Feed Effects on Water–Gas Shift Activity of M/Co₃O₄-ZrO₂ (M = Pt, Pd, and Ru) and Potassium Role in Methane Suppression

Abstract

Water–gas shift (WGS) is an industrial process to tackle CO abatement and H₂ upgradation. The syngas (CO and H₂ mixture) obtained from steam or dry reformers often has unreacted (from dry reforming) or undesired (from steam reforming) CO₂, which is subsequently sent to downstream WGS reactor for H₂ upgradation. Thus, industrial processes must deal with CO₂ and H₂ in the reformate feed. Achieving high CO₂ or H₂ selectivities become challenging due to possible CO and CO₂ methanation reactions, which further increases the separation costs to produce pure H₂. In this study, M/Co₃O₄-ZrO₂ (M = Ru, Pd and Pt) catalysts were prepared using sonochemical synthesis. The synthesized catalysts were tested for WGS activity under three feed conditions, namely, Feed A (CO and steam), Feed B (CO, H₂ and steam) and Feed C (CO, H₂, CO₂ and steam). All the catalysts gave zero methane selectivity under Feed A conditions, whereas the methane selectivity was significant under Feed B and C conditions. Among all catalysts, PtCZ was found to be the best performing catalyst in terms of CO conversion and CO₂ selectivity. However, it still suffered with low but significant methane selectivity. This best performing catalyst was further modified with an alkali component, potassium to suppress undesirable methane selectivity. All the catalysts were well characterized with BET, SEM, TEM to confirm the structural properties and effective doping of the noble metals. Additionally, the apparent activation energies were obtained to showcase the best catalyst.

1. Introduction

The development of efficient and zero emission vehicles is a major challenge for automobile industries. It is also challenging for process industries to generate high purity H₂ for proton exchange membrane fuel cells (PEMFC) to further address the clean energy requirements. H₂ production route map in the process industries have series of steps such as reforming, water–gas shift, preferential CO oxidation and pressure swing adsorption to capture CO₂. Among these steps, water–gas shift reaction is a route for hydrogen upgradation using the reformate feed obtained from reformers [1,2]. Using H₂ as feed, the PEMFCs can generate clean energy. However, PEMFCs demand high purity H₂ with extremely low CO impurities (less than 10 ppm) and other hydrocarbons to avoid the poisoning of Pt membrane. Thus, all the catalysts in the downstream processes after reforming must be designed to maximize the H₂ purity. Typically, the dry reformate gas mixture contains H₂, CO at high compositions, and the other components such as CO₂, unreacted hydrocarbons at low compositions [1]. The CO composition is minimized with water–gas shift (WGS) reaction by facilitating a reaction with superheated steam (often in excess quantity). The typical WGS reaction for upgrading H₂ and minimizing CO is as follows,

$$CO\hspace{0.33em}+\hspace{0.33em}{H}_{2}O\stackrel{\hspace{0.33em}}{\rightleftarrows }\hspace{0.33em}C{O}_2\hspace{0.33em}+\hspace{0.33em}{H}_2·\mathsf{\Delta }{\mathrm{H}}_{298\mathrm{K}}=-41kJ{mol}^{-1}$$

(1)

Industries implement two stages for converting CO into CO₂ from H₂ rich feed by conducting high temperature and low temperature water–gas shift (HTS and LTS). HTS operates in a temperature range of 350–500 °C to reduce CO composition below 4%, whereas LTS operates in the range of 150–300 °C to bring down the CO compositions below 0.2% [1,3]. These two stages (HTS and LTS) significantly improve the H₂ composition. After that, CO₂ will be removed using pressure swing adsorption to obtain pure H₂.

The industrial catalysts used for HTS and LTS are Fe-Cr oxides and Cu-ZnO-Al₂O₃ [1,4,5,6,7]. Fe₂O₃/Cr₂O₃/CuO is used as commercial catalyst for high temperature water gas shift reaction in which Cr becomes carcinogenic as it is oxidized to Cr⁶⁺ from Cr³⁺ [8]. To overcome these problems, various metal oxide catalysts with noble metal impregnation or substitution were developed. In recent years, a significant use of reducible supports (such as ceria) in conjunction with noble metals for LTS was observed [9]. With the usage of reducible oxides, the interplay of lattice oxygen vacancies and active metal sites in the reaction mechanism is significant. Oxygen vacancies play a crucial role in the adsorption of water and/or CO molecule, active metal sites can enhance the activation of the molecules leading to the formation of intermediates such as carboxyl and formates [10]. CeO₂ is one of such metal oxides that can function as oxygen buffer under exhaust conditions by releasing/acquiring oxygen through redox processes involving the Ce⁴⁺/Ce³⁺ couple [11]. Senanayake et al. [12] also tested ceria alone as a catalyst and showed that reduced Ce³⁺ and oxygen vacancies contribute to the production of hydrogen through the reaction. Other than CeO₂, low temperature water–gas shift is a well explored reaction on various reducible and non-reducible catalysts such as TiO₂, ZrO₂ and Al₂O₃ [13,14,15]. The effect of noble metals and transition metals on reducible oxides such as ceria were widely studied [16,17,18,19]. Pt- and Au-based catalysts on other reducible catalysts showed high catalytic activity for LTS, whereas Ni- and Fe-based catalysts were reported for HTS [6,20,21,22,23]. The water–gas shift activity of a support majorly depends on the reducible nature of the catalysts, strong metal support interaction (SMSI), oxygen storage capacity (OSC) and surface nature of the support [24,25,26,27]. By acquiring the mentioned properties, the high catalytic activity and stability can be achieved.

Mierczynski et al. [28] studied the activity of monometallic (Pd, Ru, Ni and Cu) catalysts supported on ZnAl₂O₄ spinel support. While Ru catalysts showed the highest conversion at high temperature, the Cu catalysts were shown to be good at low temperatures. Pd catalysts showed poor performance due to intermetallic PdZn compounds at intermediate temperatures due to interaction between the metal and the support. The lowest activity was observed for Ni catalysts because only a small amount of nickel species dispersed on the support surface. The terminal OH groups bound to active metal of the ZnAl₂O₄ spinel reacted with carbon monoxide and formed formate bridges and obtained H₂ and CO₂ as the products. Hu et al. [29] modified the Pd-based catalysts with Zn and showed that addition of Zn weakened the electron-accepting ability of the catalyst and increased the electron-donating ability and promoted the LTS activity. The reaction pathway is an associative mechanism through COOH intermediate, which is same as the Pd catalyst.

The effect of the noble metal Pt on ceria support was also shown by Jacobs et al. [30]. In this study, Pt facilitated partial reduction in ceria and promoted formate decomposition through enhanced C-H bond rupture rate, and ceria helped bridging the OH groups such that formates could be formed upon CO adsorption. Mandapaka et al. [10] studied the WGS over Pt supported on ceria. An associative carboxyl mechanism was proposed. The rate limiting step was considered as the dissociation of surface carboxyl species over Pt to give surface bound CO₂ and H. DFT studies for ceria catalyst suggested the reaction mechanism steps as reversible adsorption of CO on ionic platinum site and of H₂O on the surface vacancy on ceria. The adsorbed H₂O then disintegrates to give OH surface species, which then react with adsorbed CO species to form surface COOH and H [27]. The COOH surface species then disintegrate to form CO₂ and H. The products then desorb and complete the catalytic cycle with 90% conversion at 250 °C. It was also shown that the use of noble metal catalysts supported on zinc and platinum co-doped ceria can convert the WGS into a single-stage process. Both metal and support were found to be equally effective in the reaction mechanism in which CO adsorbs on the Pt site while H₂O is adsorbed and activated on the ceria site. Shinde et al. [31] studied Pd-modified Ni/CeO₂ to prevent CO methanation while improving H₂O dissociation. Vignatti et al. [32] showed that Pt/Ce_xZr_1−xO₂ catalysts with x ≥ 0.5 were more active than Pt/CeO₂ or Pt/ZrO₂ as Zr tends to increase the surface area as well as the reducibility of ceria, while also increasing the concentration of surface OH groups that are formed on Ce³⁺ reduced sites. These terminal OH groups participate in the WGS reaction resulting in an associative mechanism via formate formation. Ding et al. [33] provided comparative results for Pt single atoms and Pt nanoparticles as Pt is believed to be an exceptionally active catalyst for the WGS. It was observed that only Pt nanoparticles showed activity for LTS while the Pt single atoms on HZSM-5 behaved as spectators due to the strong binding of the CO molecules. This shows the poisonous effect of CO on Pt atoms. Improvement in performance and CO conversion was observed with the lattice substitution. Studies conducted by Petallidou et al. [34] showed the catalytic activity showed better catalytic performance with lattice substituted Pt than the impregnation. The performance trend was observed as Pt/Ce_0.9Ti_0.2O_2−x > Pt/CeO₂ > Pt/TiO₂.

The variety reducible supports with noble and transition metals as catalysts may satisfy the requirements for PEMFCs in eliminating entire CO in the gas mixture under the feed conditions of CO and steam. However, under real conditions, the reformate gas mixture (from the steam or dry reforming unit) contains CO along with other components such as CO₂, H₂ as shown in Scheme 1. This often results in CH₄ formation as a side product, that too in prominent compositions. The WGS catalysts under reformate feed conditions can result side reactions such as CO and CO₂ methanation which decreases the H₂ yield and CO conversion. Thus, the study of catalytic-inhibiting effects of CO₂ and H₂ in the feed is an interesting aspect for practical applications of WGS.

Providing strong metal support interaction (SMSI) and oxygen storage capacity (OSC) are merely not sufficient to reduce the methanation. Thus, alkali modifications were investigated to suppress the methanation affinity. The addition of alkalis (electron donors) create electron cloud surrounding surface and increases the chemisorption of electron acceptor species such as CO and O₂ and suppresses the chemisorption of electron donor species such as H₂ and olefins [35]. Maneerung et al. [35] showed that LaNiO₃ perovskite materials were not equally effective for HTS under reformate feed conditions. These catalysts were favorable for methane selectivity through CO₂ methanation. However, methanation was successfully prevented by the addition of K on the perovskite surface with an optimum of 5 wt% K over La₂O₃. K occupies the interface between La₂O₃ and Ni nanoparticles. K, La₂O₃ and Ni act as adsorbents for water, CO₂ and H₂, respectively. K aids in providing hydroxyl groups to combine with the CO and form HCOO intermediates. The doped catalyst was found to stable for an operating period of 5 h and resulted in 90% CO conversion at 350 °C. Ang et al. [22] attributed the methanation suppression using 5 wt% K doping on ceria supported nickel (Ni/5K/CeO₂) to the inhibition of the formation of nickel sub-carbonyls. These are precursors for methanation reaction and are prevented due to the interaction between Ni and K. Additionally, as K is hygroscopic, K was found to enhance a reduction in CeO₂ and promote water dissociation on reduced CeO₂ to form hydroxyl (OH) groups, which dissociate further into adsorbed oxygen that reacts with adsorbed CO on Ni to form adsorbed carbon dioxide CO₂. Ni/5K/CeO₂ was found to be stable for 100 h of time-on-stream condition.

Noble metals such as Pt and Pd were proved to be effective catalysts for the WGS. Watanabe et al. [36] evaluated the activities of Pd/alkali/Fe₂O₃ catalysts with different ratios of alkali/Pd (alkali: Li, Na, K or Cs). The optimum K/Pd molar ratio was found to be 2, and the reaction proceeded through the redox mechanism using the lattice oxygen, which was regenerated by H₂O, while reducing the Fe₂O₃ to Fe₃O₄. Order of reducibility of these catalysts was K-impregnated > Cs-impregnated > Na-impregnated > Pd/Fe₂O₃ ≈ Li-impregnated alkali metals such as Na (51.2% conversion), K (83.1% conversion) and Cs (72.2% conversion) drastically enhanced WGS activity, except for Li-loaded (only 11.5% conversion) catalyst. The role of alkalis such as K and Na was studied on various reducible supports [22,29,30].

This study focuses on the development of thermally stable composite supports using sonochemical synthesis and the effect of noble metals on WGS activity. A mixed metal composite support containing reducible Co₃O₄ and non-reducible ZrO₂ was prepared and the role of noble metals (Pt, Pd and Ru) was investigated for WGS. The catalytic activity of the M/Co₃O₄-ZrO₂ (M = Pt, Pd and Ru) was studied with three different dry feed gas mixtures, to investigate the role of H₂ and CO₂ in the feed on WGS activity. The suppression of methanation ability under realistic reformate gas feed condition using K as promoter was also investigated. All the catalysts were characterized with XRD, XPS, TEM, BET and TPR studies.

2. Results and Discussion

2.1. X-ray Diffraction and BET Measurements

XRD patterns of the synthesized catalysts are shown in Figure 1, which confirmed the presence of both Co₃O₄ and ZrO₂ phases by comparing with JCPDS database files 00–042–1467 and 00–003–0640, respectively. The CZ composite consisted of cubic Co₃O₄ (a = b = c = 8.084 Å, space group: Fd-3 m) and cubic ZrO₂ (a = b = c = 5.103 Å, space group: Fm-3 m) phases [37,38]. Monoclinic ZrO₂ and cubic CoO phases were possible in the synthesis of composites, but these phases were not detected in the synthesized materials. The most possible prominent peaks associated with metals Pt (111), Pd (111) and Ru (101) at 39.67°, 40.42° and 44.37° were not found from the XRD of PtCZ, PdCZ and RuCZ. As the doping % was limited to 2%, the detection was not possible from XRD. However, a slight shift in 2θ values was observed because of the different ionic radius of metals and variation in lattice parameters [37,38]. This further indicates the possibility of ionic catalysts, where dopant replaces Co or Zr and bonds with lattice oxygen upon the nucleation and growth. In KPtCZ, the presence of K was not detectable, but a very small peak at 39.7° corresponding to Pt (111)/PtO₂ (101) was noticed. In the synthesis of KPtCZ, the K impregnation was implemented using reducing agents. Due to the reducing agents, weakly bound Pt ion in the lattice may diffuse to surface and form PtO₂ cluster and it was possible during K impregnation on PtCZ. Upon investigating XPS analysis, no metallic Pt was found in KPtCZ and these results were explained in Section 2.4.

The adsorption–desorption isotherms for all the materials indicated Type II isotherms (as shown in Figure 2) and offered unrestricted monolayer–multilayer adsorption. Type H3 hysteresis was noticeable for the synthesized catalysts, indicating the possibility of meso- and micro-porous materials. The specific surface areas, pore volumes, mean pore diameters and crystallite sizes (using Debye–Scherrer relation) of CZ, PtCZ, KPtCZ, PdCZ and RuCZ are listed in

Table 1. Characteristics of CZ, PtCZ, KPtCZ, PdCZ and RuCZ from BET and XRD analysis.

Catalyst	Surface Area, m2/g	Total Pore Volume, cm3/g	Mean Pore Diameter, nm	Crystallite Size, nm
CZ	34	0.133	15.5	18.1
RuCZ	40	0.150	15.1	20.4
PdCZ	31	0.130	16.9	21.5
PtCZ	33	0.116	13.4	19.8
KPtCZ	74	0.248	13.5	24.2

. In general, the sharper XRD peaks indicate larger crystallite sizes, whereas the broader peaks correspond to smaller crystallite sizes. The most intense peak of ZrO₂ (111) at 30.1° was merged with Co₃O₄ (220) at 31.2°. Thus, the most intense sharper peak corresponds to Co₃O₄ (311) at 36.8° and was considered for crystallite size using the Debye–Scherrer equation. Interestingly, the crystallite size of KPtCZ was found to be high compared to all the materials. On the contrary, its surface area and pore volumes were also found to be higher. This could be due to the additional chemical reduction step associated with K impregnation. The chemical reduction was able to create more pores and effective K dispersion, which led to achieve enhanced surface area and pore volumes (from Figure 2e and Table 1).

2.2. Scanning Electron Microscopy and Transmission Electron Microscopy Studies

The scanning electron microscope (SEM) images shown in Figure 3 indicates the fine and homogenous particles formation with less porosity. The pore volume of KPtCZ was improved, as shown in Table 1. The elemental analysis using energy dispersive X–ray diffraction (EDX) analysis of the RuCZ, PdCZ, PtCZ and KPtCZ on Figure 4 confirmed the presence of dopants of Ru, Pd, Pt and K.

From Figure 5, ZrO₂ and Co₃O₄ phases were confirmed with HRTEM results by analyzing fringes of the d–spacing of the particles. The large particles indicate the ZrO₂ phase whereas smaller particles were confirmed as Co₃O₄. The presence of Pt (111), Pd (111) and Ru (110) was found in the lattice fringes of the particles. The (111), (222), (220) planes of ZrO₂ and (311), (422) planes of Co₃O₄ were confirmed by analyzing diffraction rings (Figure 5a–f). Figure 5e,h showed very small clusters of Pt, Pd and Ru dispersed in a solid solution. The cluster embedded in the support were around 2–3 nm, as shown in Figure 5e,h,k,n. These clusters shared complete boundary with the support material due to the substitution. This is an advantageous feature to enhance the metal support interaction and keep the noble metal isolated at elevated temperatures by avoiding thermal sintering. The resistance towards sintering and effective dispersion lead to high catalytic activity. The ionic substitution of Pt and Pd was due to the atomically dispersed Pt, Pd and Ru in the support, which can oxidize by utilizing bulk oxygen or oxygen vacancies in the support [39,40]. As the active metals were substituted in the support material, slight deviations in the interplanar spacing values were possible compared to noble metal reference data. Figure 5e,k,n shows the HRTEM images of PtCZ, KPtCZ, PdCZ and RuCZ and the d–spacing of lattice fringes for Pt, PD and Ru were observed as 2.25 Å, 1.94 Å and 2.04 Å, respectively. This d–spacing values were closely matched with the reference Pt (111), Pd (200) and Ru (100) planes.

2.3. Catalytic Activity Studies

The synthesized catalysts were subjected to catalytic activity studies under different feed conditions. To understand the behavior of feed on the surface of CZ composite catalysts, different feed compositions were maintained. The total flow rate of 100 mL/min (at NTP) dry gas flow mixed with superheated steam at 150 °C and introduced to the reactor. Dry gas concentrations of CO: N₂ = 2%: 98% (Feed A), CO: H₂: N₂ = 2%: 35%: 63% (Feed B) and CO: H₂: CO₂: N₂ = 2%: 35%: 14%: 49% (Feed C) were maintained in the catalytic activity studies. The equilibrium analysis was performed by minimizing the Gibbs free energy under different feed conditions. The Peng–Robinson equation of state was implemented to assess the real behavior of the gases. From this analysis, both conversions and selectivities were benchmarked at all operating temperatures under different feeds. Further, two sets of reactions were implemented. In one set, only WGS reaction was implemented, whereas in another set, both CO and CO₂ methanation were considered along with WGS.

From the results shown in Figure 6a, it is evident that the role of side reactions was insignificant for Feed A. The equilibrium conversion at 300 °C was 99.92% and 99.94% with and without side reactions. Though the CO conversions were similar, a significant change in methane selectivity was noticed due to side reactions. At 300 °C, the methane selectivity was found to be 7.3% with side reactions (shown in Figure 6b), whereas no methane selectivity was possible in absence of side reactions. Thus, 100% methane selectivities were obtained for Feed A conditions. Under Feed B (H₂ rich stream) and Feed C (H₂ and CO₂ rich stream), the higher conversions of CO are possible due to side reactions. From Figure 6c,e, the CO conversions were found to be 98.7% and 89.3% at 300 °C for Feed B and C in absence of side reactions. Due to CO and CO₂ methanation, the equilibrium CO conversions were increased to 100% and 99.48% for Feed B and C at 300 °C. High methanation trend was clearly evident from Figure 6d,f as well. For Feed B, the CH₄ selectivities can go up to 100% due to side reactions at temperatures below 325 °C, whereas the CH₄ selectivity was found to be 49.4% at 300 °C and it decreased with temperature under Feed C. The equilibrium conversions and selectivities were compared with the experimental data in Table 2 for benchmarking.

Figure 7a shows the catalytic activity of CZ composite under different feed conditions. CZ composites started showing activity from 220 °C and the conversions increased with temperature. At 300 °C, CZ composites showed CO conversion of 16%, 11% and 8% under different feed conditions (Feed A, B and C). The presence of hydrogen in the feed inhibited the catalytic activity due to its reduction ability. Additionally, the presence of both H₂ and CO₂ in feed further decreases the catalytic activity due to blockage of active sites and allow CO to compete for the adsorption sites [41]. In all three feed conditions, no methanation was observed below 320 °C (from Figure 7b). These experiments provided information about the role of support (CZ composite) for WGS activity within the range of 180–320 °C. The effect of reformate feed condition on RuCZ and PdCZ catalysts is shown in Figure 7c–f. The CO conversions greater than 95% were observed at 320 °C in both Feed A and Feed B conditions for RuCZ, whereas under Feed C condition of CO₂ and H₂ rich feed, the CO conversions increased to a maximum of 84% at 300 °C and continued to decrease as the temperature rose. Interestingly, the methane selectivity was found to be very high prominent under both Feed B and C conditions. For instance, at 320 °C, the methane selectivity was found to be 38% and 47%, respectively, under both Feed B and Feed C conditions. Under Feed C condition, the methane selectivity was continued to increase up to 77% at 400 °C, indicating the methanation ability of the Ru active sites in a catalyst. This could be due to CO₂ methanation resulted from CO₂ rich feed. It indicates that Ru-based catalysts are not potential candidates for suppressing methanation in water–gas shift reaction under reformate feed and our results are in line with Utaka et al. [42] At 320 °C, PdCZ was able to achieve 47%, 73% and 56% of CO conversions for Feed A, B and C, as seen in Figure 7e. These conversions were significantly lower than RuCZ catalyst. Especially under Feed B condition, the CO conversions were high compared to other feed conditions. This was due to high affinity of Pd in dissociating H₂ to atomic hydrogen [37]. However, the atomic hydrogen was more prone to form methane, the methane selectivities were found to be significantly higher in 220–440 °C. The maximum conversions of 96% at 400 °C and 94% at 360 °C were observed on PdCZ catalysts under Feed A and Feed B conditions. The methanation was negligible under Feed A condition but it is significant in other cases. Methane formation was even observed at low temperatures of 220 °C under both Feed A and B conditions. From these studies, PdCZ was not an active catalyst for WGS under both reformate and pure feed conditions. Additionally, it was found to have poor methane suppression ability.

Figure 7g shows the role of Pt for WGS activity under all feed conditions. With PtCZ catalysts, the WGS activity was observed even at 180 °C, unlike CZ composite catalysts that showed minimum activity at 220 °C. A 99% CO conversion was observed at 280 °C under Feed A condition. However, the CO conversion of 95% and 56% were observed at 300 °C under Feed B and C conditions. These conversions were exceptionally higher than PdCZ under all feed conditions, even higher than RuCZ catalyst except for Feed C condition. The presence of H₂ and CO₂ in feed inhibited the catalytic activity and allowed unfavorable side reactions such as methanation to occur. A maximum of 26% of methane selectivity was observed at 300 °C, whereas it was limited to 16% under Feed B condition. Like palladium, Pt is also an excellent catalyst for H₂ dissociation into atomic hydrogen [37]. Thus, the selectivity towards methane was significant, but it was found to be lower compared to PdCZ as well as RuCZ. For the application of PEMFC, the H₂ purity must be high with permissible concentrations of both CO and CH₄ [43]. Thus, minimizing methanation is essential in the WGS step. The catalysts offering methanation activity were not suitable for WGS to target PEMFC application. As PtCZ was found to be the best candidate among RuCZ and PdCZ in achieving high CO conversions and low methane selectivities, a further surface modification on catalyst was performed on PtCZ using K impregnation.

The impregnation of K on PtCZ catalyst suppressed methanation and allowed high CO conversions under all feed conditions, as shown in Figure 7i. ~100% CO conversion and rate of 9.09 μmol g⁻¹ s⁻¹ was observed at 300 °C under Feed A condition, whereas 98% and 93% (8.9 and 8.45 μmol g⁻¹ s⁻¹) were observed under both Feed B and C conditions. The structural features of high surface area and pore volume also complimented its high catalytic activity. The catalytic activity in the presence of H₂ and CO₂ was found to be higher than the feed condition B in the presence of H₂ in feed. Interestingly, methanation was found to be negligible and methane selectivity of <4% was observed with KPtCZ catalyst under Feed B and C conditions and, like all other catalysts, no methane selectivity was observed under Feed A. A similar behavior was even observed in the case of WGS under H₂ rich feed as well as realistic reformate feed conditions, thus promoting the undesired methanation by utilizing H₂ activation. Promoting PtCZ catalyst with K offers electron density on catalysts surface, such a way that to increase the chemisorption affinity towards electron acceptor species such as CO and O₂ [44]. The alkali promoter further suppresses the chemisorption affinity of electron donor species such as H₂ [45]. The catalyst performance in comparison with literature and equilibrium data is shown in Table 2. KPtCZ alone was close to the equilibrium CO conversion and minimum CH₄ selectivity at 300 °C, this indicates KPtCZ as the best candidate for WGS even under reformate feed conditions. All the experimental methane selectivities were found to be well below the maximum selectivity obtained from equilibrium studies.

Table 2. Literature comparison for water–gas shift studies.

Catalyst	Feed Details	Method of Synthesis	Temperature, °C	GHSV, h−1	% Conversion of CO and % CH4, CO2 Selectivity	Reaction Rate, μmol g−1 s−1	Ref.
Mn2.94Pt0.06O4-ö	CO:N2 = 2:98 vol % Flowrate at NTP: 100 mL/min Water flowrate at NTP = 0.1 mL/min 100 mg catalyst	Sonochemical	250	48,000 (dry basis)	XCO = 100% SCH4 = 0 SCO2 = 100%	12.24	[46]
Ti0.84Pt0.01Fe0.15O4-ö	CO:N2 = 2:98 vol % Flowrate at NTP: 100 mL/min Water vapor flowrate at 150 °C = 55 mL/min 300 mg catalyst	Sonochemical	280	48,000 (dry basis)	XCO = 56% SCH4 = 0 SCO2 = 100%	2.59	[47]
1% Pt/Ceria	CO:H2O:H2:N2 = 1.5:50:40:8.5 vol % 300 mg catalyst	Incipient wet impregnation	325	-	XCO = 56% SCH4 = - SCO2 = -	-	[48]
Pd/Ce0.83Zr0.15O2	CO:CO2:H2:N2 = 2:10:40:48 vol% Flowrate at NTP: 100 mL/min Water flowrate at NTP = 0.1 mL/min 500 mg catalyst	Solution combustion	250	95,000 (dry basis)	XCO = 94% SCH4 = - SCO2 = -	-	[49]
Ce0.93Zn0.05Pt0.02O2-ö	CO:N2 = 2:98 vol% Flowrate at NTP: 100 mL/min Water flowrate at NTP = 0.05 mL/min 200 mg catalyst	Sol-gel	300	48,000 (dry basis)	XCO = 94% SCH4 = - SCO2 = -	8.33	[27]
RuCZ (Feed A)	CO:N2 = 2:98 vol% Flowrate at NTP: 100 mL/min Water flowrate at NTP = 0.05 mL/min 150 mg catalyst	Sonochemical	320	48,000 (dry basis)	XCO = 92% SCH4 = 0% SCO2 = 100%	8.36	Present Study
RuCZ (Feed B)	CO:H2:N2 = 2:35:63 vol% Flowrate at NTP: 100 mL/min Water flowrate at NTP = 0.05 mL/min 150 mg catalyst	Sonochemical	320	48,000 (dry basis)	XCO = 96% SCH4 = 37% SCO2 = 63%	8.72	Present Study
RuCZ (Feed C)	CO:H2:CO2:N2 = 2:35:14:49 vol% Flowrate at NTP: 100 mL/min Water flowrate at NTP = 0.05 mL/min 150 mg catalyst	Sonochemical	320	48,000 (dry basis)	XCO = 84% SCH4 = 47% SCO2 = 53%	7.63	Present Study
PdCZ (Feed A)	CO:N2 = 2:98 vol% Flowrate at NTP: 100 mL/min Water flowrate at NTP = 0.05 mL/min 150 mg catalyst	Sonochemical	320	48,000 (dry basis)	XCO = 47% SCH4 = 0% SCO2 = 100%	4.27	Present Study
PdCZ (Feed B)	CO:H2:N2 = 2:35:63 vol% Flowrate at NTP: 100 mL/min Water flowrate at NTP = 0.05 mL/min 150 mg catalyst	Sonochemical	320	48,000 (dry basis)	XCO = 73% SCH4 = 18% SCO2 = 82%	6.63	Present Study
PdCZ (Feed C)	CO:H2:CO2:N2 = 2:35:14:49 vol% Flowrate at NTP: 100 mL/min Water flowrate at NTP = 0.05 mL/min 150 mg catalyst	Sonochemical	320	48,000 (dry basis)	XCO = 56% SCH4 = 13% SCO2 = 87%	5.09	Present Study
PtCZ (Feed A)	CO:N2 = 2:98 vol% Flowrate at NTP: 100 mL/min Water flowrate at NTP = 0.05 mL/min 150 mg catalyst	Sonochemical	300	48,000 (dry basis)	XCO = 100% SCH4 = 0% SCO2 = 100%	9.09	Present Study
PtCZ (Feed B)	CO:H2:N2 = 2:35:63 vol% Flowrate at NTP: 100 mL/min Water flowrate at NTP = 0.05 mL/min 150 mg catalyst	Sonochemical	300	48,000 (dry basis)	XCO = 96% SCH4 = 17% SCO2 = 83%	8.72	Present Study
PtCZ (Feed C)	CO:H2:CO2:N2 = 2:35:14:49 vol% Flowrate at NTP: 100 mL/min Water flowrate at NTP = 0.05 mL/min 150 mg catalyst	Sonochemical	300	48,000 (dry basis)	XCO = 57% SCH4 = 26% SCO2 = 74%	5.18	Present Study
KPtCZ (Feed A)	CO:N2 = 2:98 vol% Flowrate at NTP: 100 mL/min Water flowrate at NTP = 0.05 mL/min 150 mg catalyst	Sonochemical	300	48,000 (dry basis)	XCO = 100% SCH4 = 0% SCO2 = 100%	9.09	Present Study
KPtCZ (Feed B)	CO:H2:N2 = 2:35:63 vol% Flowrate at NTP: 100 mL/min Water flowrate at NTP = 0.05 mL/min 150 mg catalyst	Sonochemical	300	48,000 (dry basis)	XCO = 98% SCH4 = 4% SCO2 = 96%	8.9	Present Study
KPtCZ (Feed C)	CO:H2:CO2:N2 = 2:35:14:49 vol% Flowrate at NTP: 100 mL/min Water flowrate at NTP = 0.05 mL/min 150 mg catalyst	Sonochemical	300	48,000 (dry basis)	XCO = 93% SCH4 = 3.3% SCO2 = 96.7%	8.45	Present Study
Feed A	CO:N2 = 2:98 vol% Flowrate at NTP: 100 mL/min Water flowrate at NTP = 0.05 mL/min	No Catalyst	300	-	(with WGS alone) XCO = 99.94% SCH4 = 0% SCO2 = 100% (with side reactions) XCO = 99.92% SCH4 = 7.3% SCO2 = 92.7%	-	Equilibrium
Feed B	CO:H2:N2 = 2:35:63 vol% Flowrate at NTP: 100 mL/min Water flowrate at NTP = 0.05 mL/min	No Catalyst	300	-	(with WGS alone) XCO = 98.7% SCH4 = 0% SCO2 = 100% (with side reactions) XCO = 100% SCH4 = 100% SCO2 = 0%	-	Equilibrium
Feed C	CO:H2:CO2:N2 = 2:35:14:49 vol% Flowrate at NTP: 100 mL/min Water flowrate at NTP = 0.05 mL/min	No Catalyst	300	-	(with WGS alone) XCO = 89.3% SCH4 = 0% SCO2 = 100% (with side reactions) XCO = 99.48% SCH4 = 49.4% SCO2 = 50.6% (with WGS alone)	-	Equilibrium

Table 2. Literature comparison for water–gas shift studies.

Catalyst	Feed Details	Method of Synthesis	Temperature, °C	GHSV, h−1	% Conversion of CO and % CH4, CO2 Selectivity	Reaction Rate, μmol g−1 s−1	Ref.
Mn2.94Pt0.06O4-ö	CO:N2 = 2:98 vol % Flowrate at NTP: 100 mL/min Water flowrate at NTP = 0.1 mL/min 100 mg catalyst	Sonochemical	250	48,000 (dry basis)	XCO = 100% SCH4 = 0 SCO2 = 100%	12.24	[46]
Ti0.84Pt0.01Fe0.15O4-ö	CO:N2 = 2:98 vol % Flowrate at NTP: 100 mL/min Water vapor flowrate at 150 °C = 55 mL/min 300 mg catalyst	Sonochemical	280	48,000 (dry basis)	XCO = 56% SCH4 = 0 SCO2 = 100%	2.59	[47]
1% Pt/Ceria	CO:H2O:H2:N2 = 1.5:50:40:8.5 vol % 300 mg catalyst	Incipient wet impregnation	325	-	XCO = 56% SCH4 = - SCO2 = -	-	[48]
Pd/Ce0.83Zr0.15O2	CO:CO2:H2:N2 = 2:10:40:48 vol% Flowrate at NTP: 100 mL/min Water flowrate at NTP = 0.1 mL/min 500 mg catalyst	Solution combustion	250	95,000 (dry basis)	XCO = 94% SCH4 = - SCO2 = -	-	[49]
Ce0.93Zn0.05Pt0.02O2-ö	CO:N2 = 2:98 vol% Flowrate at NTP: 100 mL/min Water flowrate at NTP = 0.05 mL/min 200 mg catalyst	Sol-gel	300	48,000 (dry basis)	XCO = 94% SCH4 = - SCO2 = -	8.33	[27]
RuCZ (Feed A)	CO:N2 = 2:98 vol% Flowrate at NTP: 100 mL/min Water flowrate at NTP = 0.05 mL/min 150 mg catalyst	Sonochemical	320	48,000 (dry basis)	XCO = 92% SCH4 = 0% SCO2 = 100%	8.36	Present Study
RuCZ (Feed B)	CO:H2:N2 = 2:35:63 vol% Flowrate at NTP: 100 mL/min Water flowrate at NTP = 0.05 mL/min 150 mg catalyst	Sonochemical	320	48,000 (dry basis)	XCO = 96% SCH4 = 37% SCO2 = 63%	8.72	Present Study
RuCZ (Feed C)	CO:H2:CO2:N2 = 2:35:14:49 vol% Flowrate at NTP: 100 mL/min Water flowrate at NTP = 0.05 mL/min 150 mg catalyst	Sonochemical	320	48,000 (dry basis)	XCO = 84% SCH4 = 47% SCO2 = 53%	7.63	Present Study
PdCZ (Feed A)	CO:N2 = 2:98 vol% Flowrate at NTP: 100 mL/min Water flowrate at NTP = 0.05 mL/min 150 mg catalyst	Sonochemical	320	48,000 (dry basis)	XCO = 47% SCH4 = 0% SCO2 = 100%	4.27	Present Study
PdCZ (Feed B)	CO:H2:N2 = 2:35:63 vol% Flowrate at NTP: 100 mL/min Water flowrate at NTP = 0.05 mL/min 150 mg catalyst	Sonochemical	320	48,000 (dry basis)	XCO = 73% SCH4 = 18% SCO2 = 82%	6.63	Present Study
PdCZ (Feed C)	CO:H2:CO2:N2 = 2:35:14:49 vol% Flowrate at NTP: 100 mL/min Water flowrate at NTP = 0.05 mL/min 150 mg catalyst	Sonochemical	320	48,000 (dry basis)	XCO = 56% SCH4 = 13% SCO2 = 87%	5.09	Present Study
PtCZ (Feed A)	CO:N2 = 2:98 vol% Flowrate at NTP: 100 mL/min Water flowrate at NTP = 0.05 mL/min 150 mg catalyst	Sonochemical	300	48,000 (dry basis)	XCO = 100% SCH4 = 0% SCO2 = 100%	9.09	Present Study
PtCZ (Feed B)	CO:H2:N2 = 2:35:63 vol% Flowrate at NTP: 100 mL/min Water flowrate at NTP = 0.05 mL/min 150 mg catalyst	Sonochemical	300	48,000 (dry basis)	XCO = 96% SCH4 = 17% SCO2 = 83%	8.72	Present Study
PtCZ (Feed C)	CO:H2:CO2:N2 = 2:35:14:49 vol% Flowrate at NTP: 100 mL/min Water flowrate at NTP = 0.05 mL/min 150 mg catalyst	Sonochemical	300	48,000 (dry basis)	XCO = 57% SCH4 = 26% SCO2 = 74%	5.18	Present Study
KPtCZ (Feed A)	CO:N2 = 2:98 vol% Flowrate at NTP: 100 mL/min Water flowrate at NTP = 0.05 mL/min 150 mg catalyst	Sonochemical	300	48,000 (dry basis)	XCO = 100% SCH4 = 0% SCO2 = 100%	9.09	Present Study
KPtCZ (Feed B)	CO:H2:N2 = 2:35:63 vol% Flowrate at NTP: 100 mL/min Water flowrate at NTP = 0.05 mL/min 150 mg catalyst	Sonochemical	300	48,000 (dry basis)	XCO = 98% SCH4 = 4% SCO2 = 96%	8.9	Present Study
KPtCZ (Feed C)	CO:H2:CO2:N2 = 2:35:14:49 vol% Flowrate at NTP: 100 mL/min Water flowrate at NTP = 0.05 mL/min 150 mg catalyst	Sonochemical	300	48,000 (dry basis)	XCO = 93% SCH4 = 3.3% SCO2 = 96.7%	8.45	Present Study
Feed A	CO:N2 = 2:98 vol% Flowrate at NTP: 100 mL/min Water flowrate at NTP = 0.05 mL/min	No Catalyst	300	-	(with WGS alone) XCO = 99.94% SCH4 = 0% SCO2 = 100% (with side reactions) XCO = 99.92% SCH4 = 7.3% SCO2 = 92.7%	-	Equilibrium
Feed B	CO:H2:N2 = 2:35:63 vol% Flowrate at NTP: 100 mL/min Water flowrate at NTP = 0.05 mL/min	No Catalyst	300	-	(with WGS alone) XCO = 98.7% SCH4 = 0% SCO2 = 100% (with side reactions) XCO = 100% SCH4 = 100% SCO2 = 0%	-	Equilibrium
Feed C	CO:H2:CO2:N2 = 2:35:14:49 vol% Flowrate at NTP: 100 mL/min Water flowrate at NTP = 0.05 mL/min	No Catalyst	300	-	(with WGS alone) XCO = 89.3% SCH4 = 0% SCO2 = 100% (with side reactions) XCO = 99.48% SCH4 = 49.4% SCO2 = 50.6% (with WGS alone)	-	Equilibrium

The experimental data in the given conditions follows the Weisz–Prater correlation [50] for mass transfer limitation, indicating the validity of differential flow reactor assumption. Thus, the reaction rates were calculated by, $r_{CO}\hspace{0.33em}=\hspace{0.33em}\frac{X_{CO}}{\left(\raisebox{1ex}{$W$}\!\left/ \!\raisebox{-1ex}{$F_{CO}$}\right.\right)}$. Figure 8 shows the logarithm of rate (obtained from the slope of CO fractional conversion versus $\raisebox{1ex}{$W$}\!\left/ \!\raisebox{-1ex}{$F_{CO}$}\right.$) with reciprocal of temperature, which follows the Arrhenius relationship, and the activation energies were calculated from the slopes of the linear fittings. The activation energy of KPtCZ catalyst under reformate feed condition C was found to be lowest (66 kJ mol⁻¹) compared with all other catalysts. Even under Feed B condition, the activation energy of KPtCZ was 78 kJ mol⁻¹, which was almost close to the activation energy of PtCZ. Overall, KPtCZ was found to be the best catalyst for WGS activity under reformate feed conditions. In our previous studies, the mechanistic studies of CO abatement on CO₃O₄ catalysts indicated the carbonates as key intermediates during the reaction and its dissociation to CO₂ was critical [51,52]. Under H₂ rich conditions, formate and hydroxyl species were also detected [52]. K impregnation suppresses the chemisorption affinity of electron donor species such as H₂ and allows scission of C–H bond in formate species [35,53,54]. Thus, in the present study, CO and CO₂ methanation reactions may be insignificant on the KPtCZ catalyst, leading to very low methane selectivities, as shown in Figure 7j. The lowest activation energy of KPtCZ under Feed C condition further indicates its high catalytic ability and successful resistance towards methanation. The activation energies and maximum CO conversions of the catalysts are listed in

Table 3. Apparent activation energy comparison with literature.

Catalyst	Maximum CO Conversion	Apparent Activation Energy, Ea kJ/mol	Temperature Range and Flow Rate, °C and mL/min	Catalyst Loading, mg	Ref.
Pt (NO)/CeO2	~92	~73.84	250–400, 100	200	[55]
Pt/SrHAP-11	~95	70	250–450, 100	75	[56]
Cu/Pd-Ag	~95	10	200–300, 190	1500	[57]
Pt-Re (2:3)	~90	20	250, 500	120	[20]
Pd/Ceria	-	49	180	100	[58]
Mn2.94Pt0.06O4-ö	100	59	180–450	100	[46]
CZ (Feed A)	~28	70 ± 5	180–300, 100	150	Present Study
CZ (Feed B)	~16	140 ± 7	180–300, 100	150	Present Study
CZ (Feed C)	~12	78 ± 4	180–300, 100	150	Present Study
PtCZ (Feed A)	~100	63 ± 2	180–300, 100	150	Present Study
PtCZ (Feed B)	~98	75 ± 3	180–300, 100	150	Present Study
PtCZ (Feed C)	~50	72 ± 3	180–300, 100	150	Present Study
KPtCZ (Feed A)	~100	75 ± 4	180–300, 100	150	Present Study
KPtCZ (Feed B)	~100	78 ± 5	180–300, 100	150	Present Study
KPtCZ (Feed C)	~90	66 ± 3	180–300, 100	150	Present Study

.

2.4. X-ray Photoelectron Spectroscopy Studies

Figure 9a shows Co 2p spectra of the catalyst. The presence of Co₃O₄ phase as well as the presence of negligible CoO phase was observed. The ratio of Co³⁺/Co²⁺ was found to be 2:1. The binding energies of core level Co 2p_3/2, 2p_1/2 peaks were observed at 779.7 eV, 794.8 eV for Co³⁺ and 781.3 eV, 796.4 eV for Co²⁺. The difference between Co 2p_3/2 and Co 2p_1/2 of Co³⁺ and Co²⁺ was 15.1 eV [59,60]. Co 2p_3/2 and Co 2p_1/2 shakeup peaks were observed at 783.7 and 797.9 eV, a peak corresponded to 789.4 eV in Figure 8a, which was because of the CoO satellite [61]. Figure 9b shows the Zr 3d spectra and deconvoluted according to 3d_5/2 and 3d_3/2 split orbitals. The peaks at 181.7 eV and 184.1 eV were assigned to 3d_5/2 and 3d_3/2 [62]. In KPtCZ catalyst, Pt was found in only +4 ionic states, indicating the ionic substitution (as shown in Figure 9c) and formation PtO₂ clusters. No metallic Pt was detected in KPtCZ. Deconvoluted spectra showed 74.3 eV and 77.6 eV for Pt 4f_7/2 and Pt 4f_5/2 core level spectra. From Figure 9, K 2p_3/2 and K 2p_1/2 core level peaks were observed at 292.6 eV and 295.4 eV, respectively, thus confirming the potassium presence.

2.5. H₂–Temperature Programming Reduction (H₂–TPR) Analysis

As shown in Figure 10, the TPR profiles started at ~202 °C in the case of CZ, but the starting temperatures were 102 °C, 107 °C and 120 °C with PtCZ, PdCZ and RuCZ. However, the reduction peak started at 95 °C due to K deposition in the KPtCZ catalyst. RuCZ and PdCZ had a wide reduction over the temperature range of 100–900 °C [63]. This could be due to overlap with Co₃O₄ reduction peaks, which is usually limited to temperatures below 450 °C. Pd, Pt and Ru incorporation in metal oxide caused structural changes and enhanced the oxygen mobility for the reduction in supports [64,65]. The reduction peaks of Co₃O₄ also shifted to low temperatures compared with the reduction peaks in the CZ composite. The broad reduction peaks were observed at 352 °C and 358 °C for PdCZ and RuCZ. All these supports had wide reduction distribution over a temperature range up to 900 °C. However, in the case of KPtCZ similar behavior was not observed. As K impregnation itself having a chemical reduction step, the reduction peaks were noticed at 141 °C and 417 °C, which were lower temperatures than for PtCZ. This could be another reason for the good catalytic performance of KPtCZ for WGS than other catalysts, as the reduction lead to more lattice oxygen vacancies as well. Even for high temperature WGS, the operating temperatures fell below 500 °C. All synthesized catalysts showed reducible behavior greater than 600 °C, indicating its ability towards WGS. As evident from Figure 7, KPtCZ is a potential candidate for low temperature WGS.

3. Materials and Methods

3.1. Materials

Cobaltous nitrate hexahydrate (Co(NO₃)₂·6H₂O, 98%), zirconium(IV) oxynitrate hydrate (ZrO(NO₃)₂·xH₂O, 99%), palladium chloride (PdCl₂, 99%), hexachloroplatinic acid (H₂PtCl₆, ~40%) and ruthenium chloride trihydrate (RuCl₃·3H₂O, 99%) are procured from Sigma-Aldrich. Diethylene triamine (DETA), ammonia solution and polyethylene glycol powder (PEG-600) were procured from SD Fine chemicals, Maharashtra, India.

3.2. Catalyst Synthesis

M/Co₃O₄–ZrO₂ (MCZ, M = Pt, Pd and Ru) composite catalysts were synthesized by sonochemical synthesis. The synthesis protocols for PtCZ and PdCZ were adopted from our previous work [37]. A similar protocol was implemented to prepare RuCZ. In a typical synthesis, 100 mL of 1.2 mM Co(NO₃)₂·6H₂O was added dropwise to 65 mL of 1.2 mM of ZrO(NO₃)₂·xH₂O under rigorous stirring. 2 g of PEG−600 was added to the solution and allowed 30 min stirring. 22 mg of RuCl₃ dissolved in 10 mL and added dropwise to the solution. 5 mL of diethylene triamine (DETA) was added to the solution to ensure effective doping. To this solution 0.1 M NH₄OH was added to maintain the pH of 8 under ultrasonic radiation (25 kHz, 125 W, Electrosonic Industries, Mumbai, India). The effective sonication continued for 3 h and the precipitate was separated by centrifugation. The collected precipitate was washed thrice with ethanol and dried at 120 °C for 6 h and kept for calcination at 500 °C for 2 h. The effect of K as promoter was studied for WGS activity on the best performing catalyst (PtCZ). 0.5 g of PtCZ was taken and dispersed in 500 mL of water. Under rigorous stirring, 50 mL of 5.2 mM of KOH solution was slowly added and then reduced using 2 mL of hydrazine hydrate. Further, the catalyst was allowed to mix for 12 h, the suspended nanoparticles were separated and dried on a hot plate at 120 °C for 12 h.

3.3. WGS Experimental Conditions

The catalysts were made into pellets using hydraulic press and were then crushed and a 60/80 mesh size was collected using sieves. In a typical experiment, 150 mg catalyst was packed with ceramic glass wool plugs, at the center of 4 mm ID quartz reactor. The bed length of 1 cm was maintained by diluting the catalysts with silica. The catalyst loading varied from 50 to 200 mg. To study the effect of reformate gas mixture, different feed conditions with dry gas volume ratios of CO:N₂ = 2:98 (Feed A), CO:H₂:N₂ = 2:35:63 (Feed B) and CO:H₂:CO₂:N₂ = 2:35:14:49 (Feed C) were used at 1 atm. The dry gas flow rate was maintained at 100 mL/min. Then, 0.05 mL/min of water was passed using a HPLC pump through the boiler maintained at 150 °C and superheated steam to the reactor along with the dry feed. Isothermal reaction conditions were maintained at each temperature in the range of 150–400 °C and product gases were analyzed at steady state operation of 30 min. A condenser was connected at the exit of the reactor to remove the steam and the dry gases alone were allowed to pass through online gas chromatograph to analyze the outlet gas composition. The conversion of CO and selectivity of undesirable product methane were calculated using the following formulae,

$$\%\hspace{0.33em}CO\hspace{0.33em}Conversion\hspace{0.33em}=\hspace{0.33em}\left(\frac{F_{CO,\hspace{0.33em}in}\hspace{0.33em}-\hspace{0.33em}{F}_{CO,\hspace{0.33em}out}}{F_{CO,\hspace{0.33em}in}}\right)\times 100$$

(2)

$$\%\hspace{0.33em}C{H}_4\hspace{0.33em}Selectivity\hspace{0.33em}=\hspace{0.33em}\left(\frac{F_{C{H}_4,\hspace{0.33em}out}}{F_{C{H}_4,\hspace{0.33em}out}\hspace{0.33em}+\hspace{0.33em}{F}_{C{O}_2,\hspace{0.33em}out}}\right)\times 100$$

(3)

$$\%\hspace{0.33em}C{O}_2\hspace{0.33em}Selectivity\hspace{0.33em}=\hspace{0.33em}\left(\frac{F_{C{O}_2,\hspace{0.33em}out}}{F_{C{H}_4,\hspace{0.33em}out}\hspace{0.33em}+\hspace{0.33em}{F}_{C{O}_2,\hspace{0.33em}out}}\right)\times 100$$

(4)

Here, $F_{CO,\hspace{0.33em}in}$ and $F_{CO,\hspace{0.33em}out}$ are the molar flowrates of CO at the inlet and exit of the reactor, $F_{C{H}_4,\hspace{0.33em}out}$ and $F_{C{O}_2,\hspace{0.33em}out}$ are the exit molar flowrates of CH₄ and CO₂.

3.4. Characterization Methods

All the catalysts were characterized using XRD, XPS, TEM and BET. X-ray diffraction patterns were collected by using Rigaku X-ray diffraction equipment with 0.35°/min in the scan range of 20–80° with a Cu Kα source. XPS data were collected using AXIS ULTRA instrument with Al-Kα as radiation source and all samples were calibrated using graphic carbon at 284.8 eV. A surface area analyzer (Microtrac Bel, BELSORP Mini II, Osaka, Japan) was used to determine the surface properties of the catalysts at 77 K using liquid N₂. The scanning electron micrographs (SEM) and energy dispersive X-ray diffraction (EDX) studies for all catalysts were recorded with a high vacuum and 20 kV using an APREOS field-emission scanning electron microscope (ThermoFisher Scientific, Waltham, MA, USA). TEM micrographs for the prepared catalysts were recorded by using FEI F30 instrument (FEI Tecnai F30, Atlanta, GA, USA) operating at 200 kV.

3.5. H₂-Temperature Programmed Reduction

Temperature programmed reduction (TPR) with H₂ was performed with 10 mg of catalyst loaded in a quartz reactor. A flow rate of 30 mL/min of 5% H₂ in Ar (Chemix gases, Karnataka, India) was maintained in temperature range of 50 to 900 °C with a ramping rate of 10 °C/min using a PID controller. Hydrogen consumption during the reduction was analyzed with thermal conductivity detector connected (Mayura Analytical Private Limited, Karnataka, India) to outlet stream.

4. Conclusions

The present study focused on the water–gas shift activity of noble metal-substituted Co₃O₄–ZrO₂ (CZ) support under reformate feed conditions. To obtain maximum yield of H₂, the side reactions such as CO and CO₂ methanation must be avoided in water–gas shift stage. However, it is a challenging task to suppress the methanation under reformate feed (H₂ and CO₂ rich feed) conditions. In this study, we developed CZ, RuCZ, PdCZ, PtCZ by sonochemical synthesis. The electron donor compound, potassium, was impregnated on the surface of PtCZ supports to suppress the methanation with high CO conversions at low temperatures. The best catalyst, PtCZ, showed high CO conversions of 100% and rate of 9.09 μmol g⁻¹ s⁻¹ at 300 °C. However, the conversions were observed as 96% and 57% CO conversions at 300 °C with prominent methane selectivity of 17% and 26% under Feed B and C conditions, whereas the KPtCZ catalyst showed 100% and 98% of CO conversions and rates of 9.09 and 8.9 μmol g⁻¹ s⁻¹ at 300 °C and no significant methane selectivity (<4%) was observed. The H₂ activation on the catalyst surface can be negligible due to K impregnation and, furthermore, it reduced the methane formation. By tuning the surface properties, stable conversions were achieved. On overall, PtCZ and KPtCZ were found to be potential candidates for low temperature water–gas shift reaction; however, KPtCZ was found to be best, with the lowest activation energy of 66 ± 3 kJ mol⁻¹ under reformate feed (H₂ and CO₂ rich feed) conditions.