Ni-sepiolite and Ni-todorokite as efficient CO2 methanation catalysts: Mechanistic insight by operando DRIFTS

Journal Pre-proof Ni-Sepiolite and Ni-Todorokite as efficient CO2 methanation catalysts: Mechanistic insight by operando DRIFTS ´ Cristina Cerda-Moreno (Investigation) (Data curation) (Writing review and editing)Funding aquisation), Antonio Chica (Conceptualization)Writing review and editing) (Supervision), Sonja Keller (Data curation), Christine Rautenberg (Investigation) (Data curation), Ursula Bentrup (Conceptualization) (Writing - original draft) (Supervision)

PII:

S0926-3373(19)31292-5

DOI:

https://doi.org/10.1016/j.apcatb.2019.118546

Reference:

APCATB 118546

To appear in:

Applied Catalysis B: Environmental

Received Date:

9 July 2019

Revised Date:

16 December 2019

Accepted Date:

18 December 2019

´ Please cite this article as: Cerda-Moreno C, Chica A, Keller S, Rautenberg C, Bentrup U, Ni-Sepiolite and Ni-Todorokite as efficient CO2 methanation catalysts: Mechanistic insight by operando DRIFTS, Applied Catalysis B: Environmental (2019), doi: https://doi.org/10.1016/j.apcatb.2019.118546

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Ni-Sepiolite and Ni-Todorokite as efficient CO2 methanation catalysts: Mechanistic insight by operando DRIFTS

Cristina

Cerdá-Morenoa,

Antonio

Chicaa,

Sonja

Kellerb,

Christine

Rautenbergb,

Ursula Bentrupb,*

a

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Instituto de Tecnología Química, Universitat Politècnica de València-Consejo Superior de

Investigaciones Científicas, Avenida de los naranjos s/n, 46022 Valencia, Spain b

Leibniz-Institut

für

Katalyse

re

Corresponding author. E-mail address: [email protected]

Jo

ur

na

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*

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Albert-Einstein-Str. 29a, 18059 Rostock, Germany

e.V.

Graphical abstract

(LIKAT),

2 Highlights: 

Sepiolite- and todorokite-based Ni catalysts were tested in CO2 methanation.



Based on operando DRIFTS studies, specific reaction pathways were proposed for both types of catalysts, respectively.



The ability of Ni-todorokite catalysts to activate CO2 via a dissociative as well as

an



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associative mechanism explains their excellent catalytic performance. The influence of the applied preparation method on the catalytic performance was

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demonstrated for the Ni-sepiolite catalysts.

Abstract

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The hydrogenation of CO2 to methane was studied on Ni-sepiolite and Ni-todorokite catalysts. Catalytic testing and operando DRIFTS studies revealed the excellent performance of in

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particular todorokite-based catalysts. The catalytic activity is related to specific reaction pathways depending on the support material. Based on operando DRIFTS studies, different

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mechanisms are proposed for the methanation reaction. Over Ni-sepiolite catalysts, a dissociative adsorption of CO2 is observed. Linearly and bridged bonded CO on Ni0 was identified as intermediate. On Ni-todorokite this dissociative mechanism is accompanied by an associative one. Here, carbonate species were identified as additional intermediates, the formation of which is facilitated by the MnOx support. The ability of Ni-todorokite catalysts to activate CO2 via a dissociative as well as an associative mechanism explains their excellent

3 catalytic performance. The influence of the applied preparation method on the catalytic performance was demonstrated for the Ni-sepiolite catalysts.

Keywords: operando DRIFTS, CO2 hydrogenation, mechanism, Ni-sepiolite, Ni-todorokite

1. Introduction

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The reduction of CO2 emissions, which contribute to greenhouse effect and thus to climate change, is a major challenge in the 21st century. Besides diminishing CO2 emissions, its use as a feedstock in chemical processes offers a complementary strategy to close the anthropogenic

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carbon cycle [1-4]. The hydrogenation of CO2 to CH4 using sustainable H2 is an advantageous reaction with respect to thermodynamics and is highly important from an industrial viewpoint

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because there are several uses of methane within the commercial infrastructure. But it has to be

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considered that an effective and economically attractive method for the production of hydrogen is an essential requirement to mitigate CO2 emissions by CO2 hydrogenation processes. In the past, various methanation catalysts have been investigated. Besides other metals (e.g. Ru, Rh,

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Fe, Co), in particular Ni supported on metal oxides (e.g. Al2O3, TiO2, CeO2, Ce0.5Zr0.5O2) cat-

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alyze the methanation of CO2 [5-8].

Various studies, often based on in situ infrared spectroscopy, have been carried out in the past

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to unravel the CO2 methanation mechanism. In principle, two types of mechanisms are discussed, the associative and dissociative CO2 methanation mechanism [7,8]. The first one involves the associative adsorption of CO2 and H2 with accompanied formation of oxygenate species like carbonates and formates which are then hydrogenated to form methane. On the other hand, CO2 directly dissociates to CO and O both adsorbed at the catalyst surface, while adsorbed CO is subsequently methanated. Although CO2 methanation processes have been extensively studied on various catalyst systems (vide infra) the exact mechanism is still under

4 debate, which seems to be related to the variabilities of reaction pathways associated with the specific catalyst properties. Studying the CO2 methanation at Ru/TiO2, Marwood et al. [9] discussed the existence of interfacial formate as precursor for CO as reaction intermediate in the pathway to methane besides unreactive formates on the support. In a similar manner, Panagiotopoulou et al. [10] explained the mechanism of CO2 hydrogenation on the same type of catalyst. They found that CO2 is adsorbed on the support on which it reacts on the metal-support interface with adsorbed hydro-

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gen to form formate and carbonyl species.

A nondissociative mechanism with formate as the pivotal surface intermediate was also proposed for the CO2 hydrogenation reaction over nickel zirconia catalysts while doubly and singly

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bound adsorbed CO was detected besides, and identified as precursor for methane formation

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[11]. In contrast, Aldana et al. [12] concluded from operando FTIR spectroscopic studies on Ni-based ceria-zirconia catalysts that the main CO2 methanation mechanism does not neces-

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sarily require CO as reaction intermediate. Thus, H2 would be dissociated on Ni0 sites while CO2 is activated on the support to form carbonates which are hydrogenated into formates and

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further into methoxy species and methane. Weak basic sites are postulated to be involved in the adsorption of CO2, implying a stable metal-support interface. In this way, the much better ac-

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tivity of these catalysts compared to Ni-silica could be explained by the fact that both, CO2 and H2 are activated on Ni0 particles.

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A promotional effect of medium basic sites was found by comparing the CO2 methanation reaction over Ni/-Al2O3 and Ni/Ce0.5Zr0.5O2 [13,14]. The higher activity of the latter catalyst was explained by the preferred formation of monodentate carbonate species on medium basic sites which can be more quickly hydrogenated than bidentate formate species derived from hydrogen carbonate as formed on Ni/-Al2O3.

5 Westermann et al. [15] studied the CO2 methanation over Ni/USY. This ultra-stable Y-zeolite was chosen as support to limit the adsorption sites for CO2, because of its high acidity. While in absence of hydrogen CO2 is not adsorbed over the acidic zeolite, formates and carbonyls were detected when H2 is present. Thus, the authors concluded that CO2 hydrogenation mechanism does not probably pass through carbonate formation, but rather through formate dissociation on Ni0 particles. Hence, considering the absence of basic sites, CO seems to be the true intermediate in the CO2 methanation reaction.

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The dissociative mechanism for CO2 hydrogenation at low temperature was discussed for Rh/Al2O3 catalysts [16]. According operando DRIFTS studies, CO2 adsorption and dissociation proceeds readily over the catalysts when H2 is present, resulting in the formation of different

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Rh carbonyl species. The latter are the precursors of methane, while additional formate species, rather formed by reaction of CO with OH groups of support, were proposed to be spectator

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species, which do not significantly contribute to methane formation. Similar conclusions have

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been made from in situ infrared spectroscopic studies of the CO2 methanation over Rh/SiO2 as well as Ru/Al2O3 catalysts [17,18]. The reaction proceeds via the dissociation of CO2 into ad-

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sorbed CO which is then methanated, while the formation and decomposition of surface formates plays only a minor role.

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Density functional theoretical (DFT) studies of CO2 methanation on Ni(111) surfaces have been carried out by Ren et al. [19] with and without the formation of CO as an intermediate. Three

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possible mechanisms were identified, where the path starting with CO2 dissociation into CO and O, CO decomposition into C and O species and C species hydrogenation to form methane was proved to be the optimum. Scrutinizing the reaction mechanism of CO2 methanation on Pd-MgO/SiO2 [20], a bifunctional mechanism was proposed based on DFT studies which revealed different roles of MgO and Pd

6 nanoparticles. Thus, it was demonstrated that MgO binds CO2 by formation of carbonate species, while atomic H supplied by H2 activation on Pd is essential for the further hydrogenation of the carbonate species to methane. Summarizing the mentioned literature data concerning the mechanism of CO2 hydrogenation in particular of Ni-containing catalysts, it seems that the nature of the support plays an important role because it not only influences the dispersion of the metal particles but also acts as a catalyst component for CO2 activation. Comparing the CO2 methanation activity of different Ni-sup-

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ported catalysts [11-15, 21-27] it is evident that rather basic support materials like CeO2/ZrO2, or modified hydrotalcites facilitate the adsorption and activation of CO2. But it is also evident that the CO2 methanation mechanism can differ depending on the used support.

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With the intention of bringing together two aspects, the necessity of developing methanation

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catalysts with high activity and selectivity for possible industrial application as well as mechanistic insights needed for a rational catalyst design, we report here the use of Ni-sepiolite and

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Ni-todorokite as methanation catalysts. Because of the totally different nature of the support materials, natural sepiolite is a fibrous magnesium silicate clay mineral [28] and todorokite an

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octahedral molecular sieve of manganese oxide [29], the respective Ni catalysts show different catalytic performance. As demonstrated by operando DRIFTS studies, the catalytic activity is

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related to specific reaction pathways depending on the support material, which contributes to CO2 activation in different manner. Furthermore, we show for the Ni-sepiolite catalysts by

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means of several characterization methods and operando DRIFTS that the applied preparation method influences the catalytic performance.

2. Experimental 2.1. Catalyst preparation

7 Ni-sepiolite catalysts were prepared by two different methods: incipient wetness impregnation (IWI) and precipitation (P). For IWI method, an aqueous solution containing the required amount of Ni(NO3)2·6H2O (Sigma-Aldrich) to achieve a nominal concentration of 5 wt% of Ni in the catalyst was prepared. Then, this solution was added to sepiolite (Pangel S9, supplied by Tolsa) dropwise and the material was dried at 100°C. Regarding P method, a suspension of sepiolite in water was prepared and a solution with the required amount of Ni(NO3)2·6H2O to obtain 5 wt% of Ni in the catalyst was added to the previous sepiolite suspension under agita-

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tion. Afterwards, Ni was precipitated using a NaOH (1M) solution. The solid was washed with distilled water until pH 7 and dried at 100°C. All samples were finally calcined at 450°C for 3 h.

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Concerning Ni-todorokite catalyst, the synthesis of the todorokite and Ni incorporation occurred simultaneously, following the procedure described by Onda et al. [30]. Briefly, the lay-

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ered precursor Ni-birnessite was firstly obtained by adding an aqueous solution containing

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MnCl2·4H2O (Sigma-Aldrich) and Ni(NO3)2·6H2O (Sigma-Aldrich) into another aqueous solution of KMnO4 (Sigma-Aldrich) and NaOH (Scharlab S.L.) under stirring and aging the mix-

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ture for 24 h. After aging, the material was washed until pH 7 and ion-exchanged with a solution of Ni(NO3)2·6H2O (0.5 M) for 24 h at room temperature resulting in Ni-buserite. Finally, the

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solid was filtered and transferred to a PTFE-Lined stainless-steel autoclave and kept at 160°C for 48 h obtaining, after washing and drying at 60°C, Ni-todorokite. In addition, Ni-free birnes-

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site was also prepared following a procedure similar to that already described for Ni-todorokite. An aqueous solution containing MnCl2·4H2O (Sigma-Aldrich) was added to another aqueous solution of KMnO4 (Sigma-Aldrich) and NaOH (Scharlab S.L.) under stirring. The mixture was aged for 24 h and washed until pH 7. The Ni-free birnessite was dried at 100°C and calcined at 450°C for 3 h.

8 The Ni/Al2O3 catalysts with 5 wt% and 15 wt% of Ni were prepared as reference catalysts by the IWI method. These catalysts were dried at 100°C and calcined at 450°C for 3 h. An overview of all prepared catalysts, the applied preparation methods, Ni contents, and BET surface

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areas is given in Table 1.

Table 1. Overview of studied catalysts: preparations, Ni contents and BET surface areas Preparation method

Ni content (wt%)

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Catalyst

Surface area (m2 g-1)

precipitation

6.4

5Ni/Sep (IWI)

IWI

5.5

5Ni/Al2O3

IWI

15Ni/Al2O3

IWI

15Ni-Tod (nc)*

Ref. 30

15.1

13

15Ni-Tod

Ref. 30

16.6

26

Birnessite

Ref. 30



28

141 291 196

15.9

160

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5.2

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* non-calcined

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5Ni-Sep (P)

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2.2. Catalytic activity measurements

Catalytic tests were carried out in a fixed bed reactor at atmospheric pressure, temperature be-

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tween 250 and 450°C, GSHV of 9000 and 36000 mL·gcat-1·h-1 and H2 : CO2 = 4. The different catalysts were pelletized, crushed and sieved into particles of 250-425 μm and diluted with SiC to obtain a bed volume of 5 cm3. Before reaction, the catalysts were activated in situ in H2 flow at 450°C for 2 h and cooled down to 250°C in N2 flow. Afterwards, a 18 vol% CO2, 72 vol% H2 and 10 vol% N2 flow was fed into the reactor. The composition of the outlet stream was analyzed online with a Varian 3800 gas chromatograph, equipped with two columns (HayeSep

9 Q and MolSieve 13X) and two detectors (TCD and FID). The CO2 conversion was calculated using eq. 1 from the inlet and outlet molar flows, while CO and CH4 selectivities were estimated considering the molar flows of these products (eq. 2). The molar flows of each compound were quantified using N2 as internal standard, and the C balance considering the C-containing compounds in the inlet and outlet of the reactor was calculated.

𝑆𝑖 (%) = ∑

𝑛̇ 𝐶𝑂2 ,0 −𝑛̇ 𝐶𝑂2 ,𝑓 𝑛̇ 𝐶𝑂2 ,0 𝑛̇ 𝑖,𝑓 −𝑛̇ 𝑖,0 (𝑛̇ 𝑖,𝑓 −𝑛̇ 𝑖,0 )

∙ 100

(1)

∙ 100

(2)

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𝑋𝐶𝑂2 (%) =

The molar flows of CO2 and products (CO and CH4) are 𝑛̇ 𝐶𝑂2 and 𝑛̇ 𝑖 , respectively, being i either

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CO or CH4, while the subscripts 0 and f refer to the values in the inlet and outlet of the reactor.

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2.3. Catalyst characterization

For determining the composition of the prepared catalysts, inductively coupled plasma optical

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emission spectrometry (ICP-OES) was applied using a Varian 715-ES ICP-Optical Emission

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spectrometer.

Nitrogen adsorption-desorption isotherms were collected at 77 K on a Micromeritics ASAP

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2420. The specific surface area of fresh catalysts was calculated from the corresponding isotherms applying the Brunauer, Emmett, and Teller equation for the N2 relative pressure range

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of 0.05 < P/P0 < 0.30.

The XRD pattern of the fresh samples were measured on a theta/theta diffractometer (X’Pert Pro from Panalytical, Almelo, Netherlands), with CuK radiation ( = 1.5418 Å). The phase composition of the samples was determined using the program suite WinXPOW by STOE&CIE with inclusion of the Powder Diffraction File PDF2 of the ICDD (International Centre of Diffraction Data).

10 The temperature-programmed reduction measurements (H2-TPR) were performed on a Micromeritics Autochem 2910. The sample (50 mg) was pretreated in Ar flow at room temperature, then the gas was switched to H2:Ar (10:90) flow and the temperature increased to 900°C at a heating rate of 10°C·min-1. The H2 consumption was monitored using a TCD. The H2 chemisorption experiments were carried out on a Quantachrome Autosorb-1C. Prior to adsorption, the samples were reduced in situ at the same conditions used before catalytic tests (H2 flow, 10°C/min up to 450°C for 2 h). After reduction, the samples were degassed at the

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same temperature for 2 h and, afterwards, the temperature was lowered to 30°C for isotherms measurement. The metallic surface area were calculated from chemisorbed H2 assuming a stoichiometry of Ni/H = 1.

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Transmission electron microscopy (TEM) measurements were performed on a JEOL-JEM-

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2100F operating at 200 kV in order to obtain the distribution of Ni0 particle size. Previously, the catalysts were reduced in H2 flow at 450°C for 2 h. In addition, field-emission scanning

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electron microscope (FESEM) (Zeiss Ultra 55) was used to inspect the morphology of the different catalysts.

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UV-vis spectra were measured in diffuse reflection at room temperature using an AvaSpec 2048 fiber optic spectrometer (Avantes) equipped with an AvaLight-DHS light source and a FCR-

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7UV400-2-ME reflection probe.

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In situ FTIR spectra of adsorbed CO were recorded in transmission mode on a Nicolet iS10 spectrometer (Thermo Scientific) equipped with a DTGS detector and an in-house developed reaction cell with CaF2 windows connected to a gas-dosing and evacuation system. The sample powders were pressed into self-supporting wafers with a diameter of 20 mm and a weight of 50 mg. The samples were pretreated external by heating in synthetic air at 400°C for 1h. Then, the wafer was transferred into the low-temperature cell. Before adsorbing CO, the samples were

11 pretreated again in synthetic air for 30 min at 200°C. After cooling down to -160°C, a background spectrum of the sample was recorded. Then, a mixture of 5 vol% CO in He was pulsed until saturation, as checked by the respective measured spectra. Before recording the CO adsorbate spectrum, the physisorbed CO was removed by evacuating the cell. Subsequently, also the CO desorption under vacuum was followed by continuous heating the sample and measuring a spectrum at respective temperatures. The spectra were collected at 4 cm-1 resolution and

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64 scans. Generally, background-subtracted spectra are shown.

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2.4. Operando DRIFTS measurements

For the DRIFTS measurements a commercial reaction cell (Harrick) with CaF2 windows was

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implemented into a Nicolet 6700 FTIR spectrometer (ThermoFischer Scientific) equipped with a MCT detector. The spectra were collected at 4 cm-1 resolution and 64 scans. For product

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analytics the gas outlet of the reaction cell was connected to a quadrupole mass spectrometer (OmniStar, Pfeiffer Vacuum). Samples with a defined particle size of 250-315 μm were placed

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in the cell, which practically acts as fixed-bed flow reactor. After pretreatment in H2 (30 mL/min) at 600°C for 2 h, the sample was cooled down in He to the first reaction temperature

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at 250°C and exposed to a gas mixture containing 18 vol% CO2 and 72 vol% H2 and 0.1 vol%

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Ne as internal standard balanced with He with a total gas flow rate of 30 mL/min for 30 min. During subsequent flushing with He for 20 min the sample was heated to the next reaction temperatures 300 and 350°C, respectively and exposed to the methanation feed again. For the transient experiments, the samples were exposed after reductive pretreatment firstly to 18 vol% CO2/He for 45 min and subsequently treated with 72 vol% H2/He for 45 min at respective temperatures. It has to be mentioned that due to the temperature gradient in the Harrick cell the temperatures at the surface of the catalyst bed are lower than that measured at the bottom. Thus,

12 the measured surface temperatures are 170°C, 204°C, 240°C at the set temperatures 250°C, 300°C, 350°C, respectively. In the following the set temperatures are indicated.

3. Results and discussion 3.1. Catalytic activity Figure 1 shows the conversion of CO2 and the selectivity of CH4 measured for the different

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catalysts between 250 and 450°C at the same GHSV. Compared with 5Ni/Al2O3, which was prepared as reference catalyst, the differently prepared sepiolite-based catalysts are much more active (Figure 1a). The Ni catalyst prepared by precipitation, 5Ni-Sep (P), shows a distinct

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better performance than that prepared by incipient wetness impregnation, 5Ni/Sep (IWI). Under the same reaction conditions, the performance of the 15Ni-Tod catalyst was also superior to

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that of the respective reference catalyst 15Ni/Al2O3 with the same Ni content (Figure 1b). The 15Ni-Tod catalyst was also tested without previous calcination (15Ni-Tod (nc)) and shows

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higher values of CO2 conversion at those temperatures at which equilibrium is not reached (Figure S1). For the sake of completeness, it should be mentioned that the Ni-free birnessite

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catalyst was catalytically inactive.

There is clear evidence that the catalytic performance depends on the used support as well as

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the preparation method. Therefore, for deeper understanding this different behavior, the cata-

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lysts have been comprehensively characterized and studied by operando DRIFT spectroscopy as described in the following.

3.2. Catalyst characterization The sepiolite-based catalysts were characterized in un-reduced form by XRD, TPR, as well as UV-vis-DRS and DRIFT spectroscopy. The XRD pattern (Figure S2) of the sample 5Ni/Sep

13 (IWI) shows the typical reflections of sepiolite (ICDD 01-080-5781), while in the pattern of 5Ni-Sep (P) besides the sepiolite peaks also intensive reflections of Mg8Si12O30(OH)4 (ICDD 00-026-1227) are detectable. It is known that sepiolite loses zeolitic water between 350 and 450°C and forms sepiolite ”anhydride” which is accompanied by changes of the lattice parameters [33]. However, although both catalysts were calcined at the same temperature, the sepiolite structure remains intact in the case of 5Ni/Sep (IWI). The catalysts show a different reducibility as indicated by the respective TPR profiles (Figure

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S3). Thus, the reduction of 5Ni-Sep (P) starts at lower temperatures in comparison to 5Ni/Sep (IWI). The different reducibility of both catalysts seems to be related to different NiO particles sizes obtained by the different methods applied for the preparation of the fresh catalysts. Indi-

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cations for this conclusion are provided by the UV-vis-DR spectra of the calcined, un-reduced samples (Figure 2a). The broad band below 450 nm results from O2(2p)  Ni2+(3d) charge

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transfer (CT) transition, and the low intensity band around 745 nm from d-d transition [34,35].

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The fact that pure sepiolite show a CT band similar to that of 5Ni/Sep (IWI) is due to most probably Fe impurities (0.5% Fe detected by ICP in the uncalcined sepiolite). The spectrum of

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5Ni-Sep (P) looks different and is characterized by a rather narrow CT band, and the bands at 423 and 740 nm, which are typical for NiO-like Ni2+ in octahedral surrounding, are seen more

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clearly. This indicates the presence of larger NiO particles dominating the shape of the UV-visDRS spectrum.

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Inspecting the (OH) region of the DRIFT spectra recorded after reductive pretreatment several well resolved bands are visible (Figure 2b). It has to be mentioned here that similar spectra have been obtained after oxidative treatment at the same temperature. According literature data [36,37], the bands at 3734/3702 cm-1 can be assigned to (SiOH), the intensive band at 3671 cm-1 to (MgOH), and the bands at 3590/3529 to (OH) of Mg-coordinated water. The bands

14 at 3017/2980 cm-1 are also related to (OH), most probably stemming from strongly coordinated OH groups as also observed in oxide hydroxides [38,39]. While the spectrum of 5Ni-Sep (P) looks very similar to that of the pure sepiolite sample, the bands in the spectrum of 5Ni/Sep (IWI) are not well resolved indicating the involvement of Mg-coordinated water within the impregnation process. Additionally, new OH groups are created indicated by the shoulders around 3702 and 3635 cm-1. Taking into account both, the UV-vis-DRS and DRIFTS results, it seems that in the case of 5Ni-Sep (P) the larger NiO particles are mainly present at the outer

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surface, while in 5Ni/Sep (IWI) rather NiO agglomerates are present in the pores. The XRD patterns of the uncalcined and calcined 15Ni-Tod samples exhibit a different phase composition, which could explain the different catalytic activity (cf. Figure S1). The non-cal-

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cined sample 15Ni-Tod (nc) (Figure S4) shows the typical pattern of todorokite [30,40], while in the calcined sample NiMnO3 (ICDD 00-048-1330) and Mn5O8 (ICDD 00-039-1218) are de-

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tectable formed by a structural transformation induced by the calcination process.

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For characterizing the nature, oxidation state and accessibility of the Ni species in the calcined, un-reduced samples, FTIR spectroscopic measurements of the CO adsorption at low tempera-

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ture were applied. Thus, also species which are not stable at room temperature can be detected [41,42]. After oxidative pretreatment the CO adsorption was carried out at -160°C, followed by

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subsequent temperature programmed desorption of CO. The obtained CO adsorbate spectra for the different Ni catalysts and pure sepiolite at -150°C as well as -100°C are shown in Figure 3.

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The bands around 2193, 2184, 2173, 2163 cm-1 can be assigned to (Ni2+CO) vibrations resulting from Ni2+ located at different sites [41-45], while the bands appearing in the sepiolite samples below 2160 cm-1 are related to CO interacting with SiOH and MgOH groups as well as to physisorbed, liquid-like CO (Figure 3a) [46,47]. Because of the rather weak interactions, these bands vanish at higher temperature as can be seen from the respective CO-TPD spectra of the sepiolite samples (Figure S5). According to that, these bands are completely vanished at

15 -100°C. Hence, comparing the CO adsorbate spectra at -100°C (Figure 3b), the remaining bands at 2192, 2183, 2173, and 2162 cm-1 indicate Ni2+ sites in different surroundings [43,45]. In the case of pure sepiolite the band at 2186 cm-1 stems from (Mg2+CO) [48]. From the intensities of the bands the amount of accessible Ni2+ sites can be estimated. Thus, the comparable high percentage of such sites occurring in the 5Ni/Sep (IWI) sample is most probably due to the presence of small NiO agglomerates. In contrast, the 5Ni-Sep (P) sample contains a distinct lower concentration of accessible Ni sites, which might be due to the pres-

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ence of larger NiO particles. The spectra of 5Ni-Sep (P) and 15Ni-Tod (nc) are dominated by (Ni2+CO) bands around 2192 and 2183/2181 cm-1 (Figure 3b) indicating the presence of similar NiO-like species in theses samples. The additional bands at 2173 and 2162 cm-1 which are

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clearly seen on 5Ni/Sep (IWI) seem to be related to rather isolated Ni2+ sites present in the pores

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of the sepiolite structure, as it has also been concluded from the UV-vis-DR and DRIFTS spectra (cf. Figure 2).

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As mentioned before, the different NiO particles sizes and location of the Ni sites, created by the applied different preparation methods of the fresh catalysts, influence their reducibility at

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selected temperatures. Thus, additional H2 chemisorption experiments on the reduced sepiolitebased catalysts revealed metallic surface areas of 0.84 m2/g for 5Ni/Sep (IWI) and 4.84 m2/g

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for 5Ni-Sep (P). Taking into account the higher metallic surface area of 5Ni-Sep (P), the good catalytic performance of this catalyst is explainable. This is in accordance with recently pub-

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lished results of CO2 methanation over Ni-Al2O3, showing a linear correlation between CO2 conversion and Ni surface area [49]. Differences in the particles size of Ni species for reduced sepiolite-based catalysts were also detected by TEM (Figure 4a, b). Although for both samples the vast majority of Ni0 particles show sizes in the range between 1 and 5 nm, only in the case of the 5Ni-Sep (P) catalyst, particles larger than 8 nm were observed. Regarding 15Ni-Tod (nc) catalyst, a narrow particles size

16 distribution was found. Despite the fact that Ni content was threefold higher in comparison with the sepiolite-based catalysts, only a slight increase in Ni0 particles size was detected for the 15Ni-Tod (nc) catalyst. In addition, the inspection of the prepared catalysts by FESEM revealed that sepiolite and todorokite-based catalysts show a completely different morphology (Figure 4c). While the former type of materials can be described as “fibers”, the latter presents a plate-

3.3. Operando DRIFTS studies: Sepiolite-based Ni catalysts

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lets-like morphology.

For deeper understanding the different catalytic behavior of the Ni catalysts operando DRIFTS studies were carried out. The operando DRIFT spectra measured during methanation reaction

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over 5Ni-Sep (P) and 5Ni/Sep (IWI) at different temperatures are shown in Figure 5.

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The spectra are dominated by the intense bands of gaseous CO2 around 2349 cm-1 [50]. The formation of methane is clearly seen by means of its characteristic gas phase spectrum with

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main bands at 3016 and 1305 cm-1 [50]. The bands at 2893/2882, 1587/1607, 1382/1393 cm-1, well observable for both samples in particular at 250°C, stem from adsorbed formates [51,52],

na

while the band at 1634 cm-1 is related to (H2O) vibration of adsorbed water. The bands around 2048 and 1922 cm-1, clearly seen on 5Ni-Sep (P) only, are related to (Ni0CO) vibrations [53-

ur

56]. Thus, the band around 2048 cm-1 can be assigned to linearly adsorbed CO on Ni0, and the

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band around 1922 cm-1 stems from bridged bonded CO. Comparing the spectra of the two catalysts at respective temperatures, a different behavior becomes visible. Thus, in accordance with the catalytic testing results (cf. Figure 1), an essential higher amount of methane is formed over the 5Ni-Sep (P) catalyst as can be seen from the DRIFT spectra as well as from the MS profiles. Consequently, also an enhanced formation of water is seen. What is also apparent is the different ability for Ni0 carbonyl formation which is

17 clearly observed only over 5Ni-Sep (P). This observation is related to the higher metallic surface area of this catalyst compared to 5Ni/Sep (IWI), as revealed by H2-chemisorption experiments. Therefore, over 5Ni-Sep (P) catalyst hydrogen activation is promoted and a higher H2 surface coverage is reached which increases the ability for dissociative adsorption of CO2 and subsequent methane formation. On the 5Ni/Sep (IWI) merely weak bands of gaseous CO are observed, but practically no characteristic features of CO adsorbed on Ni0 sites, which might be due to the low metallic surface area as revealed by H2-chemisorption.

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The formation of formate species is probably related either to the direct reaction of CO2 with activated H2 on Ni0 [57] or the reaction of adsorbed CO with neighbored OH groups of the support [58], because no hydrogen carbonate or carbonate species were detectable. The OH

-p

groups of the catalysts are only slightly involved in the reaction (Figure S6), merely for 5Ni/Sep

might be stem from adsorbed water.

re

(IWI) the formation of additional OH groups around 3610 and 3545 cm-1 was observed which

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The methanation reaction was also studied using the pure sepiolite. As can be seen from the operando DRIFT spectra as well as the MS product analytics (Figure S7), methane formation

na

occurs also over the pre-reduced sepiolite, but the formation of CO according the water-gasshift (RWGS) reaction is preferred. This is apparently due to the Fe impurities existing in this

ur

material which was also proved by EPR spectroscopy (Figure S7). To get deeper insight into the activation of CO2 and the involvement of adsorbed carbonyls into

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methane formation, transient experiments have been carried out. For this purpose, the pre-reduced catalyst was firstly exposed to CO2/He for 45 min, and then H2/He was dosed also for 45 min. In Figure 6a the spectra obtained after different exposure times are exemplarily depicted for the 5Ni-Sep (P) catalyst. During first exposure to CO2/He, besides gaseous CO2 and small amounts of gaseous CO, intensive bands of linearly (2045 cm-1) and bridged bonded (1908 cm-

18 1

) CO on Ni0 can be seen. Additionally, bands of adsorbed water (1632/1611 cm-1) are observ-

able, while no bands of formate species were detected. The formation of adsorbed carbonyls proceeds very fast, being complete already after 3 min. The subsequent dosing of H2/He provokes immediate formation of methane by reaction of activated H2 with adsorbed CO which is stopped when all adsorbed CO is consumed. It has to be taken into account that dosing of H2/He causes also a “flushing” effect as can be seen by the removal of excessive gaseous CO2 within 3 min. For checking the influence of this

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effect we have made another experiment, in which after CO2 adsorption only He was dosed (Figure S8). Comparing the spectra measured after 0.5 min exposure to He and H2/He, respectively, it is seen that, although gaseous CO2 and a part of adsorbed CO were removed, essen-

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tially more adsorbed CO is consumed in the presence of H2, confirming the finding that ad-

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sorbed CO reacts with activated H2 and forms methane.

Comparing the intensity changes of the Ni0CO bands from linearly and bridged bonded CO

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for both Ni catalysts after 0.5 min exposure to H2/He, it is seen (Figure 6b) that only the band around 2045/2050 cm-1, being characteristic for linearly bonded CO, significantly loses inten-

na

sity. This effect is, as expected, more pronounced for the 5Ni-Sep (P), but can also be seen in the case of 5Ni/Sep (IWI). The intensity of the other band keeps more or less constant, which

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suggests that rather linearly adsorbed CO reacts with activated H2 to form methane. For the sake of completeness, it was also proved if and how the catalysts interact with CO2

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when pretreated with synthetic air instead of H2 at 600°C. But, neither on 5Ni-Sep (P) nor on 5Ni/Sep (IWI), adsorption of CO2 or the formation of carbonate species was observed. Hence, the findings of the transient experiments lead to the following conclusions: i) CO2 is only activated over pre-reduced sepiolite-based Ni catalysts, where, as consequence, CO is immediately formed and adsorbs on Ni0 sites and ii) the activation of CO2 proceeds exclusively via a dissociative mechanism.

19

3.4. Operando DRIFTS studies: Todorokite-based Ni catalysts In contrast, the 15Ni-Tod catalysts behave totally different as demonstrated in the following. Before discussing the results of the transient experiments in more detail, the results of the operando DRIFTS study of the methanation reaction over 15Ni-Tod (nc) should be presented and compared with those obtained for the 5Ni-Sep (P) catalyst. It is seen from the DRIFT spectra (Figure 7a) as well as from the respective MS profiles (Figure 7b) that essentially more methane

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is formed over the 15Ni-Tod (nc) catalyst already at 250°C. This finding is in accordance with the catalytic testing results (cf. Figure 1). On both catalysts bands in the region 2050-1760 cm1

are observable stemming from CO adsorbed on Ni0 species [56]. But, while no adsorbate

-p

bands are detected in the region 1600-1050 cm-1 over 5Ni-Sep (P), several intensive bands are

re

visible in the case of 15Ni-Tod (nc), more pronounced at lower temperature. These bands are assigned to different adsorbed carbonate and formate species (vide infra) [51].

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For elucidation the role of these carbonate and formate species in the methanation reaction, transient experiments have been carried out on non-calcined 15Ni-Tod (nc) and calcined 15Ni-

na

Tod catalysts, too. Because the catalytic tests revealed a better performance of the non-calcined catalyst compared to the calcined one (cf. Figure S1), this kind of experiment was applied to

ur

find possible reasons for this different behavior. The pre-reduced catalysts were firstly exposed to CO2/He and then to H2/He (Figure 8a). During exposure to CO2/He, bands of linearly (2052

Jo

cm-1) and bridged bonded (1924/1759 cm-1) CO on Ni0 can be seen on both catalysts as well as broad features in the spectral region 1600-1280 cm-1. The latter bands are related to adsorbed monodentate carbonate, bidentate carbonate, and formate species as well as possibly ionic carbonate [51,59]. Because of the superimposed features, a clear band assignment is difficult. As already observed for the sepiolite-based catalysts, also over the Ni-Tod catalysts, the formation of adsorbed carbonyls proceeds very fast, being complete already after 3 min. Although

20 the nature of the formed carbonyl species is the same on both catalysts, the amount is essentially lower on the calcined 15Ni-Tod catalyst, which might be related to the structural changes of this catalyst induced by calcination (vide supra). In parallel, also the amount of adsorbed carbonates is lowered. By subsequent dosing of H2/He the immediate formation of methane (bands at 3016/1305 cm-1) is seen, while the intensities of the Ni0CO bands as well as of the carbonate bands decrease due to the reaction with activated H2. This effect is more pronounced for the 15Ni-Tod (nc) catalyst as can be seen more clearly from the subtracted spectra (Figure 8b).

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Comparing the intensity changes of the carbonate/formate bands it is evident that in particular the bands of adsorbed monodentate carbonate species (bands around 1532/1488/1314/1063 cm1

) lose intensity, while the bands of adsorbed formate species (2805/1582/1351 cm-1) become

-p

more pronounced. This suggests that the amount of formate species rather increases as consequence of the carbonate reduction. On the other hand, the intensity of the band assigned to

re

linearly adsorbed CO on Ni0 at 2052 cm-1 decreases to a higher extent than that of bridged

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bonded CO (1924 cm-1). This effect was also observed for the sepiolite-based Ni catalysts (cf. Figure 6b). Besides intensity changes of the Ni0CO bands during H2/He exposure, a shift to

na

lower wavenumbers is observed which might be related to the lower coverage of CO and/or the lower concentration of adsorbed carbonate and formate species in the vicinity of the Ni0 parti-

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cles.

The above mentioned effects could also be observed during respective transient experiments at

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lower temperatures (Figure S9). But, after exposing the catalyst to H2 at 250°C, the formation of adsorbed formates is more pronounced, which might be related to their higher thermal stability at this temperature. Hence, the characteristic formate bands are observable even after 45 min exposure to H2 at 250°C, while at 350°C only an intensive band around 1387 cm-1 remains, most probably stemming from ionic carbonate.

21 The excellent ability of the manganese oxide octahedral molecular sieve support for binding CO2 was also proved by transient experiments of CO2 and subsequent H2 adsorption. For this purpose, a birnessite-type manganese oxide [30] was used as model compound consisting of birnessite (ICDD 01-075-8311) as main component, MnO2 (ICDD 01-072-1982), and Mn5O8 (ICDD 00-039-1218) as proved by XRD (cf. Figure S4). At first CO2 was adsorbed on birnessite at 250°C after pretreatment in He for 2 h at 600°C (Figure 9a). The adsorption of CO2 proceeds very quickly, so that already after 1 min an ad-

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sorbate spectrum is seen which does not change markedly within 30 min. By inspecting the subtracted spectrum measured after 30 min exposure to CO2/He (Figure 8b), the adsorbate bands can be seen more clearly. The bands at 1563/1516, 1325 and 1056 cm-1 can be assigned

-p

to monodentate carbonate species [51]. The other bands are related to ionic carbonate. This assignment is based on the fact that besides the typical vibration of the CO32 anion at 1452 cmalso the corresponding combination bands at 1773, 2115, 2584/2487, 2952/2845 cm-1 are ob-

re

1

lP

servable as also described for K2CO3 and CaCO3 [60,61]. It should be noted that the same adsorbate spectrum is obtained when CO2 was adsorbed on birnessite at 250°C after reductive

na

pretreatment at 600°C. But, importantly, in both cases no changes of these adsorbate bands were observed during subsequent exposure to H2/He (Figure 9b). This finding explains the

ur

missing catalytic activity of birnessite in the methanation reaction. On the other hand, when comparing the behaviour of the respective 15Ni-Tod catalysts (cf. Figure 8), this finding nicely

Jo

demonstrates the influence of Ni, which affect the CO2 activation and adsorption as well as the activation of H2. In particular, the ability of the reduced 15Ni-Tod catalyst for H2 activation facilitates the reduction of adsorbed carbonates. As consequence, the adsorbed carbonates are partly reduced to formates, the extent of which mainly depends on reaction temperature. It can be assumed that a part of adsorbed formate species are involved in methane formation, but possible intermediates like methoxy species as observed over Pt/ZrO2 [58] could not be detected

22 during the transient experiments. At higher reaction temperatures (350°C) the partly decomposition of formate species into CO2 and/or CO has to be taken into account which can also be activated/hydrogenated to methane.

3.5. Mechanistic considerations The proposed simplified reaction pathways for the hydrogenation of CO2 over the studied Ni

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catalysts are presented in Scheme 1. As confirmed by the operando DRIFTS results, CO2 adsorbs dissociatively on the pre-reduced Ni catalysts. The formed CO is singly and doubly bound on Ni0 as indicated by respective Ni0CO bands observed for sepiolite-based Ni catalysts as well as for the Ni-Tod catalysts. By subsequent exposure to H2, which is activated by dissocia-

-p

tion into H atoms on Ni0 particles, methane is immediately formed. It can be assumed that the

re

methane formation proceeds on the same way as CO is methanated [6], namely by disproportionation of CO into C and O followed by their hydrogenation. The exact way cannot be deter-

lP

mined because no further intermediates could be detected.

On the Ni-Tod catalysts carbonate and formate species are additionally formed, mainly ad-

na

sorbed on the MnOx support as concluded from CO2 adsorption experiments on a birnessitetype manganese oxide. Such adsorbates were not detected on the sepiolite-based catalysts, nei-

ur

ther in oxidized nor in pre-reduced state. During subsequent exposure to H2, the carbonate spe-

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cies are reduced to formate species, but only over the Ni-Tod catalysts, not over the birnessitetype manganese oxide support. This implicates an activation of H2 over the Ni0 particles by provoking the dissociation of hydrogen into reactive H atoms which are essential for further hydrogenation. However, for hydrogenating adsorbed carbonate species, these H atoms generated on Ni0 particles have to migrate to the metal-support interface, which is known as hydrogen spillover effect [62]. This in turn means that only carbonate species in the vicinity of Ni0 can be reduced, which are most probably the adsorbed monodentate carbonate species, because the

23 transient DRIFTS experiments revealed the preferred diminishing of the characteristic bands from these species. To which extent the formate species, generated by hydrogenation of monodentate carbonate species, are involved in methane formation is difficult to estimate because, in particular at lower reaction temperatures, the formates are stable adsorbed and no further intermediates like methoxy species could be observed in the transient experiments. At higher temperatures the decomposition of adsorbed formate species plays a role, leading to CO2 and CO which can be activated

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and hydrogenated on Ni0 particles as described above. However, taking into account the essential higher catalytic activity of the todorokite-based compared to the sepiolite-based Ni catalysts, the involvement of carbonate/formate species as intermediates in methanation reaction

-p

has to be considered.

re

Summarizing the findings from the operando DRIFTS results, the methanation of CO2 over the sepiolite-based Ni catalysts proceeds exclusively according pathway A. In the case of the Ni-

lP

todorokite catalysts the manganese oxide support causes an additional binding of CO2 which is due to its basic properties and, thus, offers a second methanation pathway B in parallel to path-

na

way A. The finding that both methanation pathways are realized over Ni-Tod catalysts most probably explains their higher activity compared to that of the Ni-sepiolite catalysts. The ability

ur

of the support to activate CO2 via formation of carbonates which are further hydrogenated might be the reason for the generally observed higher activity of such type of catalysts as in particular

Jo

observed on ceria-zirconia-based Ni catalysts [12-14].

4. Conclusions Based on operando DRIFTS studies, different mechanisms are proposed for the methanation reaction studied on two types of supported Ni catalysts which are mainly influenced by the

24 nature of the support. Over Ni-sepiolite catalysts, where the support material is not able to adsorb CO2, a dissociative adsorption of CO2 in the presence of H2, activated by dissociation into H atoms on Ni0 particles, is observed. As consequence, linearly and bridged bonded CO on Ni0 was identified as intermediate, where preferentially linearly bonded CO is hydrogenated to methane. In the case of Ni-todorokite catalysts, differently bound CO on Ni0 was found, too. However, because CO2 easily adsorbs on the basic MnOx support, carbonate species were identified as

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additional intermediates. In particular, monodentate carbonate species can be hydrogenated to formates which are, depending on reaction temperature, directly hydrogenated to methane or via a decomposition step including CO2 and CO formation. The demonstrated ability of Ni-

-p

todorokite catalysts to activate CO2 via a dissociative as well as an associative mechanism explains its excellent catalytic performance. Furthermore, this finding confirms, once again, the

re

significant influence of the support material.

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By the example of sepiolite-based Ni catalysts, the influence of the preparation method was demonstrated. Comparing the accessible Ni2+ sites, as proved by low-temperature adsorption of

na

CO, the 5Ni/Sep (IWI) sample possesses an essential higher amount compared to that of 5NiSep (P). This can be explained by the occurrence of smaller NiO agglomerates in 5Ni/Sep

ur

(IWI), rather present in the pores, and larger NiO particles in 5Ni-Sep (P), mainly present at the outer surface. As a consequence, the latter sample exhibits a better reducibility and higher me-

Jo

tallic surface area. Thus, operando DRIFTS results reveal the different ability for Ni0 carbonyl formation which is essentially higher over 5Ni-Sep (P). In summary, the nature of the support plays a crucial role in terms of nickel-support interaction, CO2 activation, as well as Ni0 particle formation and dispersion. The latter property, important for H2 and CO2 activation, is also influenced by the method applied for the synthesis of the respective Ni catalysts.

25 Declaration of interests The authors declare that they have no known competing financial interests or personal relationships that could have appeared to influence the work reported in this paper.

Author Contribution Conceptualization: A. Chica, U. Bentrup Investigation: C. Cerdá-Moreno, C. Rautenberg Data Curation: C. Cerdá-Moreno, S. Keller, C. Rautenberg

Writing-review and editing: C. Cerdá-Moreno, A. Chica Supervision: A. Chica, U. Bentrup

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Funding aquisation: C. Cerdá-Moreno

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Writing-original draft preparation: U. Bentrup

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Acknowledgment. Financial support by the Spanish Government-MINECO (Project

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ENE2014-57651-R) is gratefully acknowledged. C. Cerdá-Moreno thanks the Spanish Government-MINECO for the predoctoral fellowship from “Severo Ochoa Program” (SVP-2014-

na

068713). The authors thank H. Lund (LIKAT) for performing the XRD measurements and J. Rabeah (LIKAT) for the EPR measurement of sepiolite. The Electron Microscopy Service of

Jo

tion.

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the Universitat Politècnica de València is acknowledged for their help in sample characteriza-

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33 Figure captions Figure 1. Conversion of CO2 (filled symbols) and selectivity of CH4 (open symbols) obtained for sepiolite- and alumina-based catalysts (a) and todorokite and alumina-based catalysts (b) as well as the equilibrium curves [31,32]. Reaction conditions: 18 vol% CO2, 72 vol% H2 balanced with N2; GHSV = 9000 mL·gcat-1·h-1.

Figure 2. UV-vis-DR spectra of the calcined, un-reduced sepiolite-based samples measured at

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room temperature (a) and DRIFT spectra of these samples measured at 250°C after reductive pretreatment at 600°C (b).

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Figure 3. CO adsorbate spectra of different calcined, un-reduced catalyst samples measured at

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-150°C (a) and -100°C (b). Insets: enlarged spectra of 15Ni-Tod (nc).

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Figure 4. TEM images of the different pre-reduced catalysts (a) and respective Ni particle size

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distributions (b) as well as FESEM micrographs of the un-reduced catalysts (c).

Figure 5. Operando DRIFT spectra recorded after 30 min exposure of pre-reduced 5Ni-Sep (P)

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and 5Ni/Sep (IWI) to the methanation feed (18 vol% CO2/72 vol% H2/He) at different temper-

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atures (a) and simultaneously measured MS profiles of CO2, CH4 and H2O (b).

Figure 6. Operando DRIFT spectra of pre-reduced 5Ni-Sep (P) recorded after exposure firstly to 18 vol% CO2/He at 350°C followed by short exposure to 72 vol% H2/He (a) and comparison of the Ni0CO bands arising on pre-reduced 5Ni-Sep (P) and 5Ni/Sep (IWI) after pre-adsorption of CO2/He and subsequent exposure to H2/He (b).

34 Figure 7. Operando DRIFT spectra recorded after 30 min exposure of pre-reduced 5Ni-Sep (P) and 15Ni-Tod (nc) to the methanation feed (18 vol% CO2/72 vol% H2/He) at 250 and 350°C (a) as well as the MS profiles of CO2, CH4 and H2O simultaneously measured during reaction at 250°C (b).

Figure 8. Operando DRIFT spectra of pre-reduced non-calcined 15Ni-Tod (nc) and calcined 15Ni-Tod catalysts recorded after 45 min exposure firstly to 18 vol% CO2/He at 350°C and

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subsequent 0.5 min exposure to 72 vol% H2/He (a) and comparison of the respective subtracted spectra, obtained by subtraction of the spectrum measured after CO2 adsorption from that meas-

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ured after H2 exposure (b).

Figure 9. Operando DRIFT spectra of birnessite measured at 250°C after 2h pretreatment in

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He at 600°C and subsequent exposure to 18 vol% CO2/He (a) and subtracted spectra obtained

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after 30 min exposure to 18 vol% CO2/He and subsequent exposure to 72 vol% H2/He for 30 min at 250°C (b).

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Scheme 1. Proposed reaction pathways for CO2 methanation over sepiolite-based and

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todorokite-based Ni catalysts.

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Figure 4

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Scheme 1