Novel functions of composite catalysts derived from Pd and support metal oxides-mechanism and utilization

在各种负载型Pd催化剂中，Pd/ZnO对甲醇水蒸气重整反应和脱氢制甲酸均表现出较高的催化性能。水蒸气重整和脱氢制甲酸的选择性分别达到99.5%和80%。相比之下，其它负载型Pd催化剂对水蒸气重整和脱氢制甲酸的选择性分别为0 - 40%和0%。在较高温度下对Pd/ZnO催化剂进行预还原，可大大提高Pd/ZnO的水蒸气重整和脱氢催化性能。催化性能的提高归因于催化剂在前一次还原中形成的PdZn合金。XP谱表明，在PdZn合金的形成过程中，发生了电子从Pd向Zn的转移。在合金化催化剂上，反应中生成的甲醛被-OH或CH_3O-有效地攻击，转化为CO_2和H_2O。 ...更多信息 2或HCOOCH_3。在其它含非合金Pd的催化剂上，甲醛在水蒸气重整和甲醇脱氢反应中选择性地分解为CO和H_2。结果表明，在PdZn合金上，甲醛物种以eta^1（O）-形式稳定在带正电荷的Pd位上，并与-OH或CH_3O-发生亲核加成反应.根据提出的Pd新催化功能的产生机理，开发了新型的Pd_Zn_2和PtZn合金催化剂。<3.1><6.9>这些合金对甲醇脱氢制HCOOCH_3和水蒸气重整反应具有很高的选择性。对不同制备方法制备的Pd/ZnO催化剂进行了实验研究。Pd/ZnO（10wt%Pd）催化剂是由Pd（Ⅱ）和Zn（Ⅱ）经Na_2CO_3沉淀而成，其水蒸气重整反应性能最好。Pd/ZnO催化剂的催化活性是Cu/SiO_2催化剂（30wt%Cu）的2倍。少

Pd/ZnO exhibited anomalously high catalytic performance for the steam reforming of methanol and the dehydrogenation to HCOOCH_3 among various supported Pd ctalysts. The selectivities for the steam reforming and the dehydrogenation to HCOOCH_3 were achieved at 99.5% and 80%, respectively. In contraast, the other supported Pd catalysts showed the selectivities of 0-40% and 0%, respectively for the steam reforming and the dehydrogenation to HCOOCH_3. The catalytic performance of Pd/ZnO for the steam reforming and the dehydrogenation was greatly improved by previously reducing the catalyst at higher temperatures. The increased catalytic performace was attributed to PdZn alloys formed in the previous reduction of the catalyst. By XP spectroscopy, it was revealed that electron transfer took place from Pd to Zn in the formation of PdZn alloys. Over the alloyed catalysts, formaldehyde species formed in the reactions were effectively attacked by -OH or CH_3O-, being transformed into CO_2 and H_ … More 2 or HCOOCH_3. Over the other catalysts containing unalloyed Pd, the formaldehyde species decomposed selectively to CO and H_2 in the steam steam reforming and the dehydrogenation of methanol. It was suggested that formaldehyde species were stabilized on positively charged Pd sites in eta^1 (O) -form in the reactions over PdZn alloys, and subjected to the nucleophilic addition of -OH or CH_3O-. On the basis of the proposed mechanism of the generation of new catalytic function of Pd, novel Pd_<3.1>Zn_<6.9> and PtZn alloy catalysts were developed. These alloys showed high selectivity for the dehydrogenation of methanol to HCOOCH_3 as well as the steam reeforming. Experimemts were carried out over Pd/ZnO catalysts prepared by various preparation methods. Pd/ZnO (10wt% Pd) obtained by precipitation of Pd (II) and Zn (II) with a Na_2CO_3 solution exhibited best performance for the steam reforming among the Pd catalysts so far studied. The activity of Pd/ZnO was twice that for Cu/SiO_2 (30wt% Cu) previously studied. Less