The high nozzle temperatures used in combination with CO 2 seeded in H 2 raises a potential concern about the presence of Reverse Water-Gas Shift (RWGS) reaction, which can adversely impact our measurements. Specifically, there is a possibility that the RWGS reaction could produce CO and H 2 O, which might interfere with our S 0 measurements. In particular, the dissociative chemisorption of H 2 O can also lead to oxygen atom buildup on the Cu(110) surface in addition to CO 2 dissociation. To assess this situation better, we used a Quadrupole Mass Spectrometer (QMS) to monitor any increase in CO levels with increasing T n . As the CO signal on QMS also results from the residual gases in the UHV chamber and the electron impact fragmentation of CO 2 in the QMS, the ratio of CO to CO 2 partial pressure was chosen as a measure for detecting small changes in CO levels as T n was varied. Our detection sensitivity, capable of discerning changes in the CO to CO 2 partial pressure ratio as small as 4%, revealed no significant variation in this ratio, indicating that CO production via RWGS was minimal even at the highest T n used in this work. Further confirmation comes from the measurement of S 0 at T n = 750 K for a mixture containing 30% CO 2 in H 2 (Figure (a), red hollow circle #1, E i = 0.66 eV). Here, any additional O-atom coverage buildup due to RWGS is expected to show up as an increase in the estimated S 0 . However, the observed reaction probability is 3.9 × 10 -4 , consistent with the overall trend showing no anomalous increase. Additionally, the measurement with a beam of 1% CO 2 in He with E i = 0.6 eV (black star, Figure ), where no RWGS reaction is possible, is also consistent with that observed at similar E i using H 2 seeded beam. Based on these considerations, we rule out any overestimation of S 0 caused by RWGS, despite using the heated nozzle in our measurements.