Compression ignition engines cause combustion of a hydrocarbon by injecting the hydrocarbon into compressed air and can be fuelled by diesel fuel, biodiesel fuel, blends of diesel and biodiesel fuels and compressed natural gas. The purpose of the present invention is different from the invention claimed in UK patent application no. 1003244.9 filed on 26 Feb. 2010 entitled “Filter”. The purpose of the invention in that patent application is a filter for particulate matter in exhaust gas of a positive ignition engine.
Ambient PM is divided by most authors into the following categories based on their aerodynamic diameter (the aerodynamic diameter is defined as the diameter of a 1 g/cm3 density sphere of the same settling velocity in air as the measured particle):                (i) PM-10—particles of an aerodynamic diameter of less than 10 μm;        (ii) Fine particles of diameters below 2.5 μm (PM-2.5);        (iii) Ultrafine particles of diameters below 0.1 μm (or 100 nm); and        (iv) Nanoparticles, characterised by diameters of less than 50 nm.        
Since the mid-1990's, particle size distributions of particulates exhausted from internal combustion engines have received increasing attention due to possible adverse health effects of fine and ultrafine particles. Concentrations of PM-10 particulates in ambient air are regulated by law in the USA. A new, additional ambient air quality standard for PM-2.5 was introduced in the USA in 1997 as a result of health studies that indicated a strong correlation between human mortality and the concentration of fine particles below 2.5 μm.
Interest has now shifted towards nanoparticles generated by diesel and gasoline engines because they are understood to penetrate more deeply into human lungs than particulates of greater size and consequently they are believed to be more harmful than larger particles, extrapolated from the findings of studies into particulates in the 2.5-10.0 μm range.
Size distributions of diesel particulates have a well-established bimodal character that corresponds to the particle nucleation and agglomeration mechanisms, with the corresponding particle types referred to as the nuclei mode and the accumulation mode respectively (see FIG. 1). As can be seen from FIG. 1, in the nuclei mode, diesel PM is composed of numerous small particles holding very little mass. Nearly all diesel particulates have sizes of significantly less than 1 μm, i.e. they comprise a mixture of fine, i.e. falling under the 1997 US law, ultrafine and nanoparticles.
Nuclei mode particles are believed to be composed mostly of volatile condensates (hydrocarbons, sulfuric acid, nitric acid etc) and contain little solid material, such as ash and carbon. Accumulation mode particles are understood to comprise solids (carbon, metallic ash etc.) intermixed with condensates and adsorbed material (heavy hydrocarbons, sulfur species, nitrogen oxide derivatives etc.). Coarse mode particles are not believed to be generated in the diesel combustion process and may be formed through mechanisms such as deposition and subsequent re-entrainment of particulate material from the walls of an engine cylinder, exhaust system, or the particulate sampling system. The relationship between these modes is shown in FIG. 1.
The composition of nucleating particles may change with engine operating conditions, environmental condition (particularly temperature and humidity), dilution and sampling system conditions. Laboratory work and theory have shown that most of the nuclei mode formation and growth occur in the low dilution ratio range. In this range, gas to particle conversion of volatile particle precursors, like heavy hydrocarbons and sulfuric acid, leads to simultaneous nucleation and growth of the nuclei mode and adsorption onto existing particles in the accumulation mode. Laboratory tests (see e.g. SAE 980525 and SAE 2001-01-0201) have shown that nuclei mode formation increases strongly with decreasing air dilution temperature but there is conflicting evidence on whether humidity has an influence.
Generally, low temperature, low dilution ratios, high humidity and long residence times favour nanoparticles formation and growth. Studies have shown that nanoparticles consist mainly of volatile material like heavy hydrocarbons and sulfuric acid with evidence of solid fraction only at very high loads.
Particulate collection of diesel particulates in a diesel particulate filter is based on the principle of separating gas-borne particulates from the gas phase using a porous barrier. Diesel filters can be defined as deep-bed filters and/or surface-type filters. In deep-bed filters, the mean pore size of filter media is bigger than the mean diameter of collected particles. The particles are deposited on the media through a combination of depth filtration mechanisms, including diffusional deposition (Brownian motion), inertial deposition (impaction) and flow-line interception (Brownian motion or inertia).
In surface-type filters, the pore diameter of the filter media is less than the diameter of the PM, so PM is separated by sieving. Separation is done by a build-up of collected diesel PM itself, which build-up is commonly referred to as “filtration cake” and the process as “cake filtration”.
It is understood that diesel particulate filters, such as ceramic wallflow monoliths, may work through a combination of depth and surface filtration: a filtration cake develops at higher soot loads when the depth filtration capacity is saturated and a particulate layer starts covering the filtration surface. Depth filtration is characterized by somewhat lower filtration efficiency and lower pressure drop than the cake filtration.
Selective catalytic reduction (SCR) of NOx by nitrogenous compounds, such as ammonia or urea, was first developed for treating industrial stationary applications. SCR technology was first used in thermal power plants in Japan in the late 1970s, and has seen widespread application in Europe since the mid-1980s. In the USA, SCR systems were introduced for gas turbines in the 1990s and have been used more recently in coal-fired powerplants. In addition to coal-fired cogeneration plants and gas turbines, SCR applications include plant and refinery heaters and boilers in the chemical processing industry, furnaces, coke ovens, municipal waste plants and incinerators. More recently, NOx reduction systems based on SCR technology are being developed for a number of vehicular (mobile) applications in Europe, Japan, and the USA, e.g. for treating diesel exhaust gas.
Several chemical reactions occur in an NH3 SCR system, all of which represent desirable reactions that reduce NOx to nitrogen. The dominant reaction is represented by reaction (1).4NO+4NH3+O2→4N2+6H2O  (1)
Competing, non-selective reactions with oxygen can produce secondary emissions or may unproductively consume ammonia. One such non-selective reaction is the complete oxidation of ammonia, shown in reaction (2).4NH3+5O2→4NO+6H2O  (2)
Also, side reactions may lead to undesirable products such as N2O, as represented by reaction (3).4NH3+5NO+3O2→4N2O+6H2O  (3)
Various catalysts for promoting NH3-SCR are known including V2O5/WO3/TiO2 and transition metal/zeolites such as Fe/Beta (see U.S. Pat. No. 4,961,917) and transition metal/small pore zeolites (see WO 2008/132452).
EP 1663458 discloses a SCR filter, wherein the filter is a wallflow monolith and wherein an SCR catalyst composition permeates walls of the wallflow monolith. The specification discloses generally that the walls of the wallflow filter can contain thereon or therein (i.e. not both) one or more catalytic materials. According to the disclosure, “permeate”, when used to describe the dispersion of a catalyst slurry on the wallflow monolith substrate, means the catalyst composition is dispersed throughout the wall of the substrate.
WO 2008/136232 A1 discloses a honeycomb filter having a cell wall composed of a porous cell wall base material and, provided on its inflow side only or on its inflow and outflow sides, a surface layer and satisfying the following requirements (1) to (5) is used as DPF: (1) the peak pore diameter of the surface layer is identical with or smaller than the average pore diameter of the cell wall base material, and the porosity of the surface layer is larger than that of the cell wall base material; (2) with respect to the surface layer, the peak pore diameter is from 0.3 to less than 20 μm, and the porosity is from 60 to less than 95% (measured by mercury penetration method); (3) the thickness (L1) of the surface layer is from 0.5 to less than 30% of the thickness (L2) of the cell wall; (4) the mass of the surface layer per filtration area is from 0.01 to less than 6 mg/cm2; and (5) with respect to the cell wall base material, the average pore diameter is from 10 to less than 60 μm, and the porosity is from 40 to less than 65%. See also SAE paper 2009-01-0292.
NOx absorber catalysts (NACs) are known e.g. from U.S. Pat. No. 5,473,887 and are designed to adsorb nitrogen oxides (NOx) from lean exhaust gas (lambda>1) and to desorb the NOx when the oxygen concentration in the exhaust gas is decreased. Desorbed NOx may be reduced to N2 with a suitable reductant, e.g. gasoline fuel, promoted by a catalyst component, such as rhodium, of the NAC itself or located downstream of the NAC. In practice, control of oxygen concentration can be adjusted to a desired redox composition intermittently in response to a calculated remaining NOx adsorption capacity of the NAC, e.g. richer than normal engine running operation (but still lean of stoichiometric or lambda=1 composition), stoichiometric or rich of stoichiometric (lambda<1). The oxygen concentration can be adjusted by a number of means, e.g. throttling, injection of additional hydrocarbon fuel into an engine cylinder such as during the exhaust stroke or injecting hydrocarbon fuel directly into exhaust gas downstream of an engine manifold.
A typical NAC formulation includes a catalytic oxidation component, such as platinum, a significant quantity, i.e. substantially more than is required for use as a promoter such as a promoter in a TWC, of a NOx-storage component, such as barium, and a reduction catalyst, e.g. rhodium. One mechanism commonly given for NOx-storage from a lean exhaust gas for this formulation is:NO+½O2→NO2  (4); andBaO+NO2+½O2→Ba(NO3)2  (5),wherein in reaction (4), the nitric oxide reacts with oxygen on active oxidation sites on the platinum to form NO2. Reaction (5) involves adsorption of the NO2 by the storage material in the form of an inorganic nitrate.
At lower oxygen concentrations and/or at elevated temperatures, the nitrate species become thermodynamically unstable and decompose, producing NO or NO2 according to reaction (6) below. In the presence of a suitable reductant, these nitrogen oxides are subsequently reduced by carbon monoxide, hydrogen and hydrocarbons to N2, which can take place over the reduction catalyst (see reaction (5)).Ba(NO3)2→BaO+2NO+ 3/2O2 or Ba(NO3)2→BaO+2NO2+½O2  (6); andNO+CO→½N2+CO2  (7);(Other reactions include Ba(NO3)2+8H2→BaO+2NH3+5H2O followed by NH3+NOx→N2+yH2O or 2NH3+2O2+CO→N2+3H2O+CO2 etc.).
In the reactions of (4)-(7) above, the reactive barium species is given as the oxide. However, it is understood that in the presence of air most of the barium is in the form of the carbonate or possibly the hydroxide. The skilled person can adapt the above reaction schemes accordingly for species of barium other than the oxide and sequence of catalytic coatings in the exhaust stream.
In Europe, since the year 2000 (Euro 3 emission standard) emissions are tested over the New European Driving Cycle (NEDC). This consists of four repeats of the previous ECE 15 driving cycle plus one Extra Urban Driving Cycle (EUDC) with no 40 second warm-up period before beginning emission sampling. This modified cold start test is also referred to as the “MVEG-B” drive cycle. All emissions are expressed in g/km.
The Euro 5/6 implementing legislation introduces a new PM mass emission measurement method developed by the UN/ECE Particulate Measurement Programme (PMP) which adjusts the PM mass emission limits to account for differences in results using old and the new methods. The Euro 5/6 legislation also introduces a particle number emission limit (PMP method), in addition to the mass-based limits.
Emission legislation in Europe from 1 Sep. 2014 (Euro 6) requires control of the number of particles emitted from both diesel and gasoline passenger cars. For diesel EU light duty vehicles the allowable limits are: 500 mg/km carbon monoxide; 80 mg/km nitrogen oxides (NOx); 170 mg/km total hydrocarbons+NOx; 4.5 g/km particulate matter (PM); and particulate number standard of 6.0×1011 per km. The present specification is based on the assumption that this number will be adopted in due course.
A difficulty in coating a filter with a catalyst composition is to balance a desired catalytic activity, which generally increases with washcoat loading, with the backpressure that is caused by the filter in use (increased washcoat loading generally increases backpressure) and filtration efficiency (backpressure can be reduced by adopting wider mean pore size and higher porosity substrates at the expense of filtration efficiency).