Hydrodechlorination Activity Research Articles

Catalytic hydrodechlorination of chlorophenols over facet-specific Pd nanocrystals

Clarifying the reaction mechanism of catalytic hydrodechlorination (HDC) and designing Pd surface structures with high catalytic activity are crucial for maximizing its ability to remove chlorophenol pollutants. In this study, single-crystalline Pd nanocrystals enclosed by {110}, {100} or {111} facets with average particle diameters ∼ 6.5 nm were prepared and then originally evaluated their activities for catalytic HDC of chlorophenols. The experimental results indicate that dodecahedral Pd nanocrystals enclosed with {110} facets show the highest catalytic activity for HDC of chlorophenols with a turnover frequency up to 19.1 min−1, which is ∼ 3.1 times as high as that of cubic Pd nanocrystals and 12.7 times higher than that of octahedral Pd nanocrystals. Theoretic calculations reveal that Pd (110) facets have the strongest activation ability for chlorophenol and the lower energy barrier in dechlorination and hydrogenation steps. Moreover, dodecahedral Pd nanocrystals exhibit high HDC activity for various chlorophenolic derivatives and structural stability. We believe that the study could provide insights into the understanding and design of highly efficient Pd systems for various HDC reactions.

Enhanced electrocatalytic hydrodechlorination of 2,4-dichlorophenol over palladium nanoparticles loading on dendrite-like Ni-Cu-P interlayer

A new electrode with high electrocatalytic hydrodechlorination activity was prepared by pulse electrodeposition of Pd nanoparticles on a dendrite-like Ni-Cu-P interlayer with foam nickel as the substrate. The physical and chemical properties of the composite electrode were systematically investigated by various characterizations. The dechlorination performance was evaluated with 2,4-dichlorophenol as the model pollutant. The results indicated that in 150 min the dechlorination efficiency over the electrode without the Ni-Cu-P interlayer was only 52.7 %, while 100 % of 2,4-dichlorophenol could be removed by the composite electrode. The enhanced dechlorination performance was mainly attributed to the synergic effect between Pd nanoparticles and the Ni-Cu-P interlayer. The dendrite morphology of Ni-Cu-P facilitated Pd nanoparticle dispersion, thus providing more active sites and reducing the consumption of Pd. Furthermore, the Ni-Cu-P interlayer assisted Pd0 to produce more active hydrogen and increased the content of Pd2+ to promote CCl bond cleavage. Consequently, the dechlorination performance was improved.

Single-atom Pd loaded on nitrogen-doped carbon as a bifunctional catalyst for the electrochemical degradation of 2,4-dichlorophenol

A nitrogen-doped carbon-anchored single atom Pd (SA-Pd/NC) electrocatalyst was prepared by pyrolysis of Pd-doped zeolite imidazolite framework (ZIF) precursors. The SA-Pd/NC catalyst was characterized by high-resolution transmission electron microscopy, aberration-corrected high-angle annular dark field scanning transmission electron microscopy, X-ray diffraction, X-ray photoelectron spectroscopy and Raman spectrometry and applied in the degradation of 2,4-dichlorophenol (2,4-DCP). We demonstrate that the SA-Pd/NC catalyst has dual functions, with both electrocatalytic hydrodechlorination (ECH) activity and in situ heterogeneous electro-Fenton (hetero-EF) activity even at low current density. The ECH process was the main pathway of 2,4-DCP degradation without oxygen aeration. Under aerobic conditions, hetero-EF coupling with ECH occurred simultaneously on the SA-Pd/NC cathode, i.e., the dechlorination product phenol was oxidatively degraded with the reactive oxygen species produced on the electrode. The degradation rate of 2,4-DCP and TOC removal rate reached 95.1% and 37.6%, respectively, with a current density of 2.5 mA cm−2 and an initial pH of 3.0. This work provides the design of bifunctional single-atom catalysts for 2,4-DCP degradation, including dechlorination and mineralization.

Ultra-low-loading Pd nanocrystals modified Ni foam electrode for efficient electrochemical hydrodechlorination

Pd nanocrystals modified Ni foam electrode (Pd NCs/Ni) with Pd loading of 12.5 μg cm−2 was prepared via electrodeposition and exhibited higher mass activity in the electrochemical hydrodechlorination. With the assistance of polyvinylpyrrolidone and bromide ion, Pd nanocrystals with abundant catalytic sites were uniformly anchored on both the inner and surfaces of the Ni foam. Six chlorophenols were rapidly dechlorinated to phenol using Pd NCs/Ni with high conversion (> 98%) and selectivity (∼ 100%). Pd NCs/Ni showed broad applicability to various reaction conditions and maintained stable activity over 12 consecutive cycles. The electrochemical hydrodechlorination follows the indirect dechlorination mechanism. Hydrogenolysis between adsorbed H and chlorophenols is the rate-controlling step. Low coordination Pd atoms on the faces of Pd (220) and Pd (311) are the main catalytic sites. The study provides a valuable insight of high performance and low cost Pd modified electrode and its catalytic mechanism in electrochemical hydrodechlorination.

Enhanced performance of an in-situ synthesized Pd/N-TiO2/Ti cathode for electrocatalytic hydrodechlorination

The generation capacity of adsorbed reactively atomic hydrogen (Hads*) and the electrocatalytic hydrodechlorination (ECHD) activity can be improved by adjusting the electronic structure of Pd during the metal-support interaction. In addition, the durability and conductivity of the cathode significantly affect the ECHD efficiency. In this study, nitrogen-doped titanium dioxide (N-TiO2) is synthesized in situ using Ti sheet as matrix. Pd is loaded on N-TiO2/Ti electrode by hydrothermal synthesis self-assembly. The in-situ synthesized Pd/N-TiO2/Ti electrode can improve the atomic utilization rate of Pd and omit the insulating polymer binder during preparation. Moreover, it has unique advantages, such as large electrochemically active area, high stability and electrical conductivity. Under the condition of Pd loading of 0.75 mg, 84.09 % removal of 2,4-DCP (0.61 mM) is achieved on the electrode within 4 h at a potential of − 1.24 V (vs Hg/Hg2SO4). Finally, the Pd/N-TiO2/Ti cathode with good durability and high specific catalytic activity has a promising application prospect in electrocatalytic dehalogenation.

Palladium (Pd0) Loading-Controlled Catalytic Activity and Selectivity for Chlorophenol Hydrodechlorination and Hydrosaturation.

Reductive catalysis by zero-valent palladium nanoparticles (Pd0NPs) has emerged as an efficient strategy for promoting the detoxification of chlorophenols (CPs) via hydrogenation. Most studies achieved hydrodechlorination of CP to phenol for detoxification, but it requires considerably high energy input and harsh conditions to further hydrosaturate phenol to cyclohexanone (CHN) as the most desired product for resource recovery. This study documented 4-CP hydrodechlorination and hydrosaturation catalyzed by Pd0NPs deposited on H2-transfer membranes in the H2-based membrane catalyst-film reactor, which yielded up to 99% CHN selectivity under ambient conditions. It was further discovered that the Pd0 morphology and size, both determined by Pd0 loading, were the key factors controlling the catalytic activity and selectivity: while sub-nano Pd particles catalyzed only 4-CP hydrodechlorination, Pd0NPs were able to catalyze the subsequent hydrosaturation that requires more Pd0 reactive sites than hydrodechlorination. In addition, better dispersion of Pd0, caused by lower Pd0 loading, yielded higher activity for hydrodechlorination but lower activity for hydrosaturation. During the 15 day continuous tests, the substantial product from 4-CP hydrogenation was constantly phenol (>98%) for 0.2 g-Pd/m2 and CHN (>92%) for 1.0 g-Pd/m2. This study opens the door for selectively manipulating desired products from Pd0-catalyzed CP hydrogenation under ambient conditions.

Electronic structure engineering of bimetallic Pd-Au alloy nanocatalysts for improving electrocatalytic hydrodechlorination performance

Regulating the electronic structure of Pd catalyst for promoting electrocatalytic hydrodechlorination is promising but still challenging. Herein, we propose an electronic structure engineering for accelerating the hydrodechlorination process by desigining bimetallic Pd-Au alloy nanoparticles (ANs) with electron-rich Pd as high-efficiency electrocatalysts. The Pd7Au3 ANs exhibits the highest electrocatalytic hydrodechlorination activity, the 4-chlorophenol hydrodechlorination efficiency reaches 98.35 % within 4 h, and the mass activity is 0.47 h−1 mgPd−1, which is 7.83 times as high as that of pure Pd nanoparticles. The experimental characterization and theoretical calculation reveal that Pd atoms “obtain” electrons and gold atoms “lose” electrons in Pd-Au ANs, resulting in the formation of electron-rich Pd, which promotes the desorption of the dechlorination product phenol and accelerates the renewal of active sites. This work provides an electronic structure engineering strategy of Pd-based electrocatalysts for electrocatalytic hydrodechlorination, which opens a new opportunity for rational design and construction of high-performance hydrodechlorination catalysts.

Generation of atomic hydrogen by Ni-Fe hydroxides: Mechanism and activity for hydrodechlorination of trichloroethylene

Atomic hydrogen (H•) is highly reactive for the hydrodechlorination of trichloroethylene (TCE). In this work, we found that the coprecipitation of Ni2+ and Fe2+ at neutral pH led to an unprecedented catalytic generation of H•. The generated H• effectively dechlorinate TCE to nontoxic ethylene and ethane, and Fe2+ is the only electron donor. The abundant adsorbed H• produced with a Ni/Fe ratio of 0.4 enhances hydrogen evolution reaction causing a low efficiency for hydrodechlorination. In contrast, the active absorbed H• is generated in the crystal lattice of Ni-Fe hydroxides with a Ni/Fe ratio of 3.0 causing highly efficient hydrodechlorination of TCE. This work not only reveals the mechanism of catalytic hydrodechlorination by Ni-Fe hydroxides at neutral pH, but also provides a novel approach to detoxify TCE in contaminated water using facile precipitated Ni-Fe hydroxides.

Designing an Electron-Deficient Pd/NiCo2O4 Bifunctional Electrocatalyst with an Enhanced Hydrodechlorination Activity to Reduce the Consumption of Pd.

Reducing the Pd loading on electrodes is critical in the electrocatalytic hydrodechlorination (EHDC) of chlorinated organic compounds (COCs). The EHDC reaction of COCs on Pd involves three steps: H* formation, H* adsorption, and dechlorination. It has been established that the initial hydrogen evolution reaction (HER) occurs on Pd0 and the dechlorination steps occur on Pd2+. A strategy is proposed to design new electrodes by adding a reducible HER-active interlayer to replace Pd0, fulfilling the responsibility of producing hydrogen, and to facilitate the formation of more Pd2+ for following C-Cl bond cleavage. Keeping the atomic hydrogen adsorption energy on the Pd/interlayer similar to that on pure Pd is also necessary for H* adsorption as well as to maintain a high EHDC activity. For the first time, the NiCo2O4-interlayer-modified Pd/Ni-foam electrode was applied in the EHDC of COCs, which enhanced the EHDC efficiency to 100% within 90 min and reduced 88.6% of Pd consumption. The Pd/NiCo2O4/Ni-foam electrode with enhanced EHDC activity was also observed with almost 100% product selectivity and good stability. A synergistic mechanism is proposed for the enhanced EHDC activity on the Pd/NiCo2O4/Ni-foam. This work offers a simple and useful strategy to design robust electrocatalysts for the EHDC of COCs.

Ordered Mesoporous Carbon as a Support for Palladium-Based Hydrodechlorination Catalysts

Ordered mesoporous carbon (OMC) was employed as a support for palladium nanoparticles in catalysts for the gas phase hydrodechlorination (HDC) of trichloromethane (TCM). 1 wt% palladium was incorporated using three methods: incipient wetness (IW); a dilute solution (DS) method; and a solid-liquid (SL) method. The effect of the preparation method on catalyst structure and activity was investigated. Catalyst composition and nanostructure were studied using gas physisorption, high specification transmission electron microscopy and X-ray photoelectron spectroscopy. Catalytic conversion and product selectivities were determined in steady-state activity tests at temperatures between 70 and 300 °C. Two of the catalysts (IW and DS) showed excellent dispersion of fine Pd nanoparticles of average diameter ~2 nm. These materials showed excellent activity for HDC of TCM which compares favourably with the performance reported for Pd on amorphous carbon catalysts. In addition, they showed relatively high selectivities to the more valuable higher hydrocarbons. However, the SL method gave rise to catalysts with larger particles (~3 nm) and a less uniform palladium distribution. This resulted in lower conversion and lower selectivities to higher hydrocarbons and in more severe catalyst deactivation at the highest reaction temperatures.

In-Situ doping-induced crystal form transition of amorphous Pd–P catalyst for robust electrocatalytic hydrodechlorination

Developing catalysts with high activity and high atom utilization as well as exploring catalytic active sites are the biggest challenges for the electrocatalytic hydrodechlorination technology. Herein, a novel strategy of Pd crystal transformation induced by in-situ doping was proposed, and a series of amorphous Pd–P nanoparticles (NPs) with controllable coordination environment were synthesized successfully. The amorphous Pd–P NPs catalyst exhibits the highest activity for electrocatalytic hydrodechlorination and good cycling stability when the Pd–P coordination number is 3 and the Pd–Pd coordination number is 4. The 4-chlorophenol hydrodechlorination efficiency of Pd–P-60 NPs reaches 100 % within 2 h, and the mass activity is 8.58 min−1 g−1, which is 5.57 times as high as that for crystalline Pd NPs catalyst. Theoretical calculation shows that Pd–P catalyst facilitates the desorption of phenol and weakens the toxic effect on active sites. Density of states indicate that the doping of P results in a downshift of d-band center, facilitating the desorption of phenol. This work discovers the crystal effect and coordination effect of amorphous Pd–P NPs for electrocatalytic hydrodechlorination, which lays an important theoretical foundation for the design and development of high-performance Pd-based catalysts for electrocatalytic hydrodechlorination.

Hydrodechlorination of CHClF2 (HCFC-22) over Pd–Pt Catalysts Supported on Thermally Modified Activated Carbon

Commercial activated carbon, pretreated in helium at 1600 °C and largely free of micropores, was used as a support for two series of 2 wt.% Pd–Pt catalysts, prepared by impregnating the support with metal acetylacetonates or metal chlorides. The catalysts were characterized by temperature-programmed methods, H2 chemisorption, X-ray diffraction (XRD), X-ray photoelectron spectroscopy (XPS), and scanning transmission electron microscopy (STEM) with energy dispersive spectroscopy (EDS). Overall, the results confirmed the existence of well-dispersed Pd–Pt nanoparticles in the bimetallic catalysts, ranging in size from 2 to 3 nm. The catalysts were investigated in the gas phase hydrodechlorination of chlorodifluoromethane (HCFC-22). In this environmentally relevant reaction, both the ex-chloride and ex-acetylacetonate Pd–Pt/C catalysts exhibited better hydrodechlorination activity than the monometallic catalysts, which is consistent with the previous results of hydrodechlorination for other chlorine-containing compounds. This synergistic effect can be attributed to the electron charge transfer from platinum to palladium. In general, product selectivity changes regularly with Pd–Pt alloy composition, from high in CH2F2 for Pd/C (70–80%) to the selective formation of CH4 for Pt/C (60–70%).

Hydrodechlorination and deep hydrogenation on single-palladium-atom-based heterogeneous catalysts

Halophenols are widely present in wastewater and groundwater, which threaten global water safety and human health. Importantly, hydrodechlorination (HDC) via heterogeneous catalysis is efficient and environmentally friendly. To further improve the catalytic performance and atomic economy, Pd-based single atom catalysts (SACs) were prepared through electrostatic attraction and used in the catalytic HDC for the first time. By substituting different defect-rich supports, the coordination environments of atomically dispersed Pd-atoms were mediated with different catalytic performance. The results demonstrate that the Pd/CeO2 SACs have the highest HDC activity and remain stable during cycle tests. The moderately intensive metal-support interactions (MIMSI) effect was identified as a critical factor for the higher activity of SACs, unlike the traditional strong metal-support interaction effect. The HDC process on Pd SACs was further investigated by combining kinetic experiments with density functional theory calculations, which facilitated the recognition of HDC intermediates and an understanding of the reaction mechanism.

Catalytic properties of transition metals modified nanoscale zero-valent iron for simultaneous removal of 4-chlorophenol and Cr(VI): Efficacy, descriptor and reductive mechanisms

Since chlorophenols (CPs) and Cr(VI) are two types of common pollutants in the environment, developing an effective approach to remove these contaminants has important benefits for public health. However, few efforts have been made so far. In this study, we prepared nanoscale zero-valent iron (nZVI) and a series of bimetallic nanoparticles (transition-metal modified nZVI) to investigate their catalytic properties for the simultaneous removal of 4-chlorophenol (4-CP) and Cr(VI). While nZVI enabled a fast removal of Cr(VI), it had a poor dechlorination ability. However, effective simultaneous removal of 4-CP and Cr(VI) was achieved with the transition metal modified nZVI, especially in the Pd/Fe bimetallic system. The enhanced catalytic activity of transition metal modified nZVI was primarily attributed to the formations of numerous nano-galvanic cells and atomic hydrogen species that facilitated electron transfer in the reaction system and played a key role in triggering the C-Cl bond cleavage, respectively. According to the dechlorination ability, the transition-metal catalysts examined in this study can be divided into three groups in descending order: the first being Pd and Ni, the second including Cu and Pt, while the last consisting of Au and Ag. The catalytic hydrodechlorination activity of bimetals can be well described by the volcano curve and rationally explained by the hydrogen adsorption energies on the metals, and was severely impaired by increasing Cr(VI) concentrations. Characterization results validated the formations of Fe(III)-Cr(III) hydroxide/oxyhydroxide on the bimetals surface after reacting with 4-CP and Cr(VI). This work provides the first insight into the catalytic properties of transition-metal modified nZVI for the effective removal of combined pollutants.

Recyclable heterogeneous Pd nanoparticles supported on plant polyphenol-modified γ-Al2O3 for hydrodechlorination of 2,4-dichlorophenols

Heterogeneous Pd catalysts were developed by immobilizing Pd nanoparticles (Pd NPs) onto plant polyphenol (bayberry tannin, BT) decorated γ-Al2O3. The abundant hydroxyls of plant polyphenols were capable of stabilizing the Pd NPs. Transmission electron microscopy observation confirmed that the Pd NPs with the diameter of 3.75 ± 0.5 nm were highly dispersed in the catalyst. The as-prepared Al2O3–BT–Pd catalysts were found to be highly active in mild hydrodechlorination (HDC) of 2,4-dichlorophenols (DCPs) using formic acid as a hydrogen source. The 2,4-DCPs were completely dechlorinated in 4 h at 30°C and under atmospheric pressure. During the catalytic HDC, the stabilizing capability of BT successfully prevented the leakage and aggregation of Pd NPs, thus ensuring a high cycling stability with stable and high catalytic activity. The Al2O3–BT–Pd catalysts were recycled six times, without obvious loss of activity. In the sixth cycle, the catalytic HDC yield still reached 98.29% under the same reaction conditions, superior to the control catalysts, including γ-Al2O3 supported Pd NPs (Al2O3–Pd) and powdered activated carbon supported Pd NPs (AC–Pd). Furthermore, the Al2O3–BT–Pd also showed high activity in the mild catalytic HDC of 2,4,6-trichlorophenols and chlorobenzene derivatives. Our results demonstrated efficient catalysts to address the environmental issue of chlorophenol pollution.

The role of nickel traces in fine chemicals for hydrodechlorination reactions with zero-valent iron

Two different types of nanoscale zero-valent iron (nZVI) were synthesized according to the same standard procedure: reduction of dissolved Fe(II) with sodium borohydride – one starting from a commercial iron salt having pro analysi (p.a.) purity, and another from microscale iron with an extraordinary degree of purity, derived from the decomposition of iron pentacarbonyl. The ZVI samples exhibited similar hydrodechlorination activities towards hexachlorobenzene as an example for higher chlorinated aromatics and carbon tetrachloride as an aliphatic model substance. However, only the sample derived from the iron salt (p.a.) was able to dechlorinate monochlorobenzene (MCB) at a measurable rate whereas the nZVI derived from the very pure precursor (nZVIpure) did not show this ability. Closer inspection lead us to the finding that the observed activity could be attributed to a Ni content as small as 0.005 wt-% of the former nZVI sample which was named nZVIimpure accordingly. It was demonstrated that such a small amount of Ni develops its catalytic activity for the hydrodechlorination of MCB only under acidic conditions. Although acidic conditions might not be common for studies with ZVI, higher Ni contents (e.g. of reagent grade iron sources) could have an influence under ambient conditions.

Improving the activity in hydrodechlorination of Pd/C catalysts by nitrogen doping of activated carbon supports

Aqueous phase 4-chlorophenol hydrodechlorination reaction was used to study the effect of N-doping of activated carbon support on the catalytic activity of Pd catalysts. Activated carbon was doped using pyridine and 1,10-phenantroline, reaching nitrogen contents of 0.42–1.22 and 1.35–4.19 % (w), respectively. All catalysts (0.75 % Pd w, carbon basis) showed relatively large Pd nanoparticles (35−55 nm), but they exhibited fast and complete 4-chlorophenol disappearance in batch experiments. In runs at 30 °C 4-chlorophenol disappearance was mainly ascribed to hydrodechlorination, although N-doping of the support also increased adsorption. Catalysts with supports doped with pyridine yielded higher 4-chlorophenol disappearance rate in spite of lower bulk nitrogen content, however they showed higher concentration of nitrogen species at the external surface and lower loss of surface area during the doping. 4-chlorophenol disappearance rate was boosted at 60 °C, with minor differences between catalysts with undoped and N-doped supports, but generation of cyclohexanone was only observed for the ones with doped support. Phenol generation simultaneous to 4-chlorophenol disappearance was observed with all the catalysts. However, subsequent hydrogenation to cyclohexanone ocurred only with the catalysts supported on N-doped activated carbon.

Chloroform Hydrodechlorination on Palladium Surfaces: A Comparative DFT Study on Pd(111), Pd(100), and Pd(211)

Palladium has been shown to be an effective catalyst for chloroform hydrodechlorination, which serves as a promising treatment method for industrial chloroform waste. To investigate the structure sensitivity of this chemistry on Pd surfaces, we performed a density functional theory (DFT, GGA-PW91) study of the chloroform hydrodechlorination reaction on three distinct facets: Pd(111), Pd(100), and Pd(211). Based on the DFT results, the binding strengths of most surface intermediates generally increase in the following order: Pd(111) < Pd(100) < Pd(211). On all three Pd facets, methane is formed as the preferred reaction product through a pathway in which CHCl3* is fully dechlorinated to CH* first, and then hydrogenated to CH4. We constructed potential energy diagrams (PED) and compared the reaction energetics for chloroform hydrodechlorination on the three Pd facets. We propose that the competition between the desorption of chloroform and its initial dechlorination to form CHCl2* likely determines the hydrodechlorination activity of the catalyst. On Pd(111), the desorption of chloroform is energetically favored over its dechlorination while the dechlorination barriers are lower than the desorption barriers on Pd(100) and Pd(211). On the other hand, Pd(100) and Pd(211) bind atomic chlorine stronger and also catalyze the formation of atomic carbon effectively; both are potential site-blocking species. Our results suggest that the more open facets and step edge sites of a Pd nanoparticle may carry higher intrinsic activity towards chloroform hydrodechlorination than the close-packed facets, yet these under-coordinated sites are more prone to catalyst poisoning.

Carbon-Supported Palladium–Cobalt Catalysts in Chlorobenzene Hydrodechlorination

Bimetallic and monometallic composite materials containing ultra-microporous carbon, the structure of which is similar to that of activated carbon, Pd0, CoO, and possibly PdCoO2 nanoparticles are synthesized via the pyrolysis of sawdust impregnated with a solution of palladium nitrate and cobalt nitrate. According to TEM, the size distribution of metal-containing particles in the bimetallic catalysts is slightly broader than the one in monometallic catalysts, and the average particle size of the former is larger than that of the latter. Some metal-containing particles are localized on the support surface, while others are pressed into the support matrix or coated with a carbon shell. Bimetallic catalysts exhibit high stability and activity in gas-phase chlorobenzene hydrodechlorination (HDC) to form mostly benzene; in the presence of all cobalt-containing catalysts, chlorobenzene conversion is greater than 85% throughout the investigated range of temperatures (150–300°C). XPS shows that during HDC, the metals are chlorinated and the carbon shells of the metal particles are partially degraded under the action of hydrogen chloride. This is accompanied by the emergence of active metal particles on the catalyst’s surface. Introducing cobalt helps reduce the deactivation of the catalysts’ palladium-containing sites and ensures the formation of additional active sites based on CoO.

Catalytic Performance of Nitrogen-Doped Activated Carbon Supported Pd Catalyst for Hydrodechlorination of 2,4-Dichlorophenol or Chloropentafluoroethane.

Nitrogen-doped activated carbon (N-AC) obtained through the thermal treatment of a mixture of HNO3-pretreated activated carbon (AC) and urea under N2 atmosphere at 600 °C was used as the carrier of Pd catalyst for both liquid-phase hydrodechlorination of 2,4-dichlorophenol (2,4-DCP) and gas-phase hydrodechlorination of chloropentafluoroethane (R-115). The effects of nitrogen doping on the dispersion and stability of Pd, atomic ratio of Pd/Pd2+ on the surface of the catalyzer, the catalyst’s hydrodechlorination activity, as well as the stability of N species in two different reaction systems were investigated. Our results suggest that, despite no improvement in the dispersion of Pd, nitrogen doping may significantly raise the atomic ratio of Pd/Pd2+ on the catalyst surface, with a value of 1.2 on Pd/AC but 2.2 on Pd/N-AC. Three types of N species, namely graphitic, pyridinic, and pyrrolic nitrogen, were observed on the surface of Pd/N-AC, and graphitic nitrogen was stable in both liquid-phase hydrodechlorination of 2,4-DCP and gas-phase hydrodechlorination of R-115, with pyridinic and pyrrolic nitrogen being unstable during gas-phase hydrodechlorination of R-115. As a result, the average size of Pd nanocrystals on Pd/N-AC was almost kept unchanged after liquid-phase hydrodechlorination of 2,4-DCP, whereas crystal growth of Pd was clearly observed on Pd/N-AC after gas-phase hydrodechlorination of R-115. The activity test revealed that Pd/N-AC exhibited a much better performance than Pd/AC in liquid-phase hydrodechlorination of 2,4-DCP, probably due to the enhanced stability of Pd exposed to the environment resulting from nitrogen doping as suggested by the higher atomic ratio of Pd/Pd2+ on the catalyst surface. In the gas-phase hydrodechlorination of R-115, however, a more rapid deactivation phenomenon occurred on Pd/N-AC than on Pd/AC despite a higher activity initially observed on Pd/N-AC, hinting that the stability of pyridinic and pyrrolic nitrogen plays an important role in the determination of catalytic performance of Pd/N-AC.