Ti

Article pubs.acs.org/JPCC

Water−Gas Shift Reaction on Pt/Ce1−xTixO2−δ: The Effect of Ce/Ti Ratio Klito C. Petallidou,† Kyriaki Polychronopoulou,*,‡ Soghomon Boghosian,§,∥ Sergio Garcia-Rodriguez,⊥ and Angelos M. Efstathiou*,† †

Department of Chemistry, Heterogeneous Catalysis Laboratory, University of Cyprus, University Campus, P.O. Box 20537, CY 1678, Nicosia, Cyprus ‡ Department of Mechanical Engineering, Khalifa University of Science, Technology and Research, P.O. Box 127788, Abu Dhabi, UAE § Department of Chemical Engineering, University of Patras, GR-26500 Patras, Greece ∥ Institute of Chemical Engineering Sciences (FORTH/ICE-HT), Stadiou Str., Platani P.O. Box 1414, GR-26504 Patras, Greece ⊥ Instituto de Catálisis y Petroleoquimica (CSIC), Marie Curie 2, Cantoblanco, 28049 Madrid, Spain S Supporting Information *

ABSTRACT: Pt nanoparticles (1.2−2.0 nm size) supported on Ce1−xTixO2−δ (x = 0, 0.2, 0.5, 0.8, and 1.0) carriers synthesized by the citrate sol−gel method were tested toward the water−gas shift (WGS) reaction in the 200−350 °C range. A deep insight into the effect of two structural parameters, the chemical composition of support (Ce/Ti atom ratio), and the Pt particle size on the catalytic performance of Pt-loaded catalysts was realized after employing in situ X-ray diffraction (XRD), high-resolution transmission electron microscopy (HR-TEM) and HAADF/STEM, scanning electron microscopy (SEM), in situ Raman and diffuse reflectance infrared Fourier transform (DRIFT) spectroscopies under different gas atmospheres, H2 temperature-programmed reduction (H2-TPR), and temperature-programmed desorption (NH3-TPD and CO2-TPD) techniques. The 0.5 wt % Pt/Ce0.8Ti0.2O2−δ solid (dPt = 1.7 nm) was found to be by far the best catalyst among all the other solids investigated. In particular, at 250 °C the CO conversion over Pt/Ce0.8Ti0.2O2−δ was increased by a factor of 2.5 and 1.9 compared to Pt/TiO2 and Pt/CeO2, respectively. The catalytic superiority of the Pt/Ce0.8Ti0.2O2−δ solid is the result of the support’s (i) robust morphology preserved during the WGS reaction, (ii) moderate acidity and basicity, and (iii) better reducibility at lower temperatures and the significant reduction of “coking” on the Pt surface and of carbonate accumulation on the Ce0.8Ti0.2O2−δ support. Several of these properties largely influenced the reactivity of sites (k, s−1) at the Pt−support interface. In particular, the specific WGS reaction rate at 200 °C expressed per length of the Pt−support interface (μmol CO cm−1 s−1) was found to be 2.2 and 4.6 times larger on Pt supported on Ce0.8Ti0.2O2−δ (Ti4+-doped CeO2) compared to TiO2 and CeO2 alone, respectively.

1. INTRODUCTION The water−gas shift (WGS) reaction (CO + H2O ↔ CO2 + H2, ΔH° = −41.2 kJ/mol) is an industrial catalytic chemical process applied in several important chemical technologies such as H2 and NH3 production.1 The WGS is a slightly exothermic reaction which is favored at low temperatures ( 300 °C according to the observed shift in the main XRD peak. The specific surface area (SSA, m2 g−1), pore volume (cm3 −1 g ), and average pore size (nm) of the Ce1−xTixO2−δ solid supports are reported in Table S2 (Supporting Information). The mixed metal oxides have higher specific surface area, pore volume, and similar or lower average pore size compared to the single metal oxides of CeO2 and TiO2. SEM images obtained for the Ce0.8Ti0.2O2−δ and TiO2 solids loaded with 0.5 wt % Pt (fresh) and after the WGS reaction are given in Figure S2 (Supporting Information). Differences in morphology after 50 h in the WGS were not seen over the most active (Pt/Ce0.8Ti0.2O2−δ) catalyst, which was not the case with the least active (Pt/TiO2) catalyst (see Section 3.2.1). This result is linked to the stability behavior of the catalysts discussed in the following Section 3.2.2. Figure 1a shows an HR-TEM image of the 0.5 wt % Pt/TiO2 catalyst, whereas Figure 1b is the resulting Pt particle size

composition. The purity of the gases used in all catalytic activity experiments (e.g., H2, He, CO, Ar; Linde Gas) was higher than 99.95%. The activity of the catalysts was estimated in terms of CO conversion (XCO, %) and specific WGS reaction rate, rCO (μmol CO gcat−1 s−1), using the following relationships 2 and 3, respectively XCO (%) =

in out − FCO (FCO ) in FCO

× 100 (2)

in rCO (μmol CO gcat−1 s−1) = FCO XCO/Wcat

(3) −1

where and are the molar flow rates (μmol CO s ) of CO at the reactor’s inlet and outlet, respectively, and Wcat is the mass of catalyst sample used. Initial rates (at XCO → 0) at a given temperature were estimated from the slope of the linear in relationship XCO vs W/FCO . The variation of XCO was performed by adjusting the mass of the catalyst and keeping the total gas flow rate constant. 2.10. Characterization of Carbonaceous Deposits. 2.10.1. Transient Experiments. The amount and reactivity toward oxygen of the carbon-containing intermediates formed on the surface of supported 0.5 wt % Pt catalysts during the WGS reaction were studied by two different kinds of transient experiments. In the first experiment, following reaction at 325 °C for 50 h, the catalyst was heated to 800 °C in He flow (HeTPSR) to remove adsorbed water, CO2, and/or carbon deposits that could thermally decompose in He gas flow. In the second experiment, after He-TPSR the reactor was cooled quickly in He flow to 25 °C, and the gas flow was then switched to a 2 vol % O2/He gas mixture for performing temperatureprogrammed oxidation (TPO, β = 30 °C min−1) during which the H2 (m/z = 2), CO (m/z = 28), and CO2 (m/z = 44) MS signals were recorded continuously. Quantification of the CO and CO2 MS signals was made after accounting for the contribution of CO2 to the 28 (m/z) MS signal and using standard calibration gas mixtures for CO and CO2. Transient experiments were conducted in a specially designed gas-flow system described in detail elsewhere.40 2.10.2. Raman Spectroscopy. Approximately 90−100 mg of catalyst in powder form was pressed into a wafer disc and mounted by gold wiring on a sample holder of a homemade in situ Raman cell.42 The cell consisted of a double-walled quartzglass transparent tube furnace mounted on an xyz plate allowing it to be positioned on the optical table. The inner furnace tube (23 mm o.d.; 20 mm i.d., and 10 cm long) was kanthal wire-wound for heating the cell. The cell had a gas inlet and outlet as well as a thermocouple sheath possessing a sample holder at its tip. In situ Raman spectra were recorded at 300 °C under flowing (15 mL min−1) N2 (inert atmosphere). Prior to recording the Raman spectra, each catalyst sample was subjected to a 15 mL min−1 flow of N2 gas for 2 h. The 488.0 nm line of a Spectra Physics Stabilite 2017 Ar+ laser operated at 40 mW was used for exciting Raman vibrational modes. The incident light was slightly defocused to reduce sample irradiance. The scattered light was collected at 90° (horizontal scattering plane), analyzed with a 0.85 m Spex 1403 double monochromator, and detected by a −20 °C cooled RCA PMT interfaced with a Labspec data acquisition software. FinCO

Fout CO

Figure 1. (a) HR-TEM image and (b) Pt particle size distribution of 0.5 wt % Pt/TiO2 catalyst.

distribution. Most Pt particles exhibit sizes in the 1.5−2.5 nm range, and only a small fraction exhibits sizes in the 3−13 nm range. The mean diameter of Pt particles was estimated to be 2.0 nm (±0.2 s.d.) after counting 100 particles. HAADF/STEM images were also obtained over the 0.5 wt % Pt/CeO2 catalyst,13 where the mean Pt particle size was 1.2 nm (±0.3 s.d.). These results are in good agreement with those obtained based on H2 chemisorption performed at 25 °C and after using 0.3 vol % H2/He for 15 min followed by TPD (see Supporting Information). It is observed that the increase of Ce/Ti atom ratio led to an increase in Pt dispersion and in turn to a smaller Pt particle size, thus a higher concentration (μmol Pts g−1) of surface metal sites. The effect of Pt loading on the platinum dispersion and particle size was investigated over the Pt/ Ce0.8Ti0.2O2−δ catalytic system. It was found that the increase of Pt loading from 0.1 to 0.5 wt % led to a decrease of Pt

3. RESULTS AND DISCUSSION 3.1. Structural, Textural, and Morphological Properties of Pt/Ce1−xTixO2−δ Catalysts. XRD patterns, primary D

dx.doi.org/10.1021/jp406059h | J. Phys. Chem. C XXXX, XXX, XXX−XXX

The Journal of Physical Chemistry C

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Table 1. Metal Dispersion (D, %) and Mean Pt Particle Size (dPt, nm) Estimated from HR-TEM, HAADF/STEM, and H2 Chemisorption Followed by TPDa 0.5 wt % Pt/support CeO2 Ce0.8Ti0.2O2−δ Ce0.5Ti0.5O2−δ Ce0.2Ti0.8O2−δ TiO2 0.1 wt % Pt/Ce0.8Ti0.2O2−δ

D (%) b

c

69 /92 65c 65b 61b 44b/55d 96b

dPt (nm) b

c

1.6 /1.2 (±0.3 s.d.) 1.7 (±0.2 s.d.)c 1.7b 1.8b 2.5b/2.0 (±0.2 s.d.)d 1.1b

rCO (μmol gcat−1 s−1)

Io (cm gcat−1)

0.24 0.78 0.41 0.45 0.20 0.29

× × × × × ×

1.9 1.3 9.7 8.6 7.0 2.3

11

10 1011 1010 1010 1010 1011

RCO (μmol cm−1 s−1) 1.3 6.0 4.2 5.2 2.8 1.3

× × × × × ×

10−12 10−12 10−12 10−12 10−12 10−12

Specific initial kinetic rates of WGS reaction per gram of catalyst, rCO (μmol gcat−1 s−1), and per length of the perimeter of the Pt−Ce1−xTixO2−δ interface (Io, cm gcat−1), RCO (μmol cm−1 s−1), measured at 200 °C. b0.3 vol % H2/He chemisorption at 25 °C for 30 min; dPt (nm) = 1.1/D(%) × 100. cAccording to HAADF/STEM studies. dAccording to HR-TEM studies. a

compared to Pt/TiO2 and Pt/CeO2, respectively. On the basis of the XRD results (Figure S1a and Table S1, Supporting Information), a solid solution for the Ce0.8Ti0.2O2−δ composition was indeed formed. The latter phase showed a relatively large line broadening suggesting that the oxide ion sublattice might be distorted leading to a change in the local coordination environment around Ti4+ ions. This in turn could lead to shorter Ti−O bond lengths compared to Ce−O ones (CeO2 case). Thus, for the oxygen ion it is easier to move away from the shorter Ti−O bond lengths, and an increase in the oxygen ionic mobility within the bulk and surface structure should therefore be expected, as reported in the case of a Ce1−xZrxO2−δ system.44 The higher electronegativity of Ti (1.54) compared to Ce (1.12)45 could also contribute to the higher mobility of oxygen ions in the Ce1−xTixO2−δ solid (Ti4+-doped CeO2), given that an increased electron attraction around Ti4+ compared to Ce4+ cations could facilitate diffusion of O2− anions toward Ti4+ cations (shorter Ti−O bond length). Table 1 presents specific initial kinetic rates (see Section 2.9) of the WGS reaction at 200 °C per gram of catalyst (rCO, μmol CO gcat−1 s−1) and per length of the perimeter of the Pt− Ce1−xTixO2−δ interface (RCO, μmol CO cm−1 s−1). The length of the perimeter of the Pt−Ce1−xTixO2−δ interface (Io, cm gcat−1)11 is also reported in Table 1; RCO values are estimated after dividing the rCO by the corresponding Io value. The specific kinetic rate was found to be larger for the mixed metal oxides (0.41−0.78 μmol gcat−1 s−1) compared to the single ones (0.20−0.24 μmol gcat−1 s−1). In particular, the specific kinetic rate, rCO, increased by a factor of 3.9 and 3.25 in the case of Pt/ Ce0.8Ti0.2O2−δ (0.78 μmol gcat−1 s−1) compared to Pt/TiO2 (0.20 μmol gcat−1 s−1) and Pt/CeO2 (0.24 μmol gcat−1 s−1) catalysts, respectively. The presence of 20 atom % Ce4+ in the Ce0.2Ti0.8O2−δ solid composition resulted in an increase in the kinetic rate of WGS by a factor of 2.2 when compared to the TiO2-supported Pt (0.20 μmol gcat−1 s−1). These results show the important effect of the Ce/Ti atom ratio in the support composition on the specific kinetic rate. The specific kinetic rate of WGS per length of the perimeter of Pt−Ce1−xTixO2−δ interface was found to be the highest over the 0.5 wt % Pt/Ce0.8Ti0.2O2−δ catalyst and the lowest one over the 0.5 wt % Pt/CeO2 catalyst. These two catalytic systems have similar Io values (Table 1), where the number of Pt sites per length of metal−support interface is only slightly modified by the Pt particle size and metal support composition, e.g., Pt− Pt and Mn+−OL distance; Mn+ and OL are the support surface metal cation and lattice oxygen, respectively. This result indicates that the 0.5 wt % Pt/Ce0.8Ti0.2O2−δ possesses larger site reactivities (k, s−1) along the Pt−support interface compared to the other catalysts (Table 1). The specific

dispersion (96 vs 65%) and to an increase in the mean Pt particle size (1.1 vs 1.7 nm, Table 1). 3.2. Catalytic Activity. 3.2.1. Effects of Support Chemical Composition. Figure 2 presents WGS catalytic activity results

Figure 2. Effect of support chemical composition on the conversion of CO (XCO, %) as a function of WGS reaction temperature over 0.5 wt % Pt/Ce1−xTixO2−δ (x = 0.0, 0.2, 0.5, and 1.0) catalysts; GHSV = 40 000 h−1 (L/Lcat/h).

in terms of CO conversion (XCO, %) as a function of reaction temperature in the 200−350 °C range obtained over the 0.5 wt % Pt supported on Ce1−xTixO2−δ carriers (x = 0.0, 0.2, 0.5, 1.0). The Pt/Ce0.2Ti0.8O2−δ solid gave a CO-conversion profile (not shown) very similar to that of Pt/Ce0.5Ti0.5O2−δ. The Ce1−xTixO2−δ (x = 0.2, 0.5, 0.8)-supported Pt catalysts present higher activity in the 200−275 °C range compared to Pt supported on the single metal oxides of CeO2 and TiO2. The Pt/TiO2 presents the lowest catalytic activity (200−350 °C) and at the same time the largest mean primary titania support crystal size (∼25 nm, Table S1, Supporting Information). Panagiotopoulou et al.32 reported that the catalytic performance of Pt/TiO2 was improved when Pt was deposited on a TiO2 carrier with small primary crystallite size (high surface area), due to an improvement in the reducibility of support. This result is in harmony with the fact that the “redox” mechanism largely controls the overall WGS reaction rate on Pt/CeO2,6 Pt/TiO2,26 and the present Pt/Ce0.8Ti0.2O2−δ catalysts,43 where CO adsorbed on Pt reacts with oxygen of support (at the metal−support interface) to form CO2, leaving behind an oxygen vacant site. Water can reoxidize the support to form dihydrogen gas. The 0.5 wt % Pt/Ce0.8Ti0.2O2−δ solid presents the highest CO conversion in the entire temperature range of 200−350 °C (Figure 2). In particular, at the low temperature of 250 °C the CO conversion was increased by a factor of 2.5 and 1.9 E



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the Pt particle size (nm) was also reported over the Pt/CeO26 and Pt/Ce1−xLaxO2−δ12,13 catalytic systems. This dependence was explained after considering the fact that as the Pt particle size decreases Pt atoms with lower coordination number and different local electron density are formed. This in turn is expected to influence the electron density on adjacent oxygen atoms of support. Both Pt sites (adsorption of CO) and M− O−M (M = Ce4+ or Ti4+) (adsorption site of H2O) bond strength at the interface are considered key factors for the control of the rate of WGS via the redox mechanism.6,12,15 On the basis of the above offered discussion, the fact that the integral specific rate, rCO (μmol CO gPt−1 s−1), vs T profile of the 0.1 wt % Pt-loaded catalyst lies above that of the 0.5 wt % Pt-loaded catalyst, as opposed to the case of the CO-conversion profile (Figure 3), can be explained as follows. The estimated WGS reaction rate, rCO (μmol CO gPt−1 s−1), should be regarded as the product of the rate per site of Pt across the metal−support interface (μmol CO μmol sites−1 s−1) multiplied by the number of sites per length of the Pt−support interface (μmol sites cm−1), the length of the Pt−support interface per gram of Pt (cm gPt−1), and the loading of Pt (gPt gcat−1). Thus, all these important kinetic and structural parameters will dictate the specific rate of WGS based on the Pt loading. The superiority of the 0.5 wt % Pt/Ce0.8Ti0.2O2−δ compared to the 0.5 wt % Pt/CeO213 and 0.5 wt % Pt/TiO2 catalysts reflects also its prolonged high CO conversion with time on stream as shown in Figure S4a (Supporting Information). After 50 h on the WGS reaction stream at 325 °C, Pt/Ce0.8Ti0.2O2−δ exhibited significantly less deactivation compared to the Pt/ TiO2 catalytic system. The “carbon” buildup was only 0.17 mg “C” gcat−1 in the case of Pt supported on Ce0.8Ti0.2O2−δ compared to 1.2 mg “C” gcat−1 on Pt/TiO2 (Figure S5b, Supporting Information). The type of this carbon was “amorphous” as revealed by Raman studies (Figure S6, Supporting Information). On the other hand, after 50 h of WGS reaction at 325 °C the flake-like morphology of Ce0.8Ti0.2O2−δ support (Figure S2c, Supporting Information) was preserved as opposed to that of TiO2 support (Figure S2d, Supporting Information). This result may suggest that one of the main reasons for the significant deactivation of Pt/TiO2 observed is due to changes in support structural morphology that might lead to some loss of Pt active sites, in addition to “carbon” deposition. 3.3. Understanding the Catalytic Activity of Pt/ Ce1−xTixO2−δ in Relation to the Surface Physicochemical Characteristics of Support. Figure 4 presents H2-TPR traces (μmol H2 g−1 min−1 vs T) obtained over Ce1−xTixO2−δ (x = 0.0, 0.2, 0.5, and 0.8) solids following calcination (20 vol % O2/ He) at 600 °C for 2 h. It is seen that when Ti4+ at the level of 20 atom % is introduced in the ceria lattice surface reduction of Ce0.8Ti0.2O2−δ starts at ∼300 °C compared to 380 °C in the case of pure ceria, and the whole H2-TPR trace is shifted to lower temperatures, in agreement with the literature.37,46−48 These results are also in agreement with the in situ XRD measurements obtained in 20% H2/He gas flow (Figure S1b, Supporting Information). On the other hand, by further increasing the Ti4+ content to 50 and 80 atom % no progressive trend in both the M−O bond strength (shift of TPR trace) and the concentration of labile oxygen species formed (area of TPR trace) was observed. The Ce0.5Ti0.5O2−δ solid exhibits the highest concentration of labile oxygen (surface and bulk, 552 μmol g−1) and Ce0.2Ti0.8O2−δ the lowest one (308 μmol g−1),

reaction rate for the mixed metal oxide-supported Pt catalysts (4.2−6.0 × 10−12 μmol cm−1 s−1) is larger compared to that obtained on the single metal oxide-supported Pt (1.3−2.8 × 10−12 μmol cm−1 s−1), suggesting that the Ce/Ti ratio significantly affects the reactivity of sites at the Pt−support interface. The different site reactivity at the Pt−TiO2 and Pt− Ce0.8Ti0.2O2−δ interfaces is strongly supported by the present in situ DRIFTS studies, where the bonding of adsorbed CO on the Pt metal and along the Pt−support interface is influenced by the support chemical composition (see Section 3.2.2). Furthermore, transient isotopic work performed on the present Pt catalysts supported on CeO2, TiO2, and Ce0.8Ti0.2O2−δ carriers43 demonstrated that doping of CeO2 with Ti4+ (20 atom %) resulted in a significant increase in the concentration of the active C-pool and H-pool of intermediates formed under steady-state WGS compared to Pt supported on the undoped CeO2. 3.2.2. Effect of Pt Particle Size. The WGS activity in terms of XCO (%) and integral (large conversions) specific rate of CO conversion per gram of Pt metal, rCO (μmol CO gPt−1 s−1), is given in Figure 3 for the 0.1 and 0.5 wt % Pt supported on

Figure 3. Effect of Pt loading (0.1 and 0.5 wt %) on the conversion of CO (XCO, %) and specific integral rate of CO conversion per gram of Pt metal, rCO (μmol CO gPt−1 s−1), as a function of WGS reaction temperature over the x wt % Pt/Ce0.8Ti0.2O2−δ (x = 0.1, 0.5) catalysts.

Ce0.8Ti0.2O2−δ in the 200−350 °C range. It is noted that these Pt loadings correspond to 1.1 and 1.7 nm mean Pt particle size (Table 1). It is seen that the CO conversion is significantly higher over the 0.5 wt % Pt-loaded catalyst in the 250−325 °C range. In particular, at 275 °C an increase by a factor of 2.2 was observed in the XCO (%) by increasing the Pt loading from 0.1 to 0.5 wt %. It is noted that the amount of surface Pt metal per gram of catalyst (Pts gcat−1) for the 0.5 wt % Pt-loaded catalyst is 3.8 times larger than in the case of 0.1 wt % Pt-loaded catalyst. To better understand the effect of Pt loading on the XCO (%) and rCO vs T profiles shown in Figure 3, the following are noted. At 200 °C, the intrinsic initial kinetic rate of WGS expressed per length of the perimeter of the Pt−support interface was estimated to be 1.3 × 10−12 and 6.0 × 10−12 μmol CO cm−1 s−1 for the 0.1 wt % Pt and 0.5 wt % Pt/Ce0.8Ti0.2O2−δ catalysts, respectively (Table 1). Considering the fact that the number of Pt sites per length (no. Pt cm−1) of the Pt−support interface is similar for both catalytic systems, it is concluded that the 0.5 wt % Pt/Ce0.8Ti0.2O2−δ with a mean Pt particle size of 1.7 nm possesses sites of higher intrinsic reactivity (k, s−1) than the 0.1 wt % Pt/Ce0.8Ti0.2O2−δ catalyst, the latter having a smaller Pt particle size (1.1 nm). The dependence of the intrinsic rate of WGS per length of perimeter of the Pt−support interface on F



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Table 2. Amounts (μmol g−1) of Basic and Acid Sites Estimated by CO2-TPD and NH3-TPD Studies, Respectivelya sample

a

Figure 4. H2-TPR traces obtained over Ce1−xTixO2−δ solids (x = 0.0, 0.2, 0.5, and 0.8).

CO2 (μmol g−1)

CeO2

47.0

Ce0.8Ti0.2O2

TM, CO2 (°C)

NH3 (μmol g−1)

TM, NH3 (°C)

27.0

185, 220

15.2

68, 128, 160, 250, 650 83, 800

28.5

Ce0.5Ti0.5O2

20.6

68, 750

35.5

Ce0.2Ti0.8O2

3.6

78, 620

71.6

120, 160, 260 120, 200, 310 130, 210, 330

Peak maximum desorption temperatures (TM, °C) are also provided.

solids. The peak maximum desorption temperatures (TM, °C) of CO2 and NH3 are also reported. On the basis of these results it is seen that all solid supports present weak (low-temperature peak) and strong (high-temperature peak) basic sites. By comparing the TM values observed in the CO2-TPDs, it can be stated that the introduction of Ti4+ into the CeO2 lattice induced a redistribution of basic site strengths. A significant change in the population of strong basic sites is observed after comparing the Ce4+-poor support (Ce0.2Ti0.8O2−δ) with the Ce4+-rich ones (Ce0.5Ti0.5O2−δ and Ce0.8Ti0.2O2−δ). The total concentration of surface basic sites was found to be 15.2, 20.6, 3.6, and 47.0 μmol g−1 for Ce0.8Ti0.2O2−δ, Ce0.5Ti0.5O2−δ, Ce0.2Ti0.8O2−δ, and CeO2 solids, respectively. It has been reported53,54 that TiO2 hardly exhibits CO2 desorption peaks. These results explain well the lower amount of CO2 desorption obtained over the Ce0.2Ti0.8O2−δ solid. Surface basicity promotes carbon gasification through an enhanced water chemisorption: C + 2H2O ↔ CO2 + 2H2.51,52,55−57 This is an intrinsic feature of particular importance for a carbon-free catalytic surface. The Ce0.8Ti0.2O2−δ support was found to exhibit the lowest specific concentration (μmol m−2) of basic sites and the highest specific desorption rates (μmol m−2 min−1) of CO2 in the low-T region of 50−200 °C. High and strong basicity is expected to lead to strong CO2 chemisorption, thus making the surface energy demanding for releasing CO2 into the gas product stream. The basicity, which is related to the oxygen anion charge and to the Mx+−Oy− bond strength, implicitly determines also the reducibility of the metal oxide support and the oxygen anion mobility, important factors for both the predominance of the “redox” mechanism in the WGS and the gasification of “carbon” deposits. Regarding the acidity measurements (NH3-TPDs) conducted over the Ce1−xTixO2−δ solids (x = 0.0, 0.2, 0.5, 0.8), CeO2 exhibits two desorption peaks at 185 and 220 °C, whereas mixed oxides exhibit three desorption peaks (Table 2). The introduction of Ti4+ ions in the ceria lattice led to the formation of new acid sites, not present in the case of pure CeO2. The new acid sites are likely Ti4+ centers (Lewis acid sites).58 By increasing the Ti4+ content from 20 to 80 atom % in the Ce1−xTixO2−δ solid, desorption of NH3 shifted to higher temperatures (Table 2). Manriquez et al.59 reported that the maximum rate of NH3 desorption over TiO2 prepared by the sol−gel method occurred at 310 °C, and the concentration of acid sites was 173 μmol g−1 (1.57 μmol m−2). In the present work, the concentration of surface acid sites was found to be 27.0, 28.5, 35.5, and 71.6 μmol g−1 for CeO2, Ce0.8Ti0.2O2−δ, Ce0.5Ti0.5O2−δ, and Ce0.2Ti0.8O2−δ, respectively (Table 2). It is

whereas Ce0.8Ti0.2O2−δ and CeO2 exhibit intermediate values (418 and 340 μmol g−1, respectively). It was reported that the reducibility of CeO249,50 and TiO232 solids increases after decreasing the corresponding primary crystallite size. This is consistent with the present H2-TPR results (Figure 4), where Ce0.8Ti0.2O2−δ and Ce0.5Ti0.5O2−δ solids exhibit much smaller primary crystal sizes than the other supports (Table S1, Supporting Information). On the other hand, the Ce0.8Ti0.2O2−δ support exhibits the highest rate of H2 uptake (extent of reduction) at T < 500 °C (Figure 4) and the highest CO conversion (Figure 2), which corroborates with the availability of labile oxygen species. The latter can potentially migrate from the support to the Pt metal,6,15,51,52 leading eventually to the oxidation of CO to CO2 over the Pt surface under WGS. This result must be also related to the fact that the Ce0.8Ti0.2O2−δ composition was the only one which led to the formation of a uniform solid solution among the other support compositions investigated (Table S1, Supporting Information). The best reducibility exhibited by the Ce0.8Ti0.2O2−δ support among all the other support compositions at low temperatures is in harmony with that reported by Baidya et al.,37 where the Ti4+ ion (0.74 Å) being much smaller than Ce4+ (0.97 Å) prefers six instead of eight coordination for Ce4+, and since Ti−O bond lengths are shorter than those of Ce−O (2.34 Å) found in ceria, the remaining two oxygen ions move away to longer distances, thus becoming more reactive toward H2. When Pt nanoparticles (dPt = 1.7 nm) are deposited on the Ce0.8Ti0.2O2−δ support, electron transfer from Pt(5d) to Ti(3d) than Pt(5d) to Ce(4f) is favored energetically.37 The extent of this electron charge transfer depends, therefore, on the composition of Ce0.8Ti0.2O2−δ in Ce3+ and Ti3+ defect sites. Considering the fact that on Pt/Ce1−xTixO2−δ catalysts the redox WGS reaction mechanism largely applies,6,15,26,43 water dissociation on M3+−□L−M3+ (M = Ti, Ce) sites is an important step for the reoxidation of support and the formation of H2(g) during WGS. It is, therefore, reasonable to expect that an appropriate ratio of Ce3+/Ti3+ would determine optimum surface electron density on Pt thus an optimum CO−Pt bond strength at the Pt−support interface. The latter is related to the fact that Pt/Ce0.8Ti0.2O2−δ exhibits the highest specific WGS reaction rate expressed per length of the Pt−support interface (Table 1), as a result of an increased Pt site reactivity at the metal−support interface. Table 2 reports the total concentration (μmol g−1) of basic and acid sites estimated by CO2-TPD and NH3-TPD, respectively, for the Ce1−xTixO2−δ (x = 0.0, 0.2, 0.5, and 0.8) G



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Figure 5. In situ DRIFTS spectra recorded in the 2250−1800 cm−1 (a), (c) and 1800−1200 cm−1 (b), (d) range after (A) 3% CO/10% H2O/Ar; (B) 3% CO/10% H2O/40% H2/Ar; and (C) 3% CO/10% H2O/40% H2/10% CO2/Ar gas treatments at 325 °C for 30 min over the 0.5 wt % Pt/ Ce0.8Ti0.2O2−δ (a, b) and 0.5 wt % Pt/TiO2 (c, d) catalysts.

seen that the introduction of 80 atom % Ti4+ in Ce0.2Ti0.8O2−δ caused a dramatic increase in the acidity of the resulting solid compared to pure CeO2. On the other hand, the Ce0.8Ti0.2O2−δ support exhibited the largest percentage of acid sites having weak and moderate acid strength among the other support compositions, and this result correlates with the highest WGS activity exhibited by the corresponding supported Pt catalyst (Figure 2). The high surface acidity exhibited by Ce0.2Ti0.8O2−δ would compete with the oxygen vacant sites for water adsorption, the latter being considered key active sites in the redox mechanism of WGS. This result could explain the lower activity of Pt supported on Ce0.2Ti0.8O2−δ than Ce0.8Ti0.2O2−δ. It is pointed out that there is no correlation between the BET surface area (Table S2, Supporting Information) and the total concentration of acid sites, suggesting that the site density of acid sites is different for each Ce1−xTixO2−δ solid. 3.4. Adsorbed WGS Reaction Intermediates Formed on Pt/TiO2 and Pt/Ce0.8Ti0.2O2. In situ DRIFTS studies were conducted over the 0.5 wt % Pt/TiO2 and 0.5 wt % Pt/ Ce0.8Ti0.2O2−δ catalytic systems under different gas atmospheres. The goal of these studies was to assess the impact of support chemical composition and copresence of reaction products (H2 and CO2) on the chemical composition and surface concentration of the adsorbed species formed during WGS. The reason for choosing these catalytic systems for DRIFTS investigation was the fact that Pt/TiO2 and Pt/ Ce0.8Ti0.2O2−δ exhibited the worst and best catalytic activity, respectively, according to the results shown in Figure 2.

Figure 5 shows in situ DRIFTS spectra recorded in the 2250−1800 (a, c) and 1800−1200 cm−1 (b, d) range after 30 min exposure of the catalysts at 325 °C in different gas atmospheres: (A) 3% CO/10% H2O/Ar, (B) 3% CO/10% H2O/40% H2/Ar, and (C) 3% CO/10% H2O/40% H2/10% CO2/Ar. The infrared bands recorded at 2067, 2047, 2019, and 1974 cm−1 on Pt/Ce0.8Ti0.2O2−δ (Figure 5a) correspond to different kinds of linearly adsorbed carbon monoxide (COL), whereas those at 2180 and 2110 cm−1 correspond to gas-phase CO. The infrared band at 2067 cm−1 corresponds to CO adsorbed on Pt metal crystallites,12 where its integral band intensity is little affected by the presence of 40% H2 or 40% H2/ 10% CO2 in the WGS reaction feed stream. On the other hand, the integral band intensities of the IR bands at 2019 and 1974 cm−1 are largely reduced when H2 or CO2/H2 is present in the feed gas stream. A similar IR band recorded in the 1940−1980 cm−1 region was reported on various other noble metal catalysts dispersed on reducible supports,21 including ceria and titania. This IR band was attributed to CO adsorption on metal sites strongly interacting with the metal oxide support (metal− support interface). In fact, an IR band recorded at 1940 cm−1 was observed on Pt/TiO232 and attributed to terminal CO adsorbed on Pt sites in contact with Ti3+ at the metal−support interface. The structure of this site was later proposed to be Pt−□s−Ti3+ based on 18O-transient isotopic experiments.26 The very small infrared band recorded at 1837 (Figure 5a) and that at 1741 cm−1 (Figure 5b) correspond to two different kinds of bridged adsorbed carbon monoxide, COB. It was H



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cm−1 is reduced significantly and is shifted to higher wavenumbers when H2 or H2/CO2 was present in the CO/ H2O feed gas stream. On the other hand, in the case of Pt/ Ce0.8Ti0.2O2−δ, the corresponding IR bands shifted to lower wavenumbers (Figure 5a), and their intensities were not reduced to the extent appearing in the Pt/TiO2 catalyst. This demonstrates the important role of support in determining Pt− CO bond strengths due to metal−support electronic interactions as previously discussed. Furthermore, the IR bands corresponding to adsorbed COB (1837 and 1736 cm−1) and to the vibrational modes of O−C−O in formates, carbonates, or carboxylates (1650−1300 cm−1) appear with progressively increased intensity when the reaction mixture is switched from CO/H2O to CO/H2O/H2 or CO/H2O/H2/ CO2 (Figure 5d). This relative increase was found to be larger on Pt/TiO2 than Pt/Ce0.8Ti0.2O2−δ catalyst (compare Figures 5b and 5d). This is an important result which shows that the Ce0.8Ti0.2O2−δ support is less prone to the accumulation of carbonates (inactive species of WGS)6,12,13,15 under WGS, where carbonates potentially block active catalytic sites (see Figure S5a, Supporting Information).

suggested21 that the latter IR band is associated with COB at the metal−support interface, where COB interacts with the support via either the carbon (bonding with On− of MO) or the oxygen atom (bonding with Mn+ of MO, where MO = metal oxide). The IR band recorded at 1741 cm−1 (COB) appears more intense when H2 and CO2 were cofed in the WGS feed gas stream (CO/H2O); practically no effect was seen when only H2 was fed in the WGS feed gas stream. These results suggest that the reverse WGS reaction (CO2 + H2 ↔ H2O + CO) promotes the formation of this type of adsorbed COB at the Pt−support interface. The infrared band observed at 2067 cm−1, and which corresponds to COL on Pt metal crystallites, shifts to lower wavenumbers (2057−2052 cm−1) when H2 or H2/CO2 are present in the WGS feed gas stream. This could be explained on the basis of reduced dipole−dipole interactions induced by the lower CO surface coverage (θCO) in the presence of adsorbed H-s, thus leading to a higher binding energy for the adsorbed CO. Also, the presence of H2 in the WGS feed gas stream might be expected to cause reduction of Ce0.8Ti0.2O2−δ support (Figure 4), thus leading to a defect enrichment along the Pt−support interface. A higher concentration of defects in the vicinity of Pt crystallites might therefore be expected, facilitating the charge transfer for highly dispersed Pt,60 thus altering the Pt site reactivity at the Pt−support interface. The latter parameter was argued for explaining the specific activity order of the various Pt/Ce1−xTixO2−δ catalysts (see Section 3.2.1). The IR band at 1643 cm−1 is due to the bending mode of the adsorbed water molecule. This band was increased when H2 or H2/CO2 was present in the WGS reaction feed stream due to the reverse WGS reaction. Two of the most intense IR bands shown in Figure 5b are recorded at 1580 and 1371 cm−1, which are attributed to the O−C−Oas and O−C−Os vibrational modes of formate (HCOO−).32,61 The IR band at 1513 cm−1 corresponds to carboxylate species,62 whereas those at 1463 and 1662 cm−1 correspond to carbonate and carboxylic acid type species.62 The IR band at 1417 cm−1 is due to bicarbonate-type chemisorbed carbon dioxide61,63 and that at 1235 cm−1 to bridged carbonate.62,63 The fact that in the presence of CO2 in the WGS feed gas stream these IR bands were significantly larger in intensity than those formed under CO/H2O, where CO2 is also formed, strongly supports these assignments. It is worth mentioning that based on transient isotopic studies conducted on Pt/CeO 2 6 and Pt/Ce 1−x Ti x O 2−δ 43 formate species (HCOO−) were considered active reaction intermediates in the 250−300 °C range. The IR band recorded at 1691 cm−1 was formed only when 40% H2/10% CO2 was cofed in the WGS feed gas stream. This IR band should be rather due to a carboxylic acid type of adsorbed species, the surface coverage of which was largely enhanced in the presence of H2/CO2 as compared to the ordinary WGS feed gas stream; a band shift (1691−1662 cm−1) can be justified due to the surface coverage effect. Figure 5c, d presents in situ DRIFTS spectra recorded in the 2250−1800 cm−1 (c) and 1800−1200 cm−1 (d) range after 30 min exposure of the 0.5 wt % Pt/TiO2 catalyst at 325 °C to the same feed gas compositions A−C used in the case of Pt/ Ce0.8Ti0.2O2−δ. The IR bands recorded at 2074 and 2014 cm−1 correspond to adsorbed COL on reduced Pt, whereas that at 1837 cm−1 corresponds to bridged adsorbed COB. It is observed that the intensity of the IR band recorded at 2074

4. CONCLUSIONS Pt (0.1 and 0.5 wt %) loaded on Ce1−xTixO2−δ (x = 0, 0.2, 0.5, 0.8 and 1.0) carriers synthesized by the citrate sol−gel route was tested toward the WGS reaction in the 200−350 °C range. The best performed synthesized catalyst was that of 0.5 wt % Pt/Ce0.8Ti0.2O2−δ demonstrating the pronounced effect of support chemical composition (Ce/Ti atom ratio) and Pt particle size on the specific kinetic rate of WGS and catalyst’s stability with time-on-stream. The Ce0.8Ti0.2O2−δ support of this particular catalytic system holds an appropriate combination of intrinsic characteristics, such as enhanced reducibility at low temperatures and relatively low acidity and basicity associated with weak to medium acid or basic strength sites. These features are appropriate for the enhancement of WGS reaction rate through an increased availability of labile oxygen (On−/ OH), sites of high reactivity toward CO along the Pt−support interface and within a reactive zone around Pt nanoparticles, and basic sites that promote the steam gasification of the “carbon” formed. The above features, as proven herein, play a significant role in the establishment of appropriate surface coverage and chemical bond strength for Pt−CO (DRIFTS studies) and surface sustainability in terms of self-cleaning from “carbon” deposits (DRIFTS, Raman, and TPO studies). In addition, the 0.5 wt % Pt/Ce0.8Ti0.2O2−δ catalytic system exhibited morphology preservation and hydrothermal stability after 50 h of continuous WGS reaction.

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ASSOCIATED CONTENT

* Supporting Information S

Catalyst structural, textural, and morphological properties; Pt dispersion and mean particle size estimated via H 2 chemisorption/TPD measurements; characterization of “coke” deposits; and in situ DRIFTS and Raman studies. This material is available free of charge via the Internet at http://pubs.acs.org.

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AUTHOR INFORMATION

Corresponding Authors

*Tel.: +357 22 892776. Fax: +357 22 892801. E-mail efstath@ ucy.ac.cy (A.M.E.). I



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*Tel.: +971 02 401 8211. Fax: +971 02 447 2442. E-mail [email protected] (K.P.).

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Notes

The authors declare no competing financial interest.

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ACKNOWLEDGMENTS The European Regional Development Fund, the Republic of Cyprus, the Research Promotion Foundation of Cyprus, and the Research Committee of the University of Cyprus are gratefully acknowledged for their financial support through the project TEXNO/0308(BE)/05. S.B. acknowledges financial support from the COST Action CM1104.

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