Role of CO₂ During Oxidative Dehydrogenation of Propane Over Bulk and Activated-Carbon Supported Cerium and Vanadium Based Catalysts

Abstract

CeO₂, V₂O₅ and CeVO₄ were synthesised as bulk oxides, or deposited over activated carbon, characterized by XRD, HRTEM, CO₂-TPO, C₃H₈-TPR, DRIFTS and Raman techniques and tested in propane oxidative dehydrogenation using CO₂. Complete oxidation of propane to CO and CO₂ is favoured by lattice oxygen of CeO₂. The temperature programmed experiments show the ~ 4 nm AC supported CeO₂ crystallites become more susceptible to reduction by propane, but less prone to re-oxidation with CO₂ compared to bulk CeO₂. Catalytic activity of CeVO₄/AC catalysts requires a 1–2 nm amorphous CeVO₄ layer. During reaction, the amorphous CeVO₄ layer crystallises and several atomic layers of carbon cover the CeVO₄ surface, resulting in deactivation. During reaction, V₂O₅ is irreversibly reduced to V₂O₃. The lattice oxygen in bulk V₂O₅ favours catalytic activity and propene selectivity. Bulk V₂O₃ promotes only propane cracking with no propene selectivity. In VO_x/AC materials, vanadium carbide is the catalytically active phase. Propane dehydrogenation over VC proceeds via chemisorbed oxygen species originating from the dissociated CO₂.

Graphic Abstract

1 Introduction

Propene is an important commodity chemical in the petrochemical industry and is used for the synthesis of polypropylene, propylene oxide, acrylonitrile, cumene, butyraldehyde, acrylic acid, etc. It is currently produced in the amount of ~ 100 Mt/year, mainly by naphtha steam cracking and FCC (fluidised catalytic cracking) technologies. In recent years, a growing gap between the demand and supply of propene is becoming apparent, which calls for the implementation of additional propene production pathways [1]. Catalytic oxidative dehydrogenation of propane using O₂ or CO₂ emerge as possible solutions. The propane-O₂ ODH reaction is thoroughly researched and supported VO_x catalysts have been the focus of many experimental [2, 3] and theoretical studies [4]. The support strongly influences the performance of these catalysts and precise active site architecture is required to promote propane dehydrogenation instead of total combustion to CO_x [1]. Also, propane conversion and propene selectivity are strongly inversely connected due to facile activation of sp² hybridised C=C bond in propene, which promotes its further oxidation.

The propane-CO₂ ODH reaction (reaction 1) has some benefits, as well as drawbacks. It reduces CO₂ to CO and could be implemented for CO₂ valorisation and reduction of anthropogenic CO₂ emissions.

$${\text{C}}_{{3}} {\text{H}}_{{8}} + {\text{CO}}_{{2}} \leftrightarrow {\text{ C}}_{{3}} {\text{H}}_{{6}} + {\text{ CO }} + {\text{ H}}_{{2}} {\text{O}}\,\,\,\,\,\,\,\,\,\,\, \Delta {\text{H}}_{{{\text{298K}}}} = {\text{167 kJ}}/{\text{mol}}$$

(1)

The reaction is endothermic, thermodynamically restricted, and favours carbon formation in a broad range of reaction conditions [5]. Thermodynamic analysis has shown that surplus CO₂ in the feed has the potential to alleviate the restricted equilibrium propane conversion and also lowers the tendency for carbon accumulation [6].

Propene weight time yield (gC₃H₆/g_cat h) is about two orders of magnitude lower with propane-CO₂ ODH compared to propane-O₂ ODH, which calls for the development of more active catalysts [7, 8]. Catalyst optimisation should start with understanding the active sites (requirements for propane and CO₂ activation), and continue by steering the catalyst synthesis towards their highest abundance.

Significantly higher propene selectivity (85 vs 20%) at 50% propane conversion demonstrates CO₂ as a superior oxidant in the propane ODH reaction [7, 9]. The high selectivity to the desired alkene originates from a selective formation of nucleophilic monoatomic O²⁻ ions when CO₂ dissociates on the catalyst’s surface. Contrary, dissociation of O₂ leads to the formation of short-lived electrophilic oxygen species (O₂⁻ and O⁻) before they are transformed to O²⁻. These electrophilic oxygen species favour deep alkane/alkene oxidation [10, 11].

Several metal oxides (bulk and supported Ga₂O₃, Cr₂O₃, and Fe₂O₃) were tested by Michorczyk and Ogonowski [12] in the propane-CO₂ ODH reaction. Ga₂O₃ was identified as the most effective, with propene yield equal to 30% at 600 °C.

Zou et al. [13] investigated the effect of chromium loading in CrO_x/SiO₂ catalysts and established the following decreasing order of propane conversion and propene selectivity: 2.5 wt% CrO_x/SiO₂ > 5 wt% CrO_x/SiO₂ > 10 wt% CrO_x/SiO₂. UV–Vis spectroscopic analysis indicated that ODH activity correlates with highly dispersed chromates, such as Cr⁶⁺ ions in the form of mono, di and polychromates. With increasing chromium loading above 3 wt%, the presence of bulk Cr₂O₃ was identified. Deactivation of the catalyst with time on stream was observed and related to coke accumulation and changes in chromate speciation from oligomeric to bulk crystalline Cr₂O₃.

Martinez Huerta et al. [14] studied catalysts containing 2 and 5 wt. % vanadium dispersed over CeO₂ for ethane-O₂ ODH reaction. The operando Raman-GC analysis showed stepwise dispersion of V₂O₅ into surface VO_x, before the formation of the amorphous CeVO₄ phase. In the CeVO₄ phase, vanadium maintains its 5 + oxidation state and redox activity is related only to Ce⁴⁺ ↔ Ce³⁺. The initially formed amorphous CeVO₄ converts into a well crystalline CeVO₄ phase as the reaction temperature increases. These authors also suggest that the V–O–Ce bond must be the active phase. Increased crystallinity decreases the number of exposed V–O–Ce bonds due to the lower surface-to-volume ratio, which accounts for catalyst deactivation.

Ascoop et al. [15] studied a WO_x–VO_x/SiO₂ catalyst and using isotopic labelling confirmed that in parallel to an oxidative dehydrogenation pathway (reaction 1), also reverse water gas shift reaction (H₂ + CO₂ ↔ CO + H₂O) and a direct, non-oxidative propane dehydrogenation reactions (C₃H₈ → C₃H₆ + H₂) occur over the catalyst at 600 °C. According to their DFT calculations, C-H bond activation in propane is the rate-limiting reaction step, whereas catalyst re-oxidation (V³⁺ to V⁴⁺) with CO₂ occurs much faster.

Turakulova et al. [16] observed that the interaction between supported VO_x and Ce_0.46Zr_0.54O₂ results in the formation of CeVO₄, which is the active phase responsible for propane ODH. The full oxidation of propane to CO_x is reported to be catalysed at the CeZrO₂ surface, whereas propene is formed over CeVO₄. The oxygen exchange properties of the catalyst play an essential role in the ODH reaction. Contrary to Ascoop et al. [15], Turakulova et al. report that re-oxidation of the active sites is the rate-limiting step of the process, which is associated with the redox properties of the VO_x/Ce_0.46Zr_0.54O₂ catalyst.

Nowicka et al. [17] studied propane-CO₂ ODH reaction over Pd/CeZrAlO_x catalysts and showed that over ceria-based catalysts, the reaction proceeds via the Mars van Krevelen mechanism in parallel with the RWGS reaction. The high propene selectivity (95% at ~ 3% propane conversion) was attributed to the exclusive formation of selective oxide (O²⁻) ions when CO₂ dissociates at the surface oxygen vacancy sites of CeO₂. The role of Pd is to enhance the reducibility and to accelerate the re-oxidation of the CeO_2−x surface. Strong catalyst deactivation was attributed to carbon accumulation which blocks the active sites and incomplete catalyst oxidation by CO₂.

Carbon materials with a high surface area and large pore volume have an increasing number of applications as catalyst supports and adsorbents [18,19,20]. They can be synthesized in a variety of morphologies (nanotubes, ordered mesoporous structures, etc.) and their surfaces can be functionalized with adatoms (V and N) or oxygen containing groups, giving rise to catalytic activity in many reactions, including oxidative dehydrogenation of propane [21,22,23].

Our common observation in many propane-CO₂ ODH studies is that catalytic tests were run at relatively high reaction temperatures (600–700 °C) in order to activate CO₂ and ensure reasonable conversions. Under such conditions, thermal (noncatalytic) reactions strongly contribute to the observed propane conversion and influence the distribution of reaction products [7, 15].

This work is focused on the systematic analysis of propane and CO₂ interaction with bulk and activated carbon (AC) supported VO_x, CeO₂ and CeVO₄ catalysts. Vanadia based materials are very often investigated catalysts in the oxidative dehydrogenation of parafines [1], whereas ceria is known for its reactivity towards CO₂ [24]. Consequently, these materials, as well as their mixed oxide phase (CeVO₄) were the subject of this investigation. Structural, redox, and chemical nature of the active phases were varied by changing their loading on the AC support and were correlated to activity, stability, and olefin selectivity in the propane-CO₂ ODH reaction.

2 Experimental

2.1 Synthesis of Catalysts

Bulk V₂O₅ was synthesised by calcination of vanadium acetyl-acetonate (Sigma Aldrich, purity 99%). Bulk CeVO₄ was synthesised by completely dissolving 92 mg of NH₄VO₃ (Sigma Aldrich, p.a.) in 65 ml of ultrapure water (resistivity of 18.2 MΩ, Elga Purelab Option-Q), while the solution was heated to 80 °C and stirred with a magnetic stirrer. To prevent water evaporation, the glass beaker was covered with a petri dish. Once dissolved, 1 drop of concentrated HNO₃ was added. In a separate glass beaker, 340 mg of Ce(NO₃)₃ × 6H₂O (Sigma Aldrich, purity 99%) was dissolved in 10 ml of ultrapure water. After complete dissolution, the prepared solutions were mixed, resulting in an instantaneous formation of a yellow–brown precipitate. The suspension was stirred at 80 °C for an additional 2 h. After cooling, filtering and washing 3 times with ultrapure water, the suspension was dried overnight at 70 °C in a laboratory drier. Pure CeO₂ was synthesised by dissolving 4.9 g of Ce(NO₃)₃ × 6H₂O (Sigma Aldrich, purity 99%) in 84 ml of ultrapure water. This solution was mixed with 140 ml of 0.1 M aqueous NaOH and stirred for 30 min. Afterwards, it was transferred to Teflon® lined autoclaves (~ 35 ml volume), which were placed into a laboratory drier preheated to 180 °C and kept at this temperature for 24 h. The precipitate was filtered, washed 3 times with ultrapure water, and dried overnight at 70 °C in a laboratory drier. All bulk catalysts were calcined 4 h at 600 °C (Nabertherm, model LT 9/11, heating ramp of 2 °C/min) in static air.

Synthesis of catalysts containing CeVO₄ (10–40 wt% CeVO₄) supported on activated carbon (AC) was similar to the one of bulk CeVO₄ with the following exceptions: 0.7 g of activated carbon, finely ground in an agate mortar, was added to the aqueous NH₄VO₃ solution. After filtration the samples were dried overnight at 70 °C. The VO_x (2–12 wt% vanadium) was deposited over AC by dissolving appropriate amounts of NH₄VO₃ in 10 ml of ultrapure water at 80 °C. Afterwards, 0.7 g of AC was added, stirred for another 2 h at room temperature, filtered and dried overnight at 70 °C in a laboratory drier. CeO₂ (7–17 wt% cerium) was deposited over AC by dissolving appropriate amounts of Ce(NO₃)₃ × 6H₂O in 1 ml ultrapure water. Afterwards, 0.7 g of AC was added, mixed for 2 h and dried overnight at 70 °C. All AC supported catalysts were calcined (4 h at 600 °C, 2 °C/min heating ramp) in a tubular oven (Carbolite, model HST 12/400) in Argon (20 l/h) to prevent AC oxidation. The CeVO₄ content in the CeVO₄/AC catalysts was selected so the actual content of vanadium or cerium in the CeVO₄ phase covered a similar range as in CeO₂/AC or VO_x/AC catalysts. Actual content of vanadium, cerium and CeVO₄ is shown in Table S1.

2.2 Characterisation and Catalytic Testing

XRD analyses were performed on a PANalytical X’pert PRO diffractometer using Cu Kα radiation (λ = 0.15406 nm) between 2theta angles of 10° and 80° with the step size of 0.034°. The BET specific surface area, total pore volume and average pore size were determined using N₂ adsorption/desorption isotherms at 77 K (Micromeritics, model TriStar II 3020). The samples were degassed before measurements using a SmartPrep degasser (Micromeritics) in a N₂ stream 1 h at 90 °C followed by 4 h at 180 °C. Actual vanadium, cerium and CeVO₄ contents were determined from the mass of oxidic residue remaining after complete oxidation of the AC support (TGA apparatus, model STA6000 by Perkin Elmer) by heating the samples in air (25 ml/min) with a heating ramp of 10 °C/min to 800 °C. The carbon content, accumulated on the bulk catalyst during reaction was analysed by CHNS analytical technique (Series II CHNS analyser, model 2400 from Perkin Elmer).

Prior to TEM analysis, samples were dispersed in absolute ethanol and sonicated for 30 s, then directly transferred to Cu lacey carbon support grids. As prepared samples were analysed by 200 kV transmission electron microscope (TEM, JEM-2010F, Jeol Inc.) and by C_s-corrected (TEM, Titan Themis G³, FEI Inc., equipped with CEOS CETCOR aberration corrector) operating at 300 kV. Amount of carbon deposited on the bulk unsupported catalysts during the catalytic reaction was analysed using CHNS elemental analyser (Perkin Elmer, model 2400).

Catalyst reduction with propane (C₃H₈-TPR) and oxidation with CO₂ (CO₂-TPO) was analysed in the Micromeritics Autochem 2920 apparatus. The powdered samples (100 mg) were positioned on a flock of quartz wool inside an U-shaped quartz reactor. The samples were pre-treated with He (Linde, purity 5.0) for 30 min at 300 °C, followed by cooling to 10 °C, switching to a 50% C₃H₈/He flow (10 ml/min) and heating to 600 °C with a 10 °C/min ramp. The samples were again cooled to 10 °C, and re-oxidation by CO₂ (Linde, purity 5.3) was started by increasing the temperature until 700 °C with a 10 °C/min ramp. The analytical protocol is shown in Fig. S1. Reaction products were monitored by mass spectrometry (model Thermostar^®, Pfeiffer Vacuum) propane (m/z = 29), propene (m/z = 41), ethene and ethane (m/z = 27), methane (m/z = 15), water (m/z = 18), CO₂ (m/z = 44), He (m/z = 4) and H₂ (m/z = 2).

Transient interaction of bulk CeVO₄, CeO₂ and V₂O₅ with propane and CO₂ was analysed with Diffuse Reflectance Infrared Furrier Transformed Spectroscopy (DRIFTS) analysis (Perkin Elmer, model Frontier) equipped with DiffusIR cell from Pike Scientific. Approximately 10 mg of finely powdered sample was pre-treated in 20%O₂/He stream at 550 °C for 15 min. Afterwards, the sample was exposed to the following sequence of atmospheres: 20% O₂/He → 20% C₃H₈/He → 20% CO₂/He. Sample spectra were recorded continuously (4 s between scans) in the range between 500 and 4000 cm⁻¹, 8 accumulations per scan and spectral resolution of 4 cm⁻¹.

Raman analysis of the powdered catalysts was performed in the spectral range from 70 to 3700 cm⁻¹ using Witec Alpha 300 spectrometer that employed green laser with excitation wavelength of 532 nm and resolution of 4 cm⁻¹ at 30 mW laser power.

Catalytic experiments were performed in a PID Eng&Tech reactor system using a quartz tubular reactor (10 mm I.D.). The powdered catalyst (300 mg) was positioned between two flocks of quartz wool. Reaction temperature was measured with a K-type thermocouple, which was covered by a quartz sleeve, thus minimising the heated volume inside the reactor above the catalyst bed. A quartz rod was used to minimise the reactor volume after the catalyst bed. This reduced the occurrence of thermal, non-catalytic reactions (verified with a blank experiment which showed < 0.5% propane conversion at 550 °C. Before the catalytic reaction, the catalyst was heated using a heating ramp of 10 °C/min in a flow of CO₂ (Linde, purity 5.3) and He (Linde, purity 5.0) with a flowrate of 10 ml/min each, until reaching the reaction temperature of 550 °C. Afterwards, the propane flow (Linde, purity 3.5, 10 ml/min) was added and 15 min were allowed before starting the GC analysis to allow for stabilization of gas concentrations. Agilent 7890A gas chromatograph (equipped with Molesieve 5A and Poraplot Q columns and two TCD detectors) was used for qualitative and quantitative analysis of gas stream. The carbon mass balances were in all cases between 85 and 104%. Operation in kinetic regime was ensured by plotting the propane reaction rate versus progressively increasing C₃H₈/CO₂/He flowrate over a constant mass of catalyst (Fig. S2). Details on calculation of propane conversion and product selectivities are provided in the Supplementary information.

3 Results

3.1 Catalytic Activity of Bulk CeO₂, V₂O₅ and CeVO₄

Catalytic performance of all materials during propane-CO₂ ODH reaction is summarized in Tables 1 and 2.

Table 1 Initial propane conversion, propane reaction rate, propene selectivity and propene yield at 550 °C. Numbers in parentheses represent values after 240 min of reaction

Table 2 Product selectivity of AC supported catalysts after 240 min of reaction. See Supplementary Information file for details on calculation of these values

Main reaction product over bulk CeO₂ was CO, along with CH₄ and H₂ (71, 10 and 13% selectivity, respectively). The olefin selectivity was low (12% for propene and 4% for ethene).

The bulk V₂O₅ deactivated quickly: the initial propane conversion of 9.5% stabilised at 3.2% after 240 min TOS (Fig. 1a). Also, C₃H₆ selectivity decreased from 27 to 0% (Table 1 and Fig. 1b) and CO dropped to zero in 140 min (not shown). Contrary, selectivities for H₂ and CH₄ increased slowly during the experiment from 9 to 25% and 31 to 61%, respectively. This transient behaviour of the V₂O₅ catalyst indicates drastic changes in contribution of several possible reaction pathways occurring during propane-CO₂ ODH reaction: (i) oxidative dehydrogenation with participation of lattice oxygen producing propene and water, (ii) oxidative dehydrogenation with CO₂ acting as the oxidant, producing propene, CO and water (iii) total oxidation of propane to CO_x and water, (iv) nonoxidative dehydrogenation pathway producing propene and hydrogen and (v) propane cracking to CH₄, H₂ and carbon which deposits over the catalyst’s surface (reactions 1–4). The relevance of these reactions will be discussed in detail in the following sections.

$${\text{C}}_{{3}} {\text{H}}_{{8}} \to {\text{ C}}_{{2}} {\text{H}}_{{4}} + {\text{ CH}}_{{4}}$$

(2)

$${\text{C}}_{{3}} {\text{H}}_{{8}} \to {\text{ C}}_{{3}} {\text{H}}_{{6}} + {\text{ H}}_{{2}}$$

(3)

$${\text{C}}_{{2}} {\text{H}}_{{4}} \to {\text{ CH}}_{{4}} + {\text{ C}}$$

(4)

Fig. 1

Propane conversion (a) and propene selectivity (b) as a function of TOS over bulk V₂O₅, CeVO₄ and CeO₂ catalysts during propane-CO₂ ODH reaction. Please refer to online version of this manuscript for colour figure

Activity of bulk CeVO₄ was stable, as well as selectivity for all reaction products: C₃H₆, CH₄, H₂ and C₂H₄ at 25, 31, 26 and 18%, respectively. No CO was formed, which indicates the lattice oxygen or CO₂ participation (Reaction 1) and RWGS reaction (CO₂ + H₂ ↔ H₂O + CO) are not occurring over this catalyst. The non-oxidative dehydrogenation and propane/propene cracking dominate the reaction product distribution over bulk CeVO₄. Results of a blank experiment (Fig. S3) show that no conversion is taking place at 550 °C in the empty reactor. This confirms the non-oxidative dehydrogenation reaction is a consequence CeVO₄.

3.2 Catalytic Activity of AC Supported Catalysts

3.2.1 CeO₂/AC Catalysts

Propane conversions and propene selectivities were notably higher over CeO₂/AC catalysts compared to bulk CeO₂ (Table 1). A notable (~ 50%) decrease in propane conversion in the initial 60 min of reaction for 7CeO₂/AC (Fig. 2a) was accompanied by an increase in propene selectivity.

Fig. 2

Propane conversion (a) and propene selectivity (b) as a function of TOS over 7CeO₂/AC, 5VO_x/AC and 30CeVO₄/AC catalysts during propane-CO₂ ODH reaction. Please refer to online version of this manuscript for colour figure

3.2.2 VO_x/AC Catalysts

Catalytic stability of VO_x/AC catalysts is notably improved compared to bulk V₂O₅ (Table 1, Figs. 1A and 2A). Propene selectivities ranged between 40 and 51% and were marginally influenced by vanadium content, which was varied between 2 to 12 wt%. Selectivities for CO were between 41 and 50%, for H₂ ranged between 8 and 10%, for CH₄ between 5–6% and for C₂H₄ between 1–2% (Table 2). Contrary to the complete loss of propene selectivity over bulk V₂O₅, the propene selectivity over VO_x/AC catalysts was stable during the 240 min of reaction (Fig. 2b). This indicates markedly different catalytic behaviour of supported vanadium species compared to bulk V₂O₅.

3.2.3 CeVO₄/AC Catalysts

Testing of CeVO₄/AC catalysts revealed a positive correlation between CeVO₄ content and catalytic activity, as well as propene selectivity. The 30CeVO₄/AC sample achieved 11.4% propane conversion and 57% propene selectivity at 60 min TOS (Table 1 and Fig. 2). This is clearly superior compared to all tested bulk and supported VO_x, CeO₂ and CeVO₄ materials. For all CeVO₄/AC catalysts, a slow continuous deactivation was observed with time on stream. Selectivities for CO ranged between 22 and 59%, H₂ ranged between 8 and 18%, for CH₄ between 5 and 11% and for C₂H₄ up to 4% (Table 2).

Catalytic propane dehydrogenation reaction often suffers from poor stability, which is usually caused by carbon build-up on the catalyst, which blocks the active sites [6]. A long-term catalytic test was performed on the 20CeVO₄/AC catalyst (Fig. 3). Continuous deactivation was observed in the first 30 h of reaction; the catalyst lost 71% of its initial activity (based on the drop of propane conversion). Catalyst deactivation was accompanied by a slow rise in propene, CH₄, C₂H₄ and H₂ selectivities (Fig. 3), as well as decrease of CO selectivity (not shown). This indicates a slow transition from an oxidative propane-CO₂ to a non-oxidative propane dehydrogenation pathway. Finally, conversions of propane and CO₂ stabilised at 3% each. Very similar behaviour was observed over Pd/CeZrAlO_x catalysts during propane-CO₂ ODH reaction at 600 °C by Nowicka et al.[17]

Fig. 3

Catalytic performance of 20CeVO₄/AC catalyst during the propane-CO₂ ODH stability test. Please refer to online version of this manuscript for colour figure

An important aspect for discussion of catalytic performance of AC supported catalysts is experimental verification that support gasification does not contribute to measured reaction products. Under simulated reaction conditions (4 h at 550 °C in a 30 ml/min total flow consisting of 33% C₃H₈, 33% CO₂ and 33% He) inside a thermogravimetric apparatus, the mass of 20CeVO₄/AC catalyst increased by 0.58 wt. % (Fig. S4A). This experiment confirmed a small amount of carbon was deposited on the catalyst during the 4 h of reaction. Additionally, the TGA-TPO experiment of fresh and spent 20CeVO₄/AC catalysts after 45 h of reaction showed the weight fraction of carbon in the sample increased by 0.62 wt% (Fig. S4B). These two experiments revealed no gasification of the AC support during catalytic reaction and that carbon accumulation on the catalyst is low and occurs mainly in the initial 4 h of reaction. Spent bulk catalysts after 4 h of propane-CO₂ ODH reaction contain 1.6, 1.4 and 1.4 wt% of carbon for CeVO₄, V₂O₅ and CeO₂, respectively.

Propene selectivity at comparable propane conversions is shown in Fig. S5 for all catalysts, which revealed increasing selectivity in the following order: CeVO₄ > V₂O₅ > CeO₂.

3.3 Catalyst Characterisation

3.3.1 N₂ Physisorption and XRD Analysis

Morphological properties analysed by N₂ physisorption technique are compiled in Table 2. The V₂O₅ and CeVO₄ are mesoporous with total low pore volumes and BET specific surfaces, whereas specific surface area of CeO₂ is much higher. After 4 h of reaction, the specific surface area of CeO₂ drops by about 50%, whereas changes for V₂O₅ and CeVO₄ were much smaller. The specific surface area and pore volume of supported catalysts are dominated by the microporous activated carbon support. With increasing content of the active phase (VO_x, CeO₂ or CeVO₄), a continuous decrease of specific surface area and pore volume are observed, which is in line with a progressively larger contribution from the deposited oxides.

XRD analysis was performed on the fresh and spent catalysts (Figs. 4, 5 and S6). In bulk CeVO₄ and CeVO₄/AC samples (Fig. 4a), only a CeVO₄ phase (PDF 00-012-0757) was observed. No diffraction lines of crystalline CeO₂, V₂O₅ or V₂O₃ could be identified. The Scherrer equation was applied and the average scattering domain size of the CeVO₄ crystallites was calculated based on its most intense diffraction line at 2θ = 24°. For bulk CeVO₄, the calculated average scattering domain size was 80 nm, whereas for the AC supported CeVO₄ catalysts this value was lower and increased from 25 nm in the 10CeVO₄/AC to 33 nm in the 40CeVO₄/AC sample. These results indicate the AC supports limits the crystal growth of the CeVO₄. The XRD results of 20CeVO₄/AC sample before, and after 4 and 45 h of propane-CO₂ ODH reaction (Fig. S6) showed no structural changes in the CeVO₄ and negligible sintering since the average crystallite size measured 24, 25 and 26 nm, respectively. The average pore diameter of AC supported catalysts is below 2 nm, which indicates that CeVO₄ crystallites observed by XRD, which are more than an order of magnitude larger, reside in the interparticle voids of the AC support.

Fig. 4

a XRD patterns of fresh bulk CeVO₄ and CeVO₄/AC catalysts and b V₂O₅ before and after 4 h of propane-CO₂ ODH reaction. Please refer to online version of this manuscript for colour figure

Fig. 5

a XRD patterns of VO_x/AC and b CeO₂/AC catalysts before and after 4 h propane-CO₂ ODH tests. Please refer to online version of this manuscript for colour figure

XRD analysis of bulk V₂O₅ sample before and after propane-CO₂ ODH reaction (Fig. 4b) shows that the initially present V₂O₅ phase (PDF 01-089-0612), is reduced and quantitatively transformed to V₂O₃ (PDF 01-071-0342).

Results for the VO_x/AC samples before and after catalytic tests are shown in Fig. 5a. The broad reflection peaks between 20°–30° and 40°–50° are visible on all samples and originate from activated carbon. The diffraction lines at 21.8°, 26.6° and 35.8° can be ascribed to SiO₂ cristobalite*, quartz^# and graphite^$ (PDF 01-089-3435, 01-085-1780 and 01-075-2078, respectively). These crystalline phases are present in the pristine activated carbon. The amount of SiO₂ in the AC was determined to be 1.1 wt% (thermogravimetric heating of pristine AC sample in air, followed by SEM-EDXS analysis of inorganic, non-combustible residue). No diffraction peaks from vanadium containing crystalline phases could be identified in the 2VO_x/AC and 5VO_x/AC samples.

In the fresh 12VO_x/AC sample weak diffraction lines at characteristic positions for V₂O₃ became apparent (PDF 01-071-0342). Formation of V₂O₃ is triggered by sample calcination in argon (see experimental section). The intensity of diffraction lines belonging to V₂O₃ increased after 4 h of reaction, revealing its sintering.

Figure 5b shows XRD results of bulk and supported CeO₂/AC catalysts. In all samples, only diffraction lines belonging to CeO₂ (PDF 00-034-0394) were identified. Average crystallite size in bulk CeO₂ was 15 nm (calculated by the Scherrer equation).

Broad diffraction lines characteristic of CeO₂ were identified in the CeO₂/AC catalysts with the average crystallite size of 4 nm which did not change with increasing CeO₂ content from 7 to 17 wt%. Negligible growth of CeO₂ crystallites occurred during the catalytic reaction, as average crystallite size of CeO₂ after reaction increased to 5 nm (Table 3). The AC support efficiently prevented sintering of the deposited CeO₂ during the reaction.

Table 3 Specific surface area, total pore volume, average pore diameter and average crystallite size of fresh AC supported and bulk catalysts

3.3.2 TEM Analysis

To explore the rapid drop of the catalytic performance, the 20CeVO₄/AC sample was analysed before and after 45 h of reaction (Fig. 6). In the fresh sample, the individual CeVO₄ crystals are agglomerated in larger clusters, deposited on the outer surface of AC support. The average CeVO₄ particle size measured 24 nm (Fig. S7A), which is in line with XRD estimation (Table 2). Selective area electron diffraction (SAED) pattern analysis confirmed the presence of a crystalline CeVO₄ phase only (Fig. 6, inset). The CeVO₄ crystallites have a plate-like polyhedral shape and well developed 3D morphology (bulges and cavities) and are additionally covered with a thin (1–2 nm) amorphous layer of CeVO₄.

Fig. 6

TEM micrographs of 20 CeVO₄/AC catalyst before (upper row) and after (lower row) 45 h propane-CO₂ ODH test. SAED simulation (inset) was calculated using crystal structure data for tetragonal CeVO₄ (SG141, I41/amd) [25]

After the 45 h catalytic test, negligible increase in CeVO₄ crystallite size could be measured (Fig. S7B), which is in line with the XRD analysis. However, two changes were observed on the large majority of visualized CeVO₄ crystallites: (i) the amorphous layer present in the fresh sample was absent and (ii) CeVO₄ crystallites were covered with 2–3 atomic layers of amorphous carbon, encapsulating the surface of the CeVO₄ NPs. More details on the characterization of the amorphous CeVO₄ layer and carbon layers are provided in the supplementary information and Figs. S8–S10.

In order to visualize and analyse the catalytically active phase in the 5VO_x/AC catalyst, TEM-SAED analyses were performed (Fig. 7). It was observed that the activated carbon was covered by cubic crystallites measuring 3–10 nm in size. Selective area electron diffraction (SAED) pattern analysis identified these crystals as a cubic vanadium carbide phase (Fig. 7d). No vanadium oxides (crystalline V₂O₃ or V₂O₅) could be identified.

Fig. 7

TEM micrographs of fresh 5VO_x/AC catalyst at different magnifications (a–c); comparison of experimental and simulated SAED for cubic VC (d)

3.3.3 Temperature Programmed Reduction with Propane (C₃H₈-TPR)

Catalyst interaction with propane (Fig. 8) and CO₂ (Fig. 9) was tested according to the protocol shown in Fig. S1. These experiments investigated: (a) the ability of lattice oxygen to react with propane, (b) the re-oxidation of the reduced catalyst by CO₂, and how these processes are influenced by the morphology of the VC, VO_x, CeO₂ and CeVO₄ phases. During the C₃H₈-TPR experiments, CO_x and water were identified in parallel to propane conversion (drop of propane signal in Fig. 8), indicating participation of lattice oxygen in the propane conversion.

Fig. 8

The MS signal of propane during C₃H₈-TPR experiments over: a CeO₂ and CeO₂/AC; b CeVO₄ and CeVO₄/AC and c V₂O₅ and VO_x/AC catalysts. Vertical lines at 550 °C represent the propane-CO₂ ODH reaction temperature. d–f show MS signals of C₃H₈, C₃H₆, H₂, H₂O and CH₄ obtained over 17CeO₂/AC, 30CeVO₄/AC and 12VO_x/AC catalysts during C₃H₈-TPR, respectively. Please refer to online version of this manuscript for colour figure

Fig. 9

CO signal measured during CO₂-TPO experiments over: a CeO_2−x and CeO_2-x/AC; b CeVO₄ and CeVO₄/AC and c V₂O₅ and VO_x/AC catalysts. Vertical lines at 550 °C represent the propane-CO₂ ODH reaction temperature. Please refer to online version of this manuscript for colour figure

Propane signal as a function of temperature over CeO₂ containing catalysts is shown in Fig. 8a. Several broad, low-intensity bands are apparent between 150 and 500 °C in CeO₂/AC samples, which is absent in bulk CeO₂. These are likely related to propane reacting with coordinatively unsaturated surface lattice oxygen, which are more reactive in 4 nm CeO₂ crystallites (present in all CeO₂/AC samples), compared to bulk CeO₂ (15 nm). Propane conversion lights-off between 495 and 508 °C for AC supported samples, whereas a higher temperature (530 °C) is required for propane activation over bulk CeO₂.

Bulk CeVO₄ shows negligible activity for reduction by propane up to 600 °C (black line in Fig. 8b). However, propane is able to reduce the CeVO₄/AC samples already between 480 and 505 °C, producing propene and water.

Over bulk V₂O₅, propane is oxidized extensively in the low temperature region between 350 and 500 °C producing propene and water. These results are consistent with the oxidative dehydrogenation pathway involving lattice oxygen and reduction of V₂O₅ to V₂O₃. The second propane consumption signal between 500 and 600 °C produces methane and especially hydrogen. This is consistent with catalytic tests at TOS ˃ 140 min (Fig. 1). Propane cracking inevitably produces carbon deposition, but this could not be analysed. The VO_x/AC catalysts containing either only VC (2VO_x/AC and 5VO_x/AC) or a combination of VC and V₂O₃ (12VO_x/AC) are starting to decompose propane at about 485 °C. In all cases, propane conversion produces propene and hydrogen (Fig. 8c).

In all analysed samples, higher propane conversion during C₃H₈-TPR experiments (Fig. 8) generally correlates with higher catalytic activity (Table 1).

3.3.4 Temperature Programmed Oxidation with CO₂ (CO₂-TPO)

In situ oxidation experiments with CO₂ were performed on the catalysts previously exposed to the C₃H₈-TPR protocol (Fig. S1). During CO₂-TPO, CO₂ is dissociated over the catalyst to CO and O. The latter oxidizes the catalyst and CO desorbs. CO desorption profiles over different catalysts are shown in Fig. 9.

The re-oxidation of bulk CeO_2-x by CO₂ took place with appearance of two intense CO peaks centred at 460 and 620 °C (Fig. 9a). On the other hand, the re-oxidation of CeO_2-x/AC samples was initiated at between 400 and 450 °C with a slow, continuous rise of the CO signal until the final temperature of 700 °C was reached. This reveals very different dynamics of CeO₂ oxidation which is strongly related to the size of CeO₂ crystallites.

Bulk CeVO₄ does not react with CO₂ up to 700 °C (Fig. 9b), which is in line with inertness of this sample during C₃H₈-TPR and catalytic experiment showing no CO among reaction products. Over CeVO₄/AC catalysts, CO starts to appear at 500 °C and its amount continuously increases with increasing CeVO₄ loading.

Oxidation of bulk V₂O₃ (V₂O₅ catalyst after C₃H₈-TPR) and VO_x/AC catalysts with CO₂ was initiated at 570 °C (Fig. 9c), which reveals that at the reaction temperature of 550 °C, V₂O₃ cannot be re-oxidized with CO₂. This is in line with results of Ascoop et al. [15] who report that oxidation of WO_x–VO_x/SiO₂ catalysts with CO₂ at 600 °C can only transform V³⁺ to V⁴⁺. Also, the VC phase is inert towards CO₂ at 550 °C.

The C₃H₈-TPR and CO₂-TPO results revealed that the reduction and re-oxidation of CeO₂, and CeVO₄ phases are strongly dependent on their morphology. Smaller CeO₂ crystals (4 nm) in CeO₂/AC are more readily reduced by propane compared to bulk CeO₂ (15 nm), whereas during re-oxidation by CO₂, the situation is reversed. Both processes are possible at 550 °C, which was also the temperature during propane-CO₂ ODH reaction.

The CeVO₄/AC samples can be reduced by propane and oxidized by CO₂ at 550 °C. This ability is absent in bulk CeVO₄, which has negligible activity for propane and CO₂ activation.

In the case of VO_x/AC catalysts, the vanadium containing phase depends on the vanadium content (V₂O₅ in bulk sample, VC in 2VO_x/AC and 5VO_x/AC and a mixture of V₂O₃ and vanadium carbide in 12 VO_x/AC sample). All VO_x/AC and V₂O₅ samples can dehydrogenate propane: bulk V₂O₅ already at 350 °C through the oxidative dehydrogenation pathway with lattice oxygen participation, whereas the crystalline VC is active above 485 °C through a non-oxidative pathway. Re-oxidation with CO₂ takes place above 570 °C.

3.3.5 Time Resolved Isothermal DRIFTS Experiments

Transient behaviour of CeO₂, CeVO₄ and V₂O₅ during isothermal reduction with propane and re-oxidation with CO₂ was investigated with an in situ DRIFTS analysis at 550 °C (Figs. 10, 11 and S11–S17). Only bulk oxides were analysed due to the black colour and total absorbance of all AC supported catalysts. No interaction of propane or CO₂ with CeVO₄ was observed and these results are consequently not shown.

Fig. 10

a Reduction of CeO₂ with propane and time resolved signal intensity changes for propane (2967 cm⁻¹), CO₂ (2340 cm⁻¹), CO (2180 cm⁻¹), and carbonates (1454 cm⁻¹). b Re-oxidation of CeO_2−x with CO₂ and time resolved signal intensity changes of CO₂ (2340 cm⁻¹), CO (2180 cm⁻¹), and carbonates (1454 cm⁻¹). Please refer to online version of this manuscript for colour figure

Fig. 11

A) Reduction of V₂O₅ with propane and time resolved signal intensity changes for propane (2967 cm⁻¹), CO₂ (2340 cm⁻¹), CO (2180 cm⁻¹), and V–O overtones (2005 cm⁻¹). B) Re-oxidation of V₂O₃ with CO₂ and temporal signal intensity changes of CO₂ (2340 cm⁻¹), CO (2180 cm⁻¹), V–O overtone (2005 cm⁻¹). Please refer to online version of this manuscript for colour figure

Exposure of CeO₂ to propane (Fig. 10a) showed instantaneous formation of CO and CO₂ which decayed slowly with prolonging TOS. Also, a rapid increase of the broad polydentate carbonate bands (1454 cm⁻¹) was observed (Fig. S12A) [26, 27].

Exposure of reduced CeO_2-x to CO₂ (Figs. 10b, S13 and S14) leads to a fast increase of two broad polydentate carbonate bands centred at 1454 and 1350 cm⁻¹ (Fig. S14A). These carbonates are observed regularly over ceria and are thermally very stable, as bands remain stable during the re-oxidation experiment. Also, instantaneous formation of CO is observed; it goes through a maximum and slowly starts to decline after 300 s of CO₂ exposure. This confirms CO₂ dissociation and oxygen vacancy replenishment, leading to oxidation of CeO_2−x. Existence of the band at 1765 cm⁻¹ (CO adsorbed on reduced Ce³⁺ sites [28]) after 10 min of oxidation at 550 °C (Fig. S14A) suggests oxidation of CeO_2-x with CO₂ does not proceed to completion.

Upon exposure of V₂O₅ to propane (Figs. 11a, S15 and S16), the V=O vibration overtones at 2005 and 1969 cm⁻¹ diminish after 53 s, indicating surface reduction [29]. In parallel to propane introduction, the CO₂ signal (2340 cm⁻¹) lights off, goes through a maximum at 135 s and declines slowly. Formation of gas phase CO (characteristic band at 2180 cm⁻¹) is much smaller compared to CO₂ and appears only after the surface V=O overtone signal disappears at 100 s. This time-resolved experiment reveals the oxidation of propane to CO is possible only in the presence of a partly reduced surface, most likely containing V⁴⁺. On the reduced V₂O₃ sample, the envelope of signals between 1300 and 1550 cm⁻¹ appears (Fig. S16A), which belong to gas phase propane; no carbonates are formed. Absence of bands above 1600 cm⁻¹ indicates no carboxylates or bicarbonates are formed.

Re-oxidation of V₂O₃ with CO₂ occurs only marginally, since only about 3% of initial V=O band intensity was achieved after 400 s of oxidation, Fig. 11b, S17).

To summarize, time resolved DRIFTS experiments showed that reduction of CeO₂ with propane at 550 °C is fast, and that instantaneous surface population with polydentate carbonates takes place. Re-oxidation with CO₂ is substantial, but does not proceed to completion. This suggests the working state of the CeO₂ catalyst during the propane-CO₂ reaction is partly reduced.

Reduction of V₂O₅ is slower (CO₂ peak intensity is reached 60 s after propane introduction, compared to 5 s for CeO₂), re-oxidation by CO₂ is negligible. This suggests a slow irreversible transformation of initially present V₂O₅ into V₂O₃. Also, the inability of V₂O₅ and V₂O₃ to dissociate CO₂ shows the crucial role of lattice oxygen (nucleophilic O²⁻ species) for enabling the oxidative dehydrogenation reaction pathway for propene formation. These findings are in line with catalytic tests (stable activity and low propene selectivity over CeO₂, compared to fast deactivation and total loss of propene selectivity over V₂O₅).

3.3.6 Raman Analysis

In the fresh and spent 2VO_x/AC and 5VO_x/AC catalysts, no signal below 1000 cm⁻¹ is visible (Fig. 12a). This is due to the absence of V–O bonds in these samples and shows that only vanadium carbide (VC) is present [30]. Also, the VC phase is resistant towards oxidation during propane-CO₂ ODH reaction, thus eliminating this possibility for the observed catalyst deactivation (Table 1). In the fresh 12VO_x/AC sample, the bands at 996, 700, 529, 478, 410, 287 and 142 cm⁻¹ are visible, which is consistent with the presence of V₂O₃ [31]. This is in line with the XRD results (Fig. 5a). In the spent 12VO_x/AC sample, four additional bands at 927, 877, 840 and 161 cm⁻¹ are seen, which are characteristic for multi-valent vanadium states as present in V₆O₁₃ [32, 33]. The formation of newly formed V₆O₁₃ phase is likely related to exposure of finely dispersed V₂O₃ to highly reducing conditions during the propane-CO₂ ODH reaction.

Fig. 12

a Stacked Raman spectra of fresh and spent VO_x/AC and b fresh and spent CeVO₄/AC catalysts. Please refer to online version of this manuscript for colour figure

In the CeVO₄/AC samples (Fig. 12b), only characteristic CeVO₄ Raman bands appear at 217, 257, 369, 463, 775 and 845 cm⁻¹ [14]. In the fresh 30CeVO₄/AC and 40CeVO₄/AC samples, weak bands at 998 and 705 cm⁻¹ are visible, which suggests presence of V₂O₃ in these samples. Their fraction is very likely minor, as they could not be identified through XRD.

4 Discussion

The initial propene selectivity of CeO₂/AC catalysts is low (16–27%) and stabilizes after 60 min of reaction at notably higher values (38–50%, Table 1). The re-oxidation of small CeO_2-x crystallites (4 nm), as present in CeO₂/AC catalysts with CO₂ occurs at higher temperatures compared to bulk CeO₂ (15 nm), revealing it is more difficult to dissociate CO₂ as the CeO₂ size decreases (Fig. 9a). This is consistent with structure sensitivity for CO₂ activation, namely the π-bond in C=O and TOF increases with increasing particle size, as a certain degree of site coordination is required [34, 35].

The propene selectivity appears to be influenced by the oxidation degree of CeO_2−x. Lower abundance of reactive surface oxygen sites in partly reduced CeO_2−x crystallites, when supported over AC compared to bulk CeO₂, provides less active sites for the activation of the C=C bond in propene. Consequently, higher propene selectivities are achieved over nanosized CeO_2-x.

Nowicka et al. [17] ascribed the initial deactivation of Pd/CeAlO_x catalysts and a concomitant propene selectivity increase to the consumption of reactive oxygen species stored within the ceria lattice. These authors also suggest that the nonselective oxygen species, which are likely electrophilic, are singly charged interstitial oxygen anions. Once these O⁻ species are consumed, the catalyst can be re-oxidized by CO₂.

The fast deactivation and complete loss of propene selectivity over bulk V₂O₅ is related to its quantitative reduction from V⁵⁺ to V³⁺, which leads to a progressive shift from propane ODH to propane cracking reaction. The CO₂-TPO experiment (Fig. 9c) revealed that bulk V₂O₃ cannot be re-oxidized by CO₂ at 550 °C. This shows the participation of CO₂ in the propane ODH reaction is negligible and the dominant ODH reaction pathway is governed by the availability of lattice oxygen in V₂O₅ and V₂O₄.

The reduction and re-oxidation dynamics of CeO₂ and V₂O₅ differ considerably, as could be observed by transient isothermal DRIFT spectroscopy analyses (Figs. 10 and 11). Propane interaction with CeO₂ leads to instantaneous and simultaneous CO₂ and CO formation which tail off slowly. Such behaviour is likely connected to the diffusion of lattice oxygen from the bulk to the surface [36], where it participates in the oxidation reactions.

Upon re-oxidation of CeO_2−x with CO₂, the polydentate carbonate signal stabilizes after 10 s of CO₂ addition, suggesting a kinetic preference for these species. The instantaneous appearance of a CO signal confirms facile CO₂ disproportionation and catalyst re-oxidation at 550 °C.

During V₂O₅ reduction with propane, the CO₂ signal lights off slowly and reaches a maximum at about 70 s after propane introduction. This suggests higher propane oxidation rates with lattice oxygen of CeO₂ compared to V₂O₅, which correlates also with notably higher initial propene selectivity over bulk catalysts (10% over CeO₂ and 27% over V₂O₅). CO appears after V=O overtone vibrations disappear (suggesting the absence of V⁵⁺ on the surface) [37].

At higher vanadium loadings in the VO_x/AC catalysts (12 wt%), a separate V₂O₃ phase is formed which is inert towards CO₂ and does not promote propane dehydrogenation. All VO_x/AC catalysts exhibit some deactivation with TOS, but propene selectivity remains constant (Table 1). Also, CO was identified as the reaction product during the entire duration of the catalytic tests and its concentration followed that of propene (Fig. S18), whereas the concentration of all other reaction products remained stable. This suggests that propene and CO are produced by the same oxidative dehydrogenation pathway (reaction 1). Considering that the fresh and spent catalysts contains no lattice oxygen (TEM-SAED, XRD and Raman analyses confirm a VC phase is formed exclusively at vanadium loadings up to 5 wt. %, whereas VC and V₂O₃ coexist at 12 wt. % vanadium loading), CO₂ remains as the only possible oxygen source. By combining the C₃H₈-TPR, CO₂-TPO and catalytic results we can postulate that CO₂ activation over the VC crystals is assisted by the adsorbed propane (protons at the methylene and methyl positions). In the absence of these electrophilic species which facilitate CO₂ decomposition [38] and H₂O formation, CO₂ activation requires temperatures close to 600 °C (Fig. 9c), which is above the reaction temperature used in this work. The deactivation of VO_x/AC catalysts is accompanied by a stable propene selectivity, which indicates that the total number of active sites, and not their nature, is changed with TOS. Oxidation of the VC phase during the propane-CO₂ ODH reaction does not occur, due to absence of V–O bonding in Raman spectra of the spent 2VO_x/AC and 5VO_x/AC catalysts. As a result, deactivation is probably related to blocking of active sites by carbon. Recently, Thakur et al. [39] report of in situ formation of oxycarbide phases (V₂O₃ and V₈C₇) during exposure of vanadium containing MXene catalysts to CH₄ and CO₂ atmosphere at much higher temperatures (800 °C).

The CeVO₄/AC catalysts showed the highest catalytic performance in terms of propene yields. Upon dispersion of CeVO₄ over activated carbon, a thin (1–2 nm) amorphous phase is formed over the CeVO₄ crystals. Its formation is likely connected to calcination in argon. The amorphous layer is intrinsically defective and the reactivity of oxygen in amorphous mixed metal oxides is higher than in any of its crystalline components [40]. Ruth et al. [41], report that the amorphous part of the multiphase Mo–V–Nb oxide catalyst is particularly important during oxidative dehydrogenation and partial oxidation of ethane.

During oxidative dehydrogenation of ethane, a transformation of VO_x/CeO₂ to CeVO₄ was identified by Martinez-Huerta et al. [14]. Based on the operando Raman analysis of the catalyst’s structure, they identified the bridging oxygen atom (Ce–O–V) present in the ill-defined CeVO₄ phase as the active site for the rate determining step in the ODH reaction. Exposure to reaction temperatures above 500 °C favours crystallization of the CeVO₄ phase, making it inactive in the ODH reaction [14].

An increase of propane ODH catalytic activity scaled with the CeVO₄ loading. Catalytic activity is in line with the redox ability of the supported CeVO₄ phase, probed by propane and CO₂ (Figs. 8 and 9), which revealed lattice oxygen abstraction and re-oxidation are feasible under the reaction conditions. TEM analysis of the CeVO₄/AC catalyst after reaction revealed elimination of the amorphous CeVO₄ layer and the deposition of a few layers of carbon on the surface. This change is likely correlated to the observed catalyst deactivation. Since propene selectivity does not alter drastically with TOS (Figs. 2 and 3) despite notable deactivation, the number and not the nature of the active sites is decreased. A recent review of propane ODH with CO₂ by Atanga et al. [5] suggests that indium and especially gallium based catalysts are superior in activity and selectivity compared to chromium, platinum and vanadium based ones. When benchmarking the performance of best performing sample in this work (30CeVO₄/AC, 15.3% C₃H₈ conversion, 42% C₃H₆ selectivity, 6.4% C₃H₆ yield, C₃H₆ productivity of 9.5 × 10^–5 mol/(g_cat min) at 550 °C, Table 2) with those reported over Ga₂O₃/Al₂O₃ by Michorczyk et al. [42], at 550 °C. A similar initial activity was observed (18% C₃H₈ conversion), but C₃H₆ selectivity was much higher (90%), corresponding to C₃H₆ productivity of 7.2 × 10^–5 mol/(g_cat min). Xu et al. [43] tested several supports (TiO₂, SiO₂, ZrO₂ and MgO) for dispersing the active Ga₂O₃ phase. At 600 °C, the most active was Ga₂O₃/Al₂O₃, which enabled 26% conversion at 94% C₃H₆ selectivity, resulting in C₃H₆ productivity of 11.2 × 10^–5 mol/(g_cat min).

Chen et al. [44] investigated how different supports (Al₂O₃, SiO₂ and ZrO₂) influence the propane ODH activity of 10 wt% In₂O₃ phase. At 600 °C, the highest propene selectivity of 85% was obtained In₂O₃/Al₂O₃ catalyst at a propane conversion of 20%, resulting in a propene productivity of 5.6 × 10^–5 mol/(g_cat min).

Based on the above, the propene selectivity of best catalyst (30CeVO₄/AC) in this work is about twofold lower compared to those of Ga₂O₃ or In₂O₃ catalysts, but propene productivities are comparable.

5 Conclusions

The re-oxidation of small CeO_2−x crystallites (4 nm) with CO₂ occurs at higher temperatures compared to bulk CeO₂ (15 nm), revealing it is more difficult to dissociate CO₂ as the CeO₂ size decreases. Catalyst reduction by propane and re-oxidation by CO₂ are fast under reaction conditions and the propane dehydrogenation reaction proceeds via lattice oxygen participation. The Main reaction pathway over cerium containing catalysts is the total oxidation of propane.

Under propane-CO₂ ODH reaction conditions, V₂O₅ is irreversibly reduced to V₂O₃, which leads to a progressive shift from propane ODH to the propane cracking reaction. The participation of CO₂ in the ODH reaction is negligible and the reaction pathway is governed by the availability of lattice oxygen in V₂O₅ and V₂O₄. V₂O₃ preferentially catalyses propane cracking.

In the VO_x/AC catalysts, a VC phase is formed exclusively at vanadium loadings up to 5 wt%, whereas VC and V₂O₃ coexist at 12 wt% vanadium loading. By combining the C₃H₈-TPR, CO₂-TPO, and catalytic results we can postulate that CO₂ activation over the VC crystals is assisted by the adsorbed propane. As a result, the propane-CO₂ ODH reaction, over vanadium carbide, proceeds through the Langmuir–Hinshelwood mechanism. The deactivation of VO_x/AC catalysts is accompanied with a stable propene selectivity, which indicates the total number of active sites and not their nature is changed with TOS. Oxidation of the VC phase during the propane-CO₂ ODH reaction does not occur due to the absence of any V–O bonding in spent catalysts. As a result, deactivation is probably related to blocking of the active sites by carbon.

The active site for the propane ODH reaction in the CeVO₄/AC catalysts is a thin (1–2 nm) amorphous CeVO₄ phase which covers the CeVO₄ crystals. Propane and CO₂-TPR experiments confirmed that during the propane ODH reaction lattice oxygen abstraction and re-oxidation are feasible, indicating active participation of CO₂ and a Mars van Krevelen reaction mechanism. During reaction, crystallization of the amorphous CeVO₄ layer and surface covering with carbon, which are the causes of catalyst deactivation.