Preparation of nitrogen-doped microporous modified biochar by high temperature CO₂–NH₃ treatment for CO₂ adsorption: effects of temperature

Abstract

Nitrogen-rich agricultural waste, soybean straw, was used as a raw material to prepare high efficiency CO₂ adsorbents (nitrogen-doped porous modified biochars). Three different modification methods for the preparation of these adsorbents were compared, i.e. activation with carbon dioxide, ammonification with ammonia (NH₃) and high temperature treatment with the mixture of CO₂ and NH₃. Effects of modification temperature on physicochemical properties of the modified biochars and influences of adsorption temperature on their CO₂ capture performances were both investigated. Activation with CO₂ obviously developed the pore structure of modified biochars, especially micropores, while the ammonification with NH₃ and modification with the mixture not only developed porosity, but also introduced nitrogen functional groups, and the modification with the mixture was better than the ammonification with NH₃. As the modification temperature increased, the micropore surface area and N/C ratio of the modified biochars by the modification with the mixture both increased first, and reached the maximum at 800 °C, and then decreased. Furthermore, at the lower adsorption temperature, the micropore structure played an important role to influence the CO₂ capture performance, while at the higher adsorption temperature, the chemical properties, especially the nitrogen functional groups, contributed more to the CO₂ capture.

---

1. Introduction

Increasing CO₂ concentration in the atmosphere is considered a main contributor to global warming.¹ CCUS (namely, CO₂ capture, usage and storage), is a promising technology for large-scale CO₂ reduction, where the CO₂ capture step dominates in determining the cost of the technology.^2–5 Among various CO₂ capture methods, adsorption has received wide attention and research, benefiting from its low energy consumption and low equipment cost.⁶ Developing adsorbents with high performance and low cost is crucial for CO₂ adsorption technology. Over recent years, wide arrays of solid adsorbents have been used for CO₂ capture. A screening of the possible adsorbents for CO₂ capture indicates that carbonaceous adsorbents represent a good option to balance all the criteria (adsorption capacity, process cost, process efficiencies and engineering feasibilities) due to their promising properties such as low preparation cost, chemical inertness, and easy regeneration.^7–9

In previous researches, many attempts have been made to modify textural characteristics and surface chemistry of carbonaceous adsorbents to improve their adsorption performance.¹⁰ With regard to modifying the surface chemistry, it has been validated that introducing basic functional groups is an effective way to enhance the adsorption performance for acidic gases.^6,11 There exits three major methods to introduce basic functional groups to carbon surface, including chemical impregnation in amine solution,¹² plasma treatment in N₂ or NH₃ atmosphere,¹³ and high temperature treatment in ammonia.^14–16 However, chemical impregnation in amine solution runs the risk of deteriorating pore structures. Because the impregnation agent is inclined to crystallize on the carbon surface and block the pore structures, which is unfavorable for gas adsorption. Meanwhile, plasma treatment in N₂ or NH₃ atmosphere needs to bear the high cost of plasma equipment. Therefore, high temperature treatment in ammonia seems to be more promising.

In the scope of modifying the textural characteristics, several methods have been attempted, including mainly, KOH, K₂CO₃ and H₃PO₄ activation,^8,17,18 steam activation,^19,20 and high temperature CO₂ activation.²⁰ These conventional activation methods mainly focus on the development of the pore structure, which can effectively increase the CO₂ filled spaces.^21,22 In particular, the development of the narrow micropores (<0.8 nm) can significantly contribute to the selectivity for CO₂ on the conventional operation conditions (i.e., T ∼ 0–25 °C and P_CO2 ∼ 0–1 bar).^23–26 However, the effect of the pore properties of carbon-based adsorbents on the CO₂ adsorption capacity become less important when increasing the adsorption temperature.²⁷ At the moment the surface chemical property of carbonaceous adsorbent will become more important for the CO₂ adsorption capacity, including CO₂ selectivity, because the base sites are stronger than that of the pure porous structure for the CO₂ adsorption.^6,27 If the conventional activation can be integrated with the high temperature NH₃ modification process in a single step, it will not only develop the physical property of adsorbent, but also the chemical property, which will be inevitably beneficial to CO₂ capture.

Therefore, it would be worthy to dig out what happens if high temperature CO₂ activation and ammonia treatment are integrated to modify biochar. In this work, biochar derived from soybean straw by fast pyrolysis was modified by high temperature CO₂–NH₃ mixture, and compared with those modified with CO₂ activation, NH₃ treatment and the non-modified. The physiochemical properties and CO₂ adsorption performance of biochar before and after modification were characterized and compared. The CO₂ adsorption kinetics of modified biochars was discussed with deactivation model and the effect of the physiochemical properties on the CO₂ adsorption capacity of modified biochars was investigated.

2. Experimental

2.1. Sample

Raw soybean straws collected locally were adopted as raw materials, and the proximate and ultimate analyses were shown in Table 1. Before carbonization, the collected soybean straw was ground and sieved, and a particle size between 0.6 and 1 mm was selected for preparing biochar at 500 °C in N₂ (400 mL min⁻¹). The derived biochar was labeled as R-char.

Table 1 The proximate and ultimate analysis of soybean straw

Sample	Proximate analysis (wt%, ad)				Ultimate analysis (wt%, ad)
	Moisture	Volatile	Ash	Fixed carbon	C	H	N	S	O^a
a Calculated by difference.
Soybean straw	4.61	74.32	4.03	17.04	44.94	4.25	1.52	0.34	40.31

---

2.2. Modification of biochar

R-char was subjected to three different modification approaches: physical activation with CO₂, ammonification with NH₃ and treatment with their gas mixture (CO₂–NH₃). During all modification treatments, batches of around 3 g R-char in a quartz reactor were treated in a vertical tube furnace. R-char was heated with a heating rate of 10 °C min⁻¹ in a N₂ atmosphere (400 mL min⁻¹) until it reached the preset temperature (500–900 °C), and then, N₂ was replaced by CO₂ (20%; 500 mL min⁻¹), NH₃ (16%; 500 mL min⁻¹) or CO₂–NH₃ (CO₂ is 20% and NH₃ is 16%; 500 mL min⁻¹) for 30 min. Finally, the reactive gas was changed back to N₂ until the furnace was cooled to the ambient temperature. According to the modification atmospheres and temperature, the modified biochars were labeled as C500–C900, N500–N900 and CN500–CN900.

2.3. Physicochemical properties

Textural properties of all samples were measured by N₂ adsorption isotherms obtained at 77 K and CO₂ adsorption isotherms obtained at 273 K with an automatic adsorption equipment (ASAP2020, Micromeritics, USA). Prior to any adsorption measurements, all samples were first degassed at 120 °C under vacuum for 24 h. The apparent surface areas (S_BET) of all samples were calculated from the N₂ adsorption isotherms by the Brunauer–Emmett–Teller (BET) equation. According to the CO₂ adsorption isotherms, the micropore surface area (S_mic) and micropore volume (V_mic) were calculated by the Dubinine–Radushkevitch (DR) equation, and the average width of the micropore system (L_mic) by Dubinin–Astakhov (DA) method. A CHNS elementary analyzer (Vario Micro Cube, Germany) was used to determine the carbon, hydrogen, and nitrogen contents of the samples.

2.4. CO₂ adsorption experiments

The CO₂ adsorption performance of R-char and all modified biochars was characterized by a fixed bed adsorption system (shown in Fig. 1). The adsorption column was fabricated from quartz pipe of 12 mm inner diameter. The feed inlet of N₂ and CO₂ were controlled with mass flow meters which were calibrated by a wet gas meter. The concentration of the feed and the effluent were measured by using a mass spectrometer (Omnistar™ GSD 320, Pfeiffer Vaccum, Germany). CO₂ breakthrough curves of the studied samples were obtained at 30 °C and 120 °C, respectively representing the indoor temperature and exhaust gas temperature in the power plant.

Fig. 1 The setup of CO₂ adsorption experiments in a fixed-bed.

First, about 2 g of the sample was loaded in the fixed bed reactor and heated up to 150 °C under N₂ atmosphere (100 mL min⁻¹) and held isothermally for 1 h to remove moisture and other gases. Second, stopped heating until the temperature decreased to targeted adsorption temperature of 30 °C/120 °C; once the targeted temperature was reached, the atmosphere was switched to CO₂ (10%, 100 mL min⁻¹) and the sample was hold isothermally at 30 °C/120 °C for the entire CO₂ adsorption experiment. The concentration of the feed and the effluent was the average of three experimental data to reduce the relative errors, which were within ±5%.

2.5. Model description

To fit the breakthrough curve of CO₂ adsorption, the deactivation model (DM) was introduced in the sight of adsorption dynamics.²⁸ In this model, the activity of R-char and modified biochars is considered as an activity term a, which is lumped into a reducing activation factor to represent the deactivation due to a decrease of the active site concentration, the textural changes and so on during CO₂ adsorption. Neglecting the axial dispersion and pore diffusion resistance, species conservation equation for the adsorption column is given as eqn (1):where Q is volumetric flow rate (mL min⁻¹), C_A is the CO₂ concentration in the outlet (mol L⁻¹), W is the mass of R-char or modified biochar (g), and k_o is the initial adsorption rate constant (cm³ min⁻¹ g⁻¹). The rate equation for the activity change of R-char or modified biochar (a) can be expressed as eqn (2):where t is the adsorption time (min) and k_d is the deactivation rate constant (min⁻¹). Using an iterative procedure, the solution of this model gives eqn (3) for the breakthrough curve:where C_A0 is the CO₂ concentration in the inlet (mol L⁻¹). According to the breakthrough curves, the CO₂ adsorption capacity (q) is calculated from eqn (4):where M is the relative molecular mass of CO₂, and t_0.95 is the time when the C_A/C_A0 is 0.95.

3. Results and discussion

3.1. Physical characteristics

Fig. 2 shows the N₂ adsorption isotherms at 77 K of R-char and all modified biochars, and all of them present the type I adsorption isotherms, indicating characteristic of microporous materials. Fig. 3 presents the CO₂ adsorption isotherms at 273 K of all samples. Due to the existence of diffusion limitations to the entrance of N₂ molecule at 77 K in narrow micropores, the BET surface area (S_BET) of R-char and all modified biochars were calculated by the N₂ adsorption isotherms, but the micropore surface area (S_mic), micropore volume (V_mic) and average width of the micropore system (L_mic) by the CO₂ adsorption isotherms, and the results have been shown in Table 2. The S_mic of R-char is much larger than its S_BET, which indicates that micropores dominate in its pore structure. After modification, the S_BET increase continuously with the temperature from 500 °C to 900 °C for the three different modification atmospheres. When the modification temperature is low (<700 °C), the increase of S_BET are slight, but the modification temperature over 700 °C, they increase faster and faster. The reason is that at lower temperature, the gas–solid reaction is not intense enough, but when the temperature exceeds the limit value, the reaction becomes more and more intense. It suggests that the higher modification temperature is beneficial to the improvement of S_BET.

Fig. 2 The N₂ adsorption isotherms at 77 K of R-char (a) and all modified biochars: CO₂ activation (a), NH₃ ammonification (b) and CO₂–NH₃ modification (c).

Fig. 3 The CO₂ adsorption isotherms at 273 K of R-char (a) and all modified biochars: CO₂ activation (a), NH₃ ammonification (b) and CO₂–NH₃ modification (c).

Table 2 Textural properties and CN elementary analysis results

Sample	N2 adsorption	CO2 adsorption			C (wt%)	N (wt%)	N/C
	SBET (m^2 g^−1)	Smic (m^2 g^−1)	Vmic (cm g^−1)	Lmic (nm)
R-char	0.04	250	0.10	1.02	72.21	1.37	0.019
C500	5.5	300	0.12	1.02	75.24	1.36	0.018
C600	2.6	342	0.14	1.01	75.13	1.35	0.018
C700	22	398	0.16	1.00	75.86	1.34	0.018
C800	346	473	0.19	1.03	71.83	1.46	0.020
C900	397	445	0.18	1.06	68.54	1.34	0.020
N500	1.5	311	0.13	1.02	72.02	1.93	0.027
N600	5.8	339	0.14	1.01	73.31	2.74	0.037
N700	221	433	0.17	1.01	71.23	5.52	0.077
N800	365	479	0.19	1.04	65.90	6.11	0.093
N900	469	461	0.19	1.06	65.00	4.07	0.063
CN500	2.0	318	0.13	1.02	77.79	2.49	0.032
CN600	1.2	370	0.15	1.00	74.03	3.13	0.042
CN700	41	439	0.18	1.00	70.89	5.93	0.084
CN800	491	534	0.21	1.06	64.19	6.61	0.103
CN900	764	489	0.20	1.16	62.51	5.00	0.080

---

When the modification temperature is increased, the S_mic and V_mic of these three kinds of modified biochars increase at the beginning, reaching the maximum at 800 °C, and then decrease, but L_mic decreases firstly, reaching a minimum at 700 °C. At low modification temperature (≤700 °C), the hot corrosion of biochar from the gas–solid reaction can only make a tiny damage on the surface of biochar, which can weakly improve meso- and macroporosity, but obviously creates more narrow micropores that result in the increase of S_mic and V_mic and the decrease of L_mic. As the modification temperature increases, the hot corrosion of biochar becomes very intense, and it can cause that new pores (narrow micropores) are created and the pores formed at lower temperature are widen, giving rise to wider micropores, mesopores and macropores. However, when the modification temperature increases to 900 °C, the intense hot corrosion can provoke the coalescence between micropores, the micropore and mesopore, and the micropore and macropore, which can lead to the decrease of micropore²⁹ and the increase of mesopore and macropore.

Among the three different modification approaches, the CO₂–NH₃ modification is the best on the improvement of the pore structure, followed by the ammonification with NH₃ and the physical activation with CO₂. The reason is that at high temperatures, the mixture CO₂–NH₃ can produce the largest amount of free radicals which can participate in the gas–solid reactions, followed by NH₃ ammonification and CO₂ activation.

3.2. Chemical characteristics

Table 2 also shows the chemical analysis of R-char and all modified biochars. With the increase of modification temperature, regardless of the modifying atmosphere used, the carbon content decreases due to the hot corrosion from gas–solid reactions.²⁹ In addition, at the lower modification temperatures (≤700 °C), the carbon content still keeps a relatively high level, but drops to a lower level when the modification temperature increases. It exactly indicates that the hot corrosion to the carbon skeleton of biochars, coming from the gas–solid reactions, is not intense enough at the lower modification temperature, but with the increase of the temperature, it becomes stronger and stronger, due to the more intense gas–solid reactions.

In the case of the nitrogen content and N/C ratio, they are all low for the biochars modified by CO₂ activation (C500–C900), and very similar to R-char. As the modification temperature increases (from 500 °C to 900 °C), there is almost no change in the nitrogen content and N/C ratio for the modified biochars with CO₂ activation. However, the high temperature NH₃ ammonification and CO₂–NH₃ modification influence significantly the nitrogen content and N/C ratio of modified biochars. With the increase of modification temperature, both of them first increase and then decrease; when the temperature is 800 °C, they reach the maxima (6.11 wt% and 0.093 for N800 and 6.61 wt% and 0.103 for N800). As can be seen from those changes in the nitrogen content and N/C ratio of modified biochars, nitrogen functional groups are successfully grafted into the structure of modified biochars by the NH₃ treatment and CO₂–NH₃ modification.³⁰ Furthermore, at the same modification temperature, content and N/C ratio of the biochars modified with the CO₂–NH₃ mixture are higher than those of the biochars modified with NH₃. It indicates that during the high temperature ammonification, the presence of CO₂ promotes the introduction of the nitrogen functional groups, which is attributed to the gas–solid reaction between CO₂ and biochars that provides more active sites for the ammonification reaction.^30,31

3.3. Assessment of CO₂ adsorption

3.3.1. CO₂ adsorption properties at different temperatures. The CO₂ breakthrough curves are obtained from the fit of the experimental CO₂ adsorption results by the deactivation model (DM). Fig. 4 shows the breakthrough curves of CO₂ on R-char and all modified biochars at 30 °C. It can be observed that the modified biochars with CO₂–NH₃ modification have the longest CO₂ breakthrough time (the time when the CO₂ concentration at the outlet reached 10% allowable breakthrough concentration),³² followed by CO₂ activation and NH₃ treatment. It indicates that among these three different modifications, the CO₂–NH₃ modification was the most efficient treatment methods for CO₂ adsorption at 30 °C. The results of regression analysis of the experimental data, using eqn (3), are reported in Table 3. It can be found that all correlation coefficients (R²) are more than 0.97, which indicates a good fit of the DM to the experimental data. The two important regression coefficients, namely initial adsorption rate constant (k_o) and deactivation rate constant (k_d), reflect the CO₂ adsorption performance of modified biochars.³³ With an increase in the modification temperature, the deactivation rate constant values of the modified biochars using the CO₂ activation and NH₃ ammonification both present an increasing trend, and the initial adsorption rate constant values show a first increasing and then decreasing trend. In the case of the CO₂–NH₃ modification, both of k_o and k_d first reduce and then increase with the temperature increasing. When the modification temperature is 800 °C or 900 °C, these three kinds of modified biochars all have high k_o and low k_d values. It indicates that the higher modification temperature is conducive to the CO₂ adsorption at 30 °C.

Fig. 4 Breakthrough curves of CO₂ on R-char and all modified biochars at 30 °C: (a) R-char, (b) CO₂ activation, (c) NH₃ ammonification and (d) CO₂–NH₃ modification.

Table 3 Regression parameters of deactivation model for CO₂ on R-char and all modified biochars at different temperatures

Samples	CO2 adsorption at 30 °C			CO2 adsorption at 120 °C
	ko (cm^3 min^−1 g^−1)	kd (min^−1)	R^2	ko (cm^3 min^−1 g^−1)	kd (min^−1)	R^2
R-char	127.107	0.558	0.989	146.263	1.115	0.997
C500	128.842	0.554	0.978	125.629	0.917	0.983
C600	128.217	0.446	0.991	104.320	0.747	0.953
C700	136.713	0.443	0.988	110.121	0.794	0.957
C800	163.330	0.434	0.997	99.308	0.597	0.952
C900	126.555	0.360	0.979	96.812	0.612	0.955
N500	134.694	0.559	0.993	140.212	0.967	0.990
N600	134.384	0.475	0.970	136.818	0.851	0.986
N700	132.116	0.409	0.988	107.292	0.555	0.963
N800	150.949	0.384	0.998	114.037	0.527	0.981
N900	127.740	0.337	0.990	115.681	0.604	0.975
CN500	134.366	0.476	0.975	108.896	0.619	0.994
CN600	132.815	0.432	0.984	97.085	0.510	0.950
CN700	125.678	0.372	0.983	103.569	0.486	0.979
CN800	126.277	0.297	0.986	98.845	0.381	0.971
CN900	140.755	0.337	0.981	116.277	0.539	0.989

---

To acquire the CO₂ adsorption amount of R-char and all modified biochars, the breakthrough curve is integrated by eqn (4), and the definite integral results are presented in Fig. 5. It can be seen that for the three kinds of modified biochars, the CO₂ adsorption capacities increase first, and then decrease with the rising of the modification temperature. At 800 °C, all three reach their maxima (C800 for 76.31 mg g⁻¹, N800 for 79.19 mg g⁻¹ and CN800 for 88.89 mg g⁻¹). It indicates that 800 °C is the most favorable temperature for the modification of biochars to adsorb CO₂ at 30 °C. The reason is that when the modification temperature reaches 800 °C, the S_mic and V_mic of the three kinds of modified biochars are also the maxima, and the improvement of the narrow microporosity can promote the CO₂ adsorption capacity under ambient conditions.^22,25 On the other hand, at the same modification temperature, the CO₂ adsorption capacity of modified biochars using the CO₂–NH₃ modification is the highest, followed by the NH₃ ammonification and the CO₂ activation, and it is consistent with the previously proposed result that the mixed gas modification to improve the narrow microporosity is superior to their individual modification.

Fig. 5 CO₂ adsorption capacities of R-char and all modified biochars at 30 °C.

The change of the adsorption temperature necessarily results in the change of the diffusion rate of CO₂ and the thermodynamic adsorption equilibrium, which certainly leads to the change of the CO₂ adsorption rate and amount. Fig. 6 presents the breakthrough curves of CO₂ on R-char and all modified biochars at 120 °C, and Table 3 also reports the results of regression analysis of the experimental data, using the deactivation model. It can be seen from Table 3 that the correlation coefficients (R²) are more than 0.95, which demonstrates that the deactivation model can also be used to the CO₂ adsorption of R-char and all modified biochars at higher temperature. With increasing adsorption temperature, the initial adsorption rate constant (k_o) presents a decreasing tendency, but the deactivation rate constant (k_d) significantly goes up. Higher adsorption temperatures lead to higher rates of CO₂ transport through the pore structure of modified biochars and therefore reaching more active sites, which lead to the increase of deactivation rate, but then the initial adsorption rate drops due to the major driving force of adsorption shifts from physical adsorption to chemical adsorption when the adsorption temperature is raised.^34,35 Furthermore, at the higher modification temperature, the initial adsorption rate constant slumps by even more, and the enhancement of the deactivation rate constant is more obvious at the lower modification temperature.

Fig. 6 Breakthrough curves of CO₂ on R-char and all modified biochars at 120 °C: (a) R-char, (b) CO₂ activation, (c) NH₃ ammonification and (d) CO₂–NH₃ modification.

Fig. 7 shows the CO₂ adsorption capacities of R-char and all modified biochars at 120 °C. As the adsorption temperature increases, the CO₂ adsorption performance of biochars deteriorates, due to the major driving force of adsorption shifts from physical adsorption to chemical adsorption. Among the three modifications, the CO₂ activation decreases the most in the CO₂ adsorption capacity, followed by the NH₃ ammonification and the CO₂–NH₃ modification. The possible reason is that at 120 °C, chemical adsorption is the dominant driving force for CO₂ adsorption, which can be promoted by the enriched nitrogen functional groups,^36,37 and many nitrogen functional groups are introduced into the modified biochars during the CO₂–NH₃ modification and NH₃ ammonification, whereas none in the case of CO₂ activation. When the adsorption temperature is 120 °C, with the increasing modification temperature, the CO₂ adsorption capacities of the modified biochars using the NH₃ ammonification and CO₂–NH₃ modification also first increase and reach the maximum at 800 °C, and then decrease. In addition, CN800 still has the largest CO₂ adsorption capacity (49.87 mg g⁻¹) among all modified biochars. It suggests that around 800 °C is the best modification temperature for the modified biochars to adsorb CO₂, especially CN800 which has the best micropore structure and the highest nitrogen content.

Fig. 7 CO₂ adsorption capacities of R-char and all modified biochars at 120 °C.

3.3.2. Relationship between adsorption capacity and physicochemical properties. In order to explore the key controlling factors of CO₂ adsorption on modified biochars, the linear fitting is used to analyze relationships between the CO₂ adsorption capacities and physicochemical properties. The linear relevant fitting results are shown in Fig. 8. When the adsorption temperature is 30 °C, the linearly dependent coefficient (R²) between the CO₂ adsorption capacity and the apparent surface area (S_BET) is 0.732, and more than the N/C concentration ratio (R² = 0.469), which demonstrates that there is an approximately linear relationship between the textural characterization and the CO₂ adsorption capacity at the lower temperature. However, the relationship between the micropore surface area (S_mic) and the CO₂ adsorption capacity displayed a stronger linear correlation than the S_BET. It indicates that at the lower temperature, the micropore structure of modified biochars mainly influences their CO₂ capture performance, and physical adsorption is the important adsorption driving force.^31,38

Fig. 8 Relationship between CO₂ adsorption capacity and physicochemical properties: (a) S_BET, (b) S_mic and (c) N/C.

Compared with 30 °C, the relationship between the N/C ratio and the CO₂ adsorption capacity of modified biochars at 120 °C displays significantly better regression coefficient than that at 30 °C, but the BET and micropore surface area of modified biochars respectively present weaker linear correlation with their CO₂ adsorption capacity at 120 °C than that at 30 °C (shown in Fig. 8). In addition, the N/C ratio of modified biochars exhibits a significantly better linear regression coefficient with their CO₂ adsorption capacity at 120 °C than that of the BET and micropore surface area. These would seem to indicate that the influence of porosity development becomes less important with the adsorption temperature increasing, and the surface chemistry characteristics of modified biochars, particularly the formation of nitrogen functional groups, play a more active role at higher temperatures.^31,39–42

According to the relationship between the CO₂ adsorption capacity of modified biochars and their physicochemical properties, the modified biochars by the CO₂–NH₃ modification, obtaining the best micropore structure and the most nitrogen functional groups among these three different modifications, should also own the best CO₂ capture performance not only at the lower adsorption temperature, but at the higher. It has been confirmed by the results of CO₂ adsorption on modified biochars (shown in Fig. 5 and 7).

3.4. Comparisons with literature results

The comparisons of the CO₂ adsorption capacities for the different adsorbents are represented in Table 4. Different adsorbents, such as activated carbon, carbon nanotube, MCM-41, activated alumina, titanium oxide and amine-based sorbent modified by various kinds of agents (i.e. KOH, APTS, TEPA, DEA, DETA, etc.) have been reported. It can be seen that the CO₂ adsorption capacities of different adsorbents can be usually be enhanced after the improvement of physical and chemical properties by the modification. In this work, the modification of CO₂–NH₃ mixture gas commendably combines the advantages of both CO₂ activation and high temperature NH₃ treatment, not only improving the micropore structure, but also introducing the nitrogen functional groups, which is certainly conducive to the CO₂ adsorption. It may be helpful for the development of novel CO₂ adsorbents.

Table 4 Comparisons on the CO₂ adsorption capacities of different adsorbents

Adsorbent	Agents	SBET (m^2 g^−1)	Smic (m^2 g^−1)	T (°C)	P (KPa)	Cin of CO2 (vol%)	qe of CO2 (mg g^−1)	Reference
a APTS is 3-aminopropyl-triethoxysilane.b TEPA is tetraethylenepentamine.c DEA is diethanolamine.d DETA is diethylenetriamine.
Activated carbon	KOH	1260	1230	25	101	100%	212	22
Activated carbon	KOH	1260	1230	50	101	100%	158	22
Activated carbon	KOH	2400	—	23	101	100%	202	25
Carbon nanotube	APTS^a	198	—	25	101	50%	96	43
Carbon nanotube	APTS^a	198	—	25	101	10%	41	43
MCM-41	TEPA^b	5.3	—	70	101	15%	108	44
Activated alumina	DEA^c	205	—	35	101	10%	56	45
Titanium oxide	DETA^d	1037	—	75	101	10%	116	46
Titanium oxide	TEPA^b	28.6	—	60	101	15%	192	47
Amine-based sorbent	—	144	—	30	101	1%	85	48
CN800	CO2–NH3	491	534	30	101	10%	89	This work
CN800	CO2–NH3	491	534	120	101	10%	50	This work

---

4. Conclusions

The CO₂–NH₃ modification combines the advantages of the CO₂ activation and the NH₃ ammonification, which not only promotes the development of micropore structure of soybean straw modified biochars, but also increased the introduction of nitrogen functional groups. With the modification temperature increasing, the micropore surface area, micropore volume, nitrogen content and N/C ratio of the modified biochars by the CO₂–NH₃ modification all increase first and then decrease, and reach the maximum at 800 °C.

At the lower adsorption temperature, among the physicochemical properties of modified biochars, the micropore structure is the most important characteristic to influence their CO₂ capture performance. However, as the adsorption temperature increases, the effect of micropore structure gradually becomes less important, and the chemical properties, particularly the presence of nitrogen functional groups, are more noticeable at the higher adsorption temperatures.

Acknowledgements

The authors wish to express their sincere thanks to the financial support from the National Basic Research Program of China (2013CB228102), the National Natural Science Foundation of China (51576088 and 51506071), Special Fund for Agro-scientific Research in the Public Interest (201303095), and Fundamental Research Funds for the Central Universities. The authors are also grateful for the technical support from Analytical and Testing Center in Huazhong University of Science & Technology.

References

1. R. K. Pachauri and A. Reisinger, Climate change 2007 synthesis report: Summary for policymakers, IPCC Secretariat, 2007.
2. G. T. Rochelle, Science, 2009, 325, 1652–1654.
3. I. Pfaff, J. Oexmann and A. Kather, Energy, 2010, 35, 4030–4041.
4. B. Belaissaoui, D. Willson and E. Favre, Chem. Eng. J., 2012, 211–212, 122–132.
5. N.-S. Kwak, J. H. Lee, I. Y. Lee, K. R. Jang and J.-G. Shim, Energy, 2012, 47, 41–46.
6. M. S. Shafeeyan, W. M. A. W. Daud, A. Houshmand and A. Shamiri, J. Anal. Appl. Pyrolysis, 2010, 89, 143–151.
7. N. Sun, C. Sun, J. Liu, H. Liu, C. E. Snape, K. Li, W. Wei and Y. Sun, RSC Adv., 2015, 5, 33681–33690.
8. D. Adinata, W. M. A. W. Daud and M. K. Aroua, Bioresour. Technol., 2007, 98, 145–149.
9. X. Zhang, S. Zhang, H. Yang, J. Shao, Y. Chen, Y. Feng, X. Wang and H. Chen, Energy, 2015, 91, 903–910.
10. W. M. A. W. Daud and A. H. Houshamnd, J. Nat. Gas Chem., 2010, 19, 267–279.
11. J. P. Boudou, M. Chehimi, E. Broniek, T. Siemieniewska and J. Bimer, Carbon, 2003, 41, 1999–2007.
12. C. S. Lee, Y. L. Ong, M. K. Aroua and W. M. A. W. Daud, Chem. Eng. J., 2013, 219, 558–564.
13. A. Bhatnagar, W. Hogland, M. Marques and M. Sillanpää, Chem. Eng. J., 2013, 219, 499–511.
14. J. Przepiórski, M. Skrodzewicz and A. W. Morawski, Appl. Surf. Sci., 2004, 225, 235–242.
15. X. Zhang, S. Zhang, H. Yang, J. Shao, Y. Chen, Y. Feng, X. Wang and H. Chen, RSC Adv., 2015, 5, 106280–106288.
16. T. Horikawa, N. Sakao, T. Sekida, J. i. Hayashi, D. D. Do and M. Katoh, Carbon, 2012, 50, 1833–1842.
17. A. H. Basta, V. Fierro, H. El-Saied and A. Celzard, Bioresour. Technol., 2009, 100, 3941–3947.
18. L. Chunlan, X. Shaoping, G. Yixiong, L. Shuqin and L. Changhou, Carbon, 2005, 43, 2295–2301.
19. M. Fan, W. Marshall, D. Daugaard and R. Brown, Bioresour. Technol., 2004, 93, 103–107.
20. C. A. Toles, W. E. Marshall, L. H. Wartelle and A. McAloon, Bioresour. Technol., 2000, 75, 197–203.
21. B. Adeniran, E. Masika and R. Mokaya, J. Mater. Chem. A, 2014, 2, 14696.
22. M. Sevilla and A. B. Fuertes, Energy Environ. Sci., 2011, 4, 1765.
23. B. Adeniran and R. Mokaya, Chem. Mater., 2016, 28, 994–1001.
24. M. Sevilla, J. B. Parra and A. B. Fuertes, ACS Appl. Mater. Interfaces, 2013, 5, 6360–6368.
25. N. P. Wickramaratne and M. Jaroniec, J. Mater. Chem. A, 2013, 1, 112–116.
26. N. P. Wickramaratne and M. Jaroniec, Adsorption, 2013, 20, 287–293.
27. Z. Yong, V. Mata and A. r. E. Rodrigues, Sep. Purif. Technol., 2002, 26, 195–205.
28. S. Yaşyerli, I. Ar, G. Doğu and T. Doğu, Chem. Eng. Process., 2002, 41, 785–792.
29. M. G. Plaza, C. Pevida, C. F. Martín, J. Fermoso, J. J. Pis and F. Rubiera, Sep. Purif. Technol., 2010, 71, 102–106.
30. R. J. J. Jansen and H. van Bekkum, Carbon, 1994, 32, 1507–1516.
31. M. S. Shafeeyan, W. M. A. W. Daud, A. Houshmand and A. Arami Niya, Appl. Surf. Sci., 2011, 257, 3936–3942.
32. X. Zhang, S. Zhang, H. Yang, Y. Feng, Y. Chen, X. Wang and H. Chen, Chem. Eng. J., 2014, 257, 20–27.
33. Y. Suyadal, M. Erol and H. Oğuz, Fuel, 2005, 84, 1705–1712.
34. S. Hosseini, I. Bayesti, E. Marahel, F. Eghbali Babadi, L. Chuah Abdullah and T. S. Y. Choong, J. Taiwan Inst. Chem. Eng., 2015, 52, 109–117.
35. V. P. Mulgundmath, R. A. Jones, F. H. Tezel and J. Thibault, Sep. Purif. Technol., 2012, 85, 17–27.
36. H. Yang, Y. Yuan and S. C. E. Tsang, Chem. Eng. J., 2012, 185–186, 374–379.
37. M. G. Plaza, C. Pevida, A. Arenillas, F. Rubiera and J. J. Pis, Fuel, 2007, 86, 2204–2212.
38. C. Pevida, M. G. Plaza, B. Arias, J. Fermoso, F. Rubiera and J. J. Pis, Appl. Surf. Sci., 2008, 254, 7165–7172.
39. M. G. Plaza, F. Rubiera, J. J. Pis and C. Pevida, Appl. Surf. Sci., 2010, 256, 6843–6849.
40. T. Chen, S. Deng, B. Wang, J. Huang, Y. Wang and G. Yu, RSC Adv., 2015, 5, 48323–48330.
41. S. Deng, T. Chen, T. Zhao, X. Yao, B. Wang, J. Huang, Y. Wang and G. Yu, Chem. Eng. J., 2016, 286, 98–105.
42. X. Zhang, S. Zhang, H. Yang, J. Shao, Y. Chen, X. Liao, X. Wang and H. Chen, Proc. Combust. Inst., 2016 DOI:10.1016/j.proci.2016.06.062.
43. C. Y. Lu, H. L. Bai, B. L. Wu, F. S. Su and J. Fen-Hwang, Energy Fuels, 2008, 22, 3050–3056.
44. X. Wang, L. Chen and Q. Guo, Chem. Eng. J., 2015, 260, 573–581.
45. M. Auta and B. H. Hameed, Chem. Eng. J., 2014, 253, 350–355.
46. L. Ma, R. Bai, G. Hu, R. Chen, X. Hu, W. Dai, H. F. M. Dacosta and M. Fan, Energy Fuels, 2013, 27, 5433–5439.
47. F. Song, Y. Zhao, Y. Cao, J. Ding, Y. Bu and Q. Zhong, Appl. Surf. Sci., 2013, 268, 124–128.
48. L. He, M. Fan, B. Dutcher, S. Cui, X.-d. Shen, Y. Kong, A. G. Russell and P. McCurdy, Chem. Eng. J., 2012, 189–190, 13–23.

---