CeO2-supported Pt/Ni catalyst for the renewable and clean H2 production via ethanol steam reforming
Graphical abstract
Highlights
► Pt/Ni/CeO2 showed promising performance at low temperature. ► The products distribution was close to the equilibrium. ► Few carbon depositions could occur, but not linked to the active sites. ► The water promotes the gasification reactions. ► The reaction pathway includes 6 reactions.
Introduction
Hydrogen plays an important role as an energy carrier for the transition to the green power engineering [1], [2]. It is the preferred fuel in fuel cells, and its production from methane, methanol, and gasoline has been extensively studied [3], [4].
Steam reforming of natural gas is currently the dominant technology but, recently, the rising concern with the reduction of greenhouse gas emissions and atmospheric pollution increased the interest in clean and renewable feedstocks [5].
Steam reforming of ethanol (ESR) is seen as a sustainable route to feed on-line proton exchange membrane fuel cells (PEMFC) with hydrogen. This reaction is drawing much attention due to the high H2 yield at low temperature and the non-toxic character of the reactant, which can be prepared from the renewable biomass [6], [7]. It is directly usable in the ESR reaction as an aqueous solution, avoiding the costs related to the water separation processes. In addition, bio-ethanol provides a renewable carbon cycle when it is used as the feedstock for hydrogen production and the thermodynamics properties allow high ethanol conversion at low temperature; moreover, ethanol is significantly less toxic than methanol and gasoline. Ultimately, the economic future of ethanol production looks even more favourable when one considers the likely increasing in the price of petroleum and other fossil fuels as the world's reserves are depleted [8], [9].
As compared to partial oxidation (POX) and oxidative steam reforming (OSR), the ESR reaction is characterized by higher hydrogen yields, lower reaction rates and a marked endothermicity (Eq. (1))
This multi-molecular reaction can lead to a complex equilibrium involving several reactions giving off hydrogen and numerous by-products such as carbon oxides, methane, ethylene, acetaldehyde, acetone and coke [6], [7] (Eqs. (2), (3), (4), (5), (6), (7), (8), (9), (10), (11), (12), (13), (14), (15)).
In this work, the thermodynamic analysis of the ESR reaction has received considerable attention, to provide guidelines for the selection of optimal operational parameters. The equilibrium composition of the gases, in the temperature range 100–1000 °C, has been calculated considering nine gaseous species, C2H5OH, H2O, CH4, CO, CO2, H2, C2H4O, C3H6O, C2H4 and one in solid phase, carbon, as potential products. It is also assumed that the carbon formed is elemental, in the graphitic form, with a negligible vapour pressure in the overall analyzed temperature range. In this way, the amount of carbon can be included in the elemental mass balance [10].
Confirming the literature results, since the reaction takes place with an increase in the number of moles, the equilibrium conversion decreases as the reaction pressure increases. As a consequence, high pressure ESR is undesirable [11]. The effect of H2O/EtOH (S/E) ratio and reaction temperature on the equilibrium ethanol conversion showed that at S/E = 3, the reaction reached nearly 100% ethanol conversion at temperatures higher than 577 °C. Excess H2O improved the equilibrium conversion, reaching a value higher than 95% with S/E = 5 at 277 °C, and reducing the yield of undesirable products such as CO, CH4 and carbon [12]. Carbon formation occurs only at low water to ethanol ratios (<2) and low temperatures (<600 °C) [13].
At temperatures higher than 600 °C the ESR is thermodynamically favoured, but it is also promoted the formation of a large amount of CO, which is a poison for the PEM fuel cells. [14], [15]. As a consequence, the water gas shift (WGS) reaction (Eq. (14)) must be performed in one or more later steps on the reformate, to purify hydrogen from CO. Thus high temperature operations raise an obvious concern over the energy efficiency of the ESR, as well as the overall energy cycle [16], [17], [18].
As a result, it is of great interest to lower the ESR reaction temperature, with the objective of minimizing both the CO formation and the thermal duty [19], [20]. Several recent papers have proposed different process loops options at low temperatures [21], [22] In many cases, it has also been considered to assemble the functions of the traditional reformer and the gas cleaning unit into a membrane reactor (MR) [23], [24].
Nevertheless, at temperatures lower than 600 °C, the hydrogen yield strongly decreases, since the conditions are in favour of the formation of unwanted by-products, which may in turn be coke precursors [25]. The efficiency of hydrogen production from ESR depends on the control of the formation of these products. Therefore, a properly designed catalysts for the steam reforming of ethanol at low temperature (LT-ESR) is important, to achieve high efficiency and selectivity for hydrogen production [18].
Both noble metals (e.g., Pd, Pt, Ru, Rh) and non-noble metals (e.g., Co, Ni, Cu) have been studied for ESR reaction [26], [27], [28]. Some preliminary works have been published recently on the use of Rh/Al2O3, Ni/La2O3, ZnO, Co/Al2O3, Cu/SiO2 combined with Ni/MgO, Cu, Ni, Pt, or Rh on various supports, potassium promoted Ni/Cu, and dual bed Pd/C Ni/alumina systems [29], [30], [31], [32].
The catalytic performance of supported noble metal catalysts has been investigated with respect to the nature of the active metallic phase (Rh, Ru, Pt, Pd), the nature of the support (Al2O3, MgO, TiO2) and the metal loading. Rh/CeO2–ZrO2 is very active for ESR at low temperature (450 °C) [33], [34], and for low-loaded catalysts, seems to be more active and selective towards hydrogen formation compared to Ru, Pt and Pd. Rhodium is also resistant to sintering among the oxide-supported noble metals catalysts for ethanol reforming [14], [35], [36], [37], [38], [39], [40], [41] but its performance is strongly dependent on oxide support. Platinum is considered very promising for the ethanol reforming to hydrogen, even if typically promoted via ethanol decomposition and dehydration [41]: it is possible to take advantage of the natural aptitude of Pt for the CO removal, enhancing its activity and selectivity by using the synergic effect of Co or Ni [11]; on the other hand, the activity of platinum in WGS reaction can be used to improve to catalytic performance of other metals.
Among the non-noble metals, cobalt and nickel are very common active species for the desired reactions [42], [43], [44], [45]. Nickel catalysts are well known for their activity in reforming reactions and low cost. However, they have a troublesome problem, such as deactivation tendency, due to a high coke deposition. As a result, catalysts have been developed that limit the coke formation; i.e., Ni supported on Y2O3, La2O3 and CeO2 to reduce carbon deposition, and ZrO2 with Y2O3 and La2O3 for CO reduction, [45] or Cu/Ni/K/γ-Al2O3 [46], [47], [48] and (Ni/La2O3)/Al2O3 [49], [50] for enhancing activity and long-term stability for hydrogen production.
In the present study the reaction of ethanol steam reforming at temperature lower than 600 °C on a commercial CeO2-supported Pt/Ni catalyst has been carried out. The dependence of the catalytic activity and selectivity on the reaction temperature, the H2O/C2H5O molar ratio and the contact time were studied. Time on stream experiments were also performed in order to evaluate catalyst stability and coke formation. A preliminary kinetic approach was realized, leading to the mathematical modelling of the reaction pathway.
Section snippets
Catalyst preparation and characterization
The catalyst 3 wt%Pt–10 wt%Ni/CeO2 was prepared by calcination of commercially available CeO2 (Aldrich, BET = 80 m2/g) at 600 °C (heating rate = 10 °C/min), followed by sequential wet impregnations using respectively, aqueous nickel acetate C4H6O4Ni·4H2O (Aldrich) and platinum chloride PtCl4 (Carlo Erba Reactants) solutions, with an intermediate and a final calcination at 600 °C.
The metal load has been previously optimized through a preliminary screening of different relative amounts of the noble and
Catalyst characterization before the ESR reaction
The experimental metal load obtained by XRF analysis was in agreement with the nominal content, as reported in Table 1. The result of SSA measurement, reported in the same table, showed that the deposition of the active species causes a specific surface area decrease with respect to the pure support, probably due to crystallites rearrangement and the metals–support interaction during impregnation and calcination steps [54].
XRD diffractograms of calcined support and catalyst samples have been
Conclusions
The performances of a bimetallic catalytic system, based on Pt (3 wt.%) and Ni (10 wt.%) and supported on CeO2, were investigated for the ethanol steam reforming at 250–600 °C.
The catalyst Pt/Ni revealed a good ability in the C–C bond rupture of the ethanol molecule, and this is ascribed to the well-known function of the nickel, enhanced by the platinum, that is directly available on the catalyst surface [79].
The sample showed high activity in the desired reaction, with a products distribution in
Acknowledgments
This work has been partly funded by the European Commission through the FCH JU Project CoMETHy: Compact Multifuel-Energy To Hydrogen converter (GA No. 279075).
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