Monolithic hierarchical gold sponges for efficient and stable catalysis in a continuous-flow microreactor

Abstract

Monolithic hierarchical gold sponges are prepared by the polymer-assisted metal deposition method under air- and moisture-compatible conditions. These sponges feature excellent catalytic properties, robustness and chemical recyclability for the reduction reaction of 4-nitrophenol, and are also available for the scale-up catalysis of the same reaction in a continuous-flow microreactor.

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Metal-based materials with large surface area-to-volume ratios have received considerable attention due to their potential applications in the fields of energy storage and conversion,¹ optical sensors,² catalysis chemistry³ and so on.⁴ In particular, metal particles and wires at the nano-/micro-scale have been demonstrated as promising candidates for catalysing many chemical reactions.⁵ However, these small particles and wires irreversibly aggregate in solution, thereby leading to a gradual decrease in catalytic activity as time goes on. To address these issues, one main strategy is to decorate these small metal catalysts with polymers,⁶ graphene sheets⁷ and others,⁸ or immobilize them on matrices of metal–organic frameworks,⁹ particles,¹⁰ fibres,¹¹ and aerogels.¹² The stability of these catalysts is therefore significantly improved and the catalytic reactivity is maintained at high levels throughout the reaction. Nevertheless, the fabrication as well as the separation process of these modified metal catalysts usually requires sophisticated and multiple steps,¹³ which are always tedious and undesirable in scale-up applications.

To get rid of any tedious fabrication and separation processes, another strategy is to fabricate monolithic metals with nano- and micro-pores, namely metal sponges that feature porous structures to provide superior catalytic properties (>90%) and kinetic constants (>0.5 × 10⁻³ s⁻¹). These porous structures can not only make the metal more chemically reactive compared to their bulks,¹⁴ but can also avoid complicated preparation and time-consuming separation processes. With excellent catalytic properties toward chemical reactions, metal sponges are free standing (or self-supported) and highly conductive, showing broad applications in the field of materials science, such as promising electrode candidates for hydrogen revolution,¹⁵ water splitting,^4d,16 and fabricating enzyme-free detectors,¹⁷ and energy conversion¹⁸ and storage devices.¹⁹ However, it should be noted that the fabrication of metal sponges often adopts either expensive bottom-up approaches in a vacuum²⁰ or top-down removal strategies that require a high-temperature and/or corrosive environment,²¹ which does not allow ambient and mild fabrication at low cost. Even though vacuum-free electrochemical deposition can realize relatively low-cost metal deposition,²² it requires conductive substrates which significantly limit the wider use of non-conductive polymeric materials. Moreover, the as-deposited metal coatings often crack easily and detach from the substrate surface during catalysis due to weak physical adhesion between the metal and the substrate.²³ As a consequence, the stability and catalytic properties of these metals are always poor.

To tackle the adhesion problem between the metal and the substrate, recently, it was found that long-chain functional polymers are helpful for immobilizing stable and robust metal nanoparticles (or thin films) on different non-conductive substrates.²⁴ For example, by using the surface-initiated polymerization method, polymer brushes were covalently grafted from the surface of mesoporous silica films or nanoporous anodic aluminium oxide membranes.²⁵ Subsequent metal ion loading and reduction could yield high-performance and durable metal thin films on the substrate surface. The as-prepared metal thin films showed high catalytic efficiency and stability for typical catalytic reactions. Still, polymer brushes are grafted via surface-initiated-atom transfer radical polymerization (SI-ATRP) which must be performed in an inert atmosphere for several hours. Therefore, it is difficult to scale-up the whole fabrication process. To date, how to fabricate highly efficient, reactive and stable metal sponges with scale-up potential in a facile, ambient and time-saving manner still remains a significant challenge.

Herein, we report a solution-processed, air- and moisture-compatible approach to prepare free-standing monolithic gold sponges by the polymer-assisted metal deposition method (PAMD). Functional polymers are grafted onto the supporting polyurethane (PU) sponges in a dip-coating fashion, followed by electroless deposition to form a uniform Au layer with excellent robustness and chemical stability. The Au sponge possesses an ultrahigh catalytic efficiency (∼95% conversion in several minutes) for the reduction reaction of 4-nitrophenol (4-NP) even after 100 times of repeated use. We also demonstrate a continuous-flow system made of the Au sponge, which allows separation-free, continuous catalytic reduction of flow-in liquid chemicals for a long time.

The fabrication process is illustrated in Fig. 1a, which includes three steps, namely co-polymer coating, ion exchange and gold deposition. The copolymer, i.e., poly[2-(methacryloyloxy)ethyl trimethylammonium chloride-co-3-(trimethoxysilyl)propyl methacrylate] (p(METAC-co-MPTS)), used in this study contains two kinds of functional groups: one is MPTS which is used to anchor the polymers onto the PU via a self-condensation reaction, while the other is METAC which acts as the reservoir of precursors for the subsequent electroless deposition of Au. In a typical fabrication, pristine PU sponges were immersed in an ethanol solution of the co-polymer and incubated in an ammonia atmosphere, which resulted in a uniform wrapping of p(METAC-co-MPTS) on the surface of the PU sponges. After that, the PdCl₄²⁻ precursors for electroless deposition were loaded via an ion exchange process, where the Cl⁻ in METAC was replaced by PdCl₄²⁻ due to the high affinity of PdCl₄²⁻ to the residue of quaternary ammonium in METAC. Au sponges were finally obtained by sequentially immersing the PdCl₄²⁻-loaded PU in electroless deposition solutions of copper then gold. As shown in Fig. 1b, the PU sponge was successfully coated with Au. The corresponding SEM images in Fig. 1c–e indicate that a 350 nm-thick Au film was uniformly and densely coated on the PU matrices. In addition, the successful fabrication of Au sponges was monitored by energy-dispersive X-ray spectroscopy (EDS). As shown in Fig. 1f and Fig. S1a (ESI†), after co-polymer coating, two new peaks, respectively, for Si (from MPTS) and Cl (from METAC) indicate that the co-polymer was successfully immobilized onto the PU surface. After ion exchange with PdCl₄²⁻, a series of peaks belonging to PdCl₄²⁻ moieties were observed. Finally, three new peaks corresponding to gold signals were generated simultaneously and confirmed the successful deposition of Au. X-ray diffraction (XRD) was also used to further confirm the presence of gold (Fig. 1g and Fig. S1b, ESI†) with characteristic peaks ranging from 30° to 100°. Remarkably, the as-prepared Au sponges feature high conductivity (300 S cm⁻¹) and comparable porosity with the pristine PU sponges. The reason for this can be ascribed to the fact that PAMD is a full solution-processed method to realize metal deposition under air- and moisture-compatible conditions. The polymers and small molecules used in each step can freely penetrate into the microstructures of the sponges by diffusion. Theoretically, it is feasible to fabricate metal sponges of different pore sizes.

Fig. 1 (a) Scheme for the preparation of Au sponges by the polymer-assisted metal deposition method. The inset shows the commercial PU sponge and the chemical structure of the functional polymer p(METAC-co-MPTS). (b–d) Digital and SEM images of the PU (b and c) and as-prepared Au (b and d) sponges, respectively. The scale bars in (b), (c) and (d) are 4 mm, 400 μm and 400 μm, respectively. (e) The cross-section of Au sponges. The dashed lines show the as-deposited thin gold film on PU. (f and g) EDS and XRD spectra of the PU and Au sponges, respectively.

Importantly, the whole fabrication process is time-saving and only takes ∼60 min to complete. Therefore, it shows great potential for fabricating Au sponges in a large scale.

As a proof-of-concept, the as-fabricated Au sponges were employed to catalyse the reduction of 4-NP using sodium borohydride as a reducing agent. The product of the reaction, 4-aminophenol (4-AP), is one important chemical in drug and dyeing agent manufacturing and also in the photograph developing sector. By monitoring the decreased characteristic absorbance of 4-NP (392 nm) or the increased characteristic absorbance of 4-AP (292 nm) using UV-vis spectroscopy, the reduction progress can be traced.^6c,26 Here, the as-fabricated Au sponges were immersed in an aqueous solution of 4-NP and sodium borohydride. The UV-vis spectra show that the intensity of the characteristic absorbance of 4-NP at 392 nm gradually decreased as the reduction proceeded (Fig. 2a). At the same time, a new peak was simultaneously generated at 292 nm, indicating that the reduction of 4-NP to 4-AP could be catalysed by the Au sponges as predicted.

Fig. 2 (a) UV-vis absorption spectra during the catalytic reduction of 4-NP over Au sponges. The inset digital image shows the colour change of solutions before and after reduction. (b) ln(A₀/A_t) at 392 nm vs. reaction times for the reduction of 4-NP with different catalyst systems. PU sponge: 13 mm × 30 mm × 4 mm. Au film: 13 mm × 30 mm. Au sponge 1: 13 mm × 30 mm × 4 mm. Au sponge 2: 13 mm × 4 mm × 4 mm. (c–e) AFM and SEM images of thermally-evaporated gold films (c) and Au sponges (d and e), respectively. The scale bars are 500 nm in (c) and (e), and 200 μm in (d), respectively. (f) The absorbance of reaction solutions at 392 nm with (or without) Au sponges at predetermined times. (g) The conversion of 4-NP to 4-AP vs. cycle numbers.

To further understand the catalytic mechanism of 4-NP using the Au sponge as a catalyst, the reduction of 4-NP was systematically studied. ln(A₀/A_t) at 392 nm was plotted against reaction times and the reduction of 4-NP to 4-AP was found to follow a first-order kinetics when excessive sodium borohydride was used (Fig. 2b). Therefore, the obtained rate constants (k) were utilized as the criterion to evaluate the catalytic properties in the following studies. When the reduction was carried out using pristine PU sponges as a catalyst, no decrease in absorbance at 392 nm was observed upon stirring for a long time (120 min), showing that there was no reduction of 4-NP (k = 0 s⁻¹). However, when the as-prepared Au sponge 1 was immersed in the same solution, the characteristic absorbance peak of 4-NP disappeared within a few minutes as shown in Fig. 2a and b (k = 0.304 s⁻¹). This result demonstrated that Au sponges played a key role in this catalytic reaction and further supported our prediction mentioned above. We also evaluated the effect of different sizes of the Au sponges on the catalytic rate of 4-NP. When a smaller Au sponge 2 (13.3% in volume ratio to 1) was employed, the reduction rate of 4-NP significantly decreased and the value of k was only 5.6% to that of Au sponge 1 (Fig. 2b). It is easy to understand the reason for that: the smaller sponge possesses a smaller surface area which decreases both the transmission rate of electrons and the possibility of Au–H bond formation between Au sponges and sodium borohydride.^26,27 If the surface area of the sponge decreases further, the rate will sharply decrease. To confirm that we evaporated gold films on a flat substrate to obtain a smooth gold surface with a roughness of ∼1.2 nm to catalyse the same reaction (Fig. 2c). The reaction rate was found to be only 1.5% of that of Au sponge 1.

Table 1 lists the catalytic efficiencies for reducing 4-NP to 4-AP using various gold-based catalytic systems. The as-prepared Au sponges show ultrahigh efficiency (∼95% conversion in several minutes) and a kinetic constant of 5.0 × 10⁻³ s⁻¹, which are among the best performing systems. The outstanding catalytic performance of the Au sponges is mainly attributed to the following reasons. First, the surface of the Au film on the sponge deposited by ELD is rough (roughness ∼28.3 nm), where there are lots of clusters of gold nanoparticles with a size of 100–400 nm aggregated on the surface of Au sponges (Fig. 2e). Second, the structures of the Au sponges are interconnected with each other, featuring numerous micropores that increase the specific surface area (Fig. 2d). Such a hierarchical structure of Au made them more chemically reactive than that of the corresponding macroscopic metals (i.e., flat gold films), promoting the reaction rates (Fig. S2, ESI†).^4c,26,27 Compared with these listed strategies, Au sponges demonstrated here can be readily prepared under very mild conditions, and the sophisticated processes for fabricating and separating catalysts are not required. In addition, the on/off catalytic reaction can be easily controlled by immersing the Au sponges in or pulling them out from the solutions (Fig. 2f). More importantly, the as-prepared Au sponges are stable throughout the reaction and can provide excellent catalytic efficiency of more than 90% even after performing the catalytic reaction 100 times (Fig. 2g).

Table 1 Comparison of kinetic constant (k_app) for the reduction reaction of 4-NP using various catalytic systems

Entry	k app (×10^−3 (s^−1))	Structures	Separation	m Au/[4-NP]
1	0.7–14	Au NPs with ligands^26	Not mentioned	Not mentioned
2	1.3–8.5	Au clusters^18b	Not mentioned	30 μg/1.67 mM
3	6.1	Au NPs@GO sheets^7b	Centrifugation	4 μg/1.54 mM
4	0.68–12	Au NPs@fibrous silica microspheres^10d	Centrifugation	0.5 mg/0.06 mM
5	14	Au NPs@Fe3O4@SiO2^10e	Magnetic force	Not mentioned/0.08 mM
6	8.0	AuNPs@fiber^11b	Decantation	0.5 mg/3 mM
7	6.3	Au NPs@porous films^6c	Filtration	1.8 mg/1.67 mM
8	1.1	Au NPs@hydrogel^12b	Not mentioned	12 μg/0.6 mM
9	4.0	Au foam@diatom^23d	Filtration	0.3 mg/8 μM
10	5.0	Monolithic Au sponges (this work)	Pulling out	3 mg/1.67 mM

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Taking advantage of the outstanding catalytic efficiency, excellent chemical stability and recyclability of the Au sponges, we designed a continuous-flow catalytic system to realize the scale-up reduction of 4-NP (Fig. 3a). To this end, a Au sponge with a thickness of 4 mm and a width by length of 5 × 60 mm² was embedded in a fluidic channel where a total of 500 mL of 4-NP solution slowly passed through pores within the sponge and was continuously reduced to colourless 4-AP (Fig. 3a). In this system, it was found that the flow rate had a profound influence on the reduction efficiency of 4-NP in which the efficiency increased with decreased flow rate (Fig. 3b). The specific conversion was up to 92% at a flow rate of 0.24 mL min⁻¹, which was very close to that in the intermittent way (shown in Fig. 2). Moreover, this continuous-flow process was monitored by UV-vis spectroscopy, and it was found that there was no obvious difference detected between the two samples that were obtained after 2 h and 12 h of reduction (Fig. 3c). This result indicates that the Au sponges featured excellent stability and high efficiency for the reduction reaction of 4-NP in the continuous-flow reaction system.

Fig. 3 (a) The Au sponge-based continuous-flow catalytic system for the reduction of 4-nitrophenol. Inset digital images show the solution of 4-NP before and after reduction via the continuous chemical reaction (volume = 500 mL). (b) The conversion of 4-NP to 4-AP with different flow rates. (c) UV-vis spectra of the starting solution and the samples obtained at different reaction times.

In conclusion, we have prepared free-standing monolithic Au sponges via the PAMD method under air- and moisture-compatible conditions. The as-prepared Au sponges are very stable, and feature high catalytic efficiency and excellent recyclability for the reduction of 4-NP. Moreover, they possess the ability to scale-up catalytic reactions in a continuous-flow fashion. By using commercial porous supports and the PMAD method, this kind of Au sponge can be fabricated without using expensive vacuum equipment and sophisticated chemistry, showing great potential applications in industrial fields. Not limited to PU matrices, Au can be stabilized onto arbitrary substrates with the assistance of functional polymer layers, especially for substrates that cannot withstand high temperature, organic solvent, and extreme acid (and/or alkaline) conditions. Furthermore, many other noble metal sponges can also be prepared by this approach. It is anticipated that this novel strategy and the as-prepared Au sponges can be used in sensors, water-purifiers, chemical catalysis and other related fields.

We acknowledge the Natural Science Foundation of Shaanxi Province (2016JQ5035), NFFTBS (No. J1210057), the Research Grant Council of Hong Kong (PolyU5036/13P, Poly5030/12P), and the NSFC (21125316, 21434009, 21604069) for the financial support of this work.

Notes and references

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Footnote

† Electronic supplementary information (ESI) available: Experimental section and supporting figures. See DOI: 10.1039/c6qm00115g

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