Branko Zugic

The oxidative dehydrogenation of cyclohexane by cobalt oxide nanoparticles was studied via temperature programmed reaction combined with in situ grazing incidence X-ray absorption spectroscopy and grazing incidence small-angle X-ray scattering and theoretical calculations on model Co3O4 substrates. Both 6 and 12 nm Co3O4 nanoparticles were made through a surfactant-free preparation and dispersed on an Al2O3 surface formed by atomic layer deposition. Under reaction conditions the nanoparticles… Show more

The oxidative dehydrogenation of cyclohexane by cobalt oxide nanoparticles was studied via temperature programmed reaction combined with in situ grazing incidence X-ray absorption spectroscopy and grazing incidence small-angle X-ray scattering and theoretical calculations on model Co3O4 substrates. Both 6 and 12 nm Co3O4 nanoparticles were made through a surfactant-free preparation and dispersed on an Al2O3 surface formed by atomic layer deposition. Under reaction conditions the nanoparticles retained their oxidation state and did not sinter. They instead underwent an assembly/disassembly process and could reorganize within their assemblies. The selectivity of the catalyst was found to be size- and temperature-dependent, with larger particles preferentially producing cyclohexene at lower temperatures and smaller particles predominantly resulting in benzene at higher temperatures. The mechanistic features thought to control the oxidative dehydrogenation of cyclohexane and other light alkanes on cobalt oxide were established by carrying out density functional theory calculations on the activation of propane, a surrogate model alkane, over model Co3O4 surfaces. The initial activation of the alkane (propane) proceeds via hydrogen abstraction over surface oxygen sites. The subsequent activation of the resulting alkoxide intermediate occurs at a second surface oxygen site to form the alkene (propene) which then desorbs from the surface. Hydroxyl recombination results in the formation of water which desorbs from the surface. Oxygen is necessary to regenerate the surface oxygen sites, catalyze C–H activation steps, and minimize catalyst degradation. Show less

In the present study we have found that gold supported on CeO2 and ZnO with well-defined crystal structures or shapes is an excellent catalyst for the low-temperature (175−225 °C) steam reforming of methanol (SRM). We have compared the active nature of gold dispersed on {0001} surfaces of ZnO nanorods and polyhedra to our previous work, which demonstrates that TEM-invisible gold dispersed on the {110} surfaces of CeO2 catalyzes the SRM reaction in a cooperative mechanism with CeO2. Similar to… Show more

In the present study we have found that gold supported on CeO2 and ZnO with well-defined crystal structures or shapes is an excellent catalyst for the low-temperature (175−225 °C) steam reforming of methanol (SRM). We have compared the active nature of gold dispersed on {0001} surfaces of ZnO nanorods and polyhedra to our previous work, which demonstrates that TEM-invisible gold dispersed on the {110} surfaces of CeO2 catalyzes the SRM reaction in a cooperative mechanism with CeO2. Similar to Au−CeO2, we have found that Au−O bonds are essential species for SRM over Au−ZnO and the same apparent activation energy (110−120 kJ·mol−1) of the reaction was calculated for both catalysts. On the basis of temperature-programmed surface reaction/mass spectrometry analysis, we determined that the SRM reaction on both Au−ZnO and Au−CeO2 involves methanol dehydrogenation, methyl formate hydrolysis, and formic acid decomposition steps to produce CO2 and H2. Better than 95% catalyst selectivity to CO2 was found over the temperature range from 175 to 250 °C for both catalysts. In the presence of methanol, the water−gas shift reaction is suppressed and is not part of the mechanism at temperatures below 250 °C. The SRM stability of the Au−ZnO and Au−CeO2 systems is good for practical application of this type catalyst. Show less

Crystallization experiments have been carried out in microfluidic devices to screen for polymorphs by crystallization on a range of surfaces. These devices consist of PDMS (polydimethylsiloxane) patterned with microchannels and then bonded to self-assembled monolayers (SAMs) of organic molecules on gold substrates. Barbital was crystallized in microchannels over five different SAMs functionalized with polar and nonpolar organic groups. Growth of polymorphs was examined under thermodynamic… Show more

Crystallization experiments have been carried out in microfluidic devices to screen for polymorphs by crystallization on a range of surfaces. These devices consist of PDMS (polydimethylsiloxane) patterned with microchannels and then bonded to self-assembled monolayers (SAMs) of organic molecules on gold substrates. Barbital was crystallized in microchannels over five different SAMs functionalized with polar and nonpolar organic groups. Growth of polymorphs was examined under thermodynamic conditions from solutions at room temperature and under kinetic conditions by rapid cooling. The results of these experiments and the influence of chemically modified surfaces in microchannels in controlling polymorphism are discussed. Show less