Investigating the Impact of Na₂WO₄ Doping in La₂O₃-Catalyzed OCM Reaction: A Structure–Activity Study via In Situ XRD-MS

Abstract

The La₂O₃ catalyst exhibits good performance in OCM reactions for its promising C₂ selectivity and yield. Previous studies have affirmed that the formation of carbonates in La₂O₃ impedes the catalyst’s activity as a result of poisoning from CO₂ exposure. In this study, a series of Na₂WO₄-impregnated La₂O₃ catalysts were synthesized to investigate the poisoning-resistant effect. The bulk phase and kinetics of the catalysts were analyzed in reactors employed with in situ XRD-MS and online MS, focusing on the CO₂ adsorption on La₂O₃ and the phase transition process to La₂O₂CO₃ in temperature zone correlated to OCM light-off. In situ XRD analysis revealed that, with Na₂WO₄ doped, CO₂ exposure at elevated temperatures formed La₂O₂CO₃ in tetragonal crystal phases, exhibiting distinctive differences from the hexagonal phase carbonates in undoped commercial La₂O₃. The ability to develop tetragonal or monoclinic La₂O₂CO₃ was suggested as a descriptor to assess the sensitivity of La₂O₃ catalysts to CO₂ adsorption, a tunable characteristic found in this study through varying Na₂WO₄ doping levels. Coupled XRD-MS analysis of CO₂ adsorption uptake and phase change further confirmed a positive dependence between the resistivity of La₂O₃ catalyst to CO₂ adsorption and its low-temperature C₂ selectivity. The results extended the previous CO₂ poisoning effect from multiple perspectives, offering a novel modification approach for enhancing the low-temperature performance of La₂O₃ catalysts in OCM.

1. Introduction

The oxidative coupling of methane (OCM) reaction is deemed a promising industrial process, as it enables the direct conversion of CH₄ into C₂H₄ and C₂H₆, which are essential precursors in the industrial realm [1,2,3,4]. The obstacle preventing the large-scale industrialization of the OCM reaction is the restricted C₂ yield and selectivity at high reaction temperatures [5,6,7,8]. Hence, catalysts exhibiting high catalytic activity at low temperatures in OCM reactions are indispensable to meet industrial requirements. Following the groundbreaking contributions by Keller and Bhasin, numerous catalytic materials have undergone investigation for the OCM reaction [8,9]. Currently, La₂O₃ has undergone extensive research and is acknowledged as one of the most effective catalysts for OCM reactions [2,3,8,10]. In general, most OCM reaction systems, including La₂O₃, require very high temperatures around or above 800 °C [11], which is important to accomplish the cleavage of C-H bond and CH₃ radical desorption to form methyl radicals. On the other hand, the overall OCM reaction is highly exothermic [12]. While OCM is generally recognized as a catalytic process, its efficiency is influenced by synergistic gas-phase chemistry, which is affected by factors, such as residence time, pressure, and the ratio of CH₄ to O₂. Experimental studies indicate that the rate of C₂ formation is heavily reliant on CH₃ concentrations [13]. The formation rate of methyl radicals is only mildly affected by temperature but varies significantly depending on the specific catalyst employed. Considering both the thermodynamics and kinetics, it is imperative to improve the performance of OCM at low temperatures.

As a single metal oxide species, La₂O₃ demonstrates susceptibility to carbonation by CO₂ exposure, either from the atmosphere or the OCM reaction circumstance, resulting in the formation of La₂O₂CO₃, which is exclusively generated at temperatures exceeding 500 °C [11,14,15,16,17,18,19]. Several relevant studies have focused on La₂O₂CO₃, regarding whether CO₃²⁻ exerts an inhibitory or promotive effect on the OCM reaction activity of La₂O₃ [15,17,20,21,22,23]. CO₂ is the primary by-product of La₂O₃-catalyzed OCM [24], of which the yield is around 10% accompanied by a 25% CH₄ conversion rate and nearly 50% selectivity of C₂ in the reaction [12]. The in situ surface and bulk structure formation of La₂O₂CO₃ from pure La₂O₃ under a variety of OCM-correlated conditions have been investigated by our group in detail using XPS and XRD [11,25].

Taylor et al. conducted a comparison of various synthesized lanthanum salts and suggested that the rank of C₂ activity is as follows: La₂O₂CO₃ > La₂(CO₃)₃ > La(OH)₃ > La₂O₃ [26]. Other studies also agreed that synthesized lanthanum catalyst containing carbonate exhibits excellent performance [27]. However, under high-temperature OCM reaction conditions, most La₂O₂CO₃ will undergo thermal decomposition and be reduced back into purified La₂O₃ on the catalytic surface. On the other hand, most in situ structure or real-time activity-related studies, in which the CO₂ input is applied as a reaction parameter, concluded that carbonates have a negative impact in La₂O₃-catalyzed OCM reaction. Through ¹⁶O and ¹⁸O isotope and SSITKA analysis, Lacombe et al. concluded that, when exposed to CO₂ at 750 °C, La₂O₃ will strongly inhibit surface oxygen dissociation [28,29]. Yide Xu et al. observed a negative impact on the CH₄ conversion rate, C₂ selectivity, and ${\mathrm{C}}_2^{=}$/${\mathrm{C}}_2^0$ ratio when CO₂ was added to the OCM reaction catalyzed by 10% La₂O₃/ZnO [30]. In our previous publication, by employing in situ XRD-MS, La₂O₂CO₃ to La₂O₃ ratio was quantitatively estimated and controlled by adjusting CO₂ exposure. Characterization afterward revealed that the higher the initial La₂O₂CO₃ level in the catalyst bulk phase, the higher the OCM light-off temperature of both CO_x and C₂ products in OCM, which is a typical poisoning descriptor [11]. XPS results after in situ CO₂-correlated treatment also revealed that the sample pre-carbonated by CO₂ exhibits an activation temperature nearly 65 °C higher than the one with a pristine La₂O₃ surface [25]. Combined with in situ kinetic correlated structure studies and detailed DFT modeling, it revealed a thermodynamically favored peroxide structure as the active oxygen site. On La₂O₃, this peroxide is formed on the subsurface six-coordinated oxygen site, which is also the same site of carbonate formation. The model provided a reasonable preliminary explanation from the perspective of chemical pathways that the La₂O₂CO₃ acts as a poisoning species in La₂O₃-catalyzed OCM reaction; however, La₂O₂CO₃ is a very robust species. In an inert environment, such as in a vacuum or an Ar-filled reactor, the complete decomposition of La₂O₂CO₃ back to purified La₂O₃ requires a high temperature of around 800 °C [11]. In an atmosphere with CO₂ at the same high temperatures, the catalyst sample could still contain both components [11,12,15,23,31,32,33,34]. This explains why OCM requires a very high reaction temperature as it is a necessity to decompose the poisoned sites.

Further in situ studies correlated the higher resistance to carbonation of La₂O₃ catalysts to its better low-temperature OCM activity [35]. In situ XRD-MS was applied to investigate the CO₂ adsorption behavior and OCM reactivity of two La₂O₃ catalysts, a lab-synthesized nanorod La₂O₃ [23,33,34] and a commercial La₂O₃ supplied by [11]. In the identical temperature range, the apparent activation energy for the OCM reaction of nanorod La₂O₃ is approximately 60 kJ mol⁻¹ lower than that observed for commercial La₂O₃ [35]. The online MS CO₂ adsorption curve and the coupled in situ XRD indicate that, under the same exposure condition of CO₂ concentration and heating temperature, nanorod La₂O₃ always absorbs less CO₂ than commercial La₂O₃. This case by itself strongly suggests that increasing the resistivity of CO₂ adsorption of La₂O₃ is an effective method to enhance the low-temperature OCM performance of La₂O₃, which is also a natural consequence of the above theory that CO₂ acts as the poison in La₂O₃.

On the other hand, Na₂WO₄ is widely acknowledged as a highly effective additive in catalyst formulations, particularly in the case of the Mn_xO_y–Na₂WO₄/SiO₂ catalyst, which demonstrates excellent performance in the OCM reaction [36,37,38,39,40,41,42]. Pengwei Wang et al. synthesized a TiO₂-doped Mn₂O₃–Na₂WO₄/SiO₂ catalyst and revealed that it has accepted CH₄ conversion of 22% and C_2–3 selectivity of ~62% at a lower temperature (650 °C) and established a reaction mechanism model supported by the active oxygen transforming on W⁴⁺–W⁶⁺ [42]. Recently, Shihui Zou et al. innovatively developed a novel two-part catalyst comprising Na₂WO₄/SiO₂ and La₂O₃; this catalyst involves modifying the La₂O₃ catalyst with additional Na₂WO₄, achieving a C₂ yield of 10.9% at 570 °C [43]. Studies indicate the potential of incorporating Na₂WO₄ as an active additive in modified catalysts. Motivated by the Na₂WO₄-modified catalyst, Na₂WO₄ is introduced into the catalyst as an approach of optimization, especially on the low-temperature performance, in this study.

Based on the above research foundation of Na₂WO₄ and La₂O₃, we used in situ XRD-MS characterization to investigate the effect of doping Na₂WO₄ into La₂O₃ samples, focusing on CO₂-adsorption-correlated structure behavior and the low-temperature OCM reaction activity. In situ XRD allows real-time monitoring of the pure oxide to dioxymonocarbonate phase changes in La₂O₃ during the linear heating process under CO₂ exposure, while the coupled online MS collects the real-time CO₂ adsorption from reaction outlet gas providing a quantitative estimation of the uptake. By combining the changes in bulk structure and gas composition through the time temperature correspondence, the resistance of the samples to CO₂ adsorption can be directly compared through the experiment results. It was found that, after doping with Na₂WO₄, a series of new behaviors were induced in the commercial La₂O₃ catalysts. First, the online MS revealed that the CO₂ adsorption temperature on the doped La₂O₃ sample is significantly increased. The in situ XRD also observed that the formed La₂O₂CO₃ crystal structure after CO₂ exposure changed from hexagonal to monoclinic/tetragonal with Na₂WO₄ doped over La₂O₃. Both the bulk phase crystal structure of the formed dioxymonocarbonates and the CO₂ uptake temperature are characteristics of the nanorod La₂O₃ [23]. In the end, the OCM reaction activity results were acquired in a microreactor coupled with online MS, and the doped samples were found to have higher conversion and selectivity at 500–650 °C than the undoped La₂O₃. Combining the CO₂ adsorption and OCM activity results, it becomes evident that the CO₂ adsorption resistance of the La₂O₃ catalyst is critical for better low-temperature OCM performance. The results of this article provide a new approach and possibility for improving La₂O₃.

2. Results and Discussion

2.1. Differences in La₂O₃ and La₂O₂CO₃ Phase Structure

After in situ calcination at 800 °C, the results comparison of bulk phase analysis obtained by XRD from M-La₂O₃ samples with different loading levels of Na₂WO₄ are shown in Figure 1. Little different from the undoped M-La₂O₃ sample, the XRD patterns of all three M-La₂O₃_nW catalysts exhibit similar crystal structures after high-temperature purification. The diffraction patterns all align well with the hexagonal phase of La₂O₃. As generally reported, it is expected that Na₂WO₄ exists in the bulk phase as a cubic crystal structure [44]. However, the XRD pattern of all three M-La₂O₃_nW samples at room temperature did not reveal any signal correlated to Na₂WO₄. This absence could be attributed to the low content and effective dispersion of Na₂WO₄, so the grain size is not adequate to generate diffraction patterns.

To confirm that Na₂WO₄ was indeed doped into the sample, XPS analysis was performed for M-La₂O₃_1W, 3W, and 5W samples (calcined at 800 °C) revealing the surface elements, their content, and electronic structure. As shown in Figure 1b (left), W 4f spectra exhibit the main W 4f_7/2 peak at 35.5 eV confirming the +6 oxidation state for all samples [45]. While the only possible La oxidation state is +3 (metallic La is not expected at these conditions), different La³⁺ compounds are known to demonstrate various La 3d_5/2 multiplet splitting. The La 3d_5/2 spectra presented in Figure 1b (right) show the splitting of 4.5 eV, which is clearly associated with La₂O₃ (without hydroxide or carbonate) [46,47]. With the increase in Na₂WO₄ doping amount, the W 4f peaks intensity grows being in agreement with the samples preparation procedure. The atomic percentages of W and La (normalized to the total of W and La, other elements not considered) given in

Table 1. W and La atomic percentages (other elements are not taken into consideration) for M-La₂O₃_1W, 3W, and 5W samples.

SAMPLES	XPS ATOMIC PERCENTAGES		XRD
	W	La	Grain Size (nm)
M-La2O3_1W	0.8	99.2	27.12
M-La2O3_3W	2.9	97.1	37.85
M-La2O3_5W	4.2	95.8	39.33

demonstrate the increase in W content on the catalyst surface in agreement with the increase in Na₂WO₄ loading.

As mentioned in previous studies, it was observed that the La₂O₃ catalysts with identical bulk structures have different OCM performance, whereas the nanorod La₂O₃ catalyst shows better low-temperature performance than the M-La₂O₃ catalyst. Distinctions of the bulk phases between these two catalysts were only found after carbonate formed under identical CO₂ treatment conditions [11,35]. In the subsequent experiments, the three M-La₂O₃_nW catalysts and M-La₂O₃ catalysts were exposed to identical CO₂ flow.

2.2. Adsorption of CO₂ and OCM Reaction Performance

As mentioned in the introduction, previous in situ XRD-MS studies [35] indicated that, under the same exposure condition of CO₂ concentration and heating temperature, the same in situ XRD-MS was applied for simultaneously monitoring of CO₂ adsorption processes over the four catalyst samples during linear heating. To compare with this previous result, constant CO₂ exposure treatments are performed on the M-La₂O₃_5W sample with 10% CO₂ in the first in situ measurement. The XRD signal intensity in this region is plotted in a topographical map style (Figure 2a), and the XRD patterns clearly show the peaks at around 29.1° and 30.1° in this range, representing hexagonal La₂O₃ (002) and (101) are not reduced. The results showed that the doped M-La₂O₃_5W is always robust to CO₂ exposure, remaining oxidic under the same 10% CO₂ exposure when the temperature reaches 600 °C. Furthermore, it still shows no bulk phase change after additional heating to temperature region up to 740 °C (Figure 2a). The simultaneously obtained online MS signal also shows little change in CO₂ output, confirming that there is no CO₂ adsorption rate significant enough to change its partial pressure in the in situ XRD-MS cell during the whole process (Figure 2b). In previous studies [35], the optimized nanorods with better low-temperature activity also showed a stable La₂O₃ phase while exposed to 10% CO₂ both throughout the 600 °C isotherm and after cooling down to room temperature. In this study, the M-La₂O₃ sample only after a simple Na₂WO₄ doping modification shows a similar behavior of resistance to carbonation.

Complete carbonation over the M-La₂O₃_nW samples with significant CO₂ uptake (MS) and carbonate phase change can only be more easily detected under nearly 100% CO₂ exposure. In this way, their CO₂ adsorption behavior can be directly compared to the undoped M-La₂O₃ sample. The calibrated MS signal, presented as CO₂ consumption percentage normalized to the input during the adsorption process, was plotted vs. real-time linear temperature profile, as illustrated in Figure 3a. The simultaneously obtained oxide to carbonate full phase change, represented by normalized intensities of La₂O₃ (011) at 30.1° and the La₂O₂CO₃ (103) peak at 29.9°, is plotted in Figure 3b. In this series of measurements, as the temperature reaches 500 °C, the M-La₂O₃ sample initiates CO₂ adsorption at approximately 510 °C, reaching its peak value at 527 °C. M-La₂O₃_1W sample initiates CO₂ adsorption at 556 °C, reaching its peak at 574 °C. In comparison to M-La₂O₃, the temperature for CO₂ adsorption is approximately 47 °C higher. M-La₂O₃_3W sample starts CO₂ adsorption around 570 °C, with the maximum value achieved at 585 °C, which is about 14 °C higher than the 1% wt Na₂WO₄-doped La₂O₃ sample. M-La₂O₃_5W sample initiates CO₂ adsorption at approximately 570 °C, reaching its maximum at 589 °C. For all four samples, after reaching 700 °C, CO₂ adsorption saturated, reaching around zero rate. By integrating each online MS peak, the yielded total CO₂ uptake molar amounts are 208, 265, 283, and 307 μmol for the M-La₂O₃, M-La₂O₃_1W, M-La₂O₃_3W, and M-La₂O₃_5W samples, respectively. As the sample loading is around 300 μmol (100 mg, with molar mass around 326 g/mol), this CO₂ uptake represents almost full carbonation (CO₂ uptake to La₂O₃ loading ratios are 68%, 86%, 92%, and 100%, respectively), which can be represented by the following equation:

$${\mathrm{La}}_2{\mathrm{O}}_3+{\mathrm{CO}}_2\to {\mathrm{La}}_2{\mathrm{O}}_2{\mathrm{CO}}_3$$

(1)

The result shows that adding Na₂WO₄ slightly raises the CO₂ uptake amount, approaching a 1:1 molar ratio. In addition to the CO₂ uptake amount increase, there is a noticeable increasing trend in the CO₂ uptake peaking temperature. In the case of the three doped M-La₂O₃_nW samples, the CO₂ adsorption temperature is 47 °C–62 °C higher than that of M-La₂O₃. Furthermore, at equal temperatures below 650 °C, the estimated adsorbed CO₂ quantities in sequential of the four samples are as follows: M-La₂O₃ > 1% > 3% > 5%, revealing a singular positive dependent correlation with the increment in Na₂WO₄ doping (Figure S1). As a solid base, it may be expected that Na₂WO₄ add-on species will decrease the uptake temperature. But instead, it actually increases the temperature. It could be because Na₂WO₄ is only doped at an overall low level, not as the main component.

The preceding results have indicated that, in comparison to M-La₂O₃, the three doped La₂O₃ M-La₂O₃_nW catalysts display more resistance to CO₂ adsorption. Based on this observation, OCM reactivity evaluation was conducted over these four samples in the microreactor. The exhaust gas from the reaction was collected for online MS analysis of C₂ and CO_x products, and the reaction properties, including conversion, product yields, and selectivity, were extracted. All results are presented in Figure 4. Figure 4a reveals that the OCM reaction light-off temperature of M-La₂O₃ is higher than the other three samples and at low temperatures below 500 °C, conversion from this system is also the lowest. The CH₄ conversion rate, C₂ yield, C₂ selectivity, and CO_x yield results at 600 °C and 650 °C are illustrated in the column charts of C₂ yield and C₂ activation temperature for the four samples, as depicted in Figure 4b,c. For all four samples, the C₂ selectivity always shows a singular positive dependence on the Na₂WO₄ loading level. The CH₄ conversion rates of M-La₂O₃, after its later light-off, increased faster than all the other systems to 17.8% at 650 °C, while the M-La₂O₃_1W and M-La₂O₃_3W are notably lower at 14.0% and 13.0%, respectively. The overall C₂ product yields of M-La₂O₃_1W and M-La₂O₃_3W are also lower than that (7.8%) of M-La₂O₃, measuring 6.3% and 5.8%, respectively. However, the C₂ selectivity of the undoped M-La₂O₃ remained below 50% and its excessive conversion mostly turned into unwanted CO_x by-products. For the two most important OCM evaluation properties, C₂ yield and selectivity, the M-La₂O₃_5W surpasses the undoped M-La₂O₃ system from light-off to 650 °C, reaching 9% and 58.2%, respectively. The total CH₄ conversion rate of M-La₂O₃_5W also rebounds to 17.3%. Consequently, tuning the Na₂WO₄ level doped on La₂O₃ shows a positive effect on the catalyst OCM performance, especially in the low-temperature region.

The C₂ yield did directly increase as the loading of Na₂WO₄ on the catalyst increased. However, there is an obvious singular dependence between the light-off temperatures and the Na₂WO₄ doping level on M-La₂O₃, which is plotted in Figure 5. For C₂ products, light-off temperature of M-La₂O₃ is 481 °C, while those of the M-La₂O₃_1W, M-La₂O₃_3W, and M-La₂O₃_5W samples are 475 °C, 460 °C, and 415 °C, respectively. The light-off temperatures of CO_x also have the same dependence but are all around 70 °C lower. These two significant decreasing trends vs. the Na₂WO₄ doping level clearly contrast the reversed trend of CO₂ adsorption uptake temperature. As mentioned in the introduction, CO₂ adsorption results in carbonate formation, which poisons the active oxygen sites. In OCM, CO₂ is the main by-product, which will cause sample carbonation as an autocatalysis process, especially with the high temperature of the reaction. The Na₂WO₄ doping will increase the catalyst resistance to carbonation under CO₂ exposure, as already proved in Figure 3. For those doped samples, fewer catalyst surface sites will be poisoned by the CO₂ products, and a significant reaction rate becomes sustainable at lower temperatures. As a result, doping La₂O₃ with Na₂WO₄ is a worthy approach to reduce the activation temperature of C₂ products and achieve better low-temperature (<650 °C) OCM reaction activity as compared to traditional M-La₂O₃.

2.3. Differences in Carbonate Bulk Phase between Doped and Undoped Samples

During the CO₂ uptake measurements by online MS, simultaneous in situ time-resolved XRD experiments were performed to validate phase changes under CO₂ exposure during linear heating on the three doped M-La₂O₃_nW samples and undoped M-La₂O₃. The wide range full-scan in situ XRD patterns were collected immediately after the specific treatment, as illustrated in Figure 5. The diffraction peak angle at high temperature (Figure 5a) exhibits a 2θ decrease of 0.18° from the room temperature (Figure 1) due to lattice thermal expansion. After the sample cooled down, direct exposure to CO₂ (99% balanced with 1% Ar) at room temperature did not change (Figure 5b) the bulk structure so there was no bulk CO₂ uptake. Figure 5c demonstrates the M-La₂O₃_nW samples under CO₂ exposure at 800 °C and the bulk phase completely converted to La₂O₂CO₃. Interestingly, the dioxymonocarbonates formed in M-La₂O₃_nW samples evidently exhibit a tetragonal La₂O₂CO₃ crystal phase, as can be directly compared with the PDF database plotted in the same figure. After cooling to room temperature, all three tetragonal La₂O₂CO₃ phases transformed into monoclinic La₂O₂CO₃ (Figure 5d), characterized by the single diffraction peak at 31.5° splitting into two peaks at 2θ = 30.7° and 31.3°. On the other hand, the undoped M-La₂O₃ after complete carbonate formation only shows a hexagonal phase, either at high or room temperature, which was identical to our previous published results [11,35]. The results suggest that this phase change to tetragonal structure dioxymonocarbonate is strongly correlated to the high resistance to CO₂ adsorption.

The time-resolved XRD rapid scan collected during the heating of the M-La₂O₃_nW samples in the CO₂ flow is plotted in Figure 6a–c. These data are collected simultaneously with the online MS data collected in Figure 3. The 28.5~33.5° window perfectly captured the phase change from hexagonal La₂O₃ to tetragonal La₂O₂CO₃ as it covers the characteristic peaks of La₂O₃ (011) at 30.1° and the La₂O₂CO₃ (103) and (110) peaks at around 29.9 and 31.5°, respectively. As explained in Section 2.2, the phase change temperature can only be more roughly estimated as the scan time resolution sets a low limit of the temperature range of each scan at 38 °C. However, it still indicates that the higher doped M-La₂O₃_3W and M-La₂O₃_5W samples have a phase change temperature (570–606 °C) higher than that of the lower doped M-La₂O₃_1W sample (530–570 °C). It can also be noted from this figure that the phase change temperature difference is within 38 °C. This phase change temperature result agrees with the MS analysis results on the CO₂ uptake temperature of the four samples.

Previous research also indicates that after complete carbonation, the M-La₂O₃ sample only shows a uniform hexagonal phase. The tetragonal/monoclinic phase carbonates formation could be found on M-La₂O₃, but only after a partial phase change, which can be controlled by heating in low CO₂ concentration (<30%) to 600 °C for a limited time [35]. On the other hand, the nanorod La₂O₃ catalyst, with a higher low-temperature OCM reactivity compared to the M-La₂O₃ catalyst, also formed tetragonal La₂O₂CO₃ at high temperatures after carbonation and subsequently transformed into monoclinic La₂O₂CO₃ crystal phase upon cooling to room temperature. In a word, after the M-La₂O₃ was doped with Na₂WO₄, its carbonate formation behavior became similar to the nanorod La₂O₃. The previous studies, based on the comparison between the nanorod La₂O₃ and M-La₂O₃ catalysts behavior, already suggest that the special tetragonal carbonate phase change correlates with the higher CO₂ adsorption resistance. In this study, as the La₂O₃ materials are all the same for the four catalysts, the analysis data certainly strengthen this conclusion as it can be correlated as follows:

Tetragonal carbonate
↓
CO₂ adsorption/poisoning resistance
↓
Low-temperature OCM activity

As a result, we strongly recommend that being able to form tetragonal carbonate is an experimental descriptor for a La₂O₃ catalyst with better low-temperature activity in OCM.

3. Materials and Methods

3.1. Catalyst Preparation

Through impregnation in Na₂WO₄ solution, La₂O₃ can be modified to obtain samples with diverse weight loading percentages. Both Na₂WO₄ (S859551, ≥98%) and commercial La₂O₃ (L812319, ≥99.99%, denoted as M-La₂O₃) were from Shanghai Macklin Biochemical Co., Ltd. (Shanghai, China). Previous BET measurement (Kr) of M-La₂O₃ yields a relatively low specific surface area of 3.4 m² g⁻¹ (56 μmol g⁻¹ assuming lattice density of 10¹⁵ cm⁻²) [11]. Initially, 5, 10, and 15 mg of Na₂WO₄ were precisely weighed and introduced into separate 20 mL portions of deionized water. Subsequently, 495, 490, and 485 mg of La₂O₃ were gradually added to the solution under continuous magnetic stirring. The solution was fully dried at 70 °C for 24 h and then calcined at 800 °C in a muffle furnace for 4 h with a heating rate of 5 °C min⁻¹. The contents of Na₂WO₄ were calculated to be 1%, 3%, and 5% wt, respectively (denoted as M-La₂O₃_nW, where n refers to the weight percentage of Na₂WO₄-loaded). The precipitation does not significantly change the BET surface area results and dispersion. All freshly prepared samples were characterized immediately. Prior to commencing the CO₂ absorption and OCM experiment, the sample was pretreated at 800 °C in an Ar atmosphere for 30 min in the in situ XRD or online MS microreactors, which aims to prevent sample hydroxylation or carbonation before the introduction of experimental gases.

3.2. In Situ XRD-MS

The XRD employs the Bruker D8 Advance powder X-ray diffractometer equipped with a Cu K α target with a wavelength of λ = 0.154 nm (8.05 KeV). The in situ heating reaction cell (Anton Paar XRK 900) is capable of conducting testing at 900 °C and 10 bar. The specific information on the relevant equipment is mentioned in the literature, including the design of the reaction gas pathway and coupled online mass spectrometry [11]. Before further characterization, the loaded 0.1 g sample (~300 μmol) was heated in situ to 800 °C at a rate of 10 °C min⁻¹ under a 20 sccm Ar airflow, followed by a 30 min dwelling at 800 °C. Previous studies [25,47] show that this treatment effectively depletes the bulk and surface impurities including hydroxyl and carbonates, leaving only purified La₂O₃.

For online CO₂ adsorption uptake studies, once cooling back to room temperature, the sample was exposed to CO₂-contained gas at 20 sccm. After a 20 min gas flushing, the XRD pattern was collected at room temperature with a step size of 0.02° and a sensor exposure time of 1.5 s, spanning from 15 to 60°. Subsequently, the temperature was raised from room temperature to 800 °C at a rate of 10 °C min⁻¹, and the XRD pattern at 800 °C was collected using the same parameters. During the heating process, the continuous fast scanning XRD patterns were collected in the range of 28.5~33.5 °, with a step size of 0.05 ° and a sensor exposure time of 1.0 s each for M-La₂O₃_nW samples, totally 100 s for each scan. The hardware still requires extra time after each scan to re-calibrate the initial alignment of the arms; so for the linear heating at the rate of 10 °C min⁻¹, each scan covers a temperature variation of about 38 °C. The custom-designed coupled online mass spectrometry system (SRS RGA 200) collects real-time data of the exhaust gas released from the XRD in situ reaction cell during the heating process using capillary tubes [48]. In situ XRD-MS measurements were performed with two CO₂ input conditions, 10% and nearly 100% (99% CO₂ balanced with 1% Ar). The 10% condition is close to the practical CO₂ concentration under real OCM reaction conditions above 400 °C. The nearly 100% condition makes it easier to capture full phase change from La₂O₃ to La₂O₂CO₃ as shown in the data results section. Due to the inability of mass spectrometry to detect the consumption of pure gas uptake signal, such as 100% CO₂, 1% Ar gas was mixed with 99% CO₂ for labeling. In this way, accompanied by the CO₂ partial consumption in an adsorption process, mass spectrometry will detect the partial pressure increase in the Ar signal from the sampled mixture. The relative CO₂ uptake ratio thus can be re-calibrated from the Ar signal change.

The grain size on a specific face in the XRD result was calculated by Debye–Scherrer Formula:

$$\mathrm{D}\left(\mathrm{nm}\right)=\frac{\mathrm{K}·\mathsf{\gamma }}{\mathrm{B}·\cos \mathsf{\theta }}$$

(2)

where K in Equation (2) is 0.89 for the (100) face, which is calculated using the FWHM of the diffraction peak.

3.3. Online MS Microreactor

OCM reaction kinetics data for all samples were obtained using a specialized online mass spectrometry (MS) microreactor designed to operate under high-temperature and high-pressure conditions. The sample was sealed in a ¼-inch quartz tube using sealing rings and sleeves to ensure airtightness. The gas manifold offers diverse reaction input options, enabling real-time online sampling and gas characterization via a mass spectrometer (Pfeiffer PrismaPlus). More detailed information about the experimental setup is described in our previous publications [49].

For online OCM reactivity evaluation, 0.1 g samples were loaded in the center of quartz tubes plugged with quartz wool on both ends. Before the OCM reaction, the sample undergoes calcination in Ar up to 800 °C to eliminate carbonates and hydroxides. After a 15 min OCM reaction gas (CH₄:O₂:Ar = 5:1:4, GHSV = 8000 mL·h⁻¹ gcat⁻¹) purging at room temperature, the reactor is linearly heated to 740 °C at a rate of 10 °C min⁻¹. The CH₄ conversion rate, C₂ selectivity, and yields of C₂ and CO_x are calculated by the following equation, derived from carbon balance:

$$\mathrm{Conversion}\left(\%\right)=\frac{\left[\mathrm{C}{\mathrm{H}}_{4\mathrm{input}}\right]-[\mathrm{C}{\mathrm{H}}_{4\mathrm{out}}]}{\left[{\mathrm{CH}}_{4\mathrm{input}}\right]}\times 100$$

(3)

the selectivity of the product is determined using the following equation:

$$\mathrm{Selectivity}\left(\%\right)=\frac{\mathrm{n}·\left[\mathrm{Product}\right]}{\left[{\mathrm{CH}}_{4\mathrm{converted}}\right]}\times 100$$

(4)

the yield of the product is determined using the following equation:

$$\mathrm{Yield}\left(\%\right)=\frac{\mathrm{n}·\left[\mathrm{Product}\right]}{\left[{\mathrm{CH}}_{4\mathrm{input}}\right]}\times 100$$

(5)

where n in Equations (4) and (5) is the carbon number of the product molecules.

The TOF of the product is estimated using the following equation:

$$\mathrm{TOF}\left({\mathrm{s}}^{-1}\right)=\frac{\mathrm{Activity}}{\mathrm{a}·\mathrm{S}}=\frac{\mathrm{Yield}·\left[{\mathrm{C}\mathrm{H}}_{4\mathrm{i}\mathrm{n}\mathrm{p}\mathrm{u}\mathrm{t}}\right]}{\mathrm{m}·\mathrm{a}·\mathrm{S}}$$

(6)

where $a$ is the surface La atom density on the (001) surface of La₂O₃, which is calculated in average one La atom per 0.134 nm² (about 7.5 × 10¹⁴ cm⁻²), applying the La₂O₃ (001) surface model; $S$ is the specific surface area acquired from BET measurement (3.4 m²/g). The product of $\mathrm{a}·\mathrm{S}$ yields the catalyst surface La site density of 42 μmol/g_cat; m is the loading quality of the sample; yield is calculated by Equation (5).

3.4. XPS Measurement

The XPS surface analysis is performed utilizing a ThermoFischer ESCALAB 250Xi photoelectron spectrometer, applying monochromatic X-ray irradiation AlKα (hv = 1486.7 eV) and using a 180° double-focusing hemispherical analyzer with a six-channel detector. For each sample, C 1s, O 1s, La 3d, La 4d, W 4d, W 4f, and Na 1s core level spectra are collected with 30 eV pass energy and 0.1 eV step. The survey XPS scans are monitored as well to demonstrate no impurities on the sample surfaces. The binding energy scale of the collected spectra is calibrated to the adventitious carbon C 1 s peak at 284.8 eV. The atomic percentages for W and La are calculated from the corresponding photoelectron peaks after background subtraction taking into account transmission function and atomic sensitivity factors.

4. Conclusions

The carbonation behavior of La₂O₃ catalysts, as an inherent phenomenon during the OCM reaction process, is studied and correlated with OCM activity. This study carried out a comprehensive investigation of the bulk structure and kinetic analysis of CO₂ adsorption on M-La₂O₃_nW catalysts. The CO₂ kinetic adsorption curve indicated that an elevated Na₂WO₄ doping level raised the temperature at which CO₂ absorption occurred, at least 46 °C higher than that of commercial La₂O₃. These results suggest that Na₂WO₄-doped La₂O₃ is more difficult to form La₂O₂CO₃. Furthermore, online MS results indicated that at low temperatures (650 °C), the M-La₂O₃_nW catalyst exhibited higher C₂ yield, and C₂ selectivity, and produced less CO₂ compared to commercial La₂O₃. In situ XRD-MS results also revealed that with Na₂WO₄ doped on the M-La₂O₃_nW catalyst, CO₂ induced transform into tetragonal La₂O₂CO₃ after high-temperature CO₂ treatment. This behavior diverges from the hexagonal carbonation pattern observed in undoped M-La₂O₃ at elevated temperatures. The results correlate the tetragonal La₂O₂CO₃ formation upon CO₂ exposure to higher resistance to CO₂ poisoning and better low-temperature OCM activity. Coupled the XRD measurement with simultaneous kinetic analysis of CO₂ adsorption, this observation provides a novel approach for the enhancement of La₂O₃ catalysts through structure descriptor obtained from in situ characterization associated with real-time activity.