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Mechanistic study of low temperature CO2 hydrogenation over modified Rh/Al2O3 catalysts Denise Heyl, Uwe Rodemerck, and Ursula Bentrup ACS Catal., Just Accepted Manuscript • DOI: 10.1021/acscatal.6b01295 • Publication Date (Web): 08 Aug 2016 Downloaded from http://pubs.acs.org on August 9, 2016

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Mechanistic study of low temperature CO2 hydrogenation over modified Rh/Al2O3 catalysts Denise Heyl, Uwe Rodemerck, and Ursula Bentrup* Leibniz-Institut für Katalyse e.V. an der Universität Rostock (LIKAT), Albert-Einstein-Str. 29a, 18059 Rostock, Germany KEYWORDS operando DRIFTS, CO2 hydrogenation, mechanism, Rh catalyst, K, Ni ABSTRACT The hydrogenation of CO2 on Rh/Al2O3 catalysts modified with Ni and K was studied by in situ and operando DRIFT spectroscopy comprising transient and isotopic exchange experiments to study the influence of this modification on the catalytic performance in CO and methane formation at 250-350°C and to gain mechanistic insight. Catalytic testing and spectroscopic studies revealed that the modification with particularly K promotes the formation of CO being the highest over Rh,K,Ni/Al2O3, while methane formation is preferred over the unmodified catalyst. It was found that CO2 does not dissociatively adsorb, but is adsorbed at the support forming mainly hydrogen carbonate and, in the presence of K, also carbonate species. The dissociative adsorption of H2 proceeds on Rh. The activated H2 reacts mainly with the hydrogen carbonate species forming CO adsorbed on Rh and formate (F1) species stable adsorbed on the support. On the K-containing catalysts an additional formate species (F2) was identified being more reactive than F1 formate and can act as reaction intermediate in the CO formation pathway. Furthermore, adsorbed formyl species were detected which are assumed to be intermediates in the

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methanation reaction. The modifying additives change the surroundings of the Rh particles. This influences the strength of CO adsorption and the activation ability of Rh for H2 dissociation. Thus, desorption of the formed CO from the catalyst surface is favored and the methanation of CO is hindered. The modification with K enhances the ability for CO2 fixation by formation of additional carbonate species which cover adsorption sites for unreactive F1 formate species and favors the formation of reactive F2 formate species. 1. Introduction Because carbon dioxide is one of the main contributors to greenhouse effect and thus to climate change, there is a growing interest concerning the use of CO2 as a feedstock in chemical processes.1,2,3 From the industrial point of view the hydrogenation of CO2 to CH4 using sustainable H2 is highly important because there are several uses of methane within the existing gas distribution system and commercial infrastructure. On the other hand the catalytic conversion of CO2 to CO via the reverse water gas shift (RWGS) reaction is a key reaction being the first step in producing fuels by CO2 hydrogenation. From the thermodynamic point of view, the synthesis of CH4 is favored at normal conditions, while at elevated temperatures (> 500°C) and pressures the equilibrium shifts to CO formation.1 So, it has been demonstrated that CO2 can react with H2 over Rh/γ-Al2O3 to produce CH4 with high selectivity even at room temperature and atmospheric pressure.4 The mechanism of CO2 methanation has been mainly investigated using Ni and noble metals (e.g. Rh, Ru) as active species supported on various oxides (e.g. TiO2, Al2O3, CeO2, ZrO2). In general, it can be stated that CO2 adsorbed on the catalyst surface reacts with H2 activated on the metal to produce CH4.5 It is most probably that CO2 methanation proceeds via CO as intermedi-


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ate which is subsequently methanated. In the literature two mechanisms are discussed for CO formation from CO2 (i) the dissociation of CO2 into adsorbed CO and O6,7,8 and (ii) the RWGS reaction with formate as intermediate.9,10,11 There are also different views concerning identification of active sites, the function of carbonate and formate species as possible intermediates, the CH4 formation process and the role of the support material for the stabilization of the active metal as well as the intermediate species. Studying the interaction of CO2/H2 mixtures on Rh supported on MgO, TiO2, SiO2, and Al2O3 the adsorption measurements showed that, with exception of Rh/SiO2, the presence of H2 enhances the uptake of CO2 in the order Rh/Al2O3 > Rh/MgO > Rh/TiO2 which was related to the formation of formate species produced by the reaction of activated hydrogen with hydrogencarbonate adsorbed at the support.12 In situ surface and gas phase analysis for kinetic studies under transient conditions revealed that CO and formates are formed as reaction intermediates during CO2 hydrogenation over Ru/TiO2.10 While CO was identified as key intermediate for CH4 formation, the formates correspond to a side product adsorbed on the support. Other mechanistic investigations of CO2 hydrogenation over Ru/TiO2 lead to the conclusion that hydrogen adsorbed on Ru migrates to the metal-support interface where it reacts with CO2 adsorbed on TiO2 to yield formate and Ru−CO species.13 To identify reaction intermediates and side products in the methanation of CO and CO2 over supported Ru/Zeolite and Ru/Al2O3 in situ DRIFTS measurements using quantitative steady-state isotope transient kinetic analysis (SSITKA) techniques have been applied. Based on the correlation between band intensities of adsorbed species, CO coverage, and CH4 formation rate under steady-state conditions an adsorbed formyl (HCO) species was identified as reaction intermediate in the dominant reaction pathway for CO and CO2 methanation on the Ru/Al2O3 catalyst.9


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Comprehensive studies were carried out to get mechanistic insight into the low temperature (50150°C) CO and CO2 methanation reaction over Rh/Al2O3.14,15 Results of operando DRIFTS experiments showed that CO2 is dissociatively adsorbed in the presence of H2, forming adsorbed CO and O. Only linear Rh−CO and geminal-dicarbonyl Rh−(CO)2 have been observed where the latter species are more reactive. Evidence was provided that the adsorbed CO species are the precursors of methane. Formates were proposed to be spectator species which do not significantly contribute to methane formation. The turnover frequency for CH4 formation was found to be dependent on the Rh particle size. At low temperature (135-150°C) larger Rh particles are significantly more active than smaller ones, but at higher temperatures (200°C) the turnover frequencies are similar for all particle sizes. By comparing the CO2 methanation reaction over Ni/γ-Al2O3 and Ni/Ce0.5Zr0.5O2 a promotion effect of medium basic sites was found.16 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 derived from hydrogen carbonate as formed on Ni/γ-Al2O3. An influence of basic promotors like K and Ba in CO2 hydrogenation was described for Ru/Al2O3, Rh/Al2O3, and Cu/Al2O3 catalysts.17,18,19 In the case of Ru/Al2O3 the promotion effect of alkali metals was explained by a modification of the local electron density of Ru metal by electron donation, facilitating the dissociation of CO which was assumed as rate-determining step for CO2 hydrogenation.17 DRIFTS measurements of CO adsorption on Ba- and K-containing Rh/Al2O3 revealed a vastly different CO adsorption on these catalysts. For Ba-containing Rh/Al2O3 a high CH4 selectivity was found in CO2 hydrogenation, while K-containing Rh/Al2O3 converts CO2 selectively to CO between 300 und 700°C, and no CH4 was formed.18


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Summarizing the mentioned literature data concerning the mechanism of CO2 hydrogenation the following aspects were differently discussed: (i) the CO formation from CO2 (ii) the role of adsorbed formate and carbonate species, and (iii) the impact of basic additives in terms of promoting CH4 formation or RWGS reaction. But it should be taken into account that the variety of investigated catalyst systems and applied reaction conditions, in particular reaction temperature ranges hinder an objective evaluation. It is known that Rh/Al2O3 as well as Ni/Al2O3 catalysts promote the methanation of CO2,14,15 while K-containing Rh/Al2O3 converts CO2 selectively to CO.18 Therefore, the present work aims to clarify the role of basic promotors towards the reaction pathway in the hydrogenation of CO2. For gaining mechanistic insights, we modified Rh/Al2O3 catalysts with Ni and K to vary their acidity/basicity and redox behavior, and to study the influence of this modification on the catalytic performance in CO and methane formation at 250-350°C. In the mechanistic studies we focus on the identification and characterization of possible reaction intermediates and their reactivity by means of operando DRIFTS measurements including isotopic transient experiments in which the feed is switched from

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CO2/H2 to

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CO2/H2 feed under chemical steady state. The

latter type of experiments allows distinguishing between intermediate species playing a main (active) or minor (spectator) role. 2. Experimental section 2.1. Catalyst preparation. Rh/Al2O3 catalysts were prepared by incipient wetness impregnation using rhodium chloride trihydrate (Acros Organics, 38%), potassium nitrate (Alfa Aesar), nickel nitrate hexahydrate (Aldrich) and γ-alumina (PURALOX® Sasol) obtaining metal loadings of 0.5 wt% Rh, 2 wt% K, and 3 wt% Ni. The respective salts were dissolved in a small amount of dis-


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tilled water and then suspended with the γ-Al2O3. The prepared catalysts were dried for 24 h at 120 °C and subsequently calcined for 5 h at 700 °C in inert gas atmosphere (Ar). In this way four catalysts have been prepared: Rh/A, Rh,K/A, Rh,Ni/A, and Rh,K,Ni/A (A = Al2O3). The obtained Rh, K, ad Ni loadings are given in Table 1. 2.2. Catalyst characterization. For determining the metal contents inductively coupled plasma optical emission spectrometry (ICP-OES) was applied using a VARIAN 715-ES. Nitrogen adsorption-desorption isotherms were collected at 77 K on a BELSORP-mini II (BEL Japan, Inc.). 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 of 0.05 < P/P0 < 0.30. The temperature-programmed reduction measurements (H2-TPR) were performed in a parallel operating 8-channel fixed-bed continuous-flow reactor system. Each quartz reactor (i.d. 6 mm) was filled with 50 mg of the respective sample (sieve fraction of 315-710 µm), which were pretreated at 700 °C in an Ar flow (80 cm3 min-1) with a heating rate of 10 K min-1. The reduction was carried out in H2/Ar flow (5:95) with a temperature ramp of 10 K min-1. The total gas flow per reactor was 10 cm3 min-1, fulfilling the criterion of Monti and Baiker.20 Hydrogen consumption and water formation were monitored by an on-line quadrupole mass spectrometer (OmniStar, Pfeiffer Vacuum). Transmission electron microscopy (TEM) measurements were performed at 200 kV with an aberration-corrected JEM-ARM200F (JEOL, Corrector: CEOS). The microscope is equipped with a JED-2300 (JEOL) energy-dispersive x-ray-spectrometer (EDXS) for chemical analysis. The aberration corrected STEM imaging (High-Angle Annular Dark Field (HAADF) and Annular Bright Field (ABF)) were performed under the following conditions. Both, HAADF and ABF


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were done with a spot size of approximately 0.13 nm, a convergence angle of 30-36 ° and collection semi-angles for HAADF and ABF of 90-170 mrad and 11-22 mrad respectively. The samples were deposited without any pretreatment on a holey carbon supported Cu-grid (mesh 300) and transferred to the microscope. In situ FTIR spectra of adsorbed CO were recorded on a Nicolet 6700 FTIR spectrometer (ThermoFischer Scientific). The sample powder was pressed into self-supporting wafer (50 mg, d = 20 mm) which was placed in a heatable transmission IR cell equipped with CaF2 windows and connected to a gas dosing system. After pretreatment in 20 vol% H2/He at 400 °C for 30 min, the samples were exposed to 5 vol% CO + 5 vol% H2/He at 100 °C for 30 min. The total gas flow rate was 50 ml min-1. After flushing with He (50 ml min-1) for 10 min the adsorbate spectrum was recorded. Generally, difference spectra were evaluated, obtained by subtracting the respective spectrum of the pretreated sample from the adsorbate spectrum. 2.3. Operando DRIFTS measurements. For the DRIFTS measurements a commercial reaction cell (Harrick) fitted with CaF2 windows was implemented into a Nicolet 6700 FTIR spectrometer (ThermoFischer Scientific) equipped with a MCT detector. The gas outlet of the reaction cell was connected to a quadrupole mass spectrometer (OmniStar, Pfeiffer Vacuum) for product analytics. Samples with a defined particle size of 315-710 µm were placed in the cell, which practically acts as fixed-bed flow reactor. After pretreatment (He, 400°C, 30 min) the samples were cooled down to the initial reaction temperature. The feed gas composition was 20 vol% CO2, 20 vol% H2, 5 vol% Ar, 55 vol% He with a total gas flow rate of 30 ml min-1. Argon was used as internal standard for MS analytics. In a sequential experiment CO2 (20 vol% CO2/He) was pre-adsorbed for 30 min at 300°C, followed by a fast switch to 20 vol% H2/He.


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To perform experiments under SSITKA conditions a four way valve was installed to realize a quick switch between

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CO2/H2 and

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CO2/H2 (20 vol% CO2 + 20 vol% H2 balanced with He),

maintaining a constant flow rate of 30 ml min-1. After the above mentioned pretreatment (He, 400°C, 30 min) the sample was exposed to the unlabeled feed. After reaching steady-state,21 it was switched from the normal non-labeled gas mixture to the respective isotopomer-containing feed. The intensities of the mass spectroscopic signals (m/z = 15, 17, 28, 29, 44, and 45) were normalized to the Ar signal intensity. Considering the fragmentation of CO2 to CO, experiments without catalyst were performed and the respective coefficients have been determined. 2.4. Catalytic activity measurements. Catalytic tests were carried out in a parallel operating 50channel fixed-bed continuous-flow reactor system at ambient pressure and 250-350 °C. For studying activation/deactivation effects a catalytic test at 750°C was included followed by tests at 250-350°C again. Each quartz tube reactor (i.d. 4 mm) was filled with 50 mg of catalyst diluted with 125 mg SiC. Activation was performed at 400 °C in N2 with a flow rate of 18 ml min-1 per reactor. After cooling down to reaction temperature, a CO2/H2/N2 = 1/1/3 mixture was fed to the reactors with a flow rate of 30 ml min-1 per reactor. The reaction products and feed components were analyzed by an on-line GC (Agilent 7890) equipped with both FID (FFAP & AL/M) and TCD (HP-Plot Q & Molsieve 5A). The conversion of CO2 was calculated from the inlet and outlet mole flow as shown in equation 1. Selectivity and yield of CO and CH4 were calculated on CO2 basis using equations 2 and 3. ܺ௙ = ܻ௜ =

௡ሶ ೑,బ ି௡ሶ ೑ ௡ሶ ೑,బ ௡ሶ ೔ ି௡ሶ ೔,బ ௡ሶ f,బ

,

(1)

,

(2)


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௒

ܵ௜ = ೔ , ௑

(3)

೑

The mole flows of feed components and reaction products are ݊ሶ ௙ and ݊ሶ ௜ , respectively. The subscript 0 is used for inlet mole flows. 3. Results and discussion 3.1. Catalyst characterization. The metal contents determined by ICP and the BET surface areas of the studied catalysts as well as the νRh−CO band intensities are summarized in Table 1. Besides TPR studies (Figure 1) for studying the reducibility of the prepared catalysts, CO adsorption experiments have been carried out (vide infra) to characterize the oxidation state of the formed Rh particles and to determine their accessibility. The latter can be evaluated by comparing the respective νRh−CO band intensities which are included in Table 1.

Table 1. Rh, K, and Ni contents, BET surface areas, and νRh−CO band intensities of the different catalysts.

Rh wt% Rh/A Rh,K/A Rh,Ni/A Rh,K,Ni/A a

0.45 0.40 0.34 0.50

K wt%

Ni wt%

Surface area m2 g-1

Band intensitya νRh−CO

2.30 3.10

141 139 117 134

16.0 8.2 5.2 15.1

1.47 2.07

integral band intensity (a. u.), normalized to the Rh content


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Figure 1. TPR profiles of the Rh catalysts showing hydrogen consumption and water evolution. The TPR profiles (Figure 1) show, that the rhodium particles on alumina are easily reduced at low temperature, where for Rh,K/A no signal could be observed around 100°C. This suggests that in this case the rhodium particles are completely reduced already after calcination in inert gas. Such effect that addition of alkali metals reduces the reduction temperature was also described for Ru/Al2O3 catalysts.17 In contrast, the NiO particles in the Ni-containing catalysts need higher temperatures to be reduced. The observed peak maximum at 790 °C indicates strong metal-support interactions, presumably by the formation of NiAl2O4.22,23 Frequently, decreased reduction temperatures were observed for metal oxides in the presence of a noble metal, e.g. rhodium, on the same support, which is explained by hydrogen spillover effects.24 However, in the case of the Ni-modified Rh/A catalysts such effect was not observed. It has to be assumed that during calcination a NiAl2O4 phase is formed at the surface which hinders an easier reduction by rhodium-aided spillover of hydrogen to the nickel particles on this type of catalysts. FTIR spectroscopic measurements of the CO adsorption were used to characterize the nature of the formed Rh particles. While position and shape of the Rh carbonyl bands provide information


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concerning the oxidation state, comparing of band intensities is indicative for the accessibility of Rh. After pretreatment in H2/He at 400°C the catalysts were exposed to CO/H2 at 100°C. The obtained adsorbate spectra after flushing with He are shown in Figure 2.

Figure 2. FTIR spectra of CO adsorbed at 100°C on different Rh catalysts pretreated in 20 vol.% H2/He at 400°C for 30 min. A couple of bands with comparable intensities can be seen which are formed at each catalyst. These bands can be assigned to the asymmetric (higher frequency) and symmetric (lower frequency) vibrations of geminal dicarbonyl Rh+−(CO)2 species.25,26,27,28,29 The existence of such bands is typical for CO adsorbed on low coordinated highly dispersed metallic Rh surface atoms. This was also confirmed by STEM investigation of the catalysts revealing single Rh atoms in the case of Rh/A and small particles (2 nm) in the Rh,K,Ni/A catalyst (Figure S1). In the latter a superlattice structure of high contrast atoms is observed forming a surface decoration which was also observed in K/Ni systems if K is deposited on Ni surfaces.30


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The formation of Rh+ sites during CO adsorption is caused by a reaction of Rh0 with surface OH groups from the alumina support leading to the formation of the geminal dicarbonyl complex.26,28,29 Compared to the Rh+−(CO)2 band position of the Rh/A catalyst, the positions of the respective bands of all modified catalysts are shifted to lower wavenumbers in the order Rh,Ni/A > Rh,K/A > Rh,K,Ni/A. This band shift might be caused by the presence of Ni and/or K in the vicinity of the Rh particles affecting the adsorption sites of CO on Rh. In the spectrum of Rh,Ni/A the Rh carbonyl bands are broader showing additional shoulders at 2094 and 2021 cm-1 the position of which is shifted to slightly higher wavenumbers compared to those observed for Rh/A. This suggests that due to different surroundings two types of Rh+ particles are present in this catalyst. While the nature of the formed Rh particles is the same in all catalysts, the accessibility of Rh seems to be hindered in the Rh,Ni/A and Rh,K/A catalysts because the Rh+−(CO)2 band intensities (cf. Table 1) are distinctly lower compared to those observed for Rh/A and Rh,K,Ni/A. 3.2. Catalytic activity. Figure 3 shows the catalytic activity of the studied catalysts at 350 °C before and after additional test at 750°C, which approximately complies with the reaction temperatures normally used in RWGS reaction.31 Generally, the methanation markedly dominates the RWGS reaction on the Rh/A catalyst and less significantly on Rh,K/A and Rh,Ni/A. In contrast, CO formation is preferred on the Rh,K,Ni/A catalyst. The high temperature treatment enhances the activity and, thus, the CO2 conversion of the Ni containing catalysts. This can be explained by the progressive reduction of the Ni particles at 750°C, which could obviously not be attained by calcination at 700°C in Ar and the usual pretreatment at 400 °C in inert gas. In the case of Rh/A and Rh,K/A, the conversion of CO2 was lower after treatment at 750°C, whereas no strong influence of this procedure on the product selectivities could be observed.


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Figure 3. CO2 conversion and yields of CH4 and CO obtained for the different Rh catalysts before (a) and after (b) high temperature treatment (750°C). Reaction conditions: p = 1 atm, T = 350 °C, τ = 1.67 gcat·min·L-1, CO2/H2/N2=1/1/3. In other studies of low temperature hydrogenation of CO2 on Rh/Al2O315,32 using a H2:CO2 ratio of 4:1, CO2 conversions around 6.2% and a high CH4 selectivity was found already at 200 °C. Considering the influence of temperature, it can be assumed that the performance of the Rh/A catalyst used in this study shows a comparable performance although the utilized H2:CO2 ratio was only 1:1. It is known that supported Ni catalysts catalyse rather the methanation of CO2 than the RWGS reaction.5,23 Thus, over Ni/SiO2 and Ni/CexZr1-xO2 catalysts33 CO2 conversions of 25-80% were found at 350 °C depending on time on stream and the used support. Methane selectivities up to 99% were observed and a maximum of 14% CO selectivity. The Ni-containing Rh catalysts used in this study do not achieve this performance. The conversions are lower and especially Rh,K,Ni/A shows a drastically deviating selectivity, forming more CO than CH4. Hence, it can be stated, that the prepared catalysts do not behave like typical Ni-containing methanation cata-


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lysts. But, it should be noted at this point, that the aim of the present study was not to develop an optimized methanation or RWGS catalyst. Instead, the influence of the modifying additives K (possibly present as potassium oxide) and Ni (occurring mainly as NiAl2O4 at the surface as concluded from TPR experiments and XRD) on the catalytic performance of Rh/A catalyst concerning methane and CO formation at 250-350°C should be elucidated, in particular from a mechanistic point of view. 3.3. In situ DRIFTS studies. The in situ DRIFT spectra measured for all catalysts after 30 min exposure to the hydrogenation feed (CO2:H2 = 1:1) at 250 and 300°C are displayed in Figure 4. In the νC−H region bands at 3016, 2998, 2907/2906/2901/2898, 2866/2855, 2766 and 2744 cm-1 appear stemming from gaseous CH4 (3016 cm-1), clearly recognizable by the rotational bands, and adsorbed formate species.34,35,36 The bands around 2000 cm-1 are characteristic for Rh carbonyls.25,26 In the O−C−O stretching region between 1650 and 1200 cm-1 several bands are observable resulting from adsorbed carbonate34 and formate species. Comparing the spectra of Rh/A and Rh,Ni/A with those of the K-containing catalysts, characteristic differences concerning CH4 formation, nature and amount of Rh carbonyls as well as adsorbed carbonate/formate species are apparent. While on Rh/A and Rh,Ni/A methane is formed at 350°C and Rh carbonyls can be detected, only a low amount of methane as well as Rh carbonyls are formed on Rh,K/A and Rh,K,Ni/A. Generally, the spectra measured at 250°C differ distinctly from those measured at 350°C. Thus, in the case of Rh/A and Rh,Ni/A catalysts geminal dicarbonyl species Rh+−(CO)2 are observable at 250°C, but linear Rh0−CO species at 350°C.26 The bands at 1644/1647, 1441/1440, and 1228 cm-1 which can be assigned to hydrogen car-


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bonate34 appear at 250°C, but they vanish during He flushing, and were not formed at temperatures above 250°C.

Figure 4. In situ DRIFT spectra of the different Rh catalysts recorded after 30 min exposure to the hydrogenation feed (CO2/H2/He = 1/1/3) at 250 °C (red) and 350 °C (blue). The K-containing catalysts are characterized by broad intensive bands around 1620 and 1330 cm1

which can be assigned to adsorbed bidentate carbonate species which are stable also at 350°C.

The Rh carbonyl band intensities are very low, but intense bands are observable around 2866/2855, 2766, and 1650 at 250°C besides those at 2998, 2901/2898, 1598, 1392, and 1374 cm-1. While the latter bands stem from formate species most likely adsorbed at the alumina support35,36 the additional bands observed at 250°C only might stem from adsorbed formates, too. In the presence of K it seems likely that potassium formates are formed which should show fundamental formate bands at 2833, 1590 and 1385/1350 cm-1.35 However, it can also be assumed that no pure potassium formates are formed but rather formates which are adsorbed at the alumina


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surface in the vicinity of K which is in accordance with the observed shift of the formate band positions to lower wavenumbers. Such effect, that alkali promotion influences the formate adsorption on alumina, has also been reported for modified Cu/Al2O3 catalysts used for CO2 hydrogenation at high pressure.19 Thus, we assume that two kinds of formate species are present at the surface of the K-containing catalysts Rh,K/A and Rh,K,Ni/A: i) formates adsorbed on pure alumina support (F1) and ii) formates adsorbed on the support, but interacting with neighboring K (F2). The F2 species are not strongly adsorbed being only stable under the hydrogenation feed. They are not stable at temperatures above 250°C because they already vanish during He flushing at 250°C (Figure S2). In the case of Rh,K/A and Rh,K,Ni/A an additional band is observed at 1794 and 1780 cm-1, respectively, more pronounced at 350°C. This band is only observable under RWGS conditions and vanishes during flushing with He. The existence of such band was also reported for Ru/Al2O3 catalysts studied in the methanation of CO and CO2 in H2-rich reformate gases and was attributed to an adsorbed formyl (HCO) species.8 Looking at the bands of the Rh carbonyl species in more detail (Figure 5), it is obvious that they change under exposure to the hydrogenation feed which is due to the progressing reduction of Rh+ species and agglomeration effects. Depending on temperature a band around 2020 cm-1 becomes dominant. While on Rh/A and Rh,Ni/A geminal dicarbonyl Rh+−(CO)2 species are present under hydrogenation feed at 250°C, Rh0−CO species are observable at 300°C and 350°C on Rh/A and Rh,Ni/A, respectively, and already at 250°C on Rh,K,Ni/A and Rh,K/A (not shown). The Rh0−CO band around 2020 cm-1 appears at comparable low wavenumbers because linear Rh0−CO species normally give a band around 2050 cm-1.12,26 However, considering that under


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CO2 hydrogenation conditions the surface is enriched with adsorbed hydrogen, the formation of normal linear carbonyls can be hindered, and CO as well as H will be adsorbed at the same Rh site as demonstrated in Scheme 1. Because chemisorbed H is electron donating, the π donation from Rh into the antibonding orbital of CO increases and thus, the carbonyl band appears at lower frequencies.12

Figure 5. In situ DRIFT spectra of CO adsorbed on different catalysts. The spectra were recorded after 30 min exposure to the hydrogenation feed (CO2/H2/He = 1/1/3) at different temperatures, respectively. The Rh carbonyl band intensities observed on Rh,K,Ni/A and Rh,K/A are low compared with those of Rh/A and Rh,Ni/A and do not change markedly depending on temperature. The band intensities at 350°C, normalized to the Rh content, are 2.1 and 2.8 for Rh,K/A and Rh,K,Ni/A, respectively, but 10.0 for Rh/A and 13.5 for Rh,Ni/A. This finding reflects a distinct lower cov-


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erage of CO on the K-containing catalysts which seems to be associated with a lower stability of the formed carbonyl species in this case.

Scheme 1. Surface Rh carbonyl species formed on Rh/A, and their characteristic infrared absorption frequencies.12, 26 The formation of agglomerated Rh particles during CO2 hydrogenation reaction could also be confirmed by STEM investigations (Figure S3). The used Rh/A catalyst exhibits small heaps of Rh atoms with first near order, whereas in the used Rh,Ni/A sample nano particles (ca. 1-2 nm) were detected which contain Rh and Ni. After hydrogenation reaction the Rh,K,Ni/A catalyst exhibit nano particles being larger as those in Rh/A (ca. 2 nm) and contain Rh as well as Rh and Ni in varying proportions. For elucidating the role of carbonate species transient experiments have been carried out. Thus, the catalysts, as well as the pure alumina support were pretreated with CO2/He at 300°C and then exposed to H2/He. The respective spectra obtained after pretreatment with CO2 and subsequent feeding with H2 are displayed in Figure 6. After preadsorption of CO2 the characteristic bands of adsorbed hydrogen carbonate (1647/1440/1228 cm-1) and monodentate carbonate (1527 cm-1) are seen on Rh/A and Rh,Ni/A, as well as on the support (Figure 6a). The spectra of Rh,K/A and Rh,K,Ni/A are dominated by broad bands of bidentate and monodentate carbonate species.34


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Under these conditions, as expected, no dissociative adsorption of CO2 proceeds. However, after switching to H2/He, the formation of Rh carbonyls is immediately detected on all Rh-containing catalysts (Figure 6b), while methane formation (band at 3016 cm-1) is mainly observed at Rh/A and Rh,K/A and less pronounced at Rh,Ni/A (Figure 6c).

Figure 6. In situ DRIFT spectra of the different catalysts and the support recorded after 30 min exposure to 20 vol% CO2/He at 300°C (a), and subsequent short (1 min) exposure to 20 vol% H2/He in the spectral range 2200-1300 cm-1 (b) and 3100-2750 cm-1 (c). The appearance of a band around 2900 cm-1 at all Rh-containing catalysts points to the formation of formates, while no formates were formed on the pure support. Because the intensities of the hydrogen carbonate and carbonate bands decrease in the case of the Rh-containing catalysts after exposure to H2/He, a reaction of H2 with these adsorbed species has to be assumed leading to CO and formates. As already reported,12 Rh activates the hydrogen molecule and enables its dissociative adsorption. Hence, the hydrogen activated on Rh can migrate to the adjacent acceptor site


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on the support and react with the hydrogen carbonate species which are adsorbed there. Consequently, the formed formates (F1) are mainly adsorbed on the support. 3.4. Isotopic transient experiments. To unravel the role of the different surface species present on the catalyst surface isotopic transient experiments were carried out. For this purpose it was switched from the

12

CO2/H2/He feed after reaching steady state to the

13

CO2/H2/He feed. By

comparing the isotopic exchange rates of the surface species monitored by DRIFTS and gas phase species measured by MS (operando DRIFTS) it is possible to discriminate between active and spectator species.37,38

Figure 7. In situ DRIFT spectra of the different catalysts obtained after exposure to 12CO2/H2/He (black) and

13

CO2/H2/He (red) at 300°C. For better clarity, only the respective spectra recorded

after 30 min exposure time are shown.


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The DRIFT spectra of the catalysts Rh/A, Rh,K/A, and Rh,K,Ni/A measured after switching from the 12CO2/H2/He to the 13CO2/H2/He feed are displayed in Figure 7. After switching to the labelled feed, in principle all bands shift to lower wavenumbers which means that all Ccontaining species are involved in the reaction. However, for discrimination between potential reaction intermediates and spectator species it is necessary to analyze the specific rates of isotopic exchange of the surface and gas phase species. Comparing the respective intensities of the Rh−12CO/Rh−13CO bands (2016/1969 cm-1) and formyl bands (1780/1742 cm-1 and 1794/1754 cm-1) a rapid exchange takes place being complete after 2 min on all catalysts (Figure S4).

Figure 8. Time-dependent normalized band intensities of 12C-containing and 13C-containing Rh carbonyls and formate species observed over Rh/A and Rh,K,Ni/A after switching from 12

CO2/H2/He to 13CO2/H2/He at 300°C.

Compared to the rapid exchange of carbonyls, the exchange rate of the formate species is different which is exemplarily demonstrated for the Rh/A and Rh,K,Ni/A catalyst in Figure 8. Here, the variation of the band intensities of the corresponding

12

C-containing carbonyl and formate


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species as function of time following the isotopic switch is displayed. For calculation of the formate band intensities the bands in the spectral region 2910 - 2850 cm-1 have been selected because in the O−C−O stretching region the formate bands are overlaid by carbonate bands which makes the assignment difficult. While the

12

C-containing formate species adsorbed on Rh,K,Ni/A are exchanged very fast, the

respective formate species on Rh/A are slowly and not completely replaced by their isotopomers (Figure 8). The corresponding MS data reveal a fast formation of 13CO over Rh,K,Ni/A while no 13

CO was observed in the case of Rh/A which agrees with the results obtained from catalytic

testing (cf. Figure 3). Instead, a fast and constant formation of 13CH4 is seen over Rh/A (Figure S5). The formation of 13CO over Rh,K,Ni/A proceeds at a timescale similar to that of formates (Figure 8), and therefore, these species are potentially main reaction intermediates. In contrast, over Rh/A the exchange rate of formates monitored by DRIFTS does not correlate with the fast 13CH4 formation rate as measured by MS (Figure S5). Here, Rh−12CO followed by DRIFTS is exchanged by its isotopomer in the same timescale as 13CH4 is formed. This finding suggests that CO is the main reaction intermediate for CH4 formation. As mentioned above, the exchange rate of the formyl species observed on Rh,K,Ni/A (Figure 7, bands at 1780/1742 cm-1) is also very fast (within 2 min) and correlates with the formation rate of

13

CO as detected by MS. Therefore, it has to be assumed that formyl species are also main

reaction intermediates. 3.5. Mechanistic considerations. The adsorption experiments of CO2 in the absence of H2 revealed no dissociative adsorption of CO2. Instead, adsorbed hydrogen carbonate and monoden-


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tate carbonate species are formed on Rh/A and Rh,Ni/A, as well as on the alumina support (cf. Figure 6a), where on Rh,K/A and Rh,K,Ni/A additional bidentate and monodentate carbonate species were detectable. The formation of Rh carbonyls, CH4, and formate species could only be observed after subsequent feeding of H2, and only on the Rh-containing catalysts. This suggests that Rh activates the dissociative adsorption of H2 while the pure support as well as the support modified with K and Ni is responsible for CO2 fixation. The activated H2 reacts mainly with the hydrogen carbonate species forming CO adsorbed on Rh and formate (F1) species adsorbed on the support (Scheme 2). These mechanistic suggestions are in accordance with experimental findings and conclusions described also for other catalyst systems.9,10,12,13,39

Scheme 2. Identified species and proposed mechanism for CO2 hydrogenation over Rh/A and Rh,Ni/A. It is obvious that this reaction needs the close vicinity of the support-bound hydrogen carbonate species and metal adsorbed hydrogen and, thus, proceeds at the interface. This suggests that the formates are initially formed at the interface from which they migrate to the support where they are stable adsorbed as detected by DRIFTS.


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During exposure the catalysts to the hydrogenation feed (CO2/H2/He = 1/1/3), besides F1 formates a second type of formate species (F2) was detected at the surface of the K-containing catalysts Rh,K/A and Rh,K,Ni/A, which was not observable on Rh/A and Rh,Ni/A (cf. Figure 4). These species are not strongly adsorbed, because they vanish during He flushing at 250°C. This might be related to a hampered adsorption of formates at the support in the neighborhood of K, which implicates a possible higher reactivity of these F2 formates species compared to the F1 formates being rather inactive. Under hydrogenation feed geminal dicarbonyl Rh+−(CO)2 species are present on Rh/A and Rh,Ni/A at 250°C, whereas on the K-containing catalysts Rh,K,Ni/A and Rh,K/A already linear Rh0−CO species are formed (cf. Figure 5). The effect of K can be explained in two ways: i) a better reducibility of the Rh particles, or ii) covering of surface OH groups from the alumina support, which hampers the Rh+ formation by reaction of Rh0 with these hydroxyl groups. At 350°C the band intensities of Rh0−CO normalized to the Rh content are distinct lower on Rh,K/A and Rh,K,Ni/A compared to that observed on Rh/A and Rh,Ni/A, which suggests an influence of Ni and particularly K on the strength of CO adsorption under reaction conditions. Thus, the lower coverage of CO on the K-containing catalysts reflects a lower stability of the formed carbonyl species. That means, in turn, that consecutive reactions of CO, particularly methanation, are hindered, which goes along with the experimental findings (cf. Figures 3, 4) showing mainly CO formation over Rh,K/A and Rh,K,Ni/A, while over Rh/A and Rh,Ni/A the formation of CH4 is preferred. Isotopic transient experiments revealed for Rh/A the fast exchange of 12CO by its isotopomer as observed by DRIFTS. The exchange of CO proceeds in the same timescale as 13CH4, monitored


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by MS, is formed. This suggests that CO is the main reaction intermediate for CH4 the formation of which proceeds very fast. Because the methanation of CO is the dominating reaction, no CO could be detected as product in the gas phase by MS. On the other hand, the formate species are slowly and not completely replaced by their isotopomers as detected by DRIFTS, and the exchange rate of formates does not correlate with the

13

CH4 formation rate as analyzed by MS,

suggesting that formates are only spectator species. On the K-containing catalysts the situation is different. The studies on Rh,K,Ni/A reveal a fast exchange of 12C-containing formate species by its isotopomers as well as a fast exchange of CO and formyl species. Simultaneously, the formation of 13CO was detected by MS proceeding at a timescale similar to the formation of

13

C-containing formate and formyl species and, therefore,

these species are potentially main reaction intermediates for CO formation. However,

13

CH4 is

also formed very fast at the beginning, but its concentration decreases with time on stream while the concentration of 13CO keeps nearly constant. This suggests that two competing reactions (CO and CH4 formation) proceed at the beginning of reaction, obviously via the same reaction intermediates. It is notable that, in contrast to the Rh/A catalyst, the formate species adsorbed on Rh,K,Ni/A play an active role. This is apparently because of the presence of formate species (F2) at the surface of the K-containing catalysts Rh,K/A and Rh,K,Ni/A which are, as mentioned above, not strongly adsorbed and only visible under hydrogenation feed (Scheme 3). This implicates that F2 formate species are more reactive than F1 formates and can act as reaction intermediate for CO formation.


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Scheme 3. Identified species and proposed mechanism for CO2 hydrogenation over Rh,K,Ni/A. This also applies for the formyl species, the formation of which is favored on the K-containing catalysts. However, it cannot unambiguously be determined if the formyl species are intermediates in the CO or CH4 formation pathway, because the exchange rates of CO and formyl species as detected by DRIFTS are comparable and goes along with

13

CO and

13

CH4 formation rates

determined by MS. Considering the fact that on Ru/Al2O3 such formyl species have been identified as reaction intermediate species in the dominant reaction pathway for CO and CO2 methanation,8 it can be assumed that also over Rh,K/A and Rh,K,Ni/A the formyl species are rather intermediates in the methanation reaction. 4. Conclusions Based on in situ and operando DRIFTS studies comprising transient measurements and SSITKA type exchange technique a mechanism is proposed for the hydrogenation of CO2 on alumina supported Rh catalysts modified with Ni and K which is presented schematically in Scheme 3. It was found that no dissociative adsorption of CO2 takes place at all investigated catalysts, but


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CO2 is adsorbed at the support forming mainly hydrogen carbonate and, in the presence of K, also carbonate species. The dissociative adsorption of H2 proceeds on Rh. The activated H2 reacts mainly with the hydrogen carbonate species forming CO adsorbed on Rh and formate (F1) species adsorbed on the support, where the formates are initially formed at the interface from which they migrate to the support. On the K-containing catalysts a further formate species (F2) was identified, the strong adsorption of which at the support is hampered in the neighborhood of K, which leads to the conclusion that F2 formate species are more reactive than F1 formates and can act as reaction intermediate in the CO formation pathway. Furthermore, K favors the formation of formyl species which are assumed to be intermediates in the methanation reaction. At the beginning of reaction both reaction pathways compete with each other, while under steady state conditions the CO formation is preferred. Consistently, catalytic testing and spectroscopic studies revealed that the modification of Rh/A with particularly K promotes the formation of CO being the highest over Rh,K,Ni/A. The influence of the modifiers can be summarized as follows: The modifiers, in particular K, change the surroundings of the Rh particles affecting the adsorption sites of CO on Rh and influence the activation ability for H2 dissociation as well as the strength of CO adsorption. Thus, an easier desorption of the formed CO from the catalyst surface is favored and the methanation of CO is hindered. Furthermore, K affects the oxidation state of the Rh particles under hydrogenation feed by covering of surface OH groups from the alumina support and increases the fixation of CO2 adsorbed as bidentate carbonate species. This hampers formation of Rh+ by the reaction of Rh0 with hydroxyl


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groups of the alumina support and covers adsorption sites for unreactive F1 formate species. Thus, the formation of reactive F2 formate species is favored. ASSOCIATED CONTENT Supporting Information. HAADF-STEM images of fresh and used Rh/A and Rh,K,Ni/A catalysts, in situ DRIFT spectra of Rh/A and Rh,K,Ni/A under reaction conditions and after flushing with He, time-resolved in situ DRIFT spectra from the isotopic exchange experiment. This material is available free of charge via the Internet at http://pubs.acs.org. AUTHOR INFORMATION Corresponding Author *Email [email protected] Notes The authors declare no competing financial interest. ACKNOWLEDGMENT The authors thank Dr. Marga-Martina Pohl for carrying out catalyst characterization by HAADFSTEM analysis and Anja Simmula for ICP-OES analysis. REFERENCES (1) Centi, G.; Perathoner, S. Catal. Today 2009, 148, 191-205. (2) Wang, W.; Wang, S.; Ma, X.; Gong, J. Chem. Soc. Rev. 2011, 40, 3703-3727. (3) Kondratenko, E. V.; Mul, G.; Baltrusaitis, J.; Larrazábal, G. O.; Pérez-Ramírez, J. Energy Environ. Sci. 2013, 6, 3112-3135.


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(4) Ruiz, P.; Jacquemin, M.; Blangenois, N. Catalytic CO2 Methanation Process. Patent WO 2010006386, 2010. (5) Tada, S.; Kikuchi, R. Catal. Sci. Technol. 2015, 5, 3061-3070. (6) Karelovic, A.; Ruiz, P. J. Catal.2013, 301, 141-153. (7) Fisher, I. A.; Bell, A. T. J. Catal.1996, 162, 54-65. (8) Eckle, S.; Anfang, H.-G.; Behm, R. J. J. Phys. Chem. C 2011, 115, 1361-1367. (9) Marwood, M.; Doepper, R.; Renken, A. Appl. Catal. A: General 1997, 151, 223-246. (10) Panagiotopoulou, P.; Kondarides, D. I.; Verykios, X. E. Catal. Today 2012, 181, 138147. (11) Schild, C.; Wokaun, A.; Koeppel, R. A.; Baiker, A. J. Phys. Chem. 1991, 95, 63416346. (12) Solymosi, F.; Erdöhelyi, A.; Bánsági, T. J. Chem. Soc. Faraday Trans. I 1981, 77, 2645-2657. (13) Panagiotopoulou, P.; Kondarides, D. I.; Verykios, X. E. J. Phys. Chem. C 2011, 115, 1220-1230. (14) Beuls, A.; Swalus, C.; Jaquemin, M. ; Heyen, G.; Karelovic, A.; Ruiz, P. Appl. Catal. B: Environmental 2012, 113-114, 2-10. (15) Karelovic, A.; Ruiz, P. Appl. Catal. B: Environmental 2012, 113-114, 237-249. (16) Pan, Q.; Peng, J.; Sun, T.; Wang, S.; Wang, S. Catal. Comm. 2014, 45, 74-78. (17) Li, D.; Ichikuni, N.; Shimazu, S.; Uematsu, T. Appl. Catal. A: General 1998, 172, 351358. (18) Büchel, R.; Baiker, A.; Pratsinis, S. E. Appl. Catal. A: General 2014, 477, 93-101.


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(19) Bansode, A.; Tidona, B.; von Rohr, P. R.; Urakawa, A. Catal. Sci. Technol. 2013, 3, 767-778. (20) Monti, D. A. M.; Baiker, A. J. Catal. 1983, 83, 323-335. (21) Tibiletti, D.; Goguet, A.; Reid, D.; Meunier, F. C.; Burch, R. Catal. Today 2006, 113, 94-101. (22) Li, C.; Chen, Y. W. Thermochim. Acta 1995, 256, 457-465. (23) Hilli, Y.; Kinnunen, N. M.; Suvanto, M.; Savimäki, A.; Kallinen, K. Appl. Catal., A 2015, 497, 85-95. (24) Sermon, P. A.; Bond, G. C. Chem. Rev. 1974, 8, 211-239. (25) Yang, A. C.; Garland, C. W. J. Phys. Chem. 1957, 61, 1504-1512. (26) Solymosi, F.; Lancz, M. J. Chem. Soc. Faraday Trans. 1 1986, 82, 883-897. (27) Zaki, M. I.; Kunzmann, G.; Gates, B. C.; Knözinger, H. J. Phys. Chem. 1987, 91, 14861493. (28) van´t Blik, H. F. J.; van Zon, J. B. A. D.; Huizinga, T.; Vis, J. C.; Konigsberger, D. C.; Prins, R. J. Am. Chem. Soc. 1985, 107, 3139-3147. (29) Gélin, P.; Dutel, J.-F.; Taârit, Y. B. J. Chem. Soc., Chem. Commun. 1990, 1746-1747. (30) Ogawa, K.; Harada, S.; Nakanishi, K.; Namba, H. Appl. Surf. Sci. 2008, 254, 76427646. (31) Schwab, E.; Milanov, A.; Schunk, S. A.; Behrens, A.; Schödel, N. Chem. Ing. Tech. 2015, 87, 347-353. (32) Karelovic, A.; Ruiz, P. ACS Catal. 2013, 3, 2799-2812. (33) Ussa Aldana, P. A.; Ocampo, F.; Kobl, K.; Louis, B.; Thibault-Starzyk, F.; Daturi, M.; Bazin, P.; Thomas, S.; Roger, A. C. Catal. Today 2013, 215, 201-207.


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