Catalyst for purifying exhaust gas and a process for purifying exhaust gas

Abstract

An exhaust-gas-purifying catalyst is made by depositing on a support iridium serving as a catalyst active substance, sulfur for improving the catalyst activity of iridium and, if necessary, platinum. The sulfur is preferably contained as a sulfate. The exhaust-gas-purifying process of the present invention is a process in which exhaust gas from an internal combustion engine is allowed to pass through the exhaust-gas-purifying catalyst with the exhaust-gas temperature being set in the range of 200° C. to 700° C. at the inlet of the exhaust-gas-purifying catalyst. The above-mentioned composition and process provide an activity for purifying exhaust gas, especially for eliminating nitrogen oxides in an oxidizing atmosphere, in a wide temperature range, allow high heat-resistance and durability, and consequently, are superior in practical use.

1 . A catalyst for purifying exhaust gas comprising iridium and sulfur as catalyst active substances.  
     
     
         2 . The catalyst for purifying exhaust gas as defined in    claim 1   , wherein iridium is deposited on a support containing sulfur.  
     
     
         3 . The catalyst for purifying exhaust gas as defined in    claim 1   , wherein sulfur is provided as a sulfate.  
     
     
         4 . A catalyst for purifying exhaust gas comprising: 
 a fire-resistant inorganic compound having at least one element selected from the group consisting of platinum, palladium and rhodium deposited thereon; and    a metallic sulfate having iridium deposited thereon.    
     
     
         5 . The catalyst for purifying exhaust gas as defined in    claim 4   , wherein: among the catalyst ingredients of the catalyst for purifying exhaust gas, the fire-resistant inorganic compound having at least one element selected from the group consisting of platinum, palladium and rhodium deposited thereon is provided as a lower layer and the metallic sulfate having iridium deposited thereon as an upper layer is placed on the lower layer.  
     
     
         6 . The catalyst for purifying exhaust gas as defined in    claim 4   , wherein among the catalyst ingredients of the catalyst for purifying exhaust gas, the metallic sulfate having iridium deposited thereon is placed on an upstream side of an exhaust gas flow, while the fire-resistant inorganic compound having at least one element selected from the group consisting of platinum, palladium and rhodium deposited thereon is placed on a downstream side thereof.  
     
     
         7 . The catalyst for purifying exhaust gas as defined in    claim 4   , wherein: the catalyst for purifying exhaust gas is divided into a plurality of parts, and those parts containing the metallic sulfate having iridium deposited thereon are placed at a preceding stage, while those parts containing the fire-resistant inorganic compound having at least one element selected from the group consisting of platinum, palladium and rhodium deposited thereon are placed at a succeeding stage.  
     
     
         8 . The catalyst for purifying exhaust gas as defined in    claim 4   , wherein the metallic sulfate is a sulfate of an alkaline earth metal.  
     
     
         9 . The catalyst for purifying exhaust gas as defined in    claim 4   , wherein onto the metallic sulfate having iridium deposited thereon is further deposited at least one element selected from the group consisting of tin, gallium, germanium and silicon.  
     
     
         10 . A catalyst for purifying exhaust gas comprising: 
 iridium and sulfur; and    at least one element selected from the group consisting of calcium, strontium and barium, as catalyst active substances.    
     
     
         11 . The catalyst for purifying exhaust gas as defined in    claim 10   , wherein iridium forms a complex oxide with said at least one element selected from the group of said elements.  
     
     
         12 . The catalyst for purifying exhaust gas as defined in    claim 10   , wherein iridium is deposited on a support containing sulfur.  
     
     
         13 . The catalyst for purifying exhaust gas as defined in    claim 10   , further comprising a fire-resistant inorganic compound.  
     
     
         14 . A catalyst for purifying exhaust gas comprising iridium, a rare-earth metal and sulfur.  
     
     
         15 . The catalyst for purifying exhaust gas as defined in    claim 14   , wherein the rare-earth metal is contained as an oxide containing at least one element selected from the group consisting of cerium, lanthanum, yttrium, neodymium and praseodymium.  
     
     
         16 . The catalyst for purifying exhaust gas as defined in    claim 14   , wherein the rare-earth metal is contained as a composite oxide containing at least one element selected from the group consisting of cerium, lanthanum, yttrium, neodymium and praseodymium and at least one element selected from the group consisting of manganese, iron, cobalt, nickel, copper and zinc.  
     
     
         17 . The catalyst for purifying exhaust gas as defined in    claim 14   , further comprising at least one element selected from the group consisting of tin, gallium, germanium and silicon.  
     
     
         18 . An exhaust-gas purifying process comprising the steps of: 
 preparing any one of the catalysts for purifying exhaust gas according to    claims 1    to    17   ;    setting the exhaust-gas temperature in the range of 200° C. to 700° C. at the inlet of the catalyst for purifying exhaust gas; and    allowing exhaust gas from an internal combustion engine to pass through the catalyst for purifying exhaust gas so as to reduce nitrogen oxides in the exhaust gas.    
     
     
         19 . An exhaust-gas purifying process comprising the steps of: 
 preparing any one of the catalysts for purifying exhaust gas according to    claims 4    to    9   ;    setting the exhaust-gas temperature in the range of 200° C. to 700° C. at the inlet of the catalyst for purifying exhaust gas; and    allowing exhaust gas from an internal combustion engine to pass through the catalyst for purifying exhaust gas so as to reduce hydrocarbons, carbon monoxide and nitrogen oxides in the exhaust gas from the internal combustion engine.