Correction to “To Alloy or Not to Alloy? The Unexpected Power of Pd–Au Catalyst Physical Mixtures in Efficient HMF Oxidation to FDCA”

Effect of the coverage of modulated Au(Pd) atoms over bimetallic Pd-Au catalysts on catalytic performance for direct oxidative esterification of methacrolein into methyl methacrylate

Direct synthesis of H2O2 in AuPd coated micro channels: An in-situ X-Ray absorption spectroscopic study

Efficient oxidation of 5-hydroxymethylfurfural to 2,5-furandicarboxylic acid by a two-enzyme system: Combination of a bacterial laccase with catalase

The direct synthesis of H2O2 at low temperature (2 degrees C) from H2 and O2 using carbon-supported Au, Pd and Au-Pd catalysts is described and contrasted with data for TiO2, Al2O3 and Fe2O3 as supports. The Au-Pd catalysts all perform significantly better than the pure Pd/TiO2 and Au/ TiO2 materials. The Au Pd/carbon catalysts gave the highest rate of H2O2 production, and the order of reactivity observed is: carbon > TiO2 > Al2O3. Catalysts were prepared by co-impregnation of the supports using incipient wetness with aqueous solutions of PdCl2 and HAuCl4, and following calcination at 400 degrees C the catalysts were stable and could be reused several time without loss of metal. The method of preparation is critical, however, to achieve stable catalysts. No promoters are required (e.g. halides) to achieve the high rates of hydrogen peroxide synthesis. The surface and bulk composition of the gold palladium nanoparticles was investigated by STEM-XEDS spectrum imaging. For TiO2 and Al2O3 as supports the Au Pd particles were found to exhibit a core-shell structure, Pd being concentrated on the surface. In contrast, the Au-Pd/carbon catalyst exhibited Au Pd nanoparticles which were homogeneous alloys and X-ray photoelectron studies were consistent with these observations. The origin of the enhanced activity for the carbon supported catalysts is a result of higher H2 selectivity for the formation of hydrogen peroxide which is due to the surface composition and size distribution of the nanoparticles. The key problem remaining is the sequential hydrogenation of hydrogen peroxide which limits the utilisation of the direct synthesis methodology and this is discussed in detail.

ConspectusThe enhanced catalytic activity of Pd-Au catalysts originates from ensemble effects related to the local composition of Pd and Au. The study of Pd-Au planar model catalysts in an ultrahigh vacuum (UHV) environment allows the observation of molecular level catalytic reactions between the Pd-Au surface and target molecules. Recently, there has been progress in understanding the behavior of simple molecules (H2, O2, CO, etc.) employing UHV surface science techniques, the results of which can be applied not only to heterogeneous catalysis but also to electro- and photochemical catalysis.Employing UHV methods in the investigation of Pd-Au model catalysts has shown that single Pd atoms can dissociatively adsorb H2 molecules. The recombinative desorption temperature of H2 varies with Pd ensemble size, which allows the use of H2 as a probe molecule for quantifying surface composition. In particular, H2 desorption from Pd-Au interface sites (or small Pd ensembles) is observed from 150-300 K, which is between the H2 desorption temperature from pure Au (∼110 K) and Pd (∼350 K) surfaces. When the Pd ensembles are large enough to form Pd(111)-like islands, H2 desorption occurs from 300-400 K, as with pure Pd surfaces. The different H2 desorption behavior, which depends on Pd ensemble size, has also been applied to the analysis of dehydrogenation mechanisms for potential liquid storage mediums for H2, namely formic acid and ethanol. In both cases, the Pd-Au interface is the main reaction site for generating H2 from formic acid and ethanol with less overall decomposition of the two molecules (compared to pure Pd).The chemistry behind O2 activation has also been informed through the control of Pd ensembles on a gold model catalyst for acetaldehyde and ethanol oxidation reactions under UHV conditions. O2 molecules molecularly adsorbed on continuous Pd clusters can be dissociated into O adatoms above 180 K. This O2 activation process is improved by coadsorbed H2O molecules. It is also possible to directly (through a precursor mechanism) introduce O adatoms on the Pd-Au surface by exposure to O2 at 300 K. The quantity of dissociatively adsorbed O adatoms is proportional to the Pd coverage. However, the O adatoms are more reactive on a less Pd covered surface, especially at the Pd-Au interface sites, which can initiate CO oxidation at temperatures as low as 140 K. Acetaldehyde molecules can be selectively oxidized to acetic acid on the Pd-Au surface with O adatoms, in which the selectivity toward acetic acid originates from preventing the decarboxylation of acetate species. Moreover, the O adatoms on the Pd-Au surface accelerate ethanol dehydrogenation, which causes the increase in acetaldehyde production. Hydrogen is continuously abstracted from the formed acetaldehyde and remaining ethanol molecules, and they ultimately combine as ethyl acetate on the Pd-Au surface.Using Pd-Au model catalysts under UHV conditions allows the discovery of molecular level mechanistic details regarding the catalytic behavior of H and O adatoms with other molecules. We also expect that these findings will be applicable regarding other chemistry on Pd-Au catalysts.

A kinetic study of vinyl acetate synthesis over Pd-based catalysts: kinetics of vinyl acetate synthesis over Pd–Au/SiO 2 and Pd/SiO 2 catalysts

Facile preparation of three-dimensional porous Pd–Au films and their electrocatalytic activity for methanol oxidation

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.

Reactivity trends for oxidation of various alcohols and polyols have been examined for carbon-supported Au, Pd, and Au–Pd catalysts. A Hammett σρ approach was used to study substituent effects, with Hammett factors (ρ) of 1.27, 1.31, and 0.40 obtained for Pd, Au, and Au–Pd catalysts, suggesting the formation of a net negative charge at the transition state of the rate limiting step. The lower ρ for the Au–Pd catalyst versus Au and Pd monometallic catalysts indicates the ability of the Au–Pd catalyst to stabilize the negative charge at the transition state, explaining the improved performance of Au–Pd bimetallic catalysts for alcohol oxidation. Hammett–Taft factors were used to explain the low selectivity of terminal diols and polyols to diacids.

Atomic layer deposition of Pt nanoparticles on low surface area zirconium oxide for the efficient base-free oxidation of 5-hydroxymethylfurfural to 2,5-furandicarboxylic acid

To Alloy or Not to Alloy? The Unexpected Power of Pd–Au Catalyst Physical Mixtures in Efficient HMF Oxidation to FDCA

The present study focused on developing a stable basic MnOx support for Ru (RuMn) for the efficient oxidation of 5-hydroxymethylfurfural (HMF) to 2,5-furandicarboxylic acid (FDCA) in water in the absence of an external base. A series of MnOx supports, synthesized via hydrothermal approach using urea as precipitant, was prepared by thermal treatment at various temperatures (300-800 °C) before doping with Ru. The RuMn-2 (1 wt % Ru, MnOx calcined at 400 °C) possessed a large number of basic sites (1.72 mmol g-1 ) based on CO2 temperature-programmed desorption analysis, affording an FDCA yield of 87 % with a turnover frequency of 22 h-1 . Transmission electron microscopy energy-dispersive X-ray spectroscopy elemental mapping of RuMn-2 showed a high dispersion of Ru over the surface of MnOx, contributing to the efficient HMF oxidation. Moreover, X-ray diffraction, X-ray photoelectron spectroscopy, and H2 temperature-programmed reduction indicated that the predominant MnO2 phase (ϵ-MnO2 ) played a vital role in HMF oxidation. RuMn-2 was recyclable for up to four runs without significant loss in the activity and retained its structural integrity.

We report the competitive adsorption of CO, ethylene and acetate on a Pd–Au (100) surface in the reaction system of vinyl acetate synthesis with DFT method. The effect of CO on the catalytic performance of Pd–Au in the vinyl acetate system was also studied. Moreover, the method of reducing the content of CO in the industrial process and the reaction path of CO on the Pd–Au catalyst was investigated. Furthermore, the effect of CO on the acetoxylation of ethylene to vinyl acetate was reflected in two aspects. Firstly, the adsorption of CO on a Pd atom is more stable than that of ethylene, in which the adsorption of ethylene is blocked and the catalytic activity of the Pd–Au alloy decreases. Secondly, the Pd–Au alloy can catalyze the CO oxidation reaction.

Pd–Au bimetallic catalysts have shown promising performance in numerous reactions that involve hydrogen. Fundamental studies of hydrogen interactions with Pd–Au surfaces could provide useful insights into the reaction mechanisms over Pd–Au catalysts, which may, in turn, guide future catalyst design. In this study, the interactions of hydrogen (i.e., adsorption, absorption, diffusion, and desorption) with Pd/Au(111) model surfaces were studied using temperature-programmed desorption (TPD) under ultrahigh-vacuum conditions. Our experimental results reveal Pd–Au bimetallic surfaces readily dissociate H2 and yet also weakly bind H adatoms, properties that could be beneficial for catalytic reactions involving hydrogen. The presence of contiguous Pd sites, characterized by reflection–absorption infrared spectroscopy using CO as a probe molecule (CO-RAIRS), was found to be vital for the dissociative adsorption of H2 at 77 K. The H adatom binds to Pd–Au alloy sites more strongly than to Au(111) but more weakly th...

Deep aromatics hydrogenation in the presence of DBT over Au–Pd/γ-alumina catalysts