Nitrogen doped heat treated and activated hydrothermal carbon: NEXAFS examination of the carbon surface at different temperatures

Abstract

Hydrothermal carbons have been shown to have controllable surface functionalization through various post-treatment techniques, which indicates these materials may be tuned for specific applications. For this reason, Near Edge X-ray Absorption Fine Structure (NEXAFS) studies have been conducted on a series of nitrogen doped and non-doped heat treated and activated hydrothermal carbons to further understand the changes in surface functionality with treatment. The NEXAFS carbon K-edge spectrum of the non-doped samples displayed a loss of oxygen functionalities (CO and COH) as well as the furan ring structure with increasing temperature, while CC bonds from graphitic groups increased. This effect was amplified further upon the addition of phosphoric acid (H₃PO₄) during activation. The doped hydrothermal carbons displayed similar functionality to the non-doped, although the effect of both heat treatment and activation was diminished. The nitrogen K-edge displayed characteristic peaks for pyridine and imines/amides, with pyrroles located under the broad ionization step. This work represents the first time a series of heat treated and activated hydrothermal carbons have been examined via NEXAFS spectroscopy. Additionally, difference analysis has been applied to the NEXAFS spectra to obtain a deeper understanding in the changes in surface functionality, a previously unused technique for these materials.

Introduction

The incorporation of nitrogen into carbon structures has been shown to improve the performance of carbon materials in CO₂ sequestration [1], catalysis and catalyst supports [2], [3] and electrochemical capacitors [4], [5]. Currently, several approaches have been developed for selectively doping nitrogen into carbon structures, such as electric arc, pyrolysis, hydrothermal and pre/post treatments with nitrogen sources (e.g., ammonia) [6], [7], [8], [9], [10]. Of these, hydrothermal carbonization has received increased attention due to its simplicity to form environmentally friendly, low cost, low temperature, nitrogen-doped, functionalized carbonaceous materials [8], [11], [12], [13], [14], [15], [16]. Unfortunately, the surface area of hydrothermal carbons has to date been observed to be considerably under-developed (10–100 m² g⁻¹) [16], [17], limiting their use in applications that are dependent on high surface areas (e.g., electrochemical capacitors) [18]. Thus, increasing their specific surface area through either physical or chemical activation is generally required before hydrothermal carbons are suitable for these applications.

The most commonly used activation method for hydrothermal carbons is chemical activation with potassium hydroxide (KOH) [12], [19], [20], [21], [22], [23], [24], [25], [26], [27], [28], [29], [30], [31], although sodium hydroxide (NaOH) [20], [32], phosphoric acid (H₃PO₄) [20], and physical activation via CO₂ [33], [34] have also been examined. Generally, the surface area of hydrothermal carbons increases with increasing temperature under activation, as well as concentration of activation agent [20], [21], [27], [35]. With regards to specific surface area, KOH and NaOH have been shown to produce the highest specific surface areas at 3420 [31] and 2455 [20] m² g⁻¹, respectively. Unfortunately, alkali hydroxides are not as economical as other activation methods, as the activation agents are not easily recovered in industry. Additionally, these activation methods have the potential to form potassium and sodium cyanides when activating nitrogen doped materials. Although the surface areas obtained are lower than alkali hydroxides, from an environmental and economical perspective phosphoric acid is the preferred activation agent. This is due to activation conditions being milder along with the ability to easily recover phosphoric acid after activation [36], [37].

With regards to nitrogen incorporation, the issue with activating nitrogen-doped precursors is the loss of surface functional groups with increasing temperature and activation agent. Thus, obtaining both high surface area and high levels of nitrogen incorporation is considerably challenging and a compromise between the two is typically required [38]. In addition to this, the analysis of nitrogen doped hydrothermal carbon is inherently difficult for lab-based techniques such as FT-IR and XPS. This is predominantly due to poor resolution in the spectra of the amorphous carbon structures, which typically predominate in hydrothermal carbons, and the tendency of peaks from oxygen and nitrogen functionalities to be superimposed [11], [16].

For this reason, the synchrotron-based technique Near Edge X-ray Adsorption Fine Structure (NEXAFS) technique constitutes an increasingly important spectroscopic technique to resolve surface functionalities on carbon structures. These publications have examined carbons in soil environments [39], absorbed molecules [40], chars [41], [42], [43], amorphous carbons [44], [45], carbon containing compounds [46], [47], films [44], [45], [48], [49], [50] and various carbon allotropes [51], [52]. Considering the large number of studies examining activated carbon for applications that are surface sensitive (i.e., electrochemical capacitors [53], there are currently very few publications utilizing NEXAFS to examine these materials.

A NEXAFS experiment consists of focusing intense, tuneable and highly polarized X-ray radiation onto the hydrothermal carbon to excite a core electron to an unoccupied or partly occupied valence level. The absorption of an X-ray photon is measured by following the annihilation of core holes via Auger or fluorescence yield, with Auger being the main processes typically examined in NEXAFS experiments. Since the precise energy at which the excitation process occurs is a function of the energy level of the valence band, the resulting spectra yields elemental-specific information on the local bonding environment within the material at a far greater resolution than XPS or FTIR. This technique has been previously shown to be highly effective at determining the functionality on the surface of hydrothermal carbon [54]. In this study, NEXAFS has been utilized to examine the effect of heat treatment and activation with H₃PO₄ on the surface of nitrogen doped and non-doped hydrothermal carbon.