation approximation, and implements selfconsistent field potentials for the determination of the Fermi level and the charge transfer. Thus, ∆µ can be calculated by performing the same difference as that obtained experimentally. Figure 1 shows the total ∆µ (minus the first term in eq 1) obtained from FEFF8 calculations on the Pt6 clusters illustrated in Figure 1. As in previous work,6 the ∆µo contribution was not included in Figure 1 because it is exaggerated for these very small clusters utilized in the FEFF8 calculations. More details of the Delta XANES technique can be found elsewhere.9 The experimental XANES data have to be carefully aligned before taking these differences. This alignment procedure has been outlined in detail previously. First the zero of energy is set to the energy that falls at 0.6 in each of the normalized L2 spectrum. Then the EXAFS features in each L2 and corresponding L3 spectrum are aligned using a computer routine that minimizes the square of the difference µL3 - µL2 in an energy range usually between 30 and 100 eV above the edge. This energy range has been varied some in our previous work, depending on the data.9 This entire energy alignment procedure, used in all of our previous gas phase work, is critical to obtaining systematic ∆µ spectra because of the large cancellations involved in the differences, leaving ∆µ typically only around 0.03-0.06 in magnitude. 3. Results 3.1. XPS of O 1s. The influence of the modifier on the electron charge of the support oxygen atoms is determined by measuring with XPS the binding energy of the O 1s orbital. It can be seen in Figure 2 that the 1s BE systematically increases with decreasing ionicity. This demonstrates that the modification of the ionicity with modifiers using the sol-gel method changes the electron charge of the support oxygen atoms with the lowest value for the most acidic support (modifier Si) and the highest for the most basic (modifier Cs). 3.2. FTIR of Chemisorbed CO. The diffuse reflection FTIR data from these samples are displayed in Figure 3. Two regions of adsorbed CO can be assigned. At higher wave numbers (2200-2000 cm-1), an adsorption band due to linearly coordinated CO, and at lower wave numbers (2000-1700 cm-1),

Support Influence on CO and H

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Figure 1. Previously reported ∆µ theoretical signatures obtained from FEFF8 calculations on the clusters indicated for atop, 2-fold bridged, and 3-fold fcc H.

Figure 4. (a) Relative intensity of linear coordinated CO (L/LSi) and relative ratio of bridged to linear coordinated CO (B/L) and (b) the wavenumber of the linear and bridged coordinated CO band on Pt as a function of the O 1s B. E of the support as measured by XPS.

Figure 2. XPS binding energy of oxygen 1s of the Al2O3 supports with different modifiers.

Figure 3. IR drift spectra of CO adsorbed on Pt particles supported on Al2O3 with different modifiers, measured at 20 °C in vaccum.

adsorption due to CO in bridging coordination is visible. Integrating the curve areas of the data collected at 20 °C and using the XPS data displayed in Figure 2, the ratio of bridging

bonded to linear bonded CO (B/L) can be plotted as a function of the O 1s BE of the support. Figure 4a shows a strong increase with decreasing O 1s BE, i.e., with increasing electron charge or increasing ionicity of the support (see Figure 4a). The intensity of the linear CO band of Pt/Si-Al2O3 (LSi) is normalized to 1. It can be seen that the relative intensity of the linear CO band (L/LSi) decreases by about 50% with increasing ionicity. Although it is often not valid to compare the absolute intensities of diffuse reflection FTIR data from sample to sample because of changing reflectivities, in this case we are only changing the acid/base properties of the support so that the signal due to CO/Pt appears to be reasonably systematic. Further, the noise level in L/LSi is not much greater than that for the B/L ratio, suggesting that the 50% decrease from the acidic to basic supports is indeed representative of the changing Pt dispersion consistent with the Delta XANES and EXAFS data to be discussed below. Also, a large influence of the support ionicity on the C-O vibrational frequency is observed as shown in Figure 4b. Both adsorption bands shift to higher wave numbers as the support ionicity decreases. 3.3. XAS Spectroscopy. 3.3.1. Delta XANES. The Pt L3 and L2 X-ray absorption data are displayed in Figure 5 (Pt/Cl-Al2O3 (panels a and c) and Pt/Rb-Al2O3 (panels b and d)) as a function of the desorption of chemisorbed hydrogen. The effect of H desorption is larger for Pt/Cl-Al2O3 pointing to a higher Pt dispersion in this sample. Figure 6 gives the results of the Delta XANES technique. A systematic decrease in amplitude of the L2 edges is observed as a function of the H desorption. The change in the shape of the L3 edges due to H desorption is more complicated and different for both samples as will be discussed below. Optimal fits of the expression

∆µtheo ) Cat∆µat (E + δat) + C3f∆µ3f(E + δ3f)

(2)

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Figure 5. Pt L3 and L2 X-ray absorption spectra (after alignment) of Pt/Cl-Al2O3 (a, b) and Pt/Rb-Al2O3 (c, d) after reduction at 300 °C and evacuation at 200 °C.

Figure 6. ∆µL3 (a, c) and ∆µL2 (b, d) Delta XANES spectra of Pt/Cl-Al2O3 and Pt/Rb-Al2O3. Solid line: Red-Vac473. Dotted line: Red 373Vac473. Dashed-dotted line: Red 423-Vac473. In panels a and c, the heavy lines represent the data, and the light lines represent the optimal fits of eq 2 to ∆µL3 with the parameters given in Table 3 as described in the text. For clarity, the y-axis of the dotted line in Figure 6a and of the dashed-dotted line in Figure 6c is shifted by -0.3 and -0.1, repectively.

are also shown where the ∆µat and ∆µ3f are the FEFF8 calculated signatures shown in Figure 1 for the atop and 3-fold binding sites respectively. In eq 2 the δ’s are small energy shifts of the theoretical signatures, which are required for optimal agreement with experiment. Small shifts are usually required since FEFF8 does not accurately give the absolute energies for such small Pt particles. The coefficients C will be discussed

below but reflect the relative coverage and the dispersion of the Pt particles. The optimal values for C and δ are given in Table 3. 3.3.2. EXAFS and AXAFS. The raw EXAFS data from Pt/ Cl-Al2O3 and Pt/Rb-Al2O3 after reduction (solid lines) and evacuation (dotted lines) are presented in Figure 7a,c. The S/N ratio allows for an EXAFS data-analysis up to about k ) 14

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TABLE 3: Optimal Fit Parameters Used to Fit Eq 2 to the Experimental ∆µL3 Results in Figure 6 Pt/Rb-Al2O3 Cat δat

C3f

RED-VAC473 0.0 - 0.45 VAC373-VAC473 0.0 - 0.14 VAC423-VAC474 0.0 - 0.16

Pt/Cl-Al2 O3

δ3f cova Cat δat C3f δ3f cova 2 0 0

.45 .14 .16

1.0 -3 0.7 4 5.0 -3 0.0 2.5 -3 0.0 -

.37 .08 .04

a Calculated by scaling ∆µatop by a factor of 1/30 and assuming dispersion for the Cl samples is 0.5.

Å-1. It can be seen that the EXAFS oscillations above k ) 8 Å-1 (dominated by the Pt-Pt contribution) for Pt/Rb-Al2O3 are larger and extend to higher values of k pointing to larger Pt particles for this sample. The changes in the nodal positions of the Pt-Pt oscillations due to the evacuation treatment are larger for Pt/Cl-Al2O3, while the influence of evacuation on the amplitude of the Pt-Pt EXAFS is smaller for this sample. The corresponding Fourier transforms (k1, ∆k ) 3-14 Å-1) of the EXAFS spectra are displayed in panels b and d, respectively, of Figure 7. The shoulders at both the low- and high-R sides of the first shell Pt-Pt peak in the Fourier transform (2 < R < 3 Å) are due to the nonlinear Pt-Pt phase shift and the k dependence of the Pt backscattering amplitude.15 The amplitude of the first shell Pt-Pt peak is much larger for Pt/ Rb-Al2O3, while the relative contribution of the low-R part of the FT (1 < R < 2 Å, dominated by low Z contributions like oxygen) is more pronounced for Pt/Cl-Al2O3. It can be seen that the influence of hydrogen desorption on the FT is very different for both samples. The shift of the nodes of the imaginary part of the Pt-Pt FT peak to lower values of R is larger for Pt/Cl-Al2O3, whereas a decrease in amplitude of the Pt-Pt peak can be observed only for Pt/Pt/Rb-Al2O3. Multiple-shell fitting in R-space (FT: ∆k ) 3-14 Å-1, ∆R ) 1.5-3.2 Å) together with the use of the difference-file technique resulted in the identification of Pt and O backscatterers. The resulting EXAFS coordination parameters are given in Table 4. The number of free fit parameters used to analyze the EXAFS data obtained after reduction and after evacuation at 200 °C is 12 and 8, respectively. This number is allowed since it is lower than the number of independent parameters according to the Nyquist theorem15 (Nindp ) 13.9). Minimization of the variances in imaginary and absolute parts led to the best fit. The variances of the fit of the imaginary part and absolute part are less than 1% for all samples. The quality of the fit obtained after reduction of Pt/Rb-Al2O3, as shown in Figure 8a, is representative for all samples. By subtracting the total fit from the raw EXAFS data, the total difference file is obtained, which contains only the AXAFS oscillations and the higher shell Pt-Pt peaks. Figure 8b displays the FT of the difference files (raw data minus fitted [Pt-Pt + Pt-OS]) obtained for Pt/Cl-Al2O3 (solid line) and Pt/Rb-Al2O3 (dotted line) after evacuation at 200 °C. It can be seen that the AXAFS peak around 0.5 Å is well resolved due to the proper determination of the Pt-O EXAFS parameters; this led to a FT of the difference file which has no contributions between 2 < R < 3 Å. The amplitude of the FT AXAFS peak for Pt/ClAl2O3 (solid line) is larger than for Pt/Rb-Al2O3 (dotted line) showing the effect of the modifier on the electronic structure of the Pt (see further discussion). The second and third higher shell Pt-Pt peaks are well resolved in the FT of the EXAFS data of Pt/Rb-Al2O3. It should be noted here again that the AXAFS peak and the third shell Pt-Pt peak in the FT of the difference file have comparable amplitudes, revealing a reliable detectability of the AXAFS. The higher shell peaks visible in

the FT of the difference file for Pt/Cl-Al2O3 have a lower amplitude, and more importantly the imaginary part of the FT is different (see further discussion). The FT of the Pt-O difference files (raw data minus the fitted Pt-Pt EXAFS) are given in panels a and b of Figure 9 for Pt/ Cl-Al2O3 and Pt/Rb-Al2O3, respectively. The data obtained after reduction are given with a solid line and after evacuation with a dotted line. The peak at around 0.5 Å can be ascribed to the Atomic XAFS contribution (already indicated above). The peak at around 1.5 Å is due to the Pt-O contributions arising from the metal-support interface (see further discussion). The difference file obtained after reduction shows a peak intensity at higher values of R (2.0 < R < 2.5 Å) than those for the data obtained after evacuation. Moreover, differences can be observed in the imaginary parts pointing to changes after evacuation. As can be seen in Table 4, the difference file of the data obtained after reduction can be fitted only when both long (PtOL) and short (Pt-OS) oxygen distances are included, while after evacuation only the short Pt-OS distance is detected. Both the long and short Pt-O distances show a systematic decrease with decreasing ionicity of the support oxygens (see further discussion). Details of the fit procedure are given here only for the analysis of the EXAFS data of the Pt/Cl-Al2O3 sample. The fitted total Pt-O contributions (Pt-OL + Pt-OS) are given in Figure 9c with a dotted line. After evacuation only the short (Pt-OS) contribution (dotted line) can be observed in the difference file (see Figure 9d). It can be seen that the FT AXAFS peak is tailing into the FT Pt-O peaks. By starting the multiple shell fit at 0.5 Å, it becomes possible to separate the Pt-O contributions from the AXAFS. Evidence for the optimization of the Pt-O EXAFS parameters is provided by a full isolation of the AXAFS contribution as demonstrated in Figure 8b It can be seen that the amplitude of the Pt-O peak(s) (both for reduction and evacuation) is higher for Pt/Cl-Al2O3, implying that a larger fraction of Pt is detecting interfacial support oxygen neighbors. This also points to a higher dispersion for this sample. A strong increase of the Pt-O peak is observed for the Pt/Rb-Al2O3 after evacuation. This suggests a noticeable change in particle morphology due to evacuation with more Pt atoms in the metal-support interface detecting neighboring oxygen atoms of the support. 4. Discussion 4.1. Geometrical Structure of the Supported Pt Clusters. Table 4 shows that after reduction the Pt-Pt first shell coordination number is larger for Pt/Rb-Al2O3 (7.7) than for Pt/Cl-Al2O3 (5.9), pointing to significantly larger Pt particles on the Rb-Al2O3 support. Since interfacial Pt-O coordination can be also observed (see section 4.2), a significant fraction of the Pt atoms must be in contact with the support, suggesting that the shape of the Pt particles is much less than completely spherical. More information about the morphology of the Pt particles will be obtained below from changes in the Pt-Pt EXAFS coordination parameters with (i) decreasing Pt particle size and (ii) desorption of hydrogen. It has been shown previously that the electronic structure of the metal atoms in small metal particles changes from the bulk due to the dehybridization of the spd metal orbitals.23 This dehybridization results in an increased local electron density (less delocalization) between the atoms in the small particle, which in turn results in a contraction of the interatomic Pt-Pt distance. At the same time, the phonon spectrum softens causing a broader potential well. This in turn leads to a larger disorder which is reflected in a larger Debye Waller factor.

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Figure 7. (a, c) Raw EXAFS data and (b, d) corresponding Fourier transforms (k1, ∆k ) 3-14 Å-1) of (a, b) Pt/Cl-Al2O3 and (c, d) Pt/Rb-Al2O3. Solid lines: reduction at 573K. Dotted lines: evacuation at 473K.

TABLE 4: EXAFS Coordination Parameters Obtained from R-Space Fit (k1, ∆k ) 3-14 Å-1, ∆R)1.5-3.2 Å) variance N R(Å) ∆σ2(Å2) E0(eV) catalyst scatter ((10%) ((0.02) (103, (5%) ((10%) im. red vac

red vac

Pt OL OS Pt OS

5.9 0.7 0.8 5.8 1.0

Pt/Cl-Al2O3 2.70 6.0 2.53 -2.9 2.01 0.0 2.66 6.4 2.00 0.0

8.7 -1.6 0.8 7.7 4.5

Pt OL OS Pt OS

7.7 0.3 0.4 6.8 0.6

Pt/Rb-Al2O3 2.75 2.0 2.63 2.0 2.13 0.5 2.74 3.0 2.08 1.1

1.5 -13.3 -13.3 1.2 -5.2

abs.

0.49 0.26 0.69 0.38

0.29 0.19 0.34 0.62

The Pt structural parameters in Table 4 for those samples under evacuation are fully consistent with these previous theoretical and experimental results (specifically for Pt/γ-Al2O324 and Ir/γ-Al2O325). The first-shell Pt-Pt distance for the larger Pt particles supported in Rb-Al2O3 is closer to that in bulk Pt (2.74 versus 2.77 Å) with a smaller Debye-Waller factor (0.003 Å2). The smaller Pt particles in Cl-Al2O3 show a considerable contraction (2.66 Å) with a larger Debye-Waller factor (0.0063 Å2). Hydrogen chemisorption reduces the contraction of the PtPt distance upon going from large to small Pt particles (i.e., it effectively makes the small clusters bigger). This can be explained by the fact that hydrogen chemisorption leads to an increase in the amount of neighboring atoms for the surface Pt. Hydrogen is now sharing Pt valence electron density in the Pt-H bond, making the neighboring Pt-Pt bonds weaker. In EXAFS this effect is averaged over the whole particle, effectively leading to an observed larger Pt-Pt distance. The firstshell Pt-Pt distance in Pt/Rb-Al2 O3 after reduction is only slightly affected by hydrogen chemisorption (increase from 2.74 to 2.75 Å), and a small decrease in disorder is observed (0.003 to 0.002 Å2). The effect of hydrogen chemisorption on the smaller Pt particles supported on Cl-Al2 O3 is much larger as expected; the Pt-Pt distance relaxes back from 2.66 to 2.70 Å.

Figure 8. (a) Fourier transform (k1, ∆k ) 3-14 Å-1) of the raw EXAFS data (solid line) and total fit (Pt-Pt + Pt-OS + Pt-OL) (dotted line) for Pt/Rb-Al2O3 after reduction at 573K. (b) Fourier transform (k1, ∆k ) 3-9.5 Å-1) of the total difference file [raw EXAFS minus fitted (Pt-Pt + Pt-OS)] of Pt/Cl-Al2O3 (solid line) and Pt/Rb-Al2O3 (dotted line), both after evacuation at 473K. Centroid of AXAFS peak of Pt/Cl-Al2O3 shifted to lower values of R.

However, no significant change is observed in the Pt-Pt disorder. This is a good indication that the Pt particles are not only smaller on Cl-Al2 O3, but they must also have a different Pt particle morphology, as will be explained below. Desorption of hydrogen leads to a noticeable decrease in the first shell Pt-Pt coordination number (from 7.7 to 6.8) for Pt/ Rb-Al2O3, which can be explained by some flattening out of the Pt particle onto the support after H leaves the Pt surface that causes a decrease in Pt coordination. The increase in surface

Support Influence on CO and H

J. Phys. Chem. C, Vol. 111, No. 10, 2007 3945

Figure 9. (a, b) Fourier transforms (k1, ∆k ) 3-9.5 Å-1) of Pt-O difference file (raw EXAFS minus fitted Pt-Pt) of (a) Pt/Cl-Al2O3 and (b) Pt/Rb-Al2O3 after reduction at 573K (solid lines and after evacuation at 473K (dotted lines). (c, d) Fourier transforms (k1, ∆k ) 3-9.5 Å-1) of Pt-O difference files (raw EXAFS minus fitted Pt-Pt) (solid lines) of Pt/Cl-Al2O3 (c) after reduction at 573K and (d) after evacuation at 473K. Dotted lines indicate corresponding Fourier transforms of (c) fitted Pt-OL + Pt-OS after reduction and of (d) fitted Pt-OS contribution after evacuation.

free energy due to the loss of H is compensated for by an increase in coordination with support oxygen atoms, which can be maximized by the flattening out of the Pt particles. Thus, the Pt particles on Rb/Al2O3 change morphology from more spherical to half spherical due to desorption of hydrogen. The increase in Debye-Waller factor due to hydrogen desorption fully confirms this. However, the situation for the Pt particles supported on ClAl2O3 is totally different. No change in Pt-Pt coordination and a smaller change in Debye-Waller factor is observed after hydrogen desorption. This can only be explained by a particle morphology that is already flatter or raft like even in the presence of chemisorbed hydrogen. The much higher Pt-O coordination as well as the totally different scattering intensities of the FT higher Pt-Pt shell EXAFS peaks in Cl-Al2O3 are fully consistent with a raft like structure. The peak amplitudes of the higher coordination shells are not only lower (see Figure 8b), but the different nodal positions in the imaginary part points to the presence of higher Pt-O shell contributions, which originate from the interfacial oxygen atoms. The reason why the support ionicity strongly influences the particle geometrical structure will be discussed in section 4.6. This change in shape and dispersion between the two catalysts is entirely consistent with the full H and CO adsorbate coverages found at low temperature. Indeed, the magnitudes of the ∆µ ) µRED - µVAC473 data in Figure 6 for the two catalysts indicate that that the H coverage at full coverage, and hence the dispersion of the Pt/Rb-Al2O3 sample, is about 50% lower than that of Pt/Cl-Al2O3. This is in full agreement with the EXAFS results as discussed in section 4.1. and the 50% reduction in intensity (L/LSi) of the linear IR CO band of the same samples (see Figure 4b). 4.2. Detection of Interfacial Pt-O Contributions. In Table 4 both short (∼2.0 Å) and long metal-oxygen distances (∼2.5

Å) can be observed. Previous EXAFS studies on several supported metal catalysts (metals Rh, Ir, Pt; supports Al2O3, TiO2, MgO, LTL, Y)2,25,26,27 show metal-oxygen interactions characterized by both a short (2.10-2.20 Å) and long (2.52.8 Å) distance.25 By combining the results of TPD and XAFS experiments, Vaarkamp et al.27 concluded that the long metal oxygen distance is due to the presence of hydrogen in the metal-support interface, since evacuation leads to a shortening of this distance. Further proof was obtained from the EXAFS experiments carried out on Ir/Al2O3 catalysts.25 The long metal oxygen distance was shown to be reversible: admission of hydrogen after the evacuation treatment restored the originally observed long metal oxygen distance. The behavior of the Pt-O contributions as a function of particle morphology and evacuation treatment as observed in this work is fully consistent with the earlier assignment of the presence of support oxygen scatters in the Pt-support interface. 4.3. Influence of the Support Ionicity on Pt Electronic Structure. As mentioned in the introduction, the support ionicity also influences the Pt electronic structure.1,2,3,4,5,6,7,8,9 The effect of support ionicity (depending on the acid/base properties) and geometry (macroporous flat and zeolitic) on the Pt particles has been extensively studied by our group. In this study, the ionicity of the Al2O3 support was systematically varied over a very large range by introducing modifiers (M), and the sol-gel method allows a relatively large M/Al ratio. In fact, the large range in ionicity of the Al2O3 support in this study makes it possible for the first time to monitor the systematic decrease in the electron charge of the support oxygen atoms with decreasing support ionicity by XPS (see Figure 2). This experimental observation can now be directly combined with the increase of the amplitude of the FT AXAFS peak and the shift of the centroid to lower values of R of Pt/Cl-Al2O3, as shown in Figure 8b. As previously observed for Pt/LTL,1 Pt/Y,2 MgO-Al2O3, and

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Stakheev et al.

SiO2-Al2O3,28 the highest amplitude of the FT AXAFS peak and a shift of the centroid to lower values of R is detected for Pt on the support with the lowest ionicity (most acidic). The results obtained in this study fully support the Madelung potential model as was introduced earlier by our group to describe metal-support interaction.1,2,3,28 The ionization potential of the Pt 5d orbitals increases and the Pt interstitial (6s, 6p) IBO orbitals4 move from the Pt surface to the Pt-support interface with decreasing support ionicity, thereby influencing the chemisorption properties of Pt29 as further discussed below. 4.4. Correlation of the Delta XANES Data with a ThreeSite Langmuir Model for H Chemisorption. Previous application of the new Delta XANES technique on the L3 data from an acidic Pt/Cl-Al2O3 and basic Pt/K-A2O3 catalyst, as well as other work reviewed elsewhere,30 led us to conclude that three different hydrogen adsorption sites on Pt are evident.9 The strongest bonded H at low coverage is chemisorbed in an atop position. With increasing coverage only n-fold (n ) 2 or 3) sites are occupied. At high coverage the weakest bonded H is positioned in an “ontop” site, defined as Pt atop sites surrounded by stronger bonded H atoms in the neighboring 3-fold sites. It can indeed be seen in Figure 6a that at high temperature in vacuum (low coverage) the ∆µ signature of the Pt/Cl-Al2O3 data points to the presence of H in an atop position as indicated by the optimal fit parameters in Table 3. Under the same conditions H is present in the n-fold site in Pt/RbAl2O3 (Figure 6c). At RT for the Pt/Cl-Al2O3 ontop H (combination of signatures from both atop and n-fold) can be observed. The Delta XANES results obtained in this study can be correlated with previous quantitative estimates of the adsorption enthalpies, ∆Hi, (i.e., ∆Hi for the reaction 1/2H2 + Pt f H/Pt) for each site in the three-site Langmuir adsorption model. These estimates were obtained from quantitative fits of Langmuir adsorption isotherms:8

θH )

PH21/2e-∆GH0/RT 1+

PH21/2e -∆GH0/RT

(3)

to H chemisorption experiments on a basic Pt/NaY and an acidic Pt/H-USY data7 and confirmation of the atop to n-fold conversion using L3 and L2 X-ray absorption edge data on these same samples. These quantitative fits led both to values of the ∆Hi as well as fractions per Pt atom, Ni, of each type of site:

θH ) Natθat + Nnfθnf + Notθot

(4)

These quantities along with eq 4 then allow us to calculate the coverage at any temperature and pressure. The range of the acid/base properties of the Al2O3 supports in this study is expected to be large due to the applied sol-gel method used in this work. Assuming that the acid/base character of Pt/Rb-Al2O3 versus Pt/Cl-Al2O3 will approximately equal that of the Pt/NaY vs Pt/H-USY samples used in the previous work allows us to calculate the total hydrogen coverage as a function of T (see Figure 10a). Table 5 gives the values for the reaction enthalpies (∆H1) and the number (Ni) of the atop, n-fold and ontop sites used for this calculation. In order to combine the results of this calculation with the ∆µ data obtained in this work, a pressure of 10-5 atm. H2 in vacuum was used since the pressure shown on the pressure gauge during the XANES measurements was about 10-5 atm. The calculated total H coverage can be correlated with values derived for the total H coverage in vacuum at 373 and 423 K from the ∆µ L2 data

Figure 10. Total hydrogen coverage per Pt surface atom (Θ/Pts) (top) and individual components (bottom) as a function of temperature. Calculations were carried out with the three-site Langmuir isotherm model using input values given in Table 5. Experimental points indicated by the dots were derived from the δ L2 XANES data at the indicated temperatures relative to that full coverage (reduced). The dominant coverage at the two temperatures (horizontal lines) is atop H for the Cl and 3-fold H for the Rb consistent with the ∆µL3 theoretical fits.

TABLE 5: Summary of Fit Parameters Using Eq 4 and the Three-Site Model PT/H-USY

atop n-fold On top (weak)

Pt/NaY

Nia

∆Hi (kJ/mol)b

Nia

∆Hi (kJ/mol)b

0.32 0.52 0.73

-57 -40 -23

0.35 0.51 0.56

-80 -53 -32

a Ni values are those given in ref 8 appropriate for increasing temperature such as that used for obtaining the XANES data. b ∆H for the reaction 1/2H2 + Pt f H/Pt.

given in Figure 6. The ∆µL2 ) µRED - µVAC intensity around 8 eV in Figure 6 is assumed to correspond to full coverage at 100 K, since they were collected at 100 K under 1 atm H in the in-situ EXAFS cell. In Figure 10 the ∆µL2 ) µRED - µVAC473 data are indicated by the dots on the curves at 100 K and are taken as reference values for full coverage. The corresponding data points for the H coverage in vacuum at 373 and 423 K can be determined from the ∆µL2 data given in Figure 6 using the ∆µL2 ) µRED - µVAC473 as reference (see Figure 10a). The agreement is quite good suggesting that indeed the ionicities of the Rb-Al2O3 and Cl-Al2O3 catalysts are widely different, nearly having the same magnitude and cover the same ionicity range as NaY and H-USY, which are known to be widely different in acidity. As already mentioned in the Introduction, it can be seen in Figure 10a that the support ionicity has a large influence on the H chemisorption properties of the Pt particles. The total H coverage is larger for Pt on supports with a high ionicity (basic), which is caused by a higher Pt-H bond strength.4,6 The coverage of the separate components present on Pt/ClAl2O3 and Pt/Rb-Al2O3 as determined by the model is also plotted as a function of temperature in Figure 10b. It can be seen that at low-temperature both the ontop and n-fold sites are filled with chemisorbed hydrogen. With increasing temperature the weakest bonded ontop sites become empty first. At

Support Influence on CO and H the point where the temperature increases enough for desorption of hydrogen from the n-fold sites, atop sites also become filled, which continues until H is completely desorbed from the n-fold sites. With further increase of the temperature, the atop sites finally loose their chemisorbed hydrogen. By comparing the separate components in Figure 10b, it can also be observed that the temperature dependence of the coverage of each individual H adsorption site on Pt depends strongly on the ionicity of the support All of the desorption events occur at a lower temperature and with a lower H coverage on Pt in an acid support compared with the basic, showing again that a decrease in ionicity of the support leads to a decrease in Pt-H bond strength. The optimal fits of the theoretical line shape, eq 2, shown in Figure 6 and the optimal parameters coming from these fits given in Table 3 can now be related to the relative fractions for atop/ontop and n-fold at each temperature shown in Figure 10b. The ∆µL3 ) µRED - µVAC473 line shape, which corresponds to full coverage at about 100K, shows a combination of ontop and n-fold signatures for Pt/Cl-Al2O3, while the signature for Pt/ Rb-Al2O3 is dominated by the n-fold sites. The ∆µL3 ) µVAC373 - µVAC473 and ∆µL3 ) µVAC423 - µVAC473 line shapes (see Figure 6 and Table 3) in Pt/Cl-Al2O3 both show an atop signature exactly as expected from the data points at 373 and 423 K in Figure 10b. The higher temperature ∆µ’s for Pt/Rb-Al2O3 have signatures which reflect more the n-fold sites as predicted by the adsorption model in Figure 10b. As indicated previously,8 the ∆µL2 spectral signatures more directly show the relative absolute H coverages as illustrated in Figure 10a; however, these same relative coverages can be obtained from the ∆µL3 spectral shapes (the latter showing the different binding sites more clearly) if the theoretical line shape for the atop signature is divided by 30. The relative coverage (cov) given in Table 3 shows the calculated coverage when including this factor and accounting for the dispersion factor of 0.5 for Pt/Cl-Al2O3 catalysts as discussed above. The underestimate of the magnitude of the ∆µat signature by the FEFF8 code has been discussed extensively elsewhere,8 so we do not discuss that further here. The δ shifts given in Table 3 reflect in part the inaccuracy of the FEFF8 absolute energy and the slight shifts that result from different Pt-H bond strengths as discussed previously.29 For example δ gets larger in ∆µ3f for the reduced samples when the coverage is larger and the Pt-H bond strength is weaker due to lateral interactions. A negative shift for ∆µat has typically been required in our previous fits 30, but this too can vary with the coverage. The enthalpy values for the three different H adsorption sites used in the calculation of the H coverage shown in Figure 10 were derived from fitting H/M data for Pt/NaY and Pt/H-USY samples, which have similar Pt particle sizes.7,8 The Pt/Al2O3 samples used in this study do not have the same Pt particle sizes. However, the agreement between the signatures of the ∆µL3 data and that predicted by the model in Figure 10, as well as the agreement in magnitudes between the calculated H coverage and the estimated values derived from the temperature dependence of the ∆µ data, show the general applicability of the three-site Langmuir isotherm model.8 4.5. Influence of the support ionicity on Pt CO chemisorption. In our earlier work,1 FTIR data of chemisorbed CO on a series of Pt and Pd catalysts (Pd/LTL, Pt/LTL and Pt/ SiO2) ranging from acidic (LTL:H+, SiO2 :Al) to basic (using different ratio’s of K/Al) showed a systematic decrease in the bridged to linear, B/L, ratio of adsorbed CO with decreasing ionicity of the supports. This has also been reported in a later

J. Phys. Chem. C, Vol. 111, No. 10, 2007 3947 more extensive study for Pt particles with similar sizes dispersed in a series of zeolites with different ionicities.31 For Pt on all supports, the IR CO bands shift to higher wavelengths and the B/L ratio decreases with decreasing ionicity of the support. Temperature-dependent CO desorption studies on the same samples as studied in this work will reveal more detailed insights into the mechanism and influence of the support ionicity on the Pt-CO chemisorption properties.32 4.6. Influence of the Support Ionicity on the Pt Geometrical Structure. It is striking that the morphology of the Pt particles is totally different in the Rb-Al2O3 versus the ClAl2O3 samples despite the fact that the preparation procedure for all samples is identical. We wish to correlate the differences in Pt morphology with the differences in support ionicity. Our earlier work1,2,3 and the results presented in this study have shown that the electronic structure of the Pt particles is influenced by the ionicity of the support. The change in electronic structure leads in turn to differences in chemisorption properties not only for H and CO, but also for O.29 It is shown elsewhere29 that chemisorption of O leads to stronger Pt-O bonds for Pt on supports with a low ionicity. The shift of the Pt VB to lower energy (higher binding energies) causes the Pt-O bond to become more covalent and therefore stronger.29 Systematic shorter Pt-O bonds with lower Debye-Waller factors are observed in this study at the metal-support interface in Cl-Al2O3, which indeed points to stronger Pt-O bonds. This can explain why the morphology of the Pt particles is more raft like even in the presence of chemisorbed hydrogen. Stronger Pt-O bonds can compensate for the increase in surface free energy due to the loss of neighboring Pt atoms when going to smaller Pt particles. Thus, there is a higher probability for raft like Pt particles on supports with a lower ionicity. 4.7. Implications of the Influence of the Support Ionicity on Pt Catalyzed reactions. Since Pt particles on acidic supports are the most active in hydrogenation and hydrogenolysis reactions,1,2,3,6 it seems logical to relate the difference in catalytic activity to the differences in H2 coverage and type of H binding sites as found in this study. The results presented here demonstrate that the three-site Langmuir isotherm model is generally applicable. In an earlier study,8 the model was combined with actual hydrogen chemisorption data and results obtained with the Delta XANES technique. It was possible to calculate the H coverage and to determine which types of H chemisorption sites are available for hydrogen dissociation during relevant catalytic conditions. We have previously reported a full analysis of the kinetic data for hydrogenolysis reactions of alkanes catalyzed by Pt particles on supports with different ionicity and acid/base properties. 6 This analysis has led to a basic understanding of the metal-support interaction on the kinetics of hydrogenolysis reactions which are mediated by the H coverage. As will be discussed in subsequent papers, the influence of the ionicity of the support on both the H coverage and the type of Pt site available for hydrogen splitting fully determines the kinetics of hydrogenation reactions (aromatic saturation). Summary The increase in the support oxygen 1s BE with decreasing ionicity of the Al2O3 support (increase in acidity) can be directly correlated with a decrease in electron charge on the support oxygen atoms introduced by the modifiers in the order Cs < Rb < K < parent < Cl < Si. The decrease in electron charge of the support oxygen atoms leads to an increase in the amplitude of the Pt FT AXAFS peak and a shift of the centroid

3948 J. Phys. Chem. C, Vol. 111, No. 10, 2007 to lower R values. These changes are directly related to a change in the electronic structure of Pt as described by the Madelung potential model introduced previously. The Pt valence band (VB) shifts to higher binding energy due to this increased Madelung potential brought about by a decrease in the support oxygen electron charge. This Pt 5d band shift results in a stronger bond between the interfacial Pt atoms and the support oxygen atoms leading to a change in morphology of the Pt particles from more (half) spherical to more raft like. This is confirmed by the results of the EXAFS data-analysis of Pt/ClAl2O3 and Pt/Rb-Al2O3 and by the H and CO coverage at low temperatures. The results of the Delta XANES technique combined with a previously derived three-site Langmuir isotherms model shows that three different H adsorption sites (ontop, n-fold, and atop with increasing order of Pt-H bond strength) can describe the Pt hydrogen chemisorption properties on these catalysts. The total H and CO coverage and the Pt-H and Pt-CO bond strengths are highest for Pt on supports with a high ionicity. The results imply that the type of Pt site available for hydrogen splitting during Pt catalyzed reactions and for CO oxidation over Pt are determined by the ionicity of the support. Acknowledgment. The authors would like to thank the scientific staff in HASYLAB Beamline X1.1 for their continuing interest and stimulating support. References and Notes (1) Mojet, B. L.; Miller, J. T.; Ramaker, D. E.; Koningsberger, D. C. J. Catal. 1999, 186, 373. (2) Koningsberger, D. C.; de Graaf, J.; Mojet, B. L.; Ramaker, D. E.; Miller, J. T. Appl. Catal. A 2000, 191, 205. (3) Ramaker, D. E.; de Graaf, J.; van Veen, J. A. R.; Koningsberger, D. C. J. Catal. 2001, 203, 7. (4) Oudenhuijzen, M. K.; van Bokhoven, J. A.; Ramaker, D. E.; Koningsberger, D. C. J. Phys. Chem. B 2004, 108, 20247. (5) Ramaker, D. E.; Oudenhuijzen, M. K.; Koningsberger, D. C. J. Phys. Chem. B 2005, 109, 5605. (6) Koningsberger, D. C.; Oudenhuijzen, M. K.; van Bokhoven, J. A.; Ramaker, D. E. J. Catal. 2003, 125, 178. (7) Ji, Y.; van der Eerden, A. M. J.; Koot, V.; Kooyman, P. J.; Weckhuysen, B. M.; Koningsberger, D. C. J. Catal. 2005, 234, 376. (8) Ji, Y.; Koot, V.; van der Eerden, A. M. J.; Weckhuysen, B. M.; Koningsberger, D. C.; Ramaker, D. E. J. Catal. 2007, 245, 413-425.

Stakheev et al. (9) Oudenhuijzen, M. K.; van Bokhoven, J. A.; Miller, J. T.; Ramaker, D. E.; Koningsberger, D. C. J. Amer. Chem. Soc. 2005, 127, 1530. (10) Yoldas, B. E. Am. Ceramic. Soc. Bull. 1995, 54, 286. (11) Scofield, J. H. J. Electron. Spectrosc. 1976, 9, 29. (12) Vaarkamp, M.; Mojet, B. L.; Kappers, M. J.; Miller; J. T.; Koningsberger, D. C. J. Phys. Chem. B 1995, 99, 633. (13) Vaarkamp, M.; Linders, J. C.; Koningsberger, D. C. Phys. B 1995, 208/209, 159. (14) van Dorssen, G. E.; Koningsberger, D. C.; Ramaker, D. E. J. Phys. Condens. Matter 2002, 14, 13529. (15) Koningsberger, D. C.; Mojet, B. L.; van Dorssen, G. E.; Ramaker, D. E. Top. Catal. 2000, 10, 143. (16) Vaarkamp, M.; Modica, F. S.; Miller, J. T.; Koningsberger, D. C. J. Catal. 1993, 144, 611. (17) Lytle, F. W.; Greegor, R. B.; Marques, E. C.; Biebesheimer, V. A.; Sandstrom, D. R.; Horsley, A.; Via, G. H.; and Sinfelt, J. H. ACS Symp. Ser. 1985, 288, 280. (18) Kubota, T.; Asakura, K.; Ichikuni N.; Iwasawa, Y. Chem. Phys. Lett. 1996, 256, 445. (19) Reifsnyder, S. N.; Otten, M. M.; Sayers, D. E.; Lamb, H. H. J. Phys. Chem. B 1997, 101, 4972. (20) Ramaker, D. E.; Mojet, B. L.; Garriga Oostenbrink, M. T.; Miller, J. T.; Koningsberger, D. C. Phys. Chem. Chem. Phys. 1999, 1, 2293. (21) Ramaker, D. E.; Koningsberger, D. C. Phys. ReV. Lett. 2002, 89, 139701. (22) Ankudinov, A. L.; Ravel B.; Rehr, J. J.; Conradson, S. D. Phys. ReV. B 1998, 58, 7565. (23) Delley, B.; Ellis, D. E.; Freeman, A. J.; Baerends, E. J.; Post, D. Phys. ReV. B 1983, 27, 2132. (24) Oudenhuijzen, M. K.; Bitter, J. H.; Koningsberger, D. C. J. Phys. Chem. B 2001, 105, 4616. (25) Koningsberger, D. C.; van Zon, F. B. M.; Vaarkamp, M.; Mun˜ozPaez, A. X-ray Absorption Fine Structure for Catalysts and Surfaces, edited by Iwasawa, Y., Tokyo, 1995, P257. (26) de Graaf, J.; van Dillen, A. J.; de Jong, K. P.; Koningsberger, D. C. J. Catal. 2001, 203, 307. (27) Vaarkamp, M.; Modica, F. S.; Miller, J. T.; Koningsberger, D. C. J. Catal. 1993, 144, 611. (28) Koningsberger, D. C.; Oudenhuijzen, M. K.; Ramaker, D. E.; Millet, J. T. Stud. Surf. Sci. Catal. 2000, 130A, 317. (29) Ramaker, D. E.; Teliska, M; Zhang, Y.; Stakheev, A. Y.; Koningsberger, D. C. Phys. Chem. Chem. Phys. 2003, 5, 4492. (30) Teliska, M.; O’Grady, W. E. O.; Ramaker, D. E. J. Phys. Chem. B 2004, 108, 2333. (31) Visser, T.; Nijhuis, T. A.; van der Eerden, A. M. J.; Jenken, K.; Bras, W.; Nikitenko, S.; Ikeda, Y.; Lepage, M.; Weckhuysen, B. M. J. Phys. Chem. B 2005, 109, 3822. (32) Stakheev, A. Y; Zhang, Y.; Ramaker, D. E.; Koningsberger, D. C. to be published.