Catalysts Promoted with Niobium Oxide for Air Pollution Abatement

Abstract

Pt-containing catalysts are currently used commercially to catalyze the conversion of carbon monoxide (CO) and hydrocarbon (HC) pollutants from stationary chemical and petroleum plants. It is well known that Pt-containing catalysts are expensive and have limited availability. The goal of this research is to find alternative and less expensive catalysts to replace Pt for these applications. This study found that niobium oxide (Nb₂O₅), as a carrier or support for certain transition metal oxides, promotes oxidation activity while maintaining stability, making them candidates as alternatives to Pt. The present work reports that the orthorhombic structure of niobium oxide (formed at 800 °C in air) promotes Co₃O₄ toward the oxidation of both CO and propane, which are common pollutants in volatile organic compound (VOC) applications. This was a surprising result since this structure of Nb₂O₅ has a very low surface area (about 2 m²/g) relative to the more traditional Al₂O₃ support, with a surface area of 150 m²/g. The results reported demonstrate that 1% Co₃O₄/Nb₂O₅ has comparable fresh and aged catalytic activity to 1% Pt/γ-Al₂O₃ and 1% Pt/Nb₂O₅. Furthermore, 6% Co₃O₄/Nb₂O₅ outperforms 1% Pt/Al₂O₃ in both catalytic activity and thermal stability. These results suggest a strong interaction between niobium oxide and the active component—cobalt oxide—likely by inducing an oxygen defect structure with oxygen vacancies leading to enhanced activity toward the oxidation of CO and propane.

1. Introduction

Carbon monoxide (CO) is produced by incomplete combustion of carbon-containing fuels. When this deadly toxic gas combines with hemoglobin in blood, oxygen (O₂) cannot be delivered to vital organs essential for life. Propane (C₃H₈) is produced during the process of combusting liquefied petroleum gas, but can also be considered a model of volatile organic compounds (VOCs). An active low-temperature oxidation catalyst has attracted immense attention to meet ever-changing stringent environmental regulations for oxidation of volatile organic compounds in chemical plants, petroleum refineries, pharmaceutical plants, automobile manufacturing, etc. [1,2].

Volatile organic compounds are toxic and mainly contribute to the formation of photochemical smog with a with a negative impact on air quality [3]. Catalytic oxidation is a main technology used commercially in their reduction [4,5]. The basic catalytic oxidation reactions of CO (1) and hydrocarbon (C_xH_y) (2) are shown as follows [2]:

$$\mathrm{C}\mathrm{O}+1/2{\mathrm{O}}_2\stackrel{Catalyst}{\to }\mathrm{C}{\mathrm{O}}_2$$

(1)

$${\mathrm{C}}_{\mathrm{x}}{\mathrm{H}}_{\mathrm{y}}+(\mathrm{x}+\mathrm{y}/4)\left({\mathrm{O}}_2\right)\stackrel{Catalyst}{\to }\mathrm{x}\mathrm{C}{\mathrm{O}}_2+\mathrm{y}/2{\mathrm{H}}_2\mathrm{O}$$

(2)

Many environmental abatement catalysts are precious metals, such as Pt and Pd, due to their excellent performance and superior life in abating real exhausts. However, their high price is a disadvantage, and therefore alternative materials are always being sought. However, this is very challenging. For low temperature applications, such as VOCs removal in indoor air, transition metal oxides have been proposed as replacements for precious metals [6]. Among transition metal oxides, cobalt oxide (Co₃O₄) shows very high CO oxidation activity in CO/O₂ mixtures even at ambient temperature [7]. Co₃O₄ is also highly effective for the total oxidation of propane under conditions relevant for VOC emission control [8]. Moreover, the evasive “holy grail” for catalytic applications is a precious metal–free catalyst for three-way automobile gasoline exhaust catalysts [9].

It is an essential to disperse the catalytic components on a carrier with a high surface area, such as Al₂O₃, TiO₂, SiO₂, ZrO₂, or SiO₂-Al₂O₃, in order to maximize active sites available for reactants. Furthermore, supported catalysts are deposited (as washcoats) on high cell density monoliths (ceramic and metal) to minimize pressure drop and volume relative to packed beds [2]. Niobium pentoxide (Nb₂O₅) has been reported to show strong metal support interaction (SMSI) with certain metals [1,10,11]. However, no commercial VOC applications that include Nb₂O₅ are known.

The goal of this study was to investigate Nb₂O₅’s promoting effects on base metal catalysts for CO and propane oxidation. Nb₂O₅ has the ability to form defect structures with oxygen vacancies when combined with base metal oxide materials to enhance catalytic activity [12,13]. The active oxidation state for cobalt is +3 in Co₃O₄ and has been reported to be a prime candidate for precious metal replacement in VOC applications due to its high catalytic activity [14,15]. Retaining Co in the active +3 state will enhance its thermal stability. The current study was designed to further explore the performance of Co supported on Nb₂O₅ and to investigate whether other transition metal oxides combined with Nb₂O₅ could also have a beneficial effect for VOC applications. This feasibility study compared the oxidation performance of Co₃O₄/Nb₂O₅ with traditional Co₃O₄/Al₂O₃. Furthermore, the study expanded to evaluate other base metal oxides such as iron oxide (Fe₂O₃), copper oxide (CuO), and nickel oxide (NiO), all of which were deposited on Nb₂O₅ relative to Al₂O₃ with the aim to broaden the understanding of the promoting effects of Nb₂O₅ in VOC applications. Finally, the catalytic performance of Co₃O₄/Nb₂O₅ was compared to that of Pt/γ-Al₂O₃.

The catalytic protocols used in establishing performance characteristics were fresh and aging activity tests. Thermal gravimetric analysis (TGA) was used to establish minimum time and temperature necessary for complete decomposition of precursor to the respective oxides.

2. Result and Discussion

2.1. TGA Results for Precursor of Cobalt (Co)

Thermal gravimetric analysis (TGA) measures the weight change of a material upon heating in various gaseous environments. Figure 1 shows the TGA results of three samples: the precursor Co(NO₃)₂·6H₂O, the impregnated precursor on carriers, 6% Co(NO₃)₂/Nb@800 (the term Nb@800 = pre calcination temperature of Nb₂O₅ at 800 °C in air for 2 h), 6% Co(NO₃)₂/Al@800 (pre calcination of Al₂O₃ in air for 2 h). The precursor Co(NO₃)₂·6H₂O decomposes completely to Co₃O₄ at about 300 °C where no additional weight loss occurs [16] upon continued heating as shown in Figure 1. The impregnated catalysts also achieve constant weight at 300 °C. Thus, all impregnated catalysts were calcined at 300 °C. The cobalt oxide content was 6% by weight on supported carriers.

2.2. Preparative Details of Co/Nb and Co/Al

2.2.1. Pre-Calcination Temperature of Carriers Nb₂O₅ and Al₂O₃

Nb₂O₅ has different crystal phases at different temperature [17]. The pre-calcination temperature affects the crystal phase of Nb₂O₅, which in turn affects the chemical and physical properties of Nb₂O₅. The structure of Al₂O₃ is similarly affected. The monohydrate and tri-hydrate alumina structures change as a function of the temperature (°C) in air [18]. The impact of pre-calcination temperature on carriers and final catalysts were therefore explored.

All carriers were pre-calcined at specified temperatures for 2 h in air. After pre-calcination of the carriers, Co(NO₃)₂·6H₂O was deposited on each carrier and calcined at 300 °C in air for 2 h to give 6% Co@300/Nb and 6% Co@300/Al. This nomenclature indicates 6% cobalt oxide calcined at 300 °C after deposited on Nb₂O₅ or Al₂O₃.

Table 1. Six percent Co@300/Nb with the number following @ = pre-calcination temperatures of the carrier for CO oxidation.

Catalyst	T20 (°C)	T50 (°C)	T90 (°C)
6% Co@300/Nb@500	190	200	220
6% Co@300/Nb@700	150	155	160
6% Co@300/Nb@800	130	145	160
6% Co@300/Nb@900	130	155	185

demonstrates catalytic performances of fresh catalysts of 6% Co/Nb with different pre-calcination temperatures of Nb₂O₅. In Table 1, all carriers were Nb₂O₅ pre-calcined from 500 °C to 900 °C. T₂₀, T₅₀ and T₉₀ values (temperature for 20%, 50% and 90% conversion) are compared. T₂₀ and T₅₀ are indicative of chemical kinetic control, while T₉₀ often reflects some pore diffusion control. It is clear that Co on Nb@700 and Nb@800 show better catalytic performance of CO oxidation with the Nb@800 showing the best performance.

Table 2. Six percent Co/Al with @ = pre-calcination temperatures of the carrier for CO oxidation.

Catalyst	T20 (°C)	T50 (°C)	T90 (°C)
6% Co@300/Al@500	205	225	245
6% Co@300/Al@700	200	215	-
6% Co@300/Al@800	185	205	240
6% Co@300/Al@900	215	240	270

provides catalytic results of fresh 6% Co/Al with different pre-calcination temperatures of Al₂O₃. The T₂₀, T₅₀ and T₉₀ indicate that Al@800, pre-calcined at 800 °C, yielded the best results.

One can conclude that the optimal pre-calcination temperature for both Nb₂O₅ and Al₂O₃ was 800 °C. Therefore, both Nb₂O₅ and Al₂O₃ were pre-calcined at 800 °C for all experiments conducted henceforth.

Further, Figure 2 compares the activity of cobalt catalysts as a function of the pre-calcination temperature of the two carriers. Clearly, 6% Co@300/Nb@800 yielded the best performance of all for CO oxidation.

To further quantify the advantages offered by fresh 6% Co@300/Nb@800 relative to fresh 6% Co@300/Al@800 turn over frequencies (TOF) are presented in

Table 3. Turnover Frequencies (TOF) for CO oxidation for fresh 6% Co@300/Nb@800 relative to fresh 6% Co@300/Al@800.

Catalyst	TOF@150 °C (h−1 × 105)	TOF@170 °C (h−1 × 105)	TOF@190 °C (h−1 × 105)	TOF@210 °C (h−1 × 105)
6% Co@300/Nb@800	897.8	1508	1508	1508
6% Co@300/Al@800	53.48	129.9	496.6	1241

for different temperatures.

$$T=\frac{\left(\frac{gram}{hr}ofCO\right)•\left(fractionalconversionofCO\right)}{gramofC{o}_3{O}_4}$$

2.2.2. Catalytic Performance of Fresh and Aged Co Catalysts for CO Oxidation

Based on the data in Table 1, Table 2 and Table 3, the pre-calcination temperature of carriers was set at 800 °C with the precursor decomposition at 300 °C for 2 h in air.

Figure 3 shows that fresh 6% Co@300/Nb@800 outperforms fresh 6% Co@300/Al@800 for CO oxidation over the entire conversion-temperature profile including the kinetic control regime up to fractional conversion approaching of 1.0 (100% conversion). Catalytic performance of each catalysts aged at 400 °C for 12 h in reaction gases is also shown in this figure. This temperature is approximately the maximum experienced in abating VOC from some stationary chemical plants. Clearly, 6% Co@300/Nb@800 outperformed its counterpart however, both catalysts suffered some deactivation after aging. Six percent Co@300/Nb@800 seemed to experience some reaction rate limited by pore diffusion (slightly lower slope at T₅₀ region), while the chemical kinetic control region (T₂₀) was not affected. For 6% Co@300/Al@800 both kinetic control (T₂₀ region) and pore diffusion control (T₅₀ region) were negatively affected as evidenced by the large shift to higher temperature and the decrease in their respective slopes. One can speculate that the shift to higher temperature and a slightly lower slope reflects a loss of accessibility to the Co active sites due to its reaction rate limited by pore diffusion.

2.3. Various Loadings of Co₃O₄ Supported on Nb₂O₅

2.3.1. Catalysts with Various Co₃O₄ Loadings: Propane Oxidation

Various Co₃O₄ loadings were studied to optimize Co₃O₄ content. The oxidation of propane was used as the metric because it is more difficult to oxidize than CO. In Figure 4 (left), three fresh catalysts with cobalt contents, 1%, 3%, and 6% are compared. All three completed propane oxidation up to 400 °C. Not surprisingly 6% Co@300/Nb@800 shows the best catalytic performance. The sensitivity of performance to cobalt loading confirms that the activity is in the kinetic regime suitable for activity comparison.

In Figure 4 (right), all cobalt oxides on niobium oxide were aged at 400 °C for 12 h in reaction propane gas.

Detailed information of these Co@300/Nb@800 are provided in

Table 4. Co@300/Nb@800 with various cobalt loadings for propane oxidation: fresh and aged.

Catalyst	T20 (°C)	T50 (°C)	T90 (°C)
Fresh 6% Co@300/Nb@800	250	280	305
Aged 6% Co@300/Nb@800	290	310	340
Fresh 3% Co@300/Nb@800	260	270	310
Aged 3% Co@300/Nb@800	285	320	360
Fresh 1% Co@300/Nb@800	290	315	345
Aged 1% Co@300/Nb@800	315	355	390

. It is clear that 6% Co@300/Nb@800 presents the lowest T₂₀, T₅₀, and T₉₀ over the entire conversion profile.

2.3.2. Propane Oxidation with and without Moisture

Results for both fresh 6% Co@300/Nb@800 and 6% Co@300/Al@800, are presented in Figure 5. Once again, the superiority of fresh 6% Co@300/Nb@800 relative to the baseline 6% Co@300/Al@800 is demonstrated.

Moisture (from upstream combustion) is always present and typically has a poisoning or inhibiting effect on the performance of the catalyst. Tests were conducted with a feed containing 5% H₂O (steam) for both 6% Co@300/Nb@800 and 6% Co@300/Al@800. In Figure 6, fresh and aged 6% Co@300/Nb@800 and 6% Co@300/Al@800 are compared for propane oxidation with 5% H₂O present. 6% Co@300/Nb@800 retains much of its activity, and is far more resistant to deactivation than 6% Co@300/Al@800. However, moisture does inhibit 6% Co@300/Nb@800, but far less than for the 6% Co@300/Al@800.

This figure also demonstrates the obvious advantage of 6% Co@300/Nb@800 over 6% Co@300/Al@800 when both catalysts were aged for 12 h in feed gas containing H₂O at 5 vol %. The aged 6% Co@300/Al@800 lost the majority of its active sites achieving only about 40% (0.4) conversion at 400 °C. Aged 6% Co@300/Nb@800 showed significant catalytic activity even after being aged in steam containing reaction gas at 400 °C for 12 h.

2.4. Comparison of Co₃O₄ and Pt for Propane Oxidation

The state of the art catalyst used in VOC abatement is typically 1% Pt/Al₂O₃. For the purpose of a practical application, the behavior of 1% Co@300/Nb@800 and 6% Co@300/Nb@800 was compared with 1% Pt/Nb@800 and 1% Pt/Al@800 for propane oxidation. The result shown in Figure 7 (left) indicates that 6% Co@300/Nb@800 shows a performance advantage over 1% Pt@500/Nb@800 and 1% Pt@500/Al@800. Some advantage for 1% Co@300/Nb@800 is noted at both low and higher conversions however, diffusional effects may be operative. 1% Pt@500/Nb@800 shows no advantage over 1% Pt@500/Al@800. Figure 7 (right) shows results of 1% Co@300/Nb@800, 6% Co@300/Nb@800 and 1% Pt@500/Nb@800 subjected to aging test at 400 °C for 12 h in air. The 6% cobalt system suffers slightly, relative to the Pt system, at light off but recovers its performance advantage as 100% conversion is approached. From these encouraging results, monolith-supported catalysts will be prepared in the future and comparison extended closer to an actual system.

Table 5. T₂₀, T₅₀, and T₉₀ of fresh Co catalysts and Pt catalysts for propane oxidation.

Catalysts	T20 (°C)	T50 (°C)	T90 (°C)
6% Co@300/Nb@800	250	280	305
1% Co@300/Nb@800	290	315	345
1% Pt@500/Nb@800	275	325	370
1% Pt@500/Al@800	275	340	370

and

Table 6. T₂₀, T₅₀, and T₉₀ of aged Co catalysts and Pt catalysts for propane oxidation.

Catalysts	T20 (°C)	T50 (°C)	T90 (°C)
6% Co@300/Nb@800	290	310	340
1% Co@300/Nb@800	315	355	390
1% Pt@500/Nb@800	290	335	375

exhibit T₂₀, T₅₀, and T₉₀ of Co catalysts and Pt catalysts showed in Figure 7. The advantages 6% Co@300/Nb@800 over 1% Pt@500/Nb@800 are clear to see.

2.5. Other Base Metal Oxides for Propane Oxidation

Based on the encouraging results with 6% Co@300/Nb@800 for both CO and C₃H₈ the study was expanded to include nickel (Ni), copper (Cu) and iron (Fe) each deposited on Nb@800 and Al@800. A comparison is shown in Figure 8 (left) for propane oxidation. All four base metal oxides were prepared with a loading of 6% on Nb@800. It is clear that Co₃O₄ on Nb₂O₅ has lowest light-off and full oxidation performance. Aging for 12 h at 400 °C in reacting gases also showed the superiority of the cobalt system. Figure 8 (right) shows results that Nb₂O₅ has a much greater promoting effects on Co₃O₄ than on NiO.

2.6. Cobalt Oxide Catalyst Characterization

BET Tests for Catalysts and Carriers

Surface area of the carrier is among the most fundamentally important properties in catalysis because the active sites are present or dispersed throughout the internal surface through which reactants and products are transported [2]. Quantachrome ChemBET Pulsar TPR/TPD is used to measure surface area of Al₂O₃ and Nb₂O₅.

A 50 mg sample is used for Al₂O₃ and 6% Co₃O₄/Al₂O₃; 200 mg sample is used for Nb₂O₅ and 6% Co₃O₄/Nb₂O₅. Figure 9 shows that Nb₂O₅ pre-calcined at 500 °C has a surface area of 53 m²/g. When pre-calcined at 700 °C, Nb₂O₅ has a surface area decrease to 5 m²/g, while at 800 °C it, decreased to 2 m²/g. Meanwhile, Al₂O₃ has a 100 times larger surface area than Nb₂O₅. After pre-calcined at 800 °C Al₂O₃ still has a surface area of about 120 m²/g. Nb₂O₅ has no surface area stability advantages over Al₂O₃, indicating some other interaction of Nb₂O₅ and Co₃O₄ gives rise to the enhancement of activity. In Figure 10 (left), when 6% Co₃O₄ is impregnated onto Nb₂O₅, the total surface area increases. But the final surface area is still very small compared to the surface area of Al₂O₃. Figure 10 (right) shows no change in surface area after Co impregnation.

3. Materials and Methods

3.1. Carrier Preparation

Niobium Oxide Hydrate (Nb₂O₅·5H₂O) was received from CBMM Brazil (Sao Paolo, Brazil). Alumina (gamma-Al₂O₃) was provided by BASF (Iselin, NJ, USA). Both Nb₂O₅ and Al₂O₃ were pre-calcined at various temperatures prior to deposition of cobalt nitrate hydrate (Co(NO₃)₂·6H₂O).

3.2. Catalyst Preparation

Incipient wetness was used for metal impregnations. In order to determine the required volume to fill the pores, water was slowly added to the calcined carrier until saturation was achieved. The precursor salt was dissolved in this precise amount of water, which was then added to the carrier. It was dried in air at 130 °C for one hour and then calcined in air to various temperatures as shown in the Results and Discussion section.

3.2.1. Co-Containing Catalyst Preparation

The cobalt nitrate impregnated catalyst was dried at 130 °C for 1 h in air to remove all the water and then calcined at 300 °C in air to completely decompose the precursor salt for 2 h. The heating rate was 10 °C/min. Cobalt catalysts were prepared with loadings from 1%, 3% to 6% on both carriers. Percentages are based on the metal oxide component.

3.2.2. Pt-Containing Catalysts Preparation

A water soluble platinum amine salt, which is alkali and halide free, provided by BASF (Iselin, NJ, USA), was used for preparation of the Pt catalyst using incipient wetness. 1% Pt/Nb₂O₅ and 1% Pt/Al₂O₃ were prepared with the 800 °C pre-calcined carriers. Drying was accomplished 130 °C in air and the final calcination at 500 °C for 2 h in air at a heating rate of 10 °C/min.

3.3. Reactor Test

3.3.1. Reactor Preparation

The 0.1 mL (about 0.08–0.09 g) of finished catalyst was uniformly mixed with about 0.75 g of diluent quartz to maintain the bed temperature approximately constant. Air and propane (1% C₃H₈ in N₂ were introduced into the reactor. The reactor quartz tubing was 10.5 mm (ID) × 12.75 mm (OD) with a length of 50 cm with XC-course quartz frit provided by Quartz Scientific, Inc. Harbor, OH, USA to support the catalyst.

3.3.2. Reactor Gas Flow Rate

The volume of catalyst used for all experiments was 0.1 mL and the total volumetric flow rate was 108 mL/min. Thus the GHSV was set at 64,800 h⁻¹.

$$GHSV=\frac{FlowRate\left(STP\right)}{CatalystVolume}=\frac{108\mathrm{mL}/\min }{0.1\mathrm{mL}}=1080{\min }^{-1}=64,800{\mathrm{h}}^{-1}$$

Reactor Gas Flow without Steam for CO Oxidation

21% O₂/N₂ with flow rate 75 mL/min and 4.94% CO/N₂ with flow rate 33 mL/min was used as reactant gases. Flow rates are measured at room temperature of 25 °C and pressure of 1 atm. The GHSV here is: 64,800 h⁻¹. Rotameters were calibrated with a soap film bubble meter and used to control flow rates.

Reactor Gas Flow without Steam for Propane Oxidation

21% O₂/N₂ at flow rate of 97.2 mL/min and 1% C₃H₈/N₂ at a flow rate of 10.8 mL/min was used as reactant gases giving C₃H₈ = 0.1% C₃H₈ or 1000 ppm. All flow rates were measured at room temperature of 25 and pressure of 1 atm. The GHSV here was: 64,800 h⁻¹. Rotameters were calibrated with a soap film bubble meter and used to control flow rates.

Reactor Gas Flow with Steam for Propane Oxidation

Twenty-one percent O₂/N₂ at a flow rate of 91.8 mL/min, 1% C₃H₈/N₂ at a flow rate of 10.8 mL/min and H₂O (steam) was introduced giving a flow rate of 5.4 mL/min. This gives 0.1% C₃H₈ with 5% steam and balance air. Liquid water was injected into the reactor connected into a heated transfer line. The heating tape was set at 150 °C; sufficiently high to generate a homogenous gaseous mixture. The GHSV here is 64,800 h⁻¹. Rotameters were calibrated with a soap film bubble meter and used to control flow rate.

3.4. Data Management

3.4.1. Data Acquisition

Catalytic oxidations were carried out in a fixed-bed quartz flow reactor at atmospheric pressure. A cold trap was placed at the outlet to condense water produced during the reaction prior to entering the gas chromatography (GC). Effluent gases of O₂, CO, CO₂, C₃H₈ and N₂ were analyzed by the micro GC. The temperature of the catalyst bed was measured by a K-type thermocouple and controlled by Omega CN7800 series temperature controller. An INFICON 3000 Micro GC, East Syracuse, NY, USA was used to analyze gas composition. Five single runs were conducted at each temperature. The GC was calibrated using 4 different certified standard gases before measurement.

3.4.2. Data Processing

Raw data from the GC was managed and analyzed in Excel to obtain plots of conversion as a function of temperature. The inlet catalyst temperature was increased and held until steady conversion was achieved. The temperature was controlled by a thermocouple located inside the reactor immediately above the catalyst inlet and connected to the MELLEN TEMPERATURE CONTROLLER. The temperature was increased in increments of 10 °C and the conversion measured.

Conversion of CO and C₃H₈ oxidation at each temperature point is as follows:

$$CO(Conversion)\%=\frac{C{O}_{\left(initial\right)}-C{O}_{\left(exit\right)}}{C{O}_{\left(inital\right)}}\times 100\%$$

(3)

$$C_3{H}_8(Conversion)\%=\frac{C_3{H}_{8\left(initial\right)}-C_3{H}_{8\left(exit\right)}}{C_3{H}_{8\left(initial\right)}}\times 100\%$$

(4)

The oxidation reaction of CO and C₃H₈ to CO₂ is as follows:

$$CO+\frac{1}{2}{O}_2\to C{O}_2$$

(1)

$$C_3{H}_8+5O_2\to 3C{O}_2+4H_{2}O$$

(2)

3.5. Aging Test

The aging temperature was maintained at 400 °C for 12 h in the reactant gases, followed by the generation of a conversion vs. temperature profile.

4. Conclusions

The pre-calcination temperature of the Nb₂O₅ carrier of 800 °C with cobalt calcined at 300 °C gives the best fresh and aged performance for both CO and propane oxidation. Nb₂O₅ shows a significant advantage over Al₂O₃ as the carrier. Six percent Co@300/Nb@800 has better fresh catalytic performance, higher thermal stability and greater resistance to steam than its counterpart, 6% Co@300/Al@800. Co₃O₄/Nb₂O₅ demonstrates increasing catalytic performance with Co₃O₄ loading from 1% to 6%. 6% Co@300/Nb@800 outperforms 1% Pt@500/Nb@800 for both fresh and aged states. Nb₂O₅ promotes Co₃O₄ and fresh NiO. However there is no advantage for CuO and Fe₂O₃ compared to the traditional Al₂O₃ carrier. Additionally, 6% Ni@350/Nb@800 has poor thermal stability relative to 6% Co@300/Nb@800. Therefore, 6% Co@300/Nb@800 is a viable candidate for replacing 1% Pt@500/Al@800 even though some small decrease in GHSV may be necessary to insure equal performance. However, this will not negatively affect the cost advantage of Co₃O₄ compared to Pt-containing catalysts. Naturally, candidate catalysts must be deposited on monolith and subjected to real VOC feed streams before a definitive recommendation can be made regarding replacement for Pt.