Promotion impact of different strontium doping on Ni‐9La+Zr catalyst for dry reforming of methane

The potential dry reforming of methane technology allows for the synthesis of syngas from methane and carbon dioxide. Numerous investigations have been conducted on developing catalysts with exceptional activity and stability. Carbon deposition causes severe deactivation in typical nickel‐based catalysts, which is one of the most prevalent and important problems. In this study, methane was dry‐reformed over 5 wt.% Ni+xSr‐9La+Zr (x = 0–4 wt.%) catalysts for 7.5 h at 700°C and ambient pressure in a tubular fixed‐bed reactor. According to the weight percent of Sr loading, the features of the material's texture, morphology, and catalysis were investigated. N2‐physisorption, H2‐temperature programmed reduction, X‐ray diffraction, Raman, and TEM were used to evaluate the physicochemical characteristics of the catalysts. N2‐physisorption research revealed that changing the weight percentage loading of the Sr promoter had little effect on the textural qualities. The overall number of reducible NiO‐interacting species over the catalyst surface increased with increasing Sr loading. The 5Ni+2Sr‐9La+Zr catalyst exhibited the optimum CH4 and CO2 conversions of 62.9%–65.9% and 69.1%–70.3%, respectively, and the lowest deactivation factor of 4.7.


| INTRODUCTION
2][3][4] Alternatively, the deterioration of worldwide warming and the goal of carbon neutrality posesnew difficulties for the use of carbon dioxide and methane, promoting sustainable energy as methane is the principal element in natural gas. 5These greenhouse gases are converted via the dry reformation of methane (DRM) process into synthesis gas (a mixture of CO and H 2 ), which reduces their atmospheric concentrations and lessens their contribution to worldwide warming. 67][8][9] Their principal reforming reactions for the synthesis gas productions are: 1][12] The DRM has attracted a lot of consideration owing to its suitability for tackling greenhouse gas emissions. 13The synthesis gas is employed as input feed for the Fischer-Tropsch technique to produce precious products similar to methanol. 14,15Promising catalysts are based on nickel because they are more accessible and affordable than noble metals. 168][19] Application of supports to create a suitable catalyst that can withstand sintering and suppression of carbon deposition are two strategies to reduce the deposition of coke. 20,21Metal oxide supports such as ZrO 2 were employed because of their favorable redox characteristics and oxygen mobility. 22,23Supports that provide O 2 to metals, such as ZrO 2 , are preferable to irreducible oxide (SiO 2 or Al 2 O 3 ). 24zkara et al. 25 examined Pt supported on ZrO 2 and found it to have the maximum stability and catalytic activity during DRM (86%).Similarly, Therdthianwong et al. 26 displayed that the use of ZrO 2 resulted in good resistance to carbon deposition and catalytic stability.On the other hand, Chong et al. reported that DRM over Zr promoted Ni/SBA-15, where Zr enhanced catalytic behavior due to the formation of small NiO crystallite size and a moderate number of basic sites. 27Likewise, Liu et al. reported high catalytic activity and long-term stability in DRM owing to the CO 2 partial activation by Zr +4 together with the strong anchoring effect of Zr4 for Ni-Zr-modified MCM-41 support. 28Solid solutions and exsolution process strengthen the active metal's contact with the support, which plays a crucial part in lowering carbon on the catalyst. 29,30It was reported that lanthana serves to stabilize Al 2 O 3 thermally at very high temperatures, 31 increases metal-support interaction or improves Ni dispersion, 32 increases the fundamental properties that might stimulate CO 2 adsorption for the oxidation of carbon deposits, 33 and optimizes the size of nickel particles for DRM. 34The increased redox potential of La 2 O 3 /La 2 O 2 CO 3 , which could also gasify the carbon species, 35 and reduced H 2 consumption by inhibiting the reverse water gas shift reaction (RWGS) 36 all have an impact on the Boudouard equilibrium and shift toward CO formation, which leads to the removal of carbon deposits.Ni-supported powders have an elevated rate of carbon production, nevertheless this may be mitigated by using the proper promoters. 33,34,37Effective promoters demonstrated higher catalytic stability and decreased deposition of carbon.Promoters influence a sample's stability because they are able to change the composition and catalytic performance of the catalyst that is promoted compared to the unpromoted one. 35To circumvent homogeneous cracking and coking of hydrocarbons, highly active metals such as Sr, Ce, Sm, and Cs are selected. 36,38,39The influence of the Sr promoter on the Ni/ La 2 O 3 catalyst in reforming reaction displayed that the Srdoped Ni/La 2 O 3 catalyst produces the highest conversion of CH 4 and CO 2 , the highest H 2 production, and the least carbon deposition. 40Likewise, because of the reduced acidity and the stronger contact between the support and Ni, the SrO-modified Ni/Ce+Al 2 O 3 catalyst displayed a higher hydrogen yield and a reduced amount of carbon deposit. 41Similar outcomes were discovered by Mota et al., where the addition of Sr promoter improved the basicity of the catalyst and promoted the better dispersion of Ru. 42 To produce synthesis gas, the influence of partial La substitution by Sr on perovskite catalysts was examined.The basicity of the catalysts was changed by replacing the A site with Sr, which altered the catalytic activity under the tested conditions. 43Strontium-promoting behavior on the activity of the nickel-based catalyst was examined in research by Amir et al. 44 The Ni/10% Sr-Al 2 O 3 catalyst demonstrated great stability and catalytic activity, according to the study's findings.The goal of this research is to create supported Nibased catalysts with high activity, stability, and a reduced tendency to produce coke during the DRM.We'll look into how the Sr promoter affects the activity, stability, and coke formation of catalysts which is constituted by 5%Ni supported on commercial material "9La+Zr."Different characterization methods will be used to support the conclusions.

| Preparation of catalysts
All chemicals were purchased from commercial sources and used without further treatment.Commercial lanthanum oxide and zirconia support were obtained as a present from Saka, Japan's Daiichi Kigenso Kagaku Kogyo Co. Ltd., its specific surface areas (BET) value is 67.3 m 2 /g and D 50 of 4.04 μm.The nickel nitrate hexahydrate Ni (NO 3 ) 2 •6H 2 O) were purchased from Alfa Aesar.A 5wt.% of Ni plus 1%-4% Sr was synthesized by a simple wet impregnation method.Typically, 0.95-0.91g of support is dispersed in 100 mL of deionized water at room temperature for 20 min.Then, an appropriate amount of nickel nitrate hexahydrate was added, and the output product was stirred at 80°C until dry.The obtained solid sample was first calcined in air at 600°C for 3 h.The catalysts will be referred for simplicities as: 5Ni+xSr-9La+Zr (x = 0-4).Further details of catalyst characterization and catalytic activity evaluation are provided in the Supporting Information.

| RESULTS AND DISCUSSION
Figure 1A depicts the 5Ni+xSr-9La+Zr (x = 0-4) catalysts' N 2 adsorption-desorption isotherms.The inflection points at P/P°= 0.7 is the beginning of capillary pore condensation of the adsorbed N 2 which reaches saturation at P/P°= 0.99.N 2 desorption starts after saturation.At the end of N 2 desorption, all the catalysts exhibited a typical irreversible type IV adsorption isotherm with H1 hysteresis loop which is characteristic of well-defined 2D mesopores. 45,46After modification of the catalysts with Sr, the isotherms remained the same and the nonpromoted catalyst keeps similar mesoporous morphology. 47Table 1 displays the textual features of the salient catalysts.The BET surface area decreases with the incorporation of the Sr promoter.The pore volume remains constant until high loading of Sr (3 and 4wt.%) is added.Alternatively, the pore slightly increases with the addition of Sr. but it decreases doping with high Sr loading.The rate of change of pore volume in relation to pore width is shown in Figure 1B, which is mainly unimodal pore size distribution.The pore size distribution curves in Figure 1B confirmed the existence of mesoporous cavities inside the as synthesized catalysts, with diameters between 40 and 300 Å.The maximum is achieved when the pore size is 173 Å.Table 1 also exhibits the values of the deactivation factor which is the ratio of the difference between the initial CH 4 and the final CH 4 conversions to the initial CH 4 conversion multiplied by 100.and denotes that the addition of Sr reduces its value and hence promotes the performance of the catalysts with respect to the nonpromoted catalyst.The decrease in surface area with the increase of Sr loading depicts the pore blockage of support.The H 2 -temperature programmed reduction analysis of the prepared catalysts offers information about the reduction pattern and temperature of metal oxides interacting with the respective support (Figure 2).The reduction peaks at 350°C are attributed to weakly immobilized NiO species, while the reduction peaks between 500°C and 600°C is attributed to the moderate interaction of NiO with the support. 48In the small peaks at high temperatures above 600°C, the Ni is probably diffused into the bulk of the support, coordinated tetrahedrally and octahedrally.The peaks are assigned to the reduction of complex NiO located in the subsurface layers of the ZrO 2 and La 2 O 3 lattices, showing an intimate interaction between NiO and ZrO 2 and La 2 O 3 . 49,50Doping 5Ni-9La+Zr with Sr slightly improved the reducibility of NiO crystallites as evidenced by the increase of both hydrogen consumption and the % reduction given in Table 2. Table 2 displays the H 2 consumption quantities of the various promoted and nonpromoted Sr catalysts and the calculation of the % reduction of all catalysts.
F I G U R E 2 H 2 -temperature programmed reduction of fresh 5Ni+xSr-9La+Zr (x = 0-4) catalysts.The increase of Sr loading from 1 wt.% to 4 wt.%Sr does not alter the crystallinity of catalyst samples except that at the highest of 4 wt.%., where a slim peak appears at 2θ = 25.1°which can be ascribed to orthorhombic SrCO 3 phase (JCPDS reference number: 00-001-0556). 51igure 4 displays the CO 2 -TPD for the present catalysts.It is branded that CO 2 is an inert compound; thus, highly active metal catalysts are necessary for its chemical activation.The addition of the Sr element is expected to alter the surface basicity and electron properties of the Nibased catalysts which contributes to CO 2 activation.In the CO 2 -TPD profile, It is established that CO 2 -desorption peaks that appear at low-temperature (100-250°C) are ascribed to weak Brønsted basic sites such as surface -OH groups, CO 2desorption peaks at intermediate temperature (250-400°C) are related to medium-strength Lewis base sites, and CO 2desorption peaks at high temperature (400-600°C) are linked to adsorption on low-coordination oxygen anions acting as strong basic sites.The nonpromoted sample displays the existence of weak and intermediate basic sites whereas the Sr-promoted samples produced peaks in all three ranges indicating the presence of strong basic sites.The 5Ni+1Sr-9La+Zr produced the lowest intensity peaks at high temperatures.Increasing the Sr loading increases the intensities.The 5Ni+2Sr-9La+Zr sample gave the optimum F I U E 3 X-ray diffraction of calcined 5Ni+xSr-9La+Zr (x = 0-4) catalysts.

T A B L E 2
T A B L E 3 The amount of CO 2 desorbed for 5Ni+xSr-9La+Zr (x = 0-4) samples.values.Further increase of Sr loading over 2% enhanced the carbon deposition and contribute to the deactivation as depicted in the TGA analyses.Therefore, the incorporation of 2% Sr promoter optimally increases the basicity and the CO 2 activation.Table 3 shows the amount CO 2 desorbed for different samples.Figure 5 displays the catalytic performance of all catalysts based on experimental results of CH 4 and CO 2 conversions.The CH 4 conversion denotes a somewhat decreasing profile due to the deactivation of the catalysts, while the CO 2 conversion displays a pretty steady response within 7.5 h.The higher values of CO 2 conversions than those of CH 4 reveal the impact of the RWGS reaction during the DRM. 52The conversions of CH 4 and CO 2 were 50.5%-58.8%and 59.8%-64.0%,respectively, over 5Ni-9La+Zr within 7.5 h at 700°C.Incorporation of the Sr promoter and increasing its loading to 4.0 wt.%, the CH 4 and CO 2 conversions increased gradually.The optimum conversions of CH 4 and CO 2 were 62.9%-65.9%and 69.1%-70.3%,respectively, over 5Ni+2Sr-9La+Zr within 7.5 h at 700°C.Further loading above 2.0 wt.% loading resulted in a reduction in CH 4 conversion and an increase in deactivation factor as can be seen in Table 1.This could be related to the excessive basicity as depicted in Figure 4. Adsorbed CO 2 combines with adsorbed CH 4 species across the catalyst surface in the DRM reaction, according to the Langmuir-Hinshelwood model. 53In comparison to an unpromoted catalyst, the oxidizing gas CO 2 can oxidize a wide spectrum of carbon deposits at high temperatures.More stable Ni particles and more extensive carbon deposit oxidation by CO 2 results in higher and consistent catalytic activity over the catalyst.Figure 6 illustrates the recorded Raman spectra, the dominant peaks appear at ca. 1344 cm −1 (D band), ca.1580 cm −1 (G band), and ca.2670 cm −1 .The G band associates to the stretching vibration mode with E2g 54 symmetry in the graphite aromatic layer, while the D band, the maximum important defect band, is linked to graphitic lattice vibration mode A1g. 55The intensity ratio of the D band to the G band (I D /I G ), is usually related to the degree of the graphitization degree of the carbon deposit.The numerical value of I D /I G ratio of carbon deposit over 5Ni+xSr-9La+Zr (x = 0-4) catalysts is 2.06, 2.00, 0.88, 1.07, and 0.89, 0.89, and 1.04 respectively.Generally, the higher value of the I D /I G ratio, the more disordered the carbon deposit is, and the lower the T max is.Clearly, the addition of Sr slightly favors the formation of carbon deposits with a more disordered structure.
Figure 7 shows the catalyst weight loss of the used catalysts.operated for 7.5 h.No weight loss below 400°C was observed, indicating the absence of oxidation of volatile organic compounds and desorption of strongly adsorbed water.The coke was oxidized between 500°C and 700°C suggesting that the carbon deposit was graphitic in nature.For the 5Ni-9LaZr, 5Ni+1Sr-9La+Zr, 5Ni+2Sr-9La+Zr, 5Ni+3Sr-9La+Zr, and 5Ni+2Sr-9La+Zr catalysts, the weight loss was about 30%, 45%, 51%, 56%, and 60% respectively.The catalyst that wasn't promoted produced the least amount of coke.The poor activity of the catalyst may be to blame for this.On the other hand, because of the higher extent of reaction, the promoted catalysts produced more coke.This was estimated by measuring mass loss associated with defined peaks in the differential thermal gravimetric analysis (DTGA) profiles, above 500°C where C(s) combustion takes place.Figure 7A displays the TGA profile while Figure 7B shows the DTGA of catalysts.
The investigation of sample morphology was conducted using transmission electron microscopy (TEM).Figure 8 in particular demonstrates the uniform dispersion of oxides of the active metals on the catalyst surface.The size of the oxide particles is in the nanometer range, while the size of the oxide particles on the spent catalysts is somewhat larger.In the case of the used catalyst, the amount of carbon is likewise greater.
The temperature-programmed oxidation (TPO) was carried out to compute the total amount, reactivity of the carbon deposit on the surface of catalysts.The developed peaks correspond to CO 2 evolution.Figure 9

| CONCLUSIONS
The DRM activity test was carried out at 700°C for 7.5 h, with a GSHV of 42,000 mL/g/h.In the beginning, the activity comparison between the 5Ni-9La+Zr catalyst and Sr-promoted catalysts showed that the 5Ni-9La+Zr catalyst exhibited inferior catalytic activity which was attributed to its higher deactivation factor and lower basicity.The 5Ni+2Sr-9La+Zr catalyst produced an F I G U R E 9 oxidation (TPO) of catalysts obtained after 7.5 h reaction at 700°C.excellent promotional effect and the least deactivation factor.The experimental results displayed that the optimum conversions of CH 4 and CO 2 were 62.9%-65.9%and 69.1%-70.3%,respectively, over 5Ni+2Sr9La+Zr within 7.5 h at 700°C.These findings were well revealed by the results of the characterization techniques used in this work: BET, temperature programmed reduction, X-ray diffraction, TGA, and TEM.

F
I G U R E 1 N 2 adsorption-desorption isotherms of the catalysts.(A) nitrogen sorption isotherms.(B) pore size distribution.T A B L E 1 Textural properties of the catalysts.

F
I G U R E 6 Raman spectra of the used catalysts.F I G U R E 7 TGA (A) and DTGA (B) of catalysts obtained after 7.5 h reaction at 700°C.
displays the O 2 -TPO profile of spent-5Ni+xSr-9La+Zr (x = 0-4 wt.%) catalysts.The different types of carbon on the Ni surface is confirmed by TPO measurements.The peaks at about 450-650°C are for moderately oxidizable carbon species.The present spent catalyst surfaces have predominately a single type of carbon specie (amorphous).
The analysis of H 2 -consumption during H 2 -TPR.