CeO2-X catalyst for

Subscriber access provided by OCCIDENTAL COLL

Kinetics, Catalysis, and Reaction Engineering

Reaction mechanism and catalytic impact of Ni/ CeO2-X catalyst for low temperature CO2 methanation Sangmoon Lee, Ye Hwan Lee, Dea Hyun Moon, Jeong Yoon Ahn, Dinh Duc Nguyen, Soon Woong Chang, and Sungsu Kim Ind. Eng. Chem. Res., Just Accepted Manuscript • DOI: 10.1021/acs.iecr.9b00983 • Publication Date (Web): 30 Apr 2019 Downloaded from http://pubs.acs.org on April 30, 2019

Just Accepted “Just Accepted” manuscripts have been peer-reviewed and accepted for publication. They are posted online prior to technical editing, formatting for publication and author proofing. The American Chemical Society provides “Just Accepted” as a service to the research community to expedite the dissemination of scientific material as soon as possible after acceptance. “Just Accepted” manuscripts appear in full in PDF format accompanied by an HTML abstract. “Just Accepted” manuscripts have been fully peer reviewed, but should not be considered the official version of record. They are citable by the Digital Object Identifier (DOI®). “Just Accepted” is an optional service offered to authors. Therefore, the “Just Accepted” Web site may not include all articles that will be published in the journal. After a manuscript is technically edited and formatted, it will be removed from the “Just Accepted” Web site and published as an ASAP article. Note that technical editing may introduce minor changes to the manuscript text and/or graphics which could affect content, and all legal disclaimers and ethical guidelines that apply to the journal pertain. ACS cannot be held responsible for errors or consequences arising from the use of information contained in these “Just Accepted” manuscripts.

is published by the American Chemical Society. 1155 Sixteenth Street N.W., Washington, DC 20036 Published by American Chemical Society. Copyright © American Chemical Society. However, no copyright claim is made to original U.S. Government works, or works produced by employees of any Commonwealth realm Crown government in the course of their duties.

Page 1 of 26 1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 18 19 20 21 22 23 24 25 26 27 28 29 30 31 32 33 34 35 36 37 38 39 40 41 42 43 44 45 46 47 48 49 50 51 52 53 54 55 56 57 58 59 60

Industrial & Engineering Chemistry Research

Reaction mechanism and catalytic impact of Ni/CeO2-X catalyst for low temperature CO2 methanation Sang Moon Lee, Ye Hwan Lee, Dea Hyun Moon, Jeong Yoon Ahn, Dinh Duc Nguyen, Soon Woong Chang, Sung Su Kim* Department of Environmental Energy Engineering, Kyonggi University, 94-6 San, Iui-dong, Youngtong-ku, Suwon-si, Gyeonggi-do 443-760, Republic of Korea *E-mail

address; [email protected]

KEYWORDS: CO2 methanation, Bridged carbonate, Oxygen vacancies, Ni/CeO2-X.

1

ACS Paragon Plus Environment

Industrial & Engineering Chemistry Research 1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 18 19 20 21 22 23 24 25 26 27 28 29 30 31 32 33 34 35 36 37 38 39 40 41 42 43 44 45 46 47 48 49 50 51 52 53 54 55 56 57 58 59 60

ABSTRACT: Ni supported on calcined ceria nitrate catalyst is highly active and stable for low temperature CO2 methanation reaction (CO2 conversion: 70% at 180 °C, 0.05 bar and gas hourly space velocity (GHSV) of 14,400 lkg-1h−1). We investigated CO2 adsorption and CO2 + H2 reaction on the surface of Ni/CeO2 and Ni/CeO2-X catalysts to examine the structure and strength of adsorbed species using diffuse reflection infrared fourier transform spectroscopy (DRIFTS). At temperature of 180 °C, weakly adsorbed bridged carbonate was generated on the surface of CeO2-X support by new active sites of oxygen vacancies created by addition of H2. High reducibility of Ni/CeO2-X catalyst played an important role in increasing low temperature CO2 methanation catalytic activity.

1. Introduction Methane synthesis reaction using CO2 as a reactant is a promising technology in industries because it reduces greenhouse gas and produces clean fuel. CO2 methanation (also referred to as Sabatier reaction) has been studied extensively using different types of metals and supports to enhance its catalytic activity. Many studies have used noble metal catalysts (such as Ru, Rh, and Pd) and transition metal catalysts (such as Ni and Cu). These metal catalysts are supported on metal oxides such as Al2O3, SiO2, TiO2, CeO2, and CeO2–ZrO2.1 Ni or Ru based CeO2 catalysts have emerged as promising catalysts for CO2 methanation reaction due to abundant oxygen vacancy and oxygen storage ability in CeO2 that can improve catalytic activity. Recently, considerable efforts have been made to determine the mechanism of CO2 methanation reaction over various catalysts including Ni or Ru supported catalysts.2-5 Generally, two main different CO2 methanation reaction pathways have been proposed: (1) CO2 associative, and (2) CO2 dissociative. In the CO2 associative mechanism, CO2 is associatively adsorbed with adsorbed H to produce CH* which is subsequently hydrogenated 2


Page 2 of 26

Page 3 of 26 1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 18 19 20 21 22 23 24 25 26 27 28 29 30 31 32 33 34 35 36 37 38 39 40 41 42 43 44 45 46 47 48 49 50 51 52 53 54 55 56 57 58 59 60


to CH4. CO2 dissociative mechanism describes that CO2 is directly dissociated to carbonyl (COad) and Oad as intermediates during methanation process. COad is subsequently hydrogenated or further dissociated to Cad and Oad in the next step.6 Heine et al.7 have studied methanation reaction on Ni(111) surface and revealed the reaction mechanism with ambient pressure X-ray photoelectron spectroscopy. They found that NiO was formed from CO2 dissociation into CO and atomic oxygen while carbonate was present on the surface from further reaction of CO2 with NiO. Pan et al.8 have investigated methanation reaction on Ni/Ce0.5Zr0.5O2 catalyst and revealed the reaction mechanism with In-situ FT-IR spectroscopy. They reported that formate species were the main intermediate species during the reaction and proposed that Ce3+ sites were active sites for their hydrogenation. Upham et al.9 have investigated CO2 methanation mechanism on Ru-doped ceria catalyst by DRIFT study and they proposed that methane is formed through reaction of hydrogen with surface carbonates, not through CO or formate intermediate. Many studies have investigated CO2 methanation reaction pathway and presence of intermediates by density functional theory (DFT) calculations. Lu et al.10 have investigated the initial reduction of CO2 on perfect and O-defective CeO2 (111) surfaces via direct dissociation and hydrogenation and reported that O-vacancy on CeO2 (111) surface assists reductive dissociation of CO2 to CO. Ren et al.11 have reported that CO route is more favorable energetically for CO2 methanation on Ni(111) surface through the following steps: CO2 → CO + O → C + O + 4H2 → CH2 + 2H → CH3 + H → CH4. Sharma et al.12 have also investigated reaction mechanism on Ru-substituted Ce0.95Ru0.05O2 catalyst by TPR, DRIFT, and DFT calculation. They proposed that surface CO* species was more likely to act as a key intermediate for CH4 production, rather than formate. The reaction proceeded via CO2 → CO → OCH2 → OCH3 → CH4. Although many recent studies have focused on reaction mechanism of Ni or Ru supported on CeO2 catalysts, to the best of our knowledge, reaction 3



mechanism of low temperature CO2 methanation at 180 °C or adsorption and desorption properties of CeO2-X supported Ni based catalyst has not been reported yet. Thus, we prepared Ni supported on calcined ceria nitrate and CeO2 catalysts to compare oxygen vacancy, activities, and reaction mechanism of Ni/CeO2-X catalysts. This study provides an insight into the mechanism of low temperature CO2 reaction on the basis of Ni/CeO2 catalyst structure.

2. Experiments 2.1 Preparation of catalysts. Nickel based catalysts supported on CeO2 and CeO2-X were prepared by wet impregnation method. These supports were employed as two kinds of CeO2 materials. One support was pigment CeO2 (Sigma Aldrich Co.) while the other support was prepared by calcination of (Ce(NO3)3·6H2O) at 500 °C for 4 h in air. They are named as CeO2 (A) and CeO2 (B), respectively. Calculated amount of nickel nitrate hexahydrate (Ni(NO3)2·6H2O; (Sigma Aldrich Co.) was dissolved in distilled water at 80 °C. Ni loading amount was fixed at 10wt%. After impregnation, moisture was evaporated at 70 °C using a rotary vacuum evaporator followed by drying at 103 °C overnight. Samples were then calcined at 400 °C for 2 hr. Obtained samples were ground and sieved using a 40-50 mesh.

2.2 Activity Test. The CO2 methanation experimental apparatus consisted of a continuous flow–type fixed-bed reaction system comprising of a quartz tube (inner diameter: 8 mm; height: 650 mm) and a catalytic bed that was fixed using quartz wool. To measure gas inlet temperature, another K-type thermocouple was installed at the top of the catalytic bed. Prior to the experiment, catalysts were pretreated at 300 °C for 1 h with 30% H2/N2 at a flow rate of 100 cc/min. Feed gases comprised of 16.67% CO2, 66.66% H2, and 16.67% N2. Total flow 4


Page 4 of 26

Page 5 of 26 1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 18 19 20 21 22 23 24 25 26 27 28 29 30 31 32 33 34 35 36 37 38 39 40 41 42 43 44 45 46 47 48 49 50 51 52 53 54 55 56 57 58 59 60


through the reactor was 120 cc/min and the reaction pressure was 0.05 bar. A space velocity of 14,400 lkg-1h−1 was obtained. These premixed gases were supplied by a mass flow controller (MKS). The entire gas-supply pipe was made of stainless steel and wrapped with a heating band set at 180 °C. Concentrations of reactants and products were measured as follows. Inlet and outlet gas concentrations were analyzed using a gas chromatograph (GOW-MAC, series 580) equipped with a thermal conductivity detector and carboxen 1000 column.

2.3 Characterizations. Surface areas of Ni based catalysts used in this study were measured via physical nitrogen adsorption at −196 °C using an ASAP 2010 instrument (Micrometrics). Specific surface area was analyzed using a Brunauer-Emmett-Teller (BET) model. X-Ray diffraction measurements were carried out using Cu Kα (λ = 1.5056 Å) radiation. Catalysts were run at 2θ = 20°–90° with a step size of 0.1° and time step of 1.0 s using an X’Pert PRO MRD instrument (PANalytical). Temperature-programmed reduction (TPR) of H2 was measured using 10% H2/Ar and 0.3 g of catalyst at a total flow rate of 50cc/min. Before H2 TPR measurement, the catalyst was pretreated in a flow of air at 400 °C for 0.5 h followed by cooling to 50°C. The catalyst was placed in dilute hydrogen and consumption of hydrogen was monitored using Autochem 2920 (Micrometrics) by increasing the temperature to 900°C at a rate of 10°C/min. Temperature-programmed desorption (TPD) of CO2 was also measured with 10% CO2/Ar using 0.3 g of the catalyst at a total flow rate of 50cc/min. Before TPD measurements, the catalyst was pretreated under a 10% H2/Ar flow at 300 °C for 0.5 h and cooled to 50 °C. Samples were treated with 10% CO2/Ar for 0.5 h. Adsorbed CO2 was removed using Ar flow for at least 1 h before starting TPD experiments. Dispersion and crystallite size of catalysts were characterized via H2 chemisorption at 35 °C using an Autochem 2920 (Micrometrics). Catalyst samples were activated with 10% H2 at 300 °C for 0.5 h, cooled to 50 5



°C, and purged by He gas until the baseline was stable and saturated with H2 pulses. Fouriertransform infrared (FT-IR) spectroscopy experiments were conducted in a diffuse reflection cell equipped with a CaF2 window using a 660 plus FT-IR spectrometer (Nicolet IS 10, Thermo Fisher, USA). A diffuse reflectance (DR) 400 accessory was used for solid reflectance analysis. Spectra included 30 accumulated scans at resolutions of 4 cm−1. They were obtained using a mercury-cadmium-telluride (MCT) detector.

3. Results and Discussion 3.1. Catalytic activities. Fig. 1 (a) shows CO2 methanation activities of 10wt% Ni supported on CeO2 catalyst (A) and CeO2 catalyst (B) at different reaction temperatures. A similar CO2 conversion rate was observed at 280 °C for Ni/CeO2 (A) and Ni/CeO2 (B) catalysts. However, the conversion on Ni/CeO2 (A) catalyst gradually decreased until 220 °C and CO2 conversion was nearly zero at reaction temperature under 200 °C. While Ni/CeO2 (B) catalyst showed superior activity until 180 °C, a different behavior was observed on Ni/CeO2 (A) catalyst. CO2 conversion reached about 76.7 and 70.5 % at 220 and 200 °C, respectively. Long-term stabilities of these two catalysts were also investigated. Results obtained for a period of 50 hon-stream are presented in Fig. 1 (b) where CO2 conversion to CH4 is plotted as a function of time-on-stream at 220 °C for Ni/CeO2 (A) catalyst and at 180 and 200 °C for Ni/CeO2 (B) catalyst. The catalytic activity of Ni/CeO2 (A) catalyst was slowly decreased from 66.0% to 59.0% at 220 °C. Although long-term stability of Ni-based catalyst differed with operational conditions such as temperature, space velocity, and inlet gas composition, general results were similar to results of previously reported studies by other researchers.13-15 Cai et al.16 have reported that CO2 conversions range from 62.18% to 72.21% using 5% Ni/CexZr1-xO2 catalyst at 390 °C for 60 h. The higher reducibility of Ce-rich supported on dispersed Ni catalyst was 6


Page 6 of 26

Page 7 of 26 1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 18 19 20 21 22 23 24 25 26 27 28 29 30 31 32 33 34 35 36 37 38 39 40 41 42 43 44 45 46 47 48 49 50 51 52 53 54 55 56 57 58 59 60


considered an important factor for long-term stability.17-19 Interestingly, CO2 conversion was kept constant at 180 and 200 °C for Ni/CeO2 (B) catalyst for 50 h. It showed relatively very stable activity with time on stream. XRD patterns of calcined Ni/CeO2 (A) and Ni/CeO2 (B) catalysts are shown in Fig.2. The X-ray diffraction pattern of CeO2 showed main peaks associated with face-centered cubic (fcc) fluorite structure located at 28.78, 33.18, 47.58, 56.28, 59.28, 69.78, 76.78, and 79.38 corresponding to (1 1 1), (2 0 0), (2 2 0), (3 1 1), (2 2 2), (4 0 0), (3 3 1) and (4 2 0) planes, respectively.20 CeO2 crystallite peaks of Ni/CeO2 catalyst (B) were broader and weaker than those of Ni/CeO2 catalyst (A). This result indicated that lattice distortion occurred within the CeO2 structure, resulting in smaller particle size and/or lower crystallization by calcination of Ce(NO3)3·6H2O.21 However, Corresponding peaks in NiO of Ni/CeO2 (A) and Ni/CeO2 (B) catalysts were 20.2 and 21.3 nm from XRD analysis, respectively, meaning that there was no major difference in crystallite size between the NiO species of two catalysts. Physicochemical characteristics of these two catalysts are summarized in Table 1. Both Ni/CeO2 (A) and Ni/CeO2 (B) catalysts exhibited similar Ni dispersions and active particle diameters from H2 chemisorption. The particle diameter of Ni0 of Ni/CeO2 (A) and Ni/CeO2 (B) catalysts were 20.5 and 19.4 nm from H2 chemisorption analysis, respectively, which means that there was no major difference in crystallite size between the NiO and metallic Ni species. However, surface area and total pore volume of Ni/CeO2 catalyst (B) were larger than those of Ni/CeO2 catalyst (A). These results confirmed that CeO2 structural properties were affected by ceria precursor while Ni size or structure was not affected by CeO2 crystallite structure in this study. 3.2. Temperature Programmed studies. H2-TPR tests were conducted to investigate oxygen behavior and reducibilities of supports and catalysts. Results are shown in Fig. 3 (a). CeO2 reduction to Ce might have proceeded in the following steps: CeO2→Ce2O3→CeO→Ce. 7



The H2 consumption peaks corresponding to CeO2 support (B) emerged at 390, 490, and 800 °C. The first peak at 390 °C could be attributed to reduction of surface oxygen of stoichiometric ceria of type Ce4+–O–Ce4+ 22 while the second peak at 490 °C could be attributed to reduction of nonstoichiometric ceria of type Ce3+–O–Ce4+.23 The last peak at 800 °C might be attributed to reduction of CeO to Ce. CeO2 support (A) showed maximum reduced peaks at 345, 490, and 800 °C. These peaks might be due to a slight reduction peak of Ce4+ to Ce3+ in the bulk CeO2. The trend of reducing peak in two supports is similar, there is significant difference in the reducible amount between the two supports. With increasing temperature, there were three steps of reduction reaction (CeO2→Ce2O3→CeO→Ce). CeO2 support (A) made it more difficult to carry out oxygen due to strong ceria and oxygen interaction. Compared with reduction properties of Ni/CeO2 catalysts and pure CeO2 supports, the reduction temperature of Ni/CeO2 catalyst shifted to lower regions due to interaction between Ni and CeO2 probably. There might be a weaker interaction between Ni and CeO2 phase. For this reason, the reduction of Ni/CeO2 catalysts occurred at a lower temperature compared to CeO2 supports. For Ni/CeO2 catalyst (B), four reduction peaks at 190, 235, 300, and 800 °C were observed. The first low temperature peak could be assigned to reduction of weakly adsorbed surface oxygen such as – OH or O2 species. The signal at 190°C can be assigned to –OH groups and/or O2 on the Ni surface. Because these reducing peaks were not confirmed at low temperature in CeO2 support. The second peak at 235 °C might be due to reduction of surface NiOx species having weak interaction with the support. The third broad peak at 300 °C might be attributed to co-reduction of Ni-O-Ce oxides. The last peak at 800 °C was assigned to reduction of bulk CeO2.24 Similar reduction trends were also observed for Ni/CeO2 catalyst (A). Interestingly, Ni/CeO2 catalyst (A) showed relatively very low reduction peaks at low temperatures (190 and 235 °C) than Ni/CeO2 catalyst (B). Ni/CeO2 (A) and (B) catalysts had similar temperature and shape of the 8


Page 8 of 26

Page 9 of 26 1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 18 19 20 21 22 23 24 25 26 27 28 29 30 31 32 33 34 35 36 37 38 39 40 41 42 43 44 45 46 47 48 49 50 51 52 53 54 55 56 57 58 59 60


third reduction peak at 300 °C, suggesting that metal dispersion and particle size of Ni were similar, in good agreement with results of XRD and H2 pulse chemisorption analysis. Some differences such as temperature and amount in the third reduction peak could be observed if dispersion and particle size of Ni were different. It can be concluded that Ni supported on calcined ceria nitrate can enhance partial reduction of surface oxygen at low temperatures (H2TPR studies), resulting in higher active site densities of weakly adsorbed surface -OH or oxygen groups associated with ceria. In brief, CeO2 support’s oxygen binding structure has a significant influence on catalyst reducibility. The formation of surface oxygen sites may influence low temperature CO2 methanation activity. CO2 TPD experiments were carried out in order to obtain information about relative amounts and adsorption strengths of CO2. CO2 TPD profiles of the two catalyst are shown in Fig. 3 (b). These two catalysts contained two broad CO2 desorption peaks at 50~200 °C and 200~500 °C attributed to weak and medium basic sites, respectively.25 Chemisorptions at weak basic site for temperatures at about 50~200 °C were shown to be more important than desorption from medium basic sites at temperature of approximately 350~600 °C. Therefore, CO2 coordinated on weak basic site had crucial influence on activity while medium basic sites were not necessary for low temperature CO2 methanation reaction. It was assumed that CO2 species were only adsorbed on Ni metal. If dispersion and particle size of Ni are similar for two catalyst, CO2 desorption amount and temperature are almost the same for the two catalysts. However, the amount of weak CO2 adsorbed onto Ni/CeO2 (B) was much larger than that onto Ni/ CeO2 (A). The coverage of Ni/CeO2 by CO2-derived species might be large, leading to improvement of low temperature CO2 methanation. This result indicates different CO2 adsorption and activation sites of Ni/CeO2 (A) and (B) catalysts. More details on CO2 ad-species are given in the forthcoming in-situ DRIFTS section. 9



3.3. DRIFT Study. To examine CO2 adsorption properties and reaction pathway for low temperature, FT-IR measurements were carried out under CO2 and CO2/H2 mixtures from 180 and 300 °C. According to the above activity result, Ni/CeO2 (B) catalyst was active while Ni/CeO2 (A) catalyst was not active in low CO2 methanation at ~200 °C. To compare CO2 adsorption properties taking place on the surface of catalyst, CO2 adsorption experiments of Ni/CeO2 (A) and (B) catalysts were performed using in situ FT-IR spectroscopy. Results are shown in Fig. 4 CO2 adsorption modes identified in previous literatures are summarized in Scheme 1. Prior to analysis, Ni/CeO2 (A) and (B) catalysts were pretreated at 300 °C for 1 h with a 30% H2/N2 as shown in Figs. 3a~3b. In case of reduced Ni/CeO2 (A) catalyst, bands observed at 1603, 1540, and 1220 cm-1 could be assigned to hydrogen carbonates species while bands centered at 1562 and 1273 cm-1 were assigned to bidentate carbonate species. Monodentate carbonate band was also detected at 1395 cm-1.21 Spectra of CO2 adsorption on Ni/CeO2 (B) catalyst were very similar to those of Ni/CeO2 (A) catalyst. One interesting feature of Ni/CeO2 (B) catalyst was that a new band was observed at 1150 cm-1, indicating the formation of bridged carbonate species. From previous H2 TPR result, reduction of NiO to metallic Ni behavior can occur at both Ni/CeO2 (A) and (B) catalysts and 300 °C under 30% H2/N2 atmosphere. It was known that Ni/CeO2 (B) catalyst had abundant weakly adsorbed surface oxygen such as –OH or O2 species. If bridged carbonate species was adsorbed on reduced Ni species, bridged carbonate species might be detected on Ni/CeO2 (A) catalyst. However, it was only observed on Ni/CeO2 (B) catalyst, meaning that the bridged carbonate species was adsorbed on oxygen vacancy of the surface of CeO2 as weakly adsorbed surface oxygen, not Ni species. In this study, bands of CO were not observed on Ni (2077, 2035, and 1920 cm-1) or CeO2 (2121 cm-1) during CO2 adsorption for the two catalysts.26-27 This indicates that CO2 could not decompose into CO at low temperature. Fig. 5 shows DRIFT spectra 10


Page 10 of 26

Page 11 of 26 1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 18 19 20 21 22 23 24 25 26 27 28 29 30 31 32 33 34 35 36 37 38 39 40 41 42 43 44 45 46 47 48 49 50 51 52 53 54 55 56 57 58 59 60


obtained under CO2 methanation reaction conditions over reduced Ni/CeO2 (A) and (B) catalysts at increasing temperature range from 180 to 300 °C. When introducing CO2 and H2, formate (1545 cm-1), hydrogen carbonates (1214 and 1604 cm-1), bidentate (1291 and 1588 cm1),

and monodentate (1370 and 1392 cm-1) species were found to be main adsorbed species

during reaction at low temperature, suggesting that strongly-adsorbed CO2 species were for Ni/CeO2 (A) catalyst. With increasing reaction temperature, there was a decrease in hydrogen carbonates and bidentate carbonate bands. When reaction temperature was over 260 °C, only formate and monodentate carbonate bands remained well. Both formate and monodentate carbonate species were found to be the main intermediate species at high temperature. At this high temperature, Ce4+ species can be reduced to Ce3+ by introducing H2. It was confirmed by H2 TPR results. Thus, Ce3+ sites were proposed to be active sites for hydrogenation.28 Formate (1586 cm-1), hydrogen carbonates (1607 cm-1), bidentate (1300 cm-1), and monodentate (1370 cm-1) species were also observed for Ni/CeO2 (B) catalyst. Bands of bridged carbonate species were also observed at low temperature. Peak intensities of all adsorbed bands were all relatively low. At 180 °C, Ni/CeO2 (B) catalyst was active for CO2 methanation caused by bridged carbonate species adsorbed on oxygen vacancy on the surface of CeO2. This result suggests that weakly adsorbed bridged carbonate is generated on the surface of CeO2-X support by new active sites of oxygen vacancies created by addition of H2. The high reducibility of Ni/CeO2-X catalyst is the most important factor for low temperature CO2 methanation reaction. The peak attributed to linear CO adsorbed on Ni species and nickel carbonyl hydride (H2-CO) species appeared at higher wavenumbers (2019 and 1917 cm-1) above 260 °C and the peak arising from formate species was more intense. Bidentate carbonates were not observed on Ni/CeO2 (B) catalyst when CO2 and H2 were introduced. This indicates that these adsorbed species are very unstable and that these species have already been desorbed before formate species starts to form. It is 11



believed that formate species are derived from hydrogen carbonates and monodentate carbonates.29 This was because hydrogen carbonates and monodentate carbonates species vanished while the formation of formats species appeared. However, some bidentate carbonates and monodentate carbonates species remained even at 300 °C for Ni/CeO2 (B) catalyst. It is known that bidentate carbonates species are very stable. These species started to vanish at 280 °C while hydrogen carbonates species started to vanish at 240 °C. These results strongly suggest that adsorbed bidentate carbonates species are difficult to transform into formate species on Ce4+ support. These sites can have higher activation energy for the formation of formate. According to previous reports, Ce3+ sites might serve as medium basic sites for CO2 adsorption and conversion while Ce3+ sites with oxygen vacancies are active sites for the formation of formate species.30 The deactivation of Ni/CeO2 (A) catalyst as shown in Fig. 1 (b) might be caused by strongly adsorbed bidentate carbonates species on Ce4+ support. Decomposition of strongly adsorbed bidentate carbonates species might be the reason for the deactivation of Ni/CeO2 (A) catalyst. To better describe the reaction pathway, proposed reaction mechanism of CO2 methanation for Ni/CeO2 (B) catalysts at low and high temperatures is depicted in Scheme 2. At 180 °C, CO2 would be activated on Ce3+ sites with oxygen vacancies support to form bridged carbonates which would be hydrogenated into formates. Bridged carbonates site can lower the activation energy for the formation of formate. The dissociated hydrogen on Ni0 can react with the weakly adsorbed bridged carbonates on Ce3+ site to produce methane and reduced ceria can be oxidized by CO2 at this temperature. At 180 ≤ T ≤ 240 °C, CO2 would be activated on the ceria support to form various carbonates that would be hydrogenated into formates. At 260 ≤ T ≤ 300 °C, linear CO adsorbed on Ni species and nickel carbonyl hydride (H2-CO) species were observed. At this high temperature, activation energy starts to be high enough to dissociate and then hydrogenate CO directly until 12


Page 12 of 26

Page 13 of 26 1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 18 19 20 21 22 23 24 25 26 27 28 29 30 31 32 33 34 35 36 37 38 39 40 41 42 43 44 45 46 47 48 49 50 51 52 53 54 55 56 57 58 59 60


the formation of methane.31 Potential energy barriers for hydrogenation of CO and production of CH4 on Ni/CeO2 (B) catalyst were lower than those on Ni/CeO2 (A) catalyst. This explains the high activity of Ni/CeO2 (A) catalyst. The lower potential energy barriers can be caused by weak charge transfer between Ni atoms and adsorbed species on CeO2 supported Ni particles.13 Electron charge transfer from CO to adsorbing Ni atoms on Ni/CeO2 (B) was less than that on Ni/CeO2 (A), indicating that the interaction between adsorbed species and Ni atoms on Ni/CeO2 (B) catalyst was weaker. Therefore, the adsorption of CO and CH3 on Ni/CeO2 (B) is less energetically favorable than that on Ni/CeO2 (A). Thus, the potential energy barriers of hydrogenation reaction of CO and the production of CH4 are smaller.

4. Conclusions Difference in catalytic behavior between Ni/CeO2 and Ni/CeO2-X catalysts was discussed. Both Ni/CeO2 (A) and Ni/CeO2 (B) catalysts exhibited similar Ni dispersions and active particle diameters. However, different behaviors were observed for reducibility and CO2 adsorption properties of catalysts. At low temperature (180 °C), H2 would be dissociated on Ni0 sites while CO2 would be activated on Ce3+ sites with oxygen vacancies support to form bridged carbonates which would be hydrogenated into formates and methane. At high temperature (260~300 °C), the activation energy started to be high enough to dissociate which then directly hydrogenated CO until the formation of methane for Ni/CeO2 (B) catalyst. This mechanism involves weak basic sites of the CeO2-X support for the adsorption of CO2. We also confirmed that decomposition of strongly adsorbed bidentate carbonates species was the reason for the deactivation of Ni/CeO2 (A) catalyst.

13



FIGURES Figure 1. (a) CO2 conversion during the CO2 methanation reaction over Ni based CeO2 catalysts. The experimental value was obtained at a total flow of 120 cc/min, WHSV of 14,400 lkg-1h−1, 16.67% CO2 and gas composition of 66.66% H2 and 16.67% N2. (b) Long-term stability test of Ni/CeO2 catalysts for CO2 methanation reaction at a temperature of 200 and 220 °C. Figure 2. XRD patterns of CeO2 supports and Ni based CeO2 catalysts. Figure 3. (a) H2 TPR profiles of CeO2 supports and Ni/CeO2 catalysts. (b) CO2 TPD profiles of Ni/CeO2 catalysts. Figure 4. In situ FTIR spectra of CO2 adsorbed on various Ni based CeO2 catalysts as function of time at 180 °C. (a) 300 °C reduced Ni/CeO2 (A), (b) 300 °C reduced Ni/CeO2 (B). Figure 5. In situ FTIR spectra of CO2 methanation reaction (CO2 + 4H2) on various Ni based CeO2 catalysts as function of temperature. (a) 300 °C reduced Ni/CeO2 (A), (b) 300 °C reduced Ni/CeO2 (B).

SCHEMES

Scheme 1. Surface CO2 species formed on Ni/CeO2 catalyst and their characteristic infrared absorption frequencies. Scheme 2. Proposed reaction mechanism on Ni/CeO2-x catalyst. (a) CO2 adsorption at 180 °C, (b) CO2 methanation reaction (CO2 + 4H2) at 180 °C and (c) CO2 methanation reaction (CO2 + 4H2) over 260 °C.

14


Page 14 of 26

Page 15 of 26 1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 18 19 20 21 22 23 24 25 26 27 28 29 30 31 32 33 34 35 36 37 38 39 40 41 42 43 44 45 46 47 48 49 50 51 52 53 54 55 56 57 58 59 60


Figure 1. 15



Figure 2.

16


Page 16 of 26

Page 17 of 26 1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 18 19 20 21 22 23 24 25 26 27 28 29 30 31 32 33 34 35 36 37 38 39 40 41 42 43 44 45 46 47 48 49 50 51 52 53 54 55 56 57 58 59 60


Figure 3.

17



Page 18 of 26

Figure 4.


18

Page 19 of 26 1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 18 19 20 21 22 23 24 25 26 27 28 29 30 31 32 33 34 35 36 37 38 39 40 41 42 43 44 45 46 47 48 49 50 51 52 53 54 55 56 57 58 59 60


Figure 5.


19


Page 20 of 26

Scheme 1.

Scheme 2.


20

Page 21 of 26 1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 18 19 20 21 22 23 24 25 26 27 28 29 30 31 32 33 34 35 36 37 38 39 40 41 42 43 44 45 46 47 48 49 50 51 52 53 54 55 56 57 58 59 60


TABLE

Table 1 Physicochemical characteristics of Ni/CeO2 (A) and Ni/CeO2 (B) catalysts.

Surface area (m2/g)

Total pore volume (cm3/g)

Average pore diameter (nm)

Metal dispersion (%)a

Active particle diameter (nm)a

Ni/CeO2 (A)

33.17

0.065

7.816

4.932

20.528

Ni/CeO2 (B)

65.95

0.181

10.98

5.209

19.438

a calculated

by H2 pulse chemisorption.

AUTHOR INFORMATION Corresponding Author *E-mail: [email protected] Notes The authors declare no competing financial interest. ACKNOWLEDGMENT This research was supported by Basic Science Research Program through the National Research Foundation of Korea(NRF) funded by the Ministry of Education(2017R1D1A1B03036192).


21


Page 22 of 26

REFERENCES

(1) Thampi, K. R. Methanation and photo-methanation of carbon dioxide at room temperature and atmospheric pressure. Nature 1987, 327, 506. (2) Sharma, S.; Hu, Z.; Zhang, P.; McFarland, E. W.; Metiu, H. CO2 methanation on Ru-doped ceria. J. Catal. 2011, 278, 297. (3) Tada, S.; Ochieng, O. J.; Kikuchi, R.; Haneda, T.; Kameyama, H. Promotion of CO2 methanation activity and CH4 selectivity at low temperatures over Ru/CeO2/Al2O3 catalysts. Int. J. Hydrogen Energy 2014, 39, 10090. (4) Garbarino, G.; Bellotti, D.; Riani, P.; Magistri, L.; Busca, G. Methanation of carbon dioxide on Ru/Al2O3 and Ni/Al2O3 catalysts at atmospheric pressure: catalysts activation, behaviour and stability. Int. J. Hydrogen Energy 2015, 40, 9171. (5) Karelovic, A.; Ruiz, P. Mechanistic study of low temperature CO2 methanation over Rh/TiO2 catalysts. J. Catal. 2013, 301, 141. (6) Park, J.-N.; McFarland, E. W. A highly dispersed Pd–Mg/SiO2 catalyst active for methanation of CO2. J. Catal. 2009, 266, 92. (7) Fujita, S.; Nakamura, M.; Doi, T.; Takezawa, N. Mechanisms of methanation of carbon dioxide and carbon monoxide over nickel/alumina catalysts. Appl. Catal. A: Gen. 1993,104, 87. (8) Garbarino, G.; Riani, P.; Magistri, L.; Busca, G. A study of the methanation of carbon dioxide on Ni/Al2O3 catalysts at atmospheric pressure. Int. J. Hydrogen Energy 2014, 39, 11557. (9) Weatherbee, G. D.; Bartholomew, C. H. Hydrogenation of CO2 on group VIII metals: I. Specific activity of NiSiO2. J. Catal. 1981, 68, 67.


22

Page 23 of 26 1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 18 19 20 21 22 23 24 25 26 27 28 29 30 31 32 33 34 35 36 37 38 39 40 41 42 43 44 45 46 47 48 49 50 51 52 53 54 55 56 57 58 59 60


(10) Weatherbee, G. D.; Bartholomew, C. H. Hydrogenation of CO2 on group VIII metals: II. Kinetics and mechanism of CO2 hydrogenation on nickel. J. Catal. 1982, 77, 460. (11) Graҫa, I.; González, L. V.; Bacariza, M. C.; Fernandes, A.; Henriques, C.; Lopes, J. M.; Ribeiro, M. F. CO2 hydrogenation into CH4 on NiHNaUSY zeolites. Appl. Catal. B: Environ. 2014, 147, 101. (12) Westermann, A.; Azambre, B.; Bacariza, M. C.; Graҫa, I.; Ribeiro, M. F.; Lopes, J. M.; Henriques, C. Carbon dioxide methanation over Ni catalysts supported on various metal oxides. Appl. Catal. B: Environ. 2015, 174–175, 120. (13) Tada, S.; Shimizu, T.; Kameyama, H.; Haneda, T.; Kikuchi, R. Ni/CeO2 catalysts with high CO2 methanation activity and high CH4 selectivity at low temperatures. Int. J. Hydrogen Energy 2012, 37, 5527. (14) Aldana, P. A. U.; Ocampo, F.; Kobl, K.; Louis, B.; Thibault-Starzyk, F.; Daturi, M.; Bazin, P.; Thomas, S.; Roger, A. C. Catalytic CO2 valorization into CH4 on Ni-based ceria-zirconia. Reaction mechanism by operando IR spectroscopy. Catal. Today 2013, 215, 201. (15) Petala, A.; Panagiotopoulou, P. Methanation of CO2 over alkali-promoted Ru/TiO2 catalysts: I. Effect of alkali additives on catalytic activity and selectivity. Appl. Catal. B: Environ. 2018, 224, 919. (16) Zhou, G.; Liu, H.; Cui, K.; Xie, H.; Jiao, Z.; Zhang, G.; Xiong, K.; Zheng, X. Methanation of carbon dioxide over Ni/CeO2 catalysts: Effects of support CeO2 structure. Int. J. Hydrogen Energy 2017, 42, 16108. (17) Kim, A.; Debecker, D. P.; Devred, F.; Dubois, V.; Sanchez, C.; Sassoye, C. CO2 methanation on Ru/TiO2 catalysts: On the effect of mixing anatase and rutile TiO2 supports. Appl. Catal. B: Environ. 2018, 220, 615.


23


Page 24 of 26

(18) Panagiotopoulou, P.; Verykios, X. E. Mechanistic study of the selective methanation of CO over Ru/TiO2 catalysts: Effect of metal crystallite size on the nature of active surface species and reaction pathways. J. Phys. Chem. C 2017, 121, 5058. (19) Miao, B.; Ma, S. S. K.; Wang, X.; Su, H.; Chan, S.H. Catalysis mechanisms of CO2 and CO methanation. Catal. Sci. Technol. 2016, 6, 4048. (20) Heine, C.; Lechner, B. A. J.; Bluhm, H.; Salmeron, M. Recycling of CO2: Probing the chemical state of the Ni(111) surface during the methanation reaction with ambient-pressure Xray photoelectron spectroscopy. J. Am. Chem. Soc. 2016, 138, 13246. (21) Pan, Q.; Peng, J.; Wang, S.; Wang, S. In situ FTIR spectroscopic study of the CO2 methanation mechanism on Ni/Ce0.5Zr0.5O2. Catal. Sci. Technol. 2014, 4, 502. (22) Upham, D. C.; Derk, A. R.; Sharma, S.; Metiu, H.; McFarland, E.W. CO2 methanation by Ru-doped ceria: the role of the oxidation state of the surface. Catal. Sci. Technol. 2015, 5, 1783. (23) Lu, X.; Wang, W.; Wei, S.; Guo, C.; Shao, Y.; Zhang, M.; Deng, Z.; Zhu, H.; Guo, W. Initial reduction of CO2 on perfect and O-defective CeO2 (111) surfaces: towards CO or COOH?. RSC Adv. 2015, 5, 97528. (24) Ren, J.; Guo, H.; Yang, J.; Qin, Z.; Lin, J.; Li, Z. Insights into the mechanisms of CO2 methanation on Ni(111) surfaces by density functional theory. Appl. Surf. Sci. 2015, 351, 504. (25) Sharma, S.; Kumar, K. B. S.; Chandnani, Y. M.; Kumar, V. S. P.; Gangwar, B. P.; Singhal, A.; Deshpande, P.A. Mechanistic insights into CO2 methanation over Ru-substituted CeO2. J. Phys. Chem. C 2016, 120, 14101. (26) Ocampo, F.; Louis, B.; Roger, A. C. Methanation of carbon dioxide over nickel-based Ce0.72Zr0.28O2 mixed oxide catalysts prepared by sol–gel method. Appl. Catal., A 2009, 369, 90.


24

Page 25 of 26 1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 18 19 20 21 22 23 24 25 26 27 28 29 30 31 32 33 34 35 36 37 38 39 40 41 42 43 44 45 46 47 48 49 50 51 52 53 54 55 56 57 58 59 60


(27) Cai, W.; Zhong, Q.; Zhao, Y. Fractional-hydrolysis-driven formation of non-uniform dopant concentration catalyst nanoparticles of Ni/CexZr1− xO2 and its catalysis in methanation of CO2. Catal. Commun. 2013, 39, 30. (28) Guo, M.; Lu, G. The effect of impregnation strategy on structural characters and CO2 methanation properties over MgO modified Ni/SiO2 catalysts. Catal. Commun. 2014, 54, 55. (29) Trovarelli, A. Catalysis by Ceria and Related Materials, World Scientific Publishing Co., Singapore, 2002. (30) Zangeneh, F. T.; Sahebdelfar, S.; Ravanchi, M. T. Conversion of carbon dioxide to valuable petrochemicals: an approach to clean development mechanism. J. Nat. Gas Chem. 2011, 20, 219. (31) Farias, A. M. D.; Nguyen-Thanh, D.; Fraga, M. A. Discussing the use of modified ceria as support for Pt catalysts on water–gas shift reaction. Appl. Catal. B: Environ. 2010, 93, 250-258. (32) Wang, L.; Liu, H.; Liu, Y.; Chen, Y.; Yang, S. Influence of preparation method on performance of Ni-CeO2 catalysts for reverse water-gas shift reaction. J. Rare Earths 2013, 31, 559. (33) Damyanova, S.; Perez, C. A.; Schmal, M.; Bueno, J. M. C. Characterization of ceria-coated alumina carrier. Appl. Catal. A Gen. 2002, 234, 271. (34) Murugan, B.; Ramaswamy, A. V. Chemical states and redox properties of Mn/CeO2−TiO2 nanocomposites prepared by solution combustion route. J. Phys. Chem. C 2008, 112, 20429. (35) Zhu, H.; Qin, Z.; Shan, W.; Shen, W.; Wang, J. Pd/CeO2–TiO2 catalyst for CO oxidation at low temperature: a TPR study with H2 and CO as reducing agents. J. Catal. 2004, 225, 267. (36) Bian, L.; Zhang, L.; Xia, R.; Li, Z. Enhanced low-temperature CO2 methanation activity on plasma-prepared Ni-based catalyst. J. Nat. Gas Sci. Eng. 2015, 27, 1189.


25


Page 26 of 26

(37) Bera, P.; Cámara, A. L.; Hornés, A.; Martínez-Arias, A. Comparative in situ DRIFTS-MS study of 12CO- and 13CO-TPR on CuO/CeO2 catalyst. J. Phys. Chem. C 2009, 113, 10689. (38) Aziz, M. A. A.; Jalil, A. A.; Triwahyono S.; Ahmad, A. CO2 methanation over heterogeneous catalysts: recent progress and future prospects. Green Chem. 2015, 17, 2647. (39) Wang, F.; Wei, M.; Evans, D. G.; Duan, Xue. CeO2-based heterogeneous catalysts toward catalytic conversion of CO2. J. Mater. Chem. A. 2016, 4, 5773.

Contents Graphic


26

CeO2-X catalyst for

Recommend Documents