Dissociative Adsorption Of Methanol Research Articles

Understanding methanol dissociative adsorption and oxidation on amorphous oxide films.

Interactions between a transition metal (oxide) catalyst and a support can tailor the number and nature of active sites, for instance in the methanol oxidation reaction. We here use ambient pressure X-ray photoelectron spectroscopy (AP-XPS) to identify and compare the surface adsorbates that form on amorphous metal oxide films that maximize such interactions. Considering Al(1-x)MxOy (M = Fe or Mn) films at a range of methanol : oxygen gas ratios and temperatures, we find that the redox-active transition metal site (characterized by methoxy formation) dominates dissociative methanol adsorption, while basic oxygen sites (characterized by carbonate formation) play a lesser role. Product detection, however, indicates complete oxidation to carbon dioxide and water with partial oxidation products (dimethyl ether) comprising a minor species. Comparing the intensity of methoxy and hydroxyl features at a fixed XPS chemical shift suggests methanol deprotonation during adsorption in oxygen rich conditions for high transition metal content. However, increasing methanol partial pressure and lower metal site density may promote oxygen vacancy formation and the dehydroxylation pathway, supported by a nominal reduction in the oxidation state of iron sites. These findings illustrate that AP-XPS and mass spectrometry together are powerful tools in understanding metal-support interactions, quantifying and probing the nature of catalytic active sites, and considering the link between electronic structure of materials and their catalytic activity.

In Situ Studies of Methanol Decomposition Over Cu(111) and Cu2O/Cu(111): Effects of Reactant Pressure, Surface Morphology, and Hot Spots of Active Sites

The dissociative adsorption of methanol was investigated on Cu(111) and ultrathin Cu2O films. We employed synchrotron-based Ambient Pressure X-ray Photoelectron Spectroscopy (AP-XPS) and Scanning T...

Mechanism of Methanol Decomposition over Single-Site Pt1/CeO2 Catalyst: A DRIFTS Study.

Single-site catalysts have drawn broad attention in catalysis because of their maximum atomic utilization and unique catalytic performance. Early work in our group has shown a 40-fold higher activity of methanol decomposition over single-site Pt1/CeO2 catalyst than CeO2 supported 2.5 nm Pt nanoparticles, while a molecular-level understanding of such enhancement is lacking. Herein, the reaction mechanism of methanol decomposition over Pt1/CeO2 was carefully investigated using in situ DRIFTS, and a reaction pathway was proposed. Methanol molecules were dissociatively adsorbed on nanoceria support first, followed by the diffusion of as-formed methoxy species onto Pt single sites where the dehydrogenation occurs and results in the weakly bonded CO. The ease of methanol dissociative adsorption on nanoceria support and the tailored electronic property of Pt1 via the metal-support interaction are believed to be strongly correlated with the high activity of Pt1/CeO2.

Can Supported Reduced Vanadium Oxides form H2 from CH3OH? A Computational Gas-Phase Mechanistic Study.

A detailed density functional theory study is presented to clarify the mechanistic aspects of the methanol (CH3OH) dehydrogenation process to yield hydrogen (H2) and formaldehyde (CH2O). A gas-phase vanadium oxide cluster is used as a model system to represent reduced V(III) oxides supported on TiO2 catalyst. The theoretical results provide a complete scenario, involving several reaction pathways in which different methanol adsorption sites are considered, with presence of hydride and methoxide intermediates. Methanol dissociative adsorption process is both kinetically and thermodynamically feasible on V-O-Ti and V═O sites, and it might lead to form hydride species with interesting catalytic reactivity. The formation of H2 and CH2O on reduced vanadium sites, V(III), is found to be more favorable than for oxidized vanadium species, V(V), taking place along energy barriers of 29.9 and 41.0 kcal/mol, respectively.

Synthesis and Characterization of Three‐Dimensional Porous Cu@Pt and Cu@Pt‐Ru Catalysts for Methanol Oxidation

AbstractHighly porous Cu foams comprised of interconnected branched dendrites were used as sacrificial templates for the fabrication of pseudo core–shell Cu@Pt and Cu@Pt‐Ru unsupported electrodes by galvanic replacement. The as‐prepared materials presented a three‐dimensional structure with pores between 17 and 45 μm made of superimposed layers of ramified dendrites. TEM analysis showed that the dendrites were composed of agglomerates of grains of about 4 nm in size and mesopores of ca. 30 nm in diameter. The as‐prepared 3D electrodes were tested for methanol oxidation in acid media at different temperatures. The results showed that the catalytic activities of Cu@Pt and Cu@Pt‐Ru foams normalized to the electrochemical surface area are almost 50 % higher than that of a commercial Pt−Ru/C material, with poisoning rates that are reduced by half in the same potential range. The enhanced behavior of the as‐prepared foams is believed to be the result of the influence of copper atoms on the reactivity of platinum sites, the highly defective structure of the electrodes, as well as the facilitated diffusion of methanol molecules and the products formed during the reaction throughout the highly porous three‐dimensional structure of the electrode. The apparent activation energies (Ea,app) for the methanol oxidation reaction (MOR) were determined by potentiostatic experiments. Ea,app values of 29.07 and 24.20 kJ mol−1 were calculated for Cu@Pt and Cu@Pt‐Ru foams at 0.3 V, respectively. The results suggested that the MOR is governed by the dissociative adsorption of methanol as a result of the multifunctional nature of the catalyst and the facilitated diffusion of the products formed during the reaction.

Method for studying high temperature aqueous electrochemical systems: Methanol and glycerol oxidation

A method for high purity aqueous electrochemical experiments at temperatures above the normal boiling point of water and at temperatures up to 140°C is described. A three-electrode cell in a self-pressurized glass autoclave is heated in an oil bath. Slow ramping of the temperature allows efficient acquisition of kinetic parameters such as activation energies, oxidation onset potentials and Tafel slopes by using cyclic voltammetry. The oxidation of two organic alcohols with different volatilities, methanol (high volatility) and glycerol (low volatility), are studied to demonstrate the capabilities of the method. Methanol oxidation on platinum is found to have a similar mechanism at all temperatures, with either dissociative adsorption of water or dissociative adsorption of methanol as the rate-determining step. In the case of glycerol oxidation on platinum, the mechanism changes at 110°C. At low temperatures dissociative adsorption of water or dissociative adsorption of glycerol is suggested to be the rate-determining step. At higher temperatures, a significant decrease in onset potential was observed and the glycerol is suggested to selectively oxidize to glyceraldehyde or dihydroxyacetone, with dissociative glycerol adsorption as the rate-determining step.

A novel theoretical study of thermally-induced reaction and vibration dynamics of methanol dissociative adsorption onto a Si(001) surface

The thermally-induced reaction and vibration dynamics of methanol dissociative adsorption onto a Si(001) surface is studied by combining temperature-dependent DFT-based MD simulations with molecular adsorption sampling and wavelet transform.

A Relevant Estimation of the TOF for Methanol Oxidation Over Au/CeO2: A Combined SSITKA and FTIR Operando Contribution

The importance to eliminate methanol from the environment arises from its classification as a volatile organic compound. For this purpose, it is advantageous to use effective catalysts that operate under ambient conditions. The cerium oxide doped with gold is a good candidate thanks to the high reducibility of ceria and the gold activity at low temperatures. Indeed, in previous studies it was evidenced that Au/CeO2 are highly effective not only for room temperature methanol trapping (upon adsorption) but also for catalytic full oxidation with air. Aiming at understanding the reaction mechanism, the FTIR operando survey of the catalyst under reaction flow conditions was performed. Distinct adsorbed species were observed and among them both spectators and reaction intermediates ones. More precisely, methoxy species formed upon methanol dissociative adsorption over CeO2 seemed to behave both as spectator and intermediate species most probably depending on their proximity with gold nanoparticles. Combining the Steady State Isotopic Transient Kinetic Analysis method and Fourier Transform Infrared spectroscopy operando setup, a more precise description of the methanol oxidation was possible thanks to the observation of the delayed evolution of distinct species at the catalyst surface. Moreover, in order to optimize the reaction conditions when moving from the laboratory scale to the industrial scale (pilot) it is of primary importance to know the intrinsic reaction rate parameters. The kinetic constants and the fraction of involved superficial sites were thus computed allowing an evaluation of a relevant turn over frequency.

Oxidation of methanol to methyl formate over supported Pd nanoparticles: insights into the reaction mechanism at low temperature

The mechanism of methyl formate formation at low temperature was studied using DRIFT-operando. The first reaction step would be the formation of methoxy groups by dissociative adsorption of methanol and then transformation into formaldehyde. The final step would be the condensation of the formaldehyde species with gaseous methanol.

Dehydration of methanol to dimethyl ether over γ‐Al2O3 catalyst: Intrinsic kinetics and effectiveness factor

AbstractDehydration of methanol to dimethyl ether (DME) over a commercial γ‐Al2O3 catalyst was studied at the temperature interval 513–613 K, liquid hourly space velocity (LHSV) of 0.9–6.0 h−1 and pressures between 0.1 and 1.0 MPa. The effect of different operation conditions on the dehydration of methanol was investigated in an isothermal fixed bed reactor. A kinetic equation which describes a Langmuir–Hinshelwood surface controlled reaction with dissociative adsorption of methanol was found to fit the experimental results quite well. An activation energy of 62.4 kJ/mol was obtained for the catalyst. A two‐dimensional reaction–diffusion model was established for a cylindrical‐shaped methanol dehydration catalyst. The internal effectiveness factor and the concentration distribution of methanol in the catalyst were obtained by the finite element method in MATLAB. The reaction–diffusion model was verified by the global kinetics data. The calculation data agreed well with the experimental data, so the model can be used to describe the processes of reaction and diffusion in the cylindrical‐shaped catalyst.

The influence of surface defects in methanol dissociative adsorption and CO oxidation on Pt(1 1 0) probed by nonlinear vibrational SFG spectroscopy

We report the study of methanol electro-oxidation on ordered and defective Pt(110) surfaces using infrared-visible sum-frequency generation vibrational spectroscopy in the COads internal stretch spectral region (1700–2150cm−1). Comparison of the spectra as a function of the structural quality of the electrodes and of the applied potential shows that: (i) dissociative chemisorption occurs primarily at low potentials on defect sites such as steps and (ii) the signal of CO adsorbed on terraces decreases more sharply at higher potentials than the signal of CO adsorbed on defects. This latter behavior appears to contradict the fact that dense CO adlayers on terraces block the oxidation process. This apparent paradox is solved by considering the interplay of preferential oxidation of CO at defects sites and transfer of adsorbed CO from terraces reservoir to defects.

Conceptual chemistry approach towards the support effect in supported vanadium oxides: Valence bond calculations on the ionicity of vanadium catalysts

The concept of bond ionicity, obtained via a valence bond analysis, is invoked in the interpretation of the catalytic activity of supported vanadium oxides, in analogy with previous work conducted within the framework of conceptual DFT. For a set of model clusters representing the vanadium oxide supported on SiO 2, Al 2O 3, TiO 2, ZrO 2, the ionic character of the vanadium-oxygen bond, involved in the dissociative adsorption of methanol on the catalyst, was quantified. Detailed scrutiny shows that this ionicity increases from the Al through the Zr support, in agreement with the increasing catalytic activity through this series; the case of the Si supported oxide is found to be an exception however, giving rise to the most ionic V–O bond of the different compounds studied. This finding is confirmed by calculations on smaller clusters focusing on detail in the π back bonding.

Enhanced activity of rare earth doped PtRu/C catalysts for methanol electro-oxidation

PtRuRE/C catalysts were prepared through doping the commercial JM PtRu/C catalyst with rare earth (RE = La, Eu, Gd, Y, Sm and Er). The doping was conducted by chemical reduction and sintering treatment methods. The catalysts were characterized by transmission electron microscopy (TEM), energy dispersive X-ray (EDX) spectroscopy, X-ray diffraction (XRD) and X-ray photoelectron spectroscopy (XPS). The results demonstrated that the rare earth doping did not significantly change the average size of the JM PtRu/C particles, and modified the PtRu surface with two element states of the metal and oxide. Studies of cyclic voltammetry and chronoamperometry indicated that the electrocatalytic activity of PtRuRE/C catalysts was higher than that of the JM PtRu/C for methanol oxidation (except for PtRuGd/C), generating a decreasing order of PtRuEu/C > PtRuEr/C > PtRuY/C > PtRuSm/C > PtRuLa/C > JM PtRu/C > PtRuGd/C. It has revealed that among the PtRuRE/C catalysts the PtRuEu/C exhibited the best performance. In addition, the reaction process of methanol electrocatalytic oxidation on several catalysts was investigated by in situ FTIR spectroscopy at molecule level. The results demonstrated that the adsorbates derived from methanol dissociative adsorption on the catalysts were the linear-bonded CO (CO L), and the CO-tolerance performance of PtRuRE/C catalysts was better than JM PtRu/C. It has revealed also that the electronic effect of rare earth plays an important role in the catalytic performance of PtRuRE/C catalysts for methanol electrooxidation.

Dissociation of methanol on hydroxylated TiO 2-B (1 0 0) surface: Insights from first principle DFT calculation

The adsorption of methanol on hydroxylated TiO 2-B (1 0 0) surface with bridging and terminal hydroxyl groups has been studied by first principle calculations. On both clean and hydroxylated surfaces with bridging OH group (OH br), the O–H bond scission is the most favorable dissociation of methanol and the C–O bond scission is also feasible. This indicates the OH br has little influence on the adsorption of methanol. The terminal OH group (OH t) plays a major role in the C–H scission of methanol, which is important for the applications associated with the direct use of hydrogen, such as in situ hydrogenation, and hydrogen generation via the photocatalytic reaction. The dissociative adsorption of methanol via C–H scission, which is an endothermic adsorption on other TiO 2 surfaces, is identified as exothermic adsorption with adsorption energy in the range of −1.54 eV to −1.91 eV around OH t on TiO 2-B (1 0 0) surface. The lowest activation barrier for C–H scission is ∼0.80 eV, which is lower than the release heat of molecular adsorption. Moreover, the hydrogen atoms in methanol are easily transferred to the OH t and then move to nearby O 2c sites to regenerate the hydroxyl group. This proton migration process could result in extra stable chemi-sorption of methanol with an adsorption energy as low as −2.23 eV, which is above twice that of methanol molecularly adsorbed on the surface. Thus, the proton channel feature of OH t on the surface is borne out by our calculations.

Direct kinetic evidence for the electronic effect of ruthenium in PtRu on the dissociative adsorption of methanol

Dissociative adsorption of methanol on platinum in a PtRu bimetallic electrocatalyst has been studied by an accelerated degradation test and normal pulse voltammetry. PtRu electrodes with a minimum number of potential cycles show the highest electrode rate constant values while the one with platinum-like surface characteristics show the lowest rate constant values, evidencing the promotional effect of Ru.

Effect of Methanol on the Lewis Acidity of Rutile TiO2Nanoparticles Probed through Vibrational Spectroscopy of Coadsorbed CO

Infrared spectroscopy of adsorbed CO has been used to characterize the effect of adsorbed methanol on the Lewis acidity of 4 nm rutile TiO(2) nanoparticles. Measurements of CO absorbance and vibrational frequency have revealed that CO adsorbs primarily at one class of Lewis acid sites on clean TiO(2) particles, where evidence for lateral interactions between neighboring molecules suggests dense coverage occurs near saturation. The response of the CO infrared intensities and frequencies to methanol exposure has shown that methanol uptake occurs primarily at the Lewis acid sites and through hydrogen bonding to surface OH groups. These surface sites appear to be responsible for driving both molecular and dissociative adsorption of methanol on the titania. Most importantly, these studies have revealed that the parent methanol and associated methoxy products lower the Lewis acidity of neighboring sites on TiO(2) nanoparticles.

Study of reaction intermediates of methanol decomposition and catalytic partial oxidation on Pt/Al2O3

Abstract Methanol is considered a promising liquid fuel for hydrogen fuel cell systems. Catalytic partial oxidation (CPO) of methanol offers the potential of producing hydrogen for these systems, and is capable of high hydrogen production rates with no heat addition. However, the mechanism of methanol CPO is not completely understood due to its complexity. In this work, methanol CPO on Pt/Al 2 O 3 catalysts was studied using in situ diffuse reflectance infrared Fourier transform spectroscopy (DRIFTS) and mass spectrometry. Methanol adsorption experiments were conducted on different catalyst states. Methoxy species, CH 3 O s , were formed during dissociative adsorption of methanol. As temperature was increased, formate adspecies, likely formed by reaction of methoxy species with surface hydroxyls, were increasingly formed. Ignition of CPO was also studied, and it was found to occur at room temperature on both reduced and oxidized powder catalysts. Formate disappeared during this process, while CO species and CO 2 were detected in the products. CPO was studied at different temperatures from 723 K to 973 K, and for methanol to oxygen mole ratios from 2 to 4. It was found that CO 2 productivity increased at lower temperatures and H 2 and CO had a higher selectivity at higher temperatures. Fuel lean conditions favored H 2 and HCOOH production. CO 2 increased when O 2 concentration increased in the reaction mixture. These results suggest that formate was one of the important intermediates in the methanol partial oxidation reaction pathway. Indirect formate decomposition is dominant for CO 2 production in the reactions at higher temperatures.

The roles of Cu, Zn and Mn in Cu 0.5Zn 0.5Mn 2O 4 spinel-lattice catalyst for methanol decomposition

Cu 0.5Zn 0.5Mn 2O 4 spinel-lattice catalyst has been studied for methanol decomposition by in situ infrared (IR) spectroscopy analysis and micro reactor study. Methoxy species is formed by dissociative adsorption of methanol which subsequently converts into formate species via step-wise dehydrogenation process. The activities of surface intermediates on reduced CuO, ZnO and Mn 2O 3 catalysts are strongly affected by the ease of hydrogen desorption from the site, the strength of C–H bonding and the availability of oxygen. A synergism between Cu and MnO whereby the Cu serves as a sink for reverse hydrogen atoms spillover from MnO has been proposed. The micro reactor analysis correlates well with the IR results.

IN-SITU FT-IR SPECTROSCOPIC STUDIES OF FUEL CELL ELECTRO-CATALYSIS: FROM SINGLE-CRYSTAL TO NANOPARTICLE SURFACES

The work presented in this article shows the power of the variable temperature, in-situ FT-IR spectroscopy system developed in Newcastle with respect to the investigation of fuel cell electro-catalysis. On the Ru(0001) electrode surface, CO co-adsorbs with the oxygen-containing adlayers to form mixed [CO + (2 × 2)–O(H)] domains. The electro-oxidation of the Ru(0001) surface leads to the formation of active (1 × 1)–O(H) domains, and the oxidation of adsorbed CO then takes place at the perimeter of these domains. At 20°C, the adsorbed CO is present as rather compact islands. In contrast, at 60°C, the COads is present as a relatively looser and weaker adlayer. Higher temperature was also found to facilitate the surface diffusion and oxidation of COads. No dissociation or electro-oxidation of methanol was observed at potentials below approximately 950 mV; however, the Ru(0001) surface at high anodic potentials was observed to be very active. On both Pt and PtRu nanoparticle surfaces, only one linear bond CO adsorbate was formed from methanol adsorption, and the PtRu surface significantly promoted both methanol dissociative adsorption to CO and its further oxidation to CO2. Increasing temperature from 20° to 60°C significantly facilitates the methanol turnover to CO2.

Special features of methanol interaction with adsorbed oxygen at platinized platinum electrode: Transients of the open-circuit potential

Open-circuit potential transients are measured under the conditions of methanol interaction with the pre-adsorbed oxygen at platinized platinum electrode. The time necessary for complete removal of the adsorbed oxygen monolayer appeared being shorter by a factor of ∼1.5 as compared with smooth polycrystalline platinum. The dependence of platinum surface coverage with adsorbed oxygen on the potential during its decay is found. It was shown that the reaction of methanol with the adsorbed oxygen is most slow at a high coverage (1–0.8). It is suggested that at these coverages, like the case of polycrystalline platinum, the adsorbed oxygen directly interacts with the methanol molecules from the solution. At moderate coverages (0.8–0.2), the reaction of the adsorbed oxygen with methanol at the platinized platinum is better described by the “conjugated reactions” mechanism. The specific rates of the methanol dissociative adsorption at the platinized platinum turned out to be close to those observed earlier for the polycrystalline platinum.