Rapid extraction of quantitative kinetic information from variable-temperature reaction profiles

Abstract

Analysis of variable-temperature reaction profiles, measured in an isothermal packed-bed reactor (PBR) whose temperature increases during the experiment, has the potential to yield accurate and precise kinetic parameters quickly for some heterogeneous catalysts. The method is demonstrated here for a typical supported nanoparticle catalyst, 2 wt% Pd/Al2O3, in the oxidation of H2, C3H8 and CO by O2. These reactions do not exhibit major changes in activation energy as a function of conversion over the range of reaction conditions analyzed. Reliable and quantitative information about rate laws was extracted readily from the shapes and positions of these profiles, as an alternative to more laborious conventional kinetic analyses. Temperature and pressure gradients were minimized by the use of sieved catalyst particles and large amounts of inert diluent for both the catalyst and feed gas. Curve-fitting of analytical expressions with as few as two adjustable parameters results in remarkable agreement between models and data. First-order profiles are indeed kinetically-limited, without mass and heat transfer effects, while inverse-firstorder profiles deviate from kinetically-controlled behavior at intermediate-to-high conversions. The activation energy and reaction order with respect to the limiting reactant obtained from a single reaction profile (with appropriate data truncation for non-kinetic phenomena, as necessary) are at least as accurate and precise as those obtained from a conventional Arrhenius analysis conducted with data obtained under differential conditions, and are measured in a fraction of the experimental time. Information about more elaborate rate laws can be obtained by global curve-fitting of a family of such profiles recorded with different volumetric flow rates.

Origin of the Normal and Inverse Hysteresis Behavior during CO Oxidation over Pt/Al2O3

“…14 The observed variations due to the different temperature ramp rates seem to be more related to the properties induced by the Pt particle size under these conditions, since the experiments were conducted with the same catalyst bed (strongly diluted) and diluted gases and the difference between the inlet and outlet temperatures was for all of the experiments below 4 °C over the entire temperature window, which shows the minor influence of heat transfer effects in our setup. 51 Nevertheless, as the heating is achieved with a tubular furnace, the presence of small radial temperature gradients that depend on the heating rate cannot be excluded and needs more consideration in the future.…”

Low-Temperature Ammonia Production during NO Reduction by CO Is Due to Atomically Dispersed Rhodium Active Sites

“…Differences in the light-off curve shapes suggests different rate laws for NO reduction over Rh clusters compared with atomically dispersed Rh species. 55 However, future steady-state measurements are necessary to unravel the underlying kinetics. It is clear that Rh clusters on Al 2 O 3 are more effective at catalyzing the reduction of NO by CO under dry conditions, although only by a small amount.…”

“…All catalysts produced 1:1 stoichiometric amounts of CO 2 and N 2 with high selectivity, although a small amount of N 2 O was observed at low temperatures, which was independent of Rh loading, see Figure S3. Differences in the light-off curve shapes suggests different rate laws for NO reduction over Rh clusters compared with atomically dispersed Rh species . However, future steady-state measurements are necessary to unravel the underlying kinetics.…”

Spatiotemporal insights into forced dynamic reactor operation for fast light-off of Pd-based methane oxidation catalysts

“…Towards the end of the catalytic channel, the reactivity declines again, which may be related to the fact that most of the methane is already oxidized and thus CH 4 diffusion limitation in the presence of water may come into play; such a decline in the conversion rate has been reported especially in the presence of water and at elevated temperatures. 63 In addition, hydroxyl groups that intrinsically form during CH 4 oxidation on the surface of the noble metal particles likely accumulate with increasing CH 4 conversion, which has been reported to be even more detrimental than humidity from the gas phase. 16 That the TOFs in the front part of the catalyst are very similar in static and SRP mode despite the 220 °C higher reactor temperature in static reactor operation mode can also be explained by the blockage of active surface sites by hydroxyl groups: hydroxyl accumulation takes place unhindered in static operation, whereas the reducing phases in forced dynamic reactor operation remove hydroxyls from the catalyst surface, hereby making active surface sites available for CH 4 adsorption, activation, and conversion.…”