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CeO2 catalysts
Journal of Energy Chemistry 25 (2016) 1051–1057
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Identification of relevant active sites and a mechanism study for reverse water gas shift reaction over Pt/CeO2 catalysts Xiaodong Chen a,b,1, Xiong Su a,1, Binglian Liang a,b, Xiaoli Yang a,b, Xinyi Ren a, Hongmin Duan a, Yanqiang Huang a,∗, Tao Zhang a a b
State Key Laboratory of Catalysis, Dalian Institute of Chemical Physics, Chinese Academy of Sciences, Dalian 116023, Liaoning, China University of Chinese Academy of Sciences, Beijing 100049, China
a r t i c l e
i n f o
Article history: Received 19 August 2016 Revised 18 October 2016 Accepted 19 October 2016 Available online 16 November 2016 Keywords: RWGS reaction Pt/CeO2 catalyst Formate intermediate Mechanism
a b s t r a c t Reverse water gas shift (RWGS) reaction can serve as a pivotal stage in the CO2 conversion processes, which is vital for the utilization of CO2 . In this study, RWGS reaction was performed over Pt/CeO2 catalysts at the temperature range of 20 0–50 0 °C under ambient pressure. Compared with pure CeO2 , Pt/CeO2 catalysts exhibited superior RWGS activity at lower reaction temperature. Meanwhile, the calculated TOF and Ea values are approximately the same over these Pt/CeO2 catalysts pretreated under various calcination conditions, indicating that the RWGS reaction is not affected by the morphologies of anchored Pt nanoparticles or the primary crystallinity of CeO2 . TPR and XPS results indicated that the incorporation of Pt promoted the reducibility of CeO2 support and remarkably increased the content of Ce3+ sites on the catalyst surface. Furthermore, the CO TPSR-MS signal under the condition of pure CO2 flow over Pt/CeO2 catalyst is far lower than that under the condition of adsorbed CO2 with H2 -assisted flow, revealing that CO2 molecules adsorbed on Ce3+ active sites have difficult in generating CO directly. Meanwhile, the adsorbed CO2 with the assistance of H2 can form formate species easily over Ce3+ active sites and then decompose into Ce3 + –CO species for CO production, which was identified by in-situ FTIR. © 2016 Science Press and Dalian Institute of Chemical Physics, Chinese Academy of Sciences. Published by Elsevier B.V. and Science Press. All rights reserved.
1. Introduction The accelerated emission of CO2 due to the combustion of fossil fuels has triggered a series of global concerns (global warming, glaciers melting, ocean acidification, and so on), which seriously threatened the sustainable development and survival of our human beings [1,2]. Catalytic reduction of CO2 to value-added commodity is a promising way that can offer a solution to the reuse of CO2 as environmental friendly carbon resource, and is necessary for the development of long term fossil–C industry [3–8]. However, the activation of CO2 is a great challenge because of its inherent thermodynamic inertness, with the standard Gibbs free energy (G0 ) of −394.38 kJ/mol. Hydrogen, produced from water splitting through solar photon or renewable electric driving energy, is considered as an ideal intermediate energy carrier for the conversion of CO2 [9,10]. Many hydrogenation processes of CO2 to produce liquid carbon-based fuels, such as HCOOH, CH3 OH, and hydrocarbons,
∗
1
Corresponding author. Fax: +86 411 84685940. E-mail address: [email protected] (Y. Huang). These authors contributed equally to this work.
are the preferred options [11,12]. For most of these processes, a direct concern of transforming CO2 into CO (known as the RWGS reaction) is one of the most promising processes in the CO2 resource utilization, because the produced CO can be applied widely as a pivotal resource for C1 chemistry in the sustainable coal chemical industry [13–15]. Besides that, RWGS reaction is an essential intermediate step of many vital CO2 hydrogenation processes [16]. Kim et al. reported that the CO2 hydrogenation to light olefins was associated with RWGS reaction and then followed by Fischer–Tropsch synthesis [17]. Hu et al. exploited the CAMERE process of producing methanol from CO2 , which also involved the intellectual RWGS process [18]. Therefore, it is of great importance to separately investigate the RWGS reaction which not only facilitates the CO2 conversion but also helps looking deep insight into the CO2 hydrogenation reaction mechanisms. In recent studies, great efforts have been dedicated to the design of reducible oxide supported metal catalysts, which often show high activity for CO2 activation than representative mixed oxides or inert oxide supported metal catalysts, due to their structures with dual functionalities of the supported metal and adjacent vacancy sites [19–24].
http://dx.doi.org/10.1016/j.jechem.2016.11.011 2095-4956/© 2016 Science Press and Dalian Institute of Chemical Physics, Chinese Academy of Sciences. Published by Elsevier B.V. and Science Press. All rights reserved.
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A number of reports have indicated that the abundant oxygen vacancies over reducible supports are in favor of the activation of CO2 from C=O bond scission [13,19,25,26]. In addition, an effective site for CO desorption, either at the metal site or metal-support intersection region, is vastly important [27,28]. As RWGS is a reversible reaction, Pt/CeO2 catalysts, which are active in the WGS reaction, should be a promising candidate for the RWGS reaction [29–32]. In this study, we report that Pt/CeO2 catalysts exhibited highly desired activities for RWGS reaction. We identified that the TOF values of CO yields were approximately the same over these Pt/CeO2 catalysts, irrespective of their morphologies (the anchored Pt particles and primary crystallinity of CeO2 support). Meanwhile, combined with in-situ diffuse reflectance infrared Fourier transform spectroscopy and mass spectrometric analysis, we found that the RWGS activity was correlated with the reducibility of Pt/CeO2 catalysts. Ce3+ sites, in combination with the adjacent Pt particles, were recognized as the active sites for the RWGS reaction. 2. Experimental 2.1. Chemicals H2 PtCl6 (37 wt% aqueous solution) and Ce(NO3 )3 (≥ 99.0%, AR grade) were purchased from Sinopharm Chemical Reagent Co., Ltd. Na2 CO3 (≥ 98.8%, AR grade) and NaOH (≥ 96.0%, AR grade) were purchased from Tianjin Kermel Chemical Reagent Co., Ltd.. All chemicals were used as received without further purification. Deionized water was used throughout the experiments. 2.2. Catalyst preparation The Pt/CeO2 catalysts were synthesized using a co-precipitation method as described in previous work [23]. In a typical synthesis procedure, the calculated amounts of H2 PtCl6 and Ce(NO3 )3 were dissolved in 500 mL deionized water. Na2 CO3 and NaOH (with a molar ratio of 1:1) were dissolved in another 500 mL deionized water to make a mixed solution with the total molar concentration of 2.0 mol/L. The Pt content in CeO2 was calculated to be 1%. The hydrolysis of metal salts was achieved by controlled slowly dropwise of the two solutions into deionized water with stirring, until a pH value between 9 and 10 was achieved. The precipitates were stirred for 6 h and then aged overnight at ambient temperature. Then, the precipitates were filtered and washed several times with deionized water until the pH value was reached less than 7.5. Next, the samples were dried at 80 °C for 12 h and calcined at 500 °C, 650 °C, or 800 °C for 4 h, which were then reduced online in the reactor with the feed gas. Samples with different pretreatment conditions are denoted as PC–M–N for abbreviation in the following. PC represents the co-precipitated 1% Pt/CeO2 catalyst, M and N denotes the calcination and reduction temperatures of the samples, respectively. Bare CeO2 catalyst was prepared using similar procedures but without the addition of Pt source. The CeO2 was calcined in air under 500 °C for 4 h. 2.3. Characterization The structures of samples were measured by X-ray diffraction on a PANalytical X’Pert-Pro powder X-ray diffractometer with Cu Kα monochromatized radiation (λ = 0.1541 nm). Diffraction peaks were recorded in a 2θ range between 10° and 80°, with a scanning speed of 4 °/min. The specific surface area was examined on an ASAP 2460 (Micromeritics), and calculated by BET (Brunauer– Emmett–Teller) equation. Before analyzing, the samples were degassed under vacuum at 110 °C for 1 h and 300 °C for 5 h. The H2 chemisorptions over these Pt/CeO2 catalysts were measured by H2 pulse adsorption method on a Micromeritics Au-
toChem II 2920 automated catalyst characterization system. Typically, the samples were reduced under different temperatures as implemented in the activity tests. Then, the temperature of the system was increased by 10 °C and the freshly reduced catalysts were flushed with Ar for 30 min to remove the adsorbed hydrogen on the catalyst surface. Afterwards, pure H2 was injected until saturation when the temperature was cooled to 50 °C. The amount of H2 adsorption was calculated from the pulse results monitored by TCD detector. Temperature-programmed reduction (TPR) experiments were carried out on a Micromeritics AutoChem II 2920 automated catalyst characterization system. Prior to the measurement, about 50 mg of the catalyst bullets (20–40 mesh) were loaded into a U-shape quartz reactor and pretreated with Ar flow at 300 °C for 30 min to remove the adsorbed water. After cooling to 50 °C, 10 vol% H2 /Ar mixed gas was passed through the catalyst bed and then heated to 900 °C with a ramping rate of 10 °C/min. TPSR-MS experiments were performed in a U-shape quartz fixed-bed micro reactor at atmospheric pressure using 0.2 g PC-500 at 300 °C. The reactor was placed in a vertical electrical furnace for heating and the effluent products were monitored online with a PRISMA quadrupole mass spectrometer (MS) system. A K-type thermocouple was located inside the quartz reactor for monitoring reaction temperature. Switching procedures between the 5% H2 /He and 5% CO2 /He gases were used during the experiments. The total gas flow rate was kept at 20 mL/s and the switching stepwise was 20 min. Between the reduction and re-oxidation periods of the TPSR, the reactor was flushed with pure helium for 20 min to prevent the mixing of H2 and CO2 . High-resolution scanning transmission electron microscopy (HR-STEM) images were recorded on a JEM-2100F field emission electronic microscope equipped with STEM dark-field (DF) detector, which was operated at an acceleration voltage of 200 keV. Samples for field emission TEM measurements were prepared by suspending an ultrasonically dispersed powder in ethanol and by placing a drop of the suspension onto a Cu grid. X-ray photoelectron spectra (XPS) were determined on an ESCALAB 250Xi apparatus to obtain the surface composition and the binding energies of the catalysts. Mg Kα radiation at an energy scale calibrated versus adventitious carbon (C 1s peak at 284.6 eV) was used. Fourier transform infrared (FTIR) experiments were carried out using a BRUKER Equinox 55 spectrometer, equipped with a diffuse reflectance (DRIFT) and an MCT detector with a resolution of 4 cm−1 . The inlet gas into the cell was directly connected to a flow system, equipped with the mass flow controller and a set of valves that allow selection and control of feed gas composition in a typical experiment. The catalysts were pretreated under He flow at 300 °C for 60 min and then reduced in a flow of 5% H2 /He at 300 °C for 60 min. The reduced catalyst was then exposed to Ar at 300 °C for 30 min. After the pretreatment, the reference background spectrum of the catalyst was detected and subtracted automatically from the subsequently recorded spectra. Prior to measurements, the feed gas, which was composed of H2 :CO2 :N2 (45:45:10, v:v:v), was introduced into the in-situ cell at a total flow rate of 30 mL/min. 2.4. Activity test The RWGS catalytic activities over Pt/CeO2 catalysts were evaluated in a continuous-flow fixed-bed reactor under ambient pressure. The flow of feed gas was controlled by DZ47-63 mass flow controllers. The quartz reactor (11 mm, i.d.) was heated in a furnace which was connected to a proportional–integral–derivative (PID) temperature controller. The temperature of reactor was measured and controlled with a K-type thermocouple that was located
X. Chen et al. / Journal of Energy Chemistry 25 (2016) 1051–1057
2 Theta (degree) Fig. 1. XRD patterns of the PC catalysts under different pretreatment conditions over the Pt/CeO2 catalysts.
at the central axis of the reactor. Prior to the experiments, catalyst pellets (20–40 mesh) with the calculated amount of metal loading were packed into the reactor and then purged with a He flow (30 mL/min) under 300 °C for 30 min. Subsequently, the catalysts were pre-reduced at 400 °C for 1 h by H2 with a flow rate of 10 mL/min (denotes PC-50 0-40 0, PC-650-40 0, and PC-80 0-40 0). During the experiments, the reaction temperature range was set to 20 0–50 0 °C and the feed gas (CO2 /H2 /N2 = 45%/45%/10%) was introduced to the catalyst bed at a flow rate of 50 mL/min (GHSV of 30,0 0 0 mL/gcat /h). The ice-bath unit was used to remove the water vapor of the effluent gas from the reactor and then analyzed on line with an Agilent 7890B gas chromatograph with a TDX-01 column connected to a TCD detector. The catalytic performance was evaluated by the conversion of CO2 based on different concentrations between inlet and outlet, which is defined as:
Conversion (CO2 % ) =
CO2 (in ) − CO2 (out) ×
N2 (in) N2 (out)
CO2 (in)
× 100%
The selectivity of CO is defined as:
Selectivity (CO% ) =
CO(out ) × CO(out ) ×
N2 (in ) CO(in )
N2 (in ) N2 (out )
+ CH4 (out ) ×
N2 (in ) CH4 (in )
× 100%
The CO yield is calculated as follows: Yield (CO%) = Conversion (CO2 %) × Selectivity(CO%)/100 × 100% Where, CO2 (in), CH4 (in) and N2 (in) are the concentrations of CO2 , CH4 and N2 at inlet; CO2 (out), CH4 (out) and N2 (out) are the concentrations of CO2 , CH4 and N2 at outlet, respectively. During the entire catalytic testing, CH4 or other by-products were not detected. Meanwhile, the turnover frequencies (TOF) of CO2 conversion were defined as: n = moles of converted CO2 /(number of active sites determined by H2 chemisorption × time of reaction). 3. Results and discussions Fig. 1 shows the powder XRD patterns of Pt/CeO2 samples with different pretreatment conditions. There were no reflections of metallic Pt in the samples PC-50 0-40 0, PC-650-40 0, indicating
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that the anchored Pt nanoparticles with mean size less than 2 nm were beyond the detection limit of the XRD technique. When the prepared sample was calcined at 800 °C, we observed that the XRD pattern of PC-80 0-40 0 showed characteristic broadened signals of metallic Pt at 2θ = 39.8° and 46.3°. These results were consistent with the average Pt particle size counted from HAADF-STEM images based on the particle size distribution (see Fig. 2 and Table 1). The statistical mean size of Pt nanoparticles in PC-650-400 samples was 1.8 nm; whereas the average particle size of Pt over PT-80 0-40 0 was enlarged to 2.3 nm. The BET surface areas collected from N2 physical adsorption and the primary crystalline size of CeO2 substrate estimated from the XRD patterns of these corresponding Pt/CeO2 catalysts are also listed in Table 1. The conditioned physical structures of these catalysts showed that a variation in the reduced surface area (from 54.0 to 0.5 m2 /g) and enlarged primary crystalline size of CeO2 (from 8.0 to 19.5 nm) were achieved during the increased tougher heat processing of calcination or reduction treatments. These results can be further verified by the representative HAADF-STEM images of these corresponding Pt/CeO2 catalysts shown in Fig. 2(a)–(c). The H2 -TPR profiles of the bare CeO2 and Pt/CeO2 catalysts pretreated at different calcination temperatures were displayed in Fig. 3. Interestingly, the H2 consumption volumes of the first reduction peak over all the Pt/CeO2 catalysts were quantified as 494, 205, 128 μmol/gcat , exceeding the required amount of H2 consumption for the reduction of PtO2 phase (102 μmol/gcat ). The exceeded amount of H2 consumption for PtO2 reduction suggested that partial reduction of surface CeO2 was included in the scope of the first reduction peak. As increasing the calcination temperature of Pt/CeO2 catalysts, the crystallinity of CeO2 became more integrated with the reduced catalyst surface area, which caused the declined reduction degree of the reducible CeO2 phase. Moreover, the temperature of the first reduction peak tended to decrease through Pt incorporation, which was caused by H2 spillover effect. Interestingly, as the calcination temperature increased, those PC catalysts showed a large decline in the initiate temperature of the first reduction peak. The possible reason is that a weaker interaction between large Pt particles and CeO2 support was induced under higher calcination temperature and thus the reducibility of PtO2 was enhanced [33]. To be noted, the second peak in H2 -TPR profiles represents the reduction of bulk CeO2 phase. However, since it is not directly correlated with the catalyst activity, we will not discuss in detail here. In order to quantify the surface compositions of these PC catalysts, XPS measurements were conducted. The curve fitting results were shown in Fig. 4 and the deconvoluted data were listed in Table 2. Normally, the O1s spectra showed two obvious characteristic peaks, as shown in Fig. 4(a). The peak at 531.0–531.4 eV was attributed to the surface-adsorbed oxygen (O2 2− or O− ). The other one at lower binding energy of 529.0–530.0 eV was assigned to the lattice oxygen (O2− ). The percentage of the two categories of oxygen atoms were almost the same, even if with the Pt incorporation. Moreover, the Ce3d spectra were fitted with eight peaks, which were referred as Ce3+ and Ce4+ components. As shown in Fig. 4(b), the peaks signed as v, v”, v”’ belong to the 3d5/2 core level of Ce4+ and those labeled as u, u”, u”’ were assigned to that of Ce3+ . The other double peaks also correspond to the presence of Ce3+ [34]. Compared to pure CeO2 , the component of Ce3+ sites increased obviously over the PC catalysts, especially in PC-50 0-40 0. This result indicated that the addition of Pt apparently increased the content of surface Ce3+ species over these PC catalysts. From the deconvoluted results shown in Table 2, the detected surface composition of Pt declined with the rise of calcination temperature, which was attributed to the reduction of Pt dispersion caused by the growth of Pt particles.
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X. Chen et al. / Journal of Energy Chemistry 25 (2016) 1051–1057
Frequency (%)
50 PC-650-400
40
AD = 1.81 0.36 nm 30 20 10 0 1
Frequency (%)
50
2 3 4 Diameter (nm)
5
PC-800-400
40
AD = 2.29 0.53 nm
30 20 10 0 1
2 3 4 Diameter (nm)
5
Fig. 2. HAADF-STEM images and statistical particles distributions of the Pt/CeO2 catalysts: (a) PC-50 0-40 0, (b) PC-650-400 and (c) PC-800-400. Table 1. Physicochemical characterizations of the PC catalysts with different pretreatment conditions and the corresponding CO2 hydrogenation performance in the RWGS reaction. Reaction conditions: 0.2 g catalyst; CO2 /H2 /N2 = 45/45/10 (v/v/v); reaction temperature is 300 °C and the feed gas flow rate is 50 mL/min. Catalysts
Crystalline size of CeO2 (nm)
BET surface area (m2 /g)
H2 chemisorption volume (μmol/g)
Average Pt particle size (nm)
CO2 conversion (%)
PC-50 0-40 0 PC-650-400 PC-80 0-40 0
8.0 15.8 19.5
54.0 7.5 0.5
51.3 28.5 21.0
-a 1.81 2.29
6.7 4.1 3.2
a
TOF values (10−1 s−1 ) 0.560 0.550 0.555
Not determined.
Table 2. The deconvoluted XPS results of surface composition of CeO2 and Pt/CeO2 catalysts. Catalyst
CeO2 -50 0-40 0 PC-50 0-40 0 PC-650-400 PC-80 0-40 0
Pt (at%)
– 0.46 0.17 0.14
Ce (at%)
O (at%)
Ce3+
Ce4+
Osur + Oads
Olatt
2.60 6.44 5.53 5.06
21.1 21.5 19.9 20.1
37.4 42.4 42.4 38.9
38.9 29.2 32.0 35.8
The catalytic activity and the corresponding calculated Arrhenius plots of RWGS reaction over the bare CeO2 and Pt/CeO2 catalysts as a function of reaction temperature were shown in Fig. 5. Fig. 5(a) shows that the CO2 conversion increased monotonically as increasing the reaction temperature. A preferable catalytic performance over these 1% Pt/CeO2 catalysts was achieved when the catalyst was pretreated under lower calcination temperature. The PC-50 0-40 0 catalyst exhibited the best catalytic performance than
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Fig. 3. H2 -TPR profiles of the bare CeO2 and Pt/CeO2 catalysts.
any other catalysts due to its high Pt dispersion. Interestingly, the calculated TOF values were nearly the same over all these Pt/CeO2 catalysts (about 0.055 s−1 , see Table 1), indicating that the RWGS reaction activity seems to be not influenced by the morphologies of catalysts, i.e., the anchored Pt particles and the primary crystallinity of CeO2 . We also noticed that RWGS reaction occurred over the bare CeO2 catalyst, but only became observable at the reaction temperature higher than 400 °C, suggesting that oxygen vacancies generated over the CeO2 catalyst surface could be regarded as active sites for CO formation. In addition, a series of Arrhenius plots of CO yield were calculated and shown in Fig. 5(b), which displayed reasonable linearity in the reaction temperature range. To obtain more reliable assessments, the CO2 conversion was confined to an extent of no more than 10% in order to eliminate the effect of mass transfer. Based on these calculated slopes, the apparent activation energies (Ea ) of CO yield were estimated. The Ea values of CO yield over Pt/CeO2 catalysts were approximately the same (∼68 kJ/mol), which were prominently lower than that over the bare CeO2 catalyst (∼77 kJ/mol). According to the report of Philip et al. [13], the catalytic activity of Pt/TiO2 catalyst in the RWGS reaction largely depends on the reducibility of TiO2 support. Lu et al. also proposed that the portion of the reducible support that with good intimacy to the metal particles could be in highly reduced state, which is thought to be effective in the reduction of CO2 to CO [24]. Similarly, the variations in the activities of Pt/CeO2 catalysts can be explained from the TPR data (see Fig. 3), which shows that the incorporation of Pt and the increase of calcination temperature could enhance the reducibility of CeO2 support. Therefore, the easily generated oxygen vacancy sites over the catalyst surface could accelerate the activation of CO2 in RWGS reaction. To determine the reaction route of RWGS process, we performed transient TPSR-MS experiments on Pt/CeO2 catalyst by detecting the products under input gas-flow switching procedures. The intensities of the MS signals of the effluent gases from the switched gas-flow over the PC-500 sample were shown in Fig. 6(a). During the first experiment stage, the feed gas was alternately switched by purging H2 , He and CO2 streams at 300 °C. When the gas flow was switched from He to CO2 over the reduced Pt/CeO2 sample, only a weak signal of CO was detected instantaneously as the CO2 molecules arrived at the sample. But this signal disappeared quickly to a steady level after 2 min. This result supports the fact that the redox mechanism involved in the RWGS reaction over the Pt/CeO2 catalyst, but with only a small extent. In the fol-
Fig. 4. XPS spectrum of the bare CeO2 and Pt/CeO2 catalysts.
lowing test, the gas stream was switched to He flow for 20 min in order to remove the residual CO2 gas in the pipe line and the physically adsorbed species on the catalyst surface. After that, the gas-flow was switched to H2 and the effluent gas signals were recorded. We observed a surge appearance of the CO signal but it disappeared quickly. Within a short time, the CO signal emerged again and dropped to a steady level in 20 min, as seen from Fig. 6(b). The generated CO product in this stage was prominently more than the amount produced during the first stage. Meanwhile, the first sharp peak was attributed to the desorption of CO from Ce3+ sites or Pt particles, because H atoms could preferentially adsorb on those sites in the flowing H2 atmosphere. The second broad peak was assigned to CO generated from the dissociation of intermediates which were formed through the H2 assisted adsorption of CO2 over Pt/CeO2 catalyst surface. These results further indicated that CO2 adsorbed over the reduced Pt/CeO2 catalyst have difficulty in generating CO product directly; whereas the activated CO2 can be facilely decomposed into CO via H2 -assisted pathway. Therefore, although the redox mechanism
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X. Chen et al. / Journal of Energy Chemistry 25 (2016) 1051–1057 0.20
CH4
Concentration (%)
0.15
(a)
CO H2
0.10
0.05
0.00
-0.05
-0.10 8000
8100
8200
8300
8400
8500
Real time (s)
0.20
CH4
(b)
CO2
Concentration (%)
0.16
CO
0.12
0.08
0.04
1/T (1/K) Fig. 5. (a) CO2 conversions and (b) Arrhenius plots of CO2 hydrogenation reactions over the bare CeO2 and Pt/CeO2 catalysts as a function of reaction temperature. Reaction conditions: 0.1 g catalyst loading; CO2 /H2 /N2 = 45/45/10 (v/v/v); feed gas flow rate is 50 mL/min.
and dissociation mechanism coexisted in the RWGS reaction, the dissociation mechanism, involving the formation of intermediates, has been recognized as the major route for CO production in the RWGS reaction. To get further insight into the reaction mechanism and analyzing the formed intermediates during RWGS reaction, in-situ reflectance FTIR experiments were conducted. The attenuated reflectance FTIR spectra of RWGS reaction over the PC-500 catalyst were collected after its exposure to the feed gas (CO2 /H2 /N2 = 45/45/10, v/v/v). After a reaction period of 1 h in the feed gas at 300 °C, the intensity of collected signals reached stable. The incoming gas stream was then switched to pure He flow for scrubbing and the reflectance spectra were recorded at once, see Fig. 7(a). At the very beginning, the peaks at ν = 1772 and 2045 cm−1 were assigned to the bridge- and linear-bonded CO on the surface of reduced Pt particles. Another peak at ν = 1940 cm−1 was ascribed to the linearly adsorbed CO at the interfacial sites of Pt and CeO2 . The band situated at 2130 cm−1 was consistent with the assignment of Ce3 + –CO, where the CO is adsorbed on Ce3+ sites [24,35–37]. Moreover, the peaks at 2830 and 1600 cm−1 were assigned to the
0.00 12500
13000
13500
14000
14500
15000
15500
Real time (s) Fig. 6. The evolution of MS signals of CO product in the gas-flow switching experiments over the pre-reduced PC-500 catalyst: (a) when He flow was switched to CO2 flow and (b) when He flow was switched to H2 /He after the treatment procedure of (a). Treatment conditions: 0.2 g catalyst; reaction temperature is 300 °C; the feed gas flow rate is 20 mL/min.
formate and carbonate species, respectively. When purged with He flow, the intensity of the peaks attributed to the linearly adsorbed CO at the interfacial sites and the Pt–CO species attenuated slowly, while the signals of the formates and CO adsorbed on Ce3+ sites attenuated quickly. This means that the generated CO was strongly adsorbed on these interfacial or metal sites even in the flow of inert gas. Meanwhile, the formed formate species and CO adsorbed on Ce3+ sites were weakly bonded. We further detected IR signals by changing the input gas stream to H2 –He flow. Again, the intensity of the peaks attributable to the linearly adsorbed CO at the interfacial sites and the Pt–CO species attenuated slowly. However, the signals of formate species and CO adsorbed on Ce3+ sites emerged again. This can be interpreted as that, when purged in the H2 flow, formates firstly formed on the oxygen vacancies located at the vicinity of Pt particles. Then, these species decomposed into CO and adsorbed weakly on Ce3+ sites,
X. Chen et al. / Journal of Energy Chemistry 25 (2016) 1051–1057
enhanced and thus oxygen vacancies were generated more easily, which accelerated the activation of CO2 . In addition, through insitu FTIR and TPSR-MS experiments, we identified that the redox mechanism and dissociation mechanism coexisted in the RWGS reaction over the Pt/CeO2 catalyst. The CO2 molecules adsorbed on Ce3+ active sites have difficult in generating CO directly (surface redox mechanism). Meanwhile, the adsorbed CO2 with H2 -assisted pathway facilitates the formation of formate species as main intermediates (dissociative mechanism) and then decomposes into CO.
(a) 0.05 2045
Absorbance (a.u.)
1772 1940
He
2130
2830
Acknowledgments 1600
4000
3500
3000
2500
2000
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The authors acknowledge the National Natural Science Foundation of China (nos. 21476226 and 21506204), National Key Projects for Fundamental Research and Development of China (2016YFB0600902), the Strategic Priority Research Program of the Chinese Academy of Sciences (XDB17020400) and the Youth Innovation Promotion Association CAS for financial support. 1000
References
-1
Wavenumber (cm )
(b)
Absorbance (a.u.)
0.05
2045 1772 1940
H2
2130
2830
1600
4000
3500
3000
1057
2500
2000
1500
1000
-1
Wavenumber (cm ) Fig. 7. Stacked in-situ FTIR spectra collected on the PC-500 catalyst. (a) The first purging process in pure He gas with a flow rate of 30 mL/min was conducted after the catalyst was exposed to a feed gas (CO2 /H2 /N2 = 45/45/10, v/v/v) and reacted at 300 °C for 1 h; (b) in the second stage, the catalyst was scrubbed in the 5% H2 –He with a flow rate of 30 mL/min at 300 °C.
which accelerated the generation of CO over the Pt/CeO2 catalyst in a H2 -assisted reaction pathway. These results also indicated that formates only presented on the surface of reduced ceria (Ce3+ active sites) and worked as the main intermediates for CO formation. 4. Conclusions In summary, RWGS reaction was performed over Pt/CeO2 and bare CeO2 catalysts in the temperature range of 20 0–50 0 °C. The activation energies of CO yields over Pt/CeO2 catalysts were prominently lower than that over pure CeO2 catalyst. The calculated TOF values were approximately the same over these Pt/CeO2 catalysts, indicating that the RWGS reaction is insensitive to the morphologies of catalysts, i.e., the size of the anchored Pt particles and primary crystallinity of CeO2 support. TPR and XPS results showed that, with the addition of Pt, the reducibility of CeO2 support was
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Identification of relevant active sites and a mechanism study for reverse water gas shift reaction over Pt/CeO2 catalysts Xiaodong Chen a,b,1, Xiong Su a,1, Binglian Liang a,b, Xiaoli Yang a,b, Xinyi Ren a, Hongmin Duan a, Yanqiang Huang a,∗, Tao Zhang a a b
State Key Laboratory of Catalysis, Dalian Institute of Chemical Physics, Chinese Academy of Sciences, Dalian 116023, Liaoning, China University of Chinese Academy of Sciences, Beijing 100049, China
a r t i c l e
i n f o
Article history: Received 19 August 2016 Revised 18 October 2016 Accepted 19 October 2016 Available online 16 November 2016 Keywords: RWGS reaction Pt/CeO2 catalyst Formate intermediate Mechanism
a b s t r a c t Reverse water gas shift (RWGS) reaction can serve as a pivotal stage in the CO2 conversion processes, which is vital for the utilization of CO2 . In this study, RWGS reaction was performed over Pt/CeO2 catalysts at the temperature range of 20 0–50 0 °C under ambient pressure. Compared with pure CeO2 , Pt/CeO2 catalysts exhibited superior RWGS activity at lower reaction temperature. Meanwhile, the calculated TOF and Ea values are approximately the same over these Pt/CeO2 catalysts pretreated under various calcination conditions, indicating that the RWGS reaction is not affected by the morphologies of anchored Pt nanoparticles or the primary crystallinity of CeO2 . TPR and XPS results indicated that the incorporation of Pt promoted the reducibility of CeO2 support and remarkably increased the content of Ce3+ sites on the catalyst surface. Furthermore, the CO TPSR-MS signal under the condition of pure CO2 flow over Pt/CeO2 catalyst is far lower than that under the condition of adsorbed CO2 with H2 -assisted flow, revealing that CO2 molecules adsorbed on Ce3+ active sites have difficult in generating CO directly. Meanwhile, the adsorbed CO2 with the assistance of H2 can form formate species easily over Ce3+ active sites and then decompose into Ce3 + –CO species for CO production, which was identified by in-situ FTIR. © 2016 Science Press and Dalian Institute of Chemical Physics, Chinese Academy of Sciences. Published by Elsevier B.V. and Science Press. All rights reserved.
1. Introduction The accelerated emission of CO2 due to the combustion of fossil fuels has triggered a series of global concerns (global warming, glaciers melting, ocean acidification, and so on), which seriously threatened the sustainable development and survival of our human beings [1,2]. Catalytic reduction of CO2 to value-added commodity is a promising way that can offer a solution to the reuse of CO2 as environmental friendly carbon resource, and is necessary for the development of long term fossil–C industry [3–8]. However, the activation of CO2 is a great challenge because of its inherent thermodynamic inertness, with the standard Gibbs free energy (G0 ) of −394.38 kJ/mol. Hydrogen, produced from water splitting through solar photon or renewable electric driving energy, is considered as an ideal intermediate energy carrier for the conversion of CO2 [9,10]. Many hydrogenation processes of CO2 to produce liquid carbon-based fuels, such as HCOOH, CH3 OH, and hydrocarbons,
∗
1
Corresponding author. Fax: +86 411 84685940. E-mail address: [email protected] (Y. Huang). These authors contributed equally to this work.
are the preferred options [11,12]. For most of these processes, a direct concern of transforming CO2 into CO (known as the RWGS reaction) is one of the most promising processes in the CO2 resource utilization, because the produced CO can be applied widely as a pivotal resource for C1 chemistry in the sustainable coal chemical industry [13–15]. Besides that, RWGS reaction is an essential intermediate step of many vital CO2 hydrogenation processes [16]. Kim et al. reported that the CO2 hydrogenation to light olefins was associated with RWGS reaction and then followed by Fischer–Tropsch synthesis [17]. Hu et al. exploited the CAMERE process of producing methanol from CO2 , which also involved the intellectual RWGS process [18]. Therefore, it is of great importance to separately investigate the RWGS reaction which not only facilitates the CO2 conversion but also helps looking deep insight into the CO2 hydrogenation reaction mechanisms. In recent studies, great efforts have been dedicated to the design of reducible oxide supported metal catalysts, which often show high activity for CO2 activation than representative mixed oxides or inert oxide supported metal catalysts, due to their structures with dual functionalities of the supported metal and adjacent vacancy sites [19–24].
http://dx.doi.org/10.1016/j.jechem.2016.11.011 2095-4956/© 2016 Science Press and Dalian Institute of Chemical Physics, Chinese Academy of Sciences. Published by Elsevier B.V. and Science Press. All rights reserved.
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A number of reports have indicated that the abundant oxygen vacancies over reducible supports are in favor of the activation of CO2 from C=O bond scission [13,19,25,26]. In addition, an effective site for CO desorption, either at the metal site or metal-support intersection region, is vastly important [27,28]. As RWGS is a reversible reaction, Pt/CeO2 catalysts, which are active in the WGS reaction, should be a promising candidate for the RWGS reaction [29–32]. In this study, we report that Pt/CeO2 catalysts exhibited highly desired activities for RWGS reaction. We identified that the TOF values of CO yields were approximately the same over these Pt/CeO2 catalysts, irrespective of their morphologies (the anchored Pt particles and primary crystallinity of CeO2 support). Meanwhile, combined with in-situ diffuse reflectance infrared Fourier transform spectroscopy and mass spectrometric analysis, we found that the RWGS activity was correlated with the reducibility of Pt/CeO2 catalysts. Ce3+ sites, in combination with the adjacent Pt particles, were recognized as the active sites for the RWGS reaction. 2. Experimental 2.1. Chemicals H2 PtCl6 (37 wt% aqueous solution) and Ce(NO3 )3 (≥ 99.0%, AR grade) were purchased from Sinopharm Chemical Reagent Co., Ltd. Na2 CO3 (≥ 98.8%, AR grade) and NaOH (≥ 96.0%, AR grade) were purchased from Tianjin Kermel Chemical Reagent Co., Ltd.. All chemicals were used as received without further purification. Deionized water was used throughout the experiments. 2.2. Catalyst preparation The Pt/CeO2 catalysts were synthesized using a co-precipitation method as described in previous work [23]. In a typical synthesis procedure, the calculated amounts of H2 PtCl6 and Ce(NO3 )3 were dissolved in 500 mL deionized water. Na2 CO3 and NaOH (with a molar ratio of 1:1) were dissolved in another 500 mL deionized water to make a mixed solution with the total molar concentration of 2.0 mol/L. The Pt content in CeO2 was calculated to be 1%. The hydrolysis of metal salts was achieved by controlled slowly dropwise of the two solutions into deionized water with stirring, until a pH value between 9 and 10 was achieved. The precipitates were stirred for 6 h and then aged overnight at ambient temperature. Then, the precipitates were filtered and washed several times with deionized water until the pH value was reached less than 7.5. Next, the samples were dried at 80 °C for 12 h and calcined at 500 °C, 650 °C, or 800 °C for 4 h, which were then reduced online in the reactor with the feed gas. Samples with different pretreatment conditions are denoted as PC–M–N for abbreviation in the following. PC represents the co-precipitated 1% Pt/CeO2 catalyst, M and N denotes the calcination and reduction temperatures of the samples, respectively. Bare CeO2 catalyst was prepared using similar procedures but without the addition of Pt source. The CeO2 was calcined in air under 500 °C for 4 h. 2.3. Characterization The structures of samples were measured by X-ray diffraction on a PANalytical X’Pert-Pro powder X-ray diffractometer with Cu Kα monochromatized radiation (λ = 0.1541 nm). Diffraction peaks were recorded in a 2θ range between 10° and 80°, with a scanning speed of 4 °/min. The specific surface area was examined on an ASAP 2460 (Micromeritics), and calculated by BET (Brunauer– Emmett–Teller) equation. Before analyzing, the samples were degassed under vacuum at 110 °C for 1 h and 300 °C for 5 h. The H2 chemisorptions over these Pt/CeO2 catalysts were measured by H2 pulse adsorption method on a Micromeritics Au-
toChem II 2920 automated catalyst characterization system. Typically, the samples were reduced under different temperatures as implemented in the activity tests. Then, the temperature of the system was increased by 10 °C and the freshly reduced catalysts were flushed with Ar for 30 min to remove the adsorbed hydrogen on the catalyst surface. Afterwards, pure H2 was injected until saturation when the temperature was cooled to 50 °C. The amount of H2 adsorption was calculated from the pulse results monitored by TCD detector. Temperature-programmed reduction (TPR) experiments were carried out on a Micromeritics AutoChem II 2920 automated catalyst characterization system. Prior to the measurement, about 50 mg of the catalyst bullets (20–40 mesh) were loaded into a U-shape quartz reactor and pretreated with Ar flow at 300 °C for 30 min to remove the adsorbed water. After cooling to 50 °C, 10 vol% H2 /Ar mixed gas was passed through the catalyst bed and then heated to 900 °C with a ramping rate of 10 °C/min. TPSR-MS experiments were performed in a U-shape quartz fixed-bed micro reactor at atmospheric pressure using 0.2 g PC-500 at 300 °C. The reactor was placed in a vertical electrical furnace for heating and the effluent products were monitored online with a PRISMA quadrupole mass spectrometer (MS) system. A K-type thermocouple was located inside the quartz reactor for monitoring reaction temperature. Switching procedures between the 5% H2 /He and 5% CO2 /He gases were used during the experiments. The total gas flow rate was kept at 20 mL/s and the switching stepwise was 20 min. Between the reduction and re-oxidation periods of the TPSR, the reactor was flushed with pure helium for 20 min to prevent the mixing of H2 and CO2 . High-resolution scanning transmission electron microscopy (HR-STEM) images were recorded on a JEM-2100F field emission electronic microscope equipped with STEM dark-field (DF) detector, which was operated at an acceleration voltage of 200 keV. Samples for field emission TEM measurements were prepared by suspending an ultrasonically dispersed powder in ethanol and by placing a drop of the suspension onto a Cu grid. X-ray photoelectron spectra (XPS) were determined on an ESCALAB 250Xi apparatus to obtain the surface composition and the binding energies of the catalysts. Mg Kα radiation at an energy scale calibrated versus adventitious carbon (C 1s peak at 284.6 eV) was used. Fourier transform infrared (FTIR) experiments were carried out using a BRUKER Equinox 55 spectrometer, equipped with a diffuse reflectance (DRIFT) and an MCT detector with a resolution of 4 cm−1 . The inlet gas into the cell was directly connected to a flow system, equipped with the mass flow controller and a set of valves that allow selection and control of feed gas composition in a typical experiment. The catalysts were pretreated under He flow at 300 °C for 60 min and then reduced in a flow of 5% H2 /He at 300 °C for 60 min. The reduced catalyst was then exposed to Ar at 300 °C for 30 min. After the pretreatment, the reference background spectrum of the catalyst was detected and subtracted automatically from the subsequently recorded spectra. Prior to measurements, the feed gas, which was composed of H2 :CO2 :N2 (45:45:10, v:v:v), was introduced into the in-situ cell at a total flow rate of 30 mL/min. 2.4. Activity test The RWGS catalytic activities over Pt/CeO2 catalysts were evaluated in a continuous-flow fixed-bed reactor under ambient pressure. The flow of feed gas was controlled by DZ47-63 mass flow controllers. The quartz reactor (11 mm, i.d.) was heated in a furnace which was connected to a proportional–integral–derivative (PID) temperature controller. The temperature of reactor was measured and controlled with a K-type thermocouple that was located
X. Chen et al. / Journal of Energy Chemistry 25 (2016) 1051–1057
2 Theta (degree) Fig. 1. XRD patterns of the PC catalysts under different pretreatment conditions over the Pt/CeO2 catalysts.
at the central axis of the reactor. Prior to the experiments, catalyst pellets (20–40 mesh) with the calculated amount of metal loading were packed into the reactor and then purged with a He flow (30 mL/min) under 300 °C for 30 min. Subsequently, the catalysts were pre-reduced at 400 °C for 1 h by H2 with a flow rate of 10 mL/min (denotes PC-50 0-40 0, PC-650-40 0, and PC-80 0-40 0). During the experiments, the reaction temperature range was set to 20 0–50 0 °C and the feed gas (CO2 /H2 /N2 = 45%/45%/10%) was introduced to the catalyst bed at a flow rate of 50 mL/min (GHSV of 30,0 0 0 mL/gcat /h). The ice-bath unit was used to remove the water vapor of the effluent gas from the reactor and then analyzed on line with an Agilent 7890B gas chromatograph with a TDX-01 column connected to a TCD detector. The catalytic performance was evaluated by the conversion of CO2 based on different concentrations between inlet and outlet, which is defined as:
Conversion (CO2 % ) =
CO2 (in ) − CO2 (out) ×
N2 (in) N2 (out)
CO2 (in)
× 100%
The selectivity of CO is defined as:
Selectivity (CO% ) =
CO(out ) × CO(out ) ×
N2 (in ) CO(in )
N2 (in ) N2 (out )
+ CH4 (out ) ×
N2 (in ) CH4 (in )
× 100%
The CO yield is calculated as follows: Yield (CO%) = Conversion (CO2 %) × Selectivity(CO%)/100 × 100% Where, CO2 (in), CH4 (in) and N2 (in) are the concentrations of CO2 , CH4 and N2 at inlet; CO2 (out), CH4 (out) and N2 (out) are the concentrations of CO2 , CH4 and N2 at outlet, respectively. During the entire catalytic testing, CH4 or other by-products were not detected. Meanwhile, the turnover frequencies (TOF) of CO2 conversion were defined as: n = moles of converted CO2 /(number of active sites determined by H2 chemisorption × time of reaction). 3. Results and discussions Fig. 1 shows the powder XRD patterns of Pt/CeO2 samples with different pretreatment conditions. There were no reflections of metallic Pt in the samples PC-50 0-40 0, PC-650-40 0, indicating
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that the anchored Pt nanoparticles with mean size less than 2 nm were beyond the detection limit of the XRD technique. When the prepared sample was calcined at 800 °C, we observed that the XRD pattern of PC-80 0-40 0 showed characteristic broadened signals of metallic Pt at 2θ = 39.8° and 46.3°. These results were consistent with the average Pt particle size counted from HAADF-STEM images based on the particle size distribution (see Fig. 2 and Table 1). The statistical mean size of Pt nanoparticles in PC-650-400 samples was 1.8 nm; whereas the average particle size of Pt over PT-80 0-40 0 was enlarged to 2.3 nm. The BET surface areas collected from N2 physical adsorption and the primary crystalline size of CeO2 substrate estimated from the XRD patterns of these corresponding Pt/CeO2 catalysts are also listed in Table 1. The conditioned physical structures of these catalysts showed that a variation in the reduced surface area (from 54.0 to 0.5 m2 /g) and enlarged primary crystalline size of CeO2 (from 8.0 to 19.5 nm) were achieved during the increased tougher heat processing of calcination or reduction treatments. These results can be further verified by the representative HAADF-STEM images of these corresponding Pt/CeO2 catalysts shown in Fig. 2(a)–(c). The H2 -TPR profiles of the bare CeO2 and Pt/CeO2 catalysts pretreated at different calcination temperatures were displayed in Fig. 3. Interestingly, the H2 consumption volumes of the first reduction peak over all the Pt/CeO2 catalysts were quantified as 494, 205, 128 μmol/gcat , exceeding the required amount of H2 consumption for the reduction of PtO2 phase (102 μmol/gcat ). The exceeded amount of H2 consumption for PtO2 reduction suggested that partial reduction of surface CeO2 was included in the scope of the first reduction peak. As increasing the calcination temperature of Pt/CeO2 catalysts, the crystallinity of CeO2 became more integrated with the reduced catalyst surface area, which caused the declined reduction degree of the reducible CeO2 phase. Moreover, the temperature of the first reduction peak tended to decrease through Pt incorporation, which was caused by H2 spillover effect. Interestingly, as the calcination temperature increased, those PC catalysts showed a large decline in the initiate temperature of the first reduction peak. The possible reason is that a weaker interaction between large Pt particles and CeO2 support was induced under higher calcination temperature and thus the reducibility of PtO2 was enhanced [33]. To be noted, the second peak in H2 -TPR profiles represents the reduction of bulk CeO2 phase. However, since it is not directly correlated with the catalyst activity, we will not discuss in detail here. In order to quantify the surface compositions of these PC catalysts, XPS measurements were conducted. The curve fitting results were shown in Fig. 4 and the deconvoluted data were listed in Table 2. Normally, the O1s spectra showed two obvious characteristic peaks, as shown in Fig. 4(a). The peak at 531.0–531.4 eV was attributed to the surface-adsorbed oxygen (O2 2− or O− ). The other one at lower binding energy of 529.0–530.0 eV was assigned to the lattice oxygen (O2− ). The percentage of the two categories of oxygen atoms were almost the same, even if with the Pt incorporation. Moreover, the Ce3d spectra were fitted with eight peaks, which were referred as Ce3+ and Ce4+ components. As shown in Fig. 4(b), the peaks signed as v, v”, v”’ belong to the 3d5/2 core level of Ce4+ and those labeled as u, u”, u”’ were assigned to that of Ce3+ . The other double peaks also correspond to the presence of Ce3+ [34]. Compared to pure CeO2 , the component of Ce3+ sites increased obviously over the PC catalysts, especially in PC-50 0-40 0. This result indicated that the addition of Pt apparently increased the content of surface Ce3+ species over these PC catalysts. From the deconvoluted results shown in Table 2, the detected surface composition of Pt declined with the rise of calcination temperature, which was attributed to the reduction of Pt dispersion caused by the growth of Pt particles.
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X. Chen et al. / Journal of Energy Chemistry 25 (2016) 1051–1057
Frequency (%)
50 PC-650-400
40
AD = 1.81 0.36 nm 30 20 10 0 1
Frequency (%)
50
2 3 4 Diameter (nm)
5
PC-800-400
40
AD = 2.29 0.53 nm
30 20 10 0 1
2 3 4 Diameter (nm)
5
Fig. 2. HAADF-STEM images and statistical particles distributions of the Pt/CeO2 catalysts: (a) PC-50 0-40 0, (b) PC-650-400 and (c) PC-800-400. Table 1. Physicochemical characterizations of the PC catalysts with different pretreatment conditions and the corresponding CO2 hydrogenation performance in the RWGS reaction. Reaction conditions: 0.2 g catalyst; CO2 /H2 /N2 = 45/45/10 (v/v/v); reaction temperature is 300 °C and the feed gas flow rate is 50 mL/min. Catalysts
Crystalline size of CeO2 (nm)
BET surface area (m2 /g)
H2 chemisorption volume (μmol/g)
Average Pt particle size (nm)
CO2 conversion (%)
PC-50 0-40 0 PC-650-400 PC-80 0-40 0
8.0 15.8 19.5
54.0 7.5 0.5
51.3 28.5 21.0
-a 1.81 2.29
6.7 4.1 3.2
a
TOF values (10−1 s−1 ) 0.560 0.550 0.555
Not determined.
Table 2. The deconvoluted XPS results of surface composition of CeO2 and Pt/CeO2 catalysts. Catalyst
CeO2 -50 0-40 0 PC-50 0-40 0 PC-650-400 PC-80 0-40 0
Pt (at%)
– 0.46 0.17 0.14
Ce (at%)
O (at%)
Ce3+
Ce4+
Osur + Oads
Olatt
2.60 6.44 5.53 5.06
21.1 21.5 19.9 20.1
37.4 42.4 42.4 38.9
38.9 29.2 32.0 35.8
The catalytic activity and the corresponding calculated Arrhenius plots of RWGS reaction over the bare CeO2 and Pt/CeO2 catalysts as a function of reaction temperature were shown in Fig. 5. Fig. 5(a) shows that the CO2 conversion increased monotonically as increasing the reaction temperature. A preferable catalytic performance over these 1% Pt/CeO2 catalysts was achieved when the catalyst was pretreated under lower calcination temperature. The PC-50 0-40 0 catalyst exhibited the best catalytic performance than
X. Chen et al. / Journal of Energy Chemistry 25 (2016) 1051–1057
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Fig. 3. H2 -TPR profiles of the bare CeO2 and Pt/CeO2 catalysts.
any other catalysts due to its high Pt dispersion. Interestingly, the calculated TOF values were nearly the same over all these Pt/CeO2 catalysts (about 0.055 s−1 , see Table 1), indicating that the RWGS reaction activity seems to be not influenced by the morphologies of catalysts, i.e., the anchored Pt particles and the primary crystallinity of CeO2 . We also noticed that RWGS reaction occurred over the bare CeO2 catalyst, but only became observable at the reaction temperature higher than 400 °C, suggesting that oxygen vacancies generated over the CeO2 catalyst surface could be regarded as active sites for CO formation. In addition, a series of Arrhenius plots of CO yield were calculated and shown in Fig. 5(b), which displayed reasonable linearity in the reaction temperature range. To obtain more reliable assessments, the CO2 conversion was confined to an extent of no more than 10% in order to eliminate the effect of mass transfer. Based on these calculated slopes, the apparent activation energies (Ea ) of CO yield were estimated. The Ea values of CO yield over Pt/CeO2 catalysts were approximately the same (∼68 kJ/mol), which were prominently lower than that over the bare CeO2 catalyst (∼77 kJ/mol). According to the report of Philip et al. [13], the catalytic activity of Pt/TiO2 catalyst in the RWGS reaction largely depends on the reducibility of TiO2 support. Lu et al. also proposed that the portion of the reducible support that with good intimacy to the metal particles could be in highly reduced state, which is thought to be effective in the reduction of CO2 to CO [24]. Similarly, the variations in the activities of Pt/CeO2 catalysts can be explained from the TPR data (see Fig. 3), which shows that the incorporation of Pt and the increase of calcination temperature could enhance the reducibility of CeO2 support. Therefore, the easily generated oxygen vacancy sites over the catalyst surface could accelerate the activation of CO2 in RWGS reaction. To determine the reaction route of RWGS process, we performed transient TPSR-MS experiments on Pt/CeO2 catalyst by detecting the products under input gas-flow switching procedures. The intensities of the MS signals of the effluent gases from the switched gas-flow over the PC-500 sample were shown in Fig. 6(a). During the first experiment stage, the feed gas was alternately switched by purging H2 , He and CO2 streams at 300 °C. When the gas flow was switched from He to CO2 over the reduced Pt/CeO2 sample, only a weak signal of CO was detected instantaneously as the CO2 molecules arrived at the sample. But this signal disappeared quickly to a steady level after 2 min. This result supports the fact that the redox mechanism involved in the RWGS reaction over the Pt/CeO2 catalyst, but with only a small extent. In the fol-
Fig. 4. XPS spectrum of the bare CeO2 and Pt/CeO2 catalysts.
lowing test, the gas stream was switched to He flow for 20 min in order to remove the residual CO2 gas in the pipe line and the physically adsorbed species on the catalyst surface. After that, the gas-flow was switched to H2 and the effluent gas signals were recorded. We observed a surge appearance of the CO signal but it disappeared quickly. Within a short time, the CO signal emerged again and dropped to a steady level in 20 min, as seen from Fig. 6(b). The generated CO product in this stage was prominently more than the amount produced during the first stage. Meanwhile, the first sharp peak was attributed to the desorption of CO from Ce3+ sites or Pt particles, because H atoms could preferentially adsorb on those sites in the flowing H2 atmosphere. The second broad peak was assigned to CO generated from the dissociation of intermediates which were formed through the H2 assisted adsorption of CO2 over Pt/CeO2 catalyst surface. These results further indicated that CO2 adsorbed over the reduced Pt/CeO2 catalyst have difficulty in generating CO product directly; whereas the activated CO2 can be facilely decomposed into CO via H2 -assisted pathway. Therefore, although the redox mechanism
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1/T (1/K) Fig. 5. (a) CO2 conversions and (b) Arrhenius plots of CO2 hydrogenation reactions over the bare CeO2 and Pt/CeO2 catalysts as a function of reaction temperature. Reaction conditions: 0.1 g catalyst loading; CO2 /H2 /N2 = 45/45/10 (v/v/v); feed gas flow rate is 50 mL/min.
and dissociation mechanism coexisted in the RWGS reaction, the dissociation mechanism, involving the formation of intermediates, has been recognized as the major route for CO production in the RWGS reaction. To get further insight into the reaction mechanism and analyzing the formed intermediates during RWGS reaction, in-situ reflectance FTIR experiments were conducted. The attenuated reflectance FTIR spectra of RWGS reaction over the PC-500 catalyst were collected after its exposure to the feed gas (CO2 /H2 /N2 = 45/45/10, v/v/v). After a reaction period of 1 h in the feed gas at 300 °C, the intensity of collected signals reached stable. The incoming gas stream was then switched to pure He flow for scrubbing and the reflectance spectra were recorded at once, see Fig. 7(a). At the very beginning, the peaks at ν = 1772 and 2045 cm−1 were assigned to the bridge- and linear-bonded CO on the surface of reduced Pt particles. Another peak at ν = 1940 cm−1 was ascribed to the linearly adsorbed CO at the interfacial sites of Pt and CeO2 . The band situated at 2130 cm−1 was consistent with the assignment of Ce3 + –CO, where the CO is adsorbed on Ce3+ sites [24,35–37]. Moreover, the peaks at 2830 and 1600 cm−1 were assigned to the
0.00 12500
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Real time (s) Fig. 6. The evolution of MS signals of CO product in the gas-flow switching experiments over the pre-reduced PC-500 catalyst: (a) when He flow was switched to CO2 flow and (b) when He flow was switched to H2 /He after the treatment procedure of (a). Treatment conditions: 0.2 g catalyst; reaction temperature is 300 °C; the feed gas flow rate is 20 mL/min.
formate and carbonate species, respectively. When purged with He flow, the intensity of the peaks attributed to the linearly adsorbed CO at the interfacial sites and the Pt–CO species attenuated slowly, while the signals of the formates and CO adsorbed on Ce3+ sites attenuated quickly. This means that the generated CO was strongly adsorbed on these interfacial or metal sites even in the flow of inert gas. Meanwhile, the formed formate species and CO adsorbed on Ce3+ sites were weakly bonded. We further detected IR signals by changing the input gas stream to H2 –He flow. Again, the intensity of the peaks attributable to the linearly adsorbed CO at the interfacial sites and the Pt–CO species attenuated slowly. However, the signals of formate species and CO adsorbed on Ce3+ sites emerged again. This can be interpreted as that, when purged in the H2 flow, formates firstly formed on the oxygen vacancies located at the vicinity of Pt particles. Then, these species decomposed into CO and adsorbed weakly on Ce3+ sites,
X. Chen et al. / Journal of Energy Chemistry 25 (2016) 1051–1057
enhanced and thus oxygen vacancies were generated more easily, which accelerated the activation of CO2 . In addition, through insitu FTIR and TPSR-MS experiments, we identified that the redox mechanism and dissociation mechanism coexisted in the RWGS reaction over the Pt/CeO2 catalyst. The CO2 molecules adsorbed on Ce3+ active sites have difficult in generating CO directly (surface redox mechanism). Meanwhile, the adsorbed CO2 with H2 -assisted pathway facilitates the formation of formate species as main intermediates (dissociative mechanism) and then decomposes into CO.
(a) 0.05 2045
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The authors acknowledge the National Natural Science Foundation of China (nos. 21476226 and 21506204), National Key Projects for Fundamental Research and Development of China (2016YFB0600902), the Strategic Priority Research Program of the Chinese Academy of Sciences (XDB17020400) and the Youth Innovation Promotion Association CAS for financial support. 1000
References
-1
Wavenumber (cm )
(b)
Absorbance (a.u.)
0.05
2045 1772 1940
H2
2130
2830
1600
4000
3500
3000
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2500
2000
1500
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-1
Wavenumber (cm ) Fig. 7. Stacked in-situ FTIR spectra collected on the PC-500 catalyst. (a) The first purging process in pure He gas with a flow rate of 30 mL/min was conducted after the catalyst was exposed to a feed gas (CO2 /H2 /N2 = 45/45/10, v/v/v) and reacted at 300 °C for 1 h; (b) in the second stage, the catalyst was scrubbed in the 5% H2 –He with a flow rate of 30 mL/min at 300 °C.
which accelerated the generation of CO over the Pt/CeO2 catalyst in a H2 -assisted reaction pathway. These results also indicated that formates only presented on the surface of reduced ceria (Ce3+ active sites) and worked as the main intermediates for CO formation. 4. Conclusions In summary, RWGS reaction was performed over Pt/CeO2 and bare CeO2 catalysts in the temperature range of 20 0–50 0 °C. The activation energies of CO yields over Pt/CeO2 catalysts were prominently lower than that over pure CeO2 catalyst. The calculated TOF values were approximately the same over these Pt/CeO2 catalysts, indicating that the RWGS reaction is insensitive to the morphologies of catalysts, i.e., the size of the anchored Pt particles and primary crystallinity of CeO2 support. TPR and XPS results showed that, with the addition of Pt, the reducibility of CeO2 support was
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