Theoretical Study of As2O3 Adsorption Mechanisms on CaO surface

Emission of hazardous trace elements, especially arsenic from fossil fuel combustion, have become a major concern. Under an oxidizing atmosphere, most of the arsenic converts to gaseous As2O3. CaO has been proven effective in capturing As2O3. In this study, the mechanisms of As2O3 adsorption on CaO surface under O2 atmosphere were investigated by density functional theory (DFT) calculation. Stable physisorption and chemisorption structures and related reaction paths are determined; arsenite (AsO33−) is proven to be the form of adsorption products. Under the O2 atmosphere, the adsorption product is arsenate (AsO43−), while tricalcium orthoarsenate (Ca3As2O8) and dicalcium pyroarsenate (Ca2As2O7) are formed according to different adsorption structures.


Introduction
Arsenic is a hazardous element existing in fossil fuels such as coal and petroleum [1]. According to the properties of arsenic and its compounds, it has been classified as volatile trace element by Clark and Sloss [2]. During combustion or chemical industry processes, gaseous arsenic is released into the environment. Excess amounts of arsenic can pollute water and soil. The exposure of arsenic to human may lead to hyperpigmentation, keratosis, skin and lung cancers with high possibility [3,4]. Arsenic compounds (including inorganic arsine) have been identified as hazardous air pollutants by the US government since 1990 [5]. The concentration of atmospheric arsenic in China is 51.0 ng/m 3 , which is much higher than the limit of NAAQS (6.0 ng/m 3 , GB 3095-2012) and the limit of WHO (6.6 ng/m 3 , WHO) [6].
Combustion of fossil fuels, especially coal, is one of the main sources for anthropogenic emission of atmospheric arsenic [7]. It was estimated that 335.5 tons of atmospheric arsenic were emitted from Chinese coal-fired plants in 2010 [8]. In 2011, the US Environmental Protection Agency issued the Mercury and Air Toxics Standards (US, MATS, updated in 2016). An arsenic emission limit of 3.0 × 10 −3 lb/MWh (approximately 0.41 µg/m 3 ) was set for coal-fired power plants [9]. In Chinese coal-fired power plants, the control of arsenic still remains scarce, but there are increasing interests in understanding its transformation in flue gas and developing emission reduction techniques.

. Stable Chemisorption Structures
Ten chemisorption structures were obtained, with Eads ranging from −198.5 kJ/mol to −391.4 kJ/mol, which implies strong chemisorption. Superficial Ca is close to As2O3's O, the bond length is about 2.269 Å to 2.528 Å, while superficial O is close to As2O3's O, the bond length is 1.788 Å to 2.086 Å. According to electron density cloud and bong length, chemisorption active sites are superficial O atoms that interact with As of As2O3. According to the adsorption energy and structure (i.e., the positions of As and O), four categories were classified in Table 3: Category I: As2O3's As is located on the hollow site Category II: All of As2O3's O is located on or close to superficial Ca top site Category III: As2O3 transforms into a spoon-shaped structure Category IV: All of As2O3's As is located on two neighboring superficial O top site

. Stable Chemisorption Structures
Ten chemisorption structures were obtained, with Eads ranging from −198.5 kJ/mol to −391.4 kJ/mol, which implies strong chemisorption. Superficial Ca is close to As2O3's O, the bond length is about 2.269 Å to 2.528 Å, while superficial O is close to As2O3's O, the bond length is 1.788 Å to 2.086 Å. According to electron density cloud and bong length, chemisorption active sites are superficial O atoms that interact with As of As2O3. According to the adsorption energy and structure (i.e., the positions of As and O), four categories were classified in Table 3: Category I: As2O3's As is located on the hollow site Category II: All of As2O3's O is located on or close to superficial Ca top site Category III: As2O3 transforms into a spoon-shaped structure Category IV: All of As2O3's As is located on two neighboring superficial O top site

Stable Chemisorption Structures
Ten chemisorption structures were obtained, with E ads ranging from −198.5 kJ/mol to −391.4 kJ/mol, which implies strong chemisorption. Superficial Ca is close to As 2 O 3 's O, the bond length is about 2.269 Å to 2.528 Å, while superficial O is close to As 2 O 3 's O, the bond length is 1.788 Å to 2.086 Å. According to electron density cloud and bong length, chemisorption active sites are superficial O atoms that interact with As of As 2 O 3 . According to the adsorption energy and structure (i.e., the positions of As and O), four categories were classified in Table 3: Category I: As 2 O 3 's As is located on the hollow site Category II: All of As 2 O 3 's O is located on or close to superficial Ca top site Category III: As 2 O 3 transforms into a spoon-shaped structure Category IV: All of As 2 O 3 's As is located on two neighboring superficial O top site

. Stable Chemisorption Structures
Ten chemisorption structures were obtained, with Eads ranging from −198.5 kJ/mol to −391.4 kJ/mol, which implies strong chemisorption. Superficial Ca is close to As2O3's O, the bond length is about 2.269 Å to 2.528 Å, while superficial O is close to As2O3's O, the bond length is 1.788 Å to 2.086 Å. According to electron density cloud and bong length, chemisorption active sites are superficial O atoms that interact with As of As2O3. According to the adsorption energy and structure (i.e., the positions of As and O), four categories were classified in Table 3: Category I: As2O3's As is located on the hollow site Category II: All of As2O3's O is located on or close to superficial Ca top site Category III: As2O3 transforms into a spoon-shaped structure Category IV: All of As2O3's As is located on two neighboring superficial O top site According to electron density cloud and bong length, chemisorption active sites are superficial O atoms that interact with As of As2O3. According to the adsorption energy and structure (i.e., the positions of As and O), four categories were classified in Table 3: Category I: As2O3's As is located on the hollow site Category II: All of As2O3's O is located on or close to superficial Ca top site Category III: As2O3 transforms into a spoon-shaped structure Category IV: All of As2O3's As is located on two neighboring superficial O top site According to electron density cloud and bong length, chemisorption active sites are superficial O atoms that interact with As of As2O3. According to the adsorption energy and structure (i.e., the positions of As and O), four categories were classified in Table 3: Category I: As2O3's As is located on the hollow site Category II: All of As2O3's O is located on or close to superficial Ca top site Category III: As2O3 transforms into a spoon-shaped structure Category IV: All of As2O3's As is located on two neighboring superficial O top site According to electron density cloud and bong length, chemisorption active sites are superficial O atoms that interact with As of As2O3. According to the adsorption energy and structure (i.e., the positions of As and O), four categories were classified in Table 3: Category I: As2O3's As is located on the hollow site Category II: All of As2O3's O is located on or close to superficial Ca top site Category III: As2O3 transforms into a spoon-shaped structure Category IV: All of As2O3's As is located on two neighboring superficial O top site According to electron density cloud and bong length, chemisorption active sites are superficial O atoms that interact with As of As2O3. According to the adsorption energy and structure (i.e., the positions of As and O), four categories were classified in Table 3: Category I: As2O3's As is located on the hollow site Category II: All of As2O3's O is located on or close to superficial Ca top site Category III: As2O3 transforms into a spoon-shaped structure Category IV: All of As2O3's As is located on two neighboring superficial O top site According to electron density cloud and bong length, chemisorption active sites are superficial O atoms that interact with As of As2O3. According to the adsorption energy and structure (i.e., the positions of As and O), four categories were classified in Table 3: Category I: As2O3's As is located on the hollow site Category II: All of As2O3's O is located on or close to superficial Ca top site Category III: As2O3 transforms into a spoon-shaped structure Category IV: All of As2O3's As is located on two neighboring superficial O top site According to electron density cloud and bong length, chemisorption active sites are superficial O atoms that interact with As of As2O3. According to the adsorption energy and structure (i.e., the positions of As and O), four categories were classified in Table 3: Category I: As2O3's As is located on the hollow site Category II: All of As2O3's O is located on or close to superficial Ca top site Category III: As2O3 transforms into a spoon-shaped structure Category IV: All of As2O3's As is located on two neighboring superficial O top site According to electron density cloud and bong length, chemisorption active sites are superficial O atoms that interact with As of As2O3. According to the adsorption energy and structure (i.e., the positions of As and O), four categories were classified in Table 3: Category I: As2O3's As is located on the hollow site Category II: All of As2O3's O is located on or close to superficial Ca top site Category III: As2O3 transforms into a spoon-shaped structure Category IV: All of As2O3's As is located on two neighboring superficial O top site According to electron density cloud and bong length, chemisorption active sites are superficial O atoms that interact with As of As2O3. According to the adsorption energy and structure (i.e., the positions of As and O), four categories were classified in Table 3: Category I: As2O3's As is located on the hollow site Category II: All of As2O3's O is located on or close to superficial Ca top site Category III: As2O3 transforms into a spoon-shaped structure Category IV: All of As2O3's As is located on two neighboring superficial O top site

Adsorption Process
Due to the continuity of energy, the adsorption process can be characterized as an energy-drop process, including both physisorption and chemisorption.

Transformation Process of Physisorption Structures to Chemisorption Structures
In the following part, the transition state number n is abbreviated as TSn, and the intermediate position number n is abbreviated as IPn, for short.
As shown in Figure 1, when As2O3 approaches the surface with vibration along the surface, the physisorption structure transforms into a chemisorption structure during one or two transition state. For instance, P1 to C7 (Figure 1a), P2 to C8 ( Figure 1b) and P3 to C8 (Figure 1c). The energy barrier is low, from 1.4 kJ/mol to 13.9 kJ/mol, suggesting that the physisorbed As2O3 is not stable enough and could be easily transformed into chemisorption structures by thermal vibration.

Adsorption Process
Due to the continuity of energy, the adsorption process can be characterized as an energy-drop process, including both physisorption and chemisorption.

Transformation Process of Physisorption Structures to Chemisorption Structures
In the following part, the transition state number n is abbreviated as TSn, and the intermediate position number n is abbreviated as IPn, for short.
As shown in Figure 1, when As2O3 approaches the surface with vibration along the surface, the physisorption structure transforms into a chemisorption structure during one or two transition state. For instance, P1 to C7 (Figure 1a), P2 to C8 ( Figure 1b) and P3 to C8 (Figure 1c). The energy barrier is low, from 1.4 kJ/mol to 13.9 kJ/mol, suggesting that the physisorbed As2O3 is not stable enough and could be easily transformed into chemisorption structures by thermal vibration.

Adsorption Process
Due to the continuity of energy, the adsorption process can be characterized as an energy-drop process, including both physisorption and chemisorption.

Transformation Process of Physisorption Structures to Chemisorption Structures
In the following part, the transition state number n is abbreviated as TSn, and the intermediate position number n is abbreviated as IPn, for short.
As shown in Figure 1, when As2O3 approaches the surface with vibration along the surface, the physisorption structure transforms into a chemisorption structure during one or two transition state. For instance, P1 to C7 (Figure 1a), P2 to C8 ( Figure 1b) and P3 to C8 (Figure 1c). The energy barrier is low, from 1.4 kJ/mol to 13.9 kJ/mol, suggesting that the physisorbed As2O3 is not stable enough and could be easily transformed into chemisorption structures by thermal vibration.

Adsorption Process
Due to the continuity of energy, the adsorption process can be characterized as an energy-drop process, including both physisorption and chemisorption.

Transformation Process of Physisorption Structures to Chemisorption Structures
In the following part, the transition state number n is abbreviated as TSn, and the intermediate position number n is abbreviated as IPn, for short.
As shown in Figure 1, when As2O3 approaches the surface with vibration along the surface, the physisorption structure transforms into a chemisorption structure during one or two transition state. For instance, P1 to C7 (Figure 1a), P2 to C8 ( Figure 1b) and P3 to C8 (Figure 1c). The energy barrier is low, from 1.4 kJ/mol to 13.9 kJ/mol, suggesting that the physisorbed As2O3 is not stable enough and could be easily transformed into chemisorption structures by thermal vibration.

Adsorption Process
Due to the continuity of energy, the adsorption process can be characterized as an energy-drop process, including both physisorption and chemisorption.

Transformation Process of Physisorption Structures to Chemisorption Structures
In the following part, the transition state number n is abbreviated as TSn, and the intermediate position number n is abbreviated as IPn, for short.
As shown in Figure 1, when As2O3 approaches the surface with vibration along the surface, the physisorption structure transforms into a chemisorption structure during one or two transition state. For instance, P1 to C7 (Figure 1a), P2 to C8 ( Figure 1b) and P3 to C8 (Figure 1c). The energy barrier is low, from 1.4 kJ/mol to 13.9 kJ/mol, suggesting that the physisorbed As2O3 is not stable enough and could be easily transformed into chemisorption structures by thermal vibration.

Adsorption Process
Due to the continuity of energy, the adsorption process can be characterized as an energy-drop process, including both physisorption and chemisorption.

Transformation Process of Physisorption Structures to Chemisorption Structures
In the following part, the transition state number n is abbreviated as TSn, and the intermediate position number n is abbreviated as IPn, for short.
As shown in Figure 1, when As2O3 approaches the surface with vibration along the surface, the physisorption structure transforms into a chemisorption structure during one or two transition state. For instance, P1 to C7 (Figure 1a), P2 to C8 ( Figure 1b) and P3 to C8 (Figure 1c). The energy barrier is low, from 1.4 kJ/mol to 13.9 kJ/mol, suggesting that the physisorbed As2O3 is not stable enough and could be easily transformed into chemisorption structures by thermal vibration.

Adsorption Process
Due to the continuity of energy, the adsorption process can be characterized as an energy-drop process, including both physisorption and chemisorption.

Transformation Process of Physisorption Structures to Chemisorption Structures
In the following part, the transition state number n is abbreviated as TSn, and the intermediate position number n is abbreviated as IPn, for short.
As shown in Figure 1, when As2O3 approaches the surface with vibration along the surface, the physisorption structure transforms into a chemisorption structure during one or two transition state. For instance, P1 to C7 (Figure 1a), P2 to C8 ( Figure 1b) and P3 to C8 (Figure 1c). The energy barrier is low, from 1.4 kJ/mol to 13.9 kJ/mol, suggesting that the physisorbed As2O3 is not stable enough and could be easily transformed into chemisorption structures by thermal vibration.

Adsorption Process
Due to the continuity of energy, the adsorption process can be characterized as an energy-drop process, including both physisorption and chemisorption.

Transformation Process of Physisorption Structures to Chemisorption Structures
In the following part, the transition state number n is abbreviated as TSn, and the intermediate position number n is abbreviated as IPn, for short.
As shown in Figure 1, when As2O3 approaches the surface with vibration along the surface, the physisorption structure transforms into a chemisorption structure during one or two transition state. For instance, P1 to C7 (Figure 1a), P2 to C8 ( Figure 1b) and P3 to C8 (Figure 1c). The energy barrier is low, from 1.4 kJ/mol to 13.9 kJ/mol, suggesting that the physisorbed As2O3 is not stable enough and could be easily transformed into chemisorption structures by thermal vibration.

Adsorption Process
Due to the continuity of energy, the adsorption process can be characterized as an energy-drop process, including both physisorption and chemisorption.

Transformation Process of Physisorption Structures to Chemisorption Structures
In the following part, the transition state number n is abbreviated as TSn, and the intermediate position number n is abbreviated as IPn, for short.
As shown in Figure 1, when As2O3 approaches the surface with vibration along the surface, the physisorption structure transforms into a chemisorption structure during one or two transition state. For instance, P1 to C7 (Figure 1a), P2 to C8 (Figure 1b) and P3 to C8 (Figure 1c). The energy barrier is low, from 1.4 kJ/mol to 13.9 kJ/mol, suggesting that the physisorbed As2O3 is not stable enough and could be easily transformed into chemisorption structures by thermal vibration.

Adsorption Process
Due to the continuity of energy, the adsorption process can be characterized as an energy-drop process, including both physisorption and chemisorption.

Transformation Process of Physisorption Structures to Chemisorption Structures
In the following part, the transition state number n is abbreviated as TSn, and the intermediate position number n is abbreviated as IPn, for short.
As shown in Figure 1, when As2O3 approaches the surface with vibration along the surface, the physisorption structure transforms into a chemisorption structure during one or two transition state. For instance, P1 to C7 (Figure 1a), P2 to C8 (Figure 1b) and P3 to C8 (Figure 1c). The energy barrier is low, from 1.4 kJ/mol to 13.9 kJ/mol, suggesting that the physisorbed As2O3 is not stable enough and could be easily transformed into chemisorption structures by thermal vibration.

Adsorption Process
Due to the continuity of energy, the adsorption process can be characterized as an energy-drop process, including both physisorption and chemisorption.

Transformation Process of Physisorption Structures to Chemisorption Structures
In the following part, the transition state number n is abbreviated as TSn, and the intermediate position number n is abbreviated as IPn, for short.
As shown in Figure 1, when As2O3 approaches the surface with vibration along the surface, the physisorption structure transforms into a chemisorption structure during one or two transition state. For instance, P1 to C7 (Figure 1a), P2 to C8 (Figure 1b) and P3 to C8 (Figure 1c). The energy barrier is low, from 1.4 kJ/mol to 13.9 kJ/mol, suggesting that the physisorbed As2O3 is not stable enough and could be easily transformed into chemisorption structures by thermal vibration.

Adsorption Process
Due to the continuity of energy, the adsorption process can be characterized as an energy-drop process, including both physisorption and chemisorption.

Transformation Process of Physisorption Structures to Chemisorption Structures
In the following part, the transition state number n is abbreviated as TSn, and the intermediate position number n is abbreviated as IPn, for short.
As shown in Figure 1, when As2O3 approaches the surface with vibration along the surface, the physisorption structure transforms into a chemisorption structure during one or two transition state. For instance, P1 to C7 (Figure 1a), P2 to C8 (Figure 1b) and P3 to C8 (Figure 1c). The energy barrier is low, from 1.4 kJ/mol to 13.9 kJ/mol, suggesting that the physisorbed As2O3 is not stable enough and could be easily transformed into chemisorption structures by thermal vibration.

Adsorption Process
Due to the continuity of energy, the adsorption process can be characterized as an energy-drop process, including both physisorption and chemisorption.

Transformation Process of Physisorption Structures to Chemisorption Structures
In the following part, the transition state number n is abbreviated as TSn, and the intermediate position number n is abbreviated as IPn, for short.
As shown in Figure 1, when As2O3 approaches the surface with vibration along the surface, the physisorption structure transforms into a chemisorption structure during one or two transition state. For instance, P1 to C7 (Figure 1a), P2 to C8 (Figure 1b) and P3 to C8 (Figure 1c). The energy barrier is low, from 1.4 kJ/mol to 13.9 kJ/mol, suggesting that the physisorbed As2O3 is not stable enough and could be easily transformed into chemisorption structures by thermal vibration.

Adsorption Process
Due to the continuity of energy, the adsorption process can be characterized as an energy-drop process, including both physisorption and chemisorption.

Transformation Process of Physisorption Structures to Chemisorption Structures
In the following part, the transition state number n is abbreviated as TSn, and the intermediate position number n is abbreviated as IPn, for short.
As shown in Figure 1, when As2O3 approaches the surface with vibration along the surface, the physisorption structure transforms into a chemisorption structure during one or two transition state. For instance, P1 to C7 (Figure 1a), P2 to C8 (Figure 1b) and P3 to C8 (Figure 1c). The energy barrier is low, from 1.4 kJ/mol to 13.9 kJ/mol, suggesting that the physisorbed As2O3 is not stable enough and could be easily transformed into chemisorption structures by thermal vibration.

Adsorption Process
Due to the continuity of energy, the adsorption process can be characterized as an energy-drop process, including both physisorption and chemisorption.

Transformation Process of Physisorption Structures to Chemisorption Structures
In the following part, the transition state number n is abbreviated as TSn, and the intermediate position number n is abbreviated as IPn, for short.
As shown in Figure 1, when As2O3 approaches the surface with vibration along the surface, the physisorption structure transforms into a chemisorption structure during one or two transition state. For instance, P1 to C7 (Figure 1a), P2 to C8 (Figure 1b) and P3 to C8 (Figure 1c). The energy barrier is low, from 1.4 kJ/mol to 13.9 kJ/mol, suggesting that the physisorbed As2O3 is not stable enough and could be easily transformed into chemisorption structures by thermal vibration.

Adsorption Process
Due to the continuity of energy, the adsorption process can be characterized as an energy-drop process, including both physisorption and chemisorption.

Transformation Process of Physisorption Structures to Chemisorption Structures
In the following part, the transition state number n is abbreviated as TSn, and the intermediate position number n is abbreviated as IPn, for short.
As shown in Figure 1, when As2O3 approaches the surface with vibration along the surface, the physisorption structure transforms into a chemisorption structure during one or two transition state. For instance, P1 to C7 (Figure 1a), P2 to C8 (Figure 1b) and P3 to C8 (Figure 1c). The energy barrier is low, from 1.4 kJ/mol to 13.9 kJ/mol, suggesting that the physisorbed As2O3 is not stable enough and could be easily transformed into chemisorption structures by thermal vibration.

Adsorption Process
Due to the continuity of energy, the adsorption process can be characterized as an energy-drop process, including both physisorption and chemisorption.

Transformation Process of Physisorption Structures to Chemisorption Structures
In the following part, the transition state number n is abbreviated as TSn, and the intermediate position number n is abbreviated as IPn, for short.
As shown in Figure 1, when As2O3 approaches the surface with vibration along the surface, the physisorption structure transforms into a chemisorption structure during one or two transition state. For instance, P1 to C7 (Figure 1a), P2 to C8 (Figure 1b) and P3 to C8 (Figure 1c). The energy barrier is low, from 1.4 kJ/mol to 13.9 kJ/mol, suggesting that the physisorbed As2O3 is not stable enough and could be easily transformed into chemisorption structures by thermal vibration.

Adsorption Process
Due to the continuity of energy, the adsorption process can be characterized as an energy-drop process, including both physisorption and chemisorption.

Transformation Process of Physisorption Structures to Chemisorption Structures
In the following part, the transition state number n is abbreviated as TSn, and the intermediate position number n is abbreviated as IPn, for short.
As shown in Figure 1, when As2O3 approaches the surface with vibration along the surface, the physisorption structure transforms into a chemisorption structure during one or two transition state. For instance, P1 to C7 (Figure 1a), P2 to C8 (Figure 1b) and P3 to C8 (Figure 1c). The energy barrier is low, from 1.4 kJ/mol to 13.9 kJ/mol, suggesting that the physisorbed As2O3 is not stable enough and could be easily transformed into chemisorption structures by thermal vibration.

Adsorption Process
Due to the continuity of energy, the adsorption process can be characterized as an energy-drop process, including both physisorption and chemisorption.

Transformation Process of Physisorption Structures to Chemisorption Structures
In the following part, the transition state number n is abbreviated as TSn, and the intermediate position number n is abbreviated as IPn, for short.
As shown in Figure 1, when As2O3 approaches the surface with vibration along the surface, the physisorption structure transforms into a chemisorption structure during one or two transition state. For instance, P1 to C7 (Figure 1a), P2 to C8 (Figure 1b) and P3 to C8 (Figure 1c). The energy barrier is low, from 1.4 kJ/mol to 13.9 kJ/mol, suggesting that the physisorbed As2O3 is not stable enough and could be easily transformed into chemisorption structures by thermal vibration.

Adsorption Process
Due to the continuity of energy, the adsorption process can be characterized as an energy-drop process, including both physisorption and chemisorption.

Transformation Process of Physisorption Structures to Chemisorption Structures
In the following part, the transition state number n is abbreviated as TSn, and the intermediate position number n is abbreviated as IPn, for short.
As shown in Figure 1, when As2O3 approaches the surface with vibration along the surface, the physisorption structure transforms into a chemisorption structure during one or two transition state. For instance, P1 to C7 (Figure 1a), P2 to C8 (Figure 1b) and P3 to C8 (Figure 1c). The energy barrier is low, from 1.4 kJ/mol to 13.9 kJ/mol, suggesting that the physisorbed As2O3 is not stable enough and could be easily transformed into chemisorption structures by thermal vibration.

Adsorption Process
Due to the continuity of energy, the adsorption process can be characterized as an energy-drop process, including both physisorption and chemisorption.

Transformation Process of Physisorption Structures to Chemisorption Structures
In the following part, the transition state number n is abbreviated as TSn, and the intermediate position number n is abbreviated as IPn, for short.
As shown in Figure 1, when As2O3 approaches the surface with vibration along the surface, the physisorption structure transforms into a chemisorption structure during one or two transition state. For instance, P1 to C7 (Figure 1a), P2 to C8 (Figure 1b) and P3 to C8 (Figure 1c). The energy barrier is low, from 1.4 kJ/mol to 13.9 kJ/mol, suggesting that the physisorbed As2O3 is not stable enough and could be easily transformed into chemisorption structures by thermal vibration.

Adsorption Process
Due to the continuity of energy, the adsorption process can be characterized as an energy-drop process, including both physisorption and chemisorption.

Transformation Process of Physisorption Structures to Chemisorption Structures
In the following part, the transition state number n is abbreviated as TS n , and the intermediate position number n is abbreviated as IP n , for short.
As shown in Figure 1, when As 2 O 3 approaches the surface with vibration along the surface, the physisorption structure transforms into a chemisorption structure during one or two transition state. For instance, P 1 to C 7 (Figure 1a), P 2 to C 8 ( Figure 1b) and P 3 to C 8 (Figure 1c). The energy barrier is low, from 1.4 kJ/mol to 13.9 kJ/mol, suggesting that the physisorbed As 2 O 3 is not stable enough and could be easily transformed into chemisorption structures by thermal vibration. position number n is abbreviated as IPn, for short.
As shown in Figure 1, when As2O3 approaches the surface with vibration along the surface, the physisorption structure transforms into a chemisorption structure during one or two transition state. For instance, P1 to C7 (Figure 1a), P2 to C8 (Figure 1b) and P3 to C8 (Figure 1c). The energy barrier is low, from 1.4 kJ/mol to 13.9 kJ/mol, suggesting that the physisorbed As2O3 is not stable enough and could be easily transformed into chemisorption structures by thermal vibration.

Transformation Process of Chemisorption Structures
Chemisorbed As2O3 gradually transforms into more stable structures. Different possible reaction paths were calculated. The four categories of chemisorption structures can be sorted by the Eads of each as Category IV < Category III ≈ Category II < Category I.
Category I has relatively high energy, i.e., relatively low stability, its transformation to Category II, III and IV could be triggered by molecular thermal vibration.
The pathway that Category I transforms to Category II is shown in Figure 2. Firstly, C1 transforms into C6 (Category II) and then C5 (Category III), with the energy barrier of 10.8 kJ/mol, 16.7 kJ/mol, and 6.7 kJ/mol, respectively. As shown in Figure 2, Category I transforms into Category IV along with another reaction path, the related energy barrier is 7.4 kJ/mol. The relatively low energy barrier suggests that Category I is not stable enough, and could easily transform to Category II, III and IV.

Transformation Process of Chemisorption Structures
Chemisorbed As 2 O 3 gradually transforms into more stable structures. Different possible reaction paths were calculated. The four categories of chemisorption structures can be sorted by the E ads of each as Category IV < Category III ≈ Category II < Category I.
Category I has relatively high energy, i.e., relatively low stability, its transformation to Category II, III and IV could be triggered by molecular thermal vibration.
The pathway that Category I transforms to Category II is shown in Figure 2. Firstly, C 1 transforms into C 6 (Category II) and then C 5 (Category III), with the energy barrier of 10.8 kJ/mol, 16.7 kJ/mol, and 6.7 kJ/mol, respectively. As shown in Figure 2, Category I transforms into Category IV along with another reaction path, the related energy barrier is 7.4 kJ/mol. The relatively low energy barrier suggests that Category I is not stable enough, and could easily transform to Category II, III and IV.
The pathway that Category I transforms to Category II is shown in Figure 2. Firstly, C1 transforms into C6 (Category II) and then C5 (Category III), with the energy barrier of 10.8 kJ/mol, 16.7 kJ/mol, and 6.7 kJ/mol, respectively. As shown in Figure 2, Category I transforms into Category IV along with another reaction path, the related energy barrier is 7.4 kJ/mol. The relatively low energy barrier suggests that Category I is not stable enough, and could easily transform to Category II, III and IV.  The reaction path of Category II is shown in Figure 3. C 3 firstly transforms into intermediate and then converts to C 9 . The corresponding energy barrier is 16.1 kJ/mol and 83.0 kJ/mol, proving that Category II transforms to Category IV with the special direction of thermal vibration. The reaction path of Category II is shown in Figure 3. C3 firstly transforms into intermediate and then converts to C9. The corresponding energy barrier is 16.1 kJ/mol and 83.0 kJ/mol, proving that Category II transforms to Category IV with the special direction of thermal vibration. Structures of Category III can transform into Category II, as shown in Figure 4. The spoonshaped structure of As2O3 in C7 disappears and then overcomes a 48.3 kJ/mol energy barrier to transform to C5, indicating the conversion of Category III to Category II. Category IV is the most stable category. C8, C9, and C10 can transform into each other (shown in Figure 5). As2O3's As does not move during the transformation. When all of As2O3's O in C8 vibrate, C8 converts to C9, and the energy barrier is 41.6 kJ/mol (Figure 5a). When one of As2O3's O in C8 vibrates, C8 converts to C10, and the energy barrier is 153.3 kJ/mol (Figure 5b). When one of the oxygen Structures of Category III can transform into Category II, as shown in Figure 4. The spoon-shaped structure of As 2 O 3 in C 7 disappears and then overcomes a 48.3 kJ/mol energy barrier to transform to C 5 , indicating the conversion of Category III to Category II. The reaction path of Category II is shown in Figure 3. C3 firstly transforms into intermediate and then converts to C9. The corresponding energy barrier is 16.1 kJ/mol and 83.0 kJ/mol, proving that Category II transforms to Category IV with the special direction of thermal vibration. Structures of Category III can transform into Category II, as shown in Figure 4. The spoonshaped structure of As2O3 in C7 disappears and then overcomes a 48.3 kJ/mol energy barrier to transform to C5, indicating the conversion of Category III to Category II. Category IV is the most stable category. C8, C9, and C10 can transform into each other (shown in Figure 5). As2O3's As does not move during the transformation. When all of As2O3's O in C8 vibrate, C8 converts to C9, and the energy barrier is 41.6 kJ/mol (Figure 5a). When one of As2O3's O in C8 vibrates, C8 converts to C10, and the energy barrier is 153.3 kJ/mol (Figure 5b). When one of the oxygen Category IV is the most stable category. C 8 , C 9 , and C 10 can transform into each other (shown in Figure 5). As 2 O 3 's As does not move during the transformation. When all of As 2 O 3 's O in C 8 vibrate, C 8 converts to C 9 , and the energy barrier is 41.6 kJ/mol (Figure 5a). When one of As 2 O 3 's O in C 8 vibrates, C 8 converts to C 10 , and the energy barrier is 153.3 kJ/mol (Figure 5b). When one of the oxygen atoms of As 2 O 3 in C 9 vibrates, C 9 converts to C 10 , and the energy barrier is 154.4 kJ/mol (Figure 5c). The difference in energy barrier is caused by the movement distance of As 2 O 3 's O being motivated by thermal vibration. In the first reaction, the movement distance of As 2 O 3 's O is shorter than that in the second and third reactions, which demands relatively lower energy to overcome the energy barrier.

Path of the Reaction
According to above-mentioned processes, the reaction paths can be concluded as follows; firstly, the isolated As2O3 is physisorbed on a CaO surface (As2O3's O weakly interacts with superficial Ca); secondly, the physisorbed As2O3 transforms to chemisorbed As2O3. (As2O3's As interacts with superficial O); and thirdly, due to thermal vibration, the chemisorbed As2O3 transforms into more stable chemisorbed As2O3 (the position of As2O3's O changed).
The adsorption path of As2O3 was summarized as the process shown in Figure 6. These reactions could be classified as three types according to the energy barrier with the aim to reflect the intensity of the required reaction temperature. The number of superficial CaO occupied by As2O3 is also considered in order to describe the adsorption reaction equation.
Blue arrow: energy barrier is in the range of 0-40 kJ/mol, suggesting that reaction is likely to

Path of the Reaction
According to above-mentioned processes, the reaction paths can be concluded as follows; firstly, the isolated As 2 O 3 is physisorbed on a CaO surface (As 2 O 3 's O weakly interacts with superficial Ca); secondly, the physisorbed As 2 O 3 transforms to chemisorbed As 2 O 3 . (As 2 O 3 's As interacts with superficial O); and thirdly, due to thermal vibration, the chemisorbed As 2 O 3 transforms into more stable chemisorbed As 2 O 3 (the position of As 2 O 3 's O changed). The adsorption path of As 2 O 3 was summarized as the process shown in Figure 6. These reactions could be classified as three types according to the energy barrier with the aim to reflect the intensity of the required reaction temperature. The number of superficial CaO occupied by As 2 O 3 is also considered in order to describe the adsorption reaction equation.
The adsorption path of As2O3 was summarized as the process shown in Figure 6. These reactions could be classified as three types according to the energy barrier with the aim to reflect the intensity of the required reaction temperature. The number of superficial CaO occupied by As2O3 is also considered in order to describe the adsorption reaction equation.
Blue arrow: energy barrier is in the range of 0-40 kJ/mol, suggesting that reaction is likely to occur under a relatively low-temperature condition.
Yellow arrow: energy barrier is in the range of 40-100 kJ/mol, suggesting that reaction is likely to occur under a relatively medium-temperature condition.
Red arrow: energy barrier is in the range of 100-200 kJ/mol, suggesting that reaction is likely to occur under a relatively high-temperature condition.  Blue arrow: energy barrier is in the range of 0-40 kJ/mol, suggesting that reaction is likely to occur under a relatively low-temperature condition.
Yellow arrow: energy barrier is in the range of 40-100 kJ/mol, suggesting that reaction is likely to occur under a relatively medium-temperature condition.
Red arrow: energy barrier is in the range of 100-200 kJ/mol, suggesting that reaction is likely to occur under a relatively high-temperature condition. Figure 6 reveals that three main reaction paths may exist: 1. As 2 O 3 → P 1 → C 7 → C 9 →C 10 ; 2.
Under a relatively low-temperature condition (blue arrow, 0-40 kJ/mol), the main products are C 7 and C 8 (blue grid). Three superficial Ca and one or two superficial O are involved in the reaction, representing three CaO participates in the adsorption. The adsorption equation could be written as: Under a relatively medium-temperature condition (yellow arrow, 40-100 kJ/mol), the main products are C 9 . Two superficial Ca and two superficial O participate in the structure. The adsorption equation could be written as: Under a relatively high-temperature condition (red arrow, 100-200 kJ/mol), the main product is C 10 . Three superficial Ca and two superficial O are involved in the reaction (hollow Ca represents 1/2 Ca atom). The adsorption equation could be written as: With the reaction temperature increases, adsorption product changes from Ca 3 As 2 O 6 to Ca 2 As 2 O 5 and back to Ca 3 As 2 O 6 again. Different microcosmic adsorption structures lead to different macroscopic products and reaction equation.
Besides, as shown in Figure S2, the paths of C 1 transforming to other structures have been also been found. However, no possible paths which isolated or physisorbed As 2 O 3 transforms to C 1 has been found, implying C 1 is unstable or nonexistent.

Partial Density of States (PDOS)
The change PDOS of As 2 O 3 and CaO was put in the Supplementary Materials ( Figure S3). For As 2 O 3 , the PDOS of physisorption structure 1, 2, and 3 are similar to each other. As the physisorption structure transforms to C 7 , the p state orbitals near Fermi level (from −0.6 eV to 1.9 eV) drift to lower energy level, meanwhile get energy splitting and orbital reorganization, caused by the changing of As 2 O 3 structure and the combination between As 2 O 3 's As and superficial O. When C 7 transforms to C 5 , s state orbital (−17.2 eV) energy level splits into two peaks of −18.0 eV and −16.9 eV, which is caused by the As-O bond breaking and the bonding between As and superficial O. When C 5 transform to C 8 , the p state orbital (3.7 eV) and s state orbital (−17.9 eV) energy level both split slightly. This is the result of the slight change in the surface distribution of As 2 O 3 . As the adsorption products have close energies and structures, PDOS of C 9 and C 10 are basically similar to C 8 .
For CaO slab surface, when an As 2 O 3 molecule is physisorbed on the surface, little change of PDOS is detected. When As 2 O 3 is chemisorbed, it can be seen that the superficial p orbitals around Fermi level (from −2.7 eV to 0.4 eV) drift to a lower energy range (from −5.8 eV to 0.2 eV). Moreover, a small peak (−16.8 eV) is separated from s orbitals (peak at −14.6 eV), proving that s orbitals participate in the chemisorption to some extent. Superficial p state orbitals near Fermi level play an important role in the chemisorption of As 2 O 3 . It suggests that the CaO surface's property of capturing As 2 O 3 might be improved by increasing the quantities of superficial p orbitals near Fermi level.

Influence of O 2 on Adsorbed As 2 O 3
Under the flue gas atmosphere, especially O 2 -containing atmosphere, O 2 reacts with chemisorbed As 2 O 3 ; i.e., arsenite (AsO 3 3− ) is oxidized to arsenate (AsO 4 3− ). As an example, two stable chemisorption structures (C 5 , C 9 ) identified previously were presented in  energy level, meanwhile get energy splitting and orbital reorganization, caused by the changing of As2O3 structure and the combination between As2O3's As and superficial O. When C7 transforms to C5, s state orbital (−17.2 eV) energy level splits into two peaks of −18.0 eV and −16.9 eV, which is caused by the As-O bond breaking and the bonding between As and superficial O. When C5 transform to C8, the p state orbital (3.7 eV) and s state orbital (−17.9 eV) energy level both split slightly. This is the result of the slight change in the surface distribution of As2O3. As the adsorption products have close energies and structures, PDOS of C9 and C10 are basically similar to C8. For CaO slab surface, when an As2O3 molecule is physisorbed on the surface, little change of PDOS is detected. When As2O3 is chemisorbed, it can be seen that the superficial p orbitals around Fermi level (from −2.7 eV to 0.4 eV) drift to a lower energy range (from −5.8 eV to 0.2 eV). Moreover, a small peak (−16.8 eV) is separated from s orbitals (peak at −14.6 eV), proving that s orbitals participate in the chemisorption to some extent. Superficial p state orbitals near Fermi level play an important role in the chemisorption of As2O3. It suggests that the CaO surface's property of capturing As2O3 might be improved by increasing the quantities of superficial p orbitals near Fermi level.

Influence of O2 on Adsorbed As2O3
Under the flue gas atmosphere, especially O2-containing atmosphere, O2 reacts with chemisorbed As2O3; i.e., arsenite (AsO3 3− ) is oxidized to arsenate (AsO4 3− ). As an example, two stable chemisorption structures (C5, C9) identified previously were presented in Table 4. The distance between As2O3's As and O2's O is 1.763-1.764 Å, which is close to the As-O bond length of As2O3 (1.628 Å). The distance between O2's O and superficial Ca is 2.247-2.263 Å. According to the electron density cloud, one of O2's O overlaps with As2O3's As. The other O of O2 overlaps slightly with the superficial Ca. Based on Figure 6 and Table 4, the reaction equation of adsorption under O2 atmosphere can be written as Equations (5)-(7), corresponding to low-temperature, medium-temperature, and hightemperature adsorption, respectively.
With the increase of reaction temperature, adsorption product changed from Ca3As2O8 to Materials 2019, 12, x FOR PEER REVIEW 10 of 13 energy level, meanwhile get energy splitting and orbital reorganization, caused by the changing of As2O3 structure and the combination between As2O3's As and superficial O. When C7 transforms to C5, s state orbital (−17.2 eV) energy level splits into two peaks of −18.0 eV and −16.9 eV, which is caused by the As-O bond breaking and the bonding between As and superficial O. When C5 transform to C8, the p state orbital (3.7 eV) and s state orbital (−17.9 eV) energy level both split slightly. This is the result of the slight change in the surface distribution of As2O3. As the adsorption products have close energies and structures, PDOS of C9 and C10 are basically similar to C8. For CaO slab surface, when an As2O3 molecule is physisorbed on the surface, little change of PDOS is detected. When As2O3 is chemisorbed, it can be seen that the superficial p orbitals around Fermi level (from −2.7 eV to 0.4 eV) drift to a lower energy range (from −5.8 eV to 0.2 eV). Moreover, a small peak (−16.8 eV) is separated from s orbitals (peak at −14.6 eV), proving that s orbitals participate in the chemisorption to some extent. Superficial p state orbitals near Fermi level play an important role in the chemisorption of As2O3. It suggests that the CaO surface's property of capturing As2O3 might be improved by increasing the quantities of superficial p orbitals near Fermi level.

Influence of O2 on Adsorbed As2O3
Under the flue gas atmosphere, especially O2-containing atmosphere, O2 reacts with chemisorbed As2O3; i.e., arsenite (AsO3 3− ) is oxidized to arsenate (AsO4 3− ). As an example, two stable chemisorption structures (C5, C9) identified previously were presented in Table 4. The distance between As2O3's As and O2's O is 1.763-1.764 Å, which is close to the As-O bond length of As2O3 (1.628 Å). The distance between O2's O and superficial Ca is 2.247-2.263 Å. According to the electron density cloud, one of O2's O overlaps with As2O3's As. The other O of O2 overlaps slightly with the superficial Ca. Based on Figure 6 and Table 4, the reaction equation of adsorption under O2 atmosphere can be written as Equations (5)-(7), corresponding to low-temperature, medium-temperature, and hightemperature adsorption, respectively.
With the increase of reaction temperature, adsorption product changed from Ca3As2O8 to Materials 2019, 12, x FOR PEER REVIEW 10 of 13 energy level, meanwhile get energy splitting and orbital reorganization, caused by the changing of As2O3 structure and the combination between As2O3's As and superficial O. When C7 transforms to C5, s state orbital (−17.2 eV) energy level splits into two peaks of −18.0 eV and −16.9 eV, which is caused by the As-O bond breaking and the bonding between As and superficial O. When C5 transform to C8, the p state orbital (3.7 eV) and s state orbital (−17.9 eV) energy level both split slightly. This is the result of the slight change in the surface distribution of As2O3. As the adsorption products have close energies and structures, PDOS of C9 and C10 are basically similar to C8. For CaO slab surface, when an As2O3 molecule is physisorbed on the surface, little change of PDOS is detected. When As2O3 is chemisorbed, it can be seen that the superficial p orbitals around Fermi level (from −2.7 eV to 0.4 eV) drift to a lower energy range (from −5.8 eV to 0.2 eV). Moreover, a small peak (−16.8 eV) is separated from s orbitals (peak at −14.6 eV), proving that s orbitals participate in the chemisorption to some extent. Superficial p state orbitals near Fermi level play an important role in the chemisorption of As2O3. It suggests that the CaO surface's property of capturing As2O3 might be improved by increasing the quantities of superficial p orbitals near Fermi level.

Influence of O2 on Adsorbed As2O3
Under the flue gas atmosphere, especially O2-containing atmosphere, O2 reacts with chemisorbed As2O3; i.e., arsenite (AsO3 3− ) is oxidized to arsenate (AsO4 3− ). As an example, two stable chemisorption structures (C5, C9) identified previously were presented in Table 4. The distance between As2O3's As and O2's O is 1.763-1.764 Å, which is close to the As-O bond length of As2O3 (1.628 Å). The distance between O2's O and superficial Ca is 2.247-2.263 Å. According to the electron density cloud, one of O2's O overlaps with As2O3's As. The other O of O2 overlaps slightly with the superficial Ca. Based on Figure 6 and Table 4, the reaction equation of adsorption under O2 atmosphere can be written as Equations (5)-(7), corresponding to low-temperature, medium-temperature, and hightemperature adsorption, respectively.
With the increase of reaction temperature, adsorption product changed from Ca3As2O8 to energy level, meanwhile get energy splitting and orbital reorganization, caused by the changing of As2O3 structure and the combination between As2O3's As and superficial O. When C7 transforms to C5, s state orbital (−17.2 eV) energy level splits into two peaks of −18.0 eV and −16.9 eV, which is caused by the As-O bond breaking and the bonding between As and superficial O. When C5 transform to C8, the p state orbital (3.7 eV) and s state orbital (−17.9 eV) energy level both split slightly. This is the result of the slight change in the surface distribution of As2O3. As the adsorption products have close energies and structures, PDOS of C9 and C10 are basically similar to C8. For CaO slab surface, when an As2O3 molecule is physisorbed on the surface, little change of PDOS is detected. When As2O3 is chemisorbed, it can be seen that the superficial p orbitals around Fermi level (from −2.7 eV to 0.4 eV) drift to a lower energy range (from −5.8 eV to 0.2 eV). Moreover, a small peak (−16.8 eV) is separated from s orbitals (peak at −14.6 eV), proving that s orbitals participate in the chemisorption to some extent. Superficial p state orbitals near Fermi level play an important role in the chemisorption of As2O3. It suggests that the CaO surface's property of capturing As2O3 might be improved by increasing the quantities of superficial p orbitals near Fermi level.
With the increase of reaction temperature, adsorption product changed from Ca3As2O8 to Materials 2019, 12, x FOR PEER REVIEW 10 of 13 energy level, meanwhile get energy splitting and orbital reorganization, caused by the changing of As2O3 structure and the combination between As2O3's As and superficial O. When C7 transforms to C5, s state orbital (−17.2 eV) energy level splits into two peaks of −18.0 eV and −16.9 eV, which is caused by the As-O bond breaking and the bonding between As and superficial O. When C5 transform to C8, the p state orbital (3.7 eV) and s state orbital (−17.9 eV) energy level both split slightly. This is the result of the slight change in the surface distribution of As2O3. As the adsorption products have close energies and structures, PDOS of C9 and C10 are basically similar to C8. For CaO slab surface, when an As2O3 molecule is physisorbed on the surface, little change of PDOS is detected. When As2O3 is chemisorbed, it can be seen that the superficial p orbitals around Fermi level (from −2.7 eV to 0.4 eV) drift to a lower energy range (from −5.8 eV to 0.2 eV). Moreover, a small peak (−16.8 eV) is separated from s orbitals (peak at −14.6 eV), proving that s orbitals participate in the chemisorption to some extent. Superficial p state orbitals near Fermi level play an important role in the chemisorption of As2O3. It suggests that the CaO surface's property of capturing As2O3 might be improved by increasing the quantities of superficial p orbitals near Fermi level.
With the increase of reaction temperature, adsorption product changed from Ca3As2O8 to Materials 2019, 12, x FOR PEER REVIEW 10 of 13 energy level, meanwhile get energy splitting and orbital reorganization, caused by the changing of As2O3 structure and the combination between As2O3's As and superficial O. When C7 transforms to C5, s state orbital (−17.2 eV) energy level splits into two peaks of −18.0 eV and −16.9 eV, which is caused by the As-O bond breaking and the bonding between As and superficial O. When C5 transform to C8, the p state orbital (3.7 eV) and s state orbital (−17.9 eV) energy level both split slightly. This is the result of the slight change in the surface distribution of As2O3. As the adsorption products have close energies and structures, PDOS of C9 and C10 are basically similar to C8. For CaO slab surface, when an As2O3 molecule is physisorbed on the surface, little change of PDOS is detected. When As2O3 is chemisorbed, it can be seen that the superficial p orbitals around Fermi level (from −2.7 eV to 0.4 eV) drift to a lower energy range (from −5.8 eV to 0.2 eV). Moreover, a small peak (−16.8 eV) is separated from s orbitals (peak at −14.6 eV), proving that s orbitals participate in the chemisorption to some extent. Superficial p state orbitals near Fermi level play an important role in the chemisorption of As2O3. It suggests that the CaO surface's property of capturing As2O3 might be improved by increasing the quantities of superficial p orbitals near Fermi level.

Influence of O2 on Adsorbed As2O3
Under the flue gas atmosphere, especially O2-containing atmosphere, O2 reacts with chemisorbed As2O3; i.e., arsenite (AsO3 3− ) is oxidized to arsenate (AsO4 3− ). As an example, two stable chemisorption structures (C5, C9) identified previously were presented in  energy level, meanwhile get energy splitting and orbital reorganization, caused by the changing of As2O3 structure and the combination between As2O3's As and superficial O. When C7 transforms to C5, s state orbital (−17.2 eV) energy level splits into two peaks of −18.0 eV and −16.9 eV, which is caused by the As-O bond breaking and the bonding between As and superficial O. When C5 transform to C8, the p state orbital (3.7 eV) and s state orbital (−17.9 eV) energy level both split slightly. This is the result of the slight change in the surface distribution of As2O3. As the adsorption products have close energies and structures, PDOS of C9 and C10 are basically similar to C8. For CaO slab surface, when an As2O3 molecule is physisorbed on the surface, little change of PDOS is detected. When As2O3 is chemisorbed, it can be seen that the superficial p orbitals around Fermi level (from −2.7 eV to 0.4 eV) drift to a lower energy range (from −5.8 eV to 0.2 eV). Moreover, a small peak (−16.8 eV) is separated from s orbitals (peak at −14.6 eV), proving that s orbitals participate in the chemisorption to some extent. Superficial p state orbitals near Fermi level play an important role in the chemisorption of As2O3. It suggests that the CaO surface's property of capturing As2O3 might be improved by increasing the quantities of superficial p orbitals near Fermi level.

Influence of O2 on Adsorbed As2O3
Under the flue gas atmosphere, especially O2-containing atmosphere, O2 reacts with chemisorbed As2O3; i.e., arsenite (AsO3 3− ) is oxidized to arsenate (AsO4 3− ). As an example, two stable chemisorption structures (C5, C9) identified previously were presented in Table 4. The distance between As2O3's As and O2's O is 1.763-1.764 Å, which is close to the As-O bond length of As2O3 (1.628 Å). The distance between O2's O and superficial Ca is 2.247-2.263 Å. According to the electron density cloud, one of O2's O overlaps with As2O3's As. The other O of O2 overlaps slightly with the superficial Ca. Based on Figure 6 and Table 4, the reaction equation of adsorption under O2 atmosphere can be written as Equations (5)-(7), corresponding to low-temperature, medium-temperature, and hightemperature adsorption, respectively. 3CaO + As2O3 + O2 → Ca3As2O8 (5) 3CaO + As2O3 + O2 → Ca3As2O8 (6) 3CaO + As2O3 + O2 → Ca3As2O8 (7) With the increase of reaction temperature, adsorption product changed from Ca3As2O8 to Ca2As2O7 and then to Ca3As2O8 in an O2-containing atmosphere. According to this research, the Based on Figure 6 and Table 4, the reaction equation of adsorption under O 2 atmosphere can be written as Equations (5)-(7), corresponding to low-temperature, medium-temperature, and high-temperature adsorption, respectively.
With the increase of reaction temperature, adsorption product changed from Ca 3 As 2 O 8 to Ca 2 As 2 O 7 and then to Ca 3 As 2 O 8 in an O 2 -containing atmosphere. According to this research, the product under low-temperature and high-temperature conditions is Ca 3 As 2 O 8 with different structures, i.e., crystalline form. Under a medium-temperature condition, the main product is Ca 3 As 2 O 7 .
Previous experimental research consistently reflected that the adsorption product with O 2 existence is AsO 4 3− , while different opinions existed regarding the adsorption structures. The study of Jadhav [12] found that the adsorption product obtained under 500 • C was mainly Ca 3 As 2 O 8 (JCPDS No.01-0933). Under 700 • C and 900 • C, the product was Ca 2 As 2 O 7 (JCPDS No.17-0444). When the temperature increased to 1000 • C, the reaction product was Ca 3 As 2 O 8 (JCPDS No.26-0295). Mahuli [41] (600 • C and 1000 • C) and Sterling [11] (800 • C) found that the adsorption product was Ca 3 As 2 O 8 (JCPDS No. 26-0295), while the sorbent used by Mahuli was Ca(OH) 2 . Li [13] found that the product obtained under 600 • C mainly belonged to Ca 3 As 2 O 8 crystal structure (JCPDS No. 01-0933), and another kind of Ca 3 As 2 O 8 crystal (JCPDS No. 73-1928) was identified for the products obtained under 800 • C and 1000 • C. The role of temperature on adsorption product transformation is qualitatively described. The more detailed description of the product layer development is associated with many other factors, such as the concentration and flow rate of As 2 O 3 and O 2 , and the quantity and granular size of CaO. The quantitative description of the adsorption process is still a very difficult challenge. Nevertheless, the DFT calculation findings revealed by this study could directly explain the experimental results obtained by previous researchers, which might provide some meaningful insight to understand the process of As 2 O 3 adsorption on CaO.

Conclusions
The mechanisms of As 2 O 3 adsorption on a CaO surface have been studied by using DFT calculation; conclusions are as follows: (1) Physisorption active sites are composed of superficial Ca atoms that interact with O of As 2 O 3 .
Chemisorption active sites are superficial O atoms that interact with As of As 2 O 3 ; (2) The adsorption process can be described as follows: the isolated As 2 O 3 molecule is firstly adsorbed on the CaO surface by physisorption, and then physisorbed As 2 O 3 will transform to chemisorbed As 2 O 3 . Due to thermal vibration, the chemisorbed As 2 O 3 would overcome the energy barrier and transform to a more stable chemisorbed As 2 O 3 state. The adsorption product is AsO 3 3− ; (3) The adsorption products of As 2 O 3 under an O 2 -containing atmosphere are AsO 4 3− .
The adsorption product's structure is influenced by the adsorption temperature. Under relatively low-temperature, the product is Ca 3 As 2 O 8 ; under relatively medium-temperature, the product is Ca 3 As 2 O 7 ; and under relatively high-temperature, the product is Ca 3 As 2 O 8 .
The consistency between DFT calculation and the previous experiments proves high possibilities to design and optimized the CaO-based adsorbents by modifying O sites or other elements. Besides, other flue gases such as SO 2 or CO 2 can be involved in the following study to achieve materials design under real flue gas conditions. The optimized CaO-based adsorbents should be of high industrial value, could be applied in the injection of limestone into the furnace, CaO looping reactor, and dry desulfurization, etc.
Supplementary Materials: The following are available online at http://www.mdpi.com/1996-1944/12/4/677/s1. Table S1: Changes in physical and chemical properties of different surface size; Table S2: Changes in physical and chemical properties of different layers; Figure S1: Initial adsorbate structures; Figure S2: Paths and structures of