Precisely Engineered Supported Gold Clusters as a Stable Catalyst for Propylene Epoxidation

Abstract Designing a stable and selective catalyst with high H2 utilisation is of pivotal importance for the direct gas‐phase epoxidation of propylene. This work describes a facile one‐pot methodology to synthesise ligand‐stabilised sub‐nanometre gold clusters immobilised onto a zeolitic support (TS‐1) to engineer a stable Au/TS‐1 catalyst. A non‐thermal O2 plasma technique is used for the quick removal of ligands with limited increase in particle size. Compared to untreated Au/TS‐1 catalysts prepared using the deposition precipitation method, the synthesised catalyst exhibits improved catalytic performance, including 10 times longer lifetime (>20 days), increased PO selectivity and hydrogen efficiency in direct gas phase epoxidation. The structure‐stability relationship of the catalyst is illustrated using multiple characterisation techniques, such as XPS, 31P MAS NMR, DR‐UV/VIS, HRTEM and TGA. It is hypothesised that the ligands play a guardian role in stabilising the Au particle size, which is vital in this reaction. This strategy is a promising approach towards designing a more stable heterogeneous catalyst.


Introduction
Propylene oxide (PO) is ah igh value-added commodity chemical, because it serves as astarting material to synthesise polyether polyols and propene glycol, which are subsequently used to produce polyurethane foams and polyesters,r espectively. [1] Thec urrent annual PO production is more than 10 million tons world-wide,a nd demand is increasing remarkably. [2] However,t he conventional methods to produce PO (chlorohydrin and the hydroperoxide process) suffer from major drawbacks,l ike toxic waste generation, complicated multistep processing, and formation of by-products in afixed ratio,w hich rely heavily on market demands to be profitable. [1b, 3] DOW-BASF and EVONIK-ThyssenKrupp have independently commercialised the HPPO (hydrogen peroxide to propylene oxide) process,w hich employs hydrogen peroxide as an oxidant, using TS-1 as ac atalyst, demonstrating the economic feasibility of utilising hydrogen, provided the hydrogen is used efficiently as sacrificial reductant in this hydro-epoxidation. [4] TheHPPO process is aground-breaking route to produce propylene oxide from propylene,b ut the main disadvantage of this method is that it employs multiple reactors,a nd requires as eparate hydrogen peroxide production plant. [3a, 4a, 5] Ever since Haruta and Hutchings discovered the impressive catalytic properties of gold nanoparticles,t he direct gas phase epoxidation of propylene using H 2 and O 2 has gained considerable attention as agreen, simple and environmentally benign route for PO synthesis. [6] It is known that H 2 reacts with O 2 over the surface of highly dispersed gold nanoparticles to generate in situ peroxo species,a long with tetrahedrally coordinated Ti 4+ sites,which enable subsequent epoxidation of propylene to PO. [6b,c, 7] As ingle-step route is highly desirable to achieve ac ost-effective and environmentally friendly process.M oreover,u sing H 2 and O 2 as reactants is astep towards arenewable route,asboth H 2 and O 2 can be produced from renewable electricity by electrolysis. Numerous reports of various gold nanoparticles supported on Ti-containing materials showcase good activity and selectivity;h owever, they do not exhibit ap articularly long catalyst lifetime. [8] Al ong catalyst lifetime means that the catalytic performance,i ncluding activity and selectivity,s hould be stable over an appreciable time-on-stream, or the catalyst should easily be regenerated to as imilar performance level. Thed irect epoxidation of propylene with Au/Ti-containing catalysts could eventually become an alternative route to other, indirect or multistep production methods to propylene oxide,b ut two significant impediments are poor hydrogen utilisation and low catalyst stability.T here is still al ong way towards commercialisation, but this work represents as tep towards improving catalyst stability,hydrogen utilisation, and PO selectivity.
Catalyst stability is an important parameter that cannot be neglected, especially in industrial processes.Aconsiderable amount of time and resources are spent on catalyst replace-ment. [9] This process can cost billions of dollars per year to the industry. [10] Them aximum catalyst stability reported for the direct gas phase reaction in literature so far is ca. 250 hi n am icroreactor setup with periodic regenerations,u sing Au/ TS-1 catalyst prepared by ad eposition precipitation (DP) method. [5a] Thus,l ow catalyst stability and poor hydrogen efficiency are key issues that still need to be addressed to make the single-step process using Au-based catalysts economically viable.T he deactivation of these catalysts mainly originates from blocking of active sites by adsorption of oxygenate species or metal particle sintering. [5a, 11] Although catalyst deactivation is inevitable,i tc an be delayed or reduced, by carefully designing the catalyst architecture using different synthetic approaches and tuning the metal-support interaction to impart extra stability. [12] TheD Pm ethod is commonly employed to deposit gold on supports and is based on the isoelectric point of the support material. [13] This catalyst synthesis strategy also presents some disadvantages, such as nanoparticle aggregation, non-homogeneous particle size distributions,a nd weak affinity toward the support, making it ad ifficult to reproduce technique. [13,14] Therefore, designing as table,y et selective catalyst becomes an area of key importance from both academic and industrial points of view.
Herein, we report af acile one-pot methodology to synthesise very stable,s ub-nanometre gold clusters (ca. 0.8 nm core size) using triphenylphosphine as as tabilising ligand. These pre-formed gold clusters can be directly immobilised onto the zeolitic support (in this work, TS-1) without any notable change in the shape and size of the clusters.W edemonstrate anon-thermal plasma technique as af ast, efficient, and effective treatment for the removal of bound ligands that provides considerable advantages over thermally driven oxidative procedures.T he catalytic behaviour of this catalyst and untreated Au/TS-1 catalysts are subsequently compared in direct propylene epoxidation with H 2 and O 2 .T he catalyst demonstrates ag ood activity and selectivity over several regeneration cycles;h owever, longer testing will be required in ap ilot scale reactor to assess its total lifetime in the future. Figure 1a illustrates the synthesis procedure for Au nanoclusters.T hese nanoclusters are prepared using gold (III) chloride trihydrate (HAuCl 4 ·3 H 2 O) as gold precursor. Theg old precursor is dissolved in ethanol and stirred for 15 minutes.A fterwards,a ne thanolic solution of triphenylphosphine (TPP) is added to the gold solution. TPP ligands are nucleophiles,which possess alone pair of electrons;they play an important role in this reaction, acting as ar educing agent before helping to stabilise the Au nanocluster intermediate. [15] Theg old precursor,w hich is yellow in colour, changes to colourless after the addition of TPP,d ue to reduction of Au III to Au I ,leading to formation of acoordination complex (Cl-Au I -PPh 3 )w ith TPP ligands.S ubsequently, sodium borohydride (NaBH 4 ), as trong reducing agent, is added to the above Au I complex, converting Au I to Au 0 ,and resulting in the formation of sub-nanometre Au clusters stabilised by phosphine ligands.V arious concentrations of NaBH 4 (1, 4, 10 and 20 equiv.) are tested for optimisation and 4equivalents proves to be the most suitable concentration for the synthesis of the smallest gold clusters,a sc onfirmed by UV/Vis spectroscopy (Supporting Information, Figure S1). Figure S1 shows photographs and UV/Vis spectra of different Au nanoparticle solutions recorded immediately after synthesis.Asharp peak at 420 nm (Figure 1b,F igure S1) confirms the presence of sub-nanometre clusters,c ontaining approximately 11 Au atoms,i ndicating the quantized electronic structure,w hile other spectra contain ab road peak at higher wavelengths (500-600 nm), due to polydispersity of the Au nanoparticle size distribution. [12a, 16] Theg old clusters formed are stable for at least 30 days.Tovalidate these results, the size of the Au clusters is confirmed by high resolution transmission electron microscopy (HRTEM). TheH RTEM micrograph and corresponding particle size distribution (Figure 1c,d)demonstrates that the average core diameter of the phosphine stabilised gold clusters is ca. 0.8 nm. This methodology does not require any complicated purification of the Au I precursor and directly produces in situ phosphine-stabilised Au clusters.

Results and Discussion
Thetitanium silicalite-1 (TS-1, Si/Ti = 60) zeolite support is hydrothermally synthesised according to Nijhuis et al. [1a] Figure 2a shows atypical HRTEM image of TS-1, indicating ah ighly crystalline material. Then itrogen sorption isotherm ( Figure S2) is of IUPAC Ty pe I, characteristic for purely microporous materials,and both micropore volume and BET surface area are typical for TS-1 zeolites. [17] TheU V/Vis spectrum shown in Figure 2creveals that the titanium present in the zeolite sample is mostly in tetrahedral coordination, as evident in the peaks at 205 nm attributed to tetrahedral titanium species. [18]  Sub-nanometre Au clusters are prepared in ethanol and mixed with TS-1 in around bottom flask to obtain 1wt. %A u loading. Thee thanol solvent is evaporated, and an orange coloured powder is obtained after drying and purification. Thefinal size of the clusters after immobilisation is calculated using HRTEM and the average particle size of the clusters remains ca. 0.8 nm after immobilisation, as illustrated in Figure 2b.A fter deposition of the gold clusters on the TS-1, no significant structural changes in the zeolite are evident in XRD ( Figure S3) and DR-UV/Vis spectroscopy ( Figure 2c). TheD R-UV/Vis spectrum shows as harp peak at 420 nm, indicating the presence of sub-nanometre gold clusters,a nd the lack of asurface plasmon resonance band at 500-600 nm further confirms the absence of any larger gold particles (aggregates). [12a,16] Furthermore,t he colour of the powder does not change to purple or black after immobilisation, as shown in Figure S4. [19] TheA uc lusters are stabilised by triphenylphosphine ligands and, in order to increase accessibility to the Au surface,itisimportant to remove the stabilising agent. These capping agents prevent the aggregation of the nanoparticles but at the same time block the access to the active sites. [20] The traditional routes employed for the removal of ligands are thermal and oxidative treatments. These techniques induce inevitable mobility of metal atoms on the support, which leads to an increase in particle size and loss of monodispersity. [21] This makes it very difficult to control catalytic activity through aspecific particle size.Itiswell known that small Au nanoparticles,a nd particularly those with ac ore diameter below 5nm, are crucial to achieve catalytic activity in propylene epoxidation. [7a,22] Thus,inaneffort to avoid harsh ligand removal methods,w e demonstrate as imple way to remove ligands by using an on-thermal O 2 plasma technique that does not induce an increase in the size of the Au nanoparticles. [23] Figure 2d schematically illustrates the O 2 plasma removal procedure for Au/TS-1 materials.A u/ TS-1 powder is placed in av acuum chamber and irradiated with aplasma for 30 minutes.The energetic O 2 plasma species remove the triphenylphosphine ligands under vacuum. To monitor any particle size changes of Au/ TS-1 after plasma treatment, particle size is measured via TEM. AT EM image of Au/TS-1 PT is shown in Figure 2e and the corresponding particle size distribution (Figure 2f)c onfirms that the average particle size has increased slightly to 1.5 nm, which is considered an appropriate size for catalytic applications. [24] It is evident from the TEM images that the particles remain uniformly dispersed onto the zeolite after the ligand removal. To quantify the amount of TPP removed from the sample,t hermogravimetric analysis (TGA) is performed (Figure 2g). TheTGA curves show that the total weight loss of the plasma treated Au/TS-1 (1.8 %) is lower than that of the untreated sample (7.3 %), further suggesting that 74 wt. %of the total amount of ligands have been removed. The tetrahedrally and octahedrally coordinated Ti sites in the TS-1 framework remain unchanged after ligand removal, which is confirmed using DR-UV/Vis spectroscopy (Figure S5). TheUV/Vis spectrum also shows the peak at 420 nm, which indicates that the characteristic absorption band of small gold NPs remains unchanged after the plasma treatment. Forc omparison with thermal methods,aAu/TS-1c atalyst was also calcined at 300 8 8Ci na ir to remove triphenylphosphine ligands.F igure S6a illustrates the TGA curve,s howing 80 %l igand removal after calcination. DR-UV/Vis spectroscopy ( Figure S6b) and TEM ( Figure S6c,d) further confirm that the particle size increases to 11.7 AE 3.7 nm, which shows that high temperature leads to an increased particle size,d ue to agglomeration of the gold nanoparticles.Hence,itcan be concluded from the employed characterisation techniques that the non-thermal plasma treatment removes as ubstantial amount of bound TPP ligands from the gold nanoparticles without greatly altering their particle size and morphology,b oth factors having important implications on catalytic performance. [25] TheA u/TS-1 PT (plasma treated, 1wt. %A u) catalyst is tested (Figure 3a)inaquartz tubular reactor for 5-hour cycles at ar eaction temperature of 200 8 8C. Thec atalyst is regenerated after every cycle by heating at 300 8 8Cu nder 10 % oxygen in helium for 1h to remove organic species which accumulate on its surface over time. [8b, 26] Forc omparison, am ore conventional Au/TS-1 (1 wt. %A u) catalyst is prepared as well, in which Au is directly dispersed on the support using the deposition precipitation (DP) method (characterisation data in Figure S7) and tested under the same reaction conditions. [1a] Thec atalytic performance of both catalysts is compared in Figure 3b-d. In Figure 3b,t he Au/TS-1 DP catalyst shows ah igher initial PO formation rate ( ), which could be because of the ligand-free

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Au nanoparticles incorporated into active Ti IV sites.T he initially low PO formation rate of Au/TS-1 PT can be attributed to the lower accessibility of the active sites,d ue to partial coverage of the Au surface with TPP ligands;however,asthe time-on-stream (TOS) increases,t here is an appreciable increase of the PO formation rate (from 10 to 20 g PO h À1 kg cat À1 ), which reaches am aximum of 22.5 g PO h À1 kg cat À1 (propylene single pass conversion ca. 1.3 %, hydrogen single pass conversion ca. 8.2 %, oxygen single pass conversion ca. 6.2 %) by the 11th cycle.A st he reaction temperature is 200 8 8C, the residual protecting ligands present in the catalystsvicinity are removed during reaction, since the decomposition temperature of triphenylphosphine is around 180 8 8C. [16] TheTPP removal under reaction conditions leads to increased accessibility of Au-Tia ctive sites,w hich results in higher activity.O nt he other hand, the Au/TS-1 DP catalyst starts to deactivate with increasing time-on-stream, with the rate decreasing from 22 to 18.8 g PO h À1 kg cat À1 (propylene single pass conversion ca. 1.2 %h ydrogen single pass conversion ca. 11.6 %, oxygen single pass conversion ca. 7.7 %). [27] Apart from the activity,t he PO selectivity and H 2 efficiency are the two other important factors defining the catalytic performance. [28] Theselectivity towards PO for both catalysts are shown in Figure 3c.T he Au/TS-1 DP catalyst exhibits PO selectivity of ca. 82 %, whereas the Au/TS-1 PT catalyst shows ah igher selectivity of ca. 88 %o ver the timeon-stream. Figure 3d shows the hydrogen efficiencyf or both catalysts,and it can be easily seen that Au/TS-1 PT has,overall, amuch higher H 2 efficiency than the traditional catalyst. The initial increase of H 2 efficiencyfrom 15 to 23 %inthe case of Au/TS-1 PT is related to the availability of Au active sites over time,which are partially obstructed in the beginning. On the contrary,the H 2 efficiency value constantly drops for Au/TS-1 DP from 12 to 8%,i ndicating that al arge amount of H 2 is directly converted to water, hence making the process economically unviable.I tc an be concluded that Au/TS-1 PT exhibits better hydrogen efficiency,h igh PO selectivity,a nd remarkably improved stability between subsequent cycles,as compared to the Au/TS-1 DP catalysts.
Tw of actors contribute to deactivation in this reaction: one is the adsorption of carbonates/carboxylate species on the catalyst surface,w hich is easily reversible by periodic regenerations to remove such species,w hile the second is the sintering of gold nanoparticles to form larger aggregates, which is an irreversible phenomenon. [5a, 29] Although most of the deactivating species are removed by regeneration, sintering can still occur during the reaction, which is irreversible and thus could account for adecreasing activity.T oassess this, both spent catalysts are analysed using TEM (Figure 3e-h) to observe changes in catalyst morphology.I ti se vident from Figure 3g,hthat gold particles in the Au/TS-1 DP catalyst are sintered to ah igher degree over the total time-on-stream, with an average particle size of ca. 7.3 nm. On the other hand, the average particle size of the gold in the spent Au/TS-1 PT catalyst (Figure 3e,f)isonly ca. 3nmand large agglomerates are not observed, which is one of the possible reasons for the enhanced catalytic stability.T herefore,t he main reason behind the decreased activity can be attributed to the sintering of gold particles,d ue to the absence of protecting agents,w hich play an important role in stabilisation during the catalyst synthesis. [30] Ther esults show that Au/TS-1 PT outperforms the Au/TS-1 DP in terms of activity and hydrogen efficiency over al onger time in the direct gas-phase epoxidation of propylene.
In the above study,i ti so bserved that the Au/TS-1 PT catalyst possesses improved catalytic performance compared to the Au/TS-1 DP catalyst. Theconventional catalyst gradually deactivates over time-on-stream, losing activity,s electivity, and hydrogen efficiency,inaccordance with previous reports in the literature. [6e, 29a] In contrast, the Au/TS-1 PT does not show any deactivation under the same reaction conditions. Subsequently,A u/TS-1 PT is tested for al onger time-onstream, without the periodical regeneration, to evaluate its durability and stability,w hich are crucial indicators for industrial catalysts.T he catalytic results of Au/TS-1 PT (1 wt. %A u) are illustrated in Figure 3i-k. Thep ropylene epoxidation involves in situ peroxo species formation from H 2 and O 2 ,which can either react with propylene to form PO or can decompose or be hydrogenated to produce water. Figure 3ireveals that there is an initial increase in PO production rate,related to the higher accessibility of Au sites,due to the slow removal of the bound ligands at 200 8 8Cthat facilitates the formation of more peroxo species.Insmall gold nanoparticles most of the atoms are on the periphery,l ying in ac lose proximity to Ti 4+ sites,a llowing easier access to reaction intermediates to produce PO. [31] PO production rate reaches am aximum on day 11 (16.9 g PO h À1 kg cat À1 ,p ropylene single pass conversion ca. 1.1 %hydrogen single pass conversion ca. 12.7 %, oxygen single pass conversion ca. 9.4 %) and then as light decrease in the PO production rate is evident. The catalyst demonstrates ah igher PO selectivity ca. 84 %w ith as maller amount of side products (Figure 3j)t han the untreated Au/TS-1 DP catalyst. [1a] TheH 2 efficiency ( Figure 3k)d ecreases from 23 to 10 %i n6days.A st he timeon-stream increases,ahigher number of Au sites become available,w hich produces more peroxo species,c ausing an increase in both PO formation and water formation. After some time,H 2 O 2 formed on the larger Au nanoparticles travels al onger way to nearby Ti 4+ sites,w here it rapidly decomposes to form more H 2 O, resulting in lower hydrogen efficiency values. [22b,31] Theepoxidation rate and the hydrogen efficiency are determined by ad elicate balance between the rate of peroxo formation and its consumption by the epoxidation reaction. If the peroxo formation becomes too fast, the epoxidation cannot keep up and the hydrogen efficiency drops.L owering the gold loading is,t herefore, ah ighly effective way to further increase the hydrogen efficiency,although at the expense of activity. [32] Theoptimum gold loading and its spatial location should be determined based on an economic evaluation of the process. [33] Furthermore,t he productivity of the catalyst could be enhanced by using core-shell structures,alkali metal promoters or employing bimetallic nanoparticles. [8d, 34] From the previous discussion, it is apparent that there is no significant decrease in activity and PO selectivity during along lifetime test without regeneration, which indicates the remarkable stability of these catalysts.N otably,t here is no change in the TS-1s tructure during the reaction, which is evident from the TEM and DR-UV/Vis ( Figure S8, S9) measured after the reaction.
It is well understood from previous reports that Au catalysts suffer from deactivation because of the agglomeration of particles and accumulation of carbonaceous species on the catalyst surface over the course of ar eaction. [29a, 35] However,the reported Au/TS-1 PT material shows remarkable stability and high selectivity towards PO formation over the time on stream. To understand the mechanism(s) behind the enhanced stability of Au/TS-1 PT ,weutilize multiple characterisation techniques,such as X-ray photoelectron spectroscopy (XPS), 31 PMAS NMR spectroscopy,TGA, DR-UV/Vis,and TEM to investigate changes in the catalyst structure and the deactivation mechanism(s) during the propylene epoxidation reaction.
X-ray photoelectron spectroscopy (XPS) investigates changes in the chemical state and the size of gold clusters deposited on TS-1, since the 4f 7/2 orbital of gold provides as ensitive measure of its electronic state and the full-widthhalf-maximum (FWHM) of the gold peak is related to its particle size. [36] The4 f 7/2 signal of the untreated Au/TS-1s ample is ca. 84.2 eV (Figure 4a), av alue characteristic of gold in metallic state (Au 0 ). [36,37] After treatment with oxygen plasma (Figure 4a), the 4f 7/2 peak of gold shifts to ca. 83.4 eV, indicating the coexistence of metallic gold and undecomposed Figure 4. Investigations to elucidates tructure-stability relationships. a) XPS spectra of gold for Au/TS-1 before reaction. b) XPS spectra of gold for Au/TS-1 PT after reaction. c) TEM of Au/TS-1 PT after 2days of reaction. d) Particle size distribution histogram of (c). e) TEM of Au/TS-1 PT after 20 days of reaction. f) Particle size distributionh istogram of (e). g) 31 PMAS NMR of Au/TS-1 PT before and after reaction.
gold-phosphine complex (Au(PPh 3 )(Cl)). [38] In the case of Au/ TS-1 PT that had twenty days on-stream, additional peaks of gold appear at ca. 85.1 and ca. 87.8 eV,revealing the presence of oxidised gold species (Au d+ ,0 < d < 3). [39] Thea tomic percentage of oxidised gold is ca. 41 %, suggesting the partial oxidation of gold clusters after twenty days of continuous reaction. Furthermore,adecrease in the value of the FWHM of the 4f 7/2 peak of gold is observed (Table 1) for Au/TS-1 PT after reaction, indicating the formation of agglomerates. [36] This interpretation is verified by DR-UV/Vis spectroscopy ( Figure S9), where asurface plasmon band at 520 nm appears for both samples,again indicating alarger particle size.TEM images (Figure 4c,d)s how that the size of the Au particle is ca. 3nmafter 2days and reaches ca. 4nm (Figure 4e,f)after 20 days of reaction. This partial agglomeration and oxidation of gold clusters results in as light decrease in catalytic activity [40] as the time-on-stream increases (Figure 3i-k). [41] Furthermore,t he comparison of Ti 2p spectra of untreated and post reaction Au/TS-1 samples ( Figure S10) reveals no shift in the binding energy,indicating the high stability of Ti in such oxidizing environment. [42] Titania plays ac rucial role in the overall stability of Au/TS-1 catalyst, since there is strong interaction between Au and Ti via adsorption of Au nanoparticles on Ti defect sites of the TS-1 lattice. [42,43] These Ti sites act as nucleating sites for Au nanoparticles (in the range of 1-10 nm) resulting in ah ighly stable Au/TS-1 catalyst. [44] Additionally,t he treatment of aA u/TS-1 sample with oxygen plasma alters its surface properties.The untreated Au/ TS-1 sample has ap hosphorus peak at ca. 131.8 eV (Figure S11a), assigned to phosphine ligands bonded to the gold cluster. [22a] However,this phosphorus peak shifts to ca. 133 eV (Figure S11b-d) after treatment of the sample with oxygen plasma, demonstrating the formation of phosphorus oxide bonded on the surface of TS-1, due to the dislodging of phosphine ligands from the gold clusters and their subsequent binding and oxidation via interaction with the surface of the metal oxide support. [22a] 31 PM AS NMR spectroscopy is applied to analyse the nature of the phosphorus on TS-1 in the catalyst, before and after the reaction, to better comprehend the structure-stability relationship (Figure 4g). The 31 PNMR spectra of all the Au/TS-1 samples are shown in Figure 4g.As depicted, the Au/TS-1 sample exhibits strong signals at 52 and 36 ppm, which are ascribed to surface bound PPh 3 to Au and aphosphine-gold complex, Au(PPh 3 )Cl, respectively. [45] After plasma treatment, there is ad ecrease in the intensity of the signal observed at 52 ppm with simultaneous appearance of ap eak at À4ppm, corresponding to physiosorbed phosphine, [46] which indicates that some fraction of the bound PPh 3 is moved onto the TS-1 support. Thep resence of as mall amount of ligand in the catalyst is ap lausible reason for the long induction period for the Au/TS-1 PT catalyst. After 2days of reaction, the peak at 36 ppm (Au(PPh 3 )Cl) completely vanishes, which is attributed to the decomposition of the ligand within the reactor at high reaction temperature.A dditionally,t he signals appearing at ca. 13.3 and À3ppm are linked to the phosphine species migrated to the support. [46,47] Thepeak at 4.6 ppm is ascribed to some phosphate species. [48] However, after 20 days of reaction, most of the phosphine ligands are oxidised to phosphates,c orresponding to the peak at 2.6 ppm, also confirmed by XPS analysis (Figure S11c, d). This observation is corroborated by TGA data (Figure S12), where the total weight loss for the catalyst after 2days of reaction is double that of the spent catalyst after 20 days.T his indicates that most of the phosphine species have decomposed, while the remaining phosphine species migrated and adsorbed onto the TS-1, and slowly oxidise to phosphates with increasing timeon-stream. It has been previously reported that phosphate grafting onto TS-1 can be used to enhance epoxidation. [49] Therefore,t he increasing trend in the PO production rate is linked to more accessible Au-Tisites with the gradual release of the bound phosphine species at 200 8 8Cfrom the Au surface. Thes ubsequent decrease in the PO production rate after 11 days can be associated to partial aggregation of Au particles caused by the loss of ligands.O verall, the catalyst exhibited as ignificantly prolonged lifetime compared to the conventional Au/TS-1 catalyst, which is hypothesised to be due to the stabilising effect of residual phosphine ligands on the gold particles.T he residual bound phosphine ligands prevent sintering and significantly improve its stability in the harsh reaction environment. Moreover,t he importance of gold particle size and phosphine ligand is further confirmed on testing ac alcined Au/TS-1 catalyst, where 80 %o ft he ligand was removed prior to the reaction. Thec alcined Au/ TS-1 samples showed al ower PO production rate of 10.4 g PO h À1 kg cat À1 (propylene conversion 0.9 %h ydrogen single pass conversion ca. 8.2 %, oxygen single pass conversion ca. 5%)after 30 hofreaction ( Figure S13a). Thecalcined catalyst exhibits alow PO selectivity of 56.9 %( Figure S13b) with propanal as the other primary product, along with CO 2 and water. This lower PO selectivity is linked to the much larger gold nanoparticles that favour direct combustion. The calcined catalyst shows very low H 2 efficiencyv alues of ca. 3% ( Figure S13c) initially,w hich is indicative of more water formation accompanied by low PO production. As the reaction proceeds,t his value increases to ca. 6% on 20 h time-on-stream.

Conclusion
Sub-nanometre gold clusters with ac ore diameter of approximately 0.8 nm, stabilised by phosphine ligands,a re synthesised using af acile one-pot methodology.T he clusters can be easily immobilised onto the support with minimal change in particle size.Afast technique using non-thermal O 2 plasma has been developed to remove around 74 %o f

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Research Articles phosphine ligands,l eading to ah ighly stable catalyst. The catalyst prepared using this approach demonstrates better catalytic performance in direct gas phase propylene epoxidation based on PO selectivity of ca. 89 %a nd stability over 20 days.Comparing these results to traditional Au/TS-1 catalysts in direct gas-phase epoxidation, [29a] there is significant improvement in overall lifetime,i ncreased selectivity,h igher hydrogen utilisation, and less nanoparticle sintering.T riphenylphosphine plays an important role as as acrificial labile ligand in preventing nanoparticle agglomeration, while keeping the overall particle size small (< 5nm) and simultaneously improving the size specific catalytic activity.Apart from these phosphine ligands,a na dditional factor contributing to the enhanced stability of Au nanoparticles during the epoxidation reaction is the presence of Ti defect sites,w hich act as nucleating sites for Au nanoparticles,preventing their oxidation and improving the stability of Au/TS-1. [42][43][44] Although this work focused on propylene epoxidation, the insights provided herein can be used to rationally design stable catalysts with alonger lifetime for other reactions,which is an important aspect in the design of catalysts for industrial applications.