Process for removing nitrogen oxides in exhaust gases

Abstract

A process for removing nitrogen oxides in exhaust gases is provided. The process comprises mixing an exhaust gas generated during the chemical vapor deposition process for producing semiconductors and containing dinitrogen monoxide (N 2 O), nitrogen monoxide (NO) and nitrogen dioxide (NO 2 ) with ammonia (NH 3 ) in an amount 0.5-3 times the sum of the stoichiometric amount corresponding to nitrogen monoxide and the stoichiometric amount corresponding to nitrogen dioxide; and then contacting said mixed gas with a noble metal catalyst at a sufficiently high temperature to decompose dinitrogen monoxide, nitrogen monoxide and nitrogen dioxide. Dinitrogen monoxide (N 2 O), nitrogen monoxide (NO) and nitrogen dioxide (NO 2 ) in exhaust gases can be treated in one step. No N 2 O is produced as a by-product.

What is claimed is:  
     
         1 . A process for removing nitrogen oxides in an exhaust gas, comprising the steps of: 
 mixing an exhaust gas generated during the chemical vapor deposition process for producing semiconductors and containing dinitrogen monoxide (N 2 O), nitrogen monoxide (NO) and nitrogen dioxide (NO 2 ) with ammonia (NH 3 ) in an amount 0.5-3 times a sum of a stoichiometric amount corresponding to the nitrogen monoxide and a stoichiometric amount corresponding to the nitrogen dioxide; and    contacting said mixed gas with a noble metal catalyst at a sufficiently high temperature to decompose dinitrogen monoxide, nitrogen monoxide and nitrogen dioxide.    
     
     
         2 . The process of  claim 1  further comprising, before said contacting step, the step of heating said exhaust gas or said mixed gas at a temperature ranging from 200° C. to said high temperature selected for said contacting step.  
     
     
         3 . The process of  claim 1  wherein said exhaust gas is mixed with ammonia (NH 3 ) in an amount 0.6-2 times the sum of the stoichiometric amount corresponding to the nitrogen monoxide and the stoichiometric amount corresponding to the nitrogen dioxide during said mixing step.  
     
     
         4 . The process of  claim 1  wherein said mixed gas contains 1 mole part or more of ammonia (NH 3 ) per 1 mole part of an oxygen gas (O 2 ).  
     
     
         5 . The process of  claim 1  wherein said exhaust gas contains 1 part by weight of the dinitrogen monoxide (N 2 O), 0.01-3 parts by weight of the nitrogen monoxide (NO) and 0.01-100 parts by weight of the nitrogen dioxide (NO 2 ).  
     
     
         6 . The process of  claim 1  wherein said exhaust gas contains 70% by weight or more of a nitrogen gas (N 2 ).  
     
     
         7 . The process of  claim 1  wherein said exhaust gas is substantially free from sulfur oxides.  
     
     
         8 . The process of  claim 1  wherein said noble metal catalyst comprises palladium or platinum.  
     
     
         9 . The process of  claim 1  wherein said noble metal catalyst comprises a carrier in a particle form and palladium or platinum supported on said carrier.  
     
     
         10 . The process of  claim 1  wherein said contacting step takes place at 250° C.-600° C.  
     
     
         11 . The process of  claim 1  wherein said contacting step takes place at 350° C.-400° C.  
     
     
         12 . The process of  claim 1  wherein the outlet of said exhaust gas and the outlet of said ammonia face to each other during said mixing step.  
     
     
         13 . The process of  claim 1  further comprising the step of passing said mixed gas through a tortuous path after said mixing step and before said contacting step.  
     
     
         14 . The process of  claim 1  further comprising the step of removing excessive ammonia after said contacting step.