The LaCo1−xVxO3 Catalyst for CO Oxidation in Rich H2 Stream

Abstract

The LaCo1−xVxO3 catalysts for CO oxidation in H2 rich stream were studied. These perovskite-type oxides with variable amounts of vanadium (x = 0, 0.05 and 0.1) were synthesized by modified citrate method. It was found that the modification in LaCoO3 perovskite after vanadium addition affected directly the structure and morphology leading to decrease in the crystallite size and increase in BET surface area. XPS results confirmed quantitatively the higher presence of V4+ species at the surface of the modified perovskite. The existence of Co2+ was reported to facilitate the interplay between Co2+/Co3+ and V5+/V4+ redox couples, which can enhance the lattice oxygen mobility and create interfacial active sites of CoOx/V2O5. Tests during 20 h on stream at 250 °C showed excellent stability, high resistance to coke deposition and no sintering.

Graphic Abstract

Perovskite modified by addition of vanadium LaCo1−xVxO3 for preferential CO oxidation in H2-rich stream.

Introduction

Proton exchange membrane fuel cells (PEMFCs) are an attractive power sources for transport and portable application due to their high energy density, easy start-up and low emissions of pollutants [1]. PEMFCs usually operate in the temperature range of 60–80 °C and require hydrogen as fuel and usually use a Pt-based anode that is susceptible to CO poisoning [2]. Three different approaches to remove CO from reformate gases, including Pd-membrane separation [3], catalytic methanation [4] and preferential oxidation of CO (CO-PROX) [5], have been proposed and investigated intensively. Among these, the CO-PROX reaction is regarded as one of the most promises and effective process to hydrogen purification [6].

The most important requirements for CO-PROX catalysts are high CO oxidation rate at low temperatures without consuming H2, resistance to deactivation and inhibiting the undesired reactions [7, 8]. A variety of catalysts have been reported in the literature for the CO-PROX reaction. They include supported noble metals (Pt, Pd, Ru and Au) [9,10,11,12] and supported transition metals (Co, Ni and Cu) [13,14,15]. The catalysts based on noble metals present good activity and excellent resistance to carbon formation, but high cost limit their large-scale application. Conversely, catalysts containing transition metals have a low cost compared to expensive noble metals, but have a tendency to catalyze undesired reaction of hydrogenation of CO2 and CO with consequent formation of methane, besides being prone to coke formation. Thus, the development of non-noble metal catalysts for CO-PROX with high activity, stability and suppression of undesired side reaction is of great importance and represents a meaningful challenge in industrial application.

In this context, LaCoO3 perovskite oxides have been extensively studied as a possible substitute for noble metals based catalysts due to their low cost, high CO conversion and O2 selectivity to CO2 under low temperatures ranges. These oxides can have their catalytic activity strongly modified by doping with appropriate amounts of transition metals, while still maintaining the perovskite structure [16]. This structural modification result in an improvement of the catalytic activity due to creation of structural defects, such as oxygen vacancies in the network [1]. These defects are able to activate the gas-phase O2 molecules to produce active oxygen species makes them attractive materials as catalysts in oxidation reaction [17].

Several studies have described the advantages of the addition of small amounts of transition metals into perovskite lattice, including Mg [18], Cu [19], Sr [20], and more recently Cr [21]. In all cases, the partial substitution of Co for other metals showed improves in the catalytic activity at low temperatures as compared to undoped LaCoO3 perovskite. Sun et al. [18] reported that catalytic activities of Mg-doped LaCoO3 particles were clearly enhanced by the increase of the surface area and the amount of adsorbed oxygen. Mg-doped LaCoO3 particles with large surface areas exhibited high catalytic activity for CO oxidation. Seyfi et al. [19] verified that the partial substitution of Co for Cu in the LaCoO3 catalyst synthesized by citrate method was efficient for improving the CO oxidation activity, due to a collaborating effect induced between the metals. Pereñíguez et al. [20] had studied the Sr2+ modified LaCoO3−δ, indicating that the Sr substitution promotes the chemisorbed oxygen species over the LaCoO3−δ, hence improving its catalytic performance in the toluene oxidation. Recently, Ali and Al-Otaibi [21] had doped Cr3+ into LaCoO3 structure, which favored the oxygen vacancies and O2− storage and hence enhanced the rates of the electro-oxidation reactions. Likewise, Zhang et al. [22], investigated the effect of A-site substitution by Sr, Mg and Ce on the catalytic performance of LaMnO3 catalysts for the oxidation of vinyl chloride emission. They reported a higher average oxidation state of manganese was achieved by the substitution of Mg and Ce, resulting in an improved redox ability of the Mn4+/Mn3+ redox couple as well as the increase of the oxygen vacancies in the perovskite oxides. Thus, the partial substitution could be exploited to develop suitable material for CO-PROX process operating at low and moderate temperatures.

Although the literature reported several studies about incorporation of a wide variety metals into perovskite structure [18, 23,24,25,26,27,28,29,30], there are still too few studies doping LaCoO3 with vanadium [31,32,33], mainly for CO-PROX in H2-rich stream. Vanadium is very interesting because of its high number of different oxidation states between V2+ and V5+ [34]. Thus, the substitution of Co3+ species in LaCoO3 structure by V species introduces excess of positive and/or negative charges in the lattice. This one must be compensated either by creation of oxygen vacancies, which considered as the most active species for the catalytic oxidation reaction [27]. In this study, for the first time we investigated the effect of partial substitution of cobalt by vanadium in the perovskite-type structure LaCo1−xVxO3 (x = 0, 0.05 and 0.1) on the structural, morphology, catalytic performances and stability for CO-PROX in H2-rich stream.

Experimental

Preparation of Perovskites

LaCo1−xVxO3 perovskites-type oxides with variable amount of vanadium (x = 0, 0.05 and 0.1) were synthesized by a modified citrate method starting from the La(NO3)36H2O, Co(NO3)26H2O and NH4VO3 as precursors [1]. Firstly, the appropriate amounts of metal precursors were dissolved in a minimum quantity of deionized water. Then, citric acid (CA) solution with molar ration of total metals and CA of 1:3 was added to this solution. The resulting mixture was stirred at 90 °C until complete evaporation of water and formation of solid sponge, characteristic of metal citrate. The obtained powder was calcined in two steps: the first one was performed at 600 °C for 1 h to eliminate the organic constituents and the second at 700 °C for 6 h to promote the organization of the solid and formation of the perovskite structure, both under static atmosphere. These temperatures were defined by preliminary experiments. From now on the LaCoO3, LaCo0.95V0.05O3 and LaCo0.9V0.1O3 catalysts are designated as LC, LCV5 and LCV10, respectively.

Characterizations of Perovskites

Chemical composition analysis was performed by X-ray fluorescence spectroscopy (XRF) using a Rigaku spectrometer RIX 3100 model apparatus. Textural properties were obtained from the N2 adsorption/desorption isotherms at liquid nitrogen temperature (− 196 °C), using an ASAP model 2020 equipment of Micromeritics. Prior to the measurements, the catalysts were degassed under high vacuum at 300 °C overnight in order to remove adsorbed species. The specific surface area was obtained using the Brunauer–Emmett–Teller (BET) method in a relative pressure range of 0.05–0.3. The pore size distribution was determined from desorption branches by the Barrett–Joyner–Halenda (BJH) method.

X-ray powder diffraction (XRD) measurements were performed at room temperature in a Miniflex Rigaku diffractometer operated at 20 kV and 15 mA, using CuKα (λ = 1.5406 Å) as radiation source. The angular interval of 10 to 90° was varied in 0.01° steps, using a 1 s counting time per step and reproduced here without any background or smoothing treatment. Crystalline phases were identified by using JCPDS (Joint Committee on Powder Diffraction Standards) database. The lattice parameters and the structure of the catalysts have been estimated from Rietveld’s powder structure refinement analysis employing FullProf Suite® package and graphical interface WinPLOTR.

Temperature-programmed reduction with hydrogen (H2-TPR) experiments were performed on a system equipped with a thermal conductivity detector (TCD). A gas mixture 10 vol.% in H2/Ar (30 cm3 min−1) was used to reduce the catalysts by heating from room temperature up to 1000 °C at 10 °C min−1. Before starting the H2-TPR analyses, the catalysts were pretreated by flowing Ar (30 cm3 min−1) at 150 °C for 30 min.

The morphology and microstructure of the catalysts was examined by scanning electron microscopy (SEM) using a FEG-SEM (FEI company) model QUANTA 400. For sample preparation, the powders were poured as thin layers on double-sided adhesive carbon discs on the sample holder.

The surface chemical state of the atoms and their relative abundance were evaluated by X-ray photoelectron spectroscopy (XPS) using an Escalab 250Xi Thermo Scientific spectrometer with a monochromatic AlKα (1486.6 eV) X-ray source. The XPS spectra were acquired in constant analyzer energy mode (CAE) with pass energy of 100 eV for survey and 25 eV for high resolution. The C 1s signal at 284.6 eV was binding energy reference.

Catalytic Evaluation

The catalytic tests were carried out in a glass U-shaped reactor under atmospheric pressure and equipped with a temperature programmed controller. The feed gas mixture consisted of 1 vol.% CO, 1 vol.% O2, 60 vol.% H2 and He balance and the total feed flow rate was held constant at 100 cm3 min−1, with a weight hour space velocity of 40,000 cm3 (gcat h)−1. The outlet gases were analyzed on line by a gas chromatograph (Varian, model CP3800) equipped with Carboxen®-1010 column with helium as a carrier and then quantified using TCD and FID detectors. A nickel catalytic converter was used to detect trace amount of CO (below 10 ppm). Prior the tests, the catalysts (150 mg) were dried under helium flow (50 cm3 min−1) at 200 °C for 30 min, followed by cooling down to the initial reaction temperature in helium. The temperature was varied between 50 and 300 °C in steps of 50 °C. CO and O2 conversions and CO2 selectivity were calculated according to our previous studies [35]. The stability tests were conducted at 250 °C keeping the catalyst under reaction conditions for 30 h.

Results

The physicochemical properties of the catalysts are summarized in Table 1. The chemical composition results are close to the nominal value. The N2 adsorption/desorption isotherms are presented in Fig. S1 (see in supplementary information, SI). According to IUPAC classification, all the catalysts showed isotherms similar to type II with hysteresis H3-type typical of macro-porous materials with non-uniform size and/or shape [36]. Textural data from N2 adsorption/desorption isotherms (Table 1) show that the LC catalyst exhibited low value of BET surface area (< 10 m2 g−1) that are ascribed to the high calcination temperature (700 °C) employed in the synthesis that causes sintering. Interestingly, it was observed that the partial substitution of Co by V into perovskite structure resulted in an increase of the BET surface area (from 3 to 15 m2 g−1), as shown in Table 1. This result could be indicative of changes in the crystallite growth of perovskite induced by the insertion of V in its lattice, as will be discussed further in the XRD results.

Table 1 Physicochemical properties of the catalysts

Information about crystalline structure and phase formation were obtained by XRD measurements. The XRD patterns are shown in Fig. 1. All patterns presented diffraction peaks that agree well with those expected for single crystalline phase of the LaCoO3 perovskite with rhombohedral unit cell and R-3c space group (JCPDS48-0123). No other diffraction peaks due to precursors or the formation of segregated phases corresponding to La, Co or V oxides were detected. Nevertheless, the presence of La/Co phases at a very low concentration and/or an amorphous state cannot be discarded because of the detection limit of the XRD technique. The XRD pattern of the LVC5 and LCV10 catalysts also shows that the perovskite structure was still well maintained after the vanadium insertion. This fact is indicative of a high degree of substitution of Co by V into perovskite structure, suggesting that the synthesis method was appropriated.

Fig. 1
figure1

XRD patterns of the LC, LCV5 and LCV10 catalysts

Rietveld refinement analysis of diffraction patterns was performed to evaluate if vanadium was successively inserted into lattice of perovskite structure. The results are presented in Fig. 2 and show a very good correspondence between experimental data and the simulated curve, confirmed by observing the difference patterns. Some structural properties inferred by Rietveld refinement are summarized in Table 2.

Fig. 2
figure2

Rietveld refinement of XRD data for LC, LCV5 and LCV10 catalysts

Table 2 Crystallite structure properties of the catalysts obtained by Rietveld refinement

Table 2 shows a very small change in the size unit cell of the lattice of LC catalyst after incorporation of vanadium (LCV5 and LCV10 catalysts), suggesting that the V species were inserted into the perovskite structure. These results can be attributed to the almost identical ionic radii of Vn+ (rv3+  = 0.65 Å; rv4+  = 0.61 Å and rv5+  = 0.59 Å) and Co3+ (rCo3+  = 0.61 Å). Quantitative estimation of the crystallite size of the catalysts was also evaluated by applying the Scherrer equation (Table 2). From these results, it was observed that the mean crystallite size was affected by the degree of Co substitution in the perovskite structure. The crystallite size decreased with the increasing degree of incorporation of the V species into the perovskite structure. These observations are in good agreement with the textural analysis, as previously presented in Table 1.

The H2-TPR profiles are displayed in Fig. 3. The LC catalyst exhibited a reduction profile formed by two consecutive regions: at temperatures around 300–600 °C, corresponding to the reduction from Co3+ to Co2+ species and at 620–800 °C, attributed to the reduction of Co2+ species to metallic cobalt (Co0), typical for conventional LaCoO3 catalyst, as reported by several studies in literature [1, 35, 37]. In addition, the position of the afore-mentioned peaks depends on the kinetics of the reduction, which is mainly influenced by crystallite size and oxygen defects in the perovskite lattice [38]. On the other hand, the reduction profiles of LCV5 and LCV10 catalysts display the presence of a small reduction peak at 930 and 955 °C, respectively, besides the reduction peaks relative to cobalt species. This additional H2 consumption is attributed to the vanadium reduction inserted in the perovskite structure. Based on typical reduction profiles for bulk V2O5 in literature [39,40,41], these peaks were assigned to reduction of V4+ to V3+ species. It must be pointed out that the reduction of V5+ species (from V5+ to V4+) probably occurring simultaneously with Co species, since the reduction of this species of vanadium occurs in temperature range 400–700 °C [42,43,44].

Fig. 3
figure3

H2-TPR profiles for LC, LCV5 and LCV10 catalysts

In order to complement the structural analysis, the catalysts were examined by microscopy and the representative SEM micrographs are displayed in Fig. 4. The LC catalyst exhibited a typical morphology with spherical particles and uniform grain size distribution, which have an agglomeration tendency due to the high calcination temperature employed during synthesis. These observations are consistent with those reported in literature for LaCoO3 catalyst obtained by citrate method [1, 35, 37]. Conversely, the LCV5 and LCV10 catalysts showed substantial changes in the microstructure and porosity, showing irregular shapes and a morphology of spongy nature. These changes are probably due to the partial substitution of Co by V species into the perovskite lattice. The presence of large pores in the LCV5 and LCV10 catalysts is clearly observed in the images. These cavities result from the rapid evolution of gases during the calcination process. These findings are in accordance with the N2 adsorption/desorption results, which revealed significant changes in the BET surface area (Table 1).

Fig. 4
figure4

Representative SEM images of the catalysts

The surface species of all catalysts were investigated by XPS measurements and the XP spectra show the Co 2p, V 2p, La 3d species and O 1s in different regions. No extra peak related with any impurities was detected. The XP spectra of LC (not show) are presented in Fig. S2 (SI) displaying typical XPS peaks for the LaCoO3 phase. In the case of LCV5 catalyst (Fig. 5), the XP spectrum of cobalt exhibited two main peaks at 779.4 and 794.6 eV associated to the doublet Co 2p3/2 and Co 2p1/2 regions, respectively. In addition, the energy difference (ΔE) of the 2p doublet is 15.2 eV (Fig. 5). Both observations confirm the presence of Co3+ species on the surface [45]. On the other hand, two other peaks less intense can be observed at 781.4 and 796.9 eV and are attributed to the low Co2+ species content [17]. The spectrum of vanadium was fitted to three components, with a main peak at 516.9 eV, corresponding to the V4+ species, followed by two less intense peaks at 515.9 and 518.4 eV, characteristics of V3+ and V5+ species, respectively [34, 46,47,48]. This result shows that the LCV5 surface is enriched by V4+ species. These findings indicate a similar oxidation state of the vanadium at the surface and in the bulk, as evidenced by H2-TPR and XRD results. The complex spectrum of La 3d was decomposed into eight contributions. The peaks located at 832.9, 834.8, 836.9 and 838.6 eV are ascribed to the La 3d5/2 line, whereas the peaks at 849.5, 851.4, 853.5 and 855.3 eV correspond to the La 3d3/2 line [45, 49]. It suggests that the LCV5 surface present La3+ at the surface.

Fig. 5
figure5

XP spectra of the LCV5 catalyst

The O 1s spectrum can be decomposed into seven components, indicating that oxygen is present in different chemical environments. It is well accepted in literature [26, 50,51,52,53,54,55] that the peak at the binding energy of 527.5–530 eV corresponds to the lattice oxygen species (e.g., O2−, designated as Olatt) which reflect the redox behavior of the metal; and the peak at 530–531.5 eV can be attributed to the chemisorbed oxygen species (e.g., O2, designated as Oads), whose content reflects the concentration of oxygen vacancy in the compound. Whereas the peak at 531.5–534 eV is generally assigned to the physically adsorbed oxygen-containing groups (e.g., hydroxyl and/or carbonate species well as adsorbed molecular water, designated as Oocg). Also, the presence of small quantities of extra VOx phases located on surface (undetected by our XRD measurements) could contribute to peaks previously attributed to oxygen lattice (Olatt) [56]. The atomic percentage of the surface species obtained from XPS data are summarized in Table 3. The XP spectra of LCV10 are illustrated in Fig. 6. No significant differences of these spectra were observed when compared to the XPS spectra of the LCV5 (Fig. 5).

Table 3 Relative percentages of the different species on the surface for all catalysts
Fig. 6
figure6

XP spectra of the LCV10 catalyst

Catalytic Activity

The catalytic activity in the CO-PROX in H2-rich feed was evaluated in the temperature range of 50–300 °C. The conversion of CO as a function of the reaction temperature is shown in Fig. 7a. For comparison, the catalytic activity of LC catalyst was included. As seen, the CO conversion increase by raising the temperature, attaining maxima located at 250 °C for all the catalysts. A further increase in reaction temperature to 300 °C resulted in a decrease of the CO conversion for all the catalysts, accompanied by a gradual fall in CO2 selectivity (see Fig. 7c), due to the competitive oxidation of H2 to H2O that becomes favorable at high temperatures [14]. Figure 7b shows that the O2 conversion for the catalysts following the same tendency, reaching a maxima conversion at 300 °C for all the catalysts. Maxima oxygen consumption at higher temperatures was accompanied by significantly drop in CO conversion. As the CO/O2 ratio employed in the reactant mixture is 1/1 (representing an O2 concentration higher than that stoichiometric required, i.e. 1/0.5), it indicates the occurrence of side reaction of H2 oxidation. In addition, the formation of H2O (verified during our catalytic test) was more significant when O2 reached maximum consumption, evidencing the increasing H2 oxidation. Moreover, an appreciable formation of CH4 also was detected at high temperatures (above 200 °C).

Fig. 7
figure7

CO conversion (a), O2 conversion (b) and CO2 selectivity (c) as a function of the reaction temperature

Noteworthy is that at 200 °C the CO conversion for LCV10 catalyst was of 55%, reaching the maximum conversion at 250 °C (94%). In turn, the CO conversions over LC catalyst were of 25% at 200 °C and about 80% at 250 °C. This result indicates that the modified with vanadium catalyst was improved, since two times more active at 200 °C. However, above 200 °C, the CO conversion decreased favoring the hydrogen oxidation, and decreasing the CO2 selectivity.

The CO2 selectivity profiles are presented in Fig. 7c. The LCV10 catalyst exhibited maximum selectivity (100%) until 100 °C, whereas the LC and LCV5 catalysts up to 150 °C. Above these temperatures, the CO2 selectivity decreased almost equally for all the catalysts, being this behavior more significant for the LCV10 catalyst. This observation can be attributed to occurrence of the side reactions of H2 oxidation, the RWGS and methanation reactions [1, 35, 57,58,59].

The catalytic activity is usually associated with the temperature corresponding to 50% of CO conversion to CO2 (T50%CO). Hence, T50%CO values were obtained from the light-off curves (Fig. 7a) and are summarized in Table 4. Interestingly, the vanadium insertion into LaCoO3 structure (LCV10 catalyst) resulted in an additional improvement of T50%CO value from 220 to 194 °C. A similar result has been reported in our previous studies [59], where we evaluated the effect of partial substitution of La by Sr into LaCoO3 perovskite on the surface properties and activity/selectivity for CO-PROX reaction. The T50%CO for La0.75Sr0.25CoO catalyst was at 145 °C, whereas the same activity was achieved only at 170 °C for LaCoO3 catalyst. This behavior was attributed to the increase in the amount of surface oxygen vacancies caused by Sr insertion.

Table 4 Light-off temperatures for CO-PROX reaction in H2-rich condition

The stability of the catalysts for long-term operation under the reducing conditions of the CO-PROX reaction is important to determine the suitability of the catalyst for a possible industrial application. To investigate the stability in the CO-PROX reaction, additional experiments were carried out at 250 °C during 20 h on stream, as displayed in Fig. 8. As can be seen, the LC catalyst showed a relatively high stability in terms of O2 conversion with a loss in initial value from 71 to 67% after 14 h on stream. Concerning to the CO conversion, the LC catalyst exhibited initially 83% and after 3 h on stream this value increased to about 90%. Whereas CO2 selectivity this catalyst exhibited an increased within the first 4 h on stream (from 66 to 73%), without showing any significant variations in the subsequent time-on-stream. Over LCV5 catalyst, the initial conversion of CO and O2 was about 78 and 74%, respectively. Interestingly, a slightly activation was observed after 3 h on stream before the activity became stable in subsequent hours. The initial CO2 selectivity (about 60%) over this catalyst was practically stable throughout time-on-stream experiment. In turn, the LCV10 catalyst showed a small increase in the initial CO conversion and CO2 selectivity from 93 to 100% and from 59 to 67%, respectively, during the first 4 h on stream. Then, CO conversion remained almost unchanged during the 20 h on stream test, whereas CO2 selectivity exhibited a decline returning to its initial value after 12 h on stream. Conversely, the O2 conversion, initially 83%, presented a fall after the first 4 h over this catalyst, returning to its initial value only after 12 h. In summary, all catalysts presented good stability (in different levels of conversion and selectivity) during 20 h on stream. This was due to the absence of sintering and coke deposits, as discussed further.

Fig. 8
figure8

Stability test with time on stream under isothermal conditions at 250 °C

The spent catalysts (after 20 h on stream at 250 °C) were analyzed by XRD measurements in order to identify possible changes in the morphology and structure after reaction and the XRD patterns are displayed in Fig. S3 (SI). We verified that the perovskite structure was preserved after the course of CO-PROX reaction for all the catalysts. Mean crystallite size was estimated by applying the Scherrer equation to the XRD patterns of the spent catalysts (Fig. S3) and results are summarized in Table S1. Similar values are observed with respect to those determined for the fresh catalysts.

Discussion

The partial substitution of Co by V into LaCoO3 perovskite led to structural and textural modifications in the perovskite. The structural characterization results of the modified perovskite obtained by XRD analysis (Fig. 1) showed the presence of pure crystalline perovskite phase, suggesting that all vanadium has been successfully incorporated into LaCoO3 lattice. The insertion of V into the perovskite structure is also supported by Rietveld analysis (Table 2) that revealed a small changes in lattice parameter from 5.43 Å (in pure perovskite) to 5.44 Å after insertion of vanadium in both catalysts (LCV5 and LCV10). As expected, the incorporation of V species was accompanied by maintaining of the perovskite structure, since the multiple valence states of V species have the ionic radii similar to the Co3+ species, as previously discussed. Note also that the crystalline domains presents in the XRD patterns of perovskite decreased with the increase in the partial substitution of cobalt by vanadium.

The textural properties were also affected by substitution degree of V into the perovskite structure. The increase in BET surface area observed for the V substituted catalyst can be directly linked to the decrease in the crystallite size of the perovskite, as shown by XRD analysis (Table 2) and the increase in the porosity (Table 1) of the particles after insertion of vanadium into perovskite structure. This last one is also supported by SEM results (Fig. 4). The insertion of vanadium into the perovskite structure had also influence on their reduction behavior. The TPR profile of LCV5 and LCV10 catalysts indicated clearly the presence of additional reduction peaks at higher temperatures (higher than 900 °C) in comparison to the LC catalyst (without vanadium). Based on typical reduction profile for bulk V2O5 in literature [39,40,41], this H2 consumption was attributed to the V species in a + 4 valence state. This is consistent with the observation reported by Crapanzano et al. [32] who studied the effect of V addition into the LaNiO3 lattice. In addition, TPR result also indicates that all perovskite catalysts are stable (in oxide form) within the temperature range (50–300 °C) and atmosphere reducing used in our catalytic tests.

XPS measurement reveal the higher concentration of V4+ species at the surface and the existence of a lower content of V with valence states + 5 and + 3 (Table 3) for both catalysts (LCV5 and LCV10). These species must exist in the form of additional oxides phases (VOx), which are too small to be detected by XRD. Note that this hypothesis is consistent with the multiple peaks observed in the complex XP spectrum of O 1s (Figs. 5 and 6). These fitted peaks (indicated by a dotted line in the Figs. 5 and 6) are quite difficult to analyze because of the overlapping contribution of oxygen from various component oxides, including vanadium with different valence states.

Results presented in Table 4 indicated an improved in the T50%CO value after insertion of vanadium into LaCoO3 structure. This result is directly related to the increased of oxygen vacancies and presence of vanadium surface species. In fact, the insertion of vanadium affected the surface vacancies due to its tetravalent state, as evidenced in the XPS in Figs. 5 and 6. To maintain the charge balance of the perovskite oxides, vanadium should promote the excessive oxide anion on the surface and, thereby, oxygen is easily supplied from the sub-surface for CO oxidation, in agreement with the literature [29, 60,61,62]. Thus, the changes in the oxygen species surface concentration after adding vanadium could be responsible, at least part, for improving the catalytic activity by favoring the adsorption of reactants from the gas phase [63].

The existence of Co2+ was reported to facilitate the interplay between Co2+/Co3+ and V5+/V4+ redox couples, which can enhance the lattice oxygen mobility and create interfacial active sites of CoOx/V2O5. A possible explanation is that the Co2+ in Co-based catalysts can react with the high V5+ to produce Co3+ species and V4+ species, which is enriched at the surface.

$${\text{Co}}^{2 + } + {\text{V}}^{5 + } \to {\text{Co}}^{3 + } + {\text{V}}^{4 + }$$

In fact, the calculated concentration in Table 3 for the chemically adsorbed oxygen (e.g., O2) and lattice oxygen species (e.g., O2−), respectively, indicate increasing concentration of the oxygen in vacancies from 71.2% for the LC to 75.4% for the LCV10. The concentration of Co2+ signal was observed the Co 2p analysis. According to Zhou et al. [64], Co2+ in tetrahedral sites is considered to be inactive for CO oxidation. Therefore, the lower Co2+ concentration on the surface of LCV10 catalyst favors the CO oxidation conversion at lower temperature.

Stabilization Test

The deactivation of catalysts during the CO-PROX process is mainly caused by carbon deposition and particle sintering. As seen in Fig. 8, the activity and selectivity remained high and stable throughout the 20 h on stream at 250 °C. In order to investigate the structural properties after stability tests, the spent catalysts were characterized by XRD technique and the diffractograms were presented in Fig. S3. All the spent catalysts exhibited intense diffraction peaks which are characteristic to the single crystalline phase of the perovskite structure. As expected, no carbon was detected (about 2θ = 26.2°) that reveals that no deactivation by carbon formation or graphite. This result confirms the high resistance to carbon deposition and can be attributed to increase of the electronic and oxygen mobility properties induced upon introduction of the V into the perovskite lattice. Previous studies have shown that addition of second metal to perovskite oxides has been considered as a way to improves stability and resistance to coke deposition [37, 65,66,67,68], which is in good agreement with our assumptions. Also, the spent catalysts exhibited high and well defined peak intensity which was also assigned to the maintenance of perovskite structure after reaction. Noteworthy is also that the used sample didn’t show isolated vanadium or cobalt oxides after reaction that confirms stable perovskite structure, preserving its catalytic activity. The sharp intensities evidence well recrystallization and not growth of crystallites or sintering (see Table S1). This result is in agreement with H2-TPR experiments that confirms its stable structure under the reducing condition of the CO-PROX reaction. These results also suggest that the vanadium perovskite structure is competitive with the perovskite structure promoted with Sr (La0.75Sr0.25CoO) reported in our previous work [59], principally in relation to coke deposition. This thought is important to confirm that perovskite B sites promote vacancies due to the distortion of the crystal structure, which indeed, promote the oxygen in sub-surfaces or bulk that enhances the stability of the structure without affecting the surface sites of the Co oxides as active surface sites.

Conclusions

Perovskite modified by addition of vanadium with single phase have been successfully synthesized by acid citric method. The incorporation of vanadium into LaCoO3 perovskite lattice modified its physicochemical characteristics, as shown by characterization results. It was found that the modification in LaCoO3 due to the partial substitution of Co by V directly affect the structure and morphology leading to decrease in the crystallite size and increase in the BET surface area. The positive effect observed in CO conversion (T50%CO) for LCV5 and LCV10 catalysts were induced by changes in the oxygen species surface concentration after the partial substitution of Co by V in the perovskite lattice. The existence of Co2+ was reported to facilitate the interplay between Co2+/Co3+ and V5+/V4+ redox couples, which can enhance the lattice oxygen mobility and create interfacial active sites of CoOx/V2O5. In addition, all the catalysts exhibited high resistance to coke deposition and particles sintering after 20 h on stream at 250 °C.

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Acknowledgements

The authors gratefully thank CNPq (Conselho Nacional de Desenvolvimento Científico) and FAPERJ (Fundação de Amparo à Pesquisa do Estado do Rio de Janeiro) for financial support this work. The authors would like to thank LAQUIS (Laboratório de Química de Superfícies) for the XPS analysis.

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Chagas, C.A., Magalhães, R.N.S.H. & Schmal, M. The LaCo1−xVxO3 Catalyst for CO Oxidation in Rich H2 Stream. Catal Lett 151, 409–421 (2021). https://doi.org/10.1007/s10562-020-03303-y

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Keywords

  • LaCoO3 perovskite
  • Vanadium substitution
  • CO oxidation
  • Hydrogen purification