Publications

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.

The low temperature CO oxidation activity of Pd doped MnOx-CeO2 (MC) solid solution catalyst is evaluated to determine the role of Pd speciation and oxygen transfer. Dynamic reduction behavior of Pd2+ species in the presence of CO at low temperatures is characterized by in-situ diffuse reflectance infrared Fourier transform spectroscopy (DRIFTs) and X-ray absorption spectroscopy (XAS). H2 temperature-programmed reduction (TPR) and CO/O2 transient pulse experiments are conducted to evaluate the lattice oxygen reducibility of Pd/CeO2 and Pd/MC. Our results show that highly dispersed PdO species form on the freshly calcined CeO2 and MC supports. Despite the fact that similar Pd species form on the CeO2 and MC supports, PdO can be reduced immediately on CeO2 in the presence of CO, while the MC support can preserve the oxidized Pd species during CO reduction. In the case of CO oxidation, Pd2+ species are maintained through lattice oxygen transfer facilitated by the MC support. CO/O2 transient pulse experiments confirm the higher reducibility of the MC support, which can favor CO oxidation at 50°C.

We present a simple, though uncommonly used, method to produce versatile, well-ordered, nanoscale arrays of metal oxides such as MgO, Al2O3, TiO2, MnO2, Fe2O3, Co3O4, NiO, CuO, ZnO, ZrO2, RuO2, SnO2, or Ce2O3 by decoupling metal oxide precursor incorporation from block copolymer (BCP) template formation. In this work, neat BCP thin films were cast and annealed, using standard techniques, to generate templates. The templates were immersed in a precursor solution and formed metal-polymer complexes in one polymer domain. Finally, the organics were removed in an oxidative environment to leave the templated metal oxides. As a concrete example of the methods applicability, we show that the templating method produced ordered TiO2 arrays that exhibited a 13% increase in photocatalytic activity over TiO2 produced by EISA. Furthermore, the addition of gold nanoparticles further improved photocatalytic activity by 43% on our templated TiO2, whereas gold nanoparticles on EISA TiO2 exhibited no improvement. The simplicity and modularity of the templating method makes it amenable to additional applications in catalysis, optics, and sensors.

Pt and Gd loaded ZSM-5 catalysts were synthesized by ion exchange method to investigate the effect of metal promoters on catalyst activity, coking and regeneration during military aviation fuel (JP-8) cracking into petroleum gas (PG) at 723K. Multiple cracking and regeneration cycles were performed over the ZSM-5 based catalysts and their crystalline structure, oxidation profile, coke band and acidity were characterized. It was revealed that addition of Gd metal to the ZSM-5 catalyst prevented formation of complex aromatic coke and increased the number of Lewis acid sites, while Pt promoted ZSM-5 catalyst showed a decrease in the coke oxidation temperature. The effect of Pt and Gd promoters enhanced the coke burn-off ability, formed hydrogen rich carbon species and reduced oxidation temperature of coke substantially. Furthermore, agglomeration of Pt particles was partially impeded by coexisting Gd metal on the regenerated ZSM-5 catalyst. Synergetic effects of Pt and Gd promoters stabilized the PG yield and product distribution over the Pt–Gd/ZSM-5 catalyst during the cracking and regeneration cycles.

Pd deposited on CeO2, MnOx–CeO2 and SnO2–MnOx–CeO2 solid solution were tested for low temperature CO oxidation activity. Among them, Pd–MnOx–CeO2 (Pd–MC) and Pd–SnO2–MnOx–CeO2 (Pd–SMC) showed excellent low temperature activity and over 90% of CO conversion was obtained at room temperature. SO2 poisoning tests with 200ppm at room temperature showed that Pd–SMC could keep high CO conversion as compared to Pd–MC. The high CO oxidation activity of Pd–SMC at low temperatures could be attributed to strong oxidation ability of highly oxidized Pd species and interactions between Pd and solid solution support.

In this study, we demonstrate the production of long-chain hydrocarbons (C8+) from 2-methylfuran (2MF) and butanal in a single step reactive process by utilizing a bi-functional catalyst with both acid and metallic sites. Our approach utilizes a solid acid for the hydroalkylation function and as a support as well as a transition metal as hydrodeoxygenation catalyst. A series of solid acids was screened, among which MCM-41 demonstrated the best combination of activity and stability. Platinum nanoparticles were then incorporated into the MCM-41. The Pt/MCM-41 catalyst showed 96% yield for C 8+ hydrocarbons and the catalytic performance was stable over four reaction cycles of 20 hour each. The reaction pathways for the production of long-chain hydrocarbons is probed with a combination of infrared spectroscopy and steady-state reaction experiments. It is proposed that 2MF and butanal go through hydroalkylation first on the acid site followed by hydrodeoxygenation to produce the hydrocarbon fuels. This journal is © the Owner Societies 2014.