Influence of CO on the Activation, O-Vacancy Formation, and

Letter pubs.acs.org/JPCL

Cite This: J. Phys. Chem. Lett. 2019, 10, 3645−3653

Influence of CO on the Activation, O‑Vacancy Formation, and Performance of Au/ZnO Catalysts in CO2 Hydrogenation to Methanol Ali M. Abdel-Mageed,† Alexander Klyushin,‡,§,∥ Axel Knop-Gericke,‡,§ Robert Schlögl,‡,§ and R. Jürgen Behm*,† †

Institute of Surface Chemistry and Catalysis, Ulm University, Albert-Einstein-Allee 47, D-89081 Ulm, Germany Fritz-Haber-Institut der Max-Planck-Gesellschaft, Faradayweg 4-6, D-14195 Berlin, Germany § Max Planck Institute for Chemical Energy Conversion, Heterogeneous Reactions, Stiftstrasse 34-36, D-45470 Mülheim, Germany ∥ Helmholtz-Zentrum Berlin für Materialien und Energie, BESSY II, Albert-Einstein-Straße 15, D-12489 Berlin, Germany

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‡

S Supporting Information *

ABSTRACT: The impact of CO on the activation and reaction characteristics of Au/ ZnO catalysts in methanol synthesis from a CO2/H2 mixture was studied by kinetic, near ambient pressure X-ray photoelectron spectroscopy and X-ray absorption spectroscopy at the O K-edge, together with in situ Foureir transform infrared measurements. Transient measurements under up to industrial reaction conditions (50 bar, 240 °C) show a pronounced transient increase of the activity for methanol formation from CO2/H2 after exposure to a CO/H2 reaction gas mixture, while the steady-state activity is similar to that observed directly after oxidative pretreatment. For the reaction in CO/H2, the much longer activation phase is accompanied by formation of CO2 due to reaction of CO with the ZnO catalyst support. This leads to O-vacancy formation on the support at an extent significantly higher than in CO2/H2. The consequences of these findings on the mechanistic understanding of methanol formation from CO2/H2 on Au/ZnO and for ZnO-supported catalysts in general are discussed.

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mixture with time on stream, we recently demonstrated that the catalyst passes through an initial activation period (from 120 to 70 min at total pressures between 5 and 50 bar), which goes along with the formation of O-vacancies in the ZnO support during reaction.9 Operando Fourier transform infrared (FTIR) measurements at up to industrial conditions (up to 50 bar, 240 °C) indicated that the formation of these O-defects goes along with the formation of negatively charged Au species, whose amount correlated well with the formation of methanol both during time on stream and with the increase of pressure.9 On the basis of these findings, one may expect that the activation of the catalyst and its activity depend sensitively on the reducing character of the reaction gas mixture. Considering earlier findings for oxide-supported Au catalysts, where H2 was found to be significantly less reducing than CO8,10 and where CO2 was observed to be mildly oxidative for the oxide support, here specifically for Au/CeO2 catalysts,11 one may expect that in particular exposure to CO will sensitively affect the catalytic performance of the Au/ZnO catalyst. This is the topic of the present Letter, where we present results of a comparative study

he catalytic hydrogenation of CO2 to methanol (CO2 + 3 H2 → CH3OH + H2O), by reaction with H2 generated using renewable energy sources, has recently attracted considerable interest as part of concepts for the chemical storage of renewable energies.1 While methanol synthesis in general is a widely established industrial process, the fluctuating and delocalized nature of renewable energies requires operation and catalyst concepts that are optimized for the specific conditions of this problem. It was demonstrated earlier2−4 and explored in more detail recently5−8 that oxide-supported Au catalysts, Au/ZnO in particular, show a high activity for methanol formation from a CO2/H2 reaction gas mixture, which is at least comparable to that of a commercial Cu/ZnO catalyst. Furthermore, Au/ZnO catalysts show a significantly higher selectivity for methanol formation in this reaction, with a rather low activity for the undesired reverse water gas shift reaction (RWGS: CO2 + H2 → CO + H2O).7,8 Considering also their high stability against oxidation, these catalysts may be interesting candidates for such kinds of “power-to-liquid” processes. The performance of these catalysts is governed by a variety of structural and reaction parameters, such as Au particle size,6 oxide basicity,3 reaction conditions,7 and gas composition.8 For instance, increasing the total pressure was found to favor the selectivity for methanol.7 Following the activity of a Au/ZnO catalyst under realistic reaction conditions in a CO2/H2 (1:3) © 2019 American Chemical Society

Received: April 1, 2019 Accepted: June 13, 2019 Published: June 13, 2019 3645

DOI: 10.1021/acs.jpclett.9b00925 J. Phys. Chem. Lett. 2019, 10, 3645−3653

Letter

The Journal of Physical Chemistry Letters on the influence of CO on the activation and reaction characteristics of Au/ZnO catalysts in the methanol formation reaction from CO2/H2 gas mixtures. This includes kinetic measurements under up to industrial reaction conditions (up to 50 bar, 240 °C) as well as in situ X-ray spectroscopy and FTIR measurements at pressures in the millibar to bar range, where we monitored in different ways modifications of the ZnO support and of the catalytic performance during time on stream and the dynamics of these processes, e.g., upon transient changes in the composition of the reaction gas atmosphere. Considering the ongoing intense debate about mechanistic details of the methanol synthesis reaction on the industrial standard Cu/ZnO catalyst,12−18 where the role of a partial reduction of the ZnO support is one of the major controversial issues, we expect the present findings to be valuable also for the mechanistic understanding of methanol formation on Cu/ZnO catalysts. We start by following the time evolution of the methanol formation rates in CO2/H2 (1:3) (15% CO2, 45% H2, balance Ar) and CO/H2 (1:3) (15% CO2, 45% H2, balance Ar) reaction atmospheres, which will henceforth be denoted as CO2/H2 and CO/H2 mixtures, on the Au/ZnO catalyst at 5 and 50 bar at 240 °C (see Figures 1a,b). They were recorded after an oxidative pretreatment (O400 pretreatment 1 h in 10% O2/Ar at 1 bar, 400 °C). In the CO2/H2 gas mixture, the methanol formation rate was found to increase slowly during an initial activation phase, until steady-state conditions were reached, with the duration of this activation phase decreasing from 150 min at 5 bar total pressure to 75 min at 50 bar. Performing a similar set of experiments in a CO/H2 reaction gas mixture shows a comparable tendency of the reaction rate, but in this case the methanol formation rate increased much slower. At 5 bar, steady-state was not reached even at 500 min; at 50 bar, this took about 200 min. In the latter case the resulting steady-state methanol formation rate was slightly smaller than that obtained from CO2/H2, while at 5 bar such comparison is not possible because steady-state conditions were not reached in CO/H2. Next, we performed a similar set of experiments in the CO2/ H2 gas mixture, but after a preceding measurement in the CO/ H2 gas mixture. In this case, the rate started much higher and steeply increased in the first few minutes, until reaching a distinct maximum after 4 min (5 bar) or 8 min (50 bar). Subsequently, the rate decayed equally steeply and then reached a steady-state value, which closely resembles that obtained when running the reaction under similar conditions directly after O400 pretreatment (Figure 1a,b). This decay took about 80 min, independent of the pressure; for reaction at 5 bar, the rate passed through a subtle minimum. The respective maximum and steady-state reaction rates are 12.2/ 3.1 μmolMeOH gAu−1 s−1 at 5 bar and 34.8/10.1 μmolMeOH gAu−1 s−1 at 50 bar. Similar measurements were performed also at additional, higher pressures (10, 20, 40 bar). The resulting rate versus time curves are shown in Figure S1 in the Supporting Information. The steady-state rates obtained in all measurements in the CO2:H2 gas mixture are plotted in Figure 1c, once for reaction on the O400 pretreated catalyst (CO2/H2-1) and once after a preceding reaction in CO/H2 (CO2/H2-2). This figure indicates an increasingly higher activity of the Au/ZnO catalyst for methanol formation with increasing pressure (black bars), except for the last step from 40 to 50 bar. Furthermore,

Figure 1. Time-on-stream Au mass-normalized methanol formation rates on a 1.0 wt % Au/ZnO catalyst at 240 °C and pressures of 5 bar (a) and 50 bar (b) in CO2/H2 (30 Nml min−1) and in CO/H2 (30 Nml min−1). Methanol formation rates in CO2/H2-1 and CO/H2 are measured directly after calcination (O400); the rate for CO2/H2-2 was measured after reaction in CO-ref for 550 min. (c) Comparison of the methanol steady-state rates in CO2/H2-1 (black dashed bars) and CO2/H2-2 (red bars) at different pressures.

this plot also shows an enhanced activity as a consequence of the reductive treatment in CO/H2 as compared to reaction directly after the oxidative O400 pretreatment, and this enhancement increases with increasing total pressure at pressures from 10 to 40 bar. Also in this case the effect disappears almost completely at 50 bar. Reasons for the sudden change in trends at 50 bar are not yet clear, but obviously the catalyst can more easily reach steady-state conditions which are independent of the pretreatment. Finally, in order to gain more information on the effect of a reducing pretreatment, we performed a similar experiment in CO2/H2 reaction gas at 5 bar, but otherwise the same conditions, on a catalyst that was reduced at 400 °C in 10% H2/Ar for 1 h (1 bar) instead of the O400 pretreatment. The resulting time dependence of the methanol formation rate (Figure S2) shows a similar activation behavior and activation period as observed after O400 pretreatment. This result differs considerably from the activation behavior observed after reduction in a CO/H2 mixture (Figure 1a,b), where the activity passed through a steep maximum, followed by a pronounced decay to finally reach steady-state conditions. Apparently, there is a distinct difference in the state of the catalyst after reductive pretreatment in H2 (at 400 °C) and after reduction in the CO/H2 reaction mixture (at 240 °C), 3646


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gAu−1 s−1. This latter amount (0.137 × 1015 CO2 molecules cm−2) is much more than the background level obtained for reaction at 5 bar, indicating ongoing CO2 formation, in addition to transient CO2 formation in the initial peak. It is possible that this results from reaction with oxygen from deeper regions, which at higher depletion levels at the surface can diffuse to the surface, although CO2 formation via the Boudouard reaction cannot be ruled out. More definite conclusions on the physical origin of this CO2 formation are, however, not possible at present. Hence, in both cases transient CO2 formation seems to be limited to the activation phase, which is more than 500 min at 5 bar and about 200 min at 50 bar. The formation of CO2 under these conditions and in this range is most convincingly explained by the removal of lattice oxygen from the ZnO support, i.e., by the creation of Ovacancies under reaction conditions.5,19,20 For a quantitative evaluation of the amount of O-vacancies created during this activation period (0−500 min), we integrated the area under the curves in Figure 2a, which is a direct measure of the amount of CO2 formed per gram of catalyst. For the measurement at 50 bar, we subtracted the background indicated by the black dashed line in Figure 2a. Assuming a density of 1.2 × 1015 surface oxygen atoms per cm2 for ZnO and using the active surface area of the catalyst of 45 m2gcat−1, about 0.23 × 1015 CO2 molecules per cm2 catalyst were formed at 5 bar, which is equivalent to ∼19% of the surface lattice oxygen (0.19 monolayers (MLs)). At 50 bar, the corresponding numbers are 0.40 × 1015 CO2 molecules cm−2 and ∼33% of a monolayer. Here one should keep in mind that the number of O-vacancies derived from this measurement is a lower limit, as part of the CO2 produced in this process can be consumed for the production of methanol. On the other hand, the amount of O-vacancies created during reaction at 5 or 50 bar is significantly higher than the number of surface lattice oxygen vacancies that could be created in temporal analysis of products (TAP) reactor measurements on a Au/ZnO catalyst by CO pulsing at 120 °C, where we obtained a value of 0.9 × 1018 O atom gcat−1, equivalent to 0.0018 × 1015 O atoms cm−2 in a previous study.19 Furthermore, it is 1−2 orders of magnitude higher than the total number of Au-ZnO perimeter sites/atoms (0.0044 × 1015 atom cmcat−2).6,19,20 These latter sites had been predicted as active sites for CO oxidation, by formation and replenishment of oxygen surface vacancies at the perimeter of the Au nanoparticles,20−22 but were also proposed as the active sites for methanol formation from CO/CO2/H2 on Au/ZnO catalysts.5 The relatively high number of oxygen vacancies indicates that under present reaction conditions these are not limited to surface species at the Au-ZnO perimeter but are present also further apart from the Au NPs at the surface; considering the total amount, they are likely to extend also into the subsurface and bulk regions of the ZnO support material. This observation of a significant amount of O-vacancy defects created is in good agreement with previous TAP reactor measurements titrating the removable surface oxygen on an identical Au/ZnO catalyst.23 These vacancies may affect the activity of the methanol formation reaction by electronic metal−support interactions, as it had been described by Campbell.24 Such effects have been demonstrated recently in our group also for CO oxidation on Au/TiO2.25 In order to learn more about the effect of CO2 on the state of the catalyst during reaction, in particular on the buildup of relatively stable adsorbed reaction intermediates, we also followed the change in the methanol formation rate upon

with a much more pronounced reduction effect in the latter case. These differences cannot result from changes in the Au nanoparticle size, which is similar after O400 and after both reaction steps in CO2/H2, and after CO/H2 gas mixtures (2.4 ± 0.1 nm, dispersion = 44%), but must mainly be due to a modification of the ZnO support.7 Interestingly, during the activation phase in the CO/H2 mixture in Figure 1b we also observed a continuous formation of CO2 over more than 500 min. The time dependence of the CO2 formation rate under these conditions is depicted in Figure 2a. Both for reaction at 5 bar and at 50 bar, the CO2

Figure 2. (a) Time-dependent Au-mass-normalized CO2 formation rate during methanol synthesis in a CO/H2 mixture (30 Nml min−1) at 240 °C and pressures of 5 bar (red) and 50 bar (black). (b) Absolute quantity of O-vacancies created in Au/ZnO during reaction at 5 and 50 bar. (c) Changes in the methanol formation rate at 5 bar upon switching from CO2/H2 mixture (A) to 45% H2 /Ar gas mixture (B) and back again to CO2/H2 (C) at 240 °C and 5 bar.

level steeply increased at the onset of the reaction, passed through a maximum, and then decayed about exponentially to a steady-state level, which we tentatively associate with the background level under these conditions. At 5 bar, the maximum rate is about 1.2 μmolCO2 gAu−1 s−1, which was reached after 40 min. It then decayed to a very low value of 0.08 μmolCO2 gAu−1 s−1, which is essentially the background level. At 50 bar, the maximum rate, which was reached after a slightly longer time (60 min), is more than double as high with 2.7 μmolCO 2 gAu−1s−1, and it decreased to 0.58 μmolCO 2 3647


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O-vacancy formation compared to CO was indeed reported recently for Au/TiO226 and Au/CeO2.10 For more detailed information on the chemical state of the catalyst during reaction, we performed in situ X-ray absorption near-edge spectroscopy (XANES) measurements at the O Kedge (Figure 3a). These measurements probe changes in the

switching from a CO2/H2 mixture to a 45% H2/Ar gas mixture, which contains a similar amount of H2 but no CO2, and back again (240 °C, preact = 5 bar) (Figure 2c). As expected, the methanol formation rate dropped steeply upon changing to the H2:Ar mixture, from 3.2 to about 1.0 μmolMeOH gAu−1 s−1. Subsequently, the rate decreased continuously with time, until reaching values below the detection limit after 600 min in H2/ Ar atmosphere. Hence, also after removal of CO2 from the supply feed methanol formation is still observed for a rather long time. Initially, this can be explained by the presence of CO2 in the reactor, because under the present reaction conditions it takes about 8 min to exchange the gas content of the reactor line. Nevertheless, after 20 min the CO2 content in the reaction gas phase should be negligible, and further methanol formation can only be due to reactive desorption of adsorbed reaction intermediates. On the basis of the integration of the area under the curve, starting 20 min after switching to 45% H2/Ar, the total amount of methanol formed during this time is estimated to be 2 × 1020 methanol molecules (200 mg Au/ZnO). This corresponds to 0.22 × 1015 molecules per cm2 catalyst surface, equivalent to 0.18 ML (oxygen surface atom density of 1.2 × 1015 atoms cm−2) of adsorbed methanol or precursor species, which had at least been present on the surface when terminating the reaction by switching to the CO2-free feed (see Figure S3). This rather high value, which is only a lower limit, also means that the reaction intermediates cannot be (solely) adsorbed on the Au nanoparticles with their much smaller surface area but must be adsorbed (also) on the ZnO support. Upon switching back to the CO2/H2 mixture, the methanol formation rate instantaneously shot up, reaching a value of 6.9 μmolMeOH gAu−1 s−1 within 2 min (see data point directly on the dashed line in Figure 2c). Subsequently it decreased and passed through a minimum ∼5 min after changing the gas mixture (0.8 μmolMeOH gAu−1 s−1). Following this initial phase, the catalyst showed an activation behavior which closely resembled that observed after the oxidative pretreatment, with an increase in rate over ∼120 min, to finally reach the steadystate activity. For the latter we obtained a value of 3.0 μmolMeOH gAu−1 s−1, which also closely resembles that obtained after O400 pretreatment (Figures 1a and 2c). Considering the significantly higher initial maximum rate and in particular the longer time of the high-rate peak in the CO2/H2 reaction after the preceding reaction in CO/H2 (12 μmol gAu−1 s−1, peak width about 36 min, Figure 1b) compared to after exposure to H2 (6.9 μmol gAu−1 s−1, peak width a few minutes, Figure 2c), CO seems to be much more effective for the (transient) activation of Au/ZnO catalysts in the methanol formation reaction from CO2/H2 than H2. Tentatively, we assign this difference to a combination of two effects: a more effective reduction of the ZnO support by reaction with CO than with H2, as evidenced by the observation of significant CO2 formation during the activation phase in CO/H2 (see Figure 2a), and the accumulation of adsorbed reaction intermediates, which is possible during exposure to CO/H2 but not during H2 exposure. The very short high-activity peak after changing from H2 to CO2/H2 (Figure 2c), compared to the change from CO/H2 to CO2/H2 (Figure 1b), and also its lower intensity would be compatible with either of these two reaction models, because exposure to H2 would not support the buildup of adsorbed reaction intermediates and is also expected to be less efficient for reduction of the ZnO support in Au/ZnO catalysts. A significantly lower efficiency of H2 for

Figure 3. (a) Normalized XANES spectra (Auger electron yield (AEY) detection) of the O K-edge recorded in 0.3 mbar O2/Ar (1:1) at 240 °C after calcination at 400 °C in the same gas mixture (spectrum 1) and during methanol synthesis in 0.3 mbar CO2/H2 (spectrum 2; 300 min after gas exchange), followed by reaction in 0.3 mbar CO/H2 (spectrum 3; 330 min after gas exchange) and back again in 0.3 mbar CO2/H2 (spectrum 4; 120 min after gas exchange). (b) Atomic surface ratio of Zn:O derived from Zn 2p and O 1s NAPXPS measurements on the different catalyst surfaces described in panel a (for normalization of the intensity of spectrum 1, see text; spectra in Figure S4).

unoccupied density of states (UDOS), which depends sensitively on the oxidation state of the ZnO and on the formation of O-vacancies.27−29 In these spectra, the feature A at 526−528 eV is related to an excitation of the O 1s electrons into an O 2p state hybridized with a Zn 3d state.28 Features B (531.5 eV) and C (533.3 eV) result from the excitation of O 1s electrons into O 2p states hybridized with Zn 4p and 4f states, respectively.27−29 The intensity of these features is proportional to the UDOS. For the absolute comparison of spectra taken during pretreatment and during subsequent reaction steps, all XANES data were normalized to identical intensity at an energy of 532.9 eV. Normalized spectra, recorded at different stages of a reaction sequence in the respective gas mixtures, are shown in Figure 3a. Spectrum 1 was recorded in 0.3 mbar O2/Ar (1:1), after 1 h calcination in the same gas mixture at 400 °C; spectrum 2 was recorded during subsequent exposure to a CO2/H2 reaction gas mixture, 300 min after gas exchange. Spectrum 3 was acquired after changing to a CO/H2 reaction gas mixture, 330 min after gas exchange, and finally spectrum 4 was recorded after 3648


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The Journal of Physical Chemistry Letters changing back again to the CO2/H2 mixture, 120 min after gas exchange (all at 240 °C, 0.3 mbar total pressure). Comparing spectra 1 and 2 reveals a significant decrease of the intensity of the pre-edge feature (A) and of the intensity of the two features B and C during reaction, which points to a decrease of the UDOS during exposure to the reaction gas and hence to the formation of O-vacancy defects. After switching to the CO/H2 gas mixture (spectrum 3), the pre-edge feature (A) disappeared almost completely and the intensity of features B and C decreased by ∼50% compared to spectrum 2. Obviously, the density of O-vacancies increased substantially in the presence of CO/absence of CO2. Changing again to a CO2/H2 gas mixture (spectrum 4), the features A−C were restored to about their initial intensity in spectrum 2. Obviously, O-vacancies are created during reaction in CO/ H2 or CO2/H2 gas mixtures and the formation of O-vacancies is significantly more pronounced in the presence of CO than in CO2/H2. CO2 can even partly refill the O-vacancies created during reaction in CO/H2. For more quantitative information on the amount of Ovacancies and changes therein in different gas mixtures, we also performed in situ near-ambient pressure X-ray photoelectron spectroscopy (NAP-XPS) measurements during the transient steps presented in Figure 3a. It should be noted that because the difference in the Zn 2p binding energy between metallic Zn and ZnO is only 0.3 eV (1021.4 and 1021.7 eV), one would not expect to see a significant shift of the Zn 2p BE upon partial reduction of ZnO.13 From the XP spectra of the Zn 2p and the O1s regions (see Figure S4) we determined the atomic Zn:O ratio in the surface region, using orbital symmetry factors and photoionization cross section corrected intensities of the Zn 2p3/2 peak at 1022.3 ± 0.1 eV and of the O 1s peak component at 530.3 ± 0.1 eV, which is commonly related to ZnO lattice oxygen species (see Figure S4).30,31 As outlined in more detail in the Supporting Information, this is only a small contribution to the total O 1s peak, where the latter is dominated by a component from adsorbed OH at 531.5 eV,32 possibly with a minor contribution from lattice oxygen in the vicinity of O-vacancies,30 or from surface carbonates,33,34 and an about equally large component from adsorbed H2O at 532.8 eV.32 The latter may also contain contributions from surface carbon containing species such as adsorbed methoxy or adsorbed formate species. On the basis of the intensity of the related C 1s signal (see below), however, these contributions must be rather small. This resulted in a Zn:O atomic surface ratio of about 1.22 ± 0.05 for sample 1. Considering that this sample is fully oxidized and the approximation we had to make in the deconvolution of the lattice oxygen O 1s signal, we normalized this to 1. This value increased from 1.0 to 1.17 ± 0.06 upon switching to the CO2/H2 reaction gas (Figure 3b). After changing to the CO/H2 gas mixture, it further increased to 1.28 ± 0.07. Returning again to the CO2/H2 gas mixture finally resulted in a decrease of the ratio to 1.09 ± 0.06. Hence, the trend in these data (ratio of the Zn 2p:O 1s intensity using the component of lattice oxygen) agrees fully with that derived from the changes in the oxygen K-edge region (Figure 3a) and also with the variation in the amount of O-vacancies calculated from CO2 formation during reaction in CO/H2 (Figure 2b). In all cases we find a significant increase in the number of oxygen vacancies compared to the fully oxidized state, after O400 pretreatment, upon exposure to the reaction gas, and in all cases this is most pronounced during reaction in CO/H2 mixture and significantly less during reaction in a CO2/H2

reaction gas mixture. Finally, both in the XANES and in the NAP-XPS data CO2 was found to partly reoxidize ZnO that had been reduced before by exposure to a CO/H2 gas mixture. The similar trends in the spectroscopic measurements compared to the reaction measurements also indicate that the respective tendencies for catalyst reduction are effective already at much lower pressures than applied in the reaction measurements (5−50 bar). Even though it is not possible to measure methanol formation rates at the pressure employed in the in situ XANES and NAP-XPS measurements (0.3 mbar), the effects on the catalyst oxidation state are identical at the lower pressure, at least on a qualitative scale. It should be noted that a more quantitative comparison of the titration data in Figure 2a, which were obtained under realistic reaction conditions, and the in situ spectroscopy data recorded at 0.3 mbar is hindered not only by the different pressures, but first of all by the differences in surface sensitivity of the methods applied. This is very high (NAP-XPS) or rather high (XANES) in the in situ spectroscopic detection, with kinetic energies of the emitted electrons of about 150 eV (NAP-XPS) or 530 eV (XANES), while the titration measurements (Figure 2b) integrate over all O vacancies formed, independent of whether they are near-surface or deeper in the bulk. Nevertheless, the trends in O vacancy formation are similar. Additional information on the electronic structure of ZnO and changes therein upon the interaction with CO2, CO, and H2 at low temperature (30 °C) was obtained from in situ FTIR spectroscopy in the diffuse reflectance (DRIFTS) mode, where the measurements were performed at close-to-realistic conditions, at 1.0 bar. Here, we make use of the sensitivity of the conduction electrons of semiconductor oxides such as ZnO toward IR absorption in the midfrequency range.35,36 Figure 4a shows the change in the reflected FTIR intensity at 30 °C, which is inversely proportional to the absorption. In Figure 4a we plotted the range from 650 to 1650 cm−1, which is sensitive to changes in the band structure.35 Spectrum 0 was recorded in Ar on the fully oxidized catalyst after the O400 pretreatment and shall be used as background for normalization of the spectra measured subsequently. All other spectra were recorded 1 h after the respective gas exchange (for timeresolved spectra see Figure S5). After admitting 2% CO2/Ar (spectrum 1), the reflected intensity decreased by 45%, resulting in a reflectance of 0.55, as indicated in Figure 4b (see description in the caption). Changing then to 2% CO/Ar, the reflected intensity decreased by more than 1 order of magnitude, leading to a total reflectance of only 0.044 (spectrum 2). After returning to CO2/Ar (spectrum 3), the reflectance increased again to 0.35. Finally, upon replacing CO2/Ar by another purely reductive gas mixture, by 45% H2/ Ar (spectrum 4), the reflectance decreased once again to 0.055. Interestingly, the reduction process in CO/Ar is much faster than the reoxidation of the catalyst in CO2/Ar, which is illustrated in Figure S5. In combination, these observations further support that CO2 can reoxidize prereduced Au/ZnO, in this case even at room temperature, but the rate is much slower than that for reduction by CO. This result closely resembles previous findings for Au/CeO2, where a similar discrepancy between reduction by CO and reoxidation by CO2 was reported.11 In total, these spectroscopic findings fit well with the observations under reaction conditions, specifically to the observation of CO2 formation in the initial activation phase (Figure 2a,b), and indicate that also at pressures in the bar 3649


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Figure 4. (a) In situ FTIR spectra (diffuse reflected intensity) recorded on a Au/ZnO catalyst in different gas mixtures: (0) in Ar after O400 pretreatment; (1) in 2% CO2/Ar; (2) in 2% CO/Ar; (3) in 2% CO2/Ar; (4) in 45% H2/Ar. (b) Integrated diffuse reflectance in the range 650−1650 cm−1 (reflectance = Ix/I0; x = 1−4). All gas mixtures were admitted at a total flow of 30 N ml min−1 and a pressure of 1.0 bar.

Figure 5. (a) NAP-XP spectra of the C 1s region recorded at 240 °C in (a) 0.3 mbar of O2/Ar (1:1); (b−d) during exposure to a methanol synthesis reaction gas mixture, (b) in 0.3 mbar CO2/H2 (1:3) (after 290 min reaction), (c) in 0.3 mbar CO/H2 (1:3) (after 320 min reaction), and (d) again in 0.3 mbar CO2/H2 (after 110 min reaction).

range interaction with CO/H2 is much more reductive than interaction with H2/CO2. Furthermore, these data demonstrate that changes in the concentration of O-vacancies (oxidation−reduction of ZnO) result in a significant modification of the electronic properties of the oxide support. We had recently reported that exposure to a H2/CO2/ reaction gas mixture also leads to a slight structural modification, indicated by a slight increase in the Zn 2p:Au 4f intensity ratio.9 This, however, is not a unique indication of a flattening of the Au NPs on the partially reduced ZnOx surface, but may equally result from the formation of AuZnx (surface) alloys, as proposed for Cu/ZnO catalysts (see below).13 Finally, considering the recent debate on the role of adsorbed formate species in methanol synthesis on Cu/ZnO catalysts,16−18 we tried to identify possible correlations between the oxidation state of the catalyst, surface formate concentration, and reactivity for methanol formation. This was explored by in situ NAP-XPS and post mortem XPS measurements. Spectra of the C 1s region, recorded after 30 min in different reaction mixtures at 240 °C/0.3 mbar, are shown in Figure 5. After O400 pretreatment (Figure 5a), where the catalyst is fully oxidized, the spectra showed three different contributions to the C 1s peak: a dominant one at 284.8 ± 0.1 eV, which is related to adventitious carbon species and which was used for energy calibration; a second one at 286.2 ± 0.1 eV, which is attributed to oxygenated carbon species, e.g., in methoxy species (CH3−O−);32 and a third very weak peak at 288.9 ± 0.2 eV, which can be related to surface carboxyl species such as surface formate spe-

cies17,34,37,38 or to surface carbonates.39 The existence and assignment of this third peak with its low intensity is supported also by a clearly resolved similar feature in XPS data recorded ex situ, after the reaction (see below), and by the appearance of a weak signal characteristic for formate related features in the DRIFTS data.8 During exposure to the CO2/H2 gas mixture (Figure 5b), the intensity of the component at 286.2 ± 0.1 eV (peak 2, Figure 5b) increased. In the subsequent changes to the CO/H2 gas mixture (Figure 5c) and back to the CO2/H2 gas mixture (Figure 5d), the intensity of this peak component did not change significantly. For the weak peak at 288.9 ± 0.1 eV, which had tentatively been related to surface formate/ carbonate species (see above), we did not find significant differences in the relative intensity after changing to CO/H2 (peak 3, Figure 5c) and to CO2/H2 reaction gas mixtures (peak 3, Figure 5d) in these experiments, considering the scatter in the spectra. In total, the XP spectra do not provide any evidence for significant effects of the composition of the reaction gas mixture on the abundance of surface species on the catalyst surface during reaction. Comparable results were obtained from another set of measurements, where the reaction was performed under completely realistic conditions: in a flow reactor at 5 bar and 240 °C for 1000 min on stream; the composition of the resulting catalyst surface was analyzed 3650


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conditions) a state is passed that is more active, but unstable, under these reaction conditions. Furthermore, the partial reduction of the ZnO support under reaction conditions may lead to the formation of AuZnx surface alloys, as has been shown for Cu/ZnO catalysts.13 However, so far our attempts to verify this have been unsuccessful. In conclusion, we have demonstrated that the presence of CO considerably affects the activation and catalytic performance of Au/ZnO catalysts in the formation of methanol from CO2/H2, where CO may be present either as reactant, in COcontaining reaction gas mixtures, or result from a side reaction, from conversion of CO2 to CO via the RWGS reaction, in COfree mixtures, or a mix of both. Reduction of the catalyst by reaction in CO/H2 leads to a transient high activity for methanol formation in CO2/H2, which decreases, however, soon after gas exchange to CO2/H2. Obviously, the activity of Au/ZnO for methanol formation from CO2/H2 is sensitively affected by the oxidation state of the ZnO support, i.e., by the presence of oxygen vacancies in the ZnO support. It is highest for a concentration of oxygen vacancies which is comparable or slightly lower than that obtained after reaction in a CO/H2 gas mixture. The considerably higher presence of oxygen vacancies during reaction in CO/H2 than in CO2/H2 gas mixture is confirmed by in situ XANES measurements at the O K-edge and NAP-XPS measurements of the atomic surface ratio of Zn:O. The relatively long time required for reaching steadystate conditions indicates that oxygen vacancies are formed not only at the surface but also in near-surface or even bulk regions. This is supported also by the amount of oxygen vacancies, around 0.3 ML equivalent, that are formed upon reaction in the CO/H2 gas mixture as derived from the transient formation of CO2, which is much higher than realistically possible for surface oxygen vacancies. It is furthermore supported by the distinct difference in IR reflectivity of the catalyst during reaction in CO2/H2 and CO/H2. We speculate that bulk vacancy defects, which exist in a dynamic equilibrium between O vacancy formation by CO (and H2) and reoxidation by reaction with CO2 (and H2O), affect the reaction via electronic metal−support interactions (EMSIs), which also modify the charge state on the Au nanoparticles and possibly also the Au particle shape, as observed for Cu/ZnO. In total, not only are CO and CO2 important as a carbon source but also their ratio is decisive for the oxidation state of the support and the charge state and adsorption properties of the Au NPs. Similar trends are proposed also for Cu/ZnO and possibly also for other metal/ ZnO catalysts, such as Pd/ZnO.

by XPS under UHV conditions afterward, in the same system as used above. This involved a complex sample transfer procedure, which ensured that the catalyst did not get into contact with air between reaction measurement and the XPS measurement (see details in the Supporting Information). After reaction in CO/H2 at 5 bar, the C 1s spectrum showed, in addition to the dominant signal of ubiquitous carbon at 284.8 eV, a clearly resolved peak at 289.6 eV which can be associated with a C 1s formate signal (see insets in Figure S6). After reaction in CO2/H2, we found a comparable peak at 289.5 eV. This feature seems to be slightly more intense after reaction in CO/H2 than after reaction in the CO2/H2 gas mixture (see insets in Figure S6), but the difference in the relative intensity of these features with 1.7 ± 0.25% of the total C 1s intensity after reaction in CO/H2 and 1.3 ± 0.2% after reaction in CO2/H2 is at the limits of these measurements. In total, these XPS data provide clear evidence for the presence of surface carboxylates such as surface formates and of oxygenated carbon-containing surface species such as methoxy species, both during and after exposure to the reaction gas atmosphere, independent of the reaction pressure and the composition of the reaction gas mixture. The total amounts of these species and in particular the differences between the two reaction gas mixtures are, however, small. Comparable observations of a transient activation in the methanol synthesis reaction upon reductive treatment in CO/ H2 mixtures have been reported also for Cu/ZnO catalysts for reaction in syngas.40 Furthermore, these authors observed a synergistic effect, with a 1:1 CO:H2 mixture leading to more reactive catalysts (higher transient activity) than reduction in either CO or H2. They suggested gas-dependent modifications in the Cu nanoparticle morphology as the physical origin for the transient activation of these catalysts for methanol production, assuming a shape−activity relationship as described in ref 41. The present results for methanol synthesis over Au/ZnO catalysts are consistent with these conclusions, but are more far-reaching. For Au/ZnO, our findings indicate that reductive pretreatment leads to the formation of Ovacancies, and these in turn lead to stronger metal−support interaction. The latter are likely to modify the shape of the Au nanoparticles, possibly to flatter shapes as indicated for Cu in the above references, but also lead to a change in the adsorption properties, as evidenced by a stronger binding of adsorbed CO and a change in absorption frequency for adsorbed CO to regions typical for COad on negatively charged Au nanoparticles.9 While these latter effects may be more pronounced for Au/ZnO than for Cu/ZnO, because of the more electronegative nature of Au compared to Cu, the general trend is expected to be comparable. In that case, the oxidative/ reductive nature of the reaction atmosphere would result in a composition-specific reduction of the ZnO support, which in turn defines the metal−support interactions with the Cu nanoparticles. These in turn are decisive for the Cu particle shape under reaction conditions, with flatter particles (enhanced wetting) resulting under more reductive reaction or pretreatment atmospheres. Furthermore, these interactions may also lead to subtle changes in the charge state of the Cu nanoparticles or the active Cu sites. Changes in the gas atmosphere will result in a transient activation for methanol synthesis if on the way to the new dynamic equilibrium state (particle shape, electronic/chemical properties of the Cu nanoparticle) of the Cu/ZnO catalyst determined by the new reduction state of the ZnO support (under steady-state

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ASSOCIATED CONTENT

* Supporting Information S

The Supporting Information is available free of charge on the ACS Publications website at DOI: 10.1021/acs.jpclett.9b00925. Details of the experimental procedures and parameters and additional data from kinetic, IR, and XPS measurements (PDF)

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AUTHOR INFORMATION

Corresponding Author

*E-mail: [email protected]. ORCID

R. Jürgen Behm: 0000-0002-7565-0628 3651


Letter

The Journal of Physical Chemistry Letters Notes

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The authors declare no competing financial interest.

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ACKNOWLEDGMENTS We thank the HZB (Berlin/Germany) for the allocation of synchrotron radiation beam time.

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Influence of CO on the Activation, O-Vacancy Formation, and

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