Si (100) Surface: A Reflection

Oct 6, 2007 - Department of Inorganic, Physical and Materials Chemistry, and NIS “Center of Excellence”, University of Torino, Via P. Giuria 7, I-...
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J. Phys. Chem. C 2007, 111, 16437-16444

16437

Reactivity of Cr Species Grafted on SiO2/Si(100) Surface: A Reflection Extended X-ray Absorption Fine Structure Study down to the Submonolayer Regime G. Agostini,† E. Groppo,† S. Bordiga,† A. Zecchina,† C. Prestipino,‡ F. D’Acapito,§ E. van Kimmenade,| P. C. Thu1 ne,| J. W. Niemantsverdriet,| and C. Lamberti*,† Department of Inorganic, Physical and Materials Chemistry, and NIS “Center of Excellence”, UniVersity of Torino, Via P. Giuria 7, I-10125 Torino Italy, ESRF, BP220 F-38043 Grenoble, France, CNR-INFM-OGG c/o ESRF, Gilda CRG, 6 Rue Jules Horowits F-38043 Grenoble, France, and Schuit Institute of Catalysis, EindhoVen UniVersity of Technology, 5600 MB EindhoVen, The Netherlands ReceiVed: May 25, 2007; In Final Form: July 24, 2007

In situ X-ray absorption near-edge spectroscopy/extended X-ray absorption fine structure (XANES/EXAFS) experiments are conducted for the first time on a highly diluted Cr/SiO2/Si(100) system (2 Cr/nm2, representing a model of the Phillips catalyst for the ethylene polymerization) by exploiting the reflection EXAFS (ReflEXAFS) geometry. This experiment, aimed to give a contribution in bridging the gap between surface science and catalysis, demonstrates that it is possible to follow the reversible red-ox reactivity of surface species grafted on a single well-defined surface, at a concentration limit that is far below the monolayer coverage level and for a highly sensitive sample. A further improvement on the impurity level of the ReflEXAFS chamber is however required in order to be able to follow in situ the polymerization reaction. Our results demonstrate that the red-ox ability of the isolated surface Cr species is not enough to make a polymerization active species.

1. Introduction X-ray absorption spectroscopy, in both the EXAFS and XANES regions, being able to discriminate oxidation states, coordination states, and the local environment of a selected atomic species, is a useful technique to probe the reactivity of surface species.1-11 Most of the quoted examples refer to transmission or fluorescence experiments performed on samples where the reactive species were grafted (hosted) on a high surface area material (oxides, zeolites, active carbons, etc...). However, the development of beamlines ad hoc conceived to measure grazing angle geometry has allowed the surface science community to investigate the surface of single crystals and thin films. The high photon flux available on third generation synchrotron radiation sources has been able to push the surface detection limit down to the monolayer and submonolayer regime.12-18 In this regard, only a few examples are present where the same experiment combines the monolayer sensitivity of grazing angle geometry acquisitions with the control of the sample environment (temperature and reaction atmosphere) needed to perform chemical reactions.19-30 The Cr/SiO2 Phillips catalyst,31 although used in industrial plants since the 1960s for C2H4 polymerization, is still one of the most debated systems, concerning both the molecular structure of the active sites and the related initiation mechanism, for which a unifying picture is still missing.32-35 The main reasons why these two strictly connected questions are not properly addressed are the high intrinsic heterogeneity of the Cr sites formed at the surface of amorphous silica and the high Cr dilution (typically less than 1 wt % Cr). The second point is related to the formation of catalytically inactive Cr2O3 clusters * Corresponding author. Phone: +39011-6707841. Fax: +390116707855. E-mail: [email protected]. † University of Torino. ‡ ESRF. § CNR-INFM-OGG. | Eindhoven University of Technology.

at higher Cr loadings.11 Attempts to reduce the complexity of the catalyst surface have been continuously made over the last decades. In this respect, the annealing method of McDaniel and co-workers,32 further developed by Groppo et al.,35 represents a way to fine-tune the relative population of Cr(II) sites. The use of organometallic precursors has been proposed by the group of Scott.36,37 Recently, a method to completely remove the heterogeneity of the CrII/SiO2 system has been reported.38-40 With the use of the complex TAC (1,3,5-tribenzylhexahydro 1,3,5-triazine) ligand, as a surface-modifying agent, a single site Cr species was made, resulting in the formation of polyethylene with a very low polydispersity index. It has recently been found that the much simpler CH2Cl2 molecule acts as a surfacemodifying agent, having the dual function to selectively enhance the catalytic activity of a small fraction of Cr sites, while poisoning the remaining ones. This approach represented a much simpler method to reduce the Cr heterogeneity, increasing at the same time the catalytic performances of the system.41 An alternative approach to simplify the CrOx/SiO2 system is to replace the porous silica substrate by a flat silicon wafer with thermal oxide layers,42-49 hereafter SiO2/Si(100). While the preparation chemistry remains unaltered with respect to conventional Phillips catalysts, these model systems feature an almost atomically flat silicon oxide surface, which renders the active surface equally accessible for surface characterization and for the reactants during polymerization. This in turn allows one to correlate surface chemistry47 to intrinsic catalytic reactivity48 and polymerization kinetics.49 In this work we want to extend the XANES and EXAFS investigation to the SiO2/Si(100) model catalysts. The extremely low number of Cr atoms grafted on the support for a load of 2 Cr atom/nm2 implies that we are below the threshold of 1000500 ppm concentration commonly accepted to obtain reasonable quality EXAFS spectra. As a consequence, the choice of ReflEXAFS geometry has to be considered mandatory. In fact,

10.1021/jp074066t CCC: $37.00 © 2007 American Chemical Society Published on Web 10/06/2007

16438 J. Phys. Chem. C, Vol. 111, No. 44, 2007 the grazing incidence geometry of the X-ray beam on the sample (few mrad), implies that the beam does not penetrate inside the bulk and thus that the only atoms excited are those lying a few nm below the surface. This fact implies a reduction of many orders of magnitude of the background photons, both those due to the bulk fluorescence and those due to the elastic scattering of the primary beam (positioning the fluorescence detector on top of the surface). Moreover, also the photons coming from the Cr fluorescence, representing the EXAFS signal, can be maximized by the adopted grazing incidence geometry that makes it possible to illuminate a very long sample (up to 8 cm), thus increasing the total number of surface atoms effectively probed. Summarizing, the ReflEXAFS mode allows the S/N ratio of the X-ray fluorescence signal emitted by surface atoms to increase by several orders of magnitude.50,51 These challenging requirements can be fulfilled only on an ad hoc conceived reaction chamber mounted on a third generation synchrotron radiation source like the ReflEXAFS chamber available at the GILDA BM8 beamline at the ESRF.50 2. Experimental Section 2.1 Sample Preparation, in Situ Activation, and Reduction and Oxidation Cycles. CrOx/SiO2/Si(100) samples with a Cr loading of two Cr atoms each 100 Å2 were prepared by spincoating impregnation as described elsewhere.42 Because of the extreme sensitivity to poisoning shown by the atomically dispersed Cr atoms, the ReflEXAFS chamber has been subjected to a rigorous cleaning procedure. For this reason, before introducing the CrOx/SiO2/Si(100) samples, it was first baked at 70 °C, with the sample holder heated at 800 °C in a dynamical vacuum. O2 was then dosed, and an equilibrium pressure of 20 mbar was maintained for 5 min, before evacuation down to 10-5 mbar. This procedure has been repeated three times. Finally, the ReflEXAFS chamber was baked at 70 °C overnight with the sample holder at 150 °C in a dynamical vacuum of 10-5 mbar. Cr(VI)/SiO2/Si(100) samples have been obtained in situ in the ReflEXAFS chamber by dosing O2 at a particular temperature on the CrOx/SiO2/Si(100) precursor, thus simulating the well-known procedure usually adopted in the case of the powdered Cr/SiO2 system.32,34,52-56 Two different oxidation temperatures (450 and 550 °C) and three different O2 equilibrium pressures (10, 100, and 200 mbar) have been investigated, see Table 1. The adopted calcination temperatures have been kept lower than those usually adopted for the powdered Phillips catalyst32,34,52-56 in order to avoid a significant Cr desorption from the surface, which has been observed when the calcinations treatment is performed at higher temperatures (see Figure 3 of ref 48 for the 2 Cr atoms/nm2 sample). According to the literature,32,34,52-56 Cr(II)/SiO2/Si(100) samples have been obtained in situ in the ReflEXAFS chamber, from Cr(VI)/SiO2/ Si(100) samples by reducing the oxidized samples in CO at 350 °C. In this case, two different CO equilibrium pressures (10 and 40 mbar) have been probed, see Table 1. Two wafers of CrOx/SiO2/Si(100) samples, prepared in the same way (see above), hereafter labeled as A and B, have been subjected to successive oxidation/reduction cycles, according to the temporal order listed in Table 1 (time evolution from top to bottom). As reference compounds for isolated Cr(VI) and Cr(II) species, we have adopted the standard powdered Cr/SiO2 Phillips catalyst34 (4 Cr wt % on a 400 m2/g silica support, which is equivalent to a nominal chromium density of 1.2 Cr/nm2) after calcination in O2 or reduction in CO, hereafter Cr(VI)/SiO2 and

Agostini et al. TABLE 1: Summary of the Treatment and Measurement Conditionsa treatment conditions sample name A Aox450LP Ared350LP Aox550LP Aox550RT Aox550 Ared350 Box550 Bred350 Bred550 Box550bis BredC2H4

gas

measurements conditions

no. of P (mbar) T (°C) gas P (mbar) T (°C) spectra

O2 CO O2 O2

10 10 10 200

450 350 550 550

CO O2 CO CO O2 C2H4

10 100 40 40 100 500

350 550 350 550 550 160

O2 CO O2 O2 O2 CO O2 CO CO O2

10-5 10 10 10 200 200 10 100 40 40 100 10-5

30 350 350 350 30 350 350 350 350 350 350 160

1 4 3 4 2 1 2 2 2 2 1 3

a Two starting grafted CrOx/SiO2/Si(100) samples have been used (A and B), that have been subjected to successive oxidation-reduction cycles, according to the temporal order listed below (time evolution from top to bottom). O2/CO have been used as oxidant/reducing agents. On B samples, C2H4 reduction has also been performed.

Cr(II)/SiO2. R-Cr2O3, where chromium is in the +3 oxidation state, has been used as a model for clustered chromium. The spectra of these samples have been acquired in the transmission mode on the same beamline as described elsewhere.11 Because of the high Cr loading for a powdered system, a fraction of clustered Cr species is present in the Cr(II)/SiO2 sample (38 ( 4%).11 Note that higher Cr loading (up to 2 Cr/nm2) can be reached on the CrOx/SiO2/Si(100) samples without the appearance of clustered species.47 This important difference is due to a better dispersion of the Cr precursor during the impregnation step performed on a single flat surface with the spin-coating method. 2.2. ReflEXAFS Acquisition. In situ X-ray absorption spectra have been acquired in the ReflEXAFS mode, using a 13 element germanium detector to collect the Cr fluorescence, in the ReflEXAFS chamber50 of the GILDA BM8 beamline at the ESRF, equipped with a Si(111) double crystal monochromator operating in the dynamic sagittal focusing mode.57 The sampling steps are 5.0 and 0.5 eV in the preedge and edge regions, respectively, while the EXAFS region has been sampled up to 6550 eV with a variable energy step resulting in ∆kmax < 0.05 Å-1. A constant integration time of 15 s/points was used. Acquisition was made with a incident angle φ of the X-ray photons on the Si(100) surface of about 2.3 mrad. To minimize the risk of poisoning, most out of the acquisitions have been performed in the oxidizing or reducing atmosphere and at 350 °C (see Table 1). A variable number n of spectra (ranging from 1 to 4, see Table 1) has been collected in the same experimental condition to increase the statistics. We will call a “run” this single acquisition; thus, for each sample we will have run 1, run 2, run n. For each run, we collect 13 fluorescence spectra, which makes for each sample a total number of 13n spectra. The raw data have been treated according to the complex procedure described below. (1) For each run, each single spectrum has been checked and those containing Bragg peaks from the Si(100) substrate have been corrected (when the Bragg peak influenced less than three experimental points) or eliminated (otherwise). To appreciate the S/N ratio of a single spectrum see top curves in Figure 1. (2) The selected spectra of each run are then averaged (middle curves in Figure 1). (3) A preliminary normalized XANES spectrum and a preliminary χ(k) extraction will be obtained for each run. (4) Comparison among the XANES spectra and the χ(k) functions obtained in the different

Reactivity of Cr Species Grafted on SiO2/Si(100)

Figure 1. Improvement of the S/N ratio of the ReflEXAFS spectra upon increasing the statistics for sample Aox450LP. From top to bottom, single element spectrum; single run spectrum (averaged over 13 elements); final spectrum (averaged over 4 runs). Part a is the whole spectrum, and part b is the XANES region.

runs is made to check the possible, undesired, sample evolution along the measurement. Note that a single run is almost 2 h long, making the sample poisoning possible. Moreover, being the geometrical conditions very critical, the incident angle φ can vary only in a very limited range (few mrad) to allow a correct ReflEXAFS collection. Consequently, any beam instability can have relevant effects on the quality of the spectra. In the set of measured samples (see Table 1), no time evolution of the Cr signal has been observed. Conversely, one spectrum of the Aox450LP, Ared350LP, Aox550LP, and Aox550RT samples has been removed as the physical conditions of the acquisition were evidently altered after the electron injection in the storage ring. (5) Once check 4 has been done, the spectra corresponding to the accepted runs have been further averaged (see bottom curves in Figure 1). Figure 1 shows, as an example, the increase of the S/N ratio for the Aox450LP sample obtained by moving from a single element spectrum (top curve), through a single run spectrum (middle curve), and to the final spectrum (bottom curve). XANES spectra have been normalized to the edge jump according to the classical procedure.58,59 The EXAFS χ(k) functions have been extracted with the Athena code,60 using the Rbkg parameter in the 1.0-1.1 Å range for the different samples. χ(k) functions were then Fourier transformed in the 2.5-10.0 Å-1 range with a k2-weight and a Hanning apodization window. 3. Results and Discussion 3.1. O2/CO Oxidation/Reduction Cycles Followed by XANES. XANES spectra of sample A after different oxidationreduction cycles are reported in Figure 2a. To understand these spectra, a comparison with Cr(VI)/SiO2, Cr(II)/SiO2, and R-Cr2O3 model compounds (see Experimental Section) is needed, see Figure 2b. The XANES spectrum of the film oxidized at 450 °C (sample Aox450LP), full gray line in Figure 2a, is characterized by a preedge peak at 5993.5 eV, with an intensity (I) of 0.39 in normalized units, and a full width at half-maximum (FWHM) of 2.8 eV. This feature is the fingerprint of Cr(VI) in Td-like local geometry,1,11,34,40,61-66 where the lack of inversion center makes the A1 f E electronic transition Laporte allowed, as happens also with other d0 systems like Ti(IV).67-69 After the edge, we observe a white line at 6011.4 eV (I ) 1.16), with an evident low energy shoulder a few eV below. The maximum

J. Phys. Chem. C, Vol. 111, No. 44, 2007 16439 of the first EXAFS oscillation is observed at 6025 eV. When comparison is made with the standard Cr(VI)/SiO2, see full gray line in Figure 2b, it is clear that in the film the preedge feature occurs at the same energy but with a lower intensity and broader character (I ) 0.84 and FWHM ) 2.0 eV, for the model system) and that the match does not hold in the postedge region, where the Cr(VI)/SiO2 sample exhibits an almost featureless behavior. The postedge features described for the Aox450LP sample are very similar to those observed for R-Cr2O3 (see dotted line in Figure 2b), exhibiting a double white line at 6007.3 eV (I ) 1.42) and 6011.3 eV (I ) 1.54) and the maximum of the first EXAFS oscillation at 6025 eV. These considerations lead to the straightforward conclusion that on the Aox450LP sample, together with isolated Cr(VI) ions in Td-like local geometry, aggregated Cr atoms forming an oxidic phase very close to that of R-Cr2O3 are present. Interaction with CO at 350 °C (sample Ared350LP, full black line spectrum in Figure 2a), results in the following spectral evolutions: (i) a significant red shift of the absorption edge of 2.5 eV (arbitrarily evaluated at I ) 0.75); (ii) the removal of the preedge peak due to Cr(VI) species in Td-like local geometry; (iii) an increase of the white line intensity (I ) 1.32), accompanied by a red shift of 1.0 eV; (iv) an important modification of the first EXAFS oscillation. These modifications suggest that the fraction of isolated Cr(VI) species is reduced by CO to isolated Cr(II) sites (see Scheme 1), with the fraction of clustered Cr(III) almost unaffected. This interpretation is supported by the smaller edge shift with respect to what is observed on the powdered sample (2.5 vs 6.5 eV), compare parts a and b of Figure 2. In a more detailed discussion of the XANES features, starting from the low-energy side, the spectrum of sample Ared350LP is characterized by a weak and featureless absorption in the 5989-5995 eV range. These features are assigned to the dipole forbidden transitions of Cr(III) species in an octahedral-like geometry of the oxide phase. These transitions result in two weak, but well defined, bands at 5990.5 and 5993.6 eV in crystalline R-Cr2O3 (black dotted curve in Figure 2b). The disappearance of the strong absorption of Cr(VI) ions in Tdlike local geometry makes these weak features visible in the reduced sample. The loss of resolution with respect to the model R-Cr2O3 may indicate the amorphous nature of the aggregated oxidic phase. The white line intensity, feature, and position agree well with the copresence of isolated Cr(II) and clustered Cr(III) species, see full black line and dotted curves in Figure 2b, for the Cr(II)/SiO2 and R-Cr2O3 models, respectively. Successive oxidation (sample Aox550LP) and reduction (sample Ared350) steps, see dashed gray and black curves in Figure 2a, respectively, prove the reversibility of the red-ox process on the fraction of isolated Cr species, according to the loop shown in Scheme 1. These results indicate that once Cr species are clustered they are no longer involved in the redox mechanism. Once a fraction of a cluster is formed, the remaining isolated Cr sites are no longer poisoned. The irreversible formation of chromium oxide clusters, in the form of R-Cr2O3, is a well-known phenomenon occurring on the powdered Cr/SiO2 catalyst for Cr loadings higher than 1 wt % and/or in presence of water poisoning.11 However, the redox behavior observed inside the ReflEXAFS chamber differs significantly from earlier observations based on calcination experiments performed inside a dedicated microreactor attached to a UHV preparation chamber.42,44,47 Under extremely dry conditions in a flowing gas, we observed the exclusive formation of silica bound monochromates with a maximum chromium

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Figure 2. (a) Normalized XANES spectra of sample A after successive red-ox cycles; see Table 1 for details. (b) Normalized XANES spectra, collected in the transmission mode, of the standard Cr/SiO2 Phillips catalyst in oxidized (full gray curve) and reduced (full black curve) states and of R-Cr2O3 model compound (dotted black curve).

SCHEME 1

loading depending on the calcination temperature. Superficial chromium desorbes from the flat silica surface at calcination temperatures above 450 °C within 30 min. Only under oxygen deficiency we observe Cr(III) clusters with XPS. Since in the present case we are dealing with a very diluted sample, the presence of a fraction of R-Cr2O3 suggests that the adopted experimental conditions are not good enough to guarantee a perfect waterless activation of the sample. Under the adopted reaction conditions, the impurities (e.g., water) desorbing from the walls of the ReflEXAFS chamber can accumulate in the sample atmosphere. This might explain the irreversible formation of Cr2O3 consuming a fraction of the supported chromium. Such deactivation would occur during the first heating cycle only. The discussion made so far, based on a simple qualitative comparison of the XANES features, allowed us to observe the presence of a clustered phase and of isolated sites, without being able to determine quantitatively their relative fraction. In principle, when two or more phases are present in a sample, a quantitative determination of their relative fractions is possible by fitting the sample spectrum with a linear combination of the XANES spectra of each pure phase.70-72 This method is reliable when the pure phases are representative of the species present on the sample. Unfortunately, this is not the case for both the Cr(VI)/SiO2 and Cr(II)/SiO2 samples. In particular, for Cr(VI)/ SiO2, the intensity and FWHM of the preedge peak at 5993.5 eV is strongly influenced by the Td local geometry of the Cr(VI) species which, in turn, depends upon the SiO2 surface

strain, i.e., upon the degree of dehydroxylation.34 As a consequence, Cr(VI)/SiO2 samples obtained in different activation conditions (O2 pressure, calcination temperature, etc.) can show different XANES features in the preedge region. This is demonstrated by comparing the spectra of Aox450LP and Aox550LP samples (full and dashed gray lines in Figure 2a), showing that a calcination at higher temperature causes a small increase in the intensity of the preedge peak. It is clear that the local structure of the Cr(VI) precursors influences the geometry of the Cr(II) sites resulting from CO reduction. This means that also the XANES spectrum of the Cr(II)/SiO2 sample cannot be used for a quantitative evaluation of the fraction of the Cr(II) isolated sites present on the film. Worthy to note is the absence in the Ared350LP spectrum of the component at 5996 eV (low energy shoulder of the edge) attributed to the fraction of highly uncoordinated Cr(II) sites able to polymerize ethylene on the Cr(II)/SiO2 sample.11,73 3.2. O2/CO Oxidation/Reduction Cycles Followed by EXAFS. The conclusions reached by XANES spectroscopy are supported by the EXAFS data. The quality of the EXAFS data can be appreciated in Figure 3, where the k2χ(k) functions of an oxidized sample and a CO-reduced sample are reported (gray and black full curves). Beside the low S/N ratio of the experimental data, the spectrum of the oxidized phase shows an evident beat at around k ) 4 Å-1, reflecting the copresence of two significantly different chromium oxygen first shell bonds: CrdO and Cr-O contributions, see Scheme 1. A second beat at lower wavenumbers (k ≈ 1.5 Å-1) reflects the presence of higher shell contributions. Reduction in CO significantly modifies the EXAFS spectrum (full black curve in Figure 3), that now possesses, for k > 2.5 Å-1, an almost unique frequency, demonstrating that the heterogeneity of first shell chromium oxygen distances is much smaller and no more appreciable with the small k interval and the high S/N ratio of the acquired data. Conversely, the beat at lower k is still present, testifying that the species responsible for the higher shell contributions are not affected by the treatment in CO. The effects of CO reduction are more evident in Figure 4a,b reporting the k2-weighted, phase uncorrected, FT of the EXAFS function (modulus and imaginary parts, respectively) of sample

Reactivity of Cr Species Grafted on SiO2/Si(100)

Figure 3. k2-Weighted EXAFS functions of Aox450LP, Ared350 LP, and BredC2H4 samples, see Table 1. Vertical lines evidence the k region used to perform the FT reported in Figure 4a,b and Figure 6.

Figure 4. Parts a and b as Figure 2a, for the k2-weighted, phase uncorrected FT of the EXAFS function: modulus and imaginary part, respectively. Parts c and d as Figure 2b, for the k2-weighted, phase uncorrected FT of the EXAFS function: modulus and imaginary part, respectively. The vertical lines allow one to compare the position of the maxima of the imaginary part of the FT of the films with those of the models: Cr(VI)/SiO2 (full line, light gray) and R-Cr2O3 (dotted black line). The quantitative values of both maxima and minima are reported in Table 2.

A after the same oxidation/reduction treatments described for the XANES curves reported in Figure 2 (see Table 1). In the following, all reported distances are phase uncorrected and thus shorter than the actual ones. From a simple observation of the curves, it emerges that the features of the FT of the EXAFS data are completely different for the oxidized (gray curves) and

J. Phys. Chem. C, Vol. 111, No. 44, 2007 16441 the reduced (black curves) forms of the sample in the 0-2 Å region, while for R > 2 Å the same features are observed independently from the sample treatment. These observations support the conclusions reached from the XANES study, that is, the presence of an unreactive aggregated phase of Cr2O3 (dominating the FT spectra for R > 2 Å) not involved in the red-ox mechanism, together with a reactive phase (dominating the FT spectra for R < 2 Å), able to undergo the red-ox cycle as described in Scheme 1. In more detail, the |FT| spectra of the oxidized samples exhibit a first shell contribution of low intensity with two maxima at around 0.95 and 1.6 Å and a complex higher shell contribution with a maximum around 2.5 Å (see Figure 4a, gray curves). The higher shell contribution is located where the same signal appears for R-Cr2O3 model (see dotted curve in Figure 4c). Conversely, the |FT| of the Cr(VI)/SiO2 model is totally different, showing a single intense maximum at a distance (1.06 Å) intermediate to the two peaks observed in the |FT| of the oxidized films. In order to attribute in a straightforward manner the contributions observed on the oxidized CrOx/SiO2/Si(100) films, a careful investigation of the imaginary parts is mandatory. The direct correspondence of the maxima and of the minima with those observed for the R-Cr2O3 model in the 1.8-3.2 Å range (Table 2 and vertical lines in Figure 4b,d) allows us to conclude that a fraction of aggregated Cr(III) species is present on the films. The phase responsible for the low R signal is ascribed to isolated Cr(VI) species grafted to the surface of the SiO2/Si(100) films, owing to the correspondence with the features of the imaginary parts observed on Cr(VI)/SiO2 model, compare Figure 4b and Figure 4d and see Table 2: first minimum around 0.68-0.69 Å, followed by a maximum at 0.94-0.95 Å, and a second minimum at 1.17-1.18 Å. This means that the isolated Cr species in samples Aox450LP and Aox550LP are Cr(VI) species in the form of monochromates, characterized by two single Cr-O bonds with the silica surface and by two shorter CrdO bonds (see top part of Scheme 1). The much lower intensity of the |FT| of the oxidized films, when compared to that of the Cr(VI)/SiO2 model, and the splitting of the |FT| into two maxima, is due to a destructive interference between the imaginary parts of the isolated Cr(VI) phase and the clustered R-Cr2O3, that are totally out of phase in the 0.851.75 Å region (compare full gray and dotted black curves in Figure 4d). Note that several cases, where two different contributions are out of phase in the EXAFS signal can be found in the literature.74-76 The |FT| of the reduced samples exhibits two contributions: the first one around 1.49 Å and the second one around 2.5 Å (see Figure 4a black curves). The complete disappearance of the low R peak (see gray curves in the same Figure) proves that the oxygen atoms contributing to the short CrdO distances have been removed in the CO reduction step (see Scheme 1). This results in an important increase of the |FT| in the first shell region (peak at 1.49 Å), not due to an increase of the number of atoms in the first coordination shell of Cr (number that actually decreases, see Scheme 1), but to the removal of the destructive interference between the CrdO signal of the isolated phase and the Cr-O one of the clustered R-Cr2O3 phase. As the |FT| of the Cr(II)/SiO2 and R-Cr2O3 models show a maximum at 1.50 and 1.52 Å, respectively (see black curves in Figure 4c), and as the imaginary part of the reduced sample and of these two models exhibit a first maximum around 1.291.32 Å, followed by a minimum at 1.57-1.59 Å (see Figure 4b and Figure 4d), we conclude that the experimental EXAFS data on the reduced form of the film are compatible with the

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TABLE 2: Positions of the Relevant Minima and Maxima of the Imaginary Part of the k2-Weighted, Phase Uncorrected, FT Reported in Figure 4a CrdO first shell

higher shells of the R-Cr2O3 phase

Cr-O first shell

shell feature

min/Å

max/Å

min/Å

max/Å

min/Å

max/Å

min/Å

max/Å

min/Å

max/Å

min/Å

max/Å

Aox450LP Ared350LP BredC2H4 Cr(VI)/SiO2 Cr(II)/SiO2 R-Cr2O3

0.68

0.95

1.18

1.34 1.31 1.32

1.81 1.82 1.83

2.03 1.97 2.01

2.25 2.22 2.21

2.48 2.54 2.50

2.73 2.77 2.74

2.94 2.97 2.95

3.12 3.16

0.69

0.94

1.17

1.56 1.57 1.58 1.47 1.59 1.58

1.84 1.82

2.04 2.02

2.29 2.23

2.56 2.48

2.79 2.68

2.99 2.90

1.29 1.32

3.08

a

Owing to the reproducibility of the red-ox cycles, the differences among different CO-reduced or oxidized films (see Table 1) are irrelevant. Consequently, only samples Aox450LP and Ared350LP have been reported here as a representation for the two systems. As discussed elsewhere,11 also the reduced Phillips catalysts, samples Cr(II)/SiO2, contain a fraction of R-Cr2O3.

Figure 5. (a) Normalized XANES spectra of sample B after successive oxidation (full gray line) and reduction (full black line) treatments and after interaction of the oxidized phase with C2H4 at 160 °C (dashed black curve), see Table 1 for details, and normalized XANES spectrum of Cr(VI)/SiO2/Si(100) films after polymerization and measured ex situ in the ReflEXAFS chamber (dotted black curve). (b) Normalized XANES spectra, collected in the transmission mode, of the standard Cr/SiO2 Phillips catalyst in oxidized (full gray curve) and reduced (full black curve) states and after polymerization obtained by contacting the Cr(VI)/SiO2 system with C2H4 at 250 °C (dotted black curve).

copresence of two phases, i.e., isolated Cr(II) and an almost unreactive clustered Cr2O3. The presence of the Cr2O3 phase is supported by the correspondence of the positions of the maxima and minima of the imaginary part in the 1.8-3.2 Å range (Table 2 and vertical lines in Figure 4b,d). A slight increase of the clustered fraction is observed after the reduction step (compare gray and black curves in Figure 4a,b), a phenomenon already observed in a highly Cr loaded (4 wt %) Cr/SiO2 Phillips system,11 here considered in first approximation as a model for isolated Cr. The fraction of clustered Cr on that sample is well visible in the imaginary part of its FT (full black curve in Figure 4d). 3.3. C2H4 Reduction Followed by XANES and EXAFS. Sample B has been subjected to a similar red-ox cycling, although less reiterated than for sample A (see Table 1). In both XANES (Figure 5a) and EXAFS (Figures 3 and 6 in k and R spaces, respectively) regions, the reduced (full black curves) and oxidized (gray curves) versions of sample B exhibit the same features already observed and discussed for sample A (see Figures 2-4). The only minor difference consists of a slightly minor fraction of the aggregated phase, qualitatively appreciable from the intensity of the preedge feature in sample Box550 (I ) 0.55). The difference with sample A can be explained by considering that a higher temperature (550 vs 450 °C) and a higher O2 equilibrium pressure (100 vs 10 mbar) have been

Figure 6. As in Figure 4a, the k2-weighted, phase uncorrected FT of the EXAFS function: modulus and imaginary parts, parts a and b, respectively.

adopted in the first oxidation treatment (see Table 1). Also a better cleaning of the ReflEXAFS chamber, reached for the second loaded sample, can have played a positive role. Once it has been demonstrated that the isolated Cr species grafted on the surface of the SiO2/Si(100) films are able to react reversibly with O2 and CO according to the loop shown in Scheme 1, it has been verified, in situ in the ReflEXAFS chamber, whether isolated Cr(VI) species are able to polymerize ethylene or not. With this aim, the oxidized Box550bis sample has been subjected to interaction with ethylene (500 mbar) at 160 °C. The resulting XANES spectrum (dashed line in Figure 5a) is almost indistinguishable from the XANES spectra of the CO reduced Cr(VI)/SiO2/Si(100) films (full line in Figure 5a), demonstrating that the treatment in ethylene has reduced the isolated Cr(VI) species. However, it is completely different from what is expected in the case of a successful polymerization.11,34 In fact, it has been already shown that, under similar conditions, ethylene polymerization strongly perturbs the XANES features of the oxidized Phillips catalyst, resulting in (i) the complete depletion of the preedge peak characteristic of Cr(VI) in a Td-like geometry; (ii) a shift of the edge toward lower energy values; and (iii) an impressive increase of the white line intensity (compare full gray and dotted black curves in Figure 5b), reflecting a high increase of the average chromium coordination.11,34 A similar XANES spectrum is obtained ex situ in the ReflEXAFS mode also on the Cr(VI)/SiO2/Si(100) film when the polymerization is performed in the prechamber of the XPS instrument in Eindhoven (see dotted black curve in Figure 5a). The FT of the EXAFS function, in both the modulus (Figure 6a) and imaginary part (Figure 6b), indicates that interaction with ethylene at 160 °C causes a strong variation of the Cr local

Reactivity of Cr Species Grafted on SiO2/Si(100) environment (compare gray and dashed curves). In particular, the total depletion of the CrdO component at 0.97 Å confirms that the treatment in ethylene reduces the isolated Cr(VI) species. The resulting reduced Cr species exhibits a local environment virtually indistinguishable from that obtained after CO reduction (compare full and dashed black curves in Figure 6). In fact, it has been demonstrated that, when polymerization occurs, the first shell component increases by about 60%, due to the constructive interference of Cr-O (of the support) and Cr-C (of the polymer) bonds.11,34 The absence of such an expected increase in the EXAFS spectra reported in Figure 6a implies that no significant polymerization occurs on the BredC2H4 sample, thus confirming what is observed in the XANES spectra (see Figure 5). These results imply that the ability of surface Cr species to undergo a red-ox cycle is not sufficient to guarantee the presence of sites active in ethylene polymerization. Moreover, the presence/absence of the preedge feature around 5996 eV (low energy shoulder of the edge) in the XANES spectra is diagnostic in discriminating between active and inactive isolated Cr(II) sites, in agreement with a recent combined in situ IR and XANES study.11,73 4. Conclusions The present work describes the red-ox behavior of a highly diluted Cr/SiO2/Si(100) system, which has been shown to behave as a model of the Phillips catalyst for ethylene polymerization characterized by a lower heterogeneity of the Cr active sites.42 Such a system represents a challenge for in situ XAFS experiments aimed to bridge the gap between surface science and catalysis. In this contribution, it has been demonstrated that, operating in the ReflEXAFS mode near the critical angle, it is possible to suppress by several orders of magnitude the background due to the fluorescence of the bulk and to the elastically scattered photons. This allowed us to collect a measurable fluorescence XAFS signal from 2 Cr/nm2 distributed on a flat SiO2/Si(100) surface. Operating under the cleanest condition of the ReflEXAFS chamber (10-5 mbar), we were unable to avoid the clusterization of a fraction of Cr atoms. However, the complementary fraction of isolated Cr atoms has been able to undergo, in a reversible way, several O2/CO oxidation-reduction loops. This observation implies that, using the high photon flux available at a third generation synchrotron radiation source like the ESRF, it is possible to follow the red-ox reactivity of surface species grafted on a single well-defined surface at a concentration limit that is far below the monolayer coverage level. The Cr(VI) oxidized phase of the sample was successfully reduced to Cr(II) also using ethylene, even if no polymerization was observed. This means that the red-ox ability of surface Cr species grafted on silica is a condition necessary but not sufficient to make the isolated Cr sites active in the ethylene polymerization reaction. Finally, XANES spectroscopy can be diagnostic in discriminating between active and inactive isolated Cr(II) sites owing to the presence/absence of the preedge feature around 5996 eV (low energy shoulder of the edge). The present result supports the attribution previously made on the basis of a combined in situ IR and XANES study.11,73 An improvement of the impurity level of the ReflEXAFS chamber will strongly increase the potentiality of such a facility in the field of in situ surface reactivity investigation. Acknowledgment. This experiment has been performed within the ESRF public beamtime (Experiment CH-1542). We

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