Growth of Au-Pd2Sn nanorods via galvanic replacement and their catalytic performance on hydrogenation and sonogashira coupling reactions

Colloidal Pd2Sn and Au–Pd2Sn nanorods (NRs) with tuned size were produced by the reduction of Pd and Sn salts in the presence of size- and shape-controlling agents and the posterior growth of Au tips through a galvanic replacement reaction. Pd2Sn and Au–Pd2Sn NRs exhibited high catalytic activity toward quasi-homogeneous hydrogenation of alkenes (styrene and 1-octene) and alkynes (phenylacetylene and 1-octyne) in dichloromethane. Au–Pd2Sn NRs showed higher activity than Pd2Sn for 1-octene, 1-octyne, and phenylacetylene. In Au–Pd2Sn heterostructures, X-ray photoelectron spectroscopy evidenced an electron donation from the Pd2Sn NR to the Au tips. Such heterostructures showed distinct catalytic behavior in the hydrogenation of compounds containing a triple bond such as tolan. This can be explained by the aurophilicity of triple bonds. To further study this effect, Pd2Sn and Au–Pd2Sn NRs were also tested in the Sonogashira coupling reaction between iodobenzene and phenylacetylene in N,N-dimethylformamide. At low concentration, this reaction provided the expected product, tolan. However, at high concentration, more reduced products such as stilbene and 1,2-diphenylethane were also obtained, even without the addition of H2. A mechanism for this unexpected reduction is proposed.Peer ReviewedPostprint (author's final draft

Mn3O4@CoMn2O4-CoxOy nanoparticles : partial cation exchange synthesis and electrocatalytic properties toward the oxygen reduction and evolution reactions

Mn3O4@CoMn2O4 nanoparticles (NPs) were produced at low temperature and ambient atmosphere using a one -pot two-step synthesis protocol involving the cation exchange of Mn by Co in preformed Mn3O4 NPs. Selecting the proper cobalt precursor, the nucleation of CoxOy crystallites at the Mn3O4@a CoMn2O4 surface could be simultaneously promoted to form Mn3O4@CoMn2O4-CoxOy NPs. Such heterostructured NPs were investigated for oxygen reduction and evolution reactions (ORR, OER) in alkaline solution. Mn3O4@ CoMn2O4-Cox0y NPs with [Co]/[Mn] = 1 showed low overpotentials of 0.31 Vat(-3) mA.cm(-2) and a small Tafel slope of 52 mV.dec(-1) for ORR, and overpotentials of 0.31 V at 10 mAPeer ReviewedPostprint (author's final draft

Tuning branching in ceria nanocrystals

Branched nanocrystals (NCs) enable high atomic surface exposure within a crystalline network that provides avenues for charge transport. This combination of properties makes branched NCs particularly suitable for a range of applications where both interaction with the media and charge transport are involved. Herein we report on the colloidal synthesis of branched ceria NCs by means of a ligand-mediated overgrowth mechanism. In particular, the differential coverage of oleic acid as an X-type ligand at ceria facets with different atomic density, atomic coordination deficiency, and oxygen vacancy density resulted in a preferential growth in the [111] direction and thus in the formation of ceria octapods. Alcohols, through an esterification alcoholysis reaction, promoted faster growth rates that translated into nanostructures with higher geometrical complexity, increasing the branch aspect ratio and triggering the formation of side branches. On the other hand, the presence of water resulted in a significant reduction of the growth rate, decreasing the reaction yield and eliminating side branching, which we associate to a blocking of the surface reaction sites or a displacement of the alcoholysis reaction. Overall, adjusting the amounts of each chemical, well-defined branched ceria NCs with tuned number, thickness, and length of branches and with overall size ranging from 5 to 45 nm could be produced. We further demonstrate that such branched ceria NCs are able to provide higher surface areas and related oxygen storage capacities (OSC) than quasi-spherical NCs

Síntesi i caracterització de nanopartícules mono- i bimetàl∙liques per aplicacions catalítiques

[cat] Els objectius inicials d’aquesta tesi eren: 1) desenvolupar nanocatalitzadors amb composició, geometria i propietats estructurals ben controlades, 2) estudiar vies per a la incorporació de les nanopartícules prèviament sintetitzades en suports d’elevada àrea superficial i 3) estudiar i correlacionar les propietats i el comportament catalític dels nanocatalitzadors model sintetitzats. Per aquest motiu en una primera etapa s’ha estudiat la preparació de nanopartícules metàl·liques disperses i accessibles, a partir de les quals s’han fet créixer matrius mesoporoses amb porositat controlada. Concretament s’ha estudiat el mètode mitjançant la síntesi col·loïdal de nanopartícules d’or com a fase activa i en òxid de titani com a suport model i d’ interès. La porositat i àrea superficial del suport (TiO2) s’ha controlat mitjançant derivats carboxilats aromàtics, els quals actuen com a espaiadors físics entre centres de titani, generant una xarxa híbrida orgànica- inorgànica intermèdia. Per tal d’eliminar els lligands orgànics i estabilitzar el material, aquestes mostres s’han calcinat en aire. Mitjançant difracció de raigs-X i àrea B.E.T, s’ha confirmat la formació d’una xarxa mesoporosa abans i després de l’etapa de calcinació. Aquests resultats corroboren la funció dels lligands orgànics com a directors d’estructura, ja que proporcionen estabilitat tèrmica al material i el prevenen contra el col·lapse de l’estructura mesoporosa. Els materials mesoporosos finals eviten i bloquegen el creixement i l’agregació de les nanopartícules d’or, per tant minimitzen la sinterització que normalment té lloc en l’etapa de calcinació i incrementen les interaccions entre les nanopartícules metàl·liques i el suport. Finalment s’ha estudiat el comportament catalític d’aquests materials en dues reaccions d’interès: reacció d’oxidació de CO i la reacció de Water Gas Shift (WGS). La bona accessibilitat de les nanopartícules d’or ha estat confirmada amb una elevada conversió de CO a COR2R a baixa temperatura en la mostra Au@TiOR2R(b). Els resultats obtinguts per espectroscòpia DRIFT mostren una banda única i ben definida en el catalitzador Au@TiOR2R(b), que s’associa a espècies AuP característic dels enllaços Au-O-Ti de la interfície metall-suport. En una segona etapa s’han sintetitzat nanopartícules de coure, cobalt i s’ha desenvolupat una síntesi per a la preparació de nanopartícules bimetàl·liques de cobalt-coure amb estructura core shell, Co@Cu. Aquesta síntesi es basa en la substitució d’àtoms de cobalt per àtoms de coure mitjançant una reacció de desplaçament galvànic, que ha permès modular la composició en la nanopartícula. Per a la preparativa del nanocatalitzador, les nanopartícules han estat incorporades en una matriu mesoporosa de SiOR2R a través del mètode de la inclusió capil·lar. Els elevats valors de dispersió obtinguts, confirmen la mida nanomètrica de les nanopartícules i l’eficiència del mètode per a la seva incorporació. Mitjançant l’anàlisi de reducció tèrmica programada, s’ha observat un únic pic de reducció en les mostres Co@Cu-SiOR2 en un rang de temperatures intermedis als catalitzadors monometàl·lics. Aquests perfils mostren una una elevada homogeneïtat de la mostra i un contacte íntim entre el cobalt i el coure en les nanopartícules Co@Cu, així com un efecte sinèrgic en la reducció del cobalt i del coure. L’estudi de l’evolució de les nanopartícules durant els diferents tractaments ha mostrat una segregació de les fases en la nanopartícula, Co@Cu, en funció del tractament aplicat: la reducció a pressió i temperatura ha mostrat la segregació del cobalt en superfície donant una estructura core-shell inversa a la de les nanopartícules inicials. Finalment s’ha estudiat el comportament catalític en la reacció d’hidrogenació de CO2. Els nanocatalitzadors han mostrat capacitat per a la formació d’enllaços C-C i la formació d’alcohols, fet que posa de manifest un comportament sinèrgic i bifuncional en els centres bimetàl·lics. Tot i que , els resultats obtinguts en les conversions de CO2 R indiquen que la presència de cobalt en superfície és necessari per poder millorar el rendiment catalític d’aquest materials.[eng] The goal set at the beginning of this thesis was the preparation of metallic nanoparticles via colloidal synthesis and its use in catalytic applications. The first point of interest was the synthesis of disperse and accessible Gold nanoparticles, that were used in the growth of a mesoporous Titanium oxide lattice. The porosity and superficial area of the support were controlled by carboxylic derivatives that act like physical spacers between de titanium centers and create a hybrid organic-inorganic lattice. In order to remove the organic ligands and to stabilize the material, samples were calcined. The formation of the mesoporous lattice, both before and after the calcinations, was confirmed by X-ray diffraction and B.E.T. area measurements. These results indicate that the carboxylic ligands provide the mesoporous structure with thermal stability and prevent its collapse. The final mesoporous materials efficiently blocked the aggregation and growth of the gold nanoparticles, minimizing their sintering during the calcination treatment and increasing the interaction strength between the metal nanoparticles and the oxide support. The catalytic behavior of these materials was studied in two reactions of interest: CO oxidation and Water Gas Shift (WGS). The obtained results confirmed the catalytic activity and its dependence with the size of the nanometric Gold particles and the interaction established between the metal and the support. The second point of interest was the preparation, using the colloidal synthesis, of Copper and Cobalt nanoparticles, as well as Copper-Cobalt bimetallic nanoparticles with core-shell structure (Co@Cu). The synthesis of these Co@Cu nanoparticles is based on the substitution of Cobalt by Copper atoms as a result of a redox reaction that permitted the synthesis of nanoparticles with different Cu/Co ratio. Moreover, the obtained nanoparticles were incorporated in a SiO2 mesoporous support by means of capillary inclusion. The final catalysts were chemically and structurally characterized with DRX, ICP, TEM, HRTEM, TPR and B.E.T. area

Growth of Au-Pd2Sn nanorods via galvanic replacement and their catalytic performance on hydrogenation and sonogashira coupling reactions

Colloidal Pd2Sn and Au–Pd2Sn nanorods (NRs) with tuned size were produced by the reduction of Pd and Sn salts in the presence of size- and shape-controlling agents and the posterior growth of Au tips through a galvanic replacement reaction. Pd2Sn and Au–Pd2Sn NRs exhibited high catalytic activity toward quasi-homogeneous hydrogenation of alkenes (styrene and 1-octene) and alkynes (phenylacetylene and 1-octyne) in dichloromethane. Au–Pd2Sn NRs showed higher activity than Pd2Sn for 1-octene, 1-octyne, and phenylacetylene. In Au–Pd2Sn heterostructures, X-ray photoelectron spectroscopy evidenced an electron donation from the Pd2Sn NR to the Au tips. Such heterostructures showed distinct catalytic behavior in the hydrogenation of compounds containing a triple bond such as tolan. This can be explained by the aurophilicity of triple bonds. To further study this effect, Pd2Sn and Au–Pd2Sn NRs were also tested in the Sonogashira coupling reaction between iodobenzene and phenylacetylene in N,N-dimethylformamide. At low concentration, this reaction provided the expected product, tolan. However, at high concentration, more reduced products such as stilbene and 1,2-diphenylethane were also obtained, even without the addition of H2. A mechanism for this unexpected reduction is proposed.Peer Reviewe

Pd2Sn [010] nanorods as a highly active and stable ethanol oxidation catalyst

The development of highly active, low cost and stable electrocatalysts for direct alcohol fuel cells remains a critical challenge. While Pd2Sn has been reported as an excellent catalyst for the ethanol oxidation reaction (EOR), here we present DFT analysis results showing the (100) and (001) facets of orthorhombic Pd2Sn to be more favourable for the EOR than (010). Accordingly, using tri-n-octylphosphine, oleylamine (OLA) and methylamine hydrochloride as size and shape directing agents, we produced colloidal Pd2Sn nanorods (NRs) grown in the [010] direction. Such Pd2Sn NRs, supported on graphitic carbon, showed excellent performance and stability as an anode electrocatalyst for the EOR in alkaline media, exhibiting 3 times and 10 times higher EOR current densities than that of Pd2Sn and Pd nanospheres, respectively. We associate this improved performance with the favourable faceting of the NRs.Peer ReviewedPostprint (published version

Pd2Sn [010] nanorods as a highly active and stable ethanol oxidation catalyst

The development of highly active, low cost and stable electrocatalysts for direct alcohol fuel cells remains a critical challenge. While Pd2Sn has been reported as an excellent catalyst for the ethanol oxidation reaction (EOR), here we present DFT analysis results showing the (100) and (001) facets of orthorhombic Pd2Sn to be more favourable for the EOR than (010). Accordingly, using tri-n-octylphosphine, oleylamine (OLA) and methylamine hydrochloride as size and shape directing agents, we produced colloidal Pd2Sn nanorods (NRs) grown in the [010] direction. Such Pd2Sn NRs, supported on graphitic carbon, showed excellent performance and stability as an anode electrocatalyst for the EOR in alkaline media, exhibiting 3 times and 10 times higher EOR current densities than that of Pd2Sn and Pd nanospheres, respectively. We associate this improved performance with the favourable faceting of the NRs.Peer Reviewe

Fe3O4@NiFexOy nanoparticles with enhanced electrocatalytic properties for oxygen evolution in carbonate electrolyte

The design and engineering of earth-abundant catalysts that are both cost-effective and highly active for water splitting are crucial challenges in a number of energy conversion and storage technologies. In this direction, herein we report the synthesis of Fe3O4@NiFexOy core shell nanoheterostructures and the characterization of their electrocatalytic performance toward the oxygen evolution reaction (OER). Such nanoparticles (NPs) were produced by a two-step synthesis procedure involving the colloidal synthesis of Fe3O4 nanocubes with a defective shell and the posterior diffusion of nickel cations within this defective shell. Fe3O4@NiFexOy NPs were subsequently spin-coated over ITO-covered glass and their electrocatalytic activity toward water oxidation in carbonate electrolyte was characterized. Fe3O4@NiFexOy catalysts reached current densities above 1 mA/cm(2) with a 410 mV overpotential and Tafel slopes of 48 mV/dec, which is among the best electrocatalytic performances reported in carbonate electrolyte.Postprint (published version

Mn3O4@CoMn2O4-CoxOy nanoparticles : partial cation exchange synthesis and electrocatalytic properties toward the oxygen reduction and evolution reactions

Mn3O4@CoMn2O4 nanoparticles (NPs) were produced at low temperature and ambient atmosphere using a one -pot two-step synthesis protocol involving the cation exchange of Mn by Co in preformed Mn3O4 NPs. Selecting the proper cobalt precursor, the nucleation of CoxOy crystallites at the Mn3O4@a CoMn2O4 surface could be simultaneously promoted to form Mn3O4@CoMn2O4-CoxOy NPs. Such heterostructured NPs were investigated for oxygen reduction and evolution reactions (ORR, OER) in alkaline solution. Mn3O4@ CoMn2O4-Cox0y NPs with [Co]/[Mn] = 1 showed low overpotentials of 0.31 Vat(-3) mA.cm(-2) and a small Tafel slope of 52 mV.dec(-1) for ORR, and overpotentials of 0.31 V at 10 mAPeer Reviewe

Fe3O4@NiFexOy nanoparticles with enhanced electrocatalytic properties for oxygen evolution in carbonate electrolyte

The design and engineering of earth-abundant catalysts that are both cost-effective and highly active for water splitting are crucial challenges in a number of energy conversion and storage technologies. In this direction, herein we report the synthesis of Fe3O4@NiFexOy core shell nanoheterostructures and the characterization of their electrocatalytic performance toward the oxygen evolution reaction (OER). Such nanoparticles (NPs) were produced by a two-step synthesis procedure involving the colloidal synthesis of Fe3O4 nanocubes with a defective shell and the posterior diffusion of nickel cations within this defective shell. Fe3O4@NiFexOy NPs were subsequently spin-coated over ITO-covered glass and their electrocatalytic activity toward water oxidation in carbonate electrolyte was characterized. Fe3O4@NiFexOy catalysts reached current densities above 1 mA/cm(2) with a 410 mV overpotential and Tafel slopes of 48 mV/dec, which is among the best electrocatalytic performances reported in carbonate electrolyte