Zn-Al Mixed Oxides Decorated with Potassium as Catalysts for HT-WGS: Preparation and Properties

: A set of ex-ZnAl-LDHs catalysts with a molar ratio of Zn / Al in the range of 0.3–1.0 was prepared using co-precipitation and thermal treatment. The samples were characterized using various methods, including X-ray ﬂuorescence spectroscopy (XRF), X-ray photoelectron spectroscopy (XPS), X-ray powder di ﬀ raction (XRD), Fourier transform infrared spectroscopy FT-IR, N 2 adsorption, Temperature-programmed desorption of CO 2 (TPD-CO 2 ) as well as Scanning electron microscopy (SEM-EDS). Catalyst activity and long-term stability measurements were carried out in a high-temperature water–gas shift (HT-WGS) reaction. Mixed oxide catalysts with various Zn / Al molar ratios decorated with potassium showed high activity in the HT-WGS reaction within the temperature range of 330–400 ◦ C. The highest activity was found for the Zn / Al molar ratio of 0.5 corresponding to spinel stoichiometry. However, the catalyst with a stoichiometric spinel molar ratio of Zn / Al (ZnAl_0.5_K) revealed a higher tendency for surface migration and / or vaporization of potassium during overheating at 450 ◦ C. The correlation of the activity results and TPD-CO 2 data show that medium basic sites enhance the progress of the HT-WGS reaction.


Introduction
The water-gas shift (WGS) reaction CO + H 2 O ↔ CO 2 + H 2 (∆H • 298K = −41.1 kJ/mol) is now, and will remain in the future, a very important method for the production of hydrogen [1,2]. The WGS reaction is also of great importance in the production of ammonia throughout steam reforming processes (expressed as CH 4 + H 2 O ↔ CO + 3H 2 , ∆H • 298K = 205.8 kJ/mol) [3]. Usually, the WGS reaction is performed via a two-stage process to overcome thermodynamic and kinetic limitations [4]. The well-known Fe-Cr-Cu-based catalysts for HT-WGS have numerous significant disadvantages, including the presence of environmentally toxic Cr +6 with cancerogenic and mutagenic properties. Moreover, in the case of those catalysts, there is also the necessity of conducting the steam reforming process (preceding the HT-WGS process at syngas plants) at a H 2 O/C ratio not lower than ≈2.8 [5][6][7]. The decrease in this value, which is very favorable technologically, is connected with the risk of forming iron carbide-catalyzing side reactions (Fischer-Tropsch reactions), as the result of which hydrogen is consumed, causing a number of unfavorable consequences and leading to a decrease in process efficiency. Moreover, these systems are gradually deactivated by thermal recrystallization of Fe x O y enhanced by steam [7]. The development of a new formula of HT-WGS catalyst without the above drawbacks, i.e., without the content of cancerogenic chromium and phyrophoric Fe x O y and having

Results and Discussion
The results of chemical analysis of the ZnAl_X_K (fresh) and ZnAl_X_K AT (after long-term stability tests) catalysts are shown in Table 1. Additionally, in Table 1, the surface concentrations of potassium in the ZnAl_X_K catalysts, determined by XPS, have been presented. The atomic concentrations of potassium (Ks c ) were converted into weight percentages to compare them with the values obtained from XRF and SEM-EDS analysis ( Table 1). The XPS analysis of the fresh ZnAl_1.0_K catalyst confirmed a good agreement between the bulk and surface concentrations of potassium. As one can see from Table 1, the values of bulk and surface concentrations of potassium for ZnAl_0.3_K and ZnAl_0.5_K are not the same. In the case of the ZnAl_0.5_K catalyst, the surface concentration of K was higher than the bulk.
The XRF analysis of the ZnAl_X_K catalysts confirmed a good agreement between the real and nominal Zn/Al molar ratios. In addition, Table 1 shows the composition of potassium (K a ) determined by SEM-EDS for the fresh catalysts (ZnAl_X_K) and for the catalysts after long-term stability tests (ZnAl_X_K AT ). For both series, a good agreement between the bulk (K b -determined by XRF) and the content of potassium determined by SEM-EDS (K a ) was confirmed. For the catalysts with a non-stoichiometric Zn/Al molar ratio (ZnAl_0.3_K AT and ZnAl_1.0_K AT ), after thermal stability tests, slight differences in potassium content were observed. However, they were more significant for the sample of ZnAl_0.5_K AT . It seems that this can be associated with the higher tendency for surface migration and/or vaporization of potassium during overheating at 450 • C. Comparing the total content of potassium in the ZnAl_X_K AT series catalysts, determined by the XRF technique, with composition determined by SEM-EDS, it can be concluded that during long-term stability tests, potassium was just slightly evaporated. Similar conclusions come from the work of Bieniasz et al. [27]. According to the authors of this work, alkali promoters may agglomerate or be removed from the catalyst surface due to their volatility at higher temperatures. Moreover, the onset of potassium desorption was observed at 400 • C, meaning that during the HT-WGS process at temperatures below 400 • C, potassium does not escape from the surface of catalyst. When the HT-WGS process was carried out above 400 • C, potassium became mobile.
The comparison of diffraction patterns for ZnAl_X_LDHs precursors with different molar ratio of Zn/Al is presented in Figure 1A,B, respectively. As shown in Figure 1A, the presence of characteristics for the LDH-phase peaks is observed. In the set of diffraction patterns, the first band (in the range 2θ: 11-12 • ) is the strongest reflex coming from the hydrotalcite-like LDH of the formula Zn 0.63 Al 0.37 (OH) 2 (CO 3 ) 0.185 ·xH 2 O. However, the intensity of the characteristics bands at angle 2θ: 11.79 • (003), 23 Figure 1B). Moreover, it was shown that with the increase in the Zn/Al molar ratio in precursors, the parameter "c" decreased and, at the same time, the parameter "LDH interlamellar distance" increased (see Table S1).
The FT-IR spectra of ZnAl-LDHs with variable Zn/Al molar ratios are presented in Figure 3. In all of the samples, three major regions of bands were identified for wavenumbers cm −1 : 400-800 cm −1 , 1550-1250 cm −1 , and 3750-2500 cm −1 . In the FT-IR spectra, for wavenumbers 3750-2500 cm −1 , wide bands with a similar intensity, which are characteristic of stretching bands of O-H in water particles present in interlayer spaces of LDHs and in brucite-like layers, were identified. Moreover, with the increase in the molar ratio of Zn/Al, a shift of bands towards higher wavenumbers (ZnAl_1.0, maximum 3354 cm −1 ) was observed.   The FT-IR spectra of ZnAl-LDHs with variable Zn/Al molar ratios are presented in Figure 3. In all of the samples, three major regions of bands were identified for wavenumbers: 400-800 cm −1 , 1550-1250 cm −1 , and 3750-2500 cm −1 . In the FT-IR spectra, for wavenumbers 3750-2500 cm −1 , wide bands with a similar intensity, which are characteristic of stretching bands of O-H in water particles present in interlayer spaces of LDHs and in brucite-like layers, were identified. Moreover, with the increase in the molar ratio of Zn/Al, a shift of bands towards higher wavenumbers (ZnAl_1.0, maximum 3354 cm −1 ) was observed.  For wavenumbers 1550-1300 cm −1 , bands characteristic of antisymmetric stretching vibrations of CO3 2− present for interlayer spaces of LDHs were visible. The intensity of these bands changed with increases in the Zn/Al molar ratio, with the band of the highest intensity (maximum ~1350 cm −1 ) at ZnAl_1.0. The contribution of CO2 in LHDs is in agreement with the data presented in Table S1. In Figure 4, the FT-IR spectra of ZnAl_X_K are presented. For the set of ZnAl_X_K catalysts, four major regions for wavenumbers cm −1 were evidenced: 400-700 cm −1 , 700-850 cm −1 , 1550-1250 cm −1 , and 3750-2500 cm −1 . In the case of the ZnAl_X_K samples (re-calcined at 450 °C after impregnation with aqueous solution of potassium nitrate), wide bands at wavenumbers in the range of 3750-2500 cm −1 corresponding to the stretching of O-H bonds are visible in FT-IR spectra. These For wavenumbers 1550-1300 cm −1 , bands characteristic of antisymmetric stretching vibrations of CO 3 2− present for interlayer spaces of LDHs were visible. The intensity of these bands changed with the increase in the Zn/Al molar ratio, with the band of the highest intensity (maximum~1350 cm −1 ) at ZnAl_1.0. The contribution of CO 2 in LHDs is in agreement with the data presented in Table  S1. In Figure 4, the FT-IR spectra of ZnAl_X_K are presented. For the set of ZnAl_X_K catalysts, four major regions for wavenumbers were evidenced: 400-700 cm −1 , 700-850 cm −1 , 1550-1250 cm −1 , and 3750-2500 cm −1 . In the case of the ZnAl_X_K samples (re-calcined at 450 • C after impregnation with aqueous solution of potassium nitrate), wide bands at wavenumbers in the range of 3750-2500 cm −1 corresponding to the stretching of O-H bonds are visible in FT-IR spectra. These bands are characteristic of water intercalated between the layers of the rehydrated ex-hydrotalcite-like mixed Zn-Al oxides. Moreover, with the increase of the Zn/Al molar ratio, the shift of bands towards higher wavenumbers (ZnAl_1.0, maximum 3420 cm −1 ) was observed. For wavenumbers of 1550-1400 cm −1 , bands characteristic of stretching vibrations of CO 3 2− intercalated between the layers of the rehydrated ex-hydrotalcite-like mixed Zn-Al oxides were evidenced. The CO 3 2− stretching vibration became weaker with a slight shift to a higher frequency on decreasing the Zn/Al molar ratio, although it was still indicative of the CO 3 2− groups' decomposition and subsequent CO 2 removal from the interlayer space. Additionally, in the ZnAl_1.0_K sample, at 2922 and 2900 cm −1 , bands characteristic of stretching vibrations of CO 3 2 were detected. In addition, the FT-IR results showed bands of high intensity at 1374-1378 cm −1 owing to stretching vibrations of CO 3 2− intercalated between the layers of reconstructed ex-hydrotalcite-like mixed Zn-Al oxides.
Catalysts 2020, 10, x FOR PEER REVIEW 6 of 14 bands are characteristic of water intercalated between the layers of the rehydrated ex-hydrotalcite-like mixed Zn-Al oxides. Moreover, with the increase of the Zn/Al molar ratio, the shift of bands towards higher wavenumbers (ZnAl_1.0, maximum 3420 cm −1 ) was observed. For wavenumbers of 1550-1400 cm −1 , bands characteristic of stretching vibrations of CO3 2− intercalated between the layers of the rehydrated ex-hydrotalcite-like mixed Zn-Al oxides were evidenced. The CO3 2− stretching vibration became weaker with a slight shift to a higher frequency on decreasing the Zn/Al molar ratio, although it was still indicative of the CO3 2− groups' decomposition and subsequent CO2 removal from the interlayer space. Additionally, in the ZnAl_1.0_K sample, at 2922 and 2900 cm −1 , bands characteristic of stretching vibrations of CO3 2 were detected. In addition, the FT-IR results showed bands of high intensity at 1374-1378 cm −1 owing to stretching vibrations of NO3 − intercalated between the layers of reconstructed ex-hydrotalcite-like mixed Zn-Al oxides. Moreover, for the ZnAl_1.0_K sample, a single peak centered at 1066 cm −1 is attributed to NO3 − anions. For wavenumbers 700-400 cm −1 , bands characteristic of stretching and bending vibrations of the Al-O bond with octahedral coordination (AlO6) were visible. Bands at 688 cm −1 and 533 cm −1 corresponded to stretching vibrations of Al-O, whereas the band at 479 cm −1 could be attributed to bending O-Al-O bonds with octahedral coordination (AlO6). However, with the increase in the Zn/Al molar ratio, a decrease in the relative intensity of these bands was observed. Moreover, for ZnAl_0.3_K and ZnAl_0.5_K, bands with a low intensity, in the range of wavenumbers 700-850 cm −1 , coming from Al-O bonds with tetrahedral coordination (AlO4), were identified. The presence of these bonds indicates that the spinel formed under the applied conditions is a zinc-deficient spinel. According to the literature, preparation via coprecipitation and moderate calcination (<600 °C) of mixed oxides (also with a stoichiometric spinel molar ratio of Zn/Al) usually leads to zinc-deficient spinel [19][20][21]. In the ZnAl_1.0_K sample, the intensity of the bands attributed to tetrahedral aluminum (AlO4) was higher. The presence of bands characteristic of tetrahedral aluminum (AlO4) indicates the partial inversion of the ZnAl2O4 spinel structure. Figure 5 shows N2 adsorption-desorption isotherms of the ZnAl_X_K and ZnAl_X_KAT samples with different Zn/Al molar ratios. All of the isotherms correspond to H3-type hysteresis according to IUPAC [31], which is a characteristic feature of mesoporous solids. The specific surface area and porous structure parameters of the ZnAl_X_K and ZnAl_X_KAT catalysts are given in Table 1. These results show that specific surface areas of the fresh ZnAl_X_K catalysts were in the range of 77-154 m 2 /g. With increased Zn/Al molar ratios, significantly lower specific surface areas with a Moreover, for the ZnAl_1.0_K sample, a single peak centered at 1066 cm −1 is attributed to NO 3 − anions. For wavenumbers 700-400 cm −1 , bands characteristic of stretching and bending vibrations of the Al-O bond with octahedral coordination (AlO 6 ) were visible. Bands at 688 cm −1 and 533 cm −1 corresponded to stretching vibrations of Al-O, whereas the band at 479 cm −1 could be attributed to bending O-Al-O bonds with octahedral coordination (AlO 6 ). However, with the increase in the Zn/Al molar ratio, a decrease in the relative intensity of these bands was observed. Moreover, for ZnAl_0.3_K and ZnAl_0.5_K, bands with a low intensity, in the range of wavenumbers 700-850 cm −1 , coming from Al-O bonds with tetrahedral coordination (AlO 4 ), were identified. The presence of these bonds indicates that the spinel formed under the applied conditions is a zinc-deficient spinel. According to the literature, preparation via coprecipitation and moderate calcinations (<600 • C) of mixed oxides (also with a stoichiometric spinel molar ratio of Zn/Al) usually leads to zinc-deficient spinel [19][20][21]. In the ZnAl_1.0_K sample, the intensity of the bands attributed to tetrahedral aluminum (AlO 4 ) was higher. The presence of bands characteristic of tetrahedral aluminum (AlO 4 ) indicates the partial inversion of the ZnAl 2 O 4 spinel structure. Figure 5 shows N 2 adsorption-desorption isotherms of the ZnAl_X_K and ZnAl_X_K AT samples with different Zn/Al molar ratios. All of the isotherms correspond to H3-type hysteresis according to IUPAC [31], which is a characteristic feature of mesoporous solids. The specific surface area and porous structure parameters of the ZnAl_X_K and ZnAl_X_K AT catalysts are given in Table 1. These results show that specific surface areas of the fresh ZnAl_X_K catalysts were in the range of 77-154 m 2 /g. With increased Zn/Al molar ratios, significantly lower specific surface areas with a concomitant lower pore volume were observed. A radically lower value of specific surface area (at about 50%) was determined for ZnAl_1.0_K compared to the ZnAl_0.3_K sample. For the ZnAl_X_K AT series, a significant decrease in the specific surface area was observed (by 20% as compared to the fresh samples) due to the progressing, sintering, and concomitant pore collapsing. Recrystallization processes were clearly visible for ZnAl_1.0_K, the evidence of which is a decrease in S BET by 40% and in total pore volume (V c ) by over 60% as compared to fresh sample. The materials of the ZnAl_X_K AT series are characteristic of a lower total pore volume as compared to ZnAl_X_K ones.
Catalysts 2020, 10, x FOR PEER REVIEW 7 of 14 concomitant lower pore volume were observed. A radically lower value of specific surface area (at about 50%) was determined for ZnAl_1.0_K compared to the ZnAl_0.3_K sample. For the ZnAl_X_KAT series, a significant decrease in the specific surface area was observed (by 20% as compared to the fresh samples) due to the progressing, sintering, and concomitant pore collapsing. Recrystallization processes were most visible for ZnAl_1.0_K, the evidence of which is a decrease in SBET by 40% and in total pore volume (Vc) by over 60% as compared to fresh sample. The materials of the ZnAl_X_KAT series are characteristic of a lower total pore volume as compared to ZnAl_X_K ones.  Figure S1 shows the pore size distribution of the ZnAl_X_K and ZnAl_X_KAT samples with different Zn/Al molar ratios.
Pore size distributions prove that pores with a diameter of 20-60 nm account for the majority of the porous structure of the obtained materials. The porous structure comprises three types of pores: mesopores with diameter ranging from 2 to 10 nm, wide mesopores with a dominant diameter of 20-50 nm, and macropores with a dominant diameter of 140-150 nm. The materials of the ZnAl_X_KAT series are characteristic of lower total pore volume as compared to materials of the ZnAl_X_K series and the dominant pore diameter increased due to overheating.
The surface morphology of the ZnAl_X_K series (fresh catalysts with different Zn/Al molar ratios) were examined by SEM ( Figure 6). Figure 6A,B show that the ZnAl_0.3_K and ZnAl_0.5_K catalysts are built with tightly connected rough quasi-spherical particles and, thus, the hexagonal plate-like morphology typical for LDH materials was not detected. It evidences that the lamellar structure of LDHs was destroyed during thermal treatment. The morphology of the fresh ZnAl_1.0_K catalyst was significantly different (Figure 6C,D). The ZnAl_1.0_K catalyst exhibited a hexagonal platelet morphology of randomly oriented grains. Moreover, aggregates composed of round-edged nanoparticles on their surfaces were also present. It can be seen that, for higher Zn/Al molar ratios, the platelet-like morphology was not totally transformed into a spherical one. However, during the thermal stability test of the ZnAl_1.0_KAT catalyst, the hexagonal plates became aggregated and the presence of larger particles with rounded edges is evidenced on SEM images ( Figure 6E).
Analysis of the SEM images proved that sintering of mixed oxides, with concomitant loss of the platelet morphology, occurred during stability tests at 450 °C. Figure 7 shows TPD-CO2 data for the ZnAl_X_K catalysts with their deconvoluted three Gaussian peaks. The peak area can be considered as the amount of basic sites of different strengths. Analysis of the TPD-CO2 results obtained for the ZnAl_X_K catalysts showed three ranges of CO2  Figure S1 shows the pore size distribution of the ZnAl_X_K and ZnAl_X_K AT samples with different Zn/Al molar ratios.
Pore size distributions prove that pores with a diameter of 20-60 nm account for the majority of the porous structure of the obtained materials. The porous structure comprises three types of pores: mesopores with diameter ranging from 2 to 10 nm, wide mesopores with a dominant diameter of 20-50 nm, and macropores with a dominant diameter of 140-150 nm. The materials of the ZnAl_X_K AT series are characteristic of lower total pore volume as compared to materials of the ZnAl_X_K series and the dominant pore diameter increased due to overheating.
The surface morphology of the ZnAl_X_K series (fresh catalysts with different Zn/Al molar ratios) were examined by SEM ( Figure 6). Figure 6A,B show that the ZnAl_0.3_K and ZnAl_0.5_K catalysts are built with tightly connected rough quasi-spherical particles and, thus, the hexagonal plate-like morphology typical for LDH materials was not detected. It evidences that the lamellar structure of LDHs was destroyed during thermal treatment. The morphology of the fresh ZnAl_1.0_K catalyst was significantly different ( Figure 6C,D). The ZnAl_1.0_K catalyst exhibited a hexagonal platelet morphology of randomly oriented grains. Moreover, aggregates composed of round-edged nanoparticles on their surfaces were also present. It can be seen that, for higher Zn/Al molar ratios, the platelet-like morphology was not totally transformed into a spherical one. However, during the thermal stability test of the ZnAl_1.0_K AT catalyst, the hexagonal plates became aggregated and the presence of larger particles with rounded edges is evidenced on SEM images ( Figure 6E). desorption bands from the mixed oxides surface which demonstrates the presence of alkali centers with various strengths: (a) weak centers (α peak); CO2 desorption at 75-180 °C; (b) medium centers (β peak); CO2 desorption at 180-300 °C; (c) strong centers (γ peak); CO2 desorption above 300 °C.
Moreover, for the ZnAl_0.5_K catalyst, the shift of bands (α, β, γ) towards higher maximum peak temperature was observed. Weak basic centers correspond to O-H groups on the surface of the catalysts; medium strength sites are associated to the oxygen in Zn 2+ -O 2− , Al 3+ -O 2− and K + -O 2− pairs, and strong basic sites correspond to low coordination oxygen atoms [32]. Table S2 shows the contribution of basic sites to ZnAl_X_K samples and Tmax desorption. Deconvolution results (see Table S1 and Figure 7) show that medium centers (β peak) are a dominant in all the tested samples.
For ZnAl_0.3_K and ZnAl_1.0_K, similar levels of CO2 desorption are observed which correspond with weak alkali centers (α) and medium centers (β). Concomitantly, for ZnAl_1.0_K, the increase in concentration of strong alkali centers (γ) on the catalyst surface was observed as compared to ZnAl_0.3_K. For the sample with a stoichiometric spinel molar ratio of Zn/Al (ZnAl_0.5_K), the desorption of a larger (compared to the other two samples) amount of CO2 at 180-400 °C was observed, which indicates that the number of surface active sorption sites is higher for stoichiometric ZnAl2O4 spinel compared to other studied systems. Moreover, it seems reasonable that the excessive amount of free ZnO hinders CO2 adsorption on medium alkali centers present on the surface of the ZnAl_1.0_K catalyst. It also seems reasonable to assume that CO2 is less efficiently adsorbed on the zinc-deficient spinel ZnAl_0.3_K for the entire range of studied temperatures. Figure 8A shows kinetic data for the set of ZnAl_X_K fresh catalysts with different Zn/Al molar ratios. The results reveal that the tested catalysts exhibited high catalytic activity in the HT-WGS reaction at temperatures up to 400 °C. The activity of the ZnAl_0.3_K and ZnAl_1.0_K catalysts containing a defective spinel phase decorated with potassium was high. However, the highest activity was revealed for the sample with a stoichiometric spinel molar ratio of Zn/Al (ZnAl_0.5_K). Moreover, the ZnAl_0.5_K catalyst exhibited relatively high activity at a low temperature range of 330-350 °C. The catalytic activity of the ZnAl_X_K catalysts in the HT-WGS reaction is associated with the surface basicity. This confirms well-evidenced observations indicating a correlation Analysis of the SEM images proved that sintering of mixed oxides, with concomitant loss of the platelet morphology, occurred during stability tests at 450 • C. Figure 7 shows TPD-CO 2 data for the ZnAl_X_K catalysts with their deconvoluted three Gaussian peaks. The peak area can be considered as the amount of basic sites of different strengths. Analysis of the TPD-CO 2 results obtained for the ZnAl_X_K catalysts showed three ranges of CO 2 desorption bands from the mixed oxides surface which demonstrates the presence of alkali centers with various strengths: (a) weak centers (α peak); CO 2 desorption at 75-180 • C; (b) medium centers (β peak); CO 2 desorption at 180-300 • C; (c) strong centers (γ peak); CO 2 desorption above 300 • C.
Moreover, for the ZnAl_0.5_K catalyst, the shift of bands (α, β, γ) towards higher maximum peak temperature was observed. Weak basic centers correspond to O-H groups on the surface of the catalysts; medium strength sites are associated to the oxygen in Zn 2+ -O 2− , Al 3+ -O 2− and K + -O 2− pairs, and strong basic sites correspond to low coordination oxygen atoms [32]. Table S2 shows the contribution of basic sites to ZnAl_X_K samples and T max desorption. Deconvolution results (see Table S1 and Figure 7) show that medium centers (β peak) are a dominant in all the tested samples.
For ZnAl_0.3_K and ZnAl_1.0_K, similar levels of CO 2 desorption are observed which correspond with weak alkali centers (α) and medium centers (β). Concomitantly, for ZnAl_1.0_K, the increase in concentration of strong alkali centers (γ) on the catalyst surface was observed as compared to ZnAl_0.3_K. For the sample with a stoichiometric spinel molar ratio of Zn/Al (ZnAl_0.5_K), the desorption of a larger (compared to the other two samples) amount of CO 2 at 180-400 • C was observed, which indicates that the number of surface active sorption sites is higher for stoichiometric ZnAl 2 O 4 spinel compared to other studied systems. Moreover, it seems reasonable that the excessive amount of free ZnO hinders CO 2 adsorption on medium alkali centers present on the surface of the ZnAl_1.0_K catalyst. It also seems reasonable to assume that CO 2 is less efficiently adsorbed on the zinc-deficient spinel ZnAl_0.3_K for the entire range of studied temperatures. Figure 8A shows kinetic data for the set of ZnAl_X_K fresh catalysts with different Zn/Al molar ratios. The results reveal that the tested catalysts exhibited high catalytic activity in the HT-WGS reaction at temperatures up to 400 • C. The activity of the ZnAl_0.3_K and ZnAl_1.0_K catalysts containing a defective spinel phase decorated with potassium was high. However, the highest activity was revealed for the sample with a stoichiometric spinel molar ratio of Zn/Al (ZnAl_0.5_K). Moreover, the ZnAl_0.5_K catalyst exhibited relatively high activity at a low temperature range of 330-350 • C. The catalytic activity of the ZnAl_X_K catalysts in the HT-WGS reaction is associated with the surface basicity. This confirms well-evidenced observations indicating a correlation between the number of medium basic sites and catalytic activity in the HT-WGS reaction (see TPD-CO 2 results).   The highest activity of ZnAl_0.5_K in the HT-WGS reaction is caused by the presence of larger (compared to the other two samples) amount of medium basic centers (see Table S2). Moreover, the reasons for lower catalyst activity of ZnAl_1.0_K may be associated with partially disordered structure of ZnAl2O4 phase in Zn-Al mixed oxide catalysts decorated with potassium. According to the FT-IR results (see Figure 4A,B), the occurrence of characteristic tetrahedral coordination bands (AlO4) indicates a partial inversion of the ZnAl2O4 spinel structure.
Despite the high activity, the K-decorated Zn-Al systems differ in thermal stability depending on the Zn/Al molar ratio ( Figure 8B). The decrease of activity by ca. 20% and 40% for ZnAl_0.5_K and ZnAl_1.0_K, respectively, was observed as a result of the samples being overheated in HT-WGS process conditions. This may be caused by recrystallization (see Table 1) during the catalyst performance. In spite of recrystallization, the ZnAl_1.0_K catalyst maintained high catalytic activity in the HT-WGS process. The best thermal stability was found for the catalyst with a stoichiometric spinel phase as the dominant one (ZnAl_0.5_KAT), despite the visible escape of potassium. Table 2 shows the HT-WGS reaction rate (r) of the ZnAl_X_K and ZnAl_X_KAT catalysts with   The highest activity of ZnAl_0.5_K in the HT-WGS reaction is caused by the presence of larger (compared to the other two samples) amount of medium basic centers (see Table S2). Moreover, the reasons for lower catalyst activity of ZnAl_1.0_K may be associated with partially disordered structure of ZnAl2O4 phase in Zn-Al mixed oxide catalysts decorated with potassium. According to the FT-IR results (see Figure 4A,B), the occurrence of characteristic tetrahedral coordination bands (AlO4) indicates a partial inversion of the ZnAl2O4 spinel structure.
Despite the high activity, the K-decorated Zn-Al systems differ in thermal stability depending on the Zn/Al molar ratio ( Figure 8B). The decrease of activity by ca. 20% and 40% for ZnAl_0.5_K and ZnAl_1.0_K, respectively, was observed as a result of the samples being overheated in HT-WGS process conditions. This may be caused by recrystallization (see Table 1) during the catalyst performance. In spite of recrystallization, the ZnAl_1.0_K catalyst maintained high catalytic activity in the HT-WGS process. The best thermal stability was found for the catalyst with a stoichiometric spinel phase as the dominant one (ZnAl_0.5_KAT), despite the visible escape of potassium. The highest activity of ZnAl_0.5_K in the HT-WGS reaction is caused by the presence of larger (compared to the other two samples) amount of medium basic centers (see Table S2). Moreover, the reasons for lower catalyst activity of ZnAl_1.0_K may be associated with partially disordered structure of ZnAl 2 O 4 phase in Zn-Al mixed oxide catalysts decorated with potassium. According to the FT-IR results (see Figure 4A,B), the occurrence of characteristic tetrahedral coordination bands (AlO 4 ) indicates a partial inversion of the ZnAl 2 O 4 spinel structure.
Despite the high activity, the K-decorated Zn-Al systems differ in thermal stability depending on the Zn/Al molar ratio ( Figure 8B). The decrease of activity by ca. 20% and 40% for ZnAl_0.5_K and ZnAl_1.0_K, respectively, was observed as a result of the samples being overheated in HT-WGS process conditions. This may be caused by recrystallization (see Table 1) during the catalyst performance. In spite of recrystallization, the ZnAl_1.0_K catalyst maintained high catalytic activity in the HT-WGS process. The best thermal stability was found for the catalyst with a stoichiometric spinel phase as the dominant one (ZnAl_0.5_K AT ), despite the visible escape of potassium. Table 2 shows the HT-WGS reaction rate (r) of the ZnAl_X_K and ZnAl_X_K AT catalysts with different Zn/Al molar ratios. Additionally, the activity of Fe-based reference HT-WGS catalysts has been presented. The results reveal that, for the tested range of temperatures, the ZnAl_0.5_K catalyst showed higher activity by approx. 20% as compared to the (Fe-Cr-Cu) reference HT-WGS catalyst. A comparable activity level was observed for ZnAL_1.0_K. A lower activity, as compared to Fe-Cr-Cu, was observed for ZnAL_0.3_K (activity lowered by approx. 10% at 400 • C).
The comparison of data collected in Table 2 also shows the substantial differences of E a between the Fe-based reference catalysts and the series of ZnAl_X_K and ZnAl_X_K AT catalysts. As shown in the presented data, the ZnAl_X_K are characteristic of clearly lower E a values as compared to the Fe-Cr-Cu (162.2 ± 1.1) applied in the HT-WGS. The presented data for ZnAl_X_K in the kinetic regime are 91.8-115.3 kJ/mol. For the ZnAl_X_K AT catalysts, slightly lower E a values were observed at 99.4-118.74 kJ/mol.

Materials and Synthesis
A series of Zn-Al-LDH precursors with different molar ratios of Zn/Al were prepared by co-precipitation. Zn-Al-LDH precursors with nominal compositions of (Zn/Al) mol = 0.3, 0.5, and 1.0 were prepared by a simultaneous dosing of two streams of Zn(NO 3 ) 2 ·6H 2 O (Chempur, PiekaryŚląskie, Poland), Al(NO 3 ) 3 ·9H 2 O (99% purity, Chempur, PiekaryŚląskie, Poland) aqueous solution and Na 2 CO 3 /NaOH (99% purity, Chempur, PiekaryŚląskie, Poland) aqueous solution to the precipitation reactor under vigorous stirring. Synthesis was carried out at a temperature of 75-80 • C and pH in the range of 7.5-8. After complete precipitation, the slurry was aged at 80 • C for 0.5 h under vigorous stirring. In the next step, the sodium and nitrate ions were thoroughly removed by washing with redistilled water to reach a filtrate conductivity below 150 µS/cm, and were then separated, dried, and calcined at 500 • C for 4 h. The obtained solids were ground to 0.16-0.25 mm fraction. In the next stage, the materials were subjected to incipient wetness impregnation with an aqueous solution of potassium nitrate (99.0% purity, Sigma Aldrich, Poznań, Poland) with concentrations selected on the basis of the water absorption of Zn-Al oxides, allowing to obtain catalysts with a nominal K content equal to 3 wt. %. After impregnation, the samples were dried and recalcined at 450 • C for 2 h. The resulting solids were indicated: ZnAl_X_K, where X is the Zn/Al molar ratio.

Characterization
The chemical composition of the samples was determined by the WD XRF method using a Axios mAx spectrometer (Malvern Panalytical, Malvern, UK), equipped with a lamp Rh SST-mAx 4 kW. The specific surface area and total pore volume were determined, using ASAP ® 2050 Xtended Pressure sorption analyzer (Micromeritics Instrument Co., Norcross, GA, USA), from N 2 adsorption and desorption isotherms at a temperature of −196 • C for p/p0 ≤ 0.99, using the BET adsorption model and BJH (Barret-Joyner-Halenda) transformation for mesopore characteristics. The XRD measurements were performed on an Empyrean (Malvern Panalytical, Malvern, UK) system (Bragg-Brentano geometry) equipped with a PIXcel3D detector, using Cu Kα radiation (λ = 1.542 Å) and operating at 40 kV and 40 mA. The crystallite size of the Zn-Al-LDH's phase was calculated using Scherrer's formula from the diffraction broadening on planes (003), (006), (012), and (015). Fourier-transform infrared spectroscopy (FT-IR) measurements were taken from Nicolet 8700A (Thermo Scientific, USA) equipped with a diamond crystal (Smart Orbit TR diamond ATR). Infrared spectra were obtained in a 4000-400 cm −1 spectral range. A high resolution FEI Quanta3D FEG microscope (Waltham, MA, USA) with an electron beam energy of 20 kV was used in order to analyze the surface morphology by scanning electron microscopy (SEM) using an EDX Octane Elect Plus detector. Analysis of surface composition was carried out for 5 areas (150 µm × 150 µm). The specified surface composition was the average derived from results for five randomly chosen separated sub-areas. The surface basicity of the ZnAl_X_K catalysts was measured by temperature programmed desorption of CO 2 (TPD-CO 2 ) using a AutoChem 2950 HP analyzer (Micromeritics Instrument Co., Norcross, GA, USA). Each sample (0.2 g) was pre-treated in a He flow at 500 • C for 30 min (10 • C/min); the samples were then cooled to 200 • C and CO 2 adsorption was performed, and then the samples were cooled to 150 • C. In the next stage, the CO 2 stream was replaced with a He purge to remove the physisorbed CO 2 and samples were cooled down to 20 • C. Finally, the desorption process was performed at a heating rate of 10 • C/min to 500 • C. Surface elemental analyses were performed by X-ray photoelectron spectrometry (multi-chamber analytical system UHV, Prevac, Rogów, Poland) at a base pressure better than 1.0 × 10 −8 mbar, using a polychromatic Al K X-ray source (1486.7 eV).
The HT-WGS reaction rate at the kinetics regime was calculated based on a mass balance of the reactor: r = 0.01 · V · ∆C CO · m cat The catalytic effect's durability was evaluated on the basis of activity changes as a result of overheating the catalysts at 450 • C over 24 h in a stream of process gas. The ZnAl_X_K catalysts, after long-term stability tests, were indicated as ZnAl_X_K AT , where X is the Zn/Al molar ratio.

Conclusions
Mixed oxide catalysts (ZnO/ZnAl 2 O 4 ) derived from LDH precursors and K-decorated showed high specific surface areas. Increasing the Zn/Al molar ratio resulted in decreases in the specific surface area and total pore volume of catalysts. The amount and strength of surface basic sites on ex-ZnAl-LDH decorated with potassium depended on the (Zn/Al) mol ratio. The correlation of activity results and TPD-CO 2 data show that medium basic sites enhance HT-WGS reaction.
The catalytic activity of ZnAl_X_K in the HT-WGS reaction was relatively high and depended on the Zn/Al molar ratio. The highest activity was observed for the catalyst with a stoichiometric spinel molar ratio of Zn/Al (ZnAl_0.5_K). The catalyst containing a zinc-deficient spinel phase decorated with potassium (ZnAl_0.3_K) and the catalyst with partial inversion of the ZnAl 2 O 4 spinel phase in Zn-Al mixed oxide decorated with potassium (ZnAl_1.0_K) showed lower catalytic activity in HT-WGS process conditions.
Sintering of ZnO-ZnAl 2 O 4 mixed oxides, with a concomitant loss of platelet morphology, occurred even during the preparation of calcinations step (500 • C). Nevertheless, in spite of recrystallization as a result of overheating, the ZnAl_1.0_K catalyst maintained high catalytic activity in the HT-WGS process. The catalyst with a stoichiometric spinel molar ratio of Zn/Al (ZnAl_0.5_K showed a higher level of activity by about 20% compared to the Fe-Cr-Cu catalyst.
Supplementary Materials: The following are available online at http://www.mdpi.com/2073-4344/10/9/1094/s1: Table S1: The comparison of the physicochemical properties and lattice parameters of ZnAl_X_LDHs; Table S2: Contribution of basic sites for ZnAl_X_K catalysts. Peak area ratio is defined as the ratio of the sum of α, β and δ peak area of ZnAl_X_K catalysts to ZnAl_0.5_K catalyst; Figure S1: Pore size distribution of ZnAl_X_K and ZnAl_X_K AT samples with different Zn/Al molar ratios.
Author Contributions: K.A.-J., methodology, supervision and writing-original draft; P.K., writing-review and editing; K.M., investigation.; W.P., investigation.; R.B., investigation. All authors have read and agreed to the published version of the manuscript.