Mechanism of Methanol Synthesis on Cu through CO2 and CO Hydrogenation
Mechanism of Methanol Synthesis on Cu through CO2 and CO Hydrogenation
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DOI：
10.1021/cs200055d
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发表时间：
2011-04-01
期刊：
影响因子：
12.9
通讯作者：
Mavrikakis, M.
中科院分区：
文献类型：
--
作者：
Grabow, L. C.;Mavrikakis, M.
我们提出了一个全面的平均场微观动力学模型的甲醇合成和水煤气变换（WGS）反应，包括新的反应中间体，如甲酸（HCOOH）和羟甲氧基（CH3O2），并允许形成甲酸（HCOOH），甲醛（CH2O），甲酸甲酯（HCOOCH3）作为副产物。所有输入的模型参数最初来自周期性的，自洽的，GGA-PW91密度泛函理论计算的Cu（111）表面，随后拟合公布的实验甲醇合成速率数据，这是收集在现实条件下的商业Cu/ZnO/Al2O3催化剂。我们发现，WGS反应遵循羧基（COOH）介导的路径，CO和CO2加氢途径是积极的甲醇合成。在典型的工业甲醇合成条件下，CO2加氢约占所生产甲醇的2/3。用于甲醇合成的CO2途径的中间体包括HCOO *、HCOOH *、CH3O2 *、CH2O * 和CH3O *。在Cu（111）上由CO2 * 和H * 形成甲酸盐（HCOO *）不涉及中间碳酸盐（CO3 *）物质，并且HCOO * 的氢化导致HCOOH * 而不是二甲醛（H2CO2 *）。CO的作用不仅是促进的; CO * 还大量氢化为HCO *、CH（2）Q *、CH3O * 和CH3OH *。我们考虑了CO促进的两种可能性：（a）通过COOH * 去除OH * 以形成CO2和氢气（WGS），以及（B）CO辅助的各种表面中间体的氢化，HCO * 是H-供体。只有前一种机制有助于甲醇的形成，但它的影响是小的，与直接CO加氢制甲醇。总的来说，甲醇合成速率受到在低CO2/（CO + CO2）比率下甲氧基（CH3O *）形成和富CO2进料中CH3O * 氢化的限制。CH3O * 氢化是CO和CO2甲醇合成路线的共同缓慢步骤;每种路线的相对贡献由它们各自的缓慢步骤HCO *+ H *-> CH2O *+* 和HCOOH *+ H *-> CH3O2 *+* 以及进料组成和反应条件决定。对工业Cu/ZnO/Al_2O_3催化剂的拟合参数的分析表明，更开放的Cu表面，例如Cu（110）、Cu（100）和Cu（211）部分被氧覆盖，可以提供更好的甲醇合成活性中心模型，但我们的研究不能排除与ZnO载体的协同效应。
We present a comprehensive mean-field microkinetic model for the methanol synthesis and water-gas-shift (WGS) reactions that includes novel reaction intermediates, such as formic acid (HCOOH) and hydroxymethoxy (CH3O2) and allows for the formation of formic-acid (HCOOH), formaldehyde (CH2O), and methyl formate (HCOOCH3) as byproducts. All input model parameters were initially derived from periodic, self-consistent, GGA-PW91 density functional theory calculations on the Cu(111) surface and subsequently fitted to published experimental methanol synthesis rate data, which were collected under realistic conditions on a commercial Cu/ZnO/Al2O3 catalyst. We find that the WGS reaction follows the carboxyl (COOH)-mediated path and that both CO and CO2 hydrogenation pathways are active for methanol synthesis. Under typical industrial methanol synthesis conditions, CO2 hydrogenation is responsible for similar to 2/3 of the methanol produced. The intermediates of the CO2 pathway for methanol synthesis include HCOO*, HCOOH*, CH3O2*, CH2O*, and CH3O*. The formation of formate (HCOO*) from CO2* and H* on Cu(111) does not involve an intermediate carbonate (CO3*) species, and hydrogenation of HCOO* leads to HCOOH* instead of dioxymethylene (H2CO2*). The effect of CO is not only promotional; CO* is also hydrogenated in significant amounts to HCO*, CH(2)Q*, CH3O*, and CH3OH*. We considered two possibilities for CO promotion: (a) removal of OH* via COOH* to form CO2 and hydrogen (WGS), and (b) CO-assisted hydrogenation of various surface intermediates, with HCO* being the H-donor. Only the former mechanism contributes to methanol formation; but its effect is small compared with that of direct CO hydrogenation to methanol. Overall, methanol synthesis rates are limited by methoxy (CH3O*) formation at low CO2/(CO + CO2) ratios and by CH3O* hydrogenation in CO2-rich feeds. CH3O* hydrogenation is the common slow step for both the CO and the CO2 methanol synthesis routes; the relative contribution of each route is determined by their respective slow steps HCO* + H* -> CH2O* + * and HCOOH* + H* -> CH3O2* + * as well as by feed composition and reaction conditions. An analysis of the fitted parameters for a commercial Cu/ZnO/Al2O3 catalyst suggests that a more open Cu surface, for example, Cu(110), Cu(100), and Cu(211) partially covered by oxygen, may provide a better model for the active site of methanol synthesis, but our studies cannot exclude a synergistic effect with the ZnO support.