Laurent Delannoy - Academia.edu (original) (raw)

Papers by Laurent Delannoy

J Phys Chem C, 2010

Diffuse optical reflectance of Au/TiO 2 powder catalysts, prepared by the deposition-precipitatio... more Diffuse optical reflectance of Au/TiO 2 powder catalysts, prepared by the deposition-precipitation method, is measured in the UV-visible range in controlled atmosphere. An intense optical absorption observed around 550 nm is interpreted by the excitation of plasmon resonances in the 4 nm gold nanoparticles (NPs). The location, intensity, and width of the absorption can be reproduced theoretically by using a distribution of shapes of the NPs. The changes of the reflectance upon exposure of the Au/TiO 2 catalyst to different atmospheres (O 2 /He, H 2 /He, CO/He) are measured in real-time by use of a homemade differential diffuse reflectance (DDR) spectrometer. The derivative-like DDR spectrum shows that the exposure to oxygen leads to the adsorption of oxygen species at the surface of the Au NPs. By modeling the DDR spectrum, we analyze different possible processes that could occur during the oxygen exposure and we find that the main effect is a charge transfer from gold to the adsorbed oxygen species, combined with a slight flattening of the Au NPs. The exposure to CO gives a similar but much smaller effect than the one of oxygen. Finally, real-time optical measurements performed during the exposures to oxygen indicate that two different sets of particles are probably present in the catalyst, and react with different kinetics, slightly flat three-dimensional NPs and very flat almost two-dimensional NPs.

Journal of Catalysis, 2013

ABSTRACT Combined Density Functional Theory (DFT) calculations and Diffuse Reflectance Infrared F... more ABSTRACT Combined Density Functional Theory (DFT) calculations and Diffuse Reflectance Infrared Fourier Transform Spectroscopy (DRIFTS) were performed to study the distribution of Pd atoms in bimetallic AuPd nanoparticles in the presence of adsorbed CO. Compared to vacuum condition, the results showed evidence of Pd surface enrichment where both Pd monomers and Pd dimers could exist. The energetic stability calculated for several alloy configurations evidenced the preference of Pd to occupy under-coordinated edge sites in the presence of CO gas. Moreover, the calculation of the vibrational frequencies of adsorbed CO for the first time allowed the fine assignment of the complex experimental DRIFTS bands of CO interacting with the bimetallic nanoparticles and their evolution with time exposure. Electronic structure analysis shows preponderant it-back-donation from under-coordinated Pd to CO inducing strong bonding on edge sites.

Gold Bulletin, Jun 1, 2008

The catalytic gas-phase (473 K) hydrodechlorination (HDC) of 2,4-dichlorophenol has been investig... more The catalytic gas-phase (473 K) hydrodechlorination (HDC) of 2,4-dichlorophenol has been investigated over Ni/SiO 2 , Ni/TiO 2 , Ni-Au/SiO 2 , and Ni/Au-TiO 2 (Ni loading ca. 5 wt%; bulk Ni/Au atomic ratio = 10). The samples were prepared by either (co-)impregnation or (co-)depositionprecipitation. The catalyst samples were characterized in terms of BET surface area, TPR, H 2 chemisorption, and TEM-EDX measurements. The impregnated Ni/SiO 2 and Ni/TiO 2 samples had a similar narrow Ni particle size distribution (1-6 nm). The addition of Au to both Ni/support samples lowered the temperature requirements for Ni II reduction and suppressed H 2 chemisorption. The impregnated Ni-Au/SiO 2 (10-150 nm) and Ni-Au/TiO 2 (2-95 nm) were characterized by a wider range of particle sizes than the monometallic nickel catalysts and a variable surface Ni/Au atomic ratio (<1-40). In comparison, Ni-Au/TiO 2 prepared by deposition-precipitation exhibited a narrower particle size range (2-60 nm) and a more uniform Ni and Au surface distribution (Ni/Au atomic ratio of <1-15). The titania-supported catalysts delivered significantly higher specific HDC rates and distinct HDC selectivities than the silica systems, with catalytic responses discussed in terms of metal-support and reactant-surface interactions. The incorporation of Au, regardless of the support or method of preparation, resulted in higher HDC activity, an effect attributed to a surface Ni-Au synergism. Hydrogen thermal treatment of the bimetallic catalysts after reaction resulted in appreciable surface reconstruction, notably a more homogeneous combination of Ni and Au in smaller particles, which enhanced HDC performance.

Phys. Chem. Chem. Phys., 2015

Iridium was added to the Au/TiO2 system to try to enhance its catalytic activity in the reaction ... more Iridium was added to the Au/TiO2 system to try to enhance its catalytic activity in the reaction of propene oxidation, performed under conditions close to those used in the studies of decomposition of volatile organic compounds (1200 ppm propene and 9 vol% O2 in He). Titania supported Ir-Au (Ir/Au = 1) was prepared by sequential deposition-precipitation with urea (DPU) of Ir then Au. The effect of the activation conditions (hydrogen or air at 400 °C) was investigated. The study of the activation conditions of Ir-Au/TiO2 showed that activation under hydrogen at 400 °C generated a catalyst more active than the monometallic ones, while Ir-Au/TiO2 activated in air remained as poorly active as Au/TiO2. TEM characterization showed the formation of metallic particles of similar size (2-3 nm) in both monometallic Au/TiO2 and bimetallic Ir-Au/TiO2. Characterization especially by DRIFTS using CO as a probe molecule suggests the presence of Ir-Au interaction, IrO2-Au(0) interaction when the sample is calcined and Ir(0)-Au(0) bimetallic particles when it is reduced. XPS and TPR characterization techniques showed that gold hinders to some extent the reoxidation of iridium in the reduced bimetallic Ir-Au/TiO2 catalyst. The enhanced catalytic activity of the reduced bimetallic Ir-Au/TiO2 catalyst is attributed to a surface Ir(0)-Au(0) synergism.

Catal. Sci. Technol., 2016

catalytic materials. The common problem associated with the TEM analysis of Au/CeO2 system is tha... more catalytic materials. The common problem associated with the TEM analysis of Au/CeO2 system is that the gold nanoparticles are barely visible due to poor contrast between gold and CeO2 ; this is especially the case when the size of the CeO2 particles is below 10 nm and when gold particles are also a few nanometer size (6,7). The characterization of metal phases dispersed on cerium oxide (CeO2) and related oxide supports is therefore a challenging problem. In the present investigation, Energy-Filtered Transmission Electron Microscopy (EFTEM) was performed to characterize gold catalysts supported on CeO2-Al2O3 mixed oxides, in order to overcome the drawbacks associated with regular TEM analysis. Such Au/xCeO2/Al2O3 cata- lysts are aimed to be used in reactions of decomposition of volatile organic compounds (VOC), such as propene as in this study, into CO2, These VOCs arise from domestic or industrial activities, and lead to air contamination that is harmful for public health. xCeO2-Al2...

Chem. Commun., 2015

This work provides the first experimental evidence of an H2 effect in C3H6-SCR over an Au/Al2O3 c... more This work provides the first experimental evidence of an H2 effect in C3H6-SCR over an Au/Al2O3 catalyst. This effect could only be observed when the number of Au catalytic sites was decreased. The N2 turnover rate estimated for the first time for the Au catalytic sites for H2-C3H6-SCR was found to be similar to that estimated for Ag ones supported on Al2O3.

Gold Bulletin, Dec 1, 2009

This article is about the reaction of selective hydrogenation of 1,3-butadiene performed in an ex... more This article is about the reaction of selective hydrogenation of 1,3-butadiene performed in an excess of alkenes (propene/butadiene ratio = 100) over supported gold catalysts, so as to mimic the conditions of purification of light alkenes (C4 cuts) from the presence of impurities (highly unsaturated compounds, 1-3%). Gold was found highly selective (close to 100%, no propane or butane formation) for the hydrogenation of butadiene to butenes at 100% conversion of butadiene. The influence of the concentrations of the various reactants, i.e. hydrogen, butadiene and propene, on the rates of butadiene and propene hydrogenation was investigated, and the apparent reaction orders were deduced from the dependence of rate on partial pressure in the gas phase. The reason for the high selectivity of gold catalysts is that the reaction of hydrogenation of alkenes occurs at much higher temperature (0.1% conversion at 300°C) than that of selective hydrogenation of butadiene (100% at T <170°C).

ACS Catalysis, 2013

ABSTRACT Incorporating small amounts of Pd into supported Au catalysts has been shown to have ben... more ABSTRACT Incorporating small amounts of Pd into supported Au catalysts has been shown to have beneficial effects on selective hydrogenation reactions, particularly 1,3-butadiene hydrogenation and the hydrogenation of nitroaromatics, especially p-chloronitrobenzene. Appropriate Pd incorporation enhances hydrogenation activity while maintaining the desirable high selectivity of supported Au catalysts. To better understand this phenomenon, a series of alumina- and titania-supported Au and dilute Pd–Au catalysts were prepared via urea deposition–precipitation. The catalysts were studied with infrared spectroscopy of CO adsorption, CO oxidation catalysis, and cyclohexene hydrogenation catalysis with the goal of understanding how Pd affects the catalytic properties of Au. CO adsorption experiments indicated a substantial amount of surface Pd when the catalyst was under CO. Adsorption experiments at various CO pressures were used to determine CO coverage; application of the Temkin adsorbate interaction model allowed for the determination of adsorption enthalpy metrics for CO adsorption on Au. These experiments showed that Pd induces an electronic effect on Au, affecting both the nascent adsorption enthalpy (ΔH0) and the change in enthalpy with increasing coverage. This electronic modification had little effect on CO oxidation catalysis. Michaelis–Menten kinetics parameters showed essentially the same oxygen reactivity on all the catalysts; the primary differences were in the number of active sites. The bimetallic catalysts were poor cyclohexene hydrogenation catalysts, indicating that there is relatively little exposed Pd when the catalyst is under hydrogen. The results, which are discussed in the context of the literature, indicate that a combination of surface composition and Pd-induced electronic effects on Au appear to increase hydrogen chemisorption and hydrogenation activity while largely maintaining the selectivities associated with catalysis by Au.

Phys. Chem. Chem. Phys., 2014

Oxide supported copper and gold catalysts are active for the selective hydrogenation of polyunsat... more Oxide supported copper and gold catalysts are active for the selective hydrogenation of polyunsaturated hydrocarbons but their low activity compared to palladium catalysts and the deactivation of copper catalysts limit their use. There are only a very limited number of studies concerned with the use of bimetallic Au-Cu catalysts for selective hydrogenation reactions and the aim of this work was to prepare TiO2-supported monometallic Au and Cu and bimetallic AuCu (Cu/Au atomic ratio of 1 and 3) catalysts and to evaluate their catalytic performance in the selective hydrogenation of butadiene. Small gold, copper and gold-copper nanoparticles (average particle size &amp;amp;amp;amp;amp;amp;amp;amp;amp;amp;amp;amp;amp;amp;amp;amp;amp;amp;amp;amp;amp;amp;lt; 2 nm) were obtained on TiO2 using the preparation method of deposition-precipitation with urea followed by reduction under H2 at 300 °C. Very small clusters were observed for Cu/TiO2 (∼1 nm) which might result from O2 induced copper redispersion, as also supported by the XPS analyses. The alloying of copper with gold was found to inhibit its redispersion and also limits its reoxidation, as attested by XPS. The bimetallic character of the AuCu nanoparticles was confirmed by XPS and EDX-HAADF. Cu/TiO2 was initially more active than Au/TiO2 in the selective hydrogenation of butadiene at 75 °C but it deactivated rapidly during the first hours of reaction whereas the gold catalyst was very stable up to 20 hours of reaction. The bimetallic AuCu/TiO2 catalysts displayed an activation period during the first hours of the reaction, which was very pronounced for the sample containing a higher Cu/Au atomic ratio. This initial gain in activity was tentatively assigned to copper segregation at the surface of the bimetallic nanoparticles, induced by the reactants. When the AuCu/TiO2 catalysts were pre-exposed to air at 75 °C before butadiene hydrogenation, surface copper segregation occurred, leading to higher initial activity and the suppression of the activation period. Under the same conditions, Cu/TiO2 totally lost its activity, probably due to irreversible copper oxidation.

Journal of the American Chemical Society, Mar 1, 2007

ChemCatChem, 2013

ABSTRACT A TiO2-supported AuPd bimetallic catalyst with an Au/Pd atomic ratio of 8 was prepared b... more ABSTRACT A TiO2-supported AuPd bimetallic catalyst with an Au/Pd atomic ratio of 8 was prepared by deposition-precipitation with urea, and its activity in CO oxidation at room temperature was compared to the one of a monometallic Au/TiO2 catalyst. X-ray photoelectron spectroscopy (XPS) and diffuse reflectance infrared Fourier transform spectroscopy (DRIFTS) analyses suggest that Au-Pd/TiO2 contains bimetallic nanoparticles after reduction under H2 at 500 °C although the presence of monometallic Au particles cannot be totally excluded. The evolution of the AuPd nanoparticles surface composition during exposure to O2 and CO/O2 was studied in situ by environmental high resolution electron microscopy (ETEM) and DRIFTS. Pd segregation at the surface of the bimetallic nanoparticles was evidenced by DRIFTS and directly observed by ETEM under O2 and CO/O2 with the formation of Aucore-Pdshell structure. The changes in the surface composition of the Au-Pd nanoparticles under CO/O2 was paralleled with the higher rate of deactivation in the first reaction stages observed for Au-Pd/TiO2 compared to Au/TiO2, which could be related to the possible replacement of Au in low coordination sites, at the origin of the high activity in CO oxidation, by Pd atoms. These results noticeably underline the modifications induced by the reactant that can undergo a bimetallic AuPd catalyst.

Applied Catalysis B: Environmental, 2014

ABSTRACT The addition of H-2 has been reported to promote drastically the selective catalytic red... more ABSTRACT The addition of H-2 has been reported to promote drastically the selective catalytic reduction of NOx by hydrocarbons (HC-SCR). Yet, the influence of the Ag loading on the H-2-promoted HC-SCR has been the subject of a very limited number of investigations. The H-2-HC-SCR earlier studies reported mostly on Ag/Al2O3 samples containing about 2 wt% Ag, since this particular loading has been shown to provide optimum catalytic performances in the HC-SCR reaction in the absence of H-2. The present study highlights for the first time that the H-2-C3H6-SCR catalytic performances of Ag/Al2O3 samples improved in the 150-550 degrees C temperature domain as the Ag loading (Ag surface density: x (Ag/nm(2),1203)) decreased well below 2 wt%. A detailed kinetic study of H-2-C3H6-SCR was performed in which the reaction orders in NO, C3H6 and H-2, and the apparent activation energies were determined for the reduction of NOx to N-2 on a Ag(x)/Al2O3 catalysts series, for which Ag was found to be in a highly dispersed state by TEM and HAADF-STEM. Remarkably, changes in these kinetic parameters were found to occur at an Ag surface density close to 0.7 Ag/nm(Al2O3)(2) (Ag loading of 2.2 wt%) coinciding with the changes observed earlier in the NOx uptakes of the Al2O3 supporting oxide [18]. Interpretation of the activity and kinetic data led us to conclude that the H-2-C3H6-SCR reaction proceeds via the activation of H-2 and C3H6 on Ag species and their further reaction with NOx adspecies activated on the Al2O3 support. The unexpected higher catalytic performances of the Ag samples with the lower Ag surface densities was attributed to the higher concentration of active sites on the Al2O3 supporting oxide able to chemisorb NOx species, in agreement with the NOx uptake data. The kinetic data obtained for Ag surface densities lower than 0.7 Ag/nm(Al2O3)(2),3 also suggest that the interaction between NOx and C3H6 adspecies would be rate determining in the C-3-H-6-SCR process.

Applied Catalysis A: General, 2015

ABSTRACT In a former study, we showed that bimetallic Au-Ag nanoparticles supported on TiO2 activ... more ABSTRACT In a former study, we showed that bimetallic Au-Ag nanoparticles supported on TiO2 activated under H2 at 550 °C present the highest activity and a high stability in the reaction of CO oxidation due to a compromise between the particle size and the bimetallic character of the nanoparticles. This paper focuses on the chemical phenomena occurring during the preparation and on the evolution of the Au and Ag species during the activation of the catalysts leading to the formation of bimetallic particles. To characterize the samples, three in situ techniques were used: in situ UV-visible, DRIFTS coupled with CO adsorption and XPS after thermal treatment in adjacent treatment chamber. UV-visible characterization proves that during the preparation of Au-Ag/TiO2 by sequential deposition-precipitation (silver first and then gold), silver reduced during the first step suffers a reoxidation due to the chlorides added during the deposition of gold and the formation of AgCl. The UV-visible study during in situ reduction indicates that the bimetallic particles gradually form as the reduction temperatures increases. The evolution of the gold and silver species present at the surface of the metal particles studied by XPS and CO-DRIFTS reveals an increasing proportion of silver atoms in the bimetallic particles during activation under hydrogen while chemical analysis indicates a gradual elimination of the chlorides with temperature. We conclude that the optimum temperature of activation of 550 °C for the reaction of CO oxidation is the result of a competition between these positive and negative effects.

Catalysis Science & Technology, 2013

Topics in Catalysis, 2007

The catalytic reduction of nitrogen monoxide by propene in the presence of excess oxygen over gol... more The catalytic reduction of nitrogen monoxide by propene in the presence of excess oxygen over gold based ceria catalyst was studied. Adsorption and temperature programmed desorption of NO/O 2 on Au/CeO 2 reveal that the catalyst adsorbs and desorbs NO over a ...

Topics in Catalysis, 2007

The origin of CO oxidation performance variations between three different supported Au catalysts ... more The origin of CO oxidation performance variations between three different supported Au catalysts (Au/CeO 2 , Au/Al 2 O 3 , Au/ TiO 2 ) was examined by in situ XAFS and DRIFTS measurements. All samples were prepared identically, by depositionprecipitation of an aqueous Au(III) complex with urea, and contained the same gold loading ($1 wt %). The as-prepared supported Au(III) precursors exhibited different reduction behaviour during exposure to the CO/O 2 /He reaction mixture at 298 K. The reducibility of the Au(III) precursor was found to decrease as a function of the support material in the order: titania > ceria > alumina. The as-prepared samples were inactive catalysts, but Au/TiO 2 and Au/CeO 2 developed catalytic activity as the reduction of Au(III) to metallic Au proceeded. Au/Al 2 O 3 remained inactive. The developed catalytic CO oxidation activity at 298 K varied as a function of the support as follows: titania > ceria > alumina $ 0. The EXAFS of samples pretreated in air at 773 K and in H 2 at 573 K reveals the presence of only metallic particles for Au/TiO 2 and Au/Al 2 O 3 . Au(III) supported on CeO 2 remains unreduced after calcination, but reduces during the treatment with H 2 . CO oxidation experiments performed at 298 K with the activated samples show that the presence of metallic gold is necessary to obtain active catalysts (Au/CeO 2 is not active after calcination) and that the reducible supports facilitate the genesis of active catalysts, while metallic gold particles on alumina are not active.

J Phys Chem C, 2010

Diffuse optical reflectance of Au/TiO 2 powder catalysts, prepared by the deposition-precipitatio... more Diffuse optical reflectance of Au/TiO 2 powder catalysts, prepared by the deposition-precipitation method, is measured in the UV-visible range in controlled atmosphere. An intense optical absorption observed around 550 nm is interpreted by the excitation of plasmon resonances in the 4 nm gold nanoparticles (NPs). The location, intensity, and width of the absorption can be reproduced theoretically by using a distribution of shapes of the NPs. The changes of the reflectance upon exposure of the Au/TiO 2 catalyst to different atmospheres (O 2 /He, H 2 /He, CO/He) are measured in real-time by use of a homemade differential diffuse reflectance (DDR) spectrometer. The derivative-like DDR spectrum shows that the exposure to oxygen leads to the adsorption of oxygen species at the surface of the Au NPs. By modeling the DDR spectrum, we analyze different possible processes that could occur during the oxygen exposure and we find that the main effect is a charge transfer from gold to the adsorbed oxygen species, combined with a slight flattening of the Au NPs. The exposure to CO gives a similar but much smaller effect than the one of oxygen. Finally, real-time optical measurements performed during the exposures to oxygen indicate that two different sets of particles are probably present in the catalyst, and react with different kinetics, slightly flat three-dimensional NPs and very flat almost two-dimensional NPs.

Journal of Catalysis, 2013

ABSTRACT Combined Density Functional Theory (DFT) calculations and Diffuse Reflectance Infrared F... more ABSTRACT Combined Density Functional Theory (DFT) calculations and Diffuse Reflectance Infrared Fourier Transform Spectroscopy (DRIFTS) were performed to study the distribution of Pd atoms in bimetallic AuPd nanoparticles in the presence of adsorbed CO. Compared to vacuum condition, the results showed evidence of Pd surface enrichment where both Pd monomers and Pd dimers could exist. The energetic stability calculated for several alloy configurations evidenced the preference of Pd to occupy under-coordinated edge sites in the presence of CO gas. Moreover, the calculation of the vibrational frequencies of adsorbed CO for the first time allowed the fine assignment of the complex experimental DRIFTS bands of CO interacting with the bimetallic nanoparticles and their evolution with time exposure. Electronic structure analysis shows preponderant it-back-donation from under-coordinated Pd to CO inducing strong bonding on edge sites.

Gold Bulletin, Jun 1, 2008

The catalytic gas-phase (473 K) hydrodechlorination (HDC) of 2,4-dichlorophenol has been investig... more The catalytic gas-phase (473 K) hydrodechlorination (HDC) of 2,4-dichlorophenol has been investigated over Ni/SiO 2 , Ni/TiO 2 , Ni-Au/SiO 2 , and Ni/Au-TiO 2 (Ni loading ca. 5 wt%; bulk Ni/Au atomic ratio = 10). The samples were prepared by either (co-)impregnation or (co-)depositionprecipitation. The catalyst samples were characterized in terms of BET surface area, TPR, H 2 chemisorption, and TEM-EDX measurements. The impregnated Ni/SiO 2 and Ni/TiO 2 samples had a similar narrow Ni particle size distribution (1-6 nm). The addition of Au to both Ni/support samples lowered the temperature requirements for Ni II reduction and suppressed H 2 chemisorption. The impregnated Ni-Au/SiO 2 (10-150 nm) and Ni-Au/TiO 2 (2-95 nm) were characterized by a wider range of particle sizes than the monometallic nickel catalysts and a variable surface Ni/Au atomic ratio (<1-40). In comparison, Ni-Au/TiO 2 prepared by deposition-precipitation exhibited a narrower particle size range (2-60 nm) and a more uniform Ni and Au surface distribution (Ni/Au atomic ratio of <1-15). The titania-supported catalysts delivered significantly higher specific HDC rates and distinct HDC selectivities than the silica systems, with catalytic responses discussed in terms of metal-support and reactant-surface interactions. The incorporation of Au, regardless of the support or method of preparation, resulted in higher HDC activity, an effect attributed to a surface Ni-Au synergism. Hydrogen thermal treatment of the bimetallic catalysts after reaction resulted in appreciable surface reconstruction, notably a more homogeneous combination of Ni and Au in smaller particles, which enhanced HDC performance.

Phys. Chem. Chem. Phys., 2015

Iridium was added to the Au/TiO2 system to try to enhance its catalytic activity in the reaction ... more Iridium was added to the Au/TiO2 system to try to enhance its catalytic activity in the reaction of propene oxidation, performed under conditions close to those used in the studies of decomposition of volatile organic compounds (1200 ppm propene and 9 vol% O2 in He). Titania supported Ir-Au (Ir/Au = 1) was prepared by sequential deposition-precipitation with urea (DPU) of Ir then Au. The effect of the activation conditions (hydrogen or air at 400 °C) was investigated. The study of the activation conditions of Ir-Au/TiO2 showed that activation under hydrogen at 400 °C generated a catalyst more active than the monometallic ones, while Ir-Au/TiO2 activated in air remained as poorly active as Au/TiO2. TEM characterization showed the formation of metallic particles of similar size (2-3 nm) in both monometallic Au/TiO2 and bimetallic Ir-Au/TiO2. Characterization especially by DRIFTS using CO as a probe molecule suggests the presence of Ir-Au interaction, IrO2-Au(0) interaction when the sample is calcined and Ir(0)-Au(0) bimetallic particles when it is reduced. XPS and TPR characterization techniques showed that gold hinders to some extent the reoxidation of iridium in the reduced bimetallic Ir-Au/TiO2 catalyst. The enhanced catalytic activity of the reduced bimetallic Ir-Au/TiO2 catalyst is attributed to a surface Ir(0)-Au(0) synergism.

Catal. Sci. Technol., 2016

catalytic materials. The common problem associated with the TEM analysis of Au/CeO2 system is tha... more catalytic materials. The common problem associated with the TEM analysis of Au/CeO2 system is that the gold nanoparticles are barely visible due to poor contrast between gold and CeO2 ; this is especially the case when the size of the CeO2 particles is below 10 nm and when gold particles are also a few nanometer size (6,7). The characterization of metal phases dispersed on cerium oxide (CeO2) and related oxide supports is therefore a challenging problem. In the present investigation, Energy-Filtered Transmission Electron Microscopy (EFTEM) was performed to characterize gold catalysts supported on CeO2-Al2O3 mixed oxides, in order to overcome the drawbacks associated with regular TEM analysis. Such Au/xCeO2/Al2O3 cata- lysts are aimed to be used in reactions of decomposition of volatile organic compounds (VOC), such as propene as in this study, into CO2, These VOCs arise from domestic or industrial activities, and lead to air contamination that is harmful for public health. xCeO2-Al2...

Chem. Commun., 2015

This work provides the first experimental evidence of an H2 effect in C3H6-SCR over an Au/Al2O3 c... more This work provides the first experimental evidence of an H2 effect in C3H6-SCR over an Au/Al2O3 catalyst. This effect could only be observed when the number of Au catalytic sites was decreased. The N2 turnover rate estimated for the first time for the Au catalytic sites for H2-C3H6-SCR was found to be similar to that estimated for Ag ones supported on Al2O3.

Gold Bulletin, Dec 1, 2009

This article is about the reaction of selective hydrogenation of 1,3-butadiene performed in an ex... more This article is about the reaction of selective hydrogenation of 1,3-butadiene performed in an excess of alkenes (propene/butadiene ratio = 100) over supported gold catalysts, so as to mimic the conditions of purification of light alkenes (C4 cuts) from the presence of impurities (highly unsaturated compounds, 1-3%). Gold was found highly selective (close to 100%, no propane or butane formation) for the hydrogenation of butadiene to butenes at 100% conversion of butadiene. The influence of the concentrations of the various reactants, i.e. hydrogen, butadiene and propene, on the rates of butadiene and propene hydrogenation was investigated, and the apparent reaction orders were deduced from the dependence of rate on partial pressure in the gas phase. The reason for the high selectivity of gold catalysts is that the reaction of hydrogenation of alkenes occurs at much higher temperature (0.1% conversion at 300°C) than that of selective hydrogenation of butadiene (100% at T <170°C).

ACS Catalysis, 2013

ABSTRACT Incorporating small amounts of Pd into supported Au catalysts has been shown to have ben... more ABSTRACT Incorporating small amounts of Pd into supported Au catalysts has been shown to have beneficial effects on selective hydrogenation reactions, particularly 1,3-butadiene hydrogenation and the hydrogenation of nitroaromatics, especially p-chloronitrobenzene. Appropriate Pd incorporation enhances hydrogenation activity while maintaining the desirable high selectivity of supported Au catalysts. To better understand this phenomenon, a series of alumina- and titania-supported Au and dilute Pd–Au catalysts were prepared via urea deposition–precipitation. The catalysts were studied with infrared spectroscopy of CO adsorption, CO oxidation catalysis, and cyclohexene hydrogenation catalysis with the goal of understanding how Pd affects the catalytic properties of Au. CO adsorption experiments indicated a substantial amount of surface Pd when the catalyst was under CO. Adsorption experiments at various CO pressures were used to determine CO coverage; application of the Temkin adsorbate interaction model allowed for the determination of adsorption enthalpy metrics for CO adsorption on Au. These experiments showed that Pd induces an electronic effect on Au, affecting both the nascent adsorption enthalpy (ΔH0) and the change in enthalpy with increasing coverage. This electronic modification had little effect on CO oxidation catalysis. Michaelis–Menten kinetics parameters showed essentially the same oxygen reactivity on all the catalysts; the primary differences were in the number of active sites. The bimetallic catalysts were poor cyclohexene hydrogenation catalysts, indicating that there is relatively little exposed Pd when the catalyst is under hydrogen. The results, which are discussed in the context of the literature, indicate that a combination of surface composition and Pd-induced electronic effects on Au appear to increase hydrogen chemisorption and hydrogenation activity while largely maintaining the selectivities associated with catalysis by Au.

Phys. Chem. Chem. Phys., 2014

Oxide supported copper and gold catalysts are active for the selective hydrogenation of polyunsat... more Oxide supported copper and gold catalysts are active for the selective hydrogenation of polyunsaturated hydrocarbons but their low activity compared to palladium catalysts and the deactivation of copper catalysts limit their use. There are only a very limited number of studies concerned with the use of bimetallic Au-Cu catalysts for selective hydrogenation reactions and the aim of this work was to prepare TiO2-supported monometallic Au and Cu and bimetallic AuCu (Cu/Au atomic ratio of 1 and 3) catalysts and to evaluate their catalytic performance in the selective hydrogenation of butadiene. Small gold, copper and gold-copper nanoparticles (average particle size &amp;amp;amp;amp;amp;amp;amp;amp;amp;amp;amp;amp;amp;amp;amp;amp;amp;amp;amp;amp;amp;amp;lt; 2 nm) were obtained on TiO2 using the preparation method of deposition-precipitation with urea followed by reduction under H2 at 300 °C. Very small clusters were observed for Cu/TiO2 (∼1 nm) which might result from O2 induced copper redispersion, as also supported by the XPS analyses. The alloying of copper with gold was found to inhibit its redispersion and also limits its reoxidation, as attested by XPS. The bimetallic character of the AuCu nanoparticles was confirmed by XPS and EDX-HAADF. Cu/TiO2 was initially more active than Au/TiO2 in the selective hydrogenation of butadiene at 75 °C but it deactivated rapidly during the first hours of reaction whereas the gold catalyst was very stable up to 20 hours of reaction. The bimetallic AuCu/TiO2 catalysts displayed an activation period during the first hours of the reaction, which was very pronounced for the sample containing a higher Cu/Au atomic ratio. This initial gain in activity was tentatively assigned to copper segregation at the surface of the bimetallic nanoparticles, induced by the reactants. When the AuCu/TiO2 catalysts were pre-exposed to air at 75 °C before butadiene hydrogenation, surface copper segregation occurred, leading to higher initial activity and the suppression of the activation period. Under the same conditions, Cu/TiO2 totally lost its activity, probably due to irreversible copper oxidation.

Journal of the American Chemical Society, Mar 1, 2007

ChemCatChem, 2013

ABSTRACT A TiO2-supported AuPd bimetallic catalyst with an Au/Pd atomic ratio of 8 was prepared b... more ABSTRACT A TiO2-supported AuPd bimetallic catalyst with an Au/Pd atomic ratio of 8 was prepared by deposition-precipitation with urea, and its activity in CO oxidation at room temperature was compared to the one of a monometallic Au/TiO2 catalyst. X-ray photoelectron spectroscopy (XPS) and diffuse reflectance infrared Fourier transform spectroscopy (DRIFTS) analyses suggest that Au-Pd/TiO2 contains bimetallic nanoparticles after reduction under H2 at 500 °C although the presence of monometallic Au particles cannot be totally excluded. The evolution of the AuPd nanoparticles surface composition during exposure to O2 and CO/O2 was studied in situ by environmental high resolution electron microscopy (ETEM) and DRIFTS. Pd segregation at the surface of the bimetallic nanoparticles was evidenced by DRIFTS and directly observed by ETEM under O2 and CO/O2 with the formation of Aucore-Pdshell structure. The changes in the surface composition of the Au-Pd nanoparticles under CO/O2 was paralleled with the higher rate of deactivation in the first reaction stages observed for Au-Pd/TiO2 compared to Au/TiO2, which could be related to the possible replacement of Au in low coordination sites, at the origin of the high activity in CO oxidation, by Pd atoms. These results noticeably underline the modifications induced by the reactant that can undergo a bimetallic AuPd catalyst.

Applied Catalysis B: Environmental, 2014

ABSTRACT The addition of H-2 has been reported to promote drastically the selective catalytic red... more ABSTRACT The addition of H-2 has been reported to promote drastically the selective catalytic reduction of NOx by hydrocarbons (HC-SCR). Yet, the influence of the Ag loading on the H-2-promoted HC-SCR has been the subject of a very limited number of investigations. The H-2-HC-SCR earlier studies reported mostly on Ag/Al2O3 samples containing about 2 wt% Ag, since this particular loading has been shown to provide optimum catalytic performances in the HC-SCR reaction in the absence of H-2. The present study highlights for the first time that the H-2-C3H6-SCR catalytic performances of Ag/Al2O3 samples improved in the 150-550 degrees C temperature domain as the Ag loading (Ag surface density: x (Ag/nm(2),1203)) decreased well below 2 wt%. A detailed kinetic study of H-2-C3H6-SCR was performed in which the reaction orders in NO, C3H6 and H-2, and the apparent activation energies were determined for the reduction of NOx to N-2 on a Ag(x)/Al2O3 catalysts series, for which Ag was found to be in a highly dispersed state by TEM and HAADF-STEM. Remarkably, changes in these kinetic parameters were found to occur at an Ag surface density close to 0.7 Ag/nm(Al2O3)(2) (Ag loading of 2.2 wt%) coinciding with the changes observed earlier in the NOx uptakes of the Al2O3 supporting oxide [18]. Interpretation of the activity and kinetic data led us to conclude that the H-2-C3H6-SCR reaction proceeds via the activation of H-2 and C3H6 on Ag species and their further reaction with NOx adspecies activated on the Al2O3 support. The unexpected higher catalytic performances of the Ag samples with the lower Ag surface densities was attributed to the higher concentration of active sites on the Al2O3 supporting oxide able to chemisorb NOx species, in agreement with the NOx uptake data. The kinetic data obtained for Ag surface densities lower than 0.7 Ag/nm(Al2O3)(2),3 also suggest that the interaction between NOx and C3H6 adspecies would be rate determining in the C-3-H-6-SCR process.

Applied Catalysis A: General, 2015

ABSTRACT In a former study, we showed that bimetallic Au-Ag nanoparticles supported on TiO2 activ... more ABSTRACT In a former study, we showed that bimetallic Au-Ag nanoparticles supported on TiO2 activated under H2 at 550 °C present the highest activity and a high stability in the reaction of CO oxidation due to a compromise between the particle size and the bimetallic character of the nanoparticles. This paper focuses on the chemical phenomena occurring during the preparation and on the evolution of the Au and Ag species during the activation of the catalysts leading to the formation of bimetallic particles. To characterize the samples, three in situ techniques were used: in situ UV-visible, DRIFTS coupled with CO adsorption and XPS after thermal treatment in adjacent treatment chamber. UV-visible characterization proves that during the preparation of Au-Ag/TiO2 by sequential deposition-precipitation (silver first and then gold), silver reduced during the first step suffers a reoxidation due to the chlorides added during the deposition of gold and the formation of AgCl. The UV-visible study during in situ reduction indicates that the bimetallic particles gradually form as the reduction temperatures increases. The evolution of the gold and silver species present at the surface of the metal particles studied by XPS and CO-DRIFTS reveals an increasing proportion of silver atoms in the bimetallic particles during activation under hydrogen while chemical analysis indicates a gradual elimination of the chlorides with temperature. We conclude that the optimum temperature of activation of 550 °C for the reaction of CO oxidation is the result of a competition between these positive and negative effects.

Catalysis Science & Technology, 2013

Topics in Catalysis, 2007

The catalytic reduction of nitrogen monoxide by propene in the presence of excess oxygen over gol... more The catalytic reduction of nitrogen monoxide by propene in the presence of excess oxygen over gold based ceria catalyst was studied. Adsorption and temperature programmed desorption of NO/O 2 on Au/CeO 2 reveal that the catalyst adsorbs and desorbs NO over a ...

Topics in Catalysis, 2007

The origin of CO oxidation performance variations between three different supported Au catalysts ... more The origin of CO oxidation performance variations between three different supported Au catalysts (Au/CeO 2 , Au/Al 2 O 3 , Au/ TiO 2 ) was examined by in situ XAFS and DRIFTS measurements. All samples were prepared identically, by depositionprecipitation of an aqueous Au(III) complex with urea, and contained the same gold loading ($1 wt %). The as-prepared supported Au(III) precursors exhibited different reduction behaviour during exposure to the CO/O 2 /He reaction mixture at 298 K. The reducibility of the Au(III) precursor was found to decrease as a function of the support material in the order: titania > ceria > alumina. The as-prepared samples were inactive catalysts, but Au/TiO 2 and Au/CeO 2 developed catalytic activity as the reduction of Au(III) to metallic Au proceeded. Au/Al 2 O 3 remained inactive. The developed catalytic CO oxidation activity at 298 K varied as a function of the support as follows: titania > ceria > alumina $ 0. The EXAFS of samples pretreated in air at 773 K and in H 2 at 573 K reveals the presence of only metallic particles for Au/TiO 2 and Au/Al 2 O 3 . Au(III) supported on CeO 2 remains unreduced after calcination, but reduces during the treatment with H 2 . CO oxidation experiments performed at 298 K with the activated samples show that the presence of metallic gold is necessary to obtain active catalysts (Au/CeO 2 is not active after calcination) and that the reducible supports facilitate the genesis of active catalysts, while metallic gold particles on alumina are not active.