Interesting. Is this how catalysts work? By bringing the atoms/molecules reacting together and providing a path for taking away the excess energy more easily?

Catalysts work principally by lowering the activation energy of the reaction.

https://en.m.wikipedia.org/wiki/Activation_energy

They don't necessarily bring atoms or molecules together, per se, but some do.

Yes but activation energy is still a "system" level concept. It's not the actual atomic level mechanism that results in the reaction.

There are lots of different types of catalysts: Homogeneous (molecule, typically small, in solution), heterogeneous (solid or porous surface) and enzymes (biological molecules.) Many do work by bringing reactants together so that the relevant fragments can combine. Chemical reactions are usually described as going through an energetic transition state and many catalysts stabilize this state so that the reaction can "get over the hill" and become product. Of course, it also stabilizes it for the reverse reaction so the products can become reactants again. The overall equilibrium is not changed. Both reactions speed up. If, however, one extracts the product as it is formed, more product will be formed and hence catalysts are incredibly useful since, by definition, they are not consumed by the reaction - i.e. you get the catalyst back so it can continue to do its work.

Here's a good example of how enzymes catalyze reactions: https://pdb101.rcsb.org/learn/videos/how-enzymes-work

The last 1/3 of this old video from Shell describes how catalytic cracking, a crucial part of oil refining, works. It covers practical reactor considerations, including getting the products out so more are formed and 'poisoning' where a catalyst's efficiency is reduced by contamination and how that's dealt with in a continuous process. https://www.youtube.com/watch?v=hC1PKRmiEvs

We still don’t know exactly how catalysts work, because they operate at high temperature and pressure, so it is very hard to study.

That doesn't seem right. Cars have catalysts where the air flows through. Also 2 H2 + O2 -> 2 H2O (+ a characteristic bang :-)) at room temperature and pressure (initially) using a platinum catalyst is shown in school chemistry classes.

Just learned on Wikipedia (en/Platinum) that the 2007 Nobel price in Chemistry was awarded for explaining how that works. doi:10.1002/anie.200800480

Car's catalyst work at high temperatures, not at room temperature.

In fact, at room temperature they don't work at all, which is exactly the problem with catalysts, they don't work when you start your car.

They need to heat with the hot combustion gases to start working, and that takes a while.

This is one of the reasons that laboratory's measurements of particles and gases like nitrogen oxides do not apply to real emissions in cities, because most people in cities will use the cars, for example, diesels in Europe, for very short periods of time, then let the car cool down and use it again to go back less than 10 kilometers away.

There are many kinds of catalysts beyond materials surfaces. Organic chemists use metal complexes as catalysts in organic synthesis. This is especially important in asymmetric catalysis where you want to form high excesses of one isomer of a molecule, which can have drastically different effects in medicine for example. Catalysis also plays an important role in biology I believe.

This is completely wrong. Many catalysts work at room temperature and pressure and in many cases we understand very well how they work.

And our bodies are full of those. Enzymes are catalysts.

Couldn’t they poot off a photon?

Can they? Normally[1] emission of a photon would be a result of an electron moving from a high-energy shell to a low-energy shell. Going from two Cl to one Cl_2 is also a transition to a lower-energy state, but it's not immediately obvious that this would be able to generate a photon in the same way as an electron shell transition. You can do the molecule merge with atoms that already had their electrons in the lowest state... can't you?

[1] According to my grade-school chemistry. :/

Yeah I was thinking that too. Given the observations they describe I can only assume that releasing a photon is too slow of a process compared to the speed in which the molecule breaks apart.

Can't you make two atoms form a molecule by smashing them together fast enough (where electrons might fly off)?

If you smash them together faster, you just increase their energy, i.e. the opposite of what you want. If electrons fly off, you are likely to be left with an unstable molecule.

But in a covalent bonding you're sharing electrons, which means you need less electrons than in the atom state.

I sort of see what you are trying to say, but I think you're slightly misunderstanding what is going on. It is true that you are sharing electrons in a covalent bond, but it is not correct to therefore say that you need less electrons than in the atom state. A simple example: say you have to H atoms. Each has one electron. You put them together and form H2, which has two electrons combined. In both cases you need a total of two electrons, in order to have a neutral system. If you change that in any way, you will end up with highly reactive species (e.g. H+, H-, H2+, H2-). H+ is of course just an acid.

What you are probably thinking of, is the fact that if you put NaOH, or HCl in water, you get Na+, OH-, etc., and these are more stable than their neutral counterparts. But this is because they are stabilised by (polar) water molecules. Outside of water (or some other stabilising solvent), Na+, OH-, H+ and Cl- are all very reactive species.