CN104254386A - NOx捕集阱组合物 - Google Patents

NOx捕集阱组合物 Download PDF

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CN104254386A
CN104254386A CN201380021647.6A CN201380021647A CN104254386A CN 104254386 A CN104254386 A CN 104254386A CN 201380021647 A CN201380021647 A CN 201380021647A CN 104254386 A CN104254386 A CN 104254386A
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collecting trap
magnesia
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F-M·麦肯娜
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Johnson Matthey PLC
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Abstract

公开了NOx捕集阱组合物，和其在内燃机用排气系统中的用途。NOx捕集阱组合物包含铂族金属、钡、钴和氧化镁-氧化铝载体。NOx捕集阱组合物不易于储存失活，并且表现出减少的N2O形成。

Description

NOx捕集阱组合物
发明领域
本发明涉及NOx捕集阱组合物，其在内燃机用排气系统中的用途，和用于处理内燃机废气的方法。
发明背景
内燃机产生含有多种污染物的废气，包括烃、一氧化碳、氮氧化物(“NOx”)、硫氧化物和颗粒物质。越来越严格的国家和地区法规已经降低了可以从这些柴油机或汽油机排放的污染物的量。已经对排气系统施用了许多不同的技术来在废气进入大气之前对其进行净化。
一种用来净化废气的这种技术是NOx捕集阱(或称“NOx吸附器催化剂”)。NOx捕集阱是在贫燃废气条件下吸附NOx，在富燃条件下释放吸附的NOx，并且还原释放的NOx形成N2的装置。NOx捕集阱通常包括用于储存NOx的NOx吸附剂和氧化/还原催化剂。
NOx吸附剂组分通常为碱土金属(如Ba、Ca、Sr和Mg)、碱金属(如K、Na、Li和Cs)、稀土金属(如La、Y、Pr和Nd)或其组合。这些金属通常以氧化物的形式存在。氧化/还原催化剂通常为一种或多种贵金属，优选铂、钯和/或铑。通常，包括铂以实现氧化功能，包括铑以实现还原功能。氧化/还原催化剂和NOx吸附剂通常负载在载体材料如无机氧化物上，用于排气系统。
NOx捕集阱实现三种功能。首先，在氧化催化剂存在下一氧化氮与氧反应生成NO2。然后，NO2以无机硝酸盐的形式由NOx吸附剂吸附(例如，BaO或BaCO3在NOx吸附剂上转化为Ba(NO3)2)。最后，当发动机运行在富燃条件下时，储存的无机硝酸盐分解形成NO或NO2，其然后在还原催化剂存在下通过与一氧化碳、氢和/或烃反应而还原形成N2(或者经由NHx或NCO中间体)。通常，在废物流中的热、一氧化碳和烃存在下，氮氧化物转化为氮、二氧化碳和水。
现有技术中已经描述了NOx捕集阱。例如，美国专利7,811,536描述了NOx储存催化剂，其包含钴、钡和。催化剂可以含有铂，或者可以不含铂。载体是氧化铝、二氧化硅、二氧化钛、氧化锆、硅铝酸盐及其混合物，其中优选氧化铝。
对于任何汽车系统和过程，需要在废弃处理系统中获得更进一步的改进。已经发现具有改进的老化特性的新NOx捕集阱组合物。
发明概述
本发明是一种NOx捕集阱组合物，其包含铂族金属、钡、钴和氧化镁-氧化铝载体。本发明还包括一种NOx捕集阱，其包含负载在基底上的NOx捕集阱组合物，以及其在排气系统中的用途。该NOx捕集阱组合物不易于储存失活，并且表现出经过老化后减少的N2O形成。
发明详述
本发明的NOx捕集阱组合物包含铂族金属、钡、钴和氧化镁-氧化铝载体。铂族金属(PGM)优选为铂、钯、铑或其混合物；最优选地，PGM为铂、钯或其混合物。
氧化镁-氧化铝载体优选为尖晶石、氧化镁-氧化铝混合金属氧化物、水滑石或类水滑石材料及其两种或更多种的组合。更优选地，氧化镁-氧化铝载体是尖晶石。
优选地，氧化镁-氧化铝载体包含5-40wt％的氧化镁，更优选地10-30wt％。如果氧化镁-氧化铝载体是水滑石，该载体优选与氧化铝例如勃姆石混合，以将总体氧化镁含量保持在5-40wt％。
尖晶石优选地为铝酸镁尖晶石，优选地Mg与Al的原子比为约0.17-约1，更优选地为约0.25-约0.75，和最优选地为约0.35-约0.65。最优选的实施方案包括MgAl2O4。
氧化镁-氧化铝混合金属氧化物包含Al2O3和MgO。该Al2O3和MgO的部分可以是经化学反应的或未经反应的。氧化镁-氧化铝混合金属氧化物中的Mg/Al比可以优选地为约0.25-10，更优选地为约0.5-约2，和最优选地为约0.75-约1.5。
氧化镁-氧化铝载体也可以是水滑石或类水滑石(HTL)材料。水滑石或HTL可以是收缩的(collapsed)、脱水的或脱羟基的。用于制造各种类型水滑石或HTL的非限定性实例和方法在美国专利4,866,019、4,946,581、4,952,382、6,028,023、6,479,421、6,929,736和7,112,313中描述；其在此通过参考整体引入。
优选地，氧化镁-氧化铝载体在高于600℃、更优选高于700℃和最优选高于800℃的温度煅烧，然后包含在NOx捕集阱组合物中。煅烧通常在含氧气体(例如空气)的存在下进行多于1小时。高温煅烧使氧化镁-氧化铝载体中形成尖晶石。
本发明的NOx捕集阱组合物可以通过任何适宜手段制备。优选地，通过任何已知手段将铂族金属、钴和钡负载在氧化镁-氧化铝载体上，以形成NOx捕集阱组合物，添加方式不认为是特别重要的。例如，可以通过浸渍、吸附、离子交换、初始润湿、沉淀等将PGM化合物(例如硝酸铂)、钴化合物(例如硝酸钴)和钡化合物(例如硝酸钡)负载在氧化镁-氧化铝载体上。
向氧化镁-氧化铝载体添加PGM、钴和钡化合物的顺序不认为是重要的。。例如，可以将铂、钴和钡化合物同时添加到氧化镁-氧化铝载体，或者可以以任意顺序依次添加。优选地，将钴和钡化合物添加到氧化镁-氧化铝载体，然后添加PGM化合物。
NOx捕集阱组合物优选包含0.1-10wt％的PGM，更优选0.5-5wt％的PGM，和最优选1-3wt％的PGM。NOx捕集阱组合物优选包含2-20wt％的钴，更优选5-15wt％的钴，和最优选7-12wt％的钴。NOx捕集阱组合物优选包含1-10wt％的钡，更优选2-8wt％的钡，和最优选3-7wt％的钡。优选地，钴:钡的重量比大于1，更优选地为2或更高。
本发明还包括一种NOx捕集阱。NOx捕集阱包含负载在陶瓷基底或金属基底上的NOx捕集阱组合物。陶瓷基底可以由任何难熔材料制成，例如氧化铝、二氧化硅、二氧化钛、氧化铈、氧化锆、氧化镁、沸石、氮化硅、碳化硅、硅酸锆、硅酸镁、硅铝酸盐和金属硅铝酸盐(例如堇青石和锂辉石)，或者其任意两种或更多种的混合物或混合氧化物。特别优选堇青石(一种硅铝酸镁)和碳化硅。
金属基底可以由任何适宜金属制成，特别是耐热金属和金属合金，例如钛和不锈钢以及含有铁、镍，铬，和/或铝还有其他痕量金属的铁素体合金。
基底优选为流通式基底或过滤器基底。最优选地，基底为流通式基底。具体地，流通式基底为优选具有蜂窝状结构的流通式整料，该蜂窝状结构具有许多轴向穿过该基底和延伸遍及该基底的小的、平行的薄壁通道。基底的通道横截面可以为任何形状，但优选为正方形、正弦曲线形、三角形、矩形、六角形、梯形、圆形或椭圆形。
优选地，NOx捕集阱通过使用活化涂覆程序将NOx捕集阱组合物沉积在基底上来制备。使用活化涂覆程序制备NOx捕集阱的代表性方法如下所述。将会理解的是，根据本发明的不同实施方案，下述方法可以不同。
活化涂覆优选通过以下来进行：首先将NOx捕集阱组合物的磨碎颗粒在适合的溶剂(优选为水)中制浆，以形成浆料。该浆料优选含有5至70wt％、更优选10至50wt％的固体。优选地，将该颗粒研磨或经过其他粉碎方法以确保在形成浆料之前基本上全部固体颗粒具有平均直径小于20微米的粒度。可以将另外的组分如稳定剂或促进剂掺入浆料作为水溶性或水分散性化合物或配合物的混合物。
然后可以用该浆料将基底涂覆一遍或多遍，以使基底上沉积有所需负载量的NOx捕集阱组合物。
也可以在基底上形成NOx捕集阱组合物以制备NOx捕集阱。在这种程序中，将氧化镁-氧化铝载体的浆料如上所述活化涂覆到基底上。氧化镁-氧化铝载体已经沉积在基底上(和任选地煅烧)之后，再可以将铂族金属、钴和钡添加到氧化镁-氧化铝活化涂层。可以通过任何已知手段添加PGM、钡和钴，包括浸渍、吸附或PGM化合物(例如硝酸铂)、钡化合物(例如硝酸钡)和钴化合物(例如硝酸钴)的离子交换。该添加的顺序不认为是重要的，因而铂族金属化合物、钡化合物和钴化合物可以同时添加，或者以任意顺序依次添加。
优选地，基底的整个长度涂覆有NOx捕集阱组合物，从而NOx捕集阱组合物的活化涂层覆盖基底的整个表面。
将NOx捕集阱组合物沉积在基底上之后，通常通过在优选80-150℃的升高的温度加热来干燥NOx捕集阱，然后通过在升高的温度加热来煅烧。优选地，煅烧在400至600℃进行约1-8小时。
本发明还包括内燃机用排气系统，其包括本发明的NOx捕集阱。优选地，该排气系统包括具有氧化催化剂和/或微粒过滤器的NOx捕集阱。这些后处理装置在本领域中是公知的。微粒过滤器是减少内燃机废气中的颗粒物的装置。微粒过滤器包括催化烟灰过滤器(CSF)和单纯(非催化)微粒过滤器。催化烟灰过滤器(用于柴油和汽油应用)包含金属和金属氧化物组分(如Pt、Pd、Fe、Mn、Cu和氧化铈)，以在破坏由该过滤器捕集的烟灰之外氧化烃和一氧化碳。
特别优选的排气系统包括NOx捕集阱，然后是CSF，二者均为紧耦合的；紧耦合的NOx捕集阱和地板下的CSF；和紧耦合的柴油机氧化催化剂/CSF和地板下的NOx捕集阱。
本发明还包括处理内燃机废气，特别是处理来自机动车贫燃内燃机(如柴油机、贫燃汽油机或由液化石油气或天然气供能的发动机)的废气。该方法包括使该废气与本发明的NOx捕集阱接触。
下面的实施例仅示例性说明本发明。本领域技术人员将会领会，许多变化处于本发明精神和权利要求书范围之内。
实施例1：制备催化剂
催化剂1A(Pt-Pd-Ba-Co/氧化镁-氧化铝载体)：
使用微热将硝酸钴(II)(4.92g)和醋酸钡(0.93g)溶解于软化水(～15mL)中。然后将该Co-Ba溶液逐渐添加到氧化镁-氧化铝载体(10g)，然后在105℃干燥2-3小时，然后在500℃煅烧2小时，以形成Ba-Co/氧化镁-氧化铝。使Ba-Co/氧化镁-氧化铝与铂和钯盐的水溶液(～7g溶液)接触，以将1.5wt％Pt和0.5wt％Pd添加到最终的催化剂上，然后在105℃干燥2-3小时，然后在500℃煅烧2小时，以形成催化剂1A。催化剂1A含有10wt％Co、5wt％Ba、1.5wt％Pt和0.5wt％Pd。
对比催化剂1B(Pt-Pd-Ba/氧化镁-氧化铝载体)：
根据催化剂1A的程序制备对比催化剂1B，只是不使用硝酸钴。对比催化剂1B含有5wt％Ba、1.5wt％Pt和0.5wt％Pd。
对比催化剂1C(Pt-Pd-Ba-Co/氧化铝载体)：
根据催化剂1A的程序制备对比催化剂1C，只是使用氧化铝代替氧化镁-氧化铝载体。对比催化剂1C含有10wt％Co、5wt％Ba、1.5wt％Pt和0.5wt％Pd。
实施例2：NOx储存测试程序
在含NO气体中将催化剂(0.4g)在200℃储存5分钟，然后以20℃/分钟的升温速率将温度升至290℃，以获得275℃的床温，并将催化剂保持在275℃床温5分钟。然后在富气体存在下，使催化剂经过15秒富吹扫，然后在程序升温脱附气体的存在下进行程序升温脱附(TPD)，直到床温达到约500℃，以出新鲜催化剂的NOx储存和N2O选择性(“新鲜周期”)。
然后催化剂在800℃在空气中热老化24小时，并在富气体存在下在500℃的温度进行富活化2分钟。
重复该程序，以测量经热老化的催化剂的NOx储存和N2O选择性(“老化周期”)。
含NO气体包含10.5vol％O2、50ppm NO、6vol％CO2、1500ppm CO、100ppm烃和6.3vol％H2O。
富气体包含1.5vol％O2、6vol％CO2、43,200ppm CO、1830ppm烃和6.3vol％H2O。
TPD气体包含10.5vol％O2、6vol％CO2、1500ppm CO、100ppm烃和6.3vol％H2O。
NOx储存结果示于表1中。
N2O选择性结果示于表2中。
结果显示，本发明的催化剂(催化剂1A)与对比催化剂1B和1C相比，具有更高的NOx储存和良好的N2O选择性。在800℃高温老化后，催化剂1A也保持良好的NOx储存和N2O选择性，相比之下，对比催化剂1B和1C经过老化后NOx储存低得多，N2O选择性增大。
表1：NOx储存结果
*对比例
表2：贫N2O选择性结果
*对比例

Claims (13)

1.一种NOx捕集阱组合物，其包含铂族金属、钡、钴和氧化镁-氧化铝载体。
2.根据权利要求1所述的NOx捕集阱组合物，其中铂族金属选自铂、钯、铑及其混合物。
3.根据权利要求1所述的NOx捕集阱组合物，其中氧化镁-氧化铝载体为铝酸镁尖晶石。
4.根据权利要求1所述的NOx捕集阱组合物，其中氧化镁-氧化铝载体包含5-40wt％的氧化镁。
5.根据权利要求1所述的NOx捕集阱组合物，其中NOx捕集阱组合物包含0.1-10wt％的铂族金属。
6.根据权利要求1所述的NOx捕集阱组合物，其中NOx捕集阱组合物包含2-20wt％的钴。
7.根据权利要求1所述的NOx捕集阱组合物，其中NOx捕集阱组合物包含1-10wt％的钡。
8.根据权利要求1所述的NOx捕集阱组合物，其中氧化镁-氧化铝载体在高于600℃的温度预煅烧。
9.一种NOx捕集阱，其包含负载在金属或陶瓷基底上的根据权利要求1所述的NOx捕集阱组合物。
10.根据权利要求9所述的NOx捕集阱，其中该基底为流通式整料。
11.一种内燃机用排气系统，其包括根据权利要求9所述的NOx捕集阱。
12.根据权利要求11所述的排气系统，其进一步包括氧化催化剂、微粒过滤器或其组合。
13.一种处理内燃机废气的方法，其包括使该废气与根据权利要求9所述的NOx捕集阱接触。
CN201380021647.6A 2012-04-26 2013-04-22 NOx捕集阱组合物 Pending CN104254386A (zh)

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