Abstract

As a source of energy, fossil fuels have a tremendous impact on the planet in different ways, including human welfare and the environment. In order to deal with the problem of petroleum derived as a source of energy, and also as a source of pollution, scientists are investigating the means to improve the yield of energy with a mimimun level of toxic effluents. 

Investigations in projecting optimum fuel consumption to reduce the energy demand and lowering pollutant emissions that have to meet the regulations. 

During the last three decades there are growing concerns about the shortage in energy supply in the world, as well as about the increase of pollution including that of green house gases. This has spurred the development of fuel cells that convert chemical energy to electrical energy using hydrogen as fuel with low to zero pollutant emission.

Thousands of leading world scientists conclude that there were discerning evidences of the global warming taking place and increasing progressively within the next century, the world community has been exhorted to consider mitigating this effect by reducing the greenhouse gases emission (in particular CO2) to the atmosphere. This alert has pushed governments of industrialised countries to refocus they energy policies and strategies with the dual purpose of improving security of supply and reducing greenhouse gases emissions.

In the last few years, extensive literature dealing with different options for cleaner energy sources, their advantages, handicaps and economic aspects has appeared. Among the bundle of proposed alternatives there is a common statement: in the long-term hydrogen seems too be the most suitable large-scale fuel because it has the advantages of its clean combustion, allowing generation from any imaginable source of energy and storage over time.

However, it must be stressed that hydrogen is not an energy source but an energy carrier. The advantage of using hydrogen as fuel, as far as security of supply or greenhouse gases emission is concerned, will depend on how it is produced. In the long term, a hydrogen-based energy system would have to use renewable primary energy sources for meeting sustainability goals. Reaching this scenario will require significant cost and performance improvements in production, conversion, storage, transportation, distribution and end- use technologies. 

However, it must be stressed that hydrogen is not an energy source but an energy carrier. Hydrogen is the ultimate clean energy carrier. When it is combusted, heat and water are the only products. When it is used as fuel for fuel cells, then it has the potential of providing much higher efficiencies compared to combustion. Thus, hydrogen offers a potentially non-polluting and efficient fuel for today’s rising energy demands.

Fuel cells are electrochemical devices that directly convert chemical energy into electrical energy with high efficiency. Hydrogen and fuel cell technologies have the potential to revolutionize the way we produce and use energy. The development of a "economy based on hydrogen" implies its use as combustible and the use of the electrochemical batteries like devices of transformation of energy, being a this one way would revolutionize to produce and to use the energy. 

Although hydrogen is abundant on earth as an element, hydrogen combines readily with other elements and is almost always found as part of some other substance, such as water, hydrocarbons, or alcohols. It is also found in biomass, which includes all plants and animals. Therefore, the advantages of using hydrogen as fuel will depend on the raw material from which it is obtained and of the technology used for its production.

The advantage of using hydrogen as fuel, as far as security of supply or greenhouse gases emission is concerned, will depend on how it is produced. In the long term, a hydrogen-based energy system would have to use renewable primary energy sources for meeting sustainability goals. Reaching this scenario will require significant cost and performance improvements in production, conversion, storage, transportation, distribution and end- use technologies. The transition to a fully developed “hydrogen economy” will need structural changes, which will span over many decades. 

In the near and mid-term, hydrogen production from hydrocarbons seems to be the best option to achieve a gradual transition, given that the present infrastructure can be used.
Currently, steam reforming of natural gas, which is composed mainly of methane, is used to produce most of hydrogen in the U.S. and about half of the world’s hydrogen supply. Naphtha fractions with a final boiling point of less than 220ºC are also considered as suitable feedstock. However, supported Ni catalysts suffer from catalyst deactivation by coke formation more severely when higher hydrocarbons are reformed at low steam/carbon ratios. Although supported precious metals such Pd, Pt, and Rh have been reported to be active and stable for steam reforming of hydrocarbons, cost of the precious metals is still a major issue. The low-cost and long-proven performance of Ni-based catalysts, therefore, warrant the efforts to optimize these catalysts for more demanding steam reforming applications. On-board steam reforming of hydrocarbons for fuel cell-powered vehicles has attracted much attention. Widespread applications of fuel cell in transportation will depend on the development of an effective and efficient fuel processing technology from existing liquid fuels such as gasoline and diesel especially during transition to hydrogen economy. Therefore, development of novel Ni-based catalysts with superior performance and stability for the steam reforming process is essential.

Steam reforming process transforms a hydrocarbon stream into a gaseous mixture constitued by CO2, CO, CH4 and H2. Reaction product composition is determined by the thermodynamic equilibrium between gaseous species according to operation conditions at which this process takes place (pressure, temperature, steam/carbon ratio and space velocity). Thus, for producing a hydrogen-rich gas effluent is suitable working at low pressure, high temperature and with a high steam/carbon ratio, with the purpose of moving the thermodynamic equilibrium, which determines the composition of the gas, towards hydrogen formation. 

For many years, nickel has been the most suitable metal for steam reforming of hydrocarbons. The current steam reforming catalysts are mainly nickel supported on refractory alumina and ceramic magnesium aluminate. These supports provide high crush strength and stability [14]. However, coke formation is the major problem associated with nickel catalyst, coking is an even more serious problem when reforming heavy hydrocarbons fuels such us gasoline and diesel [18-22]. The risk towards carbon formation increases as increases hydrocarbon molecular weight. Therefore, catalysts with improved coke resistance for steam reforming of available hydrocarbons, such as gasoline and diesel are highly desirable. Such catalysts must also have high activity, selectivity, and durability.
In addition to coke formation, the necessity to operate at high temperature and with the presence of high steam partial pressure introduces several potential problems and other deactivation mechanisms can act during this process due to the severe operating conditions. For example, some of they are metallic nickel oxidation, nickel-support reaction forming hardly reducible compounds (as NiAl2O4) and the presence of steam tending to enhance catalyst and support sintering. 
There is a necessity of developing new materials with improved characteristics such as higher resistance to carbon deposition and high stability in the reaction conditions to avoid catalyst deactivation.

Layered Double Hydroxides (LDHs), or hydrotalcite-like (HT) compounds with general formula [M2+1-x M3+x (OH)2]x+ [Ax/n]n- • m H2O, are lamellar materials of Brucite-like layers with positive charge and anionic compounds in the interlayer to form neutral compounds which possess versatile acid-basic and  redox properties and have potential applications in various catalytic fields. These materials may contain divalent (M2+) and trivalent (M3+) cations then various kinds of transition metals could be introduced into the Brucite-like layer having a high potential function as the catalytic active centre.
HT compounds easily decompose into a mixed oxide of the M2+M3+(O2-) type upon calcinations. Further reduction of HT compounds or mixed oxides, containing reducible cation yields supported metal catalysts. Catalysts obtained by calcination/reduction of hydrotalcite-type materials are characterized by highly dispersed metallic crystallites stabilized inside an inert matrix with high surface area.
In a previous work  we have studied HT-based catalysts (prepared from a hydrotalcite-like precursor), and they show higher activity than do conventional supported catalysts due to the higher BET and metal areas, higher metal dispersions and smaller nickel particle size. Other studies have been realized using these materials in reforming reactions. Some authors have been used HT-materials in steam reforming or partial oxidation processes using methane or light paraffins as fuels. 
The properties of supported catalysts can be fine-tuned by use of the metal-support interaction.
There is practically no limitation with regard to the nature of cations belonging to the structures, which is a great advantage for the applications of the catalysts obtained from HT materials. One major consequence is that LDH containing transition or novel metal cations are precursors leading, after appropriate activation treatment, to supported metal catalysts.(parte nueva)
Thermal treatments lead to synergetic effects between the elements in mixed oxide structures, and after appropriate activation treatment, give rise to well dispersed metal particles like a supported metal catalysts, with the possibility of controlling metal-support interaction during the synthesis stages. So that, rational design of adaptable multifunctional nanostructured catalysts based on LDH precursors becomes a real possibility which offers the opportunity of controlling the nature of the active sites and their environment, the texture and the stability of the catalysts. Due to their wide variety of compositions, LDHs are attractive precursors of multicomponent nanostructured catalysts, highly functionalized at atomic level.
Structure and reactivity of mixed oxides obtained by thermal decomposition of LDHs is greatly influenced by the preparation procedure (co-precipitation, impregnation, ionic exchange, sol-gel, hydrothermal, etc.) and the synthesis variables (composition, nature and amount of the elements, metal precursor, reaction temperature, stirring speed, ageing, drying, calcinations, reduction, etc.)

In order to develop an efficient liquid hydrocarbon steam reforming catalyst we have studied the design of nickel-based hydrotalcite-derived catalysts. This work reports synthesis, physicochemical characterization and catalytic testing of three-element mixed oxides obtained by coprecipitation method and varying several synthesis parameters. We have studied the effect of synthesis parameters on structure and reactivity of these materials in the mentioned reaction.

The studied variables have been: the composition of the precursory material, the method of incorporation of the active metallic species, the temperatures of calcination and reduction, the speed of addition of the dissolutions, the temperature of aging of the gel, the ion concentration in the dissolutions, pH of the synthesis, the interlaminar anion and the dissolvent used in the case of the impregnated samples. Also it has been studied the effect of the introduction of small amounts of promoters such as Ce, La, Fe, Cr, Ca, Zn, Mn, Li, Cu, Co. Finally, with the best catalysts its stability has studied and also a kinetic and mecanistic study of the process has been made. 

Synthesis parameters such as method of introduction of active species, amount of components and thermal treatments (calcinations and reduction), could be optimized to obtain a material which presents high catalytic activity and high resistance to carbon formation. Ideal situation would be that which combines all these variables to obtain a material with a suitable combination of textural properties (dispersion, form and metallic particle size, specific area and proportion of different superficial metallic planes) to get recombination of C1 species with oxygen species coming from the water dissociation taking place at the same rate at which they have been formed from C-C ?-scission, and thus to avoid C1 species remaining on active sites sufficient time to form coke.