Effect of activated carbon on the production selectivity of Ni/MoC catalyst for methanol steam reforming

Technical Summary: Effect of Activated Carbon on the Production Selectivity of Ni/MoC Catalyst for Methanol Steam Reforming

Problem Statement
Hydrogen is a critical energy carrier for proton exchange membrane fuel cells (PEMFCs), particularly for mobile transportation. However, challenges in hydrogen storage and transportation (low volumetric density, high-pressure requirements) have hindered widespread adoption. Methanol steam reforming (MSR) offers a promising solution due to methanol's high gravimetric hydrogen density and mild reforming conditions. The core challenge lies in developing catalysts that simultaneously achieve high methanol conversion, high hydrogen production rates, and low carbon monoxide (CO) selectivity (essential for PEMFCs), while maintaining long-term stability against sintering and coking. While non-noble metal catalysts like Cu/ZnO/Al2O3 are common, they suffer from poor thermal stability and high CO selectivity. Molybdenum carbide (MoC) has emerged as a superior support due to its metal-like conductivity and ability to activate water O-H bonds, yet optimizing its interaction with active metals and suppressing CO formation remains a significant hurdle.

Methodology
The study synthesized a series of Ni/MoC catalysts modified with varying loadings of activated carbon (AC) using a novel precursor preparation method.

• Catalyst Synthesis: Two distinct MoC supports were prepared via temperature-programmed reaction (TPR). MoC-1 was derived from ammonium molybdate calcination, while MoC-2 utilized a hydrothermal precursor step. Ni was loaded onto these supports via incipient wetness impregnation.
• AC Modification: Activated carbon was introduced into the Ni/MoC system (denoted as xCNi/MoC, where x is the AC weight percentage) and CyNi/MoC (where y is the Ni weight percentage with fixed 1% AC) to investigate the effects of AC content and Ni loading.
• Characterization: The physicochemical properties were analyzed using ICP-AES (Ni content), N2 physisorption (surface area and pore volume), H2 chemisorption (Ni dispersion), XRD and Raman spectroscopy (phase composition), SEM and STEM/EDX (morphology and particle distribution), and XPS (surface chemical states).
• Activity Testing: Catalytic performance was evaluated in a micro quartz tube reactor at temperatures ranging from 200°C to 300°C. Key metrics included methanol conversion, hydrogen production rate, and product selectivity (CO, CO2, CH4). Stability was assessed over 40 hours of continuous operation.

Key Contributions and Results

1. Optimization of AC Loading: The incorporation of a small amount of activated carbon (specifically 1 wt%, denoted as 1CNi/MoC) significantly enhanced catalytic performance compared to unmodified Ni/MoC and catalysts with higher AC loadings.

   • Activity: The optimal 1CNi/MoC catalyst achieved 95.6% methanol conversion and a hydrogen production rate of 164.3 mmol·gcat⁻¹·h⁻¹ at 250°C.
   • Selectivity: Crucially, the CO selectivity dropped to 0.8%, a substantial improvement over the 10.8% observed for the unmodified Ni/MoC catalyst.
   • Mechanism: The addition of AC facilitated the formation of the active α-MoC1−x phase and enhanced the dispersion of Ni species on the MoC surface. XRD and Raman data confirmed that 1CNi/MoC possessed the highest content of the α-MoC1−x phase. STEM analysis revealed that Ni particles were highly dispersed in 1CNi/MoC, whereas excess AC (3–5 wt%) caused pore blockage and Ni particle aggregation, leading to reduced activity.

2. Effect of Ni Loading: An optimal Ni loading of 10 wt% (C10Ni/MoC) was identified. Lower loading (5%) resulted in insufficient active sites, while higher loading (15%) promoted particle agglomeration, both reducing efficiency. The 10% loading provided the best balance between active site density and dispersion.

3. Temperature and Stability:

   • Temperature: 250°C was identified as the optimal operating temperature. While higher temperatures (280–300°C) did not significantly increase conversion, they promoted unwanted side reactions (methanation), increasing CH4 formation.
   • Stability: The 1CNi/MoC catalyst demonstrated superior stability, maintaining consistent activity and low CO selectivity over 40 hours of continuous operation. In contrast, unmodified catalysts showed initial deactivation and higher CO formation. The structural integrity provided by the optimal AC loading helped maintain the active α-MoC1−x phase and dispersed Ni sites.

Significance and Claims
The authors claim that this work provides a practical concept and technical strategy for the rational design of MSR catalysts. By introducing an appropriate amount of activated carbon, the study successfully achieved a synergistic effect that:

• Promotes the formation of the highly active α-MoC1−x phase.
• Enhances Ni dispersion and prevents sintering.
• Drastically suppresses CO formation, making the catalyst suitable for fuel cell applications.
• Ensures long-term operational stability.

The study concludes that the 1CNi/MoC catalyst represents a viable, high-performance, and stable solution for hydrogen production via methanol steam reforming, offering a pathway to overcome the limitations of traditional non-noble metal catalysts. The findings lay a theoretical and experimental foundation for further development of MoC-based catalysts in the MSR field.