## 3 Experimental
### Capturing in-situ by-Product (CO\({}_{2}\))  
As shown in Equation (1), the formation of nitrate by anion exchange leads to the formation of carbonic acid, which exists in equilibrium with gaseous CO\({}_{2}\). Clearly, the creation of 1 mol CO\({}_{2}\) per 2 mol absorbed NO\({}_{2}\) does not make much environmental sense if the purpose of capturing NO\({}_{x}\) is to mitigate initially existing NO\({}_{x}\) emissions to minimise vehicle ICE-choice-driven transitions in CO\({}_{2}\) emission. Nor does it make sense to release CO\({}_{2}\) when, in the future, it will be necessary to meet the ICE and industrial NO\({}_{x}\) emission from idealised sustainable hydrogen burning, as discussed in the introduction to this paper. Therefore, capturing in situ production of CO2 within the filter must be seriously considered. There are a number of mineral options able to fulfil this function. Among them are intercalating minerals, such as sepiolite (porous hydrous magnesium silicate, palygorskite, best known as white \"clay\" for traditional pipe-making and conveniently able to be sourced locally in Serbia), or gas condensing microporous and mesoporous minerals, such as fused perlite (a glassy volcanic residue, sodium potassium aluminium silicate). The action of sepiolite in CO2 capture relies on the function of its -OH groups on the surface leading to acid-base intercalation of CO2, as shown in Equation (8). \[-(\text{OH}^{-})_{2}+\text{CO}_{2}\rightarrow-\text{CO}_{3}{}^{2-}+\text{H}_{ 2}\text{O} \tag{8}\]  
In contrast, fused perlite, with its myriad of ultrafine pores, induces gas condensation when confined within the pore network following the Thomson, Lord Kelvin effect [33]. Figure 28 shows SEM micrographs of the two minerals (sepiolite and fused perlite). Both the high porosity of fused perlite and the combination of the high aspect ratio needle-like particle shape of sepiolite and its high porosity result in increased volume fraction occupancy per unit weight addition when using either of these materials when compared to solid rhombohedral calcitic GCC. Therefore, the volume swept out during particle rotation during flow in aqueous suspension is also greatly increased when including proportions of such particles, such that the suspension exhibits increased viscosity, viscoelasticity, initial thixotropy after yield, and dilatancy under high shear. Such rheological properties would be unsuitable for most continuous flow metering systems but remain usable in the context of needle/pin coating and enable high loading volumes to be achieved across the localised pixelated coating distribution. Examples of CO2 capture in fused perlite and sepiolite when mixed at 10 w/w% with GCC are shown in Figure 29a,b, respectively. Given the two different mechanisms between the CO2 capturing materials proposed, sepiolite is expected to act selectively due to the chemical reaction required, whereas perlite adsorbs purely physically. These differences will manifest depending on the gas species under exposure. For example, sepiolite intercalates NO [34]. In this context, the preference of sepiolite for NO means that any NO initially intercalated in the mineral becomes exchanged for CO2. The apparatus set-up used to run sequential exposure to CO2 and subsequent flushing with synthetic air is shown in Figure 30.  
Figure 28: SEM images of (**a**) sepiolite, and (**b**) fused perlite with a schematic insert that demonstrates the pore condensation of gas. Figure 31 shows the air quality detector data since CO\({}_{2}\) is introduced into a filter previously exposed to NO\({}_{x}\)-containing ambient air using the method illustrated in Figure 30. The active filter coating comprised 90:10 by weight GCC: sepiolite in which the released selectively intercalated NO, flushed using synthetic air following the exchange with introduced CO\({}_{2}\), could be clearly observed, and there was little to no evidence of NO\({}_{2}\).