Catalytic reduction of NOx

Abstract

A system for NO x reduction in combustion gases, especially from diesel engines, incorporates an oxidation catalyst to convert at least a portion of NO to NO 2 , a particulate filter, a source of reductant such as NH 3 and an SCR catalyst. Considerable improvements in NO x conversion are observed.

1 .- 14 . (canceled)  
   
   
       15 . A method of reducing NO x  in a gas stream containing NO and particulates, comprising: 
 passing the gas stream over an oxidation catalyst thereby converting at least a portion of the NO in the gas stream to NO 2  to form a converted gas stream;    adding a reductant fluid to the converted gas stream to form a gas mixture; and    passing the gas mixture over an SCR catalyst.    
   
   
       16 . The method of  claim 15 , wherein the ratio of NO to NO 2  in the gas mixture is from about 4:1 to 1:3 by volume.  
   
   
       17 . The method of  claim 15 , wherein the gas stream comprises exhaust from sources selected from the group consisting of: heavy duty diesel engines, light duty diesel engines, gasoline direct injection engines, compressed natural gas engines, ships, and stationary sources.  
   
   
       18 . The method of  claim 17 , wherein the gas stream comprises exhaust from a heavy duty diesel engine.  
   
   
       19 . The method of  claim 17 , wherein the gas stream comprises exhaust from a light duty diesel engine.  
   
   
       20 . The method of  claim 17 , wherein the gas stream comprises exhaust from a gasoline direct injection engine.  
   
   
       21 . The method of  claim 17 , wherein the gas stream comprises exhaust from a compressed natural gas engine.  
   
   
       22 . The method of  claim 15 , wherein the SCR catalyst is selected from the group consisting of transition metal/zeolite catalysts, rare earth-based catalysts and transition metal catalysts.  
   
   
       23 . The method of  claim 15 , wherein the SCR catalyst comprises a transition metal/zeolite catalyst.  
   
   
       24 . The method of  claim 15 , wherein the oxidation catalyst comprises a platinum catalyst.  
   
   
       25 . The method of  claim 24 , wherein the platinum catalyst is deposited on a ceramic through-flow honeycomb support.  
   
   
       26 . The method of  claim 24 , wherein the oxidation catalyst comprises platinum deposited on a metal through-flow honeycomb support.  
   
   
       27 . The method of  claim 24 , wherein the platinum catalyst comprises a Pt/Al 2 O 3  catalyst.  
   
   
       28 . The method of  claim 15 , wherein the reductant fluid is selected from the group consisting of NH 3 , urea, ammonium carbamate, hydrocarbons, and diesel fuel.  
   
   
       29 . The method of  claim 15 , wherein the reductant fluid comprises NH 3 .  
   
   
       30 . The method of  claim 15 , wherein the reductant fluid comprises urea.  
   
   
       31 . The method of  claim 15 , wherein the reductant fluid comprises ammonium carbamate.  
   
   
       32 . The method of  claim 15 , wherein the reductant fluid is injected into the gas stream.  
   
   
       33 . The method of  claim 32 , wherein the supply of the reductant fluid is controlled using a mass controller.  
   
   
       34 . The method of  claim 32 , wherein the reductant fluid is injected into the gas stream through an injection ring.  
   
   
       35 . The method of  claim 34 , wherein the injection ring is an annular injection ring mounted in an exhaust pipe of a vehicle.  
   
   
       36 . The method of  claim 15 , wherein the SCR catalyst is maintained at a temperature of from 160° C. to 450° C.  
   
   
       37 . The method of  claim 36 , wherein the temperature of the catalyst is maintained using a cooling means.  
   
   
       38 . The method of  claim 37 , wherein the cooling means comprises water injection.  
   
   
       39 . The method of  claim 37 , wherein the cooling means comprises air injection.  
   
   
       40 . The method of  claim 15 , wherein the ratio of the reductant fluid to NO added to the converted gas stream comprises 0.6:1 to 1:1.  
   
   
       41 . The method of  claim 40 , wherein the reductant fluid comprises NH 3 .  
   
   
       42 . The method of  claim 15 , wherein the ratio of the reductant fluid to NO 2  added to the converted gas stream comprises 0.8:1 to 4:3.  
   
   
       43 . The method of  claim 42 , wherein the reductant fluid comprises NH 3 .  
   
   
       44 . The method of  claim 41 , further comprising removing any NH 3  and derivatives thereof downstream of the SCR catalyst.  
   
   
       45 . The method of  claim 44 , wherein the removing any NH 3  and derivatives thereof downstream of the SCR catalyst includes incorporation of a clean-up catalyst downstream of the SCR catalyst.  
   
   
       46 . A method according to  claim 15 , wherein a space velocity of the exhaust gas over the SCR catalyst is in the range 40,000 to 70,000 hr −1 .  
   
   
       47 . A method of reducing NO x  in a gas stream containing NO and particulates, comprising: 
 passing the gas stream over an oxidation catalyst thereby converting at least a portion of the NO in the gas stream to NO 2  to form a converted gas stream;    removing at least a portion of the particulates from the converted gas stream;    adding a reductant fluid to the converted gas stream to form a gas mixture; and    passing the gas mixture over an SCR catalyst.    
   
   
       48 . The method of  claim 47 , wherein the particulates are removed from the converted gas stream using a particulate filter or particulate trap.  
   
   
       49 . The method of  claim 48 , wherein the particulates are removed without causing accumulation and resulting blockage and back pressure problems.  
   
   
       50 . The method of  claim 48 , wherein the particles are removed from the particulate trap by combustion in the presence of NO 2 .  
   
   
       51 . The method of  claim 48 , wherein the particulate trap comprises a wall-flow filter.  
   
   
       52 . The method of  claim 48 , wherein the particulate trap is manufactured from ceramic.  
   
   
       53 . The method of  claim 48 , wherein the particulate trap is manufactured from woven knitted heat resistant fabrics.  
   
   
       54 . The method of  claim 48 , wherein the particulate trap is manufactured from non-woven heat resistant fabrics.  
   
   
       55 . The method of  claim 48 , wherein the ratio of NO to NO 2  in the gas mixture is from about 4:1 to 1:3 by volume.  
   
   
       56 . The method of  claim 48 , wherein the gas stream comprises exhaust from sources selected from the group consisting of: heavy duty diesel engines, light duty diesel engines, gasoline direct injection engines, compressed natural gas engines, ships, and stationary sources.  
   
   
       57 . The method of  claim 56 , wherein the gas stream comprises exhaust from a heavy duty diesel engine.  
   
   
       58 . The method of  claim 56 , wherein the gas stream comprises exhaust from a light duty diesel engine.  
   
   
       59 . The method of  claim 56 , wherein the gas stream comprises exhaust from a gasoline direct injection engine.  
   
   
       60 . The method of  claim 48 , wherein the SCR catalyst is selected from the group consisting of transition metal/zeolite catalysts, rare earth-based catalysts and transition metal catalysts.  
   
   
       61 . The method of  claim 48 , wherein the SCR catalyst comprises a transition metal/zeolite catalyst.  
   
   
       62 . The method of  claim 48 , wherein the oxidation catalyst comprises a platinum catalyst.  
   
   
       63 . The method of  claim 62 , wherein the platinum catalyst is deposited on a ceramic through-flow honeycomb support.  
   
   
       64 . The method of  claim 62 , wherein the platinum catalyst is deposited on a metal through-flow honeycomb support.  
   
   
       65 . The method of  claim 62 , wherein the platinum catalyst comprises a Pt/Al 2 O 3  catalyst.  
   
   
       66 . The method of  claim 48 , wherein the reductant fluid is selected from the group consisting of NH 3 , urea, ammonium carbamate, hydrocarbons, and diesel fuel.  
   
   
       67 . The method of  claim 66 , wherein the reductant fluid comprises NH 3 .  
   
   
       68 . The method of  claim 66 , wherein the reductant fluid comprises urea.  
   
   
       69 . The method of  claim 66 , wherein the reductant fluid comprises ammonium carbamate.  
   
   
       70 . The method of  claim 48 , wherein the SCR catalyst is maintained at a temperature of from 160° C. to 450° C.  
   
   
       71 . The method of  claim 70 , wherein the temperature of the catalyst is maintained using a cooling means.  
   
   
       72 . The method of  claim 71 , wherein the cooling means comprises water injection.  
   
   
       73 . The method of  claim 71 , wherein the cooling means comprises air injection.  
   
   
       74 . The method of  claim 48 , wherein the ratio of the reductant fluid to NO added to the gas stream comprises 0.6:1 to 1:1.  
   
   
       75 . The method of  claim 54 , wherein the reductant fluid comprises NH 3 .  
   
   
       76 . The method of  claim 48 , wherein the ratio of the reductant fluid to NO 2  added to the gas stream comprises 0.8:1 to 4:3.  
   
   
       77 . The method of  claim 76 , wherein the reductant fluid comprises NH 3 .  
   
   
       78 . The method of  claim 75 , further comprising removing any NH 3  and derivatives thereof downstream of the SCR catalyst.  
   
   
       79 . The method of  claim 64 , wherein the removing any NH 3  and derivatives thereof downstream of the SCR catalyst includes incorporation of a clean-up catalyst downstream of the SCR catalyst.  
   
   
       80 . A method according to  claim 48 , wherein the space velocity of the exhaust gas over the SCR catalyst is in the range 40,000 to 70,000 hr −1 .  
   
   
       81 . A method of reducing NO x  in a gas stream containing NO and particulates, comprising: 
 passing the gas stream over an oxidation catalyst thereby converting at least a portion of the NO in the gas stream to NO 2  to form a converted gas stream having a ratio of NO to NO 2  adjusted according to the type of SCR catalyst to improve NO x  reduction;    adding a reductant fluid to the converted gas stream to form a gas mixture; and    passing the gas mixture over an SCR catalyst.    
   
   
       82 . The method of  claim 81 , wherein the ratio of NO to NO 2  in the gas mixture is from about 4:1 to 1:3 by volume.  
   
   
       83 . The method of  claim 81 , wherein the gas stream comprises exhaust from sources selected from the group consisting of: heavy duty diesel engines, light duty diesel engines, gasoline direct injection engines, compressed natural gas engines, ships, and stationary sources.  
   
   
       84 . The method of  claim 81 , wherein the SCR catalyst comprises a transition metal/zeolite catalyst.  
   
   
       85 . The method of  claim 81 , wherein the oxidation catalyst comprises a platinum catalyst deposited on a ceramic through-flow honeycomb support.  
   
   
       86 . The method of  claim 85 , wherein the platinum catalyst comprises a Pt/Al 2 O 3  catalyst.  
   
   
       87 . The method of  claim 81 , wherein the reductant fluid comprises urea.  
   
   
       88 . A method of reducing NO x  in a gas stream containing NO and particulates, comprising: 
 passing the gas stream over an oxidation catalyst thereby converting at least a portion of the NO in the gas stream to NO 2  to form a converted gas stream having a ratio of NO to NO 2  adjusted according to the type of SCR catalyst to improve NO x  reduction;    removing at least a portion of the particulates from the converted gas stream;    adding a reductant fluid to the converted gas stream to form a gas mixture; and    passing the gas mixture over an SCR catalyst.    
   
   
       89 . The method of  claim 88 , wherein the ratio of NO to NO 2  in the gas mixture is from about 4:1 to 1:3 by volume.  
   
   
       90 . The method of  claim 88 , wherein the gas stream comprises exhaust from sources selected from the group consisting of: heavy duty diesel engines, light duty diesel engines, gasoline direct injection engines, compressed natural gas engines, ships, and stationary sources.  
   
   
       91 . The method of  claim 88 , wherein the SCR catalyst comprises a transition metal/zeolite catalyst.  
   
   
       92 . The method of  claim 88 , wherein the oxidation catalyst comprises a platinum catalyst deposited on a ceramic through-flow honeycomb support.  
   
   
       93 . The method of  claim 92 , wherein the platinum catalyst comprises a Pt/Al 2 O 3  catalyst.  
   
   
       94 . The method of  claim 88 , wherein the reductant fluid comprises urea.  
   
   
       95 . A method of improving NO x  conversion in an SCR system, comprising: 
 passing the gas stream over an oxidation catalyst thereby converting at least a portion of the NO in the gas stream to NO 2  to form a converted gas stream;    adding a reductant fluid to the converted gas stream to form a gas mixture; and    passing the gas mixture over an SCR catalyst.    
   
   
       96 . The method of  claim 95 , wherein the ratio of NO to NO 2  in the gas mixture is from about 4:1 to 1:3 by volume.  
   
   
       97 . The method of  claim 95 , wherein the gas stream comprises exhaust from sources selected from the group consisting of: heavy duty diesel engines, light duty diesel engines, gasoline direct injection engines, compressed natural gas engines, ships, and stationary sources.  
   
   
       98 . The method of  claim 95 , wherein the SCR catalyst comprises a transition metal/zeolite catalyst.  
   
   
       99 . The method of  claim 95 , wherein the oxidation catalyst comprises a platinum catalyst deposited on a ceramic through-flow honeycomb support.  
   
   
       100 . The method of  claim 99 , wherein the platinum catalyst comprises a Pt/Al 2 O 3  catalyst.  
   
   
       101 . The method of  claim 95 , wherein the reductant fluid comprises urea.  
   
   
       102 . A method of improving NO x  conversion in an SCR system, comprising: 
 passing the gas stream over an oxidation catalyst thereby converting at least a portion of the NO in the gas stream to NO 2  to form a converted gas stream;    removing at least a portion of the particulates from the converted gas stream;    adding a reductant fluid to the converted gas stream to form a gas mixture; and    passing the gas mixture over an SCR catalyst.    
   
   
       103 . The method of  claim 102 , wherein the ratio of NO to NO 2  in the gas mixture is from about 4:1 to 1:3 by volume.  
   
   
       104 . The method of  claim 102 , wherein the gas stream comprises exhaust from sources selected from the group consisting of: heavy duty diesel engines, light duty diesel engines, gasoline direct injection engines, compressed natural gas engines, ships, and stationary sources.  
   
   
       105 . The method of  claim 102 , wherein the SCR catalyst comprises a transition metal/zeolite catalyst.  
   
   
       106 . The method of  claim 102 , wherein the oxidation catalyst comprises a platinum catalyst deposited on a ceramic through-flow honeycomb support.  
   
   
       107 . The method of  claim 106 , wherein the platinum catalyst comprises a Pt/Al 2 O 3  catalyst.  
   
   
       108 . The method of  claim 102 , wherein the reductant fluid comprises urea.  
   
   
       109 . A process for reducing NO x  present in a lean exhaust gas from an internal combustion engine by selective catalytic reduction on a reduction catalyst using ammonia, comprising oxidizing some of the NO present in the exhaust gas to NO 2  so that the ratio of NO to NO 2  in the exhaust gas is from about 4:1 to 1:3 by volume before contact with the reduction catalyst, wherein oxidation of the NO present in the exhaust gas takes place in the presence of an oxidation catalyst, passing the exhaust gas, together with ammonia, over said reduction catalyst, wherein the reduction catalyst comprises a transition metal/zeolite catalyst.  
   
   
       110 . The process according to  claim 109 , wherein the oxidation catalyst comprises platinum on aluminum oxide.  
   
   
       111 . The process according to  claim 110 , wherein the oxidation catalyst is deposited on a honeycomb carrier.  
   
   
       112 . A method of reducing levels of regulated pollutants in a gas stream comprising: 
 passing the gas stream over an oxidation catalyst thereby converting at least a portion of the NO in the gas stream to NO 2  to form a converted gas stream;    adding a reductant fluid to the converted gas stream to form a gas mixture; and    passing the gas mixture over an SCR catalyst.    
   
   
       113 . The method of  claim 112 , wherein the ratio of NO to NO 2  in the gas mixture is from about 4:1 to 1:3 by volume.  
   
   
       114 . The method of  claim 112 , wherein the gas stream comprises exhaust from sources selected from the group consisting of: heavy duty diesel engines, light duty diesel engines, gasoline direct injection engines, compressed natural gas engines, ships, and stationary sources.  
   
   
       115 . The method of  claim 112 , wherein the SCR catalyst comprises a transition metal/zeolite catalyst.  
   
   
       116 . The method of  claim 112 , wherein the oxidation catalyst comprises a platinum catalyst deposited on a ceramic through-flow honeycomb support.  
   
   
       117 . The method of  claim 116 , wherein the platinum catalyst comprises a Pt/Al 2 O 3  catalyst.  
   
   
       118 . The method of  claim 112 , wherein the reductant fluid comprises urea.  
   
   
       119 . The method according to  claim 112 , wherein the regulated pollutants comprise particulates.  
   
   
       120 . The method according to  claim 112 , wherein the regulated pollutants comprise hydrocarbons.  
   
   
       121 . A method of reducing levels of regulated pollutants in a gas stream comprising: 
 passing the gas stream over an oxidation catalyst thereby converting at least a portion of the NO in the gas stream to NO 2  to form a converted gas stream;    removing at least a portion of the particulates from the converted gas stream;    adding a reductant fluid to the converted gas stream to form a gas mixture; and    passing the gas mixture over an SCR catalyst.    
   
   
       122 . The method according to  claim 121 , wherein the regulated pollutants comprise particulates.  
   
   
       123 . The method according to  claim 121 , wherein the regulated pollutants comprise hydrocarbons.  
   
   
       124 . The method of  claim 121 , wherein the ratio of NO to NO 2  in the gas mixture is from about 4:1 to 1:3 by volume.  
   
   
       125 . The method of  claim 121 , wherein the gas stream comprises exhaust from sources selected from the group consisting of: heavy duty diesel engines, light duty diesel engines, gasoline direct injection engines, compressed natural gas engines, ships, and stationary sources.  
   
   
       126 . The method of  claim 121 , wherein the SCR catalyst comprises a transition metal/zeolite catalyst.  
   
   
       127 . The method of  claim 121 , wherein the oxidation catalyst comprises a platinum catalyst deposited on a ceramic through-flow honeycomb support.  
   
   
       128 . The method of  claim 127 , wherein the platinum catalyst comprises a Pt/Al 2 O 3  catalyst.  
   
   
       129 . The method of  claim 121 , wherein the reductant fluid comprises urea.  
   
   
       130 . A method of improving NO x  conversion in an SCR system, comprising: 
 passing the gas stream over an oxidation catalyst thereby converting at least a portion of the NO in the gas stream to NO 2  to form a converted gas stream;    removing at least a portion of the particulates from the converted gas stream;    minimizing the level of hydrocarbons in the gas stream;    adding a reductant fluid to the converted gas stream to form a gas mixture; and    passing the gas mixture over an SCR catalyst.    
   
   
       131 . The method of  claim 130 , wherein the ratio of NO to NO 2  in the gas mixture is from about 4:1 to 1:3 by volume.  
   
   
       132 . The method of  claim 130 , wherein the gas stream comprises exhaust from sources selected from the group consisting of: heavy duty diesel engines, light duty diesel engines, gasoline direct injection engines, compressed natural gas engines, ships, and stationary sources.  
   
   
       133 . The method of  claim 130 , wherein the SCR catalyst comprises a transition metal/zeolite catalyst.  
   
   
       134 . The method of  claim 130 , wherein the oxidation catalyst comprises a platinum catalyst deposited on a ceramic through-flow honeycomb support.  
   
   
       135 . The method of  claim 134 , wherein the platinum catalyst comprises a Pt/Al 2 O 3  catalyst.  
   
   
       136 . The method of  claim 130 , wherein the reductant fluid comprises urea.  
   
   
       137 . A method of reducing NO x  in a gas stream comprising: 
 passing the gas stream over an oxidation catalyst thereby incompletely converting NO in the gas stream to NO 2  to form a converted gas stream;    adding a reductant fluid to the converted gas stream to form a gas mixture; and    passing the gas mixture over an SCR catalyst.    
   
   
       138 . The method of  claim 137 , wherein the ratio of NO to NO 2  in the gas mixture is from about 4:1 to 1:3 by volume.  
   
   
       139 . The method of  claim 137 , wherein the gas stream comprises exhaust from sources selected from the group consisting of: heavy duty diesel engines, light duty diesel engines, gasoline direct injection engines, compressed natural gas engines, ships, and stationary sources.  
   
   
       140 . The method of  claim 137 , wherein the SCR catalyst comprises a transition metal/zeolite catalyst.  
   
   
       141 . The method of  claim 137 , wherein the oxidation catalyst comprises a platinum catalyst deposited on a ceramic through-flow honeycomb support.  
   
   
       142 . The method of  claim 141 , wherein the platinum catalyst comprises a Pt/Al 2 O 3  catalyst.  
   
   
       143 . The method of  claim 137 , wherein the reductant fluid comprises urea.  
   
   
       144 . A method of reducing volume and/or weight of an exhaust gas after-treatment system of a light duty diesel engine comprising: 
 attaching an SCR system to the light duty diesel engine, the SCR system providing reduction of NO x  in the exhaust gas by: 
 passing the exhaust gas over an oxidation catalyst thereby converting at least a portion of the NO in the exhaust gas to NO 2  to form a converted exhaust gas;  
 adding a reductant fluid to the converted gas stream to form a gas mixture; and  
 passing the gas mixture over an SCR catalyst.  
   
   
   
       145 . The method of  claim 144 , wherein the ratio of NO to NO 2  in the gas mixture is from about 4:1 to 1:3 by volume.  
   
   
       146 . The method of  claim 144 , wherein the gas stream comprises exhaust from sources selected from the group consisting of: heavy duty diesel engines, light duty diesel engines, gasoline direct injection engines, compressed natural gas engines, ships, and stationary sources.  
   
   
       147 . The method of  claim 144 , wherein the SCR catalyst comprises a transition metal/zeolite catalyst.  
   
   
       148 . The method of  claim 144 , wherein the oxidation catalyst comprises a platinum catalyst deposited on a ceramic through-flow honeycomb support.  
   
   
       149 . The method of  claim 148 , wherein the platinum catalyst comprises a Pt/Al 2 O 3  catalyst.  
   
   
       150 . The method of  claim 144 , wherein the reductant fluid comprises urea.  
   
   
       151 . A method of reducing NO x  in a gas stream containing NO and particulates, comprising: 
 passing the gas stream over an oxidation catalyst thereby converting at least a portion of the NO in the gas stream to NO 2  to form a converted gas stream;    removing at least a portion of the particulates from the converted gas stream; and    passing the gas mixture over an SCR catalyst.    
   
   
       152 . The method of  claim 151 , wherein the particulates are removed from the converted gas stream using a particulate filter or particulate trap.  
   
   
       153 . The method of  claim 151 , wherein the ratio of NO to NO 2  in the gas mixture is from about 4:1 to 1:3 by volume.  
   
   
       154 . The method of  claim 151 , wherein the gas stream comprises exhaust from sources selected from the group consisting of: heavy duty diesel engines, light duty diesel engines, gasoline direct injection engines, compressed natural gas engines, ships, and stationary sources.  
   
   
       155 . The method of  claim 151 , wherein the SCR catalyst comprises a transition metal/zeolite catalyst.  
   
   
       156 . The method of  claim 151 , wherein the oxidation catalyst comprises a platinum catalyst deposited on a ceramic through-flow honeycomb support.  
   
   
       157 . The method of  claim 156 , wherein the platinum catalyst comprises a Pt/Al 2 O 3  catalyst.  
   
   
       158 . The method of  claim 151 , wherein the reductant fluid comprises urea.  
   
   
       159 . A method of reducing pollutants, including particulates and NO x , in a gas stream, comprising passing said gas stream over an oxidation catalyst under conditions effective to convert at least a portion of NO in the gas stream to NO 2  thereby enhancing the NO 2  content of the gas stream, removing at least a portion of said particulates in a particulate trap, reacting trapped particulate with NO 2 , adding reductant fluid to the gas stream to form a gas mixture downstream of said trap, and passing the gas mixture over an SCR catalyst under NO x  reduction conditions.  
   
   
       160 . A method according to  claim 159 , wherein said gas stream is the exhaust from a diesel, GDI or DNG engine.  
   
   
       161 . A method according to  claim 159 , wherein the gas stream or gas mixture is cooled before reaching the SCR catalyst.  
   
   
       162 . A method according to  claim 159 , wherein the NO to NO 2  ratio of the gas mixture is adjusted to a level pre-determined to be optimum for the SCR catalyst, by oxidation of NO over said oxidation catalyst.  
   
   
       163 . A method according to  claim 159 , wherein the SCR catalyst is maintained at a temperature from 160° C. to 450° C.  
   
   
       164 . A method according to  claim 159 , wherein the SCR catalyst includes a component selected from the group consisting of a transition metal and a rare-earth metal.  
   
   
       165 . A method according to  claim 164 , wherein the transition metal is selected from the group consisting of copper and vanadium.  
   
   
       166 . A method according to  claim 159 , wherein the SCR catalyst is V 2 O 5 /WO 3 /TiO 2 .  
   
   
       167 . A method according to  claim 162 , wherein substantially all NO is converted to NO 2 .  
   
   
       168 . A method according to  claim 162 , wherein the ratio of NO:NO 2  is adjusted to about 4:3.  
   
   
       169 . A method according to  claim 159 , wherein the reductant fluid is a hydrocarbon.  
   
   
       170 . A method according to  claim 159 , wherein the reductant fluid is selected from the group consisting of ammonia, ammonium carbamate and urea.  
   
   
       171 . A method according to  claim 168 , comprising contacting the gas mixture leaving the SCR catalyst with a clean-up catalyst to remove NH 3  or derivatives thereof.  
   
   
       172 . A method according to  claim 171 , wherein the space velocity of the exhaust gas over the SCR catalyst is in the range 40,000 to 70,000 hr −1 .  
   
   
       173 . An SCR system for reducing NO x  in exhaust gases comprising: 
 an oxidation catalyst that converts at least a portion of NO in the exhaust gas to NO 2 ;    a reductant fluid source downstream from the oxidation catalyst; and    an SCR catalyst downstream from the reductant fluid source.    
   
   
       174 . The system of  claim 173 , wherein the ratio of NO to NO 2  in the exhaust gas just prior to the SCR catalyst is from about 4:1 to 1:3 by volume.  
   
   
       175 . The system of  claim 173 , wherein the exhaust gas comprises exhaust from sources selected from the group consisting of: heavy duty diesel engines, light duty diesel engines, gasoline direct injection engines, compressed natural gas engines, ships, and stationary sources.  
   
   
       176 . The system of  claim 173 , wherein the SCR catalyst comprises a transition metal/zeolite catalyst.  
   
   
       177 . The system of  claim 173 , wherein the oxidation catalyst comprises a platinum catalyst deposited on a ceramic through-flow honeycomb support.  
   
   
       178 . The system of  claim 177 , wherein the platinum catalyst comprises a Pt/Al 2 O 3  catalyst.  
   
   
       179 . The system of  claim 173 , wherein the reductant fluid comprises urea.  
   
   
       180 . The system of  claim 173 , wherein the ratio of the reductant fluid to NO in the exhaust gas stream just prior to the SCR catalyst comprises 0.6:1 to 1:1.  
   
   
       181 . The system of  claim 173 , wherein the ratio of the reductant fluid to NO 2  in the exhaust gas stream just prior to the SCR catalyst comprises 0.8:1 to 4:3.  
   
   
       182 . The system of  claim 173  further comprising a clean-up catalyst downstream of the SCR catalyst.  
   
   
       183 . The system of  claim 173 , wherein a space velocity capacity of the SCR catalyst is in the range of 40,000 to 70,000 hr −1 .  
   
   
       184 . An SCR system for reducing NO x  in exhaust gases comprising: 
 an oxidation catalyst that converts at least a portion of NO in the exhaust gas to NO 2 ;    a particulate filter or particulate trap downstream from the oxidation catalyst;    a reductant fluid source downstream from the particulate filter or particulate trap; and    an SCR catalyst downstream from the reductant fluid source.    
   
   
       185 . The system of  claim 184 , wherein the ratio of NO to NO 2  in the exhaust gas just prior to the SCR catalyst is from about 4:1 to 1:3 by volume.  
   
   
       186 . The system of  claim 184 , wherein the exhaust gas comprises exhaust from sources selected from the group consisting of: heavy duty diesel engines, light duty diesel engines, gasoline direct injection engines, compressed natural gas engines, ships, and stationary sources.  
   
   
       187 . The system of  claim 184 , wherein the SCR catalyst comprises a transition metal/zeolite catalyst.  
   
   
       188 . The system of  claim 184 , wherein the oxidation catalyst comprises a platinum catalyst deposited on a ceramic through-flow honeycomb support.  
   
   
       189 . The system of  claim 184 , wherein the platinum catalyst comprises a Pt/Al 2 O 3  catalyst.  
   
   
       190 . The system of  claim 184 , wherein the reductant fluid comprises urea.  
   
   
       191 . The system of  claim 184 , wherein the ratio of the reductant fluid to NO in the exhaust gas stream just prior to the SCR catalyst comprises 0.6:1 to 1:1.  
   
   
       192 . The system of  claim 184 , wherein the ratio of the reductant fluid to NO 2  in the exhaust gas stream just prior to the SCR catalyst comprises 0.8:1 to 4:3.  
   
   
       193 . The system of  claim 184  further comprising a clean-up catalyst downstream of the SCR catalyst.  
   
   
       194 . The system of  claim 184 , wherein a space velocity capacity of the SCR catalyst is in the range of 40,000 to 70,000 hr −1 .