Partial Oxidation of Methane to Syngas Over γ-Al₂O₃-Supported Rh Nanoparticles: Kinetic and Mechanistic Origins of Size Effect on Selectivity and Activity

Abstract

A series of supported Rh/γ-Al 2 O 3 catalysts with an overall metal loading of 0.005 wt % was synthesized by impregnation of γ-Al 2 O 3 with a toluene solution containing colloidally prepared well-defined (1.1, 2.5, 2.9, 3.7, and 5.5 nm) Rh nanoparticles (NP). The size of NP was not found to change after their deposition on γ-Al 2 O 3 and even after performing partial oxidation of methane (POM) to synthesis gas at 1073 K for 160 h on stream. Apparent CO formation turnover rates and CO selectivity strongly decrease with an increase in this size. Contrarily, the overall scheme of POM is size-independent, i.e. CO and H 2 are mainly formed through reforming reactions of CH 4 with CO 2 and H 2 O at least under conditions of complete oxygen conversion. The size effect on the activity and selectivity was related to the kinetics of interaction of CH 4 , O 2 , and CO 2 with Rh/γ-Al 2 O 3 as concluded from our microkinetic analysis of corresponding transient experiments in the temporal analysis of products reactor. The rate constants of CH 4 , O 2 , and CO 2 activation decrease with an increase in the size of supported Rh NP thus influencing both primary (methane combustion) and secondary (reforming of methane) pathways within the course of POM.

Forty years of temporal analysis of products

“…Table 2 summarizes reactions and catalysts studied, kinetic models used and type of reaction parameters derived. 38,63,71,108,110,114,[120][121][122][123][124] The kinetics of CO adsorption and oxidation have been studied, 40,71,120 using the TZTR approach. The main focus was put on the role of supported metals (Au or Pt) and the kind of active oxygen species on performance of Au/SiO 2 , Pt/CeO 2 , CuMnOx and Au/CuMnOx.…”

“…A good practice is to apply the moment based approaches for determining the type (reversible vs. irreversible) of interaction of feed components and reaction products and to use such knowledge for developing kinetic schemes for numerical calculations. 63,108,111,123,124 On the basis of the order of appearance and shapes of transient responses, a detailed reaction scheme has been developed for oxidative propane dehydrogenation over VOx/TiO 2 in the absence of gas-phase O 2 . 123 It consisted of thirteen near to elementary reaction steps.…”

“…In the multi-pulse approach, the catalyst is usually first adjusted to a reproducible state by appropriate pretreatments. [62][63][64][65] Examples of such pre-treatments are an oxidation of the catalyst (usually atmospheric pressure flow treatment) or the irreversible adsorption of one reactant. In the second step, a single reactant is repeatedly pulsed (several 1000 s of pulses) to produce a controlled change in the catalyst state.…”

Utilization of Volatile Organic Compounds as an Alternative for Destructive Abatement

“…POM is thermodynamically an exothermic reaction, but most of the catalysts are active and selective at higher temperatures 700-1000 °C (shown in Table 3). At higher temperatures, deactivation of the catalysts takes place due to active metal sintering or agglomeration [55,70]. Developing new catalytic materials, which are active at low temperatures can be a possible solution to avoid metal sintering and also make the process less energy intensive.…”

“…Most of the catalysts used in the literature achieved the conversion and selectivity close to equilibrium values ( Table 3). Durability of the catalyst is still a challenge, since to the best of our knowledge, only a few catalysts performed excellently when the time-on-stream was longer than 150 h [70]. Table 3.…”