Noble metal-free bimetallic nanoparticles for the hydrogenation of sugars – NobleFreeCat

The MATCAT team from UCCS has undertaken the preparation of supported Fe-Ni and Fe-Cu nanoparticles (NPs) following routes that have been seldom tested in the literature, in order to form NPs whose size (a few nm) and composition (metal ratio, reduction degree of Fe) are as uniform as possible. The fine characterization of NPs formation, structure and composition can be carried out through a unique combination of advanced techniques (X-ray absorption spectroscopy, Mössbauer spectroscopy, low-energy ion scattering).
The team «Engineering of metal nanoparticles« from LCC, that is an expert team in the preparation of metal particles by organometallic routes, optimizes the synthesis of Fe@Ni, Ni@Fe, Fe@Cu and Cu@Fe core-shell NPs, in which one of the two metals preferentially appears at the surface of the other metal. This strategy requires the selection and synthesis of adequate organometallic precursors that decompose with distinct kinetics. These NPs will serve as standards to establish catalysis - structure correlations.
Given the highly reducing character of iron and aluminium, the TEMIC team from LCP will explore an alternative route for the manufacturing of Fe-Ni, Fe-Cu and Fe-Al systems: radiolytic synthesis, in which the reducing agents are solvated electrons resulting from solvent irradiation. It will be checked if NPs of controlled size and composition, and metastable structures (Fe-Cu alloys) can be quantitatively obtained in mild conditions.
Finally, the MATCAT and VAALBIO teams from UCCS, and the «Catalysis and non-conventional media« from IC2MP will screen the catalytic performances and stability of the bimetallic NPs in the hydrogenation of xylose to xylitol, and maltose to maltitol, in aqueous phases (UCCS) and in green, non-aqueous deep-eutectic liquid solvents (IC2MP).

During the first 18 months, the project was developed mostly at UCCS (teams MATCAT and VAALBIO) through Duchao Shi's PhD (CSC scholarship) on supported Fe-Ni catalysts. Dichao Shi has optimized the synthesis of Fe-Ni/SiO2 catalysts by deposition-precipitation with urea (DPU) and has obtained the following results:
(i) understanding of the formation process of Fe-Ni bimetallic NPs: precipitation of an ill-crystallized (Fe(III), NI(II)) phyllosilicate; reduction of Fe(III) to Fe(II) within the phyllosilicate; reduction of Ni(II) to Ni metal, triggering the destruction of the silicate and the reduction of Fe(II) to Fe metal; progressive insertion of Fe into particles of cubic face-centered Fe-Ni alloys;
(ii) slightly preferential deposition of nickel with respect to iron, compared with the composition of the DPU solution;
(iii) homogeneity in size (4-7 nm, metal dispersion: 15-20%) and composition (standard deviation on Fe: 8 at%) of the bimetallic NPs whatever the tested formulation, from Fe65Ni35 to Fe8Ni92;
(iv) evidence for an excess of iron at the surface of the NPs both in the oxidized and in the reduced states;
(v) reversibility of the surface structural changes, between the oxidized state in air and the reduced state resulting from an activation in H2;
(vi) correlation between the formulation of the NPs and their catalytic performances in the hydrogenation of a test aldehyde, furfural. It was shown that NPs containing between 60 and 75 at% Ni presented a maximum of activity and selectivity toward furfuryl alcohol (selective hydrogenation of the aldehyde function), while a higher content in Fe considerably decreased the activity, and a higher content in Ni progressively favoured side-reactions of alcohol etherification, hydrogenolysis, and hydrogenation of the furan ring, that becomes predominant for a monometallic Ni catalyst.

Synthesizing Fe-Ni NPs with controlled structure, size and composition by deposition-precipitation over a large range of compositions is a progress with respect to standard preparation methods. Understanding the process of formation of these NPs, which was made possible by the use of in situ advanced characterization techniques, has as of today led to one publication (Catalysis Today) and to oral presentations in national and international congresses. These results allow one to unambiguously correlate catalytic performances and composition of the NPs, through kinetic studies that have not been yet reported in the literature. The surface characterization of NPs by LEIS spectroscopy should even give access to a correlation between catalytic performances and the proportions of the two metals at the surface of the particles.
In the next period, Achraf Sadier, post-doctoral researcher at UCCS, will extend the synthesis of supported catalysts to the Fe-Cu system, will verify the stability of the catalysts in hydrothermal conditions under H2, and will test them in the selective hydrogenation of xylose and maltose in aqueous phase. A post-doctoral researcher recruited at IC2MP in January 2020 will develop catalysis in deep-eutectic solvents, in order to explore non-aqueous media in which sugars are soluble and the hydrogenation reaction can take place.
As far as alternative synthesis procedures of Fe-based bimetallic particles are concerned, the team from LCC «Engineering of metal particles« has started the project in November 2018, with the recruitment of a PhD researcher, François Robert. A post-doctoral researcher recruited in September 2019 at LCP will prepare Fe-Ni, Fe-Cu and Fe-Al NPs via radiolytic routes, at ambient temperature, as described in the Methods and methodology section.

Bimetallic Fe-Ni/SiO2 catalysts for furfural hydrogenation: Identification of the interplay between Fe and Ni during deposition-precipitation and thermal treatments – D. Shi, Q. Yang, C. Peterson, A. F. Lamic-Humblot, J. S. Girardon, A. Griboval-Constant, L. Stievano, M. T. Sougrati, V. Briois, P. A. J. Bagot, R. Wojcieszak, S. Paul, E. Marceau* – Catalysis Today, accepted, 2018, doi.org/10.1016/j.cattod.2018.11.041

The hydrogenation of sugars into polyols is often carried out on noble metals such as ruthenium Ru or on Raney nickel, with little work reported on supported non-noble metals. However, the cost of noble metals is high, their abundance is low, and both their cost and availability are prone to large fluctuations. Substitutes to noble metals should be found in the first period of transition metals, which encompasses especially abundant and cheap metals, but that may deactivate upon reaction or, like nickel, induce hydrogenolysis side-reactions. In this context, the potential of iron-based bimetallic combinations deserves to be fully explored. However, the hydrogenation/hydrogenolysis balance of the catalyst is not simply governed by the ratio between the two metals, but also by their degree of interplay, i.e., alloyed vs. segregated metals, a crucial point to take into account for a metal sensitive to oxidation like Fe. The possibility to synthesize precisely controlled Fe-based bimetallic nanoparticles appears to be limited when the catalyst is prepared by standard methods, with particles of different size, structure or composition coexisting in the catalyst. As a consequence, no firm conclusion can currently link the catalyst formulation and the catalytic properties of Fe-based systems owing to a lack of control of the nature of the nanoparticles at the nanometer level, leading to difficulties to interpret, reproduce, extrapolate and rationalize catalytic results.
The NobleFreeCat project brings together four academic teams with complementary expertise in synthesis of metal nanoparticles and catalysis (Unité de Catalyse et Chimie du Solide, Laboratoire de Chimie de Coordination, Institut de Chimie des Milieux et Matériaux de Poitiers, Laboratoire de Chimie Physique), to develop scalable synthesis routes towards bimetallic Fe-based supported nanoparticles well-controlled in terms of size, chemical composition and chemical order, for their use as tunable and low-cost substitutes to noble metals in the catalytic hydrogenation of sugars. Three systems whose activity in selective hydrogenation has been recently described in the literature will be explored: Fe-Ni, Fe-Cu and intermetallic Fe-Al compounds. Supported catalysts on silica and zirconia will be prepared by deposition-precipitation, impregnation in the presence of organic additives, and radiolytic methods. Alloyed or core-shell systems prepared by organometallic route or mechanochemistry will be used as standards for the determination of the alloys stability, for structural characterization of supported systems, and as references for catalytic testing. The hydrogenation of a monosaccharide, xylose, to xylitol and that of a disaccharide, maltose, to maltitol, seldom tackled in the academic literature but of great interest for the industry, will be tested both in aqueous phases under hydrothermal conditions and in water-free deep-eutectic solvents. A unique combination of characterization techniques (in situ X-ray diffraction and X-ray absorption spectroscopy, STEM-EDX and -EELS subnanometric mapping, XPS/LEIS, Mössbauer spectroscopy) will be implemented to overcome the difficulty to distinguish poorly Z-contrasting elements and to probe both the bulk and surface composition of individual nanoparticles, along their whole cycle of use: reduction, reaction, exposure to air, catalyst regeneration. The establishment of correlations between synthesis route, chemical composition and catalytic activity will allow the consortium to lay the groundwork for subsequent technological maturation, furthered by the participation of representatives of sugar-derivatives producer Tereos in the steering committee of the project.