The diesel part iculat e filters (DPF s) technology ha s impressively  advanced espe cially  dur ing th e last year s, drive n by th e int eres t in th e red uction of aut omobile part iculat e emissions。 This paper is concern ed with th e effect of NO2 as an active oxidation agent in th e regenerat ion process of th e soot accumu lat ed in th e part iculat e filter。 Experiment s at realistic conditions using a diesel engine equipped with a stan dar d oxidat ion cata lyst an d a part iculat e filter ar e carr ied out at a wide ran ge of operat ing conditions。 These results ar e used to validat e an already available math emat ical model of th e NO2-assisted regenerat ion phenomena in th e part iculat e filter。 The combined use of expe rimenta l an d modeling res ults provides int eres ting conclusions regar ding th e significance an d th e chemistr y  of  th e  reaction of  soot  with  NO2。  The  advanta ges  an d dra wbacks of such an app roach compar ed to stan dar d laborat ory synth etic gas stu dies ar e discussed 。 The agreement betwee n expe rimenta l an d simu lat ion res ults in term s of engineering int eres t (rat e of soot accumu lat ion or depletion) is quite sat isfactory an d indicat es that such a type of  model could be  a  promising design tool。83967

Introd uc tio nPart iculat e filtrat ion in th e exhau st system of diesel engines is increasingly gaining in importan ce for both light- an d heavy-dut y applicat ions。 Pa sse nger car s equipped with diesel part iculat e filters (DPF s) already appe ar ed in th e mark et as a mean s to achieve th e low part iculat e emission stan dar ds in Eur ope。1 The par- ticulat e filter technology is also considered th e most promising solut ion towar d atta ining th e emission stan - dar ds of heavy-dut y vehicles。2-5

Filter regenerat ion, i。e。, th e process of soot removal from th e filter to avoid excessive backpr ess ur e buildup, can be accomplished by th erma l soot oxidat ion by th e exhau st gas oxyge n。 This reaction occur s with noticeable rat es at temperatur es above 500-550 °C,  which  ar e rar ely met un der typical operat ing conditions。 On th e oth er han d, NO2 is highly reactive with soot。6 NO2  is able to oxidize soot at temperatur es as low as 250 °C, which can be encount ered in diesel exhau st dur ing norma l driving cycles。 Howeve r, NO2 is prese nt in diesel ra w exhau st at very low concentrat ions (5-15% of tota l NOx, or less than 50 pp m), which ar e not sufficie nt to provide th e  required reaction rat es。

The concentrat ion of NO2 in th e exhau st gas ent ering th e filter can be increased by placing ups tr eam of th e filter an oxidat ion cata lyst, which oxidizes NO to NO2。 This is th e ma in idea un derlying th e cont inuously regenerat ing tra p (CRT) concep t, which is th e tra de nam e of a system composed of  a  selective  oxidat ion cata lyst placed directly ups tr eam of th e part iculat e filter。7 At temperatur es of 300-350 °C, th e oxidat ion cata lyst oxidizes a proport ion of th e NO in th e exhau st str eam to form NO2, increasing th e NO2 fraction to about  50% of tota l NOx。8-10

The increased int eres t in part iculat e filter regenera- tion systems an d th e promising res ults of NO2-assisted

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systems ha ve motivat ed a num ber of recently publis hed rese arch work s。11-16 Howeve r, th ese work s ar e rath er few, compar ed to th e high comm ercial int eres t of such systems。 Moreover, th ese work s deal mostly with reac- tion rat e stu dies at laborat ory scale using synth etic soot or synth etic gas。 Har dly an y publis hed data  can be foun d on th e reactivity of NO2 with soot in real engine conditions。