CO₂ Self-Poisoning and Its Mitigation in CuO Catalyzed CO Oxidation: Determining and Speeding up the Rate-Determining Step

Abstract

Cu-based catalysts are promising for CO oxidation applications with catalyst deactivation being a major barrier. We start with a CuO/Al₂O₃ catalyst and find that while the CO conversion decreases, CO₂ accumulates and the average Cu chemical state stays the same. It suggests CO₂ self-poisoning, i.e., CO₂ desorption is the rate-determining step. Subsequently, experiments are performed to prove this hypothesis by showing (1) CO₂ adsorption inhibits O₂ adsorption, (2) complete desorption of CO₂ regenerate the catalyst, (3) pre-adsorbed CO₂ quenches catalyst activity which recovers during the reaction and (4) the apparent activation energy is consistent with CO₂ desorption. It is further evidenced by using a stronger CO₂ adsorbing support CeO₂ to speed up CO₂ desorption from the CuO sites resulting in a superior CuO/CeO₂ catalyst. It provides an example for experimentally deciding and speeding up the rate-determining step in a catalytic reaction.

1. Introduction

Catalytic CO oxidation is an important reaction in automotive exhaust catalysis [1,2,3,4,5]. The current commercial catalysts are mainly noble metal (Pt, Pd and Rh) containing catalysts. However, due to the high prices and the limited resources of noble metals, noble metal-free catalysts are much more desired. Among all base metal oxides, cobalt and copper-based oxides are potential candidates [6,7,8,9,10]. A major drawback of Cu based catalysts is high susceptibility to deactivation by water and sulfur, which hinders their applications [11,12]. Specifically, there is no consensus on what the active sites and deactivation mechanism are for Cu based catalysts.

Many studies suggest that reduced species such as Cu⁰ or Cu⁺ species are active species while Cu²⁺ species have poor activity. It is believed that the Cu oxidation state changes among Cu, Cu₂O and CuO depending on the pretreatment or reaction condition [11,13,14,15,16]. Several characterization techniques, including XRD, XPS and TEM, have been used to characterize fresh and spent Cu catalysts under ex situ conditions. However, there is always concern of proper preservation of chemical state and catalyst structure when the spent catalysts are exposed to air in these experiments [17]. To overcome these problems, in situ characterization has also been carried out. However, due to the constraint imposed by in situ techniques, new issues arise with the most significant one probably being that the chemical environments are quite different from the real reaction conditions, for example, much lower reactant partial pressures in in situ XPS, [18]. different gas flow patterns in in situ XRD [19,20] etc.

The extensive efforts in determining catalyst structure or active sites are well reflective of the current development in mechanistic and kinetics studies of heterogeneous catalysis, i.e., a surface reaction involving the active site is presumably the rate determining step. A catalysis mechanism consists of many elementary steps and the quasi-equilibrium approximation has it that the slowest elementary step determines the overall reaction rate (thus called “the rate determining step”) while all other steps are sufficiently fast that they can be considered as being in quasi-equilibrium when deriving kinetics in heterogeneous catalysis [21]. Product desorption are often assumed to be fast, such as in Langmuir–Hinshelwood–Hougen–Watson (LHHW) kinetics [22]. There are few reports suggesting that the product desorption can be the rate determining step such as in the NH₃ decomposition and hydrogen desorption reactions, which were inferred from the N₂ desorption rate measurements for the former and DFT calculation of activation energy for the latter [23,24].

In CO oxidation, CO₂ is the reaction product. There are limited research efforts on CO₂ inhibiting effect on the reaction evidenced by feeding additional CO₂ gas in the system [3,25,26], which, however, may well result from a change in thermodynamic equilibrium of the reaction. The effect was believed to be due to the accumulation of surface carbonate species evidenced by IR characterization. However, it is not clear which sites these carbonates accumulate on and it is often argued that the observed species may be a spectator species. When a reaction product strongly adsorbs on the catalytic active sites, it inhibits further adsorption of reactants and slows down the catalytic reaction; this phenomenon is called self-poisoning in catalysis. In these cases, product desorption is the rate determining step. There are a few reports in the literature on CO₂ self-poisoning of Au based catalysts in the literature [27,28,29,30]. To the best of our knowledge, there has been no report regarding CO₂ self-poisoning of Cu based catalysts.

In this work, we develop a method to in situ measure the Cu chemical state and the adsorption characteristics of the reactants (CO and O₂) and the product (CO₂). The average Cu chemical state is measured by a difference method. After ruling out chemical change in the catalyst itself and CO poisoning and separating temperature effects, CO₂ self-poisoning is identified as the main reason for the catalyst deactivation and CO₂ desorption is the rate-determining step on the CuO/Al₂O₃ catalyst, which is further proved by a set of experiments.

2. Results and Discussion

2.1. Catalyst Deactivation Occurs on a CuO/Al₂O₃ Catalyst in CO Oxidation

To reveal intrinsic catalytic activity, CO conversion was tested at different furnace temperatures until steady state was reached according to our recently published work [31]. The temperatures were selected in such a way that there was a good spacing in the initial conversion covering low (~20%) to ~100%. In all catalytic performance tests, CO conversion decreases over time as shown in Figure 1a. At each set furnace temperature, it takes over 10 h to reach steady state. Meanwhile, the catalyst front temperatures were also recorded and they were constantly above the set furnace temperatures due to the exothermicity of the reaction. The trends of CO conversion and the temperature difference are very similar (see Figure 1a,b), which raises a question as to whether the observed decreasing of CO conversion is merely a temperature effect or indeed due to catalyst deactivation, which may involve factors such as physical or chemical change of the catalyst itself, adsorption of CO and O₂, or desorption of CO₂.

In order to assess the effect of catalyst structure on the catalytic surface reaction, we reliably measured the average Cu chemical state using a difference method (see Figure 1c and experimental Section 3.5). We applied the procedure on the catalyst after every reaction we performed in this work. All tests show the same result that after reaction Cu in the catalyst remains at the same average chemical state, i.e., 94% in oxide (CuO) form. Since CuO doesn’t have multiple phases at the reaction temperatures [32], it is inferred that the physical property of the CuO catalyst does not change during the reaction. Therefore, the catalyst bulk structure remains stable and is expected to have little contribution to the observed decreasing of CO conversion. Note that the catalyst surface under reaction conditions can still be dynamic as the catalyst is continuously consumed and regenerated. A cycle such as Cu²⁺ -> Cu⁺ -> Cu²⁺ is very likely and when the reaction ends the catalyst returns to Cu²⁺ state.

In catalytic CO oxidation, temperature dependence of the apparent CO conversion is through the rate determining step. In other words, when the catalyst temperature decreases the rate determining step slows down accordingly resulting in lower CO conversion. The activation energy is estimated to be 66 ± 4 kJ/mol (see Figure 1d). With the activation energy, we determine the relative conversion changes due to these temperature differences being 2.3–6.2%, as shown in

Table 1. Relative conversion changes due to temperature and experimentally determined relative conversion loss at different furnace temperatures.

Furnace Temp./°C	Initial CO Conv.	Steady-State CO Conv.	Relative Conv. Loss *	Initial Cat. Temp. (Ti)/°C	Steady-State Cat. Temp. (Ts)/°C	Relative Conv. Change Due to Temp. **	CO2 Adsorp./μmol
100	30.2%	3.6%	89.4%	105.1	104.1	5.6%	10.5
113	32.4%	6.3%	80.2%	118.9	117.8	5.6%	15.38
125	48.5%	17.3%	63.9%	131.6	131.1	2.3%	25.1
138	39.3%	27.8%	30.5%	146.4	145.1	5.8%	39.8
150	82.1%	45.9%	43.8%	159.5	158.0	6.2%	30.0
175	90.1%	70.2%	22.2%	186.1	184.5	5.9%	66.7

* Relative conversion loss is defined as $\frac{\left(Initial CO conv.-Steady state CO conv.\right)}{Initial CO conv.}\times 100\%$; ** Relative conversion change due to temperature is calculated from Arrhenius equation: $\frac{exp\left\{-Ea/\left[R\times \left(Ti+273.15\right)\right]\right\}-exp\left\{-Ea/\left[R\times \left(Ts+273.15\right)\right]\right\}}{exp\left\{-Ea/\left[R\times \left(Ti+273.15\right)\right]\right\}}\times 100\%$.

. These values are much smaller than the relative conversion losses observed in the experiments (22.2–89.4%). Thus, the change in the catalyst temperature has a minor effect on the performance.

2.2. CO₂ Adsorption Is a Major Cause for the Observed Catalyst Deactivation

As we have ruled out temperature and the catalyst structure as major sources of deactivation, we now turn our attention to other possible correlating factors such as the adsorption of the reactants and the desorption of the reaction product. In preparation of the catalyst for the experiments to measure Cu oxidation state, the procedure provided us an opportunity to look at the adsorbed species on the catalyst surface after each reaction. We found no CO, but substantial amount of CO₂ desorbing from the catalyst as shown in Figure 2, suggesting CO₂ other than CO poisoning may be the reason for the observed catalyst deactivation. Figure 2a shows the evolution of CO conversion in CO oxidation reaction at 150 °C for 21 h. The three shorter reactions for 0.8, 5 and 10 h under the same reaction conditions were performed so that we were able to measure the adsorption of CO, CO₂ and O₂ at different times during the reaction. These three shorter reactions overlap very well with the 21-h run with the initial CO conversion at 80 ± 2%. In the last few hours of the 21-h test, the reaction reached steady state with a 46% CO conversion. Subsequently, the adsorptions of CO₂ gases were quantified (see

Table 2. CO₂ physical and chemical adsorption on spent and CO₂ pre-adsorption catalysts.

Sample	Initial Conv.	Final Conv.	CO2 Phys. Adsorp./μmol	CO2 Chem. Adsorp./μmol	Total Adsorp./μmol
0.8 h	78.8%	67.4%	10.4	4.4	14.8
5 h	77.5%	55.5%	17.5	5.7	23.2
10 h	81.3%	49.1%	21.8	6.6	28.4
21 h	82.1%	45.6%	22.0	8.0	30.0
CO2 pre-adsorp.	67.9%	46.7%	10.9	4.0	14.9

). The relative measurement errors are estimated to be less than 1%. The correlation between the CO conversion and the CO₂ adsorption is well illustrated in Figure 2d. The CO conversion appears to decrease linearly with the CO₂ adsorption, which manifests our theory that CO₂ adsorption is a major cause for the observed catalyst deactivation.

2.3. CO₂ Adsorption Inhibits O₂ Adsorption

To further corroborate the theory, additional experiments were performed to elucidate how CO₂ adsorption affects the catalytic reaction network. The first experiment was to show direct evidence that pre-adsorbed CO₂ adversely affects catalyst performance. The regenerated catalyst was exposed to 0.5% CO₂ at the reaction temperature (150 °C) for 1 h and it was determined experimentally that 14.9 μmol CO₂ was adsorbed. This CO₂ pre-adsorption quantity is close to CO₂ adsorption during a reaction at the same temperature for 0.8 h (14.8 μmol, see Table 2). As shown in Figure 3a, with the CO₂ pre-adsorption catalyst the CO conversion starts off from 41%, much lower than the 80% conversion with the fresh catalyst. It then reaches its maximum conversion at 68% in about an hour and then gradual decrease is observed with time. Throughout the investigation, we confirmed multiple times that complete desorption of CO₂ off used catalysts brought back their activity. These results support our earlier assumption that catalyst structure contributes little to the observed loss of performance. Specifically, the observed decreasing CO conversion may be mostly due to the accumulations of CO₂ adsorbed species on Al₂O₃ surface, which can be completely desorbed at high temperatures reversibly. Interestingly, the catalyst with CO₂ pre-adsorption seems to deactivate in a slower pace than without CO₂ pre-adsorption. This is somehow puzzling and deserves further investigation. What is less surprising is that regardless of CO₂ pre-adsorption, the steady-state CO conversion appears to be very similar.

To better understand CO₂ self-poisoning in CO oxidation, the second experiment was to determine what reactant adsorption is adversely affected by CO₂ adsorption. In this case, since CO does not adsorb, CO₂ supposedly competes with O₂ for active catalyst sites. We measured the O₂ adsorption (0.25% O₂ for 1 h) on the fresh CuO/Al₂O₃ catalyst with and without CO₂ pre-adsorption (0.5% CO₂ for 1 h) and the titration profiles are shown in Figure 3b. O₂ adsorption is lower on the catalyst with pre-adsorbed CO₂ adsorption. This result confirms that CO₂ competes with O₂ for adsorption sites leading to less adsorbed O₂ on the CuO/Al₂O₃ catalyst.

Since the rate determining step limits the overall reaction rate, the apparent activation energy of the reaction should correspond to CO₂ desorption. The apparent activation energy of the reaction has been estimated to be 66 kJ/mol (see Figure 1d), which is similar than the activation energy of carbonate decomposition on some CuO/CeO₂ catalysts [33]. During the reaction, the number of active sites is decreasing; strictly speaking, the value of activation energy determined in Figure 1d is not conceptually accurate as no catalyst deactivation was implied. We attempted to take into account the effect of CO₂ self-poisoning on the determination of activation energy (see Supplementary Materials) and the value would be slightly higher at 71 kJ/mol. The magnitude of the activation energy further supports the conclusion that CO₂ desorption is the rate-determining step and CO₂ self-poisoning, the catalyst deactivation mechanism.

2.4. CO₂ Self-Poisoning Is Mitigated by Introducing a Strong CO₂ Adsorbent as the Support

Since CO₂ self-poisoning has been positively identified as the main factor for the CuO/Al₂O₃ catalyst deactivation, it is now possible to improve CuO catalysts by rational design. Previous tests showed that CO₂ adsorbed more on the CuO component than the Al₂O₃ support; this brings us an idea that a better CO₂ adsorbing support would possibly help free CuO from CO₂ poisoning by providing a dump-field for CO₂ before it desorbs into the gas phase (see Figure 4a). To test the idea, we chose a new support to synthesize another supported CuO catalyst that has the same molar ratio between Cu and the support metal (Cu/Al = 8.17 atom.%) and the same CuO loading per support surface area (0.011 mmol Cu/m²) as in the CuO/Al₂O₃ catalyst. With a special CeO₂ support, a 3.9wt% CuO/CeO₂ catalyst meeting the criteria was synthesized. We tested its performance for CO oxidation using 0.61 g CuO/CeO₂ catalyst so that the CuO quantity was the same as in the 0.20 g CuO/Al₂O₃ catalyst. The CO₂ adsorption capacities of the Al₂O₃ and CeO₂ supports are depicted in Figure 4b and the CeO₂ has much higher CO₂ adsorption capacity than the Al₂O₃ support (116.7 μmol vs. 8.4 μmol).

The activity test shows much better catalytic performance for the CuO/CeO₂ catalyst than that of the CuO/Al₂O₃ catalyst. As shown in Figure 4c, the former has an initial CO conversion of 94% at a furnace temperature of 63 °C, while the latter has an initial CO conversion of 82% at a furnace temperature of 150 °C. Moreover, there is only a small decrease in CO conversion for the former (2%) in 7 h while within the same time the latter experiences a much more significant loss in CO conversion (28%). The results of steady-state CO conversion as a function of the catalyst front temperature over the two catalysts are compared in Figure 4d. Undoubtedly, replacing the Al₂O₃ support with CeO₂, a stronger CO₂ adsorbent, greatly improves the performance of the CuO catalysts. Importantly, the CuO/CeO₂ catalyst has exceeded one current US DOE technical goal by having a T100 at 98 °C (catalyst temperature required to reach 100% CO conversion) much lower than 150 °C [34,35]. To clarify the effect of pure Al₂O₃ and CeO₂ supports on the performance, we further tested their catalytic activities. Both pure supports showed no activity under 200 °C, which have also been found elsewhere [36,37,38]. Apparently, the activity comes from CuO. The better activity of the CuO/CeO₂ catalyst is due to the higher affinity of CO₂ to CeO₂ but not necessarily its redox property [39]

3. Experimental Section

3.1. Catalyst Synthesis

A 10.0 wt.% Cu/γ-Al₂O₃ (or 12.2 wt.% CuO/γ-Al₂O₃, S_BET = 141 m²/g) catalyst was prepared by the incipient wetness impregnation method. 2.03 g Cu(NO₃)₂·2.5H₂O (Alfa Aesar Co. Inc., Shanghai, China, 98%) was dissolved in 4.6 mL distilled water and then mixed with 5 g γ-Al₂O₃ (Hongwu Nano Co. Ltd., Guangdong, China, 99.99%) powder. After achieving the complete impregnation by stirring, the sample was dried at 80 °C overnight and then calcined at 500 °C in air for 4 h. Finally, the catalyst was ground and then crushed into 40–60 mesh. A 3.2 wt.% Cu/CeO₂ (or 3.9 wt.% CuO/CeO₂, S_BET = 46 m²/g) catalyst was prepared using the same method to keep the same molar fraction of Cu/Ce and Cu/Al for comparison.

3.2. Catalytic Performance Evaluation

The CO oxidation reaction was performed under a stoichiometric feed condition (λ = 1.0). The catalyst reaction system was described elsewhere [31,40,41]. The feed gas composition was 0.5 mol.% CO and 0.25 mol.% O₂ and N₂ as balance. The total flow rate was 0.5 L/min (or 22.33 mmol/min) and the gaseous hourly space velocity, 150,000 mL/g_cat-h. All the activities were evaluated under steady state conditions. CO and CO₂ were identified and their molar fractions quantified by an online FT-IR spectrometer (Nicolet iS50, Thermo Scientific, Madison, WI, USA). The FT-IR spectrometer was equipped with an MCT detector. A 2-m gas cell (PIKE Technologies, Inc., Madison, WI, USA) with a KBr window was placed in the beam path and maintained at 120 °C. A spectral resolution of 0.25 cm⁻¹ was used and every 4 scans were averaged to generate one spectrum. These settings produced an absorbance spectrum every 0.187 min. To minimize system error, the CO conversion was calculated using the following equation (please see Supplementary Materials for detail):

$$X_{CO} (\%)=\frac{F_{CO,in}-\frac{F_{N_{2,in}}}{1-\frac{3}{2}{C}_{CO,out}-C_{C{O}_2,out}}\times C_{CO,out}}{F_{CO,in}}\times 100\%$$

where $C_{CO,out}$ and $C_{C{O}_2,out}$ are the outlet concentrations of CO and CO₂ measured by IR, respectively. $F_{CO,in}$ and $F_{N_{2,in}}$ are the flow rates of CO and N₂ in the influent.

3.3. Catalyst Activation Process

Typically, about 200 mg catalyst was used. To activate the fresh catalyst or re-generate the spent catalyst, a sequential reduction and oxidation process was carried out prior to CO oxidation reaction or other tests in this work. The gases evolved in the process were quantified by the IR spectrometer and the average relative measurement errors are within 1%. The first step was to heat up the catalyst to 400 °C in flowing N₂ (500 mL/min, STP) to desorb gases adsorbed on the catalyst during catalyst preparation (in the case of a fresh catalyst) or after reaction or pre-adsorption and the thermal desorption procedure would go on until no further gas evolution was detected by IR. After the desorption, catalyst reduction was performed in 500 mL/min 1 mol.% H₂/N₂ gas at 400 °C for 1 h. Finally, 500 mL/min 4 mol.% O₂/N₂ gas was introduced to oxidize the catalyst for 2 h. To confirm the repeatability of the procedure, the reduction and oxidation were repeated 5 times on a fresh catalyst.

3.4. In-Situ Quantification of Adsorption

To investigate CO oxidation mechanism, the effects of CO, CO₂ and O₂ adsorption on the catalyst performance were investigated. The adsorption usually took place after the catalyst surface was properly prepared such as described above. Quantification of CO adsorption was performed by first exposing the catalyst to 0.5 mol.% CO/N₂ gas at a flow rate of 500 mL/min for 1 h at desired adsorption temperature such as 150 °C. Afterwards, the flow was switched to 500 mL/min N₂ to remove physically adsorbed CO molecules. TPD spectrum was then collected at a heating rate of 20 K/min and this process would go on until no CO was detected by IR. CO desorption rate was determined from its IR measured molar fraction and the flow rate of the N₂ gas (see Supplementary Materials). Integration of the CO desorption rate gave the overall CO desorption amount, which was equal to the CO adsorption amount. Quantification of CO₂ adsorption was performed using the same procedure except that 500 mL/min 0.5 mol.% CO₂/N₂ was used for a fresh catalyst or in case of a spent catalyst no pre-adsorption of CO₂ would be performed.

Quantification of O₂ adsorption was quite different since O₂ molecules are IR inactive and cannot be detected by IR directly. Thus, the amount of adsorbed O₂ was measured indirectly by H₂-O₂ titration, i.e., by IR measurement of H₂O produced in the titration. Since the catalyst itself was also reducible by H₂, the O₂ adsorption was quantified by the difference in H₂O production with and without O₂ adsorption. After catalyst surface was properly prepared as described before, O₂ adsorption was performed in 500 mL/min 0.25 mol.% O₂/N₂ flow on the fresh catalyst at 150 °C for 1 h. Afterwards, the gas was switched to pure N₂ while maintaining the same molar flow rate to remove physically adsorbed O₂ molecules. H₂ was introduced into the reactor at 150 °C for 0.5 h and then the furnace temperature was raised up to 400 °C with a ramp rate of 20 K/min to titrate the adsorbed O₂. The same H₂ titration procedure was carried out on the fresh catalyst without O₂ adsorption to establish baseline H₂ consumption by the catalyst.

To further probe the influence of CO₂ adsorption, CO oxidation was also carried out on the fresh catalyst with pre-adsorbed CO₂. The fresh catalyst was exposed to 500 mL/min 0.5 mol.% CO₂/N₂ mixture at 150 °C for 1 h. CO₂ desorption was quantified before and after the reaction using the same procedure described above.

3.5. In-Situ Measurement of Average Cu Chemical State

The average Cu chemical state was measured by a difference method, which requires that the (used) catalyst be reduced to the same extent under certain reduction conditions (i.e., 1% H₂, 400 °C, 1 h). As a reference, the fresh or regenerated catalyst with dominantly Cu²⁺ was reduced three separate times for reliability test. The results are shown in Figure 5. The reduction kinetics is fast and the major feature appears within the first 10 min as evidenced by the H₂O production rate. Integration of the kinetic curves gives the quantities of the total produced H₂O and according to the reduction chemistry stoichiometry this is the amount of oxygen being reduced from the initial Cu catalyst. In all three cases, the reduction quantity is 0.288 ± 0.001 mmol, which is very close to the theoretical Cu loading in the fresh catalyst (~0.307 mmol). Evidently, about 94% CuO is reduced. Considering the activation, regeneration and reduction conditions, most probably in the fresh Cu catalyst 94% Cu is in CuO form and 6% Cu in the CuAlO₂ form existing as interface between the reducible CuO phase and the Al₂O₃ support [42]. The Cu catalyst could be completely reduced to Cu by H₂ reduction at 400 °C. To determine the average Cu oxidation state, the catalyst with certain reaction history was reduced in situ by H₂ at 400 ^oC and the reduction product, H₂O, quantified by IR. From the amount of H₂O production, the average Cu chemical state was determined. The H₂ reduction procedure was performed as described above. The reduction would continue until no reduction product was detected by IR. H₂O production rate was derived from its IR measured molar fraction and the flow rate of the N₂ gas. Integration of the H₂O production rate resulted in the overall H₂O production amount, which determined the difference in chemical state between the measured and Cu²⁺.

4. Conclusions

We have successfully executed experiments to show conclusively the rate determining step in CO oxidation over the CuO/Al₂O₃ catalyst. With the initial show of correlation between CO₂ adsorption and the catalyst performance, we experimentally proved the inferences from the CO₂ self-poisoning hypothesis, including (1) CO₂ adsorption inhibits O₂ adsorption, (2) complete desorption of CO₂ regenerate the catalyst, (3) pre-adsorbed CO₂ quenches catalyst activity which recovers during the reaction and (4) the apparent activation energy is consistent with CO₂ desorption. It is further evidenced by using a stronger CO₂ adsorbing support CeO₂ to promote CO₂ desorption from the CuO sites resulting in a superior CuO/CeO₂ catalyst. This work sets an example for experimentally deciding the rate-determining step in a catalytic reaction. It is also a vivid example of fundamental catalysis research leading to discovery of better catalysts.