Letter pubs.acs.org/JPCL
Reactivity of Carbon Dioxide on Nickel: Role of CO in the Competing Interplay between Oxygen and Graphene Enrico Monachino,†,⊥ Mark Greiner,‡ Axel Knop-Gericke,‡ Robert Schlögl,‡ Carlo Dri,§,∥ Erik Vesselli,*,§,∥ and Giovanni Comelli§,∥ †
Physics Department, University of Trieste, via Valerio 2, I-34127 Trieste, Italy Abteilung Anorganische Chemie, Fritz-Haber-Institut der Max-Planck-Gesellschaft, Faradayweg 4-6, 14195 Berlin, Germany § Physics Department and CENMAT, University of Trieste, via Valerio 2, I-34127 Trieste, Italy ∥ IOM-CNR Laboratorio TASC, Area Science Park, S.S. 14 km 163.5, I-34149 Basovizza, Trieste, Italy ‡
ABSTRACT: The catalytic conversion of carbon dioxide to synthetic fuels and other valuable chemicals is an issue of global environmental and economic impact. In this report we show by means of X-ray photoelectron spectroscopy in the millibar range that, on a Ni surface, the reduction of carbon dioxide is indirectly governed by the CO chemistry. While the growth of graphene and the carbide-graphene conversion can be controlled by selecting the reaction temperature, oxygen is mainly removed by CO, since oxygen reduction by hydrogen is a slow process on Ni. Even though there is still a consistent pressure gap with respect to industrial reaction conditions, the observed phenomena provide a plausible interpretation of the behavior of Ni/Cu based catalysts for CO2 conversion and account for a possible role of CO in the methanol synthesis process. SECTION: Surfaces, Interfaces, Porous Materials, and Catalysis
O
important point, since CO is added to the CO2+H2 feedstock also for the synthesis of MeOH at the industrial level.1,6−8 On the Ni(110) termination, ultrahigh vacuum (UHV) and density functional theory (DFT) studies show that CO2 adsorption at the Ni surface is accompanied by a large charge transfer.9,21,22 The reaction of adsorbed CO2 with hydrogen then yields an abundant surface intermediate (formate), which is not further hydrogenated, and thus acts as a spectator, while hydrocarboxyl species directly take part in the reaction.10,12,15,23 Under methanation conditions at higher pressure, in which CO is hydrogenated, the reactivity of the Ni surface is strongly influenced by carbon accumulation and conversion between carbidic and graphitic species.24−26 In the present paper, we unveil significant insights into the role of CO in the complex process of CO2 reduction on Ni. We find that on a Ni(110) single crystal the surface conditions and reactivity are ultimately defined by the CO chemistry at a given sample temperature, governing the delicate interplay among several simultaneously occurring processes, such as accumulation of oxygen (originating from the conversion of CO2), surface reconstruction, and graphene growth. The latter process is additionally influenced by the equilibrium among other processes (diffusion, dissolution, segregation) involving carbon and carbide surface and subsurface species.24−30
ne of the most significant issues faced by our society today is how to deal with the carbon dioxide emissions produced by the massive use of chemical fuels. One strategy for handling waste CO2 is its catalytic reduction to synthesize useful chemicals, for example methanol (MeOH) and urea, in the reforming and in the trireforming of methane,1−5 and in the clean synthesis of dimethylcarbonate (DMC) as an alternative to toxic reactants such as phosgene and chlorine.1 The reactions involving CO2, CO, and H2 have attracted significant attention due to their importance in MeOH synthesis. This process is presently performed on Cu-based ZnO/Al2O3 supported catalysts.1,6 Great efforts have been devoted to understanding, at the atomic-level detail, the mechanisms of the involved reaction pathways.6−17 In particular, the role of surface crystallographic planes and the activity of formate, carboxyl species, and water have been studied in detail.16−20 The addition of Ni to Cu catalysts is known to increase the CO2 conversion rate.6−8 The effect of Ni-doping on Cu single crystals has been investigated on several accounts.6−8,13,14,16,17 In particular, the enhanced CO2 conversion rate on the Nidoped Cu(100) surface, at 1 bar and 543 K, was linked to the diffusion and segregation of Ni to the catalyst’s surface when CO is present in the gas phase.6−8,13,14 It was found, however, that CO is not directly involved in the conversion of CO2 to MeOH, as the carbon in MeOH mainly originates from CO2.6−8 This is not the case when working with supported Cu catalysts at higher pressures (6 bar) and lower temperatures (below 475 K), where CO is also converted.18,19 This is an © 2014 American Chemical Society
Received: April 16, 2014 Accepted: May 19, 2014 Published: May 19, 2014 1929
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CO2+H2, and CO+CO2+H2 with the Ni surface was therefore studied in a sequence of separate experiments. The clean surface was first exposed to the individual reactants (CO2 and CO) separately, at a pressure of 0.03 mbar. Selected results for the CO2 case are reported in Figure 1 for T = 420
By means of near ambient pressure (mbar range) X-ray photoelectron spectroscopy (NAP-XPS), the evolution of the oxygen and carbon 1s core levels was monitored as a function of surface temperature (350−670 K) for different mixtures of the gas phase reactants (CO, CO2, H2). Measurements were carried out at a total pressure of 0.3 mbar and an H2 partial pressure of 0.27 mbar. The total pressure of CO and CO2 reactants was 0.03 mbar. The measurements were performed at the ISISS end-station of the BESSY synchrotron radiation facility at the Helmholtz Zentrum Berlin (Germany).31 The sample was a Ni(110) single crystal, and was mounted on a sapphire holder by means of Ta support and Ta screws. Temperature was measured with a K-type thermocouple, and sample heating was performed by laser irradiation on the back of the sample. After each reactivity experiment, the Ni(110) surface was cleaned by standard cycles of ion sputtering and annealing in high vacuum. Before each experimental run, the absence of contaminants was verified by measuring the S 2p, C 1s, and O 1s core level XPS regions, together with the Ni 2p line shape in order to ensure absence of NiO phases. Unlike Ni−O bonding, Ni−C bonding induces only small changes in the Ni 2p line shape and position.32 Therefore, we focused on C and O core levels to characterize the surface in the temperature-resolved experiments. The flux and pressure of the gases introduced into the reaction cell were governed by means of mass-flow controllers and a motorized valve. Gas purity was monitored using a quadrupole mass spectrometer. After normalization and subtraction of a Shirley background,33 NAP-XPS spectra were analyzed by least-squares fitting of the data with Doniach-Šunjić profiles,34 convoluted with a Gaussian envelope to account for inhomogeneity and thermal broadening. Binding energies were calibrated with respect to the Fermi level for each oxygen (hν = 695 eV) and carbon (hν = 450 eV) core level spectrum. A very detailed and careful analysis of the C and O 1s core level spectra was performed in order to obtain a unique and reliable identification of the contributions originating from different species. This was achieved by identifying a single set of line shape parameters that allowed optimal fitting of all the C or O 1s spectra. Within this set, in order to reduce the number of fitting parameters, identical line shapes were used for similar species (e.g., bridge and on-top CO, interacting and free-standing graphene, etc.). The resulting binding energy values were assigned on the basis of literature data (see Table 1 for a complete list of data and references). The experimental strategy consisted of investigating the evolution of surface intermediate species, as a function of the sample temperature, by selectively adding the reactants to the feedstock. The interaction of CO, CO2, CO+H2,
Figure 1. Oxygen (a,b) and carbon (c,d) 1s spectra at selected reaction temperatures (top: 520 K, bottom: 420 K) for CO2/Ni(110) under steady state conditions [p(CO2) = 0.03 mbar].
and 520 K. It is found that atomic oxygen accumulates at the surface as a result of CO2 decomposition. The atomic oxygen leads to the formation of a chemisorbed oxygen layer (529.6− 529.8 eV) at 420 K (Figure 1, panel a), and to a precursor to surface oxide phases (530.2−530.4 eV) at 520 K (panel b). In both cases, higher binding energy peaks, assigned to C−O and CO species, are present with lower intensity. In summary, upon exposure to CO2, the Ni(110) surface becomes increasingly covered with oxygen and is eventually oxidized. In agreement with previous results,35 we observed that atomic O species could hardly be removed by reduction with hydrogen. However, reduction of atomic O species by CO proceeds at a much higher rate. Therefore, after each experiment any surface oxide phase was reduced by CO. At 420 K, decomposition and reaction of CO2 results in the formation of several adsorbed carbon species, as seen in the C 1s spectrum (Figure 1, panel c). The surface becomes covered
Table 1. Binding Energies (in eV) of Relevant Observed Species species carbide graphene (free) graphene (ep) CO bridge CO on-top carbonate chemisorbed O surface oxide
O 1s
529.6−8 530.2−4
C 1s
literature
283.2−5 284.0−3 284.5−8 285.4 285.8 288.3−5
283.227,37,38 284.427,37 284.7−827,37,38 285.4−536 285.836 288−28939,40 529.3−49,35,41 530.441 1930
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spectra it is apparent that, following CO decomposition, carbide and graphene are formed on the surface (panel c). The accumulation of carbon at the nickel surface can be ascribed both to direct CO decomposition occurring at defective sites, and to the 2CO →CO2+C Boudouard mechanism.5 At higher temperature (520 K), the O 1s region shows small peaks (panel b) corresponding to low O and CO coverage, while a strong increase in the surface carburization and graphitization is observed (panel d: note the data rescaling to fit the panel). After the characterization of the interaction of CO and CO2 with the Ni(110) surface, we investigated the reactivity of the observed adsorbed species with hydrogen. In Figure 3, the signal intensities (color scale) of the oxygen and carbon 1s core level spectra are shown as a function of temperature, during a linear temperature ramp from 350 to 670 K. The top panels (a and b) show the evolution of the surface species when 0.3 mbar of CO2+H2 [p(H2)/p(CO2) = 9] is introduced into the cell. The temperature ramping rate was 1 K/min and the NAP-XPS acquisition time was 5 min, thus corresponding to one data point every 5 K. As previously mentioned, hydrogen does not remove the atomic oxygen species that accumulate from the CO2 decomposition/reduction reaction (panel a). The surface is graphene free for temperatures above 520 K, where only carbide species are present (see the region delimited by the white dashed lines in panel b). Conversely, when exposing the Ni(110) surface to 0.3 mbar of CO+H2 [p(H2)/p(CO) = 9], (panels c and d in Figure 3), a low coverage of atomic oxygen species is observed in the 400−620 K temperature range (white dashed lines in panel c). In addition, conversion of graphene species to carbide and back to graphene is observed between 540 and 660 K for increasing temperature (panel d). These observations suggest that the delicate interplay between the accumulation of oxygen originating from the conversion of CO2, the growth of graphene, and the surface reconstruction can be controlled by adding CO to the gas stream, which is indeed the approach adopted both at the industrial level,1,8 and in the cited model studies on a Ni-doped Cu catalyst.6−8 In order to further address this crucial point, the same timeresolved NAP-XPS experiment was repeated in the 350−670 K range by introducing a CO2+H2+CO stream [p(H2)/p(CO2+CO) = 9] into the reaction cell. The spectroscopy results are reported in panels e and f of Figure 3. Interestingly, under these reaction conditions, no graphene and only little atomic oxygen are present on the surface in the 520−550 K temperature range (panels f and e, respectively). The dashed line in Figure 3e,f indicates the temperature of 543 K. This temperature corresponds to the temperature with the highest CO2 conversion rate on the Ni/Cu(100) model system, where the roles of Ni doping and of CO were first addressed.6−8 The present results suggest that the highest conversion rate occurs when the surface is in a metallic state, and free from both graphene and oxide. It is interesting to note that, on a totally different catalyst, Cu/ZnO/Al2O3, the conventional MeOH synthesis reaction from a CO2+H2+CO stream at elevated pressures is performed in a very similar temperature range (500−550 K).8 Finally, the present results are also in line with previous observations on Ni powders at 1 bar (i.e., on a significantly different model system at higher pressure): it was observed that hydrogen titration of oxygen species is the rate limiting process in CO2 reduction, since the active Ni catalyst phase is the metallic one.15 On the other hand, concerning the role of carbon, it is known that it acts as an efficient oxidationresistant coating on Ni.43 Similarly, graphitic carbon passivates
by a carbide species (283.2−283.5 eV), together with both freestanding (284.0−284.3 eV) and strongly interacting (284.5− 284.8 eV) graphene islands. Carbon is also found in oxygencontaining species. The presence of CO adsorbed in bridge sites (285.4 eV) and of a carbonate-containing (CO3−) intermediate (288.3−288.5 eV) is proposed, based on literature assignments (Table 1).9,27,35−41 At higher temperature (520 K, panel d), no carbon signal is evident, indicating that at higher temperatures C-containing species are effectively removed. In contrast to the case of CO2, when exposing the surface to CO at 420 K, almost no atomic oxygen is present (Figure 2,
Figure 2. Oxygen (a,c) and carbon (b,d) 1s spectra at selected reaction temperatures (top: 520 K, bottom: 420 K) for CO/Ni(110) under steady state conditions [p(CO) = 0.03 mbar].
panel a). This is attributed to the fast reaction of O with CO, yielding CO2, and to the CO disproportionation mechanism yielding C and CO2, known as Boudouard reaction.5 At this temperature, CO preferentially adsorbs to on-top (285.8 eV) sites (panel c), while at lower temperatures CO mainly adsorbs to bridge sites, and moves to on-top sites upon annealing (not shown). This interpretation is consistent with a previous report that demonstrated surface restructuring of the Ni(110) surface upon exposure to CO at 2.3 bar above 400 K, resulting in (111) faceting of the single crystal surface.42 On Ni(111) CO indeed occupies on-top sites.36 Returning to Figure 2, from the C 1s 1931
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lation (both in graphenic and carbidic forms) is prevented by reaction with hydrogen and by the presence of CO2. In parallel, Ni oxide is reduced by CO, stemming both from the gas phase and from CO2 decomposition. Similar mechanisms may be relevant also in the CO2 reforming process on Ni-based catalysts, where carbon monoxide obtained via the carbon dioxide reforming (CDR) and reverse water−gas shift (RWGS) processes yields carbon accumulation via the Boudouard reaction, thus contributing to the catalyst deactivation.2−5 In conclusion, by means of NAP-XPS measurements at 0.3 mbar on the Ni(110) surface, we have demonstrated that oxidation, graphene growth, and surface restructuring can be tailored by adding CO to the gas phase reactants stream in the carbon dioxide hydrogenation process. Our results provide a new picture of the dynamic processes that govern the activity of a Ni surface during CO2 hydrogenation. This picture can provide insight for understanding the behavior of the previously studied Ni/Cu model catalyst for carbon dioxide conversion into MeOH, thereby identifying the crucial role played by carbon monoxide. Even though a direct extension of the validity of our findings to industrial conditions (higher pressures) is by no means straightforward, it is likely that similar effects also occur under industrial synthesis environments.
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AUTHOR INFORMATION
Corresponding Author
*E-mail:
[email protected]. Present Address ⊥
Zernike Institute for Advanced Materials, University of Groningen, Nijenborgh 4, 9747 AG - Groningen, The Netherlands.
Author Contributions
The manuscript was written through contributions of all authors. All authors have given approval to the final version of the manuscript. Notes
The authors declare no competing financial interest.
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ACKNOWLEDGMENTS E.V. and C.D. acknowledge financial support from MIUR through Futuro in Ricerca FIRB 2010 Project Nos. RBFR10J4H7 and RBFR10FQLB, respectively. The research leading to these results has received funding from the European Community’s Seventh Framework Programme (FP7/20072013) under Grant Agreement No. 226716. We acknowledge the Helmholtz-Zentrum Berlin for provision of synchrotron radiation beamtime at beamline ISISS of BESSY II. Fondazione Kathleen Foreman Casali, Beneficentia Stiftung, and Consorzio per l’Incremento degli Studi e delle Ricerche dei Dipartimenti di Fisica dell’Università degli Studi di Trieste are also acknowledged for their contribution. We thank K. C. Prince for carefully reading the manuscript.
Figure 3. Temperature dependence of the intensity of the NAP-XPS signal in the carbon and oxygen 1s regions for the CO2+H2 (a,b), CO +H2 (c,d), and CO2+CO+H2 reactions (e,f). Total pressure: 0.3 mbar [p(H2)/p(CO) = p(H2)/p(CO2) = p(H2)/p(CO2+CO) = 9]. The horizontal dashed line in the bottom panels indicates the methanol synthesis temperature reported in the literature for the Ni/Cu catalyst.6−8 The dashed lines in panel b (c) delimitate the temperature interval in which the surface is almost graphene (oxygen) free. For clarity, energy positions of the relevant species (from the literature data in Table 1), are reported at the top of the figure.
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