Ni-based catalysts for H2 production from ethanol steam reforming: Effect of the support and use of CO2 as regenerating agent and reactant

Abstract

In this work, the effect of CO2 on the hydrogen production under substoichiometric ethanol steam reforming (ESR) conditions over nickel based catalysts was studied. A special emphasis was focused on the application of carbon dioxide as a reactant under the CO2-assisted substoichiometric ethanol steam reforming. Moreover, CO2 was applied as regenerating agent to reduce the amount of carbonaceous deposits, which formation is a main drawback of the nickel catalysts. The obtained results were related with the physical-chemical properties of the catalysts.

A series of nickel catalysts supported on single metal oxides (MgO, Al2O3, Y2O3, La2O3 and ZrO2), and on binary (ZrO2-Y2O3, ZrO2-La2O3) and ternary systems (Y2O3-ZrO2-La2O3) was prepared. The supports of the catalysts were prepared by a pseudo sol-gel method using active carbon as templating agent.

The catalysts were characterized by ICP-AES for chemical analysis, physisorption of nitrogen (BET), powder X-ray diffraction (XRD), X-ray photoelectron spectroscopy (XPS) and temperature-programmed reduction with H2 (H2-TPR). The surface density and strength of basic centres were determined from calorimetry of CO2 adsorption and temperature programmed desorption of CO2 (TPD-CO2) experiments.

A relationship between the surface area of Ni/ZrO2-Y2O3, Ni/ZrO2-La2O3 and Ni/ZrO2-Y2O3-La2O3 and the Zr content in the catalysts was found. Catalysts with higher Zr content showed higher values of surface area, independently of the components of the support.

The analysis of the reducibility of the catalysts showed that, for yttrium- and lanthanum-containing catalysts, the H2-consumption values largely exceeded those expected for NiO reduction to Ni0. This was related with the partial reduction of the supports due to the presence of superficial oxygen vacancies and/or with the presence of La2O2CO3.

Catalytic behaviour of all catalysts was studied under ESR conditions (ethanol/water = 1.0/1.6, molar ratio), at 600 ° C. The catalysts supported on single metal oxides were also tested under substoichiometric CO2-assisted ethanol steam reforming (CDESR) (ethanol/CO2/water = 1.0/1.6/1.6, molar ratios), at 600 ° C. In all cases, CO2 was also applied as a regenerating agent in order to remove carbon deposits.

The catalysts were active under the tested conditions; the main product was hydrogen (40-70% mol/mol). Carbon monoxide, methane, ethene, carbon dioxide and acetaldehyde were also obtained as a function of the catalyst and the reaction conditions used. Application of CO2-treatments to partially deactivated catalysts allowed to recover partially or totally the catalytic performance.

The spent catalysts were characterized in order to determine changes of the crystalline phases (powder XRD) and surface composition (XPS). The amount and the characteristics of the carbon deposits formed under the reaction conditions were determined using temperature programmed oxidation with mass spectrometer (TPO-MS) and Raman and diffuse reflectance infrared Fourier transform spectrometry analysis (DRIFTS).

For catalysts supported on single oxides, the effect of CO2 introduction as reactant is related with the basic characteristics of catalysts. After CDESR, Ni/La2O3 showed a lower amount and less ordered carbon deposits than after ESR. This was related with possible formation of La2O2CO3, which could participate in gasification of the carbon deposits.

Application of CO2-treatments during ESR resulted an effective way in the regeneration of Ni/La2O3, Ni/Y2O3, Ni/ZrO2-Y2O3, Ni/ZrO2-La2O3 and Ni/ZrO2-Y2O3-La2O3 catalysts. The partial removal of carbon deposits through the reverse Boudouard reaction is related with the surface density of rather strong basic sites of the catalysts. Ni/12Zr29Y13La exhibited the smallest carbon formation under substoichiometric ESR with intermediate CO2-treatments and the formed carbon deposits were poorly ordered. This catalyst was tested under long-time substoichiometric ESR test (280 h). Using periodic CO2-treatments for regeneration, a continuous operation of Ni/12Zr29Y13La under 100% ethanol conversion with a H2 yield of about 65% during a long-term ESR experiment was achieved.

En este trabajo se han estudiado catalizadores de níquel soportado para la producción de hidrógeno a partir del reformado subestequiométrico de etanol con vapor. En el mismo se analiza el efecto de la introducción de CO2 como reactivo, realizando un reformado asistido con CO2 en condiciones subestequiometricas de agua/etanol. El objetivo principal del estudio fue relacionar el comportamiento catalítico de los catalizadores con las propiedades físico-químicas de los soportes usados. Además, se hizo énfasis en la aplicación del dióxido de carbono como agente regenerador de los catalizadores con el fin de reducir la presencia de depósitos carbonosos, que es el problema más común de los catalizadores de níquel.

Atendiendo a los objetivos mencionados, se preparó y caracterizó una serie de catalizadores de níquel soportado sobre óxidos simples (MgO, Al2O3, Y2O3, La2O3 y ZrO2), y sobre sistemas binarios (ZrO2-Y2O3, ZrO2-La2O3) y ternarios (ZrO2-Y2O3-La2O3), con diversas composiciones. Para los catalizadores preparados se determinó las propiedades fisicoquímicas de superficie, como el área BET expuesta (fisisorción de N2), la densidad y fuerza de los centros básicos (TPD-CO2, calorimetría de adsorción de CO2) y la composición atómica (XPS). Se determinó las diferentes fases cristalinas presentes (XRD) y su reducibilidad (H2-TPR). El comportamiento catalítico de todos los catalizadores se estudió en condiciones de reformado subestequiométrico de etanol con vapor (etanol/agua = 1.0/1.6, ratio molar), a 600 °C. Adicionalmente, con los catalizadores soportados sobre óxidos simples se hicieron ensayos bajo condiciones de reformado subestequiométrico de etanol con vapor y asistido con CO2 (etanol/CO2/agua = 1.0/1.6/1.6, ratio molar).

Todos los catalizadores preparados resultaron activos bajo las condiciones de reacción; el producto principal fue H2. Se obtuvo también CO, CH4, C2H4, CO2 y acetaldehído, en diferentes proporciones en función del catalizador y las condiciones de reacción. Los catalizadores usados fueron caracterizados mediante XRD, XPS, espectroscopias Raman e infrarroja y experimentos de oxidación a temperatura programada (TPO-MS), determinando la cantidad y las características de los depósitos carbonosos formados bajo las condiciones de reacción.

El efecto en el comportamiento catalítico de la introducción de CO2 con reactivo se ha podido relacionar con las características básicas de los catalizadores. La aplicación de CO2 como agente de reactivación resultó efectiva en la disminución de la cantidad de depósitos carbonosos para los catalizadores Ni/La2O3, Ni/Y2O3, Ni/ZrO2-Y2O3, Ni/ZrO2-La2O3 y Ni/ZrO2-Y2O3-La2O3. El catalizador Ni/12Zr29Y13La mantuvo conversión completa de etanol, y rendimiento hacia H2 de alrededor de 65%, bajo condiciones de reformado subestequiométrico de etanol mediante la aplicación de tratamientos periódicos de regeneración con CO2.