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Article

Effect of Ba Addition on the Catalytic Performance of NiO/CeO2 Catalysts for Methane Combustion

by
Xiuhui Huang
1,*,
Wenkai Yang
1 and
Junfeng Li
2
1
School of Energy and Power Engineering, University of Shanghai for Science and Technology, Shanghai 200093, China
2
Shanghai MCC20 Construction Co., Ltd., Shanghai 201999, China
*
Author to whom correspondence should be addressed.
Processes 2024, 12(8), 1630; https://doi.org/10.3390/pr12081630
Submission received: 8 July 2024 / Revised: 24 July 2024 / Accepted: 30 July 2024 / Published: 2 August 2024
(This article belongs to the Special Issue Progress in Catalysis Technology in Clean Energy Utilization)
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Abstract

:
Methane catalytic combustion, a method for efficient methane utilization, features high energy efficiency and low emissions. The key to this process is the development of highly active and stable catalysts. This study involved the synthesis of a range of catalysts, including NiO/CeO2, NiO–M/CeO2, and NiO-Ba/CeO2. In order to modify the NiO/CeO2 catalysts to improve their catalytic activity, various alkaline earth metal ions were introduced, and the catalysts were characterized to evaluate the impact of different alkaline earth metal ion doping. It was found that the introduction of Ba as a dopant yielded the highest catalytic activity among the dopants tested. Based on this, the influence of the impregnation sequence, the Ba loading amount, and other factors on the catalytic activity of the NiO/CeO2 catalysts doped with Ba were investigated, and comprehensive characterization was conducted using a variety of analytical techniques, including N2 adsorption/desorption, X-ray diffraction, Fourier transform infrared, hydrogen temperature-programmed reduction, methane temperature-programmed surface reaction, and oxygen temperature-programmed oxidation. The H2–TPR characterization results suggest that Ba introduction partially enhances the reducing property of NiO/CeO2 catalysts, and improves the surface oxygen activity in the catalysts. Meanwhile, the CH4–TPSR and O2–TPO results indicate that Ba introduction also boosts the bulk-phase oxygen liquidity in the catalysts, renders the migration of bulk-phase oxygen to surface oxygen, and increases the surface oxygen number in the catalysts. These results provide evidence of the effectiveness of this catalyst in methane catalytic combustion.

1. Introduction

Amid escalating concerns regarding energy scarcity and environmental pollution, natural gas, renowned for its abundant reserves, cost-effectiveness, and recognition as a clean energy source, is currently attracting considerable attention. Natural gas is primarily composed of methane; the direct flame combustion of methane yields significant high temperatures but with lower efficiency and generates a substantial amount of harmful substances, such as nitrogen oxides and carbon monoxide, leading to environmental pollution. Moreover, when methane is directly combusted to a certain extent, the gas becomes too diluted to sustain ongoing direct combustion, resulting in a substantial portion of underutilized methane. If these incompletely burned methane emissions were to be released directly into the atmosphere, the profoundly potent greenhouse effect of methane gas would once again impact the environment [1]. Catalytic combustion represents one of the most straightforward, feasible, and efficient methods for utilizing natural gas. It not only reduces the utilization temperature of methane, thereby decreasing energy consumption and enhancing methane efficiency, but also diminishes emissions of gases such as CO2, NOx, and CH4. The catalytic combustion of methane is an exothermic process (Equation (1)) that generates water vapor, thus imposing stringent requirements on the catalyst. Suitable catalysts for methane catalytic combustion typically possess the following characteristics: high catalytic activity, superior hydrothermal stability, and resistance to sulfur poisoning. Current research primarily focuses on enhancing these three aspects of catalyst performance.
C H 4 + O 2 → C O 2 + H 2 O ,   ∆ H 298 = − 192   k c a l / m o l
In recent years, there has been a growing interest in the catalytic combustion of methane due to its high energy efficiency, reduced pollutant emissions, and high heat release [2,3,4]. However, the activation of the C-H bond in methane presents a notable obstacle in the development of catalysts with high activity and stability. Generally, noble-based metals such as Pd and Pt demonstrate excellent catalytic activity in the context of low-temperature CH4 combustion [5,6]. Building on this, Wang [7] introduced Au-Pd on the catalyst and found that the synergistic interaction between the support material and Au-Pd resulted in a catalyst with good catalytic activity and optimized hydrothermal stability. Nevertheless, Sadokhina [8] found that Pd-based catalysts suffered from a loss of catalytic activity when exposed to conditions involving SO2 poisoning. Furthermore, when H2O and SO2 coexisted, the loss of activity was even greater, while the addition of Pt enhanced the resistance of Pd-based catalysts to withstand water inhibition and SO2 poisoning. But the widespread applications are still constrained by their high cost, easy poisoning, and sintering at high temperatures. As a result, considerable research efforts have been directed toward exploring transition metal-based catalysts in CH4 combustion due to their good stability and affordability [9,10,11]. A few studies have focused on CH4 combustion using nickel-based catalysts, especially nickel-containing CeO2 catalysts [12,13,14]. The findings indicate that the incorporation of NiO can augment the concentration of oxygen vacancies and facilitate the mobility of lattice oxygen in CeO2, which is favorable for the combustion of hydrocarbons. For example, Ce1-XNiXO2 catalysts prepared via the sol–gel method exhibit superior reducibility and catalytic activity compared to pure CeO2 for CH4 combustion.
CeO2 is a crucial support material in CH4 combustion due to its notable oxygen storage capacity (OSC). In our previous research [15], we prepared NiO/CeO2 catalysts that exhibited high activity and resistance to water vapor, and we investigated the reasons underlying the high activity of NiO/CeO2 catalysts. However, the CeO2 support material is susceptible to sintering at high temperatures, leading to a substantial reduction in the specific surface area of the catalyst and particle growth, leading to catalyst deactivation [16,17]. It is often necessary to load CeO2 onto more stable carriers, such as Phosphate [18] or CO3O4 nanosheets [19], or to dope CeO2 with other metal ions. For example, the introduction of Zr can enhance the sintering resistance of CeO2, elevate its concentration of oxygen vacancies, facilitate the mobility of oxygen, and enhance its redox ability. For instance, Junjie Chen [20] found that the addition of Zr dopants in catalysts can enhance their reducibility and thermal stability, thus improving their combustion activity. Long-term stability tests have demonstrated that H2O exhibits a more severe inhibitory effect on catalysts than CO2. Nevertheless, when H2O and CO2 are eliminated from the raw materials, the catalyst’s performance can be restored. Similarly, Ya-Qiong Su [21] demonstrated that the stable structure formed by replacing one Ce4+ ion with two Pd2+ ions can effectively activate CH4, resulting in high activity. Furthermore, the incorporation of other metals such as Ru-Re, Pt, and Mo has also demonstrated the potential to enhance the activity and stability of cerium-based catalysts to some extent [22,23]. Nevertheless, the applications of these catalysts remain constrained due to their elevated cost, susceptibility to poisoning, and tendency to undergo sintering at high temperatures.
In addition, the introduction of alkaline earth metals and alkali metals is also a viable method to enhance the activity and thermal stability of Ni-based catalysts in methane catalytic combustion. As demonstrated by Qiao [24], the incorporation of a small amount of Ca (Ca:Ce < 0.1) can enhance the activity of NiO–CeO2 catalysts; however, the doping of Ca does not improve the thermal stability of the catalyst. Zhao [25] found that the introduction of Sr can enhance the oxygen mobility in Ce–Zr catalysts, thereby improving the oxygen storage/release capacity, consequently enhancing the activity and thermal stability of the catalyst. Similarly, Du [26] discovered that the catalytic activity and hydrothermal stability of Ba-loaded catalysts were significantly improved, and the reduction performance of the catalysts and the ability to activate O2 were also enhanced. With respect to water resistance, Ida Fribe [27] found that loading Ba onto the catalyst could enhance the CH4 oxidation activity under steam conditions. Furthermore, the incorporation of other elements such as La and K [28,29] can, to a certain extent, enhance the activity and stability of cerium-based catalysts.
Ba, a commonly used alkaline earth metal, is rarely reported in the literature concerning its influence on the catalytic activity of NiO/CeO2 for methane combustion. The mechanisms by which Ba affects the properties of the catalyst remain unclear. In this study, we investigated the impact of doping different alkaline earth metals on the activity and thermal stability of NiO/CeO2 catalysts, aiming to identify the alkaline earth metal with the most significant influence on catalyst activity. Building upon this foundation, we investigated the effects of different impregnation sequences, varying calcination temperatures, and different amounts of alkaline earth metal doping on the performance of NiO/CeO2 catalysts. Characterization techniques such as XRD, FT–IR spectroscopy, O2–TPO, H2–TPR, and CH4–TPSR were employed to examine the cause of the influence of alkaline earth metal introduction on the structure and performance of NiO/CeO2 catalysts.

2. Experimental

2.1. Catalysts Preparation

The NiO/CeO2 catalysts were synthesized using D–CeO2–450 as the support material to prepare 10 wt.% NiO/CeO2 catalysts according to the impregnation method. The catalysts were named NiO/CeO2-X based on different calcination temperatures. Prepared samples underwent tableting, trituration, and filtering with a 40–60-mesh sieve before use. The detailed procedure for the preparation of the NiO/CeO2 catalyst is comprehensively described in our previous study [15].
Ba and Ni were loaded according to the impregnation method, and the preparation methods were classified as three types according to the distinct impregnation sequences.
In Method 1, the Ba precursor was impregnated first, followed by impregnation of the Ni precursor. Using D–CeO2–450 as the support, the required concentration of barium nitrate (Ba(NO3)2) solution was calculated based on the Ba loading. An equivalent volume of Ba(NO3)2 solution with varying concentrations was used for impregnation. After impregnation, the sample was dried at 100 °C and then calcined in a muffle furnace at 450 °C for 4 h in an air atmosphere. Subsequently, the sample was impregnated with nickel nitrate (Ni(NO3)2) solution, dried at 100 °C, and calcined in a muffle furnace at 450 °C for 4 h in an air atmosphere. The resulting sample was named the NiO/Ba/CeO2 catalyst. The catalyst was then subjected to tableting, grinding, and sieving to obtain 40–60 mesh particles for use.
Method 2 involved impregnating the Ni precursor first, followed by impregnation of the Ba precursor, with a preparation method similar to that of Method 1. Using D–CeO2–450 as the support, the sample was first impregnated with nickel nitrate solution, dried at 100 °C, and calcined at 450 °C for 4 h. Subsequently, the sample was impregnated with barium nitrate solution, dried at 100 °C, and calcined in a muffle furnace at 450 °C for 4 h in an air atmosphere. The resulting sample was named the Ba/NiO/CeO2 catalyst. The catalyst was then subjected to tableting, grinding, and sieving to obtain 40–60 mesh particles for use.
Method 3 involved simultaneous impregnation of Ni and Ba: Using D–CeO2–450 as the support, the sample was impregnated with an equivalent volume of nickel nitrate and barium nitrate solutions, dried at 100 °C, and calcined in a muffle furnace at 450 °C for 4 h in an air atmosphere. The resulting sample was named the NiO–Ba/CeO2–X (where X represents the calcination temperature) catalyst. The catalyst was then subjected to tableting, grinding, and sieving to obtain 40–60 mesh particles for use.
The synthesis procedure utilized for the NiO–M/CeO2 catalyst is the same as that of the NiO–Ba/CeO2 catalyst, that is, the simultaneous impregnation method was employed to fabricate the two-component supported NiO–M/CeO2 catalyst, and the calcination and treatment conditions remained unchanged.

2.2. Characterization

XRD patterns of all samples were recorded on a Bruker D8 Focus diffractometer using Cu Kα radiation of wavelength 1.541 Å (40 kV, 40 mA, canning step = 0.02°). The FT–IR absorption spectra were recorded on a Nicolet NEXUS 670 FT–IR spectrometer, with 32 scans at an effective resolution of 4 cm–1. The sample to be measured was ground with KBr and pressed into thin wafer for analysis. H2–TPR was conducted with a TCD detector. Sample (50 mg) was loaded in quartz tube reactor. A total of 5% H2/N2 mixture gas of 40 mL·min–1 was used, and the reactor was heated from room temperature to 800 °C with a heating rate of 10 °C·min–1. Methane temperature-programmed surface reaction (CH4–TPSR) of samples was performed in a quartz micro-reactor. A total of 200 mg catalyst in the reactor was pretreated at 500 °C for 30 min in 50 mL·min–1 20 vol.% O2/He mixture gas. After the sample was cooled to the room temperature, the sample was heated from the room temperature to 800 °C in 50 mL·min–1 1 vol.% CH4/He at a heating rate of 10 °C·min–1. The outlet gas was analyzed by a quadrupole mass spectrometer (INFICON Transpecter 2). The signals of CH4, H2O, CO, O2, and CO2 were recorded at m/z = 15, 18, 28, 32, and 44, respectively. O2–TPO was taken on the sample after treatment by CH4–TPSR. The samples were pretreated by He at room temperature for 1 h, and then heated from the room temperature to 800 °C in 40 mL·min–1 5 vol.% O2/He at a heating rate of 10 °C·min–1. The outlet gas was analyzed by the quadrupole mass spectrometer and the signal of O2 was recorded at m/z = 32.

2.3. Catalytic Activity Tests

The CH4 combustion catalytic capabilities of various NiO–Ba/CeO2 catalysts were evaluated using a quartz tube reactor (Ø 6 mm) and operation under atmospheric pressure conditions. The reagent gas mixture employed in the investigation comprised 1 vol.% CH4 + 4 vol.% O2 in Ar, with a collective stream rate of 50 mL·min–1. The gas mixture was passed through a catalyst (200 mg, 20–40 mesh) bed, and the weight hourly space velocity (WHSV) utilized in this trial was established at 15,000 mL·g–1·h–1. The reactants and products were subjected to real-time analysis using a gas chromatograph (GC) that was equipped with TCD. The catalyst’s performance was assessed by conducting programmed-temperature measurements spanning a temperature range from 200 °C to 700 °C, with a heating rate of 5 °C·min–1. The catalyst activity was assessed based on three temperature points: T10, T50, and T90, which, respectively, indicate the temperatures at which methane conversion reaches 10%, 50%, and 90%.

3. Results and Discussion

3.1. Thermal Stability of the NiO/CeO2 Catalysts

Figure 1a presents the activity curves of the NiO/CeO2 catalysts for methane combustion under different calcination temperatures. The figure reveals that the activity of the NiO/CeO2–450 and NiO/CeO2–550 catalysts showed little difference. However, upon raising the temperature of calcination to 650 °C, the performance of the NiO/CeO2–650 catalyst exhibited a notable decrease in activity. Notably, T10, T50, and T90 temperatures elevated by 38 °C, 37 °C, and 64 °C, respectively (as shown in Table 1), as compared to the NiO/CeO2–450 catalyst. Furthermore, as the calcination temperature continued to increase, the catalyst’s activity showed a decreasing trend. The T10, T50, and T90 temperatures for the NiO/CeO2–1000 catalyst were determined to be 442 °C, 545 °C, and 611 °C, respectively. The results indicate that, at a low calcination temperature (<550 °C), the catalyst’s activity remained unchanged with a change in calcination temperature. However, at a high calcination temperature (>600 °C), the catalyst’s activity decreased significantly with an increase in calcination temperature. These observations suggest that the thermal stability of NiO/CeO2 catalysts needs further improvement.
Figure 1b depicts the XRD spectral patterns of the NiO/CeO2 catalysts obtained at various calcination temperatures. According to the figure, with the increase in calcination temperature, the CeO2 diffraction peak intensity was gradually enhanced and became sharpened, suggesting that the CeO2 particles began to grow after high-temperature calcination (Table 1). Notably, after calcination at 1000 °C (NiO/CeO2–1000), the catalyst’s specific surface area was only 3 m2·g–1, and the CeO2 developed severe sintering, resulting in the CeO2 diffraction peak becoming extremely sharp. The NiO diffraction peak also became more distinct, as the NiO particles originally dispersed onto the CeO2 surface began to aggregate and grow due to the significant decrease in the specific surface area of CeO2 following the calcination process at 1000 °C. Consequently, the declined catalyst-specific surface area and the particle growth were the major reasons behind the decreased activity of the NiO/CeO2 catalyst. To address this issue, a third component was introduced to delay the sintering of NiO/CeO2 catalysts at high temperatures, thereby improving its activity and thermal stability during the catalytic combustion of methane.

3.2. Effects of Different Metal Ion Doping on the Activity of NiO/CeO2

To enhance NiO/CeO2 catalysts’ thermal stability, we investigated the effects of introducing various metal elements, including alkaline metal K, alkaline earth metals Ca, Mg, Sr, and Ba, and the lanthanide La, on the catalysts’ activity. The catalysts were fabricated using the simultaneous impregnation method and subsequently calcined at 650 °C. They were named NiO–M/CeO2 (M = K, Ca, Mg, Sr, Ba, La), and their activities in catalytic methane combustion were examined (Figure 2). The experimental findings indicated that the introduction of alkaline metals, alkaline earth metals, and lanthanide greatly altered the activities of NiO/CeO2 catalysts. Among them, the introduction of K and Mg noticeably reduced the catalysts’ activity, with T10 values of 441 °C and 448 °C, respectively, which increased by 78 °C and 83 °C compared to the NiO/CeO2 catalyst without a third component. The introduction of Ca, Sr, and La had a negligible effect on the catalysts’ activity, slightly decreasing the activities of the NiO–M/CeO2 catalysts. In contrast, the introduction of Ba had a remarkable positive effect on the catalysts’ activity, leading to a significant improvement, as indicated by the T10, T50, and T90 values of 346 °C, 431 °C, and 505 °C, respectively. These values decreased by 25 °C, 21 °C, and 26 °C, respectively, compared to those of the NiO/CeO2 catalyst. Thus, the catalysts with Ba introduction were selected for further research.

3.3. Effect of Ba Impregnation Sequence on the Activity of NiO–Ba/CeO2

Catalytic Activity

In this study, three methods were adopted to prepare the NiO–Ba/CeO2 catalysts (calcinated at 450 °C), as mentioned in Section 2.1. After evaluating various catalysts’ activities in the combustion of methane, it was observed that the impregnation sequence significantly affected the catalysts’ activity. According to Figure 3, the impregnation sequence greatly affected the catalysts’ activity. The NiO–Ba/CeO2 catalyst achieved the most activity, exhibiting T10, T50, and T90 values of 337 °C, 406 °C, and 462 °C, correspondingly. In comparison, the performance of the NiO/Ba/CeO2 catalyst ranked in second place, with T10, T50, and T90 values of 353 °C, 424 °C, and 499 °C, separately. Finally, the Ba/NiO/CeO2 catalyst exhibited the lowest level of activity, as evidenced by the comparatively higher T10, T50, and T90 values as high as 372 °C, 450 °C, and 539 °C, respectively. The poor activity of the Ba/NiO/CeO2 catalyst might be ascribed to the fact that Ba was impregnated later, which totally enveloped the outer surface of the catalyst in the form of BaCO3. On the one hand, BaCO3 covered the active site of the catalyst and affected the methane molecule adsorption and activation; on the other hand, BaCO3 led to excessive alkalinity on the catalyst’s outer surface. According to the literature report [30], the weakly alkaline catalyst promotes CO2 production, which enhances the methane conversion rate. However, the excessive alkalinity of the catalyst hampers the catalytic combustion of methane. In our investigation, the NiO–Ba/CeO2 catalyst, synthesized through the simultaneous impregnation method, exhibited the highest level of catalytic activity, which might be because the Ni and Ba were evenly dispersed at the time of simultaneous impregnation, rendering the relatively moderate interactions between Ni, Ba, and the CeO2 support.

3.4. Effect of Ba Loading Amount on the Activity of NiO–Ba/CeO2

3.4.1. Catalytic Activity

Figure 4 investigates the impact of Ba loading on the activity of the NiO–Ba/CeO2 catalyst in the methane combustion reaction, with all catalysts calcined at 650 °C. The findings demonstrate that Ba addition enhances the catalyst’s activity, with the NiO–Ba/CeO2 catalyst exhibiting the highest activity at a Ba loading of 1 wt.%, with T10, T50, and T90 values of 346 °C, 431 °C, and 505 °C, respectively. These values were observed to decrease by 25 °C, 21 °C, and 26 °C, respectively, when compared to those of the NiO/CeO2 catalysts. However, as the Ba loading increased, a decline in the activity of the NiO–Ba/CeO2 catalyst was observed, which could be attributed to the formation of BaCO3. The presence of BaCO3 can cover the catalyst’s active sites and lead to excessive alkalinity on the catalyst surface, which is unfavorable for methane catalytic combustion.

3.4.2. XRD

Figure 5 presents the XRD spectral illustrating the variations observed in NiO–Ba/CeO2 catalysts with varying amounts of Ba loading. As shown in the figure, obvious CeO2 characteristic diffraction peaks were detected in all samples. Moreover, the weak cubic phase NiO characteristic diffraction peaks were also observed in all samples. According to the literature report, Ba forms the perovskite-like structures like BaPtO3 [31], BaZrO3 [32], and BaCeO3 [33] with Pt, Zr, and Ce, respectively. No Ba compound diffraction peak was observed in Figure 5. The ionic radius of Ba2+ was greatly different from that of Ce4+. Therefore, Ba hardly entered the CeO2 lattice. As the quantity of Ba loading increased, the diffraction peak positions of CeO2 and NiO in the NiO–Ba/CeO2 catalyst remained unchanged. Furthermore, the diffraction peak intensities of CeO2 and NiO in the catalyst remained largely unchanged. As a result, it remained clear that Ba introduction did not change the NiO dispersion. It is reported in the literature that Ba is likely to form BaCO3 on the CeO2 catalyst’s surface, and no BaCO3 characteristic diffraction peak was observed on the XRD spectral diagram of the NiO–Ba/CeO2 sample at the Ba loading amount of 5 wt.%.

3.4.3. H2–TPR

Figure 6 investigates the influence of varying Ba loading on the reduction performance of NiO–Ba/CeO2 catalysts. The figure indicates that there were four major reducing peaks observed in the NiO–Ba/CeO2 catalysts, which were akin to those observed in the NiO/CeO2 catalyst. Among them, the two reducing peaks before 260 °C were indexed to the reduction of the absorbed oxygen on the catalyst’s surface, the reducing peak at ~780 °C was related to the reduction of bulk-phase lattice oxygen in the CeO2, and the change in the Ba loading amount had little influence on these three reducing peaks. Notably, changes in the Ba loading amount had minimal influence on these three reducing peaks. In addition, all NiO–Ba/CeO2 catalysts displayed a strong reducing peak at around 330 °C, which was associated with the reduction of NiO that interacted with the CeO2support. Changes in the Ba loading amount exerted a certain influence on the NiO-reducing performance, but such influence was not obvious. When the quantity of loading Ba was 1 wt.%, the NiO–Ba/CeO2 catalyst had a slightly better reducing performance; the NiO reduction temperature was about 326 °C, which was close to that with the Ba loading amount of 2 wt.%, and was slightly lower than the reducing peaks of the catalysts with the Ba loading amounts of 0.5 wt.%, 3 wt.%, and 5 wt.% (Figure 6). The obtained outcomes are in accordance with the activity sequence of NiO–Ba/CeO2 catalysts. According to the literature report [34], the incorporation of a suitable quantity of Ba to CeO2 enhances the oxygen hole concentration of the latter [35], thus improving its reducing properties. Moreover, the findings in Figure 6 suggest that the introduction of an appropriate amount of Ba partially promoted the reduction of NiO. When the Ba loading is too low (<0.5 wt.%), the influence of Ba is not adequately manifested. Conversely, at high loadings, BaCO3 hinders the reduction of NiO.

3.4.4. FT–IR

As evidenced in Figure 7, all NiO–Ba/CeO2 catalysts displayed a weak vibration absorption peak at ~1630 cm–1. This particular peak corresponds to the characteristic vibration of hydroxyl O-H, indicating the existence of a certain amount of absorbed water on the catalyst’s surface. Moreover, all specimens exhibited distinct vibration adsorption peaks at 1417 ccm−1, 1061 ccm−1, and 857 ccm−1, respectively, which were the characteristic vibration peaks of monodentate carbonate [36]. At a Ba loading amount exceeding 3 wt.%, the NiO–Ba/CeO2 catalysts exhibited a pronounced absorption vibration peak at 692 ccm−1, which was the characteristic peak of BaCO3 [37]. As the Ba loading amount increased, the adsorption vibration peaks of carbonates on the NiO–Ba/CeO2 catalysts became more conspicuous, indicating that the catalysts had growing alkalinity. This was because Ba was likely to form BaCO3 on a CeO2 surface, and the BaCO3 content on the catalyst surface was higher when the Ba loading amount was higher, leading to the stronger alkalinity on the catalyst surface. The existence of an excessive amount of carbonate contents on the catalyst surface has the potential to cover the active sites, resulting in a decline in catalyst activity. This observation aligns with the findings from the activity evaluation.

3.5. Effect of Ba Introduction on the Lattice Oxygen of the Catalysts

3.5.1. CH4–TPSR

To further investigate the influence of Ba introduction on the NiO/CeO2 catalyst properties, we carried out CH4–TPSR and O2–TPO characterizations on the catalysts.
Figure 8 illustrates the CH4–TPSR spectral diagram of the NiO/CeO2 and NiO–Ba/CeO2 catalysts, and the signal values of CH4 and CO2 are traced. Clearly, CO2 was produced on the NiO/CeO2 catalysts at about 320 °C, which was ascribed to the reaction between catalyst surface oxygen (NiO and surface CeO2) and methane, and this was close to the catalyst ignition temperature. As for the NiO/CeO2 catalysts, there was an obvious CO2 production peak at 580–680 °C, which was related to the reduction of bulk-phase oxygen in the catalyst (bulk phase CeO2). In the CH4–TPSR spectral diagram of the NiO–Ba/CeO2 catalyst, the low-temperature CO2 production peak and low-temperature CO2 production peak intensity were observed, which were indexed to the reactions of catalyst surface oxygen and bulk-phase oxygen with methane, respectively. The position of the low-temperature peak of the NiO–Ba/CeO2 catalyst was close to those of the NiO/CeO2 catalysts, but the low-temperature peak intensity of the former was far higher than those of the latter. Such findings revealed that the surface oxygen number of the NiO/CeO2 catalysts was markedly elevated after introducing Ba. Based on the research, the introduction of appropriate components in CeO2 may promote the bulk-phase oxygen liquidity in CeO2 [38,39], and enhance the migration of bulk-phase oxygen (transformation of bulk-phase oxygen to surface oxygen), thus increasing the surface oxygen number on CeO2. We believed that the introduction of Ba enhanced the bulk-phase oxygen liquidity of NiO/CeO2 catalysts, and improved the activity of bulk-phase oxygen [40,41]. Consequently, the bulk-phase oxygen in the NiO–Ba/CeO2 catalyst reacted with methane at 535 °C to produce CO2 and H2O, which was about 45 °C lower than the reaction temperature of bulk-phase oxygen in the NiO/CeO2 catalysts. Additionally, the introduction of Ba also increased the surface oxygen number on the catalyst. Consequently, the low-temperature CO2 production peak area of the NiO–Ba/CeO2 catalyst was greater than that of the NiO/CeO2 catalyst.
It is generally believed that lattice oxygen transfer is an important step in the methane catalytic combustion reaction. Ba introduction enhanced the liquidity of bulk-phase oxygen so that the bulk-phase oxygen migrated to the surface oxygen, thus increasing the surface oxygen number and improving the bulk-phase oxygen activity, which accounted for the high activity of the NiO–Ba/CeO2 catalyst.

3.5.2. O2–TPO

To better demonstrate the influence of gas-phase oxygen on the methane catalytic combustion reaction process, we conducted O2–TPO characterization on the NiO–Ba/CeO2 catalysts tested by CH4–TPSR. The results are presented in Figure 9. According to the figure, after CH4–TPSR reduction, the surface oxygen and bulk-phase oxygen of the NiO–Ba/CeO2 catalyst were consumed; therefore, there was O2 consumed in O2-TPO. Different from the literature report [42] on the O2-TPO curve of Ba-based catalysts, the NiO–Ba/CeO2 catalyst began to show O2 consumption at 190 °C, which peaked at 365 °C and continued to 500 °C. A part of the consumed O2 was oxidized into NiO by Ni, while the other part was oxidized by the CeO2 on the reducing surface. Moreover, the NiO–Ba/CeO2 catalyst displayed an obvious O2 consumption peak at 622 °C, which occurred because the previously reduced bulk phase CeO2 was reoxidized.

3.6. Effect of Different Calcination Temperatures on the Activity of NiO–Ba/CeO2

The calcination temperature exerts a significant influence on the Ba-based catalysts. This section investigates the thermal stability of NiO–Ba/CeO2 catalysts during the process of methane combustion. The catalysts were prepared with 1 wt.% and 10 wt.% loading amounts of Ba and NiO, respectively.

3.6.1. XRD

Figure 10 presents the XRD spectral diagram of NiO–Ba/CeO2 catalysts calcinated at various temperatures. The findings indicate that the crystalline grain size of NiO/CeO2–450 was approximately 8.9 nm, whereas that of NiO/CeO2–650 had increased to 13.1 nm. At 800 °C, the XRD diffraction peak intensity and sharpness increased, the CeO2 support underwent sintering, the catalyst particles grew, and the crystalline grain size of the NiO/CeO2 catalysts was about 20 nm. The crystalline grain size of the NiO–Ba/CeO2 catalysts calcined at low temperatures (450 °C and 650 °C) was similar to that of the catalysts without Ba. However, compared with NiO/CeO2–800, the NiO–Ba/CeO2–800 diffraction peak slightly shifted to the lower angle, which occurred because a small amount of Ba entered the CeO2 lattice and led to expansion of the CeO2 lattice since the Ba2+ ionic radius was greater than Ce4+. The introduction of Ba partially relieved the sintering of the NiO/CeO2 catalysts, and the crystalline grain size of NiO–Ba/CeO2-800 was about 17 nm, indicating the partial alleviation of sintering due to Ba introduction.

3.6.2. H2–TPR

To examine the influence of calcination temperature on the reducing property of NiO–Ba/CeO2 catalysts, the H2-TPR technique was utilized. The findings are demonstrated in Figure 11. The reduction peaks of the NiO–Ba/CeO2 catalysts were similar to those of the NiO/CeO2 catalysts, but the former had slightly shifted. As the calcination temperature increased, the reduction peak positions of the absorbed oxygen on the NiO–Ba/CeO2 surface were not markedly changed, but the peak areas were gradually reduced. This was because the growth of catalyst particles decreased the specific surface area, which resulted in the lower absorbed oxygen content on the catalyst surface. Moreover, as the calcination temperature was elevated, there was a gradual rise in the reduction temperature of NiO within the NiO–Ba/CeO2. When the calcination temperature reached 800 °C, a shoulder peak at high temperature in NiO was observed, in addition to the major reduction peak, which was ascribed to the reduction of the larger particle NiO.
In comparison to NiO/CeO2, the absorbed oxygen reduction peak of NiO–Ba/CeO2–450 and the NiO reduction peak (320 °C) shifted to the low-temperature direction, indicating that Ba introduction enhanced the catalyst reducing property. Furthermore, the reduction peak of NiO–Ba/CeO2–450 bulk-phase lattice oxygen also shifted to the low-temperature direction compared with NiO/CeO2–450; such an observation suggested that Ba introduction boosted the liquidity and activity of lattice oxygen in the catalysts. Similarly, the several major reduction peaks in the NiO–Ba/CeO2–650 catalyst also shifted to the low-temperature direction compared with the NiO/CeO2 catalysts, but no great difference was observed. The major reduction peak of the NiO–Ba/CeO2–800 catalyst displayed almost no obvious difference from the NiO/CeO2 catalysts, and the NiO reduction shoulder peak area in NiO–Ba/CeO2–800 was even greater. Therefore, the addition of Ba improves the reducibility of NiO–Ba/CeO2 catalysts at the temperatures below 800 °C. However, at the temperature of >800 °C, Ba introduction had an insignificant influence on the catalyst reducing property.

3.6.3. FT–IR

Figure 12 presents the FT–IR spectral analysis of NiO–Ba/CeO2 catalysts that underwent calcination at varying temperatures. Apart from the characteristic vibration peak of hydroxyl at ~1630 cm–1, all NiO–Ba/CeO2 catalysts also displayed the characteristic vibration peaks of monodentate carbonate at 1417 cm–1, 1061 cm–1, and 857 cm–1, as seen in the figure. The intensity of the characteristic vibration peak associated with carbonate exhibited a decrease as the calcination temperature increased. This phenomenon can be attributed to the catalysts undergoing sintering at elevated temperatures, resulting in the growth of crystalline grains, reduction in specific surface area, and subsequent reduction in the contents of surface groups on the catalysts. Notably, the NiO–Ba/CeO2–800 sample (calcination at 800 °C) exhibited a weak absorption vibration peak at 692 cm–1, which corresponds to the characteristic peak of BaCO3, while such a peak was not observed in NiO–Ba/CeO2–450 or NiO–Ba/CeO2–650. This was because, after high-temperature calcination, the CeO2 crystalline grain grew, and the BaCO3 particles on the CeO2 surface also gradually grew. Therefore, the BaCO3 characteristic peak was observed on the infrared spectrum. The excessively high BaCO3 content on the catalyst surface covered part of the active sites, which was one of the factors that contributed to the decrease in activity of the NiO–Ba/CeO2–800 catalyst.

3.6.4. Catalytic Activity

Figure 13 presents the impact of calcination temperature on the performance of NiO–Ba/CeO2 catalysts in the context of methane combustion, with Ba and NiO loading amounts of 1 wt.% and 10 wt.%, respectively. Table 2 is the summary of all activity data in Figure 13. Based on the above results, the thermal stability of the NiO–Ba/CeO2 catalysts was markedly superior to that of the NiO/CeO2 catalysts. The T10, T50, and T90 of the NiO/CeO2–450 catalyst were 333 °C, 415 °C, and 464 °C, respectively. After introducing Ba, the specific surface area and activity of the NiO–Ba/CeO2–450 catalyst were slightly improved. With the continuous increase in the calcination temperature, the NiO/CeO2 catalysts started to exhibit signs of sintering; for instance, the specific surface area of NiO/CeO2–800 was merely 25.4 m2·g–1, with the T10, T50, and T90 as high as 376 °C, 483 °C, and 561 °C, respectively, and such values were 44 °C, 68 °C, and 97 °C higher, respectively, compared to the NiO/CeO2–450 catalyst. Ba introduction partially relieved the sintering of the NiO/CeO2 catalysts; for example, the T10, T50, and T90 of the NiO–Ba/CeO2–650 catalyst were 346 °C, 431 °C, and 504 °C, separately, which were 25 °C, 21 °C, and 27 °C lower than those of the NiO/CeO2–650 catalyst, respectively.

4. Conclusions

(1)
The development of catalysts with high activity and stability is crucial for achieving efficient methane catalytic combustion and enhancing methane resource utilization. In this study, a series of catalysts were synthesized and evaluated, and the conclusions are presented as follows: we examined the thermal stability of 10 wt.% NiO/CeO2 catalysts in methane catalytic combustion reactions and discovered that the NiO/CeO2 catalysts were likely to develop sintering at high temperatures, thus declining the catalyst activity.
(2)
Therefore, metal ions were introduced to modify the catalysts. Based on our observation, the introduction of Ba ion (NiO/CeO2 catalyst) delays the sintering of catalysts, and improves the catalyst activity at high temperatures; Ba has the most obvious influence on the catalyst at the content of 1 wt.%.
(3)
The activities of NiO/CeO2 catalysts slightly decline after high-temperature sintering, among which the NiO/CeO2 calcinated at 650 °C has the T10, T50, and T90 of 346 °C, 431 °C, and 505 °C, respectively, which are reduced by 25 °C, 21 °C, and 26 °C compared with those of the NiO/CeO2–650 catalyst.
(4)
The H₂-TPR characterization results indicate that the introduction of Ba enhances the reducibility of the NiO/CeO₂ catalyst and increases the activity of surface oxygen. Additionally, the CH4–TPSR and O2–TPO results indicate that the introduction of Ba in the catalysts enhances the mobility of bulk-phase oxygen and promotes the migration of bulk-phase oxygen to the catalyst surface. This in turn leads to an increase in the number of surface oxygen species in the catalysts.
To summarize, the NiO–Ba/CeO2 catalyst has demonstrated its efficiency for methane combustion catalysts at high temperatures. In future research, we plan to carry out supplementary examinations encompassing EDX and long-term stability tests. Our primary focus will be directed toward the catalytic mechanism of CH4 combustion over NiO–Ba/CeO2.

Author Contributions

X.H.: Formal analysis; investigation; methodology; writing—review and editing; writing—original draft; W.Y.: investigation; software; writing—original draft; writing—review and editing; J.L.: conceptualization; data curation; writing—original draft; writing—review and editing. All authors have read and agreed to the published version of the manuscript.

Funding

This work was funded by the National Natural Science Foundation of China (Grant No. 22308217 and Grant No. 62203291).

Data Availability Statement

The data presented in this study are available upon request from the corresponding author.

Conflicts of Interest

Author Junfeng Li was employed by the company Shanghai MCC20 Construction Co. Ltd. The remaining authors declare that the research was conducted in the absence of any commercial or financial relationships that could be construed as a potential conflict of interest.

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Figure 1. (a) The thermal stability test of NiO/CeO2 catalysts in methane combustion; (b) XRD patterns of NiO/CeO2 catalysts calcined at different temperatures.
Figure 1. (a) The thermal stability test of NiO/CeO2 catalysts in methane combustion; (b) XRD patterns of NiO/CeO2 catalysts calcined at different temperatures.
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Figure 2. Light-off curves of methane catalytic combustion over NiO–M/CeO2 catalysts (M = K, Mg, Ca, Ba, Sr, and La).
Figure 2. Light-off curves of methane catalytic combustion over NiO–M/CeO2 catalysts (M = K, Mg, Ca, Ba, Sr, and La).
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Figure 3. The effect of the impregnation sequence on the catalytic activity in methane combustion.
Figure 3. The effect of the impregnation sequence on the catalytic activity in methane combustion.
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Figure 4. The effect of Ba loading amount on the catalytic performance for methane combustion of NiO–Ba/CeO2 catalysts.
Figure 4. The effect of Ba loading amount on the catalytic performance for methane combustion of NiO–Ba/CeO2 catalysts.
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Figure 5. XRD patterns of NiO–Ba/CeO2 catalysts with different Ba loading amounts.
Figure 5. XRD patterns of NiO–Ba/CeO2 catalysts with different Ba loading amounts.
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Figure 6. H2–TPR profiles of NiO–Ba/CeO2 catalysts with different Ba loading amounts.
Figure 6. H2–TPR profiles of NiO–Ba/CeO2 catalysts with different Ba loading amounts.
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Figure 7. FT–IR spectra of NiO–Ba/CeO2 catalysts with different Ba loading amounts.
Figure 7. FT–IR spectra of NiO–Ba/CeO2 catalysts with different Ba loading amounts.
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Figure 8. MS signals of CO2 (m/z = 44) and CH4 (m/z = 15) in the CH4–TPSR on NiO/CeO2 and NiO–Ba/CeO2 catalysts.
Figure 8. MS signals of CO2 (m/z = 44) and CH4 (m/z = 15) in the CH4–TPSR on NiO/CeO2 and NiO–Ba/CeO2 catalysts.
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Figure 9. O2–TPO profiles of NiO–Ba/CeO2 catalyst after treatment in CH4–TPSR process (Ba:1%, NiO:10%).
Figure 9. O2–TPO profiles of NiO–Ba/CeO2 catalyst after treatment in CH4–TPSR process (Ba:1%, NiO:10%).
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Figure 10. XRD patterns of NiO/CeO2 and NiO–Ba/CeO2 catalysts calcined at different temperatures.
Figure 10. XRD patterns of NiO/CeO2 and NiO–Ba/CeO2 catalysts calcined at different temperatures.
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Figure 11. H2–TPR profiles of NiO/CeO2 and NiO–Ba/CeO2 catalysts calcined at different temperatures.
Figure 11. H2–TPR profiles of NiO/CeO2 and NiO–Ba/CeO2 catalysts calcined at different temperatures.
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Figure 12. FT–IR spectra of NiO–Ba/CeO2 catalysts calcined at various temperatures.
Figure 12. FT–IR spectra of NiO–Ba/CeO2 catalysts calcined at various temperatures.
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Figure 13. The light curves of CH4 combustion over NiO–Ba/CeO2 catalysts calcined at different temperatures.
Figure 13. The light curves of CH4 combustion over NiO–Ba/CeO2 catalysts calcined at different temperatures.
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Table 1. Textual structure properties and catalytic performance of NiO/CeO2 catalysts calcined at different temperatures.
Table 1. Textual structure properties and catalytic performance of NiO/CeO2 catalysts calcined at different temperatures.
SampleCrystallite Size
of CeO2 (nm) a		Crystallite Size
of NiO (nm) b		BET Surface
Area (m2·g–1)		Catalytic Activity (°C)
T10T50T90
NiO/CeO2–4508.9-58.1333415467
NiO/CeO2–5509.4-52.2337408463
NiO/CeO2–65013.1-49.9371452531
NiO/CeO2–80020-25.4376483561
NiO/CeO2–10003822.73442545611
a Crystallite size was performed utilizing the Scherrer formula (calculated based on the CeO2 (111) crystal plane). b Crystallite size was carried out using the Scherrer formula (calculated based on the NiO (200) crystal plane).
Table 2. The influence of calcined temperature on the textual structure properties and activities of NiO/CeO2 and NiO–Ba/CeO2 catalysts.
Table 2. The influence of calcined temperature on the textual structure properties and activities of NiO/CeO2 and NiO–Ba/CeO2 catalysts.
SampleCrystallite Size
of CeO2 (nm) a		BET Surface
Area (m2·g–1)		Catalytic Activity (°C)
T10T50T90
NiO/CeO2–4508.958.1333415467
NiO–Ba/CeO2–4508.965.3337408463
NiO/CeO2–65013.149.9371452531
NiO–Ba/CeO2–65012.956.2346431504
NiO/CeO2–8002025.4376483561
NiO–Ba/CeO2–8001730378459530
a Crystallite size is calculated according to the Scherrer formula (calculated based on the CeO2 (111) crystal plane).
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Huang, X.; Yang, W.; Li, J. Effect of Ba Addition on the Catalytic Performance of NiO/CeO2 Catalysts for Methane Combustion. Processes 2024, 12, 1630. https://doi.org/10.3390/pr12081630

AMA Style

Huang X, Yang W, Li J. Effect of Ba Addition on the Catalytic Performance of NiO/CeO2 Catalysts for Methane Combustion. Processes. 2024; 12(8):1630. https://doi.org/10.3390/pr12081630

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Huang, Xiuhui, Wenkai Yang, and Junfeng Li. 2024. "Effect of Ba Addition on the Catalytic Performance of NiO/CeO2 Catalysts for Methane Combustion" Processes 12, no. 8: 1630. https://doi.org/10.3390/pr12081630

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