New Insight into the Interplay of Method of Deposition, Chemical State of Pd, Oxygen Storage Capability and Catalytic Activity of Pd-Containing Perovskite Catalysts for Combustion of Methane

: Elaboration of Pd-supported catalysts for catalytic combustion is, nowadays, considered as an imperative task to reduce the emissions of methane. This study provides new insight into the method of deposition, chemical state of Pd and oxygen storage capability of transition metal ions and their effects on the catalytic reactivity of supported catalysts for the combustion of methane. The catalyst with nominal composition La(Co 0.8 Ni 0.1 Fe 0.1 ) 0.85 Pd 0.15 O 3 was supported on SiO 2 -modiﬁed/ γ -alumina using two synthetic procedures: (i) aerosol assisted chemical vapor deposition (U-AACVD) and (ii) wet impregnation (Imp). A comparative analysis shows that a higher catalytic activity is established for supported catalyst obtained by wet impregnation, where the PdO-like phase is well dispersed and the transition metal ions display a high oxygen storage capability. The reaction pathway over both catalysts proceeds most probably through Mars–van Krevelen mechanism. The supported catalysts are thermally stable when they are aged at 505 ◦ C for 120 h in air containing 1.2 vol.% water vapor. Furthermore, the experimentally obtained data on La(Co 0.8 Ni 0.1 Fe 0.1 ) 0.85 Pd 0.15 O 3 —based catalyst, supported on monolithic substrate VDM ® Aluchrom Y Hf are simulated by using a two-dimensional heterogeneous model for monolithic reactor in order to predict the performance of an industrial catalytic reactor for abatement of methane emissions.


Introduction
Methane is one of the most abundant air pollutants (second after carbon dioxide), responsible for global warming [1,2] which requires the reduction of its emissions. At present time there is an increased interest in the application of methane (as the main component of natural gas) as a fuel in the internal combustion engines. Bordelanne and co-workers [3] reported that GHG emissions from vehicles on compressed natural gas (CNG) fuel may produce significantly lower emissions than of gasoline vehicles and in case of hybrid CNG vehicles the reduction of 51% can be reached. In the case of the Toyota Prius CNG Hybrid prototype fuelled by biomethane produced from waste (in comparison to a gasoline vehicle), the emissions are lowered by 87% [3]. Therefore the use of biomethane can lead to reduction of GHG emissions below the minimum specified by the European Directive on the Promotion of Renewable Energy Sources (2009/28/EC). However, the wide use of methane as a fuel for the internal combustion engines meets problems originating from the unburned methane in the exhaust gases.
The aim of the present study was to obtain new data on the preparation and the catalytic behavior of Pd-containing perovskite catalysts supported on SiO 2 -modified/γ-alumina for further application in abatement of methane emissions. The nominal composition of the Pd-containing perovskite La(Co 0.8 Ni 0.1 Fe 0.1 ) 0.85 Pd 0.15 O 3 is selected since nickel and iron ions have opposite effects on the redox properties of cobalt-based perovskite, that allows to modify and improve their catalytic performance, as it was previously shown [13,14]. Regarding the deposition of the catalytically active phase, two synthetic procedures have been applied: (i) impregnation and (ii) aerosol assisted chemical vapor deposition.

Structure and Texture of Pd-Containing Perovskite Supported on SiO 2 -Modified/γ-Alumina
The powder X-ray diffraction patterns of deposited catalysts are dominated by diffraction peaks due to γ-alumina support, which makes difficulties in the determination of phase composition. In this case, the more informative is the SEM/EDS study. The deposition of the catalysts on the SiO 2 -modified/γ-alumina proceeds in a different way when the process is carried out by impregnation and by U-AACVD-pyrolysis ( Figure 1). opment of a suitable preparation method for catalysts deposition is highly needed in respect of the catalysts utilization in different technological sectors.
The aim of the present study was to obtain new data on the preparation and the catalytic behavior of Pd-containing perovskite catalysts supported on SiO2-modified/γ-alumina for further application in abatement of methane emissions. The nominal composition of the Pd-containing perovskite La(Co0.8Ni0.1Fe0.1)0.85Pd0.15O3 is selected since nickel and iron ions have opposite effects on the redox properties of cobalt-based perovskite, that allows to modify and improve their catalytic performance, as it was previously shown [13,14]. Regarding the deposition of the catalytically active phase, two synthetic procedures have been applied: (i) impregnation and (ii) aerosol assisted chemical vapor deposition.

Structure and Texture of Pd-Containing Perovskite Supported on SiO2-Modified/γ-Alumina
The powder X-ray diffraction patterns of deposited catalysts are dominated by diffraction peaks due to γ-alumina support, which makes difficulties in the determination of phase composition. In this case, the more informative is the SEM/EDS study. The deposition of the catalysts on the SiO2-modified/γ-alumina proceeds in a different way when the process is carried out by impregnation and by U-AACVD-pyrolysis ( Figure  1). It seems that the impregnation technique leads to a deposition of plate-like aggregates, on the surface of which small particles are visible (Figure 1 top). The distribution of elements in aggregates, determined by EDS, shows that the small particles are mainly composed of Pd, while the aggregates contain all the other elements such as La, Co, Ni, Fe and Pd in ratio, which does not coincide with that for the nominal perovskite composition. This means that the mixture of palladium, lanthanum and transition metal oxides rather than pure perovskite phase is formed during the impregnation technique. It is of importance that the ratio between Si and Al remains constant, thus indicating a homogeneous distribution of SiO2 over the γ-alumina. The plate-like aggregates are also formed during the U-AACVD-pyrolysis process, but the aggregates are rather It seems that the impregnation technique leads to a deposition of plate-like aggregates, on the surface of which small particles are visible (Figure 1 top). The distribution of elements in aggregates, determined by EDS, shows that the small particles are mainly composed of Pd, while the aggregates contain all the other elements such as La, Co, Ni, Fe and Pd in ratio, which does not coincide with that for the nominal perovskite composition. This means that the mixture of palladium, lanthanum and transition metal oxides rather than pure perovskite phase is formed during the impregnation technique. It is of importance that the ratio between Si and Al remains constant, thus indicating a homogeneous distribution of SiO 2 over the γ-alumina. The plate-like aggregates are also formed during the U-AACVD-pyrolysis process, but the aggregates are rather nonhomogeneously distributed over the surface (Figure 1). The distribution of elements in aggregates corresponds to the requirement for the nominal perovskite composition, i.e., La(Co 0.8 Ni 0.1 Fe 0.1 ) 0.85 Pd 0.15 O 3 . Both the SEM images and the EDS analysis reveal that the perovskite phase is mainly deposited over SiO 2 -modified/γ-alumina support during U-AACVD-pyrolysis process, which is an opposite fact to that established for the impregnation process. It is noticeable that, irrespective of the synthesis procedure, the mean Pd-content in supported catalysts is one and the same: 1.7 ± 0.2 wt % for Pd-LMO/Imp and 1.6 ± 0.4 wt % Pd-LMO/U-AACVD.
The formation of well-dispersed Pd-rich particles is demonstrated by TEM analysis. Figure 2 gives SAED and HRTEM images for Pd-LMO/Imp. non-homogeneously distributed over the surface (Figure 1). The distribution of elements in aggregates corresponds to the requirement for the nominal perovskite composition, i.e., La(Co0.8Ni0.1Fe0.1)0.85Pd0.15O3. Both the SEM images and the EDS analysis reveal that the perovskite phase is mainly deposited over SiO2-modified/γ-alumina support during U-AACVD-pyrolysis process, which is an opposite fact to that established for the impregnation process. It is noticeable that, irrespective of the synthesis procedure, the mean Pd-content in supported catalysts is one and the same: 1.7 ± 0.2 wt % for Pd-LMO/Imp and 1.6 ± 0.4 wt % Pd-LMO/U-AACVD.
The formation of well-dispersed Pd-rich particles is demonstrated by TEM analysis. Figure 2 gives SAED and HRTEM images for Pd-LMO/Imp. The SAED of particles with dimensions of about 10 nm displays an occurrence of Pd-rich phases such as La2PdO4. In addition, HRTEM gives an indication that the small particles are composed of domains containing Pd-rich (such as La2Pd2O5) and perovskite phases. The same picture is observed with Pd-LMO/U-AACVD sample (not shown here).
The state of Pd species in the supported catalysts is probed by means of XPS. Figure  3 shows the Pd 3d3/2 and Pd 3d5/2 core levels spectra for the supported catalysts Pd-LMO/Imp and Pd-LMO/U-AACVD. The SAED of particles with dimensions of about 10 nm displays an occurrence of Pd-rich phases such as La 2 PdO 4 . In addition, HRTEM gives an indication that the small particles are composed of domains containing Pd-rich (such as La 2 Pd 2 O 5 ) and perovskite phases. The same picture is observed with Pd-LMO/U-AACVD sample (not shown here).
The state of Pd species in the supported catalysts is probed by means of XPS. Figure 3 shows the Pd 3d3/2 and Pd 3d5/2 core levels spectra for the supported catalysts Pd-LMO/Imp and Pd-LMO/U-AACVD.  The Pd 3d5/2 spectra of both catalysts consist of a broad asymmetric envelope, which can be deconvoluted into two overlapping signals: one narrow signal centered at around 336 eV and other broader signal centered above 337 eV. The deconvoluted parameters including peak position, line width and the ratio between signals are summarized in Table 1. The comparison shows that the positions and the line widths for the two signals are relatively the same for the catalysts prepared by the impregnation and . XPS spectra in the Pd 3d3/2 and Pd 3d5/2 regions for fresh Pd-LMO/Imp (a) and Pd-LMO/U-AACVD (b), as well as for catalytic tested Pd-LMO/Imp (c) and Pd-LMO/U-AACVD (d). The peak deconvolution in the Pd 3d5/2 region is indicated with blue and red lines. The Shirley's type of background is also shown (green lines). The Pd 3d5/2 spectra of both catalysts consist of a broad asymmetric envelope, which can be deconvoluted into two overlapping signals: one narrow signal centered at around 336 eV and other broader signal centered above 337 eV. The deconvoluted parameters including peak position, line width and the ratio between signals are summarized in Table 1. The comparison shows that the positions and the line widths for the two signals are relatively the same for the catalysts prepared by the impregnation and by the U-AACVD-pyrolysis, but the ratio between them displays a clear dependence on the synthesis history: the broad high-energy peak is more intensive for the catalysts obtained by the U-AACVD-pyrolysis, while the narrow low-energy signal-for the impregnated catalysts ( Figure 3, Table 1). Based on the numerous XPS studies on the oxidation state of Pd in palladiumcontaining perovskite powders and catalysts [15,17,27], the broad high energy peak above 337 eV can be assigned to Pd 3+ or Pd 4+ species inserted inside the B-site of the perovskite structure, whereas the narrow low-energy peak within the range of 335.9-336.4 eV is probably associated with Pd 2+ in PdO [15,17,27]. The ratio between signals reveals that the surface of impregnated catalysts is enriched with low-oxidized Pd 2+ ions stabilized in the PdO-like phase, while highly oxidized Pd 3+/4+ ions included in the perovskite-like phase occur predominantly on the surface of spray-U-AACVDolysed catalysts. The XPS results coincide well with the SEM observation for the deposition of oxide mixture during impregnation, while the Pd-containing perovskite phase is formed after U-AACVD-pyrolysis.
The surface Pd-content in supported catalysts is calculated from corresponding XPS spectra. The results show that the surface Pd-content in two supported catalysts is relatively the same: 4.0 wt % for Pd-LMO/Imp and 4.2 wt % for Pd-LMO/U-AACVD. It is worth mentioning that the surface Pd-content estimated by XPS is higher (more than 2 times) than that determined by EDS analysis, which is a more deeply penetrating method. The difference between XPS and EDS data for Pd content means that the Pd ions occur mainly on the catalyst's surface.
The BET surface area of supported catalysts Pd-LMO/Imp and Pd-LMO/U-AACVD, as well as the supports γ-alumina and SiO 2 -modified γ-alumina, is further analyzed by low-temperature nitrogen adsorption. All the samples are mesoporous with predominant pores sizes between 4 and 15 nm, which are demonstrated by the type IV hysteresis loops. The specific surface area, the total pore volume and the mean pore diameter are listed in Table 2. The comparison shows that the texture parameters of γ-Al 2 O 3 support are only slightly perturbed after its modification with SiO 2 . A tendency for reduction in the surface area and the total pore volume is concomitant with an increase in the mean pore diameter and it is observed during the deposition of Pd-containing perovskites. This trend is more pronounced for the catalysts obtained by the impregnation technique.
The observed texture changes are in good agreement with the morphology observations (Figure 1), where non-homogeneous distribution of supported Pd-containing perovskite is established when the U-AACVD-pyrolysis technique is used. To check this supposition, Table 2 gives also the texture parameters for perovskite supported catalysts without containing any Pd. As in the case of the Pd-containing catalysts, the specific surface area and the total pore volume decrease more strongly for the catalyst obtained by the impregnation. Table 2. Specific surface area (S), total pore volume (V t ) and mean pore diameter (D) for γ-Al 2 O 3 , SiO 2 -modified γ-Al 2 O 3 , Pd-LMO/U-AACVD and Pd-LMO/Imp. For comparison, the corresponding perovskites without Pd are also given (LMO/Imp and LMO/U-AACVD, respectively).

Sample
S, m 2 /g V t , cm 3  The reducibility of supported catalysts is monitored using temperature programmed reduction with both strong and mild agents such as H 2 and CH 4 . Figure 4A compares the H 2 -TPR curve profiles for Pd-LMO/Imp and Pd-LMO/U-AACVD samples. The comparison indicates that the reduction of both supported catalysts is accomplished in three steps in the well separated temperature ranges: between 50 and 160 • C, 250 and 470 • C and above 580 • C, respectively. To rationalize the reduction properties of supported Pd-containing catalysts, the same figure demonstrates the H 2 -TPR curve for perovskite supported catalyst, which does not contain any Pd, i.e., LMO/Imp. As one can see, the LMO/Imp exhibits only two broad peaks of hydrogen consumption: between 250 and 500 • C and above 550 • C, respectively. This reveals clearly that the low-temperature peaks of hydrogen consumption are related to the Pd species. Depending on the method of synthesis, the Pd-containing catalysts displays in the low temperature range different H 2 -TPR curve profiles: Pd-LMO/Imp exhibits one dominating peak of H 2 consumption centred at 72 • C with a shoulder at around 120 • C, while two peaks at 75 • C and 115 • C are clearly resolved for Pd-LMO/U-AACVD. This is a H 2 -TPR evidence for the variation in the distribution of Pd species over the supported catalysts. The peak at 75 • C can be attributed to the reduction of dispersed PdO-like phase, while the peak at 115 • C can be interpreted by the reduction of Pd species included inside the perovskite-like phase or by the strong interaction between PdO-like phase with SiO 2 -modified alumina support [28,29]. The effect of the synthesis procedure on the distribution of Pd species over the supported catalysts is in good agreement with the above SEM and XPS observations (Figures 1 and 3a,c): the impregnation method yields well dispersed PdO-like phase, while Pd-containing perovskite is a main product during U-AACVD deposition method. It is worth mentioning that the negative peak at around 220 • C (especially pronounced for Pd-LMO/U-AACVD) could be tentatively related to the decomposition of palladium hydride, which is previously observed for PdO-supported Ce, Zr, alumina [28].  To analyze the reduction of supported catalysts by CH4, EPR measurements were undertaken. Figure 5 gives the ex-situ EPR spectra of partially reduced Pd-LMO/Imp sample with CH4. The EPR spectrum of fresh catalysts displays low intensity signal at g = 4.28 due to the impurity Fe 3+ ions in the alumina support. After the partial reduction of Pd-LMO/Imp sample with CH4 at 200 °C, the EPR spectrum does not undergo any significant changes. At 500 °C, the reduction of Pd-LMO/Imp with CH4 results in the appearance of a strong asymmetric signal with an apparent g-value of =2.17 and line width In the middle temperature range between 250 and 500 • C, the reduction peak for Pd-LMO/Imp shifts slightly to 335 • C in comparison with that for Pd-LMO/U-AACVD at 355 • C Catalysts 2021, 11, 1399 7 of 20 ( Figure 4A). In this case, the consumption of H 2 is due to the reduction of transition metal ions from oxide/perovskite deposited phases. The reduction of powders LaCo 1-x Ni x O 3 and LaCo 1-x Fe x O 3 perovskites were well examined previously [12]. It has been found out that the reduction of LaCo 1-x Ni x O 3 perovskites with H 2 proceeds above 520 • C to Co/Ni metals and La 2 O 3 via the formation of intermediate oxygen deficient Brownmillerite-type of phase between 320 • C and 500 • C due to Co 3+ /Ni 3+ → Co 2+ /Ni 2+ [12]. The interaction of LaCo 1-x Fe x O 3 with H 2 occurs by reduction of Co 3+ to Co 2+ prior to the Fe 3+ ions [12]. In general, Ni-containing perovskites LaCo 1-x Ni x O 3 are more easily reduced than those containing some Fe, i.e., LaCo 1-x Fe x O 3 . In comparison with perovskites, the reduction of Co 3 O 4 and Ni x Co 3-x O 4 spinels proceeds between 300 and 400 • C to metals [29]. Based on the redox properties of perovskite and spinel powders, it can be supposed that the consumption of H 2 between 250 and 500 • C is due to the uncompleted reduction of transition metal ions (i.e., Co 3+ and Ni 3+ to Co 2+ and Ni 2+ ). The slightly lower reduction temperature observed for Pd-LMO/Imp is related to a higher amount of separate phase of transition metal oxides, that is more easily reduced to metals. The consumption of H 2 by supported catalysts above 580 • C is associated with a complete reduction of transition metal ions to metals by evolving La 2 O 3 phase. It should be mentioned that the complete reduction of the supported catalysts starts at higher temperatures in comparison with the powder perovskites [12]. This indicates a possible interaction of transition metal ions with SiO 2 -modified alumina support.
Although the strong H 2 reagent allows differentiating the reducibility of supported catalysts with respect to the method of synthesis, the mild CH 4 reagent makes equalizing of the catalysts ( Figure 4B). The consumption of CH 4 by supported catalysts starts above 200 • C by developing a reduction peak centered at around 600 • C. It is noticeable that the intensity of this peak is higher for Pd-LMO/Imp. In comparison with Pd-containing catalysts, the supported perovskite LMO/Imp sample that does not contain any Pd exhibits a negligible consumption of CH 4 within this temperature range. The comparison of CH 4 -TPR curves of supported catalysts indicates that the peak at around 600 • C can be attributed to the consumption of CH 4 by dispersed PdO-like phase, whose amount is higher in Pd-LMO/Imp. The dispersed PdO-like phase is also responsible for low-temperature consumption of H 2 ( Figure 4B).
To analyze the reduction of supported catalysts by CH 4 , EPR measurements were undertaken. Figure 5 gives the ex-situ EPR spectra of partially reduced Pd-LMO/Imp sample with CH 4 . The EPR spectrum of fresh catalysts displays low intensity signal at g = 4.28 due to the impurity Fe 3+ ions in the alumina support. After the partial reduction of Pd-LMO/Imp sample with CH 4 at 200 • C, the EPR spectrum does not undergo any significant changes. At 500 • C, the reduction of Pd-LMO/Imp with CH 4 results in the appearance of a strong asymmetric signal with an apparent g-value of =2.17 and line width of 115 mT. The signal intensity increases when the reduction temperature is raised from 500 to 700 • C. The detection of a strong EPR signal for partially reduced catalysts can be related to an appearance of Pd particles due to the consumption of CH 4 by Pd-LMO/Imp sample. To support this supposition, Figure 5 gives also the EPR spectrum of completely reduced Pd-LMO/Imp with H 2 at 700 • C. In this case, the EPR spectrum consists of strong EPR asymmetric signal with an apparent g-factor close to that of CH 4 -reduced Pd-LMO/Imp: g = 2.20 versus g = 2.17, respectively. The important feature of the EPR spectrum of H 2reduced Pd-LMO/Imp is the strong signal broadening: 145 mT and 115 mT for H 2 -and CH 4 -reduced Pd-LMO/Imp, respectively. The broadening of the EPR signal after H 2 treatment of Pd-LMO/Imp sample can be associated with a complete reduction at 700 • C of both palladium and transition metal ions to metals, as it was observed by H 2 -TPR curves ( Figure 4A). In comparison with H 2 -reduction, the methane is partially consumed by Pd-LMO/Imp at 700 • C ( Figure 4B) as a result of which only the palladium ions are reduced leading to an appearance a narrow EPR signal. The comparative EPR study provides experimental evidence for the reduction of palladium ions during CH 4 treatment of Pd-LMO/Imp at temperatures higher than 200 • C, while transition metal ions to higher extent remain inactive event at 700 • C. It is of importance that Pd-LMO/U-AACVD reduced with CH 4 at 700 • C displays the same EPR features: there is a strong asymmetric signal with an apparent g-value of =2.17 and line width of 115 mT. The difference comes from the intensities of the signals detected for reduced with CH 4 at 700 • C samples: the normalized intensity is 1.0 for Pd-LMO/Imp versus 0.7 for Pd-LMO/U-AACVD. For the sake of comparison, the completely reduced Pd-LMO/Imp with H2 at 700 °C is also shown (e).

Pre-Treatment Tests
In order to compare the reactivity of oxygen species on both catalytic surfaces, experiments on the so-called "depletive" oxidation were performed [30,31]. The test consists of stopping the supply of oxygen in the feed gas mixture after establishing steady-state conditions and monitoring the formation of oxidation products. The duration of "depletive" oxidation experiment was fixed to 70 min, a period within which the emission of oxidation products from both samples was almost completed. The results from "depletive" oxidation of both fresh catalysts are represented in Figure 6. The on-line gas-analysis shows that the "depletive" oxidation of both catalysts leads to the formation of CO2 and CO products. It appears that CO2 is evolved immediately after the termination of oxygen supply and this process stops within a period of 5-6 min. The CO formation starts only after the CO2 evolution is ended and it passes through a maximum at about 5-6 min, followed by a slow diminishing for a longer time period. All these observations reveal the redox ability of catalysts that depend on the method of synthesis. As a measure of redox ability, Table 3 gives the amount of oxygen from catalysts involved in the reaction with CH4.

Pre-Treatment Tests
In order to compare the reactivity of oxygen species on both catalytic surfaces, experiments on the so-called "depletive" oxidation were performed [30,31]. The test consists of stopping the supply of oxygen in the feed gas mixture after establishing steady-state conditions and monitoring the formation of oxidation products. The duration of "depletive" oxidation experiment was fixed to 70 min, a period within which the emission of oxidation products from both samples was almost completed. The results from "depletive" oxidation of both fresh catalysts are represented in Figure 6.

Pre-Treatment Tests
In order to compare the reactivity of oxygen species on both catalytic surfaces, experiments on the so-called "depletive" oxidation were performed [30,31]. The test consists of stopping the supply of oxygen in the feed gas mixture after establishing steady-state conditions and monitoring the formation of oxidation products. The duration of "depletive" oxidation experiment was fixed to 70 min, a period within which the emission of oxidation products from both samples was almost completed. The results from "depletive" oxidation of both fresh catalysts are represented in Figure 6. The on-line gas-analysis shows that the "depletive" oxidation of both catalysts leads to the formation of CO2 and CO products. It appears that CO2 is evolved immediately after the termination of oxygen supply and this process stops within a period of 5-6 min. The CO formation starts only after the CO2 evolution is ended and it passes through a maximum at about 5-6 min, followed by a slow diminishing for a longer time period. All these observations reveal the redox ability of catalysts that depend on the method of synthesis. As a measure of redox ability, Table 3 gives the amount of oxygen from catalysts involved in the reaction with CH4. The on-line gas-analysis shows that the "depletive" oxidation of both catalysts leads to the formation of CO 2 and CO products. It appears that CO 2 is evolved immediately after the termination of oxygen supply and this process stops within a period of 5-6 min. The CO formation starts only after the CO 2 evolution is ended and it passes through a maximum at about 5-6 min, followed by a slow diminishing for a longer time period. All these observations reveal the redox ability of catalysts that depend on the method of Catalysts 2021, 11, 1399 9 of 20 synthesis. As a measure of redox ability, Table 3 gives the amount of oxygen from catalysts involved in the reaction with CH 4 . The quick generation of CO 2 reveals that at 500 • C the methane reduces the catalyst by reaction with surface oxygen species: CH 4 + O 2surf(lattice) → CO 2 + H 2 O. At this stage of examination, it is not possible to relate undoubtedly the oxygen species with palladium or transition metal ions. However, the previous data have demonstrated that the PdO supported on γ-Al 2 O 3 is reduced with CH 4 to Pd 0 in the temperature range of 240-290 • C depending on the particle sizes [32], while Co 3 O 4 with different morphologies undergoes two steps reduction starting in the temperature range of 540-590 • C [33]. In addition, the ex-situ EPR spectra indicate that the palladium ions are reduced by CH 4 prior to the transition metal ions within the temperature range of 500-700 • C (Figure 4). From the comparative point of view, it is of importance that the process of CO 2 formation is more effective for Pd-LMO/Imp as it was demonstrated by the evaluated amount of O 2 participating in the reaction with CH 4 ( Table 3). This is related to different catalyst surface properties: more dispersed PdO phase and higher content of transition metal oxides are established for the catalysts obtained by the impregnation. In comparison with CO 2 , the observed continuation of CO formation after the specified test duration is a more complex process and it can be explained either by the reaction of methane with oxygen from the catalyst surface or by the reforming of CH 4 with CO 2 (or H 2 O) evolved or by cracking of CH 4 over the metal particles (as it was previously found out for the reduction of Co 3 O 4 with CH 4 to CO [33]), but some influence of oxygen traces (10-15 ppm) in the nitrogen gas cannot be excluded. Even in this complex case, the evaluated oxygen amount involved in the reaction with CH 4 to CO is higher for the Pd-LMO/Imp sample. The comparative data of "depletive" oxidation of fresh catalysts evidence clearly that Pd-LMO/Imp contains more mobile oxygen species, which give rise to its higher redox ability.
After the "depletive" oxidation test both samples showed remarkable activation once the oxygen supply was restored. In terms of reaction rate constants (pseudo-first order kinetics), a twofold increase was observed (more specifically k 2 /k 1 (500 • C) = 1.85 for Pd-LMO/U-AACVD and 1.90 for Pd-LMO/Imp) after the first cycle. It was reported that after the 110 h of testing in methane combustion over 2% Pd/Al 2 O 3 (at 420 • C) the conversion degree decreased from 100% to 88%, but the following treatment in hydrogen (at 200 • C for 1 h) resulted in a restoration of the initial activity [34]. Although the reductive treatment is enhancing the activity of the catalyst, this enhancement is only temporary and it can be lost in oxidizing conditions [35].

Reaction Kinetics
The inlet concentrations of reagents were varied and the corresponding kinetic parameters were calculated by applying the method, described by Duprat [36]. The temperature dependencies of the conversion degree of methane during the reaction of complete oxidation are presented in Figure 7. Details on the calculation procedure were published previously [37,38].
The effect of the external mass transfer limitations was minimized by tests at relatively high values for the hourly space velocity. Preliminary experiments on varying the GHSV( STP ) showed that the reaction rate (calculated by integration alongside the reactor length) increased gradually until we reached 40,000 h −1 and a further increase in the value of the gas film coefficient had no effect on the reaction rate. In addition, calculations on the effect of higher gas velocity at the reactor wall showed a decrease in the conversion within the range of 2-4%, which was compensated during the calculation procedure. Further, in order to decrease the effect of the internal mass transfer limitations, the catalytic active phase was supported in the form of a thin layer and the values of the effectiveness factor at conversions below 40% were within the range of 0.95-0.99. between the experimental data for the conversion and the predicted values by the model. The applied optimization procedure is based on searching for the lowest value for the squared sum of the residuals (RSS) between the measured conversions and the model predictions, as well as the value of the square correlation coefficient (R 2 ) within the entire set of varied experimental data.
Based on the experimental results (conversions below 40-45%), an estimation of the kinetic parameters was performed by solving the material balance in an isothermal plug flow type of reactor (PFR) applying a numerical nonlinear optimization procedure, based on iterative reduction of the gradients. The fitting program was applied under different sets of reaction conditions in searching for the global minimum. As a first approximation a power law kinetic model (PWL) has been applied. The calculated kinetics parameters are represented in Tables 4-6. The parameters calculated based on the power law kinetic model (PWL) suppose almost the same inhibition effect of water vapor on both catalysts. Similar negative reaction orders towards the water were discussed earlier [39]. It should be pointed out that the water vapor is present in all exhaust gas compositions and its impact on the performance of the catalytic converter is very significant. The applied rate equations contain the following kinetic parameters: pre-exponential coefficient (k o ), reaction orders towards the methane, oxygen and water vapor (m, n, p), the rate constant (k), activation energy (E a ), enthalpy of adsorption (∆H), and adsorption equilibrium constant (K). The corresponding dimensions for each kinetic parameter and kinetic model are shown below in Tables 4-6. The calculation procedure is based on an integrated computer program where the material balance of the reactor model is solved simultaneously using the suggested multivariate analysis kinetics parameters. The consistency of the obtained results of each run is checked by comparison between the experimental data for the conversion and the predicted values by the model. The applied optimization procedure is based on searching for the lowest value for the squared sum of the residuals (RSS) between the measured conversions and the model predictions, as well as the value of the square correlation coefficient (R 2 ) within the entire set of varied experimental data.
Based on the experimental results (conversions below 40-45%), an estimation of the kinetic parameters was performed by solving the material balance in an isothermal plug flow type of reactor (PFR) applying a numerical nonlinear optimization procedure, based on iterative reduction of the gradients. The fitting program was applied under different sets of reaction conditions in searching for the global minimum.
As a first approximation a power law kinetic model (PWL) has been applied. The calculated kinetics parameters are represented in Tables 4-6. The parameters calculated based on the power law kinetic model (PWL) suppose almost the same inhibition effect of water vapor on both catalysts. Similar negative reaction orders towards the water were discussed earlier [39]. It should be pointed out that the water vapor is present in all exhaust gas compositions and its impact on the performance of the catalytic converter is very significant.  In general, the oxi-redox pathway of complete methane oxidation on palladium catalysts proceeds through the dissociation of a methane molecule to form a hydroxyl group and methyl fragment [40,41]. The mechanism of the interaction between the water molecules and the active sites on Pd-containing catalysts is still unclear. A process of slow recombination of hydroxyls (and following water desorption) occurs on the palladium based catalytic surface. It was observed that the isotopic exchange of oxygen with the palladium active centers occurs before desorption of the water [42]. Schwartz et al. [43] reported that catalyst deactivation during methane oxidation can be related to hydroxyl radicals accumulation on the oxide support. The formed hydroxyls affect the oxygen exchange and decrease the Pd catalyst activity [44].
The values of the reaction order with respect to oxygen (ranged at 0.2) suppose that the reaction pathway of oxygen passes through interaction with the catalytic surface (adsorption step with dissociation), which is just the opposite in the case of methanevalues approaching unity reveal the possibility of a direct reaction from the gas phase (that is because of the consideration for Eley-Rideal mechanism [45], which is also included). When the oxygen undergoes a dissociative adsorption [O 2 + 2Z → 2Z(O)], its partial pressure should appear in the rate equation at the power of 1/2 [37,38].
The close values of the apparent activation energies and the reaction orders enable the opportunity to conclude that the reaction mechanism on both catalysts should be the same one and therefore to compare the values of the pre-exponents. Although such an analysis is only tentative, it is obvious that over the Imp sample reaction of complete methane oxidation proceeds with a three times higher rate than on the U-AACVD sample.
Based on the parameters with the PWL model the following mechanistic models were selected for fitting with the experimental results: Mars-van Krevelen (MVK) [46], water molecules compete with the methane molecules for the oxidized or reduced sites; Eley-Rideal (ER) mechanism, water molecules compete with the oxygen for the same type of adsorption sites and methane molecules react directly from the gas phase.
As it can be seen in Tables 4-6, the best fit was obtained with Mars-van Krevelen (MVK-1), where the water molecules compete with the methane molecules for the oxidized adsorption sites. Comparative data on model predicted and experimentally measured conversions are presented in Figure 8.
Calculations show that the rate-determining step (RDS) is the reduction of the active sites, except at low concentration of oxygen (below 1.0 vol.% for Pd-LMO/Imp and 0.3% for Pd-LMO/U-AACVD), where the re-oxidation stage is a more slow process. Despite the relatively higher values for RSS in Table 6, the mechanism of Eley-Rideal, where the methane reacts from the gas phase cannot be completely neglected.
Eley-Rideal (ER) mechanism, water molecules compete with the oxygen for the same type of adsorption sites and methane molecules react directly from the gas phase.
As it can be seen in Tables 4-6, the best fit was obtained with Mars-van Krevelen (MVK-1), where the water molecules compete with the methane molecules for the oxidized adsorption sites. Comparative data on model predicted and experimentally measured conversions are presented in Figure 8. Calculations show that the rate-determining step (RDS) is the reduction of the active sites, except at low concentration of oxygen (below 1.0 vol.% for Pd-LMO/Imp and 0.3% for Pd-LMO/U-AACVD), where the re-oxidation stage is a more slow process. Despite the relatively higher values for RSS in Table 6, the mechanism of Eley-Rideal, where the methane reacts from the gas phase cannot be completely neglected.
Regarding the thermodynamic consistency of the equilibrium adsorption constants for methane, oxygen and water, the values calculated by the models were constrained within the limits, defined by the guidelines, given by Boudard [47], Vannice et al. [48] and Troops et al. [49]. The following criteria were applied to the calculated values of the enthalpies [49]: −ΔHads > 0 (Qads > 0); 0 < −ΔS° ads < S°g; 10 ≤ −ΔS°ads ≤ 12.2 − 0.0014. ΔHads where ΔHads is in kcal/mol, S°g-standard entropy of the gas at 1 atm.

Ex-Situ XPS Analysis
To understand the observed changes in the catalytic activity of supported catalysts, ex-situ XPS measurements are undertaken. Figures 3 and 9 show the XPS spectra in the Pd 3d and La 3d regions for Pd-LMO/U-AACVD and Pd-LMO/Imp samples. The Pd Regarding the thermodynamic consistency of the equilibrium adsorption constants for methane, oxygen and water, the values calculated by the models were constrained within the limits, defined by the guidelines, given by Boudard [47], Vannice et al. [48] and Troops et al. [49]. The following criteria were applied to the calculated values of the enthalpies [49]: −∆Hads > 0 (Qads > 0); 0 < −∆S • ads < S • g; 10 ≤ −∆S • ads ≤ 12.2 − 0.0014. ∆Hads where ∆H ads is in kcal/mol, S • g -standard entropy of the gas at 1 atm.

Ex-Situ XPS Analysis
To understand the observed changes in the catalytic activity of supported catalysts, ex-situ XPS measurements are undertaken. Figures 3 and 9 show the XPS spectra in the Pd 3d and La 3d regions for Pd-LMO/U-AACVD and Pd-LMO/Imp samples. The Pd 3d5/2 core level spectra consist of two overlapping high-and low-energy components as in the case of the fresh catalysts ( Figure 3). The only parameter that is changed is the ratio between the intensities of the two components: there is a decrease in the intensity of the high-energy components, which is more pronounced for the catalysts obtained by U-AACVD-pyrolysis ( Figure 3, Table 1). This means that the highly-oxidized surface Pd 3+/4+ ions are extracted from the structure of the perovskite-like phase during the catalytic reaction, while Pd 2+ in PdO phase remains unchanged. The changes in the chemical state are also observed for La (Figure 9). The typical La 3d core level spectrum displays two separated spin-orbit components (3d3/2 and 3d5/2), where each one of them is further on split by multiplet splitting. This is what we observe with the fresh catalysts. The binding energy for La 3d5/2 is around 834 eV with energy difference between the 3d3/2 and the 3d5/2 levels of 17 eV, which correspond to La 3+ ions. Comparing Pd-LMO/Imp and Pd-LMO/U-AACVD, it appears that the multiplet structure is more well resolved for the Pd-LMO/Imp catalyst, which is related to the slightly different local structure of La in both catalysts: although at Pd-LMO/Imp a mixture of oxides is mainly deposed, Pd-LMO/U-AACVD contains relatively higher amounts of the perovskite phase. After the activity catalytic test, the multiplet splitting becomes less resolved and the 3d5/2 level is slightly shifted to 835 eV without changing the energy difference between the 3d3/2 and the 3d5/2 levels. This reflects, most probably, the difference in the local structure of La in both the fresh and tested catalysts. 3d5/2 core level spectra consist of two overlapping high-and low-energy components as in the case of the fresh catalysts (Figure 3). The only parameter that is changed is the ratio between the intensities of the two components: there is a decrease in the intensity of the high-energy components, which is more pronounced for the catalysts obtained by U-AACVD-pyrolysis ( Figure 3, Table 1). This means that the highly-oxidized surface Pd 3+/4+ ions are extracted from the structure of the perovskite-like phase during the catalytic reaction, while Pd 2+ in PdO phase remains unchanged. The changes in the chemical state are also observed for La (Figure 9). The typical La 3d core level spectrum displays two separated spin-orbit components (3d3/2 and 3d5/2), where each one of them is further on split by multiplet splitting. This is what we observe with the fresh catalysts. The binding energy for La 3d5/2 is around 834 eV with energy difference between the 3d3/2 and the 3d5/2 levels of 17 eV, which correspond to La 3+ ions. Comparing Pd-LMO/Imp and Pd-LMO/U-AACVD, it appears that the multiplet structure is more well resolved for the Pd-LMO/Imp catalyst, which is related to the slightly different local structure of La in both catalysts: although at Pd-LMO/Imp a mixture of oxides is mainly deposed, Pd-LMO/U-AACVD contains relatively higher amounts of the perovskite phase. After the activity catalytic test, the multiplet splitting becomes less resolved and the 3d5/2 level is slightly shifted to 835 eV without changing the energy difference between the 3d3/2 and the 3d5/2 levels. This reflects, most probably, the difference in the local structure of La in both the fresh and tested catalysts.
Furthermore, one can suppose that the different local structures of La can be induced by the extraction of Pd from the perovskite structure during the catalytic reaction. In general, Pd-LMO/U-AACVD undergoes more significant changes under the catalytic activity test due to the extraction of Pd 3+/4+ ions from the perovskite structure and the formation of PdO-like phase.
This coincides well with the "depletive" oxidation of spent catalysts ( Figure 6, Table  3). The formation of CO2 starts immediately after the stop of O2 supply as in the case of fresh catalysts, but its formation proceeds in an extended time period (up to 20 min of the experiment, Figure 6). The formation of CO during the interaction of CH4 with spent catalysts retains its specific features: CO is formed after CO2 and it is evolved in a longer time period (Figure 6). The comparison between the calculated amounts of oxygen uptake during "depletive" oxidation within the first cycle and the followed "working" cycle treatments show more significant changes within the U-AACVD sample. More specifically, there is a remarkable increase in the amount of oxygen capable to convert me- Furthermore, one can suppose that the different local structures of La can be induced by the extraction of Pd from the perovskite structure during the catalytic reaction. In general, Pd-LMO/U-AACVD undergoes more significant changes under the catalytic activity test due to the extraction of Pd 3+/4+ ions from the perovskite structure and the formation of PdO-like phase.
This coincides well with the "depletive" oxidation of spent catalysts ( Figure 6, Table 3). The formation of CO 2 starts immediately after the stop of O 2 supply as in the case of fresh catalysts, but its formation proceeds in an extended time period (up to 20 min of the experiment, Figure 6). The formation of CO during the interaction of CH 4 with spent catalysts retains its specific features: CO is formed after CO 2 and it is evolved in a longer time period (Figure 6). The comparison between the calculated amounts of oxygen uptake during "depletive" oxidation within the first cycle and the followed "working" cycle treatments show more significant changes within the U-AACVD sample. More specifically, there is a remarkable increase in the amount of oxygen capable to convert methane to CO and CO 2 (an increase of the order of 2-3 times), while the changes with the Pd-LMO/Imp sample are in the margin of the experimental error. It is worth mentioning that the amounts of oxygen species involved in the CH 4 reaction become nearly the same for both supported catalysts. This result supports once again the XPS data on the extraction of Pd 3+/4+ ions from the perovskite structure concomitant with the formation of PdO-like phase over Pd-LMO/U-AACVD surface during the catalytic test (Figure 9).
According to the CH 4 -TPR data for both catalysts, the process of oxygen consumption begins at temperatures above 200 • C and proceeds with a very high rate at temperatures above 500 • C. Therefore one could suppose that at the temperature range of 200-500 • C the methane from gas phase reacts with the lattice oxygen with higher reactivity (thus producing CO 2 mainly), while at higher temperatures a process of incomplete oxidation of CH 4 to CO occurs. The highly reactive lattice oxygen is most probably associated with palladium ions from dispersed PdO-like phase, while oxygen species bonded to transition metal ions from other oxide phases are not enough reactive to convert methane to CO 2 (and H 2 O). However, their role could be related to the oxygen supply to Pd-active sites during the methane oxidation. Supporting this statement, the H 2 -TPR curves revealed the different reducibility of transition metal ions depending on the method of synthesis: the reduction peak for Pd-LMO/Imp at 335 • C is shifted with 20 • C to the lower temperatures in comparison with Pd-LMO/U-AACVD ( Figure 4A). This indicates an occurrence of more mobile oxygen species linked to transition metal ions for Pd-LMO/Imp. These oxygen species are able to restore the oxygen vacations after methane reaction with the Pd-bonded oxygen.
Due to the well dispersed PdO-like phase and of oxygen storage capability of transition metal ions, Pd-LMO/Imp sample displays a better catalytic performance. Catalytic activity tests under the basic conditions (20% oxygen, absence of an additional amount to that originated from the reaction water vapor) demonstrate that the light-off temperature (T50) for Pd-LMO/Imp is lower with about 44 • C in comparison with that of Pd-LMO/U-AACVD (T50 is at 436 • C). The so proposed scheme of participation of oxygen species in CH 4 oxidation is in agreement with Mars-van Krevelen mechanism: adsorbed CH 4 reacts with lattice oxygen species leading to a formation of adsorbed products such as CO 2 and H 2 O concomitant with the creation of oxygen vacancies. The next step is associated with desorption of CO 2 and H 2 O molecules together with regeneration of lattice oxygen species by refilling of oxygen vacancies. The intimate contact between PdO-like phase and transition metal oxides favors the oxygen exchange between them (see also Figure 2b).
In comparison with Pd-LMO/Imp, the Pd-LMO/U-AACVD sample contains relatively higher amounts of the perovskite phase and it undergoes a significant change after the catalytic reaction due to the extraction of Pd from the perovskite structure during the catalytic reaction. As it was reported [15], a significant improvement in the oxidation activity of the materials was observed when the noble metal is not completely reintegrated into the perovskite crystal. Supports that possess high oxygen mobility can behave as oxygen suppliers to the Pd-active sites [42,50]. As it was reported earlier, the Co 2+ ions can be active sites for dissociative adsorption of oxygen molecules thus forming activated oxygen species [51].

Preliminary Data on Possible Practical Application
The thermal stability of supported catalysts was examined in order to test their potential practical applicability. The results show that the catalyst exhibited higher activity (Imp) was examined by aging treatment at 505 • C for 120 h in air, containing 1.2 vol.% water vapor. The tests result showed an increase in T50 with 5 • C, which difference is in the experimental error margin and therefore it can be considered as promising regarding the possible practical application of the synthesized material.
On the basis of the results from tests with a fixed bed reactor a metal monolithic substrate was examined for possible practical application in the development of an industrial catalyst for methane combustion by assembling it with the synthesized Pd-containing perovskite active phase. The catalytic support has been obtained by using of VDM ® Aluchrom Y Hf foil (ferritic chrome steel alloyed with yttrium and hafnium, thickness 0.20 mm), thermally treated in air at 925 • C and rolled to create tubes with circular cross-section with length of 110 mm and diameter of 3.5 mm so representing the geometrical configuration of a single monolithic channel. Experimental data from the catalytic tests with the monolithic sample, prepared on the basis on the wet impregnated sample (Pd-LMO/Imp/Aluchrom) were used for model calculations, performed by using of a two-dimensional heterogeneous model for monolithic reactor, accounting for both external and internal mass transfer effects. The main principles of the applied model are published by Belfiore [52]. Finite difference method was used and the computation code was written in standard Excel ® (Microsoft) program. Figure 10a shows the results from experiments with the monolithic element. The model calculations on the needed amount of catalyst for achieving a specified degree of waste gases purification (for example if 98% of the methane is to be combusted in humid air at adiabatic conditions (Figure 10b,c). published by Belfiore [52]. Finite difference method was used and the computation code was written in standard Excel ® (Microsoft) program. Figure 10a shows the results from experiments with the monolithic element. The model calculations on the needed amount of catalyst for achieving a specified degree of waste gases purification (for example if 98% of the methane is to be combusted in humid air at adiabatic conditions ( Figure  10b,c). Due to the adiabatic effect of the reaction (inlet concentration of methane 0.5 vol.% methane), the temperature at the outlet of the reactor increased by 132 °C. The numerical investigation showed that the desired conversion of 98% can be realized when the length of the monolithic element is about 4.5 times longer than the used for the described above laboratory experiments.

Catalyst Preparation
The modification of γ-alumina with SiO2 is accomplished by impregnation with silica sol (colloidal silica, 40% water solution). The alumina particle sizes varied between 0.5 and 0.8 mm. The content of SiO2 with respect to γ-alumina equals 2 wt. %. The SiO2-modified/γ-alumina is used as a support for catalytic phase deposition. Due to the adiabatic effect of the reaction (inlet concentration of methane 0.5 vol.% methane), the temperature at the outlet of the reactor increased by 132 • C. The numerical investigation showed that the desired conversion of 98% can be realized when the length of the monolithic element is about 4.5 times longer than the used for the described above laboratory experiments.

Catalyst Preparation
The modification of γ-alumina with SiO 2 is accomplished by impregnation with silica sol (colloidal silica, 40% water solution). The alumina particle sizes varied between 0.5 and 0.8 mm. The content of SiO 2 with respect to γ-alumina equals 2 wt. %. The SiO 2 -modified/γalumina is used as a support for catalytic phase deposition.
The first method of active phase deposition consists in conventional wet impregnation of SiO 2 -modified/γ-alumina with a citrate solution containing all metal ions (La, Co, Ni, Fe and Pd A clear solution with a concentration of 0.5 M in regard to La is obtained after stirring at 60 • C, then the solution is cooled down to room temperature. The impregnated support was heated at 400 • C for 3 h in air in order to decompose the citrate salts, and then the obtained solid residue was annealed at 600 • C for 3 h in air. The temperature treatment procedure is suitable for the preparation of substituted perovskites with high specific surface area, as was previously shown [13]. The weight amount of the deposed perovskite is 10%. In addition, the impregnation method is also used for the preparation of Pd-free perovskite LaCo 0.8 Ni 0.1 Fe 0.1 O 3 , supported on SiO 2 -modified/γ-alumina. The second method of active phase deposition is based on the aerosol assisted chemical vapor deposition (AACVD) [53,54] with ultrasonic vaporization at atmospheric pressure [55]. The reaction between the reactant in the gas phase and the alumina support was intensified by pyrolysis at maintaining conditions close to isothermal (400 • C) by operation in a drilled large metal block (Al-alloy). In order to compare the supported catalysts obtained by two preparation methods, the same citric solution of La, Co, Ni, Fe and Pd ions is used. The final heat treatment is also accomplished at 600 • C for 3 h.
For the sake of convenience, the supported catalysts obtained by two methods will be further on denoted as Pd-LMO/Imp and Pd-LMO/U-AACVD, respectively. The Pd-free perovskite LaCo 0.8 Ni 0.1 Fe 0.1 O 3 supported on SiO 2 -modified/γ-alumina is indicated by LMO/Imp.

Catalyst Characterization
The powder X-ray structural analysis was conducted on a Bruker Advance 8 diffractometer with a LynxEye detector (CuKα radiation). All XRD patterns were recorded at 0.02 • 2θ steps of 2 s duration. The computer program WinPLOTR was used for XRD patterns calculation with the pseudo-Voigt function applied to model the peak shape.
The morphologies of catalyst surfaces were observed using a JEOL JSM-5510 scanning electron microscope. The elemental composition of the precursors and target phosphoolivines was determined by energy dispersive X-ray spectroscopy (EDS) using a JSM 6390 scanning electron microscope and an INCA Oxford EDS detector.
The TEM investigations were performed on a TEM JEOL 2100 instrument at accelerating voltage of 200 kV. The specimens were prepared by grinding and dispersing them in ethanol by ultrasonic treatment for 6 min. The suspensions were dripped on standard holey carbon/Cu grids. The TEM micrographs were made using digital image analysis of reciprocal space parameters. The analysis was carried out by the Digital Micrograph software.
The porous texture of the samples was examined by low-temperature (77.4 K) nitrogen adsorption using Quantachrome (Boynton Beach, FL, USA) NOVA 1200e instrument. The specific surface area was evaluated by the BET method at a relative pressure p/p o within the range of 0.10-0.30. The total pore volume is calculated by the Barett-Joyner-Halenda method.
The XPS study was carried out using an ESCALAB MkII (VG Scientific, East Grinstead, UK) electron spectrometer at a base pressure in the analysis chamber of 5 × 10 −10 mbar (during the measurement 1 × 10 −8 mbar), using a Mg Kα X-ray source (excitation energy hν = 1253.6 eV) and an Al Kα X-ray source (excitation energy hν = 1486.6 eV). The instrumental resolution measured as the full width at the half-maximum (FWHM) of the Ag 3d5/2 photoelectron peak is about 1 eV. The energy scale is corrected with respect to the C 1s peak maximum at 285 eV for electrostatic charging. The fitting of the recorded XPS spectra was performed, using a symmetrical Gaussian−Lorentzian curve fitting after Shirley's type of subtraction of the background.
The EPR spectra in the form of a first derivative of the absorption signal was detected by a Bruker EMX plus EPR spectrometer operating in the X-band (9.4 GHz) within the temperature range of 100-500 K. The powdered samples were homogenized and dried at 100 • C before every experiment. The amount of each measurable sample was about 30-40 mg.
Temperature programmed reduction (TPR) experiments were carried out in the measurement cell of a differential scanning calorimeter (DSC), model DSC-111 (SETARAM), directly connected to a gas chromatograph (GC), within the 300-973K range at a 10 K/min heating rate in a flow of Ar:H 2 = 9:1or He:CH 4 = 9:1, the total flowrate being 20 mL/min. A cooling trap between DSC and GC removed the water and CO 2 obtained during the reduction. To perform the ex-situ EPR measurements of the partially reduced oxides, the reduction process was interrupted at selected temperatures and then the samples were cooled down to room temperature in an Ar:H 2 or He:CH 4 flow followed by Ar treatment for 10 min.

Catalytic Activity Measurements
The catalytic activity tests were carried out in a continuous-flow type of reactor and the following testing conditions were applied: catalyst bed volume of 0.5 cm 3 , irregularly shaped particles with an average size of 0.65 mm (fraction of 0.5-0.8 mm), reactor diameter of 7.0 mm, gaseous hourly space velocity (GHSV) of 120,000 h −1 . The effects of the internal diffusion limitations were neglected as the particles of γ-alumina were supported by a thin layer with a thickness of 20-30 µm and results up to conversion degree of about 40% were used for reaction rate calculations. The characteristics of the catalyst bed correspond to a chain of more than 10 ideal-mixing cells along the reactor axis and therefore the flow conditions can be considered to be close to the behavior of an isothermal plug flow reactor (PFR). The inlet concentrations were varied as follows: methane feed concentrations were set at levels of 0.003, 0.010 and 0.023 vol.%, oxygen at levels of 0.3, 1.0, 8.0 and 20.0 vol.%, water vapour at levels of 0, 1.0 and 2.0 vol.%. All gas mixtures were balanced to 100% with nitrogen (4.0). The reproducibility of the measured conversion degrees was established by repeating the tests under identical conditions and the calculated value for the standard deviation (+/−1.75%) was taken from the average values from six measurements. The gas analysis was performed using the mass-spectrometer of the CATLAB (Hiden Analytical, Warrington, UK) system and in addition to it the on-line gas-analyzers of CO/CO 2 /O 2 (Maihak, Hamburg, Germany) and THC-FID (total hydrocarbon content with a flame ionization detector, Horiba, Kyoto, Japan). The catalysts after the catalytic test are subjected to ex-situ XPS analysis.

Conclusions
The wet impregnation and the aerosol assisted chemical vapor deposition are effective chemical techniques for the preparation of supported catalysts for CH 4 combustion. The impregnation of SiO 2 -modified/γ-alumina support with citrate solutions of La, Co, Ni, Fe and Pd leads to a deposition of an intimate mixture of well dispersed PdO-like phase, transition metal oxides and Pd-containing perovskite on the support surface, while the aerosol assisted chemical vapor deposition of the same citrate solution yields a Pd-containing perovskite phase as the main product. The occurrence of dispersed PdO-like phase determines the higher redox ability of catalysts obtained by the impregnation method-the catalyst obtained by impregnation method contains mobile oxygen species in higher content in comparison with that obtained by modified chemical vapor deposition.
The observed better catalytic performance of Pd-LMO/Imp can be attributed to the appearance of well dispersed PdO-like phase, as well as to the higher oxygen storage capability of transition metal ions. The catalyst Pd-LMO/U-AACVD contains relatively higher amounts of the perovskite phase and it undergoes a significant change after the catalytic reaction due to the extraction of Pd from the perovskite structure.
Based on the results from the kinetic models fitting, the reaction pathway over both Pd-LMO/Imp and Pd-LMO/U-AACVD catalysts proceeds most probably through Marsvan Krevelen mechanism (lowest values for RSS, Table 5). The intimate contact between PdO-like phase and transition metal oxides favors the oxygen mobility. The inhibiting effect of water molecules is due to the competition with the oxygen molecules for chemisorption on the oxidized active sites. In addition, the mechanism of Eley-Rideal, where the methane molecules react from gas phase cannot be completely neglected.
The tests results on a hydrothermally aged catalyst that showed higher initial activity (Imp) reveal noticeably weak deactivation, which result can be considered as promising regarding the possible practical application of the synthesized material. The established correlations between synthesis history, structure and morphology of catalysts and their catalytic activity are of significance in order to design more effective monolith catalysts for the reaction of methane combustion at low temperatures.
Preliminary data on the practical applicability of the La(Co 0.8 Ni 0.1 Fe 0.1 ) 0.85 Pd 0.15 O 3based catalyst, supported on monolithic substrate VDM ® Aluchrom Y Hf are obtained. Taking into account the overall high thermal stability of the active phase and the carrier, one may conclude that the prepared catalyst could contribute to response the currently growing demands aimed at reducing methane emissions. Therefore, future research will focus on further optimizing the active phase composition in order to attain high sulfur resistance and new monolithic substrates such as mullite and various shaped stainless steels should be investigated for their resistance to alkali metal oxides (as present in some flue gases.

Data Availability Statement:
The data presented in this study are available on request from the corresponding author.

Conflicts of Interest:
The authors declare no conflict of interest.