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

The rise in atmospheric CO2 concentration and the concomitant rise in global surface temperature have prompted massive research effort in designing catalytic routes to utilize CO2 as a feedstock. Prime among these is the hydrogenation of CO2 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 CO2 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 m2g–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 CO2 conversion to methanol than on ZnO. Our approach is applicable to the discovery and design of improved catalysts for CO2 hydrogenation and will aid future catalyst discovery.

Initially, higher surface area ZnO was prepared in accordance with the method published by Farag et al., 1 with some modifications. Firstly, as described in the original method, stock solutions of Zn(CH 3 CO 2 ) 2 and (NH 4 ) 2 CO 3 (both 0.5 M) were prepared. A 600 mL glass beaker was charged with Zn(CH 3 CO 2 ) 2 (100 mL), then (NH 4 ) 2 CO 3 solution (100 mL) was added quickly yet cautiously with vigorous stirring. The mixture was heated in an oil bath to 60 °C and aged for 1 hour, stirring all the time. Once the ageing period was complete, the precipitate was filtered under vacuum, washed with 2 L deionised water and dried in an oven at 110 °C for 16 hours. The zinc hydroxycarbonate was then collected and ground in a pestle and mortar, then calcined under flowing air at 450 °C for 3 hours to give the desired zinc oxide product. The resulting surface area was measured to be 25 m 2 /g. without further purification. Other materials were provided as described above.
The co-precipitation method tested was based on the two ZnO preparations outlined in parts 1.1. and 1.2. In each case, once the Zn precursor was charged into the reaction vessel, and Pd(NO 3 ) 2 solution was then added. The precipitating agent, Na 2 CO 3 , was added and then the 3 mixture was aged for 1 hour and filtered, washed, dried and calcined as previously described.
For deposition-precipitation the Zn oxide was charged into the reaction vessel, Pd(NO 3 ) 2 solution was then added. The method then proceeded as before, with the precipitating agent added, the mixture aged for 1 hour and filtered, washed, dried and calcined as previously described.    7 Figure S3. STEM images of 5 wt.% Pd/ZnO (catalyst 1) after calcination, but before reduction.
There is no alloy formation and the Pd is present as PdO.
8 Figure S4 a) STEM images for sample 6, 5% Pd/MFZ, Pd deposited by CVI. The sample was reduced at 500 C for 1 hour. b) the same, except reduced at 200 C 9 3. Reactor data. Here we give estimates of the uncertainties in the reactor data values for conversion and selectivity. Figure S5 shows data for one particular type of catalyst, prepared by author JRE, made by CVI of Pd onto Sigma Aldrich ZnO with 5% loading. The catalysts were made in two separate batches at different times and loaded in two different runs, into different beds of the 16-bed reactor. It can be seen that the mean deviation in conversion is ~0.5%, while for selectivity it is ~3% for the data at 230 °C (left panel), while at 270 °C it is ~0.6% for conversion and 0.8% for selectivity. In turn, each of these data points is taken from an average of four measurements over a period of 24 hours, where the mean deviation in conversion for a single sample was 1% at 230 °C and 0.6% at 270 °C, and in selectivity was 0.3% at each temperature.    The turnover number was calculated as follows.
Using the data for catalyst 1 and the known average particle diameter, as shown in the main text, fig. 3 of the main text, of around 3.6nm. The metal surface area (MSA) then can be approximated by the following

MSA = 3W/r
Where W is the weight of metal (g),  is its density (g. m -3 ) and r is the average particle radius.
Since we use the radius of the alloy particle, then we use an average density of 9.5 x 10 6 g m -3 , and the weight W of the metal particle must be more than that of the original Pd in the sample (0.025g) and is increased by the addition of equimolar amounts of Zn, adding to this weight by (65/106) x 0.075 g = 0.015 g, giving a total weight of metal particles of 0.04 g. This results in a surface area of 7 m 2 . We do not know the exact number of sites per unit area, since the surface will be composed of a number of different exposed planes and steps etc, but we approximate this to 1x10 19 sites m -2 . So, with that approximation in mind we have around 7 x 10 19 surface metal sites.
The conditions for the data of fig. 1, sample 1, at 250 C reaction temperature then conversion of CO2 is 13.5% giving 0.9 ml min -1 of CO 2 converted or 0.015 ml s -1 . If we now convert this to molecular units, then this equates to (0.015 ml s -1 x 6 x 10 23 molecules mol -1 /24000 ml) which is 3.8 x 10 17 molecules s -1 . The overall turnover frequency is then given by the following