Dissociation Of Methanol Research Articles

Identifying the role of excess electrons and holes for initiating the photocatalytic dissociation of methanol on a TiO2(110) surface.

As a prototype for the catalytic oxidation of organic contaminants, photocatalytic methanol dissociation on rutile TiO2(110) has drawn much attention, but its reaction mechanism remains elusive. While polarons are ubiquitous in photovoltaics and heterogeneous catalysis, how surface polarons influence adsorption remains unclear. In this paper, density functional theory is used to study the effect of excess electrons and holes on methanol dissociation on rutile TiO2(110). The effect of excess carriers for three types of methanol dissociation on the metal oxides are compared. The results show that the excess electron and hole play different roles in the dissociation reactions even though they present similar adsorption behaviors. The excess electron is easily trapped in the lattice Ti atom, and it decreases the dissociation barrier of methanol to 0.13 eV. Furthermore, the excess hole prefers to stick with the hydroxyl radical, which increases the energy barrier of methanol dissociation up to 0.43 eV. It was found that the height of the dissociation barrier is dependent on the orientation of the methanol molecules, not on the distance to the modified electron. This study identifies the essential roles of excess electrons and holes for promoting O-H dissociation. Our findings contribute considerably to broadening the understanding of photocatalytic chemistry.

Dual Lewis site creation for activation of methanol on Fe3O4(111) thin films.

Despite a wide application in heterogeneous catalysis, the surface termination of Fe3O4(111) remains controversial. Herein, a surface with both Lewis acid and base sites is created through formation of an Fe3O4(111) film on α-Fe2O3(0001). The dual functionality is generated from a locally nonuniform surface layer of O adatoms and Fetet1 sites. This reactive layer is reproducibly formed even in oxygen-free environments because of the high mobility of ions in the underlying α-Fe2O3(0001). The atomic structure of the Fe3O4(111) surface was identified by scanning tunneling microscopy (STM) and density functional theory (DFT) using the registry of the overlayers with the surface and the distinct electronic structure of oxygen adatom (Oad) and uncovered lattice Fetet1. The surface is dominated by the interface of Oad and Fetet1, a Lewis acid–base pair, which favors methanol dissociation at room temperature to form methoxy. Methoxy is further oxidized to yield formaldehyde at 700 K in temperature programmed reaction spectra, corresponding to an approximate activation barrier of 179 kJ mol−1. The surface termination of Fe3O4(111) is fully recovered by rapid heating to 720 K in vacuum, demonstrating the high mobility of ions in this material. The work establishes a clear fundamental understanding of a unique magnetite surface and provides insights into the origin of selective oxidation of alcohols on magnetite-terminated catalysts.

Water-promoted surface basicity in FeO(OH) for the synthesis of pseudoionones (PS) and their analogues

Use of Iron oxyhydroxide (γ-FeO(OH)) as a robust catalyst for the synthesis of important intermediates like pseudoionones and their analogues through the C-C bond formation reactions like knoevenagel and aldol condensation is explored. These motifs are the building blocks for the construction of the sesquiterpenes as well as the diterpenes such as retinoic acid, Vitamin A etc. Iron oxyhydroxide (γ-FeO(OH)) was synthesized and well characterized using XRD, FT-IR, TEM, XPS and adsorption studies to establish the catalytic activity. A thorough investigation on the nature of basic sites and the role of water as a promoter was explored based on dye adsorption, in situ methanol dissociation and CO2 adsorption studies. The catalyst also showed a wide range of substrate scope with active methylene groups involving various functional groups such as cyanides, esters and acetophenones along with its stability and reproducibility.

DFT Study of Methanol Adsorption on Defect‐Free CeO2 Low‐Index Surfaces

Methanol decomposition is a promising method for hydrogen production. However, the performance of current catalysts for this process is not sufficient for commercial applications. In this work, methanol adsorption on the CeO2 low-index surfaces is studied by density functional theory (DFT). The results show that methanol always dissociates spontaneously on the (100) surface, whereas dissociation on the (110) surface is site-selective; dissociation does not occur at all on the (111) surface, where only weak physisorption is found. The results confirm that surfaces with higher energies are more catalytically active. Analysis of the surface geometries shows that the dominant factors for the dissociation of methanol are the degree of undercoordination and the charges of the surface ions. The adsorption energy of each methanol molecule decreases with increasing coverage and there is a transition threshold between dissociative and associative adsorption. The present work indicates that a strategy to design catalysts with high activity is to maximize exposure of surfaces on which the ions have a high degree of undercoordination and a strong tendency to donate/accept electrons. The results demonstrate the importance of appropriately selecting and controlling exposed facets and particle morphology for optimizing catalyst performance.

Dynamics of hydroxyl dissociation of methanol and water on silicon surfaces

Molecular beam techniques show how reaction dynamics depend on bonding of intermediate state and functional groups involved.

Methanol dissociation and oxidation on single Fe atom supported on graphitic carbon nitride

Considering environmental protection and sustainable development, fuel cells have always been an ideal choice for clean energy. For the performance of fuel cells, the anode catalysts are a key consideration. In the work reported, we designed a new type of anode catalyst, in which graphitic carbon nitride was used as the substrate and transition metal iron as the supported atom. The calculation results were characterized by adsorption energy, reaction energy barrier, potential energy surface and charge analysis. Based on density functional theory, the gradual decomposition of methanol molecules on the catalyst is realized; the decomposition products are oxidized to reduce the formation of CO, and the efficiency of anode catalysis is improved, which provides a new idea for the design of anode materials for methanol fuel cells.

Deuterium Kinetic Isotope Effect in the Photocatalyzed Dissociation of Methanol on TiO2(110)

Deuterium kinetic isotope effect (KIE) in the photochemistry of methanol on TiO2(110) has been studied to find the rate-determining step (RDS) and understand the reaction mechanism using two-photon photoemission spectroscopy. Deuterium substitution of the methyl hydrogen has little effect on the kinetics of this reaction, suggesting that neither the break of the C–H(D) bond nor the transfer of the H(D) atoms to the bridging sites is the RDS in the transformation of methanol into formaldehyde. In contrast, the reaction rate of MeOH is ∼1.3 times that of MeOD, suggesting that the cleavage of O–H(D) is the RDS in the photocatalyzed dissociation of methanol on TiO2(110). The results contradict the common fact that C–H(D) is more difficult to break than O–H(D) based on ground-state energetics, implying the involvement of photogenerated charge carriers in the reaction of the C–H breakage, whereas the cleavage of O–H is likely a thermal reaction. Difference in the activation energy of O–H and O–D dissociation reaction in the methanol/TiO2(110) system has been calculated based on the KIE measurements. Our work is consistent with the fact that methoxy is photocatalytically more reactive than methanol and suggests that the conversion of methanol into methoxy is crucial in the photochemistry of methanol on TiO2(110) and probably other metal oxide semiconductor surfaces.

Dissociation and oxidation mechanism of methanol on Al12N12 cage: a DFT study

The density functional theory has been used to investigate the methanol adsorption, decomposition and oxidation as well as the water adsorption and decomposition on the clean and Pt-encapsulated Al12N12 cages. It is shown that platinum metal atom can be encapsulated within the Al12N12 cage, thus forming Pt-encapsulated Al12N12 cage electrocatalyst. According to the reaction results, CH3OH and H2O both prefer to be adsorbed on the Al atom top site. The O–H bond scission is the significantly more favorable reaction pathway than C–O bond scission on the cage surface. Furthermore, the structures of the intermediates, transition states and products, the corresponding energy barriers and reaction energies are confirmed. The results also show that the adsorption energies and energy barriers of CH3OH and H2O dissociation are cut down slightly with the aid of the platinum atom. Nevertheless, Pt-encapsulated Al12N12 cage makes the potential energy surface changes smoother than pure Al12N12 cage.

DENSITY FUNCTIONAL THEORY STUDY OF METHANOL ADSORPTION AND DISSOCIATION ON 2Cu/ZnO CATALYST SURFACE

The adsorption and dissociation of methanol (CH3OH) on the cluster with two copper atoms (2Cu) supported over ZnO catalyst surface were investigated using the density functional theory (DFT). The performance of 2Cu adsorbed on ZnOsurface to obtain model of the 2Cu/ZnO catalyst surface. The most stable site of CH3OH with configuration and energy of adsorption was investigated. The reaction pathway of CH3OH dissociation via bond scission of the O–H, C–H and C–O to form methoxide, hydroxymethyl and methyl was examined, respectively. The obtained calculations pointed out that O–H bond scission is found to be the most favorable pathway on 2Cu/ZnO catalyst surface.

Methoxy Formation Induced Defects on MoS2

We find that exposure of the MoS2 basal plane to methanol leads to the formation of adsorbed methoxy and coincides with sulfur vacancy generation. The conversion of methanol to methoxy on MoS2 is temperature dependent. Density functional theory simulations and experiment indicate that the methoxy moieties are bound to molybdenum, not sulfur, while some adsorbed methanol is readily desorbed near or slightly above room temperature. Our calculations also suggest that the dissociation of methanol via O–H bond scission occurs at the defect site (sulfur vacancy), followed subsequently by formation of a weakly bound H2S species that promptly desorbs from the surface with creation of new sulfur vacancy. Photoluminescence and scanning tunneling microscopy show clear evidence of the sulfur vacancy creation on the MoS2 surface, after exposure to methanol.

Acid-Promoter-Free Ethylene Methoxycarbonylation over Ru-Clusters/Ceria: The Catalysis of Interfacial Lewis Acid-Base Pair.

The interface of metal-oxide plays pivotal roles in catalytic reactions, but its catalytic function is still not clear. In this study, we report the high activity of nanostructured Ru/ceria (Ru-clusters/ceria) in the ethylene methoxycarbonylation (EMC) reaction in the absence of acid promoter. The catalyst offers 92% yield of MP with TOF of 8666 h-1, which is about 2.5 times of homogeneous Pd catalyst (∼3500 h-1). The interfacial Lewis acid-base pair [Ru-O-Ce-Vö], which consists of acidic Ce-Vö (oxygen vacancy) site and basic interfacial oxygen of Ru-O-Ce linkage, acts as active site for the dissociation of methanol and the subsequent transfer of hydrogen to the activated ethylene, which is the key step in acid-promoter-free EMC reaction. The combination of 1H MAS NMR, pyridine-IR and DFT calculations reveals the hydrogen species derived from methanol contains Brönsted acidity. The EMC reaction mechanism under acid-promoter-free condition over Ru-clusters/ceria catalyst is discussed.

Methanol activation catalyzed by Pt7, Pt3Cu4, and Cu7 clusters: A density functional theory investigation

Metal clusters were considered as excellent catalysts for methanol dissociation. In this work, two main decomposition mechanisms of methanol on Pt7, Pt3Cu4, and Cu7 clusters were investigated by the density functional theory. One was methanol direct dehydrogenation, and the other was non‐CO‐involved oxidation. Stable adsorption configurations, elementary reaction barriers, the potential energy surface (PES), and the charge analysis were elucidated. The results showed that on Pt7 cluster, methanol was favorable for direct decomposition. On Pt3Cu4 and Cu7 clusters, methanol was inclined to the pathway of non‐CO‐involved oxidation. All the transition‐state energies and the final‐state energies were related in a linear, including those for the clusters. The results may be useful for computational design and catalysts optimization.

Highly selective conversion of CO2 into ethanol on Cu/ZnO/Al2O3 catalyst with the assistance of plasma

A novel and efficient way for conversion of CO2 and H2O into ethanol over a commercial Cu/ZnO/Al2O3 catalyst with the assistance of negative corona discharge plasma was presented in this work. The low-temperature production rate and selectivity of ethanol were effectively enhanced by combining catalyst with plasma. The production rate of ethanol increased with increasing of input power and water vapor content, while decreased with the increase of flow rate of CO2. XRD and XPS results revealed that metallic copper was partly or completely oxidized after reaction and FTIR results uncovered that on the oxidized copper CO2 adsorbed as carboxylate (COO−) which finally hydrogenated to ethanol. The synergistic effect of partly oxidized copper combined with plasma could promote the dissociation of methanol to form ethanol, which resulted in the high selectivity of ethanol.

Methanol on Anatase TiO2 (101): Mechanistic Insights into Photocatalysis.

The photoactivity of methanol adsorbed on the anatase TiO2 (101) surface was studied by a combination of scanning tunneling microscopy (STM), temperature-programmed desorption (TPD), X-ray photoemission spectroscopy (XPS), and density functional theory (DFT) calculations. Isolated methanol molecules adsorbed at the anatase (101) surface show a negligible photoactivity. Two ways of methanol activation were found. First, methoxy groups formed by reaction of methanol with coadsorbed O2 molecules or terminal OH groups are photoactive, and they turn into formaldehyde upon UV illumination. The methoxy species show an unusual C 1s core-level shift of 1.4 eV compared to methanol; their chemical assignment was verified by DFT calculations with inclusion of final-state effects. The second way of methanol activation opens at methanol coverages above 0.5 monolayer (ML), and methyl formate is produced in this reaction pathway. The adsorption of methanol in the coverage regime from 0 to 2 ML is described in detail; it is key for understanding the photocatalytic behavior at high coverages. There, a hydrogen-bonding network is established in the adsorbed methanol layer, and consequently, methanol dissociation becomes energetically more favorable. DFT calculations show that dissociation of the methanol molecule is always the key requirement for hole transfer from the substrate to the adsorbed methanol. We show that the hydrogen-bonding network established in the methanol layer dramatically changes the kinetics of proton transfer during the photoreaction.

Methanol adsorption and dissociation on LaMnO3 and Sr doped LaMnO3 (001) surfaces

Using density functional theory, we investigate methanol adsorption and dissociation on the MnO2- and LaO-terminated LaMnO3 (001) surface as a function of Sr dopant enrichment in and near the surface. In response to bulk cleavage, we find electron depletion of the negatively charged MnO2 surface layer that is enhanced by Sr doping in the subsurface. In contrast, we observe electron accumulation in the positively charged LaO surface layer that is reduced by Sr doping in the surface layer. Methanol adsorbs dissociatively on the LaO termination of the LaMnO3 (001) surface. Methanol adsorption on the LaO termination is strongly preferred over adsorption on the MnO2 termination. While moderate doping has a small influence on methanol adsorption and dissociation, when 100% of La is replaced by Sr in the surface or subsurface, the adsorption preference of methanol is reversed. If the surface is highly dopant enriched, methanol favours dissociative adsorption on the MnO2-terminated surface.

Effect of Ca and Sr in MgO(100) on the activation of methanol and methyl acetate

The activation of methanol (CH3OH) via HO and of methyl acetate (CH3COOCH3) via CO and CH bond dissociations, has been studied using periodic density functional theory to probe the effect of small amount of higher basic alkaline earth metals in MgO(100) surface. The interaction of methyl acetate, methanol and ionic dissociation products including methoxide, acetyl, acetate, proton, enolate, methylene acetate, methylene are studied on MgO(100)-1Ca/1Sr/2Sr/2Ca surfaces in terms of adsorption, activation, dissociation and association energies. The strength of methanol interaction increases progressively with the basicity and concentration of alkaline earth oxides in the surface: MgO(100)<MgO-1Ca<MgO-1Sr<MgO-2Ca<MgO-2Sr. This increase of basicity and concentration do not affect in the same manner the activation of methyl acetate. The adsorption energies of methyl acetate and the activation energies of CO and CH bond dissociations are only slightly reduced compared to pure MgO surface. Only the stability of fragments that bind with the oxygen on these surfaces is increased when increasing the basicity and concentration, fact reflected in the dissociation energy and the activation energy of the reverse bond formation. These results were discussed in the context of transesterification and intersetrification reactions with application in biodiesel production.

Interplay between Methanol and Anatase TiO2(101) Surface: The Effect of Subsurface Oxygen Vacancy

The dissociation of methanol plays a vital role in many reactions of C1 chemistry. The effect of defects on methanol dissociation is unclear. The adsorption and reactivity of methanol on the anatase TiO2(101) surface with or without a subsurface oxygen vacancy (VOsub) have been studied using first-principles calculations. The adsorbed methanol on the perfect surface prefers the molecular state with a dissociation energy barrier of 0.56 eV. Interestingly, the adsorption and reactivity of methanol are greatly promoted by VOsub. Upon methanol adsorption, the predominant VOsub on the clean surface becomes active, which is comparable to the surface oxygen vacancy (VOsur). The VOsub migrates from the subsurface to the surface layer with a low energy barrier of 0.17 eV, and then the adsorbed methanol molecule easily dissociates through a barrierless pathway facilitated by VOsur. Such results clearly reveal that the VOsub plays a vital role in the dissociation of methanol. These results may provide new insights i...

Identifying the Role of Photogenerated Holes in Photocatalytic Methanol Dissociation on Rutile TiO2(110)

As an important model reaction, photocatalytic methanol dissociation on rutile TiO2(110) has drawn much attention, but its reaction mechanism remains elusive. Using DFT+U calculations, we investigate the whole dissociation process of methanol into formaldehyde with and without photogenerated holes, aiming to illustrate how the hole is involved in the dissociation. We find that the O–H dissociation of methanol is a heterolytic cleavage process and is likely to be thermally driven; the presence of a hole has no promotion on the barrier and enthalpy change. In contrast, the subsequent C–H bond cleavage follows the homolytic cleavage mode and is likely to be photochemically driven; great enhancement can be made in both kinetics and thermodynamics when holes are introduced. The essential roles of holes in promoting C–H dissociation are identified, and what kinds of catalytic reactions can or cannot be facilitated by holes is discussed. Our findings may considerably broaden the understanding of photocatalytic c...

Splitting methanol on ultra-thin MgO(100) films deposited on a Mo substrate.

The dissociation reaction of methanol on metal-supported MgO(100) films has been studied by employing density functional theory calculations. As far as we know, the dissociation of a single methanol molecule over inert oxide insulators such as MgO has not yet been successfully realized without the introduction of defects or low coordinated atoms. By depositing ultra-thin oxide films on a Mo substrate, we have successfully proposed the dissociative state of methanol. The dissociation reaction is energetically exothermic and nearly barrierless. The lattice mismatch between ultra-thin MgO(100) films and metal substrates plays a crucial role in the heterolytic dissociation of adsorbates, while the electronic effect of the Mo(100) substrate plays a non-ignorable role in the homolytic dissociation of methanol. The metal-supported ultra-thin oxide films studied herein provide a versatile approach to enhance the surface reaction activity and properties of oxides.

Partial Oxidation of Methanol on MoO3 (010): A DFT and Microkinetic Study

Methanol oxidation is employed as a probe reaction to evaluate the catalytic properties of the (010) facets of molybdenum trioxide (MoO3), a reducible oxide that exhibits a rich interplay of catalytic chemistry and structural transformations. The reaction mechanism is investigated with a combination of electronic structure calculations, using the BEEF-vdW and HSE06 functionals, and mean-field microkinetic modeling. Considered pathways include vacancy formation and oxidation, monomolecular dehydrogenation of methanol on reduced and nonreduced surfaces, bimolecular reactions between dehydrogenated intermediates, and precursor steps for hydrogen molybdenum phase (HyMoO3–x) formation. Methanol dissociation begins with C–H or O–H scission, with the O–H route found to be kinetically and thermodynamically preferred. Dehydrogenation of CH2O* to CHO* is slow in comparison to desorption, leading to complete selectivity toward CH2O. C–H scission of CH3O* and recombination of dissociated OH* to form H2O* are kinetica...