Formation Of Oxygenates Research Articles

(Invited) Developing Carbon Catalysts for Selective Electro-Reduction of Carbon Dioxide into Target Products

Electrocatalytic reduction of CO2 into value added chemicals or fuels is a promising technique towards a carbon-neutral chemical process. The electrochemical reduction of CO2 is a complicated process involving multiple protons coupled electron transfer, theoretically resulting in a variety of products (e.g. CO, HCOOH, CH4, C2H4 and C2H5OH). Therefore, the major challenge in CO2 reduction lies in the manipulation of the selectivity towards a specific product as demanded. However, the study on CO2 reduction has not substantially advanced primarily because of the lack of fundamental understanding of the reaction mechanism and the challenge of discovering efficient and robust catalysts for the various multi-electron transfer processes. Researchers have screened a wide range of metal-based materials for electrochemical reduction of CO2, and found only copper-based metals exhibit selectivity towards formation of hydrocarbons and oxygenates at fairly high efficiencies while most others favor production of carbon monoxide or formate. Here we present the development of carbon materials as an alternative to Cu for efficient and high-rate electro-reduction of CO2 into hydrocarbons and oxygenates. We will discuss the key structural and electronic factors that govern the selectivity of carbon catalysts towards production of CO, CH4 and C2 products (e.g. C2H4 and C2H5OH). Three categories of carbon catalysts were developed based on the primary products of CO, CH4 and C2H4 in our group. The first carbon catalyst featuring the metal-nitrogen-carbon structure exclusively catalyzes CO2 electro-reduction into CO. The second catalyst called functionalized carbon nanostructure can selectively reduce CO2 into CH4 with Faradaic efficiency up to 90% while the third one namely doped carbon nanostructure (e.g. N-doped graphene quantum dots) can yield C≥2 products with a Faradaic efficiency up to 70%. Both carbon nanostructure can achieve partial current density at the scale of 100 mA cm-2 for target product at fairly low overpotentials. This study provides in-depth insights into developing high-performance carbon-based catalysts for electrochemical reduction of CO2.

Thermal Pyrolysis of Municipal Solid Waste (MSW)

Plastics are very desired components, used in a lot of fields, from domestic packaging, agrarian, automotive, to building structure. The frequent use of plastic products related to their lightweight, high chemical stability and low cost.The present work aims to illustrating the thermal pyrplysis of two types of plastic: Polyethylene terephthalate (PET) and polystyrene (PS) in a batch tubular reactor. The reaction indicated that PS and PET were converted more meaningfully into useful liquid products out of thermally route compared to PET pyrolysis.The resulted liquid could be used as fuels or raw materials in chemical industries. Formation of oxygenates and aromatics were noticed.The maximum liquid produced at optimum temperature and optimum heating time (90 min) is 77%, 67.56%, 54% and 37.48% for PS, 10%PET + PS, 20%PET + PS and 30%PET + PS respectively. The liquid product is analysed by GC-MS at optimum condition. The compositions of the liquid produced are styrene monomer, toluene, alpha-methylstyrene, 1,3 Diphenylpropane, 1-Phenylnaphthalene, 4-phenyl-1 butene. The main product in the pyrolysis of PS is styrene monomer, 10%PET+PS, 20%PET+PS and 30%PET+PS.

C2 oxygenates formation from syngas over the promoter M(M = Rh, Co) monolayer-modified Cu(111) surface: Probing into the role of monolayer promoter on the selectivity

Abstract The promoter metal monolayer deposited over other metal substrate exhibits excellent catalytic performance, in this study, the mechanism of syngas conversion to C2 oxygenates over the promoter M(M = Rh, Co) monolayer-modified Cu catalysts have been systematically investigated to identify the role of monolayer promoter M and its effect on the selectivity using density functional theory calculations. Here, two key steps are examined, including the formation of key CHx intermediates and the C C bond formation of C2 oxygenates. The results show that compared to the pure Cu catalyst, the promoter Co monolayer-modified Cu catalyst exhibits higher selectivity towards CHx(x = 1–3) formation instead of methanol, the most favored CHx monomer is CH2; whereas CHx(x = 1,2) is the most favored CHx monomer on the promoter Rh monolayer-modified Cu(111), which is competitive with methanol formation. The favored CHx monomer originates from the C O bond cleavage of CHxO(x = 1,2) and CHxOH(x = 1,2) species formed by CO hydrogenation. Starting from the favored CHx monomer, compared to the pure Cu catalyst, C2 oxygenates formed by CHO insertion into CHx is the most favorable over the promoter Rh and Co monolayer-modified Cu catalysts. In general, the promoter Co monolayer-modified Cu catalyst exhibits higher selectivity towards syngas conversion to C2 oxygenates than the promoter Rh monolayer-modified Cu, as well as the pure Cu, Rh and Co catalysts, the reasons is attributed to that the d-band center of Co monolayer-modified Cu is the closest to Fermi level leading to the enhancement of adsorption capacity of the intermediates, and promoting the C O bond activation as well as reducing the repulsion interaction between CHx and CHO to facilitate the formation of C2 oxygenates. The present study provides useful information for the design of highly efficient catalysts with the metal promoter monolayer-modified metal substrate in syngas conversion to C2 oxygenates.

Insight into activation of CO and initial C2 oxygenate formation during synthesis of higher alcohols from syngas on the model catalyst K2O/Cu(111) surface

Alkali-metal-modified Cu-based catalysts are important in low-temperature synthesis of methanol and higher alcohols from syngas. Spin-polarized density functional theory calculations with periodic slab models were performed for a K2O/Cu(111) model catalyst to investigate the interactions between K2O and the Cu(111) surface, and activation of CO. The results show that K2O interacts strongly with the Cu(111) surface via electron transfer between K2O and the Cu(111) surface. This changes CO activation and modulates the mechanism of formation of the initial C2 oxygenates. Although K2O significantly promotes CO direct dissociation, H-assisted dissociation of CO is still the most energetically favorable route. The optimal reaction pathway for C2 oxygenate formation is proposed based on a climbing-image nudged elastic band calculation. The reactions starts with CO hydrogenation to yield the preferred monomers, i.e., CHO, CH2O, and CH2, through the pathways CO + H → CHO + H → CH2O + H → CH2 + OH, with reaction barriers of 0.92, 0.97, and 0.58 eV. The reaction of CH2 with CH2O is the main C2 oxygenate formation route on the K2O/Cu(111) surface. These results clarify the effects of K2O on Cu-based catalysts and elucidate the mechanism of C2 oxygenate formation during alcohol synthesis from syngas.

Emerging Carbon-Based Heterogeneous Catalysts for Electrochemical Reduction of Carbon Dioxide into Value-Added Chemicals.

The electrocatalytic reduction of CO2 provides a sustainable way to mitigate CO2 emissions, as well as store intermittent electrical energy into chemicals. However, its slow kinetics and the lack of ability to control the products of the reaction inhibit its industrial applications. In addition, the immature mechanistic understanding of the reduction process makes it difficult to develop a selective, scalable, and stable electrocatalyst. Carbon-based materials are widely considered as a stable and abundant alternative to metals for catalyzing some of the key electrochemical reactions, including the CO2 reduction reaction. In this context, recent research advances in the development of heterogeneous nanostructured carbon-based catalysts for electrochemical reduction of CO2 are summarized. The leading factors for consideration in carbon-based catalyst research are discussed by analyzing the main challenges faced by electrochemical reduction of CO2 . Then the emerging metal-free doped carbon and aromatic N-heterocycle catalysts for electrochemical reduction of CO2 with an emphasis on the formation of multicarbon hydrocarbons and oxygenates are discussed. Following that, the recent progress in metal-nitrogen-carbon structures as an extension of carbon-based catalysts is scrutinized. Finally, an outlook for the future development of catalysts as well as the whole electrochemical system for CO2 reduction is provided.

Effect of the Nature of the Catalyst on Catalytic Activity and Selectivity in the Formaldehyde Hydrogenation

The effect the nature of the carrier and supported metal on the activity and selectivity of the catalyst in the reaction of formaldehyde hydrogenation to methanol is studied. The formation of such oxygenates as ethanol, formic acid, and diethyl formal is observed. It is found that ethanol forms on Fe-containing alloyed catalyst, while formic acid forms on the catalysts containing Au. Thermodynamic calculations are performed for a series of side reactions that confirm the formation of the resulting oxygenates.

Deactivation of Cu–Mg–Al mixed oxide catalysts for liquid transportation fuel synthesis from biomass-derived resources

A “platform molecule” (2-hexanol) obtained from the primary conversion of sugars was upgraded to liquid transportation fuel precursors under gas phase conditions at 573 K and 101.3 kPa. Reaction was promoted by Cu-Mg-Al mixed oxides with different copper loading (0.3–61.2%) and a Mg/Al = 1.5 (molar ratio). Products were mainly low oxygen content C9-C24 oxygenates and hydrocarbons. The product pool average molecular weight and the oxygenates/hydrocarbons ratio increase with the catalyst copper loading, but the latter might be diminished by augmenting the contact time. A slow catalyst deactivation process occurs in the first 2 h of reaction. Temperature-programmed oxidation, BET surface are measurements and X-ray photoelectron and Auger electron spectroscopies of the spent catalysts indicated that the main reasons for the activity decay during reaction are carbon deposition on the active sites and, to a lesser degree, partial oxidation of the surface copper particles. Oxygenates (reactant or products) are the chemical species responsible for deactivation. The initial deactivation rate (rd0) depends on the copper content and contact time. On catalysts with low Cu content, rd0 is higher at short contact times, which is consistent with coke formed directly from the reactant. Contrarily, at high Cu loadings rd0 increases with contact time and parallels formation of heavy unsaturated oxygenates. Oxidation/reduction/catalytic test cycles of spent Cu-Mg-Al mixed oxides were implemented to explore catalyst reusability.

Ceria‐Zirconia/Zeolite Bifunctional Catalyst for Highly Selective Conversion of Syngas into Aromatics

AbstractCeria‐zirconia solid solutions with tunable surface oxygen vacancy have been used in the direct conversion of syngas into aromatics after coupling with ZSM‐5 zeolite. A high selectivity of 83.1 % to aromatics has been achieved from syngas with low H2/CO ratio over Ce0.2Zr0.8O2/ZSM‐5 bifunctional catalyst. The surface oxygen vacancy of Ce0.2Zr0.8O2 is supposed to facilitate the formation of oxygenates which act as intermediates for C−C coupling and aromatization on ZSM‐5, and thus contribute to the well kinetic matching between C−O activation and C−C coupling over these two components of catalyst. The ceria‐zirconia with abundant surface oxygen vacancies and enhanced lattice oxygen mobility intimately mixed with ZSM‐5 zeolite with moderate acidity and crystal size play an important role in the catalytic performance.

Hydrogenation of carbon dioxide over iron carbide prepared from alkali metal promoted iron oxalate

Iron carbide was prepared by the carburization of iron oxalate doped with various alkali metals in flowing CO at 400 °C. The addition of alkali metal (Li, Na, K, Rb or Cs) to iron oxalate was found to decrease the bulk iron carbides formation. However, the amount of carbon deposited on Fe as inferred from temperature programmed hydrogenation and high-resolution transmission electron microscopic studies show an increasing trend with the increase of the basicity of alkali added. The CO2 and H2 conversions of all alkali doped iron catalysts were initially higher but they declined over time irrespective of alkali. The benefits of addition of alkali to iron without any structural and reduction promoters, such as copper, is found to lower the selectivity for methane, higher olefins and oxygenates formation for CO2 hydrogenation.

In Situ Formation of FeRh Nanoalloys for Oxygenate Synthesis

Early and late transition metals are often combined as a strategy to tune the selectivity of catalysts for the conversion of syngas (CO/H2) to C2+ oxygenates, such as ethanol. Here we show how the use of a highly reducible Fe2O3 support for Rh leads to the in situ formation of supported FeRh nanoalloy catalysts that exhibit high selectivity for ethanol synthesis. In situ characterizations by X-ray diffraction (XRD) and X-ray absorption spectroscopy (XAS) reveal the coexistence of iron oxide, iron carbide, metallic iron, and FeRh alloy phases depending on reaction conditions and Rh loading. Structural analysis coupled with catalytic testing indicates that oxygenate formation is correlated to the presence of FeRh alloys, while the iron oxide and carbide phases lead mainly to hydrocarbons. The formation of nanoalloys by in situ reduction of a metal oxide support under working conditions represents a simple approach for the preparation bimetallic catalysts with enhanced catalytic properties.

Fischer-Tropsch synthesis: Effect of CO conversion on CH4 and oxygenate selectivities over precipitated Fe-K catalysts

The explanation for CH4 selectivity for iron based Fischer-Tropsch catalysts in the low conversion region (i.e., <50%) remains elusive. In this contribution, the CO conversion effect was carefully examined over four K promoted Fe catalysts (100 Fe/5.1Si/2Cu/ x K, where x = 1–5) over a wide range of CO conversion (i.e., 4–85%). Moreover, the effect of CO conversion on oxygenate selectivity of the Fe-K catalysts was carefully studied as well. The change in CH4 selectivity with CO conversion was found to resemble asymmetric “V” shaped curves, with the minimum values occurring at approximately 50% CO conversion. Adding greater than x = 2 K significantly alleviated the CO conversion effect which was attributed to the high K loading greatly decreasing the surface H coverage while improving CO adsorption. The unique CH4 selectivity trend suggests a complicated CH4 formation process that results from different aspects of the catalyst (i.e., chain growth and hydrogenation rates), and process conditions. Oxygenate selectivity was in the range of 0.7–2.8% and varied with the CO conversion and K loading. The addition of K up to x = 3 was found to promote oxygenate formation and chain growth. The overall oxygenate distribution up to C17 follows an Anderson-Schulz Flory (ASF) distribution, with ethanol being the dominant oxygenate. Mechanisms of oxygenate formation different from that of hydrocarbon formation (e.g., CO insertion versus CO dissociation) were proposed to explain the experimental results.

Selectivity of Synthesis Gas Conversion to C2+ Oxygenates on fcc(111) Transition-Metal Surfaces

Using a combined density functional theory and descriptor based microkinetic model approach, we predict production rate volcanos for higher oxygenate formation on (111) transition-metal surfaces. Despite their lower activity for CO conversion compared to stepped surfaces, (111) transition metal surfaces bring the potential for selectivity toward C2+ oxygenates. The volcano plots can be used to rationalize and predict activity and selectivity trends for transition-metal-based catalysts.

Oxygenate formation over K/β-Mo2C catalysts in the Fischer–Tropsch synthesis

Potassium promoted β-Mo2C catalysts were studied in the hydrogenation of CO to oxygenates. The effect of synthesis procedures, promotion levels and reaction conditions on catalyst activity and selectivity was investigated in detail.

Insight into the effect of promoter Co on C2 oxygenate formation from syngas on CoCu(100) and Cu(100): A comparative DFT study

Density functional theory calculations have been employed to investigate the effect of promoter Co on C2 oxygenate formation from syngas on pure Cu(100) and two kinds of Co-Cu bimetallic surfaces. Based on the results of previous studies that CH3O is a key intermediate in ethanol formation on Cu catalyst, five reactions starting from CH3O were taken into consideration. Different from the results on Cu(100) that CH3OH is the most favorable product on pure Co sites, CH3 formation is more favorable both kinetically and thermodynamically than CH3OH formation, which leads to more CH3 available for CHO insertion to form C2 oxygenates. On the other hand, Co-Cu bimetallic sites can facilitate CHO insertion into CH3 energetically, which is favorable for carbon chain growth. And the addition of Co can make the barrier of CH3 hydrogenation and CH3 coupling to CH4 and CH3CH3 higher, making CH3CHO much more selective than hydrocarbons. In conclusion, by introducing Co into Cu catalyst, the productivity and selectivity of C2 oxygenate precursor of ethanol can be effectively improved. The optimum Co-Cu catalyst should contain Cu ensembles and Co ensembles with proper sizes, and offer enough Co-Cu bimetallic sites at the same time.

C2 Oxygenate Synthesis via Fischer–Tropsch Synthesis on Co2C and Co/Co2C Interface Catalysts: How To Control the Catalyst Crystal Facet for Optimal Selectivity

Density functional theory (DFT) analysis is used to shed light on the intricate effects of the Co2C and Co/Co2C catalyst crystal facets on the selectivity of the C2 oxygenate and hydrocarbon formation in Fischer–Tropsch synthesis. Three representative low-index Co2C (101), (110), and (111) surfaces, varying in surface energy from low and medium to high, are model examples of different Co2C exposed crystal facets. Since CHx (x = 1–3), CO, and H species are the key intermediates critical to the C2 oxygenate selectivity, all Fischer–Tropsch reactions related to CHx (x = 1–3) species, including CO insertion into CHx (x = 1–3) and CHx + CHy (x, y = 1–3) coupling to form C2 species (C2Hx and C2HxO), as well as the hydrogenation and dissociation of CHx (x = 1–3) to form C1 species (CH4 and C), are used as examples examined at a typical FTS temperature of 493 K. On Co2C (101) and (110) surfaces, CH and CH2 species are dominant form of the CHx species, CH self-coupling to C2H2 and CH coupling with CH2 to CH2CH is ...

Direct synthesis of long-chain alcohols from syngas over CoMn catalysts

CoMn model catalysts were prepared by co-precipitation and evaluated for higher alcohol synthesis (HAS) via syngas. The selectivity to oxygenates (mainly alcohols and aldehydes) was found to be higher than 20C% for the Na-promoted CoMn catalyst. Among the oxygenates, C2+ and C6+ fractions accounted for >90wt% and ∼50wt%, respectively. Metallic Co0 particles and MnCO3 were found in both Na-promoted and unpromoted CoMn catalysts after the reaction, whereas Co2C nanoparticles could only be observed in the spent Na-promoted catalyst. The addition of Na benefited the carbonization of cobalt and increased the selectivity to oxygenates and CO2. Our studies thus suggested that Co/Co2C were the bifunctional dual-sites for the oxygenates formation over the Na-promoted CoMn catalyst, where Co catalyzed CO dissociation and chain propagation, while Co2C was responsible for CO non-dissociative activation and subsequent insertion. In addition, the sole Co2C nanoparticles with certain exposed facets may also act as another kind of active dual-sites for oxygenates formation.

Carbon Based Atomic Catalysts for Carbon Dioxide Reduction

Technological development and human mobility depends heavily on fossil-fuel-based energy infrastructure. The byproduct of this is a surplus generation of carbon dioxide (CO2) which causes major environmental challenges, and addressing this problem is technologically challenging. The mitigation of CO2 emissions involve separation, capture and sequestration of a significant fraction of the billion tons of CO2 currently produced worldwide each year. A more promising and direct alternative is a chemically recycling process that converts CO2 to carbon-neutral fuels or commodity chemicals employing water as the hydrogen source via electrochemical catalysis. This technology not only reduces greenhouse gases but also provides pathways to sustainable energy. The chemical reduction of CO2 is a complicated process involving multiple proton coupled electron transfer, theoretically resulting in a variety of products (e.g. CO, HCOOH, CH3OH, C2H4 and C2H5OH). Therefore the major challenge in CO2 reduction lies in the manipulation of the selectivity towards a specific product as demanded. However, the study on CO2 reduction has not substantially advanced primarily because of the lack of fundamental understanding of the reaction mechanism and the challenge of discovering efficient and robust catalysts for the various multi-electron transfer processes. Researchers have screened a wide range of materials for electrochemical reduction of CO2, including metals, alloys, organometallics, layered materials and carbon nanostructures, only copper (Cu) exhibits selectivity towards formation of multi-carbon hydrocarbons and oxygenates at fairly high efficiencies while most others favor production of carbon monoxide or formate. However, Cu suffers from the poor selectivity and large overpotential in the reactions. Compared to traditional metal nanoparticle catalysts, sub-nanometer and single atom metal catalysts (atomic catalysts) possess enhanced catalytic activity. Beyond metal-based atomic catalysts, we have recently shown metal free atomic catalysts of nitrogen (N) doped carbon sp2 sheets (graphene) as possible CO2 reduction catalysts. We designed N-incorporated carbon nanostructures (N-doped carbon nanotubes and N-doped graphene) for selective and efficient electro-reduction of CO2 into CO with high efficiency (~80%) and at low overpotential.1-3 We further demonstrated that the N-doped carbon materials can be atomically engineered to achieve the yield of high order (C2 and C3) products. when enriching the N-doping at the edge of carbon nanostructures, the N-doped graphene quantum dots (NGQDs, thickness ~ 1nm and diameter ~ 2 nm) exhibit exceptional activity towards formation of C2 products (C2H4 and C2H5OH) with high Faradaic efficiency of 40%, and the current density is enhanced to the order of magnitude of 100 mA/cm2 at this potentialOc.4 This is for the first time the metal-free electrocatalyst has been discovered to steer the CO2reduction to produce C2 hydrocarbons and oxygenantes at a relatively high yield comparable to those obtained with copper nanoparticle-based electrocatalysts.

Cyclohexane oxidation with an O2–H2 mixture in the presence of a two-component Pt/C–heteropoly acid catalyst and ionic liquids

Approaches to increase the efficiency of Pt/C–heteropoly acid catalyst in a liquid-phase oxidation of cyclohexane using an O2–H2 mixture were studied. It was shown that small additives of ionic liquid (BMImBr, Bu4NBr, or Bu4NHSO4) significantly improve the catalytic effect of the Pt/C–H3PMo12O40–CH3CN system at 35°C, by slowing the rate of side reactions resulting in water formation, increasing the rate of oxygenate formation, and inhibiting their secondary oxidation reactions. The efficiency of H2 consumption increases from 2 to 18–25%, while the selectivity of cyclohexane conversion is 92–98%. The substitution of one or two Mo(VI) ions by V(V) in the structure of the heteropoly acid decreases these parameters. In the presence of Bu4NHSO4, a Pt/C catalyst can be used many times. During the reaction, the heteropoly acid present in the solution is in a reduced state under the action of the reaction medium and undergoes reversible redox transformations. The nature of the catalytic action of the studied system is explained from the viewpoint of the effect of ionic liquids on the properties of a Pt/C catalyst in activating O2, heteropoly molybdate chemistry, and the known mechanisms of the peroxide oxidation of hydrocarbons.

Analysis of oxidised heavy paraffininc products by high temperature comprehensive two-dimensional gas chromatography

Heavy petroleum fractions are produced during crude and synthetic crude oil refining processes and they need to be upgraded to useable products to increase their market value. Usually these fractions are upgraded to fuel products by hydrocracking, hydroisomerization and hydrogenation processes. These fractions are also upgraded to other high value commercial products like lubricant oils and waxes by distillation, hydrogenation, and oxidation and/or blending. Oxidation of hydrogenated heavy paraffinic fractions produces high value products that contain a variety of oxygenates and the characterization of these heavy oxygenates is very important for the control of oxidation processes. Traditionally titrimetric procedures are used to monitor oxygenate formation, however, these titrimetric procedures are tedious and lack selectivity toward specific oxygenate classes in complex matrices. Comprehensive two-dimensional gas chromatography (GC×GC) is a way of increasing peak capacity for the comprehensive analysis of complex samples. Other groups have used HT-GC×GC to extend the carbon number range attainable by GC×GC and have optimised HT-GC×GC parameters for the separation of aromatics, nitrogen-containing compounds as well as sulphur-containing compounds in heavy petroleum fractions. HT-GC×GC column combinations for the separation of oxygenates in oxidised heavy paraffinic fractions are optimised in this study. The advantages of the HT-GC×GC method in the monitoring of the oxidation reactions of heavy paraffinic fraction samples are illustrated.

Engineering Cu surfaces for the electrocatalytic conversion of CO2: Controlling selectivity toward oxygenates and hydrocarbons

In this study we control the surface structure of Cu thin-film catalysts to probe the relationship between active sites and catalytic activity for the electroreduction of CO2 to fuels and chemicals. Here, we report physical vapor deposition of Cu thin films on large-format (∼6 cm2) single-crystal substrates, and confirm epitaxial growth in the <100>, <111>, and <751> orientations using X-ray pole figures. To understand the relationship between the bulk and surface structures, in situ electrochemical scanning tunneling microscopy was conducted on Cu(100), (111), and (751) thin films. The studies revealed that Cu(100) and (111) have surface adlattices that are identical to the bulk structure, and that Cu(751) has a heterogeneous kinked surface with (110) terraces that is closely related to the bulk structure. Electrochemical CO2 reduction testing showed that whereas both Cu(100) and (751) thin films are more active and selective for C-C coupling than Cu(111), Cu(751) is the most selective for >2e- oxygenate formation at low overpotentials. Our results demonstrate that epitaxy can be used to grow single-crystal analogous materials as large-format electrodes that provide insights on controlling electrocatalytic activity and selectivity for this reaction.