Elucidation of water promoter effect of proton conductor in wgs reaction over pt-based catalyst: An operando drifts study

catalysts Article Elucidation of Water Promoter Effect of Proton Conductor in WGS Reaction over Pt-Based Catalyst: An Operando DRIFTS Study Lole Jurado 1,2,*, Nuria García-Moncada 1,3 , Luis F. Bobadilla 1, Francisca Romero-Sarria 1and JoséA. Odriozola 1,* 1 Instituto de Ciencia de Materiales de Sevilla and Departamento de Qu í mica Inorg á nica, Centro Mixto CSIC Universidad de Sevilla, Av. Américo Vespucio 49, 41092 Sevilla, Spain; [email protected] (N.G.-M.); [email protected] (L.F.B.); [email protected] (F.R.-S.) 2 Institut de chimie et proc é d é s pour l’ é nergie, l’environnement et la sant é (ICPEES), d é partement de Catalyse et Matériaux, Universitéde Strasbourg, CNRS, 25 rue Becquerel, CEDEX 02, 67087 Strasbourg, France 3 Laboratoire Catalyse et Spectrochimie, ENSICAEN, Universit é de Caen, CNRS, 6 Boulevard Mar é chal Juin, 14050 Caen, France *Correspondence: [email protected] (L.J.); [email protected] (J.A.O.) Received: 29 June 2020; Accepted: 23 July 2020; Published: 25 July 2020   Abstract: A conventional Pt/CeO 2 /Al 2 O 3 catalyst physically mixed with an ionic conductor (Mo- or Eu-doped ZrO 2 ) was tested at high space velocity (20,000 h −1 and 80 L h −1 g cat−1 ) under model conditions (only with CO and H 2 O) and industrial conditions, with a realistic feed. The promoted system with the ionic conductor physically mixed showed better catalytic activity associated with better water dissociation and mobility, considered as a rate-determining step. The water activation was assessed by operando diffuse reflectance infrared fourier transformed spectroscopy (DRIFTS) studies under reaction conditions and the Mo-containing ionic conductor exhibited the presence of both dissociated (3724 cm −1 ) and physisorbed (5239 cm −1 ) water on the Eu-doped ZrO 2 solid solution, which supports the appearance of proton conductivity by Grotthuss mechanism. Moreover, the band at 3633 cm −1 ascribed to hydrated Mo oxide, which increases with the temperature, explains the increase of catalytic activity when the physical mixture was used in a water gas shift (WGS) reaction. Keywords: WGS reaction; protonconductor; wateractivation; Grotthuss’mechanism; operando DRIFTS 1. Introduction The applicability of a water gas shift (WGS) reaction (Equation (1)) in fuel cells and mobile devices as important step to remove CO pollutants and produce clean H 2 is still challenging This arises because the reaction is thermodynamically favored at low temperatures and implies low space velocities and kinetics which require large volume reactors [1,2]. CO +H2OCO2+H2∆H0298K =−41.1 kJ mol−1(1) During the last decades, numerous efforts have been made to design better catalysts which work at high space velocity, keeping high activity and reducing the reactor volume significantly. In this sense, noble metals such as Pt, Pd, Ru or Au have shown optimal performances at low-intermediate temperatures, good behavior in start-up/shut-down cycles and good tolerance to impurities [ 3 – 7 ]. Despite no agreement about the WGS reaction mechanism [ 8 – 11 ], also influenced by the experimental conditions and the used active metal as catalyst, it seems clear the water activation and diffusion to the active sites is a rate-determining step [ 12 , 13 ]. Particularly, noble metals supported on reducible Catalysts 2020,10, 841; doi:10.3390/catal10080841 www.mdpi.com/journal/catalysts Catalysts 2020,10, 841 2 of 18 oxides like CeO 2 have shown better performances than those on unreducible oxides like Al 2 O 3 or SiO 2 [ 14 – 17 ], since the redox cycles of the reducible supports develop the water activation. For instance, Vignatti et al. [ 18 ] observed that the catalytic activity in WGS reaction increased in the order Pt/SiO2<Pt/CeO2<Pt/TiO2 . Likewise, Panagiotopoulou et al. [ 16 ] studied different noble metal-support combinations and identified Pt-based catalysts were more active, exhibiting higher activity when supported on reducible oxides. Accordingly, in order to improve the kinetics at low temperatures and work at high space velocity, it is mandatory to promote the water activation step. In this context, a physical mixture between a typical noble-based WGS catalyst and a ZrO 2 -based ionic conductor to promote water activation and mobility was studied in our group, obtaining in all caseshigher activitywith thepromotedsystem inrespecttothebarecatalystwithout theionic conductor, whatever the catalyst or the ionic conductor [ 19 ]. Particularly, we demonstrated that Eu-doped ZrO 2 mixed oxide as an ionic conductor exhibited proton conductivity via Grotthuss mechanism [ 20 – 22 ] in the WGS reaction temperature range [ 23 ]. The Eu doping creates oxygen vacancies in the formed solid solution (Eu y Zr 1−y O 2−0.5y ) which are able to activate the water molecules, confirmed by in situ diffuse reflectance infrared fourier transformed spectroscopy (DRIFTS) measurements under WGS reaction conditions. The higher activity in WGS reaction was observed in the same temperature range that this proton conductivity occurred. The optimal Eu content according to better mobility of the oxygen vacancies and higher conductivity measured by impedance spectroscopy (95:5 mol.% of ZrO 2 :Eu 2 O 3 ) also agreed with the best catalyst-ionic conductor mixture’s performance in the catalytic tests in the WGS reaction. Mo-containing compounds have been reported as excellent ionic conducting materials. In general, Mo-doped oxides present mixed ionic-electronic conductivity (MIEC), where the temperature and the Mo content shift the conducting character [ 24 – 27 ]. Thus, high Mo content decreases the crystal symmetry performing electronic conductivity mainly, but decreasing the Mo loading the proton conductivity is favored [ 28 , 29 ]. In addition, Eu-doped Mo oxide is reported to form bronze-type structures capable to retain H 2 or water molecules [ 30 – 32 ]. In bronze formation, H 2 is dissociated and bonded as –OH or –OH 2 groups, depending on the H 2 concentration [ 30 , 33 , 34 ]. This hydrogen addition disturbs the crystal structure affecting the hydrogen mobility, where protons can diffuse through the oxide lattice and where –OH or –OH 2 groups are the bridges on the surface inducing the proton conductivity [34–37]. In the present work, a typical Pt/CeO 2 /Al 2 O 3 catalyst was physically mixed with a Zr-based ionic conductor and tested in a WGS reaction under realistic conditions at high space velocity. The formed species during the reaction were analyzed by operando DRIFTS measurements mainly focused on studying the water activation step. The used ionic conductor is based on the optimal Eu-doped ZrO 2 solid solution (95:5 mol.% of ZrO 2 :Eu 2 O 3 ) already studied in our group, which has been prepared by doping with a small amount of Mo in order to increase the water activation effect. 2. Results and Discussion 2.1. Physicochemical and Textural Properties Figure 1shows the diffractograms of the prepared samples. As can be observed in Figure 1a, both γ –Al 2 O 3 (JCPDS: 01-077-0396) and CeO 2 (JCPDS: 00-034-0394) phases were identified on the Pt/CeAl catalyst. However, Pt phases were hardly detected, likely due to Pt particles being very small and highly dispersed. On the other hand, Figure 1b includes the diffractogram of the ZrEuMo_5 sample and the X-ray diffraction (XRD) pattern of a cubic Fm 3 m structure associated to Eu y Zr 1−y O 2−0.5y solid solution (JCPDS: 01-078-1303) for comparison. The ZrEuMo_5 sample contains a loading of 5 mol.% of Eu 2 O 3 (y  0.1 in the solid solution), and it is noticeable that the diffractogram (Figure 1b) exhibits all peaks associated to Eu-doped ZrO 2 solid solution phase whereas any phase of molybdenum was evidenced. Moreover, molybdenum presence does not modify the diffraction pattern of the solid solution, which indicates Mo is not entering in the lattice or disturbing the Eu-doped ZrO 2 solid Catalysts 2020,10, 841 3 of 18 solution structure. Further analysis on Mo addition to the Eu-doped ZrO 2 solid solution using different amounts of Mo by Garc í a-Moncada et al. showed that molybdenum is not incorporated in the mixed oxide but forms highly dispersed small clusters of MoO x which preferentially are located on Eu sites [38]. Catalysts 2020, 10, x FOR PEER REVIEW 3 of 18 pattern of the solid solution, which indicates Mo is not entering in the lattice or disturbing the Eudoped ZrO 2 solid solution structure. Further analysis on Mo addition to the Eu-doped ZrO 2 solid solution using different amounts of Mo by García-Moncada et al. showed that molybdenum is not incorporated in the mixed oxide but forms highly dispersed small clusters of MoO x which preferentially are located on Eu sites [38]. Figure 1. Diffractograms of (a) Pt/CeAl catalyst and (b) ZrEuMo_5 sample. Eu-doped ZrO 2 solid solution pattern (Eu y Zr 1−y O 2−0.5y ) is included for comparison. Figure 2 evidences the adsorption-desorption N 2 isotherm of the synthesized materials. The N 2 isotherm related to Pt/CeAl catalysts (Figure 2a) can be described as type IV according to Brunauer- Deming-Deming-Teller (BDDT) classification [39] or, as pseudo-type II with a type H 3 loop according to International Union of Pure and Applied Chemistry (IUPAC) classification, characteristic of slitshaped type pores or materials with a low degree of pore curvature [40,41]. This pseudo-type II character is related to the metastability of the absorbed multilayer and delayed capillary condensation. Concerning the ionic conductor, ZrEuMo_5, the resulting isotherm (Figure 2b) can be classified as type IV characteristic of mesoporous solids (2–50 nm) [40]. Additionally, the high convexity of the curve for relative pressures (P/P 0 ) below 0.5 might suggest the existence of pores with a radius close to that of micropores. Likewise, the textural properties of the catalyst and the ionic conductor have been summarized in Table 1. The catalyst possesses a high Brunauer-Emmett-Teller (BET) specific surface area attributed to the γ–Al 2 O 3 phase, and Pt loading keeps the same order of magnitude of the pores’ volume and size with respect to the bare CeO 2 /Al 2 O 3 [42], suggesting that metal is highly dispersed on the porous support. Regarding to the ZrEuMo_5 sample (Table 1), the BET surface area is increased by Mo addition onto the bare Eu-doped ZrO 2 (42 m 2 /g, reported by García-Moncada et al. [23]), and monomodal pore size distribution centered in 2.4 nm. The chemical composition of the ionic conductor as well as the Pt loading on the catalyst determined by X-ray fluorescence spectrometry (XRF) have been also shown in Table 1. Thus, the desired amount of Pt (2 wt.%) was achieved by the wet impregnation method. Furthermore, the composition of the commercial Puralox CeO 2 /Al 2 O 3 reported in its technical data [42] (mass ratio of 20:80 in CeO 2 :Al 2 O 3 ) was confirmed. The XRF analysis of the ZrEuMo_5 sample shows that the desired composition is achieved during co-precipitation synthesis. Table 1. Textural properties and chemical composition of the synthesized samples. Sample S BET (m 2 g −1 ) V p (cm 3 g −1 ) D p (nm) Chemical Composition a (wt.%) Pt/CeAl 110 0.26 8.84 1.8% Pt (2%) 18.8% CeO 2 (19.6%) 79.4% Al 2 O 3 (78.4%) ZrEuMo_5 74 0.06 2.4 58.4% Zr (60.9%) 11.9% Eu (10.7%) Figure 1. Diffractograms of ( a ) Pt/CeAl catalyst and ( b ) ZrEuMo_5 sample. Eu-doped ZrO 2 solid solution pattern (EuyZr1−yO2−0.5y) is included for comparison. Figure 2evidences the adsorption-desorption N 2 isotherm of the synthesized materials. The N 2 isotherm related to Pt/CeAl catalysts (Figure 2a) can be described as type IV according to Brunauer-Deming-Deming-Teller (BDDT) classification [ 39 ] or, as pseudo-type II with a type H 3 loop according to International Union of Pure and Applied Chemistry (IUPAC) classification, characteristic of slit-shaped type pores or materials with a low degree of pore curvature [ 40 , 41 ]. This pseudo-type II character is related to the metastability of the absorbed multilayer and delayed capillary condensation. Concerning the ionic conductor, ZrEuMo_5, the resulting isotherm (Figure 2b) can be classified as type IV characteristic of mesoporous solids (2–50 nm) [ 40 ]. Additionally, the high convexity of the curve for relative pressures (P/P 0 ) below 0.5 might suggest the existence of pores with a radius close to that of micropores. Likewise, the textural properties of the catalyst and the ionic conductor have been summarized in Table 1. The catalyst possesses a high Brunauer-Emmett-Teller (BET) specific surface area attributed to the γ –Al 2 O 3 phase, and Pt loading keeps the same order of magnitude of the pores’ volume and size with respect to the bare CeO 2 /Al 2 O 3 [ 42 ], suggesting that metal is highly dispersed on the porous support. Regarding to the ZrEuMo_5 sample (Table 1), the BET surface area is increased by Mo addition onto the bare Eu-doped ZrO 2 (42 m 2 /g, reported by Garc í a-Moncada et al. [ 23 ]), and monomodal pore size distribution centered in 2.4 nm. The chemical composition of the ionic conductor as well as the Pt loading on the catalyst determined by X-ray fluorescence spectrometry (XRF) have been also shown in Table 1. Thus, the desired amount of Pt (2 wt.%) was achieved by the wet impregnation method. Furthermore, the composition of the commercial Puralox CeO 2 /Al 2 O 3 reported in its technical data [ 42 ] (mass ratio of 20:80 in CeO 2 :Al 2 O 3 ) was confirmed. The XRF analysis of the ZrEuMo_5 sample shows that the desired composition is achieved during co-precipitation synthesis. Table 1. Textural properties and chemical composition of the synthesized samples. Sample SBET (m2g−1) Vp(cm3g−1)Dp(nm) Chemical Composition a(wt.%) Pt/CeAl 110 0.26 8.84 1.8% Pt (2%) 18.8% CeO2(19.6%) 79.4% Al2O3(78.4%) ZrEuMo_5 74 0.06 2.4 58.4% Zr (60.9%) 11.9% Eu (10.7%) 4.5% Mo (3.6%) 25.2% O (26.8%) aNominal value between parentheses. Catalysts 2020,10, 841 4 of 18 Catalysts 2020, 10, x FOR PEER REVIEW 4 of 18 4.5% Mo (3.6%) 25.2% O (26.8%) a Nominal value between parentheses. Figure 2. Adsorption-desorption N 2 isotherm of (a) Pt/CeAl catalyst and (b) ZrEuMo_5 sample. Figure 3 shows the transmission electron microscopy (TEM) images of the studied materials. The inspection of the TEM image of Pt/CeAl catalyst (Figure 3a) does not evidence Pt particles, which could suggest a high dispersion of the Pt phase. However, it has to be highlighted that Pt cannot be distinguished due to the low contrast between Ce and Pt signals in TEM. As can be noted in Figure 3a, two areas (dotted yellow boxes) allow to measure the interplanar spacing of 3.1 Å associated to the (111) planes of the cubic Fm3 m phase of the CeO 2 . The cubic phase and the polycrystalline character of the Pt/CeAl catalyst can be also distinguished by the electron diffraction pattern (Figure 3a, inset). The ZrEuMo_5 mixed oxide (Figure 3b) evidences monocrystalline and polycrystalline zones as well as cubic structure. In addition, the pore size distribution of the prepared ionic conductor determined by the Barrett-Joyner-Halenda (BJH) method is confirmed by TEM, where a high porosity of the sample with a homogeneous pore diameter between 2–3 nm is clearly seen in Figure 3b. Figure 2. Adsorption-desorption N2isotherm of (a) Pt/CeAl catalyst and (b) ZrEuMo_5 sample. Figure 3shows the transmission electron microscopy (TEM) images of the studied materials. The inspection of the TEM image of Pt/CeAl catalyst (Figure 3a) does not evidence Pt particles, which could suggest a high dispersion of the Pt phase. However, it has to be highlighted that Pt cannot be distinguished due to the low contrast between Ce and Pt signals in TEM. As can be noted in Figure 3a, two areas (dotted yellow boxes) allow to measure the interplanar spacing of 3.1 Å associated to the (111) planes of the cubic Fm 3 m phase of the CeO 2 . The cubic phase and the polycrystalline character of the Pt/CeAl catalyst can be also distinguished by the electron diffraction pattern (Figure 3a, inset). The ZrEuMo_5 mixed oxide (Figure 3b) evidences monocrystalline and polycrystalline zones as well as cubic structure. In addition, the pore size distribution of the prepared ionic conductor determined by the Barrett-Joyner-Halenda (BJH) method is confirmed by TEM, where a high porosity of the sample with a homogeneous pore diameter between 2–3 nm is clearly seen in Figure 3b. Catalysts 2020, 10, x FOR PEER REVIEW 5 of 18 Figure 3. TEM micrograph and electron diffraction of (a) Pt/CeAl catalyst and (b) ZrEuMo_5 sample. Pore size distribution was added for ZrEuMo_5 sample. 2.2. Catalytic Performance Two catalytic systems were tested: (i) Pt/CeAl diluted with inert quartz and (ii) a physical mixture of Pt/CeAl and ZrEuMo_5 (mass ratio of 1:5 Pt/CeAl:ZrEuMo_5) to verify the promoting effect of the ionic conductor on the catalyst. The catalytic activity of these systems was assessed using two different feeds: model conditions (4.5% CO, 30% of H 2 O and N 2 as balance) and industrial conditions (9% CO, 30% H 2 O, 11% CO 2 , and 50% H 2 ). Figure 4 shows the catalytic activity in terms of CO conversion versus temperature for both catalytic systems under model (Figure 4a) and industrial (Figure 4b) conditions. It is noteworthy that the promoted system formed by the physical mixture of Pt/CeAl catalyst and ZrEuMo_5 ionic conductor provides higher activity in terms of CO conversion in both feeding conditions. For comparison, the ionic conductor (ZrEuMo_5) alone was also tested in both feed conditions confirming that this material is catalytically inactive for WGS reaction (data not shown). Nevertheless, it is worth mentioning that CO 2 concentration slightly decreased at low temperatures under industrial conditions without changes in the other components of the feed and returning to its initial value above 300 °C, suggesting that a small capture of CO 2 molecules occurs into the pores of the ionic conductor. Figure 3. TEM micrograph and electron diffraction of ( a ) Pt/CeAl catalyst and ( b ) ZrEuMo_5 sample. Pore size distribution was added for ZrEuMo_5 sample. Catalysts 2020,10, 841 5 of 18 2.2. Catalytic Performance Two catalytic systems were tested: (i) Pt/CeAl diluted with inert quartz and (ii) a physical mixture of Pt/CeAl and ZrEuMo_5 (mass ratio of 1:5 Pt/CeAl:ZrEuMo_5) to verify the promoting effect of the ionic conductor on the catalyst. The catalytic activity of these systems was assessed using two different feeds: model conditions (4.5% CO, 30% of H 2 O and N 2 as balance) and industrial conditions (9% CO, 30% H2O, 11% CO2, and 50% H2). Figure 4shows the catalytic activity in terms of CO conversion versus temperature for both catalytic systems under model (Figure 4a) and industrial (Figure 4b) conditions. It is noteworthy that the promoted system formed by the physical mixture of Pt/CeAl catalyst and ZrEuMo_5 ionic conductor provides higher activity in terms of CO conversion in both feeding conditions. For comparison, the ionic conductor (ZrEuMo_5) alone was also tested in both feed conditions confirming that this material is catalytically inactive for WGS reaction (data not shown). Nevertheless, it is worth mentioning that CO 2 concentration slightly decreased at low temperatures under industrial conditions without changes in the other components of the feed and returning to its initial value above 300 ◦ C, suggesting that a small capture of CO2molecules occurs into the pores of the ionic conductor. Catalysts 2020, 10, x FOR PEER REVIEW 6 of 18 Figure 4. Catalytic activity in terms of CO conversion against temperature for water gas shift (WGS) reaction over Pt/CeAl catalyst and Pt/CeAl + ZrEuMo_5 promoted system at 80 L h −1 g cat−1 and 20,000 h −1 under (a) model conditions feed (4.5% CO, 30% H 2 O, balanced with N 2 ) and (b) industrial conditions feed (9% CO, 11% CO 2 , 30% H 2 O, 50% H 2 ). Apparently, the differences observed in both catalytic systems could be associated to higher thermal diffusion by the presence of a dilutant. However, the Pt catalyst was diluted in both systems, either ionic conductor or quartz, using the same amount of catalyst (0.1g of Pt/CeAl) and sieved in the same particle sizes. Thereby, both systems were tested in the same weight and volumetric space velocities (same catalyst weight and catalytic bed volume). This indicates that an additional feature exists for explaining the superior activity of Pt/CeAl + ZrEuMo_5 promoted catalyst. As can be underlined, the effect by ZrEuMo_5 presence is observable from ~250–270 °C, which is in good agreement with the temperature from which the Eu-doped zirconia ionic conductor starts to show proton conductivity, according to impedance spectroscopy measurements reported by García- Moncada et al. [23]. In the literature, Pt-based catalysts for WGS reaction have been reported to provide high CO coverage, with water activation as one of the slowest steps [43,44]. In this context, ceria support improves the fast oxygen storage/release cycles via a reversible conversion between Ce 4+ and Ce 3+ [45–47]. The reduction process from Ce 4+ to Ce 3+ provides a high mobility of oxygen/hydroxyl groups promoting the hydroxyl groups formation on the surface. Thus, the higher activity obtained by the proposed catalytic system including the ionic conductor, can be explained by higher concentration and mobility of hydroxyl groups and molecular water on the surface. Hence, the ionic conductor does not catalyze the reaction itself, but from temperatures (250–270 °C) where the proton conductivity starts to happen according to Grotthuss mechanism [20,22], ZrEuMo_5 acts as a promoter of Pt catalyst increasing water activation and mobility. Moreover, it should be pointed out that the incremental effect by ZrEuMo_5 observed in industrial conditions was more pronounced than in model conditions. This could be related to the capacity of MoO x to create hydrated bronzes from 300 °C, since the industrial conditions feed includes 50% H 2 , in contrast to model conditions [38], and the presence of these structures could enhance the water activation. 2.3. Operando DRIFTS Study In order to gain insights into the mechanistic aspects of WGS reaction on Pt/CeAl surface catalyst and elucidate the promoter role of ionic conductor, both solids were independently studied under reaction conditions by means of DRIFT spectroscopic measurements. 2.3.1. Initial Pretreatment of the Samples Prior to reaction, the catalyst was in situ pretreated as described in Materials and Methods (Section 3.). Figure 5a displays the initial spectrum at RT, the spectrum of the activated sample at 350 Figure 4. Catalytic activity in terms of CO conversion against temperature for water gas shift (WGS) reaction over Pt/CeAl catalyst and Pt/CeAl +ZrEuMo_5 promoted system at 80 L h −1 g cat−1 and 20,000 h −1 under ( a ) model conditions feed (4.5% CO, 30% H 2 O, balanced with N 2 ) and ( b ) industrial conditions feed (9% CO, 11% CO2, 30% H2O, 50% H2). Apparently, the differencesobservedinbothcatalytic systemscould beassociated tohigher thermal diffusion by the presence of a dilutant. However, the Pt catalyst was diluted in both systems, either ionic conductor or quartz, using the same amount of catalyst (0.1g of Pt/CeAl) and sieved in the same particle sizes. Thereby, both systems were tested in the same weight and volumetric space velocities (same catalyst weight and catalytic bed volume). This indicates that an additional feature exists for explaining the superior activity of Pt/CeAl +ZrEuMo_5 promoted catalyst. As can be underlined, the effect by ZrEuMo_5 presence is observable from ~250–270 ◦ C, which is in good agreement with the temperature from which the Eu-doped zirconia ionic conductor starts to show proton conductivity, according to impedance spectroscopy measurements reported by Garc í a-Moncada et al. [ 23 ]. In the literature, Pt-based catalysts for WGS reaction have been reported to provide high CO coverage, with water activation as one of the slowest steps [ 43 , 44 ]. In this context, ceria support improves the fast oxygen storage/release cycles via a reversible conversion between Ce 4+ and Ce 3+ [ 45 – 47 ]. The reduction process from Ce 4+ to Ce 3+ provides a high mobility of oxygen/hydroxyl groups promoting the hydroxyl groups formation on the surface. Thus, the higher activity obtained by the proposed catalytic system including the ionic conductor, can be explained by higher concentration and mobility of hydroxyl groups and molecular water on the surface. Hence, the ionic conductor does not catalyze the reaction itself, but from temperatures (250–270 ◦ C) where the proton conductivity starts to happen according to Catalysts 2020,10, 841 6 of 18 Grotthuss mechanism [ 20 , 22 ], ZrEuMo_5 acts as a promoter of Pt catalyst increasing water activation and mobility. Moreover, it should be pointed out that the incremental effect by ZrEuMo_5 observed in industrial conditions was more pronounced than in model conditions. This could be related to the capacity of MoO x to create hydrated bronzes from 300 ◦ C, since the industrial conditions feed includes 50% H 2 , in contrast to model conditions [ 38 ], and the presence of these structures could enhance the water activation. 2.3. Operando DRIFTS Study In order to gain insights into the mechanistic aspects of WGS reaction on Pt/CeAl surface catalyst and elucidate the promoter role of ionic conductor, both solids were independently studied under reaction conditions by means of DRIFT spectroscopic measurements. 2.3.1. Initial Pretreatment of the Samples Prior to reaction, the catalyst was in situ pretreated as described in Materials and Methods (Section 3.). Figure 5a displays the initial spectrum at RT, the spectrum of the activated sample at 350 ◦ C in H 2 /Ar, and the spectrum of the solid after activation at 180 ◦ C in Ar before starting reaction. Figure 5b also includes the difference spectra of the latter two with respect to the former one. Herein, the positive bands indicate the formed species whereas the negative ones are associated to species that disappear during the activation step. Consequently, the disappearance of the broad band centered at 3470 cm −1 coupled to the peak at 1642 cm −1 , and the appearance of two new features at 3732 and 3680 cm −1 clearly indicate the desorption of physisorbed water during the activation step, whose vanishing leads to the observation of stretching vibrations of hydroxyl groups over the ceria/alumina support [48]. Catalysts 2020, 10, x FOR PEER REVIEW 7 of 18 °C in H 2 /Ar, and the spectrum of the solid after activation at 180 °C in Ar before starting reaction. Figure 5b also includes the difference spectra of the latter two with respect to the former one. Herein, the positive bands indicate the formed species whereas the negative ones are associated to species that disappear during the activation step. Consequently, the disappearance of the broad band centered at 3470 cm −1 coupled to the peak at 1642 cm −1 , and the appearance of two new features at 3732 and 3680 cm −1 clearly indicate the desorption of physisorbed water during the activation step, whose vanishing leads to the observation of stretching vibrations of hydroxyl groups over the ceria/alumina support [48]. Figure 5. (a) DRIFT spectra of Pt/CeAl catalyst: initial at RT (pink line), activated after 30 min in H 2 atmosphere (orange line), and after activation at 180 °C in Ar before reaction conditions (green line); (b) difference spectra of activated sample at 350 °C in H 2 (orange line) and at 180 °C in Ar before reaction (green line) with respect to the initial spectrum. On the other side, the bands appearing at 2020 and 1980 cm −1 are associated with the creation of Pt–CO species, which are formed upon decomposition of carboxylates whose disappearance is assessed by the intensity decrease of the bands at 1517 and 1401 cm −1 assigned to υ OCO vibrations [49]. Not surprisingly, an intensity increase of a band at 2138 cm −1 in H 2 atmosphere is also observed. This is attributed to the typical 2 F 5/2 → 2 F 7/2 electronic transition of Ce 3+ [48,50]. Identical activation pre-treatment was carried out on the ZrEuMo_5 proton conductor. Similarly, the spectra before and after the activation process, and the respective difference spectra in respect to that at RT before the activation step are shown in Figure 6. Prior to activation step, the spectrum is dominated by a broad band placed at 3438 cm −1 caused by the stretching vibration (υ OH ) of species resulting from the interaction between molecular water and hydroxyl groups via hydrogen bonds. Moreover, this feature is related with the bands at 1636 and 5162 cm −1 corresponding to the scissoring bend (δ HOH ) and combination band (υ OH + δ HOH ), respectively, and both typical of physisorbed water (Figure 6a). After H 2 treatment, it is noticeable the disappearance of the broad band associated to nondissociated water and the appearance of vibrations in the 3900–3500 cm −1 region corresponding to hydroxyl species (shown in the difference spectra, Figure 6b). The interaction via hydrogen bonds between the water molecules and the hydroxyl species is responsible for the non-appearance of these hydroxyl related bands before activation. Figure 5. ( a ) DRIFT spectra of Pt/CeAl catalyst: initial at RT (pink line), activated after 30 min in H 2 atmosphere (orange line), and after activation at 180 ◦ C in Ar before reaction conditions (green line); ( b ) difference spectra of activated sample at 350 ◦ C in H 2 (orange line) and at 180 ◦ C in Ar before reaction (green line) with respect to the initial spectrum. On the other side, the bands appearing at 2020 and 1980 cm −1 are associated with the creation of Pt–CO species, which are formed upon decomposition of carboxylates whose disappearance is assessed by the intensity decrease of the bands at 1517 and 1401 cm −1 assigned to υOCO vibrations [ 49 ]. Not surprisingly, an intensity increase of a band at 2138 cm −1 in H 2 atmosphere is also observed. This is attributed to the typical 2F5/2→2F7/2electronic transition of Ce3+[48,50]. Identical activation pre-treatment was carried out on the ZrEuMo_5 proton conductor. Similarly, the spectra before and after the activation process, and the respective difference spectra in respect to Catalysts 2020,10, 841 7 of 18 that at RT before the activation step are shown in Figure 6. Prior to activation step, the spectrum is dominated by a broad band placed at 3438 cm −1 caused by the stretching vibration ( υOH ) of species resulting from the interaction between molecular water and hydroxyl groups via hydrogen bonds. Moreover, this feature is related with the bands at 1636 and 5162 cm −1 corresponding to the scissoring bend ( δHOH ) and combination band ( υOH + δHOH ), respectively, and both typical of physisorbed water (Figure 6a). After H 2 treatment, it is noticeable the disappearance of the broad band associated to non-dissociated water and the appearance of vibrations in the 3900–3500 cm −1 region corresponding to hydroxyl species (shown in the difference spectra, Figure 6b). The interaction via hydrogen bonds between the water molecules and the hydroxyl species is responsible for the non-appearance of these hydroxyl related bands before activation. Catalysts 2020, 10, x FOR PEER REVIEW 8 of 18 Figure 6. (a) Direct spectra of ZrEuMo_5 sample: initial at RT (pink line), activated after 30 min in H 2 atmosphere (orange line), and after activation at 180 °C in Ar before reaction conditions (green line); (b) difference spectra of activated sample at 350 °C in H 2 (orange line) and at 180 °C in Ar before reaction (green line) with respect to the initial spectrum. To facilitate the analysis of hydroxyl species, the 3900–3500 cm −1 region is displayed in Figure 7a. The new bands appearing at 3749, 3722, and 3671 cm −1 are assigned to monodentate (linear, type I) and tricoordinate bridged OH groups over ZrO 2 [51]. These observations are in agreement with that found for Eu-doped zirconia solid solution, without Mo addition, as previously reported [23]. Furthermore, the region from 2000 to 900 cm −1 has been also magnified (Figure 7b), where besides the negative band at 1636 cm −1 due to the elimination of the physisorbed water, two other bands appear at 980 cm −1 and 1936 cm −1 , both associated to Mo=O bond stretch and its overtone, respectively [52]. The increase of these bands after the activation step may be related to the dehydration of superficial Mo oxide of the sample according to Scheme 1. Figure 7. Difference spectra of activated ZrEuMo_5 sample at 350 °C in H 2 (green line) and at 180 °C in Ar before WGS reaction (orange line) in the (a) OH and (b) hydrated Mo oxide ranges. Figure 6. ( a ) Direct spectra of ZrEuMo_5 sample: initial at RT (pink line), activated after 30 min in H 2 atmosphere (orange line), and after activation at 180 ◦ C in Ar before reaction conditions (green line); ( b ) difference spectra of activated sample at 350 ◦ C in H 2 (orange line) and at 180 ◦ C in Ar before reaction (green line) with respect to the initial spectrum. To facilitate the analysis of hydroxyl species, the 3900–3500 cm −1 region is displayed in Figure 7a. The new bands appearing at 3749, 3722, and 3671 cm −1 are assigned to monodentate (linear, type I) and tricoordinate bridged OH groups over ZrO 2 [ 51 ]. These observations are in agreement with that found for Eu-doped zirconia solid solution, without Mo addition, as previously reported [ 23 ]. Furthermore, the region from 2000 to 900 cm −1 has been also magnified (Figure 7b), where besides the negative band at 1636 cm −1 due to the elimination of the physisorbed water, two other bands appear at 980 cm −1 and 1936 cm −1 , both associated to Mo=O bond stretch and its overtone, respectively [ 52 ]. The increase of these bands after the activation step may be related to the dehydration of superficial Mo oxide of the sample according to Scheme 1. On the other side, there are also some bands between 1600 and 1100 cm −1 (Figure 7b) which can be ascribed to residual carbonates/carboxylates like compounds. The presence of these species is unavoidable due to the atmospheric CO 2 adsorption on Eu-doped zirconia during the calcination process, forming thereby these carbon-species which remain occluded into the pores according to the high porosity of the material [53]. Finally, the new bands also formed during the activation in the range of 2400–2150 cm −1 are more detailed in Figure 8. At these vibrational frequencies can be observed low-energy f-f transitions of Eu 3+ [ 54 ]. In particular, the band at 2236 cm −1 is attributed to 7 F 0→7 F J transition, which is shifted on reducing temperature to 2228 cm −1 , since these transitions are highly sensitive to temperature changes [ 23 ]. Moreover, in this region are bands (between 2400–2300 cm −1 ) associated to occluded CO 2 inside the pores of the sample. Hence, O=C=O ···· M n+ interactions produce splitting of the bands [ 55 ]. In addition, due to the molecular CO2presence, the attribution of these bands result complex. Catalysts 2020,10, 841 8 of 18 Catalysts 2020, 10, x FOR PEER REVIEW 8 of 18 Figure 6. (a) Direct spectra of ZrEuMo_5 sample: initial at RT (pink line), activated after 30 min in H 2 atmosphere (orange line), and after activation at 180 °C in Ar before reaction conditions (green line); (b) difference spectra of activated sample at 350 °C in H 2 (orange line) and at 180 °C in Ar before reaction (green line) with respect to the initial spectrum. To facilitate the analysis of hydroxyl species, the 3900–3500 cm −1 region is displayed in Figure 7a. The new bands appearing at 3749, 3722, and 3671 cm −1 are assigned to monodentate (linear, type I) and tricoordinate bridged OH groups over ZrO 2 [51]. These observations are in agreement with that found for Eu-doped zirconia solid solution, without Mo addition, as previously reported [23]. Furthermore, the region from 2000 to 900 cm −1 has been also magnified (Figure 7b), where besides the negative band at 1636 cm −1 due to the elimination of the physisorbed water, two other bands appear at 980 cm −1 and 1936 cm −1 , both associated to Mo=O bond stretch and its overtone, respectively [52]. The increase of these bands after the activation step may be related to the dehydration of superficial Mo oxide of the sample according to Scheme 1. Figure 7. Difference spectra of activated ZrEuMo_5 sample at 350 °C in H 2 (green line) and at 180 °C in Ar before WGS reaction (orange line) in the (a) OH and (b) hydrated Mo oxide ranges. Figure 7. Difference spectra of activated ZrEuMo_5 sample at 350 ◦ C in H 2 (green line) and at 180 ◦ C in Ar before WGS reaction (orange line) in the (a) OH and (b) hydrated Mo oxide ranges. Catalysts 2020, 10, x FOR PEER REVIEW 9 of 18 Scheme 1. Hydration/dehydration process by activation in H 2 at 350 °C on MoO x phase. On the other side, there are also some bands between 1600 and 1100 cm −1 (Figure 7b) which can be ascribed to residual carbonates/carboxylates like compounds. The presence of these species is unavoidable due to the atmospheric CO 2 adsorption on Eu-doped zirconia during the calcination process, forming thereby these carbon-species which remain occluded into the pores according to the high porosity of the material [53]. Finally, the new bands also formed during the activation in the range of 2400–2150 cm −1 are more detailed in Figure 8. At these vibrational frequencies can be observed low-energy f-f transitions of Eu 3+ [54]. In particular, the band at 2236 cm −1 is attributed to 7 F 0 → 7 F J transition, which is shifted on reducing temperature to 2228 cm −1 , since these transitions are highly sensitive to temperature changes [23]. Moreover, in this region are bands (between 2400–2300 cm −1 ) associated to occluded CO 2 inside the pores of the sample. Hence, O=C=O····M n+ interactions produce splitting of the bands [55]. In addition, due to the molecular CO 2 presence, the attribution of these bands result complex. Figure 8. Difference spectra of activated ZrEuMo_5 sample at 350 °C in H 2 (green line) and at 180 °C in Ar before WGS reaction (orange line). After the activation pre-treatment, the WGS reaction was performed under model and industrial conditions in both solids. 2.3.2. WGS Reaction under Model Conditions Figure 9 shows the evolution of the spectra during WGS reaction under model conditions from 180 to 350 °C on Pt/CeAl catalyst. As can be noticed, the evolution of bands associated to OH groups are displayed in the 4000–3400 cm −1 region. In particular, negative bands are observed at 3759, 3730, and 3682 cm −1 corresponding to isolated OH groups bonded to the ceria/alumina support [48], which means that these species disappear once the reaction starts. At the same time a broad band peaking at ca. 3400 cm −1 appears. This indicates H-bonded molecular water to the initially isolated OH species. Scheme 1. Hydration/dehydration process by activation in H2at 350 ◦C on MoOxphase. Catalysts 2020, 10, x FOR PEER REVIEW 9 of 18 Scheme 1. Hydration/dehydration process by activation in H 2 at 350 °C on MoO x phase. On the other side, there are also some bands between 1600 and 1100 cm −1 (Figure 7b) which can be ascribed to residual carbonates/carboxylates like compounds. The presence of these species is unavoidable due to the atmospheric CO 2 adsorption on Eu-doped zirconia during the calcination process, forming thereby these carbon-species which remain occluded into the pores according to the high porosity of the material [53]. Finally, the new bands also formed during the activation in the range of 2400–2150 cm −1 are more detailed in Figure 8. At these vibrational frequencies can be observed low-energy f-f transitions of Eu 3+ [54]. In particular, the band at 2236 cm −1 is attributed to 7 F 0 → 7 F J transition, which is shifted on reducing temperature to 2228 cm −1 , since these transitions are highly sensitive to temperature changes [23]. Moreover, in this region are bands (between 2400–2300 cm −1 ) associated to occluded CO 2 inside the pores of the sample. Hence, O=C=O····M n+ interactions produce splitting of the bands [55]. In addition, due to the molecular CO 2 presence, the attribution of these bands result complex. Figure 8. Difference spectra of activated ZrEuMo_5 sample at 350 °C in H 2 (green line) and at 180 °C in Ar before WGS reaction (orange line). After the activation pre-treatment, the WGS reaction was performed under model and industrial conditions in both solids. 2.3.2. WGS Reaction under Model Conditions Figure 9 shows the evolution of the spectra during WGS reaction under model conditions from 180 to 350 °C on Pt/CeAl catalyst. As can be noticed, the evolution of bands associated to OH groups are displayed in the 4000–3400 cm −1 region. In particular, negative bands are observed at 3759, 3730, and 3682 cm −1 corresponding to isolated OH groups bonded to the ceria/alumina support [48], which means that these species disappear once the reaction starts. At the same time a broad band peaking at ca. 3400 cm −1 appears. This indicates H-bonded molecular water to the initially isolated OH species. Figure 8. Difference spectra of activated ZrEuMo_5 sample at 350 ◦ C in H 2 (green line) and at 180 ◦ C in Ar before WGS reaction (orange line). After the activation pre-treatment, the WGS reaction was performed under model and industrial conditions in both solids. 2.3.2. WGS Reaction under Model Conditions Figure 9shows the evolution of the spectra during WGS reaction under model conditions from 180 to 350 ◦C on Pt/CeAl catalyst. As can be noticed, the evolution of bands associated to OH groups Catalysts 2020,10, 841 9 of 18 are displayed in the 4000–3400 cm −1 region. In particular, negative bands are observed at 3759, 3730, and 3682 cm −1 corresponding to isolated OH groups bonded to the ceria/alumina support [ 48 ], which means that these species disappear once the reaction starts. At the same time a broad band peaking at ca. 3400 cm −1 appears. This indicates H-bonded molecular water to the initially isolated OH species. This supports the important role of the support hydroxyl groups in the reaction mechanism. On increasing the reaction temperature these negative bands become less intense and the positive broad band almost disappears. This likely means that free OH groups regenerate, which also indicates that molecular water no longer remains adsorbed on the support surface. Catalysts 2020, 10, x FOR PEER REVIEW 10 of 18 This supports the important role of the support hydroxyl groups in the reaction mechanism. On increasing the reaction temperature these negative bands become less intense and the positive broad band almost disappears. This likely means that free OH groups regenerate, which also indicates that molecular water no longer remains adsorbed on the support surface. Figure 9. Evolution of difference spectra during WGS reaction under model conditions from 180 to 350 ˚C on Pt/CeAl in the (a) OH and (b) carbonates, formates and/or carboxylates ranges. The spectrum recorded at 180 ˚C after activation was used as reference. On the other side, the interaction of CO with Pt during the reaction as well as the formation/elimination of carbonate-carboxylate species are also considered (Figure 9). The minimum observed at 2143 cm −1 corresponds to gaseous CO present in the infrared (IR) cell volume. In addition, bands at 2073, 2052, and 2020 cm −1 are associated to the stretching vibration (υ CO ) of carbonyl species formed on metallic Pt. The presence of several bands indicates that CO molecules are adsorbed on Pt with different electron density which also depends on the Pt particle sizes. The chemical bond between CO and Pt involves the 5σ and 2π* orbitals of the CO molecule. When the coordination of Pt particles is low, its electron density is high, which increases the 2π*, the back-donation phenomenon that consequently debilitates the C–O bond. Hence, the associated υ CO band shifts to lower frequencies [56]. Accordingly, the observed band at 2073 cm −1 corresponds to Pt particles with low electron density, which can be associated to bigger particle sizes or particles with higher coordination. In the same sense, the intense band at 2052 cm −1 is assigned to reduced Pt particles (Pt 0 ) with a small particle size around 1.5 nm or the CO is adsorbed on terrace sites. The band at 2020 cm −1 can be related to the linear CO adsorbed on the smallest reduced Pt particles (d p < 1.5 nm) [57]. However, this band at 2020 cm −1 can also be explained by the adsorption of terminal CO on metallic Pt in contact with the Ce 3+ in the metal-support interface (Pt- □ s -Ce 3+ ) [58]. Furthermore, band shifts and changes in intensities are noticeable when increasing the temperature during the WGS reaction. This may indicate a variation of the Pt electron density, a different Pt coordination, a different Pt oxidation state or a variation of Pt particle size. Actually, at 350 °C (Figure 9), a new band at 2007 cm −1 is observed. This band can be related to a shift from the band at 2020 cm −1 by a reduction of Pt particles (d p < 1.5 nm) [56] or can be attributed to adsorbed CO in presence of superficial carbon on Pt; this latter caused by CO dissociation [49]. On the other side, in the region of 1700–1300 cm −1 are shown the symmetric and asymmetric stretching modes characteristic of superficial carbonates, formates or carboxylates. The bands at 1591 and 1371 cm −1 have been attributed to monodentate carbonates [59,60], whereas the shoulder at 1640 cm −1 is associated to physisorbed water. The intensities of both bands decrease on increasing the temperature during the reaction as is expected. Nevertheless, it is observed the appearance and increase with the temperature of a band at 1444 cm −1 , which can be assigned to C–H vibrational mode (υ C–H ) and/or C=C bond stretch (υ C=C ) due to the slightly amorphous adsorbed carbon as cyclic system Figure 9. Evolution of difference spectra during WGS reaction under model conditions from 180 to 350 ◦ C on Pt/CeAl in the ( a ) OH and ( b ) carbonates, formates and/or carboxylates ranges. The spectrum recorded at 180 ◦C after activation was used as reference. On the other side, the interaction of CO with Pt during the reaction as well as the formation/elimination of carbonate-carboxylate species are also considered (Figure 9). The minimum observed at 2143 cm −1 corresponds to gaseous CO present in the infrared (IR) cell volume. In addition, bands at 2073, 2052, and 2020 cm −1 are associated to the stretching vibration ( υCO ) of carbonyl species formed on metallic Pt. The presence of several bands indicates that CO molecules are adsorbed on Pt with different electron density which also depends on the Pt particle sizes. The chemical bond between CO and Pt involves the 5 σ and 2 π * orbitals of the CO molecule. When the coordination of Pt particles is low, its electron density is high, which increases the 2 π *, the back-donation phenomenon that consequently debilitates the C–O bond. Hence, the associated υCO band shifts to lower frequencies [ 56 ]. Accordingly, the observed band at 2073 cm −1 corresponds to Pt particles with low electron density, which can be associated to bigger particle sizes or particles with higher coordination. In the same sense, the intense band at 2052 cm −1 is assigned to reduced Pt particles (Pt 0 ) with a small particle size around 1.5 nm or the CO is adsorbed on terrace sites. The band at 2020 cm −1 can be related to the linear CO adsorbed on the smallest reduced Pt particles (d p <1.5 nm) [ 57 ]. However, this band at 2020 cm −1 can also be explained by the adsorption of terminal CO on metallic Pt in contact with the Ce 3+ in the metal-support interface (Pt- s -Ce 3+ ) [ 58 ]. Furthermore, band shifts and changes in intensities are noticeable when increasing the temperature during the WGS reaction. This may indicate a variation of the Pt electron density, a different Pt coordination, a different Pt oxidation state or a variation of Pt particle size. Actually, at 350 ◦ C (Figure 9), a new band at 2007 cm −1 is observed. This band can be related to a shift from the band at 2020 cm −1 by a reduction of Pt particles (d p <1.5 nm) [ 56 ] or can be attributed to adsorbed CO in presence of superficial carbon on Pt; this latter caused by CO dissociation [49]. On the other side, in the region of 1700–1300 cm −1 are shown the symmetric and asymmetric stretching modes characteristic of superficial carbonates, formates or carboxylates. The bands at 1591 and 1371 cm −1 have been attributed to monodentate carbonates [ 59 , 60 ], whereas the shoulder at Catalysts 2020,10, 841 16 of 18 22. Malavasi, L.; Fisher, C.A.J.; Islam, M.S. Oxide-ion and proton conducting electrolyte materials for clean energy applications: Structural and mechanistic features. Chem. Soc. Rev. 2010 ,39, 4370–4387. [CrossRef] [PubMed] 23. Garc í a-Moncada, N.; Bobadilla, L.F.; Poyato, R.; L ó pez-Cartes, C.; Romero-Sarria, F.; Centeno, M. Á .; Odriozola, J.A. 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