Gaining insight into the kinetics of partial oxidation of light hydrocarbons on Rh, through a multiscale methodology based on advanced experimental and modeling techniques

A. Beretta, A. Donazzi, G. Groppi, M. Maestri, E. Tronconi and P. Forzatti

DOI: 10.1039/9781849737203-00001

This chapter updates previous reviews on the catalytic partial oxidation of methane and light hydrocarbons over noble metals; specifically, it focuses on the development of experimental and modeling tools that in recent years allowed to measure with accuracy and formalize the kinetics of the surface process, thus setting the basis for the engineering of short contact time CPO reformers. Such advanced tools include special micro-reactor designs for the kinetic investigation under isothermal conditions, first-principle microkinetic schemes, techniques for the spatially resolved measurement of temperature and concentration profiles inside working adiabatic reactors, detailed reactor models accounting for the role of transport phenomena in structured catalysts as well as that of homogeneous reactions. These contributions pave a multi-scale path, that runs from the fundamentals of surface kinetics to the reactor optimal design.

1 Introduction

Small scale reformer designs are currently being developed for the distributed and small scale production of H2. Future and futuristic applications are: the fuelling of H2-vehicles, the use of cogeneration systems based on fuel cells for residential heat and power supply, the valorization of CH4-rich bio-gas, the on-board generation and utilization of H2-rich streams as co-fuels, reducing agents and/or feed of Auxiliary Power Unit systems based on SOFC.

Fuel reformers produce hydrogen or hydrogen/COX mixtures by exploiting the catalytic conversion of gaseous or liquid fuels in the presence of steam, CO2 and/or O2 or air. Among various processes and reactor solutions that have been proposed (externally heated reformers, exo-endo reformers, autothermal reformers), the catalytic partial oxidation (CPO) of hydrocarbon fuels in insulated short contact time reactors has several advantages: flexibility to scale up and down, fast light off, resistance to extinction after load changes, high H2 yields at millisecond contact times, and the possibility of fully autothermal operation after light off. This is important in systems of a smaller scale than conventional hydrogen or syngas plants, particularly if restrictions on weight and/or volume are tight.

The CPO process consists of the selective conversion of a hydrocarbon fuel and O2 into CO and H2 using metals (Ni, Pd, Pt, Rh but also Fe, Co and other transition metals), typically deposited in the form of a catalytic wash-coat over high-void fraction structures such as foams or honeycomb monoliths. Lanny Schmidt and his group at Minnesota University have pioneered the process and during the last twenty years largely demonstrated its potential over a wide range of fuels, from gaseous to liquid to solid ones.

In recent years the development of novel experimental techniques and theoretical methods based on DFT has made possible the advancement of fundamental studies of CH activation on metals, which have led to the development of reliable kinetic schemes that can be used to interpret the complex behavior of short contact time-CPO reformers and address their engineering. The surface kinetics of the partial oxidation of light hydrocarbons have been investigated to a lesser extent, but recent studies have shown important analogies between the paths of activation of hydrocarbon fuels of different nature.

The aim of this chapter is to exemplify such recent advances, by reviewing findings from our and other laboratories that, along a multi-scale methodology, run from the surface kinetics to the reactor optimal design. Other previous review papers, in particular the recent paper by Enger, Rune and Holmen, have given a broad view of mechanicistic findings (based also on the development of in-situ or operando characterization techniques) for various catalyst systems. In this work, the emphasis is on the development of experimental and modeling tools that have allowed to recognize and formalize the kinetics of the surface reaction, and also to identify the roles of surface chemistry, gas-phase chemistry, heat and mass transfer phenomena in short contact time CPO reformers.

Kinetic studies on Rh are mainly treated in this chapter, the reason being that experimental, theoretical and modeling studies have presently reached a maturity and a comprehension of the process drivers that is still missing in the case of other metals. However, investigations on other catalysts, mostly Pt, will be also commented because of their important methodological contributions. First (Section III), we illustrate the kinetic findings from isothermal reactor designs, wherein the surface chemistry was isolated and kinetically controlled data were collected on CH4, C2H6 and C3 H8 activation in the presence of O2 and steam. Then (Section IV) we present the microkinetic modeling of CH4 partial oxidation on Rh based on a priori estimations of the kinetic parameters; the model was extremely useful in identifying the prevalent reaction paths behind the observed kinetic dependences. Concerning then the more complex and representative scale of adiabatic, short contact time CPO-reformers, a state of the art technique for the experimental investigation is represented by the axially resolved sampling technique; this will be also presented (Section IV). It is an extremely powerful technique that provides detailed information on the axial evolution of the process stoichiometry and of the corresponding temperature evolution. However, a thorough interpretation of the data needs the means of reactor modeling, which in turn needs the best possible description of the transport phenomena characteristic of the specific reactor configuration (Section V).

Finally, the chapter presents (Section VI) some case-studies wherein the spatially resolved measurements of temperature and composition, in combination with predictive modeling, have driven important advances in the comprehension of the CPO process and set the basis for the rational design of improved CPO-reformers. We conclude by presenting recent results on the optimal design of CPO-reformers, aimed at improving the thermal management of the reactor and...