Abstract
Strong Electrostatic Adsorption (SEA) has been demonstrated as a simple, scientific method to prepare well dispersed Pt nanoparticles over typical forms of carbon: activated, black, and graphitic carbons. Many varieties of specialty carbons have been invented in the last few decades including multi-walled nanotubes, nanofibers, graphene nanoplatelets, etc. In this work, we explore whether SEA can be applied to these specialty carbons for the synthesis of Pt nanoparticles. Over a number of oxidized and unoxidized multiwalled nanotubes and nanofibers, the point of zero charge (PZC) was measured and the uptake of anionic Pt complexes (Pt hexachloride, [PtCl6]2−, and cationic Pt complexes (platinum tetraammine, [Pt(NH3)4]2+) as functions of final pH were surveyed. Pt nanoparticles on the various supports were synthesized at the optimal pH and were characterized by x-ray diffraction (XRD) and scanning transmission electron microscopy (STEM). The specialty carbons displayed volcano-shaped uptake curves typical of electrostatic adsorption for both Pt anions at low pH and Pt cations at high pH. However, the regimes of uptake often did not correspond to the measured PZC, probably due to surface impurities from the carbon manufacturing process. This renders the measured PZC of these specialty carbons unreliable for predicting anion and cation uptake. On the other hand, the anion and cation uptake curves provide an “effective” PZC and do indicate the optimal pH for the synthesis of ultrasmall nanoparticle synthesis. High resolution STEM imaging also showed that with SEA it is possible to disperse nanoparticles on the surface as well as the inner walls of the specialty carbons.
Highlights
Carbon materials in the form of nanofibers and nanotubes have great potential as catalyst supports owing to their high specific surface area, mechanical strength, and flexibility [1,2,3] and have been increasingly used as such [4]
With a variety of specialty carbons, comprised of multi-walled carbon nanotubes and nanofibers with varying surface functional groups and orientation of graphene sheets, we demonstrate that Strong Electrostatic Adsorption (SEA) can be extended for anionic and cationic metal precursor adsorption over carbons of all types
The carbons used in the study as listed in Table 1: for multi-walled carbon nanotube (MWCNT) I (OD × ID × L: 10 nm × 4.5 nm × (3–4) micrometer) and HB nanofibers were obtained from Sigma-Aldrich, Saint Louis, MO, USA and the other carbons: MWCNT II (OD × ID × L:
Summary
Carbon materials in the form of nanofibers and nanotubes have great potential as catalyst supports owing to their high specific surface area, mechanical strength, and flexibility [1,2,3] and have been increasingly used as such [4]. The most common method of preparation involves reductive deposition of soluble Pt precursors using ethylene glycol or sodium borohydride as the reducing agent [5,6,7,8,9,10,11,12,13,14,15,16]. These methods generally produce particles around 3–5 nm in diameter. A host of other methods including as simple as incipient wetness and as complicated as radio frequency magnetron sputtering, have yielded nanoparticles in the range of 2–5 nm [21,22,23,24,25,26,27], with the smallest stemming from electrodeposition (1.5 nm) [26], and atomic layer deposition (2 nm) [22]
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