Synergistic Effect of Alkali Na and K Promoter on Fe-Co-Cu-Al Catalysts for CO 2 Hydrogenation to Light Hydrocarbons

: Alkali metal K- and/or Na-promoted FeCoCuAl catalysts were synthesized by precipitation and impregnation, and their physicochemical and catalytic performance for CO 2 hydrogenation to light hydrocarbons was also investigated in the present work. The results indicate that Na and/or K introduction leads to the formation of active phase metallic Fe and Fe-Co crystals in the order Na < K < K-Na. The simultaneous introduction of Na and K causes a synergistic effect on increasing the basicity and electron-rich property, promoting the formation of active sites Fe@Cu and Fe-Co@Cu with Cu 0 as a crystal core. These effects are advantageous to H 2 dissociative adsorption and CO 2 activation, giving a high CO 2 conversion with hydrogenation. Moreover, electron-rich Fe@Cu (110) and Fe-Co@Cu (200) provide active centers for further H 2 dissociative adsorption and O-C-Fe intermediate formation after adsorption of CO produced by RWGS. It is beneﬁcial for carbon chain growth in C 2+ hydrocarbons, including oleﬁns and alkanes. FeCoCuAl simultaneously modiﬁed by K-Na exhibits the highest CO 2 conversion and C 2+ selectivity of 52.87 mol% and 89.70 mol%, respectively.


Introduction
With the development of industrialization and urbanization, a large amount of CO 2 with a greenhouse effect has been discharged due to excessive use of fossil fuels, such as petroleum, coal and natural gas. The negative effects associated with increasing atmospheric concentrations of CO 2 , climate change and ocean acidification, are considered as the most challenging issues of the 21st century [1,2]. Many efforts must be put forth to decrease CO 2 direct emissions, permanently sequester CO 2 and convert CO 2 to valuable products [3]. In recent years, CO 2 catalytic conversion has been a rapidly growing field because CO 2 is an abundant, non-toxic and low-cost C 1 feedstock that can be converted to many valuable chemicals and energy storage substances, such as methane [4], methanol [5][6][7], low-carbon alkanes [8] and olefins [9,10]. Among these, CO 2 selective hydrogenation to light hydrocarbons, including alkanes and olefins, is the most promising route [10]. For example, ethylene and propylene are the top two petrochemicals produced worldwide with a high demand for the production of plastics, polymers, solvents and cosmetics.
In the process of CO 2 hydrogenation to C 2 + hydrocarbons, a reverse water-gas shift reaction (RWGS) to CO over the reported Fe and Co-based catalysts is the key step, followed by a Fischer-Tropsch synthesis (FTS) reaction [11,12]. From the reported results, it has been found that FTS mainly produces light hydrocarbons over Fe-based catalysts and heavier hydrocarbons over Co-based catalysts exhibiting better chain growth potential during CO 2 hydrogenation [13][14][15]. Meanwhile, a Fe or Co monometallic catalyst gives a poor C 2 + hydrocarbon selectivity, and produces more undesired CO and CH 4 [16,17]. Therefore, Fe-Co or Fe-Cu bimetallic catalysts are adopted, and corresponding promoters are also introduced to increase catalytic activity and C 2 + selectivity [18][19][20]. This is because the bimetallic catalysts can produce metallic alloy active sites after reduction, favoring H 2 activation for RWGS. The introduced promoters mainly include transition metals such Zn, Ti and Zr [14,17,21], and alkali metals, especially K [19,22,23]. The introduced transition metals increase the electron-rich property of the catalytic active metallic phase, which is helpful for the adsorption of H 2 and CO activation followed by C-C coupling. Additionally, the alkali metal K ions can provide surface basic sites favoring CO 2 adsorption, interacting with the metallic phase and inhibiting the deep hydrogenation of carbon chains to alkanes during FTS [17,24,25]. Meanwhile, K modification is found to result in the formation of an Fe 5 C 2 intermediate that enhances the RWGS reaction and then undergoes the FTS process and suppresses the secondary reaction and methane formation to obtain more hydrocarbons [23,26]. Similarly, it is also found that the presence of Na, another common alkaline metal, helps to increase CO 2 conversion and product selectivity, which contributes to the formation and stabilization of intermediate Fe x C y as an active species to enhance adsorption of CO 2 and weaken further hydrogenation of olefins to alkanes [27,28].
As is well known, hydroxide and carbonate of sodium are usually used as precipitants for preparing precursors of metal oxide catalysts from precipitation, in which there are often some residual sodium ions. To date, it is not clear whether the co-existence of Na and K affects CO 2 hydrogenation to light hydrocarbons over Fe-and/or Co-based catalysts. Therefore, FeCoCu-based catalysts with Al as a support component are synthesized by precipitation using Na 2 CO 3 -NaOH and K 2 CO 3 -KOH as precipitants, respectively. The obtained FeCoCuAl catalysts before and after K and/or Na modification are characterized by X-ray diffraction (XRD), N 2 adsorption-desorption, H 2 temperatureprogrammed reduction (H 2 -TPR), CO 2 temperature-programmed desorption (CO 2 -TPD), H 2 temperature-programmed desorption (H 2 -TPD) and X-ray photoelectron spectroscopy (XPS). Combining catalytic tests, the effect of Na and/or K modification on the physicochemical and catalytic properties of FeCoCuAl in CO 2 hydrogenation to light hydrocarbons are investigated in the present work.

Textual and Structural Properties
The obtained FeCoCuAl catalyst, using sodium alkali as a precipitant, was found by ICP-OES to contain low residual Na with a Na/Fe molar ratio of 0.16%, and using potassium alkali as a precipitant gives a residual amount of K of the K/Fe molar ratio of 0.18%. From Table 1, the BET specific surface area, pore volume and average pore diameter of FeCoCuAl oxide catalysts obtained by using Na alkali solution (NaOH-Na 2 CO 3 ) are bigger than those with K alkali solution (KOH-K 2 CO 3 ) as a precipitant. At the same time, it is clear that Na modification can further increase the BET specific surface area, pore volume and average pore diameter of FeCoCuAl catalysts. K modification has a reverse function, which can be also proved by N 2 adsorption-desorption results of FeCoCuAl catalysts with different K and/or Na amount (see Tables S1 and S2 in Supplementary Materials). This mainly because the Na ion has a small ionic radius and easily occupies macropores to form new micropores, providing a larger surface, which can be proved by the decrease in the average pore diameter. However, the introduced K ions with a bigger ionic radius possibly block pores, especially some micropores.  [29]. Meanwhile, the weak diffraction peaks that appear at 2θ = 43.5 • and 50.3 • are characteristic of metallic Cu (pdf No. 1-1241), and the diffraction peaks assigned to metallic Fe (pdf No. 3-1050) or Fe-Co alloy (pdf No. 65-6829) are found to exhibit weak signals. This indicates that iron and cobalt oxides in catalysts are difficult to reduce to a metallic state. When promoter Na is introduced, these characteristic diffraction peaks of metallic Cu, Fe 3 O 4 or CoFe 2 O 4 weaken, and a new diffraction peak assigned to the (110) crystal face of metallic Fe at 2θ = 44.5 • appears. Additionally, these changes are more obvious after K modification. Moreover, the simultaneous modification of Na and K makes characteristic diffraction peaks of Fe and Co oxides nearly disappear, and further increases the intensity of the diffraction peak at 2θ = 44.5 • , along with an obvious new peak at 65.3 • assigned to the (200) crystal face of the metallic Fe-Co alloy. These show that the modification of alkali metals, Na and K ions, is helpful in the reduction of iron and cobalt species to metallic Fe or Fe-Co alloy in FeCoCuAl catalysts [30]. From XRD results, it can also be found that the reduction of copper oxide is easier than that of iron and cobalt oxides. The weakness and disappearance of metal Cu diffraction peaks after Na and/or K modification also show that copper species are reduced first. During catalyst reduction, the produced metallic Fe and Co crystals grow on the surface of metallic Cu cores. Therefore, active species of the present FeCoCuAl catalyst with K and/or Na modification are deduced to be metallic Fe or Fe-Co alloy deposited on Cu as a crystal core, which are denoted as Fe@Cu and Fe-Co@Cu, respectively. These can also be proved by XRD results of FeCoCuAl catalysts with different K and/or Na amount (see Figures S1-S3 in Supplementary Materials). The formation of Co@Cu is difficult because of a low Co content with an Fe/Co molar ratio of 3. On the other hand, no distinct diffraction signals assigned to Al-containing species are observed in the XRD pattern, indicating that Al species in reduced catalysts perhaps exist in the form of highly dispersed amorphous aluminum oxide species. Therefore, it can be concluded that the simultaneous modification of alkali metals K and Na gives rise to a synergistic effect when producing Fe@Cu and Fe-Co@Cu crystals on FeCoCuAl samples during reduction.
atalysts 2021, 11, x FOR PEER REVIEW Figure 1. XRD pattern of reduced FeCoCuAl catalysts before and after K and/or Na Catalyst reduction at 400 °C for 1 h in pure H2.

H 2 -TPR
As shown in H 2 -TPR profiles of FeCoCuAl catalysts ( Figure 2), both FCCA-N and FCCA-K exhibit a sharp reduction peak at about 256 • C, which is perhaps from H 2 consumption due to the reduction of Cu 2+ → Cu 0 [31], Fe 3+ → Fe 2+ [32] and Co 3+ → Co 2+ [33]. Additionally, these reduced Fe and Co species are highly dispersed due to ready occurrence of their reduction only at a low temperature. When the Na promoter is introduced into FCCA-K, this reduction peak becomes broad and slightly shifts to a high temperature. This is mainly because the presence of Na ions promotes the reduction of iron and cobalt oxides to a metallic state. Moreover, the modification of K and K-Na can further make the reduction peak broader and shift to a higher temperature. Additionally, the simultaneous introduction of K and Na brings forth a new broad reduction peak near 500 • C, which is possibly from the reduction of iron and cobalt species in CoFe 2 O 4 . These results indicate that Na and K modification is beneficial to the easy reduction of Fe 2+ and Co 2+ , forming metallic Fe and Fe-Co alloy, which can be further proved by H 2 -TPR profiles of FeCoCuAl catalysts with different K and/or Na amount (see Figures S4-S6 in Supplementary Materials). The improvement order is Na < K < K-Na modification. From H 2 -TPR results, it can be proved that Cu 0 crystal core should be produced first, and followed by the formation of Fe@Cu and Fe-Co@Cu crystals during reduction by H 2 , in agreement with the above XRD results.

H2-TPR
As shown in H2-TPR profiles of FeCoCuAl catalysts (Figure 2), both FCCA-K exhibit a sharp reduction peak at about 256 °C, which is perhaps sumption due to the reduction of Cu 2+ → Cu 0 [31], Fe 3+ → Fe 2+ [32] and Co Additionally, these reduced Fe and Co species are highly dispersed due t rence of their reduction only at a low temperature. When the Na promoter into FCCA-K, this reduction peak becomes broad and slightly shifts to a hig This is mainly because the presence of Na ions promotes the reduction of i oxides to a metallic state. Moreover, the modification of K and K-Na can fu reduction peak broader and shift to a higher temperature. Additionally, the introduction of K and Na brings forth a new broad reduction peak near 50 possibly from the reduction of iron and cobalt species in CoFe2O4. These r that Na and K modification is beneficial to the easy reduction of Fe 2+ and metallic Fe and Fe-Co alloy, which can be further proved by H2-TPR profile catalysts with different K and/or Na amount (see Figures S4-S6 in Supplem rials). The improvement order is Na < K < K-Na modification. From H2-TPR be proved that Cu 0 crystal core should be produced first, and followed by of Fe@Cu and Fe-Co@Cu crystals during reduction by H2, in agreement w XRD results.

H 2 -and CO 2 -TPD
H 2 -TPD curves of all reduced FeCoCuAl catalysts are shown in Figure 3. FCCA-N without K modification exhibits a broad H 2 desorption peak in a temperature range of 110-300 • C, which can be assigned to the desorption of H 2 species bonded with O atoms of iron oxide and CoFe 2 O 4 species exposed on the catalyst surface [34]. However, FCCA-K without Na modification exhibits a wide H 2 desorption band in a higher temperature range of 200-470 • C. This is perhaps because FeCoCuAl synthesized by using a Na alkali solution as a precipitant contains more highly dispersed iron and cobalt oxides on the catalyst surface. When K alkali solution is used as a precipitant, the synthesized FeCoCuAl oxides are dense particles with a small surface area and pore diameter (see Table 1). This is perhaps due to the formation of Fe 3 O 4 -CoFe 2 O 4 solid solution [29]. The ferric and cobalt oxide species in solid solution provide fewer surface O atoms. H 2 adsorption mainly occurs on the surface of metallic Cu 0 and metallic oxide solid solution, which is difficult to reduce. Additionally, FCCA-N modified with K as a promoter exhibits a similar H 2 desorption behavior to FCCA-K. These results indicate that the obtained FeCoCuAl catalysts, whether with K modification or using K alkali as a precipitant, contain more CoFe 2 O 4 or Fe 3 O 4 -CoFe 2 O 4 solid solution. Na-modified FCCA-K exhibits two obvious H 2 desorption peaks at 220 • C and near 400 • C, respectively. The similar results are also observed from H 2 -TPD results of FeCoCuAl catalysts with different K and/or Na amount (see Figures S7-S9 in Supplementary Materials). Additionally, these two peaks become more distinct for FCCA-N simultaneously modified by Na and K as promoters. In any case, both XRD and H 2 -TPR results have proved that Na and/or K modification leads to the easy formation of metallic Fe@Cu or Fe-Co@Cu alloy in FeCoCuAl catalysts, because alkali Na + and K + ions can provide an electron-rich environment [35]. Therefore, the obvious and broad H 2 adsorption peak bands in a temperature range of 200-420 • C are attributed to the desorption of active H formed by dissociative adsorption on metallic Fe@Cu or Fe-Co@Cu alloy species. Combined with XRD results, it can be deduced that Fe@Cu (110) or Fe-Co@Cu alloy (200) perhaps provides the main sites for H 2 dissociative adsorption.
to reduce. Additionally, FCCA-N modified with K as a promoter exhibit desorption behavior to FCCA-K. These results indicate that the obtained F lysts, whether with K modification or using K alkali as a precipitant, contain or Fe3O4-CoFe2O4 solid solution. Na-modified FCCA-K exhibits two obvio tion peaks at 220 °C and near 400 °C, respectively. The similar results are from H2-TPD results of FeCoCuAl catalysts with different K and/or Na amo S7-S9 in Supplementary Materials). Additionally, these two peaks become for FCCA-N simultaneously modified by Na and K as promoters. In any c and H2-TPR results have proved that Na and/or K modification leads to the e of metallic Fe@Cu or Fe-Co@Cu alloy in FeCoCuAl catalysts, because alk ions can provide an electron-rich environment [35]. Therefore, the obvious adsorption peak bands in a temperature range of 200-420 °C are attributed tion of active H formed by dissociative adsorption on metallic Fe@Cu or F species. Combined with XRD results, it can be deduced that Fe@Cu (110) alloy (200) perhaps provides the main sites for H2 dissociative adsorption. CO2-TPD results ( Figure 4) indicate that all reduced FeCoCuAl catalyst desorption peak at 110 °C, which is attributed to weakly adsorbed CO2. Me be observed that Na-modified FCCA-K shows a new and weak CO2 deso 350-600 °C, along with a slight shift to a low temperature. This shows that sodium promoter can provide more base sites. K-and K-Na-promoted sa two obvious new CO2 desorption peaks at about 180 °C and 630 °C, respect ilar results are also observed from CO2-TPD results of FeCoCuAl catalysts K and/or Na amount (see Figures S10-S12 in Supplementary Materials). CO 2 -TPD results ( Figure 4) indicate that all reduced FeCoCuAl catalysts exhibit a CO 2 desorption peak at 110 • C, which is attributed to weakly adsorbed CO 2 . Meanwhile, it can be observed that Na-modified FCCA-K shows a new and weak CO 2 desorption band at 350-600 • C, along with a slight shift to a low temperature. This shows that the introduced sodium promoter can provide more base sites. K-and K-Na-promoted samples exhibit two obvious new CO 2 desorption peaks at about 180 • C and 630 • C, respectively. The similar results are also observed from CO 2 -TPD results of FeCoCuAl catalysts with different K and/or Na amount (see Figures S10-S12 in Supplementary Materials). The former is attributed to CO 2 weakly adsorbed on moderate base sites. Additionally, the latter should be from the decomposition of bidentate carbonate species produced by the strong interaction between CO 2 and potassium or sodium ferrite (K 2 Fe 2 O 4 or Na 2 Fe 2 O 4 ) with strong base sites [19]. This is because the alkali K + possesses a stronger alkalinity than Na + . For the same reason, the introduction of alkali metal ions can lead to the formation of electron-rich Fe@Cu and Fe-Co@Cu alloy crystals, which can also activate CO 2 molecules. All these data show that simultaneous promotion of K and Na exhibits a synergistic effect when activating H 2 and CO 2 . same reason, the introduction of alkali metal ions can lead to the formatio rich Fe@Cu and Fe-Co@Cu alloy crystals, which can also activate CO2 molec data show that simultaneous promotion of K and Na exhibits a synergist activating H2 and CO2.

XPS
From XPS results ( Figure 5) of reduced samples, both FCCA-N and signals at electron-binding energies of about 932.5 eV, 780.2 eV and 711.2 e Cu 2p3/2, Co 2p3/2 and Fe 2p3/2 [18,30], respectively. Na modification gives decrease in all electron-binding energies of FCCA-K, and K modification l vious decrease in those of FCCA-N. The simultaneous introduction of K-N biggest shift to a low binding energy value. This is possibly because the int metal ions exist in the form of oxides, which can provide more electrons for Co@Cu alloy phase [18,20]. Meanwhile, K ions have a bigger ionic radius th corresponding oxides have a stronger capacity to provide electrons [30]. Al indicates that the introduced alkali ions promote the formation of electron active phases in the following order: Na < K < K-Na. This shows that the modification of alkali metals K and Na gives a synergistic effect of increasin rich property of active phase metallic Fe@Cu and Fe-Co@Cu crystals.

XPS
From XPS results ( Figure 5) of reduced samples, both FCCA-N and FCCA-K give signals at electron-binding energies of about 932.5 eV, 780.2 eV and 711.2 eV, assigned to Cu 2p 3/2 , Co 2p 3/2 and Fe 2p 3/2 [18,30], respectively. Na modification gives rise to a slight decrease in all electron-binding energies of FCCA-K, and K modification leads to an obvious decrease in those of FCCA-N. The simultaneous introduction of K-Na exhibits the biggest shift to a low binding energy value. This is possibly because the introduced alkali metal ions exist in the form of oxides, which can provide more electrons for Fe@Cu or Fe-Co@Cu alloy phase [18,20]. Meanwhile, K ions have a bigger ionic radius than Na, and the corresponding oxides have a stronger capacity to provide electrons [30]. All this evidence indicates that the introduced alkali ions promote the formation of electronrich metallic active phases in the following order: Na < K < K-Na. This shows that the simultaneous modification of alkali metals K and Na gives a synergistic effect of increasing the electron-rich property of active phase metallic Fe@Cu and Fe-Co@Cu crystals. data show that simultaneous promotion of K and Na exhibits a synergistic effe activating H2 and CO2.

XPS
From XPS results ( Figure 5) of reduced samples, both FCCA-N and FCCA signals at electron-binding energies of about 932.5 eV, 780.2 eV and 711.2 eV, ass Cu 2p3/2, Co 2p3/2 and Fe 2p3/2 [18,30], respectively. Na modification gives rise to decrease in all electron-binding energies of FCCA-K, and K modification leads t vious decrease in those of FCCA-N. The simultaneous introduction of K-Na exh biggest shift to a low binding energy value. This is possibly because the introduce metal ions exist in the form of oxides, which can provide more electrons for Fe@C Co@Cu alloy phase [18,20]. Meanwhile, K ions have a bigger ionic radius than Na, corresponding oxides have a stronger capacity to provide electrons [30]. All this e indicates that the introduced alkali ions promote the formation of electron-rich active phases in the following order: Na < K < K-Na. This shows that the simul modification of alkali metals K and Na gives a synergistic effect of increasing the e rich property of active phase metallic Fe@Cu and Fe-Co@Cu crystals.

Catalytic Performances
Catalytic activities of obtained K and/or Na-modified FeCoCuAl samples in CO 2 hydrogenation are listed in Table 2. It can be seen that a similar CO 2 conversion occurs on FeCoCuAl catalysts prepared with Na or K alkali aqueous solution as a precipitant. However, the introduction of 3.4 mol% Na, 9.3 mol% K and 3.4 mol% Na-9.3 mol% K relative to Fe leads to a sharp increase in CO 2 conversion. This is mainly because the presence of alkali K and/or Na improves the formation of Fe@Cu and Fe-Co@Cu alloy crystals and base sites, which has been proved by the XRD and CO 2 -TPD results above. The formed Fe@Cu and Fe-Co@Cu alloy crystals act as catalytic active centers to activate hydrogen through dissociative adsorption. The formation of more basic sites is helpful for CO 2 adsorption and activation [36]. Additionally, the co-existence of K and Na leads to production of alkali metal ferrite, such as K 2 Fe 2 O 4 and Na 2 Fe 2 O 4 , which can also activate CO 2 to form intermediate bidentate carbonate species [19]. Moreover, the presence of alkali K and/or Na ions also improves the electron-rich capacity of the (110) and (200) crystal surface of active Fe@Cu and Fe-Co@Cu, which is favorable for H 2 dissociative adsorption, proved by the H 2 -TPD results above [36]. These factors accelerate CO 2 conversion to CO through a reverse water-gas shift reaction (RWGS). Meanwhile, it can be also found from Table 2 that the K promotion seems weaker than Na modification in improving CO 2 conversion, which is mainly due to the fact that the alkalinity of K ions is stronger than that of Na ions. The presence of strong base sites will promote the adsorption of more CO 2 , which can occupy the space for adsorption of H 2 over the catalyst surface. Although the simultaneous introduction of 3.4 mol% Na and 9.3 mol% K can provide more strong base sites, it can also increase the formation of Fe@Cu and Fe-Co@Cu species (see XRD results) for adsorbing H 2 . Therefore, it is not difficult to understand that 9.3K-3.4Na-FCCA-N exhibits the highest CO 2 conversion of 52.87 mol%. RWGS is the first step for CO 2 hydrogenation. The produced CO can continue to react with the activated hydrogen over a catalyst. The next one is methanation to CH 4 , and another is FTS reaction to C 2 + products including olefin and alkanes [16]. From Table 2, it is found that all catalysts give a low CO selectivity. Additionally, K modification makes FCCA-N exhibit a decreasing CH 4 selectivity and an increasing C 2 + hydrocarbon selectivity. Na modification also leads to a similar tendency for FCCA-K. They are also found from the catalytic property of FeCoCuAl catalysts modified by different K and/or Na amount in CO 2 hydrogenation (see Tables S1-S3 in Supplementary Materials). This is perhaps because the (110) crystal face of the formed Fe@Cu provides the active sites for dissociative adsorption of CO followed by the formation of CO-like species such as O-C-Fe@Cu and O-C-Fe-Co@Cu intermediates [24,30], which are beneficial for the carbon-chain growth for the FTS reaction to C 2 + hydrocarbons. At the same time, both C 2 -C 4 alkanes (39.98 mol%) and CH 4 selectivity are found to decrease along with an increasing C 2 -C 4 olefin selectivity (32.14 mol%) when K and Na are introduced into corresponding FeCoCuAl catalysts. This is because the introduced alkali metals ions interact with active sites in the metallic Fe and Fe-Co phase, and weaken the activity of the active hydrogen. The deep hydrogenation of CO can be inhibited to produce CH 4 , and the complete hydrogenation of carbon chains to saturated hydrocarbons is prevented simultaneously. From these results, the same conclusion as that reported in [12,19] has been drawn, that the introduced K plays a role in promoting C-C coupling and suppressing CH 4 formation during CO 2 hydrogenation. Although FCCA-N simultaneously promoted by K and Na exhibits the strongest basicity, it also has more metallic Fe@Cu and Fe-Co@Cu active sites. In particular, the increase in Fe (110) and the formation of new Fe-Co (200) crystal surfaces (see XRD results) further accelerate H 2 and CO dissociative adsorption. All these factors are advantageous for the carbon chain growth. From these catalytic data, it is found that the simultaneous modification of alkali metal K and Na provides a synergistic effect for promoting CO 2 conversion and low hydrocarbon and C 2 + olefin formation over FeCoCuAl catalysts during CO 2 hydrogenation.

Catalyst Preparation
FeCoCuAl (FCCA) catalysts were synthesized by precipitation using the corresponding nitrates as raw materials with a molar ratio of Fe 3+ :Co 2+ :Cu 2+ :Al 3+ = 18:6:1:6. Typically, the desired amounts of Fe(NO 3 and Al(NO 3 ) 3 ·9H 2 O (c.p.) were dissolved in distilled water to obtain a mixture solution (total concentration of metal ions being 0.5 mol·L −1 ). In order to ensure the modification accuracy of the introduced promoter, K or Na, two mixtures of alkali aqueous solutions including 0.6 mol·L −1 NaOH-0.4 mol·L −1 Na 2 CO 3 and 0.6 mol·L −1 KOH-0.4 mol·L −1 K 2 CO 3 were used as precipitants. During precipitation, the precipitant was added dropwise into the abovementioned metal ion-containing liquid mixture under stirring at room temperature until it reached pH 8.5. After finishing precipitation, precursor slurry was aged overnight, filtered and washed with distilled water until it reached pH 7.0. The obtained filter cake was dried at 110 • C, and then promoter K or Na (K 2 CO 3 or NaNO 3 as raw material) was introduced to the dried FCCA catalyst precursor by impregnation, followed by calcination at 500 • C for 5 h in air atmosphere. The K-modified FCCA catalyst was denoted as 9.3K-FCCA-N with an amount of K of the K/Fe ratio of 9.3 mol%, and N as a NaOH-Na 2 CO 3 mixture alkali was used as a precipitant during precursor preparation. Similarly, the Na-modified FCCA catalyst was denoted as 3.4Na-FCCA-K with a percentage of Na relative to Fe of 3.4 mol%, and the K used as a precipitant was a KOH-K 2 CO 3 mixture alkali aqueous solution. The precursors of FCCA catalysts simultaneously modified by K and Na were obtained by using a NaOH-Na 2 CO 3 mixture alkali as a precipitant. The modified amounts of K and Na were confirmed to be 9.3 mol% and 3.4 mol% relative to Fe, respectively, according to the optimal values reported in the literature [28,37]. This sample simultaneously modified by K and Na was denoted as 9.3K-3.4Na-FCCA-N.

Catalyst Characterization
XRD analysis of the reduced samples was carried out on a DX-2700 powder diffractometer (Dandong Fangyuan Co., China), using Cu Kα radiation (λ = 0.15406 nm) with an operating condition of 40 kV, 30 mA and a scan step of 0.03 • ·min −1 .
Textural properties of all samples were measured from N 2 adsorption-desorption isotherms (−196 • C) on an SSA-4200 micromeritics instrument (Beijing Builder Co., Beijing, China). The specific surface area was calculated by the BET method according to adsorption isotherms in a relative pressure (P/P 0 ) range of 0.05-0.35, and pore distribution and pore diameter were calculated by the BJH method according to desorption isotherms. H 2 -TPR experiments were carried out on a PCA-1200 chemisorption analyzer equipped with a thermal conductivity detector (TCD). About 200 mg of catalysts were pretreated at 300 • C for 1 h in 30 mL·min −1 N 2 flow. After cooling down to room temperature, the sample was purged with 30 mL·min −1 5% H 2 -N 2 mixture gas and heated to 800 • C with a rate of 10 • C·min −1 . The hydrogen consumption was monitored by the TCD. CO 2 -and H 2 -TPD profiles were produced on the same instrument as for H 2 -TPR. For CO 2 -TPD, the catalyst sample was first reduced at 400 • C for 1 h in a 5% H 2 -N 2 mixture gas, followed by cooling down to 50 • C. The reduced catalyst was exposed for 30 min in Catalysts 2021, 11, 735 9 of 11 30 mL·min −1 CO 2 gas flow, and then flushed with He flow for 30 min to remove physically adsorbed CO 2 . After cooling to room temperature, CO 2 desorption proceeded by heating from room temperature to 800 • C in 30 mL·min −1 He flow as a carrier gas, and the desorbed CO 2 amount was monitored by the TCD. For H 2 -TPD, the procedure was similar to that for CO 2 -TPD. The used carrier gas was Ar flow.
The chemical compositions of prepared catalysts were determined by inductively coupled plasma-optical emission spectroscopy (ICP-OES, Agilent 725 ICP). X-ray photoelectron spectroscopy (XPS) was used to analyze the chemical states of prepared catalysts on an X-ray photoelectron spectrometer system (Thermo Scientific K-Alpha, ThermoFisher, Waltham, MA, USA) equipped with a monochromated Al Kα X-ray source. The binding energy (BE) was calibrated to C 1 s peak at 284.8 eV.

Catalytic Test
The catalytic performance of Na-and/or K-modified FeCoCuAl was investigated in a high-pressure fixed-bed reactor. Typically, 6 g samples (apparent volume 5.8 mL) were loaded in a stainless-steel tube reactor (internal diameter 15 mm) and reduced at 400 • C for 1 h in 50 mL·min −1 pure H 2 flow under atmospheric pressure. CO 2 hydrogenation was carried out under reaction conditions of temperature 300 • C, pressure 20 atm, H 2 /CO 2 ratio 3 and gas hourly space velocity (GHSV) 3000 h −1 . Liquid products were collected through a 0 • C cold trap connected to the reactor outlet. The distribution of H 2 , CO, CH 4 and CO 2 in tail gas was found by using a gas chromatographer (GC) equipped with a TCD and TDX-01 column (1 m × 3 mm). The produced light hydrocarbons (C 1 -C 5 ) in product gas were analyzed on-line by another GC equipped with a flame ionization detector (FID) and Kromat KB-Al 2 O 3 /Na 2 SO 4 capillary column (30 m × 0.53 mm × 20 um). The collected liquid products, including water and heavy hydrocarbons, were analyzed off-line by using a GC equipped with a TCD and Paropak-Q column (4 m × 3mm). Based on the obtained GC data, CO 2 conversion and selectivity of hydrocarbons were calculated according to carbon balance. All data were obtained by averaging the values from three parallel catalytic tests.

Conclusions
From investigating the effect of Na and/or K modification on the physicochemical and catalytic properties of FeCoCuAl catalysts in CO 2 hydrogenation, metallic Fe@Cu and Fe-Co@Cu alloy are found to be catalytic active sites for CO 2 to CO and FTS reactions. The co-existence of Na and K in FeCoCuAl catalysts provides a synergistic effect in increasing surface basicity and the formation of Fe@Cu (110) and Fe-Co@Cu (200). Basic species provide active sites for CO 2 adsorption, which is the key step for CO 2 hydrogenation to CO through RWGS. H 2 and CO dissociative adsorption easily occur on the crystal surface of Fe@Cu (110) and Fe-Co@Cu (200), especially in the form of an electron-rich state, improved by the co-existence of K and Na. All factors accelerate carbon chain growth through C-C coupling during FTS reactions to more C 2 + products. K and/or Na modification is found to improve the catalytic performance of FeCoCuAl in CO 2 hydrogenation to light hydrocarbons in the following order: Na < K < K-Na. FeCoCuAl with the co-existence of K and Na exhibits the highest CO 2 conversion and C 2 + selectivity of 52.87 mol% and 89.70 mol%, respectively.
Supplementary Materials: The following are available online at https://www.mdpi.com/article/ 10.3390/catal11060735/s1, Figure S1: XRD pattern of the reduced FCCA-N modified with different amount of promoter K, Figure S2: XRD pattern of the reduced FCCA-K modified with different amount of promoter Na, Figure S3: XRD pattern of reduced K-Na-FCCA-N catalysts (K/Fe 9.3mol%) modified by different Na content, Figure S4: H 2 -TPR profiles of FCCA-N catalysts promoted with different amount of K, Figure S5: H 2 -TPR profiles of FCCA-K catalysts promoted with different amount of Na, Figure S6: H 2 -TPR profiles of K-Na-FCCA-N catalysts (K/Fe 9.3mol%) promoted with different amount of Na, Figure S7: H 2 -TPD profiles of FCCA-N catalysts promoted with different amount of K, Figure S8: H 2 -TPD profiles of FCCA-K catalysts promoted with different amount of Na, Figure S9: H 2 -TPD profiles of K-Na-FCCA-N catalysts (K/Fe 9.3mol%) promoted with different amount of Na, Figure S10: CO 2 -TPD profiles of FCCA-N catalysts promoted with different amount of K, Figure S11: CO 2 -TPD profiles of FCCA-K catalysts promoted with different amount of Na, Figure S12: CO 2 -TPD profiles of K-Na-FCCA-N catalysts (K/Fe 9.3mol%) promoted with different amount of Na, Table S1: Textural and catalytic properties of K-promoted FCCA-N in CO 2 hydrogenation, Table S2: Textural and catalytic properties of Na-promoted FCCA-K in CO 2 hydrogenation, Table S3: Textural property and catalytic performance of Na-promoted K-FCCA-N catalysts (K/Fe 9.3mol%) in CO 2 hydrogenation.
Author Contributions: Methodology, C.X.; investigation, X.Z.; writing-original draft preparation, Y.Z. and Q.W.; writing-review and editing, J.L.; supervision, C.X. All authors have read and agreed to the published version of the manuscript.