Catalytic activation and reforming of methane on supported palladium clusters
Catalytic activation and reforming of methane on supported palladium clusters
复制标题
DOI：
10.1016/j.jcat.2010.06.001
复制
发表时间：
2010-08
影响因子：
7.3
通讯作者：
A. Yamaguchi;E. Iglesia
中科院分区：
文献类型：
--
作者：
A. Yamaguchi;E. Iglesia
反应物和产物浓度对转化率、同位素示踪和动力学同位素效应的影响导致了甲烷在负载型Pd催化剂上与CO2和H2O反应的一系列基本步骤。与动力学相关的C-H键活化步骤在Pd上的速率常数比在其他金属(Ni、Ru、Rh、Ir、Pt)上大得多。因此，这些步骤在催化过程中是可逆的，因为CH4解离的产物迅速耗尽了由CO2或H2O和共反应物活化形成的所需氧共反应物，并且在CH4转化所需的时间尺度内，水-气变换反应保持不可逆。H_2和CO产物分别通过影响CH_4和CO的解离步骤来抑制CH_4反应。这些力学结论与反应物和产物对转化率的动力学效应一致，与H2O和CO2共反应物测得的CH4/CD4动力学同位素效应相似且正常，没有H2O/D2O同位素效应，与CH4重整催化过程中CH4与CD4、12C16O与13C18O、13CO与12CO之间的同位素抢占速率一致。这种催化顺序，但不是其基本步骤的可逆性，与在其他第VIII族金属上报道的相同。不同载体(Al_2O_3、ZrO_2、ZrO_2、−、La_2O_3)上Pd团簇的转化率相似，且与Pd在重整条件下可达的窄范围内的分散度无关，因为动力学相关的C-H键活化步骤主要发生在Pd表面。具有可检测的CO_2和H_2O活化活性的ZrO_2和ZrO_2−La_2O_3载体可以逆转罕见的碳覆层的形成并抑制失活，但对稳态催化重整速率没有贡献。Pd表面在C-H键活化中的高活性反映了它们与C和H的强结合，以及与动力学相关的C-H活化步骤过渡态的稳定，并导致了观察到的CH4和CO解离步骤中形成的化学吸附碳物种的动力学抑制。
The effects of reactant and product concentrations on turnover rates and isotopic tracing and kinetic isotope effects have led to a sequence of elementary steps for CH4reactions with CO2and H2O on supported Pd catalysts. Rate constants for kinetically-relevant C–H bond activation steps are much larger on Pd than on other metals (Ni, Ru, Rh, Ir, Pt). As a result, these steps become reversible during catalysis, because the products of CH4dissociation rapidly deplete the required oxygen co-reactant formed from CO2or H2O and co-reactant activation, and water–gas shift reactions remain irreversible in the time scale required for CH4conversion. H2and CO products inhibit CH4reactions via their respective effects on CH4and CO dissociation steps. These mechanistic conclusions are consistent with the kinetic effects of reactants and products on turnover rates, with the similar and normal CH4/CD4kinetic isotope effects measured with H2O and CO2co-reactants, with the absence of H2O/D2O isotope effects, and with the rate of isotopic scrambling between CH4and CD4,12C16O and13C18O, and13CO and12CO during CH4reforming catalysis. This catalytic sequence, but not the reversibility of its elementary steps, is identical to that reported on other Group VIII metals. Turnover rates are similar on Pd clusters on various supports (Al2O3, ZrO2, ZrO2−La2O3) and independent of Pd dispersion over the narrow range accessible at reforming conditions, because kinetically-relevant C–H bond activation steps occur predominantly on Pd surfaces. ZrO2and ZrO2−La2O3supports, with detectable reactivity for CO2and H2O activation, can reverse the infrequent formation of carbon overlayers and inhibit deactivation, but do not contribute to steady-state catalytic reforming rates. The high reactivity of Pd surfaces in C–H bond activation reflects their strong binding for C and H and the concomitant stabilization of the transition state for kinetically-relevant C–H activation steps and causes the observed kinetic inhibition by chemisorbed carbon species formed in CH4and CO dissociation steps.