Selective hydrogenation of CO on Fe 3 S 4 {111}: a computational study

Fischer – Tropsch (FT) synthesis has been a recursive method to form valuable molecules from syngas. Metal surfaces have been extensively studied as FT catalysts; among them, iron presents several phases under reaction conditions, oxides and carbides, as active sites for the FT and reverse water gas shift reaction. We present CO reduction on an iron sul ﬁ de phase with spinel structure, Fe 3 S 4 , also considering the pathways where C – O dissociates leaving CH x species on the surface, which may feed longer aliphatic chains via the FT process. We analysed the thermodynamic and kinetic availability of each step leading to O and OH species co-adsorbed on the surface as well as the formation of H 2 O from the hydrogenation of the alcohol group in the molecule. This detailed analysis led to energy pro ﬁ les on both active sites of the surface, and we conclude that this Fe 3 S 4 surface is highly selective towards the formation of methanol, in full agreement with experimental results. These ﬁ ndings point out that the C – C bond formation on greigite takes place through a hydroxycarbene FT mechanism.


Introduction
2][3] It is considered one of the most attractive routes to convert syngas into liquid fuels and chemicals, which consists of mainly paraffins and olens, and low levels of oxygenates including alcohols, aldehydes, ketones and acids. 4Traditionally, four types of chain growth mechanisms have been proposed: the carbene mechanism, 5 the hydoxycarbene mechanism, 6 the CO-insertion mechanism, 7 and the CH 2 -insertion mechanism. 8he mechanism for methanol synthesis on metal catalysts has been extensively investigated with both experimental [9][10][11] and theoretical methods, [11][12][13] in part because it is considered to be an important alternative fuel for future transportation needs, either for methanol fuel cells or for the on-board generation of H 2 for proton-exchange membrane fuel cells. 14,15Comparing the formation enthalpies of methanol and methane, fully reduced compounds, it is obvious that the latter is thermodynamically more favourable and therefore should hinder the formation of methanol.Methane is generated through the C-O bond ssion, which may take place in adsorbed intermediate species (CH x O and CH x OH) somewhere along the CO hydrogenation pathway. 13,16Indeed, minor CH x species have been detected in methanol decomposition on metal surfaces, particularly at high pressures, 17 while they may desorb rather than react under ultra-high vacuum conditions. 18Among the catalysts containing metals, iron-based catalysts have considerable merits due to their potential activity for the water-gas shi (WGS) reaction as well as their high FT activity and low cost. 3,19][22][23][24][25] A sulde isomorphic to Fe 3 O 4 , greigite, has recently showed low turnover to electro-reduce CO 2 to small molecules, methanol among them. 26However, the presence of S on FT catalysts has been considered as a poison, which raises the queries of the actual mechanism and the role of sulfur in the sulde catalysts.In this work, we account for the lack of CH 4 in the experimental analysis and provide evidence of the selectivity of Fe 3 S 4 between hydrogenation pathways from CO to CH 3 OH and CH x on the surface, for this is crucial to complete and understand the intricate reaction networks that can be intercrossed at any stage of the reaction.which improves the description of localized states in this type of systems where standard LDA and GGA functionals fail. 58The orbital spin-splitting in the valence region results in localized outermost 3d-electrons and in ordered magnetism 45,59,60 in good agreement with experimental evidence. 50,51We have prepared the Fe 3 S 4 {111} surface as a slab model by cutting the bulk structure with the METADISE code, 61 which considers periodicity in the plane direction and provides the atomic layer stacking resulting in non-dipolar reconstructions. 62The slab contains 56 atoms (24 Fe and 32 S) per unit cell exposing an area of 81.0 Å2 and a thickness of sufficient size to relax the two uppermost layers (four Fe 3 S 4 units), keeping the bulk structure frozen at the bottom.We added a vacuum width of 12 Å between periodic slabs, big enough to avoid interaction between periodic images.The molecules in the gas phase were relaxed in an asymmetric box of 15 Â 16 Â 17 Å3 .
We employed the improved dimer method (IDM), to nd the transition states. 63,64We dened the adsorption energy as the difference between the isolated species and the combined system.The activation energy (E A ) of a certain step is the energy required to surmount the potential barrier characteristic of the transition state.We have dened this energy barrier as the difference between the initial state and transition state energy for the forward process.The reaction energy (E R ) of each step is calculated as the total energy difference between the nal state (product(s)) and the initial state (reactant(s)).

Results and discussion
Greigite (Fe 3 S 4 ) is an inverse spinel-structured material whose particles produced in hydrothermal synthesis expose {001} and {111} surfaces. 26We focused on the active {111} surface and, due to the presence of non-equivalent Fe ions, it has two distinct terminations depending on the relative Fe A position with respect to the uppermost sulfur layer.The most stable termination contains two adsorption sites, Fe A and Fe B , originally occupying tetrahedral and octahedral bulk positions. 65The top S layer has a bulk-like structure consisting of rows along the [0À11] direction with rhombohedral packing.
We have explored the adsorption of reactants and products on Fe 3 S 4 {111}.While CO adsorbs on Fe A and Fe B with a binding energy of À0.68 and À1.07 eV respectively, the adsorption of CH 3 OH is 20% weaker and CH 4 does not chemisorb.Hence, all the CH 4 molecules produced from the CO reduction will desorb from the surface leading to an overall exothermic process driven by the production of H 2 O.The co-adsorption of CO and four H ad-atoms is also higher in energy than adsorbed CH 3 OH (DE ¼ 0.6 eV), indicating a favourable process.
We have represented the optimised structures of CO and CH 3 OH in Fig. 1 and displayed the C-O and Fe-C distances.The C-O distances in the adsorbed states on Fe A and Fe B are 1-2% longer than in isolated CO and CH 3 OH molecules respectively.The molecules are placed practically on top of the metal sites at a distance according to their binding energy: the distances on Fe A are longer than on Fe B , indicating a weaker bond.The elongation of the C-O bond is also reected in the n C-O stretching mode, which changes by $100 and $50 cm À1 for CO and CH 3 OH respectively compared to the gas phase molecules.
While the redox property of the surface determines the capacity to transfer electrons to the adsorbates, the acid-base character controls the gain and loss of  hydrogens and therefore the yields of hydrogenation products.We have explored the co-adsorption and reaction of CO and H ad-atoms on the Fe 3 S 4 {111} surface leading to competition between hydrogenation and C-O ssion.
The adsorption of these species on transition metal surfaces has been previously investigated leading to a wide range of data.Using a similar methodology to the one we employed here, CO adsorption is reported between À0.23 and À3.21 eV (corresponding to Cu and Pd respectively), 13 while on oxides such as Fe 3 O 4 {111} the adsorption energies are À1.09 and À1.94 eV on Fe A and Fe B  respectively, 66 which are stronger than on the sulde counterpart.These reports also indicated a C-O stretching red-shi, in agreement with our calculations, except that on pure metals it is considerably lower ($1900 cm À1 ). 12 Methanol adsorbs on top of fcc metals with an ideal surface releasing $0.5 eV, while methoxy was found to be unstable on Pt{111}. 67,68On Fe 3 O 4 {111}, the adsorption of CH 3 OH is weaker than of CO, which is similar to what we found on the thio-  spinel, Fe 3 S 4 {111}, where the E B difference between CH 3 OH and CO is $0.1 eV. 17his is due to the fact that Fe 3 O 4 is reluctant to modify its electronic structure while the sulde has a milder ionic character. 50sorption of H x COH and H x CO surface species (x ¼ 0-3) We have adsorbed the species involved in the generation of CH 3 OH and characterised the most stable congurations (see Table 1).We found progressive changes between the different species along the reduction process: (i) the C-O distance increases by 0.3 Å, which on average is only 0.06 Å shorter in H x CO than in the H x COH species ($1.392 Å), (ii) the Fe-C distance also increases by >1 Å, and (iii) the C-O stretching vibrational mode shis by $1000 cm À1 .All these changes are related to the change in carbon orbital hybridisation from sp 1 to sp 3 .It is noticed that the saturation of the C by hydrogens makes the molecule bind to the surface through the O(H).This implies charge transfer from the surface and makes the distances and frequencies lay slightly out from the trend line, as in the case of H 2 CO (q ¼ À0.5e) and more drastically in the Fe-C distance for the methoxy bonded through the oxygen (q ¼ À0.35e).The relative stability of the intermediates is expressed in Table 1 by the energy difference between the intermediate in question, and the CO with the corresponding number of hydrogens on the Fe 3 S 4 {111} surface.It shows that although the formation of CH 3 OH is thermodynamically favourable, there are less favourable intermediates making the process less likely, especially on Fe A .
Although the CH 3 OH formation from CO is still a matter of discussion, it is well documented on fcc metals.Cu showed good catalytic performance to reduce CO though most of the reaction intermediates (those in Table 1) adsorb weakly or endothermically. 69This is not the case for heavier metals, of which the general trend is that adsorption weakens in line with the C-orbitals hybridisation, 13 i.e. the greater hybridisation the weaker the adsorption.The geometrical changes taking place on the different species, such as the increase in bond length and red-shi of the C-O stretching vibrational mode, are also common features. 12ssociation of H x COH and H x CO species (x ¼ 0-3) We have studied the C-O scission of each intermediate species lying on Fe A and Fe B taking into account that the products may be displaced to a nearby Fe site; for instance, the C-O splitting of H 2 COH adsorbed on Fe A , leads to CH 2 remaining on Fe A while OH binds both Fe A and Fe B , see Fig. 2. Once the C-O bond is completely dissociated, the CH x species lie an average of 0.4 Å closer to Fe than before dissociation.This short distance is also a consequence of the electrophilic character of the co-adsorbed O and OH groups, which withdraw electrons from nearby atoms; a co-adsorbed O ad-atom makes the Fe-C distance 0.09 Å shorter than a co-adsorbed hydroxyl on Fe A , while it remains practically invariable on Fe B (Dd Fe-C ¼ 0.01 Å).
Table 2 summarises the activation and reaction energies for the direct C-O scission.On Fe A sites, the reaction energy (E R ) related to C-O bond breaking is (i) endothermic (E R > 1 eV) for a2, a3 and a5 species, (ii) slightly endothermic (0 < E R > 1 eV) for methoxy (a6), and (iii) exothermic (E R < 0 eV) for formaldehyde (a4) and methanol (a7).Nevertheless, the high energy barriers indicate that the process is kinetically hindered and the C-O scission is unlikely; the lowest E A is for H 2 CO but still above 1 eV.The electronic and geometrical differences between sites make Fe B behave differently: the process is endothermic for all the intermediates and E R is lower than 1 eV only for H 3 COH, while all the energy barriers are well above 1.5 eV.
The energy data in Table 2 shows that breaking the C-O bond is unfavourable under mild conditions.Although the activation and reaction energies have been found to be much smaller on pure metallic systems such as Pd{111}, 13 they follow a similar trend, showing the C-O splitting to be more favourable on HCOH and H 2 COH intermediates.
We have explored an alternative mechanism for C-O bond breaking driven by H 2 O generation, which is schematically represented in Fig. 3. Table 3 contains the activation and reaction energies for that process on Fe A and Fe B , respectively.The energy trends show decreasing values as the number of hydrogens in the molecule is increased, i.e. the C-orbital hybridisation changes from sp 1 to sp 3 .In spite of this, the process is endothermic and kinetically unfavourable.
Among the different reaction mechanisms explored, only R3 and R6 are exothermic.Despite this, they are kinetically disabled and therefore the presence of CH x groups should be very scarce on Fe 3 S 4 {111}.Fig. 4 shows the energy prole for the reduction of CO to methanol (black lines) and the different paths studied describing the C-O dissociations for the direct and H 2 O driven pathways (red and blue lines, respectively).Most of these pathways are hindered by the high energy barrier, however, we noticed that for H x CO(H) species with x > 2, the dissociation of the C-O bond requires the same or less energy than some steps undertaken during the CO reduction.Therefore, if the system has enough energy to overcome the barrier of producing H x CO(H) intermediates, the C-O dissociation is accessible and some pathways become competitive, e.g.R4, R6, R11, R12, R14, R15, R17 and R18.
The reaction network for CH 3 OH formation on Fe A may take place through H x CO and H x COH intermediates; nonetheless, the path for the formation of a6 from a4 requires more energy to overcome the transition energy than that leading to a5.However, R14 is more kinetically feasible leading to CH x + H 2 O, although the process is thermodynamically unfavourable.Thus, a5 (H 2 COH) is crucial for the production of methanol, the desorption of which is >1 eV below the H 3 C-OH scission energy barriers.On the other hand, the C-O dissociations on Fe B have a higher energy barrier: the stabilisation of b4 means that both H 3 CO and H 3 COH will be formed, the latter desorbing from the catalyst prior to dissociation.These results are in full agreement with the fact that CH 4 molecules were not detected during the experiments using gas-chromatography and 1 H-NMR techniques. 26

Conclusions
We have studied systematically the competitive reactions of CO hydrogenation and C-O dissociation on both adsorption sites, Fe A and Fe B , of Fe 3 S 4 {111}.All the calculations were performed using DFT+U, including the long-range dispersion forces.We found a correlation of the C-O bond strength with the C orbital hybridization according to the bond distances and vibrational analysis, which is similar to the trends reported on metals and metal oxides.The examination of relative energies between the reactants and intermediates showed that although the reactions leading to the formation of methanol and methane are thermodynamically favorable, driven by the high energy of the H 2 molecules (and the formation of H 2 O), it is necessary to overcome a reaction barrier.There are intermediates of higher energy, HCOH and H 2 CO, that determine the formation of products, especially on Fe A .The lower energy reaction pathway leads exclusively to methanol through HCOH on Fe A while both the HCOH and H 2 CO intermediates are of competing stability in the reaction mechanism on Fe B .Thus, the C-O scission is generally an endothermic or kinetically disabled process under mild conditions on Fe 3 S 4 {111}; for this reason, this surface presents a selective CO reduction towards a low level of oxygenated molecules while iron oxides may lead to totally reduced molecules.
facilities, the UK's national high-performance computing service, which is provided by UoE HPCx Ltd at the University of Edinburgh, Cray Inc. and NAG Ltd, funded by the Office of Science and Technology through EPSRC's High End Computing Programme and provided via our membership of the HPC Materials Chemistry Consortium (EPSRC EP/L000202).The authors also acknowledge the use of the UCL Legion High Performance Computing Facility and the Advanced Research Computing @ Cardiff (ARCCA) at Cardiff University, and associated support services, in the completion of this work.All data created during this research is openly available from the University of Cardiff Research Portal at http://doi.org/10.17035/d.2016.0011347963.

Fig. 1
Fig. 1 Schematic representation of CO, CH 3 OH and CH 4 on Fe A and Fe B sites of Fe 3 S 4 {111}.Inset distances are in Å. Colour scheme: light-grey is Fe, yellow is S, red is O, white is H and dark-grey is C.

Fig. 2
Fig. 2 Top and side views of C-O dissociation on the Fe 3 S 4 {111} surface: H 2 COH on Fe A and H 2 CO on Fe B .Reactants are on the right, the transition states are in the centre and the products are on the left.Colour scheme: light-grey is Fe, yellow is S, red is O, white is H and dark-grey is C.All distances in Å.

Fig. 3
Fig. 3 Top and side views of H 2 O driven C-O dissociation on the Fe 3 S 4 {111} surface: H 2 COH on Fe A and on Fe B sites.Reactants are on the right, transition states are in the centre and products are on the left.Colour scheme: light-grey is Fe, yellow is S, red is O, white is H and dark-grey is C.All distances in Å.

Fig. 4
Fig. 4 Energy profile for the reduction mechanism of CO to CH 3 OH (black line) on Fe A (top) and Fe B (bottom) sites of Fe 3 S 4 {111} considering the direct scission of C-OH x bond (red line) and its dissociation assisted by an H ad-atom leading to H 2 O (blue line).Intermediates and C-O dissociation reactions are labelled according to Tables 1-3.

Table 1
Relative energies (DE), distances (d) and C-O stretching vibrational modes (n C-O ) of the different CH x OH and CH x O intermediates on Fe A and Fe B sites of the Fe 3 S 4 {111} surface.The energies are relative to adsorbed CO, taking into account the corresponding number of H ad-atoms Open Access Article.Published on 08 November 2016.Downloaded on 09/25/2018 20:10:37.This article is licensed under a Creative Commons Attribution 3.0 Unported Licence.

Table 2
Activation (E A ) and reaction (E R ) energies for breaking the surface species C-O bond on Fe A and Fe B sites of the Fe 3 S 4 {111} surface

Table 3
Activation (E A ) and reaction (E R ) energies for the reaction H x COH / H x C + H 2 O on Fe A and Fe B sites of the Fe 3 S 4 {111} surface