text: The first efforts that can be considered photogeochemical research can be traced to the "formaldehyde hypothesis" of Adolf von Baeyer in 1870, in which formaldehyde was proposed to be the initial product of plant photosynthesis, formed from carbon dioxide and water through the action of light on a green leaf. This suggestion inspired numerous attempts to obtain formaldehyde in vitro, which can retroactively be considered photogeochemical studies. Detection of organic compounds such as formaldehyde and sugars was reported by many workers, usually by exposure of a solution of carbon dioxide to light, typically a mercury lamp or sunlight itself. At the same time, many other workers reported negative results. One of the pioneer experiments was that of Bach in 1893, who observed the formation of lower uranium oxides upon irradiation of a solution of uranium acetate and carbon dioxide, implying the formation of formaldehyde. Some experiments included reducing agents such as hydrogen gas, and others detected formaldeyhde or other products in the absence of any additives,  although the possibility was admitted that reducing power may have been produced from the decomposition of water during the experiment. In addition to the main focus on synthesis of formaldehyde and simple sugars, other light-assisted reactions were occasionally reported, such as the decomposition of formaldehyde and subsequent release of methane, or the formation of formamide from carbon monoxide and ammonia.
  In 1912 Benjamin Moore summarized the main facet of photogeochemistry, that of inorganic photocatalysis: "the inorganic colloid must possess the property of transforming sunlight, or some other form of radiant energy, into chemical energy." Many experiments, still focused on how plants assimilate carbon, did indeed explore the effect of a "transformer" (catalyst); some effective "transformers" were similar to naturally occurring minerals, including iron(III) oxide or colloidal iron hydroxide; cobalt carbonate, copper carbonate, nickel carbonate; and iron(II) carbonate. Working with an iron oxide catalyst, Baly concluded in 1930 that "the analogy between the laboratory process and that in the living plant seems therefore to be complete," referring to his observation that in both cases, a photochemical reaction takes place on a surface, the activation energy is supplied in part by the surface and in part by light, efficiency decreases when the light intensity is too great, the optimal temperature of the reaction is similar to that of living plants, and efficiency increases from the blue to the red end of the light spectrum.
  At this time, however, the intricate details of plant photosynthesis were still obscure, and the nature of photocatalysis in general was still actively being discovered; Mackinney in 1932 stated that "the status of this problem [photochemical CO reduction] is extraordinarily involved."  As in many emerging fields, experiments were largely empirical, but the enthusiasm surrounding this early work did lead to significant advances in photochemistry. The simple but challenging principle of transforming solar energy into chemical energy capable of performing a desired reaction remains the basis of application-based photocatalysis, most notably artificial photosynthesis (production of solar fuels).
  After several decades of experiments centered around the reduction of carbon dioxide, interest began to spread to other light-induced reactions involving naturally occurring materials. These experiments usually focused on reactions analogous to known biological processes, such as soil nitrification, for which the photochemical counterpart "photonitrification" was first reported in 1930.