Carbon nanotubes are promising materials for various applications. In recent years, progress in manufacturing and functionalizing carbon nanotubes has been made to achieve the control of bulk and surface properties including the wettability, acid–base properties, adsorption, electric conductivity and capacitance. In order to gain the optimal benefit of carbon nanotubes, comprehensive understanding on manufacturing and functionalizing carbon nanotubes ought to be systematically developed. This review summarizes methodologies of manufacturing carbon nanotubes via arc discharge, laser ablation and chemical vapor deposition and functionalizing carbon nanotubes through surface oxidation and activation, doping of heteroatoms, halogenation, sulfonation, grafting, polymer coating, noncovalent functionalization and nanoparticle attachment. The characterization techniques detecting the bulk nature and surface properties as well as the effects of various functionalization approaches on modifying the surface properties for specific applications in catalysis including heterogeneous catalysis, photocatalysis, photoelectrocatalysis and electrocatalysis are highlighted.

Carbon nanotube catalysts: recent advances in synthesis, characterization and applications.

Higher alcohols are important compounds with widespread applications in the chemical, pharmaceutical and energy sectors. Currently, they are mainly produced by sugar fermentation (ethanol and isobutanol) or hydration of petroleum-derived alkenes (heavier alcohols), but their direct synthesis from syngas (CO + H2) would comprise a more environmentally-friendly, versatile and economical alternative. Research efforts in this reaction, initiated in the 1930s, have fluctuated along with the oil price and have considerably increased in the last decade due to the interest to exploit shale gas and renewable resources to obtain the gaseous feedstock. Nevertheless, no catalytic system reported to date has performed sufficiently well to justify an industrial implementation. Since the design of an efficient catalyst would strongly benefit from the establishment of synthesis–structure–function relationships and a deeper understanding of the reaction mechanism, this review comprehensively overviews syngas-based higher alcohols synthesis in three main sections, highlighting the advances recently made and the challenges that remain open and stimulate upcoming research activities. The first part critically summarises the formulations and methods applied in the preparation of the four main classes of materials, i.e., Rh-based, Mo-based, modified Fischer–Tropsch and modified methanol synthesis catalysts. The second overviews the molecular-level insights derived from microkinetic and theoretical studies, drawing links to the mechanisms of Fischer–Tropsch and methanol syntheses. Finally, concepts proposed to improve the efficiency of reactors and separation units as well as to utilise CO2 and recycle side-products in the process are described in the third section.

Status and prospects in higher alcohols synthesis from syngas

Hydrothermal processing, a thermochemical approach, is an excellent method of converting energy-rich biomass into useful products. This approach offers the advantage of handling biomass with relatively high moisture content by precluding an energy-intensive pretreatment step. Hydrothermal processing is of world-wide interest in view of depleting fossil-fuel reserves and increased environmental greenhouse gas emissions. There is potential to develop this novel technology at demonstration scale. This paper reviews the three hydrothermal technologies, namely hydrothermal liquefaction, gasification and carbonization, to provide insight into the likelihood of commercialization. The study discusses the role of different process parameters that have key impacts on the quality and yield of the desired products. This study also identifies the gaps in the literature including the need to establish a baseline to develop key process models and to perform a techno-economic assessment to get a better sense of the viability of the technology in future.

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A review on the current status of various hydrothermal technologies on biomass feedstock

Graphene is considered as one of the most promising materials in a wide range of applications because of its outstanding electronic, optical, thermal, and mechanical properties. Given its large specific surface area, two-dimensional structure, facile decoration, and high adsorption capacity, numerous graphene-based nanomaterials with unprecedented characteristics have been designed, prepared, and applied in catalysis. In this article, we first reviewed common synthetic methods to prepare graphene-based catalysts followed by critical comments and possible solutions. We then briefly summarized the characterization techniques that were relevant to catalysis applications and their applications in energy conversion, environmental protection, and several other typical fields. Finally, we discussed the challenges and opportunities for the future development of graphene-based nanomaterials in sustainable catalysis. This review provides key information to the catalysis community to design and fabricate graphene-ba...

Graphene-Based Nanomaterials for Catalysis

C1 catalysis refers to the conversion of simple carbon-containing compounds, such as carbon monoxide, carbon dioxide, methane, and methanol into high-value-added chemicals, petrochemical intermediates, and clean fuels. Because of the rising oil price and the apprehension of fossil fuel depletion in the future, C1 catalysis has been attracting widespread academic and industrial interest and became one of the most attractive research fields in heterogeneous catalysis. Especially in recent years, benefiting from advanced technology development, precise and controllable material synthesis methods, and powerful computational simulation capabilities, C1 catalysis has achieved remarkable progress in many aspects, including insights into the reaction mechanism, identification of active-site structures, highly efficient catalysts and reaction process, and the reactor designs. This Review highlights the latest developments (from 2012 to 2018) in highly efficient catalyst systems and reaction processes in this field...

Significant Advances in C1 Catalysis: Highly Efficient Catalysts and Catalytic Reactions

Graphene and CNTs-supported cobalt catalyst, is introduced for Fischer–Tropsch synthesis (FTS). The catalysts were prepared using wet impregnation method with cobalt loading of 15.0 wt%. The catalysts were characterized by TGA, BET, XRD, XPS, H 2 chemisorption, TPR, and TEM techniques. The activity, selectivity and stability of the catalysts in FTS were assessed in a fixed-bed micro-reactor, and the results were compared with each other. Comparing TEM images showed dispersion of cobalt clusters in graphene-supported catalyst is higher and the average clusters size decreased from 7.5 to about 6.0 nm. Graphene as the support shifted the reduction temperature of cobalt oxide species to lower temperatures. Degree of CO conversion increased from 61 to 74%, FT synthesis rate increased by about 22%; the product distribution shifted to the higher molecular weight hydrocarbons, compared to CNTs-supported catalyst. On the graphene-supported cobalt catalyst, 480 h continuous FT synthesis decreased 22% of the initial CO conversion. However, at the same time for CNTs-supported catalyst the CO conversion decreased 34%.

Cobalt supported on Graphene – A promising novel Fischer–Tropsch synthesis catalyst

Functionalization of carbon nanotubes (CNTs) was performed, during preparation of Fischer–Tropsch synthesis (FTS) cobalt nanocatalysts, to modify the surface properties of CNTs support. Common CNTs and functionalized CNTs supported cobalt nanocatalysts were prepared using microemulsion technique with cobalt loading of 15 wt%. The catalysts were characterized by Brunauer–Emmett–Teller (BET), Fourier transform infrared spectroscopy (FTIR), X-ray diffraction (XRD), temperature program reduction (TPR), H2 chemisorption, X-ray photoelectron spectroscopy (XPS) and transmission electron microscopy (TEM) techniques. Most of the cobalt particles were homogeneously distributed inside the tubes and the rest on the outer surface of the functionalized CNTs. Functionalization of CNTs shifted the TPR reduction peaks to lower temperatures, improved the reduction degree, and increased the dispersion of cobalt particles. The proposed cobalt catalyst supported on functionalized CNTs increased the FTS rate (g HC/g cat./h), and percentage CO conversion from 0.1476, and 32.4% to 0.24, and 54%, respectively. Catalysts supported on functionalized CNTs showed better stability.

Enhancement of activity, selectivity and stability of CNTs-supported cobalt catalyst in Fischer–Tropsch via CNTs functionalization

Solution combustion synthesis (SCS) is a preparation technique that can be used to synthesize a variety of inorganic nanomaterials and structured catalysts. It is based on a self-propagating exothermic redox reaction between organic salts and a fuel mixed together in an aqueous solution, which results in the formation of nanocrystalline and highly pure solid nanomaterials. SCS can be considered as an attractive synthesis method for catalysts due to the simple nature of the synthetic route and short reaction times. The process is easily scaled up to any kind of application which makes it economically attractive. This mini-review provides a short overview on the synthesis of structured catalysts by SCS and their recent utilization for energy applications and pollution control.

Solution combustion synthesis for preparation of structured catalysts: A mini-review on process intensification for energy applications and pollution control

Catalytic combustion of residual methane on alumina monoliths and open cell foams coated with Pd/Co3O4

The effect of morphology and structure of graphene nanosheets (GNS) and carbon nanotubes (CNT) as support on the stability of cobalt catalyst for Fischer–Tropsch synthesis (FTS) was investigated using a fixed bed micro-reactor.15.0 wt.% of cobalt was loaded on the supports by impregnation method. The deactivation of the two catalysts was studied at 220 °C, 1.8 MPa and 45 STP mL/min feed flow rate. Characterization of the supports, calcined fresh and used catalysts were studied by Raman spectroscopy, BET, XRD, TEM, TPR and H 2 chemisorption techniques. XRD confirmed formation of cobalt oxides on the used catalysts. According to the TEM and H 2 chemisorption tests, 480 h continuous FT synthesis increased the average cobalt particle size from about 6 to 6.8 nm for Co/GNS catalyst and from about 7.5 to 9.5 nm for Co/CNT catalyst. The initial CO conversion of the Co/GNS catalyst was 12% higher than that of the Co/CNT. For the Co/GNS, 480 h continuous FT synthesis decreased the CO conversion by 3.9%, whereas, under the same reaction conditions the CO conversion for Co/CNT decreased by 20.7%. Regeneration of the Co/GNS and Co/CNT recovered 99.3 and 92.8% of the initial activities of the catalysts, respectively. Significant stability of Co/GNS catalyst in FT synthesis, introduces graphene as an excellent support for the cobalt catalysts.

Saba Karimi

Papers

Cobalt supported on Graphene – A promising novel Fischer–Tropsch synthesis catalyst

Enhancement of activity, selectivity and stability of CNTs-supported cobalt catalyst in Fischer–Tropsch via CNTs functionalization

Solution combustion synthesis for preparation of structured catalysts: A mini-review on process intensification for energy applications and pollution control

Catalytic combustion of residual methane on alumina monoliths and open cell foams coated with Pd/Co3O4

Enhancement of cobalt catalyst stability in Fischer-Tropsch synthesis using graphene nanosheets as catalyst support