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Investigating deposition sequence during synthesis of Pd/Al2O3 catalysts modified with organic monolayers

机译：在使用有机单层修饰的PD/AL2O3催化剂合成过程中研究沉积序列

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Modification of supported metal catalysts with self-assembled monolayers (SAMs) has been shown to improve selectivity and turnover frequencies (TOFs) for many catalytic reactions. However, these benefits are often accompanied by a decrease in overall mass activity due to partial blocking of active sites by the monolayers. Therefore, a potential method for increasing site accessibility is to deposit the active metal (e.g., Pd) after deposition of a SAM modifier on the support material. In this work, Pd/Al2O3 catalysts were synthesized using both a conventional, “metal-first” sequence in which the phosphonic acid (PA) SAM was deposited on Pd/Al2O3 and a “SAM-first” technique in which the ligands were deposited onto the support prior to deposition of the metal. Although metal-first catalysts showed a significant increase in TOF during benzyl alcohol HDO, they exhibited only a modest increase in overall mass activity due to a decrease in the number of active sites. Meanwhile, SAM-first catalysts showed similar improvements in TOF and toluene selectivity but exhibited significantly improved HDO rates compared to metal-first catalysts. The rate increase was attributed to reduced site blocking as evidenced by CO chemisorption measurements. Additionally, analysis of catalysts having high Pd loadings using transmission electron microscopy (TEM) showed that SAM-first catalysts generally resulted in lower average particle sizes than traditionally modified and unmodified catalysts, suggesting that precoating the support surface with PAs changes the way that Pd is deposited during incipient wetness, leading here to improved activity.
机译：已证明用自组装单层（SAM）修饰支持的金属催化剂（SAM）可改善许多催化反应的选择性和周转频率（TOF）。但是，这些好处通常伴随着由于单层对主动部位的部分阻断而导致的总体活动减少。因此，增加站点可访问性的潜在方法是沉积在支持材料上的SAM修饰符后沉积活性金属（例如PD）。在这项工作中，使用常规的“金属优先”序列合成PD/AL2O3催化剂，其中将磷酸（PA）SAM沉积在PD/AL2O3上，并沉积了配体的“ SAM-FIRR”技术。在沉积金属之前，请在支撑上。尽管在苄醇HDO期间，金属优先催化剂显示出TOF的显着增加，但由于活性位点的数量减少，它们仅显示出适度的总体质量活性增加。同时，与金属优先的催化剂相比，SAM-率先催化剂的TOF和甲苯选择性相似，但HDO速率显着提高。速率增加归因于CO化学吸附测量结果所证明的位点阻塞减少。此外，对使用透射电子显微镜（TEM）进行高PD载荷的催化剂的分析表明，SAM-催化剂通常会导致平均颗粒尺寸低于传统改性和未修饰的催化剂，这表明PAS将支撑表面预先涂抹，而PAS会改变PD的方式。在初期湿润期间沉积，在这里导致活动改善。

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