Plasma-enhanced direct conversion of CO 2 to CO over oxygen-deficient Mo-doped CeO 2 †

Plasma CO 2 splitting to CO over oxygen-deficient Mo-doped CeO 2 under mild conditions was investigated for the first time, showing B 20 times higher CO 2 conversion compared to pure CeO 2 , which can be attributed to the increased oxygen vacancies (V O ) and the formation of Ce 3+ –V O –Mo on the catalyst surface. Importantly, V O sites showed excellent catalytic stability.

only a few catalysts (e.g., Ni/SiO 2 , NiO/TiO 2 and Rh/TiO 2 ) have been evaluated (Table S1, ESI †). [17][18][19][20] Recently, surface oxygen vacancies (V O ) have been suggested to be the active sites in plasma-catalytic CO 2 splitting to CO. Mei et al. reported that higher CO 2 conversion was achieved when coupling plasma with BaTiO 3 , which can be explained by the formation of more V O sites on the surfaces of BaTiO 3 compared with TiO 2 . 20 Chen et al. found the V O on Ni/TiO 2 contributed to the enhanced CO 2 dissociation. 18 In fact, the coupling of catalysts and plasma is a promising strategy to improve CO 2 conversion and energy efficiency. However, the knowledge in selection of appropriate catalysts for highly efficient CO 2 splitting to CO using NTP was still very limited until now.
Herein, CO 2 splitting to CO over M-doped CeO 2 catalysts (M = Fe, Co, Ni, Cu, Cr, V, Mn or Mo) has been carried out in a dielectric barrier discharge (DBD) reactor ( Fig. S2-S5, ESI †). CeO 2 not only serves as a support to anchor and disperse the metal oxide particles but also generates V O active sites through the interaction with metal oxides. Significant differences were observed among the M-doped CeO 2 catalysts in terms of CO 2 conversion, and the Mo-doped CeO 2 exhibited the best activity in CO 2 conversion (Fig. S3, ESI †). Therefore, Mo-doped CeO 2 has been selected for further studies.
Comprehensive catalyst characterization was carried out to understand the physicochemical properties of Mo-doped CeO 2 . As shown in Fig. 1, the X-ray diffraction (XRD) pattern of CeO 2 exhibits characteristic peaks of a cubic fluorite phase (JCPDS, 34-0394). For Mo-doped CeO 2 , small peaks of a-MoO 3 (JCPDS, 05-0508) and Mo 4 O 11 (JCPDS, 05-0337) are observed, revealing the coexistence of Mo(VI) and Mo(V) species. Notably, the characteristic peaks of CeO 2 downshift compared to pure CeO 2 , which suggests that Mo doping leads to the expansion of the CeO 2 unit cell. Usually, inserting Mo ions into CeO 2 induces shrinkage of the CeO 2 unit cell since the radius of Mo ions is much smaller than that of Ce ions, resulting in upshifting of CeO 2 peaks, rather than downshifting. Thus, Mo ions do not insert into the CeO 2 unit cell, and there might be other reasons responsible for this downshift. X-ray photoelectron spectroscopy (XPS) was employed to analyze surface properties of Mo-doped CeO 2 . The deconvoluted Ce 3d XPS spectra are presented in Fig. 2(a). The peaks labeled as v, v 00 , v 00 0 and u, u 00 , u 00 0 are assigned to 3d 5/2 and 3d 3/2 electrons of Ce 4+ , respectively, while the peaks of v 0 , u 0 and v o , u o correspond to 3d 5/2 and 3d 3/2 electrons of Ce 3+ , respectively. 21 Clearly, Ce 3+ exists in CeO 2 and Mo-doped CeO 2 , suggesting the formation of V O in both samples. More importantly, the proportion of Ce 3+ in Mo-doped CeO 2 is 40.2%, higher than that in CeO 2 (30.0%) ( Fig. S6 and S7, ESI †). This finding suggests that Mo doping induces partial transformation of Ce 4+ to Ce 3+ and creates more V O on Modoped CeO 2 . As reconfirmed by the O 1s XPS spectra in Fig. 2(b) and Fig. S6 (ESI †), a higher surface V O (30.4%) is achieved in the Mo-doped CeO 2 in comparison to pure CeO 2 (21.0%). Furthermore, the transformation of Ce 4+ to Ce 3+ leads to the expansion of the CeO 2 unit cell since the ion radius of Ce 3+ (1.23 Å) is higher than that of Ce 4+ (0.97 Å), 22 which explains the reason for the downshifting of CeO 2 peaks in Fig. 1. More interestingly, the binding energies of Ce 3d shift significantly towards higher values after Mo doping, revealing that the electron density of the surface CeO 2 species is lower in Mo-doped CeO 2 compared with pure CeO 2 , which might be induced by the electron transfer from Ce to Mo, due to the higher electronegativity of Mo. These results suggest different properties of V O sites in the form of Ce 3+ -V O in CeO 2 and Ce 3+ -V O -Mo in Mo-doped CeO 2 , as well as the strong interaction between Mo and CeO 2 , which agrees with the results of H 2 -temperature programmed reduction (H 2 -TPR), scanning electron microscopy (SEM) and transmission electron microscopy (TEM) (Fig. S8-S11, ESI †). Fig. 2(c) shows the deconvoluted Mo 3d spectra, in which the Mo-doped CeO 2 sample exhibits typical doublet peaks of Mo 6+ with an energy gap of ca. 3.1 eV, indicating the formation of MoO 3 in Mo-doped CeO 2 . 23 The two smaller peaks observed, however, are identified to be 3d 3/2 and 3d 5/2 electrons of Mo(V), demonstrating the formation of the non-stoichiometric MoO 3Àx , 23 which is consistent with the Mo 4 O 11 species confirmed by the XRD analysis ( Fig. 1). Fig. 2(d) shows the Raman spectra of CeO 2 and Mo-doped CeO 2 . For pure CeO 2 , the intense band at 465 cm À1 is well-indexed to the typical F2g modes of a cubic CeO 2 fluorite structure, and the weak bands at 262, 597 and 1171 cm À1 are assigned to V O , reconfirming the XPS results in Fig. 2(a and b). 23 For Mo-doped CeO 2 , the emerging Raman bands at 673, 824 and 997 cm À1 are assigned to MoO 3 crystallites. 24 However, the band at 955 cm À1 is associated with Mo suboxides (MoO 3Àx ). 24,25 These results indicate that the valence state of Mo in suboxides is Mo 5+ . Moreover, the presence of Mo 5+ and Ce 3+ indicates that more V O sites are created through the strong interaction between Mo and CeO 2 , as well as the calcining atmosphere with deficient oxygen and rich energetic Ar species, which agrees with the results reported by Chen et al. 18 Fig . 3 shows the effect of different operating conditions on CO 2 conversion. Clearly, no reaction occurred without plasma (catalyst only, 400 1C). The conversion of CO 2 was 3.8% in the plasma reaction without a catalyst (plasma only). In the plasma reaction coupled with pure CeO 2 , the CO 2 conversion dropped to 1.2%, which suggests that pure CeO 2 is unfavorable for CO 2 splitting to CO despite CeO 2 being O-deficient. Using Mo-doped   CeO 2 instead of CeO 2 , however, results in a significant increase of CO 2 conversion by a factor of 9 at 400 1C. In addition, the Mo-doped CeO 2 showed stable CO 2 conversion for at least 10 h (Fig. S12, ESI †). Interestingly, the reaction performance can be further improved by using a lower reaction temperature (30 1C) and an additive gas (Ar or N 2 ). This promotional effect was more pronounced when adding N 2 . The highest CO 2 conversion of 23.2% and energy efficiency of 14.3% were achieved in the plasma splitting of CO 2 with N 2 addition over Mo-doped CeO 2 at 30 1C (Table S1, ESI †), while the corresponding formation rate of CO and O 2 was 24.9 mmol h À1 and 12.4 mmol h À1 , respectively (Table S2, ESI †). The optimal energy efficiency achieved in this study is comparable to that reported in previous works (Table S1, ESI †).
Regarding surface reactions, the improved activity over Modoped CeO 2 , on one hand, is mainly attributed to the increased formation of V O (Fig. 2 and Fig. S6, ESI †), since V O serve as adsorption centers for CO 2 dissociative adsorption, 17,26 i.e.,  Fig. S8, ESI †), which leads to a stronger binding strength of CO 2 with the V O of Ce 3+ -V O -Mo. As discussed above, V O is the active site for CO 2 activation; thus, the stability of V O greatly influences the subsequent catalytic cycle. It is well recognized that oxygen can be produced in plasma CO 2 splitting, and the produced O species could poison the catalyst through filling V O sites to form stable lattice oxygen species, resulting in termination of the catalytic cycle.
Therefore, the spent Mo-doped CeO 2 catalysts were further characterized by XPS, Raman, XRD and H 2 -TPR (Fig. 4). Interestingly, compared with the fresh catalyst, the spent Mo-doped CeO 2 catalysts show an increased amount of Ce 3+ and MoO 3Àx , which can be confirmed by the higher intensities of Ce 3+ peaks and the Raman band at B950 cm À1 in Fig. 4(a and b), respectively. The enhancement effect is more pronounced for the catalyst used at 400 1C. Correspondingly, more V O sites were created in the high-temperature reaction (Table S3, ESI †). In addition, the color of the catalyst changed from gray/blue (MoO 3Àx ) to yellow (MoO 3Ày ) with x 4 y after the reaction at 30 1C, while no visible changes were observed on the phase structure of Mo-doped CeO 2 before and after the reaction (30 1C), as shown in Fig. 4(c). These results indicate that some O atoms from CO 2 splitting are adsorbed onto the catalyst, as seen by the slightly increased H 2 consumption in Fig. 4(d). Even so, the V O concentration of Mo-doped CeO 2 used at 30 1C remained at a similar level with that of the fresh sample (Table S3, ESI †). These findings suggest that V O -rich Modoped CeO 2 is stable after the plasma reaction, and hightemperature reactions facilitate the formation and recovery of V O sites, resulting from accelerating recombinative desorption of adsorbed O atoms. 27 In addition to surface reactions, gas-phase reactions also play a crucial role in the plasma-catalytic process. In a pure CO 2 DBD, CO 2 splitting to CO mainly proceeds through the electron impact dissociation of CO 2 (CO 2 + e -CO + O + e), which can be confirmed by plasma chemical kinetic modeling, 28 as well as the formation of CO bands and O atomic lines detected by optical emission spectra of CO 2 DBD (Fig. 5(a)). As shown in Fig. 5(a), the presence of strong N 2 (C 3 P u -B 3 P g , B 3 P g -A 3 S + u ) molecular bands and Ar atomic lines suggests the formation of excited nitrogen species (N 2 *) and metastable Ar species (Ar*). 29,30 These species create an additional reaction route for CO 2 dissociation (N 2 * (or Ar*) + CO 2 -CO + O + N 2 (or Ar)), supported by the increased intensity of O atomic lines and CO bands when adding N 2 or Ar, which contributes to the enhanced CO 2 conversion. Furthermore, 150 ppm NO x was detected by Fourier transform infrared (FTIR) in the case of N 2 addition (Fig. S13, ESI †), revealing that N 2 can be regarded as an alternative scavenger of O species. 12 The elimination of partial O species can effectively limit the reverse reaction, i.e., O + CO + M -CO 2 + M, and thus enhance the CO 2 conversion. This could explain why adding N 2 has a stronger promotion on the CO 2 conversion compared with Ar.
Interaction between reactive species in the gas phase and catalyst is also crucial in plasma-catalytic reactions. 31 Compared to plasma only, packing Mo-doped CeO 2 into the  discharge zone decreased the current (Fig. S14, ESI †), lowering the contribution of the gas-phase reactions. In this case, the CO 2 conversion, however, was still improved, which might be attributed to the interaction between active species and V O -rich Mo-doped CeO 2 , accelerating V O recovery. Therefore, a possible reaction mechanism is proposed in Fig. 5(b). Firstly, CO 2 is activated to species of CO 2 + , CO 2(v) , CO and O radicals as shown in Fig. 5(a)  In conclusion, plasma-catalytic CO 2 splitting over M-doped CeO 2 catalysts (M = Fe, Co, Ni, Cu, Cr, V, Mn or Mo) has been investigated. Mo-doped CeO 2 exhibited the best activity; this is attributed to the increased oxygen vacancies created by strong interaction between Mo and CeO 2 , as well as the calcining atmosphere being oxygen-deficient and rich in Ar metastable species. Furthermore, oxygen vacancies were stable during the reaction, which is ascribed to the interaction between active O produced in the gas phase and the adsorbed O on the oxygen vacancy site, resulting in desorbing as O 2 molecules and recovering oxygen vacancy sites. These findings suggest that introducing proper doping on CeO 2 offers a potential route to tune properties of oxygen vacancy in CeO 2 . Additionally, adding N 2 and Ar into the plasma process enhanced the CO 2 conversion, especially when adding N 2 . This promotional effect is mainly attributed to the new reaction routes induced by the presence of metastable species. We found that N 2 can be used as an O scavenger to forward the chemical equilibrium and inhibit the reverse reaction to form CO 2 .
This work was financially supported by the NSFC (No.

Conflicts of interest
There are no conflicts to declare.