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Papers by Riccardo Pellegrini

Research paper thumbnail of Investigation of carbon and alumina supported Pd catalysts during catalyst preparation

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.

Research paper thumbnail of Effect of Pre-Reduction on the Properties and the Catalytic Activity of Pd/Carbon Catalysts: A Comparison with Pd/Al 2 O 3

ACS Catalysis, 2014

The effect of pre-reduction in solution with chemical reagents on the catalytic performance and c... more The effect of pre-reduction in solution with chemical reagents on the catalytic performance and catalyst properties of Pd/carbon catalysts was systematically investigated with a multitechnique approach. The results are critically discussed in comparison to those recently obtained on analogous Pd/alumina catalysts. It was proved that the Pd phase on the carbon surface is characterized by a high mobility, opposite to what occurs on alumina. As a result, the Pd particles on carbon aggregate together during pre-reduction, with a consequent decrease in available metal surface. Pd particles remain aggregated also in reaction conditions; the decreased Pd dispersion negatively affects the catalyst activity in the debenzylation of 4benzyloxyphenol.

Research paper thumbnail of Preparation of Supported Pd Catalysts: From the Pd Precursor Solution to the Deposited Pd2+ Phase

Langmuir, 2010

The preparation by the deposition-precipitation method (using Na 2 PdCl 4 as a palladium precurso... more The preparation by the deposition-precipitation method (using Na 2 PdCl 4 as a palladium precursor and Na 2 CO 3 as a basic agent) of Pd catalysts supported on γ-Al 2 O 3 and on two different types of active carbons has been followed by several techniques (UV-vis, EXAFS, XRPD, and TPR). This work consists of four successive parts: the investigation of (i) the palladium precursor liquid solution (in the absence of substrate), (ii) the solid precipitated phase (in the absence of substrate), (iii) the precipitated Pd 2þ -phase on the supports as a function of Pd loading from 0.5 to 5.0 wt % (i.e., the final catalyst for debenzylation reactions), and (iv) the Pd 0 -phase formed upon reduction in H 2 atmosphere at 393 K. A time/pH-dependent UV-vis experiment indicates that Pd 2þ is present in the mother solution mainly as PdCl 2 (H 2 O) 2 ] and [PdCl(H 2 O) 3 ] þ . Upon progressive addition of NaOH (3.0 < pH < ∼3.8), the concentration of the two complexes is almost constant and then they rapidly disappear because of the precipitation of an amorphous aggregation of Pd 2þ -polynuclearhydroxo complexes. This phase represents a model material for the active supported phase. Thermal treatments at increasing temperature of this phase cause progressive water loss and resulted in a progressive increase in crystallinity typical of a defective PdO-like phase. The EXAFS spectrum of the final catalysts has been found to be intermediate between that of the unsupported amorphous Pd 2þ -polynuclearhydroxo complexes and that of the PdOlike phase. Independent of the support, EXAFS was not able to evidence any fraction of reduced metallic Pd, meaning that all Pd is in the 2þ oxidation state within the sensitivity of the technique (a few percent). Analogously, independent of the support, XRPD was not able to detect the presence of any crystalline supported phase. The Pd local environment of the as-precipitated samples changes slightly as a function of Pd loading from 0.5 to 2.0 wt %: at higher loadings, no further modification has been observed. After reduction in an H 2 atmosphere, two trends have been observed: (i) the dispersion of Pd nanoparticles tends to decrease with increasing Pd concentration, less significantly on Al 2 O 3 -supported samples and more significantly on carbon-supported ones and (ii) the dispersion depends on the carrier following the sequence Al 2 O 3 . Cp > Cw according to the increasing palladium-support interaction strength.

Research paper thumbnail of Pd supported catalysts: Evolution of the support during Pd deposition and K doping

Studies in Surface Science and Catalysis - STUD SURF SCI CATAL, 2010

The changes in the support morphology, at nano- and micro-scale level, have been investigated alo... 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 SiO2-Al2O3 (SA) catalyst: Pd deposition and K2CO3 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 29Si and 27Al solid-state (SS) NMR indicate that the average local environment around both Si and Al atoms remains unaltered.

Research paper thumbnail of Effect of Different Face Centered Cubic Nanoparticle Distributions on Particle Size and Surface Area Determination: A Theoretical Study

The Journal of Physical Chemistry C, 2014

Nanoparticles research represents one of the most active fields in science due to the importance ... more Nanoparticles research represents one of the most active fields in science due to the importance of nanosized materials in a wide variety of applications. Their characterization needs the comparison of data coming from different experimental techniques, but the peculiar properties of the nanosystem that each technique points out are not always properly taken into account and misleading results have been often reported. In this work, we generated transmission electron microscopy like (TEM-like) data to predict the extended X-ray absorption fine structure (EXAFS) and chemisorption-like typical outputs as the average coordination numbers up to fourth shell of the particles distribution and the surface area. The aim of the simulations is to explore the dependence of the calculated average coordination number (ACN) and average dispersion (AD) values from each parameter characterizing a particle size distribution (PSD), as the mean diameter, the width, the shape, and the profile, and shows that a range of distributions is compatible with given values of ACN and AD. In this way, we have established a general method to properly take into account the above-mentioned parameters and to allow for an accurate analysis and comparison of results. Furthermore, it will be shown that unfavorable distribution shape makes the comparison among techniques critical and potentially misleading if performed with an oversimplified model of the PSD such as those using the average diameter only.

Research paper thumbnail of Preparation of Supported Pd Catalysts: From the Pd Precursor Solution to the Deposited Pd 2+ Phase

Langmuir, 2010

The preparation by the deposition-precipitation method (using Na 2 PdCl 4 as a palladium precurso... more The preparation by the deposition-precipitation method (using Na 2 PdCl 4 as a palladium precursor and Na 2 CO 3 as a basic agent) of Pd catalysts supported on γ-Al 2 O 3 and on two different types of active carbons has been followed by several techniques (UV-vis, EXAFS, XRPD, and TPR). This work consists of four successive parts: the investigation of (i) the palladium precursor liquid solution (in the absence of substrate), (ii) the solid precipitated phase (in the absence of substrate), (iii) the precipitated Pd 2þ -phase on the supports as a function of Pd loading from 0.5 to 5.0 wt % (i.e., the final catalyst for debenzylation reactions), and (iv) the Pd 0 -phase formed upon reduction in H 2 atmosphere at 393 K. A time/pH-dependent UV-vis experiment indicates that Pd 2þ is present in the mother solution mainly as PdCl 2 (H 2 O) 2 ] and [PdCl(H 2 O) 3 ] þ . Upon progressive addition of NaOH (3.0 < pH < ∼3.8), the concentration of the two complexes is almost constant and then they rapidly disappear because of the precipitation of an amorphous aggregation of Pd 2þ -polynuclearhydroxo complexes. This phase represents a model material for the active supported phase. Thermal treatments at increasing temperature of this phase cause progressive water loss and resulted in a progressive increase in crystallinity typical of a defective PdO-like phase. The EXAFS spectrum of the final catalysts has been found to be intermediate between that of the unsupported amorphous Pd 2þ -polynuclearhydroxo complexes and that of the PdOlike phase. Independent of the support, EXAFS was not able to evidence any fraction of reduced metallic Pd, meaning that all Pd is in the 2þ oxidation state within the sensitivity of the technique (a few percent). Analogously, independent of the support, XRPD was not able to detect the presence of any crystalline supported phase. The Pd local environment of the as-precipitated samples changes slightly as a function of Pd loading from 0.5 to 2.0 wt %: at higher loadings, no further modification has been observed. After reduction in an H 2 atmosphere, two trends have been observed: (i) the dispersion of Pd nanoparticles tends to decrease with increasing Pd concentration, less significantly on Al 2 O 3 -supported samples and more significantly on carbon-supported ones and (ii) the dispersion depends on the carrier following the sequence Al 2 O 3 . Cp > Cw according to the increasing palladium-support interaction strength.

Research paper thumbnail of Effect of reduction in liquid phase on the properties and the catalytic activity of Pd/Al2O3 catalysts

Journal of Catalysis, 2012

The change in structural and surface properties of Pd/Al 2 O 3 catalysts as the effect of reducti... more The change in structural and surface properties of Pd/Al 2 O 3 catalysts as the effect of reduction in liquid phase during preparation is investigated by a multi-technical approach. It has been found that, although the treatment leaves an external shell of oxidized Pd because of the exposure to air of final wet catalyst, it affects both the catalyst reducibility and the properties of the supported Pd nanoparticles, in terms of dispersion, particle size, morphology, interaction with the support, structural order, and surface properties. Consequently, unreduced and pre-reduced catalysts show different performances in the debenzylation of 4-benzyloxyphenol reaction.

Research paper thumbnail of Investigation of carbon and alumina supported Pd catalysts during catalyst preparation

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.

Research paper thumbnail of Effect of Pre-Reduction on the Properties and the Catalytic Activity of Pd/Carbon Catalysts: A Comparison with Pd/Al 2 O 3

ACS Catalysis, 2014

The effect of pre-reduction in solution with chemical reagents on the catalytic performance and c... more The effect of pre-reduction in solution with chemical reagents on the catalytic performance and catalyst properties of Pd/carbon catalysts was systematically investigated with a multitechnique approach. The results are critically discussed in comparison to those recently obtained on analogous Pd/alumina catalysts. It was proved that the Pd phase on the carbon surface is characterized by a high mobility, opposite to what occurs on alumina. As a result, the Pd particles on carbon aggregate together during pre-reduction, with a consequent decrease in available metal surface. Pd particles remain aggregated also in reaction conditions; the decreased Pd dispersion negatively affects the catalyst activity in the debenzylation of 4benzyloxyphenol.

Research paper thumbnail of Preparation of Supported Pd Catalysts: From the Pd Precursor Solution to the Deposited Pd2+ Phase

Langmuir, 2010

The preparation by the deposition-precipitation method (using Na 2 PdCl 4 as a palladium precurso... more The preparation by the deposition-precipitation method (using Na 2 PdCl 4 as a palladium precursor and Na 2 CO 3 as a basic agent) of Pd catalysts supported on γ-Al 2 O 3 and on two different types of active carbons has been followed by several techniques (UV-vis, EXAFS, XRPD, and TPR). This work consists of four successive parts: the investigation of (i) the palladium precursor liquid solution (in the absence of substrate), (ii) the solid precipitated phase (in the absence of substrate), (iii) the precipitated Pd 2þ -phase on the supports as a function of Pd loading from 0.5 to 5.0 wt % (i.e., the final catalyst for debenzylation reactions), and (iv) the Pd 0 -phase formed upon reduction in H 2 atmosphere at 393 K. A time/pH-dependent UV-vis experiment indicates that Pd 2þ is present in the mother solution mainly as PdCl 2 (H 2 O) 2 ] and [PdCl(H 2 O) 3 ] þ . Upon progressive addition of NaOH (3.0 < pH < ∼3.8), the concentration of the two complexes is almost constant and then they rapidly disappear because of the precipitation of an amorphous aggregation of Pd 2þ -polynuclearhydroxo complexes. This phase represents a model material for the active supported phase. Thermal treatments at increasing temperature of this phase cause progressive water loss and resulted in a progressive increase in crystallinity typical of a defective PdO-like phase. The EXAFS spectrum of the final catalysts has been found to be intermediate between that of the unsupported amorphous Pd 2þ -polynuclearhydroxo complexes and that of the PdOlike phase. Independent of the support, EXAFS was not able to evidence any fraction of reduced metallic Pd, meaning that all Pd is in the 2þ oxidation state within the sensitivity of the technique (a few percent). Analogously, independent of the support, XRPD was not able to detect the presence of any crystalline supported phase. The Pd local environment of the as-precipitated samples changes slightly as a function of Pd loading from 0.5 to 2.0 wt %: at higher loadings, no further modification has been observed. After reduction in an H 2 atmosphere, two trends have been observed: (i) the dispersion of Pd nanoparticles tends to decrease with increasing Pd concentration, less significantly on Al 2 O 3 -supported samples and more significantly on carbon-supported ones and (ii) the dispersion depends on the carrier following the sequence Al 2 O 3 . Cp > Cw according to the increasing palladium-support interaction strength.

Research paper thumbnail of Pd supported catalysts: Evolution of the support during Pd deposition and K doping

Studies in Surface Science and Catalysis - STUD SURF SCI CATAL, 2010

The changes in the support morphology, at nano- and micro-scale level, have been investigated alo... 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 SiO2-Al2O3 (SA) catalyst: Pd deposition and K2CO3 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 29Si and 27Al solid-state (SS) NMR indicate that the average local environment around both Si and Al atoms remains unaltered.

Research paper thumbnail of Effect of Different Face Centered Cubic Nanoparticle Distributions on Particle Size and Surface Area Determination: A Theoretical Study

The Journal of Physical Chemistry C, 2014

Nanoparticles research represents one of the most active fields in science due to the importance ... more Nanoparticles research represents one of the most active fields in science due to the importance of nanosized materials in a wide variety of applications. Their characterization needs the comparison of data coming from different experimental techniques, but the peculiar properties of the nanosystem that each technique points out are not always properly taken into account and misleading results have been often reported. In this work, we generated transmission electron microscopy like (TEM-like) data to predict the extended X-ray absorption fine structure (EXAFS) and chemisorption-like typical outputs as the average coordination numbers up to fourth shell of the particles distribution and the surface area. The aim of the simulations is to explore the dependence of the calculated average coordination number (ACN) and average dispersion (AD) values from each parameter characterizing a particle size distribution (PSD), as the mean diameter, the width, the shape, and the profile, and shows that a range of distributions is compatible with given values of ACN and AD. In this way, we have established a general method to properly take into account the above-mentioned parameters and to allow for an accurate analysis and comparison of results. Furthermore, it will be shown that unfavorable distribution shape makes the comparison among techniques critical and potentially misleading if performed with an oversimplified model of the PSD such as those using the average diameter only.

Research paper thumbnail of Preparation of Supported Pd Catalysts: From the Pd Precursor Solution to the Deposited Pd 2+ Phase

Langmuir, 2010

The preparation by the deposition-precipitation method (using Na 2 PdCl 4 as a palladium precurso... more The preparation by the deposition-precipitation method (using Na 2 PdCl 4 as a palladium precursor and Na 2 CO 3 as a basic agent) of Pd catalysts supported on γ-Al 2 O 3 and on two different types of active carbons has been followed by several techniques (UV-vis, EXAFS, XRPD, and TPR). This work consists of four successive parts: the investigation of (i) the palladium precursor liquid solution (in the absence of substrate), (ii) the solid precipitated phase (in the absence of substrate), (iii) the precipitated Pd 2þ -phase on the supports as a function of Pd loading from 0.5 to 5.0 wt % (i.e., the final catalyst for debenzylation reactions), and (iv) the Pd 0 -phase formed upon reduction in H 2 atmosphere at 393 K. A time/pH-dependent UV-vis experiment indicates that Pd 2þ is present in the mother solution mainly as PdCl 2 (H 2 O) 2 ] and [PdCl(H 2 O) 3 ] þ . Upon progressive addition of NaOH (3.0 < pH < ∼3.8), the concentration of the two complexes is almost constant and then they rapidly disappear because of the precipitation of an amorphous aggregation of Pd 2þ -polynuclearhydroxo complexes. This phase represents a model material for the active supported phase. Thermal treatments at increasing temperature of this phase cause progressive water loss and resulted in a progressive increase in crystallinity typical of a defective PdO-like phase. The EXAFS spectrum of the final catalysts has been found to be intermediate between that of the unsupported amorphous Pd 2þ -polynuclearhydroxo complexes and that of the PdOlike phase. Independent of the support, EXAFS was not able to evidence any fraction of reduced metallic Pd, meaning that all Pd is in the 2þ oxidation state within the sensitivity of the technique (a few percent). Analogously, independent of the support, XRPD was not able to detect the presence of any crystalline supported phase. The Pd local environment of the as-precipitated samples changes slightly as a function of Pd loading from 0.5 to 2.0 wt %: at higher loadings, no further modification has been observed. After reduction in an H 2 atmosphere, two trends have been observed: (i) the dispersion of Pd nanoparticles tends to decrease with increasing Pd concentration, less significantly on Al 2 O 3 -supported samples and more significantly on carbon-supported ones and (ii) the dispersion depends on the carrier following the sequence Al 2 O 3 . Cp > Cw according to the increasing palladium-support interaction strength.

Research paper thumbnail of Effect of reduction in liquid phase on the properties and the catalytic activity of Pd/Al2O3 catalysts

Journal of Catalysis, 2012

The change in structural and surface properties of Pd/Al 2 O 3 catalysts as the effect of reducti... more The change in structural and surface properties of Pd/Al 2 O 3 catalysts as the effect of reduction in liquid phase during preparation is investigated by a multi-technical approach. It has been found that, although the treatment leaves an external shell of oxidized Pd because of the exposure to air of final wet catalyst, it affects both the catalyst reducibility and the properties of the supported Pd nanoparticles, in terms of dispersion, particle size, morphology, interaction with the support, structural order, and surface properties. Consequently, unreduced and pre-reduced catalysts show different performances in the debenzylation of 4-benzyloxyphenol reaction.