Nanoparticle Selection Criteria

The nanoparticle’s thermal stability plays an important role in this selection process. The nanoparticle is considered to be thermally stable if it does not decompose under the influence of temperature. In this case, the solvent regeneration usually occurs at temperatures above 100 °C. One method in determining the thermal stability of the nanoparticle is to use a thermogravimetric analyser (TGA). For this selection process, any weight loss due to moisture is neglected and only the weight loss of the material due to decomposition is reported. A scale of three was used to evaluate the thermal stability and is represented as low (T < 333 °C), medium (334 °C < T < 666 °C) or high (T > 667 °C).

Next, the toxicity of chemicals. This review focuses on the acute toxicity of the chemical. Acute toxicity is defined as the adverse changes that occur either immediately or a short time after a single point or short period of exposure to a chemical [94]. The term LD₅₀ was first introduced in 1927 [95], where it defined toxicity as a “median lethal dose”, which is the dose that would kill 50% of a large animal group. The classification for the toxicity of a chemical is based on the Recommended Classification of Pesticides of the World Health Organization (WHO) [96] and can be seen in Table 1. The classification distinguishes between the different levels of hazards based on the toxicity of chemicals. It is also based on the acute oral and dermal toxicity in rats, since their use is the standard in toxicology. The larger the value for LD50 is, the less toxic the chemical is, and, therefore, the smaller the value, the more toxic. For this review, the LD₅₀ is retrieved from the supplier of different nanoparticles and is categorized according to the classification in Table 1. For the summarization of the toxicity of chemicals, the terms “Extreme”, “Highly”, “Moderately” and “Slightly” are implemented.”

The major concern regarding the employment of nanoparticles in a base fluid is its stability. The stability of nanoparticles can be defined as its ability to agglomerate and has different key aspects to it, such as core composition, shape, size and surface chemistry [98]. The most common indicators of instability that occur are sedimentation and agglomeration [20]. Agglomeration is the act of the clustering of particles. For particles that are smaller than 100 nm in size, this can cause collisions, which can result in either an attachment or repulsion [99]. Surface chemistry, which is the variation caused by surface atomic and molecular density, chemical composition and potential, also leads to aggregation [99]. The types of interaction forces between the nanoparticles can be classified into two: the van der Waals attractive force (V_A) and the repulsive force (V_R). Both are functions of the distance that is between two particles in the base fluid. When the total potential energy is zero, the particles are then considered to be stable, as they are arranged at a stable distance. According to Wang et al. [100], the nanoparticle suspensions are more unstable at high CO₂ loading. As explained in the study, when the CO₂ loading increases, an ion that is formed from the reaction of amine and CO₂ interacts with the functional group on the nanoparticle’s surface. This leads to the negative-positive reversal of Zeta potential. Additionally, when the CO₂ loading increases, the ion concentration will compress the electric double layers of nanoparticles, thus reducing the stability of the nanoparticle. Improving the stability of the nanoparticle is an important issue to tackle, especially if the nanoparticles are utilized at an industrial scale. This is because the stability of the nanoparticles would directly affect the mass transfer performance [101]. The solvent can lose its ability of heat transfer if unstable, due to the nanoparticles being prone to agglomeration. The current methods proposed to improve the stability of the nanoparticles are the use of surfactants and surface modification techniques [102]. These methods are known to be easy and economically viable in order to improve the stability of the nanoparticles, but the effects on the CO₂ absorption and desorption are still undetermined [103]. Choi et al. [57] have studied the effects of adding surfactants and concluded that they can enhance the CO₂ absorption rate but interrupt the CO₂ desorption rate. Other methods to enhance the stability, other than the addition of surfactants, are the modification of the surface, pH control of the nanoparticles and stirring and ultrasonic agitation [21]. Apart from the stability of the nanoparticle in terms of its agglomeration, it is also important to evaluate the nanoparticle’s stability in terms of its core composition. Instability, in this case, refers to the chemical composition variations or coordination number changes [99]. Oxidation is a process that leads to a reduction in the catalytic performance of nanoparticles. Another aspect of nanoparticle stability is its morphology (shape and size) [104]. The instability of this aspect comes from its cluster size, as particles in colloidal solutions usually come into contact with each other, and clusters are formed due to the unstable particle collisions [59]. This alters the thermo-physical properties of nanoparticles for its application. The effect of interparticle distance, as reported by Ono et al. [105], reported that the sample with the largest interparticle distance showed higher stability against agglomeration. Therefore, apart from the initial nanoparticle size, their distribution on a substrate’s surface can improve the stability and lifetime of a nanoparticle. The shape also affects the nanoparticle’s stability, as it is related to the conservation of the original local structure and radius of the curvature of the nanoparticles [98]. It was reported that the morphological changes caused variations in surface facet percentages, which led to a reduction in the surface energy, due to the shape variation. It is important to study the stability of the nanoparticles, in terms of their morphology, as it directly influences the physiochemical properties of the nanoparticles.

Another important criterion is the recycling ability of the nanoparticle. Different nanoparticles display different recycling abilities, which would determine the nanoparticle’s reusability or the lifetime of the solvent. For this to be achieved, nanoparticles would be selected for the further study of the absorption-regeneration cycle. In order to identify the reasons for the reduction in the catalytic performance of the material, the recycled nanoparticle would be sent for characterization so that the structural properties of the nanoparticles could be proven to have been maintained [93]. Another method is also to compare the cyclic capacity of each cycle and if there is no significant decrease, then the nanoparticle is said to possess excellent stability and can be implemented in CO₂ capture [93]. Another study reported on the amount of CO₂ released (mol) for each regeneration cycle and is an indicator regarding the particle’s stability and ability to be recycled [106]. The changes in heat duty or energy consumption reported in each cycle can also be another indicator [107]. This is also closely related to the cyclic capacity of the material.

Foaming in amine plants causes an increase in the operating costs and would reduce the separation efficiency [108]. Pure amines do not actually lead to the formation of stable foams; therefore, one or more components in the solvent must be present in order to form a persistent foam [109]. The components added into the solution can impact either the foaming tendency or foaming stability. In the case of MDEA solvent, it has a lower foaming tendency but the employment of any additives that reduce surface tension can increase foam stability. Since foam tendency is also highly correlated to the type of solvent used, this important nanoparticle selection criteria will touch on foaming stability. Nanoparticles which do not enhance foaming stability will be reported in this review.”