Propylene Production via Oxidative Dehydrogenation of Propane with Carbon Dioxide over Composite MxOy-TiO2 Catalysts

The CO2-assisted oxidative dehydrogenation of propane (ODP) was investigated over titania based composite metal oxides, 10% MxOy-TiO2 (M: Zr, Ce, Ca, Cr, Ga). It was found that the surface basicity of composite metal oxides was significantly higher than that of bare TiO2 and varied in a manner which depended strongly on the nature of the MxOy modifier. The addition of metal oxides on the TiO2 surface resulted in a significant improvement of catalytic performance induced by a synergetic interaction between MxOy and TiO2 support. Propane conversion and propylene yield were strongly influenced by the nature of the metal oxide additive and were found to be superior for the Cr2O3-TiO2 and Ga2O3-TiO2 catalysts characterized by moderate basicity. The reducibility of the latter catalysts was significantly increased, contributing to the improved catalytic performance. This was also the case for the surface acidity of Ga2O3-TiO2 which was found to be higher compared with Cr2O3-TiO2 and TiO2. A general trend was observed whereby catalytic performance increased significantly with decreasing the primary crystallite size of TiO2. DRIFTS studies conducted under reaction conditions showed that the adsorption/activation of CO2 was favored on the surface of composite metal oxides. This may be induced by the improved surface basicity observed with the MxOy addition on the TiO2 surface. The Ga2O3 containing sample exhibited sufficient stability for about 30 h on stream, indicating that it is suitable for the production of propylene through ODP with CO2 reaction.


Introduction
Propylene is a versatile precursor for the formation of various derivatives used in our daily life (e.g., polypropylene, isopropanol, acrylic acid, acrylonitrile, propylene oxide, butyraldehyde, cumene, etc.) and, thus, it is considered as a key component of the chemical industry [1,2].One of the traditional methods used for propylene production is the reaction of propane dehydrogenation (1), which is strongly endothermic and equilibrium limited [3].The reaction requires high temperatures, and, therefore, high energy consumption, suffering from fast catalyst deactivation as well as low C 3 H 8 conversions and C 3 H 6 selectivities [4].Oxidative dehydrogenation of propane in the presence of molecular oxygen has been proposed as an alternative pathway.This is an exothermic reaction, with no thermodynamic limitations, and operable at low reaction temperatures.The main drawback of this process is the deep oxidation of both C 3 H 8 and C 3 H 6 towards CO and CO 2 , resulting in low propylene yields [4].Thus, the replacement of molecular oxygen by a milder and readily available oxidant, such as CO 2 , has recently gained interest as an alternative approach for selective propylene production [1,[3][4][5][6][7].This approach has the advantage that CO 2 participates both in (a) propane conversion towards propylene (2) and (b) hydrogen consumption via the reverse water-gas shift (RWGS) reaction (3) [8].Removing hydrogen from the gas stream can overcome the equilibrium limitations of propane dehydrogenation, resulting in higher propylene yields [1,6,9].Moreover, carbon monoxide produced via both reactions is a valuable byproduct, which can be utilized in chemical synthesis [5,6].Depending on the catalyst and reaction conditions employed, the reactions of propane hydrogenolysis (4) and (5), and propane or propylene decomposition ( 6)- (9), may also take place, resulting in a decrease in propylene yield and possibly surface carbon formation (8) and (9) [3,4,7].Dry reforming of propane may also run in parallel leading to syngas production (CO/H 2 ) (10) [10,11].(10) Carbon dioxide may also be involved in the reverse Boudouard reaction (11), removing coke from the catalyst surface and, thus, improving catalyst stability [5].
CO 2 + C ↔ 2CO ∆H 0 298K = 172.4kJ/mol (11) The major benefit of the ODP process is the utilization of CO 2 , of which emissions into the atmosphere have increased rapidly during recent decades and nowadays is considered as one of the main greenhouse gases resulting in global warming and, therefore, major climate change [4,12].However, CO 2 is a thermodynamically stable compound (∆G f = −394 kJ•mol −1 ), the reduction of which requires high energy reactants combined with active and selective catalysts as well as optimal reaction conditions to gain a thermodynamic driving force.Thus, in order for the proposed process to be effective, (a) a suitable catalyst must be applied to selectively promote both the ODP with CO 2 and RWGS reactions and be able to retard C 3 H 8 and/or C 3 H 6 decomposition and hydrogenolysis reactions, and (b) operating conditions should be optimized.
Oxidative dehydrogenation of propane with CO 2 has been investigated over various single or composite metal oxides, including MnO [13], Cr 2 O 3 /SiO 2 [14,15], Cr 2 O 3 /ZrO 2 [16], Cr 2 O 3 /Al 2 O 3 [14,16], V 2 O 5 /SiO 2 [17], Ga 2 O 3 [18], Ga 2 O 3 /TiO 2 [15], Ga 2 O 3 /Al 2 O 3 [15,19], Ga 2 O 3 /ZrO 2 [15,20], Ga 2 O 3 /SiO 2 [15], Ga 2 O 3 /MgO [15], noble metal catalysts supported on metal oxides (e.g., Pt/Al 2 O 3 [7], Au/ZnO [3], Pd/CeZrAlO x [21]), as well as zeolites with different frameworks [4].The beneficial effect of CO 2 on catalytic performance varies depending on the catalyst employed.For example, in the case of Ga 2 O 3 based catalysts, CO 2 (a) suppresses catalyst deactivation by carbon deposition due to the occurrence of the reverse Boudouard reaction, (b) enhances propylene yield by removing H 2 via the RWGS reaction [6,18], and (c) favors the desorption of propylene from the catalyst surface [22].The beneficial effect of CO 2 over Cr 2 O 3 based catalysts has been related to CO 2 involvement in subsequent reduction-oxidation cycles between Cr 6+ and Cr 3+ , which have been found to be crucial in the propane dehydrogenation pathway [6,23].In the case of ceria based catalysts, CO 2 has the dual role of regenerating selective oxygen species, and shifting the equilibrium for propane dehydrogenation by consuming H 2 through the RWGS [21].In particular, the lattice oxygen ions abstract hydrogen from propane molecules to form propylene and H 2 O, while CO 2 replenishes these selective oxygen species, releasing CO in the gas phase.Clearly, the design and development of new catalytic materials for the ODP reaction requires a detailed investigation of CO 2 interaction with catalytic active sites in order to elucidate the exact role of CO 2 on the reaction pathway.
In the present study, the production of propylene through oxidative dehydrogenation of propane with CO 2 was investigated over composite metal oxides M x O y -TiO 2 (M: Ce, Zr, Ca, Cr, Ga).The influence of the nature of the M x O y additive on the physicochemical properties of TiO 2 was also explored, employing detailed characterization of the catalysts.An attempt was made to correlate these properties with catalytic performance in order to develop active and selective catalysts towards propylene production.DRIFTS studies were also carried out aiming to identify the surface intermediate species formed under reaction conditions and determine the beneficial effect of M x O y modifier on reactants' activation.
Nitrogen adsorption at 77 K (B.E.T. method) was applied to measure the specific surface area (SSA) of composite metal oxides using a Gemini III 2375 instrument (Micromeritics, Norcross, GA, USA).The X-ray diffraction (XRD) patterns of M x O y -TiO 2 samples were carried out on a Bruker D8 Advance instrument (Billerica, MA, USA) operating with Cu K a radiation (λ = 0.15496 nm, 40 kV, 40 mA).All samples were scanned from 20 to 80 • with a scan rate of 0.05 • /s and a step size of 0.015 • .The diffraction peaks were identified by comparing them with those provided by the JCPDS database.Scherrer's Equation (12) was used to estimate the mean crystallite size of TiO 2 (d TiO2 ): where λ = 0.15406 nm is the X-ray wavelength corresponding to CuK a radiation, B is the peak width at half maximum intensity (in radians) and θ is the diffraction angle corresponding to the peak broadening.The anatase content (x A ) of TiO 2 was estimated using the following equation [24]: where I A and I R denote the integral intensities of the peaks corresponding to (1 0 1) and (1 1 0) Miller reflections of anatase and rutile, respectively.The basicity of metal oxides was investigated by temperature programmed desorption of CO 2 (CO 2 -TPD) using an apparatus which consisted of a flow measuring and control system, and an electrical furnace where a fixed bed quartz reactor was placed with its outlet being directly connected to an Omnistar (Pfeiffer Vacuum, Asslar, Germany) mass spectrometer (MS).The experimental procedure involved the heating of 150 mg of catalyst at 450 • C in He where it remained for 15 min.The temperature was then decreased to 25 • C and the flow was switched to 1% CO 2 /He mixture for 30 min.A 30 min purging period with He was then followed before temperature was increased up to 750 • C using a linear heating rate of 10 • C/min.Similar experiments were carried out employing in situ diffuse reflectance infrared Fourier transform spectroscopy (in situ DRIFTS).These experiments were conducted in a FTIR (Nicolet iS20, Thermo Fischer Scientific, Waltham, MA, USA) spectrometer equipped with an MCT detector, a KBr beam splitter and a diffuse reflectance cell (Specac, Orpington, UK).An apparatus consisting of mass flow controllers and a set of valves was directly connected into the inlet of the DRIFT cell.In these experiments, the catalyst powder was placed in the DRIFT cell and heated at 450 • C under He flow for 60 min.The temperature was then decreased to 25 • C under the same atmosphere and the catalyst was exposed to 5% CO 2 (in He) for 30 min followed by purging with He for 10 min.The first FTIR spectrum was then collected and the temperature was subsequently stepwise increased to 450 • C. The catalyst remained at each temperature for 3 min prior to spectrum recording.All spectra were normalized by subtracting background spectra recorded in the He flow at the corresponding temperature during cooling of the catalyst.A total flow rate of 30 cm 3 /min was used in all stages of the experiment.
Temperature programmed reduction with hydrogen (H 2 -TPR) was performed to investigate the reducibility of the synthesized composite metal oxides using the apparatus described above for the CO 2 -TPD experiments.An amount of 200 mg of catalyst was loaded in a fixed bed quartz reactor and heated at 450 • C in He flow for 15 min followed by treatment at 300 • C using a mixture consisting of 20.5% O 2 /He.After being maintained under these conditions for 30 min, the temperature was increased to 450 • C in He flow and then decreased to 25 • C. A mixture of 3% H 2 /He was then introduced into the reactor and a heating program was initiated (after remaining at 25 • C for 15 min), increasing up to 750 • C using a temperature rising rate of 10 • C/min.The transient-MS signal at m/z = 2 (H 2 ) was continuously monitored by the aforementioned mass spectrometer.The total flow rate in all stages was 30 cm 3 /min.
The acidity of the synthesized catalysts was investigated by conducting thermogravimetric analysis (TGA) experiments which were carried out using a TA Q50 thermal analysis instrument (TA instruments/WATERS, New Castle, Delaware).An amount of 40 mg of dried catalyst was suspended in 10% NH 3 /H 2 O solution (Merck KGaA, Darmstadt, Germany) for 1 h at 25 • C until saturation, followed by filtration and drying under vacuum at 60 • C for 1 h in order to remove water and weakly adsorbed ammonia.Thermogravimetric analysis (TGA) was then initiated by increasing temperature from 25 to 600 • C under N 2 atmosphere using a heating rate of 10 • C/min.

In Situ FTIR Spectroscopy under Reaction Conditions
In situ DRIFTS studies were also performed under conditions of oxidative dehydrogenation of propane with CO 2 using the experimental setup described above.In these experiments, the following procedure was employed: heating in He flow at 500 • C for 30 min → cooling at 25 • C in He flow → switching of the flow to 1% C 3 H 8 + 5% CO 2 (in He) → stepwise increasing of temperature up to 500 • C.An equilibration time of 15 min at each temperature took place prior to spectrum collection.

Catalytic Performance Experiments
Catalytic performance tests were carried out in a tubular fixed-bed quartz reactor (O.D.: 6mm) in the temperature range of 570-750 • C and atmospheric pressure.The catalyst sample (particle diameter: 0.15 < d p < 0.25 mm) was placed in an expanded section of 5 cm length (O.D.: 12 mm) in the middle of the reactor, whereas a K-type thermocouple running through the reactor served for measuring the temperature of the catalyst bed.The reactor was placed in an electric furnace with its inlet being connected with a flow measuring and control system.The feed gases were provided by high-pressure gas cylinders and controlled by mass flow controllers.The outlet of the reactor was directly connected with a gas chromatograph (Shimadzu 2014, Kyoto, Japan) equipped with TCD and FID detectors and two packed columns (Porapak-Q, Carboxen) for the analysis of the effluent gases.A carboxen column was used for separation of CO, CO 2 and CH 4 in the TCD detector, whereas a Porapak-Q column was used for the separation of C 3 H 8 , C 3 H 6 , C 2 H 6 and C 2 H 4 in the FID detector.In these experiments, 0.5 g of catalyst was introduced to the reactor and heated under He flow at 450 • C where it remained for 1 h.The catalyst was then exposed to the feed gas mixture consisting of 5% C 3 H 8 + 25% CO 2 /He using a total flow rate of 50 cm3 /min, and the concentration of gas products and unreacted C 3 H 8 and CO 2 were analyzed by the gas chromatograph described above.Similar measurements were obtained at selected temperatures up to 750 • C.
The propane conversion (X C 3 H 8 ), product selectivity (S Cn ), and propylene yield (Y C 3 H 6 ) were calculated according to the following equations:

Catalyst Characterization
The SSAs measured following the Brunauer-Emmett-Teller (BET) method for the synthesized 10% M x O y -TiO 2 and bare TiO 2 samples are presented in Table 1.It was observed that the addition of a metal oxide on the surface of TiO 2 generally resulted in a slight decrease in the SSA from 36.9 m2 /g (bare TiO 2 ) to 33.8 m 2 /g (CeO 2 -TiO 2 ), with the exception of ZrO 2 -TiO 2 and Ga 2 O 3 -TiO 2 catalysts which exhibited an increase in the SSA of up to 47.9 m 2 /g.The decrease in the SSA was most possibly due to the partial blockage of the titania pores induced by the presence of M x O y on its surface, in agreement with previous studies over composite metal oxides [25][26][27][28].On the other hand, the observed increase in the SSA for the ZrO 2 -TiO 2 and Ga 2 O 3 -TiO 2 catalysts was previously reported to be related to the additional porosity of the particles of the metal oxide additive which may have smaller interaction with the support, as suggested by Daresibi et al. [29] and Shimizu et al. [30] over alumina-supported gallium oxide catalysts.The X-ray diffractograms obtained for the TiO 2 -based oxides are presented in Figure 1.In the case of bare TiO 2 , the XRD pattern (trace a) consisted of peaks located at 2θ equal to 25.36  , attributed to (1 0 1), (1 0 3), (0 0 4), (1 1 2), (2 0 0), (1 0 5), (2 1 1), (2 0 4), (2 2 0), (2 1 5) and (3 0 1) indices, respectively, of tetragonal anatase (JCPDS Card No. 4-477).Crystallographic peaks at 27.42 diffraction angles corresponding to (1 1 0), (1 0 1), (2 0 0), (1 1 1), (2 1 0) (2 1 1), (2 2 0), (0 0 2), (3 1 0), (3 0 1) and (1 1 2) planes, respectively, of tetragonal rutile (JCPDS Card No. 21-1276) were also recorded.The same peaks were also detected for all the investigated composite M x O y -TiO 2 samples (traces b-f) indicating that both anatase and rutile phases still coexisted upon the addition of metal oxide particles on the TiO 2 surface (Figure 1).It should be noted that the intensity of certain peaks was too low for some composite metal oxides, that were hardly discernable in the obtained diffractograms.Furthermore, the XRD pattern of CaO-TiO 2 (trace d) was also characterized by peaks at 2θ equal to 33.18  Anatase and rutile crystallite sizes were estimated employing the Scherrer Equation (12) using data from the peaks located at 25.36° and 27.42° diffraction angles, respectively, and the estimated values are listed in Table 1.It was observed that the mean crystallite size of the rutile phase ( , ) decreased from 36.8 to 14.9 nm in the order TiO2 (bare) < CeO2-TiO2 < CaO-TiO2 < ZrO2-TiO2 < Cr2O3-TiO2 < Ga2O3-TiO2.A smaller decrease from 22.5 (bare TiO2) to 18.3 nm (Ga2O3-TiO2) following the same order was found for the mean anatase crystallite size ( , ), with the exception of the Cr2O3-TiO2 sample where a similar crystallite size (22.7 nm) to that of TiO2 (bare) and CeO2-TiO2 samples was estimated.The addition of metal oxides on the TiO2 surface also influenced the anatase content, xA, calculated via Equation (13), which ranged between 59% (for bare TiO2) and 83% (for ZrO2-TiO2 and Cr2O3-TiO2).

Adsorption/Desorption Characteristics of CO2 by In Situ DRIFTS
The adsorption/desorption characteristics of CO2 were investigated by conducting in Anatase and rutile crystallite sizes were estimated employing the Scherrer Equation (12) using data from the peaks located at 25.36 • and 27.42 • diffraction angles, respectively, and the estimated values are listed in Table 1.It was observed that the mean crystallite size of the rutile phase (d TiO 2 ,R ) decreased from 36.8 to 14.9 nm in the order TiO 2 (bare) < CeO 2 -TiO 2 < CaO-TiO 2 < ZrO 2 -TiO 2 < Cr 2 O 3 -TiO 2 < Ga 2 O 3 -TiO 2 .A smaller decrease from 22.5 (bare TiO 2 ) to 18.3 nm (Ga 2 O 3 -TiO 2 ) following the same order was found for the mean anatase crystallite size (d TiO 2 ,A ), with the exception of the Cr 2 O 3 -TiO 2 sample where a similar crystallite size (22.7 nm) to that of TiO 2 (bare) and CeO 2 -TiO 2 samples was estimated.The addition of metal oxides on the TiO 2 surface also influenced the anatase content, x A , calculated via Equation (13), which ranged between 59% (for bare TiO 2 ) and 83% (for ZrO 2 -TiO 2 and Cr 2 O 3 -TiO 2 ).
An increase in temperature to 100 • C (trace b) resulted in a significant decrease in the intensities of the bands at 1668, 1572, 1405 and 1247 cm −1 , while the band at 1222 cm −1 disappeared probably due to desorption of the corresponding species from the catalyst surface.In addition, two new bands appeared at 1522 and 1447 cm −1 due to monodentate carbonate [31,34,35,37,39] and bicarbonate species [32,36,38,39], respectively.The latter bands may also have been present in the spectrum recorded at 25 • C but not able to be distinguished due to overlapping, or may have developed as a result of conversion of carboxylates and/or bidentate carbonates.
Further increase in temperature to 200 • C (trace d) resulted in a progressive decrease in the intensity of all bands apart from that located at 1635 cm −1 which increased and maximized at 200 • C, most possibly due to conversion of the aforementioned species to bicarbonates.All bands disappeared from the spectra detected above 350 • C indicating their complete desorption from the TiO 2 surface.
Similar experiments were conducted over M x O y -TiO 2 (M: Ga, Cr, Zr, Ce, Ca) catalysts, and results obtained are presented in Figure 2b-f.As can be seen, adsorbed surface species over the ZrO 2 -TiO 2 (Figure 2b) sample seemed to be similar to those discussed above over bare TiO 2 catalyst, with the main differences being related to the variation of the relative intensities of the corresponding bands.Specifically, the spectrum recorded at 25 • C (trace a) was characterized by bands attributed to carboxylate (1670 and 1246 cm −1 ), bicarbonate (1653, 1417 and 1223 cm −1 ), bidentate carbonate (1578 and 1339 cm −1 ) and monodentate carbonate (1517 cm −1 ) species adsorbed on TiO 2 .It should be noted that bidentate and monodentate carbonates as well as bicarbonates and bridged polydentate carbonate species adsorbed on a ZrO 2 surface give rise to the development of bands at similar wavenumbers, indicating that these species may partially contribute to the bands detected at 1655, 1578, 1517, 1417, 1339 and 1222 cm −1 [43-45].Interestingly, the relative intensity of the bands due to bicarbonates and monodentate carbonates over 10% ZrO 2 -TiO 2 catalyst was higher compared with bare TiO 2 , which may be related to the creation of more basic sites upon ZrO 2 addition on the TiO 2 surface [40].However, adsorbed surface species on 10% ZrO 2 -TiO 2 catalyst seemed to desorb at similar temperatures with bare TiO 2 .carbonates were formed on a TiO2 surface through CO2 interaction with acid-base pair sites (cus Ti 4+ -O 2-centers).An increase in temperature to 100 °C (trace b) resulted in a significant decrease in the intensities of the bands at 1668, 1572, 1405 and 1247 cm −1 , while the band at 1222 cm −1 disappeared probably due to desorption of the corresponding species from the catalyst surface.In addition, two new bands appeared at 1522 and 1447 cm −1 due to monodentate carbonate [31,34,35,37,39] and bicarbonate species [32,36,38,39], respectively.The latter In the case of the Cr 2 O 3 -TiO 2 (Figure 2c) catalyst, bicarbonate (1633 and 1416 cm −1 ) and bidentate (1556 and 1391 cm −1 ) carbonate species associated with TiO 2 were detected on the catalyst surface at 25 • C (trace a).Specific bands (1633, 1591, 1556, 1416 and 1358 cm −1 ) detected in the spectra obtained across the entire temperature range may also be related to species associated with Cr 2 O 3 [46,47].For example, Zecchina et al. [46] conducted CO 2 adsorption experiments over a-Cr 2 O 3 and attributed similar bands detected at 1620 and 1425 cm −1 to bicarbonate species, and bands at 1635, 1590, 1560 and 1340 cm −1 to bidentate carbonate species adsorbed on Cr 2 O 3 .Regarding desorption of surface species from the Cr 2 O 3 -TiO 2 surface (Figure 2c), it seems that it was completed at higher temperatures (~400 • C) compared with TiO 2 (Figure 2a) and ZrO 2 -TiO 2 (Figure 2b) catalysts.
The main adsorbed surface species detected at 25 • C (trace a) on the surface of Ga 2 O 3 -TiO 2 (Figure 2e) were bicarbonates (1649 and 1416 cm −1 ) and bidentate carbonates (1580 and 1320 cm −1 ) associated with a TiO 2 [31][32][33][34][35][36][37] and/or Ga 2 O 3 [49,[53][54][55] surface.Although the same surface species were detected over Cr 2 O 3 -TiO 2 catalysts (Figure 2c), their relative population was significantly higher, indicating that CO 2 adsorption was favored over Ga 2 O 3 -TiO 2 .No bands due to carboxylates could be discerned.It was possible, however, that the high frequency band (~1670 cm −1 ) of carboxylate species may have been overlapped by the broad band at 1649 cm −1 .This was also the case for the band at 1538 cm −1 assigned to monodentate carbonate species on TiO 2 which was clearly discerned at 150 • C but probably pre-existed in the lower temperature spectra.The intensity of all bands decreased with increasing temperature.However, bands due to bicarbonates and bidentate carbonates were still present in the spectrum obtained at temperatures as high as 450 • C (trace i), implying that the adsorption strength of CO 2 on the Ga 2 O 3 -TiO 2 catalyst is high.
The population of adsorbed surface species was found to be higher over the CaO-TiO 2 sample (Figure 2f).Carboxylates (1676 and 1241 cm −1 ), bicarbonates (1642 and 1213 cm −1 ), bidentate (1560 and 1335 cm −1 ) and monodentate (1388 cm −1 ) carbonates adsorbed on TiO 2 were detected on the catalyst surface following CO 2 adsorption at 25 • C. Partial adsorption of bicarbonates and monodentate carbonates on CaO may have occurred in agreement with previous studies [56][57][58].A weak band discerned at 1769 cm −1 (Figure 2f) was attributed to the C=O stretching vibrational mode of a bridged-bonded carbonate species adsorbed on the CaO surface [56].A band located at 1515 cm −1 was discerned in the spectrum collected at 200 • C and was assigned to an asymmetrical mode of monodentate carbonates associated with a TiO 2 and/or CaO surface [56,59,60].The results of Figure 2f show that a significant part of adsorbed surface species remained on the catalyst surface up to 450 • C, implying that they were strongly adsorbed on the surface of CaO-TiO 2 .
Comparison between the DRIFT spectra (Figure 2) of the investigated catalysts shows that both the population and desorption temperature of the surface species formed via interaction of catalyst with CO 2 increased following the sequence TiO 2 (bare) ~ZrO 2 < Cr 2 O 3 < CeO 2 < Ga 2 O 3 < CaO.Taking into account the acidic character of CO 2 , it is expected to be preferentially adsorbed on the basic sites of metal oxides [32].Therefore, the basicity of the investigated catalysts seems to follow the above ranking.

Temperature-Programmed Desorption of CO 2
The results of the CO 2 -TPD experiments obtained over bare TiO 2 and M x O y -TiO 2 catalysts are presented in Figure 3.As can be seen, CO 2 was desorbed from bare TiO 2 exhibiting a low temperature (LT) peak centered at 72 • C, which was previously attributed to weak basic sites, and a weak high temperature (HT) peak at ca. 510 • C due to CO 2 desorption from strong [61,62] and/or medium [63,64] basic sites.The addition of metal oxides on the TiO2 surface resulted in a significant increase in the intensity of the LT peak accompanied by a shift of its maximum (by ~10 °C) towards higher temperatures following the order TiO2 (bare) < Cr2O3 < ZrO2 < CeO2 < Ga2O3 < CaO.Moreover, the amount of desorbed CO2 estimated by integrating the area below the LT peak was found to increase from 4.1 µmol g −1 for TiO2 to 32.6 µmol g −1 for CaO-TiO2 (Table 2), providing evidence that the number and strength of weak basic sites increases, following the aforementioned order.The HT desorption peak was clearly distinct for all composite metal oxides where in certain cases (Cr2O3, CeO2 and Ga2O3) two peaks were evolved in the high temperature range (500-750 °C).Results indicated that the number of medium and/or strong basic sites was remarkably higher in the presence of MxOy on the TiO2 surface.The amount of CO2 desorbed at high temperatures was found to increase significantly from 0.23 µmol/g for TiO2 to 34.1 µmol/g for CaO-TiO2 (Table 2).The high strength of basic sites of CaO towards CO2 was also demonstrated by Constantinou et al. [60] who reported that CO2 was desorbed from a CaO surface above 700 °C during CO2-TPD experiments.In the case of the CaO-containing sample, an additional broad CO2 desorption peak was observed at medium temperatures (~360 °C) (Figure 3), possibly corresponding to medium basic sites.
The total amount of desorbed CO2 was calculated by integrating the total area below the CO2 response curve and was found to vary in the range of 4.3-66.7 µmol/g following the sequence TiO2 (bare) < ZrO2 < Cr2O3 ~ CeO2 < Ga2O3 < CaO (Table 2).As can be seen in Table 2, a similar trend was observed by comparing the amount of CO2 evolved per unit specific surface area (in µmol/m 2 ), indicating that the observed variations in CO2 evolution with respect to the nature of the MxOy additive was not a matter of SSA variation.The results of Figure 3 are in excellent agreement with the results of the DRIFTS studies discussed above (Figure 2), clearly implying that the basicity of the composite metal oxides was significantly higher than that of bare TiO2 and varied in a manner which depended strongly on the nature of the MxOy additive.The addition of metal oxides on the TiO 2 surface resulted in a significant increase in the intensity of the LT peak accompanied by a shift of its maximum (by ~10 • C) towards higher temperatures following the order TiO 2 (bare) < Cr 2 O 3 < ZrO 2 < CeO 2 < Ga 2 O 3 < CaO.Moreover, the amount of desorbed CO 2 estimated by integrating the area below the LT peak was found to increase from 4.1 µmol g −1 for TiO 2 to 32.6 µmol g −1 for CaO-TiO 2 (Table 2), providing evidence that the number and strength of weak basic sites increases, following the aforementioned order.The HT desorption peak was clearly distinct for all composite metal oxides where in certain cases (Cr 2 O 3 , CeO 2 and Ga 2 O 3 ) two peaks were evolved in the high temperature range (500-750 • C).Results indicated that the number of medium and/or strong basic sites was remarkably higher in the presence of M x O y on the TiO 2 surface.The amount of CO 2 desorbed at high temperatures was found to increase significantly from 0.23 µmol/g for TiO 2 to 34.1 µmol/g for CaO-TiO 2 (Table 2).The high strength of basic sites of CaO towards CO 2 was also demonstrated by Constantinou et al. [60] who reported that CO 2 was desorbed from a CaO surface above 700 • C during CO 2 -TPD experiments.In the case of the CaO-containing sample, an additional broad CO 2 desorption peak was observed at medium temperatures (~360 • C) (Figure 3), possibly corresponding to medium basic sites.The total amount of desorbed CO 2 was calculated by integrating the total area below the CO 2 response curve and was found to vary in the range of 4.3-66.7 µmol/g following the sequence TiO 2 (bare) < ZrO 2 < Cr 2 O 3 ~CeO 2 < Ga 2 O 3 < CaO (Table 2).As can be seen in Table 2, a similar trend was observed by comparing the amount of CO 2 evolved per unit specific surface area (in µmol/m 2 ), indicating that the observed variations in CO 2 evolution with respect to the nature of the M x O y additive was not a matter of SSA variation.The results of Figure 3 are in excellent agreement with the results of the DRIFTS studies discussed above (Figure 2), clearly implying that the basicity of the composite metal oxides was significantly higher than that of bare TiO 2 and varied in a manner which depended strongly on the nature of the M x O y additive.
An improvement of surface basicity was previously reported over titania-supported metal oxides.In particular, a shift of the CO 2 desorption peak maximum towards higher temperatures and an increase in the amount of CO 2 desorbed were observed by Xu et al. [15] with the addition of Ga 2 O 3 on TiO 2 .An increase in the total basicity was also found to occur by modifying TiO 2 with CeO 2 [65].Similarly, Makdee et al. [66] reported that addition of Zr improved the basicity of the Ni/TiO 2 catalyst, favoring CO 2 adsorption.
Regarding the strength of basic sites, Al-Shafei et al. [63] studied the basicity of ZrO 2 -TiO 2 catalysts and observed three temperature regions of CO 2 desorption during CO 2 -TPD, corresponding to weak (50-325 • C), medium (325-725 • C) and strong basic sites (>725 • C).They suggested that CO 2 desorption in the low temperature range corresponded to bicarbonate species and CO 2 evolution at medium temperatures was due to desorption of bidentate carbonates, whereas CO 2 detection above 725 • C was related to oxycarbonates.The formation of bicarbonates on weak strength basic sites via CO 2 interaction with OH groups was also reported by Kumar et al. [67].However, these authors suggested that high-strength basic sites favored the formation of unidentate carbonates.This was in agreement with previous studies over TiO 2 and ZrO 2 -TiO 2 catalysts [40,68] where it was found that the formation of monodentate carbonates following CO 2 adsorption occurred over strong basic sites and those of bidentate carbonates over medium basic sites, whereas bicarbonates were mainly formed over weak basic sites.Based on the results of Figure 2 bicarbonates, bidentate and monodentate carbonates were detected on the surface of all catalysts investigated following their interaction with CO 2 , with the exception of Cr 2 O 3 where only bicarbonates and bidentate carbonates were discerned.This indicates that all types of basic sites (weak, medium and strong) possibly coexist on an M x O y -TiO 2 surface.However, as discussed above for the results of Figure 3, CO 2 is desorbed at two main temperature regions-low and high regions.Taking into account that the identification of the CO 2 desorption temperature range from medium and strong basic sites is unclear in the literature [61][62][63][64]69], we cannot safely assign CO 2 desorbed above 500 • C to medium or strong basic sites.The only certainty is that the basicity of titania is improved with the addition of metal oxides on its surface.

Effect of the Nature of M x O y Additive on Catalytic Performance
The results of the catalytic performance experiments obtained over 10% M x O y -TiO 2 catalysts for the oxidative dehydrogenation of propane with CO 2 are presented in Figure 4, where the conversion of propane (Figure 4a) and propylene yield (Figure 4b) are plotted as a function of reaction temperature.The equilibrium conversion of propane predicted by thermodynamics was also calculated using the Outokumpu HSC Chemistry ® program and found to be 100% in the entire temperature range (570-750 • C) investigated (Figure 4a).According to the results of Figure 4a, bare TiO 2 was activated above 600 • C and reached X C 3 H 8 = 50% and Y C 3 H 6 = 18% at 750 • C. The addition of a metal oxide on the TiO 2 surface resulted in all cases in a significant improvement of catalytic performance with the propane conversion curve being shifted (by ~50 • C) towards lower temperatures.The Ga 2 O 3 -and Cr 2 O 3 -containing samples were found to be the most active catalysts, exhibiting measurable propane conversions above 550 • C and reaching X C 3 H 8 equal to 80% at 745 • C. Titaniasupported ZrO 2 and CaO samples exhibited intermediate performance and were similar to each other.Although CeO 2 -TiO 2 was found to be more active than bare TiO 2 below 700 • C, they presented similar propane conversions at higher temperatures.
of acidic and basic properties.Lavalley et al. [76] demonstrated that the higher surface basicity of α-Ga2O3 favored the activation of CO2 compared with γ-Ga2O3.According to the results presented in Figures 2 and 3, and as will be discussed below concerning surface acidity, the population of both basic and acid sites were increased with the addition of Ga2O3 on TiO2.Therefore, the high activity of the Ca2O3-TiO2 catalyst observed in Figure 4 can be partially attributed to the interactions between Ga2O3 and TiO2, which may result in an optimum number of acidic/basic sites, which seems to benefit the CO2-assisted ODP reaction by promoting CO2 and C3H8 activation on the catalyst surface.On the other hand, the high activity and selectivity of CrOx based catalysts for the ODP with CO2 reaction was assigned to the structural and redox properties of CrOx oxides, with the catalytic performance being significantly affected by the Cr 3+ /Cr 6+ ratio and  Propylene yield was found to be strongly influenced by the nature of the metal oxide modifier and increased from 5.5 to 16% at 700 • C in the order TiO 2 (bare) < CaO-TiO 2 CeO 2 -TiO 2 < ZrO 2 -TiO 2 < Cr 2 O 3 -TiO 2 ~Ga 2 O 3 -TiO 2 .The achievement of high yields over Ga and Cr promoted catalysts was previously reported for the production of propylene via ODP reaction [9,15,[26][27][28][29][70][71][72][73][74][75].The high activity of the Ga 2 O 3 -TiO 2 catalyst was previously attributed to the higher number of medium strong acidic sites and the strong interaction between Ga 2 O 3 and TiO 2 [15].Similarly, Xia et al. [71] found that the high surface area and the large amount of tetrahedral Ga ions which were correlated with the medium-strong Lewis acid sites, were responsible for the superior activity of the Ga 2 O 3 -Al 2 O 3 catalyst.Moreover, Daresibi et al. [29] synthesized Ga 2 O 3 -Al 2 O 3 catalysts using the atomic layer deposition method, and found that the dispersion of Ga 2 O 3 on Al 2 O 3 and the interaction between them was enhanced, leading to the formation of a higher number of Ga-O-Al linkages and higher surface moderate acidity.The abundance of weak acid sites induced by the synergy between Ga 2 O 3 and Al 2 O 3 in the spinel-type structure of Ga 2 O 3 -Al 2 O 3 solid solutions as well as the creation of a higher population of surface Ga sites with relative weak acidity were also reported to be responsible for the high activity and stability of the Ga 2 O 3 -Al 2 O 3 catalyst [75].Moreover, it was found that the activation of CO 2 during CO 2 conversion processes requires an optimum combination of acidic and basic properties.Lavalley et al. [76] demonstrated that the higher surface basicity of α-Ga 2 O 3 favored the activation of CO 2 compared with γ-Ga 2 O 3 .According to the results presented in Figures 2 and 3, and as will be discussed below concerning surface acidity, the population of both basic and acid sites were increased with the addition of Ga 2 O 3 on TiO 2 .Therefore, the high activity of the Ca 2 O 3 -TiO 2 catalyst observed in Figure 4 can be partially attributed to the interactions between Ga 2 O 3 and TiO 2 , which may result in an optimum number of acidic/basic sites, which seems to benefit the CO 2 -assisted ODP reaction by promoting CO 2 and C 3 H 8 activation on the catalyst surface.
On the other hand, the high activity and selectivity of CrO x based catalysts for the ODP with CO 2 reaction was assigned to the structural and redox properties of CrO x oxides, with the catalytic performance being significantly affected by the Cr 3+ /Cr 6+ ratio and the facile switch of Cr 3+ and Cr 6+ at elevated temperatures [26,70,73,74].In the case of supported CrO x materials, three different types of chromium oxides were found to exist, namely, isolated Cr 6+ , polymeric Cr 6+ and crystalline Cr 2 O 3 , of which the activity and selectivity towards propene formation depends on the nature of the support.For example, Wang et al. [74] demonstrated that although the polymeric Cr 6+ oxides were more active than the isolated Cr 6+ oxides for the ODP with CO 2 reaction over CrO x /silicalite-1 catalysts, they were less selective.Polymeric Cr 6+ oxides were also found to be more active when Al 2 O 3 was used as a support, contrary to SBA-15 supported CrO x catalysts where Cr 6+ oxides exhibited higher activity than crystalline Cr 2 O 3 [77].Moreover, it was found that catalytic activity of chromium oxide-based catalysts for the ODP reaction increased with increasing chromium oxide dispersion.Although the oxidation state of Cr could not be revealed based on the characterization results of the present study, the absence of XRD peaks related to CrO x species from Figure 1 indicated that their dispersion on TiO 2 surface was high and maybe (at least in part) responsible for the observed superior catalytic activity (Figure 4).Comparison of the results of the present study with the literature results over composite metal oxides is shown in Table S1.The observed differences in catalytic activity compared with the results of the present study may be attributed to the different reaction conditions used including the mass of catalyst, the total flow rate and the CO 2 :C 3 H 8 ratio, as well as the different catalysts' composition, precursor compounds and synthesis method.
The distribution of products results are presented in Figure 5, where it was observed that C 3 H 6 , CO, C 2 H 4 and CH 4 were detected for all catalysts examined, whereas in certain cases (CaO-TiO 2 , Cr 2 O 3 -TiO 2 and Ga 2 O 3 -TiO 2 ) traces of C 2 H 6 were also produced.In the case of bare TiO 2 (Figure 5a), selectivity towards C 3 H 6 (S C 3 H 6 ) decreased from 60 to 35% with the temperature increasing from 605 to 750 • C.This was also the case for CO selectivity (S CO ) which decreased from 30 to 5%.Production of both CO and C 3 H 6 indicated that, under the present experimental conditions, the desired reaction of oxidative dehydrogenation of propane took place, whereas part of the produced CO may have been due to the RWGS reaction (3) and/or the reverse Boudouard reaction (11).Selectivities towards C 2 H 4 (S C 2 H 4 ) and CH 4 (S CH 4 ) exhibited similar behavior with temperature and increased from 10 to 40% and from 0 to 20%, respectively, in the temperature range of 605-750 • C, most possibly due to enhancement of their production via C 3 H 8 hydrogenolysis and C 3 H 8 or C 3 H 6 decomposition reactions (4)-( 9) [4,78].
The addition of metal oxides on the TiO 2 surface led to certain variations in the distribution of products with respect to the nature of the metal oxide additive.Titania-supported CeO 2 and CaO catalysts exhibited significantly lower S C 3 H 6 than S CO , which remained almost stable up to 650 • C and then decreased rapidly with further increase in temperature.This implies that the RWGS and/or reverse Boudouard reactions may dominate below 650 • C over these catalysts against the ODP reaction resulting in higher production of CO in agreement with previous studies [79,80].It should be noted that although S C 3 H 6 was lower below 650 • C than S CO over CeO 2 -TiO 2 and CaO-TiO 2 , it increased with increasing temperature up to 750 • C contrary to the rest of the catalysts investigated.S C 2 H 4 and S CH 4 were also lower (S C 2 H 4 < 20%, S CH 4 < 10%) below 650 • C over CeO 2 -TiO 2 and CaO-TiO 2 , indicating that C 3 H 8 hydrogenolysis and C 3 H 8 or C 3 H 6 decomposition were hindered.Selectivity towards C 3 H 6 for the most active Ga 2 O 3 -TiO 2 , Cr 2 O 3 -TiO 2 and ZrO 2 -TiO 2 materials was found to be similar (~40% at 650 • C) to that of bare TiO 2 indicating that despite the increase in both propane conversion and propylene yield achieved over these composite metal oxides, propylene selectivity remained almost constant.
It is of interest to note that S C 2 H 4 was always higher than S CH 4 , implying that in addition to propane cracking via reaction (7) which results in stoichiometric production of C 2 H 4 and CH 4 , propane cracking via reaction (6) runs in parallel, producing an excess of C 2 H 4 [3,4].Propane decomposition through reaction (9) and/or hydrogenolysis reactions (4) and (5) as well as propylene decomposition (8) cannot be excluded.The hydrogenolysis of propane through reaction (4) was confirmed by the detection of C 2 H 6 traces over the samples containing CaO, Cr 2 O 3 and Ga 2 O 3 (Figure 5c,e,f).It is of interest to note that  was always higher than  , implying that in addition to propane cracking via reaction (7) which results in stoichiometric production of C2H4 and CH4, propane cracking via reaction (6) runs in parallel, producing an excess of C2H4 [3,4].Propane decomposition through reaction ( 9) and/or hydrogenolysis reactions (4) and ( 5) as well as propylene decomposition ( 8) cannot be excluded.The hydrogenolysis of propane through reaction (4) was confirmed by the detection of C2H6 traces over the samples containing CaO, Cr2O3 and Ga2O3 (Figure 5c,e,f).
In an attempt to clarify the beneficial effect of the addition of metal oxides on the TiO2 surface, the CO2-assisted oxidative dehydrogenation of propane was also conducted over bare Ga2O3 and Cr2O3 catalysts.Results showed that 10% Ga2O3-TiO2 catalyst exhibited significantly higher  and  compared with bare TiO2 and Ga2O3 (Figure S1a,b).This was also the case for the 10% Gr2O3-TiO2 catalyst which was found to be more active towards propylene formation compared with bare TiO2 and Cr2O3 (Figure S1c,d).Results provide evidence that a synergistic effect exists between MxOy and TiO2 leading to an improvement of the catalytic activity and process efficiency concerning propylene production.This effect may be related to the basic properties of 10% Ga2O3-TiO2 and 10% Gr2O3-TiO2 which were found to be enhanced compared with bare TiO2 (Figures 2 and 3) as well as when compared with bare Ga2O3 or Gr2O3 as evidenced by the results of the CO2-DRIFTS studies presented in Figure S2.As can be seen in this graph, the relative populations of bicarbonate (1633 and 1235 cm −1 for Cr2O3 [47,81], 1617 and 1224 for Ga2O3 [53][54][55]) and bidentate carbonate (1587 cm −1 for Cr2O3 [81], 1580 and 1332 cm −1 for Ga2O3  In an attempt to clarify the beneficial effect of the addition of metal oxides on the TiO 2 surface, the CO 2 -assisted oxidative dehydrogenation of propane was also conducted over bare Ga 2 O 3 and Cr 2 O 3 catalysts.Results showed that 10% Ga 2 O 3 -TiO 2 catalyst exhibited significantly higher X C 3 H 8 and Y C 3 H 6 compared with bare TiO 2 and Ga 2 O 3 (Figure S1a,b).This was also the case for the 10% Gr 2 O 3 -TiO 2 catalyst which was found to be more active towards propylene formation compared with bare TiO 2 and Cr 2 O 3 (Figure S1c,d).Results provide evidence that a synergistic effect exists between M x O y and TiO 2 leading to an improvement of the catalytic activity and process efficiency concerning propylene production.This effect may be related to the basic properties of 10% Ga 2 O 3 -TiO 2 and 10% Gr 2 O 3 -TiO 2 which were found to be enhanced compared with bare TiO 2 (Figures 2 and 3) as well as when compared with bare Ga 2 O 3 or Gr 2 O 3 as evidenced by the results of the CO 2 -DRIFTS studies presented in Figure S2.As can be seen in this graph, the relative populations of bicarbonate (1633 and 1235 cm −1 for Cr 2 O 3 [47,81] , 1617 and 1224 for Ga 2 O 3 [53][54][55]) and bidentate carbonate (1587 cm −1 for Cr 2 O 3 [81], 1580 and 1332 cm −1 for Ga 2 O 3 [53,55]) species were significantly lower for both bare Cr 2 O 3 and Ga 2 O 3 catalysts compared with 10% Ga 2 O 3 -TiO 2 , 10% Gr 2 O 3 -TiO 2 and bare TiO 2 , implying weaker adsorption of CO 2 and, therefore, lower basicity.
Comparing the results of Figure 4 with catalyst characterization results (Table 1), a correlation between the catalytic performance and the crystallite size of TiO 2 support was found to exist.This is clearly depicted in Figure 6a where propane conversion and propylene yield measured at 700 • C are plotted as a function of the crystallite size of the rutile phase of TiO 2 .It was observed that both X C 3 H 8 and Y C 3 H 6 increased from 15 to 45% and from 6.5 to 16%, respectively, with decreasing the d TiO 2 ,R from 36.8 to 14.9 nm.A similar general trend but to a lesser degree was also found between X C 3 H 8 and Y C 3 H 6 , and d TiO 2 ,A , with the exception of the Cr 2 O 3 -containing sample which was characterized by a similar d TiO 2 ,A as that estimated for bare TiO 2 .This finding indicates that propylene production via ODP with CO 2 reaction was favored over small TiO 2 crystallites.The rutile content was also found to be generally lower for composite metal oxides compared with bare TiO 2 without, however, presenting any trend with respect to X C 3 H 8 and/or Y C 3 H 6 .[53,55]) species were significantly lower for both bare Cr2O3 and Ga2O3 catalysts co pared with 10% Ga2O3-TiO2, 10% Gr2O3-TiO2 and bare TiO2, implying weaker adsorpt of CO2 and, therefore, lower basicity.
Comparing the results of Figure 4 with catalyst characterization results (Table 1 correlation between the catalytic performance and the crystallite size of TiO2 support w found to exist.This is clearly depicted in Figure 6a where propane conversion and p pylene yield measured at 700 °C are plotted as a function of the crystallite size of the tile phase of TiO2.It was observed that both  and  increased from 15 to 4 and from 6.5 to 16%, respectively, with decreasing the  , from 36.8 to 14.9 nm similar general trend but to a lesser degree was also found between  and  ,  , , with the exception of the Cr2O3-containing sample which was characterized b similar  , as that estimated for bare TiO2.This finding indicates that propyl production via ODP with CO2 reaction was favored over small TiO2 crystallites.The tile content was also found to be generally lower for composite metal oxides compa with bare TiO2 without, however, presenting any trend with respect to  and  . (a) (b) Moreover, based on the results of the DRIFTS (Figure 2) and CO2-TPD (Figur studies, the surface basicity was improved with the addition of metal oxides on the T surface, whereas according to the results of Figure 4, catalytic activity was higher o composite metal oxides characterized by moderate basicity.This is illustrated in Fig 6b where 𝑋 and  obtained at 700 °C are presented as a function of the t amount of CO2 desorbed during CO2-TPD experiments (Table 2).It was observed t both propane conversion and propylene yield increased with the increase in surface sicity, exhibiting maximum values for the Cr2O3and Ga2O3-TiO2 catalysts and then creased rapidly for the CaO-TiO2 catalyst which was found to contain the largest num of basic sites.
As mentioned above, in addition to surface basicity, acidity may also influence catalytic activity for the ODP with CO2 reaction.According to previous studies, the t amount of the surface acid sites of TiO2 and especially, those characterized by me um-strong acid strength can be significantly increased with the addition of Ga2O3 [15 ZrO2 [40,82], whereas low or no influence on the TiO2 acidity is expected by modify TiO2 with CeO2 [83], Cr2O3 [73,84] or CaO [60,85,86].This was further confirmed conducting TGA experiments following ammonia adsorption at 25 °C over selected alysts and specifically, over TiO2, Ga2O3-TiO2 and Cr2O3-TiO2.The results obtained presented in Figures 7a and S3 where the weight loss (%) and the TGA derivative cur respectively, are plotted as a function of temperature.In all cases, two weight loss regi Moreover, based on the results of the DRIFTS (Figure 2) and CO 2 -TPD (Figure 3) studies, the surface basicity was improved with the addition of metal oxides on the TiO 2 surface, whereas according to the results of Figure 4, catalytic activity was higher over composite metal oxides characterized by moderate basicity.This is illustrated in Figure 6b where X C 3 H 8 and Y C 3 H 6 obtained at 700 • C are presented as a function of the total amount of CO 2 desorbed during CO 2 -TPD experiments (Table 2).It was observed that both propane conversion and propylene yield increased with the increase in surface basicity, exhibiting maximum values for the Cr 2 O 3 -and Ga 2 O 3 -TiO 2 catalysts and then decreased rapidly for the CaO-TiO 2 catalyst which was found to contain the largest number of basic sites.
As mentioned above, in addition to surface basicity, acidity may also influence the catalytic activity for the ODP with CO 2 reaction.According to previous studies, the total amount of the surface acid sites of TiO 2 and especially, those characterized by mediumstrong acid strength can be significantly increased with the addition of Ga 2 O 3 [15] or ZrO 2 [40,82], whereas low or no influence on the TiO 2 acidity is expected by modifying TiO 2 with CeO 2 [83], Cr 2 O 3 [73,84] or CaO [60,85,86].This was further confirmed by conducting TGA experiments following ammonia adsorption at 25 • C over selected catalysts and specifically, over TiO 2 , Ga 2 O 3 -TiO 2 and Cr 2 O 3 -TiO 2 .The results obtained are presented in Figures 7a and S3 where the weight loss (%) and the TGA derivative curves, respectively, are plotted as a function of temperature.In all cases, two weight loss regions were observed.The initial weight loss appearing in the temperature range of 150-250 • C could be attributed to NH 3 desorption from weak to moderate acid sites and the weight loss initiated above 300 • C was due to NH 3 desorption from strong acid sites [15,83,87].A weight loss observed below 120 • C may be due to the removal of residual physisorbed water [82].Interestingly, the weight loss was significantly higher and extended above 350 • C for the Ga 2 O 3 -TiO 2 catalyst indicating that this sample consisted of more and stronger acid sites compared with TiO 2 and Cr 2 O 3 -TiO 2 .This was also reflected by the acidity values (Table S2) estimated according to Equation (S1) following the procedure described elsewhere [88], which were found to be 310.1 µmol/g for TiO 2 , 318.8 µmol/g for Cr 2 O 3 -TiO 2 and 510.3 µmol/g for Ga 2 O 3 -TiO 2 , clearly indicating that the total surface acidity increased with the addition of Ga 2 O 3 on TiO 2 but was not practically affected by modifying TiO 2 with Cr 2 O 3 .Based on the above and taking into account that both Cr 2 O 3 -TiO 2 and Ga 2 O 3 -TiO 2 catalysts exhibited superior activity, it can be suggested that the surface acidity may affect catalytic activity for certain catalyst formulations but is not the only parameter that determines the conversion of propane towards propylene via the ODP with CO 2 reaction at least under the present experimental conditions.s 2024, 14, x FOR PEER REVIEW 18 of 26 mated from the area below the corresponding hydrogen response curves and found to be 31.1 µmol/g for TiO2, 51.2 µmol/g for Cr2O3-TiO2 and 106.5 µmol/g for Ga2O3-TiO2.This indicates that the reducibility was notably enhanced with the addition of Cr2O3, and especially, Ga2O3, on the TiO2 surface.It is worth mentioning that although these two catalysts presented similar catalytic activity (Figure 4), the reducibility of Ga2O3-TiO2 was twice that of Cr2O3-TiO2, implying that ODP activity is not solely determined by the redox properties of TiO2 based catalysts.
(a) (b) Summarizing, the synergistic effect between MxOy and TiO2 seems to involve modification of the physicochemical properties of catalysts including the variation of acid/base and redox properties, as well as the anatase/rutile content and the primary crystallite size of TiO2 support which influence catalytic activity and propylene yield.It should be noted that the optimization of catalytic activity of the Ga2O3-TiO2 and Cr2O3-TiO2 samples, which presented superior activity, is currently under investigation by varying the CO2:C3H8 ratio and/or WGHSV as well as the MxOy content in order to achieve higher propane conversions and propylene yields at temperatures of practical interest.

Time-On-Stream (TOS) Stability Test
The influence of reaction time on the activity, propylene yield and products selectivity for the ODP with CO2 reaction was investigated over the 10% Ga2O3-TiO2 catalyst which was among those presenting superior performance.Measurements were obtained at 710 °C and the results showed that propane conversion fluctuated between 52 and 57% during the first 25 h on stream, whereas it progressively increased up to 66.5% with further remaining of catalyst under reaction conditions up to 32 h (Figure 8a).On the other hand, propylene yield was stable for 32 h on stream ranging between 17 and 19% (Figure 8a).Products' selectivity remained constant for 25 h on stream, taking the following values:  = 32.5-36%,SCO = 0.5-2%,  = 40-43%  = 20-22.5% and  ~1% (Figure 8b).A slight and progressive increase in both  and  up to 45.5 and 24%, respectively, was observed after 32 h on stream accompanied by a parallel decrease in  to 28.5%.This implies that the undesired reactions of propane decomposition or hydrogenolysis (4)-( 7) and ( 9) were enhanced after prolonged catalyst interaction with the reaction mixture hindering to some extent the oxidative dehydrogenation of propane (2).In general, 10% Ga2O3-TiO2 catalyst exhibited sufficient stability with time on stream Based on the above, it may be proposed that a balance of surface acid/base characteristics is required as was previously suggested over various CO 2 -assisted catalytic reactions [9,89].For example, Burri et al. [89] found that the number and strength of both acidic and basic sites were higher over the TiO 2 -ZrO 2 catalyst compared with those measured for bare TiO 2 or ZrO 2 .According to these authors, the optimum surface acidity and basicity were responsible for the superior activity of the TiO 2 -ZrO 2 catalyst for the oxidative dehydrogenation of ethylbenzene to styrene.In a subsequent publication, the same authors reported that the promotion of TiO 2 -ZrO 2 with Na or K enhanced the catalyst surface basicity, with the Na-doped sample exhibiting the optimum balance of acid/base properties leading to higher catalytic activity [90].In addition, Sui et al. [91] prepared Cr/Na-ZSM-5 catalysts of variable Cr content for the reaction of the oxidative dehydrogenation of ethane and demonstrated that the redox properties of catalysts were influenced by Cr 2 O 3 and Na-ZSM-5 interactions, which resulted in an increase in the number of basic sites.
It is well known that catalyst reducibility is among the physicochemical properties that were found to affect ODP activity [70].Therefore, the redox properties of selected catalysts and particularly, the least active bare TiO 2 and the most active Ga 2 O 3 -TiO 2 and Cr 2 O 3 -TiO 2 samples were examined by conducting H 2 -TPR experiments.The results (Figure 7b) showed that the TPR profile of bare TiO 2 was characterized by a peak centered at 359 • C and a weak feature extending between 515 and 635 • C, which was previously assigned to the reduction of the surface TiO 2 [92][93][94].The addition of Cr 2 O 3 on TiO 2 support resulted in the appearance of a sharp H 2 consumption peak with its maximum located at 256 • C, which was followed by a weaker peak centered at ~343 • C. According to previous studies, the low temperature peak was due to the reduction of Cr 6+ to Cr 5+ and the high temperature peak to the reduction of Cr 5+ to Cr 3+ [95] or the reduction of chromium located deeper in the catalyst lattice [96].On the other hand, Wang et al. [26,74] demonstrated that the low temperature peak detected in the H 2 -TPR profiles of CrO x /silicalite-1 and CrO x dispersed on dealuminated b zeolite was due to the reduction of isolated Cr 6+ , whereas that detected at higher temperatures was due to the reduction of polymeric Cr 6+ .An additional peak was also discerned over Cr 2 O 3 -TiO 2 (Figure 7b) at ~365 • C (overlapped with the peak centered at 343 • C) as well as a broad feature above 400 • C which, as discussed above, may be related to the reduction of surface TiO 2 .Modification of TiO 2 support with Ga 2 O 3 led to significant variations of the H 2 -TPR profile which consisted of two intense peaks at 338 • C and 598 • C. Similar peaks observed over Ga 2 O 3 based catalysts were previously attributed to the reduction of well dispersed Ga species and bulk or larger Ga 2 O 3 particles, respectively [97][98][99].It should be noted that the peaks discussed above due to the reduction of the TiO 2 surface may be overlapped by the high intensity peaks of Ga 2 O 3 reduction.The total amount of consumed H 2 was estimated from the area below the corresponding hydrogen response curves and found to be 31.1 µmol/g for TiO 2 , 51.2 µmol/g for Cr 2 O 3 -TiO 2 and 106.5 µmol/g for Ga 2 O 3 -TiO 2 .This indicates that the reducibility was notably enhanced with the addition of Cr 2 O 3 , and especially, Ga 2 O 3 , on the TiO 2 surface.It is worth mentioning that although these two catalysts presented similar catalytic activity (Figure 4), the reducibility of Ga 2 O 3 -TiO 2 was twice that of Cr 2 O 3 -TiO 2 , implying that ODP activity is not solely determined by the redox properties of TiO 2 based catalysts.
Summarizing, the synergistic effect between M x O y and TiO 2 seems to involve modification of the physicochemical properties of catalysts including the variation of acid/base and redox properties, as well as the anatase/rutile content and the primary crystallite size of TiO 2 support which influence catalytic activity and propylene yield.It should be noted that the optimization of catalytic activity of the Ga 2 O 3 -TiO 2 and Cr 2 O 3 -TiO 2 samples, which presented superior activity, is currently under investigation by varying the CO 2 :C 3 H 8 ratio and/or WGHSV as well as the M x O y content in order to achieve higher propane conversions and propylene yields at temperatures of practical interest.

Time-On-Stream (TOS) Stability Test
The influence of reaction time on the activity, propylene yield and products selectivity for the ODP with CO 2 reaction was investigated over the 10% Ga 2 O 3 -TiO 2 catalyst which was among those presenting superior performance.Measurements were obtained at 710 • C and the results showed that propane conversion fluctuated between 52 and 57% during the first 25 h on stream, whereas it progressively increased up to 66.5% with further remaining of catalyst under reaction conditions up to 32 h (Figure 8a).On the other hand, propylene yield was stable for 32 h on stream ranging between 17 and 19% (Figure 8a).Products' selectivity remained constant for 25 h on stream, taking the following values: S C 3 H 6 = 32.5-36%,S CO = 0.5-2%, S C 2 H 4 = 40-43% S CH 4 = 20-22.5% and S C 2 H 6 ~1% (Figure 8b).A slight and progressive increase in both S C 2 H 4 and S CH 4 up to 45.5 and 24%, respectively, was observed after 32 h on stream accompanied by a parallel decrease in S C 3 H 6 to 28.5%.This implies that the undesired reactions of propane decomposition or hydrogenolysis (4)-( 7) and ( 9) were enhanced after prolonged catalyst interaction with the reaction mixture hindering to some extent the oxidative dehydrogenation of propane (2).In general, 10% Ga 2 O 3 -TiO 2 catalyst exhibited sufficient stability with time on stream suggesting that is a promising material for the production of propylene via CO 2 -assisted ODP reaction.

Oxidative Dehydrogenation of Propane with CO2 Studied by In Situ DRIFTS
The identification of reaction intermediates formed on the catalyst surface under reaction conditions was investigated by conducting in situ DRIFTS experiments.In the case of bare TiO2 (Figure 9a), the spectrum recorded at 25 °C following catalyst exposure to 1% C3H8 + 5% CO2/He mixture was characterized by three negative bands located at 3718, 3677 and 3611 cm −1 due to surface hydroxyl groups originally existing on the TiO2 surface, two bands at 1565 and 1357 cm −1 due to bidentate carbonates, two bands at 1415 cm −1 and 1253 cm −1 assigned to bicarbonates and carboxylates, respectively, and a broad band at ca. 1657 cm −1 containing contributions from both the latter two species [31,32,34,[36][37][38].Several bands were also discerned in the C-H stretching (v) region assigned to the different vibrations of propane and/or its derivatives, which can be better seen in Figure 9d.In particular, spectral features attributed to asymmetric (2980 and 2967 cm −1 ) and symmetric (2960 cm −1 ) C-H stretching vibrations in methyl groups (CH3,ad), as well as asymmetric (2902 cm −1 ) and symmetric (2875 cm −1 ) C-H stretching vibrations in methylene groups (CH2,ad) were detected [3,25,100].Regarding the band located at 2886 cm −1 , it was previously assigned to νs(CH2)/νas(CH3) of gaseous propane [3,100].
An increase in temperature to 100 °C resulted in better distinguishment of the two overlapping bands at ~1630-1660 cm −1 , confirming, as suggested above, the contribution from both carboxylate (1667 cm −1 ) and bicarbonate (1639 cm −1 ) species.Further increase in temperature resulted in an increase in the relative intensity of the 1639 cm −1 band at the expense of the bands at 1667 and 1565 cm −1 , implying that either an interconversion of carboxylates and bidentate carbonates towards bicarbonates occurs under reaction conditions or bicarbonates are thermally more stable.This is in agreement with results presented in Figure 2 where bicarbonates were found to have survived on the TiO2 surface at higher temperatures.Most of the bands detected below 1700 cm −1 almost disappeared from the spectra collected above 450 °C.This was also the case for the spectral features discerned in the C-H stretching region (Figure 9d).It is of interest to note that a new band appeared at 3414 cm −1 in the spectrum recorded at 150 °C, the intensity of which increased significantly with progressive increase in temperature to 400 °C.A similar band was previously assigned to OH surface groups raised by H2O adsorption which may be produced either by the main reaction of CO2-assisted ODP (2) and/or the RWGS reaction (3) [39,101,102].The identification of reaction intermediates formed on the catalyst surface under reaction conditions was investigated by conducting in situ DRIFTS experiments.In the case of bare TiO 2 (Figure 9a), the spectrum recorded at 25 • C following catalyst exposure to 1% C 3 H 8 + 5% CO 2 /He mixture was characterized by three negative bands located at 3718, 3677 and 3611 cm −1 due to surface hydroxyl groups originally existing on the TiO 2 surface, two bands at 1565 and 1357 cm −1 due to bidentate carbonates, two bands at 1415 cm −1 and 1253 cm −1 assigned to bicarbonates and carboxylates, respectively, and a broad band at ca. 1657 cm −1 containing contributions from both the latter two species [31,32,34,[36][37][38].Several bands were also discerned in the C-H stretching (v) region assigned to the different vibrations of propane and/or its derivatives, which can be better seen in Figure 9d.In particular, spectral features attributed to asymmetric (2980 and 2967 cm −1 ) and symmetric (2960 cm −1 ) C-H stretching vibrations in methyl groups (CH 3,ad ), as well as asymmetric (2902 cm −1 ) and symmetric (2875 cm −1 ) C-H stretching vibrations in methylene groups (CH 2,ad ) were detected [3,25,100].Regarding the band located at 2886 cm −1 , it was previously assigned to ν s (CH 2 )/ν as (CH 3 ) of gaseous propane [3,100].
An increase in temperature to 100 • C resulted in better distinguishment of the two overlapping bands at ~1630-1660 cm −1 , confirming, as suggested above, the contribution from both carboxylate (1667 cm −1 ) and bicarbonate (1639 cm −1 ) species.Further increase in temperature resulted in an increase in the relative intensity of the 1639 cm −1 band at the expense of the bands at 1667 and 1565 cm −1 , implying that either an interconversion of carboxylates and bidentate carbonates towards bicarbonates occurs under reaction conditions or bicarbonates are thermally more stable.This is in agreement with results presented in Figure 2 where bicarbonates were found to have survived on the TiO 2 surface at higher temperatures.Most of the bands detected below 1700 cm −1 almost disappeared from the spectra collected above 450 • C.This was also the case for the spectral features discerned in the C-H stretching region (Figure 9d).It is of interest to note that a new band appeared at 3414 cm −1 in the spectrum recorded at 150 • C, the intensity of which increased significantly with progressive increase in temperature to 400 • C. A similar band was previously assigned to OH surface groups raised by H 2 O adsorption which may be produced either by the main reaction of CO 2 -assisted ODP (2) and/or the RWGS reaction (3) [39,101,102].Similar experiments were conducted for all the investigated catalysts.Representative results for the most active 10% Cr2O3-TiO2 and 10% Ga2O3-TiO2 catalysts are shown in Figure 9b and Figure 9c, respectively.The main difference observed for the Cr2O3-containing sample compared with bare TiO2 was that the population of adsorbed carboxylates (1676 cm −1 ), bicarbonates (1633 and 1418 cm −1 ) and bidentate carbonates (1555 and 1360 cm −1 ) was significantly higher and progressively increased with increasing temperature up to 500 °C.It is of interest to note that the relative intensity of the bands assigned to bidentate carbonates seems to be higher compared with those due to bicarbonate and carboxylate species contrary to what was observed for bare TiO2.This may be related to the higher anatase content observed for 10% Cr2O3-TiO2 (Figure 1, Table 1) and agrees well with results reported by Su et al. [32] who demonstrated that adsorbed CO2 on anatase phase led mainly to bidentate carbonates formation whereas on rutile phase produced mainly bicarbonates.A significantly higher intensity was also observed for the bands in the C-H stretching region, which can be clearly discerned up to 500 °C.Moreover, two new bands were detected at 1761 and 1718 cm −1 in the spectra obtained between 150 and 350 °C which were previously attributed to bridged carbonate species [32,103].Similar experiments were conducted for all the investigated catalysts.Representative results for the most active 10% Cr 2 O 3 -TiO 2 and 10% Ga 2 O 3 -TiO 2 catalysts are shown in Figure 9b and Figure 9c, respectively.The main difference observed for the Cr 2 O 3containing sample compared with bare TiO 2 was that the population of adsorbed carboxylates (1676 cm −1 ), bicarbonates (1633 and 1418 cm −1 ) and bidentate carbonates (1555 and 1360 cm −1 ) was significantly higher and progressively increased with increasing temperature up to 500 • C. It is of interest to note that the relative intensity of the bands assigned to bidentate carbonates seems to be higher compared with those due to bicarbonate and carboxylate species contrary to what was observed for bare TiO 2 .This may be related to the higher anatase content observed for 10% Cr 2 O 3 -TiO 2 (Figure 1, Table 1) and agrees well with results reported by Su et al. [32] who demonstrated that adsorbed CO 2 on anatase phase led mainly to bidentate carbonates formation whereas on rutile phase produced mainly bicarbonates.A significantly higher intensity was also observed for the bands in the C-H stretching region, which can be clearly discerned up to 500 • C.Moreover, two new bands were detected at 1761 and 1718 cm −1 in the spectra obtained between 150 and 350 • C which were previously attributed to bridged carbonate species [32,103].
The relative intensity of the bands below 1700 cm −1 was also found to be higher over the Ga 2 O 3 -TiO 2 catalyst (Figure 9c) as well as over CaO-TiO 2 (Figure S4a), CeO 2 -TiO 2 (Figure S4b) and ZrO 2 -TiO 2 (Figure S4c) compared with bare TiO 2 (Figure 9a).This implies that the adsorption/activation of CO 2 was enhanced with the addition of metal oxides on the TiO 2 surface most possibly due to the improved basicity observed in the results of Figures 2 and 3.Besides the preferential adsorption of CO 2 on the basic sites of metal oxides, the surface basicity has been suggested to have a beneficial effect for oxidative dehydrogenation reactions because it hinders the adsorption of the produced alkenes on the catalyst surface and consequently their deep oxidation to carbon oxides or oxygenates [104].As in the case of the Cr 2 O 3 -TiO 2 catalyst, the relative population of bidentate carbonate species was higher than that of bicarbonates species for all composite metal oxides which as discussed above may be related to the higher content of the anatase phase.It should be also noted that CO produced via ODP reaction (Figure 5) may also be partially responsible for the formation of adsorbed carbonate-like species on the catalyst surface.The detection of bands due to CH 3,ad and CH 2,ad species at temperatures as low as 25 • C provides evidence that propane is dissociatively adsorbed on the catalyst surface where it interacts with the adsorbed CO 2 producing the reaction products.Based on previous studies, both non-oxidative dehydrogenation and oxidative dehydrogenation may occur during catalyst interaction with the CO 2 /C 3 H 8 mixture in a manner which depends strongly on the catalyst reducibility and/or the type of active sites [4].In the case of catalysts containing reducible metal oxides (e.g., Cr 2 O 3 , CeO 2 , etc.) the reaction has been suggested to proceed via the one-step oxidative route (2) with CO 2 participating in the re-oxidation of reduced metals according to the Mars-Van Krevelen mechanism.On the other hand, when irreducible metal oxides are used (e.g., Ga 2 O 3 ) the reaction occurs through the non-oxidative dehydrogenation reaction (1) in combination with the RWGS reaction (3) where the role of CO 2 is to remove the produced H 2 and shifts the equilibrium position towards higher propylene yields.
Although results of Figure 9 contributed to the identification of the surface intemediates produced under reaction conditions and demonstrated that the activation of CO 2 is facilitated on composite metal oxides, they cannot reveal the reaction pathway.Clearly, detailed mechanistic studies are required in order to further explore the reaction mechanism and determine the active sites and the elementary steps of the ODP with CO 2 reaction at the M x O y -TiO 2 surface with respect to the nature of the M x O y additive.

Conclusions
The addition of various M x O y (M: Zr, Ce, Ca, Cr, Ga) additives on TiO 2 surface for the production of propylene via the ODP with CO 2 reaction was reported herein in an attempt to determine the role of the type of M x O y additive on both the propane conversion and propylene yield.A considerable increase in catalytic performance was found over M x O y -TiO 2 catalysts with the Y C 3 H 6 being increased by a factor of 2.9 following the order TiO 2 (bare) < CaO ~CeO 2 < ZrO 2 < Cr 2 O 3 ~Ga 2 O 3 .A synergistic effect between M x O y and TiO 2 seems to occur, resulting in modification of the surface basicity and reducibility of the investigated catalysts as well as the anatase/rutile ratio and the primary crystallite size of TiO 2 support.Moderate surface basicity and small TiO 2 crystallite size were found to be crucial for the efficient conversion of propane towards propylene.DRIFTS studies carried out under reaction conditions provided evidence that CO 2 adsorption in the form of carbonate-like species is enhanced over composite metal oxides, implying that CO 2 activation may benefit by the presence of certain metal oxide modifiers on TiO 2 surface, leading to higher propylene yields.

Figure 6 .
Figure 6.Propane conversion and propylene yield obtained at 700 °C as a function of (a) the m crystallite size of rutile TiO2 and (b) the total amount of desorbed CO2 during CO2-TPD exp ments of the indicated catalysts.

Figure 6 .
Figure 6.Propane conversion and propylene yield obtained at 700 • C as a function of (a) the mean crystallite size of rutile TiO 2 and (b) the total amount of desorbed CO 2 during CO 2 -TPD experiments of the indicated catalysts.

Figure 8 .
Figure 8. Thirty-two hour TOS stability test of the 10% Ga2O3-TiO2 catalyst conducted at 710 °C under conditions of oxidative dehydrogenation of C3H8 with CO2.Alterations of (a) XC3H8 and YC3H6, and (b) products selectivity with time-on-stream.Experimental conditions: same as in Figure 4. Dashed vertical black lines indicate shutting down of the system overnight where the catalyst remained under He flow.

Table 1 .
Physicochemical characteristics of the synthesized oxides.

Table 2 .
Total amount of desorbed CO 2 during CO 2 -TPD experiments.
Figure 4. (a) Conversions of C 3 H 8 and (b) yields of C 3 H 6 as a function of reaction temperature obtained over TiO 2 and 10% M x O y -TiO 2 catalysts.Experimental conditions: mass of catalyst, 500 mg; particle diameter, 0.15 < d p < 0.25 mm; feed composition, 5% C 3 H 8 , 25% CO 2 (balance He); total flow rate, 50 cm 3 •min −1 .Dashed line corresponds to the equilibrium conversion of propane predicted by thermodynamics.
Figure 8. Thirty-two hour TOS stability test of the 10% Ga 2 O 3 -TiO 2 catalyst conducted at 710 • C under conditions of oxidative dehydrogenation of C 3 H 8 with CO 2 .Alterations of (a) X C3H8 and Y C3H6 , and (b) products selectivity with time-on-stream.Experimental conditions: same as in Figure 4. Dashed vertical black lines indicate shutting down of the system overnight where the catalyst remained under He flow.3.6.Oxidative Dehydrogenation of Propane with CO 2 Studied by In Situ DRIFTS