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The Human Development Index, a measure of the quality of life developed by the United Nations Development Program, is plotted against the per capita energy consumption of 103 of the world’s most populous nations, representing a total of 5.763 billion people. The data refer to 2004 [2]. 

The Human Development Index, a measure of the quality of life developed by the United Nations Development Program, is plotted against the per capita energy consumption of 103 of the world’s most populous nations, representing a total of 5.763 billion people. The data refer to 2004 [2]. 

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The photocatalytic reduction of CO2 with water vapour and catalysts under UV irradiation to yield hydrocarbons is a potential way of decreasing greenhouse gas and it represents an attractive alternative energy source to fossil fuels. However, this process still has to overcome several hurdles, because it involves the activation of two stable molecu...

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... use of fossil fuels results in a dilemma for society. On one hand, the combustion of coal, natural gas and oil satisfies about 90 per cent of our current energy needs. However, it produces, and releases into the atmosphere, more than 3.5 t of carbon dioxide (CO ) for each 1 t of equivalent carbon burned [1]. Moreover, the Earth's surface temperature has risen by approximately 0.6 K over the past century, with particularly significant increases over the past 20 years. There are many reasons why fossil fuels remain so popular: they are accessible, in different forms, in almost all of the world; humans have learned how to use them effectively to provide energy for a variety of applications; they are relatively cheap and easily transportable. It is clear that, in our current global energy economy, a higher standard of living means increasing energy consumption [2]. This situation is shown in Figure 1 , where the human development index (HDI) is plotted against the per capita energy consumption, with energy expressed in units of kilogram equivalents of oil. The current approaches towards the reduction of CO emissions are mainly focused on carbon capture and storage (CCS). Carbon capture refers to the removal of CO 2 from flue gas, by means of gas separation, before it is released into the atmosphere, while carbon sequestration (storage) refers to the isolation of CO from the atmosphere. At present, the most commonly used technologies for the capture of CO 2 are gas absorption into chemical solvents, permeation through membranes, gas adsorption into a solid sorbent and cryogenic distillation, but none of these are economically convenient. A reduction in CO 2 emissions can be achieved through three approaches: post combustion capturing, pre-combustion capturing and oxy fuel combustion [3]. A fourth option, which is the topic of this work, is to use captured CO 2 to synthesize sustainable hydrocarbon fuels [4]. The great advantage of liquid fuels ( e.g. petroleum, diesel and others) is their intrinsic chemical energy content and the easiness by which they are stored and transported. It is possible to reduce CO 2 with hydrogen or even electricity, to synthesize sustainable fuels, but the renewable source that has to be used as the vector to transform CO 2 into fuels would not contribute to the net CO emissions. An idealized energy cycle is shown in Figure 2 , where CO 2 is transformed into carbon neutral liquid fuels and sustainable or renewable electricity is used to produce hydrogen and the resulting Fischer-Tropsch process liquid hydrocarbon fuels [5]. The energy requirements for the production of such renewable fuels depend on the methods that are used to capture CO and to produce hydrogen. In a recent survey of their work, Olah et al . have stated: "Carbon dioxide can be chemically transformed from a detrimental greenhouse gas causing global warming into a valuable, renewable and inexhaustible carbon source of the future allowing environmentally neutral use of carbon fuels and derived hydrocarbon products" [6]. Therefore, the aim throughout the world is to improve capture and storage technologies; this will allow large amount of CO 2 to be obtained, which will then become available as feedstock for innovative conversion to synthetic fuels. CO 2 is a highly stable molecule. Therefore, for the chemical conversion of carbon dioxide to a synthetic fuel, it is necessary to assure a high energy input, optimized reaction conditions and very active catalysts. A key aspect of the thermodynamics of any possible CO conversion is illustrated in Figure 3 . However, it is important that any chemical reaction is driven by the differences in Gibbs free energy between the reactants and the products of a chemical reaction, as can be seen from the Gibbs-Helmotz ...
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... situation is shown in Figure 1, where the human development index (HDI) is plotted against the per capita energy consumption, with energy expressed in units of kilogram equivalents of oil. ...

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