Photocatalytic nitrogen fixation

The photocatalytic processes that could have been involved in the synthesis of prebiotic molecules are not limited to iron sulphides. Photocatalytic processes occurring on the surface of other sulphide and oxide materials should be considered as well. 

Kisch et al. reported photoreduction of N2 at iron titanate films with a Ti Fe molar ratio 1 1 [21, 22], The postulated new semiconducting phase Fe2Ti2Q7 

Ammonia and hydrazine formation was also observed when a neat semiconductor or semiconductor (Ti02, W03) doped with metal ions other than Fem (Cr, Mn, Co) were used as photocatalyst [23,24], In the presence of oxygen nitric oxides and nitrates can also be formed. 

Reduction of C02 photocatalyzed by semiconducting materials may lead to formation of formaldehyde, formic acid, methanol, methane, and oxalate among other products [7], The earliest report on photocatalytic reduction of C02 with water at Ti02 was published by Inoue et al. [25], Almost at the same time the photoreduction of C02 to methane at SrTi03 was reported by Hemminger et al. [26], The same process can also be performed on AgCl/zeolites [27], 

Metal(0) deposited at the Ti02 surface may enhance yields and selectivity of the reduction products [28-30], In the presence of copper(0)-loaded Ti02 formaldehyde and methanol were detected as main products [31]. Mercury- and platinum-loaded Ti02 favoured formation of carbon and formaldehyde [28,29], Methane was obtained with a high selectivity in the case of palladium-doped Ti02, whereas in the presence of Rh, Au, and Ru metals acetic acid was also produced [30], 

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Laboratory-scale processes for nitrogen fixation II. Heterogeneous photocatalytic ammonia synthesis... 

Despite many studies there is available, at this time, no unequivocal report of artificial photocatalytic synthesis of ammonia from nitrogen and water on heterogeneous catalysts. The chemical reaction 1, shown at the beginning of this chapter, remains speculative 17 years after it was initially reported. No rational start has yet been made on determining a mechanism for the reported nitrogen fixation process, despite the fact that a clear demonstration and understanding of any such process would be of great importance. Despite 17 years of research, the reported yields of ammonia from artificial photosynthesis remain at or near the limits of detection by routine analytical methods. As we stated earlier in this chapter, the concept of a major discovery that remains forever on the borderline of detectability is internally contradictory. The current state of the field is one where serious scientific skepticism is appropriate. 

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