Weiyu
Song
,
Peng
Liu
and
Emiel J. M.
Hensen
*
Inorganic Materials Chemistry, Eindhoven University of Technology, P. O. Box 513, 5600 MB Eindhoven, The Netherlands. E-mail: e.j.m.hensen@tue.nl
First published on 23rd May 2014
The catalytic oxidation of bio-ethanol to acetaldehyde entails a promising route for valorization of biomass into many important chemicals that are currently mainly being produced from fossil-based ethylene feedstock. We employ here DFT calculations to understand the unprecedented synergy between gold clusters and a MgCuCr2O4 spinel support, which shows excellent catalytic performance for the oxidation of ethanol to acetaldehyde (space-time yield of 311 gacetaldehyde ggold−1 h−1 at 250 °C). The investigations support a mechanism involving catalytic reactions at the gold–support interface. Dissociative adsorption of ethanol is facilitated by cooperative action of a gold atom at the metal cluster–support interface and a basic oxygen atom of the support. The most difficult step is the recombinative desorption of water from the surface. The oxygen vacancy formation energy is found to be a good performance descriptor for ethanol oxidation of Au/MgMeCr2O4 (Me = Cu, Ni, Co) catalysts. The high selectivity towards acetaldehyde stems from the facile desorption of acetaldehyde as compared to the cleavage of the remaining α-C–H bond in the product. The opposite holds for methanol oxidation, explaining why experimentally we observe complete methanol oxidation over Au/MgCuCr2O4 under conditions where ethanol is selectively converted to acetaldehyde.
The details of the model are shown in Fig. 1. It includes two stoichiometric layers (Fig. 1a) with the bottom layer kept frozen. The vacuum spacing between two slabs is 15 Å. For the Au cluster model, we first optimized a bilayer Au10 cluster on the MgCr2O4(111) surface and froze a selection of the Au atoms at their optimized positions during subsequent calculations (Fig. 1b). This choice is made to have a suitable model for the Au nanoparticle–support interface. Inclusion of a rigid nanoparticle or other suitable models thereof is not possible given the required size of the unit cell. The gold atoms in the cluster close to the surface are fully relaxed during the computations. The approach involving a partially frozen model for the Au cluster has been used before in theoretical studies to study reactions at the interface between metal clusters and oxidic supports.30
Additional calculations were done for Cu(111) and Au(111) surfaces. To model these surfaces, 3 × 3 unit cells with dimensions of 7.67 × 7.67 Å for Cu and 8.87 × 8.87 Å for Au containing four metal layers were employed. The top two layers were relaxed and the bottom two layers were frozen. The thickness of the vacuum between the slabs was 10 Å. A 3 × 3 × 1 k-point mesh was employed for these calculations. The most stable hcp site was considered for O adsorption on Cu(111). For the H migration on the Au(111), the most stable three-fold hollow sites, i.e., fcc and hcp, were considered.
In all calculations, atoms were relaxed until forces were smaller than 0.05 eV Å−1. The location and energy of the transition states were calculated with the climbing-image nudged elastic band method.31
To close the first part of the catalytic cycle, the H atoms need to be removed as water. For water formation, the H atom on the Au cluster first migrates close to the interface with the support (state vi, Fig. 2) with a nearly negligible reaction energy (ΔE = 7 kJ mol−1). The barrier for this process is rather high because some of the Au atoms in the cluster were frozen. To obtain a reasonable value for the activation barrier, we studied H migration from an fcc to an hcp site on the Au(111) surface, representative of the stable surfaces on Au nanoparticles. The barrier for this H migration step is 10 kJ mol−1 (Fig. 3). Before its reaction with this H atom, the OH group on the support migrates from the initial site connected to the Cu cation (state vi, Fig. 2) to an adjacent site connected to a neighboring Mg cation (state vii, Fig. 2). This reaction is endothermic by 45 kJ mol−1 and it does not involve a notable activation barrier. Formation of water adsorbed to the support requires overcoming a barrier of 45 kJ mol−1. The reaction is thermodynamically very favorable and releases 123 kJ mol−1. Desorption of water into the gas phase takes 115 kJ mol−1. Thus, the overall water formation process from the H atom on the Au cluster with the support OH group is thermodynamically favorable.
These reaction events generate one oxygen vacancy on the support surface close to the Au cluster and suggest that ethanol can be oxidized by the support oxygen atoms. This is consistent with the experimental finding that ethanol can also be converted to acetaldehyde and water in the absence of oxygen.15 It results, however, in catalyst deactivation, which should at least in part be due to the depletion of the O atoms of the support. Under aerobic reaction conditions, one O2 molecule will reduce two ethanol molecules to two acetaldehyde molecules. We found that O2 adsorbs on the oxygen vacancy site with an energy of 42 kJ mol−1 (state x, Fig. 2). The O–O bond distance in adsorbed O2 is elongated to 1.33 Å compared to the gas-phase value of 1.23 Å. It points to the formation of O2−. In contrast, we did not find any other adsorption sites for O2 close to the interface between the Au cluster and the stoichiometric MgCuCr2O4 support. The reason is that Cu is fully coordinated by O atoms.
The adsorption energy of ethanol on the Au atom located close to the interface is 73 kJ mol−1. Ethanol reacts with the O2− species to dissociate its hydroxyl group, forming an OOH* group on the support and an ethoxy group on the Au cluster (state xii, Fig. 2). This reaction has again a negligible barrier and is endothermic by 24 kJ mol−1. The bond distance of O–O after formation of OOH* is further elongated to 1.47 Å. The enthalpy change starting from ethanol in the gas phase to dissociated ethoxy and OOH* (from state x to state xii, Fig. 2) is −49 kJ mol−1, which is not as favorable as for the ethanol activation process by surface O (from state i to state ii, Fig. 2) is −117 kJ mol−1. Bader charge analysis34 of the O atoms on the pristine surface and of the top O atom of adsorbing O2 molecule on the defect surface predicts charges of −0.99 e for the former and −0.21 e for the latter. The difference underpins the higher basicity of the pristine surface, explaining the more exothermic activation step of gas-phase ethanol. Similar to the earlier explored pathway, the ethoxy group dissociates one of its α-C–H atoms to the Au cluster and, subsequently, desorbs as acetaldehyde.
Two different reaction pathways exist for the OOH* group adsorbed on the support. The first one involves its migration to the Au cluster and further recombination with H adsorbed to Au, whereas the second one involves O–OH dissociation and OH recombination with the H atom adsorbed to Au to close the reaction cycle. The migration of OOH from the support to the Au cluster is highly endothermic (ΔE > 100 kJ mol−1) and can therefore be excluded. During geometry optimization, the OOH group adsorbed on Au migrates spontaneously to the support vacancy. The preference for adsorption on the support is the much stronger Cu–O bond compared to the Au–O bond. In contrast, the alternative dissociation of OOH is spontaneous and exothermic by 147 kJ mol−1. The resulting OH group bridges between two Mg ions of the support (state xvi, Fig. 2) and recombines with the H atom from the Au cluster with a barrier of 45 kJ mol−1. This water formation reaction is again strongly exothermic by 123 kJ mol−1 and it takes 98 kJ mol−1 to desorb water into the gas phase, closing the whole reaction cycle.
As it was experimentally observed that substituting Mg with Cu in the spinel support is essential to obtain highly active catalysts (Au/MgCuCr2O4 exhibits an order of magnitude higher activity than Au/MgCr2O4 (ref. 15)), we investigated in more detail the role of Cu for the reaction mechanism of ethanol oxidation. For Au/MgCr2O4, we found that the dissociation of ethanol to ethoxy and OH surface species is spontaneous and exothermic by −118 kJ mol−1. This value is the same as the one computed for Au/MgCuCr2O4. It implies similar basicity of the O atoms in both support materials. The α-C–H bond cleavage to form acetaldehyde takes place on the Au cluster and we therefore do not expect any influence of Cu substitution in the support. We then found that the water formation step from H on the Au cluster with OH on the support is strongly dependent on the presence of substitutions in the support surface. Whereas the overall reaction energy for water formation is +37 kJ mol−1 for Au/MgCuCr2O4, it amounts to +136 kJ mol−1 for Au/MgCr2O4. This substantial energy difference suggests that regeneration of the active site by removal of water is facilitated by the presence of Cu. It can be understood by the stronger binding of OH to Mg2+ than to Cu2+. We should like to note here that the Au cluster is close to the oxygen vacancy site. This may influence the binding energy of surface O atoms of the support, as elegantly shown by Wahlström et al.35 As we will show below, the substitution of Mg for Cu is the dominant factor for the decrease in the oxygen vacancy formation energy.
On this basis, we speculated that the oxygen vacancy formation energy might be a useful descriptor for the reactivity in ethanol oxidation. To explore this, we defined the oxygen vacancy formation energy as E(vacancy formation) = E(defective surface) + 0.5 × E(O2) − E(pristine surface). This provides values for MgCr2O4 and MgCuCr2O4 of 160 and 102 kJ mol−1, respectively, correlating well with the difference in the energy for water formation. Substitutions of Mg in the MgCr2O4 support with Ni or Co only had a very minor effect on the ethanol oxidation activity, much less than that observed for the Cu-substituted support.15 Consistent with this, we compute oxygen vacancy formation energies of 170 kJ mol−1 for MgNiCr2O4 and 152 kJ mol−1 for MgCoCr2O4, respectively. Table 1 lists the oxygen vacancy formation energies of Au nanoparticles on substituted MgMeCr2O4 with Me = Co, Ni, Cu, Mg and experimentally measured catalytic activities of gold nanoparticles on such supports for oxidation of ethanol to acetaldehyde. When the energy needed to vacate the support surface is high, the catalytic activity is low. It relates to the removal of water as a difficult step in the overall reaction mechanism. We then attempted to apply this concept to understand the synergetic effect in Au–Cu alloy catalysts for selective alcohol oxidation.7,8 The synergy is attributed to the presence of reduced Au and Cu, the latter being possibly present as a thin CuOx film.7 Such Au–Cu catalyst systems typically deactivate, which has been correlated to the deep oxidation of Cu to bulk CuO. Our simple concept is able to explain the Au–Cu synergy and the deactivation. The binding energy for an O atom on the Cu(111) surface amounts to 120 kJ mol−1 (Fig. 4), which is only marginally higher than the energy for MgCuCr2O4. We speculate that O atoms adsorbed on the Cu metal surface will act as basic sites for ethanol activation, followed by further reaction on the Au cluster and OH removal as water. It is likely that O2 adsorption on the vacated Cu surface will be dissociative. However, the oxidative reaction conditions will lead to formation of CuO. Formation of CuO leads to a very high vacancy formation energy. For oxygen vacancy formation energy on the CuO(111) surface, we turn to the very recent results reported by Maimaiti et al.36 The value is in the range of 258–340 kJ mol−1 based on the same functional (GGA-PBE). Although the combination between Au and CuO will be able to activate ethanol into an ethoxy group and OH, the recombinative removal of OH with H as water will be prohibited by the strong binding of O in CuO. This is consistent with the experimentally observed deactivation of Au–Cu alloys due to formation of CuO.7
Catalyst | d Au (nm) | Ratea (10−7 mol s−1) | TOFb (s−1) | E vacancy (kJ mol−1) |
---|---|---|---|---|
a Reaction conditions: 0.1 g of catalyst, ethanol/O2/He = 1/3/63, T = 250 °C, GHSV = 100![]() |
||||
Au/MgCr2O4 | 3.3 | 3.8 | 0.20 | 160 |
Au/MgCoCr2O4 | 3.2 | 4.2 | 0.21 | 153 |
Au/MgNiCr2O4 | 3.3 | 3.4 | 0.18 | 170 |
Au/MgCuCr2O4 | 3.1 | 17.2 | 0.89 | 102 |
Given the industrial importance of the oxidation of methanol to formaldehyde, we explored the reaction pathway from methanol to formaldehyde on Au/MgCuCr2O4 by computational modeling. We found that a similar pathway as found for ethanol oxidation to acetaldehyde is viable. Dissociative adsorption of methanol on the Au atoms located at the interface with the support (state ii, Fig. 5a) is strongly exothermic (−118 kJ mol−1) and barrierless. The reaction energy is similar to the dissociative adsorption of ethanol on this surface model. After rotation, the methoxy group dissociates one of its C–H bonds on the Au cluster (state iv, Fig. 5a) with a barrier of 38 kJ mol−1 (ΔE = 31 kJ mol−1). The adsorption energy of formaldehyde is 28 kJ mol−1, which is higher than that of acetaldehyde (17 kJ mol−1). The further reaction sequence leading to closure of the reaction cycle by water formation and desorption is similar to that of the ethanol oxidation case. Thus, we tested the optimized Au/MgCuCr2O4 catalyst for methanol oxidation but found that, instead of formaldehyde, CO2 is the main product at 250 °C. Fig. 6 shows methanol conversion and product selectivities as a function of the reaction temperature. At very low conversion at low reaction temperatures, the main products formed are formaldehyde and methyl formate. Formation of the latter product indicates that further oxidation of formaldehyde to formic acid occurs, consistent with the formation of formic acid at intermediate reaction temperatures. With increasing temperature, the formaldehyde selectivity decreases and the methyl formate selectivity goes through a maximum. The decreasing selectivities of desirable products are caused by complete oxidation of methanol to CO2. These results are very different from what was observed for ethanol oxidation, where acetaldehyde was the main product, even at a high temperature of 250 °C.
![]() | ||
Fig. 5 (a) Mechanism of oxidation of methanol to formaldehyde for the Au10/MgCuCr2O4 model. From state v, the recombinative desorption of water is similar to the mechanism found for ethanol oxidation (states v–ix in Fig. 2); (b) α-C–H bond cleavage in adsorbed formaldehyde and (c) α-C–H bond cleavage in adsorbed acetaldehyde. |
To better understand the significant difference between methanol and ethanol oxidation, we explored further decomposition of adsorbed formaldehyde (Fig. 5b) and acetaldehyde (Fig. 5c) by quantum-chemical modeling. The cleavage of the remaining α-C–H bond in adsorbed formaldehyde has a barrier of 19 kJ mol−1 and is exothermic by 18 kJ mol−1. These energetics are more favorable than desorption of formaldehyde, explaining the preference for its further oxidation. Comparatively, the cleavage of the α-C–H bond in adsorbed acetaldehyde is much more difficult. The activation energy is 37 kJ mol−1. Consistent with the higher activation energy, the reaction energy is endothermic by 15 kJ mol−1. Thus, desorption of acetaldehyde is more favorable than further α-C–H bond cleavage to products that can lead to complete oxidation.
Footnote |
† Electronic supplementary information (ESI) available. See DOI: 10.1039/c4cy00462k |
This journal is © The Royal Society of Chemistry 2014 |