Activating effect of cerium in hydrotalcite derived Cu–Mg–Al catalysts for selective ammonia oxidation and the selective reduction of NO with ammonia

Hydrotalcite originated mixed metal Cu–Mg–Al oxide system was doped with various amounts of cerium (0.5 or 3.0 wt%) and tested in the role of catalysts for the selective catalytic oxidation of ammonia to dinitrogen (NH3-SCO) and the selective catalytic reduction of NO with ammonia (NH3-SCR). The activating effect of cerium was observed in both studied processes. However, the CeO2 loading is a very important parameter determining catalytic performance of the studied samples. It was shown that an introduction of cerium into Cu–Mg–Al mixed oxide resulted in its significant activation in the low-temperature NH3-SCR process, independently of the CeO2 loading and a decrease in the efficiency of the NO reduction at higher temperatures, which was more significant for the catalyst with the lower cerium content. In the case of the NH3-SCO process, the introduction of cerium into Cu–Mg–Al mixed oxide resulted in the activation of the low temperature reaction, which was more intensive for the catalyst with lower cerium content. These effects were related to the presence of cerium in the form of crystallites of various size and therefore their different reducibility.


Introduction
Nitrogen containing pollutants, such as nitrogen oxides and ammonia, belong to the most serious environmental problems. Nitrogen oxides, mainly NO and NO 2 , are produced as the side products of fuel combustion processes as well as in the chemical industry (e.g. N 2 O is a side product in nitrogen fertilizer industry). On the other hand, the majority of ammonia emission is related to agricultural activities (e.g. volatilization from livestock wastes, losses from agriculture crops) and additionally to industrial processes and biomass burning [1]. Moreover, increasing ammonia emission is expected in the transportation sector due to the common use of cars with the systems of flue gases purification (e.g. Adblue system in diesel cars and TWC in cars with spark-ignition engines) [2].
The most promising methods for the elimination of nitrogen oxides and ammonia from flue gases are based on their catalytic conversion to the non-toxic products. The NO and NO 2 emitted from stationary and mobile sources are selectively catalytically reduced with ammonia to dinitrogen (NH 3 -SCR, DeNO x ) [3,4]. On the other hand, the technology based on the selective catalytic oxidation of ammonia to dinitrogen (NH 3 -SCO) has been reported to be the most promising for the elimination of NH 3 from oxygen containing flue gases [1].
Among various catalytic systems studied in the processes mentioned above, the hydrotalcite originated mixed metal oxides containing various transition metals seem to be very promising [5][6][7]. Hydrotalcites, also called layered double hydroxides (LDHs), are minerals characterized by the brucite-like network, Mg(OH) 2 , where octahedra of Mg 2? are six-coordinated to OH -. Part of Mg 2? cations is substituted by trivalent aluminum cations, what results in the positive charging of the brucite-like layers compensating by anions (typically CO 3 2-), which, together with the water molecules, are located in the interlayer space of hydrotalcite. It is possible to synthesize materials with the hydrotalcite structure relatively easily under laboratory conditions. In such hydrotalcite-like materials, Mg 2? as well as Al 3? ions can be partially or completely replaced by various di-(e.g. Cu 2? , Co 2? , Ni 2? , Zn 2? ) and/or trivalent (e.g. Fe 3? , Cr 3? ) cations [8]. The range of various metal cations that can be incorporated into the brucite-like layers is relatively broad and is determined by their size, which should be similar to that of Mg 2? in the case of divalent cations and to Al 3? in the case of trivalent cations [8]. Thermal decomposition of hydrotalcite-like materials results in the formation of mixed metal oxides, which due to their relatively high surface area, porosity and homogenous distribution of metal cations are very promising for potential applications in catalysis [9][10][11], including also the processes of selective catalytic reduction of NO with ammonia-DeNO x , NH 3 -SCR [12,13] as well as selective catalytic oxidation of ammonia to dinitrogen-NH 3 -SCO [1,[5][6][7].
Our previous studies have shown that the modification of the hydrotalcite originated Cu-Mg-Al mixed oxide with noble metals (Pt, Pd, Rh) resulted in its significant activation in the low-temperature NH 3 -SCO process [14]. This effect was explained by the activation of oxygen by noble metals for the reaction of ammonia oxidation. Moreover, it was shown that the process of the selective ammonia oxidation proceeds in the presence of the hydrotalcite-based catalysts according to the so called internal selective catalytic reduction (i-SCR) [1,14]. Because of the relatively high cost of noble metals, a cheaper additive, cerium, known to be active in oxygen activation [15,16], was tested in the frame of the presented studies.

Experimental Catalyst preparation
The hydrotalcite-like sample with the intended Cu/Mg/Al molar ratio of 5/62/33 was synthesized by a co-precipitation method using aqueous solutions of the following metal nitrates: . A solution of NaOH (POCh) was used as a precipitating agent. The solutions of nitrates and NaOH were simultaneously added to a vigorously stirred solution containing Na 2 CO 3 (POCh). The pH was maintained constant at 10.0 ± 0.2 by the dropwise addition of NaOH solution. The obtained slurry was stirred at 60°C for a further 120 min, filtered, washed with distilled water and dried. Finally, the sample was calcined in an air atmosphere at 600°C for 12 h and then was kept in a desiccator in order to avoid the possible reconstruction of the hydrotalcite structure. The obtained sample is labelled as Cu 5 Mg 62 Al 33 .
The Cu 5 Mg 62 Al 33 catalyst was doped with cerium by the wet impregnation method using an aqueous solution of Ce(NO 3 ) 3 Á6H 2 O. After impregnation, the samples were dried and calcined in an air atmosphere at 600°C for 12 h. The catalysts doped with 0.5 and 3.0 wt% of cerium are labelled as Cu 5 Mg 62 Al 33 -Ce0.5% and Cu 5 Mg 62 Al 33 -Ce3.0%, respectively.

Characterization of the catalysts
The thermal decomposition of the hydrotalcite-like samples was studied using thermogravimetry coupled with QMS analysis of evolved gases and in situ high temperature XRD. The TGA-DTG-QMS measurements were carried out using a Mettler Toledo 851 e operated under a flow of synthetic air (80 mL min -1 ) in the temperature range of 25-1000°C with a linear heating rate of 10°C min -1 . The gases evolved during the thermal decomposition process were continuously monitored by the quadrupole mass spectrometer ThermoStar (Balzers) connected on-line to the microbalance. In-situ high temperature XRD (HT-XRD) measurements were carried out at 25°C and then from 100 to 900°C with steps of 100°C in air using a PANalytical-Empyrean diffractometer (Cu K a1/2 radiation, k = 1.54060 Å ). Measurements were performed with a sequential temperature increase of 5°C min -1 and with no temperature holding time before each analysis.
The structure and phase composition of the calcined samples were studied by XRD method. The X-ray diffraction patterns of the as-synthesized and calcined samples were recorded in the range of 8°-80°2h with steps of 0.02°2h by a D2 Phaser diffractometer (Bruker) using Cu K a1 radiation (k = 1.54060 Å ).
The chemical composition of the calcined hydrotalcite-like samples (cationic ratio) was determined by an electron microprobe analysis performed on a JEOL JCXA 733 Superprobe (electron probe microanalysis, EPMA).
The specific surface area of the calcined samples was determined by N 2 adsorption at -196°C using a 3Flex (Micromeritics) automated gas adsorption system. Prior to the analysis, the samples were outgassed under vacuum at 350°C for 24 h. The specific surface area was determined using the BET equation. The total volume of pores (at p/p 0 = 0.98) was calculated using the single point model.
The reducibility of the calcined samples was studied by temperature-programmed reduction method (H 2 -TPR). Experiments were carried out in a fixed-bed flow microreactor system starting from room temperature to 1000°C, with a linear heating rate of 5°C min -1 . H 2 -TPR runs were carried out in a flow (10 mL min -1 ) of 5 vol% H 2 diluted in Ar (N5 quality, Messer). The evolution of hydrogen was detected by micro volume TCD (Valco). Prior to the H 2 -TPR runs, the samples were outgassed in a flow of pure helium (20 mL min -1 ) at 600°C for 1 h.
The DR UV-Vis spectra were recorded using an Evolution 600 (Thermo) spectrophotometer. The measurements were performed in the range of 190-900 nm with a resolution of 2 nm.

Catalytic studies
The calcined samples were tested as catalysts for the selective oxidation of ammonia to nitrogen and water vapor (HN 3 -SCO) and for selective catalytic reduction of NO with ammonia (DeNO x , NH 3 -SCR). Catalytic tests were done in a fixed-bed flow microreactor system. The analysis of the reaction products was performed using QMS detector (PREVAC). Prior to the activity tests, the sample of the catalyst (200 mg) was outgassed at 600°C for 1 h in a flow of pure helium (20 mL min -1 ). The SCO-NH 3

Results and discussion
The structure of the hydrotalcite-like sample as well as the transformations of its structure occurring during thermal treatment was analyzed by in situ high temperature XRD measurements and thermogravimetric method coupled with QMS analysis of evolved gases. The results of HT-XRD studies are presented in Fig. 1. It can be seen that the diffractogram recorded at 25°C for the dried sample is typical of the hydrotalcite structure without any reflections of the other phases [8]. Cell parameters of a and c are 0.31 and 2.31 nm, respectively, while the average crystallite size is about 75 nm. The hydrotalcite structure was not damaged after heating the sample to 100°C. The c parameter after heating the sample to 100°C was not changed indicating that water molecules located in the interlayer space of the hydrotalcite-like materials was not removed within this temperature range. An increase in temperature from 100 to 200°C significantly reduced the c parameter from 2.31 to 2.07 nm. Thus, it could be concluded that water molecules were intensively evacuated form the interlayer space of the sample in this temperature range. A further increase in temperature to 300°C resulted in a gradual decrease and broadening of the reflections characteristic of the hydrotalcite structure, which is related to the decreasing long distance ordering of the brucite-like layers and their gradual degradation. At higher temperatures, broad reflections at 2h values about 35°, 43°and 64°, characteristic of MgO (periclase), were formed. The intensity of these reflections increased with an increase in temperature, which is related to the progressive growth of this crystalline phase. At 900°C, new reflections at 19°, 31°, 36°, 45°, 59°, 65°and 78°related to the spinel phases (possibly MgAl 2 O 4 and CuAl 2 O 4 ) appeared in diffractograms [5,17]. Moreover, the reflections at 31°, 36°, 61°, 73°and 78°could be possibly assigned to cuprite (Cu 2 O) [18]. However, it should be noted that the reflections at 31°, 36°, 78°are probably superpositions of the reflections characteristic of Cu 2 O and spinel phases, while the reflection at 61°is possibly a superposition of the reflections characteristic of Cu 2 O and MgO. Thus, only the weak and broad reflection located at 73°could individually represent the Cu 2 O phase and therefore the presence of cuprite in the sample cannot be fully confirmed by in situ XRD studies.
Thermogravimetry coupled with the QMS (quadrupole mass spectrometry) analysis of the released gases (TG-QMS) was another technique used in the studies of thermal deposition of the hydrotalcite-like materials into mixed metal oxides. The results of these studies are presented in Fig. 2. The thermal behavior of hydrotalcites is generally characterized by two main transitions: (i) the first one is related to the loss of interlayer water without collapse of the hydrotalcite structure at relatively low temperatures, while (ii) the second one is associated with the elimination of hydroxyl groups from the brucite-like layers and the decomposition of interlayer anions at higher temperatures [8]. The temperature ranges of these two transitions depend mainly on the composition of the hydrotalcite-like materials.
The DTG peak related to the release of interlayer water from the Cu 5 Mg 62 Al 33 sample is spread from room temperature to about 230°C with two minima located at 106 and 204°C. The low temperature minimum is possibly related to the water adsorbed on the outer surface of the sample grains or crystallites, while the second one, with a minimum at 204°C, could be attributed to the release of interlayer water molecules [18]. These results are fully consistent with the HT-XRD studies, which showed that the c parameter determined for the Cu 5 Mg 62 Al 33 sample thermally treated at 25, 100 and 200°C is 2.32, 2.31 and 2.07 nm.
The second stage of the hydrotalcite-like samples decomposition, including dehydroxylation of the brucite-like layers as well as decomposition of interlayer anions, is represented by the DTG peaks and maxima of H 2 O and CO 2 evolution located in the range of 230-500°C. The release of CO 2 and H 2 O from the studied sample is represented in this temperature range by double maxima at about 335-350°C and 385-405°C. Thus, the OHanions in the brucite-like layers as well as interlayer carbonates are differently stabilized in the studied sample. The comparison of the locations for these peaks with temperatures of pure Al(OH) 3 and Mg(OH) 2 dehydroxylation, which was reported to be about 300°C [19] and 380°C [20], respectively, may suggest that the low-temperature peak could be related to the release of OHanions attached to aluminum, while the high-temperature peak to the dehydroxylation of OHbounded to Mg 2? cations. It should be noted that small amount of CO 2 is released at temperatures about 640°C. A similar effect of the carbonate stabilization in the copper containing hydrotalcite-like samples was observed in our previous studies [18,21]. Thus, it seems that the formation of such stable carbonates is characteristic of hydrotalcite-like materials containing copper.
Based on the HT-XRD and TG-QMS studies, it was decided to calcine the hydrotalcite-like sample at 600°C for 12 h. As it was shown under such conditions of thermal treatment, the hydrotalcite-like sample was thermally decomposed to mixed metal oxide system. Fig. 3 presents diffractograms recorded for the dried hydrotalcite-like sample and its calcined form (600°C/12 h) as well as the calcined samples doped with cerium (0.5 and 3.0 wt%). The diffractogram recorded for the calcined Cu 5 Mg 62 Al 33 sample contains the reflections characteristic of periclase (MgO) at 2h values of 36°, 43°and 64°. The deposition of cerium resulted in an appearance of the reflection characteristic of CeO 2 at about 29°, 47°and 56°. It should be noted that these reflections are present in the samples with as low cerium content as 3 wt%, which could be explained by the tendency of cerium to the formation of aggregated crystallites on the surface of the calcined hydrotalcite sample. In the case of the Cu 5 Mg 62 Al 33 -Ce3.0% sample, the average size of CeO 2 crystallites, determined using the Scherrer method, is about 2.9 nm. The reflections characteristic of CeO 2 in diffractogram recorded for Cu 5 Mg 62 Al 33 -Ce0.5% are characterized by very low intensity, and therefore the determination of the average size of CeO 2 crystallites in this case is impossible. However, the size of CeO 2 crystallites in the Cu 5 Mg 62 Al 33 -Ce0.5% sample is significantly lower in comparison the size of crystallites in Cu 5 Mg 62 Al 33 -Ce3.0%.
The chemical composition and specific surface area of the calcined hydrotalcitelike samples are presented in Table 1. The determined cation ratios in the studied samples are very close to the intended theoretical values. In the case of the studied samples, very similar values of specific BET surface areas of about 240 m 2 g -1 were determined.
The form and aggregation of copper in the Cu 5 Mg 62 Al 33 sample and its modifications with cerium were studied by UV-Vis-DR spectroscopy (Fig. 4). The spectra recorded for all the studied samples consist of the intensive asymmetric band, which is a superposition of the peaks indicating the charge-transfer between mononuclear Cu 2? ion and oxygen as well as copper in the form of oligomeric   [22][23][24]. Moreover, a very broad band centered at about 745-790 nm, is attributed to the d-d transition in Cu 2? located in a distorted octahedral coordination [25,26]. The insert in Fig. 4 presents the result of deconvolution of the original spectrum recorded the Cu 5 Mg 62 Al 33 sample in the range characteristic of mononuclear and oligomeric copper species. The sharp band centered at about 260 nm is assigned to mononuclear Cu 2? ions, while the less intensive peak at about 345 nm is related to the presence of copper in the form of oligomeric copper oxide species. A comparison of the intensity of these bands leads to the conclusion that copper in the form of mononuclear Cu 2? cations dispersed in the Mg-Al oxide matrix dominates in the studied sample. Thus, it could be concluded that copper is present in the well dispersed forms in the studied samples: monomeric and small oligomeric species dispersed in the Mg-Al oxide matrix. It should be also noted that introduction of cerium to the calcined hydrotalcite sample did not influence significantly the type and distribution of coppers species.
The reducibility of the hydrotalcite-based catalysts was studied by temperatureprogrammed reduction (H 2 -TPR) method (Fig. 5). The only peak in the H 2 -TPR profile of the Cu 5 Mg 62 Al 33 sample centered at about 230°C is related to the reduction of Cu 2? to Cu 0 [18,27]. The introduction of cerium to the Cu 5 Mg 62 Al 33 sample resulted in a shift of this peak into higher temperatures by about 8-10°C and the appearance of two additional peaks. The first one located at about 110°C is possibly attributed to the reduction of surface Ce 4? to Ce 3? by atomic hydrogen species [28,29]. Such species are formed by the dissociation of the H 2 molecules on the copper surface and then are spilled over to reduce the ceria surface oxygen [29]. The second peak located at 740 and 790°C for the samples doped with 0.5 and 3.0 wt% of cerium is related to the reduction of Ce 4? to Ce 3? in CeO 2 [29]. A comparison of the peak intensities and positions leads to the conclusions that there is only a small contribution of surface cerium reduced at low temperatures by atomic The Cu 5 Mg 62 Al 33 sample and its modifications with cerium were tested in the role of the catalysts for the selective catalytic reduction of NO with ammonia (NH 3 -SCR, DeNO x ) and the selective catalytic oxidation of ammonia (NH 3 -SCO).
In the case of the DeNO x process, dinitrogen is a desired reaction product, while N 2 O is an undesired side product. The results of the catalytic studies of this process are presented in Fig. 6. The NO reduction by ammonia in the presence of the Cu 5 Mg 62 Al 33 catalyst started at about 125°C and gradually increased to 92% at 325°C. Above this temperature, the efficiency of the DeNO x process decreased due to the side process of direct ammonia oxidation by oxygen present in the reaction mixture. It should be noted that the selectivity to dinitrogen in the whole studied temperature range is very high (above 90%). The introduction of cerium to the Cu 5 Mg 62 Al 33 sample resulted in its significant activation in the low-temperature DeNO x process. In the case of the Cu 5 Mg 62 Al 33 -Ce05% and Cu 5 Mg 62 Al 33 -Ce3.0% catalysts, the NO conversion profiles were shifted in the direction of lower temperatures by about 30-50°C in comparison with the Cu 5 Mg 62 Al 33 sample. This effect was practically independent of the content of introduced cerium. The maximum of the NO conversion on the level of 96% was obtained for both cerium modified catalysts at about 275°C. Above this temperature, a significant decrease in the efficiency of the NO conversion, much more intensive in comparison with the Cu 5 Mg 62 Al 33 catalyst and related to the side process of direct ammonia oxidation by  Reac Kinet Mech Cat oxygen, was observed. It should be noted that there are significant differences in the catalytic performances of the samples with various content of cerium in the high temperature range. In the case of the catalyst with lower cerium content (Cu 5 Mg 62 Al 33 -Ce0.5%), a decrease in the efficiency of the NO conversion was significantly more significant in comparison to the sample with higher cerium loading (Cu 5 Mg 62 Al 33 -Ce3.0%). It should also be noted that the selectivity to dinitrogen, obtained in the presence of the cerium doped catalysts, is very high (above 95%) in the whole studied temperature range, including also the high temperature region, where the process of direct ammonia oxidation dominated. Thus, it could be concluded that in this range, ammonia was oxidized nearly exclusively to dinitrogen. The activating role of cerium for the low-temperature DeNO x process could be explained by supplying reactive oxygen as a result of the Ce 4? to Ce 3? reduction occurring on the surface of CeO 2 crystallites being in the contact with copper species (low temperatures) as well as in bulky CeO 2 (high temperatures). As it was shown by H 2 -TPR studies (Fig. 5), the size of CeO 2 crystallites is a very important factor influencing the reduction temperature of Ce 4? to Ce 3? . The Ce 4? cations in larger CeO 2 crystallites are significantly more effectively stabilized in comparison with such cations in smaller CeO 2 crystallites. The majority of the mechanisms suggested for the DeNO x process include ammonia chemisorption and activation as a crucial step [30]. In such mechanisms, oxygen could react with chemisorbed ammonia molecules resulting in proton abstraction and the formation of -NH 2 adsorbed species, which could be converted by NO to dinitrogen and water molecules (-NH 2 ? NO ? N 2 ? H 2 O) in the next step. Such a mechanism was suggested in the scientific literature for various catalytic systems [30]. However, it needs verification in the case of the studied catalysts. Active oxygen supplied by CeO 2 could also oxidize NO to very reactive NO 2 . The reduction of the mixture of NO and NO 2 with ammonia, known as fast-SCR [31,32], typically proceeds much more effectively in comparison to the reduction of NO with ammonia. The possible oxidation of NO to NO 2 over the studied catalysts was tested by temperature programmed reaction of NO with oxygen. The results of these studies are presented in Fig. 7. It can be seen that the oxidation of NO to NO 2 occurred for all the studied catalysts. However, in the case of the samples doped with cerium, this effect is more significant. Therefore, it is possible that the activation role of cerium is related to the intensification of the process of NO to NO 2 oxidation. Of course, the activation of the chemisorbed ammonia molecules by cerium cannot be excluded in this stage of the studies. A decrease in the efficiency of the NO 2 formation observed at higher temperatures is due to thermodynamic limitation (the equilibrium profile is presented in Fig. 7). A more intensive decrease in the NO conversion observed for the samples with the lower cerium loading  crystallites in the process of direct ammonia oxidation than larger crystalizes of cerium oxide. Taking into account the results of H 2 -TPR studies (Fig. 5), this effect is not surprising because the smaller crystallites were reduced (Ce 4? ? Ce 3? ) at significantly lower temperature than larger crystallites of CeO 2 . The second studied process was the selective catalytic oxidation of ammonia (NH 3 -SCO) to dinitrogen, which is a desired reaction product. Nitrogen oxides (NO, N 2 O and NO 2 ) are the undesired side products. The results of the catalytic studied of this process are presented in Fig. 8. The NH 3 conversion in the presence of the Cu 5 Mg 62 Al 33 catalyst started at about 275°C and continuously increased up to 500°C, when ammonia was completely converted in the reaction stream. The introduction of a small amount of cerium (0.5 wt%) significantly activated the Cu 5 Mg 62 Al 33 sample for the low-temperature ammonia oxidation. The ammonia conversion profile of the Cu 5 Mg 62 Al 33 -Ce0.5% catalyst was shifted to lower temperatures by about 25°C relative to Cu 5 Mg 62 Al 33 . An increase in cerium loading to 3.0 wt% only very slightly increased the ammonia conversion in comparison to the Cu 5 Mg 62 Al 33 catalyst. Such a significant difference in the activation effect observed for the samples doped with various amounts of cerium is probably, similarly to the DeNO x process, a result of different stabilization of oxygen in smaller and larger CeO 2 crystallites. The catalyst containing CeO 2 in the form of smaller, and therefore easier reduced (Ce 4? ? Ce 3? ) crystallites was found to be more active in the low temperature range in comparison of the samples with larger crystallites of cerium oxide, which needs higher temperature to reduce Ce 4? to Ce 3? . N 2 and NO were the main products of ammonia oxidation. Moreover, small amounts of N 2 O were formed (not shown in Fig. 8). The formation of any other nitrogen containing products, including NO 2 , was not detected. Dinitrogen and water vapor were the only products of ammonia oxidation at temperatures below 450°C. The formation of NO started in the presence of the most active Cu 5 Mg 62 Al 33 -Ce0.5% catalyst at 450°C, while in the case of Cu 5 Mg 62 Al 33 and Cu 5 Mg 62 Al 33 -Ce3.0%, the temperature was about 500°C. This effect, similarly to the differences in the ammonia conversion, could be explained by various size of CeO 2 crystallites, which can release reactive oxygen at different temperatures. Therefore, in the case of the Cu 5 Mg 62 Al 33 -Ce0.5% sample oxidation of ammonia can be ''deeper'' and results in the formation of NO at lower temperatures than for Cu 5 Mg 62 Al 33 -Ce3.0%.

Conclusions
The hydrotalcite originated mixed metal Cu-Mg-Al oxide system was found to be effective catalyst for the selective reduction of NO with ammonia (DeNO x , NH 3 -SCR) and selective oxidation of ammonia to dinitrogen (NH 3 -SCO). The further catalytic activation of such metal oxide system, especially for the low-temperature processes, was possible by the deposition of small amounts of cerium (0.5 or 3.0 wt%) using the impregnation method. It was shown that in the case of the DeNO x process doping of mixed metal Cu-Mg-Al oxide with cerium resulted in its significant activation in the low-temperature range independently of the cerium loading. On the other hand, a decrease in the NO conversion, related to the side process of direct ammonia oxidation, was less intensive for the sample with higher cerium loading. In the case of the NH 3 -SCO process, only doping of the calcined hydrotalcite with a small amount of cerium (0.5 wt%) resulted in a significant activation of the catalyst for the low-temperature process. These differences in the catalytic properties of the samples loaded with various amounts of cerium were explained by the different size of deposited CeO 2 crystallites. It was shown that the reduction of Ce 4? to Ce 3? occurs in smaller CeO 2 crystallites at temperatures significantly lower in comparison with the larger crystallites of cerium oxide. It seems that the redox properties of the catalysts are crucial for their performance in the studied processes.