Adsorbed Methoxy Species Research Articles

Enhanced performance and selectivity of CO2 methanation over g-C3N4 assisted synthesis of Ni[sbnd]CeO2 catalyst: Kinetics and DRIFTS studies

The hydrogenation of CO2 on CeNi catalyst modified with g-C3N4 (CeNiCN) as a sacrificial and protective template was studied by in-situ DRIFTS and Kinetics experiments to investigate the influence of modification on the catalytic activity and selectivity to gain mechanistic insight. After modification, the catalyst showed higher catalytic activity and selectivity. H2-TPR, CO2-TPD, TEM and XPS confirmed that this modification could enhance the interaction of nickel and ceria and decrease the particle size of nickel, which is in favor of the dissociation of H2 and adsorption of CO2. The in-situ DRIFTS experiments demonstrated that CO2 is adsorbed on ceria sites, forming carboxylate (CO2δ−), unidentate carbonate and bicarbonates, which, in turn, react with adsorbed and dissociated H on Ni to produce formate species. Furthermore, adsorbed methoxy species were observed, which are recognized to be intermediates in the methanation process. In-situ transient DRIFTS confirm that the adsorbed CO is not a reaction intermediate, but a by-product, which originates from the decomposition of weak-binding formate species on Ce3+ sites. The unmodified catalyst has more weak-binding formate species, which are more inclined to decompose into CO accounting for the low selectivity. Furthermore, the adsorbed CO on Ce3+ sites cannot react with the adsorbed hydrogen to produce methane. Kinetics studies are consistent with a Langmuir-Hinshelwood type mechanism in which the formation of bicarbonate is the rate-determining step (RDS).

Methanol Adsorption and Reaction on Samaria Thin Films on Pt(111).

We investigated the adsorption and reaction of methanol on continuous and discontinuous films of samarium oxide (SmOx) grown on Pt(111) in ultrahigh vacuum. The methanol decomposition was studied by temperature programmed desorption (TPD) and infrared reflection absorption spectroscopy (IRRAS), while structural changes of the oxide surface were monitored by low-energy electron diffraction (LEED). Methanol dehydrogenates to adsorbed methoxy species on both the continuous and discontinuous SmOx films, eventually leading to the desorption of CO and H2 which desorbs at temperatures in the range 400–600 K. Small quantities of CO2 are also detected mainly on as-prepared Sm2O3 thin films, but the production of CO2 is limited during repeated TPD runs. The discontinuous film exhibits the highest reactivity compared to the continuous film and the Pt(111) substrate. The reactivity of methanol on reduced and reoxidized films was also investigated, revealing how SmOx structures influence the chemical behavior. Over repeated TPD experiments, a SmOx structural/chemical equilibrium condition is found which can be approached either from oxidized or reduced films. We also observed hydrogen absence in TPD which indicates that hydrogen is stored either in SmOx films or as OH groups on the SmOx surfaces.

Esterification of acetic acid with methanol to methyl acetate on Pd-modified zeolites: effect of Brønsted acid site strength on activity

The effect of the amount of Bronsted acid sites with their strengths on bare and palladium-modified H-ZSM5 and H-Ferrierite on the catalytic activity in the esterification of acetic acid with methanol was investigated by comparing the variation of Bronsted acid site strength. The strong acid sites on zeolites are responsible for the low reaction rate of esterification and low selectivity to methyl acetate by the strong adsorption properties of reactants. The stronger Bronsted acid site strengths, which are more abundant on bare H-Ferrierite than H-ZSM5, were properly modified with Pd promoter without significant loss of total acid sites. The esterification rates were largely increased with a concomitant increase of methyl acetate selectivity after Pd modification. The strongly adsorbed methoxy species by forming surface dimeric alcohols can be attributed to the suppressed rate of esterification. The roles of Bronsted acid sites and their strengths to catalytic activity were verified through NH3-TPD and adsorbed pyridine or methanol through FT-IR analyses.

Kinetic and mechanistic study of the photocatalytic reforming of methanol over Pt/TiO2 catalyst

The kinetics and mechanism of photo-reforming reaction of methanol have been studied over Pt/TiO2 suspensions irradiated with near-UV light (λmax=365nm). The effects of initial methanol concentration in solution (CMeOH), photocatalyst content in suspension (Ccat) and incident light intensity (I0) on the reaction rate have been investigated in detail. It has been found that the reaction rate depends strongly on methanol concentration and increases by more than two orders of magnitude with increase of CMeOH from zero to 1.0M. The dependence of the reaction rate on CMeOH can be described by saturation-type kinetics according to the Langmuir–Hinselwood model. Increase of photocatalyst content in suspension results in an increase of the reaction rate, which tends to stabilize for Ccat>3gL−1. The rate of hydrogen evolution varies with incident light intensity in a manner which depends on the initial methanol concentration in solution. In particular, the order of the reaction with respect to I0 is unity for CMeOH=100mM and 0.5 for CMeOH=3.0mM. Analysis of reaction intermediates in solution and on the photocatalyst surface indicates that, under the present experimental conditions, the reaction proceeds via progressive oxidation of adsorbed methoxy species to formaldehyde, dioxymethylene and formate. These species do not desorb from the photocatalyst surface but remain in the adsorbed mode until they are completely reformed to H2 and CO2.

Methanol dehydration to dimethylether over Al2O3 catalysts

The vapor-phase dehydration of methanol to dimethylether (DME) has been investigated over a set of eight commercial and self-prepared Al2O3 catalysts with different physicochemical characteristics. Materials were characterized with respect to their textural properties (B.E.T. and B.J.H. methods), acidity (TPD of ammonia), crystallinity, phase composition and morphology (XRD, TEM). Catalytic activity and selectivity have been evaluated in the temperature range of 150–400°C in the absence and in the presence of water in the feed. The adsorption/desorption properties of catalysts toward methanol and water as well as some aspects of the reaction mechanism have been investigated with the use of transient-MS and in situ DRIFTS techniques. Results show that catalytic activity per gram of catalyst varies by two orders of magnitude from one sample to another in a manner that depends on the physicochemical characteristics of Al2O3. All samples investigated are highly selective to DME at temperature lower than ca. 325°C, whereas small amounts of CO and CH4 are formed at higher reaction temperatures. As a general trend, activity per gram of catalyst increases with increase of SSA (amount of acidic surface sites) and is enhanced over samples characterized by high crystallinity and relatively high porosity. The presence of water vapor in the feed suppresses conversion of methanol at a given temperature due to competition for active sites and thermodynamic limitations. Methanol adsorbs on the Al2O3 surface both molecularly and in the form of methoxy species. Evidence is provided that formation of DME involves weakly adsorbed methoxy species whereas methoxy species that are bonded more strongly on the Al2O3 surface are converted to surface formates, which decompose to yield CO, H2 and CH4 in the gas-phase.

Comparative study on the photocatalytic decomposition of methanol on TiO2 modified by N and promoted by metals

The photo-induced vapor-phase reaction of methanol was investigated on Pt metals deposited on pure and N-doped TiO2. Infrared spectroscopic measurements revealed that illumination of the CH3OH–TiO2 and CH3OH-M/TiO2 systems led to the conversion of adsorbed methoxy species into adsorbed formate. In the case of metal-promoted TiO2 catalysts CO bonded to the metals was also detected. Pure titania exhibited very little photoactivity, its efficiency, however, increased with the narrowing of its bandgap by N-doping, a feature attributed to the prevention of electron–hole recombination. Deposition of Pt metals on pure and N-doped TiO2 dramatically enhanced the extent of photoreaction of methanol even in visible light: hydrogen and methyl formate with selectivities of 83–90% were the major products. The most active metal was Pt followed by Pd, Ir, Rh, and Ru. The effect of metal was explained by a better separation of charge carriers induced by illumination and by enhanced electronic interaction between metal nanoparticles and TiO2.

Elucidating the role of Cu species in the oxidative carbonylation of methanol to dimethyl carbonate on CuY: An in situ spectroscopic and catalytic study

CuY catalysts with varied Cu content obtained by incipient-wetness-impregnation of commercial NH 4-Y zeolite with copper nitrate solution and calcination at 400 °C in air were examined by in situ FTIR investigations to characterize the Cu species and elucidate their specific role in the oxidative carbonylation of methanol. The adsorption of CO was applied to check the state of Cu(I) species whereas the acidity was characterized by pyridine adsorption. Depending on the Cu content different intensity ratios of the characteristic Cu(I)–CO bands at 2160 and 2146 cm −1 were observed which give information about the distribution of Cu(I) cations at different positions near the supercage. Lewis sites of different strengths are created by introduction of Cu resulting from Cu(I)/Cu(II) cations positioned in the super cages. MeOH adsorbs dissociatively on preferable Cu(I) Lewis sites by formation of methoxy species without additional supply of oxygen. Methoxy species and CO adsorb at the same Cu sites. Oxygenated products (monomethyl carbonate, dimethyl carbonate, CO 2) are formed during simultaneous adsorption of MeOH and CO with and without oxygen which indicates a participation of lattice oxygen of CuO x aggregates in the oxidation process and points to a Mars-van-Krevelen mechanism. Adsorbed formate species more pronounced at higher Cu loadings were additionally found. CO reacts with adsorbed methoxy species by formation of mainly monomethyl carbonate whereas the simultaneous presence of formate-like species promotes the formation of dimethyl carbonate. This implicates a specific role of adsorbed carbonate-like species for dimethyl carbonate formation. High Cu loadings were found to be beneficial because the additional formation of CuO x agglomerates in the super cages favors oxidation and oxocarbonylation reactions of methanol and enhances the formation of dimethyl carbonate which was also confirmed by the catalytic tests.

Time-resolved infrared absorption study of nine TiO 2 photocatalysts

Electron kinetics of nine TiO 2 catalysts were compared in a microsecond time domain. Each catalyst was band-gap excited with an UV light pulse, and electron-induced absorption of mid infrared light was observed as a function of time delay. The probability of electron–hole recombination in the bulk, electron attachment to adsorbed oxygen, and hole attachment to adsorbed methoxy species was estimated.

Decomposition of methanol on Au(310)

Experimental results on the interaction of methanol with a Au(310) surface studied using X-ray photoelectron spectroscopy (XPS) and temperature programmed desorption (TPD) are reported. At 80 K methanol forms a physisorbed condensed layer above 5 L of exposure. Interestingly, O-H bond scission takes place above 150 K on the surface resulting in an adsorbed methoxy species which is stable till 500 K. This is in contrast with other gold surfaces like Au(111) and Au(110) which showed no evidence for decomposition. The difference in reactivity of Au(310) surface towards methanol is interpreted in terms of the presence of special sites formed by (110) steps and (100) terraces on this surface.

Oxidation of Methanol to Formaldehyde on Vanadia Films Supported on CeO2(111)

The reaction of methanol to formaldehyde on both V2O3 and V2O5 films supported on CeO2(111) was studied by temperature programmed desorption (TPD). Methanol was found to react on monolayer and submonolayer vanadia films to produce formaldehyde, while multilayer films were found to be inactive for this reaction. The kinetics and mechanism of the dehydrogenation of adsorbed methoxides was found to be dependent on the oxidation state of the vanadium cations in the supported vanadia layer. On films that contained V3+ (i.e., V2O3), adsorbed methoxy species produced via dissociative adsorption of methanol underwent dehydrogenation to formaldehyde at 600 K. In contrast, on films that contained predominantly V5+ (i.e., V2O5), adsorbed methoxy species underwent dehydrogenation to produce formaldehyde at 540 K. Kinetic parameters for the methoxide dehydrogenation to formaldehyde on both V2O3 and V2O5 films were determined from the TPD data. The TPD results also provide insight into the thermal stability of supporte...

Hydrogen production by methanol reforming: post-reaction characterisation of a Cu/ZnO/Al2O3 catalyst by XPS and TPD

Post-reaction X-ray photoelectron spectroscopy (XPS) and temperature-programmed desorption (TPD) experiments have been carried out after exposure of a commercial Cu/ZnO/Al2O3 catalyst to methanol reforming conditions in the presence of O2 and H2O. A dependence of the copper oxidation state on the temperature was observed, with low temperatures favouring a higher oxidation state. In the presence of water, Cu(II) was formed at T≤485K, giving rise to a very low catalytic activity. Under methanol dry partial oxidation conditions at T≥530K a decrease of the Cu surface concentration was observed. Adsorbed methoxy species were found on the surface after all catalytic treatments, with the amount decreased by the presence of water in the feed. The presence of O2 favoured the formation of surface formate.

A Kinetic Study for Methanol Decomposition on Plate-Type Nickel Catalyst Prepared by Electroless Plating.

A plate-type nickel catalyst on an aluminum plate prepared by electroless plating, which consisted of displacement of aluminum with zinc and the deposition of nickel by chemical reduction, has high activity and high selectivity to form hydrogen and carbon monoxide for methanol decomposition. This nickel catalyst, then would be suitable for a plate-type catalyst for the construction of a wall reactor system, which has an effective exchange of heat energy, a quick load response due to a low pressure loss and a downsized dimension of the reactor. In this study, in order to accumulate any kinetic information and derive the reaction rate equation for constructing the wall reactor system, a kinetic study for methanol decomposition on the plate-type nickel catalyst prepared by electroless plating was conducted at 533–623 K and atmospheric pressure.The reaction rate equation revealed that a methanol molecule is strongly adsorbed on the plate-type catalyst surface, forming a methoxy species (CH3O-), and the rate-determining step involves the abstraction of hydrogen from the adsorbed methoxy species. Furthermore, the reaction rate was depressed by the introduction of carbon monoxide in the reaction feed, but was not affected by that of hydrogen. By assuming the Langmuir–Hinshelwood mechanism, the integrated rate equation is derived from the obtained kinetic data in fitting parameters, such as the constant of the reaction rate and adsorption equilibrium for methanol, hydrogen, and carbon monoxide, by means of the Simplex method. The derived rate equation is well expressed with each experimental result.

Methanol as an IR probe to study the reduction process in ceria–zirconia mixed compounds

Adsorbed methoxy species were observed on Ce x Zr 1− x O 2 ceria–zirconia mixed compounds, with x ranging from 0 to 1. Surface cationic sites were so investigated looking at the ν(OC) and ν s(CH 3) methoxy vibrations. From the ν(OC) band due to methoxy species on-top coordinated with Zr 4+ ions, it was concluded that mixed compounds were solid solutions with no segregation of the cations at the surface. Mixed compounds were H 2-reduced in the temperature range 473–873 K. No reduction of Zr 4+ ions was observed. Conversely, the formation of surface O-vacancies in the vicinity of cerium ions was accompanied by the Ce 4+/Ce 3+ reduction. Cerium surface ions were completely reduced at 673 K except for samples for which x=0.8 and 0.68. This is in accordance with an increased O-bulk mobility towards the surface as zirconia is incorporated into ceria. Re-oxidation by O 2 amounts, volumetrically measured, has shown that the reducibility of cerium cations increased with the zirconium content. This was correlated with a deeper reduction of the crystallites.

In situ DRIFTS study of the adsorption–oxidation of CH 3OH on V 2O 5

The oxidation of CH 3OH on V 2O 5 has been studied from room temperature to 250°C. The reaction products were analyzed by on-line gas chromatography (GC) and the adsorbed species were characterized by “in situ” diffuse reflectance infrared Fourier transform spectroscopy (DRIFTS). Both isothermal and non-isothermal experiments were carried out in order to identify adsorbed species in the oxidation of CH 3OH on V 2O 5 catalysts. The interaction of CH 3OH with theV 2O 5 surface results in the formation of methoxy groups and V–OH species as stated by DRIFTS. The infrared spectra suggest that more than one catalytic site are involved in the adsorption process. The formation of V–OH and the resulting reduction of the vanadia catalyst leading to V 4+⎓O and V 3+ groups is inferred from DRIFTS data and X-ray diffraction patterns of the used catalysts showing the presence of reduced oxide phases. As the reaction proceeds, the adsorbed methoxy species are oxidized to H 2CO, formate species, partial oxidation products, CO and CO 2.

Thermal evolution of the adsorbed methoxy species on Ce xZr 1− xO 2 solid solution samples: a FT-IR study

Thermal reduction of a series of Ce x Zr 1− x O 2 solid solution samples by methanol adsorbed at room temperature was investigated using infrared spectroscopy (IR). Methoxy species resulting from methanol dissociation and adsorbed as on-top or bridging species, either on Zr 4+ or Ce 4+ ions, are well differentiated. Upon the thermal treatment, Ce 4+ sites appear to be exclusively the reactive ones through their reduction into Ce 3+. First, on-top methoxy species adsorbed on Ce 4+ sites are oxidised to mobile formate species in the 423–473 K temperature range, with the subsequent partial cerium reduction. Second, bridging methoxy species and formate ones decomposed in the 473–523 K temperature range through processes involving CO, CO 2, H 2, H 2O gaseous evolution. The reduction of surface cerium ions is then complete. The production of H 2 molecules was established by a complementary temperature programmed desorption study. The different temperature range at which mixed oxide reduction and hydrogen evolution take place demonstrates that the catalysts are not reduced by H 2 uptake, but by framework oxygen consumption, due to methoxys oxidation into formate species.

Methanol adsorption and reactivity at uranium and UO 2 surfaces

The interaction of methanol (CH 3OD) with polycrystalline uranium and UO 2 has been studied by X-ray photoelectron spectroscopy (XPS), thermal desorption–mass spectrometry (TDMS) and secondary-ion mass spectrometry (SIMS) over the temperature range 90–500 K. Low-temperature (~90 K) adsorption on uranium resulted in the formation of methoxy species along with condensed-phase adsorbed methanol. Room-temperature adsorption of methanol (or annealing an adsorbed methanol adlayer to 300 K) on uranium produces a mixture of methoxy and a uranium oxycarbide. Further heating to 400 K completely decomposes the adsorbed methoxy species, with 10% desorbing as methane and the remaining methoxide irreversibly converting to an uranium oxycarbide overlayer of nominal composition UO 0.66C 0.34. Concomitant with these carbon-fragment-conversion processes, desorption of hydrogen and deuterium are also seen over a wide temperature regime (325–450 K). Methanol–d adsorption on UO 2 also produces a methoxy surface species at surfaces temperatures <150 K. Adsorbate decomposition following thermal desorption releases gaseous CH 4, H 2, HD, D 2 and CO. Most of the oxygen derived from the methanol molecule (>90%) was incorporated into the UO 2 layer and all of the adsorbed carbon desorbs from the surface as one of the molecular species identified above. The reactive adsorption and thermal decomposition of methanol at uranium and UO 2 surfaces is compared with that observed at other metal and metal oxide surfaces.

Use of methanol as an IR molecular probe to study the surface of polycrystalline ceria

A. Badri, C. Binet and J. Lavalley, J. Chem. Soc., Faraday Trans., 1997, 93, 1159 DOI: 10.1039/A606628C

A quantum-chemical study of para/ortho-toluene alkylation by adsorbed methoxy species on zeolites

MNDO and PM3 semiempirical methods have been employed to study the mechanism of toluene alkylation by methanol on zeolites, by means of the cluster (H) 3SiO(CH 3)T(H) 2OSi(H) 3 in were T  Al, B, Ga. Two possible reaction mechanisms have been found. The first one corresponds to the classical process of aromatic electrophilic substitution, in which the transition state of the reaction shows distances and angles according to the precursor of the Wheland intermediate. The second mechanism proceeds in one concerted step, in which the transfer of the proton from the toluene to the basic oxygen of the zeolite is simultaneous to the transfer of the methyl cation from the zeolite to the toluene. The effect of chemical composition of the cluster on regioselectivity has been studied, and perturbed molecular orbital (PMO) analysis of the system at the ab initio HF STO-3G level has shown that covalent interactions are important. The energy of the covalent interaction between the molecular orbitals of the toluene and the cluster mainly HOMO-LUMO, are more important in para than in ortho positions.

Identification of two tilted adsorbed μ 2-methoxy species on Ni{110} using RAIRS

Two different adsorbed methoxy species on Ni{110} are identified using reflection absorption infrared spectroscopy both of which are twofold coordinated and tilted with respect to the surface normal. One methoxy species is adsorbed onto short bridge sites on the close-packed rows, with a large tilt angle in the [001] direction. The other methoxy species is adsorbed at a long bridge site across the close-packed rows and tilted with a smaller angle in the [1 1 0] direction. We also report a remarkable similarity between the infrared data for methoxy adsorption on the same single crystal faces of Ni and Cu, indicating that the nature of the chemisorbed species is determined by the site symmetries on the different planes, and not by the metal itself.

Photodecomposition of adsorbed methoxy species by UV light and formaldehyde adsorption on silicon(111) studied by XPS and UPS

ADVERTISEMENT RETURN TO ISSUEPREVArticleNEXTPhotodecomposition of adsorbed methoxy species by UV light and formaldehyde adsorption on silicon(111) studied by XPS and UPSKatsumi Tanaka, Shigeo Matsuzaki, and Isamu ToyoshimaCite this: J. Phys. Chem. 1993, 97, 21, 5673–5677Publication Date (Print):May 1, 1993Publication History Published online1 May 2002Published inissue 1 May 1993https://doi.org/10.1021/j100123a035RIGHTS & PERMISSIONSArticle Views211Altmetric-Citations17LEARN ABOUT THESE METRICSArticle Views are the COUNTER-compliant sum of full text article downloads since November 2008 (both PDF and HTML) across all institutions and individuals. These metrics are regularly updated to reflect usage leading up to the last few days.Citations are the number of other articles citing this article, calculated by Crossref and updated daily. Find more information about Crossref citation counts.The Altmetric Attention Score is a quantitative measure of the attention that a research article has received online. Clicking on the donut icon will load a page at altmetric.com with additional details about the score and the social media presence for the given article. Find more information on the Altmetric Attention Score and how the score is calculated. Share Add toView InAdd Full Text with ReferenceAdd Description ExportRISCitationCitation and abstractCitation and referencesMore Options Share onFacebookTwitterWechatLinked InReddit PDF (602 KB) Get e-Alerts