Catalytic cooperation at the interface of physical mixtures of CaO and MgO catalysts during steam gasification of naphthalene

Abstract

Oxides CaO, MgO and a physical mixture of 10% CaO + 90% MgO were used as catalysts for the steam gasification of naphthalene. A synergistic effect in the conversion was observed with the mixture. Catalysts were characterized by diffuse reflectance infrared Fourier transform spectroscopy (DRIFTS) and XPS techniques. Basicity was significant for CaO as compared to MgO and the mixture, but did not correlate with catalytic activity. Bidentate carbonate-type formation, which remained even in the presence of steam at high temperature (1033 K), was observed on CaO. Carbonates on MgO and on the mixture are unidentate and disappeared when steam was present. Formates, carboxylic and aldehydic groups also were formed, at 723 K on MgO and at 423 K on the mixture and on CaO. These species disappeared at 873 and 1033 K for the mixture and CaO, respectively, thus indicating their greater stability on CaO than on the mixture and on the MgO. Deposition of carbonaceous matter on solids was observed. The order of the deposited content is mixture < MgO < CaO. No indication of mutual contamination by the metal composing the oxides was observed. A higher carbonate removal and less carbonaceous matter deposition were observed in CaO when in the mixture, as compared with CaO alone, suggesting a beneficial effect of MgO in the mixture. The synergistic effect observed using the 10% CaO + 90% MgO mixture was related to the inhibition of carbonaceous matter and carbonate formation on the catalysts. This inhibition is due to a catalytic cooperation between both oxides, as two separate phases. The presence of MgO is assumed to avoid the formation of stable bidentate carbonates on CaO, promoting instead unidentate species formation; this facilitates the gasification of naphthalene by steam.

Evolution of dolomite composition and reactivity during biomass gasification

“…Figure 6. Chemical mechanism of benzene steam reforming over dolomite proposed based on the experimental results of the present study and on previous works [45,68,74,94,97].…”

“…Based on our results and on literature analysis [45,68,74,94,97], an overall mechanism of benzene steam reforming over dolomite is proposed (Figure 6). Figure 6.…”

“…In a deep study Alarcon et al have proposed a detailed mechanism for the steam reforming of aromatics over CaO and MgO [68]. These authors have proposed, by analogy with the mechanism of hydrocarbons steam-reforming over metal supported catalysts [96], that aromatic hydrocarbons reforming over CaO or MgO catalysts would take place in two main step [94]: (i) the decomposition of the hydrocarbon and the water on the catalyst surface, and then (ii) the gasification of the deposited carbon with steam. The overall process could be written according to the following simplified mechanism:…”

“…where ArH is an aromatic hydrocarbon specie, is a M 2+ O 2unit (the active site in dolomite). The deposit (CxHy-) may also decompose into smaller fragments and further form H2, CH4, and Cx [68,94].…”

“…The XPS results are presented in Table 4. Binding energy (BE) and atomic concentration (%) were ascribed to the element according to published data [45,94,95].…”