Electronic structure modulating for supported Rh catalysts toward CO2 methanation

Journal Pre-proof Electronic structure modulating for supported Rh catalysts toward CO2 methanation Yueyue Jiang (Methodology) (Investigation) (Writing - original draft), Junyu Lang (Methodology), Xuechen WuData analysis), Yun Hang Hu (Supervision)conceptualization) (Writing review and editing)

PII:

S0920-5861(20)30033-X

DOI:

https://doi.org/10.1016/j.cattod.2020.01.029

Reference:

CATTOD 12648

To appear in:

Catalysis Today

Received Date:

14 August 2019

Revised Date:

7 January 2020

Accepted Date:

24 January 2020

Please cite this article as: Jiang Y, Lang J, Wu X, Hu YH, Electronic structure modulating for supported Rh catalysts toward CO2 methanation, Catalysis Today (2020), doi: https://doi.org/10.1016/j.cattod.2020.01.029

This is a PDF file of an article that has undergone enhancements after acceptance, such as the addition of a cover page and metadata, and formatting for readability, but it is not yet the definitive version of record. This version will undergo additional copyediting, typesetting and review before it is published in its final form, but we are providing this version to give early visibility of the article. Please note that, during the production process, errors may be discovered which could affect the content, and all legal disclaimers that apply to the journal pertain. © 2020 Published by Elsevier.

Electronic structure modulating for supported Rh catalysts toward CO2 methanation

Yueyue Jiang1, Junyu Lang1, Xuechen Wu1, Yun Hang Hu1,2,*

1

ro of

School of Environmental Science and Engineering, Shanghai Jiao Tong University, 800

Dongchuan Road, Shanghai 200240, China 2

Department of Materials Science and Engineering, Michigan Technological University,

-p

Houghton, Michigan 49931-1295, United States

Jo

ur

na

lP

re

Graphical abstract

Highlights 

TiO2 is the best support to constitute an effective Rh-based catalyst for CO2 hydrogenation.



The negatively charged Rh dispersed on TiO2 plays a critical role in the CO2 1

hydrogenation. 

A possible mechanism of catalytic CO2 hydrogenation is proposed.

*Corresponding author: [email protected].

Abstract Methanation of carbon dioxide is a promising approach to ameliorate greenhouse

ro of

effect. Rhodium based catalysts have been intensively investigated due to its ability in cleavage of C-O bond, but the role of support in the catalysts was underestimated. In

-p

this regard, we explored the methanation of CO2 over Rh catalysts with three metal oxide supports (TiO2, Al2O3, and ZnO). It was found that Rh/TiO2 exhibited the

re

highest catalytic activity with product yield of 455 mmol g-1cat h-1 (CH4 and CO) at 370 o

lP

C and 2 MPa, which is 2 and 14 times higher than Rh/A12O3 and Rh/ZnO,

respectively. Moreover, the CH4 selectivity over Rh/TiO2 was higher than 95%. The

na

superior catalytic performance of Rh/TiO2 can be mainly attributed to its unique electronic structure associated with stronger Rh-TiO2 interaction and the existence of

ur

Ti3+ ions on TiO2.

Jo

Keywords: CO2 methanation; Rh catalysts; oxide supports; electronic structure.

1. Introduction Excessive anthropogenic carbon dioxide (CO2) emission is a main cause for greenhouse effect and the associated climate change, leading to global warming, sea level rising, and land desertification. The conversion of CO2 into value-added 2

chemicals and fuels is a potential strategy to solve the issues. Additionally, CO2 is a cheap, nontoxic and abundant C1 feedstock. Therefore, research interest has been evoked in terms of environmental and economic viewpoints [1-3]. There are three main routes for CO2 chemical transformation: (i) reaction with hydrogen to produce carbon monoxide [4], methane [5], methanol [6], dimethyl ether [7], alkene [8], and so on; (ii) reaction with hydrocarbons, such as reforming of methane to carbon

ro of

monoxide [9] and oxidizing of propane hydrocarbon oxygenates [10]; (iii) reaction with oxy-organics to produce dimethyl carbonate from CO2 and methanol [11].

Among these, CO2 methanation (Eq. 1), namely, conversion of CO2 and renewable H2

re

chemical and petrochemical industry [12-14].

-p

into value-added methane, is a promising way to facilely synthesize a substitute in

(1)

CO2 +H2 →CO+H2 O, ∆Ho = 41.1 kJ/mol

(2)

CO+3H2 →CH4 +H2 O, ∆Ho = -206 kJ/mol

(3)

na

lP

CO2 +4H2 →CH4 +2H2 O, ∆Ho = -165 kJ/mol

However, huge amount of energy was required to activate CO2 since the average

ur

energy of C=O bond is 804.4 kJ·mol-1. The application of CO2 methanation was also obstructed due to the low selectivity towards CH4. It is widely accepted that the

Jo

production of CH4 is followed by the formation of CO via the reverse water-gas shift (RWGS) reaction (Eq. 2) and CO hydrogenation (Eq. 3). However, portion of generated CO is desorbed to the gas phase without being further methanated, leading to low CH4 selectivity [15-17]. Considering these, searching and developing effective catalysts is urgent to concurrently achieve CO2 activation and high selectivity towards

3

methane. Metal/oxide catalytic system has been widely used for CO2 methanation because it is effective to overcome the high thermodynamic and kinetic stability of CO2 molecules [18, 19]. Extensive research work has conducted on different transition metals such as Ni [20, 21], Co [22, 23], Ru [24, 25], Rh [26-28], and Pd [1, 29] supported on different metal oxides (e.g. Al2O3 [21, 29], CeO2 [24], TiO2 [28, 30],

ro of

SiO2 [1, 23], ZrO2 [26]). Ni, Ru and Rh have been demonstrated as active metals for CO2 methanation [31-33]. However, Ni based catalysts suffer from metal sintering

and deactivation. Rh-based catalysts exhibited significantly higher CO2 conversion,

-p

but lower selectivity to CH4 than Ru-based catalysts [17]. It is generally recognized

re

that supports can alter the catalytic performance of active metals. Particularly, a support can modulate the electronic structure of metal, tuning bonds between the

lP

metal and the adsorbate (Rh-CO in our case) and thus enhancing the catalytic

na

performance. However, a mechanistic understanding of the electronic effects is not yet in position to design a more active and selective catalyst.

ur

In this study, we aim to investigate the effects of supports including TiO2, ZnO and Al2O3 with the same content of rhodium for CO2 methanation. The results showed

Jo

that Rh/TiO2 was the best candidate among the three catalysts for CO2 methanation with the highest activity and selectivity towards CH4. Various characterization methods (XRD, BET, TPR, TEM, and XPS) were exploited to demonstrate the changes in physicochemical properties, morphology, and electronic structure. Furthermore, the catalytic mechanism of CO2 methanation was also evaluated by in-

4

situ DRIFTS measurement. 2. Experimental 2.1. Catalyst preparation Materials. All chemicals in this work were commercially available and used without further purification unless otherwise stated. Rhodium chloride hydrate

ro of

(RhCl3·xH2O) was supplied by Alfa Aesar. TiO2 (Aeroxide P25, Lot No. MKCD8503), aluminum oxide, and ZnO were purchased from Sigma Aldrich,

Sinopharm Chemical Reagent Co., Ltd, and from Macklin, respectively. The feed gas

-p

(H2 : CO2 = 3:1) was supplied by Air Liquide.

re

Synthesis of catalysts. All samples were prepared via the impregnation method. Specifically, for synthesis of 1 wt.% Rh/TiO2 catalyst, an aqueous solution of

lP

RhCl3·xH2O (0.4 ml, 0.1 M) was added into TiO2 powder (0.4 g). The formed paste was ground and dried at 80 oC for 16 h and calcined in muffle furnace at 500 oC for 4

na

h. The preparation of other supported Rh samples was similar to the above method with different metal oxides. In this paper, Rh/TiO2, Rh/Al2O3 and Rh/ZnO were

ur

referred to the catalysts with Rh loading of 1.0 % in weight.

Jo

2.2. Evaluation of catalytic performance Catalytic performance was evaluated in a continuous-flow reactor. Prior to all

experiments, the catalyst (5 mg) was reduced in situ at 240 °C in flowing H2 (30 ml/min) under atmospheric pressure for 3 h and then cool down to room temperature. Feed gas (H2 : CO2 = 3:1) was subsequently introduced into the reactor via a mass

5

flow controller (HORIBA METRON, MT-52) at a GHSV of 120000 cm3·g-1cat·h-1). A back-pressure valve (Tescom, 26-1765-24) was used to ensure reaction pressure (2 MPa). A K-type thermocouple was inserted into the center of the reactor to monitor the reaction temperature. The outlet gas was analyzed online using a gas chromatography (GC9800) equipped with a thermal conductivity detector (TCD) and flame ionization detector (FID). The FID was connected to a PEG-20M column,

ro of

whereas the TCD detector was connected to a TDX-01 column. CO, CO2, and CH4 were analyzed by TCD, while trace amount of CH3OH and C2H6 were detected by FID.

-p

The CO2 conversion (XCO2) and CH4 selectivity (SCH4) were calculated by using

re

the Eq. 4 and Eq. 5, respectively. R

sXCO2 = 0.25V

⁄22.4

R CH4

lP

SCH4 = 𝑅

CH4 +𝑅co

×100%

×100%

(4) (5)

na

Where R is the total yield rate of CH4 and CO (mmol·g-1·h-1), V is the gas hourly space velocity (GHSV, cm3·g-1cat·h-1), and 0.25 was the molar ratio of CO2 in feed gas.

ur

RCH4 and RCO are the yield rates of CH4 and CO.

Jo

2.3. Catalyst Characterization XRD measurements. The crystal structure of the fresh samples was studied by

X-ray diffraction (XRD) using a SHIMADZU Lab XRD-6100 diffractometer with Cu-Kα radiation equivalent to 0.15418 nm. The 2θ range scanned between 10 and 80o by steps of 0.02o with an acquisition time of 2 s at each step. Crystalline phases were identified by comparing the observed reflections with the reference ones from ICDD6

JCPDS database. SEM measurements. The morphology and elemental composition of the prepared samples were characterized by scanning electron microscopy energy dispersive spectroscopy (SEM-EDS, Sirion 200). BET surface area analysis. The specific surface area of the catalysts was determined by nitrogen sorption at 77 K (Micromeritics ASAP 2010) using the

dried in vacuum at 250 °C for 6 h before the measurement.

ro of

Brunauer Emmet and Teller (BET) method. Prior to the analysis, the samples were

H2-TPR measurements. The temperature-programmed reduction (TPR) with

-p

hydrogen for various catalyst samples was performed with an automated catalyst

re

characterization system (XianQuan, TP-5076). 40 mg of the catalyst sample was placed in the center of the reactor tube and held in place by the two quartz wool plugs,

lP

degassed with N2 flow at 120 oC for 30 min to remove moisture and cooled to room

na

temperature. The sample was then exposed to 30 sccm flow of 5% H2/ Ar gas mixture, as the reactor temperature was ramped at 10 oC/min from room temperature to 700 oC.

ur

TCD signal corresponding to H2 consumption was recorded as a function of temperature.

Jo

CO-TPD measurements. Catalyst sample (50 mg) of was pre-reduced at 240 oC

for 3 h in H2 flow and cooled down to room temperature. Then it was dried with He flow at 200 oC for 30 min. CO was allowed to flow through the prepared sample for 1 h at room temperature and then the system was purged with He flow to remove the unabsorbed CO until the mass spectrometer cannot detect any CO signal. Finally, the

7

desorption of CO was performed by heating in the same He flow from 30 and 650 oC at a heating rate of 5 oC/min. Even though CO is adsorbed in bridged, twinned, and the linear forms, a 1/1 stoichiometry of CO adsorption on Rh was assumed. Thus, the number of Rh surface atoms (NRh, mmol/g) supported on different oxides can be calculated from the amount of CO by using Eq 6. Where the Vads (ml/g) is the total amount of CO absorbed per mass of catalyst. Vads

(6)

22.4

ro of

NRh =

XPS measurements. The surface elemental composition and chemical state of

catalysts were determined by X-ray photoelectron spectroscopy (XPS) investigations

-p

on EscaLab 250Xi spectrometer equipped with an Al Ka (1486.6 eV) radiation as the

re

excitation radiation. Binding energies were corrected by setting the binding energy of the adventitious carbon (C 1s) at 284.6 eV.

lP

TEM measurements. Transmission electron microscopy (TEM) images were

na

obtained with a FEI Tecnai G2 F20 microscope, operating at 120 kV. The samples were prepared by ultrasonic dispersion of the powders in ethanol and a droplet of the

ur

dispersion was then placed onto a carbon-coated copper grid. In-situ DRIFTS measurements. The in-situ DRIFTS experiments were

Jo

conducted on Nicolet 6700 to evaluate intermediate species. The IR reactor cell was connected to a gas flow system. Prior to the experiments, 40 mg around 1%Rh/TiO2 was reduced and then flushed with He to remove residual H2. Reaction background was recorded after introducing the CO2/H2 at room temperature. Then, the IR reactor cell was heated to 395 oC with the interval of 25 oC. The absorbance spectra were

8

recorded by 32 scans with a resolution of 4 cm-1. 3. Results and discussions 3.1. Physicochemical properties The structure, composition, and phase identification of calcined catalysts were analyzed by XRD characterization as shown in Fig. 1. The XRD pattern of the

ro of

Rh/TiO2 was essentially the same as that of the pure TiO2 sample. There were two phases of TiO2 consisting of 88% anatase (#73-1764) and 12% rutile (#76-1938) in

the Rh/TiO2 catalyst. The diffraction peaks of Rh/ZnO can be indexed to the crystal

-p

phases of zinc oxide (#89-0510). Notably, the crystallinity of Rh/ZnO was enhanced

re

after being loaded with Rh while the phase composition remained unchanged. And amorphous Al2O3 (#29-0063) in Rh/Al2O3 was also identified. The raw support

lP

material contained some boehmite (AlO(OH), #88-2112), which was decomposed into Al2O3 after calcination. The diffraction lines of the supported Rh catalysts are

na

similar to those of the bare metal oxides, indicating the independence of the supports structure from the Rh loading. As shown in Table 1, loading Rh onto a support with

ur

calcination at 500 oC caused negligible change of crystalline size for TiO2 and Al2O3,

Jo

but a remarkable increase for ZnO. This happened because the calcination at 500 oC could lead to the growing of ZnO crystal particles and the doped-Rh further promoted the growing (Fig. S1 in supplementary information). In all cases, no diffraction peak assignable to Rh was observed, possibly due to their low loadings and fine particle sizes. Morphology of catalysts can be obtained from SEM images. As shown in Fig. 9

2a, single particle in agglomerates possesses uniform spherical shape with a rough surface. The as prepared particles vary in sizes range from 10 to 35 nm, which is consistent with the crystallite sizes of TiO2 determined from the XRD measurement (Table 1). TiO2 was solely observed instead of Rh, probably because the Rh particles are too small for observation by SEM. However, the EDS data of Rh/TiO2 (Fig. 2b) showed the presence of different chemical elements, especially Rh, which was not

ro of

identified by XRD analysis. The molar ratio of Rh and Ti is 0.0079, which is very close to the predicted value (0.0078) of 1 wt.% Rh/TiO2, suggesting an accurate control of the Rh loading amount during the preparation process.

-p

The specific surface areas measured by N2 absorption (77 K) were 49.2, 192.6

re

and 10.1 m2/g for Rh/TiO2, Rh/Al2O3 and Rh/ZnO, respectively (Table 1). Adsorption-desorption isotherms of all samples belong to a Type IV classification

lP

[40] (Supplementary, Fig. S2). The surface areas of Rh/TiO2 and Rh/Al2O3 catalysts

na

are similar to those of their corresponding supports (TiO2 and Al2O3). In contrast, the surface area of Rh/ZnO was much smaller than bare ZnO, because the calcination and

ur

Rh-loading could cause the growing of ZnO crystal particles (Fig. S1 in supplementary information).

Jo

The reducibility of supported Rh catalysts were obtained from TPR

measurements. As shown in Fig. 3, the reduction peaks of Rh/TiO2, Rh/Al2O3, and Rh/ZnO were located at 77, 84, and 117 oC, respectively, which were attributed to the reduction of surface Rh oxide species (RhOx → Rh0) [35]. Additionally, the hightemperature reduction peaks belonged to the reduction of larger RhOx particles [39].

10

The amount of consumed hydrogen from TPR results were summarized in Table 1. The lower reduction temperature and hydrogen consumption of Rh/TiO2 indicated that Rh nanoparticles (NPs) dispersed on TiO2 were more easily reduced to metallic phase. CO-TPD results were presented in Fig. 3 and Table 1. The CO adsorbed on Rh started to desorb at 115, 310, and 290 oC for Rh/TiO2, Rh/Al2O3 and Rh/ZnO,

ro of

suggesting different magnitude interactions of CO with surface Rh dispersed on

different oxides. The peak area indicated the amount of Rh surface atoms, which

-p

decreased in the order: Rh/Al2O3 > Rh/TiO2 > and Rh/ZnO. 3.2. Catalytic performance

re

In the CO2 methanation process, massive CH4, CO and trace CH3OH, C2H6 were

lP

detected. Therefore, the CH4 and CO were regarded as the major products to evaluate the activity of various catalysts. It should be noted that, with the exception of

na

temperature, the experimental conditions (e.g. mass of catalyst, CO2/H2 ratio, flow rate, pressure etc.) were identical. Meanwhile, the absolute yield rate of catalysts

ur

divided by their corresponding amount of surface Rh atoms, i.e. turnover frequency

Jo

(TOF), was used for comparison of catalytic activity to eliminate the influence of specific surface area. As shown in Fig. 4 (a-b), and disproportionate magnitude of changes in activity and selectivity are observed. The activity of oxides supported Rh catalysts, measured by CO2 conversion, followed the sequence: Rh/TiO2 > Rh/Al2O3 > Rh/ZnO. Specifically, the maximum difference was at 370 oC where the activity of Rh/TiO2 was 2.0 and 14.5 times larger than Rh/Al2O3 and Rh/ZnO, 11

respectively. Even decreased at the high temperature (470 oC), the activity of Rh/TiO2 was 1.3 and 3.5 times of that over Rh/Al2O3 and Rh/ZnO, respectively. In addition to the difference in TOF over the supported Rh catalysts, the products distribution among the three was also in remarkably divergence (Fig. 4b). In comparison with higher selectivity towards CO of Rh/ZnO (91.6%), higher selectivity to CH4 were observed from Rh/Al2O3 (87.8%) and Rh/TiO2 (95.8%). For its superior activity and

ro of

selectivity towards methane, Rh/TiO2 was employed to further uncover the

temperature influence on the catalytic performance in CO2 methanation process. As shown in Fig. 4c, the RWGS activity was higher than CO2 methanation at a low

-p

temperature (220 oC), and it reversed when temperature went higher. Rh/TiO2 shows

re

the highest activity (455.6 mmol·h-1·g-1 CH4 and CO yield) and CH4 selectivity (95.8%) up to 370 oC. Over this temperature, the CH4 production declines and the

lP

selectivity towards CO increases. Furthermore, as shown in Fig. 4d, as reaction time

na

increased, the production rate of CH4 increased to the maximum at 3 h and then decreased, suggesting variations of the surface structure.

ur

3.3. Effect of electronic structure

Jo

In all cases, Rh/TiO2 shows the highest activity and selectivity towards methane. The results presented above indicate that a support plays an important role in enhancing catalytic performance of CO2 methanation. To gain a deeper insight into the effect of electronic structure on CO2 methanation, the catalysts were subjected to characterizations. XPS measurements are commonly utilized to evaluate the electronic structure 12

properties of catalysts, since the charge transfer between metal and support may lead to binding energy (B.E.) shifts or line shape changes. XPS results were shown in Fig. 5 and the detailed peak information was summed up in Table 2. As shown in Fig. 5a and Fig. 5c, for all catalysts, Rh 3d5/2 peak at 306.6-307.4 eV is attributed to Rh0, a peak at 308.2-308.9 eV to Rh+, and a peak at 309.5-309.6 eV to Rh3+ [40-42]. The Rh 3d spectra of Rh/TiO2 shifted to lower value (306.6 eV for Rh 3d 5/2) compared to that

ro of

of metallic bulk Rh (307.0-307.1 eV for Rh 3d 5/2 [40-42]), indicating a charge

transfer from the partially reduced TiO2 support to the Rh NPs. In contrast, a positive shift of Rh 3d 5/2 line by 0.4 eV was observed for ZnO support, which can be

-p

attributed to the charge transfer from Rh NPs to ZnO. As shown in Fig. 5b, a shoulder

re

at lower binding energy of 307.8 eV, near the typical peak of Ti3+ [40-42], appeared in the Ti 2p spectra, suggesting partially reduced TiO2 after H2 treatment. For Al2O3

lP

and ZnO supports in the Rh-based catalysts, we could not see any significant changes

na

in the Al 2p, and Zn 2p spectra (Fig. 5d). It had been demonstrated that the Zn 2p doublet is not very sensitive to the chemical environment changes around Zn atoms

ur

[43].

TEM characterization was employed to concurrently uncover the morphology of

Jo

the supports and the Rh particles due to aforementioned inefficiency of the SEM measurements in detection of Rh particles. The Rh NPs were not identified in the TEM images of fresh and reduced Rh/TiO2, whereas the TEM images of 3h-used and 6h-used Rh/TiO2 clearly showed Rh NPs. Furthermore, the size of Rh NPs increased with increasing reaction time, which would be a reason for the declined production of

13

methane (Fig. 4d). The Rh NPs on TiO2 surface after 6 h used were in an average diameter of around 1.6 nm (Supplementary, Fig. S3), which can explain the absence of Rh diffraction peak in XRD measurement. As illustrated in HRTEM images in Fig. 6, the lattice fringes with d = 0.352 nm and d = 0.190 nm can be assigned to the (101) plane of anatase TiO2 and (200) plane of Rh, respectively [44, 45]. Notably, the

was in line with Ti3+ obtained from XPS measurement.

ro of

amorphous TiOx (x < 2) species located at the perimeter of TiO2 was observed, which

From the consistency of the above results of XPS and TEM characterization, the superior performance of Rh/TiO2 was attributed to the electronic interaction between

-p

the loaded metal and oxide support. The fresh, reduced, 3h-used and 6h-used Rh/TiO2

re

with a different proportion of Rh0 and Ti3+ were calculated and presented in Table 2. During the catalytic conversion process, the contents of Rh0 and Ti3+ reached the

lP

maximum at 3 h and then decrease, which is consistent with the yield of methane.

process.

na

This indicated that the Rh0 and Ti3+ played an essential role in CO2 methanation

ur

3.4. Mechanisms of the effect of electronic structure

Jo

The above reported results demonstrate that the support can highly influence the electronic structure of Rh NPs, playing a significant role in CO2 methanation process. Specifically, Rh dispersed on three types of support oxides display three different phenomena of electron transfer. The electrons are strongly localized within Rh/Al2O3 catalyst since Al2O3 is insulator with large band gap energy (8.8 eV) [42]. Consequently, only when the reaction temperature is extremely high (e.g., > 1000 oC) 14

would motivate the electron transfer over Al2O3 [43]. With respect to relatively narrow band gap oxide supports, the direction of charge transfer between metal and support depends on the relative position of initial fermi level (EF) of two components. In the Rh/ZnO catalyst, the electrons tended to transfer from the Rh NPs to the ZnO support since the EF of Rh is higher than that of ZnO, which as a result, the B.E. of Rh 3d shifted to a higher value (307.4 eV). In contrast, EF of the Rh is lower than that of

ro of

the TiO2, driving the electrons transferred from reduced TiO2 support to Rh NPs and then accumulate on Rh NPs, which fit well with a negative B.E. shift of Rh 3d core level in XPS results.

-p

Electron transfer usually occurs at the interfaces of the metal and support, where

re

the already existing chemical bonds are broken and new ones are formed, resulting in the formation of new phases. The amorphous TiOx species located at the perimeter of

lP

TiO2 can be observed in the HRTEM images, which is consistent with the XPS results

na

showing the formation of Ti3+ after reduction treatment of Rh/TiO2. The formation of Ti3+ species can be explained by the hydrogen spillover from Rh NPs to TiO2 support

ur

during the reduction process, which was still originated from electron transfer from essentially speaking. From the TPR results (Fig. 3), it was clearly to see that H2 began

Jo

to be activated over Rh/TiO2 as low as 77 oC. The H atoms donated electrons (e-) to Ti4+ ions forming Ti3+ at the metal-support interface, and the remaining protons (H+) bonded to O2- forming OH- at the TiO2 interface [39, 46]. The resulted Ti3+ in Rh/TiO2 may play a direct role as an active site for CO2 methanation [40, 47]. In-situ DRIFTS, which is an effective technique to evaluate the active surface

15

species [48, 49], was obtained for the hydrogenation of CO2 over reduced 1%Rh/TiO2 catalyst. As shown in Fig. 8, one can see IR absorption in region of 3000 – 3700 cm-1, which can be attributed to hydroxyl bands [50]. Furthermore, the broad peak around 1800 cm-1, which is associated with bridge-bonded CO on Rh in the form of (Rh0)2CO [51], was also observed, indicating the dissociation of CO2. The IR peak appearing in the region of 1200 – 1600 cm-1 reveals the formation of formate and carbonate-like

ro of

species [52]. The adsorbed CO and formate species increased with increasing temperature, suggesting that formate species is the key intermediate in the

methanation of CO2 over Rh catalysts. Therefore, the possible reaction mechanism is

-p

schematically illustrated in Fig. 9. CO2 is adsorbed on the support and reacts with OH

re

group to form hydrogen carbonate (-HCO3) species, while H2 is absorbed and dissociated into H atoms on Rh metal. The activated H atoms react with hydrogen

lP

carbonate to generate formate intermediate (-COOH) species at the interface of metal

na

and support. The produced formate species can further react with localized H atoms to form CO or migrate and bond to the support. Part of the CO species adsorbed on Rh

ur

can be subsequently hydrogenated to CH4 (g), whereas the rest ones desorbed to the gas phase. The electronic effect was likely to enhance the overall activity by

Jo

promoting the CO dissociation, which is a rate-determining step for CO2 methanation. The charge transfer between Rh and TiO2 support can increase the Rh-C bond strength, namely, weaken the C-O bond. In addition, the existence of Ti3+ was able to donate electrons to adsorbed CO on the Rh NPs, which was favorable for the cleavage of C-O bond [53]. In addition to the growth of Rh NPs, the gradual oxidation of Ti3+

16

by CO2 and H2O was also likely to reduce the yield rate of methane [54]. 4. Conclusions Rh supported on three different oxide supports (TiO2, Al2O3, and ZnO) were synthesized and assessed for CO2 methanation. A significant effect of the support on the catalytic activity was observed, where the order is Rh/TiO2 > Rh/A12O3 >

ro of

Rh/ZnO. The difference in catalytic activity of oxide supported Rh catalysts was attributed to the divergence in electronic structure of metallic Rh. For Rh/TiO2, electrons transferred from the TiO2 support to Rh NPs and the existence of Ti3+

-p

facilitated the dissociation of CO enhanced the catalytic activity. In contrast, for Rh/ZnO and Rh/Al2O3 catalysts, electrons were transferred to support or stably

lP

re

localized, leading to poor catalytic performance.

Credit Author Statement

na

Yun Hang Hu: Supervision, conceptualization, writing-reviewing and editing. Yueyue Jiang: Methodology, investigation, writing-original draft preparation. Junyu Lang: Methodology. Xuechen

ur

Wu: Data analysis.

Jo

Declaration of Author Financial Interest

The authors declare no competing financial interest.

17

References [1] J.N. Park, E.W. McFarland, Journal of Catalysis, 266 (2009) 92-97. [2] D. Theleritis, S. Souentie, A. Siokou, A. Katsaounis, C. Vayenas, ACS Catalysis, 2 (2012) 770-780. [3] K.R. Thampi, J. Kiwi, M. Graetzel, Nature, 327 (1987) 506.

ro of

[4] C.S. Chen, W.H. Cheng, Catalysis Letters, 83 (2002) 121-126. [5] F. Wang, S. He, H. Chen, B. Wang, L. Zheng, M. Wei, D.G. Evans, X. Duan, Journal of the American Chemical Society, 138 (2016) 6298-6305.

Chemistry C, 118 (2014) 24452-24466.

-p

[6] J. Qu, X. Zhou, F. Xu, X.Q. Gong, S.C.E. Tsang, The Journal of Physical

re

[7] G. Bonura, M. Cordaro, C. Cannilla, A. Mezzapica, L. Spadaro, F. Arena, F.

lP

Frusteri, Catalysis Today, 228 (2014) 51-57.

[8] R.W. Dorner, D.R. Hardy, F.W. Williams, H.D. Willauer, Catalysis

na

Communications, 15 (2011) 88-92.

[9] Q.J. Chen, J. Zhang, Q.W. Jin, B.R. Pan, W.B. Kong, T.J. Zhao, Y.H. Sun,

ur

Catalysis Today, 215 (2013) 251-259.

Jo

[10] R.G. Hu, S.H. Zhong, Progress In Chemistry, 16 (2004) 751-757. [11] B. Lu, X. Wang, Y. Li, J. Sun, J. Zhao, Q. Cai, Journal of CO2 Utilization, 3 (2013) 98-101.

[12] A. Westermann, B. Azambre, M.C. Bacariza, I. Graça, M.F. Ribeiro, J.M. Lopes, C. Henriques, Applied Catalysis B: Environmental, 174 (2015) 120-125. [13] A. Karelovic, P. Ruiz, Applied Catalysis B: Environmental, 113-114 (2012) 23718

249. [14] C. Swalus, M. Jacquemin, C. Poleunis, P. Bertrand, P. Ruiz, Applied Catalysis B: Environmental, 125 (2012) 41-50. [15] P. Panagiotopoulou, D.I. Kondarides, X.E. Verykios, The Journal of Physical Chemistry C, 115 (2011) 1220-1230. [16] P. Panagiotopoulou, D.I. Kondarides, X.E. Verykios, Catalysis today, 181 (2012)

ro of

138-147.

[17] P. Panagiotopoulou, Applied Catalysis A: General, 542 (2017) 63-70.

[18] B. Andreas, J. Schwarz, C.U. Maier, Journal of the Electrochemical Society, 140

-p

(1993), 1006-1008.

Cells, 122 (2014) 183-189.

re

[19] M.M. Gui, S.P. Chai, B.Q. Xu, A.R. Mohamed, Solar Energy Materials and Solar

na

249 (2007) 370-379.

lP

[20] G. Du, S. Lim, Y. Yang, C. Wang, L. Pfefferle, G.L. Haller, Journal of Catalysis,

[21] J. Yang Lim, J. McGregor, A.J. Sederman, J.S. Dennis, Chemical Engineering

ur

Science, 141 (2016) 28-45.

[22] J. Janlamool, P. Praserthdam, B. Jongsomjit, Journal of Natural Gas Chemistry,

Jo

20 (2011) 558-564.

[23] G. Zhou, T. Wu, H. Xie, X. Zheng, International Journal of Hydrogen Energy, 38 (2013) 10012-10018. [24] S. Sharma, K.B. Sravan Kumar, Y.M. Chandnani, V.S. Phani Kumar, B.P. Gangwar, A. Singhal, P.A. Deshpande, The Journal of Physical Chemistry C, 120

19

(2016) 14101-14112. [25] S. Eckle, M. Augustin, H.G. Anfang, R.J. Behm, Catalysis today, 181 (2012) 4051. [26] T. Inoue, T. Iizuka, K. Tanabe, Applied catalysis, 46 (1989) 1-9. [27] D. Heyl, U. Rodemerck, U. Bentrup, ACS Catalysis, 6 (2016) 6275-6284. [28] J.C. Matsubu, V.N. Yang, P. Christopher, Journal of the American Chemical

ro of

Society, 137 (2015) 3076-3084.

[29] J.H. Kwak, L. Kovarik, J. Szanyi, ACS Catalysis, 3 (2013) 2094-2100. [30] A. Karelovic, P. Ruiz, Journal of Catalysis, 301 (2013) 141-153.

-p

[31] M. Younas, L. Loong Kong, M.J.K. Bashir, H. Nadeem, A. Shehzad, S.

re

Sethupathi, Energy & Fuels, 30 (2016) 8815-8831.

[32] S. Matthischke, M. Schlüter, M. Götz, J. Lefebvre, P. Prabhakaran, S. Bajohr,

lP

Fuel, 166 (2016) 276-296.

na

[33] A. Galadima, O. Muraza, Renewable and Sustainable Energy Reviews, 115 (2019) 109333.

ur

[34] J. Broekhoff, Studies in Surface Science and Catalysis, 3 (1979) 663-684. [35] J.C. Vis, H.F.J.v.t. Blik, T. Huizinga, J.V. Grondelle, R. Prins, Journal of

Jo

Catalysis, 95 (1985) 333-345.

[36] R. State, M. Scurtu, A. Miyazaki, F. Papa, I. Atkinson, C. Munteanu, I.C. Balint, Arabian Journal of Chemistry,10 (2017) 975-984. [37] J.F. Moulder, Physical electronics, 1995, 230-232. [38] K. Dohmae, T. Nonaka, Y. Seno, Surface and Interface Analysis, 37 (2005) 115-

20

119. [39] S. Suárez, M. Yates, A.L. Petre, J.A. Martín, P. Avila, J. Blanco, Applied Catalysis B: Environmental, 64 (2006) 302-311. [40] M. Ahmadi, H. Mistry, B. Roldan Cuenya, The Journal of Physical Chemistry Letters, 7 (2016) 3519-3533. [41] X. Wang, H. Shi, J.H. Kwak, J. Szanyi, ACS Catalysis, 5 (2015) 6337-6349.

ro of

[42] J. Robertson, Journal of vacuum science & technology B, 18 (2000) 1785-1791. [43] Q. Fu, T. Wagner, Surface Science Reports, 62 (2007) 431-498.

[44] T. Kawahara, Y. Konishi, H. Tada, N. Tohge, J. Nishii, S. Ito, Angewandte

-p

Chemie, 114 (2002) 2935-2937.

118 (2006) 1310-1314.

re

[45] N. Zettsu, J.M. Mclellan, B. Wiley, Y. Yin, Z. Li, Y. Xia, Angewandte Chemie,

lP

[46] R. Prins, Chemical Reviews, 112 (2012) 2714-2738.

na

[47] J.C. Matsubu, S. Zhang, L. DeRita, N.S. Marinkovic, J.G. Chen, G.W. Graham, X. Pan, P. Christopher, Nature chemistry, 9 (2016) 120-127.

ur

[48] J. Ashok, M. Ang, S. Kawi, Catalysis Today, 281 (2017) 304-311. [49] Y. Yu, Y.M. Chan, Z. Bian, F. Song, J. Wang, Q. Zhong, S. Kawi, International

Jo

journal of hydrogen energy, 43 (2018) 15191-15204.

[50] G. Busca, Physical Chemistry Chemical Physics, 1 (1999) 723-736. [51] I.A. Fisher, A.T. Bell, Journal of Catalysis, 162 (1996) 54-65. [52] S. Eckle, Y. Denkwitz, R.J. Behm, Journal of Catalysis, 269 (2010) 255-268. [53] J. Rasko, F. Solymosi, The Journal of Physical Chemistry, 98 (1994) 7147-7152.

21

Jo

ur

na

lP

re

-p

ro of

[54] C. Deleitenburg, A. Trovarelli, Journal of Catalysis, 156 (1995) 171-174.

22

ro of -p re lP

Jo

ur

na

Fig. 1. XRD patterns of supported Rh catalysts and their corresponding oxide supports.

Fig. 2. (a) SEM image and (b) Corresponded EDS spectrum of fresh 1%Rh/TiO2.

23

Jo

ur

na

lP

re

-p

ro of

Fig. 3. (a) TPR and (b) CO-TPD curves of 1%Rh/TiO2, 1%Rh/Al2O3, and 1%Rh/ZnO.

Fig. 4. (a) Catalytic activity of CO2 methanation over Rh supported on different oxides; (b) Selectivity of CH4 on oxide-supported Rh catalysts measured at 270, 370 and 470 oC under 2 MPa; (c) Dependence of yield rate of 1%Rh/TiO2 on temperature under 2 MPa; (d) CH4 production rate versus reaction time over 1%Rh/TiO2 under the condition of 2 MPa and 370 o C.

24

ro of -p re lP na ur Jo

Fig. 5. XPS spectra of catalysts: (a) Rh 3d and (b) Ti 2p core level spectra of fresh, reduced, 3h-used and 6h-used 1%Rh/TiO2; (c) Rh 3d core level spectra and (d) Ti 2p, Al 2p, Zn 2p spectra of reduced 1%Rh/TiO2, 1%Rh/Al2O3 and1% Rh/ZnO.

25

ro of -p re lP na ur Jo

Fig. 6. TEM (left) and HRTEM (right) images of 1%Rh/TiO2: (a) fresh, (b) reduced, (c) 3hused, and (d) 6h-used.

26

ro of -p re

Jo

ur

na

lP

Fig. 7. HAADF-STEM image of 6h-used 1%Rh/TiO2 and the corresponding EDX mappings, indicating the particular distributions of Ti, O, and Rh element.

Fig. 8. In-situ DRIFTS for the methanation of CO2 to CH4 over 1%Rh/TiO2 catalyst.

27

ro of

Fig. 9. (a) Electronic structure changes in Rh/TiO2 catalyst and (b) Proposed mechanism of CO2 methanation over supported Rh catalysts.

na

lP

re

-p

Table 1. Physicochemical characteristics of prepared catalysts. crystallite sizea BET surface H2 consumptionb catalyst (nm) area (m2/g) (umol/g) TiO2 21.9 55.7647 / 1%Rh/TiO2 22.1 49.2007 71.25 Al2O3 11.9 190.7745 / 1%Rh/Al2O3 13.8 192.6518 103.8 ZnO 13.9 26.9052 / 1%Rh/ZnO 49.2 10.0894 200.2 a Calculated from the XRD results in Fig. 1 using Scherrer equation. b Quantified by integrating the H2-TPR peaks in Fig. 3a. c Estimated by employing CO-TPD method.

exposed Rhc (mmol/g) / 0.00662 / 0.01 / 0.00643

Table 2. Summary of detailed XPS information from Fig. 5.

Rh+

Rh0

309.5

308.2

0

reduced

308.2

3h-used

ur

Rh3+

Rh0/Rh (%) -

Catalyst

E (Rh 3d5/2), eV

E (Ti 2p3/2), eV

Ti3+/Ti (%)

Ti4+

Ti3+

0

458.5

0

0

306.6

42.3

458.5

457.8

7.6

308.2

306.6

49.7

458.5

457.8

11.7

6h-used

308.2

306.6

45.9

458.5

457.8

10.3

1%Rh/Al2O3 309.6

308.6

307.1

31.3

1%Rh/ZnO

308.9

307.4

44.0

Jo

fresh

1%Rh/TiO2

reduced

28

29

ro of

-p

re

lP

na

ur

Jo