Acid treated biochar for CO2 capture

“It has been reported that acid-treatment generates a higher micropore volume than the alkali activation due to the stronger solubility of the volatile organic matters in biomass precursors (Mallesh et al., 2020). The resultant well-developed microporous structure benefits the CO2 adsorption (Sevilla et al., 2012a). Commonly used acids include H3PO4, H2SO4 and HNO3 (Román et al., 2008).

As an effective activation agent, H3PO4 can modify the pyrolytic pathways of biopolymers, enhance the porosity and decorate the carbon surface with P-containing functional groups (Hulicova-Jurcakova et al., 2009de Yuso et al., 2014Liu et al., 2010). The proposed activation mechanism of H3PO4 was as below: P2O5 derived from the dehydration of H3PO4 reacted with carbon to release P and CO2; meanwhile, the decomposition of polyphosphoric acid generated O, P and steam in gaseous states (Olivares-Marín et al., 2006). During the activation process, bond cleavage and phosphate cross-linking reactions (e.g., condensation and cyclisation) were both promoted (Jagtoyen and Derbyshire, 1998), generating gaseous products, such as CH4, CO2 and CO. Recently, when H3PO4 was added to date stems with a ratio of 2:1, a low activation temperature (500°C) was realized and a high surface area of 1455 m2/g was obtained accordingly (Hadoun et al., 2013). Similarly, a series of ACs were prepared from biomass precursors using H3PO4 as the activator, exhibiting a high surface area up to 2450 m2/g, which was a result of the abundant micropore formation during the extensive reaction with the biopolymers of a considerably reduced particle size in the presence of H3PO4 (Quesada-Plata et al., 2016). In addition, the enhanced microporosity could also be attributed to the intercalation of H3PO4 into the carbon lattice, which was expanded and exhibited a higher porosity after being washed. As a result, more cavities were observed at the external surface of carbon adsorbents, possessing a twice surface area and micropore volume compared with the commercial AC (Nowrouzi et al., 2018). As reported in most works, the formation of both mesopores and micropores were facilitated with an increase of the H3PO4 content (Teng et al., 1998) since more sites would be penetrated by the acid molecules, benefiting the widening and opening of pores. However, excessive amount of H3PO4 might not be effective as expected considering the possible formation of insulating layers (Molina-Sabio and Rodriguez-Reinoso, 2004).

Sulfuric acid (H2SO4) is also a common acid activator, which can dissolve impurities in the precursor and effectively remove carbon ash (Olivares-Marín et al., 2012). When it was applied to treat materials containing organic compounds, the dehydration and oxidation of the carbon precursors would be promoted, leading to a 250-fold increase of surface area from the raw carbon (571 vs. 2.31 m2/g). Also, the AC surface after H2SO4 treatment turned to be rich in oxygenated groups (Vithanage et al., 2015). Similarly, the carbon activated by HNO3 contained more –OH and –COOH groups on the surface, which might improve the CO2 adsorption (Jiang et al., 2013).”

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