Hydroxyl Radical Formation on Metal-Loaded Ga2O3 Photocatalysts for Dehydrogenative Coupling of Methane to Ethane with Water
Hydroxyl Radical Formation on Metal-Loaded Ga2O3 Photocatalysts for Dehydrogenative Coupling of Methane to Ethane with Water
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DOI：
10.1021/acs.energyfuels.2c00401
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发表时间：
2022-05
期刊：
影响因子：
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通讯作者：
Fumiaki Amano;Mizuki Ishimaru
中科院分区：
文献类型：
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作者：
Fumiaki Amano;Mizuki Ishimaru
甲烷的光催化非氧化偶联反应（photo-NOCM; 2CH 4 → C2 H6 + H2）可以在室温下将甲烷直接转化为乙烷和氢气。然而，在不存在氧化剂的情况下，光NOCM的表观量子效率（AQE）非常低。我们观察到，在水蒸气的存在下，使用Ga 2 O3基光催化剂在紫外光（UV）照射下的甲烷的光催化分解偶联（photo-DHCM）进行。在Pt/Ga 2 O3和Pd/Ga 2 O3光催化剂上，可以有效地诱导光DHCM，并伴随有甲烷水蒸气重整反应（photo-SRM）.对于C2 H6的产生，光-DHCM与水蒸气的AQE是超过2个数量级的高于常规的光-NOCM。然而，Ga 2 O3负载的金属助催化剂在光催化DHCM反应中对C2 H6和H2生成的作用尚不清楚。在水蒸气的作用下，CH 4被一个羟基自由基（·OH）活化，两个甲基自由基发生了自偶联反应.因此，·OH的生成影响了光催化甲烷转化的产率和选择性。本文采用电子自旋共振（ESR）方法研究了Ga 2 O3光催化剂上负载的金属助催化剂（Au、Ag、Ru、Rh、Pd和Pt）对·OH生成的影响。紫外光照射下的ESR测量表明，Au/Ga 2 O3中·OH浓度高，表现出高的C2 H6生成速率（1140 μmol gcat-1h-1; 254 nm处的AQE = 4.3%）和高的C2 H6选择性（92.2%）。相比之下，Rh/Ga 2 O3具有最低的·OH浓度，与光DHCM相比，其对光SRM和水裂解反应表现出高的选择性。
Photocatalytic non-oxidative coupling of methane (photo-NOCM; 2CH4→ C2H6+ H2) can directly convert methane into ethane and hydrogen at room temperature. However, the apparent quantum efficiency (AQE) of photo-NOCM is very low in the absence of oxidants. We observed that photocatalytic dehydrogenative coupling of methane (photo-DHCM) proceeds in the presence of water vapor using Ga2O3-based photocatalysts under ultraviolet (UV) light irradiation. Photo-DHCM is efficiently induced over Pt/Ga2O3and Pd/Ga2O3photocatalysts, accompanied by steam reforming of methane (photo-SRM). For C2H6production, the AQE of photo-DHCM with water vapor was more than 2 orders of magnitude higher than that of conventional photo-NOCM. However, the role of the metal co-catalyst supported on Ga2O3for the production of C2H6and H2in photo-DHCM is unclear. The reaction mechanism accelerated by water vapor was assumed in which CH4was activated by a hydroxyl radical (•OH) and homocoupling occurred by the two methyl radicals. Therefore,•OH formation affects the productivity and selectivity of the photocatalytic CH4conversion. In this study, we investigated the effect of metal co-catalysts (Au, Ag, Ru, Rh, Pd, and Pt) supported on a Ga2O3photocatalyst on•OH formation by an electron spin resonance (ESR) method using a spin-trapping agent. ESR measurements upon UV irradiation revealed that the•OH concentration was high in Au/Ga2O3, which exhibited a high C2H6production rate (1140 μmol gcat–1h–1; AQE = 4.3% at 254 nm) and high C2H6selectivity (92.2% on a carbon basis). In contrast, Rh/Ga2O3, which had the lowest•OH concentration, exhibited high selectivity for photo-SRM and water-splitting reactions compared to photo-DHCM.