Methane Decomposition to Hydrogen Over Zirconia‐Supported Fe Catalysts–Effects of the Modified Support

Abstract Methane decomposition is a promising route to synthesize COx‐free hydrogen and carbon nanomaterials (CNMs). In this work, the impregnation method was employed for the preparation of the catalysts. Systematic investigations on the activity and stability of Fe‐based catalysts were carried out in a packed‐bed micro‐activity reactor at 800 °C with a feed gas flow rate of 18 mL/min. The effect of doping Y2O3, MgO, SiO2 and TiO2 over ZrO2 on the catalytic performance was also studied. BET revealed that the specific surface areas and pore volumes are increased after SiO2, TiO2, and Y2O3 are added to ZrO2 while MgO had a negative impact and hence a little decrease in specific surface area is observed. The catalytic activity results showed that the Fe‐based catalyst supported over TiO2‐doped ZrO2 that is, Fe−TiZr, demonstrated the highest activity and stability, with a maximum methane conversion of 81.3 % during 180 min time‐on‐stream. At 800 °C, a maximum initial methane conversion of 73 %, 38 %, 64 %, and 69 % and a final carbon yield of 121 wt. %, 55 wt. %, 354 wt. %, and 174 wt. % was achieved using Fe−MgZr, Fe−SiZr, Fe−TiZr and Fe−YZr catalysts, respectively. Moreover, bulk deposition of uniform carbon nanotubes with a high degree of graphitization and different diameters was observed over the catalysts.


Introduction
Hydrogen is an inspiring green energy alternative and can be used in many commercial applications (such as ammonia production, in refineries, and as a feedstock for chemicals).Since hydrogen produces more energy than any fossil fuel (per unit weight) and can replace fossil fuel. [1]The difficulties posed by high CO 2 emissions from methane's conventional conversion have led to the development of emission-free hydrogen production from methane. [2]Natural gas is often used to generate hydrogen via catalytic decomposition of methane (CDM), steam reforming of methane (SRM), partial oxidation of methane (POM), and dry reforming of methane (DRM). [3]6][7] Methane is the best source of hydrogen because it has the maximum ratio of hydrogen-carbon in between all hydrocarbon, is more abundant than other fossil fuels, and as well as has a simpler conversion process. [8]Methane can be decomposed into H 2 and C in a simple reaction (Equation ( 1)) as shown in the following [9] CH 4 At temperatures more than 550 °C, methane cracking is spontaneous and can happen even without the use of a catalyst. [10]To achieve total breakdown, the reaction needs temperatures higher than 1000 °C. [11]Nickel and cobalt catalysts are efficient for the CDM reaction at temperatures of 500 and 600 °C.However, these catalysts would deactivate quickly at higher temperatures.Ni and Co are also quite expensive and toxic.However, Fe catalysts are less expensive and non-toxic, and it can be used with the produced nano carbon for a variety of applications without harming the environment. [4,12]Although the Fe catalysts have a lower activity than Ni and Co-based catalysts, the Fe catalysts can operate over wider temperature ranges, which may result in higher conversion.On the contrary, the catalytic activity performance of the active metals could be influenced by the textural promoters [4] and support types used in the catalyst. [13]DM has been studied using silica-supported Fe catalysts.A variety of Fe/SiO 2 catalysts with Fe loadings ranging from 25 to 100 wt.% were created.Comparatively more activity was shown by all supported catalysts than by the unsupported 100 wt.% Fe.Additionally, as the Fe loading increased, the catalytic activity increased as well. [14]reviewed the CMD and emphasized the need for industrial-scale CDM in light of the current high CO x emission levels and the advancement of Fe-based catalysts.The most recent developments in the kinetic analysis of metal catalyst reactions are described, the effects of several parameters, like metal loading effect and support influences, on CDM catalyst performance, [8] highlighted the need for a green alternative energy source because CH 4 decomposition results in zero carbon emissions.They concentrate on the use of GAD reactors and discussed the impact of several parameters, including operation conditions, additives, geometric configuration, and catalysts on the performance. [15]irconia-based oxides offer acid/base properties, redox properties, and tunable phase composition that has attracted the interest of the scientific community.However, in the thermal decomposition of methane, there aren't many studies that focus on the use of ZrO 2 -supported catalysts with Fe and Ni oxides. [16]Metal oxides like lanthanum, ceria, and zirconia were studied by Pudukudy et al. [17] as catalyst support for Ni catalyst.High catalytic activity and stability were shown by all of the catalysts for methane decomposition.At 700 °C, lanthanum, ceria, and zirconia, supported catalysts produced an initial hydrogen yields of 58, 62, and 61 %, respectively, and final carbon yields of 1360 wt.%, 1159 wt.%, and 1576 wt.%.Metal oxides doping to zirconia is a strategy used to enhance a number of characteristics and the presence of the additive causes zirconia to crystallize in the tetragonal phase, which has a higher specific surface area as compared with the more stable monoclinic ZrO 2 .Al-Fatesh et al. [18] studied the impact of doping lanthanum (La 2 O 3 ) and tungsten oxide (WO 3 ) to ZrO 2 on the performance of Fe-containing catalysts.They found that the catalyst's stability was increased by the addition of La 2 O 3 , however, the use of WO 3 -ZrO 2 as support material enhanced its stability significantly.The catalyst with the label 20 wt.% Fe/ WO 3 -ZrO 2 maintained its activity for 240 min that was assigned to the stronger metal-support interaction, that is, interaction of Fe 2 O 3 particles with the support (WO 3 -ZrO 2 ).
Silica is reported to prevent ZrO 2 from crystallization at elevated temperatures that facilitates reservation of the higher specific surface area of the amorphous structure, [19] whereas yttrium oxide with a valence lower than + 4 favors the formation of oxygen vacancies, helping to store oxygen and return it at low temperatures. [20]Additionally, it was shown that adding MgO to the ZrO 2 support serves as a thermally stable catalyst support while effectively preserving the tetragonal ZrO 2 phase. [21]Gac et al. [22] studied the effect of MgO addition on the activity of Ni/Al 2 O 3 and found that the active surface area of Ni/ Al 2 O 3 increases as MgO is added to the system.The addition of MgO to the catalyst caused the growth of smaller nickel crystallites with more potent adsorption sites as a result of strong metal-support interaction and the microstructure organization of the catalyst.The rate of methane decomposition increased during the initial stages of the reaction as the content of MgO increased.TiO 2 has poor thermal stability and a low specific surface area.In addition, under high-temperature and high-pressure conditions, the surface area of TiO 2 would drastically decrease. [23]Fortunately, it has been established by numerous researchers that doping titania with a different metal (such as Si, Fe, La, Zr, etc.) can effectively increase both its thermal stability and catalytic activity. [24,25]Among various titaniasupported catalysts reported earlier, [26,27] Fakeeha et al. [26] examined the effect of various types of support materials, such as titania and magnesia on the performance of iron-based catalysts.The results of the catalytic activity tests showed that all titanium-supported catalysts exhibited less activity as well as deactivation.The FeÀ Mg catalyst displayed the best activity (65 % conversion) for the duration of 3 h on the stream.TiO 2 À ZrO 2 has attracted more attention as catalyst support because of the high specific area and high thermal stability compared to the relative single oxide.The addition of titanium to zirconium not only increases the specific surface area and thermal stability but also improves the acidity and basicity of zirconium and titanium. [28]he objective of this study is to develop a FeÀ Zr catalyst with high activity and stability for the catalytic decomposition of methane.For this purpose, the catalytic performance of FeÀ Zr with different additives (Y 2 O 3 , MgO, SiO 2 , and TiO 2 ) was developed and reported in this work.The fresh and spent catalysts are characterized to justify the findings using different techniques such as BET, H 2 -TPR, O 2 -TPO, XRD, TGA, and TEM.

Characterization of fresh catalysts
The compositions of the prepared catalysts were verified by EDX.For Instance, Figure 1S displays the spectra for 30 %Fe/ 10 %TiO 2 À ZrO 2 catalyst.The result establishes that the prepared catalysts approximately assumed the desired composition as shown in Table 1S.

Textural properties
The textural characteristics of the as-synthesized catalysts were evaluated using N 2 adsorption/desorption isotherms as shown in Figure 1.According to IUPAC sorting, the catalysts showed a type IV isotherm with a type H3 hysteresis loop, which is typical of a mesoporous structure with slit-shaped pores of various sizes and shapes. [29]As shown in Table 1, FeÀ SiZr has a high surface area (127.2 m 2 /g) and an average pore size of 5.35 nm, which is beneficial in the metal additive's dispersion at high loading. [30]ompared with FeÀ Zr, the specific surface area and pore volume of Fe-mZr (m = Si , Ti, Y) is increased after SiO 2 , TiO 2 , and Y 2 O 3 are added except FeÀ MgZr that shows a little decrease.Nevertheless, the FeÀ SiZr sample shows the highest surface area (127.2 m 2 /g) and lowest pore size (5.35 nm), while the FeÀ MgZr sample displays the lowest surface (19.1 m 2 /g) and highest pore size (36.3 nm).The surface area of the unmodified FeÀ Zr catalyst is 20.2 m 2 /g and remarkably increased to 22.8 m 2 /g and 50.9 m 2 /g for FeÀ TiZr and FeÀ YZr catalysts, respectively.Also, the pore volume of FeÀ TiZr is found to be 0.173 cm 3 /g, which is higher than that of the unmodified FeÀ Zr (0.157 cm 3 /g).

X-ray diffraction (XRD)
Powder XRD was conducted to study the crystal structures of the Fe-loaded samples.The XRD patterns of zirconia-supported Fe catalysts are presented in Figure 2 Typically, this behavior showed that the addition of Y 2 O 3 had a significant impact on the phase composition of the zirconia support.Thus, we can infer that rather than the monoclinic ZrO 2 (m-ZrO 2 ) phase, the growth of the tetragonal zirconium phases (t-ZrO 2 ) phase was enhanced by the addition of Y 2 O 3 .The degree of crystallinity of the zirconia-supported Fe catalyst was calculated to be 12 %, 10.3 % and 10.2 % for the FeÀ TiZr, FeÀ YZr and FeÀ Zr catalysts respectively.Moreover, the low intensity of the Iron peaks in the FeÀ TiZr and FeÀ YZr catalysts further confirms its fine surface dispersion on the catalyst support.

H 2 -TPR
The H 2 -TPR (Temperature-programmed reduction) is an important technique for characterizing the catalyst's metal-support interactions and reducibility.It may also be used to verify the catalyst's appropriate reduction temperatures. [31]H 2 -TPR curves of the fresh catalysts are shown in Figure 3.The peaks between 300 and 500 °C are ascribed to the reduction of Fe 2 O 3 to Fe 3 O 4 .The unmodified and SiO 2 modified samples showed identical profiles in terms of intensity and position, however, the rest of the modified samples displayed lower intensities, particularly MgO and TiO 2 modified samples, and their peaks are slightly shifted to higher temperatures denoting stronger interactions.The broad peaks between 550 and 750 °C could be assigned to the further reductions of Fe 3 O 4 to FeO, and Fe.The addition of modifiers diminished the intensities with respect to the [a] S.A. = surface area; P.V. = pore volume; P.S. = pore size, all evaluated from BET. unmodified sample.The peak for Y 2 O 3 modified sample exhibited the highest intensity, while the rest of the modified samples exhibited similar intensities.At around 800 °C temperature, a weak H 2 consumption peak appeared related to the FeÀ TiZr sample.This could be attributed to the reduction of mixed oxide species to metallic species. [18]This further enhances the dispersion of the active metals.

TEM analysis
The inner microstructure of the freshly calcined FeÀ TiZr catalyst was investigated by TEM analysis and the images are shown in Figure 4.As illustrated in Figure 4a, the catalyst particles were observed to be closer to the form that is spherical and their boundaries are extremely clear.Besides, the dark spots shown in the TEM images represent the Fe 2 O 3 particles were significantly smaller and much more separated, which further confirmed the greater dispersion associated with active Fe 2 O 3 on surface of TiZr support (Figure 4a).The particle size of Fe 2 O 3 nanoparticles was within the range of 24-32 nm.In addition, high-resolution TEM images showed that Fe 2 O 3 and ZrO 2 particles exhibit well-defined lattice spacing visible from the lattice edges of the catalyst particles (Figure 4b).These observations were very consistent with the XRD results.

Catalyst activity
The CDM reaction over Fe-based catalysts was carried out at 800 °C under atmospheric pressure with a flow rate of 18 mL/ min.The space velocity of the feed was maintained at 7200 mL h À 1 g c À 1 . Figure 5 shows the effect of different additives (Y 2 O 3 , MgO, SiO 2 , and TiO 2 ) on the methane conversion for 180 min time on-stream (TOS) using 30 wt. % Fe supported on ZrO 2 catalysts.The results exhibited that the FeÀ TiZr sample gave the highest conversions of methane at about 81.3 % and kept increasing over the period of the investigation (3 h).In contrast, FeÀ MgZr, FeÀ SiZr, and FeÀ Zr catalysts began with initial CH 4 conversions of about 74 %, 38 % and 50 % respectively, but suffered fast deactivation to attain as low values as 29 %,17 % and 11.5 %, respectively after running 180 min of TOS (Figure 5).
A high methane conversion of about 69 % was achieved in the FeÀ YZr catalyst at the temperature of 800 °C.The addition of yttria (Y 2 O 3 ) in the catalyst structure led to an increase in its catalytic activity.However, its activity decreased with time to about 50.1 % due to carbon deposition on active Fe surfaces. [32]he catalytic activity of FeÀ Zr improved significantly by adding Titanium and the methane conversion of FeÀ TiZr increased from about 64 % to 81.3 % after 180 min time onstream.The high dispersion of Fe 2 O 3 particles, which resulted from their proper interaction with the TiZr support, may be primarily responsible for the FeÀ TiZr catalyst's pronounced catalytic activity and durability.The addition of Titanium could also accelerate the growth of carbon fiber or carbon nanotubes   and stop the formation of amorphous carbon over active sites. [31]Consequently, the FeÀ TiZr catalyst's lifetime was prolonged, due to the availability of active sites with FeÀ Zr.The low methane conversion and hydrogen yield at the beginning of the reaction of FeÀ TiZr can be attributed to the in-situ consumption of H 2 released during the decomposition of methane, which continued the reduction of unreduced Fe active sites left over from the pre-reduction step.This in-situ reduction increases the total number of active sites available for the reaction, which in turn promotes methane decomposition and carbon formation until equilibrium is reached. [33,34]rom the results, it can be inferred that among all the dopants used with ZrO 2 , TiO 2 enhanced the catalyst's performance.On the other hand, the weak interaction between the catalyst's components can be used to explain the low catalytic activity and stability of other zirconia-supported This behaviour causes a metal sintering that produces large metal particles, especially with higher calcination temperatures. [18]he performance comparison between the current work and others in the literature is summarized in Table 2. Furthermore, in contrast to Ref. [10], where 20 % Fe was sustained with Zr stabilized with La and W, in this work 30 % Fe was supported with Zr stabilized with Ti, Mg, Si, and Y.Alternatively, in Ref. [23], the CMD was performed using promoted 60 % Ni catalyst supported only by Al 2 O 3 .

Temperature programmed oxidation (TPO)
TPO is one of the useful methods that can be used to identify the type of carbon deposited onto the surface of used catalysts and it has also been performed to evaluate the spent catalysts' thermal stability (Figure 6).From the profile curves, it is first observed that there was a slight gain in the mass at around 400 °C, which can be attributed to the oxidation of the metallic species present in the catalysts, followed by a broad gain in mass between 500 and 700 °C.The low-temperature peak (< 400 °C) corresponds to the coke-like carbon (amorphous carbon) which covers the active metal species resulting in catalyst deactivation.The high-temperature peak (500-700 °C) corresponds to the highly ordered carbon nanostructures which have the crystalline graphitic structures. [39,40]The result is indicating a highly ordered crystal structure (600-750 °C) with the absence of amorphous carbon (~400 °C).For FeÀ TiZr catalyst, the TPO curve (Figure 6) had a weight loss peak at ~700 °C where the highest temperature peak was dominant.The increase in the amount of carbon deposited by titania doping agreed with the result obtained in the stability test after 180 min of on-stream reaction at 800 °C, which means that more amounts of hydrogen and carbon were produced from methane decomposition. [41]

TGA
The amount of deposited carbon is frequently calculated using thermogravimetric analysis (TGA). [42]The weight loss of the carbon deposits over zirconia-supported Fe catalysts was shown in Figure 7.The total carbon accumulation on the spent catalysts was expressed as weight loss percentages.Different types of carbon oxidize at different temperatures, and the combustion of amorphous carbon is primarily responsible for weight loss in TGA curves below 450 °C. [43]At 800°C reaction temperature, the catalyst FeÀ TiZr exhibited the highest weight loss of 83 %, followed by FeÀ YZr , FeÀ MgZr and FeÀ Zr catalysts that presented 78 %, 74 % and 71 % weight loss respectively, and ultimately FeÀ SiZr catalyst displayed the lowest carbon weight loss of 63 % and hence the minimum carbon formation on it.In addition, the carbon yield of 121 wt.%, 55 wt.%,   354 wt.%, and 174 wt.% was achieved using MgO, SiO 2 , TiO 2 , and Y 2 O 3 modified over zirconia-supported catalysts respectively (Figure 8).The weight loss peak observed in the FeÀ YZr sample at around 900 °C is likely due to the weight gain resulting from the oxidation of residual Fe, which takes place after the oxidation of all carbon deposits. [33]The FeÀ TiZr catalyst showed the highest weight loss of deposited carbon, which was in good agreement with the catalyst's high activity.These findings infer that the TiO 2 addition promoted the efficiency of the Fe/ZrO 2 catalyst and thus increased the accumulation of carbon on the catalyst surface.

Raman spectra
Raman spectra of the used catalysts are presented in Figure 9.
All the spent catalysts displayed D, G band peaks.The D band peaks at around 1350 cm À 1 is assigned to the disordered carbon, including amorphous carbon or defective graphite sheets.On the contrary, the G band peaks at around 1580 cm À 1 correspond to CÀ C stretching vibrations that are characteristic of graphite. [44]The integral intensity ratio I D /I G is widely used to express the degree of graphitization for the carbon, that is, the lower the I D /I G ratio, the higher the crystalline order of the carbon species.The I D /I G values of all samples means that the carbon is ordered, with minor contributions from disordered particles. [45]As seen from Figure 9, the values of I D /I G are different, indicating that the deposited carbon on FeÀ TiZr and FeÀ YZr catalysts had less graphitization degree than other zirconia-supported Fe catalysts, which is in accordance with the formation of more carbon on the surface of FeÀ TiZr catalysts.

SEM and TEM study
The microstructural changes and morphology of the deposited carbon over the most active FeÀ TiZr catalyst at 800 °C reaction temperature and 3 h time on stream were studied by SEM and TEM analysis and the images are shown in Figures 10 and 11, respectively.SEM image of FeÀ TiZr catalyst indicates the deposition of filamentous carbon during methane decomposition reaction.It is obvious that carbon nanomaterials with various aspect ratio are formed over FeÀ TiZr.Due to intermingled nanomaterials, it is challenging to identify the exact length and/or diameter of nanomaterials however, some of the nanomaterials were of millimeter range.As illustrated in Figure 11a, the deposited carbon material on the surface of FeÀ TiZr was multi-walled carbon nanotubes (MWCNTs) with different diameters.A mixture of large-diameter MWCNTs with hollow and full cores was obtained, Moreover, the MWCNTs have some defects in the outer walls (Figure 11a). [18]The dark spots shown in Figure 11b represent the metallic Fe nanoparticles with spherical shapes that were clearly observed inside  the hollow core of MWCNTs.The high-resolution TEM image in Figure 11c, further confirms the presence of MWCNTs with a clear hollow core, where the diameter of 25.5 nm, wall thickness of 8 nm and an internal channel space of 10 nm was observed. [46]

Conclusions
This work reports wet-impregnated Fe-based zirconia-supported catalysts for catalytic decomposition of methane.Zirconia was modified by doping with metal oxides including MgO, SiO 2 , TiO 2 , and Y 2 O 3 .From the results of catalyst activity, TiO 2 À ZrO 2 incorporated is found to be an excellent support for Fe-based catalyst towards CH 4 decomposition reaction on time on stream, the catalytic performance of the FeÀ MgZr catalyst drops fast than FeÀ YZr catalyst.Even after 180 min, CH 4 conversion over FeÀ YZr catalyst does not drop below 50 %.It is also found that the FeÀ TiZr catalyst was more stable than other zirconia-supported catalysts and it could be responsible for the highest carbon yield over the FeÀ TiZr catalyst, even it showed low surface area.The high stability of the titanium-supported catalyst could be attributed to the formation of nanosized active Fe and its fine dispersion on the titanium zirconia matrix after the reduction of spinel solid solution.

Experimental Section Materials
Zirconium oxide and other oxide used were obtained from DAIICHI KIGENSO KAGAKU KOGYO CO., LTD.OSAKA -JAPAN.The precursor for iron was ferric nitrate nonahydrate (Fe (NO 3 ) 3 • 9H 2 O, purity 99 %, laboratory reagents & fine chemicals).Millipore system of water purification was used to obtain ultrapure water.

Catalysts preparation
The wetness impregnation route was chosen to synthesize ironbased catalysts supported over ZrO 2 (Figure 12).The active metal loading of iron was fixed at 30 wt. %.Firstly, stoichiometric quantity of active metal precursor was dissolved in 20 mL of double-distilled water followed by addition of oxide supports.Subsequently the mixture was subjected to magnetic stirring for 3 h at 80 °C.Later, the drying of slurry samples was done using a furnace at 120 °C for 16 h.The final step was to calcine the samples at 600 °C for 3 h.

Catalytic activity test
The testing of as-synthesized catalysts was conducted at a reaction temperature of 800 °C at 1 bar pressure using a fixed-bed tubular reactor purchased from PID Eng & Tech.Firstly, glass wool was placed into the reactor to serve as the catalyst bed over which 0.15 g of catalyst was loaded.A K-type thermocouple was axially located in the reactor to measure the temperature of the catalyst bed during CDM.The fresh catalyst was reduced under hydrogen for 1 h at 800 °C prior to flowing reaction gas.The gaseous feedstock containing 12 mL/min of CH 4 and 6 mL/min of N 2 was used for CDM investigation.The reactor outlet was analysed using an online Gas Chromatography (GC-Shimadzu 2014) equipped with a thermal conductivity detector.The conversion of methane (X CH4 ) and Carbon yield were calculated using following Equations ( 2

Figure 1 .
Figure 1.The textural properties of fresh supported Fe catalysts.

Figure 4 .
Figure 4. TEM images of the fresh FeÀ TiZr catalyst.Figure 5. Methane conversion of fresh Fe-supported catalysts at 800 °C.

Figure 5 .
Figure 4. TEM images of the fresh FeÀ TiZr catalyst.Figure 5. Methane conversion of fresh Fe-supported catalysts at 800 °C.

Figure 6 .
TPO profiles of the spent supported Fe catalysts.

Figure 7 .
Figure 7.The weight loss of the carbon deposits over zirconia-supported Fe catalysts.

Figure 8 .Figure 9 .
Figure 8. Carbon yield as a function of mixed metal oxide catalysts composition.Figure 9. Raman spectra of the used catalysts.

Figure 12 .
Figure 12.The scheme of the catalyst preparation.

Table 1 .
Textural properties and description of the used catalysts.

Table 2 .
Comparison of the catalytic performance of Fe-based catalysts with previously reported results.[a]