The Critical Role of βPdZn Alloy in Pd/ZnO Catalysts for the Hydrogenation of Carbon Dioxide to Methanol

Abstract

The rise in atmospheric CO 2 concentration and the concomitant rise in global surface temperature have prompted massive research effort in designing catalytic routes to utilize CO 2 as a feedstock. Prime among these is the hydrogenation of CO 2 to make methanol, which is a key commodity chemical intermediate, a hydrogen storage molecule, and a possible future fuel for transport sectors that cannot be electrified. Pd/ZnO has been identified as an effective candidate as a catalyst for this reaction, yet there has been no attempt to gain a fundamental understanding of how this catalyst works and more importantly to establish specific design criteria for CO 2 hydrogenation catalysts. Here, we show that Pd/ZnO catalysts have the same metal particle composition, irrespective of the different synthesis procedures and types of ZnO used here. We demonstrate that all of these Pd/ZnO catalysts exhibit the same activity trend. In all cases, the β-PdZn 1:1 alloy is produced and dictates the catalysis. This conclusion is further supported by the relationship between conversion and selectivity and their small variation with ZnO surface area in the range 6–80 m 2 g –1 . Without alloying with Zn, Pd is a reverse water-gas shift catalyst and when supported on alumina and silica is much less active for CO 2 conversion to methanol than on ZnO. Our approach is applicable to the discovery and design of improved catalysts for CO 2 hydrogenation and will aid future catalyst discovery.

Methanol synthesis from CO2and H2using supported Pd alloy catalysts

“…A linear relationship between CO2 conversion and methanol selectivity has recently been demonstrated for Pd/ZnO catalysts prepared through a variety of techniques, 24 this linear relationship is shown by the PdZn line on Figure 2. In addition to the alloy, a recent study has shown the presence of a ZnO phase to be important for achieving high methanol productivity, 25 which may explain the high activity of the Pd/ZnO catalysts.…”

“…Furthermore, the importance of controlling PdZn particle size was highlighted, with methanol selectivity decreasing from 60 % to 20 % upon increasing the PdZn particle size from 3 to 7 nm. More recently, Pd/ZnO catalysts prepared by a range of techniques were investigated for CO2 hydrogenation; 24 here it was shown that the β-PdZn alloy was formed for all materials, regardless of preparation method. This was attributed to the favourable heat of mixing for the β-PdZn 1:1 alloy, as determined by DFT studies.…”

“…In the Pd/ZnO sample, alloying of Pd and Zn can be observed in the displayed EDX maps, as has been observed previously. 24 A clear ordered structure can also be seen in HAADF-STEM images (Figure S2a), which is indicative of an ordered PdZn alloy.…”

Design of Pd–Zn Bimetal MOF Nanosheets and MOF-Derived Pd3.9Zn6.1/CNS Catalyst for Selective Hydrogenation of Acetylene under Simulated Front-End Conditions

“…As shown in Figure 5 , there is a broad peak at 2θ = 20°−25°, in all XRD patterns that is attributed to the characteristic diffraction peak of amorphous carbon [ 45 ]. For the patterns of samples were carbonized at 650 and 800°C, i.e., Pd–Zn-ins/CNS-650, Pd–Zn-ins/CNS-800, the diffraction peaks, located at 2θ = 26.8°, 30.8°, 41.2°, 44.1°, 55.2°, 64.2°, 72.9 and 79.2° correspond to the lattice planes of (001), (110), (111), (200), (002), (112), (310) and (311) of PdZn, respectively [ 34 , 35 , 36 , 37 , 38 , 39 , 40 , 41 ]. No diffraction peaks of individual palladium and zinc or their oxides were detected.…”

“…Recently, it was found that supported palladium–zinc alloy catalysts exhibited high performance for the hydrogenation of acetylene in the front-end hydrogenation process [ 33 , 34 , 35 , 36 , 37 ]. According to the literature, the type of Pd–Zn intermetallics (Pd–Zn-ins) in catalysts dramatically affected the activity and selectivity of specific catalytic reactions [ 38 , 39 , 40 , 41 ]. However, to the best of our knowledge, the synthesis of a serial of Pd–Zn-ins and the discussion of the effect of the intermetallic type on selective hydrogenation of acetylene have not been reported.…”