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Give examples of heterogeneous catalysis (i) in gas phase and (ii) in liquid phase.

Now, I came to know that heterogeneous catalysis involves different phases between the catalyst used and the reactants. That way, either the catalyst can be in solid phase where the reactants can be in gas/liquid phase. But in the above question, what does "in gas phase" mean? Is it about the gas phase of the catalyst or the gas phase of the reactants?

Also, it would be of great help if you could provide me with examples of the above question.

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  • $\begingroup$ Look up the “Haber process” in wiki and scroll down to “Catalysts”. One of the most important catalyzed processes in the world. $\endgroup$
    – Ed V
    Apr 25 '20 at 0:36
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    $\begingroup$ "in gas phase" refers just to the phase of the reactants and products. The catalyst in these heterogeneous systems is usually solid phase. $\endgroup$
    – Andrew
    Apr 25 '20 at 13:11
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"Gas phase" means that the reactants are gases

The basic answer is that heterogeneous catalysis "in the gas phase" just means that the materials being reacted are gases. They catalysts can be liquids or solids (more common).

A simple, but very, very important example is the reaction used to convert ammonia into nitric acid (vital for fertiliser production, important for many organic chemistry building blocks and explosives). In this reaction gaseous mixtures of air and ammonia are passed over platinum gauze and, initially, produce NO (nitric oxide) which is further oxidised homogeneously to dinitrogen tetroxide which ultimately reacts with water to give nitric acid. A good summary of the reaction and its history is given in this Johnson Matthey article.

So gas-phase reactants + solid catalyst -> product. Gas phase heterogeneous catalysis.

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Here is a liquid phase example, 'Iron–cobalt mixed oxide nanocatalysts: Heterogeneous peroxymonosulfate activation, cobalt leaching, and ferromagnetic properties for environmental applications, to quote, in part:

(i) need of heterogeneous activation of sulfate salts using transition metal oxides, (ii) nanoscaling of the metal oxide catalysts for high catalytic activity and promising properties with respect to leaching, and (iii) easy removal and recovery of the catalytic materials after their applications for water and wastewater treatments. In this study, we report a novel approach of using Fe–Co mixed oxide nanocatalysts for the heterogeneous activation of peroxymonosulfate (PMS) to generate SRs targeting the decomposition of 2,4-dichlorophenol...

where SRs refers to sulfate radicals.

For gas phase, I cite: 'The importance of heterogeneous and homogeneous reactions in oxidative coupling of methane over chloride promoted oxide catalysts'. To quote:

The formation of ethene from ethane and methane in a silica reactor has been studied both in the presence and in the absence of chloride-containing catalysts.

Key to these mixed catalysts is surface chemistry activation reactions.

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