PacSurf2018 Session NM-MoE: NanoCatalysis

Transition metal carbides (TMCs) are known to display catalytic properties similar with those of noble metals. The high activity of TMCs compared with their parent metals originates from a modification of the electronic properties from the addition of carbon, which in turn affects the binding strength and the reactivity of adsorbates. In the present talk, special interest will be focused on employing TMCs as a substrate to load another metal and the strong interactions between the admetal and the carbide. Several reactions involved will be discussed, including dry reforming of methane, water gas shift and reverse water gas shift reactions [1,2], as well as liquid phase reforming of methanol for H₂ production [3].

References:

[1] X. Zhang, X. Zhu, L. Lin, S. Yao, M. Zhang, X. Liu, X. Wang, Y.W. Li, C. Shi*, D. Ma, “Highly Dispersed Copper over β-Mo2C as an Efficient and Stable Catalyst for the Reverse Water Gas Shift (RWGS) Reaction”, ACS Catalysis 7 (2017) 912-918.

[2] S. Yao, X. Zhang, W. Zhou, R. Gao, W. Xu, Y. Ye, L. Lin, X. Wen, P. Liu, B. Chen, E. Crumlin, J. Guo, Z. Zuo, W. Li, J. Xie, L. Lu, C.J. Kiely, L. Gu, C. Shi*, J.A. Rodriguez*, D. Ma*, “Atomic-layered Au clusters on α-MoC as catalysts for the low-temperature water-gas shift reaction”, Science, (2017). Science, 357, (2017) 389.

[3] L. Lin, W. Zhou, R. Gao, S. Yao, X. Zhang, W. Xu, S. Zheng, Z. Jiang, Q. Yu, Y.W. Li, C. Shi*, X.D. Wen, D. Ma*, “Low-temperature hydrogen production from water and methanol using Pt/alpha-MoC catalysts”, Nature, 544 (2017) 80-83

Ethylene, a major petrochemical building block, is produced industrially by endothermic steam cracking of ethane and petroleum naphtha at 800-850°C. Catalytic oxidative dehydrogenation (ODH) of ethane offers huge potential for reductions in energy consumption and greenhouse gas emissions; however, ethane ODH using co-fed O₂ also requires costly cryogenic air separation [1]. As an alternative, we are investigating redox catalysts that operate in a cyclic mode (chemical looping, CL) and utilize lattice oxygen (O^2-) as the sole oxidant [2-3]. One monolayer (1 ML) equivalent MoO₃/Al₂O₃ catalysts prepared by conventional impregnation and calcination methods contain highly dispersed supported molybdate species and exhibit high ethylene selectivity (>95%) in CL-ODH at 500-550°C. In contrast, at higher MoO₃ loadings the Al₂(MoO₄)₃ bulk phase predominates, as evidenced by Raman spectroscopy and x-ray diffraction (XRD); these catalysts have somewhat higher activity albeit 10–12% lower ethylene selectivity under equivalent conditions. X-ray photoelectron spectroscopy (XPS) studies indicate that high ethylene selectivity correlates with reduction of Mo^+VI to Mo^+V. The presence of Mo in lower oxidation states (+V, +IV) triggers H₂ and CH₄ generation resulting in selectivity loss. Interaction of molybdate species with the Al₂O₃ support appears to decrease reducibility and enhance ethylene selectivity in CL-ODH. Even after extended reduction in ethane at 550°C, the monolayer catalyst retained about 25% Mo^+VI. In contrast, for 3 ML equivalent catalysts, surface Mo^+VI is rapidly consumed, and its concentration eventually approaches zero. To better elucidate the nature of the catalytically active sites, model catalysts consisting of nanostructured MoO₃ films on c-plane sapphire were deposited at 580°C by molecular beam epitaxy using a conventional Knudsen cell. The sapphire substrate after annealing at 700°C in UHV exhibited a streaky reflection high-energy electron diffraction (RHEED) pattern. Films deposited at short times (<1 min) were polycrystalline with relatively smooth surfaces (1.1 nm RMS roughness by atomic force microscopy). XPS revealed that MoO₃ films deposited in vacuo were oxygen-deficient. Surface asperities grew with deposition time and with annealing at 700°C in UHV. The catalytic properties of the films are currently under investigation using molecular beam methods.

References:

1. C.A. Gärtner, A.C. van Veen, J.A. Lercher, ChemCatChem 5 (2013) 3196–3217.

2. S. Yusuf, L. Neal, V. Haribal, M. Baldwin, H.H. Lamb, F. Li, Appl. Catal. B 232 (2018) 77-85.

3. P. Novotný, S. Yusuf, F. Li, and H.H. Lamb, Catal. Today (2018) in press.

Designing multiphase or multicomponent semiconductor heterojunctions is a promising strategy to enhance the performance of photocatalytic materials. Titanium dioxide (TiO₂) is a material of interest as a photocatalytic material because of its availability, stability, and cost-effectiveness. However, one major drawback of TiO₂ is its wide band gap resulting to photocatalytic activity in the UV region only. Through heterojunction coupling of TiO₂ with a lower band gap semiconductor such as cuprous oxide (Cu₂O) with a direct band gap of 2.17 eV, visible light active photocatalytic activity can be achieved. TiO₂/Cu₂O heterojunction thin films were assembled in a configuration wherein the junction of the coupled semiconductors is exposed to light irradiation for photocatalysis. This design ensures that photocatalytic reaction can occur in both semiconductors since they are exposed to the light source. A layer of TiO₂ thin film was deposited on (100) silicon (Si) substrate via reactive RF magnetron sputtering coupled with post deposition thermal annealing at 500°C. To form the heterojunction, ordered Cu clusters were patterned on top of the TiO₂ layer through a mask. The deposited Cu were transformed into Cu₂O by thermal oxidation at 200°C in air atmosphere followed by slow cooling at a rate of 0.48°C/min. The TiO₂/Cu₂O heterojunction films were characterized using XRD and FE-SEM with EDS mapping. Photocatalytic performance was evaluated by visible light photodegradation of methylene blue dye as the test analyte. The XRD pattern of the TiO₂/Cu₂O heterojunction film indicated the existence of rutile and anatase phases of TiO₂ as well as the existence of mostly Cu₂O phase. SEM image of the TiO₂/Cu₂O showed a patterned thin film design composed of a TiO₂ thin film layer decorated with grain-like Cu₂O clusters that are approximately 500 μm in diameter. At higher magnification, the TiO₂ thin film exhibited a porous columnar structure while the Cu₂O is characterized by terraced grain-like structures. Using an effective thin film surface area of 1.0 cm x 2.5 cm, around 50% removal rate of methylene blue was observed in 6 h of visible light irradiation. This promising approach of designing immobilized heterojunction thin films offers a potential for wastewater treatment applications using visible light irradiation.