ja4123889_si_001.pdf (2.19 MB)
Probing the Low-Temperature Water–Gas Shift Activity of Alkali-Promoted Platinum Catalysts Stabilized on Carbon Supports
journal contribution
posted on 2014-02-26, 00:00 authored by Branko Zugic, Shiran Zhang, David
C. Bell, Franklin (Feng) Tao, Maria Flytzani-StephanopoulosWe report on the
direct promotional effect of sodium on the water–gas
shift activity of platinum supported on oxygen-free multiwalled carbon
nanotubes. Whereas the Na-free Pt catalysts are shown to be completely
inactive, the addition of sodium is found to improve the water–gas
shift
activity to levels comparable to those obtained with highly active
Pt catalysts on metal oxide supports. The structure and morphology
of the catalyst surface was followed using aberration-corrected HAADF-STEM,
which showed that atomically dispersed platinum species are stabilized
by the addition of sodium. In situ atmospheric-pressure X-ray photoelectron
spectroscopy (AP-XPS) experiments demonstrated that oxidized platinum
Pt–OHx contributions in the Pt
4f signal are higher in the presence of sodium, providing evidence
for a previously reported active-site structure of the form Pt–Nax–Oy–(OH)z. Pt remained oxidized in all redox experiments,
even when a H2-rich gas mixture was used, but the extent
of its oxidation followed the oxidation potential of the gas. These
findings offer new insights into the nature of the active platinum-based
site for the water–gas shift reaction. A strong inhibitory
effect of hydrogen was observed on the reaction kinetics, effectively
raising the apparent activation energy from 70 ± 5 kJ/mol (in
product-free gas) to 105 ± 7 kJ/mol (in full reformate gas).
Increased hydrogen uptake was observed on these materials when both
Pt and Na were present on the catalyst, suggesting that hydrogen desorption
might limit the water–gas shift reaction rate under such conditions.