Hydrothermal carbonization

On the other hand, the porous cokes with their adjustable capacity of nutrient and water storage are discussed for their ability to condition or to recultivate degraded soil. Bringing coke into soils is also discussed as a potential contribution to carbon sequestration in connection with the CO2 emissions trading system. 

The result of a comprehensive literature review by Behrendt consistently confirmed that the hydrothermal treatment of biomass coals by dehydration and decarboxylation yields a product with calorific value of about 30 MJ/kg whose elemental composition is similar to that of lignite [10]. The carbon content and the properties of such cokes strongly depend on the starting materials and the process paramters applied, namely temperature and residence times of 4—14h. The coke yields can reach values of up to 70% (g/g) and even higher values if catalysts such as citric acid are used. Those products have an increased calorific value of 15-20%, relative to the dry weight. About 80-85% of the carbon remains in the coke, about 10-15% remains in the aqueous phase in the form of organic acids and other 

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Shikazono, N., Hoshino, M., Utada, M., and Ueda, A. (1998) Hydrothermal carbonates in altered wall rocks at the Uwamuki Kuroko deposits, Japan. Mineralium Deposita, 33, 346-358. 

Richter S, Goldberg SA, Mason PB, Traina AJ, Schwieters JB (2001) Linearity tests for secondary electron multipliers used in isotope ratio mass spectrometry. Inti J Mass Spectrom 206 105-127 Rihs S, Condomines M, Sigmarsson O (2000) U, Ra, and Ba incorporation dining precipitation of hydrothermal carbonates imphcations for Ra-Ba dating of impure travertines. Geochim Cosmochim Acta 64 661-671... 

Markus Antonietti, Li Zhao, and Maria-Magdalena Titirici 7 Sustainable carbon hybrid materials made by hydrothermal carbonization and their use in energy applications... 

The technique of hydrothermal carbonization was recently rediscovered by several working groups [9 11]. Since then, it became an important technique for the production of various carbonaceous materials and hybrids, usually applying mild temperatures (< 200 °C) in water inside closed recipients and under self-generated pressure. 

In the first part of this chapter we will describe some general aspects of hydrothermal carbonization, using either carbohydrates or complex biomass to control structure formation in the presence of various catalysts and/or templates. In the second part, we then describe some of the most promising applications of these car-bon/hybrid materials in energy applications. 

In order to gain some information about the fundamentals of the hydrothermal carbonization process, the hydrothermal carbonization of different carbohydrates and carbohydrate products was examined [12, 13]. For instance, hydrothermal carbons synthesized from diverse biomass (glucose, xylose, maltose, sucrose, amylopectin, starch) and biomass derivatives (HMF and furfural) were treated under hydrothermal conditions at 180 °C and were analyzed with respect to their chemical and morphological structures by SEM,13 C solid-state NMR and elemental analysis. This was combined with GC-MS experiments on residual liquor solutions to analyze side products... 

The hydrothermal carbons obtained in the end from soluble, non-structural carbohydrates are micrometer sized, spherically shaped particle dispersions, containing a sp2 hybridized backbone (also responsible for the brown to black color) decorated with a dense layer of polar oxygenated functionalities still remaining from the original carbohydrate. The presence of these surface groups offers the possibility of further functionalization and makes the materials more hydrophilic and well-dispersible in water. The size of the final particles depends mainly on the carbonization time and precursor concentration inside the autoclave, as well as additives and stabilizers potentially added to the primary reaction recipe. An SEM image of a model reaction illustrating this dispersion state is shown in Fig. 7.1. 

It was also found that the presence of some metal ions and borates can effectively accelerate the hydrothermal carbonization of starch, which shortens the reaction time to some hours. Thus, iron ions and iron oxide nanoparticles were shown to effectively catalyze the hydrothermal carbonization of starch (< 200 °C) and also had a significant influence on the morphology of the formed carbon nanomaterials [10]. In the presence of Fe2+ ions, both hollow and massive carbon microspheres could be obtained. In contrast, the presence of Fe203 nanoparticles leads to very fine, rope-like carbon nanostructures, reminding one of disordered carbon nanotubes. 

A similar approach for the production of hydrothermal carbon nanotubes is the hydrothermal carbonization of glucose in the macrochannels of anodic alumina membranes [16]. Depending on the pore size of the membrane different hollow hydrother-... 

The coating of templates already indicated that carbonaceous hybrid materials on the nanoscale length can be produced when leaving the second material phase within the carbon frame. In the case of functional nanoparticles and nanostructures, this has tremendous importance for various applications in science and technology [18]. Consequently, hydrothermal carbonization has been also intensively used for the production of various hybrids. 

Li et al. reported first on the decoration of hydrothermal carbon spheres obtained from glucose with noble metal nanoparticles [19]. They used the reactivity of as-prepared carbon microspheres to load silver and palladium nanoparticles onto then-surfaces, both via surface binding and room-temperature surface reduction. Furthermore, it was also demonstrated that these carbon spheres can encapsulate nanoparticles in their cores with retention of the surface functional groups. Nanoparticles of gold and silver could be encapsulated deep in the carbon by in situ hydrothermal reduction of noble-metal ions with glucose (the Tollens reaction), or by using silver nanoparticles as nuclei for subsequent formation of carbon spheres. Some TEM images of such hybrid materials are shown in Fig. 7.4. 

The production of Ag Carbon nanocables was reported by Yu et al. [20]. In this case the authors used the hydrothermal carbonization of starch in the presence of Ag N03 leading to the one-step formation of carbon/Ag hybrid nanocables. Such silver-carbon nanocables can have a length as long as 10 mm and overall diameters of 1 micron with a 200-250 nm silver lining. When made at higher concentrations, they tend to fuse with each other (Fig. 7.5(a) and (b)). This method was extended to a polyvinyl alcohol(PVA)-assisted synthesis of flexible noble metal (Ag, Cu) carbon composite microcables [21]. 

Simultaneous to the understanding of some basics of hydrothermal carbonization using pure carbohydrate models, the synthesis of hydrothermal carbon materials using raw biomass was continued. It has been analyzed whether complex biomass - hy-drothermally carbonized - can also be directed to complex structural motifs with distinct surface polarities. Ideally, for this purpose one can use the structures and functionalization components already included in the biomass. We specifically selected waste biomass for material synthesis, starting products which are known to be hard to use otherwise, rich in ternary components, and applied different HTC conditions [29]. That way, one can avoid the food-raw materials competition, a prerequisite we regard as crucial for the development of a fully sustainable chemistry. 

Fig. 7.7 HRSEM of pine needles (a) before and (b) after being hydrothermally carbonized at 200 °C for 12 h (c) low-magnification SEM overview of a HTC-treated oak leaf (d) high-magnification picture of the same HTC-treated oak leaf indicating its nanostructure.

Energy applications of hydrothermal carbons and their hybrids... 

The first application of HTC as an anode in Li-ion batteries was first reported by Huang et al. [30], After the hydrothermal carbonization of sugar, the resulting... 

Fig. 7.8 TEM images of the Si SiOx/C nanocomposite nanoparticles produced by hydrothermal carbonization of glucose and Si and further carbonization at 750 °C under N2. (a) Overview of the Si SiOx/C nanocomposites and a TEM image at higher magnification (in the inset) showing uniform spherical particles (b) HRTEM image clearly showing the core/shell structure (c), (d) HRTEM image displaying details of the silicon nanoparticles coated with SiOxand carbon.

Despite quite some progress reported in improving the performance and lifetime of anode materials, a great deal of research needs to be dedicated to the improvement of the cathode in Li-ion batteries. This task was addressed by hydrothermal carbon coating techniques. Thus, Olivine LiMP04 (Me = Mn, Fe, and Co) cathodes with a thin carbon coating have been prepared by a rapid, one-pot, microwave-assisted hy-... 

Fig. 7.13 UV-VIS diffuse reflectance spectra of modified and pure samples (a) Ti02, (b) C Ti02, (c) physical mixture of Ti02 and hydrothermal carbon and (d) pure carbon.

Hydrothermal Carbonization (HTC), an alternative chemical pathway leading to a variety of carbonaceous materials, was presented in context with the generation of energy materials and energy hybrids. HTC is sustainable, as at least a majority of the precursors are biomass based, and the reaction takes place in pure water at mild tem-... 

After the description of chemical structure and control of meso-architecture and surface area, selected applications of such carbon materials as battery electrodes, supercapacitors, and in the design of controlled hybrid heterojunctions were presented. In the Li battery, coating or hybridization with hydrothermal carbon brought excellent capacities at simultaneous excellent stabilities and rate performances. This was exemplified by hybridization with Si, Sn02 (both anode materials) as well as LiFeP04 (a cathode material). In the design of supercapacitors, porous HTC carbons could easily reach the benchmark of optimized activated traditional carbons, with better stability and rate performance. 

Sustainable carbon hybrid materials made by hydrothermal carbonization... 

Figure 3.12 presents 5 C and 5 0-values of hydrothermal carbonates from the Pb-Zn deposits of Bad Grund and Lautenthal, Germany. The positive correlation between and 0/ 0-ratios can be explained either by calcite precipitation... 

Figure 2.4.2 Two other relevant nanoarchitectures of hydrothermal carbons (A) disperse spherical morphology, and (B) carbon nanofibers.

Figure 2.4.4 (A) Time-dependent electric current generated from the oxidation of HTC coal in an indirect carbon fuel cell. Solutions of Fe 111 and Vv were prepared in 0.5 mol L 1 H2SO4. (B) Development of open-circuit potential Eoc (up) and current I (down) due to Fe2+ formation in the anodic half-cell via oxidation of HTC coal, indicating the reducing potential of bare hydrothermal carbon (HC) dispersions. Charge equalization between the two half-cells was assured by a salt bridge containing a saturated KC1 solution. Carbon felt was used as electrodes. (C) Comparison of hydrothermal and fossil carbon sources in the same setup.

Titirici MM, Thomas A, Yu SH, Antonietti M. A direct synthesis of mesoporous carbon with bicontinuous pore morphology from crude plant material by hydrothermal carbonization. Chem Mater. 2007 19 4205-12. 

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