Novel Rh-based structured catalysts for the catalytic partial oxidation of methane

Abstract

Novel Rh-based structured catalysts were prepared by the electrosynthesis of Rh/Mg/Al hydrotalcite-type (HT) precursors on a FeCrAlY foam. The catalysts obtained by the calcination of HT compounds were investigated in the catalytic partial oxidation (CPO) of CH₄. The effects of the electrosynthesis conditions (potential and pH of the plating solution) on the surface morphology and chemical composition of the samples as well as on the catalytic activity were investigated. The control of the pH of the solution favoured the precipitation of Rh as hydroxide rather than of metallic particles, whereas the potential applied determined the pH value reached near to the foam. By increasing the cathodic potential from −1.2 to −1.3 V, but keeping the synthesis time constant (1000 s), the required conditions to obtain HT precursors were achieved faster and, therefore, a larger amount of them precipitated. Due to the different coverage and chemical nature of the electrosynthesized species, catalytic performances depended on the synthesis conditions, the best values being achieved by the catalyst obtained from the HT precursor prepared at −1.3 V for 1000 s.

Introduction

The catalytic partial oxidation (CPO) of methane to syngas (CO + H₂) is a slightly exothermic reaction in which high methane conversion and syngas selectivity are achieved at short contact times, making it possible to use small reactors [1], [2], [3]. For these reasons, CPO may be used to obtain syngas in small-medium scale plants, i.e. to produce it for distribution. Pelletized catalysts consisting of Ni, Co and noble metals on several supports such as Al₂O₃, MgAl₂O₄, CeO₂–ZrO₂ are widely used [4]. However, due to both the high gas-hourly-space-velocity (GHSV) values adopted and the high temperatures reached in the catalytic bed, the mechanical stability of the catalyst and its thermal conductivity play a key role in the development of the process. In this sense, structured catalysts [5] can be useful thanks to their large geometric areas, low pressure drop and high mechanical stability. In addition, by selecting structured supports with high void fractions, thermal conductivity, and convective heat transfer, both the formation of hot spots and the “run away” of the reaction may be avoided [6], as the large amount of heat generated on the upper part of the catalytic bed can be diffused by the metallic support and consumed by further reforming reactions [7].

Bulk metal structured catalysts, such as noble metal gauzes or sponges [8], [9], [10] and nickel foams [11], [12] have been applied in CPO, although they show low surface area values. On the other hand, structured catalysts in which the active catalyst is deposited on a ceramic or metallic support – such as alumina foams [13], [14], [15], [16], [17], [18], [19], extruded cordierite or α-alumina honeycomb monoliths [9], [20], [21], [22], [23], [24], [25], [26], felts [27], FeCrAlloy and Nicrofer metallic monoliths [28], [29], [30] as well as a FeCrAlloy metallic foam [31] - lead to a significant reduction of the amount of catalyst, without a decline in the catalytic performances. In structured catalysts, the amount and dispersion of the active phase can be controlled by changing either the thickness of the coating or the metal loading. Furthermore, the performance of the catalyst is determined by the synthesis procedure, morphology and stability of the film. Different synthetic procedures have been used to coat the supports and obtain CPO catalysts: (i) impregnation or coprecipitation of the salts of the active metals (Rh, Pt or Ni) on alumina-structured supports [15], [18], [19], [22] or impregnation of the elements to form Zr_0.8Ce_0.2O₂ and LaNi_0.9Pt_0.1O_x phases [26]; (ii) wash-coating of a primer and subsequent impregnation or coprecipitation of the active phase [9], [17], [20], [25]; (iii) wash-coating of a ready made catalyst [27]; (iv) microwave-assisted combustion synthesis of Pt nanoparticles on Al₂O₃ foams [32]. The coating of metallic supports with a ceramic catalyst is not straightforward, since the adhesion of the layer to the support is low and problems can arise during drying and calcination; therefore, the major challenge is in achieving a homogeneous and well-adhered catalyst layer on the monolith walls [33]. In some cases it has been reported that the addition of a primer or the treatment of the support at a high temperature in an oxidizing atmosphere improve the adherence of the catalyst to the support [34], [35].

The electrodeposition can be used to deposit hydroxides and/or oxides on metallic supports through the base-electrogeneration method, which consists of the generation of a basic pH near the support by the reduction of an easily reducible anion, like NO₃⁻ or ClO₄⁻ [36], [37]. This technique, largely used for the preparation of modified electrodes, has been extended to the preparation of catalysts [38], [39], [40]. Recently, some of the authors proposed the electrosynthesis and deposition of hydrotalcite (HT)-type compounds, precursors of Ni-based catalysts active in the steam methane reforming [41], [42], [43]. HT compounds are layered materials [44], [45] that, after calcination, lead to mixed oxides with the active phase well distributed and after reduction small and stable metallic particles are obtained. In particular, HT compounds containing Ni, Co and noble metals have been used as precursors for the CPO of methane [46], [47]. In the present study, the base-electrogeneration method has been extended to the preparation of Rh/Mg/Al HT compounds on a FeCrAlY alloy foam, which are precursors of Rh-based catalysts for the CPO of methane [48]. The effects of the pH of the plating solution and potential applied on both the coating properties and the catalytic activity are investigated. Moreover, the aim of this work was to compare the activity of the electrosynthesized catalysts with that of a Rh-based pelletized catalyst with the same structure, it means, obtained from HT compounds.