Publications

The synthesis of nanomaterials with well-controlled morphologies and surface orientations has opened new avenues towards increasing catalytic performance and understanding of fundamental catalytic pathways. Here, we illustrate how tailoring surface orientations of Co3O4 catalysts on the nanoscale results in control over catalytic performance via the preferential formation of active surface species during CO2 hydrogenation. This results in a significant increase in the methane yield on Co3O4 nanorods, as opposed to conventional nanoparticle catalysts, where Co3O4 nanorods inhibit the formation of formate spectator species via the preferential formation of bridged CO as an intermediate species. This design approach provides a new dimension for the development of next generation catalysts and opens new, more efficient strategies for the conversion of carbon dioxide into useful hydrocarbons.

Passive selective catalytic reduction (SCR) is a promising approach for the control of NOX emissions in lean burn gasoline exhausts. It requires ammonia (NH3) to be produced over a three-way catalyst (TWC) during periods of fuel-rich operation for the reduction of NOX during periods of fuel-lean operation. Previous research has shown the viability of this system but has not examined the effects of hydrothermal degradation. This work is focused on evaluating the effects of hydrothermal aging on the TWC in a passive SCR system. Two catalysts were studied: a Pd-TWC, and a NOX storage and reduction (NSR) TWC. Samples were aged at 920 °C for 100 h using a four-mode hydrothermal aging procedure. This causes the catalyst to be oxidized and reduced, as it would in a real system. The effects of aging were evaluated using simulated exhaust under both steady state and lean-rich cycling conditions. Hydrothermal aging caused significant changes in catalyst activity, leading to a decrease in low temperature conversion of carbon monoxide (CO) and propane (C3H8) on both catalysts, and degradation of oxygen storage and NOX storage components. However, the catalysts maintained their activity for NOX conversion and NH3 production, showing sufficient activity for the operation of a passive SCR with an optimum projected fuel consumption of 92–98% compared to stoichiometric operation.

Corn residue pellets were torrefied with wet flue gas, simulated by steam (0–21% v/v), CO2 (12% v/v), and O2 (4% v/v), balanced with N2 as reactive gas, in a fixed bed reactor at 260 °C of temperature and at 10–40 min of residence time. The distribution and yields of torrefied pellets, liquid, and gas products were examined. The microstructural changes of torrefied pellets were evaluated by Raman spectroscopy and scanning electron microscopy, while the components of gas products were analyzed by mass spectrometry. Residence time and steam concentration in the reactive gas were found to have significant effects on the products yield distribution, the porosity of the torrefied pellets, and the concentrations of CO, CH4, H2, and CO2 in the gas products. At high steam concentrations, the decomposition reaction of hemicellulose and lignin in the raw pellets, and the formation of the graphene structures in torrefied pellets occurred faster.

Tri-reforming of methane (TRM) produces syngas by directly utilizing flue gas from a fossil fuel-fired power plant without requiring post-combustion CO2 separation. In this work, different yolk sizes of a NiCe@SiO2 multi–yolk–shell nanotube catalyst were prepared and their catalytic properties were evaluated at different oxidizer (CO2 + H2O + O2) to methane (O/M) feed ratios for TRM. The NiCe@SiO2 multi–yolk–shell nanotube catalyst can exhibit longer stability than the conventional NiCe/SiO2Imp catalyst synthesized by impregnation method due to its controlled morphology and synergetic interactions of Ni–Ce and Ni–Si species. At a low O/M feed ratio of 1.0, NiCe@SiO2 with smaller yolks (< 20 nm) shows higher resistance to carbon deposition than NiCe@SiO2 with larger yolks due to the facile oxidation of carbon. On the other hand, NiCe@SiO2 with larger yolks (> 30 nm) presents stable TRM activity at a high O/M feed ratio of 1.1, whereas NiCe@SiO2 consisting of smaller yolks deactivates. Deactivation of NiCe@SiO2 with smaller yolks can be explained by the re-oxidation of active Ni species, in which carbon formation and oxidation rates, and Ce3+/Ce4+ redox properties play a crucial role. Our results indicate that the NiCe@SiO2 multi–yolk–shell nanotube structures can provide high TRM activity, yet their structure should be tuned for stable performance by considering the yolk sizes and interaction of Ni–Ce species.

In this work, a statistical design and analysis platform was used to develop cobalt oxide based oxidation catalysts prepared via one pot metal salt reduction. An emphasis was placed upon understanding the effects of synthesis conditions, such as heating regimen and Co2+ concentration on the metal salt reduction mechanism, the resultant nanomaterial properties (i.e., size, crystal structure, and crystal faceting), and the catalytic activity in CO oxidation. This was accomplished by carrying out XRD, TEM, and FTIR studies on synthesis intermediates and products. Additionally, high-throughput experimentation was employed to study the performance of Co3O4 oxidation catalysts over a wide range of reaction conditions using a 16-channel fixed bed reactor equipped with a parallel infrared imaging system. Specifically, Co3O4 nanomaterials of varying properties were evaluated for their performance as CO oxidation catalysts. Figure-of-merits including light-off temperatures and activation energies were measured and mapped back to the catalyst properties and synthesis conditions. Statistical analysis methods were used to elucidate significant property-activity relationships as well as the design rules relevant in the synthesis of active catalysts. It was found that the degree of grain boundary consolidation and anisotropic growth in fcc and hcp CoO intermediates significantly influenced the catalytic activity. By utilizing the discovered synthesis-structure-activity relationships, CO oxidation light off temperatures were decreased to < 90°C.

Deoxydehydration (DODH) is an emerging biomass deoxygenation process whereby vicinal OH groups are removed. Based on DFT calculations and microkinetic modeling, we seek to understand the mechanism of the Re-catalyzed deoxydehydration supported on CeO2(111). In addition, we aim at understanding the promotional effect of Pd in a heterogeneous ReOx-Pd/CeO2 DODH catalyst system. We disentangle the contribution of the oxide support, the oxide-supported single ReOx species, and a co-adsorbed Pd promoter that has no direct interaction with the Re species. In the absence of a nearby Pd cluster, a Re site is able to reduce subsurface Ce-ions of a hydroxylated CeO2(111) surface, leading to a catalytically active Re +6 species. The effect of Pd is twofold: (i) Pd catalyzes the hydrogen dissociation and spillover onto CeO2, which is an indispensable process for the regeneration of the Re catalyst, and (ii) Pd adsorbed in close proximity to Re on CeO2(111) facilitates the oxidation of Re to a +7 oxidation state, which leads to an even more active Re species than the Re +6 site present in the absence of Pd. The latter promotional effect of Pd (and change in oxidation state of Re) disappears with increasing Pd-Re distance and in the presence of oxygen defects on the ceria support. Under these conditions, the ReOx-Pd/CeO2 catalyst system exhibits appreciable activity consistent with recent experiments. The established mechanism and role of various species in the catalyst system help to better understand the deoxydehydration catalysis. Also, the importance of the Re oxidation state and the identified oxidation state modification mechanisms suggest a new pathway for tuning the properties of metal-oxide supported catalysts.

The catalytic performance of Mo8V2Nb1-based mixed-oxide catalysts for ethane partial oxidation is highly sensitive to the doping of elements with redox and acid functionality. Specifically, control over product distributions to ethylene and acetic acid can be afforded via the specific pairing of redox elements (Pd, Ni, Ti) and acid elements (K, Cs, Te) and the levels at which these elements are doped. The redox element, acid element, redox/acid ratio, and dopant/host ratio were investigated using a three-level, four-factor factorial screening design to establish relationships between catalyst composition, structure, and product distribution for ethane partial oxidation. Results show that the balance between redox and acid functionality and overall dopant level is important for maximizing the formation of each product while maintaining the structural integrity of the host metal oxide. Overall, ethylene yield was maximized for a Mo8V2Nb1Ni0.0025Te0.5 composition, while acetic acid yield was maximized for a Mo8V2Nb1Ti0.005Te1 catalyst.

The high activity observed on Pd impregnated MnOx-CeO2 solid solution catalysts for low temperature CO oxidation is investigated through in situ extended X-ray absorption fine structure (EXAFS) and diffuse reflectance infrared Fourier transform spectroscopy (DRIFTS) experiments. The change in the Pd local structure on CeO2 and MnOx-CeO2 is studied to identify the role of oxidized Pd nanoparticles during CO oxidation. EXAFS analysis of the calcined samples confirms the formation of PdO structures on CeO2 and MnOx-CeO2 supports. The structural model applied to the Ce1-xPdxO2-$δ$ interaction phase could not predict the second and third near-neighbor coordination shells of Pd. Sintering and re-dispersion of Pd is observed on CeO2 during H2 reduction and subsequent oxidation with air. During CO oxidation, PdO species are reduced by CO on CeO2, forming a mixture of Pdn+/Pd0 species. These reduced Pd particles can be re-oxidized and re-dispersed on the CeO2 surface forming larger PdO crystallites. In the case of Pd/MnOx-CeO2, Pdn+ species can be stabilized during the reaction and no obvious Pd0 formation could be detected. Due to the formation of similar PdO species after CO oxidation on both CeO2 and MnOx-CeO2 supports, the different low temperature CO oxidation activities can be associated with the oxygen storage properties and oxygen mobility of the support.