SOURCE: Sheridan Institutional Repository SOURCE: Sheridan Institutional Repository Investigation of Co3O4 and LaCoO3 interaction by performing Investigation of Co3O4 and LaCoO3 interaction by performing N2O decomposition tests under Co3O4-CoO transition N2O decomposition tests under Co3O4-CoO transition temperature temperature

Interaction Abstract: The research presented in this paper addresses the question: How does the addition of a small amount of LaCoO 3 impact the activity of a Co 3 O 4 catalyst? By testing such a catalyst in N 2 O decomposition under conditions at which the thermal decomposition of Co 3 O 4 to CoO is possible, one gains unique insight into how the two phases interact. The activity of such a catalyst increases in the entire studied temperature range, unlike the activity of the undoped cobalt catalyst which is lower at 850 ◦ C than at 800 ◦ C due to the reduction of Co 3 O 4 to CoO. XRD measurements showed that CoO was also the main cobalt oxide present in the Co3.5La catalyst after operating at 850 ◦ C, as did the XPS measurements, but there was no drop of activity associated with this change. The inﬂuence of NO, O 2 and H 2 O on the activity of the new catalyst, Co3.5La, was determined. Lack of positive effect of NO, a known oxygen scavenger, on the activity was noticed at all temperatures, showing that the effect of LaCoO 3 is probably due to increased oxygen desorption. Temperature programed oxidation (TPO) tests showed that the beneﬁcial effects of the presence of LaCoO 3 on the activity of cobalt oxide at 850 ◦ C were probably caused by enhanced diffusion of O 2 − anions through the entire catalyst, which facilitates desorption of oxygen molecules from the surface. catalyst. Two recent interesting studies on high temperature N 2 O decomposition have shown the signiﬁcance of parameters such as extrudate size [25] and pressure [27] on N 2 O decomposition. In fact, in measurements carried out at 890 ◦ C with the commercial catalyst [25] it was found that an increase of diameter of the extrudates leads to a decrease of conversions of N 2 O from approx. 80% for 2 mm diameter to 60% for 5 mm extrudates. Experiments carried out on monoliths in the temperature region 700–900 ◦ C [27] show that the conversion of N 2 O increases substantially not only with the increase in temperature, but with increase of pressure in the regime which is currently found in nitric acid plants. For the lowest studied pressure, the following N 2 O conversions were obtained for 700 ◦ C, 800 ◦ C and 900 ◦ C: 10%, 30% and 65%, but at 900 ◦ C it can reach to over 90% for the highest studied pressure [27].


Introduction
Cobalt oxides, especially Co 3 O 4 , continue to be of interest to scientists from different disciplines due to the effective redox properties of this compound ( [1] and references therein).Co 3 O 4 is one of the catalysts still currently studied in deNO x reactions [2,3].As a catalyst, Co 3 O 4 is commonly promoted with other oxides, such as ceria [4][5][6][7][8][9] and lanthanum cobaltate [10][11][12][13].Studies on the cobalt catalyst doped with CeO 2 [4][5][6][7][8][9] show a beneficial effect of this compound on the activity in high-temperature decomposition of N 2 O, as well as combustion of propene, methane and toluene, even though ceria alone does not catalyze these reactions.Cerium oxide, especially when doped with gadolinium, is a solid electrolyte applied in solid oxide fuel cell (SOFC) applications [14][15][16] because of its high ionic conductivity.However, the limitations of ceria in SOFCs are due to the fact that at high temperatures or low oxygen partial pressures ceria exhibits mixed ionic and electronic conductivity [17].On the other hand, perovskite type oxides conduct only ions, and compounds such as lanthanum cobaltate, LaCoO 3 (LCO), are often used in SOFCs as the cathode material [18][19][20].
Previously performed studies on Co 3 O 4 -LaCoO 3 systems have shown that even a small amount of lanthanum cobaltate has a substantial impact on the properties and hence on the catalytic activity of the obtained system in propene combustion [11].Therefore, in the current study, the activity of a cobalt catalyst containing 3.5 mol % of lanthanum (3.5 mol % = mol La/(mol La + mol Co)) in the form of LaCoO 3 , which exhibits the perovskite structure, is investigated.The novelty of the present work is probing the interaction between Co 3 O 4 and LaCoO 3 further, this time in a different reaction (N 2 O decomposition), at a temperature at which the thermal reduction of Co 3 O 4 is possible, as well as using both characterization studies and the addition of probe molecules in the inlet stream to gain insight into this interaction.In order to mitigate the impact of the perovskite addition on the surface area [11] and to avoid any additional sintering during the high temperature activity tests, the catalysts were exposed to prolonged treatment at 850 • C, as in the case of our previously studied series of CexCo catalysts for nitrous oxide decomposition [5].Lanthanum cobaltate is stable at high temperatures and also known for high conductivity [21][22][23].Extensive characterization studies, including XPS, XRD, nitrogen physisorption, SEM imaging and Temperature Programmed Oxidation (TPO) measurements, have been performed in order to attempt to explain the impact of LaCoO 3 .
The decomposition of nitrous oxide, whose main industrial source is nitric acid plants, continues to be a topic investigated by several research groups [24][25][26].Catalytic decomposition seems to be the best available solution and preferably as a second stage directly downstream of the platinum-rhodium gauze in the high-temperature zone [25,27].It has been reported in literature that the activity of catalysts for the decomposition of nitrous oxide is reduced by the addition of water vapor to the reaction mixture [28][29][30].The influence of water vapor on the N 2 O conversion rates was tested on various types of catalysts [31].The water vapor content most commonly used in experiments is 5 or 10 vol %.The presence of water vapor in the reaction mixture is known to cause a decrease of the activity of the nitrous oxide decomposition catalysts regardless of the temperature of the reaction [32][33][34][35][36].The negative impact of water vapor on the activity in the decomposition of N 2 O was also found in the presence of the undoped cobalt catalyst in the temperatures range: 700-850 • C [37].In contrast, the opinion on the effect of nitric oxide is not univocal.A negative effect of nitric oxide on the activity in nitrous oxide decomposition has been noted on catalysts consisting of noble metals on supports, mixed oxides of nickel or copper obtained from hydrotalcite-type anionic clays [29], Fe 2 O 3 supported on alumina [38], Co-MgO [39] and catalysts containing CeO 2 and Co 3 O 4 with high Ce/Co ratios [40].However, in the case of various types of zeolite catalysts a positive effect of nitric oxide (II) in the inlet stream on nitrous oxide decomposition was observed [41][42][43].The beneficial effect of the presence of nitric oxide on the activity of the undoped cobalt catalyst was observed at 850 • C [37].It was stated that the effect probably results from the impact of NO on the N 2 O decomposition mechanism, i.e., a scavenger effect, not the state of the catalyst [37].

Results and Discussion
Figure 1 contains a schematic of the preparation of the catalyst which contains 3.5 mol % La as calculated using Equation 1. Due to the fact that lanthanum oxide is highly hygroscopic, it was dried prior to use.Afterwards, nitric acid was added to the oxide, so that the Co 3 O 4 would be impregnated with a solution of lanthanum nitrate.The nitrate was then decomposed to lanthanum oxide (Figure 1), which then reacted with some of the Co 3 O 4 to form lanthanum cobaltate.The X-ray diffraction pattern of the fresh catalyst confirmed that the reaction consumed all of the available lanthanum oxide (Figure 2).It revealed the presence of signals from two phases, namely, Co 3 O 4 and LaCoO 3 .The following reference diffraction patterns of the catalyst are shown below for clarity: Co 3 O 4 (71-0816) and LaCoO 3 .The XRD results show that both phases have crystallites with the average size above 100nm before and after the catalytic tests due to the prolonged high-temperature treatment during the synthesis (Figure 1).
0816) and LaCoO3 .The XRD results show that both phases have crystallites with the average size above 100nm before and after the catalytic tests due to the prolonged high-temperature treatment during the synthesis (Figure 1).
CLa = [nLa/(nLa+nCo)] × 100 (1)  The nitrogen physisorption measurements revealed that the surface of the lanthanum-doped cobalt catalyst and that of the undoped cobalt catalyst is very small, not exceeding 2m 2 •g −1 , i.e., 1.78 and 0.63 m 2 •g −1 , respectively.Moreover, both values slightly decrease to around 0.5 m 2 •g −1 during operation in the reactor.Figure 3a shows the pore size distribution of the fresh catalyst in comparison to that of undoped Co3O4.A similar distribution was found in both cases.The SEM images of Co3.5La before and after catalytic tests can be seen in Figure 3b,c, respectively.The morphology of both samples is comparable.0816) and LaCoO3 .The XRD results show that both phases have crystallites with the average size above 100nm before and after the catalytic tests due to the prolonged high-temperature treatment during the synthesis (Figure 1).CLa = [nLa/(nLa+nCo)] × 100 (1)  The nitrogen physisorption measurements revealed that the surface of the lanthanum-doped cobalt catalyst and that of the undoped cobalt catalyst is very small, not exceeding 2m 2 •g −1 , i.e., 1.78 and 0.63 m 2 •g −1 , respectively.Moreover, both values slightly decrease to around 0.5 m 2 •g −1 during operation in the reactor.Figure 3a shows the pore size distribution of the fresh catalyst in comparison to that of undoped Co3O4.A similar distribution was found in both cases.The SEM images of Co3.5La before and after catalytic tests can be seen in Figure 3b,c, respectively.The morphology of both samples is comparable.The nitrogen physisorption measurements revealed that the surface of the lanthanumdoped cobalt catalyst and that of the undoped cobalt catalyst is very small, not exceeding 2m 2 •g −1 , i.e., 1.78 and 0.63 m 2 •g −1 , respectively.Moreover, both values slightly decrease to around 0.5 m 2 •g −1 during operation in the reactor.Figure 3a shows the pore size distribu- tion of the fresh catalyst in comparison to that of undoped Co 3 O 4 .A similar distribution was found in both cases.The SEM images of Co3.5La before and after catalytic tests can be seen in Figure 3b,c, respectively.The morphology of both samples is comparable.From previously performed studies of the undoped cobalt catalyst and Co-Ce systems [5], it is known that two factors affect the performance of cobalt-based systems in the studied reaction at high temperatures.First of all, it is the oxidation state of cobalt in the oxide.At 850 °C CoO dominates, which leads to lower N2O conversions.At 850 °C after the addition of 5 vol % O2 to the inlet stream, Co3O4 is the dominant cobalt oxide, which leads to higher N2O conversions than in an inlet stream without oxygen in the case of the undoped cobalt oxide as shown in [5].The second parameter which influences N2O conversion is the Rate Determining Step (RDS) of the reaction, namely desorption of molecular oxygen, which can be improved by the addition of NO to the inlet mixture.
The Mars van Krevelen mechanism, which is typically used to describe oxidation, is composed of the following steps (CO oxidation is used as an example): (1) adsorption of an oxygen molecule on the surface of the catalyst (filling two anion vacancies) and adsorption of two CO molecules by the oxygen atom, (2) reaction of each CO molecule with an oxygen anion, (3) desorption of two molecules of CO2 with the regeneration of the oxygen ion vacancies on the surface.In the case of the decomposition of nitrous oxide, the following steps occur: (1) adsorption of a N2O molecule by the oxygen atom on the catalyst surface, (2) cleavage of the N-O bond and departure of N2, leaving an oxygen anion on the surface of the catalyst, (3) adsorption of another N2O molecule directly onto a surface oxygen anion to form N2 and O2 (Eley-Rideal mechanism), or onto another spot, cleavage of the N-O bond and subsequent migration of oxygen anions to form an O2 molecule (Langmuir-Hinshelwood mechanism) From previously performed studies of the undoped cobalt catalyst and Co-Ce systems [5], it is known that two factors affect the performance of cobalt-based systems in the studied reaction at high temperatures.First of all, it is the oxidation state of cobalt in the oxide.At 850 • C CoO dominates, which leads to lower N 2 O conversions.At 850 • C after the addition of 5 vol % O 2 to the inlet stream, Co 3 O 4 is the dominant cobalt oxide, which leads to higher N 2 O conversions than in an inlet stream without oxygen in the case of the undoped cobalt oxide as shown in [5].The second parameter which influences N 2 O conversion is the Rate Determining Step (RDS) of the reaction, namely desorption of molecular oxygen, which can be improved by the addition of NO to the inlet mixture.
The Mars van Krevelen mechanism, which is typically used to describe oxidation, is composed of the following steps (CO oxidation is used as an example): (1) adsorption of an oxygen molecule on the surface of the catalyst (filling two anion vacancies) and adsorption of two CO molecules by the oxygen atom, (2) reaction of each CO molecule with an oxygen anion, (3) desorption of two molecules of CO 2 with the regeneration of the oxygen ion vacancies on the surface.
In the case of the decomposition of nitrous oxide, the following steps occur: (1) adsorption of a N 2 O molecule by the oxygen atom on the catalyst surface, (2) cleavage of the N-O bond and departure of N 2 , leaving an oxygen anion on the surface of the catalyst, (3) adsorption of another N 2 O molecule directly onto a surface oxygen anion to form N 2 and O 2 (Eley-Rideal mechanism), or onto another spot, cleavage of the N-O bond and subsequent migration of oxygen anions to form an O 2 molecule (Langmuir-Hinshelwood mechanism) (4) desorption of oxygen from the surface of the catalyst (rate determining step [6]).
From the point of the catalytic systems, in both types of reactions, a redox cycle occurs.In the case of CO oxidation (see [44] and Figure 3) a cobalt ion in an octahedral site with unsaturated coordination is the active site.It is possible that the redox cycle in N 2 O decomposition also occurs on this site.The electron transfer from the catalyst to either CO or N 2 O molecule occurs, and a vacancy on the catalyst surface is filled.This is followed by the formation of a bond between an oxygen anion on the surface and either the adsorbed CO or the oxygen anion left after the N-O bond cleavage.In both cases, an additional oxygen anion is needed to form CO 2 or O 2 , respectively.Finally, electron density is transferred back to the catalyst surface when carbon dioxide or, in the case of N 2 O decomposition, molecular oxygen, desorbs from the surface and the oxygen vacancy is restored.
At 850 • C the addition of NO does not change the oxidation state of cobalt, but it is known for its "oxygen scavenger" properties which facilitate the regeneration of the anion vacancies on the catalyst surface by removing the oxygen anions from the surface.The facilitation of the RDS of N 2 O decomposition is the reason for its favorable influence on the activity of the undoped cobalt catalyst [37].Similarly, the evolution of oxygen, and hence regeneration of the original adsorption site, can be improved by the addition of potassium to Co 3 O 4 , as reported by Asano et al. [45].
Figure 4a shows the N 2 O conversion on the studied catalyst (blue bars) in the inlet stream with the composition 5 vol % N 2 O in Ar in comparison with the results obtained under the same conditions for the undoped cobalt catalyst (grey bars).It can be seen that at temperatures of 700 and 750 • C, the undoped cobalt catalyst exhibits an activity that is similar to that of the catalyst containing LaCoO 3 .At 800 • C the activity of both catalysts is the same.In contrast, at the highest temperature, i.e., 850 • C, the Co3.5La is significantly more active than the undoped catalyst.Two recent interesting studies on high temperature N 2 O decomposition have shown the significance of parameters such as extrudate size [25] and pressure [27] on N 2 O decomposition.In fact, in measurements carried out at 890 • C with the commercial catalyst [25] it was found that an increase of diameter of the extrudates leads to a decrease of conversions of N 2 O from approx.80% for 2 mm diameter to 60% for 5 mm extrudates.Experiments carried out on monoliths in the temperature region 700-900 • C [27] show that the conversion of N 2 O increases substantially not only with the increase in temperature, but with increase of pressure in the regime which is currently found in nitric acid plants.For the lowest studied pressure, the following N 2 O conversions were obtained for 700 • C, 800 • C and 900 • C: 10%, 30% and 65%, but at 900 • C it can reach to over 90% for the highest studied pressure [27].
Catalysts 2021, 11, x FOR PEER REVIEW 5 of 14 (4) desorption of oxygen from the surface of the catalyst (rate determining step [6]).
From the point of the catalytic systems, in both types of reactions, a redox cycle occurs.In the case of CO oxidation (see [44] and Figure 3) a cobalt ion in an octahedral site with unsaturated coordination is the active site.It is possible that the redox cycle in N2O decomposition also occurs on this site.The electron transfer from the catalyst to either CO or N2O molecule occurs, and a vacancy on the catalyst surface is filled.This is followed by the formation of a bond between an oxygen anion on the surface and either the adsorbed CO or the oxygen anion left after the N-O bond cleavage.In both cases, an additional oxygen anion is needed to form CO2 or O2, respectively.Finally, electron density is transferred back to the catalyst surface when carbon dioxide or, in the case of N2O decomposition, molecular oxygen, desorbs from the surface and the oxygen vacancy is restored.
At 850 °C the addition of NO does not change the oxidation state of cobalt, but it is known for its "oxygen scavenger" properties which facilitate the regeneration of the anion vacancies on the catalyst surface by removing the oxygen anions from the surface.The facilitation of the RDS of N2O decomposition is the reason for its favorable influence on the activity of the undoped cobalt catalyst [37].Similarly, the evolution of oxygen, and hence regeneration of the original adsorption site, can be improved by the addition of potassium to Co3O4, as reported by Asano et al. [45].
Figure 4a shows the N2O conversion on the studied catalyst (blue bars) in the inlet stream with the composition 5 vol % N2O in Ar in comparison with the results obtained under the same conditions for the undoped cobalt catalyst (grey bars).It can be seen that at temperatures of 700 and 750 °C, the undoped cobalt catalyst exhibits an activity that is similar to that of the catalyst containing LaCoO3.At 800 °C the activity of both catalysts is the same.In contrast, at the highest temperature, i.e., 850 °C, the Co3.5La is significantly more active than the undoped catalyst.Two recent interesting studies on high temperature N2O decomposition have shown the significance of parameters such as extrudate size [25] and pressure [27] on N2O decomposition.In fact, in measurements carried out at 890 °C with the commercial catalyst [25] it was found that an increase of diameter of the extrudates leads to a decrease of conversions of N2O from approx.80% for 2 mm diameter to 60% for 5 mm extrudates.Experiments carried out on monoliths in the temperature region 700-900 °C [27] show that the conversion of N2O increases substantially not only with the increase in temperature, but with increase of pressure in the regime which is currently found in nitric acid plants.For the lowest studied pressure, the following N2O conversions were obtained for 700 °C, 800 °C and 900 °C: 10%, 30% and 65%, but at 900 °C it can reach to over 90% for the highest studied pressure [27].The N2O conversion related to the Co3O4 content in the fresh catalyst bed is presented in Figure 4b.The activity at the lowest temperatures is the same on both catalysts.This might indicate that the active phase in Co3.5La is the spinel phase.At 750 and 800 °C the activity is similar or slightly higher in the case of Co3.5La.At 850 °C, a pronounced dif- The N 2 O conversion related to the Co 3 O 4 content in the fresh catalyst bed is presented in Figure 4b.The activity at the lowest temperatures is the same on both catalysts.This might indicate that the active phase in Co3.5La is the spinel phase.At 750 and 800 • C the activity is similar or slightly higher in the case of Co3.5La.At 850 • C, a pronounced difference is even more visible.The conversion obtained on Co3.5La is higher than that obtained on the undoped cobalt catalyst.The results noted for the two higher temperatures are the same as those observed when oxygen is added to the inlet stream introduced onto the undoped cobalt catalyst [37].
Summing up the results of activity in an inlet stream composed of 5 vol % N 2 O in Ar, it should be emphasized that the catalytic activity of Co3.5La does not exhibit a decrease in activity at 850 • C (Figure 4).In the case of the undoped cobalt catalyst a drop in the activity occurs at 850 • C and is a consequence of the thermal decomposition of Co 3 O 4 to CoO [46].The addition of oxygen to the inlet stream shifts the temperature of the thermal decomposition of Co 3 O 4 to higher values, which is why the undoped cobalt catalyst exhibits higher N 2 O conversions in a stream with the composition: 5 vol % N 2 O + 5 vol % O 2 in Ar [37].Therefore, the next step was to check if the positive effect of LaCoO 3 on the activity of Co3.5La at 800 • C is a result of its effect on the oxidation state of cobalt in the cobalt oxide.
Co 3 O 4 is a mixed oxide and contains cobalt ions on two oxidation states, namely +2 and +3.The only type of cobalt ions present in CoO are Co 2+ ions.In contrast cobalt in LaCoO 3 is present solely as Co 3+ .From literature it is known that the Co 2p regions of the XPS spectra of Co 2+ and Co 3+ ions differ considerably.In the case of ions in the lower oxidation state intensive shake-up satellites located approximately 5.4 eV [47] to 6.3 eV [48] away from the main signals are visible in the spectrum.In the spectrum of cobalt +3, the satellites are far less intense and more distant from the main signals, by about 9-10 eV [48,49].Figure 5 shows the Co 2p region of Co3.5La and the undoped cobalt catalyst [5] after operating at 850 • C in the inlet stream: 5 vol % N 2 O in Ar.In the case of the undoped catalyst (Figure 5 spectrum 1), the obtained spectrum is characteristic of Co 2+ ions.Thus, cobalt is present in the form of CoO.In the spectrum of the catalyst containing LaCoO 3 (Figure 5 spectrum 2) operating at 850 • C in an inlet stream consisting only of nitrous oxide in an inert gas (Ar), similar satellites can be seen, which are intense and distant by about 6 eV from the main signals.Due to the presence of Co 3+ ions in the lanthanum cobaltate, which does not undergo changes under these conditions, one can expect a very small signal corresponding to Co 3+ ions in the Co 2p region of the spectrum of Co3.5La, even at a total reduction of Co 3 O 4 to CoO.As seen in Figure 5, line 2 there is a small contribution from a Co 3+ satellite, which shows that on the surface Co Figure 6 summarizes the results of activity tests carried out on the cobalt catalyst containing lanthanum in streams with various composition.Figure 6a shows the effect of oxygen concentration in the inlet stream on N2O conversion.It can be seen that there is a negative effect of the additional oxygen on the activity of the catalyst, except at 850 °C, at The results of the XPS studies are in line with the conclusions drawn on the thermodynamic calculations.They clearly show that the presence of LaCoO 3 in the catalytic system does not influence the oxidation state of cobalt in the oxide in streams with the following inlet compositions: 5 vol % N 2 O + 95 vol % Ar and 5 vol % N 2 O + 5 vol % O 2 + 90 vol % Ar (Figure 5 spectrum 3) because it is the same as in the undoped cobalt catalyst [37].Summing up this part of the study, it can be concluded that the positive effect of lanthanum on the catalyst activity at 850 • C does not result from the impact of LaCoO 3 on the oxidation state of cobalt.
Figure 6 summarizes the results of activity tests carried out on the cobalt catalyst containing lanthanum in streams with various composition.Figure 6a shows the effect of oxygen concentration in the inlet stream on N 2 O conversion.It can be seen that there is a negative effect of the additional oxygen on the activity of the catalyst, except at 850 • C, at which there is a slight increase of the N 2 O conversion, which is probably due to the fact that more cobalt is in the Co 3 O 4 form, as indicated in the XPS results.Such a theory can be verified by XRD studies of the bulk composition of the spent catalysts.Figure 6 summarizes the results of activity tests carried out on the cobalt catalyst containing lanthanum in streams with various composition.Figure 6a shows the effect of oxygen concentration in the inlet stream on N2O conversion.It can be seen that there is a negative effect of the additional oxygen on the activity of the catalyst, except at 850 °C, at which there is a slight increase of the N2O conversion, which is probably due to the fact that more cobalt is in the Co3O4 form, as indicated in the XPS results.Such a theory can be verified by XRD studies of the bulk composition of the spent catalysts.Figure 7 shows the diffraction patterns obtained from the two samples of Co3.5La after operating at 850 °C in two streams, namely: 5 vol % N2O + 5 vol % O2 in Ar and 5 vol % N2O in Ar, respectively.It can be seen that the sample operating in a mixture containing Figure 7 shows the diffraction patterns obtained from the two samples of Co3.5La after operating at 850 • C in two streams, namely: 5 vol % N 2 O + 5 vol % O 2 in Ar and 5 vol % N 2 O in Ar, respectively.It can be seen that the sample operating in a mixture containing 5 vol % oxygen has cobalt mostly in the form of Co 3 O 4 , whereas the one operating without an addition of oxygen has cobalt present mostly as CoO.These findings correspond to the results obtained for the undoped cobalt catalyst operating in mixtures with these compositions [37].Comparing these data with those from XPS measurements (Figure 5 spectrum 3) and the ones obtained previously for the undoped cobalt catalyst, it is interesting that, although LaCoO 3 does not lead to a higher degree of oxidation of cobalt in the catalyst, the activity of Co3.5La is so high even without the addition of oxygen to the inlet stream.Moreover, the positive effect of the addition of oxygen to the feed at 850 • C is much less pronounced than in the undoped cobalt catalyst.When considering a possible reason for this difference, it should be taken into account that a certain amount of oxygen is always present in the reaction mixture as it is one of the products of N 2 O decomposition.
3) and the ones obtained previously for the undoped cobalt catalyst, it is interesting that, although LaCoO3 does not lead to a higher degree of oxidation of cobalt in the catalyst, the activity of Co3.5La is so high even without the addition of oxygen to the inlet stream.Moreover, the positive effect of the addition of oxygen to the feed at 850 °C is much less pronounced than in the undoped cobalt catalyst.When considering a possible reason for this difference, it should be taken into account that a certain amount of oxygen is always present in the reaction mixture as it is one of the products of N2O decomposition.The influence of the presence of water vapor in the inlet stream on the nitrous oxide conversion on Co3.5La is shown in Figure 6b.As can be seen, the addition of 5 vol % water vapor to the reactant mixture reduces the activity to a certain extent in the entire investigated temperature range, except at 850 °C, at which the conversion is the same with or without the addition of water vapor.A further increase of the H2O(g) concentration has no effect on the N2O conversion.
The addition of 5 vol % of the nitric oxide to the inlet stream does not affect the degree of conversion of nitrous oxide under the tested conditions (Figure 6c).The N2O conversion on Co3.5La is the same in the stream with the initial composition 5 vol % N2O + 5 vol % NO in Ar and 5 vol % N2O in Ar.In the case of the undoped cobalt catalyst, the presence of NO in the stream led to higher levels of conversion of nitrous oxide at the temperature of 850 °C.It is known from literature that the rate limiting step for N2O decomposition is the formation of molecular oxygen from two anions deposited on the surface by two molecules of N2O, i.e., desorption of O2 from the surface of the catalyst [6].Nitric oxide is known for its "scavenger" properties, which enables the transport of oxygen anions away from the surface.A pronounced positive effect was observed on Cu-zeolites especially when the distance between Cu cations was increased [41].Therefore, the same conversions obtained in the presence of Co3.5La in the mixture without and with NO provide evidence of the positive impact of LaCoO3.A positive effect of doping Co3O4-based catalysts with either ceria [5] or cesium [27] has been attributed to the shortening of the Co-O The influence of the presence of water vapor in the inlet stream on the nitrous oxide conversion on Co3.5La is shown in Figure 6b.As can be seen, the addition of 5 vol % water vapor to the reactant mixture reduces the activity to a certain extent in the entire investigated temperature range, except at 850 • C, at which the conversion is the same with or without the addition of water vapor.A further increase of the H 2 O (g) concentration has no effect on the N 2 O conversion.
The addition of 5 vol % of the nitric oxide to the inlet stream does not affect the degree of conversion of nitrous oxide under the tested conditions (Figure 6c).The N 2 O conversion on Co3.5La is the same in the stream with the initial composition 5 vol % N 2 O + 5 vol % NO in Ar and 5 vol % N 2 O in Ar.In the case of the undoped cobalt catalyst, the presence of NO in the stream led to higher levels of conversion of nitrous oxide at the temperature of 850 • C. It is known from literature that the rate limiting step for N 2 O decomposition is the formation of molecular oxygen from two anions deposited on the surface by two molecules of N 2 O, i.e., desorption of O 2 from the surface of the catalyst [6].Nitric oxide is known for its "scavenger" properties, which enables the transport of oxygen anions away from the surface.A pronounced positive effect was observed on Cu-zeolites especially when the distance between Cu cations was increased [41].Therefore, the same conversions obtained in the presence of Co3.5La in the mixture without and with NO provide evidence of the positive impact of LaCoO 3 .A positive effect of doping Co 3 O 4 -based catalysts with either ceria [5] or cesium [27] has been attributed to the shortening of the Co-O bond and hence facilitation of the desorption of oxygen from the catalyst surface.It may be assumed that in the case of LaCoO 3 the positive effect is also caused by easier O 2 desorption.Since LaCoO 3 is known to exhibit high mobility of oxygen and an increased mobility of oxygen in the catalyst might affect the formation and desorption of molecular oxygen, the observed behavior may be due to an overall increased mobility of oxide anions.This would account for a negative effect of the increase in the concentration of oxygen in the reaction mixture, which causes an increased amount of oxygen anions occupying sites and making them unavailable to N 2 O molecules.It would also explain why NO is not beneficial despite the fact that the majority of cobalt on the surface is present in the form of CoO.Thanks to the high oxygen mobility due to the presence of LaCoO 3 , the catalyst would not need the "scavenger" to transport oxygen anions so that O 2 is desorbed as a molecule.
In order to validate the hypothesis that the reason for a positive effect of the LaCoO 3 perovskite on the activity of the Co3.5La catalyst is facilitating the diffusion of oxygen ions throughout the entire catalytic system, a series of temperature-programed oxidation measurements was performed.TPO measurements were carried out after the reduction of the catalyst in hydrogen-rich stream.Then, the sample was oxidized in a reaction mixture with 5 vol % oxygen in an inert gas and the weight change resulting from the oxygen uptake was simultaneously monitored.Figure 8 shows the curves obtained in the tests as a function of time of the experiment.The dashed and dotted curve shows the temperature profile and refers to the right axis.The continuous curve shows the signal received during the oxidation of a sample of Co3.5La whose weight was 0.475 g.The curve with long dashes refers to the sample of the undoped catalyst of the same weight.Cobalt oxidation occurs in two stages.Line 1 in Figure 8 marks the beginning of oxidation of Co3.5La.It can be seen that at that temperature neither of the cobalt samples begins oxidation.The first step of cobalt oxidation is the oxidation of metal cobalt to CoO.Next, CoO is oxidized to Co 3 O 4 .In the presence of lanthanum, the oxidation begins at a lower temperature (Figure 8).The second step is particularly interesting in terms of this study, because under the conditions of the activity measurements both CoO and Co 3 O 4 can be present, and the oxidation of CoO to Co 3 O 4 is very important for the activity.The TPO results show a significant difference in the oxidation step for Co3.5La and the undoped cobalt catalyst.The beginning of oxidation in Co3.5La is marked with line 1.It can be seen that the undoped cobalt catalyst has not begun to oxidize.What is more, in the case of Co3.5La the oxidation does not slow down, as it does in the undoped cobalt catalyst.The undoped cobalt catalyst finishes oxidation only during the isothermal fragment, which starts at line 2. In order to verify whether this effect is not due to a slightly larger content of cobalt in the sample of the undoped catalyst and, therefore, to a larger demand for oxygen, another measurement was performed with less than half of this quantity, i.e., 0.210 g, of the undoped cobalt catalyst.The curve obtained during this measurement can be seen in Figure 8 as the line with short dashes.Although the amount of cobalt in this sample is considerably smaller than that in the Co3.5La sample, the second stage of oxidation slows down and ends after the isothermal step started (Figure 8, line 2).

Catalyst Preparation
The catalyst was obtained from Co3O4 (POCH, Gliwice, Poland, pure) and lanthanum nitrate.In order to obtain lanthanum nitrate, lanthanum oxide (Fluka, St. Gallen, Switzerland, pure) was calcined at 800 °C for 12 h, cooled and dissolved in a 1:1 aqueous solution of nitric acid (POCH, Gliwice, Poland, 65% pure for analysis).Cobalt oxide was added to the solution.The resulting mixture was heated to approximately 800 °C for 3h until lanthanum nitrate decomposed.Next, the mixture was heated in static air for approximately

Catalyst Preparation
The catalyst was obtained from Co 3 O 4 (POCH, Gliwice, Poland, pure) and lanthanum nitrate.In order to obtain lanthanum nitrate, lanthanum oxide (Fluka, St. Gallen, Switzer-L.F.L., J.P. and K.K.; visualization, E.M.I. and Z.K.; project administration, J.P., E.M.I., K.K. and L.F.L.; funding acquisition, J.P. All authors have read and agreed to the published version of the manuscript.
Funding: These studies were funded by the Ministry of Science and Higher Education of Poland, within the Research Project NN205 012434.

Figure 3 .
Figure 3. Results of characterization studies: (a) porosimetry test results for the fresh undoped cobalt catalyst (black) and Co3.5La (blue) and SEM images of Co3.5La(b) fresh and (c) after catalytic tests.

Figure 3 .
Figure 3. Results of characterization studies: (a) porosimetry test results for the fresh undoped cobalt catalyst (black) and Co3.5La (blue) and SEM images of Co3.5La(b) fresh and (c) after catalytic tests.

Figure 4 .
Figure 4. Nitrous oxide conversions in a stream composed of 5% N2O in Ar on the Co3.5La system (blue columns) and the undoped cobalt catalyst (grey columns): (a) conversion without accounting for difference in mass of Co3O4 in catalyst bed, (b) conversion relative to the mass percentage of Co3O4 in fresh catalyst bed.

Figure 4 .
Figure 4. Nitrous oxide conversions in a stream composed of 5% N 2 O in Ar on the Co3.5La system (blue columns) and the undoped cobalt catalyst (grey columns): (a) conversion without accounting for difference in mass of Co 3 O 4 in catalyst bed, (b) conversion relative to the mass percentage of Co 3 O 4 in fresh catalyst bed.

Figure 5 .
Figure 5. Results of XPS measurements: the Co 2p region of the undoped cobalt catalyst and Co3.5La after tests in a stream with the inlet composition: 5%N 2 O + 95% Ar (1 and 2, respectively) and Co 3.5La after tests in stream with inlet composition 5%N 2 O + 5%O 2 + 90%Ar (3).

Figure 8 .
Figure 8. Temperature Programmed Oxidation (TPO) results of the undoped cobalt catalyst (long dashes: 475 mg; short dashes: 210 mg) and Co3.5La (solid line: 475 mg) and the function of temperature in time (dotted line); line 1 indicates the onset of oxidation of Co3.5La, line 2 indicates the beginning of the isothermal segment.

Figure 8 .
Figure 8. Temperature Programmed Oxidation (TPO) results of the undoped cobalt catalyst (long dashes: 475 mg; short dashes: 210 mg) and Co3.5La (solid line: 475 mg) and the function of temperature in time (dotted line); line 1 indicates the onset of oxidation of Co3.5La, line 2 indicates the beginning of the isothermal segment.