Efficient Sn Recovery from SnO2 by Alkane (CxHy=2x+2, 0 ≤ x ≤ 4) Reduction

We study the mechanism of alkane reduction of SnO2 for efficient low-temperature recovery of Sn from SnO2. Based on thermodynamic simulation results, we comparatively analyze the reduction behavior and the efficiency of SnO2 reduction by H2 and alkanes (CxHy=2x+2, 0 ≤ x ≤ 4). We found that alkanes (n·CxHy) with the higher nx generally complete the reduction of SnO2 (T100) at the lower temperature. Moreover, the T100 of the SnO2 reduction by alkanes (n·CxHy) was decreased from the T100 of pure hydrogen with the same amount of hydrogen atoms (n·Hy). We found that the concentration of a gas phase product mixture, the amount of the produced solid carbon, and the T100 complementary vary as a function of the nx and ny, the total amount of carbon and hydrogen atoms in the reducing gas phase molecules. Our results demonstrate a viability of the low temperature reduction method of SnO2 by alkanes for efficient recovery of Sn from SnO2, which can be applied for Sn recovery from Sn containing industrial wastes or Sn ores with economic value added that is held by the co-produced H2.

Recovering (extracting) valuable metallic elements from industrial wastes is technically important for the efficient recycle of earth unabundant resources. However, the current dry-smelting, hydro-smelting, or combined smelting-electrolytic refining technologies, which are commonly applied for extraction of high purity metals from ores or used oxidized scraps, are not environmentally friendly [1][2][3][4][5][6][7][8][9] . Because the high quality minable ores deplete first, designing environmentally friendly techniques are necessary to set up ecofriendly recovery processes of used metals.
Although Sn (Tin) is relatively earth unabundant among the industrially demanded metals, Sn and Sn oxides play a key role in several electronic devices and products such as sensor 10,11 , Pb-free solder 12,13 , or transparent electrode [14][15][16] . Despite of the high LME (London Metal Exchange) market price ($16,450.00/metric ton as of October 4, 2019) of Sn 17 , which is more than 3 and 1.5 times expansive than Cu and Ni, respectively, 70% of the annually consumed Sn is not appropriately recycled 18 . Recovery of Sn from used SnO 2 or oxidized metal scrap proceeds in a similar process with the ore smelting in the presence of a proper and strong reducer, usually cokes. However, a poor solid-solid contact between SnO x and cokes and a high operation temperature lowers the overall efficiency 19 .
In our previous report 20, 21 , we have designed a methane reduction (MR) method, an ecofriendly and simple versatile process of Sn recovery from SnO x containing industrial wastes, which also can be potentially applied for Sn ore smelting. A direct facile contact between gas phase methane and SnO x improves the efficiency of the reaction. Moreover, multiple reductants provided by methane (hydrogen and carbon) sequentially reduce SnO x , producing H 2 O, H 2 , CO, or CO 2 depending on the reduction conditions 21 . The geometry of a SnO 2 bound methane inhibits the initial participation of the carbon of methane to the reduction process 21 . Rather, the released hydrogen atoms from methane attribute to the initial reduction power of methane 21 . The extended release of two kinds of reducing agents from methane assures the versatility of the MR of SnO x and the increased economic efficiency 21 . Another interesting finding was that the H 2 /CO ratio in the produced gas varies as a function of the CH 4 /SnO 2 ratio. We found that the H 2 /CO ratio increases if the MR of SnO x proceeds under oxygen depleted conditions because the late released more oxophilic carbon takes up oxygen atoms from SnO x and gas phase H 2 O 21 .
Unique chemistry between Sn and CH 4 has reported in a recent study by the Metiu and McFarland groups 22 . They found that molten Sn and other transition metals can directly dissociate CH 4 into solid C and H 2 and suggested that such direct H 2 production from CH 4 without CO 2 formation as an advanced H 2 production method www.nature.com/scientificreports www.nature.com/scientificreports/ from hydrocarbons 22 . Based on our previous findings on the CH 4 reduction of SnO 2 21 , we have reached to a hypothesis that use of alkanes with more carbon and hydrogen contents per mole (C x H y=2x+2 , 0 ≤ x ≤ 4) would accelerate the SnO 2 reduction and also lower the reaction temperature. Moreover, if molten Sn, which is produced upon SnO 2 reduction by alkanes, assists dissociation of alkanes into carbon and hydrogen, the SnO 2 reduction would occur under the stronger reduction atmosphere so that the overall SnO 2 reduction will be greatly accelerated.
In this letter, we use a combined study of thermodynamic simulations and density functional theory (DFT) calculations to study the effect of the C/H 2 ratio in the reducing gas on the efficiency of the reduction of SnO 2 . To provide a fundamental insight into the mechanism of SnO 2 reduction by alkanes with the higher carbon and hydrogen contents per mole and further clarify the reduction potential of the applied alkanes, we introduce commercially available alkanes (C x H y=2x+2 , 0 ≤ x ≤ 4) as a reducing agent for SnO 2 reduction. The efficiency of the alkane reduction of SnO 2 is evaluated by the reduction complete temperature, T 100 , and compared with the T 100 of mole-balanced pure hydrogen. We find that the T 100 is an inverse exponential function of the amount of supplied reducing agent (H 2 or alkane) and that the addition of carbon as a form of alkane significantly lowers the T 100 from that of the H 2 reduction of SnO 2 . Our findings predict that the operation temperature of the alkane reduction of SnO 2 can be adjusted by controlling the composition and the x/y ratio of the reducing gas, suggesting an easy and industrially highly accessible recycling process of SnO x containing industrial wastes.

Results and Discussion
H 2 reduction of SnO 2 . Figure 1 shows the equilibrium concentrations of the mixture of SnO 2 and n•H 2 (n = 2, 4, or 6) at between 0 to 1200 °C. Obviously, H 2 reduces SnO 2 into Sn through a two-step process. In all cases, SnO 2 was first transferred to SnO. SnO was formed at below the melting temperature of Sn (231.9 °C) and further transferred to metallic Sn upon temperature increase. Under the stoichiometric condition (H 2 /SnO 2 = 2, 2 moles of H 2 is required to reduce a mole of SnO 2 to Sn and 2H 2 O), the reduction does not complete even at 1200 °C and SnO survives. The T 100 , at which SnO 2 and SnO were completely reduced to Sn, was significantly decreased upon increase of the amount of supplied H 2 up to 4 or 6 moles (H 2 /SnO 2 = 4 or 6, respectively, Table 1). Because consistent two moles of H 2 were used for SnO 2 reduction to Sn, irrespective to the initial H 2 /SnO 2 ratio, the decrease of the T 100 is presumably due to the increased chemical potential of gas phase H 2 upon increase in the H 2 /SnO 2 ratio.
Alkane reduction of SnO 2 : methane and ethane. Figure 2 shows the equilibrium concentrations of the mixture of SnO 2 and n•CH 4 (Fig. 2a,b) or n•C 2 H 6 ( Fig. 2c,d) (n = 2 or 4) at between 0 to 1200 °C. Like the cases of the H 2 reduction of SnO 2 , the T 100 is equal to the point at which the SnO and SnO 2 are completely depleted. Because a mole of CH 4 supplies total five units of reducing agents (one C and four H), a mole of SnO 2 can be easily reduced to metallic Sn. The increase of H 2 , C, and H 2 O above 200 °C shows that CH 4 was decomposed into C and H 2 and the released H 2 from CH 4 initially reduces SnO 2 . The delayed increase of CO 2 compared to the increase of C confirms that the reduction by C occurs at the higher temperature than the reduction by H 2 . In both cases (CH 4 /SnO 2 = 2 or 4) the decrease of H 2 O, C, and CO 2 is coupled with the increase of CO and H 2 , meaning that C takes up oxygen from SnO 2 under C and H 2 rich conditions. Despite the active role of hydrogen in the early stage of the reduction, carbon completes the reduction and hydrogen of CH 4 was released as gas phase H 2 . The T 100 of CH 4 reduction of SnO 2 also decreases response to the increase of the CH 4 /SnO 2 ratio (Table 2).  www.nature.com/scientificreports www.nature.com/scientificreports/ The overall reduction process, initial active reduction of SnO 2 by H 2 and complete reduction by C, was consistently appeared in the C 2 H 6 reduction of SnO 2 . C 2 H 6 decomposes rapidly into C and H 2 and the overall reduction occurs under highly reducible conditions. However, although the reduction occurs under C and H 2 rich conditions, SnO was also appeared as an intermediate, showing that the reduction of SnO 2 occurs through a two-step process. The rapidly increased H 2 upon C 2 H 6 decomposition gradually decreased as H 2 was transformed to H 2 O. Like the case of CH 4 reduction of SnO 2 , C takes up oxygen, being transformed to CO 2 and eventually, to CO. Most of the H 2 transformed to H 2 O was released upon CO formation. When the C 2 H 6 /SnO 2 increases to 4, the excess C was remained as solid state carbon even after complete reduction of SnO 2 . The T 100 values of C 2 H 6 reduction of SnO 2 were generally lower than the values of CH 4 reduction of SnO 2 ( Table 2). The effect of the amount of C and H 2 in reducing alkanes on the T 100 will be discussed below.
Alkane reduction of SnO 2 : propane and butane. C 3 H 8 , propane, and C 4 H 10 , butane, are commercially widely available alkanes and a component of liquid petroleum gas. No meaningful changes in the reduction behavior was observed in the C 3 H 8 and C 4 H 10 reduction of SnO 2 (Fig. 3). Decomposition of C 3 H 8 and C 4 H 10 caused a rapid increase of H 2 and C in the initial state of the reduction. Gradual increase of H 2 O coupled with the increase of reduced Sn and SnO represents the initial reduction of SnO 2 by H 2 released from alkanes. Because the excess amount of H 2 was supplied, even in the presence of solid state carbon, H 2 takes up oxygen from SnO 2 . Subsequent reactions between H 2 O and solid state carbon produce CO 2 , CO, and H 2 . Eventually, all of the oxygen from SnO 2 was converted to CO at high temperatures and the excess C and all of H 2 from alkanes were released as is. Upon increase of the supplied C 3 H 8 and C 4 H 10 , the T 100 was also significantly reduced (Table 3). Interestingly, the formation of SnO was suppressed at C 3 H 8 /SnO 2 = 2 and C 4 H 10 /SnO 2 = 2. Direct reduction of SnO 2 could become available under H 2 rich conditions.
Modelling the reduction trend in alkanes (n·c x H y=2x+2 , 0 ≤ x ≤ 4). The equilibrium concentration diagrams presented in Figs 1 and 2 show that the overall reduction process of SnO 2 by H 2 and alkanes (n·C x-H y=2x+2 , 0 ≤ x ≤ 4) does not differ a lot. Vigorous release of H 2 at low temperatures from alkanes generates the similar reducing atmosphere with the reduction by pure H 2 . Addition of the released C from alkanes induces the  Table 2. T 100 of n•C x H y=2x+2 (x = 1 or 2) reduction of SnO 2 as a function of the amount of supplied alkanes, n.
gas phase conversion of H 2 O into H 2 . Moreover, as the alkane/SnO 2 ratio increases from 1 to 2, the T 100 decreases. The response of the T 100 as a function of the total amount of supplied C, nx, and H, ny, is presented in Tables 2, 3 and Fig. 4a.
Considering that the T 100 values are exponentially decreasing upon increase of nx and ny and that the addition of C affects to the T 100 , we presented the T 100 values as a function of nx (Fig. 4b) or ny (Fig. 4c). Figure 4b shows that once the amount of C, nx, is given, the T 100 just slightly varies upon change in ny, presenting the quite prominent and dominant effect of C on the T 100 of SnO 2 reduction by alkanes, as predicted by thermochemical data: the standard formation enthalpy of CO 2 , H K (CO , 298 15 ) 393 474 kJ/mol , is greater than that of water, 298 15 ) 285 830 kJ/mol f 0 2 23 . Because single C atom can take over two O atoms from SnO 2 , whereas two H atoms are required to remove one O atom from SnO 2 , C of alkanes will naturally more aggressively reduce SnO 2 .
In Fig. 4c, to more intensively compare the effect of C on the T 100 of SnO 2 , we presented a pair of dataset, the T 100 values of H 2 or alkane reduction of SnO 2 as a function of ny. The control group data, the T 100 values acquired from H 2 reduction of SnO 2 , gradually decrease as a function of ny: 615 °C at H 2 /SnO 2 = 8 and 464 °C at H 2 / SnO 2 = 20. The filled square data points in Fig. 4c represent the T 100 of SnO 2 reduction by n·CH 4 , n·C 2 H 6 , n·C 3 H 8 , and n·C 4 H 10 . For the cases where two or more combinations of alkanes are available to match the total amount of supplied hydrogen, ny, we took the case with the higher nx. For example, we took the T 100 from 2C 2 H 6 (nx = 4) rather than that from 3CH 4 (nx = 3), to compare with the T 100 from 6H 2 (ny = 12). The T 100 values of alkane reduction of SnO 2 , fitted to an exponential function, show a significantly decrease in T 100 (Fig. 4c). Replacing a reducing agent from 4 moles of H 2 (ny = 8) to a mole of C 3 H 8 (nx = 3, ny = 8) decreased the T 100 of a mole of SnO 2 from 615 °C to 494 °C (Fig. 4c). The fitted exponential curves of T 100 as a function of nx or ny (solid lines in Fig. 4b,c) show that the T 100 of the alkane or H 2 reduction can be fit to simple exponential functions (refer to Tables 4 and 5 for fitting constants and R 2 -values).
Interestingly, we found that the ΔT 100 (T 100 -alkane -T 100 -hydrogen), an indicator of the effect of carbons from alkanes on the reduction of SnO 2 was −121 °C at ny = 3 and rapidly saturated to −101 °C at ny = 4 and beyond (Fig. 4c). The ΔT 100 calculated from the two fitted exponential curves predicts the slightly fluctuating ΔT 100  Table 3. T 100 of n•C x H y=2x+2 (x = 3 or 4) reduction of SnO 2 as a function of the amount of supplied alkanes, n.
centered at −105 °C (Fig. 4d). Because the ΔT 100 was estimated comparing the (0, ny) and (nx, ny) data points with the maximum nx value, it naturally represents the maximum effect of C addition to the T 100 of SnO 2 reduction. The overall increase of nx and ny is beneficial for SnO 2 reduction because the lower T 100 assures the higher economic efficiency. However, the effect of additional C to the T 100 is limited to ΔT 100 ≈ −105 °C. The vertically separated three data points in Fig. 4c, (0, 8), (2,8), and (3,8), show that the effect of C on the T 100 increase as a function of C addition. Because C released from alkanes aggressively attack H 2 O and liberate hydrogens of H 2 O, the presence of excess C may increase the chemical potential and the reducing potential of gas phase H 2 21 .
Reaction mechanism of alkane reduction of SnO 2 . As a prototypical example of alkane reduction of SnO 2 , DFT-calculated reaction mechanism of CH 4 reduction of SnO 2 is presented in Fig. 5a. The original DFT-calculated reaction energy values were adopted from our previous publication (Under Creative Commons  Table 4. T 100 of alkane reduction of SnO 2 fitted to an exponential function of nx.

Reference function T 100 = exp[a + b (ny) + c(ny) 2 ], (0 ≤ ny ≤ 20)
Fitting  www.nature.com/scientificreports www.nature.com/scientificreports/ Attribution 4.0 International License) 21 . The initial CH 4 dissociative adsorption (Process #1, Fig. 5a) initiates the CH 4 reduction of SnO 2 . Because a CH 4 molecule was dissociated into a -CH 3 methyl group and a hydrogen atom, which are independently bound to surface lattice oxygen atoms of SnO 2 , the SnO 2 surface will be strongly hydrogenated upon exposure to CH 4 . The sequential combined processes of dehydrogenation of -CH 3 to -CH (Processes #2 to #5) and water formation (process #3 and #4) are energetically uphill. This is because two hydrogen atoms produced upon dehydrogenation of single CH 4 molecule were used for water formation. As we discussed above, under the CH 4 rich reduction conditions, the surface oxygen ions of SnO 2 will be eventually hydrogenated and thus the endothermic dehydrogenation of -CH 3 and water formation will not hinder the overall reduction of SnO 2 . The second water formation (Process #6 and #7) and CO 2 production (Process #8) are Figure 5. DFT-estimated mechanism of alkane reduction of SnO 2 . (a) CH 4 was introduced as a prototypical alkane molecule to explore the atomistic mechanism of alkane reduction of SnO 2 . The reduction reaction proceeds clockwise following the numerical order. ΔE n presents the energetics of the n th process. White and grey spheres denote hydrogen and carbon atoms, respectively. Sn and oxygen atoms are colored in deep green and red, respectively. Because of the protruded hydrogen atoms of a methyl group, -CH 3 , formed upon dissociative adsorption of CH 4 , initially participate in the reduction process, H 2 O is the initial product. The original data for the reaction pathway and the morphology of each reaction stage are adopted from our previous report (ref. 21 ). Dissociative adsorption of (b) C 2 H 6 , (c) C 3 H 8 , and (d) C 4 H 10 on SnO 2 (100). Dissociative adsorption of alkanes of 2 ≤ x ≤ 4 produces multiple -OH and -CH 3 groups. Because each -CH 3 group was eventually dissociated into -CH 2 and -OH, the mechanism and the energetic of the subsequent reactions follow the case presented in panel (a).
www.nature.com/scientificreports www.nature.com/scientificreports/ strongly thermodynamically preferred. The overall reduction of SnO 2 by CH 4 shows that the hydrogen atoms of CH 4 participate in the reduction process first and the residual carbon atom finally reduces SnO 2 . This finding is consistent with the equilibrium concentration diagrams (Figs 2 and 3) showing that H 2 O always forms first to CO and CO 2 .
Interestingly, upon initial adsorption of C 2 H 4 , C 3 H 8 , and C 4 H 10 , multiple -OH and -CH 3 groups were formed as a result of dissociative adsorption of alkanes (Fig. 5b-d). Later, each -CH 3 group was eventually dissociated into -CH 2 and -OH, therefore the subsequent -CH 2 dissociation, water formation, and CO 2 formation processes would saturate into the same processes presented in Fig. 5a. The overall reaction mechanism of SnO 2 reduction by alkanes (C x H y=2x+2 , 0 ≤ x ≤ 4), therefore, is identical to each other except for the detailed energetics of the initial dissociative binding step. Interestingly, the initial dissociative adsorption of C 3 H 8 and C 4 H 10 are energetically endothermic (Fig. 5c,d). However, considering that the alkane reduction would occur under the high alkane partial pressure conditions 20,21 , the highly negative entropic contribution to the Gibbs free energy of binding, −TΔS, will definitely compensate the positive ΔE of dissociative adsorption (ΔE 1 in Fig. 5c,d) 24,25 , making the ΔG of dissociative C 3 H 8 and C 4 H 10 binding negative (exothermic). The roughly calculated highly negative − TΔS 0 at standard state 26 of propane (−0.83 eV) and butane (−0.95 eV) confirm that the ΔG values of dissociative alkane bindings (ΔG = ΔE − TΔS) are negative. The DFT-calculated mechanism of alkane (C x H y=2x+2 , 0 ≤ x ≤ 4) reduction of SnO 2 shows that the overall reaction mechanism is consistent within the alkanes that we applied (C x H y=2x+2 , 0 ≤ x ≤ 4) for SnO 2 reduction, irrespective to x and y. This result confirms that the significantly accelerated reduction potential of alkanes upon increase in nx is due to the quantitatively excessive supply of reducing agents by alkanes with the higher nx. As we have noticed in the introducing part, the presence of the already reduced liquid Sn metal may assist the direct reduction of alkanes. If this process occurs, the overall reaction will proceed under the higher H 2 partial pressure conditions (under the total pressure greater than 1 atm) with excessive solid state carbon supply. Results on the mechanism of SnO 2 reduction by alkanes under the various conditions (partial pressure and carbon content) will be reported in due course.

Conclusions
We study the mechanism of alkane reduction of SnO 2 for efficient low-temperature recovery of Sn from SnO 2 using combined study of thermodynamic simulations and DFT calculations. Through a comparative analysis of the reducing power of H 2 and commercially available alkanes (C x H y=2x+2 , 0 ≤ x ≤ 4) toward SnO 2 reduction, we scaled the reducing potential of studied reductants with T 100 , the temperature at which SnO 2 is completely converted to metallic Sn. The alkanes with the higher nx and ny quickly complete the reduction at low T 100 . Moreover, the positive effect of nx on the T 100 was quite prominent in all studied cases of alkane reduction of SnO 2 . The T 100 of the SnO 2 reduction by alkanes (n·C x H y ) was significantly decreased from the T 100 of pure hydrogen with the same amount of hydrogen atoms (n·H y ). The fitted exponential curves of T 100 plotted as a function of ny, presents that the effect of C on the T 100 being saturated to ΔT 100 ≈ −105 °C.
The C and H atoms released from alkanes sequentially reduce SnO 2 to Sn and eventually to metallic Sn. The initial stage of SnO 2 reduction by alkane is identical to the H 2 reduction of SnO 2 ; H 2 takes up oxygen from SnO 2 . However, in the presence of the released C from alkanes, H 2 of H 2 O is released as a gas phase molecule as C takes up oxygen from H 2 O. Because the gas phase redistribution between H 2 O, H 2 , CO, and CO 2 , caused by solid C occurs at above the T 100 , the role of the solid C released from alkanes is likely to adjust the chemical potential of hydrogen of H 2 O and H 2 , accelerating the reduction of SnO 2 by H 2 . The DFT-calculated atomic scale mechanism of alkane reduction of SnO 2 confirmed that the overall reaction mechanism is consistent within applied alkanes (C x H y=2x+2 , 0 ≤ x ≤ 4).
Our results show that the alkane reduction of SnO 2 is an effective recovery method of metallic Sn from SnO 2 or SnO containing industrial wastes or from Sn ores. The low T 100 values of alkane reduction and the maximum ΔT 100 of −105 °C suggest that the alkane reduction of SnO 2 assures high economically efficiency with economic value added that is held by the co-produced H 2 and carbons.

Methods
Thermodynamic simulation. Thermodynamic simulations were performed with the HSC 6.0 code (Outotec Research, www.hsc-chemistry.com). The relative thermodynamic stability of various Sn, C, O, and H containing chemical compounds was estimated at temperatures between 0 °C and 1,200 °C. The initial equilibrium simulations were performed with 1 kmol of SnO 2 balanced with increasing amount of H 2 or alkanes. The T 100 of several commercially accessible alkanes (C x H y=2x+2 , 0 ≤ x ≤ 4), methane (CH 4 ), ethane (C 2 H 6 ), propane (C 3 H 8 ), and butane (C 4 H 10 ), were measured and compared with that of pure H 2 to estimate the effect of carbon addition on the reducing power of a gas phase reductant. To generalize the effect of carbon, the measured T 100 values were fitted to exponential curves.
Density functional theory calculation. We performed density functional theory calculations with the Vienna ab-initio simulation package (VASP) 27 with the Perdew-Burke-Ernzerhof (PBE) 28 exchange-correlation functional to study the reaction pathway and the corresponding energetics of alkane (C x H y=2x+2 , 0 ≤ x ≤ 4) reduction of SnO 2 . The most bottom SnO 2 triple layer was fixed during the optimization to ensure the structural robustness of the slab models. The interaction between the ionic cores and the valence electrons was described with the projector augmented-wave method 29 . The valance-electron wave functions were expanded in the plane-wave basis set up to the energy cutoff of 400 eV. The convergence criteria for the electronic structure and the atomic geometry were 10 −4 eV and 0.03 eV/Å, respectively.
The datasets generated during and/or analyzed during the current study are available from the corresponding author on reasonable request.