Part I: A Comparative Thermal Aging Study on the Regenerability of Rh/Al₂O₃ and Rh/Ce_xO_y-ZrO₂ as Model Catalysts for Automotive Three Way Catalysts

Abstract

The rhodium (Rh) component in automotive three way catalysts (TWC) experiences severe thermal deactivation during fuel shutoff, an engine mode (e.g., at downhill coasting) used for enhancing fuel economy. In a subsequent switch to a slightly fuel rich condition, in situ catalyst regeneration is accomplished by reduction with H₂ generated through steam reforming catalyzed by Rh⁰ sites. The present work reports the effects of the two processes on the activity and properties of 0.5% Rh/Al₂O₃ and 0.5% Rh/Ce_xO_y-ZrO₂ (CZO) as model catalysts for Rh-TWC. A very brief introduction of three way catalysts and system considerations is also given. During simulated fuel shutoff, catalyst deactivation is accelerated with increasing aging temperature from 800 °C to 1050 °C. Rh on a CZO support experiences less deactivation and faster regeneration than Rh on Al₂O₃. Catalyst characterization techniques including BET surface area, CO chemisorption, TPR, and XPS measurements were applied to examine the roles of metal-support interactions in each catalyst system. For Rh/Al₂O₃, strong metal-support interactions with the formation of stable rhodium aluminate (Rh(AlO₂)_y) complex dominates in fuel shutoff, leading to more difficult catalyst regeneration. For Rh/CZO, Rh sites were partially oxidized to Rh₂O₃ and were relatively easy to be reduced to active Rh⁰ during regeneration.

1. Introduction

When the gasoline engine is operated around the stoichiometric air-to-fuel ratio (14.6 wt.%, ±2%), a three way catalyst (TWC) allows simultaneous conversions (~98%) of CO, HCs and NO_x to innocuous compounds [1]. Specifically, the oxidation of CO and HCs (non-methane HCs) to CO₂ and steam (H₂O) is catalyzed by Pd, while NO_x is reduced to N₂ catalyzed by Rh [2]. Modern TWC uses supported bimetallic Pd-Rh catalysts deposited on stabilized γAl₂O₃ washcoated on a ceramic or metallic monolithic substrate [3,4,5]. A cartoon of a washcoated monolith is shown as Figure 1.

Figure 1. A washcoated monolith automotive TWC catalyst.

Figure 1. A washcoated monolith automotive TWC catalyst.

The TWC conversion profile is shown as Figure 2. CO and HCs are essentially fully oxidized at lean (excess O₂) of the stoichiometric air-to-fuel ratio (right side of stoichiometric). NO_x reduction occurs when little or no O₂ is present, as in the rich operating mode (left side of stoichiometric). The ratio of air-to-fuel in the exhaust to the air-to-fuel at stoichiometric is defined as the lambda point (λ). At stoichiometric operation, λ equals 1. The λ value is controlled via a feedback control system through a signal received from the O₂ sensor as shown in Figure 3.

Gamma-Al₂O₃ (γ-Al₂O₃), stabilized by incorporation of small percentages of La₂O₃ and/or BaO is most widely used as a support for the catalytic components due to its excellent hydrothermal stability, and high specific surface area and porosity, which provide adequate metal dispersion of the precious metals [6]. Cerium oxide (Ce_xO_y), well known for its high oxygen storage capacity (OSC) due to the function of Ce⁴⁺/Ce³⁺ redox pair, is also included in modern TWC composition. The air-to-fuel ratio reversibly oscillates during lean/rich perturbations as a consequence of the feedback control strategy. The main function of the Ce_xO_y is to provide O₂ when λ < 1 for oxidation, and storing O₂ when λ > 1 to allow reduction to occur [2,7,8,9]. A schematic of the catalytic emission abatement system, with feedback control, is shown in Figure 3. Note a small TWC converter close couple catalyst (next to the engine) gets hot faster than the main catalyst and initiates conversion more quickly. The O₂ sensor, after the main TWC, is for on-board diagnostics to inform the driver of a malfunction in the converter.

Figure 2. The TWC conversion profile as a function of air-to-fuel ratio.

Figure 2. The TWC conversion profile as a function of air-to-fuel ratio.

Figure 3. A schematic of the unit operations in the exhaust system for a TWC with feed back control of air-to-fuel ratio (λ).

Figure 3. A schematic of the unit operations in the exhaust system for a TWC with feed back control of air-to-fuel ratio (λ).

The redox chemistry for the CeO_x is shown as Reactions (1) and (2). Further incorporation of zirconium oxide (ZrO₂) into Ce_xO_y crystallite structure (denoted as CZO) improves the thermal stability of Ce_xO_y, and enhances the mobility of lattice oxygen through the formation of oxygen vacancies [10,11,12,13]. Other proprietary elements are also added to further enhance performance.

$$2{\text{CeO}}_2+\text{CO}\to {\text{Ce}}_2{\text{O}}_3+{\text{CO}}_2\text{ (at slightly fuel rich, λ < 1)}$$

(1)

$${\text{Ce}}_2{\text{O}}_3+\frac{1}{2}{\text{O}}_2\to 2{\text{CeO}}_2\text{ (at slightly fuel lean, λ > 1)}$$

(2)

Fuel shutoff has been practiced for many years for enhancing fuel economy by 2%–4%. This operational mode is implemented when the vehicle is coasting down hill, usually lasting a short period (no more than a few seconds or few minutes). During this operational mode, fuel injection is discontinued and air flows into the TWC converter, exposing catalyst to high surface temperatures (up to 1050 °C), resulting in severe catalyst deactivation [14,15,16].

Rhodium needs to be maintained in its metallic state (Rh⁰) to maintain its activity for NOx reduction [2,17]. The deactivation modes include metal and/or support sintering [18,19], metal and/or support oxidation [20], and metal-support interactions [19,20,21,22,23]. It is widely accepted that the interaction between Rh and γ-Al₂O₃ during thermal oxidative exposure leads to the formation of stable and inactive Rhodium Aluminate Rh(AlO₂)_y (Reaction 3) [2,17].

$${\text{Rh}}_2{\text{O}}_3+y{\text{Al}}_2{\text{O}}_3+\frac{y-3}{2}{\text{O}}_2\stackrel{800℃,\text{ air}}{\to }2\text{ Rh}{\left({\text{AlO}}_2\right)}_y$$

(3)

Interactions between Rh and Ce_xO_y in combination with ZrO₂ have been discussed previously. High surface energy of Ce_xO_y favors metal-support interactions. The dissolution of Rh into the bulk ceria was observed after catalyst calcination at 550 °C [24,25]. Meanwhile, Rh⁺−O−Ce and [Rh−O₂]²⁻ species are likely formed in an oxidative environment [26,27,28,29]. At high temperature, treatment of Rh/CeO₂ in air leads to the formation of Rh₂O₃ [30], slight Rh metal sintering [31,32], and segregation of Rh cations into the CeO₂ lattice [33,34]. Rh may also incorporate into the sublattice ZrO₂, leading to decrease in metal redox behavior [35].

Two practical approaches have been employed to solve (or partially solve) the deactivation problem: (1) stabilizing the support by using/adding refractory materials to prevent negative Rh-support interactions; and (2) regenerating the deactivated catalyst after fuel shutoff by operating engine at the fuel rich (λ < 1) condition [2,15,36,37,38,39,40,41,42]. For the first approach, refractory materials as supports in place of gamma Al₂O₃ include zirconia, titania, denser forms of alumina, and alkaline metal oxides [43,44] have been suggested. The second approach returns the operational mode to slightly fuel rich at ~500 °C, which allows the creation of a reducing engine exhaust atmosphere for partially reversing catalyst deactivation [2,45].

At slightly fuel rich conditions, the O₂ concentration is very low while considerable amounts of HCs and CO, along with excess steam and CO₂, are present in the exhaust. The oxidized catalytic components can be reduced by H₂ generated mainly through catalytic steam reforming (SR). The SR reaction, with propane as a model compound for the exhaust HC being reformed, is shown in Reaction (4) [46,47]. Fuel rich regeneration allows Rh³⁺ to be reduced to active Rh⁰, and released from the interaction with the support. Reaction (5) represents the reverse of Rh-Al₂O₃ interaction by H₂. It is also possible for some dry reforming to occur where H₂O is replaced with CO₂.

$${\text{C}}_3{\text{H}}_8+3{\text{H}}_2\text{O}\to 3\text{CO}+7{\text{H}}_2$$

(4)

$$2\text{Rh}{\left({\text{AlO}}_2\right)}_y+{\text{yH}}_2\stackrel{\text{Fuel rich }}{\to }2\text{Rh}+{\text{yAl}}_2{\text{O}}_3+y{\text{H}}_2\text{O}$$

(5)

One advantage of this process is that the endothermic SR reaction (which is both thermodynamcially and kinetically favorable at high temperature) can be catalyzed by the TWC catalyst itself. In other words, the deactivated components in TWC can be regenerated in situ by the H₂ produced through SR, as catalyzed by the precious metal sites (mainly Rh) remaining active after aging.

The present work shows new data of the effects of fuel shutoff and subsequent regeneration on the catalytic performance and properties of Rh-TWC, with 0.5% Rh/Al₂O₃ and 0.5% Rh/Ce_xO_y-ZrO₂ (Ce:Zr atomic ratio of 1:2) as model catalysts. Fuel shutoff was simulated by aging fresh catalysts in flowing air at high temprature (800 °C, 950 °C, or 1050 °C) for a short period (5 min), while catalyst regeneration was performed by exposing the aged catalysts to a reducing atmosphere (500 vppm propane, 10% steam, 8% CO₂, and N₂ balance at 550 °C for 1 h), simulating a slightly rich exhaust composition which is close to normal engine operation. Catalyst regenerability was examined by comparing the activity of fresh, aged, and regenerated catalysts via H₂ generation. By combining various characterization techniques including BET, CO chemisorption, TPR, and XPS, the roles of catalyst properties were examined. The study (i) provides a mechanism study of catalyst deactivation during simulated fuel shutoff process; and (ii) explores the aging and support effects on catalyst regenerability during simulated fuel rich operation. The paper highlights the maintenance of catalyst performance through cyclic fuel shutoff-fuel rich operation.

2. Results and Discussions

2.1. Thermodynamic Model for TWC Conversion at Simulated Engine Fuel Rich Condition

The main reaction pathways occurring during fuel rich operation are listed in Table 1. Specifically, endothermic steam reforming of propane reactions (Reactions a and b) are thermodynamically and kinetically favorable at high temperature. Exothermic water gas shift (Reaction c) and methanation of CO₂ (Reaction d) reactions are thermodynamically favorable at relative low temperature, where reaction kinetics are slow. The CH₄ produced undergoes steam reforming (Reaction e). The reaction thermodynamics and kinetics are also largely dependent on the feed. When 8 vol-% CO₂ is considered (as present in the exhaust), it can react with CH₄ in what is referred to dry reforming.

Table 1. Main reaction pathways during catalytic conversions of simulated exhaust feed at fuel rich condition.

Reaction #	Reaction Pathways	Reaction Type (Forward Direction)	ΔHr0 (298 K) (kJ/mol)
a	${\text{C}}_3{\text{H}}_8+3{\text{H}}_2\text{O}\to 3\text{CO}+7{\text{H}}_2$	Propane steam reforming	+497.70
b	${\text{C}}_3{\text{H}}_8+6{\text{H}}_2\text{O}\to 3{\text{CO}}_2+10{\text{H}}_2$	Propane steam reforming	+374.29
c	$\text{CO}+{\text{H}}_2\text{O}\rightleftharpoons {\text{CO}}_2+{\text{H}}_2$	Water gas shift	−41.14
d	${\text{CO}}_2+4{\text{H}}_2\rightleftharpoons {\text{CH}}_4+2{\text{H}}_2\text{O}$	Methanation of CO2	−165.02
e	${\text{CH}}_4+{\text{H}}_2\text{O}\rightleftharpoons \text{CO}+3{\text{H}}_2$	Methane steam reforming	+206.16

Table 1. Main reaction pathways during catalytic conversions of simulated exhaust feed at fuel rich condition.

Reaction #	Reaction Pathways	Reaction Type (Forward Direction)	ΔHr0 (298 K) (kJ/mol)
a	${\text{C}}_3{\text{H}}_8+3{\text{H}}_2\text{O}\to 3\text{CO}+7{\text{H}}_2$	Propane steam reforming	+497.70
b	${\text{C}}_3{\text{H}}_8+6{\text{H}}_2\text{O}\to 3{\text{CO}}_2+10{\text{H}}_2$	Propane steam reforming	+374.29
c	$\text{CO}+{\text{H}}_2\text{O}\rightleftharpoons {\text{CO}}_2+{\text{H}}_2$	Water gas shift	−41.14
d	${\text{CO}}_2+4{\text{H}}_2\rightleftharpoons {\text{CH}}_4+2{\text{H}}_2\text{O}$	Methanation of CO2	−165.02
e	${\text{CH}}_4+{\text{H}}_2\text{O}\rightleftharpoons \text{CO}+3{\text{H}}_2$	Methane steam reforming	+206.16

The Gibbs free energy for main reactions (Reactions a–e) are plotted in Figure 4. The extent of methanation of CO₂ (Reaction d) and reverse water gas shift (reverse of Reaction c) increase in the presence of 8 vol-% CO₂ in the feed. Both of these reactions decrease H₂ content. It is clear SR is favored above about 450 °C.

Figure 5 shows the main equilibrium products (H₂, CO, and CH₄) and reactant (propane and water) equilibrium mole fractions at fuel rich condition (500 vppm propane, 10 vol-% steam, 8 vol-% CO₂, N₂ in balance). In the low temperature regime (T < 350 °C), steam reforming, water gas shift, and methanation reactions dominate. In the high temperature regime (T > 350 °C), WGS and methanation reactions become less favorable.

Figure 4. Reaction Gibbs free energy as a function of reaction temperature (25 °C to 700 °C) at 1 atm. Assume ideal gas behavior for the reactant and product gas components. Compound thermodynamic data with temperature and pressure inputs is collected from I. Barin, Thermochemical Data of Pure Substances (3rd Edition) [48].

Figure 4. Reaction Gibbs free energy as a function of reaction temperature (25 °C to 700 °C) at 1 atm. Assume ideal gas behavior for the reactant and product gas components. Compound thermodynamic data with temperature and pressure inputs is collected from I. Barin, Thermochemical Data of Pure Substances (3rd Edition) [48].

Figure 5. (a) Main mole fractions of H₂, CO, and CH₄; and (b) theoretical reactant (propane and water) conversions as a function of reaction temperature (200 °C to 550 °C) at thermodynamic equilibrium conditions. Reactant feed: 500 vppm propane, 10 vol-% steam, 8 vol-% CO₂, N₂ in balance.

Figure 5. (a) Main mole fractions of H₂, CO, and CH₄; and (b) theoretical reactant (propane and water) conversions as a function of reaction temperature (200 °C to 550 °C) at thermodynamic equilibrium conditions. Reactant feed: 500 vppm propane, 10 vol-% steam, 8 vol-% CO₂, N₂ in balance.

2.2. Catalyst Deactivation and Regeneration

After aging in air at three temperatures (800 °C, 950 °C, or 1050 °C), the activity of fresh and aged Rh/Al₂O₃ and Rh/CZO are compared as the H₂ mole fraction in the propane rich feed gas (Figure 6). Fresh Rh/CZO (b) shows higher activity than Rh/Al₂O₃ (a) under fresh and all aged conditions.

Figure 6. Catalyst activity of fresh and aged (a) 0.5% Rh/Al₂O₃ and (b) 0.5% Rh/CZO Catalyst activity is plotted in terms of H₂ mole fraction as a function of reaction temperature (200 °C–550 °C).

Figure 6. Catalyst activity of fresh and aged (a) 0.5% Rh/Al₂O₃ and (b) 0.5% Rh/CZO Catalyst activity is plotted in terms of H₂ mole fraction as a function of reaction temperature (200 °C–550 °C).

Not surprisingly, deactivation of both catalysts increases with aging temperature from 800 °C to 1050 °C. The most difficult regeneration was expected after aging at elevated temperatures. This is in agreement with a previous report [49]. Compared to Rh/Al₂O₃, fresh and aged Rh/CZO showed higher catalytic activity in converting propane to H₂.

The activity of fresh, aged, and regenerated catalysts are compared in Figure 7. The regeneration method was very effective in recovering full activity of Rh catalysts. This is also shown in Table 2, which compares the T₅₀ of regenerated catalysts with those of fresh and aged. T₅₀ is the temperature at which 50% maximum equilibrium H₂ production is achieved. The temperatures of Rh/CZO were always 40 °C lower than Rh/Al₂O₃, indicating a more rapid response to in situ regeneration.

Figure 7. Activity of fresh, aged, and regenerated (a) 0.5% Rh/Al₂O₃ and (b) 0.5% Rh/CZO in converting simulated engine exhaust gas at fuel rich condition. Catalyst activity is plotted in terms of H₂ product mole fraction as a function of reaction temperature (200 °C to 550 °C). Aged catalysts were obtained by treating fresh ones in air at 1050 °C for 5 min, followed by cooling to room temperature in air. Catalyst regenerations were performed by at rich condition at 550 °C for 1 h.

Figure 7. Activity of fresh, aged, and regenerated (a) 0.5% Rh/Al₂O₃ and (b) 0.5% Rh/CZO in converting simulated engine exhaust gas at fuel rich condition. Catalyst activity is plotted in terms of H₂ product mole fraction as a function of reaction temperature (200 °C to 550 °C). Aged catalysts were obtained by treating fresh ones in air at 1050 °C for 5 min, followed by cooling to room temperature in air. Catalyst regenerations were performed by at rich condition at 550 °C for 1 h.

Table 2. T₅₀ (temperature at which 50% maximum equilibrium H₂ production is reached during activity tests, °C) of fresh, aged, and regenerated catalysts.

Catalyst		T50 (°C)
	Fresh	After Aging in air for 5 min at Different temp.			After Regeneration *
		800 °C	950 °C	1050 °C
0.5% Rh/Al2O3	375	415	460	470	375
0.5% Rh/CZO	334	365	431	435	330

* Catalyst regenerations were performed at 550 °C in propane-containing feed gas.

Table 2. T₅₀ (temperature at which 50% maximum equilibrium H₂ production is reached during activity tests, °C) of fresh, aged, and regenerated catalysts.

Catalyst		T50 (°C)
	Fresh	After Aging in air for 5 min at Different temp.			After Regeneration *
		800 °C	950 °C	1050 °C
0.5% Rh/Al2O3	375	415	460	470	375
0.5% Rh/CZO	334	365	431	435	330

* Catalyst regenerations were performed at 550 °C in propane-containing feed gas.

2.3. Catalyst Stability during Simulated Fuel Shutoff Aging-Fuel Rich Regeneration Cycle Tests

Figure 8 shows the rapid response (increased slope of H₂ production) achieved during regeneration for the Rh/CZO catalyst relative to the slower recovery of Rh/Al₂O₃.

The activity of both catalysts after every 5 cycles of aging-regeneration cycles are plotted in Figure 9. Significant losses of catalyst activity were observed in the first 5 cycles. The initial deactivations (greater for the Rh/Al₂O₃) are believed caused by Rh metal sintering and dissolution of oxidized Rh into the sintered support materials (metal-support interactions). After the first 5 cycles, the performance stabilized. At this condition the major deactivation modes have been completed and no further permanent deactivation is noted after repeated cycles. The aged catalysts could then be regenerated but to a lesser extent than after 5 cycles.

Figure 8. H₂ generations during regeneration processes in simulated fuel shutoff aging- fuel rich regeneration cycle tests (First 5 cycles out of total 25 cycles) with (a) 0.5% Rh/Al₂O₃ and (b) 0.5% Rh/CZO. In each cycle, the catalyst sample was first aged in air at 1050 °C for 5 min, followed by in situ regeneration at propane rich condition at 550 °C for 1 h.

Figure 8. H₂ generations during regeneration processes in simulated fuel shutoff aging- fuel rich regeneration cycle tests (First 5 cycles out of total 25 cycles) with (a) 0.5% Rh/Al₂O₃ and (b) 0.5% Rh/CZO. In each cycle, the catalyst sample was first aged in air at 1050 °C for 5 min, followed by in situ regeneration at propane rich condition at 550 °C for 1 h.

Figure 9. Activity of regenerated (a) 0.5% Rh/Al₂O₃ and (b) 0.5% Rh/CZO catalysts after 1, 5, 10, 15, 20, and 25 cycles in the aging-regeneration cycle tests. After every five cycles of aging/regeneration. The catalyst activity is plotted in terms of H₂ product mole fraction as a function of SR temperature from 250 °C to 550 °C.

Figure 9. Activity of regenerated (a) 0.5% Rh/Al₂O₃ and (b) 0.5% Rh/CZO catalysts after 1, 5, 10, 15, 20, and 25 cycles in the aging-regeneration cycle tests. After every five cycles of aging/regeneration. The catalyst activity is plotted in terms of H₂ product mole fraction as a function of SR temperature from 250 °C to 550 °C.

2.4. Catalyst Deactivation and Regeneration Mechanisms

Figure 10 shows the BET surface areas of fresh and aged 0.5% Rh/Al₂O₃ and 0.5% Rh/CZO at different aging treatments. In agreement with the catalyst activity result as shown in Figure 6, most significant losses in catalyst surface areas occurred at 950 °C and 1050 °C. The Al₂O₃ support exhibited higher intrinsic surface area, but a slightly higher percentage of sintering relative to the CZO support (with 41.4% and 35.9% for Rh/Al₂O₃ and Rh/CZO respectively after 1050 °C aging). ZrO₂ in CZO support is likely the main contributor to the thermostability of Rh/CZO [50].

Figure 10. BET surface areas of fresh and aged (a) 0.5% Rh/Al₂O₃ and (b) 0.5% Rh/CZO, as a function of aging conditions. Aged samples were obtained by aging fresh catalysts (I) in air at the following conditions: (II) 800 °C for 5 min; (III) 800 °C for 1 h; (IV) 950 °C for 5 min; or (V) 1050 °C for 5 min. The aging processes were followed by cooling in air to room temperature. As a reference, BET surface areas of support materials were measured 142.9 m²/g and 60.3 m²/g respectively for fresh Al₂O₃ and CZO.

Figure 10. BET surface areas of fresh and aged (a) 0.5% Rh/Al₂O₃ and (b) 0.5% Rh/CZO, as a function of aging conditions. Aged samples were obtained by aging fresh catalysts (I) in air at the following conditions: (II) 800 °C for 5 min; (III) 800 °C for 1 h; (IV) 950 °C for 5 min; or (V) 1050 °C for 5 min. The aging processes were followed by cooling in air to room temperature. As a reference, BET surface areas of support materials were measured 142.9 m²/g and 60.3 m²/g respectively for fresh Al₂O₃ and CZO.

The metal dispersions of fresh and aged catalysts are shown in Table 3. Fresh Rh catalysts showed higher metal dispersions on CZO than on Al₂O₃. Soria and Duarte et al. [51,52] reported that the enhancement of metal dispersion in Rh/CeO₂ system was achieved by the Rh-Ce interaction (ceria stabilized Rh⁺ species formed on the support). The higher metal dispersions likely enhance its activity in catalytic steam reforming of propane. After aging, loss of active metal sites occurs for both Rh catalysts, and is accelerated with increasing aging temperature from 800 °C to 1050 °C. Barbier et al. [53], reported that the decrease of Rh surface area in Rh/Al₂O₃ system was mainly linked to the diffusion of Rh³⁺ into the alumina matrix, while the presence of Ce_xO_y stabilizes Rh and prevents Rh³⁺ from dissolving into the support.

Table 3. Metal dispersions (%) of fresh and aged 0.5% Rh/Al₂O₃ and 0.5% Rh/CZO catalysts as measured by room temperature CO chemisorption ^a. After simulated fuel shutoff, loss of active metal sites occurred for both Rh catalysts, and was accelerated with aging temperature from 800 °C to 1050 °C.

Catalyst	Metal Dispersion (%)
	Fresh	Aged @ 800 °C	Aged @ 1050 °C
0.5%Rh/Al2O3	30.1	15.8	8.0
0.5%Rh/CZO	80.7	40.2	27.9

^a CO can only be chemisorbed on Rh⁰ in Rh/Al₂O₃ and Rh/CZO. CO chemisorption was negligible on non-reduced catalyst samples, and was zero on Al₂O₃ and CZO support-only material.

Table 3. Metal dispersions (%) of fresh and aged 0.5% Rh/Al₂O₃ and 0.5% Rh/CZO catalysts as measured by room temperature CO chemisorption ^a. After simulated fuel shutoff, loss of active metal sites occurred for both Rh catalysts, and was accelerated with aging temperature from 800 °C to 1050 °C.

Catalyst	Metal Dispersion (%)
	Fresh	Aged @ 800 °C	Aged @ 1050 °C
0.5%Rh/Al2O3	30.1	15.8	8.0
0.5%Rh/CZO	80.7	40.2	27.9

^a CO can only be chemisorbed on Rh⁰ in Rh/Al₂O₃ and Rh/CZO. CO chemisorption was negligible on non-reduced catalyst samples, and was zero on Al₂O₃ and CZO support-only material.

However, there likely exists an overestimation of the metal particle dispersion by measuring the CO chemisorption of Rh/CZO, due to the formation of carbonate species on CeO₂ surface even at low temperature (323 K) and by the likelihood of multiple CO molecules adsorbing on the Rh itself. Some preliminary TEM result as below (Table 4). After aging and regeneration, negligible metal crystallite size grow was observed with both Rh/Al₂O₃ and Rh/CZO, which supports our result that metal sintering was not the major deactivation mode in Rh-TWC. The dramatically reduced CO chemisorption capacity of the aged Rh-TWC together with the TEM images suggests that the main deactivation mode during 1050 °C aging was metal-support interaction.

Table 4. Mean metal particle sizes (nm) of fresh and aged TWCs as measured using TEM images.

Catalyst	Active Particle Mean Size (nm) *
	Fresh	After Air Aging @ 1050 °C	After Regeneration
0.5% Rh/Al2O3	4.4	5.7	5.4
0.5% Rh/CZO	8.5	9.2	9.3

* Feret diameter of active metal/metal oxide particle.

Table 4. Mean metal particle sizes (nm) of fresh and aged TWCs as measured using TEM images.

Catalyst	Active Particle Mean Size (nm) *
	Fresh	After Air Aging @ 1050 °C	After Regeneration
0.5% Rh/Al2O3	4.4	5.7	5.4
0.5% Rh/CZO	8.5	9.2	9.3

* Feret diameter of active metal/metal oxide particle.

During air aging, the oxidation state of Rh increased in both Rh/Al₂O₃ and Rh/CZO, i.e., Rh⁰ → Rh³⁺ (Reaction 6). Meanwhile, strong metal-support interactions with the formation of Rhodium Aluminate (Rh(AlO₂)_y) took place in Rh/Al₂O₃ sample (Reaction 3) [50].

$$2{\text{Rh}}^0+\frac{3}{2}{\text{O}}_2\stackrel{>800℃}{\to }{\text{ Rh}}_2{\text{O}}_3$$

(6)

$${\text{Rh}}_2{\text{O}}_3+y{\text{Al}}_2{\text{O}}_3+\frac{y-3}{2}{\text{O}}_2\stackrel{800℃,\text{ air}}{\to }2\text{ Rh}{\left({\text{AlO}}_2\right)}_y$$

(3)

H₂-TPR was used to study the catalyst redox property after aging at different conditions (Figure 11). The lower the temperature of the H₂ consumption peak, the easier the reduction. The reductions of Rh³⁺ to Rh⁰ in both catalysts occurred around 100 °C. H₂ reaction pathways on Rh sites include H₂ spill over and dissociation on Rh⁰ sites (Reaction 7), and subsequent reduction of Rh³⁺ → Rh⁰ (Reaction 8) [54,55,56].

$${\text{Rh}}^0+\frac{x}{2}{\text{H}}_2\rightleftharpoons \text{Rh}\cdots {\text{H}}_x$$

(7)

$${\text{Rh}}_2{\text{O}}_3+6\text{H}\to 2{\text{Rh}}^0+3{\text{H}}_2\text{O}$$

(8)

Since Al₂O₃ is non-reducible, H₂ consumption peaks in fresh and aged Rh/Al₂O₃ are only assigned to Rh reductions. The results indicate that Rh in fresh Rh/Al₂O₃ was already partially oxidized with a H₂ consumption peak at 90 °C. After aging in air with (800–1050 °C), the TPR profiles of Rh/Al₂O₃ samples shifted to higher reducing temperatures, while the H₂ consumption peak area continued to increase. Rh was “released” from metal-support interaction by H₂ (Reaction 5), but this became increasingly difficult at higher aging temperature.

$$2\text{Rh}{\left({\text{AlO}}_2\right)}_y+y{\text{H}}_2\stackrel{\text{Fuel rich }}{\to }2\text{Rh}+y{\text{Al}}_2{\text{O}}_3+y{\text{H}}_2\text{O}$$

(5)

Figure 11. Normalized H₂ consumption in H₂-Temperature Programmed Reduction (H₂-TPR) measurements of fresh and aged (a) 0.5% Rh/Al₂O₃ and (b) 0.5% Rh/CZO, as a function of reducing temperature. Aged samples for measurements were respectively achieved by aging the fresh ones in air at the following conditions: 800 °C for 5 min, 800 °C for 1 h, 950 °C for 5 min, or 1050 °C for 5 min.

Figure 11. Normalized H₂ consumption in H₂-Temperature Programmed Reduction (H₂-TPR) measurements of fresh and aged (a) 0.5% Rh/Al₂O₃ and (b) 0.5% Rh/CZO, as a function of reducing temperature. Aged samples for measurements were respectively achieved by aging the fresh ones in air at the following conditions: 800 °C for 5 min, 800 °C for 1 h, 950 °C for 5 min, or 1050 °C for 5 min.

The precise stoichiometry of Rh:O is dependent on many factors including metal loading, metal dispersion, aging temperature, and aging oxygen partial pressure. Hwang et al. [50], reported the phase diagram for the variation of rhodium oxide species on the dispersion of rhodium samples and the oxidation temperature.

The quantitative H₂ consumption for the fresh and aged 0.5% Rh/Al₂O₃ samples during H₂-TPR is shown in Table 5. The H₂ consumptions correspond well to our statement that the fresh 0.5% Rh/Al₂O₃ sample was already partially oxidized. N(H₂)/N(Rh), i.e., the ratio between consumed H₂ molecules and reduced Rh atoms, increases with increasing aging temperature, and approaches 1.5 after high temperature (950 °C) aging, suggesting almost complete oxidation of Rh⁰ to Rh³⁺ in severely aged 0.5% Rh/Al₂O₃. The H₂ consumption by rhodium oxides in 0.5% Rh/CZO is very difficult to quantify because the reduction of Ce⁴⁺ to Ce³⁺ also consumes H₂. For 0.5% Rh/CZO samples, qualitative TPR analysis and semi-quantitative XPS analysis (later text) are sufficient for the Rh oxidation state study.

Table 5. Reduction temperature (T_R) and H₂ consumption during H₂-TPR for fresh and aged 0.5% Rh/Al₂O₃ samples.

Sample	TR (°C)	H2 Consumption (μmol H2/gcat)	N (H2)/N (Rh)
Fresh	90	31.78	0.65
Aged @ 800 °C for 5 min	118	59.78	1.23
Aged @ 800 °C for 1 h	135	72.85	1.50
Aged @ 950 °C for 5 min	150	70.49	1.45
Aged @ 1050 °C for 5 min	162	71.57	1.47

Table 5. Reduction temperature (T_R) and H₂ consumption during H₂-TPR for fresh and aged 0.5% Rh/Al₂O₃ samples.

Sample	TR (°C)	H2 Consumption (μmol H2/gcat)	N (H2)/N (Rh)
Fresh	90	31.78	0.65
Aged @ 800 °C for 5 min	118	59.78	1.23
Aged @ 800 °C for 1 h	135	72.85	1.50
Aged @ 950 °C for 5 min	150	70.49	1.45
Aged @ 1050 °C for 5 min	162	71.57	1.47

For fresh CZO support, one broad reduction peak is shown at 400–660 °C, which is assigned to bulk surface Ce⁴⁺ to Ce³⁺, with the following global reaction (Reaction 9) [57,58,59].

$$2{\text{CeO}}_2+{\text{H}}_2\to {\text{Ce}}_2{\text{O}}_3+{\text{H}}_2\text{O}$$

(9)

The reduction of Rh³⁺ to Rh⁰ on CZO (71 °C) is easier than for fresh Rh/Al₂O₃ (91 °C). The presence of Rh in fresh Rh/CZO allows the reduction of Ce⁴⁺ to Ce³⁺ to occur at a lower temperature (100–420 °C), with the Ce⁴⁺ reduction peak split into two side peaks (at 110 °C and 308 °C respectively). The lower reduction temperature of Ce⁴⁺ (at 110 °C) occurs following the surface reduction of Rh³⁺, suggesting that the Ce⁴⁺ sites being reduced were most likely the ones in close contact with the Rh sites. It has been reported that the redox properties of both Rh and Ce are enhanced when Rh is deposited on Ce_xO_y [60]. The Rh–O–Ce bond is likely formed, creating Rh^δ+/Rh⁰ (0 < δ < 1) and Ce⁴⁺/Ce³⁺ redox couple. Electrons transfer more efficiently during H₂ reduction [61,62,63]. The introduction of Zr into the Ce_xO_y crystal lattice, now widely practiced for OSC, stabilizes the Rh–Ce interaction via improving mobility of oxygen in Ce_xO_y or maintaining Ce_xO_y dispersion in nanometer scale [64,65,66,67,68,69,70,71]. Furthermore, the Rh–O–Ce bond can be very easily dissociated [60], which makes the interaction between Rh and Ce_xO_y much weaker than that between Rh and Al₂O₃. After complete reduction of Rh^δ+ to Rh⁰, electrons transfer from dissociated H₂ on Rh⁰, allowing easier reduction of the bulk Ce⁴⁺ to Ce³⁺. The schematic mechanism of the redox reaction pathways and the promotional metal-support interaction within 0.5% Rh/CZO during H₂-TPR are sketched in Figure 12.

In agreement with previous literature [72], increasing the aging temperature, the reduction of both Rh and Ce in Rh/CZO shifted to higher temperature values, suggesting decreases in hydrogen dissociation capability after aging.

The XPS Rh 3d electron orbitals were used to identify and semi-quantify the Rh oxidation states, by comparing the binding energy values and relative ratio of the corresponding states. Figure 13 profiles the XPS Rh 3d spectra of both catalysts, and Table 6 summarizes the peak information details. Rh 3d_3/2 and Rh 3d_5/2 peaks, resulting from spin-orbital splitting, with different binding energies (BE) corresponding to the Rh valence states were assigned [73,74,75,76,77,78,79,80]. For a fresh sample, Rh 3d_5/2 peak with BE of at 307.5 eV–308.4 eV is attributed to Rh⁰ valence state, while Rh 3d_5/2 peak at 309.2 eV–310.1 eV is attributed to Rh³⁺ valence state.

Figure 12. Speculative schematic of proposed redox reaction mechanism and interaction between Rh^δ+/Rh⁰ and Ce⁴⁺/Ce³⁺ redox couple during H₂-TPR of 0.5% Rh/CZO. The redox reactions followed the order described below. (I) After simulated fuel shutoff aging in air at 800 °C, 950 °C, or 1050 °C, surface Rh sites are oxidized to Rh₂O₃, while the Rh sites in close contacts with Ce_xO_y remained in reduced states (Rh^δ+, 0 < δ < 1), with Rh^δ+/Rh⁰ and Ce⁴⁺/Ce³⁺ redox couple formed for enhancing electron transfer efficiency; (II) H₂ flow through the sample; (III) At low temperature regime around 100 °C to 120 °C, H₂ was chemisorbed and dissociated on the Rh⁰ sites, followed by (IV) Reduction of Rh³⁺ to Rh⁰; (V) Reduction of surface Ce⁴⁺ sites to Ce³⁺ promoted by the Rh^δ+/Rh⁰ and Ce⁴⁺/Ce³⁺ redox couple; (VI) Reduction of bulk Ce⁴⁺ sites to Ce³⁺ when more H₂ molecules were chemisorbed and dissociated on Rh⁰.

Figure 12. Speculative schematic of proposed redox reaction mechanism and interaction between Rh^δ+/Rh⁰ and Ce⁴⁺/Ce³⁺ redox couple during H₂-TPR of 0.5% Rh/CZO. The redox reactions followed the order described below. (I) After simulated fuel shutoff aging in air at 800 °C, 950 °C, or 1050 °C, surface Rh sites are oxidized to Rh₂O₃, while the Rh sites in close contacts with Ce_xO_y remained in reduced states (Rh^δ+, 0 < δ < 1), with Rh^δ+/Rh⁰ and Ce⁴⁺/Ce³⁺ redox couple formed for enhancing electron transfer efficiency; (II) H₂ flow through the sample; (III) At low temperature regime around 100 °C to 120 °C, H₂ was chemisorbed and dissociated on the Rh⁰ sites, followed by (IV) Reduction of Rh³⁺ to Rh⁰; (V) Reduction of surface Ce⁴⁺ sites to Ce³⁺ promoted by the Rh^δ+/Rh⁰ and Ce⁴⁺/Ce³⁺ redox couple; (VI) Reduction of bulk Ce⁴⁺ sites to Ce³⁺ when more H₂ molecules were chemisorbed and dissociated on Rh⁰.

Figure 13. X-ray Photoelectron Spectroscopy (XPS) multiplex spectra in Rh 3d region (with BE of 318 eV–304 eV) of fresh, aged, and regenerated (a) 0.5% Rh/Al₂O₃ and (b) 0.5% Rh/CZO powder catalysts, and with aging temperature varied. Aged samples were achieved by aging the fresh catalysts in air at 800 °C, 950 °C or 1050 °C for 5 min. Regenerated samples were achieved by regenerating the aged ones (1050 °C for 5 min) using the method as described in Section 3.3.

Figure 13. X-ray Photoelectron Spectroscopy (XPS) multiplex spectra in Rh 3d region (with BE of 318 eV–304 eV) of fresh, aged, and regenerated (a) 0.5% Rh/Al₂O₃ and (b) 0.5% Rh/CZO powder catalysts, and with aging temperature varied. Aged samples were achieved by aging the fresh catalysts in air at 800 °C, 950 °C or 1050 °C for 5 min. Regenerated samples were achieved by regenerating the aged ones (1050 °C for 5 min) using the method as described in Section 3.3.

Table 6. Summary of detailed information of XPS spectra as shown in Figure 12, i.e., values of binding energy Rh 3d_3/2 for Rh³⁺ and Rh⁰ oxidation states, and Rh³⁺/Rh⁰ (or Rh³⁺/Rh^δ+) ratios ^a for fresh, aged, and regenerated 0.5% Rh/Al₂O₃ and 0.5% Rh/CZO samples.

Catalyst		E (Rh 3d3/2), eV		E (Rh 3d5/2), eV		Rh3+/Rh0 (Rh3+/Rhδ+) b
		Rh3+	Rh0	Rh3+	Rh0
0.5% Rh/Al2O3	Fresh	314.10	311.69	309.18	307.46	1.29
	800 °C aged for 5 min	314.27	312.59	309.81	308.16	1.51
	950 °C aged for 5 min	315.39	313.87	310.02	308.85	2.14
	1050 °C aged for 5 min	315.55	314.03	310.55	309.13	3.21
	Regenerated (I) c	313.83	312.78	309.35	307.34	1.41
	Regenerated (II) d	314.87	313.12	309.55	308.89	2.18
0.5% Rh/CZO	Fresh	313.49	311.39	310.10	308.36	0.48
	800 °C aged for 5 min	313.64	311.79	310.23	308.89	0.75
	950 °C aged for 5 min	313.74	311.90	311.02	309.10	0.81
	1050 °C aged for 5 min	314.17	312.30	311.21	309.31	0.99
	Regenerated (I) c	313.35	311.40	310.29	308.11	0.64
	Regenerated (II) d	314.01	311.70	310.70	308.44	0.70

^a Rh³⁺/Rh⁰ ratio was calculated by comparing the integrated area under the corresponding fitted curves of Rh³⁺ 3d_3/2 and Rh⁰ 3d_3/2 in Figure 6; ^b In Rh/Al₂O₃, Rh³⁺ and Rh⁰ coexist, and Rh³⁺/Rh⁰ ratios are compared. In Rh/CZO, Rh³⁺ and Rh^δ+ (0 ≤ δ < 1) coexist, and the Rh³⁺/Rh^δ+ ratios are compared; ^c Regenerated samples (I) were achieved by performing in situ regeneration using propane steam reforming with aged samples after aging at 1050 °C for 5 min; ^d Regenerated samples (II) were achieved after 25 cycles in the simulated fuel shutoff-regeneration cycle tests.

Table 6. Summary of detailed information of XPS spectra as shown in Figure 12, i.e., values of binding energy Rh 3d_3/2 for Rh³⁺ and Rh⁰ oxidation states, and Rh³⁺/Rh⁰ (or Rh³⁺/Rh^δ+) ratios ^a for fresh, aged, and regenerated 0.5% Rh/Al₂O₃ and 0.5% Rh/CZO samples.

Catalyst		E (Rh 3d3/2), eV		E (Rh 3d5/2), eV		Rh3+/Rh0 (Rh3+/Rhδ+) b
		Rh3+	Rh0	Rh3+	Rh0
0.5% Rh/Al2O3	Fresh	314.10	311.69	309.18	307.46	1.29
	800 °C aged for 5 min	314.27	312.59	309.81	308.16	1.51
	950 °C aged for 5 min	315.39	313.87	310.02	308.85	2.14
	1050 °C aged for 5 min	315.55	314.03	310.55	309.13	3.21
	Regenerated (I) c	313.83	312.78	309.35	307.34	1.41
	Regenerated (II) d	314.87	313.12	309.55	308.89	2.18
0.5% Rh/CZO	Fresh	313.49	311.39	310.10	308.36	0.48
	800 °C aged for 5 min	313.64	311.79	310.23	308.89	0.75
	950 °C aged for 5 min	313.74	311.90	311.02	309.10	0.81
	1050 °C aged for 5 min	314.17	312.30	311.21	309.31	0.99
	Regenerated (I) c	313.35	311.40	310.29	308.11	0.64
	Regenerated (II) d	314.01	311.70	310.70	308.44	0.70

^a Rh³⁺/Rh⁰ ratio was calculated by comparing the integrated area under the corresponding fitted curves of Rh³⁺ 3d_3/2 and Rh⁰ 3d_3/2 in Figure 6; ^b In Rh/Al₂O₃, Rh³⁺ and Rh⁰ coexist, and Rh³⁺/Rh⁰ ratios are compared. In Rh/CZO, Rh³⁺ and Rh^δ+ (0 ≤ δ < 1) coexist, and the Rh³⁺/Rh^δ+ ratios are compared; ^c Regenerated samples (I) were achieved by performing in situ regeneration using propane steam reforming with aged samples after aging at 1050 °C for 5 min; ^d Regenerated samples (II) were achieved after 25 cycles in the simulated fuel shutoff-regeneration cycle tests.

Fresh 0.5% Rh/Al₂O₃ displayed (1) an intense Rh³⁺ 3d_5/2 peak at 309.2 eV; (2) a small Rh⁰ 3d_5/2 side peak at 307.5 eV; and (3) Rh³⁺/Rh⁰ ratio of 1.29. Consistent with the TPR result, the XPS data suggests the Rh sites in the fresh samples were partially oxidized. With increasing aging temperature, Rh 3d peaks shift to higher binding energy, together with increases in Rh³⁺/Rh⁰ ratio (1.29 → 1.51 → 2.14 → 3.21), suggesting a transition to a higher Rh oxidation state, i.e., Rh⁰ → Rh³⁺. It is known that the oxidation process increases Rh oxidation states while the reduction process has an opposite effect [48]. The non-reducible Rh phase was reported resulting from a diffusion of Rh³⁺ ions in subsurface regions of the alumina matrix. The binding energy of the new Rh phase is greater than that in Rh₂O₃, indicating a different state from that of Rh³⁺ in Rh₂O₃, which is ascribed to metal-support interaction [81].

Different from 0.5% Rh/Al₂O₃, low Rh valence state (Rh^δ+, 0 < δ < 1) dominates in fresh 0.5% Rh/CZO (Rh³⁺/Rh^δ+ ratio of 0.48). For 0.5% Rh/CZO, Rh⁰ 3d_5/2 peaks display higher BE values. The small but definite electropositive shifts detected for Rh⁰ peaks are ascribed to the existence of both Rh⁰ and Rh^δ+ species, giving evidence to the existence of Rh^δ+/Rh⁰ and Ce⁴⁺/Ce³⁺ redox couple [63]. This assignment is in agreement with previous FT-IR result [82,83], which shows the existence of surface electron deficient Rh^δ+ species present on CZO support. Like Rh/Al₂O₃, the Rh³⁺/Rh^δ+ ratio in Rh/CZO increased with aging temperature. It is also important to note that the way Rh3d peak is interpreted largely affects the result. The XPS Rh 3d spectra for Rh/CeO₂ system studied by Force et al. [84] was deconvoluted into three peaks, respectively assigned to Rh⁰ (306.8 eV), Rh⁺ (307.8 eV), Rh³⁺ (309.2 eV) states. While other systems have different interpretations [85]. In our Rh/CZO system, assigning XPS peaks to Rh⁰ and Rh^δ+ species is easier for comparison.

Furthermore, the reduced areas under Rh 3d peaks for aged samples suggests Rh sintering and/or Rh dissolution into sintered support during simulated fuel shutoff aging. The characterization result is in agreement with the findings by Kang et al. [20]. In their study, the effect of aging atmosphere on the sintering behavior of commercial Pd- or Rh-TWC as well as the TWC performance were investigated under straight oxidizing, reducing, and periodic cycling aging conditions. For Rh-TWC, the diffusion of Rh₂O₃ into the support along with the agglomeration of the Rh metal were found the main causes of catalyst deactivation during high temperature oxidative aging.

XPS Rh 3d spectra of regenerated 0.5% Rh/Al₂O₃ and 0.5% Rh/CZO in both show that after the first in situ regeneration, the oxidation state of Rh was significantly lowered, exposing more active Rh⁰ species to the reactant atmosphere. This explains the enhanced reforming activity resulting from H₂ reduction (regeneration).

In summary, different types of interactions between Rh and support materials exist in Rh/Al₂O₃ and Rh/CZO during fuel shutoff aging. It is well known that strong interaction between Rh and Al₂O₃ with the formation of Rhodium Aluminate occurs in oxidative aging of Rh/Al₂O₃ [2]. Compared to aged Rh/Al₂O₃, the metal-support interaction in aged Rh/CZO occurs to a much lesser extent. Haneda et al. [86] reported that high-temperature aging can alter the surface properties of Ce_xO_y-ZrO₂ to inhibit the formation of formate species poisoning the catalytic active Rh sites.

The superior regenerability of 0.5% Rh/CZO was believed mainly contributed by the co-existence of Ce⁴⁺/Ce³⁺ and Rh⁰/Rh^δ+ redox couple [87,88,89,90]. Wang et al. [62], investigated the interaction between Rh and Ce_xO_y in Rh-Ce_xO_y/Al₂O₃ catalyst system, with enhanced electron transfer efficiency during catalytic CO₂ dry reforming of CH₄. Similar promotional effect likely occurred with 0.5% Rh/CZO catalyst during regeneration, as confirmed by TPR and XPS results. The electron transfer pathways during catalyst regeneration are proposed in Figure 14.

Figure 14. Proposed reaction mechanism and electron transfer pathways for steam reforming of propane on (a) Rh/Al₂O₃ and (b) Rh/CZO catalysts.

Figure 14. Proposed reaction mechanism and electron transfer pathways for steam reforming of propane on (a) Rh/Al₂O₃ and (b) Rh/CZO catalysts.

For Rh/Al₂O₃, electrons are first donated by hydrocarbons (reactant C₃H₈, and product C₂H₆ and CH₄), and then transferred through the redox circle of Rh⁰ ⇌ Rh⁺, and finally accepted by H₂O. Electron transfer is accompanied by redox reactions and the formation of H₂, CO, and intermediate products. For Rh/CZO, the coexistence of the Ce⁴⁺/Ce³⁺ and the Rh⁰/Rh^δ+ redox couple allows availability of Rh^δ+ species, to accept the electrons donated by HC more easily. The efficient electron transfer pathway results in the significant catalytic steam reforming performance of Rh/CZO.

3. Experimental Section

3.1. Catalyst Materials

The model catalysts studied were 0.5% Rh/Al₂O₃ and 0.5% Rh/Ce_xO_y-ZrO₂ (denoted as CZO with Ce:Zr atomic ratio of 1:2). The catalysts and reference support materials were supplied by BASF Iselin, NJ, USA. After impregnation of the precursor salts (proprietary) onto the support (γ-Al₂O₃ or CZO), a 25% solid slurry was created, ball milled, and calcined at 550 °C in N₂ to generate a catalyst powder sample with average particle size less than 30 µm, as estimated by SEM, Figure 15). The samples were stored in ambient air. XRD of 0.5% Rh/γ-Al₂O₃ showed a well-defined γ-Al₂O₃ support structure, with all the peaks indexed to a cubic unit cell (a = b = c = 7.900 Å, space group symmetry Fd3⁻m(227)) [91]. For 0.5% Rh/CZO, most of the support zirconia was observed incorporated into the ceria fluorite structure, with the formation of cubic symmetric Ce_xZr_(1−x)O₂ solid solution [92,93].

Figure 15. Scanning Electron Microscopic (SEM) images of fresh (a) 0.5% Rh/Al₂O₃ and (b) 0.5% Rh/CZO at µm scale. SEM measurement condition: beam voltage of 20 kV, beam current of 10 µm, working distance of 12 mm, and 30 µm in scale.

Figure 15. Scanning Electron Microscopic (SEM) images of fresh (a) 0.5% Rh/Al₂O₃ and (b) 0.5% Rh/CZO at µm scale. SEM measurement condition: beam voltage of 20 kV, beam current of 10 µm, working distance of 12 mm, and 30 µm in scale.

3.2. Simulated Fuel Shutoff Aging and Fuel Rich Regeneration Processes

Immediately prior to fuel shutoff aging, the TWC catalyst bed temperature is around 1000 °C generated under high load conditions. Upon fuel shutoff the introduction of air from the cylinder causes a short time increase in catalyst temperature by 15 °C to 20 °C due to the exothermic oxidation of adsorbed hydrocarbons on the catalyst surface. The catalyst bed temperature then begins to fall to about 800 °C in less than 10–15 s. During this high temperature-oxidizing environment the Rh reacts with the Al₂O₃ causing deactivation of the NO_x activity. Some of the US automobile companies use 1050 °C aging in air as a simulation of what is experimentally observed to insure stable catalyst performance for 150,000 miles of driving with periodic fuel shut off. We have adapted this procedure in our paper consistent with current practice. For more details about the catalyst temperature profiles during fuel shutoff, please refer to a previous SAE Technical Paper [16].

The schematic reactor system is sketched in Figure 16. During aging, 0.0702 mL (around 0.05 g) of powdered catalyst, well mixed with 0.25 mL quartz sand as a diluent, was loaded into a quartz tube reactor (ID of 10.5 mm, OD of 12.7 mm) with a quartz frit fused in the middle to hold the sample in place. The reactor was housed in an infrared furnace. Air flowed into the reactor system at 3400 mL/h through a calibrated gas flow controller (MKS 647 C, MKS Instrument Inc., Andover, MA, USA) with multiple gas channels. Reaction temperature of the catalyst bed was monitored by a thermocouple (Omega K type) placed at the inlet to the catalyst.

Figure 16. Schematic of the packed bed flow reactor and analysis system. (MFC: Mass Flow Controller, GC: Gas Chromatography, TI: Temperature Indicator).

Figure 16. Schematic of the packed bed flow reactor and analysis system. (MFC: Mass Flow Controller, GC: Gas Chromatography, TI: Temperature Indicator).

Catalyst regeneration was performed in situ by exposing the aged catalyst to reducing conditions. During regeneration the Rh catalyzes steam reforming (SR), generating H₂, which reduces Rh³⁺ to its active metallic state Rh⁰.

The regeneration feed gas mixture of 500 vppm propane, 10 vol-% steam, 8 vol-% CO₂, and N₂ in balance, with a total GHSV of 120,000 h⁻¹ was used to simulate the engine exhaust at slightly fuel rich conditions. Propane is commonly used as a model compound [30,31,94,95] for the HC species. Liquid water was injected at 0.68 mL/h by a syringe pump (Cole Parmer), vaporized at around 120 °C, and mixed with the incoming gas feed. The regeneration temperature was maintained 550 °C for 1 h with H₂ production continuously monitored. Temperatures of the water evaporator T_W and catalyst bed T_bed were monitored by thermocouples. A cold trap was placed downstream to condense the unreacted water, and a calibrated micro GC (Inficon 3000, INFICON Inc., New York, NY, USA, equipped with 10 m Molsieve 5A column, 8 m Plot U column, and thermal conductivity detectors) was used for online analysis of the gas products every three minutes. The regenerated sample was then cooled in air to room temperature, and preserved in ambient air.

3.3. Catalyst Regenerability as Measured at Simulated Fuel Rich Condition

Activity tests were performed with the same reaction feed as that in regeneration, but with temperature scans from 200 °C to 550 °C, with 50 °C increments, and a 30 min-hold at each temperature. The catalytic conversions were conducted far from equilibrium.

3.4. Catalyst Stability during Simulated Fuel Shutoff Aging-Fuel Rich Regeneration Cycle Tests

Fuel shutoff aging-fuel rich regeneration cycle tests (25 cycles in total) were performed to simulate the automotive engine operation cycles, as shown in Figure 17a,b respectively. Simulated fuel shutoff and regeneration conditions were maintained as described in Section 3.2, except the aging temperature of 1050 °C was used. Activity of the regenerated catalysts after every five cycles were measured, as described in Section 3.3.

Figure 17. Schematic process flow diagrams of (a) simulated fuel shutoff aging-fuel rich regeneration cycle and activity test; and (b) on-board gasoline engine fuel shutoff-fuel rich operation cycles.

Figure 17. Schematic process flow diagrams of (a) simulated fuel shutoff aging-fuel rich regeneration cycle and activity test; and (b) on-board gasoline engine fuel shutoff-fuel rich operation cycles.

3.5. Data Analysis for Catalyst Activity Tests

During catalyst activity tests, the mole flow rate Q_i (mol/h) of each gas product component (H₂, N₂, CO, CH₄, CO₂, C₃H₈, except for water, which was condensed before GC analysis) was determined by Equation (1).

$$Q{}_{\text{i}}=Q_{{\text{N}}_2}\times \frac{F_{\text{i}}}{F_{{\text{N}}_2}}$$

(1)

where $Q_{{\text{N}}_2}$ is the mole flow rate of the carrier gas N₂ (also the internal standard, 0.3383 mol/h). F_i is the mole percentage of compound i in the gas product mixture as analyzed by the online micro GC. Thermodynamic modeling at the same reaction condition was performed by HSC Chemistry 5.

3.6. Catalyst Characterization

SEM images were taken with a Hitachi S-4700 I Cold Field Emission Scanning Electron Microscope (Hitachi High Technologies America Inc., Schaumburg, IL, USA. Accelerating voltage of 20 kV, emission current of 10 µA, and working distance of 12 mm were used. Multiple pictures at different spots within the same measured sample were collected in each measurement.

XRD patterns of catalyst samples were generated with a Scintag XDS-2000 unit (Scintag Inc., Cupertino, CA, USA). The powdered samples were scanned between 15° and 80° with an incremental step of 0.02° and preset time of 2 s.

The Brunauer-Emmett-Teller (BET) surface areas of catalysts were determined using a Quantachrome ChemBET Pulsar TPR/TPD unit (Quantachrome Instrument, Boynton Beach, FL, USA), equipped with a TCD detector. About 0.05 g of catalyst sample was outgassed in pure N₂ at 200 °C for 2 h, while subsequent N₂ adsorption was performed using 30% N₂/He at liquid N₂ temperature (−195.6 °C). The TCD signal was calibrated using an external standard method, and monolayer N₂ adsorption was evaluated by single point BET method.

The metal dispersions of catalysts were measured by CO- selective chemisorption using the same Quantachrome unit. About 0.1 g of catalyst sample was heated in pure He at 200 °C for 120 min, followed by pre-reduction in 10% H₂/Ar at 400 °C for 120 min. CO (99.9% purity) adsorption, with automatic injection volume of 285 µL per pulse, was performed at 40 °C after pre-reduction. CO chemisorption capacity of each catalyst sample was evaluated based on the total volume of adsorbed CO at standard condition (V_CO,std, L). Metal dispersion (D_M) is calculated according to Equation (2):

$$\text{Dispersion }\left(\text{\%}\right)=\frac{1}{n}\times \frac{V_{\text{CO},std}}{22.4}\times \frac{M_M}{m_s\times y_M}\times 100\text{\%}$$

(2)

where M_M is the metal atomic weight (102.9 g/mol for Rh). The catalyst weight and metal content are designed by m_s and y_M respectively. The CO-to-metal site stoichiometry n was assumed to be 2 in accordance with current literature [20,96].

The Transmission Electron Microscopic (TEM) observations of the fresh, aged, and regenerated samples were taken with a JEOL 100CX-II TEM unit (JOEL Inc., Peabody, MA, USA). The TEM measurements were operated at an accelerating voltage of 100 kV. The catalyst powder sample was dispersed in pure ethanol (200 proof), followed by sonication for 3 h, and deposition on a Lacey carbon film supported Cu grid (200 mesh). For each sample, 50~60 TEM images with different magnifications at multiple spots were taken, and no less than 400 individual palladium particles were counted with ImageJ software. The mean surface area-weighted palladium particle size is calculated using Equation (3):

$$d_{TEM}=\frac{{\sum }_i{n}_i{d}_i^3}{{\sum }_i{n}_i{d}_i^2}$$

(3)

where n_i is the number of particles in ferret diameter d_i and ${\sum }_i{n}_i>400$.

The redox properties of catalysts were studied by TPR. The measurements were carried out with the same Quantachrome unit as above. About 0.1 g of catalyst sample was first outgassed in pure Helium at 150 °C for 2 h, and cooled to room temperature. TPR analysis was performed subsequently by heating a sample located in a U-tube reactor to 800 °C at 5 °C/min, with 4% H₂/N₂ flowing through the sample. The TCD signal (corresponding to H₂ uptake) was then normalized to per gram of catalyst.

Ex situ XPS spectra of catalysts were measured with a Perkin-Elmer PHI 5500 XPS instrument (Physical Electronics Inc., Chanhassen, MN, USA) equipped with a Mg Kα monochromatic source. The samples were prepared by fixing catalyst powder onto a double-sided carbon sticky tape. The XPS main chamber was evacuated to 10⁻⁹ Torr. C 1s peak with standard binding energy of 284.6 eV was used for peak position calibration. AugerScan and Origin software were used for spectra data analysis. NIST XPS online database and other literature sources were used for peak assignments.

4. Conclusions

Automotive three way catalysts (TWC) experience severe deactivation during fuel shutoff when exposed to an oxidizing environment at temperatures up to 1050 °C. A return of air-to-fuel ratio to slight rich (λ < 1) allows in situ catalyst regeneration by H₂ generated mainly through catalytic steam reforming. The paper examined the effects of oxidative fuel shutoff and simulated fuel rich regeneration on the activity and chemistry of the Rh component in TWC. 0.5% Rh/Al₂O₃ and 0.5% Rh/CZO are compared.

For both supported Rh catalysts, deactivation is accelerated with increasing oxidative aging temperature. Metal and support sintering, reversible metal oxidation, and metal-support interactions were found in both catalysts but to widely different extents. Compared to the strong metal-support interactions (with the formation of rhodium aluminate) in Rh/Al₂O₃, the interactions in Rh/CZO during simulated fuel shutoff was weaker and more readily reversible.

Partial catalyst regeneration is accomplished by reducing Rh to its active metallic state (Rh⁰) thereby “releasing” it from the metal-support interactions. Stable catalytic performance is achieved by periodic aging-regeneration cycle tests (25 cycles in total). Compared to Rh/Al₂O₃, Rh/CZO showed more rapid response to regeneration and maintained higher stability. The existence of Rh^δ+/Rh⁰ and Ce³⁺/Ce⁴⁺ redox pair in Rh/CZO was confirmed by TPR and XPS, which was believed promoting the catalyst regenerability by enhancing the electron transfer efficiency during catalytic steam reforming.

This study is consistent with the known practice, used in gasoline vehicles, of fuel shutoff followed by a slightly rich mode to regenerate the Rh component and the NO_x activity.