Using a newly designed and developed parallelized photoreactor and colorimetric detection method, a large sampling of bimetallic cocatalysts (Pd/Sn, Pd/Mo, Pd/Ru, Pd/Pb, Pd/Ni, Ni/Sn, Mo/Sn, and Pt/Sn) for photocatalytic water reduction have been tested. Of these cocatalysts, the combination of palladium and tin showed the highest synergistic behavior and peak hydrogen gas production at a low relative fraction of palladium. The resulting palladium/tin bimetallic cocatalysts were characterized, and specifically, and scanning transmission electron microscopy energy-dispersive X-ray spectroscopy indicated that palladium and tin elements reside within the same particle. The experimental catalytic activity for the palladium/tin mixture was compared to density functional theory-derived energy values associated with the adsorption of hydrogen onto a surface. This comparison demonstrated that the typical peak found in electrochemical Sabatier volcano plots at ΔG H* = ∼0 eV were replicated in the experimental photocatalytic system with a peak activity observed at ΔGH* = −0.036 eV. Computational confirmation of the results expressed here demonstrates the efficacy of colorimetric detection of hydrogen in parallel and presents a model for increasingly rapid catalyst screening.

Noble-metal photosensitizers and water reduction co-catalysts (WRCs) still present the highest activity in homogeneous photocatalytic hydrogen production. The search for earth-abundant alternatives is usually limited by the time required to screen new catalyst combinations; however, here, we utilize newly designed and developed high-throughput photoreactors for the parallel synthesis of novel WRCs and colorimetric screening of hydrogen evolution. This unique approach allowed rapid optimization of photocatalytic water reduction using the organic photosensitizer Eosin Y and the archetypal cobaloxime WRC [Co­(GL1)2pyCl], where GL1 is dimethylglyoxime and py is pyridine. Subsequent combinatorial synthesis generated 646 unique cobalt complexes of the type [Co­(LL)2pyCl], where LL is a bidentate ligand, that identified promising new WRC candidates for hydrogen production. Density functional theory (DFT) calculations performed on such cobaloxime derivative complexes demonstrated that reactivity depends on hydride affinity. Alkyl-substituted glyoximes were necessary for hydrogen production and showed increased activity when paired with ligands containing strong hydrogen-bond donors.

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One consistent challenge of both computational and empirical catalyst screening is ensuring that the variables chosen for the screen are driving the performance analyzed. Here, we compare photocatalytic hydrogen evolution from in situ formed Au and Au/Cu nanoparticles to nanoparticles of the same composition synthesized prior to being used as catalysts, as well as compare them to in situ formed Au and Au/Cu nanoparticles in the presence of exogeneous ligand. For all experiments, we observed that ligand-terminated nanoparticles performed better than un-stabilized in situ formed nanoparticles. We tested the generality of this result by studying Co, Ni, and Pd in the same system and also observed that the introduction of nanoparticle ligand leads to enhanced catalytic activity. Taken together, these results suggest 1) nanoparticle ligands can be beneficial, and even necessary, to produce catalysts with sustained activity 2) For computational prediction of these catalysts, factors relating to particle formation and stability need to be considered when both generating predictions as well as interpreting experimental results based on those predictions.

Photocatalytic hydrogen (H2) evolution, particularly though water reduction, presents an enticing alternative to current fossil fuel intensive methods of hydrogen production. While this field has been active for decades and advances have been made, it has been limited to serial experimentation due to the solar-mimicking lamps used and data collection techniques. With the democratization of machine and instrument design through the ever-decreasing prices of computers and sensing equipment, paired with the availability of high-power light emitting diodes (LEDs) as viable replacement light sources, reactor design has seen significant changes in recent years. After the advent of the first LED-illuminated parallel reactors for gas evolving photocatalytic reactions, a host of research groups around the world have matched the design and used their own creative means of studying H2 evolving reactions in parallel. Here we report select cases of research utilizing parallelized reactors for light-driven H2 evolution, highlighting the benefits of parallel and high-throughput experimentation. Lastly, changes to reactor design and sensing methodology, specifically how colorimetric measurement enabled the development of a 108-well parallelized reactor, are described, and these state-of-the-art reactors are compared to alternative, nonparallelized approaches using serial, robotic automation.