Selective Partial Oxidation Research Articles

Hydrogen production via methanol oxidation on platinum oxide thin film catalyst: Influence of methanol-to-oxygen ratio

Platinum oxide (PtOx) thin films deposited by means of radio frequency (rf) magnetron sputtering on oxidized Si substrate were investigated under realistic conditions as efficient catalysts promoting oxidation of methanol to produce hydrogen. The main focus was given to the role of reactant mixture (methanol + oxygen) composition at different surface temperatures. Freshly deposited PtOx changes its chemical state and morphology during the initial reaction leading to strongly corrugated metallic Pt layer with small amounts of stable Pt2+ cations providing a crucial ingredient for its specific catalytic properties. With growing temperature or methanol-to-oxygen molar ratio the reaction selectivity of such catalyst generally shifts from pathways producing water to those generating hydrogen. The efficiency for a given reaction route can be thus maximized via independent tuning of the fuel-oxygen ratio and surface temperature. The most intensive hydrogen evolution with the lowest relative amount of CO by-product (but relatively high abundance of methanolic residuals) was achieved with mildly oxygen lean feed above 500 K. The highest selectivity for partial oxidation of methanol pathway (>80%) can be, on the other hand, obtained with stoichiometric or oxygen rich mixture.

Methane to Methanol: Structure-Activity Relationships for Cu-CHA.

Cu-exchanged zeolites possess active sites that are able to cleave the C-H bond of methane at temperatures ≤200 °C, enabling its selective partial oxidation to methanol. Herein we explore this process over Cu-SSZ-13 materials. We combine activity tests and X-ray absorption spectroscopy (XAS) to thoroughly investigate the influence of reaction parameters and material elemental composition on the productivity and Cu speciation during the key process steps. We find that the CuII moieties responsible for the conversion are formed in the presence of O2 and that high temperature together with prolonged activation time increases the population of such active sites. We evidence a linear correlation between the reducibility of the materials and their methanol productivity. By optimizing the process conditions and material composition, we are able to reach a methanol productivity as high as 0.2 mol CH3OH/mol Cu (125 μmol/g), the highest value reported to date for Cu-SSZ-13. Our results clearly demonstrate that high populations of 2Al Z2CuII sites in 6r, favored at low values of both Si:Al and Cu:Al ratios, inhibit the material performance by being inactive for the conversion. Z[CuIIOH] complexes, although shown to be inactive, are identified as the precursors to the methane-converting active sites. By critical examination of the reported catalytic and spectroscopic evidence, we propose different possible routes for active-site formation.

Cu doped amine functionalized graphene oxide and its scope as catalyst for selective oxidation

We describe the synthesis of Cu doped amine functionalized graphene oxide (Cu@AGO) and its effective application as recyclable heterogeneous catalyst for selective partial oxidation of benzyl alcohols to corresponding benzaldehydes under mild conditions with acetonitrile as solvent. The Cu@AGO was fully characterized by XRD, FT-IR, Raman spectra, XPS, SEM and TEM analysis. FT-IR validated the successful grafting of organic amine with graphene oxide, while TEM confirmed the uniform distribution of metal particles on amine functionalized graphene oxide. The catalyst gave excellent conversion (92%) and selectivity (99%) towards benzaldehyde, with good reusability up to 4cycles.

Titanium Dioxide/Reduced Graphene Oxide Hybrid Photocatalysts for Efficient and Selective Partial Oxidation of Cyclohexane

Partial oxidation of cyclohexane (CHA) to cyclohexanone (CHA-one) with molecular oxygen (O2) is one of the most important reactions. Photocatalytic oxidation has been studied extensively with TiO2-based catalysts. Their CHA-one selectivities are, however, insufficient because the formed CHA-one is subsequently decomposed by photocatalysis involving the reaction with superoxide anion (O2●–) produced by one-electron reduction of O2 on TiO2. Here we report that TiO2, when hybridized with reduced graphene oxide (rGO), catalyzes photooxidation of CHA to CHA-one with enhanced activity and selectivity under UV light (λ > 300 nm). The TiO2/rGO hybrids produce CHA-one with twice the amount formed on bare TiO2 with much higher selectivity (>80%) than that on bare TiO2 (ca. 60%). The conduction band electrons photoformed on TiO2 are transferred to rGO, promoting efficient charge separation and enhanced photocatalytic cycles. The trapped electrons on rGO selectively promote two-electron reduction of O2 and suppress one-electron reduction. This inhibits the formation of O2●–, which promotes photocatalytic decomposition of the CHA-one formed. These properties of rGO therefore facilitate efficient and selective formation of CHA-one on the hybrid catalyst.

Comparative Analysis of Reactant and Product Adsorption Energies in the Selective Oxidative Coupling of Alcohols to Esters on Au(111)

Gold-based heterogeneous catalysts have attracted significant attention due to their selective partial oxidation capabilities, providing promising alternatives for the traditional industrial homogeneous catalysts. In the current study, the energetics of adsorption/desorption of alcohols (CH3OH/methanol, CH3CH2OH/ethanol, CH3CH2CH2OH/n-propanol) and esters (HCOOCH3/methyl formate, CH3COOCH3/methyl acetate, and CH3COOCH2CH3/ethyl acetate) on a planar Au(111) surface was investigated in conjunction with oxidative coupling reactions by means of temperature programmed desorption (TPD) and dispersion-corrected density functional theory (DFT) calculations. The results reveal a complex interplay between inter-molecular and surface-molecule interactions, both mediated by weak van der Waals forces, which dictates their relative stability on the gold surface. Both experimental and theoretical adsorption/desorption energies of the investigated esters are lower than those of the alcohols from which they originate through oxidative coupling reactions. This result can be interpreted as an important indication in favor of the selectivity of Au surfaces in alcohol oxidative coupling/partial oxidation reactions, allowing facile removal of partial oxidation products immediately after their generation preventing their complete oxidation to higher oxygenates.

Insight on the Interaction of Methanol-Selective Oxidation Intermediates with Au- or/and Pd-Containing Monometallic and Bimetallic Core@Shell Catalysts.

Using density functional theory (DFT), the interaction of crucial molecules involved in the selective partial oxidation of methanol to methyl formate (MF) with monometallic Au and Pd and bimetallic Au/Pd and Pd/Au core@shell catalysts is systematically investigated. The core@shell structures modeled in this study consist of Au(111) and Pd(111) cores covered by a monolayer of Pd and Au, respectively. Our results indicate that the adsorption strength of the molecules examined as a function of catalytic surface decreases in the order of Au/Pd(111) > Pd(111) > Au(111) > Pd/Au(111) and correlates well with the d-band center model. The preadsorption of oxygen is found to have a positive impact on the selective partial oxidation reaction because of the stabilization of CH3OH and HCHO on the catalyst surface and the simultaneous intensification of MF desorption. On the basis of a dynamical matrix approach combined with statistical thermodynamics, we propose a simple route for evaluating the Gibbs free energy of adsorption as a function of temperature. This method allows us to anticipate the relative temperature stability of molecules involved in the selective partial oxidation of methanol to MF in terms of catalytic surface.

An Overview of Recent Advances of the Catalytic Selective Oxidation of Ethane to Oxygenates

The selective partial oxidation of short chain alkanes is a key challenge within catalysis research. Direct ethane oxidation to oxygenates is a difficult aim, but potentially rewarding, and it could lead to a paradigm shift in the supply chain of several bulk chemicals. Unfortunately, low C–H bond reactivity and kinetically labile products are just some reasons affecting the development and commercialisation of such processes. Research into direct ethane oxidation is therefore disparate, with approaches ranging from oxidation in the gas phase at high temperatures to enzyme catalysed hydroxylation under ambient conditions. Furthermore, in overcoming the barrier posed by the chemically inert C–H bond a range of oxidants have been utilised. Despite years of research, this remains an intriguing topic from both academic and commercial perspectives. Herein we describe some recent developments within the field of catalytic ethane oxidation focusing on the formation of oxygenated products, whilst addressing the key challenges which are still to be overcome.

Partial Oxidation of Alcohols on Visible-Light-Responsive WO3 Photocatalysts Loaded with Palladium Oxide Cocatalyst

Particles of tungsten oxide loaded with a palladium oxide cocatalyst (PdOx/WO3) exhibit higher selectivity in comparison to other photocatalysts, such as Pt/WO3, Pd/TiO2, and Pt/TiO2, in the partial oxidation of alcohols such as 2-propanol to the corresponding aldehydes or ketones (e.g., 80% selectivity for acetone production with ca. 96% conversion of 2-propanol) in water containing molecular O2. A detailed investigation of 2-propanol oxidation as a model reaction revealed significant differences between the reactivities of the WO3 and TiO2 systems. On TiO2 photocatalysts, complete decomposition to CO2 proceeded readily, due to the occurrence of direct oxidation of 2-propanol and acetone adsorbed by holes, resulting in significantly low selectivity for partial oxidation. On the other hand, the rates of acetone peroxidation on WO3 photocatalysts were much lower than those on TiO2 due to the low affinity of the WO3 surface to the substrates, particularly acetone. The low affinity of the WO3 surface also en...

Designing for selectivity: weak interactions and the competition for reactive sites on gold catalysts.

A major challenge in heterogeneous catalysis is controlling reaction selectivity, especially in complex environments. When more than one species is present in the gas mixture, the competition for binding sites on the surface of a catalyst is an important factor in determining reaction selectivity and activity. We establish an experimental hierarchy for the binding of a series of reaction intermediates on Au(111) and demonstrate that this hierarchy accounts for reaction selectivity on both the single crystal surface and under operating catalytic conditions at atmospheric pressure using a nanoporous Au catalyst. A partial set of measurements of relative binding has been measured by others on other catalyst materials, including Ag, Pd and metal oxide surfaces; a comparison demonstrates the generality of this concept and identifies differences in the trends. Theoretical calculations for a subset of reactants on Au(111) show that weak van der Waals interactions are key to predicting the hierarchy of binding strengths for alkoxides bound to Au(111). This hierarchy is key to the control of the selectivity for partial oxidation of alcohols to esters on both Au surfaces and under working catalytic conditions using nanoporous gold. The selectivity depends on the competition for active sites among key intermediates. New results probing the effect of fluorine substitution are also presented to extend the relation of reaction selectivity to the hierarchy of binding. Motivated by an interest in synthetic manipulation of fluorinated organics, we specifically investigated the influence of the -CF3 group on alcohol reactivity and selectivity. 2,2,2-Trifluoroethanol couples on O-covered Au(111) to yield CF3CH2O-C([double bond, length as m-dash]O)(CF3), but in the presence of methanol or ethanol it preferentially forms the respective 2,2,2-trifluoroethoxy-esters. The ester is not the dominant product in any of these cases, though, indicating that the rate of β-H elimination from adsorbed trifluoroethoxy is slower than that for either adsorbed methoxy or ethoxy, consistent with their relative estimated β-C-H bond strengths. The measured equilibrium constants for the competition for binding to the surface are 2.9 and 0.38 for ethanol and methanol, respectively, vs. 2,2,2-trifluoroethanol, indicating that the binding strength of 2,2,2-trifluoroethoxy is weaker than ethoxy, but stronger than methoxy. These results are consistent with weakening of the interactions between the surface and the alkyl group due to Pauli repulsion of the electron-rich CF3 group from the surface, which offsets the van der Waals attraction. These experiments provide guiding principles for understanding the effect of fluorination on heterogeneous synthesis and further demonstrate the key role of molecular structure in determining reaction selectivity.

Ferroelectrics: A pathway to switchable surface chemistry and catalysis

It has been known for more than six decades that ferroelectricity can affect a material's surface physics and chemistry thereby potentially enhancing its catalytic properties. Ferroelectrics are a class of materials with a switchable electrical polarization that can affect surface stoichiometry and electronic structure and thus adsorption energies and modes; e.g., molecular versus dissociative. Therefore, ferroelectrics may be utilized to achieve switchable surface chemistry whereby surface properties are not fixed but can be dynamically controlled by, for example, applying an external electric field or modulating the temperature. Several important examples of applications of ferroelectric and polar materials in photocatalysis and heterogeneous catalysis are discussed. In photocatalysis, the polarization direction can control band bending at water/ferroelectric and ferroelectric/semiconductor interfaces, thereby facilitating charge separation and transfer to the electrolyte and enhancing photocatalytic activity. For gas–surface interactions, available results suggest that using ferroelectrics to support catalytically active transition metals and oxides is another way to enhance catalytic activity. Finally, the possibility of incorporating ferroelectric switching into the catalytic cycle itself is described. In this scenario, a dynamic collaboration of two polarization states can be used to drive reactions that have been historically challenging to achieve on surfaces with fixed chemical properties (e.g., direct NOx decomposition and the selective partial oxidation of methane). These predictions show that dynamic modulation of the polarization can help overcome some of the fundamental limitations on catalytic activity imposed by the Sabatier principle.

Honeycomb-structured catalysts for the selective partial oxidation of H2S

The reaction of H2S selective partial oxidation to elemental sulfur was studied on cordierite honeycomb-structured catalysts in the range of temperature of 150–200 °C.The preparation procedure of catalytic cordierite monoliths has been studied, from the washcoating with CeO2 to the deposition of V2O5 by wet impregnation.Two different preparation washcoating procedures deposition were studied: in one case the washcoat had been already added with the salts precursors of the active species (joint impregnation method), in other case the active species were added only after the deposition step of the washcoat on the monolith, by impregnation in a solution of the salt precursor (distinct impregnation method).The catalysts prepared with the two different methods were characterized and the stability was investigated in catalytic activity tests. The catalysts prepared with the “joint impregnation” method have shown a poor catalytic activity and tendency to the deactivation.Very different results were found for the catalysts prepared with the “distinct impregnation” method, for which the effect of the vanadium content (2–19 V2O5 wt %) was also studied at 200 °C. Good catalytic performances were obtained for both samples that have shown a high H2S conversion (∼90%), low SO2 selectivity (3%) and a high stability.The catalytic tests performed to varying the contact time with the temperature have allowed to identify the possible reaction pathway and the optimal operating conditions for which were obtained good catalytic performance.

Ozone-Activated Nanoporous Gold: A Stable and Storable Material for Catalytic Oxidation

We report a new method for facile and reproducible activation of nanoporous gold (npAu) materials of different forms for the catalytic selective partial oxidation of alcohols under ambient pressure, steady flow conditions. This method, based on the surface cleaning of npAu ingots with ozone to remove carbon documented in ultrahigh vacuum conditions, produces active npAu catalysts from ingots, foils, and shells by flowing an ozone/dioxygen mixture over the catalyst at 150 °C, followed by a temperature ramp from 50 to 150 °C in a flowing stream of 10% methanol and 20% oxygen. With this treatment, all three materials (ingots, foils, and shells) can be reproducibly activated, despite potential carbonaceous poisons resulting from their synthesis, and are highly active for the selective oxidation of primary alcohols over prolonged periods of time. The npAu materials activated in this manner exhibit catalytic behavior substantially different from those activated under different conditions previously reported. On...

Ab initio atomistic thermodynamics study on the oxidation mechanism of binary and ternary alloy surfaces.

Utilizing a combination of ab initio density-functional theory and thermodynamics formalism, we have established the microscopic mechanisms for oxidation of the binary and ternary alloy surfaces and provided a clear explanation for the experimental results of the oxidation. We construct three-dimensional surface phase diagrams (SPDs) for oxygen adsorption on three different Nb-X(110) (X = Ti, Al or Si) binary alloy surfaces. On the basis of the obtained SPDs, we conclude a general microscopic mechanism for the thermodynamic oxidation, that is, under O-rich conditions, a uniform single-phase SPD (type I) and a nonuniform double-phase SPD (type II) correspond to the sustained complete selective oxidation and the non-sustained partial selective oxidation by adding the X element, respectively. Furthermore, by revealing the framework of thermodynamics for the oxidation mechanism of ternary alloys through the comparison of the surface energies of two separated binary alloys, we provide an understanding for the selective oxidation behavior of the Nb ternary alloy surfaces. Using these general microscopic mechanisms, one could predict the oxidation behavior of any binary and multi-component alloy surfaces based on thermodynamics considerations.

Control of selectivity in allylic alcohol oxidation on gold surfaces: the role of oxygen adatoms and hydroxyl species.

Gold catalysts display high activity and good selectivity for partial oxidation of a number of alcohol species. In this work, we discuss the effects of oxygen adatoms and surface hydroxyls on the selectivity for oxidation of allylic alcohols (allyl alcohol and crotyl alcohol) on gold surfaces. Utilizing temperature programmed desorption (TPD), reactive molecular beam scattering (RMBS), and density functional theory (DFT) techniques, we provide evidence to suggest that the selectivity displayed towards partial oxidation versus combustion pathways is dependent on the type of oxidant species present on the gold surface. TPD and RMBS results suggest that surface hydroxyls promote partial oxidation of allylic alcohols to their corresponding aldehydes with very high selectivity, while oxygen adatoms promote both partial oxidation and combustion pathways. DFT calculations indicate that oxygen adatoms can react with acrolein to promote the formation of a bidentate surface intermediate, similar to structures that have been shown to decompose to generate combustion products over other transition metal surfaces. Surface hydroxyls do not readily promote such a process. Our results help explain phenomena observed in previous studies and may prove useful in the design of future catalysts for partial oxidation of alcohols.

Improved selectivity for partial oxidation of methane to methanol in the presence of nitrite ions and BiVO4 photocatalyst.

Nitrite ions are shown to have significant influence on the selectivity of the photocatalytic oxidation of methane to methanol. An almost complete inhibition of undesired CO2 has been achieved with BiVO4 in the presence of a low concentration of nitrite, which might act both as a UV filter and as a hydroxyl radical scavenger.

Partial oxidation of light alkanes by periodate and chloride salts.

The efficient and selective partial oxidation of light alkanes using potassium periodate and potassium chloride is reported. Yields of methane functionalization in trifluoroacetic acid reach >40% with high selectivity for methyl trifluoroacetate. Periodate and chloride also functionalize ethane and propane in good yields (>20%).

Modulation of the selectivity in partial oxidation of methanol over CuZnAl catalysts by adding CO2 and/or H2 into the reaction feed

The influence of the addition of the reaction products (CO2 and H2) to the feed during the partial oxidation of methanol (POMeOH) was studied over a Cu/ZnO/γ–Al2O3 catalyst. The addition of the reaction products influences in a significantly way the selectivity to hydrogen during the reaction. The observed changes are not due to the changes suggested by the thermodynamics when promoter gases are added and might be interpreted by considering modifications in the physicochemical properties of the catalysts, particularly in the kinetic of the reactions involved in POMeOH process. In some cases CO-free hydrogen (namely highly pure, CO is not observed in analysis) could be obtained by POMeOH. The processes consider the modulation of the selectivity by a controlled amount of CO2 and/or H2 into the reaction flux. The presence of promoter gases strongly influenced the Cu oxidation state. It is concluded that the inhibition in CO formation is related to the high content of Cu0. It is suggested that in the presence of metallic copper the kinetic of the several reactions involved during POMeOH, facilitates the formation of H2 and CO2 and inhibits the CO formation. Results are useful for processes where the CO/H2 ratio has to be reduced drastically (fuel cells) or finely modulated (Fischer Tropsch, methanol synthesis, etc.).

Electrochemical activation of Au nanoparticles for the selective partial oxidation of methanol

An electrochemical catalyst based on Au nanoparticles dispersed in an yttria-stabilized zirconia (YSZ) matrix was prepared by reactive cosputtering of zirconium–yttrium and Au targets on a K-βAl2O3 solid electrolyte. The Au/YSZ catalyst film deposited on K-βAl2O3 was found to be active in the partial oxidation of methanol with a high selectivity toward methyl formate. This configuration allowed to electrochemically promote a highly dispersed Au catalyst by K+ ions and allowed to decrease the amount of metal used in the solid electrolyte cell. The back-spillover of K+ ions sharply increased the H2 and HCOOCH3 production rates up to more than 9 and 5 times, respectively. A number of experiments confirmed that the observed electro-promotional effect did not depend on the rate of K+ supply nor on the operation mode (galvanostatic or potentiostatic) and only depended on the promoter coverage (total applied charge). The stability of the Au–YSZ catalyst film under the explored conditions was also verified.

H2S purification from biogas by direct selective oxidation to sulfur on V2O5–CeO2 structured catalysts

The complete preparation procedure of catalytic cordierite monoliths has been studied, from the washcoating with CeO2–ZrO2 to the deposition of the active phase (V2O5) for the abatement of H2S from biogas by selective partial oxidation to sulfur at 150–200°C.The washcoat deposition on the cordierite was carried out by dip coating method, while the active phase was deposited by wet impregnation after washcoat deposition and stabilization.The washcoat coating quality was evaluated with adhesion tests and SEM analysis; furthermore the washcoat was characterized by X-ray Diffraction, X-ray Fluorescence (XRF) and Hg porosimetry.The effect on the catalytic performances of the vanadium loading variable between 2wt% and 19wt% was investigated in the temperature range of 150–200°C in terms of H2S conversion and SO2 production.

An Efficient Oxygen Activation Route for Improved Ammonia Oxidation through an Oxygen‐Permeable Catalytic Membrane

AbstractUpon using a reactant/oxygen mixture as co‐feed in partial oxidation, adsorbed surface molecular oxygen species can cause low selectivity. We propose a concept different from the conventional co‐feed partial oxidation process in packed‐bed reactors. In this new configuration, the activation of oxygen is separated from the catalytic oxidation by using an oxygen‐permeable membrane to suppress the formation of nonselective surface molecular oxygen species. A continuous flux of lattice oxygen through the membrane allows highly selective partial oxidation. In the oxidation of ammonia to NO, the NO selectivity was improved from 77 to 95 % if a La0.6Sr0.4Co0.2Fe0.8O3–δ oxygen‐permeable catalytically active membrane was used at 850 °C instead of a co‐feed fixed bed reactor.