Steam reforming of methanol over Cu/ZrO/CeO catalysts: a kinetic study

Abstract

Steam reforming of methanol (SRM) was investigated over Cu/ZrO 2 /CeO 2 (CZC) catalysts prepared via a novel synthetic method based on coprecipitation and polymer templating. Structural characterization of the samples was performed by N 2 adsorption-desorption, N 2 O decomposition, and X-ray diffraction. The variation of the Cu loading resulted in an increased Cu crystallite size and a decreased specific surface area of the active particles. Catalytic investigations were carried out in a fixed bed reactor at 10 5 Pa, by applying a CH 3 OH:H 2 O = 1:1 ratio. The samples with Cu contents higher than 5 % exhibited good long-term stabilities and low CO levels during continuous operation. The kinetic model suggested for the transformation involved the reverse water-gas shift (RWGS) and methanol decomposition (MD), in addition to the SRM reaction. Kinetic measurements were accomplished in the temperature range 503-573 K and the experimental results could be well simulated. The highest methanol conversions and the lowest CO levels were observed in the temperature range 523-543 K. The apparent activation energies for the individual reactions were found to depend on the Cu content of the catalyst. Since the influence of mass transport limitations on the kinetic data could be excluded, it was established that the variation of the Cu concentration in the precursor material altered the microstructure of the Cu particles and, accordingly, the active Cu surface, which resulted in the formation of significantly different catalysts.Keywords: steam reforming of methanol, copper, zirconia, ceria, N 2 O chemisorption, long term stability, CO formation, kinetic model, reverse water-gas shift reaction, methanol decomposition, activation energy 1.IntroductionIn the past decade, considerable attention has been focused on the reduction of the significant emissions originating from mobile sources, such as internal combustion engines [1][2][3][4]. For environmental reasons, the development of proton-exchange membrane fuel cells (PEMFCs) has gained in increasing importance [5,6]. As compared with conventional heat engines, several advantages of fuel cell application have been established, including a higher efficiency and a more convenient operation, the absence of moving parts and the low emission of hazardous compounds [1,5]. The combustion of hydrogen in a fuel cell is regarded as a clean process, releasing energy and providing only water as an exhaust material [4,7,8]. However, hydrogen is not a natural energy source and must be generated by consuming a large amount of energy, either from natural gas or via the electrolysis of water [4]. Furthermore, for a fuel cell vehicle, the storage and the supply of hydrogen, a volatile and explosive gas, imposes mechanical problems and safety hazards on a commercial level [1,4,8].Several liquid fuel candidates have been discussed for on-board reforming, including methanol, ethanol, gasoline and diesel [1], of which methanol is considered the most favourable alternative [1,9]. Although mo...

Methanol steam reforming over Cu/CeO2 catalysts: influence of zinc addition

“…7,33 Different reaction mechanisms have been reported in literature which differ in the CO formation step: some of them consider that CO is a primary product formed from methanol decomposition (MD) followed by WGS; 34 others suggest that CO is a secondary product formed from reverse WGS 5,6 and those where the three reactions (SRM, WGS, MD) are included into the mechanism. 35 In Figure 7, methanol conversion as a function of W/F MeOH is illustrated. The values at 300 °C correspond to 400 min in reaction.…”

“…This value is near data in literature. 6,35,36 Purnama et al 6 assuming a scheme of SRM and r-WGS reactions have reported Ea = 76 kJ mol -1 over a commercial CuO/ZnO/Al 2 O 3 catalyst ; and Mastalir et al 35 have informed activation energy between 109.2 and 60.7 kJ mol -1 depending on Cu/ZrO 2 /CeO 2 catalyst composition. These authors have assumed three reactions to describe the mechanism: SRM, r-WGS and MD.…”

Methanol steam reforming behavior of copper impregnated over CeO 2 –ZrO 2 derived from a surfactant assisted coprecipitation route

“…They have studied the effect of delayed time on stream MSR behavior by continuing the process for a long period as high as 16 days. This treatment revealed the decrease in activity from ~35% to ~17% up to 9 days [7]. The coprecipitation made CuO/ZnO/ CeO2eZrO2 catalyst is shown to exhibit greater activity and long term stability and the CeO2eZrO2 (50:50) support has a greater promotional effect than pure CeO2, ZrO2 and Al2O3 [29].…”

“…The decrease in activity during long term tests is a general observation for this kind of catalytic systems [7,23,25,29]. To investigate in detail the possibility of activity recovery and the catalyst reuse after an intermediate oxidation step, we carried out regeneration tests as described in the experimental section.…”

“…Several catalytic systems based on Cu [6,7,11] and Pd [5] have been reported in the literature for MSR; they clearly show that the Cu-based catalysts are highly active and selective at lower temperatures. The commercial MSR catalyst Cu/ZnO/Al2O3 contains a high loading of Cu and ZnO and a relatively lower amount of Al2O3 [12].…”