131. Z. Zhao, K.N. Heck, P. Limpornpipat, H. Qian, J.T. Miller, M.S. Wong, "Hydrogen-generating behavior of Pd-decorated gold nanoparticles via formic acid decomposition" Catalysis Today (2019) DOI:10.1016/j.cattod.2018.06.044
Formic acid is a promising hydrogen storage material where hydrogen is generated via metal-catalyzed decomposition. Bimetallic catalysts are active for this reaction, but the mechanism has not been fully proven. Palladium metal supported on gold nanoparticles (Pd-on-Au NPs) has structural properties that are advantageous for studying aqueous-phase catalytic reactions. In this work, a series of Pd-on-Au NPs of varying Pd loadings (calculated in terms of Pd surface coverage, sc%) were synthesized, immobilized onto carbon, and studied for formic acid decomposition at room temperature. Pd-on-Au NPs were catalytically active, with a reaction rate constant as high as 137 mL-H2/gPd/min (corresponding to an initial turnover frequency TOF of 123 h-1) at a Pd loading of 300 sc%. In contrast, Au NPs were inactive, and Pd NPs were slightly active (5 mL-H2/gPd/min and TOF of 38 h-1). The Pd metal of Pd-on-Au catalysts are partially oxidized, and is readily reduced without changing the metal-on-metal structure during reaction, according to in situ x-ray adsorption spectroscopy measurements. CO formation was inhibited at a Pd loading of 300 sc%, suggesting that three-dimensional Pd ensembles favored the desired dehydrogenation pathway while single-atom and small two-dimensional Pd ensembles are active for the undesired dehydration pathway.
130. Y.B. Yin, K.N. Heck, C.L. Coonrod, C.D. Powell, S. Guo, M.A. Reynolds, and M.S. Wong, "PdAu-catalyzed oxidation through in situ generated H2O2 in simulated produced water" Catalysis Today (2019) DOI:10.1016/j.cattod.2019.05.001
Most wastewater recovered from hydraulically fractured oil and gas wells (i.e. produced water) is transported to government-permitted, salt-water disposal units (SWDs) and discarded via downhole injection. However, there is a limited availability of disposal wells in some states and growing interest over future options for beneficial reuse. One alternative to using SWD facilities is to recycle the water for further use in oilfield operations. Residual oil and grease are one contaminant class in produced water where cost-effective treatment technologies are lacking. In this work, we studied the ability of alumina-supported bimetallic PdAu to degrade organic compounds at room temperature and atmospheric pressure via the catalytic formation of H2O2. Similar to monometallic Pd and Au catalysts, the PdAu catalyst produced H2O2 and hydroxyl radicals in the presence of oxygen and formic acid. The bimetallic catalyst was the most active in terms of initial OH formation rate, and when phenol was present, PdAu showed the highest rate of phenol degradation. We assessed the promotional and inhibitory effects of other species present in produced water including ferrous ion concentration, pH and salt concentration on catalytic phenol oxidation. PdAu was catalytically active for phenol degradation in simulated produced water at salinities as high as ˜0.3 M (˜16,000 ppm). The combination of air-formic acid-bimetallic catalyst is an intriguing approach for the degradation of organics in contaminated water at low pH and moderate salinity.
129. Y.B. Yin, C.L. Coonrod, K.N. Heck, F. Lejarza, and M.S. Wong, "Microencapsulated Photoluminescent Gold for ppb-level Chromium(VI) Sensing" ACS Applied Materials and Interfaces (2019) DOI:10.1021/acsami.9b04699
Luminescent gold nanoclusters (Au NCs) are a promising probe material for selective chemical sensing. However, low luminescent intensity and an incomplete understanding of the mechanistic origin of the luminescence limit their practical implementation. We induced glutathione-capped Au NCs to aggregate within silica-coated microcapsular structures using polymer-salt aggregate (PSA) self-assembly chemistry. The encapsulated NCs have a 5× luminescence enhancement compared to free Au NCs, and can detect Cr(VI) at concentrations as low as 6 ppb (= 0.12 µM CrO42-) through luminescence quenching, compared to free Au NCs which have a limit of detection (LOD) of 52 ppb (= 1 μM CrO42-). The LOD is 16× lower than the US EPA maximum contaminant level for total chromium (Cr(III) + Cr(VI), 100 ppb) in drinking water. No pH adjustment is needed using the encapsulated Au NCs, unlike the case for free Au NCs. The luminescent microcapsule material can sense Cr(VI) in simulated drinking water with a ~20-30 ppb LOD, serving as a possible basis for a practical Cr(VI) sensor.
128. T. Zhang, G.V. Lowry, N.L. Capiro, J. Chen, W. Chen, Y. Chen, D.D. Dionysiou, D.W. Elliott, S. Ghoshal, T. Hofmann, H. Hsu-Kim, J. Hughes, C. Jiang, G. Jiang, C. Jing, M. Kavanaugh, Q. Li, S. Liu, J. Ma, B. Pan, T. Phenrat, X. Qu, X. Quan, N. Saleh, P.J. Vikesland, Q. Wang, P. Westerhoff, M.S. Wong, T. Xia, B. Xing, B. Yan, L. Zhang, D. Zhou, and P.J.J. Alvarez, "In situ remediation of subsurface contamination: opportunities and challenges for nanotechnology and advanced materials" Enviornmental Science: Nano (2019) DOI:10.1039/C9EN00143C
Complex subsurface contamination domains and limited efficacy of existing treatment approaches pose significant challenges to site remediation and underscore the need for technological innovation to develop cost-effective remedies. Here, we discuss opportunities for nanotechnology-enabled in situ remediation technologies to address soil and groundwater contamination. The discussion covers candidate nanomaterials, applications of nanomaterials to complement existing remediation approaches and address emerging contaminants, as well as the potential barriers for implementation and strategies and research needs to overcome these barriers. Promising nanomaterials in subsurface remediation include multi-functional nanocomposites for synergistic contaminant sequestration and degradation, selective adsorbents and catalysts, nano-tracers for subsurface contaminant delineation, and slow-release reagents enabled by stimuli-responsive nanomaterials. Limitations on mixing and transport of nanomaterials in the subsurface are severe constraints for in situ applications of these materials. Mixing enhancements are needed to overcome transport limitations in laminar flow environments. Reactive nanomaterials may be generated in situ to remediate contamination in low hydraulic conductivity zones. Overall, nano-enabled remediation technologies may improve remediation performance for a broad range of legacy and emerging contaminants. These technologies should continue to be developed and tested to discern theoretical hypotheses from feasible opportunities, and to establish realistic performance expectations for in situ remediation techniques using engineered nanomaterials alone or in combination with other technologies.
127. C.A. Clark, K.N. Heck, C.D. Powell, and M.S. Wong, "Highly Defective UiO-66 Materials for the Adsorptive Removal of PFOS" ACS Sustainable Chemistry & Engineering (2019) DOI:10.1021/acssuschemeng.8b05572 (Virtual Special Issue)
Perflorooctane sulfonate (PFOS) is a persistent organic pollutant that is bioaccumulative and toxic. While its use in most countries has been restricted to certain industrial applications due to environmental and health concerns, chrome plating and semiconductor manufacturing facilities are industrial point sources of PFOS-containing wastewater. Current remediation technologies are ineffective at treating these highly concentrated industrial effluents. In this work, UiO-66 metal-organic frameworks (MOFs) of several defect concentrations were studied as sorbents for the removal of PFOS from concentrated aqueous solutions. PFOS sorption isotherms indicated that defective UiO-66, prepared with HCl as a modulator, had a maximum Langmuir sorption capacity of 1.24 mmol/g, which was ~2× greater than powdered activated carbon (PAC), but ~2× less than that of a commercial ion exchange resin. Defective UiO-66 adsorbed PFOS two orders of magnitude faster than the ion exchange resin. Large pore defects (~16 and ~20 Å) within the framework were critical to the increased adsorption capacity due to higher internal surface area and an increased number of coordinatively unsaturated Zr sites to bind the PFOS head groups. Of the common co-contaminants in chrome plating wastewaters, chloride ions have a negligible effect on PFOS sorption, while sulfate and hexavalent chromium anions compete for cationically charged adsorption sites. These materials were also effective adsorbents for the shorter-chain homologue, perfluorobutane sulfonate (PFBS). The enhanced PFOS and PFBS adsorptive properties of UiO-66 highlight the advantage of structurally defective MOFs as a water treatment approach towards environmental sustainability.
Fresh water demand is driven by human consumption, agricultural irrigation, and industrial usage and continues to increase along with the global population. Improved methods to inexpensively and sustainably clean water unfit for human consumption are desired, particularly at remote or rural locations. Heterogeneous catalysts
offer the opportunity to directly convert toxic molecules in water to nontoxic products. Heterogeneous catalytic reaction processes may bring to mind large-scale industrial production of chemicals, but they can also be used at the small scale, like catalytic converters used in cars to break down gaseous pollutants from fuel combustion. Catalytic
processes may be a competitive alternative to conventional water treatment technologies. They have much faster kinetics and are less operationally sensitive than current bioremediation-based methods. Unlike other conventional water treatment technologies (i.e., ion exchange, reverse osmosis, activated carbon filtration), they do not transfer
contaminants into separate, more concentrated waste streams.
In this Account, we review our efforts on the development of heterogeneous catalysts as advanced reduction technologies to treat toxic water contaminants such as chlorinated organics and nitrates. Fundamental understanding of the underlying chemistry of catalytic materials can inform the design of superior catalytic materials. We discuss the impact of the catalytic structure (i.e., the arrangement of metal atoms on the catalyst surface) on the catalyst activity and selectivity for these aqueous reactions. To explore these aspects, we used model metal-on-metal nanoparticle catalysts along with state-of-the-art in situ spectroscopic techniques and density functional theory calculations to deduce the catalyst surface structure and how it affects the reaction pathways and hence the activity and selectivity. We also discuss recent developments in photocatalysis and electrocatalysis for the treatment of nitrates, touching on fundamentals and surface reaction mechanisms.
Finally, we note that despite over 20 years of growing research into heterogeneous catalytic systems for water contaminants, only a few pilot-scale studies have been conducted, with no large-scale implementation to date. We conceive of modular, on- or off-grid catalytic units that treat drinking water at the household tap, at a community well, or for larger-scale reuse of agricultural runoff. We discuss how these may be enhanced by combination with photocatalytic or electrocatalytic processes and how these reductive catalytic modules (catalytic converters for water) can be coupled with other modules for the removal of potential water contaminants.