<p>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<sup>-1</sup> indicating formation of highly uniform
and stable Pd+2 ions on anatase titania. The 0.05 wt% Pd/TiO<sub>2</sub> 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<sup>+2</sup> cations and switches off
methane steam reforming activity. Moreover, 0.05 wt% Pd/TiO<sub>2</sub> is a
competent CO oxidation catalyst in the presence of water steam with 90% CO
conversion and TOF ~ 4,000 hr<sup>-1</sup> at 260 ⁰C. </p><p>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<sup>-1</sup>. 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.</p>
Differences in the Nature of Active Sites for Methane Dry Reforming and Methane Steam Reforming over Nickel Aluminate Catalysts
