Activated carbon nanofibers incorporated metal oxides for CO₂ adsorption: Effects of different type of metal oxides

Highlights

• Effects of incorporation of different types of metal oxides on physicochemical properties of ACNFs composites.

• ACNFs/MgO exhibits the highest SBET and CO₂ adsorption capacity.

• CO₂ adsorption on the resultant ACNFs incorporated metal oxides involved physisorption and chemisorption, simultaneously.

Abstract

Activated carbon nanofibers (ACNFs) incorporated with four different types of metal oxides; magnesium oxide (MgO), manganese dioxide (MnO₂), zinc oxide (ZnO), and calcium oxide (CaO) were successfully prepared via a simple electrospinning and pyrolysis process. Optimum electrospinning and pyrolysis parameters were performed to obtain porous ACNFs composites for CO₂ capture. The porous and textural characteristics of the resultant ACNFs composites were performed using N₂ adsorption isotherms at 77 K, while the features and morphologies were observed using TEM and FE-SEM. The EDX and Raman analysis were used to determine and analyse the elemental composition in the ACNFs. It was observed that ACNFs incorporated MgO (ACNF2) exhibited the largest surface area (413 m²/g) and the highest micropore volume (0.1777 cm³/g) as compared to pristine ACNF (ACNF1) and other ACNFs composites. ACNF2 also possessed the smallest fiber diameter of 357.8 ± 16.7 nm as compared to other samples. The successful incorporation of all metal oxides in electrospun fibers were proven by EDX analysis. All resultant ACNFs exhibited D- and G-peaks in Raman spectra indicating the carbon-based materials structure. As expected, the ACNF2 attained the highest CO₂ adsorption of 60 cm³/g at 298 K as compared to other ACNFs samples which is correspond to N₂ adsorption capacity. The CO₂ adsorption/desorption isotherms of the best composite sample (ACNF2) was measured at three different temperatures (273, 298, and 318 K) at 1 bar through a volumetric adsorption process and this result was compared to ACNF1. It shown that the CO₂ adsorption capacity is inversely proportional to the increasing temperature in which as the adsorption temperature increased, the adsorbed amounts of CO₂ decreased. These results indicated that the incorporation of MgO into ACNFs shows the best improvement in their physicochemical properties for enhanced adsorption performance of CO₂ under practical conditions.

Introduction

Industrial processes and burning of fossil fuels in transportation sectors have become the main contributors of the anthropogenic carbon dioxide (CO₂) saturating in the atmosphere which contributing up to 82 % of total greenhouse gases (GHGs) that simultaneously increases every year since these past 40 years [1]. The increment of CO₂ gas percentage in the atmosphere can be toxic and very harmful to the humans which causes suffocation of living organisms as well as increases the Earth’s atmosphere temperature that leads to global warming and threaten the existence of life on this planet [2]. In early October 2020, Mauna Lao Observatory has reported that the estimated global daily emission of CO₂ is 413.17 ppm [3], which is believed at alarming rate and above the safe level as it is supposed to be below than 350 ppm. Since the COVID-19 outbreak earlier this year, there are speculations on the reduction of CO₂ concentration in the atmosphere and a report by Global Monitoring Laboratory stated that there is small reduction in CO₂ emission by about 0.2 ppm from March to April 2020 [4]. However, the open burning and forest fires are producing CO₂ at perhaps a similar rate as the modest lowering emissions resulting from pandemic. It does look like CO₂ continues to increase at the same rate as in previous years. These problems indicate that all parties should gather up their efforts in finding convenient solutions to tackle the global heating emergency.

Due to concerns on the excessive emissions of CO₂, major interests were focused in reducing the percentage of this gas before it was released to the atmosphere. Up to now, several ways have been implemented in order to minimize the CO₂ emissions from industrial and transportation sectors, including energy efficiency, fuel switching, combined heat and power, use of renewable energy, and the more efficient use the recycling of materials [5]. Many industrial processes and the use of fossil fuels in electricity generation have no existing low-emission alternative and will require carbon capture, storage, and utilization (CCSU) to reduce emissions over the long term. The CO₂ capture and storage are the technology that capture up to 90 % of CO₂ while CO₂ utilization involves the conversion of CO₂ into various carbon-containing chemicals and fuels such as CO₂ reduction through electrochemical methods [6,7]. In this study, for maximum reduction of emission, CO₂ need to be captured from its source point before it was emitted into the atmosphere such as from the exhaust of a combustion process known as post-combustion capture. There are several effective alternatives of CCSU that have been implemented in order to reduce CO₂ emissions including membrane separation, cryogenic separation, absorption, and adsorption [8]. Amongst those available methods, absorption of CO₂ using suitable solvent such alkaloamine was commonly used on industrial scale for decades. Regardless of its effectiveness, this method has a number of drawbacks such as high energy requirement and cost for amine regeneration, high equipment corrosion, and toxic to the ecosystems are the main concern for the continuous utilization of this technology [9]. In addition, most of the commercially used technologies are costly and energy intensive [10].

Adsorption that using porous solid adsorbent has attracted considerable attention for these past few decades as a viable alternative to the conventional CO₂ absorption. Adsorption was selected due to its cost-effectiveness, energy efficiency, and the ability to regenerate the adsorbents [11], which able to solve the problems associated with the previous technologies. Different porous adsorbents such as silica-based, zeolites-based, lithium orthosilicate (Li₄SiO₄), magnesium oxide (MgO), graphite oxide (GO), carbon-based, metal organic frameworks (MOFs) have been widely utilized in gas adsorption applications [12]. These porous adsorbents are more preferable due to their easiness of recovery and good recyclability [13]. Carbon-based adsorbents including activated carbon (AC), carbon nanotubes (CNTs), graphene, and carbon nanofibers (CNFs) have been widely utilized in gas adsorption applications due to their high specific surface area (S_BET), wide range of porous structures, and high adsorption capacity [14]. Out of those mentioned carbon-based adsorbents, activated carbon nanofibers (ACNFs) (fibrous form) have shown promising results in the gas adsorption performance contributed by their high S_BET, total pore volume (S_total), and micropore volume (S_micro). ACNFs is the combination of AC and CNFs which consists properties of both materials [15]. Their thread-like structure and small diameter with large S_BET and wide range of pore size distribution (PSD) have been found to be part of the main contributors for high adsorption capability [16].

In spite of enhanced properties of the newly developed ACNFs, however, the adsorption capacity of the pristine ACNFs is found to be lower or comparable to AC and other carbon-based adsorbents due their lower S_BET and S_total. Due to this, it is believed with some modifications on the ACNFs, their physicochemical properties can significantly be improved and consequently, enhance their adsorption performance. There are various works have been done in order to improve the properties of ACNFs and it was found that the incorporation of suitable types and amounts of nanofillers or additives can significantly enhance its physicochemical properties. Previously, it has been reported that metal oxides are highly sensitive to different gases along with high stability [17]. However, these metal oxides are poor in selectivity [18] and due to this, many alternatives have been proposed to improve the selectivity towards specific gas. Groups of metal oxides such as iron oxide (Fe₂O₃) [19], manganese oxide (MnO₂) [20], aluminium oxide (Al₂O₃) [21], magnesium oxide (MgO) [22], calcium oxide (CaO) [23], and zinc oxide (ZnO) [24] are widely known for their porous structure and high S_BET. Lately, numerous researchers have established metal oxides covering ACNFs to escalate liquid and gas adsorption by incorporating metal oxides to form composites ACNFs. Previous study conducted by Rafiq et al. (2014) [25] revealed that the utilization of MgO and ZnO nanoparticles owing to their high adsorption capacities, which is believed their incorporation in ACNFs in this study would significantly enhance their adsorption capacities toward CO₂ [25]. On one hand, the MgO is attractive due to its relatively low regeneration temperature (573−773 K) [26] and non-toxicity [27].

Meanwhile, Yang et al. (2018) has successfully fabricated composite adsorbents made of graphene oxide and MnO₂ with enhanced adsorption capacity [28]. CaO-based adsorbents have been selected due to abundance availability of its natural precursors such as limestones (CaCO₃) as well as its high reactive sorption capacity [29]. Additionally, the selection of these oxides is due to their excellent properties including cheap, environmental friendliness, abundance, and high stability. The incorporation of thermally stable metal oxides also improved the stability of the ACNFs, resulting ACNFs with higher yield as it can withstand higher temperature treatment or can be implemented in high temperature applications. The ACNFs composites were tested at low to moderate temperature to understand their CO₂ adsorption performance, similar to real condition for flue gases treatment.

Up to now, there are still limited studies focusing on the incorporation of metal oxides into ACNFs for gas adsorption applications. In this study, MgO and ZnO, and MnO₂ and CaO have been selected based on their different groups in periodic table, which is alkaline metal earth and transition metals, respectively to compare which metal group will produced the best composite ACNFs for CO₂ adsorption. Alkaline earth metal oxides such as MgO serve as a good candidate for CO₂ capture at moderate temperature due to their thermal-stable, low-cost wide availability and act as pore booster agent [30]. This current work is aiming to fabricate and characterize the physicochemical properties enhancement of the newly modified ACNFs composites incorporated with MgO as excellent CO₂ adsorbents. Pristine ACNFs and other metal oxides incorporated ACNFs were also used for comparison. The adsorption performance towards CO₂ was tested using a simple volumetric method. Their potential towards CCS (1 bar; 313−353 K) was also evaluated by varying their adsorption temperatures at 273, 298, and 318 K.