Just to pin a date on it, platinum was declared an element in Europe in 1751, according to the Northwest Territorial Mint As platinum was initially discovered in the New World, I figure that's a decent source.

That doesn't really answer how it became known as "more valuable than gold", which - if you ask me - is a dubious status considering that many things are technically more valuable than gold, and the price of raw materials fluctuates all the time. But if that's how it became known in popular culture for a time, I'm certain the answer lies after 1751

Platinum has been known for quite a while. Though already used by the Egyptians, it first became available in notable quantities as a by-product of gold mining and refining in Spain's American colonies, most notably in Colombia. Like gold, platinum does not oxidise nor get dull. It is also rather bright, hence it is attractive for being used in jewelry. The disadvantage in using platinum is its extremely melting point of 1768 °C = 3215 °F. This fact at first prevented it being used and in fact the metal was seen as a nuisance, as "unripe gold" or "small silver". Even the discovery in 1751 by Scheele, who also determined that platinum is an element, that one could melt is with the additiion of arsenic did not produce a practical solution for this problem.

The turning point for platinum's use was the discovery of processes to produce pure oxygen and of practical ways to feed high-temperature flames with the gas. These discoveries took place in the mid- to late 19th century mainly by Lavoisier and the first practical melting process for platinum was developed in 1862 by Deville and Debray. Since then, platinum is not just being used for jewelry but also increasingly employed as a catalyst or catalyst component in industrial processes. It is this industrial use together with its rarity (~30 times less abundant than gold) that gives it its high value.