Tuning CO2 hydrogenation selectivity to obtain targeted value‐added chemicals and fuels has attracted increasing attention. However, a fundamental understanding of the way to control the selectivity ...is still lacking, posing a challenge in catalyst design and development. Herein, we report our new discovery in ambient pressure CO2 hydrogenation reaction where selectivity can be completely reversed by simply changing the crystal phases of TiO2 support (anatase‐ or rutile‐TiO2) or changing metal loadings on anatase‐TiO2. Operando spectroscopy and NAP‐XPS studies reveal that the determining factor is a different electron transfer from metal to the support, most probably as a result of the different extents of hydrogen spillover, which changes the adsorption and activation of the intermediate of CO. Based on this new finding, we can not only regulate CO2 hydrogenation selectivity but also tune catalytic performance in other important reactions, thus opening up a door for efficient catalyst development by rational design.
CO2 hydrogenation selectivity can be tuned by changing the crystal phase of TiO2 or simply changing the metal loadings on anatase‐TiO2 supported Ru catalysts. The essential reason for the distinctly different selectivity is the charge transfer under reaction conditions, probably as a result of hydrogen spillover, which leads to changes in adsorption and activation of intermediate CO.
Pd–Ga bimetallic nanoparticles are formed under CO2 hydrogenation to methanol (523K, 3MPa) and under pure hydrogen reduction (523K, 0.1MPa) on a Pd (1wt.%)/Ga2O3 catalyst. The main roles of the ...bimetallic particles in the reaction mechanism are to hydrogenate the carbonaceous species adsorbed on the Ga2O3 support and to inhibit CO production. Display omitted
► Pd–Ga bimetallic nanoparticles are formed upon hydrogen reduction. ► Pd–Ga bimetallic nanoparticles enhance the selectivity to methanol. ► In situ infrared experiments show that Pd–Ga/Ga2O3 behaves as truly bifunctional. ► Encapsulation of Pd by Ga2O3 is observed by electron microscopy after air exposure.
The effect of palladium–gallia interaction in Pd(1wt.%)/β-Ga2O3 during selective methanol synthesis by CO2 hydrogenation was studied. A detailed quasi-in situ transmission electron microscopy analysis of the as-prepared H2-reduced catalyst, without exposing it to air, showed that Ga–Pd bimetallic (nano)particles were formed under a reductive atmosphere at or above 523K. However, these particles were unstable; upon air exposure, a dramatic and extensive encapsulation of the metallic crystallites by Ga2O3 occurred. In addition, the function of the bimetallic particles in the mechanism of methanol synthesis was investigated by in situ infrared spectroscopy at 0.7MPa. The results confirmed those of previous studies in which the stepwise hydrogenation of (bi)carbonate to formate and then to methoxy groups on the Ga2O3 surface took place via a bifunctional pathway. In this pathway, the role of the Ga–Pd bimetallic crystallites was to provide atomic hydrogen, via spillover, to the oxidic surface and to hamper both CH3OH decomposition and CO production.
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•Reversible deactivation on gold-supported catalysts.•Oxidation of adsorbed CO on gold is retarded by the presence of CO2.•Some adsorbed CO2 species – carbonates – inhibits the CO ...oxidation.•CO2 adsorbs on the oxygen sites able to oxidize Au–CO at the metal–support interface.
Highly active supported gold catalysts frequently undergo substantial decrease in activity due to deactivation. The aim of this paper is to investigate a reversible inhibition phenomenon of the CO oxidation in an Au/Ce0.62Zr0.38O2 (Au/CZ) catalyst. The results of a systematic study of CO oxidation conditions (CO, O2, and CO2) are discussed. By identifying CO2 as an Au/CZ poison using isotopic transient analysis (13CO/12CO2), we also investigate the origin of inhibition activity by transient and modulated infrared spectroscopy in DRIFT mode. Results demonstrate that carbonate species are formed by reaction of CO2 with reactive lattice oxygen at the metal–support interphase, which in turn inhibits the replenishment of the vacancies by molecular oxygen. A microkinetic model that accounts for the reaction mechanism and inhibition by CO2 is proposed.
Reducible oxides have been shown to greatly improve the activity of water gas shift (WGS) catalysts. The precise mechanism for this effect is a matter of intense debate, but the dissociation of water ...is generally considered to be the key step in the reaction. We present here a study of the water activation on oxygen vacancies at the support as part of the mechanism of the WGS reaction on Pt supported on pure and gallium-doped ceria. Doping the ceria with gallium allows tuning the vacancies in the support while maintaining constant the metal dispersion. An inverse relationship was found between the catalytic activity to WGS and the amount of oxygen vacancies. In situ time-resolved X-ray diffraction, mass spectrometry, and diffuse reflectance infrared spectroscopy (DRIFT) showed that the oxygen vacancy filling by water is always fast in either Pt/CeO2 or Pt/CeGa. DFT calculation provides molecular insights to understand the pathway of water reaction with vacancies at the metal–oxide interface sites. Our results suggest that the activation of the water molecule in the WGS mechanism is not the rate-limiting step in these systems. Concentration-modulation spectroscopy in DRIFT mode under WGS reaction conditions allows the selective detection of key reaction intermediates, a monodentate formate (HCOO) and carboxylate (CO2 δ−) species, which suggests the prevalence of a carboxyl (HOCO) mechanism activated at the oxide–metal interface of the catalyst.
The adsorption of CO2 over a set of gallium (III) oxide polymorphs with different crystallographic phases (α, β, and γ) and surface areas (12−105 m2 g-1) was studied by in situ infrared spectroscopy. ...On the bare surface of the activated gallias (i.e., partially dehydroxylated under O2 and D2 (H2) at 723 K), several IR signals of the O−D (O−H) stretching mode were assigned to mono-, di- and tricoordinated OD (OH) groups bonded to gallium cations in tetrahedral and/or octahedral positions. After exposing the surface of the polymorphs to CO2 at 323 K, a variety of (bi)carbonate species emerged. The more basic hydroxyl groups were able to react with CO2, to yield two types of bicarbonate species: mono- (m-) and bidentate (b-) νas(CO3) = 1630 cm-1; νs(CO3) = 1431 or 1455 cm-1 (for m- or b-); δ(OH) = 1225 cm-1. Together with the bicarbonate groups, IR bands assigned to carboxylate νas(CO2) = 1750 cm-1; νs(CO2) = 1170 cm-1, bridge carbonate νas(CO3) = 1680 cm-1; νs(CO3) = 1280 cm-1, bidentate carbonate νas(CO3) = 1587 cm-1; νs(CO3) = 1325 cm-1, and polydentate carbonate νas(CO3) = 1460 cm-1; νs(CO3) = 1406 cm-1 species developed, up to approximately 600 Torr of CO2. However, only the bi- and polydentate carbonate groups still remained on the surface upon outgassing the samples at 323 K. The total amount of adsorbed CO2, measured by volumetric adsorption (323 K), was ∼2.0 μmol m-2 over any of the polymorphs, congruent with an integrated absorbance of (bi)carbonate species proportional to the surface area of the materials. Upon heating under flowing CO2 (760 Torr), most of the (bi)carbonate species vanished a T > 550 K, but polydentate groups remained on the surface up to the highest temperature used (723 K). A thorough discussion of the more probable surface sites involved in the adsorption of CO2 is made.
The adsorption of methanol was studied on three gallia polymorphs (α, β, and γ), pretreated under oxygen or hydrogen at 723 K. Their Brunauer−Emmett−Teller surface areas were in the range 12−105 m2 ...g−1. Methanol (or methanol-d3) chemisorbs on the gallium oxides both molecularly, as CH3OHS (or CD3OHS,) and dissociatively, as methoxy (CH3O or CD3O) species, at 373 K. The quantification of the total amount of chemisorbed methanol at this temperature allowed us to determine the number of available surface active sites per unit area (N S), which is in the range 1−2 μmol m−2 for the oxygen pretreated oxides at 723 K. The density of active sites was moderately smaller (∼25%) after pretreating the oxides under hydrogen at 723 K. The temperature-programmed surface reaction of adsorbed methanol and methoxy was followed by mass spectrometry and infrared spectroscopy under He flow, up to 723 K. It was found that, upon heating above 473 K, methoxy oxidized to methylenbisoxi (H2COO) and, then, to formate (HCOO) species, and traces of dimethyl ether were also detected. Surface formate species further decompose to give CO(g) and CO2(g) at temperatures higher than 573 K, with the concurrent generation of OH and H species over the surface, which react toward H2(g). It is suggested that the CO2 production implies the removal of lattice oxygen, generating a surface oxygen vacancy, which can be restored by water molecules from the gas phase. Thus, gallia can be envisaged as a promising support for the steam reforming of methanol, as long as a (noble) metal officiates/acts as a rapid H2 releaser from the surface.
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•A Flow-through cell-microreactor in Attenuated Total Reflection (ATR) mode was used.•Hydrogenation of acetonitrile to amines over a platinum catalyst.•Imine surface intermediate ...(CH3CH = NH) was identified.•A microkinetic mechanism was proposed and kinetic constants were obtained.
The mechanism of the acetonitrile hydrogenation to amines over a platinum/alumina catalyst was investigated by in situ infrared spectroscopy. The reaction was studied under realistic conditions -liquid phase using toluene as solvent- using a flow-through cell-microreactor in Attenuated Total Reflection (ATR) mode. Acetonitrile linearly chemisorbed on platinum sites is stepwise hydrogenated to form an imine surface intermediate (CH3CH = NH), which in turn is hydrogenated to ethylamine. Using deuterium, the sequential formation of CD and ND bonds was observed. Then, imine intermediate can condense producing diethylamine and triethylamine, giving ammonia as a by-product. The temporal evolution of the IR signals was modeled using a proposed microkinetic mechanism and intrinsic kinetic constants were obtained under chemical control.
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•CO2 hydrogenation to methanol on Ga-Pd/SiO2 catalysts is highly selective.•Pd2Ga nanoparticles are produced after reduction at 723 K.•Bimetallic particles are in contact with Ga2O2 ...thin layers.•Carbonaceous species on Ga2O3 are hydrogenated via spillover.
A series of palladium (2 wt.%) catalysts supported on silica (301 m2/g) and loaded with increasing amount of gallium – ratio of Ga/Pd = 2, 4 and 8 atom/atom – were investigated for CO2 hydrogenation to methanol. The turnover frequency to methanol (H2/CO2 = 3; 523 K, 3 MPa), based on surface palladium, showed a 200-fold enhancement as compared to the monometallic Pd/SiO2 catalyst. Additionally, the apparent activation energy for methanol synthesis decreased from 60 kJ/mol on Pd/SiO2 to ∼40 kJ/mol on the supported Ga-Pd catalysts. Characterization of the Pd-Ga catalyst series by X-ray absorption spectroscopy and high resolution transmission electron microscopy indicates the formation of Pd2Ga bimetallic nanoparticles partially covered by a thin layer of Ga2O3 on the silica surface. In situ infrared spectroscopy was employed to examine the reaction mechanism during the CO2 adsorption and hydrogenation at 0.7 MPa. It is proposed a bifunctional pathway where the carbonaceous species bound to the gallium oxide surface are hydrogenated, stepwise, to formate and methoxy groups by atomic hydrogen, which spillovers from the Pd-Ga bimetallic nanoparticles.
Blends of fully and partially hydrogenated soybean oil (80:20 wt %) were enzymatically interesterified employing a commercial immobilized lipase (Novozym 435) to obtain bioparaffins of industrial ...interest. The reaction was performed at 90 °C, shortening the reaction time, and the reutilization of the lipase can be done for up to 9 cycles. The synthesized materials were characterized by gas chromatography, differential scanning calorimetry, X-ray diffraction, polarized light microscopy, and by a penetration test. Interesterification modified the composition of triacylglycerols reducing the concentration of trisaturated and triunsaturated and increasing the amount of monounsaturated-disaturated. Minor quantities of mono- and diacylglycerols were also produced. The change in the triacylglycerols composition lowers the melting point and modifies the crystallization behavior. Textural characterization reveals that interesterified blends decrease in crystal size and improve plasticity as compared with the original blends. Bioparaffins produced by this procedure possess a similar range of melting points and textural behavior as that of mineral paraffins widely used in the industry, representing a more sustainable and eco-friendly alternative.
Operando DRIFT spectroscopy is used to elucidate the role of key reaction intermediates during toluene oxidation on a Pd/CeO2 catalyst and bare CeO2 support. Selective identification of active ...surface species and their discrimination from spectators were carried out by combining concentration-modulation excitation spectroscopy (c-MES) experiments with phase-sensitive detection (PSD) spectral analysis. The resolution of highly overlapped infrared bands in MES-PSD spectra was performed by a chemometric multivariate curve resolution-alternating least squares (MCR-ALS) method. Pd/CeO2 catalyst completely oxidizes toluene to CO2 (T50 = 235 ºC), while pure CeO2 activates the toluene molecule but incomplete combustion to CO and formaldehyde is observed. Our results revealed that the methyl group of adsorbed toluene on the ceria surface is activated by subtraction of an H atom by a lattice O−2, forming benzyl (C6H5CH2-) species intermediate. Then, the benzyl species is stepwise oxidized to benzyl alcohol, benzaldehyde, and benzoate. Subsequent decarboxylation of benzoate and oxidation of the aromatic ring produces formate and aldehyde-like species, which are finally oxidized to CO2 and water. Alternatively, the benzoate intermediate can be oxidized to anhydride species (maleic anhydride or succinic anhydride) that accumulates on the surface and are slowly oxidized to CO2. The main role of the palladium metal nanoparticles is to facilitate the replenishing of the lattice oxygen via a metal-assisted Mars-van Krevelen mechanism. These findings provide molecular insights into a key environmental catalysis process, which will improve the rational design of optimized catalytic systems.
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•Pd/CeO2 catalyst selectively oxidizes toluene to CO2.•Operando DRIFT shows adsorption and oxidation of toluene on ceria support.•Modulation excitation experiments allow identification of surface intermediates.•Highly overlapped bands are resolved using a chemometric (MCR-ALS) method.•Benzyl, benzaldehyde, benzoate and aldehyde-like species are observed.
