question

also, can we apply previously online? or that is just for 144 hour visa free permit?

Your itinerary is definitely eligible for 24 hour visa free transit in Beijing area and beside the printout of your accommodation and your flight tickets you don't need any additional documentation. Also, you don't need any kind of visa.

You will declare at check-in in Hanoi that you will travel to Barcelona via Beijing using 24 hour visa free transit. After a short check of your flight tickets, you will be allowed to board the plane.

Upon arrival you will just approach 24 hour visa free transit counter where you will apply for a temporary entry permit to be allowed to exit the airport. Since your layover is quite long and you will have accommodation booking confirmation, your chances to be granted a permit are practically certain.

You don't need to apply for anything online. Sometimes, this is quite counterproductive.