摘要
由于缺乏对废尾气催化剂的全面表征,导致对其物理化学性质及失效原因的了解不完全,影响其中铂族金属的回收。 为此,对报废尾气催化剂进行了化学组成和物相、表面元素、微观形貌、粒度、比表面积和热稳定性等一系列性质表征分析,得出其失效原因主要包括铂族金属硫化及磷化中毒、催化剂表面积碳吸附、铂族金属氧化、烧结团聚及铂族金属在载体内的包裹、比表面积和总孔体积缩小等。
铂族金属(PGMs),尤其是铂(Pt)、钯(Pd)、铑(Rh),因其独特的物理化学性能,如催化活性等,被广泛应用于汽车催化剂行
另一方面,汽车尾气催化剂在使用过程中会吸附一些重金属(如Pb、Cr等)、有机物等有害物质,会对环境造成严重影响,因此被列为危险废
废汽车尾气催化剂中PGMs的回收主要采用火法冶金和湿法冶金两种技
采用X射线荧光光谱仪 (XRF,瑞士赛默飞世尔科技 ARL Advant’X IntellipowerTM 3600X)和电感耦合等离子体发射光谱仪(安捷伦ICP‒OES 5110)分析废汽车尾气催化剂的化学组成,借助德国元素分析系统公司的Vario ELIII有机元素分析仪分析其中的C、N、S元素的含量。ICP‒OES测试前,为使实验样品消解完全,将样品加入含有HCl和H2O2的聚四氟乙烯压力消解罐中,然后放入200 ℃的马弗炉中溶解12
目前市场上较为常用的多为蜂窝状三元催化剂,主要由载体、涂层和活性组分3部分组成。载体多为陶瓷堇青石,主要成分为2MgO∙2A12O3∙5SiO2 或2FeO∙2Al2O3∙5SiO
成分 | Al2O3 | SiO2 | MgO | Fe2O3 | SO3 | TiO2 | P2O5 | CeO2 | NiO | PtO2 | 其他 |
---|---|---|---|---|---|---|---|---|---|---|---|
质量分数/% | 73.67 | 20.33 | 3.74 | 0.52 | 0.87 | 0.25 | 0.06 | 0.09 | 0.04 | 0.17 | 0.26 |

图 1 废汽车催化剂 X 射线衍射图
Fig. 1 X-ray diffraction of waste automobile catalysts
表 2 废汽车催化剂元素分析及PGMs质量分数分析结果
Tab. 2 Results of elemental analysis and PGMs
content analysis of waste automobile
catalysts
元素 | C | N | S | Pt | Pd |
---|---|---|---|---|---|
质量分数/% | 0.472 | 0.026 | 0.307 |
95 |
1 03 |
注: *表示单位为g·
由于X射线衍射仪检测限较高,并不能确定PGMs在废汽车催化剂中的赋存状态,但PGMs具有极强的耐高温特性和化学惰性,即使在失效后也大多保持金属单质的形式。尽管致密的金属铂在任何温度下都不会被空气氧化,也不会失去表面的金属光泽,但其细粉在高温高压条件下能与氧发生作用,形成黑色粉状物PtO和PtO2。而钯粉也会在800~840 ℃下与氧气发生反应,转化为PdO或PdO

图 2 废汽车催化剂 X 射线光电子能谱分析
Fig. 2 X-ray photoelectron spectroscopy analysis of waste automobile catalysts

图 3 废汽车催化剂 SEM图
Fig. 3 SEM of waste automobile catalysts

图 4 废汽车催化剂AFM形貌及高度图
Fig. 4 AFM morphology and height of waste automobile catalysts
废汽车尾气催化剂的氮气吸附‒脱附曲线如

图 5 废汽车催化剂氮气吸附‒脱附曲线
Fig. 5 Nitrogen adsorption-desorption of waste automobile catalysts
废汽车尾气催化剂的粒度对后续的湿法冶金工艺有较为明显的影响,当样品粒度处于一个较高的范围之内时,会明显延长其溶解时间。因此,本研究对接收到的样品进行了粉碎预处理,并对粉碎后的样品经过200目筛分,然后对样品的粒度分布情况进行了分析,结果如

图 6 废汽车催化剂粒度分布曲线
Fig. 6 Particle size distribution of waste automobile catalyst
对废汽车尾气催化剂的热行为分析,可以获得其表面吸附的积碳等有机物的热解规律,为后续提取工艺中预处理方法的选择提供一定参考。本实验利用TG‒DSC研究了废汽车催化剂在空气气氛下的热化学行为,测试温度范围为室温至1 000 ℃,选取的升温速率为10 ℃·mi
FeS2(s) → Fe(s) + 2S(g) | (1) |
4MgS(s) → 2Mg2S(s) + S2(g) | (2) |
同时,部分PGMs氧化物如PdO也会自820 ℃起开始分解为Pd与O2,当温度高于870 ℃时,完全分解为单质

图 7 废汽车催化剂热重曲线
Fig. 7 Thermogravimetric curve of waste automobile catalysts
通过对该废尾气催化剂的表征分析,得知其失效原因是多方面的,具体包括PGMs硫化及磷化中毒、催化剂表面积碳吸附、PGMs氧化、烧结团聚及 PGMs的包裹、比表面积和总孔体积缩小等。而这些物理化学性质的变化均会严重影响废尾气催化剂中PGMs的提取效率:① PGMs的硫化或磷化反应会使部分铂族金属单质转化为性质更加稳定的化合物,进而增加了PGMs提取的难度。② 催化剂表面吸附的积碳等有机物会影响其中目的金属颗粒与药剂的相互作用以及颗粒之间的解离度来降低PGMs的溶解分离效率。③ PGM氧化物PtO2、PdO2的性质极其稳定,几乎都不溶于强酸,甚至不溶于王水,这将为废汽车催化剂中PGMs的浸出增加一定难
对某报废汽车尾气催化剂的化学组成和物相、表面元素、微观形貌、粒度、比表面积和热稳定性等性质进行了分析表征,主要结论如下:
(1) 该废汽车催化剂中存在含S和P的化合物,同时有积碳附着,导致该催化剂中毒失效。另外,其中贵金属Pt和Pd的总含量接近2 000 g·
(2) 部分Pt、Pd发生氧化反应,分别转化为难溶的PtO2和PdO2,不仅使汽车催化剂失效,还会给其浸出增加一定难度。
(3) 该废汽车催化剂不仅发生了严重的烧结团聚反应,而且造成了PGMs在氧化铝涂层及载体内的包裹,一方面使汽车催化剂失效,另一方面也会影响后续PGMs的提取。
(4) 该废汽车催化剂的比表面积和总孔体积大幅缩小,也是其失效的一个重要原因。为了使PGM充分解离,同时提高其浸出效率,需保证一定的粒度范围。
(5) 废汽车催化剂热化学性质稳定,没有明显的热分解行为,这也使得废汽车催化剂湿法冶金工艺中的预处理条件更为苛刻。
(6) 通过对汽车催化剂失效原因的全面分析,认为需在湿法冶金前有针对性地采取一定的预处理方法,消除含S和P的化合物及积碳等的污染,破坏其烧结团聚现象,打开载体对PGMs的包裹,以提高废汽车催化剂中PGMs的提取效率,并同时关注金属单质和金属氧化物的回收。
作者贡献声明
孙士强:负责材料表征分析及论文初稿的撰写。
靳晨曦:负责论文初稿的整体修改及论文的格式、排版。
贺文智:负责论文总体设计,提出需求与思路,统筹论文写作与修改。
李光明:负责论文中表征方法的设计和指导。
朱昊辰:负责论文中部分表征结果的分析指导。
黄菊文:对论文提出修改建议。
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