Skip to main content

Hydrogen production by catalytic reforming for a new generation of solid oxide fuel cells directly operating with biogas at intermediate temperatures

Final Activity Report Summary - BIOGASFUELCELL (Hydrogen production by catalytic reforming for a new generation of solid oxide fuel cells directly operating with biogas at intermediate temperatures)

This project aimed at developing highly effective anodes for solid oxide fuel cell (SOFC), working at high temperature under direct feeding of waste biogas. The presence of methane (CH4), carbon dioxide (CO2) and water (H2O) in biogas made possible an internal reforming at the SOFC anode for hydrogen (H2) production.

This required the development of new catalytic materials, highly resistant to carbon (C) deposition and showing a high tolerance to eventual impurities which might be present in the fuel. The catalysts had to be thermally stable and have good redox properties able to insure the required mixed electronic and ionic conductivity through the fuel cell. In this work, gadolinium doped cerium oxide (CGO) was selected as the starting material for the development of high performance CGO-based catalysts. The methodology consisted in studying the catalytic activity in CH4 steam reforming under water deficient conditions and in CH4, mixed with CO2 and H2O, reforming for varying H2O concentrations in the feed. Mechanistic aspects based on step reactions studies were also performed to elucidate the reforming process of each anode.

Catalytic tests were carried out between 750 °C and 900 °C, for H2O to CH4 ratios varying between 0.1 and 1, pretreated in H2O and N2, N2, and H2 and N2. Above 800 °C, a slight deactivation of the stream with time was observed except for the H2-pretreated sample. Surface area measurements, oxygen (O2) adsorption at room temperature and O2 temperature programmed oxidation experiments were performed after catalytic testing. Changes in both surface area and redox properties of CGO were observed and related to catalytic deactivation. H2 was thought to play a key role in the catalytic activity and deactivation process.

Afterwards, the catalytic behaviour of CGO-supported iridium (Ir) catalyst (0.1 wt % Ir/CGO) in the steam reforming of CH4 was investigated at temperatures between 750 °C and 900 °C under H2O deficient conditions, i.e. with ratio of H2O to CH4 equal to 0.1 in order to evaluate its potential use as anode material in SOFC directly fed with CH4 and to integrate a gradual internal reforming (GIR) concept. Doping Gd-doped ceria with Ir led to a material with impressive catalytic performances in CH4 steam reforming compared to CGO, while unreactive C could not form. For comparison, Ir/Al2O3 exhibited much lower catalytic activity and significant deactivation with time on stream. The formation of weakly reactive C deposits on Ir/Al2O3 would be responsible for the deactivating behaviour of this catalyst.

Moreover, an iridium-based catalyst (Ir/CGO) was studied in mixed dry and steam reforming of CH4 under varying feed composition, consisting of 25 % CH4, 12.5 % CO2 and X % H2O, with X varying between 0 and 11 %). Ir/CGO was tested between 600 °C and 800 °C and was remarkably active and stable. Increasing steam concentration decreased CO2 conversion and increased C deposition. Addition of H2O to the CH4 and CO2 feed allowed simultaneous dry and steam reforming over Ir/CGO catalyst; however the processes were not additive. The inhibitory effect of water was related to:

1. a competitive adsorption of H2O and CO2 evidenced by Fourier transform infrared (FTIR) spectroscopy and
2. a simultaneous reactivity of H2O and CO2 as shown in catalytic measurements. C deposition remained low and did not affect the catalytic activity. Ir/CGO appeared as an excellent anode material with a catalytic reforming function for a potential use in SOFCs working with biogas.

A mechanistic study of catalytic steam reforming (SR) of CH4 was undertaken over 0.1 wt % Ir/CGO using various techniques, such as steady-state rate measurements, transient responses to CH4 or H2O step changes in isothermal conditions, temperature programmed reaction with CH4 (TP-CH4), temperature programmed reaction with H2O (H2O-TP). The results were compared to a reference 0.1 wt % Ir/Al2O3 and CGO. The methane SR reaction over Ir/CGO proceeded through a dual-site, i.e. bifunctional, mechanism involving:

1. Ir sites as active sites for the cracking of CH4 into reactive C species
2. reducible (Ce4+) sites in CGO responsible for a red-ox mechanism involving Ce4+/Ce3+ sites, being reduced by reaction with reactive C into CO (CO2) and re-oxidised by H2O.