Impact of ammonia treatment and platinum group or nickel metal decoration on the activated carbon storage of carbon dioxide and methane

Greenhouse gases, chiefly carbon dioxide (CO2) and methane (CH4), emission is responsible for the global warming and heat waves which strike the world causing floods and droughts everywhere with more CO2 attributions. The adsorption and desorption capacities of CO2 and CH4 at room temperature and up to 5.0 and 100 bar, respectively, were investigated for the untreated and ammonia-treated activated carbons (ACs), metal-anchored (metal: Ru, Rh, Pd, Ir or Ni) samples. We merged ammonia treatment and metal decoration to discover their influences on the CO2 and CH4 storage capability of ACs and the potential use of such modified ACs for capturing greenhouse gases and purifying natural gas from CO2. The CO2 storage capacities ranged between 25.2 and 27.7 wt% at 5.0 bar with complete regeneration upon desorption, while the uptakes for CH4 were in the range of 9.6 − 12.6 wt% at 35 bar with hysteresis behavior of the adsorbed gas. The highest adsorption capacities were achieved for the pristine samples, showing that metal decoration reduced slightly the adsorption. Ammonia-treated samples showed minor enhancing effect on the CH4 adsorption in comparison to the CO2 adsorption. The higher adsorption capacities of CO2 than those of CH4 could be employed for upgrading the natural gas, while the 9.6 wt% (2.2 mmol g−1) CO2 storage capacity would allow for its removal from the flue gases at ambient temperature and pressure. The higher adsorption capacity and preferentiality of CO2 over CH4 could be attributed mainly to its higher quadrupole moment and its higher clustering above the AC surfaces, while a minor effect, if any, would be attributed to the modifications of the ACs, implying that physisorption mechanism acted significantly in the adsorption process in comparison to chemisorption mechanism at the studied conditions.


Introduction
The increase in the concentrations of greenhouse gases [carbon dioxide (CO 2 ), methane (CH 4 ) and nitrous oxide (N 2 O)] is the main reason for global warming as they trap the Sun's radiation within atmosphere [1,2]. Their concentrations were increased dramatically in 2020 in comparison to the pre-industrial levels in 1750 by 149% for CO 2 to reach 413.2 ppm, while the increase was by 262% for CH 4 and 123% for N 2 O [3]. Although the global warming decreased from 3.5°C in 2009 to 2.7°C in 2015 and to 2.6°C in 2020, but it is still higher than its target of 2.1°C [1].
In Paris summit in 2015 [4], the participating governments agreed on the target of net zero emission for reducing the global warming to 1.5°C. For achieving this target, many countries announced different green controlled through chemically or physically activated high carbon contents precursors such as petroleum coke [46], coal [47], agricultural waste materials of rice husk [48], peanut shell [49], fruit stones and seeds [50,51] and industrial wastes of used tires [52] and polymers [53]. In addition, they can be synthesized from unconventional materials such as sewage sludge, which have positive impact on environment in many aspects as waste disposal and greenhouse gases adsorbents [54].
The ACs are composed of flat hexagonal array of aromatic carbon rings, forming the basal plane sheets (graphene layers) which are broken in different locations to form pores with different shapes and are arranged in a three-dimensional crystalline or mostly amorphous structure [55]. The graphene layers have high electron density regions [56] due to the un-hybridized 2p orbitals, which participate in the π-bonding, providing the AC with its Lewis base character [57]. ACs could be synthesized and modified to enhance their adsorption capacity [58] by doping with heteroatoms such as nitrogen, sulfur, and oxygen, etc, or changing their surface acidity or basicity by attaching acidic functional groups such as carboxylic acid groups through oxidation reactions or basic functional groups of nitrogen moieties such as amine groups through reduction reactions [59,60]. Furthermore, decorating the ACs surfaces with metal particles may introduce additional active adsorption sites for gas molecules [61,62]. The CO 2 fixations or adductions with transition metals were reported for the CO 2 gas molecules, where the CO 2 molecule was inserted within the structure to form metal carbon complexes (M-CO 2 ) [63][64][65][66][67]. These interactions were reported for the platinum group metals such as rhodium (Rh) [61,68], ruthenium (Ru) [69], iridium (Ir) [70], and palladium (Pd) [71,72] as well as nickel (Ni) [73][74][75]. They all showed good interactions with CO 2 , but their interactions could be considered as electron acceptor-donor interactions [76], where the CO 2 carbon atom was susceptible to nucleophilic attack [67]. On the basis of the above literature, we combined ammonia treatment and metal decoration and to explore their effects on the carbon dioxide and methane storage capacity of activated carbon and the potential use of such modified activated carbon for capturing greenhouse gases and upgrading natural gas by the selective removal of carbon dioxide.

Experimental section
The preparation and characterization of all adsorbents were published in our previous article papers [32,33] and were investigated for hydrogen adsorption and desorption measurements at both room and cryogenic temperatures [32]. In brief, the commercially available Norit AC was doped by 2.0 wt% nitrogen via heat treatment under ammonia flow at 850°C. A platinum group metal (Ru, Rh, Pd, Ir) or Ni was loaded as 5.0 wt% on either untreated or ammonia-treated ACs by wet impregnation, followed by reduction under hydrogen atmosphere at 350°C . They were characterized by using a JEOL X-ray fluorescence (XRF; JSX-3202M), Japan. Nitrogen isotherms were generated by using a Micromeritics Gemini VII, 2390 analyzer, USA. X-ray powder diffraction (XRPD) was recorded on a Rigaku MiniFlex 600, Japan. Elemental analysis (CHNS) was accomplished by using a PerkinElmer 2400 Series instrument, USA. Morphology was studied by using a JSM-7600F Schottky Field Emission Scanning Electron Microscope (FE-SEM), Japan, equipped with INCAx-act energy-dispersive x-ray (EDX) spectroscopy, Oxford instrument, England. The storage capacity measurements were evaluated with high-pressure volumetric analyzer HPVAII (Micromeritics), USA. The CO 2 and CH 4 storage uptakes were assessed by measuring the pressure-composition-isotherms (PCI) at RT and up to 5.0 bar for CO 2 and 100 bar for CH 4 . The PCI curves were measured 5 times for validity and repeatability of the results, where the details of characterizations were elucidated in [32,33].

Results and discussion
Carbon dioxide uptake Figures 1-5 show the PCI adsorption-desorption isotherms at 298 K, before and after ammonia treatment, for CO 2 adsorption and desorption isotherms of the Ru-, Rh-, Pd-, Ir-and Ni-anchored ACs, respectively. Table 1 summarizes the excess CO 2 storage capacities for all of the samples at both 1.0 and 5.0 bar. All the PCI isotherms were type-I of IUPAC classification [77,78], where the storage capacities increased gradually with pressure, without the observation of saturation limit up to 5 bar, indicating higher achievable storage capacities at higher pressures. The shape of the isotherms could represent the shape of the empirical Freundlich isotherm model [79,80], supported by Freundlich data fitting for CO 2 adsorption, reported by Singh et al [81]. Total reversibility, without hysteresis, was achieved after removing the pressure during the desorbing cycle, indicating complete regeneration of CO 2 .
Ammonia treatment, which eliminated the surface impurities and amorphous carbon species from the carbonaceous structure [102][103][104], showed minor improvement of carbon dioxide uptake with very slight difference from one sample to another. Anchoring the transition metals on the ACs skeleton did not improve their CO 2 uptake, but uptake reduction was observed, which could be attributed to the counter-effect of metal pore-blocking and the higher density of the metal-anchored ACs than the pristine ones [40,105]. The high strength of the double bonds in CO 2 molecule of 803.75 kJmol −1 [9] would prevent its cleavage over the  transition metals surfaces, at the studied conditions, which supported the presence of CO 2 configuration as unactivated flat geometry onto the porous materials without being modified [106]. Even though, our metalanchored samples showed superior CO 2 storage capacity in comparison to 0.856 mmol g −1 storage capacity for some metal oxides−decorated ACs [107]. Figure 6−10 show the PCI adsorption-desorption isotherms at 298 K, before and after ammonia treatment, for CH 4 adsorption and desorption of the Ru-, Rh-, Pd-, Ir-and Ni-anchored ACs, respectively. The excess CH 4 storage capacities at 5, 35 bar, and at saturation plateau are briefed in table 2. All the PCI isotherms were of type-I of IUPAC classification [77,78], where the storage capacities increased with pressure until the adsorption plateaus started at ∼50 bar.  The hysteresis phenomenon, with no tangential trajectories between the adsorption and desorption, could be attributed to the meta-stability of CH 4 gas molecules within the pore network [77]. Retaining of some CH 4 gas upon pressure removal could be attributed to some chemical interactions as cushion gases with AC's surfaces [29]. At 35 bar, the CH 4 storage capacities of our various ACs ranged between 9.6 to 12.6 wt% (an average of ∼11.1 wt%), which were equivalent to 6.0-7.9 mmol g −1 (an average of ∼6.9 mmol g −1 ). Our reported CH 4 storage values were superior as 7 folds as of the storage capacities of ACs, reported by Kumar et al [108]. They were in consistence with the reported values of different ACs [29,92,94,[109][110][111]. In addition, they showed better storage capacities values than the 8.9 wt% storage capacity value, reported for MOFs [112]. Our values were lower than the storage capacities values of 8.3 and 9.0 mmol g −1 , reported for the higher surface area Maxsorbs ACs samples [91,94]. Ammonia treatment showed a little bit enhancement, above 20 bar, for almost all the samples, which could be accredited to its erosion effect on the AC surface [102][103][104]. Anchoring the transition metals on the ACs decreased the CH 4 uptake, attributed to the counter-effect of metal pore-blocking and the higher density of the metal−anchored ACs in comparison to the pristine ones [40,105]. The high bond strength for CH 4 molecule, with a value of 429 kJmol −1 [113], would result in high thermodynamic stability of the CH 4 bonds to be dissociated by the transition metals, as it was supported by the difficulty of its dissociation  on nickel surfaces [114]. Furthermore, the high tetrahedral symmetry of the CH 4 gas molecules may prevent the formation of polar interactions on the surface of the transition metals [115] . At 5.0 bar, the storage capacity of CO 2 was as∼6 folds as that of CH 4 and as 530 folds as that of hydrogen at ambient temperature [32], a feature enables the upgrading of natural gas as clean fuel sources after the removal of CO 2 contamination by physical adsorption. While purifying hydrogen from CO 2 impurities is essential, it would be so beneficial for numerous industries such as steam reforming of methane (SRM) [116,117], water gas shift (WGS) [118,119], and ammonia synthesis [120,121].

Methane uptake
This high adsorption preference of CO 2 in comparison to CH 4 cannot be justified through the Lewis acidbase interactions between the acid gases and the Lewis base sites of graphene basal plans [57,122,123]. The CO 2 has a basicity of 515.8 kJ mol −1 and a proton affinity of 540.5 kJ mol −1 [124,125], which are little lower than the basicity of CH 4 of 520.6 kJ mol −1 and proton affinity of 543.5 kJ mol −1 [124,126]. Such small difference in acidity between the two gases would not give CO 2 gas such adsorption superiority over CH 4 gas in this context. Even though, a proportional correlation was reported between the acidity of C-H bond and its electrostatic interactions with the resonating π electrons of the graphene basal plane [127,128], which would increase upon its coordination with the 3d metal ions [129]. However, in our study, the case did not appear to be the same due  to the absence of any interactions between the anchored metals and the CH 4 molecules. In addition, the high critical temperature (T C ) of CO 2 gas of 304 K, in comparison to the lower critical temperature of 190 K for CH 4 [130], would allow CO 2 molecules to be easily liquefied and to act as a condensed liquid instead of volatile supercritical gas on the AC's surfaces. Nevertheless, CO 2 did not condense, in our study, because we performed our experiments at pressure much lower than the critical pressure (P C ) of 64 bar for CO 2 gas at 298 K, where the CO 2 gas molecules existed in their vapor form [131].
In spite of the overall non-polarity of both CO 2 and CH 4 molecules due to their molecular symmetry with 0.0 Debye dipole moments [131,132], the large difference between the carbon electronegativity of 2.5 and that of oxygen of 3.5 in comparison to the tiny difference between carbon and that of hydrogen of 2.1 [133] would result in high molecular quadrupole moment of 4.3 (D.Å) for CO 2 molecule in comparison to 0.02 (D.Å) for CH 4 molecule [62]. This CO 2 high quadrupole moment may result in the formation of a high CO 2 molecular clustering geometry, where a positively charged pole on the carbon atom of one CO 2 molecule would interact with a negatively charged pole on the oxygen atom of a second CO 2 molecule and so on to form large clusters over the AC surfaces. This high stacking of the linear CO 2 molecules in a flat parallel orientation was simulated over the aromatic carbon ring [134] and was reported by Meconi [135] by using the classical force-fields theory, where the optimized geometry showed that the flat CO 2 molecules would stack with its carbon atom along the middle half above the two carbons of the aromatic ring, while the two oxygen atoms of the CO 2 molecules were located in the middle empty center above the carbon aromatic ring. This high clustering of CO 2 molecules in comparison to CH 4 molecules may be supported by the high van der Waals forces between CO 2 molecules, as  reflected by their higher T C than the lower T C for CH 4 molecules. This clustering state may also be supported by the negative values of the entropy (ΔS°), reported upon CO 2 adsorption on activated carbon surfaces, favoring the conversion of randomizations state to ordered state upon adsorption [136]. On the other hand, the stacking of the tetrahedral CH 4 gas molecules over the aromatic ring was reported [137] to be located on one of the atoms of the C-C bond in a bidentate or tridentate rather than mono-dentate fashion. The linear flat geometry stacking of CO 2 in comparison to the tetrahedral stacking would allow higher assembling and accumulations of the CO 2 molecules over the graphene sheets. This favorable CO 2 clustering could also explain the steeped adsorption behavior, reported by Krishna et al [138], of the CO 2 in comparison with the mild increase of CH 4 adsorption and support the higher adsorption energy of -25.1 kJ mol −1 for CO 2 molecules than the lower adsorption energy of −16.5 kJ mol −1 for CH 4 above graphene sheets [135]. Moreover, the smaller kinetic diameter for the CO 2 molecule of 3.3 Å than the 3.8 Å for CH 4 molecule [62] may explain the higher response to pressure on the graphene layers due to the higher diffusivity of the smaller CO 2 molecules within the pore structure of the ACs.

Conclusion
High storage capabilities were achieved for both carbon dioxide and methane storages for the ACs before and after their modifications with an average values of∼26.5 wt% at 5 bar and 11.1 wt% at 35 bar for CO 2 and CH 4 , respectively. Such high storage would make them appealing materials for the removal of both CO 2 and CH 4 . At 5 bar, the adsorbents showed∼as much 6 folds as predilection towards carbon dioxide in comparison to methane, a feature working beneficially for natural gas upgrading and decreasing greenhouse gasses. This high adsorption capacity and preferentiality of CO 2 over CH 4 could be attributed to its physicochemical nature, mainly quadrupole moment, and higher clustering effect.
The dominant adsorption mechanism for CO 2 and CH 4 is mainly the physical adsorption with little attribute, if any, toward the chemical adsorption. This behavior would have its good economical aspect as no extra costs would be needed for expensive metals decoration of the AC's surfaces for biased enhancement of the CO 2 and CH 4 adsorptions under the studied conditions. In addition, their CO 2 storage capacities were as 530 folds as of their H 2 storage capacities, which would be so beneficial for generating hydrogen with low-carbon contents, as one of the most crucial issues in hydrogen production.