Interactions between oxygen and silver are important in many areas of science and technology, including materials science, medical, biomedical and environmental applications, spectroscopy, photonics, and physics. In the chemical industry, identification of oxygen structures on Ag catalysts is important in the development of environmentally friendly and sustainable technologies that utilize gas-phase oxygen as the oxidizing reagent without generating byproducts. Gas-phase oxygen adsorbs on Ag atomically by breaking the O–O bond and molecularly by preserving the O–O bond. Atomic O structures have Ag–O vibrations at 240–500 cm–1. Molecular O2 structures have O–O vibrations at significantly higher values of 870–1150 cm–1. In this work, we identify hybrid atomic-molecular oxygen structures, which form when one adsorbed O atom reacts with one lattice O atom on the surface or in the subsurface of Ag. Thus, these hybrid structures require dissociation of adsorbed molecular oxygen into O atoms but still possess the O–O bond. The hybrid structures have O–O vibrations at 600–810 cm–1, intermediate between the Ag–O vibrations of atomic oxygen and the O–O vibrations of molecular oxygen. The hybrid O–O structures do not form by a recombination of two adsorbed O atoms because one of the O atoms in the hybrid structure must be embedded into the Ag lattice. The hybrid oxygen structures are metastable and, therefore, serve as active species in selective oxidation reactions on Ag catalysts.

The development of improved technologies for biomass processing into transportation fuels and industrial chemicals is hindered due to al ack of efficient catalysts for selectiveo xygen removal.H ere we report that platinum nanoparticles decorated with subnanometer molybdenum clusters can efficiently catalyzeh ydrodeoxygenation of acetica cid, which serves as am odel biomass compound. In contrast with monometallic Mo catalysts that are inactive and monometallic Pt catalysts that have low activities and selectivities, bimetallic Pt-Mo catalysts exhibit synergistic effects with high activities and selectivities. The maximum activity occurs at aP tt oM om olar ratio of three. Although Mo atoms themselvesa re catalytically inactive, they serve as preferentialb inding anchors for oxygen atoms while ac atalytic transformation proceeds on neighboring surfaceP ta toms. Beyond biomass processing, Pt-Mo nanoparticles are promisingc atalysts for aw ide variety of reactionst hat requireatransformation of molecules with an oxygen atom and, more broadly, in other fieldso fs cience and technology that require tuning of surface-oxygen interactions.Promising new technologiesf or biomassc onversion into fuels and chemical feedstocks rely on production of bio-oils, which need to be upgraded to removeo xygen-containing hydrocarbons (oxygenates) andw ater. [1] In contrast with fossil fuels that are usually contaminatedw ith sulfur and nitrogen-containing compounds but are free of oxygenates,a ll biomass contains oxygen. Removalo fo xygenates is required because ah igh oxygen concentration makes bio-oils acidic and corrosive, unstable during storage, and less energetically valuable per unit weightt han petroleum-derived hydrocarbons. Althoughe fficient pyrolysis and liquefaction processes have been recently developed for the production of bio-oils,o xygen removal from these bio-oils remains challenging. Current technologies for removingo xygen in the presence of hydrogen (hydrodeoxygenation) rely mostly on traditionalp etroleum refining catalysts, which are not optimizedf or biomass processing and, as a result, suffer from insufficienta ctivity, selectivitya nd stability. More efficientu pgrading technologiesa re, therefore, needed for the developmento fs ustainable energy and chemical resources. [1b, d-f] Among major oxygenate types in bio-oils, carboxylic acids are some of the most challenging because their hydrodeoxygenationr ates are usuallya no rder of magnitudel ower. [2] Therefore, acetic acid, the simplest carboxylic acid, is often used as am odel compound in the development of new hydrodeoxygenation processes. [2a,c, 3] Furthermore, acetic acid itself is as ignificant component of bio-oils with ac oncentration from typically5 -8 wt % [1d, 4] to as high as 32 wt %, depending on the type of the initial biomass. [5] In the development of improved catalysts, multiple metals, supports and reaction conditions were evaluated for acetic acid hydrodeoxygenation. For example, in additiont ot raditionals ulfidedN ia nd Mo-based ca...

Temperature programmed reaction (TPR) measurements with propane over silica-supported Ni, NiÀ Sn and Sn catalysts show that the reaction products change significantly from mostly methane, hydrogen and surface carbon over Ni to propylene and hydrogen over NiÀ Sn. Propylene formation over NiÀ Sn starts at a moderate temperature of 630 K. Since the activity of Sn by itself is low, Sn serves as a promoter for Ni. The promoter effects are attributed to a lower adsorption energy of molecularly adsorbed propylene and suppression of propylidyne formation on NiÀ Sn based on temperature programmed desorption (TPD) and infrared reflection absorption spectroscopy (IRAS) measurements as well as density functional theory (DFT) calculations for propylene adsorption on Ni(110) and c(2 × 2)-Sn/Ni(110) single-crystal surfaces. On Ni, propylene forms a π-bonded structure with ν(C=C) at 1500 cm À 1 , which desorbs at 170 K, and a di-σ-bonded structure with ν(C=C) at 1416 cm À 1 , which desorbs at 245 K. The di-σ-bonded structure is asymmetric, with the methylene C atom being in the middle of the NiÀ Ni bridge site, and the methylidyne C atom being above one of these Ni atoms. Therefore, this structure can also be characterized as a hybrid between di-σ-and π-bonded structures. Only a fraction of propylene desorbs from Ni because propylene can convert into propylidyne, which decomposes further. In contrast, propylene forms only a π-bonded structure on NiÀ Sn with ν(C=C) at 1506 cm À 1 , which desorbs at 125 K. The low stability of this structure enables propylene to desorb fully, resulting in high reaction selectivity in propane dehydrogenation to propylene over the NiÀ Sn catalyst.

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Guaiacol (2-methoxyphenol, C6H4(OH)­(OCH3)) adsorption and reactions on a Pt(100) surface were studied with infrared reflection–absorption spectroscopy (IRAS) and temperature programmed desorption (TPD) measurements at different surface coverage values from 100 to 800 K. In addition, density functional theory (DFT) calculations were used to determine geometries, adsorption energies, and vibrational frequencies for adsorption structures. Depending on surface coverage, guaiacol formed one or two physisorbed states. At low coverage, a single state with a desorption peak at 225 K was observed. At high coverage, two physisorbed states were observed with desorption peaks at 195 and 225 K. At temperatures above 225 K, after the desorption of physisorbed layers, a dissociatively adsorbed structure, C6H4O­(OCH3) + H, was observed. Recombinative molecular guaiacol desorption was detected at 320 K. The dissociatively adsorbed structure was stable up to 337 K when C–O bonds began to break. Molecularly adsorbed guaiacol in horizontal (flat-lying) configurations bound through its benzene ring was not observed under all tested conditions. Similarities of vibrational spectra and desorption measurements for a Pt(100) surface in this study and a Pt(111) surface reported previously demonstrate that the obtained results are generally valid for low-index Pt crystal planes and, more importantly, for catalytic Pt nanoparticles.

Kinetic measurements for silica-supported Pt and Pt−Mo catalysts were collected in vapor-phase acetic acid hydrodeoxygenation by varying the hydrogen partial pressure between 18 and 72 kPa and the acetic acid partial pressure between 7 and 18 kPa at 423−473 K. Under all testing conditions, the Pt− Mo catalyst was more active and selective. In addition, the apparent activation energy for Pt−Mo of 76 ± 3 kJ/mol was lower than that of 84 ± 4 kJ/mol for Pt. The apparent reaction orders were also different. The order in hydrogen of 0.8 ± 0.1 for Pt−Mo changed to zero at higher hydrogen partial pressures while that for Pt remained constant at 0.6 ± 0.1. A near-zero order in acetic acid for Pt−Mo changed to −2.1 at higher acetic acid pressures while that for Pt remained constant at −2.9 ± 0.3. These differences in reaction kinetics as well as in selectivity trends with changes in temperature and feed composition indicated a change in the reaction mechanism for Pt−Mo. The catalysts were characterized with hydrogen temperature programmed desorption, oxygen temperature programmed oxidation and transmission electron microscopy with energy-dispersed X-ray spectroscopy elemental mapping. Mo was present in the form of subnanometer-size clusters on the surface of Pt nanoparticles. Both Pt and Pt−Mo catalysts were stable under the reaction conditions for 10 h, and the size and structure of Pt and Pt−Mo particles remained mostly unchanged, without coke accumulation. Density functional theory calculations show that neighboring Pt−Mo surface atoms act as a single active site where Mo serves as a preferential binding anchor for O atoms. The presence of Mo changes the structure and reactivity of hydrocarbon surface species, leading to a change in the reaction mechanism. In contrast with Pt, C−O bond breaking reactions become more favorable on Pt−Mo and, conversely, C−C bond breaking reactions become less favorable on Pt−Mo, explaining the experimentally observed higher activities and selectivities of Pt−Mo.

The O-H stretching vibration of surface hydroxyls remained at 3691 cm-1 for gold structures ranging in size from clusters to nanoparticles, to non-flat bulk surfaces. In contrast, this vibration was...

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