Simultaneous activation and N-doping of hydrothermal carbons by NaNH₂: An effective approach to CO₂ adsorbents

Highlights

• N-doped porous carbons were synthesized by treating HTCs with NaNH2.

• NaNH2 enables simultaneous activation and N-doping of HTCs at 400˜600 ℃.

• NaNH₂-treated HTCs show excellent performance for selective CO2 capture.

Abstract

Synthesis of carbon materials with both large surface area and abundant heteroatoms is an important task in scientific research. Traditional approaches mostly proceed at extremely high temperature, and suffer from complex routes and/or expensive raw materials. NaNH₂ has been recently found to be able to not only etch C atoms to create nanochannels, but also substitute O species to introduce N species. In this work, an effective approach to N-doped porous carbons was realized by treating hydrothermal carbons (HTCs) with sodium amide (NaNH₂). The considerable amount of O species preserved in pristine HTCs enables the achievement in simultaneous activation and N-doping of HTCs by NaNH₂ at relatively moderate temperature (400˜600 ℃). The porous and chemical structure of NaNH₂-treated HTCs were characterized systematically. Although the pristine HTCs are almost non-porous and N-free, the specific surface areas and total N contents of prepared NaNH₂-treated HTCs reach 190˜2430 m²/g and 0.78˜6.57 wt.% respectively. Furthermore, the CO₂ capture performance of NaNH₂-treated HTCs was also examined considering their highly porous and N-doped nature. Interestingly, NaNH₂-treated HTCs exhibit high CO₂ capacities, large CO₂/N₂ selectivities, fast CO₂ adsorption rate and excellent recyclability, endowing them with potential application as solid adsorbents for CO₂ capture.

Introduction

Carbon materials have been extensively applied in research fields such as electrochemistry [1], catalysis [2] and separation [3], because they have many unique features such as high thermal stability, controllable porosity and tunable surface functionality [4]. An important target in this respect is to design and synthesize carbons with not only large surface area but also abundant heteroatoms. The increase in surface area is critical to enhance the exposure of active sites on carbons to guest molecules, while the inclusion of heteroatoms can change the surface electron distribution on carbons, thereby endowing them with specific functionality. Generally, there are two different approaches to heteroatom-doped porous carbons: (1) pyrolyzing carbon precursors containing heteroatoms, followed by chemical activation or physical activation [[5], [6], [7], [8], [9]]; (2) pre-synthesizing porous carbons, followed by treatment with compounds containing heteroatoms [[10], [11], [12], [13], [14]]. Traditional approaches mostly proceed at extremely high temperature (>600 ℃), and suffer from complex routes and/or expensive raw materials. Therefore, a more effective approach to heteroatom-doped porous carbons is highly demanded.

In our previous work, it was found that sodium amide (NaNH₂) can act as both chemical activation and N-doping agents for phenolic resin-based mesoporous carbons prepared by a soft-template method [15,16]. Inspired by our pioneering work, some researchers also utilized NaNH₂ for low-temperature activation and N-doping of other porous carbons very recently [[17], [18], [19]]. In principle, the activation mechanism mainly lies in the direct etching of C atoms, while the N-doping mechanism mainly lies in the substitution of O species. However, the activation effect is considerable only at relatively high temperature (>400 ℃), because the etching of C atoms proceeds more efficiently at elevated temperature. On the other hand, the N-doping effect is considerable only at relatively low temperature (200˜400 ℃), because NaNH₂ is subjected to severe decomposition at elevated temperature. The conflict defers the achievement in simultaneous activation and N-doping of carbon materials by NaNH₂. Furthermore, the carbon substrates previously used for NaNH₂ treatment are inherently mesoporous [15,18,19]; another question remains to be answered is whether the activation and N-doping functions of NaNH₂ still work effectively for non-porous carbon substrates?

Hydrothermal carbons (HTCs) are a class of carbon materials having experienced renaissance in materials science research, as they can be prepared from a wide range of renewable biomass under mild conditions [[20], [21], [22]]. Owing to the relatively low carbonization temperature (150˜250 ℃), considerable amount of O species can be preserved in resultant HTCs. Given that O species play an important role in the N-doping of carbons by NaNH₂, it is anticipated that HTCs are more appropriate to be used as substrates for NaNH₂ treatment, to overcome the gap existing between activation temperature and N-doping temperature. With this in mind, we proposed an effective approach to N-doped porous carbons through simultaneous activation and N-doping of HTCs by NaNH₂, as depicted in Scheme 1. The porous and chemical structure of NaNH₂-treated HTCs were characterized systematically in this work.

One of the most important applications of N-doped porous carbons is CO₂ capture [[17], [18], [19],[23], [24], [25], [26], [27], [28], [29], [30], [31], [32], [33], [34], [35], [36], [37]]. The emission of CO₂ from fossil fuels combustion has been a widely concerned issue, because the accumulation of CO₂ in atmosphere may contribute to a number of environmental issues such as rising of sea level and desertification of green land [38]. Traditional amine-scrubbing technology suffers from significant volatile loss of solvents, intensive energy consumption for desorption and strong corrosion to facilities [39,40]. Utilizing carbon materials as solid adsorbents for CO₂ capture is a promising option, to address the challenges confronted by amine scrubbing. Therefore, the CO₂ capture performance of NaNH₂-treated HTCs was also investigated systematically in this work.