Research Article pubs.acs.org/acscatalysis
How Temperature Affects the Mechanism of CO Oxidation on Au/ TiO2: A Combined EPR and TAP Reactor Study of the Reactive Removal of TiO2 Surface Lattice Oxygen in Au/TiO2 by CO Daniel Widmann,† Anke Krautsieder,† Patrick Walter,† Angelika Brückner,‡ and R. Jürgen Behm*,† †
Institute of Surface Chemistry and Catalysis, Ulm University, Albert-Einstein-Allee 47, D-89081 Ulm, Germany Catalytic in situ Studies, Leibniz Institute of Catalysis, D-18059 Rostock, Germany
‡
ABSTRACT: Despite enormous breakthroughs in our understanding of the reaction mechanism of the low-temperature CO oxidation on gold catalysts, in particular on Au/TiO2 and down to temperatures as low as −150 °C, there are still many contradictory proposals about the dominant reaction pathway. In this work, we will demonstrate that these discrepancies often originate from the rather different reaction conditions applied in numerous studies, most notably from different reaction temperatures. By combining temporal analysis of products reactor measurements with electron paramagnetic resonance spectroscopy, we will show that removal of TiO2 surface lattice oxygen from a Au/TiO2 catalyst upon exposure to CO (i) readily takes place at 120 °C, where it represents the active oxygen species for CO oxidation, (ii) is still possible at −20 °C, although much slower and to a much lower extent, and (iii) is completely inhibited at −90 °C. Consequences of these findings for our understanding of the dominant reaction pathway for the CO oxidation on Au/TiO2 catalysts, in particular its dependency on the reaction temperature, will be discussed. KEYWORDS: heterogeneous catalysis, Au catalysis, Au/TiO2, CO oxidation, reaction mechanism, active oxygen, temporal analysis of products (TAP), electron paramagnetic resonance (EPR)
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INTRODUCTION Supported Au catalysts, consisting of Au nanoparticles a few nanometers in size deposited on metal oxides, are most wellknown for their high activity for low-temperature CO oxidation.1,2 The term “low temperature”, however, covers a rather broad range, from temperatures as low as approximately −240 °C3 to 80 °C or even higher.4 At temperatures below room temperature, the CO oxidation reaction is mainly interesting from a scientific point of view. The Au-catalyzed CO oxidation at and above room temperature, in contrast, is regularly proposed to represent an attractive candidate for practical applications, e.g., for air purification or the removal of CO impurities from H2-rich feed gases.5 Recently, several mechanistic aspects of CO oxidation on supported Au catalysts have been unraveled, in particular for Au/TiO2, which among different oxide-supported catalysts has been investigated most often. On the basis of these findings, a rather detailed picture of the dominant reaction processes under certain reaction conditions has emerged.4,6−11 For the CO oxidation on Au/TiO2 at temperatures of ≥80 °C (80−400 °C), we recently demonstrated that TiO2 surface lattice oxygen close to the perimeter of the interface between Au NPs and the TiO2 substrate represents the active oxygen species.4,12 In the absence of Au NPs, i.e., on pure TiO2, in contrast, removal of TiO2 surface lattice oxygen by CO is completely inhibited under these reaction conditions. Accord© XXXX American Chemical Society
ingly, the reaction was proposed to predominantly proceed via a Au-assisted Mars-van Krevelen (MvK) mechanism, where surface oxygen vacancies at the perimeter of the Au−TiO2 interface are continuously formed (upon reaction with adsorbed CO) and replenished (upon interaction with O2) during the reaction.4,12 In the meantime, these findings have been confirmed both by experimental7 and by theoretical studies.10,11,13,14 A similar mechanism was confirmed later also for CO oxidation on other Au catalysts supported on reducible metal oxides, such as Au/FeOx,8 Au/ZnO,15 Au/ZrO2,15 and Au/CeO2,16−18 as well as for the H2 oxidation on Au/TiO2.19,20 For CO oxidation on Au/TiO2 at very low temperatures (from −140 to −160 °C), in contrast, molecular oxygen adsorbed at dual perimeter sites (also located at the perimeter of the Au− TiO2 interface) has been demonstrated to represent the active oxygen species for CO oxidation.6,21 This can react either with CO adsorbed on the TiO2 support (dominant below approximately −140 °C) or with CO adsorbed on the Au nanoparticles (dominant at higher temperatures) via a [CO·O2] co-adsorption complex.6,21 Comparison of the studies described above, which have been performed at very different temperReceived: April 29, 2016 Revised: June 23, 2016
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DOI: 10.1021/acscatal.6b01219 ACS Catal. 2016, 6, 5005−5011
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ACS Catalysis
present on the catalyst surface, as recently demonstrated in TPD measurements directly after calcination.30 However, considering the small amount and the fact that there is no further insertion of water during reaction, because of the use of moisture filters for all gases, we describe this as dry reaction conditions. The Au loading of the resulting catalyst was 2.7 wt %, as determined by optical emission spectroscopy (ICP-OES), with a mean (surface area-normalized) Au particle size of 2.7 ± 0.6 nm, as determined by transmission electron microscopy (TEM). A representative TEM image and the Au particle size distribution after calcination of the catalyst (O400) are shown in Figure 1.
atures, indicates that the dominant reaction pathway for CO oxidation changes with temperature, as proposed previously.22 Finally, the presence or absence of water in the reaction atmosphere and/or on the catalyst surface was also demonstrated to change the dominant reaction pathway,9,23−27 leading to the proposal of a water-mediated reaction mechanism at room temperature with a very low energy barrier for O2 activation in the presence of adsorbed water and CO.9 Returning to the influence of the reaction temperature, we note that little is known about the transition between the “lowtemperature regime” (below −140 °C), where the reaction is dominated by the formation of a [CO·O2] co-adsorption complex, and the higher-temperature regime (≥80 °C) with the Au-assisted MvK mechanism. One may envision that with a decreasing reaction temperature the removal of surface lattice oxygen becomes increasingly inhibited, until it is even no longer possible. Such a behavior would directly point to a change in the dominant reaction mechanism of the CO oxidation on Au/ TiO2. So far, however, experimental proof of a barrier in the reactive removal of lattice oxygen, which would increasingly inhibit this in the intermediate-temperature range, is missing. Furthermore, so far, there has been no direct spectroscopic evidence for the participation of the surface reduction of TiO2/ formation of surface oxygen vacancies coupled with Ti3+ formation in the reaction mechanism of the CO oxidation on Au/TiO2 at temperatures of ≤80 °C. Previous assignments have been based on the thermal stability of the active oxygen species, and the temperature dependency of its removal/ replenishment,4 or on changes in the electric conductivity of Au/TiO2 upon interaction with CO.7 These two aspects are topics of the work presented here, where we (i) unravel the temperature dependence in the reactive removal of surface lattice oxygen on a Au/TiO2 catalyst, by reaction with CO, and (ii) present a direct spectroscopic evidence of the formation of Ti3+ species on Au/TiO2 upon exposure to CO (in the absence of gas phase oxygen) at different characteristic temperatures (−90, −20, and 120 °C) by operando electron paramagnetic resonance (EPR) spectroscopy. These measurements were combined with temporal analysis of products (TAP) reactor measurements to also quantify the removal of TiO2 surface lattice oxygen at the respective temperatures. To minimize effects due to moisture (see above), the experiments were performed under strictly dry reaction conditions (see below). On the basis of these results, we will discuss whether and the extent to which the Au-assisted Mars-van Krevelen mechanism contributes to the CO oxidation reaction also at low temperatures and, hence, whether there is a change in the dominant reaction mechanism with temperature upon going to ambient temperatures and below.
Figure 1. (a) Representative TEM image of the Au/TiO2 catalyst after calcination in 10% O2/N2 at 400 °C (O400, 20 NmL min−1, 30 min) and (b) corresponding particle size distribution of the Au nanoparticles.
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Temporal Analysis of Products (TAP). The pulse experiments were performed in a homemade TAP reactor, which is described in ref 29. Gas pulses of typically 1 × 1016 molecules per pulse (generated by piezo-electric pulse valves) were directed into the quartz tube microreactor containing the catalyst bed, which consisted typically of 5.0 mg of the Au/ TiO2 catalyst diluted with inert quartz powder (1:1). This was packed between two layers of quartz particles and fixed in the center of the tubular micro reactor (90 mm long, 4.0 mm inside diameter) by metal sieves. This dilution with quartz powder and the additional layers are necessary to obtain a manageable catalyst packing (total length of ∼10 mm) and thus to ensure reproducible reaction conditions, and to adjust the residence time of gases within the catalyst bed. It was checked in separate
EXPERIMENTAL SECTION Preparation and Characterization of the Catalysts. All TAP and EPR measurements were performed on a homemade Au/TiO2 catalyst prepared by a deposition−precipitation procedure with a nominal Au loading of 3.0 wt %, which had been described in more detail in ref 28. Commercial, nonporous TiO2 with a surface area of 56 m2 g−1 was used as support material (P25 from Degussa). Prior to all measurements, the as-prepared catalyst was first dried at 100 °C in a flow of N2 for 15 h and subsequently pretreated by in situ calcination in a flow of 20 NmL min−1 10% O2/Ar at 400 °C for 30 min (O400). Note that even after this pretreatment there are still strongly adsorbed water and/or hydroxyl groups 5006
DOI: 10.1021/acscatal.6b01219 ACS Catal. 2016, 6, 5005−5011
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ACS Catalysis measurements that both quartz powder and metal sieves are inert for the CO oxidation and have no influence on CO and O2 adsorption under the reaction conditions described above. After passing through this catalyst bed, effluent gases are analyzed by a quadrupole mass spectrometer located in the analysis chamber. For in situ calcination at atmospheric pressure, the reactor can also be separated from the analysis chamber and connected directly to an adjustable roughing pump. To reactively remove active oxygen from the catalyst surface, sequences of CO/Ar pulses (1/1 CO/Ar, ∼5 × 1015 molecules CO per pulse) were subjected to the fully oxidized Au/TiO2 catalyst at various temperatures between −90 and 120 °C (separation between individual CO/Ar pulses of 6 s). After removal of all available active surface oxygen by reaction with CO (eq 1), the removed oxygen was replenished by a subsequent sequence of O2/Ar pulses (eq 2, separation between pulses of 6 s, 1/1 O2/Ar, ∼5 × 1015 molecules O2 per pulse). This procedure was repeated at least three times on all samples to distinguish between reversible and irreversible removal of active oxygen by CO.30 The starting point for all TAP reactor measurements was a Au/TiO2 catalyst that had been fully (re)oxidized by O2/Ar pulses at 120 °C. This way, oxygen species present after calcination that participate only in a noncatalytic CO oxidation (because of their irreversible removal upon the first exposure to CO under reaction conditions) do not have to be considered.30 Au/TiO2 + CO→Au/TiO2 − x + CO2 Au/TiO2 − x
1 + O2 →Au/TiO2 2
Figure 2. MS signals of CO (blue, top panel) and CO2 (black, bottom panel) recorded during the first 50 CO/Ar pulses on the fully oxidized Au/TiO2 catalyst at 120 °C.
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consumption. This results from a delay in CO2 desorption, due to the interaction of CO2 with Au/TiO2, which is indicated also by the upshift of the baseline for the CO2 signal during the initial 10 pulses. The total amount of CO2 formed during the whole pulse sequence, however, is identical to the amount of CO consumed, indicating that there is no accumulation of carbon-containing species, such as surface carbonates, on the catalyst surface under the reaction conditions used in this work. Because no gas phase O2 is present during the CO/Ar pulses, all oxygen for CO oxidation/CO2 formation has to originate from the Au/TiO2 catalyst itself. This result agrees fully with findings in previous studies that had proven that under these conditions TiO2 surface lattice oxygen at the perimeter of the Au−TiO2 interface represents the active oxygen species for CO oxidation.4,7,12 Integrating over the whole CO/Ar pulse sequence leads to the removal of 2.4 × 1018 O atoms gcat−1 in total. This equals ∼1% of the totally available TiO2 surface lattice oxygen in Au/TiO2 and fits well to an oxygen removal close to the Au−TiO2 perimeter sites only, in good agreement with previous findings from TAP reactor measurements4 and from in situ electrical conductivity measurements.7 Note that similar experiments with pure TiO2 under identical reaction conditions demonstrated that in the absence of Au NPs removal of surface lattice oxygen by CO is inhibited.4,12 Moreover, inspecting the O2 consumption in a subsequent sequence of O2/Ar pulses demonstrates that the removal of TiO2 surface lattice oxygen from Au/TiO2 is completely reversible upon exposure of the (partially) reduced catalyst to O2. Hence, under reaction conditions, in the simultaneous presence of CO and O2 in the reaction atmosphere, there is a continuous formation and replenishment of surface oxygen vacancies by CO and O2, respectively. Considering that the reoxidation process by O2 is faster than the reduction by CO, this results in an almost fully oxidized catalyst surface during reaction under the reaction conditions used in this work.7
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Electron Paramagnetic Resonance (EPR). In situ EPR spectra were recorded in X-band on a CW spectrometer ELEXSYS 500-10/12 (Bruker, microwave frequency ν ≈ 9.5 GHz) equipped with a heatable homemade quartz plug flow reactor, which was connected to a gas dosing device with mass flow controllers (Bronckhorst) at the inlet. EPR spectra were recorded with a microwave power of 20.0 mW, a modulation frequency of 100 kHz, and an amplitude of 0.5 mT. For the measurements presented in this study, 35−50 mg of the Au/TiO2 catalyst was first pretreated at 400 °C (O400) and afterward exposed to a flow of 5% CO in He (10 NmL min−1) at various temperatures. All spectra were recorded at room temperature or below. The exact temperatures at which each spectrum was recorded are mentioned together with the results (see below). The total flow for all gases and gas mixtures was always 10 NmL min−1. Each gas was passed through a moisture filter; Ar and the 5% CO/He mixture were additionally passed through an oxygen filter to remove any traces of oxygen, which have been demonstrated to falsify the results.
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RESULTS AND DISCUSSION Removal of TiO2 Surface Oxygen from Au/TiO2 at 120 °C. Mass spectrometric signals for CO and CO2 recorded during exposure of the previously oxidized (by O2/Ar pulses at 120 °C) Au/TiO2 catalyst to CO/Ar pulses at 120 °C (Figure 2) clearly show the consumption of CO, indicated by the missing intensity during the first pulses as well as the simultaneous formation of CO2 on Au/TiO2. After ∼20 CO/ Ar pulses, there is no more CO conversion visible in the raw data. CO2 formation, in contrast, is detected during the first 30−40 pulses and, hence, even after there is no more CO 5007
DOI: 10.1021/acscatal.6b01219 ACS Catal. 2016, 6, 5005−5011
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ACS Catalysis For spectroscopic characterization, we measured EPR spectra of the freshly calcined Au/TiO2 catalyst (O400) after (i) treatment in Ar at 400 °C for 15 min and (ii) exposure of the catalyst to a flow of 5% CO/He at 120 °C for 10 min (spectra are recorded after subsequent cooling to room temperature). The EPR spectrum after treatment in an inert atmosphere at 400 °C shows an axial signal with g⊥ = 1.978 and g∥ = 1.964, which is characteristic for Ti3+ single sites in TiO2 and may have been present already in the bulk of the as-prepared catalyst (Figure 3).31 After subsequent exposure of this Au/TiO2
Figure 4. MS signals of CO (blue) and CO2 (black) recorded during the first 50 CO/Ar pulses on the fully oxidized Au/TiO2 catalyst at −20 °C.
the baseline of the CO signal, indicating that not all CO could desorb from the catalyst before the arrival of the next CO pulse(s) (always 6 s later). Second, there is CO consumption during the initial five CO/Ar pulses, which is indicated by the lower intensity measured compared to that at the end of the pulse sequence and, hence, after saturation. At the same time we cannot detect any CO2 formation, either in the initial stage, accompanying CO consumption, or in later stages of the pulse sequence. This seems to indicate that reaction of CO with surface lattice oxygen is not possible at −20 °C. One has to consider, however, also the possibility that the CO2 formed upon exposure to CO and its subsequent reaction with TiO2 surface lattice oxygen remains adsorbed on the surface, at least for some time, and does not desorb under the reaction conditions used in this work or does so at only a slow rate. To check for this possibility, we additionally performed a temperature-programmed desorption (TPD) measurement directly after the CO pulses (start of the TPD temperature ramp 2 min after the last CO pulse). Neither CO desorption nor CO2 desorption was detected (not shown), demonstrating that CO2 formation during the preceding CO pulses is very slow at −20 °C, below the detection limit of the TPD measurement described above. In contrast to the apparent absence of CO2 formation, there is, however, some oxygen uptake during a sequence of O2/Ar pulses at −20 °C on the previously CO-treated catalyst (not shown), which may be attributed to the replenishment of previously formed oxygen vacancies and, hence, indicate active oxygen removal during the previous exposure to CO. The total amount of oxygen consumed during such a pulse sequence of 100 O2/Ar pulses (0.7 × 1018 O atoms gcat−1) is, however, significantly lower than that at 120 °C (2.4 × 1018 O atoms gcat−1) and already close to the detection limit. A second explanation for this oxygen consumption may be that this originates (at least in part) from reaction of O2 with COad still present on the catalyst surface after CO pulsing. Although we cannot detect COad in the TPD measurement recorded after CO pulsing at −20 °C, small amounts of COad below the TPD detection limit may still be present on the surface. (It should be noted that the sensitivity of the TPD measurements is significantly lower than that of the pulse experiments.) Overall, these results show that reaction of CO with TiO2 surface lattice oxygen may still be possible at −20 °C, but in any case with a
Figure 3. EPR spectra of the freshly calcined Au/TiO2 catalyst (O400) after exposure to Ar at 400 °C (black) and after subsequent exposure of the catalyst to 5% CO/He for 10 min at 120 °C (red). All spectra were recorded at room temperature.
catalyst to CO (5% CO/He) at 120 °C for 10 min, the intensity of this Ti3+ signal increases significantly, directly showing the reduction of TiO2 by CO. The slightly larger Δg∥/ Δg⊥ ratio of 1.69 in comparison to a value of 1.46 before exposure to CO (Δg∥ = g∥ − ge and Δg⊥ = g⊥ − ge, where ge = 2.0023) points to a slightly more distorted local environment of the Ti3+ sites created upon exposure to CO, which may be explained by their location on the surface. After a subsequent exposure of the reduced catalyst sample to air at 80 °C, the Ti3+ signal intensity decreased again strongly (not shown here). This is in agreement to previous findings from Okumura et al., who also studied the interaction of Au/TiO2 with CO and O2 by EPR spectroscopy (at room temperature).32 They did, however, not consider Ti3+ formation/surface oxygen removal as part of the reaction mechanism. Overall, the EPR data indicate that the removal and replenishment of TiO2 surface lattice oxygen upon interaction with CO and O2, respectively, are indeed reversible under the exposure/reaction conditions used in this work, providing direct spectroscopic evidence of the previously proposed removal of surface lattice oxygen/ surface reduction of TiO2 in a Au/TiO2 catalyst upon exposure of the catalyst to CO under these conditions. Removal of TiO2 Surface Oxygen from Au/TiO2 at −20 °C. Next we performed similar combined TAP reactor and EPR measurements at −20 °C, i.e., in a temperature range in which the ongoing catalytic CO oxidation is still active, but the rate is rather low.33 The raw data recorded by the mass spectrometer during a sequence of CO/Ar pulses on a fully (re)oxidized Au/ TiO2 catalyst at −20 °C (Figure 4) reveal distinct differences compared to the reaction behavior at 120 °C (Figure 2). First, the CO pulses are rather broad because of the interaction between Au/TiO2 and CO. This even results in an upshift of 5008
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ACS Catalysis significantly lower efficiency compared to that at 120 °C, in agreement with a substantial activation barrier for surface lattice oxygen removal (see above). More precise information about the removal of TiO2 surface lattice oxygen under the conditions used in this work comes from the Ti3+ signal observed in the corresponding EPR spectra, which were recorded after exposure of the freshly calcined catalyst (i) to Ar at 400 °C and (ii) after subsequent exposure to 5% CO/He at −20 °C (Figure 5) (both spectra
oxygen from a fully oxidized Au/TiO2 catalyst, which would result in an oxygen removal rate of ∼2 × 1017 O atoms gcat−1 s−1. Considering that the absolute amount of active oxygen present on the fully oxidized catalyst surface is 2.4 × 1018 O atoms gcat−1 (determined in the TAP reactor measurements at 120 °C), and assuming (i) first-order kinetics for its removal and (ii) saturation coverage for COad (which is reasonable considering the low temperature of −20 °C and the rather high CO partial pressure of 50 mbar during the EPR measurements), its complete removal (99%) upon interaction with CO should take much less than 1 min. This comparison and the results of the EPR measurements described above, which showed a rather slow formation of Ti3+ at −20 °C, clearly demonstrate that the Au-assisted MvK mechanism does not dominate the overall reaction any more at −20 °C and represents at the most a minor reaction pathway under these conditions, in good agreement with the findings for the simultaneous CO/Ar and O2/Ar pulse sequences. Removal of TiO2 Surface Oxygen from Au/TiO2 at −90 °C. Finally, the same approach was used to study the interaction of the catalyst with CO at −90 °C. This temperature was determined by experimental limitations in the cooling system of the TAP reactor. It also comes close to the temperatures at which the mechanistic studies by Green et al. were performed previously (−150 °C).6 As expected from the results of the TAP reactor measurements at −20 °C, we did not detect any CO2 formation upon exposure of fully (re)oxidized Au/TiO2 to CO pulses (Figure 6a). The missing intensity in the CO signals during the first CO pulses is assigned to the adsorption of CO. Compared to CO pulses at −20 °C, a much larger amount of CO is adsorbed at −90 °C. This is also visible from the CO desorption detected in a subsequent TPD measurement, which showed significant CO desorption in the temperature range between −90 °C and room temperature, with a maximal desorption rate at approximately −20 °C (Figure 6b). Comparable to the TPD measurement after CO pulsing at −20 °C, there is again no CO2 desorption detected during the TPD subsequent to the CO pulses at −90 °C (up to 120 °C with a heating ramp of 10 K/min). From these measurements, it is obvious that there is no CO2 formation upon exposure of the fully (re)oxidized catalyst to CO and, thus, that the removal of TiO2 surface lattice oxygen is not possible at that low temperature. Here it should be noted that the catalyst is nevertheless still active for CO2 formation, but only in the presence of oxygen in the gas phase. Exposure of the catalyst to simultaneous CO and O2 pulses at −90 °C resulted in the formation of adsorbed CO2, which could be detected in subsequent TPD measurements (not shown). Also in that case, however, no gaseous CO2 was detected directly during the CO/O2 pulses, indicating that all CO2 formed is stably adsorbed on the catalyst surface. Accordingly, most studies of the low-temperature CO oxidation on Au/TiO2 actually do not investigate the continuous formation of gaseous CO2, but the formation of stable adsorbed CO2, e.g., by in situ infrared spectroscopy.6,34 Kinetic measurements at atmospheric pressure (1% CO, 1% O2, and balance N2) at −90 °C performed in this work still show some (residual) formation of gaseous CO2. The measured CO2 formation rate is, however, very low and only ∼2 × 10−6 molCO2 gAu−1 s−1 (3.5 × 10−6 and 1.5 × 10−6 molCO2 gAu−1 s−1 in the initial state and after reaching a steady-state situation, respectively), which is ∼10% of that at −20 °C and
