Showing papers on "Photocatalysis published in 1975"

Generation of catalysts by photolysis of transition metal complexes

Abstract: The concepts of photocatalysis and photoassistance are defined and recent experimental results involving transition metal carbonyls as photocatalysts and photoassistance agents are described Photolysis of Fe(CO)5 is shown to lead to olefin hydrogenation under mild conditions (H2 1 atm, 25°), and d8 —+ d6 oxidative addition to photogenerated coordinatively unsaturated iron carbonyl intermediates is shown to be a possible pathway both for hydrogenation and olefin isomerization Olefin isomerization, hydrogenation, and hydrosilation via photolysis of Cr(CO)6, Mo(CO)6 and W(CO)6 are also described, and d6 -÷ d4 oxidative additions to photogenerated coordinatively unsaturated intermediates are invoked to account for the olefin activations Photoisomerization of the coordinated styrylpyridines in M(CO)jstyrylpyridine)6 (n =5, 4; M W, Mo, Cr) and XRe(CO)3(trans-4-styrylpyridine)2 (X = Cl, Br) is discussed in terms of model photoassistance situations Metal—metal bond cleavage is shown to dominate the photochemistry of the compounds M2(CO)10 (M = Mn, Re), Mn2(CO)9PPh3, Mn2(CO)8(PPh3)8, and [h5-C5H5M(CO)3]2 (M = Mo, W) The resulting mononuclear metal fragments react efficiently with other photogenerated metal fragments, '2or alkyl halides INTRODUCTION We define the term photocatalysis to be the photogeneration of a catalyst from a thermally inert precursor such that substrate transformations result which are both catalytic with respect to the number of photons absorbed and with respect to the actual catalyst Photocatalytic systems such as that shown in Scheme I are the subject of this paper For Scheme I the following points are to be noted: (1) the only excited state chemistry is the primary act of catalyst formation; (2) the catalyst which is generated behaves as a thermal Metal complex --* [Catalyst] (1) Metal complex + Substrate —-÷ No reaction (2) [Catalyst] + Substrate —-+ Transformed substrate + [Catalyst] (3) [Catalyst] —'-+ Poisoning (4)

Ultraviolet-gas phase and -photocatalytic synthesis from CO and NH3.

Abstract: Ammonium cyanate is identified as the major product of the photolysis of gaseous NH3-CO mixtures at 206.2 or 184.9 nm. Lesser amounts of urea, biurea, biuret semicarbazide, formamide and cyanide are observed. A series of 18 reactions underlying the formation of photolysis products is presented and discussed. Photocatalytic syntheses of C-14-urea, -formamide, and -formaldehyde are carried out through irradiation of (C-14)O and NH3 in the presence of Vycor, silica gel, or volcanic ash shale surfaces. The possible contributions of the relevant reactions to the abiotic synthesis of organic nitrogen compounds on Mars, the primitive earth, and in interstellar space are examined.

Ultraviolet-gas phase and -photocatalytic synthesis from CO and NH3. [photolysis products]

Abstract: SummaryThe major photoproduct obtained on irradiation of gaseous NH3 and CO mixtures is ammonium cyanate; lesser amounts of urea, biurea, biuret semi-carbazide, formamide and cyanide were observed. The formation of the major gas phase photolysis product may be rationalized by the following reaction sequence: Urea is probably formed from NH4NCO in a thermal reaction while formamide may result from the disproportionation of NH2CO.Photocatalytic syntheses of14C-urea, -formamide, and -formaldehyde are effected by irradiation of14CO and NH3 in the presence of Vycor, silica gel, or volcanic ash shale surfaces. These syntheses are catalyzed by ultraviolet wavelengths longer than those absorbed by the gaseous reactants. The syntheses are also effected when the surface material is first irradiated in the presence of CO followed by a dark incubation with NH3. Apparently, the initiating step is a light dependent formation of a reactive form of CO on the surface.A discussion is given on the possible contribution of these reactions to the abiotic synthesis of organic nitrogen compounds on Mars, on the primitive Earth and in interstellar space.