Strontium‐promoted Ni/ZrO2–Al2O3 catalysts for dry reforming of methane

Dry reforming of methane (DRM) is an established process that utilizes CH4 and CO2 to produce syngas, which is subsequently used to produce liquid fuel. Developing an optimized catalyst with favorable physicochemical properties is essential to enhance the effectiveness of the DRM process. In this study, we report novel findings on Ni/ZrO2 + Al2O3 catalyst performance promoted with strontium (Sr) for DRM reaction. The characteristics of fresh and spent catalysts were evaluated via a suite of analytical characterization techniques, including physisorption analysis, temperature‐programmed reduction, transmission electron microscopy, X‐ray diffractometry, Raman spectroscopy, temperature‐programmed desorption, temperature‐programmed oxidation, and thermogravimetric analysis before actual DRM performance analysis. The integration of Sr essentially enhanced the basicity, imparted CO2 adsorption stability, and improved the reducibility of the catalyst. An optimal concentration of 3.0 wt% Sr promoted on the catalyst afforded the highest CH4 and CO2 conversions. The work presented in this contribution affords an understanding of optimum Sr loading and provides insights into the synergistic role of Sr on catalytic performances as applied to the DRM process.


| INTRODUCTION
Dry reforming of methane (DRM) is a process that uses CH 4 and CO 2 to produce syngas with a mole ratio of H 2 /CO that is suitable for Fischer-Tropsch synthesis to produce liquid fuel. 1,2However, the DRM process can be hindered by the well-known problem of catalyst deactivation by carbon deposition, which can be resolved by modifying the catalyst structure. 3Therefore, highly active metals should be selected for DRM to prevent homogeneous cracking and coking of hydrocarbons. 4,5The most recent advancements in anti-coking Ni-based catalysts in the application of known reforming techniques to convert CO 2 and CH 4 thermally were summarized. 6Ni catalysts based on various substrates were examined by Damyanova et al. 7 It has been demonstrated that this type of support δ,θ- Due to their favorable properties, noble and rareearth metals such as platinum (Pt), rhodium (Rh), and palladium (Pd) have been extensively studied in the context of DRM, but their applications are not costeffective. 8The effects of 0.2 wt% of Au and Pt addition on the catalytic performances of 4 wt% Ni catalyst supported over Al 2 O 3 and Al 2 O 3 -10 wt% MO x (M = Ce or Mg) oxides in DRM were explored. 9The catalytic performance of monometallic Ni catalyst is also greatly improved by the effect of alumina dopants, along with the kind and quantity of carbon deposits. 10The reactions that occurred during the DRM procedure are described below: (3) Consequently, transition metals would be a more viable, affordable, and accessible option.For instance, nickel (Ni) has been extensively studied and demonstrated favorable syngas selectivity in DRM. 11,124][15] Lou et al. 16 reported that Ni/ZrO 2 catalyst with a 1.1 nm particle diameter exhibited excellent stability in DRM, whereby almost 90% of the initial activity was maintained after 60 h time-on-stream (TOS).Ni's tendency to sinter and coke under DRM conditions currently inhibits its widespread use.Charisiou et al. 17 have studied the carbon deposition during the dry reforming of biogas on a Ni catalyst based on (ZrO 2 , La 2 O 3 + ZrO 2 , and CeO 2 + ZrO 2 ).It was discovered that the La 2 O 3 and CeO 2 modifiers positively affected the Ni/ZrO 2 catalyst's performance and TOS stability.On the modified catalysts, it was discovered that the deposited carbon is rapidly oxidized at high temperatures.Similarly, the dry reforming of biogas using perovskite catalysts indicated that the Lanthanide promotion resulted in lower coke formation due to the interaction of the rare-earth oxide with the more reactive surface carbonaceous matter in redox reactions. 18he cerium-supported nickel catalysts with 15 wt% Ni synthesized via microemulsion, sol-gel, and auto combustion for DRM were investigated. 19Their findings show that the monoclinic NiO phase gives maximum activity to the Ni/CeO 2 catalyst.Tang and Gao 20 reviewed nanostructured CeO 2 to showcase its attractive properties, such as high surface areas, tunable pore sizes, abundant defects, and adjustable surface chemistry, greatly expanding their potential applications in energy and environmental catalysis.It was revealed that ceria was good for dispersing and stabilizing active metals in many catalytic systems.On the other hand, it can function as an active component by participating in reactions with lattice oxygens.
Using bimetallic catalysts, which provide synergistic effects via metal-to-metal interactions to circumvent this issue, appears to be an effective DRM solution.Concerning the observed long-term activity profiles, the impacts of metal loading and particle size of NiCo alloy placed on Ce 0.75 Zr 0.25 O 2 carrier on the carbon reaction routes of DRM at 750°C were clarified. 21The higher activity of cobalt-rich catalysts is related to the higher activity of this metal for methane decomposition; however, given that these catalysts also produce non-deactivating carbon deposits, their remarkable stability appears to be caused by the presence of large particles involved in long-term conversion. 22The reaction mechanism and coke behavior of the bimetallic catalysts of Ni and Co for methane dry reforming were investigated. 23It was found that the characteristics of metallic Ni and Co decrease while the electronic synergy of Ni and Co increases.The AIMD modeling results have good thermal stability at 1000 K.
Moreover, the analysis of the electronic structure of the coked catalyst surface demonstrates that Ni can remain active even after carbon buildup-this is attributed to Co and Ni's electronic synergy. 24Alternatively, Foo et al. 25 elaborated on the kinetics of carbon deposition and removal on catalysts for dry reforming that is encouraged by lanthanides.It was found that the carbon deposition rate diminished with temperature.Due to the interaction of the rare-earth oxide with the more reactive surface carbonaceous materials in a redox reaction, lanthanide promotion led to a decrease in coke production. 22The dynamic oxygen on the catalyst's surface in DRM, which functions as an intermediary to react with the carbon species from dissociated CH 4 , was examined by Damaskinos et al. 26 It has been demonstrated that preserving a specific quantity of surface oxygen can maximize the use of CH 4 and enhance coking resistance.
The addition of strontium (Sr) to Ni-based catalysts (alloying) may increase dispersion, improve selectivity, and increase resistance to carbon deposition and sintering. 27For example, Zhang et al. 28 investigated the application of Ni/ZrO 2 catalyst and discovered that adsorbed oxygen species on the catalyst surface could improve its ability to activate CO 2 and dissociate CH 4 , which, in turn, enhanced resistance to carbon deposition.Modifying the catalyst support would also aid in enhancing the stability of the catalyst by reducing carbon deposition.Impregnation is a standard method for preparing catalysts that is widely used in the production of industrial catalysts.
For the DRM process to be effective, it is crucial to develop an optimized catalyst with favorable physicochemical properties.In recent years, numerous basic metal oxides used as supports or promoters in the DRM process have been thoroughly investigated.For example, Shin et al. 29 reported that Ni/ZrO 2 -Al 2 O 3 catalyst was an effective DRM catalyst with high resistance to carbon formation.The performance of a Ni/ZrO 2 + Al 2 O 3 catalyst promoted with strontium (Sr) for DRM reaction is evaluated in this study.There is currently a knowledge gap regarding optimal Sr loading and the role of Sr in catalyst performance, and this forms the scientific motivation for our research.To the best of our knowledge, the incorporation of Sr into the overall Ni/ZrO 2 + Al 2 O 3 catalytic system and its effect on the DRM process have not been fully or holistically explored by previous researchers.Characterizations of fresh and spent catalysts have also been examined to shed light on the physicochemical properties of nickel dispersion and carbon deposition.

| Catalyst preparation
The catalyst 5Ni-xSr/10ZrO 2 + 90Al 2 O 3 (x = 0, 1, 2, 3, and 4 wt%) was synthesized by impregnation of the required amounts of Ni(NO 3 ) 2 •6H 2 O by 5.0 wt% loading of Ni as the active metal, Sr(NO 3 ) 2 as the promoter, and 10ZrO 2 -90Al 2 O 3 as support.Nickel nitrate hexahydrate is dissolved in 10 mL of distilled water and thoroughly mixed-strontium nitrate is added to obtain a homogeneous solution.To this solution, 1 g of 10ZrO 2 -90Al 2 O 3 support was added.The solution was heated at 80°C for 5 h and stirred until a slurry was formed.The excess water was evaporated by placing the sample in an oven, dried at 120°C for 16 h, and then calcined at 600°C with a heating rate of 3°C/min for 3 h.Subsequently, the prepared catalysts were ground into powder.The description of the catalyst characterization is provided in the supplementary information.

| Catalyst performance evaluation
The catalysts were tested for DRM at 800°C reaction temperature under atmospheric pressure.A packed bed reactor stainless steel reactor (0.91 mm internal diameter; 30 cm height) was used to conduct the experiment.An amount of 0.10 g of catalyst was placed in the reactor on top of glass wool.Stainless steel, sheathed thermocouple K-type, axially positioned close to the catalyst bed, was utilized to establish the temperature during the reaction.Before the start of the reaction, activation of the catalysts was conducted at 800°C with H 2 .This lasted for 60 min, and the residual H 2 was purged with N 2 .During the dry reforming reaction, the feed volume ratio was maintained at 3:3:1 for CH 4 , CO 2 , and N 2 gases, respectively, with a space velocity of 42,000 mL/g cat h −1 .The outlet gas from the reactor was connected to an online gas chromatography with a thermal conductivity detector to determine its composition.CH 4 and CO 2 conversions were calculated by using the equations shown below: The reproducibility and repeatability of experiments were tested by preparing fresh catalysts and testing them at least three times from the beginning.Thus, the reported results present an average of triplicate runs, and the results were within 2% accuracy.

| Characterizations
Figure 1 shows the nitrogen physisorption isotherms of the studied catalysts, which exhibit a feature typical of IUPAC Type IVa isotherm classification.In our case, there is the presence of a final saturation plateau which commences at a relative pressure of slightly less than 0.9 before the inflection point is reached.The Sr loading appears to have a marginal to nonexistent effect on the physisorption characteristics of the catalyst samples.The presence of hysteresis loops indicates mesoporosity of the catalysts, which suggests the possible occurrence of capillary condensation-that is, gas condenses into a quasi-liquid phase within a pore at a pressure less than the saturation pressure of the bulk liquid. 30This may happen when the pore width in the catalysts is higher than a certain critical width, which is dependent on the adsorption temperature. 31The IUPAC classification for the hysteresis loops in our study is a hybrid H2(b)/H5 type, which is associated with pore-blocking. 31This is rather expected due to the effect of the Ni impregnation and/or Sr loading present in our study.
In our study, the hydrogen reduction temperature is a vital indication of the catalytic oxidation capabilities of the metal oxide support. 32Figure 2 shows the hydrogen reduction temperature profiles of the catalysts as influenced by the varying loadings of Sr.There is a broad reduction peak between 450°C and 600°C for the catalyst without Sr, which is representative of Ni-ZrO 2based catalyst. 33There is a marked absence of peaks at temperatures lower than 400°C, implying the lack of free NiO. 34The inverted peaks at around 200°C imply the occurrence of intensely interacting metal-support species-the interaction between the metal species and the support material could potentially lead to a more complex reduction behavior, resulting in the observed inverted peaks.All the catalysts exhibit distinct bimodal profiles, though such profiles are evident at temperatures higher than 500°C with reduction peaks clearly observable at around 550°C and 850°C for catalysts with infused Sr.There is a palpable trend in which the amount of added Sr is directly correlated with the intensity of the reduction peaks at 500°C-that is, the increase of the added amount of Sr is proportional with enhanced reduction capabilities.A similar trend is also observed for the peaks at 850°C, whereby the addition of Sr imparts enhanced reduction intensity, but such enhancement is not in a precise incremental manner.There is marginal or no apparent shifting of peaks between temperatures observed with the addition of Sr.The reduction peaks observed at 700°C could possibly relate to nickel oxide with strong interaction with Al 2 O 3 due to increased loading of Sr with an eventual reduction of nickel aluminate at 800°C. 35he XRD spectra of the catalysts with different Sr loadings are shown in Figure 3.The diffraction peaks at around 30°and 50°can be attributed to zirconia (ZrO 2 ). 36I G U R E 1 Nitrogen physisorption isotherms of the catalysts.
F I G U R E 2 H 2 -TPR profiles of the catalysts with different loadings of Sr from 0 to 4 wt%.
Figure 4 shows the CO 2 -TPD profiles of the catalysts as impacted by the varying loadings of Sr, which are used to establish the basicity of the catalysts.It seems that three CO 2 desorption peaks appear between 200°C and 900°C.These peaks are illustrative of the existence of basic sites possessing varying strength: weak Brønsted basic sites attributed to surface OH groups (low temperature), medium-strength Lewis acid-base sites (intermediate temperature), and low-coordination oxygen anions acting as strong basic sites (high temperature). 37ll five samples show the existence of weak basic sites, as evidenced by the very prominent peaks at around 250°C and no apparent medium basic sites.Interestingly, increases in weight % of Sr (i.e., 2-4 wt% Sr) induce stronger basic sites in the three catalyst samples, as evidenced by the peaks at around 650°C, while very strong basic sites are observed for the 3 and 4 wt% Sr samples.It should be noted that the very strong basic sites give rise to stable adsorption of CO 2 and are characterized by desorption temperatures exceeding 700°C. 38The presence of very strong basic sites for 3 and 4 wt% Sr samples suggests that these two samples may not be the most optimized catalysts as excessive basicity of the catalyst is detrimental to catalytic activity since it promotes more CO 2 dissociation (CO 2 → C + O 2 ), triggering deactivation of the catalyst.Indeed, this unfavorable effect intensifies accordingly with the initiation of the Boudouard reaction (2CO → C + CO 2 ) at high temperatures, resulting in a greater amount of coke deposition on the catalyst's surface. 34The performance of the work is compared with the literature (previously reported findings), as shown in Table S1 of the support information.The result confirms the suitability of the present work.Figure S3 shows the long TOS result of the best catalyst, providing information regarding the catalyst's durability and its suitability for reuse after simple regeneration.Table S2 indicates the deactivation factors of the catalysts.The addition of the Sr promoter reduces the factor values.
Figure 5 shows the effect of Sr loadings on the CH 4 and CO 2 conversion percentages concerning TOS at a reaction temperature of 800°C.The addition of Sr evidently enhances the conversion of CH 4 and CO 2 .Based on the conversion curves, the unloaded (pristine) 5Ni/10Zr-Al catalyst already exhibits relatively high CH 4 and CO 2 conversion percentages within the 72%-82% range.The gradual increment of Sr additions enhances both the conversion percentages to more than 90% conversions (for 3 wt% Sr).This may be attributed to a lowering in CH 4 and CO 2 conversion activation energies.
Nonetheless, the conversion experiences a slight drop to the 80% range for the 4 wt% Sr, which implies that the 3 wt% Sr may well be the optimized loading concentration.It should be noted that there is a gradual reduction in CH 4 and CO 2 conversion percentages concerning TOS, which was also observed by Zhang et al. 12  catalysts.It appears that the CO 2 conversions are significantly greater (about 5% more) than CH 4 conversions at corresponding TOS for pristine 5Ni/10Zr-Al catalyst, while such discrepancies for the Sr-promoted catalysts are comparatively marginal.For the pristine sample, this is attributed to the generation of H 2 as a result of the reforming participation in a side reaction via parallel reverse water gas shift reaction (RWGS) (CO 2 + H 2 → CO + H 2 O). 39,40igure S1 displays the H 2 /CO ratio concerning TOS and its influence on various Sr loading.The addition of Sr enhanced the ratio toward unity and showed less variation with time.The 3% Sr sample assumes the highest and the best stability trend.
The weight loss of spent catalysts is shown in Figure S2.TGA curve (a) presents three segments: the weight loss between 200°C and 500°C is predominantly brought on by the oxidation of amorphous carbon types, which are more volatile than graphitic carbon, while the weight loss below 200°C is mostly attributed to the removal of H 2 O and other chemisorbed species.The gasification of graphite carbon is the key factor contributing to the weight loss of over 600°C.As shown in Figure S2, the 5Ni-1Sr/10Zr-Al and 5Ni-3Sr/10Zr-Al catalysts exhibit the least weight loss, while the 5Ni-2Sr/ 10Zr-Al catalyst incurs the most weight loss.Figure S2b also shows the rate of change of TGA against the temperature.All samples started carbon gasification at around 500°C, whereas the main gasification process occurred at 800°C.This phenomenon explains the variation of carbon type formed.Figure 6 shows the Raman spectra of the catalysts with different Sr loadings.A Raman shift band at 1300-1400 cm −1 , representing the D band, can be assigned to all the samples. 41It is widely regarded as a structural flaw of defective carbon materials.A Raman shift band at around 1600 cm −1 can be assigned to the G band.The addition of 1 wt% Sr has a negligible effect on the molecular composition and molecular structure (i.e., graphitization degree) of the catalyst judging by the position of the Raman shift bands.At higher wt% (2 and 3 wt%), however, there is a band shift to 1800 cm −1 for both loadings and an additional band at approximately 1900-1950 cm −1 range.Interestingly, such bands disappear for the highest loading at 4 wt%.It was possible that the addition of Sr at 2 and 3 wt% modified the structure of the basic catalyst (5Ni/10Zr + Al), which could result in the establishment of stable ZrO 2 phases and other mixed phases of zirconium promoter oxide. 34or the TPO studies, the result is shown in Figure 7, where the deposited coke can be classified according to its oxidation temperature as Cα, Cβ, and Cγ. 42Cα, or active carbon, is evident, though it may be overlapped with physisorbed CO 2 .Inactive species (Cβ and Cγ) are supposed to be responsible for catalyst deactivation.Their higher degasification temperatures suggest a lower reactivity.Complete oxidation of the deposited coke was not reached below 900°C.The TPO curve of the Ni-based catalysts evidenced the presence of two or three peaks at 101°C, 210°C, and 650-755°C, respectively.5][46][47] The broad peak at around 475°C for 1 wt% Sr sample can be attributed to the oxidation of carbonaceous species.The presence of the Sr promoter induces the appearance of the third peak, enhancing the development of amorphous and graphitic carbon.When strontium is integrated into the catalyst, it can synergize with the surface of the support material (ZrO 2 + Al 2 O 3 ) and the active metal (Ni) to develop phases with varying properties compared to the unpromoted metal.Such compounds can have a stabilizing effect on the active metal particles.For example, strontium-containing compounds could perhaps synergize with the active sites and afford a protective layer that prevents coalescence.The negative peak around 300°C is related to the oxidation of the metallic nickel.
Figure 8 shows the transmission electron micrographs of fresh and spent 5Ni/10Zr-Al, 5Ni-3Sr/10Zr-Al, and 5Ni-4Sr/10Zr-Al samples calcined at 600°C, respectively.In TEM images of metal nanoparticles, the observed dark and light spots are attributed to the interaction of electrons with the nanoparticles and the surrounding medium.Dark regions in TEM images often correspond to areas where electrons have been scattered more strongly or absorbed by the sample, while lighter regions in TEM images generally correspond to areas where electrons have passed through with less scattering or absorption.The micrographs show the existence of dark spots of ZrO 2 , 48 which are dispersed throughout the "light" features of the surrounding matrix.It can be observed that the Ni nanoparticles are rather dispersed in the support areas, which is consistent with findings on Ni-ZrO 2 /Al 2 O 3 catalysts reported by Liu et al.The spent catalytic samples seem to be slightly more dispersed after the process.Interestingly, our findings somewhat tallied with Gupta and Deo, in which the dark spots in their reported micrographs were due to Ni metal particles while the lighter "gray" spots were attributed to γ-Al 2 O 3 support. 49

| CONCLUSIONS
The study elaborated on the effects of Sr incorporation into a 5Ni/10Zr-Al catalyst for dry methane reforming.The optimal loading for enhancing the activity and stability of the 5Ni/10Zr-Al catalyst was determined by preparing catalysts with varying amounts of Sr loadings from 0 to 4 wt%.In terms of BET and porosity, the addition of Sr has a negligible to nonexistent effect on the morphology according to the characterization results.In contrast, the predominant effect of Sr was associated with its capacity to absorb CO 2 at high temperatures (800°C) and enhance the reducibility of the catalyst.The highest CH 4 and CO 2 conversions were observed in catalysts containing 3.0 wt% Sr, as indicated by these noteworthy observations.The incorporation of Sr increased the basicity of the catalyst and led to the stable adsorption of CO 2 and its reducibility.The addition of Sr improved the catalyst's performance in terms of activity, prevented catalyst sintering, and reduced coke formation, which had a significant positive effect on the catalyst's stability.In addition, the presence of Sr promoter altered the type of carbon produced, favoring the formation of amorphous and graphitic carbon.

Al 2 O 3 ,
MgAl 2 O 4 , SiO 2 -Al 2 O 3 , and ZrO 2 -Al 2 O 3 significantly impacts the structure and catalytic efficiency of the catalysts.The best activity and stability were displayed by the Ni catalyst supported on MgAl 2 O 4 due to the existence of tiny and evenly scattered Ni particles.It was established that the accumulation of Ni particles and carbon accumulation were the root causes of the low activity.
for Ni/ZrO 2 -Al 2 O 3 -based F I G U R E 3 XRD spectra of the catalysts by varying Sr loadings from 0 to 4 wt%.F I G U R E 4 CO 2 -TPD profiles of the catalysts as affected by varying loadings of Sr from 0 to 4 wt%.

F
I G U R E 5 The effect of Sr loadings on the (A) CH 4 conversion % and (B) CO 2 conversion % concerning time-on-stream (TOS) at a reaction temperature of 800°C.F I G U R E 6 Raman spectra of the used catalysts with varying Sr loadings.

F
I G U R E 7 Temperature-programmed oxidation (TPO) thermograms of the spent samples subsequent to 420 min of reaction time in DRM at 800°C.F I G U R E 8 Transmission electron micrographs of (A, B) 5Ni/10Zr-Al, (C, D) 5Ni-3Sr/10Zr-Al, and (E, F) 5Ni-4Sr/10Zr-Al samples calcined at 600°C, where (A, C, and E) are for fresh and (B, D, and F) are for spent samples, respectively.
Table S1 shows comparative catalytic performances concerning maximum CH 4 and CO 2 conversions of other nickel-based catalysts with the current study.It is evident from Table S1 that our currently studied strontium-promoted Ni/ZrO 2 + Al 2 O 3 catalysts generally outperformed other nickel-based catalysts in terms of CH 4 and CO 2 conversions.