Pd single-atom sites on the surface of PdAu nanoparticles: A DFT-based Topological search for suitable compositions

Structure of model bimetallic PdAu nanoparticles is analyzed aiming to find Pd:Au ratios optimal for existence of Pd1 single-atom surface sites inside outer Au atomic shell. The analysis is performed using density-functional theory (DFT) calculations and topological approach based on DFT-parameterized topological energy expression. The number of the surface Pd1 sites in the absence of adsorbates is calculated as a function of Pd concentration inside the particles. At low Pd contents none of the Pd atoms emerge on the surface in the lowest-energy chemical orderings. However, surface Pd1 sites become stable, when Pd content inside a Pd-Au particle reaches ca. 60%. Further Pd content increase up to almost pure Pd core is accompanied by increased concentration of surface Pd atoms, mostly as Pd1 sites, although larger Pd ensembles as dimers and linear trimers are formed as well. Analysis of the chemical orderings inside PdAu nanoparticles at different Pd contents revealed that enrichment of the subsurface shell by Pd with predominant occupation of its edge positions precedes emergence of Pd surface species

Near-Ambient Pressure XPS and MS Study of CO Oxidation over Model Pd-Au/HOPG Catalysts: The Effect of the Metal Ratio

In this study, the dependence of the catalytic activity of highly oriented pyrolytic graphite (HOPG)-supported bimetallic Pd-Au catalysts towards the CO oxidation based on the Pd/Au atomic ratio was investigated. The activities of two model catalysts differing from each other in the initial Pd/Au atomic ratios appeared as distinctly different in terms of their ignition temperatures. More specifically, the PdAu-2 sample with a lower Pd/Au surface ratio (~0.75) was already active at temperatures less than 150 °C, while the PdAu-1 sample with a higher Pd/Au surface ratio (~1.0) became active only at temperatures above 200 °C. NAP XPS revealed that the exposure of the catalysts to a reaction mixture at RT induces the palladium surface segregation accompanied by an enrichment of the near-surface regions of the two-component Pd-Au alloy nanoparticles with Pd due to adsorption of CO on palladium atoms. The segregation extent depends on the initial Pd/Au surface ratio. The difference in activity between these two catalysts is determined by the presence or higher concentration of specific active Pd sites on the surface of bimetallic particles, i.e., by the ensemble effect. Upon cooling the sample down to room temperature, the reverse redistribution of the atomic composition within near-surface regions occurs, which switches the catalyst back into inactive state. This observation strongly suggests that the optimum active sites emerge under reaction conditions exclusively, involving both high temperature and a reactive atmosphere

Spatially resolved NMR spectroscopy of heterogeneous gas phase hydrogenation of 1,3-butadiene with parahydrogen

Magnetic resonance-based methods such as nuclear magnetic resonance (NMR) and magnetic resonance imaging (MRI) are widely used to provide in situ/operando information of chemical reactions. However, the low spin density and magnetic field inhomogeneities associated with heterogeneous catalytic systems containing gaseous reactants complicate such studies. Hyperpolarization techniques, in particular parahydrogen-induced polarization (PHIP), increase significantly the NMR signal intensity. In this study, we test 16 glass tube reactors containing Pd, Pt, Rh or Ir nanoparticles dispersed on a thin layer of TiO2, CeO2, SiO2 or Al2O3 for the hydrogenation of 1,3-butadiene using parahydrogen. The catalytic coatings of Ir and Rh gave hydrogenation products with the highest nuclear spin polarization while the coatings of Pd are the most selective ones for the semihydrogenation of 1,3-butadiene to 1- and 2-butenes. Spatially resolved NMR spectroscopy of the reagent and the product distribution along the reactor axis provided further mechanistic insight into the catalytic function of these reactive coatings under operando conditions.ISSN:2044-4753ISSN:2044-476

<i>In Situ</i> NAP-XPS and Mass Spectrometry Study of the Oxidation of Propylene over Palladium

The oxidation of propylene over a Pd(551) single crystal has been studied in the millibar pressure range using near-ambient pressure X-ray photoelectron spectroscopy and mass spectrometry. It has been shown that, irrespective of the O₂/C₃H₆ molar ratio in the range 1–100, the total oxidation of propylene to CO₂ and water and the partial oxidation of propylene to CO and H₂ occur when the catalyst is heated above the light-off temperature; increasing the partial pressure of O₂ leads to decreasing the catalytic activity. The selectivity toward CO₂ is at least two times higher than the selectivity toward CO, indicating that the total oxidation is the main reaction route. The normal hysteresis with a light-off temperature higher than the extinction temperature is observed in the oxidation of propylene between 100 and 300 °C. According to NAP-XPS, the main reason for the hysteresis appearing is a competition between two surface processes: carbonization and oxidation of palladium. At low temperatures, the adsorption and following decomposition of propylene dominate, which results in accumulation of carbonaceous deposits blocking the palladium surface. Increasing the catalyst temperature leads to burning the carbonaceous deposits which initiates the following oxidation of propylene. The highest conversion of propylene is observed when both free surface sites and adsorbed oxygen atoms exist in a large amount on the catalyst surface. As the partial pressure of O₂ increases, the catalyst surface gets covered by clusters of surface 2D palladium oxide, which is accompanied by a decrease in the catalytic activity. The mechanism of the oxidation of propylene over palladium is discussed

An Investigation into the Bulk and Surface Phase Transformations of Bimetallic Pd-In/Al2O3 Catalyst during Reductive and Oxidative Treatments In Situ

A series of oxidative treatments of PdIn-supported intermetallic nanoparticles at different temperatures were performed. The bulk and surface structure of catalyst during phase transformation was investigated by bulk- and surface-sensitive techniques (in situ XAFS, DRIFTS of adsorbed CO). It was found that comparison of palladium and indium fractions in bulk and on the surface suggests the formation of a «core-shell» structure. According to obtained results, the core consists of In-depleted intermetallic compound or inhomogeneous bimetallic phase with the inner core of metallic Pd, when a mixture of indium oxide, metallic palladium and small part of PdIn is present on the surface

Comparative Study of the Photocatalytic Hydrogen Evolution over Cd1−xMnxS and CdS-β-Mn3O4-MnOOH Photocatalysts under Visible Light

A series of solid solutions of cadmium and manganese sulfides, Cd1−xMnxS (x = 0–0.35), and composite photocatalysts, CdS-β-Mn3O4-MnOOH, were synthesized by precipitation with sodium sulfide from soluble cadmium and manganese salts with further hydrothermal treatment at 120 °C. The obtained photocatalysts were studied by the X-ray diffraction method (XRD), UV-vis diffuse reflectance spectroscopy, transmission electron microscopy (TEM), X-ray photoelectron spectroscopy (XPS), and N2 low temperature adsorption. The photocatalysts were tested in hydrogen production using a Na2S/Na2SO3 aqueous solution under visible light (λ = 450 nm). It was shown for the first time that both kinds of photocatalysts possess high activity in hydrogen evolution under visible light. The solid solution Cd0.65Mn0.35S has an enhanced photocatalytic activity due to its valence and conduction band position tuning, whereas the CdS-β-Mn3O4-MnOOH (40–60 at% Mn) samples were active due to ternary heterojunction formation. Further, the composite CdS-β-Mn3O4-MnOOH photocatalyst had much higher stability in comparison to the Cd0.65Mn0.35S solid solution. The highest activity was 600 mmol g−1 h−1, and apparent quantum efficiency of 2.9% (λ = 450 nm) was possessed by the sample of CdS-β-Mn3O4-MnOOH (40 at% Mn)

Sustainable Hydrogen Production from Starch Aqueous Suspensions over a Cd0.7Zn0.3S-Based Photocatalyst

We explored the photoreforming of rice and corn starch with simultaneous hydrogen production over a Cd0.7Zn0.3S-based photocatalyst under visible light irradiation. The photocatalyst was characterized by UV–vis diffuse reflectance spectroscopy, X-ray diffraction, and X-ray photoelectron spectroscopy. The influence of starch pretreatment conditions, such as hydrolysis temperature and alkaline concentration, on the reaction rate was studied. The maximum rate of H2 evolution was 730 μmol·h−1·g−1, with AQE = 1.8% at 450 nm, in the solution obtained after starch hydrolysis in 5 M NaOH at 70 °C. The composition of the aqueous phase of the suspension before and after the photocatalytic reaction was studied via high-performance liquid chromatography, and such products as glucose and sodium gluconate, acetate, formate, glycolate, and lactate were found after the photocatalytic reaction

Hydrothermal solubilization-hydrolysis-dehydratation of cellulose to glucose and 5-hydroxymethylfurfural over solid acid carbon catalysts

Solid acid catalysts based on graphite-like mesoporous carbon material Sibunit were developed for the one-pot solubilization–hydrolysis–dehydration of cellulose into glucose and 5-hydroxymethylfurfural (5-HMF). The catalysts were produced by treating Sibunit surface with three different procedures to form acidic and sulfo groups on the catalyst surface. The techniques used were: (1) sulfonation by H2SO4 at 80–250 °C, (2) oxidation by wet air or 32 v/v% solution of HNO3, and (3) oxidation-sulfonation what meant additional sulfonating all the oxidized carbons at 200 °C. All the catalysts were characterized by low-temperature N2 adsorption, titration with NaOH, TEM, XPS. Sulfonation of Sibunit was shown to be accompanied by surface oxidation (formation of acidic groups) and the high amount of acidic groups prevented additional sulfonation of the surface. All the Sibunit treatment methods increased the surface acidity in 3–15 times up to 0.14–0.62 mmol g−1 compared to pure carbon (0.042 mmol g−1). The catalysts were tested in the depolymerization of mechanically activated microcrystalline cellulose at 180 °C in pure water. The main products 5-HMF and glucose were produced with the yields in the range of 8–22 wt% and 12–46 wt%, respectively. The maximal yield were achieved over Sibunit sulfonated at 200 °C. An essential difference in the composition of main products obtained with solid acid Sibunit carbon catalysts (glucose, 5-HMF) and soluble in water H2SO4 catalysts (formic and levulinic acids) as well as strong dependence of the reaction kinetics on the morphology of carbon catalysts argue for heterogenious mechanism of cellulose depolymerization over Sibunit