Cobalt based catalysts on alkali-activated zeolite foams for N2O decomposition

catalysts Article Cobalt Based Catalysts on Alkali-Activated Zeolite Foams for N2O Decomposition Zdenˇek Tišler 1, Anna Klegová2, Eliška Svobodová1,* , Jan Šafáˇr 1, Kateˇrina Strejcová1, Jan Kohout 1, Stanislav Šlang 3, Kateˇrina Pacultová2, Daily Rodríguez-Padrón4and Roman Bulánek 5 1Unipetrol Centre for Research and Education, a.s, Areál Chempark 2838, Záluží1, 436 70 Litvínov, Czech Republic; [email protected] (Z.T.); [email protected] (J.Š.); [email protected] (K.S.); [email protected] (J.K.) 2Institute of Environmental Technology, VSB-Technical University of Ostrava, 17. listopadu 15/2172, 708 00 Ostrava, Czech Republic; [email protected] (A.K.); [email protected] (K.P.) 3Center of Materials and Nanotechnologies, Faculty of Chemical Technology, University of Pardubice Nam. Cs. Legii, 530 02 Pardubice, Czech Republic; stanislav[email protected] 4Departamento de Química Orgánica, Campus de Rabanales, Universidad de Córdoba, Edificio Marie Curie (C-3), Ctra Nnal IV-A, Km 396, E14014 Cordoba, Spain; [email protected] 5Department of Physical Chemistry, Faculty of Chemical Technology, University Pardubice, Studentská573, 532 10 Pardubice, Czech Republic; r[email protected] *Correspondence: [email protected] Received: 16 October 2020; Accepted: 27 November 2020; Published: 30 November 2020   Abstract: In this work, we studied the effect of alkali-activated zeolite foams modifications on properties and catalytic activity of cobalt phases in the process of catalytic decomposition of N 2 O. The zeolite foam supports were prepared by alkali activation of natural zeolite followed by acid leaching and ion exchange. The cobalt catalysts were synthesised by a different deposition technique (direct ion exchange (DIE) and incipient wetness impregnation (IWI) method of cobalt on zeolite foams. For comparison, catalysts on selected supports were prepared and the properties of all were compared in catalytic tests in the pellet form and as crushed catalysts to determine the effect of internal diffusion. The catalysts and supports were in detail characterized by a variety of techniques. The catalyst activity strongly depended on the structure of support and synthesis procedure of a cobalt catalyst. Ion exchange method provided active phase with higher surface areas and sites with better reducibility, both of these factors contributed to higher N 2 O conversions of more than 80% at 450 ◦ C. A large influence can also be attributed to the presence of alkali metals, in particular, potassium, which resulted in a modification of electronic and acid base properties of the cobalt oxide phase on the catalyst surface. The promotional effect of potassium is better reducibility of cobalt species. Keywords: naturalzeolite;clinoptilolite;zeolitefoam;alkaliactivation;cobaltcatalyst;N 2 Odecomposition 1. Introduction The alkali-activated materials have properties very similar to zeolites but do not form crystalline structures. These materials can be prepared by activating a variety of aluminosilicate components. Raw materials for the preparation of these materials, e.g., natural zeolites, most often occur as compact fine-grained rocks formed by sedimentary or volcanic activity, whose main component is zeolite clinoptilolite, belonging to the group of heulandite, accompanied by minerals from the group of feldspars, clays and mica. Due to their properties, natural zeolites are widely used as sorbents for water purification (Cs + , Rb + and NH 4+ ) or gases (NH 3 ) [ 1 , 2 ]. They do not have suitable properties for use in catalysis, which requires their modification by processes such as dealumination [ 3 , 4 ], desilication or Catalysts 2020,10, 1398; doi:10.3390/catal10121398 www.mdpi.com/journal/catalysts Catalysts 2020,10, 1398 2 of 21 ion exchange [ 5 ]. These modifications improve the properties in the desired direction, e.g., increase the specific surface area or reduce the content of accompanying elements (Fe, K, Na, Ca and Mg). The same modification procedures can be used to modify the properties of zeolite foams which, in addition to almost any desired shape (pellets, blocks, crumb) and macroporosity not present in the original zeolite material, impart the desired properties of the zeolite material. Macroporosity is advantageous not only for diffusion reasons but also for structural reasons, as this material reduces weight of catalyst/sorption bed and thus the complexity of the equipment and its robustness requirements [6]. Alkali-activation of natural zeolite produces a foam material consisting of the original zeolite (clinoptilolite) structure and the N/K-A-S-H binder matrix (Na 2 O/K 2 O-Al 2 O 3 -SiO 2 -H 2 O) that binds the individual grains of aluminosilicate component. This binder matrix is formed by the reaction of the alkaline activator with an aluminosilicate (natural zeolite). These materials with the additional macroporous structure are prepared by alkaline activation of natural zeolite with a mixed activator based on potassium hydroxide and sodium silicate. The macroporous structure is obtained by foaming an activated mixture of metal powders (Al, Mg, Zn) [ 6 – 8 ], solutions containing hydrogen peroxide [ 8 , 9 ], silica fume or silicon powder [ 10 , 11 ]. Subsequent reaction of these components with the alkaline activator results in the evolution of a gas which foams the activated mixture. The foaming of the alkali-activated mixture and following post-synthesis modification of prepared materials e.g., by acid leaching create a well permeable foam structure of low density in which micro-, meso-andmacroporesarepresent. Acidleachingisoftenusedforpost-synthesismodificationofzeolites to remove framework aluminium and, in the case of alkali-activated materials, other accompanying elements (K, Na, Ca, Fe, Mg), presented in the basic zeolite grains or binder matrix. This leaching of the material creates additional porosity, which is important for the easier permeability of foams [ 8 ]. The second option for post-synthesis modification of foams is the ion exchange (IE). This method is widely used for zeolites and due to the similarity of alkali-activated materials, this technique can also be used for the removing of Na and K ions or exchanging another cation [8]. Nitrous oxide (N 2 O) is considered as an important pollutant contributing to the greenhouse effect even though it is not the major contributor. However, unlike the main contributors (CO 2 and CH 4 ), it is much more efficient greenhouse gas [ 12 ]. The largest industrial sources of N 2 O emissions are waste gases from nitric acid production plants in a global amount of more than 400 kt of N 2 O per year [ 12 ]. Catalytic decomposition of N 2 O can be performed directly in the NH 3 burner (temperature around 900 ◦ C, catalysts based on oxides, spinels and perovskite materials) or behind the burner, when the gas temperature is only 250–500 ◦ C. In addition to the possibility of thermal decomposition of N 2 O (by increasing the gas temperature to 750–1000 ◦ C), non-selective or selective catalytic reduction of N2O (requiring the addition of reducing agents such as hydrogen, natural gas, LPG, NH3, etc.) is the most economically advantageous direct catalytic decomposition of N 2 O. The low-temperature catalytic decomposition of N 2 O (up to 450 ◦ C) to nitrogen and oxygen offers an attractive way to decrease N 2 O emissions in tail gas from nitric acid production plants. Results from a number of catalytic systems have been published. For example, catalysts based on transition metals (Cu, Co, Ni, Fe) or catalysts based on precious metals (Rh, Ru, Pd) using various catalyst supports (ZnO, CeO 2 , Al 2 O 3 , TiO 2 , ZrO 2 , zeolites, hydrotalcites and perovskites). This study is focused on cobalt-based catalysts that show excellent catalytic activities at low temperature (≤450 ◦C) in the decomposition of N2O [12–30]. Catalysts based on synthetic zeolites are successfully used and studied in a number of catalytic applications including catalytic decomposition of nitrous oxide. Synthetic cobalt modified zeolites (Beta, Y, ZSM-5, etc.) [ 31 ], eventually modified by other metals like iron or copper [ 32 ], were often studied. These catalysts show very good results in the catalytic decomposition of N 2 O, conversions reach up to 100% at 425 ◦C and given experimental conditions [33]. The disadvantage in comparison with natural zeolites and materials formed therefrom, for example, alkali-activated foams, is in particular that their synthesis is relatively time-consuming and economically demanding. On the other hand, the use of leached modified natural zeolite with subsequent ion exchange (IE) encounters some limitations, such as unsatisfactory textural properties or limited IE capacity [34]. Catalysts 2020,10, 1398 3 of 21 In addition to zeolite catalysts, other types of catalysts prepared, for example, by impregnation of other supports, precipitated catalysts or catalysts prepared by other techniques based on the presence of spinel phases of cobalt or mixed Co-Mn-Al phases are studied for catalytic N 2 O decomposition [11,24,27,35] . Modification of the cobalt spinels with a small amount of alkali metals significantly increases the activity of the catalyst, alkali metal promoter lowers the work function of the cobalt spinel facilitating redox processes that occur between the catalyst surface and the reaction oxygen intermediatesproduced during the N 2 Odecompositionleadingtothesignificantincreaseinthecatalytic activity[ 14 , 17 , 23 , 27 , 35 – 38 ]. SynthesisconditionsoftheCospineloxidecouldsignificantlyaffectcatalytic properties. Different methods were described for the preparation of cobalt oxide-based catalysts: (co)-precipitation from cobalt nitrate solution using different precipitation agents like Na 2 CO 3 [ 20 , 39 ], K 2 CO 3 [ 40 , 41 ], (NH 4 ) 2 CO 3 [ 16 ], KOH [ 42 ] and NH 3· H 2 O, NaOH [ 39 ], by thermal treatment of cobalt nitrate [ 24 , 43 , 44 ], cobalt carbonate [ 14 , 24 ] or cobalt benzoate-dihydrazinate complex [ 45 ] and by solution combustion synthesis from cobalt nitrate and urea [ 43 ]. Equally, the deposition of active phase on suitable support has a significant impact. Therefore, the selection of suitable support could lead to more efficient utilization of active phase. It is achieved by applying a thin active layer to the support material. Supported cobalt oxide N 2 O catalysts were studied mainly on different monoliths [ 46 – 49 ], sieves [32,50], tablets, pellets or extrudates [29,51] and ceramic foams [52]. The work aims to prepare cobalt catalysts based on alkali-activated zeolite foams and determine the influence of catalyst support modifications and cobalt oxide loading method on physic-chemical properties and catalytic activity in N 2 O decomposition. Acid leaching and ion exchange of alkali activated zeolite foam (AZF) were used to prepare catalyst supports, which were subsequently impregnated with cobalt nitrate, and after annealing, cobalt oxide catalysts were obtained. The same method was used to prepare comparative catalysts using alumina as a support. The effect of different preparation on the properties of the active phase was studied on catalysts prepared by direct ion exchange (DIE) of basic alkali activated zeolite foam, as well as the promotional effect of manganese in Co catalyst in catalytic decomposition of N 2 O. The effect of Mn, was monitored in our study because CoMnAl HTC 4:1:1 catalysts are often reported to have very good results.Studied catalytic materials showed significant differences in the textural properties and chemical composition of the active phase resulting in significant differences in activity depending on the chosen modification method. All catalysts were characterized by various instrumental techniques and tested in the N 2 O catalytic decomposition in inert gas. The obtained results showed the further possible use of catalysts based on alkali-activated zeolite foams in catalytic applications. In the field of industrial catalysis, in addition to the catalytic results, the ease of synthesis, affordability and the price of the catalysts used are also important, and therefore, these materials have a lot to offer. 2. Results and Discussion 2.1. Characterisation of the Catalysts and Supports Zeolite foam used as catalyst support was prepared by foaming of alkali-activated natural zeolite mixture. Obtained porous solid foam material was post-synthesis modified (i) by acid leaching with 3M HCl or (ii) by ion-exchange treatment using 1M NH4NO3. As-prepared zeolite foam (AA-S, Table 1) had a higher alkali metals content than natural zeolites due to the use of mixed alkaline activator KOH +Na 2 SiO 3 . Chemical composition of Al 2 O 3 can be seen in Appendix ATable A1. A part of the alkaline activator occurred in a form of mixed Na/K silicate, which can be seen in images from SEM equipped with EDS analyser as a leaf-like particles (Figure 1). Ion-exchange treatment of AA-S sample using NH 4 NO 3 solution (AA-N) reduced predominantly content of K and Na cations. A part of K + and Na + ions was in “unchangeable positions” trapped inside of the zeolite and/or other aluminosilicates framework [ 4 , 8 ]. Results of chemical composition analysis (Table 1) showed a noticeable effect of HCl leaching (AA-D). HCl mostly reduced the alkali Catalysts 2020,10, 1398 4 of 21 content (K, Na) in addition to other elements (Fe, Ca, Mg and especially Al), which resulted into higher Si/Al ratio (up to 11.8 from the original 5.8). Table 1. Chemical composition and specific surface area of catalysts and zeolite foam supports. Sample Chemical Composition (wt.%) Si/Al Ratio (mol/mol) Co/AM * Ratio SSA ** (m2/g) Si Al K Na Ca Fe Co Mn AA-S 33.4 5.5 7.2 2.9 2.3 0.9 - - 5.8 - 15.0 AA-N 37.8 6.3 1.6 0.3 1.7 1.1 - - 5.8 - 23.4 AA-D 42.0 3.4 1.1 0.2 0.5 0.7 - - 11.8 - 111.7 AA-S-Co 30.5 5.1 6.6 2.6 2.1 0.9 6.4 - 5.7 0.4 13.5 AA-N-Co 35.0 6.0 1.4 0.2 1.6 1.0 5.7 - 5.6 2.1 27.7 AA-D-Co 38.7 3.5 1.1 0.2 0.4 0.7 5.4 - 10.6 2.5 49.9 AA-IE-Co 32.0 5.2 4.1 0.5 1.7 0.9 6.3 - 5.9 0.9 115.8 AA-IE-CoMn 31.9 5.2 4.2 0.5 1.7 0.9 5.4 0.8 5.9 0.7 118.6 * AM =alkali metals (Na +K), ** SSA =specific surface area (BET). Catalysts 2020, 10, x FOR PEER REVIEW 4 of 22 Na) in addition to other elements (Fe, Ca, Mg and especially Al), which resulted into higher Si/Al ratio (up to 11.8 from the original 5.8). Figure 1. Macroporous structure of zeolite foam AA-S (a), crystalline Na/K silicate (white arrow) in AAS (b) and leaves of Na/K silicates in AA-S-Co catalyst (c). Table 1. Chemical composition and specific surface area of catalysts and zeolite foam supports. Sample Chemical Composition (wt.%) Si/Al Ratio (mol/mol) Co/AM * Ratio SSA ** (m2/g) Si Al K Na Ca Fe Co Mn AA-S 33.4 5.5 7.2 2.9 2.3 0.9 - - 5.8 - 15.0 AA-N 37.8 6.3 1.6 0.3 1.7 1.1 - - 5.8 - 23.4 AA-D 42.0 3.4 1.1 0.2 0.5 0.7 - - 11.8 - 111.7 AA-S-Co 30.5 5.1 6.6 2.6 2.1 0.9 6.4 - 5.7 0.4 13.5 AA-N-Co 35.0 6.0 1.4 0.2 1.6 1.0 5.7 - 5.6 2.1 27.7 AA-D-Co 38.7 3.5 1.1 0.2 0.4 0.7 5.4 - 10.6 2.5 49.9 AA-IE-Co 32.0 5.2 4.1 0.5 1.7 0.9 6.3 - 5.9 0.9 115.8 AA-IE-CoMn 31.9 5.2 4.2 0.5 1.7 0.9 5.4 0.8 5.9 0.7 118.6 * AM = alkali metals (Na + K), ** SSA = specific surface area (BET). These three types of support based on zeolite foams (AA-S, AA-N, AA-D) and one commercial supports (γAl2O3) were used to prepare cobalt catalysts with 5 wt.% Co content. The actual cobalt content ranged from 4.8 to 6.4 wt.%, as determined by XRF the slightly higher cobalt content was due to the non-stoichiometric water content of the cobalt nitrate hydrate. The last two catalysts tested (AAIE-Co and AA-IE-CoMn) were prepared by a direct ion-exchange method using a solution of cobalt nitrate, and mixture of cobalt nitrate with manganese nitrate, respectively. The cobalt content of AA-IE-Co catalysts was 6.3 wt.% and the cobalt content of the AA-IE-CoMn catalyst was 5.4 wt.% and manganese 0.8 wt.%, respectively. The cobalt content was average for the whole pellet, so the cobalt content in the active layer of AA-IE-Co catalyst is higher than 6.3 wt.% (theoretically up to about 13 wt.% or about up 11 wt.% for AA-IE-CoMn catalyst). The molar ratio of Co/Mn is 6.5:1, the difference compared to the target ratio (Co/Mn 4:1) is due to the different course of precipitation of Co and Mn ions during the formation of the layer in the pellet. Catalysts prepared using this method had a significantly higher content of potassium and slightly higher content of sodium in comparison with a catalyst impregnated on AA-N support. This showed that cobalt precipitation precluded complete ion-exchange of Na+ and K+ ions by Co2+ ion. Higher Na and K contents were observed only for the AA-S-Co catalyst, but the alkalis were present here in the form of Na/K silicates. As Table 1 shows, the presence of these components had a very negative effect on the specific surface area (SSA) because the silicates closed the pores. Additionally, due to pore opening and cleaning, there was also an increase in specific surface area (SSA) two times for (AA-N) and more than seven times for AA-D. SSA of catalysts did not change significantly compare to original supports. The decrease of SSA was observed only for alumina and AA-D supported catalyst, which could have been caused by partially blocking the pores with cobalt, more precisely, its oxide. This decrease in AA-D was related to blockade of clinoptilolite micropores which have been released by acid leaching [4,8]. The catalysts AA-IE-Co and AA-IE-CoMn prepared in this way exhibit relatively high specific surface (SSA) areas compared to catalysts employing differently modified zeolite foam supports (Table 1). This was probably partially caused by c ) Figure 1. Macroporous structure of zeolite foam AA-S ( a ), crystalline Na/K silicate (white arrow) in AA-S (b) and leaves of Na/K silicates in AA-S-Co catalyst (c). These three types of support based on zeolite foams (AA-S, AA-N, AA-D) and one commercial supports ( γ - Al 2 O 3 ) were used to prepare cobalt catalysts with 5 wt.% Co content. The actual cobalt content ranged from 4.8 to 6.4 wt.%, as determined by XRF the slightly higher cobalt content was due to the non-stoichiometric water content of the cobalt nitrate hydrate. The last two catalysts tested (AA-IE-Co and AA-IE-CoMn) were prepared by a direct ion-exchange method using a solution of cobalt nitrate, and mixture of cobalt nitrate with manganese nitrate, respectively. The cobalt content of AA-IE-Co catalysts was 6.3 wt.% and the cobalt content of the AA-IE-CoMn catalyst was 5.4 wt.% and manganese 0.8 wt.%, respectively. The cobalt content was average for the whole pellet, so the cobalt content in the active layer of AA-IE-Co catalyst is higher than 6.3 wt.% (theoretically up to about 13 wt.% or about up 11 wt.% for AA-IE-CoMn catalyst). The molar ratio of Co/Mn is 6.5:1, the difference compared to the target ratio (Co/Mn 4:1) is due to the different course of precipitation of Co and Mn ions during the formation of the layer in the pellet. Catalysts prepared using this method had a significantly higher content of potassium and slightly higher content of sodium in comparison with a catalyst impregnated on AA-N support. This showed that cobalt precipitation precluded complete ion-exchange of Na + and K + ions by Co 2+ ion. Higher Na and K contents were observed only for the AA-S-Co catalyst, but the alkalis were present here in the form of Na/K silicates. As Table 1 shows, the presence of these components had a very negative effect on the specific surface area (SSA) because the silicates closed the pores. Additionally, due to pore opening and cleaning, there was also an increase in specific surface area (SSA) two times for (AA-N) and more than seven times for AA-D. SSA of catalysts did not change significantly compare to original supports. The decrease of SSA was observed only for alumina and AA-D supported catalyst, which could have been caused by partially blocking the pores with cobalt, more precisely, its oxide. This decrease in AA-D was related to blockade of clinoptilolite micropores which have been released by acid leaching [ 4 , 8 ]. The catalysts Catalysts 2020,10, 1398 5 of 21 AA-IE-Co and AA-IE-CoMn prepared in this way exhibit relatively high specific surface (SSA) areas compared to catalysts employing differently modified zeolite foam supports (Table 1). This was probably partially caused by the formation of porous cobalt and cobalt-manganese oxides originating from precipitated hydroxides on the zeolite foam surface, or by partial cleaning of the zeolite foam surface (an increase of crystallinity or intensity respectively was observed, Figure 2) and accessibility of pores. The increase in the surface area has not yet been satisfactorily explained but has been verified by repeated measurements. Rutkowska [ 53 ] states that the increase in surface area of the samples (Cu, Fe and Co beta zeolites) could be related to the secondary recrystallization of zeolites during the preparation of catalysts. In this case, it could also be caused by changes in the binder N/K-A-S-H phase or by the generation of additional porosity between the cobalt and manganese hydroxides deposited on the sample surface. Catalysts 2020, 10, x FOR PEER REVIEW 5 of 22 the formation of porous cobalt and cobalt-manganese oxides originating from precipitated hydroxides on the zeolite foam surface, or by partial cleaning of the zeolite foam surface (an increase of crystallinity or intensity respectively was observed, Figure 2) and accessibility of pores. The increase in the surface area has not yet been satisfactorily explained but has been verified by repeated measurements. Rutkowska [53] states that the increase in surface area of the samples (Cu, Fe and Co beta zeolites) could be related to the secondary recrystallization of zeolites during the preparation of catalysts. In this case, it could also be caused by changes in the binder N/K-A-S-H phase or by the generation of additional porosity between the cobalt and manganese hydroxides deposited on the sample surface. Since the samples are predominantly macroporous materials, textures of catalytic supports and prepared catalysts were determined via Hg porosimetry. The obtained results (Figure 2) showed that other supports contain predominantly bigger pores. Zeolite foams have broad pore spectrum from mesopores to macropores to supermacropores with a diameter of tenths to hundreds of micrometres with corresponding pore mean size thousands nm. Total intrusion volume of zeolite foams varies from 0.65 (AA-S) to 0.75 mL/g (AA-D). Intrusion volume of their “mesopores” is from 0.06 mL/g (AA-S), 0.07 mL/g (AA-N) to 0.09 mL/g (AA-D). Increase in total intrusion volume and share of AA-D support mesopores was caused by cleansing the surface from the excess alkali activator and mineral sediments inside the pores of natural zeolite. In the case of cobalt catalysts as well of used catalysts (after catalytic reaction of N2O decomposition), there were no big changes (decreases) observed in intrusion volume. Changes were only due to blocking the pores with cobalt oxide. In contrast, an increase of the mesopore volume was observed in AA-IE-Co and AA-IE-CoMn catalysts (both 0.09 mL/g), which can be attributed to cobalt oxides formed during the catalyst synthesis. Alumina as comparative support is purely mesoporous material with the overall intrusion volume of 0.5 mL/g and mean pore diameter 5.6 nm, Co-Al2O3 catalyst shows a partial reduction in intrusion volume due to cobalt oxides deposited on the surface (Appendix A Figure A1). Figure 2. Pore distribution of supports, fresh and used cobalt catalysts. SEM-EDS mapping of catalyst pellet cross-section (Figure 3) showed identical macroporous structure of the catalysts (every first figure in the row) and homogeneous distribution of cobalt on the Figure 2. Pore distribution of supports, fresh and used cobalt catalysts. Since the samples are predominantly macroporous materials, textures of catalytic supports and prepared catalysts were determined via Hg porosimetry. The obtained results (Figure 2) showed that other supports contain predominantly bigger pores. Zeolite foams have broad pore spectrum from mesopores to macropores to supermacropores with a diameter of tenths to hundreds of micrometres with corresponding pore mean size thousands nm. Total intrusion volume of zeolite foams varies from 0.65 (AA-S) to 0.75 mL/g (AA-D). Intrusion volume of their “mesopores” is from 0.06 mL/g (AA-S), 0.07 mL/g (AA-N) to 0.09 mL/g (AA-D). Increase in total intrusion volume and share of AA-D support mesopores was caused by cleansing the surface from the excess alkali activator and mineral sediments inside the pores of natural zeolite. In the case of cobalt catalysts as well of used catalysts (after catalytic reaction of N 2 O decomposition), there were no big changes (decreases) observed in intrusion volume. Changes were only due to blocking the pores with cobalt oxide. In contrast, an increase of the mesopore volume was observed in AA-IE-Co and AA-IE-CoMn catalysts (both 0.09 mL/g), which can be attributed to cobalt oxides formed during the catalyst synthesis. Alumina as comparative support is purely mesoporous material with the overall intrusion volume of 0.5 mL/g and mean pore diameter Catalysts 2020,10, 1398 6 of 21 5.6 nm, Co-Al 2 O 3 catalyst shows a partial reduction in intrusion volume due to cobalt oxides deposited on the surface (Appendix AFigure A1). SEM-EDS mapping of catalyst pellet cross-section (Figure 3) showed identical macroporous structure of the catalysts (every first figure in the row) and homogeneous distribution of cobalt on the surface of the modified zeolite foam catalyst support in the case of AA-N-Co and AA-D-Co catalysts (Figure 3b,c). In case of impregnation of the AA-S support (Figure 3a), an area non-covered by cobalt was observable in the middle of the pellet, which could be caused by precipitation of cobalt hydroxide during impregnation process by alkali agents in the zeolite foam. This as-created layer was blocking further impregnation inside the pellet. Certainly, this will also be affected by the inhomogeneity of the pellets caused by the synthesis method used. A similar effect was observed during preparations via the direct ion-exchange method. As can be seen in Figure 3d,e cobalt was observed only on the outer surface of the pellet in the form of a ring. The depth of cobalt penetration reached approximately 0.7 mm. The difference in depth compared to AA-S-Co was due to different catalyst preparation conditions (solution concentration, temperature, etc.). Manganese distribution showed a decline in content from the surface to the core, but in comparison with cobalt, there was not a strong borderline. There was a significant decline in content of sodium and potassium (observable from grey and yellow colour intensity on Figure 3) in the following order: AA-S-Co, AA-IE-Co/AA-IE-CoMn, AA-N-Co and AA-D-Co and distribution in the whole cross-section is in agreement with results of chemical analysis (XRF). Catalysts 2020, 10, x FOR PEER REVIEW 6 of 22 surface of the modified zeolite foam catalyst support in the case of AA-N-Co and AA-D-Co catalysts (Figure 3b,c). In case of impregnation of the AA-S support (Figure 3a), an area non-covered by cobalt was observable in the middle of the pellet, which could be caused by precipitation of cobalt hydroxide during impregnation process by alkali agents in the zeolite foam. This as-created layer was blocking further impregnation inside the pellet. Certainly, this will also be affected by the inhomogeneity of the pellets caused by the synthesis method used. A similar effect was observed during preparations via the direct ion-exchange method. As can be seen in Figure 3d,e cobalt was observed only on the outer surface of the pellet in the form of a ring. The depth of cobalt penetration reached approximately 0.7 mm. The difference in depth compared to AA-S-Co was due to different catalyst preparation conditions (solution concentration, temperature, etc.). Manganese distribution showed a decline in content from the surface to the core, but in comparison with cobalt, there was not a strong borderline. There was a significant decline in content of sodium and potassium (observable from grey and yellow colour intensity on Figure 3) in the following order: AA-S-Co, AA-IE-Co/AA-IE-CoMn, AA-N-Co and AA-D-Co and distribution in the whole cross-section is in agreement with results of chemical analysis (XRF). Figure 3. SEM-EDS elemental mapping of sodium (gray), potassium (yellow), cobalt (green) and manganese (brown) on cross section area of AA-S-Co (a), AA-N-Co (b), AA-D-Co (c), AA-IE-Co (d) and AA-IE-CoMn (e) catalysts. The crystalline structure of materials was determined using X-ray powder diffraction (XRD). In case of zeolite foam supported catalysts, the analysis showed clinoptilolite to be a main crystalline phase Figure 3. SEM-EDS elemental mapping of sodium (gray), potassium (yellow), cobalt (green) and manganese (brown) on cross section area of AA-S-Co ( a ), AA-N-Co ( b ), AA-D-Co ( c ), AA-IE-Co ( d ) and AA-IE-CoMn (e) catalysts. Catalysts 2020,10, 1398 7 of 21 The crystalline structure of materials was determined using X-ray powder diffraction (XRD). In case of zeolite foam supported catalysts, the analysis showed clinoptilolite to be a main crystalline phase with typical reflections at 2theta 9.9 ◦ , 11.2 ◦ , 17.5 ◦ and 22.4 ◦ (PDF 83-1261) along with other minor phases in the natural zeolite, belonging to the mineralogic group of clays and feldspars (Figure 4a). Thediffractogramsshowedthedifference in crystallinity or intensities of main reflectionsofclinoptilolite respectively depending on the modification method used. The crystallinity of alkali-activated material was lower in comparison with the original natural zeolite [ 8 ]. This may be caused by a partial structure disruption during the alkaline activation. There was a significant increase in crystallinity after IE using NH 4 NO 3 caused by surface cleansing. In contrast, after leaching with 3M HCl, there was a decrease in crystallinity connected with disruption of the zeolite structure due to removing Al from the clinoptilolite crystalline matrix. During the modification process, there was a change in intensities at 2theta 11.2 ◦ /9.9 ◦ that corresponded to planes (020) and (200), caused by the difference in electron density in that plane. From the original ratio 0.5 in the natural zeolite, the value increases after alkali-activation to 0.8 and after leaching with HCl or IE treatment, it decreases to 0.4 [ 8 ]. Comparative catalysts showed typical broad reflections for γ-Al2O3(2theta 46.5◦and 67.3◦, PDF 74-2206) (Appendix AFigure A2). Catalysts 2020, 10, x FOR PEER REVIEW 7 of 22 with typical reflections at 2theta 9.9°, 11.2°, 17.5° and 22.4° (PDF 83-1261) along with other minor phases in the natural zeolite, belonging to the mineralogic group of clays and feldspars (Figure 4a). The diffractograms showed the difference in crystallinity or intensities of main reflections of clinoptilolite respectively depending on the modification method used. The crystallinity of alkali-activated material was lower in comparison with the original natural zeolite [8]. This may be caused by a partial structure disruption during the alkaline activation. There was a significant increase in crystallinity after IE using NH4NO3 caused by surface cleansing. In contrast, after leaching with 3M HCl, there was a decrease in crystallinity connected with disruption of the zeolite structure due to removing Al from the clinoptilolite crystalline matrix. During the modification process, there was a change in intensities at 2theta 11.2°/9.9° that corresponded to planes (020) and (200), caused by the difference in electron density in that plane. From the original ratio 0.5 in the natural zeolite, the value increases after alkali-activation to 0.8 and after leaching with HCl or IE treatment, it decreases to 0.4 [8]. Comparative catalysts showed typical broad reflections for γ-Al2O3 (2theta 46.5° and 67.3°, PDF 74-2206) (Appendix A Figure A2). Figure 4. Powder diffraction patterns of supports (a) and fresh catalysts (b). Cobalt catalysts prepared using these supports showed significant crystalline structures corresponding to Co3O4 (2theta 31.5°, 36.9°, 45.0°, 55.7°, 59.6° and 65.6°, PDF 65-3103). These were the most apparent in case of using alumina as support (Appendix A Figure A2). In case of other supports (Figure 4b), there was only one reflection visible (36.9° 2theta), while AA-D supported catalysts showed the reflections corresponding to clinoptilolite (2theta 9.9° and 11.2°) and other minor phases (clays and feldspars) with no significant intensities corresponding to cobalt oxide. This indicated homogeneous distribution of cobalt on the surface without the formation of a crystalline Co3O4 phase. Crystallite size of Co3O4 calculated using the Scherrer equation from the reflection 36.9° 2theta is 12 nm for catalysts with alumina support. It was not possible to evaluate the crystallite size in other catalysts due to overlap of cobalt oxide diffraction with line assigned to the clinoptilolite, quartz or anorthite. Analysis of the used catalysts (Appendix A Figure A3), samples labelled with additional –U, catalysts after their usage for N2O decomposition test) did not show any significant changes in the crystalline structure compared with fresh catalysts. Powder diffraction patterns of Al2O3 can be seen in Appendix A Figure A2. After using the catalysts in the reaction test only slight decreases in intensity are noticeable. Only in the case of the AA-D-Co-U catalyst, there was a significant decrease in intensities 2theta 27.8°, 28.0° and others corresponding to anorthite (PDF 71-0748). Temperature-programmed ammonia desorption (NH3-TPD) showed significant differences in the amount and strength of acid sites (Figure 5a) of each catalyst (Table 2). While the catalyst supported on industrial support (Al2O3-Co) showed low acidity (Appendix A Table A2, Figure A4a), the zeolite foam supported catalysts showed changes in acidity depending on the support modification method during catalyst preparation. In general, peaks at low temperatures, below 200 °C are attributed to weakly acidic sites or physisorbed ammonia, and peaks at higher temperatures in the range of 200–400 °C are associated with the interaction of NH3 with acidic Brønsted sites attributed to strongly acidic sites. Peaks Figure 4. Powder diffraction patterns of supports (a) and fresh catalysts (b). Cobalt catalysts prepared using these supports showed significant crystalline structures corresponding to Co 3 O 4 (2theta 31.5 ◦ , 36.9 ◦ , 45.0 ◦ , 55.7 ◦ , 59.6 ◦ and 65.6 ◦ , PDF 65-3103). These were the most apparent in case of using alumina as support (Appendix AFigure A2). In case of other supports (Figure 4b), there was only one reflection visible (36.9 ◦ 2theta), while AA-D supported catalysts showed the reflections corresponding to clinoptilolite (2theta 9.9 ◦ and 11.2 ◦ ) and other minor phases (clays and feldspars) with no significant intensities corresponding to cobalt oxide. This indicated homogeneous distribution of cobalt on the surface without the formation of a crystalline Co 3 O 4 phase. Crystallite size of Co 3 O 4 calculated using the Scherrer equation from the reflection 36.9 ◦ 2theta is 12 nm for catalysts with alumina support. It was not possible to evaluate the crystallite size in other catalysts due to overlap of cobalt oxide diffraction with line assigned to the clinoptilolite, quartz or anorthite. Analysis of the used catalysts (Appendix AFigure A3), samples labelled with additional –U, catalysts after their usage for N 2 O decomposition test) did not show any significant changes in the crystalline structure compared with fresh catalysts. Powder diffraction patterns of Al 2 O 3 can be seen in Appendix AFigure A2. After using the catalysts in the reaction test only slight decreases in intensity are noticeable. Only in the case of the AA-D-Co-U catalyst, there was a significant decrease in intensities 2theta 27.8◦, 28.0◦and others corresponding to anorthite (PDF 71-0748). Temperature-programmed ammonia desorption (NH 3 -TPD) showed significant differences in the amount and strength of acid sites (Figure 5a) of each catalyst (Table 2). While the catalyst supported on industrial support (Al 2 O 3 -Co) showed low acidity (Appendix ATable A2, Figure A4a), the zeolite foam supported catalysts showed changes in acidity depending on the support modification method during catalyst preparation. In general, peaks at low temperatures, below 200 ◦ C are attributed to weakly Catalysts 2020,10, 1398 8 of 21 acidic sites or physisorbed ammonia, and peaks at higher temperatures in the range of 200–400 ◦ C are associated with the interaction of NH 3 with acidic Brønsted sites attributed to strongly acidic sites. Peaks at temperatures above 400 ◦ C correspond to the interaction of NH 3 with Lewis acids and are attributed to Co-O species that form very strongly acidic sites [54–56] Catalysts 2020, 10, x FOR PEER REVIEW 8 of 22 at temperatures above 400 °C correspond to the interaction of NH3 with Lewis acids and are attributed to Co-O species that form very strongly acidic sites [54–56] The highest total acidity exhibited catalyst prepared via impregnation of ion-exchange modified support (AA-N-Co), which contained a high population of strong acid sites, i.e., sites with high NH3 desorption temperature. The acidity of AA-S-Co and AA-D-Co catalysts is about one-third in comparison and very similar to each other, which is probably due to poor availability of acid sites in the base zeolite–clinoptilolite for AA-S-Co and in case of AA-D-Co due to acid centre removal during acid leaching. Samples prepared using direct ion-exchange treatment showed slightly lower acidity than AA-N-Co, but no strong acid sites. Table 2. Acidity of cobalt catalysts determined by ammonia TPD. Sample cSUM (μmol/g) Tmax1 (°C) cmax1 (μmol/g) Pmax1 * (%) Tmax2 (°C) cmax2 (μmol/g) Pmax2 * (%) Tmax3 (°C) cmax3 (μmol/g) Pmax3 * (%) AA-S-Co 501 175 501 100 - - - - - - AA-N-Co 1542 173 1313 85 390 229 15 - - - AA-D-Co 593 158 446 75 255 70 12 382 77 13 AA-IE-Co 1446 178 1379 95 247 27 2 362 40 3 AA-IE-MnCo 1211 187 1211 100 - - - - - - * P = population of the relevant sites, index 1–first deconvolution peak, 2–second deconvolution peak and 3–third deconvolution peak. Figure 5. Acidity (a) and basicity (b) of cobalt catalysts determined by NH3-TPD and CO2-TPD, respectively. The basicity of the catalysts was determined by temperature programmed desorption of CO2 (CO2TPD). The data obtained (Table 3, Figure 5b) showed significant low temperature peaks around 110 °C for ion-exchange treated (AA-N-Co) catalyst, direct ion-exchange treated (AA-IE-Co and AA-IE-CoMn) catalysts and a similar maximum is also observed for the Al2O3-Co catalyst (Appendix A Table A3, Figure A4b). The basicity is related to the alkali content, i.e., alkali metals (Na + K) in the sample (Table 3). The AA-S-Co catalyst had the highest alkali content, but a large part of the alkali metals was inaccessible to the interaction (they were enclosed in the binder N(A)-A-S-H phase), while in the case of AA-IE-Co and AA-IE-CoMn catalysts which had also higher SSA, alkali metals were more available and the samples showed high overall basicity, especially also the content of weakly basic sites. On the other hand, the basicity of the samples may also be related to the adsorption of CO2 on the polar adsorption centres, i.e., on all coordination unsaturated cations present in the sample. Figure 5. Acidity ( a ) and basicity ( b ) of cobalt catalysts determined by NH 3 -TPD and CO2-TPD, respectively. Table 2. Acidity of cobalt catalysts determined by ammonia TPD. Sample cSUM (µmol/g) Tmax1 (◦C) cmax1 (µmol/g) Pmax1 * (%) Tmax2 (◦C) cmax2 (µmol/g) Pmax2 * (%) Tmax3 (◦C) cmax3 (µmol/g) Pmax3 * (%) AA-S-Co 501 175 501 100 - - - - - - AA-N-Co 1542 173 1313 85 390 229 15 - - - AA-D-Co 593 158 446 75 255 70 12 382 77 13 AA-IE-Co 1446 178 1379 95 247 27 2 362 40 3 AA-IE-MnCo 1211 187 1211 100 - - - - - - * P =population of the relevant sites, index 1–first deconvolution peak, 2–second deconvolution peak and 3–third deconvolution peak. The highest total acidity exhibited catalyst prepared via impregnation of ion-exchange modified support (AA-N-Co), which contained a high population of strong acid sites, i.e., sites with high NH 3 desorption temperature. The acidity of AA-S-Co and AA-D-Co catalysts is about one-third in comparison and very similar to each other, which is probably due to poor availability of acid sites in the base zeolite–clinoptilolite for AA-S-Co and in case of AA-D-Co due to acid centre removal during acid leaching. Samples prepared using direct ion-exchange treatment showed slightly lower acidity than AA-N-Co, but no strong acid sites. The basicity of the catalysts was determined by temperature programmed desorption of CO 2 (CO 2 -TPD). The data obtained (Table 3, Figure 5b) showed significant low temperature peaks around 110 ◦ C for ion-exchange treated (AA-N-Co) catalyst, direct ion-exchange treated (AA-IE-Co and AA-IE-CoMn) catalysts and a similar maximum is also observed for the Al 2 O 3 -Co catalyst (Appendix A Table A3, Figure A4b). The basicity is related to the alkali content, i.e., alkali metals (Na +K) in the sample (Table 3). The AA-S-Co catalyst had the highest alkali content, but a large part of the alkali metals was inaccessible to the interaction (they were enclosed in the binder N(A)-A-S-H phase), while in the case of AA-IE-Co and AA-IE-CoMn catalysts which had also higher SSA, alkali metals were more available and the samples showed high overall basicity, especially also the content of weakly basic sites. On the other hand, the basicity of the samples may also be related to the adsorption of CO 2 on the polar adsorption centres, i.e., on all coordination unsaturated cations present in the sample. Catalysts 2020,10, 1398 9 of 21 Table 3. Basicity of cobalt catalysts determined by CO2-TPD. Sample cSUM (µmol/g) Tmax1 (◦C) cmax1 (µmol/g) Pmax1 * (%) Tmax2 (◦C) cmax2 (µmol/g) Pmax2 * (%) Tmax3 (◦C) cmax3 (µmol/g) Pmax3 * (%) AA-S-Co 213 105 42 20 186 161 76 321 10 14 AA-N-Co 125 110 82 66 363 14 11 437 30 23 AA-D-Co 156 112 36 23 204 44 28 289 76 49 AA-IE-Co 306 111 215 70 338 91 30 - - - AA-IE-MnCo 408 113 227 56 273 181 44 - - - * P =population of the relevant sites, index 1–first deconvolution peak, 2–second deconvolution peak and 3–third deconvolution peak. The results of the H 2 -TPR analysis performed on the catalyst pellets show significant differences in H 2 consumption (Table 4), which corresponds to the content of various forms of Co oxides present on the surface of the catalyst support. Co 3 O 4 reduction takes place in two steps, most often parameters for reduction of Co 3+ to Co 2+ are the temperature range of 250–300 ◦ C and higher than 400 ◦ C for reduction of Co 2+ to Co 0 [ 9 , 10 ]. From the obtained TPR profiles (Figure 6) it can be seen that the reduction took place in the two steps, but significant differences are observed for the individual catalysts. The results of the H2-TPR analysis performed on the Al2O3 can be seen in Appendix ATable A4 and Figure A5. Table 4. Reducibility of cobalt catalysts determined by TPR. Sample cSUM (µmol/g) Tmax1 (◦C) cmax1 (µmol/g) Pmax1 * (%) Tmax2 (◦C) cmax2 (µmol/g) Pmax2 * (%) Tmax3 (◦C) cmax3 (µmol/g) Pmax3 * (%) AA-S-Co 360 289 77 21 330 218 61 478 65 19 AA-N-Co 875 304 704 80 483 171 20 - - - AA-D-Co 260 284 76 29 306 184 71 - - AA-IE-Co 176 222 60 34 418 116 66 - - - AA-IE-MnCo 500 239 337 67 303 163 33 - - - * P =population of the relevant sites, index 1–first deconvolution peak, 2–second deconvolution peak and 3–third deconvolution peak. Catalysts 2020, 10, x FOR PEER REVIEW 9 of 22 Table 3. Basicity of cobalt catalysts determined by CO2-TPD. Sample cSUM (μmol/g) Tmax1 (°C) cmax1 (μmol/g) Pmax1 * (%) Tmax2 (°C) cmax2 (μmol/g) Pmax2 * (%) Tmax3 (°C) cmax3 (μmol/g) Pmax3 * (%) AA-S-Co 213 105 42 20 186 161 76 321 10 14 AA-N-Co 125 110 82 66 363 14 11 437 30 23 AA-D-Co 156 112 36 23 204 44 28 289 76 49 AA-IE-Co 306 111 215 70 338 91 30 - - - AA-IE-MnCo 408 113 227 56 273 181 44 - - - * P = population of the relevant sites, index 1–first deconvolution peak, 2–second deconvolution peak and 3–third deconvolution peak. The results of the H2-TPR analysis performed on the catalyst pellets show significant differences in H2 consumption (Table 4), which corresponds to the content of various forms of Co oxides present on the surface of the catalyst support. Co3O4 reduction takes place in two steps, most often parameters for reduction of Co3+ to Co2+ are the temperature range of 250–300 °C and higher than 400 °C for reduction of Co2+ to Co0 [9,10]. From the obtained TPR profiles (Figure 6) it can be seen that the reduction took place in the two steps, but significant differences are observed for the individual catalysts. The results of the H2-TPR analysis performed on the Al2O3 can be seen in Appendix A Table A4 and Figure A5. Table 4. Reducibility of cobalt catalysts determined by TPR. Sample cSUM (μmol/g) Tmax1 (°C) cmax1 (μmol/g) Pmax1 * (%) Tmax2 (°C) cmax2 (μmol/g) Pmax2 * (%) Tmax3 (°C) cmax3 (μmol/g) Pmax3 * (%) AA-S-Co 360 289 77 21 330 218 61 478 65 19 AA-N-Co 875 304 704 80 483 171 20 - - - AA-D-Co 260 284 76 29 306 184 71 - - AA-IE-Co 176 222 60 34 418 116 66 - - - AA-IE-MnCo 500 239 337 67 303 163 33 - - - * P = population of the relevant sites, index 1–first deconvolution peak, 2–second deconvolution peak and 3–third deconvolution peak. Figure 6. H2-TPR profiles of cobalt catalysts. The shift of the reduction peaks may occur due to (i) the presence of alkali [35] which generally lowers temperatures of reduction of cobalt species, or (ii) due to the interaction between the Co3O4 spin Figure 6. H2-TPR profiles of cobalt catalysts. The shift of the reduction peaks may occur due to (i) the presence of alkali [ 35 ] which generally lowers temperatures of reduction of cobalt species, or (ii) due to the interaction between the Co 3 O 4 spin phase and the support used [ 51 ]. In case of the alumina supported catalyst, only one wide reduction peak was evident with two maxima at 435 ◦ C and 479 ◦ C, indicating cobalt species interaction with Al 2 O 3 to form Co aluminates with stronger oxygen bonds that decrease reducibility (Appendix A Table A4, Figure A5) [ 11 ]. The catalyst using the AA-S basic support exhibited a wide reduction peak Catalysts 2020,10, 1398 16 of 21 Table A2. Acidity of cobalt catalyst determined by ammonia TPD. Sample cSUM (µmol/g) Tmax1 (◦C) cmax1 (µmol/g) Pmax1 * (%) Tmax2 (◦C) cmax2 (µmol/g) Pmax2 * (%) Tmax3 (◦C) cmax3 (µmol/g) Pmax3 * (%) Al2O3-Co 296 173 178 60 283 118 40 - - - * P =population of the relevant sites, index 1–first deconvolution peak, 2–second deconvolution peak and 3–third deconvolution peak. Table A3. Basicity of cobalt catalyst determined by CO2TPD. Sample cSUM (µmol/g) Tmax1 (◦C) cmax1 (µmol/g) Pmax1 * (%) Tmax2 (◦C) cmax2 (µmol/g) Pmax2 * (%) Tmax3 (◦C) cmax3 (µmol/g) Pmax3 * (%) Al2O3-Co 145 109 31 21 144 89 61 263 25 18 * P =population of the relevant sites, index 1–first deconvolution peak, 2–second deconvolution peak and 3–third deconvolution peak. Table A4. Reducibility of cobalt catalyst determined by TPR. Sample cSUM (µmol/g) Tmax1 (◦C) cmax1 (µmol/g) Pmax1 * (%) Tmax2 (◦C) cmax2 (µmol/g) Pmax2 * (%) Tmax3 (◦C) cmax3 (µmol/g) Pmax3 * (%) Al2O3-Co 109 - - - 435 85 80 479 24 20 * P =population of the relevant sites, index 1–first deconvolution peak, 2–second deconvolution peak and 3–third deconvolution peak. Catalysts 2020, 10, x FOR PEER REVIEW 16 of 22 Table A2. Acidity of cobalt catalyst determined by ammonia TPD. Sample cSUM (μmol/g) Tmax1 (°C) cmax1 (μmol/g) Pmax1 * (%) Tmax2 (°C) cmax2 (μmol/g) Pmax2 * (%) Tmax3 (°C) cmax3 (μmol/g) Pmax3 * (%) Al2O3-Co 296 173 178 60 283 118 40 - - - * P = population of the relevant sites, index 1–first deconvolution peak, 2–second deconvolution peak and 3–third deconvolution peak. Table A3. Basicity of cobalt catalyst determined by CO2 TPD. Sample cSUM (μmol/g) Tmax1 (°C) cmax1 (μmol/g) Pmax1 * (%) Tmax2 (°C) cmax2 (μmol/g) Pmax2 * (%) Tmax3 (°C) cmax3 (μmol/g) Pmax3 * (%) Al2O3-Co 145 109 31 21 144 89 61 263 25 18 * P = population of the relevant sites, index 1–first deconvolution peak, 2–second deconvolution peak and 3–third deconvolution peak. Table A4. Reducibility of cobalt catalyst determined by TPR. Sample cSUM (μmol/g) Tmax1 (°C) cmax1 (μmol/g) Pmax1 * (%) Tmax2 (°C) cmax2 (μmol/g) Pmax2 * (%) Tmax3 (°C) cmax3 (μmol/g) Pmax3 * (%) Al2O3-Co 109 - - - 435 85 80 479 24 20 * P = population of the relevant sites, index 1–first deconvolution peak, 2–second deconvolution peak and 3–third deconvolution peak. Figure A1. Pore distribution of Al2O3 support, fresh and used cobalt catalyst. Figure A1. Pore distribution of Al2O3support, fresh and used cobalt catalyst. Catalysts 2020,10, 1398 17 of 21 Catalysts 2020, 10, x FOR PEER REVIEW 17 of 22 Figure A2. Powder diffraction patterns of Al2O3 support, fresh and used cobalt catalyst. Figure A3. Powder diffraction patterns of used cobalt catalysts. Figure A2. Powder diffraction patterns of Al2O3support, fresh and used cobalt catalyst. Catalysts 2020, 10, x FOR PEER REVIEW 17 of 22 Figure A2. Powder diffraction patterns of Al2O3 support, fresh and used cobalt catalyst. Figure A3. Powder diffraction patterns of used cobalt catalysts. Figure A3. Powder diffraction patterns of used cobalt catalysts. Catalysts 2020, 10, x FOR PEER REVIEW 18 of 22 Figure A4. Acidity (a) and basicity (b) of Al2O3 cobalt catalyst determined by NH3-TPD and CO2-TPD, respectively. Figure A5. H2-TPR profiles of Al2O3 cobalt catalyst. Figure A4. Acidity ( a ) and basicity ( b ) of Al 2 O 3 cobalt catalyst determined by NH 3 -TPD and CO2-TPD, respectively. Catalysts 2020,10, 1398 18 of 21 Catalysts 2020, 10, x FOR PEER REVIEW 18 of 22 Figure A4. Acidity (a) and basicity (b) of Al2O3 cobalt catalyst determined by NH3-TPD and CO2-TPD, respectively. Figure A5. H2-TPR profiles of Al2O3 cobalt catalyst. Figure A5. H2-TPR profiles of Al2O3cobalt catalyst. Catalysts 2020, 10, x FOR PEER REVIEW 19 of 22 Figure A6. N2O conversion of Al2O3 cobalt catalyst. References 1. Choya, A.; De Rivas, B.; Gutiérrez-Ortiz, J.I.; Velasco, J.R.G.; López-Fonseca, R. Synthesis, Characterization and Kinetic Behavior of Supported Cobalt Catalysts for Oxidative after-Treatment of Methane Lean Mixtures. Materials 2019, 12, 3174, doi:10.3390/ma12193174. 2. Lin, H.-K.; Wang, C.-B.; Chiu, H.-C.; Chien, S.-H. In situ FTIR Study of Cobalt Oxides for the Oxidation of Carbon Monoxide. Catal. Lett. 2003, 86, 63–68, doi:10.1023/a:1022659025068. 3. Zhang, J.; Li, Y.; Wang, L.; Zhang, C.; He, H. Catalytic oxidation of formaldehyde over manganese oxides with different crystal structures. Catal. Sci. Technol. 2015, 5, 2305–2313, doi:10.1039/c4cy01461h. 4. 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