True, they already have a good amount of Euros, as it's the currency they've been using.
However, there are several practical problems. Catalonia could happily use the currency for their own transactions, just as they could use seashells or rocks. There's no issues there. The issues are all international, very important given how much of Catalonia's business is internationally focused.
1) Currency has a lifespan. As a result the EU is constantly printing new bills to replace old bills that are no longer usable. I believe Euros last longer than dollars, but US money has a lifespan of between 5-10 years depending on denomination. Beyond this lifespan point - and possibly much sooner depending on how new their Euros are, Catalonia will need to create their own currency or start buying (which would likely be made impossible).
2) EU can print new bills and coins with modified designs and do trade-ins (this has already been happening throughout 2015 in the new Europa series). Don't offer trade-ins to Catalonians and refuse the use of old bills after a certain date. EU countries are already used to this procedure, as they all did it quite recently when they traded in their old national currencies for Euros and then made old bills invalid.
3) Less effective, but cheaper and easier, the EU could offer a trade-in program just for bills printed in Spain (which are /probably/ most of the bills in Catalonia), which are marked by a V in their serial number. Allow other Spaniards (and other EU members) to trade in their V series bills for a new Spanish A series and mark V series bills as invalid.
All of these would have the effect of making the Euros Catalonia already has useless for international transactions as "Euros." This does at least mean they have a usable currency and don't need to instantly spend money to create their own, but they still will need to eventually, since money wears out (see step 1).
But beyond all this I chuckle at how much Catalonia whines about the economics. They get shorted €1,500 per capita (if we're being generous and using their numbers, not Spain's numbers) by helping out poorer parts of Spain. In the US? New Jersey gets shorted $3,900 per capita in federal taxes paid vs federal spending received. Mississippi gets an extra $7,900 per capita. I don't see New Jersey trying to secede because the US is stealing their money to help Mississippi. And this pattern is seen all over the world from richer areas to poorer areas, often in much worse terms than Catalonia.
If they want to secede for language and culture, whatever I guess. They do build human pyramids so clearly they need to be independent /s. But framing it economically is silly. If they deserve to secede on economics then Germany should leave the EU, New Jersey should leave the US, and Bavaria should leave Germany, etc.
The resulting geography is laughable and in no one's best interest.