McGill Chemistry Students Invited Lecture: Emily A. Weiss - Visible-Light Photocatalysis within Dispersions of Colloidal Quantum Dots

The advancement of next generation catalysts that operate at lower temperatures, in ambient
conditions and that utilize renewable energy sources such as sunlight is of paramount importance to
our society since the chemical industry, which depends on catalysts for most reactions, is a large energy
consumer in many countries (for example, the chemical industry consumes ~7% of the energy
produced in the US). Heterogeneous photocatalysts benefit from good photo-stability, and high
absorption coefficients at the band-edge, and they are often made from relatively cheap materials that
are easy to synthesize into the desired crystal structure and surface chemistry. Quantum dots (QDs)
have the potential to combine these benefits of heterogeneous catalysts with the solution dispersability
of homogeneous catalysts. This paper describes the visible-light photocatalysis of the proton-coupled
six-electron reduction of nitrobenzene to aniline at room temperature within dispersions of CdS
quantum dots (QDs) in 80:20 water:methanol. The QDs act as direct photocatalysts – there is no
intervening molecular catalyst present. Over 54 hours of illumination with visible light at pH 2.6, each
QD transfers 4.5x10⁶ electrons which reduces 8.3x10⁵ nitrobenzene molecules to aniline and the other
isolable intermediate, phenylhydroxylamine. 3-mercaptopropionic acid serves as (i) a solubilizing ligand
for the QDs, (ii) the terminal reductant that regenerates the QD catalyst and (iii) the proton donor in
each reduction step. If left unprotonated, aniline adsorbs to the QD surface and limits the reaction rate
and yield by inhibiting electron transfer to nitrobenzene. The activity (electrons transferred x g catalyst-
1 x J photons-1) of the high-surface-area QDs used in this study is a factor of 2.5x10⁴ greater than that
previous reported for CdS micropowder.