Dissociative Adsorption Of Ethanol Research Articles

In Situ ATR-FTIR Investigation of the Effects of H2O and D2O Adsorption on the TiO2 Surface

We have investigated the behavior of H2O and D2O adsorption on TiO2 surfaces in the dark and under UV irradiation using in situ ATR-FTIR spectroscopy. The influence of an electron scavenger (oxygen) and a hole scavenger (ethanol) on the hydroxyl group and/or hydration water behavior on the TiO2 surface were also investigated. Adsorption of H2O–D2O mixtures revealed an isotopic exchange reaction occurring in the dark onto the surface of the TiO2 material. Under UV(A) irradiation, the amount quanitity of both OH and OD groups was found to be increased by the presence of molecular oxygen. On the other hand, the ATR-FTIR study of the ethanol adsorption in H2O and D2O reveals revealed a stronger adsorption ability ofcapacity for ethanol compared to both H2O and D2O resulting in molecular and dissociative adsorption of ethanol on the TiO2 surface. When the system was subsequently illuminated with UV(A) light, the surface becomes enriched with adsorbed water. Different possible mechanisms and hypotheses are discussed in terms of the effect of UV irradiation on the TiO2 particle network for the photocatalytic reaction and photoinduced hydrophilicity.

A mechanism of gas-phase alcohol oxidation at the interface of Au nanoparticles and a MgCuCr2O4spinel support

The catalytic oxidation of bio-ethanol to acetaldehyde entails a promising route for valorization of biomass into many important chemicals that are currently mainly being produced from fossil-based ethylene feedstock. We employ here DFT calculations to understand the unprecedented synergy between gold clusters and a MgCuCr2O4 spinel support, which shows excellent catalytic performance for the oxidation of ethanol to acetaldehyde (space-time yield of 311 gacetaldehyde ggold−1 h−1 at 250 °C). The investigations support a mechanism involving catalytic reactions at the gold–support interface. Dissociative adsorption of ethanol is facilitated by cooperative action of a gold atom at the metal cluster–support interface and a basic oxygen atom of the support. The most difficult step is the recombinative desorption of water from the surface. The oxygen vacancy formation energy is found to be a good performance descriptor for ethanol oxidation of Au/MgMeCr2O4 (Me = Cu, Ni, Co) catalysts. The high selectivity towards acetaldehyde stems from the facile desorption of acetaldehyde as compared to the cleavage of the remaining α-C–H bond in the product. The opposite holds for methanol oxidation, explaining why experimentally we observe complete methanol oxidation over Au/MgCuCr2O4 under conditions where ethanol is selectively converted to acetaldehyde.

Near ambient pressure XPS investigation of the interaction of ethanol with Co/CeO2(1 1 1)

Near ambient pressure X-ray photoelectron spectroscopy was applied to study the interaction of ethanol (CH3CH2OH) with a well-ordered CeO2(111) film on Cu(111) and with a Co/CeO2(111)/Cu(111) model catalyst. The oxidation state of the surface and the chemical nature of reaction intermediates were analyzed. At 300K, the oxidation state of ceria decreased gradually with increasing ethanol pressure. At a constant pressure of 0.1mbar, the reduction of Ce4+ to Ce3+ increased significantly between 320 and 600K due to a higher mobility of oxygen or Ce3+ centers at elevated temperatures. The main intermediate, ethoxide, was formed by dissociative adsorption of ethanol at room temperature. No coke formation was observed up to 600K on CeO2. Upon deposition of metallic cobalt, partial reduction of ceria was observed, leading to the formation of Co2+ sites but still leaving metallic Co in the metal particles. During the reaction of ethanol with the Co/CeO2(111) model catalyst, the amount of Co2+ decreased drastically with increasing temperature, and at 600K, the majority of Co was metallic. This process was accompanied by the severe reduction of ceria and the formation of significant carbon deposits.

Isotopic transient analysis of the ethanol coupling reaction over magnesia

Isotopic transient analysis of ethanol coupling to butanol over MgO in a fixed-bed reactor at 673K revealed a surface coverage of adsorbed ethanol equivalent to about 50% of the exposed MgO atomic pairs. DRIFTS of ethanol reaction at 673K confirmed that the surface was populated primarily with adsorbed ethoxide and hydroxide, presumably from the dissociative adsorption of ethanol. The coverage of reactive intermediates leading to butanol was an order of magnitude lower than that of adsorbed ethanol, and about half the surface base sites counted by adsorption of CO2. The intrinsic turnover frequency for the coupling reaction at 673K determined by isotopic transient analysis was 0.04s−1, which is independent of any assumptions about the nature of the active sites. Although the ethanol coupling reaction appears to involve aldol condensation of an aldehyde intermediate, the high coverage of ethanol under steady-state conditions apparently inhibits unproductive CC coupling reactions that deactivate the catalyst at high temperature.

Electrocatalytic Activity of Pd, Pt, and Rh for the Electro-oxidation of Ethanol in Alkaline Electrolyte: An Online DEMS Study

The electro-oxidation of ethanol was investigated on electrodeposited layers of Pd, Pt, and Rh in alkaline electrolyte. The reaction products were monitored by experiments of online differential electrochemical mass spectrometry (DEMS). Potentiodynamic curves for the ethanol electro-oxidation catalyzed by these three different metal electrocatalysts showed similar onset potentials, but the highest Faradaic current peak was observed for the Pt electrocatalyst. Online DEMS experiments evidenced similar amounts of CO2 for the three different materials, but Pd presented the higher production of ethylacetate (acetic acid). This indicated that the electrochemical oxidation of ethanol on the Pd surface occurred to a higher extent. The formation of methane, which was observed for Pt and Rh, after potential excursions to lower potentials, was absent for Pd. On the basis of the obtained results, it was stated that, on Pt and Rh, the formation of CO2 occurs mainly via oxidation of CO and CHx,ad species formed after dissociative adsorption of ethanol or ethoxy species that takes place only at low potentials. This indicates that the dissociative adsorption of ethanol or ethoxy species is inhibited at higher potentials on Pt and Rh. On the other hand, on the Pd electrocatalyst, the reaction may occur via nondissociative adsorption of ethanol or ethoxy species at lower potentials, followed by oxidation to acetaldehyde and, after that, by a further oxidation step to acetic acid on the electrocatalyst surface. Additionally, in a parallel route, the acetaldehyde molecules adsorbed on the Pd surface can be deprotonated, yielding a reaction intermediate in which the carbon–carbon bond is less protected, and therefore, it can be dissociated on the Pd surface, producing CO2, after potential excursions to higher potentials.

Kinetic study of ethanol dehydrogenation to ethyl acetate promoted by a copper/copper-chromite based catalyst

A kinetic study of the ethanol dehydrogenation to ethyl acetate on a copper/copper-chromite catalyst has been performed. The used catalyst, in cylindrical pellets, contained also alumina as a support and barium chromate as a promoter. Support and promoter have the effect of increasing the activity, the selectivity and the stability of the catalyst, as shown in a previous work. The kinetic runs were carried out in a packed bed tubular reactor, alternatively filled with 2 or 50g of catalyst, approximately isothermal, by feeding pure ethanol together with a mixture of nitrogen and hydrogen as carrier gas. Kinetic runs have been made by changing the temperature, in the range of 200–260°C, the pressure between 10 and 30bar and the space time from 1 to 100ghmol−1. We have verified, at first, that inter-phase and intra-phase mass transfer limitations were negligible in the adopted conditions. Then, a Langmuir–Hinshelwood–Hougen–Watson kinetic model has been used for interpreting all the experimental data collected. This model corresponds to a mechanism in which the first step is the dissociative adsorption of ethanol on the surface, giving an adsorbed ethoxy group. Then, two other consecutive steps give place to respectively acetaldehyde as intermediate and ethyl acetate. This kinetic model allows a satisfactory fitting of all the performed experimental runs with a standard error below 15% for the runs performed with 2g of catalyst and less than 12% for the runs made with 50g of catalyst.

Steam and CO2 reforming of ethanol over Rh/CeO2 catalyst

The performance of ceria supported Rh catalysts prepared with high and low surface areas for steam reforming and dry reforming of ethanol was investigated. According to results of diffuse reflectance infrared Fourier transform spectroscopy of both reactions, dissociative adsorption of ethanol over ceria gives rise to ethoxy species and bridging OH groups. Oxidation of ethoxy species proceeds by addition of O provided by the support from species such as Type II bridging OH groups (i.e., steam reforming) or O adatoms (e.g., CO2 reforming). Rh promotes demethanation of acetate to carbonate and the steam/dry reforming of CHx species.Catalyst deactivation occurs from the deposition of carbon on the catalysts during both reactions. However, no carbon formation is observed on the Rh/CeO2 high surface area catalyst during steam reforming due to a higher surface fugacity of O from species adsorbed on the support that react to remove carbon.

Ethanol Steam Reforming over Rh(1%)MgAl2O4/Al2O3: A Kinetic Study

A kinetic study of ethanol steam-reforming reaction on Rh catalyst supported over a spinel structure (MgAl2O4/Al2O3) was carried out. From the analysis of products distribution, the following four reactions were proposed as the reaction scheme: C2H5OH → CO + CH4 + H2 (ED), C2H5OH + H2O → CO2 + CH4 + 2H2 (ER), water−gas shift reaction, and methane steam-reforming reaction. When the initial rate method was applied, it was demonstrated that in the rate-determining step (RDS) two active sites of the same type are involved. With use of our experimental results and data obtained from the literature, a sequence of 14 elementary steps were proposed, in agreement with the reaction scheme. Taking into account both the reaction mechanism and the occurrence of two active sites in the RDS, four different kinetic expressions can be formulated in which the RDS is (1) dissociative adsorption of ethanol, (2) dehydrogenation of ethoxide, (3) C−C bond scission, or (4) reaction between two adsorbed species. Finally, when discrimination models were applied, it was verified that the mechanism that fits experimental data is that in which the RDS is the surface reaction.

The effect of oxygen containing species on the catalytic activity of ethanol oxidation for PtRuSn/C catalysts

The effect of oxygen containing species (OCS) on the electrochemical activities of carbon supported PtRu, PtSn, and PtRuSn electrocatalysts has been investigated. The catalysts are prepared by the precipitation–deposition method and applied for the ethanol oxidation reaction (EOR). The oxidation treatment is applied to generate the OCS for these catalysts in order to realize their influences on the mechanism of EOR. The distinct effects of OCS on EOR performance are observed. RuO2 enhances the activity of EOR only in high potential region due to the promotion of CO oxidation, while Pt3Sn, SnO or SnO2 can promote both dissociative adsorption of ethanol on Pt surface and the CO oxidation reaction, thus the EOR activity increases in whole potential region. However, for the oxidized ternary PtRuSn catalysts where RuO2 and SnO2 coexist and cover the Pt active sites, the EOR activity deteriorates. Herein, the best catalyst for EOR is the as-reduced PtRuSn/C among all samples owing to the surface OCS (SnO) and an appropriate Ru/Sn surface ratio. For the oxidized sample, PtRu/C displays high EOR activity. Models for the mechanism of EOR on the PtRuSn/C catalysts are also proposed on the basis of the results presented here.

Reactivity of FeO(111)/Pt(111) with Alcohols

We report on the reactivity of a FeO(111) monolayer grown on Pt(111) toward aliphatic alcohols. Using a combination of temperature-programmed desorption, infrared reflection−absorption spectroscopy, and scanning tunneling microscopy techniques, we show that the alcohols dissociate primarily at FeO(111) step edges and their oxidation leads to the removal of the FeO(111) film. Upon annealing, FeO(111) lattice oxygen is incorporated into the reaction products, and reduced iron left behind dissolves into the underlying Pt(111) substrate. Ethanol is employed in a more detailed spectroscopic study to follow the reaction products and surface intermediates as the removal of FeO(111) proceeds. The ethoxy species formed upon dissociative adsorption of ethanol at the FeO(111) step edges undergo partial oxidation to acetaldehyde and a complete oxidation to CO and H2O. Other products, CH4 and H2, associated with the reactions occurring on Pt(111) are also observed as the bare Pt(111) surface appears. A similar etching...

Promoting role of unalloyed Sn in PtSn binary catalysts for ethanol electro-oxidation

The electro-oxidation of ethanol has been studied on PtSn catalysts obtained by galvanostatic codeposition technique. It was observed that by suitably varying the relative concentration of the metal precursors in the deposition bath both PtSn alloy catalysts as well as Pt rich phase with Sn and/or its oxides dispersed in an unalloyed state could be obtained. The onset potential of ethanol oxidation was lowered by ca. 300 mV for PtSn catalysts as compared to Pt demonstrating that surface Sn and/or its oxides can provide oxygen species at much lower potentials compared to Pt. Cyclic voltammetry provided evidence of lowering of the onset potential on PtSn catalysts corresponding to the appearance of an additional electro-oxidation peak at potentials where Pt surface oxides could not have been formed. Higher reversibility of the formation and reduction of surface oxides for the unalloyed PtSn catalyst formulation compared to other PtSn binary systems presumably facilitates easier transfer of oxygen species for oxidation of adsorbed CO resulting from the dissociative adsorption of ethanol. Potentiostatic polarization and chronoamperometric studies further confirmed the superior performance of this catalyst. However, alloying Pt with Sn relatively lowered the activity of PtSn catalysts possibly owing to the inhibition of ethanol adsorption/dehydrogenation on Pt.

Mechanism study of the ethanol oxidation reaction on palladium in alkaline media

In this work, the mechanism of the ethanol oxidation reaction (EOR) on a palladium electrode was studied using the cyclic voltammetry method. The dissociative adsorption of ethanol was found to proceed rather quickly and the rate-determining step was the removal of the adsorbed ethoxi by the adsorbed hydroxyl on the Pd electrode. The Tafel slope was found to be 130 mV dec −1 at lower potentials, which suggests that the adsorption of OH − ions follows the Temkin-type isotherm on the Pd electrode. In comparison, the Tafel slope increased gradually to 250 mV dec −1 at higher potentials. The change in the Tafel slope indicated that, at higher potentials, the kinetics is not only affected by the adsorption of the OH − ions, but also by the formation of the inactive oxide layer on the Pd electrode.

X-ray Absorption Fine Structure Combined with X-ray Fluorescence Spectroscopy. Monitoring of Vanadium Sites in Mesoporous Titania, Excited under Visible Light by Selective Detection of Vanadium Kβ5,2Fluorescence

The photocatalytic role of vanadium doped in mesoporous TiO2 has not been clarified. Valence state-sensitive V Kbeta5,2-selecting (5462.9 eV) X-ray absorption fine structure (XAFS) was used to monitor the V sites in mesoporous TiO2 for ethanol dehydration under equilibrium in situ conditions and visible light-illumination. First, the feasibility of discriminating V(IV) sites from a 1:1 physical mixture of standard V(IV) and V(V) inorganic compounds was demonstrated, by tuning the secondary fluorescence spectrometer to 5459.0 eV. The chemical shift of V Kbeta5,2 emission between V(IV) and V(V) sites was 1.0 eV. The selection of valence states V(IV) and V(V) was 100% and 80%, respectively. The redox states for ethanol dehydration over mesoporous TiO2 excited in visible light were suggested to be V(III) and V(IV). The chemical shift between valence states V(III) and V(IV) was greater (3.2 eV). On the basis of V Kbeta5,2 emission and V Kbeta5,2-selecting XAFS spectra tuned to the V Kbeta5,2 peak, we determined that the fresh mesoporous V-TiO2 catalyst has a valence state of V(IV). The vanadium sites were partially reduced by the dissociative adsorption of ethanol under visible light, but they still stay within the emission-energy ranges for standard V(IV) compounds. These partially reduced vanadium sites were reoxidized in oxygen under visible light. Finally, direct XAFS observation of photoreduced V(III) sites was attempted by tuning the fluorescence spectrometer to 5456.3 eV for partially reduced mesoporous V-TiO2. Valence state V(III) was selected for 60% of the spectrum in the mixture of V(III) (minor) and V(IV) (dominant) valence states.

Ethanol Oxidation on Carbon Supported Pt-Sn Electrocatalysts Prepared by Reduction with Formic Acid

Carbon supported alloy catalysts were prepared by reduction of Pt and Sn precursors with formic acid and characterized by X-ray diffraction, high-resolution transmission electron microscopy, and X-ray absorption spectroscopy techniques. Their electrocatalytic activity for ethanol oxidation was compared with commercial and electrocatalysts and the differences discussed in terms of intrinsic properties of the materials. The activity for ethanol oxidation in linear sweep voltammetry experiments at room temperature is greatly enhanced, mainly at low potentials, on (3:1) and (2:1) catalysts prepared by the formic acid method, while only a slight improvement is observed on (9:1) prepared with formic acid and on from E-TEK. The commercial catalyst showed a higher current performance as anode material in a direct ethanol fuel cell operating at 90 and . At the low temperatures of the electrochemical experiments, the rate determining step (rds) of ethanol oxidation is the removal of -species, which is enhanced by the presence of Sn oxides. At the higher temperatures of the fuel cell, instead, the rds is the dissociative adsorption of ethanol and/or the oxidation of acetaldehyde to acetic acid: both these reactions are favored by an increase of the Sn content in the alloyed state.

Kinetics of the Oxidative Dehydrogenation of Ethanol to Acetaldehyde on V2O5/TiO2−SiO2 Catalysts Prepared by Grafting

Catalysts that are very active and selective in the oxidative dehydrogenation of ethanol to acetaldehyde have been prepared by grafting vanadyl triisopropoxide onto a silica support whose surface was coated with TiO2. Together with acetaldehyde, small amounts of byproducts were obtained, including acetic acid, acetals, and CO2. The kinetic behaviors of the catalysts were studied in the temperature range 100−180 °C, by changing the ethanol residence time, the molar ratio between the reagents, and the vanadium load. A four-step mechanism was adopted, consisting of the following steps: (i) dissociative adsorption of ethanol onto vanadium, (ii) α-hydrogen withdrawal by the metal to form acetaldehyde and a hydride group, (iii) oxidation of the formed hydride, and (iv) dehydration of the vanadium site to restore the original active site. This mechanism can be simplified, and the resulting kinetic expression is similar to a Mars and van Krevelen kinetic expression. The reaction is well-described by this kinetic...

Heterogeneous photocatalytic dehydrogenation from ethanol and heavy water

An isotopic method was used, with ethanol as hydrogen source and heavy water as deuterium source, to investigate the effect of ethanol concentration and pH value of the solution on the gaseous products (D 2, HD and H 2) and H/D ratio during the photocatalytic dehydrogenation on Pd-CdS powdered photocatalyst. In a neutral medium of equimolar concentration, H/D=2.08 is observed. In media with a wide range of pH values, H/D>1 and a maximum H/D ratio of 2.5 is obtained at pH 12. This demonstrates that a larger number of effectively dissociated adsorptive ions are obtained from ethanol than from D 2O, and that water acts as a reactant not a mere mediator. The occurrence of a maximum H/D value indicates the competition between dissociative adsorption of ethanol and D 2O onthe CdS surface. A detailed reaction mechanism is proposed. At a reaction temperature above room temperature using M-CdS, the rate-determining step is considered to be the transfer of photoproduced holes; this occurs when they are captured by adsorbed alcoholate ion RO −, followed by α-H separation, in acidic, neutral or weak basic media, or when they are captured by adsorbed hydroxyl ion OH −, followed by free radical OH· formation, in strongly basic (pH⩾12) medium.

Quantum chemical interpretation of the mechanism of ethanol dehydrogenation driven by active oxygen bridges in silicate-1 and silica

The reaction mechanism of ethanol dehydrogenation on silicalite-1 or highly dehydrated silica has been investigated by ab initio molecular orbital calculations. The model of the intermediate is constructed by a combination of (HO) 3SiOH and C 2H 5OSi(OH) 3 fragments, which would be formed by dissociative adsorption of ethanol on the active oxygen bridge existing in silicalite-1 or silica. On the basis of the calculation, the following mechanism is proposed: an interaction between the hydroxyl group of SiOH and the silicon atom of the ethoxide results in an increase in the acidity of the hydroxyl group. Simultaneously, the bond between the ethoxyl group and the silicon atom weakens and the reactivity of the ethoxyl group toward the acidic hydroxyl group increases; then acetaldehyde and hydrogen are formed.

ChemInform Abstract: Selective Dehydrogenation of Ethanol to Acetaldehyde Over Silicalite‐1.

Abstract Silicalite-1 catalyzes ethanol dehydrogenation to acetaldehyde. On the basis of the IR spectra of ethanol adsorbed on its surface, the active sites are believed to be the active oxygen bridges on which dissociative adsorption of ethanol can take place. The active sites are stable compared with the active oxygen bridges on highly dehydrated silica, which also catalyze the selective ethanol dehydrogenation. Since there were trace amounts of acid site on the Silicalite-1, simultaneous ethanol dehydration was observed. However, almost all the acid sites disappeared by heating at 1070 K in the case of Silicalite-1 containing 0.7 wt% of sodium ions, and ethanol was selectively converted to acetaldehyde. In this treatment, the sodium ions partly poison the active sites for dehydrogenation, but their activity is restored by leaching excess sodium ions from the silicalite-1 under reflux in water.

Selective dehydrogenation of ethanol to acetaldehyde over silicalite-1

Silicalite-1 catalyzes ethanol dehydrogenation to acetaldehyde. On the basis of the IR spectra of ethanol adsorbed on its surface, the active sites are believed to be the active oxygen bridges on which dissociative adsorption of ethanol can take place. The active sites are stable compared with the active oxygen bridges on highly dehydrated silica, which also catalyze the selective ethanol dehydrogenation. Since there were trace amounts of acid site on the Silicalite-1, simultaneous ethanol dehydration was observed. However, almost all the acid sites disappeared by heating at 1070 K in the case of Silicalite-1 containing 0.7 wt% of sodium ions, and ethanol was selectively converted to acetaldehyde. In this treatment, the sodium ions partly poison the active sites for dehydrogenation, but their activity is restored by leaching excess sodium ions from the silicalite-1 under reflux in water.

ChemInform Abstract: Selective Dehydrogenation of Ethanol over Highly Dehydrated Silica

Abstract Silica dehydrated above ca. 1000 K selectively converts ethanol to acetaldehyde, and the dehydrogenation proceeds according to Langmuir-Hinshelwood kinetics. Measurement of phosphorescence spectra confirmed that new sites are formed on silica by preheating above ca. 1000 K, but the dehydrated silica showed no acidity or basicity which might have been responsible for the dehydrogenation activity. Infrared spectra indicated dissociative adsorption of ethanol on the dehydrated silica and we infer that the adsorption site is an active SiOSi oxygen bridge formed by the dehydration of silica at high temperature, and that the reaction intermediate is formed by the dissociative adsorption of ethanol accompanying cleavage of that oxygen bridge.