Formic Acid Dehydrogenation on Au-Based Catalysts at Near-Ambient Temperatures†
Formic Acid Dehydrogenation on Au-Based Catalysts at Near-Ambient Temperatures†
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DOI：
10.1002/ange.200805723
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发表时间：
2009-05
影响因子：
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通讯作者：
Manuel Ojeda Dr. and;Enrique Iglesia Prof.
中科院分区：
文献类型：
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作者：
Manuel Ojeda Dr. and;Enrique Iglesia Prof.
甲酸（HCOOH）是设计用于便携式使用的燃料电池中的方便的氢载体。[1-4]最近的研究表明，钌基配合物在接近环境温度下分解HCOOH水溶液。[5，6] Pt是HCOOH分解最活跃的固体催化剂，至少是大的微晶和扩展的表面。[7]表面原子的身份和氧化态影响脱氢的选择性（HCOOH！H2+ CO2）和脱水（HCOOH！H2O+ CO）路线和形成适用于低温燃料电池的无CO H2流的能力。贵金属选择性地催化脱氢，而贱金属和氧化物直接或通过随后的水煤气变换（WGS）催化这两种途径。甲酸盐作为HCOOH分解的中间体;其形成限制了贵金属（Au、Ag）的分解速率，并限制了其他金属的分解速率。[10]Au催化剂的面积速率比其他金属低，因为它在HCOOH解离中的惰性，从它的一级HCOOH分解动力学可以看出。[12]氧化物载体上的小Au簇（< 5 nm）以比大Au簇更高的转换速率催化许多反应，包括HCOOH氧化，这显然是因为Au金属、阴离子或阳离子的配位不饱和物质比低指数Au金属表面更强地结合分子。[13-17]在这里，我们表明，分散良好的Au物质分解HCOOH的金属时间产率（每Au原子的速率）[18]甚至大于Pt簇。HCOOH在接近环境温度（% 350 K）下分解，仅形成H2和CO2（< 10 ppm CO），适用于燃料电池。这种前所未有的反应性来自于分散的Au物质，在显微照片中未检测到，其在热处理时生长，而不是来自可见的金属簇（3- 4 nm），其催化CO氧化并在热处理期间保持稳定。在343-383 K下，Au/Al 2 O3上的HCOOH分解金属-时间产率远高于Pt/Al 2 O3（图1）。这些差异并不反映明显的金属分散（根据透射电子显微镜（TEM）中可见的簇估计，Au为0.28，Pt为0.21）。在零级动力学范围内，Au和Pt上的活化能分别为53 ~ 22 kJ mol·1和72 ~ 44 kJ mol·1，与以前的数据（Au为40-60 kJ mol·1，Pt为58- 73 kJ mol·1）一致。
Formic acid (HCOOH) is a convenient hydrogen carrier in fuel cells designed for portable use.[1–4] Recent studies show that Ru-based complexes decompose aqueous HCOOH solutions at near-ambient temperatures.[5, 6] Pt is the most active solid catalyst for HCOOH decomposition, at least as large crystallites and extended surfaces.[7] The identity and oxidation state of surface atoms influence the selectivity to dehydrogenation (HCOOH! H2+ CO2) and dehydration (HCOOH! H2O+ CO) routes and the ability to form CO-free H2 streams suitable for low-temperature fuel cells. Noble metals catalyze dehydrogenation selectively, while base metals and oxides catalyze both routes, either directly or via subsequent water-gas shift (WGS).[8–11] Formates act as intermediates in HCOOH decomposition; their formation limits rates on the nobler metals (Au, Ag) and their decomposition on the others.[10] Au catalysts give lower areal rates than other metals because of its inertness in HCOOH dissociation, evident from its first-order HCOOH decomposition kinetics.[12] Small Au clusters (< 5 nm) on oxide supports catalyze many reactions, including HCOOH oxidation, at higher turnover rates than larger Au clusters, apparently because coordinatively unsaturated species of Au metal, anions, or cations bind molecules more strongly than low-index Au metal surfaces.[13–17] Here, we show that well-dispersed Au species decompose HCOOH with metal-time yields (rates per Au atom)[18] even larger than on Pt clusters. HCOOH decomposes at near ambient temperatures (% 350 K) to form only H2 and CO2 (< 10 ppm CO), suitable for use in fuel cells. This unprecedented reactivity arises from dispersed Au species, undetected in micrographs, which grow upon thermal treatment, and not from visible metal clusters (3–4nm), which catalyze CO oxidation and remain stable during thermal treatment. HCOOH decomposition metal-time yields on Au/Al2O3 are much higher than on Pt/Al2O3 at 343–383 K (Figure 1).These differences do not reflect a distinct metal dispersion (0.28 for Au and 0.21 for Pt, estimated from clusters visible in transmission electron micrographs, TEM). Activation energies in the zero-order kinetic regime were 53 Æ2 kJ molÀ1 and 72 Æ 4 kJ molÀ1 on Au and Pt, respectively, consistent with previous data (40–60 and 58–73kJmolÀ1 for Au and