Hydrochar (OW250) was activated at 500, 600 and 700 °C. The final temperature was hold for 30 min in each case. As reported in Table 4-2, although the considerable burn-off (41 wt%), the initial temperature chosen (500 °C) was not sufficiently high to imply a significant increase of surface area of the starting char (see Table 4-9). This might be because when temperature was 500 °C the reaction between char and CO2 was too slow. As seen in Table 4-2,
600 °C maximized the hydrothermally carbonized oak’s surface area, and therefore it was deemed as optimal temperature. This condition corresponded to a burn-off of 46 wt%. It seems that the extra 7% burn-off opened up the (micro)porosity in the sample. Isotherm measured for this sample showed a significant micropore filling at the very beginning of the pressure range. In addition to that, the slope observed over mid pressure range can be attributed to multilayer adsorption process on the non-microporous surface (i.e. mesopores, macropores and external surface) [283].
Conversely, activation at the highest temperature tested (700 °C) was found to be too severe for the hydrochar, leading to higher material burn-off but lower porosity. This was probably because the sample had not experienced any previous high temperature treatment. As a result, a significant drop of pore volume was measured, likely caused by a collapse of the pore walls. This is shown by the lower volume of nitrogen adsorbed at the very low pressure and the milder slope in the mid-pressure range (see Figure 4-1). However, the isotherm shape of all activated carbons is closer to a type II [242], thus indicating a limited micropore percentage (up to 62.5% for OW250PA_600_0.5h). The latter can be calculated from values reported in Table 4-2 using (4.1)).
(𝑽𝒎𝒊
𝑽𝒕𝒐𝒕) · 𝟏𝟎𝟎 (4.1)
The ineffectiveness of the lowest activation temperature (500 °C) was further backed by the non-local density functional theory (NLDFT) pore size distribution measured for OW250PA_500_0.5h, as no distinct peaks were observed. This confirmed that at 500 °C CO2 oxidation of char was too slow to
form any significant porosity. On the contrary, ACs obtained at 600 and 700 °C exhibited an intense sharp peak at ca. 0.3 nm, indicating development of ultramicroporosity (d<0.7 nm). However, peak shoulders extending up to ca. 1.2 nm were also observed, suggesting the presence of a supermicropore (d>0.7
nm) population. However, both ultra- and supermicropores decreased with increasing temperature from 600 to 700 °C. In addition to that, optimal sample (OW250PA_600_0.5h) exhibited an additional sharp peak centred at around 0.8 nm and very broad peaks for pore widths higher than 2 nm. The latter indicated a minor contribution of mesoporosity.
Table 4-2 Effect of CO2 activation temperature (T) on burn-off and textural
parameters of hydrothermally carbonized wood; t represents the activation dwell time.
Sample ID6
T t BO7
SBET8 Vtot9 Vmi10 Vme11 Vma12
ºC h wt% m2·g-1 cm3·g-1 cm3·g-1 cm3·g-1 cm3·g-1
OW250PA_500_0.5h 500 0.5 41 39 0.071 0.020 0.023 0.028
OW250PA_600_0.5h 600 0.5 46 415 0.278 0.173 0.066 0.040 OW250PA_700_0.5h 700 0.5 53 257 0.163 0.102 0.023 0.039
6 PA stands for physically activated 7 Burn-off
8 Surface area calculated by applying Brunauer–Emmett–Teller (BET) method
to N2 adsorption data
9 Total pore volume calculated by applying Gurvitsch’s rule at P/P0=0.99
10 Micropore volume calculated by applying Dubinin-Radushkevich (DR) model
to N2 adsorption data
11 Mesopore volume calculated by applying Barrett-Joyner-Halenda (BJH)
model to N2 adsorption data
Figure 4-1 N2 adsorption isotherms - Effect of temperature on CO2
activation of hydrothermally carbonized oak wood
Figure 4-2 NLDFT pore size distributions - Effect of temperature on CO2
activation of hydrothermally carbonized oak wood
4.3.1.1.2 Pyrolyzed wood (OW800)
Higher temperatures were chosen for the activation of the pyrolyzed oak wood, as this was already subjected to a more severe heat-treatment. In particular, runs were carried out at 750, 800 and 850 °C. In this case, sample was held to activation temperature for 1 h. Very similar textural properties and carbon burn-offs were obtained at 750 and 800 °C. However, the latter slightly prevailed over the former in terms of texture development. Isotherms measured for these samples may be considered as an intermediate between types I and II [264]. Although there was a pronounced upswing exhibited in the microporous pressure range, the mild slope in the mid pressure range indicated the presence of mesoporosity to some extent.
0.0 0.2 0.4 0.6 0.8 1.0 0 20 40 60 80 100 120 140 160 180 200 N2 adsorbed (cm 3 g -1 , STP ) P/P0 (-) BET (m2 g-1 ) OW250 6 OW250PA_500_0.5h 39 OW250PA_600_0.5h 415 OW250PA_700_0.5h 257 0.5 1.0 1.5 2.0 2.5 3.0 3.5 4.0 4.5 0.00 0.05 0.10 0.15 0.20 0.25 dV(w) (c m 3 nm -1 g -1 ) Pore Width (nm) OW250 OW250PA_500_0.5h OW250PA_600_0.5h OW250PA_700_0.5h
Table 4-3 Effect of CO2 activation temperature on burn-off and textural
parameters of pyrolyzed wood
Sample ID T t BO SBET Vtot Vmi Vme Vma ºC h wt% m2·g-1 cm3·g-1 cm3·g-1 cm3·g-1 cm3·g-1 OW800PA_750_1h 750 1 32 505 0.257 0.193 0.037 0.026 OW800PA_800_1h 800 1 34 627 0.313 0.240 0.042 0.031 OW800PA_850_1h 850 1 89 0 0.000 0.000 0.000 0.000
Figure 4-3 N2 adsorption isotherms - Effect of temperature on CO2
activation of pyrolyzed oak wood
In contrast, a further increase of activation temperature (up to 850 °C) of the initial char was detrimental, textural parameters being dramatically reduced down to effectively zero as given in Table 4-3. Accordingly, 800 °C resulted to be the optimal activation temperature for the pyrolysis-derived oak char. This result disagreed with the optimal condition reported by Sanchez et al. [160], who found increased surface area when increasing temperature from 800 to 840 °C. Yet, this might be ascribed to the lower burn-off (68.3 wt%) reported by these authors compared to that experienced by the oak wood char tested in this work under similar conditions (89 wt%).
An isotherm measured for OW800PA_850_1h showed evidence for the complete degradation of its texture (see Figure 4-3). A negative trend of the adsorbate volume with increasing pressure was observed. This clearly indicated a total destruction of the porous structure for this sample. It is likely that when temperature was 850 °C, activation went far beyond the limit of volatiles evolution, thereby leading to the damage of any porosity previously created.
0.0 0.2 0.4 0.6 0.8 1.0 -20 0 20 40 60 80 100 120 140 160 180 200 220 N2 adsorbed (cm 3 g -1 , STP ) P/P0 (-) BET (m2 g-1 ) OW800 164 OW800PA_750_1h 505 OW800PA_800_1h 627 OW800PA_850_1h 0
As seen in Figure 4-4, a narrower distribution of pores was obtained when increasing activation temperature from 750 to 800 °C. In particular, it seemed that ultramicroporosity increased at the expense of supermicroporosity. As given in Table 4-3, this is consistent with the increase of surface area (from 505 up to 627 m2·g-1) with increasing temperature.
In contrast, further increase of temperature up to 850 °C led to a collapse of the porosity created at 800 °C. This was shown by the flat NLDFT distribution exhibited OW800PA_850_1h.
Figure 4-4 NLDFT pore size distributions - Effect of temperature on CO2
activation of pyrolyzed oak wood