## 3 Results and Discussion
### Acetaldehyde Removal Mechanisms  
Based on the 2D-GC TOF-MS analyses, we may propose the following formation reaction mechanism for acetanilide (\(\circleddelta\) in Figure 4):  
1. \(\mathrm{CH_{3}CHO_{(g)}\to CH_{3}CHO_{(AD)}}\) [adsorption]
2. \(\mathrm{CH_{3}CHO_{(AD)}+\frac{1}{2}O_{2}\to CH_{3}COOH}\) [Oxidation]
3. \(2\mathrm{CH_{3}COOH}\to(\mathrm{CH_{3}CO})_{2}O+H_{2}O\) [Dehydration]
4. \((\mathrm{CH_{3}CO})_{2}O+C_{6}H_{5}NH_{2}\to C_{6}H_{5}NCH_{3}CO-CH_{3}COOH\)  
First, the acetaldehyde can be physically adsorbed onto the active sites in the A@CACF-H. The active sites with oxygen (\"O\") and nitrogen (\"N\") on the A@CACF-H are allowed to make a hydrogen bonding with hydrogen in the water vapor and with the acetaldehyde molecules. In general, the water vapor in air may occupy the pores of the carbon material, which tend to hinder gas removal. In this study, the concentration of acetaldehyde is 2 ppm, and so the number of acetaldehyde molecules is significantly lower than that of the H\({}_{2}\)O molecules under a relative humidity of 40%. The acetaldehyde and H\({}_{2}\)O molecules competitively adsorb onto the micropores of the carbon material. As a result, the active sites in the micropores of the carbon material are blocked by H\({}_{2}\)O molecules. However, due  
Figure 5: Aldol condensation of acetaldehyde under basic conditions and formation of 2-methyl-quinoline followed by the Skraup–Doebner–Von [PERSON] and Mannich reactions.  
to its hydrophilicity, acetaldehyde tends to be caught in moisture (the thin water film on the surface) that penetrates into the micropores. Then, it can be oxidized into acetic acid on the active sites in the micropores. This acetic acid from the micropores may then spill over the surface, and react with the functionalized amine to form amide structures such as N-phenyl-acetamide or acetanilide [38].  
To verify the amide structure formation through the acetaldehyde-aniline reaction, the experimental conditions were changed from (a) O\({}_{2}\) 21% v/v under RH of 40%, to (b) O\({}_{2}\) 21% v/v under RH of 0%, and (c) O\({}_{2}\) 0% v/v under RH of 0%, as shown in Figure 6. Under the conditions of (c), the acetaldehyde-aniline reaction may not occur. A certain amount of the acetaldehyde was removed even without the O\({}_{2}\), indicating that a small amount of acetaldehyde can be captured by the functionalized amine. However, compared to the conditions with O\({}_{2}\) and/or H\({}_{2}\)O, it is not affordable to effectively remove acetaldehyde without an oxidation reaction.  
Hence, the dominant behavior of acetaldehyde adsorption over the adsorbent can be confirmed by the formation of amide species such as acetanilide. Generally, it has been speculated and reported, so far, that the chemical reaction between amine and aldehyde formed mainly amine species [33; 36; 37; 39]. In this study, we attempted to confirm the detailed and objective mechanism of acetaldehyde adsorption, using the high resolution 2D TOF-MS over the surface functionalized (amine) microporous media. As a result, amide species can also be considered as the main intermediates during the adsorption of acetaldehyde over the media.