The influence of Ni stability, redox, and lattice oxygen capacity on catalytic hydrogen production via methane dry reforming in innovative metal oxide systems

Finding a robust catalytic system for hydrogen production via dry reforming of methane (DRM) remains a challenge. Herein, MNi0.9Zr1−xYxO3 (M = Ce, La, and La0.6Ce0.4; x = 0.00, 0.05, 0.07, and 0.09) catalyst was prepared by the sol–gel method, tested for DRM and characterized by surface area and porosity, X‐ray diffraction, H2‐temperature programmed reduction, thermogravimetry, and transmission electron microscopy. In La0.6Ce0.4NiO3 catalyst, the substitution of Ni by 0.1% Zr results in a constant high catalytic activity (83% hydrogen yield at 800°C) due to the presence of reducible “NiO‐species interacted strongly with the support” (stable metallic Ni over reduced catalyst) and redox input by ceria phase for laying instant lattice oxygen during lag‐off period of CO2. Substitution of Ni by Zr and Y in the CeNiO3 catalyst system nurtures Ni3Y (providing highly stable metallic Ni for CH4 decomposition) and cerium yttrium oxide phases (providing strong redox input). CeNi0.9Zr0.01Y0.09O3 shows 85% H2 yield at 800°C.


| INTRODUCTION
Dry reforming of methane (DRM) is an endothermic high-temperature reaction of CH 4 and CO 2 , where the successful industrialization of this reaction would reduce the concentration of those two greenhouse gases. DRM yields hydrogen-rich syngas with significant synthetic utility, where hydrogen is a promising energy source with three times the gravimetric energy density of gasoline (143 MJ/kg vs. 46.4 MJ/kg). 1 The syngas serves as feedstock for Fischer-Tropsch process, acetic acid synthesis, ethyl acetate synthesis, and dimethyl synthesis. [2][3][4] For the DRM reaction, the catalytic community utilized highly ordered perovskite material. In the literature, the responsible site for the reaction is extensively discussed. The general formula for perovskite is ABO 3 , where A is the site at the corner of the unit cell, B is the site at the center of the unit cell, and oxygen is fitted over the faces of the unit cell. Generally, the cation at the B sites is the active site for CH 4 decomposition, whereas the cations at the A sites are basic sites involved in CO 2 interaction. BaZrO 3 is completely inactive toward DRM, indicating that Zr is not involved in the reaction. However, after replacing the B site (Zr) with noble metals, such as Pt, Ru, and Rh, DRM-active sites are developed on the surface of the catalyst. 5 Among the three BaZr 1−x Pt x O 3 , BaZr 1−x Ru x O 3 , and BaZr 1−x Rh x O 3 catalysts, the BaZr 1−x Rh 1−x O 3 catalyst is readily reducible and thus highly active, as evidenced by its 68% H 2 yield with the 0.82 H 2 /CO ratio. Basic La 2 O 3 interacts more strongly with acidic CO 2 than barium. La 2 O 3 forms dissociable La 2 O 2 CO 3 species (La 2 O 3 + CO 2 → La 2 O 2 CO 3 ) by combining with CO 2 . The La 2 O 3 oxidizes carbon via the following reaction La 2 O 2 CO 3 + C → 2CO + La 2 O 3 6 and La 2 O 2 CO 3 + CH 4 → La 2 O 3 + 2CO + 2H 2 . 7 LaRhO 3 perovskite, however, possessed both Rh at the A site for CH 4 dissociation and La at the B site for CO 2 interaction. Nevertheless, this perovskite possesses little DRM reaction activity. 8 It indicates that other factors also contribute to the high DRM catalytic performance. Co may be an option for those pursuing inexpensive active metal sites for DRM. Similar to noble metals, step sites of Co (111) are found to be active for CH 4 dissociation. 9 Once more, Co (211) undergoes surface oxidation; therefore, perovskite LaCoO 3 was examined for DRM. It requires an activation time for the reduction of Co +3 to Co, and it does not form the La 2 O 2 CO 3 phase during the reaction. 10,11 Moreover, Ni is a more affordable and preferred option for DRM than Co. Metal Ni has a significantly lower steric repulsion with CH 4 than Co does. Therefore, the interaction energy of CH 4 with Ni is 25 times greater than that of CH 4 with Co. 12 Ni possessed lower CH 4 dissociation energy than noble metals likewise. 13 It is interesting to note that substituting Co by Ni in cobalt-rich perovskite (LaNi 1−x Co x O 3 ; x ≥ 0.6); La 2 O 2 CO 3 species are not formed in spent catalyst, whereas in Ni-rich perovskite (LaNi 1−x Co x O 3 ; x ≤ 0.4); La 2 O 2 CO 3 species is formed. 7 Thus, Ni-rich perovskite had the additional advantage of the presence of the La 2 O 2 CO 3 phase, which is a key element for carbon removal or CH 4 oxidation into syngas. 7 Due to the in situ formation of metallic Ni Co active sites highly dispersed on the La 2 O 2 CO 3 matrix, LaNi 1−x Co x O 3 (x ≤ 0.8) showed more than 80% H 2 selectivity close to thermodynamic equilibrium (~100%). If Ni completely replaces Co, the LaNiO 3 perovskite structure is formed. LaNiO 3 drew the most attention because it perfectly initiates the DRM reaction by interacting with CO 2 at the A site (at La) and CH 4−x (x = 0-4) at the B site (at Ni).
In addition, this structure allows for numerous substitutions at the B site (Ni) by the cations Mg, Al, Mn, Fe, Co, Fe, Cu, Zn, Ce, Rh, and Ru, as well as at the A site (La) by the following cations Ca, Ba, Sr, Ce, and Pr. 7,12, Substitution of A site ions with those of a lower oxidation state can affect the stability in a reducing environment and the movement of oxygen ion vacancies. Thus, a decrease in valency is compensated by the formation of oxygen vacancies. 41 Khalesi et al. demonstrated that substitution of La with Sr in LaNi 0.3 Al 0.7 O 3 causes the organization of solid solution, enhanced porosity, and reduction of Ni +2 at higher reduction temperature (due to strong metal support interaction). The catalyst (with prior reduction) resulted in 80% H 2 yields at H 2 /CO > 1 at 750°C. 39 The catalytic activity of perovskite-type oxides is directly related to their reduction properties. 42 It was reported that the substitution at the A site in perovskites such as La 1−x Ce x NiO 3 enhanced the activity of methane combustion. 43 Individual metal oxide phases exist in place of the perovskite phase when La is replaced by Ce at the A site of the LaNiO 3 catalyst, as demonstrated by Su et al., and the reducibility of the La 1−x Ce x NiO 3 catalyst increases with substitution. 31 However, among different metal oxides, such as alumina, 44 silica, 45 ceria, 46 lanthana, 47 and zirconia; zirconia was found to be a superior metallic Ni carrier due to its heat resistance and unique properties, such as its reduction-oxidation and acid-base features. 48 Santamaria et al. 41 demonstrated that ZrO 2 is an appropriate support for Ni because it limits coke formation with low carbon combustion temperatures. In the CeNi 0.9 Zr 0.1−x (x = 0, 0.03, 0.05, and 0.07) catalyst system, Lanre et al. demonstrated the absence of perovskite phase, presence of individual metal oxide phases, and the presence of mixed yttrium cerium oxide. 49 Dezvareh et al. showed that on substitution La by 0.1% Ce at the A site and Ni by 0.2% Zr at the B site, metal support interaction increased, resulting in increased activity. 40 The synergic effect of two metal oxides (ceria-lanthana) on redox and basicity function was previously utilized in the catalytic combustion of methane 50 and oxidation of CO. 51 Overall, "CH 4 decomposition over Ni," "strong interaction of CO 2 with lanthanum," "redox cycle and CO 2 activation with ceria," and "instant lattice oxygen availability by Zr and Y" are some catalyst merits in favor of DRM. For DRM reaction, "CeNiO 3 and LaNiO 3 " catalysts and substituted catalyst MNi 0.9 Zr 1−x Y x O 3 (M = Ce, La, and La 0.6 Ce 0.4 ; x = 0.00, 0.05, 0.07, and 0.09) are considered in light of these properties. The catalyst is synthesized following the propionic acid-assisted sol-gel method. This catalyst series is characterized by surface area and porosity, X-ray diffraction (XRD), H 2 -temperature programmed reduction (TPR), thermogravimetry, and transmission electron microscopy. The fine tuning of DRM catalytic activity in terms of H 2 yield is established using characterization results.

| Catalyst preparation
The perovskite catalysts were prepared by the sol-gel method, with propionic acid acting as a solvent to dissolve the nitrates of each metal. In the preparation, La (NO 3 ) 3 ·6H 2 O, Ni(NO 3 ) 2 ·6H 2 O, Ce (NO 3 ) 3 ·6H 2 O, Y(NO 3 ) 3 · 6H 2 O, ZrO(NO 3 ) 2 ·6H 2 O, and propionic acid (C 3 H 6 O 2 ) were all obtained from Sigma-Aldrich. The nitrates were separately dissolved in propionic acid, stirred, and heated at T = 90°C with oil as a heating medium in a closed vessel. Afterwards, the solutions were mixed and stirred continuously for about 2 h at T = 130°C. After that, the propionic acid was evaporated using a rotary evaporator at T = 70°C until a gel was formed. The gel obtained was dried at T = 90°C overnight and calcined at 725°C for 4 h. The calcined catalysts were ground into powder and used for the DRM reaction.

| Catalyst characterization
The perovskite catalysts' surface area, as well as the pore size distribution, was measured by N 2 adsorption-desorption at −196°C using a Micromeritics Tristar II 3020 for porosity and surface area analyzer. For H 2 -TPR analysis, 70 mg of the sample was loaded in the sample holder (Micromeritics apparatus). H 2 -TPR measurements were performed at 150°C using Ar gas for 30 min. Afterwards, it was cooled to ambient temperature. The next step involved heating by the furnace up to 800°C, ramping at 10°C/min in an atmosphere of H 2 / Ar mixture (1:9 vol%) flowing at 40 mL/min. The H 2 O molecules are removed from a gas stream by using a cold trap. The thermal conductivity unit recorded the consumption of H 2 during the operation. The XRD patterns of the perovskite catalysts were recorded on a Miniflex Rigaku diffractometer that was equipped with CuKα X-ray radiation. The device was run at 40 kV and 40 mA. The quantity of carbon deposit on the spent catalysts was measured by the thermogravimetric analysis (TGA). Here, a platinum pan was filled with 10-15 mg of the used catalysts. Heating was done at room temperature up to 1000°C at a 20°C/min temperature ramp. Change in mass was constantly monitored as the heating progressed.

| Catalytic activity test
The catalysts were tested for DRM at 700°C, 750°C, and 800°C reaction temperatures under atmospheric pressure. A packed bed stainless steel reactor (internal diameter, 0.0091 m; height, 0.3 m) was used to perform the experiments. A catalyst mass of 0.10 g was positioned in the reactor over a ball of glass wool. Stainless steel, sheathed thermocouple K-type, axially positioned close to the catalyst bed was used to determine the temperature during the reaction. Before reaction, activation of the perovskite catalysts was performed at 700°C in an atmosphere of H 2 . This lasted for 60 min, and the remnant H 2 was purged with N 2 . During the dry reforming reaction, the feed volume ratio was kept at 3:3:1 for CH 4 , CO 2 , and N 2 gases, respectively, with a space velocity of 42 L/h/g cat . The outlet gas from the reactor was connected to an online Gas Chromatography with a thermal conductivity detector to analyze its composition. The hydrogen yield was computed thus

| Catalyst activity results
Hydrogen yield versus time on stream plot for dry reforming reaction over different catalyst samples is shown in Figure Figure 1C). DRM is a highly endothermic reaction that favors high-temperature reactions. Here, the effect of Ni substitution with 0.1% Zr can be seen clearly. La 0.6 Ce 0.4 NiO 3 exhibited an initial H 2 yield of 80%, but the activity decreased to 75% after 420 min time on steam at 800°C reaction temperature ( Figure 1D), whereas over La 0.6 Ce 0.4 Ni 0.9 Zr 0.1 O 3 , the activity not only increased but also remained constant at~83% up to 420 min time on stream at 800°C. This indicates that catalytic active sites for CH 4 decomposition are unaffected by adding 0.1% Zr and that stable catalytic performance is maintained. Furthermore, upon substitution of Ni by 0.05% Zr-0.05% Y, 0.03% Zr-0.07% Y, and 0.01% Zr-0.09% Y at B site in  Figure 1E). Again, for CeNi 0.9 Zr 0.05 Y 0.05 O 3 , CeNi 0.9 Zr 0.03 Y 0.07 O 3 , and CeNi 0.9 Zr 0.01 Y 0.09 O 3 catalysts, the H 2 yield is 74%, 80%, and 83%, respectively. The H 2 /CO ratio over CeNi 0.9 Zr 0.1−x Y x O 3 (x = 0.05, 0.07, and 0.09) and CeNi 0.9 Zr 0.1−x Y x O 3 (x = 0.05, 0.07, and 0.09) catalysts system was always above one as well as it increases notably on progressive substitution of Zr by Y (Supporting Information Figure S1A,B). This indicates that reverse water gas shift reaction (which is DRM competitive reaction and H 2 consuming reaction) is retarded efficiently over these catalysts systems. Other activity parameters, like, CH 4 conversion and CO 2 conversion, are also comparable with H 2 yield (Supporting Information Figure S1C-H). The CO 2 conversion remains relatively higher than CH 4  yield are noticed. Clearly, the catalytic performance was enhanced by a higher level of Zr substitution with Y, but this enhancement is more pronounced in CeNi 0.9 Zr 0.1−x Y x O 3 (x = 0.05, 0.07, and 0.09) catalysts system than La 0.6 Ce 0.4 Ni 0.9 Zr 0.1−x Y x O 3 (x = 0.05, 0.07, and 0.09) catalyst system (Supporting Information Figure S2).

| Catalyst characterization results
Nitrogen physisorption isotherms, surface parameter specific surface area (SSA), pore volume (PV), pore diameter (PD), and pore size distribution plot of catalyst samples are shown in Figure 2. The SSA of the catalysts based on the Brunauer-Emmett-Teller method range from 1 to 9 m 2 /g which is comparable with reported values in the literature. 52 All the samples are mesoporous and characterized by a type IV adsorption isotherm having a hysteresis loop between H 3 to H 4 . CeNi 0.9 Zr 0.1 O 3 shows the H 3 hysteresis loop, which confirms the presence of a plat-like pore having unlimited adsorption at the high-pressure range (Figure 2A).  Figure 2B-D). Among CeNi 0.9 Zr 0.03 Y 0.07 O 3 and CeNi 0.9 Zr 0.01 Y 0.09 O 3 catalysts, the latter has improved surface parameters (surface area, PV, and PD). The samples have average particle sizes ranging from 12 to 35 nm. 54 When Ni is replaced with 0.1% Zr in La 0.6 Ce 0.4 NiO 3 catalyst, the XRD peak intensity decreased very rapidly (Supporting Information Figure S3), but catalytic activity and stability both increased (discuss in Section 3.1). It has been determined that La 0. 6  It is interesting to note that on substitution of Ni by 0.05% Zr and 0.05% Y (in La 0.6 Ce 0.4 Ni 0.9 Zr 0.05 Y 0.05 O 3 catalyst), the diffraction peak pattern remains unchanged, but the diffraction peak width has increased due to developing in homogeneous strain within a crystallite after yttria introduction ( Figure 3A,B). 53 Upon substitution of Ni by 0.03% Zr and 0.07% Y in La 0.6 Ce 0.4 Ni 0.9 Zr 0.03 Y 0.07 O 3 catalyst, the intensity of every phase has grown many times as well as new La 2 NiZrO 6 phase has appeared (2θ = 24.9°, 32.06°, 37.15°, 46.01°, and 74.44°; JCPDS reference number 00-044-0624) ( Figure 3C,D). Once more, with the substitution of Ni by 0.01% Zr and 0.09% Y, the crystallinity of La 0.6 Ce 0.4 Ni 0.9 Zr 0.01 Y 0.09 O 3 catalyst sample was further increased (Figure 3C,D). That indicates that the diffraction intensity/crystallinity is multiplied by a significant amount when the proportion of Y is greater than 0.05% (La 0.06 Ce 0.04 Ni 0.9 Zr 1−x Y x O 3 ; x = 0.07 and 0.09 catalyst system).
The XRD profiles of Ce 0.4 Ni 0.9 Zr 0.1−x Y x O 3 catalyst (x = 0.00, 0.05, 0.07, and 0.09) are shown in Figure 3E-H. The H 2 -TPR profiles of different catalyst systems are shown in Figure 4. The reduction peaks are shown at more than one temperature region. In literature, reduction peaks at 360°C, 395°C, and 540°C were claimed for progressive reduction of LaNiO 3 into Ni (Ni +3 → Ni +2 → Ni o ), 17 whereas the intensity of the reduction peak at 540°C was higher than rest two reduction peaks. XRD profile has shown the absence of perovskite phases in CeNi 0.9 Zr 0.1−x Y x O 3 (x = 0.00, 0.05, 0.07, and 0.09) catalyst system. As discussed in the XRD results, the perovskite phase is only possible in the lanthanum-rich phase. Consequently, in this instance, the H 2 -TPR peak cannot be correlated with such sequential perovskite-based reduced phases in CeNi 0.9 Zr 0.1−x Y x O 3 (x = 0.00, 0.05, 0.07, and 0.09) catalyst system. Here, the H 2 -TPR profile can be correlated to reduction peaks of reducible species interacting with support in different strengths. CeNi 0.9 Zr 0.1 O 3 catalyst had one intense reduction peak at 346°C for reduction of "NiO-species that interacted weakly with support" as well as reduction of Ce +4 to Ce +3 , 55,56 a shoulder peak at 436°C for reduction of "NiOspecies that interacted moderately with support" and a reduction peak at 536°C for reduction of "NiO-species that interacted strongly with support." Interestingly on substitution of Ce with 0.6% La; in La 0.6 Ce 0.4 Ni 0.9 Zr 0.1 O 3 catalyst system, reduction peak intensity for a low-temperature peak is greatly decreased, and the high-temperature peak is admirably increased ( Figure 4A). It indicates that a lower temperature reduction peak of about 346°C is majorly contributed by the reduction of Ce +4 to Ce +3 than the reduction of NiO-species in CeNi 0.  Figure 4B). XRD of lanthana-rich catalyst system reveals the presence of perovskite and NiO phases. Therefore, the reduction peak in the intermediate temperature region may be a merged reduction peak of Ni +3 → Ni +2 → Ni o , 17 or it may be a reduction peak of "NiO-interacting moderately with the support." 57,58 Similar reduction profiles are observed for CeNi 0.9 Zr 1−x Y x O 3 (x = 0.07 and 0.09) catalysts. If Ce is substituted by La and Ni by Zr and Y, the reduction peak is broadened to the higher reduction temperature. It indicates that substitution at Ce and Ni sites enhances the metal-support interaction.
The 0.07, and 0.09); weight loss increases. The same trend is observed in CeNi 0.9 Zr 0.1−x Y x O 3 (x = 0.00, 0.05, 0.07, and 0.09) catalyst system by Lanre et al. 49 The transmission electron microscope image of La 0.6 Ce 0.4 Ni 0.9 Zr 0.01 Y 0.09 O 3 catalyst (fresh and spent) and corresponding average Ni particle size distribution are shown in Figure 5B,C(b,c). Significant filamentous coke formation is observed over the spent La 0.6 Ce 0.4 Ni 0.9 Zr 0.01 Y 0.09 O 3 catalyst. The average Ni particle size over the spent catalyst is found to be larger (8.05 nm) than the fresh catalyst (5.68 nm).

| DISCUSSION
Generally, in a longer time on stream at high-temperature conditions, active metallic Ni particles are sintered, and the catalyst system is slowly deactivated toward DRM. Substitu  59 Interestingly, on substitution of La by 0.4% Ce; La 0.6 Ce 0.4 Ni 0.9 Zr 0.1 O 3 catalyst has a higher surface area (1.5 times), PV (2.4 times), and PD (1.36 times) than LaNi 0.9 Zr 0.1 O 3 catalyst. La 0.6 Ce 0.4 Ni 0.9 Zr 0.1 O 3 catalyst has La 2 O 3 phase (known for enhanced interaction with CO 2 ), 60 CeO 2 phase (known for lattice oxygen mobility) 61 and LaNiO 3 perovskite phases (known for enhanced lattice oxygen mobility as well as generating exsolved stable metallic Ni). 16 In search of higher catalytic activity, Ni in the La 0.6 Ce 0.4 NiO 3 catalyst is substituted by both "Zr and Y." It should be noted that upon substitution of Ni by both "Zr and Y," surface area and porosity of all catalyst systems had decreased sharply and "NiO-species that strongly interacted with support" has depleted over the catalyst surface. The hydrogen yield of La 0.6 Ce 0.4 Ni 0.9 Zr 0.05 Y 0.05 O 3 catalyst was~72% (against 83% over La 0.6 Ce 0.4 Ni 0.9 Zr 0.1 O 3 at 800°C reaction temperature). Upon further substitution of Ni by "0.03-0.01 Zr and 0.07%-0.09% Y" in La 0.6 Ce 0.4 Ni 0.9 Zr 0.1 O 3 , La 2 NiZrO 6 phase is nurtured, and the catalytic activity progressed relatively. La 0.6 Ce 0.4 Ni 0.9 Zr 0.03 Y 0.07 O 3 and La 0.6 Ce 0.4 Ni 0.9 Zr 0.01 Y 0.09 O 3 catalysts show 76% and 81% H 2 yield at 800°C, respectively. However, the limitation of catalytic activity over La 0.6 Ce 0.4 Ni 0.9 Zr 0.1−x Y x O 3 (x = 0.05, 0.07, and 0.09) seems to be caused by higher weight loss (over spent catalyst surface) upon increasing the proportion of Y in the catalyst sample. The yttrium-containing catalyst sample had a substantial carbon deposit (as per the TGA results), which may have contributed to the shading of the catalytic active site. Therefore, the catalytic activity is found unstable and relatively inferior when Y is incorporated along with Zr in the La 0.6 Ce 0.4 -NiO 3 catalyst.
The higher catalytic activity has been noticed upon substituting of Ni by "Zr and Y" site in the CeNiO 3 catalyst. In CeNi 0.9 Zr 1−x Y x O 3 (x = 0.05, 0.07, and 0.09) series also; for x > 0.05, the intensity of all diffraction peaks is increased several times as like as in La 0.6 Ce 0.4 -Ni 0.9 Zr 0.1−x Y x O 3 (x = 0.05, 0.07, and 0.09) catalyst system. Catalytic active Ni 3 Y alloy, Y 2 O 3 phase and better redox cerium yttrium oxide phases are grown additionally. During the DRM reaction, the Ni 3 Y alloy enhances the metallic Ni stability for dissociation of CH 4 , and redox cerium yttrium oxide endows lattice oxygen instantly for carbon deposit oxidation. It is reported that the addition of Y 2 O 3 in ZrO 2 caused the enrichment of the oxygen layer over the catalyst surface which was potentially consumed during the oxidation of the carbon deposit. 63  . On the basis of the discussion, the DRM reaction mechanism is seemed to be oriented toward two facts: (1) stability of metallic Ni during DRM and (2) redox input by ceria/cerium yttrium oxide phases during DRM (Figure 6). The major attention over the F I G U R E 6 Proposed reaction mechanism over (A) La 0. 6  *, catalyst name is presented as numerical value followed by an element symbol in most of the entries. La 0.6 Ce 0.4 Ni 0.9 Zr 0.1 O 3 catalyst is the presence of the large amount of reducible "NiO-species that interacted strongly with support" which leads "metallic Ni stability" over reduced catalyst during DRM ( Figure 6A). It results in 83% H 2 yield at 800°C. Over La 0.6 Ce 0.4 Ni 0.9 Zr 0.1−x Y x O 3 (x = 0.07 and 0.09) catalyst, there is a loss of reducible "strongly interacted NiO-species" or loss of stability of metallic Ni, which turns into relatively inferior catalyst performance. CeNi 0.9 Zr 0.1−x Y x O 3 (x = 0.07 and 0.09) catalyst is characterized by stable metallic Ni in the form of Ni 3 Y alloy as well as wide redox input by mixed oxide cerium yttrium oxide ( Figure 6B). Highly stable metallic Ni turns CH 4 decomposition whereas redox mixed-metal-oxide/ metal-oxide endows instant lattice oxygen between lag-off periods of CO 2 molecule for carbon deposit oxidation. CeNi 0.9 Zr 0.01 Y 0.09 O 3 catalyst shows the highest 85% H 2 yield at 800°C. On comparing the H 2 yield of our catalyst system with the recent catalyst system, it is found that La 0.6 Ce 0.4 Ni 0.9 Zr 0.1 O 3 . CeNi 0.9 Zr 0.01 Y 0.09 O 3 catalysts are quite competent with 5Ni 2.5 Ce/9La 91 Zr, 5Ni/15La 85 Zr, and 5Ni 4 Ho/13Y 77 Zr and superior to the rest of the catalyst system shown in Table 1.

| CONCLUSION
Substitution of Ni by 0.1% Zr in the La 0.6 Ce 0.4 NiO 3 catalyst system stabilizes the active metal Ni in a long time on stream against high-temperature DRM condition and brings 45% H 2 yield constantly in long TOS. On the other hand, substituting La by 0.4% Ce in LaNi 0.9 Zr 0.1 O 3 , the catalyst gains enhanced surface parameters as well as high redox potential phases, like, CeO 2 and perovskite. Even La 0.6 Ce 0.4 Ni 0.9 Zr 0.1 O 3 has a higher amount of "strongly interacted reducible NiO-species" than CeNi 0.9 Zr 0.1 O 3 . Such ceriacontaining catalyst does not need initial activation times at low temperature (700°C) and it performed optimum 83% H 2 yield at 800°C reaction temperature. However, upon substitution of Ni by "Zr and Y" in La 0.6 Ce 0.4 NiO 3 , loss of reducible "NiO-species that strongly interacted with support" with substantial carbon deposit leads to inferior catalyst performance (than La 0.6 Ce 0.4 Ni 0.9 Zr 0.1 O 3 catalyst) toward DRM. Ceria-based catalyst exhibits enhanced catalytic performance upon substituting Ni by both "Zr and Y" (CeNi 0.9 Zr 0.1−x Y x O 3 ; x = 0.05, 0.07, and 0.09 catalyst). CeNi 0.9 Zr 1−x Y x O 3 (x = 0.07 and 0.09) catalyst has gained Ni 3 Y alloy phase, cerium yttrium oxide phase, and Y 2 O 3 phase. Stability of metallic Ni in the alloy phase, redox input by cerium yttrium oxide, surface oxygen enrichment by Y 2 O 3 phase, and relatively higher surface parameters of CeNi 0.9 Zr 0.01 Y 0.09 O 3 turns into optimum 85% H 2 yield at 800°C.