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Pd tio2 Sep 2020 Chemrxiv version.pdf (677.95 kB)

Ultra-Low Amounts of Palladium (0.005-0.05 Wt% Pd) Supported on Titania: Remarkable Low-Temperature Activity for NO Reduction with CO and Structure-Function Property Relationships in Methane Oxidation

preprint
submitted on 26.12.2020, 07:48 and posted on 29.12.2020, 06:01 by Konstantin Khivantsev, Libor Kovarik, Nicholas R. Jaegers, János Szanyi, Yong Wang

Atomically dispersed Pd +2 cations with ultra-dilute loading of palladium (0.005-0.05 wt%) were anchored on anatase titania and characterized with FTIR, microscopy and catalytic tests. CO infrared adsorption produces a sharp, narrow mono-carbonyl Pd(II)-CO band at ~2,130 cm-1 indicating formation of highly uniform and stable Pd+2 ions on anatase titania. The 0.05 wt% Pd/TiO2 sample was evaluated for methane combustion under dry and wet (industrially relevant) conditions in the presence and absence of carbon monoxide. Notably, we find the isolated palladium atoms respond dynamically upon oxygen concentration modulation (switching-on and switching off). When oxygen is removed from the wet methane stream, palladium ions are reduced to metallic state by methane and catalyze methane steam reforming instead of complete methane oxidation. Re-admission of oxygen restores Pd+2 cations and switches off methane steam reforming activity. Moreover, 0.05 wt% Pd/TiO2 is a competent CO oxidation catalyst in the presence of water steam with 90% CO conversion and TOF ~ 4,000 hr-1 at 260 ⁰C.

More importantly, we find that diluting 0.05 wt% Pd/titania sample with titania to ultra-low 0.005 wt% palladium loading produces a remarkably active material for nitric oxide reduction with carbon monoxide under industrially relevant conditions with >90% conversion of nitric oxide at 180 ⁰C (~460 ppm NO and 150 L/g*hr flow rate in the presence of >2% water steam) and TOF ~6,000 hr-1. Pd thus outperforms state-of-the-art rhodium containing catalysts with (15-20 times higher rhodium loading; rhodium is ~ 3 times more expensive than palladium). Furthermore, palladium catalysts are more selective towards nitrogen and produce significantly less ammonia relative to the more traditional rhodium catalysts due to lower Pd amount nd lower water-gas-shift activity. Our study is the first example of utilizing ultra-low (0.05 wt% and less) noble metal (Pd) amounts to produce heterogeneous catalysts with extraordinary activity for nitric oxide reduction. This opens up a pathway to study other Pd, Pt and Rh containing materials with ultra-low loadings of expensive noble metals dispersed on titania or titania-coated oxides for industrially relevant nitric oxide abatement.

Funding

The research at PNNL was supported by the U.S. Department of Energy, Energy Efficiency and Renewable Energy, Vehicle Technology Office. Experiments were conducted in the Environmental Molecular Sciences Laboratory (EMSL), a national scientific user facility sponsored by the Department of Energy’s Office of Biological and Environmental Research at Pacific Northwest National Laboratory (PNNL). PNNL is a multi-program national laboratory operated for the DOE by Battelle Memorial Institute under Contract DE-AC06-76RL01830. We acknowledge the support of CLEERS (Crosscut Lean Exhaust Emissions Reduction Simulations). CLEERS is an initiative funded by the U.S. Department of Energy (DOE) Vehicle Technologies Office to support the development of accurate tools for use in the design, calibration, and control of next generation engine/emissions control systems that maximize efficiency while complying with emissions regulations.

History

Email Address of Submitting Author

Konstantin.Khivantsev@pnnl.gov

Institution

Pacific Northwest National Laboratory

Country

United States

ORCID For Submitting Author

0000-0002-4810-586X

Declaration of Conflict of Interest

The authors filed for a patent.

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