Transition metal/zeolite scr catalysts

Abstract

A method of converting nitrogen oxides in a gas to nitrogen by contacting the nitrogen oxides with a nitrogenous reducing agent in the presence of a zeolite catalyst containing at least one transition metal, wherein the zeolite is a small pore zeolite containing a maximum ring size of eight tetrahedral atoms, wherein the at least one transition metal is selected from the group consisting of Cr, Mn, Fe, Co, Ce, Ni, Cu, Zn, Ga, Mo, Ru, Rh, Pd, Ag, In, Sn, Re, Jr and Pt.

1 . An exhaust system for treatment of an exhaust gas stream comprising NO x  and particulate matter, the exhaust system comprising:
 an SCR catalyst substrate containing a catalyst comprising a Cu-containing silicoaluminophosphate having a CHA framework structure; and   an ammonia injector or an ammonia precursor injector positioned upstream of the SCR catalyst substrate;   wherein the catalyst is effective to promote the reaction of ammonia with nitrogen oxides to form nitrogen and H 2 O selectively.   
     
     
         2 . The exhaust system of  claim 1 , wherein the Cu-containing silicoaluminophosphate comprises Cu-SAPO-34. 
     
     
         3 . The exhaust system of  claim 2 , wherein the catalyst contains a binder selected from the group consisting of alumina, silica, (non-zeolite) silica-alumina, naturally occurring clays, TiO 2 , ZrO 2 , and SnO 2 . 
     
     
         4 . The exhaust system of  claim 3 , wherein the binder comprises ZrO 2 . 
     
     
         5 . The exhaust system of  claim 2 , wherein the Cu-SAPO-34 contains between about 0.5 wt. % and about 5 wt. % Cu. 
     
     
         6 . The exhaust system of  claim 5 , wherein the Cu-SAPO-34 contains about 3 wt. % Cu. 
     
     
         7 . The exhaust system of  claim 1 , wherein the Cu-containing silicoaluminophosphate having a CHA framework structure is combined with a metal-containing zeolite SCR catalyst selected from Beta zeolite, zeolite Y, and ZSM-5. 
     
     
         8 . The exhaust system of  claim 7 , wherein the Cu-containing silicoaluminophosphate having a CHA framework structure comprises Cu-SAPO-34. 
     
     
         9 . The exhaust system of  claim 1 , further comprising means for metering urea flowing into the system. 
     
     
         10 . The exhaust system of  claim 1 , further comprising a second substrate interposed between the ammonia injector or ammonia precursor injector and the SCR catalyst substrate. 
     
     
         11 . The exhaust system of  claim 10 , wherein the second substrate is selected from a honeycomb flow through substrate, a wall flow substrate, or a honeycomb wall flow substrate. 
     
     
         12 . The exhaust system of  claim 1 , further comprising an oxidation catalyst upstream of the ammonia injector or ammonia precursor injector. 
     
     
         13 . An exhaust system for treatment of an exhaust gas stream comprising NO x  and particulate matter, the exhaust system comprising:
 a SCR catalyst substrate containing a catalyst comprising a Cu-containing silicoaluminophosphate having a CHA framework structure and a Cu-loading so that the catalyst is effective to selectively reduce nitrogen oxides with ammonia in the presence of oxygen in an exhaust gas stream at low temperature; and   an ammonia injector or an ammonia precursor injector positioned upstream of the SCR catalyst substrate for injecting ammonia or an ammonia precursor;   wherein the catalyst is effective to promote the reaction of ammonia with nitrogen oxides to form nitrogen and H 2 O selectively.