Elena Groppo | University of torino (original) (raw)

Papers by Elena Groppo

Journal of Physical Chemistry C, 2007

The vibrational properties of adsorbed carbonyls on heterogeneous Pd/oxide systems (SiO 2 -Al 2 O... more The vibrational properties of adsorbed carbonyls on heterogeneous Pd/oxide systems (SiO 2 -Al 2 O 3 , Al 2 O 3 , and MgO) are compared and discussed. On the basis of the surface science literature, the complex IR spectra of carbonyls formed on Pd particles are interpreted in terms of what occurs on well-defined Pd and Pd(100) faces and on edge/corner defects, typical of metal nanoparticles, thus bridging the gap between single crystals and high surface area systems. Furthermore, the influence of the electronic properties of the support (i.e., surface acidity/basicity) in determining the spectral features of carbonyls formed on Pd particles is clearly

Journal of Physical Chemistry B, 2005

This work reports the first complete FTIR characterization of H 2 , N 2 and C 2 H 4 molecular com... more This work reports the first complete FTIR characterization of H 2 , N 2 and C 2 H 4 molecular complexes formed on the Cr(II) sites in the Phillips catalyst. The use of a silica aerogel as support for Cr(II) sites, substituting the conventional aerosil material, allowed us to obtain a remarkable increase in the signal-to-noise ratio of the IR spectra of adsorbed species. The improvement is directly related to an increase of the surface area of the support (∼700 m 2 g -1 ) and to an almost complete absence of scattering [Groppo et al., Chem. Mater. 2005, 17, 2019-2027. The use of this support and the adoption of suitable experimental conditions results, for the first time, in the clear observation of H 2 and N 2 adducts formed on two different types of Cr(II) sites, thus yielding important information on the coordinative state of the Cr(II) ions, which well agrees with the evidences provided in the past by other probe molecules. Furthermore, we report the first complete characterization of the C 2 H 4 π-complexes formed on Cr(II) sites. These results are particularly important in the view of the understanding of the polymerization mechanism, since the C 2 H 4 coordination and the formation of π-bonded complexes are the first steps of the reaction.

Chemistry of Materials, 2005

We report the first Raman spectra of molecular adducts (CO and N 2 ) formed on Cr(II) sites dispe... more We report the first Raman spectra of molecular adducts (CO and N 2 ) formed on Cr(II) sites dispersed on silica surface (Phillips catalyst for ethylene polymerization), thus obtaining indirect information about the metal anchored species. These results have been achieved using an ad hoc selected laser line (able to excite a ligand to Cr charge-transfer transition that does not relax in a radiative channel), and adopting as a support a silica aerogel behaving as an optically uniform medium in the region of work. These two combined strategies, never simultaneously applied before, allowed us to obtain great quality Raman spectra of surface species, demonstrating that Raman spectroscopy can have great sensitivity toward surface species present in small concentration. Raman spectroscopy thus becomes a powerful tool complementary to IR and UV-vis DRS spectroscopies for surface investigation. The improvement can be quantified by comparing published spectra of the oxidized Cr(VI)/SiO 2 system (the only available in the literature) with the more intense and much richer one obtained under the present experimental conditions, where new spectroscopic features (never observed before) are clearly observed.

Physical Chemistry Chemical Physics, 2007

E r l a n g e n -Nü r n b e r g ) , a n d Ad r i a n o Z e c c h i n a ( Un i v e r s i t y o f T... more E r l a n g e n -Nü r n b e r g ) , a n d Ad r i a n o Z e c c h i n a ( Un i v e r s i t y o f T o r i n o ) , a n d p u b l i s h e d i n s e l e c t e d 2 0 0 7 p r i n t i s s u e s o f P CCP , a l l p a p e r s a r e c o l l e c t e d o n l i n e o n a d e d i c a t e d we b s i t e : www. r s c . o r g / p c c p / a l t f u e l V i s i t t h e we b s i t e f o r b o t h c u t t i n g e d g e r e s e a r c h p a p e r s a n d a u t h o r i t a t i v e r e v i e w a r t i c l e s b y l e a d e r s i n a r a n g e o f f i e l d s o f c r i t i c a l i mp o r t a n c e t o t h e wo r l d t o d a y

Studies in Surface Science and Catalysis, 2010

The preparation by a deposition-precipitation method (using Na 2 PdCl 4 as palladium precursor an... more The preparation by a deposition-precipitation method (using Na 2 PdCl 4 as palladium precursor and Na 2 CO 3 as basic agent) of Pd catalysts supported on γ-Al 2 O 3 and on two different types of activated carbons has been studied by EXAFS and TPR as a function of Pd loading from 0.5 to 5.0 wt%. Independently from the support, neither reduced metal fraction nor crystalline phase has been observed. EXAFS shows that the Pd local environment of the final catalysts changes slightly as a function of the Pd loading from 0.5 to 2.0 wt%: at higher loadings no further modification has been observed.

Studies in Surface Science and Catalysis, 2010

The changes in the support morphology, at nano-and micro-scale level, have been investigated alon... more The changes in the support morphology, at nano-and micro-scale level, have been investigated along the two-step processes involved in the preparation of a Pd supported on SiO 2 -Al 2 O 3 (SA) catalyst: Pd deposition and K 2 CO 3 doping. During the latter step part of the support dissolves and re-precipitates (partially outside the pores) covering the Pd particles. XRPD shows a significant rearrangement of the support at the long range order scale, while 29 Si and 27 Al solid-state (SS) NMR indicate that the average local environment around both Si and Al atoms remains unaltered.

Topics in Organometallic Chemistry, 2005

A relevant fraction of the polyethylene produced in the world (about 30%) is obtained with the Ph... more A relevant fraction of the polyethylene produced in the world (about 30%) is obtained with the Phillips process. Many efforts in the last 30 years have been devoted to establish the valence state and the structure of the catalytically active species formed by reduction with ethylene. However, no certain conclusions have been obtained so far, even using a CO-prereduced simplified system. In this

The Journal of Physical Chemistry A, 2009

In this article, we present a computational study of the structure and vibrational properties of ... more In this article, we present a computational study of the structure and vibrational properties of the species formed by the interaction between Cr sites of Phillips catalyst and probe molecules (CO, N(2)). The vibrational properties of these surface species, intensively investigated in the past, form a very rich and ideal set of experimental data to test computational approaches. By adopting the X(4)Si(2)O(3)Cr (X = H, OH, F) cluster as a simplified model of the ([triple bond]SiO)(2)Cr(II) species present at the surface of the real catalyst, we found that the B3LYP hybrid functional (containing 20% of Hartree-Fock exchange), when applied to this model, is unable to reproduce with reasonable accuracy the currently available experimental data (principally coming from IR spectroscopy). Better agreement is obtained when the percentage of Hartree-Fock exchange is increased (up to 35-40%).

The Journal of Physical Chemistry C, 2008

A systematic in situ FTIR investigation of the interaction of several molecules characterized by ... more A systematic in situ FTIR investigation of the interaction of several molecules characterized by a different electron accepting ability (or acidity) with the basic pyridine units contained in a commercial poly(4vinylpyridine) system characterized by a high surface area is here reported, with the final goal to fully understand the role of basicity in the catalytic processes. This approach, for the first time applied to polymeric materials, has only some similarities with the method usually adopted in the investigation of catalytic acid sites hosted inside oxidic porous materials (such as zeolites), where the probes are molecules characterized by an increased basicity. The experimental results are confirmed by ab initio theoretical calculations conducted on the 4-methylpyridine, which is the simplest molecule resembling the pyridine moiety in the P4VP system. . Time evolution of the FTIR spectra of the P4VP/NO2 system. Bold black spectrum: pure P4VP (outgassed at room temperature for 1 h); light-gray spectra: time evolution upon exposure to NO 2 (PNO 2 ) 25 mbar); bold gray spectrum: after 30 min. Part (a) reports the full spectra, whereas part (b) shows an enlargement of the frequency interval related with the vibrations of the Py rings.

Journal of Physical Chemistry C, 2007

In situ X-ray absorption near-edge spectroscopy/extended X-ray absorption fine structure (XANES/E... more 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/SiO 2 /Si(100) system (2 Cr/nm 2 , 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.

The Journal of Physical Chemistry C, 2009

In this work we used several complementary techniques (TEM, TPR, CO chemisorption, EXAFS and FTIR... more In this work we used several complementary techniques (TEM, TPR, CO chemisorption, EXAFS and FTIR spectroscopy) to understand the effects of the activation temperature and activation atmosphere (air or H 2 ) on the particle size distribution, the fraction, and the type of exposed surface sites of Pd nanoparticles supported on a high surface area SiO 2 -Al 2 O 3 (SA) support. Pd particle distribution has been carefully determined by a high statistic TEM study, from which the cuboctahedral-like shape of the metal particles is demonstrated. Assuming a model of perfect cuboctahedral particles, from the TEM particle size distribution we estimated the expected average Pd first shell coordination number. This value is slightly larger than that directly found by EXAFS owing to the fraction of very small Pd particles (d < 6-8 Å) that basically escape TEM detection. The same geometrical model allows prediction, from TEM particle size distribution, of the metal dispersion observed by CO chemisorption (S/V Chemi ). The S/V Chemi value drops significantly upon increasing the H 2reduction temperature. According to TEM, the sintering process can account only for a very small fraction of the S/V Chemi decrease, suggesting an important poisoning of the potentially available Pd surface. This hypothesis is supported by a parallel experiment of thermal decomposition at the same temperature (in absence of H 2 ), showing a S/V Chemi value almost unchanged. FTIR spectroscopy of adsorbed CO, probing the nature of the Pd surface available for adsorption, confirms the hypothesis.

The Journal of Physical Chemistry C, 2009

Chromocene (Cp 2 Cr) hosted inside the supercage cavities of a NaY zeolite undergoes a structural... more Chromocene (Cp 2 Cr) hosted inside the supercage cavities of a NaY zeolite undergoes a structural distortion induced by the strong local electric fields generated by charge-balancing counterions. This effect, clearly observed by an in situ Cr K-edge extended X-ray absorption fine structure (EXAFS) study, is the key factor in enhancing the reactivity of Cp 2 Cr toward CO. The Cp 2 Cr(CO) adducts initially formed are not as stable as when hosted in nonpolar environments such as toluene solution or polystyrene. The presence of strong anionic/ cationic pairs (Y -/Na + ) favors, in a CO atmosphere, the loss of a Cp ring driven by an electron transfer mechanism (accompanied by ligand rearrangement) that results in the formation of the charged [CpCr(CO) 3 ]and [Cp 2 Cr(CO)] + carbonyl species that are stabilized by Na + and Ypairs. Shape selectivity of the supercage cavity of the Y zeolite is necessary for this reaction, as it can host the two Cp 2 Cr molecules needed for disproportionation. Fast Fourier transform infrared (FTIR) spectroscopy, working in operando conditions, allows us to follow the time evolution of the IR stretching modes peculiar of reactants and products and thus to infer a reaction mechanism. The combination of quantum mechanical calculation with an in situ EXAFS study supports the hypothesis made on the basis of IR results. The work is further demonstration that zeolitic voids act as "nanoscale reaction chambers", where the reactivity of guest organometallic complexes can provide molecular insights into the elementary steps of heterogeneous catalysis. In this context, the investigation of metallocene reactivity inside a polar matrix can be extremely useful to understanding their properties in polymerization conditions, where they are usually found as part of an ion pair, together with the anionic form of the activator (e.g., MAO).

The Journal of Physical Chemistry B, 2003

This paper reports a structural study of 3 and 10 monolayer (ML) thick NiO films grown on Ag(001)... more This paper reports a structural study of 3 and 10 monolayer (ML) thick NiO films grown on Ag(001). Polarization-dependent X-ray absorption spectroscopy at the Ni K edge allowed us to obtain an accurate description of the local atomic environment of the Ni atoms up to the seventh coordination shell, including the determination of the in-plane and out-of-plane strains for the NiO films (determined with an accuracy better than 1%, resulting from the determination of Ni-O bond distances with a statistical error lower than 0.01 Å). Multishell multiple scattering calculations have been used to fit the data. Within the experimental errors, the 3 ML film has both in and out-of-plane Ni-O distances compatible with those expected in the case of perfect pseudomorphism on Ag(001). A rough film morphology, possibly including also the presence of NiO islands, is also suggested by the optimized coordination numbers. Conversely the 10 ML film is partially relaxed without significant deviation from ideal bidimensional film. The present EXAFS study allows us to rule out any significant atomic interdiffusion process between the substrate and the NiO film. Comparison with previous literature data allows us to make the hypothesis of a two step growth mechanism for NiO films on Ag(001). The NiO-Ag(001) interface distance determined in this EXAFS study (d interface ) 2.36 ( 0.05 Å) is in remarkable agreement with theoretical calculations (d interface ) 2.38 Å).

Physical Review Letters, 2003

Geometric parameters of NiO films epitaxially grown on Ag(001) were determined using two independ... more Geometric parameters of NiO films epitaxially grown on Ag(001) were determined using two independent experimental techniques and ab initio simulations. Primary beam diffraction modulated electron emission experiments determined that the NiO films grow with O on top of Ag and that the oxide/metal interface distance is d 2:3 0:1 A. Polarization-dependent x-ray absorption, at the Ni-K edge, determined the tetragonal strain (r k 2:046 0:009 A, r ? 2:12 0:02 A) and d 2:37 0:05

Physical Review B, 2004

In a recent contribution [Phys. Rev. Lett. 91, 046101 (2003)] we used polarization-dependent, Ni ... more In a recent contribution [Phys. Rev. Lett. 91, 046101 (2003)] we used polarization-dependent, Ni K edge, x-ray absorption spectroscopy (XAS) to probe the structure of ultrathin NiO epilayers deposited on Ag͑001͒. In that experiment samples were measured ex-situ and a 5 ML-thick MgO cap was used to avoid the hydroxylation of the NiO film. In the present paper we report complementary O K edge XAS data on the same system; NiO epilayers, in the 3 -50 ML thickness range, were grown in situ in the end station of the ALOISA beamline of the ELETTRA facility. A quantitative analysis of the data in the extended energy range has been performed using multiple scattering simulations. We found that, even in the ultrathin limit, the local structure of the film is rock-salt and we obtained a quantitative evaluation of the average in-plane and out-of-plane film strain as a function of the film thickness T. An in-plane compressive strain, due to lattice mismatch with the Ag substrate, is clearly present for the 3 ML film, being the in-and out-of-plane nearest neighbor distances equal to r ʈ = 2.048± 0.016 Å and r Ќ = 2.116± 0.018 Å. These values are in agreement with the expected behavior for a tetragonal distortion of the unit cell. The growth-induced strain is gradually released with increasing T: it is still appreciable for 10 ML but is completely relaxed at 50 ML. Any significant intermixing with the Ag substrate has been ruled out. Combining O and Ni K edge results we can conclude that NiO films grow on Ag͑001͒ in the O-on-Ag configuration, with an interface distance d = 2.28± 0.08 Å. This expansion of the interplanar distance is in agreement with recent ab initio simulations. A comparison with the similar MgO/ Ag͑001͒ system is also performed.

The Journal of Physical Chemistry C, 2009

The microporous metal-organic framework Ni 2 (dhtp) (H 4 dhtp ) 2,5-dihydroxyterephtalic acid) is... more The microporous metal-organic framework Ni 2 (dhtp) (H 4 dhtp ) 2,5-dihydroxyterephtalic acid) is a 3D network that maintains crystallinity and porosity after solvent removal. A mild thermal treatment in high vacuo at 393 K removes completely water allowing to obtain a MOF material with honeycomb structure able to strongly coordinate CO at room temperature. In this contribution, the reactivity toward CO of CPO-27-Ni has been studied by the combined use of infrared, extended X-ray absorption fine structure, X-ray absorption near edge structure, and UV-vis spectropies and microcalorimetry. Most of the Ni 2+ sites are involved in the interaction (about 80%), forming 1:1 linear adducts whose interaction enthalpy is slightly above 50 kJ/mol. In the presence of CO, a large modification of the vibrational and electronic properties of the material with respect to the dehydrated one has been observed.

Physical Review B, 2004

We determined the local atomic structure of MgO epilayers on Ag͑001͒ by means of polarization-dep... more We determined the local atomic structure of MgO epilayers on Ag͑001͒ by means of polarization-dependent x-ray absorption spectroscopy measurements at the Mg and O K edges. A quantitative analysis of the data in the extended energy range has been performed using multiple scattering simulations. We found that, even in the ultrathin limit, the local structure of the films is rocksalt and we obtained a quantitative evaluation of the average in-plane and out-of-plane film strain at the different thicknesses investigated. An in-plane compressive strain, due to lattice mismatch with the Ag substrate, is clearly present for the 3 ML film. The out-of-plane lattice constant is found to be expanded, in agreement with the expected behavior for a tetragonal distortion of the unit cell. This growth-induced strain is gradually released with increasing thickness and it is almost completely relaxed at 20 ML. Any significant intermixing with the Ag substrate can be ruled out. An expansion of the interplanar distance at the MgO-Ag interface is detected and its sign and magnitude are found to be in agreement with recent ab initio simulations. This work provides previously unavailable input for modeling the physical properties of the system and supports the hypothesis that the different electronic properties of MgO films on Ag͑001͒ are not related to structural or compositional differences at the ultrathin limit.

Journal of Physical Chemistry C, 2007

The vibrational properties of adsorbed carbonyls on heterogeneous Pd/oxide systems (SiO 2 -Al 2 O... more The vibrational properties of adsorbed carbonyls on heterogeneous Pd/oxide systems (SiO 2 -Al 2 O 3 , Al 2 O 3 , and MgO) are compared and discussed. On the basis of the surface science literature, the complex IR spectra of carbonyls formed on Pd particles are interpreted in terms of what occurs on well-defined Pd and Pd(100) faces and on edge/corner defects, typical of metal nanoparticles, thus bridging the gap between single crystals and high surface area systems. Furthermore, the influence of the electronic properties of the support (i.e., surface acidity/basicity) in determining the spectral features of carbonyls formed on Pd particles is clearly

Journal of Physical Chemistry B, 2005

This work reports the first complete FTIR characterization of H 2 , N 2 and C 2 H 4 molecular com... more This work reports the first complete FTIR characterization of H 2 , N 2 and C 2 H 4 molecular complexes formed on the Cr(II) sites in the Phillips catalyst. The use of a silica aerogel as support for Cr(II) sites, substituting the conventional aerosil material, allowed us to obtain a remarkable increase in the signal-to-noise ratio of the IR spectra of adsorbed species. The improvement is directly related to an increase of the surface area of the support (∼700 m 2 g -1 ) and to an almost complete absence of scattering [Groppo et al., Chem. Mater. 2005, 17, 2019-2027. The use of this support and the adoption of suitable experimental conditions results, for the first time, in the clear observation of H 2 and N 2 adducts formed on two different types of Cr(II) sites, thus yielding important information on the coordinative state of the Cr(II) ions, which well agrees with the evidences provided in the past by other probe molecules. Furthermore, we report the first complete characterization of the C 2 H 4 π-complexes formed on Cr(II) sites. These results are particularly important in the view of the understanding of the polymerization mechanism, since the C 2 H 4 coordination and the formation of π-bonded complexes are the first steps of the reaction.

Chemistry of Materials, 2005

We report the first Raman spectra of molecular adducts (CO and N 2 ) formed on Cr(II) sites dispe... more We report the first Raman spectra of molecular adducts (CO and N 2 ) formed on Cr(II) sites dispersed on silica surface (Phillips catalyst for ethylene polymerization), thus obtaining indirect information about the metal anchored species. These results have been achieved using an ad hoc selected laser line (able to excite a ligand to Cr charge-transfer transition that does not relax in a radiative channel), and adopting as a support a silica aerogel behaving as an optically uniform medium in the region of work. These two combined strategies, never simultaneously applied before, allowed us to obtain great quality Raman spectra of surface species, demonstrating that Raman spectroscopy can have great sensitivity toward surface species present in small concentration. Raman spectroscopy thus becomes a powerful tool complementary to IR and UV-vis DRS spectroscopies for surface investigation. The improvement can be quantified by comparing published spectra of the oxidized Cr(VI)/SiO 2 system (the only available in the literature) with the more intense and much richer one obtained under the present experimental conditions, where new spectroscopic features (never observed before) are clearly observed.

Physical Chemistry Chemical Physics, 2007

E r l a n g e n -Nü r n b e r g ) , a n d Ad r i a n o Z e c c h i n a ( Un i v e r s i t y o f T... more E r l a n g e n -Nü r n b e r g ) , a n d Ad r i a n o Z e c c h i n a ( Un i v e r s i t y o f T o r i n o ) , a n d p u b l i s h e d i n s e l e c t e d 2 0 0 7 p r i n t i s s u e s o f P CCP , a l l p a p e r s a r e c o l l e c t e d o n l i n e o n a d e d i c a t e d we b s i t e : www. r s c . o r g / p c c p / a l t f u e l V i s i t t h e we b s i t e f o r b o t h c u t t i n g e d g e r e s e a r c h p a p e r s a n d a u t h o r i t a t i v e r e v i e w a r t i c l e s b y l e a d e r s i n a r a n g e o f f i e l d s o f c r i t i c a l i mp o r t a n c e t o t h e wo r l d t o d a y

Studies in Surface Science and Catalysis, 2010

The preparation by a deposition-precipitation method (using Na 2 PdCl 4 as palladium precursor an... more The preparation by a deposition-precipitation method (using Na 2 PdCl 4 as palladium precursor and Na 2 CO 3 as basic agent) of Pd catalysts supported on γ-Al 2 O 3 and on two different types of activated carbons has been studied by EXAFS and TPR as a function of Pd loading from 0.5 to 5.0 wt%. Independently from the support, neither reduced metal fraction nor crystalline phase has been observed. EXAFS shows that the Pd local environment of the final catalysts changes slightly as a function of the Pd loading from 0.5 to 2.0 wt%: at higher loadings no further modification has been observed.

Studies in Surface Science and Catalysis, 2010

The changes in the support morphology, at nano-and micro-scale level, have been investigated alon... more The changes in the support morphology, at nano-and micro-scale level, have been investigated along the two-step processes involved in the preparation of a Pd supported on SiO 2 -Al 2 O 3 (SA) catalyst: Pd deposition and K 2 CO 3 doping. During the latter step part of the support dissolves and re-precipitates (partially outside the pores) covering the Pd particles. XRPD shows a significant rearrangement of the support at the long range order scale, while 29 Si and 27 Al solid-state (SS) NMR indicate that the average local environment around both Si and Al atoms remains unaltered.

Topics in Organometallic Chemistry, 2005

A relevant fraction of the polyethylene produced in the world (about 30%) is obtained with the Ph... more A relevant fraction of the polyethylene produced in the world (about 30%) is obtained with the Phillips process. Many efforts in the last 30 years have been devoted to establish the valence state and the structure of the catalytically active species formed by reduction with ethylene. However, no certain conclusions have been obtained so far, even using a CO-prereduced simplified system. In this

The Journal of Physical Chemistry A, 2009

In this article, we present a computational study of the structure and vibrational properties of ... more In this article, we present a computational study of the structure and vibrational properties of the species formed by the interaction between Cr sites of Phillips catalyst and probe molecules (CO, N(2)). The vibrational properties of these surface species, intensively investigated in the past, form a very rich and ideal set of experimental data to test computational approaches. By adopting the X(4)Si(2)O(3)Cr (X = H, OH, F) cluster as a simplified model of the ([triple bond]SiO)(2)Cr(II) species present at the surface of the real catalyst, we found that the B3LYP hybrid functional (containing 20% of Hartree-Fock exchange), when applied to this model, is unable to reproduce with reasonable accuracy the currently available experimental data (principally coming from IR spectroscopy). Better agreement is obtained when the percentage of Hartree-Fock exchange is increased (up to 35-40%).

The Journal of Physical Chemistry C, 2008

A systematic in situ FTIR investigation of the interaction of several molecules characterized by ... more A systematic in situ FTIR investigation of the interaction of several molecules characterized by a different electron accepting ability (or acidity) with the basic pyridine units contained in a commercial poly(4vinylpyridine) system characterized by a high surface area is here reported, with the final goal to fully understand the role of basicity in the catalytic processes. This approach, for the first time applied to polymeric materials, has only some similarities with the method usually adopted in the investigation of catalytic acid sites hosted inside oxidic porous materials (such as zeolites), where the probes are molecules characterized by an increased basicity. The experimental results are confirmed by ab initio theoretical calculations conducted on the 4-methylpyridine, which is the simplest molecule resembling the pyridine moiety in the P4VP system. . Time evolution of the FTIR spectra of the P4VP/NO2 system. Bold black spectrum: pure P4VP (outgassed at room temperature for 1 h); light-gray spectra: time evolution upon exposure to NO 2 (PNO 2 ) 25 mbar); bold gray spectrum: after 30 min. Part (a) reports the full spectra, whereas part (b) shows an enlargement of the frequency interval related with the vibrations of the Py rings.

Journal of Physical Chemistry C, 2007

In situ X-ray absorption near-edge spectroscopy/extended X-ray absorption fine structure (XANES/E... more 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/SiO 2 /Si(100) system (2 Cr/nm 2 , 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.

The Journal of Physical Chemistry C, 2009

In this work we used several complementary techniques (TEM, TPR, CO chemisorption, EXAFS and FTIR... more In this work we used several complementary techniques (TEM, TPR, CO chemisorption, EXAFS and FTIR spectroscopy) to understand the effects of the activation temperature and activation atmosphere (air or H 2 ) on the particle size distribution, the fraction, and the type of exposed surface sites of Pd nanoparticles supported on a high surface area SiO 2 -Al 2 O 3 (SA) support. Pd particle distribution has been carefully determined by a high statistic TEM study, from which the cuboctahedral-like shape of the metal particles is demonstrated. Assuming a model of perfect cuboctahedral particles, from the TEM particle size distribution we estimated the expected average Pd first shell coordination number. This value is slightly larger than that directly found by EXAFS owing to the fraction of very small Pd particles (d < 6-8 Å) that basically escape TEM detection. The same geometrical model allows prediction, from TEM particle size distribution, of the metal dispersion observed by CO chemisorption (S/V Chemi ). The S/V Chemi value drops significantly upon increasing the H 2reduction temperature. According to TEM, the sintering process can account only for a very small fraction of the S/V Chemi decrease, suggesting an important poisoning of the potentially available Pd surface. This hypothesis is supported by a parallel experiment of thermal decomposition at the same temperature (in absence of H 2 ), showing a S/V Chemi value almost unchanged. FTIR spectroscopy of adsorbed CO, probing the nature of the Pd surface available for adsorption, confirms the hypothesis.

The Journal of Physical Chemistry C, 2009

Chromocene (Cp 2 Cr) hosted inside the supercage cavities of a NaY zeolite undergoes a structural... more Chromocene (Cp 2 Cr) hosted inside the supercage cavities of a NaY zeolite undergoes a structural distortion induced by the strong local electric fields generated by charge-balancing counterions. This effect, clearly observed by an in situ Cr K-edge extended X-ray absorption fine structure (EXAFS) study, is the key factor in enhancing the reactivity of Cp 2 Cr toward CO. The Cp 2 Cr(CO) adducts initially formed are not as stable as when hosted in nonpolar environments such as toluene solution or polystyrene. The presence of strong anionic/ cationic pairs (Y -/Na + ) favors, in a CO atmosphere, the loss of a Cp ring driven by an electron transfer mechanism (accompanied by ligand rearrangement) that results in the formation of the charged [CpCr(CO) 3 ]and [Cp 2 Cr(CO)] + carbonyl species that are stabilized by Na + and Ypairs. Shape selectivity of the supercage cavity of the Y zeolite is necessary for this reaction, as it can host the two Cp 2 Cr molecules needed for disproportionation. Fast Fourier transform infrared (FTIR) spectroscopy, working in operando conditions, allows us to follow the time evolution of the IR stretching modes peculiar of reactants and products and thus to infer a reaction mechanism. The combination of quantum mechanical calculation with an in situ EXAFS study supports the hypothesis made on the basis of IR results. The work is further demonstration that zeolitic voids act as "nanoscale reaction chambers", where the reactivity of guest organometallic complexes can provide molecular insights into the elementary steps of heterogeneous catalysis. In this context, the investigation of metallocene reactivity inside a polar matrix can be extremely useful to understanding their properties in polymerization conditions, where they are usually found as part of an ion pair, together with the anionic form of the activator (e.g., MAO).

The Journal of Physical Chemistry B, 2003

This paper reports a structural study of 3 and 10 monolayer (ML) thick NiO films grown on Ag(001)... more This paper reports a structural study of 3 and 10 monolayer (ML) thick NiO films grown on Ag(001). Polarization-dependent X-ray absorption spectroscopy at the Ni K edge allowed us to obtain an accurate description of the local atomic environment of the Ni atoms up to the seventh coordination shell, including the determination of the in-plane and out-of-plane strains for the NiO films (determined with an accuracy better than 1%, resulting from the determination of Ni-O bond distances with a statistical error lower than 0.01 Å). Multishell multiple scattering calculations have been used to fit the data. Within the experimental errors, the 3 ML film has both in and out-of-plane Ni-O distances compatible with those expected in the case of perfect pseudomorphism on Ag(001). A rough film morphology, possibly including also the presence of NiO islands, is also suggested by the optimized coordination numbers. Conversely the 10 ML film is partially relaxed without significant deviation from ideal bidimensional film. The present EXAFS study allows us to rule out any significant atomic interdiffusion process between the substrate and the NiO film. Comparison with previous literature data allows us to make the hypothesis of a two step growth mechanism for NiO films on Ag(001). The NiO-Ag(001) interface distance determined in this EXAFS study (d interface ) 2.36 ( 0.05 Å) is in remarkable agreement with theoretical calculations (d interface ) 2.38 Å).

Physical Review Letters, 2003

Geometric parameters of NiO films epitaxially grown on Ag(001) were determined using two independ... more Geometric parameters of NiO films epitaxially grown on Ag(001) were determined using two independent experimental techniques and ab initio simulations. Primary beam diffraction modulated electron emission experiments determined that the NiO films grow with O on top of Ag and that the oxide/metal interface distance is d 2:3 0:1 A. Polarization-dependent x-ray absorption, at the Ni-K edge, determined the tetragonal strain (r k 2:046 0:009 A, r ? 2:12 0:02 A) and d 2:37 0:05

Physical Review B, 2004

In a recent contribution [Phys. Rev. Lett. 91, 046101 (2003)] we used polarization-dependent, Ni ... more In a recent contribution [Phys. Rev. Lett. 91, 046101 (2003)] we used polarization-dependent, Ni K edge, x-ray absorption spectroscopy (XAS) to probe the structure of ultrathin NiO epilayers deposited on Ag͑001͒. In that experiment samples were measured ex-situ and a 5 ML-thick MgO cap was used to avoid the hydroxylation of the NiO film. In the present paper we report complementary O K edge XAS data on the same system; NiO epilayers, in the 3 -50 ML thickness range, were grown in situ in the end station of the ALOISA beamline of the ELETTRA facility. A quantitative analysis of the data in the extended energy range has been performed using multiple scattering simulations. We found that, even in the ultrathin limit, the local structure of the film is rock-salt and we obtained a quantitative evaluation of the average in-plane and out-of-plane film strain as a function of the film thickness T. An in-plane compressive strain, due to lattice mismatch with the Ag substrate, is clearly present for the 3 ML film, being the in-and out-of-plane nearest neighbor distances equal to r ʈ = 2.048± 0.016 Å and r Ќ = 2.116± 0.018 Å. These values are in agreement with the expected behavior for a tetragonal distortion of the unit cell. The growth-induced strain is gradually released with increasing T: it is still appreciable for 10 ML but is completely relaxed at 50 ML. Any significant intermixing with the Ag substrate has been ruled out. Combining O and Ni K edge results we can conclude that NiO films grow on Ag͑001͒ in the O-on-Ag configuration, with an interface distance d = 2.28± 0.08 Å. This expansion of the interplanar distance is in agreement with recent ab initio simulations. A comparison with the similar MgO/ Ag͑001͒ system is also performed.

The Journal of Physical Chemistry C, 2009

The microporous metal-organic framework Ni 2 (dhtp) (H 4 dhtp ) 2,5-dihydroxyterephtalic acid) is... more The microporous metal-organic framework Ni 2 (dhtp) (H 4 dhtp ) 2,5-dihydroxyterephtalic acid) is a 3D network that maintains crystallinity and porosity after solvent removal. A mild thermal treatment in high vacuo at 393 K removes completely water allowing to obtain a MOF material with honeycomb structure able to strongly coordinate CO at room temperature. In this contribution, the reactivity toward CO of CPO-27-Ni has been studied by the combined use of infrared, extended X-ray absorption fine structure, X-ray absorption near edge structure, and UV-vis spectropies and microcalorimetry. Most of the Ni 2+ sites are involved in the interaction (about 80%), forming 1:1 linear adducts whose interaction enthalpy is slightly above 50 kJ/mol. In the presence of CO, a large modification of the vibrational and electronic properties of the material with respect to the dehydrated one has been observed.

Physical Review B, 2004

We determined the local atomic structure of MgO epilayers on Ag͑001͒ by means of polarization-dep... more We determined the local atomic structure of MgO epilayers on Ag͑001͒ by means of polarization-dependent x-ray absorption spectroscopy measurements at the Mg and O K edges. A quantitative analysis of the data in the extended energy range has been performed using multiple scattering simulations. We found that, even in the ultrathin limit, the local structure of the films is rocksalt and we obtained a quantitative evaluation of the average in-plane and out-of-plane film strain at the different thicknesses investigated. An in-plane compressive strain, due to lattice mismatch with the Ag substrate, is clearly present for the 3 ML film. The out-of-plane lattice constant is found to be expanded, in agreement with the expected behavior for a tetragonal distortion of the unit cell. This growth-induced strain is gradually released with increasing thickness and it is almost completely relaxed at 20 ML. Any significant intermixing with the Ag substrate can be ruled out. An expansion of the interplanar distance at the MgO-Ag interface is detected and its sign and magnitude are found to be in agreement with recent ab initio simulations. This work provides previously unavailable input for modeling the physical properties of the system and supports the hypothesis that the different electronic properties of MgO films on Ag͑001͒ are not related to structural or compositional differences at the ultrathin limit.