(617fr) Transient Kinetic Screening of Active Sites and Their Consumption: Pt on Molybdenum Carbide Nanotubes

Transient
kinetic pulse response experiments were used to characterize reaction
pathways and active sites in platinum catalysts prepared via atomic
layer deposition on a Mo₂C
nanotube support. CO multiplulse TAP (Temporal Analysis of Products)
experiments combined with slow temperature programming revealed a
complex dependence of CO surface storage and CO₂
release. The primary kinetic characteristics were calculated
directly from experimentally measured exit-flow moments without a
priori
assumption of a kinetic mechanism using the theoretical methodology
developed by Shekhtman and Yablonsky [1]. This ‘model-free’
approach allows one to compare materials based on intrinsic rate
constants and regulation between the surface and gas phase. A
variety of kinetic models are then imposed and rate constants of
elementary steps presented.

The
CO/CO₂
transient kinetic results presented here, combined with detailed
structural characterization presented separately, suggest that Pt
particles in the unique size range (<2.7 nm) present an additional
high temperature pathway for CO₂
formation. A detailed analysis of the temperature dependence allowed
active sites to be distinguished kinetically as their consumption
incrementally progressed and the material transitioned from one
dominant site to another. A rate/rate coherency technique was used
to distinguish reaction pathways observed on pure support and
Pt-modified materials. These results present a new experimental
strategy for identifying and distinguishing pathways and kinetic
regimes in complex catalytic materials.

[1]
S.O. Shekhtman, G.S. Yablonsky, J.T. Gleaves, R. Fushimi, “State
defining” experiment in chemical kinetics—primary
characterization of catalyst activity in a TAP experiment, Chem. Eng.
Sci. 58 (2003) 4843-4859.