Chemisorption Of Methanol Research Articles

Understanding surface (un)reactive sites of titania supports towards propyl-phosphonic acid surface modification

Organophosphonic acids (PAs) surface modification on metal oxides is important for applications, nevertheless, a detailed study evaluating the influence of metal oxides surface properties on PAs modification is lacking. This work presents a method to qualitatively probe surface (un)reactive sites of titania towards propyl-phosphonic acid (3PA) grafting by methanol. It identifies the more or less reactive sites, i.e., hydroxyl groups and Lewis acid sites, allowing to understand relative differences in maximum modification degrees of 3PA among different titania supports. Therefore, three different types of titania were used, while all other 3PA modification conditions were kept constant. A clear difference in the maximum modification degree on the three titania supports was observed. In-situ diffuse reflectance Fourier transform infrared spectroscopy revealed that not all surface OH groups had reacted with 3PA at the highest modification degree, which were similar to most of the OH groups remaining after methanol chemisorption. Also the adsorption capacities of the strongly bonded chemisorbed methanol elucidated differences in the three titania, which was related to the maximum modification degree of 3PA. Methanol chemisorption can thus aid in the understanding of the kinds of reactive surface sites that play a role in the divergence of 3PA coverage on different types of titania supports.

Methanol assisted water electrooxidation on noble metal free perovskite: RRDE insight into the catalyst’s behaviour

In this work, we have hypothesized that noble metal-free perovskites are an essential class of oxygen evolution reaction (OER) catalysts in an alkaline medium and thus, they are a suitable candidate for the assisted water oxidation catalysts. Herein, we demonstrate that the origin of the methanol-assisted OER activity at near thermodynamic potential on perovskite electrode arises due to the involvement of additional hydroxyls as a result of dissociative chemisorption of methanol. When the perovskite electrode is screened for methanol electrooxidation reaction in 0.5 M KOH + 0.5 M methanol electrolyte, it delivers a two times higher current density. This imparts an 82 % increase in the evolution of oxygen gas moles with complete oxidation of methanol to carbon dioxide. Along with the electrochemical characterization to understand the electrocatalyst property, Rotating ring disk electrode (RRDE) technique is explored for the first time in literature to validate the catalyst’s involvement during OER. RRDE is effective in understanding the lattice oxygen behaviour and methanol-assisted water electrooxidation during OER. Our results suggest new insights and ideas towards the oxygen evolution reaction process and the mechanistic insight into the elevated OER due to assisted methanol electrooxidation.

Pressure-induced postsynthetic cluster anion substitution in a MIL-53 topology scandium metal-organic framework.

Postsynthetic modification of metal-organic frameworks (MOFs) has proven to be a hugely powerful tool to tune physical properties and introduce functionality, by exploiting reactive sites on both the MOF linkers and their inorganic secondary building units (SBUs), and so has facilitated a wide range of applications. Studies into the reactivity of MOF SBUs have focussed solely on removal of neutral coordinating solvents, or direct exchange of linkers such as carboxylates, despite the prevalence of ancillary charge-balancing oxide and hydroxide ligands found in many SBUs. Herein, we show that the μ2-OH ligands in the MIL-53 topology Sc MOF, GUF-1, are labile, and can be substituted for μ2-OCH3 units through reaction with pore-bound methanol molecules in a very rare example of pressure-induced postsynthetic modification. Using comprehensive solid-state NMR spectroscopic analysis, we show an order of magnitude increase in this cluster anion substitution process after exposing bulk samples suspended in methanol to a pressure of 0.8 GPa in a large volume press. Additionally, single crystals compressed in diamond anvil cells with methanol as the pressure-transmitting medium have enabled full structural characterisation of the process across a range of pressures, leading to a quantitative single-crystal to single-crystal conversion at 4.98 GPa. This unexpected SBU reactivity - in this case chemisorption of methanol - has implications across a range of MOF chemistry, from activation of small molecules for heterogeneous catalysis to chemical stability, and we expect cluster anion substitution to be developed into a highly convenient novel method for modifying the internal pore surface and chemistry of a range of porous materials.

Number of surface sites and turnover frequencies for oxide catalysts

Unlike for metal catalysts where selective chemisorption probes (H2, O2, CO and N2O) have been successfully applied to count the number of active surface sites and allow determination of TOF values, the analogous situation for oxide catalysts has developed very slowly. The reason for the sluggish development of selective chemisorption probes for oxides is that the same chemisorption probes that work well with metals aren’t compatible with oxides since these chemical probes weakly adsorb on oxides. Methanol, however, is a very reactive molecule that has been found to readily chemisorb on oxides and allows for quantitative determination of the number of active surface sites (Ns). In this mini-review, the determination of the number of active sites by methanol chemisorption is reviewed with examples for pure oxides (one-component oxides), supported oxides (submonolayer, monolayer and above monolayer) and bulk mixed oxides (surface enriched oxide present in submonolayer, monolayer and in excess of monolayer coverages).

Probing Acid–Base Properties of Anatase TiO2 Nanoparticles with Dominant {001} and {101} Facets Using Methanol Chemisorption and Surface Reactions

In the present study, we investigate the surface acid–base properties of anatase TiO2 nanomaterials with dominant {101} and {001} facets via methanol titrations. Two anatase nanoparticles, TiO2(101...

An AIMD study of dissociative chemisorption of methanol on Cu(111) with implications for formaldehyde formation.

An important industrial process is methanol steam reforming, which is typically used in conjunction with copper catalysts. However, little agreement exists on the reaction mechanisms involved on a copper catalyst. Therefore, we have performed research yielding additional insight into the reaction mechanism for dissociative chemisorption of methanol on Cu(111) using ab initio molecular dynamics, supported by static calculations of the molecule-surface interaction with density functional theory. Our work predicts that after the initial dissociation, formaldehyde is formed through three different mechanisms. Additionally, it is observed that at high energy, CH cleavage is the dominant pathway instead of the formerly presumed OH cleavage pathway. Finally, in order to describe the interaction of methanol with the metal surface, the SRP32-vdW functional is used, which has been previously developed and tested for CHD3 on Ni(111), Pt(111), and Pt(211) using the Specific Reaction Parameter (SRP) approach. In this work, the SRP32-vdW functional is applied to methanol on Cu(111) as well, in the hope that future experiments can validate the transferability of the SRP32-vdW functional to chemically related molecule-metal surface systems.

Vibrational control of selective bond cleavage in dissociative chemisorption of methanol on Cu(111)

Controlling product branching ratios in a chemical reaction represents a desired but difficult achievement in chemistry. In this work, we demonstrate the first example of altering the branching ratios in a multichannel reaction, i.e., methanol dissociative chemisorption on Cu(111), via selectively exciting specific vibrational modes. To this end, we develop a globally accurate full-dimensional potential energy surface for the CH3OH/Cu(111) system and perform extensive vibrational state-selected molecular dynamics simulations. Our results show that O–H/C–H/C–O stretching vibrational excitations substantially enhance the respective bond scission processes, representing extraordinary bond selectivity. At a given total energy, the branching ratio of C–O/C–H dissociation can increase by as large as 100 times by exciting the C–O stretching mode which possesses an unprecedentedly strong vibrational efficacy on reactivity. This vibrational control can be realized by the well-designed experiment using a linearly polarized laser.

First principles studies on the selectivity of dimethoxymethane and methyl formate in methanol oxidation over V2O5/TiO2-based catalysts.

First principles calculations using both molecular cluster and periodic slab models were performed to reveal the mechanism for the formation of dimethoxymethane (DMM) from methanol over V2O5/TiO2-based catalysts. Two different pathways were found, and the formation of DMM was predicted to be initiated by methanol chemisorption followed by a dehydration reaction with hemiacetal catalyzed by acidic sites. For unpromoted V2O5/TiO2 catalysts, we predicted the energy barrier for the rate determining step (RDS) to follow the order formaldehyde (FA) > methyl formate (MF) > DMM, consistent with the experimental observation for the preferential formation of DMM at a relatively low temperature and that of MF at a relatively high temperature. For sulfate-promoted catalysts, the energy barriers were calculated to follow the order FA > DMM > MF, so the sulfate promoter was predicted to mainly enhance the selectivity of MF, consistent with our previous experiment in which very high yield of MF was obtained with the sulfate-promoted catalyst. Calculated rate constants for the RDS were further used for semi-quantitative predictions of the product selectivities, which were found to be in quite good agreement with some of the recent experimental data in the literature, showing the validity of our approach. We also investigated the effects of the titania support and the polymerization of the vanadia species on the reactivity of the V2O5/TiO2 catalyst. Finally, we benchmarked several popular exchange-correlation functionals for calculating the reaction energies for the formation of FA, MF, and DMM from methanol oxidation, and the M06 hybrid functional was found to be superior to other semi-local and hybrid functionals studied.

Highly Efficient and Stable Vanadia–Titania–Sulfate Catalysts for Methanol Oxidation to Methyl Formate: Synthesis and Mechanistic Study

Vanadia–titania–sulfate nanocatalysts for methanol oxidation to methyl formate (MF) were prepared by coprecipitation. When calcinated at 400 °C, both methanol conversion and MF selectivity reached ∼98.5% at reaction temperatures of 140–145 °C. Characterizations with several experimental techniques revealed the catalysts as highly dispersed vanadia supported by anatase titania with acidic sites of significant strength and density. The catalysts also showed very high stability with lifetime exceeding 4500 h. Extensive density functional theory calculations using both cluster and surface models revealed MF to form via the hemiacetal mechanism, involving the condensation of methanol and formaldehyde at acidic sites and methanol and hemiacetal oxidations at redox sites. The alternative formyl mechanism was predicted kinetically much less favorable, showing these catalysts to work in a distinct mechanism from the rutile titania photocatalyst. Interestingly, methanol chemisorption at redox sites leads to the formation of acidic sites capable of catalyzing the condensation reaction.

Methyl formate synthesis from methanol on titania supported copper catalyst under UV irradiation at ambient condition: Performance and mechanism

Selective oxidation of methanol to methyl formate (MF) with high MF formation rate and simple by-products at ambient conditions is a challenging process. Herein, we report the methyl formate synthesis from methanol on a titania supported copper catalyst in a flow reactor at 15–45°C under UV irradiation. The MF formation rate reaches 56.4mmolg−1h−1. Carbon dioxide is the only by-product below 30°C, and very small amount of dimethoxymethane besides carbon dioxide can be detected at 45°C. CuO on titania in the as-prepared catalyst is reduced to Cu to form an Ohmic contact between Cu and titania during the reaction under UV irradiation, which facilitates the photo-excited electron transfer from the conduction band of titania to Cu and reduces the recombination of the photo-excited electron–hole pairs. The negative charge on the Cu surfaces promotes the dissociation of oxygen. The coordinated methoxy resulted from the dissociative chemisorption of methanol on the surface of titania is oxidized by the photo-generated holes to form coordinated surface formaldehyde. The oxygen dissociation and the oxidation of coordinated methoxy are parallel reactions, and both of them are rate-controlled by light intensity. This study may contribute to design new photocatalytic systems and provide an applicable green route to MF synthesis from methanol.

The surface chemistry of nanocrystalline MgO catalysts for FAME production: An in situ XPS study of H2O, CH3OH and CH3OAc adsorption

An in situ XPS study of water, methanol and methyl acetate adsorption over as-synthesised and calcined MgO nanocatalysts is reported with a view to gaining insight into the surface adsorption of key components relevant to fatty acid methyl esters (biodiesel) production during the transesterification of triglycerides with methanol. High temperature calcined NanoMgO-700 adsorbed all three species more readily than the parent material due to the higher density of electron-rich (111) and (110) facets exposed over the larger crystallites. Water and methanol chemisorb over the NanoMgO-700 through the conversion of surface O2− sites to OH− and coincident creation of Mg-OH or Mg-OCH3 moieties respectively. A model is proposed in which the dissociative chemisorption of methanol occurs preferentially over defect and edge sites of NanoMgO-700, with higher methanol coverages resulting in physisorption over weakly basic (100) facets. Methyl acetate undergoes more complex surface chemistry over NanoMgO-700, with C–H dissociation and ester cleavage forming surface hydroxyl and acetate species even at extremely low coverages, indicative of preferential adsorption at defects. Comparison of C 1s spectra with spent catalysts from tributyrin transesterification suggest that ester hydrolysis plays a key factor in the deactivation of MgO catalysts for biodiesel production.

A Theoretical Study of Methanol Oxidation Catalyzed by Isolated Vanadia Clusters Supported on the (101) Surface of Anatase

A theoretical model has been developed for describing isolated vanadate species dispersed on the (101) surface of anatase that takes into account the equilibration of the supported species with gas-phase oxygen. The lowest energy of the combined solid and gas phases identifies the VOx species with the optimal structure and composition. This model of VOx species supported on the surface of anatase is then used to analyze the reaction path for methanol oxidation to formaldehyde. The chemisorption of methanol is found to proceed preferentially by addition across a V–O–Ti bond to form V–OCH3 and Ti–OH species. The rate-limiting step for the formation of formaldehyde takes place via the transfer of a hydrogen atom from V–OCH3 bound to an oxygen atom bridging two Ti atoms, i.e., a Ti–O–Ti group located adjacent to the supported vanadate species. This step is found to have the lowest apparent activation energy of all pathways explored for the formation of formaldehyde.

Tetrahexahedral Pt Nanocrystal Catalysts Decorated with Ru Adatoms and Their Enhanced Activity in Methanol Electrooxidation

Tetrahexahedral Pt nanocrystals (THH Pt NCs) bound by well-defined high index crystal planes offer exceptional electrocatalytic activity, owing to a high density of low-coordination surface Pt sites. We report, herein, on methanol electrooxidation at THH Pt NC electrodes studied by a combination of electrochemical techniques and in situ FTIR spectroscopy. Pure THH Pt NC surfaces readily facilitate the dissociative chemisorption of methanol leading to poisoning by strongly adsorbed CO. Decoration of the stepped surfaces by Ru adatoms increases the tolerance to poisoning and thereby reduces the onset potential for methanol oxidation by over 100 mV. The Ru modified THH Pt NCs exhibit greatly superior catalytic currents and CO2 yields in the low potential range, when compared with a commercial PtRu alloy nanoparticle catalyst. These results are of fundamental importance in terms of model nanoparticle electrocatalytic systems of stepped surfaces and also have practical significance in the development of surface tailored, direct methanol fuel cell catalysts.

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.

Investigation of the Nature and Number of Surface Active Sites of Supported and Bulk Metal Oxide Catalysts Through Methanol Chemisorption

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A density functional study of water and methanol chemisorption on MgO(1 1 0)

Periodic density functional calculations have been used to study the adsorption of water and methanol on the (1 1 0) surface of MgO. A brief re-investigation of the (1 0 0) surface confirms previous results that the dissociative adsorption of H 2O on MgO(1 0 0) is energetically unfavourable. We find the same to be true of CH 3OH. On the (1 1 0) surface, both water and methanol are strongly adsorbed, with high energies of adsorption. Water is dissociatively adsorbed with Δ E ranging from −290 kJ/mol at low coverage to −240 kJ/mol at a full monolayer. The adsorption of water is found not to be activated. Δ E for adsorption of methanol ranges from −255 to −240 kJ/mol. A mixed water–methanol adsorption layer is also possible, with a similar binding energy.

First-principles calculations on the surface electronic and reactive properties of M/SnO 2 (M=Ge, Mn) (110)

First-principles calculations based on density functional theory within the generalized gradient approximation (GGA) have been performed for the rutile M/SnO 2 (M=Ge, Mn) (110) surfaces. The difference in the surface electronic and reactive properties between the stoichiometric and reduced surfaces was investigated and compared to the SnO 2 surfaces. It is found that the close heat of formation of the bulk rutile MO 2 to that of bulk SnO 2 provides the energetic favorable substitution of M atoms for the 5-fold coordinated tin cations in both the stoichiometric and reduced surfaces. The semiconducting Ge/SnO 2 and metallic Mn/SnO 2 surfaces suggested by the band calculations show the increasing and decreasing surface energies relative to the SnO 2 surface, respectively. Using the oxidation of nitrogen monoxide (NO) with the topmost-layer bridging oxygen O b to NO 2, the reduced surfaces served as an oxidation catalyst which can easily generate by less than half of the energies of the thermal reduction process, which is specially effective for the Mn/SnO 2 surfaces with loosely bound O b. In addition, the very weak adsorption of methanol on the Mn/SnO 2 surfaces favors the generation of gaseous methanol before further oxidation. It is expected that those results will be useful to design a catalyst for the selective oxidation of hydrocarbons using the oxidative and reductive reactions on the reduced surface at a lower temperature than the one currently in use. The detailed molecular mechanics of the chemisorption of NO and methanol on the surfaces are also discussed.

A mechanistic approach to phenol methylation on Cu 1− xCo xFe 2O 4: FTIR study

The interaction of phenol, methanol, and possible reaction products of phenol methylation with the Cu 1− x Co x Fe 2O 4 system has been studied by FTIR spectroscopy in the temperature range between 373 and 623 K. The spectra obtained from the chemisorption of methanol onto Cu 1− x Co x Fe 2O 4 above 373 K indicate progressive oxidation to formate and/or dioxymethylene and then to CO, CO 2, and H 2. Phenol is predominantly adsorbed as phenolate species by the dissociative adsorption on an acid–base site and the phenyl ring of phenol is perpendicular to the catalyst surface, facilitating selective ortho methylation by methyl cations. Characteristic ν C O bands observed for ortho-methylated phenols on the catalyst surface allow the identification of the same from the reaction mixture adsorbed on catalysts at 473 K, well below the optimum reaction temperature of 623 K, on Cu-containing catalysts. However, CoFe 2O 4 shows little interaction of phenol with MeOH, when they are coadsorbed, and might be a limiting factor to the overall reaction. Coadsorption of acidity probes with phenol and methanol indicates that the same acid–base sites are responsible for the reaction. Methylated phenols show a weak interaction with the surface compared to phenol and are susceptible to desorption at reaction temperatures and thus facilitate efficient methylation.

In-Situ X-ray Absorption Spectroscopy Study of Pt and Ru Chemistry during Methanol Electrooxidation

Methanol electrooxidation in a 0.5 M sulfuric acid electrolyte containing 1.0 M CH3OH was studied on 30% Pt/carbon and 30% PtRu/carbon (Pt/Ru = 1:1) catalysts using X-ray absorption spectroscopy (XAS). Absorption by Pt and Ru was measured at constant photon energy in the near edge region during linear potential sweeps of 10-50 mV/s between 0.01 and 1.36 V vs rhe. The absorption results were used to follow Pt and Ru oxidation and reduction under transient conditions as well as to monitor Ru dissolution. Both catalysts exhibited higher activity for methanol oxidation at high potential following multiple potential cycles. Correlation of XAS data with the potential sweeps indicates that Pt catalysts lose activity at high potentials due to Pt oxidation. The addition of Ru to Pt accelerates the rate of methanol oxidation at all potentials. Ru is more readily oxidized than Pt, but unlike Pt, its oxidation does not result in a decrease in catalytic activity. PtRu/carbon catalysts underwent significant changes during potential cycling due to Ru loss. Similar current density vs potential results were obtained using the same PtRu/carbon catalyst at the same loading in a membrane electrode assembly half cell with only a Nafion (DuPont) solid electrolyte. The results are interpreted in terms of a bifunctional catalyst mechanism in which Pt surface sites serve to chemisorb and dissociate methanol to protons and carbon monoxide, while Ru surface sites activate water and accelerate the oxidation of the chemisorbed CO intermediate. PtRu/carbon catalysts maintain their activity at very high potentials, which is attributed to the ability of the added Ru to keep Pt present in a reduced state, a necessary requirement for methanol chemisorption and dissociation.

Adsorbate lone-pair-electron stimulated charge transfer between surface dangling bonds: methanol chemisorption on Si(1 1 1)-7 × 7

Unrestricted spin density functional theory in the flavor of B3LYP is used to explore the reaction pathways for CH 3O–H dissociation over an adatom–rest atom pair site on Si(1 1 1)-7 × 7. We find that an adatom–rest atom pair site should be best regarded as a di-radical pair and that adsorbate lone-pair electrons stimulate efficiently the charge transfer from Si adatoms to Si rest atoms and vice versa. Our calculations show that rest atoms are more active than adatoms towards the dissociation of a lone-pair electron donor adsorbate.