Multifunctional Core-Shell Microgels as Pd-Nanoparticle Containing Nanoreactors With Enhanced Catalytic Turnover

Abstract

In this work, we present core-shell microgels with tailor-made architecture and properties for the incorporation of palladium nanoparticles. The microgel core consists of poly-N-isopropylacrylamide (PNIPAM) copolymerized with methacrylic acid (MAc) as anchor point for the incorporation of palladium nanoparticles. The microgel shell is prepared by copolymerization of NIPAM and the UV-sensitive comonomer 2-hydroxy-4-(methacryloyloxy)-benzophenone (HMABP). The obtained core-shell architecture was analyzed by means of photon correlation spectroscopy, while the incorporated amount of HMABP was further confirmed via Fourier transform infrared spectroscopy. Subsequently, the microgel system was used for loading with palladium nanoparticles and their size and localization were investigated by transmission electron microscopy. The catalytic activity of the monodisperse palladium nanoparticles was tested by reduction of 4-nitrophenol to 4-aminophenol. The obtained reaction rate constants for the core-shell system showed enhanced activity compared to the Pd-loaded bare core system. Furthermore, it was possible to recycle the catalyst several times. Analysis via transmission electron microscopy revealed, that the incorporated palladium nanoparticles emerged undamaged after the reaction and subsequent purification process since no aggregation or loss in size was observed.

Keywords: catalysis; core-shell structure; microgels; nanoparticles; recycling; responsive material.

FIGURE 1

(A): Angle-dependent PCS measurements for the core (hollow squares) and core-shell particles (filled squares) at pH 4 and pH 10 in the swollen state (20°C). The slopes obtained from the linear fits were used to calculate the hydrodynamic radius by the Stokes-Einstein equation. Red data points were masked and not used for the fitting procedure. (B): Hydrodynamic radii of the core and core-shell microgels as function of temperature. The measurements were performed at a fixed angle at pH 4 and pH 10. The blue line represents the temperature at which the angle-dependent measurement were performed. The error bars correspond to the standard deviation.

FIGURE 2

TEM micrographs of the core-shell microgels containing Pd nanoparticles. Two different magnifications with the respective scale bars are shown.

FIGURE 3

Histogram of the relative frequencies plotted against the nanoparticle radius. The size distribution was fitted with a single Gaussian function. A total of 2,300 palladium nanoparticles were analyzed. A mean particle radius of (6.4 ± 1.4) nm was obtained.

FIGURE 4

Catalytic reduction of 4-nitrophenolate to 4-aminophenol with an excess of sodium borohydride in the presence of palladium nanoparticle containing core-shell microgels. Due to the hydrolysis of sodium borohydride, an initial pH value of 9.1 is present and the product 4-aminophenol with a pK _a of 10.3 remains in its protonated state (Haynes, 2017).

FIGURE 5

Absorbances plotted against the wavelength (left) for the catalytic degradation of 4-nitrophenolate to 4-aminophenol with palladium loaded microgels as catalyst at different times. The time-dependent absorbance intensities at wavelengths of 300 and 400 nm are plotted on the right. While 4-nitrophenolate, which absorbs mainly at 400 nm, decays, the product 4-aminophenol at 300 nm arises. Since the microgel has a wavelength-dependent scattering, the baseline intensity increases with decreasing wavelength.

FIGURE 6

On the left y-axis the absorbance at a wavelength of 400 nm is plotted against the time for the three catalytic cycles. Each cycle has an own horizontal axis scaling, while all three share the same ordinate axis. The shown decay represents a conversion up to 95%. A mono-exponential pseudo-first order fit was applied (solid red line). The resulting baseline absorbance A _i and the initial absorbance A ₀ were applied for the calculation of ln ((A− A _i)/(A ₀− A _i)). On the right ln ((A− A _i)/(A ₀− A _i)) is plotted against the time and fitted with a straight line (solid black line) in the respective area.

FIGURE 7

Magnified view of the linearized absorbance decay at a wavelength of 400 nm for the three cycles of the free hybrid system reactions. The three distinct regions of the catalytic reaction can be clearly identified.