Effect of Pyrolysis Temperature on Acidic Oxygen-Containing Functional Groups and Electron Storage Capacities of Pyrolyzed Hydrochars

Abstract

Hydrothermal carbonization (HTC) is a thermochemical process, where biomass is treated with subcritical water. Hydrochar, the solid product of HTC, is a carbon-rich material containing acidic functional groups. In order to increase total surface area, one of the common practices is to pyrolyze it. However, dehydration occurs during pyrolysis, which may affect the acidic functional groups on hydrochar. In addition, biochars have recently been shown to possess significant electron storage capacities (ESC), but it was unknown whether pyrolyzed hydrochars also possess ESC and to what extent. In this article, the effect of pyrolysis temperature on acidic oxygen-containing functional groups and ESC of pyrolyzed hydrochars is evaluated. Hydrochars were prepared from cellulose and wood at 220 and 260 °C. These hydrochars were then pyrolyzed at 400, 500, and 600 °C under an N2 atmosphere. Afterward, the changes in functional groups were evaluated by BET analysis, ultimate analysis, ESC measurement, pH, pH at point of zero charge (pHPZC), Boehm titration, and FTIR analysis. The hydrochars showed relatively low surface areas mostly due to the lack of pores or pores being clogged with volatiles. The surface area was increased by an order of magnitude after pyrolysis; however, acidic oxygen-containing functional groups decreased significantly with increasing pyrolysis temperature. ESC was also decreased with increasing pyrolysis temperature, ranging from 1.44 (cellulose at 600 °C) to 3.25 (wood at 400 °C) mmol/g. This result suggests that a portion of the ESC of the pyrolyzed wood hydrochars originated from cellulose. A linear correlation between ESC and lactonic group was observed for pyrolyzed hydrochars.

Synopsis of Factors Affecting Hydrogen Storage in Biomass-Derived Activated Carbons

“…This could be explained by means of findings from Fourier-Transform Infrared Spectroscopy (FTIR) of various such ACs in the literature, where a decreased peak intensity or an even complete disappearance of certain bands, for example, C-N, C-NH-C, hydroxylic O-H, aromatic carbonyl/carboxyl C=O, and aliphatic ether and alcohol O-H, were noted [79][80][81][82]. Such a loss of functional groups results from the formation of a fused ring structure that aids in pore formation, by-producing volatile products like CO, H2O, and CO2 [83][84][85]. Removal of hydrogen (H) and oxygen (O) from the precursor during the chemical activation process leads to aromaticity development [86][87][88].…”

Cationic Dye Adsorption on Hydrochars of Winery and Citrus Juice Industries Residues: Performance, Mechanism, and Thermodynamics

“…A backward titration method was used to measure the OCAFGs, while the forward method was used to measure the BFGs. Detailed experimental procedures including calculation techniques of these methods are discussed elsewhere [36,37].…”

“…However, CW hydrochar showed a minimal difference in pH and pH PZC , while WW showed higher pH compared to its pH PZC at all conditions. According to the literature, the surface of the hydrochar is typically negatively charged when the pH is higher than its pH PZC and it is positively charged if the pH is lower than its pH PZC [36], which indicates that the WW hydrochars are negatively charged while CW hydrochars are mostly neutral. Furthermore, Figure 2 provides quantitative information on the acidic and basic groups on the hydrochars' surfaces.…”

Abiotic reduction of 3-nitro-1,2,4-triazol-5-one (NTO) and other munitions constituents by wood-derived biochar through its rechargeable electron storage capacity

“…35 Biochar ESC can range from 0.2 to 7 mmol e − g −1 , is distributed over a broad range of reduction potential ( E H ), and is highly reversible over multiple redox cycles. 30,34,36–39…”