Single and Dual Metal Oxides as Promising Supports for Carbon Monoxide Removal from an Actual Syngas: The Crucial Role of Support on the Selectivity of the Au–Cu System

Abstract

A catalytic screening was performed to determine the effect of the support on the performance of an Au–Cu based system for the removal of CO from an actual syngas. First, a syngas was obtained from reforming of ethanol. Then, the reformer outlet was connected to a second reactor, where Au–Cu catalysts supported on several single and dual metal oxides (i.e., CeO₂, SiO₂, ZrO₂, Al₂O₃, La₂O₃, Fe₂O₃, CeO₂-SiO₂, CeO₂-ZrO₂, and CeO₂-Al₂O₃) were evaluated. AuCu/CeO₂ was the most active catalyst due to an elevated oxygen mobility over the surface, promoting CO₂ formation from adsorption of C–O* and OH⁻ intermediates on Au⁰ and CuO species. However, its lower capacity to release the surface oxygen contributes to the generation of stable carbon deposits, which lead to its rapid deactivation. On the other hand, AuCu/CeO₂-SiO₂ was more stable due to its high surface area and lower formation of formate and carbonate intermediates, mitigating carbon deposits. Therefore, use of dual supports could be a promising strategy to overcome the low stability of AuCu/CeO₂. The results of this research are a contribution to integrated production and purification of H₂ in a compact system.

1. Introduction

Synthesis gas (syngas) is used as a chemical building block in the synthesis of commodity chemicals and for energy applications. Specifically, syngas can be used in combustion processes [1], gas turbines [2], or hydrogen fuel cells (H₂-FC) [3] to produce energy. The H₂-FC are promising systems to provide sustainable energy for households, industry, transportation, and small devices. Likewise, the use of H₂-FC has been proposed as an alternative to supply energy in places that are not connected to the electrical network and for remote installations [4].

The syngas composition varies depending on the production source, but mostly contains H₂, carbon monoxide (CO), and light hydrocarbons. Bioethanol reforming is one of the most used pathways to produce syngas due to its high yield to H₂ [5]. In a previous study [6], we obtained a syngas containing H₂, CO, CO₂, CH₄, and H₂O from ethanol steam reforming (ESR) using a RhPt/CeO₂-SiO₂ catalyst. Syngas production remained stable for 72 h of continuous operation and on/off cycles. This syngas could be used for sustainable energy production in H₂-FC. However, CO must be removed from the syngas because of its harmful effect on fuel cell electrodes [7].

One of the most used strategies of CO removal from syngas is via chemical pathways, which includes preferential oxidation of CO (CO-PROX) [8,9], water gas shift reaction (WGSR) [10], and selective CO methanation (CO-SMET) [10]. Traditionally, the objective of the CO cleanup step is to ensure CO concentrations below 10 ppm, which requires several catalytic reactors in series [11] and presents a high operating cost. However, recent research studies have allowed the development of H₂-FC systems that tolerate CO concentrations above 100 ppm [12,13,14]. These contributions facilitate the use of less complex systems for syngas purification, which could lead to the development of more compact and economic H₂ technology.

Anticipating the commercialization of a new generation of more CO-tolerant H₂-FC, it has been proposed to redesign the CO removal stage to reduce the number of process units in syngas purification. The new approach seeks to carry out CO removal using a single catalytic reactor, where several reactions occur simultaneously (i.e., CO-PROX, WGSR, and CO-SMET). Kugai et al. [15] studied Pt–Cu and Pd–Cu bimetallic catalysts supported on CeO₂ for oxygen-enhanced water gas shift (OWGS), where WGSR and CO-PROX occur concurrently, reporting higher CO removal from a model reformate gas (synthetic syngas) in the OWGS compared to the WGSR carried out individually. Similarly, Xu and Zhang [16] reported that the presence of CO-SMET during CO-PROX on a commercial Ru/Al₂O₃ catalyst allows for wider temperature windows that ensure the CO removal of a synthetic syngas. Despite these valuable contributions, the CO removal from syngas in a compact system is still at laboratory scale. Among the limitations for evaluation at the pilot scale is the lack of consensus regarding the catalyst and the most appropriate operating conditions to carry out the syngas purification.

Au is recognized as a promising catalyst in the three cleaning reactions of syngas (i.e., CO-PROX, WGSR, and CO-SMET) [17,18]. Reina et al. [19] evaluated bimetallic catalysts of Au–M (M = La, Ni, Cu, Fe, Cr, Y), reporting that CO oxidation is favored by the Au–Cu combination because Cu interacts strongly with the support, favoring the oxygen mobility in the catalyst. Also, in a previous study [20], we evaluated Au–Cu bimetallic catalysts supported on CeO₂ for CO removal from a syngas obtained from ESR. It was possible to reduce the CO concentration below 100 ppm, but the catalyst showed rapid deactivation after 40 h. Deactivation was related to structural changes in the support and to the accumulation of carbonaceous compounds during continuous operation. Thus, this study illustrated that the support plays a key role in CO removal from an actual syngas, and led us to evaluate different supports for CO removal from a syngas in the search for a stable material.

Figure 1 shows the supports most used in the CO removal processes (i.e., WGSR, CO-PROX, CO-SMET, or their combinations). CeO₂, Fe₂O₃, ZrO₂, TiO₂, and Al₂O₃ are the most commonly used single supports in CO removal from synthetic syngas. However, there is a growing interest in mixed supports (dual metal oxides), because they may have characteristics not observed in individual supports [21]. Most combinations of dual metal oxides include CeO₂ in the matrix, usually combined with supports that provide larger surface area, such as Al₂O₃ [22] and SiO₂ [23], or with basic oxides, such as ZrO₂, to generate new active sites [24]. TiO₂ is mainly used in CO removal by photocatalytic processes [25] and was not considered in this study. On the other hand, although La₂O₃ is not among the most used supports in CO removal, it was recently reported that La₂O₃ is effective for avoiding carbon deposits during CO-SMET [26].

Although several supports for syngas cleanup have been proposed, each investigation was carried out under different experimental conditions and using synthetic syngas, which makes it difficult to select the most suitable support for the CO removal. Therefore, the objective of this work was to study the CO removal from an actual syngas using bimetallic catalysts of AuCu-supported on single and dual metal oxides. Specifically, CeO₂, ZrO₂, La₂O₃, Fe₂O₃, Al₂O₃, and SiO₂ were selected as single metal oxides, and CeO₂-SiO₂, CeO₂-ZrO₂, and CeO₂-Al₂O₃ as dual metal oxides. The catalytic performance of the supports with and without active metals (i.e., Au and Cu) was evaluated. Then, the activity, selectivity, and stability were established as criteria for selecting the most suitable support for the CO elimination. In addition, characterization tests were conducted, such as temperature programmed reduction (TPR), surface area tests using the Brunauer-Emmett-Teller (BET) method, oxygen storage capacity (OSC) tests, thermogravimetric analysis (TGA), and in situ diffuse reflectance infrared Fourier transform spectroscopy (DRIFTS).

2. Results and Discussion

2.1. Activity, Selectivity, and Stability

Figure 2 shows the CO conversion in the cleanup reactor on the bare supports (i.e., without Au and Cu) and Au–Cu-supported catalysts. CeO₂ and ZrO₂ display the larger CO conversion between single metal oxides (Figure 2a). Indeed, the presence of oxygen vacancies on the surface of an oxide could favor a support showing high activity in the CO oxidation, despite the absence of active metals [27]; on the other hand, supports with low OSC, such as Al₂O₃ [28], present lower activity. The use of dual metal oxides has been proposed as a strategy to overcome the deficiencies of single supports [21]. Figure 2b shows that CeO₂-SiO₂ increases the CO conversion compared to SiO₂, which could be associated with the interaction between the two oxides. However, no significant improvement in the CO conversion with CeO₂-Al₂O₃ was observed, and even for CeO₂-ZrO₂, the combination of the two metal oxides leads to a less active material. Furthermore, below 260 °C the dual metal oxides showed less activity that single CeO₂, suggesting that the combination of several metal oxides does not always lead to more active materials in the syngas cleaning.

On the other hand, catalytic systems based on Au, Cu and Au–Cu have been studied extensively for the CO oxidation, CO-PROX, WGSR, and CO-SMET. In-depth descriptions for Cu/CeO₂ [9], AuCu/CeO₂ [29], AuCu/SiO₂ [30], AuCu/Al₂O₃ [31], Au/Fe₂O₃ [32], Au/La₂O₃/Al₂O₃ [33], and Au/CeO₂-ZrO₂ [34,35] are available in the literature. In general, Au favors the CO conversion through a mechanism that involves Au–CO and Au–OOH species [36], where the formation of C–O* intermediates determines the selectivity of the process [20], while CuO acts through a redox mechanism [8], promoting oxygen mobility in the oxide lattice [37] and facilitating the CO oxidation. A synergistic Au–Cu effect has also been proposed [19,29,38]. Therefore, the inclusion of 1 wt% Au and 1 wt% Cu in the single and dual metal oxides promotes greater CO conversion (Figure 2). Despite having the same active metals (i.e., Au and Cu), the catalysts showed maximum CO conversion at different temperatures, indicating that the properties of the support have a key role in the syngas cleaning.

Table 1. Minimum concentration of CO obtained in syngas, apparent active metal dispersion (H/M ratio), surface area, OSC, and OSCC of Au–Cu catalysts supported on single and dual supports.

Catalysta	Minimum CO Concentration in Outlet Gas (ppm)b	H/M Index	BET Surface Area (m2/gcat)		OSC in AC Samples (μmol O2/gcat)		OSCC at 300 °C (μmol O2/gcat)
			AC	Spent	100 °C	300 °C	Fresh	Spent
AuCu/CeO2	75 at 210 °C	0.9	60	58 (U)	41	91	230	121 (U)
				50 (S)				93 (S)
AuCu/SiO2	8320 at 240 °C	0.7	364	277 (U)	21	37	45	41 (U)
AuCu/ZrO2	507 at 225 °C	0.8	58	47 (U)	39	76	185	84 (U)
AuCu/Al2O3	745 at 180 °C	0.8	90	65 (U)	31	35	75	41 (U)
AuCu/La2O3	5365 at 225 °C	0.4	19	18 (U)	21	41	90	24 (U)
AuCu/Fe2O3	9416 at 140 °C	0.4	16	5 (U)	NR	NR	NR	NR
AuCu/CeO2-SiO2	861 at 230 °C	1.6	110	75 (U)	34	78	146	121 (U)
				74 (S)				126 (S)
AuCu/CeO2-ZrO2	941 at 210 °C	0.9	42	30 (U)	42	94	210	162 (U)
AuCu/CeO2-Al2O3	1521 at 260 °C	1.2	65	56 (U)	32	79	155	121 (U)

^a Nominal metal loadings: 1 wt% Au and 1 wt% Cu. ^b Value includes the carrier gas. AC: activated catalyst, which were reduced with H₂ and stabilized in air before activity tests. U: sample used to obtain light-off curves. S: sample evaluated in the stability test. Note: NR = Not reported; OSC = oxygen storage capacity; OSCC = oxygen storage complete capacity; BET: Brunauer-Emmett-Teller test.

shows that only AuCu/CeO₂ reached CO concentrations below 100 ppm in the actual syngas at 210 °C, whereas minimum CO concentrations of the other catalysts were above 500 ppm.

On the other hand, the selectivity in the CO removal has been attributed to the support rather than the active metal [39], being the consumption of H₂ an important criterion in catalyst selection [39]. Figure 3 shows that H₂ consumption increases with temperature, particularly in the supports and catalysts based on ZrO₂. The deficiency of ZrO₂ to adsorb/desorb bidentate carbonates above 150 °C has been associated with a promotion of the H₂ combustion over the CO oxidation [40]. Likewise, H₂ loss increases in the majority of the supported Au–Cu catalysts (Figure 3c,d) compared to their respective bare support (Figure 3a,b), possibly due to affinity of the Au–Cu system to form intermediates in the H₂ oxidation (e.g., hydroxyl groups [29,41]) and methane formation (e.g., C–O* species [18,20,42]). Also, the most active catalysts in the CO removal (i.e., AuCu/CeO₂, AuCu/ZrO₂, AuCu/CeO₂-SiO₂, and AuCu/CeO₂-ZrO₂) promote higher H₂ consumption. That is, an active catalyst in the CO conversion possibly has an inherent tendency to consume H₂. The high H₂ consumption, which in some cases exceeds 20%, could be associated with the syngas composition [20,37], specifically with the H₂/CO ratio.

Table 2. Comparison of various catalytic systems for the CO removal using Au–Cu catalysts.

Catalyst	Syngas Type	H2/CO	T (°C)	CO Conversion (%)	H2 Loss (%)	Ref.
AuCu/CeO2	Synthetic	30	220	90	2	[29]
AuCu/SBA-15	Synthetic	>50	25	~100	~ 5a	[47]
Au/CuO-CeO2/Al2O3	Synthetic	4.5	350	~75	NR	[17]
Au/CeO2-CuO2/Al2O3	Synthetic	50	110	~95	~3a	[19]
Au/Al2O3	Synthetic	>50	80	~99	~2a	[36]
Au/CeO2-ZrO2	Actual	30	100	99	~2a	[35]
AuCu/CeO2	Actual	4	210	99	17	This work
AuCu/CeO2-SiO2	Actual	4	230	97	19	This work

^a Calculated by O₂ mass balance. NR: Not reported.

shows the results obtained in the CO removal with catalytic systems based on Au–Cu. High H₂/CO ratios (>>10 [43]) are used in CO-PROX with synthetic syngas to favor CO oxidation [44] and reduce the H₂ consumption. To achieve such high H₂/CO ratios before CO-PROX several WGSR reactors are required, however [11]. Thence, aiming at reducing the number of units used in the traditional process, it has been proposed to carry out CO removal reactions in a single reactor using the syngas that comes directly from the reformer [15,20,35]. Nevertheless, the syngas obtained directly from the ESR contains larger amounts of CO. H₂/CO ratios around 4 have been reported for syngas obtained from ESR using Ir/CeO₂ [45] and RhPd/CeO₂ [46] catalysts. Thus, the low H₂/CO ratio in the actual syngas (e.g., the syngas used in this work has an H₂/CO = 4) could conduce to a high H₂ loss in the cleanup reactor. Simultaneous production of CO₂ and CH₄ was observed in all catalysts evaluated (Figure A1 and Figure A2 in Appendix A), suggesting that CO-SMET and CO₂ methanation occur together with CO-PROX and WGSR. Then, H₂ oxidation and carbon hydrogenation would be the main causes of H₂ loss during the CO removal from an actual syngas.

Although CH₄ formation implicitly involves an undesirable H₂ consumption, it has been reported that a combination of CO-PROX and methanation improves CO removal compared to the CO-PROX alone, because of favoritism in the activation of adsorbed CO [16]. Then, C and H mass balances were carried out to determine the effect of CH₄ production on H₂ consumption and CO conversion. Figure 4 shows the H₂ and CO converted with respect to the CH₄ formed in the cleanup reactor. CH₄ formation appears to be directly proportional to H₂ loss (Figure 4a), but the amount of H₂ consumed is larger than the amount of H₂ contained in the formed CH₄ (yellow line); moreover, in most catalysts, H₂ loss is larger than the H₂ required by CO₂ methanation (green line). Hence, the remnant of H₂ loss may be associated with the production of water or hydrogenated compounds not detected by GC, indicating that methanation would have a secondary role in the H₂ loss during the syngas cleanup. On the other hand, CO conversion grows faster compared to the contribution of methanation (Figure 4b). Xu et al. [16] studied a Rh/Al₂O₃ catalyst and proposed that at temperatures above 150 °C, the methanation of CO₂ formed during the CO-PROX facilitates the CO oxidation caused by changes in the C–O* and H* adsorbed species. This possible beneficial effect of CO-PROX and subsequent CO₂ methanation seems to be stronger in some catalysts (e.g., AuCu/CeO₂-Al₂O₃ and AuCu/CeO₂-SiO₂), which would explain their higher activity at high temperatures (Figure 2d), where most CH₄ was produced (Figure A2 in Appendix A). AuCu/CeO₂ and AuCu/La₂O₃ show an atypical trend (Figure 4b), where the CO conversion decreases with the CH₄ formation, which could depend on the intermediates of C–O* formed on theses catalysts, as will be discussed later. Therefore, these results would confirm the beneficial effect of CO₂ methanation during the CO-PROX proposed in [16], but it was also identified that this effect depends on the support and composition of the syngas.

Although the main objective in the cleaning of the syngas is the CO removal, differences in the activity and selectivity could lead to changes in product distribution over prolonged periods of operation. Therefore, the stability of Au–Cu catalysts loaded on the best single (CeO₂) and dual support (CeO₂-SiO₂) was evaluated. Figure 5 shows the product distribution over time obtained from a system consisting of ESR and cleanup reactors, the latter of which is packed with either AuCu/CeO₂ or AuCu/CeO₂-SiO₂. In both cases, a H₂-rich stream is obtained. However, AuCu/CeO₂ shows more variability in product distribution, and after around 42 h of operation deactivation was observed, at which point the test was stopped. In contrast, the AuCu/CeO₂-SiO₂ catalyst ensures a stable operation for longer periods of time (at least 30% more time-on-stream, Figure 5b) with CO concentration of about 1000 ppm. The results of the stability test show that the use of dual metal oxides leads to less active (i.e., CO concentration of 1000 ppm versus 75 ppm) but more stable materials, which could be more interesting in extended processes.

Activity, H₂ consumption, and stability were used as criteria for comparison among the Au–Cu-supported catalysts for the CO removal from an actual syngas. Now, catalytic properties, such as reducibility, surface area, OSC, carbon deposit formation, and the CO-support interactions, will be related to the activity, selectivity, and stability of the Au–Cu catalysts supported in single and dual metal oxides.

2.2. Catalysts Characterization

2.2.1. TPR

The redox properties of catalysts have a significant effect on CO oxidation and metal-support interactions [17]. Figure 6 shows the H₂-TPR profiles for the Au–Cu catalysts supported on single and dual metal oxides. Deconvolution peaks are presented to identify possible individual contributions in each reduction zone, but they are not intended to be exact. Contrary to bare supports (Figure A3 in Appendix A), discrepancies are observed between supported Au–Cu catalysts. The specific reduction temperatures for Au and Cu are very diverse in the literature, possibly because the reduction of metals strongly depends on the interaction with other species [48]. In this study, a first zone (<130 °C) observed was attributed to the reduction of Au³⁺ and Au⁺ nanoparticles [41]. The second zone (130 to 430 °C) was associated with the reduction of Cu, where at least three species [49] can be identified: (α) easily reducible CuO nano particles, (β) particles of CuO dispersed that interact moderately with the support, and (γ) isolated particles of Cu [50]. In the last zone (>430 °C), the reduction of surface layers and bulk of the support is likely happening [34]. The α and β species promote the formation of oxygen vacancies [51], contributing to the CO oxidation. Thus, preferential formation of CuO species in single and dual metal supports would explain the increase in CO₂ production over Au–Cu-supported catalysts compared to bare supports (Figure A1 and Figure A2 in Appendix A).

On the other hand, the displacement of the reduction peaks to lower temperatures has been associated with changes in metal-support interactions [48]. CeO₂ shows an exceptional ability to facilitate the reduction of Cu and the formation of (mostly) β species. This effect has been previously studied [9,52], correlating a stronger CuO-CeO₂ interaction with high activity during CO-PROX. However, the increase in the contribution of γ-species and a slight shift of reduction peaks to higher temperatures could indicate a variation of the CuO-CeO₂ interaction in the Au–Cu catalysts supported in dual oxides. Thus, a change in the redox properties of the support caused by the presence of a second metal oxide could explain why the Au–Cu catalysts supported on dual oxides (i.e., AuCu/CeO₂-SiO₂, AuCu/CeO₂-ZrO₂, and AuCu/CeO₂-Al₂O₃) showed less activity compared to AuCu/CeO₂ (Figure 2). However, an exceedingly strong CuO-support interaction could also mitigate the formation of selective Au–Cu alloys [31]. In fact, AuCu/ZrO₂ and AuCu/CeO₂-ZrO₂ show a significant contribution of α species, which could be related to the high H₂ loss observed in these catalysts (Figure 3c,d). On the contrary, the combination of inert metal oxides such as Al₂O₃ with CeO₂ could facilitate the migration of CuO towards Au particles [31], leading to lower H₂ loss compared to single CeO₂ support. Then, the change in redox properties of CeO₂ by the presence of inert metal oxides (e.g., SiO₂) could lead to less active but more selective materials during CO removal.

Table 1 shows the H/M index, which has been associated with apparent active metal dispersion [53]. The H/M index in AuCu/Fe₂O₃ and AuCu/La₂O₃ is particularly low, indicating that these catalysts are not as effective for dispersing active metals [53]. In the other catalysts, the H/M index was close to or larger than 1.0 (i.e., complete reduction of Au and Cu), which could be associated with a higher dispersion of Au and Cu on the catalytic surface. However, a high H/M value could also indicate an additional effect of superficial reduction of the supports by the interaction between metal oxides and active metals [54]. Au–Cu catalysts supported on dual metal oxides showed higher H/M index compared to their respective single supports, which could be associated with a favoring in the reduction of both active metals and support due to the interaction between metal oxides. If so, then the redox properties of Au–Cu catalysts supported on dual oxides would depend on several interactions: (i) active metal-active metal; (ii) active metal-support, and (iii) oxide I-oxide II. The variation of these interactions influences catalytic performance during CO removal.

2.2.2. BET Area

The surface area of the catalysts is key to the availability of the active sites and catalytic performance [55]. The BET area (Table 1) of the catalysts supported on basic oxides (i.e., CeO₂, ZrO₂ and La₂O₃ [56]) is larger than their respective bare supports (

Table A1. Surface area, OSC, OSCC, and thermogravimetric analysis (TGA) results of single and dual supports.

Support	BET Surface Area (m2/gcat)		OSC in Fresh Samples at 300 °C (μmol O2/gcat)	OSCC in Fresh Samples at 300 °C (μmol O2/gcat)	Weight Loss (%)
	Fresh	Used			Fresh	Used
CeO2	52.4	55.4	61	135	1.1	0.6
SiO2	466.5	410.6	41	49	1.4	1.1
ZrO2	51.6	44.9	55	99	1.6	0.6
Al2O3	96	68.6	36	55	1.7	0.2
La2O3	14.1	15.3	21	68	5.5	2.1
Fe2O3	38.1	36.7	5	16	0.7	0.8
CeO2-SiO2	163.2	155.2	54	105	−2.7	1.8
CeO2-ZrO2	44.3	40.5	46	110	0.2	2.0
CeO2-Al2O3	72.7	69.1	41	120	2.0	1.0

in Appendix B), which has been previously associated with the formation of high disperse β species [20,57]. On the other hand, AuCu/Fe₂O₃ and AuCu/La₂O₃ show low surface areas, which match to the low capacity of these metal oxides to disperse active metals (low H/M index, Table 1). The synthesis method of the catalysts could influence the surface area of the support, overcoming some drawbacks of metal oxides such as Fe₂O₃ by using alternative synthesis methods [32,58]. In contrast, the higher surface area of AuCu/CeO₂-Al₂O₃ and AuCu/CeO₂-SiO₂ could favor the dispersion of Au and Cu, which is reflected by a larger H/M index (Table 1).

Although an increase in the surface area could contribute to improving the catalytic activity [55], the trend for surface area of the catalysts does not match their activity (Figure 2), indicating that the supports have other features that could be more relevant during the CO removal. Figure 7 shows the conversion rate of CO normalized by the surface area of catalysts. AuCu/La₂O₃ has a high normalized activity, possibly because the basic supports promote the formation of Au nanoparticles [33] and formation of β species [20,57], which are active in the CO conversion. In fact, basic oxides, such as CeO₂ and ZrO₂, also have higher normalized activity compared to less basic supports, such as Al₂O₃ and SiO₂. Also, the normalized activity of the AuCu/CeO₂-ZrO₂ catalyst increases compared to their respective single supports. Recently, it was reported that the replacement of Zr⁴⁺ ions in the lattice of the CaO-CeO₂ system leads to the formation of highly basic sites [59]. Then, we speculate that the high interaction in CeO₂-ZrO₂ observed by TPR could lead to the formation of sites with greater basicity. However, the low surface area of basic supports is a well-known limitation that affects their activity [6]. So, because of the possible role of basic sites in CO removal, the design of catalysts for CO removal should include a support with both a high surface area and elevated basicity. Modifications in the morphology of metal oxides have been proposed as a successful strategy to achieve this objective in other catalytic processes [60]. Then, preparation of Au–Cu catalysts supported on single and dual metal oxides can be optimized to improve their catalytic properties during CO removal.

2.2.3. OSC Measurements

The OSC of the support plays a central role in the oxidation of CO adsorbed on active sites [27]. Table 1 shows the OSC of Au–Cu catalysts supported on single and dual metal oxides. In general, the OSC of supported Au–Cu catalysts is higher than that of the bare supports (Table A1 in Appendix B), indicating that the presence of Au and Cu favors greater oxygen mobility in the catalyst. Also, the presence of α and β species has been associated with the formation of oxygen vacancies on the catalytic surface [52]. Catalysts that have a higher OSC at 300 °C (i.e., AuCu/CeO₂-ZrO₂, AuCu/CeO₂, AuCu/CeO₂-SiO₂, and AuCu/ZrO₂) were the most active (Figure 2), but also those that showed the highest consumption of H₂ (Figure 3). However, the OSC depends strongly on the temperature: at 100 °C, all catalysts except AuCu/Al₂O₃ showed an OSC up to 60% lower compared to 300 °C, which could be related to the lower activity of catalysts at low temperatures (Figure 2).

Likewise, the CO₂ formation depends on the availability of surface oxygen [15]. The first CO pulse (OSC) in AuCu/CeO₂ only corresponds to 39% of its oxygen storage complete capacity (OSCC), indicating that oxygen adsorbed on CeO₂ may not be easily released. The possible deficiency of CeO₂ to release the oxygen absorbed on its surface could limit the oxidation of carbon intermediates, which could, in turn, be related to the atypical trend observed in Figure 4b. The OSC in supports with larger surface area (i.e., AuCu/SiO₂, AuCu/Al₂O₃, AuCu/CeO₂-SiO₂, and AuCu/CeO₂-Al₂O₃), on the other hand, corresponds to more than 50% of their OSCC. A higher availability of surface oxygen (> OSC/OSCC) could be associated with the strong effect of CO₂ methanation on the CO removal for AuCu/CeO₂-SiO₂ and AuCu/CeO₂-Al₂O₃, as previously discussed. If so, then the beneficial effect of methanation during the CO-PROX proposed by [16] could be enhanced in catalysts that combine a high OSC and readiness to release their adsorbed oxygen (i.e., high OSC/OSCC ratio), which would require a high surface area.

On the other hand, the OSCC of the catalysts used decreases with respect to the fresh, activated ones (AC samples), reaching up to 73% reduction with AuCu/La₂O₃. This reduction could be associated with progressive oxidation of the catalyst surface by the presence of oxidants in the gas stream and deposits on the catalytic surface [20], conducive of a progressive deactivation. To clarify this, a TGA study was conducted.

2.2.4. TGA

Table 3. Weight loss of Au–Cu catalysts supported on single and dual supports evaluated in CO removal from an actual syngas.

Catalyst	Total Weight Loss (%)		Weight Loss of Spent Catalyst Samples by Temperature Intervals (mg of C/gcat*h)
	AC	Spent	40–250 °C	250–600 °C	600–1000 °C
AuCu/CeO2	0.7	3.8 (U)	17.1 (U)	6.8 (U)	11.8 (U)
		5.6 (S)	14.5 (S)	15.1 (U)	18.1 (U)
AuCu/SiO2	0.3	3.7 (U)	35.5 (U)	3.4 (U)	3.9 (U)
AuCu/ZrO2	0.9	1.6 (U)	9.2 (U)	2.1 (U)	4.7 (U)
AuCu/Al2O3	0.5	3.7 (U)	28.9 (U)	2.1 (U)	9.4 (U)
AuCu/La2O3	0.6	2.1 (U)	9.2 (U)	7.5 (U)	2.4 (U)
AuCu/Fe2O3	−0.3	2.5 (U)	18.4 (U)	8.9 (U)	NR
AuCu/CeO2-SiO2	−0.9	0.3 (U)	3.9 (U)	4.8 (U)	NR
		1.3 (S)	15.8 (S)	4.1 (U)	NR
AuCu/CeO2-ZrO2	0.5	1.7 (U)	9.2 (U)	6.2 (U)	0.8 (U)
AuCu/CeO2-Al2O3	0.6	2.6 (U)	17.1 (U)	2.7 (U)	7.1 (U)

Note: AC = activated catalyst, which were reduced with H₂ and stabilized in air before activity tests; U = sample used to obtain light-off curves; S = sample evaluated in the stability test; NR = Not reported.

shows the weight loss of Au–Cu catalysts supported on single and dual metal oxides. Most AC samples show a weight loss of less than 1% that could correspond to a remnant of the precursors of the active metals. However, AuCu/Fe₂O₃ and AuCu/CeO₂-SiO₂ show an increase in weight that can be associated with an oxygen adsorption; specifically, the CeO₂-SiO₂ system can form a Ce_9.33(SiO₄)·6O₂ phase that is susceptible to consume oxygen above 600 °C [6]. The used catalysts have a higher weight loss than the fresh, activated ones (AC samples), indicating the presence of compounds deposited on the catalytic surface during the reaction. To determine the nature of the deposits, the TGA results were analyzed by weight loss in terms of rate of carbon equivalent formed in each temperature interval (Table 3). In the first interval (40–250 °C), light compounds, such as water, and adsorbed OH⁻ and gases are released [28]; in this interval, AuCu/SiO₂ and AuCu/Al₂O₃ showed the highest weight loss, which could be related to their high surface area, which favors moisture adsorption. In the second interval (250–600 °C), light hydrocarbons are oxidized [26]; AuCu/La₂O₃ and AuCu/Fe₂O₃ had the highest rate of carbon formation in this interval, which would explain the strong decrease in the OSCC and surface area, respectively, observed in these samples (Table 1). In the last interval (600–1000 °C), heavy hydrocarbons are oxidized, which are the type of deposits that could favor a faster deactivation of the catalyst [56]; in this zone, AuCu/CeO₂ showed a higher rate of carbon formation. Thus, rapid deactivation observed in AuCu/CeO₂ (Figure 5) could be associated with the decrease in surface area (17%, Table 1) and OSCC (59%, Table 1) promoted by the accumulation of deposits on the catalytic surface (Table 3). The formation of stable deposits could be associated with the formation of intermediates during the CO removal [28]; therefore, in situ DRIFTS was carried out to identify how the interaction between CO and support affects the performance of the supported Au–Cu catalysts.

2.2.5. In Situ DRIFTS

Figure 8 shows the DRIFTS spectra of CO adsorption on bare supports and supported Au–Cu catalysts. CeO₂ and ZrO₂ show higher intensity in the area associated with hydroxyl groups (~3500 cm⁻¹) that contributes to the CO conversion [36], which would explain their high activity among single metal oxides (Figure 2). Although the CO pulses were free of H₂ or water, hydroxyl groups may be formed from the interaction of H₂ with the surface of the support [61], which could occur during the H₂ reduction that was performed on the AC samples. In fact, Zhou et al. [62] studied the CO adsorption on bare ZrO₂ by DRIFTS and Fourier Transform Infrared Spectroscopy (FTIR), identifying up to three families of hydroxyl groups in the zone from 3675 to 3772 cm⁻¹, which are activated by the adsorption of CO, even at room temperature, and have an active role in the formation of surface intermediates. CeO₂ favors the formation of hydroxyl groups even with the first pulse of CO, which could be decisive in ensuring a syngas with a lower CO concentration. In the C–O* zone (1200 to 1700 cm⁻¹ [63]), the formation of bidentate carbonates (1600 cm⁻¹) and formates (1300 and 1500 cm⁻¹) are observed, which are also intermediates in the CO conversion [20,51,63]. The formation of hydroxyl groups and C–O* species were lower than dual supports when compared to CeO₂; specifically, CeO₂-Al₂O₃ shows a significant reduction in the formation of C–O* intermediates, which would correspond to its lower activity among the dual supports (Figure 2).

The inclusion of Au–Cu in the single oxides (Figure 8c) favors the presence of hydroxyls and the formation of C–O* intermediates, possibly due to the ability of Au to form Au–CO and Au–OOH species [36]. In fact, most catalysts show an increase in CO adsorbed (2100 cm⁻¹), which is associated with CO–Au⁰ species [64], indicating that Au could be present mostly as Au⁰ on the catalytic surface, as previously reported for systems such as Au/CeO₂ [20] and Au/La₂O₃/Al₂O₃ [33], evaluated by XPS. However, in AuCu/CeO₂-SiO₂ and AuCu/CeO₂-ZrO₂, a weak peak of CO adsorption between 2075 and 2050 cm⁻¹ is also observed, which has been associated with the formation of CO–Au^δ− species [65]. In the case of AuCu/CeO₂-ZrO₂, the formation of these species only occurs after several CO pulses. The presence of Au^δ− has been related to a stronger support-metal interaction, which could be ascribed to the high stability of AuCu/CeO₂-SiO₂ (Figure 5).

The formation of C–O* intermediates may occur on different active sites, including Au⁰, Au^δ, and CuO, but the formation of carbonate species at approximately 1470 cm⁻¹ occurs preferably on Cu⁺ species [66], which are very active in CO-PROX [67]. The peak associated with Cu⁺ is well defined in AuCu/CeO₂. Furthermore, the formation of active Cu⁺ species due to the high affinity in CuO-CeO₂ has been extensively studied by XPS and DRIFTS [25,68]. Thus, a smaller amount of Cu⁺ species on the other catalysts could explain their inability to ensure CO concentrations below 100 ppm (Table 1). Besides, the peaks associated with formate species, which are related to CH₄ formation, are better defined on CeO₂. It is accepted that CH₄ formation is promoted on several oxides (e.g., Al₂O₃, ZrO₂, Y₂O₃, MgO, and CeO₂ [69]), but the special ability to adsorb and activate carbon species makes CeO₂ an adequate support in CO₂ methanation and CO-SMET [70]. Nevertheless, during the CO removal the Boudouard reaction and the CH₄ decomposition could contribute to the production of carbon deposits [26], favoring the catalyst deactivation. Then, the ability of CeO₂ to form C–O* intermediates (Figure 8) assisted by Cu⁺ species and its lower capacity to release the surface oxygen (low OSC/OSCC) could contribute to the generation of stable carbon deposits, as was observed by TGA, leading to its rapid deactivation (Figure 5). Besides, the deficiency of AuCu/CeO₂ to mitigate carbon deposition due to the excessive formation of C–O* intermediates could be also related to the atypical behavior of CH₄ formation (Figure 4b). However, the less active materials show low formation of intermediates (e.g., AuCu/La₂O₃, AuCu/Fe₂O₃, and AuCu/SiO₂). Thus, the selection of the support for the CO removal from a syngas must consider the balance between activity and stability.

The results of DRIFTS support the notion that the use of dual metal oxides favors less active but more stable catalysts. Therefore, in this study, CeO₂ is presented as the most promising support for developing a compact system to carry out the CO removal from an actual syngas. However, the selectivity and stability of CeO₂ require improvements. Furthermore, it was shown that the use of dual supports, specifically CeO₂-SiO₂ and CeO₂-ZrO₂, could be a promising strategy to overcome the deficiencies presented by CeO₂.

3. Materials and Methods

3.1. Support Selection

The supports evaluated in this work were selected according to a literature review, and are summarized in Figure 1. Scientific articles published between 2012 and 2019 that included at least one of the following reactions were reviewed: CO-PROX, WGSR, and CO-SMET. The detailed list of reviewed articles can be consulted in

Table A2. Reviewed articles for the selection of supports evaluated in the CO removal.

Date	Active Metals	Metal Oxide I	Metal Oxide II	Journal	Digital Object Identifier (DOI)
2012	CuO	Fe2O3	-	Chemical Engineering Journal	10.1016/j.cej.2012.01.017
2012	Pt	Other	-	Electrochimica Acta	10.1016/j.electacta.2012.04.150
2012	-	Fe2O3	-	Applied Surface Science	10.1016/j.apsusc.2011.10.092
2012	-	NiO2	-	Journal of Molecular Catalysis A: Chemical	10.1016/j.molcata.2012.05.001
2013	Ni, Co	Co3O4	-	Journal of Alloys and Compounds	10.1016/j.jallcom.2013.04.053
2013	CuO	TiO2	Al2O3	Surface and Coatings Technology	10.1016/j.surfcoat.2012.10.031
2013	Co	Fe2O3	-	Chemical Engineering Journal	10.1016/j.ces.2013.02.002
2014	Co	MgO	-	Process Safety and Environmental Protection	10.1016/j.psep.2013.12.003
2014	Pt	CeO2	-	Chemical Engineering Journal	10.1016/j.cej.2014.06.058
2014	Pd	Fe2O3	-	Journal of Catalysis	10.1016/j.jcat.2014.06.019
2014	Ag	Zeolite	-	Fuel	10.1016/j.fuel.2014.07.011
2014	Au	NiO2	-	Applied Catalysis A: General	10.1016/j.apcata.2014.02.003
2014	CuO	SiO2	CeO2	Journal of Environmental Chemical Engineering	10.1016/j.jece.2014.03.021
2015	Co, Fe, Cr	CeO2	-	International Journal of Hydrogen Energy	10.1016/j.ijhydene.2015.03.044
2015	-	Co3O4	-	Applied Catalysis A: General	10.1016/j.apcata.2014.10.024
2015	CuO	Fe2O3	-	Chinese Journal of Catalysis	10.1016/S1872-2067(15)60922-6
2015	Au	Zeolite	-	Catalysis Communications	10.1016/j.catcom.2015.06.018
2015	Pt	CeO2	-	Catalysis Today	10.1016/j.cattod.2014.12.038
2015	Au, Cu	CeO2	ZrO2	Catalysis Today	10.1016/j.cattod.2014.08.035
2015	-	PtO2	-	Applied Surface Science	10.1016/j.apsusc.2015.03.108
2015	CuO	CeO2	ZrO2	Journal of Industrial and Engineering Chemistry	10.1016/j.jiec.2015.06.038
2015	-	MnO2	CeO2	Applied Catalysis B: Environmental	10.1016/j.apcatb.2014.06.038
2016	Pd	Fe2O3	-	Journal of Environmental Chemical Engineering	10.1016/j.jece.2016.10.019
2016	-	CeO2	ZrO2	Applied Catalysis B: Environmental	10.1016/j.apcatb.2016.02.023
2016	Au	Zn2SnO4	-	Chinese Journal of Catalysis	10.1016/S1872-2067(16)62468-3
2016	-	Co3O4	-	Catalysis Communications	10.1016/j.catcom.2016.08.020
2016	Au	CeO2	-	Applied Catalysis B: Environmental	10.1016/j.apcatb.2016.02.025
2016	Pd	CeO2	-	Journal of Molecular Catalysis A: Chemical	10.1016/j.molcata.2016.08.035
2016	-	SiO2	Al2O3	Journal of Molecular Graphics and Modelling	10.1016/j.jmgm.2016.08.005
2016	Ag	SiO2	-	Catalysis Today	10.1016/j.cattod.2016.05.033
2016	-	PdO	-	Surface Science	10.1016/j.susc.2015.08.043
2016	-	Co3O4	-	Applied Catalysis A: General	10.1016/j.apcata.2016.03.027
2016	CuO	TiO2	-	Catalysis Communications	10.1016/j.catcom.2016.02.001
2016	Pt	CeO2	-	Applied Catalysis B: Environmental	10.1016/j.apcatb.2016.01.056
2016	CuO	MnO2	-	Journal of Molecular Catalysis A: Chemical	10.1016/j.molcata.2016.08.024
2016	CuO	Peroskita	-	Applied Clay Science	10.1016/j.clay.2015.08.034
2016	Pd	ZnO	-	Catalysis Today	10.1016/j.cattod.2015.05.021
2016	-	Fe2O3	-	Chemical Engineering Journal	10.1016/j.cej.2016.04.136
2016	Au	TiO2	-	Catalysis Today	10.1016/j.cattod.2015.09.040
2016	Au	Fe2O3	CeO2	Catalysis Today	10.1016/j.cattod.2016.05.059
2016	-	Co3O4	-	Materials Letters	10.1016/j.matlet.2016.06.108
2016	-	Co3O4	-	Chinese Journal of Catalysis	10.1016/S1872-2067(15)60969-X
2016	Au	TiO2	-	Applied Surface Science	10.1016/j.apsusc.2016.01.285
2016	-	Fe2O3	-	Journal of Molecular Catalysis A: Chemical	10.1016/j.molcata.2016.01.003
2016	Au	Other	-	Journal of Colloid and Interface Science	10.1016/j.jcis.2016.06.072
2016	Au	LaPO4	-	Journal of the Taiwan Institute of Chemical Engineers	10.1016/j.jtice.2016.01.016
2016	Pt	Al2O3	-	International Journal of Hydrogen Energy	10.1016/j.ijhydene.2016.08.170
2016	Pt	Other	-	Surface Science	10.1016/j.susc.2015.08.024
2017	CuO	Nb2O5	-	Catalysis Communications	10.1016/j.catcom.2017.04.008
2017	Zn, Pt	CeO2	-	Applied Catalysis B: Environmental	10.1016/j.apcatb.2017.04.044
2017	Pt, Fe	Fe2O3	Co3O4	Chinese Journal of Catalysis	10.1016/S1872-2067(17)62838-9
2017	CuO	MnO2	CeO2	Catalysis Communications	10.1016/j.catcom.2017.05.016
2017	Pt	MnO2	-	Journal of Electroanalytical Chemistry	10.1016/j.jelechem.2016.09.031
2017	Au	LaPO4	-	Chinese Journal of Chemical Engineering	10.1016/j.cjche.2017.08.008
2017	Fe, Mn	CeO2	-	Catalysis Today	10.1016/j.cattod.2016.11.046
2017	Mn	Co3O4	-	Solid State Sciences	10.1016/j.solidstatesciences.2017.07.006
2017	Mn	Co3O4	-	Fuel	10.1016/j.fuel.2017.04.140
2017	Au	CeO2	-	Applied Surface Science	10.1016/j.apsusc.2017.04.158
2017	-	MgO	-	Applied Catalysis B: Environmental	10.1016/j.apcatb.2016.11.043
2017	CuO	CeO2	Zeolite	Microporous and Mesoporous Materials	10.1016/j.micromeso.2017.02.016
2017	-	Zeolite	-	Applied Catalysis B: Environmental	10.1016/j.apcatb.2017.06.083
2017	Co	ZnO	-	Ceramics International	10.1016/j.ceramint.2017.06.157
2017	Pd	TiO2	SnO2	Applied Catalysis B: Environmental	10.1016/j.apcatb.2017.02.017
2017	Pd	Fe2O3	-	Fuel Processing Technology	10.1016/j.fuproc.2017.02.037
2017	CuO	CeO2	-	Journal of Power Sources	10.1016/j.jpowsour.2017.01.127
2017	Mn	CeO2	-	Applied Catalysis B: Environmental	10.1016/j.apcatb.2017.03.049
2017	Co	Co3O4	-	Chemical Physics Letters	10.1016/j.cplett.2017.02.085
2017	Au	TiO2	-	Catalysis Today	10.1016/j.cattod.2016.05.056
2017	CuO	CeO2	-	International Journal of Hydrogen Energy	10.1016/j.ijhydene.2017.02.088
2017	CuO	CeO2	-	Journal of Rare Earths	10.1016/j.jre.2017.05.015
2017	Pd	Al2O3	-	Applied Catalysis B: Environmental	10.1016/j.apcatb.2017.02.038
2017	Pt	TiO2	-	Molecular Catalysis	10.1016/j.mcat.2017.01.014
2017	-	CeO2	Other	Catalysis Today	10.1016/j.cattod.2017.06.017
2017	-	Al2O3	SnO2	Applied Surface Science	10.1016/j.apsusc.2017.01.058
2017	Ag	Zeolite	-	Fuel	10.1016/j.fuel.2016.10.037
2017	Au	TiO2	-	Applied Surface Science	10.1016/j.apsusc.2016.10.076
2017	-	Carbon	-	Molecular Catalysis	10.1016/j.molcata.2016.12.007
2017	Ag	SiO2	-	Microporous and Mesoporous Materials	10.1016/j.micromeso.2017.01.016
2017	Pd, Rh	Al2O3	-	Catalysis Today	10.1016/j.cattod.2016.10.010
2017	Au, Cu	SiO2	-	Catalysis Today	10.1016/j.cattod.2016.08.003
2017	Pd	CeO2	MnO	Applied Catalysis B: Environmental	10.1016/j.apcatb.2017.01.020
2017	-	CeO2	-	Catalysis Today	10.1016/j.cattod.2016.04.016
2017	Pd	Co3O4	-	Applied Catalysis A: General	10.1016/j.apcata.2016.12.021
2017	Pt	CeO2	-	Applied Catalysis A: General	10.1016/j.apcata.2017.08.012
2017	Mn	Co3O4	-	Solid State Sciences	10.1016/j.solidstatesciences.2017.07.006
2017	Ni	ZrO2	-	Applied Catalysis A: General	10.1016/j.apcata.2017.02.001
2018	-	SiO2	Co3O4	Microporous and Mesoporous Materials	10.1016/j.micromeso.2017.07.016
2018	Pt	Fe2O3	-	Applied Catalysis A: General	10.1016/j.apcata.2018.09.014
2018	Pd	SiO2	Al2O3	Applied Catalysis B: Environmental	10.1016/j.apcatb.2018.06.059
2018	Cu	CeO2	-	Catalysis Today	10.1016/j.cattod.2018.10.037
2018	Cu -Ni	CeO2	Al2O3	International Journal of Hydrogen Energy	10.1016/j.ijhydene.2018.12.127
2018	Ru	TiO2	ZrO2	International Journal of Hydrogen Energy	10.1016/j.ijhydene.2018.10.061
2018	Ni	ZrO2	-	International Journal of Hydrogen Energy	10.1016/j.ijhydene.2018.06.173
2018	-	ZrO2	-	Applied Catalysis B: Environmental	10.1016/j.apcatb.2018.03.001
2018	Ni	ZrO2	-	Applied Catalysis B: Environmental	10.1016/j.apcatb.2018.06.045
2019	Au	TiO2	-	International Journal of Hydrogen Energy	10.1016/j.ijhydene.2018.11.050
2019	Cu	Co3O4	-	Molecular Catalysis	10.1016/j.mcat.2019.01.020
2019	-	Other	-	Applied Catalysis B: Environmental	10.1016/j.apcatb.2018.12.022
2019	Pt	Zeolite	-	Applied Catalysis A: General	10.1016/j.apcata.2018.12.034
2019	Ni	ZrO2	-	Applied Catalysis B: Environmental	10.1016/j.apcatb.2018.11.024

(see Appendix B).

3.2. Catalyst Synthesis

The single supports of CeO₂, ZrO₂, and Fe₂O₃ were obtained by calcination at 500 °C for 2 h of Ce(NO₃)₃·6H₂O (CAS: 10294-41-4, Sigma Aldrich, Saint Louis, MO, USA), ZrO(NO₃)₂·xH₂O (CAS: 14985-18-3, Sigma Aldrich, Saint Louis, MO, USA), and Fe(NO₃)₃·9H₂O (CAS: 7782-61-8, Merck, Darmstadt, HE, Germany), respectively. Also, commercial oxides of La₂O₃ (CAS: 1312-81-8, Sigma Aldrich, Saint Louis, MO, USA), Al₂O₃ (CAS: 1344-28-1, Sigma Aldrich, Saint Louis, MO, USA), and SiO₂ (CAS: 60676-86-0, Merck, Darmstadt, HE, Germany) were used, which were also calcined at 500 °C in a muffle for 2 h.

Dual supports of CeO₂-ZrO₂, CeO₂-Al₂O₃, and CeO₂-SiO₂ were obtained from aqueous solutions of Ce Ce(NO₃)₃·6H₂O (CAS: 10294-41-4, Sigma Aldrich, Saint Louis, MO, USA) with ZrO(NO₃)₂·H₂O (CAS: 14985-18-3, Sigma Aldrich, Saint Louis, MO, USA), Al₂O₃ (CAS: 1344-28-1, Sigma Aldrich, Saint Louis, MO, USA), and SiO₂ (CAS: 60676-86-0, Merck, Darmstadt, HE, Germany), respectively, ensuring a molar ratio of Ce/M = 1 (M = Si, Zr and Al). Each solution was dried at 80 °C for 24 h and calcined at 500 °C in a muffle for 4 h. All supports (i.e., single and dual metal oxides) were screened with a 140-mesh sieve.

Bimetallic Au–Cu catalysts supported on each single and dual metal oxide were prepared according to the procedure described in [20], ensuring active metal loads of Au (1 wt%) and Cu (1 wt%). Au was first impregnated on each support by the precipitation-deposition method at pH 6 and 80 °C, using a solution of HAuCl₄·3H₂O (CAS: 16961-25-4 Sigma Aldrich, MO, USA). The filtered solid was dried at 80 °C for 24 h. Subsequently, Cu was included in the Au catalysts by the incipient wetness impregnation method, using a solution of Cu(NO₃)₂·3H₂O (CAS: 10031-43-3, Sigma Aldrich, Saint Louis, MO, USA). The catalyst obtained was dried at 80 °C for 24 h, calcined at 500 °C in a muffle for 2 h, and screened with a 140-mesh sieve.

The RhPt/CeO_2-SiO₂ catalyst for ESR was prepared according to the methodology described in [6]. Briefly, CeO_2-SiO₂ support was obtained from Ce(NO₃)₃·6H₂O (CAS: 10294-41-4, Sigma Aldrich, Saint Louis, MO, USA) and SiO₂ (CAS: 60676-86-0, Merck, Darmstadt, HE, Germany) solutions, ensuring a molar ratio of Ce/Si = 2. Rh and Pt were deposited on the CeO₂-SiO₂ support by the incipient wetness co-impregnation method, using RhCl₃·H₂O (CAS: 20765-98-4, Sigma Aldrich, Saint Louis, MO, USA) and H₂PtCl₆·6H₂O (CAS: 10025-65-7, Sigma Aldrich, Saint Louis, MO, USA) solutions. The catalyst obtained was dried at 80 °C for 24 h, calcined at 700 °C for 2 h, and screened with a 140-mesh sieve.

3.3. Obtaining Syngas

The syngas was obtained from ESR with a RhPt/CeO_2-SiO₂ catalyst at 700 °C in the first reactor (ESR reactor). The plug flow conditions in the ESR reactor were maintained ensuring L/Dp >50 ratios (i.e., catalytic bed height (L) and catalyst particle size (Dp)) and D/Dp >60 (i.e., diameter internal to the reactor (D)), as recommended in [71]. The catalyst bed consisted of 0.050 g of RhPt/CeO_2-SiO₂ and 0.250 g of inert quartz. The reactor feed consisted of 0.3 L/min of a mixture of ethanol (1.8 mol%), water (5.4 mol%), and Ar as carrier gas. The space velocity (SV) was set at 6.4 ± 0.2 L/g_cat*min. The syngas obtained in the ESR reactor, containing H₂ (8.4 mol%), CO (2.2 mol%), H₂O (1.6 mol%), CO₂ (0.6 mol%), CH₄ (0.3 mol%), and Ar (86.9 mol%), remained stable, with a variation <6.8%.

3.4. Catalytic Test

The supports and Au–Cu catalysts for the CO removal from the syngas were evaluated in a second reactor (cleanup reactor) between 100 and 300 °C. For this, the ESR reactor outlet was mixed with dry air, ensuring an excess oxygen factor (λ) of 1.8 ± 0.05 [20], and connected to the cleanup reactor inlet. The plug flow conditions in the cleanup reactor were maintained as previously described for the ESR reactor. The catalyst bed consisted of 0.050 g of catalyst (i.e., supports or Au–Cu catalysts) and 0.250 g of inert quartz. The SV in the cleanup reactor was set at 6.5 ± 0.3 L/g_cat^*min. Before the reaction, the supports and Au–Cu catalysts were pretreated in situ at 300 °C with streams of 8% H₂/Ar for 1 h, followed by Ar for 0.5 h, and finally 10% air/Ar for 0.5 h. These samples were labeled as “activated catalyst” (AC). Also, the samples used to obtain the light-off curves were labeled “U”, while those used in the stability test were labeled “S”.

The species at the outlet of each reactor (i.e., ESR reactor and cleanup reactor) were quantified by gas chromatography (GC) in a Clarus 580 (Perkin Elmer, Waltham, MA, USA), equipped with a Carboxen 1010 plot column (30 m, 0.53 mm ID, Restek, Bellefonte, PA, USA) connected to a thermal conductivity detector (TCD). Ar was used as carrier gas and N₂ as internal reference. The reaction conditions and GC data processed in Excel can be consulted in detail and downloaded from [72].

The conversion of CO ($x_{CO}$), the production of the main products ($Y_{C{O}_2} \mathrm{and} Y_{C{H}_4}$), and the H₂ obtained ($Y_{H_2}$) from the integrated system were obtained considering the molar flows ($F_i$) to the output of each reactor (i.e., ESR reactor and cleanup reactor), according to Equations (1) and (3). Production of CO, CH₄, and H₂ were normalized with the amount of carbon entering the system ($F_{C, inlet to the system}$), which remained constant at 5.2*10⁻⁴ mol/min of C.

$$x_{CO}=\frac{F_{CO, ESR-reactor} - F_{CO, cleanup-reactor}}{F_{CO, ESR-reactor}}$$

(1)

$$Y_{C{H}_4;C{O}_2}=\frac{ F_{C{H}_4;C{O}_2, cleanup-reactor - F_{C{H}_4;C{O}_2, ESR-reactor}}}{F_{C, inlet to the system}}$$

(2)

$$Y_{H_2}=\frac{ F_{H_2, cleanup-reactor}}{F_{C, inlet to the system}}$$

(3)

3.5. Characterization Tests

The reducibility of supports and Au–Cu catalysts was determined by TPR. The experiments were carried out in a ChemBET Pulsar unit (Quantachrome Instruments, Boynton Beach, FL, USA) equipped with a TCD. Prior to the reduction, 0.07 ± 0.01 g of AC samples was pretreated with N₂ (0.02 L / min) at 120 °C for 1 h and then cooled to room temperature. Subsequently, 5 % H₂/N₂ was passed, and the temperature was increased to 700 °C (5 °C/min). The H₂ uptake was calculated by integrating the peaks associated with the reduction of active metals (i.e., Au and Cu). The apparent active metal dispersion (H/M ratio) was also determined [53], assuming that the adsorption stoichiometry is one hydrogen atom for one active metal atom (Au + Cu).

The surface area of the samples was determined by standard physisorption of N₂ in a ChemBET Pulsar unit (Quantachrome Instruments, Boynton Beach, FL, USA). For this, 0.06 ± 0.01 g of sample was pretreated with N₂ (0.02 L/min) at 100 °C for 1 h and then cooled to room temperature for 0.5 h. Subsequently, the sample was immersed in a liquid N₂ bath. The BET area was measured with a single point, using 30% N₂/He (0.02 L/min). The measurements were repeated until deviations lower than 5% were obtained.

The OSC values of the samples were measured in a ChemBET Pulsar unit (Quantachrome Instruments, Boynton Beach, FL, USA), according to the procedure described in [41]. Briefly, 0.06 ± 0.01 g of sample was degassed in Ar (0.02 L/min) at 300 °C for 1 h. OSC was measured at 300 and 100 °C with independent samples. For this, 10 pulses of pure O₂ (0.25 mL) were injected to oxidize the sample, followed by a 20 min purge with Ar. Then, pulses of a 5 % CO/Ar mixture (0.25 mL) were injected until a constant signal was obtained. The OSC value was calculated by the CO consumed in the first pulse, and the OSCC value was determined by the total CO consumed.

The weight loss, associated with the presence of impurities, moisture, and carbon deposition in samples, was measured by TGA. The change in mass was determined using a thermogravimetric analyzer (Mettler Toledo, Columbus, OH, USA). For this, 0.02 ± 0.01 g of sample was pretreated with a N₂ (0.1 L/min) at 100 °C for 1 h and then cooled to 40 °C for 0.5 h. Subsequently, the sample was heated to 1000 °C (5 °C/min) in a dry air stream (0.1 L/min). Then, the rate of carbon formation was calculated according to Equation (4).

$$\mathrm{Rate} \mathrm{of} \mathrm{carbon} \mathrm{formation} =\frac{Weight loss in term of C \left(mg\right)}{mass of catalyst \left(g\right)*TGA test time \left(h\right)}$$

(4)

The CO adsorption on supports and catalysts was studied by in situ DRIFTS in a Nicolet iS10 spectrum device (Thermo Scientific, Waltham, MA, USA) equipped with a diffuse reflection attachment DRK-3 Praying Mantis (Harrick Scientific Products, New York, NY, USA). Spectra were taken between 400 and 4000 cm⁻¹, with 64 scans per minute and a resolution of 4 cm⁻¹. The sample holder was sealed with an airtight hood with ZeSn windows. In addition, the airtight hood was isolated with an Ar stream to avoid interference from the environment. Approximately 0.02 g of AC samples were degassed in Ar (15 mL/min) at 50 °C for 30 min. Then, 10 pulses of 30 μL of CO, obtained from a certified 5% CO/Ar mixture, were injected into the cell; between each pulse, Ar (15 mL/min) was passed for 10 min.

Raw and processed Excel data for characterization tests can be downloaded from [72].

4. Conclusions

Several single and dual metal oxides were investigated as supports in a catalytic system based on Au–Cu for the CO removal from an actual syngas. The use of a syngas obtained directly from the ESR affects the effectiveness in the CO removal; specifically, a low H₂/CO ratio could favor greater H₂ loss. AuCu/CeO₂ was identified as the most active catalyst in the CO removal, but it also contributes to a higher H₂ consumption. H₂ is lost mainly by the formation of water and CH₄, where the occurrence of CO₂ methanation affected the CO removal differently. Over CeO_2-Al₂O₃ and CeO_2-SiO₂, methanation seems to improve CO removal because the CO-PROX product, CO₂, is constantly consumed to produce CH₄. On the contrary, methanation has a negative effect on CeO₂ and La₂O₃ because the formed CH₄ favors carbon deposition.

Differences among the catalysts were evaluated by several characterization techniques. DRIFTS spectra of CO adsorption showed that CeO₂ has a superior activity because it favors the formation of C–O* and OH⁻ intermediates, but it promotes the formation of carbon deposits that lead to its deactivation. Similarly, TPR showed that ZrO₂ has a high interaction with active metals (Au–Cu), which makes it active but less selective, favoring a high H₂ oxidation. In addition, the low OSC of Al₂O₃ and SiO₂, and the lower surface area of Fe₂O₃ and La₂O₃ make these metal oxides less active. Regarding dual supports, the inclusion of a second metal oxide weakens the interaction of CeO₂ with the active metals, reducing activity. However, dual metal oxides are more selective and stable than single CeO₂ because they mitigate the excess of C–O* species, as was observed by DRIFTS; specifically, CeO_2-SiO₂ mitigates the formation of stable carbon deposits that deactivate the catalyst. Thus, AuCu/CeO₂ was identified as a promising catalyst for carrying out the CO removal from a syngas using just one catalytic reactor, but improvements in CeO₂ stability are still required. Therefore, the use of dual supports (e.g., CeO_2-SiO₂) could be a strategy to overcome single CeO₂ deficiencies. Thus, the development of more compact systems for the purification of H₂ suitable for FC implicitly promotes greater H₂ consumption. The results of this work aim to contribute to the development and establishment of sustainable energies based on H₂.