Comparison of CH4 and CO2 Adsorptions onto Calcite(10.4), Aragonite(011)Ca, and Vaterite(010)CO3 Surfaces: An MD and DFT Investigation.

The interaction between greenhouse gases (such as CH4 and CO2) and carbonate rocks has a significant impact on carbon transfer among different geochemical reservoirs. Moreover, CH4 and CO2 gases usually associate with oil and natural gas reserves, and their adsorption onto sedimentary rocks may influence the exploitation of fossil fuels. By employing the molecular dynamics (MD) and density functional theory (DFT) methods, the adsorptions of CH4 and CO2 onto three different CaCO3 polymorphs (i.e., calcite(10.4), aragonite(011)Ca, and vaterite(010)CO3) are compared in the present work. The calculated adsorption energies (E ad) are always negative for the three substrates, which indicates that their adsorptions are exothermic processes and spontaneous in thermodynamics. The E ad of CO2 is much more negative, which suggests that the CO2 adsorption will form stronger interfacial binding compared with the CH4 adsorption. The adsorption precedence of CH4 on the three surfaces is aragonite(011)Ca > vaterite(010)CO3 > calcite(10.4), while for CO2, the sequence is vaterite(010)CO3 > aragonite(011)Ca > calcite(10.4). Combining with the interfacial atomic configuration analysis, the Mulliken atomic charge distribution and overlap bond population are discussed. The results demonstrate that the adsorption of CH4 is physisorption and that its interfacial interaction mainly comes from the electrostatic effects between H in CH4 and O in CO3 2-, while the CO2 adsorption is chemisorption and the interfacial binding effect is mainly contributed by the bonds between O in CO2 and Ca2+ and the electrostatic interaction between C in CO2 and O in CO3 2-.

Introduction

Calcium carbonate (CaCO3) extensively exists as sedimentary rocks in the earth’s crust.[1,2] The carbonate rocks are expected to influence and regulate the carbon transfer between different geochemical reservoirs.[1,3] It is reported that these minerals might be helpful in converting atmospheric greenhouse gases (such as carbon dioxide and methane) into solid carbonate.[4−6] For instance, CaCO3 can be regarded as the products of CO2 capture reactions with CaO and can also be decarbonated to CaO;[7] vaterite CaCO3 microspheres can be synthesized and used as a novel CO2 storage material;[8] and mixed alkali metal salt MgO–CaCO3 sorbents are capable of adsorbing CO2 at an ultrafast rate, high capacity, and good stability.[9] Thus, carbonate rocks may have an impact on global climate change.

On the other hand, a considerable proportion of the world’s oil and natural gas reserve is found associated with carbonate sedimentary rocks, such as limestone, chalk, and dolomite, in which CaCO3 is the main constituent.[10−13] Meanwhile, the injection of carbon dioxide (CO2) is utilized as an approach to enhance fuels’ recovery. So, the interaction between the gas constituents and sedimentary rocks may have an impact on fossil fuels’ exploiting efficiency. Until now, the studies on CH4 and CO2 adsorptions on carbonate rocks are insufficient. Aiming to make a further clarification of the adsorptions, we have investigated the competitive adsorption of CH4 and CO2 onto different polymorphs of CaCO3, which will be helpful in enhancing oil[12] and natural gas[14] recovery rates.

Calcium carbonate (CaCO3) can exist as different polymorphs, in which the three phases of calcite, aragonite, and vaterite are commonly reported, and their thermodynamic stability decreases as per the sequence.[15] Although vaterite is reported to not commonly found in geological conditions, it is an important precursor in several carbonate-forming systems.[15] So, in the present work, all of the three CaCO3 polymorphs are considered.

In recent decades, the methods of DFT calculation and MD simulations have been successfully implemented in the study of molecule adsorption onto carbonate substrates. For instance, for the interaction between water and the calcite(10.4) surface, the dissociated and associated H2O molecules were compared and the dissociated ones were confirmed as a metastable state.[16] The adsorptions of several organic molecules (hexane, cyclohexane, and benzene) are studied on the surface (10.4) of dolomite CaMg(CO3)2, and the adsorption energies of these organic molecules are compared with that of the water molecule.[12] Chun et al.[17] characterized the adsorption of benzoate and stearate on the surface of calcite(10.4), and the binding energies of adsorbed molecules were investigated in the presence of water and oil phases. The adsorption energies of a series of small molecules (i.e., water, several alcohols, and acetic acid) were determined and compared on three synthetic CaCO3 polymorphs (calcite, aragonite, and vaterite).[18] Ataman et al.[13,19] investigated the adsorptions of some functional groups on calcite(10.4), including oxygen-, nitrogen-, and sulfur-containing molecules and nonpolar organic molecules.

Narrowly, for the adsorptions of CH4 and CO2 onto CaCO3 rocks, several important studies[20−23] can be noted and classified into experimental and theoretical sides:

On the experimental side, the competitive adsorption of CH4 and CO2 onto limestone was investigated in the temperature range of 50–150 °C and the higher affinity of CO2 to the rock was confirmed, which can be ascribed to the strong electrostatic attraction between the CO2 molecule and limestone.[23] Mixing of 10% CO2 into CH4 would enhance the adsorption of methane at 150 °C. Due to the high adsorption affinity of CO2, the total uptake increased, depending on the CO2 partial pressure. The adsorption of CO2 on limestone was confirmed to be four times higher than that of CH4. The higher natural selectivity of carbonate toward CO2 was thermodynamically supported by the lower adsorption heat of CO2.

For theoretical studies of the CH4[21,22] and CO2[20−22] adsorptions onto calcite substrates, the methods of MD simulations[20−22] and grand canonical Monte Carlo (GCMC)[21] have been employed. The adsorptions of H2O, CO2, CH4, and N2 gases on calcite(11̅0) are compared, and the preferential order is confirmed as follows: H2O > CO2 > CH4 > N2; CO2 molecules could form an adsorbed layer on the surface, while no significant feature indicates that CH4 molecules would be adsorbed on calcite(11̅0).[22] Furthermore, the adsorption and diffusion of CH4 and CO2 in calcite nanosized pores (width ∼22 Å) were compared, and it was confirmed that CO2 has much higher adsorption capacity and much less diffusion capacity compared with CH4.[21] Finally, the adsorption behavior of CO2 molecules on calcite(10.4) was investigated, which demonstrates that CO2 molecules would be adsorbed perpendicularly at the sites of Ca ions and the desorption of CO2 molecules would be positively correlated with temperature.[20]

Based on our literature availability, the hierarchical comparisons of CH4 and CO2 adsorptions onto different CaCO3 polymorphs are still insufficient and need further clarification. In the present work, by combining MD and DFT methods, the CH4 and CO2 adsorptions onto various CaCO3 polymorphs (i.e., calcite, aragonite, and vaterite) are investigated and compared. The interactions between adsorbents (CaCO3 polymorphs) and adsorbates (gas molecules) are emphasized and compared; therefore, the adsorption systems are specifically studied in vacuum environments. All our DFT calculations are performed on a static molecular system, and the temperature effect is out of discussion in this work (the temperature is fixed at 0 K).

First, bulk unit cells of three CaCO3 polymorphs are established and relaxed with DFT calculations, and the bulk properties, such as lattice parameters and bulk modulus, are calculated. Then, various surfaces are created based on these relaxed unit cells, and the surface energies are examined. After that, the interface systems are established by putting gas molecules on the surfaces, and MD geometry optimizations are conducted for the interface models to achieve rough estimates of atomic configurations. Finally, the rough estimates are continually relaxed with the DFT method to reach their ground states, and based on these final atomic configurations, the remaining properties, such as adsorption energy, electron distribution, and density of states, are determined with the DFT method.

Results and Discussion

Bulk Calculations

The space groups of calcite and aragonite are experimentally identified as R3̅c (167)[13] and Pmcn (62),[24] respectively. However, the space group of vaterite is still in controversy, and based on previous literature works,[25,26] the space group Pbnm (62) is adopted in this work. The unit cells of these three polymorphs are depicted in Figure .

Figure 1

Unit cells of CaCO3 polymorphs: (a) calcite, (b) aragonite, and (c) vaterite. Different color spheres denote Ca (green), O (red), C (dark gray), and H (white) atoms.

The calculated lattice constants (a, b, and c) and bulk moduli (B) of bulk calcite, aragonite, and vaterite are listed in Table . Our results are in good accordance with the previous experimental and theoretical data. Especially for the lattice constants, they agree well with the experimental data for bulk calcite, aragonite, and vaterite.

Table 1

Lattice Constants (a, b, and c) and Bulk Moduli (B) of Bulk Calcite, Aragonite, and Vaterite

					lattice constants (Å)
phases	space group	Pearson symbol	Strukturbericht designation	data sources	a	b	c	B (GPa)
calcite	R3̅c (167)	hR10	G01	present work	5.0527	5.0527	17.2510	71.8
				previous calculation	4.797	4.797	17.482[27]	69.6[28]
				previous calculation	5.06	5.06	17.25[13]	75.6[29]
				previous calculation	4.933	4.933	17.242[30]	85.7[31]
				previous calculation	5.039	5.039	17.456[32]
				experimental	4.99	4.99	17.06[13]	78.0[33]
				experimental	4.988	4.988	17.061[30]	73.5[34]
				experimental	4.991	4.991	17.062[35]
aragonite	Pmcn (62)	oP20	G02	present work	5.0180	8.0388	5.8155	67.8
				previous calculation	4.8314	7.8359	5.7911[27]	67.7[31]
				previous calculation	5.003	8.047	5.659[30]	66.8[36]
				previous calculation	5.112	8.230	5.915[32]
				previous calculation	4.9609	7.9936	5.7020[37]
				experimental	4.9633	7.9703	5.7441[24]	66.8[33]
				experimental	4.961	7.967	5.741[30]	64.8[38]
				experimental	4.962	7.969	5.743[39]
				experimental	4.9614	7.9671	5.7404[40]
vaterite	Pbnm (62)	oP28	S12	present work	4.5423	6.6792	8.5127	70.8
				previous calculation	4.531	6.640	8.477[41]	69.1[31]
				previous calculation	4.43	6.62	8.04[27]
				previous calculation	4.531	6.640	8.477[41]
				experimental	4.13	7.15	8.48[25]	63.8[42]

Surface and Interface Models

Surface stability can be characterized by surface energy (γs) values; the surface with smaller γs is thermodynamically more stable. The surface energies of CaCO3 polymorphs have been investigated and compared by Leeuw and Parker,[27] Sekkal and Zaoui,[31] Massaro et al.[37] Based on these data (refer to Table ), the calcite(10.4) and CO32– terminated vaterite(010) can be identified as the most stable surfaces for both polymorphs. For aragonite, although the literature works[27,37,43] consent that the (011) plane is the most stable surface, it is still controversial for its termination, namely, CO32–[27] and Ca2+ [27,37,43] terminations (illustrated in Figure ) are both reported as the most stable configurations. Therefore, we recalculated the surface energies of both terminations. The surface energy (γs) of aragonite(011) is ascertained aswhere As denotes the surface area, nCaCO is the number of CaCO3 formula contained in the surface slab, and Earagonite(011)slab and Earagonitebulk are total energies of aragonite(011) surface slab and bulk aragonite per formula, respectively. Our calculated data are also listed in Table , and the values are 0.636 and 0.469 J/m2 for CO32– and Ca2+ terminations, respectively. This result indicates that Ca2+ termination will be more stable for aragonite(011), which is in line with the data of Massaro et al.[37]

Table 2

Surface Energies (J/m2) of Calcite, Aragonite, and Vaterite Reported in Literature Works and Calculated Values in This Work

surfaces	Leeuw et al.[27]	Sekkal et al.[31]	Massaro et al.[37]	Bano et al.[43]	Rohl et al.[44]	Massaro et al.[45]	Aquilano[46]	Bruno[47]	present work
calcite(10.4)	0.59	0.71		0.7113	0.534	0.536	0.536	0.503
aragonite(011)CO3	0.69	0.90	0.801						0.636
aragonite(011)Ca			0.578						0.469
aragonite(011)				0.8406
vaterite(010)CO3	0.62	0.75

Figure 2

Two different terminations of aragonite(011): (a) terminated with Ca2+ and (b) terminated with CO32–. Different color spheres denote Ca (green), O (red), C (dark gray), and H (white) atoms.

As aforementioned, the surfaces calcite(10.4), aragonite(011)Ca, and vaterite(010)CO3 are identified as stable surfaces for the three polymorphs. Consequently, the adsorptions of CH4 and CO2 are compared on these three surfaces in the following parts.

The surface slabs are created on the basis of optimized bulk structures. The supercells of calcite(10.4), aragonite(011)Ca, and vaterite(010)CO3 are modeled as (1 × 2), (2 × 1), and (1 × 2) slabs, respectively. Their surface areas are 8.2 × 10.1, 10.0 × 9.9, and 8.5 × 9.1 Å2, respectively. To avoid the imaginary interaction between top and bottom sides, a 30 Å vacuum space is inserted in all surface models in depth. The calcite(10.4) slab contains four layers of CO32– and Ca2+; the top two layers are free to relax, and the bottom two layers are constrained. For the Ca-terminated aragonite(011), eight layers of Ca2+ and eight layers of CO32– are included, and the bottom part (four layers of Ca2+ and four layers of CO32–) is frozen and the rest are free to move. For the CO3-terminated vaterite(010), four layers of Ca2+ and eight layers of CO32– are used to mimic the surface, and the bottom two layers of Ca2+ and four layers of CO32– are fixed to exhibit a bulklike interior.

The interface models are established by putting one molecule on the surfaces. Assuming that each Ca atom on the surface is an adsorption site for a molecule, all of the above configurations correspond to 0.25 ML coverage. First, the interface models are optimized using the MD method by employing COMPASS forcefield. Then, the DFT geometry optimizations are continually conducted to achieve equilibrium states of the adsorption systems.

Equilibrium Configurations and Adsorption Energies

After the calculations of MD and DFT procedures, the equilibrium configurations of adsorption systems can be achieved (as shown in Figure ).

Figure 3

Fully relaxed configurations of CH4 and CO2 adsorbed on CaCO3 polymorphs: (a) CH4 on calcite(10.4), (b) CO2 on calcite(10.4), (c) CH4 on aragonite(011)Ca, (d) CO2 on aragonite(011)Ca, (e) CH4 on vaterite(010)CO3, and (f) CO2 on vaterite(010)CO3. Different color spheres denote Ca (green), O (red), C (dark gray), and H (white) atoms.

For all three CaCO3 polymorphs, surface reconstruction can be observed in the equilibrium models. Especially for aragonite(011)Ca and vaterite(010)CO3, surface reconstruction is more obvious. This phenomenon is consistent with the previous studies that the absorbed molecules (CO2[20] and H2O[48−50]) influence the surface reconstruction of calcite(10.4).

Based on these fully optimized interfacial configurations, the adsorption energies (Ead) can be ascertained as[51]where Einterface, Esurface, and Emolecule denote the energies of the interface slab, surface slab, and adsorbed molecule, respectively. The calculated results are listed in Table . Our calculated Ead is −51.04 kJ/mol for the system of CO2 adsorbed on calcite(10.4), which is very close to the previous experimental study (52–67 kJ/mol).[52] So, our calculation results are accurate and reasonable.

Table 3

Calculated Adsorption Energies (Ead) of CH4 and CO2 on Calcite(10.4), Aragonite(011)Ca, and Vaterite(010)CO3 Surfaces

	Ead of molecules (kJ/mol)
surfaces	CH4	CO2
calcite(10.4)	–22.75	–51.04
aragonite(011)Ca	–35.01	–52.94
vaterite(010)CO3	–27.94	–74.79

If the adsorption energy is negative, the adsorption process will occur spontaneously, which also means that it is an exothermic process. Moreover, the relation between adsorption energy (Ead) and binding energy (Eb) can be expressed as Eb = – Ead.[53][53] So, if the adsorption energy is more negative, then the adsorption process will have larger potential (or driving force) and will tend to form stronger binding with the adsorbent. By examining our calculated results within this theorem, the following statements can be concluded:

The adsorption energies of CH4 and CO2 on calcite(10.4), aragonite(011)Ca, and vaterite(010)CO3 surfaces are always negative, which suggests that the adsorptions are spontaneous and exothermic processes.

The Ead of CO2 are more negative than those of CH4; therefore, compared with CH4, the interfacial interaction between CO2 and CaCO3 surfaces should be stronger. This statement is also consistent with the previous studies.[22,23,54]

By comparing Ead values of the same molecule on different surfaces, the adsorption precedence of the three surfaces can be described as follows: (i) for CH4, the sequence is aragonite(011)Ca > vaterite(010)CO3 > calcite(10.4); (ii) for CO2, the sequence is vaterite(010)CO3 > aragonite(011)Ca > calcite(10.4).

Mulliken Population and Atomic Configuration Analyses

The Mulliken population analysis is commonly employed in the DFT adsorption investigations.[55−57] Although the absolute values of the atomic charges generated by the population analysis are regarded to have a little physical meaning, these values are strongly influenced by the atomic basis set with which the DFT calculations were conducted.[58] However, consideration of their relative values can yield useful information.[59−61]

In the present work, Mulliken populations have been calculated for these equilibrium interfacial models. By examining the final atomic configurations together with the Mulliken population analysis, some implications can be obtained, and more importantly, these implications are helpful to shed light on the adsorption mechanism. The interfacial atomic configurations are shown in Figure . The results of the Mulliken atomic charge and overlap population are summarized in Tables and 5, respectively. The overlap population may be useful to assess the bond nature; the bond’s covalency level increases with an increase in positive values, while negative values suggest an antibonding state.[59,60] To clarify the interfacial interaction nature, the analysis is focused on the adsorbed molecules and some atoms in neighboring surface ions.

Figure 4

Interfacial atomic configurations of six models: (a) CH4 on calcite(10.4), (b) CH4 on aragonite(011)Ca, (c) CH4 on vaterite(010)CO3, (d) CO2 on calcite(10.4), (e) CO2 on aragonite(011)Ca, and (f) CO2 on vaterite(010)CO3. Different color spheres denote Ca (green), O (red), C (dark gray), and H (white) atoms.

Table 4

Mulliken Charge Distribution in Adsorbed Molecules (CH4, CO2) and Some Atoms in Neighboring Surface Ions (Ca2+, CO32–)a

			atomic population
absorption systems	atoms’ origin	atom no.	s	p	d	total	charge (e)
CH4 on calcite(10.4)	CH4	C17	1.48	3.61		5.09	–1.09
		H1	0.74	0.00		0.74	0.26
		H2	0.73	0.00		0.73	0.27
		H3	0.71	0.00		0.71	0.29
		H4	0.77	0.00		0.77	0.23
	CO32–	O32	1.81	4.93		6.74	–0.74
		O47	1.81	4.88		6.69	–0.69
CH4 on aragonite(011)Ca	CH4	C17	1.47	3.61		5.07	–1.07
		H1	0.76	0.00		0.76	0.24
		H2	0.78	0.00		0.78	0.22
		H3	0.71	0.00		0.71	0.29
		H4	0.73	0.00		0.73	0.27
	CO32–	O8	1.81	4.89		6.70	–0.70
		O20	1.82	4.85		6.67	–0.67
		O44	1.83	4.88		6.71	–0.71
CH4 on vaterite(010)CO3	CH4	C17	1.46	3.59		5.05	–1.05
		H1	0.80	0.00		0.80	0.20
		H2	0.75	0.00		0.75	0.25
		H3	0.73	0.00		0.73	0.27
		H4	0.73	0.00		0.73	0.27
	CO32–	O8	1.81	4.91		6.71	–0.71
		O18	1.81	4.89		6.70	–0.70
		O32	1.81	4.90		6.71	–0.71
		O36	1.81	4.90		6.71	–0.71
CO2 on calcite(10.4)	CO2	C17	0.69	2.33		3.02	0.98
		O49	1.83	4.63		6.46	–0.46
		O50	1.82	4.69		6.51	–0.51
	Ca2+	Ca1	2.12	6.00	0.46	8.58	1.42
	CO32–	O47	1.81	4.90		6.71	–0.71
CO2 on aragonite(011)Ca	CO2	C17	0.68	2.32		3.00	1.00
		O49	1.84	4.59		6.43	–0.43
		O50	1.81	4.71		6.53	–0.53
	Ca2+	Ca2	2.10	6.00	0.48	8.57	1.43
CO2 on vaterite(010)CO3	CO2	C17	0.71	2.33		3.03	0.97
		O49	1.82	4.68		6.50	–0.50
		O50	1.82	4.66		6.48	–0.48
	Ca2+	Ca6	2.10	6.00	0.51	8.61	1.39
		Ca10	2.10	6.00	0.51	8.61	1.39
	CO32–	O8	1.81	4.88		6.69	–0.69
		O44	1.81	4.92		6.73	–0.73

The atom no. is labeled in Figure .

Table 5

Mulliken Bond Population for the Atom Pairs between Adsorbed Molecules (CH4, CO2) and Neighboring Surface Ions (Ca2+, CO32–)a

absorption systems	atom pairs	overlap population	interatomic length (Å)
CH4 on calcite(10.4)	H1–O32	–0.01	2.9149
	H2–O47	0.00	2.9543
CH4 on aragonite(011)Ca	H1–O32	–0.01	2.9112
	H1–O36	0.00	2.8975
	H2–O8	0.00	2.8950
	H2–O18	–0.01	2.8372
	H3–O32	–0.01	2.8784
CH4 on vaterite(010)CO3	H1–O20	–0.01	2.5088
	H1–O44	–0.01	2.9252
	H2–O8	–0.01	2.7314
CO2 on calcite(10.4)	O50–Ca1	0.05	2.6361
	C17–O47	0.01	2.7357
CO2 on aragonite(011)Ca	O50–Ca2	0.06	2.5128
CO2 on vaterite(010)CO3	O49–Ca10	0.03	2.7892
	O50–Ca6	0.03	2.8444
	C17–O8	0.01	2.6498
	C17–O44	0.00	2.6256

The atom no. is shown in Figure .

The Mulliken charge distribution (Table ) indicates that the H atoms in CH4 act as charge donors and O atoms act as charge acceptors. For the atoms in the surface ions, the O atoms in CO32– act as charge acceptors and Ca atoms act as charge donors. So, the interfacial interactions between H in CH4 and O in CO32– (likewise between O in CO2 and surface Ca2+) can be assumed.

The bond population results in Table support the above assumption. Especially for the atom pairs of O in CO2 and surface Ca, their bond populations are positive values (0.03–0.06), which indicates that weak bonds form between the atoms. As for the atom pairs of H in CH4 and O in CO32–, the bond population values are basically negative around zero, so the interactions between them are mainly electrostatic effects. Therefore, the adsorptions of CH4 and CO2 on CaCO3 polymorphs can be featured as physisorption and chemisorptions, respectively. This is also the reason why CO2 adsorptions have larger Ead compared with that of CH4 adsorptions.

Furthermore, for the models of CO2 on calcite(10.4) and vateriate(010)CO3, we noticed that the interactions between C in CO2 and O in CO32– also contribute to their interfacial binding effect. However, their overlap bond populations are around zero, and the interactions between these atoms are mainly electrostatic effects.

Summarily, for the interfacial interaction nature of absorbed molecule and CaCO3 surfaces, the following statements can be deduced:

CH4 adsorptions on the three CaCO3 polymorphs can be characterized as phsisorptions, and the interfacial interactions are mainly contributed by the electrostatic effects between H in CH4 and O in CO32–.

CO2 may form chemisorption on the three surfaces, and the interfacial binding effect mainly comes from the bonds between O in CO2 and Ca2+ ions. Besides that, the electrostatic interactions between C in CO2 and O in CO32– also make some contributions.

Conclusions

The adsorptions of CH4 and CO2 onto calcite(10.4), aragonite(011)Ca, and vaterite(010)CO3 are investigated and compared by employing MD and DFT calculations.

In the equilibrium atomic configurations, surface reconstruction is observed.

The calculated adsorption energies (Ead) of CH4 and CO2 are always negative for the three substrates, which indicates that the adsorptions are exothermic processes and spontaneous in thermodynamics.

Comparing with that of CH4, the Ead of CO2 is more negative, which suggests that there is a stronger driving force for the adsorption of CO2, leading to stronger interfacial interactions after adsorption.

The adsorption precedence for the three surfaces can be confirmed as follows: for CH4, aragonite(011)Ca > vaterite(010)CO3 > calcite(10.4); while for CO2, the sequence is vaterite(010)CO3 > aragonite(011)Ca > calcite(10.4).

Combining with interfacial atomic configuration analysis, the Mulliken charge distribution and population suggest that the adsorption of CH4 is physisorption and the interfacial interaction mainly comes from the electrostatic effect between H in CH4 and O in CO32–, while the adsorption of CO2 is chemisorption and the interfacial binding effect is mainly contributed by the bonds between O in CO2 and Ca2+ and the electrostatic interaction between C in CO2 and O in CO32–.

Computation Methodology

All DFT calculations were performed with the CASTEP package,[62,63] wherein planewave ultrasoft pseudopotentials[64,65] were employed to describe the cores. The electronic exchange correlation effects were treated within generalized gradient approximation (GGA) in the formalism of the Perdew–Burke–Ernzerhof (PBE) functional.[66] The van der Waals dispersion corrections were included using the semiempirical DFT-D approach with the Tkatchenko and Scheffler (TS) scheme,[67] which can generate accurate results for the adsorption of small molecules on solid surfaces.[68] The Brillouin zone is sampled by the Monkhorst–Pack k-point grid.[69] Convergence tests have been conducted to determine the cutoff energy and k-point grid separation. The details are described in the Supporting Information.

To ensure that the calculation results of different adsorption systems are comparable, the computation parameters employed in the DFT calculations should be the same (or as close to each other as possible). For this reason, the cutoff energy is fixed as 450 eV in all DFT calculations. Similarly, for the different adsorption systems, the k-point grid separation (or actual grid spacing) should be as close to each other as possible, so the grid separation is fixed as 0.03 Å–1 in all DFT calculations, and the k-points are automatically generated as 8 × 8 × 2, 6 × 4 × 6, and 8 × 4 × 4 for bulk unit cells of calcite, aragonite, and vaterite, respectively. Likewise, the k-points are automatically set as 4 × 3 × 1, 3 × 3 × 1, and 4 × 4 × 1 for the adsorption systems on the substrates of calcite(10.4), aragonite(011)Ca, and vaterite(010)CO3 respectively. We also noticed that the cutoff energy of 450 eV and k-point grid separation of around 0.03 Å–1 have been successfully implemented in the adsorption system containing the calcite(10.4) surface[70] and calcium carbonate hydrates.[71]