32 33 D I A M O N D L I G H T S O U R C E A N N U A L R E V I E W 2 0 1 9 / 2 0 D I A M O N D L I G H T S O U R C E A N N U A L R E V I E W 2 0 1 9 / 2 0 Structures and Surfaces Group Beamline B07 & ePSIC Insights into the structure and reactivity of Ni nanoparticles for catalytic asymmetric hydrogenation Related publication title and DOI: Arrigo R., Gallarati S., Schuster M. E., Seymour J. M., Gianolio D., da Silva I., Callison J., Feng H., Proctor J. E., Ferrer P.,Venturini F. &Held G. Influence of synthesis conditions on the structure of nickel nanoparticles and their reactivity in selective asymmetric hydrogenation, ChemCatChem 12 , 1491-1503 (2019). DOI: 10.1002/cctc.201901955 Publication keywords: Nanoparticles; Ni phosphides; Hot-injection synthesis; hydrogenation; XPS ; XAFS; HRTEM C hiral organic molecules occur in two different enantiomeric (mirror image) forms. In pharmaceutical applications, these two forms can have different effects. To maximise the effectiveness of a drug, and to reduce side effects, it is essential to synthesise only the desired form. A team of researchers studied the synthesis of methyl‐3‐hydroxybutyrate (MHB), an important intermediate step in the synthesis of a treatment for the eye disease glaucoma, using a nickel (Ni) catalyst. The biggest challenge of this reaction is selecting the ( R )-MHB form. Understanding howenantioselectivity is realised over nickel catalysts is paramount to tackle this challenge. The team studied the impact of synthesis conditions on the size, shape and electronic structure of the Ni nanoparticles on their catalytic performance.To achieve this goal, the researchers used theVersatile Soft X-ray (VERSOX) beamline, also known as B07, at Diamond Light Source toperformsurface sensitive X-ray photoelectronandabsorption spectroscopy todetermine the electronic structureof theNi nanoparticles, and electronmicroscopy at the electron Physical Science Imaging Centre (ePSIC) to determine the size and shape of the nanoparticles. Their results are relevant for asymmetric hydrogenation reactions and for the catalysis over Ni nanoparticles in general. The enantioselective asymmetric hydrogenation of unsaturated molecules is broadlyappliedforthesynthesisofpharmaceuticalsandfinechemicals,wherethe biggestchallenge isattainingtotalselectivitytooneenantiomer 1 .Oneofthemost studied examples of such reactions is the hydrogenation of the carbonyl double bond of methyl acetoacetate (MAA) to methyl-3-hydroxybutyrate (MHB) over nickel catalysts (Fig. 1). This reaction can be carried out with high enantiomeric selectivitytowardsthedesired( R )-MHBproduct ifthesurfaceofthenickelcatalyst is modified with a chiral molecule, typically ( R,R )-tartaric acid (TA). However, the long-term stability of heterogeneous Ni catalysts for this reaction is poor. Understanding of the much-debated mechanism by which enantioselectivity is realised at the Ni surface aided by modifiers is beneficial for the design of improved catalysts 2 . Three possible pathways have been proposed: a) a one-to- one molecular interaction of adsorbed TA controls the adsorption geometry of MAA 3 such that H 2 is delivered to the desired side of the carbonyl double bond; b) TA molecules form a supramolecular chiral arrangement on the Ni surface which desorb leaving a chiral enantioselective Ni surface 4 ; c) the interaction of TA with oxidised Ni particles facilitates etching of Ni cations from the surface leaving chiral Ni kink sites 5 . In this work we explore whether achiral round-like small nanoparticles (NPs) with high-index surface sites enable more opportunities for the generation of enantioselective surface ensembles upon chiral modification. Herein, supported and unsupported nickel nanoparticles are synthesised by means of hot-injection colloidal synthesis. The impact of the synthesis conditions on the physicochemical properties, such as morphology, size and structure are investigated using synchrotron-based X-ray Photoelectron Spectroscopy (XPS), high angle annual dark field scanning transmission electron microscopy (HAADF- STEM) and X-ray diffraction (XRD). The reactivity of these NPs in enantioselective hydrogenation is investigated to determine a structure-activity correlation, which will enable new mechanistic insights into the selective chiral function of the catalytic system and the structural transformations responsible for performance degradation. To control the size of the NPs the ratio between the reducing (oleylamine, OAm) and protecting agents (trioctylphosphine,TOP) was changed (in the sample notationsusedlateronxandycorrespondtotherelativeamountsofOAmandTOP, respectively). While OAm is expected to control the nucleation rate and growth, TOP acts as capping agent and provides surface stabilisation through coordination with the Ni surface, thus hindering the growth of the NPs. The HAADF-STEM image of Ni_5x1.5y (Fig. 2a) shows a core shell crystalline Ni NP S composed of a metallic core encapsulated within a shell of lower contrast, presumably of organic nature. Moreover, Ni nanocrystallites are embedded in this organic shell (Fig. 2b). Twinned metallic Ni nanoparticles are formed when the amount of the reducing agent (x) is 5 and above (Fig. 2a and 2b). However, when the amount of OAm (x) is relatively lower (Ni_2.5x1.5y), round amorphous core-shell NPs are obtained (Fig. 2c). Average particle sizes as determined by statistical analysis of the HAADF-TEM are 8.0±1.4 nm, 7.0±1.3 nm and 10.6±1.9 nm for Ni_2.5x1.5y, Ni_5x1.5y, Ni_10x1.5y, respectively.The trend in particle size observed for Ni_5x1.5y and Ni_10x1.5y is consistent with the expectation: the smaller NPs in the Ni_5x1.5y catalyst are due to the relatively more abundant capping agentTOPwhich hinders aggregation and growth. The Ni2p XPS spectra of the as synthesised samples recorded using the ambient pressure end station of beamline B07 (VERSOX) are also reported in Fig. 2d-f. XPS was particularly useful in this study to identify not only the electronic structure of Ni in these nanoparticulate systems but also to clarify the nature of the Ni amorphous phase in the sample Ni_2.5x1.5y. The XPS spectra were characterised by a main metallic Ni 2p 3/2 peak at the binding energy of 852.6 eV (Ni1), a relatively less abundant component Ni2 at 853.7 eV due to Ni(II) in NiO, and amore abundant component Ni3 at 856.1 eV due to amixed Ni(II)/Ni(III) oxy- hydroxide phase. A closer inspection of the spectrum of the Ni_2.5x1.5y sample reveals the presence of an additional Ni δ+ component Ni4 at 852.95 eV attributed to Ni phosphides. The presence of a phosphide phase was also confirmed by energy dispersive X-ray elemental mapping and Raman spectroscopy. Therefore, underthesynthesisconditionsrealisedduringtheNi_2.5x1.5ysynthesis,thelarge amount of TOP not only acts as capping agent to prevent NPs’growth but also as a source of phosphorous. Fig. 3a and 3b report the instantaneous conversion of MAA and selectivity as a functionofthereactiontimefortheTAmodifiedunsupportedandSiO2-supported NPs, respectively. The activities of the catalysts are steadily increasing for the unsupported Ni NP s indicating that new surface is continuously exposed during the reaction due to the detachment of the C overlayer. However, the increase of the activity is not reflected in an increase of the selectivity towards the ( R )-MHB. Particularly, for the unsupported Ni_5x1.5y and Ni_10x1.5y the selectivity towards the ( R )-MHB is maintained constant within the time frame investigated regardless of theMAA conversion achieved. Interestingly, the unsupported Ni_2.5x1.5y is highly selectivity towards the ( R )-MHB at the beginning of the reaction but deteriorates very rapidly to reach a similar value characteristic for these unsupported NPs systems, for which the TA modification appears to no longer exist. Amongst the SiO 2 -supported Ni NP s , Ni/ SiO 2 _5x0.75y shows interesting results. This sample is initially present as a NiO phase which explain the low conversion achieved, since the active hydrogenation Ni surface is metallic. However, this catalyst shows also the highest selectivity towards ( R )-MHB observed in this study. These results allow to draw a structure function correlation according to which high selectivity towards ( R )-MHB is obtainedwhen cationic Ni species are present which can strongly bind carboxylate groups of TA. It is therefore evident that to design highly active and ( R )-selective hydrogenation catalysts, a combination of large Ni metallic surface domains is needed,togetherwiththeexistenceofNicationic islandswherethechiralmodifier can strongly chemisorb to assist the side-by-side selective hydrogenation of MAA. References: 1. Kyriakou G. et al. Aspects of Heterogeneous Enantioselective Catalysis by Metals. Langmuir. 27 , 9687-9695 (2011).DOI: 10.1021/la200009w 2. Tsaousis P. et al. Combined Experimental andTheoretical Study of Methyl Acetoacetate Adsorption on Ni{100}. J. Phys. Chem. C. 122 , 6186-6194 (2018). DOI:10.1021/acs.jpcc.8b00204 3. HumblotV. et al. FromLocal Adsorption Stresses to Chiral Surfaces:( R,R )- Tartaric Acid on Ni(110). J. Am. Chrm. Soc. 124 , 503-510 (2002). DOI: 10.1021/ja012021e 4. Baddeley C. J. et al. Fundamental Investigations of Enantioselective Heterogeneous Catalysis. Top. Catal. 54 , 1348-1356 (2011). DOI: 10.1007/s11244-011-9761-3 5. KeaneM. A. et al. Asymmetric hydrogenation of methyl acetoacetate using Ni/ SiO 2 modifiedwith tartaric acid and alanine. J. Mol. Catal. 73 , 91-95 (1992). DOI: 10.1016/0304-5102(92)80064-N Funding acknowledgement: Simone Gallarati acknowledges fellowship from the Year in Industry placement scheme at Diamond Light Source (2017-18,“Chirally modified catalyst nanoparticles”project). Haosheng Feng is grateful for the Summer Placement student program at Diamond Light Source. The UK Catalysis Hub is kindly thanked for resources and support provided via our membership of the UK Catalysis Hub Consortium and funded by EPSRC (portfolio grants EP/K014706/1, EP/K014668/1, EP/K014854/1, EP/K014714/1 and EP/ I019693/1). Corresponding author: Dr Rosa Arrigo, University of Salford,
[email protected] Figure 1: Hydrogenation of pro-chiral methyl acetoacetate (1) gives two enantiomers, (R)- and (S)-methyl-3-hydroxybutyrate (2 and 3, respectively). When the surface of the nickel catalyst is modified with (R,R)-tartaric acid (also in combination with an inorganic salt such as NaBr), the (R)-enantiomer is preferably obtained. Figure 2: (a) and (b): HR-HAADF-STEM characterisation of Ni_5x1.5y. The arrow indicates the twinned lamellae of the NPs in (a) and a smaller Ni NPs embedded in the organic shell in (b); (c) HR- HAADF-STEM characterisation of Ni_2.5x1.5y; Fitted Ni 2p XP spectra (KE 570 eV) for Ni_10x1.5y (d); Ni_2.5x1.5y (e) and Ni_5x1.5y (f). Fitting: Ni1 (852.6 eV) is Ni 0 ; Ni2 (853.7 3 V) is Ni 2+ and Ni3 (856.1 eV) is Ni 2+ /Ni 3+ in oxide and oxyhydroxide, Ni4 (852.9 eV) is NiP; respectively. Figure 3: Instantaneous conversion of MAA vs. time of reaction (a) and corresponding instantaneous selectivity to (R)-MHB (b).