The steam reforming of phenol over natural calcite materials Domna A. Constantinou, Angelos M. Efstathiou * Heterogeneous Catalysis Laboratory, Department of Chemistry, University of Cyprus, P.O. Box 20537, CY 1678 Nicosia, Cyprus 1. Introduction Hydrogen is receiving considerable attention as an important energy carrier as well as a clean fuel for transportation, especially for fuel cell applications [1,2]. The catalytic steam gasification of biomass could be considered as a promising technology for the production of hydrogen-rich gas streams resulting to limited CO 2 and other greenhouse gas emissions. However, one of the important issues associated with the practical application of biomass gasification is the elimination of tars, which leads to clogging of fuel lines and blockage of filters and heat exchangers [3]. The addition of steam was found to produce a lower amount of refractory tars and to enhance the formation of phenol that can be easier reformed catalytically. Therefore, phenol appears to be one of the main constituent molecules of tar composition, in particular when wood-biomass is used in the gasification process [4–6]. A limited number of research works were reported in the literature [7–10] regarding the steam reforming of phenol towards hydrogen production. Supported nickel catalysts are in general the widely used industrial catalysts for gasification/reforming reac- tions of aromatics [7] for fixed-bed catalytic reactor applications. However, the ability of using appropriate natural materials should be considered very efficient in fluidized-bed reactor applications because of the non-negligible loss of material occurred. Calcined dolomites (CaMg(CO 3 ) 2 ) and olivines ((Mg,Fe) 2 SiO 4 ) are the most investigated natural materials for tar conversion in biomass gasification processes, whereas calcite materials (CaCO 3 ) are the least investigated ones [11–13]. These naturally occurring materials are inexpensive, non-toxic, and abundant with good activity at high temperatures [11]. The use of natural calcites appears as an interesting innovative class of catalytic materials for enhancing the hydrogen yield in conventional biomass steam gasification processes. The calcium oxide (CaO)-containing natural materials can act both as catalysts and as absorbents of CO 2 , thus combining the reforming, water–gas shift, and the CO 2 adsorption individual reaction steps into a single Catalysis Today 143 (2009) 17–24 ARTICLE INFO Article history: Available online 9 December 2008 Keywords: Phenol steam reforming Hydrogen production Natural calcites CO 2 -TPD CO 2 -DRIFTS ABSTRACT Five natural calcite materials of different geographical origin were studied towards the steam reforming of phenol reaction and the activity results obtained were correlated with some of their physico-chemical properties. It was found that the specific catalytic activity (mmol/(m 2 s)) of the calcite material following calcination at 850 8C was not correlated with the BET area and the primary crystal size of calcium oxide (CaO). The metal impurity levels of the natural calcite materials investigated were found to be very low as evidenced by EDX and XPS measurements, leading to the conclusion that their effect on the catalytic chemistry of phenol steam reforming and surface adsorption properties of CaO was minor. These results suggest that the reaction at hand depends on the heterogeneity and site reactivity present in each CaO surface examined. On the other hand, based on diffuse reflectance infrared Fourier transform spectroscopy (DRIFTS) CO 2 chemisorption and TPD-CO 2 mass spectrometry studies, it was found that the catalytic activity can be correlated with the amount of CO 2 adsorbed at 25 8C in the unidentate form. The latter species was found to be significantly more thermally stable than the bicarbonate adsorbed species also populated on the CaO surface after CO 2 chemisorption (15 Torr) at 25 8C. The least concentration of unidentate carbonate species accumulated on the catalyst surface the higher the catalytic activity of CaO to be expected. It was found that the presence of hydrogen in the feed stream results in a significant decrease of steam reforming of phenol activity over CaO. This result was probed to be likely related to the reduction in the rate of water dissociation to form –OH species, as revealed by the significant decrease in the concentration of adsorbed bicarbonate and –OH species on the surface of CaO according to in situ DRIFTS-CO 2 adsorption experiments in the presence of water and of varying hydrogen concentration in the feed. The inhibiting role of H 2 during steam reforming of phenol over CaO is discussed. ß 2008 Elsevier B.V. All rights reserved. * Corresponding author. Tel.: +357 22 892776; fax: +357 22 892801. E-mail address: efstath@ucy.ac.cy (A.M. Efstathiou). Contents lists available at ScienceDirect Catalysis Today journal homepage: www.elsevier.com/locate/cattod 0920-5861/$ – see front matter ß 2008 Elsevier B.V. All rights reserved. doi:10.1016/j.cattod.2008.10.032