1. Field of Invention
The present invention relates to a catalyst composition for selective catalytic reduction of exhaust gases and a process for its preparation. More specifically, such a catalyst composition may be used in catalyst application for the removal of NOx, especially in exhaust gas after treatment of diesel- and lean burn engines of automotive. In addition the catalyst composition may also be used in stationary applications (e.g. removal of NO in power plant stations).
2. Related Technology
The most widely employed technology for removal of NOx out of exhaust gases is S(elective) C(atalytic) R(eduction), e.g. O. Kröcher, Chapter 9 in <<Past and Present in DeNOx Catalysis>>, edited by P. Granger et al., published by Elsevier 2007. Accordingly, NOx is removed by Ammonia into Nitrogen and water according to the reaction:4NO+4NH3+O2=4N2+6H2O
For the stationary application mainly V2O5 doped TiO2/WO3 compositions are used commercially since 1970 to remove nitrogen oxide emissions of fossil fuel power plants.
Already 15 years ago, the idea was discussed to apply SCR also on mobile diesel engines, which is now an emerging technology for the NOx reduction from heavy duty diesel vehicles, passenger cars and off road machines.
A typical SCR system consists of a reduction catalyst, urea injection and dosing components, pipes and a storage tank. A huge number of modern catalysts are extruded substrates or coated substrates with Vanadium Pentaoxide (V2O5) as the catalytically active component.
Catalytic coatings of zeolite based SCR are currently being developed because of the limited temperature stability of the vanadium catalysts and the fact that Vanadium Pentaoxide (V2O5) has been classified as a health risk in some countries. Increased temperature stability is especially important in SCR catalysts installed downstream a particulate filter since relatively high temperatures are generated during filter regeneration (M. Rice, R. Mueller at al., Development of an Integrated NOx and PM Reduction Aftertreatment System: SCRi for Advanced Diesel Engines, SAE technical paper 2008-01-132, SAE World Congress Detroit, Mich. Apr. 14-17, 2008)
There are further reports on the commercially applied well known V2O5/WO3—TiO2 material for exhaust gas aftertreatment of automotives by O. Kröcher in Chapter 9, p. 267f in <<Past and Present in DeNOx Catalysis>>, edited by P. Granger et al.
According to the presentation given by Dirk Vatareck (Catalysts Automotive Applications, Argillon) at the 3rd CTI Forum SCR Systems (Bonn 9 Apr. 2008) Titania-tungsten oxide based catalysts containing V2O5 as active component are applied for making automotive catalysts in a huge number (approx. 4 Mio. Catalysts/year).
The preparation of V2O5 containing materials based on TiO2/WO3 which additionally may contain oxides out of transition metals, Rare Earths and other elements and the application in SCR is widely disclosed in numerous publications, patent applications and patents. For example GB 1 495 396 describes a catalyst composition containing as active ingredients oxides from titanium, at least one of molybdenum, tungsten, iron, vanadium, nickel, cobalt, copper, chromium and uranium, and as optional component(s) tin and/or at least one of silver, beryllium, magnesium, zinc, boron, aluminium, yttrium, rare earth metal, silicon, niobium, antimony, bismuth, manganese, thorium and zirconium, which oxides are present as an intimate mixture.
EP 787 521 describes the preparation of several V-containing catalysts based on TiO2/WO3 containing additional dopants such as Y2O3, B2O3, PbO, SnO2 the vanadium being present as Vanadiumpentaoxide V2O5.
U.S. Pat. No. 4,221,768 reports on V2O5 containing materials based on TiO2 as a support material and additional oxides out of transition metals as dopants. Additional SCR-Materials which contains V2O5 supported on TiO2 are also described in GB 1 430 730.
V2O5 containing materials which contain TiO2, SiO2, S and Oxides of Ce, Sn, Mo and W are reported in UK Appl. GB 2 149 680.
U.S. Pat. No. 4,466,947 describes V-containing denitration catalysts in which the vanadium is present in form of an oxide or a sulfate.
EP 1 145 762 A1 describes a process for the preparation of a vanadia SCR-catalyst supported on titania.
The main disadvantage of the V-based catalyst type is the limited stability at temperatures above 600° C.
A report on the stability of a SCR system based on TiO2/WO3/V2O5 is given by Jan M T et al. in Chemical Engineering & Technology, Vol. 30, No 10, 1440-1444, 2007. Accordingly a deactivation of the catalyst occurs since V2O5 melts at approx. 650° C.
A TiO2/WO3/V2O5 suitable SCR catalyst for NOx removal of Diesel-powered vehicles is described in U.S. Pat. No. 6,805,849 B1, column 2. Although such a catalyst has shown good performance it was found that sustained high temperature operation can cause catalyst deactivation. Heavy duty diesel engines, which are almost exclusively charged, can produce exhaust gases at greater than 500° C. Under conditions of high load and/or high speed, and such temperatures deactivation of the catalyst may occur.
In the presentation of Dirk Vatareck (Catalysts Automotive Applications, Argillon) at the 3rd CTI Forum SCR Systems (Bonn 9 Apr. 2008) there were reported maximum operation temperatures for a TiO2/WO3/V2O5 catalyst of 550° C. and 580° C. for short term basis.
In view of the application under EURO 6 an improvement in thermal stability of a V2O5 containing TiO2/WO3 based catalyst was reported in the presentation of Dirk Vatareck (Catalysts Automotive Applications, Argillon) at the 3rd CTI Forum SCR Systems (Bonn 9 Apr. 2008). Accordingly a catalyst containing V2O5 as active component and a support material consisting of TiO2/WO3 and additionally containing Si may be operated at max. temp. of 600° C. and 650° on a short term basis.
Vanadium based systems with improved thermal stability are also reported by James. W. Girard et al., “Technical Advantages of Vanadium SCR Systems for Diesel NOx Control in Emerging Markets”, SAE technical paper 2008-01-132, SAE World Congress Detroit, Mich. Apr. 14-17, 2008. After ageing at 600° C./50 hrs the catalyst is still active. However, because of high exhaust gas temperatures possible during active Diesel Particulate Filter (DPF) regeneration, vanadium SCR catalysts are usually not considered for these applications.
With increasingly stringent automobile exhaust emission regulations for Diesel vehicles (US 2010 and EURO 6 regulations) aftertreatment systems containing a Diesel Particulate Filter (DPF) as well as a SCR catalyst will be required in future. Such systems will demand a higher temperature stability of the SCR catalyst and V2O5 based systems are not considered to be viable for such an application as additionally to the problem of heat stability V2O5 may be exhausted also to the environment (J. Munch et al “Extruded Zeolite based Honeycomb Catalyst for NOx Removal from Diesel Exhaust, SAE Paper 2008-01-1024). Since the activity of SCR catalysts in the temperature range of 180-350° C. is important in the diesel application there have been established systems to improve catalytic activity in the low temperature range.
For example with the aid of a diesel oxidation catalyst (DOC) attached in front of the SCR system (NO being present as a majority (=more than 90%) in the raw emission of the diesel engine) is oxidized to form NO2. The NO2 can be used to burn up particulates and to improve the low temperature activity (in the range 180-350° C.), see M. Rice, R. Mueller at al., Development of an Integrated NOx and PM Reduction Aftertreatment System: SCRi for Advanced Diesel Engines, SAE technical paper 2008-01-132, SAE World Congress Detroit, Mich. Apr. 14-17, 2008.
In the same publication there are summarized the design parameters of two engine/aftertreatment scenarios for US 2010/Euro 6. One concept will lead to high particulate matters/low NOx with active regeneration of the PM-filter. The SCR catalyst proposed for the described scenario is a zeolite. Zeolites must be used since higher heat resistance of the SCR system is required due to the active regeneration of the PM filter.
The second concept comprises an engine concept which will lead to low concentrations of PM and low NOx concentrations. The SCR catalyst may consist of a vanadium based material or a zeolite. Both concepts will use a diesel oxidation catalyst (DOC) prior to the SCR treatment. The problem of low temperature activity of SCR catalysts is also addressed in US 2008/0234126 A1. It is described a method for the preparation of a vanadium/titania based catalyst with enhanced low temperature activity for removing nitrogen oxides at a window of 300° C. and lower. However, US 2008/0234126 A1 does not address the problem of stability of the catalyst >600° C.
An improvement in terms of the thermal stability of a V-containing TiO2/WO3/SiO2 “SCR catalyst” is reported in the WO 2005/046864 A1. According to a preferred embodiment the vanadium in the formulations based on TiO2/WO3/(SiO2) is present not in form of Vanadiumpentaoxide (V2O5) but in form of a Rare Earth Vanadate (REVO4). The Rare Earth vanadate may be introduced to the support material (TiO2/WO3/(SiO2)) as a powder by simple mixing route (of the support and the Rare Earth Vanadate) following by calcination of the mixture.
Alternatively the Rare Earth Vanadates may be formed in the composition also in situ during the preparation (calcination) of the catalyst composition from precursors eg. Rare Earth acetate and Ammoniummetavanadate. The presence of the Rare Earth Vanadates in the catalyst is proved by XRD.
The catalyst compositions referred in WO 2005/046864 A1 exhibit good NOx conversion activity after being heat treated at 750° C./10 hrs, whereas in contrast the reference material containing V2O5 on the TiO2/WO3/SiO2 support may be considered to be almost inactive after being heat treated (aged) at 750° C./10 hrs.
However, WO 2005/046864 A1 does not describe any NOx conversion rates below 250° C., eg at 230° C. and 200° C. which is important for the automotive SCR systems. As shown in comparative example 2 an ErVO4 doped TiO2/WO3/SiO2 composition, which refers to Example 18, Table 2b in WO 2005/046864 A1, was subjected to NOx conversion test at temperatures at 200° C. and 230° C. NOx conversion was found to be zero at 200° and 230° C. and 25% at 250° C. for the “fresh” material.
After heat treatment of the compound at 700° C./10 hrs there was found an increase of catalytic activity exhibiting a relatively low NOx conversion at 200° C. and 230° C. being 6% at and 20% respectively; at 250° C. there was measured a conversion rate of 55%.
Comparative example 1 refers to a commercially available catalyst containing V2O5 supported on TiO2/WO3/SiO2 which is currently applied in heavy duty diesel SCR. After heat treatment at 650° C./2 hrs the material still exhibits activities. However the activity in the range 200-250° C. is already less than 50%; the activity drops significantly after heat treatment 700° C./10 hrs.
In addition in comparative example 1.1 it is shown, that a TiO2/WO3/SiO2: V2O5 catalyst is almost inactive after being heat treated at 750° C./10 hrs.
As a summary out of the state of the art review it may be concluded that RE-Vanadate doped TiO2/WO3/SiO2 materials are more heat stable compared to the V2O5 doped materials but show a weakness in NOx conversion at operating temperatures below 300° C. V2O5 containing TiO2/WO3/SiO2 materials seem to be operable up to 650° C. but loose already activity.