The Interplay between Homogeneous and Heterogeneous Phases of PdAu Catalysts for the Oxidation of Alcohols

Highly efficient blackberry-like trimetallic PdAuCu nanoparticles with optimized Pd content for ethanol electrooxidation

The rational design of highly efficient catalysts for ethanol electrooxidation is extremely challenging for developing direct ethanol fuel cells (DEFCs). Herein, a facile one-pot method has been developed to prepare blackberry-like PdAuCu nanoparticles (NPs) with tunable composition and surface structures. Among PdAuCu NPs with different Pd contents (1.6-22 mass%), PdAuCu NPs-0.5 (contained Pd at 2.5 mass%) delivered one of the highest catalytic activities of Pd-based catalysts towards ethanol electrooxidation, exhibiting a mass activity of 23.0 A mg_Pd^-1. Kinetic analysis, electrochemical impedance spectroscopy and CO stripping test results suggested that the excellent electrocatalytic activity may originate from the optimized balance between Pd content and surface structure of PdAuCu NPs-0.5. The optimization of the balance between composition and surface structure would contribute to the further design of multimetallic nanoparticles for fuel cells and other applications.

Bimetallic PdAu Catalysts within Hierarchically Porous Architectures for Aerobic Oxidation of Benzyl Alcohol

Hierarchically porous (HP) zeotype materials (possessing both micropores and mesopores) offer improved diffusional access to intra-framework active sites, analogous to mesoporous materials, yet retain the high selectivity of the microporous (MP) bulk. We have recently designed crystalline hierarchically porous silicoaluminophosphates (SAPOs) with enhanced mass-transport characteristics, which can lead to significant improvement in catalytic activity and catalyst lifetime. In this study, we have prepared PdAu bimetallic nanostructures supported on HP-SAPO frameworks by an incipient impregnation of metal precursors followed by H₂ reduction at 300 °C, for the aerobic oxidation of benzyl alcohol to benzaldehyde. PdAu NPs supported on HP framework displayed significantly enhanced catalytic activities, when compared with their MP analogues, clearly highlighting the benefits of introducing hierarchical porosity in the SAPO support matrix.

Selective oxidation of crotyl alcohol by AuxPd bimetallic pseudo-single-atom catalysts

Pseudo single-atom Pd catalysts dispersed in gold nanoparticle matrices show high selectivity and activity for room temperature crotyl alcohol oxidation.

Controllable Synthesis of Surface Pt-Rich Bimetallic AuPt Nanocatalysts for Selective Hydrogenation Reactions

Bimetallic nanocatalysts, with efficient and controllable catalytic performance, have a promising application in chemical production. In this study, surface Pt-rich bimetallic AuPt nanoparticles with different Pt/Au ratios were prepared and tested in selective hydrogenation reactions of substituted nitroaromatics. Au nanoparticles were first prepared with n-butyllithium as a rapid reducer, which were further used as seeds in the slow growth process of Pt atoms. Because of the employed sequential reduction method and the following atom diffusion, surface Pt-rich bimetallic AuPt nanoparticles were obtained. Compared with the uniform AuPt alloy nanocatalysts synthesized by the co-reduction method with n-butyllithium as the reducer and monometallic Pt nanocatalysts, the obtained surface Pt-rich AuPt bimetallic nanocatalysts presented an enhanced catalytic selectivity or activity. The performance enhancement is assigned to the optimized Au/Pt interaction in the surface Pt-rich bimetallic nanostructures. This work demonstrates that the optimization of the stoichiometry and construction of bimetallic materials is a feasible method to synthesize controllable and efficient nanocatalysts.

Mechanism of atom economical conversion of alcohols and amines to amides using Fe(ii) pincer catalyst. An outer-sphere metal-ligand pathway or an inner-sphere elimination pathway?

In this present theoretical study, we investigated the reaction mechanism of atom-economical amide formation from alcohols and amines mediated by iron(ii) hydride complex (^iPrPNP)Fe(H)(CO) (^iPrPNP = N[CH₂CH₂(PiPr₂)]₂) using state-of-the-art density functional theory. Two scenarios of mechanistic pathways were considered, the inner-sphere and the outer-sphere pathways. In former case, the reaction of encounter complex of formaldehyde with amine is the rate-determining step with ΔG_{298 K} = 33.75 kcal mol⁻¹ while as in latter case dehydrogenation from trans-hydride is the rate-determining step having ΔG_{298 K} = 21.34 kcal mol⁻¹. Both the mechanistic scenarios operate through stepwise ionic pathways. The assessment of computational results demonstrate that inner-sphere pathway is energetically demanding and thus rendering outer-sphere pathway to be the most plausible mechanism of amide formation. Ligand modifications reveal that electron-withdrawing groups like CF₃ near N of PNP ligand reduce the catalytic efficiency of the catalyst. Furthermore, changing the isopropyl moiety of phosphine scaffold with CH₃ has a minimal impact on catalytic activity of the catalyst. Overall, our computational results provide new insights for the design and development of new Fe(ii) based pincer catalysts for atom economical amide formation from alcohols and amines.

The schematic representation depicting the difference in inner and outer-sphere pathways for amide synthesis from alcohols and amines mediated by Fe(ii) hydride complex.

Reaction mechanisms at the homogeneous–heterogeneous frontier: insights from first-principles studies on ligand-decorated metal nanoparticles

The frontiers between homogeneous and heterogeneous catalysis are progressively disappearing.

Initial Stages in the Formation of Nickel Phosphides

Metal phosphides have emerged as a new powerful class of materials that can be employed as heterogeneous catalysts in transformations mainly to generate new energy vectors and the valorization of renewables. Synthetic protocols based on wet techniques are available and are based on the decomposition of the organic layer decorating the nanoparticles. For nickel, the phosphine of choice is trioctylphosphine, and this leads to the formation of NiP_x materials. However, the temperature at which the decomposition starts has been found to depend on the quality of the nickel surface. Density functional theory, DFT, holds the key to analyze the initial steps of the formation of these phosphide materials. We have found how clean nickel surfaces, either (111) or (100), readily breaks the ligand P-C bonds. This triggers the process that leads to the replacement of a surface nickel atom by P and concomintantly forms a Ni adatom on the surface surrounded by two methyl groups, thus starting the formation of the NiP_x phase. The whole process requires low energies, in agreement with the low temperature found in the experiments, 150 °C. In contrast, if the surface is oxidized, the reaction does not proceed at low temperatures and oxygen vacancies need to be created first to start the P-C bond breaking on the Ni-clean patches. Our results show that the cleaner the surface is, the milder the reactions are required for the NiP_x formation, and thus they pave the way for gentler synthetic protocols that can improve the control of these materials.

Synthesis and characterization of Co3O4 immobilized on dipeptide-functionalized silica-coated magnetite nanoparticles as a catalyst for the selective aerobic oxidation of alcohols

A supermagnetic core–shell nanocatalyst functionalized with a dipeptide ligand as a recyclable catalyst for the selective oxidation of alcohols.

Recyclable cobalt(0) nanoparticle catalysts for hydrogenations

Small Co(0) nanoparticles catalyze hydrogenations of alkenes, alkynes, imines, and heteroarenes; the magnetic properties enabled catalyst separation and multiple recyclings.

Synergistic effect of bimetallic PdAu nanocrystals on oxidative alkyne homocoupling

A bimetallic Pd–Au heterogeneous catalyst with synergistic effects is developed to efficiently catalyze oxidative homocoupling of terminal alkynes.

Pd/Cu-Oxide Nanoconjugate at Zeolite-Y Crystallite Crafting the Mesoporous Channels for Selective Oxidation of Benzyl-Alcohols

Solid-state grinding of palladium and copper salts allowed the growth of palladium/copper oxide interface at the zeolite-Y surface. The hybrid nanostructured material was used as reusable heterogeneous catalyst for selective oxidation of various benzyl alcohols. The large surface area provided by the zeolite-Y matrix highly influenced the catalytic activity, as well as the recyclability of the synthesized catalyst. Impregnation of PdO-CuO nanoparticles on zeolite crystallite leads to the generation of mesoporous channel that probably prevented the leaching of the metal-oxide nanoparticles and endorsed high mass transfer. Formation of mesoporous channel at the external surface of zeolite-Y was evident from transmission electron microscopy and surface area analysis. PdO-CuO nanoparticles were found to be within the range of 2-5 nm. The surface area of PdO-CuO-Y catalyst was found to be much lower than parent zeolite-Y. The decrease in surface area as well as the presence of hysteresis loop in the N₂-adsoprtion isotherm further suggested successful encapsulation of PdO-CuO nanoparticles via the mesoporous channel formation. The high positive shifting in binding energy in both Pd and Cu was attributed to the influence of zeolite-Y framework on lattice contraction of metal oxides via confinement effect. PdO-CuO-Y catalyst was found to oxidize benzyl alcohol with 99% selectivity. On subjecting to microwave irradiation the same oxidation reaction was found to occur at ambient condition giving same conversion and selectivity.

Toward a Comprehensive Understanding of Enhanced Photocatalytic Activity of the Bimetallic PdAu/TiO2 Catalyst for Selective Oxidation of Methanol to Methyl Formate

Photocatalytic selective oxidation of alcohols over titania supported with bimetallic nanoparticles represents an energy efficient and sustainable route for the synthesis of esters. Specifically, the bimetallic PdAu/TiO₂ system was found to be highly active and selective toward photocatalytic production of methyl formate (MF) from gas-phase methanol. In the current paper, we applied the electronic structure density functional theory method to understand the mechanistic aspects and corroborate our recent experimental measurements for the photocatalytic selective oxidation of methanol to MF over the PdAu/TiO₂ catalyst. Our theoretical results revealed the preferential segregation of Pd atoms from initially mixed PdAu nanoclusters to the interface of PdAu/TiO₂ and subsequent formation of a unique structure, resembling a core@shell architecture in close proximity to the interface. The analysis of the calculated band gap diagram provides an explanation of the superior electron-hole separation capability of PdAu nanoparticles deposited onto the anatase surface and hence the remarkably enhanced photocatalytic activity, in comparison to their monometallic counterparts. We demonstrated that facile dissociation of molecular oxygen at the triple-point boundary site gives rise to in situ oxidation of Pd. The in situ formed PdO/TiO₂ is responsible for total oxidation of methanol to CO₂ (no MF formation) in the gas phase. Our investigation provides theoretical guidance for designing highly selective and active bimetallic nanoparticles-TiO₂ catalysts for the photocatalytic selective oxidation of methanol to MF.

Simultaneous enzymatic activity modulation and rapid determination of enzyme kinetics by highly crystalline graphite dots

The research field in enzyme-based biotechnology urgently requires the discovery of new materials and methods with high-performance. Here we report that highly crystalline graphite dots (GDs) can modulate enzyme activities, and simultaneously allow for real-time measurements on enzyme kinetics in combination with mass spectrometry (MS). A well-defined modulation of lipolytic activities from inhibition to enhancement can be realized by selectively coupling lipase enzymes with GDs containing specific functional groups on the surface. As a unique feature of our approach, GDs in the enzyme reaction can simultaneously serve as a versatile matrix for rapid and sensitive detection of the residual enzyme substrate, the intermediate or final product of lipolytic digestion using MS technology. Therefore, enzyme kinetic data can be collected in a real-time, high-throughput format. This work provides a new platform for enzymological research in hybrid bio-catalytic processes with advanced nanotechnology.

Structural Rearrangement of Au-Pd Nanoparticles under Reaction Conditions: An ab Initio Molecular Dynamics Study

The structure, composition, and atomic distribution of nanoalloys under operating conditions are of significant importance for their catalytic activity. In the present work, we use ab initio molecular dynamics simulations to understand the structural behavior of Au-Pd nanoalloys supported on rutile TiO₂ under different conditions. We find that the Au-Pd structure is strongly dependent on the redox properties of the support, originating from strong metal-support interactions. Under reducing conditions, Pd atoms are inclined to move toward the metal/oxide interface, as indicated by a significant increase of Pd-Ti bonds. This could be attributed to the charge localization at the interface that leads to Coulomb attractions to positively charged Pd atoms. In contrast, under oxidizing conditions, Pd atoms would rather stay inside or on the exterior of the nanoparticle. Moreover, Pd atoms on the alloy surface can be stabilized by hydrogen adsorption, forming Pd-H bonds, which are stronger than Au-H bonds. Our work offers critical insights into the structure and redox properties of Au-Pd nanoalloy catalysts under working conditions.