3.2. Adsorption–Desorption Behavior
The mechanisms of the adsorption processes of ZnAc2
/ZnO/CAC_DCM and ZnAc2
/ZnO/CAC_OS were investigated in this work using multiple characterization techniques, such as SEM-EDX, BET, and FTIR. Characterization was performed three times for each grunting procedure. Each sample was labelled as described in Section 2.4
The morphological structures of fresh ZnAc2
/ZnO/CAC_DCM and ZnAc2
/ZnO/CAC_OS and the presence of elements on the adsorbents’ surfaces were performed using micrographic pictures and SEM mapping, respectively. Figure 3
depicts the morphological structure of each adsorbent below the 2.5K× magnification range and the 2-micron scale. The existence of colorful spots that form layers as a basis for the presence of chemical compounds on the surface of the adsorbent may be noticed in the ensuing image of the morphological structure observation. In general, the red spot on Figure 3
a refers to the deposited overall elements including Zn and C, while Figure 3
b shows the deposits of Zn, O, Si and C as presented by the blue spot. Additionally, the map shows the overall elements that were observed in detail, as presented by several other color spots.
The deposition of coating layers on the surface of the adsorbent is believed to enhance gas adsorption. Thus, adsorbent addition via the saturation approach alters the morphological structure, which is believed to reduce the natural pores (based on the raw CAC) that are capable of enhancing the H2
S-adsorbing surface area. In addition, observations of the adsorption–desorption cycle process were performed via data collected using EDX for the investigation of elements on the surfaces of DCM/CAC and CS/CAC for three complete cycles. Table 3
and Table 4
represent the elements that exist on the surface of the adsorbent depending on the mass percentage of the fresh adsorbent until the third adsorption cycle.
Among the elements found were carbon, calcium, zinc, oxygen, potassium, titanium, and sulfur. In accordance with the adsorbent obtained from carbon-type mesopores and the primary source of agricultural products, each adsorbent has carbon and calcium as components [43
]. Carbon is the primary medium for this adsorbent; therefore, the amounts of carbon present on the surfaces of both adsorbents were considerably larger than the amounts of other elements.
For the first cycle, the saturated adsorbent materials had a drop in zinc content (18.8% for ZnAc2/ZnO/CAC DCM and 25.9% for ZnAc2/ZnO/CAC OS). However, the total amounts of zinc elements required for breakdown dropped by 26% (ZnAc2/ZnO/CAC DCM) and 14.5% (ZnAc2/ZnO/CAC OS) in the third cycle. This drop is the result of the formation of ZnS on the surface of the adsorbent, which causes the decomposition of the decomposition cycle in preparation for the subsequent cycle.
The presence of moisture and oxygen has a substantial impact on effective adsorption. The reduction in the amount of oxygen element from fresh ZnAc2/ZnO/CAC_DCM to the saturated adsorbent in the first cycle was 28.6%, and the overall reduction in the amount of oxygen element in the third adsorption cycle was around 27%. The loss of oxygen during the first cycle in the saturated ZnAc2/ZnO/CAC_OS was 27%, whereas the loss of oxygen element during the entire process was only 15%. Compared with ZnAc2/ZnO/CAC_DCM, ZnAc2/ZnO/CAC_OS exhibited a lower oxygen element reduction, which is an indication that ZnAc2/ZnO/CAC_OS can be utilized more than once.
The presence of potassium and titanium elements was only detectable in ZnAc2/ZnO/CAC_OS when KOH and TiO compounds were used to synthesize the adsorbents During adsorption, K2S and TiS were formed, resulting in a decrease in saturation level. The drop was 16% for potassium element and 42% for element titanium element in the adsorption phase. This outcome is likely due to the fact that the TiO2 layer is the outermost layer following the TEOS layer, which is strongly bonded to the sulfur material. This result indicates that other layers may be reused multiple times and adsorb H2S gas over time. In addition, the fact that the silicon element on the outer layer decreased by 59% during the third adsorption cycle suggests that hazardous H2S gas is likely to damage the silica layer.
One may see an increase in ZnS production with each desorption step with increasing adsorption–desorption cycles. However, the ZnS increase in the ZnAc2
/ZnO/CAC_DCM was more than that in ZnAc2
/ZnO/CAC_OS (4.04% and 1.38%, respectively). The increase in ZnS on the surface of ZnAc2
/ZnO/CAC_DCM may indicate a decline in performance over the subsequent cycle. Even after the adsorption process was completed, the sulfur substance, which affects the surface of the adsorbent, was difficult to remove. The presence of sulfur following the adsorption process was likewise consistent with the results of a previous study [43
depicts the adsorption of N2
gas as a function of pore volume in ZnAc2
/ZnO/CAC_DCM. According to the IUPAC classification, the isothermal adsorption of N2
gas exhibits type I(b) isotherm with type IV hysteria, indicating that the adsorbent is formed by micropores [15
]. Fresh adsorbents have a lesser volume of N2
adsorption than saturated and diabetic adsorbents. The adsorbent becomes saturated and increases in surface area, pore volume, and pore size during H2
S gas adsorption, because the micropore area is blocked with metal compounds and H2
The desorbed adsorbents (D1, D2, and D3) showed a decrease in the volume of N2
adsorption compared with the saturated adsorbents (T1, T2, and T3). The procedure of dissolving at 150 °C and the combination of air regeneration agents and N2
demonstrated a positive influence on the next cycle’s adsorption efficiency. This decrease correlates with the findings of Shi et al. [47
], who observed a decrease in adsorbent (AC) decomposition throughout the N2
adsorption study. In addition, pore structure was detected via the computation of isothermal N2
adsorption, and average pore volume was estimated using N2
adsorption at the relative pressure of 0.98. Micropore volume was also determined using the t-plot method. Table 5
provides a summary of the comprehensive pore structure, including the BET surface area (SBET
), average pore volume, micropore area, and pore size.
A study of the BET surface areas and adsorbent pore counts for each of the adsorption–desorption processes revealed a decrease in comparison with ZnAc2
/ZnO/CAC_DCM (F), with the exception of D1 and D3 adsorbents. The considerable drop in N2
adsorption volume was probably caused by metal particles obstructing the surface of the pores in the deepest pores [48
]. Various essential issues must be addressed, such as the assessment of adsorption material regeneration, to keep the effectiveness of H2
S gas adsorption for a period of the adsorption-desorption cycle. This finding is a result of the vapor’s probable performance degradation with each grunt cycle. The loss of surface area and obstruction of active sites on the surface of adsorbents because of multiple vapor–pore cycles may lead to a potential reduction in the efficiency of H2
S gas adsorption.
The micropore area of the T1 adsorbent increased compared with the ZnAc2/ZnO/CAC_DCM (F) adsorbent owing to the presence of H2S particles within the pores of the adsorbent. The rise in micropore area and pore volume is indicative of H2S gas adsorption. The adsorbent in T1 phase was then degraded, and a decrease in surface characteristics was observed in the D1 adsorbent. The reduction in micropore area was 26% compared with that of fresh raw CAC. This result indicates that the adsorbents of type ZnAc2/ZnO/CAC_DCM may sustain 74% adsorption capability in the subsequent cycle. Compared with ZnAc2/ZnO/CAC_DCM, the adsorbent’s performance degraded through the micropore area at 1.2% and 3.1% during the second and third adsorption cycles, respectively.
The presence of metal in an adsorbent has no effect on the pore size factor. However, in this study, things are different. H2S gas adsorption is impacted by variations in pore size. In comparison with fresh and dehydrated adsorbent, the saturated adsorbents (T1, T2 and T3) had smaller pores. This finding demonstrates how well this adsorbent works. The results of the adsorption–desorption cycle, which are deteriorated for each cycle, can be matched by the low adsorption capability of this low pore size.
Furthermore, surface characterization and N2
gas adsorption investigations were performed on ZnAc2
/ZnO/CAC_OS materials to investigate its adsorption–desorption processes. Its N2
isotherm adsorption based on IUPAC was identical to the type 1 investigation. ZnAc2
/ZnO/CAC_OS (F) was compared with its saturated (T1, T2 and T3) and desorbed counterparts (D1, D2, D3). All saturated materials (T1, T2 and T3) exhibited higher N2
adsorption than the fresh adsorbent as displayed in Figure 5
. The result indicates that the H2
S gas adsorbent on the ZnAc2
/ZnO/CAC_OS surface has a higher surface area, micropore area, and pore volume for adsorption [37
adsorption–desorption isotherm has acted upon the pore structure characteristics of ZnAc2
/ZnO/CAC_OS that experienced three adsorption–desorption cycles. Each adsorption-desorption procedure of ZnAc2
/ZnO/CAC_OS was calculated using BET analysis. Nitrogen intake at P/P0
= 0.98 was used to calculate the pore volume. The volume of micropores was determined using the t-plot approach. Table 6
provides a summary of the detailed pore structure of the parameter, including SBET
, micropores volume, and pore size.
SBET increased after H2S gas adsorption. The increase in SBET for ZnAc2/ZnO/CAC_OS (T1) was 13% more than that for ZnAc2/ZnO/CAC_OS (T2) compared to ZnAc2/ZnO/CAC_OS (F). Among the exhausted adsorbents, ZnAc2/ZnO/CAC_OS (T1) exhibits a 19.1% decrease in SBET compared with ZnAc2/ZnO/CAC_OS (D1). This phenomenon can be observed in the next adsorption–desorption cycle. Furthermore, the ZnAc2/ZnO/CAC_OS (D1) had an 8.6% reduction in active site, which decreased the SBET values compared with ZnAc2/ZnO/CAC_OS (F) and decreased the pore size by 0.21%. Compared with ZnAc2/ZnO/CAC_OS (F), ZnAc2/ZnO/CAC_OS (D2) and ZnAc2/ZnO/CAC_OS (D3) were diminished by 5.1–5.7%. Hence, the reduction in ZnAc2/ZnO/CAC_OS in three adsorption–desorption cycles was stable and not excessive.
/ZnO/CAC_OS, in particular, is able to maintain a micropore area of 93.7% following a regeneration procedure compared with the micropore of fresh adsorbents, and its reduction in the next 2–3 cycles was negligible. This micropore area was substantial in the H2
S gas adsorption process because it contains active sites that function effectively for effective H2
S adsorption. Furthermore, the amount adsorbed in ZnAc2
/ZnO/CAC_OS pores contributed to the reduction in surface area. Thus, chemical interactions with a reduction in micropore area and physical adsorption influence the H2
S removal capability of the ZnAc2
/ZnO/CAC_OS surface, which resulted in a decrease in pore size. This result is consistent with the findings of Wang et al. [49
], Shi et al. [47
], and De Oliveira et al. [15
], who discovered that the regeneration of this adsorbent is dependent on the micropore area.
After the adsorption–desorption process, a reduction in pore volume makes the chemical adsorption process for H2
S hazardous to the adsorbent [15
]. Chemical adsorption is influenced by a functional group of hydrophilic carbon, surface pH, and the presence of moisture film on the surface of the adsorbent; therefore, the reduction in pore volume can be demonstrated by a number of other characterization methods [15
This chemical adsorption can be demonstrated by identifying the functional groups on the surface of the adsorbent using FTIR analysis. The comparison of these functional group pathways is attributable to the H2
S gas adsorption mechanism of two adsorbents: ZnAc2
/ZnO/CAC_DCM (Figure 6
) and ZnAc2
/ZnO/CAC_OS (Figure 7
). Samples were collected at each adsorption (T1, T2, T3) and desorption step (D1, D2 and D3) to identify the functional groups on the surface of the adsorbent.
Few repeating peaks were identified and previously addressed. These peaks correspond to the hydroxyl group (OH), alkene (C–H), and carbonyl groups (C=O). Additionally, the chemical impregnated onto the adsorbent surface with metal oxide materials, such as ZnO, was also discovered at peaks between 1400 and 1600 cm−1
]. The presence of oxygen produced peak differences in the chemical adsorbents of ZnO, NH3
, TEOS, KOH, and TiO2
, showing that functional groups, such as carboxylic acid (O–H), ether (strain C–O–C), hydroxyl (O–H), acid anhydride (RC [=O] OC [=O] R), and the aromatic group (C=C ring) are at the peak wavelengths of 1550–1200 cm−1
The peak detection at 405 cm−1
for the Zn–C bond shows evidence of the interaction between the phase of metal material that was linked with the CAC [54
]. The 470 cm−1
apex corresponds to Zn–O vibration [55
]. Furthermore, the peaks at 418 cm−1
and 1400–1600 cm−1
are reserved for O–H and Zn–O, respectively [51
]. The Zn–O path, however, vanished after the desorption process. After the adsorption procedure, the Zn–S functional group was found on the vibration band at 961 cm−1
]. However, the metal oxide route returns in the FTIR spectrum of the adsorbed substance. This finding demonstrates that the chemical link between the metal atom and atomic oxygen is broken during the desorption process, which is consistent with the earlier EDX research.
The carbon surface’s hydroxyl groups (OH) promote SO2
binding through ion–dipole interactions with COS. Therefore, the activity and selectivity of the concentrated compounds on precursors, such as AC, are remarkably impacted by the presence of functional groups in the oxidation reaction of H2
S catalysts [57
]. The Zn–S vibration band may be seen at 961 cm−1
]. The peak also supports the decryption of the EDX characterization of the sulfur element on the adsorbent, which states that the chemical link between the metal and oxygen atom is changed to the metal and sulfur atoms during the sulfidation process.
Adsorbents that have adsorbed gas under saturated circumstances (T1, T2 and T3) can be found in a variety of functional groups when compared with fresh adsorbents, particularly ZnAc2/ZnO/CAC_DCM (F). H2S gas adsorption exhibits numerous peaks involving sulfur bonds, including sulfate (S=O), sulfur oxide (S=O), and sulfonate (S=O), having peaks of 1415–1380, 1070–1030, and 1372–1335 cm−1, respectively. Peaks for weak aromatic groups with C–H swelling developed along the 2000–1650 cm−1 peak. The wavenumbers for saturated and diabetic adsorbents (D1, D2, and D3) were identical in terms of the functional groups present but differ in terms of wavenumber percentage of intensity. The percentage of intensity for saturated adsorbents was often higher than that for the adsorbed adsorbent.
Furthermore, additional functional groups for ZnAc2
/ZnO/CAC_OS were added, such as N–H bonding of amine salt (3000–2800 cm−1
), C–N bond strain weak for nitrile group (2260–2222 cm−1
), N–H swelling band at 1650–1580 cm−1
, the path of thiocyanate group with strong S–CN stretching (2175–2140 cm−1
), and isotiocyanate (2140–1990 cm−1
). The 1086 cm−1
strip depicts cyclic Si–O–Si bonding, whereas the 473 cm−1
strip depicts the SiO pathway [58
]. The ZnAc2
/ZnO/CAC_OS adsorbent had a lower intensity band than the ZnAc2
/ZnO/CAC_DCM adsorbent, which has an amine functional group on the adsorbent. This amine group is thought to protect the Si–OH group from stretching vibrations and improve the adsorbent’s thermal stability on the adsorbent’s surface. It is associated with asymmetric bending and primary amine symmetry (–NH2
) in the 1564 and 1480 cm−1
bands and secondary amine bending (–N[R]H) in the 1652 cm−1
]. Finally, the functional group was analyzed in this adsorption process to illustrate the gas adsorption process via chemical adsorption, in which the groups interact with sulfur material on each saturated adsorbent and decompose.
3.3. Adsorbent Performance in H2S Adsorption–Desorption
The adsorption–desorption process was analyzed using ZnAc2
/ZnO/CAC_DCM and ZnAc2
/ZnO/CAC_OS to determine adsorption through the regeneration of saturated adsorption materials. Each adsorbent underwent desorption at 150 °C. The profile of ZnAc2
/ZnO/CAC_DCM in three adsorption–desorption cycles is shown in Figure 8
Each of these adsorbents performed well in the first adsorption–desorption cycle. This outcome is most likely due to the adsorbent’s surface pores having a high active site. In the first cycle, the amounts of H2S gas adsorption were approximately 1.75, 1.00, and 0.75 mg H2S/g for ZnAc2/ZnO/CAC_DCM, ZnAc2/ZnO/CAC_OS, and raw CAC, respectively. The second adsorption performance was observed after the first desorption process. The adsorption capacity in the second cycle decreased by 35.4% for ZnAc2/ZnO/CAC_DCM. While, 25% and 22.7% decrease were recorded in the second cycles of ZnAc2/ZnO/CAC_DCM. However, in the third cycle, the H2S gas adsorption of ZnAc2/ZnO/CAC_DCM and raw CAC kept decreasing by another 20.4% and 6.9%, respectively, whereas ZnAc2/ZnO/CAC_OS showed no changes in adsorption capability for current cycles.
In conclusion, ZnAc2
/ZnO/CAC_DCM had the capability to adsorb more H2
S gas compared with other adsorbents; however, this adsorbent seems difficult to regenerate, as its adsorption capacity dropped drastically and was obviously not stable for longer adsorption–desorption cycles. However, ZnAc2
/ZnO/CAC_OS performed better and was more stable compared with ZnAc2
/ZnO/CAC_DCM and raw CAC. The decrease in adsorption capability with increasing adsorption–desorption cycles is caused by chemical interactions that reduce the adsorbent’s surface catalytic activity [59
]. The interaction between sulfur and the adsorbent surface is one of factors that reduces H2
S adsorption capability. Additionally, the production of stable sulfur polymers impairs adsorbent regeneration.
Ozekmekci et al. [60
] suggested the application of a higher temperature in the desorption process to increase the adsorption capability in the next cycle as a good regeneration procedure that demands high temperatures of 500–600 °C in inert conditions. However, the researcher also advised to keep the adsorbent’s surface structure and effectiveness at temperatures below 400 °C [60
]. Furthermore, based on the TG analysis of Zulkefli et al. [37
], this temperature range might change the structure of the adsorbent’s surface, which could lessen its ability to adsorb H2
The successful adsorption–desorption of H2
S in three cycles motivated us to observe further the capability of adsorbents until the maximum cycles. Raw CAC, ZnAc2
/ZnO/CAC_DCM, and ZnAc2
/ZnO/CAC_OS were evaluated for adsorption–desorption until they were saturated and could no longer be regenerated. Figure 9
depicts the comparison profile of the adsorbent up to the last adsorption–desorption cycle.
According to Figure 9
, the raw CAC was capable of regenerating up to the 9th cycle with a final adsorption capability of 0.04 mg H2
S/g. In comparison, ZnAc2
/ZnO/CAC_DCM and ZnAc2
/ZnO/CAC_OS had outstanding adsorption capabilities with 12 and 30 cycles, respectively, and their final adsorption capacities were 0.04 mg H2
S/g. Hence, ZnAc2
/ZnO/CAC_OS showed the most prominent stabilization with a large number of cycles in the regeneration process. The stability of its adsorption–desorption cycles were 70% and 60% longer than those of raw CAC and ZnAc2
/ZnO/CAC_DCM, respctively. As a result, ZnAc2
/ZnO/CAC_OS had a higher number of regeneration cycles. Its adsorption capability can be further investigated to enhance its adsorption capacity in a longer regeneration cycle.