A challenge in converting CO₂ into hydrocarbons (HC) via methanol (MeOH) is the gap between the optimal temperature for each step (i.e. ≤250 °C for MeOH and ≥350 °C for HC). The focus of this study is to elucidate the cause of the limitations associated to oxygenate and hydrocarbon formation in combined PdZn and H-ZSM-5 catalysts at 250 to 350 °C. Starting with two different chemical states of Pd and Zn from two preparation approaches (physical mixture and surface organometallic chemistry grafting), operando X-ray absorption spectroscopy (XAS) and powder X-ray diffraction (PXRD) studies revealed similar PdZn alloy active phase formed during pretreatment in flowing H₂/Inert at 400 °C. The physical mixture performed better than the grafted analogue, with up to 8.8% yield to oxygenates (MeOH and dimethyl ether (DME); MeOH+) at 300 °C, close to the estimated thermodynamic yield (9.0%). The space–time yield (STY) of oxygenates increased from 250 to 300 °C, reaching 8.5 mol_(MeOH+) kg_(PdZn)⁻¹ h ⁻¹. The catalyst performance surpassed other reported yields in similar systems, which activity declined with temperature even below 300 °C. Operando XAS and PXRD experiments further showed that the PdZn phase active for MeOH formation was maintained during testing in the 250–350 °C range. InfraRed (FT-IR) and XAS experiments revealed the poisoning of Brønsted acid sites in H-ZSM-5 by Zn(II) exchange, thereby rendering it inactive for hydrocarbon formation. Overall, to avoid biasing the hybrid catalyst performance, a careful and balanced choice of the compositional characteristics will be crucial in designing an improved multi-functional catalytic system for CO₂ valorisation.

中文翻译：

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在复合PdZn和H-ZSM-5催化剂上将CO2转化为增值产品时：Zn相对Pd而言过量，是折中还是不利？

通过甲醇（MeOH）将CO ₂转化为碳氢化合物（HC）的一个挑战是每个步骤的最佳温度之间的差距（即MeOH≤250°C，HC≥350°C）。这项研究的重点是阐明在250至350°C下PdZn和H-ZSM-5组合催化剂中与氧化物和碳氢化合物形成相关的限制原因。从两种制备方法（物理混合物和表面有机金属化学接枝）开始，Pd和Zn具有两种不同的化学状态，操作X射线吸收光谱（XAS）和粉末X射线衍射（PXRD）研究表明形成了相似的PdZn合金活性相在预处理过程中流动的H ₂/在400°C下惰性。物理混合物的性能优于接枝类似物，在300°C下含氧化合物（MeOH和二甲醚（DME）； MeOH +）的产率高达8.8％，接近估算的热力学产率（9.0％）。含氧化合物的时空产率（STY）从250升高至300°C，达到8.5 mol _{（MeOH +）} kg _（PdZn）^-1 h ^-1。在类似的系统中，催化剂的性能超过了其他报道的产率，甚至在低于300°C的温度下，活性也下降了。Operando XAS和PXRD实验进一步表明，在250–350°C范围内的测试过程中，对甲醇形成有效的PdZn相得以保持。红外（FT-IR）和XAS实验表明Zn（II）污染了H-ZSM-5中布朗斯台德酸位）交换，从而使其对烃的形成失去活性。总体而言，为避免偏倚杂化催化剂的性能，在设计用于CO ₂气化的改进型多功能催化体系时，对组成特征进行仔细而平衡的选择至关重要。