CO oxidation activity of the AT and SEA catalysts

Figure 1 shows CO oxidation light-off curves for the catalysts synthesized by AT and SEA before activation (called as-synthesized) and the corresponding catalysts that were further reduced at 275 °C in CO (called activated). The activity of the as-synthesized catalysts is very low, both showing a T 90 of ~280 °C which has been observed previously on isolated ionic Pt species supported on ceria10,12. The activated AT catalyst shows significantly higher reactivity and reaches 90% conversion at 64 °C, while the activated SEA catalyst achieves 90% conversion at 120 °C. The same trend was observed for the AT and SEA catalysts synthesized using CPA as precursor (Supplementary Fig. 1). Furthermore, the activated AT catalyst shows no deactivation after numerous CO oxidation runs (Supplementary Fig. 2). It is also important to consider that this catalyst was synthesized by heating in air at 800 °C which satisfies the requirements for accelerated aging set by the U.S. DRIVE partnership for DOCs9. Moreover, it is worth noting that the support alone does not contribute to the reactivity as shown in Supplementary Fig. 3.

Fig. 1 CO oxidation light-off curves for 1wt.%Pt/CeO 2 catalysts. The activity of the catalysts synthesized by AT and SEA methods was measured before and after activation. TAPN was used as precursor Full size image

Supplementary Table 1 shows the turnover frequency (TOF) (calculated with respect to the total amount of Pt) at 80 °C and activation energy for the as-synthesized and activated AT and SEA catalysts. Both activated catalysts have higher TOF than the as-synthesized catalysts. This is more noticeable in the AT catalyst in which the TOF of the activated catalyst (at 80 °C) is ~20 times while the SEA is only ~6 times higher than the corresponding as-synthesized catalysts. Moreover, the catalyst prepared by AT exhibits a decrease in activation energy from 53.5 to 30.1 kJ/mol after activation. This suggests that the nature of the active site and the reaction mechanism on the activated AT catalyst could be different. The SEA catalyst also shows a decrease in the activation energy after activation, but to a lesser extent.

Atomic-scale images of catalysts after activation

Figure 2 shows HAADF-STEM images of the as-synthesized (CPA) and activated (CPA and TAPN) catalysts. In the as-synthesized state, the AT and SEA catalysts show only Pt single atoms. After activation, Pt transforms into nanoparticles, exhibiting very similar particle size for AT and SEA catalysts. Supplementary Figs. 4 and 5 show the particle size distribution (PSD) for the activated TAPN and CPA catalysts. Results indicate that in the TAPN catalysts, Pt nanoparticles have a mean particle size and standard deviation of 1.68 ± 0.3 and 1.58 ± 0.33 nm, for the AT and SEA catalysts, respectively, while in the case of the CPA catalysts, these values are 1.05 ± 0.2 nm and 1.72 ± 0.36 nm, respectively. Therefore, differences in reactivity between AT and SEA catalysts cannot be attributed to differences in the Pt particle size, which are comparable, and all under 2 nm, the size at which Cargnello et al.19 observed the highest reactivity in their study of particle size effects. Negative effects of chloride on the activity of ceria-supported transition metal catalysts have been reported previously30,31. Hence, to avoid incorrect conclusions due to the effects of Cl in the catalysts, the main focus for the rest of the manuscript will be on the TAPN catalysts.

Fig. 2 HAADF-STEM images of the 1wt.% Pt/CeO 2 catalysts: a as-synthesized AT (CPA), b as-synthesized SEA (CPA), c activated AT (CPA), d activated SEA (CPA), e activated AT (TAPN), f activated SEA (TAPN) Full size image

Probing the Pt-CO interaction with infrared spectroscopy

CO oxidation was performed, and the adsorbed species were monitored via DRIFTS to probe the surface of the as-synthesized and activated catalysts. The as-synthesized AT catalyst shows a well-defined peak at 2091 cm−1 with a small shoulder at 2041 cm−1 (Fig. 3a). The as-synthesized SEA catalyst is similar, with a prominent peak at 2105 cm−1 and a shoulder at 2056 cm−1 (Fig. 3b). Based on previous work10, the peaks at high wavenumber (2091 and 2105 cm−1) can be assigned to CO adsorbed on ionic Pt species corresponding to Pt single atoms supported on CeO 2 , but the difference in peak position between catalysts suggests there is a different interaction between the metal and the support. There is little change in the peak intensity after CO was stopped and helium or oxygen were flowed, indicating that CO is strongly bound to the ionic Pt. This results in low activity for both catalysts in the as-synthesized state (CO poisoning)32, which matches the activity results observed in Fig. 1. This strong bond between CO and ionic Pt single atoms has been observed previously33. Even though the peaks at low wavenumber (2046 cm−1 and 2051 cm−1) fall in the range for CO on Pt nanoparticles, the CO adsorbs strongly to Pt on these sites and is not removed even after helium or oxygen were flowed. On the other hand, the fact that CO adsorbed on metallic Pt nanoparticles reacts readily with oxygen suggests the peaks at low wavenumber can also be assigned to oxidized Pt sites17,33.

Fig. 3 CO oxidation reaction monitored by DRIFTS on the 1wt% Pt/CeO 2 TAPN catalysts: a as-synthesized AT (125 °C), b as-synthesized SEA (at 125 °C), c activated AT (50 °C), d activated SEA (50 °C) Full size image

Figure 3c and d show spectra for CO oxidation at 50 °C for the activated AT and SEA catalysts, respectively. At this temperature, the AT catalyst shows ~20% conversion of CO, while the SEA catalyst shows <3% CO conversion. The activated AT catalyst shows two prominent bands observed at 2093 cm−1 and 2078 cm−1 due to CO adsorption on metallic Pt nanoparticles33,34,35,36,37,38,39,40,41,42,43,44,45,46,47. These peaks disappear readily as soon as the CO flow is stopped, and helium is flowed, indicating that CO is weakly bound to Pt nanoparticles. A smaller peak is observed at 2109 cm−1 and is assigned to CO adsorbed on ionic Pt. This site binds CO very strongly since the peak does not disappear after helium or oxygen is flowed. Nevertheless, the nature of these ionic Pt species is likely to be different from the ionic Pt single atoms observed in the as-synthesized state. This is supported by the blueshift in the wavenumber compared to the peak in the as-synthesized state for ionic Pt single atoms and an observable redshift during desorption. Similar results have been obtained previously18, and the authors attributed this peak to Pt oxide clusters. Spectra during the activation treatment at 275 °C with CO (Supplementary Fig. 6) and spectra for CO adsorption at 125 °C before CO oxidation (Supplementary Fig. 7) do not show the same peak as observed in Fig. 3c. This suggests that the peak at 2109 cm−1 is formed during CO oxidation due to exposure to oxygen, supporting its assignment to Pt oxide clusters. The activated SEA catalyst shows similar behavior to the activated AT catalyst. Peaks for CO on Pt nanoparticles are observed at 2095 cm−1 and 2080 cm−1, while a peak for CO on ionic Pt is also observed at 2109 cm−1. The main difference between the two catalysts is that the intensity of the peaks for CO on Pt nanoparticles decreases more slowly for the SEA catalyst when the gas is switched to helium (Supplementary Table 2). CO adsorption on Ce4+ seen at 2172 cm−134 is present in the activated SEA catalyst after helium or oxygen is flowed at 50 °C (Fig. 3d). This agrees with the activity differences since at 50 °C, the activated AT catalyst is able to provide oxygen at low temperatures, compared to the SEA catalyst. When the same experiment was performed at 125 °C (Supplementary Fig. 8), there was no significant difference in the disappearance of CO between the two catalysts, since both catalysts provide very high conversion of CO at this temperature. Therefore, DRIFTS results show that the increase in the reactivity after CO reduction is related to the formation of Pt nanoparticles and the ease with which adsorbed CO can be reacted away. These results are consistent with previous reports that CO on metallic Pt reacts readily with oxygen, unlike the CO on ionic Pt which does not react unless heated to higher temperatures3,10,19,33. Since the Pt particle size, as seen in Fig. 2, is similar on both catalysts, this difference in reactivity between the AT and SEA catalyst must be related to the catalyst support.

Surface oxygen reactivity monitored by reduction with CO

Previous studies on ceria-supported Pt catalysts have observed that CO oxidation follows a Mars-Van Krevelen (MvK) reaction mechanism with positive effect of surface oxygen activation on the oxidation of CO23,35,48. It has also been observed that the relevant step during CO oxidation in a MvK reaction mechanism is the reaction between CO adsorbed on Pt and oxygen from the lattice42. Furthermore, previous work found that the amount and type of Pt species deposited on the support can play a role during the activation of surface oxygen49,50. Considering that the AT and SEA catalysts are synthesized by exposing them to significantly different temperatures, it is reasonable to expect a different interaction between Pt and CeO 2 for the two catalysts. Therefore, CO-TPR was used to determine if there is a difference between the two catalysts in the reaction between CO and oxygen from the catalyst.

The as-synthesized AT and SEA catalysts were exposed to CO to study the initial reduction of the catalyst, as reported in Supplementary Fig. 9. The ease of reducibility of the AT catalyst is evident in the as-synthesized catalyst. A separate aliquot of the catalyst was next activated via CO reduction at 275 °C, then exposed to an oxidation at 200 °C to remove any adsorbed species and to replenish the oxygen on the support. Figure 4 shows the formation of CO 2 during CO-TPR for the activated catalysts. The AT catalyst shows CO 2 formation at lower temperature than the SEA catalyst. Since both catalysts were exposed to an oxidative treatment prior to the CO-TPR, the CO 2 must come from reactive oxygen species accessible during reaction. The results shown here demonstrate that the AT catalyst contains ceria sites that are reducible at low temperatures where the AT catalyst is active for CO oxidation. The SEA catalyst is only able to do this at higher temperatures. Previous work by Gatla et al.14 suggested that when there is a strong interaction between Pt and CeO 2 , the energy to remove the oxygen between Pt and Ce is reduced. Therefore, our results would suggest the interaction between Pt and CeO 2 in the AT catalyst is stronger than in the SEA. This is not completely unexpected considering that the interaction between Pt and CeO 2 has to be strong enough to keep Pt as single atoms during the calcination at 800 °C. These results are different from previous works in which the activation of oxygen is related to the previous formation of nanoparticles that help activate the reductant to further react with surface oxygen12,13. Here we show that high synthesis temperature is important to modify the ceria, allowing activation of surface oxygen at lower temperatures, which subsequently results in higher reactivity for CO oxidation at low temperature. Nevertheless, the high-temperature synthesis is beneficial only when Pt is already present on the support and the synthesis is made in an oxygen rich environment like air. Previous work by Bunluesin et al.51 showed that calcining only CeO 2 at high-temperature (900 °C) and further depositing a metal, leads to low catalytic activity due a large increase in crystallite size. Therefore, it is not about exposing CeO 2 to high temperatures to help activate oxygen at low temperature but rather promoting the interaction between Pt and CeO 2 at high temperature. This interaction leads to volatile PtO 2 being trapped on CeO 2 10, creating the active sites for oxygen activation at low temperature.

Fig. 4 CO-TPR of the 1wt.%Pt/CeO 2 catalysts. The reducibility of the catalysts synthesized by AT and SEA methods was monitored by observing the formation of CO 2 . Both catalysts were synthesized using TAPN as precursor and were activated prior to the experiment Full size image

Surface oxygen reactivity monitored by NAP-XPS

To better understand the difference in activity between the AT and SEA catalysts, XPS was performed on the as-synthesized and activated states. Supplementary Fig. 10a, b show the Pt4f and Ce3d regions for the AT and SEA catalysts in the as-synthesized state. For both catalysts, only Pt2+ and Pt4+ species are observed52,53, confirming that the as-synthesized state only contains ionic Pt species. The percentage of Pt2+ species is very similar with 80.5% and 72.4% for the AT and SEA, respectively. On the other hand, the Ce3+ amount observed54 in the AT catalyst is almost double that of the SEA catalyst with 10.7% and 5.8%, respectively. Figure 5a, b show the Pt4f and Ce3d regions, respectively, for the activated AT catalyst after exposing it to different gas environments during NAP-XPS measurements (progress of experiment from top to bottom). Spectra after in-situ activation at 275 °C and then exposure to CO at 50 °C show that Pt0 is the primary species (% Pt0 ~ 83%). Continuously, exposing the catalyst to a mixture of CO and O 2 at 50 °C leads to the reoxidation of some of the Pt species (% Pt0 = 68%). This is due to the small particle size of the Pt nanoparticles and supports the assignment of the 2109 cm−1 peak (Fig. 3c) to Pt oxide clusters. Further exposure to CO and CO + O 2 environments at 50 °C do not change significantly the fraction of Pt0 species in the catalyst. On the other hand, the amount of Ce3+ species after activation with CO at 275 °C is ~28%. After exposing the catalyst to the reaction mixture at 50 °C, this fraction of Ce3+ decreases to 8.6%. Exposing the catalyst to CO at 50 °C increases the amount of Ce3+ to 14.3%, suggesting that even at 50 °C oxygen can be easily removed leaving behind vacancies and forming Ce3+ species. Further exposure to the reaction mixture at 50 °C reduces the fraction of Ce3+ species to 7%, suggesting these changes are reversible at 50 °C. The composition of the gas during the NAP-XPS experiments was monitored by mass spectrometry (MS) (Supplementary Fig. 11a) and it confirms the formation of CO 2 when the catalyst is exposed to the reaction mixture at 50 °C.

Fig. 5 NAP-XPS results for the 1wt.%Pt/CeO 2 TAPN catalyst synthesized by AT after activation in CO at 275 °C: a Pt4f region, b Ce3d region. The catalyst was sequentially exposed to different gas environments, starting at the top. The Pt/Ce ratio remained at ~0.030 throughout the experiment Full size image

Figure 6a, b show the Pt4f and Ce3d regions, respectively, for the SEA catalyst activated with CO at 275 °C, after exposure to different gas environments during NAP-XPS (progress of experiment from top to bottom). The SEA catalyst also has Pt0 as the primary species (% Pt0 = 74.6%), however, compared to the AT catalyst, the SEA catalyst shows more reoxidation of the Pt species (% Pt0 = 40.8%) which remain stable after further exposure to CO and CO + O 2 environments at 50 °C. The biggest difference between the AT and SEA catalysts is the reducibility of CeO 2 . The amount of Ce3+ after activation is similar to the AT catalyst (25.4%). Once the SEA catalyst is exposed to the reaction mixture at 50 °C, the Ce3+ species decrease to 9% and successive exposure to CO at 50 °C does not lead to a significant increase of Ce3+ (9.7%), suggesting that oxygen vacancies are not easily formed in the SEA catalyst under these low temperature reaction conditions, unlike on the AT catalyst. Further exposure to the reaction mixture leads to similar amount of Ce3+ species (8.7%). Since the SEA catalyst is less active than the AT, the temperature was increased to 100 °C and the catalyst was exposed to the reaction mixture. Results showed that the amount of Ce3+ decreases to 7.6%. Mass spectrometry (Supplementary Fig. 11b) confirms that a negligible amount of CO 2 is formed at 50 °C while raising the temperature to 100 °C leads to a clearer formation of CO 2 , suggesting the catalyst is active at this temperature but not at 50 °C like the AT catalyst.

Fig. 6 NAP-XPS results for the 1wt.%Pt/CeO 2 TAPN catalyst synthesized by SEA after activation in CO at 275 °C: a Pt4f region, b Ce3d region. The catalyst was sequentially exposed to different gas environments, starting at the top. The Pt/Ce ratio remained at ~0.015 throughout the experiment Full size image

To probe the strength of the interaction between Pt and CeO 2 for both catalysts, a harsh reduction treatment with CO at 450 °C for 8 h was performed (Supplementary Fig. 10c, d). Similar results are observed regarding the amount of Ce3+ species formed, however, the AT catalyst shows that a fraction of Pt2+ species is still present, while the SEA catalyst only shows Pt0 species. Therefore, the XPS results indicate that the interaction between Pt and CeO 2 in the AT catalyst is stronger than in the SEA catalyst.