Hydrogen reactions with catalysts

Question

I have a question about reactions involving hydrogen using catalysts. I'm a layman, so forgive me if my questions seem elementary.

Reacting hydrogen plus oxygen and a flame reacts, unleashing energy (in the form of an explosion) and creating water.

My questions:

1. Why does it need a "flame or spark "? Is there something unique about the fire that causes the reaction? Or is it merely the heat of the flame? In other words, if you put hydrogen and oxygen in, say, a pressurized vessel and added energy in the form of heat (but not a flame) would it still react and create water? Would it do it without an explosion?

2. I understand catalysts "speed up" the reaction, or put another way, reduces the amount of energy required to cause it. I've read that in this particular reaction ($\ce{H2 + O2}$) copper works as a catalyst. Is there a way to determine by looking at the chemical formulas if other catalysts would work as well or better or is it just trial and error? For example, could you tell if silver would also act as a catalyst just by looking at the formula? Or would you need to actually add silver and see for yourself?

3. When a catalyst is used, is it inexhaustible or does it get "used up"? If you were using copper to react these two, could you use it forever? If not, is there a way to "recharge" the catalyst?

Thanks in advance, I'm trying to learn about this particular reaction for a project I'm working on ($\ce{H2 + O2}$) but without a formal science background, some of the scientific papers I'm reading are a little hard to grasp.

Answer 1

Adding a bit of information for your question a). The reactin we are looking at is:

$$\ce{2 H2 + O2 -> H2O}$$

As you can clearly see, the final product water has one oxygen atom for 2 hydrogen atoms, while in the beginning, we only had diatomic molecules. This means that during the reaction, at least one fo the diatomics had to break to single atoms. In this case, this happens with the reaction:

$$\ce{H2 -> 2H^.}$$

From this step, we have a nice reaction scheme called a radical chain reaction. However, we first need to break the $\ce{H-H}$ bond somehow, and this is what flame is doing. If you break a significant amount, then you don't need to provide flame anymore, the reaction is self-supporting — the flame is ignited.

Now, a catalyst is doing exactly this. Many metals, including copper, can adsorb hydrogen molecules to the surface, and this also results in breaking the $\ce{H-H}$ bond — so the explosion can start this way too.

Answer 2

Why does it need a "flame or spark "? Is there something unique about fire that causes the reaction?

Yes and no. Hydrogen/oxygen flame is a branched chain reaction, so after injecting enough free radicals it may propagate on its own without heat applied. This, however, not the case for many flames, as they may be not branched (hydrogen/chlorine flame) or require heat to produce combustible gases (common wood/oxygen flame)

if you put hydrogen and oxygen in, say, a pressure vessel and added energy in the form of heat (but not a flame) would it still react and create water? Would it do it without an explosion?

Most combustible compounds have self-ignition temperature at which they ignite without flame. Hydrogen/oxygen mixture has self-ignition temperature too. However if heated below it it may react slowly and nicely... just very slow.

Is there a way to determine by looking at the chemical formulas if other catalysts would work as good or better? Or is it just trial and error?

Sometimes, but generally trial and error. In most cases a family of catalysts catalize a family (or several families) of reactions. Once families are established, an educated guess is possible. However, it is usually not possible to guess catalytic activity without previous experience.

For example. I know, that late transition metals usually are good catalysts for hydrogenation. Hence, I can guess that I can use platinum as hydrogenation catalyst for most alkenes. However, I the knowledge was obtained by trial and error (and explanations are not obvious even after it was found). Finding the best match for the given pair of process and catalyst is also mostly a trial and error, though typically for each family a good educated guess may be made for the several most likely candidates.

When a catalyst is used, is it inexhaustible or does it get "used up"?

Ideally it should be inexaustible. In reality various processes lead to gradual loss of activity.

It not, is there a way to "recharge" the catalyst?

sometimes. For example, catalytic cracking of hydrocarbons uses strong solid acidic catalysts. In the process, coke is formed on the catalyst, leading to loss of activity. Since the catalysts employed are usually pretty stable, it is possible to simply burn the coke and regenerate the catalyst. However, the catalyst may be too fragile for burning off the coke or it may loose activity in other irreversible way.

Answer 3

I always get a knee jerk reaction about SAFETY when I read such questions. Putting a spark in a mixture of hydrogen and oxygen is known to the FBI as detonating a BOMB.

(1) Given a mixture of hydrogen and oxygen: (a) A spark would set off an explosion which would make water as the product and give off heat.

(b) If you "gently" heat a mixture of hydrogen and oxygen enough then the molecules would acquire enough energy to form water. There would be an equilibrium which would depend on temperature and pressure. Heating too vigorously could cause an explosion.

(2) As Jan pointed out there has been a lot of theoretical work on catalysts. However we still don't have the ability to calculate a priori how a particular catalyst will perform. Chemists rather use calculations to make educated guesses, then must actually do an experiment to determine the result.

(3) (a)In theory a catalyst will last forever. In practice catalysts tend to get "poisoned" for one reason or another - typically by contaminates in the reactants. (b) It might be possible to recharge a catalyst. For noble metals like platinum you might just reclaim the metal and start over.