*3.2.3 SEM-EDX analysis of biochar*

As shown in **Figure 8(a)**, the unactivated biochar particles' surfaces showed more, larger (40 × 60 μm) hill-like structures than the surfaces of the activated biochars. H2O and CO2 activate the biochar via C + H2O → CO + H2 and C + CO2 → 2CO, respectively. However, its larger size meant activation by CO2 was limited to the surface of biochar, resulting in small structures (15 × 15 μm), which can be seen in **Figure 8(c)**. However, as illustrated by the structure shown in **Figure 8(b)**, H2O, as well as H/O/OH radicals, was able to alter the surface morphology (creating structures of 20 × 20 μm) and infiltrate into the particle's carbon matrix to produce new larger pore structures from the inside out. According to Wu et al. [62], interactions between radicals and metal species take place on the surface of internal pores or inside the char matrix. **Table 4** shows little change in the biochar's internal metal (K, Ca, Mg, Fe, and Al) contents before and after H2O/CO2 activation. Thus, the effect of H2O/CO2 may be more focused on changing the distribution of metal species within the biochar samples. As we reported previously [43], the effect of K in biochar on tar reforming is stronger than that of Ca and other species. Thus, the surface content and distribution of K were studied, as shown in **Figure 8(a)–(c)**. The surface content of K significantly increased from 0.18% in the unactivated biochar to 0.35% in the H2O-activated biochar and 0.21% in CO2-activated biochar. In addition, an obvious enrichment occurred on the surface of H2O-activated biochar. Klinghoffer et al. [63] reported that during thermal treatment the metal species migrated to the biochar surface, some of which formed clusters that then acted as an active site for catalytic reactions. During H2O/CO2 activation, an increase in surface O content occurred alongside the migration of AAEM species from the interior of the particles to the surface, forming metal-carbon complexes. The redox properties of these metal-carbon complexes may have had implications for the biochar's catalytic properties. Also, highly dispersed metal species in a highly porous carbon matrix could have effectively acted as active adsorption sites that

### **Figure 8.**

*SEM-EDX analysis of biochar samples. (a) Pyrolysis biochar, (b) H2O activated biochar, and (c) CO2 activated biochar.*

**59**

**Figure 9.**

*Mechanism of In-Situ Catalytic Cracking of Biomass Tar over Biochar with Multiple Active Sites*

promote volatile hydrocarbon condensation reactions to form coke [25], which were caused by hydrodeasphalting (HDA) reactions. Thus, the H2O/CO2 activation of biochar impacted the biochar surface's morphology and metal content, both of

The biochar samples' N2-absorption/desorption isotherms at 77 K during H2O/ CO2 activation and biomass tar reforming are shown in **Figure 9**. Compared with those of the original pyrolysis biochar, the pore systems of H2O/CO2-activated biochar samples and that from tar H2O reforming over H2O-activated biochar were better developed. Conversely, the other conditions exhibited pore structures that were somewhat blocked, especially for reforming over pyrolysis biochar in Ar and

To further investigate the microphysical structures of the biochar samples, their BET surface properties were evaluated and are presented in **Table 4**. The unactivated pyrolysis biochar presented a BET surface area of 195.35 m2

efficient tar adsorption, resulting in a good residence time of the tar reacting with the catalyst [46]. **Table 5** also shows that the concentration of micropores (<2 nm), mesopores (2–50 nm), and macropores (>50 nm) varied between the samples. Thus, the ratio of micropores (<2 nm) to mesopores and macropores (>2 nm) (SMic./SExt.) was employed. The H2O-activated biochar showed a lower value of this ratio (2.28) than that of the CO2-activated biochar (4.57) indicating that activation/ gasification under a CO2 atmosphere produced a higher relative micropore content, whereas under an H2O atmosphere mesopores were favored. This may be explained by considering that H2O removes carbon atoms from the particle's interior, enlarging open micropores and opening closed micropores, promoting the formation of mesopores. Meanwhile, CO2 causes changes in the biochar surface that create more micropores. According to Klinghoffer et al. [63], the higher biochar surface area was the main reason for better catalyst performance, but pore size distribution also affected its activity, and evidence of diffusion limitations in microporous biochar was observed. Elsewhere, it has been confirmed that mesopores significantly

*N2-absorption/desorption isotherms at 77 K for biochar obtained from different conditions: (A) in Ar over pyrolysis biochar; (B) in Ar over H2O-activated biochar; (C) in Ar over CO2-activated biochar; (D) in 15 vol.%* 

*H2O over H2O-activated biochar; and (E) in 29 vol.% CO2 over CO2-activated biochar.*

/g. Activation by H2O and CO2 increased the BET surface

/g, respectively. The biochar's porous structure enabled

/g and a

*3.2.4 BET analysis of biochar during H2O/CO2 activation and tar reforming*

which influenced the reforming of biomass tar over biochar.

*DOI: http://dx.doi.org/10.5772/intechopen.91380*

for the CO2-activated biochar.

pore volume of 0.0999 cm3

area to 307.45 and 237.71 m2

promote volatile hydrocarbon condensation reactions to form coke [25], which were caused by hydrodeasphalting (HDA) reactions. Thus, the H2O/CO2 activation of biochar impacted the biochar surface's morphology and metal content, both of which influenced the reforming of biomass tar over biochar.
