The College of Arts + Sciences
Department of Chemistry

Hybrid Nanomaterials for Environmental and Chemical Catalysis

 

The CO2 Challenge.  Like other small molecule feedstocks (e.g. N2), CO2 is thermodynamically quite stable (DHof = -394 kJ/mol), however it is susceptible to acid-base thermal (heterogeneous and homogeneous), electrochemical, and photoredox activation schemes in the presence of functional catalysts. Within these schemes, the general properties associated with CO2 activation are distortion of the linear structure, redistribution of charge to create relative acidic (carbon) and basic (oxygen) sub-sites, reduction of the molecule by electron transfer into the lower-lying LUMO of the distorted structure, and proton transfer to the compromised molecule. On surfaces, the adjacent atoms of a close-packed structure can serve to dissociate CO2 to generate reduced CH4, or partially reduced products such as CO and formate, as well as surface carbonate byproducts. Advanced catalysts will need to use these and other building blocks of this type in atom economical ways to assemble higher-order, value-added chemicals.

In stark contrast to kinetically challenging, 8-electron reduction to methane strategies for CO2 repurposing, nature has taken a unique approach to managing the critical greenhouse gas CO2.The Lewis acid (Mg2+) enzyme Rubisco catalyzes the carboxylation of ribulose-1,5-bisphosphonate as the primary approach to recycling CO2.  Rather than a catalytic cycle based on metal-based reduction of the substrate at Mg2+, this transformation is carried by acid-base incorporation of CO2 at the non-redox active metal and internal electron push to CO2 without the need to manage multiple redox equivalents externally and high energy, one-electron steps.  Thus, our vision is to use insertion (Lewis acid-base) strategies at heterogeneous catalytic active sites guided by computational AI to repurpose CO2 into value-added chemicals that when uncatalyzed, require high energetic costs to construct.  Moreover, we will develop photothermal activation methods of non-equilibrium catalyst heating to learn to drive transformations at reduced external temperatures.

We have also adhered gold nanorods to glass coverslips through electrostatic interactions by treating the glass coverslips with (3-aminopropyl)trimethoxysilane and polystyrene sulfonate. By exploring a condition matrix approach, we were able to select specific nanorod (NR) catalyst loadings that give optimal single monolayer coverage.  These Au NR-functionalized coverslips exhibit sustainable, catalytic reduction of 4-nitroaniline by sodium borohydride.  Moreover, Au NR coverslips were also found to have pseudo-first-order photochemical rate constants two orders of magnitude faster than the only other affixed catalyst, Au NP in polymer. Au NR coverslips are also recyclable at ~65% of the original rate after 5 cycles and can be carried from solution to solution with no catalyst mobility or loss of function.

Figure 1. TEM images of AgPd
nanodendrite-modified Au nanoprisms
with different amounts of Ag coating (A,F,K)
and H2PdCl4 (A-E, F-J, K-O).

Of higher profile than the other two reactions, we have also demonstrated that our synthetic routes to multifunctional, hybrid nanoparticles in the form of Au/Ag/Pd prisms, octahedra, and hexagonal plates produce excellent catalysts for electocatalytic CO2 reduction.  By controlling the under-potential deposition of Ag/Pd, novel nanostructures with photoactive Au cores and reactive dendritic tips can be controllably produced.  The prisms exhibit very low potential for CO2 reduction at the nanoscale and form high yields of CO (~87%), HCOOH (~50% in aq. soln.), as well as C-C coupling products (Figure 1).  These hybrid materials combine multiple functionalities into single particle catalysts and are clearly the direction of the future, especially for tandem reactions and in-line processes.

 

Figure 2. Methodology for
hybrid nanocatalyst development.

Multi-Metallic Satellite Nanocatalysts for C(sp)-CO2 Transformations. Our experience in generating satellite-based, multi-metallic nanoarchitectures such as Au@AgPd and Fe3O4-M (M = Au, Pd, Ag, and PtAg) for electro and photo-chemical reduction of CO2 and nitroaromatics has taught us that these catalytically active sites are potent for small molecule reduction/insertion and alkyne transformations.  Literature precedence also demonstrates the adsorption properties of small molecules (e.g. CO2, NH3) and activation of alkynes on Pd, Cu, Ag (111) and Ni(110) metal facets, and Fe3+/Fe2+ active sites.  This allows for design of nanocatalysts for small molecule alkyne insertions to generate diverse products despite the paucity of published examples.

We are currently refining our surfactant free, hydrothermal method for 3D growth of metallic silver with Fe3O4 octahedra (Fe3O4@Ag) in bulk scale (0.5-0.8 g).  Microscopy (SEM/EDX) analysis and elemental mapping demonstrate formation of 0.5-1 mm Fe3O4@Ag octahedra upon reaction of FeCl3 and AgNO3 (2-40 wt%) in the presence of NaOH in aqueous ethylene glycol at 200 °C for 12 h (Figure 2).  Detailed SEM/EDX characterization of bulk particles as well as microtome and FIB slices illustrate that Ag(0) is present both inside the particle (low [Ag]) and on the octahedral surface facets, corners, and edges (high [Ag]).

Figure 3. Fe3O4@Ag catalyzed CO2 insertion in
alkynes to afford lactone isomer derivatives (top).
Control table for substrate R = H; Ar = Ph (bottom).

With a sound structural characterization of catalyst architecture in hand, our efforts focused on demonstrating the composition/function relationship for the catalyst toward CO2 feedstock insertion into alkynes with consequential cyclization to yield value-added, higher-order lactone products.  Reaction of functionalized phenylacetylenes (Hammett substitutions) with modestly reactive 3-phenylpropargyl chloride in the presence of Fe3O4@Ag with varying Ag (2-40 wt%) amounts at 90 °C for 8 h yields major (43-55%) and minor (15-30%) lactone isomers in up to 85% (Figure 3).  In the presence of just the Fe3O4 Lewis acid component of the catalyst, the reaction generates the diyne coupled product as the major species with a low amounts of the CO2 insertion isomers. Alternatively, if the reaction is performed using just Ag NPs as the active catalyst, the only products detected are the CO2 insertion isomers, but with markedly lower yields under the same reaction conditions.  These critical control experiments suggest that there is a synergistic or at least cooperative effect between Fe3O4 and Ag that contributes to a catalyst system that affords significant reaction yields with good selectivity for the CO2 inserted products under modest conditions.

Figure 4. Proposed mechanism
for lactone formation at Fe3O4@Ag.

These key observations have allowed us to generate a unifying catalytic cycle that accounts for the reactivity observed using the different catalyst components.  Since diyne formation is the dominant, while not exclusive product in the Fe3O4-only condition, this suggests that Fe-acetylide formation with oxidative addition to form the diyne (Figure 4, Path I) is a more facile pathway than CO2 insertion (Figure 4, Path II).  Additionally, the observation that only lactone isomers are detected in Ag NP-catalyzed trials, albeit in low yields, also illuminates the CO2 activation capability of Ag as a key mechanistic determinant. Thus, Fe3O4 and Ag clearly complement each other as catalyst components for lactone isomer formation via CO2 insertion. We propose that Fe3O4 serves as the Lewis acid to form the key Fe-acetylide bond, which may or may not be buffered by electron density from neighboring metallic Ag to enhance the subsequent CO2 insertion step from activated CO2 on Ag. While the presence of metallic Ag alone can carry out both Ag-acetylide and CO2 insertion step, the overall reaction is clearly sluggish but yet exhibits no diyne formation. Thus, the CO2 insertion step must be facile with the slow step most likely involving binding of the anionic phenylacetylide to metallic Ag. In further support of the role of each catalyst component, subsequent SN2 reaction of the catalyst-bound carboxylate with the 3-phenylpropargyl chloride gives the ester, which subsequently cyclizes to the lactone. While catalyst may assist in these last steps, control experiments have shown that reactions proceed efficiently in the absence of catalyst, indicating that the role of the catalyst is paramount only for the initial steps in the transformation.  Therefore, this hybrid catalyst offers complementary Lewis acidic and CO2 activating sites that enhance the rate of the initial steps in the formation of tandem reaction products under moderate conditions.

Related Publications

  1. Development of magnesium oxide–silver hybrid nanocatalysts for synergistic carbon dioxide activation to afford esters and heterocycles at ambient pressure, Upasana Gulati, Ummadisetti Chinna Rajesh, Diwan S. Rawat, and Jeffrey M. Zaleski; Green Chem. (2020), 22, 3170-3177.
  2. Designing Synergistic Nanocatalysts for Multiple Substrate Activation: Interlattice Ag-Fe3O4 Hybrid Materials for CO2-Lactones, Ummadisetti Chinna Rajesh, Yaroslav, Losovyj, Chun-Hsing Chen and Jeffrey M. Zaleski; ACS Catal. (2020), 10, 3349-3359.
  3. The Outliers: Inorganic Radical Reagents for Biological Substrate Degradation, Meghan R. Porter, Joan M. Walker, Jeffrey M. Zaleski, Accts. Chem. Res., (2019), 52(7) 1957-1967.
  4. Site-Selective Growth of AgPd Nanodendrite-Modified Au Nanoprisms: High Electrocatalytic Performance for Carbon Dioxide Reduction, Changsheng Shan, Erin Martin, Dennis G. Peters and Jeffrey M. Zaleski Chem. Mater., (2017), 29, 6030-6043.
  5. A Simple Route to Diverse Noble Metal-Decorated Iron Oxide Nanoparticles for Catalysis, Joan M. Walker, and Jeffrey M. Zaleski, Nanoscale, (2016), 8, 1535-1544.
  6. Expansion and Contraction: Shaping the Porphyrin Boundary via Diradical Reactivity, Leigh J.K. Boerner, David F Dye, Tillmann Köpke, Jeffrey M. Zaleski, Coord. Chem. Rev., (2013), 257(2), 599-620.
  7. PtCl2 Catalyzed Benzannulation of Nickel(II) 2,3-dialkynylporphyrins to Form Unusual Phenanthroporphyrins, Mahendra Nath, Maren Pink, Jeffrey M. Zaleski, Organometallic Chem., (2011), 696(25), 4152-4157.
  8. ZnS quantum-dot based nanocomposit scintillators for thermal neutron detection, C. L. Wang, L. Gou, J. M. Zaleski, D. L. Friesel, Nucl. Instr. and Meth. A., (2010), 622(1), 186-190, doi:10.1016/j.nima.2010.07.032.