CO2 Hydrogenation: Na Doping Promotes CO and Hydrocarbon Formation over Ru/m-ZrO2 at Elevated Pressures in Gas Phase Media

Sodium-promoted monoclinic zirconia supported ruthenium catalysts were tested for CO2 hydrogenation at 20 bar and a H2:CO2 ratio of 3:1. Although increasing sodium promotion, from 2.5% to 5% by weight, slightly decreased CO2 conversion (14% to 10%), it doubled the selectivity to both CO (~36% to ~71%) and chain growth products (~4% to ~8%) remarkably and reduced the methane selectivity by two-thirds (~60% to ~21%). For CO2 hydrogenation during in situ DRIFTS under atmospheric pressure, it was revealed that Na increases the catalyst basicity and suppresses the reactivity of Ru sites. Higher basicity facilitates CO2 adsorption, weakens the C–H bond of the formate intermediate promoting CO formation, and inhibits methanation occurring on ruthenium nanoparticle surfaces. The suppression of excessive hydrogenation increases the chain growth probability. Decelerated reduction during H2-TPR/TPR-MS and H2-TPR-EXAFS/XANES at the K-edge of ruthenium indicates that sodium is in contact with ruthenium. A comparison of the XANES spectra of unpromoted and Na-promoted catalysts after H2 reduction showed no evidence of a promoting effect involving electron charge transfer.


Introduction
There is a worldwide effort underway to find methods of recycling CO 2 , and one method is through its hydrogenation, a process that produces a substitute natural gas [1,2] or transportation fuels, either by Fischer-Tropsch synthesis (FTS) [3,4] or by methanol synthesis (used to produce alternative gasoline [5,6]). Sources of CO 2 include power generation plants [7], quarry activities [8], biogas [9], direct air capture systems [10], and ocean water [11]. The H 2 required to produce useful fuels from CO 2 can be generated in a renewable manner through the electrolysis of H 2 O using electricity harvested from solar energy [12], wind power [13], or nuclear plants [14]. Although H 2 has high level of energy per mass, because it is a gas, it possesses a low energy content per unit volume [15]. Because of this serious issue, there is an impetus to add hydrogen to carbon chains to produce liquid fuels with a high energy content as well as synthetic chemicals.
A practical route to the production of liquid fuels is to first produce CO from CO 2 via the heterogeneously catalyzed reverse water-gas shift (RWGS) reaction [16]. Carbon is acidic) and (2) a suppression of the activity of Pt. WGS catalytic testing using a fixed bed reactor showed that the addition of 2.5 wt.% sodium increased the conversion of CO by up to 5-6 times compared with that of unpromoted platinum/zirconia at temperatures of between 235 • C and 250 • C. Because Na facilitates C-H bond scission for the forward WGS reaction, according to the principle of microscopic reversibility, sodium is expected to promote C-H bond formation for the reverse reaction. As such, our recent research is aimed at exploring sodium-promoted metal/zirconia catalysts for CO 2 hydrogenation.
As previously discussed, Pt was substituted for Ru, which possesses CO hydrogenation activity that is important for reactions such as methanation and FTS. The loading of ruthenium at 1% by weight was chosen due to its atomic equivalency with platinum at 2% by weight used in prior forward WGS research. The presence of Na has been found to improve the activity during WGS [32,33] or modify the selectivity of ruthenium catalysts, including an increase in the WGS activity [34], a decrease in the methanation activity [34], or an enhancement in chain growth [35] during aqueous-phase FTS and a decrease in methanation as well as an increase in chain growth during FTS over Ru/TiO 2 catalysts [36] and a thin-film ruthenium catalyst that was supplied with sodium by electro-pumping from a solid electrolyte (Na-β -alumina) [37]. Other elements that have been found to decrease methanation activity during CO hydrogenation over Ru and Co catalysts include Group 11 elements, such as Ag [38,39] and Au [39,40]. The addition of Au to a cobalt FTS catalyst was also found to temporarily promote WGS [41].
In the current contribution, we investigated how Na doping influences product selectivity during CO 2 hydrogenation at a higher pressure of 20 bar. As such, we examined how an increase in Na doping inhibits CH 4 production in favor of CO and determine whether any chain growth products are formed. In this scenario, we used gas-phase media and a fixed-bed reactor system. Prior to working at a higher pressure, we first examined the effect of the H 2 :CO 2 ratio. In our prior work, we operated under atmospheric pressure and utilized a H 2 :CO 2 ratio of 4:1. However, in this work, we first explored whether lowering the H 2 :CO 2 ratio to 3:1 (i.e., the stoichiometric ratio for RWGS/Fischer-Tropsch) would increase the CO selectivity (and decrease the CH 4 selectivity). To shed light on the mechanism, the catalysts were characterized by in situ DRIFTS of the CO 2 hydrogenation reaction at a low pressure using these realistic H 2 :CO 2 ratios (3:1 and 4:1 rather than 15:1, as used previously [19]). To gain insight into the question of whether CO 2 hydrogenation occurs directly or via a CO intermediate, the catalysts were subjected to a temperature programmed reaction by preadsorbing a H 2 :CO 2 mixture at the preferred ratio of 3:1. The most promising catalysts were further tested using a fixed bed reactor at 20 bar in gasphase media. For the purpose of experimental control, unpromoted and sodium-promoted 1%Ru/m-zirconia catalysts were prepared using a similar method to that used in our prior investigation conducted at a low pressure [19]; the new batches were characterized by the Brunauer-Emmett-Teller (BET) method, TPR/TPR-MS, TPR-EXAFS/XANES, and TPD-MS of CO 2 . These experiments confirmed that the catalysts were similar to prior batches (see the Supplementary Information for these routine characterization studies).

Materials and Methods
Zirconia extrudates (monoclinic, Alfa Aesar, Haverhill, MA, USA) were ground and sieved to a range of particles of between 63 and 125 microns in size. A mother batch of m-zirconia supported 1%Ru was first made by using the incipient wetness impregnation (IWI) of ruthenium nitrosyl nitrate (Alfa Aesar, Haverhill, MA, USA) as the precursor. After drying under ambient conditions for 8 h, the material was placed in a muffle furnace and dried at 110 • C for an additional 8 h. The temperature was then increased to 350 • C, and the catalyst was calcined in air for 4 h. This mother batch was separated into multiple batches to dope the catalyst with Na to different loadings (0.5, 1.0, 1.8, 2.5, and 5 wt.% Na). The Na was added by IWI with sodium nitrate (Alfa Aesar, Haverhill, MA, USA) used as the salt. The material was calcined at 350 • C for 4 h. The specific surface area and pore volume were measured using N 2 physisorption and the BET method on a Micromeritics 3-Flex instrument (Norcross, GA, USA). Samples were degassed for 12 h at 160 • C to below 0.05 Torr.
Temperature programmed reduction (TPR) was performed using an Altamira AMI-300R (Pittsburgh, PA, USA) catalyst characterization system connected to a Hiden Analytical quadrupole mass spectrometer (MS) (Warrington, UK). As the temperature was increased from 50 to 1000 • C, 10%H 2 /Ar (San Antonio, TX, USA) flowed at a ramp rate of 10 • C/min. The MS signals recorded were H 2 , H 2 O, CO, and CO 2 . Temperatureprogrammed desorption (TPD) was carried out using the Altamira AMI-300R instrument (Pittsburgh, PA, USA). Each catalyst was first reduced at 300 • C using 33%H 2 /He (Airgas, San Antonio, TX, USA) flowing at 30 mL/min using a thermal ramp of 1 • C/min. After activation, the temperature was lowered to 50 • C in flowing helium (Airgas, San Antonio, TX, USA) at 30 mL/min. After that, 4%CO 2 /He (Airgas, San Antonio, TX, USA) flowing at 30 mL/min was passed through the sample for 15 min. CO 2 -TPD was performed under a flow of He (30 mL/min) to 800 • C using a thermal ramp of 10 • C/min, and the mass spectrometry signal of carbon dioxide was monitored.
H 2 TPD was performed using the Altamira AMI-300R unit (Pittsburgh, PA, USA) connected to the Hiden mass spectrometer (Warrington, UK). Each catalyst was activated at 300 • C using 33%H 2 /Ar (Airgas, San Antonio, TX, USA) flowing at 30 mL/min using a thermal ramp of 1 • C/min. After activation, the temperature was lowered to 75 • C in the flowing H 2 /Ar mixture. After that, Ar (Airgas, San Antonio, TX, USA) flowing at 30 mL/min was passed through the sample for 45 min. H 2 -TPD was performed under a flow of Ar (30 mL/min) to 350 • C using a thermal ramp of 10 • C/min, and the mass spectrometry signal of H 2 was also monitored. After TPD, pulse calibration was carried out by sending 5 pulses of hydrogen (Airgas, San Antonio, TX, USA) from a 500 microliter sample loop into a pure Ar (Airgas, San Antonio, TX, USA) stream flowing at 30 mL/min. X-ray absorption spectroscopy (XAS) experiments were conducted at Beamline 10 BM, which is managed by the Materials Research Collaborative Access Team at the Advanced Photon Source at Argonne National Laboratory. In situ H 2 -TPR combined with EX-AFS/XANES spectroscopies was carried out. X-ray energies were tuned using a silicon (111) monochromator. A Rh-coated mirror was used to remove undesired harmonics of the beam energy. Details about the system can be found in reference [42]. Experiments were performed on six samples concurrently using a stainless-steel cylinder with 3 mm i.d. holes for supporting up to six samples. Pinhole-free sample disks consisted of 13 to 16 mg of sample (i.e., optimized for the Ru K-edge) and~3 mg SiO 2 . The cylinder was inserted into a clamshell furnace installed on a positioning table. Kapton ports allowed the quartz tube to be viewed. Ports were provided for flowing gases as well as the placement of a thermocouple. Samples were aligned to within 20 µm to allow for precise repeated scanning. Prior to the TPR experiment, 100 mL/min of He was flowed for at least 5 min to purge the chamber. Then, pure H 2 with a flow rate of 100 mL/min was applied, and the temperature was increased to 300 • C under a thermal ramp of 1.0 • C/min. Scans at and near the K-edge of Ru were recorded in transmission mode, and a ruthenium metal foil was used to calibrate the energy. XANES and EXAFS spectra were processed using WinXAS [43] (Berlin, Germany). EXAFS fits were performed with the software suite (Seattle, Washington, USA) consisting of Atoms [44], as well as FEFF and FEFFIT [45] using eight averaged spectra taken at the end of the TPR run. The fitting range was 3 to 10 Å −1 in k-space and 1.5 Å to 3.0 Å in R-space.
To assess the effects of the H 2 :CO 2 ratio and the Na-doping level on the surface species present, as well as the methanation activity, infrared spectroscopy was employed during CO 2 hydrogenation. In situ DRIFTS data were recorded using a Harrick (Pleasantville, NY, USA) praying mantis apparatus coupled to a Nicolet iS-10 infrared spectrometer (Waltham, MA, USA). Samples were first activated for 1 h using a 1:1 H 2 /He mixture (200 mL/min) (Airgas, San Antonio, TX, USA) at 300 • C, and 512 scans were recorded at a resolution of 4 cm −1 . After cooling to 50 • C in 100 mL/min of flowing He (Airgas, San Antonio, TX, USA), another spectrum was recorded. Next, a blend of either 4%CO 2 : 12%H 2 : 84%He or 4%CO 2 : 16%H 2 : 80%He (Airgas, San Antonio, TX, USA) was introduced to the reaction cell with a flow rate of 80 mL/min, and scans were recorded at 50 • C. The temperature was stepped in increments of 25 • C up to 400 • C and spectra were recorded. For the purpose of experimental control, scans in 4%CO 2 (balance He) (Airgas, San Antonio, TX, USA) were also recorded for unpromoted and 2.5 wt.% sodium-promoted catalysts.
As with our previous work at a lower pressure [19], the principle of microscopic reversibility underpinned the current investigation. To ensure experimental control with the new batches of the catalyst, it was necessary to ensure that Na-doping facilitated the formation of formate, a proposed intermediate in the forward/reverse water-gas shift, and that it promoted the rate of forward formate decomposition in steam for forward WGS by weakening the formate C-H bond (as observed in our prior work). Unpromoted and Na-doped catalysts were treated in 200 mL/min of 1:1 H 2 :He mixture (Airgas, San Antonio, TX, USA) at 300 • C for 1 h and then cooled to 225 • C in H 2 (Airgas, San Antonio, TX, USA). Next, 75 mL/min of He (Airgas, San Antonio, TX, USA) saturated with H 2 O (saturator temperature of 31 • C) was flowed for 8 min to ensure that defect-associated OH groups were formed [46][47][48]. Then, H 2 (Airgas, San Antonio, TX, USA) was flowed for 15 min at 100 mL/min, and the chamber was purged in 100 mL/min He. Then, 4%CO/He (Airgas, San Antonio, TX, USA) was flowed at 50 mL/min, and the catalyst was cooled to 130 • C. A spectrum was recorded at this temperature to obtain the maximum band intensities for surface formates and Ru carbonyls. It is well known that these species are quite stable when H 2 O is not present. However, if a low concentration of H 2 O is introduced, the decomposition of formate and Ru carbonyl species can be monitored at a low temperature (e.g., 130 • C). Helium (Airgas, San Antonio, TX, USA) was flowed through a bubbler that was situated within a water bath held at 31 • C. Vapor (4.4% H 2 O, balance He) was flowed through the reaction chamber at 75 mL/min. The results of forward formate decomposition in steam are provided in the Supplementary Information section.
To shed further light on the mechanism, a temperature-programmed surface reaction with the mass spectrometry (TP-rxn/MS) of CO 2 hydrogenation was conducted in the AMI-300R unit (Pittsburgh, PA, USA) using the Hiden mass spectrometer (Warrington, UK). Each catalyst was first reduced at 300 • C using 33%H 2 /Ar (Airgas, San Antonio, TX, USA) at a flow rate of 30 mL/min with a thermal ramp of 1 • C/min. After activation, the catalyst was purged in Ar (Airgas, San Antonio, TX, USA) flowing at 50 mL/min at 300 • C for 15 min, and then the temperature was decreased to 50 • C in Ar (Airgas, San Antonio, TX, USA) flowing at 30 mL/min. After that, 4%CO 2 /12%H 2 /balance He (Airgas, San Antonio, TX, USA) was flowed at 30 mL/min through the reaction chamber for 15 min. TP-rxn/MS was performed under Ar (Airgas, San Antonio, TX, USA) flowing at 30 mL/min to 800 • C using a thermal ramp of 10 • C/min. The mass spectrometry signals of CO and CH 4 were monitored. CO 2 hydrogenation catalytic tests were conducted using two different fixed bed reactor systems, a low-pressure reactor (to explore the effect of H 2 :CO 2 on the selectivity) and a high-pressure reactor (to favor the formation of some chain growth products). With the Universal Synthetic Gas Reactor (USGR ® ), the Ru-based catalysts were mixed with an alumina diluent in a 1:1 ratio and then screened for activity at 1 bar at 300 • C with H 2 :CO 2 ratios of 4:1 and 3:1 at a gas hourly space velocity of 60,000 mL/g cat /h in 15% carbon dioxide and a 60-45% hydrogen balance in nitrogen. Gas species, including carbon dioxide, carbon monoxide, methane, ethene, propane, propene, and propane, were monitored at a frequency of 1 Hz with FTIR using a Thermo Scientific Antarias IGS utilizing a Thermo Nicolet 2-m gas cell. In a high-pressure reactor, catalysts were mixed with alumina diluent in an alumina/catalyst mass ratio of 5 to diminish the formation of hot spots. Catalysts were reduced in pure hydrogen from 25 • C to 300 • C using a thermal ramp of 10 • C/min. The reactor was operated at 300 • C with a fixed H 2 :CO 2 ratio of 3:1 (60%H 2 , 20%CO 2 , balance nitrogen) and a space velocity of 80,000 mL/g cat ·h. The products were analyzed after 2 h of time on-stream through the gas chromatography of bag samples. CO 2 conversion (X CO2 , Equation (1)) and selectivity to the different carbon-containing products on a carbon basis (S x , Equation (2)) were calculated by the following equations: where F in and F out are the reactant molar flow at the exit and entrance of the reactor, respectively; S is selectivity of species x; c is the number of carbon atoms in species x; and n is the number of moles of species x.

BET Surface Area and Porosity
BET surface area and porosity data are tabulated in Table S1. The addition of 1%Ru decreased the specific surface area from 95.4 m 2 /to 90.4 m 2 /g. This was more than expected decrease due to the addition of weight. Here, it is assumed that Ru was converted into RuO 2 after calcination. Adding sodium (here, assumed to be sodium carbonate with varying degrees of hydration, resulting in a range of "expected surface areas") reduced the specific surface area further, from 90.4 m 2 /g for the undoped catalyst to 44.7 m 2 /g for the catalyst with 5.0 wt.% sodium. The reduction in the specific surface area caused by the addition of sodium was greater than the change anticipated with no pore blocking (i.e., by adding weight only, which impacts the denominator). Therefore, the addition of Na results in a degree of pore blocking in zirconia, which becomes more significant at higher Na loadings. Moreover, the pore volume (calculated using the BJH method) diminishes by the addition of Ru or Na or by increasing the amount of Na. The average pore diameter was not significantly altered by the addition of Ru or Na, and all values were within the range of 9.1-10.1 nm. The slight increasing trend in the pore diameter above 1%Na may indicate preferential pore filling of narrower pores.

Chemisorption by H 2 -TPD
H 2 chemisorption was conducted for the 1%Ru/m-zirconia catalyst as well as the 2.5%Na and 5%Na-doped catalysts. Using the TCD signal (and confirming with the mass spectrometer signal) for H 2 , the Ru metal nanoparticles were found to be completely dispersed for the unpromoted catalyst. Using the TCD signal and assuming a near spherical morphology and an H:Ru ratio of 1:1, the average Ru 0 diameter of the unpromoted catalyst was estimated to be 0.6 nm. Because Na (e.g., a Na species) is likely in contact with Ru surfaces (to be discussed), the average diameter could not be estimated for the Na-promoted catalysts. Nevertheless, the site capacities for the promoted catalysts with 2.5 wt.% Na and 5 wt.% Na were found to be 55% and 46%, respectively, that of the unpromoted catalyst. Moreover, as shown in Figure S1, there were three H 2 desorption peaks labelled a, b, and c. With an increase in Na loading, the total TCD or H 2 MS signal was diminished. The higher temperature peaks b and c were more greatly impacted than the low temperature peak a.

Catalyst Activation
The results of hydrogen TPR are provided in Figure 1, and the primary peak temperatures are recorded in Table 1. The results are qualitatively similar those produced in our previous work, indicating good experimental control [19]. The profiles of the unpromoted 1%Ru/m-ZrO 2 and 0.5%Na-1%Ru/m-ZrO 2 catalysts exhibit one sharp peak and a broad shoulder extending up to 450 • C. Increasing the loading of sodium caused the peaks to move to higher temperatures (e.g., from 187 • C for the unpromoted catalyst to between 238 • C and 402 • C for the 5%Na-doped catalyst). When the Na loading was increased above 0.5%, new peaks appeared ( Table 1). The reduction process first involves the reduction of Ru oxide species to Ru metal, as corroborated by the TPR-XANES/EXAFS results. Once Ru was reduced, H 2 easily dissociated and spilled over to the support to produce active OH groups at reduced defect sites in zirconia. In addition, O-vacancies formed with the evolution of H 2 O, as confirmed in the TPR-MS profiles of H 2 O shown in Figure S2. Finally, surface carbonates species decomposed, with the assistance of dissociated hydrogen, to CO X gases, and the production of these gases increased with the Na content. Although a small signal for CO 2 was detected in TPR-MS at Na doping loadings of 1% and higher, the predominant CO X signal was that of CO, indicating the decomposition of carbonate via Ru-catalyzed decarbonylation. The addition of Na hindered the reduction processes involved during activation in H 2 . This likely indicates a direct interaction between sodium and ruthenium particles. shoulder extending up to 450 °C. Increasing the loading of sodium caused the peaks to move to higher temperatures (e.g., from 187 °C for the unpromoted catalyst to between 238 °C and 402 °C for the 5%Na-doped catalyst). When the Na loading was increased above 0.5%, new peaks appeared ( Table 1). The reduction process first involves the reduction of Ru oxide species to Ru metal, as corroborated by the TPR-XANES/EXAFS results. Once Ru was reduced, H2 easily dissociated and spilled over to the support to produce active OH groups at reduced defect sites in zirconia. In addition, O-vacancies formed with the evolution of H2O, as confirmed in the TPR-MS profiles of H2O shown in Figure S2. Finally, surface carbonates species decomposed, with the assistance of dissociated hydrogen, to COX gases, and the production of these gases increased with the Na content. Although a small signal for CO2 was detected in TPR-MS at Na doping loadings of 1% and higher, the predominant COX signal was that of CO, indicating the decomposition of carbonate via Ru-catalyzed decarbonylation. The addition of Na hindered the reduction processes involved during activation in H2. This likely indicates a direct interaction between sodium and ruthenium particles.    The TPR-XANES spectra are provided in Figure 2, and the trends closely match those shown in our previous investigation at a low pressure [19]. In agreement with the TPR and TPR-MS profiles previously shown, increasing the Na dopant content systematically resulted in inhibited reduction. This can be most clearly observed in the linear combination fittings of the profiles with the Ru oxide and Ru 0 reference spectra (see Supplementary Information, Figure S3). The point at which half of the Ru oxide was converted into Ru 0 occurred at higher temperatures with the following increases in Na doping levels: no Nanomaterials 2023, 13, 1155 8 of 26 sodium, 121 • C; 0.5 wt.% Na, 129 • C; 1 wt.% Na, 149 • C; 1.8 wt.% Na, 168 • C; 2.5 wt.% Na, 203 • C; and 5 wt.% Na, 246 • C. As shown in Figure S4, there was no significant difference in the normalized XANES intensity following a reduction in H 2 , suggesting electron charge transfer [49] caused by the presence of the alkali.  TPR-EXAFS spectra provided in Figure S5 also confirm the slower reduction of Ru oxide to Ru metal nanoparticles with an increase in sodium loading. During the reduction in H 2 with an increasing temperature, the Ru-O coordination peak centered at 1-2 Å in phase-uncorrected Fourier transform magnitude spectra decreased, while the peak for metal Ru-Ru coordination at 2-3 Å increased. As the Na doping level increased, the Ru-O coordination peak was systematically retained at higher temperatures, while the peak for Ru-Ru coordination representing the formation of Ru 0 nanoparticles appeared at higher temperatures after the disappearance of Ru-O. For the case of unpromoted 1%Ru/m-ZrO 2 , the Ru-Ru coordination peak started to form at~130 • C, while it began to appear at~245 • C for the 5 wt.% Na-promoted catalyst. This inhibited reduction can likely be attributed to the partial covering of ruthenium oxide particles by Na + species, as suggested in our previous work [19].
Theoretical fits of EXAFS spectra are compiled in Figure 3 and Table 2. The addition of Na increased the Ru-Ru coordination number from 2 for the undoped catalyst to the range of 3.1-4.5 with Na-doping. Nominally, a coordination number of 2 for the undoped 1%Ru/m-ZrO 2 should correlate with a 3-atom cluster. The estimates provided in Table 2 range from 7 to 11 atoms for Na-doped catalysts. They were obtained by extrapolation assuming an approximately spherical cluster morphology. Using this extrapolation method, the average Ru diameter was found to be slightly smaller for the unpromoted catalyst (0.31 nm), while that of Na-doped catalysts ranged from 0.47 to 0.69 nm, and no clear trend in Ru 0 cluster size was observed with Na loading. The average Ru cluster sizes for all catalysts were slightly smaller than those observed in our previous work [19], indicating slight variations in the Ru size between parent batches prior to Na doping. Both H 2 chemisorption and EXAFS detected subnanometer particle sizes for the unpromoted catalyst. Because the sizes predicted by EXAFS for all catalysts should result in greater dispersion, the fact that the site capacities predicted by H 2 chemisorption for the 2.5 wt.% Na and 5 wt.% Na catalysts were significantly lower that of the unpromoted catalyst suggests contact of a Na-containing species with Ru metal surfaces.

Thermal Desorption of CO 2
TPD-MS of carbon dioxide was performed to gain insight into the impact of adding Na on the surface basicity, as CO 2 is an acidic molecule, and its adsorption is the first step in the RWGS catalytic cycle. Figure 4 shows the TPD-MS of the CO 2 profiles for all catalysts, including a fitting with twelve Gaussian peaks from~50 to 800 • C. Table 3 compiles the percentages of Gaussian peaks with peak maxima located within three different temperature ranges, including below 250 • C, between 250 • C and 400 • C, and above 400 • C. The undoped 1%Ru/m-ZrO 2 catalyst exhibited facile removal of CO 2 , as 65% of the peaks had peak maxima falling below 250 • C. With the addition of Na and with an increase in Na loading, the CO 2 desorption peaks shifted to higher temperatures. At a dopant level of 5%Na, only 10% of the peaks had peak maxima below 250 • C, while 21% of the peaks had maxima between 250 • C and 400 • C, and 69% were positioned above 400 • C. While this may be attributed, in part, to higher catalyst basicity, the inhibition of Ru-catalyzed CO 2 removal caused by Na blocking Ru sites cannot be ruled out as a contributor. Systematic attenuation of Ru metal activity by increasing the Na content diminished the methanation activity in favor of RWGS [19] and, in this work, this was expected to increase the probability of C-C coupling resulting in the formation of some chain growth products. As detailed in our previous work [19], increased basicity is expected to increase the strength of bonding between the O-C-O functional group of formate and the surface, thus weakening the C-H bond. Since, as previously discussed, scission of this bond is the proposed ratedetermining step (RDS) of the surface formate mechanism (i.e., the associative mechanism) for the forward water-gas shift, weakening this bond could (according to the Principle of Microscopic Reversibility) aid in the formation of formate carbon-hydrogen bonds for the reverse reaction.

Thermal Desorption of CO2
TPD-MS of carbon dioxide was performed to gain insight into the impact o Na on the surface basicity, as CO2 is an acidic molecule, and its adsorption is the in the RWGS catalytic cycle. Figure 4 shows the TPD-MS of the CO2 profiles for lysts, including a fitting with twelve Gaussian peaks from ~50 to 800 °C. Table 3 the percentages of Gaussian peaks with peak maxima located within three diffe perature ranges, including below 250 °C, between 250 °C and 400 °C, and abov probability of C-C coupling resulting in the formation of some chain growth products. As detailed in our previous work [19], increased basicity is expected to increase the strength of bonding between the O-C-O functional group of formate and the surface, thus weakening the C-H bond. Since, as previously discussed, scission of this bond is the proposed rate-determining step (RDS) of the surface formate mechanism (i.e., the associative mechanism) for the forward water-gas shift, weakening this bond could (according to the Principle of Microscopic Reversibility) aid in the formation of formate carbon-hydrogen bonds for the reverse reaction.  Table 3). , and (f) 5%Na, including Gaussian fitting peaks having temperature maxima in the following ranges: (green) < 250 • C, (blue) 250 • C to 400 • C, and (red) > 400 • C (see Table 3).

In Situ Infrared Spectroscopy
Prior to conducting RWGS reactor testing, it was important to first verify that Na doping resulted in electronic modification of the formate CH bond during forward WGS, as was shown in our previous investigation at a low pressure [19]. To that end, formate was produced by reacting CO with defect-associated bridging OH groups and then subsequently decomposed in H 2 O at 130 • C. As shown in Figure S6, which shows intensities normalized by height, the position of the ν(CH) band in DRIFTS indicates that Na-doping weakened the formate bond, as there was a shift in the main formate ν(CH) band to lower wavenumbers, as follows: 0%Na, 2868 cm −1 (with shoulders at 2900, 2884, and 2858 cm −1 ); 0.5%Na, 2861 cm −1 (with shoulders at 2892, 2861, 2837, and 2834 cm −1 ); 1%Na, 2828 cm −1 (with a minor peak at 2853 cm −1 and shoulders at 2879 and 2798 cm −1 ); 1.8%Na, 2801 cm −1 (with a shoulder at 2850 cm −1 ); 2.5%Na, 2803 cm −1 (with a shoulder at 2848 cm −1 ); and 5%Na, 2815 cm −1 (with a shoulder at 2852 cm −1 ). Figure S7 shows that the intensity of the formate ν(CH) band at 2868 cm −1 was low for the undoped catalyst, as well as for loadings of 0.5%Na and 1%Na. At Na loadings of 1.8% and 2.5%, however, the formate signal was intensified. Additionally, at those loadings, formate underwent rapid forward decomposition in H 2 O. At 5%Na loading, the formate intensity was lower than that observed in the 1.8-2.5%Na range, and the formate decomposition rate in steam was slow, indicating an excessive loading of Na when forward WGS is desired. These results are consistent with those of our previous investigation [19], indicating good experimental control. Figure S8, the addition of Na attenuated the Ru carbonyl ν(CO) bands, indicating that Na disrupts Ru 0 sites. Using 0%Na as a reference for an Ru carbonyl band area of 100%, the area of the overall Ru carbonyl band region decreased with the addition of Na, as follows: undoped, 100%; 0.5 wt.% sodium, 65%; 1 wt.% and 1.8 wt.% sodium, 60%; 2.5 wt.% sodium, 26%; and 5 wt.% sodium, 4%. Comparing Figure S7 with Figure S9 shows that the formate ν(CH) band for the 1.8 wt.% sodium and 2.5 wt.% sodium doped catalysts (i.e., close to the optimal sodium doping level) decreased faster (i.e., via the WGS reaction) in comparison with the Ru carbonyl ν(CO) band. Although the proposed catalytic cycle remains unproven, this result favors a surface formate mechanism [27] over a support-mediated redox mechanism [50]. In the latter case, Ru-CO is proposed to react with O adatoms of m-ZrO 2 , producing CO 2 , with H 2 O replenishing O-vacancies on the surface of zirconia with O and liberating H 2 in the process. In our previous investigation at a low pressure [19], the attenuation of the activity of the Ru metal function helped to hinder the interception of CO and the subsequent secondary reaction of methanation, improving the RWGS selectivity significantly. In the current study, in addition to this, we expect that the attenuation of the CO X hydrogenation activity resulting from contact between the Na and Ru nanoparticles may have allowed for an increase in the probability of C-C coupling, resulting in the production of some chain growth products from Fischer-Tropsch synthesis.

As shown in
Before conducting CO 2 hydrogenation experiments using in-situ DRIFTS, control experiments of CO 2 in He were performed (Figures S10 and S11 for 1%Ru/m-zirconia and 2.5%Na-1%Ru/m-zirconia, respectively). The ν(OCO) infrared bands of surface carbonates and/or bicarbonate species were observed over the entire temperature range. CO 2 hydrogenation experiments were then carried for the purpose of characterizing the surface species formed and the temperature and the degree of methanation. The infrared spectra recorded during CO 2 hydrogenation are shown in Figures 5-12 for all catalysts. The Na doping effect can be observed by comparing Figures 5, 7-10 and 12, which were conducted at an H 2 :CO 2 ratio equal to 3:1. The effect of the H 2 :CO 2 ratio (i.e., 3:1 versus 4:1) can be examined by comparing Figures 5 and 6 for undoped 1%Ru/m-zirconia and Figures 11 and 12 for 2.5%Na-1%Ru/m-zirconia. The wavenumber assignments of the bands for the various species detected on the catalysts are provided in Table 4. The DRIFTS spectra revealed that carbonates and/or bicarbonate species were formed at 50 • C. With an increase in temperature, formate species were formed from the hydrogenation of surface carbonates, presumably at the Ru-support junction. The intensities of these bands typically reached close to the maximum prior to the detection of gas-phase CO and CH 4 , as observed by the ν(CO) of CO and the ν(CH) band of CH 4 .   Figure 13 shows the formate ν(CH) band with the intensity normalized to the height for the purpose of comparing positions. Table 4 shows the band positions. The results are in good agreement with the observations of our low pressure investigation [19]; however, in that study, the H 2 /CO 2 ratio was 15:1, whereas in the current study the same H 2 /CO 2 ratio as that used in fixed-bed reaction tests was employed. As observed in forward WGS experiments, the formate ν(CH) band position in RWGS was located at lower wavenumbers when the Na doping reached 1.8%Na (which is close to the optimal doping level for promoting the WGS reaction). From a loading of 1.8%Na to 2.5%Na, the band position changed very little (∆ < 1 cm −1 ) and then broadened at 5%Na. Table 4 also provides the formate band positions for the asymmetric and symmetric ν(OCO) modes. The difference in the band positions of the asymmetric and symmetric ν(OCO) bands provides information about the strength of bonding with the catalyst surface. With an increase in Na loading, there was an increase in the strength of this interaction, as the differences exhibited an increasing trend, as follows (note that, in some cases, multiple formate species were detected, and we report the primary bands observed): undoped, ∆ = 199 and 232 cm −1 ; 0.5 wt.% Na, ∆ = 239 and 306 cm −1 ; 1 wt.% Na, ∆ = 325 cm −1 ; and 1.8 wt.% Na, ∆ = 336 cm −1 . Above 1.8 wt.% Na, the difference decreased somewhat: 2.5 wt.% Na, ∆ = 327 cm −1 and 5 wt.% Na, ∆ = 306 cm −1 . These results align well with the trend observed with CO 2 TPD, where the higher basicity of Na-doped catalysts led to shifts in the CO 2 TPD profiles to higher temperatures. However, while that conclusion was likely affected by the inhibiting effect of Na on Ru-catalyzed CO 2 removal, the splitting of the formate ν(OCO) bands was not impacted by the effect of Ru present in CO 2 TPD, and as such, this may be a better indication of higher basicity with the addition of Na. Thus, as shown in our prior investigation at a low pressure [19], the DRIFTS and CO 2 TPD results provide strong evidence that the catalyst basicity is increased with the addition of Na. With the addition of Na, there is an increase in the strength of the interaction between the O-C-O functional group and the catalyst surface. This is proposed to be the cause of C-H bond weakening, as strongly suggested by the formate ν(CH) band shift to lower wavenumbers. To reiterate, in the absence of other effects, formate C-H bond weakening should promote C-H bond scission in forward WGS (i.e., it is the proposed rate limiting step in forward WGS), accelerating the catalytic cycle. For CO 2 hydrogenation via RWGS, according to the Principle of Microscopic Reversibility, Na doping should therefore (once again, in the absence of other effects) tend to promote C-H bond formation when producing the formate intermediate and, as a result, promote the formation of CO.
In in situ DRIFTS studies, the general mechanistic trend is that carbonate species are formed at low temperature; with an increasing temperature, carbonate is hydrogenated to formate intermediates, which decompose to CO with the aid of Ru. Once CO is formed, Na plays an important role in determining whether the CO is subsequently hydrogenated to CH 4 by a secondary reaction, or whether it desorbs. The attenuation of methanation may also lead to a greater probability of chain growth at higher pressures.
Nanomaterials 2023, 13, x FOR PEER REVIEW 15 of 27 Figure 5. DRIFTS of CO2 hydrogenation with 4%CO2 + 12%H2 (balance He) over 1%Ru/m-zirconia activated at 300 °C in hydrogen, purged in helium, and then cooled to 50 °C prior to the reaction. Figure 5. DRIFTS of CO 2 hydrogenation with 4%CO 2 + 12%H 2 (balance He) over 1%Ru/m-zirconia activated at 300 • C in hydrogen, purged in helium, and then cooled to 50 • C prior to the reaction. Figure 5. DRIFTS of CO2 hydrogenation with 4%CO2 + 12%H2 (balance He) over 1%Ru/m-zirconia activated at 300 °C in hydrogen, purged in helium, and then cooled to 50 °C prior to the reaction. Figure 6. DRIFTS of CO2 hydrogenation with 4%CO2 + 16%H2 (balance He) over 1%Ru/m-zirconia activated at 300 °C in hydrogen, purged in helium, and then cooled to 50 °C prior to the reaction. Figure 6. DRIFTS of CO 2 hydrogenation with 4%CO 2 + 16%H 2 (balance He) over 1%Ru/m-zirconia activated at 300 • C in hydrogen, purged in helium, and then cooled to 50 • C prior to the reaction.
Nanomaterials 2023, 13, x FOR PEER REVIEW 18 of 27 Figure 11. DRIFTS of CO2 hydrogenation with 4%CO2 + 16%H2 (balance He) over 2.5%Na-1%Ru/mzirconia activated at 300 °C in hydrogen, purged in He, and then cooled to 50 °C prior to reaction. Figure 11. DRIFTS of CO 2 hydrogenation with 4%CO 2 + 16%H 2 (balance He) over 2.5%Na-1%Ru/mzirconia activated at 300 • C in hydrogen, purged in He, and then cooled to 50 • C prior to reaction. Figure 11. DRIFTS of CO2 hydrogenation with 4%CO2 + 16%H2 (balance He) over 2.5%Na-1%Ru/mzirconia activated at 300 °C in hydrogen, purged in He, and then cooled to 50 °C prior to reaction. Figure 12. DRIFTS of CO2 hydrogenation with 4%CO2 + 12%H2 (balance He) over 5%Na-1%Ru/mzirconia activated at 300 °C in hydrogen, purged in He, and then cooled to 50 °C prior to reaction. Figure 12. DRIFTS of CO 2 hydrogenation with 4%CO 2 + 12%H 2 (balance He) over 5%Na-1%Ru/mzirconia activated at 300 • C in hydrogen, purged in He, and then cooled to 50 • C prior to reaction. In in situ DRIFTS studies, the general mechanistic trend is that carbonate species are formed at low temperature; with an increasing temperature, carbonate is hydrogenated Figure 13. DRIFTS of the formate ν(CH) band, with the intensity normalized to the height, for CO 2 hydrogenation with 4%CO 2 + 12%H 2 (balance He) over catalysts activated at 300 • C in H 2 , including (a, black) unpromoted 1%Ru/m-zirconia, and the promoted catalysts with Na loadings of (b, cyan) 0.5 wt.%, (c, green) 1%, (d, red) 1.8 wt.%, (e, orange) 2.5 wt.%, and (f, dark yellow) 5 wt.%. Spectra were recorded at temperatures 25 • C below those at which the n(CH) signal of CH 4 Figure 14 shows that, as in the case of the forward water-gas shift reaction described earlier, the Ru carbonyl intensity is attenuated by the addition of Na, as follows: undoped, 100%; 0.5 wt.% sodium, 75%; 1 wt.% sodium, 69%; 1.8 wt.% sodium, 19%; 2.5 wt.% sodium, 17%; 5 wt.% sodium, 14%. As such, methanation becomes suppressed by the disruption of ensembles of Ru surface atoms by Na. By examining the initial temperature of CH 4 formation ( Table 5) through the ν(CH) band of CH 4 at 3010-3020 cm −1 , the following trend was observed: undoped, 150 • C; 0.5 wt.% sodium, 175 • C; 1 wt.% sodium, 175 • C; 1.8 wt.% sodium, 250 • C 2.5 wt.% sodium, 275 • C; and 5 wt.% sodium, 300 • C. Thus, the addition of sodium inhibited methanation remarkably, shifting its initial formation temperature by ∆ = +150 • C.   There was also a shift in the ruthenium carbonyl band to lower wavenumbers. From the geometric perspective, the addition of Na may diminish the dipole coupling of CO, resulting in a shift of the ruthenium carbonyl bands to lower wavenumbers. In that case, the role of Na may be to decrease the surface concentration and mobility of hydrogen  There was also a shift in the ruthenium carbonyl band to lower wavenumbers. From the geometric perspective, the addition of Na may diminish the dipole coupling of CO, resulting in a shift of the ruthenium carbonyl bands to lower wavenumbers. In that case, the role of Na may be to decrease the surface concentration and mobility of hydrogen adsorbed on the surfaces of Ru metal nanoparticles, as proposed by Komaya [39]. From an electronic point of view, alkali promoters have been proposed to donate electron density to the catalyst surface and, in turn, facilitate the backdonation of electron density from the catalyst surface to the 2π* antibonding molecular orbital of CO, weakening the CO bond and shifting the carbonyl bands to lower wavenumbers. Görling et al. [53] determined that about half of the shift to lower wavenumbers is due to electronic backdonation [54], while the additional shift is primarily due to electrostatic interactions between CO and the surface dipole layer modified by alkali. With the related Na-doped Pt/m-ZrO 2 system [55], we did not obtain evidence for an electron transfer effect in applying the XANES difference procedure at the the L III minus L II edges of Pt. Although we could not apply that method to the Ru system, we did not observe any change in the XANES line shape profile or observe a shift in the K-edge energy that would confirm such an electronic effect. Nevertheless, an electronic effect cannot be ruled out.
Thus, Na likely plays two important roles in controlling the selectivity. Weakening the carbon-hydrogen bond of formate tends to promote the RWGS cycle, while attenuating the Ru metal activity by the addition of Na inhibits methanation. Based on the loading of Na, the relative rates of CO and CH 4 formation can therefore be controlled to a significant degree. The inhibition of methanation also means that the probability of C-C coupling may be improved when CO 2 hydrogenation is conducted at higher pressures.

Temperature-Programmed Surface Reaction with Mass Spectrometry
The TP-rxn/MS results for the CO 2 hydrogenation of preadsorbed 4%CO 2 /12%H 2 are reported in Figure 15 and Table 6. For the undoped 1%Ru/m-zirconia catalyst and the same catalyst promoted with up to 1%Na, the profile was dominated by CH 4 (red curves), while for catalysts with 1.8%Na to 5%Na, the profile switched and was dominated by CO (blue curves). As shown in Table 6, in all cases, the maximum temperature for CO evolution was below the maximum temperature for CH 4 evolution, with the difference being in the range of 37-64 • C. Combined, these two results strongly suggest that the mechanism involves, to a significant degree, a stepwise mechanism involving the primary formation of CO (or adsorbed CO) followed by the interception of CO for secondary hydrogenation (e.g., at low P, to CH 4 ). Na thus acts to profoundly facilitate CO formation (i.e., RWGS) and inhibit the secondary hydrogenation reaction (e.g., methanation).

Catalytic Testing
Prior to carrying out reaction testing at 20 bar, the influence of the H2:CO2 ratio on the methanation selectivity was investigated for the undoped versus 2.5%Na-promoted catalysts. In our prior investigation at a low pressure, the H2:CO2 ratio was 4:1; however, we were interested in determining whether operating at a H2:CO2 ratio equal to 3:1 (stoichiometric for RWGS/Fischer-Tropsch) might diminish the methanation selectivity at a low pressure. Comparing Figure 5 (H2:CO2 = 3:1) and 6 (H2:CO2 = 4:1) for unpromoted 1%Ru/m-ZrO2, the primary difference is not the temperature at which methane forms, but rather, that the methane bands are stronger in intensity at an H2:CO2 ratio of 4:1. The same is true when comparing Figures 10 and 11 for the 2.5%Na-promoted catalyst. Testing the undoped and 2.5%Na-promoted catalyst in the fixed-bed reactor at a low pressure ( Figure  16), the CO selectivity was shown to be slightly improved (~1%) for the unpromoted catalyst and more significantly improved (~6%) for the 2.5%Na-promoted catalyst after switching to the lower H2/CO2 ratio of 3:1 from 4:1. As a result, the H2:CO2 ratio of 3:1 was used (stoichiometric for RWGS/Fischer-Tropsch) to carry out tests at the higher pressure of 20 bar.

Catalytic Testing
Prior to carrying out reaction testing at 20 bar, the influence of the H 2 :CO 2 ratio on the methanation selectivity was investigated for the undoped versus 2.5%Na-promoted catalysts. In our prior investigation at a low pressure, the H 2 :CO 2 ratio was 4:1; however, we were interested in determining whether operating at a H 2 :CO 2 ratio equal to 3:1 (stoichiometric for RWGS/Fischer-Tropsch) might diminish the methanation selectivity at a low pressure. Comparing Figure 5 (H 2 :CO 2 = 3:1) and 6 (H 2 :CO 2 = 4:1) for unpromoted 1%Ru/m-ZrO 2 , the primary difference is not the temperature at which methane forms, but rather, that the methane bands are stronger in intensity at an H 2 :CO 2 ratio of 4:1. The same is true when comparing Figures 10 and 11 for the 2.5%Na-promoted catalyst. Testing the undoped and 2.5%Na-promoted catalyst in the fixed-bed reactor at a low pressure (Figure 16), the CO selectivity was shown to be slightly improved (~1%) for the unpromoted catalyst and more significantly improved (~6%) for the 2.5%Na-promoted catalyst after switching to the lower H 2 /CO 2 ratio of 3:1 from 4:1. As a result, the H 2 :CO 2 ratio of 3:1 was used (stoichiometric for RWGS/Fischer-Tropsch) to carry out tests at the higher pressure of 20 bar.
Hybrid Na-promoted 1%Ru/m-ZrO 2 catalysts comprised of a RWGS function and an FTS function were tested at an elevated pressure of 20 bar, a temperature of 300 • C, a space velocity equal to 80,000 mL/g cat ·h, and an H 2 /CO 2 ratio 3:1. As shown in Figure 17, increasing the Na content from 2.5 wt.% sodium to 5 wt.% sodium slightly decreased the CO 2 conversion from~14% to~10%. However, the selectivity for desired products was greatly improved. Undesired methanation decreased, with CH 4 selectivity dropping from~60% to~21%, while desired CO selectivity almost doubled from~36% to~71%, and desired chain growth product selectivity (C 2 -C 4 selectivity) doubled from~4% to~8%.  Figure 16. Effect of the H2:CO2 ratio on reaction testing at 1 bar and 300 °C at a space velocity of 60,000 mL/gcat/h using a fixed-bed reactor with 1%Ru/m-zirconia versus 2.5%Na-1%Ru/m-zirconia catalysts, including ratios of (blue) 4:1 and (orange) 3:1.

Conclusions
By promoting 1%Ru/m-zirconia with sodium and adjusting the H 2 :CO 2 ratio from 4:1 to 3:1, the selectivity of the CO 2 hydrogenation reaction was turned away from methanation to favor the RWGS product, CO. At an elevated pressure of 20 bar, in addition to improvements in the CO selectivity, the selectivity to chain growth products was significantly improved as well. For example, at 20 bar, by increasing the Na content from 2.5 wt.% to 5 wt.%, the CH 4 selectivity was decreased by more than half, from 60% to 21%, while the CO selectivity increased from 36% to 71% and that of light hydrocarbons (C 2 -C 4 ) doubled from 4% to 8%. CO 2 TPD and an analysis of the splitting of the formate ν(OCO) bands during CO 2 hydrogenation showed that the addition of Na to the catalyst (as well as increasing the Na doping level) increased the basicity of the catalyst. This increase in basicity from Na doping resulted in a weakening of the formate intermediate C-H bond, as confirmed by an observed shift in the ν(CH) band of formate to lower wavenumbers. While this facilitates formate C-H bond scission during forward WGS, boosting CO conversion, during reverse WGS, this effect promotes the formation of carbon-hydrogen bonds when producing the formate intermediate, and in turn, facilitates the formation of CO at the Ru/m-zirconia junction. At the same time, increasing the amount of Na dopant attenuates the hydrogenation activity of Ru 0 at on-top sites, and systematic decreases in Ru carbonyl intensity were observed in infrared spectroscopy during CO 2 hydrogenation. Moreover, the site capacity, as measured by H 2 chemisorption, was found to be significantly lower for the 2.5 wt.% and 5 wt.% Na-promoted catalysts, despite all catalysts having small particle sizes, as measured by EXAFS, that should, in the absence of coverage by Na, otherwise have led to high dispersion. Because these Ru 0 on-top sites are responsible for intercepting CO from the Ru/m-zirconia junction and subsequently converting the CO formed to methane, the addition of sodium tends to suppress the methanation reaction. An added benefit is that suppressing the excessive hydrogenation activity of Ru 0 increases the probability of C-C chain growth, thereby boosting the selectivity of C 2 -C 4 products. Thus, if the desired product is synthetic natural gas, the addition of Na should be avoided. On the other hand, if CO is desired for making syngas (to be used in downstream Fischer-Tropsch synthesis or methanol-to-gasoline processes), then adding sodium to the catalyst is beneficial, and some chain growth products can be made during the RWGS section as well. The first primary role of Na is to enhance the catalyst basicity, and this has a direct effect on the weakening of the formate carbon-hydrogen bond by increasing the strength of the interaction between the catalyst surface and the O-C-O functional group of formate. As a result, Na doping facilitates the formation of surface formate intermediates which, in turn, promotes CO production via RWGS. The second primary role of Na is to suppress the excessive hydrogenation activity of Ru 0 metal sites, either allowing CO to escape further hydrogenation or increasing the probability of C-C chain growth, promoting the formation of C 2 -C 4 products.