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Understanding oxygen activation on nanoporous gold Wilke Dononelli, Gabriele Tomaschun, Thorsten Kluener, and Lyudmila V. Moskaleva ACS Catal., Just Accepted Manuscript • DOI: 10.1021/acscatal.9b00682 • Publication Date (Web): 24 Apr 2019 Downloaded from http://pubs.acs.org on April 24, 2019
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ACS Catalysis
Understanding oxygen activation on nanoporous gold Wilke Dononelli,a Gabriele Tomaschun,a Thorsten Klüner,a Lyudmila V. Moskalevab,c,* a Institute of Chemistry, Carl von Ossietzky University of Oldenburg, 26129 Oldenburg, Germany b Department of Chemistry, University of the Free State, PO Box 339, Bloemfontein 9300, South Africa c Institute of Applied and Physical Chemistry and Center for Environmental Research and Sustainable Technology, University of Bremen, 28359 Bremen, Germany
Abstract Nanoporous gold (np-Au) is a catalytically highly active material, prepared by selectively dealloying silver from a gold-silver alloy. It can promote aerobic CO oxidation and a range of other oxidation reactions. It has been debated whether the remarkable catalytic properties of np-Au are mainly due to its structural features or whether the residual Ag remaining in the material after dealloying is decisive for the reactivity, especially for the activation of O2. Recent theoretical studies provided evidence that Ag impurities can facilitate the adsorption and dissociation of O2 on np-Au. Yet, these studies predicted quite a high activation barrier for O2 dissociation on Au-Ag alloy catalysts, whereas experimentally reported activation energies are much lower. In this work we use the stepped Au(321) surface with Ag impurities, which is arguably a realistic model for np-Au material as well as for Au-Ag catalysts in general. We present alternative routes for O2 activation via its direct reaction with adsorbed CO or H2O. In all of the reactions considered, surface atomic O is generated via a sequence of elementary steps with calculated low activation energies of < 0.4 eV with respect to co-adsorbed reactants. Ag impurities are shown to increase the adsorption energy of O2 and hence the probability of a surface-mediated reaction versus desorption. We considered four possible mechanisms of CO oxidation in dry and humid environments in a microkinetic modeling study. We show that via the proposed mechanisms water indeed promotes O2 dissociation; nevertheless, the “dry” mechanism, in which CO directly reacts with O2 is by far the fastest route of CO2 formation on pure Au and on Au with Ag impurities. Ag impurities lead to significantly higher turnover rates; thus, calculations point to the key role of Ag in promoting the catalytic activity of Au-Ag alloy systems.
Keywords Heterogeneous catalysis, gold, nanoporous gold (np-Au), Au(321), oxygen activation, low coordinated sites, DFT, PBE
*
Corresponding author. Email address:
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Introduction The interest in using gold containing materials in catalysis has been continuously growing recently. For many reactions with commercial potential gold-based heterogeneous catalysts (including alloys of Au with other metals) showed high activity and exceptionally high selectivity under mild conditions, demonstrating unique advantages over other precious metals for certain applications 1. One of the target groups is selective oxidation in the gas and liquid phase, e.g. selective oxidation of CO in hydrogen streams (PROX) 2, CO removal from room atmospheres, 3 or selective oxidation of alcohols and polyols 4. Mechanistic description of these processes is still under development. However, atomiclevel details about the elementary reactions involved are much desirable to guide a rational design of improved catalysts. There is also a purely academic interest behind this research with the ambition to foster our fundamental understanding of the rich and unique chemistry of gold. It is generally agreed that the common “bottle-neck” of aerobic oxidation reactions on catalysts containing gold as active component is the activation of O2, i.e. the splitting of the O–O bond. However, the exact mechanisms by which O2 reacts on gold have been heavily debated. Recent studies on oxide-supported gold nanoparticles provided evidence that several mechanisms may work in parallel and the dominant one may depend on the catalyst preparation and reaction conditions, particularly on the temperature 5. In some of the proposed mechanisms dioxygen first dissociates to reactive surface atomic O, whereas in the other group of mechanisms O2 adds directly to the adsorbed reactant, and the O–O bond splits in subsequent reaction steps 5-7. In both scenarios surface atomic O is involved as a reaction intermediate. For Au nanoparticles supported on reducible oxides atomic O was proposed to originate from the surface lattice oxygen of the oxide via a gold-assisted Mars-van Krevelen (MvK) type of mechanism, whereas on non-reducible oxide supports the oxidation likely proceeds by Au-only mechanisms 7-9. In recent years, the family of gold-based catalysts has been extended to nanoporous gold (np-Au), a nanostructured bulk material with a still largely unexplored potential for catalytic applications, which could be tailored through modification by oxides, ligands, or dopants 10-11. The intrinsic activity of npAu has been attributed to residual Ag impurities, which may help generate active oxygen species. Recent theoretical studies provided evidence 12-15 that Ag impurities can facilitate the adsorption and dissociation of O2 on np-Au. While the XPS and TPD study of active np-Au foams 16 indicated that the chemically active form of oxygen on np-Au is likely atomic surface O produced via O2 dissociation, theoretical studies
12-14
predict quite a high activation barrier for O2 dissociation on Au-Ag catalysts, > 0.67 eV. It was shown that with increasing amount of Ag impurities in the top surface layer, the magnitude of the adsorption 2 ACS Paragon Plus Environment
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energy of O2 increases, whereas the activation barrier for the O2 dissociation drops
12
. However,
calculations showed that large local silver ensembles are required for lowering the activation barrier of O2 dissociation to moderate values. Recent studies by Krekeler et al.
17
and Mahr et al.
18
demonstrated that large Ag clusters could indeed be present in np-Au after dealloying and, therefore, supported the availability of large Ag ensembles on the surface of np-Au ligaments. Still there seems to be an unresolved conflict between the theoretically predicted rather high activation energy of O2 dissociation and experimentally reported low activation energies of CO and methanol oxidation by O2 on np-Au, ca. 0.3 eV 19-21. In the attempt to clarify this puzzle Montemore et al. 15 recently reported a low activation barrier, ~0.4 eV, for O2 dissociation on special sites at a bimetallic step modelled by Au(211). Their study suggests that Ag should segregate near step edges (but not on steps) and such bimetallic sites may facilitate O2 dissociation. These are interesting results that deserve further exploration. Currently, it is not clear whether they could be generalized to other types of Au-Ag surfaces. Our tests (see the Supporting Information (SI), section S3) showed that Ag impurities near the steps on Au(321) only slightly lowered the barrier of O2 dissociation and did not increase the binding energy of O2. An alternative to the dissociation of O2 as an activation process is a direct reaction of a chemical with the chemisorbed molecular oxygen on a catalyst surface forming an intermediate of X−O−O* type. (Here and below, the “*” symbol indicates adsorbed species.) In a recent review Hibbits and Iglesia proposed that on catalytic metal surfaces, especially under the reaction conditions when the coverage of adsorbates is high, the activation of strong bonds in N2, CO, NO and O2 often occurs via bimolecular routes, such as the formation of chemisorbed OOH* in the presence of water on Au catalysts
22
.
Whereas the dissociation of O2 on Au is difficult, in a peroxo-like intermediate X−O−O*, the O−O bond is weakened before cleavage lowering the activation energy of the following dissociation to X−O* + O* . This phenomenon was found even on Pt surfaces, on which O2 has a low dissociation barrier 23. According to Hibbits and Iglesia, 22 in this case, “a dearth of vacant sites causes O2* to react with CO* to form *OOCO* intermediates that undergo O−O cleavage .” Such possibility of a bimolecular reaction between CO* and O2* can also be proposed on np-Au, however, it remains questionable because of a very weak binding of O2 to gold. In contrast to supported Au nanoparticles, where the adsorption energy of O2 is sizable (the calculated adsorption energies for bare nanoparticles are in the range 0.3 − 0.4 eV and may be significantly increased by charging of nanoparticles 23-24), the adsorption energy of O2 on extended gold surfaces is nearly zero. Therefore, silver or other impurities may again be required to enhance the binding and, hence, the reaction probability of O2.
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In the following we will explore the possibility of a direct oxidation with O2 on nanoporous gold as an alternative to O2 dissociation considered previously. We will investigate the complete multistep pathways for the O2 reaction with prototype reactants CO and water on the model Au(321) stepped and kinked surface with or without Ag impurities. Although this is a relatively simple model, which does not reflect the full complexity of np-Au, we are convinced that it captures well the main probable active sites present on np-Au: under-coordinated Au atoms and Ag impurities. Additionally, possibly present large Ag-rich fragments are also considered and modeled by the Ag(321) surface. The results of DFT calculations will provide a basis for a microkinetic model, which will help us to determine which mechanism dominates at given reaction conditions and how silver impurities affect the reaction rates. With respect to the catalytic performance of np-Au, we will provide a possible explanation why Ag impurities in np-Au are beneficial for the activity.
1. Models and Computational Details A kinked Au(321) surface was chosen as a model to represent defect-rich surface of curved ligaments of nanoporous gold. This model was previously successfully employed in several studies modelling the reactivity of np-Au and other nanostructured gold-based catalysts 12, 14, 25-27. The Au(321) surface consists of (111) terraces terminated by zigzag-shaped steps, see Fig. S1 in the SI), which may be favorable as possible adsorption positions of various reactants. Periodic slab calculations were performed using a (2×1) surface unit cell consisting of 28 metal atoms in 14 atomic layers. The bulk lattice parameter was optimized to 4.173 Å. The thickness of the surface slab was ~ 7.2 Å. The vacuum space between the periodically repeated slabs in z direction was ~ 8.5 Å. While the seven uppermost layers (14 atoms) were allowed to fully relax, the remaining atoms were fixed at the bulk positions. The Ag(321) slab was constructed similarly, using an optimized bulk lattice parameter of 4.159 Å. To model the presence of silver impurities, selected atoms in the uppermost layer of Au(321) were replaced by silver. While previous theoretical studies predicted that in the absence of adsorbates thermodynamically preferred positions for Ag are in the subsurface layer15, 28, this is likely to change in the presence of adsorbates. For example, our recent work shows that adsorbed atomic O drives Ag from subsurface layers to the surface29. In particular, we found that if CO* and O2* are co-adsorbed on the surface, on-surface positions at which O2 binds become about as favorable for Ag as the most favorable subsurface positions (see the SI, section S2). Therefore, we substituted silver at those locations, where we expected the effect of the substitution to be most pronounced, e.g. at the step edge. Because bulk gold and silver have very similar lattice constants (the optimized values of a0 are 4.173 and 4.159 Å, respectively), the effects of strain due to surface alloying are negligible.
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Electronic structure calculations were performed using density functional theory (DFT) as implemented in the Vienna ab initio simulation package (VASP)
30
. The exchange-correlation
interaction was treated by the generalized gradient approximation using the PBE functional
31
. For
reactions involving O2 spin-polarized calculations were performed. The effect of the core electrons in the valence density was taken into account by means of the projector-augmented wave (PAW) method 32-33
with an energy cutoff of 415 eV and a kinetic energy cutoff of 645 eV. A 5×5×1 k point mesh and
the k point sampling scheme of Monkhorst and Pack34 was used in all calculations of slabs. The geometries of CO, OOH, O2, and H2O were optimized in a cubic unit cell of 20×20×20 Å3 size using a 5×5×5 k point mesh. The minimum energy reaction paths were determined using the climbing-image nudged elastic band method (ci-NEB) 35, an improvement of the conventional nudged elastic band method (NEB)36, and refined by the dimer method 37. The energies of the minima and transition states along the reaction paths were calculated either relative to the gas-phase reactants and the clean surface or adsorbed stable intermediates were used as reference for individual elementary reactions considered. Further details can be found in the SI, section S1.
2. Results and Discussion 2.1 CO Oxidation In an earlier study
12
we reported that the binding of O2 at the most favorable adsorption site is too
weak (Eads = -0.14 eV) and, therefore, molecular oxygen shall not stick to a pure Au(321) surface at temperatures sufficiently high for a CO oxidation reaction. We demonstrated that if silver impurities are added, the adsorption strength of O2 increases. Furthermore, for large Ag ensembles on the surface, the barrier for the dissociation of O2 decreases from 1.1 eV to 0.7 eV
12
. To achieve this barrier
lowering, almost all of the gold atoms at the surface need to be replaced by silver, which leads to a local silver concentration in the top layer of more than 80%, whereas the bulk Ag concentration in npAu is typically on the order of a few atom%. In a further study we were able to show, that the activation barrier of the O2 dissociation significantly drops if the Au(321) is pre-covered with atomic oxygen 13. The activation barrier can thus be lowered to 0.6 eV with the increasing size of Ag ensembles and the adsorption strength of molecular oxygen increases as well. Nonetheless, even this value of the activation energy seems to be too high, when compared to reported activity for CO and methanol oxidation, 19-21, 38 and the source of pre-existing atomic oxygen remains unclear.
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𝑎𝑎) 𝑂𝑂2∗ + ∗ ⟶ 𝑂𝑂∗ + 𝑂𝑂∗
𝑏𝑏) 𝐶𝐶𝐶𝐶∗ + 𝑂𝑂∗ ⟶ 𝐶𝐶𝐶𝐶2 + ∗
𝐶𝐶𝐶𝐶∗ ⟶ 𝑂𝑂𝑂𝑂𝑂𝑂𝑂𝑂∗ + ∗ ⟶ 𝐶𝐶𝐶𝐶2 + 𝑂𝑂∗
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(1) (2)
In previous theoretical studies of our group mentioned above as well as in the experimental studies 10, 19, 21, 39-41
a mechanism of CO oxidation shown in Eq. 1 was postulated, where in the first step (1a)
adsorbed atomic oxygen is formed and in the second step (1b) this active oxygen reacts with CO* to form CO2. However, a possibility of a direct reaction of CO* with O2* (Eq. 2) was not considered in our earlier studies 1) because surface science experiments showed that weakly bound O2* desorbs at very low temperature (around 100 K), whereas CO2 is formed only above 250 K and 2) because experiments provided evidence that O2 dissociates on np-Au.16 Nevertheless, it cannot be excluded that the chemistry in a catalytic reactor could be different from what is observed under ultrahigh vacuum (UHV) conditions. Also, different mechanisms may work in parallel, whereas the dominant mechanism may change with the reaction conditions. Therefore, in this study, we consider the direct reaction of CO* with O2* (Eq. 2) and we show by microkinetic modeling that this mechanism should be more plausible than the dissociative mechanism (Eq. 1) under ambient conditions. In the proposed mechanism a metastable OCOO* intermediate is formed in the first step. In the second step it decomposes to CO2 and atomic surface O* (Eq. 2). This mechanism has been proposed earlier in several DFT studies on gold nanoparticle catalysts42-44 and also more recently in theoretical studies related to nanoporous gold
45-46
although the latter works have not attempted to show why this
mechanism should be preferred over the dissociative one. At variance to extended Au surfaces, the adsorption energy of O2 on nanoparticles is sizable; in addition, some oxide support materials may stabilize the adsorption of molecular oxygen at the particle perimeter sites. In contrast, the adsorption of O2 on extended Au surfaces, even on stepped surfaces, is weak. This is one of the strong arguments suggesting that pure np-Au should not be catalytically active. Fig. 1a shows the geometries (see also the SI, Fig. S4) and Fig. 1b shows the reaction energy profiles calculated for a CO* + O2* reaction on the pure Au(321) surface, as well as on surfaces with silver impurities. In the initial state, both CO* and O2* adsorb at low-coordinated Au atoms of the step edge. The O−O bond length, 1.31 Å, is significantly elongated with respect to the bond length in the gas-phase O2, 1.23 Å. Hence, the O2* molecule is activated through an electron transfer from gold to its 2π* orbital. The bond length of 1.31 Å suggests that its charge state is close to a superoxide, O2-47.
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In the first reaction step an OCOO* intermediate is formed from the co-adsorbed CO* and O2* with an activation barrier of 0.30 eV, Fig. 1. In the resulting OCOO* species the distance between the two oxygen atoms of O2* increases further to 1.52 Å. In the second step, a metastable OCOO* intermediate dissociates to CO2 and atomic O* in a strongly exothermic process with an activation barrier of only 0.04 eV (Fig. 1 b). In the final state CO2 is considered to be desorbed irreversibly because of its very weak binding strength (Eads = -0.03 eV). The activation barriers for both steps of the described “associative” mechanism are thus low; however, the adsorption strength of molecular oxygen is also low, suggesting that there should be no reactivity on pure Au(321). As seen from Fig. 2b, the adsorption energy of O2* on a CO* pre-covered Au(321) is -0.16 eV. This is close to the value calculated earlier on the clean Au(321), -0.14 eV.12 As we will show in Section 2.3, our microkinetic model predicted a turnover frequency of 0.17 s-1 site-1 for this mechanism on pure Au(321) at 350 K, a surprisingly high value. However, this value is likely overestimated by at least 1-2 orders of magnitude because we considered only active sites at surface steps but other surface atoms are expected to be essentially inactive due to very weak binding of CO* and no binding of O2. Figure 1: Direct reaction of CO* with molecular oxygen: a) Top view of reactants, products, O-C-OO* intermediate and
CO oxidation with O2 via an initial direct reaction between CO* and O2* on Au(321), followed by a
transition states on pure Au(321). Critical bond distances are given in Å. Structures with Ag impurities are shown in the
dissociation of OCOO*, has been earlier proposed
SI. b) Reaction energy profile in eV. Energies are given with
by Fajín et al.27 These authors looked at initial
respect to the CO and O2 molecule in the gas phase and
configurations different from those considered in
Au(321). Here and below the “*” symbol indicates adsorbed species. The horizontal bars denote axis breaks. The
our work because they used a rather small 1×1
adsorption energies of CO* on bimetallic surfaces with 1
surface unit cell. Hence, the lowest co-adsorption
and 2 Ag atoms is -0.76 and -079 eV, respectively.
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energy of CO* and O2* they found was -0.75 eV, compared to -0.97 eV in our study. Qualitatively, Fajín et al. found a similar pathway, where an OCOO* intermediate is formed first and then CO2 and atomic O* are formed with activation barriers between 0.56 eV and 0.58 eV depending on the initial position of the OCOO* species. These barriers are relatively high compared to our values, which could again be explained by the usage of a 1×1 surface cell in the latter work, resulting in a higher surface coverage. Effects of coverage are further discussed in Section S5 of the SI. As the co-adsorption of molecular oxygen with CO* does not favorably influence the adsorption strength of O2*, the latter remains too weakly bound. However, as shown in earlier studies, the adsorption strength of dioxygen can be increased by silver impurities
12-13
. Even small ensembles of
one to two Ag atoms can already raise the magnitude of the O2* adsorption energy to about -0.3 eV. In fact, further increase in the size of Ag clusters only slowly affects the binding energy of O2*
12
.
Even when replacing almost 90% of the surface gold atoms by silver, one could not reach stronger adsorption than Eads = -0.4 eV 12. On Ag-substituted surfaces the activation barrier for the first reaction step, the formation of the OCOO* intermediate, is 0.25 eV for one silver atom replacing gold at the surface and 0.22 eV for an ensemble of two silver atoms (see Fig. 1b). The second step has a very low barrier of 0.01 eV in the case of one Ag impurity atom, and 0.2 eV, if two Ag atoms substitute Au at the reaction site (Fig. 1b). All critical bond distances are given in Fig. S4 of the SI. As evident from Fig. 1b, the adsorption energies of O2* on CO* pre-covered Ag substituted surfaces are -0.26 and -0.32 eV for one and two Ag atoms at the active site, respectively. Hence, by adding just a small amount of silver impurities to Au(321), active sites for the O2 adsorption and further reaction with CO* could be created. The adsorption strength of O2* at bimetallic Ag-Au sites could be increased to the extent that should be sufficient for a sizable reactivity at ambient pressure and room temperature. Furthermore, the activation barriers for the two steps of the mechanism to form CO2 remain low upon adding small amounts of Ag. Our results are in good agreement with the recent computational study of Wang et al.46 who used different surface models of np-Au ligaments (Au(111) and Au(100) with trenches). These authors reported a low-energy associative CO* + O2* pathway with activation barriers below 0.2 eV and found that the activation barriers remain low while the adsorption strength of O2* increases on Ag-substituted surfaces. Thus, for CO oxidation on np-Au (with typical surface concentration of Ag up to 10 atom %) the mechanism shown in Eq. 2 seems to be more plausible than the mechanism shown in Eq. 1. Below, in Section 2.3 we will support this statement with the results of a microkinetic analysis. On surfaces with Ag impurities the TOFs calculated for the mechanism shown in Eq. 2 (mechanism II) increase by up to 3 orders of magnitude at 350 K compared to pure Au.
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2.2 Direct reaction between O2* and H2O* 2.2.1
Reaction on a pure gold surface
In the previous section we have shown that oxidation of CO on np-Au via a direct reaction with adsorbed molecular oxygen is a conceivable pathway. However, as argued in previous studies by us and by others
12-13, 25, 48-49
, silver impurities at the surface seem to be essential, as they facilitate the
binding of O2 and also reduce its dissociation barrier. At the same time several reports in the past have shown that CO oxidation on Au can be enhanced by water.50-53
Fujitani et al. summarized those
evidences in a recent perspective
54
and classified
proposed roles of water in four categories: “(i) maintain cationic state of gold (Au3+ or Au+), (ii) direct involvement of H2O and OH− groups in CO oxidation, (iii) activation of O2 molecules, and (iv) transformation of catalytic intermediates and inhibitors (spectators) such as carbonate species”. Recent studies of Mullins group 50, 52 on the Au(111) model surface pre-covered by atomic O* revealed the involvement of OH* groups formed via a reaction between O* and H2O* and a faster oxidation of CO* by OH* than by atomic O*. Our work, in contrast, emphasizes the stabilization of O2* by water and a possible role of water as a cocatalyst in splitting O2* to atomic O*. The adsorption energy of a water molecule on pure Au(321) is calculated to be -0.21 eV, whereas the co-adsorption energy of H2O* and O2* is -0.61 eV Figure 2: Direct reaction between H2O* and O2* on
(see Fig. 2). Hence, the adsorption energy of
Au(321). a) Geometries of the initial state (H2O* + O2*),
molecular oxygen at a water pre-covered Au(321)
final state (OOH*+ OH*) and an intermediate (H2O* + O2*), see text for details. Critical bond distances are given
surface is -0.40 eV. This is 0.24 eV stronger than in
in Å. b) Energy profile of reaction between O2 and H2O.
the absence of water, -0.14 eV 12, and is also close to
Energies (in eV) are calculated relative to Au(321), O2 and
the maximum (absolute) value reached upon
H2O.
substituting the Au(321) surface with Ag (-0.45
eV).12 Although the co-adsorption of water and O2* is favorable, it is not sufficient to strongly bind 9 ACS Paragon Plus Environment
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this complex to the surface, as we initially assumed. Microkinetic modeling (Section 2.3) revealed that the equilibrium coverage of the H2O···O2* complex on Au(321) at 350 K is about three orders of magnitude lower than that of either H2O* or O2*, the latter being on the order of 10-10 (saturation coverage is taken to be 1) at partial pressures of H2O and O2 0.5 and 9.5 torr, respectively. Equation S19 in the SI explains why this is the case. The surface coverage of the complex is expressed as 𝜃𝜃𝐻𝐻2 𝑂𝑂⋯𝑂𝑂2 = 𝐾𝐾1 𝐾𝐾2 𝐾𝐾4 𝑃𝑃𝐻𝐻2 𝑂𝑂 𝑃𝑃𝑂𝑂2 𝜃𝜃∗ , where K1, and K2 are equilibrium constants of adsorption of H2O and
O2, K4 is the equilibrium constant of the complex formation from adsorbed H2O* and O2*, and 𝜃𝜃∗ is the number of empty surface sites. Because both K1 and K2 are
