An energy scenario, mainly based on renewables, requires efficient and flexible Power-to-X (P2X) storage technologies, including the methanation of CO₂. As active Ni⁰ surface sites of monometallic nickel-based catalysts are prone to surface oxidation under hydrogen-deficient conditions, we investigated iron as “protective” dopant. A combined operando X-ray absorption spectroscopy and X-ray diffraction setup with quantitative on-line product analysis was used to unravel the structure of Ni and Fe in an alloyed Ni-Fe/Al₂O₃ catalyst during dynamically driven methanation of CO₂. We observed that Fe protects Ni from oxidation and is itself more dynamic in the oxidation and reduction process. Hence, such “sacrificial” or“protective” dopants added in order to preserve the catalytic activity under dynamic reaction conditions may not only be of high relevance with respect to fine-tuning of catalysts for future industrial P2X applications but certainly also of general interest

Grunwaldt, Jan‐Dierk

Kalz, Kai F.

Lichtenberg, Henning

Saraҫi, Erisa

Serrer, Marc‐André

English

Serrer, Marc-André

Grunwaldt, Jan-Dierk

KITopen

123456789101112131415161718192021222324252627282930313233343536373839404142434445464748495051525354555657Role of Iron on the Structure and Stability of Ni3.2Fe/Al2O3during Dynamic CO2 Methanation for P2X ApplicationsMarc-André Serrer,[a, b] Kai F. Kalz,[a, b] Erisa Saraçi,[a, b] Henning Lichtenberg,[a, b] andJan-Dierk Grunwaldt*[a, b]An energy scenario, mainly based on renewables, requiresefficient and flexible Power-to-X (P2X) storage technologies,including the methanation of CO2. As active Ni0 surface sites ofmonometallic nickel-based catalysts are prone to surfaceoxidation under hydrogen-deficient conditions, we investigatediron as “protective” dopant. A combined operando X-rayabsorption spectroscopy and X-ray diffraction setup withquantitative on-line product analysis was used to unravel thestructure of Ni and Fe in an alloyed Ni  Fe/Al2O3 catalyst duringdynamically driven methanation of CO2. We observed that Feprotects Ni from oxidation and is itself more dynamic in theoxidation and reduction process. Hence, such “sacrificial” or“protective” dopants added in order to preserve the catalyticactivity under dynamic reaction conditions may not only be ofhigh relevance with respect to fine-tuning of catalysts for futureindustrial P2X applications but certainly also of general interest.Renewable energy sources, such as wind and solar, provide asustainable solution to the ever-growing demand for energy.[1]To ensure an overall grid stability, their (seasonal) fluctuationsmust be balanced.[2] P2X storage technologies represent oneoption to overcome this problem. Energy can be stored e.g. asFischer-Tropsch products, methanol or synthetic natural gas(SNG).[3] For SNG, the existing and long-ranging gas grid can beused for low-cost storage and distribution. Within the “power-to-gas” process chain, excess renewable electric power iscatalytically converted to methane, a chemical energy carrier(Sabatier reaction, [Equation (1)]).[4]CO2 þ 4H2 Ð CH4 þ 2H2ODH298 K ¼   165 kJmol  1 (1)Renewable H2 from electrochemical water splitting,[5] andCO2, e.g. from biomass gasification or industrial exhaust gases,[6]can be used as reactants for this catalytic process. Theproduction of hydrogen via electrolysis in a renewable energyscenario depends directly on weather-related fluctuations,especially when produced in decentralized plants with small H2-buffer tanks.[1b,2] As these fluctuations are transferred to thereactor bed and the catalytic system, efficient and stablecatalysts that can withstand the highly dynamic and demandingP2X conditions are required. Catalysts based on nickel arecommonly used and have been extensively studied understationary reaction conditions, providing satisfactory catalyticactivity at low cost.[7] Recently, the dynamics of these systemshave received growing attention in research.[8] It was concludedthat during short-term H2-dropouts the preservation of thecatalytically active Ni0 species and especially the removal ofoxygen from the catalytic surface were crucial for maintainingthe catalytic activity. One possible approach to protect Ni0 fromforming surface oxygen species is the addition of a second“sacrificial” or “protective” metal with a higher affinity tooxygen, such as iron. Under stationary reaction conditions, werecently found that by addition of iron the catalyst exhibitedimproved long-term stability and superior CO2 conversioncompared to a monometallic nickel catalyst.[9] This additionallymotivates to characterize the role of iron in detail in such abimetallic Ni  Fe catalyst, especially under dynamic reactionconditions, as present in P2X applications. By combining X-rayabsorption spectroscopy (XAS) and X-ray diffraction (XRD) aspowerful operando tools with quantitative product analysis in asingle experiment, we aimed at revealing the role of iron in thissystem. To our knowledge, the role of a “sacrificial” or“protective” metal in order to preserve the catalytically activespecies has not yet been investigated with in-depth operandostudies under dynamic reaction conditions for bimetallic Ni  Fe/Al2O3 catalysts in the methanation of CO2.Therefore, we prepared two model catalysts, consisting of17 wt%Ni3.2Fe/Al2O3 and 17 wt% Ni/Al2O3 by precipitation withurea[9] with similar particle size for good comparability. In orderto obtain information about the structural changes of amor-phous as well as crystalline phases, we performed alternatinglyoperando XAS- and XRD experiments at the beamline BM31(ESRF) using a micro quartz capillary setup[10] and a micro-GCfor quantitative on-line gas analysis (details in ESI, Figure S1).Prior to the catalytic experiments, the catalysts were activated[a] M.-A. Serrer, Dr. K. F. Kalz, Dr. E. Saraçi, Dr. H. Lichtenberg, Prof. J.-D. GrunwaldtInstitute for Chemical Technology and Polymer ChemistryKarlsruhe Institute of TechnologyEngesserstr. 20Karlsruhe 76131 (Germany)[b] M.-A. Serrer, Dr. K. F. Kalz, Dr. E. Saraçi, Dr. H. Lichtenberg, Prof. J.-D. GrunwaldtInstitute of Catalysis Research and TechnologyKarlsruhe Institute of TechnologyHermann-von-Helmholtz-Platz 1Eggenstein-Leopoldshafen 76344 (Germany)E-mail: grunwaldt@kit.eduSupporting information for this article is available on the WWW underhttps://doi.org/10.1002/cctc.201901425This publication is part of a Special Collection on “Advanced Microscopy andSpectroscopy for Catalysis”. Please check the ChemCatChem homepage formore articles in the collection.CommunicationsDOI: 10.1002/cctc.2019014251ChemCatChem 2019, 11, 1–5 © 2019 Wiley-VCH Verlag GmbH & Co. KGaA, WeinheimThese are not the final page numbers! ��Wiley VCH Montag, 23.09.20191999 / 147287 [S. 1/5] 1123456789101112131415161718192021222324252627282930313233343536373839404142434445464748495051525354555657by temperature-programmed reduction (TPR) with H2. Theexperiments were performed at 350 °C and atmosphericpressure. Experimental details including further time-resolveddata are given in the ESI.As an extreme case for dynamic reaction conditions duringP2X applications, we simulated H2 dropouts for 70 minutes inthe reactant feed gas mixture during the methanation of CO2.When we performed the experiment with the monometallic Ni/Al2O3 catalyst, we achieved a CO2 conversion of 28% (79%selectivity to CH4, see Figure 1b) under stationary reactionconditions (350 °C, 1 bar). The only detected by-product wasCO. Since no differences between the Ni  K XANES spectra(Figure 1a, for time-resolved see Figure S2 in the ESI) recordedafter TPR (  ) and during methanation (- - -) were observed, thecatalyst remained stable during CO2 methanation. When wesimulated hydrogen-lean conditions resulting from a H2-drop-out (***), the shift of the pre-edge feature (Ni0 at 8333 eV) andthe increasing whiteline at 8350 eV in the Ni  K XANES spectrain Figure 1a indicate that nickel is in the oxidation state 2+ .According to linear combination analysis (LCA), ~96% of the Ni0species were oxidized to Ni2+. As it is difficult to distinguishbetween oxygen and carbon species by EXAFS, we simulta-neously followed the formation of NiO or NiCO3 species by XRD.NiO (2Θ=11.7° and 19.5°, cf. Figure S2d) was the onlydetectable oxidized nickel phase, thus, a significant formationof carbonates during the H2 dropout using technical grade CO2/N2 could be excluded. During the subsequent methanationstep, ~35% of Ni0 were recovered (~ 76% Ni0 before H2dropout) resulting in a CO2 conversion of 11% (~28% before H2dropout). Since only minor sintering occurred (crystallite sizeestimated by Scherrer equation: 3.4 nm to 3.9 nm) and thedecrease in Ni0 species directly correlates to the loss in activity,we can conclude that the active species for the conventionalNi-based catalyst is the reduced Ni0. The oxygen species eitheroriginating from CO2 dissociation,[8c,11] or from oxygen impuritiesin the technical grade gas feed might be responsible for theformation of the NiO species and are therefore critical formonometallic nickel catalysts.To protect the active Ni0 species from oxidation by surfaceoxygen species, we modified the catalyst by adding a secondmetal which, compared to Ni, thermodynamically favors theformation of an oxide species. In this case iron was used, asbimetallic Ni  Fe/Al2O3 catalysts recently showed promisingactivity and long-term stability in the stationary methanation ofCO2.[9,12] In this study, the prepared 17 wt% Ni3.2Fe/Al2O3 catalystreached 56% CO2 conversion (97% selectivity to CH4, seeFigure 2d) during methanation of CO2 at 350 °C at 1 bar, whichwas significantly higher compared to the monometallic nickelcatalyst (28%, see Figure 1b). After 60 minutes of CO2methanation (full data set, cf. ESI in Figure S3), a H2-dropoutwas applied for 70 minutes using the same gases, setup andreaction conditions as for the monometallic Ni catalyst. Duringthe H2 dropout, the increasing whiteline intensities in the Ni  Kand Fe  K XANES spectra (Fe  K measured in fluorescencemode) in Figure 2 and Figure S4 demonstrated a slightoxidation of Ni0 to Ni2+ (~7% to 11%) and of Fe0 to Fe2+(~33% to 47%). This moderate oxidation is also visible in theoperando XRD pattern (Figure 2c), where the intensity of the Ni(200) reflection at 2Θ=15.95° slightly decreased during the H2dropout. A slight shift of the Ni(200) reflection was observed,which might represent the formation of a Ni-richer alloy, assome Fe migrated to the surface under formation of Fe2+.[13] Wecan assume that mainly FeO was formed, similar to theoxidation of the monometallic nickel catalyst due to oxygenspecies present during the H2 dropout. The preferentialoxidation of iron suggests that the less-noble metal acts as a“protective” element in relation to nickel. In addition, accordingto DFT calculations,[14] iron can migrate out of a Ni  Fe alloy tothe surface to form FeO, if a monolayer of oxygen is present.This explains the hint for dealloying we observed duringhydrogen-lean conditions resulting in a larger amount ofsurface oxygen species in the catalyst bed. Hence, we observethat iron protected the reduced Ni0 species from majoroxidation by FeO formation, as illustrated in Figure 3 andassumed in the concept of the “protective” less-noble metaldopant. After 70 minutes of H2 dropout conditions, the catalystwas re-exposed to methanation conditions. Surprisingly, in theFigure 1. Operando Ni  K edge XANES spectra and k2-weighted FT-EXAFS spectra after activation and after the experiment (a), simultaneously measuredcatalytic performance (b) of Ni/Al2O3 during methanation of CO2 before (I), during (II) and after (III) a simulated H2-dropout. Reaction conditions: T=350 °C,p=1 bar, 30 mLmin  1 total flow; Methanation: H2/CO2/N2=4/1/5, H2-dropout: CO2/N2=1/9.Communications2ChemCatChem 2019, 11, 1–5 www.chemcatchem.org © 2019 Wiley-VCH Verlag GmbH & Co. KGaA, WeinheimThese are not the final page numbers! ��Wiley VCH Montag, 23.09.20191999 / 147287 [S. 2/5] 1123456789101112131415161718192021222324252627282930313233343536373839404142434445464748495051525354555657bimetallic NixFey/Al2O3 catalyst both nickel and iron regainedthe initial oxidation states, as depicted by the XAS data shownin Figure 2a and Figure 2b. However, the dealloying observedduring the H2 dropout might not be entirely reversible, as theNi(200) reflection in Figure 2c seemed not to fully return to itsinitial position. Nevertheless, and in contrast to the monometal-lic Ni catalyst (Figure 1b), the activity of the bimetallic Ni  Fecatalyst was immediately regained after switching to methana-tion conditions, reaching 58% CO2 conversion (98% selectivityto CH4), as demonstrated in Figure 2d. The slightly higheractivity might be related to the increased amount of iron sitesat the surface of the catalyst after the H2 dropout which mightchange the mechanism of CO2 activation and its reactionintermediates, as observed during DRIFTS studies on Ni  Fecatalysts.[15]In conclusion, the comparison of a monometallic Ni/Al2O3and a bimetallic Ni3.2Fe/Al2O3 model catalyst under dynamicreaction conditions by using an advanced combination ofoperando XAS and XRD with quantitative on-line productanalysis elucidated the important role of iron. During thesimulated H2-dropout the monometallic nickel catalyst wasprone to form surface oxygen species resulting in an irreversibleformation of NiO leading to catalyst deactivation during thesubsequent methanation step. This provides evidence that Ni0sites e.g. at edges/corners of the particles, that are prone tooxidation, are protected or assisted in the re-reduction. Byintroducing the concept of adding a less-noble “sacrificial” or“protective” metal, such as iron, we were able to preserve thecatalytically active species and revealed in detail the role of ironby using advanced X-ray based techniques: The active Ni0species was protected from oxidation during the simulated H2dropout by preferential formation of FeO (Figure 3). Surpris-ingly, both metals were completely reduced again in thesubsequent methanation step. This resulted in a fully recoveredcatalytic activity. The use and the understanding of the role ofless-noble metals as dopants in order to preserve the activecatalytic species from deactivation due to oxidation undertransient reaction conditions represents a major step forward inFigure 2. Simultaneously recorded operando Ni  K edge XANES spectra, as well as k2-weighted FT-EXAFS spectra after activation and after the experiment (a),Fe  K fluorescence spectra (b), XRD with λ=0.4943 Å (c) and catalytic performance (d) of Ni  Fe/Al2O3 before (I), during (II) and after (III) a simulated H2-dropout; Ni (#), NiO (*), γ-Al2O3 (♦). Reaction conditions: T=350 °C, p=1 bar, 30 mLmin  1 total flow; Methanation: H2/CO2/N2=4/1/5, H2-dropout: CO2/N2=1/9.Figure 3. Comparison of the structural changes in a monometallic Ni/Al2O3and a bimetallic Ni3.2Fe/Al2O3 catalyst before, during and after a simulatedH2-dropout during methanation of CO2.Communications3ChemCatChem 2019, 11, 1–5 www.chemcatchem.org © 2019 Wiley-VCH Verlag GmbH & Co. KGaA, WeinheimThese are not the final page numbers! ��Wiley VCH Montag, 23.09.20191999 / 147287 [S. 3/5] 1123456789101112131415161718192021222324252627282930313233343536373839404142434445464748495051525354555657the design of future P2X catalysts and may be used as aconcept in other applications as well. Our results furthermoreshow that it is very important to establish such structure-activityrelationships by a) using complementary synchrotron-basedoperando techniques such as XAS and XRD to characterize bothamorphous and crystalline phases and b) combining them inone experiment with quantitative product analysis, since thestructure depends on the reaction conditions.Further in-depth operando spectroscopic studies would behelpful to deepen the investigation both with respect to therole of iron in the CO2 activation mechanism and of less noblepromotors as sacrificial metal dopants to generalize theconcept.AcknowledgementsThe experiments were performed on beamline BM31 (SNBL) at theEuropean Synchrotron Radiation Facility (ESRF), Grenoble, France.We are grateful to Dr. Hermann Emerich (ESRF) for providingassistance and technical support during the beamtime. Theauthors thank Dr. Thomas Bergfeldt (IAM-AMP, chemical ana-lytics). The German Federal Ministry of Education and Research(BMBF) is gratefully acknowledged for financial support within theKopernikus Project P2X.Conflict of InterestThe authors declare no conflict of interest.Keywords: CO2 hydrogenation · methanation of CO2 · nickel ·iron · x-ray absorption spectroscopy[1] a) G. Centi, S. Perathoner, Greenhouse Gas Sci. Technol. 2011, 1, 21–35;b) R. Schlögl, Angew. Chem. Int. Ed. 2019, 58, 343–348.[2] K. F. Kalz, R. Kraehnert, M. Dvoyashkin, R. Dittmeyer, R. Gläser, U. Krewer,K. Reuter, J.-D. Grunwaldt, ChemCatChem 2017, 9, 17–29.[3] a) F. Schüth, Chem. Ing. Tech. 2011, 83, 1984–1993; b) R. Schlögl, Angew.Chem. Int. Ed. 2015, 54, 4436–4439; Angew. Chem. 2015, 127, 4512–4516.[4] a) M. Jentsch, T. Trost, M. Sterner, Energy Procedia 2014, 46, 254–261;b) M. Götz, J. Lefebvre, F. Mörs, A. McDaniel Koch, F. Graf, S. Bajohr, R.Reimert, T. Kolb, Renew. Energ. 2016, 85, 1371–1390; c) M. Younas, L.Loong Kong, M. J. K. Bashir, H. Nadeem, A. Shehzad, S. Sethupathi,Energy Fuels 2016, 30, 8815–8831.[5] Z. W. Seh, J. Kibsgaard, C. F. Dickens, I. Chorkendorff, J. K. Nørskov, T. F.Jaramillo, Science 2017, 355, eaad4998.[6] A. W. Kleij, M. North, A. Urakawa, ChemSusChem 2017, 10, 1036–1038.[7] a) S. Abelló, C. Berrueco, D. Montané, Fuel 2013, 113, 598–609; b) S.Rahmani, M. Rezaei, F. Meshkani, J. Ind. Eng. Chem. 2014, 20, 1346–1352;c) G. Garbarino, P. Riani, L. Magistri, G. Busca, Int. J. Hydrogen Energy2014, 39, 11557–11565; d) J. Gao, Q. Liu, F. Gu, B. Liu, Z. Zhong, F. Su,RSC Adv. 2015, 5, 22759–22776; e) M. A. A. Aziz, A. A. Jalil, S. Triwahyono,A. Ahmad, Green Chem. 2015, 17, 2647–2663; f) N. M. Martin, P. Velin, M.Skoglundh, M. Bauer, P.-A. Carlsson, Catal. Sci. Technol. 2017, 7, 1086–1094.[8] a) B. Mutz, H. W. P. Carvalho, S. Mangold, W. Kleist, J.-D. Grunwaldt, J.Catal. 2015, 327, 48–53; b) B. Mutz, A. Gänzler, M. Nachtegaal, O. Müller,R. Frahm, W. Kleist, J.-D. Grunwaldt, Catalysts 2017, 7, 279; c) C. Vogt, E.Groeneveld, G. Kamsma, M. Nachtegaal, L. Lu, C. J. Kiely, P. H. Berben, F.Meirer, B. M. Weckhuysen, Nat. Catal. 2018, 1, 127–134.[9] B. Mutz, M. Belimov, W. Wang, P. Sprenger, M.-A. Serrer, D. Wang, P.Pfeifer, W. Kleist, J.-D. Grunwaldt, ACS Catal. 2017, 7, 6802–6814.[10] J.-D. Grunwaldt, N. van Vegten, A. Baiker, Chem. Commun. 2007, 4635–4637.[11] a) J. Ren, H. Guo, J. Yang, Z. Qin, J. Lin, Z. Li, Appl. Surf. Sci. 2015, 351,504–516; b) B. Miao, S. S. K. Ma, X. Wang, H. Su, S. H. Chan, Catal. Sci.Technol. 2016, 6, 4048–4058.[12] a) C. Mebrahtu, F. Krebs, S. Perathoner, S. Abate, G. Centi, R. Palkovits,Catal. Sci. Technol. 2018, 8, 1016–1027; b) T. Burger, F. Koschany, O.Thomys, K. Köhler, O. Hinrichsen, Appl. Catal. A 2018, 558, 44–54.[13] L. Vegard, Z. Angew. Phys. 1921, 5, 17–26.[14] S. M. Kim, P. M. Abdala, T. Margossian, D. Hosseini, L. Foppa, A.Armutlulu, W. van Beek, A. Comas-Vives, C. Copéret, C. Müller, J. Am.Chem. Soc. 2017, 139, 1937–1949.[15] a) D. Pandey, G. Deo, J. Mol. Catal. A 2014, 382, 23–30; b) J. Ren, X. Qin,J.-Z. Yang, Z.-F. Qin, H.-L. Guo, J.-Y. Lin, Z. Li, Fuel Process. Technol. 2015,137, 204–211.Manuscript received: August 3, 2019Accepted manuscript online: August 9, 2019Version of record online: ■■■, ■■■■Communications4ChemCatChem 2019, 11, 1–5 www.chemcatchem.org © 2019 Wiley-VCH Verlag GmbH & Co. KGaA, WeinheimThese are not the final page numbers! ��Wiley VCH Montag, 23.09.20191999 / 147287 [S. 4/5] 1123456789101112131415161718192021222324252627282930313233343536373839404142434445464748495051525354555657COMMUNICATIONSIron as protective hero: In an energyscenario based on renewables, newand efficient storage technologies,such as Power-to-Gas (PtG) areneeded. However, monometallic Nicatalysts are highly sensitive todynamic reaction conditions, aspresent in Power-to-X applications. Inthis operando study we investigatedthe deactivation of a Ni-basedcatalyst and unraveled the protectiverole of “sacrificial” iron in a Ni3.2Fecatalyst during dynamic methanationof CO2.M.-A. Serrer, Dr. K. F. Kalz, Dr. E. Saraçi,Dr. H. Lichtenberg, Prof. J.-D.Grunwaldt*1 – 5Role of Iron on the Structure andStability of Ni3.2Fe/Al2O3 duringDynamic CO2 Methanation for P2XApplicationsCommunicationsWiley VCH Montag, 23.09.20191999 / 147287 [S. 5/5] 1

Role of Iron on the Structure and Stability of Ni3.2Fe/Al2O3 during Dynamic CO2 Methanation for P2X Applications

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Role of Iron on the Structure and Stability of Ni3.2Fe/Al2O3 during Dynamic CO2 Methanation for P2X Applications

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