Al₂O₃ Nanorod with Rich Pentacoordinate Al³⁺ Sites Stabilizing Co²⁺ for Propane Dehydrogenation

Abstract

The search for inexpensive, environmentally friendly, and highly effective catalysts to activate C-H bonds in propane dehydrogenation (PDH) reactions is still a major challenge. Co-based catalysts have gained significant attention in recent years due to their excellent ability to activate C-H bonds and their high selectivity towards olefins, despite being a non-noble and environmentally unfriendly metal. However, further improvements are necessary for practical utilization, particularly in terms of activity and anti-carbon deposition capacity. In this study, we synthesized Al₂O₃ nanorods with abundant pentacoordinated Al³⁺ (Al³⁺_penta) sites. The supported Co on the Al₂O₃ nanorod (Co/Al₂O₃-NR) exhibited higher selectivity (>96% propylene selectivity) and stability (deactivation rate 0.15 h⁻¹) compared to Co supported on an Al₂O₃ nanosheet with fewer pentacoordinated Al³⁺ sites. Various characterizations confirmed that Co(II) mainly exists as CoAl₂O₄ rather than Co₃O₄ in the form of Co/Al₂O₃-NR, which inhibits the reduction of Co(II) to Co⁰ and accordingly improves catalyst stability.

1. Introduction

Propane dehydrogenation (PDH) reaction is a production process used in industry to produce propylene. This process has several advantages over the traditional steam cracking/catalytic cracking process, including high propylene selectivity, simple raw material and product composition, and easy separation [1,2,3]. The development of this process can help address the issue of increasing demand for propylene while also providing significant economic benefits.

Currently, the industry primarily utilizes Pt-Sn catalysts and Cr-based catalysts for propane dehydrogenation. The former is used in the Oleflex process, while the latter is utilized in the Catofin process [1,4]. However, due to their high cost and toxicity (in the case of CrO_X), there is ongoing research focused on developing low-cost and environmentally friendly alternatives [5,6].

Co-based catalysts are a promising candidate due to their ability to activate C-H bonds and high selectivity towards olefins [7]. It is believed that the active site of the reaction is Co(II) on the carrier, which promotes propylene desorption. Under high loading conditions, Co₃O₄ can be easily reduced to metallic Co, leading to propylene cracking [8,9,10]. Stabilizing Co(II) has received significant attention. Yihu Dai et al. [11] synthesized Co/Al₂O₃ using a one-step hydrothermal method. The separated Co²⁺ sites in the resulting catalyst showed high intrinsic activity (specific reaction rate > 16 mmol g⁻¹·h⁻¹). The tetrahedral Co²⁺ isolation site was stabilized by γ-Al₂O₃ carrier, promoting propylene desorption and inhibiting coke formation and the reduction of active Co sites. Sun et al. [12] found that, at low loadings, “surface spinel” produced by tetrahedral Co²⁺ during the reaction was the active site; large particles of crystalline-Co₃O₄ were formed when loading was high, leading to increased cracking reactions and methane formation. Designing a more suitable carrier for stable Co(II) species and reducing carbon deposition is necessary.

γ-alumina (γ-Al₂O₃) is commonly used as a catalyst carrier in thermal catalysis due to its large specific surface area and good thermodynamic and chemical stability, among other properties [13,14]. Al₂O₃ contains three different states of Al³⁺: tetrahedral Al³⁺ (Al³⁺_tetra), octahedral Al³⁺ (Al³⁺_oct), and pentacoordinate Al³⁺ (Al³⁺_penta). The unsaturated pentacoordinate Al³⁺ is located on the surface of Al₂O₃ and can effectively anchor the active phase of the catalyst [15,16,17,18]. Liu et al. [19] synthesized stable Ru-CeO_X supported on nano-flower flake Al₂O₃ spheres rich in unsaturated pentacoordinate Al³⁺, which were then used to catalyze the oxidation of propane. Therefore, this paper aims to stabilize Co(II) through the anchoring effect of pentacoordinate Al³⁺ with the goal of improving stability.

Here, a successful synthesis of Al₂O₃ nanorods (Al₂O₃-NR) rich in pentacoordinate Al³⁺ ions was achieved to serve as a support for Co in the PDH reaction. The pentacoordinate Al³⁺ present in the Al₂O₃-NR effectively anchors Co(II), resulting in the formation of the CoAl₂O₄ spinel phase that exhibits high intrinsic activity and stability. In comparison, samples without pentacoordinate Al³⁺, such as Co(II) in Al₂O₃-NS, tend to form the Co₃O₄ phase, which can be easily reduced to Co⁰, leading to rapid deactivation of the PDH reaction.

2. Results and Discussion

2.1. Catalytic Performances of Co/Al₂O₃ Catalysts

Figure 1 displays the catalytic efficiency of Co/Al₂O₃-NS and Co/Al₂O₃-NR in PDH reaction. During the initial 4 h of the reaction, propylene selectivity for both catalysts was between 90.5% to 93.5% and 96% to 97.5%, respectively. Methane, ethane, and ethylene were identified as primary by-products, with methane content being relatively high. The Co/Al₂O₃-NR catalyst exhibited an induction period of approximately one hour where conversion increased from 11% to 40.2%, followed by a slight decrease from 40.2% to 29.8% (the deactivation constant kd was found to be at a rate of 0.15 h⁻¹). In comparison with Co/Al₂O₃-NR, the conversion rate peaked at around 35% for the Co/Al₂O₃-NS catalyst before rapidly decreasing down to 17.3% (the deactivation constant kd is measured at 0.34 h⁻¹). Conversion rates for Al₂O₃-NR were slightly higher than those observed in Al₂O₃-NS, while also exhibiting better stability characteristics; indicating that carbon deposits on Al₂O₃-NR surfaces are less or have better carbon storage capacity when compared with their NS counterparts.

A Thermogravimetric TGA analysis was conducted on the used catalyst to determine its coke content. The findings are presented in Figure 2. The temperature range of 100 °C to 300 °C is attributed to the desorption of water molecules from the surface of the catalyst, while 300 °C to 600 °C corresponds to carbon removal from the catalyst surface [20]. After reaching a temperature above 600 °C, the curve remains relatively stable. As shown in Figure 2, Co/Al₂O₃-NS has a higher carbon deposition rate at 9.16% compared with the rate of Co/Al₂O₃-NR of only 7.54%. The result of catalytic properties and carbon deposition of Co/Al₂O₃ catalysts are shown in

Table 1. Catalytic properties and carbon deposition of Co/Al₂O₃ catalysts.

Catalyst	Ypropene, % [a]	kd, h−1 [b]	Carbon Deposition, wt% [c]
Co/Al2O3-NS	32.4	0.34	9.16
Co/Al2O3-NR	38.6	0.15	7.54

^[a] Ypropene represents the yield of propylene, and the maximum yield is listed in the table. ^[b] The deactivation rate constant, kd, was calculated based on the equation, ln [(1 − X_final)/X_final] = kd × t + ln [(1 − _Xinitial)/X_initial] [22]. ^[c] Carbon deposition was calculated was calculated based on the TGA profile.

. This suggests that high levels of Co₃O₄ on Co/Al₂O₃-NS’s surface lead to severe carbon accumulation and rapid deactivation of the catalyst. The same type of coke species is present on both catalysts, and according to the weight loss curve, the temperature range for coke deposition is between 350–450 °C. This is caused by the combustion of coke deposited on the surface of active metals and supports, which is known as hard coke and typically consists of polynuclear aromatic compounds [21].

Figure 3 displays the activation energies of Co/Al₂O₃-NS and Co/Al₂O₃-NR during the PDH reaction. The apparent activation energy for each was measured at different reaction temperatures: 520 °C, 550 °C, 580 °C, 600 °C, and 610 °C. The results indicate that the apparent activation energy for Co/Al₂O₃-NS was found to be 147.2 kJ/mol, while that of Co/Al₂O₃-NR was found to be 202.9 kJ/mol. It is evident from these values that the dehydrogenation energy barrier for Co/Al₂O₃-NR was lower than that of Co/Al₂O₃-NS, making it more favorable towards propane dehydrogenation.

2.2. Bulk and Surface Characterization

To determine the crystal structure and composition information of the material, XRD spectra were used to analyze the catalyst. Figure 4 shows that both Al₂O₃-NS and Al₂O₃-NR detected a peak associated with γ-Al₂O₃ (PDF#29-0063), indicating that the synthesized nanorod Al₂O₃ and nanosheet Al₂O₃ are in γ-Al₂O₃ phase. Additionally, it was observed that the peak height of Al₂O₃-NS is higher than that of Al₂O₃-NR, suggesting that the crystallinity of Al₂O₃-NS is higher. After loading Co, only a peak associated with γ-Al₂O₃ can be detected, which indicates that either introduced Co species are uniformly dispersed on the surface of carrier or beyond detection range due to the small amount loaded by X-ray diffraction [23].

The UV-Vis spectra (refer to Figure 4) provide details about the oxidation states and coordination geometry of cobalt species on different morphologies. In the case of Co/Al₂O₃-NR catalysts, two distinct peaks are observed at around 587 nm and 631 nm, which are attributed to the ⁴A₂→⁴T₁(P)d-d transition of tetrahedral (Td) Co²⁺ (the actual number of characteristic peaks is three, which may be caused by the insufficient resolution of UV-Vis spectrometers) [8,24]. No other peaks associated with other forms of Co were detected in the spectrum, indicating a normal CoAl₂O₄ spinel structure for the sample. Apart from these two peaks mentioned earlier, the Co/Al₂O₃-NS sample also exhibits two broad absorption bands at around 409 nm and 703 nm, respectively. These bands are related to coordination-metal charge transfer between O²⁻→Co²⁺ and O²⁻→Co³⁺ in Co₃O₄. Therefore, it can be concluded that while Co(II) mainly exists as CoAl₂O₄ in Co/Al₂O₃-NR samples, in addition to a small amount of CoAl₂O₄ present in them, large amounts of Co₃O₄ species along with some traces of CO(II) exist within the structure of Co/Al₂O₃-NS samples.

To analyze the element composition and chemical state of Co/Al₂O₃, X-ray photoelectron spectroscopy (XPS) was used to intercept information related to Co 2p. Figure 5 displays the XPS spectra of the Co 2p electronic energy levels in both catalysts, while

Table 2. XPS results of Co/Al₂O₃-NS and Co/Al₂O₃-NR catalysts.

Catalyst	Surface Concentration		Co2+/Co3+ Molar Ratio	Binding Energy of Co2+ (eV)	Binding Energy of Co3+ (eV)
	Co, mol %	Co, wt%
Co/Al2O3-NS	1.89	5.90	3.03	779.5	778.1
Co/Al2O3-NR	3.29	11.31	3.56	779.4	778.2

presents the binding energy and relative surface concentration of Co analyzed by XPS, along with the calculated ratio of Co²⁺/Co³⁺. No peak corresponding to metal Co⁰ was observed on either catalyst. The four separate characteristic peaks in all Co 2p nuclear energy level spectra include two spin-orbit double peaks of Co²⁺ and Co³⁺ at positions Co 2p3/2 and 2p1/2, as well as two satellite peaks. The double peaks at position Co 2p3/2 respectively correspond to characteristic peaks of both states [11]. Peak fitting results indicate that surfaces for both types of catalysts are mainly in a state of Co²⁺; however, the surface content is higher for Co/Al₂O₃-NR than for Co/Al₂O₃-NS. Additionally, there is a higher ratio between Co²⁺/Co³⁺ on the former’s surface compared to that on latter’s surface, which suggests that it tends more towards forming CoAl₂O₄.

Figure 6 displays the Raman spectrum of each catalyst. Co/Al₂O₃-NS exhibits a clear Co₃O₄ structure and five Raman activity modes: A_1g (680 cm⁻¹), F_2g (616 cm⁻¹, 523 cm⁻¹, 191 cm⁻¹), E_g (480 cm⁻¹) [25,26]. No peaks related to Al₂O₃ or other Co phases are present. In comparison, Co/Al₂O₃-NR also has peaks corresponding to the Co₃O₄ structure at 680 cm⁻¹ and 191 cm⁻¹. Additionally, there are peaks belonging to the CoAl₂O₄ structure that correspond to F_2g (607 cm⁻¹, 191 cm⁻¹) and E_g (480 cm⁻¹) [27]. The peak of CoAl₂O₄ is difficult to observe due to its weak intensity and coverage by Co₃O₄. The A_1g peak in the figure shows that the peak value of Co/Al₂O₃-NR is significantly smaller than that of Co/Al₂O₃- NS, indicating that less surface area on Co/Al₂O₃-NR contains Co₃O₄ and its main component is CoAl₂O₄.

From the above characterization, it is easy to see that in the Co/Al₂O₃-NS catalyst, Co(II) mainly exists in the form of Co₃O₄, which leads to the rapid deactivation of the catalyst. In contrast, the Co/Al₂O₃-NR catalyst has a small amount of Co₃O₄ and mostly consists of a stable spinel structure (CoAl₂O₄), leading to improved stability. To understand the difference between these two catalysts, ²⁷Al MAS NMR spectra of various catalysts with Al³⁺ coordination are shown in Figure 7. The proportion of Al³⁺ ions with different coordination numbers is shown in

Table 3. The proportion of different coordination Al³⁺ calculated from the peak area of ²⁷Al MAS NMR spectra.

Catalysts	Al3+ Coordination/%
	Tetrahedral Al	Pentacoordinate Al	Octahedral Al
Al2O3-NS	31.5	-	68.4
Al2O3-NR	17.2	18.5	64.3
Co/Al2O3-NS	33.8	-	66.1
Co/Al2O3-NR	29.4	-	70.6

. Pure Al₂O₃ without Co showed only two different Al species in Al₂O₃-NS: octahedral Al³⁺ at about 4.13 ppm and tetrahedral Al³⁺ at about 61.8 ppm. However, in addition to these peaks, there was also a characteristic peak at about 30.7 ppm in Al₂O₃-NR attributed to pentacoordinate Al³⁺ ions (~18.5%) [28,29]. With the addition of Co, the peak corresponding to the pentacoordinate Al³⁺ ion disappeared and most of the peaks were converted into tetrahedral Al³⁺ ions. This indicates that after being impregnated, the Co particles first occupied the pentacoordinate Al³⁺ position on the surface of Al₂O₃. The disappearance of the Co-loaded NMR peak confirms that this peak is designated as Al³⁺ ions located on the surface of the Al₂O₃ support rather than lattice defect sites uniformly distributed in bulk phase [30]. The pentacoordinate Al³⁺ can anchor active centers, maintain high dispersion and thermal stability. Co(II) loaded onto a pentacoordinate Al³⁺ site on an Al₂O₃ surface is well anchored to prevent the formation of a Co₃O₄ phase, which is consistent with Raman spectrum results.

The SEM images of the Al₂O₃ support and Co-based catalyst after loading are presented in Figure 8. The Al₂O₃-NR support used in this study has a regular long rod-shaped structure, while the Al₂O₃-NS exhibits a regular sheet-like structure. Upon impregnation with 5%wt Co, there was no significant change observed on the surface of the Co/Al₂O₃-NS catalyst, and its main layered structure remained intact, indicating a high dispersion which makes its surface Co species easily reducible to Co⁰, and reduces the induction period of the reaction. On the other hand, for Co/Al₂O₃-NR, the main structure still consisted of nanorods with aggregated Co oxide particles appearing at both ends of these nanorods, making them less reducible and resulting in a longer induction period compared to layered catalysts [9].

2.3. Reducing Ability and Acidity of the Catalysts

H₂-TPR was used to examine how oxidized Co species in two Co/Al₂O₃ catalysts (Figure 9) behave during reduction. The Co/Al₂O₃-NS catalyst has two reduction peaks at 450 °C and 640 °C, which correspond to the reduction of Co₃O₄ to CoO and the reduction of CoO to Co⁰, respectively. On the other hand, the peaks of Co/Al₂O₃-NR that correspond to the reduction of Co₃O₄ are located at lower temperatures of 420 °C and 598 °C. A comparison between these two catalysts shows that there is a stronger interaction between Co and Al₂O₃-NR under pentacoordinate Al³⁺ ions’ influence, as evidenced by the shift in position for peak temperature towards higher values for Co/Al₂O₃-NS. Additionally, it is evident that there is less content of Co₃O₄ on the surface of Co/Al₂O₃-NR compared with that on Co/Al₂O₃-NS, since its peak temperature for reducing this compound is much lower; thus indicating a tendency for formation into a spinel structure mainly consisting of Co(II) and CoAl₂O₄.

Figure 10 displays the results obtained from NH₃-TPD, while

Table 4. The proportion of different acid sites calculated from the peak area of NH₃-TPD.

Catalysts	Acid Sites/%
	Strong Acid Sites	Medium Strong Acid Sites	Weak Acid Sites
Al2O3-NS	74.8	-	25.2
Al2O3-NR	83.4	-	16.6
Co/Al2O3-NS	79.2	12.4	8.4
Co/Al2O3-NR	64.5	22.4	13.1

presents the ratios of various acid strengths. The Al₂O₃ support exhibits weak and strong acids at approximately 160 °C and 450 °C, respectively. Upon Co loading, a medium-strength acid emerges on the surface of Co/Al₂O₃-NR and Co/Al₂O₃-NS around 215 °C. Furthermore, the strong acid present on the surface of Co/Al₂O₃-NS is significantly stronger than that found on the surface of Co/Al₂O₃-NR’s, which could be another explanation for their varying carbon deposition rates. The H₂-TPR results for the two types of Al₂O₃ did not reveal any H₂ reduction peaks.

2.4. Stability of the Catalyst

The experiment involved testing a catalyst at 580 °C for 2 h, followed by regeneration. The regenerated catalyst was then reused for six cycles, with regeneration after each cycle to study its multi-cycle catalytic activity. To regenerate the spent catalyst, a mixture of 20% O₂ and balance N₂ was used at 450 °C, followed by H₂ treatment at 580 °C. The data showed that the activity and stability were almost fully recovered after each cycle (as shown in Figure 11). Additionally, it can be observed from Figure 11 that both the regenerated and fresh catalysts exhibited similar reaction processes. Furthermore, there was little change in selectivity of the reaction conversion rate throughout the experiment.

2.5. Discussion

The Co/Al₂O₃-NR catalyst exhibits superior selectivity and stability in the propane dehydrogenation reaction compared to Co/Al₂O₃-NS. During a 4-h activity test, the inactivation rate of Co/Al₂O₃-NR was only 0.15 h⁻¹, significantly lower than that of Co/Al₂O₃-NS, indicating higher stability. To investigate this difference in stability between the two catalysts, we analyzed carbon deposition on the spent catalyst using thermogravimetry (TGA). The results showed that carbon deposition was 7.54% for Co/Al₂O₃-NR and 9.16% for Co/Al₂O₃-NS. Additionally, NH₃-TPD characterization revealed that Co/Al₂O₃-NS had a higher proportion of strong acid sites, which led to greater carbon deposition and faster deactivation compared to Co/Al₂O₃-NR. These characterizations demonstrate that surface carbon deposition is the primary cause of catalyst deactivation.

The UV-Vis and Raman spectra indicate that Co(II) in Co/Al₂O₃ undergoes partial oxidation to form Co₃O₄. This is subsequently reduced to CoO, then to metallic Co⁰ (as evidenced by H₂-TPR data). The presence of metallic Co⁰ leads to propylene cracking and carbon deposition on the catalyst surface. In contrast, when interacting with Al₂O₃, Co(II) in Co/Al₂O₃-NR forms a stable spinel phase called CoAl₂O₄. Compared to Co₃O₄, this spinel phase is less easily reduced (the reduction peak for the spinel in H₂-TPR occurs above 700 °C). The UV-Vis spectrum shows that while there is some amount of the spinel phase present on the surface of the catalyst in both types (NS and NR), it mainly exists as Co₃O₄. This conclusion is supported by an A_1g peak at 680 cm⁻¹ observed in the Raman spectrum. Based on H₂ consumption during H₂-TPR analysis, only a small amount of Co₃O₄ exists in the NR catalyst; most of it exists as stable structured-CoAl₂O₄. Therefore, two states exist for cobalt: one as unstable metallic-Co⁰ from easily reducible-Co₃O₄ mostly found in NS catalysts leading to deactivation due to carbon deposition; and another state as stable structured-CoAl₂O₄ from less-reducible-spinels mostly found in NR catalysts which do not cause deactivation.

We have analyzed the reasons behind the formation of different Co(II) species. Our analysis suggests that the variation in surface coordination of Al₂O₃ carriers results in distinct binding energies between Co(II) and Al₂O₃, leading to two different structures: CoAl₂O₄ and Co₃O₄. The ²⁷Al MAS NMR spectrum indicates two states of Al³⁺ on the Al₂O₃-NS carrier—tetrahedral and octahedral—while pentacoordinate Al³⁺ is present as a defect site on the surface of the Al₂O₃-NR carrier. Upon loading, Co(II) preferentially binds to pentacoordinate Al³⁺, resulting in spinel phase formation of CoAl₂O₄. This phase is more stable than the Co₃O₄ phase as an active catalyst and less susceptible to reduction by H₂ gas.

3. Methods and Materials

3.1. Catalyst Preparation

Al₂O₃ nanosheet was created using a hydrothermal method that had been previously reported. Aluminum nitrate (Al(NO₃)₃·9H₂O, provided by Aladdin Biochemical Technology Co., Ltd., Shanghai, China) and urea (CO(NH₂)₂, provided by Guangfu Technology Development Co., Ltd., Tianjin, China) were dissolved in deionized water and stirred for 15 min until a uniform solution was formed. The concentration of Al(NO₃)₃ was 0.09 mol/L, with the ratio of Al(NO₃)₃ to CO(NH₂)₂ being 1:9. The solution, which had a pH of approximately 4, was then transferred to a PTFE-lined stainless steel autoclave and kept at 100 °C for 48 h. After natural cooling, the white precipitate was filtered and washed several times with deionized water and anhydrous ethanol. The powder was dried overnight at 100 °C before being calcined in air at a heating rate of 1 °C/min for two hours at a temperature of 600 °C. This resulted in the creation of Al₂O₃ nanosheet, which is referred to as Al₂O₃-NS.

The process for creating Al₂O₃ nanorods is similar to that of Al₂O₃-NS. The ratio of Al(NO₃)₃ to CO(NH₂)₂ remains at 1:9, but the concentration of Al(NO₃)₃ is adjusted to 0.2 mol/L. The resulting product is referred to as Al₂O₃-NR.

The method used to load Co is the impregnation method. To do this, a specific amount of cobalt nitrate (Co(NO₃)₂·6H₂O, provided by Fuchen Chemical Reagent Co., Ltd., Tianjin, China) was dissolved in a small quantity of deionized water and loaded onto two different Al₂O₃ carriers using the constant volume impregnation method. The sample was then left at room temperature for 2 h, dried overnight at 70 °C, and roasted in air at 600 °C for 2 h with a heating rate of 1 °C/min. The amount of Co that was impregnated was 5 wt%. The resulting materials were named Co/Al₂O₃-NS and Co/Al₂O₃-NR, respectively.

3.2. Catalyst Characterization

The X-ray diffraction (XRD) measurement was conducted on a Rigaku C/max-2500 diffractometer (provided by Rigaku Corporation, Tokyo, Japan.) using CuKα radiation filtered through graphite (λ = 1.5406 Å), with a 2θ range of 5 to 90°.

The Raman spectrum was acquired using a RenishawinViaReex Raman spectrometer (provided by Renishaw plc, Gloucestershire, UK) with an Ar ion laser beam of 532 nm at room temperature. The data collection process involved an exposure time of 10 s and 1 accumulation. In the in situ reaction tank, the inner reaction tank is heated while the outer shell is cooled by cooling water. The gas enters into the void inside the sample tank, flows over the surface of the catalyst in a flooding manner, and then exits. Spectra are collected through the quartz glass window located on top of the sample.

Thermogravimetric analysis (TGA) is conducted using STA449F3 NETZSCH Corp (provided by NETZSCH-Gerätebau GmbH, Wittelsbacherstr, German) to examine the carbon deposition of used catalysts. The sample is first preheated in N2 (50 mL/min) at 80 °C for half an hour, and then heated in air (100 mL/min) at a speed of 10 °C/min until it reaches 800 °C.

The laboratory used self-made equipment to test H₂-TPR and NH₃-TPD. A desiccant is added before the reaction tail gas enters the TCD detector to absorb water produced by the reaction. For H₂-TPR, an appropriate amount of catalyst is treated at 350 °C under 30 mL/min N₂ for 20 min during the pretreatment stage. Then, high-purity helium is used and the sample temperature is lowered to 50 °C. During the program temperature reduction stage, a helium switch with a 10% H₂/N₂ mixture maintains a flow rate of 30 mL/min and cuts into the inlet channel of the TCD detector after stabilization of its baseline signal. The heating furnace heats up at a rate of 10 °C/min until it reaches 900 °C while detecting changes in H₂ signals online using TCD. For NH₃-TPD, dry pretreatment involves placing the sample in a reaction tube and programming the temperature from room temperature to reach up to 300 °C at a rate of 10 °C/min. He gas flow (30 mL/min) purges for about twenty minutes and then cools down to around fifty degrees Celsius before introducing mixed gas consisting of ten percent NH₃/He (30 mL/min) for thirty minutes until saturation occurs; finally, desorption attachment takes place in He atmosphere heated at ten degrees Celsius per minute until reaching seven hundred degrees Celsius while detecting outgassing through the TCD detection method.

XPS is mainly used to determine the chemical states and elemental composition of a catalyst’s surface. The experiment utilizes ESCALAB220iXL (provided by Thermo Fisher Scientific Inc., Waltham, MA, USA) with monochromatic Al Kα radiation. Charge compensation is accomplished by using low-energy electrons at 10 eV, and electron binding energy is determined based on NIST’s recommendation of carbon as a reference measurement at 284.8 eV.

The ²⁷Al MAS NMR was measured using a Bruker 400M high-resolution solid-state nuclear magnetic resonance (NMR) spectroscopy (provided by Bruker, Billerica, MA, USA) with a MAS spin rate of 10 Khz, a recovery time of 4 s, and a pre-scan delay of 6.5 μs.

3.3. Catalyst Tests

The PDH catalytic reaction was conducted in a fixed bed reactor under atmospheric pressure. The quartz tubular reactor had an inner diameter of 6 mm and a wall thickness of 1 mm. A catalyst loading mass of 0.2 g was used, with the catalyst being loaded in the middle of the quartz tube. Quartz cotton was filled at both ends of the catalyst, and 0.5 g of quartz sand was added to the front end. The catalyst underwent heating in nitrogen at a rate of 15 °C/min until it reached 580 °C, followed by pretreatment in H₂ atmosphere for one hour at 580 °C before conducting the PDH reaction. During this reaction, C₃H₈:N₂ = 1:19 and the total gas flow rate were maintained at 20 mL/min, respectively. An online gas chromatograph equipped with a TCD detector and flame ionization detector (FID) analyzed C₃H₈, C₃H₆, C₂H₄, C₂H₆, and CH₄ gasses; propane conversion rates as well as propylene selectivity were accordingly calculated.

Chromatographic data were analyzed using the FID area normalization method. The propane conversion rate X(C₃H₈), propylene selectivity S(C₃H₆), propylene yield Y(C₃H₆), and propylene space-time yield STY(C₃H₆) were determined using the following formula.

$$\begin{gathered} \mathrm{X}\left({\mathrm{C}}_3{\mathrm{H}}_8\right)=\frac{\mathrm{n}{\left({\mathrm{C}}_3{\mathrm{H}}_8\right)}_{\mathrm{in}}-\mathrm{n}{\left({\mathrm{C}}_3{\mathrm{H}}_8\right)}_{\mathrm{out}}}{\mathrm{n}{\left({\mathrm{C}}_3{\mathrm{H}}_8\right)}_{\mathrm{in}}}\times 100\% \\ \mathrm{S}\left({\mathrm{C}}_3{\mathrm{H}}_6\right)=\frac{\mathrm{n}{\left({\mathrm{C}}_3{\mathrm{H}}_6\right)}_{\mathrm{out}}}{\mathrm{n}{\left({\mathrm{C}}_3{\mathrm{H}}_8\right)}_{\mathrm{in}}-\mathrm{n}{\left({\mathrm{C}}_3{\mathrm{H}}_8\right)}_{\mathrm{out}}}\times 100\% \\ \mathrm{Y}\left({\mathrm{C}}_3{\mathrm{H}}_6\right)=\mathrm{S}\left({\mathrm{C}}_3{\mathrm{H}}_6\right)\times \mathrm{X}\left({\mathrm{C}}_3{\mathrm{H}}_8\right) \\ \mathrm{S}\mathrm{T}\mathrm{Y}\left({\mathrm{C}}_3{\mathrm{H}}_6\right)=\frac{\mathrm{n}{\left({\mathrm{C}}_3{\mathrm{H}}_8\right)}_{\mathrm{i}\mathrm{n}}\times \mathrm{Y}({\mathrm{C}}_3{\mathrm{H}}_6)\times \mathrm{M}({\mathrm{C}}_3{\mathrm{H}}_6)}{{\mathrm{V}}_{\mathrm{m}}\times {\mathrm{m}}_{\mathrm{c}\mathrm{a}\mathrm{t}}} \end{gathered}$$

where n(C₃H₈)in represents the molar concentration of propane in the reactants, n(C₃H₈)out represents the molar concentration of propane in the product, n(C₃H₆)out represents the molar concentration of propylene in the product, M(C₃H₆) represents the relative molecular weight of propylene, and V_m represents the molar volume of gas (22,400 mL/mol). m_cat represents the mass of catalyst used in the reaction and STY is measured in kg(C₃H₆)·h⁻¹·kg⁻¹.

4. Conclusions

Two different morphologies of Al₂O₃ were synthesized using the hydrothermal method and then loaded with 5 wt% Co for propane dehydrogenation reaction. The results showed that Co/Al₂O₃-NR exhibited higher selectivity and stability compared to Al₂O₃-NS. This is because Al₂O₃-NR contains a higher amount of pentacoordinate Al³⁺, which can stabilize Co(II) particles, forming a more stable active phase CoAl₂O₄ spinel that is not easily reduced by H₂. On the other hand, the surface of Al₂O₃-NS does not contain pentacoordinate Al³⁺, mainly resulting in Co₃O₄ as an active phase. However, this phase is easily reduced to metallic Co⁰ leading to cracking reactions and carbon deposition. In conclusion, modifying the support morphology can further improve the activity and stability of Co-based PDH catalysts.