Role of promoters over yttria‐zirconia supported Ni catalyst for dry reforming of methane

Dry reforming of methane (DRM) bears great hope for the catalytic community as well as environmentalists for its potential to convert two greenhouse gases, CH4 and CO2, together into synthetic feedstock “syngas”. The stable tetragonal zirconium yttrium oxide phase over the “Yttria‐zirconia supported Ni” catalyst (Ni/YZr) brings >70% CH4 conversion against 50% CH4 conversion over zirconia supported Ni catalyst) in 7 h time‐on‐stream (TOS). The use of the second metal oxide (MOx; M = Ho, Ga, Gd, Ba, Cs) in a small amount (4 wt%) over Ni/YZ catalyst is found to promote the catalytic activity further. Herein, we have prepared such metal‐promoted yttria‐zirconia supported Ni catalyst, employed them for DRM and characterized them with surface area porosity, X‐ray diffraction, spectroscopic techniques, temperature programmed techniques and transmission electron microscopy. A fine correlation of characterization results with catalytic activity brings out various useful information that would be useful for establishing yttria‐zirconia supported Ni catalyst for DRM. Ni stabilized over cubic zirconium holmium oxide phase in 5Ni4Ho/YZr catalyst, cubic zirconium gadolinium oxide phase in 5Ni4Gd/YZr catalyst and cubic zirconium barium oxide phase in 5Ni4Ba/YZr catalyst perform excellent toward DRM. Catalytically, 5Ni4Ho/YZr catalyst achieves CH4 conversion as high as ~85% whereas 5Ni4Ba/YZr and 5Ni4Gd/YZr show CH4 conversions of about ~80%. Even in 30 h TOS study, 5Ni4Ho/YZr catalyst showed >81% CH4 conversion with retaining highest H2/CO (0.97).


| INTRODUCTION
Global warming gases (CH 4 and CO 2 ) are converted into syngas (H 2 and CO) over the appropriate catalyst at hightemperature conditions.The process has synthetic utility on the expense of depleting the concentration of global warming gases.The reaction is popularly known as dry reforming of methane (DRM).Methane is dissociated over Pt, Pd, and Ni active metals where Ni has minimal methane dissociation energy. 1 In a detailed DFT calculation, Niu et al. showed that the step/edge site of Ni particle has less energy barrier for C-H dissociation than a flat surface. 2Further, the dissociated carbon species is oxidized into CO by CO 2 .Additionally, the low cost of Ni has drawn huge interest from the DRM community.However, Ni undergoes serious sintering at hightemperature endothermic DRM reactions which leads to severe coke deposition and finally deactivation.5][6] Silica is just a neutral support providing a large surface area (from micropores to mesopores range).][9][10] Addition of <5 wt.% Zr along with 6.14 wt.% Ni over MgAl 2 O 4 support had gained coke-resistant property as there was no plugging.The CH 4 conversion was retained >50% up to 25 h TOS over this catalyst system. 11The current research trend has shifted toward zirconia-supported catalysis due to oxygen-endowing capacity and subsequent vacancy formation in zirconia rather than silica and alumina.Among different ZrO 2 phases, the energy barrier of CO 2 dissociation on the defective surfaces of cubic-ZrO 2 , tetragonal-ZrO 2 , and monoclinic-ZrO 2 was found to reduce 1/2, 1/4, and 1/5 of the perfect surface. 12A series of metal oxide-promoted "zirconia" or "metal oxidezirconia" supported Ni-based catalysts are used for DRM (Figure 1).
The addition of alkali and alkaline earth metals was expected to increase the basicity of the catalyst which induces the CO 2 interaction with the catalyst surface.Apart from basicity, it may also induce surface parameters such as Ni dispersion, metal-support interaction, NiO reducibility, coke resistance, and support stability.0.6Wt.% Na promotional addition caused strong metalsupport interaction due to the formation of NiO x H y species. 13Barium addition induces self-coking of carbon deposits by -OH and -O species and strong sinter resistance by NiO x particles. 14It showed >65% conversion, >0.8 H 2 /CO up to 33 h.0.4Wt.%Mg presence over zirconia supported Ni catalyst which brought higher surface area, stabilized the t-ZrO 2 phase, and coke resistance. 15Using 2:1 ratio of Ni (0.04 mol) and Ca over 0.1 mol ZrO 2 enriched the surface parameters (surface area, pore volume and pore diameter), strengthened metal-support interaction, and induced a low degree of graphitization.It resulted into >70 conversion % 0.7 H 2 / CO ratio and 3 × 10 3 H 2 molar production in 350 min.Upon addition of a high amount of Mg (7Wt.%) over ZrO 2 supported Ni catalyst, a reducible dispersed NiO-MgO solid solution was formed which resulted into 85% CH 4 conversion and 92% CO 2 conversion 0.94 H 2 / CO in 7 h TOS. 16Further, the promotional addition of 0.5Wt.% K over a MgO-ZrO 2 (5:2) supported Ni catalyst caused an increase in surface area and high Ni/Mg ratio.The catalyst retained about 90% CH 4 conversion at 750°C up to 14 h TOS. 17 Promotional addition of p/d/f-block elements in ZrO 2 supported Ni catalysts was found to stabilize zirconia phases, optimize Ni crystallite size, induce the formation of adsorbed oxygen species, and tune the reducibilitybasicity profile.It was reported that Si promotional addition in ZrO 2 support optimized Ni size up to 9-12 nm. 18,19"Ni covered with porous silica" and then sequentially coated with a porous ZrO 2− shell had extra optimized Ni (6 nm).The catalyst caused higher adsorption energy and a lower dissociation barrier of CO 2 than the catalyst without ZrO 2 coating. 19So, it showed more about 60% CH 4 conversion, and a 0.7H 2 /CO ratio up to 150 h at 700°C.5Wt.%Al-5Wt.%Mn modified ZrO 2 supported Ni catalyst had small Ni crystallite, dense basic sites and adsorbed oxygen species.This resulted in >85% conversion up to 5 h TOS. 2015Wt.% Ni incorporated "80%CeO 2 and 20% ZrO 2 " catalyst gave more than 97% CH 4 at 800°C up to 100 h. 215Wt.% Pr, Y, and La promoted ceria-zirconia supported Ni catalysts had similar basicity and similar size of metallic Ni (after reaction) which gave very similar catalytic activity (~90%) with >0.9 H 2 /CO ratio. 22An equal proportion of Mn and Ni over ceria-zirconia (50:50) support was found to facilitate CH 4 decomposition and CO 2 activation through -CH 2 OH species at low temperatures. 23It gave a 27% conversion with a 0.75 H 2 /CO ratio at 550°C.All ceria-zirconia catalyst systems were found excellent, but the high weight percentage of Ni is needed to decrease from an economic point of view.
10 mol%Pr 2 O 3 -zirconia, 10 mol%Nd 2 O 3 -zirconia, 10 mol %La 2 O 3 -zirconia, 10 mol%ceria-zirconia, and 10 mol%Y 2 O 3zirconia supported systems having 1Wt.% Ni dispersion was applied for DRM under an electric field (up to 5 W) and external heat of 150°C. 24The reaction was presumed to initiate the activation of CH 4 and formate species.10 mol% La 2 O 3 -zirconia supported Ni catalyst showed 33% CH 4 conversion, 43% CO 2 conversion and 0.7 H 2 /CO ratio without coke decomposition.Lanthana-zirconia, Samaria-ZrO 2 supported, and yttria-zirconia supported systems were able to optimize Ni particle size to 13.5 nm and to attain conversion of more than >50% with a high H 2 /CO (>0.8) ratio. 25La 2 O 3 promoter over ZrO 2 supported Ni catalyst had increased mesoporosity, increased CO 2 interaction (by La +3 and CO 2 coordination), and increased metal-support interaction (than unsupported one). 26,273Wt.% La addition over ZrO 2 supported Ni catalyst resulted in 70% conversions and >0.9 H 2 /CO at 700°C in 50 h TOS.Ca-promoted lanthanazirconia supported Ni catalyst brought additional basicity, more CO 2 interaction, efficient carbon deposit oxidation, and minimum carbon deposition. 28So, it resulted into ~78% CH 4 conversion, 83% CO 2 conversion and a 0.9 H 2 /CO ratio.Ce promotional addition over this support induced prominent CH 4 decomposition and enhanced mobility of lattice oxygen which resulted into >80% CH 4 conversion. 29,30Ce promotional addition over "tungsten-zirconia supported Ni catalyst" had the additional redox capability and basic sites. 31,32It showed conversion close to 80%.Cr or Gd-promoted catalyst systems had an excellent regeneration capability of reducible NiO whereas Ga-promoted catalyst had increased metalsupport interaction.Cr-promoted catalyst had a low energy gap and enhanced reducibility whereas Gd-promoted catalyst had a stable zirconium-gadolinium phase and strong 3d-4f electron exchange and spin-orbit interaction between Gd and Ni. 28,33,34Cr, Gd, and Ga are quite competitive and shows >80% conversion and >0.9 H 2 /CO ratios.Yttria addition brought additional surface adsorbed oxygen species causing coke gasification.It showed 50-60% conversion and ~64% Hydrogen yield with >0.8 H 2 /CO ratio.6][37] Cs and Sr promoters were applied to a 15Wt.%Y85Wt%ZrO 2 supported Ni system where earlier one showed 56% H 2 yield (and 2.6% weight loss) whereas later one showed 63.4% H 2 yield (and 9.7% weight loss) against 55.3% H 2 -yield over a nonpromoted catalyst (and 25.6% weight loss). 38he yttria-zirconia-supported Ni catalysts had all the essential qualities like temperature sustainability, lattice oxygen endowing capability, Ni size optimization, and adequate basic sites in favor of DRM.The promotional addition of metal oxide over yttria-zirconia supported Ni catalyst may further enhance the activity.Herein, we have prepared metal oxides (like Ho, Ba, Ga, Gd, and Cs) promoted yttria-zirconia-supported Ni catalysts.These catalysts were investigated for DRM reaction.Surface area and porosity, X-ray diffraction, RAMAN, UV-vis spectroscopy, thermogravimetry, temperature programmed study and transmission electron microscopy were used to characterize these catalysts properly.The fine-tuning characterization results and catalytic activities would set a concluding remark for DRM over metal oxide-promoted "yttria-zirconia supported Ni catalysts."

| Catalyst preparation
Yttria-zirconia support is prepared by mechanically mixing the respective metal oxides (8 wt.% yttrium oxide and 92 wt.% zirconium oxide).Nickel nitrate aqueous solution (equivalent to 5 wt% Ni) and 4 wt.%promoter-based metal nitrate aqueous solution (Metal = Cs, Ga, Ba, Ho, Gd) are added to the Yttria-zirconia support by impregnation method.The prepared mixture is stirred at 80°C, and dried at 120°C overnight in an oven.Subsequently, the dried product is calcined at 600°C for 3 h.Zirconia-supported nickel catalyst, yttriazirconia-supported Ni catalyst, metal oxide-promoted yttria-zirconia supported nickel catalysts are abbreviated as 5Ni/Zr, 5Ni/YZr and 5Ni4M/YZr (M = Cs, Ga, Ba, Ho, Gd) respectively.

| Catalyst characterization
Detailed information on characterization technique, equipment, specification and methods are mentioned in Table 1.

| Catalyst activity test
0.1 g 5Ni4M/YZr catalyst is packed in a tubular stainlesssteel reactor (Diameter = 9.1 mm and height = 300 mm) supplied by PID Eng. and Tech Micro activity Reference Company.To monitor the temperature of the catalyst bed, a K-type thermocouple is injected axially at the catalyst bed.First, the reductive pretreatment is carried out over a catalyst under H 2 (flow rate 30 mL/min) for 1 h at 700°C.Further, the reactor is flushed with N 2 for 15 min to remove the physiosorbed H 2 .DRM reaction is performed at 800°C by passing CO 2 , CH 4 , and N 2 at 30, 30, and 10 mL/min (to generate 42,000 mL/h•g cat of gas hourly space velocity) T A B L E 1 Detailed information on characterization technique, equipment, specification and methods.

Techniques
Equipment Methods/specification Other information

Surface area and porosity
Micromeritics Tristar II 3020 using dewar and liquid nitrogen Brunauer-Emmett-Teller (BET) method for surface area and pore size Barrett-Joyner-Halenda (BJH) method for pore size distribution The N 2 adsorption-desorption isotherm is plotted up to (3) 3 | RESULTS
The UV-vis spectra of the catalyst samples show a charge transfer band and d-d transition band (Figure 6).It is interesting to note that on the introduction of yttria along with zirconia as support, the intensity of charge transfer bands at 230 nm for O 2− → Zr 4+ , 30 257 nm for O 2− → Ni 2+4 and 290 nm for O 2− → Ni 2+ /Zr 4+ ) are suppressed largely (Figure 6A) as well as the bandgap between the valance band and conduction band is decreased to 2.23 eV (against 3.15 eV in 5Ni/Zr) (Supporting Information: Figure S4).On promotional addition of Ba, Gd and Ho, bandgap has increased and remains about 3.13 to 3.17 eV.In the case of the promotional addition of 4 wt% Ba, again the charge transfer band for O 2− → Ni 2+ /Zr 4+ is restored.In the previous study, it was also found that with the increase of Ba 2+ concentration, the intensity of the bands in the charge transfer region also increased 42 due to charge transfer between Ba 2+ -O 2− .So, it can be expected that due to the combined charge transfer of O 2− → Ni 2+ /Zr 4+ /Ba 2+ , the charge transfer band at 295 nm is restored in 5Ni4Ba/YZr.That means the incorporation of Ba +2 does not inhibit the charge transfer of O 2− than Ho and Gd.Cs-promoted yttriazirconia Ni catalyst has additional UV peaks in the charge transfer regions (Figure 6B).
The H 2 -TPR profile of 5Ni/Zr, 5Ni/YZr, and 5Ni4M/YZr (M = Ho, Gd, Ba,) catalyst is mostly centered about 425°C (Figure 7A).The reduction peak below 425°C is attributed to the reduction of "weakly interacted NiO-species" whereas the reduction peak > 425°C is attributed to the reduction of "moderately interacted NiO-species." 43In zirconia-supported Ni catalyst, the intensity of the reduction peak for "moderately interacted NiO species" is much higher than for "weakly interacted NiO species" (Figure 7A).On incorporating yttria in support along with zirconia, the intensity of reduction peak for "moderately interacted NiO species" is greatly suppressed and the 5Ni/ YZr catalyst has a comparable amount of both types of reducible NiO species ("weakly interacted NiO species" and "moderately interacted NiO species").Upon incorporation of 4Wt.%Ho promoter as well as 4Wt.% Ba promoter, a relatively lower temperature reduction peak is marginally larger than a higher temperature reduction peak.4Wt.% Gd promotional addition in yttria-zirconia supported Ni catalyst, lower temperature reduction peak became more prominent than the higher temperature reduction peaks.It indicates that "weakly interacted reducible NiO species" are grown relatively more than "moderately interacted reducible NiO species" over 5Ni4Gd/YZr, 5Ni4Ba/YZr and 5Ni4Ho/YZr catalysts.In the case of 4Wt.% Ga incorporation over yttria-zirconia supported Ni catalyst was unique (Supporting Information: Figure S5A).The reduction peak profile is shifted to a higher temperature such that the lower temperature peak completely vanishes and a new peak centered about 600°C appears which is attributed to the reduction of "strongly interacted NiO-species" over the catalyst surface.4Wt.% Cs promoted yttria zirconia catalyst had merged reduction peaks for Cs 2 O (about 354°C and 374°C) along with reduction peaks for interacted NiO species 44 (Supporting Information: Figure S5A).It is also noticeable that, upon Cs addition, the reduction peak position for "interacted NiO-species" was extended to a relatively higher temperature (than Gd, Ba, Ho promoted catalyst) indicating enhanced metal-support interaction upon Cs addition.
The CO 2 -TPD profiles of 5Ni/Zr, 5Ni/YZr, and 5Ni4M/ YZr (M = Ho, Gd, Ba, Cs, Ga) catalysts can be broadly classified into three regions (Figure 7B).The peaks below 175°C for weak basic site regions (attributed to basicity due to surface hydroxyl), from 175°C to 450°C for intermediate strength basic site regions (attributed to basicity due to isolated O 2-species) and above 450°C for strong basic sites (attributed to basicity due to unidentate carbonates) 33,45 Most of the catalysts had CO 2 -TPD peaks in the low and intermediate temperature regions claiming weak and intermediate strength basic sites.Zirconia-supported Ni catalyst has the highest density of weak basic sites than the rest catalyst system.4Wt.% Gd promoted yttria-zirconia supported Ni catalyst is characterized by the highest density of intermediate strength basic sites among all catalysts.Cs promoted as well as Ba promoted yttria-zirconia supported Ni catalyst had strong basic sites.The thermogravimetric profile of 5Ni/Zr, 5Ni/YZr, and 5Ni4M/YZr (M = Ho, Gd, Ba, Cs, and Ga) catalyst is shown in Supporting Information: Figure S5B.Zirconia supported Ni and yttria zirconia supported Ni catalysts had 18%-19% weight loss.On incorporation of promoters like Ho into Yttria-supported Ni catalyst, 14%-15% weight loss is observed.Further, 5Ni 4Ga/YZr, 5Ni 4Ba/YZr, and 5N4Gd/YZr showed 7.8%, 6.8%, and 5.8% weight loss.Cs promoted yttria-zirconia supported Ni catalyst has no weight loss.
The TEM image fresh and spent 5Ni/Zr, 5Ni/YZr and 5Ni4M/YZr (M = Cs, Ga, Ba, Gd, Ho) is shown in Figure 8.The carbon deposit in the form of nanotubes is found in all spent catalyst systems.The particle size distribution plots and particle size in fresh and spent catalyst samples are calculated by TEM image on a 200 nm scale (Supporting Information: Figures S6 and S7, Table S1).The particle size of Ni species is found smallest in the case of Ho-promoted catalyst.It is interesting to note that thermogravimetry analysis of 5Ni4Cs/YZr shows no weight loss but the HRTEM image of the spent-5Ni4Cs/YZr catalyst (Figure 7D) evidence the presence of carbon nanotube.In the F I G U R E 6 (A) UV-vis spectra of 5Ni/Zr, 5Ni/YZr and 5Ni4M/YZr (M = Ho, Gd, Ba) (B) UV-vis spectra of 5Ni4M/YZr (M = Cs, Ga).
F I G U R E 7 (A) H 2 -TPR of 5Ni/Zr, 5Ni/YZr, 4Ni4M/YZr (M = Ba, Gd.Ho) catalysts (B) CO 2 -TPD of 5Ni/Zr, 5Ni/YZr, 4Ni4M/YZr (M = Ba, Gd.Ho, Cs, Ga) catalyst.thermogravimetry analysis, analysis is performed under oxygen which may oxidize the carbon nanotube as well as the metal oxide.It is reported that under a particular oxygen environment, various types of caesium-related oxides like caesium oxide, caesium peroxide, caesium suboxide, caesium superoxide as well as 80 different types of Caesium oxide clusters are nurturing. 46,47So, it can be concluded that net zero weight loss in TGA of 5Ni4Cs/YZr does not mean that there is no carbon deposit or there is no oxidation of coke over catalyst but it may be due to equivalent weight gain by caesium compounds under oxygen.

| DISCUSSION
The DRM reaction is known as the potential conversion of equimolar CH 4 and CO 2 into syngas.But here, over each catalyst, CO 2 conversion was found to be higher than CH 4 conversion.This indicates that additional CO 2 is engaged in some other reactions than DRM.Metallic Ni is also known for H 2 dissociation. 48Thereby, supported Ni catalyst may also felicitate the reverse water gas shift reaction (CO 2 + H 2 → CO + H 2 O) as a major side reaction during DRM 4 which caused higher CO 2 conversion than CH 4 conversion.The %weight loss over different catalysts during DRM reaction is found in the following order 5Ni4Cs/YZr (0% weight loss) < 5 Ni4Gd/YZr (5.8% weight loss) < 5Ni4Ba/YZr (6.8% weight loss) < 5Ni4Ga/YZr (7.8% weight loss) < 5 Ni4Ho/YZr (14%-15% weight loss) <5Ni/YZr (18%-19% weight loss) ~5Ni/Zr.However, DRM activity results over promoted catalysts are found in the given order 5Ni4Ho/ YZr (~85% CH 4 conversion) > 5Ni4Ba/YZr (~80% CH 4 conversion) ~5Ni4Gd/YZr (~80% CH 4 conversion) > 5Ni/ YZr (70% CH 4 conversion) > 5Ni4Ga/YZr (56.8% CH 4 conversion) > 5Ni4Cs/YZr (53.7% CH 4 conversion).Even at high coke deposition, the catalyst active site of 5Ni4Ho/YZr remains exposed and the catalysts perform excellently.It seems that coke deposition is not a crucial factor in affecting activity.It is possible if the rate of carbon formation (at the catalytic active sites) properly matches the rate of carbon diffusion (away from the catalytic active sites). 49Other factors like support stability, reducibility, and basicity also need to be considered in correlating the catalytic activity.
"Bare zirconia" supported Nickel has a substantial amount of basic sites (from weak to moderate strength), a higher intensity of "moderately interacted NiO-species" than "weakly interacted NiO-species" and 3.15 eV bandgap (between valance band and conduction band).However, unstable monoclinic ZrO 2 phases against hightemperature DRM reaction limit its activity (CH 4 conversion) below 50% in 420 min time-on-stream.On incorporation of 8Wt.%Y 2 O 3 along with ZrO 2 , a mixed oxide support named "tetragonal zirconium yttrium oxide" is formed which can withstand the hightemperature DRM reaction.The intensity of the charge transfer band from O 2-→M n+ (M n+ = Zr 4+ , Ni 2+ ) is suppressed and the bandgap between the valance band and conduction band is decreased to 2.23 eV.However, the intensity of "moderately interacted NiO-species" is decreased, the concentration of all types of basic sites are also decreased and coke deposition doesn't decrease.This indicates that despite high coke deposition, the catalytic active sites (interacted Ni species) over stable support remain exposed for the DRM reaction and performed constantly.Overall, it can be said that upon the incorporation of 8 wt% Y 2 O 3 along with zirconia, the 5Ni/YZr catalyst has mixed oxide (zirconium yttrium oxide) thermally stable phases and decreased bandgap (between valance band and conduction band) which results into higher and more constant catalytic performance (>70% CH 4 conversion and >80% CO 2 conversion) than 5Ni/Zr.The search for optimum catalyst performance is carried out by the use of different promoters especially Ba, Gd and Ho.Ho, Gd, and Ba promoted yttria-zirconia supported Ni catalysts have additional mixed oxide phases along with a zirconium yttrium oxide phase.5Ni4Ho/YZr, 5Ni4Gd/YZr, and 5Ni4Ba/YZr catalysts have cubic holmium zirconium oxide phase, cubic zirconium gadolinium oxide phase and cubic zirconium barium oxide phase respectively.The bandgap of these catalysts also lies between 3.13 and 3.17 eV.The reducibility profile of the top three catalysts (5Ni4Ho/ YZr, 5Ni4Gd/YZr, and 5Ni4Ba/YZr) is not found much different.All three catalysts have a relatively lower concentration of "moderately interacted NiO-species" than "weakly interacted NiO-species."In terms of basic sites distribution, 5Ni4Ho/YZr has the presence of weak basic sites and intermediate strength basic sites, 5Ni4Gd/ YZr acquires excessive intermediate strength basic sites and 5Ni4Ba/YZr may be characterized by the additional presence of strong basic sites.The basic profiles of 5Ni4Gd/YZr and 5Ni4Ba/YZr are quite different but the catalytic performance is very similar.This indicates that optimum catalytic activity requires an adequate amount of basic sites and a proper amount of stable metallic Ni sites.Excess amounts for one type of basic site are not needed to drive higher activity.Overall, it can be said that all three catalysts (5Ni4Ho/YZr, 5Ni4Gd/YZr, and 5Ni4Ba/YZr) had comparable reducibility profiles and adequate amounts of basic sites over the surface.The presence of different supports that stabilize the metallic Ni had key roles in deciding activity.5Ni4Ho/YZr had stable tetragonal zirconium yttrium oxide as well as cubic zirconium holmium oxide phases to stabilize catalytic active Ni.The particle size over 5Ni4Ho/YZr catalyst is minimum (3.67 nm) than the rest promoted catalyst and the size remains limited to a minimum (4.68 nm) during the entire DRM reaction.Previously, it was observed that Ni particle size < 9 nm undergoes stronger metal-support interaction which restricts the thermal sintering of nickel. 505Ni4Ho/YZr performs excellent toward DRM with ~85% CH 4 conversion at the end of 420 time on stream and more than 81% CH 4 conversion up to 30 h time-on-stream.As H 2 /CO ratio over 5Ni4Ho/YZr catalyst is maximum (0.97) and so it has the strongest inhibition for reverse water gas shift reaction than the rest catalysts.5Ni4M/YZr (M = Gd, Ba) has both stable tetragonal zirconium yttrium oxide and Cubic zirconium-gadolinium oxide or Cubic-zirconium barium oxide phase to stabilize catalytically active Ni.Both 5Ni4Gd/YZr and 5Ni4Ba/YZr are about equally efficient with 80% CH 4 conversion.As H 2 /CO ratio of 5Ni4Ba/YZr is higher than 5Ni4Gd/YZr and 5Ni4Ba/YZr catalyst has stronger inhibition for RWGS reaction 5Ni4Gd/YZr catalyst.
4 wt% Ga or 4 wt% Cs addition over yttria-zirconia supported Ni catalyst is not found effective in terms of catalytic activity toward DRM and catalytic activity of 5Ni4Ga/YZr and 5Ni4Cs/YZr are less than even nonpromoted catalyst (5Ni/YZr).XRD results show less dispersion of NiO over the catalyst surface.Clearly, less-dispersed-NiO generates fewer active sites over the catalyst surface upon reduction resulting in inferior activity.The comparative catalytic activity (in terms of CH 4 conversion and CO 2 conversion) of the metal-promoted "metal oxide-zirconia supported Ni" based catalyst system is shown in Figure 9.The catalytic activity results with detailed reaction conditions are also mentioned in Supporting Information: Table S2.Metal-promoted "yttria-zirconia supported Ni" is found superior to metal-promoted "tungsten-zirconia supported Ni" catalyst system and quite competitive to metal-promoted "lanthana-zirconia supported Ni" catalyst system 25,26,28,46,51,52 (Figure 9).The promotional effect over 5Ni/YZr catalyst was studied extensively studied where Hopromoted catalyst were found be the best whereas in rest case (5Ni/LaZr, 5Ni/WZr) ceria promoted catalyst showed excellency.Alumina and silica support are also thermally stable.4][55][56][57][58][59] Over Ni/Al 2 O 3 , 79-81% CH 4 conversion was quite noticeable over W (in 7.3 h TOS), Co (in 24-h TOS) and Yb (in 20 TOS) promotors.1][62][63][64][65][66][67] However, preparation of ordered mesoporous silica again needs skill and is more costly.So, these catalysts gain less consideration.Over Ni/ SiO 2 catalyst, promotors like Lathana were quite impressive.It showed 80% CH 4 conversion up to 24 h.In the current study, 5Ni4Ho/YZr catalyst is prepared by simple mechanical mixing and it outperforms others.It shows 85% CH 4 conversion in 7 h TOS and 81% CH 4 conversion in 30 h TOS with 0.97 H 2 /CO.

| CONCLUSION
The catalytic activity of ZrO 2 -supported Ni is limited by an unstable ZrO 2 phase under drastic DRM conditions.Upon incorporation of Y 2 O 3 along with ZrO 2 support, a thermostable support of zirconium yttrium oxide is formed and catalytic activity shoots to 70% CH 4 conversion.Along with adequate basic sites, 5Ni4Ho/YZr has zirconium yttrium oxide phase along with cubic zirconium holmium oxide phase to stabilize the optimum size of Ni (3.67 nm) which is minimum among other promoted catalysts.5Ni4Ho/YZr catalyst had highest catalytic activity with strongest inhibition for RWGS.It shows about 85% CH 4 conversion in 7 h time-on-stream and >81% CH 4 conversion in 30 h time-on-stream with 0.97 H 2 /CO.Including an adequate amount of basic sites, the 5Ni4Gd/YZr and 5Ni4Ba/YZr catalysts have promoter-incorporated support phases (as cubic zirconium gadolinium oxide and cubic zirconium barium oxide respectively) to stabilize Ni.Both catalysts are equally efficient ( ~80% CH 4 conversion) toward DRM.However, in terms of RWGS inhibition, 5Ni4Ba/YZr catalysts are found to be better than 5Ni4Gd/YZr catalysts.The low dispersion of catalytic active Ni sites over 5Ni4Cs/YZr and 5Ni4Ga/YZr results in inferior catalytic activity than other catalyst systems.

F
I G U R E 8 High Resolution Transmission Electron Microscope image of different fresh and spent catalyst.
31onversions, CO 2 conversions and H 2 /CO ratio at 800°C reaction temperature is shown in Figure2.The error bars of catalytic activity results are also shown in Supporting Information: FigureS1.It was noted that the error of catalytic activity is 3%-4% for all the experiments.Toward DRM, zirconia supported Ni system (5Ni/Zr) is found efficient to carry out more than 50% CH 4 conversion and ~61% CO 2 conversion whereas yttriazirconia supported Ni catalyst system (5Ni/YZr) is capable of improving the CH 4 conversion up to >70% and CO 2 conversion up to >80% respectively.The catalytic activity remains constant over 5Ni/YZr catalyst over the tested time (420 min).Upon the addition of 4 Wt.%Ga over yttria-zirconia supported Ni catalyst, the catalytic activity (CH 4 conversion) drops down to 56.8%.YZr catalyst up to 30 h time on stream.As per the DRM reaction (CO 2 + CH 4 → 2H 2 + 2CO), the H 2 /CO ratio should be equal to 1.However, over each catalyst, H 2 /CO ratio is found less than one.It indicates the presence of H 2 consuming reaction like reverse water gas shift reaction (CO 2 + H 2 → CO + H 2 O).31At the end of 420 min on stream, the H 2 /CO ratio over different catalysts is found in the following order; 5Ni4Ho/YZr (H 2 /CO = 0.97) > 5Ni4Ba/YZr (H 2 /CO = 0.94) > 5Ni4Gd/ CO = 0.71).H 2 /CO ratio closer to 1 indicates a strong inhibition of RWGS reaction.In a long-time study of up to 30 h, the H 2 /CO ratio of 5Ni4Ho/YZr and 5Ni4Cs/YZr catalyst remained constant (as like as 7 h TOS).
4 wt% Cs incorporated catalyst (5Ni4Cs/YZr) has least catalytic activity (53.7% CH 4 conversion) among the rest promoted catalyst system.In the alkaline earth metal group, Ba location is next to caesium.On adding 4Wt.% Ba over yttria-zirconia supported Ni catalyst (5Ni4Ba/ YZr), the CH 4 and CO 2 conversions is raised to 80% and ~87%, respectively up to 420 min time on stream.F I G U R E 2 Catalytic activity versus time on stream study of different catalysts at 800°C reaction temperature (A) CH 4 versus time on stream (B) CO 2 conversions versus time on stream (C) H 2 /CO ratio versus time on stream.