The Formation and Hydrolysis of Isocyanic Acid during the Reaction of NO, CO, and H2 Mixtures on Supported Platinum, Palladium, and Rhodium
The Formation and Hydrolysis of Isocyanic Acid during the Reaction of NO, CO, and H2 Mixtures on Supported Platinum, Palladium, and Rhodium
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DOI：
10.1006/jcat.2001.3359
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发表时间：
2001-11
影响因子：
7.3
通讯作者：
D. Chambers;D. Angove;N. W. Cant
中科院分区：
文献类型：
--
作者：
D. Chambers;D. Angove;N. W. Cant
异氰酸（HNCO）的程度是在反应过程中形成的NO/CO/H2的混合物在二氧化硅负载的Pt，Rh，和Pd已被研究连同随后的水解HNCO氧化物系统放置在下游。对于Pt/SiO2在315 ℃下超过35%，用标准2800/3400/1200 ppm NO/CO/H2混合物，从NO生成HNCO的产率最高。氢的消耗在220°C时完成，主要产物为氨和水，但在260°C以上，HNCO变成有利的产物，一些产物来自NH3。一旦所有的NO被消耗，HNCO水解成NH3和CO2。如果将额外的SiO2放置在下游，则水解程度会有所增加。其它氧化物体系-CeO 2/SiO2、BaO/SiO2、CeO 2/Al 2 O3和CeO 2-ZrO 2-在315 ℃下可完全水解至可利用的水的程度，并且如果进料中包括额外的水，则不残留HNCO。在Pd/SiO2上，NO+CO+H2反应期间的氢消耗在最低温度下开始，并且对于从130°C的起始温度开始的100°C，反应可以在很大程度上描述为NO+H2和NO+CO的总和。HNCO的形成在235°C下开始，在300°C下最大产率为20%。它似乎仅仅是通过利用作为NO+H2反应副产物的NH3而产生的。Rh/SiO_2催化剂对NO+ H_2反应的活性远低于Pt/SiO_2和Pd/SiO_2催化剂，但对NO+CO和NO+CO+ H_2反应的活性高于Pt/SiO_2和Pd/SiO_2催化剂。后者在HNCO形成中表现出小的尖峰，但最大产率仅为14%，这与NO的总消耗相吻合。在较高温度下形成相当多的氨，即使在NO+H2反应期间没有产生氨。HNCO被认为是通过表面氢原子与金属结合的NCO基团的结合而在每种金属上产生的，当N原子位于吸附的CO分子附近时，金属结合的NCO基团以少量存在。金属之间的行为差异可以根据CO和NO的相对吸附强度以及H2和NO的总消耗之间的温度差来合理化。如果差异很大，则HNCO可以由氨和氢产生。一般结论是，尽管在预热过程中，三效汽车催化剂孔隙内的铂族金属颗粒上可能会产生一些HNCO，但在水大量过量的氧化物涂层上水解的速度如此之快，以至于不会产生HNCO。永远不会出现。只有水解产物，NH3和CO2，将被看到。
The extent to which isocyanic acid (HNCO) is formed during the reaction of NO/CO/H2 mixtures over silica-supported Pt, Rh, and Pd has been investigated together with the subsequent hydrolysis of HNCO on oxide systems placed downstream. The yield of HNCO from NO is highest for Pt/SiO2 exceeding 35% at 315°C with a standard 2800/3400/1200 ppm NO/CO/H2 mixture. Hydrogen consumption is complete at 220°C with ammonia and water as major products, but above 260°C HNCO becomes the favoured product with some arising from NH3. Hydrolysis of HNCO to NH3 and CO2 takes over once all NO has been consumed. The extent of hydrolysis is increased somewhat if additional SiO2 is placed downstream. Other oxide systems–-CeO2/SiO2, BaO/SiO2, CeO2/Al2O3, and CeO2-ZrO2–-give complete hydrolysis to the extent of the available water at 315°C, and no HNCO remains if additional water is included in the feed. Hydrogen consumption during the NO+CO+H2 reaction commences at the lowest temperature on the Pd/SiO2, and for 100°C from the onset temperature of 130°C the reaction can be largely described as the sum of the NO+H2 and NO+CO ones. Formation of HNCO commences at 235°C, with a maximum yield of 20% at 300°C. It appears to arise solely through utilisation of NH3 made as a side-product to the NO+H2 reaction. Rh/SiO2 is much less active than Pt/SiO2 and Pd/SiO2 for the NO+H2 reaction, but more active for the NO+CO and NO+CO+H2 ones. The latter exhibits a small sharp peak in HNCO formation, but the maximum yield is only 14% and this coincides with total consumption of NO. Considerable ammonia is formed at higher temperatures, even though none is produced during the NO+H2 reaction. HNCO is believed to arise on each metal through the combination of surface hydrogen atoms with metal-bound NCO groups which exist in small numbers when N atoms are located adjacent to adsorbed CO molecules. The differences in behaviour between the metals can be rationalised in terms of the relative strengths of adsorption of CO and NO, and the temperature difference between total consumption of H2 and NO. If the difference is large, then HNCO can be produced from ammonia as well as hydrogen. A general conclusion is that, although some HNCO might be generated on platinum group metal particles within the pores of three-way vehicle catalysts during warm-up, the rapidity of hydrolysis on oxide washcoats with water in large excess is so great that no HNCO would ever emerge. Only the hydrolysis products, NH3 and CO2, will be seen.