Seog Joon Yoon

WGS Catalysis and In Situ Studies of CoO1–x, PtCon/Co3O4, and PtmCom′/CoO1–x Nanorod Catalysts

Water-gas shift (WGS) reactions on Co3O4 nanorods and Co3O4 nanorods anchoring singly dispersed Pt atoms were explored through building correlation of catalytic performance to surface chemistry of catalysts during catalysis using X-ray absorption spectroscopy, ambient pressure X-ray photoelectron spectroscopy (AP-XPS), and environmental TEM. The active phase of pure Co3O4 during WGS is nonstoichiometric cobalt monoxide with about 20% oxygen vacancies, CoO0.80. The apparent activation energy (Ea) in the temperature range of 180-240 °C is 91.0 ± 10.5 kJ mol(-1). Co3O4 nanorods anchoring Pt atoms (Pt/Co3O4) are active for WGS with a low Ea of 50.1 ± 5.0 kJ mol(-1) in the temperature range of 150-200 °C. The active surface of this catalyst is singly dispersed Pt1Co(n) nanoclusters anchored on Co3O4 (Pt1/Co3O4), evidenced by in situ studies of extended X-ray absorption fine structure spectroscopy. In the temperature range of 200-300 °C, catalytic in situ studies suggested the formation of Pt(m)Co(m) nanoclusters along with the reduction of Co3O4 substrate to CoO(1-x). The new catalyst, Pt(m)Co(m)/CoO(1-x) is active for WGS with a very low Ea of 24.8 ± 3.1 kJ mol(-1) in the temperature range of 300-350 °C. The high activity could result from a synergy of Pt(m)Co(m) nanoclusters and surface oxygen vacancies of CoO(1-x).

How Lead Halide Complex Chemistry Dictates the Composition of Mixed Halide Perovskites

Varying the halide ratio (e.g., Br(-):I(-)) is a convenient approach to tune the bandgap of organic lead halide perovskites. The complexation between Pb(2+) and halide ions is the primary step in dictating the overall composition, and optical properties of the annealed perovskite structure. The complexation between Pb(2+) and Br(-) is nearly 7 times greater than the complexation between Pb(2+) and I(-), thus making Br(-) a dominant binding species in mixed halide systems. Emission and transient absorption measurements show a strong dependence of excited state behavior on the composition of halide ions employed in the precursor solution. When excess halide (X = Br(-) and I(-)) are present in the precursor solution (0.3 M PbX2 and 0.9 M CH3NH3X), the exclusive binding of Pb(2+) with Br(-) results in the formation of CH3NH3PbBr3 perovskites as opposed to mixed halide perovskite.

Rationalizing the light-induced phase separation of mixed halide organic–inorganic perovskites

Mixed halide hybrid perovskites, CH3NH3Pb(I1−xBrx)3, represent good candidates for low-cost, high efficiency photovoltaic, and light-emitting devices. Their band gaps can be tuned from 1.6 to 2.3u2009eV, by changing the halide anion identity. Unfortunately, mixed halide perovskites undergo phase separation under illumination. This leads to iodide- and bromide-rich domains along with corresponding changes to the material’s optical/electrical response. Here, using combined spectroscopic measurements and theoretical modeling, we quantitatively rationalize all microscopic processes that occur during phase separation. Our model suggests that the driving force behind phase separation is the bandgap reduction of iodide-rich phases. It additionally explains observed non-linear intensity dependencies, as well as self-limited growth of iodide-rich domains. Most importantly, our model reveals that mixed halide perovskites can be stabilized against phase separation by deliberately engineering carrier diffusion lengths and injected carrier densities.Mixed halide hybrid perovskites possess tunable band gaps, however, under illumination they undergo phase separation. Using spectroscopic measurements and theoretical modelling, Draguta and Sharia et al. quantitatively rationalize the microscopic processes that occur during phase separation.

Preferential Oxidation of CO in H2 on Pure Co3O4−x and Pt/Co3O4−x

Driven by the development of a catalyst made of earth‐abundant elements for on‐board purification of H2 of this energy conversion technology, preferential oxidation (PROX) on pure Co3O4 nanorods and Co3O4 nanorods with supported Pt nanoparticles was explored with the aid of inu2005situ studies. This catalyst remains its 100u2009% conversion of CO in H2 at a gas hourly space velocity of 42u2009857u2005mLu2009h−1u2009g−1 at 120u2009°C for at least 96u2005h. Inu2005situ studies showed that the active surface phase during PROX is nonstoichiometric Co3O4−x. A correlation between density of surface oxygen vacancies and conversion of CO to CO2 suggest that oxygen vacancy is a necessary component of a catalytic site for PROX on Co3O4−x. Compared to pure Co3O4 nanorods, anchoring Pt nanoparticles on Co3O4 nanorods unfortunately increases selectivity for oxidation of H2 owing to the low dissociation barrier of molecular H2 on Pt. Co3O4−x exhibits much higher selectivity for CO oxidation in PROX than Pt/Co3O4−x at a temperature lower than 140u2009°C.

Author Correction: Rationalizing the light-induced phase separation of mixed halide organic–inorganic perovskites

The original version of this Article contained an error in the spelling of the author Joseph S. Manser, which was incorrectly given as Joseph M. Manser. This has now been corrected in both the PDF and HTML versions of the Article.